STRATIGRAPHY, STRUCTURE, AND
METAMORPHISM IN THE MONADNOCK
QUADRANGLE, NEW HAMPSHIRE
BY PETER J. THOMPSON
CONTRIBUTION NO. 58
DEPARTMENT OF GEOLOGY 8c GEOGRAPHY
UNIVERSITY OF MASSACHUSETTS
AMHERSTt MASSACHUSETTS
-~~=======--
STRATIGRAPHY, STRUCTURE, AND METAMORPHISM
IN THE MONADNOCK QUADRANGLE,
NEW HAMPSHIRE
by
Peter J. Thompson
Contribution No. 58
Department of Geology and Geography
University of Massachusetts
Amherst, Massachusetts
August, 1985
------
ii
©
Peter James Thompson
All Rights Reserved
This research was supported in part by
grants from the National Science Foundation:
Grants EAR-79-15246
EAR-84-10370
(to Peter Robinson)
Grant EAR-81-16197
(to Peter Robinson and J.M. Rhodes)
iii
TABLE OF CONTENTS
ABSTRACT. . .
1
INTRODUCTION.
3
Location and Physiography.
Purpose.
Previous Work. .
Methods of Study
Acknowledgments.
STRATIGRAPHY. . . . .
SWANZEY GNEISS, AMMONOOSUC VOLCANICS AND PARTRIDGE FORMATION
CLOUGH QUARTZITE AND FITCH FORMATION
RANGELEY FORMATION . . . . . . . . .
Description and Distribution of Rock Types
Sulfidic schist and gneiss.
Conglomerates . . . . . . . . .
Calc-silicate granulite . . . .
Gray schist and gneiss, granulite and augen schist.
Thickness. . . . . .
Age and Correlation.
Derivation . . . . .
PERRY MOUNTAIN FORMATION
Description and Distribution of Rock Types
Thickness. . . . . .
Age and Correlation.
Derivation . . . . .
FRANCESTOWN FORMATION.
Description and Distribution of Rock Types
Thickness . . . . . .
Age and Correlation.
Derivation . . . . .
WARNER FORMATION . . .
Description and Distribution of Rock Types
Thickness . . . . . .
Age and Correlation.
Derivation . . . . .
LITTLETON FORMATION . .
Description and Distribution of Rock Types
Thickness . . . . . .
Age and Correlation.
Derivation
INTRUSIVE ROCKS
INTRODUCTION
KINSMAN GRANITE.
SPAULDING TONALITE AND RELATED ROCKS
3
5
6
9
10
11
11
14
16
16
17
20
24
24
25
28
29
30
30
34
34
35
35
35
38
38
39
40
40
44
44
45
45
45
53
53
55
56
56
58
62
iv
FITZWILLIAM GRANITE.
MICRODIORITE DIKES .
Mineralogy . . . . .
Field Descriptions
Contact Relations.
PEGMATITE. . . .
TOURMALINE VEINS
DIABASE DIKE . .
STRUCTURAL GEOLOGY.
66
69
69
69
73
73
74
74
75
INTRODUCTION . .
75
DESCRIPTION OF MINOR STRUCTURAL FEATURES
75
Planar Features.
75
Bedding .
75
Foliation . .
98
Mylonitic foliation
98
Crenulation cleavage.
98
Joints. . . . . . .
98
Linear Features . . . . . .
99
Mineral lineations. .
99
Intersection and crenulation lineations
99
Minor folds . . . . . . . .
99
GEOMETRICAL ANALYSIS OF STRUCTURAL DATA.
100
Structural Data in Subareas. .
100
Construction of Cross Sections
100
PHASES ONE AND TWO: FOLD NAPPES AND THRUST NAPPES
100
Introduction . . .
100
Tectonic Levels . .
100
Monadnock Syncline
102
Folds on Mt. Monadnock
103
Other Map-scale Nappe-stage Folds.
108
Howe Reservoir syncline .
108
Dublin Pond syncline . . . . .
108
Gilson Pond anticline and Meade Brook syncline.
108
111
Nappe-stage folds in the Derby Hill window.
.
Nappe-stage Thrust Faults . . . . . . . . . . .
111
Brennan Hill fault . . . . . . . . . . . .
111
113
Chesham Pond fault and Derby Hill window.
115
Thorndike Pond fault zone . .
116
PHASES THREE AND FOUR: BACKFOLDING . . .
Introduction . . . . . . . . . . . . .
116
116
Intermediate Stage Folds, Mt. Monadnock.
Asymmetric folds . . .
116
Boudinage . . . . . . . . . . . . .
118
118
Thoreau Bog syncline. . . . . . . .
121
Intermediate Stage Folds, Poole Reservoir Area
Intermediate Stage Folds and Mylonitization, Gilson Pond Area. 125
125
Intermediate Stage Folds, Southeast of Thorndike Pond.
126
Beech Hill Anticline . . . . . . . .
127
Intermediate Stage Folds in the Troy Area . . . .
v
Intermediate or Dome Stage Folds in the Derby Hill Area.
Summary of Backfolding Episode
PHASE FIVE: DOMING.
Introduction . . .
Marlboro Syncline.
Rindge Area . . .
Late Open Folds, Mt. Monadnock
Cobb Hill. .
Discussion . . .
. . . . .
LATE PALEOZOIC INTRUSIONS AND DEFORMATION.
MESOZOIC FAULTING. . . . . . . . . . . . .
SUMMARY AND REGIONAL STRUCTURAL IMPLICATIONS .
METAMORPHISM. .
INTRODUCTION
PELITIC ROCKS.
Aluminum Silicate Polymorphs
Metamorphic Zones.
Zone II .
Zone III.
Zone IV
Zone V. .
Zone VI .
Assemblages in Sulfidic Schists.
Evidence for a Retrograde Episode.
Garnet Zoning in Zone III.
Assemblage (1) . . . . .
Assemblage (2) . . . . .
Coticule garnet zoning.
Temperature Estimates.
MK-432. . . .
MK-629 . . . .
Pressure Estimates
CALC-SILICATE ROCKS.
Mineralogy and Chemography of MND-8-74
Disequilibrium Textures and Reactions.
CORRELATION OF METAMORPHISM AND DEFORMATION.
Age of Prograde Metamorphism .
. . . .
Age of Retrograde Metamorphism and "Permian Disturbance"
128
128
130
130
131
131
131
132
132
134
• 134
136
138
138
138
138
140
140
144
145
146
. 147
14 7
148
150
150
157
160
161
161
164
165
166
166
170
174
174
176
CONCLUSIONS AND RECOMMENDATIONS FOR FUTURE RESEARCH
177
REFERENCES CITED.
179
APPENDIX 1.
189
APPENDIX 2.
190
vi
TABLES
la.
lb.
lc.
ld.
2.
3.
4.
Sa.
Sb.
6a.
6b.
7.
8.
9.
10.
11.
12.
13.
14.
15.
16.
Estimated modes of the Rangeley Formation: sulfidic schist
and gneiss . . • • . . . . . . . . . . . . . . •
18
Conglomerate localities in the Rangeley Formation. .
21
Estimated modes of the Rangeley Formation: conglomerates
and calc-silicate pods . . . . . •
• . . . .
22
Estimated modes of the Rangeley Formation: gray schist and
26
gneiss . .
. .
. . . . . .
.
. . •
· ·
Important exposures of the Perry Mountain Formation.
31
Estimated modes of the Perry Mountain Formation.
32
Estimated modes of the Francestown Formation . . .
36
Estimated modes of the lower part of the Warner Formation.
41
42
Estimated modes of the upper part of the Warner Formation.
Estimated modes of the lower part of the Littleton Formation
46
48
Estimated modes of the upper part of the Littleton Formation
Estimated modes of the Kinsman Granite .
. . . . .
. . .
60
Estimated modes of the Spaulding Tonalite and related rocks.
64
Estimated modes of the Fitzwilliam Granite
. .
. . . .
67
Estimated modes of microdiorite dikes. . .
. ..•...
70
Summary of structural history in the Monadnock quadrangle.
76
Electron microprobe analyses of garnet from schists. . . .
• 152
Estimated T from garnet-biotite and cordierite-garnet pairs.
162
Electon microprobe analyses from the core of calc-silicate
granulite pod MND-8-74
. .. .
168
List of station codes and numbers.
189
List of mineral abbreviations.
• 189
ILLUSTRATIONS
Figure
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
12.
13.
14.
Page
Index map of study area and adjacent parts of New England.
4
Stratigraphic correlation diagram across the Silurian
tectonic hinge . . . . . . . . . . . . . . . • .
7
12
Stratigraphic column, central Monadnock quadrangle .
Correlation chart for Silurian-Devonian rocks of New Hampshire,
showing fossil control . . . . . . . . . . . . . . . • . . 15
Outcrop sketches of penecontemporaneous sedimentary structures,
Mt. Monadnock . . . • . . . . . . . . . . . . . . .
52
Measured sections of the "Seven Quartzites", Littleton Fm.
54
Streckeisen plot of representative intrusive rocks .
57
Outcrop sketch of Spaulding-Kinsman contact, MK-482 . .
63
Map of mafic dikes on Mt. Monadnock. . . . .
. ...
72
Summary maps of structural features, Monadnock quadrangle:
77
lOa. Axial traces of major folds and faults . . . . . . .
78
lOb. Subareas, average foliation and sillimanite lineations
Equal area diagrams of structural data for the
79
Monadnock quadrangle . . . . . . .
82
Equal area diagrams of structural data for each subarea . •
101
Simplified geologic map, showing names of major structures
104
Outcrop sketch of nappe-stage folds in Seven Quartzites . .
vii
ILLUSTRATIONS (CONT'D)
Figure
15. Outcrop sketch of "Billings fold", nappe-stage syncline
near Mt. Monadnock summit . . . . . . . . .
16. Geologic map of summit area of Mt. Monadnock .
17. Proposed schematic cross section after nappe-stage
deformation, showing proposed thrust faults . .
18. Geologic map of area south of Hurricane Hill, showing
alternate fault interpretation rather than folds
19. Geologic map of south part of Derby Hill window.
20. Geologic map of north part of Derby Hill window.
21. Outcrop sketch of asymmetric backfold . . . . . .
22. Outcrop sketches of boudinaged schist beds . . .
23. Outcrop sketch of NE-plunging backfold (MK-799).
24. Geologic map of Poole Reservoir area, Monadnock State Park
25. Outcrop sketch of disharmonic folds in Perry Mountain Fm.
26. Interference pattern of south-plunging backfolds and
dome-stage folds south of Troy . . . . . . .
27. Proposed schematic backfolding sequence. . . . .
28. Comparison of equal area diagrams of sillimanite and mica
lineations from two structural domains . . .
29. Equal area diagram of quartz veins, west of Rindge
30. Metamorphic map of Monadnock quadrangle . . . . . .
31. P-T trajectories . . . . . . . . . . . . . . . . . .
32. Chemographic representation of mineral assemblages,
Zones II-VI, and the AKFM tetrahedron . . . . . .
33. Contoured chemical map of zoned garnets, MK-432 . . .
34. Zoning trend for MK-432 garnets on Fe-Mn-Mg ternary plot
35. Contoured chemical map of zoned garnet, MK-629 . . . .
36. Zoning trends for MK-629 and MK-6 garnets of Fe-Mn-Mg
ternary diagrams . . . . . . . . . . . . . . . . .
37. Bustamite and clinopyroxene compositions in the system
CaSi03-FeSi03-MnSi03-MgSi03 . . . . . . . . . . . .
38. Mineral compositions, MND-8-74, on ternary diagrams . .
39. Disequilibrium textures and proposed reactions, MND-8-74
40. T-Xc 02 equilibrium curves for calc-silicate assemblages.
41.
42.
105
107
109
110
112
114
117
119
120
122
123
127
129
133
135
139
141
142
151
156
158
159
167
171
172
173
190
191
Townships and station codes, Monadnock quadrangle.
Fence diagram, Mt. Monadnock summit . . . . . . . .
Plate
1. Bedrock Geologic Map, Monadnock Quadrangle, in four
quadrants: SE, SW, NE, NW . . . . . . . . .
2. Planar Structural Features, Monadnock Quadrangle.
3. Linear Structural Features, Monadnock Quadrangle.
4. Geologic Map and Key to Plates . . . . . . . . . .
5. Geologic Cross Sections, A-A', B-B', C-C', D-D', E-E'
Page
IN
IN
IN
IN
IN
POCKET
POCKET
POCKET
POCKET
POCKET
1
ABSTRACT
The layered rocks of the Monadnock quadrangle, New Hampshire, have
been mapped based on a stratigraphic sequence which correlates well
with the sequence recently described by Lyons and Hatch in central New
Hampshire and earlier by Moench in western Maine. Three distinctive
Silurian units separate the Silurian Rangeley Formation from the
Devonian Littleton Formation. In order from oldest to youngest these
are: the Perry Mountain Formation, thinly bedded schist and white
quartzite; the Francestown Formation, mainly sulfidic calc-silicate
granulite with subordinate sulfidic schist; and the Warner Formation,
consisting of a lower clean calc-silicate granulite and an upper feldspathic granulite. The Rangeley Formation includes sulfidic to grayweathering gritty schist with calc-silicate pods, granulite beds, quartzpebble conglomerate lenses near the top, and granulite-matrix conglomerate horizons near the lowest exposed part. The Littleton Formation
consists of gray-weathering schists and quartzites, with the proportion
of quartzite increasing upwards. A group of seven distinctively spaced
quartzite beds is folded into isoclinal folds, and it is this folded
sequence that forms the resistant summit of Mt. Monadnock.
The Silurian is represented by a much thinner sequence of rocks
along the west edge of the quadrangle, where the Clough Quartzite and
local Fitch Formation, probably correlative with the Silurian units
described above, overlie Ordovician rocks of the Keene dome. The quadrangle thus straddles a Silurian "tectonic hinge", which may have
behaved as a zone of weakness during Acadian deformation.
Intrusive rocks include the pre- or syn-tectonic (Devonian)
Kinsman Granite, syn-tectonic (Devonian) Spaulding Tonalite and related
rocks, and post-tectonic (?Mississippian) Fitzwilliam Granite. Granite
dikes and microdiorite dikes probably coeval with the Fitzwilliam cut
all generations of folds on Mt. Monadnock.
Five phases of Acadian deformation have affected the rocks of the
quadrangle. Fold nappes and then thrust faults transported rocks of the
~errimack trough (Rangeley through Littleton Formations) westward across
the tectonic hinge onto the thinner Bronson Hill sequence. The rocks
were then folded back toward the east in two complicated phases which
included mylonitization along the short limb of a major backfold, the
Beech Hill anticline. The final phase produced folds related to the
rise of the Keene dome at the west edge of the quadrangle. There is
evidence for several periods of movement along the four-kilometer-wide
Thorndike Pond fault zone, including nappe-stage ductile thrusting, eastdirected shear during the backfolding, late Paleozoic shear in the
Fitzwilliam Granite, and Mesozoic normal faulting and silicification.
A single occurrence of a presumably Mesozoic diabase dike was found in
a float block of Kinsman Granite.
2
The dominant foliation lies parallel to axial planes of nappe-stage
folds, and the peak metamorphism probably occurred during the early
backfolding. Assemblages in pelitic schists range from Zone II in the
west to Zone VI near the Kinsman Granite in the northeast, but Zones III
(sillimanite-biotite-garnet-muscovite) and IV (sillimanite-biotite-garnetmuscovite-K-feldspar) predominate in most of the quadrangle. Garnet and
biotite compositions in Zone III yield peak temperature estimates of
635-670° and evidence of re-equilibration during cooling and unloading.
A pressure of 6.3 kbar is indicated by garnet composition in equilibrium
with cordierite and sillimanite. A zoned calc-silicate pod from Zone III
contains bustamite in the core, as well as a variety of calc-silicate
minerals with interesting disequilibrium textural relationships.
3
" •• its summit is a bald rock; on some parts
of it are large piles of broken rocks • • • and
plumbago in large quantities."
-Jeremy Belknap, 1792
INTRODUCTION
Location and Physiography
The Monadnock quadrangle is located in southwestern New Hampshire
approximately twelve miles east of the Connecticut River and two miles
north of the Massachusetts state line (Figure 1, No. 13). The principle towns in the quadrangle are Jaffrey, Dublin, Harrisville,
Nelson, Marlboro, Troy, and Fitzwilliam (Appendix 1, Figure 41; Plates
1 and 4). Mt. Monadnock dominates the topography, its treeless summit
rising to 965 m (3165 ft.) near the center of the quadrangle. Three
watersheds meet at a point on the ridge extending south from the
mountain: the Ashuelot River drains the area to the west, the Contoocook River the area to the east, and tributaries of the Millers River
the area to the south. Water in the Contoocook eventually joins the
Merrimack River, while that in the other two empties into the Connecticut River.
Numerous small lakes and ponds dot the region, many of them artificially dammed so as to maintain higher water levels. In the south
there are numerous bogs and swamps, and the lakes are shallow. In the
north the lakes are mostly deeper and apparently occupy bedrock
basins.
The railroad bed from Hancock to Keene (Plates 1 NW and NE),
abandoned in 1938, affords some good bedrock exposure, as does the
more recently abandoned Boston and Maine line which parallels Rt. 12
(Plates 1 SW and SE). Four east-west highways cross the quadrangle:
Rt. 12 from Winchendon, Massachusetts, through Fitzwilliam toward
Keene; Rt. 124 from New Ipswich through Jaffrey to Marlboro; Rt. 101
from Peterborough through Dublin and Marlboro toward Keene; and Rt. 9
from Hillsboro diagonally across the northwest corner of the quadrangle. Rts. 202 and 137 run north-south through Jaffrey in the
eastern part of the quadrangle. Many secondary roads and old logging
roads form a network which provides good access to most areas.
Most of the region is heavily forested by either mixed hardwoods
and conifers, by stands of pine, or by fir and spruce. A few dairy
farms are still operating, but many pastures are growing up to
junipers and saplings. Juniper thickets, notably on Gap Mountain and
Bigelow Hill, are nearly impenetrable, and recent logging in some
areas makes for rough going. However, most outcrops are accessible,
and the summit of Mt. Monadnock provides approximately one half square
mile of continuous outcrop. The "tree line" is at about 723 m (2700
ft.), artificially lower than would be normal for these latitudes due
4
Fig. 1. Index map of areas and
quadrangles mentioned in text.
~
1. Littleton-Moosilauke
(Billings , 19 3 7)
2. Mt. Washington
(Billings, 1941)
3. Rumney SE
(Malinconico, 1982)
4. Holderness
(Englund, 1976)
5. Sunapee septum
(Dean, 1977)
6. Mt. Kearsarge
7. Alton-Berwick area
(Eusden et al., 1984)
8. Bellows Falls
(Kruger, 1946)
9. Lovewell Mtn. (Heald, 1950);
Gilsum-Marlow area
(Chamberlain, 1985)
10. Hillsboro
(Nielson, 1974)
11. Concord
(G. Duke, 1984)
12. Keene-Brattleboro
(Moore, 1949)
13. Monadnock
(Fowler-Billings, 1949)
Peterborough
14.
(Greene, 1970; E. Duke, 1984)
15. Vernon-Northfield area
(Elbert, 1984)
16. Orange area
(Robinson, 1963)
17. Tully body
(Pike, 1968)
18. Ashburnham-Ashby area
(Peterson, 1984)
19. Brooks Village breccia
(Morton, 1985)
20. Barre
(Tucker, 1977)
21. Ware
(Field, 1975)
22. Amherst block
(Jasaitis, 1983)
1«-1
l!b.J
White Mountain series
r:.J
L:::J
New Hampshire series
•
dome gneisses
5
to numerous fires in the early 1800's. The north slopes are densely
covered by young spruce trees where the forest is recovering from
extensive damage in the 1938 hurricane.
Landforms in the quadrangle are directly related to the underlying
geology (Plate 4). Mt. Monadnock itself is held up by quartzite beds
in the Littleton Formation, which are folded back on themselves to
form a thick resistant sequence. Irregular hills with elevations from
365 to 610 m and relatively thin glacial cover predominate in the
northern third of the quadrangle, where the rocks are poorly bedded
gneisses of the Rangeley Formation west of Harrisville, and Kinsman
Granite in the northeast. The two branches of Minnewawa Brook follow
east-west trending layered rocks between Dublin Pond and Marlboro
village, the south branch in a broad alluvial valley and the north
branch cutting gorges into bedrock.
The remainder of the quadrangle consists of lower topography with
moderate relief and a north-northeast grain parallel to the layering
of the metamorphic rocks, interrupted by still lower areas underlain
by intrusive igneous rocks. The Kinsman Granite, alone among the
intrusive rocks, forms more resistant hills and ridges, even in the
narrow bodies where the rock is strongly sheared (Plate 1 SE). The
ridge culminating in Little Monadnock Mountain (Plate 1 SW) is
parallel to a syncline extending south from Troy. Gap Mountain is
underlain by inclusions of metamorphic rocks surrounded by Spaulding
Tonalite. Roughly parallel to the western edge of the quadrangle, a
line of west-facing cliffs at about 300 m elevation marks the location
of the Clough Quartzite, separating the Monadnock area from the broad
valley and irregular hills to the west which are underlain by gneisses
of the Keene dome.
Drumlinoid hills of 335-365 m elevation and large areas of glacial
outwash and till obscure the bedrock in the east-central and southeast
parts of the quadrangle. A large ridge (elevation 365 m) made up of
Kinsman boulders obscures the south margin of the Cardigan pluton from
~Lake Skatutakee to the area north of Bonds Corner, east of Dublin.
Fowler-Billings (1949b) suggested this represents a moraine deposit.
The other important area of no outcrop extends from Beech Hill in
Dublin westward to north of Chesham.
Purpose
The main purpose of this thesis is to present the results of
mapping of the layered rocks in the Monadnock quadrangle, New Hampshire, based on field work during the summers of 1981, 1982 and 1983.
The area was suggested to me by Peter Robinson for several reasons.
It is a key area for stratigraphic correlations between central
Massachusetts, where Robinson and his students have been mapping
(Robinson, 1967; Pike, 1968; Field, 1975; Tucker, 1977; Peterson,
1984; Elbert, 1984), and central New Hampshire where John Lyons and
his Dartmouth students have been mapping (Nielson, 1974; Englund,
6
1974; Nelson, 1975; Lyons, 1979; Malinconico, 1982; E. Duke, 1984;
G. Duke, 1984) (Figure 1). Secondly, the Monadnock stratigraphy lies
between the Bronson Hill anticlinorium, with its Paleozoic sequence
originally defined in the Littleton quadrangle by Billings (1937), and
the Merrimack synclinorium, with its stratigraphic sequence which has
recently been correlated with rocks in Maine (Hatchet al., 1983).
The quadrangle straddles a Silurian "tectonic hinge"between the anticlinorium, where the Silurian rocks consist of a thin shelf sequence,
and the synclinorium, where the Silurian rocks thicken to form a
clastic wedge up to 2300 m thick (Hatchet al., 1983) (Figure 2).
Thirdly, in Massachusetts the map pattern consists of narrow parallel
units striking nearly north-south, apparently the root zone for major
west-verging nappes (Robinson and Hall, 1980). North of the state
line the structural grain trends more to the northeast, and the plunge
of the fold axes apparently steepens so that numerous fold hinges
intersect the earth's surface. Thus, the Monadnock quadrangle is
potentially an area in which to observe the large backfolds which
post-date the nappes, and which are deformed by folds related to the
rise of gneiss domes in the Bronson Hill anticlinorium (Thompson et
al., 1968).
" •• the upland between Treves and the Rhine is
one of the best examples qf an uplifted peneplain •
• • • Here and there it is still surmounted by low,
linear, eminences • • • These I would call 'monadnocks',
taking the name from a typical residual mountain
which surmounts the uplifted peneplain of New England
in southwestern New Hampshire."
-Davis, 1896, p.192.
Previous Work
'.:
Hitchcock (1877) distinguished four main groups of layered rocks
in the Monadnock quadrangle:
ferruginous schists, fibrolite schists,
pyritiferous schists, and the Montalban series. From his location
descriptions it is clear that the first three correspond respectively
to Rangeley, Littleton and Francestown Formations of the present
study, although his map leaves out some of the locations described in
his text. He described Mt. Monadnock itself as a "double synclinal".
Davis (1896) cited Monadnock as the type locality for resistant mountains rising above the general erosion level, thereby coining the term
"monadnock" for such isolated peaks worldwide.
Perry (1904) concentrated his mapping on the immediate area around
the mountain and recognized three units:
gray garnet-biotitesericite-fibrolite schist with pseudomorphs after andalusite
(Littleton Formation of this report), gray quartzose mica schist with
biotite and hornblende (Warner Formation), and a very rusty fissile
quartzose schist (Francestown and in part Rangeley Formations). He
recognized the gradational nature of the Littleton/Warner contact.
Perry mapped the orientation of foliation, and showed on his map the
w
KEENE
DOME
II
MONADNOCK
PETERBOROUGH
SEQUENCE
(E. DUKE, 1984)
CENTRAL
E
N.H ..
(HATCH ET AL., 1983)
0
Tectonic
Hinge"
rn
<
0
z
upper Littleton
Littleton
schist and quartzite
~kHill
----
-----
)>
z
upper, thick-bedded
Littleton
lower Littleton
schist
-----Littleton----lower, thin-bedded
Madrid
upper
•
\~:~~~··'·,,~:~
, : , ,w,; x •• ,,;, ,,~~:'': ·~"· . . . ..... "'
~"''
gran~lite~
~-----matnx
cg .
cgl.
......._........_
upPer, rusty
---Rangel ey_
lower, gray -
?---- ___ ? ___
- ----
Rangeley
------
7__
Fig. 2. Stratigraphic diagram showing proposed correlations across the tectonic hin ge , after
Hatchet al. (1983), to include the Monadnock and Peterborough sections.
Black arrows show change
in sediment source direction from Silurian to Devonian. See text for details of correlation for
each unit, and Figure 4 for fossil control.
-....J
8
large fold which dominates the mountain's summit. He attributed this
deformation to plutons pushing aside the metamorphic rocks. The
contacts of plutons are irregular, with abundant offshoots parallel to
foliation in the schist, leading Perry to conclude that a large
batholith may underlie the entire area. He recognized two phases of
folding, citing the recumbent fold near the summit as evidence for an
early deformation at depths where rock flowage occurred, and what he
thought was a systematic relationship between joints and the map-scale
syncline as evidence for a later, more brittle, deformation at less
depth, accompanying the granite intrusion.
The first detailed geologic map of the quadrangle was produced by
Fowler-Billings (1949a). For the first time the various plutons were
differentiated into members of Billings' (1937) plutonic series, and
their contacts approximated. The present study does not add much to
the geology of the plutons, but instead concentrates on the layered
rocks. Most of the pluton contacts on Plate 1 are much as FowlerBillings mapped them. Some structural data from her map (1949a) are
included on Plate 2 in plutons and in the Keene dome, but people
interested in more detail, and the location of quarries and mines,
should refer to her map.
With the exception of thin horizons of Ammonoosuc Vocanics,
Partridge Formation and Clough Quartzite along the eastern edge of the
Swanzey (Keene) dome, at the west edge of the quadrangle, FowlerBillings included all the layered rocks in the Littleton Formation.
She subdivided the Littleton into four members: a lower schist member
(Rangeley and Perry Mountain Formations in the present study), a rusty
quartzite member including actinolite granulite and biotite schist
(Francestown and Warner Formations), a middle schist member (Littleton
Formation) and an upper member of rusty-weathering schist and gneiss
(Rangeley Formation). The rusty quartzite serves as a marker horizon
and defines a large map-scale syncline on Fowler-Billings' map, as
well as smaller folds near Thorndike Pond and Hurricane Hill. She did
not attempt to separate various fold phases beyond suggesting that
large recumbent folds near the summit formed early, possibly as soft
sediment features. She correctly observed that the joints are a relatively late feature. One isolated area of rusty quartzite near Derby
Hill surrounded by the schists and gneisses of the upper schist member
was mentioned by Fowler-Billings as warranting more work (1949b,
p.31).
Some unpublished field notes by Peter Robinson and J.B. Thompson,
Jr. (1966) from the area along the edge of the Keene dome south from
Rt. 12, and along Grassy Hill, have been incorporated in the present
study. Their outcrop locations and structural data are included on
Plates 1, 2, 3, and Figure 12, Subarea 6. Thompson et al. (1968)
mapped a contact between the Littleton Formation which lies east of
the Keene dome and the sulfidic, rusty-weathering schists farther
east. They interpreted the sulfidic rocks as Partridge Formation in
the overturned limb of the Fall Mountain nappe. The axial trace of a
9
nappe-stage syncline would lie somewhere within the narrow belt of
Littleton. The present study interprets the contact between the gray
and rusty rocks differently.
Nelson (1975) made a detailed study of the area between the southeast foot of Monadnock and Thorndike Pond. He separated the rock
types in Fowler-Billings' rusty quartzite member and correlated them
with Francestown and Warner Formations, following Nielson's (1974)
nomenclature in the Hillsboro quadrangle. Graded beds show that the
biotite granulites and clean calc-silicates (Warner) are intermediate
in age between the rusty calc-silicates (Francestown) and gray schists
(Littleton). Nielson (1974) had proposed correlating the Francestown
and Warner with the Smalls Falls and Madrid Formations in Maine
(Moench and Baudette, 1970), which are Silurian. Therefore the rocks
below the Francestown had to be still older, and Nelson tentatively
called them Ordovician Partridge Formation. Nelson used this stratigraphy to map out a complicated interference pattern of folds; he
proposed that a large recumbent fold with a steeply plunging,
northwest-trending axis is refolded about isoclinal folds with axes
that plunge gently northeast. The structural interpretation in this
thesis is somewhat different.
Methods of Study
Field work in 1981 was started in the 7 1/2' Winchendon, Massachusetts, quadrangle (1:25,000, U.S.G.S., 1971), following the Coys
Hill Granite and adjacent lay~red rocks northward into the SE quadrant
of the 15' Monadnock quadrangle (1:62,500, U.S.G.s., 1949). It soon
became apparent that the geology in areas of abundant outcrop could
not be successfully mapped at the smaller scale, so pace and compass
maps were made at 1:3,000 and 1:6,000, and only isolated outcrops were
plotted directly on the u.s.G.S. base map. I also began exploring Mt.
Monadnock itself, plotting stations both on aerial photographs
(1:20,000, U.S.D.A., 1975) and on a 1:32,200 map published by the
Appalachian Mountain Club (1972). In 1982 I gained access to preliminary 1:25,000 U.S.G.S. maps with six meter contour intervals, and
all subsequent mapping was done at this scale with the exception of
work on the summit of Monadnock and in selected areas of complex
geology.
The New Hampshire portion of the Winchendon quadrangle was not
included in this study, unlike that of Fowler-Billings (1949). Two
areas of layered rocks in the Monadnock quadrangle were poorly
covered, and need more work: the southwestern corner south of
latitude 42° 46', and the entire town of Nelson.
Stations were numbered chronologically, with a two letter prefix
for the township in which they occur (Appendix 1, Table 15 and Figure
41). Sample and thin section numbers correspond to the station
numbers. In cases of multiple samples from one station, they were
designated -A, -B, etc. Two exceptions to this system include the
10
main mafic dike on Mt. Monadnock, which has the same number (MK-54)
throughout its length, and the MK (Jaffrey) series, in which the
stations on Mt. Monadnock were unfortunately not numbered in exact
chronological order with reference to stations elsewhere in the
township.
Samples were selected for thin sections on the basis of weathering
freshness, representative rock type, and geographical distribution.
110 thin sections were examined using a petrographic transmitted light
microscope. Those for microprobe analysis and reflected light microscopy were polished with 0.3 micron grit. Modal percentages were
estimated using density charts, and point counts on the basis of 2000
points per slide were made on several of the more homogeneous sections
for comparison. Anorthite contents of plagioclase were estimated
using the Michel-Levy method and Figure 17-3 of Jones and Bloss
(1980). Detailed electron microprobe analyses were made during 1984
on the three spectrometer ETEC automated electron microprobe at the
University of Massachusetts Department of Geology, using a 15 kV
accelerating potential and a 0.03 microamp beam current. Data was
corrected by the Bence and Albee (1968) procedure.
Acknowledgments
This paper is submitted in partial fulfillment of the requirements
for the Ph.D. degree in Geology at the University of Massachusetts,
Amherst. Peter Robinson suggested the project and served as my
advisor. I wish to thank him for his enthusiastic guidance through
all stages of the project, from my initial introduction to the
regional geology, to analysis and presentation of the results. I am
especially indebted to Peter for help in the interpretation of garnet
zoning and in preparation of the colored plates. John Lyons kindly
showed me the central New Hampshire stratigraphy, and loaned me two of
Carl Nelson's samples for microprobe study. Katharine Fowler-Billings
made her field maps and thin sections available to me. Discussions
with Page Chamberlain, Virginia Peterson, and Edward Duke, who were
!
all mapping in adjacent quadrangles, provided insight and incentive,
as did field and office sessions with Norman Hatch, John Lyons, Bob
Moench, Gene Boudette, Jim Thompson, David Elbert, Spike Berry,
MaryAnn Malinconico, Peter Morton, Jeff Josephson, and many other
people. Peter Robinson, Leo Hall, Don Wise, Charles Dickinson, and
Thelma Thompson reviewed the manuscript and made many helpful
suggestions. Thelma Thompson and Rachel Wing were much appreciated
field assistants. Stephen Field helped me with opaque mineral identification, and David Elbert, Kurt Hollocher, and David Leonard helped
me with microprobe procedure. Harold Robinson colored preliminary
versions of Plates 1 and 5, and helped prepare the final plates. The
hospitality of Mary Emerson and all my friends at Boulderidge made the
field seasons doubly enjoyable.
Support for field and laboratory work and preparation of publications for this project was provided by grants from the National
11
Science Foundation, Earth Sciences Division, as follows: from the
Geology Program EAR-79-15246 (to Robinson); from the Petrogenesis
Program EAR-81-16197 (to Robinson and J.M. Rhodes); and jointly from
the Crustal Structure and Tectonics Program and the Petrogenesis
Program EAR-84-10370 (to Robinson).
"At length as the craft was cast to one side, and ran
ranging along with the White Whale's flank, he seemed
strangely oblivious of its advance--as the whale sometimes will--and Ahab was fairly within the smoky
mountain mist, which thrown off from the whale's spout,
curled round his great, Monadnock hump • • • "
-Herman Melville, 1851
STRATIGRAPHY
In the present study Nelson's (1975) work is extended into the
rest of the Monadnock quadrangle, and rocks below the Francestown
Formation are subdivided into thinly bedded quartzites and schists of
the Perry Mountain Formation and a variety of rock types assigned to
the Rangeley Formation (Figure 3), largely on the basis of correlation
with similar rocks in central New Hampshire (Hatchet al., 1983). An
attempt is also made to tie in with the regional geology, especially
current work by E. Duke (1984) in the Peterborough quadrangle,
Chamberlain (1984) in the Gilsum-Marlow area, and Robinson (1963;
1967; Thompson et al., 1968) in Massachusetts.
For each of the major units in this study, a description of rock
types, mappable subunits, distribution and thicknesses is presented,
followed by a discussion of nomenclature, regional correlation and
postulated derivation. Although thickness estimates were made in
areas where structural repetition is not apparent, it should be emphasized that the high probability of either structural attenuation or
structural thickening makes these estimates useful only in a qualitative way.
I have not made any detailed study of the units along the east
edge of the Keene dome where the Ammonoosuc Volcanics, Partridge
Formation, Clough Quartzite and, locally, Fitch Formation overlie the
gneisses of the dome. Therefore the descriptions of these units are
condensed, and based mainly on previously published observations.
SWANZEY GNEISS, AMMONOOSUC VOLCANICS AND PARTRIDGE FORMATION
Fowler-Billings (1949a) and Moore (1949) described the gneiss of
the Swanzey dome (now generally called the Keene dome) as an intrusive
granodiorite to quartz diorite pluton of the Oliverian magma series.
Thompson et al. (1968) argued that the rocks exposed in the Keene dome
as well as-other domes in the Bronson Hill anticlinorium were older
than the cover rocks rather than intrusive into them. Robinson et al.
(1979) described in some detail the roadcut on Rt. 12 in East Swanzey
12
Fig. 3.
Stratigraphic Column.
I
thickness 1
(meters) ..:-:::-::;:-~.::_
1
>600
~·
upper part of LITTLETON FORMATION
gray schist and quartzite
~=-.;: :::: -.
=i~~.i:0
?E~t=~
z
<t
z
0
>
lower part of LITTLETON FORMATION
gray schist and sparse quartzite
650
w
0
~--
______
=
_ - co - \
37
45 /
J
;---upper part of WARNER FORMATION
gray granulite
~~~~~~!~~~.----lower
part of WARNER FORMATION
~ clean
calc-silicate granulite
89
62
~FRANCESTOWN
-;:_~•.~.::_- · ~
.. . . ~-
~-
_.,.
FORMATION
sulfidic calc-silicate granulite
PERRY MOUNTAIN FORMATION
thinly bedded schist and white quartzite
upper part of RANGELEY FORMATION (C?)
gritty schist with quartz pebble lenses
and calc-silicate pods
z
<t
a:
::>
_J
(/')
900
>600
middle part of RANGELEY FORMATION (B?)
sulfidic schist with calc-silicate pods
lower part of RANGELEY FORMATION (A?)
gray feldspathic schist and granulite
with calc-silicate pods and granulitematrix conglomerate
13
(on the boundary between Plate 1 NW and Plate 1 SW) which exposes the
dome gneiss and the overlying units. The predominantly coarse-graLned
biotite-plagioclase gneisses contain hornblende amphibolite layers and
boudins. Earlier workers had interpreted the amphibolite as
Ammonoosuc Volcanics intruded by the felsic rocks. Conspicuous magnetite porphyroblasts are common in some of the felsic gneisses.
Although most now agree that the contact between the dome gneiss and
the Ammonoosuc is not intrusive, there remains a controversy as to
whether it represents an unconformity or, as Naylor (1969) suggested,
a change in volcanic chemistry.
The Ammonoosuc Volcanics, which include a lower mafic unit and an
upper felsic unit, represent the remains of a Middle Ordovician island
arc (Leo, 1980; Schumacher, 1983). Some horizons in the mafic unit
contain anthophyllite, gedrite or cummingtonite (Robinson and Jaffe,
1969a and 1969b). At the Rt. 12 roadcut there are approximately 25m
of the formation (Robinson et al., 1979). Correlative rocks in Maine
contain Middle Ordovician brachiopods (Boucot, 1961). The Swanzey
Gneiss and Ammonoosuc Volcanics were probably joined to the North
American continent during the Taconian orogeny (Hall and Robinson,
1980).
The rusty-weathering schists of the Partridge Formation consist of
quartz, muscovite, biotite, plagioclase, graphite and sulfides, with
or without garnet and sillimanite (Fowler-Billings, 1949a). The
Partridge is locally absent, probably due to faulting, as for example
in Forbush Brook north of Rt. 12. However, it is exposed in enough
places above the Ammonoosuc to suggest it was formerly continuous
along the edge of the Keene dome. At the Rt. 12 roadcut it is about
18m thick (Robinson et al., 1979). Correlative rocks in Maine
contain Middle Ordovician-graptolites (Harwood and Berry, 1967).
Rusty schist dominates the southeast corner of the Monadhock
quadrangle. At least some of these rocks are on strike with belts of
sulfidic schist which extend south to Brimfield, Massachusetts, and
which contain amphibolites and small ultramafic bodies, and appear to
belong to the Partridge Formation. This correlation is strengthened
by the 440 m.y. Hedgehog Hill gneiss which intrudes the Brimfield
Group in Connecticut (Pease and Barosh, 1981). One outcrop of amphibolite occurs in a small roadcut on Rt. 12 in the Winchendon quadrangle, 100 m east of the Sip Pond belt of Kinsman Granite. No amphibolites have yet been found in the Monadnock quadrangle apart from the
Ammonoosuc, and lacking other reliable criteria for distinguishing the
Partridge from rusty rocks of the Rangeley Formation, a contact is
difficult to map. More detailed mapping in the Winchendon qudrangle
may clarify the situation, but for now no Partridge is shown on Plate
1 SE. In theory, the rusty Rangeley should be reddish-weathering,
more quartzose, less aluminous and perhaps should contain more
14
abundant calc-silicate pods (Norman Hatch, pers. comm., 1983). The
rusty weathering tends to be yellowish or greenish in the Partridge,
although very yellow-weathering rocks also occur in what I have mapped
as Rangeley in Troy and Roxbury.
CLOUGH QUARTZITE AND FITCH FORMATION
The Clough Quartzite includes metamorphosed orthoquartzite and
quartz-pebble conglomerate, micaceous quartzite, and quartz-garnetmuscovite schist (Fowler-Billings, 1949a). On Fowler-Billings' map
the Clough is shown as discontinuous lenses above the Partridge Formation. However, since the Clough is present everywhere that exposure
is adequate, a continuous thin layer (25 m thick at the Rt. 12
roadcut, Robinson et al., 1979) is shown on my Plate 1. In two of her
five measured sections across the edge of the Keene dome, FowlerBillings reported pegmatite at the position where the Clough should
be. Pegmatite intrudes the Clough as well as other units at the
Rt. 12 cut, and in fact is common at many places along the edge of the
Keene dome, obscuring contact relations and confusing thickness
estimates. However, these problems are minor when we consider that
major tectonic thinning has probably reduced the original thickness of
the dome cover sequence by as much as 90% on this part of the Keene
dome (Robinson, 1963). The Clough 9uartzite is well established as
late Llandoverian (Figure 4), based on numerous fossil localities
elsewhere in New Hampshire (Boucot and Thompson, 1963). The clean,
coarse nature of the quartzite, and the fact that in some places it
lies directly on the Ammonoosuc Volcanics or even on the dome gneiss
(Thompson et al., 1968), indicate its lower contact is probably an
unconformity.
~
Probable Fitch Formation was found north and east of Mt. Huggins,
north of Rt. 12 (Plate 1 NW). These rocks include well bedded calcsilicate granulite and biotite-feldspar-quartz granulite. They occur
in outcrops east of the Clough Quartzite on the steep south-facing
slope of a 246+ m knob north of Mt. Huggins, on a narrow 240+ m ridge
northeast of Mt. Huggins, and in the swampy headwaters of a westflowing brook east of Mt. Huggins. The Fitch is Pridolian (Late
Silurian) based on conodonts near Littleton, New Hampshire (Harris et
al., 1983). The Fitch is typically discontinuous elsewhere in the
anticlinorium (Thompson et al., 1968), perhaps due to local non-deposition or erosion prior to deposition of the Lower Devonian Littleton
Formation. In various places the Littleton lies directly on each of
the older units, including dome gneiss (Robinson, 1963; Hatchet al.,
in press). Thus, apparently the base of the Littleton also represents
an unconformity, at least where the underlying units are missing.
,,
age
fossil
ranges
central
Maine
northwestern
Maine
Monadnock
Bronson Hill
anticlinorium
~:: ~~~~~<:~ ~ .:::.-.:: ]-:-: ::.-_:-:.:::._:.-_-_-_-_-: ~-_.:
Littleton! __
Ems ian
Siegenian
Littleton
401
Carrabassett
Carrabassett
3941
t::l
t%j
<:
0
z
~
H
Gedinnian
·408
Fitch3 ______ 1_ _ _¥arner
------------
__ )i~<!£~<!- -- -'--~~1_1_..!3!'.9.9~--
----
Pridolian
414-
I
I
I
Ludlovian
I
I
I
Perry Mountain
41
Parkman Hill-----1
Smalls Falls
Francestown
Perry
Mountai~
unnamed unit
.
4
:I
! I
I
II
I
I
I
I
I
I
Sangerville - ------...lI
I
Clough 5---u
~-I
Partridge
~~;.I~;- -l-;;n~~l.-y6== =~===~ ~ ~~-----~~r----:_
~-D IIi
1
Waterville
'j -------~
7
· - V a s s a l b o r o --
I
I
l:ireenvale Cove
louimby
lnixvi 11 P 8
--u----+
I
I
421J
I
Wenlockian
en
H
t""
~
~
H
'·28Llandoverian
438
_.__--l
ORDOVICIAN
Fig. 4. Correlation chart (after Hatchet al., 1983), showing age ranges of fossil assemblages in
units believed to be equivalent to the Monadnock stratigraphy. 1. Boucot and Arndt (1960); Boucot
and Rumble (1980) 2. Boucot (1969) 3. Harris et al. (1983) 4. Pankiwskyj et al. (1976) 5. Boucot and
Thompson (1963) 6. Moench and Baudette (1970)~.1Dsberg (1980) 8. Harwood-;ndlBerry (1967).
.....
V1
16
RANGELEY FORMATION
Description and Distribution of Rock Types
Rocks assigned to the Rangeley Formation in the Monadnock quadrangle include rusty-weathering sulfidic schists, gray-weathering
schists, quartz-feldspar-biotite granulites, a variety of conglomerate
horizons and lenses, and a variety of calc-silicate granulite horizons
and lenses. Approximately 40% of the quadrangle is underlain by the
Rangeley, distributed in three main areas (Plate 4): an area surrounding Little Monadnock Mountain in the southwestern part of the
quadrangle and extending north to Marlboro, a northern area west of
the Cardigan pluton and north of the Chesham Pond fault, and an elongate northeast-trending area east of the Thorndike Pond fault zone.
In addition, the formation is well exposed below the Perry Mountain
Formation in the folds southeast of Mt. Monadnock and surrounding the
Spaulding Hill pluton. In the northern third of the quadrangle the
rocks have gneissic textures, but the same rock types occur. In these
gneisses strong foliation and bedding are hard to discern in most
outcrops, except where calc-silicate pods mark primary layering.
~
The mapping of rusty- and gray-weathering rocks within the
Rangeley Formation has not shown any clear internal stratigraphy; the
situation seems to be more complicated than simply an upper rusty part
and lower gray part, as suggested by Hatchet al. (1983) for central
New Hampshire. Because of the heterogeneity of the unit, consistent
internal stratigraphy should probably not be expected over wide areas.
In the Monadnock quadrangle west of Troy, three informal parts have
tentatively been identified (Figure 3 and Plate 1 SW).
17
The lower part is predominantly gray-weathering schist with calcsilicate pods and local granulite and granulite-matrix conglomerate.
The middle part consists of a thick sequence of predominantly sulfidic
schist with calc-silicate pods. The upper part consists of interbedded gray and sulfidic schists with gritty horizons, calc-silicate
pods, and local lenses of quartz-pebble conglomerate. Bedding is .
generally well defined in the upper part and beds up to 15 em thick
with "slow grading" are present. Further detailed mapping with
particular attention to the topping direction of graded beds is needed
to verify the proposed scheme. Unfortunately graded beds are abundant
only in the upper, gritty member. Both because of the uncertain
division into three parts, and for the sake of clarity, the rock types
in the Rangeley are described below without regard to the proposed
parts.
Sulfidic schist and gneiss. Reddish-rusty, quartz-biotitemuscovite-plagioclase schist (Table 1a), with gritty horizons and
calc-silicate pods, is the most common rock type in the Rangeley Formation of the Monadnock quadrangle. The plagioclase ranges from An 23
to An55 • Sillimanite, garnet, orthoclase, and retrograde staurolite
and Mg-chlorite may or may not be present, and zircon, ilmenite,
graphite, apatite, allanite, and tourmaline are accessory. The rusty
weathering is due to the presence of iron sulfide minerals, which in
thin section MK-564 were identified as pyrrhotite, marcasite after
pyrrhotite, and minor chalcopyrite. Although the reddish color is
characteristic, yellow and orange colors are also found, as well as
gray-weathering beds interlayered with the rusty ones. The cause of
the color variation in the rusty schists is uncertain.
Many outcrops of rusty schist in the Rangeley Formation contain
gritty horizons. What were apparently sand grains and small pebbles
ranging in size from 0.5 to 4 mm have been recrystallized. Sample
MK-1061A, for example, is a very gritty rock with abundant 1 to 2 mm
domains of quartz, and a few as large as 1 em. Thin section study of
~hese domains shows that they consist of grains finer than 1 mm.
Relict pebble boundaries are not visible in thin section. Graded beds
are locally well exposed in outcrop. Beds from 3 to 15 em thick show
a gradual decrease in grain size, or "slow" grading. The locations of
some outcrops with graded beds are shown on Plate 2 by dots above the
strike and dip symbols.
,.,
Table 1a.
Estimated modes of the Rangeley Formation:
MK
MK
564
1027
Quartz
Plagioclase
28
32
MK
1061A
47
sulfidic schist and gneiss.
TR
21
MK
_l2.L
HV
DB
165
RI
100
HV
52
52
24
52
32
25
32
36
15
13
10
22
6
7
3
10
15
(An40) (An42) (An41) (An23) (An55) (An34)(Anl7-29)(An30) (An26)
Micro cline
4
Muscovite
11
32
15
2
5
11
Biotite
11
23
12
25
46
12
2
8
X
3
Garnet
Sillimanite
25
34
4
3
5
Staurolite
29
7
4
10
54
X
1
.,
Chlorite
(Retrograde)
Opa<{Ues
Graphite
Ilmenite
Pyrrhotite
Chalcopyrite
Marcasite
Undifferent'd
X
(Mg) *
2
2
X
X
6
Apatite
X
Tourmaline
3
(Mg)
23
(Fe)
2
1
X
X
2
2
3
X
4
X
X
X
X
X
X
X
X
X
X
1
X
3
X
X
X
X
X
X
X
tr
X less than 1%, but more or less ubiquitous
tr trace
chlorite compositions estimated optically
*
10
7
(Fe-Mg) (Mg)
3
X
X
Zircon
X
(Mg)
X
X
X
X
X
.....
00
19
List of Samples in Table 1a.
ffiZ-564 Gray medium-grained graphitic sillimanite schist. Weathers
red rusty.
50m W of Stony Brook Farm Rd., 88m SW of Thorndike Pond, Jaffrey.
MK-1027 Gray massive gneiss with 2 em garnet porphyroblasts. Rustyweathering.
Fresh rock blasted from Wallace driveway E of Thorndike Pond Rd.,
due E of rocky peninsula in Thorndike Pond, Jaffrey.
MK-1061A Gritty, massive, coarse quartzose schist, with up to 2 em
garnet porphyroblasts and lenses several em long of sillimanite
schist. Garnets have clear rims around "sugary"-textured cores
which are full of inclusions: quartz, micas, tourmaline, opaques.
900m E of Thorndike Pond, 300m S of Dublin-Jaffrey town line, W of
contact with Kinsman granite.
TR-21 Gray medium- to fine-grained foliated to granular schist.
Weathers rusty and outcrop contains calc-silicate pods.
Rt. 12 roadcut 3.5 km W of Troy village
MK-192 Well foliated gray- to brown-weathering medium-grained granulite.
Gray schist beds and calc-silicate pods in same outcrop.
350m S of Rt. 124, 425m SE of Jaffrey Center Fire Station.
HV-52 Massive, medium-grained blue-gray retrograded gneiss, with
dark biotite-rich patches and 1-2 mm garnet porphyroblasts.
Weathers rusty. Local calc-silicate pods.
Cellar hole W of S\v corner of Silver Lake, Harrisville. Exposure
later covered by backfill; weathered outcrops E of house are
extensive to the lake shore.
DB-165 Greenish-gray to rusty fine-grained well foliated retrograded
~
schist, with 2-5 mm garnets which vary from fresh to completely
replaced by chlorite. Roadcut contains sparse calc-silicate beds.
N side of long roadcut, Rt. 101, E edge of Monadnock quadrangle.
RI-100 Rusty-weathering, medium-grained retrograded schist with
abundant graphite. Superficially resembles "whiteschist".
Rt. 202 roadcut 2 km N of West Rindge.
HV-36 Coarse, massive blue-gray retrograded gneiss with quartz-muscovite "augen".
Jeep road between Blood Hill and Cobb Hill, elev. 507m, Harrisville.
20
Conglomerates. Two distinct types of conglomerate occur in the
Rangeley Formation: quartz-pebble conglomerate, and granulite- matrix
conglomerate. Occurrences are shown by a separate color on Plate 4.
We will return to the significance of these horizons in a later discussion of regional correlations. Localities are listed in Table 1b
and estimated modes in Table 1c.
Quartz pebble conglomerate lenses and beds were found in several
localities, most of which are within 250 m of a contact with younger
formations. They are generally enclosed in rusty rocks, although at
MB-81 and TR-143 they are associated with gray schist. Station TR-65
is the best exposed of the quartz-pebble conglomerates. The bed is
about 50 em thick and of indeterminate lateral extent. Pebbles are
flattened in the plane of foliation at a low angle to the basal
contact with the underlying rusty micaceous quartzite. The pebbles
consist almost entirely of vein quartz and white quartzite, with rare
calc-silicate granulite clasts which give the outcrop a pitted surface
where they have weathered out. Although it is a grain-supported
conglomerate, there is a calc-silicate matrix which includes diopside,
garnet, clinozoisite, actinolite, sphene and calcic plagioclase.
Sample MK-1051 also has a calc-silicate matrix.
The granulite-matrix conglomerate occurs in a lens which is at
most 100 m thick, and which is exposed below the west-facing cliffs
ENE of Mt. Huggins (SZ-16 and SZ-23), in a brook north of Page Hill
(MB-297), and on the east-facing slope of Page Hill. A similar but
probably separate horizon is exposed approximately 400 m to the east.
The granulite-matrix conglomerates are much more feldspathic and
biotitic than the quartz-pebble conglomerates, and they are associated
with gray schist and granulite. The clasts are similar to the matrix
in mineralogy (see Table 1c, SZ-23), consisting mainly of quartz,
plagioclase, biotite, and garnet. There are some vein quartz clasts,
: and others that may have been intrusive igneous rocks. The clasts
have been flattened and elongated to shapes up to 10 X 3 X 1 em, so
that in cross sections parallel to their long axes they resemble thin
granulite beds.
21
Table lb.
List of conglomerate localities in the Rangeley Formation.
TR-65 366+m hill N of High Street, 875m W of Troy village green. In
contact with rusty gritty massive schist, about 125m below Perry
Mountain Formation. See Table lc.
TR-130 In brook, 400m S of TR-65.
rusty sillimanite schist.
15cm-thick pebble horizon in
TR-143 1600m NW of TR-65, elev. 416m, lOOm E of \.Jest Hill Rd., Troy.
Gray sillimanite schist, quartzite, and quartz pebble conglomerate
lenses 15cm thick. Rusty rocks with calc-silicate pods 20m to W.
TR-106 On power line, 600m NE of TR-143, elev. 372m, W of gully.
Quartz pebble boudin in bedded quartzite and rusty schist.
MK-701A West of Stony Brook Farm Road 850m SW of Thorndike Pond.
Horizon with 50mm quartz grains in gritty rusty schist.
MK-1051
See Table lc.
RX-147 \vest of Derby Hill window, W of Willard Hill, elev. 398m in
N-S brook. Sparse 1.5cm quartz pebbles in somewhat rusty schist.
HV-63A Float block 6m H of Perry Mountain Formation, elev. 462m,
Derby Hill. 0.5-lcm quartz pebbles and a few garnet schist clasts
in matrix-poor conglomerate.
MB-290 425m WSW of Glen Brook Pond, knobby outcrop. Quartz pebble
conglomerate boudin in red rusty schist with sharply bounded
quartzite beds, some pitted.
HB-81 450m NNE from E end of Clapp Pond, in woods west of field.
Garnet-sillimanite schist with quartzite beds and quartz pebble
conglomerate horizons. Pebbles elongated down-dip toward NW.
SZ-23
See Table lc.
SZ-16 500m E of SZ-23, 200m E of top of cliff. Granulite matrix
conglomerate with granulite and quartzite clasts, interbedded with
gray schist and feldspathic granulite.
MB-303
Small ledge in brook, 750m SE of SZ-23, similar rock to SZ-23.
Mll-295 Dip-slope ledge on E side of Page Hill. Granulite and conglomerate intruded by pegmatite. Strong pebble lineation.
MB-326 Poor outcrop south of Rt. 124, 250m W of Troy Rd., Marlboro.
Quartz pebble horizons, quartzite and schist intruded by pegmatite.
About 80m W of outcrops with graded beds topping to the west.
...
Table 1c.
Estimated modes of the Rangeley Formation:
conglomerates and calc-silicate pods.
Conglomerate
Quartz
Plagioclase
Calc-Silicate Pods
MK
sz
sz
MK
RX
23A
294
294A
DB
90
HV
23
TR
65
MK
1051
44
54
TR
93
RX
5
MB
75A
62
36
78
72
38
28
49
34
43
48
14
44
15
11
6
32
36
18
27
36
40
39
39
(An51)(An37+)(An37+)(An70+) (An73) (An73)(An65+)(An67+) (An67+) (An75) (An64+)
Diopside
20
6
Actinolite
1
Clinozoisite
3
28
11
16
2
Biotite
2
26
Huscovite
3
4
13
1
13
6
8
X
3
-,
14
4
tr
7
10
10
4
6
3
20
2
tr
19
5
Chlorite (Fe-Mg)
1
2
2
tr
X
X
X
X
X
2
X
X
X
X
Apatite
X
X
X
Zircon
X
X
Tourmaline
X
Calcite
14
2
Garnet
Opaques
Graphite
Pyrrhotite
Undifferent'd
3
5
19
Ferrian Zoisite
Sphene
1
2
tr
X
X
7
5
2
X
1
X
X
X
X
2
X
X
X
1
2
1
X
X
X
2
X
1
X
1
X
2
X
X
X
X
X
1
X
3
X less than 1%, but more or less ubiquitous
tr trace
N
N
23
List of Samples in Table lc.
MK-1051 Massive, medium-grained conglomerate with O.Scm quartz
pebbles in a weathered calc-silicate matrix. Diopside in
hand sample.
Elev. 363m, E-facing ledges W of Frost Pond, Jaffrey.
SZ-23 Gray to brownish granulite-matrix conglomerate with quartzite
and vein quartz "stretched" pebbles. SZ-23 is the matrix, SZ-23A
is a quartzite clast. Clasts most apparent on weathered surface.
Elev. 264m, W of tall cliffs, N of Page Hill, Swanzey.
TR-65 Gray to white quartz pebble conglomerate, with pebbles flattened
in foliation, up to Scm long. Vein quartz, quartzite, and garnet
schist clasts.
366+m hill N of High Street, 875m W of Troy village green.
MK-294 Fine-grained, white mottled with green and interlayered with
pink calc-silicate granulite. Pod from rusty schist.
300m N of Rt. 124, 1400m E of Cummings Pond, about lOOm E of Perry
Mountain Formation, Jaffrey. MK-294A is the pink layer.
DB-90
Fine-grained, mottled, pale gray calc-silicate granulite with
dark green chlorite and biotite, and a 2mm garnet horizon. Pod
from quartzose rusty schist. Weathers light tan to pink.
E side of 364m hill E of Rt. 137, 750m N of Jaffrey-Dublin town line.
HV-44 Fine-grained, mottled, pale green calc-silicate granulite with
dark green actinolite up to 4mm long. Center of pod from rusty
gneiss.
Small roadcut NW of Childs Bog, Harrisville.
RX-54 Medium-grained dark gray calc-silicate granulite, with up to
4mm garnets and patches of biotite. Pod :rom rusty schist.
40m W of S end, Woodward Pond, Roxbury.
TR-93 Fine-grained dark gray calc-silicate granulite with up to 2mm
t
vitreous quartz grains.
Pod in rusty schist.
Ridge NW from Troy, elev. 390m.
RX-5
Medium-grained pale gray calc-silicate granulite mottled with
actinolite, diopside and garnet, from pod in gray augen schist
(RX-2, Table ld). Weathers tan.
Elongate ridge 200m SE of Hardy Hill, Roxbury.
MB-75A Magnetic fine-grained dark gray calc-silicate granulite with
darker porphyroblasts. Sulfides visible in hand specimen.
Weathers with a punky brown crust.
Float block in brook, north of NE end, Clapp Pond, Marlboro.
24
Calc-silicate granulite. Lenses, pods, or "footballs" (Field,
1975, p.28) of calc-silicate granulite are common in both rusty and
gray rocks of the Rangeley Formation. In some areas at least one pod
can be found in every outcrop, but this is not true everywhere. They
range in size from 10 to 100 em and although most are indeed football-sized, few if any approximate feotballs in shape. Flattened
loaves of bread might be a more apt description, with the shortest
axis perpendicular to bedding in the enclosing rocks. There is
commonly a very hard, flinty, homogeneous core surrounded by a rim of
less resistant feldspathic granulite that weathers to form a shallow
depression. The cores are white, to light green, to shades of gray
and black, spotted by green and red. They are composed of quartz,
calcic plagioclase, and various amounts of diopside, grossular garnet,
clinozoisite, ferrian zoisite, biotite, calcite, sphene, and muscovite
alteration of plagioclase (Table 1c). Apatite, zircon, and opaque
minerals are accessory.
Locally there are continuous beds of calc-silicate granulite, most
notably at station TR-54, a large roadcut on Rt. 12 5.5 km west of
Troy, where calc-silicate beds up to 30 em thick are interlayered with
rusty schists. Elsewhere thinner beds can be found, but discontinuous
lenses are much more common. Where several pods are aligned the
question arises whether they were formed as discrete bodies, perhaps
as concretions, or whether once-continuous beds have been disrupted by
boudinage. The rims and rounded contours tend to favor a concretionary or~g~n. Where interlayered calc-silicate beds and schists have
obviously undergone boudinage, for example in the Francestown
Formation at HV-54 near Derby Hill, the calc-silicates have squaredoff ends and there are scar folds in the schist. Guthrie and Burnham
(1985) suggested that some pods in the Rangeley may have formed as
rip-up clasts or blocks.
~
Gray schist and gneiss, granulite, and augen schist. Not all the
schists of the Rangeley Formation show rusty weathering. Many
outcrops are monotonous gray rocks (Table 1d), and an attempt has been
made to map out the larger areas of gray-weathering rocks. It should
be emphasized, however, that many of these rocks are also interbedded
with the rusty schist. A comparison of modes from the two groups
(Tables 1a and 1d) shows very little difference in average mineralogy.
Many gray schists in the Rangeley greatly resemble rocks of the
Littleton Formation. Although the number of samples is not large
enough to draw statistically meaningful conclusions, the plagioclase
content of the Rangeley gray schists (range 0-14 modal %, average 4%)
seems to be greater than in Littleton gray schists (range 0-10%,
average 1.7%). Anorthite content in plagioclase from gray Rangeley
schists ranges from An13 to AnJ6·
Locally the schist grades into
quartz-plagioclase-biotite granulite. Granulite horizons are most
common in the western part of the quadrangle, in the lower part of the
formation. Platy minerals (biotite, muscovite and retrograde
chlorite) seem to be more abundant in the Littleton. However, these
subtle differences are not good field criteria for mapping. A more
25
reliable hallmark of the Rangeley gray schists is the presence of
calc- silicate granulite pods similar to those in the rusty schists.
By contrast, only the lowermost 30 m or so of the Littleton Formation
in the Monadnock quadrangle contains calc-silicate pods. It may be
significant that the schists near the base of the Littleton are also
more plagioclase-rich (Table 6, MK-604 and HV-135).
On Cobb Hill north of Lake Skatutakee (Plate 1 NE) there are some
exposures of extremely aluminous gray schist with as much as 53 modal
%sillimanite (Table 1d, HV-41). Fowler-Billings (1944b) described
this occurrence in some detail. The sillimanite-rich schist is about
15 m thick, with coarse sillimanite pseudomorphs after andalusite
averaging 1.5 em in length. Her samples K-116A and K-117 from Cobb
Hill also contain cordierite and alkali feldspar.
In some parts of the Rangeley Formation, but especially in the
gray-weathering schist and gneiss, segregations of quartz and
muscovite, with or without feldspar and sillimanite, form augen. Some
of these, for example FZ-30 in Table 1d, probably represent pockets of
recrystallized pegmatitic melt. Others, such as RX-2A, may have
replaced K-feldspar porphyroblasts. Heald (1950) observed orthoclase
porphyroblasts surrounded by shells of quartz and muscovite in the
Lovewell Mountain quadrangle. The augen textures may have been
accentuated by mylonitization during deformation. When I did most of
the field work, I overlooked the importance of trying to distinguish
between augen formed by these three processes. Those which appear to
be pseudomorphs are most common in the northern third of the quadrangle, but it is uncertain whether they are restricted to this area.
Northwest of Derby Hill there is a small gravel pit in saprolite that
consists of deeply weathered augen schist which somehow escaped
removal by the glaciers (location shown on Figure 20).
Thickness
It is impossible to estimate accurately the total thickness of the
Rangeley Formation in the Monadnock quadrangle, because the location
of the lower contact is highly uncertain. If there were no stratigraphic repetitions in the area west of Troy, the maximum thickness
would be approximately 2825 m. However, graded beds topping west in
outcrops south of Rt. 124 approximately 3 km southeast of Marlboro
village make this assumption suspect. There is apparently a syncline
with rocks belonging to the upper part of the Rangeley in its center.
West of this syncline there are about 900 m of the middle part of the
formation, and 600 m exposed of the lower part, below which there is a
proposed fault. The upper part of the Rangeley is about 600 m thick
at High Street in Troy. The maximum exposed Rangeley, taking into
account the syncline, would be about 2100 m. In Maine the Rangeley
Formation thickens abruptly eastwards from its westernmost outcrops to
a maximum of about 2700 m and then gradually thins again toward the
southeast, primarily by loss of coarse clastics (Moench, 1970).
,,..
Table ld.
Estimated modes of the Rangeley Formation:
gray schist and gneiss.
(Retrograded)
Quartz
RX
RX
HV
2A
FZ
30
RX
2
186
~
41
33
83
63
47
55
RX
4
K
116A
X
Plagioclase
3
Orthoclase
2
Muscovite
11
15
8
16
9
12
Biotite
31
1
17
24
11
12
X
1
5
17
9
6
53
Garnet
7
3
13
(An23)(Anl5?)(An34)
6
Sillimanite
12
5
X
Cordierite
K
117
K
102
NL
NL
MK
2
29
417
DB
130
DB
97
X
X
36
33
46
51
45
X
X
11
24
30
15
30
X
X
19
5
8
7
11
X
X
X
12
6
2
tr
8
X
X
X
5
2
X
1
X
X
X
Opaques
Graphite
Ilmenite
Undifferent'd
X
X
Apatite
X
X
Zircon
X
X
Tourmaline
Calcite
10?*
1
1
2
tr
X
Staurolite (Retro.)
Chlorite
(Retrograde)
14? *
5
5
8
(An36)(An26)(An40)(Anl3)
X
X
7
(Fe) (Mg)
X
X
X
X
X
X
X
X
2
X
X
X
X
X
X
I
4
X
X
X
23
(Mg)
3
2
(Fe)
2
X
X
X
X
2
X
X
3
X
X
X
tr
2
(Mg) (Mg-Fe)
X
X
X
X
X
X
X
X
X
X
X
X
* Strongly altered: some alteration resembles that typical of cordierite.
X less than 1%, but more or less ubiquitous, except for K-116A, K-117, K-102, where
X denotes presenc~ since mode was not estimated.
N
0\
27
List of Samples in Table ld.
RX-2
Massive, medium-grained gray sillimanite schist, with 2 mm garnets
and 2-5 em quartz-muscovite "augen" (RX-2A). Gritty 2 mm quartz and
1 em sillimanite form nubbles. Calc-silicate pod (RX-5) is in the
same outcrop.
Elongate ridge 200m SE of Hardy Hill, 1075m SSE from Woodward Pond,
Roxbury.
FZ-30 Well foliated medium-grained quartzose sillimanite schist with
strong fibrolite and mica lineations, gray with muscovite spangles
in foliation plane. 1 em-thick quartz-feldspar segregations.
40m due W from Little Monadnock summit, Fitzwilliam.
RX-186 Strongly foliated medium-grained black to gray biotitic schist
with 0.5 mm quartz clasts and 3 em quartz-feldspar segregations.
Elev. 277m, due E 425m from Roxbury Town Hall, !375m S of Otter Brook
Dam.
RX-189 Gritty, medium-grained, foliated gray sillimanite schist with
quartz clasts up to 0.5 em. Outcrop contains quartz-feldspar segregations and calc-silicate pods.
Elev. 326m, 1,.1 side of 345m knob 825m SSE from Otter Brook Dam.
HV-41 Massive,coarse, gray,unusually sillimanite-rich schist, with up
to 1 em garnet porphyroblasts and up to 3 em bundles of sillimanite,
probably pseudomorphs after andalusite.
Cobb Hill, elev. 573m, E side of western knob, Harrisville.
K-116A
Approximately same location as HV-41.
K-117
Cobb Hill, west of K-116A.
K-102
Garnet-rich schist.
S of Cobb Hill, elev. 566m.
NL-2
Gray, massive, medium to coarse gneiss, with 2 mm quartz grits and
quartz-feldspar segregations. Calc-silicate pods in same outcrop.
t 490m hilltop 150m S of road that leads SW from Nelson to Thunder Hill.
NL-29 Massive, medium-grained gray gneiss with 4-6 em quartz-feldsparmuscovite segregations. Outcrop contains calc-silicate pods.
Elev. 475m, whaleback outcrop W of road SW of Tolman Pond, Nelson.
MK-417 Medium-grained gray feldspathic schist, interlayered with
gray granulite in outcrop.
Ledge, E edge of swampy pond 750m SSE of Poole Reservoir, Jaffrey.
DB-130 Coarse blue-gray foliated muscovite-biotite gneiss.
Brook N of Hud Pond, elev. 306m, 750m N of Rt. 101, Dublin.
DB-97 Coarse gray schist with muscovite after sillimanite and garnets
up to 1. 5 em.
W-facing ledges, 450m E of Thorndike Pond Road, 75m S of marsh.
28
Age and Correlation
In central and southern New Hampshire, Hatchet al. (1983) have
proposed that monotonous gray pelitic schist and the-overlying,
slightly rusty-weathering pelitic schist with local calc-silicate
boudins and graded beds correspond to distal equivalents of the
Rangeley Formation in Maine south of the type area. In Maine the
Rangeley Formation contains a variety of clastic rock types, with
polymictic conglomerates at the base (Rangeley A), rusty metamorphosed
shale with graded interbeds of quartzite and two conglomeratic layers
(Rangeley B), and an upper pelitic member with quartz granule and pebble conglomerate beds which locally contain abundant calc-silicate
minerals (Rangeley C) (Moench and Boudette, 1970). Moench and
Boudette base a Silurian age (late Llandovery) for the Rangeley on
fossils in calcareous quartz conglomerate at Blanchard Ponds, Maine,
which does not occur in the type section, but is correlated with part
of Rangeley C.
t
In the Monadnock quadrangle the lower gray schist with granulite
and granulite-matrix conglomerate may correspond roughly with Rangeley
A, the uppermost schist containing quartz-pebble lenses with Rangeley
c, and the intervening sulfidic rocks with Rangeley B. Boone et al.
(1970) demonstrated evidence for a sediment supply to the west-or-north for the Merrimack synclinorium during the Silurian. The
presence of conglomerate horizons in the Monadnock and Gilsum-Marlow
areas suggests a more proximal site of deposition than for Rangeley
equivalents in central New Hampshire, or in the Peterborough quadrangle where no conglomerates have been reported (E. Duke, 1984)
(Figure 2). Duke mapped a lower gray schist unit and an upper rusty
schist unit which he suggested correlate respectively with Rangeley B
and C. He has locally mapped out a volcanoclastic member (Haunted
Lake Member) at the top of the Rangeley, and a banded calcareous
granulite horizon toward the base of the rusty unit. A thorough
discussion of several alternative stratigraphic interpretations was
presented by Peterson (1984) for the same rocks to the south near the
Massachusetts border.
In the Ware and Barre quadrangles, Massachusetts, some of the
schists mapped as Partridge and Littleton Formations by Field (1975)
and Tucker (1977) may be Rangeley equivalents (Robinson et al.,
1982a). The Lyon Road anticlinal belt in particular contains rocks
lithically identical to the red rusty Rangeley of the Monadnock area.
Rusty units containing amphibolites probably should retain assignment
to the Partridge Formation. Gray schist overlying them, particularly
where there is graded bedding (Tucker, 1977), might then correlate
with the lower part of the Rangeley. In light of the present study it
seems doubtful that the Littleton Formation would lie unconformably on
the Partridge Formation so far east of the Bronson Hill anticlinorium,
if the intervening Silurian formations thicken toward the east, as
believed. Most of the rocks immediately west of the Hardwick pluton
are on strike with rocks assigned to the Rangeley Formation in the
29
Monadnock quadrangle.
Chamberlain (1985) has found a variety of conglomerates in the
Gilsum-Marlow area, including massive clean quartzite,
granulite-matrix polymictic conglomerate, and quartz-pebble lenses and
beds. Some of these had previously been mapped as Clough Quartzite
(Heald, 1950; Trask and Thompson, 1967; Dean, 1977) but others are new
discoveries. It is becoming apparent that there is more than one
conglomerate horizon in the stratigraphy, and the question arises as
to how the Clough Quartzite and the Rangeley Formation correlate.
Both the Clough and the rocks at Blanchard Ponds which are correlated
with Rangeley C contain late Llandovery fossils (Figure 4), but the
fossils at Blanchard Ponds only date the Rangeley C unit, with no
direct control on the age of the lower parts of the formation. If, as
traditionally viewed (Osberg et al., 1968; Hatchet al., 1983), the
Rangeley represents an eastwardly thickening facies of the Clough,
then in the Monadnock area the Rangeley has been structurally transported into proximity with the Clough. On the other hand, the Clough
might lie at the base of a thick sequence of Rangeley-equivalent
rocks, including what is interpreted as Littleton Formation along the
east edge of the Keene dome (Plate 4), and for some reason the basal
unit of the Rangeley contains clean quartzite at this latitude rather
than the polymictic conglomerate of Rangeley A in Maine. I prefer the
former interpretation, as represented in Figure 2. According to this
scenario, lower units of the Rangeley would have been deposited in
pre-Clough time, which along the Bronson Hill axis was a period of
non-deposition. This interpretation necessitates a nappe-stage westdirected overthrust in the Monadnock quadrangle between the Bronson
Hill sequence and the Rangeley Formation.
West of the Connecticut Valley border fault there are three areas
of probable Rangeley Formation: the top of Fall Mountain, Walpole,
New Hampshire; rocks above the Ashuelot pluton northwest of
Winchester, New Hampshire; and the "Amherst block" between the
Hartford and Deerfield Mesozoic basins in Massachusetts. In the last
area, Jasaitis (1983) mapped out gray and rusty pelitic units as well
as lenses of conglomerate, some of which have a granulite matrix.
These three areas represent rocks which were transported far to the
west during the nappe stage of deformation.
Derivation
The Rangeley Formation formed as a clastic wedge along the basin
margin, proabably derived from rapid erosion of the former volcanic
island arc and Taconian Mountains to the west. The Taconic unconformity, so well documented along the axis of the Bronson Hill
anticlinorium, may be locally lacking in Maine where Upper Ordovician
rocks grade upward into the Silurian (Pavlides et al., 1968; Boone et
al., 1970, p.14; Hatchet al., 1983, p.758). Sediments were
-apparently being shed more or less continuously into the Merrimack
trough as the sea gradually transgressed toward the west during the
30
Silurian. The immature, polymictic rocks of the lower part of the
Rangeley indicate rapid sedimentation, whereas the cleaner quartz-rich
rocks near the top represent the transition to a more deeply weathered
source area with more extensive reworking of sediments. Deformation
of clasts and recrystallization of grain boundaries makes any analysis
of rounding characteristics impossible in the conglomerates of the
Monadnock quadrangle.
PERRY MOUNTAIN FORMATION
Description and Distribution of Rock Types
In the Monadnock quadrangle an interval of thinly bedded (2-5 em)
clean quartzite and gray to slightly rusty schist occurs in most
places between the Rangeley and Francestown Formations. Quartziteschist contacts are sharp, and graded beds are sparse. Where graded
beds are present the topping directions are commonly hard to read.
Because the Perry Mountain Formation has not previously been
recognized in the quadrangle, the most important exposures are listed
in Table 2. Some outcrops in the upper part of the Rangeley Formation
resemble the Perry Mountain, and indeed the contact may be
gradational. Following Hatch et al. ( 1983), the term "Perry Mountain"
is restricted to sharply bedded clean quartzite and schist. East of
the Thorndike Pond fault zone, little if any Perry Mountain Formation
can be recognized at the appropriate position.
The quartzites consist of quartz, muscovite, biotite, garnet, and
opaques, with or without sillimanite, plagioclase, retrograde
chlorite, and accessory apatite, zircon, and tourmaline (Table 3).
The schists are gray to somewhat rusty-weathering, quartz-biotiteplagioclase-garnet rocks (Table 3, DB-289A and MK-342) with or without
sillimanite, muscovite, orthoclase, and accessory opaques and zircon.
Locally there are large sillimanite pseudomorphs after andalusite.
t
Thin garnet-quartz granulite beds and lenses are locally present
in the Perry Mountain Formation. One such coticule bed is exposed on
Hurricane Hill in an outcrop of sillimanite-rich schist (DB-289A).
The mode estimated from thin section is 70% quartz, 28% garnet, and
the remainder biotite, sillimanite and opaques (DB-289B). John Lyons
and Norman Hatch (pers. comm., 1983) and Eusden et al. (1984) have
also observed coticule in the Perry Mountain Formation.
A different sort of coticule occurs within quartzite beds as
1-3 em lenses which weather out to form pits. Similar pitted quartzites also occur in the upper part of the Rangeley. The best exposure
of pitted quartzite is at MK-382 (Table 2, Location B) where it occurs
in nearly vertical beds two meters from the Francestown Formation.
One unweathered lens observed in thin section is composed of garnet,
quartz, biotite, apatite and opaques (Table 3, MK-382A). The
quartzite immediately around the lens is more quartz-rich than the
surrounding quartzite. Both the rim and the lens contain 5% apatite.
31
Table 2.
Important exposures of the Perry Mountain Formation.
A In complicated folds south of Poole Reservoir at Monadnock State
Park, especially E of foot trail E of Meade Brook, Jaffrey (see
Figure 24).
B W of Dole Brook 300m W of Whites Pond, N of Rt. 124 3 km W of
Jaffrey Center (MK-382W). Contact with Francestown Formation
is folded. Fossil-like pits in quartzite bed.
C At "Porcupine Ledges" along X-C ski trail N of Rt. 124, 1.9 km W
of Jaffrey Center (}~-309).
D N of the old Troy- Dublin Road,
(DB-190, DB-192).
1.9 km E of Gleason Brook bridge
E On SE ridge of Hurricane Hill 400m due W from Dark Pond, Dublin
(DB-289 and others) (see Figure 18).
F
On a knoll on the E shore of Howe Reservoir, Dublin, on south
limb of "Howe Reservoir syncline" (DB-283). Less well exposed
on north limb.
G N of the Old Chesham Road 1. 4 to 1. 8 km E of its junction with Rt.
101, Marlboro, where there is a small graphite prospect (MB-208).
H At Derby Hill in Harrisville, W of chimneys in clearing (HV-55-57)
and also N of summer house on NE part of hill, where there are
coticule beds and minor folds (HV-130 and -134) (see Figure 20).
I
On the ridge NW of Willard Hill (RX-18) and in the swamps W of
Willard Hill (RX-77-78) (see Figure 19).
J
On a small ridge at elevation 470m S of the E-W power lines which
cross Little Monadnock ridge 3.3 km S of Troy (TR-217).
K Several poorly exposed outcrops N of High Street 600m W of Troy.
'"'
Table 3.
Estimated modes of the Perry Mountain Formation.
Quartzite
Quartz
Coticule
I Schist
MK
587
MK
MK
MK
MK
MK
MB
677
356
382W
382A
382B
165
DB
289B
DB
289A
342
77
73
64
78
10
81
31
70
43
58
Plagioclase
tr
3
I
Orthoclase
MK
8
(An27)
5
I
Muscovite
9
15
18
2
Biotite
8
6
4
4
7
2
1
tr
22
14
Garnet
1
3
4
12
73
10
53
29
4
1
Sillimanite
2
X
22
Chlorite
(Retrograde)
Opaques
Graphite
Pyrite
Magnetite
Undifferent' d
1
2
8
(Mg) (Fe+Mg)(Fe)
2
1
1
Tourmaline
2
5
2
X
X
X
X
X
X
X
X
X
X
X
5
X
I
1
3
X
I
X
I
X
X
5
X
X
X
X
Allanite
Grunerite
2
X
X
Apatite
Zircon
16
X
I
13
w
N
33
List of Samples in Table 3.
MK-587 Gray fine-grained quartzite.
Ark Brook, 700m NE of State Park headquarters, elev. 408m, Jaffrey.
MK-677 Gray well foliated micaceous quartzite.
Woods S of Harling Trail, E of Ark Brook and Hinkley Trail, Jaffrey.
ffiZ-356 Well foliated, fine-grained gray to brown micaceous quartzite.
Chlorite and biotite intergrown. Garnet concentrated in layers.
300m N of Rt. 124, 1400m E of Cummings Pond, east of ridge held up
by Francestown Formation, Jaffrey.
MK-382W Pale gray fine-grained quartzite with pitted horizons, interbedded with gray schist. In contact with Francestown Formation to
the east (MK-382). Three modes from one thin section are listed
in Table 3 as follows: MK-382\v, quartzite; MK-382A unweathered
coticule from pit; MK-382B rim around the pit, about 3mm thick.
Dole Brook, 305m W of Whites Pond, north of former "Mountain Shade
Inn" (Nelson, 1975), Rt. 124, 3 km W of Jaffrey Center.
MB-165 Fine-grained, layered pink to gray to green quartz-garnetgrunerite granulite, from a boudin in well bedded gray quartzite
and schist.
325m SE of Horse Hill Road-Richardson Road junction, Marlboro, on
small N-S ridge north of stone wall.
DB-289B
2 em layer of fine-grained coticule in sample DB-289A.
DB-289A Mottled coarse schist with biotite-fibrolite clumps.
120m SE of Hurricane Hill summit, on ridgeline, 400m due W from
Dark Pond, Dublin.
Mk-342 Gray schist with 5mm garnets, well foliated, quartz and
feldspar somewhat segregated into streaks.
400m E of Meade Brook, 400m S of Poole Reservoir, Jaffrey.
34
Some of the pits resemble the molds of brachiopod fossils. Similar
rocks in the Clough Quartzite at Hetty Brook, Croydon Mountain, New
Hampshire, turned out to yield diagnostic fossils. There, "lenticular
masses of garnet simulate the shapes of fossils" (Boucot and Thompson,
1963). Samples from MK-382 have been sent to Arthur Boucot for study.
A third type of coticule was found in the Perry Mountain Formation
on a small ridge east of Horse Hill Road, Marlboro, as an 80 X 20 em
boudin consisting of garnet, quartz, grunerite, and opaques (MB-165).
Thickness
The Perry Mountain Formation is relatively thin in the Monadnock
quadrangle. Some maximum estimates are 61 m on Old Chesham Road
ridge; 53 m north of High Street in Troy; and 73 m from an area north
of Rt. 124, 245 m southeast of Porcupine Ledges. In the folded rocks
south of Poole Reservoir, a maximum of 18 m was estimated. The
formation is absent in most of the area east of the Thorndike Pond
fault zone. The exposed section of Perry Mountain Formation in the
type area is 370m thick (Boone, 1973). Hatchet al. (1983) estimated
a thickness of 500 min central New Hampshire, while E. Duke (1984)
estimated 120 m in the Peterborough quadrangle. Although tectonic
thinning cannot be ruled out as a factor, the greatly reduced
thickness in the Monadnock quadrangle may indicate a deposition site
much closer to the source area (Figure 2).
Age and Correlation
~
The Perry Mountain Formation in Maine, as revised by Osberg et al.
(1968), consists of cyclically interbedded white quartzite and lightgray micaceous metamorphosed shale. Cross laminations and convolute
laminations are common in the quartzite. No fossils have been found
in the Perry Mountain, but it lies between units believed to be
Silurian (Figure 4). The Sangerville Formation is at least partially
correlative and contains late Llandovery to middle Wenlock or early
Ludlow graptolites (Pankiwskyj et al., 1976).
Correlative rocks elsewhere in New Hampshire include the Roundtop
Quartzite (Englund, 1976), the upper part of the Crotched Mountain
Formation in the Hillsboro quadrangle (Nielson, 1981) and, according
to E. Duke's 1984 remapping of the Peterborough quadrangle, part of
the Crotched Mountain Member of the Littleton (Greene, 1970). The
rocks on Crotched Mountain itself are Rangeley Formation. The Perry
Mountain in the Peterborough quadrangle is not only thicker than at
Monadnock, but is also more feldspathic and contains minor calcsilicate pods toward the base. The formation is also present locally
along the west edge of the Ashuelot pluton (David Elbert, pers. comm.,
1984). No rocks lithically similar to the Perry Mountain have yet
been recognized in Massachusetts, or in the Gilsum-Marlow area.
35
Derivation
The Perry Mountain Formation in the type area is conformable and
gradational with the underlying Rangeley Formation (Boone et al.,
1970), and also thickens eastwards, indicating a similar depositional
setting. The thin bedding and clean quartzites suggest a maturing of
the erosional cycle in the source area, with more extensive chemical
weathering. If the Clough Quartzite and the Perry Mountain are
roughly correlative, the change from Rangeley to Perry Mountain may
represent the point at which rocks of the former volcanic arc became
eroded down enough to allow sediments from the more distant Taconian
mountains to reach the Merrimack basin. The Clough, Perry Mountain,
and Sangerville might represent respectively shelf, slope and rise
deposits.
FRANCESTOWN FORMATION
Description and Distribution of Rock Types
Extremely rusty-weathering, blocky calc-silicate granulite and
rusty-weathering graphitic schist make this the most distinctive unit
in the Monadnock stratigraphy. Graphite and sulfides together compose
up to 10% of the rock. The best place to observe the Francestown
Formation is at Monadnock State Park, in the campground and south of
Poole Reservoir. It is less well exposed along the northwest side of
the Monadnock syncline due to glacial cover, and it crops out discontinuously around the syncline south of Troy. Although graded beds are
rare, the topping direction of the sequence is well established, so
that where the Francestown is in association with the overlying Warner
Formation, as it is around Dublin Pond and Howe Reservoir, the topping
direction is certain. Both the upper and lower contacts are sharp.
Fine graded laminae can be seen in some thin sections. The formation
is well exposed along the Old Chesham Road ridge in Marlboro, at Derby
Hill west of Silver Lake, and west of Willard Hill in the southeast
corner of Roxbury. At Derby Hill and in several infolds of
Francestown surrounded by Rangeley east of Thorndike Pond, schist
predominates over calc-silicate granulite, whereas the reverse is true
elsewhere in the quadrangle. There are some interesting cemented till
outcrops along Ainsworth Brook, west of Poole Reservoir, developed
from pieces of Francestown held together by iron sulfides.
The calc-silicate granulites are composed of quartz, calcic
plagioclase, graphite and iron sulfide minerals with or without
sphene, actinolite, diopside, zoisite, microcline, minor muscovite,
Mg-biotite, and Mg-chlorite, and accessory apatite, zircon, tourmaline, and allanite (Table 4). The iron sulfides include pyrite,
pyrrhotite and secondary marcasite apparently replacing pyrrhotite.
Many outcrops contain enough pyrrhotite to deflect a compass needle.
Weathering of the sulfides results in red, orange, yellow, brown and
black outcrops. The rocks are hard and well jointed, breaking into
brick-sized fragments. The unweathered calc-silicates may be massive
....
Table 4.
Estimated modes of the Francestown Formation.
Calc-silicate Granulite
Quartz
Plagioclase
MK
MK
MK
MK
502
414
502A
701B
16
45
10
32
HK
726
DB
78
TR
50
TR
127
38
45
20
41
35
49
3
Hg-rich Biotite
9
Chlorite
(Retrograde)
Actinolite
RX
_R_
36
32
(An67)
16
Muscovite
tr
18
tr
2
(Mg)
47
7
35
27
3
17
7*
Diopside
4
2
43
32
23
tr
5
5
12
2
(Mg-Fe) (Mg)
Ferrian Zoisite
--
W<
MK
11.L 351
26
40
70
19
24
25
18
25
2
(An66+)(An73)(An66+)(An71)(An62)(An70+)(An61)(An63+) (An25?)
Micro cline
Sphene
Schist
10
3
2
2
6
14
20
2
2
Rutile
Opaques
7
Ilmenite
X
Graphite
X
Pyrrhotite
X
Pyrite/Marcasite
Undiff. other
X
Apatite
X
I
2
1
8
10
3
X
X
8
3
5
8
7
5
3
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
Zircon
X
X
X
X
X
X
X
X
X
tr
X
Tourmaline
X
Allanite
X
*uncertain identification
tr
(Fe)
I
X
X
w
0'1
37
List of Samples in Table 4.
MK-414 Very rusty fine-grained dark gray calc-silicate granulite,
strongly magnetic, delicately laminated: some layers richer in
muscovite, others in quartz, others in pyrrhotite, one layer
up to 5% sphene.
Ainsworth Brook, elev. 433m, S of Parker Trail, Mt. Monadnock.
MK-502 Rusty fine-grained dark gray calc-silicate granulite, strongly
magnetic. Actinolite and sulfides visible in hand sample. MK502A is a lmm sphene-rich layer.
Meade Brook below Poole Reservoir, elev. 390m, at contact with
Warner Formation, Monadnock State Park.
Rusty fine-grained gray calc-silicate granulite. Not magnetic.
6lm NW of Stony Brook Farm Road, 880m SW of Thorndike Pond, Jaffrey.
}~-701B
MK-726 Rusty fine-grained dark gray calc-silicate granulite, with
network of pyrrhotite veinlets, strongly magnetic. From a pod in
graphitic whiteschist.
Have ~ Pines State Park, on 382m hill, 1020m NNE of Jaffrey Center.
DB-78 Rusty fine-grained dark calc-silicate granulite in a small
fold hinge. Biotite and graphite have moderately developed
preferred orientation parallel to axial plane. Strongly magnetic.
250m N from Trowbridge house, on bridle path toward Hurricane Hill,
W of SH corner of Dublin Pond, Dublin.
TR-50 Very rusty actinolite-bearing light gray calc-silicate granulite, not magnetic.
Brandy Brook, small falls 200m upstream from Ashuelot River, Troy.
TR-127 Rusty light gray to green calc-silicate granulite, not
magnetic. Interlayered with graphitic whiteschist.
Brook south of High Street 750m WSW from Troy village green.
HK-125 !<'ine-grained rusty-weathering dark gray schist, not magnetic.
Jeep road 1200m due E of Jock Page Hill, Jaffrey.
MK-351 Medium-grained deeply rusty-weathering granular schist. White
micas and graphite prominent in unweathered rock. Weakly magnetic.
Field behind Adullam (old seminary) on Old Dublin Road, Jaffrey.
RX-97 Rusty-weathering medium-fine-grained white schist with graphite.
Along old wood road 900m SW of Willard Hill, Roxbury.
38
or finely laminated, and dark gray to nearly white.
Schists of the Francestown Formation weather deeply to a rusty
brown-orange-yellow rind surrounding a white interio~ flecked with
graphite. Unweathered schist is hard to find, but it is dark gray.
The schists contain quartz, muscovite, Mg-biotite, sillimanite,
graphite, iron sulfides, and secondary Mg-chlorite, with or without
plagioclase, rutile, sphene and zircon. Neither the schists nor the
calc-silicates contain black mica, and this is one of the chief
criteria for distinguishing isolated outcrops from otherwise similar
rusty schists in the Rangeley Formation. The sulfur content is apparently so high that nearly all the iron was consumed during metamorphism to form pyrrhotite, leaving little for the silicate minerals
Tracy et al., 1976a; Robinson et al., 1982a). The sphene shows
red-brown to pale yellow pleochroism in thin section.
Thickness
Maximum estimates are 137 m thick in Dole Brook at elevation
451 m, east of the old toll road on Mt. Monadnock; 53 m thick north of
High Street, Troy; and 76 m thick south of Rt. 124, 3 km west of Jaffrey Center. The thickness of the Francestown is greatly varied where
the rocks are complexly folded, such as around Poole Reservoir.
The Francestown and correlative rocks, as with the other Silurian
units, thicken eastwards away from the Bronson Hill anticlinorium
(Figure 2). Field (1975) estimated 15m thickness for correlative
rocks in the Ware quadrangle, Massachusetts. E. Duke (1984) estimated
10-50 min the Peterborough quadrangle, Englund (1974) 425 min
central New Hampshire, Hatchet al. (1983) 30-300 min northeastern
New Hampshire and 825 m at Smalls-Falls, Maine. In the Bronson Hill
anticlinorium the Fitch Formation, believed to correlate with both the
Francestown and Warner Formations, was estimated at 120-150 m thick in
the Littleton area (Billings, 1937) and 0-250 m in the Orange, Massachusetts, area (Hatchet al., in press).
Age and Correlation
The Francestown was defined as a member of the Devonian Littleton
Formation by Greene (1970), and although the units do not connect on
the ground, he suggested a correlation with the Rusty Quartzite Member
of the Littleton in the Monadnock quadrangle (Fowler-Billings, 1949a).
Nielson (1974) changed the unit to formation status, and suggested a
correlation with the Silurian Smalls Falls Formation of Maine, which
may be a more distal facies with much more pelite and less
calc-silicate granulite. At the type locality of the Smalls Falls
Formation non-calcareous quartzite is interbedded with schist, but
calc-silicate rocks occur elsewhere near the top of the formation
(Hatchet al., 1983). Fossils are not known in either the Francestown
or Smalls Falls Formations. Graptolites in the correlative Parkman
Hill Formation, central Maine, are middle Wenlockian to early
39
Ludlovian (Pankiwskyj et al., 1976).
Field (1975) mapped a rusty calc-silicate horizon in central
Massachusetts, west of the Coys Hill Granite, and identified this as
identical to the Francestown of the Monadnock area, and to rusty
calc-silicate rocks at Gee Mill in the Sunapee septum where Dean
(1977) mapped them as Fitch Formation. The regional importance of the
Gee Mill occurrence will be addressed later.
White sulfidic schist (Spsq on the Massachusetts state map, Zen et
al., 1983) is exposed about 3 km east of the sulfidic calc-silicate-horizon, along what is shown on the state map as the contact between
Partridge and Littleton Formations and between Partridge and Paxton
Formations (Field, 1975; Zen et al., 1983). Berry (1985) has extended
this belt of Smalls Falls equivalent rocks into Connecticut. Tucker
(1977) reported feldspathic and micaceous quartzites associated with
the white schist. The Francestown east of the Thorndike Pond fault
zone and at Derby Hill, as well as that in the Pack Monadnock Range in
the Peterborough quadrangle (E. Duke, 1984), has a schist to granulite
ratio more like that of the white schist in Massachusetts and the
Smalls Falls Formation in Maine, than that of the typical Francestown.
Assuming that the Smalls Falls is a more distal facies originally
deposited east of the Francestown, then the Smalls Falls-like rocks in
the Monadnock quadrangle may have been tectonically transported
westward to their present position. I think that the name
"Francestown Formation" should be retained for the Smalls Falls
correlative rocks with a high proportion of calc-silicate granulites,
to emphasize the facies differences.
Boone (1973) reported a 30-50 m transition from Perry Mountain to
Smalls Falls Formations in the Little Bigelow Mountain area, Maine.
The contact in the Monadnock quadrangle is abrupt, and the Perry
Mountain is locally missing. Whether the absence is due to local
erosion or non-deposition is unknown, but it is most apparent below
the Smalls Falls-like rocks east of the Thorndike Pond fault zone. A
po~sibility exists that the rusty white schists in this area are
stratigraphically lower than the Francestown, and occur as one or more
horizons within the Rangeley Formation. The main argument against
this interpretation is that lithically identical rocks occur above
very well bedded Perry Mountain Formation at Derby Hill. Furthermore,
Hatchet al. (1983) reported that the Perry Mountain is locally
missing below Francestown calc-silicate rocks in the Pinkham Notch
area.
Derivation
The strongly sulfidic and graphitic Francestown Formation was
deposited as calcareous mud in an environment with restricted oxygen
supply. The apparent lack of fossil remains, and the presence of fine
laminae which would have been destroyed by bioturbation, might
indicate truly anoxic conditions (Williams and Rickards, 1984). The
40
Merrimack trough may have become cut off from circulation with the
open ocean. Alternatively, anoxic conditions may have resulted from a
local poorly oxidized mass of water, as described by Williams and
Rickards (1984) in modern open oceans. The water depth had to be less
than the carbonate compensation depth, perhaps analogous to the modern
Black Sea, where thinly laminated carbonate-rich and organic-rich muds
are being deposited (Reineck and Singh, 1980, p.487-500).
WARNER FORMATION
Description and Distribution of Rock Types
The Warner Formation is exposed in all the areas described for the
Francestown. The most accessible exposure~ are in Monadnock State
Park northeast of Gilson Pond and downstream from Poole Reservoir.
Although they have not been mapped separately, two parts can be
distinguished.
The lower part of the Warner Formation consists of thinly bedded
(0.5 - 5 em) green, pink, gray or white calc-silicate granulites. The
color variation is due to differences in the relative proportions of
actinolite, diopside, garnet, clinozoisite, ferrian zoisite, sphene,
biotite and calcite (Table 5a). Quartz is present in all rocks, and
zircon, tourmaline, allanite and opaques are accessory. A pitted pink
horizon (sample MB-8A) contains calcite, idocrase, and an unknown,
colorless, prismatic mineral associated closely with garnet. This
unknown mineral is mainly in the cores of garnet, but also in the
matrix. The garnet rims are free of this mineral. Rare schist beds
with graded bedding occur in the lower part of the Warner, for example
HV-7 (Table 5a), and near the base of the formation west of Poole
Reservoir.
~
In the field the lower part of the Warner forms distinctive slabby
outcrops, or smooth water-worn surfaces in brooks. Several brooks
follow this more easily eroded unit, particularly Meade Brook south of
Poole Reservoir, the brook draining Gilson Pond, Minnewawa Brook in
Eliza Adams Gorge and Gleason Brook northwest of Mt. Monadnock.
The upper part of the Warner Formation consists of fine-grained
quartz-biotite-plagioclase granulite, with minor muscovite and sphene,
and accessory zircon, garnet, apatite, rutile, tourmaline and opaques
(Table 5b). Sphene is pleochroic red to yellow in both the granulites
and the calc-silicates. The plagioclase is andesine. Outcrops are
thickly bedded, massive, with smooth purplish-gray "salt and pepper"
surfaces. These granulites superficially resemble the fine-grained
tonalites and microdiorite dikes found in the quadrangle, so care must
be taken to distinguish them. They are also similar to the granulite
beds in the Rangeley Formation. Calc-silicate pods with mineralogy
similar to the lower part of the Warner are common in the upper part.
The pods are generally zoned with a weathered-out depression around a
very resistant core. Nelson (1975) reported wollastonite from one of
....
Table Sa.
HK
04D
green
HK
004
white gray pink pink white white
12
66
15
(An70+)
5
(An35)
Quartz
Plagioclase
Estimated modes of the lower part of the Warner Formation.
Micro cline
50
83
73
34
40
8
15
7
12
Diopside
15
20
70
49
39
15
31
39
(An66+)(An66+)(An53)
10
10
20
15
36
4
4
4
6
27
42
5
6
27
5
X
1
25
20
3
2
20
10
25
1
25
49
3
HV
NH/MND
7-74
7
green white (schist)
20
Garnet
2
tr
2
Calcite
5
1
10
3
Idocrase?
tr
Unknown mineral
15
Biotite
41
tr
30
Ferrian Zoisite
Sphene
45
16
Clinozoisite
Actinolite
73
MB
8
pink green pink pink
2
5
1
5
Muscovite
14
3
Opaques
(mainly graphite)
Apatite
Zircon
X
Tourmaline
X
Allanite
X
X
X
X
X
X
X
2
.
2
X
X
X
X
~
......
...
Table Sb.
Estimated modes of the upper part of the Warner Formation.
Calc-silicate Pods
Granulite and Schist
MK
04B
Quartz
Plagioclase
Muscovite
DB
8
40
DB
9
68
MK
MK
638
629
41
46
25
51
25
16
20
6
7
30
(An39)(An40)(An37)(An75)(An55)(An76)
3
2
20
7
25
21
10
28
NH/MND 8-74-1
core trans. rim
MK
759A
Quartz
30
X
44
Plagioclase
tr
X
24
(An64)
Clinozoisite
Zoisite
5
X
5
Diopside
20
X
5
8
Grossular
8
X
Sillimanite
6
4
Actinolite
Staurolite (Retrograde)
2
Biotite
Cordierite
Garnet
10
X
lO(Mg)
Chlorite (Retrograde)
Sphene
Opaques
Graphite
Ilmenite
Pyrrhotite
Pyrite
2
2
tr
X
X
X
X
X
2
1
Calcite
18
X
Bustamite
18
X
4
60
45
18
(An76)
29
12
4
2
tr
5
21
4
18
2
X
Sphene
1
X
3
3
2
Opaques
Graphite
X
X
X
X
X
X
X
X
X
X
1
1
X
X
X
X
X
X
X
Apatite
X
X
Zircon
X
Biotite
"A
Allanite
tr
tr
Apatite
X
X
X
Zircon
X
X
X
X
X
X
X
Tourmaline
Rutile
19
28
X
MK-759B
core rim
X
X
X
,c..
N
43
List of Samples in Table 5.
MK-04D Fine-grained greenish-gray granulite with 2 mrn actinolite.
W of old toll road, Mt. Monadnock, elev. 497rn, Jaffrey.
MK-004 Fine-grained banded calc-silicate granulite with compositional
layering on a scale of 1 to 10 rnrn. Slabby-weathering outcrop.
W of old toll road, Mt. Monadnock, elev. 497rn, Jaffrey.
MB-8
Fine-grained bedded calc-silicate, with pink, pitted, dark brownweathering horizon.
South bank, Gleason Brook, 262m W of Dublin-Marlboro to~ line.
NH/HHD-7-74 Well bedded calc-silicate collected by Carl Nelson.
Heade Brook, below Poole Reservoir, Jaffrey.
HV-7 Medium-grained gray feldspathic garnet schist, cut by mafic dike.
Eliza Adams Gorge, N bank, 160m downstream from Howe Reservoir Darn,
Harrisville.
HR-04B Fine-grained purplish-gray granulite, massive in outc!"op but
with foliation defined by biotite. ,
W of old toll road, ~1t. Monadnock, elev. 504rn, Jaffrey.
DB-8 Fine-grained light-gray granulite. Foliation defined by biotite.
iJE of Dublin Pond, at junction of Rt. 101 and Old Harrisville Road.
DD-9 Fine-grained gray feldspathic schist.
NE of DB-8 on steep south-facing slope, Dublin.
Medium-grained gray biotitic schist with 1 ern sillimanite
patches and smaller garnets. Interbedded with granulite.
Ledges on W bank of Stony Brook, elev. 402rn, 875rn SH of Gilson Pond,
Jaffrey.
~rrC-638
t
}~-629
Medium-grained gray granular schist with 0.5-1 ern garnet porphyroblasts, which weather as bumps on the outcrop.
91rn SW of SW corner of Poole Reservoir, Monadnock State Park, Jaffrey.
l~C-759A
Fine-grained biotite-quartz-feldspar granulite.
Birchtoft Trail, elev. 414rn, H of Gilson Pond, Jaffrey.
NH/}fi~D-8-74
Zoned calc-silicate pod collected by Carl Nelson.
Area east of outlet to Gilson Pond, Jaffrey.
Calc-silicate pod enclosed by granulite sample MK-759A. The thin
section includes the transition zone between the two, which is listed
in the table as the 11 rirn", and which weathers more readily than the core.
l~C-759B
44
these pods, which is actually bustamite (see section on metamorphism).
A comparison of estimated modes from Rangeley and Warner calc-silicate
pods shows somewhat higher plagioclase content in the Rangeley pods,
but otherwise a similar range of mineral assemblages.
Schist is interbedded with the granulite, the proportion of schist
increasing toward the top of the formation. The topping direction of
the sequence is confirmed by graded beds at several localities. The
contact with the overlying Littleton Formation is gradational;
calc-silicate pods and granulite beds persist into the lower part of
the Littleton. For mapping purposes, the uppermost continuous
granulite bed is taken as the upper contact of the Warner. Schists
within the Warner and lowermost Littleton are more feldspathic than
those higher in the Littleton. Horizons bearing 0.5 to 2 em garnets
are common, and the garnets result in characteristic bumpy-weathered
surfaces. The cordierite-bearing sample MK-629, described in detail
in the section on metamorphism, came from one of these feldspathic
"big-garnet schists".
Thickness
A section was measured west of the old toll road on Mt. Monadnock
between elevations 490 and 520 m. Allowing for a three-meter granite
sill, the lower part is 45 m thick, and the upper part is 37 m thick.
In Dole Brook east of the toll road, a total thickness of 90 m was
estimated for the Warner, but the upper contact is poorly exposed.
North of High Street in Troy the Warner is at least 30 but not more
than 68 m thick. This is very thin compared to maximum estimates of
450 min central New Hampshire (Hatchet al., 1983). In the
Peterborough quadrangle the Warner thins southeastward and is locally
absent (E. Duke, 1984).
Age and Correlation
~
Nielson (1974) established the Warner Formation as calc-silicate
granulite and biotite schist stratigraphically above the Francestown
in the Mt. Kearsarge and Hillsboro quadrangles. Neither the Warner
nor the correlative Madrid Formation of Maine has yielded fossils, but
Hatchet al. (1983) present a strong argument for correlation with the
Fitch Formation to the west, which bears conodonts of Pridolian age
(Harris et al., 1983). The Fitch is overlain by type Littleton Formation, and both the Warner and Madrid are overlain by Littleton-like
rocks. The age of the Warner thus hinges on the correctness of the
Littleton correlations (Figure 4).
The upper part of the Warner in central New Hampshire includes
some pervasively rusty-weathering sillimanite-biotite schist with
calc-silicate pods, which John Lyons informally calls the Andover
Member (pers. comm., 1982). Warner schists in the Monadnock quadrangle are generally gray-weathering.
45
The Warner probably correlates with part of the Paxton Formation
in central Massachusetts (Robinson, 1981; Hatchet al., 1983). Part
of the Paxton lies above the Smalls Falls rock type8; but other gray
granulites mapped as Paxton apparently lie lower in the section (Henry
Berry, pers. comm., 1984), and may be equivalent in part to the
Vassalboro Formation of Maine, which was formerly thought to be an
eastern facies of the Madrid (Osberg et al., 1968; Osberg, 1980).
Derivation
The lower part of the Warner Formation was deposited as calcareous
mud similar to the Francestown, but lacking the graphite and sulfides.
Fossils in the Fitch reflect a hi gh-energy, near shore environment
(Harris et al., 1983), and the Warner is presumably a deeper water
facies of the Fitch. The transition to the upper part of the Warner
might be attributable to an influx of detritus, perhaps in the form of
volcanic ash (McKerrow and Ziegler, 1972). Volcanism was going on
during the upper Silurian along the present-day Maine coast, and might
have provided a source of ash (Brookins et al., 1973). In northcentral Maine, volcanics are interlayered with clastic rocks and minor
limestone on the northwest flank of the Weeksboro-Lunksoos Lake
anticline (Neuman and Rankin, 1980, p.92) . The Seboomook Formation
overlies these volcanics in a relationship similar to that of the
Littleton overlying the Warner and Madrid Formations, with a gradually
increasing proportion of pelitic sediments.
LITTLETON FORMATION
Description and Distribution of Rock Types
The Littleton Formation in the Monadnock quadrangle consists predominantly of gray-weathering pelitic schist and micaceous quartzite.
The schists consist of quartz and muscovite with or without
biotite, staurolite, garnet, sillimanite, plagioclase, K-feldspar,
graphite and other opaques, retrograde chlorite, chloritoid and
staurolite, and accessory tourmaline, zircon and apatite (Table 6).
Pseudomorphs of sillimanite after andalusite are common in some parts
of the quadrangle, depending on the metamorphic history, but no relict
andalusite was found in thin section. Perry (1904) observed that some
pseudomorphs preserve chiastolite cross-shaped inclusion patterns. He
also noted that pseudomorphs which have been further replaced by
muscovite tend to weather out as pits in the rock, in contrast to the
raised lumps where sillimanite remains. Robinson (in Hatchet al.,
1983) has dubbed similar pseudomorphs in Massachusetts "andalumps".
Table 6a.
Estimated modes of the lower part of the Littleton Formation.
Zone II
Quartz
Plagioclase
Zone III
Zone III (Retrograded)
Zone IV
sz
MK
MK
MK
11
59
371
HV
162
MK
216
DB
10
DB
69
DB
171
RX
604
TR
20
RX
432
MB
172
TR
27
41
17
32
41
22
39
36
31
25
15
22
40
24
15
6
1
2
2
4
(An31)
3
2
K-feldspar
18
Cordierite?
X
Biotite
16
24
23
15
25
21
Zl
10
11
tr
Muscovite
21
27
13
28
8
28
13
30
37
54
3
16
5
2
3
10
7
8
tr
13
9
37
7
17
19
12
Garnet
Sillimanite
Chlorite
(Retrograde)
24
.
1
(Mg)
9
10
tr
25
60
1
41
39
4
7
8
tr
28
1
tr
2
17
(Fe) (Fe-Mg)(Fe)
Chloritoid
Staurolite
172
10
(Fe)
12
(Fe)
11
(Fe)
19
tr
Opaques
Graphite
1
Ilmenite
1
Hematite (weath.)
Undifferent'd
2
2
2
3*
X
X
X
X
X
X
X
X
X
Apatite
X
X
X
X
Zircon
X
X
X
X
Tourmaline
X
2
X
2
3
1
2
2
X
X
X
X
X
X
X
X
X
2
3
X
X
X
X
1
2
2
X
X
X
X
X
X
tr
X
X
X
X
X
2
Allanite
tr
X
1
X
Sphene
X
*includes pyrite inclusions in andalump
~
0'1
47
List of Samples in Table 6a.
SZ-27 Medium-grained gray to slightly rusty-weathering feldspathic
schist. Staurolite visible with a hand lens. Slightly retrograded.
Small outcrop E of roadcut N side Rt. 12, 200m E of junction with
Mill Road, East Swanzey.
MK-432 Well foliated gray garnet schist. Outcrop contains granulite
bed and contact with Warner Formation.
Meade Brook, 27m S from Poole Reservoir dam, Monadnock State Park.
MK-604 Medium-coarse gray schist with sillimanite patches and garnets
up to 1 em. 68m upstream from nearest Warner outcrop.
Meade Brook, 500m upstream from Poole Reservoir, elev. 457m, Jaffrey.
MB-172 Medium-grained gray schist with up to 3 em fibrolite bundles
and 2 mm garnets. Very rough-textured outcrop.
Glen Brook, elev. 309m, in small gorge, Marlboro.
TR-11 Medium-grained massive gray schist with up to 4 em sillimanite
pseudomorphs after andalusite with well preserved chiastolite
structure,and 2 nun garnets. Very rough "boot-grabbing" outcrop.
NW ridge of Gap Mountain, SW of little gap at 488m, Troy.
TR-20 Coarse gray micaceous schist with strong foliation cut by
crenulation cleavage.
Sewer excavation, Monadnock St ., 40m N of Rt. 12, Troy.
Medium-coarse gray schist with abundant 1 em sillimanite bundles.
E of jeep road 200m E of Woodward Pond,350m N of jeep road junction,
Roxbury.
~X-59
MK-371 Well foliated fine-grained gray schist with sillimanite clumps
and garnets up to 5nun. Slightly retrograded.
180m S of Rt. 124, 360m E of Cummings Pond, Jaffrey.
HV-162 Hedium-grained gray to slightly rusty schist with up to 3 em
sillimanite and 4 mm garnet. Rough outcrop. Slightly retrograded.
Small knob, elev. 408m, Nl5E 400m from 376m hill, BOOm E of Chesham Pond.
t
tOC-216 Well foliated gray muscovite schist with 5 mm green chlorite
patches and unaltered garnet. Strongly retrograded.
Bald Rock, elev. 798m, SW ridge of Mt. Monadnock, Jaffrey.
DB-10 Fine- to medium-grained gray-green schist with 1 em knobs of
chloritoid and elongate quartz-muscovite streaks. Locally rusty.
Strongly retro~raded.
Beech Hill, elev. 57lm, southernmost of three knobs on ridge, Dublin.
DB-69 Massive, medium-grained gray schist. Strongly retrograded.
What appears to be sillimanite in hand sample has been replaced.
W of Rt. 137, 200m SW of sharp bend in Windmill Hill Road, Dublin.
DB-171 Somewhat rusty-weathering medium-grained gray schist. Retrograded.
Hountain Brook, N side of Mt. Monadnock, elev. 436m at second old
dam site upstream from farm, Dublin.
~X-172
Hedium-grained gray schist with 5 mm sillimanite and abundant
3 mm garnet.
64m N of jeep road, 700m E of Woodward Pond dam, elev. 453m.
..•
Table 6b.
Quartzite
Schist
Retrograded Schist
Coticule
Misc.
DB
255
HV
135
MK
210
MK
210A
DB
114
DB
196
DB
253
DB
17
MK
873
6
HV
135A
DB
196A
DB
55
DB
220
DB
236
65
82
35
43
10
25
47
35
24
48
61
45
22
58
37
tr
10
(An43)
MK
Quartz
Estimated modes of the upper part of the Littleton Formation.
Plagioclase
tr
4
(An24)
1
tr
(An25)
Biotite
12
3
17
11
3
10
12
2
Muscovite
15
5
15
35
55
35
33
48
44
Garnet
1
4
7
tr
5
tr
3
12
Sillimanite
2
13
tr
13
X
7
tr
10
39
(An14)(An64+)
3
43
7
30
X
3
14
5
4
16
50
62
tr
Hornblende
9
Chlorite
(Retrograde)
1 .
3
(Fe) (Mg)
5
(Fe)
2
(Fe)
Chloritoid (Retr.) tr
3
9
Staurolite (Retr.) tr
1
6
1
1
Opaques
Graphite
Ilmenite
Undifferent'd
X
X
Apatite
X
Zircon
X
Tourmaline
2
1
2
2
X
5
(Fe)
.5
17
9
(Mg)(Fe-Mg) (Fe)
3
2
3
X
1*
X
5
tr
(Mg-Fe)(Fe)
5
(Mg)
2
X
X
2
2*
X
2
4
X
X
X
2
(Mg)
6
X
X
X
X
X
X
X
X
X
X
1
3
tr
2
X
X
X
X
3
X
X
X
tr
Allanite
Sphene
7
X
X
X
2
X
tr
X
X
*Includes chalcopyrite and pyrrhotite inclusions in garnet
X
~
00
49
List of Samples in Table 6b.
MK-873 Light gray micaceous quartzite with poorly defined laminations.
Interbedded with schist in outcrop.
S of Pumpelly Trail, E of Red Spot Trail, elev. 869m, }{t. Monadnock.
DB-255 Medium-fine-grained gray quartzite from the base of a 60 em
graded bed.
Ledges N of Col. Sewell house, S end Beech Hill, elev. 520m, Dublin.
HV-135 Medium-grained gray schist with 5 mm coticule bed (HV-135A).
Float block, NW end of Beech Hill ridge, elev. 455m, SSW of Harrisville.
MK-210 Medium-grained gray schist with biotites across the foliation,
unaltered garnet, pitted weathering (chlorite-chloritoid), and
retrograded "andalumps" (MK-210A) up to 8 em long.
Ledges S of White Cross Trail, elev. 786m, known as "The Caves",
Mt. Monadnock.
DB-114 Fine-grained gray-green schist with 1-3 em-long sillimanite after
andalusite and garnets up to 5 mm. Retrograded.
Snow Brook, elev. 468m, 2120m NW of Thorndike Pond, Dublin.
DB-196 Coarse-grained gray schist with up to 5 em altered sillimanite
pseudomorphs after andalusite and an 8 mm coticule bed (DB-169A).
Retrograded.
N side of Mt. Monadnock, elev. 533m, upper branch of Gleason Brook
above Dublin Trail.
DB-253 Rhythmically bedded gray schist and quartzite with conspicuous
1 em chlorite patches and unaltered garnets up to 1 em. Retrograded.
Float block on steep S-facing slope 620m E of Old Chesham Road, S of
Harrisville-Dublin town line.
DB-17 Rhythmically bedded gray schist and quartzite with 1.5-2 em
graded beds. Sparse garnets up to 1 em. Retrograded.
320m SW from where Monument Road crosses Dublin-Harrisville town
t
line, E of Beech Hill, Dublin.
MK-6 Fine-grained pink granulite (coticule), 2.8 em-thick lens in schist.
"Billings fold", elev. 945m, on cliffS of Smith Summit Trail, Mt.
Monadnock.
DB-55 Dense fine-grained coticule pod with pitted core, 4 X 8 X ? em.
Float block, Pumpelly Ridge, elev. 747m, 600m NE from Cascade Link Trail.
DB-220 Fine-grained gray granulite with 1-5 mm quartz-muscovite nubbles.
From a granulite bed several meters thick in schist.
NW end of boggy tarn, elev. 522m, Pumpelly Ridge, 380m S of Oak Hill.
DB-236 Fine-grained gray calc-silicate pod in gray schist, associated
with granulite beds.
Pumpelly Ridge, elev. 510m, near head of small gully on SE side, 500m
S of Oak Hill, Dublin.
50
The quartzites contain quartz, muscovite, biotite, garnet and
opaques, with or without sillimanite, plagioclase, retrograde chlorite, and accessory tourmaline, zircon and apatite (MK-873 and
DB-255).
The Littleton Formation occupies a narrow belt extending south
from Troy village along the east side of Little Monadnock, and an
irregular area centered on Bigelow Hill northeast of Troy (Plate 1
SW). A much broader belt extends from Mt. Monadnock northeast to
beyond Rt. 101 in Dublin, and then wraps around Beech Hill to form a
westerly striking belt which continues nearly to Marlboro village.
The Littleton is also found west and southwest of Derby Hill, in a
very narrow belt extending south from Poole Reservoir, and in another
belt extending southwest from Thorndike Pond. The belt of gray schist
east of the Keene dome between the Clough Quartzite and Rangeley
Formation is also assigned to the Littleton.
The proportion of quartzite to schist increases in stratigraphically higher parts of the formation, so that two parts can be
informally distinguished. These may or may not correspond to the
lower and upper parts of the Littleton of Hatchet al. (1983), and I
have chosen not to divide the Littleton of the Monadnock area into
formal members. An approximate contact between the two parts is shown
on Plates 1 and 4.
~
The lower part of the Littleton consists mostly of very thickbedded schist. Widely spaced, thin quartzite beds (less than 10 em
thick) locally define bedding. Equally thin garnet-biotite-rich
horizons which weather rusty brown are present in several locations.
Pale pink, fine-grained garnet-quartz granulite lenses and layers up
to 3 em thick are fairly common in both the lower and upper parts.
These coticules, the rusty horizons, and quartzite beds are the main
clues to original layering in what is otherwise rather monotonous
gray-weathering, aluminous schist. Generally more than one quartzite
bed is not observed in a single outcrop. However, in some areas of
good exposure, especially along the southwestern ridge of Mt.
Monadnock, quartzite beds are from one to five meters apart. The
presence of graded bedding is certain only where the thin sandy beds
grade up into schist. The thick schists may have originally shown a
gradual decrease in grain size upwards, but this is obscured by the
metamorphic minerals. Andalumps are distributed throughout the
schists, but in some cases seem especially abundant near the contact
below quartzite beds. The graded laminae which Hatchet al. (1983)
described as the hallmark of the lower part of the Littleton, are not
prevalent in the lower part in the Monadnock area.
Feldspathic granulite and rare calc-silicate pods occur mainly in
the basal 30 m of the formation. The main exception occurs about 575
m stratigraphically above the Warner contact in an overturned section
at Oak Hill, on the north end of Pumpelly Ridge. The granulite there
(Table 6b, DB-220) is quite similar to granulites in the upper part of
51
the Warner, and calc-silicate beds and pods (DB-236) are also present.
These rocks lie approximately between the lower and upper parts of the
Littleton, and may have regional significance (see "Age and
Correlation", below).
The upper part of the Littleton Formation contains more abundant
quartzite than the lower part, but is otherwise similar. It is best
exposed on the summit of Mt. Monadnock, on Pumpelly Ridge, and around
the village of Dublin. The increase in quartzite is gradual, so that
the mapping out of an exact contact is difficult. The quartzite beds
are progressively thicker and more abundant upward in the section.
Thickness of quartzite beds ranges from a few centimeters to 40 em,
and they are separated by anywhere from 5 to 100 em thick schist beds.
The quartzite to schist ratio is generally less than one except in a
distinctive set of seven quartzite beds, described in more detail
below, where the ratio exceeds one. Many of the beds show excellent
graded bedding. Some quartzites are in sharp contact with schist at
both top and bottom, but this is less common than slow grading. What
at first appears to be cross bedding in the quartzites is actually a
faintly defined foliation, which can be traced into the schists.
Weathered-out ellipsoidal chlorite and chloritoid leave pits that are
locally parallel to this weak foliation.
Some structural features may have been produced penecontemporaneously with deposition. High angle faults locally disturb bedding
over a distance of less than a meter, with beds above and below
undisturbed. These faults appear to have developed during compaction
and pre-date the pervasive foliation. Along the White Arrow Trail on
the south side of Mt. Monadnock, at an elevation of 829 m, there is a
vertical wall with what appears to be large scale cross bedding
(Figure 5a). It is more likely a fault related to sedimentary
slumping. Folds at the top of the same wall are cut off by a knifesharp surface, interpreted as a slump fault. Another fault of this
sort is exposed at elevation 913 m east of the White Arrow Trail
(Figure 5b). These faults are discrete planes without vein fillings
or breccia. The bases of some of the quartzite beds have irregular
lobes extending into the schist which might have been load casts.
Other structures that might be interpreted as sand dikes and other
dewatering structures (Shizuo Yoshida, pers. comm., 1984) have origins
that are more open to debate. Tectonic deformation, metamorphism, and
quartz veins greatly confuse the interpretation.
Coticule horizons are common in the upper part of the Littleton,
and are especially well exposed on Mt. Monadnock. Some are associated
with quartzite beds but others are completely enclosed in schist. In
cross section they average one to two em thick, and they pinch and
swell along bedding planes. At elevation 902 m the White Cross Trail
crosses an overturned bedding plane with several oval coticule lenses
on its surface. The lenses average 20 X 30 em and resemble concretionary masses. Coticules reported in the literature (Clifford, 1960;
Fermor, 1909; Huntington, 1975; Karamata et al., 1970; Kramm, 1976;
•"'
~~
Sa.
...,\0(\
\0\ \U
~~~:. ·~
Sb.
Fig. S. Soft-sediment slump faults in
upright sections of the upper part of the
Littleton Formation. Foliation cuts
across these faults.
Sa. Station MK-2: vertical cliff along
the White Arrow Trail at elevation 829 m.
Inset shows fold near the leading edge of
a second minor slump fault at the top of
the cliff.
Sb. Station MK-4: East of the White
Arrow Trail, elevation 913 m. Note that
foliation post-dates the slump fault.
l.n
N
53
Schiller and Taylor, 1965) show a wide range in garnet chemistry, but
many are spessartine-rich (33.8 to 92 weight % spessartine molecule).
Coticule from near the summit of Monadnock (Table 6b, MK-6) was
studied with the electron microprobe. Garnet in the coticule contains
73.7% almandine, 13.4% spessartine, 9.2% pyrope, and 3-7% grossular.
A distinctive set of seven quartzite beds separated by schist
beds, and having a total thickness of two to three meters, has been
very useful in tracing out the pattern of tectonic folds on Mt.
Monadnock. These light gray quartzites form regular stripes across
the outcrop surfaces. A comparison of the thicknesses of the seven
quartzite beds in four widely separated localities on the mountain
(Figure 6) shows that the first, third, and sixth from the bottom are
relatively thinner. The fourth locally grades "slowly" into the
overlying schist, whereas the others tend to have sharp contacts at
both top and bottom. It might be valuable to use this set of beds as
a starting point for a detailed description of bedding characteristics
in adjacent beds, and eventually develop an internal stratigraphy for
the upper part of the Littleton Formation. The amount of exposure
would certainly allow this, and a more complete understanding of the
detailed stratigraphy would help refine the structural interpretation.
The stratigraphically highest rocks exposed in the upper part of
the Littleton have rhythmically graded beds 5 to 10 em thick, with a
quartzite to schist ratio of about one to two. They are best exposed
along Pumpelly Ridge, but also occur on the hills south of Dublin
village.
Thickness
The lower part of the Littleton Formation has as approximate
thickness of 575 m, estimated in the overturned section northwest from
Oak Hill toward Dublin Pond. On the upright section along Meade
Brook, on the southeast slope of Monadnock, it is from 600 to 800 m
thick, depending on how much section is repeated by isoclinal folds.
T~ere is at least a comparable thickness of the upper part of the
formation, but the top of the formation is not exposed. The highest
rocks in the section lie on the northern slopes of the mountain, where
a late syncline intersects the nappe-stage Monadnock syncline (see
Figure 13).
Age and Correlation
The Littleton Formation was first described by Billings (1937) in
northern New Hampshire to include black slate and gray sandstone with
subordinate greenstone and soda-rhyolite volcanic conglomerate, and
their higher grade metamorphic equivalents. Both the type Littleton
(Billings and Cleaves, 1934; Boucot and Arndt, 1960) and the Seboomook
Formation (Boucot, 1969) contain Lower Devonian fossils. The Littleton of the Monadnock quadrangle is correlated with the type Littleton
on the basis of similarities to the Seboomook in both rock type and
54
SEVEN QUARTZITES
80
11
quartzite
schist
80"J-2m
MK 27
MK 36
MK 859
MK8
Fig. 6. Comparison of measured sectio~s of a distinctive
set of seven quartzite beds in the upper part of the
Littleton Formation from four different locations on
Mt. Monadnock, shown on Figure 16.
55
its stratigraphic position. Neither the Seboomook nor the Monadnock
Littleton contains volcanics. Lyons (1979) recognized an upper unit
with excellent graded bedding and called it the Kearsarge Member.
This probably correlates with the upper part of the Littleton on Mt.
Monadnock. Hatchet al. (1983) suggested correlating the lower and
upper units in New Hampshire respectively with the Carrabassett and
Seboomook Formations in Maine, which are separated by discontinuous
marble, calc-silicate, and granulite beds of the Hildreths Formation
(Moench, 1971). Malinconico (1982) found laminated calcareous
quartzite in the Rumney quadrangle, in approximately the correct
position to be correlative with the Hildreths, but this unit has not
been recognized elsewhere in New Hampshire. The granulite and calcsilicate rocks at Oak Hill in the Monadnock quadrangle might also
correlate with the Hildreths.
Derivation
The sedimentary precursors of the metamorphic rocks that make up
the Littleton Formation consisted of mudstone and fine-grained
quartzose sandstone. Hall et al. (1976) showed that the Seboomook
Formation was deposited largely by turbidity currents in a base-ofslope environment, with a source area to the east. Together, the
Seboomook and Matagamon Sandstone constitute an upward-coarsening,
westerly-prograding "flysch basin-margin delta system" (Hall et al.,
1976). Prodelta sloge facies grade upward into delta front facies.
They estimated a N80 W down-slope direction from the orientation of
flutes on the bases of sandstone beds, which is consistent with the
orientation of fluxoturbidite channels, and with a slope direction
estimated from the separation angle of slump fold axes plotted on an
equal area diagram (Hall, 1973). No such study has been done on the
Littleton Formation, but it has generally been assumed that the
Littleton was also derived from the east (L. Hall and Robinson, 1982).
However, the southeast-directed premetamorphic down-to-basin faults
and folds documented by Moench (1970) involve both Siliurian and
Devonian rocks. Therefore it seems likely that the Merrimack trough
Peceived sediments from both east and west during the Early Devonian.
A thorough examination of slump features on Mt. Monadnock might yield
pertinent information.
Graded beds in the lower Littleton are separated by thick pelitic
portions, suggesting that turbidites may be interbedded with "normal"
pelagic sediments. Higher in the section, entire turbidite cycles as
described by Bouma (1962) might be present, but the fine laminae and
ripple cross-laminae of Bouma's (b) and (c) intervals have been obliterated during metamorphism. The increased number and thickness of
quartzite beds upwards suggests an increasingly proximal site of
deposition through time.
It might be argued that the thick mass of the upper part of the
Littleton localized at Mt. Monadnock, although tectonically thickened,
represents a base-of-slope submarine fan of some sort. Mt. Kearsarge,
56
60 km to the north, might be another such fan. However, the bedding
in the upper part of the Littleton is continuous over hundreds of
meters and very few features have been found that might have been
distributary channels. The local presence of upper Littleton is more
likely due to the level of stratigraphy preserved in cores of
nappe-stage synclines.
The origin of quartz-garnet granulite (coticule) lenses within the
Littleton is uncertain. The fact that coticule occurs enclosed in
schist as well as in quartzite makes a detrital garnet origin
unlikely. The model preferred here involves the chemical precipitation of silica with iron and manganese oxides as Mn-rich chert
nodules, followed by rapid burial and reduction. This model would be
somewhat different from the Mn oxide nodule origin suggested by
Clifford (1960) on the basis of the trace element assemblage in
coticule from the Partridge Formation at Mill Hollow, New Hampshire.
The Mn-oxide nodules on the modern ocean floor contain manganese as
trivalent and tetravalent species. However, the reduction of Mn
species in chert nodules may have taken place by a process similar to
that reported by Burdige and Gieskes (1983). They proposed a diagenetic origin for Mn oxide nodules based on phases observed in shallow
marine drill holes. The interplay of Eh, pH, Mn2+ in pore fluids,
sedimentation rates, and oxidatiop of organic matter results in a
redox boundary which migrates gradually upwards, with the reduced
species below the boundary. Rapid burial, such as by turbidites,
might periodically bury a Mn2+ -rich zone too deeply for complete
re-equilibration. It seems that Mn-rich chert nodules might be
reduced in the same way.
!
Kramm (1976) argued for a volcaniclastic or~g~n of coticule in the
Ardennes, Belgium, whereby Mn2+ would be present at the outset,
incorporated in the structure of montmorillonite clays, and would stay
in the divalent state through diagenesis and metamorphism. Coticules
occur in a variety of geologic settings, the other important one being
associations with metamorphosed volcanics (e.g. the Hawley Formation,
Emerson, 1898). Analyses of trace element assemblages might shed some
light on coticule genesis and help distinguish between various types.
INTRUSIVE ROCKS
INTRODUCTION
Fowler-Billings (1949a) distinguished three intrusive igneous rock
units in the Monadnock quadrangle, each part of the New Hampshire
plutonic series. In order of decreasing age they are the Kinsman
"quartz monzonite", Spaulding "quartz diorite", and "Concord granite".
According to the lUGS classification scheme (Streckeisen, 1973), the
Kinsman is mostly granite and the Spaulding is mainly tonalite (Figure
7), so these rock names are used here. The name "Concord" was applied
to granite plutons which may not all be the same age (Lyons, 1979), so
the name "Fitzwilliam Granite" is substituted for the late two-mica
'"'
o
Mfg
A.
Dst
•
Dkg
•
Mmd
Q
•
D
GRANITE
A.
c
•
(jl
•
•
c
QTZ- SYENITE
KL----L._ _ _ _
•
I QTZ- MONZONITE
_j__ _ _ _
DIORITE,
_L_ _~-=--=s=~;;ABBRO
Fig. 7. Streckeisen (1973) plot of intrusive rocks, recalculated in
quartz, K-feldspar, and ~lagioclase, from modes in Tables 7-9. Open
Fitzwilliam Granite; open triangles - Spaulding Tonalite and related
closed circles - Kinsman Granite. Some are based on modes published
Billings (1949a), Duke (1984) and Shearer (1983).
terms of
squares rocks;
by FowlerVl
"'--
58
granites. Estimated modes presented in the tables include some from
Fowler-Billings (1949a), E. Duke (1984) from the Peterborough quadrangle, and Shearer's (1983) samples from the Monadnock quadrangle.
Shearer's modes are the most reliable, because he stained his thin
sections with sodium cobaltinitrite and counted 2000 points per
section. Modes are plotted in Figure 7 in terms of quartz, plagioclase, and alkali feldspar.
Inasmuch as the present study does not concern itself with the
interiors of plutons, any changes from previous mapping involve either
minor changes in the position of contacts or changes in the assignment
of isolated bodies of rock to the three units.
Fowler-Billings (1949a) reported a "biotite schist dike" east of
the summit of Mt. Monadnock at elevation 2940 ft. (896 m), which she
believed was a metasedimentary dike similar to those she had described
in the Mt. Washington area (Fowler-Billings, 1944a). I have located
four other biotitic mafic dikes, and believe they are igneous microdiorite dikes related to the Fitzwilliam Granite. A float block
containing diabase was found north of Lake Skatutakee. The dikes, as
well as pegmatites and tourmaline veins, are discussed in separate
sections following the more important intrusive rocks.
KINSMAN GRANITE
t
The Kinsman is a coarse-grained peraluminous granite with microcline or plagioclase megacrysts commonly 2-5 em long. The groundmass
consists of plagioclase, quartz, biotite, and muscovite, with or
without garnet, sillimanite, and accessory sphene, zircon, apatite,
allanite, graphite, ilmenite, and secondary chlorite and epidote
(Table 7). The Kinsman crops out over most of the northeast corner of
the quadrangle, where it is part of the Cardigan pluton (FowlerBillings, 1949a). The ratio of megacrysts to groundmass is varied. A
systematic study of the distribution of megacrysts within the Cardigan
pluton might aid in our understanding of the Kinsman and its relationship with the Bethlehem Gneiss.
At least three belts of Kinsman Granite trend southwest from the
Cardigan pluton toward the main belt of Coys Hill Granite in Massachusetts. Outcrop control is terrible for the two wider belts, but the
much narrower, westernmost belt, 5-15 m wide, is more convincingly
continuous, lending credence to the continuity of the others as they
are shown on Plate 1 SE. The westernmost belt is cut off by the Fitzwilliam pluton, the middle belt follows roughly the eastern edge of
that pluton to join Kinsman outcrops at Sip Pond (see Fowler-Billings,
1949a, Plate 1) and the eastern belt projects to the north toward
Kinsman outcrops in the Peterborough quadrangle (E. Duke, 1984) and to
the south toward outcrops at Damon Reservoir in the Winchendon quadrangle. The northernmost exposure of lithically identical Coys Hill
Granite occurs near a railroad cut in the southwest corner of the
Winchendon quadrangle, on strike 10.5 km south of the Sip Pond area.
59
In the cut there are two belts of Coys Hill Granite separated by
Francestown and Littleton, which have been interpreted as lying in a
fold hinge, partly on the basis of minor folds (Robinson, unpub. data;
Zen et al., 1983). Farther south there is one nearly continuous belt
of Coys Hill extending to within 10 km of the Connecticut border
(Field, 1975; Tucker, 1977; Zen et al., 1983).
In the Monadnock quadrangle the Kinsman in the narrow belts is
strongly foliated and the feldspar megacrysts have tapered, sheared
outlines. Biotite, flattened quartz grains and the megacrysts show a
strong preferred orientation parallel to foliation in the country
rocks. Inclusions of schist lying parallel to the plane of foliation
are common. A deeply weathered, coarse garnet-biotite horizon occurs
within the Kinsman southwest of Gilmore Pond. It may represent a
"restite" similar to one described by Clark (1972) in the Kinsman near
Bradford, New Hampshire.
Mylonite occurs near the contacts of Kinsman in the narrow belts
in several places, for example at the outlet of Mud Pond, Dublin
(Table 7, DB-133); west of "The Ark", Jaffrey Center (MK-484); and
east of Thorndike Pond (MK-1029). A mylonite covers the dip-slope
surface of a large Kinsman outcrop behind a house east of Rt. 137, 2.5
km north of Jaffrey (MK-1010). The Kinsman outcrops west of The Ark,
and strongly sheared rocks that were probably Kinsman east of Gilson
Pond, lie west of the westernmost continuous belt. The age of the
faulting that produced the mylonites is uncertain (see structure
section), but they may serve as evidence that the three belts of
Kinsman were once parts of a single body that was sliced by faults to
produce the repeated map pattern. Alternatively, a single body might
be repeated by early folds. There is no direct evidence to support
folding, but this may be due to the poor exposure. Yet another interpretation would be that of parallel sills extending southwest from the
Cardigan pluton, but this is not meant to imply any connotation of
"feeder dikes".
The origin of the Kinsman Granite has long been a matter of
debate. The current consensus favors an igneous plutonic origin, as
well as an igneous origin for the garnets as phenocrysts (Clark, 1972;
Lyons et al., 1973; Barreiro and Aleinikoff, 1985). Barreiro and
Aleinikoff reported an age from Sm-Nd whole rock and garnet data at
413 ~ 5 m.y. Pb-Pb data on zircons indicate an inherited Proterozoic
age which they suggested may reflect a Chain Lakes-type basement under
southwestern New Hampshire. A Rb-Sr whole-rock age had previously
been reported at 402 + 19 m.y. (Lyons and Livingston, 1977; revised,
Lyons et al., 1982). -There is no conclusive evidence that the Kinsman
intrudes Devonian rocks in the Monadnock quadrangle, but it apparently
does intrude bona fide Littleton Formation in the Rumney quadrangle
(Malinconico,-,g82-)-.--There is enough uncertainty in the absolute age
for the base of the Devonian, that 413 m.y. may still be Devonian
(Lyons and Livingston, 1977). Furthermore, a Silurian age would imply
that the Kinsman is pre-Acadian, which seems unlikely.
....
Table 7. Estimated modes of the Kinsman Granite. Modes for "K" samples from Fowler-Billings
(1949a) and those for "D" samples from E. Duke (1984) in the Peterborough quadrangle.
Extension
of Coys Hill pluton
HK
K
199
70
Quartz
48
Plagioclase
34
(An38)
Micro cline
Muscovite
14
20
Haunted Lake Gilson Pond
D
MK
18
1169
25
40
40
10
22
(Oligoclase-Andesine) (An15)
24
36
18
(An31)
12
(An38)
35
20
56
28
43
16
14
10
15
5
7
5
12
2
4
X
8
11
3
21
X
10
X
X*
Biotite
10
Cardigan pluton
K
D
K
111
72
113
Garnet
2
Sillimanite
X
Cordierite
X
Chlorite
(Retrograde)
Epidote
_{Retrograde)
Opaques
Ilmenite
Graphite
Undifferent'd
X
X
1
X
X
X
X
X
X
X
X
X
X
X
Zircon
X
X
X
X
X
Allanite
4
X
X
Apatite
Sphene
4
X
1
X
X
X
X
X
0.5
X
X
0.5
X
X
X
X
X
X
*small patches inside plagioclase megacrysts
0\
0
61
List of Samples in Table 7.
MK-199 Coarse, strongly foliated, dark gray porphyritic "granite" with
up to 2 em feldspar phenocrysts.
Southwest edge of swamp, 1100m NW of Hodge Pond, 900m SE of power
lines, Jaffrey.
K-70
Porphyritic quartz monzonite; phenocrysts of potash feldspar
4 em long. Large outcrop with mylonitic zones.
E of Rt. 137, 2.7 km N of Jaffrey (=MK-1010).
K-111 Porphyritic "granite"; phenocrysts of potash feldspar 5 X 1.5 em
make up 20% of rock; garnet grain size is 0.5 em.
On road ~E of Jacquith Brook, 300m SE of 338m crossroads, Hancock.
K-113 Porphyritic granite.
Halfway between Moose Brook and Hosley Brook, 350m H of edge of
quadrangle on W side of knob, 396m altitude, Hancock.
D-72 Porphyritic granite.
1 km NW of Greenfield, Peterborough quadrangle.
D-18 Sheared porphyritic granite.
Southeast of Haunted Lake, Francestown, Peterborough quadrangle.
MK-1169 Very fine-grained mylonite with 5 mm to 2 em feldspar porphyroclasts with recrystallized "tails" and sparse 5 mm garnets.
350m W of Gilson Pond, elev. 428m, Jaffrey.
62
SPAULDING TONALITE AND RELATED ROCKS
Fowler-Billings (1949a) named this unit after Spaulding Hill west
of Dublin. It includes a variety of rock types similar to those in
the Hardwick pluton of Massachusetts, which reaches the Monadnock
quadrangle southwest of Fitzwilliam. Other bodies of Spaulding may
have been continuous with the Hardwick pluton prior to intrusion of
the Fitzwilliam Granite. No effort has been made here to characterize
mineralogically the various Spaulding bodies. Shearer (1983)
described four rock types in the Hardwick pluton: biotite tonalite
and hornblende-biotite tonalite in the interior, biotite-muscovite
tonalite locally within one kilometer of the pluton's margins, and
biotite-garnet tonalite still closer to the margins. These rock types
as well as small bodies of strongly foliated granite, quartz gabbro,
and gabbro have also been assigned to the Spaulding in the Monadnock
quadrangle. Duke (1984) considered the Peterborough granite pluton,
part of which extends into the Monadnock quadrangle northeast of
Jaffrey, as related to the Spaulding. I have included it with the
Fitzwilliam Granite.
The tonalites contain plagioclase (An 27 _ 49 ), quartz and biotite,
with or without K-feldspar, muscovite, garnet, hornblende and
secondary chlorite, with accessory sphene, zircon, apatite, ilmenite,
allanite, calcite, anatase, tourmaline and clinozoisite (Table 8).
The plutons at Gap Mountain and south of Mt. Monadnock may once
have been continuous with the Hardwick pluton. The Spaulding Hill
pluton is separate, but a former connection through the area now
occupied by granite north of Bigelow Hill cannot be ruled out.
Outcrops of unusually garnet-rich tonalite (MB-64) which are between
Littleton schist and two-mica granite north of Rt. 124, may be
remnants of such a connection. There are large inclusions of stratified rocks on both summits of Gap Mountain, surrounded by tonalite.
They are apparently more resistant to weathering than the tonalite.
Lyons and Livingston (1977; revised, Lyons et al., 1982) reported
a late Early Devonian Rb-Sr whole-rock isochron age of 393 ~ 5 m.y.
for Spaulding Tonalite in central New Hampshire. Garnet-bearing
tonalite and hornblende gabbro-diorite samples from the same plutons
have much lower initial Sr 87 /Sr 86 ratios and do not plot on the 393
m.y. isochron, suggesting to the authors a mixed mantle-crustal
derivation for the unit as a whole. Shearer (1983) has written extensively on this topic.
Small elongate tonalite plutons and isolated outcrops are abundant
parallel to the Kinsman belts southeast of Mt. Monadnock. Both units
are strongly foliated and locally mylonitic, but west of The Ark
(Plate 1 SE), at map scale, the tonalite appears to cut across a small
body of Kinsman. A contact between Spaulding and Kinsman is well
exposed in an outcrop under power lines which lead to the State Park,
63
but superimposed metamorphic foliation obscures any original intrusive
relations at this scale (Figure 8).
-----
feldspar
megacryst
Fig. 8. Spaulding (Dst)-Kinsrnan(Dkg) contact, MK-482. Contact is
sharp except below hammer handle, where there is apparently an irregularity in the original contact. Mylonitic foliation is oriented N45°E,
580ffiv in both units.
It is imp ossible to determine age relations at
this outcrop.
The tonalite bodies southeast of Mt. Monadnock are texturally
different from rocks in the larger tonalite plutons, and are
informally referred to as the "Gilson Pond type". Foliation is much
more strongly developed, feldspars are typically aligned and sheared,
and quartz grains are ellipsoidal, producing a rock that resembles a
small-scale version of textures in foliated Kinsman. The feldspar
megacrysts are rarely larger than 2 em long, however, and plagioclase
greatly exceeds K-feldspar in abundance. East of Gilson Pond, sheared
examples of both the Spaulding and Kinsman are present, and they can
easily be confused. The oblong pluton of tonalite west of Gilson Pond
is less strongly foliated, but still contains the 1-2 em feldspar
phenocrysts. This distinctive type of Spaulding also occurs in small
plutons around Dublin Pond. There are numerous isolated bodies of
more typical tonalite, too small to show on Plate 1, especially in the
Rangeley Formation. South of Thorndike Pond the tonalite contains
xenoliths of calc-silicate granulite and schist. Locally tonalite
appears to grade into rocks of the Rangeley, as though it formed
through in situ melting of the schist, and the calc-silicate pods were
left intact.
...
Table 8. Estimated modes of the Spaulding Tonalite and related rocks. Modes for "K" samples
from Fowler-Billings (1949a) and those for "SM" samples from Shearer (1983).
Spaulding
DB
162
Quartz
Plagioclase
K-feldspar
Biotite
Muscovite
44
-
29W
SM
1-1
TR
14
MB
64
Hardwick "Gilson Pond"
pluton
t~pe
K
SM
MK
MK
32
5
753 1139
28
19
24
18
10
MB
Gap Mtn.
-
21
37
Gabb
RD
MK
5
195A
15
34
X
MK
703
14
42
20
38
32
67
43
35
45
47
23
25
48
(An34)(An27)(An29) (An41) (An49) (01- . (An45 (An34) (An40) (An65)(An67)(An66)
And) -26)
5
27
2
X
X
12
15
37
1
33
19
1
4
20
27
16
18
1
X
X
25
13
4
Hornblende
29
Garnet
5
38
35
11
5
Sillimanite
Staurolite
X
Chlorite
X
Opaques
Ilmenite
Pyrite
Undifferent' d
X
Sphene
1
X
1
X
Anatase
5
X
X
X
X
X
X
X
X
1
tr
X
X
X
1
X
2
X
X
X
X
6
2
3
X
X
X
6
4
X
X
4
X
X
2
X
X
X
X
X
X
X
1
X
1
X
X
X
X
X
X
Apatite
X
1
X
Zircon
X
X
X
Allanite
X
Calcite
X
Tourmaline
X
1
X
X
1
1
X
X
X
0\
+:-
X
65
List of Samples in Table 8.
DB-162 Strongly foliated, medium-coarse-grained, medium-gray biotite
tonalite.
Roadcut S side of Rt. 101 near Howe Reservoir, 5 km W of Dublin.
MB-29W Medium-grained, weakly foliated, medium-gray granite, in
contact with darker tonalite (SM1-1).
N side of Rt. 124 550m W of Old Dublin Road, Marlboro.
SM1-1 Coarse, weakly foliated, light-gray tonalite.
E end of same roadcut as MB-29W.
TR-14 Medium-coarse, weakly foliated, medium-gray biotite tonalite.
Elev. 442m on N side of Gap Mountain, along Metacornet-Honadnock
Trail, Troy.
MB-64 Atypical medium-coarse, poorly foliated, gray tonalite with
abundant 3rnrn garnets and rniuor sillimanite and staurolite.
Probably contaminated by incorporation of schist.
N of Rt. 124 300m E of Monadnock Drive, Marlboro.
K-32 Medium-grained dark gray quartz diorite.
Quarry 650m E of Fitzwilliam Depot, S of railroad bed.
SM-5 Medium, weakly foliated gray tonalite.
Quarry 200m S of railroad bed, 1.6 krn W of Fitzwilliam.
MK-753 Medium-grained rnylonitized gray biotite tonalite with 2-5 rnrn
feldspar megacrysts.
75rn E of SE corner, Thorndike Pond, behind house E of road, Jaffrey.
MK-1139
with
1.4 krn
360m
Medium-grained rnylonitized gray to brownish biotite tonalite
3 rnrn to 1 ern tapered feldspar megacrysts.
NNE of Jock Page Hill, 400m E of logging road junction, elev.
on E-facing slope.
RD-5
Fine-grained, dark gray biotite-hornblende quartz gabbro.
Concordant sill.
Elev. 495rn toward Wend of E-W ridge, 1750m W of Little Monadnock
ridge, Richmond.
MK-195A Coarse-grained, dark greenish-gray hornblende-biotite gabbro.
Power lines 875m SW of Jaffrey Center, where they turn from SW to ESE.
MK-703 ~1ediurn-grained weakly foliated dark greenish-gray hornblendebiotite quartz gabbro.
N of stonewall, edge of beaver swamp, Stony Brook, upstream from
Kinsman outcrops. 500m SW of Thorndike Pond, Jaffrey.
66
A sill of fine-grained hornblende-biotite quartz gabbro (RD-5)
crops out on the east-west hill west of Little Monadnock. A similar
sill at elevation 405 m on the east slope of Little Monadnock contains
sphene which is conspicuous in hand sample. One outcrop of mediumgrained hornblende quartz gabbro (MK-703) occurs near the east contact
of the Kinsman belt south of Thorndike Pond. Several coarser-grained
hornblende gabbro outcrops (MK-195A) also lie east of the Kinsman,
south of Rt. 124.
FITZWILLIAM GRANITE
The youngest plutons in the quadrangle are peraluminous two-mica
granite. They are massive to weakly foliated, and they cut across all
folds and earlier intrusions. They are thus clearly post-tectonic.
In detail the contacts are irregular, with many apophyses intruding
the country rocks parallel to foliation. The most strongly foliated
Fitzwilliam outcrops occur near silicified zones presumed to be
Mesozoic.
!
There are five major plutons of Fitzwilliam Granite: the Troy
Quarry pluton west of Mt. Monadnock, the Fitzwilliam pluton, the
pluton south of Marlboro village, the Babbidge Reservoir pluton north
of Marlboro, and an elongate pluton along the west edge of the quadrangle in Keene. Most of these occur in areas of low topography where
outcrop is generally poor and large rounded granite boulders are
common. The granite was apparently deeply weathered prior to
glaciation. The unit was extensively quarried in the nineteenth
century, but no quarries are currently in operation, except for
crushed stone production from waste rock in Marlboro. The weakly
foliated granite northeast of Jaffrey (MK-1010, Table 9) may have
Spaulding rather than Fitzwilliam affinities (E. Duke, 1984), but is
shown as Fitzwilliam on Plate 1 SE. There are numerous smaller
plutons, dikes and sills of two-mica granite, especially in the
western and southern parts of the quadrangle, which are not shown on
the plates.
Estimated modes are presented in Table 9, partly from previously
published data. Shearer (1983) showed that the Fitzwilliam pluton is
more heterogeneous than it would appear from hand samples alone, and
includes granite, quartz syenite, and quartz monzonite. The granite
consists of plagioclase (An 18 _34 ) and microcline in roughly equal
amounts, quartz, muscovite, and biotite, with secondary chlorite and
accessory zircon, apatite, opaques, sphene and tourmaline.
Lyons and Livingston (1977; revised, Lyons et al., 1982) estimated
a Mississippian Rb-Sr whole-rock isochron age for Concord granite in
the Sunapee pluton of 326 + 3 m.y. Hayward (1983) estimated Rb-Sr
whole-rock isochron ages of 349 m.y. for the Sunapee pluton and 383
m.y. for the Fitzwilliam pluton. Hayward's age determination for the
Fitzwilliam seems too old. The Fitzwilliam cuts across dome-stage
structural features, and yet the Prescott and Belchertown plutons of
67
Table 9. Estimated modes of the Fitzwilliam Granite.
Modes for "SM" samples from Shearer (1983).
Mar lb oro
pluton
MB
_]_Q_
Quartz
Plagioclase
Microcline
42
27
(An28)
F't
1 ZW1'11'1am
pluton
SM
SM
_3_
11
31
18
29
39
(An28) (An30-22)
area w 0 f
Mt. Monadnock
MK
MK
N of Jaffrey
MK
MK
828
1123
41B
1012
31
26
37
42
28
20
38
(An34)(An34)(An22-26)
7
(An29)
21
31
36
29
35
18
35
Biotite
8
5
7
6
11
5
3
Muscovite
2
5
2
3
8
1
12
Opaques
Ilmenite
Pyrite
Undifferent'd
X
X
X
X
X
X
X
1
X
X
X
X
X
X
X
X
Sphene
2
X
Tourmaline
X
X
Chlorite
X
X
X
X
1
68
List of Samples in Table 9.
MB-30 Medium to fine-grained, non-foliated, light-gray granite.
Harlboro Quarry, E of Rt. 124, 1. 75 km S of junction with Rt. 101.
SM-3 Coarse, non-foliated, light-gray granite.
N side of Rt. 12, roadcut 1.8 km N of Fitzwilliam, across from
landfill.
SM-11 Coarse, non-foliated, light-gray "granite".
S of Rt. 119, 1.3 km W of Fitzwilliam Depot.
MK-828 t1edium-coarse, weakly foliated, light-gray granite.
Dike parallel to mafic dike, elev. 628m, Mossy Brook, Jaffrey.
MK-1123 Hedium-fine, light-gray granite.
465m knob, 125m W of Fassetts Brook, W of Mt. Monadnock, Jaffrey.
MK-41B Fine-grained brownish-weathering granite with xenoliths of
gray schist. In association with black garnet-bearing mafic dike.
In gulch E of Marlboro Trail, elev. 704m, Mt. Monadnock, Jaffrey.
MK-1012 Medium-grained, weakly foliated, light-gray granite.
2.7 km N of Jaffrey, 750m E of Rt. 137, roadcut in housing development.
69
Massachusetts, dated respectively at 377 ~ 20 m.y. (Naylor, 1970) and
380 ~ 5 m.y. (Ashwal et al., 1979), are very strongly deformed by the
dome stage of deformation.
MICRODIORITE DIKES
Five biotitic mafic dikes have been found. In the field they
resemble lamprophyres, in the sense that they are mafic dikes containing only mafic phenocrysts. Mineralogically (Table 10) they are
biotite-rich microdiorite, or perhaps kersantite, which is a lamprophyre in which biotite and plagioclase together compose about 75% of
the rock (Tr6ger, 1935). Some geochemical analyses would aid in
understanding the unusual mineralogy of these dikes. Kersantites have
about 3.7 wt. % K2o and 51.8% Si0 2 (Metais and Chayes, 1963).
Mineralogy
Estimated modes for the microdiorite dikes are presented in Table
10. Due to the very fine-grained texture of these rocks, the modes
are not very accurate. The typical dike rocks are tan-weathering,
dark gray to black, fine-grained, biotite-rich microdiorites, with 2-5
mm clots of biotite that give the weathered rock a spotted appearance.
Sparse 2-15 mm garnets form knobs in ~ome portions of the dikes. In
thin section the garnets have irregular, corroded outlines, and
refractive indices for a garnet from sample MK-54A are greater than
1.76, suggesting almandine garnet. The garnets are probably xenocrysts from the Littleton Formation. The dike groundmass consists of
biotite and plagioclase, with or without green hornblende, K-feldspar,
quartz, and accessory ilmenite, sphene, zircon, and allanite. Most
samples show a foliation defined by biotite, although relict ophitic
texture is also preserved (MK-54N). Some biotite clots surround hornblende, and ilmenite is commonly rimmed by sphene.
Field Descriptions
One of the dikes (HV-7) is vertical and only 15 em wide. It
trends N37° E, cutting across the Warner Formation in Eliza Adams
Gorge (Plate 1 NW). The others are all on Mt. Monadnock (Figure 9).
MK-69A is the "biotite schist dike" east of the summit described by
Fowler-Billings (1949a, p.1271) as a metamorphosed sedimentary dike.
This dike is different from the others in that it appears to have
intruded along an irregular shear zone that offsets bedding in the
Littleton. It is about 20 em thick, but pinches and swells and at one
point forks into two branches. However, its mineralogy is similar to
that of other dikes in the quadrangle and an igneous origin is likely.
The most prominent dike (MK-54) is about 1.8 m thick, nearly
vertical, and trends N54° E to N33° E across the summit of Mt.
Monadnock (Figure 9). It crosses the summit ridge on the MarlboroDublin Trail about 115m north of the summit, following a bush-filled
lineament that shows up clearly on the air photo. The rock weathers
..
,
Table 10.
Estimated modes of microdiorite dikes.
,,
MAIN DIKE
contaminated"
,.....
1<:
Biotite
~~
-
--
'
37
I
38
41
20
Hornblende
IZ
~~
-X
52
(An45)
1<:
~~
I::S:
~~
- - ....,- -
23
43
)-35
I~
~~
~~
~~
8
42
30
20
54
33
(An46)(Anl4)(An26)
22
30
57
20
20
K
2
13
Ilmenite
3
3
X
X
X
5
3
6
Sphene
lrz..
-- -- --
Muscovite
Garnet xenocrysts
10
23
20
7
10
\()
I~
~~
-26
17
I
~
-
1
I
(Al
33
27
17
5
3
18
2
2
2
1
X
3
X
2
4
5
Apatite
X
1
X
X
X
X
1
X
X
1
Zircon
X
1
1
1
X
X
X
X
X
X
Allanite
X
Pis tacite
X
X
....,
0
71
List of Specimens in Table 10.
HV-7
Very fine-grained black mafic dike, weathering brown with
darker plates of biotite; vertically cross-cuts garnet schist.
Eliza Adams Gorge, N bank, 160m downstream from Howe Reservoir
Dam, Harrisville.
MK-69A Fine-grained, foliated, dark gray mafic dike.
White Cross Trail, elev. 893m, SE of Monadnock summit, Jaffrey.
MK-54N Dense, fine-grained black hornblende-bearing mafic dike,
weathering brown with darker 2mm clumps of biotite and sparse
5-lOmm garnet xenocrysts.
North of Monadnock summit, main dike, elev. 875m, Jaffrey.
MK-54A Fine-grained black mafic dike, weathering brown with darker
biotite clots and sparse garnet xenocrysts.
SW of Monadnock summit, main dike, elev. 928m, east of Smith
Summit Trail in one meter wide notch, Jaffrey.
}0C-54W Very fine-grained black mafic dike with 2-5mm clots of biotite.
SW of Monadnock summit, main dike, ' elev. 914m, Jaffrey.
l1K-54E Fine-grained medium dark gray mafic dike with 2-4mm clots
of biotite and up to 5mm long Lathes of feldspar.
SW of Monte Rosa, in Mossy Brook, elev. 613m, Jaffrey.
MK-54D Fine-grained light gray granitic dike, weathering brownish-gray.
W of Monte Roca, elev. 67lm, Jaffrey.
MK-54F Fine-grained light gray to brown granitic dike mottled by 3mm
biotite clots and sparse 2cm muscovite xenocrysts (after andalumps?).
SW of Monte Rosa, S of Mossy Brook, near trail, elev. 640m, Jaffrey.
MK-41B Fine-grained brownish-weathering granite with xenoliths of gray
schist which contain 2-3mm muscovites concentrated along contact.
In association with black mafic dike with garnet xenocrysts.
In gulch E of Marlboro Trail, elev. 704m, Jaffrey. Mafic dike crosses
the trail at the top of the gulch.
MK-1116 Fine-grained dark gray mafic dike with 2-3mm clots of biotite
and up to 5mm feldspar lathes; in contact with black mafic dike.
Marian Trail, W side of Mt. Monadnock, elev. 65lm, just S of large
W-facing open rocks, in woods, Jaffrey.
72
Fig. 9. Mafic dikes on Mt. Monadnock (shaded), showing sample locations.
Dotted line represents the Seven Quartzite beds in the Littleton Formation; random dashes represent Fitzwilliam Granite. The mafic dikes and
granite are probably contemporaneous. Contour interval 30 meters.
73
to smooth rounded brownish-gray rubble and can be observed in place
only here and there. It has been traced from elevation 838 m on the
north side of the mountain to 570 m on the south side for a length of
about 1.8 km. Mossy Brook follows the dike from elevation 642 m to
570 m (Figure 9). Locally there are two parallel mafic dikes and
there is also a vertical mafic dike trending N70° W. The dikes
clearly cut all phases of Acadian folds exposed on Mt. Monadnock.
Two other mafic dikes crop out along the west slope. One is a
typical black fine-grained dike where it crosses the Marlboro Trail at
elevation 704 m, but it gives way to granite (Table 9, MK-41B) in a
gulch east of the trail, and both die out at elevation 680 m. Another
dike, MK-1116, crosses the Marion Trail at elevation 651 m. It crops
out over a width of about nine meters, and its extent along strike is
not yet known. It was initially hoped that some of these dikes could
be traced west to the granite pluton, but glacial cover is too
extensive to permit this.
Contact Relations
Granite dikes also parallel the NE-trending mafic dikes, and, at
elevation 604 m where Mossy Brook Trail descends south away from the
brook, a contact between granite and . the main mafic dike (MK-54G) is
exposed west of a small waterfall over andalump schist. The dike has
an extremely fine-grained chill zone next to the granite. Downstream,
at elevation 497 m, there is a large float block of granite with
inclusions of mafic dike rock. The two rock types thus seem to be
coeval. Upstream from MK-54G, at elevation 640 m, a granite sill
intrudes the Littleton at a small saddle in the ridge east of the
brook. Where this granite reaches the brook (Table 9, MK-828), the
"mafic" dike is a fine-grained light brown dike with biotite clots
which approaches granite in composition (Table 10, MK-54F). Downstream, the dike is an intermediate dark gray rock (MK-54E). It
appears that the mafic dike material was somehow contaminated by the
granitic material where the two dikes cross. The Marion Trail dike
· also contains some portions of intermediate composition. In conclusion, the mafic microdiorite dikes appear to be about the same age
as the Fitzwilliam Granite (?Mississippian), and yet they were weakly
metamorphosed, perhaps in the "Permian disturbance" (see concluding
section in metamorphism chapter).
PEGMATITE
Pegmatite is common in the Monadnock quadrangle, both as foliated
bodies concordant to regional foliation and as undeformed bodies.
Foliated and sheared pegmatite in the Gilson Pond area, for example,
is deformed by the backfold stage of deformation, whereas large masses
of non-foliated pegmatite hold up the ridges of The Pinnacle and Bald
Hill in Roxbury (Plate 1 NW). The pegmatites consist primarily of
quartz, plagioclase, perthitic K-feldspar, and muscovite, with or
without biotite, tourmaline, garnet and other accessory minerals.
74
Fine examples of plumose muscovite were found on The Pinnacle. There
are distinctive tourmaline- and garnet-bearing aplite and pegmatite
dikes on Mt. Monadnock which locally contain masses of fine-grained,
pale green muscovite, which appear to be pseudomorphs after sillimanite.
Pegmatitic material is common as lenses and stringers in many of
the schistose rocks, apparently produced in situ as melt pockets
during metamorphism. Some of these may have coalesced to form larger
masses, but other pegmatites probably were intruded from deeper
sources associated with plutons. Fowler-Billings (1949a) showed the
location of the major pegmatites in the Monadnock quadrangle by small
red crosses on her map. They are especially common intruding the
Rangeley Formation in the towns of Roxbury and Sullivan, but pegmatite
also intrudes the Swanzey Gneiss at Mt. Huggins, and intrudes Littleton, Warner and Francestown Formations in the Marlboro syncline 800 m
due east of Marlboro village. Cameron et al. (1954), in their discussion of the "Keene pegmatite district", reported that pegmatites
intrude Bethlehem Gneiss, Kinsman Granite, and "Concord" Granite, but
they most commonly intrude the metamorphic rocks. These authors
agreed with the conclusions of Fowler-Lunn and Kingsley (1937), that
in the Grafton district each of the units in the New Hampshire
plutonic series gave rise to pegmat~tes. Although they favored the
"Concord" as a source for the larger pegmatite bodies in the Keene
district, they did not draw any decisive conclusions.
TOURMALINE VEINS
Thin, very fine-grained, black tourmaline veins are common
throughout the quadrangle. Cross-cutting relations and association
with non-foliated pegmatites indicate a post-tectonic age. A block of
microdiorite float was found at MK-54A in which the dike rock is cut
by a tourmaline vein, on each side of which is a 2 em thick pegmatite
layer. Tourmaline veins have not been noted cutting the granite.
! They are especially prominent on Mt. Monadnock, where tourmaline has
replaced aluminous minerals in rocks adjacent to the veins. Sillimanite-muscovite pseudomorphs seem especially susceptible to this
replacement. The source of boron may have been hydrothermal, or
somehow concentrated from the schists themselves. A geochemical
comparison of rocks far from tourmaline veins, to rocks progressively
nearer the veins, might help clarify this. If the boron came from the
schists it seems there ought to be a boron-depleted zone between the
tourmalinized zone· and the schists farther away.
DIABASE DIKE
A block of float was found in a brook north of Lake Skatutakee, at
elevation 380 m, 600 m east of Harrisville village, made up of diabase
cutting Kinsman Granite. The diabase is dark, fine-grained, with
sparse feldspar phenocrysts. It is probably Mesozoic.
75
"The rockwork is interesting and grand; --the clean
cleavage, the wonderful slabs, the quartz dikes, the
rock torrents in some parts • • • 11
-Ralph Waldo Emerson, 1866
STRUCTURAL GEOLOGY
INTRODUCTION
Five phases of Acadian deformation have affected the rocks of the
Monadnock quadrangle. The earliest, isoclinal folds, are believed to
be related to the huge west-verging nappes proposed by Thompson et al.
(1968). West-verging ductile thrust faults then developed and cut-across the axial surfaces of the earlier fold nappes. The fold nappes
and thrust faults are deformed by two phases of folds and backthrusts
related to a complicated "backfolding" episode, and by folds related
to the rise of gneiss domes. A summary of the structural history is
presented in Table 11. Figure 10a shows the axial traces of the major
folds and their relative ages. Figure 10b shows average foliation and
lineation orientations in 36 subareas in the quadrangle.
The nappes and thrusts transported hot rocks onto relatively
cooler rocks, setting up temperature and pressure gradients which led
to peak metamorphic conditions closely following the nappe stage. The
dominant foliation is parallel to axial planes of nappe-stage folds.
Reactions proposed to explain garnet zoning (see metamorphism section)
imply a declining pressure just beyond the peak of metamorphism as a
result of uplift. However, thermal equilibrium was apparently not
attained throughout the pile of nappes, resulting in an inverted metamorphic sequence, with lower grade rocks at the lowest structural
level next to the Keene dome. The boundaries between Zone II and III
assemblages and between Zones III and IV (explained in detail in the
metamorphism section) roughly follow the nappe-stage thrust faults,
~but there are no sharp discontinuities in metamorphic grade across the
faults. A large backfold (Beech Hill anticline) deforms the Zone
III-Zone IV boundary. The rocks were in the stability field of sillimanite during backfolding and doming.
DESCRIPTION OF MINOR STRUCTURAL FEATURES
Equal area diagrams summarizing data for some of the minor structural features from the whole quadrangle are presented in Figure 11.
Planar Features
Bedding. Bedding is present at some scale in all the metamorphosed sedimentary units. It is most easily seen in the Perry
Mountain, Francestown, Warner, and upper part of the Littleton
Formations. The lower part of the Littleton is commonly thickly
bedded, but with a little effort thin quartzite beds can be found in
,...,
Table 11.
Age
Summary of structural history in the Monadnock quadrangle.
~
Mesozoic
245-144
Extensional faulting; silicified zones; diabase dike.
Late Paleozoic
326-245
Continued local shearing; ?Permian metamorphic disturbance?
?Mississippian
326
<380
Devonian
393
(Acadian
orogeny)
Intrusion of Fitzwilliam Granite plutons and microdiorite dikes.
Late open folds
NW-trending, steep AP's; local crenulation cleavage.
DOMING
LATE BACKFOLDING
Various trends, steep AP's; crenulation cleavage; strong
linear fabric swirl
NE-trending fold axes, inclined AP's; linear fabric?
EARLY BACKFOLDING
W-over~E
THRUST NAPPES
E-over-W verging ductile thrust faults.
FOLD NAPPES
E-over-~:
verging reclined folds; local mylonitization and
mylonitic foliation; some linear fabric; peak of
metamorphism?
Intrusion of Spaulding Series plutons.
413-402
verging isoclinal folds; pervasive foliation;
quartz lineations?
Intrusion of Kinsman Granite.
Ordovician to
Devonian
480-415
Deposition of volcanic and sedimentary rocks.
?Proterozoic Z
to ?Ordovician
600-480
Genesis of rocks forming cores of later gneiss domes.
'-I
0'1
77
Fig. 10.
Summary of structural features in Monadnock quadrangle.
/
?
I
j I
I 41I
N
l
SCALE
I
2mi.
0
0
1
2
3km.
Fig. lOa. Axial traces of major folds and faults, showing phases of
deformation: (1) nappes, (2) thrusts,(3) early backfolds, (4) late backfolds, (5) dome-stage and other late folds, (M) Mesozoic faults. Base of
the Littleton Formation (dashed line) is shown locally for reference.
Names of structural features are shown in Figure 13.
78
Fig. lOb. Subareas with average dominant foliation orientations and
average sillimanite lineations. Major plutons are in gray. Base of
the Littleton Formation (solid line) is locally shown for reference.
79
Fig. 11. Equal area diagrams summarizing structural features.
Data is from the entire quadrangle unless indicated otherwise.
Planar features are on plots to left; linear features to right.
LINEAR FEATURES
PLANAR FEATURES
A. Poles to axial planes
of 13 nappe-stage folds
A'. 17 nappe-stage fold axes
B. Poles to axial planes
of 60 intermediate age
folds
B'. 127 intermediate age fold
axes
C. Poles to axial planes
of 15 late open folds
on Mt. Monadnock
C'. 25 fold axes of late open
folds on Mt. Monadnock
D. Poles to axial planes
of 15 late folds east
of the Keene dome
D'. 29 fold axes of late folds
east of the Keene dome
E. Poles to 7 mafic dikes
F.
224 sillimanite and 7
andalump lineations
G. Poles to 46 tourmaline
veins.
H.
101 quartz lineations
80
Early folds
••
•
•
•
•
•
•
+
•
•
•
•
••
•
•
•
+
•
••
••
•
•
•
•
I ntermedlate
symbols
as above
• •
• • :. t •• • •••
• ':
• • •• ••
•
~
• • •••
.... , . .. ....
. ... ....
......
. ••.. • • ... .
•
•••
•
•
•••
•
+
•
• •
•
•
•
• ••
•
•
•
~.
• ,• •
..
•
••
••
• • ••
•
.•·.:.•.....
•••
•
•• •
.. • .. ..'
+
•
•
• ••
••
•
•
•
Late folds
Mt. Monadnock
symbols
as above
• •
• •• •
•
•
•
•
•••
+
•
•
••
•
• •
81
Late folds
Keene dome
poles to
axial
planes
/
+
. ..
• •••
all
sillimanite
ond
onoolump
lineations
\
.
..
. .... . ...
. t:
,.·.
0
~ ·w
~·..
'2-!b•• ' • :
c! ~~·: •
. ....,... . ..,........
•
~=:
•.. : .·· .
••
•
..
all
•
•
•
•
•• •
. ..
•
•
•
• •
quartz
lineations
.• .. . ,.
.. .••
.
..
. ........ ·--~..
..... ... .... .t
....
. ·...
..
+
..
•
..
..
82
Figure 12. Equal area plots of
planar and linear features for the
subareas shown in Figure 11.
LINEAR FEATURES (on right)
+ sillimanite and mica lineations
PLAl.~AR
poles
• poles
A poles
)( poles
0 poles
0
to
to
to
to
to
FEATURES (on left)
andalump lineations
• quartz lineations
x feldspar lineations
~ intersection lineations
A crenulation lineations
o early fold axes
• intermediate fold axes
• late fold axes
¢
bedding
dominant foliation
secondary foliation
axial planes of folds
fault planes
Numbers of measurements·
PLANAR FEATURES
.,
....
c::
~
C::..lll
Gl ...
cO
C>O
c::
~
cO
~
~
~
~
.,
"tl
"tl
~
~
='
tn
""~
.,
Gl
IQ
Is<
0
Gl
~
~
0
Is<
Is<
1
The Pinnacle
5
49
2(out)
7
33 1
2 (in) Derby Hill
25
30
3(out)
61
17
21
3 (in) Willard Hill
57
4
Page Hill
22
56 3
5
Marlborough
45
8
6
Brennan Hill
12
96 2
7
Glen Brook
15
17
8
Cooper Hill
23
Minnewawa Brook
9
16
38
10
Meetinghouse Pond
ll
39
ll
Shaker Brook
13
69
12
Little Monadnock
68 164 1
13
13
Howe Reservoir
33
14
Seaver Pond
13 1
6
15
Harrisville
10
28
16
N base, Monadnock
31 1
9
17
Hurricane Hill
14
7
18
Beech Hill
16
17
Dublin
19
21
33
20
NE of Thorndike Pd. 9
30
21
Hud Pond
24
3
22
Monte Rosa
42
56 1
23
Monadnock summit* 266 100 6
24
Pumpelly Ridge
43 3
41
25
Bigelow Hill
42
5
26
Parker Trail
31
56
27
Poole Reservoir* 148 149 6
Gilson Pond
28
17
51 1
29
SE of Thorndike Pd. 1
45
30
E of Thorndike Pd.
21
4
31
Cwnmings Pond
29 1
13
32
Jaffrey Center
14
88 1
Jaffrey
33
11
34
26
Gilmore Pond
12
35
E of Pearly Pond
1
6
36
Rindge
2
23
* No.23
c::
0
cO
oc::.o
Sol cO '::1
Gl
~
0
~
.0
N
~
;;s "tl~
Gl
Sol
., .,
.,c::
c:: c::
....., ....., ""
......
., ""....
.,='
~
~
1
4
1
3
2
1
3
6
8
4
1
2
1
3
1
2
20
6
1
26
1
1
1
4
LINEAR FEATURES
Mineral
Fold Axes
Lineations
.,
c::
0
....Gl
Gl
c::
....Gl
~
0
cO
c::
....
~
~
~
~
Sol
~
u ....
"tl
C::IO
Gl
Gl
c::
., ~
p.
Gl
m~
....N ., Sol ='
~:c
~
..Ill
>.
......
Sol "tl
cO
....Gl Glc:: ~Sol ....Gl ....Gl u
~-a
"tl
~c::
Gl
c:: Sol
cO
c::
tnOS
~
H
:i ~tn
~ & Is< H u
14
2
1
2
5
3
6
6
1
4
2
1
3
5
12
17
4 2 1
6 2
7
2
3
3
29
3 3 1
8 2
1
5
3
1 2
1
2
4
2
9
6 1
20
3
3
2
30
1
1 5 1
7 3
2
75
3 4 1
9 5
2
2
4 2
2
1
2
8
1
1
4
4
2
2
8
3
2
2
1
6
2
2 1
9
1
2 3
30 3 12
5
6 15 1
20 4
1
3
5
2 4
3
1
2 1
10
1
1
16
45
4 2 1 42 7
14
2 1 2
10
1
2
1
2
4
1
2
1
2
2
12
6
3
12
1
1
4
2
3
4
11
2
.,
.,
§
.,
.,
.,
e
~
includes MK-6; No.27 includes MK-447 and MK-454.
~
83
0
•
•
.....
••
•
•. I
. • ' •. • +
0
0
•
•
•
•
2
outside
window
•
•
•
0
...
0
+
\
•
•
0
0
.
•
•
0
...
•
2
i nside
window
•
•
0
•
•
•
0
0
•
+
0
0
0
0
•
•
•
•
..•
•
•
_..
oO
0
•••
•
0
.x
0
0
84
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many outcrops. The quality of bedding in the Rangeley varies. The
more massive schists are frustrating, especially where they contain
abundant quartzo-feldspathic lenses and veinlets, in that bedding is
not preserved and only the calc-silicate pods offer any clue as to
bedding orientation. This is a major problem in the area north of the
Chesham Pond fault, where the schists rarely have good foliation let
alone bedding. Other parts of the Rangeley, for example the first
50 m below the Perry Mountain, are well bedded. Graded beds have been
found in all units, and the topping direction from one unit to the
next is certain. Locations of important outcrops where topping
directions can be observed are shown on Plate 2.
Foliation. A pervasive foliation in the schists and most of the
quartzites is defined by platy minerals. This foliation is nearly
parallel to bedding in most outcrops. It lies parallel to the axial
planes of tight isoclinal folds on Mt. Monadnock which are thought to
have formed during the nappe stage of folding. Andalusite pseudomorphs generally lie within the bedding planes, forming spectacular
"turkey track" patterns on many surfaces. Some quartzo-feldspathic
segregations and veins lie in the foliation while others cut across
it. Because the Kinsman Granite is believed, for several independent
reasons, to have intruded before or during the formation of the
nappes, the oldest foliation in t~e Kinsman is believed to be coeval
with the pervasive foliation in the schists, but this has not been
proven. In central Massachusetts, Tucker (1977) concluded that the
feldspar phenocrysts in the Coys Hill Granite predate the earliest
tectonic foliation.
Mylonitic foliation. Both the Kinsman and the Spaulding bodies
southeast of Mt. Monadnock, as well as some pegmatites, show various
degrees of mylonitization. Quartz grains are flattened and feldspars
are tapered with crushed margins. Because the Spaulding cuts across a
nappe-stage anticline north of Thorndike Pond, it is clearly yo~nger
than the nappe stage and the mylonitization must have taken place
still later, probably during backfolding. In some mylonitized Kinsman
the older nappe-stage foliation is weakly preserved. Most of the
schists in adjacent outcrops do not show two foliations.
Crenulation cleavage. Both the late backfolds and the dome stage
folds locally have an associated crenulation cleavage. This feature
ranges from a weakly defined alignment of crenulation fold limbs to a
well developed spaced cleavage in which platy minerals define cleavage
planes that are two to four centimeters apart. Crenulation cleavage
is best developed on the short limbs of outcrop-scale folds.
Joints. No systematic study of joint sets was made during this
project, although Mt. Monadnock itself offers an ideal place to study
brittle features because of the excellent exposure. Most of the large
surfaces visible from a distance are joints rather than foliation or
bedding. Many are coated by tourmaline and/or quartz. A plot of
poles to 46 tourmaline veins from throughout the quadrangle (Figure
99
11 G) shows that most strike roughly east-west and dip steeply south.
A systematic study of tourmaline vein orientations relative to joint
sets might shed some light on their origin and relative age. They are
younger than the microdiorite dikes. The most prominent lineaments on
an air photo of the mountain trend N30°E, and the main microdiorite
dike follows one of these directions. These relationships suggest
that at least some of the joints had formed by the time of the posttectonic (Fitzwilliam) intrusions. Other grominent joint sets visible
on air photos include N25°W, N40°W and N70 E. Joints approximately
parallel to the topographic surface are probably related to unloading.
In the layered rocks unloading joints are best d~veloped where bedding
dips gently, as on the west side of Mt. Monadnock. Here prominent
joints are parallel to bedding. There are also gently dipping joints
in the plutons. A photo of such sheeting joints in the granite quarry
at Marlboro was used in Billings' text on structural geology (1954,
Plate XIII, p.122).
Linear Features
Mineral lineations. Fine-grained sillimanite, biotit~ and muscovite after sillimanite commonly show a strongly preferred orientation
within planes of foliation or bedding. The platy mineral lineations
may represent the lines of intersect~on with some other planar
feature. Elongate quartz and feldspar grains form lineations in some
of the rocks, and quartz rods are common both on quartzite beds and
quartz vein surfaces, and as stretched pebbles in the conglomerates.
Andalusite pseudomorphs show a preferred orientation mainly in the
area of late fold hinges. Equal area plots summarizing data from the
entire quadrangle are shown for sillimanite and quartz lineations in
Figure 11 F and H.
Intersection and crenulation lineations. The lines of intersection of bedding on foliation planes can be seen as compositionally
different layers. The intersection of foliation with cleavage
commonly appears as a crinkle or crenulation lineation.
Minor folds. Folds were described in the field in terms of fold
axis plunge, axial plane orientation, rotation sense, tightness, and a
record of what planes are deformed. The folds range in scale from
tiny crenulations seen only with a hand lens to folds with amplitudes
in tens of meters. Axial plane and fold axis data are presented for
the entire quadrangle in Figure 11. It was not always possible to
distinguish relative ages of isolated minor folds in the field.
Isoclinal folds with axial planes parallel to the pervasive foliation
were assigned to the nappe stage (Figure 11 A). Open folds with steep
axial planes associated with crenulation cleavage, especially
prevalent in the western part of the quadrangle, were assigned to the
dome stage (Figure 11 D). Upright folds on Mt. Monadnock, similar in
style to the dome-stage folds but perhaps younger, are shown on a
separate equal area diagram (Figure 11 C). That leaves a large number
of folds that deform foliation, but whose relative ages are unclear
100
beyond "post-nappe stage".
folds" (Figure 11 B).
They are grouped together as "intermediate
GEOMETRICAL ANALYSIS OF STRUCTURAL DATA
Structural Data in Subareas
The quadrangle has been divided into 36 subareas (Figure 10b)
mainly for the purpose of presenting structural data. Equal area
diagrams for each subarea are shown in Figure 12. Some subareas
merely present data from isolated groups ot outcrops, while others
were chosen to portray specific major structures, such as the Beech
Hill anticline (Subarea 18) or the two limbs of the Thoreau Bog
syncline (Subareas 23 and 24). Many subarea boundaries had to be
arbitrarily drawn through areas with a continuum of data variation,
for example between Subareas 6 and 12, where the dip of foliation
gradually steepens from west to east. The average strike and dip
symbols and average sillimanite lineation directions in Figure 10b
were visually estimated from the subarea plots of Figure 12. They are
meant to give a general idea of the overall structure and should not
be taken too literally.
Construction of Cross Sections
Five east-west cross sections were constructed (Plate 5) at a
scale of 1:50,000 and with no vertical exaggeration. Average dips of
contacts were projected into the cross sections from nearby outcrops.
The plunge of major fold axes were projected from areas of known trend
and plunge, and adjusted according to known changes in plunge of minor
folds and lineations at the earth's surface. In most cases there were
few constraints on these adjustments, so the cross sections are highly
interpretative.
PHASES ONE AND TWO:
t
FOLD NAPPES AND THRUST NAPPES
Introduction
The interpretation here is tentative and rests heavily on
previously published ideas of the regional geology. However, there is
strong evidence, first, that the large isoclinal folds on Mt.
Monadnock are of the same age as larger isoclinal nappe-stage folds,
and second, that the nappes have been cut by major west-directed
ductile thrust faults. The assumption that the stratigraphic syncline
referred to below as the "Monadnock syncline" formed during the nappe
stage, may be incorrect. If so, much of the early structural history
proposed below will require revision.
Tectonic Levels
Three tectonic levels are present in the Monadnock quadrangle,
apparently separated by early thrust faults (the Chesham Pond fault
101
Gl
E
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·· F itzwilliam
· pluton
SCALE
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3 km .
Fig. 13. Simplified geologic map of the Monadnock quadrangle showing
the three structural levels separated by faults. and various structural
features mentioned in the text. in part outlined by the contact at the
base of the Littleton Formation.
102
and the Brennan Hill fault, Figure 13). These levels contain:
(1) an autochthonous, upright sequence overlying the Keene dome,
consisting of Ammonoosuc Volcanics, Partridge Formation, Clough
Quartzite, local Fitch Formation, and Littleton Formation, (2) the
folded Monadnock sequence consisting of the units shown in the stratigraphic column of Figure 3, intruded by Spaulding Tonalite, and (3) a
"gneissified" sequence consisting mainly of Rangeley Formation and
Kinsman Granite. Gravity studies show that the Cardigan pluton forms
a 2-3 km thick, subhorizontal sheet-like mass (Nielson et al., 1976).
Partly because the Kinsman and related Bethlehem Gneiss-are-exposed
above the Bernardston and Skitchewaug nappes to the west, they are
believed to have intruded prior to formation of the nappes (Thompson
et al., 1968). The Kinsman is apparently cut by the Chesham Pond
fault. Evidence in the Monadnock quadrangle indicates the Spaulding
Tonalite is younger than the nappes. We will return to a discussion
of the thrust faults which separate the tectonic levels after
presenting evidence for the fold nappes, which are older.
Monadnock Syncline
The Monadnock syncline is separated into two parts by intrusions.
Southwest of the intrusions, the syncline is relatively narrow, with
good symmetry across its axial trace. It has Littleton Formation in
its core, and extremely thin Perry Mountain, Francestown, and Warner
Formations on each limb, probably thinned tectonically. An isoclinal
fold in the Littleton Formation on the east slope of Little Monadnock
Mountain plunges 54° south. It is believed to be a nappe-stage fold
because the pervasive foliation is parallel to its axial plane.
However, the plunge of the Monadnock syncline on a regional scale must
be toward the north. More work is needed to determine how the south
end of the Monadnock syncline relates to the Tully body of Monson
Gneiss (Pike, 1968), and to the belt of sulfidic rocks mapped by
Fitzgerald (1960) east of the Tully body, in Massachusetts.
t
The Monadnock syncline north of the intrusions is wider, and overturned to the southeast. Although the stratigraphy is grossly symmetrical across it, the presently upright southeast limb is much
thicker than the presently overturned northwest limb. Prior to
backfolding, these topping directions are believed to have been opposite to what they are today. The southeast limb includes a thick
section of the upper part of the Littleton Formation, at least in part
thickened tectonically. The southeast limb is folded by the younger
northeast-trending Thoreau Bog syncline, but the northwest limb is not
affected by it. This younger fold is apparently disharmonic, dying
out rapidly to the north. The axial trace of the Monadnock syncline
crosses Pumpelly Ridge near a gulch at 728 m elevation. North of this
gulch graded beds are overturned, whereas most of the beds to the
south are upright. Isoclinal folds along Pumpelly Ridge south of the
gulch are overturned toward the northeast, consistent with the sense
of other nappe-stage folds on the southeast limb, discussed below.
103
Folds on Mt. Monadnock
Nappe-stage folds are abundantly exposed on Mt. Monadnock. They
consist of tight isoclinal folds with amplitudes far exceeding their
wavelengths, and axial planes parallel to the pervasive foliation.
Amplitudes are generally greater than five meters. Very few smaller
nappe-stage folds were observed. The lack of small-scale folds may
explain why nappe-stage folds were seldom observed in other parts of
the quadrangle where outcrops are smaller. Most of the nappe-stage
fold data in Figure 11 came from Mt. Monadnock.itself. Nappe-stage
fold axes plunge in various directions due to rotation around younger
folds. This is apparent in the wider spread of early fold axis orientations compared to intermediate axes in Figure 12, Subarea 23.
Figure 14 is a sketch of the Seven Quartzites folded by nappe-stage
isoclinal folds, about ten meters east of the Smith Summit Trail at
elevation 823 m (MK-1104). The amplitude to wavelength ratio is
approximately five to one. These are similar folds, in which beds are
thicker in fold noses than on the limbs. Pervasive foliation in the
schist, and ellipsoidal pits due to weathered-out chlorite in the
quartzite, lie parallel to the axial planes of the folds. Data from a
smaller nappe-stage fold exposed nearby (MK-1105) are plotted on an
equal area diagram in Figure 14. The fold axis plunges N87°E at 26°.
The most dramatic exposure of a nappe-stage fold is on a seven
meter west-facing cliff 150m west of the summit (MK-6). This fold
was pictured in the frontispiece of the first edition of Billings'
Structural Geology (1942), and so has been nicknamed "the Billings
fold". It is a recumbent downward-facing syncline with fold axis
plunging N58°E at 32°. A sketch of this fold, and data plotted on an
equal area diagram, are shown in Figure 15. The foliation is parallel
to the axial plane with strike N16°W, and dip 36°NE. A large bedding
surface at the base of the cliff contains abundant andalusite
pseudomorphs which appear to be randomly oriented in the foliation
plane. A plot of 96 "andalumps" measured within one square meter of
that plane shows no pronounced preferred orientation (Figure 12, MK-6,
~ith Subarea 23), but perhaps a slight maximum toward the NNE.
This
suggests the andalusite crystals formed under relatively static
conditions, possibly later than the nappe-stage folding. Examples of
andalumps forming a strong lineation are rare, and a special symbol is
used for these on other diagrams in Figure 12.
The Billings fold lies structurally within the uppermost of three
southeast-opening isoclinal synclines defined by the Seven Quartzites
(Figure 16). The fact that they now open southeastward is a function
of later folding, during which the originally overturned limb of the
nappe, with the isoclinal folds as "parasites", was backfolded to its
present stratigraphically upright position. The isoclinal pattern
shown in Figure 16 was mapped out by following the Seven Quartzites
back and forth across the mountain and paying close attention to the
topping directions of graded beds. One of the best places to observe
the folds is at elevation 914 m near the Smith Summit Trail, where the
i•'t
25m
3m{ zr.r=·_.:::_._::::··/ ·;.;~· .·k~~-.-~·:·: . ~:: ~
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NW
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N
Fig. 14. Schematic view of Seven Quartzites
folded by nappe-stage folds on a surface dipping
30 toward viewer. Fold axes trend nearly perpendicular to this plane. Equal area diagram shows
four beds and the axial plane from a smaller nappestage fold near MK-1104, with fold axis plunging
N87°E at 26•.
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Fig. 15. View to the east of "Billings fold", 150 meters west of
Monadnock summit, on cliff trending Nmv. Quartzose horizons define
bedding. Six measurements from a single bed plotted on the equal area
diagram give a nappe-stage fold axis of N58°E, 32°NE, plunging nearly
perpendicular to cliff. Dashed line on diagram is foliation in schist.
......
0
l/1
106
Seven Quartzites close in an anticlinal hinge just north of the mafic
dike. The Seven Quartzites can be followed in each limb north across
the mountain as they get farther and farther apart. The beds to the
southeast are upright while those to the northwest are overturned. An
isoclinal fold hinge can be observed in schists between the two limbs
where they cross the Marlboro-Dublin Trail, but outcrops are sparse to
the north in the dense fir thickets on the north side of the mountain.
The synclinal hinge structurally below this anticline is exposed near
the Marlboro-Dublin Trail 30m to the northwest (MK-55).
Another interesting area where the nappe-stage folds are exposed
is along the White Arrow Trail from elevations 836 to 910 m. East of
the trail, on a large southeast-slanting surface, one can walk around
the hinge of a recumbent anticline closing southeast (Figure 16,
MK-2A). The axial plane foliation is well developed in the schist,
and is refracted through the quartzite beds, forming a fan-shaped
array around the fold hinge. The syncline structurally above the
anticline at MK-2A is exposed on the adjacent cliff, although the
ledges have slid down the mountain and thus are not exactly in place.
The rotation sense of this fold pair (east-over-west), represented by
an S-shaped line on Figure 16, indicates they are on the presently
overturned limb of a larger scale anticline. Climbing up the trail
one soon crosses the Seven Quartzites where they are doubled in the
nose of this larger anticline. Above this hinge, east of the trail,
there are isoclinal folds with west-over-east rotation sense, on the
upright limb below the Billings fold. The rotation sense of these
minor folds is shown schematically as a Z-shaped line in Figure 16.
There may be a nappe-stage fault below the Seven Quartzites at
MK-1104. Graded beds are upright across almost continuous exposure
from MK-1104 (elevation 810 m) down to the Black Precipice, a 10m
cliff at elevation 795 m, where the Seven Quartzites reappear, also
upright. I am reasonably convinced that the quartzites in the two
places represent the same stratigraphic section. The first column on
the left in Figure 6 (MK-27) was measured on strike southeast from
MK-1104, while the second (MK-36) was measured at the Black Precipice.
If they are the same, and if graded beds are upright between the two,
the only possible explanation is a fault.
The axial traces of the isoclinal folds west of the summit, if
they could be followed east through the rhythmically bedded schists
and quartzites above the Seven Quartzites, should be exposed east of
Pumpelly Ridge. Indeed there are isoclinal folds exposed along the
Cascade Link Trail at about elevation 738 m (east of Figure 16). If
one could follow the axial traces in the other direction, one should
eventually be able to see the upper/lower Littleton contact and the
Littleton/Warner contact folded by the isoclinal folds. Unfortunately
that area has been intruded by the Troy Quarry pluton at the present
level of erosion (see Plate 5, Cross Section C-C').
107
./
·.o
·.~
·. ~.
··.~
Billings
fold
./
Smith
.!.....t... A
Bald Rock
0~
~0
·. Halfway
1 House
I
site
0
0
500
/
1000
250
1500
500
2000
2500ft.
750m.
Fig. 16. Summit area of Mt. Monadnock, showing isoclinal folds marked
by the Seven Quartzites. Station numbers refer to locations mentioned
in the text. Bedding symbols with dot toward present tops.
108
Other Map-scale Nappe-stage Folds
Besides the Monadnock syncline and the smaller-scale isoclinal
folds on Mt. Monadnock, some of the other map-scale folds on Plate
are interpreted as belonging to the nappe stage on the basis of their
isoclinal shapes and senses of rotation. Figure 17 shows the
schematic configuration of nappes and thrusts (discussed below) prior
to backfolding. It can be seen in Figure 17 that the Howe Reservoir
and Dublin Pond synclines are each paired with anticlines that close
toward the west. They are on the originally upright limb of the
Monadnock syncline. Smaller folds with the same rotation sense occur
at Mountain Brook at the north base of the mountain (Plate 1 NE) and
south of Gleason Brook in the northwest corner of Jaffrey (Plate 1
SW). The Gilson Pond anticline has the opposite sense of rotation and
is on the originally overturned limb Qf the Monadnock syncline. The
isoclinal folds defined by the Seven Quartzites, described above, are
also on this limb. Although it would appear from Figure 17 that the
Monadnock syncline represents a major nappe-stage closure, in a
regional context it too may be on the limb of a still larger-scale
nappe. This is important in that it could explain the presence of
units younger than Rangeley Formation which are exposed in synclines
associated with Kinsman Granite far to the south in Massachusetts
(Field, 1975; Robinson et al., 1982a).
Howe Reservoir syncline. Although the actual hinge of the Howe
Reservoir syncline is not exposed; the stratigraphic symmetry and isoclinal nature of the fold is clearly demonstrated by an area of
outcrops east of Howe Reservoir. Bedding and foliation strike WNW and
dip moderately NE. The Warner Formation is exposed in the core of the
fold, with Francestown and Perry Mountain on either limb. A mylonitic
zone in the Warner suggests some local shearing in the axial region of
the fold. The mylonite is deformed by a minor asymmetric backfold.
~
Dublin Pond syncline. The structure west of Dublin Pond is
complicated and the outcrop is not good enough to determine whether
repetitions in the stratigraphic sequence are due to folding or
faulting. Plate 1 NE shows a nappe-stage syncline-anticline pair east
of Hurricane Hill which is deformed by the younger Beech Hill anticline. From Hurricane Hill southeast to the base of Mt. Monadnock the
Francestown Formation is repeated four times, but in each case the
sequence tops toward the southeast (Figure 18). The Warner cannot
actually be followed around the fold noses from one belt to the next.
Thus the repetitions could be due to faults rather than folds. The
nappe-stage anticline north of Dublin Pond is based largely on two
areas of float, one consisting of Rangeley in the core of the fold,
and one of Littleton on the southwest limb.
Gilson Pond anticline and Meade Brook syncline. Another important
nappe-stage fold in the Monadnock quadrangle is the Gilson Pond
anticline. This was mapped by Nelson (1975). In its core is a 1.5 km
long belt of Perry Mountain Formation, which locally is less than
100 m wide. Several brooks run transversely across the fold and the
stratigraphic symmetry is best exposed in outcrops along "Ark Brook"
w
E
Level ( 3)
.c:._
Dkg
Sr
Level (2)
Fig. 17. Schematic configuration in cross-section of nappe-stage deformation, showing three
tectonic levels separated by faults. Only dome gneiss, Littleton and Rangeley Formations,
and Kinsman Granite are shown for clarity.
......
0
"'
110
N
1
Dublin
Pond
\.I ........ .
,,_, .... ,, <·,·.
/ - \ I "\ I
-; ./
-;1
I
, - I
- - i" ·. ·..
''---
\ -/ I .
- ""·
1- I -, -':
_,1~-.::
/I-:
,-- .I_....\
_...-\
' I I:._ \ - _..... /
.:
I.
I \
I-
I '_ , .·
\- -_.. . '--I
"\ / I I_...
I
I
..
.
".:.- '· .·
.....- \ ..... r I,.··
-
l
I...,_\<.
I - I.··
/ .·
-I.··
76~
0
1/2
I km.
Fig. 18. Alternate interpretation for area south of Hurricane Hill,
Dublin, at same scale as Plate 1 NE. Francestown Formation (north of
stippled unit) and Warner Formation (stippled) are repeated in four
places, each with tops toward the south (arrows). Heavy lines with
triangles represent possible faults. Bedding and foliation symbols
same as on Plate 2. Bedding symbols with dots represent overturned beds.
111
north of the Poole Memorial Road to the State Park. Both the Warner
and Francestown are much thicker on the northwest limb of the
anticline than on the southeast. There are isoclinal digitations of
Francestown into the Warner along the west limb of the anticline,
apparently nappe-stage structures.
Meade Brook, which drains Poole Reservoir, approximately follows
the Warner Formation along the axial trace of a tight syncline, the
Meade Brook syncline. This fold and associated nappe-stage folds are
deformed by backfolds, discussed below (see Figure 24). The Littleton
Formation appears in this syncline in a small area south of the
reservoir, and again farther south (outside the area of Figure 24),
continuing south as far as the Fitzwilliam pluton (Plate 1 SE). The
Meade Brook syncline is believed to be comparable in scale to the
Gilson Pond anticline. The axial traces of two smaller nappe-stage
anticlines are shown on Figure 24. One axial trace passes through the
digitation of Perry Mountain Formation southeast of the reservoir, and
then is folded back and forth through the large area of Francestown
north of the reservoir. The anticline west of the brook also has a
core of Perry Hountain Formation.
The syncline east of the Gilson Pond anticline is incomplete, cut
off by the Thorndike Pond fault zone. At Gilson Pond the axial trace
of the anticline is cut by mylonitic tonalite of the "Gilson Pond
type". If this tonalite is of the same age as the Spaulding Tonalite,
this indicates a post-nappe age for the Spaulding. The anticline and
the mylonitic tonalite are deformed by a younger NE-trending fold.
Nelson (1975) correctly observed that early folds with steep
NW-trending fold axes are deformed by the younger NE-trending folds.
However, most of the NW-trending fold axes do not belong to the nappe
stage. This is explained more fully in the section on backfolding.
Nappe-stage folds in the Derby Hill window. There are tight
isoclinal folds in the south part of the window, exposed in a series
of outcrops in the swampy area west of Willard Hill (Figure 19). The
axial traces of these folds are cut by faults (described below) and
folded by still younger folds. The isoclinal folds may have occupied
the same structural position relative to the Monadnock syncline as the
Howe Reservoir syncline.
Nappe-stage Thrust Faults
Brennan Hill fault. The nature of the contact between levels (1)
and (2) (Figures 13 and 17) is subject to debate. The interpretation
favored in this thesis is that a nappe-stage thrust fault separates
Littleton Formation to the west from overlying gray-weathering
Rangeley Formation to the east. The contact is difficult to map, and
the name "Brennan Hill fault" is not meant to imply that it is any
better exposed on Brennan Hill than elsewhere. The rocks to the west
(Littleton) are sillimanite-rich, locally staurolite-bearing,
monotonous gray-weathering schists. Those to the east ("lower part of
112
r I
J~
67~
(
'
Sra
soo'
0
9~------''lOOm
N
l
Sra
)(
Sro
Fig. 19. South part of Derby Hill window. Formations are abbreviated
as on Plate 1. Sra is augen schist assigned to the Rangeley, which may
in part be metamorphosed mylonite.
113
the Rangeley") are more feldspathic, with abundant quartz-feldspar
knots. In general they have a less uniform character, but they also
weather gray. These same distinctions were discussed by Robinson
(1963) between what he mapped as Littleton to the west, and the "Gray
Schist Member of the Partridge Formation" to the east. A fault
interpretation is strengthened by the rocks mapped as Fitch Formation
exposed above the Clough Quartzite near Mt. Huggins (Plate 1 NW).
Alternatively, as discussed earlier, the Littleton of level (1)
may not be Littleton at all, but rather may be Rangeley. The rocks
east of the Keene dome would thus constitute a depositional sequence
with Rangeley overlying Clough, in which case both levels (1) and (2)
would be autochthonous. The arguments favoring this interpretation
stem from observations to the north by J.B. Thompson, Jr., and Page
Chamberlain (pers. comm., 1983). On the Unity dome and in the
Marlow-Gilsum area, there are two or more conglomerate horizons with
gray Littleton-like schist between them, apparently representing the
transition from Clough Quartzite to the Rangeley Formation. The
"Clough-Rangeley" rocks thicken from one nappe level to the next as
one goes structurally higher, or toward what would have been the
Merrimack trough prior to deformation. As mentioned elsewhere in this
thesis, it seems odd that the clean quartz conglomerates of the Clough
would be the basal member here rather than the polymictic conglomerate
which is at the base of the type Rangeley section in Maine.
Chesham Pond fault and Derby Hill window. The Chesham Pond fault
separates levels (2) and (3) and is much better documented than the
contact between levels (1) and (2). The proposed configuration prior
to later deformation is shown in Figure 17. At several places north
of Minnewawa Brook and east of Chesham Pond the contact between
undoubted Littleton Formation in level (2) and the physically
overlying Rangeley Formation in level (3) can be approached to within
a few meters. The normally intervening units are thus missing or
extremely attenuated. They are exposed, perhaps in fault slices,
along Horse Hill Road northeast of Marlboro, and locally along the
west margin of the Derby Hill window (Plate 1 NW). Recrystallized
mylonite has not been identified for certain, but the texture of the
schist above the contact could have been produced by mylonitization of
pre-existing quartz-feldspar segregations. The Chesham Pond fault
cuts across the axial trace of the Monadnock syncline, in which the
youngest rocks belong to the upper part of the Littleton Formation.
Toward the northwest the fault cuts stratigraphically downward,
approaching the base of the Littleton, and eventually cuts into the
Rangeley. The point where this actually happens is intruded by the
Babbidge Reservoir pluton (Figure 13). The upright "Monadnock
sequence" may reappear to the northwest at Gee Mill. There is no
control on the transport direction of the thrust, but if we assume
that it was toward the present-day west, one possibility is that this
cutting down across stratigraphy is due to a local lateral ramp
(Butler, 1982), that descends toward the north.
114
§~
Sro
Q:
conglomerate
fsr
tl
lj
cJ
..
/
39
I
I
/
/
Sr
I
;4f'
/
301
I
I
/
'1'
/
Spm
,....
l .. ·
/
I
I
/
Sf r
~~
44
I
/.,----.
I
__
Sr
/
I
I
I
/../
\
\
'!"
I
(~
" " ......
I
'-
I
N
Sw
0(
'-- . /
~
!I
e;,
I
1
I
-Q
500ft.
0
0
lOOm.
Fig. 20. A portion of the north part of Derby Hill window.
Abbreviations as on Plate 1.
ll5
The Derby Hill window consists of an area of folded rocks of the
Monadnock sequence surrounded by augen schists and rusty gneisses of
the Rangeley Formation. Initially it was thought the rocks in the
window (Figures 19 and 20) were e~posed by a NNE-trending anticline
that folds the Chesham Pond thrust surface. However, there is no
symmetry within the window rocks across the proposed fold axial trace
to support such a hypothesis. The alternative presented in this
thesis is that the east margin of the window is an east-directed
backthrust. Cross Section A-A', Plate 5, passes through the northern
part of the window, and shows the proposed backthrust displacing the
earlier west-directed thrust. It should be emphasized that the angle
of dip on the faults is unknown. All the rocks are refolded by
NW-trending late folds (dome-stage or younger).
The proposed backthrust cuts the axial traces of nappe-stage
isoclinal folds. At the south end of the window a confusing group of
outcrops may represent fault-bounded slivers of Littleton and
Francestown surrounded by Rangeley. A contact between Francestown and
the augen schist is well exposed there. The nappe-stage folds inside
the window may have been on the upright limb of the Monadnock
syncline, in a position similar to that of the Howe Reservoir syncline
(Figure 17), and the Chesham Pond thrust has cut down close to them
(compare Cross Sections B-B' and A-fo').
Along the west side of the window and at Horse Hill Road, however,
the stratigraphic sequence locally faces downward from the Chesham
Pond fault, with the intervening units between Rangeley and Littleton
present. This can be seen on Plate 1 NW where the Warner and
Francestown crop out locally along the fault east of Woodward Pond.
These areas may represent fault-bounded slices carried along the fault
from the overturned limb of the Monadnock syncline itself, or from the
overturned limb of a subsidiary nappe-stage fold on the upright limb,
as shown in the cross sections.
~
Thorndike Pond fault zone. The contact between levels (2) and (3)
is less easily identified east of Mt. Monadnock. North of Thorndike
Pond (Plate 1 NE) the Littleton Formation lies to the west of a narrow
belt of Kinsman Granite with the Rangeley to the east. Southwest of
Gilmore Pond (Plate 1 SE) a wider belt of Kinsman separates Rangeley
on the east from an eastward-facing sequence on the west (only
Francestown and Warner are exposed; the Littleton here is speculative). One interpretation is that the fault splays into several
branches which are responsible for the repeated belts of Kinsman.
Alternatively, these repetitions may be due to younger faulting. The
latter interpretation is shown on Plates 1 and 5 and will be discussed
further in later sections. In some places the nappe-stage fault may
have been reactivated during backfolding and again during Mesozoic
normal faulting. Because of the apparent repeated episodes of
faulting over a broad area, the name "Thorndike Pond fault zone" is
adopted for the four-kilometer-wide zone between the elongate
mylonitic Spaulding Tonalite bodies at Whites Pond and Windmill Hill
116
on the west, and the widest belt of Kinsman on the east (Figure 13).
PHASES THREE AND FOUR:
BACKFOLDING
Introduction
There is a wide range of folds which deform the dominant foliation
in the Monadnock quadrangle. In several outcrops nappe-stage
isoclinal folds are deformed by younger folds, but there are very few
instances of intersecting younger folds, so that little success has
been made in sorting out the folds of intermediate age. Open folds
with steep axial planes and associated crenulation cleavage are
obviously younger and present fewer problems. They are presented
separately in Figures 11 and 12 as late or dome-stage folds.
The intermediate age folds are assigned to what Robinson (1979)
referred to as the backfold stage of deformation. Axial surfaces of
the nappes were "backfolded on a grand scale by east-directed folds"
(Robinson, 1979, p.126). Mapping in central Massachusetts indicates
that this episode was a complicated, multi-stage process involving
recumbent folds, longitudinal flowage of some gneissic basement and
mylonitization that occurred late in the episode. Intermediate age
structures in the Monadnock area will be described from various parts
of the quadrangle, and then an attempt will be made to present a
possible sequence of events during backfolding. Regional correlation
will be reserved until after the description of all phases of
deformation in the quadrangle.
Intermediate Stage Folds, Mt. Monadnock
~
Asymmetric folds. West-over-east asymmetric folds are very
commonly exposed on Mt. Monadnock. They deform the dominant foliation
as well as bedding. The short limbs are generally steep to slightly
overturned. The maximum amplitudes are on the order of one meter.
Where exposure is sufficiently good, it can be seen that these folds
usually die out over about 10 meters. Figure 21 shows the lower part
of an asymmetric fold north of MK-1104. A pegmatite vein is deformed
by the fold, and a late quartz vein cuts across it at a low angle.
The axial plane is oriented approximately N61°W, 57°NE, and the fold
axis plunges 31° in a N67°E direction. Folds such as this occur all
across the west limb of the Thoreau Bog syncline (Figures 13 and 16),
where they have a northwest-over-southeast rotation sense. One
excellent exposure of asymmetric folds deforming cyclic graded beds,
stratigraphically above the Seven Quartzites, occurs on a nearly horizontal outcrop about 10 m north of the Billings fold, northwest of the
mafic dike. On the east limb of the Thoreau Bog syncline, on Pumpelly
Ridge, asymmetric folds also have a west-over-east sense, with axes
plunging toward the northwest. They thus appear to predate the
Thoreau Bog syncline. On Pumpelly Ridge care must be taken not to
confuse them with the late upright folds, as both sets have axes
plunging northwest.
117
0
2m.
s
N
N
Fig. 21. Profile view to east
of asymmetric fold which deforms
foliation (MK-1107), at elevation
825m, west of Monadnock summit.
Equal area diagram shows six
bedding measurements taken at
corresponding points in sketch.
Dashed line on diagram is the
approximate axial plane. Fold
axis plunges N67°E at 31°.
118
Boudinage. In some places where closely spaced quartzite beds are
folded by the asymmetric folds, a curious sort of boudinage of the
schist beds seems to have occurred, allowing the quartzite to flow and
form a connection between adjacent schist beds (Figure 22). Dr.
Shizuo Yoshida (pers. comm., 1984) suggested that some of these were
sand dikes formed prior to lithification, but several factors favor a
tectonic origin. Foliation in the quartzite beds is deformed and
curves into the boudin necks from above and below. The boudin neck
lines lie parallel to asymmetric fold axes, and in some instances vein
quartz or pegmatitic material has collected along the neck lines. In
some outcrops apophyses of quartzite project from isolated quartzite
beds into the schist. Where the beds are upright and the quartzite
projects downward these can look like load casts, but in other places
they also project upwards from upright quartzite beds. Here, too, the
axial planes of such structures lie approximately parallel to the
axial planes of nearby asymmetric folds. If the schist beds are
indeed boudinaged, this indicates some rather unusual competence
contrasts between the schist and quartzite. Perhaps andalusite
crystals strengthened the schist, or the quartzites were enough wetter
than the schists to allow them to behave in a more ductile manner than
the schist during the same stage of deformation.
~
Thoreau Bog syncline. The large anticline-syncline pair which
dominates the foliaton pattern on Mt. Monadnock is responsible for the
topographic shape of the mountain. I have named the syncline for a
tarn-like bog located just north of Pumpelly Ridge near the axial
region of the syncline, known as Thoreau Bog (Figure 16). Thoreau
described the bog in his visits to Monadnock in the 1850's. Actually
there are two bogs, one in each limb of the fold, and they are
elongated parallel to foliation. The way in which the foliation
pattern controls topography in this hinge region is especially
striking on the air photo from which Figure 16 was made. Data from
the two limbs is presented on Figure 12, Subareas 23 and 24. Bedding
and foliation are shown separately for these subareas for the sake of
clarity. It is obvious that both bedding and the dominant foliation
were deformed by the Thoreau Bog syncline. A 3-D view can be
visualized by assembling the fence diagram in Appendix 1, Figure 42.
Sillimanite and mica lineations trend NW-SE on both limbs, indicating
that they are of a different age than the fold, but whether they are
older or younger is not certain.
The asymmetry of this syncline has the same west-over-east sense
as the minor folds described above. Because the northwest limb of the
Monadnock syncline is not deformed by the Thoreau Bog syncline, I had
originally thought there might be a fault across the lower north slope
of Mt. Monadnock. However, if the syncline dies out in amplitude in
the same way that the minor asymmetric folds do, a fault is not
necessary. This follows an extension of "Pumpelly's rule" (Pumpelly
et al., 1894, p.158), which seems appropriate since Pumpelly Ridge is
also named after Raphael Pumpelly. Pumpelly's rule states that axes
and axial surfaces of minor folds in an area are congruent with those
119
~
~~. ..;._~==
.===.;=-,-"~.,....._.,~~--~
~
- . : ....
"7
.
Fig. 22. Two views of boudinaged schist beds. Quartzite beds
are stippled. Upper view is part of the Seven Quartzite section, toward east, elev. 860m on the White Arrow Trail. Pits
are weathered-out chlorite, with long dimension lying parallel
to foliation. Note minor as ymmetric folds. Lower view shows
boudins in schist layers above the Seven Quartzites, elev.
893m, south of the Marlboro Trail, viewed toward the northeast.
120
NO<Ih
~
~«
PooklMooo:,:::,dlike Pond
Thoreau Bog
--"l_p_
21y~---~--=z:::::::::::-""'---=::-l
~
~-~/T,·~
N
Fig. 23. View towards the east of NEplunging backfold, along Pumpelly Trail
200m east of Monadnock summit (MK-799).
Hammer handle is approximately parallel
to the azimuth of the fold axis, which
plunges N33°E at 29°. Lines on bedding
surface are crenulations. Fine
sillimanite and quartz lineations
lie parallel to them. Symbols on
the equal area diagram are the same
as in Figure 12.
121
of the major fold structures of the same phase of deformation.
Along the Pumpelly Trail 200 m east of the summit, at elevation
920 m, there is an intermediate age syncline which plunges northeast
(MK-799). A one-by-three meter block of rock from inside the syncline
has slid down the slope, exposing a curved bedding surface that one
can walk around on. Figure 23 is a sketch of this fold with data
plotted on an equal area diagram. Bedding measurements define a fold
axis plunging N33°E at 29°. Sillimanite lineations and coarse crenulations trend nearly N-S, obliquely across the fold, which suggests
they date from an unrelated phase of deformation. They define a great
circle on the equal area diagram, and are presumablyyounger than the
fold. The axial planes of the crenulations are roughly parallel to
those of late open folds observed in nearby outcrops.
Intermediate Stage Folds, Poole Reservoir Area
The map pattern in Monadnock State Park near Poole Reservoir is
complicated (Figure 24). Despite excellent exposure, the structural
history is poorly understood. The following analysis is presented as
a possible explanation. Structural data from Subarea 27, which
includes Poole Reservoir, is presented in Figure 12 with bedding,
mineral lineations, foliation, and fpld axes shown in separate plots
due to the abundance of data. Details from single outcrops (MK-447
and MK-454) are discussed as examples of the backfold stage.
Most planar features around Poole Reservoir dip northwest except
around the areas of fold hinges (Plate 2). The spread in both planar
and linear data is due to the interference of at least three sets of
folds: nappe-stage folds, early backfolds plunging northwest, and
later backfolds plunging northeast. The relative ages between these
backfolds and those on Mt. Monadnock are unknown, but the following
discussion will attempt to show they are younger, mainly on the basis
of relationships to sillimanite lineations.
The axial surfaces of nappe-stage folds around Poole Reservoir are
deformed by backfolds with a range of plunge directions. The style of
deformation in outcrop differs greatly from one formation to the next.
Backfolds in the lower part of the Warner Formation appear to have
formed very plastically, with tight hinges and similar shapes. Some
calc-silicate layers are extremely attenuated on the fold limbs. The
Francestown, with its more massive bedding habit, behaved in a less
ductile fashion. Folds which approach concentric shapes are common.
The interbedded schists and quartzites of the Perry Mountain, by
contrast, favored the formation of disharmonic folds due to slip
within schist horizons.
An outcrop in the digitation of Perry Mountain Formation east of
Meade Brook (MK-447) has been studied in detail (Figure 25). It
serves as an example of the style of backfolds in the Perry Mountain.
Minor fold axes and quartz lineations plunge steeply west. The
122
I
I
I
./
I
I
---/
I
Sfr
N
mineral
lineations
/
quartz
/
mica
/
sillimanite
0
lOOm.
~-___J
0
500ft.
Fig. 24. Poole Reservoir area, Monadnock State Park. Axial traces of
nappe-stage folds (dotted) are folded by backfolds (solid). Folds at
MK-447 and MK-454 described in text. Samples from MK-432 and MK-629
described in section on metamorphism. Abbreviations as on Plate 1; "f1"
= nappe-stage minor folds. Perry Mountain Fm. stippled.
...
quartz
vein
N
0
0
0
0
0
..i!!..,
oo
0
·,
......
·I'.,.. .
• • !'.
•
co,..
..
+
~
0
"oo
0
"
0 0
0
...
N
1
...
"
Plan view
I ft .
lm .
Fig. 25 Outcrop sketch showing disharmonic folds in the Perry Mountain Formation
(MK-447). Equal area diagram shows data from this outcrop. Open circles -poles
to bedding; X's - poles to axial planes; closed circles - fold axes; squares quartz lineations. The outcrop occurs on the short limb of a larger fold.
.......
N
w
124
lineations are probably of the same age as the fold axes, but it is
possible that some were rotated into colinearity from some previous
(nappe-stage?) orientation. No nappe-stage fold closures were found
in this outcrop. At first sight the outcrop pattern appears to be the
result of interfering sets of folds. On the equal area diagram the
fold axes form a nearly coaxial cluster, and the poles to axial planes
lie on the same great circle as the poles to bedding. Such a pattern
could indeed be produced by two sets of coaxial folds. However, close
inspection of the outcrop sketch shows that some of the folds are box
folds, and the pattern is probably a result of disharmonic folding due
to the strong ductility contrast between the thinly bedded schists and
quartzites.
In the southern part of Figure 24, the rotation sense of
NW-trending folds is dominantly counterclockwise. MK-447 is on the
short limb of one such fold.
A detailed study was also made at MK-454, where the contact
between Francestown and Perry Mountain Formations is deformed by a
NE-trending fold. As one walks around the nose of this fold, the
sense of rotation of minor folds changes from clockwise west of the
fold hinge, to counterclockwise on the east. Quartz lineations on
many bedding surfaces plunge 21° to 70° toward the northeast, roughly
parallel to the fold axes.
~
Equal area plots of data from MK-454 and MK-447 are included in
Figure 12 for comparison with data from all of Subarea 27. It can be
seen that the distribution of data for the whole subarea is a combination of the NW- and NE-trending structures. Age relations are not
clear in the Poole Reservoir area, but in the Gilson Pond area 2.5 km
to the northeast (see below), NE-plunging lineations appear to be
younger. The fact that the quartz lineations show a greater spread
than the sillimanite and mica lineations suggests the quartz
lineations are older, or that they were not parallel to begin with.
Some may be inherited sedimentary features. Although the amount of
plunge varies greatly for the sillimanite and mica, the direction of
plunge is mostly to the NNW. Whether the sillimanite and mica lineations are related to the NW-trending backfolds, or represent a still
younger phase of deformation, is uncertain. It is important to note
that sillimanite and mica lineations trend toward the northwest all
across the eastern and central portions of the quadrangle, regardless
of the orientation of backfolds (Plate 3). They may be related to the
transport direction whereas quartz lineations are more tightly
constrained to be parallel to folds in bedding.
125
Intermediate Stage Folds and Mylonitization, Gilson Pond Area
Some important structural relationships are shown by the rocks
around Gilson Pond (Plate 1 SE). The Gilson Pond (nappe-stage)
anticline, which at this point has a core of Francestown Formation, is
cut off by the Spaulding Tonalite ("Gilson Pond type"). This may be
an intrusive relationship or a fault relationship, or both. There are
also tight isoclinal folds along the Francestown-Warner contact which
are cut off by the tonalite. The tonalite has been mylonitized to
varying degrees. Small bodies of Kinsman Granite, and a pegmatite
dike 50 to 100 em thick and about one kilometer long, are also
strongly sheared. Tails on sheared feldspar grains in the tonalite
suggest a west-side-up sense of shear.
All the rocks, stratified and intrusive, bear NW- to WSW-plunging
mineral lineations (Figure 12, Subarea 28). In the Warner Formation
the NW-plunging lineations are pervasive but faint. A stronger set of
NE-plunging mineral lineations lies parallel to NE-plunging folds with
west-over-east movement sense. The northeast lineations are
especially well developed in the Warner and Francestown, but are also
present in the Spaulding, mainly as faint biotite lineations. Along
the southeast shore of Gilson Pond, both the Warner and the Spaulding
are deformed by a large NE-plunging fold. The shoreline, concave to
the north, approximately follows the shape of the synclinal part of
this fold. Poles to bedding and foliation in Figure 12, Subarea 28,
show a spread due to folding around both NW- and NE-trending fold
axes. The younger NE-plunging linear structures are shown encircled
by a dashed line.
The large body of Spaulding Tonalite west of Gilson Pond has a
mylonitic texture, but has a less concordant shape than the Spaulding
bodies east of the pond. It cuts across the axial trace of the
nappe-stage digitation to the south, but its contacts have not been
mapped in enough detail to ascertain whether it is deformed by the
backfolds.
In summary, there is evidence that the Spaulding Tonalite intruded
across nappe-stage folds, underwent mylonitization during a
west-side-up phase of deformation, and then was deformed by
NE-plunging, west-over-east backfolds. The Spaulding also bears
NW-plunging lineations which may or may not be related to the
NW-plunging folds in the metamorphosed sedimentary rocks.
Intermediate Stage Folds, Southeast of Thorndike Pond
East of a belt of Kinsman Granite, a nappe-stage syncline with a
core of Francestown Formation is deformed by folds with axial planes
dipping northwest (Plate 1 SE; Figure 12, Subarea 29). Mineral
lineations plunging steeply northwest were observed in several
outcrops, and the folds are interpreted to be early backfolds similar
to those near Poole Reservoir. Minor folds at the scale of the
126
outcrop are rare, although there are some minor upright warps that
probably have little effect on the map pattern. The map pattern west
of the Kinsman is very different. A belt of Francestown Formation
strikes parallel to the Kinsman for hundreds of meters. Several
outcrops of mylonitic schist occur within the first 30 m east of the
Kinsman. It is difficult to tell what the protolith may have been.
Two to five millimeter feldspar grains lie par'a llel to a steeply
SE-dipping foliation. A younger mylonitic foliation dips 70° to the
northwest, and the feldspars are sheared out with tails that indicate
a west-over-east sense of shear. The younger foliation contains
steeply plunging biotite lineations. On Plate 1 SE a fault is drawn
through this area, with teeth marks indicating thrust motion toward
the east. The same line to the north and south is shown as a Mesozoic
normal fault, and this is meant to imply that thrusts which developed
during the backfold stage were later reactivated during the Mesozoic.
There is no evidence right at Thorndike Pond for normal movement, but
there may be some parallel normal faults which are not exposed.
Beech Hill Anticline
The strike of bedding and foliaton in Subareas 16 and 19 (Figure
10) is dominantly to the northeast, with dips to the northwest. In
Subarea 18, however, the strike ~s progressively more to the north
until, on Beech Hill, it turns quite abruptly to the WNW, with dips to
the north. This then defines the Beech Hill anticline (Figures 10a
and 13). West of Beech Hill there is a large area of no outcrop, but
still farther west in Subarea 13 the dominant strike is roughly E-W,
and then in Subareas 8 and 9 it is again dominantly to the northeast.
The bull's eye symbol on Figure 12, Subarea 18, is a beta maximum of
bedding and foliation intersections from the whole subarea, plunging
N46°W at 48°. The axis of the Beech Hill anticline estimated from
measurements taken just around Beech Hill plunges N57°W at 38°.
Regardless of the precise direction, the point is that this fold is
overturned toward the southeast, with a fold axis oriented in the same
direction as many of the minor folds and mineral lineations in the
central part of the quadrangle. On the overturned limb of the Beech
Hill anticline, between Beech Hill and Dublin Pond, there are minor
folds in the Warner Formation which plunge northwest and have a
counterclockwise rotation sense, consistent with the major anticline.
Sillimanite and quartz lineations also plunge northwest.
The Beech Hill anticline at first appears to be paired with the
Monadnock syncline because both are overturned toward the southeast.
However, my present interpretation is that the Monadnock syncline is
an older structure on the southeast limb of the Beech Hill anticline.
Thus, the anticline is a much larger-scale fold than is at first
apparent, deforming the Kinsman Granite as well as the metamorphosed
fault between levels (2) and (3). This fault is the Chesham Pond
fault west of Dublin, and it is folded by the Beech Hill anticline
around to the southeast and south, where its trace is lost in the
Thorndike Pond fault zone. The overturned southeast limb of the Beech
12 7
Hill anticline continues south across central Massachusetts. It is
presumably a short limb in the backfold system. Deep seismic
reflection profiling to the north (Ando et al., 1984) suggests that
the steeply dipping structures do not continue very far below the
earth's surface.
In the area west of Beech Hill, the Spaulding Hill pluton appears
to have domed the overlying rocks around it (Figure 10b). Perhaps
some of the Spaulding Tonalite plutons moved tectonically late in the
backfolding episode, in the same way that the streamlined bodies of
Monson Gneiss basement in Massachusetts are thought to have moved
(Robinson, 1963; Fitzgerald, 1960; Pike, 1968). Perry (1904) may not
have been so far off the mark when he observed that the plutons around
Mt. Monadnock appear to have pushed aside the foliation in the country
rocks, although he attributed the deformation to the time of
intrusion. If the NW-trending linear fabric formed at the same time
as the Beech Hill anticline and other NW-trending backfolds, it may
record the transport direction during backfolding. As we will see in
the discussion of the dome stage, however, late open folds have
approximately the same orientation, and some of the mineral lineations
may have formed then instead.
Intermediate Stage Folds in the Troy Area
Two sets of folds which deform foliation are exposed in an outcrop
of Rangeley Formation on the west side of a residential street, 1.7 km
SSE of Troy village (TR-174, Figure 26). The older fold has a
west-over-east sense of rotation, an axial plane which strikes N17°E
and dips 78°SE, and a fold axis plunging S09°W at 34°. It deforms a
still older foliation. The younger folds are more open, clearly
deform the first, and have an associated crenulation cleavage which
strikes N48°E and dips 60°SE. Their rotation sense is neutral to
east-over-west, and they plunge S23°W at about 36°.
N
1
crenulation
cleavage
0
lm
Fig. 26. Interference
pattern formed by two
sets of south-plunging
folds in the Rangeley
Formation (TR-174),
south of Troy. Explanation in text.
128
The younger folds are presumably dome-stage folds. This outcrop
illustrates how difficult it is to assign a relative age to folds in
the southwestern part of the quadrangle, except where folds of
different ages intersect one another. The linear fabrics which
developed during backfolding and doming both plunge south (Figure 12,
Subareas 6 and 12). The situation is similar to that in the central
part of the quadrangle (discussed below), where both linear fabrics
plunge northwest.
Intermediate or Dome Stage Folds in the · Derby Hill Area
The axial traces of nappe-stage folds and the proposed thrust
faults in the Derby Hill window are deformed by a variety of
cross-folds, all of which are shown in Figures 10a and 12 as
intermediate in age. Some of them could belong to the dome stage.
The proposed backthrust along the east side of the window may have
formed late in the nappe stage, or it may belong to the backfolding
phase. If the latter is true, then the cross-folds must be late
backfolds or younger.
The equal area diagrams (Figure 12, Subareas 2 and 3) are divided
to show data separately for structures outside and inside the window.
The plots show that there is litt~e difference in orientation inside
and outside the window. Thus the events responsible for the distribution of planar and linear orientations mainly post-date the nappes and
the Chesham Pond fault. There is a cluster of fold axes and mineral
lineations which plunge northwest. The axial planes to these
NW-trending folds dip moderately to steeply north. Some outcrops
contain W-plunging folds with upright axial planes. In addition, in
the northern part of the window around Derby Hill, there are some
S-plunging folds. The relative ages of these various folds are
uncertain. Nappe-stage foliation is deformed by all three.
Summary of the Backfolding Episode
The correlation of intermediate stage folds from one area to
another presents some problems. The simplest solution is to extend
the age relations observed in the Gilson Pond area to the rest of the
central part of the quadrangle. Folds with NW-trending axes formed
early, perhaps contemporaneous with local mylonitization, and were
followed by NE-trending folds. The early backfolds would thus include
the Beech Hill anticline with its related minor folds, the asymmetric
folds on Mt. Monadnock, and the NW-trending folds southeast of the
mountain. According to this solution, the Thoreau Bog syncline later
rotated some of the folds of this early stage away from their northwesterly orientations. However, the minor folds south of Beech Hill
and those on Mt. Monadnock have opposite senses of rotation, and if
they are the same age, this would require a backfold syncline between
the two areas.
129
a.
NW
-
SE
b.
SE
C.
..__
__ _
---
Chesham
. c\\(\e
Q(\\1
...-
-
.--
...- ..-
NW
--~
--
~ ~
~
SE
/
Fig. 27. Schematic cross-sections to show proposed backfolding sequence.
a. Asymmetric folds deform nappe-stage structures. b. Thoreau Bog syncline. c. Earlier structures rotated into short limb of the Beech Hill
anticline. Heavy arrows show regional shear sense. Small pole shows
topping direction, and X is shown as a reference point.
130
An alternative solution would have the asymmetric folds on Mt.
Monadnock as the earliest manifestation of backfolding, as shown in
Figure 27a schematically superimposed on the nappe-stage folds in the
Seven Quartzites. These folds were then carried along passively as
the Thoreau Bog syncline formed (Figure 27b). We noted earlier that
sillimanite lineations seem to be independent of the Thoreau Bog
syncline. Unless the syncline formed by passive flow, which is
doubtful in view of its disharmonic form, these lineations are
apparently younger. If the lineations are related in time to the
Beech Hill anticline, then it too must be younger than the Thoreau Bog
syncline. By incorporating the resulting pattern into the short limb
of the Beech Hill anticline, we arrive at the schematic cross section
in Figure 27c. To the east the short limb has undergone mylonitization, especially in the area of the earlier nappe-stage thrust fault.
Further compression resulted in the late NE-trending folds such as
those southeast of the mountain.
The timing of the sillimanite lineations remains a problem.
Throughout most of the eastern and northern portions of the quadrangle, fine sillimanite and mica lineations plunge down-dip toward
the northwest, or, in cases where surfaces dip east, toward the southeast (Plate 3). Whether or not they are of the same age as the backfolds is a question we will discu~s during the discussion of the dome
stage.
PHASE FIVE:
DOMING
Introduction
The Keene dome is one of the many gneiss domes along the Bronson
Hill anticlinorium, which, due to their relatively low density, rose
gravitationally late in the Acadian orogeny (Thompson et al., 1968).
These authors describe the Keene dome as having mushroom-shaped
margins, with a northeast lobe overturned northward, and a highly
compressed conical shape at the southern end. The south end is
superimposed on an earlier recumbent anticline and syncline, overturned toward the southeast, which do not appear at the surface in the
Monadnock quadrangle. Foliation and bedding next to the east edge of
the Keene dome dip moderately to the east (Figure 12, Subareas 4, 5
and 6; Plate 2). Farther east dips steepen to vertical and beyond
(Subareas 10, 11, 12). This is interpreted as an effect of doming
(Sections D-D' and E-E', Plate 5). Features believed to belong to the
dome stage are described below from various parts of the quadrangle,
followed by a general discussion.
There are abundant minor structures east of the Keene dome which
are obviously related to the dome. They are plotted on Figure 11 D.
Upright folds with an associated crenulation cleavage are abundant up
to four kilometers away from the dome. North of the latitude where
Rt. 12 crosses the edge of the dome, minor folds plunge gently north
(Figure 12, Subarea 4). At Rt. 12 they are nearly horizontal. To the
131
south, most dome-stage folds plunge south (Subarea 6). The rotation
sense nearest the dome ranges from east-over-west to more or less
neutral. This is apparent from the map pattern of contacts in the
area of Page Hill, north of Rt. 12 (Plate 1 NW). East of the Marlboro
syncline's axial trace, minor dome-stage folds have the opposite
rotation sense.
Marlboro Syncline
The domes of the Bronson Hill anticlinorium are characteristically
separated by tight synclines in the mantling metamorphic rocks
(Thompson et al., 1968). Although there is no well defined dome to
the east, the Marlboro syncline is believed to be a dome-stage feature
(Figure 10a). East of Marlboro it is well defined, where coincidentally its axial trace is roughly parallel to the zone in which
bedding and foliation are vertical. Subarea 7 on Figure 12
illustrates the axial region of the fold, which plunges northeast at
that point. To the south of the Marlboro pluton, the syncline follows
a belt of the upper part of the Rangeley Formation. The fold axis,
like those of the minor folds, presumably flattens out and eventually
plunges south. There are graded beds with tops facing west along its
east limb where it crosses Rt. 124. South of there, the syncline's
position is uncertain due to the current poor understanding of stratigraphy within the Rangeley. Close attention to the rotation sense of
minor folds should help locate the axial trace. More work is needed
to determine the relations of the Marlboro syncline to the Tully dome
in Massachusetts, if indeed it extends that far south. North of
Marlboro the syncline's trace is also uncertain. It strikes toward
the area of a high positive Bouguer gravity anomaly which is centered
on the Rangeley Formation west of the Cardigan pluton (Nielson et al.,
1976). Peter Robinson (pers. comm., 1984) has suggested that this-anomaly might be due to a deep synclinal septum of metamorphosed sedimentary rocks which would be more dense than the surrounding plutons
and domes.
! Rindge Area
Minor folds in the southeast corner of the Monadnock quadrangle
are similar in orientation and style to folds near the Keene dome.
They are upright, plunge gently north and south, and are associated
with a crenulation cleavage (Figure 12, Subarea 36). The crenulation
forms an intersection lineation on bedding and foliation, and sillimanite lineations are approximately parallel to this. This fabric is
similar to the dome-stage fabric that pervades central Massachusetts,
both in orientation and in amount of plunge (Robinson, 1979, Fig.
5-7).
Late Open Folds, Mt. Monadnock
Late symmetrical folds trending northwest are exposed in several
areas on Mt. Monadnock. They have steep axial planes which are paral-
132
lel to crenulation cleavage in schist beds. They range in wavelength
from 10 em to 10 m. Some outcrops, where minor late folds are not
obvious, have surfaces which are broadly warped about NW-trending
axes. A plot of structural data from these folds is shown in Figure
11 C. The great spread in fold axis orientations is due to the fact
that the surfaces which are folded had a wide range of previous
orientations. Along Pumpelly Ridge the folds plunge moderately to the
northwest, whereas west of Mt. Monadnock they are nearly horizontal.
The broad NNW-trending anticline centered on Monte Rosa is a large
fold of the same phase (Figures 10a, 13, and Plate 1 SE). These late
open folds may post-date the dome stage.
Cobb Hill
The contact between the Rangeley Formation and the Cardigan pluton
is folded by an open NW-trending fold across Cobb Hill north of Lake
Skatutakee (Figures 10a, 13, and Plate 1 NE). The Kinsman Granite
lies under the Rangeley south of Cobb Hill, and relationships are
unclear to the north. The fold deforms an older S-shape in the
contact, which may be a backfold, or may have been an irregularity in
the original intrusive contact.
Discussion
The problem of correlating dome-stage folds from one part of the
quadrangle to another is similar to the problem encountered with the
backfold stage, and is tied to the important question of timing and
significance of the mineral lineations. Plate 3 shows how the lineation pattern changes from a dominantly northwest trend in the central
and northern parts of the quadrangle, to a dominantly south trend in
the southern part. Figure 28 shows equal area plots of mica and
sillimanite lineations from these two large domains.
t
Two interrelated questions need to be answered: 1) Did the sillimanite and mica lineations form during backfolding, or doming, or
both? 2) Do the NW-plunging lineations and the S-plunging lineations
date from the same stage of deforma~ion?
We know from the Gilson Pond area that sillimanite and mica lineations lie parallel to the axes of two different sets of folds, one
plunging northwest and the other northeast. Perhaps a more diligent
search in the areas with nearly coaxial backfolds and dome-stage folds
would reveal cases where two nearly parallel sillimanite lineations
occur together in the same outcrop. So far this has not been
observed, at least not on the same surface. The significance of two
slightly different lineation orientations in the same outcrop may not
have been recognized in the field. Subareas 10 and 11 (Figure 12) are
in the critical region where lineations seem to fan from northwest to
south. There is quite a mixture of orientations in the area immediately south and southeast of the Marlboro pluton. However, as yet no
outcrops have been found with intersecting sillimanite lineations,
N
N
..... .
..
..
..
..
...
. ·. i. .
•
:. ·.,·j,:::.
.. . .·.:....... . ..
: ·... '.·,:.:. '...··. :.
:· -··••.• ..
• .·:.:-a:
~.
.... ·? .·.•
~·.
.
••
. . ....
.·:..; .. ...:....'
·.J
•• • • • • • ••
. +
~
:
133
..:....
.... t'·:
:. . ...
.
.
.• :··· ..:..:i~ .:··· ..
....•..·...::;.:
.-~. :i,··••• •
..
..
..
..
..
Fig. 28. Sillimanite and mica lineations from two domains:
left, 171 from the southern part of the quadrangle (Subareas
5, 6, 10, 11, 12, 25, 35 and 36); right, 208 from the central
and northern part (Subareas 1-4, 7-9, 13-24 and 26-34).
although in a few cases adjacent outcrops have mineral lineations and
crenulation lineations at an angle to each other.
One interpretation would be that all the NW-plunging lineations
formed during backfolding, and all ~he S-plunging lineations formed
during doming, or vice versa. Alternatively, they all may have formed
during one phase, and the pattern on Plate 3 may represent a swirl in
the direction of transport. A third possibility is that the lineations record a swirl in transport direction that was operative during
both backfolding and doming.
!
To the southwest in Massachusetts, east of the Warwick dome, the
map pattern of dome-stage lineations and fold axes forms a distinct
swirl (Robinson, 1979). The trace of the swirl trends approximately
N-S. West of this trace the lineations plunge north, across the trace
they plunge due east, and to the east they swing around to the south.
This S-plunging domain is continuous with the S-plunging domain in the
Monadnock quadrangle.
The swirl of lineations in the Monadnock quadrangle, if that is
what it truly represents, may in part record the transport direction
of the Spaulding Hill pluton in the core of the Beech Hill anticline,
as it moved north and then up toward the southeast. The Gap Mountain
and Cummings Pond plutons (Figure 13), which may be the north end of
the Hardwick pluton, may also have moved longitudinally. The long
axis of the Tully body, which is a zeppelin-shaped body of Monson
Gneiss plunging south (Pike, 1968), projects toward the southwest
corner of the quadrangle, west of the Monadnock syncline. The Tully
body and the Spaulding Hill pluton apparently lie in the same axial
surface. The late backfolding and doming may not have been too remote
from each other in time (Robinson, 1967), and I envision a scenario in
which plutons moved longitudinally in response to E-W compression just
as the domes began to rise. We will return to this idea in our
134
discussion of the regional context.
LATE PALEOZOIC INTRUSIONS AND DEFORMATION
The Fitzwilliam Granite plutons and the microdiorite dikes cut
across all the structures described above. As discussed in the
section on intrusive rocks, the Fitzwilliam is probably Mississippian.
Fowler-Billings (1949a, p.1269) described the granite as "slightly
foliated in many exposures". Some of this foliation may be igneous
flow texture, but the more strongly developed foliation parallel to
the Thorndike Pond fault zone suggests some late Paleozoic deformation
along this zone. The microdiorite dikes are also weakly foliated.
The possibility of late Paleozoic metamorphism is discussed in the
metamorphism section.
MESOZOIC FAULTING
Silicified zones, a breccia zone, and minor faults with slickensides are evidence for extensional faulting. They are probably of the
same age as the Connecticut Valley border fault, which passes about
four kilometers west of the Monadnock quadrangle (Moore, 1949;
Thompson et al., 1968), and similar zones in adjacent quadrangles
(Robinson-,-1963; Fitzgerald, 1960; Pike, 1968; Greene, 1970; Peterson,
1984; E. Duke, 1984).
t
Silicified zones occur at five localities in the quadrangle, shown
on Plates 1 and 4 in black. Three are in the southern part of the
quadrangle, on strike with the Thorndike Pond fault zone. FowlerBillings (1949a) described two of these, as well as two more en
echelon to the south in the Winchendon quadrangle. The zones-consist
of very fine-grained pink to gray silicified rock, cut by a network of
quartz veinlets. Because they are more resistant to weathering, they
locally form sharp linear ridges. The silicified zone northwest of
Pearly Lake strikes N30°E. Kinsman Granite crops out 150 m to the
west, and strongly foliated Fitzwilliam Granite 250 m to the east. A
newly found silicified locality lies west of the same belt of Kinsman.
The other silicified zones lie within the Fitzwilliam pluton. Poles
to 10 quartz veins measured in an outcrop 200 m east of Rt. 12, 250 m
north of the quadrangle boundary (FZ-59), suggest a minimum compression direction oriented about N81°E, 40° (Figure 29). The foliation
in the granite dips steeply west. More work is needed to see if this
foliation is most strongly developed near the silicified zones. If
so, it may record late Paleozoic deformation, suggesting a long
history of movement along this zone.
In roadcuts along Rt. 202 south of West Rindge, there are some
vuggy quartz veins and small faults. These roadcuts are about halfway
between the silicified zone at FZ-59, described above, and the
inferred extension of the Spofford Gap fault to the east (E. Duke,
1984). The poles to some of the faults and the directions of the
slickenlines are included in Figure 12, Subarea 36. Steps on the
135
N
Fig. 29. Equal area diagram
of ten quartz veins in silicified zone at station FZ-59.
Solid circles are poles to
veins. Dashed line is foliation in granite. Beta intersection probably represents
a common slip direction during
Mesozoic extension.
136
slickensided surfaces indicate normal movement in all cases, regardless of direction of dip, with a very minor right-lateral component.
A well developed network of quartz veins also cuts rocks of the
Littleton Formation at elevation 375m on an east-facing slope, 1.3 km
S30°W of Bonds Corner in Dublin (Plate 1 NE). The dominant set of
veins strikes N31°E and dips 73°SE. The schists are strongly altered,
green, chloritic rocks, but the alteration probably predates the
quartz veins, as it is very widespread in this part of the quadrangle.
Kinsman Granite crops out down the slope 100 m to the east. Although
the country rock is not silicified to the same extent as in the zones
on strike to the south, this occurrence is believed to be related to
the same Mesozoic fault system.
A fifth, spatially unrelated, silicified fault breccia occurs in
the Littleton Formation west of the Spaulding Hill pluton (MB-40).
The silicified zone does not crop out, but float can be found for
about 20 m along a N25°E trend between two big NW-facing ledges of
schist, south of the old railroad bed 75 m east from where it crosses
Minnewawa Brook. The zone is about three meters wide.
SUMMARY AND
REGIONAL STRUCTURAL IMPLICATIONS
In summary, rocks of the Merrimack trough were deformed by foldnappes and thrust-nappes directed toward the west, followed by backfolds and doming. The Brennan Hill fault transported rocks of the
"Monadnock sequence" over the thinner, autochthonous sequence of the
Bronson Hill anticlinorium. The thrust is interpreted here as a
ductile thrust near the root zone of the Bernardston nappe. The Chesham Pond thrust carried the Kinsman Granite and Rangeley Formation
westward over the Monadnock sequence, cutting across the nappe-stage
Monadnock syncline between the Fall Mountain nappe and lower nappes.
A major backfold, the Beech Hill anticline, deformed the Chesham Pond
thrust, and it is in the core of this anticline that the nappe-stage
syncline is exposed.
A similar interpretation can be extended northward. The "Kearsarge-Central Maine synclinorium" (Lyons et al., 1982), along which
the New Hampshire sequence is exposed beneath a sheet of Kinsman
Granite, is proably the same nappe-stage syncline exposed because of a
younger backfold anticline. According to this model, the Fall
Mountain nappe and the Kinsman Granite must be rooted east of the
Kearsarge-Central Maine synclinorium. Unfortunately, the latter name
is confusing. It refers to one of several stratigraphic synclines,
which are nappe-stage features with Littleton Formation in their
centers. I propose calling it simply the Kearsarge syncline, with the
understanding that it is a nappe-stage syncline equivalent to the
Monadnock syncline, and that it may extend as far north as central
Maine. The younger structural anticline should be given a different
137
name, perhaps using the name Beech Hill anticline from the Monadnock
area. The Littleton Formation at Mt.Wachusett, Massachusetts (Tucker
and Robinson, 1976-1977), in the Peterborough quadrangle (E. Duke,
1984) and in the Alton-Berwick area (Eusden et al., 1984) apparently
belong to the next higher nappe-stage syncline, above the Fall
Mountain level.
As mapping has proceeded during the past twenty years some discussion has centered around the exact lGcation of the axial surface of
the Merrimack synclinorium. Because the rocks of the Merrimack trough
were involved in nappes which were later crumpled by backfolds, there
is no clearly defined structural synclinorium between the Bronson Hill
anticlinorium and the Massabesic terrane (Lyons et al., 1982).
However, the term "Merrimack synclinorium" can still be applied to the
broad, complexly folded area of Silurian and Devonian rocks.
The Monadnock sequence does not appear at Fall Mountain because
the thrust fault under the Fall Mountain nappe has cut out, or
beheaded, the Monadnock syncline. One place where these rocks may
reappear is at Gee Mill, where rocks very similar to those at
Monadnock are exposed in an anticline flanked on either side by
Kinsman Granite and Bethlehem Gneiss (Chamberlain, 1984). Chamberlain
has mapped a continuation of the Ch~sham Pond fault into the Lovewell
Mountain quadrangle (Chamberlain, in progress), but the location of
the Brennan Hill fault north of the Monadnock quadrangle is less
certain. Another place where the Chesham Pond fault may be exposed
lies west of the Connecticut Valley border fault. There, the Ashuelot
pluton of Kinsman Granite is exposed in the hanging wall of the Mesozoic fault, in the Fall Mountain nappe level (Thompson et al., 1968).
Inverted units of the Monadnock sequence appear locally along the west
edge of the pluton and it appears that a thrust fault may separate
them from the rocks of the Bernardston nappe below (David Elbert,
pers. comm., 1984).
~
The Brennan Hill fault probably extends south into the area west
of the Tully body in Massachusetts, and more work in that area may
improve our understanding of the thrust-nappe root zone. The
Thorndike Pond fault zone probably also extends south into Massachusetts, but because of backfold-stage mylonitization such as that at
Brooks Village (Morton, 1984), the thrust-nappe root zone will be
harder to locate.
138
"Can thy style-discerning eye
The hidden-working builder spy,
Who builds, yet makes no chips, no din,
With hammer soft as snowflake's flight;.
"
-Ralph Waldo Emerson, 1847, from the poem "Monadnoc"
METAMORPHISM
INTRODUCTION
Regional metamorphism has affected all the layered rocks, as well
as some of the intrusive rocks, in the Monadnock quadrangle. The
quadrangle lies between two of the highest grade Acadian metamorphic
areas in New England, the central Massachusetts metamorphic high
(Tracy et al., 1976b; Robinson et al., 1982b; Zen et al., 1983), and a
smaller-area in south central New Hampshire (Chamberlain and Lyons,
1983). The peak of metamorphism produced predominantly "upper
sillimanite" zone mineral assemblages in the pelitic schists: Zones
III (sil-mus-gar-biot) and IV (sil-mus-gar-biot-ksp) of Tracy, 1975.
(Mineral abbreviations are explained in Appendix 1, Table 16.) Zone
II (sil-mus-st-gar-biot), Zone V (sil-gar-biot-ksp), and Zone VI
(sil-gar-biot-ksp-crd) assemblage~ are locally present, and extensive
retrograde processes have affected the rocks in much of the
quadrangle. Calc-silicate rocks contain diopside and grossular
garnet, indicating metamorphic conditions consistent with
sillimanite-bearing pelitic assemblages (Thompson and Norton, 1968).
Amphibolite facies assemblages in the Ammonoosuc Volcanics (FowlerBillings, 1949a) are also consistent. I have done no further work on
the mafic rocks.
PELITIC ROCKS
Aluminum Silicate Polymorphs
Thompson and Norton (1968) and Chamberlain and Lyons (1983) showed
the trace of a fossil isobaric surface crossing southwestern New Hampshire along the east edge of the Monadnock quadrangle. This line
marks the approximate former position of the triple point between the
three aluminum silicate polymorphs, kyanite, andalusite, and sillimanite. Kyanite was transformed to sillimanite west of the line, and
andalusite was replaced by sillimanite to the east. However, sillimanite polymorphs after andalusite ("andalumps", Robinson, in Hatch et
al., 1983) are abundant in much of the quadrangle (Figure 30).
Therefore the triple point trace must lie farther west than the
position shown by previous authors. Neither relict andalusite nor
relict kyanite has been found in the Monadnock quadrangle. Edward
Duke (1984) reported several samples from the Peterborough quadrangle
which contain both andalusite and sillimanite. Kyanite, locally with
fibrolitic overgrowths, has been found in the Ammonoosuc Volcanics at
the south end of the Keene dome (Robinson, 1963; Robinson et al.,
1982b). Sillimanite pseudomorphs after kyanite are apparently a rare
139
KEY
Roman numerals-Zones,
explained in text.
( ) strongly retrograded rocks.
* chloritoid
• retr. staur.
• Zone III
cordieritebearing assemblage.
m
m
(N)
I'il
:~--,
r•
, I'il''
'
,'
f"
'
, .....
Til\\I
N
I
1
1
:
,I
-~Derby Hill
I
I
lll
window
_......
UYJ
__
/
J
,......"'ii(
,~I/........
I,~
'-
\
\'21.
--~
-.....;
~
\
(
,..-11
0-
m
Oil)
om*
~
(IDJ*
( Jl \
\.../
m
\
m
I
"I(
~1,.;.oJ'
\
(ID)
li\
"\
'\
(ID)
n\
I
\
I
n I
*
m
m
m•*
i·" "'*
ill
I
m
I
I
...
I
I
I
I
z
f'
TIJ
0'
I
~
m
II
II
II
II,,
I
I
I
1
N
I
II
I
m
m
I
1/
2mi .
0
2
3km
II
Fig. 30. Metamorphic zones based on assemblages observed in thin section, or staurolite in hand sample (Zone II). Includes data from
Chamberlain (1981). Plutons outlined for reference.
140
phenomenon (Tracy and Robinson, 1980). Thus the trace of the fossil
isobar can only be located approximately as a line along the west edge
of the area that contains andalumps. My approximation is shown as a
heavy double line in Figure 30. Although there are nubbles of
fibrolite up to about one centimeter long west of t nis line, they are
smaller than the andalumps, and lack the microscopic mosaic texture of
subparallel sillimanite prisms described by Rosenfeld (1969) in a
sample from Gap Mountain.
Chamberlain and Lyons (1983) recognized a three-stage sequence of
metamorphism in southwestern New Hampshire: (1) early production of
andalusite-bearing assemblages, (2) peak metamorphism producing sillimanite-bearing assemblages of Zones III through VI, and (3) local
retrogression. Tracy and Robinson (1980) and Robinson et al. (1982b)
proposed P-T trajectories to show the different paths taken through
time by rocks in the Bronson Hill anticlinorium and in the Merrimack
synclinorium in central Massachusetts (Figure 31). During the
progress of metamorphism all the rocks experienced increases in
pressure and temperature, but those in the Merrimack synclinorium
followed a path on the low pressure side of the aluminum silicate
triple point while rocks farther west stayed on the high pressure
side. Stages (1) and (2) of Chamberlain and Lyons thus reflect the
trajectory from the andalusite field into the sillimanite field. The
model which Tracy and Robinson proposed to explain the different
trajectories, the trajectory proposed for the Monadnock area, and the
timing relative to deformational history, are discussed in later
sections of this chapter.
Metamorphic Zones
Mineral assemblages representing Zones II, III, and IV of Tracy
(1975) are the most important in the Monadnock quadrangle. They
reflect a general increase in metamorphic grade from west to east
(Figure 30).
Zone II. The prograde Zone II assemblage, sillimanite-muscovitestaurolite-garnet-biotite-quartz, is present mainly in the belt of
Littleton Formation east of the Keene dome (for example, Table 6a,
SZ-27). Staurolite grains attain lengths of 5 mm, and show typical
seive textures with abundant inclusions. Sillimanite is present as
tiny prisms included within quartz, muscovite, and garnet, or as
masses of fibrolite. This assemblage can be portrayed in a projection
of mineral phases from muscovite (Thompson, 1957) onto the AFM plane
of the AKFM four component tetrahedron (Figures 32a and 32h).
Staurolite also occurs as tiny prisms in several samples from elsewhere in the quadrangle, but these are apparently of retrograde origin
and are discussed later.
The sillimanite-staurolite-biotite field (Figure 32a) would shift
progressively toward more Fe-rich compositions as the prograde
reaction,
141
6
p kb
3
I
I
I
I
I
I
500
600
T
...
700
°C
Fig. 31. P-T trajectory for rocks southeast of
Mt. Monadnock, compared to trajectories for Bronson
Hill anticlinorium and Merrimack synclinorium rocks
in central Hassachusetts (after Robinson et al.,
1982b). Aluminum silicate triple point from Holdmvay
(1971). Staurolite-out reaction from Dutrow and
Holdaway (1983).
142
A
Zone II
+mus
+mus
• qtz
. . ·.· ..
Zone
.
m
+ qtz
. . ·.: ~ .
ksp
Fig. 32a. Muscovite projection,
showing Zone II assemblages, using
data from Hall, 1970, and Tracy et
al., 1976b, sample 36Y.
Zone
m
Fig. 32b. Muscovite projection,
showing Zone III assemblages,
using data from MK-432 (Table 12).
+Sil
+ qtz
+mus
+ qtz
Zone ISr
gar
F ~--""g'-o-r-ne"'"t'--------c~r~d---...--J>M .::·:.;.:,·.<·:.::··::·-::::·:-:·:-:-:=.::.:: bio·t: ::>:: :,:'::-:·."·:·::.:::.:.:;:·:':=:·::.-: :_: .
. . . . . .
. . · ,L. ·· . . . . . . . . . . ... .
ksp
Fig. 32c. Sillimanite projection,
showing Zone III assemblages from
both MK-432 and MK-629 (Table 12).
Fig. 32d. Huscovite projection,
showing Zone IV assemblages,
using data from Tracy et al.,
1976b, sample 871.
Fig. 32. Chemographic representation of mineral assemblages in Zones
II-IV, based on microprobe data from central Massachusetts (32a, d-g)
and from this paper (32b and c). AKFM tetrahedron from which the
ternary diagrams are constructed is shown in 32h.
143
Zone
nz
tkSp
Zone ll:
+Qtz
Fig. 32f. K-feldspar projection,
showing Zone V assemblages, using
data from Tracy et al., 1976b,
sample 067D. ·
Fig. 32e. K-feldspar projection,
showing same assemblages as in 32d
(Tracy et al., 1976b, 871).
Zonelll
+ ksp
+ qtz
tksp
AKFM
+ qtz
tetrahedron
M
M
Fig. 32g. K-feldspar projection,
showing Zone VI assemblages, using
data from Tracy et al., 1976b.
sample FW154.
Fig. 32 (continued)
F
Fig. 32h. AKFM tetrahedron from
which projections in 32a-g were
constructed. A = Al o , K = (K,Na)
2 3
Al0 , F = FeO + MnO, M = MgO.
2
Quartz is in excess and H 0 is
2
perfectly mobile.
144
[st + mus + qtz = Fe-richer st + biot + sil + H o]
2
proceeded.
(1)
Staurolite would finally be consumed by the reaction:
[st + mus + qtz
= biot
+ sil +gar+ H o].
2
(2)
The apparent coincidence of the staurolite-out isograd and the Littleton-Rangeley contact east of the Keene dome may not accurately reflect
a change in metamorphic conditions, but may be due to the absence of
bulk compositions appropriate for the formation of staurolite in the
Rangeley Formation. If the isograd does indeed follow the contact,
which I have interpreted as a nappe-stage fault, then the fault may
have juxtaposed rocks of different grade after the assemblages had
reached equilibrium. However, since there are no metamorphic grades
missing, the assemblages may have resulted after the faulting had set
up the necessary thermodynamic gradients.
The staurolite-out reactions are temperature dependent, and lie on
the high-temperature side of the aluminum silicate triple point. Thus
in rocks at pressures near the triple point, sillimanite would be the
polymorph involved in staurolite-destroying reactions (Figure 31). At
higher pressures, kyanite would be the stable polymorph. Kyanite and
andalusite had presumably been replaced by the time staurolite was
consumed in most of the Monadnock quadrangle.
Zone III. The Zone III assemblage, sillimanite-muscovite-garnetbiotite-quartz ~ plagioclase, is the most widespread in the quadrangle, and occurs in most of the pelitic schists in the Rangeley,
Perry Mountain, and Littleton Formations (Figure 32b). In rocks where
retrogression has destroyed most of the sillimanite (e.g. MK-216 and
TR-20 in Table 6a), inclusions in quartz and primary muscovite attest
to its former presence. Both fibrolitic and prismatic sillimanite are
present, with the prismatic proportion apparently increasing toward
the northeast.
Two samples of schist were selected for detailed analyses of
minerals using the electron microprobe on polished thin sections. One
of these, MK-432, is a typical gray-weathering schist of the Littleton
Formation with a Zone III mineral assemblage of muscovite, biotite,
quartz, garnet, sillimanite and plagioclase, and accessory graphite,
ilmenite, apatite, and zircon. This is shown in muscovite projection
in Figure 32b, and as assemblage (1) in sillimanite projection in
Figure 32c, using mineral compositions from the microprobe analyses.
Assemblage (2) in Figure 32c represents MK-629, a cordierite-bearing
schist from an outcrop approximately 120 m from MK-432 (locations
shown in Figure 24), in the upper part of the Warner Formation.
MK-629 consists of quartz, biotite, plagioclase (An 55 ), garnet,
cordierite, and sillimanite, with accessory graphite, ilmenite,
apatite and zircon. It contains neither muscovite nor K-feldspar, and
thus occupies a different chemical space in Figure 32c than MK-432,
but with tie line arrangements consistent with an identical P-T
145
history. Garnet zoning and estimates of metamorphic conditions from
these two rocks are discussed in a later section. The outlines of
garnets in both tend to be ragged, suggesting that they were involved
in garnet-consuming reactions at some point. Garnet would be consumed
by one such reaction in the more typical schist:
[gar + mus
= sil
+ biot + qtz].
(3)
A rise in temperature alone, a drop in pressure, or a rise in temperature with very gradual increase in pressure, would tend to drive this
reaction to the right. It could proceed to the left only so long as
pressure increased at a sufficiently high rate. Garnet may have been
consumed in the less typical schist by the pressure-sensitive
reaction:
[gar+ sil + qtz = crd].
(4)
Some sillimanite might also have been produced in Zone III as muscovite was depleted in paragonite content according to the reaction:
[Na-bearing mus + ab + qtz =
K-richer mus + sil + K-richer ab +H o].
2
Zone IV.
The appearance of
[mus + ab + qtz
K-f~ldspar
= ksp
(5)
by the reaction
+ sil + H o]
2
(6)
marks the orthoclase or "second sillimanite" isograd, forming the
assemblage: sillimanite-muscovite-garnet-biotite-K-feldsparplagioclase (Figures 32d and 32e). Some Zone IV assemblages were
found in rocks north of the Chesham Pond fault, and from inside the
Derby Hill window. Textures suggest that the rocks attained conditions favorable for Zone IV assemblages relatively early in the
metamorphic history. There are roundish zones of quartz and feldspar
surrounded by confused masses of biotite, sillimanite, retrograde
Mg-chlorite, and muscovite, some of which is almost certainly
retrograde. This matrix lacks any strong foliation. Garnets in many
cases are surrounded by biotite and sillimanite. There are also
roundish masses of biotite and sillimanite which appear to have
replaced some mineral, perhaps garnet or cordierite. In the thin
sections which contain orthoclase there are commonly small myrmekitic
intergrowths of plagioclase and quartz adjacent to the K-feldspar.
Biotite and quartz also occur locally in myrmekitic intergrowths. The
massive, gneissic texture may have developed while the retrograde
reactions were in progress, erasing any previous strong foliation that
may have been present. At Otter Brook Dam (KN-2, Figure 30), pristine
orthoclase augen cut across a pervasive foliation. Heald (1950)
reported small inclusions of gneiss inside feldspars, with foliation
parallel to that in the matrix.
Many of the gray schists in the Rangeley Formation north of the
Chesham Pond fault contain augen averaging four centimeters in
146
diameter. In thin section these are seen to be dominantly quartz and
muscovite (Table 1d, RX-2A). They may represent orthoclase porphyroblasts which were retrograded by reaction (6) running toward the left.
Heald (1950, p.75) reported a large area west of the Cardigan pluton
in which "all stages in the transition from orthoclase to aggregates
of muscovite and quartz" can be observed. He showed on his map that
secondary muscovite occurs throughout most of this area, except for a
fringe in the west near the orthoclase "isograd", where orthoclase
occurs with primary muscovite only. More field work and thin section
work are needed in the Monadnock quadrangle to define the sillimanite-orthoclase isograd. Identifying the first traces of K-feldspar
in pelitic schists near the isograd is commonly difficult.
The orthoclase augen at Otter Brook Dam, which lies west of the
Chesham Pond fault, indicate the isograd does not exactly follow the
fault. A comparison of Figures 13 and 30 shows that farther east it
is approximately parallel to the fault. Zone IV assemblages were not
found in the Monadnock quadrangle in the supposed level (3) area east
of the Thorndike Pond fault zone, but farther east, an isolated
locality was reported by Chamberlain and Lyons (1983) in the southwestern part of the Peterborough quadrangle (Figure 30).
It is important to distingui~h augen that were once K-feldspar
porphyroblasts from quartz-feldspar segregations (e.g. FZ-30, Table
1d), which are distributed much more widely, and probably represent
local melt pockets. Tracy (1978) interpreted partial melting and
muscovite dehydration as nearly simultaneous processes in central
Massachusetts. The fact that quartz-feldspar segregations seem to be
more widespread than augen after K-feldspar, occurring in level (2) as
well as in level (3), suggests that local melting may have occurred
earlier than the muscovite dehydration reaction (6). At least some of
the quartz-feldspar segregations are deformed by what I have
interpreted to be the earliest backfolds on Mt. Monadnock.
Some garnets in the area east of the Thorndike Pond fault zone
(e.g. in MK-1061A, Table 1d) have inclusion-rich cores surrounded by
clear rims. The rims may represent prograde overgrowths formed
perhaps during the prograde reaction,
[biot
+
sil
+
qtz
= ksp
+
gar
+
H o],
2
(7)
around garnets that had been partially resorbed in earlier,
lower-grade reactions. It would be interesting to probe some of these
garnets and compare zoning patterns to those in Zone III garnets.
Zone V. The final destruction of muscovite results in the typical
Zone V assemblage, sillimanite-garnet-biotite-K-feldspar (Figure 32f).
Because of the problems in differentiating prograde muscovite from
secondary muscovite, I am certain of only one Zone V assemblage in the
thin sections studied. This occurs in a rather peculiar coticulebearing schist from Hurricane Hill in the Perry Mountain Formation
147
(DB-289A, Table 3).
Near the K-feldspar there are some small patches of green-brown
biotite and sillimanite, which may be evidence for reaction (7) in
reverse. There are also clumps of red-brown biotite in 5 mm patches,
and garnets embayed by biotite and sillimanite. Some of these patches
might have replaced cordierite.
Zone VI. Cordierite may appear in normal pelitic schists through
the reaction:
[biot + sil + qtz = gar + crd + ksp +H o].
(8)
2
There are several occurrences of the Zone VI assemblage, sillimanitegarnet-biotite-K-feldspar-cordierite (Figure 32g) in the Monadnock
quadrangle. Two of these are Fowler-Billings' samples K-116A and
K-117 (Table 1d), both from Cobb Hill, where the western contact of
the Cardigan pluton jogs to the east and back again. The cordierite
has yellow alteration along cracks, yellow pleochroic haloes around
zircon, and is surrounded by nests of sillimanite prisms and finegrained red-brown biotite. The K-feldspar has a perthitic texture.
Both these samples lack muscovite. K-102, also from Cobb Hill, was
collected from the same sillimanite-rich unit as my sample HV-41, and
although it, too, contains retrograde muscovite and chlorite, it
contains K-feldspar and cordierite as well. K-106, from along the
contact to the southwest, is similar to K-102. Sample DB-130 may
contain some cordierite, but there is no yellow alteration or haloes
to distinguish it from plagioclase. The presence of cordierite mainly
along the Kinsman contact suggests a localized contact metamorphic
effect. This does not necessarily contradict Chamberlain and Lyons'
(1983) conclusion that the pattern of peak regional metamorphism cuts
across the Cardigan pluton.
Assemblages in Sulfidic Schists
t
Mineral assemblages in the sulfidic schists of the Rangeley
Formation are similar to those in the gray-weathering rocks, with the
addition of pyrrhotite (Table 1a). The sulfidic mica schists of the
Francestown, however, are distinctive in that they are so sulfidic
that most of the iron is taken up by the sulfides (Table 4). As a
consequence, garnet is absent and biotite is a pale brown to white
Mg-rich variety. No chemical study was made for Francestown rocks in
the Monadnock quadrangle, but Field (1975) reported pure end-member
Mg-cordierite, and biotite with only .04% FeO, from correlative rocks
in Zone VI in central Massachusetts. Robinson et al. (1982b)
discussed the effect of sulfides on pelitic schist-assemblages in
detail, with particular attention to the role of volatile components.
In bulk compositions rich in sulfur, rutile is the stable Ti-bearing
phase rather than ilmenite. Biotite in equilibrium with graphite and
pyrite becomes more and more Mg-rich as the following reaction
proceeds:
148
[py + biot + gr
= po
+ Mg-richer biot + sil + ksp +H o+ co ].
2
2
(9)
The Francestown schists from the Monadnock quadrangle contain rutile,
pyrrhotite, and a non-magnetic sulfide which is probably secondary
marcasite derived from pyrrhotite (Steven Haggerty, pers. comm.,
1982). Probably none contains the pyrite that has been reported from
sulfidic rocks of extreme composition at several localities in central
Massachusetts (Robinson et al., 1982b).
Evidence for a Retrograde Episode
Retrograde metamorphic assemblages in the Monadnock quadrangle are
of two sorts. The first sort is apparent in nearly all areas, and it
involves the partial or total replacement of sillimanite by muscovite,
and of K-feldspar by muscovite and quartz. The second sort occurs
mainly in the area northeast of Mt. Monadnock, and represents more
advanced, lower-temperature re-hydration. In the latter, Fe-chlorite
has partially replaced garnet, and secondary muscovite and chlorite
are major components of the rock. Biotite is absent in many of these
rocks (Table 1a, DB-165; Table 6, MK-216, DB-10, DB-69, DB-171,
MK-210, and DB-17). Hollocher (1981) studied the detailed chemistry
and textural relations of retrograde minerals in the Littleton
Formation near New Salem, Massachusetts. Hollocher's methods could be
applied to explain some of the apparently complex retrograde
assemblages of the Monadnock area. Only the most salient features and
problems will be discussed here.
As mentioned earlier, the coarse gneissic textures of the Zone IV
rocks north of Chesham Pond fault seem to be at least in part due to
widespread retrograde metamorphism. It would be interesting to study
the quartz-muscovite augen in some detail to see if there is chemical
evidence to support the textural evidence cited by Heald (1950) for
replacement of K-feldspar porphyroblasts. A possible reaction might
be
[ksp + sil + H2o = mus + qtz],
(10)
but the role of Na in this would have to be explored.
that plagioclase is involved in some cases.
Heald noted
Sillimanite is partially to completely replaced in much of the
quadrangle, mainly by muscovite. Hollocher (1981, p.178-179) proposed
the following reaction, balanced according to his microprobe analyses,
149
for the destruction of sillimanite:
[45 biot + 51 sil + 9 qtz + 86 H o
2
= 41
mus + 22 chl + 4 ilm].
(11)
The chlorite produced is much more Mg-rich relative to the remaining
biotite. Although no microprobe work was done on Monadnock retrograde
rocks, it was noted that the chlorite intergrown with biotite and
secondary muscovite has gray to brown interference colors, indicating
a Mg/Fe ratio greater than one. Chlorite associated with garnet, by
contrast, shows the anomalous blue interference colors characteristic
of Fe-rich chlorite. Some rocks contain both Fe- and Mg-chlorite,
implying disequilibrium, which is a different situation from that
reported by Hollocher (1981). He proposed three reactions for the
destruction of garnet, which would vary according to the garnet
zoning. These reactions are essentially the simple hydration
reaction,
[gt +H o = chl + qtz],
(12)
2
with more or less ilmenite and sphene involved depending on the
calcium content of the garnet, and minor amounts of biotite and
muscovite. These low-temperature garnet-consuming reactions involved
extensive re-hydration, and took place only locally in the Monadnock
area, in contrast to the more subtle and higher-temperature retrograde
ion exchange reactions, which may have resulted in changes in garnet
rim compositions. These are discussed in a later section.
Many of the rocks on Mt. Monadnock and the area to the northeast
contain chloritoid in addition to Fe-chlorite (Figure 30). The
chloritoid appears as dark green clots in hand sample, which commonly
weather out to form roundish pits. Many of the chloritoid-bearing
rocks also contain unaltered garnet and tiny (<0.5 mm) staurolites. A
thin section from sample MK-210 (Table 6b, MK-210A), displays an
andalump that contains relict patches of sillimanite that all have the
same optic orientation, surrounded by an intergrowth of fibrolite and
muscovite. Staurolite and chloritoid cut across the other minerals.
Plagioclase and tourmaline are more abundant within the andalump than
in the matrix. The staurolite, chlorite, chloritoid, and muscovite
are probably all retrograde reaction products from the breakdown of
biotite and sillimanite.
Outcrops on Beech Hill contain large clumps of chloritoid. In
thin section (Table 6a, DB-10) grains up to 10 mm long have ragged
boundaries and appear to be in disequilibrium, while much smaller
grains cut across the older (nappe-stage?) fabric. There were
apparently two periods of chloritoid growth, the first perhaps by
replacement of a relatively Fe-rich cordierite and the second by a
reaction involving sheet silicates. Since there is no biotite left in
the rock, the second chloritoid-producing reaction might be similar to
ones proposed by Hollocher (1981) involving garnet, chlorite and
muscovite, such as:
150
[gar+ H2o
[mus + chl
= more
and [ctd +gar+ H2o
= chl
+ ctd],
(13)
phengitic mus + ctd],
= Mn-richer
ctd + chl].
(14)
(15)
The final destruction of biotite in severely retrograded rocks may
have resulted mainly in the production of chlorite, along with celadonite-richer muscovite and ilmenite (Hollocher, 1981):
[mus + biot + qtz +H 2o
= mus
+ chl + ilm].
(16)
Garnet Zoning in Zone I I I
Detailed microprobe analyses of garnets in thin sections MK-432,
MK-6 and MK-629 revealed interesting zoning patterns which, if they
can be correctly interpreted relative to reactions involving garnet,
should theoretically provide information about metamorphic conditions
based on Fe and Mg fractionation between biotite and garnet and
between cordierite and garnet. Chemical zoning in garnets is believed
to be due to garnet's slow ionic diffusion rate, and thus should
reflect changes in metamorphic conditions through time. Representative mineral analyses are presented in Table 12. The garnet
compositions were recalculated in terms of molecular proportions of
pyrope (Mg-Al garnet), almandine (Fe-Al garnet), spessartine (Mn-Al
garnet), and grossular (Ca-Al garnet). All the iron is assumed to be
FeO.
Assemblage (1). MK-432 is a typical pelitic schist with the Zone
assemblage mus-biot-qtz-gar-sil, with minor amounts of
plagioclase, graphite, ilmenite, and accessory minerals (Table 6a).
Two garnets were probed in a thin section from sample MK-432 (Figure
33). The thin section cuts approximately through the center of garnet
X, but only through a portion near the rim of garnet Y. The garnets
are zoned, and because of the orientation of the sections, garnet X
shows the full range of compositions from core to rim, whereas
variation in garnet Y is much more restricted. Although there are
only a few data points near the biotite-free rim of garnet X, the
central part of the cut through garnet Y is believed to also represent
a biotite-free rim. Pyrope increases toward the rim from about 14 to
about 17%, except where the garnet touches biotite, and there pyrope
decreases abruptly down to as little as 9%. Almandine is fairly
constant except toward rims touching biotite, where it rises from
about 76% up to as much as 79%. Spessartine follows a pattern inverse
to that of pyrope, decreasing toward the rim from 5 to 3%, except next
to biotite, where it increases abruptly to as much as 8%. Grossular
varies from 2.8 to 3-7% with a much less clear pattern, and so was
omitted from Figure 33. The lower values of grossular tend to be
toward the rim.
III
··~
c ____ ..._
. 0~~:-y.~> ) .
((
__ _
........
.........
'
...._
'•
.
•
15
,&
/
Almandine
Pyrope
Fe
Mg
0
I
Spessartine
Mn
2mm
Fig. 33. Contoured values of atomic percent of Mg, Fe, and Mn in garnets from MK-432. Note
that some contours are omitted near the rims for clarity of presentation. Ca varies from
2.8 to 3.7 with no clear pattern. Dots show microprobe analysis points. Par::tllel lines
indicate biotite.
"'""
Vl
"'""
152
Table 12. Electron microprobe analyses from minerals in Zone III
schists and coticule.
Garnet
Sample No.
Location
~
Analysis No. Bw
Si0 2
Al 2o3
MgO
FeO
MnO
CaO
Sum
36.16
21.77
3.67
34.89
2.43
1. 22
100. 14
tfK-423
core
Ik'
39.03
20.18
3.50
35.17
2.56
1. 21
101.65
rim next
rim next
to biotite to biotite
Bl
Bv
rim
Ag
rim
Aa
38.19
20.04
4.20
34.95
1. 34
1. 22
99.94
36.57
20.43
3.86
35.63
1.49
1.15
99.13
36.97
22.09
2.63
35.76
3.64
1. 13
102.22
36.20
22.55
2.20
34.96
3.59
1.23
100.73
Structural Formulae (based on 12 oxygens)
2.915
2.069
.441
2.354
.166
.106
8.051
3.084
1.880
.413
2.325
.172
.103
7. 977
3.059
1. 893
.502
2.343
.091
.105
7.993
2.979
1. 959
.470
2.430
.103
.100
8.041
2.935
2.068
.311
2.376
.245
.097
8.032
2.912
2.139
.264
2.353
.245
.106
8.019
% Pyrope
14.4
% Amandine
76.7
% Spessartine 5.4
3.5
% Grossular
100.0
13.7
77.2
5.7
3.4
100.0
16.5
77.0
3.0
3.5
100.0
15.2
78.3
3.3
3.2
100.0
10.3
78.4
8.1
3.2
100.0
8.9
79.3
8.2
3.6
100.0
Mg/ (Hg+Fe+Mn) . 149
.142
.171
.157
.106
.092
Hg/(Hg+Fe)
.151
.176
.162
.116
.101
Si
Al
Mg
Fe
Mn
Ca
Total
Total
t
.158
153
Table 12. (cont'd)
Garnet
Sample No.
Location
core
Analysis No. A-C
Si02
Al203
MgO
FeO
UnO
CaO
Sum
36.16
21.39
6.13
32.35
2.09
1.38
99.50
MK-629
rim
rim
near crd near biot
A-s
A-a
34.61
22.77
4.56
32.87
1. 98
1. 73
98.52
36.56
21.92
3.97
34.20
1.86
2.01
100.52
MK-6
core
A4-l
rim
A4-w
38.25
21.75
2.19
30.16
6.06
1.06
99.47
37.37
21.94
1. 73
32.67
5.35
1. 55
100.61
Structural Formulae (based on 12 oxygens)
Si
Al
Mg
Fe
Mn
Ca
Total
2.900
2.023
. 733
2.171
.142
.119
8.088
2.818
2.187
.554
2.240
.137
. 151
8.087
2.921
2.066
.473
2.287
.126
.173 .
8.046
3.065
2.056
.262
2.022
.411
.091
7.907
2.997
2.074
.206
2.192
.363
.133
7.965
% Pyrope
23.2
% Almandine 68.6
% Spessartine 4.5
% Grossular
3.7
Total
100.0
18.0
72.7
4.4
4.9
100.0
15.5
74.8
4.1
5.6
100.0
9.4
72.6
14.8
3.2
100.0
7.1
75.7
12.5
4.7
100.0
Mg/(Mg+Fe+Mn).241
.182
.164
.097
.075
Mg/(Mg+Fe)
.198
.171
.115
.086
...
.253
15Lf
Table 12. (cont'd)
Biotite
Sample No.
MK-432
matrix
Location
Analysis No. C-3a
Si0 2
Ti0 2
Al 2o3
cr 2o3
FeO
MnO
MgO
ZnO
Na 2o
K2o
Sum
H2o
!
33.41
2.34
21.03
.06
20.05
.08
9.46
.14
.05
9.22
95.84
4.16
100.00
inclusions
B-1a
B-2b
37.85
1.84
21.01
.08
15.38
.02
12.31
.11
.32
8.71
97.63
2.37
100.00
36.37
3.03
21.19
.12
15.40
.01
11.79
.07
.23
9.62
97.83
2.17
100.00
Structural Formulae (based on 11 oxygens)
A-site
.800
.890
K
.895
Na
.032
.008
.045
Sum
.922
.903
.845
IV- site
2.635
Si
2.542
2. 723
1.365
Al
1.277
1.458
4.000
Sum
4.000
4.000
VI-site
.446
Al
.429
.505
.934
Fe
1. 276
.926
Mn
.001
.005
.001
Mg
1. 274
1.320
1.073
Zn
.004
.006
.008
Cr
.003
.005
.007
Ti
.165
.134
.099
Sum
2.831
2.862
2.929
Total
7.831
7.707
7.753
Mg/(Mg+Fe) .457
.588
.577
near
garnet
NB-6
HK.-629
near
garnet matrix
A-b
A-f
35.95
2.14
21.59
.02
19.47
.05
10.03
.04
.15
9.29
98.73
1. 27
100.00
36.80
1.61
19.83
.13
16.88
.04
13.39
0
.17
7.81
96.66
3.34
100.00
37.13
2.02
19.36
.11
17.43
.06
12.81
0
.06
8.28
97.26
2.74
100.00
.865
.021
.886
.729
.024
.753
. 771
.008
.779
2.623
1.377
4.000
2.689
1.311
4.000
2.709
1.291
4.000
.481
1.189
.003
1.092
.002
.001
.117
2.885
7. 771
.398
1.032
.003
1.459
0
.008
.088
2.988
7.741
.375
1.064
.004
1.394
0
.007
.111
2.955
7.734
.479
.586
.567
155
Table 12. (cont'd)
MK-629
Sample No.
Cordierite
Mineral
Cordierite
near garnet
core
Location
A-e
Analysis No. A-f
Si0 2
Al 2o 3
Tio 2
MgO
FeO
MnO
CaO
Na20
K2o
Total
48.68
32.18
0
9.19
7.53
.07
0
.18
0
100.21
46.64
31.13
0
9.44
6.81
.08
0
.09
0
99.77
MK-629
Plagioclase
C-b
Si0 2
Al 2o 3
BaO
FeO
CaO
Na 2o
K2o
Total
Structural Formulae (18 oxygens)
Si
Al
Mg
Fe
Mn
Na
Total
Mg/ (Hg+Fe)
5.020
3.914
1. 414
.650
.006
.036
11.040
4.989
3.927
1.506
.609
.007
.018
11.056
.685
.712
54.79
29.59
.38
.02
10.17
4.63
0
99.58
(8 oxygens)
IV-site
Si
Al
Sum
A-site
Ba
Fe
Ca
Na
Sum
Total
2.469
1.573
4.042
.007
.001
.491
.405
.904
4.946
Ab.452An.548
t.:
156
I
Mn
MK-432
garnet
10
rim next
to biotite
Fe
30
20
10
Mg-+
Fig. 34a. Mn-Fe-Mg composition trend for garnet from MK-432. Black
circles from core to "normal" rim; open circles from rim adjacent to
biotite.
I
Mn
~L_----~~----~~----~------~~~~~----10
20
30
40
biotite
50
Mg-+
Fig. 34b. Compositions of zoned garnet and coexisting biotite in MK-432.
Arrow points from core toward rim. Biotite compositions have a small
composition range, with slightly more Mg next to garnet.
157
The composition trend from core to rim can be seen in Figure 34
which is a portion of an MnO-FeO-MgO ternary plot. Each point corresponds to a point on the contour map of the garnets in Figure 33. One
possible interpretation for this trend involves two stages of zoning.
The decrease in MnO and increase in MgO/(MgO + FeO) toward the rim may
have resulted from garnet growth during the prograde continuous
reaction,
[ st + biot + qtz = mus +gar+ H o ],
(17)
2
for which T Mg > T Fe > T Mn (Tracy et al. , 197 6b) • After staurolite was
consumed, garnet growth stopped and the garnet was locally resorbed
according to the reaction,
[ mus +gar
= biot
+ sil + qtz ],
(3)
for which T Mn > T Fe > T Mg. This reaction took place mainly where the
garnet was in contact with muscovite and biotite, resulting in the
zoning trend toward higher MnO and lower MgO/(MgO + FeO) especially
next to what is now biotite. Tiny sillimanite prisms are locally
present in the adjacent garnet rim. The localized reversed zoning
next to biotite resembles somewhat that observed in garnets in Zone VI
in Massachusetts which has been attributed to local retrograde cation
exchange (Richardson, 1975; Tracy et al., 1976b). However, the MnO
enrichment in MK-432 garnets is much more extreme than in those
garnets, and ion exchange alone could not produce such large MnO
enrichment.
Figure 34b shows a simplified version of the garnet zoning path.
The sharp hook in the path represents the transition from Zone II to
Zone III assemblage conditions. The ragged edges of many garnets in
Zone III rocks may be . added evidence for a stage of garnet resorption.
The different biotite compositions shown in Figure 34b may not be
significant; biotite grains in the matrix have an average of 45.7
MgO/(MgO + FeO) (range 44.7 to 47.0 in 14 points analysed), compared
to 46.7 (range 45.0 to 48.7 in 17 points) in grains adjacent to
garnet.
Assemblage (2). MK-629 is a K-poor schist from an outcrop 120m
from MK-432. It contains the assemblage quartz-biotite- plagioclase-garnet-cordierite-sillimanite, but no muscovite or K-feldspar.
Garnet, biotite, cordierite, and other minerals in a thin section from
MK-629 were analysed with the electron microprobe (Table 12). A
garnet was studied in detail, as well as adjacent cordierite and
biotite (Figure 35). The zoning in the garnet suggests that it was
once two grains which have coalesced. Pyrope decreases toward the rim
from 23% in the core to about 18% at contacts with cordierite and
15.5% at contacts with biotite. There is really only one small length
of biotite-free rim along the lower right side. Almandine increases
toward the rim from 68% to 73% and 75%. Spessartine decreases toward
the rim, but then locally increases slightly at the very rim next to
biotite. Grossular increases slightly from 3.2% to 5.7% toward the
'"'
Spessartine
Almandine
Pyrope
Fe
Mg
0
I
Mn
2mm
Fig. 35. Contours of atomic percent of Mg, Fe, and Mn in garnets from MK-629. Ca varies
from 4.0 to 5.7 with no clear pattern. Dots show microprobe analysis points. Parallel
lines indicate biotite.
f-'
I.J1
00
159
20
I
MK-6
Mn
garnet
10
MK-629 garnet
~ ..... ~~
rim~
10
core
FeL_------------~L--------------~2~o~------------~3o~---­
Mg---+
Fig. 36a. Mn-Fe-Mg composition trends for garnets from MK-629
(K-poor schist) and MK-6 (coticule). Black circles from cores;
open circles from rims.
1
Mn
10~--------~~--------~----------~----------~---------
MK-629
garnet
10
20
30
40
Mg~
50
biotite 60
cordierite
Fig. 36b. Compositions from zoned garnet and coexisting cordierite
and biotite in MK-629. Arrow points from core to rim. Biotite
adjacent to garnet, Mg/(Mg +Fe) = .581, is slightly more Mg-rich
than that in matrix, Mg/(Mg +Fe) = .565. Biotite next to cordierite
is slightly more Fe-rich, Mg/(Mg +Fe) = .535.
70
160
rim.
The composition trend for MK-629 (Figure 36) is markedly different
from the MK-432 trend. The MgO/(MgO + FeO) ratio decreases continuously, and spessartine shows a distinct slight decline with decreasing
XMg• This is coupled with a slight increase in grossular content.
The cordierite grain in Figure 35 shows very little chemical variation. For 11 points analysed, MgO/(MgO + FeO) ranges from 0.675 to
0.718. Biotite compositions also show very little variation, the MgO/
(MgO + FeO) ratios averaging 0.565 in the matrix with sillimanite and
quartz. Ti per 11 oxygens ranges from 0.088 to 0.139, though better
analyses have Ti below 0.115. MgO increases slightly to 0.581 next to
the garnet, and decreases slightly to 0.535 next to cordierite,
suggesting the possibility of local late retrograde ion exchanges.
More analyses are needed to substantiate this data and test its
significance. However, this does not explain the overall composition
trend from the core outwards in the garnet.
Because the bulk composition of MK-629 is too K-poor to contain
muscovite or K-feldspar, the reactions proposed for zoning in MK-432,
or in the garnets studied by Tracy et al. (1976b), do not apply. The
garnet-consuming reaction,
[ sil +gar+ qtz = crd ],
(4)
would produce a slightly flatter composition trend on the MnO-FeO-MgO
plot than a reaction involving biotite, because the cordierite is more
Mg-rich than the biotite (Figure 36b). However, some other reaction
must have been involved to allow the spessartine content to remain so
constant rather than rising as is true of most garnet-consuming
reactions. One possibility is that FeO was provided to the garnet by
a reaction that consumed ilmenite, which is the only phase in this
rock richer in FeO than garnet. The Ti0 would have to be taken up by
2
biotite, in reactions such as
[3 Mg-Fe biot + 3 ilm + 5 sil = 3 Ti-biot + 2 gar + 5 qtz], (18)
or [3 Mg-Fe biot + 3 ilm + 9 sil = 3 Ti-biot + 3 crd].
(19)
The amounts of garnet and cordierite would depend on the progress of
reaction. 4. A second possibility is that CaO was provided to the
garnet by the reaction,
[3 an =gross+ 2 sil + qtz].
(20)
These possibilities are discussed further below.
Coticule garnet zoning. The composition trend for a garnet in
sample MK-6 (Table 6b) is also shown in Figure 36a, and two representative analyses are in Table 12. The decrease in spessartine from
161
core to rim is characteristic of garnet growth zoning, but the trend
of decreasing pyrope and increasing almandine toward the rim has not
been identified in other rocks of the region. The garnets probably
formed relatively early from chlorite or other sheet silicates, or
possibly from carbonates. There are now small amounts of biotite and
chlorite in the rock, but the exact garnet-forming reaction is
unknown. The present composition of the tiny amount of biotite is
certainly a result of retrograde reaction.
Temperature Estimates
Metamorphic temperatures were estimated from the distribution of
Fe and Mg between garnet and biotite, and between garnet and
cordierite, based on the calibrations of Thompson (1976) and Ferry and
Spear (1978). A problem in estimating temperatures of metamorphism
from distribution coefficients of mineral pairs is to decide which
pairs, if any, represent pairs that formed in equilibrium during any
stage of metamorphism. Zoned garnets provide information about
changes in composition through time, but P-T evaluation can be
difficult if the zoning resulted from slow diffusion during
garnet-consuming reactions. Only mineral pairs that truly
equilibrated during the metamorphic history will give "true" temperature estimates. Temperatures and pseudotemperatures estimated from
various mineral pair compositions are presented in Table 13. Values
for ln KD are listed, where KD is the distribution coefficient equal
to
(XGar
XBiot) I (XGar . ~iot)
-~g
11g
-r'e
'
Fe
or the analogous ratio for garnet and cordierite, followed by
temperatures from both Thompson's (1976) and Ferry and Spear's (1978)
calibration curves. Only those based on Thompson will be quoted in
the text. The Ferry and Spear estimates should be used to compare
~ with temperatures on Chamberlain and Lyons' 1983 map.
In the
-discussion below, an effort is made to decide which estimates are
reasonable, and which must be pseudotemperatures.
Sample MK-432. Figures 33 and 34 show that the garnet in this
rock has two distinct zoning trends, one toward biotite-free portions
of the rim, and a different trend toward the rim where it is in
contact with biotite. In Figure 34a, there is a range in Mg/Fe ratios
for each of the three groups labeled "core", "rim", and "rim next to
biotite". Two garnet compositions from each of these groups have been
selected for use in making temperature estimates. These six garnet
compositions are given in Table 12.
Four biotite analyses are presented in Table 12. The average
matrix biotites have Mg/(Mg +Fe) between 0.45 and 0.47, although they
range from 0.447 to 0.487. Ti per 11 oxygens ranges from 0.098 to
0.138. Analysis NB-6, Table 12, is from a more Mg-rich biotite in
162
Table 13. Estimated temperatures from garnet-biotite and garnetcordierite geothermometry.
T°C
Sample MK-432
T°C
(Ferry
(Thompson, and Spear,
1976)
1978)
XMg ln KD--
Mg-rich Garnet Core Bw
Matrix Biotite c
C3a
.158
.457
1.50
630
665
Fe-Hn-rich Garnet Core Be'
Matrix Biotite
C3a
.151
.457
1.55
610
630
Fe-Hn-rich Garnet Core Be'
Biotite Inclusion
2b
.151
.588
2.08
485
465
Mg-rich Garnet Rim Ag
Matrix Biotite
C3a
.176
.457
1. 37
670
710
Fe-rich Garnet Rim Aa
Matrix Biotite
C3a
.162
.457
1. 47
635
670
Fe-rich Garnet Rim Aa
Biotite Inclusion 1a
.577
1. 95
515
500
Garnet Rim against Biotite Bl
Matrix Biotite
C3a
.116
.457
1. 86
530
525
Garnet Rim against Biotite Bv
Matrix Biotite
C3a
.101
.457
2.01
500
480
Garnet Rim against Biotite Bv
Extreme Biotite
NB-6
.101
.479
2.10
480
455
Garnet Core
A-C
Matrix Biotite A-b
.253
.567
1.36
670
715
Garnet Core
A-C
Cordierite Core A-f
.253
.685
1.86
715
Garnet Rim Near Cordierite A-a
A....;b
Matrix Biotite
.198
.567
1. 65
580
Garnet Rim near Cordierite A-a
Cordierite Core
A-f
.198
.685
2.16
620
Extreme Garnet Rim near Biot. A-s .171
Matrix Biotite
A-b .567
1.85
535
Garnet Rim next to Cordierite A-a .198
Cordierite near Garnet
A-e .712
2.30
580
.162
SamEle HK-629
t
595
530
163
close proximity to garnet. The other two in the table are from tiny
biotite inclusions within garnet X (Figure 33). Inclusion 2b is in
the garnet core, and inclusion 1a is fairly close to the biotite-free
rim. The latter is the most Ti-rich biotite yet found in sample
MK-432.
Temperature estimates in Table 13 were made by comparing various
garnet-biotite pairs. The two garnet core analyses (Bw and Be)
compared with matrix biotite (C3a) give estimates of 630 and 610°C.
These are certainly pseudotemperatures, because the matrix biotite has
changed from what it was when the garnet core formed.
The garnet core Be' is compared with biotite inclusion 2b as well,
giving an estimate of 485°C. This is probably the result of local
retrograde Mg-Fe ion exchange between the inclusion and the
surrounding host garnet. The Ti content of the inclusion, however,
has probably not changed since the biotite was included, provided no
Ti-bearing phases were available for re-equilibration. The Ti content
of .099 Ti per 11 oxygens is typical of metamorphic Zone I in central
Massachusetts (Robinson et al., 1982b, Fig. 22).
The two biotite-free garnet rim analyses (Ag and Aa) compared with
matrix biotite (C3a) give temperature estimates of 670 and 635°C.
These may not be pseudotemperatures. Although the garnet rims have a
different composition where they to~ch biotite, these rims represent
only a tiny proportion of the 16% garnet in the mode (Table 6a)
available to react with a much larger amount of matrix biotite (24% of
the mode). Thus the Mg/(Mg +Fe) in the matrix biotite may not have
changed much from what was once in equilibrium with the biotite-free
garnet rims. By contrast, biotite inclusion 1a has a Mg/(Mg + Fe) of
about 0.577, because it has exchanged Fe for Mg with the enclosing
garnet. This biotite inclusion compared with garnet rim Aa gives a
temperature of 515°C. The high Ti per 11 oxygens of 0.165 in the
inclusion, however, is typical of ilmenite-saturated biotites in Zones
III and IV pelitic schists of central Massachusetts (Robinson et al.,
1982b, Fig. 22).
Two garnet rim analyses next to biotite (Bl and Bv) were compared
with matrix biotite C-3a, giving temperature estimates of 530 and
500°C. These are probably "true" temperatures of the last equilibrium
between biotite and garnet rims. They are much lower than the
625-660°C temperature estimates for Zone III in central Massachusetts
(Tracy et al., 1976b). The rea c tion responsible for the reversed
zoning in garnet next to biotite was probably the continuous
retrograde reaction
[gar+ mus = biot + sil + qtz], (3) which consumed garnet. The
equilibrium was apparently maintained by diffusion through the sheet
silicates rather than through a pervasive metamorphic fluid, or one
would expect the reversed zoning in all parts of the garnet rim.
Comparing garnet rim Bv with the Mg-enriched biotite NB-6 gives a
still lower temperature of 485°C. This may be the temperature at
164
which very local ion exchange took place as the diffusion rate through
biotite became too sluggish for reaction 3 to continue.
Sample MK-629. As described above, garnet zoning in this rock has
one distinct trend (Figure 36). Three garnet analyses were selected
for use in making temperature estimates, one from the core (A-C, Table
12), one from the rim near cordierite (A-a), and one from the rim near
biotite (A-s). Matrix biotite is represented by analysis A-b, and a
more Mg-rich analysis from near garnet is given for comparison.
Cordierite analysis A-f represents the core, and A-e is a single
analysis near garnet.
The question of temperature estimates for MK-629 is a difficult
one. In the previous section it was suggested that the zoning trend
in Figure 36 might be controlled by a reaction involving the breakdown
of ilmenite, whereby the biotite would become enriched in Ti, and Fe
from ilmenite and biotite could go into garnet and cordierite.
Against this is the lack of any obvious Ti enrichment in biotite,
though one might argue this effect was swamped by the large amount of
biotite present. If this ilmenite reaction were the key to the
peculiar garnet zoning in MK-629, then one must question why it was
not also important in other rocks such as MK-432.
A more promising solution lie's in the fact that the HK-629 garnet
shows about a 2% increase in grossular content in the range where
pyrope falls from 24 to 16% and spessartine falls by about 1%. The
reaction that controls the grossular content of garnet in this assemblage is
[3 an = gross + 2 sil + qtz]
(20)
An increase of P at constant T, or a decrease of T at constant P,
would cause an increase of grossular content (Newton, 1983). This
reaction can be combined with reaction 4,
[2 alm-pyr + 4 sil + 5 qtz
=3
crd],
which would favor garnet growth with increasing P at constant T
(Thompson, 1976), to yield
[2 alm-pyr + 3 qtz + 6 an
= 23
gross + 3 crd].
(21)
This implies for each amount of almandine-pyrope consumed an equal
amount of grossular is produced. In going from garnet core to garnet
rim in MK-629 almandine-pyrope falls from 91.8 to 90.3%, a drop of
1.5%; thus grossular should rise from 3.8 to 5.3%. The actual rise is
5.7%. It seems that garnet consumption from the cordierite-producing
part of the reaction is equalled or even outweighed by garnet
production from the anorthite-consuming part of the reaction, and that
this accounts for the slight decline in spessartine content with
falling XMg (Figure 36).
165
With the above discussion in mind, T estimates for garnet-biotite
and garnet-cordierite pairs have been listed in Table 13. In the
first two pairs, garnet core composition is compared with matrix
biotite and cordierite core compositions yielding estimates of 670 and
715°C. If the interpretation of garnet zoning were based entirely on
the phenomenon of retrograde ion exchange, then these values might
represent true conditions of peak metamorphism. At least the
cordierite temperature, however, is way outside the realm of
possibility for Zone III conditions. If the interpretation of garnet
zoning is based mainly on continuous almandine-pyrope- consuming
reactions that decrease the XMg of biotite and cordierite, then the
670 and 715°C must be considered pseudotemperatures.
In the second two pairs an intermediate garnet that is in contact
with cordierite is compared with matrix biotite and core cordierite,
yielding temperatures of 580 and 620°C. The second temperature could
be construed as a true temperature from cordierite grown from garnet
in an almandine-pyrope-consuming reaction, and is consistent with
inferred Zone III conditions.
The third two pairs must surely represent some form of retrograde
re-equilibration. The extreme garnet composition with matrix biotite
yields 535°C. The intermediate garnet rim and the cordierite immediately in contact with it yield 580°C.
Pressure Estimates
~
Tracy et al., 1976b, Fig. 6) presented a calibration of reaction 4
([sil + gar-+-qtz = crd]) in the Mg-Mn-Fe system for the purposes of
estimating pressures of metamorphism. Disregarding difficulties with
regard to the effect of H2 o in cordierite (Newton, 1983), it can be
used to make comparisons with other pressure estimates from central
New England. In an assemblage with all four phases, the pressure may
be estimated from a garnet composition and a temperature estimate. If
the assemblage lacks cordierite, the pressure will only be a minimum
estimate (Tracy et al., 1976b).
For sample MK-432 and an estimated temperature of 640°C, garnets
Ag and Aa give estimated minimum pressures of 5.7 and 5.6 kbar respectively. For sample MK-629, where T estimates are more uncertain,
cordierite is present, but it is not known which cordierite composition was in equilibrium with which garnet composition. The most
magnesian garnet composition and a temperature of 640°C gives an
estimated pressure of 6.3 kbar. The garnet rim composition (A-z) in
contact with cordierite, and the temperature estimated from core
cordierite composition (A-f), gives a pressure of 6.1 kbar. These
estimates seem reasonably consistent with the concept that garnet
zoning took place under conditions of falling temperature, but because
of the accepted H20 content of cordierite in the Thompson calibration,
do not require falling pressure.
166
The high anorthite content of the plagioclase in sample MK-629 and
the zoning of grossular content of garnet suggest a possibility of
applying the garnet-plagioclase barometer of Ghent et al. (1976) based
on the end member reaction
[an = sil +gross + qtz].
(20)
Application of the Ghent et al. (1976) equation as modified by Ghent
(1977) for the garnet core-with mole fraction grossular of 0.038 at
640°C yields 3.7 kbar and for the garnet with XGross of 0.057 at 580°C
yields 4.4 kbar.
Finally, for MK-432, using matrix biotite composition XMg = 0.45
and the biotite-free rim garnet composition XMg = 0.16 on the garnet
and biotite isopleth diagram of Spear and Selverstone (1984, Fig. 1)
yields an intersection at 650°C and 5.8 kbar, ignoring effects of
spessartine and grossular in the garnet.
CALC-SILICATE ROCKS
Calc-silicate pods and beds are widespread in the quadrangle, such
that assemblages could be studied through the full range of metamorphic grades seen in the pelitic rocks. Such a study is beyond the
scope of this thesis. What follqws is a discussion of calc-silicate
assemblages studied in a single zoned calc-silicate pod from the
Warner Formation near Gilson Pond (MND-8-74), originally collected by
Carl Nelson.
Mineralogy and Chemography of MND-8-74
Nelson (1975) reported wollastonite in the core of a calc-silicate
pod, and concluded its presence indicated relatively low C02 activity
in the fluid phase. This would not be surprising, considering that
the relative proportion of carbonate was originally small, and that
large quantities of H20 were produced during metamorphism in the
surrounding schists.
A new thin section was cut from Nelson's sample and microprobe
analyses were done (1) to confirm the presence of wollastonite and (2)
to place some chemical constraints on the reactions that produced the
assemblage. Representative microprobe analyses are presented in Table
14.
There is a good probablility that the pyroxenoid is actually
bustamite. The composition is approximately Ca. 82 Fe. 09 Mn. 09 Si0 3 ,
which is less calcic than expected for wollastonite, and plots in the
bustamite field shown by Brown et al. (1980) for upper amphibolite
facies conditions (Figure 37). -optical properties are similar for
wollastonite and bustamite. Both are optically negative; for wollastonite, 2V=39, r>v, and R.I. = 1.620-1.634; for bustamite, 2V:45, r<v,
and R.I. = 1.662-1.676 (Winchell and Winchell, 1951). Their powder
167
k===================~~fa
MgSi0 3
opx
FeSi0 3
Fig. 37. Pyroxenoid and clinopyroxene compositions in MND-8-74,
plotted in the system CaSi0 -FeSi0 -MnSi0 -MgSi0 after Brown et al.
3
3
3
3
(1980). Mineral abbreviations: wo- wollastonite; bu- bustamite;
cpx - clinopyroxene; rh - rhodonite; pxmn - pyroxmangite; opx orthopyroxene; fa - fayalite. Tie lines (dotted) are schematic.
168
Table 14. Electron microprobe analyses from the core of calc-silicate
granulite pod NH-MND-8-74.
Calcite
(4 pts) (1 pt)
Bustamite
(3 pts)
Si0
50.50
2
Al o
2 3 .03
Na o
. 05
2
Garnet
(5 pts)
(1 pt)
CaO
58.55
62.12
MnO
.20
.18
2
Al 2o
40.01
38.76
19.27
19.06
MgO
.03
.04
FeO
6.30
6.37
Sio
3
K0
2
CaO
0
FeO
0
0
CaO
30.54
32.82
38.61
Sum
58.78
62.34
MnO
4.34
3.84
FeO
5.45
MgO
• 07
• OS
MnO
5.21
Sum
100.53
100.90
MgO
. 18
Sum 100.03
Structural Formulae
(1 oxygen)
(3 oxygens)
t
(12 oxygens)
Si
.998
Ca
.996
.997
.
Si
Al
.001
Mn
. 003
.002
Al-site
Sum
. 999
Mg
.001
.001
Mg
Fe 2+
. 005
Fe
0
0
Al
Fe 3+
.090
Total 1.000
Mn
. 087
Ga
. 818
Na
.002
Sum
1.000
3.062
2.951
1. 739
1. 712
.261
.288
Sum
2.000
M2+-s1te
.
2.000
Ca
Fe 2+
2.506
2.678
.143
.118
1.002
Mn
.281
.248
Total 2.001
Mg
. 008
.005
Sum
2.938
3.049
Total
8.000
8.000
Andradite
13.0
Grossular
72.2
Almandine
4.9
Spessartine
9.6
Pyrope
0.3
Total
100.0
14.4
73.4
3.9
8.1
0.2
100.0
169
Table 14. (cont' d)
Zoisite
ClinoEyroxene
Sio
(3 pts)
50.62
2
(2 pts)
50.88
Ti0
.OS
2
Al o
.25
2 3
FeO
17.88
MnO
3.88
.06
. 39
(1 pt)
39.84
31.49
39.74
31.17
. 99
1. 39
2
Al o
2 3
CaO
0
BaO
3.67
CaO
24.96
25.43
.07
.10
97.35
97.83
ua o
2
Ko
2
Sum
3.69
CaO
22.94
22.90
Sum
.10
.17
100.66
100.02
(6 oxygens)
Si-site
(All Fe as Fe o )
2 3
(12.5 oxygens)
Si-site
Si
2.033
2.020
Si
3.055
3.045
Al
. 012
.018
Sum
3.055
3.045
Ti
. 001
.002
Al-site
2.046
M-2 site
2.040
2.000
2.000
Sum
2.000
3+ .
Fe -s1te
2.000
Ca
. 967
.975
Na
.008
.013
Sum
. 975
. 988
Al
. 347
.817
.064
.089
Mg
0
0
.911
.906
.606
Sum
Mn
.129
.123
Ca-site
Mg
.225
.219
Ca
2.052
2.089
Sum
. 942
. 948
Mn
.004
.006
3.963
3.976
Sum
2.056
2.095
Total
8.022
8.046
Fe 3+/Fe 3++Al
.031
Di27Hd73 Di27Hd73
44.97
34.33
20.23
n.a.
.34
n.a.
99.87
(8 oxygens)
IV-site
Si
2.083
Al
1.876
Sum
3.959
A-site
CaO
1.005
2
.030
Sum
1.035
Total
4.994
i~a o
Al
Fe 3+
M-1 site
Fe 2+
.588
Fe/Fe+Mg .022
Si0
0
MnO
Total
(1 pt)
MgO
3.84
Swn
Plagioclase
18.26
Structural Formulae
!
(1 pt)
Si0
2
Al o
2 3
FeO
MgO
Na o
2
Sum
Clinozoisite
An97Ab3
.022
.031
170
X-ray patterns are too similar for easy differentiation (Brown et al.,
1980). In the following discussion the pyroxenoid will be called-bustamite, largely on the basis of the composition and the dispersion
r<v. The other ternary diagrams in Figure 37 show the coexisting
pyroxenoid-pyroxene pair in MND-8-74 from various vantage points in
the system CaSi0 3-MgSi0 3-MnSi0 3-FeSi0 3 • Because the clinopyroxene is
relatively poor in diopside component, the relations are best observed
on the Mg-free face projection and in the schematic 3-D tetrahedron.
Estimated modes for MND-8-74 are listed in Table 5b, showing the
assemblages present in the core, transition zone, and rim. The core
is mostly quartz, calcite and bustamite, whereas the rim is mostly
quartz, calcic plagioclase, and actinolite. Grossular garnet,
zoisite, clinozoisite, and sphene make up the other important minerals
present. The electron microprobe compositions for zoisite, garnet,
bustamite and pyroxene are shown in Figure 38 on two different ternary
plots. The garnet appears within the three phase field zo-bu-px on
the ACFm plot, but because of Fe3+ the garnet and zoisite compositions
are not in the same composition plane as bustamite and pyroxene, as
seen in Figure 38b.
Disequilibrium Textures and Reactions
Disequilibrium textures wer~ observed among the minerals in the
core, especially in the form of grossular rims around zoisite (Figure
39), providing evidence for the reactions:
[2 zo
+
5 cc
+
3 qz
=3
gross
+
H2o+ 5 C0 2 ],
(22)
[2 zo
+
3 bu
+
2 cc
=3
gross
+
H2o+ 2 C0 2 ],
(23)
and [2 zo
~
=3
5 bu
+
gross
2 qz
+
+
1
H
o].
2
(24)
Because the garnet lies to the right of the zo-bu tie line in Figure
38a, reaction 23 is not supported by the probe data. The bustamite
locally separates quartz and calcite and appears to have formed
earlier than the grossular by a reaction of the form:
[cc + qz
= wo
+ C0 ].
2
(25)
Although pyroxene is also separated from zoisite by grossular,
pyroxene's role in the reactions is difficult to assess. Some
possible reactions involving pyroxene, balanced according to
microprobe compositions, are listed below and can be visualized on
Figure 38a:
[px + 3 bu + 2 zo = 3 gross + 2 qz +H 2o],
(26)
[px
+
[px
+
cc
+
qz
=3
2 cc
+
zo
+
qz
= gross
bu
C0 2 ],
+
+
co 2
(27)
+ H
o].
2
(28)
171
Fig. 38. Mineral compositions in MND-8-74 as determined from microprobe analyses (Table 14). Zoisite and garnet do not actually lie
in the plane of the upper ternary diagram because of ferric iron, as
shown in lower diagram. Abbreviations: an - anorthite; zo - zoisite;
gt - garnet; cc - calcite; bu - bustamite; px - pyroxene.
172
[on+ cc + qz
-
gr + co2]
(29)
cc
bu + co2] (25)
[ cc + qz
cc
qz
[zo + CC+ qz
~o+
o]
bu = gr+qz +H
2
~o+bU+CC =
(24)
gr. H20+ co2](23)
[cc
+
px
+ qz
= bu + co 2
J
(27)
Fig. 39. Portions of core mineral assemblages as seen in thin section
of calc-silicate pod MND-8-74. Stippled pattern is grossular garnet.
Other minerals as follows: qz -quartz; px- pyroxene; zo- zoisite;
cc - calcite; an - anorthite; bu- bustamite; sph - sphene; zr - zircon.
Textures suggest the reactions indicated.
173
t
T
I()
(\J
0
~
Fig. 40. (after Kerrick, 1970) Schematic equilibrium curves for
calc-silicate assemblages at low co partial pressures. Reactions
2
are numbered as in the text and in
Figure 39. A, B, and C are
isobaric invariant points. Mineral abbreviations same as in other
figures.
174
The reactions involving end-member wollastonite, quartz, calcite,
clinozoisite, grossular, COz and HzO define an invariant point on a
T-XCOz diagram at constant pressure, shown schematically as point "C"
in Figure 40 (after Kerrick, 1970). Compositions in MND-8-74, except
for calcite, are so far from the end-member compositions on which
Figure 40 is based, that it is only qualitatively relevant.
As temperature increased, the pyroxenoid-producing reaction 25
probably buffered the system up to the point C where grossular became
stable. The effects of Mn, Mg and Fe on the position of the invariant
points have not been explored experimentally, but we can predict
certain trends. The curve for reaction 25 would shift toward lower
temperatures for a given XCOz with added impurities in the wollastonite, and presumably the analogous reaction involving bustamite
would behave similarly. The effects on reactions involving garnet and
epidote are less clear. Kerrick (1970) states that, because the
distribution of Fe3+ is such that Fe3+ in grossularite exceeds that in
zoisite, reaction 28 will shift toward higher temperatures with
increased Fe3+ in the system. However, he suggests that the analogue
to reaction 24 would shift in the same direction, which is not true if
Fe~toss
Fe~6· The plot of grossular and zoisite compositions in
Figure 38b makes this clear.
>
Plagioclase is present in sparse amounts in the core assemblage,
and textures (Figure 39) indicate the reaction:
[an
+
2 cc
+
qz
= gross
+
2 C0 ].
2
(29)
No evidence was observed for reaction 30 (Figure 40), nor any of the
others which involve plagioclase. Why anorthite should be locally
present rather than zoisite is unclear, but it occurs adjacent to a
large patch of calcite where perhaps the co activity was locally
2
higher. We know that the fluid composition varied greatly from the
core, with its carbonate-bearing assemblage, to the rim, where hydrous
phases become increasingly abundant, but evidence for reaction 29 in
the core suggests minor variations in the fluid composition over less
than one millimeter.
CORRELATION OF METAMORPHISM AND DEFORMATION
Age of Prograde Metamorphism
The peak of metamorphism occurred sometime during the Acadian
orogeny, but various geologists have different ideas about its timing
relative to the phases of deformation. The fact that andalusite
pseudomorphs occur mainly in the higher tectonic levels of the nappe
pile has led to a general consensus that sillimanite did not make its
appearance until after the nappes' emplacement (Thompson et al., 1968;
Tracy and Robinson, 1980; Chamberlain and Lyons, 1983; Spear et al.,
1983). Kyanite formed at lower levels in the nappe pile, which in
present geographical terms translates to "west of the andalumps",
175
except where higher tectonic levels are preserved in the cores of
later synformal structures. The rocks in the upper levels were
hotter, perhaps in part due to the proximity of the Bethlehem and
Kinsman plutons, and at lower pressure, than those deeper in the pile.
Although it is the Fall Mountain nappe level that contains the
andalumps in the Bronson Hill anticlinorium (Thompson et al., 1968;
Spear et al., 1983), in the Monadnock quadrangle the fossil isobar
cuts obliquely through the rocks which were in the nappe-stage
syncline below level (3), i.e. below the Fall Mountain level (compare
Figures 13 and 30). This is especially apparent in the area of Little
Monadnock Mountain and Troy. The nappe-stage deformation probably did
not juxtapose previously andalusite-bearing rocks on top of kyanitebearing rocks, but rather set up the conditions necessary for the
growth of these minerals. The kyanite-andalusite boundary was
probably migrating upwards through the rocks as temperature and
pressure conditions changed, and the fossil isobar observed today
captured its position where sillimanite became stable.
The boundaries between Zones II, III, and IV (Figure 30) seem to
follow approximately the boundaries between levels (1), (2), and (3)
(Figure 13). At first sight this might seem to suggest that the peak
condition mineral assemblages preceded or coincided with the thrustnappes. However, the same pattern could have resulted if those rocks
which were initially hotter (i.e. in level 3) remained hotter than the
rest as rocks in all levels experienced increases in temperature and
pressure during early backfolding. The fact that K-feldspar augen cut
across foliation at Otter Creek Dam supports this.
!
Rocks in different parts of the orogen experienced different P-T
conditions depending on their locations in the backfolds and later
doming. Robinson and Tracy (1980) proposed that rocks in the Merrimack synclinorium underwent a continued pressure increase after the
peak temperature (Figure 31). In contrast, the rocks around the domes
rose, and were subjected to less pressure even as the temperature
continued to rise. The P-T estimates from garnet zoning studies
southeast of Mt. Monadnock suggest a trajectory intermediate between
the two.
On a smaller scale, Chamberlain (in progress, 1985) has studied
the detailed textural relations and mineral compositions at what he
interprets to be four cross-fold intersections in the Gilsum-Marlow
area. In brief, his early anticline-late anticline intersection shows
a gradual decrease in temperature, the early anticline-late syncline
intersection shows a temperature decrease followed by increase, the
early syncline-late anticline pair shows early temperature increase
followed by decrease, and the syncline-syncline crossing shows a
continuous increase. This sort of approach could probably be applied
in the Monadnock area, but care must be taken not to interpret
structures on the basis of the metamorphic results. Chamberlain's
"early anticline and syncline", for example, may reflect the west to
east increase in grade across the level (2)-level (3) boundary as
176
defined in the Monadnock area.
the Marlboro syncline.
His early syncline is continuous with
In the Monadnock quadrangle Zone IV assemblages are mainly
confined to rocks in level (3), except for the rocks in the Derby Hill
window. The rocks within the window were apparently subjected to
similar conditions as the rocks outside the window, which supports my
suspicion that Zone IV conditions post-date the nappe-stage structures. Although later structures brought the rocks in the window back
toward the surface, the axial trace of the Marlboro syncline trends
north-south through the area west of the window. This might be a good
area in which to do a detailed metamorphic study of the sort done by
Chamberlain, although it would be difficult to find samples of the
appropriate bulk composition which have escaped retrogression.
The peak of metamorphism certainly preceded dome-stage deformation, because quartz-feldspar segregations which presumably formed
at approximately peak conditions (Tracy, 1978) are deformed by domestage folds. However, the rocks remained hot long enough to allow the
formation of the pervasive sillimanite lineations in the dome stage.
Age of Retrograde Metamorphism and the "Permian Disturbance"
The cause of the severe retrogression in the area north and east
of Mt. Monadnock is uncertain. There does not seem to be any obvious
spatial relationship with late plutons or the mafic dikes.
Krueger and Reesman (1971) reported that K-Ar radiometric "ages"
determined from muscovite in ten samples of schist from Mt. Monadnock
appear to have been disturbed during the Permian. Zartman et al.
(1970) showed the Monadnock area as lying approximately on the-boundary between an area to the west where K-Ar ages of micas (350-260
m.y.) were partially reset during the Permian, and an area where K-Ar
mica ages (260-200 m.y.) were completely reset. They discussed
whether the ages were reset during gradual cooling from peak Acadian
conditions, or whether the disturbance was related to the Alleghenian
orogeny. The "ages" of coexisting biotite at Monadnock are consistently older than for muscovite, and Krueger and Reesman estimated a
390 m.y. age for the Acadian metamorphism based on a model whereby Ar
leakage from muscovite controls Ar leakage from biotite. The mafic
dikes on Monadnock cut all stages of deformation and thus are
presumably younger than the Acadian metamorphism, and yet the biotites
form a weak foliation and ilmenites are rimmed by sphene. It would be
interesting to compare K-Ar ages from biotite in the mafic dikes with
those from the Littleton Formation. Since the dikes were not present
during peak Acadian metamorphism, ages on biotites from the dikes and
muscovite from inclusions might shed some light on the so-called
Permian disturbance. More detailed sampling of schists from
throughout the quadrangle might show whether or not the zones with
severe re-hydration have any relationship to zones of possible late
Paleozoic deformation.
177
CONCLUSIONS AND RECOMMENDATIONS FOR FUTURE RESEARCH
The important contributions of this thesis are as follows:
(1) Stratigraphic units, which are correlated with those of
central New Hampshire, and indirectly with northwestern Maine, have
been defined and their distribution has been mapped. Only the middle
member of Fowler-Billings' (1949a) Littleton Formation is retained as
Littleton. All the other metamorphosed sedimentary rocks are older.
(2) Belts of Kinsman Granite across the southeast part of the
quadrangle are interpreted as a connection between the Cardigan pluton
and the Coys Hill Granite, which is repeated in the map pattern by
faults. The Spaulding Tonalite cuts the oldest folds whereas the
Kinsman preceded them. The Fitzwilliam Granite and several previously
unreported microdiorite dikes are post-Acadian.
(3) Major structural features have been defined, and an attempt
has been made to determine their relative ages. The Monadnock syncline closes south of Troy, and is believed to be a nappe-stage recumbent syncline now completely reoriented. Large isoclinal folds and
the pervasive foliation on Mt. Monadnock are also believed to have
formed in the nappe stage. These folds are responsible for the thick
pile of quartzite beds which hold up the summit. West-directed,
ductile thrust faults cut the nappe-stage folds, and separate the
region into three tectonic levels. The Brennan Hill fault separates
the Monadnock stratigraphy from the underlying autochthonous rocks on
the Keene dome. The Chesham Pond fault and the Thorndike Pond fault
zone separate the Monadnock stratigraphy from an overlying sheet of
chiefly Rangeley Formation and Kinsman Granite. Augen schists and
gneisses, believed to represent recrystallized mylonitic rocks, are
found along the thrusts. The nappes and thrusts were deformed by
backthrusts and backfolds, chief of which is the Beech Hill anticline,
which overturned the Monadnock syncline back toward the southeast.
The Thorndike Pond fault zone was reactivated during backfolding, with
west-over-east motion. The final phases of Acadian deformation
produced folds related to the movement of tonalite plutons, and to the
rise of gneiss domes to the west. In the Mesozoic, the Thorndike Pond
fault zone was again active, this time with normal faulting down to
the west.
(4) Metamorphic assemblages show an increase in grade from Zone II
in the autochthonous rocks, to Zone IV in the gneisses above the
Chesham Pond fault. An extensive area of Zone III rocks is characterized by sillimanite pseudomorphs after andalusite. Garnet zoning
patterns in Zone III pelitic schists suggest a P-T history
intermediate between those of the Bronson Hill anticlinorium and the
Merrimack synclinorium as outlined by Tracy et al. (1976). Retrograde
metamorphism has locally produced chlorite, staurolite, and chloritoid, the last particularly in rocks formerly rich in sillimanite.
Further work is needed in the following areas:
(1) Detailed mapping within the Rangeley Formation may yield an
internal stratigraphy beyond the tentative suggestions in this thesis.
The southwest portion of the quadrangle has the best potential for
such work, and more mapping is needed from there south to the Tully
body of Monson Gneiss in Massachusetts. There are also areas in
178
Roxbury, Nelson, and the southeasternmost corner of the quadrangle
where I have not mapped.
(2) More could probably be done to define a detailed stratigraphy
in the upper part of the Littleton Formation, starting with the Seven
Quartzites.
(3) A systematic study of metamorphic assemblages in pelitic
schists would be valuable, to refine the metamorphic map presented
here and to aid the attempt at relating metamorphic history to structural development. Zoned garnets from various structural positions
and metamorphic zones in the quadrangle could be compared.
(4) The geochemistry of the microdiorite dikes and of the
coticules should be studied.
179
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APPENDIX
Table 15.
Towns hiE
Numbers
DB
FZ
Dublin
Fitzwilliam
Hancock
Harrisville
Keene
Marlboro
Jaffrey
Nelson
Richmond
Rindge
Roxbury
Sullivan
Swanzey
Troy
1-327; 01-04
1-67
1-12
1-179
1-32
1-346
1-957; 997-1188; 001-013
1-31
1-26
101-128
1-187
1-23
1-63
1-344
HK
MK
NL
RD
RI
RX
sv
sz
TR
Table 16.
'.:
;
List of mineral abbreviations.
Mineral
Abbreviation
Mineral
ab
act
alm
and
an
albite
actinolite
almandine
andradite
anorthite
hd
ky
ksp
mt
mus
hedenbergite
kyanite
K-feldspar
magnetite
muscovite
ap
biot
bu
cc
chl
apatite
biotite
bustamite
calcite
chlorite
or
opx
py
px
qtz; qz
orthoclase
orthopyroxene
pyrite
pyroxene
quartz
crd
cpx
ctd
czo; cz
di
cordierite
clinopyroxene
chloritoid
clinozoisite
diopside
rh
sil
spess
sph
st
rhodonite
sillimanite
spessartine
sphene
staurolite
wo
zr
zo
wollastonite
zircon
zoisite
fa
fayalite
gar; gt garnet
gross; gr grossular
gph
graphite
''
List of station codes and numbers.
Code
HV
KN
MB
Abbreviation
1.
'fk
190
APPENDIX 2.
Sullivan
Nelson
NL
Hancock
HK
KN
Roxbury
RX
Harrisville
HV
Dublin
Marlboro
DB
MB
Swanzey
sz
• Mt. Monadnock
Troy
Jaffrey
TR
MK
N
Fitzwilliam
Richmond
RD
~
Rindge
Rl
I
FZ
2 mi.
0
0
2
Fig. 41. Townships in the Monadnock quadrangle, and codes
used for stations (see Table 15).
3 km.
191
. : :·.:~=.'f:
.
·.·.·:
:-,. .:-:-....,- - -,_
.,_··:·. ,. ,.._
. :_::-:·. ,_
-.; . . . ·.· .;}F {
::·~<:.' ·~:··:
w
·>-::.·. ··:::~.-:. ·......
E
Fig. 42. Two piece fence diagram to cut out and assemble of Mt.
Monadnock summit. Dotted pattern represents rocks younger than
Seven Quartzites. Instructions: (1) Trace all lines onto a sheet of
paper or light cardboard. (2) Tra~e mirror image on reverse side of
paper, using light table. (3) Cut out the two sections, and cut
along dashed lines. (4) Fit together with points of compass properly
arranged. (5) View from NE, NW, SW, and SE to visualize structures
in three dimensions .
:..
.. ... .... : : ~ :· : . . . . . . .
. .. .
..
. .. . . ·.·.~ · - · . · .·
· -: ·.-:::· ·
.. ·
..... . ~.: . .
. :: .. ... ·. : .... ~... : .
S
N