American Mineralogist, Volume 76, pages 1781-1810' 1991
The eye of the petrographer,the mind of the petrologist
Pnrpn RosrNsoN
Department of Geology and Geography,University of Massachusetts,Amherst, Massachusetts01003, U.S.A-
Ansrucr
To bring meaningful numbers out of metamorphic rocks, one must first understand
what the numbers represent.An idealized objective in metamorphic studies is to demonstrate that coexisting minerals approachedequilibrium. In a few rocks this can be done
reasonably, for example, in coarse-grainednodules from kimberlite that were rapidly
quenchedfrom mantle conditions. More typical, and more interesting, are rocks showing
disequilibrium by mineral zoning and reaction textures at various scales.Theseprovide a
view, if correctly interpreted, into a sequenceof events atd P-T conditions. In making
quantitative estimates,a problem is to decidecorrectly which minerals were in equilibrium
with which other minerals, and what types of reactionsand processesactually took place.
These decisions depend on the integration of both petrography and phase equilibrium
theory. Failing this, a false interpretation of rock history can be made. Featuresin metamorphic rocks in central Massachusetts,including schistsbearing strongly zoned garnets
produced by four different types ofreactions, and a newly describedocculTenceofwollastonite marble provide opportunities to explore theseideas.
In many examples, rock history can only be charted through study of the complete
geologic setting at outcrop or larger scale.Particularly spectacularare well-preservedpyroxene granulites,gabbros,and diabasesin the ScandinavianCaledonidesthat are locally
transformed along shear zones into eclogite and amphibolite. Lacking knowledge of the
surroundings, there would be no way to know that the original hlgh f rocks survived
eclogite-faciesoverprinting with few to no mineralogical traces. Their field setting demonstratesthe importance of structural environment, kinetics, and fluids in producing different minerals from the same rock composition along the same P'T path.
INrnonucrroN
In petrologic studies, as in field geology, the observations that are possible are conditioned by the experience
and training of the observer. Petrologic research is an
attempt to define the sequenceof eventsin the formation
ofthe rocks, the conditions under which the events happened, and why they happened. Petrology can never be
totally quantitative, i.e., "getting numbers out of rocks,"
but the petrologist's familiarity and deftnesswith measurements,calculations, and the constraints imposed by
phase equilibria are essential.Also essentialis observational and descriptive petrography from the atomic
through the microscopic and macroscopic to the petrologic map scale,providing the framework for interpretations. Maps can only be made by integrating knowledge
gainedfrom submicroscopicand microscopic scalesusing
the hand lens and unaided eye; thus the most truthful
petrologic maps are done by those most skilled in both
petrography and mapping. These ideas are illustrated by
examples where specialized knowledge or background
opened the eyes of the petrographer, leading to unexpected interpretations. These ideas are explored in three
parts, each chosento representa different scaleofobservation and thinking but having in common the use of
phaserelations to try to understandthe sequenceof meta0003-o04x/9l / l l l 2-l 78r $02.00
morphic events. The first part concernsthe textural and
phaserelations between garnet grains and their immediate surroundings. The secondpart deals with featuresat
the scaleof a whole thin section in a wollastonite marble
where variations in local bulk compositions are shown to
have a dramatic effect on phaserelations. The third section covers relations betweenrocks at the scaleof a large
outcrop or a mountainside, where an understanding of
the total setting is vital to understandingrock history.
A theme of this paper is "getting numbers out of rocks"
and concerns the need for careful and thorough petrographic observations in thin sections and in the field if
one is to make meaningful maps of igneous and metamorphic rocks and if one is to combine theory and experiment to make useful petrogeneticmodels' In the game
of getting numbers out of rocks, much of my attention
has been focused on the central MassachusettsAcadian
metamorphic high, shown on a metamorphic map of New
England in Figure I and in closer detail in Figure 2, wilh
metamorphic zonesranging from chlorite zone (C) in the
extreme northwest through Zone I (kyanite-muscovitestaurolite) through Zone YI (sillimanite-orthoclase-garnet-cordierite). In this whole areaI am familiar with only
two "direct" numerical determinations. The first (Fig. 3)
is a slightly flawed number 8 found in a granulite facies
1 7 8I
1782
ROBINSON: EYE OF THE PETROGRAPHER, MIND OF THE PETROLOGIST
t{ilA
Silliman
ite-OrthoclaseGarnet-Cordierile
Zone
',5t(
Sillimanite-Orthoclase
Zone
Sillimanite-1,/uscovite
Zone
\ r ,/ t ;
Kyanite-Slaurolite
Zone
i":#
'/-l
Andalusite-Staurol
t€ Zone
!Y,
GarnelZone
y,y
Biotileand ChloriteZon6s
WeaklyMetamorphosed
t-'.1
,r/
"3
Approximote Scole
2 O OK m
Fig. 1. Generalizedmap of Acadian and Taconian metamorphic zonesin southwesternNew England. Modified from Osberget
al. (1989) based on a compilation by Robinson (1983) and an unpublished compilation, Metamorphic Map of the U.S.-Canadian
Appalachians, by G.W. Fisher and W.E. Trzcienski. Heavy dashed line representsthe approximate boundary where peak metamorphism is Acadian to the east and Taconian to the west.
pegmatite formed by spherical aggregatesof graphite
platelets enclosedin a crystal of orthoclase.The second
(Fig. a) is a much larger number 2 discoveredin a garnetrich schist in the same area by Elaine Padovani. In
considering the question of geothermometry and geobarometry in such a region, it is natural to turn to the compositions ofgarnets in pelites as well as to consideration
of garnet stability and geothermobarometry in a newly
describedwollastonite-bearingmarble.
INrnnpnnr.q,TroN oF GARNETcoMposrrroNs
METAMORPHIC
rN
ROCKS
Aside from its aestheticvalues,garnet has capturedthe
attention of quantitative metamorphic petrologists for
three reasons:(l) It holds very high concentrationsofFe
and Mn relative to most other coexisting minerals, and
hence distribution coefficientsare sensitive to temperature. (2) It has high density compared with most coexisting minerals, and hence reactions involving it are likely
to be sensitive to pressurechanges.(3) Elemental difiilsion is relatively slow in garnet compared to most other
minerals, except at the highest temperatures,so that disequilibrium featuresare commonly preserved,and these
provide a key, if properly interpreted, to the changing
conditions of metamorphism. These disequilibrium features are the subject ofthis part ofthe paper.
The mention of metamorphic garnetsin central Massachusettsopens a whole can of worms (Fig. 5). In this
casethe heads of the wonns are the cores of the gamets,
and the tails are the rims. The roman numerals correspond to the different metamorphic zonesin central Massachusetts,and the compositional trajectories are different in each of the metamorphic zones.In the subsections
that follow, eachof the major types of garnetzoning found
in central Massachusettsis discussed,the importance of
careful petrographic observations to deduceequilibria and
reaction mechanismsare emphasized,and the particular
pitfalls involved in making geothermometric interpretations are pointed out.
In the discussion that follows, the terms "prograde"
and "retrograde" appear frequently becausethey are part
of the everydaylanguageof metamorphic petrologists,yet
they are fraught with ambiguity in the context of complex
P-7 paths. A prograde path is commonly thought of as
one involving a temperature increase,a dehydration, or
both. Yet progrademetamorphism to a high-pressureassemblagemight involve mainly a pressureincreasewith
little or no increaseof temperature.To consider prograde
as the direction in which minerals grow would be entirely
erroneous becauseit is quite possible for some minerals
to grow with decreasingtemperature.The ambiguity may
be epitomized by considering a hypothetical equilibrium
A : B with a positive P-Z slope. P-T trajectoies, also
with positive slopes,may be drawn to crossthis reaction
in different up-temperature directions so that A goes to
B or alternatively so that B goesto A. Both sensesof the
reaction could independently be described as prograde.
Thus, the best that one can say is that prograde metamorphism involves a movement more or less directly
ROBINSON: EYE OF THE PETROGRAPHER. MIND OF THE PETROLOGIST
I 783
I\
At
(({\
\\l'\\
rrt(t
-ra-
grapharranged
Fig. 3. The number8 formedby spherically
ite plateletsenclosedin orthoclaseof a pegmatitein Zone VI,
For locationseeRobinsonet al. (1986),
centralMassachusetts.
stop7.
\
z
o
z
o
E.
6
A sequenceof garnet compositions can be studied that
records garnet growth. Unless inclusions are preserved,
only garnet rim and matrix compositions can be used for
geothermometry. Inclusions may reequilibrate within
garnet,but this requiresdiffusion. Note that garnetgrowth
need not be prograde but may be retrograde, depending
Fig. 2. Generalized
map of Acadianand youngermetamorphic zonesin west-centralMassachusetts
and adjacentstates.
Modified from Schumacher
et al. (1989).ZonesI to VI after
Tracy et al. (1976).Diamondsindicatelocationsof sillimanite
pseudomorphs
afterandalusite;closedcirclesindicateorthopyroxenein metamorphosed
maficandintermediate
igneousrocks;
assemopencirclesindicatelocationsof relict granulite-facies
blagesin Pelhamgneissdome;diagonalshadingindicateszones
of retrogrademetamorphismat New Salemand QuabbinHill.
The areaof zoneI betweenthe retrograde
zonesand the Mesozoic ConnecticutValley fault appearsto be an areaof intense
latePaleozoicmetamorphicand tectonicoverprinting.
away from lhe P-T origin in a way that may or may not
involve significantlyincreasingpressure.
Zoning producedduring prograde garnet growth
In the lowest grade zones in central New England, the
garnetsare commonly well formed, are nearly euhedral,
and show a characteristic zoning pattern with Mn-rich
coreszoning to less Mn-rich rims with first an increasing
Mg/Fe ratio and then a decreasingratio (Figs. 6 and 7).
With some reservations,these may possibly be taken as
examplesof garnet gpowthunder the simplest conditions,
where diffusion in garnet is minimal to nonexistent.Only
the surfaceof the garnetis in equilibrium with the matrix.
Fig. 4. The number 2 formed by scatteredgarnet in sillischist from Zone Yl,
manite-orthoclase-garnet-cordierite-biotite
central Massachusetts.For location see Robinson et al. (1986),
stop I 1.
1784
ROBINSON: EYE OF THE PETROGRAPHER, MIND OF THE PETROLOGIST
/
Mg
+
'lo
Mg- oo
Fig. 5. Summaryplot of compositional
trajectories
of zoned
garnetsfrom centralMassachusetts
in termsof Fe,Mn, andMg.
NumeralsI-VI refer to metamorphiczonesin centralMassachusetts,
andP refersto partiallyresorbed
granulite-facies
garnet which the garnet reequilibrates. (It is interesting that in
some skarns containing mostly andradite garnet, howfrom the Pelhamdome.From Robinsonet al. (1989).
ever, Mn is not partitioned into garnet but into pyroxene.)
What are the means by which examples of prograde
critically on the P-7path and the reaction isopleths.This
concept of growth, under either a progade net-transfer growth zoning in garnet are destroyed or altered to obreaction or a retrogradenet-transfer reaction, is illustrat- scure their story? First of all, prograde or retrogradedifed in Figure 8. In aluminous rocks, Mn is most strongly fusional ion exchangebetweengarnet and its matrix (Fig.
partitioned into garnetas comparedwith any other phase. 9), here taken to be biotite, could destroy the near-rim
For this reason,it is the element that appearsmost abun- record. A second possibility is a retrograde (or possibly
dantly in early formed garnet and is also the last to be prograde) diffusional net-transfer reaction (Fig. l0) by
depleted from garnet in any garnet resorption reaction in which the outside of the garnet is partially resorbedwhile
those components retained by garnet actively diffuse toFe
zo
30
Fig. 7. Compositionaltrajectoriesoftwo typicalzonedgarnetsfrom zoneI, centralMassachusetts
in termsof Fe,Mn, and
Mg. Corecompositions
areat mostMn-richpointsnextto specimen number.Specimen908 is staurolite-garnet-biotite-muscovite schist;specimen4F5 is similar but also with kyanite.
From Tracyet al. (1976).
PROGRADE.NET TRANSFER,
GARNET GBOWTH REACTION
Fe
Mg
BETROGRADE,NET TRANSFER.
GABNETGROWTHREACTION
Fe
Fig. 6. Compositional maps of a zoned garnet in sample 908
from staurolite-garnet-biotite-muscoviteschist of zone I, central
Massachusetts.Contours based on Fel(Fe + Mg + Mn + Ca),
etc., are Fe 79-880/0,Mg 7-llo/o, Ca 2-5o/o, and Mn l-90l0.
Adapted from Tracy et al. (1976).
Fig. 8. AFM muscovite projections and Fe-Mn-Mg plots
showing the nature of prograde and retrograde garnet-growth
reactions. In each example, reaction proceedsfrom the solid to
the dashedtie lines, and in each example there is a net increase
in the modal amount of garnet. The progradereaction involves
dehydration but may not involve significant temperature increase,as shown by the nearly parallel tie lines in the AFM view.
The retrograde reaction does involve a significant temperature
drop, as shown by the crossingtie lines in the AFM view. Modified from Roll (1987).
17 8 5
ROBINSON: EYE OF THE PETROGRAPHER. MIND OF THE PETROLOGIST
1.00
t
10'C/]VIa
.96
t3
t2
t1
t6
ll
ilu
+
fY.
<- .92
+
.9)
u .88
()
lr
Gamet
%w
725 (initial)
.84
o
t6
.80
!r
-12
Distance --+-
+
A
200
L?
Biotite
800
1000
900
Fig. 9. Hypotheticalcompositionprofilesofgarnetand bioion exchange
for time lothroughl-. Diffusion
tite for retrograde
in garnetis a strongfunctionof temperature,
whereasbiotite is
assumed
to homogenize
throughoutthe periodofequilibration
with garnet.FromSpear(1991).
l0 "C/Ma
800
llli:::'::llr
B too
ward the interior. A third possibility, recently explored
by Spearand Florence(1991),is internal diffusive modification of the primary zoning profile, which would produce a new and obviously false record oforiginal garnet
growth compositions. In a thermal model (Fig. I l) of
diffusional ion exchangebetween garnet and matrix biotite done by Spearand Florence(1991), using diffusion
data from Cygan and Lasaga(1985) and assuminga quasi-infinite reservoir of biotite host, we can seethe limits
of diffusional reequilibration at a cooling rate of l0 "C
per m.y. beginning al.725 t and tailing down to a limit
closeto 500 'C.
Karabinos (1983) has describedgarnet-zonepelites in
600
400
Radius(pm)
700'
,r'
6'15
o
F 600
><..
500
B
500'
0
1000
800
600
400
Radius (p m)
Fig. I 1. Profilesof garnetcomposition(A) and calculated
garnet-biotitetemperature(B) at temperaturesfrom 725 "C to
500 'C, assumingdiftrsional retrogtadeion exchangebetween
garnet and a major biotite reservoir.Note that although the
interior of the garnet is hardly changedin composition,
in Fe/(Fe+
I(calculated)dropsslightlyasa resultofa decrease
with thegamet
Mg)of matrixbiotitecompositionasit exchanges
rim. From SpearandFlorence(1991).
2N
Vermont that preserve two cycles of apparent prograde
garnet-growthzoning (Fig. I 2). Theseare separatedby an
t3
lt
Mn-enriched rim (Fig. 13) believed to have been protUl
duced during an intervening period ofgarnet resorption.
Karabinos arguesthat the garnet itself gained an Mn-rich
rim by a resorption reaction (Fig. la) in which garnetwas
+
()
diffusively depleted in Mg and Fe for the production of
Gamet
Tr.
chlorite. My own interpretation is that diffusion in garnet
t*
at the gradeexperiencedby theserocks, which appearsto
-t;
have been at 500 "C or lessas demonstratedby apparent
t1
delicate preservation ofprograde Srowth zoning, was too
ts
slow to permit such retrograde diftrsive reequilibration.
Indeed, using available diffusion coefficients,Karabinos
Distance
could not account for it. I would argue that the garnet
Fig. 10. Hypothetical composition profiles ofgarnet and biwas partially resorbedby surface dissolution to produce
otite for a retrogtade net-transfer garnet resorption reaction for
an Mn-enriched retrograde assemblageof hydrous fertime lo through r-. Diffirsion through garnet is a strong function
romagnesian silicates, mainly chlorite. The Mn-rich inof temperature, whereasbiotite is assumedto remain homogetermediate garnet rim then grew by prograde growth from
neous throughout the period ofgarnet equilibration. Becauseof
the matrix at a very early stagein the secondepisodeof
resorption, the garnet interface retreatsin favor ofbiotite. Modgarnet growth in this rock.
ified from Spear(1991).
l 786
ROBINSON: EYE OF THE PETROGRAPHER, MIND OF THE PETROLOGIST
end of lst
p.rogrode
stoge
lmm
c
R1
end of
retrogression
Mn
F"
end of 2nd
p.rogrode
srqge
c
RzR3
Fig. 14. History ofgarnet growth, resorption, and growth for
the sample illustrated in Figures 12 and 13, as portrayed by
Karabinos(1983).
T
Fig. 12. Textural map of garnet from eastern Vermont,
showing two cyclesofprograde growth. From Karabinos (1983).
Line C-R is the compositional profile shown in Figure 13, and
TU is the point oftextural unconformity supposedby Karabinos
to be the outer limit of the original g:rnet at the end of the stage
ofretrogression.I arguethat resorption proceededslightly deeper
into the crystal.
Hollocher (1981, 1987) has describedjust such an example of surface dissolution of garnet to produce chlorite
(Figs. l5 and 16) in the New Salem retrograde zone, central Massachusetts (see Fig. 2). In this case there are detailed analyses of the full spectrum of relict primary zon-
(R2)
(R3)
UO|A
64-438
.lo
-G-_.t
o
\!
E.oo
Mn
Fe
--ar
-a-
E,
,.o2
o.l 2
i.o,
1- -o--o--q--o--a--a-'-
I
c
Mg
t tl
-A.AA'4
2mm
Distonce
Fe
I
I
TU
R
Fig. 13. Composition profile C-R in the garnet shown in Figure 12, modified from Karabinos (1983).Karabinos suggeststhat
the zone with high Mn/Fe ratio was produced during retrograde
garnet resorption; I believe it was produced early in the second
stageofgarnet growth.
oo,n.t
[.,.-...T]
lr.-Tf,stourotite
l-l
orortr,minormico
ffi
cntorire,
minormuscovife
E
ur..ouite,minorbiofire
Fig. 15. Variably resorbedgarnetsfrom the New Salem retrogradezone studied by Hollocher (1987). Numbers indicate the
variable spessartinecontent obviously left over from the period
of initial prograde garnet growth.
ROBINSON: EYE OF THE PETROGRAPHER, MIND OF THE PETROLOGIST
1787
Mg
+
oftwo typicalzonedgartrajectories
Fig. 18. Compositional
in termsof
netsfrom zonesIV and V, centralMassachusetts,
areat mostMg-richpoints
Fe,Mn, and Mg. Corecompositions
aresillimanite-orthonumber.Bothspecimens
nextto specimen
schistsin whichgarnethasbeenresorbedby
clase-garnet-biotite
the reactiongarnet+ orthoclase+ HrO : sillimanite+ biotite.
From Tracyet al. (1976).
Spear(1991)has recentlyexploredthis situationin detail
and shown that the last grown part ofthe progradezoned
garnet, which is likely to record the highest temperature,
is also the first to be destroyedby retrogradeion exchange
or resorption reactions.
llTtttl+
Somole NS 7 |
Retrogroded Gornet
Zoning producedby retrograde garnet resorption
Fig. 16. Drawingof partiallyresorbedgarnetfrom New SaIn regions of higher grade metamorphism, the record
lem retrograde
zoneby Hollocher(1981).Muchof garnetis re- of earlier history, if there was one, was destroyedby difplacedby chlorite,but whereit remainsit retainsthespessartine fusion, and the garnets are largely unzoned. What hapcontentsfrom the muchearlierstageofprogradegarnet$owth. pens
beyond the peak of metamorphism depends critically on environmental conditions and whether garnet
ing in garnets that are partially dissolved in a regional resorption reactions take place. The garnet illustrated in
hydration reaction. This demonstratesthat the volume Figure 17 is an irregularly shapedone from the sillimanof garnet that was in equilibrium with the matrix during ite-orthoclase-muscovitezone in central Massachusetts.
the retrograde reaction was zero, but the surface of the It appearsto have a continuous exterior rim of Mn and
Fe enrichment and Mg depletion wherever the garnet is
garnetwas being actively corroded to producenew matrix
touching the matrix and regardlessof what phase is diminerals.
In a more complex situation, garnet is growing, but rectly adjacent in the matrix. This is interpreted to mean
diffusion is causingreequilibration within garnetand pro- that the exterior part of the garnet was equilibrated with
gressively destroying the earlier tape recorder message. a pervasive phase,probably an aqueousfluid, that was in
g
lmm
Fig. 17. Composition maps of a garnet in sample 9338 from the sillimanite-orthoclase-muscovitezone (Zone IV), central
Massachusetts.Contours based on Fe/(Fe + Mg + Mn + Ca), etc., areFeT3-77Vo; Mg l2-180lo; Mn 5-80/0.Continuous rims of
higher Fe, lower Mg, and higher Mn around this irregular garnet are characteristic ofthis zone and indicate a continuous retrogfade,
net-transfer,hydration resorption reaction through the medium ofan interstitial fluid. Modified from Tracy et al. (1976)'
l 788
ROBINSON: EYE OF THE PETROGRAPHER, MIND OF THE PETROLOGIST
Fig. 21. Fractionalgarnetresorptionillustratedschematically asa seriesof steps.At stageI bulk compositionI consistsof
garnetGl andbiotite Bl. The innerpart of garnetGl ceases
to
equilibratewith matrix biotite and the effectivebulk composition movesto 2, whereit now consistsof garnetG2 andbiotite
82. Still more of the garnetceases
to equilibratewith matrix,
movingthe bulk compositionto 3, whereit consistsof garnet
Fig. 19. A seriesof model sectionsat constantT, P, and G3 andbiotiteB3.The process
continuesuntil theefectivebulk
humidityshowingequilibriumcompositions
garnet compositioncontainsno garnet,i.e., that difrrsionin garnetis
of coexisting
andbiotitein an Fe-Mn-Mgternary.FromRobilsonet al. (1982). so slow that the mineralis incapableof reactingwith biotite.
FromRobinsonet al. (1982).
easy communication with other matrix ferromagnesian
minerals. The fact that the Mn-content increasesoutward
(Fig. l8) suggeststhe action ofa net-transfer, fractional,
garnet-resorption,retrogradehydration reaction, as illustrated in Figure 10, such as GAR + KSP + H,O : SILL
+ BIO. Commonly the amount of fluid available for such
a reaction would be limited and, becauseof limited diffusion, would involve only a limited volume of the garnet, while involving virtually all of the matrix. The reaction is fractional because the garnet interior is
progressively removed from participation in the retrogradereaction, so that a decreasingvolume ofthe exterior
is subjected to more and more extreme reactions. The
effective volume of reequilibration of garnet continues to
fall, until it approacheszero and the garnet ceasesto be
in equilibrium with the matrix minerals, although, as
pointed out above, its surfaces may still be subject to
surfacecorrosion that could contribute to the bulk chemistry of matrix assemblages.
In 1982we modelledthis processin a very simple way
in an Mn-Fe-Mg ternary (Robinsonet al., 1982)that is
now being duplicated more elegantly in computer programs by Frank Spearand his students.The first stepwas
to select a seriesofequilibrium sections(Fig. 19) conesponding to a seriesof changingmetamorphic conditions
of temperature,pressure,and humidity. The figure shows
putative garnet-biotite tie lines at a seriesof temperatures
and a seriesof progressivelymore humid conditions with
falling temperature. A similar series of templates could
600"c
6250C
650"C
675"C
700"c
Fig. 20. Model equilibrium garnet composition Fe-Mn-Mg trajectories for two different initial garnet-biotite ratios during
progressofthe hydration resorption reaction garnet + orthoclase + H2O : sillimanite + biotite. In the upper diagram the hydration
takes place with falling temperaturefrom 700 to 600 "C; in the lower diagram at constant temperatureof 625 'C. In each casethe
equilibrium garnet composition trajectory is longer and more curved for the more biotite-rich bulk composirion. Frorn Robinson
et al. (1982).
ROBINSON: EYE OF THE PETROGRAPHER, MIND OF THE PETROLOGIST
17 8 9
Fig. 22. Comparisonsof model equilibrium composition trajectoriesand fractional crystallization trajectoriesduring retrogade
garnet resorption for two paths of cooling and hydration starting with the same initial bulk composition. Note that the fractional
paths are more strongly curved than the equilibrium ones. The actual zoning path for the garnet in 9338 (Fig. 18) is shown for
comparison.From Robinsonet al. (1982).
be constructed for retrogradehydration at constant temperature. Using such templates, a series of equilibrium
garnet-biotite tie lines (Fig. 20) has been constructed for
a series of changing conditions for two different bulk
compositions, both originally on the same gamet-biotite
tie line at the peak of metamorphism. One can see that
the equilibrium paths of garnet for the two bulk compositions are quite different and that garnet composition
changesmore for bulk compositions that are poorer in
garnet. This simple geometrical concept was well understood by Ferry and Spear(1978) when they designedtheir
geothermometricexperiments.In order to avoid disequilibrium problems with zoned garnet, they designedtheir
experimentswith large ratios of garnet to biotite so that
there would be little tendency for composition changein
garnet and they could study composition changesin biotite, which reachedequilibrium easily. It should be emphasizedthat the equilibrium paths illustrated in this figure are only indirectly related to the zoning in natural
resorbed garnet, which is produced by a fractional disequilibrium process.
The fractional aspect is illustrated in Figure 2l in a
seriesofsteps involving alternate equilibration and fractionation, in which progressivelylessand lessgarnet participates, thus decreasingthe garnet to biotite ratio. As
the fraction ofparticipating garnet decreases,the change
in composition of the remaining portion becomesmore
and more extreme, while changesin biotite composition
decreasedramatically. For this reason the fractional crystallization paths (Fig. 22) arc more strongly curved than
the equilibrium ones, but the exact details depend critically on the extent of fractionation. Obviously, this type
ofreaction ceaseswhen the diffttsion is so slow that garnet
cannot participateat all.
A seriousconsequenceofthis type ofreaction, in cases
where garnet and biotite start out in roughly equal proportions, is that none of the original biotite composition
il
I
RETROGRADE
CONTINUOUS
Fe
Fig. 23. Consequencesfor geothermometry of a diffirsional
retrograde continuous net-transfer garnet-resorption hydration
reaction, here wiewedin an orthoclaseAFM projection. As the
resorption reaction proceeds, equilibrium garnet and biotite
compositions both move toward Fe enrichment. The original
garnet composition Gl is protected from reaction in the interior,
whereasall or nearly all of the original biotite Bl in the matrix
is destroyed,thus removing the possibility of directly determining the original peak metamorphic temperature. Garnet G2 and
matrix biotite 82 may be usedto estimatethe temoeratureunder
which the retrograde reaction ceased.When garnet Gl is paired
with biotite 82, with which it was never in equilibrium, a completely false temperatureestimate is made, which can normally
be much higher, even hundreds ofdegrees, than the original peak
temperature.From Robinson et al. (1982).
1790
ROBINSON: EYE OF THE PETROGRAPHER. MIND OF THE PETROLOGIST
1.00
I
NHI
I
.96
bo
T
(r .
v() .88
.84
o
.80
I
200
400
600
R a d i u s( p m )
800
1000
800
1000
PELHAM
DOME
900
\.
^ 800
o
E 7oo
;;;i;;;^;/
F ooo
500 L
0
2N
400
600
Radius(pm)
Fig.24. Profilesof garnetcompositionandcalculated
garnetFig. 25. Map showing the Pelham dome relative to the rebiotite temperature
at temperatures
from 725 to 500qC.In the gional geology and metamorphic zones of west-central Massaparticularmodelproduced,the bulk garnet/biotiteratio began chusetts.Narrow belt shown in black in the southern part ofthe
nearl: l, anda diflirsionalretrograde
garnet-resorp- dome is the Mount Mineral Formation containing relict granunet-transfer
tion reactionwith a majorbiotitereservoirwasassumed
to take lite-facies assemblages.Adapted from Roll (1987).
placebetween725 and 675'C. This wasfollowedby pure ion
exchange
from 675 and 500qC.Note that althoughthe interior
ofthe garnetis hardlychanged
in composition,Z(calculated)
for staurolite), and they retain relics of an older granulitethe corerisesdramaticallyfrom 725 to 900'C as a resultof a facies metamorphism shown by relict high orthoclase with
major increasein Fe/(Fe + Mg) of matrix biotite as the garnet sillimanite inclusions, garnet cores with pyrope content
rim is resorbed.This is followedby a declinefrom 900 to 875 up to 350/0,coarse sillimanite, and relict associations of
in Fe/(Fe+ Mg) duringion exchange.
"C asthe biotite decreases
(1991).
FromSpearandFlorence
remains that was originally in equilibrium with the garnet
core, unlessa $ain remained in some unusual protected
location. Thus, when an unaltered garnet core composition, such as Gl in Figure 23, is paired to make a temperature estimate with a highly altered matrix composition such as B2, the result is always higher than the original
peak temperature. In some casesthe result may be hundreds of degreeshigher, as pointed out by Robinson et
al. (1975) and illustrated in a recent calculated profile
Gie.2q by Spearand Florence(1991).
The style of reaction just discussedis illustrated in a
spectacularway by zoned garnetsstudied in detail in the
Pelham gneiss dome, Massachusetts(Fig. 25), by Roll
(1987). These rocks occur in the Mount Mineral Formation within the area of Zone I (kvanite-muscovite-
Fig. 26. Schist sample Q-P25c from the lower part of the
Mount Mineral Formation, east limb of Pelham dome, central
Massachusetts.Contains a shearedrelic ofhigh orthoclaseabout
2.5 cm long with tails of secondaryplagioclaseand muscovite
set in a matrix of schist rich in muscovite, biotite, kyanite, and
garnet.
t79r
ROBINSON: EYE OF THE PETROGRAPHER, MIND OF THE PETROLOGIST
Fig. 27. Photomicrograph
of schisrsampleW67Bfrom the
upperpart of the Mount MineralFormation,eastlimb of Pelhamdome.To left is the marginof a high orthoclase
megacryst
enclosingquartz,plagioclase,
and sillimanite(stoutneedlesfar
left).The immediatelysurrounding
matrix (right)containsmuscovite,biotite,andcoarsekyanite(largegraincloseto orthoclase
right center).
garnetwith coarserutile. Theseoccur in a variety of rocks
with a mylonitic character related to late shearingin the
dome, and they commonly occur side by side with rocks
that have been so completelyreconstitutedthat they show
few or no traces of the older metamorphism and only
featuresof late mineral growth.
The sample in Figure 26 contains a sheared relic of
high orthoclase with tails of secondary plagioclaseand
muscovite set in a matrix of schist rich in muscovite,
biotite, kyanite, and garnet. Figure 27 shows the margin
of a high orthoclasemegacrystenclosingsillimanite. The
matrix in the immediate areacontains muscovite. biotite.
and coarsekyanite. In Figure 28, large garnet megacrysts
surrounded by orthoclase and sillimanite are enclosed
by thick rims of biotite produced by retrogradehydration
reactions.One of the samplesstudied in detail was a my-
Fig. 28. Schistsample l60C originally collectedby L.D. Ashwalin 1972 from the middle of the Mount Mineral Formation,
eastlimb of the Pelham dome. Garnet megacrystsup to 1.5 cm
long, surrounded by orthoclase and sillimanite, are enveloped
by thick rims ofbiotite produced by retrogradehydration reactions.
@
Fig. 29. Garnetcompositionmapsfrom samplel60X (see
previouscaption)with analyticalpointsindicatedby dots.Top
down
map showscontoursof pyropecontentrangingfrom 34olo
intervals.Bottornmap showscontoursof spessarto l2o/oat 2olo
tine contentof 2o/oand 30/0.Verticalruling indicatesareasof
brown biotite; dashedhorizontalruling indicatesareasof pale
greenbiotite.Adaptedfrom Roll (1987).
lonitic garnetgneiss,sample 160X, from which the garnet
composition map in Figure 29 and the composition trajectory diagram in Figure 30 were produced. These garnets show extreme zoning in pyrope content and Fe/Mg
ratio and a slight marginal increasein Mn. The pyrope
composition map shows that the large garnet was being
resorbedalong a seriesofcracks that look like fiords on
some island off the Norwegian Coast. The relict cores in
pyrope and l.l0lo spessartine.
sample l60X contain 350/o
From the core to the rim, the pyrope content decreases
continuously to l0-llo/o and spessartineincreases
smoothly to a maximum of about 3ol0.A secondgarnet
from a similar rock (Fig. 3l) is a more regular egg-shaped
porphyroclast showing an irregular pyrope-rich interior
and a thin pyrope-poor, spessartine-enrichedrim and is
more amenable to diffirsional modelling.
Roll modeled the time required to produce such resorption rims at 580 lC, 5 kbar using three different sets
of difusion coemcients(Fig. 32). Under such conditions
it would take 40 m.y. for diffirsional penetration of the
garnet to a depth of 0.05 cm, rather longer than appeared
to be available during Acadian metamorphism. Basedon
theseresults, she suggestedthat the retrograderesorption
zoning might have been more easily produced in two episodes,one during cooling at the end ofan early, probably
t792
ROBINSON: EYE OF THE PETROGRAPHER, MIND OF THE PETROLOGIST
a
E
o
O8O
580oC, 5 kbar
I
li
for 40 Ma
o7s
F
Fe
10
20
30
Mg_
40
Fig. 30. Compositional trajectoriesofzoned garnetsfrom the
Mount Mineral Formation. Pelham dome. central Massachusetts,in terms of Fe, Mn, and Mg. C indicates core, R indicates
rim. Composition maps for thesegarnetsare given in Figure 29
(160X), Figure 31 (W67B), and Figure 33 (X43A). All come
from porphyroclastic schists with other relics of original near
granulite-faciesconditions, in which garnet has been resorbed
by the reaction garnet + orthoclase + HrO : alurninum silicate
+ biotite. From Roll (1987).
Proterozoic, high-grade metamorphism and a second
during Acadian retrograde hydration. These views require reinterpretation based on new detailed isotopic
studies by R. D. Tucker (Tucker et al., 1988; Tucker and
Robinson, l99l; Robinson and Tucker, l99l). Zircon and
monzvite from a sillimanite-orthoclase pegmatite within
the schist in question yielded nearly concordant U-Pb
ages of 357 and 367 Ma, respectively, and monazite from
the schist gave 367 Ma, precluding an early metamor-
c
E
ooo
o 02
0,o4
005
0 0a
o 10
Distonce (cm)
Fig.32. Model penetration distancesproduced in hornogeneous garnet 2 mm in diameter of 350/opyrope composition by
retrogradereequilibration at 580 'C, 5 kbar for 40 m.y. Gradients are based on three sets ofdiffusion coefrcients: F: Freer
(1979);C: Cyganand Lasaga(1985);E : Elphick et al. (1985).
Under such conditions it would take 40 m.y. for diffusional penetration of the garnet to a depth of 0.05 cm. The model doesnot
take into accountthe movement ofthe garnetinterfaceproduced
by resorption.From Roll (1987).
phism much before 360 Ma. The quartzite at the top of
the Mount Mineral Formation contains virtually concordant single detrital zircons of a range of ages including
grains with ages of 458, 440, and 439 Ma, showing that
the quartzite itselfcould be no older than Early Silurian.
The same quartzite contains the extremely retrogradezoned, Mn-rich garnet described below and has yielded
Fig. 31. Composition maps of egg-shapedporphyroclasticgarnet from sample W67B of the Mount Mineral Formation (seeFig.
27). Contours basedon Me/(Fe + Mg + Mn + Ca), etc., are Py 16-220/0at 20lointervals, Sp 4-690 at l0lointervals,andGr 4-5o/o
at 1olointervals. Maps show rimward decreasein pyrope and increase in spessartineconsistent with resorption ofgarnet by reaction
with matrix through the medium of interstitial fluid. Adapted from Roll (1987).
ROBINSON: EYE OF THE PETROGRAPHER, MIND OF THE PETROLOGIST
garnetfrom
Fig. 33. Compositionmapsof smallfragmented
sampleX43A from theMount MineralFormationon thesouthwestlimb of the Pelhamdome.Contoursbasedon Mg/(Fe +
intervals,Sp l-40lo
Mg + Mn + Ca),etc.,arePy l4-34o/oat 20lo
at l0lointervals,Gr l-4o/oat lYointervals.Althoughonly I x 3
mm in size,it containsalmostthe samerangeof pyropecompositionsas the much largergametin Figure29. Probablythe
garnetis a shearedfragmentfrom the interior of a oncelarger
garnetthat wasonly tectonicallyexposedto the matnx over a
relativelyshortperiodnearthe end of the late reequilibration.
Adaptedfrom Roll (1987).
a recrystallization age of metamorphic monazite of 297
Ma, consistent with metamorphic sphene ages (Tucker
and Robinson, 1990),and Rb-Sr and Nd-Sm mineral isochrons (Gromet and Robinson, 1990)nearbyin the dome.
Thus, it appearsthat the relict granulite-faciesmetamorphism was Acadian and the kyanite-muscovite overprint
was late Paleozoic,not Acadian. The boundary between
this late overprint and true Acadian isogradsfarther east
is being actively investigated.
In sample X43 (Fig. 33) almost the same range of pyrope compositions was obtained (Fig. 30) in a grain I x
3 mm in size.In this caseit seemshardlv likely that such
Fe
lO
20
30
40
Mq+
Fig.34. Summary diagram of the zoning trajectoriesfor garnets in seven specimensof the Mount Mineral Formation studied by Roll (1987). X43A, 160X, and W67B are from the schists
described above. M22A is from a sillimanite-orthoclase pegmatite. The Fe-Mn-rich garnet is from a nearby garnet quartzite
shown with contours for spessartineand grossularin Figure 35.
There is little to no zoning in pyrope content, and the trajectory
is consistent with a resorption reaction involving formation of
Fe-rich biotite. Specimensl60M and, especially,Y33 belong to
rocks that appearto have been totally reconstituted,with garnet
newly grown during the late hydrous metamorphic overprint, as
discussedin the text.
t793
porphyroclastic
Fig. 35. Compositionmaps of egg-shaped
garnetfrom quartzitesampleM2l nearthe uppercontactof the
MountMineralFormationon theeastlimb of thePelhamdome.
Contoursat intervalsof l9o basedon Mn/(Fe + Mg + Mn +
Gr 6-80/0.Mapsshowrimwardincreasein
Ca),etc.,Sp 3-140/0,
with resorptionofgarnetby reactionwith
consistent
spessartine
the matrix biotite throughthe medium of interstitial fluid. Decontoursnear
pressionhachures
areomittedfrom closelyspaced
edgeofgarnet.Adaptedfrom Roll (1987).
a steepgradient could have been preservedthrough even
a single retrograde event at 580 'C, and more probably
the garnet is a sheared fragment from the interior of a
once larger garnet that was only tectonically exposedto
the matrix over a relatively short period near the end of
the late reequilibration.
Figure 34 summarizes the zoning trajectories fot garnets in seven diferent specimensin the Mount Mineral
Formation studied by Roll. Three are from the schists
already described,and a fourth is similar in form but is
from a sillimanite-orthoclasepegmatite. The Fe-Mn rich
garnet, from the quartzite at the top of the formation, is
shown in maps in Figure 35, with contours for grossular
and spessartine.In this specimenthere is little to no zoning in pyrope content and the trajectory is consistentwith
a resorption reaction involving formation of Fe-rich biotite.
In Figure 36 biotite compositions from all sevenrocks
are plotted in terms of Ti per I I O atoms and the ratio
Mg/(Mg+Fe). Ti content for a given Mg ratio has been
shown to be a function of metamorphic grade in rocks
saturatedwith ilmenite or rutile as well as aluminum silicate (Robinsonet al., 1982;Schumacheret al., 1990).In
theserocks variable Ti content seemsto be related in part
to extent of participation in retrograde resorption reactions and proximity to grains of Ti oxide minerals. The
extreme example is biotite from deep within the "fiords"
of the garnet l60X in Figure 29, which are colorless to
pale green in thin section and contain less than 0.025 Ti
per I I O atoms. Becausethese fiord biotite grains were
clearly produced by the reaction of a K-bearing matrix
solution upon the enclosing walls of garnet, it appears
t794
ROBINSON: EYE OF THE PETROGRAPHER, MIND OF THE PETROLOGIST
25
20
BIOTITE
@
@ wet
w
a
a
M21
a
a
Ti
15
1l
Ox' 1 0
o
Oa
$o
o
a
Fig.38. Compositionmapsof euhedralgarnetin sampleY33
from schistnearthe uppercontactof the Mount MineralFormationon the southwest
flankof the Pelhamdome.This schist
is believedto havebeencompletelyreconstituted
duringthelate
metamorphicreequilibration.Maps showrimwarddecrease
rn
pyrope,spessartine,
and grossularcontentconsistenlwith retrogradegarnetgrowth.Adaptedfrom Roll (1987).
%t
oao
s v ' o . 16ox
o
vrr,V_l
16oMy
-v
a
o
o5
A
Ol
O,2 O.3 04
05 S"
OZ OB 09
Mg/(Mg+Fe)
Fig. 36. Biotite compositions from all sevenrocks plotted in
terms of Ti per I I O atoms and the ratio Mg/(Mg + Fe). Samples
shown are W67 (squares),X43 (circles), l60X (rriangles),y33
and 160M (inverted triangles),M22 (small diamonds), and M2l
(large diamonds). Inclusion biotite (closed symbols) is more
magnesian than corresponding rim (open symbols) or matrix
(ruled symbols)biotite. Dotted trianglesindicate biotite in l60X
that fill cracks in garnet. From Roll (1987).
that under the ambient retrograde conditions estimated
at about 550'C, the matrix solution had not dissolvedor
did not contain enough Ti to produce a biotite with a
normal Ti content typically found in matrix locations in
close physical contact with Ti-oxide grains.
In three of the samples,M2l, W67, and X43, compositions were obtained both on matrix biotite and on
biotite included within garnets. Inclusion biotites (Fig.
36) are consistentlymore magnesianthan matrix biotites,
in agreementwith the two different processesby which
+M
\\
continuous
+
hydration
\,
ion"id"ng"
Fig. 37. SchematicAFM projection, showing the contrasting
effectson biotite compositions ofcontinuous retrogradehydration on matrix biotite and of retrogradeion exchangeon inclusion biotite. As hydration proceeds,equilibrium tie lines between garnet rims and matrix biotite move to the left, whereas
inclusion biotite moves to the right. The original peak biotite
composition is no longer preservedbut presumably had a composition intermediate betweenmatrix biotite and biotite included in garnet cores.From a diagram preparedby M.A. Roll (personal communication, I 986).
Fe
10
zo
ao
Mo*
Fig. 39. Comparisons of composition trajectories of garnet
in W67B and in Y33. In eachexample,garnet core has the most
Mg-rich composition. W67B is consistentwith a retrogradegarnet resorption reaction, whereas Y33 is consistent with retrograde growth. Adapted from Roll (1987).
ROBINSON: EYE OF THE PETROGRAPHER, MIND OF THE PETROLOGIST
t795
GAR+MUS=BlO+Alsil
=ffi
-
/-
lt
I
-
z-
a'
1
--t
--t
-'
7='j=-
---
r, oc Blo + Alsil = GAR + KSP + H2o
XFE GAR
XpE BIO
Fig. 40. P-T diagram with garnet and biotite composition
isopleths for the reaction garnet + muscovite : biotite + aluminum silicate. Note that isopleths change orientation when
crossingaluminum silicate field boundaries and when crossing
the muscovite-out reaction in lower right. Quadrants of garnet
production (P) and garnet consumption (C) are indicated for the
kyanite and sillimanite zones.Heavy lines with arrows represent
three retrogradeP-T trajectories illustrated in quasi-binary sections in Figure 41: one parallel to garnet isopleths (Fig. alA),
one parallel to biotite isopleths (Fig. 1O, and one intermediate
(Fig. 41B). Adapted from Spearand Selverstone(1983).
their compositions were achieved. The matrix biotite
formed by continuous retrograde net-transfer garnet resorption in which biotite grew in amount and became
progressivelymore Fe enriched.The inclusion biotite did
not changein amount but becameMg enriched as a result
oflocal ion exchangewith the enclosinggarnet, probably
producing a small Mg-impoverished compositional well
in the garnet,suchas that describedby Spearet al. (l 990).
It is apparent that if Fe-enriched matrix biotite is
matched with the relict core garnet composition with
which it was never in equilibrium, an artificially hightemperature estimate will be obtained (seealso Fig.2q.
In the case of sample l60X such a procedure gives an
estimated temperature of 1000 oC.By contrast, matching
of inclusion biotites with immediately surrounding garnets yields estimatesof temperatureof final ion exchange
equilibration of about 450 qC. A major problem is to
locate and analyze, or to estimate, the composition of
biotite that was originally presentin the rock at the peak
of granulite-faciesconditions. Spear et al. (1990) managed to locate biotite inclusions within sillimanite that
had apparently experienced little of the retrograde reequilibration describedhere. In the Pelham dome rocks,
the best estimate of original biotite composition was made
by choosing one intermediate between matrix and inclusion extremes(Fig. 37) and also one that would yield an
estimated temperature and pressureconsistent with the
presenceof sillimanite in the rock. On this basis,a biotite
composition chosenfor sample l60X with Mg/(Mg+Fe)
: 0.66 yields conditions of equilibration with sillimanite,
garnet, and high orthoclaseof 700 "C and 6.8 kbar (Roll,
I 987).
1796
ROBINSON: EYE OF THE PETROGRAPHER. MIND OF THE PETROLOGIST
Sill I
r.o
r
o.8
0.6
XFe
\Bio
0.4
0.8
0.6
Xre
0.4
0.6
o.8
0.4
Xre
Fig. 41. Binary representationsin terms of ?"-Pand XF" ofthe phaserelations along the three retrogradetrajectories illustrated
in Figure 40. For details seetext. Modified and expandedfrom Roll (1987).
Zoning producedduring retrograde garnet growth
The two garnet grains with short zoning trajectories in
Figure 34 are from schists of the Mount Mineral Formation that appearto have beencompletely reconstituted
during the second more hydrous metamorphism. These
garnetsare typically euhedral (Fie. 38) with Mn- and Caenriched coresand might be supposedto have formed by
prograde Efowth zoning. However, unlike such garnets,
they do not show a typical increasein the Mg/(Mg + Fe)
but rather a decrease,resemblingin this way zoning profiles produced by retrograde garnet-resorptionreactions.
Chemical trajectories of the two types of garnet are compared in Figure 39. The garnet in sample W67B shows
the typical outward increasein both Ca and Mn associated with diffusive rim resorption of the Mg component
into other ferromagnesianminerals. The garnetin sample
Y33, on the other hand, shows rimward decreaseof Ca
and Mn that can only be associatedwith garnet growth
becausethere is no other mineral in the rock that could
be proportionally absorbing more Mn than the garnet.
Peter J. Thompson (1985) has describedinteresting
garnets from the Mount Monadnock area, New Hampshire, in which there is a similar strong rimward decrease
in Mg/(Mg * Fe) accompanied by a similar decreasein
Mn but with a striking rimward increase in Ca. In this
example, it appearsthat the pyrope content ofthe garnet
was undergoingprogressiveresorption with respectto the
cordierite in the rock, but at the same time the grossular
content ofthe garnet was undergoing progressivegrowth
with respect to the anorthite component of plagioclase
that slightly overbalancedthe resorption. In this example,
as in all of the others discussedhere, the Mn profile is
the most reliable predictor of growth vs. resorption because Mn is more strongly fractionated into aluminous
garnet than any other phase.
It is a common habit of metamorphic petrologists to
think of garnet growth in terms of a prograde reaction
and resorption as a retrograde (i.e. decreasingtemperature) phenomenon.It is further habitual to visualize such
reactions in terms of binary Z-X loops. Study of composition isopleths for a reaction on a P-T diagram illustrates the true complexity even in an oversimplified quasi-binary situation. For this purpose,Ihe Xr. isopleths for
garnetand biotite derived by Spearand Selverstone(1983)
for the reaction garnet + muscovite : aluminum silicate
+ biotite serve as an admirable example, as shown in
Figure 40. In the habitual view, one may cross the isopleths by increasingpressureand increasingtemperature
slightly such that garnet is produced, biotite is consumed,
and progressivelylower X." isopleths for both minerals
are crossed. Similarly, one may cross isopleths by decreasingpressureand temperature such that garnet is resorbed, biotite grows, and progressivelyhigher Xr" isopleths for both minerals are crossed.Thesetwo directions
toward garnet production or consumption (P and C in
Fig. a0) He in only two of the possible four quadrants
between the isopleths.
Relations within the remaining two quadrantsare more
complex. In these quadrants the directions ofgarnet and
biotite composition change are opposite, and for each
P-I trajectory, garnet production or consumption will
depend critically on the garnet-biotite ratio in the rock.
The relations in thesequadrants are illustrated by PT-X-.
sectionsfrom Figure 40 taken to follow three specifictrajectories. Figure 4lA showsa trajectory constrainedalong
the Garnet Xo. : 0.85 isopleth for the kyanite zone. This
PZ-X section is oriented with increasing T at the top,
which is also the direction of decreasingP. Within the
kyanite zone,as specified,garnetcomposition is constant,
but along the same trajectory at lower temperaturein the
ROBINSON: EYE OF THE PETROGRAPHER. MIND OF THE PETROLOGIST
t797
Bio:
Fig.42. Pavement
outcropof typicalmigmatiticschistof the
zone,centralMassachusillimanite-orthoclase-garnet-cordierite
arebelievedto be the prodsetts.Largegarnetsand leucosomes
uct offluid-absentpartial melting.SeeRobinsonet al. (1986),
stop4.
garnet-chloritezone and also at higher temperaturein the
sillimanite zone, garnet is more Fe-rich. Taking a typical
bulk composition along the dashed line and following it
in Figure 4lA in the kyanite zone only, it can be seen
that garnet is consumedwith increasingtemperature(i.e.,
the bulk composition on the garnet-biotite tie line lies
progressivelycloser to the biotite end) and is produced
as temperature decreases.
Figure 4lC shows a trajectory constrained along the
biotite isopleth Xr.: 0.45 for the kyanite zone.This section is also oriented with increasing T at the top, but in
this case,this is also the direction of slightly increasing
P. Taking the same typical bulk composition along the
O.5mm
Ftg. 44. Compositionmapsof garnetin sampleFW407of
schistfrom Zone
sillimanite-orthoclase-garnet-cordierite-biotite
GarnetshowssignificantFe enrichVI, centralMassachusetts.
ment and Mg depletiononly wherein directcontactwith adjaFrom Tracyet
ion exchange.
centbiotite,permittingretrograde
al.(1976).SeeRobinsonet al. (1986),stop6.
dashedline and foltowing it in Figure 4lC,it can be seen
that garnet is produced in an up-temperature direction
(i.e., the bulk composition lies progressivelycloser to the
garnet ends of the tie lines) and is consumed in a downtemperature direction.
Figure 4lB is a PT-X*. section for a trajectory between
the two bounding examplesin 4lA and 4lC. On this
trajectory, with increasing temperature, garnet Xo. decreaseswhile biotite Xo" increases,consistent with the
decreasing distribution coefficient between these two
phases.Note that for Fe-poor compositions (X), garnet is
consumed with increasing temperature even though the
garnet itself decreasesin X.. until the bulk composition
consists only of biotite. Similarly for Fe-rich compositions (Y), garnet is produced with increasingtemperature.
With falling temperature along the same trajectory, garnet is produced in Fe-poor compositions (X) and is consumed in Fe-rich compositions (I').
The garnet-biotite two-phaseregion in a profile such as
that in Figure 4lB is divided into regions of production
and consumption by a vertical dash-dot line, which is
positioned differently for each trajectory. To the left of
this boundary, as in composition Y, garnet is produced
increasingtemperatureand consumedwith decreasMso
with
Feo
To the right of this boundary, as in coming
temperature.
Fig. 43. AFM projection showing mineral compositions in
typical assemblagesof pelitic schist of Zone VI, central Massa- position X, garnet is consumed with increasing temperchusetts.From Robinson et al. (1982).
ature and produced with decreasing temperature. By
r7 9 8
ROBINSON: EYE OF THE PETROGRAPHER, MIND OF THE PETROLOGIST
.-------.s--4.4o7
o-.?4.st.FWl54
FWt22
Ir/g
Fig. 45. Compositionaltrajectoriesof zonedgarnetsfrom
Zone Yl, centralMassachusetts,
in termsof Fe, Mn, and Mg.
Typicalcompositions
arethe mostMg rich, andzoningto more
Fe-richcompositions
occursonly wheregarnetis in directcontact with biotite or cordierite.From Tracvet al. (1976).
comparing Figures 4l B and 4lC, it will be seenthat the
more nearly parallel the P-T trajectory is to the biotite
isopleths, the larger the region for retrogradegamet consumption and the smaller the region for retrograde production, until, in the extreme example parallel to the biotite isopleth, there is no region for retrogradeproduction.
In the trajectory of Figure 4lB, the potential for retrogradegarnet production is proportional to the distance
from the production-consumption boundary. This is illustrated in two calculated examplesshowing the change
from 600 to 550 'C, in which Xo. garnet changesfrom
0.832to 0.845.For a bulk compositionXo": 0.593,the
mole percent garnet will increasefrom 30 to 33, whereas
for a bulk composition XF, : 0.507, the mole percent
garnet will increasefrom 5 to 10.5. In the caseofgarnet
growth by fractional crystallization, the interior of the
garnet will be effectively locked away from the exterior
bulk assemblageso that the effective bulk composition
will becomeprogressivelymore biotite rich. This will drive
the effective bulk composition toward lower Xo., further
into the region of retrograde production of garnet with
decreasingtemperature.If, in the above examplesof cooling from 600 to 550 oC,there is fractional crystallization
during retrograde $owth such that half of the garnet is
taken out of reaction at 575 "C (seedotted lines in Fig.
4lB), then the effective bulk composition will be even
Al
Fe
Mg
Fig. 46. AFM orthoclase projection of garnet and biotite
compositions, illustrating the relatively simple interpretation required in casesof retrograde ion exchange.From Robinson et
al. (1982).
Fig.47. Photograph
whitequartzose
of even-grained
marble
with brightly reflectingelongategrains of white wollastonite.
Specimenwascollectedfrom a gaspipelinetrenchon the east
Mountainby Berry( I 989),from the heartof
sideof Rattlesnake
zoneVI, centralMassachusetts.
more biotite rich and still more capable of growing garnet: in the two cases,from 30 to 33.7o/oor from 5 to
ll.7o/0.
In Figure 4lB, it will be seenthat the best opportunity
for retrograde growth zoning with a substantial garnet
composition changewill be in a trajectory at a slight angle
to biotite isopleths where a small biotite-rich composition region will be subject to garnet growth. Fractional
crystallization will continue to maintain the effectivebulk
composition in this biotite-rich regionwheregarnetgrowth
can continue. Such a situation of garnet-growth zoning
with falling temperature appears to be the origin of the
zoned garnetsin completely reconstituted schistssuch as
sampleY33 (Fig. 39).
Zoning producedduring retrograde ion exchange
The pelites of the sillimanite-orthoclase-garnet-cordierite zone (Zone VI) in central Massachusettsare typified
by leucosomesof quartz, feldspar, and cordierite commonly containing garnet up to 2 to 3 cm in diameter (Fig.
42). Such textural features have been described as the
product of fluid-absent melting (Tracy and Robinson,
1983).Typical assemblages
and mineral compositionsare
shown in an orthoclase projection in Figure 43. Garnet
is typically about 5-100/omore magnesianthan garnet in
Zones IV and V, indicating considerableprogressofthe
strong dehydration reaction biotite + sillimanite : orthoclase * garnet + HrO. Very locally there is evidence
of late retrograde hydration reactions between garnet or
cordierite and orthoclaseto produce sillimanite + biotite,
but this is not typical. The common four-phaseassemblage in projection may indicate that HrO was behaving
as an inert component in the absenceofan aqueousfluid.
Typically both larger and smaller garnets are homogeneousfrom core to rim exceptat points where the garnets
are in direct contact with another ferromagnesianmineral, either cordierite or biotite (Fig. aq. This style of
zoning was first describedin this region by Hess (1969,
1799
ROBINSON: EYE OF THE PETROGRAPHER. MIND OF THE PETROLOGIST
regions
Fig. 49. Thin-sectionsketchshowinggarnet-bearing
of anorthite+ calciteaway
closeto diopsideand preservation
FromBerry(1989).
from diopside.
5mm
o
pS76oroesoefilcARNEr'Jy'
Eu
GMINS Lj'^J BAARNG
REGoN
f_l
L-I
wotlAsToNm- WV)PtAGtoctlsE+
BEARING
REGIoN
I2-4
CNJCfiE
REGoN
marble,showing
Fig.48. Thin-sectionsketchof wollastonite
regions
subdivisionof the rock into regions.wollastonite-bearing
by areaswith quartz-calcite.
havea veinlikecharacter,separated
Plagioclase-calcite
regionsoccurpredominantlyawayfrom diand calciteare
opside,whereascloseto diopside,plagioclase
by garnet.From Berry(1989).
separated
l97l). In theselocations,the Fe-enrichedgarnet rim is
accompanied by zoned biotite or cordierite that is enriched in Mg as compared to biotite or cordierite that is
in the matrix away from the garnets.The local chemical
zoning trajectories (Fig. a5) are virtually flat with respect
to Mq strongly indicating that there has been neither
garnet resorption nor garnet $owth. This type of zoning
is most easily explained as the product of local retrograde
ion exchangebetween garnet and surrounding ferromagnesianminerals where they are in direct contact and without participation ofan interstitial fluid. It is a strong argument that at peak conditions pervasive fluids were not
present in most zone VI rocks. If no aqueous fluid was
present,then we must appealto straight difusion through
the matrix or localized ion exchange at grain-to-grain
contacts.The latter was first observedand studied in central Massachusettsby P. C. Hess (1969, l97l), and his
observationshave been repeatedcountlesstimes.
The interpretation of thesekinds of garnetsin terms of
geothermometry is apparently much simpler. The homogeneousgarnet composition can be compared with the
matrix biotite composition away from the garnet to estimate peak conditions (Fig. 46). The garnet rim com-
position and the adjacent biotite or cordierite composition can be used to estimate the temperature of last
retrograde ion exchange. The only question might be
whether some other processintervened betweenthe peak
metamorphism and the retrograde ion exchangethat could
have effectively changedthe typical matrix biotite composition without also producing concentric zoning in the
garnet. In my opinion this is unlikely.
Conclusion concerninggarnet zoning
It seemsfairly clear that at temperaturesbelow about
500 "C, most garnet zoning can be interpreted in terms
of simple growth zoning. At higher temperatures,however, all sorts ofcontradictions are producedby diffusion,
which commonly makes it very difficult to selectthe correct mineral compositions for quantitative geothermometry and geobarometry. If all of the conditions and considerations outlined here are correctly accountedfor, and
one happened to be confronted with appropriate rocks,
then it may be possible to construct detailed P-7 paths
such as those ofSpear et al. (1990).
Pnlsn
RELATToNSrN A
WOLLASTONITE.BEARING
MARBLE
In the next section of this paper, I want to show how
detailed petrographicobservationsin a singlethin section
led to a new conception for a petrogeneticgrid. This conception comesfrom the Ph.D. thesisof Henry Berry (1989,
l99l) and concernsone aspectofhis interpretationofthe
first documented occurrence of regional metamorphic
wollastonite from the granulite-facies in New England,
near Sturbridge, in the center of Zone Yl of Figure 2.
This section is basedon a small highlight of a more comprehensivestudy that Berry is preparingfor separatepublication.
Petrography
The rock is an even-grained,foliated, white quartzose
marble with brightly reflecting elongate grains of white
r 800
ROBINSON: EYE OF THE PETROGRAPHER. MIND OF THE PETROLOGIST
sio2
iL,
si
a
*..
g,
eN
_t
. d.'
(Fe,Mg)O
Fig. 50. Photomicrograph
showingabundantgarnetcoronas
betweenanorthiteand calcitecloseto largediopsidegrain in
lowerright and absence
of suchcoronasbetweenanorthiteand
calcitein upperleft. Dark grainsare sphene.Width of view, 3
mm approximately.
providedby H.N. Berry.
Photograph
wollastonite(Fig.a7). The rock consistsmainly of quartz,
calcite, anorthite (An98), wollastonite, calcic scapolite
(-Me86), ferroan diopside [Fe/(Fe + Mg) : 0.40-0.45],
grossular-richgarnet (Gross88-90),and sphene.Locally
wollastonite appearsto be concentratedalong veinlike
surfaces that may have allowed concentration of early
metamorphicfluids.
An unusually detailed petrographicstudy of a single
thin section by Berry indicated different local chemical
environments that require treatment in different chemical
systems(Fig. a8). In the first place,the rock can be divided into regions containing qtartz + calcite (mainly
Ar2o3
Fig. 52. The chemicaltetrahedronSiO,-CaO-AlrOr-(Fe,
Mg)O,showingthecompositions
of quartz(QZ),anorthite(AN),
wollastonite(wO), diopside(DI), calcite(CC),and the garnet
solid solutiongrossular(GR) to almandine-pyrope
(ALM, PY).
FromBerry(1989).
plagioclase+ calcite regionsof Fig. 48) and those containing qtartz + wollastonite (wollastonite regionsof Fig.
48). Clearly conditions of the calcite + qvartz: wollastonite reaction had been reachedin someparts ofthe rock
but not in others, suggestinga very locally variable XCO,
of the fluid may have obtained at the peak of metamorphism.
Secondly, the rock can be divided into regions where
plagioclaseand calcite are in equilibrium and there is no
garnet, and other regions where plagioclaseand calcite
are separatedby abundant garnet. The regions of abundant garnet are invariably close to the widely scattered
grains of ferroan diopside. This is illustrated in Figure 49
and more dramatically in Figure 50 where garnet has
grown at the expenseof anorthite + calcite only in the
vicinity of diopside. The necessaryparticipation of ferroan diopside in garnet-forming reactionsis shown by the
analysedgarnet composition: grossular88-90, alman1.3-1.8,atdradite2.Tdine 5, pyrope0.5-0.7,spessartine
3.4. An analogousreactionhas taken placein wollastonite-bearing regionsnear diopside, where wollastonite and
anorthite are not in contact and anorthite is rimmed by
garnet(Fig. 5l). Thesecritical textural observationsemphasizedthe role of the diopside component in all the
garnet-producingreactionsin this rock and led to consideration of equilibria in a more complex system and the
derivation ofa specificnew petrogeneticgrid.
Chemography
Fig. 51. Thin-section sketch in wollastonite-bearingregion
with abundant diopside, showing garnet rim between anorthite
and wollastonite. Sketch provided by H.N. Berry.
The consideration of calcite-quartz-wollastoniteequilibria requires SiO,-CaO-H,O-COr,the addition of anorthite and grossular reactions requires the addition of
t80l
ROBINSON: EYE OF THE PETROGRAPHER. MIND OF THE PETROLOGIST
REACTION
CaO
'ff,
DI+ AN +CP2
QZ+Ct+GAR
REACNON
Azog
(Fe,Mg)O
ALM. PY
QZPROJECTION
DI+AN+CC
sio2-2Al2o3
+AN
DI+AN+qZ+CC
@
CaO-AlrO,
ANORTHITE
PROJECTION
Fig. 53. Quartz projection of phase compositions onto the
baseofthe tetrahedron ofFigure 52 and anorthite projection of
the same compositions onto the plane SiOr-CaO-(Fe,Mg)O. In
the anorthite projection ALM, PY garnetsplot at negativevalues
ofCaO. From Berry (1989).
QZ+CAR+CO2
REACTION
CO2
GAR+
AR+C
O2
Fig. 54. Anorthiteprojectionsshowingthreechemographies
of the equilibriumDI + AN + QZ + CC + GAR + CO' (reaction 14 of Berry, 1989)dependingon the equilibriumcompositionofgarnet.Othernumbersindicatecontinuousreactions
in detailby H.N. Berry.From Berry(1989).
described
A new petrogeneticgrid
In Figure 54 are illustrated the chemographicrelations
of the equilibrium quartz + calcite * anorthite + diopside * garnet (reaction l4), all obviously within the stability of calcite + qtrartz. If garnet is less grossularthan
the quartz-diopside-anorthite plane, as in reaction l4A,
AlrO., and the addition of ferroan diopside and the real then the join diopside-anorthite gives way to the assemgarnet compositions requires the addition of (Fe,Mg)O, blage quartz + calcite + garnet, consuming CO', and is
here lumped for simplicity as one component. The re- thus a garnet-forming reaction that would take place with
decreasingf. If the garnet composition is slightly more
sulting chemical tetrahedron is shown in Figure 52. All
the phasesplot at point compositionsexceptgarnet,which grossularthan the quartz-diopside-anorthiteplane, as in
has a theoretically complete rangefrom pure grossularto reaction l4B, then the plane diopside-anorthite-calcite
pure almandine-pyrope.When projected from the quartz gives way to quartz + garnet * minor CO, and is a garapex onto the base of the tetrahedron, the phase com- net-forming reaction that would take place with increaspositions appear as shown in Figure 53 (top). A more ing Z. If the garnet composition is more grossular than
helpful projection is from anorthite onto the plane SiOr- the diopside-calcite-anorthiteplane, as in reaction l4C,
CaO-(Fe,Mg)O (Fig. 53 bottom), which already contains then all the phasesquartz-calcite-diopside-anorthitegive
the phases qtrarlz, wollastonite, calcite, and diopside. way to garnet + a large amount of COr. The limiting
Garnet projects as a line below the diopside-wollastonite situation would be the reaction where quartz-calcite-anjoin. The grossular end of the series projects positively orthite gives way to grossular + CO, without participabetween calcite and wollastonite, but the almandine-py- tion of diopside.
Of course all of these segmentsare part of the same
rope end projects to negative values ofCaO well outside
the triangle ofprojection. The big advantageofthis pro- continuous reaction, which must change its P-I slope
jection is that it allows direct observation of the special drastically in the presenceof CO'-bearing fluid, and each
garnet compositions that are coplanar with other phases segmentis separatedfrom the next by an invariant point
in the tetrahedron, and it is used in all discussion that characterizedby a particular coplanarity of phases.Between reaction l4A and reaction l4B, invariant point
follows.
I 802
ROBINSON: EYE OF THE PETROGRAPHER, MIND OF THE PETROLOGIST
for garnet compositions below grossular 67 to a product
that becomesincreasinglyvoluminous to a maximum for
pure grossular.In the vicinity of 167, the CO, coemcient
is vanishing and there is no AS of devolatilization, and
hence the reaction is primarily pressuredependent.
From the above considerations,Berry constructed the
P-?"loop shown in Figure 56 (left). At each of the three
invariant points the curve of reaction 14 is tangent to a
secondunivariant curve. At 167 this is the fluid-independent reaction grossular 67 * quartz: diopside + anorthite, which has a metastableextension to lower temperature. This reaction is critical in subsequentdiscussions.
l.o
At I83 the secondunivariant curve is the decarbonation
T'
r.r.
ra
-bOc
>q:
reaction diopside + calcite + anorthite: grossular83 +
Fig. 55. Plot ofreactioncoefrcientsversusXG ofgarnetfor qtrartz + COr, which has a metastableextensionto higher
reaction14of H.N. Berry.PointsI67c,I83,andIl00c represent pressure.At Il00 the secondreaction is the decarbongarnetcompositions
at invariantpointsproducedby collineari- ation reaction calcite + anorthite + qvarlz: grossular
tiesofgarnetcompositionwith otherphases.
FromBerry(1989). 100 + COr. In this casethe reaction 14 curve comes in
tangent to the other reaction at an end point, and there
I67c representsthe copianarity of quartz-diopside-anor- are no metastable extensions.To me, the resulting diathite-grossular67. Betweenreaction l48 and reaction l4C, gram far exceedsthe worst nightmares encountered in
invariant point I83 representsthe coplanarity of diop- J. B. Thompson's phase equilibrium course thirty years
side-calcite-anorthite.Invariant point I I 00 representsthe ago. My student Berry had only to experience this by
limiting coplanarity of quartz-calcite-anorthitewith gros- proxy. Figure 56 (righ| shows the same equilibria but
sular. The varied stoichiometry of reaction 14 with vari- with the addition ofgarnet composition isoplethsfor varable grossularcontent ofgarnet is illustrated in Figure 55, ious continuous reactionswithin the region of stability of
showing how CO, changesdramatically from a reactant garnet + quartz-calcite-anorthite.
r A R>
Q Z + C C + A N + C a - p o o r eG
e/
Gr(67) GAR + QZ
C a - r i c h eG
r A R+ C O 2
iri;
N
+
o
r
@
A
z
+
cc
+
o
l8
/:
lo
l+
l6
6'
J/
ol
g
CC + Dl + AN + GAR >
G r ( 8 3 ) - r i c h o rG A R + C O 2
T+
Fig. 56. P-I diagrams in presenceof COr-bearing fluid, showing relations of reaction 14 of H.N. Berry to other reactions
involving garnet in the chemical system of Figure 54. Diagram to left shows intersections of reaction 14 with other reactions
involving collinearities ofgarnet compositions with other phases.Hachures on reaction 14 indicate the three segments:l4A (triple
hachures),14B (double hachures),and l4C (singlehachures)demarcatedby invariant points I67c and I83. Diagram to right shows
isopleths ofgarnet composition for various reactionswithin the srability field ofgarnet. Adapted from Berry (1989).
ROBINSON: EYE OF THE PETROGRAPHER, MIND OF THE PETROLOGIST
REACTION164:
\/
Dl+AN
1803
m
WO+GAR+QZ
REACTION 168:
T+
GAR + QZ > WO +-AN+ Ca-poorerGAR
D l + A N+ W O
GAR+ QZ
Fig. 57. Anorthiteprojectionsshowingtwo chemographies
of the equilibriumDI + AN + QZ + WO + GAR (reaction16
on equilibriumcompositionof garnet.
of Berry,1989)depending
FromBerry(1989).
N
o
+
E
o
i
N
An analogousbut simpler reaction sequencein the field
l(,
of stability of quartz-wollastonite instead of quartz-cal- p l ,
z
cite is illustrated in Figure 57. Ifgarnet has lessgrossular
than the quartz-diopside-anorthite plane as in reaction
E
o
16A, then the join diopside-anorthite gives way to the
assemblagequartz + wollastonite + gamet, a fluid-independentreaction that would take place with increasing
Z. If the garnet composition is more grossularthan the
Fig. 58. P-Z diagramsshowingrelationsof reaction16 of
quartz-diopside-anorthiteplane, as in reaction l68, then H.N. Berryto otherreactionsinvolvinggarnetin the chemical
Diathe plane diopside-wollastonite-anorthite gives way to systemof Figure57. All equilibriaare fluid independent.
16
with
reacgram
other
intersections
ofreaction
shows
at
top
would
qvartz-garnet, a fluid-independent reaction that
with other
tions involvingcollinearitiesof garnetcompositions
take place with decreasingZ. The limiting situation would
16A
phases.
16
indicate
the
two
segments:
reaction
Hachures
on
be the reaction where wollastonite-anorthite gives way to
by
demarcated
(squarehachures)
and I 68 (triangularhachures)
grossular-quartzwith decreasingTwithout participation
invariant points I67w. Diagramat bottomshowsisoplethsof
of diopside.
garnetcompositionfor reactionswithin the stabilityfieldofgarReactions I 64, and l6B are parts ofa singlecontinuous net.Adaptedfrom Berry(1989).
reaction marked by two invariant points, as shown in
Figure 58 (top). At each of the invariant points the curve
dependent reaction quartz + calcite : wollastonite * COr.
of reaction I 6 is tangent to a secondunivariant curve. At
I67w this is the fluid-independent reaction grossular 67 These are shown in Figure 59, in which fluid-dependent
+ quartz: diopside + anorthite, which has a metastable and fluid-independent equilibria are keyed differently'
extension to higher temperature. This reaction is the same Most of the equilibria in Figure 56 are fluid dependent;
one that connectsat lower temperature toI67c in Figure those in Figure 58 are all fluid independent, but they
56. At Il00 the secondreaction is the fluid-independent share the two fluid-independent equilibria connectingl6Tc
reaction wollastonite + anorthite : grossular + quartz. to I67w and ll00c to Il00w. At low xco, (Fig. 59A),
In this casethe reaction 16 curve comesin tangent to the the fluid-dependent equilibria are at low Z. As XCO, inother reaction at an end point, and there are no metasta- creases,they and the related invariant points slide to
ble extensions.Figure 58 (bottom) shows the same equi- higher temperaturealong the fl uid-independentequilibria
libria but with the addition of garnet composition iso- (Fig. 59B, C, and D), progressivelyshrinking the stability
pleths for two continuous equilibria within the region of field of pure grossularin the center and the field of grosstability of garnet * quartz-wollastonite-anorthite.All of sular-rich garnet on either side. Eventually the two invariant points Il00c and Il00w merge along the calcite
the equilibria in Figure 58 are fluid independent.
The equilibria in Figure 56, with quartz-calcite stable, + quartz : wollastonite * CO, reaction (Fig. 59C), and
and those in Figure 58, with wollastonite stable,are crude at higher values of XCO, (Fig. 59D), progressivelyless
and lessgrossulargarnet can be in equilibrium with quartzmirror images of each other, reflected across the fluid-
1804
ROBINSON: EYE OF THE PETROGRAPHER. MIND OF THE PETROLOGIST
lozw
-
:/
/
- ---'-
GROSSULAR
/
--
REACTIONr5|SOP|.€TH
NUMSERSSI'loW GARNET
coMPosrIoN
REACTION1 CONTOUR
NUMBERSSHOWFLUID
COMPOSITloN
'l
/
I67c
:/"r**2
GROSSULAR
Fig.60. P-?rdiagram
to illustrateP-Z-fluidevolutionofwollastonitemarble.IsoplethsI illustrateCC + QZ: WO + CO,
for variousX.o,. Reaction2 is AN + WO : GR. Reaction4'
is the reactionCC + PLAG : SCAP.Reaction15is thecontinuousfluid-independent
reactionWO + AN * Ca-poorerGAR
: Ca-richerGAR + QZ. Reactionl6b definesthe limit of GAR
stabilitywith DI + AN + QZ + WO as in Figures58 and 59.
FromBerry(1989).
Irooc
I
Iroow
14A
c.
Iroocw
scapolite from plagioclase * calcite, which removed
practically all of the albite component from plagioclase.
/
Following a curye of increasing T and P while maintainD./
ing an increasingXCO, consistentwith the quartz * cal/
cite : wollastonite equilibrium, the rock eventually
Fig. 59. CompositeP-7 diagramsof phaserelationsof DIreacheda pressurewhere calcite + wollastonite + anorAN-QZ-CC-WO-GAR-CO,
in Figures56 and 58 separated
by
thite + diopside reacted to form garnet + quartz. This
the equilibriumCC + QZ : WO + CO,. Fluid-dependent
reactions(dashed)slidetoward highertemperatureagainstfluid would have been for a composition of grossular83 along
the fluid independentreaction l68, which would in effect
independentreactions(solid)as X.o, increasesfrom A to D.
grossularstabilityis markedin the cen- also be at invariant point I83cw in the terminology of
Regionof end-member
ter, and areasof stabilityof lessgrossulargarnetare indicated Figures 59C and 59D. Further reaction allowed the garnet
by stipple.Adaptedfrom Berry(1989).
to becomeslightly more grossular-rich,to about grossular
90, but beyond this it appears that either compression
and heating ceasedor reaction rates due to fluid loss or
calcite-wollastonite-anorthite-diopside.Further, as XCO,
slow diffusion did not permit garnet of higher grossular
increases,the total length of the combined reactions 14 content to form. The necessarycounterclockwiseP-Zpath
and 16 that govern the stability ofgarnet with anorthite- for this sequenceof events independently supports the
diopside and either quartz-calcite or wollastonite de- early part of the P-T path claimed for this region on the
creases.
basis of assemblagesin aluminous rocks (Schumacheret
al., 1989;Robinsonet al., 1989).
Petrogeneticinterpretation
Berry's petrogeneticinterpretation of theserocks, based Summary
on the petrographicfeaturesand the petrogeneticgrid deIn the example given above, the role of the diopside
veloped above, is illustrated in Figure 60. The rocks at- component in all garnet-producing reactions is emphatained the quartz + calcite: wollastonite reaction at rel- sized, and this leads to consideration of equilibria in a
atively low XCOr, and once encountered,the XCO, of more complex systemand the derivation of a specificnew
the fluid increased.Slightly later in this history, a fluid- petrogeneticgrid. In the interpretation of this rock, one
independent reaction was encountered that produced can see that it is not just the phasespresent in the rock
K
,roo
ROBINSON: EYE OF THE PETROGRAPHER. MIND OF THE PETROLOGIST
I 805
in metamorphosed
Fig. 62. Eclogiteboudins(dark smudges)
Swedofthe Sevenappe,southslopeofGrapesvare,
sandstones
ish Caledonides.Photographby the author.
tifully developedwith the growth of coarsemetamorphic
textures. Nearby, however, neither the eclogite nor its
country rock may show any remaining vestigesof its eclogite-facieshistory, being entirely overprinted by amphibolite-faciesassemblages.
Eclogitized diabasedikes in the Grapesvarearea'
Swedish Caledonides
My first example is from rocks of the Seve nappe in
the Grapesvarearea of the SwedishCaledonides(Fig. 6l)
describedby Andr6assonet al. (1985),Santallier(1988),
and Msrk et al. (1988) and visited personallyby me in
August 1990. The rocks in question are metamorphosed
Caledon- Proterozoic sandstonesand subordinate shales injected
Fig. 61. Geologicindex map of the Scandinavian
in this paper.Adapted bv diabase dikes. believed to have formed in an extenidesshowingeclogitelocalitiesdiscussed
from a figureprovidedby P.-G.Andr6asson.
and their compositions that are important but also the
spatial and textural relations of the minerals that explain
the reactions involved and their probable sequence.
Mnr,luonpHrc
HrsroRy UNDERSTooDoNLy
IN TERMS OF FEATURES
OF LARGER ROCK BODIES
In the last segmentof this paper, I consider petrology
on a larger scale.In many places,rock history can only
be charted through study of the complete geologicsetting
of a large outcrop or even an entire mountain. Particularly spectacularare well-preservedpyroxene granulites,
gabbros, and diabasesin the Scandinavian Caledonides
(Fig. 6l) that are locally transformed along shear zones
into eclogite and amphibolite. Here a host of rock compositions all appear to have been subjected to eclogitefacies conditions, but the exact record they give of this
was apparently severely dependent on very local stress
Fig. 63. Photogeologic map of distribution of metamorand fluid availability. In some locations, the original pro- phosed diabase layers and boudins at Grapesvare. Boudins in
toliths, such as coarse-grainedProterozoic gabbros, py- northern two-thirds of map area are eclogite;those to south (inroxene granulites, and diabases,appear to have been to- dicated D) are diabase with garnet coronas and omphacitic pytally protected or only slightly affected by Caledonian roxenerims. Modified from a figure provided by P.-G. Andr6aseclogite-faciesoverprinting. Elsewhere,eclogite is beau- son.
l 806
ROBINSON: EYE OF THE PETROGRAPHER. MIND OF THE PETROLOGIST
Fig. 65. Photomicrograph of eclogite from Grapesvare,
showing zoned garnetsabout I mm in diameter in a matrix of
quartz, omphacite, hornblende, and dark fine-grainedsymplectite of pyroxene and plagioclaseafter omphacite. Provided by
P.-G. Andr6asson.
Fig. 64. Outcropviews of eclogiteboudinsat Grapesvare.
Top: Lower contactof largeboudin with envelopingcountry
rockbelow.This boudincouldnot be seenon August7,1990,
because
ofsnowcoverfrom thepreviouswinter.Bottom:Partial
viewsof two smallboudins.Light coloredareain centerof boudin is garnet-richeclogite;darkerendsareamphibolitized.
Photographsprovidedby P.-G.Andr6asson.
GARNET
sional environment along the margin of Baltica. In this
they closely resemblethe typical rocks of the structurally
lower Siirv nappe and differ from the latter mainly in
being involved in more intensemetamorphism. A typical
view of the field areais shownin Figure62.The exposure
is about 800/0,intemrpted only by a few areas of moss,
small bogs,and lakes.The lighter-colored areasare highly
folded and lineated metamorphosed feldspathic sandstone and minor shale with rare calc-silicate.The darker
oval areas are large boudins of metamorphoseddiabase
up to about 15 m thick. The detailed distribution of mafic
boudins over an area of about 8 x l0 km is shown in
Figure 63. Some of the boudins occur in rows and are
interpreted to be dismembered portions of individual
dikes. Within the map area, there are no obvious stratigraphicor tectoniccontacts,though,ofcourse, there is a
high degreeofoverall strain.
In the northern two-thirds of the map area, all of the
mafic boudins are eclogite, usually with a sheath of retrogradeamphibolite of variablethickness(Fig. 64). Eclogite is well preserved in some very small boudins (Fig.
64, bottom), whereasother large boudins are extensively
\ .\ J d
i
EPIDOTE
a"
\
,
j
/r.:
//
Increasing P
Fig. 66. Petrographicdetail ofGrapesvare eclogite.Top: Detailed photomicrograph of gamet at extinction under crossed
nicols separatedfrom fresh omphacite by a zone ofplagioclasepyroxene symplectite. Bottom: Sketch of the same contact region. Epidote from an original low-grade assemblage was
oyerg.rownby garnet,with progressivelyincreasingXr" and with
omphacite inclusions Jd31 that were in contact with matrix omphacite Jd35. During retrogressionsome omphacite broke down
to pyroxene-plagioclasesymplectite and was replacedlocally or
elsewhere completely by amphibole. Provided by P.-G. Andr6asson.
ROBINSON: EYE OF THE PETROGRAPHER, MIND OF THE PETROLOGIST
1807
diabasefrom the
Fig. 67. Largeboudin of metamorphosed
for scaleto left.
southernpart ofFigure 63. Note two geologists
Photographby the author.
amphibolitized. The typical eclogiteconsistsof small euhedral garnets in a matrix of well-preserved omphacite,
of retrograde pyroxene-plagioclasesymplectite, and of
secondaryamphibole (Fig. 65). There is no preservation
of original igneous texture except for rare light-colored
patchesthat may be relict plagioclasephenocrysts.
A detailed study areaalong a garnet-omphacitecontact
is shown in Figure 66. The core of the garnet contains
of marginof originaligneouspyFig. 69. Photomicrograph
diabaseboudin.Rim areais suffirsed
roxenein metamorphosed
with exsolvedilmenite,andremainingpyroxenein rim areaconjadeitecontent.Providedby P.-G.Andr6asson.
tainsup to 8o/o
epidote believed to have been included during prograde
garnet growth. The garnet showsapparent prograde growth
zoning from Xr, : 0.35 to 0.40, and the garnetcontains
omphacite inclusions with composition Jd3l as compared to Jd35 in the coarsematrix omphacite. Omphacite
breakdown to symplectite of less sodic pyroxene * plagioclase is localized near the garnet contact. Closest to
the garnet, this consists of pyroxene Jd25 and An20; at
the symplectite edge farthest from garnet, it consists of
pyroxeneJdl I and An25. Very commonly the pyroxene
of this reaction symplectite is replaced by hornblende.
Santallier (1988) has estimatedconditions of eclogiteformation at about 700 "C and 19 kbar. Detailed studies of
Nd-Sm mineral isochrons in the eclogites(Mork et al.,
1988)suggestthat the eclogite-faciesmetamorphism took
place at 500 Ma in the so-calledFinnmarkian event, well
beforethe Scandianfinal emplacementof the thrust sheets.
In approximately the southern one-third of the map
area all of the mafic boudins are recognizabletexturally
as diabase,with a fine ophitic texture of former igneous
plagioclaseand, locally, plagioclasephenocrystsrecognizable in hand specimen.The form of the boudins (Fig. 67)
and the character of the enclosing rocks is the same. In
thin sections(Fig. 68) there seemsto be evidencefor two
metamorphic events, an early greenschist-faciesmetamorphism with formation of epidote in plagioclaseand
a later high-pressureevent with the formation of delicate
garnet coronasbetweenplagioclaseand mafic minerals in
the matrix. Some original Ti-rich calcic igneouspyroxene
is still preserved(Fig. 69) but has been attacked marginally with exsolution of Fe-Ti oxides and shows an increasein jadeite component up to 8o/0.It appears that
these rocks were just beginning the process of eclogite
formation but had gone through very much less of the
Fig. 68. Photomicrographsof metamorphoseddiabase.Top:
processas compared with similar former dikes just a kiOphitic texture of plagioclaselaths with abundant coronas of
garnet at extinction under crossed nicols. Bottom: Plagioclase lometer or two away to the north on the same mountain.
laths with epidote needlesseparatedfrom fine-grained areasof What was it that caused these dramatic differences in
pyroxene and amphibole by coronas of high-relief garnet. Pro- reaction progresswithin the confines of a small area ln a
single thrust sheet?Andr6asson and coworkers are now
vided by P.-G. Andr6asson.
I 808
ROBINSON: EYE OF THE PETROGRAPHER, MIND OF THE PETROLOGIST
Fig. 70. Pavementoutcrop at Vinddoladalen,Trollheim,
Fig. 7l. Zoneof Silurianeclogite(darkgray)parallelto quartz
Norway,showingprogressive
developmentof eclogitealonga
gneiss
veincuttingacrossold foliationin Proterozoic
anorthosite
shearzonein coarseophiticgabbro(shownto left).Gray areas
(1992).
from
the
Bergen
Arcs,
Norway.
From
Boundy
etal.
are dominatedby garnet,which tendsto form coronasaround
largeremainingplagioclases.
Photograph
by the author.
striving to understand the meaning of these differences
by studying mafic boudins in a transitional position midway up the slope.
Eclogitized Proterozoicgabbro, Vinddoladalen,
westernTrollheim, Norway
My secondexample comes from exposuresof Proterozoic basementrock about 100 m below a basement-cover contactin Vinddsladalen,westernTrollheim (Fig. 6l),
studiedby Tsrudbakken ( I 98 I ) and visited by me in I 98 l.
Here, there is an extensivesequenceofCaledonide thrust
sheets, and both basement and tectonic cover
have beenvery complexly refolded. Within this basement
there is a boudin of coarse gabbro and metamorphosed
gabbro about I 80 m thick. Within the core of the boudin,
the gabbro appearspristine, with large plagioclaselaths
commonly retaining finely exsolvedopaquedust. Locally
within the gabbro, there are shearzones,severalofwhich
are well exposedon polished pavements(Fig. 70). Along
the margins of the shear zones the gabbro shows garnet
coronas around plagioclase,and inward the rock changes
to a gneissic rock with some relict plagioclaseand then
to an eclogitegneiss.The outer margins of the gabbro and
eclogiteagainst Proterozoic gneissiccountry rock are everywhereconverted into gamet amphibolite or even garnet-free amphibolite, in some locations apparently produced by hydration directly from gabbro, in others from
eclogite.Tsrudbakken (1981) has estimatedconditions
of eclogiteformation at 630 .C and 9-10 kbar. At Vinddoladalen it appears that the eclogite-forming reaction
was catalyzedonly in shear zonesin the gabbro and that
eclogite was preserved only where protected from later
amphibolitization away from the margins of the gabbro
boudin.
Eclogitized Proterozoic garnet granulite,
Bergen Arcs, westernNorway
My third example comesfrom the BergenArcs of western Norway. This was brought to my attention by a talk
at the American GeophysicalUnion (Boundyet al., 1990)
and related papers (Austreim and Griffin, 1985; Austreim, 1987;Boundy eL al., 1992).The rocks in question
are exposedon Holsnoy, 30 km northwestofBergen. They
belong to a Caledonide thrust sheet of granulite-facies
gneisses
metamorphosedabout 900 Ma, which havebeen
variably subjectedto eclogite-faciesrecrystallization during the Silurian Scandianorogeny.
Of the approximately 3 x 5 km field area mapped in
detail by Boundy et al. (1990),about 300/o
consistsoflarge
ovoid areas of granulite-faciesanorthosite gneisswith a
very strong Proterozoic metamorphic foliation trending
north-northeast.Theseareascontain numerouscracksand
veins, commonly perpendicularto the old foliation, along
which the anorthosite gneisshas been converted to eclogite (Fig. 7l). About 150/oof the area consistsof wellfoliated eclogite-faciesrocks, with foliation trending irregularly east. Most of the remaining 550/oof the area,
betweenthe areasofgranulite-facies gneissand the wellfoliated eclogite-faciesrocks, consistsof eclogitebreccia,
composedof 40 to 800/ofoliated eclogite,enclosingblocks,
some up to tens of meters in diameter, of relatively wellpreservedgranulite-faciesanorthosite gneiss.
Conditions of the Proterozoic granulite-facies metamorphism are estimatedto have been 850'C and 8 kbar.
The Silurian eclogite-faciesconditions are estimated to
have reached700-750'C and 16-21 kbar, yet some of
the granulite-facies rocks show little mineralogical evidence of this later event. While it is obvious in this example that shearinghas been involved in promoting the
eclogite-forming reaction in the large areas of well-foliated eclogitic rocks, fluids moving along younger cracks
in older foliated rocks may have been highly significant
in promoting eclogite-faciesreactions.The grand scaleof
eclogite development within a very large body of fluidpoor protolith may have minimized the effectsof subsequent amphibolite-facies retrograde hydration, which is
limited to a very small part of the study area.
Summary
In the last segment of this paper I have attempted to
demonstrate the importance of structural environment,
ROBINSON: EYE OF THE PETROGRAPHER, MIND OF THE PETROLOGIST
kinetics, and fluids in producing different minerals from
the same rock composition along the same P-T path.
Lacking knowledgeofthe surroundings,there would have
been no way to know that the original high-Z rocks survived eclogite-faciesoverprinting with only very limited
or no mineralogical traces.
CoNcr,usroN
What I have been preaching above might have been
given from this pulpit 10-15 yearsago,or at any time in
the Society'shistory: namely, that if one is going to obtain
meaningful results from analysesof minerals in metamorphic rocks, one must understand the textural relations in the rocks, the field relations of the rocks, the
sequenceof reactions and diffusional processesthat the
rocks went through, the features of the chemical space
within which the rock composition lies, and the detailed
way in which the analytical results relate to the metamorphic processes.These ideas were well understood by
some petrologists going back nearly a century, but it
seemedto me they did not touch a majority until fairly
recently.It was at a meetingin Dublin, Ireland, in 1987
that I saw a homogeneousgroup nearly all thinking along
these lines. So, I think, my messageis less desperately
neededbut still worthwhile to impart.
In eachofthe examplesI have given, one can seewrong
avenuesdown which it would be easy to wander, particularly when the scopeof one's thinking or information is
limited. The most important point is to develop a stance
or viewpoint, basedon observational knowledgeand experience,that will permit one to get closer and closer to
the truth about what actually has happened.Although we
view the rocks as our friends, they can in some placestell
lies to us. To get to the truth behind these lies requires
perpetual training ofthe eye ofthe petrographerand the
mind of the petrologist.
AcxNowr-rIGMENTS
In a review paper such as this, great reliance is placed on the creative
ideas and quantitative data of others, in this case including results of
severalof my Ph.D. and M.S. students.None of this would have taken
placewithout the influenceofmy own mentors,J.B. Lyons, D.S. Coombs,
M.P. Billings, and J.B. Thompson, Jr. Early quantitative bouts with metamorphic rocks werenurtured by the persistentinterestsof colleaguesH.W.
Jaffe and Malcolm Ross. In the section on zoned gamets, many of the
ideas were developed in cooperation with R.J. Tracy over a number of
years (see Tracy, 1982), and these ideas were developed further in the
M.S. thesis of Kurt Hollocher, the Ph D. thesis of P.J. Thompson, and
especiallythe M.S. thesis of Margaret Roll. The rocks she studied were
originally discoveredbecauseofthe sharp handlens observationsofL.D.
Ashwal, then an M.S. student. Roll enlisted the enlightenedassistanceof
Peter Crowley of Amherst College conceming the modeling of diftrsion
profiles in garnet. Finally F.S. Spear and F.T. Florence of R.P.I. made
available some of their most recent diftrsion models to me and offered
interested advice and assistance.
The section on the wollastonite marble is almost totally the development of former Ph.D. student H.N. Berry, my only contribution being to
suggestthat his weeks of graphical adventure would indeed prove very
interesting.He gave permission to use a small part ofhis larger work as
an example of the importance of petrographic observations at the scale
ofa whole thin section or hand specimen.The section on Scandinavian
eclogiteswould not have been possible without the generousand timely
contributions of P.-G. Andr6asson of Lund Universitv. A.G. Krill of
1809
Trondheim University, Theresa M. Boundy and D.M. Fountain of the
University of Wyoming, and Hlkon Austreim of Oslo University.
Work in the New England Appalachians was supported by grants from
the National Science Foundation Earth Science Program, most recently
grant EAR-88-04852. My visit to Norway in 1990 was supported by a
guest research fellowship from NAVF (Norwegian Science Foundation)
and by the GeologicInstitute at Trondheim University. Figures 1,19,20,
21, 22, 23, 41, 56, 59, and 6 I were preparedwith the assistanceof Marie
Litterer. Figures 9 and 7 I are reprinted here with the permission of Blackwell Scientific Publications, Figures ll and 24 Y/ith the permission of
Elsevier,Figures 12 and 14 with the permission of the American Journal
ofscience, and Figure 15 with the permissionofKun Hollocher' Versions
of the manuscript were reviewed by P.T. Panish,V.L. Peterson,J.B. Brady, J T. Cheney, and D.C. Elbert. My wife, SuzanneA. McEnroe, provided support, encouragement,and wise counsel that was ultimately accepted. To each of these persons and institutions I offer my grateful
acknowledgment.
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the Norrbotten Caledonides,Sweden.In D.G. Geeand B.A. Sturt, Eds.,
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Austreim, Hikon, and Grifrn, W.L. (1985) Sheardeformation and eclogite formation within granulite facies anorthosites ofthe Bergen Arcs,
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of granulite-facies metamorphic rocks in the Brimfield-Sturbridge area,
Massachusettsand Connecticut, 330 p. Ph.D. thesis, University of
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-(1991)
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