JOURNAL OF PETROLOGY
VOLUME 54
NUMBER 9
PAGES 1849^1874
2013
doi:10.1093/petrology/egt033
Very Low- to Low-grade Metamorphic Processes
Related to the Collisional Assembly of Avalonia
in SE Cape Breton Island (Nova Scotia, Canada)
ARNE P. WILLNER1,2*, HANS-JOACHIM MASSONNE1,
SANDRA M. BARR3 AND CHRIS E. WHITE4
1
INSTITUT FU«R MINERALOGIE UND KRISTALLCHEMIE, UNIVERSITA«T STUTTGART, AZENBERGSTR. 18, 70174
STUTTGART, GERMANY
2
INSTITUT FU«R GEOLOGIE, MINERALOGIE UND GEOPHYSIK, RUHR-UNIVERSITA«T, 44780 BOCHUM, GERMANY
3
DEPARTMENT OF EARTH AND ENVIRONMENTAL SCIENCE, ACADIA UNIVERSITY, WOLFVILLE, NS, B4P 2R6, CANADA
4
NOVA SCOTIA DEPARTMENT OF NATURAL RESOURCES, PO BOX 698, HALIFAX, NS, B3J 2T9, CANADA
RECEIVED MAY 2, 2012; ACCEPTED MAY 13, 2013
ADVANCE ACCESS PUBLICATION JULY 3, 2013
To better understand geodynamic processes related to the assembly
of various belts of volcanic^sedimentary rocks in the Avalonian
Mira terrane of SE Cape Breton Island (protolith ages 680^560
Ma) we investigated metamorphic processes in 20 white-mica-bearing mafic and felsic metavolcanic rocks. The felsic metavolcanic
rocks are foliated and partly sheared with blastoporphyritic relict
fabric and contain the assemblage phengite^epidote^chlorite^
albite^K-feldspar^quartz^titanite stilpnomelane calcite ilmenite. In contrast, many mafic metavolcanic rocks are relatively undeformed and display relict porphyritic and amygdaloidal textures.
They contain the assemblages epidote^chlorite^albite^quartz^titanite phengite pumpellyite prehnite calcite K-feldspar and
actinolite^epidote^chlorite^albite^quartz^titanite
phengite calcite K-feldspar. Heterogeneous metamorphic overprinting is indicated by local relicts of magmatic clinopyroxene, magnetite and plagioclase. Metamorphic minerals formed by local
precipitation in clusters and are due to continuous nucleation of very
low-grade phases during pulses of variably pervasive fluids, which
were released during intense dehydration at 250^3008C. Nucleaction
rate dominated over growth rate at these conditions. Potassic white
mica in both mafic and felsic rocks is mostly phengite with a wide compositional range (3·11^3·41 Si a.p.f.u.). Maximum Si contents are typically between 3·30 and 3·41a.p.f.u. P^T pseudosections were
calculated for the range 200^4008C and 1^7 kbar. The peak metamorphic assemblages occupy P^T fields consistent with the position of
*Corresponding author.Telephone: þ49 71168581218. Fax: þ49 7116858
1222. E-mail: [email protected]
isopleths for corresponding maximum Si contents in white mica. Peak
P^Tconditions in the Mira terrane samples lie within a narrow range
of 3·5 0·4 kbar and 280 308C in samples representing all of the
assembled volcanic^sedimentary belts. The derived peak metamorphic
conditions suggest syn-collisional burial to 11^14 km depth and a low
metamorphic geotherm of 20^258C km^1. Under these conditions subsequent strike-slip deformation is attributed to the final assembly of
magmatic arc slices to form the crust of the Mira terrane.
very low-grade metamorphism; prehnite^actinolite
facies; phengite; P^T pseudosections; mineral growth kinetics;
collision; Avalonia
KEY WORDS:
I N T RO D U C T I O N
Partial pressure^temperature (P^T) paths are commonly
derived from a series of equilibria preserved in metamorphic rocks, taking advantage of the compositional
variation of the phases in the prevalent assemblage. This
approach is indispensable for understanding tectonics in
three dimensions, particularly in exhumed metamorphic
areas. Although metamorphic rocks of very low to low
grade (subgreenschist to greenschist facies) are widespread
on Earth, the metamorphic evolution of these rocks has
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JOURNAL OF PETROLOGY
VOLUME 54
not been intensively studied by equilibrium thermodynamics, in contrast to medium- and high-grade metamorphic rocks. Experiments that limited the P^T stability
of very low- to low-grade minerals (e.g. Liou, 1971a, 1971b;
Nitsch, 1971; Schiffman & Liou, 1980) have allowed P^Testimates for rocks containing those minerals. For instance,
Schreyer & Abraham (1978), Cho et al. (1986) and Cho &
Liou (1987) used these stability limits to estimate P^Tconditions during metamorphism of mafic to felsic rocks. In
addition, the experimentally determined stability of very
low- to low-grade metamorphic minerals was used to construct petrogenetic grids for low-temperature metamorphism (e.g. Brown, 1975; Liou et al., 1985, 1987; Frey et al.,
1991; Banno, 1998), which have also been used to constrain
P^T conditions (e.g. Robinson et al., 2005; McMullin
et al., 2010). With the extraction of thermodynamic data on
metamorphic minerals in general, datasets have been created (e.g. Berman, 1988; Holland & Powell, 1998) that also
include minerals of the very low- to low-grade metamorphic realm. Such datasets have been applied to estimate the P^T conditions of very low- to low-grade
metamorphic rocks. For instance, Massonne (1995) reconstructed a P^T path for mafic and felsic rocks of the
Taunus Mountains, central Europe, which had experienced peak temperatures at 3008C. This path was based
on the chemical variability of minerals, such as potassic
white mica and Na- and Ca-amphiboles in these rocks
and the augmentation of the thermodynamic dataset by
data for these minerals. Further examples of the thermodynamic treatment of rocks of various lithologies from the
very low- to low-grade metamorphic realm to construct
P^T paths were given by Dingeldey et al. (1997), Vidal &
Parra (2000), Willner et al. (2000, 2009), Parra et al. (2002),
Willner (2005), Vidal et al. (2006) and Jolivet et al. (2010).
After numerous P^T pseudosections were successfully
applied to medium- and high-grade metamorphic rocks,
Massonne & Willner (2008) and Massonne (2010) augmented the thermodynamic dataset by Holland & Powell
(1998, updated 2003) to calculate P^T pseudosections also
for very low-grade metamorphic rocks including metasedimentary rocks. Subsequently Willner et al. (2009), Cruciani
et al. (2011) and Kryza et al. (2011) estimated P^T paths
for various low-temperature metamorphic rocks on the
basis of P^T pseudosections. Applications of pseudosections at low grade so far have included a wide range of
lithologies such as mafic rocks, calcareous and non-calcareous metagreywacke, and metatrachyte. This approach
has the additional advantage that the P^T pseudosections
can also be used to understand phase relationships around
the derived peak P^Tconditions.
McMullin et al. (2010) studied very low- and low-grade
mafic metamorphic rocks in Neoproterozoic magmatic
arcs in southeastern Cape Breton Island, Nova Scotia,
part of the West Avalonia microplate. They showed by
NUMBER 9
SEPTEMBER 2013
chemography and calculation of some multivariant reactions that equilibria exist locally at very low-grade conditions and relatively low pressures in the transitional field
between prehnite^actinolite and greenschist facies. Here,
we broaden these studies by applying P^T pseudosections
at 20 mafic and felsic metavolcanic samples, including an
assessment of the growth behaviour of minerals at very
low grade. We also partly focus on white mica, which is
common in all rock types, but which was not included in
the study by McMullin et al. (2010). White mica in the samples is generally phengite, a mineral typically associated
with intermediate to high pressures. Hence, it is a challenge to show its compatibility with a relatively low-pressure environment. Our aims are to (1) provide better
understanding of processes at very low metamorphic temperatures where studies are challenged by fine grain size
and high chemical variability in the minerals, (2) improve
knowledge of the kinetics of metamorphic reactions at
very low grade, (3) show how transient equilibrium conditions are realized at very low grade, (4) show the potential
application of P^T pseudosections for geothermobarometry at very low- and low-grade conditions and (5) contribute to a better knowledge of geodynamics during
and after collisional processes. Results of dating of the
metamorphic processes and related deformation will be
presented elsewhere.
GEOLOGIC A L S ET T I NG
The Mira terrane of southeastern Cape Breton Island
(Fig. 1) consists mainly of Neoproterozoic to Cambrian
rocks typical of Avalonia (Barr & Raeside, 1989). Differences in late Neoproterozoic magmatic evolution led to
the recognition of five now-juxtaposed volcanic^sedimentary^plutonic belts: (1) Stirling; (2) East Bay Hills; (3) Coxheath Hills; (4) Sporting Mountain; (5) Coastal belt
(Barr, 1993; Bevier et al., 1993; Barr et al., 1996). The Stirling
belt contains metasedimentary and metavolcanic rocks
(Stirling Group) with a protolith age (U/Pb zircon) of 681
þ6/^2 Ma (Bevier et al., 1993), dioritic and granodioritic
plutons of the Chisholm Brook suite at 620 þ3/^2 Ma (U/
Pb zircon) and Devonian granitoid plutons (Barr & Macdonald, 1992; Fig. 1). The protoliths of the Stirling Group
were predominantly andesitic to basaltic lapilli tuffs, litharenites, and siltstones, with subordinate basaltic lava flows
and breccias, rhyolitic lapilli tuffs, and rhyolite porphyry,
intruded by comagmatic gabbroic dykes and sills and
including a massive sulphide deposit (Barr, 1993; Macdonald & Barr, 1993; Barr et al., 1996). Based on the chemical
composition of its volcanic and plutonic rocks, it probably
formed within a calc-alkaline volcanic arc, but under
marine conditions as evidenced by abundant epiclastic turbiditic deposits (Barr, 1993; Macdonald & Barr, 1993; Barr
et al., 1996).
1850
WILLNER et al.
VERY LOW- TO LOW-GRADE METAMORPHIC PROCESSES
Fig. 1. Simplified geological map of southeastern Cape Breton Island after Barr et al. (1996), Giles et al. (2010) and McMullin et al. (2010). Sample
locations are indicated. The inset map shows the location of the Mira terrane within Avalonia and the terrane assemblage of the northern
Appalachian orogen after Hibbard et al. (2006). A, Avalonia; G, Ganderia; M, Meguma; L, Laurentia; PL, peri-Laurentian margin.
The East Bay Hills, Coxheath Hills, and Sporting
Mountain belts (Fig. 1) consist mainly of subaerial metavolcanic and metavolcaniclastic rocks of basaltic, andesitic
and rhyolitic composition (East Bay Hills, Coxheath, and
Pringle Mountain groups) with protolith ages of c. 620 Ma
(U/Pb zircon; Bevier et al., 1993; Barr et al., 1996; White
et al., 2003). These belts also include c. 620 Ma dioritic to
granitic plutons considered to be comagmatic with the
host volcanic rocks (Barr et al., 1990; Bevier et al., 1993).
The three belts are considered to represent parts of a
single continental margin volcanic-arc complex (Barr
et al., 1996).
The Fourchu and Main-a'-Dieu groups form the Coastal
belt of the Mira terrane (Barr et al., 1996). The Fourchu
Group primarily consists of metamorphosed dacitic lapilli
and ash tuffs as well as rare lava flows with minor basaltic,
andesitic, and rhyolitic tuff and flow units. A tuffaceous
unit and a granitic pluton yielded igneous crystallization
ages of 574 1 Ma and 574 3 Ma, respectively (Bevier
et al., 1993). In contrast, the Main-a'-Dieu Group consists
dominantly of slightly metamorphosed tuffaceous sedimentary and epiclastic rocks with subordinate basaltic
and rhyolitic flows and tuffs with a maximum depositional
age of c. 563 Ma based on U^Pb dating of a rhyolite flow
(Bevier et al., 1993); its minimum age is constrained by
overlying lower Cambrian sedimentary rocks (Barr et al.,
1996). The two units of the Coastal belt are considered to
represent calc-alkaline volcanism in a continental margin
arc (Fourchu Group) and slightly younger intra-arc extension (Main-a'-Dieu Group). Both units are intruded by
Devonian plutons with widespread contact metamorphic
aureoles in both Neoproterozoic rocks and overlying
Cambrian units (Barr et al., 1996).
The Mira terrane locally displays subvertical folds of
variable scale (centimetre to kilometre; here summarized
as D1) and vergence that trend NE^SW and moderately
plunge to the NE. These folds, collectively assigned to D1,
vary in intensity and scale as a result of contrasts in rock
types within and among the volcanic-sedimentary-plutonic
belts (Macdonald & Barr, 1993; Barr et al., 1996). A widely
spaced and weak cleavage (S1) is associated with this deformation and is interpreted to represent an axial-planar
foliation. Deformation occurred prior to emplacement of
the Devonian plutons and after deposition of the
Cambrian sedimentary rocks; folding is probably polyphase and of different ages. The folded belts are dissected
by prominent shear zones (D2) of similar NE^SW strike,
particularly along the boundaries between the belts and
more penetratively in the Coastal and East Bay Hills
belts. Foliation (S2) in these shear zones, which have
1851
JOURNAL OF PETROLOGY
VOLUME 54
widths at a 100 m scale, dips steeply to the NW or SE. The
shear zones are predominantly sinistral strike-slip, characterized by conspicuous strain gradients at metre scale
from undeformed to mylonitic (Macdonald & Barr, 1993;
Barr et al., 1996), and were probably related to juxtaposition
of the various magmatic arcs. Folded (D1) strata of the
Stirling Group are intruded by the c. 620 Ma Chisholm
Brook suite, implying that some folding in that belt at
least is of late Neoproterozoic age (Barr et al., 1996).
Furthermore, McMullin et al. (2010) noted that many pyroclastic rocks contain schistose clasts. These observations
and weak local unconformities at the base of the Cambrian
section show that at least part of the folding occurred in
the late Neoproterozoic. The Cambrian rocks show no
penetrative deformation, but broad open folds. Some sandstone units contain detrital white mica with 40Ar/39Ar
ages of 630^550 Ma (Reynolds et al., 2009).
In the only published study of the metamorphism in the
Mira terrane, McMullin et al. (2010) estimated regional
metamorphic conditions transitional between the prehnite^actinolite and greenschist facies. Metamorphic conditions of 250^3508C and 53 kbar were deduced, and
ascribed to burial in a volcanic-arc setting. Rocks of both
facies occur in all belts and in close proximity to each
other. In the vicinity of plutons, amphibolite facies is attained locally as a result of contact metamorphism. Potter
et al. (2008a, 2008b) and Petts et al. (2012) reported a pervasive low-dO18 anomaly throughout the Mira terrane and
also in other parts of Avalonia. They interpreted this
anomaly as a result of pre-metamorphic hydrothermal
alteration during Late Neoproterozoic transtension of
Avalonia.
Devonian granites intruded at 378 þ5/^1 Ma (U/Pb
zircon; Barr & Macdonald, 1992; Bevier et al., 1993) to a
high level in the crust with some associated porphyry and
skarn-type mineralization. These plutons are probably
related to collision, with either Ganderia to the north or
the Meguma terrane to the south (Fig. 1 inset).
M ET HODS
Sampling
Diverse lithologies in all the belts of the Mira terrane were
sampled. Based on thin section studies of these samples
augmented by samples from the collection of McMullin
et al. (2010), we selected 20 samples of mafic and felsic
rocks for further petrological investigation, with preference
for those containing relatively large white mica grains
and representing the various belts of the Mira terrane
(Fig. 1). Samples potentially affected by contact metamorphism around Devonian plutons were avoided. The
qualitative modal compositions of the 20 selected samples
are given in Table 1.
NUMBER 9
SEPTEMBER 2013
Analytical methods
Mineral compositions were obtained using a CAMECA
SX 100 electron microprobe with five wavelength-dispersive systems at Universita«t Stuttgart, Germany. Operating
conditions were an acceleration voltage of 15 kV, a beam
current of 10 nA, 20 s counting time per element each on
the peak and the background, and preferably a slightly defocused beam of 5 mm to avoid loss of alkalis in mica and
amphibole. The following standards were used: natural
wollastonite (Si, Ca), synthetic periclase (Mg), synthetic
corundum (Al), synthetic rutile (Ti), natural hematite
(Fe), natural albite (Na), natural orthoclase (K), natural
rhodonite (Mn) and natural baryte (Ba). The PAP correction procedure provided by Cameca was used for matrix
corrections. Representative analyses and structural formulae of minerals used for P^T calculations, together with
the calculation procedure of the structural formulae, are
presented in Table 2. Detection limits given in Table 2
refer to the operating conditions used. Further data are
available as Supplementary Data, which can be downloaded from http://www.petrology.oxfordjournals.org and
upon request to the first author.
Whole-rock analyses for major elements were made on
fused glass disks obtained by melting rock powder and
Spectromelt in the proportion 1:9. The disks were analysed
using a Philips PW2400 X-ray fluorescence spectrometer
at Universita«t Stuttgart. CO2 and H2O were determined
by IR spectroscopy using a Leco RC-412 C/H/H2O analyzer. The analytical data used for the calculation of the
P^T pseudosections are given in Table 3.
Calculation methods
For a detailed assessment of phase relations, geothermobarometric constraints and equilibrium conditions in the
samples, we calculated P^T pseudosections in the system
K2O^Na2O^CaO^FeO^O2^MgO^Al2O3^TiO2^SiO2^
H2O at 200^4008C and 1^7 kbar using the PERPLE__X
software package (Connolly, 1990, 2005; version of August
2006 downloaded from www.perplex.ethz.ch). The
thermodynamic data of Holland & Powell (1998, updated
2003) for minerals and aqueous fluid were applied, with
the addition of end-member data for Fe2þ- and Fe3þ-pumpellyite, Fe2þ- and Mg-stilpnomelane, actinolite and magnesioriebeckite (Massonne & Willner, 2008). Calculations
were performed using the solid-solution models of Powell
& Holland (1999) and Holland & Powell (2003) for white
mica, epidote, chlorite and biotite and those of Massonne
& Willner (2008) for amphibole, sodic clinopyroxene,
pumpellyite and stilpnomelane. In relatively Na- and
Fe-rich bulk-rock compositions, preliminary calculations
indicated that end-member riebeckite may appear over a
wide P^T range. In these cases we used a modified solidsolution model for Na-amphibole as introduced by Kryza
et al. (2011). The clinopyroxene model is that of Holland &
1852
1853
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
Wm
x
x
x
x
x
x
x
x
x
(x)
x
x
Kf
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
Ab
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
Qz
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
Tt
x
x
x
x
x
x
St
x
x
x
x
x
Pr
x
x
x
Cc
x
x
x
x
x
x
x
x
x
x
Mag
x
x
x
Cp
x
cl
pc, po
pc, po
po
cl
po
pc
cl
po
pc
pc
po, am
po, am
po, am
pc
po, am
pc, po
pc
pc, am
po
fabric
x
Relic
Pl
An410
fy
fy
fy
fy
f
f
f
fy
fy
f
fy
nf
nf
nf
f
nf
nf
f
f
nf
Deformation
3·14–3·32
3·20–3·41
3·31–3·43
3·24–3·32
3·20–3·35
3·20–3·32
3·28–3·36
3·34–3·38
3·24–3·31
3·35–3·38
3·27–3·37
3·16–3·41
3·11–3·34
3·30–3·35
3·20–3·33
3·34
3·21–3·30
3·35–3·41
(a.p.f.u.)
Si in Wm
250–310
265–280
270–295
280–290
260–275
265–285
270–300
255–310
275–300
240–290
270–285
270–295
265–300
280–310
280–290
290–310
265–280
(8C)
Peak T
3·2–3·6
3·9
3·1–3·5
3·7–3·8
3·3–4·7
3·4–3·7
3·2–3·8
3·2–4·0
3·4–3·8
2·2–3·8
2·8–3·4
3·1–3·8
3·2–4·8
43·0–3·3
3·2–3·4
3·3–3·9
3·1–3·8
(kbar)
Peak P
Stirling Group
East Bay Hills Group
East Bay Hills Group
East Bay Hills Group
Fourchu Group
Fourchu Group
Fourchu Group
Fourchu Group
Fourchu Group
East Bay Hills Group
Fourchu Group
Stirling Group
Main-à-Dieu Group
Sporting Mtn. Group
East Bay Hills Group
Main-à-Dieu Group
Stirling Group
Fourchu Group
Fourchu Group
East Bay Hills Group
Lithostratigraphy
Mineral abbreviations as in Fig. 6 and Ilm, ilmenite; Mag, magnetite. f, foliated; nf, not foliated; po, porphyritic; pc, pyroclastic; am, amygdaloid; cl, compositional
layering; (x) calculated but not observed phase.
*Sample from McMullin et al. (2010).
yFoliation related to late sinistral strike slip.
x
10Ca44
x
10Ca39
x
x
10Ca42
x
x
10Ca34
10Ca37
(x)
x
x
10Ca33
x
x
x
x
x
x
M84-41b
x
x
10Ca11
x
x
x
x
x
x
x
x
x
x
M84-41c
x
x
x
x
M83-60
(x)
x
x
x
M83-57
(x)
(x)
FA8785*
Felsic rocks
(x)
FC881614*
AS467*
x
x
(x)
10Ca40
(x)
(x)
10Ca23
x
x
x
x
10Ca13
x
(x)
x
x
M84-42
M84-41a
(x)
M83-59
Mafic rocks
Ch
Ilm
Ep
Am
Pu
Primary phases
Metamorphic phases
Table 1: Assemblages in the studied rock types of the Mira terrane and their peak conditions
WILLNER et al.
VERY LOW- TO LOW-GRADE METAMORPHIC PROCESSES
1854
0·037
1·888
1·933
4·000
19·996
Na
K
Sum
OH
O*
4·100
Sum
0·030
0·417
Mg
0·008
0·020
Mn
Ba
0·519
Fe2þ
19·923
4·000
1·795
1·741
0·041
0·013
0·026
4·100
0·642
0·017
0·071
0·698
0·014
2·659
8·000
1·598
6·403
98·1
4·34
9·88
0·15
19·973
4·000
1·945
1·901
0·025
0·000
0·019
4·100
0·365
0·020
0·564
0·261
0·006
2·885
8·000
1·377
6·623
100·7
4·41
11·0
0·10
19·969
4·000
1·938
1·904
0·015
0·002
0·017
4·100
0·352
0·016
0·445
0·361
0·003
2·924
8·000
1·511
6·490
99·7
4·37
10·9
0·06
0·32
50·07
0·36
50·07
19·927
4·000
1·818
1·774
0·020
0·024
0·007
4·082
0·542
0·005
0·498
0·000
0·011
3·027
8·000
1·213
6·787
100·2
4·48
10·4
0·08
0·12
19·943
4·000
1·870
1·843
0·019
0·008
0·008
4·100
0·591
0·012
0·253
0·426
0·013
2·806
8·000
1·465
6·535
98·6
4·37
10·5
0·07
0·15
50·07
50·14
50·14
0·17
2·89
2·71
19·977
4·000
1·928
1·815
0·088
0·010
0·015
4·083
0·524
0·000
0·319
0·000
0·003
3·238
8·000
1·410
6·590
101·0
4·52
10·7
0·34
0·29
0·07
50·14
2·65
2·87
19·989
4·000
1·961
1·920
0·042
0·000
0·009
4·060
0·383
0·000
0·476
0·000
0·001
3·199
8·000
1·320
6·680
100·5
4·47
11·2
0·16
0·16
50·07
50·14
1·92
4·24
20·021
4·000
1·993
1·962
0·023
0·008
0·021
4·092
0·132
0·000
0·368
0·000
0·002
3·590
8·000
1·777
6·223
99·9
4·43
11·4
0·09
0·39
50·07
50·14
0·66
3·25
19·995
4·000
1·991
1·955
0·019
0·004
0·013
4·100
0·713
0·000
0·428
0·067
0·004
2·889
8·000
1·179
6·821
101·0
4·48
11·4
0·07
0·25
50·07
50·14
3·57
3·82
0·66
19·965
4·000
1·929
1·884
0·026
0·009
0·010
4·100
0·748
0·005
0·285
0·325
0·007
2·731
8·000
1·288
6·712
101·3
4·49
11·1
0·10
0·18
0·07
50·14
3·76
2·55
3·23
2·393
AlVI
XMg
OH
Ca
K
Na
Sum
Mg
Fe
Mn
0·648
16·000
0·012
0·022
0·000
11·884
6·084
3·302
0·101
0·004
2·214
AlIV
Ti
5·787
100·0
11·8
0·09
50·05
50·07
0·59
20·1
19·5
Si
Sum
H2O*
K2O
Na2O
CaO
MnO
MgO
FeO
50·05
0·485
16·000
0·010
0·011
0·002
11·707
4·249
4·520
0·218
0·002
2·718
2·170
5·831
98·9
0·661
16·000
0·046
0·012
0·008
11·912
6·318
3·239
0·075
0·002
2·278
2·218
5·782
100·0
11·8
0·05
50·05
0·21
0·44
20·9
19·1
50·05
18·8
28·6
M8359
(continued)
0·552
16·000
0·018
0·014
0·000
12·061
5·343
4·338
0·078
0·003
2·299
2·475
5·525
99·0
11·4
0·05
11·3
50·05
50·05
0·08
0·44
17·0
24·6
50·05
50·05
50·07
1·22
13·5
25·6
50·05
19·2
26·2
M8442
NUMBER 9
Ca
0·013
0·015
Fe3þ
3·117
AlVI
0·09
0·47
0·14
0·17
1·72
2·20
4·13
TiO2
19·6
27·6
M8441b
VOLUME 54
Ti
8·000
1·366
Sum
Al
IV
6·635
4·38
H2O*
Si
10·8
K2O
99·1
0·14
Na2O
Sum
0·06
0·56
CaO
BaO
0·15
1·80
4·45
0·00
0·07
19·3
28·5
0·17
3·12
3·88
3·50
50·05
Al2O3
MnO
SiO2
2·04
4·96
2·56
50·05
25·5
50·3
MgO
0·61
6·71
50·05
25·8
50·9
4·52
50·05
33·7
46·0
0·14
0·12
28·6
49·8
FeO*
0·11
50·05
29·8
49·7
Fe2O3*
0·06
26·4
47·6
0·13
26·9
50·7
27·4
47·3
0·12
26·6
48·7
TiO2
M83-59
26·1
M83-59
46·3
AS467
27·8
AS467
48·4
M84-42
SiO2
10Ca39
Al2O3
10Ca39
10Ca13
M84-41b
10Ca13
10Ca13
M84-41b
Chlorite
White mica
Table 2: Representative mineral analyses
JOURNAL OF PETROLOGY
SEPTEMBER 2013
50·08
23·4
50·08
23·2
1·89
MgO
CaO
H2O*
3·000
2·221
0·002
0·005
0·020
0·769
3·018
1·967
1·000
11·992
Si
Al
Ti
Mg
Mn
Fe
Sum
Ca
OH
O*
100·1
50·05
50·05
TiO2
Sum
0·14
0·33
1855
12·210
1·000
2·037
3·116
0·795
0·009
0·007
0·001
2·304
3·000
99·6
1·85
12·048
1·000
1·965
3·051
0·836
0·053
0·000
0·012
2·150
3·000
99·8
1·87
22·9
50·08
0·20
0·87
13·8
12·106
1·000
2·002
3·067
0·932
0·008
0·000
0·004
2·124
3·000
100·7
1·87
23·3
50·08
0·06
0·13
15·4
0·06
Orthoclase
Anorthite
Albite
Sum
K
Na
Ba
0·199
4·235
95·567
1·015
0·002
0·970
0·000
0·043
0·003
Fe3þ
Ca
1·046
2·949
Al
Si
0·341
0·658
99·001
0·984
0·003
0·974
0·000
0·007
0·000
1·009
2·996
100·0
50·05
100·3
K2O
Sum
11·5
50·08
0·14
11·4
50·08
0·92
50·14
50·13
Na2O
BaO
CaO
FeO
19·6
68·7
10Ca11
20·3
67·5
Mn2O3
13·0
22·5
Al2O3
12·9
SiO2
Fe2O3
22·7
37·4
23·8
37·4
37·0
37·9
SiO2
Al2O3
24·1
M8441b
M8359
M8442
10Ca13
M8441b
Feldspar
Epidote
Table 2: Continued
0·803
34·157
64·975
1·024
0·008
0·666
0·001
0·350
0·012
1·310
2·666
99·3
0·14
7·63
50·08
7·26
0·31
24·7
59·3
M8442
98·108
0·115
1·081
0·967
0·949
0·010
0·007
0·001
0·023
1·073
2·931
99·7
16·0
0·12
Sum
OH
O*
Ca
Mn
Fe
Al
Ti
Si
Sum
H2O*
CaO
MnO
Fe2O3
50·07
0·37
Al2O3
TiO2
SiO2
0·58
19·6
63·1
10Ca33
4·974
0·102
4·872
0·906
0·001
0·076
0·128
0·819
0·988
99·5
0·47
26·0
50·14
3·11
3·33
33·5
30·4
M8359
Titanite
4·951
0·113
4·838
1·000
0·003
0·024
0·202
0·777
0·992
99·3
0·52
28·8
50·14
1·00
5·28
31·9
30·6
M8441b
0·894
0·757
0·019
2·000
6·001
Mg
Ca
Na
Sum
O*
0·007
0·194
Fe2þ
Mn
0·019
0·089
Fe3þ
AlVI
Ti
2·000
0·021
Sum
1·872
0·129
AlIV
100·1
0·26
19·1
16·2
0·23
6·28
3·19
0·67
3·43
50·6
Si
Sum
Na2O
CaO
MgO
MnO
FeO*
Fe2O3*
TiO2
Al2O3
SiO2
M8339
(continued)
6·000
2·000
0·010
0·793
0·920
0·007
0·208
0·031
0·009
0·023
2·000
0·061
1·939
99·0
0·13
19·9
16·6
0·22
6·69
1·11
0·30
1·91
52·1
10Ca23
Clinopyroxene
WILLNER et al.
VERY LOW- TO LOW-GRADE METAMORPHIC PROCESSES
24·3
17·4
8·25
0·41
Al2O3
Fe2O3*
FeO*
MnO
0·000
0·227
2·155
1·133
1856
0·012
1·702
0·057
6·000
0·010
0·008
0·033
0·678
0·728
21·052
6·000
0·655
0·600
Ti
Mg
Mn
Sum
Ca
Ba
Na
K
Sum
O*
OH
XFe3þ
XMg
XFe
XMg
OH
Sum
K
Na
Ca
Sum
Mg
0·942
0·630
2·000
2·058
0·033
0·151
1·874
13·000
2·976
1·646
Fe2þ
0·051
0·054
Fe3þ
Mn
0·005
0·267
0·713
0·292
XMg
21·771
O*
7·000
3·980
0·022
3·954
1·000
0·278
0·032
0·690
5·279
1·541
0·007
3·731
XFe3þ
OH
Sum
Na
Ca
Sum
Fe2þ
Mn
Mg
Sum
Fe3þ
Ti
Al
6·191
99·3
6·13
0·07
21·6
2·70
0·22
1·94
12·0
0·05
0·126
OH
Ca
2·000
1·974
1·004
Fe3þ
Sum
4·000
0·878
Sum
AlVI
3·010
0·990
Si
98·8
4·29
26·4
2·39
22·7
43·1
AlIV
Sum
H2O*
CaO
Fe2O3
Al2O3
SiO2
*Calculated.
White mica: the proportion of cations is based on 42 negative charges neglecting the interlayer cations; the sum of octahedrally coordinated cations is set at 4·1 to allow for an
estimation of Fe3þ. Chlorite: cations based on 56 negative charges; H2O calculated on the basis of OH ¼ 16. Epidote: proportions of cations are based on normalization of Si to three
cations. Feldspar: normalization on the basis of 16 negative charges. Titanite: sum of cations ¼ 3; OH ¼ (Al þ Fe) 0·5; O ¼ [( positive charges) – OH] 0·5. Clinopyroxene: normalization to four cations to calculate Fe3þ. Stilpnomelane: the proportion of cations is based on 47·375 negative charges, neglecting K þ Na; the proportion of Fe3þ is estimated assuming
15 cations; H2O calculated on the basis of OH ¼ 6. Amphibole: the proportion of cations is based on the sum of cations ¼ 13 except for Ca, Na and K for estimation of Fe3þ and on 46
negative charges. Pumpellyite: cations are based on 49 negative charges; H2O is calculated on the basis of OH ¼ 4; estimation of Fe2þ by assuming half of X-position filled with divalent
cations. Prehnite: the structural formula is based on 20 negative charges and OH ¼ 4.
6·000
21·020
0·657
0·034
AlVI
Ti
8·000
0·264
AlIV
Sum
7·736
Si
Si
Sum
H2O*
Na2O
CaO
MgO
MnO
FeO*
Fe2O3*
TiO2
18·5
36·2
10Ca13
Prehnite
NUMBER 9
0·614
0·007
0·002
5·290
0·622
0·782
2·05
98·8
H2O*
Sum
0·41
0·53
0·17
12·0
13·7
13·5
0·49
0·05
MnO
Na2O
K 2O
CaO
MgO
FeO*
Fe2O3*
TiO2
Al2O3
SiO2
10Ca23
Pumpellyite
VOLUME 54
0·002
2·664
1·220
Fe2þ
9·000
0·430
Fe3þ
AlIV
8·570
98·9
9·000
3·763
Si
0·941
5·237
Sum
0·11
5·68
Sum
98·3
H2O*
2·00
0·17
AlVI
0·12
5·48
BaO
0·10
3·23
Na2O
K2O
3·31
50·07
6·95
50·07
MgO
CaO
4·63
20·1
8·83
3·08
52·9
0·10
TiO2
Al2O3
54·1
31·9
SiO2
SiO2
M8442
M8441b
10Ca44
0·02
Amphibole
Stilpnomelane
Table 2: Continued
JOURNAL OF PETROLOGY
SEPTEMBER 2013
1857
0·90
0·85
15·16
6·57
4·51
Al2O3
FeO þ MnO
MgO
2·80
100·0
0·04
8·96
100·0
H2O
CO2
Sum
100·0
6·96
0·04
1·56
6·96
0·04
100·0
*Corrected for CaO content in apatite.
100·0
8·96
0·04
0·58
7·95
5·96
0·04
2·75
2·32
2·03
8·17
3·52
K2O
4·56
3·68
O2
8·24
2·56
5·84
5·89
9·33
17·72
0·90
44·3
2·40
9·02
9·94
14·95
0·68
46·7
100·5
Na2O
4·19
8·02
18·38
0·71
51·9
4·42
101·1
CaO*
5·96
9·73
15·37
47·7
53·6
SiO2
TiO2
Simplified compositions
3·50
99·8
4·56
3·22
101·4
2·97
0·26
2·76
100·6
0·13
0·27
2·33
8·31
5·91
0·26
10·1
17·8
0·91
44·5
Sum
0·13
1·60
3·60
8·53
9·22
0·30
10·9
15·3
0·70
47·7
H2O
0·19
0·61
3·77
4·83
4·29
0·16
8·93
18·8
0·73
53·1
2·81
0·17
2·71
8·97
6·31
0·24
11·2
16·3
0·95
50·5
10CA40
CO2
P2O5
2·03
5·30
MgO
K2O
0·14
MnO
7·03
7·88
Fe2O3
2·42
15·3
Al2O3
Na2O
0·88
TiO2
CaO
56·3
SiO2
Whole-rock analyses
10Ca23
100·0
5·96
0·04
2·46
8·92
4·29
9·29
16·51
0·54
52·0
100·6
4·60
0·03
0·05
2·48
9·05
4·33
0·16
10·2
16·7
0·54
52·5
FA8785
100·0
9·96
0·04
1·89
2·58
8·04
3·68
7·12
19·33
0·77
46·6
100·1
3·00
0·10
1·80
2·50
9·80
5·20
0·20
8·28
18·7
0·80
49·7
AS467
100·0
7·96
0·04
0·44
2·19
10·12
6·83
9·25
16·52
0·81
45·8
99·2
1·60
0·15
0·46
2·29
10·8
7·15
0·31
10·4
17·3
0·85
48·0
FC881614
100·0
6·96
0·04
1·09
4·91
1·93
1·73
3·83
14·33
0·56
64·6
100·5
1·91
0·11
1·15
5·16
2·18
1·82
0·17
4·28
15·0
0·59
67·8
100·0
6·96
0·04
1·10
5·36
0·96
0·39
1·99
12·78
0·36
70·1
98·8
0·15
0·05
1·17
5·66
1·09
0·41
0·17
2·15
13·5
0·38
74·0
M84-41B
M83-57
10CA13
M83-59
M84-42
Felsic rocks
Mafic rocks
100·0
6·96
0·04
1·79
4·04
1·53
0·72
2·65
13·03
0·39
68·9
98·7
0·50
0·06
1·88
4·24
1·68
0·76
0·09
2·99
13·7
0·41
72·2
M84-41C
100·0
6·96
0·04
0·90
5·10
1·41
1·01
2·93
12·87
0·38
68·4
98·6
0·25
0·08
0·95
5·35
1·58
1·06
0·11
3·29
13·5
0·40
71·8
10CA11
Table 3: Whole-rock analyses and simplified compositions for the calculation of P^T pseudosections (Figs 5^10)
100·0
6·96
0·04
0·40
4·49
2·93
0·33
2·29
11·57
0·30
70·7
99·4
0·72
0·04
0·43
4·74
3·14
0·35
0·13
2·54
12·2
0·31
74·6
10CA33
100·0
6·96
0·04
1·35
3·79
2·90
1·71
5·23
14·28
0·60
63·1
100·6
2·19
0·12
1·41
3·96
3·19
1·79
0·18
5·88
14·9
0·63
66·0
10CA34
100·0
1·21
5·96
0·04
3·05
3·07
2·04
1·25
3·42
14·54
0·55
64·9
99·8
1·88
1·25
0·11
3·16
3·17
2·26
1·30
0·07
3·92
15·0
0·57
67·0
10CA37
100·0
0·57
5·96
0·04
3·27
3·82
1·37
0·70
2·07
13·99
0·29
67·9
98·2
0·59
0·06
3·41
3·98
1·50
0·73
0·06
2·33
14·6
0·30
70·7
10CA39
100·0
6·96
0·04
0·72
4·85
1·08
0·19
1·10
10·87
0·14
74·1
98·6
0·01
0·77
5·14
1·16
0·20
0·06
1·23
11·5
0·14
78·4
10CA44
WILLNER et al.
VERY LOW- TO LOW-GRADE METAMORPHIC PROCESSES
JOURNAL OF PETROLOGY
VOLUME 54
Powell (1996) supplemented by an acmite component,
which is commonly enhanced at very low-grade conditions. Albite, K-feldspar, prehnite, quartz, titanite, H2O,
ilmenite and magnetite were considered as pure phases.
Pseudosections were calculated for eight mafic and nine
felsic samples (Table 1). The major element analyses were
simplified to the above 10-component system, correcting
Ca for apatite and adding MnO to FeO. Water contents
were increased to excess water conditions that are considered to have prevailed during peak P^T conditions.
Finally analyses were normalized to 100% (Table 3). For
samples where calcite was present in considerable amounts,
CO2 was added to the 10-component system. However,
Massonne (2010) showed that even fluids released from
marly limestone at low temperature contain very little
CO2. Hence CO2 can largely be neglected as a fluid component at temperatures54008C. Oxygen contents were arbitrarily chosen (0·04 wt %) to account for trivalent iron
in epidote and pumpellyite. Magmatic relict phases occur
only in a few samples, and do not exceed 5%. Because
of their low abundances, the whole-rock chemical data
were not adjusted to accommodate their presence.
R E S U LT S
Petrographic characteristics and mineral
chemistry
The protoliths of the selected samples include both lava
flows and various pyroclastic rocks. Metamorphic mineral
assemblages and fabrics are similar in samples from all of
the volcanic^sedimentary belts of the Mira terrane, and
hence we do not distinguish samples with respect to location in the descriptive sections.
Felsic rocks
The felsic rocks are sheared and foliated but retain recognizable relict fabrics such as porphyritic and/or pyroclastic
textures, compositional layering, and rare relict flow-banding. Locally, deformation has entirely obliterated the
primary fabric. The common assemblage is phengite^epidote^chlorite^albite^K-feldspar^quartz^titanite stilpnomelane calcite amphibole ilmenite. Plagioclase and
K-feldspar phenocrysts and clasts (0·1^2 mm size) are conspicuous relicts and commonly rotated between phyllosilicate bands (s-clasts) showing pressure shadows, some
pressure solution and abundant brittle deformation.
Plagioclase composition is always albite, which pseudomorphed the magmatic relicts or grew within the matrix.
Quartz occurs in places as primary phenocrysts with corrosion embayments (0·5^1mm), but mostly in the matrix
or as fissure fillings, where it is invariably recrystallized
to a polygonal fabric, especially in pressure shadows
(0·01^0·05 mm; Fig. 2a and b).
White mica is entirely of metamorphic origin, finegrained (0·005^0·03 mm) and commonly oriented parallel
NUMBER 9
SEPTEMBER 2013
to the predominant foliation (Fig. 2a and b). It typically
forms unoriented irregular clusters about 0·1^0·2 mm in
diameter mostly across feldspar grains, but also in the
matrix. White mica composition is mostly phengite [i.e.
3·2 Si a.p.f.u. following the proposal of Massonne &
Schreyer (1986)]. Phengite is an intermediate member of a
solid-solution series between the end-members muscovite
and (Mg, Fe)-aluminoceladonite (e.g. Rieder et al., 1998).
In our samples, it has a wide variation in Si content
(3·13^3·43 Si a.p.f.u.), but similar ranges in all of the
samples. Some compositions show wide deviation from
the ideal Tschermak’s substitution 2Al ¼ Si þ (Mg þ Fe2þ)
owing to highly variable Fe3þ-substitution (calculated
Fe3þ 0·0^0·7 a.p.f.u.; Fig. 3a) and to a minor extent to
the di/trioctahedral substitution 2Al þ œ ¼ 3(Mg, Fe2þ).
Consistent with very low metamorphic grade, Na contents are low (0·005^0·045 a.p.f.u.), as are the contents
of Ba (0·016 a.p.f.u.), Ti (0·018 a.p.f.u.) and Mn
(0·014 a.p.f.u.). Mica grains with unusually high Na contents 40·1a.p.f.u. and corresponding elevated Si contents
are interpreted as analyses that included both white mica
and adjacent albite as a result of the very fine grain size
and hence are discarded as artefacts. XMg [¼ Mg/
(Mg þ Fe2þ)] varies considerably between and within
samples (0·4^0·9); Si and XMg appear to be slightly anticorrelated within each sample (Fig. 3b). Detailed X-ray
element distribution maps (Fig. 4) of typical small white
mica grains show strongly irregular zoning with both
increasing and decreasing Si contents from inner portions
to the rim. Also, neighbouring grains can have very different compositions. The absence of consistent compositional
zoning patterns in white mica is typical for the very low
metamorphic grade samples and has also been observed
in all other coexisting solid-solution phases.
Chlorite is a minor phase in the felsic rocks. It shows
little compositional variation within samples, but more
variation between samples (Si 5·5^6·9 a.p.f.u.; XMg
0·30^0·74; Ti 50·02, locally 0·12^0·19; Mn 0·08^0·35). It is
mostly intergrown with oriented white mica of similar
grain size, but also forms clusters together with titanite
(Fig. 2a and b).
Epidote grains form conspicuous clusters 0·2^0·5 mm in
diameter with single grain sizes of 0·01^0·05 mm. The clusters are slightly elongate parallel to the foliation. Contents
of Fe3þ in epidote vary considerably between 0·5 and
1·0 a.p.f.u. Mn (50·12 a.p.f.u.) and Ti (50·07 a.p.f.u.) are
minor constituents.
Stilpnomelane commonly occurs as mostly unoriented
grains (0·01^0·05 mm in length) in small clusters
0·2^0·5 mm in diameter (Fig. 2a and b). In some samples
stilpnomelane occurs in the dominant foliation defined by
oriented phengite and chlorite. Characteristically, stilpnomelane compositions indicate a strong and highly variable
reduction of the interlayer cations and a wide variation in
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Fig. 2. Photomicrographs of (a, b) felsic sample M84-41B with clusters of chlorite in albite and interstitial clusters of stilpnomelane (St) and
phengite and (c, d) mafic sample 10Ca13 showing clusters of prehnite (Pr), phengite (Ph) and epidote (Ep) within albite (Ab) and interstitial
chlorite (Ch) clusters. Scale bars represent 0·5 mm. þP, crossed polars; llP, parallel polars.
3þ
XFe
, here defined as Fe3þ/(Fe3þ þ Fe2þ), between 0·0
and 0·6 and XMg [¼ Mg/(Mg þ Fe2þ)] between 0·2 and
0·6. Most of this variation is assumed to be due to oxidation processes related to post-metamorphic alteration.
Other components include Mn (0·05^1·0 a.p.f.u.), Ti
(50·02 a.p.f.u.) and Ba (50·02 a.p.f.u.).
Titanite is a common accessory phase that occurs in
clusters 0·05^0·2 mm in diameter with single grain lengths
of 0·01^0·03 mm. Grains are elongate in laminae parallel
to the foliation. Owing to a minor vuagnatite substitution
[Ti4þ þ O2^ ¼ (Al,Fe)3þ þ (OH,F)] contents of Al
(0·12^0·27 a.p.f.u.) and Fe3þ (0·02^0·05 a.p.f.u.) are notable.
Ilmenite, presumably of magmatic origin, is present in
some samples as a second Ti phase.
Mafic rocks
In contrast to the felsic rocks, many mafic rocks are undeformed and preserve relict porphyritic, subophitic and
amygdaloidal textures. Mafic rocks with relict pyroclastic
textures show weak foliation defined by chlorite and
white mica, and alignment of small feldspar clasts and
lithoclasts. As described by McMullin et al. (2010) two
principal assemblages occur: (1) epidote^chlorite^albite^
quartz^titanite phengite pumpellyite prehnite calcite K-feldspar indicative of the prehnite^actinolite
facies; (2) actinolite^epidote^chlorite^albite^quartz^titanite phengite pumpellyite calcite K-feldspar indicative of the greenschist facies.
Heterogeneous overprinting is partly indicated by local
relicts of magmatic clinopyroxene and plagioclase.
Although plagioclase composition is generally albite,
some rare local patches of more calcic plagioclase (An14^
34) occur in phenocrysts and are probably relict. Augite
makes up to 5 vol. % in some of the samples. Its composition is variable both within and between samples (diopside0·36^0·56, hedenbergite0·06^0·22, orthopyroxene0·19^0·34,
acmite0·01^0·11, Tschermak component0·03^0·14) and also
includes minor Ti (0·004^0·025 a.p.f.u.) and Mn (0·004^
0·014 a.p.f.u.). Rare relict amygdules contain quartz, epidote and/or chlorite. Pyroclastic samples contain some
lithic clasts, which are schistose or mylonitic, indicating
pre-existing deformation.
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Fig. 3. Variation of Si content (a.p.f.u.) with (a) Al and (b) XMg [¼
Mg/(Mg þ Fe2þ)] in white mica of mafic and felsic samples. The line
of the ideal Tschermak’s substitution is indicated in (a).
White mica mainly forms 0·1^0·5 mm clusters of 0·01^
0·05 mm grains within plagioclase (Fig. 2c and d), but is
also disseminated in the matrix. Composition is mostly
phengite (i.e. Si43·2 a.p.f.u.) and its range is very similar
to that observed in felsic rocks (Si 3·11^3·41a.p.f.u.; calculated Fe3þ 0·00^0·76 a.p.f.u.; Na 0·01^0·05 a.p.f.u.; Ba
0·01a.p.f.u. with local 0·02 a.p.f.u.; Mn 0·015 a.p.f.u.;
Ti 0·00^0·01a.p.f.u.; XMg 0·17^0·93; Fig. 3a). Slight negative correlation is evident between Si and XMg [¼ Mg/
(Mg þ Fe2þ)] in each sample (Fig. 3b).
Ubiquitous chlorite with a grain size similar to that of
white mica is mostly disseminated in the matrix, and is
oriented in foliated samples. It also forms small clusters
(Fig. 2c and d) and fills fissures. It has similar contents of
Si (5·5^6·6 a.p.f.u.), Ti (50·06, locally 0·14^0·24 a.p.f.u.)
and Mn (0·04^0·21a.p.f.u.) to chlorite in felsic rocks, but
generally has higher XMg (0·44^0·71). Like white mica, it
displays weak negative correlation between Si and XMg.
Albite and K-feldspar were observed as part of the metamorphic assemblage in several mafic samples, and quartz
occurs in all samples as small pods of recrystallized
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crystals in the matrix or in fissures. Amphibole is a minor
component, occurring as very small disseminated crystals
(0·01^0·02 mm). It is actinolite (XMg 0·6) and contains
small amounts of Na (0·09^0·12 a.p.f.u.).
Epidote grains (0·05^0·1mm in size) commonly occur in
conspicuous clusters (0·5^2 mm in size) or are disseminated in the matrix and in relict plagioclase phenocrysts
(Fig. 2c and d). Epidote is similar in composition to that
in felsic rocks (Fe3þ 0·52^1·0 a.p.f.u., Mn 50·05 a.p.f.u., Ti
50·03 a.p.f.u., and Mg 50·12 a.p.f.u.).
Pumpellyite forms clusters similar to those of epidote,
but is less common. Compositional variation of pumpel3þ
[¼ Fe3þ/(Fe3þ þ Al)]
lyite in sample 10Ca23 is XFe
0·27^0·34 and XMg [¼ Mg/(Mg þ Fe2þ)] 0·52^0·71 with
rare exceptional XMg down to 0·23 or up to 0·86, presumably as a result of locally variable oxidation conditions.
According to McMullin et al. (2010) XMg varies in the
range 0·30^0·77 between samples.
Prehnite is present mostly in clusters within former
plagioclase (Fig. 2c and d) or fills fissures. It contains notable amounts of Fe3þ (0·10^0·13 a.p.f.u.), as also noted by
McMullin et al. (2010). Prehnite was observed in coexistence with pumpellyite in only three samples [FC881614,
FA8785 and AS467, all taken from the sample set of
McMullin et al. (2010)].
Titanite commonly forms diffuse clusters elongate parallel to the foliation. Low contents of Al (0·06^0·21a.p.f.u.)
and Fe3þ (0·04^0·08 a.p.f.u.) indicate a minor vuagnatite
substitution. Also present are ilmenite and magnetite,
which may be relict magmatic phases.
Phase relationships deduced by P^T
pseudosections
P^T pseudosections show the extent of P^T fields of specific index minerals varying with whole-rock composition.
Pseudosections calculated for four mafic and felsic rock
compositions from the Mira terrane were reduced to the
P^T fields of critical very low- and low-grade phases
(Fig. 5). The P^T conditions are between the P^T fields of
laumontite and lawsonite on the low-temperature side and
biotite at the high-temperature side (Fig. 5a and b), because none of these minerals was observed. This field
approximately corresponds to the temperature range of
250^3008C. Notably, considerable similarity exists between
the P^T fields of the three index minerals in mafic and
felsic compositions. Pumpellyite has a smaller P^T field
in the felsic samples of this study (Fig. 5c), whereas
both prehnite and pumpellyite occur in the mafic samples
(Fig. 5d). The P^T fields of both minerals have very restricted overlap with a P^T maximum at 3·5 kbar and
2908C. Although we calculated the pseudosections
using only pure prehnite, the slight Fe3þ-substitution
observed in prehnite would have only a minor effect
(change in temperature up to 58C calculated for multivariant reactions). The observation of a restricted
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Fig. 4. Maps of relative X-ray intensity distribution of Al, Si, Fe, Mg in white mica from very low-grade felsic samples M84-41 and 10Ca39.
Quantitative spot compositions as a.p.f.u. of Al, Si, Fe and Mg are shown. It should be noted that some increase or decrease of elements is due
to grain boundary effects.
prehnite^pumpellyite field has also been made elsewhere
by Frey et al. (1991). Owing to its limited stability field the
mineral pair should not be used to indicate its own facies,
but should be regarded as a subfacies within the prehnite^
actinolite facies, as also concluded by McMullin et al.
(2010).
Stilpnomelane and K-feldspar are also index minerals in
the studied mafic and felsic samples (Fig. 5e and f), but
with different extension of the P^T fields: stilpnomelane
appears in mafic and felsic rocks up to 250^3008C (at 2^6
kbar), whereas K-feldspar occurs only below 3·2 kbar in
mafic rocks, but also at higher pressure in very low-grade
felsic rocks. The appearance of epidote above 250^3008C
and the occasional replacement of white mica by K-feldspar at low pressure and temperature appears common in
both mafic and felsic rocks from the Mira terrane. In
general, the deduced P^T fields of the very low- and
low-grade metamorphic phases concur with earlier petrogenetic grids, which were constructed using approximated
mineral compositions (e.g. Brown, 1975; Liou et al., 1985,
1987; Frey et al., 1991; Banno, 1998).
Metamorphic conditions derived with
P^T pseudosections
Peak P^T conditions are estimated from P^T pseudosections by the position of isopleths representing the maximum observed contents of certain elements in one or two
phases in the field of the observed peak metamorphic
assemblage. For very low- and low-grade conditions, Si
contents of white mica, a common phase in the Mira terrane samples, are most variable. Hence in Fig. 6 isopleths
of Si contents are inserted into P^T pseudosections
of felsic sample 10Ca11 (Fig. 6a) and mafic sample
11Ca13 (Fig. 6b). Although Si contents often increase
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Fig. 5. P^T fields of minerals extracted from calculated pseudosections for the whole-rock compositions of mafic samples (a, c, e, g) and felsic
samples (b, d, f, h).
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Fig. 6. P^T pseudosections calculated for the whole-rock compositions of (a) felsic sample 10Ca11 and (b) mafic sample 10Ca13 with isopleths
of Si a.p.f.u. in white mica (dashed lines). Shading indicates the degree of variance as shown. The highest and lowest Si contents are indicated
as bold dashed lines within the field of the observed assemblage (hatched). White dotted lines in (a) indicate XMg [¼ Mg/(Mg þ Fe)] for chlorite. Abbreviations of minerals and end-member components in this paper are: Ab, albite; Am, amphibole; Bt, biotite; CA, calcic amphibole;
Cc, calcite; Ch, chlorite; Cp, clinopyroxene; Ep, epidote; Gt, garnet; Kf, potassic feldspar; Lm, laumontite; Lw, lawsonite; Mt, magnetite; NA,
sodic amphibole; Pa, paragonite; Ph, phengite; Pl, plagioclase; Pr, prehnite; Pu, pumpellyite; Qz, quartz; St, stilpnomelane; Tt, titanite; V,
H2O as hydrous fluid; Wm, potassic white mica; Wk, wairakite.
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with pressure, the form of the Si isopleths is irregular in
both felsic and mafic samples and may be used as a
geothermobarometer in combination with the P^T field of
the observed metamorphic assemblage.
Thus, in felsic sample 10Ca11 (Fig. 6a) peak P^T conditions are reached at 265^2858C and 3·4^3·7 kbar with
growth of phengite with 3·36 Si a.p.f.u. in an assemblage
with chlorite, epidote, potassic feldspar, albite, titanite
and quartz. The isopleth for the lowest Si content measured in this sample (3·28 a.p.f.u.) plots in the same P^T
field and indicates retrograde conditions. Also, XMg [¼
Mg/(Mg þ Fe)] isopleths for chlorite inserted in Fig. 6a
show little variation around 0·40, similar to the observed
values and to other pseudosections.
Similar peak P^T conditions at 280^2908C and 3·2^3·4
kbar are indicated for mafic sample 10Ca13 (Fig. 6b) as
shown by the isopleth of maximum Si content in phengite
(3·33 a.p.f.u.) in an assemblage with epidote, prehnite, potassic feldspar, chlorite, albite, titanite and quartz. The isopleth for the lowest Si content of 3·20 a.p.f.u. also plots in
the same P^T field, representing retrograde white mica.
However, two limitations are indicated for this example.
First, ilmenite and magnetite were observed in the
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sample, but do not occur in the calculated P^T pseudosections, as is also the case for pseudosections calculated for
the other three mafic samples. This indicates that these
accessory phases most probably represent primary
phases. On the other hand, a small amount of K-feldspar
(5 vol. %) was calculated for the likely field of the peak
assemblage, but not observed in the sample. However,
K-feldspar was observed in other mafic samples and small
amounts of K-feldspar in a fine-grained rock may have
been overlooked, or could occur in parts of the sample not
included in the thin section.
The P^T pseudosection for mafic sample AS467 that contains pumpellyite and prehnite in an assemblage with phengite, epidote, chlorite, albite and titanite is shown in Fig. 7.
Again, the very limited field for coexistence of pumpellyite
and prehnite is apparent. The peak metamorphic assemblage occurs at the upper P^T stability range for coexisting
prehnite and pumpellyite at 270^2958C and 3·2^3·8 kbar.
The isopleth for coexisting white mica with the highest Si
content (3·34 a.p.f.u.) almost coincides with this small field.
The field for coexisting prehnite and pumpellyite thus
limits the maximum P^Tconditions even in samples where
no white mica is present, such as sample FA8785 (Table 1).
Fig. 7. P^T pseudosection calculated for the whole-rock composition of mafic sample AS467 containing pumpellyite and prehnite.
Abbreviations and shading as in Fig. 6. The narrow P^T field of assemblages with coexisting prehnite and pumpellyite should be noted (6, 7,
21, 26, 19). Dashed lines indicate the Si contents in white mica. The maximum content of 3·34 a.p.f.u. is almost identical to the field of the peak
metamorphic assemblage (19; hatched). White dotted lines are XMg ¼ Mg/(Mg þ Fe2þ) in pumpellyite. Mineral abbreviations are as in Fig. 6.
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In sample AS467, with white mica Si contents lower than
3·34 a.p.f.u. but mostly above 3·21a.p.f.u. (except for one
analysis with 3·11a.p.f.u.) probably grew mainly during
retrograde conditions. However, no retrograde K-feldspar
was observed in the sample, which would be expected in accessory amounts according to the calculation and hence
might have been missed in petrographic work. Calculated
XMg isopleths for pumpellyite range from 0·58 to 0·64
(Fig. 7), compatible with measured XMg values in this
sample (0·51 0·10; McMullin et al., 2010).
Mafic sample 10Ca23 lacks white mica but has the
critical assemblage pumpellyite þ epidote (Fig. 8). This assemblage is restricted to a narrow temperature range of
280^3108C at pressures exceeding 3·0^3·3 kbar. The calculated range of XMg ¼ 0·68^0·74 for pumpellyite (Fig. 8)
corresponds well to the observed XMg of 0·52^0·71 (Table
2). Both actinolite and clinopyroxene are part of the calculated assemblage in amounts less than 10 vol. %. No actinolite was observed in the sample, and may have been
overlooked in the extremely fine-grained matrix. Some
relict clinopyroxene is present; it was previously argued by
McMullin et al. (2010) that this clinopyroxene could also
be part of the metamorphic assemblage, an inference
supported by its presence in the calculated assemblage.
The calculated clinopyroxene composition for sample
10Ca23 at 2808C and 3·0 kbar is diopside0·63,
hedenbergite0·30, orthopyroxene0·01, and acmite0·06, relatively similar to the observed compositional range (see
above).
Two special cases where prograde metamorphic phases
are still preserved are shown in Fig. 9. Sample M84-42 contains a typical greenschist assemblage (Fig. 9a). Peak P^T
conditions are 290^3208C and 3·3^3·9 kbar based on the
highest Si contents of 3·30 a.p.f.u. in the P^T field of the
observed assemblage of phengite, actinolite, epidote, chlorite, albite, quartz and titanite. This sample contains ilmenite rather than titanite and, as in sample 10Ca13, the
ilmenite is interpreted as a relict magmatic phase.
The range of maximum temperature for this sample (290^
3208C) is only slightly higher than the range of 270^2958C
for the typical very low-grade assemblage with pumpellyite
and prehnite in sample AS467 (Fig.7). This slight difference
in peak temperature may be real, but is also within limits
of error. Na contents in actinolite increase steadily towards
the transition to Na-amphibole at 5^7 kbar and 250^
3008C (Fig. 9a). It is important that in this rock accessory
Fig. 8. P^T pseudosection calculated for the whole-rock composition of mafic sample 10Ca23 containing pumpellyite and epidote. The narrow
P^T fields of assemblages with coexisting prehnite and pumpellyite should be noted. Abbreviations and shading as in Fig. 6. Dashed lines are
XMg ¼ Mg/(Mg þ Fe2þ) in pumpellyite. Mineral abbreviations are as in Fig. 6.
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Fig. 9. P^T pseudosections calculated for the whole-rock composition of (a) mafic sample M84-42 with a typical greenschist-facies assemblage
(hatched) and (b) felsic sample M84-41B with prograde growth of stilpnomelane. Abbreviations and shading as in Fig. 6. Isopleths of Si contents
in white micas are the black dashed lines. White dotted lines indicate Na contents in actinolite in (a) and XMg [¼ Mg/(Mg þ Fe)] in stilpnomelane in (b). Mineral abbreviations are as in Fig. 6.
stilpnomelane, pumpellyite and K-feldspar were observed.
These minerals grew as a prograde assemblage in field 17
of Fig. 9a of the calculated pseudosection at 2·3^3·3 kbar
and 230^2508C. This finding implies a late prograde P^T
path with little pressure increase. The P^T field for white
mica calculated for sample M84-42 is restricted to pressures
43 kbar and Si contents exceeding 3·28 a.p.f.u. However,
white mica with Si contents as low as 3·2 a.p.f.u. is present,
and probably formed under retrograde conditions during
restricted water access (e.g. Cruciani et al., 2011) and a reacting chemical composition that deviated slightly from the
whole-rock composition.
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The highest Si content (3·31a.p.f.u.) in phengite in felsic
sample M84-41B occurs in two fields with different
observed assemblages, occurring with stilpnomelane at
higher pressure and with potassic feldspar at higher temperature (Fig. 9b). Traces of sodic amphibole are also calculated in both fields, although this mineral was not observed
in thin section. In this case the possible range for peak pressure appears large. However, stilpnomelane is preserved as
a prograde phase formed around 2508C and the prograde
P^T path should pass near the isopleth with Si 3·31a.p.f.u.
up to maximum possible temperatures of 3108C. The isopleth with the lowest Si contents of 3·24 a.p.f.u. also cuts
the second field (at higher temperature), indicating retrograde growth of white mica below 3 kbar and 3008C. The
range of calculated values of 0·20^0·28 for XMg for stilpnomelane, shown as isopleths in Fig. 9b, fits well with observed
values of 0·21^0·28 (Table 2).
In a way similar to that illustrated in Figs 6^9, peak P^T
conditions were estimated for seven additional mafic and
nine additional felsic samples using maximum Si contents
in phengite (Table 1). All results strongly overlap and show
no consistent differences between samples. Remarkably
uniform peak metamorphic conditions within the range of
280 308C and 3·5 0·4 kbar are indicated for the entire
Mira terrane. Owing to the considerable range in wholerock compositions, assemblages of the prehnite^actinolite
and greenschist facies can overlap in this P^T range, but
slight local differences in temperature and/or pressure are
also possible. Such differences lie within the magnitude of
general errors of the geothermobarometric methods (0·5
kbar, 208C at 1s level). The thermometric approach
of McMullin et al. (2010), calculating multivariant reactions with mineral pairs, yielded an approximately similar
range of temperatures (244^3078C). No difference in
microfabric (grain size, lack of zonation, clustering,
degree of recrystallization) was identified between samples
in which subgreenschist- and greenschist-facies assemblages occur.
Isopleths of water content bound to solids also can be
extracted from the calculated P^T pseudosections. Calculated isopleths of water content (in wt %) of representative
mafic and felsic samples show that water was not released
continuously during metamorphism (Fig. 10). Most water
was released during breakdown of OH-bearing minerals
at very low-grade conditions. In the temperature range of
250^3008C, in which most metamorphic reactions took
place in these samples (Fig. 4), 3 wt % water in mafic
sample M84-42 and 1·5 wt % water in felsic sample
10Ca34 (Fig. 5) was released by prograde mineral reactions. This is equivalent to 40% and 25%, respectively,
of water bound to minerals at 2008C. In both cases water
release is most pronounced at around 2508C. This observation has important implications for metamorphic processes
at very low grade as discussed in the next section.
Fig. 10. Isopleths of weight per cent H2OSolid extracted from P^T
pseudosections of (a) mafic sample M84-42 and (b) felsic sample
10Ca34.
DISCUSSION
Kinetic and fluid control of metamorphic
reactions at very low-grade conditions
Before applying any geothermobarometric method the heterogeneity of the composition of the minerals at a specific
metamorphic grade has to be understood as well as the
availability of fluids (Putnis & Austrheim, 2010). This
understanding is as important as the attainment of mineral
equilibria. Prior to this work, McMullin et al. (2010)
showed with the aid of conventional chemographic projections that within single samples from the Mira terrane,
series of mineral pairs exist at thin section scale that vary
systematically (e.g. in their XMg) suggesting changing temperatures. In a similar way, the systematic variation of
XMg with changing Si content in white mica and chlorite
in the samples of this study can also be interpreted to reflect changing P^T conditions during mineral growth
within single samples.
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However, at very low-grade conditions, calculations of
mineral equilibria using multivariant reactions (e.g.Vidal &
Parra, 2000; Willner, 2005; Willner et al., 2009; Jolivet et al.,
2010) have been carried out using compositions of single
pairs of grains in close contact. Such an approach results in a
cluster of points around a P^T path.This application of equilibrium thermodynamics is based on the specific reaction behaviour at very low metamorphic grade, as follows.
Reactions at very low grade are governed (1) by grain
size and (2) by the availability of water, resulting in different mineral compositions pretending non-equilibrium conditions. However, this circumstance allows us to define
locally a series of equilibria during changing P^T conditions (Vidal et al., 2006). The very small grain size combined with the strong compositional variability of the
reaction products as observed, for instance, for white mica
(Fig. 4) reflects high nucleation and low growth rates. As
a consequence nucleation of new grains with a different
composition occurs during changing P^T conditions
rather than only further growth of the existing grains
(Fig. 11). In this way clusters are formed that can recrystallize under low-grade conditions, resulting in larger grains
grown at the expense of smaller grains. In addition, a different composition can form at the rim of larger grains
compared with the core composition. The phenomenon of
increasing grain size with metamorphism is well known
from clastic sedimentary rocks, where the fine-grained
clastic matrix vanishes by recrystallization at temperatures
of 3208C, when grain size increases by an order of magnitude (Brix et al., 2002).
Nevertheless, this specific kinetic behaviour at very lowgrade conditions favouring the crystallization of new
grains during changing P^Tconditions also results in prograde phases remaining metastably at peak P^Tconditions
and during retrograde mineral equilibration. Thus, prograde and retrograde minerals in general cannot be distinguished by their composition in the very low-grade
regime owing to a lack of the zoning that is characteristic
at higher grade (e.g. Kryza et al., 2011).
The reason for nucleation rates exceeding growth rates
is not yet clear. In rocks of very fine grain size close to contact metamorphic aureoles, Roselle et al. (1997) observed
an exponential increase in nucleation rates relative to
growth rates owing to a high reaction affinity, which is a
measure of the departure from equilibrium (excess of free
energy) or the degree of overstepping. Lasaga (1998) proposed that in the case of supersaturation the G of nucleation increases, with an increasing number of embryo
nuclei forming clusters until a maximum with a critical
cluster size is reached and nuclei become stable. The free
energy of nucleation decreases with formation of more
stable nuclei. At very low grade, such supersaturation
most probably prevails owing to the sluggishness of reactions and fluid transport at low temperature.
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Fig. 11. Sketch showing small phengite crystals (grey) in a thin section that grew in a matrix (light grey) of quartz, feldspar and other
minerals during a water pulse at very low-grade conditions (stage I).
Subsequently at stage II, another water pulse still at very low-grade
conditions caused further growth of phengite with a different composition (dark grey). Stage III represents metamorphism at low-grade
conditions at which small phengite crystals were dissolved and larger
phengite grains grew instead, resulting in a more or less concentric
zoning pattern. During all three stages I^III deformation did not
take place.
Nucleation of new grains is activated by the availability
of water, but partly also by deformation. The apparently
incomplete and variable consumption of protolith minerals
is due to variable access to water. Hence, a variable degree
of preservation of primary minerals is a direct indicator
of a variable reaction progress. Highly variable oxidation
states mirrored, for example, by strongly variable Fe3þ
contents in white mica, reflect locally varying oxidation
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VERY LOW- TO LOW-GRADE METAMORPHIC PROCESSES
conditions during pulses of fluid flux. The clustering of reaction products that dominates the metamorphic fabric in
all rock types is a prominent indication of water transport,
at least at thin-section scale. Nucleation of specific clusters
appears to be bound to specific precursor minerals such as
plagioclase, where white mica clusters form as a result of
the high local concentration of Al, or clusters of epidote
form as a result of a high local concentration of Ca. Water
drives the metamorphic reactions and forms abundant
OH-bearing minerals such as white mica, amphibole,
chlorite, pumpellyite, prehnite, stilpnomelane and epidote.
The clustering is controlled by the size and distribution of
precursor phases and hence distances between reactants
and products vary considerably. This influences the local
reaction progress. Putnis & Austrheim (2010) highlighted
the role of hydrous fluids during any metamorphic reaction (except at very high temperature) involving dissolution, material transport and reprecipitation. The extent
of material transport is variable and hence the difference
between metasomatism and metamorphism is rather semantic or a matter of scale. Also, clustering is a metasomatic phenomenon at small scale, resulting in its
characteristic reduction of number of phases. Despite the
local precipitation in clusters and the preferential nucleation of new minerals with differing composition during
changing metamorphic conditions at low grade, the metamorphic fluid causes material transport over a thin section
scale. Hence, the fluid is in contact with protolith minerals
and all phases continuously nucleated during changing
conditions; that is, with the full chemical reservoir represented by the whole-rock composition. Hence at any stage
of the reaction progress, water is interconnecting reactants
and products, ensuring transient equilibrium conditions.
Water at low temperature may be external water liberated by dehydration of neighbouring rocks during
prograde metamorphism. Prior to burial, considerable
amounts of water can be bound in alteration minerals
(e.g. illite, smectite, laumontite) formed during near-surface hydrothermal activity. No such phases were observed
in the rocks of the Mira terrane, because they would not
have survived dehydration, which is particularly intense
at very low grade.
The intensity of dehydration, particularly at very low
and low grade, is demonstrated by the isopleths for water
bound to solids extracted from the calculated results from
the P^T pseudosections (Fig. 10). By far the most water is
released between 250 and 3008C depending on pressure.
This observation was also made for various rock types
such as metapelite, metagreywacke and calcareous rocks
by Massonne & Willner (2008) and Massonne (2010), indicating that this phenomenon specifically occurs at very
low-grade conditions and low pressures. Massonne &
Willner (2008) and Massonne (2010) also showed that the
massive water release at very low grade facilitates
deformation owing to lowering of rock viscosity.
Deformation, in turn, facilitates the reaction progress.
In summary, a continuous adaptation of metamorphic
assemblages to the changing P^Tconditions takes place in
the higher temperature part of the very low-grade metamorphic regime. Each step of the reaction sequence is still
preserved in single samples.
Considerations about the P^T path
It has been shown above that all of the studied samples
from the Mira terrane experienced continuous nucleation
and growth of metamorphic minerals during changing P^
Tconditions within a limited P^T space in the higher temperature part of the very low-grade regime (at approximately 250^3008C and 2^4 kbar). Pressure conditions of
3·5 0·4 kbar were calculated for the Mira terrane samples on the basis of the Si contents of potassic white mica.
As no Si contents higher than 3·41a.p.f.u. were observed,
we conclude that these conditions represent the peak
pressure during the metamorphism. Temperatures of
280 308C were calculated, and we assume that the metamorphic evolution from peak P conditions followed an
exhumation path at these temperatures and, thus, a clockwise P^T trajectory, but we cannot deduce if peak temperatures were reached at the peak pressure, or later.
As no difference was observed in the range of results between samples of different composition (mafic and felsic)
or between samples from a single magmatic belt, we infer
a fairly homogeneous metamorphic evolution for the
entire terrane; that is, the P^T path was similar throughout the Mira terrane, within limits. A partial P^T path
for a single sample at very low-grade conditions can be
defined by a series of local equilibria calculated with multivariant reactions using coexisting mineral compositions
(Vidal & Parra, 2000; Willner, 2005; Jolivet et al., 2010).
On the other hand, the resulting cloud of P^T points
should lie within the P^T space defined by the lowest and
highest Si contents observed in white mica within the field
of the observed prograde, peak and retrograde assemblages; that is, the P^T space where metamorphic reactions continuously occurred to produce white mica of
different compositions. Such fields may have different extents in different samples, even from the same outcrop
area, because reactions at very low-grade conditions
strongly depend on local and temporary water access as
discussed above, as well as on rock composition.
Nevertheless, the white mica P^T fields of all samples
should have an optimal overlap not only at peak metamorphic conditions, but also along parts of the pro- and
retrograde P^T path, if a similar P^T history for all
rocks is assumed.
The entire range of measured phengite compositions
within the P^T fields of the observed (prograde, peak
and retrograde) assemblages are plotted on Fig. 12. For
better resolution separate plots are made for mafic and
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JOURNAL OF PETROLOGY
VOLUME 54
NUMBER 9
SEPTEMBER 2013
The prograde metamorphic path can be corroborated
by two fields with prograde stilpnomelane as discussed
above; that is, the field of stilpnomelane in an assemblage
with actinolite, pumpellyite, chlorite and K-feldspar in
mafic sample M84-42 and in an assemblage with phengite,
chlorite and epidote in felsic sample M84-41B. The prograde path between these fields and the peak P^T conditions constrained at 280 308C and 3·5 0·4 kbar can be
described as a heating path with very little pressure increase. By contrast, the subsequent early retrograde P^T
path is characterized by decompression with slight cooling.
It is constrained towards higher temperature by the upper
temperature limit of the retrograde white mica fields in
mafic samples 10Ca13 and AS467 (Fig. 10a) and towards
the lower temperature end by the lower temperature limit
of the retrograde white mica field for felsic sample 10Ca34
(Fig. 10b).
P^T evolution in a collisional setting
Fig. 12. P^T fields corresponding to the observed ranges of white
mica compositions within observed prograde, peak and retrograde assemblages in (a) mafic rocks and (b) felsic rocks. An inferred average
P^T path is indicated by the grey dashed line, which was deduced
from the optimal overlap of these P^T fields. Hatched fields represent
those of prograde stilpnomelane in assemblage (1) with actinolite,
pumpellyite, chlorite and K-feldspar in mafic sample M84-42 and
(2) with phengite, chlorite and epidote in felsic sample M84-41B. The
P^T path passes through the derived average peak conditions at
280 308C and 3·5 0·4 kbar.
felsic samples. Similar results are shown in samples from
all of the magmatic belts of the Mira terrane. This P^T
space, where all white mica (and all other metamorphic
phases) formed, is at 240^3108C; that is, that part where
most water is internally released in both mafic and felsic
rocks and hence is available for chemical reactions (see
Fig. 5). An inferred average P^T path can be inserted in
the area of optimal overlap.
The above derived peak metamorphic conditions suggest
burial to depths of 11^14 km (calculated with 2·8 g cm3
as mean crustal density) and a low transient metamorphic
gradient of 20^258C km1. Such a low geothermal gradient
at peak metamorphic conditions is incompatible with extensional settings, which are characterized by high gradients owing to compaction of isotherms. A relationship
with the collisional assembly of crustal segments represented by the magmatic belts appears more likely.
The maximum depth is comparable with those of
exhumed deeper parts of foreland thrust-and-fold belts
(e.g. Fielitz & Mansy, 1999); that is, of a crustal section
that was only moderately thickened by a collisional event.
No relicts of hydrothermal alteration at or near the surface
during the active magmatic arc stage are preserved.
Nevertheless, such processes probably happened, as evidenced by the pervasive low-dO18 anomaly throughout
the Mira terrane and other parts of West Avalonia (Potter
et al., 2008a, 2008b; Petts et al., 2012). If earlier metamorphic
phases had been formed at 200^2508C, they were erased
by subsequent events. On the other hand, the derived
metamorphic gradient appears relatively high for deeper
levels of a stacked crust. This may be due either to advective heating related to syn-collisional intrusions or to some
near-isobaric heating during thermal relaxation close to
maximum burial, as is typical for frontal accretionary
prisms (Willner et al., 2009). The Devonian granites,
which intruded at 378 þ5/^1 Ma (U/Pb zircon; Barr &
Macdonald, 1992; Bevier et al., 1993), could represent parts
of such syn-collisional magmatism. The timing of this
plutonism, and perhaps the regional metamorphism and
deformation in the Mira terrane, coincides with events in
the now-adjacent Meguma terrane to the south. The
Meguma terrane is characterized by high-temperature^
low-pressure metamorphism, collisional deformation and
substantial syn-collisional plutonism termed the
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WILLNER et al.
VERY LOW- TO LOW-GRADE METAMORPHIC PROCESSES
Neoacadian orogeny (400^365 Ma; White & Barr, 2012).
However, the part of the Meguma terrane now juxtaposed
against Mira terrane was situated much farther to the
east at that time, as terrane shuffling and horizontal translations continued by dextral strike-slip motion well into
the Carboniferous (Murphy et al., 2011).
The growth of white mica can be correlated with specific prograde deformation events in the Mira terrane.
The same pattern of white mica compositions (and the resulting metamorphic overprint) is observed in rocks that
are dominated by a foliation related to early D1 folding
(axial plane foliation that may include several generations
of folding) and in rocks, which are dominated by non-coaxial deformation D2 of a later sinistral strike-slip event.
The same pattern prevails also in apparently undeformed
rocks. These observations suggest that an early folding
and thrusting event D1 was mainly responsible for burial
of the rocks, whereas the later strike-slip event (D2)
occurred when the rocks now exposed were close to their
maximum depth of burial. Hence, maximum peak metamorphic conditions correspond to moderate crustal
thickening during a compressive deformational episode.
This episode was followed by a period of strike-slip deformation that was probably responsible for the dispersal
of the Avalonia microplate and final emplacement of the
single slices of magmatic arcs of the Mira terrane; that is,
the final assembly of the crust of the former microplate.
Our findings thus confirm suggestions by Barr et al. (1996)
that the various belts of the Mira terrane represent amalgamated volcanic arcs. It is likely that this model applies
throughout Avalonia, including the ‘type area’ in eastern
Newfoundland, which, like Mira, consists of belts of Neoproterozoic volcanic, sedimentary, and plutonic rocks of
different ages (e.g. O’Brien et al., 1996; Barr & Kerr, 1997;
Sparkes et al., 2005).
The strike-slip event may have been related to the
Acadian collision between Avalonia and the Ganderian
microplate to the north, and/or with the Meguma terrane to the south. Prior burial and D1 deformation
may have occurred during the same orogeny. The late prograde heating with little pressure increase that characterizes the estimated average P^T path (Fig. 12) may be due
to thermal relaxation along a horizontal particle path that
could be inferred for long-term propagation of duplex formation. Alternatively, some heating might have been
related to syn-collisional plutonism. The characteristic
retrograde decompression path with slight cooling might
be due to continuous cooling by newly underthrust rock
units.
CONC LUSIONS
The use of Si isopleths for potassic white mica in pseudosections for geothermobarometry has several advantages
over the calculation of multivariant reactions. The latter
approach uses the composition of minerals in mutual contact. However, although material transport between reactants and products is on the millimetre to centimetre scale
and fluids are in contact with the entire chemical reservoir
of the rocks, precipitation is localized in clusters of small
grains with differing compositions grown during a prevailing high nucleation rate at low temperature. This results
in the frequent neighbourhood of minerals with compositions that are not in equilibrium. Such mineral pairs are
usually discarded because of their strong deviation from
realistic P^T conditions. More importantly, thermometry
in calculated equilibria generally depends on the Fe^Mg
exchange between minerals (e.g. chlorite and white mica).
However, available datasets and activity models used to
calculate these exchange reactions, even when successfully
tested over a wide P^T range, can fail when applied to
other P^Tconditions and rock types. These two shortcomings inherent in the calculation of equilibria with multivariant reactions are overcome by the calculation of
pseudosections using the compositional variation of a
single phase within a calculated P^T field of an observed
assemblage.
The application of P^T pseudosections for geothermobarometry even at very low grade has several advantages:
(1) no coexisting mineral pair with assumed equilibrium
compositions has to be selected; (2) the pseudosections
can be used to study phase relationships; (3) peak P^Tconditions reached can be quantitatively derived with reasonable accuracy; (4) the P^T space, where metamorphic
reactions occurred along a partial P^T trajectory, can be
reconstructed; the overlap of such areas derived from several samples refines the P^T path; (5) important factors
driving metamorphic reactions such as release of water
along the partial P^T path can be evaluated.
However, for the successful application of pseudosections
at very low grade it is essential to use appropriate solid
solutions for all minerals stable at these conditions (e.g.
stilpnomelane, pumpellyite) and not end-member compositions (see Fagereng & Cooper, 2010). The activity models
for white mica, pumpellyite, chlorite, amphibole, epidote
and stilpnomelane, which we employed here, result in reasonable phase relations and compositional contouring of
the P^T pseudosections. Moreover, the specific reaction
kinetics at very low grade, where small grains are steadily
produced with changing composition, implies that the
whole-rock composition does not change during reactions,
as at higher grade with pronounced mineral zoning where
the composition of the cores is shielded from the reacting
rims of minerals.
In the Mira terrane, a region with a relatively uniform
metamorphic overprint, it has been shown for various
lithologies that phengite with Si contents of up to
3·41a.p.f.u. is compatible with formation within a range of
relatively low peak P^T conditions of 3·5 0·4 kbar and
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JOURNAL OF PETROLOGY
VOLUME 54
280 308C. In the very low-grade rocks of southeastern
Cape Breton Island most water was released at about
250^3008C and drove nucleation of metamorphic minerals
during changing P^T conditions. Kinetic factors at very
low-grade conditions, such as nucleation rates exceeding
growth rate, led to preservation of a series of mineral
pairs of different compositions in close proximity at thin
section scale, resulting from transient equilibration during
changing P^T conditions. Mineral formation at very low
grade depends on local access of internally generated or
externally derived water. It should be noted that at very
low-grade conditions fabrics related to the initiation of
metamorphic reactions are particularly well preserved.
Almost all metamorphic rocks pass through this grade
during their evolution.
The pronounced availability of water at very low-grade
conditions also lowers the competency and weakens the
rocks, facilitating deformation, which in turn also triggers
mineral formation. A compressional event characterized
by the folding of volcanic and sedimentary strata of the
volcanic arcs was responsible for burial to about 11^14 km
depth. At peak metamorphic conditions a transient metamorphic field gradient of 20^258C km1 prevailed, which
was probably related to collision. Stacking by deformation
similar to that of foreland fold-and-thrust belts can be predicted. The compressional deformation was followed by a
strike-slip event that partly overprinted rocks at depth,
causing the dispersal of the microplate and the final amalgamation of slices of former magmatic arcs to the crust
constituting the Mira terrane as part of the microplate of
Avalonia. The derived metamorphic conditions describe
the thermal state during this collisional assembly. Evidence
for the timing of the metamorphic processes and related
deformation will be presented in a forthcoming paper.
AC K N O W L E D G E M E N T S
David McMullin helped with insightful comments on earlier drafts of this paper. Critical reviews by J. Allaz, J.
Schumacher, C. van Staal and an anonymous reviewer as
well as careful editorial handling by R. Giere¤ improved
the paper substantially. We thank all of them for their
contributions.
F U N DI NG
This project was financed by Deutsche Forschungsgemeinschaft (grants Ma1126-27 and Wi847-9) to H.-J.M.
and A.P.W. Geological mapping by S.M.B. and C.E.W. in
southeastern Cape Breton Island during which some of
the studied samples were collected was funded by the Geological Survey of Canada through the 1984^1989 Canada^
Nova Scotia Mineral Development Agreement and the
1990^1992 Canada^Nova Scotia Cooperation Agreement,
NUMBER 9
SEPTEMBER 2013
as well as by research grants to S.M.B. from the Natural
Sciences and Engineering Research Council of Canada.
S U P P L E M E N TA RY DATA
Supplementary data for this paper are available at Journal
of Petrology online.
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VOLUME 54
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SEPTEMBER 2013
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