JOURNAL OF PETROLOGY
VOLUME 55
NUMBER 10
PAGES 2043^2066
2014
doi:10.1093/petrology/egu049
Microstructural and Metamorphic Constraints
on the Thermal Evolution of the Southern Region
of the Lewisian Gneiss Complex, NW Scotland
M. A. PEARCE1* AND J. WHEELER2
1
CSIRO MINERAL RESOURCES FLAGSHIP, 26 DICK PERRY AVENUE, KENSINGTON, WA 6151, AUSTRALIA
2
DEPARTMENT OF EARTH AND OCEAN SCIENCES, SCHOOL OF ENVIRONMENTAL SCIENCE, LIVERPOOL UNIVERSITY,
LIVERPOOL L69 3GP, UK
RECEIVED APRIL 18, 2013; ACCEPTED AUGUST 15, 2014
Felsic metagranitoids form a major part of the crust, but the metamorphic story they record is difficult to decipher because of a lack of
index minerals. The microstructures and metamorphic assemblages
of felsic gneisses and metadolerite dykes from the Lewisian
Gneiss complex, NW Scotland, have been examined to estimate
the pressure^temperature^time (P^T^t) history of the region.
Characteristic geometries and compositions of zoned epidote and
plagioclase from the gneisses and amphibole from the dykes provide
key information. Bulk-rock compositions are modelled to constrain
the likely metamorphic conditions experienced by the rocks. P^T^t
paths are refined using a novel model for fractionation during
grain-recycling of the plagioclase. In the gneisses, plagioclase grains
have relatively albitic cores (An10^12) grading to more anorthitic
rims (An20^30). The equant grain shapes of the plagioclase and
asymmetry of the zoning across grain boundaries are consistent with
the zoning having formed during coarsening, or grain-recycling, following deformation. The increase in anorthite content is due to the
breakdown of epidote to release Ca and Al. Sharp boundaries between Fe-poor cores and Fe-rich rims in epidote result from the resorption of epidote whilst the plagioclase is growing followed by
later regrowth. The possible P^T conditions for the end of deformation and start of grain-recycling are restricted to those that occur
along the plagioclase isopleth with the same value as the core compositions. These starting conditions are explored along with P^T^t
path orientations over a range of values. The results are compared
with the observed compositions and grain sizes to determine the
best-fit P^T^t path. Most of the best-fit paths are dominated by decompression rather than heating (both of which result in epidote
breakdown). Starting conditions are probably between 9 and 10·2
The average composition of the continental crust is dioritic
(Rudnick et al., 2003). However, when considering a pressure^temperature^time (P^T^t) history, these and more
granitic compositions, typical of many high-grade terranes, are overlooked in favour of pelitic or metabasic
rocks that usually contain more indicators of metamorphic
grade. Indicators include the index minerals of the
Barrovian series and minerals commonly used in thermobarometers such as garnet (e.g. Ferry & Spear, 1978; Spear
et al., 1984), pyroxene (e.g. Pattison & Newton, 1989), and
mica (e.g. Coggon & Holland, 2002). To elucidate the history of felsic crust, assumptions need to be made about
the autochthonous or interleaved nature of associated
*Corresponding author. Telephone: þ61 8 6436 8542.
E-mail: [email protected]
ß The Author 2014. Published by Oxford University Press. All
rights reserved. For Permissions, please e-mail: journals.permissions@
oup.com
kbar at around 5808C along a quasi-linear path ending at around
7·5^8 kbar at 600^6208C. The timescale of decompression is poorly
constrained owing to the uncertainty in the grain-recycling parameters. Rates of exhumation are between 0·14 and 2 mm a^1, which
are reasonable within the range of present-day processes.
Scourie dyke assemblages and mineral zoning broadly corroborate
this P^T^t path. The path is similar to those recorded in
Phanerozoic orogenic cycles but the significance of this work lies in
our new methods for elucidating the metamorphic histories of
metagranitoids.
KEY WORDS:
P^T^t path; thermodynamic modelling; zoning
I N T RO D U C T I O N
JOURNAL OF PETROLOGY
VOLUME 55
metasediments and metabasites (e.g. Smith & Lappin,
1989) at the time of metamorphism. If independent evidence of the thermal history of the metagranitic parts of
high-grade terranes can be found (e.g. peritectic mineral
assemblages in migmatites; Johnson et al., 2013), this can
be used along with the evidence from the pelitic or basic
layers and even justify assumptions about the deformation
of the rocks. Establishing a relative sequence of events and
then quantifying the rates at which metamorphic reactions, heating, and cooling occur can be complementary
to establishing P^T^t paths from a set of different
geochronometers.
To reconstruct P^T^t histories it is necessary to have a
time series record of the conditions through which the
rocks have passed. Disequilibrium microstructures such as
reaction rims and chemical zoning (Spear et al., 1984;
Florence & Spear, 1991; O’Brien, 1997) indicate the transitions in equilibrium compositions as conditions in the
rocks changed. Integrating pressure and temperature estimates obtained from thermodynamic modelling using software packages such as THERMOCALC (Powell et al.,
1998), which combine thermodynamic datasets (Holland
& Powell, 1998) with mineral activity^composition
models, with rate-dependent processes (e.g. diffusion and
grain growth) provides more complete P^T^t paths
(Konrad-Schmolke et al., 2005; Caddick et al., 2010).
In this study we use a method whereby mineral compositions predicted from thermodynamic datasets are integrated with time-dependent grain-growth data to estimate
the conditions of metamorphism of rocks from the
Lewisian complex in NW Scotland. After a short description of the rocks upon which in this study is based, we discuss the composition and geometry of mineral zoning
patterns found in plagioclase and epidote from the
Lewisian complex to elucidate the formation mechanism
of the zoning patterns [see Pearce & Wheeler (2010) for a
review of the types of chemical zoning] in each mineral.
A conceptual model for the thermal evolution of the
gneisses is constructed. This is then quantified using constraints from the petrology of a suite of mafic dykes that
cut the gneisses and by using a new fractional crystallization model (Pearce & Wheeler, 2010) to model the zoning
in the plagioclase grains. The application of this new
method to metagranitic rocks demonstrates the usefulness
of this forward modelling approach, compared with conventional equilibrium thermobarometers, to extract P^T^
t paths from high-variance mineral assemblages.
GEOLOGIC A L S ET T I NG
The Lewisian Gneiss complex is a suite of Archaean to
Proterozoic metamorphic rocks in NW Scotland (Fig. 1a).
It is composed of acid to intermediate orthogneiss
(tonalite^trondhjemite^granodiorite, TTG) with basic
and ultrabasic metaigneous bodies and small areas of
NUMBER 10
OCTOBER 2014
metasediment. A central region of granulite-facies gneisses
is flanked to the north and south by higher strain amphibolite-facies rocks (Fig. 1a). The whole complex is intruded
by a suite of NW^SE-trending basic and ultrabasic dykes
(Scourie dykes) that have been metamorphosed and deformed. Much of the work on the metamorphic conditions
experienced by the Lewisian complex has focused on the
early stage of ultrahigh-temperature (UHT) granulitefacies metamorphism that is preserved in the central
region (e.g. Barnicoat, 1987), with recent studies focusing
on the origin of partial melts (Johnson et al., 2013). There
is much debate about the relationships and timing of igneous activity, metamorphism, and deformation across the
Lewisian complex; a recent summary has been given by
Wheeler et al. (2010).
Rocks examined in this study are from the Torridon
high-strain zone (Wheeler, 2007) in the southern highstrain, amphibolite-facies region (Fig. 1a, inset). This inlier
consists of high-strain zones that separate low-strain lacunae (Wheeler et al., 1987). In the low-strain zones an older
location fabric (Turner & Weiss, 1963) or metamorphic
banding is present, which may have resulted from anatexis
during amphibolite-facies (Love et al., 2010) or granulitefacies (Cresswell & Park, 1973) metamorphism. Two shape
fabrics [Park (1997), equivalent to the orientation fabrics
of Turner & Weiss (1963)], striking NW^SE, are present in
the high-strain zones. The two shape fabrics are defined
by amphibolite-facies minerals, are subparallel in orientation, and are differentiable only where they are cut by or
affect the dykes (Peach et al., 1907; Sutton & Watson, 1950).
Recent dating of protolith and metamorphic ages of the
gneisses in the southern Lewisian complex suggests that
the metamorphic events here (and by inference the deformation events) are of different ages from those to the
north (Love et al., 2010), although the chronology of the
rocks in the Torridon high-strain zone remains largely unexplored. Two amphibolite-facies events, the Inverian
(Evans, 1965) and the Laxfordian (Sutton & Watson,
1950), are defined, based on field criteria, elsewhere in the
Lewisian complex to be pre- and post-dyke respectively.
These terms have been used to refer to the same field relationships at Torridon (e.g. Cresswell & Park, 1973;
Wheeler, 2007). However, to avoid temporal correlations
with fabrics elsewhere in the Lewisian complex, in this
study we will use the terms ‘pre-dyke’ and ‘post-dyke’,
which refer to the age of the shape fabric development
(and therefore deformation) relative to dyke intrusion.
F I E L D R E L AT I O N S H I P S A N D
P E T RO G R A P H Y
Demonstrably pre-dyke fabrics are difficult to find; the
ages of most shape fabrics in the southern Lewisian complex are ambiguous. However, the pre-dyke samples in
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PEARCE & WHEELER
SOUTHERN LEWISIAN METAMORPHISM
Fig. 1. Location of the samples used in this study. (a) Map of NW Scotland showing the outcrop of Lewisian gneiss (shaded). Relationship of
Ruadh Mheallan block and Torridon shear zone within the Torridon inliers (inset). (b) Detailed map of Diabaig inlier showing strain variation
and the location of the samples used in this study (after Wheeler, 2007).
(continued)
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Fig. 1. Continued.
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PEARCE & WHEELER
SOUTHERN LEWISIAN METAMORPHISM
this study were collected from where the NW^SE shape
fabric is cut by a deformed Scourie dyke (DB6.38.1;
Wheeler, 2007, Fig. 3c) and a locality where a pre-dyke
fabric has been transposed by localized post-dyke deformation along a dyke margin (M18, Table 1). A suite of postdyke rocks (DB6.25.1^9) was collected from Alligin where
a Scourie dyke has been deformed and isoclinally folded
(Table 1). Here the fabrics in the gneisses are folded in
with folded Scourie dykes and share a mineral aggregate
elongation lineation with the plagioclase aggregates in the
dyke (Pearce et al., 2011). Microstructures and mineral assemblages of the gneisses are variable, reflecting the heterogeneity of multiply intruded TTG crust. Below and in
Table 1 we give a brief description of the salient features of
the rock types studied.
Gneisses
All the gneiss samples used in this study are acid to intermediate in composition and have the assemblage
Pl þ Qz þ Bt þ Ep Kfs Hbl Ms Fe-oxide. In some
samples (DB6.25.3 and DB6.25.4) the biotite is distributed
throughout the rock and does not form interconnected
layers (Fig. 2a). In others (DB6.25.8 and DB6.25.9) it
makes up a larger modal proportion of the rock and
forms connected layers along with epidote (Fig. 2b).
Where the biotite is distributed, the epidote also occurs as
isolated grains. Quartz occurs as both smaller isolated
grains (50^300 mm) and larger elongate polycrystalline
aggregates (Fig. 2a and b) with a grain size of the order of
a few millimetres. Larger quartz grains show undulose extinction and subgrain boundary development, but have
not recrystallized. Plagioclase makes up 450% of the
rocks forming an interlocking framework. Plagioclase^
plagioclase grain boundaries are often straight or gently
curving and display 1208 triple junctions or are at 908 to
plagioclase^mica phase boundaries (Fig. 2c). The grain
size of the plagioclase is variable but of the order of
300 mm in post-dyke rocks and4450 mm in pre-dyke samples. Plagioclase grains are chemically zoned with more
albitic cores and anorthitic rims (reverse zoning). Where
K-feldspar is present it occurs mixed in with the plagioclase and has a similar grain size. In samples M18 (both
pre- and post-dyke fabrics) and DB05001 amphibole is present with a grain size of a few hundred microns.
Amphibole grains are generally elongate in the direction
of the foliation with an axial ratio 3. All samples are
macroscopically high strain with an S4L shape fabric
defined by aggregates of plagioclase and quartz, and
mica-rich layers. The location fabric present in the lower
strain rocks is transposed and is parallel to the shape
fabric (Fig. 2d). The plagioclase-rich aggregates are interpreted to derive from deformation of original plagioclase
grains in a coarse-grained protolith, although this was not
necessarily isochemical or isomineralic.
Scourie dykes
Scourie dyke samples were collected from close to the
gneiss samples. Deformed dyke samples (Fig. 2e and f) are
from an isoclinally folded dyke (DB6.25.6) and undeformed
samples DB5005 and 5008 (Fig. 2g) are from just north of
the Loch Roag Line (marking the northeasternmost edge
of the deformation; Fig. 1b). The undeformed dykes have
the assemblage Hbl þ Pl þ Qz þ Ilm þ Rt þTit. The deformed
dykes
consist of
Hbl þ Pl þ Qz þ green
Bt þ Ilm þTit Ep, although in places the green
biotite has been partially retrogressed to chlorite.
Titanite rims ilmenite in both deformed and undeformed
dykes. In the undeformed dykes large grains of ilmenite
are breaking down to rutile and the aggregate is rimmed
by titanite (Fig. 2h). In the deformed dykes, the grains of
ilmenite are much smaller and distributed throughout the
rock.
The deformed dykes show an S4L shape fabric defined
by elongate amphibole grains and plagioclase aggregates
(Fig. 2e). The mineral and mineral aggregate stretching
lineation in the isoclinally folded dyke is directly downdip of the foliation on the fold limbs, parallel to the fold
hinge (Pearce et al., 2011). Plagioclase aggregates are elongate but the plagioclase within them displays smoothly curving to straight boundaries and 1208 triple junctions. As
in the gneisses, plagioclase-rich aggregates are interpreted
to be derived from deformation of original plagioclase
grains in the dyke, although this was also not necessarily
isochemical or isomineralic. In addition to the plagioclase
aggregates that are derived from the original igneous
grains, several deformed dykes in the Torridon shear zone
show circular clots of coarse-grained plagioclase and biotite (the latter partly retrogressed to chlorite). The hornblende fabric is deflected around these areas, suggesting
that they are pseudomorphing a mineral that was stronger
than the hornblende during deformation. This may have
been garnet, especially as partially as well as completely
retrogressed garnets have been observed in deformed
dykes in the Loch Braigh Horrisdale area 10 km north
of this locality (Park, 2002).
Undeformed dykes show a less uniform texture than the
deformed dykes, which varies from dyke to dyke.
However, those from this study show a relict igneous texture in which the plagioclase grains are pseudomorphed
by polycrystalline aggregates and the pyroxenes are
completely metamorphosed to amphibole (Fig. 2g).
Plagioclase grains show a variety of zoning patterns
including reverse zoning and more complex patterns with
highly calcic cores and then reverse zoned rims. Relict pyroxene has been reported from these dykes (Park &
Cresswell, 1973) but none was observed in this study.
Quartz is present as both single grains and in sievetextured amphibole (Fig. 2g).
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Table 1: Sample details including locations according to the UK National Grid
Sample
Grid reference
Protolith
Remarks on field context
DB6.38.1
NG83355979
TTG gneiss
Demonstrably pre-dyke: NW–SE shape fabric is cut by a Scourie dyke
M18
NG82375961
TTG gneiss
Both pre-dyke and post-dyke parts; pre-dyke fabric is sheared along the
DB6.25.1, 3, 4, 8, 9
NG82525697
TTG gneiss
Fabrics folded with folded Scourie dykes
DB6.25.5, 6
NG82525697
Deformed Scourie dyke
Isoclinally folded dykes with folded shape fabric; L parallel to hinge
DB05001
NG82416083
TTG gneiss
Strongly foliated gneiss with cm-scale mafic and felsic bands
DB05005, DB05008
NG82756116
Undeformed Scourie dyke
Samples from thick undeformed dyke
edge of a Scourie dyke
(a)
(b)
(c)
Ep
Qz
Bt
Qz
Bt
1cm
1cm
Pl
(d)
400μm
Pl
(e)
(f)
Pl
Hbl
1cm
(g)
1cm
(h)
Tit
Hbl
Pl
Ilm
Rt
1cm
100μm
Hbl
Fig. 2. Photomicrographs and field photograph of the lithologies studied. (a) Plane-polarized light image of the gneiss texture of DB6.25.4
showing elongate quartz aggregates and plagioclase with disseminated mica grains. (b) Plane-polarized light image of the gneiss texture of
DB6.25.9 in which mica and epidote are coarser than in DB6.25.4 and are concentrated in bands. Elongate quartz and plagioclase aggregates
are still present. (c) Cross-polarized light image of DB6.25.3 showing the nature of plagioclase grain boundaries and plagioclase^mica phase
boundaries. (d) Field photograph showing the shape fabrics in the gneisses from Alligin, which are parallel to the transposed location fabric
(banding). Hammer for scale. (e) Plane-polarized light image of deformed Scourie dyke (DB6.25.6) with elongate plagioclase layers and aligned
hornblende. (f) Plane-polarized light image of deformed Scourie dyke (DB6.25.5) dominated by elongate hornblende and with less plagioclase
and quartz than DB6.25.6. (g) Plane-polarized light image of an undeformed Scourie dyke (DB5008) showing pseudomorphed igneous texture.
Plagioclase has altered cores and sieve-textured hornblende is after pyroxene. (h) Back-scattered electron image showing the relationship between the Ti-bearing phases in an undeformed Scourie dyke (DB05005). Ilmenite (Ilm) is being replaced by rutile (Ru) and then overgrown
by titanite (Tit).
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SOUTHERN LEWISIAN METAMORPHISM
M I N E R A L C H E M I S T RY DATA
Pressure^temperature data are recorded by variations in
the compositions of solid-solution minerals. In the following section we present profiles across mineral grains showing the absolute composition and the shape of any
variations for plagioclase and epidote. Amphibole grains
from the deformed dykes show no variation in composition
within single grains so their compositions are reported as
one analysis per grain; core and rim compositions are
given for zoned amphiboles from the undeformed dykes.
Measurements were made on polished thin-sections
using either a Cameca SX100 (at the University of
Manchester) or Cameca SX50 (at the University of
California, Santa Barbara), both W-filament electron
microprobes fitted with five wavelength-dispersive X-ray
(WDX) detectors, operated at an accelerating voltage of
15 keV with a beam current of 20 nA. The beam was defocused to minimize Na loss when analysing plagioclase,
thus reducing the spatial resolution, although this was not
a problem owing to the coarse grain size of the analysed
plagioclase. Raw data and recalculated mineral compositions are presented for each element in the
Supplementary Data (available for downloading at http://
www.petrology.oxfordjournals.org).
Plagioclase grains were chosen for analysis based on the
following criteria: (1) large variations in back-scatter coefficient to maximize the range of compositions recorded in
any one grain; (2) subvertical grain boundaries measured
using a universal stage; assuming a subspherical geometry
(as is reasonable for grains showing ‘equilibrium’ grain
boundary configurations) a cut through the centre of a
grain will yield subvertical boundaries and the best
chance of analysing the centre and the edge of the grain
where compositions are likely to be extreme values.
Plagioclase
Gneisses
Zoning patterns in DB6.38.1 (Fig. 3a and b) show only a
small range of variation between An20 and An25 over
large parts of the grains, even though they were selected
as grains showing the largest compositional variation in
back-scattered electron (BSE) images. Plagioclase grains
in the pre-dyke part of M18 (a sample that contains both
pre-dyke and post-dyke fabrics) show an increase from
An18 to An25 then a small decrease, before the anorthite
content continues to increase to a maximum of An30
(Fig. 3c). Many plagioclase grains from both of these samples have thin albite-rich (An5) rims that are subparallel
to the current grain boundary network.
The exact geometry of the zoning patterns in post-dyke
rocks varies from grain to grain and compositions depend
on the bulk composition of the rock, but are generally
characterized by an increase in anorthite content from
core to rim (Fig. 4). The low (An10^12) anorthite content of
the plagioclase cores is consistent with a metamorphic
origin, rather than being igneous relics. As with the predyke rocks, some grains show albitic overgrowths on the
edges of the grains. Typical compositional profiles vary
smoothly on the scale of the microprobe point spacing
(5^10 mm). The grains analysed have cores of the order
of 100 mm with flat (i.e. no variation) compositional profiles. Whereas some rimward increases in An content are
concentric with respect to the current grain boundaries,
others are not, and the patterns are not symmetrical in relation to the grain boundaries (Fig. 4b). Grains vary
in composition along contacts with other minerals
(Fig. 4a^c), especially along plagioclase^mica grain
boundaries.
Dykes
Deformed Scourie dykes also contain zoned plagioclase
(Fig. 5a), which shows an increase in anorthite content
from core (An23) to rim (An35). The zoning patterns show
the same characteristics as those from the gneisses, with
compositionally flat cores of the order of 100 mm wide and
smooth zoning that is not always concentric with respect
to the current grain boundaries (Fig. 5b). Zoning patterns
are truncated against phase boundaries with hornblende.
Undeformed dykes, however, show a variety of zoning
patterns (Fig. 5c) although many of the grains have been
destroyed by sericitization at lower temperatures. Where
the plagioclase has survived, in areas of pseudomorphed
plagioclase phenocrysts, some of the grains have profiles
similar to those in the the deformed dykes with more albitic cores grading into anorthitic rims (Fig. 5c, grain I).
Other grains have very calcic cores (up to An60) separated
by discontinuities from slightly more albitic rims (Fig. 5c,
grains II and III), and some are uniformly anorthite-rich
(Fig. 5c, grain IV). Large grains, which may be original igneous plagioclase as they are single grains that fill most of
the euhedral outline in the igneous texture and in some
cases are simply twinned (Fig. 5d), show concentric reverse
zoning with respect to the current grain boundary.
Epidote
Because epidote is the only other calcic phase in many of
the gneisses (except for those that contain amphibole),
changes in the Ca content of the plagioclase are expected
to be mirrored by variations in epidote composition.
Epidote found in the biotite gneisses is often homogeneous
in composition. However, where there is variation it is
characterized by Fe3þ (substituting for Al3þ) and minor
Ce (substituting for Ca2þ) substitutions, forming four
broad compositional divisions (summarized in Table 2).
Fe-poor cores (1) containing 0·6^0·65 Fe3þ per formula
unit (p.f.u.) are surrounded by relatively Fe-rich rims (2)
(0·8^0·9 Fe3þ p.f.u.) separated by a sharp discontinuity
(Fig. 6). In rocks where the biotite and epidote are segregated into separate layers from the other minerals there is
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(a)
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35
30
Mol % Anorthite
Ep
Pl
Bt
25
20
15
10
5
0
0
200μm
50
100
150
200
250
300
60
70
Distance, μm
35
(b)
Pl core
Pl Rim
Mol % Anorthite
30
Bt
25
20
15
10
5
0
0
100μm
10
20
30
40
50
Distance, μm
(c)
Pl core
Pl Rim
200μm
35
35
30
Mol % Anorthite
Mol % Anorthite
30
25
20
15
10
5
0
0
100
200
300
400
500
25
20
15
10
5
0
0
20
40 60 80 100 120 140 160 180
Distance, μm
Distance, μm
Fig. 3. Plagioclase zoning patterns in pre-dyke gneisses (BSE images and chemical transects starting at the end of the white line with
circle). Black is quartz, white is biotite and grey is plagioclase. (a, b) Zoning patterns from DB6.38.1, cut by a Scourie dyke, show small
variations in the cores but very albitic rims (An5). (c) Patterns from M18 show larger variation with two stages of plagioclase growth visible in
BSE images.
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(a) DB6.25.4
SOUTHERN LEWISIAN METAMORPHISM
(b) DB6.25.3
Qz
Qz
Pl
Bt
Pl
200μm
200μm
35
30
Mol % Anorthite
Mol % Anorthite
30
25
20
15
10
5
0
Grain Boundary
35
50
100
150
20
15
10
Core
5
Core
0
25
200
250
0
300
0
50
100
150
200
250
300
Distance, μm
Distance, μm
(c) DB7.2.15
(d) M18
Qz
Qz
T2
Pl
Bt
T1
Pl
200μm
200μm
35
35
30
Mol % Anorthite
Mol % Anorthite
30
25
20
15
Core
10
5
0
25
20
Core
15
10
Core
5
0
50
100
150
200
0
250
0
50
100
150
200
Distance, μm
Distance, μm
Fig. 4. Plagioclase zoning patterns from post-dyke gneisses (BSE images and chemical transects starting at the end of the white line with circle).
Black is quartz, white is biotite and grey is plagioclase. (a) Pattern from DB6.25.4 shows asymmetry within a single grain. (b) DB6.25.3 shows
compositional variation along a plag^mica boundary and across a plag^plag grain boundary. (c) DB7.2.1 shows truncation of patterns against
quartz^plag boundary. (d) Transects from M18 show flat centres and two-stage growth patterns.
a further stage of epidote growth (3) around the edges of
the large epidote grains (Fig. 7). The large epidotes show
oscillatory zoning for which explanation can be provided
based on microprobe analyses. The irregular overgrowth
is even richer in Fe3þ than the rims (1^1·2 Fe3þ p.f.u.) and
shows heterogeneously distributed Fe-rich patches that are
aligned with the neighbouring biotite cleavage (Fig. 7b).
Substitution of Ce for Ca causes the bright spots (4) seen
in Fig. 6. This variation is less systematic and is truncated
by the Fe-rich rims. In pre-dyke gneisses (DB6.38.1) epidote
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Fig. 5. Plagioclase zoning in Scourie dykes. (a) Back-scattered electron image of plagioclase zoning: white is amphibole, black is quartz, and
grey is plagioclase. White line shows microprobe traverse starting at the end with the circle. (b) Microprobe traverse showing the anorthite content of the plagioclase. The compositionally flat core and slight decrease at the edges of the grain should be noted. (c) Plagioclase zoning patterns from a plagioclase aggregate within an undeformed Scourie dyke. Black is quartz, white is amphibole and grey is plagioclase. There is a
variation in zoning pattern from concentric reverse zoning (I), to highly calcic cores (II and III) to more calcic grains that show little zoning
(IV). (d) Photomicrograph of large plagioclases in undeformed dyke sample DB05008. The large grains that show simple twinning and zonation
could be relict igneous plagioclase grains that are beginning to recrystallize.
shows more of this rare earth element (REE) substitution
(Fig. 7c and d). These are complex patterns that resemble
those of allanites that have become metamict. Around the
edge of these euhedral epidotes there is a more irregular
overgrowth (3) with a higher iron content.
Amphibole
Amphibole was analysed in the deformed and undeformed
dykes, and amphibole-bearing gneisses. After recalculation
of Fe3þ using the method outlined by Holland & Blundy
(1994), the compositions were classified using the scheme
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Table 2: Features of epidote zoning
Feature
Pre-dyke
Pre-dyke fabrics close to post-dyke
Post-dyke
deformation
(1) Low-Fe morphology
n.a.
n.a.
Smoothly curved where present
(2) High-Fe epidote
Euhedral (Fe3þ ¼ 0·83–0·88 p.f.u.)
Subhedral (Fe3þ ¼ 0·81–0·87 p.f.u.)
Subhedral (Fe3þ ¼ 0·8–0·85 p.f.u.)
(3) V. high-Fe epidote
Irregular, widely developed (Fe3þ ¼ 1·14
Irregular, localized Composition
Irregular, localized (Fe3þ ¼ 1–1·2 p.f.u.)
(Fe3þ ¼ 0·6–0·65 p.f.u.)
p.f.u.)
(4) Ce content
unknown
Large complex patterns in the cores
Small bright spots and ghosting
Small bright spots and ghosting
n.a., not applicable.
(a)
(b)
Bt
Bt
Fe-Rich
Fe-Poor
Fe-Rich
50 μm
150 μm
(d)
0.85
0.85
0.80
0.80
Fe Content, PFU
Fe Content, PFU
(c)
0.75
0.70
0.65
0.60
0.75
0.70
0.65
0
50
100
150
200
250
300
0.60
0
20
40
60
80
100
120
Distance, μm
Distance, μm
Fig. 6. Epidote zoning patterns from post-dyke gneiss DB6.25.9 (BSE images and chemical transects starting at end of white line with circle).
(a) Fe-poor cores with a Ce-rich part overgrown by more Fe-enriched epidote. (b) Smooth boundary between Fe-poor and Fe-rich epidote
suggests resorption.
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(b)
(a)
Bt
(b)
Ep
Fe-Enrichment
Fe-Oxide
Fe-Ep
Bt Cleavage
Qtz
50 μm
200 μm
(d)
(c)
Bt with Fe-Oxide
Ep
Fe-Rich Overgrowth
Ce-Rich Core
Ce-Rich Core
200 μm
Bt
100 μm
Fig. 7. Epidote zoning patterns (BSE images) showing high-Fe overgrowths. (a) Complexly zoned epidote from post-dyke gneiss DB6.25.9 with
anhedral Fe-rich epidote. (b) Close-up of high-Fe overgrowth in (a) showing heterogeneous distribution of Fe-rich blebs aligned with biotite
cleavage. (c) Complex Ce-rich core within euhedral epidote from pre-dyke gneiss DB6.38.1. (d) Euhedral epidotes with high-Fe overgrowths, especially where in contact with biotite. The partial chloritization of biotite (darker streaks) in (a) and (b) should be noted.
Amphibole Compositions from Scourie Dykes
of Leake (1978). Some amphiboles in the undeformed dykes
are zoned with more silicic cores and more aluminous
rims. The amphiboles from the deformed dykes have a uniform composition and plot on the join between the fields
of pargasite, edenite, tschermakite and hornblende (Fig. 8).
Na + K, pfu
1.0
I N T E R P R E TAT I O N O F Z O N I N G
PAT T E R N S
The geometries of chemical zoning patterns within minerals vary according to their formation mechanism
(Jessell, 2004; Pearce & Wheeler, 2010). Diffusion zoning
patterns are smoothly varying because steep chemical potential gradients drive lattice diffusion, leading to relaxation of sharp changes in mineral chemistry. Zoning
patterns formed by diffusion of elements from the grain
boundaries into once homogeneous minerals would be expected to be concentric and symmetrical with respect to
the present grain boundaries. Lattice diffusion into
Pargasite
Edenite
Tschermakite
Hornblende
0.5
0.0
5.5
6.0
6.5
7.0
Tremolite
7.5
8.0
Si, pfu
Deformed Dykes
Undeformed Dykes
Fig. 8. Amphibole compositions in deformed and undeformed dykes
plotted as a function of alkali content on the A-site vs Si content.
Undeformed dykes show a spread between hornblende and pargasite,
whereas deformed dykes show a clustering of compositions.
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SOUTHERN LEWISIAN METAMORPHISM
minerals is unimpeded by the presence of a phase boundary. Zoning patterns may also form during mineral
growth by a reaction such as garnet growing at the expense of chlorite in a metapelite. In this case, the combination of element supply to the growing interface and P^T
conditions controls the zoning patterns (KonradSchmolke et al., 2005). Because elements are supplied by
grain boundary diffusion, it is also likely that zoning patterns will vary smoothly. An exception to this is where
growth is discontinuous, such as along a P^T path in
which the mode of a mineral first increases and then decreases, but does not completely disappear, and then increases once again. Other phases may be incorporated as
inclusions as the mineral grows. In the case of immobile
elements, existing heterogeneities may also be incorporated during the formation of growth zoning (Yang &
Rivers, 2001). Grain recycling (Pearce & Wheeler, 2010)
occurs when a grain boundary moves, consuming one
grain at the expense of another. Changes in P and T
during this process cause zoning to develop as the recycling occurs. Characteristics of these zoning patterns include (1) non-concentric zoning, because different sections
of a grain boundary can move at different rates, (2) asymmetric zoning, because one side of the grain may be growing whilst the other is being consumed, and (3) truncation
of the zoning against phase boundaries because they are
immobile (e.g. Mas & Crowley, 1996).
Plagioclase
Plagioclase zoning profiles in both deformed and undeformed dykes and gneisses show features incompatible
with the zoning having formed purely by diffusion. Not
all profiles can be modelled using a one-dimensional diffusion model, variations are not concentric with respect to
the current grain boundaries, and variations are not symmetrical across grain boundaries. Moreover, grains show
variations along phase boundaries (especially with mica)
that are consistent with the boundary being pinned
(Fig. 4a). Therefore, it is proposed that the post-dyke
zoning patterns formed by grain recycling (Pearce &
Wheeler, 2010) following deformation. Formation of
zoning during deformation would be expected to produce
a preferred orientation of zoning with respect to the kinematic fabrics in the rocks. However, single plagioclase
grains and cores are equant and show no preferred orientation for the zoning and so are most likely to have formed
post-deformation.
Plagioclase from pre-dyke gneiss samples (DB6.38.1 and
M18; Fig. 3) shows a difference in zoning patterns between
the two samples. DB6.38.1 has only weakly zoned plagioclase with a sharp discontinuity between the main part of
the grain and a highly sodic (An5) rim. This lack of
zoning suggests that DB6.38.1 did not record the same
grain recycling event as the demonstrably post-dyke
gneisses.
The two-stage patterns shown by pre-dyke plagioclase
in M18 could be due to grain-recycling during one or
both amphibolite-facies events. The plagioclase zoning
geometries in the pre-dyke parts of M18 are more akin to
the post-dyke ones in M18 than the other pre-dyke patterns (from DB6.38.1). Therefore, the cores are considered
to be produced during static recrystallization of the predyke gneisses during post-dyke metamorphism, and the
zoning develops at the same time as in the gneisses with
post-dyke deformation. Static recrystallization probably
occurs in M18 but not DB6.38.1 because M18 is close to
post-dyke deformation (part of the sample is deformed),
which promotes fluid ingress into the gneisses (Beach,
1974).
In conclusion, it is hypothesized that the zoning in postdyke gneisses and deformed Scourie dykes was formed by
grain-recycling. Plagioclase in rocks with pre-dyke fabrics
close to post-dyke shearing (and probably increased fluid
flux) recrystallized during post-dyke deformation (as
noted by Cresswell & Park, 1973), and developed their
zoning by post-dyke grain-recycling.
Epidote
There is no major substitution for Ca (except for REE,
which form allanite) in epidote, so to increase the anorthite
content of the plagioclase, epidote must break down,
which will be recorded microstructurally as resorption. In
the post-dyke rocks, the boundaries between cores and
rims are smooth and rounded (Fig. 6a and b) and are interpreted to be due to partial resorption of existing low-Fe
epidote. Epidotes showing oscillatory zoning (Fig. 7a) are
probably igneous in origin (Naney, 1983) and also exhibit
partial resorption of the oscillatory zoning. Epidote breaks
down releasing ferric iron that is either incorporated into
biotite or produces an oxide phase (e.g. hematite).
Equilibrium thermodynamic modelling (see below for full
results) suggests that the extra aluminium needed to make
anorthite comes initially from the incongruent reaction of
white mica via the following reaction:
2Epidote þ Paragonite þ 2Quartz
ð1Þ
¼ 2Albite þ 4Anorthite þ Hematite þ 3Water
2Ca2 Al2 Fe3þ Si3 O12 OH þ Na2 Al6 Si6 O20 ðOHÞ4 þ 2SiO2
¼ 2NaAlSi3 O8 þ 4CaAl2 Si2 O8 þ Fe2 O3 þ 3H2 O:
Eventually, the potassic equivalent leads to the production
of K-feldspar:
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2Epidote þ Muscovite þ 2Quartz
¼ 2K feldspar þ 4Anorthite
þHematite þ 3Water
ð2Þ
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present in the reactants from which this epidote formed
(presumably Ce is now hosted in an accessory phase).
2Ca2 Al2 Fe3þ Si3 O12 OH þ K2 Al6 Si6 O20 ðOHÞ4 þ 2SiO2
¼ 2KAlSi3 O8 þ 4CaAl2 Si2 O8 þ Fe2 O3 þ 3H2 O:
Amphibole
Further growth of more Fe-rich epidote formed subhedral
rims and new grains. This probably occurred at the end of
the main episode of grain-recycling in the plagioclase by
the reverse of reactions (1) and (2). This would result in a
decrease in the anorthite content of the plagioclase, which
can be seen in some grains (e.g. Fig. 4c). Where there is
also an anhedral Fe-rich overgrowth (Fig. 7a and b) this is
interpreted to have formed by the breakdown of biotite.
The biotite is often clouded with elongate iron-oxide
grains (Fig. 7c). Alignment of the Fe-rich patches in the anhedral epidote overgrowths (Fig. 7b), parallel with cleavage in a neighbouring biotite grain, suggests that the
compositional heterogeneity in the epidote is inherited
from the phases that the epidote overgrew. This epidote
was produced under greenschist-facies conditions
when the biotite reacted with the anorthite component of
the plagioclase to give chlorite (observed as partial retrogression of the biotite in Fig. 7a and b) and epidote. Low
mobility of the Fe3þ means that the pre-existing heterogeneities in Fe distribution in the biotite were overgrown by
the epidote [similar to the Cr zoning observed in
garnet by Yang & Rivers (2001)]. The participation of
plagioclase in this reaction is recorded as the highly albitic
rims present in both the pre- and post-dyke rocks (Figs 3
and 4d).
In the pre-dyke rocks, there are no cores of low-Fe epidote and zones of high-Fe epidote have euhedral boundaries; thus, there was no resorption prior to the growth of
the anhedral greenschist-facies overgrowths. Complex
variation in Ce content, which is absent from the postdyke epidote, suggest that these are pre-dyke epidotes. The
lack of evidence for a static metamorphic overprint during
the post-dyke amphibolite-facies metamorphism is consistent with the plagioclase showing only weak zoning from
these rocks (Fig. 3b and c). However, the Fe-rich anhedral
overgrowths show that they did register greenschist-facies
retrogression (Fig. 7d).
The Ce zoning patterns [(4), Figs 6a, b and 7c, d] are inherited as the original igneous allanites broke down
during pre-dyke metamorphism. Nucleation of epidote
around allanite grains and subsequent growth of epidote
preserves the spatial variations in Ce concentration that
led to these patterns. Lattice diffusion of Ce is considered
slower than that of major elements (Carlson, 2002) and
these patterns suggest that this may also be true of grainboundary diffusion rates. The post-dyke cores show less
REE zoning because this was destroyed when the once
larger cores started reacting out. Subsequent growth of
new epidote on the outside of existing grains and as new
grains was richer in Fe3þ but lacked Ce as none was
The lack of zoning in the amphiboles from the deformed
dykes suggests that their composition was homogenized
during deformation. Therefore, the more silicic cores in
the undeformed dykes are remnants from before the postdyke deformation event, during which the homogenization
took place. Previous workers have suggested that the
dykes were intruded into crust that was at about 5008C
(O’Hara, 1961; Tarney,1973), consistent with the more silicic
core compositions observed in the undeformed dykes.
C O N C E P T UA L M O D E L F O R
M E TA M O R P H I C E VO L U T I O N
Using the relationships identified between zoning patterns
in different minerals it is possible to construct a conceptual
model for the evolution of the Lewisian rocks including
the Scourie dykes. This is essential for evaluating which
compositions should be used when applying equilibrium
thermodynamic methods to quantify this evolution.
(1) During pre-dyke deformation of the gneisses, epidote
(Fe 0·8 p.f.u.) grew, replacing allanite of possible igneous origin, at the same time as relatively chemically
homogeneous plagioclase.
(2) Soon after intrusion of the Scourie dykes, post-dyke
metamorphism produced silicic hornblende in the
dykes and variably recrystallized plagioclase to albitic
compositions.
(3) Post-dyke deformation homogenized the plagioclase
(An10^12) and epidote (Fe 0·6) compositions in the
gneisses and the plagioclase (An23) and amphibole
(to more aluminous) compositions in the dykes.
Some static recrystallization of pre-dyke rocks
occurred close to post-dyke deformation.
(4) Post-tectonic grain recycling led to coarsening of the
plagioclase aggregates as epidote broke down, producing reverse zoning. Amphibole does not preserve
this event as zoning, possibly because it did not form
(i.e. the amphibole did not grow at this time) or
owing to later re-equilibration. Subsequent growth of
less calcic plagioclase (Figs 4c and 5) produces new
epidote (Fe 0·8 p.f.u.).
(5) Greenschist-facies growth caused albite rims and formation of anhedral epidote (Fe 1·14) during biotite
breakdown.
Q UA N T I F I E D P ^ T ^ t E S T I M AT E S
Thus far, qualitative analysis of temperatures has been
based on longstanding assumptions such as plagioclase
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PEARCE & WHEELER
SOUTHERN LEWISIAN METAMORPHISM
becoming more calcic and amphibole becoming more aluminous with increasing temperature (Goldsmith, 1982).
Conventional thermometry assumes equilibrium amongst
the calibrated mineral assemblage. However, the chemical zoning in the plagioclase is a manifestly disequilibrium
microstructure so it is unclear which, if any, of the compositions are appropriate to pair with the amphibole compositions. Moreover, the amphiboles in the deformed
dykes lack any kind of chemical zoning and so do not
record the same P^T evolution that is recorded by the
plagioclase.
To quantify the changes in pressure and temperature
with time we use a new grain-recycling model in which
local equilibrium mineral compositions constrain P and T
and grain-recycling kinetics constrain the timescale.
Starting points for the P^T^t paths are taken from the
cores of the plagioclase grains. These are inferred to
be homogenized with the rest of the bulk-rock composition during the deformation that preceded
grain-recycling, and therefore can be estimated from
equilibrium thermodynamic models using bulk-rock
compositions.
Whole-rock equilibrium
To inform the starting point for P^T^t modelling, equilibrium assemblage diagrams or pseudosections have been
drawn for a Scourie dyke (Table 3, composition DB6.25.6)
and two post-dyke gneisses (Table 3, compositions
DB6.25.3 and DB6.25.9) from the Torridon shear zone.
Also calculated are the equilibrium isopleths of anorthite
content in the plagioclase for the same bulk compositions.
The bulk compositions used for this modelling have been
calculated by combining measured mineral compositions
with modal abundance data for the phases. For the
gneisses, modal abundances were calculated using image
analysis for biotite and epidote and electron backscatter
diffraction (EBSD) maps of entire thin-sections to determine the quartz^plagioclase ratio. All iron in the epidote
is assumed to be ferric and the proportion of ferric iron in
the biotite has been estimated using the method of Droop
(1987). For the deformed dykes, EBSD data gave the relative amounts of hornblende, plagioclase and quartz and
an estimate of 1% titanite and 0·5% ilmenite. Amphibole
compositions were recalculated for ferric iron using the
method of Holland & Blundy (1994). Volume-averaged
mineral compositions were used for zoned minerals based
on microprobe transects across mineral grains. Bulk compositions determined by volume averaging of mineral compositions have been shown to be comparable with those
obtained by other methods (e.g. X-ray fluorescence;
Waters & Lovegrove, 2002) and allow removal of accessory
phases (e.g. zircon in gneisses) and minor retrograde
alteration (chlorite altering biotite) to produce a robust
estimate of the bulk-rock composition (e.g. White et al.,
2014).
Pseudosections must be used with care because the rocks
exhibit disequilibrium microstructures, but can be informative about the stability fields of different minerals.
Theriak-Domino version 03.01.2012 was used to construct
the pseudosections using the internally consistent database
of Holland & Powell (1998) with the solution models of
Diener & Powell (2012) for amphiboles, Green et al. (2007)
for clinopyroxenes, White et al. (2007) for biotite and melt,
Holland et al. (1998) for chlorite, Holland & Powell (1998)
for epidote, and Holland & Powell (2003) updated by
Baldwin et al. (2005) for feldspars.
Scourie dykes
The dyke pseudosection Fig. 9 shows that plagioclase is
predicted to be absent at high pressures (above 12 kbar
at 6008C) and is replaced by albite at greenschist-facies
conditions. The titanium phases in the undeformed
Scourie dykes show ilmenite reacting to rutile and then
titanite. Ilmenite is likely to have been the igneous titanium-bearing phase. Rutile is stable with plagioclase above
10^10·5 kbar. At lower pressures, titanite is the stable Tibearing phase. The presence of all three phases in the undeformed dykes cannot be used to specify the conditions
because the phases are not in equilibrium. The replacement of ilmenite by rutile and the later overgrowth of
titanite is consistent with a P^T^t path that starts at 11
kbar and between 580 and 6308C (where rutile is stable)
and comes down pressure into the titanite stability field.
The complex zoning patterns in the plagioclase (and the
presence of zoning in the amphiboles) from the undeformed dykes suggest that mineral compositions were
not homogenized during metamorphism of the dykes and
that homogenization was accomplished by deformation.
In the deformed dykes, plagioclase cores are An23. This
isopleth transects both the rutile stable and titanite stable
fields and it is, therefore, feasible that the rocks, moving
along the P^T^t path inferred from the undeformed
dykes, crossed the isopleths to produce plagioclase of the
correct composition. Moreover, assuming that the plagioclase is continually homogenized until deformation stops,
deformation in the dykes continued until the rocks passed
into the titanite stable field.
Gneisses
Pseudosections were drawn for two bulk compositions of
deformed gneiss (DB6.25.3 and DB6.25.9) to illustrate the
possible variability in plagioclase compositions within the
felsic gneisses (Fig. 10a and b respectively). Sample
DB6.25.3 contains largely plagioclase and quartz, with biotite grains distributed throughout the sample and a few
percent epidote. In contrast, DB6.25.9 contains quartz
and plagioclase with layers of biotite (Fig. 2b) and zoned
epidote (Fig. 6). The variation in bulk composition does
not dramatically alter the gradient of the plagioclase isopleths in Fig. 10 but does change the anorthite content of
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Table 3: Bulk compositions used for thermodynamic modelling (mol %), one deformed dyke (DB6.25.6) and two gneiss
compositions (DB6.25.3 and DB6.25.9); all are modelled with excess water
Sample
SiO2
Al2O3
FeO
MgO
CaO
Na2O
K2O
TiO2
O
DB6.25.6
54·8
9·73
11·52
7·59
10·74
3·58
0
0·73
1·31
DB6.25.3
72·89
10·02
3·57
2·92
3·11
5·33
1·29
0·39
0·48
DB6.25.9
72·83
9·43
4·08
2·86
4·8
3·71
1·26
0·32
0·71
Fig. 9. Equilibrium assemblage diagram for deformed Scourie dyke sample DB6.25.6. Equilibrium mineral assemblages are shown along with
plagioclase isopleths (dotted lines) showing mol % anorthite in the plagioclase.
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PEARCE & WHEELER
SOUTHERN LEWISIAN METAMORPHISM
Fig. 10. Equilibrium assemblage diagram for two bulk compositions of deformed gneiss (a) DB6.25.3 and (b) DB6.25.9. Plagioclase isopleths
(dashed lines) show mol % anorthite in the plagioclase.
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the plagioclase at the epidote-out isopleths (i.e. the maximum attainable anorthite content in the plagioclase). The
mineral assemblage observed in the modelled samples,
and in much of the Lewisian complex, is Pl þ Qz þ Bt þ
Ep Ms Fe-oxide in varying proportions, with the addition of K-feldspar in the more granitic part produced by
early partial melting. The commonly observed assemblage
occurs in the shaded fields in Fig. 10, along with rutile.
Rutile is not observed in the equilibrium metamorphic assemblage but is predicted because the modelled biotite
composition contains less Ti than the observed composition. For the epidote-poor and epidote-rich rocks, the
maximum predicted anorthite content in the absence of
K-feldspar is 19 mol % and 25 mol%, respectively, assuming whole-rock equilibrium. Slightly higher observed maximum anorthite contents of 23 mol% and 29 mol %,
respectively, suggest that a non-equilibrium model may be
necessary to generate higher anorthite contents in the
plagioclase. The complete conversion of the igneous dyke
assemblage (even in the absence of deformation) to metamorphic amphibole is consistent with extensive water
availability during post-dyke metamorphism. Combined
with the absence of evidence for partial melting in the
gneisses, this suggests that post-dyke metamorphism
occurred in the sub-solidus region for wet melting and
therefore below 6508C. Conversely, pre-dyke gneisses are
interpreted to have undergone partial melting (Cresswell
& Park, 1973) at higher temperatures.
The model uses the mean grain size as predicted from the
power-law grain growth law with experimentally constrained parameters for plagioclase (Dresen et al., 1996).
Microstructurally constrained models of grain-recycling
(e.g. Jessell et al., 2003) show that the exact zoning geometries are a function of the rock microstructure. However,
the mean grain-size model results give the plagioclase
compositions and mean grain size achievable for a given
P^T^t path.
The increase in anorthite content from core to rim is
consistent with the plagioclase growing whilst epidote was
breaking down. From the bulk composition equilibrium assemblage diagrams this can result from both an increase
in temperature and a decrease in pressure. Because the
grain growth law is temperature- but not pressure-dependent, the ratio of temperature change to pressure change
will feed-back into how the composition changes with
grain size.
Metamorphic fractionation
Modelled P^T^t paths
We have used the model outlined by Pearce & Wheeler
(2010) that predicts, using Theriak-Domino, how plagioclase composition evolves during grain-recycling. This
model, designed for felsic rocks, fractionates the bulk-rock
composition by removing most of the plagioclase, as lattice
diffusion length-scales in plagioclase are relatively short
(of the order of 1 mm Ma^1) at amphibolite-facies temperatures. Therefore, this model assumes zero lattice diffusion
so that most of the plagioclase is isolated from the
bulk composition. Depending on the processes active
(e.g. grain boundary migration, diffusion creep) during
metamorphism, a small amount of plagioclase may be accessible to the reacting mineral assemblage. In the case of
the post-deformation grain-recycling experienced by the
gneisses, the model proceeds as follows.
A wide range of P^T^t paths were investigated for both
bulk compositions. The following parameters were
explored to produce zoning patterns, the grain size and
plagioclase compositions of which can be compared with
those observed in the natural rocks.
(3) Following a short time-step, during which grain recycling occurs, the volume of plagioclase swept by the
grain boundaries is calculated and this amount of
plagioclase is added back into the bulk composition.
A new equilibrium assemblage is then calculated
with the new ‘effective’ bulk composition.
(4) This process is repeated along a P^T^t path with the
plagioclase composition evolving as a function of P, T
and the effective bulk composition.
(1) The plagioclase compositions are homogenized
during the preceding deformation so an initial wholerock equilibrium assemblage (with mineral volumes
and compositions) can be calculated for a starting
P and T.
(2) The grain size, set by the size of the observed cores, is
used to calculate the grain volume and therefore the
number of grains in the rock at this time. All of the
plagioclase is removed from the bulk composition.
2060
(1) Temperature: varied between 555 and 6108C. Lower
temperatures mean that the grains do not grow and
higher temperatures very quickly lead to melting,
which is not observed in the post-dyke rocks at
Torridon.
(2) Pressure: correlated with temperature approximately
along the An12 isopleth. The temperature range used
gives a pressure range of 9 to 11 kbar.
(3) P^T^t path orientation: linear paths were tested for a
full range of orientations that crossed plagioclase isopleths. More complex paths could be tested but
linear paths give a reasonable fit to the observed
range of compositions and grain size. Furthermore,
the lack of microstructural constraints on zoning patterns means that details of single patterns may be due
to local fluctuations in boundary migration rates
rather than variations in P^Tconditions.
(4) Rate: the rate of movement along the path is a function of the path orientation. Rates are defined in
PEARCE & WHEELER
SOUTHERN LEWISIAN METAMORPHISM
terms of pressure change (an isothermal decompression path) and temperature change (isobaric heating)
with respect to time. Intermediate paths are a linear
combination of these two rates. The final results use
rates of movement such that the decompression path
varies from a rate of 3·5 to 8 kbar Ma^1 and the heating path has rates between 35 and 808C Ma^1.
For the range of conditions explored, only a small subset of
P^T^t paths give a combination of the correct plagioclase
rim composition (20^25 mol % anorthite for DB6.25.3 and
25^30 mol % for DB6.25.9) and grain size (280^320 mm).
Most of the successful paths are dominated by decompression rather than heating (Fig. 11a and d). Along these paths,
the pressure changes from between 8·5 and 9·3 kbar to
between 7 and 8 kbar over a small temperature range of
580^6258C. The grain growth rate is maximized by selecting the lower experimental bound for activation energy
(356 25 kJ mol^1) and highest experimental bounds for
pre-exponent [(2·59 0·52) 10^4] and grain growth exponent (2·6 1) to account for an additional driving force contribution from the chemically induced grain boundary
migration (e.g. Evans et al., 1986). This driving force is not
known for plagioclase; however, McCaig et al. (2007)
showed that grain boundary migration velocities of 150 mm
in calcite grains are the same order of magnitude as those
attributed to chemically induced grain boundary migration
by Hay & Evans (1992). Using these parameters, the length
of time taken for the grain recycling event is c.3 Ma, depending on the exact P^T^t path (Fig. 11b and e) and is likely to
be an upper bound. To give an idea of the uncertainty on
the timescales involved, using the median values of the
grain growth parameters gives timescales of the order of 50
Ma for the same P^T paths. All the successful paths produced a consistent zoning profile (Fig. 11c) although it
should be stressed that the exact shape of this profile will
almost certainly be influenced by the local microstructure
and therefore should be treated with caution.
DB6.25.3
(a)
(b)
(c)
DB6.25.9
(d)
(e)
(f)
Fig. 11. Model results exploring a range of P^T^t path orientations for samples DB6.25.3 (a^c) and DB6.25.9 (d^f). (a, d) P^T plots showing
all the modelled P^T^t paths (grey) highlighting those that produce the desired plagioclase compositions and grain sizes (280^320 mm).
(b, e) Grain-size evolution curves for successful P^T^t paths. (c, f) All modelled plagioclase zoning patterns (grey) and the successful subset
(black).
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JOURNAL OF PETROLOGY
VOLUME 55
DISCUSSION
Amphibolite-facies metagranitic rocks commonly have
high-variance mineral assemblages to which it is difficult
to apply conventional thermobarometry. Constraints from
grain-recycling zoning models incorporating metamorphic fractionating are discussed, as is the variation in
exhumation rates. The conditions are compared with existing thermobarometric data. Finally, the interaction between growth and grain-recycling is reviewed with
respect to P^T^t paths.
Interaction of growth and recycling zoning
The only major Ca-bearing phase in the gneisses other
than plagioclase is epidote, so changing the modal proportion of epidote changes the amount of Ca available to be
included in plagioclase. Modelling of the formation of
grain-recycling zoning gives information about the coexisting amount of epidote (Fig. 12). As plagioclase grows and
epidote is being consumed (Fig. 12, A, B, C) no zoning is
being recorded in the epidote. This continues (Fig. 12, B)
until the P^T^t path predicts an increase in the modal
amount of epidote (Fig. 12, C). All of this path is recorded
by the plagioclase. To grow epidote, Ca must be removed
from the plagioclase. Calcium removal is unlikely to be by
diffusion because it is too slow at these conditions, but can
be by grain-recycling, causing lower Ca rims on the outside
of the plagioclase. Further evidence that the plagioclase
and epidote zoning record a decrease in pressure followed
by cooling is the composition of the epidote overgrowths.
Following from the assumption that the plagioclase compositions were homogenized completely during the deformation that preceded grain recycling, the core
compositions of both the epidote and plagioclase are determined by the initial pressure and temperature of the modelled P^T path. Whilst epidote is being resorbed there is
no record of the equilibrium composition of epidote. Once
epidote begins to grow again, the record is resumed. If the
equilibrium composition of epidote is now different
(owing to changes in P^T since epidote was last grown) a
rim of the new composition will be produced. The contrast
in epidote and plagioclase zoning patterns highlights the
fundamental difference when considering growth zoning
(epidote) and recycling zoning (plagioclase) patterns.
Epidote records only the times when it grows, whereas
plagioclase records P^Tchanges by recycling (Fig. 12).
Implications for the tectonothermal history
of the Lewisian complex
Pre-dyke deformation and metamorphism
Pre-dyke gneisses, defined on the basis of field criteria,
show varying microstructures depending on their proximity to deformation zones. Those close to (on a thin-section
scale) zones of post-dyke shearing show recrystallization
and generation of the same chemical zoning patterns as in
Plagioclase
in P
A
lag
Mod
al
Pr e ssu r e, kbar
10.2
OCTOBER 2014
and
%E
Fe
Epidote
A
in E
p
p
B
Out
An
NUMBER 10
Epi
dote
B
C
C
7.8
D
D
580
620
Temperature, ºC
Fig. 12. Summary of the relationship between the plagioclase and epidote zoning and the modelled P^T^t path. Contours shown on the P^T
diagram (parallel to the epidote-out line) describe the anorthite content of the plagioclase (increasing with increasing temperature). These contours are also subparallel to the ferric iron content of the epidote (increasing with increasing temperature) and epidote modal abundance
(decreasing with increasing temperature). Exact values will depend on the exact P^T^t path chosen and bulk-rock composition. Diagrams represent the evolution of plagioclase and epidote. It should be noted that the plagioclase grains always become larger through grain recycling
zoning, even when the modal abundance of plagioclase is decreasing owing to anorthite removal. In contrast, epidote grains grow and shrink
as the modal abundance increases and decreases respectively.
2062
PEARCE & WHEELER
SOUTHERN LEWISIAN METAMORPHISM
the post-dyke rocks, but without the accommodation of
strain. This could be due to a low stress causing minor deformation, which provides an extra driving force in addition to any compositional disequilibrium or presence of
fluids. Conditions for pre-dyke metamorphism can be constrained by the presence of epidote and partial melt
(Cresswell & Park, 1973), with temperatures in excess of
6508C. The lack of significant plagioclase zoning in predyke gneisses further from zones of post-dyke deformation
(DB6.38.1) indicates that not all the gneisses recrystallized
during post-dyke metamorphism. The lack of plagioclase
zoning in these rocks is matched by a corresponding lack
of zoning in epidote. The rocks followed the same P^T
path as the post-dyke rocks but the plagioclase was already
coarse, reducing the driving force for grain recycling.
Grain boundary mobility may have been reduced by
there being very little free fluid along the grain boundaries. Without grain-recycling occurring in the plagioclase
the epidote does not break down, effectively inhibiting reaction. This supports the assumption that, in the absence
of deformation, nucleation in these rocks is negligible
during post-dyke metamorphism. This may be because
the rocks were already at amphibolite facies and therefore
there is only a small chemical driving force. In contrast,
nucleation rates dominate over growth during greenschistfacies metamorphism of igneous andesine in gabbros
(Jiang et al., 2000) because the original plagioclase composition is further from equilibrium.
Amphibole in undeformed dykes shows chemical zoning
from more silicic cores to more aluminous rims. This is
consistent with early replacement of igneous pyroxenes by
more silicic amphiboles at 5008C following intrusion at
depth, as suggested elsewhere in the Lewisian complex
(O’Hara, 1961; Tarney, 1973), and then further growth of
hornblende during later metamorphism.
Post-dyke deformation and grain-recycling
Continuity of the zoning between plagioclase cores and
rims suggests that there was no deformation after the
cores formed (An23 in dykes, An12^15 in gneisses). The modelled P^T^t path for the gneisses starts between 9 and
10·3 kbar at 555^5808C and ends around 7·5 kbar at
around 600^6208C. This path combines decompression
with heating and is likely to form part of a clockwise P^T
path that involves crustal thickening. The deformation
occurs at or around peak pressure, followed by the heating
and exhumation recorded by the recycling zoning. The
rocks reached slightly higher pressures than the modelled
P^T^t path, consistent with the presence of relict rutile in
the undeformed dykes. The rutile would have crystallized
at peak pressure and then reacted (along with relict igneous ilmenite) to titanite as the decompression occurred.
Although P^T^t paths have not been modelled for the deformed dykes, the pseudosection shows that these are compatible with the mineral assemblage observed in the
dykes. However, the predicted plagioclase composition is
too albitic (An14^16) compared with the observed compositions (An23). The An23 isopleth is crossed as the rocks decompress along the modelled path (about 8·5 kbar at
6008C) and this could be inferred to mean that the dykes
continued to deform (and therefore have homogenized
plagioclase compositions) whilst the gneisses were
undergoing static grain recycling. This model, in which deformation is partitioned into the dykes, is consistent with
observations that the dykes are weaker than the gneisses
(Pearce et al., 2011) and with the dykes accommodating a
large proportion of the post-dyke deformation (Wheeler,
2007).
Rates of decompression
P^T^t modelling indicates that the exhumation recorded
by the plagioclase zoning occurred over a time interval of
between 3 and 50 Myr depending on the grain growth
parameters used. The pressure change for these paths is of
the order of 2 kbar, which equates to 7·5 km change in
depth. On the fastest timescale this is equivalent to
2 mm a^1, which is within estimates for syn-orogenic tectonic exhumation for mid-crustal rocks (Searle et al., 1997;
Little et al., 2005) usually coupled with erosion (Burbank,
2002). For the same change in pressure over 50 Myr, the exhumation rate is 0·14 mm a^1, which is more of the order
of post-orogenic exhumation rates driven purely by erosion
(e.g. Gibson et al., 2007). Although both of these rates are
possible given the uncertainty in the experimentally
derived grain-growth parameters, the exhumation occurs
at a constant or slightly increasing temperature rather
than the cooling that would be expected for slow, postorogenic exhumation.
Lewisian thermochronometry
There are very few temperature constraints on metamorphism in the southern Lewisian complex. Droop et al.
(1999) calculated conditions of 530^6308C and 6·5 kbar
for metamorphism during the deformation of the Loch
Maree Group. It has been shown here that the coarse undeformed plagioclase grains developed their zoning as a
result of static metamorphism and grain growth.
However, this does not preclude concurrent deformation
in another part of the complex. The fastest exhumation
rates without deformation in this part of the complex require that rocks elsewhere are deforming. The temperature
conditions calculated here are consistent with temperatures from the Loch Maree Group but the pressures are
higher, although the lowest pressures registered here are
within error of the Droop et al. (1999) calculations. Given
that Torridon is about 10 km from the exposures of the
Loch Maree Group, a difference of 3^5 km is not excessive.
Although the contact is now tilted to the vertical, this
would imply that the Torridon area was structurally below
the Loch Maree Group, in accord with fig. 4d of Wheeler
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JOURNAL OF PETROLOGY
VOLUME 55
et al. (2010). The pressures estimated here are higher than
would be expected for the temperatures, given a typical
geothermal gradient of 258C km^1. The peak pressures
may be even higher, given the presence of rutile and possibly garnet in some of the Scourie dykes. High pressures
are consistent with the tectonic model of the Loch Maree
Group being a subduction accretion prism (Park et al.,
2001), such that the rocks at Torridon are in the footwall of
a subduction zone. The post-dyke deformation represents
collision of these rocks with, and their overthrusting by,
another continental unit so as to create the Lewisian complex in the form we see it today (apart from the later
Laxfordian folding). The grain-recycling zoning then
occurred during the post-collisional heating and exhumation characterized by a clockwise P^T^t path.
NUMBER 10
OCTOBER 2014
Gareth Seward is thanked for access to the microprobe at
the University of California, Santa Barbara. Chris Clark
and Alistair White are thanked for help with pseudosections involving multiple amphiboles. Dave Waters, Rob
White and Tim Johnson are thanked for thorough reviews
that helped improve the quality of the results.
FU NDI NG
This work was supported by the University of Liverpool
through a University of Liverpool Studentship (M.A.P.).
Continued development of the ideas was supported during
the tenure of an Office of the Chief Executive
Postdoctoral Fellowship (M.A.P.) from CSIRO.
S U P P L E M E N TA RY DATA
CONC LUSIONS
Metamorphic conditions for metamorphosed granitic
rocks are generally poorly constrained unless they are
associated with lower variance assemblages (pelites or
metabasites). It has been shown here using microstructural
and thermodynamic modelling that it is possible to extract
not only estimates of peak metamorphic conditions but
also P^T^t paths preserved in chemically zoned epidote,
amphibole and plagioclase.
(1) Recrystallization of igneous pyroxene to more silicic
amphiboles in the Scourie dykes and some gneisses
occurs at lowest amphibolite facies.
(2) Deformation occurs at relatively high pressures 410
kbar and finishes at 5808C and 10 kbar.
(3) Grain-recycling zoning records variations in stable
plagioclase composition during exhumation of amphibolite-facies gneisses from 10 to 8 kbar.
Subsequent cooling reaches greenschist facies, which
is recorded as albite-rich overgrowths.
This zoning is preserved on relatively long timescales
owing to slow lattice diffusion in plagioclase. Generation
of grain-recycling zoning in plagioclase through changes
in the modal proportion of epidote results in zoning in
both minerals. However, a more continuous record is created by the grain-recycling of plagioclase, whereas the
growth zoning in epidote records only when epidote stability increases. This study highlights the need to examine
the mechanisms by which chemical zoning forms, and
how minerals relate to each other, before embarking on
quantitative chemical modelling and provides a strategy
for the study of metagranitoids.
AC K N O W L E D G E M E N T S
Dave Plant and Kate Brodie are thanked for arranging
access to the microprobe at the University of Manchester.
Supplementary data for this paper are available at Journal
of Petrology online.
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