Microstructural and Metamorphic Constraints on the Thermal Evolution of the Southern Region of the Lewisian Gneiss Complex, NW Scotland

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1 JOURNAL OF PETROLOGY VOLUME 55 NUMBER1 PAGES 243^ doi:1.193/petrology/egu49 Microstructural and Metamorphic Constraints on the Thermal Evolution of the Southern Region of the Lewisian Gneiss Complex, NW Scotland M. A. PEARCE 1 * AND J. WHEELER 2 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, 213; ACCEPTED AUGUST 15, 214 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 (An 1^12 ) grading to more anorthitic rims (An 2^3 ). 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 1 2 kbar at around 588C along a quasi-linear path ending at around 7 5^8 kbar at 6^628C. The timescale of decompression is poorly constrained owing to the uncertainty in the grain-recycling parameters. Rates of exhumation are between 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 INTRODUCTION The average composition of the continental crust is dioritic (Rudnick et al., 23). 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, 22). To elucidate the history of felsic crust, assumptions need to be made about the autochthonous or interleaved nature of associated *Corresponding author. Telephone: þ mark.pearce@csiro.au ß The Author 214. Published by Oxford University Press. All rights reserved. For Permissions, please journals.permissions@ oup.com

2 JOURNAL OF PETROLOGY VOLUME 55 NUMBER 1 OCTOBER 214 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., 213), 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., 25; Caddick et al., 21). 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 (21) 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, 21) 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. 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., 213). 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. (21). Rocks examined in this study are from the Torridon high-strain zone (Wheeler, 27) 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., 21) 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.,197; Sutton & Watson, 195). 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., 21), 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, 195), 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, 27). 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. GEOLOGICAL SETTING 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 FIELD RELATIONSHIPS AND PETROGRAPHY 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 244

3 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, 27). (continued) 245

4 JOURNAL OF PETROLOGY VOLUME 55 NUMBER 1 OCTOBER 214 Fig. 1. Continued. 246

5 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, 27, 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., 211). 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 (DB and DB6.25.4) the biotite is distributed throughout the rock and does not form interconnected layers (Fig. 2a). In others (DB 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 (5^3 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 45% of the rocks forming an interlocking framework. Plagioclase^ plagioclase grain boundaries are often straight or gently curving and display 128 triple junctions or are at 98 to plagioclase^mica phase boundaries (Fig. 2c). The grain size of the plagioclase is variable but of the order of 3 mm in post-dyke rocks and445 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 DB51 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 DB55 and 58 (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., 211). Plagioclase aggregates are elongate but the plagioclase within them displays smoothly curving to straight boundaries and 128 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 1 km north of this locality (Park, 22). 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). 247

6 JOURNAL OF PETROLOGY VOLUME 55 NUMBER 1 OCTOBER 214 Table 1: Sample details including locations according to the UK National Grid Sample Grid reference Protolith Remarks on field context DB NG TTG gneiss Demonstrably pre-dyke: NW SE shape fabric is cut by a Scourie dyke M18 NG TTG gneiss Both pre-dyke and post-dyke parts; pre-dyke fabric is sheared along the edge of a Scourie dyke DB6.25.1, 3, 4, 8, 9 NG TTG gneiss Fabrics folded with folded Scourie dykes DB6.25.5, 6 NG Deformed Scourie dyke Isoclinally folded dykes with folded shape fabric; L parallel to hinge DB51 NG TTG gneiss Strongly foliated gneiss with cm-scale mafic and felsic bands DB55, DB58 NG Undeformed Scourie dyke Samples from thick undeformed dyke (a) (d) (g) 1cm 1cm Hbl Bt Pl Pl Qz (b) (e) (h) Qz 1cm 1cm Tit 1μm Ep Hbl Ilm Hbl Fig. 2. Photomicrographs and field photograph of the lithologies studied. (a) Plane-polarized light image of the gneiss texture of DB showing elongate quartz aggregates and plagioclase with disseminated mica grains. (b) Plane-polarized light image of the gneiss texture of DB in which mica and epidote are coarser than in DB and are concentrated in bands. Elongate quartz and plagioclase aggregates are still present. (c) Cross-polarized light image of DB 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 DB (g) Plane-polarized light image of an undeformed Scourie dyke (DB58) 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 (DB55). Ilmenite (Ilm) is being replaced by rutile (Ru) and then overgrown by titanite (Tit). Pl Rt Bt Pl (c) (f) 4μm 1cm 248

7 PEARCE & WHEELER SOUTHERN LEWISIAN METAMORPHISM MINERAL CHEMISTRY 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 SX1 (at the University of Manchester) or Cameca SX5 (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 2 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 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 DB (Fig. 3a and b) show only a small range of variation between An 2 and An 25 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 An 18 to An 25 then a small decrease, before the anorthite content continues to increase to a maximum of An 3 (Fig. 3c). Many plagioclase grains from both of these samples have thin albite-rich (An 5 ) 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 (An 1^12 ) 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 (5^1mm). The grains analysed have cores of the order of 1 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 (An 23 )torim(an 35 ). The zoning patterns show the same characteristics as those from the gneisses, with compositionally flat cores of the order of 1 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 An 6 ) 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 Fe 3þ (substituting for Al 3þ ) and minor Ce (substituting for Ca 2þ ) substitutions, forming four broad compositional divisions (summarized in Table 2). Fe-poor cores (1) containing 6^ 65 Fe 3þ per formula unit (p.f.u.) are surrounded by relatively Fe-rich rims (2) ( 8^ 9 Fe 3þ 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 249

8 JOURNAL OF PETROLOGY VOLUME 55 NUMBER 1 OCTOBER 214 (a) Ep Pl Bt (b) (c) Mol % Anorthite 2μm 2μm Bt Pl core Pl Rim 1μm Pl core Pl Rim Mol % Anorthite Mol % Anorthite Distance, μm Distance, μm Distance, μm Mol % Anorthite 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 (An 5 ). (c) Patterns from M18 show larger variation with two stages of plagioclase growth visible in BSE images. 25

9 PEARCE & WHEELER SOUTHERN LEWISIAN METAMORPHISM (a) DB Qz (b) DB Qz Mol % Anorthite Mol % Anorthite 2μm μm Distance, μm Distance, μm Mol % Anorthite Mol % Anorthite μm ( c) DB7.2.1 (d) M18 Pl Core Core Pl Qz 5 2μm Core T1 Core Grain Boundary Distance, μm Qz Core T2 5 1 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 DB shows asymmetry within a single grain. (b) DB 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. Pl Bt Pl Bt 3 2 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 Fe 3þ than the rims (1^1 2 Fe 3þ 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 251

10 JOURNAL OF PETROLOGY VOLUME 55 NUMBER 1 OCTOBER 214 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 DB58. 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 Fe 3þ using the method outlined by Holland & Blundy (1994), the compositions were classified using the scheme 252

11 PEARCE & WHEELER SOUTHERN LEWISIAN METAMORPHISM Table 2: Features of epidote zoning Feature Pre-dyke Pre-dyke fabrics close to post-dyke deformation Post-dyke (1) Low-Fe morphology n.a. n.a. Smoothly curved where present (Fe 3þ ¼ 6 65 p.f.u.) (2) High-Fe epidote Euhedral (Fe 3þ ¼ p.f.u.) Subhedral (Fe 3þ ¼ p.f.u.) Subhedral (Fe 3þ ¼ 8 85 p.f.u.) (3) V. high-fe epidote Irregular, widely developed (Fe 3þ ¼ 1 14 p.f.u.) Irregular, localized Composition unknown Irregular, localized (Fe 3þ ¼ p.f.u.) (4) Ce content Large complex patterns in the cores Small bright spots and ghosting Small bright spots and ghosting n.a., not applicable. (a) (c) Fe Content, PFU Fe-Rich Bt Fe-Poor (b) (d) Bt Fe-Rich 15 μm 5 μm Distance, μm 25 3 Fe Content, PFU Distance, μm Fig. 6. Epidote zoning patterns from post-dyke gneiss DB (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

12 JOURNAL OF PETROLOGY VOLUME 55 NUMBER 1 OCTOBER 214 (a) Bt (b) Ep (b) Fe-Ep Fe-Enrichment Fe-Oxide (c) Qtz Ep Bt with Fe-Oxide Ce-Rich Core 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). INTERPRETATION OF ZONING PATTERNS The geometries of chemical zoning patterns within minerals vary according to their formation mechanism (Jessell, 24; Pearce & Wheeler, 21). 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 Bt Cleavage 2 μm 5 μm (d) Ce-Rich Core 2 μm 1 μm Fe-Rich Overgrowth Fig. 7. Epidote zoning patterns (BSE images) showing high-fe overgrowths. (a) Complexly zoned epidote from post-dyke gneiss DB 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 DB (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. Na + K, pfu 1..5 Bt Amphibole Compositions from Scourie Dykes Pargasite Si, pfu Edenite Tschermakite Hornblende Tremolite 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. 254

13 PEARCE & WHEELER 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 (Konrad- Schmolke et al., 25). 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, 21). Grain recycling (Pearce & Wheeler, 21) 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, 21) 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 (DB and M18; Fig. 3) shows a difference in zoning patterns between the two samples. DB has only weakly zoned plagioclase with a sharp discontinuity between the main part of the grain and a highly sodic (An 5 ) rim. This lack of zoning suggests that DB 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 DB 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 2Ca 2 Al 2 Fe 3þ Si 3 O 12 OH þ Na 2 Al 6 Si 6 O 2 ðohþ 4 þ 2SiO 2 ¼ 2NaAlSi 3 O 8 þ 4CaAl 2 Si 2 O 8 þ Fe 2 O 3 þ 3H 2 O: Eventually, the potassic equivalent leads to the production of K-feldspar: 2Epidote þ Muscovite þ 2Quartz ¼ 2K feldspar þ 4Anorthite ð2þ þhematite þ 3Water 255

14 JOURNAL OF PETROLOGY VOLUME 55 NUMBER 1 OCTOBER 214 2Ca 2 Al 2 Fe 3þ Si 3 O 12 OH þ K 2 Al 6 Si 6 O 2 ðohþ 4 þ 2SiO 2 ¼ 2KAlSi 3 O 8 þ 4CaAl 2 Si 2 O 8 þ Fe 2 O 3 þ 3H 2 O: 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 Fe 3þ 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 (21)]. 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, 22) 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 Fe 3þ but lacked Ce as none was present in the reactants from which this epidote formed (presumably Ce is now hosted in an accessory phase). Amphibole 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 58C (O Hara, 1961; Tarney, 1973), consistent with the more silicic core compositions observed in the undeformed dykes. CONCEPTUAL MODEL FOR METAMORPHIC EVOLUTION 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 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 (An 1^12 ) and epidote (Fe 6) compositions in the gneisses and the plagioclase (An 23 ) 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 8 p.f.u.). (5) Greenschist-facies growth caused albite rims and formation of anhedral epidote (Fe 1 14) during biotite breakdown. QUANTIFIED P^T^t ESTIMATES Thus far, qualitative analysis of temperatures has been based on longstanding assumptions such as plagioclase 256

15 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 DB 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 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, 22) 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., 214). 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 was used to construct the pseudosections using the internally consistent database of Holland & Powell (1998) with the solution models of Diener & Powell (212) for amphiboles, Green et al. (27) for clinopyroxenes, White et al. (27) for biotite and melt, Holland et al. (1998) for chlorite, Holland & Powell (1998) for epidote, and Holland & Powell (23) updated by Baldwin et al. (25) for feldspars. Scourie dykes The dyke pseudosection Fig. 9 shows that plagioclase is predicted to be absent at high pressures (above 12 kbar at 68C) 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 1^1 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 58 and 638C (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 An 23.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 (DB and DB6.25.9) to illustrate the possible variability in plagioclase compositions within the felsic gneisses (Fig. 1a and b respectively). Sample DB contains largely plagioclase and quartz, with biotite grains distributed throughout the sample and a few percent epidote. In contrast, DB 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. 1 but does change the anorthite content of 257

16 JOURNAL OF PETROLOGY VOLUME 55 NUMBER 1 OCTOBER 214 Table 3: Bulk compositions used for thermodynamic modelling (mol %), one deformed dyke (DB6.25.6) and two gneiss compositions (DB and DB6.25.9); all are modelled with excess water Sample SiO 2 Al 2 O 3 FeO MgO CaO Na 2 O K 2 O TiO 2 O DB DB DB Fig. 9. Equilibrium assemblage diagram for deformed Scourie dyke sample DB Equilibrium mineral assemblages are shown along with plagioclase isopleths (dotted lines) showing mol % anorthite in the plagioclase. 258

17 PEARCE & WHEELER SOUTHERN LEWISIAN METAMORPHISM Fig. 1. Equilibrium assemblage diagram for two bulk compositions of deformed gneiss (a) DB and (b) DB Plagioclase isopleths (dashed lines) show mol % anorthite in the plagioclase. 259

18 JOURNAL OF PETROLOGY VOLUME 55 NUMBER 1 OCTOBER 214 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. 1, 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 658C. Conversely, pre-dyke gneisses are interpreted to have undergone partial melting (Cresswell & Park, 1973) at higher temperatures. Metamorphic fractionation We have used the model outlined by Pearce & Wheeler (21) 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. (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. (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. 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., 23) 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. Modelled P^T^t paths 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. (1) Temperature: varied between 555 and 618C. 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 An 12 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 26

19 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 to8kbarma^1 and the heating path has rates between 35 and 88C 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 (2^25 mol % anorthite for DB and 25^3 mol % for DB6.25.9) and grain size (28^32 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 58^6258C. The grain growth rate is maximized by selecting the lower experimental bound for activation energy ( kj mol^1 ) and highest experimental bounds for DB (a) DB (d) (b) (e) pre-exponent [( ) 1^4 ] 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. (27) showed that grain boundary migration velocities of 15 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 5 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. (c) (f) Fig. 11. Model results exploring a range of P^T^t path orientations for samples DB (a^c) and DB (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 (28^32 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). 261