Pressure-Temperature Evolution and Thermal Regimes in the Barrovian Zones, Scotland
Vorhies, Sarah H., Ague, Jay J., Journal of the Geological Society
Abstract: We constrain the P-T evolution of the Barrovian metamorphic zones from the southwestern to the northeastern coasts of Scotland using thermobarometry and pseudosection analysis based on mineral composition data, garnet zoning profiles and 2D garnet maps. Twenty-five samples were investigated from the garnet to the sillimanite zones. In the western half of the field area there was relatively high-P metamorphism (0.9-1.1 GPa) followed by near-isothermal decompression. In and around the Barrovian type area of Glen Clova maximum pressures were also high (c. 0.8-0.9 GPa); however, peak-T conditions were driven by a brief (of the order of 1 Ma or less) thermal pulse or pulses during exhumation at c. 0.6 GPa. Pressures at peak-T conditions along the eastern coast were the lowest observed, c. 0.4-0.5 GPa. These rocks were probably affected by the same thermal pulse activity evident around Glen Clova. All three regions initially developed during regional metamorphism associated with thermal relaxation of tectonically overthickened crust. The eastern part of the sequence, including Glen Clova, is fundamentally different from the western part because it required additional advective heat input to achieve peak thermal conditions. This heat was probably supplied by synorogenic magmas (e.g. Newer Gabbros) and the associated elevated crustal heat flow.
Supplementary material: Electron microprobe methods, mineral and rock analyses, mineral activity- composition relations, criteria for identifying prograde mineral compositions, pseudosection and diffusion modelling methods, and profiles of XAlm and XPrp from garnets in Figures 7-10 are available at http:// www.geolsoc.org.uk/SUP18491.
Barrovian-style metamorphism has been found in mountain belts across the world at least as far back in geological history as the Proterozoic. In his classic studies of the Scottish Highlands, Barrow (1893, 1912) concluded that pelitic rocks undergoing progressive regional metamorphism would exhibit systematically changing mineral assemblages as a function of metamorphic grade. The Barrovian type locality in the Scottish Highlands has been the focus of extensive investigation for over a century, yet questions remain regarding the peak pressures (P) and temperatures (T) of metamorphism, as well as regional P-T paths. It is clear from the mineralogy as well as available thermobarometric studies that different areas, from the northeastern coast near Aberdeen to the southwestern coast, were subject to different peak metamorphic P-T conditions (e.g. Tilley 1925; Chinner 1961, 1966; Harte & Johnson 1969; Atherton 1977). Moreover, strong evidence for multiple phases of metamorphism in some parts of the sequence has been discovered through study of mineral textural relationships, garnet zoning profiles, and timing and time-scale relationships (e.g. McLellan 1985; Ague & Baxter 2007).
Conventional models for Barrovian metamorphism involve conduction-dominated thermal relaxation of overthickened crust during exhumation (e.g. England & Richardson 1977; England & Thompson 1984; Thompson & England 1984). This type of model predicts that rocks will spend c. 10 Ma or more at or near peak thermal conditions, and that peak conditions will be reached at different times in different index mineral zones. Furthermore, the total time scales of orogeny needed to achieve high-grade conditions are predicted to be several tens of millions of years. Recent studies of Barrovian metamorphism in Scotland, however, are at variance with these predictions. For example, Oliver et al. (2000, 2008), Baxter et al. (2002) and Dewey (2005), among others, argued that the entire Grampian Orogeny took place over a time period as short as 10-15 Ma. Moreover, peak thermal conditions in the type locality were roughly synchronous across several index mineral zones, and almost certainly required a significant component of pulsed, advective magmatic heat transfer (Baxter et al. 2002; Ague & Baxter 2007). Heat input associated with magmatic activity has also been described in the Connemara region of Ireland to the SW (e.g. Yardley et al. 1987), in the Attic Cycladic Metamorphic Belt of Greece (Wijbrans & McDougall 1986, 1988) and in metamorphic core complexes around the world (Lister & Baldwin 1993, and references therein).
Quantitative thermobarometry provides a valuable, independent, test of the emerging picture of Barrovian metamorphism in the type locality. Classic studies of the region, however, have often not provided full mineral chemistry from analyses (see Baker 1985; Watkins 1985) so it is difficult to accurately recalculate P-T conditions using modern calibrations. In addition, the electron microprobe work in some earlier studies (e.g. Sivaprakash 1982) was carried out using energy-dispersive spectrometry (EDS), which is less accurate and precise than wavelength-dispersive spectrometry (WDS). Modern WDS analysis and the better spatial resolution for chemical maps that is now possible provide a new framework for understanding P-T evolution.
We present new mineral chemistry data, P-T estimates, and pseudosection results for a suite of 25 metapelite samples. The rocks range in grade from the garnet to the sillimanite zones. Importantly, the area sampled extends from the northeastern to southwestern extremes of the Barrovian zones, making it possible to construct an internally consistent P-T framework for the entire region (Fig. 1). Our primary goals are to: (1) estimate peak P-T conditions and test for variations in conditions across the field area using both thermobarometry and pseudosection methods; (2) use garnet chemical profiles and maps to elucidate multiple growth stages; (3) use growth zoning preserved in garnet together with pseudosection analysis in an effort to reconstruct the P-T paths of metamorphism. In addition, we re-examine the garnet diffusion profile modelling of Ague & Baxter (2007) to test those workers' conclusions regarding rapid thermal pulses of peak metamorphism.
The rocks sampled in this study are pelitic metasediments of the Dalradian Supergroup. They are from the Grampian Highlands Terrane, which lies between the Highland Boundary Fault and the Great Glen Fault (Fig. 1). The Dalradian metasediments also extend into Connemara, Ireland, where much of the metamorphic history is similar (e.g. Leake 1986; Chew et al. 2010). The original sediments were marine successions deposited on the margin of Laurentia. The margin started out as a rift associated with the break-up of Rodinia, and then was a passive margin on the Iapetus Ocean (MacDonald & Fettes 2007). Deposition continued until the initiation of the closure of Iapetus around 500 Ma. Around 480 Ma the loading of the Dalradian sediments began as Laurentia started to collide with the Midland Valley Arc, outboard microcontinents, and the Highland Border Ophiolite (Oliver et al. 2008; Chew et al. 2010).
Timing of metamorphism
Garnet-whole-rock Sm-Nd dating establishes that garnets grew over a c. 8 Ma interval from about 473 to 465 Ma (Oliver et al. 2000; Baxter et al. 2002). The time lapse between peak T attainment in the garnet and kyanite-sillimanite zones in Glen Clova was short or nonexistent, only 2.8 3.7 Ma (Baxter et al. 2002).
Breeding et al. (2004) carried out sub-micrometre-scale, ionprobe depth-profiling of zircons from a garnet zone sample studied by Ague (1997) from the Stonehaven coast. The outermost (c. 1m) edge of zircon from a vein margin had a concordant U-Pb age intercept of 462 9 Ma. Breeding et al. (2004) concluded that this age reflects zircon growth or recrystallization during fluid infiltration and metamorphism; notably, it overlaps the c. 465-468 Ma garnet results of Oliver et al. (2000) and Baxter et al. (2002).
The pre-, syn-, and post-metamorphic igneous intrusions in the region also provide constraints on the tectonic activity during the orogeny (Fig. 1). The intrusions that are broadly synmetamorphic include the Auchlee Granite (475 Ma; Oliver et al. 2008), the Morven-Cabrach Gabbro (472 Ma; Rogers et al. 1994), the Tillyfourie Granite (471 Ma, Oliver et al. 2008), the Aberdeen Granite (470 Ma; Kneller & Aftalion 1987), the Insch Gabbro (468 Ma; Dempster et al. 2002), and the Strichen Granite (467 Ma; Oliver et al. 2000).
Four major deformational events affected the Dalradian (D1 - D4; e.g. Harte et al. 1984). Based primarily on textural evidence, McLellan (1989) came to the following general conclusions regarding the relative timing of metamorphic mineral growth and deformation in the Eastern Highlands: (1) garnet growth was largely syn-D2 followed by another phase of growth syn- to post- D3; (2) staurolite growth was syn- to post-D2; (3) kyanite growth was pre- to syn-D3; (4) sillimanite growth was syn- to post-D3: It is generally accepted that the peak thermal conditions were attained roughly syn-D3 (Harte et al. 1984; McLellan 1985).
Existing thermobarometry covers what we summarize here as three major regions, delineated primarily by regional isograd patterns (Fig. 1): the eastern coast (referred to herein as Region I), Glen Clova and nearby glens including Glen Esk and Glen Muick (Region II), and the western half of the Highlands (Region III). In Region III the isograds are widely spaced, progressing from the chlorite zone adjacent to the Highland Boundary Fault into the biotite and garnet zones farther north. In Regions I and II the zones are markedly thinner near the Highland Boundary Fault. Moreover, the higher-grade zones (staurolite, kyanite, and sillimanite) are nearly restricted to Regions I and II. The highest-grade area, at the heart of Region II, was metamorphosed at upper amphibolite- to granulite-facies conditions; metapelitic rocks are characterized by sillimanite + K-feldspar-bearing mineral assemblages (Baker & Droop 1983; Baker 1985).
In Region I, on the east coast between Stonehaven and Aberdeen, T increased from c. 375 8C in the chlorite zone, to c. 535 8C in the garnet zone and to c. 585-600 8C in the staurolite zone (Ague 1997; Masters et al. 2000). Ague (1997) estimated a P of 0.38 GPa in the garnet zone, consistent with regional phase relations (e.g. Carmichael 1978; Droop & Harte 1995). The results of Viete et al. (2011a) are similar to those of previous studies, but their P estimates are somewhat higher for the staurolite and sillimanite zones (0.5-0.6 GPa).
Published thermobarometry has focused largely on Region II, which includes the Barrovian type locality of Glen Clova (Barrow 1893). In and around Glen Clova, McLellan (1985) reported T ranging from as low as 465 8C in the staurolite zone to as high as 625 8C in the kyanite zone. Pressures were found to be between c. 0.45 and c. 0.8 GPa. For the kyanite zone, Baker (1985) calculated T between c. 610 8C and c. 690 8C at c. 0.6µ0.7 GPa. Ague et al. (2001) estimated average peak-T conditions of c. 660 8C at c. 0.6 GPa for the sillimanite- muscovite zone in the northern part of Glen Clova, consistent with the earlier results of McLellan (1985) and those of Viete et al. (2011a) for nearby Glen Esk. Dempster (1985) concluded that peak T for the staurolite, kyanite, and sillimanite zones increased from c. 520 8C to c. 650 8C to the east of Glen Clova in Glens Lethnot and Esk. Pressures were estimated in the kyanite and sillimanite zones and were between about 0.55 and 0.75 GPa. Baker (1985) also investigated an area c. 30 km north of Glen Clova in the kyanite and sillimanite zones; T estimates range from 520 to 625 8C at c. 0.74 GPa. Finally, Baker (1985) reported conditions of c. 800 8C and 0.9µ1.0 GPa for the highest-grade sillimaniteµK-feldspar zone in Region II.
There have been somewhat fewer P-T studies done in Region III. For the southwesternmost Highlands, Skelton et al. (1995) examined pyrophyllite-kyanite equilibrium relations, reviewed previously published work (e.g. Graham et al. 1983; Dymoke 1988), and reported T increasing from c. 410 8C in the lower greenschist facies to 500-550 8C in the garnet zone. Pressure estimates for this area (Graham et al. 1983; Dymoke 1988; Skelton et al. 1995) are based on phengite geobarometry (e.g. Massonne & Schreyer 1987) and range from about 0.9 to 1.2 GPa. Similar high pressures have been estimated elsewhere in Region III by Sivaprakash (1982) and Watkins (1985).
Chemical zoning in Dalradian garnets has been investigated in relatively few studies, and we are unaware of any papers that compare regional zoning systematics from the SW to the NE. Additionally, the garnet profiles in many published studies lack the spatial resolution to constrain P-T histories or to perform diffusion modelling. Chemical profiles were presented and/or discussed by, for example, Atherton (1968), Baker (1985), Dempster (1985), McLellan (1985), Ayres & Vance (1997), Ague et al. (2001), Ague & Baxter (2007) and Viete et al. (2011a).
Twenty-five samples from the garnet, staurolite, kyanite, and sillimanite zones were investigated (Figs 1 and 2; Table 1). For brevity, the sample prefix JAB is omitted in the text. The low sample density in Region III is due to the relative rarity of garnet (needed for thermobarometry) there. Compositions of plagioclase, biotite, muscovite, garnet and ilmenite were acquired, as were chemical profiles of garnets. Approximately five spots were analysed for each mineral; garnet profiles consisted of 25-85 spots depending on the garnet size. Profiles were collected across the largest garnets in thin section, in an attempt to sample through the geometric cores of the crystals.
P-T conditions for most samples were estimated using the three possible equilibria (any two of which are independent) between pyrope, almandine, grossular, muscovite, phlogopite, annite, anorthite and quartz (Ferry & Spear 1978; Ghent & Stout 1981). For samples containing aluminosilicate minerals we used the garnet-aluminosilicate-plagioclase (GASP) barometer as well (Ghent 1976). Rutile and ilmenite coexist in garnet rims in sample 257A, so the garnet-rutile-ilmenite-plagioclase-quartz barometer was applied (Bohlen & Liotta 1986). P-T estimates were made using version 2.3 of the winTWQ program (Berman 1988, 1991, 2007; database 2.32) and version 3.33 of Thermo- Calc (Powell & Holland 1988; Holland & Powell 1998; Powell et al. 1998; Holland & Powell 2003).
Pseudosections and isopleths were calculated using the Theriak- Domino program, version 01.08.09 (de Capitani & Petrakakis 2010). This program allows for the use of two databases, both of which were used for comparison. One (JUN92.bs) is based largely on the winTWQ database, and the other (tcdb55c2d.txt) is based on ThermoCalc. The standard heat of formation for Mgstaurolite was changed in JUN92.bs as described by Bucholz & Ague (2010).
P-T conditions for a Glen Muick metabasite (sample CL2H; Baker & Droop 1983) were recalculated using the Kohn & Spear (1990) hornblende-garnet-plagioclase barometer and the Ravna (2000) garnet-clinopyroxene thermometer. We also used the GASP barometer reaction to recalculate the pressure for two nearby sillimaniteµK-feldspar samples (7 and 430) using the reported temperatures (Baker 1985).
Results and interpretations
P-T estimates calculated using winTWQ and ThermoCalc agree closely (Table 2). Average per cent differences between the estimates from the two databases are 5.5% for pressure (0.04 GPa) and 3.5% for temperature (20 8C); results from winTWQ are shown in the figures (Fig. 3). Table 3 presents P and T estimates for additional samples referenced in the text. Multiple equilibria for low-variance mineral assemblages from Regions I-III are plotted in Figure 4 to assess the degree of mineral equilibration. Clearly, intersections between reactions cluster tightly in P-T space, suggesting that the mineral compositions preserve a close approach to equilibrium (see Berman 1991). Standard deviations on the P-T intersections (Berman 1991) are ,0.025 GPa and ,18 8C in all cases.
Uncertainties on P-T estimates are difficult to quantify given the range of sources of error. Uncertainties on the chemical analyses, thermodynamic data, activity models and degree of equilibration all play a role. For graphical representation and petrological interpretation, we assign representative 50 8C and 0.2 GPa uncertainty ranges on all results. These ranges are comparable with the maximum differences between T and P estimates calculated using the winTWQ and ThermoCalc databases (60 8C and 0.12 GPa). It is important to note that throughout this study we are primarily concerned with comparing pressures and temperatures between regions rather than making distinctions between similar P-T conditions. Because nearly all thermobarometric calculations were carried out using the same two programs and the same internally consistent datasets, the relative uncertainties on the P-T estimates will probably be considerably smaller than those we assign here (Berman 1991; Worley & Powell 2000).
In Region I, T estimates increase from c. 375 8C in the chlorite zone (Masters et al. 2000), to c. 535 8C in the garnet zone, to 575-600 8C in the staurolite zone, and finally to c. 620 8C in the sillimanite-muscovite zone (Fig. 3). Pressure estimates are remarkably consistent, ranging mostly between c. 0.4 and c. 0.55 GPa regardless of grade. These P and T estimates are plotted as functions of distance from the Highland Boundary Fault in Figure 5. Notably, there is an extremely steep metamorphic field temperature gradient (MFTG) in excess of 110 8C kmµ1 across the first 2 km north of the fault. Consequently, the boundaries between metamorphic zones here are very closely spaced, reflecting a primary feature of the metamorphism (Dempster et al. 2000), later deformation (Harte & Hudson 1979; Harte & Dempster 1987), or some combination of these. In contrast, beyond this region temperatures vary little and increase slightly from the staurolite to the sillimanite-muscovite zone.
T and P profiles for Region II through Glen Clova and on to Glen Muick (Fig. 6) are markedly different from the profiles for Region I. Temperatures increase with distance from the Highland Boundary Fault, but relative to Region I the MFTGs are not as steep and the zone boundaries are farther apart. Temperature estimates increase from c. 500 8C in the lowermost garnet zone to c. 660 8C in the sillimanite-muscovite zone. Farther north, temperatures in the sillimanite-K-feldspar zone of Glen Muick are considerably higher, in the range of 750-800 8C (Baker 1985; this study). Calculated P values increase from c. 0.5 GPa in the garnet zone 4-5 km from the fault, to c. 0.8 GPa in the staurolite zone 8 km from the fault. This trend is disrupted by rocks in the kyanite and sillimanite-muscovite zones, which yield P estimates in the vicinity of 0.6 GPa. P estimates then return to higher values of c. 0.95 GPa in the sillimanite-Kfeldspar zone of Glen Muick.
We infer that several of the amphibolite-facies rocks in Region II preserve retrograde temperatures. The most glaring examples are 242A from the kyanite zone (,550 8C; Figs 3 and 6), and several sillimanite-muscovite zone rocks that yield estimates below 650 8C. Considerable increases in Mn and decreases in Mg/Fe at the rims of these garnets indicate retrograde equilibration (see Kohn & Spear 2000; see below), consistent with observed garnet resorption and retrogressive chlorite growth at garnet rims.
Pressure estimates for Region III are high and range from c. 0.9 to c. 1.1 GPa. Temperature estimates increase from c. 500 8C at c. 0.9 GPa to c. 630 8C at c. 1.1 GPa (Fig. 3). Even though temperatures could reach over 600 8C, all samples from Region III are petrographically in the garnet zone. We conclude that staurolite, kyanite and sillimanite did not form at these high temperatures because the pressures were also high (see Powell et al. 1998; Caddick & Thompson 2008). Our results confirm the evidence for relatively high-P conditions presented in earlier studies (see above). However, unlike some previous estimates, which predicted blueschist-facies metamorphism, our P-T estimates fall within the greenschist or amphibolite facies and are consistent with observed mineral assemblages (see Diener et al. 2007).
Several samples fall outside the general region boundaries in Figure 3 (164A, 240D1, 255A); these are examined further below in the context of regional P-T paths.
Garnet zoning profiles are discussed by region in order of increasing complexity, from Region III to I to II.
Garnet profiles: Region III. The garnet profiles from Region III are the simplest of the three regions. The garnets are characterized by high XGrs above 0.2 (Fig. 7; mineral abbreviations after Kretz 1983). The Ca-rich nature of the garnets reflects the high pressures of crystallization in this region, because although the bulk-rock composition is an important control on the absolute grossular content, high pressures generally favour elevated grossular mole fractions (Ghent 1976; Ghent & Stout 1981). The XPrp/XAlm ratios generally increase toward the rim, consistent with the Fe-Mg partitioning expected for the garnet-biotite equilibrium during progressive heating (Ferry & Spear 1978). The Mn contents of three of the four examples decrease toward the rim, as expected for Rayleigh fractionation (Hollister 1966) and for changing equilibrium partitioning values during heating and compression (Caddick & Thompson 2008). There is an upturn in Mn content at the edge of the 160A profile, which may indicate retrograde resorption at the rim (see Tracy et al. 1976; Kohn & Spear 2000). The Mn profile for 152A3 remains relatively flat. Garnets are relatively rare in this sample (,1% mode), so it is likely that bulk-rock Mn was not strongly depleted by fractionation during garnet growth.
Garnet profiles: Region I. In Region I, the cores of garnets from both 101L (garnet zone) and 18A (staurolite zone) have high XGrs values c. 0.2-0.25 (Fig. 8), similar to those in garnets from Region III (Fig. 7). However, grossular contents drop markedly toward the rims. For 101L, XGrs drops to c. 0.13 between x c. 500 and c. 300 m, and then stays relatively flat to the rim (Fig. 8). For 18A, there is a sharp drop in XGrs at x c. 700 m, followed by a more gradual decrease toward the rim (Fig 8). Grossular profiles for the other garnets investigated from Region I are simpler; they lack the high-Ca cores, are relatively flat, and vary mostly between XGrs c. 0.05 and c. 0.10 (Fig. 8). The profiles suggest that garnets in 101L and 18A record a multi-stage growth history, whereas the others do not.
The XPrp/XAlm values either increase toward the rim or remain relatively flat (Fig. 8), with the exception of 268A in which XPrp/ XAlm decreases slightly over the course of the profile. In addition, in the amphibolite facies, XPrp/XAlm can drop at the very outermost rim, consistent with partial re-equilibration during retrogression (e.g. 277D1, 268A, 269B; Fig. 8). For Mn, gradual decreases (e.g. 277D1, 269B) or more abrupt decreases (e.g. 18A) toward rims are observed, as are fairly uniform profiles (e.g. 101L). Small upturns in XSps at crystal margins, coincident with drops in XPrp/XAlm, are also consistent with retrogression (e.g. 277D1).
Garnet profiles: Region II. Garnets from Region II display the most diverse range of chemical behaviour. In the garnet and staurolite zones, two samples have XGrs values that drop relatively continuously from high values of 0.2-0.25 in their cores, to lower values ,0.1 on their rims (235A, 162A; Fig. 9). For the other two examples, XGrs is low (, c. 0.1) and the profiles are fairly flat (Fig. 9). XPrp/XAlm ratios increase and XSps values decrease from core to rim, consistent with prograde growth. Retrograde drops in XPrp/XAlm and increases in XSps may be present on rims.
Similar patterns are observed for some higher-grade garnets from Region II as well, but there are also major exceptions and we will focus on these here. Perhaps the most striking are the grossular profiles for kyanite zone sample 246A2 and sillimanite- muscovite zone samples 62A and 261B1 (Fig. 10). The garnets have core regions of low XGrs around 0.1 or less. Moving outward from the core, there are sharp increases in XGrs to values of c. 0.15-0.2. These are then followed by drops in XGrs toward the rim; rim values are similar to those measured in the cores.
A very important feature of these 'three domain' garnets are the drops in XPrp/XAlm observed with increasing radius in the low XGrs core regions (246A2, 62A, 261B1; Fig. 10). Normally, XPrp/ XAlm would be expected to increase with prograde growth, as predicted by the garnet-biotite Fe-Mg partitioning reaction (Ferry & Spear 1978). At first glance, the extended drops in XPrp/ XAlm would suggest significant growth during cooling, but this interpretation is inconsistent with the endothermic nature of nearly all garnet-producing reactions.
Another possibility is open-system reaction. The redox history of 62A was studied by Ague et al. (2001), who concluded that the rock was originally highly oxidized, but was reduced during synmetamorphic fluid infiltration. During the oxidized early history, the rock would have been rich in Fe3þ-bearing oxides, and the Mg/Fe2þ ratio of the rock and the ferromagnesian silicates would thus have been high. However, as reduction proceeded, the bulk-rock Fe2þ/Fe3þ ratio would have increased, leading to decreases in Mg/Fe2þ: Garnets and biotites that grew during this reduction would have their Mg/Fe2þ ratios shifted to lower values as more and more Fe2þ became available. Consequently, our interpretation is that the rimward decreases in XPrp/ XAlm observed in 62A, 246A2 and 261B1 are powerful indicators of synmetamorphic reduction during prograde growth. Reduction is inferred to have been more or less completed at the point in the profiles where XPrp/XAlm begins to increase toward the rim.
Mn zoning profiles also provide evidence for open-system processes. For example, for the kyanite zone garnet in 245A, Mn content actually increased and then reached a plateau before decreasing near the rim (Fig. 10). This sample was taken adjacent to a large quartz-kyanite vein. We suggest that the garnet sequestered Mn from fluids passing through the vein and, thus, the typical Rayleigh fractionation profile is not observed.
In the sillimanite-K-feldspar zone temperatures were high enough, and time scales long enough, for diffusion to smooth out much of the compositional growth zonation (284A; Figs 10 and 11). However, the XPrp/XAlm profile is irregular and shows clear evidence of retrogression; XPrp/XAlm drops as a result of retrograde Fe-Mg exchange with biotite at the garnet rim and around biotite inclusions and cracks within the garnet.
We made 2D chemical maps of representative garnets from each region using the electron microprobe (Fig. 11). We focus on Ca as it displays marked variations related to the baric history of metamorphism. The garnet from sample 159A (garnet zone; Region III) has relatively high and largely uniform grossular content. This most probably represents little variation in Ca partitioning into garnet during growth. It is unlikely that the Ca content was homogenized by intracrystalline diffusion given the peak temperature of the sample (600 8C).
In contrast, the garnet from 18A (staurolite zone; Region I) contains marked variations. The core is characterized by high Ca (Fig. 11). It has a euhedral outline, consistent with growth near equilibrium. Just outside the core, Ca drops significantly and displays irregular oscillatory zonation. We interpret these features to be growth zonation that records the crystal shape during crystallization; the irregularity of the crystal margins probably reflects rapid growth relatively far from chemical equilibrium (Wilbur & Ague 2006). Finally, the outermost rim returns to a more euhedral shape indicative of growth closer to equilibrium.
The 'three domain' garnet in sample 62A from the sillimanite- muscovite zone of Region II has the most complex zoning observed. The low-Ca core is ringed by a high-Ca annulus, which is in turn rimmed by a zone of lower Ca. The boundaries between the annulus and the surrounding areas are sharp but irregular. The annulus is unlikely to be the result of resorption, as there is no Mn increase associated with it (see profile in Fig. 10).
Less variation is evident in the map for 284A from the sillimaniteµK-feldspar zone of Region II (Fig. 11). This rock has undergone much higher-T metamorphism than the others, and thus Mn and Ca growth zoning has been largely smoothed by diffusion. Remnants of a higher Ca region are present near the rim. It is possible that this is what remains of a high-Ca annulus such as the one preserved in the three-domain garnet in sample 62A (Figs 10 and 11).
Pseudosections, garnet zoning and pressure
Pseudosections are equilibrium phase diagrams drawn for a specific bulk composition (e.g. Connolly 1990; Powell et al. 1998; Powell & Holland 2008; de Capitani & Petrakakis 2010). They are extremely useful for displaying phase relations as functions of P and T, including the compositions of coexisting minerals. Here we examine a P-T pseudosection drawn for the representative bulk composition of sillimanite-muscovite zone sample 46B (Region II) determined by Ague et al. (2001). The pseudosection has wide stability fields for garnet + plagioclase + bioitite + quartz + white mica chlorite, the most common mineral assemblages in the rocks that we used for thermobarometry (Fig. 12). Other bulk compositions would yield different stability fields, but the general chemical relations discussed below would be unchanged.
Isopleths of the grossular component of garnet and the anorthite component of plagioclase are overlain on the pseudosection in Figure 12a. These isopleths are depicted because equilibria between garnet and plagioclase are generally pressure sensitive (e.g. Ghent 1976; Ghent & Stout 1981). The results clearly show that grossular-rich garnet (XGrs . c. 0.15) would be expected to coexist with albite-rich plagioclase having XAn , c. 0.16 at high pressures in excess of c. 0.9 GPa (Fig. 12a; light grey shading). Importantly, however, grossular-rich garnet can also be stable at significantly lower pressures and somewhat lower temperatures, if it coexists with more calcic plagioclase (Fig. 12a; dark grey shading). The isopleths of Mg-number [XMg/ (XMg + XFe)] of garnet illustrate the very different effects of P and T (Fig. 12b). This ratio increases as T increases but there is little effect of P, consistent with the small volume change of garnet-biotite Mg-Fe exchange (Ferry & Spear 1978).
These general predictions