Metamorphic Evolution of Garnet Epidote Biotite Gneiss from the Moine Supergroup, Scotland, and Geotectonic Implications

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1 JOURNAL OF PETROLOGY VOLUME 42 NUMBER 3 PAGES Metamorphic Evolution of Garnet Epidote Biotite Gneiss from the Moine Supergroup, Scotland, and Geotectonic Implications A. ZEH 1 AND I. L. MILLAR 2 1 MINERALOGISCHES INSTITUT DER UNIVERSITÄT WÜRZBURG, AM HUBLAND, D WÜRZBURG, GERMANY 2 BRITISH ANTARCTIC SURVEY, c/o NERC ISOTOPE GEOSCIENCES LABORATORY, KINGSLEY DUNHAM CENTRE, KEYWORTH, NOTTINGHAM NG12 5GG, UK RECEIVED OCTOBER 26, 1999; REVISED TYPESCRIPT ACCEPTED JUNE 16, 2000 Metapelitic gneisses from the Glenfinnan Group of the Moine INTRODUCTION Supergroup, Scotland, contain sparse large and numerous small Although numerous models exist for the structural evolugarnets, associated with complex zoned epidote and plagioclase in tion of the Moine Supergroup, less is known about its a biotite matrix. The large garnets show four zones (AI AIV), metamorphic history. One reason is the scarcity within whereas the small garnets show three or fewer zones, indicating the Moine Supergroup of aluminosilicate-bearing mesuccessive garnet nucleation with increasing nucleation densities. tapelites, which can be used to unravel the P T history Garnet zones AI and AIV grew under static conditions, whereas in detail. Garnet-bearing schists and gneisses, which are the formation of AII and AIII was accompanied by deformation. designated in the Moine as metapelites, mostly contain Garnet zones AI and AII were formed in the assemblage (all + garnet, biotite, white mica, quartz and plagioclase. Locbiotite + epidote + plagioclase + quartz + fluid + apatite) ally, the Moine metasediments contain lenses of coarse garnet + chlorite + muscovite ± ilmenite ± sphene ± magnetite; garnet biotite epidote plagioclase gneiss, which may zone AIII in the assemblage garnet + muscovite + sphene ± also contain hornblende. Such rocks can also be very magnetite; and zone AIV in the assemblage garnet + sphene ± useful monitors of the metamorphic P T history, as ilmenite. The chemical zonation and microstructures of garnet A outlined by Menard & Spear (1993), because of the indicate two important discontinuities; one at the transition between preservation of zonation in garnet, plagioclase and epgarnet zones AI and AII, and a second between zones AII and idote, and the presence of associated minerals enclosed AIII, which correlate with complex zonation shown by epidote and in different zones of these minerals. In addition, the plagioclase. These discontinuities may result from polymetamorphic textures of these rocks, in combination with petrological garnet growth during different orogenic cycles affecting the Moine data, provide valuable information about their de- Supergroup. Geothermobarometric calculations and Gibbs method formation crystallization history. Successful remodelling provide evidence that garnet zone AI grew rapidly during heating from about 550 to 560 C at pressures of about 4 6 kbar. construction of the P T path from these zoned minerals In contrast, the formation of zone AII was accompanied by nearly is possible only when the reaction history of the rock is isothermal compression from 6 to 8 5 kbar ( C), indicating interpreted correctly (e.g. Spear & Selverstone, 1983; crustal stacking. After a certain period of cooling, garnet zone AIII Selverstone & Spear, 1985; Menard & Spear, 1993). grew during renewed heating at P T conditions of about 640 C In this paper we present a comprehensive petrological and pressures between 5 and 9 kbar. Growth of garnet AIV study of a garnet epidote biotite gneiss from the Glen- was accompanied by further temperature rise, reaching maximum finnan Group of the Moine Supergroup, and describe conditions of about 670 C at 5 kbar. mineral composition and zonation patterns and their relationship to the complex deformation crystallization KEY WORDS: epidote; garnet; Gibbs method; Moine Supergroup; P T history. P T conditions are calculated using conventional path thermobarometry and internally consistent datasets Corresponding author. Telephone: armin.zeh@mail.uni-wuerzburg.de Oxford University Press 2001

2 JOURNAL OF PETROLOGY VOLUME 42 NUMBER 3 MARCH 2001 ( Thermocalc: Powell & Holland, 1988; Holland & Powell, 1990). Additional P T path constraints are obtained from compositional zoning of garnet, plagioclase and epidote employing the program GIBBS (Spear & Menard, 1989; Spear et al., 1991). Finally, garnet growth modelling is carried out to explain the complex garnet zonation patterns found in the Moine samples. The data presented in this paper set new petrological constraints for the interpretation of the tectono-metamorphic history of the Moine Supergroup. GEOLOGICAL SETTING The Moine Supergroup is situated between the Great Glen Fault in the SE and the Moine Thrust in the NW ( Fig. 1), and has been subdivided into three lithostratigraphic units: from west to east, the Morar, Glenfinnan and Loch Eil groups ( Johnstone et al., 1969; Roberts et al., 1987; Holdsworth et al., 1994; Soper et al., 1998). The dominantly psammitic Loch Eil Group stratigraphically overlies interbanded pelites and psammites of the Glenfinnan Group. The Glenfinnan and Loch Eil groups structurally overlie the dominantly psammitic Morar Group, from which they are separated by the ductile Sgurr Beag thrust, assumed to be formed during the Caledonian orogeny (Tanner et al., 1970). Together, the Glenfinnan and Loch Eil groups form the Sgurr Beag nappe. The boundary between the Glenfinnan and the Loch Eil groups is broadly coincident with a rapid change in the level of strain, known as the Loch Quoich Line (Clifford, 1957; Roberts & Harris, 1983). To the east of the Loch Quoich Line, in the flat belt (Fig. 1), the dominant Loch Eil Group psammites are characterized by flat-lying foliation and recumbent isoclinal folds, formed during the D 2 event (Roberts & Harris, 1983). To the west, in the steep belt, Glenfinnan Group rocks and Loch Eil Group outliers are deformed by tight, Fig. 1. (a) Generalized geological map of the Moine Supergroup. Inset: upright, steeply plunging folds during D 3. All of the earlier geological situation of Scotland: MTZ, Moine Thrust zone; GGF, D Great Glen Fault; HBF, Highland Boundary Fault. (b) Geological 2 structures preserved in the flat belt can also be situation in the Glen Doe area. identified in the steep belt, indicating a common tectonometamorphic history. According to Roberts & Harris (1983), D 2 in both belts is pre-dated by rare isoclinal granite gneiss indicates emplacement of the gneiss protofolds, and an early migmatitic foliation (D 1 ). These D 1 liths at 873 ± 7 Ma (Friend et al., 1997). In places, the structures are restricted to sparse intrafolial F 1 isoclines, Glenfinnan and Loch Eil group metasediments contain and a bedding-parallel S 1 fabric. deformed metabasic bodies, which are in part spatially A discontinuous series of highly deformed and meta- associated with the West Highland Granite Gneiss. The morphosed granite bodies, the West Highland Granite emplacement of early, gabbroic metabasites has been Gneiss of Johnstone (1975), lies close to the boundary be- dated at 873 ± 6 Ma (Millar, 1999). A subsequent suite of tween the Glenfinnan and Loch Eil groups. The protoliths metabasite dykes has a tholeiitic composition and shows of these orthogneisses are thought to have been emplaced a mid-ocean ridge basalt ( MORB) signature. They are during the D 1 event affecting the Moine country rocks. interpreted to be the result of a continental rifting event at They are typical S-type granites, and were derived, at least in part, by partial melting of Moine metasediments (Barr et al., 1985). U Pb dating of zircons from the Ardgour >870 Ma, and may have contributed to the heat source for the generation of the West Highland S-type granites (Millar, 1999). This model conflicts with the interpretation 530

3 ZEH AND MILLAR METAMORPHIC HISTORY OF MOINE SUPERGROUP of Barr et al. (1985) and Friend et al. (1997), in which the Corrections for atomic number, fluorescence and absorption were carried out by means of the PAP program granite gneiss was emplaced during compressional deformation. The metabasites were isoclinally folded to- supplied by CAMECA. The procedures used for the gether with the Ardgour gneiss, during D 2. The timing of calculation of mineral formulae and of end-member this major deformation is not well constrained. However, activities are described in Appendix A, and representative recent P T t data of Vance et al. (1998) from the Morar analysis are presented in Appendix B. Group point to crustal stacking between 820 and 790 Ma and, therefore, indicate plate convergence during the Neoproterozoic. In marked contrast to this view, some workers Generalities believe that extensional tectonics were active in Moine and Dalradian rocks throughout the Neoproterozoic (e.g. Large euhedral and small, euhedral and xenomorphic Soper et al., 1998). At the moment, because of the lack of garnets occur within a layered matrix comprising alternating bands of biotite epidote quartz and pla- P T and geochronological data from the Loch Eil and gioclase quartz ( Fig. 2), which locally contain centimetre- Glenfinnan groups, it is not clear if these groups underwent scale isoclinal rootless folds. Euhedral garnet pora similar metamorphic evolution to the Morar Group. The phyroblasts up to 3 cm in size show symmetric and petrological data presented here will help to solve some of asymmetric, locally sigma-shaped pressure shadows, the unresolved questions. formed of plagioclase and quartz ( Fig. 2). The latter minerals show 120 triple point junctions and often straight grain boundaries, indicating static relaxation of SAMPLE LOCATIONS AND FIELD the fabric above the plagioclase recrystallization temperature (>500 C, Kruhl & Voll, 1976). Along sparse RELATIONSHIPS cracks the rocks show a greenschist-facies overprint, with Layers of unusual garnetiferous metapelitic gneisses and the formation of retrograde chlorite and rare calcite. schists occur within the Moine metasediments close to the contact of the Glenfinnan and Loch Eil groups (Fig. 1). These are particularly abundant to the south of Glen Garnet Doe (Peacock, 1977; Millar, 1990; Peacock et al., 1992), the area from which our samples come (NH ). Four types of garnet (A D) can be distinguished in the The garnetiferous gneisses here form layers of >0 3 m investigated gneiss sample, on the basis of their size, thickness within metapsammites, and can be traced over internal textures, mineral inclusions and zonation pat- several hundred metres. The flat-lying metapelitic and terns ( Figs 2 7). To avoid misinterpretations resulting metapsammitic layers seem to be flanks of decametrewell characterized before thin-section preparation and in from cutting effects, only the largest garnets, which were scale isoclinal D 2 folds. Such structures can be seen affecting deformed metabasite dykes within the Ardgour thin section, were analysed. To prove that individual gneiss >500 m east of the sample location (Millar, 1999). garnets of similar macro- and microscopic features belong Frequently, isoclinal folds of centimetre scale can be to the same garnet type, qualitative line scans over two observed in the metapelitic and metapsammitic gneiss garnets of type A and three of type B, C and D, bands. Large garnet porphyroblasts are restricted on respectively, were performed. Garnet type A forms euhed- metapelitic gneiss layers. They are up to 5 cm in diameter ral crystals up to 3 cm in size, which show complex and frequently show a euhedral shape. internal textures related to static and syndeformational garnet growth. Four zones, designated as garnet zones AI AIV, can be distinguished (Figs 2b, 3a and b, 4, 6a and 7a). Garnet type B reaches only 0 7 cm in diameter. PETROGRAPHY AND MINERAL It is hypidiomorphic, and has an inclusion-rich core and inclusion-poor rim. Garnet type B shows only three zones CHEMISTRY (BI, BII and BIII), which correlate with zones II, III and Microprobe analysis IV of garnet type A (Figs 6a and 7). Garnet type C has Electron microprobe analysis of relevant minerals was nearly the same size as garnet type B (0 5 cm), but is performed at the University of Würzburg using a CA- xenomorphic and contains only two zones (CI and CII), MECA SX-50 instrument. Operating conditions were which are similar to those of garnet AIII and AIV. 15 kv acceleration voltage, 15 na beam current, 1 μm Typically, some garnets of type C show s-shaped inclusion beam size, and an element-dependent integration time trails (Fig. 2c). Garnet type D is only 0 1 cm in size and of s. White mica and plagioclase were analysed contains one zone, equivalent to garnet AIV. In places, with a defocused beam of 5 μm size. Pure oxides, and garnet type D forms more or less completely idiomorphic natural and synthetic silicates were used as standards. atoll garnets. 531

4 JOURNAL OF PETROLOGY VOLUME 42 NUMBER 3 MARCH 2001 complex history of crystallization and deformation, which starts with the growth of the euhedral garnet core (Fig. 6b). The formation of the cores must have taken place under static conditions, as indicated by their idiomorphic shape (Figs 2b, 4 and 6a), and the random orientation of numerous apatite, sphene and quartz inclusions ( Fig. 3b). Furthermore, garnet A contains inclusions of white mica, chlorite, epidote, rare ilmenite, allanite and plagioclase ( Table 1). The composition of garnet zone AI, especially the high spessartine content (X sps ), clearly indicates that it formed before the growth of garnet B, C or D (Fig. 7). As there are relatively few garnets of type A in the rock, the nucleation density during the first growth of garnet must have been very low (>10 garnets/dm 3 ). The large size of garnet AI (>2 cm in diameter) and the low nucleation density require material transport over a relatively long distance, and may point to a high permeability of the rock during this early stage of metamorphism. Garnets AII and BI Textures, mineral inclusions and compositional zoning indicate at least three domains in garnet AII (AIIa, b and c; Figs 2b, 3a, 4, 6a and 7a). Domain AIIa forms a pressure shadow and occurs on two opposite sides of garnet A ( Figs 2b, 4 and 6a). It is characterized by numerous inclusions of quartz, sphene, apatite and rare epidote (Table 1). These are randomly oriented at the direct contact with garnet zone AI, but form trails oblique to the garnet AI surface toward the rim of garnet AIIa ( Fig. 6a). This clearly indicates increasing strain toward the rim of domain AIIa. In contrast to garnet domain AIIa, inclusions in domains AIIb and c are parallel to the surface of garnet AI (Figs 2b, 3a and 6b). Domains AIIb and AIIc are successively formed and can be distinguished by their different mineral contents. Domain AIIb forms the inner zone in direct contact with the euhedral garnet AI and contains ilmenite, apatite, sphene, epidote, biotite, muscovite and rare chlorite, whereas domain AIIc grew later and is ilmenite free (Fig. 3a). It Fig. 2. Garnet epidote biotite gneiss from the Glen Doe area. (a) is notable that garnet AIIa is directly overgrown by zone Sparse large and numerous small garnets set in a dark matrix of biotite, AIIc but never by zone AIIb (Fig. 6b). epidote and quartz, which alternates with leucocratic quartz plagioclase Generally, the compositional zoning of garnet AII is aggregates and layers. The layers locally trace rootless, isoclinal folds, characterized by an increase of X alm, X py and X grs, and a whereas the aggregates form sigma-shaped pressure shadows around large garnets. Some of the large garnets are sheared. (b) Large garnet decrease of X sps (Fig. 7a). In detail, however, the zonation porphyroblast (garnet A: central cut), showing an idiomorphic core, is much more complicated, as can be seen from the pressure shadow and strained domains, as well as an idiomorphic variation in X sps (Figs 4, 6a and 7a). In the pressure overgrowth (half-crossed Nicols). (c) Garnet type C, showing s-shaped inclusion trails, traced by quartz, apatite, epidote and biotite inclusions. shadow domain AIIa, X sps decreases rapidly from 13 to 10 8 mol % and then forms a plateau, whereas X sps decreases continuously from 13 to 4 mol % throughout domains AIIb and c (Figs 6a and 7a). The zonation Garnet AI pattern indicates that garnet domain AIIb initially grew The texture of the garnets, their compositional zoning faster than AIIa (Figs 4, 6b and 7a). The absence of and their mineral inclusions provide evidence for a very ilmenite in domain AIIa might result from differential 532

5 ZEH AND MILLAR METAMORPHIC HISTORY OF MOINE SUPERGROUP Fig. 3. Photomicrographs of garnet and epidote. (a) detail of Fig. 2b: changing textures from garnet zones AI to AIV. Inclusions in garnet zone AI show random orientation, whereas apatite, quartz, ilmenite (ilm) and/or sphene (sph) inclusions in garnet zones AIIb, AIIc and AIII are preferentially oriented. Garnet zone AIV is nearly inclusion-free. Notably, ilmenite inclusions are restricted to garnet zone AIIb. (b) Random orientation of apatite (ap), quartz (qtz) and sphene inclusions in garnet AI. (c) Zoned epidote (ep) with an allanite (all) core is mantled by biotite (crossed Nicols). (d) Zoned epidote shows quartz symplectites at the contact with plagioclase (pl), but not at that with biotite. stress during simultaneous growth of both garnet zones, (see Millar, 1990). These rather indicate simple shear leading to the formation of different assemblages in deformation during garnet AII growth. distinct domains. However, it could also result from the An additional deformation event after garnet AII fact that garnet AIIa growth started later than garnet growth is evident from different orientations between the AIIb. Interestingly, at X sps = 10 8 mol % domain AIIa pressure shadows represented by garnet AIIa (PS1 in Fig. suddenly grew much faster than AIIb, and finally zones 6a), and the sigma-shaped plagioclase quartz pressure AIIa and AIIb were overgrowth by garnet zone AIIc shadows around garnet A (PS2 in Fig. 6a). Furthermore, ( Fig. 6a and b). some garnets of type A from sample NH Taken together, the chemical composition and the were sheared either before or during garnet zone AIII textural observations clearly indicate different growth overgrowth, at the latest ( Figs 2a and 6b). rates in different domains of garnet AII, which seems to Finally, it should be noted that at least parts of garnet correlate with different magnitudes of deformation. Fast AII zonation correlates with that of garnet type B core initial growth of garnet AIIb was obviously accompanied (garnet BI, Figs 6a and 7). That indicates new garnet by high strain, and slow garnet AIIa formation with low formation in the matrix at the same time as garnet AII strain. These features indicate that at least the first period growth. In garnet BI, a foliation is weakly traced by of garnet AII growth was syntectonic. The symmetric inclusions of quartz and rare epidote ( Fig. 6a). array of the individual garnet AII domains points to deformation under pure shear conditions (Fig. 6b). However, it should be emphasized that some metres away Garnets AIII, BII and CI from sample NH in the same gneiss layer, type As is clearly shown in Fig. 5, the boundary between A garnets with typical snowball textures can be observed garnet zones AII and AIII is characterized by numerous 533

6 JOURNAL OF PETROLOGY VOLUME 42 NUMBER 3 MARCH 2001 Fig. 4. Composition maps of garnet A (Fig. 2b) showing the distribution of Ca, Fe, Mg and Mn (width of maps 30 mm; bright areas indicate high concentration, dark areas low concentration; squares indicate positions of detailed maps shown in Fig. 5; for explanation see text). complex embayments, which indicate garnet resorption Garnets AIV, BIII, CII and D before garnet AIII overgrowth. A gap in the garnet growth The formation of garnet zones AIV, BIII and CII took history at the AII AIII transition is also documented by place simultaneously with the growth of new garnet D a slight increase of X sps ( mol %) and Fe/(Fe + under static conditions (Figs 6a and 7). Garnet zone AIV Mg) ( ), as shown in Fig. 7a (point F). Increase can be distinguished from zone AIII by much lower X grs of Fe/(Fe + Mg) points to a period of cooling before and higher X alm contents (Fig. 7d and e). The rim of garnet AIII overgrowth, whereas the increase in X sps garnet zone AIV shows the lowest Fe/(Fe + Mg) ratio. results from manganese refractionation as a result of Notably, garnet D locally forms idiomorphic atoll garnets garnet resorption. Following this transition zone, garnet in the matrix, enclosing plagioclase, quartz, biotite, ep- AIII growth is characterized by a relatively abrupt in- idote, sphene and rare ilmenite ( Table 1). crease of X grs from about 31 to 39 mol % and decreasing X alm (from 63 to 57 mol %) and X sps (from 4 4 to 2 mol %, Fig. 7a). Toward garnet zone AIV X grs decreases (from 39 to 32 mol %) and X alm (from 57 to 62 mol %) increases, Epidote whereas X sps (1 2 mol %) is nearly constant. In places the Epidote forms euhedral crystals in the matrix and occurs garnet AIII zonations grade continuously into AIV (Fig. in garnets A, B, C and D. It invariably contains cores of 7b), but mostly change abruptly. Typical garnet AIII allanite ( Fig. 3c), which served as crystallization seeds. zonations can also be observed in garnet type B (zone Twinning of allanite frequently continues into the sur- BII) and in the core of garnet type C (garnet CI; Figs rounding epidotes. Quartz, apatite and biotite are com- 6a and 7). This clearly indicates formation of new garnet mon inclusions, whereas ilmenite occurs rarely. Most C simultaneously with garnet AIII and BII overgrowth. matrix epidotes are completely mantled by biotite ( Fig. Some garnets of type CI show s-shaped inclusion trails, 3c), although in some cases they occur in direct contact which indicate syntectonic garnet growth ( Fig. 2c). All with plagioclase and quartz. In such domains, epidote garnet types contain inclusions of quartz, biotite, epidote, frequently forms symplectites with quartz, rarely ac- apatite, sphene, plagioclase and very rare magnetite companied by magnetite ( Fig. 3d). Such symplectites (Table 1). were also found enclosed in garnet zones AIII, AIV, CII 534

7 ZEH AND MILLAR METAMORPHIC HISTORY OF MOINE SUPERGROUP mol %), whereas X ps increases rapidly to 29 5 mol % in garnet AIII (Ep3: Fig. 8). As epidotes with all three zones (EI EIII) are overgrown by garnet zone AIII, epidote zone EIII must be formed either before or with garnet zone AIII, at the latest (see below). Fig. 5. Distribution of Ca and Mn at the rim of garnet A (detail of Fig. 4). The element distribution indicates resorption of garnet zone AII before garnet AIII overgrowth (width of maps 3 mm; bright areas indicate high concentration, dark areas low concentration; for explanation see text). Biotite Biotite is widespread in the matrix and occurs as inclusion in garnets AII, AIII, AIV, B and C. Biotite also occurs as inclusions in epidote zones EII and EIII, and rarely is enclosed in allanite surrounded by epidote EI. Because all allanite and epidote (with X ps <20 mol %) were already formed before garnet AII growth started (see above), biotite must also have been present during garnet AI growth. Biotite in all textural domains has extremely high contents of titanium, at p.f.u., and of Al VI,at p.f.u. (see Appendices A and B). The silica contents of biotite are relatively low, ranging between 5 36 and 5 43 p.f.u. The Fe/(Fe + Mg) ratio of matrix biotites, of large biotite inclusions in garnet D and of small biotites included in epidote (EI and EII) and allanite ranges between 0 76 and In contrast, biotites enclosed in garnet AIII + IV and CI + II show large variations of Fe/(Fe + Mg) between 0 77 and The highest Fe/(Fe + Mg) ratio (0 85) was noted for biotites in contact with retrograde chlorite. A local increase of Fe/( Fe + Mg) of garnet in contact with biotite indicates retrograde, diffusional Fe Mg exchange. and D, indicating that the symplectites are either pre- or syn-genetic to AIII (see below). The symplectites were Chlorite probably formed as a result of the inverse net-reaction Numerous chlorite inclusions were observed in garnet [(R3), see below]. AI, and possibly formed during the prograde evolution. Most epidotes show three zones (EI EIII), with more All other chlorite occurrences, especially those formed or less abrupt transitions between them ( Fig. 8). Epidote along garnet cracks and in the matrix, result from ret- zone EI occurs at the direct contact to allanite inclusions rograde overprint. According to the nomenclature of and shows the lowest pistacite content of X ps = 16 mol % Hey (1954) all chlorites are daphnites, with a calculated amount of Fe 3+ [X ps = Fe 3+ /(Fe 3+ + Al)], whereas X ps within zone between 0 00 and 0 37 p.f.u. (Appendices EII varies between 20 and 23 mol %. Frequently, X ps A and B). Chlorite in garnet AI shows Fe/(Fe + Mg) decreases in zone EII from 23 to 20 mol % toward the ratios between and 0 963, and Mn contents be- rim (see Fig. 8: Ep1). In epidote zone EIII X tween and p.f.u., whereas chlorites formed ps increases to 29 7 mol % (Fig. 8: Ep2, Ep3), which is a nearly pure along cracks have a Fe/(Fe + Mg) value between pistacite end-member composition. Symplectitic textures and 0 872, and Mn contents between and 0 07 are restricted to epidote zones EII and EIII. p.f.u. Numerous epidote inclusions allow a correlation between the growth history of garnet and epidote. Epidote with X ps = 23 mol % (= EII), observed at the rim of Muscovite garnet AI indicates that epidote EI formation was already Muscovite was frequently observed in garnet zones AI finished before garnet AII growth started (Fig. 8). We and AII, rarely in AIII, but never in AIV. The Si content assume that epidote EI formation took place before or of muscovite in garnet zone AI ranges from 6 07 to 6 22 during garnet AI growth. The composition of epidotes p.f.u., and in garnet zones AII and III from 6 02 to 6 17 enclosed in garnet AII is nearly constant (X ps = p.f.u. Notably, all muscovites have very low aluminium 535

8 JOURNAL OF PETROLOGY VOLUME 42 NUMBER 3 MARCH 2001 Fig. 6. (a) Synopsis of garnet types A D, and relationships among the garnet zones. The figure is based on textural observations (Fig. 2a c), element distribution (Figs 4 and 5) and ten line scans. Black line shows profiles shown in Fig. 7; dotted line c r shows core rim profile used in Fig. 15. (b) Synopsis of the successive crystallization deformation of garnet type A, based on microstructural observations and element zoning: (i) static growth of garnet AI; (ii) pure shear deformation during growth of garnet AIIa and b; garnet AIIb initially grew faster than AIIa; (iii) final stage of garnet AII growth; (iv) garnet resorption; (v) simple shear deformation during garnet AIII growth, accompanied by formation of sigma-shaped quartz plagioclase pressure shadows and shearing of garnet AII porphyroblasts. (For explanation see text.) contents (AI muscovites: p.f.u.; AII and AIII observed in plagioclase Pl3 which, however, shows an muscovites: p.f.u.) and high iron contents, up unzoned core with X an = mol % and a rim with tot to 5 0 wt % Fe 2 O 3. As the Si content of muscovite is X an = mol %. Plagioclase Pl4 has an almost unzoned low, iron cannot be explained as ferrous iron, entering core with X an = 28 mol % and shows increasing the octahedral position by the Tschermak s exchange: anorthite contents up to 38 mol % (not shown) towards Si 1 Fe 2+ 1Al IV 1Al VI 1. It seems more likely that ferric iron the rim. substituted for octahedral aluminium via the exchange Zoned and unzoned plagioclase inclusions in garnet Al VI 1Fe 3+ VI 1. The calculated amount of ferric iron (Appendix indicate a decrease of X an from garnet zone AI rim A) ranges between 0 25 and 0 55 p.f.u. in AI (42 35 mol %) to garnet zone AII rim (22 mol %), fol- muscovites and between 0 14 and 0 30 p.f.u. in AII lowed by an increase of X an in garnet zone AIII to muscovites. The paragonite component of AI muscovites mol %. The maximum anorthite content found in varies between 1 7 and 6 0 mol %, that of AII and AIII garnet zone AIV was X an =32 mol %. The change of muscovites varies between 8 4 and 14 4 mol %. X an of the plagioclase inclusions between garnet AI rim and AIV correlates well with that found for matrix plagioclases Pl2 ( Fig. 9). Plagioclase Plagioclase occurs frequently in the matrix and forms rare inclusions in all garnet zones. Matrix plagioclase Accessories shows highly variable zonation patterns. Four typical Apatite, sphene, allanite and ilmenite are common accessories, plagioclase types (Pl1 Pl4) are presented in Fig. 9. Plagioclase whereas magnetite is relatively rare. Apatite is Pl1 first shows an increase of X an from 36 to widespread in the matrix and is enclosed in all garnet 46 mol % followed by a decrease to 26 mol % towards generations, whereas allanite forms cores of epidote or the rim. In contrast, plagioclase Pl2 is characterized by independent crystals within garnet, but is never found in a decrease of X an from 32 to 22 mol %, followed by a the matrix. Ilmenite occurs commonly in the matrix and rapid increase to 36 mol %. The same increase can be in garnet zone AIIb (Fig. 3a), and is rarely enclosed in 536

9 ZEH AND MILLAR METAMORPHIC HISTORY OF MOINE SUPERGROUP Fig. 7. Zonation patterns of garnets A (a and b), B (c), C (d), and D (e). The location of the profiles is shown in Fig. 6a. (For explanation see text.) garnet AI and in epidote. Ilmenites enclosed in garnet Zircon was observed, enclosed in allanite, in garnet, and zone AIIb show a systematic decrease of the haematite in matrix quartz and biotite. and pyrophanite components from 6 to 4 mol %, and from 3 to 2 mol %, respectively, whereas the ilmenite component increases from 92 to 96 mol % toward the garnet rim. In contrast, rare ilmenite inclusions in garnet MINERAL ASSEMBLAGES AND AI show pyrophanite contents of mol % and il- REACTION HISTORY menite contents of mol %. Matrix ilmenites have Textural relationships and mineral inclusions clearly inthe same compositional variations as ilmenites enclosed dicate that the growth of garnet AI took place in the in garnet zone AIIb. Magnetite, which has nearly stoiassemblage chiometric composition, forms idiomorphic grains in some matrix domains and very rarely in garnet BI II, garnet AI + chlorite + biotite + muscovite + plaand occurs along retrograde cracks within garnet. Sphene gioclase (X an = 35 46) + epidote (X ps = ) from all domains shows slightly elevated contents of + quartz + fluid + sphene + apatite + allanite ± aluminium ( p.f.u.) and iron ( p.f.u.). ilmenite. (P1a) 537

10 JOURNAL OF PETROLOGY VOLUME 42 NUMBER 3 MARCH 2001 Table 1: Mineral assemblages successively formed in sample (NH ) Fig. 8. Composition and zoning of matrix epidotes and of epidotes enclosed in different garnet zones. X ps = Fe 3+ /(Fe 3+ + Al); c, core; r, rim. Fig. 9. Composition and zoning of matrix plagioclase and of plagioclase enclosed in different garnet zones. The zonation patterns of the plagioclases Pl1 Pl4 represent different stages of the metamorphic history (c, core; r, rim; for explanation see text). 538

11 ZEH AND MILLAR METAMORPHIC HISTORY OF MOINE SUPERGROUP Although the texture of garnet AII is completely different from that of garnet AI, inclusions indicate that both zones grew within the same assemblage, with slight variations of the accessories ilmenite and sphene: found beside epidote symplectites ( Fig. 3d) and in BII are explainable by the inverse reaction epidote + biotite = garnet + plagioclase + muscovite + quartz + magnetite + fluid. (R3) garnet AIIb + chlorite + biotite + muscovite + plagioclase (X All three inverse reaction (R1) (R3) are in agreement an = 43 35) + epidote (X ps = ) + quartz + fluid + ilmenite + apatite ± sphene with garnet resorption before garnet AIII growth ( Figs (P1b) 5 and 6b). They support the interpretation that epidote symplectites ( Fig. 3d) were formed before garnet AIII garnet AIIa c + chlorite + biotite + muscovite + (see below). Following garnet AII resorption, growth of plagioclase (X an = 35 22) + epidote (X ps = 23 20) garnet AIII took place, via either reaction (R2) or reaction + quartz + fluid + sphene + apatite. (P1c) (R3). Subsequently, garnet AIV was formed in the muscovite-free assemblage A reaction explaining the formation of garnet AI and AII is garnet AIV + biotite + plagioclase (X an = 32 38) + epidote + quartz + fluid + sphene + apatite ± chlorite + biotite + epidote + plagioclase + quartz = garnet + muscovite + fluid. (R1) ilmenite. (P3) Reaction (R1) is in agreement with the inclusions found Assemblage (P3) requires that muscovite reacted out in garnet types AI and AII and with the observation that with the start of garnet AIV growth, at the latest. Re- cores of matrix plagioclases were consumed (Pll and Pl2; sorption of epidotes in the matrix and enclosed in garnet Fig. 9) just before a new plagioclase generation was AIV indicates that epidote was probably consumed durformed either immediately before or together with garnet ing garnet AIV growth. Finally, as a result of the sub- AIII growth. sequent retrograde overprint new epidote, plagioclase, The transition between garnet zones AII and AIII chlorite, magnetite and rare calcite were formed along forms an important discontinuity. This is evident from thin cracks, which cut through all garnet zones (Fig. 2b). resorption textures before garnet AIII growth ( Fig. 5), from the abrupt increase of X grs with the start of garnet zone AIII formation ( Fig. 7a), and from the sudden appearance of numerous epidote, biotite and plagioclase P T PATH RECONSTRUCTION inclusions in garnet AIII. Inclusions found in garnet type Geothermobarometry AIII indicate that this garnet zone grew in the chlorite- P T calculations were carried out using compositions of free assemblage coexisting zones of garnet, epidote and plagioclase, as well as ilmenite, muscovite and chlorite inclusions, assumed to garnet AIII + biotite + muscovite + plagioclase (X an = represent equilibrium conditions during different stages 30 32) + epidote (X ps = 30) + quartz + fluid + sphene + apatite ± magnetite. (P2) of garnet formation. Diffusive smoothing of the initial growth zonation of garnet, plagioclase and epidote can Zoned plagioclase (X an = 23 32) and epidote inclusions be excluded, because all these minerals still show steep (X ps = 20 30) in garnet AIII provide evidence that they compositional gradients between individual zones (e.g. were formed either before or simultaneously with garnet epidote EI EII EIII; garnet AII AIII; Pl2; see Figs 7 9). AIII. Formation of both minerals before garnet AIII The continuous decrease of Fe/( Fe + Mg) toward garnet growth seems likely, because some of the epidote and AIV rim (Fig. 7b) also indicates that a retrograde Fe Mg plagioclase inclusions are resorbed by garnet. Never- exchange of garnet with matrix biotite is insignificant. theless, it may also be possible that new epidote and Only at the direct contact (>5 10 μm) with some biotite plagioclase were formed together with garnet AIII, and chlorite inclusions does the Fe/( Fe + Mg) in garnet whereas older epidote and plagioclase, no longer in increase slightly. Furthermore, there is no evidence for chemical equilibrium with AIII, were consumed. For- any chemical change of plagioclase, epidote and ilmenite mation of plagioclase and epidote before garnet AIII compositions after their entrapment by garnet, because growth can be explained by the inverse reaction (R1), if the garnet composition at the contact with these inclusions chlorite was present. Otherwise the inverse chlorite-free is unmodified. reaction P T calculations were carried out with the program Thermocalc V2.4, which is based on the internally conepidote + biotite = garnet + plagioclase + muscovite sistent thermodynamic dataset of Powell & Holland (1988) + quartz + fluid (R2) and Holland & Powell (1990). The ln K values, slopes would also be possible (see below). Magnetite relics rarely and errors of the end-member reaction (Fig. 10) 539

12 JOURNAL OF PETROLOGY VOLUME 42 NUMBER 3 MARCH 2001 Fig. 10. P T diagram showing the results of geothermobarometry estimated from mineral assemblages enclosed in different zones of garnet A. The calculations were carried out using the program Thermocalc (Holland & Powell, 1990) and the garnet biotite geothermometer of Kleemann & Reinhardt (1994). The slopes for certain ln K values of reaction 4czo + qtz = grs + 5an + H 2 O were calculated using a water activity of 1 0. Further results are given in Table 2 (for explanation see text). 495 ± 13 C at3 5 ± 0 7 kbar were estimated, whereas the rims of garnet AIIb and AIIc give higher metamorphic conditions of 634 ± 45 C at 9 4 ± 1 0 kbar and 657 ± 59 C at 10 0 ± 1 3 kbar, respectively. The temperatures calculated with Thermocalc for AIIc are higher than that calculated with the garnet biotite geothermometer of Kleemann & Reinhardt (1994), ranging between 560 and 590 C (at 9 kbar). Coexisting garnet AIII (X grs = 0 39), epidote EIII (X ps = 29 7) and plagioclase (X an = 0 29) yield a pressure of 4 8 ± 0 8 kbar, at a temperature of 640 C. Garnet AIV matrix biotite pairs calculated with the garnet biotite geothermometer of Kleemann & Reinhardt (1994) yielded temperatures between 653 and 680 C at an assumed pressure of 5 5 kbar. At the moment, because of the lack of any geochronological data, it is not clear whether the individual P T points represent increments during a single, clockwise P T evolution, or if they result from a polymetamorphic evolution (see below). Furthermore, it should be noted that the P T calculations given above should be regarded as rough approximations. This especially concerns the temperature estimates obtained with the average pressure temperature calculations mode of Thermocalc, which is strongly dependent on the composition of the phyllosilicates chlorite, biotite and muscovite. As mentioned above, it is possible that chlorite re-equilibrated during the retrograde evolution or that the composition of biotite, although enclosed in epidote EII, changed during the prograde or retrograde evolution. In the case of retrograde chlorite re-equilibration, the temperature of garnet AI-rim is too low; in the case of prograde biotite re-equilibration the temperatures of 4 clinozoisite + quartz = grossular + 5 anorthite + garnets AIIb and c are too high. H 2 O (R4) Another important point is that the biotites and muscovites were calculated with the calculations on all reactions in our sample are complex solid solutions. Biotite, between end-members mode of Thermocalc, using for instance, contains high amounts of Ti, Mn and end-member activities of coexisting garnet Al vi and low silica contents, whereas muscovite contains epidote plagioclase triplets in garnet AI, AIIc and AIII, important amounts of Fe 3+ instead of Al vi (see above). assuming a water activity of 1 0. P T conditions and Furthermore, we have no control over the amounts of errors from coexisting minerals enclosed in garnet zone Fe 3+ of biotite. The activity models used in Thermocalc AI and AII were calculated using the average pressure calculations for the phyllosilicates (Appendix A) do not temperature calculations mode of Thermocalc ( Table consider the complex effects resulting from excess energies 2). This procedure has been described in detail by Will of mixing. In contrast, effects of Mn, Ti and Al vi in biotite (1998). Mineral end-members and activity models used and the complex garnet composition are taken into for calculations are listed in Table 2 and Appendix account in the garnet biotite geothermometer of Klee- A, respectively. The garnet biotite geothermometer of mann & Reinhardt (1994) which, therefore, should con- Kleemann & Reinhardt (1994, see below) was also emuncertainty strain the temperatures much better. A further ployed to calculate temperatures for garnet zone AII-rim is the water activity, which is assumed here and AIV-rim, assuming that biotite inclusions in epidote to be 1 0. At a water activity <1 0 all P T points estimated zone EII coexists with garnet AIIc, and matrix biotites with Thermocalc will shift to lower temperatures and coexist with garnet AIV. pressures, as shown in Table 2. The results of the P T calculations, which are sumthey Pressure estimates seem to be less problematic, because marized in Table 2, indicate different P T conditions dominantly result from reaction (R4), using end- during formation of different zones of garnet A (Fig. 10). members of plagioclase, epidote and garnet. The com- From minerals enclosed in garnet AI P T conditions of positions of these minerals are unaffected by prograde 540

13 ZEH AND MILLAR METAMORPHIC HISTORY OF MOINE SUPERGROUP Table 2: Results of geothermobarometric calculations Thermocalc V2.4 Kleemann & Reinhardt (1994) 4cz + q = garnet biotite average P T calculations gr + 5an + H 2 O geothermometry T ( C) 2σ P (kbar) 2σ fit corr. ah 2 O lnk d sln K d T ( C) AI-rim End-members: cz, ep, clin, daph, alm, gr, py, an, ilm, hem, q, sph, H 2 O AIIb-rim End-members: mu, cel, cz, alm, gr, py, an, ann, phl, east, ilm, q, sph, H 2 O AIIc-rim (at 9 kbar) biotite enclosed in EpII End-members: mu, cel, cz, alm, gr, py, an, ann, phl, east, q, H 2 O AIII calculation impossible, too few reactions, no biotite and Fe/(Fe+Mg)-Bt muscovite found in equilibrium too variable AIV calculation impossible, too few reactions, no epidote no epidote in and muscovite in equilibrium equilibrium (at 5 5 kbar) matrix biotite Endmembers: alm, gr, py, an, ann, phl, east, q, H 2 O Calculations were performed with the program Thermocalc V2.4 (Powell & Holland,1988; Holland & Powell, 1990) and the garnet biotite geothermometer of Kleemann & Reinhardt (1994). End-member abbreviations after Holland & Powell (1990). and retrograde diffusive alterations, as clearly indicated method of differential thermodynamics is employed by the preserved zonation patterns (see above). Activity (Rumble, 1974; Spear et al., 1982; Spear, 1988, 1989a, models are available that take into account excess energies 1989b). In this method, the intensive variables pressure of mixing for plagioclase and garnet (Appendix A). Never- ( P) and temperature (T ), and extensive variables, such theless, water activities below 1 0 shift the position of as phase composition (X) and phase abundance ( M ), are reaction (R4) in Fig. 10 for a given ln K to lower related by a set of differential thermodynamic, stoitemperatures, so that higher pressures at the same tem- chiometric and mass balance equations. Dependent upon peratures would be obtained. Furthermore, it should be the number of phases and system components chosen, a noted that the pressure of 4 8 ± 0 8 kbar estimated for certain number of variables is independent [monitor garnet zone AIII is significant only when the rims of variables according to Menard & Spear (1993)] and all epidote and plagioclase inclusions are really in equi- remain dependent (see Table 3 and 4). Changes of librium with AIII. For the case that they represent independent variables, e.g. X alm, X grs, X sps of garnet, result metastable relics formed before garnet AIII growth, the in a simultaneous change of all other variables, e.g. P, estimated pressure would be meaningless (see below). T, X an, etc. The advantage of the Gibbs method is that depending upon the purpose of the study, independent variables can be chosen in different ways. Gibbs method modelling Three types of Gibbs method calculations were performed To set additional P T path constraints that result from here, using the Apple Macintosh program GIBBS the zonation of garnet, plagioclase and epidote, the Gibbs (Spear & Menard, 1989; Spear et al., 1991), with activity 541

14 JOURNAL OF PETROLOGY VOLUME 42 NUMBER 3 MARCH 2001 Table 3: Starting conditions for contour and garnet zoning models and mixing models supplied with the program ( Table 3). For magnetite, thermodynamic data were added to the program using the dataset of Berman (1988). First, P T paths were computed using the technique of Spear Model: I I II-1 II-2 Garnet & Selverstone (1983). Second, P T space was contoured growth with mineral composition isopleths to determine relations Volume (cm 3 ) modelling (model I) Garnet (Grt 7 ) between P T X M in different assemblages. Third, garnet zoning models were constructed along estimated P T paths to evaluate effects resulting from different garnet nucleation densities during the metamorphic history. Biotite Chlorite Reference conditions Quartz A prerequisite for the use of the Gibbs method is to Plagioclase find at least one reference point at which the phase Epidote compositions of an equilibrium assemblage at given pres- Muscovite sures and temperatures and optionally the mineral mode Magnetite 6 1 are known. For well-crystallized rocks that contain con- H 2 O tinuously zoned minerals, this is simply done using the rim composition and the mode of all minerals in equi- T ( C): librium, from which the start P T conditions are obtained. P (bar): For discontinuously zoned minerals, as dealt with in this study, it may be necessary to define more than one X alm starting condition. This, however, is dependent upon the X sps kind of the discontinuity between the different mineral X grs zones. During a continuous P T evolution a discontinuity X py in the zonation can result from: X ann (U1) change of the mineral assemblage; X phl (U2) change of the mineral assemblage, accompanied X Mn-Bt by a temporary consumption of the zoned mineral; X daph (U3) changing nucleation density in a specific mineral X chl assemblage; X Mn-Chl (U4) changing nucleation density and changing mineral X pist assemblage. X czo During a polymetamorphic history a discontinuity ad- X an ditionally results from a skip of the P T conditions: X ab (U5) in a certain mineral assemblage; X ms (U6) accompanied by a change of the mineral as- X H2O semblage (or fluid species) in between; No. of NP (U7) accompanied by changing nucleation density in No. of NC a specific mineral assemblage; No. of react (U8) accompanied by a change of the nucleation density and the mineral assemblage. Model I is computed in the component system SiO 2 Examples for discontinuities of types (U1) (U4) have Al 2 O 3 MgO FeO MnO CaO Na 2 O K 2 O H 2 O, models II-1 and been given, for example, by Spear et al. (1990), Spear II-2 in the system SiO 2 Al 2 O 3 Fe 2 O 3 MgO FeO MnO (1993) and Menard & Spear (1993). In contrast, examples CaO Na 2 O K 2 O H 2 O. NP, phase components; NC, system components, react., reactions. for type (U5) (U8) discontinuities are rare, and without Based on point count modes, modified for subsequent garnet geochronological data difficult to prove (e.g. Vernon, growth and to allow the presence of fluid. 1996). In case of a continuous P T evolution only one P T from geothermobarometry (see text). The following set of starting P T X M conditions is required, as shown mineral models supplied with the program GIBBS are used for calculations: 1 Berman garnet; 2 biotite (Fe Mg Mn) idealmixing by Spear et al. (1990) and Menard & Spear (1993). In on one site; chlorite; 4 ideal = 1 0; 5 pure mus- this situation, change of the mineral assemblage is simply covite; 6 magnetite (above 550 C); with H, S, V, c p data from simulated by adding or removing minerals from an Berman (1988). 7 Garnet, ideal mixing on one site (as required by the program). existing assemblage at a certain P T condition, and change of the nucleation density by changing the number 542

15 ZEH AND MILLAR METAMORPHIC HISTORY OF MOINE SUPERGROUP Table 4: Results of Gibbs method P T path calculations for model I in assemblage (P1m) and (P2m) System: (SiO 2 Al 2 O 3 MgO FeO MnO CaO Na 2 O K 2 O H 2 O) Assemblage: (Pm1): Grt+Bt+Chl+Ms+Pl+Ep+Qtz+fluid (P2m): Grt+Bt+Ms+Pl+Ep+Qtz+fluid Point A Point Point Point Point start at Point Point (start) B C D E Point A F G Independent variables X alm X alm X sps X sps X grs X grs X an Dependent variables T ( C) T ( C) P (bar) P (bar) X py X py X ann X ann X phl X phl X Mn-Bt X Mn-Bt X daph X daph X chl X chl X Mn-Chl X Mn-Chl X an X ab X ab X czo X czo 1 1 X H2O X H2O 1 1 Point labels refer to Fig. 7. P T from geothermobarometry Biotite composition inferred using the garnet biotite thermometer of Ferry & Spear (1978) at the starting conditions; X Mn-Bt was inferred to be the same as in the matrix biotites and biotites enclosed in epidote EII. Chlorite composition was inferred using the garnet chlorite thermometer of Ghent et al. (1987). X Mn-Chl assumed to be similar to such retrogressively formed at the rim of garnet zone AII. of garnets per volume. However, for a real poly- Model I. As a first approach, it is assumed that the metamorphic evolution, new starting P T X M con- zonation of garnet AI AIV results from a nearly conditions are required for each zone interrupted by a tinuous P T evolution during a single orogenic event. The discontinuity. transition between garnet zones AI and AII represents a The petrological observations mentioned above clearly (U3) discontinuity, and the transition between garnet indicate important discontinuities between garnet zones zones AII and AIII is a (U4) discontinuity. AI and AII, and between zones AII and AIII, which P T path calculations and P T X M contouring for coincide with discontinuities in zoned epidote and pla- model I were started near the rim of garnet zone AIIc gioclase. Furthermore, there is evidence that the nuc- (point A in Fig. 7a), at which chlorite-out is inferred. leation density increases from garnet AI to AIV. Consequently, we have to change from the model as- Unfortunately, there are no geochronological data for semblage the several garnet zones, which could demonstrate garnet + chlorite + biotite + muscovite + plagioclase whether all garnet zones were formed during a single or + clinozoisite + quartz + H 2 O (P1m) during different orogenic events. Therefore, it is not clear whether the transition AI AII is a (U3) or (U7) discontinuity, and whether the AII AIII discontinuity is to the model assemblage of type (U4) or (U8). Because of these uncertainties two garnet + biotite + muscovite + plagioclase + models will be discussed. clinozoisite + quartz + H 2 O (P2m) 543