Origin of Grandite Garnet in Calc-Silicate Granulites: Mineral Fluid Equilibria and Petrogenetic Grids

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1 JOURNAL OF PETROLOGY VOLUME 46 NUMBER 5 PAGES doi: /petrology/egi010 Origin of Grandite Garnet in Calc-Silicate Granulites: Mineral Fluid Equilibria and Petrogenetic Grids SOMNATH DASGUPTA 1 * AND SUPRATIM PAL 2 1 DEPARTMENT OF GEOLOGICAL SCIENCES, JADAVPUR UNIVERSITY, KOLKATA , INDIA 2 DEPARTMENT OF GEOLOGY, DURGAPUR GOVERNMENT COLLEGE, DURGAPUR , WEST BENGAL, INDIA RECEIVED FEBRUARY 28, 2003; ACCEPTED DECEMBER 15, 2004 ADVANCE ACCESS PUBLICATION JANUARY 28, 2005 The role of clinopyroxene in producing grandite garnet is evaluated using data from an ultrahigh-temperature metamorphosed calcsilicate granulite occurrence in the Eastern Ghats Belt, India. Peak pressure temperature conditions of metamorphism were previously constrained from associated high Mg Al granulites as c. 09 GPa, >950 C, and the rocks were near-isobarically cooled to c. 750 C. Grandite garnet of variable composition was produced by a number of reactions involving phases such as clinopyroxene, scapolite, plagioclase, wollastonite and calcite, in closely spaced domains. Compositional heterogeneity is preserved even on a microscale. This precludes pervasive fluid fluxing during either the peak or the retrograde stage of metamorphism, and is further corroborated by computation of fluid rock ratios. With the help of detailed textural and mineral compositional studies leading to formulation of balanced reactions, and using an internally consistent thermodynamic dataset and relevant activity composition relationships, new petrogenetic grids are developed involving clinopyroxene in the system CaO Al 2 O 3 FeO SiO 2 CO 2 O 2 in T aco 2 fo 2 space to demonstrate the importance of these factors in the formation of grandite garnet. Two singular compositions in garnet-producing reactions in this system are deduced, which explain apparently anomalous textural relations. The possible role of an esseneite component in clinopyroxene in the production of grandite garnet is evaluated. It is concluded that temperature and fo 2 are the most crucial variables controlling garnet composition in calc-silicate granulites. fo 2, however, behaves as a dependent variable of CO 2 in the fluid phase. External fluid fluxing of any composition is not necessary to produce chemical heterogeneity of garnet solid solution. KEY WORDS: grandite garnet; role of clinopyroxene; internal buffering; oxidation decarbonation equilibria INTRODUCTION Calc-silicate granulites, although minor constituents of many high-grade terranes, provide powerful constraints on fluid rock interaction, fluid evolutionary history and retrograde P T trajectories (Warren et al., 1987; Motoyoshi et al., 1991; Harley & Buick, 1992; Buick et al., 1993, 1994; Dasgupta, 1993; Fitzsimons & Harley, 1994; Bhowmik et al., 1995; Cartwright & Buick, 1995; Sengupta et al., 1997; Stephenson & Cook, 1997; Sengupta & Raith, 2002). Grandite garnet and clinopyroxene are almost ubiquitous phases in such rocks.two lines of evidence show that clinopyroxene contributes the andradite content in garnet. Textural evidence indicating participation of clinopyroxene in grandite garnet-forming reactions includes: (1) occurrence of coronal garnet around clinopyroxene in contact with phases such as wollastonite, scapolite, plagioclase and calcite; (2) increase in modal garnet with increasing clinopyroxene content; (3) increase in the thickness of garnet coronae at clinopyroxene contacts producing sieve-textured garnets (Harley & Buick, 1992; Sengupta et al., 1997). Compositional evidence in favour of clinopyroxene participation in garnet-forming reactions includes: (1) absence of any other phase that could contribute the andradite component in garnet; (2) Al-depleted rims of clinopyroxene adjacent to garnet coronae (Warren et al., 1987; Sengupta et al., 1997); (3) decrease in hedenbergite content of clinopyroxene at the contact with garnet coronae (Harley & Buick, 1992); (4) decrease in esseneite component in clinopyroxene at the contact of coronal garnet (Fitzsimons & Harley, 1994); (5) antipathetic relation in the Al/Fe 3þ of coexisting garnet and clinopyroxene (Sengupta et al., 1997). *Corresponding author. sdg@cal3.vsnl.net.in # The Author Published by Oxford University Press. All rights reserved. For Permissions, please journals.permissions@ oupjournals.org

2 JOURNAL OF PETROLOGY VOLUME 46 NUMBER 5 MAY 2005 Table 1: Mineral reactions involving clinopyroxene in the formation of grandite garnet No. Reaction References 1 4Hd þ 2Cal þ O 2 ¼ 2Adr þ 2Qtz þ 2CO 2 B et al., C & B, S & A 2 Scp þ Cal þ Qtz þ CaTs þ FeTs þ Hd þ O 2 ¼ Adr þ CO 2 Bh et al. 3 Hd þ CO 2 þ O 2 ¼ Mag þ Cal þ Qtz þ Adr Bh et al. 4 2Wo þ 4Hd þ O 2 ¼ 2Adr þ 4Qtz S et al., H & B, D et al., D 5 3Alm þ 12Grs þ 15Hd þ 5Cal þ 6CO 2 ¼ 12Adr þ 5Scp þ 9Qtz H & B 6 4Hd þ 4Cal þ O 2 ¼ 2Adr þ 2Wo þ 4CO 2 S et al. 7 Hd þ O 2 ¼ Adr þ Qtz þ Mag L 8 4Cal þ 4Qtz þ 2FeTs ¼ Grs þ Adr þ 4CO 2 B et al. 9 Cal þ Wo þ Pl 1 þ Cpx 1 ¼ Grt þ Pl 2 þ Cpx 2 þ CO 2 B et al. 10 4FeTs þ 8Cal ¼ 2Grs þ 2Adr þ 8CO 2 C & B 11 Scp þ Cal þ Qtz þ FeTs þ CaTs ¼ Grt ss þ CO 2 H & B, Bh et al. 12 Wo þ Adr þ CaTs þ Me ¼ Hd þ Cal þ Grs þ O 2 D et al., D 13 CaTs þ Wo ¼ Grs W et al., H & B, H et al., D 14 Cpx þ Scp þ Wo þ Cal ¼ Grt þ CO 2 F & H 15 Cpx þ Scp þwo ¼ Grt þ Qtz þ CO 2 H & B, F & H, S & A 16 Cpx þ Scp þ Wo ¼ Grt þ Cal þ Qtz H & B, F & H 17 Cpx þ Wo þ Pl ¼ Grt þ Qtz H & B, F & H 18 Wo þ FeTs ¼ Grs þ Adr H & B, H et al. 19 Hd þ FeTs ¼ Grs þ Adr þ Alm H & B 20 Alm þ 6Wo ¼ Grs þ 3Hd H & B 21 Wo þ Cpx þ An þ CO 2 ¼ Grt þ Scp þ Qtz H & B 22 Scp þ Hd þ 3Cal þ Qtz ¼ 3Grt (Grs 89 Adr 11 ) þ 2Qtz þ CO 2 H & B 23 2Scp þ 3Hd þ 4Cal ¼ 6Grt (Grs 83 Adr 17 ) þ 6CO 2 H & B 24 FeTs þ 7Wo þ Scp ¼ 4(Grs 875 Adr 125 ) þ 2Qtz þ CO 2 H et al. 25 2CaTs þ 4Cal þ 4Qtz ¼ 2Grs þ 4CO 2 S et al. 26 FeTs þ Scp ¼ Adr þ Cal þ Qtz S & A 27 Scp ¼ Grs þ CaTs þ Qtz þ CO 2 W et al. 28 Scp þ Cal ¼ Grs þ CaTs þ CO 2 W et al. 29 An þ 6Cal þ 5Qtz þ 2Ess ¼ 3Grs 67 Adr 33 þ 6CO 2 S & C 30 An þ 4Cal þ 3Qtz þ CaTs ¼ 2Grs þ 4CO 2 S & C 31 2Cal þ 2Qtz þ Ess ¼ Grs 50 Adr 50 þ 2CO 2 S & C B et al., Buick et al. (1994); Bh et al., Bhowmik et al. (1995); C & B, Cartwright & Buick (1995); D, Dasgupta (1993); D et al., Dasgupta et al. (1992); F & H, Fitzsimons & Harley (1994); H & B, Harley & Buick (1992); H et al., Harley et al. (1994); L, Liou (1974); S & A, Shaw & Arima (1996); S & C, Stephenson & Cook (1997); S et al., Sengupta et al (1997); W et al., Warren et al. (1987). Although several workers have addressed the problems relating to the contribution of aluminous clinopyroxenes of hedenbergite diopside solid solutions to the andradite content of calc-silicate garnets (Sivaprakash, 1981; Warren et al., 1987; Harley & Buick, 1992; Dasgupta, 1993; Buick et al., 1994; Fitzsimons & Harley, 1994; Harley et al., 1994; Sengupta et al., 1997; Stephenson & Cook, 1997, and references cited therein), very little has been done to actually develop petrogenetic grids with clinopyroxene to explain the P T X evolution of the rocks. Table 1 lists all solid solid and solid fluid equilibria suggested by different workers to account for the formation of grandite garnet in calc-silicate granulites. There are broadly two possible ways to derive the andradite content in garnet from clinopyroxene solid solution. One group of workers suggests derivation of andradite from the esseneite in clinopyroxene (Harley & Buick, 1992; Buick et al., 1994; Fitzsimons & Harley, 1994; Harley et al., 1994; Bhowmik et al., 1995; Cartwright & Buick, 1995; Sengupta et al., 1997; Stephenson & Cook, 1997). Another group suggests that oxidation of Fe 2þ in clinopyroxene is essential (Sivaprakash, 1981; Harley & Buick, 1992; Dasgupta, 1993; Buick et al., 1994). Buick et al. (1994) argued for oxidation reactions caused by 1046

3 DASGUPTA AND PAL ORIGIN OF GRANDITE GARNET Fig. 1. Geological map of Narsapuram with inset map of India showing the Eastern Ghats Belt and location of Narsapuram. infiltrating, oxidizing fluids that also metasomatically introduced Fe 3þ to form very andradite-rich garnets. Indeed, andradite is a common product in metasomatic environments [reviewed by Barton et al. (1991) and Zhang & Saxena (1991)]. Despite these possibilities, P T fluid evolution of calc-silicate granulites is commonly treated in petrogenetic grids without clinopyroxene, and the andradite content in garnet is customarily taken into account by reduced activity of garnet solid solution. Here we document petrological evolution of an ultrahigh-temperature metamorphosed calc-silicate granulite occurrence from the Eastern Ghats Belt, India, evaluate the role of clinopyroxene in complex mineral fluid equilibria, develop petrogenetic grids to interpret the equilibria, and finally apply the grid to other occurrences. GEOLOGICAL BACKGROUND The calc-silicate granulite described here is from the southern part of the Eastern Ghats Belt (EGB), India (Fig. 1). The calc-silicate granulites are associated with high Mg Al granulite, khondalite (garnet perthite sillimanite quartz gneiss), leptynite (garnet quartz plagioclase perthite gneiss), enderbite (orthopyroxene plagioclase perthite quartz garnet gneiss), two-pyroxene granulite and metanorite. The EGB granulites are polymetamorphic, and a representative P T trajectory has been derived from the study of high Mg Al granulites (Dasgupta et al., 1995). During an early metamorphism, the rocks were metamorphosed at ultrahigh temperatures (c. 950 C) at lower-crustal depths (equivalent to 09 GPa pressure). This was 1047

4 JOURNAL OF PETROLOGY VOLUME 46 NUMBER 5 MAY 2005 Table 2: Summary of classification of the studied calc-silicate granulites Assn. Occurrence Mineralogy Phases absent I Intricately folded calc-silicate Scapolite clinopyroxene calcite K-feldspar titanite Wo, Grt, Qtz granulite bands at Foulkespeta plagioclase tremolite IA Thin bands at contact of quartzo-feldspathic Scapolite clinopyroxene calcite quartz wollastonite intrusions at Foulkespeta garnet K-feldspar titanite plagioclase II Dark bands at Kanaram Scapolite clinopyroxene wollastonite garnet calcite Pl quartz titanite IIA Dark bands at Kanaram Scapolite clinopyroxene calcite quartz garnet K-feldspar Pl, Wo titanite III Dark bands at Kanaram Scapolite clinopyroxene garnet plagioclase wollastonite Cal quartz titanite IIIA Dark bands at Kanaram Clinopyroxene scapolite garnet, plagioclase, quartz titanite Wo, Cal IV Quartzofeldspathic veins rich in wollastonite K-feldspar wollastonite quartz calcite Grt, Pl, Scp at Kanaram V Extremely dark bands at Kanaram Clinopyroxene calcite titanite amphibole magnetite antiperthitic plagioclase quartz Wo, Grt, Scp followed by near-isobaric cooling to c. 750 C. Cooling is also evident in metanorite and enderbite, where coronal garnet formed at the expense of orthopyroxene and plagioclase (Bose et al., 2003). According to Dasgupta et al. (1995), a later granulite metamorphism (represented by development of cordierite in Mg Al granulite) overprinted the isobarically cooled granulites, which also caused exhumation of the rocks to GPa. A later amphibolite-facies metamorphism, accompanying limited hydration, is also recorded in all the rocks. PETROLOGY OF THE CALC-SILICATE GRANULITES The calc-silicate granulites studied for this paper are gneissic with the development of alternate light bands (quartz þ calcite þ scapolite þ plagioclase þ minor K-feldspar þ minor clinopyroxene) and dark bands (clinopyroxene þ wollastonite þ scapolite þ plagioclase þ garnet þ calcite). There is however, considerable variation in the mineralogy of the dark bands in closely spaced outcrops. This has led to identification of eight mineral associations (Table 2). Out of these, Associations I, IV and V do not contain garnet, and will not be discussed further. Amphibole (compositionally tremolite actinolite) occurs in garnet-free Associations I and V (replacing clinopyroxene). As amphibole does not occur with garnet, the former is excluded from further discussion. Petrography and mineral chemistry Analytical techniques Mineral compositions were determined with a CAMECA CAMEBAX MICROBEAM Electron Probe Microanalyzer at the University of Bonn. Some analyses were carried out with a JEOL JXA 8600 SUPERPROBE at the Department of Geological Sciences, Jadavpur University. In both cases, operating conditions were 15 kv accelerating voltage, 10 na specimen current and 1 2 mm beam diameter. Natural mineral standards were used and the raw microprobe data were corrected by the PAP procedure (Pouchou & Pichoir, 1985) at the University of Bonn, and a ZAF correction scheme at Jadavpur University. Fe 3þ in clinopyroxene was recalculated from stoichiometry and charge balance method following the procedure of Papike et al. (1974). Garnet compositions were recalculated on a five cation (excluding Si) basis following Valley et al. (1983). Some of the garnet grains were analysed for Ti and F. The hydroxyl content, and hydrogrossular and flurogrossular components in garnet were calculated (where F was analysed) following the scheme (OH) þ F ¼ (3 Si) 4 in the tetrahedral site (Valley et al., 1983). Structural formulae of the remaining minerals were calculated using computer software MINFILE version 9-89 (Afifi & Essene, 1989). Representative analyses of the phases are given in Tables 3 6. Association IA This is developed in Association I, which has been invaded by quartzofeldspathic veins. Wollastonite occurs 1048

5 DASGUPTA AND PAL ORIGIN OF GRANDITE GARNET Table 3: Representative chemical analyses of scapolite Association: IA II Sample no.: 14c/1a 14c/1b R 4 R 4a R 12 Mode: R C C I R R R R SiO Al 2 O FeO MnO b.d.l b.d.l. b.d.l. b.d.l. b.d.l MgO CaO Na 2 O K 2 O S b.d.l n.d Cl n.d F n.d. n.d. n.d n.d Total Oxygen basis ¼ 25 Si Al Fe 2þ Mn b.d.l b.d.l. b.d.l. b.d.l. b.d.l Mg Ca Na K S b.d.l n.d Cl n.d F n.d. n.d. n.d n.d Total EqAn Association: IIA III Sample no.: R 15 R2 Mode: R R C R R R R R SiO Al 2 O FeO MnO b.d.l b.d.l b.d.l b.d.l. b.d.l. MgO CaO Na 2 O K 2 O S Cl F Total

6 JOURNAL OF PETROLOGY VOLUME 46 NUMBER 5 MAY 2005 Table 3: continued Association: IIA III Sample no.: R 15 R2 Mode: R R C R R R R R Oxygen basis ¼ 25 Si Al Fe 2þ Mn b.d.l b.d.l b.d.l b.d.l. b.d.l. Mg Ca Na K S Cl F Total EqAn Association: III Sample no.: R 2 B 66 B 66a Mode: R C R C R R SiO Al 2 O FeO MnO 0.01 b.d.l b.d.l b.d.l. MgO CaO Na 2 O K 2 O S n.d Cl n.d F n.d. n.d. Total Oxygen basis ¼ 25 Si Al Fe 2þ Mn b.d.l b.d.l b.d.l. Mg Ca Na K S n.d

7 DASGUPTA AND PAL ORIGIN OF GRANDITE GARNET Association: III Sample no.: R 2 B 66 B 66a Mode: R C R C R R Cl n.d F n.d. n.d. Total EqAn Association III IIIA Sample no.: B 66a R3 Mode: R R C R C R SiO Al 2 O FeO MnO b.d.l b.d.l. b.d.l MgO CaO Na 2 O K 2 O S 0.03 b.d.l Cl F n.d. n.d. n.d. n.d Total Oxygen basis ¼ 25 Si Al Fe 2þ Mn b.d.l b.d.l. b.d.l Mg Ca Na K S b.d.l Cl F n.d. n.d. n.d. n.d Total EqAn C, core of porphyroblast; R, rim of porphyroblast; I, inclusion in thick garnet corona; n.d., not detected; b.d.l., below detection limit; EqAn ¼ 100 (Al 3)/3, Al recalculated to Al þ Si ¼ 12 (after Evans et al., 1969). as porphyroblasts and contains inclusions of quartz and calcite. Porphyroblastic scapolite and calcite are separated from wollastonite by thin corona of garnet. An intergrowth of calcite and plagioclase is a replacement assemblage on scapolite rims. The EqAn [¼ (Al 3)/3 100, calculated on the basis Si þ Al ¼ 12; Evans et al., 1969] content in scapolite is (Table 3). Clinopyroxene contains negligible CaTs (0 18 mol %) and the maximum esseneite content is 31 mol %. X Mg shows little variation in 1051

8 JOURNAL OF PETROLOGY VOLUME 46 NUMBER 5 MAY 2005 Table 4: Representative chemical analyses of clinopyroxene Association: IA II Sample no.: 14c/1b R 4 R 12 Mode: R C I R R R C R C SiO Al 2 O FeO(T) MnO MgO CaO Na 2 O Total Oxygen basis ¼ 6 Si IV Al IV Al VI Fe 3þ Fe 2þ Mn Mg Ca Na Total X Mg X Fe X Hd CaTs mol % Ess mol % Association: IIA Sample no.: R 15 Mode: R R C R R R C R SiO Al 2 O FeO(T) MnO MgO CaO Na 2 O Total Oxygen basis ¼ 6 Si IV Al IV Al VI

9 DASGUPTA AND PAL ORIGIN OF GRANDITE GARNET Association: IIA Sample no.: R 15 Mode: R R C R R R C R Fe 3þ Fe 2þ Mn Mg Ca Na Total X Mg X Fe X Hd CaTs mol % Ess mol % Association: III Sample no.: B 66a R 2 Mode: R C C R R R R C SiO Al 2 O FeO(T) MnO MgO CaO Na 2 O K 2 O b.d.l b.d.l b.d.l b.d.l b.d.l b.d.l b.d.l 0.01 Total Oxygen basis ¼ 6 Si IV Al IV Al VI Fe 3þ Fe 2þ Mn Mg Ca Na Total X Mg X Fe X Hd CaTs mol % Ess mol %

10 JOURNAL OF PETROLOGY VOLUME 46 NUMBER 5 MAY 2005 Table 4: continued Association: IIIA Sample no.: R 3 Mode: R R R R R SiO Al 2 O FeO(T) MnO MgO CaO Na 2 O Total Oxygen basis ¼ 6 Si IV Al IV Al VI Fe 3þ Fe 2þ Mn Mg Ca Na Total X Mg X Fe X Hd CaTs mol % Ess mol % C, core of porphyroblast; R, rim of porphyroblast; I, inclusion in thick garnet corona; b.d.l., below detection limit. Structural formulae calculated by stoichiometry and charge balance method after Papike et al. (1974). this association, ranging between 082 and 084 (Table 4). Thin coronae of garnet grown around scapolite, wollastonite and calcite are nearly pure grossularite [Grs Adr Alm Prp Sps 03 ] (Table 5). Wollastonite, calcite and plagioclase are nearly pure phases (X Ca is 099 in calcite, and in plagioclase, Table 6). Association II A granoblastic mosaic texture is shown by porphyroblasts of scapolite, clinopyroxene and, in places, wollastonite, along with medium-grained calcite and quartz in this association. Wollastonite, clinopyroxene and scapolite in a few domains are separated by a corona of garnet and quartz (Fig. 2). Locally, scapolite porphyroblasts are separated from the wollastonite prisms by thin coronae of garnet intergrown with lobate quartz and calcite (Fig. 3). Calcite, wollastonite, scapolite and clinopyroxene are separated by garnet coronae of variable thickness (Fig. 4). Porphyroblastic wollastonite grains are replaced along their boundaries by a vermicular intergrowth of calcite and quartz. The EqAn content of scapolite ranges from 7032 to 7872 (Table 3). Clinopyroxene is less diopsidic (X Mg ¼ ) than in Association IA. In this association, clinopyroxene contains appreciable esseneite component ( mol %, Table 4). Al 2 O 3 content ranges between 053 and 209 wt %, but Al is mostly tetrahedrally coordinated. Thus the CaTs content in these clinopyroxene grains is always low (maximum 06 mol %, Table 4). A bivariate plot of Fe 3þ atoms p.f.u. in the octahedral site vs tetrahedral Al (calculated on the basis of four cations p.f.u.) depicts the variation in esseneite content in clinopyroxene (Fig. 5). Arrows indicate core to rim compositional 1054

11 DASGUPTA AND PAL ORIGIN OF GRANDITE GARNET Table 5: Representative chemical analyses of garnet Association: IA II Sample no.: 14c/1a R 12 R 4 Mode: Cr Cr Cr C R Cr C C R Cr Cr SiO TiO 2 n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. Al 2 O FeO(T) MgO MnO CaO Cl b.d.l b.d.l. b.d.l. b.d.l. b.d.l F n.d. n.d Total Si & T site Ti n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. Al Fe 3þ Fe 2þ Sum VI Ca Mn Mg Fe 2þ Sum VIII Cl b.d.l b.d.l. b.d.l. b.d.l. b.d.l F n.d. n.d OH O Adr Grs Alm Prp Sps F-Grs n.d. n.d OH-Grs Ti-Grt trends in the porphyroblastic grains. It is evident that the esseneite content decreases considerably from core to rim. Cores of the thick garnet coronae are andradite rich (Adr Grs Prp Sps 07 Alm F-Grs Hydrogrossular ), decreasing significantly to Adr Grs Prp Sps 083 Alm F-Grs Hydrogrossular at the contact of enclosing phases (Table 5). The thin coronae of garnet are also grossular rich (Adr Grs Prp Alm Sps F-Grs Hydrogrossular ). The compositional variation of garnet in its different modes of occurrence with respect to the enclosed phases is given in Table

12 JOURNAL OF PETROLOGY VOLUME 46 NUMBER 5 MAY 2005 Table 5: continued Association: II IIA Sample no.: R 4a R 15 Mode: Cr C R R R Cr Cr Cr Cr Cr Cr SiO TiO n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. Al 2 O FeO(T) MgO MnO CaO Cl n.d b.d.l b.d.l. b.d.l F n.d Total Si & T site Ti n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. Al Fe 3þ Fe 2þ Sum VI Ca Mn Mg Fe 2þ Sum VIII Cl n.d b.d.l b.d.l. b.d.l F n.d OH n.d O Adr Grs Alm Prp Sps F-Grs OH-Grs Ti-Grt 0.45 Association IIA Scapolite, clinopyroxene and quartz occur as porphyroblastic phases. Garnet preferentially develops along the margins of clinopyroxene grains as thin to thick coronae and separates the latter from scapolite and calcite (Fig. 6). Scapolite has distinctly lower EqAn ( ) than in earlier associations (Table 3). The Cl content in scapolite ( 118 wt % maximum) is appreciably high in this association. Clinopyroxene is compositionally similar to that in Association II (Table 4). The esseneite content decreases from core (12 mol %) to rim (48 mol %) (Fig. 5). K-feldspar is almost pure (Or 96 Ab 4, Table 6). Unlike in the previous association, thick garnet coronae do not show any significant compositional variation 1056

13 DASGUPTA AND PAL ORIGIN OF GRANDITE GARNET Association: III Sample no.: B 66 B 66a R 2 Mode: C R Cr C R Cr C R Cr C Cr Cr SiO TiO n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. Al 2 O FeO(T) MgO MnO CaO Cl n.d. n.d. n.d. n.d. n.d. n.d. b.d.l. b.d.l. b.d.l. b.d.l b.d.l. F n.d. n.d. n.d. n.d. n.d. n.d Total Si & T site Ti n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. Al Fe 3þ Fe 2þ Sum VI Ca Mn Mg Fe 2þ Sum VIII Cl n.d. n.d. n.d. n.d. n.d. n.d. b.d.l. b.d.l. b.d.l. b.d.l b.d.l. F n.d. n.d. n.d. n.d. n.d. n.d OH n.d n.d n.d. n.d O Adr Grs Alm Prp Sps F-Grs OH-Grs Ti-Grt from core (Adr 521 Grs 41 Alm 264 Prp 097 Sps 073 F-Grs 003 Hydrogrossular 245 ) to rim (Adr 5075 Grs 444 Alm 223 Prp 113 Sps 1 F-Grs 002 Hydrogrossular 051 ) (Table 5). The composition of the thin garnet coronae, either along the contacts of scapolite and clinopyroxene or along the contacts of scapolite, clinopyroxene and calcite, are nearly the same (Adr Grs Prp Alm Sps F-Grs Hydrogrossular in the former and Adr Grs Prp Alm Sps F-Grs 002 Hydrogrossular 334 in the latter). The composition of scapolite shows minor zoning from the cores (EqAn ¼ 7355) to rims (EqAn ¼ 6866) (Table 3). Association III Wollastonite, scapolite and clinopyroxene typically form porphyroblasts forming a mosaic texture along with plagioclase, quartz and accessory titanite. Garnet occurs as a 1057

14 JOURNAL OF PETROLOGY VOLUME 46 NUMBER 5 MAY 2005 Table 5: continued Association: IIIA Sample no.: R 3 Mode: C R Cr Cr Cr SiO TiO 2 n.d. n.d. n.d. n.d. n.d. Al 2 O FeO(T) MgO MnO CaO Cl 0.01 b.d.l. b.d.l. b.d.l F Total Si & T site Ti n.d. n.d. n.d. n.d. n.d. Al Fe 3þ Fe 2þ Sum VI Ca Mn Mg Fe 2þ Sum VIII Cl b.d.l. b.d.l. b.d.l F OH O Adr Grs Alm Prp Sps F-Grs OH-Grs Ti-Grt C, core of thick corona; R, rim of thick corona; Cr, thin corona; n.d., not detected; b.d.l., below detection limit; O, vacancy in T site; F-Grs, fluorine grossular; OH-Grs, hydrogrossular; other mineral abbreviations after Kretz (1983); structural formula calculated after Valley et al. (1983). coronitic phase with widely variable thickness. A continuous transition from thin to thick and finally to porphyroblast-looking grains with inclusions of clinopyroxene, wollastonite and plagioclase can be seen within a single thin section (Fig. 7). In general, thick garnet coronae are present in domains rich in clinopyroxene. Garnet coronae develop along the contacts of two or more of the phases scapolite, wollastonite, plagioclase and clinopyroxene (Figs 8 and 9). Garnet intergrown with quartz separates wollastonite, plagioclase and clinopyroxene (Fig. 8). 1058

15 DASGUPTA AND PAL ORIGIN OF GRANDITE GARNET Table 6: Representative chemical compositions of plagioclase, K-feldspar, calcite and wollastonite Association: IA III Sample no.: 14c/1b B 66 B 66a R 2 Phase: Pl Pl Pl Pl Mode: R R R C R C R C R R SiO Al 2 O FeO MnO b.d.l. b.d.l b.d.l b.d.l. MgO b.d.l. b.d.l b.d.l. b.d.l. b.d.l. b.d.l. b.d.l b.d.l. CaO Na 2 O K 2 O Total Oxygen basis ¼ 8 Si Al Fe Mn b.d.l. b.d.l. b.d.l b.d.l. b.d.l. b.d.l. b.d.l b.d.l. Mg b.d.l. b.d.l b.d.l. b.d.l. b.d.l. b.d.l. b.d.l b.d.l. Ca Na K Total X An X Ab X Or Association: IIIA IIA IA III Sample no.: R 3 R 15 14c/1a B 66 R 2 Phase: Pl K-fs Cal Wo Wo Mode: R C I R C R R C R SiO n.d Al 2 O n.d b.d.l FeO b.d.l MnO 0.01 b.d.l. b.d.l b.d.l MgO b.d.l. b.d.l. b.d.l. b.d.l CaO Na 2 O n.d b.d.l. b.d.l. b.d.l. K 2 O n.d Total

16 JOURNAL OF PETROLOGY VOLUME 46 NUMBER 5 MAY 2005 Table 6: continued Association: IIIA IIA IA III Sample no.: R 3 R 15 14c/1a B 66 R 2 Phase: Pl K-fs Cal Wo Wo Mode: R C I R C R R C R O ¼ 8 O ¼ 6 Si n.d Al n.d. b.d.l b.d.l. b.d.l. Fe b.d.l Mn b.d.l. b.d.l. b.d.l b.d.l Mg b.d.l. b.d.l. b.d.l. b.d.l Ca Na n.d b.d.l. b.d.l. b.d.l. K n.d b.d.l b.d.l. Total CO 3 n.d. n.d. n.d. n.d n.d. n.d. n.d. n.d. X An X Ab X Or X Cal 0.99 C, core of porphyroblast; R, rim of porphyroblast; I, inclusion in thick garnet corona; n.d., not detected; b.d.l., below detection limit. Qtz Wo Scp Qtz Cpx Scp Grt Cal Grt Wo Cpx Fig. 2. Compound corona of garnet (Grt) and quartz (Qtz) grown at interfaces of coarse clinopyroxene (Cpx), scapolite (Scp) and wollastonite (Wo) in Association II. Bar represents 400 mm. Crossed Nicols. Fig. 3. Porphyroblasts of wollastonite (Wo), clinopyroxene (Cpx) and scapolite (Scp) are separated by thin corona of garnet (Grt) intergrown with calcite (Cal) and quartz (Qtz) in Association II. Bar represents 400 mm. Crossed Nicols. Scapolite shows two distinct compositional clusters (Table 3). Scapolite coexisting with plagioclase has EqAn ¼ , whereas that in plagioclase-absent domains has EqAn ¼ Cl and F are generally low in concentration. However, the concentration of these elements is lower in more meionitic scapolite (F ¼ p.f.u., Cl ¼ p.f.u.) than in the others (F ¼ p.f.u., Cl ¼ p.f.u., Table 3). Plagioclase is also more calcic in scapolitebearing domains (X Ca ¼ 097) than in scapolite-absent 1060

17 DASGUPTA AND PAL ORIGIN OF GRANDITE GARNET Cpx Wo Grt Cal Scp Fig. 4. Thin to thick corona of garnet (Grt) developed along the interfaces of scapolite (Scp), clinopyroxene (Cpx), wollastonite (Wo) and calcite (Cal) in Association II. Bar represents 400 mm. Crossed Nicols. Fe in octahedral site Association IA Association II Association IIA Association III Association IIIA Al IV Fig. 5. Plot of Al IV vs Fe 3þ in octahedral sites of clinopyroxene (recalculated from electron microprobe analysis data on the basis of six oxygens) in different mineral associations. The points joined by lines (arrowheads indicating rimward direction) represent typical core to rim compositional variation. domains (X An ¼ ) (Table 6). Clinopyroxene is distinctly less magnesian than in Association IA (X Mg ¼ 06 08) with a maximum of 94 mol % of esseneite and 36 mol % of CaTs (Table 4, Fig. 5). Thick garnet coronae have andradite-rich cores (Adr Grs Alm Prp Sps F-Grs 035 Hydrogrossular 018 and andradite-poor rims (Adr Grs Alm Prp Sps Ti-Grt 275 ) (Table 5). The compositions of the thin garnet coronae are similar to the rim compositions of the thick garnet coronae. However, there are appreciable variations in the composition of garnet in contact with different phases in this association, which are given in Table 7. Association IIIA Mostly clinopyroxene and a few grains of scapolite, plagioclase and quartz occur as porphyroblasts forming a granoblastic mosaic fabric in this association. Garnet coronae of variable thickness separate clinopyroxene from scapolite and plagioclase porphyroblasts. Garnet is andradite rich ( mol %) with significant almandine ( mol %), pyrope ( mol %) and spessartine ( mol %), but with low fluorogrossular ( mol %) and hydrogrossular ( mol %) (Table 5). It does not show any significant change in composition from core to rim. The compositions of coronal garnet in contact with different phases are given in Table 7. Clinopyroxene has lower X Mg ( ) and higher Al 2 O 3 ( wt %) than in other associations (Table 4). Aluminium in the clinopyroxene is mostly tetrahedrally coordinated ( atoms p.f.u.). Thus despite having the highest alumina contents, these clinopyroxene grains have low octahedral Al (Al VI ) and correspondingly the CaTs (CaAl VI SiAl IV O 6 ) component varies between only 14 and 55 mol % (Table 4, Fig. 5). Correspondingly, the esseneite content is high ( mol %) in comparison with the pyroxene in other associations. The scapolite composition is homogeneous (EqAn ¼ ) (Table 3), with a Cl content per formula unit of around 011. The composition of the plagioclase is also homogeneous (X Ca ¼ , Table 6). EVOLUTION OF THE MINERAL ASSEMBLAGES The major phases found in the different associations of these calc-silicate granulites can be represented in the system CaO Na 2 O FeO MgO Al 2 O 3 Fe 2 O 3 SiO 2 (CO 2 H 2 O) (CNFMASV). In this complex system, mineral reactions deduced from textural criteria are multivariate. We begin our analysis with the simplified system CASV, ignoring clinopyroxene, titanite and Na content in scapolite and plagioclase. All mineral abbreviations are after Kretz (1983). Mineral reactions in the CASV system Association IA The inclusion of calcite and quartz in wollastonite indicates that wollastonite formed via the decarbonation type of CASV reaction (Grs, Me, An) Cal þ Qtz ¼ Wo þ CO 2 : ð1þ During a later stage of mineral reconstitution, formation of thin coronae of garnet along interfaces of 1061

18 JOURNAL OF PETROLOGY VOLUME 46 NUMBER 5 MAY 2005 Table 7: Variations in the composition of garnet in different textural modes Composition of the reactants Composition of garnet Association 1A; reaction Scp þ Wo þ Cal ¼ Grt ss þ CO 2 Scp: EqAn ¼ Adr Grs Alm Prp Sps 03 Association II; reaction Scp þ Wo þ Cpx þ O 2 ¼ Grt ss þ Qtz þ CO 2 Scp: EqAn ¼ 7569 Cpx: X Hd ¼ 023, Ess ¼ 55 mol % Scp: EqAn ¼ 7573 Cpx: X Hd ¼ 024, Ess ¼ 39 mol % Scp: EqAn ¼ 7298 Cpx: X Hd ¼ 024, Ess ¼ 39 mol % C: Adr 4925 Grs 4598 Alm 063 Prp 09 Sps 073 F-Grs 003 OH-Grs 248 R: Adr 2505 Grs 6885 Alm 187 Prp 063 Sps 083 F-Grs 041 OH-Grs 236 Cr: Adr 246 Grs 6876 Alm 21 Prp 057 Sps 097 F-Grs 049 OH-Grs 251 Association II; reaction Scp þ Wo þ Cal þ Cpx þ O 2 ¼ Grt ss þ CO 2 Scp: EqAn ¼ 7579 Cpx: X Hd ¼ 022, Ess ¼ 43 mol % Scp: EqAn ¼ 7247 Cpx: X Hd ¼ 023, Ess ¼ 43 mol % C: Adr 491 Grs 4669 Alm 037 Prp 08 Sps 07 F-Grs 007 OH-Grs 227 R: Adr 3375 Grs 6056 Alm 14 Prp 067 Sps 084 F-Grs 003 OH-Grs 275 Cr: Adr 2315 Grs 6829 Alm 317 Prp 063 Sps 073 F-Grs 037 OH-Grs 366 Association IIA; Scp þ Cal þ Qtz þ Cpx þ O 2 ¼ Grt ss þ CO 2 Scp: EqAn ¼ 7265 Cpx: X Hd ¼ 026, Ess ¼ 83 mol % Scp: EqAn ¼ 6824 Cpx: X Hd ¼ 021, Ess ¼ 54 mol % Association III; Wo þ Pl þ Cpx þ O 2 ¼ Grt ss þ Qtz Pl: X An ¼ 097 Cpx: X Hd ¼ 027, Ess ¼ 58 mol %, CaTs ¼ 09 mol % Pl: X An ¼ 087 C: Adr 521 Grs 4108 Alm 264 Prp 097 Sps 073 F-Grs 003 OH-Grs 245 R: Adr 5075 Grs 4436 Alm 223 Prp 113 Sps 1 F-Grs 002 OH-Grs 051 Cr: Adr 2845 Grs 6545 Alm 15 Prp 053 Sps 07 F-Grs 002 OH-Grs 335 C: Adr 373 Grs 5696 Alm 16 Prp 077 Sps 077 F-Grs 044 OH-Grs 216 R: Adr 269 Grs 6847 Alm 26 Prp 063 Sps 06 F-Grs 014 OH-Grs 066 Cr: Adr 1755 Grs 7831 Alm 197 Prp 08 Sps 077 Ti-Grt 06 Association III: reaction Scp þ Wo þ Cpx þ O 2 ¼ Grt ss þ Qtz þ CO 2 Scp: EqAn ¼ 8615 Cpx: X Hd ¼ 027, Ess ¼ 58 mol %, CaTs ¼ 09 mol % Scp: EqAn ¼ 8473 Cpx: X Hd ¼ 027, Ess ¼ 58 mol %, CaTs ¼ 09 mol % Scp: EqAn ¼ 8284 Cpx: C X Hd ¼ 034, Ess ¼ 94 mol %, CaTs ¼ 25 mol % R X Hd ¼ 030, Ess ¼ 38 mol %, CaTs ¼ 06 mol % C: Adr 373 Grs 5696 Alm 16 Prp 077 Sps 077 F-Grs 044 OH-Grs 216 R: Adr 2455 Grs 6961 Alm 297 Prp 047 Sps 087 F-Grs 028 OH-Grs 125 Cr: Adr 2265 Grs 7214 Alm 317 Prp 047 Sps 077 F-Grs 014 OH-Grs 066 Association III; reaction Scp þ Cpx þ Qtz þ O 2 ¼ Grt ss þ Pl þ CO 2 Scp: EqAn ¼ Pl: X An ¼ 088 Cpx: C X Hd ¼ 022, Ess ¼ 59 mol %, CaTs ¼ 24 mol % R X Hd ¼ 019, Ess ¼ 5 mol %, CaTs 010 mol % Scp: EqAn ¼ Pl: X An ¼ 082 (C) 088 (R) Cpx: C X Hd ¼ 023, Ess ¼ 85 mol %, CaTs ¼ 36 mol % R X Hd ¼ 021, Ess ¼ 43 mol %, CaTs ¼ 0 C: Adr 3815 Grs 6018 Prp 1 Sps 067 R: Adr 164 Grs 7983 Alm 217 Prp 083 Sps 077 Cr: Adr 171 Grs 7917 Alm 193 Prp 083 Sps 097 Association IIIA; Scp þ Cpx þ Qtz þ O 2 ¼ Grt ss þ Pl þ CO 2 Pl: C X An ¼ 093, R X An ¼ 092 Scp: EqAn ¼ Cpx: X Hd ¼ 029, Ess ¼ 207 mol %, CaTs ¼ 42 mol % C: Adr 5755 Grs 3191 Alm 8 Prp 103 Sps 147 F-Grs 003 OH-Grs 001 R: Adr 5315 Grs 3519 Alm 867 Prp 113 Sps 123 F-Grs 012 OH-Grs 051 Abbreviations of mineral names are after Kretz (1983); F-Grs, fluorogrossular; OH-Grs, hydrogrossular. 1062

19 DASGUPTA AND PAL ORIGIN OF GRANDITE GARNET Scp Cpx Grt Cal Wo Grt Cpx Scp Qtz Pl Qtz Cpx Cpx Wo Fig. 6. Scapolite (Scp), calcite (Cal), quartz (Qtz) and clinopyroxene (Cpx) are separated by thin to thick garnet (Grt) corona in Association IIA. Bar represents 400 mm. Crossed Nicols. Fig. 8. Double corona of garnet (Grt) and quartz (Qtz) separate clinopyroxene (Cpx), wollastonite (Wo) and plagioclase (Pl) in Association III. Bar represents 400 mm. Crossed Nicols. Wo Scp Pl Pl Wo Grt Grt Grt Cpx Cpx Fig. 7. Variation in thickness of garnet (Grt) corona grown along the interfaces of clinopyroxene (Cpx), wollastonite (Wo) and plagioclase (Pl) in Association III. The increase in thickness of the corona adjacent to clinopyroxene grains should be noted. Bar represents 400 mm. Planepolarized light. scapolite, wollastonite and calcite can be explained by the CASV univariant equilibria (An, Qtz) Me þ 3Wo þ 2Cal ¼ 3Grs þ 3CO 2 : ð2þ In this association, garnet coronae are compositionally close to end-member grossular ( 94 mol %). This implies insignificant contribution of clinopyroxene towards the formation of garnet. Scapolite later breaks down by the reaction Me ¼ 3An þ Cal: ð3þ Association II The earliest stabilized minerals in this association are scapolite, wollastonite, calcite, clinopyroxene and quartz. Fig. 9. Thin corona of garnet (Grt) grows along the contacts of porphyroblastic clinopyroxene (Cpx), scapolite (Scp), wollastonite (Wo) and plagioclase (Pl) in Association III. Bar represents 400 mm. Crossed Nicols. In the next phase of mineral reconstitution, garnet coronae of various thickness and composition appeared in different assemblages through several mineral fluid equilibria (Figs 2 4): Me þ 3Wo þ 2Cal ¼ 3Grs þ 3CO 2 Me þ 5Wo ¼ 3Grs þ 2Qtz þ CO 2 Me þ 6Wo ¼ 3Grs þ Cal þ 3Qtz: ð2þ ð4þ ð5þ Development of calcite and quartz intergrowths along the margin and cleavage traces of porphyroblastic wollastonite suggests the CASV degenerate reaction (Grs, Me, An) Wo þ CO 2 ¼ Cal þ Qtz: ð6þ 1063