Journal of Petrology Advance Access originally published online on January 28, 2005
Journal of Petrology 2005 46(5):1045-1076; doi:10.1093/petrology/egi010
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Origin of Grandite Garnet in Calc-Silicate Granulites: Mineral–Fluid Equilibria and Petrogenetic Grids
1 DEPARTMENT OF GEOLOGICAL SCIENCES, JADAVPUR UNIVERSITY, KOLKATA-700 032, INDIA
2 DEPARTMENT OF GEOLOGY, DURGAPUR GOVERNMENT COLLEGE, DURGAPUR-713 214, WEST BENGAL, INDIA
RECEIVED FEBRUARY 28, 2003; ACCEPTED DECEMBER 15, 2004
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ABSTRACT |
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TOP
ABSTRACT
INTRODUCTIONThe role of clinopyroxene in producing grandite garnet is evaluated using data from an ultrahigh-temperature metamorphosed calc-silicate 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. 0·9 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–Al2O3–FeO–SiO2–CO2–O2 in T–aCO2–fO2 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 fO2 are the most crucial variables controlling garnet composition in calc-silicate granulites. fO2, however, behaves as a dependent variable of CO2 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
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INTRODUCTION |
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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/Fe3+ of coexisting garnet and clinopyroxene (Sengupta et al., 1997).
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 Fe2+ 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 infiltrating, oxidizing fluids that also metasomatically introduced Fe3+ 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.
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GEOLOGICAL BACKGROUND |
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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 0·9 GPa pressure). This was 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 0·4–0·5 GPa. A later amphibolite-facies metamorphism, accompanying limited hydration, is also recorded in all the rocks.
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PETROLOGY OF THE CALC-SILICATE GRANULITES |
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The calc-silicate granulites studied for this paper are gneissic with the development of alternate light bands (quartz + calcite + scapolite + plagioclase + minor K-feldspar + minor clinopyroxene) and dark bands (clinopyroxene + wollastonite + scapolite + plagioclase + garnet + 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.
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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 µm 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. Fe3+ 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) + F = (3 – Si) x 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.
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Association IA
This is developed in Association I, which has been invaded by quartzofeldspathic veins. Wollastonite occurs 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 [= (Al – 3)/3 x 100, calculated on the basis Si + Al = 12; Evans et al., 1969] content in scapolite is 75·54–78·03 (Table 3). Clinopyroxene contains negligible CaTs (0–1·8 mol %) and the maximum esseneite content is 3·1 mol %. XMg shows little variation in this association, ranging between 0·82 and 0·84 (Table 4). Thin coronae of garnet grown around scapolite, wollastonite and calcite are nearly pure grossularite [Grs94·03–94·17Adr0·8–2·7Alm2·1–3·37Prp0·73–1·5Sps0·3] (Table 5). Wollastonite, calcite and plagioclase are nearly pure phases (XCa is 0·99 in calcite, and 0·96–0·97 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.
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The EqAn content of scapolite ranges from 70·32 to 78·72 (Table 3). Clinopyroxene is less diopsidic (XMg = 0·71–0·80) than in Association IA. In this association, clinopyroxene contains appreciable esseneite component (3·9–11·7 mol %, Table 4). Al2O3 content ranges between 0·53 and 2·09 wt %, but Al is mostly tetrahedrally coordinated. Thus the CaTs content in these clinopyroxene grains is always low (maximum 0·6 mol %, Table 4). A bivariate plot of Fe3+ 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 trends in the porphyroblastic grains. It is evident that the esseneite content decreases considerably from core to rim.
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Cores of the thick garnet coronae are andradite rich (Adr49·1–50·5Grs43–46·69Prp0·8–0·9Sps0·7Alm0·37–0·63F-Grs0·02–0·07Hydrogrossular2·27–4·34), decreasing significantly to Adr25·05–33·75Grs60·56–68·85Prp0·63–0·67Sps0·83Alm1·4–1·9F-Grs0·03–0·41Hydrogrossular2·36–2·75 at the contact of enclosing phases (Table 5). The thin coronae of garnet are also grossular rich (Adr15·8–29·5Grs62·95–78·25Prp0·47–0·7Alm1·83–4·03Sps0·73–1·1F-Grs0·21–0·49Hydrogrossular2·51–4·36). The compositional variation of garnet in its different modes of occurrence with respect to the enclosed phases is given in Table 7.
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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).
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Scapolite has distinctly lower EqAn (66·69–73·55) than in earlier associations (Table 3). The Cl content in scapolite (
1·18 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 (4·8 mol %) (Fig. 5). K-feldspar is almost pure (Or96Ab4, Table 6). Unlike in the previous association, thick garnet coronae do not show any significant compositional variation from core (Adr52·1Grs41Alm2·64Prp0·97Sps0·73F-Grs0·03Hydrogrossular2·45) to rim (Adr50·75Grs44·4Alm2·23Prp1·13Sps1F-Grs0·02Hydrogrossular0·51) (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 (Adr23·95–36·7Grs56·97–69·8Prp0·6–0·7Alm2·27–2·87Sps0·9–1·1F-Grs0·02–0·53Hydrogrossular1·07–2·48 in the former and Adr28·45–34·1Grs57·6–65·4 Prp0·5–0·8Alm1·5–2·9Sps0·7–1·2F-Grs0·02Hydrogrossular3·34 in the latter). The composition of scapolite shows minor zoning from the cores (EqAn = 73·55) to rims (EqAn = 68·66) (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 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).
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Scapolite shows two distinct compositional clusters (Table 3). Scapolite coexisting with plagioclase has EqAn = 84·73–86·56, whereas that in plagioclase-absent domains has EqAn = 70·45–82·84. Cl and F are generally low in concentration. However, the concentration of these elements is lower in more meionitic scapolite (F = 0·005–0·044 p.f.u., Cl = 0·016–0·029 p.f.u.) than in the others (F = 0·005–0·136 p.f.u., Cl = 0·029–0·047 p.f.u., Table 3). Plagioclase is also more calcic in scapolite-bearing domains (XCa = 0·97) than in scapolite-absent domains (XAn = 0·87–0·90) (Table 6). Clinopyroxene is distinctly less magnesian than in Association IA (XMg = 0·6–0·8) with a maximum of 9·4 mol % of esseneite and 3·6 mol % of CaTs (Table 4, Fig. 5). Thick garnet coronae have andradite-rich cores (Adr38·15–39·35Grs56·71–60·18Alm0·33–0·97Prp0·73–1Sps0·5–0·73F-Grs0·35Hydrogrossular0·18 and andradite-poor rims (Adr16·4–20·9Grs72·52–79·83Alm2·17–2·43Prp0·7–0·83Sps0·7–0·93Ti-Grt2·75) (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 (51·55–57·95 mol %) with significant almandine (7·27–11·2 mol %), pyrope (1·03–1·27 mol %) and spessartine (1·17–1·47 mol %), but with low fluorogrossular (0·03–0·33 mol %) and hydrogrossular (0·01–1·48 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 XMg (0·58–0·64) and higher Al2O3 (5·34–5·83 wt %) than in other associations (Table 4). Aluminium in the clinopyroxene is mostly tetrahedrally coordinated (0·205–0·236 atoms p.f.u.). Thus despite having the highest alumina contents, these clinopyroxene grains have low octahedral Al (AlVI) and correspondingly the CaTs (CaAlVISiAlIVO6) component varies between only 1·4 and 5·5 mol % (Table 4, Fig. 5). Correspondingly, the esseneite content is high (19·5–26·5 mol %) in comparison with the pyroxene in other associations. The scapolite composition is homogeneous (EqAn = 73·16–74·48) (Table 3), with a Cl content per formula unit of around 0·11. The composition of the plagioclase is also homogeneous (XCa = 0·92–0·93, Table 6).
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EVOLUTION OF THE MINERAL ASSEMBLAGES |
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The major phases found in the different associations of these calc-silicate granulites can be represented in the system CaO–Na2O–FeO–MgO–Al2O3–Fe2O3–SiO2–(CO2–H2O) (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)
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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)
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During a later stage of mineral reconstitution, formation of thin coronae of garnet along interfaces of scapolite, wollastonite and calcite can be explained by the CASV univariant equilibria (An, Qtz)
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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
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Association II
The earliest stabilized minerals in this association are scapolite, wollastonite, calcite, clinopyroxene and quartz. 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):
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Development of calcite and quartz intergrowths along the margin and cleavage traces of porphyroblastic wollastonite suggests the CASV degenerate reaction (Grs, Me, An)
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Association IIA
Formation of thin garnet coronae around scapolite, calcite and quartz indicates that garnet could have formed through the CASV univariant reaction (An, Wo)
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Association III
The earliest phases to be stabilized in this association are scapolite, wollastonite, plagioclase, clinopyroxene, quartz and titanite. The formation of wollastonite and scapolite can be explained by reaction (1) and the backward progress of reaction (3) described above. In the next phase of mineral reconstitution, garnet coronae of various thickness and composition appeared in different domains of this association (Figs 8 and 9) through the equilibria
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Association IIIA
Textural interpretations indicate that appearance of garnet in this association was controlled by clinopyroxene, and no CASV univariant reaction can be held responsible for its formation. These reactions are discussed below.
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P–T–aCO2 phase relationship in the system CASV |
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Calc-silicate granulites record a number of mineral fluid equilibria construed from their textural and compositional characteristics [reviewed by Harley et al. (1994)
and Sengupta et al. (1997)]. These mineral–fluid equilibria, in general, provide important constraints on petrological evolution because additional information on extensive and intensive variables (P, T) during metamorphic evolution can be acquired (Rice & Ferry, 1982; Moecher & Essene, 1991). Harley & Buick (1992) used the internally consistent dataset of Holland & Powell (1990) to derive a set of petrogenetic grids for calc-silicate granulites, in the system CaO–Al2O3–SiO2–CO2 with reduced phase activities to account for non-CASV components in real mineral compositions.
P–T relations and vapour-absent equilibria
All calculations were carried out with the internally consistent dataset of Holland & Powell (1998). The a–X relationships adopted for solid solution phases are taken from the literature as follows: scapolite from Baker & Newton (1995); garnet from Engi & Wersin (1987); plagioclase from Newton (1983). Only the vapour-absent reactions (3), (5) and (8) can be considered for P–T analysis. In Fig. 10, these reactions are plotted after adjusting for the respective phase compositions. Reaction (8) plotted for the composition of the core of thick garnet coronae [XGrs = Grs/(Grs + Adr) = 0·60], rim of thick garnet coronae (XGrs = 0·71) and thin coronae of garnet (XGrs = 0·81), and the respective compositions of associated plagioclase grains yields temperature estimates of 975°C, 930°C and 895°C, respectively, at 0·9 GPa [corresponding to the peak pressure estimate from this area (Dasgupta et al., 1995)]. Thus it can be concluded that in Association III, the cores of thick garnet coronae record an ultrahigh-temperature condition; successive stages of growth of garnet coronae are consistent with cooling. Reaction (5) was also plotted for the composition of thin coronae of garnet and scapolite. The temperature estimate from the pressure-insensitive reaction (3) is 685°C. Thus, the three reactions collectively depict a cooling trajectory to 685°C (Fig. 10).
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Isobaric T–ln aCO2 grid
For the construction of a T–ln aCO2 grid at 0·9 GPa, following the peak pressure estimate of Dasgupta et al. (1995)
, a high-pressure topology (Warren et al., 1987) has been considered where the [An], [Cal] and [Wo] invariant points are stable (Fig. 11). Most of the reactions in the studied calc-silicate granulites occur around the isobaric invariant points [An] (Association II) and [Cal] (Association III). The composition of garnet varies remarkably in its different modes of occurrence, such as the core and rim of thick and thin coronae. Therefore, separate reaction bundles emanating from the [An] and [Cal] invariant points are plotted in T–ln aCO2 space, consistent with the composition of garnet in its various modes.
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Using the most andradite-rich core composition of garnet in Association III plus associated phases {XGrs [= Grs/(Grs + Adr)] = 0·6, XMe = 0·86,
}, and Association II (XGrs = 0·49, XMe = 0·76), two reaction bundles are constructed (Fig. 11). It is evident that andradite-rich garnet developed at temperatures of 965°C over a wide range of ln aCO2 (–1·11 to –0·22) (corresponding aCO2 = 0·33–0·8). Association II equilibrated with fluid of much higher aCO2 (0·8) than Association III (aCO2 = 0·33), suggesting a strong fluid heterogeneity in closely spaced interbands of calc-silicate granulite during the initial stages of cooling.
The reaction bundles constructed for the rim composition of the same thick garnet coronae show marked dissimilarity in evolution of these two associations. In Association III, the rim of garnet coronae developed at 930°C equilibrating with a fluid having composition of ln aCO2 = –1·31(aCO2 = 0·27). On the other hand, Association II indicates a further drop in temperature to 865°C with a fluid having a composition of ln aCO2 = –0·87 (aCO2 = 0·42) (Fig. 11). Maximum grossular-rich thin garnet coronae in Association IA developed at conditions of 840°C and ln aCO2 = –1·05 (aCO2 = 0·35). Thus it may be concluded that more grossular-rich garnet developed at lower temperatures in equilibrium with more hydrous fluid. Similar reaction bundles constructed for the composition of thin garnet coronae developed in almost all garnet-bearing associations (IA, II, III) also corroborate the fact that more grossular-rich garnet formed at still lower temperature and lower aCO2.
The calculated T and aCO2 estimates, at first glance, indicate that Associations II and III equilibrated during near peak conditions at 965°C with fluid of extremely variable composition (aCO2 = 0·8 in Association II, aCO2 = 0·33 in Association III), and the initial heterogeneity was preserved during further evolution of the garnet coronae in each association (aCO2 = 0·42 in Association II, aCO2 = 0·27 in Association III). However, as the modal proportion of garnet is high in both associations, it can be concluded that the reaction progress was considerable (discussed below); the deduced T–ln aCO2 grid is thus valid for the composition of the thin coronae of garnet and rims of composition of the reactant phases; that is, when the assemblages ceased to evolve (see Fitzsimons & Harley, 1994). As the pseudounivariant equilibria plotted for core compositions of thick garnet coronae may not have been true P–T–aCO2 pseudosections, deduction of the evolution of T–aCO2 conditions from their position are artefacts and should not be taken into consideration (Fitzsimons & Harley, 1994). Moreover, mineral reactions in the system CASV cannot explain the full compositions of the garnet in the calc-silicate granulites.
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Mineral reactions in the CAFSV system |
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In the studied calc-silicate granulites, there is ample textural evidence favouring participation of clinopyroxene in grandite-producing reactions (discussed above). In this case, it is necessary to discriminate between the two processes, namely, oxidation of hedenbergite component in clinopyroxene, and participation of esseneite. As identified above, clinopyroxene contains high esseneite in Association IIIA, and this decreases towards the rims. The content of clinopyroxene is also highest in this association. Therefore, a straightforward solution is unlikely. The highest andradite content of garnet is in that which coexists with the highest esseneite-bearing clinopyroxene (Tables 5 and 7). However, garnet does not appear at all in Association V, in which clinopyroxene contains very high amounts of the esseneite component. Association IIIA is clearly stabilized in Fe-rich bulk compositions, as reflected in the high modal content of Fe-rich clinopyroxene and andradite-rich garnet. In other associations, the esseneite content is too low to account for the high andradite content in garnet. Buick et al. (1994)
obtained identical composition of clinopyroxene coexisting with both andradite-rich (as well as modally garnet-rich) and andradite-poor garnet from central Australia. This was one of the major reasons why they argued for an oxidizing, Fe-rich fluid infiltration. This is clearly not the case in the present study area. It needs to be emphasized here that all the workers who favour participation of esseneite in andradite-producing reactions are doubtful whether the Fe3+ supplied by pyroxene is sufficient to account for all the observed andradite component in their studied rocks (Buick et al., 1994; Fitzsimons & Harley, 1994). From the phase relationships in the simple system CASV, it is argued that grossular-rich garnet coronae can be produced as a result of cooling and hydrous fluid flux (Warren et al., 1987), or cooling alone (Fitzsimons & Harley, 1994). This controversy highlights the need for a proper analysis of garnet-forming equilibria as function of temperature and chemical potentials of the fugitive species. |
Grandite-forming reactions in a complex system |
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Petrographic observations demonstrate that thick to thin coronae of garnet formed by reactions involving combinations of the phases scapolite, clinopyroxene, plagioclase, wollastonite and calcite in Associations II, IIA, III and IIIA, whereas clinopyroxene was probably not involved in Association IA. Stability relations in the simple system CASV show that all the garnet-forming reactions could have ensued during cooling from higher temperatures with or without hydrous fluid flux (Fig. 11). A partial P–T grid was constructed in the simple system with three reactions, which do not involve a fluid phase (Fig. 10). This grid supports the notion of near isobaric cooling subsequent to peak metamorphism. Cooling is also indicated from petrographic studies of spinel granulites, quartzofeldspathic granulites, two-pyroxene granulites and metanorite from the study area (Dasgupta et al., 1995
; Bose et al., 2003). Against this background, we evaluate the garnet-forming reactions in the studied calc-silicate granulites. The chemical compositions of the phases in the calc-silicate granulites define the chemical system as Na2O–CaO–FeO–MgO–Al2O3–SiO2–TiO2–CO2–O2. Out of these, Ti is fixed in titanite, which does not participate in any of the reactions. Mg is fixed in clinopyroxene, and Na is a minor constituent because both scapolite and plagioclase are extremely rich in Ca. Thus, the system is effectively reduced to CAFSCO. The garnet-forming reactions in the simple system CASV, as deduced above, are multivariate in this system. We consider the garnet-forming reactions in two petrogenetic grids as functions of temperature and chemical potentials of CO2 and O2 under isobaric conditions.
We have used a 6 x 6 compositional matrix to write the balanced chemical reactions that are inferred on textural grounds, because in a six-component system, six phases will coexist at an isobarically univariant equilibrium. The compositions used are core and rim compositions of scapolite, plagioclase and clinopyroxene porphyroblasts, core and rim of thick garnet coronae, and composition of thin garnet coronae. It is admitted that an element of uncertainty is introduced in this method, by using core compositions of the reactants and the same of a thick garnet corona. It is assumed that the core compositions represent frozen-in equilibrium. It is further considered that the meionite content in scapolite, hedenbergite content in clinopyroxene, and anorthite content in plagioclase took part in the production of grandite garnet. Introduction of Mg in place of Fe2+ (in clinopyroxene), and of NaSi in place of CaAl (in plagioclase/scapolite) will not change the coefficients of the reactions. The balanced reactions are given below.
Association II
Core composition of thick garnet corona:
 | (9) |
 | (10) |
 | (11) |
 | (12) |
 | (13) |
 | (14) |
Association IIA
Core composition of thick garnet corona:
 | (15) |
 | (16) |
Association III
Core composition of thick garnet corona:
 | (17) |
 | (18) |
 | (19) |
 | (20) |
 | (21) |
 | (22) |
 | (23) |
 | (24) |
 | (25) |
Association IIIA
Core composition of thick garnet corona:
 | (26) |
|
Limitations of reduced-activity grids |
|---|
Fitzsimons & Harley (1994)
discussed in detail the limitations of activity-corrected grids in the system CASV to interpret the evolution of mineral assemblages in calc-silicate granulites. As a result of multivariance introduced by incorporation of non-CASV components, the CASV reaction boundaries become composition-dependent bands in P–T–aCO2 space, and the equilibrium P–T–aCO2 conditions of a given multivariate assemblage become dependent on the variable compositions of clinopyroxene, scapolite, plagioclase and garnet. Another important consequence is that progress of a reaction will generally cause changes in the composition of the phases. This implies that fixed composition reduced-activity grids are strictly valid only at the point in P–T–aCO2 space where an assemblage last equilibrated, and do not show the true positions of reactions away from this point. This limitation is also applicable for the reduced-activity grid constructed here (Fig. 11). Whereas the point in isobaric T–aCO2 space obtained from consideration of rims of the reactant phases and coronitic garnet at the contact has a meaning in terms of last equilibration, the same cannot be said of core compositions.
An extreme consequence of change in the composition of the product phase with reaction progress is that some of the reactants or products may change sides beyond a ‘singular composition’ of one of the products (Connolly & Trommsdorff, 1991; Sengupta & Raith, 2002). In the present case, such switchovers are noted for the reactions (An, Qtz), (Cal, Wo) and (An, Wo) mentioned above. For the (An, Qtz) reaction, balancing with the composition of thin coronae of garnet (Adr25), wollastonite occurs on the reactant side with a stoichiometric coefficient of 0·5. This value decreases to 0·28 with increasing andradite content in garnet in the rims of relatively thicker coronae (Adr36), which is presumed to have equilibrated with a different clinopyroxene composition. Wollastonite changes side and becomes a product with garnet, when the core compositions of thick garnet coronae (Adr51) and clinopyroxene are considered. It is found that for Adr50 composition, the reaction becomes (An, Qtz, Wo), which is the singular composition. Similarly, the reaction (An, Wo) becomes (An, Qtz, Wo) for Adr50 garnet. This can explain why garnet ± quartz coronae are noted against variable compositions of scapolite, plagioclase and clinopyroxene in different calc-silicate granulites (see, for example, Harley et al., 1994).
|
Balancing reactions with aluminous clinopyroxene |
|---|
Clinopyroxene is invariably highly aluminous in calc-silicate granulites, similar to that in the present case. In Association IIIA in particular, clinopyroxene is highly aluminous. We have attempted an alternative way of balancing the reactions taking the maximum Al content of clinopyroxene in each association.
 | (27) |
 | (28) |
 | (29) |
 | (30) |
|
Balancing with esseneite component |
|---|
Because of the suggested importance of an esseneite component of clinopyroxene in andradite-producing reactions by several workers (Harley & Buick, 1992
; Fitzsimons & Harley, 1994; Harley et al., 1994; Bhowmik et al., 1995), we have attempted to balance the reactions with esseneite. Esseneite may be an important factor in Association IIIA, where its content in clinopyroxene is high. Also, some amount of depletion in esseneite content from core to rim of clinopyroxene is noted for each association. The results are given below.
 | (31) |
 | (32) |
 | (33) |
 | (34) |
 | (35) |
|
TEMPERATURE–FLUID EVOLUTIONARY HISTORY OF CALC-SILICATE GRANULITES |
|---|
Textural and compositional criteria were presented above in support of oxidation–decarbonation reactions leading to the formation of andradite-rich garnets in different associations. Thermochemical parameters of all the phases were taken from Holland & Powell (1998)
. Activity–composition relationships in the different phases are taken as follows: plagioclase after Newton (1983), scapolite after Baker & Newton (1995), garnet after Engi & Wersin (1987) with modification of Fitzsimons & Harley (1994), and clinopyroxene after Wood (1979). Interpretation of P–T data and textural details in the associated rocks and from the CASV equilibria (discussed above) suggest garnet formation in response to cooling from peak metamorphic conditions. Cooling was probably isobaric, and we consider a fixed pressure of 0·9 GPa in all subsequent calculations. The mineral reactions, inferred in calc-silicate granulites, are then dependent on temperature, fO2 and aCO2/fCO2, other than the composition of the solid phases.
Quantitative log fO2–log fCO2 grid
In the six-component system CaO–FeO–Al2O3–SiO2–CO2–O2, we consider the phases meionite, anorthite, hedenbergite, wollastonite, garnetss (grossular + andradite), calcite, quartz and vapour. Magnetite could be another phase, but is not considered because of its total absence in all the assemblages. The mineral reactions listed above are all related to the invariant points [An] and [Cal]. Specifically, mineral reactions in Associations II and III are related to the invariant points [An] and [Cal], respectively. This study is an extension of the petrogenetic grid presented by Sengupta et al. (1997), who did not consider Al as a system component, but accounted for its presence by using reduced activity of andradite. Sengupta et al. (1997) showed that as a result of buffering of fO2, by complex oxidation–devolatilization reactions, fCO2 and fO2 show sympathetic variations with fO2 controlled by the ambient fCO2 in the fluid.
A partial petrogenetic grid in isobaric–isothermal log fCO2–log fO2 space involving the two invariant points [An] and [Cal] is shown in Fig. 12. Two separate grids for core and rim compositions of garnet coronae are constructed at the corresponding P–T conditions P = 0·9 GPa, T = 1000°C for the core compositions, and P = 0·9 GPa, T = 800°C for the rim compositions. The invariant point [An] is located at log fO2 = –3·4, log fCO2 = 5·4, and log fO2 = –10·4, log fCO2 = 4·95 for core and rim compositions, respectively, in Associations II and IIA. The invariant point [Cal] is located at log fO2 = –8·7, log fCO2 = 0·31 and log fO2 = –17·7, log fCO2 = 1·6 for core and rim compositions, respectively, in Association III. The fO2 values deduced here lie between the fayalite–magnetite–quartz (FMQ) and nickel–nickel oxide (NNO) buffers at the respective temperatures (except for one value that lies slightly below the FMQ buffer), and are comparable with those in similar situations elsewhere (Buick et al., 1994). Not taking into account the fall in temperature for the moment, we conclude that grossular-rich coronal garnet can be produced as a result of lowering of fO2 with only minor to insignificant changes in fCO2. This argues against hydrous fluid fluxing as a necessary mechanism for production of grossular-rich garnet. Oxygen did not, however, behave as an unbuffered component, because this would have completely destroyed the andradite content in garnet. More probably, it was a dependent variable of temperature and fCO2 (see Sengupta et al., 1997). Because the concentration of oxygen in the fluid is infinitesimally small as compared with major components, such as H2O or CO2, small variations in aCO2 could cause significant changes in fO2.
|
Quantitative T–ln aCO2 grid
We have further explored the interrelationship of temperature and aCO2 for the oxidation–decarbonation reactions involving hedenbergite in the system CaO–FeO–Al2O3–SiO2–CO2–O2 at fixed fO2 (Fig. 13). We have taken fO2 values from the log fCO2–fO2 diagram (Fig. 12). The grid shows that [An] is stabilized at 1005°C, ln aCO2 = 0·67, and [Cal] is stabilized at 1000°C, ln aCO2 = –11 because of intersection of the relevant univariant equilibria using the core compositions of thick garnet coronae and of plagioclase and scapolite, assuming that these phases were at equilibrium just after peak condition. The invariant points [An] and [Cal] shift to 790°C, ln aCO2 = –0·22, and 800°C, ln aCO2 = –8·3 when rim compositions of the phases are used (Fig. 13). The [An] invariant point shifts to lower temperature (
T = 210°C) and lower aCO2 as a result of changeover from core (XAdr = 0·51, XMe = 0·76, XHd = 0·23, at log fO2 = –3·4) to rim (XAdr = 0·26, XMe = 0·76, XHd = 0·23 with log fO2 = –10·4) compositions. The [Cal] invariant point shifts to lower temperature (T = 200°C) at nearly constant aCO2 as a result of changeover from core (XAdr = 0·4, XMe = 0·86, , XHd = 0·27) to rim (XAdr = 0·26–0·29, XMe = 0·84, , XHd = 0·27) compositions. The aCO2 for the invariant point [An] using the core compositions of the phases exceeds unity, and is clearly problematic. Moecher & Essene (1991) calculated aCO2 from a number of terranes and obtained similar high values in some cases, albeit not from calc-silicate granulites. Possible factors that could contribute towards such erroneous values are (1) lack of equilibrium among the phases, (2) retrogression, (3) errors in P–T estimation, and (4) error in calculation of activity of meionite (Moecher & Essene, 1991). In the present case, the problem may lie with the assumption that cores of thick garnet coronae equilibrated with the cores of the associated phases. Nevertheless, the results suggest relatively high aCO2 values for the invariant point [An] relative to [Cal]. In both the cases grossular-rich garnet is produced as a result of significant cooling. Stabilization of assemblages corresponding to [An] and [Cal] in closely spaced domains attests to internal buffering of fluid composition by mineral reactions. The opposite sense of movement of the two invariant points with respect to aCO2, both in equilibrium with grossular-rich garnet, is not observed in the system CASV (Fig. 11). It is important to note there is no major change in the temperature estimates for the two invariant points as compared with their locations in the simple system CASV. However, consideration of hedenbergite in the reactions in the natural system CFASV shifts the invariant point [An] to higher aCO2, and the invariant point [Cal] to much lower aCO2.
|
|
NATURE OF FLUID–ROCK INTERACTION |
|---|
The suggestion that grandite formation can be related to cooling without significant fluid flux is farther evaluated by considering the ambient fluid/rock ratio in the studied calc-silicate granulites following the general principle laid down by Ferry (1983
, 1986). We have selected the following CASV univariant equilibria to calculate a fluid/rock ratio:
 |
Molar volumes of garnet, CO2 and H2O were taken from Holland & Powell (1990). Two extremities were considered: no fluid infiltration, and restricted fluid infiltration. Both pure H2O and 
conditions were considered in the latter case. The modal volume percentage of garnet was estimated between 10 and 20% from petrographic studies. The fluid composition was calculated from the CASV T–ln aCO2 grid for the respective univariant assemblages (Fig. 11).
The calculated results are shown in Table 8. In Associations IIA and III, maximum (20%) garnet production is possible with fluid/rock ratios of 0·026 in the case of no infiltration, 0·057 in the case of hydrous fluid flux, and 0·076 for infiltration of intermediate fluid. A maximum value of 0·077 is obtained for Association II in the case of no infiltration. These results are consistent with the interpretation that the studied rocks evolved under conditions of rock-dominated metamorphism during retrogression.
|
There are several other textural and compositional indications that rule out any significant external fluid fluxing in the study area. These are: (1) high variance assemblages are not developed whereas isobaric univariant assemblages are common; (2) the presence of strong compositional gradients in microdomains; (3) systematic variations in the composition of coronal garnet with the nature of the associated phases (Table 7); (4) wide variations in calculated fluid compositions, regardless of the absolute values, in closely spaced domains. Collectively, these features indicate that fluid composition was internally buffered by mineral–fluid equilibria during the evolution of the rocks (Greenwood, 1975
; Rice & Ferry, 1982). |
APPLICATION OF THE ISOBARIC T–ln aCO2 GRID TO OTHER TERRANES |
|---|
(1) Northern Prince Charles Mountains, East Antarctica. Fitzsimons & Harley (1994)
deduced reactions related to [An] and [Cal] in the system CASV and suggested that grandite was produced largely as a result of cooling from 800–850°C to 700°C within a restricted aCO2 condition of 0·3–0·5. We have recalculated the reactions using the composition of hedenbergite, plagioclase, scapolite and grandite given in their work, and located these invariant points in the system CFASV at 825°C, ln aCO2 –9·1 [Cal] and 750°C, ln aCO2 –0·625 [An] (Fig. 14). Although the temperature estimates closely correspond to the values reported by Fitzsimons & Harley (1994), and corroborate their contention that grandite is produced because of cooling, we obtain far lower aCO2 conditions for the invariant point [Cal]. This result is, therefore, similar to what has been obtained from the present study area.
|
(2) Arunta Block, Australia. Warren et al. (1987)
deduced mineral reactions corresponding to [An] in the simple system CASV to account for grandite, and suggested cooling from 920 to 750°C at low aCO2 (0·30). Using the approach mentioned above, we obtain final equilibration of thin coronal garnet in this area at 750°C and identical aCO2 (0·32) (ln aCO2 = –1·125, Fig. 14).
(3) Anakapalle, Eastern Ghats, India. Sengupta et al. (1997) deduced the reactions (Cal, Me, V), (Cal, An), (An, Qtz) and (An, V) in the system CASV to account for grandite formation, which took place as a result of cooling from 850 to 750°C within a range of aCO2 of 0·29–0·65. In the system CFASV (Fig. 14), the core–rim compositional data of Sengupta et al. (1997) define a retrograde trajectory of cooling from 850 to 750°C (corrected for 0·6 GPa according to the original data) corroborating the predicted scenario. However, it appears that the assemblages equilibrated with very low aCO2.
(4) Rayagada, Eastern Ghats, India. Shaw & Arima (1996) deduced the reactions (An, Cal) and (An, Qtz) in the system CASV for grandite formation in two separate assemblages and predicted cooling from 920 to 815°C with aCO2 varying between 0·5 and 0·25. Consideration of hedenbergite in the system CFASV leads to the condition of final equilibration of the assemblages at 805°C, and ln aCO2= –1·94 (Fig. 14), which is lower than that calculated in the system CASV.
(5) Borra, Eastern Ghats, India. Bhowmik et al. (1995) deduced several reactions, such as (An, Wo), (An, Cal) and (An, Qtz) in the system CASV, in different mineral associations to explain the appearance of grandite garnet of variable composition. These reactions, when considered in the system CFSAV (Fig. 14), gave comparable temperatures but lower aCO2. The invariant point [An] in the system CFSAV is located at 900°C, ln aCO2 –1·1 (aCO2 = 0·33). The temperature estimate tallies well with the period of post-peak isobaric cooling postulated by Bhowmik et al. (1995).
(6) Sunkarametta, Eastern Ghats, India. Dasgupta (1993) described the mineral assemblages and reaction textures from this area. Contrary to all other occurrences in the Eastern Ghats and other areas described above, clinopyroxene shows an increase in XFe at the rims against garnet, the latter showing decrease in andradite content from core to rim. Dasgupta (1993) invoked a de-oxidation reaction involving breakdown of the andradite component in garnet to explain the compositional characteristics. This occurrence cannot be explained using the grids developed here, and is possibly a case of Fe-metasomatism (Buick et al., 1994). However, the [An] invariant point in the system CFSAV is located at 765°C, ln aCO2 = –0·8, which is in close agreement with that independently predicted by Dasgupta (1993) from associated rocks in the same area.
Therefore, for all the occurrences, the retrograde path of evolution of the assemblages and absolute temperature co-ordinates of final equilibration closely match those predicted by the respective researchers in the system CASV. However, a notable reduction in aCO2 value is noted in most of the cases when clinopyroxene is considered in the system CFSAV.
|
ACKNOWLEDGEMENTS |
|---|
We thank Pulak Sengupta for stimulating discussions and help in the field work. We thank Professor M. Raith for providing necessary analytical facilities at the University of Bonn. We acknowledge with thanks many rounds of discussions with Santanu Bhowmik. We are grateful to Professor E. J. Essene and Professor D. J. Ellis for very constructive and critical, but helpful comments on an earlier draft. Editorial comments of Professor R. J. Arculus were helpful. S.D. thanks the Alexander von Humboldt Foundation for a Research Fellowship, and S.P. thanks the Council of Scientific & Industrial Research, Government of India, for support through a Fellowship. S.D. thanks the Department of Science & Technology for financial support through a Research Project, and the Deutsche Forschungsgemeinschaft for award of a Mercator Guest Professorship.
* Corresponding author. E-mail: sdg{at}cal3.vsnl.net.in
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 J. L. Wykes, R. C. Newton, and C. E. Manning Solubility of andradite, Ca3Fe2Si3O12, in a 10 mol% NaCl solution at 800 {degrees}C and 10 kbar: Implications for the metasomatic origin of grandite garnet in calc-silicate granulites American Mineralogist, May 1, 2008; 93(5-6): 886 - 892. [Abstract] [Full Text] [PDF]
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