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
The 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
* Corresponding author. E-mail: sdg{at}cal3.vsnl.net.in
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