Origin of grandite garnet in calc-silicate granulites: mineral-fluid equilibria and petrogenetic grids

Abstract

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.