High-density carbonic fluid inclusions in charnockites from Eastern Ghats, India: petrologic implications

Abstract

Charnockite rocks, a significant litho unit in Eastern Ghats Granulite Belt (EGGB), are characterized by the presence of dry mineral assemblage. The anhydrous conditions required for the formation of ganulites are believed to be controlled by the lowering of water activity through the influx of CO 2-bearing fluids. Here we present the results of systematic petrographic and microthermometric studies characterizing the nature, composition, and density of CO 2-rich fluid inclusions in charnockitic rocks from Gangarajumadugula. In this study, we have combined mineralogic thermobarometry with microthermometric data of the carbonic fluid inclusion population of high density in the charnockite samples from Gangarajumadugula area to substantiate desiccation of lower crustal environment producing anhydrous conditions in the Eastern Ghats Granulite Belt. Close-to-peak pressure-temperature calculations for core composition using convergence technique in present study indicate that these charnockites have experienced maximum P– T around 860 °C at 9 kbar, which is broadly consistent with independent P– T estimates inferred for Gangarajumadugula area. The reaction textures preserved by these charnockites and the thermobarometric data characterize decompression followed by cooling history. High-density carbonic fluid inclusions (1.08 g/cm 3) in quartz provide potential evidence for the involvement of CO 2-rich fluid during granulite facies metamorphism in EGGB. Occurrence of CO 2-rich inclusions and elevated P– T conditions open up the possibility favoring the mechanism of vapour-deficient dehydration melting accompanied by CO 2 infiltration to account for charnockite formation from precursor crustal rocks. Our study documents that fluid inclusions in high-grade metamorphic rocks have enormous potential in retaining worthwhile information of peak metamorphic conditions and understanding the metamorphic processes.