Fluid inclusions in charnockites from Kodaikanal massif (South India): P-T record and implications for crustal uplift history

Abstract

The Kodaikanal massif is part of a Precambrian terrane characterized by granulite facies rocks. It is dominated by the widespread occurrence of charnockites. The observed textural relationships in these rocks are consistent with the following main reactions: i. Biotite+Quartz=Orthopyroxene+Alkali feldspar±Garnet+Vapour ii. Garnet+Quartz=Orthopyroxene+Plagioclase iii. Pyrope=Mg Tschermaks+Enstatite (in Opx) Garnet consuming reactions and the preservation of biotite-quartz-orthopyroxene-plagioclase symplectites are indicative of a decompression event. Progress of such reactions with decreasing pressure together with fluid inclusion data has implications for the construction of P-T vectors. Quartz from the charnockites contains the following fluid inclusions: (1) monophase high density CO₂-rich (0.968/–1.014g/cm³) as the dominant fluid phase; (2) aqueous biphase CO₂-H₂O (0.888–0.915 g/cm³) and (3) late minor aqueous H₂O inclusions with no visible CO₂. CO₂-isochores for the high density fluid inclusions yield a pressure limit of ca. 6.5 kbars, at granulite facies temperatures of ca. 800°C, which is in accordance with the estimation from mineralogical thermobarometry. The P-T path delineated from combined mineralogical and density data on carbonic inclusions is characteristically T-convex suggesting an isothermal decompression path and rapid uplift followed by cooling of a tectonically thickened crust.