Dehydration and Incipient Charnockite Formation: A Phase Equilibria and Fluid Inclusion Study from South India

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Abstract
A 3 m traverse across a charnockite lens in a biotite-garnet-graphite gneiss quarry from southern India has provided mineral equilibria that indicate charnockite formation occurred under fluid conditions of and T = 700 to 750°C. Dehydration of biotite, loss of garnet, and growth of orthopyroxene are related to a decrease in in the fluid phase within the charnockite lens. Isotopic analysis of the fluid entrapped in quartz from the charnockite indicates entrapped in quartz from the gneiss. Although the increase in fluid inclusion abundance in the charnockite can be related to oxidation of graphite without influx of is required in order to account for the shift to heavier carbon isotope values observed in both graphite and entrapped within the charnockite. Carbon isotope data across the gneiss/charnockite boundary suggest that the carbon isotope front has been decoupled from the dehydration reaction front by advection from the center of the charnockite lens.