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

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$X_{H_{2}O} = 0.2 to 0.3, at P = 5.5 \pm 1.9 kb$and T = 700 to 750°C. Dehydration of biotite, loss of garnet, and growth of orthopyroxene are related to a decrease in$X_{H_{2}O}$in the fluid phase within the charnockite lens. Isotopic analysis of the fluid entrapped in quartz from the charnockite indicates$\delta^{13}C = - 8.2\%_{0}, 3\%_{0} heavier than \delta^{13}C values from smaller volumes of CO_{2}$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$CO_{2}, an external source of CO_{2}$is required in order to account for the shift to heavier carbon isotope values observed in both graphite and entrapped$CO_{2}$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$CO_{2}$advection from the center of the charnockite lens.