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_H₂O = 0.2 to 0.3, at P = 5.5 ± 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_H₂O in the fluid phase within the charnockite lens. Isotopic analysis of the fluid entrapped in quartz from the charnockite indicates δ¹³C = - 8.2%₀, 3%₀ heavier than δ¹³C values from smaller volumes of CO₂ 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 CO₂, an external source of CO₂ is required in order to account for the shift to heavier carbon isotope values observed in both graphite and entrapped CO₂ 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 CO₂ advection from the center of the charnockite lens.