Retrogression of Charnockite to Biotite Gneiss/Carbonated Gneiss in Attur Valley, Tamil Nadu

Field and mineralogical studies, supplemented by chemical analyses of charnockite and its derivative gneisses from one of the quarries between Ramalingapuram and Seshanchavadi along Attur valley have revealed retrogression of charnockite to biotite gneiss and onto ankerite-dolomite-bearing gneiss, involving addition of potassium and loss of iron, magnesium and sodium. The presence of two kinds of plagioclase and biotite in the retrogressed gneiss is indicative of chemical inequilibrium with a tendency to recrystallize into a lower grade mineral assemblage. The retrogression could have taken place at about 350°C and a pressure of 4 kb, with the influx of fluids richer in H₂O and CO₂. The co-existence of biotite and muscovite also requires fluids richer in H₂O, and CO₂ to a limited extent. The CO₂- and H₂O rich fluids are probably linked to major carbonatite-alkaline syenite intrusions emplaced along NNE and ENE lineaments.