A Numerical Simulation of the Gneiss-Charnockite Transformations in Southern India

This paper presents a numerical simulation of the well-documented, fluid-controlled Kabbal and Ponmudi type gneiss-charnockite transformations in southern India using a free energy minimization method. The computations have considered all the major solid phases and important fluid species in the rock - C-O-H and rock - C-O-H-N systems. Appropriate activity-composition relations for the solid solutions and equations of state for the fluids have been included in order to evaluate the mineral-fluid equilibria attending the incipient charnockite development in the gneisses. The C-O-H fluid speciation pattern in both the Kabbal and Ponmudi type systems indicates that CO₂ and H₂O make up the bulk of the fluid phase with CO, CH₄ H₂ and O₂ as minor constituents. In the graphite-buffered Ponmudi-system, the abundance of CO, CH₄ and H₂ is orders of magnitude higher than that in the graphite-free Kabbal system. Simulation with C-O-H -N fluids of varying composition demonstrates the complementary role of CO₂ and N₂ as rather inert dilutants of H₂O in the fluid phase. The simulation, carried out on available whole-rock data, has demonstrated the dependence of the transfonnation X(H₂O) on P, T, and phase and chemical composition of the precursor gneiss.