Charnockite formation at Ponmudi in Southern India

Charnockites, constituents of most Precambrian high-grade terrains 1,2, are essential for understanding the evolution of the early continental crust. The arrested development of charnockite in shear veins at Kabbaldurga3, in the state of Karnataka in India, suggests flow of CO2-rich water-deficient fluids through deep-seated rocks as a mechanism of granulite grade metamorphism4,5. We report here an occurrence of arrested charnockite formation well south of Kabbaldurga, in the khondalite belt of southern Kerala, where rocks with amphibolite facies give way to the vast southern India-Sri Lanka charnockite terrain, indicating that metamorphism due to CO2-rich fluids (carbonic metamorphism) may have operated over a large area in southern India. If such localities prove to be widespread, the present level of exposure throughout much of the high-grade terrain probably does not extend far beyond an isofacial surface marking the boundary between upper and lower crust in the late Archaean. © 1985 Nature Publishing Group