Ancient crustal metamorphism at low pH2O: Charnockite formation at Kabbaldurga, South India

Arrested charnockitic conversion of amphibolitic gneiss at Kabbaldurga, Karnataka State, south India, was studied mineralogically. Iron-rich pyroxenes were generated from amphibole in patches and stringers without melting. The dark colour of charnockite arises from numerous tiny veins of chlorite and manganese-bearing calcite, particularly in feldspars. The metamorphism was effected by very local, mainly grain-boundary, migration of volatiles low in H2O, and probably dominantly CO2. This was followed by vein alteration at lower temperatures from volatiles richer in H2O. The volatiles are ascribed to massive liberation from the mantle in upwelling areas, and this may have been an important process in the evolution of the deep continental crust. © 1979 Nature Publishing Group

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