Mechanisms of Charnockite Formation and Breakdown in Southern Kerala: Implications for the Origin of the Southern Indian Granulite Terrain

Abstract

Field studies in southern Kerala have been successful in locating a Dumber of quarries in which charnockite has partially replaced garnet-biotite gneiss. Four types of charnockite formation have been recognised. Three of these involve the dehydration of gneisses by the influx of CO₂ - rich fluids. Types I and II show charnockite concentrated along shear and foliation planes. In type III, high CO₂ pressures are required by the presence of interlayered, scapolite-bearing calc-silicates. In type IV, charnockite is restricted to the margins of granitic dykes. and formed as a result of the decompression reaction gar + qtz + Na-plag = opx + Ca-plag, rather than a dehydration reaction. The widespread occurrence of arrested charnockitisation in southern Kerala suggests that the process of carbonic metamorphism invoked at Kabbaldurga, Karnataka, was not a local phenomenon, but extended over the entire southern Indian granulite terrain. In contrast to these examples of charnockite in the making, three types of charnockite breakdown have also been recognised. These involve the influx of water-rich solutions by means of granite pegmatites, leucocratic magmas, and pink granites. Present geochronologic data are not sufficient to determine whether the various types of charnockite formation and breakdown were different phases of a single metamorphic event or the result of several distinct metamorphic episodes.