The Incipient Charnockites of Transition Zone, Granulite Zone and Khondalite Zone of South India: Contrasting Mechanisms and Controlling Factors

Incipient charnockitisation in southern India is achieved through volatile action either during progressive metamorphism, or by progressive tectano-metamorphic activity, as in the Kerala khondalite belt. The initial inertness of some rocks to charnockitisation may be due to unsuitable bulk composition/mineralogy or high water activity or lack of sufficient fluid channels. Most workers advocate massive influx of CO2 from the mantle, for causing charnockite formation at lower levels and initiating anatexis at advancing front. The timing of CO2 influx and liberation of H2O and sequential reactions in the crust are important because they can account for the development of massive charnockites, the modification of supracrustals (metapelites etc.) by in-situ partial melting, or infiltration of alkali-rich fluid phases. the progressive build up of CO2 and patchy charnockite development. It is suggested here that the above processes operated in succession and are interrelated. It is further suggested that incipient charnockite formation in south India is not the result of one common mechanism or of a specific time related event, but is a local and independent response of rock types to changing P-T and fluid regime.