ARRESTED CHARNOCKITE FORMATION IN THE PALGHAT REGION, SOUTH-INDIA

Exposures of in-situ charnockitisation of gneisses in widely varying g eological associations, related to differing mechanisms of dehydration and prograde metamorphism, are recorded from the Palghat region, sout hern India. Mineral reactions in gneiss-charnockite prograde profiles indicate hornblende + biotite breakdown in the presence of quartz to p roduce orthopyroxene within structurally controlled and/or composition ally modified portions of the gneisses. Chemically the gneisses and ar rested charnockite zones are granitic in composition whereas the assoc iated banded charnockites (regional granulites) are tonalitic to grano dioritic in composition. Geothermobarometric data indicate high temper ature and high pressure early granulite-facies (regional granulite) me tamorphism at 780 - 880-degrees-C and 9-10 Kbar. The fluid species in both types of charnockites and gneisses are characterised by CO2 and m ixed CO2-H2O inclusions. The banded charnockites largely pre-date majo r deformation and the early Palaeozoic time of development of arrested charnockites. A definite time gap between the formation of banded cha rnockites and arrested charnockites is indicated. Arrested charnockite s in the Palghat region are formed along (1) structurally controlled z ones, (2) in compositionally favourable zones of modified paleosomes, (3) in melt portions of migmatitic gneisses and (4) along margins of i nvading alkaline dykes. These are interpreted as different processes t riggered by emplacement of igneous bodies and movement of CO2 enriched melts through the crust, producing sequential interrelated events lik e dehydration, partial melting, metasomatism and wall-rock alteration, leading ultimately to charnockite formation. The present field dispos itions of arrested charnockite are inferred as only an expression of v arying response of rock types to changing physico-chemical conditions (e.g. PT and fluid regime) with time during metamorphism.