Arrested Charnockite Formation in the Palghat Region, South India

Abstract

Exposures of in-situ charnockitisation of gneisses in widely varying geological associations, related to differing mechanisms of dehydration and prograde metamorphism, are recorded from the Palghat region, southern India. Mineral reactions in gneiss-charnockite prograde profiles indicate hornblende + biotite breakdown in the presence of quartz to produce orthopyroxene within structurally controlled and/or compositionally modified portions of the gneisses. Chemically the gneisses and arrested charnockite zones are granitic in composition whereas the associated banded charnockites (regional granulites) are tonalitic to granodioritic in composition, Geothermobarometric data indicate high temperature and high pressure early granulite-facies (regional granulite) metamorphism at 780 - 880°C and 9-10 Kbar. The fluid species in both types of charnockites and gneisses are characterised by CO, and mixed CO₂-H₂O inclusions.

The banded charnockites largely pre-date major deformation and the early Palaeozoic time of development of arrested charnockites. A definite time gap between the formation of banded charnockites and arrested charnockites is indicated. Arrested charnockites in the Palghat region are formed along (1) structurally controlled zones, (2) in compositionally favourable zones of modified paleosomes, (3) in melt portions of migrnatitic gneisses and (4) along margins of invading alkaline dykes. These are interpreted as different processes triggered by emplacement of igneous bodies and movement of CO₂, enriched melts through the crust, producing sequential interrelated events like dehydration, partial melting, metasomatism and wall-rock alteration, leading ultimately to charnockite formation. The present field dispositions of arrested charnockite are inferred as only an expression of varying response of rock types to changing physico-chemical conditions (e.g. PT and fluid regime) with time during metamorphism.