A tectonic perspective of incipient charnockites in East Gondwana

Abstract

Abstract Examples of spectacular occurrences of charnockite in an arrested stage of formation in crustal segments of East Gondwana (comprising India, Sri Lanka and Antarctica) are evaluated to constrain the broad tectonic framework under which the charnockite formed. Field structures, petrologic features, and information including fluids trapped within inclusions in minerals suggest that the conversion of upper amphibolite facies gneisses to charnockites was brought about by the local desiccation induced by the infiltration of CO2-rich fluids through faults and shears under brittle-quasi-brittle conditions. The stable isotopic composition of these fluids is consistent with a “juvenile”, sublithospheric source. Available geochronologic data indicate multiple events of the incipient charnockite formation at ∼2500, 1000, and 500 Ma in these crustal domains. In each case, incipient charnockite formation post-dates regional metamorphism. Hence, the process of local desiccation is inferred to have operated in cratonized crustal segments. The P-T-t paths, characterized by isothermal decompression, are consistent with incipient charnockite development during the waning stages of collisional mobile belts, when the tectonic regime changed from compressional to extensional. Crustal distension triggers the release of CO2-rich fluids from depth, which are transported to mid-crustal levels through magmatic conduits, a notion supplemented by the occurrence of alkaline intrusives of Pan-African age. Degassing of these magmas could have supplied the CO2 required for stabilizing the anhydrous assemblages in the charnockites.