2.62 Ga high-temperature metamorphism in the central part of the Eastern Dharwar Craton: implications for late Archaean tectonothermal history

Abstract

Aluminous metasediments occurring in the central part of the Eastern Dharwar Craton (EDC), southern India, develop contrasting mineral assemblages in closely spaced domains. Detailed petrographic, mineral chemical and geothermobarometry showed 'peak' metamorphic conditions of 780-820°C and ~5 kbar that resulted in partial melting of biotite-bearing protoliths. Initial bulk compositional heterogeneity and different degrees of melt extraction produced diverse restitic mineral assemblages. P-T pseudosections, constructed from effective bulk composition, and consideration of the sequence of mineral reactions, deduced from textural criteria, define a nearly isobaric heating-cooling P-T trajectory. Monazite in the metasediments has a detrital core age of 3161±78 Ma and a metamorphic age of 2625±26 Ma (EPMA chemical age), which is the first record of a pre-2500 Ma high temperature-low pressure metamorphism event in the EDC. Based on age contemporaneity with the 2.7-2.62 Ga hot juvenile magmas of tonalite-trondhjemite-granodiorite (TTG)-greenstone accretion and the deduced P-T history, we conclude that heat advecting from rising TTG magmas and mafic magmas (greenstones) as a consequence of mantle melting, resulted in a hot transient mid-crust during a major late Archaean tectonothermal event in the Eastern Dharwar Craton. All the phenomena are consistent with a recently proposed model involving an arc setting during ridge subduction and consequent opening of a slab window. This study therefore provides new constraints on the thermal evolution of the Eastern Dharwar Craton.