Anatomy of 2.57-2.52 Ga granitoid plutons in the eastern Dharwar craton, southern India: Implications for magma chamber processes and crustal evolution

Abstract

We present results of field studies for magmatic processes of 2.57-2.52 Ga calc-alkaline plutonic bodies from three corridors in the eastern Dharwar craton (EDC) corresponding to different crustal levels. At deeper levels plutons are bounded by thick zone of migmatites with numerous melt filled shear bands which often overprinted by incipient charnockite. On the other hand in the mid-to-upper crustal levels plutons show relatively sharp contacts and truncates the adjoining basement. The plutons are composite which comprises voluminous intrusive monzodiorite, quartz-monzonite and porphyritic monzogranite in the central part and minor anatectic granites or diatexite at periphery. Numerous xenoliths, Mafic Magmatic Enclaves (MME), disrupted trains of synplutonic mafic dykes are found in both intrusive and anatectic fades. The plutons show magmatic as well as solid-state plastic fabrics defined by magmatic flow banding and C-S fabrics respectively. Crustal scale shear zone network comprising early melt filled NE trending hot ductile dextral shear bands and slightly later colder NW trending sinistral shear bands defined by rotation of mafic boudins, phenocrysts and C-S fabrics. The internal architecture of plutons is attributed to the crustal scale magma chamber processes where voluminous intrusive magmas emplaced into the crust caused reworking of surrounding basement resulting in production of anatectic magmas. Crystallization of voluminous intrusive magmas in the deep crust probably caused development of fractures to mantle depth causing decompression melting of mantle and resultant mafic magmas penetrated the crystallizing host in magma chambers. Field evidences together with published ages and Nd isotope data reveal a spatial link between late Archaean magmatic accretion, reworking and cratonization.