Evolution of Mayurbhanj Granite Pluton, eastern Singhbhum, India: a case study of petrogenesis of an A-type granite in bimodal association

The A-type Mayurbhanj Granite Pluton (∼3.09 Ga), occurring along the eastern margin of the Singhbhum-Orissa Craton, eastern India, represents the final phase of acid plutonism in this crustal block of Archean age. The granite shows a bimodal association with a voluminous gabbroid body, exposed mainly along its western margin, and is associated with the Singhbhum Shear zone. The granite pluton is composed mainly of a coarse ferrohastingsite–biotite granite phase, with an early fine-grained granophyric microgranitic phase and a late biotite aplogranitic phase. Petrogenetic models of partial melting, fractional crystallisation and magma mixing have been advocated for the evolution of this pluton. New data, combined with earlier information, suggest that two igneous processes were responsible for the evolution of the Mayurbhanj Granite Pluton: partial melting of the Singhbhum Granite; followed by limited amount of mixing of acid and basic magmas in an anorogenic extensional setting. The necessary heat for partial melting was provided by the voluminous basaltic magma, now represented by the gabbroid body, emplaced at a shallow crustal level and showing a bimodal association with the Mayurbhanj Granite Pluton. The Singhbhum Shear Zone provided a possible channel way for the emplacement of the basic magma during crustal extension. It is concluded that all three phases of the Mayurbhanj Granite Pluton were derived from the same parent magma, generated by batch partial melting of the Singhbhum Granite at relatively high temperatures (∼980 °C) and low pressures (4 to