The Archaean nucleus of Singhbhum: The present state of knowledge

The supracrustal rocks of the Older Metamorphic Group (OMG), consisting of metasediments and ortho-amphibolite, constitute the oldest unit in the Arch aean nucleus of Singhbhum. However, there are indications that still older (3.4-3.8 Ga) crust of both sialic and mafic composition existed in this reg ion. The OMG ortho-amphibolites were formed by partial melting of mantle wi th near chondritic composition ca. 3.3 Ga ago, probably as a result of plum e activity. Shortly afterwards, partial melting of the underplated mafic ma terial produced a tonalitic melt (Older Metamorphic Tonalitic Gneiss - OMTG ), which intruded the OMG supracrustals and the entire suite was deformed a nd metamorphosed to upper amphibolite facies. Subsequent to this, melting o f the OMG ortho-amphibolites and the lower crustal material of probable and esitic composition produced melts varying in composition from tonalite to g ranite and these intruded in different phases to produce plutons of Singhbh um Granite, Bonai Granite and Kaptipada Granite, which together form volume trically the major part of the Archaean nucleus. The older OMG and OMTG occ ur as enclaves within these younger granitoids. The time difference between the emplacements of the OMTG and the early phases of younger granitic intr usion was of the order of 100-200 Ma. Thus, serial additions of juvenile ma terial led to the formation of a stable microcontinent by 3.2 Ga. Thermally triggered stretching in this microcontinent produced basins peripheral to the present day Singhbhum Granite pluton, and in these basins the younger s upracrustal rocks of the Iron Ore Group (IOG), consisting of BIF, associate d argillaceous and subordinate arenaceous rocks, and mafic lavas were laid down. There is inadequate field or geochronological evidence to resolve the issue of whether the different iron ore basins were coeval or not. Meagre geochronological data suggest that some of the BIFs are older than ca. 3.1 Ga. Post-IOG activity is confined to the intrusion of mafic dyke swarms and formation of intracratonic basins, the ages of both being uncertain.