Petrogenesis of the Palaeoarchean Keonjhar Granite, Singhbhum Craton, India: Product of Crustal Reworking or Subduction?

The early Archean represents an important eon in the evolution of the earth’s continental crust and could provide insights into the nature of geodynamic processes that operated during that period. The Singhbhum Craton from the Indian Shield is the only major archive of Palaeo–Mesoarchean geological processes. The Palaeoarchean granitoids from the Keonjhar area of Singhbhum Craton are potassic granites and granodiorites of calcalkaline affinity. Their age and elemental concentrations resemble the low Al₂O₃granites reported from the Eastern Pilbara Craton of Australia. The geochemical systematics of these granitoids suggests their derivation due to crustal reworking involving partial melting of a tonalitic source, possibly older metamorphic tonalitic gneiss (OMTG). The OMTG could have been derived due to the melting of an enriched basaltic source at the base of an oceanic plateau. In the second stage, the resultant underplating at crustal levels caused the reworking that led to intracrustal melting and differentiation of OMTG to form potassic granites, similar to that of Keonjhar pluton. Consolidating the evidences from the available geochemical and isotopic studies with our own data and correlating them with the geophysical evidences, we interpret that the Keonjhar granitoids are the product of intracrustal melting in an oceanic plateau setting.