Rb-Sr and Pb-Pb geochronological studies on the granite gneiss of Kuilapal, Purulia-Bankura Midnapore district, West Bengal

The number of younger granite plutons intruding the Singhbhum Mobile Belt (SMB) surrounding the Singhbhum-Orissa Craton, viz. Kuilapal granite on the East, Mayurbhanj granite, Soda granite, Arkasoni granite, Chakradharpur granite gneiss and Tamper kola granite on the west. Of these Kuilapal granite body is the largest. Besides small granite bodies intruding the low-grade meta sediments and meta volcanics which occur north of the Dalma volcanic belt. Petrographic and geochemical characters of the minor granite bodies south of Dalma volcanics are similar to those of Kuilapal granite, which is inferred to have been produced by shallow-level fractional crystallisation of a crustally derived granodiorite melt. Dominance of xenoliths in the Kuilapal granite suggest large-scale assimilation of crustal rocks. The radiometric ages vis-a-vis isotope geochemistry of the various granitoid plutons occurring within the mobile belt were used to constrain the evolutionary history of the SMB supracrustals. The Singhbhum Shear Zone which marks the boundary of the SMB with Singhbhum-Orissa Craton, shows multiple reactivation, the oldest being at ∼ 3.09Ga. Further phases of granite plutonism and/or shearing and mineralisation have been reported along this shear zone occurred in Palaeo-to Mesoproterozoic periods at ∼ 2.2, 1.8, 1.6-1.5, 1.4 and 1.0 Ga respectively. The present work pertains to the Rb-Sr and Pb-Pb whole rock geochronological studies on Kuilapal granite, which are intrusive into the metasediments of Singhbhum group in the tectonic setup of north Singhbhum Mobile Belt. Fourteen samples were collected, finely powdered form the homgenised representative which were processed further. The dissolution of the rock carried out in a class-100 clean chemical laboratory, followed by the elemental separation using exchange chromatography. The separated pure fractions of the elements were analysed on VG354 Thermal Ionisation Mass spectrometer for their isotopic ratios. The Rb-Sr data on ten samples define an isochron age of 1792±98 Ma, with an initial 87Sr/86Sr of 0.7064±0.0069 (MSWD = 13). The Pb isotopic data on these samples define a Pb-Pb isochron age of 1863 ± 80Ma (MlSWD =11.3) with a model μ1 value of 8.53±0.27. Both the Rb-Sr and the Pb-Pb ages are in agreement within the limits of experimental errors. Since the 87Sr/86Sr initial ratio and model μ1 value are higher than the contemporary mantle sources, the data indicate involvement of enriched crustal sources. This age obtained on Kuilapal granite can be taken as the age of one of those periods of reactivation which occurred during Palaeo-to Mesoproterozoic periods. (author)