2.8 Ga old anorogenic granite-acid volcanics association from western margin of the Singhbhum-Orissa Craton, eastern India

The Tamperkola granite-acid volcanics association occurring at the western margin of the Archaean Singhbhum-Orissa Iron Ore Craton (SOC), eastern India, is intrusive into the Darjing Group which represents a sequence of mobile belt metasediments in this part of the SOC. The Darjing Group rests unconformably on the Bonai Granite (similar to3.2 Ga old). Absence of any deformational imprints of the country rock metasediments on the Tamperkola granite- acid volcanics together with its undeformed and unmetamorphosed nature, its alkali feldspar dominant mineralogy, and its high SiO2 and Na2O + K2O and low MgO and CaO contents suggest that this granite-acid volcanics association is anorogenic in nature. Two representative samples-one each from the granite and the acid volcanics- have been dated by in situ Pb-207/Pb-206 zircon dating method using a small ion-microprobe. Minimum age of crystallisation of the acid volcanics is found to be 2.8 Ga. Strong peak in the Pb-207/Pb-206 frequency diagram and equality of the observed and expected errors in radiogenic Pb-207/Pb-206 ratios suggest that this age probably represents the true age of formation of the volcanics. The age data place the deposition and metamorphism of the mobile belt metasediments of the Darjing Group in between 3.2 and 2.8 Ga. Occurrence of 2.9- 2.8 Ga old small granitoid plutons, alkali-feldspar granite to syenogranite in composition, is also known from the southern margin of the SOC. Therefore, it appears that around 2.9-2.8 Ga small alkali granite bodies formed at the marginal part of this cratonic block after its stabilisation at similar to3.1 Ga.