Geochemistry and Rb-Sr geochronology of acid tuffs from the northern fringe of the Singhbhum craton and their significance in the precambrian evolution

Abstract

Acid volcanic rocks of tuffaceous nature occur interlayered with metasediments and basic - ultrabasic rocks between the Dalma volcanic belt to the south and the Chotanagpur Gneissic Complex to the north. Inspite of the low-grade metamorphism suffered by the rocks, certain textural - mineralogical attributes, abundance of high field strength and rare-earth elements and primary isotopic signature have remained unaffected. All the samples represent evolved composition but samples from Chandil and Ankro are interpreted to have crystallised from primary melts generated from variable source compositions. On the basis of the nature of isochron, the age of 1484 {+-} 44 Ma is inferred to be the age of eruption of the acid volcanics. This constrains the age of the metasediments and the basic - ultrabasic rocks occurring conformably interlayered with the acid volcanics and reports for the first time the presence of a mid-Proterozoic sequence in eastern Indian craton. The initial Sr ratio of 0.708545 for the Chandil samples is not very high. It is inferred to be largely dominated by Sr from mantle-derived components and the acid volcanics may be products of differentiation from some mantle-derived basic magma. (author)