Geochemical Changes in Basalts Across Rhe Archaean-Proterozoic Boundary-An Evaluation of Dalma Basalts from Eastern Indian Shield

Geochemical characteristics of Dalma basalts from the Proterozoic Singhbhum basin of eastern Indian shield have been evaluated in the perspective of basalt geochemistry across the Archaean-Proterozoic Boundary (APB) in greenstone belt association. Most of the incompatible elements and element ratios in Dalma basalts clearly establish their depleted geochemical signature comparable to the late Archaean greenstone belt basalts. Like their late Archaean counterparts, the Dalma basalts have La/Yb = 2.50, La/Ta = 19, Ti/V = 18 and Zr/Y = 2.6. This ultrabasic and potash-poor tholeiitic association of Singhbhum basin differs remarkably from calc-alkali basalts-potassic felsic rocks assemblage of Proterozoic greenstone belts. Convincing lithotectonic association and geochemical behaviour are strongly indicative of a back-are setting for Dalma volcanism. However, due to partly modifying influence of subduction zone elements on primary primitive composition of Dalma basalts, an apparent are association is imposed in terms of certain element ratios. Taken together, the lithoassociation and geochemical data suggest that basalt magmatism in early Proterozoic Singhbhum basin is broadly similar to those found in many late-Archaean greenstone belts. Dalma basalts do not appear to fit a generalised pattern of secular m~mt1e evolution across the APB. Further, it is observed that geochemical signatures of basalts are controlled by local tectonics which persist across a preconceived discrete time plane.