GEOCHEMISTRY AND TECTONIC SIGNIFICANCE OF THE ONGARBIRA METAVOLCANIC ROCKS, SINGHBHUM DISTRICT, INDIA

The volcanic-sedimentary sequence of Ongarbira occurs to the south of the Singhbhum Shear Zone and rests unconformably on a basement topogra phic-low between the Chakradharpur Granite and the Singhbhum Granite. Structural analysis reveals that the Ongarbira synform and other adjoi ning structures are kilometre-scale first-generation folds on E-W stri king axial planes. These structures are in marked contrast with the co rresponding kilometre-scale first-generation folds typical of the Iron Ore Group synclinorium towards the south of the Ongarbira belt. The O ngarbira metavolcanic rocks exhibit a limited range of tholeiitic basa lt compositions. The basalts are grouped according to their Mg# and th e details of their REE patterns. Groups I and II are relatively unevol ved and very similar to LREE- and LIL-depleted ocean ridge tholeiites. The most evolved Group III with LREE enrichment is most similar to LI L-enriched ocean floor basalts, but may be contaminated with crustal m aterials. In general, the Ongarbira volcanics evolve upward stratigrap hically. Petrogenetic major element modelling of the Ongarbira basalts suggests that the primary melt was generated by batch melting (30%) o f lherzolite at pressures of at least 1.5 GPa. Sequential fractionatio n of Ol, Ol + Cpx + Pl and Cpx + Pl followed. Comparison of incompatib le and compatible element distributions indicates a very similar batch melting-fractional crystallization sequence for Ongarbira Groups I an d II basalts. Group III basalts may have been derived from a batch mel t initially more enriched in incompatible elements. Trace element disc rimination of the Ongarbira basalts suggests that they were generated in an extensional environment. Primordial mantle normalized element di stributions indicate that the basalts are similar to continental rift basalts but with oceanic affinities. The chemical data along with the associated sediments indicate that the Ongarbira basalts were probably emplaced in a mature continental rift, although a back-arc origin can not be completely disregarded. It is concluded that the Ongarbira volc anics were emplaced as part of the same rifting event that resulted in outpourings of the Proterozoic Dalma and Dhanjori volcanics, implying that the Ongarbira suite is Proterozoic and not part of the Archaean Iron Ore Group. The structural geometry of the Ongarbira rocks is more conformable to that of the Singhbhum Mobile Belt than to the Iron Ore Group synclinorium in the Singhbhum Craton. It is likely that the sou thern limit of the Singhbhum Mobile Belt is defined by the interface o f its E-W structures with the NNE structures of the Iron Ore Group roc ks. Thus, the Singhbhum Shear Zone does not mark the interface between the Archaean Singhbhum Craton and the Proterozoic Singhbhum Mobile Be lt in the eastern Indian Shield.