Geochemistry and tectonic significance of the Ongarbira metavolcanic rocks, Singhbhum District, India

Abstract

The volcanic-sedimentary sequence of Ongarbira occurs to the south of the Singhbhum Shear Zone and rests uncomformably on a basement topographic-low between the Chakradharpur Granite and the Singhbhum Granite. Structural analysis reveals that the Ongarbira synform and other adjoining structures are kilometre-scale first-generation folds on E-W striking axial planes. These structures are in marked contrast with the corresponding kilometre-scale first-generation folds typical of the Iron Ore Group synclinorium towards the south of the Ongarbira belt. The Ongarbira metavolcanic rocks exhibit a limited range of tholeitic basalt compositions. The basalts are grouped according to their Mg# and the details of their REE patterns. Groups I and II are relatively unevolved and very similar to LREE- and LIL-depleted ocean ridge tholeiites. The most evolved Group III with LREE enrichment is most similar to LIL-enriched ocean floor basalts, but may be contaminated with crustal materials. In general, the Ongarbira volcanics evolve upward stratigraphically. Petrogenetic major element modelling of the Ongarbira basalts suggests that the primary melt was generated by batch melting (30%) of lherzolite at pressures of at least 1.5 GPa. Sequential fractionation of Ol, O1+Cpx+Pl and Cpx+Pl followed. Comparison of incompatible and compatible element distributions indicates a very similar batch m melting-fractional crystallization sequence for Ongarbira Groups I and II basalts. Group III basalts may have been derived from a batch melt initially more enriched in incompatible elements. Trace element discrimination of the Ongarbira basalts suggests that they were generated in an extensional environment. Primordial mantle normalized element distributions 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 cannot be completely disregarded. It is concluded that the Ongarbira volcanics 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 southern limit of the Singhbhum Mobile Belt is defined by the interface of its E-W structures with the NNE structures of the Iron Ore Group rocks. Thus, the Singhbhum Shear Zone does not mark the interface between the Archaean Singhbhum Craton and the Proterozoic Singhbhum Mobile Belt in the eastern Indian Shield.