Geochemistry and geotectonic implication of basic volcanic rocks in the Lower Gondwana sequence (Upper Palaeozoic) of the Sikkim Himalayas

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Abstract
Summary: As in Kashmir and Arunachal Pradesh (NEFA) Himalayas, the Upper Palaeozoic (Lower Gondwana) sequence in Sikkim Himalayas contains basic lavas. These are interbedded with boulder bed/pebble slate, sandstone andshale, exposed in a tectonic window. Geochemically the basic lavas are characterized by high contents of TiO2, K2O, P2O5and ‘incompatible’ trace elements. The chemistryis compatible with a small degree of partial melting under high pressure ofa titan-phlogopite-bearing peridotite mantle. The only primary quench phases are frequent apatite, magnetite and profuse ilmenite, the latter accounting for the high FeO/Fe2O3ratio of the rock, and demonstrating rapid crystallization. The geochemistry of the lavas appears to be consistent with basic volcanism associated with the initiation of continental rifting. From this study and from a consideration of the geochemistry of the volcanic rocks of similar age in Kashmir area, it is suggested that rifting of the northern continental margin of the Indian plate and formation of a transitional crust during Upper Palaeozoic times are important geotectonic features in the Himalayan region.