BAFLIAZ VOLCANICS, NW HIMALAYA - ORIGIN OF A BIMODAL, THOLEIITE AND ALKALI BASALT SUITE

The lower Palaeozoic (Ordovician ?) Bafliaz volcanics in the southern part of the NW syntaxial bend of the Himalaya yield information about the tectonic regime and the conditions in the subjacent mantle during the early Palaeozoic. The volcanics occur as a succession of flows; two intercalated sedimentary units (carbonates + clastics) indicate short periods of quiescence. The succession has been sampled systematically, and the chemistry shows a change from dominantly differentiated tholeiites in the basal flows through a mix of less differentiated tholeiites and a few alkali basalts in the middle part, to distinctly alkaline uppermost flows. The alkali basalts in the Bafliaz volcanics are the oldest known alkali basalts in the Himalaya. The tholeiites show chemical characteristics of low-Ti continental flood basalts whereas the alkali basalts show similarities with ocean island alkali basalts. High La/Yb and low Cr, Sc and Yb in alkali basalts indicate variable amounts of residual garnet and clinopyroxene during their source melting. A distinctive feature of the alkali basalts, however, is the negative Zr, Nb and P anomalies in their incompatible-element patterns, most likely reflecting a source feature of these rocks. Bulk chemistry suggests derivation of the two rock types from two different mantle sources by different degrees of melting followed by gabbro fractionation for the tholeiites and olivine + clinopyroxene fractionation for the alkali basalts. A plume tectonic setting for the volcanism is not favoured because of the small volume of the erupted magma. Instead, field relations favour a short-lived rift reactivation phase which induced asthenospheric melting, producing the tholeiites from shallow levels and the alkali basalts from relatively deeper levels.