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; tw o intercalated sedimentary units (carbonates + clastics) indicate shor t periods of quiescence. The succession has been sampled systematicall y, and the chemistry shows a change from dominantly differentiated tho leiites in the basal flows through a mix of less differentiated tholei ites and a few alkali basalts in the middle part, to distinctly alkali ne uppermost flows. The alkali basalts in the Bafliaz volcanics are th e oldest known alkali basalts in the Himalaya. The tholeiites show che mical characteristics of low-Ti continental flood basalts whereas the alkali basalts show similarities with ocean island alkali basalts. Hig h La/Yb and low Cr, Sc and Yb in alkali basalts indicate variable amou nts 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 li kely reflecting a source feature of these rocks. Bulk chemistry sugges ts derivation of the two rock types from two different mantle sources by different degrees of melting followed by gabbro fractionation for t he tholeiites and olivine + clinopyroxene fractionation for the alkali basalts. A plume tectonic setting for the volcanism is not favoured b ecause of the small volume of the erupted magma. Instead, field relati ons favour a short-lived rift reactivation phase which induced astheno spheric melting, producing the tholeiites from shallow levels and the alkali basalts from relatively deeper levels.