Petrology and Geochemistry of Bhowali-Bhimtal Volcanics

The Mesoproterozoic mafic volcanism in the Bhowali- Bhimtal and adjoining areas of outer Kumaun Lesser Himalaya, referred herewith as Bhimtal volcanics (BV) occur as alternating volcano-sedimentary sequences with Nagthat Formation. The mineral chemistry of clinopyroxenes from BV shows a subalkaline trend. The major element geochemistry of BV classified them as basalts, basaltic andesite and basaltic trachyandesite. The trace and REE geochemistry suggests their derivation probably from a plagioclase depleted source. Chondrite normalized REE patterns suggest that most of the REE were not significantly disturbed during alteration of BV and have retained primary features similar to that of fresh BV samples. Trace and REE geochemistry suggest slightly enriched mantle source and hence plume origin might be expected for BV magmatism. Moreover the geochemical similarity of BV with other volcanics of NW Himalaya provides the clue for similar source for all the volcanism in NW Himalaya. Based on melting model results it has been suggested that 5 to 15% melting of garnet lherzolite source can produce the observed LREE variations of precursor BV melt.