GEOCHEMISTRY AND PETROGENESIS OF THE PALAMPUR METAVOLCANICS, LESSER HIMACHAL HIMALAYA, INDIA

The Palampur metavolcanics (PV) in the northwest Himalaya are part of the Late Archaean (2.5 Ga) Rampur flood basalt province (RFBP) which r epresents one of the oldest manifestation of worldwide mafic magmatism . The volcanics occur as mafic lava flows with evidence of two phases of deformation. The first phase resulted in recrystallisation which al most completely obliterated the primary mineralogy, and the second pha se was of weak cataclasis. Immobile trace element ratios as well as ca tion percent Al - (Fe + Ti) - Mg indicate that the volcanics are thole iitic in composition. The chemical characteristics, such as the decoup ling between HFS and LIL, elements i.e., distinct negative Sr, Nb and Ti anomalies in the double normalisation ratios spiderdiagram together with low Ti/Y and Zr/Y ratios, testify the rocks as low-Ti continenta l flood basalts. The chemical variations in the volcanics can be relat ed to varying extents of partial melting of the mantle source(s), foll owed by fractional crystallisation (predominantly olivine and clinopyr oxene over plagioclase). Positive correlation between LREE and Fe abun dances, Ce-Nd and Y/Nd-Zr/Y data preclude any significant role of crus tal contamination in the evolution of their bulk chemistry. The REE da ta and [Mg]-[Fe] relations rather suggest that the parental magma of t he PV derived from non-pyrolitic source(s) which was heterogeneous wit h respect to enrichment in Zr, LREE and Fe/Mg ratios. Mantle metasomat ism appears to be the main process of such source enrichment, possibly caused by the addition of a volatile-rich silicate melt phase.