Petrogenesis and the Mantle Source Characteristics of the Abor Volcanic Rocks, Eastern Himalayas

The Late Palaeozoic Abor Volcanics of the eastern Himalayas have typical continental tholeiite chemistry in terms of rather high and variable Fe/Mg ratios and incompatible trace elements, except P, abundances. The compatible trace elements (Ni, Cr, Co) are low for their specific MgO levels. Variable abundances of incompatible elements at constant Mg numbers, lack of significant Eu anomalies and large range in the ratios of more incompatible to less incompatible elements (e.g. La/Yb) suggest that fractionation was not significant in their compositional evolution. Fe-Mg relations indicate that the melts were generated from a mantle source with enriched Fe/Mg ratio compared to pyrolitic mantle; melting was followed by separation of an immiscible sulphide phase which resulted in lowering of Ni and P contents, and some clinopyroxene fractionation. Model calculations on REE data suggest that the source was enriched by about l1x and 4x chondrite in light REE (La) and heavy REE (Yb) respectively. This enrichment of the source was probably due to an undersaturated silicate melt and a CO₂-rich fluid phase.