Geochemistry and petrogenesis of the mandidarla volcanics, northwestern Himalayas

Abstract

The Proterozoic Mandi-Darla Volcanics (MDV) occur as flows intercalated with the low-grade metasediments of the Sundarnagar Group of the Lesser Himalayas. These volcanics are aphyric and have quenched textures with plagioclase-clinopyroxene skeletal microphenocrysts. They are of tholeiitic composition, and are enriched in FeO^t, TiO₂, incompatible trace and rare earth elements. They have relatively high SiO₂ abundances for their MgO levels, which are attributed to high PH₂O pertaining during melt generation. Fractional crystallisation does not appear to have played any major role in the evolution of their bulk chemistry. Instead, they appear to have been generated by different extents of isobaric partial melting of a non-pyrolitic and heterogeneous source with enriched and variable Fe/Mg ratios. REE modelling suggests that the REE enriched source consisted of variable proportions of at least two end members differing in their MREE and HREE contents, but with similar LREE abundances. The enrichment of the source is attributed to mantle metasomatism by a melt-dominated phase penecontemporaneous with magma generation.