Petrological and Geochemical Constraints on the Petrogenesis of the Jaspa Granitic Pluton, Lahual Region, NW Himalaya

Abstract

Several granitic plutons were emplaced in the Lesser and Higher Himalayas of India during early Paleozoic. Jaspa pluton of Lahual region is one such pluton, where the granitoids are intrusive into the low-grade Haimantas made up of phyllitic and quartzitic rocks. The petrochemical studies of the Jaspa granitoids show two successive phases of intrusion. The early phase is dominated by Biotite-Muscovite Granite (BMG) of Ordovician age. Compositionally these rocks are similar to the biotite-granites of Shingo-La of Higher Himalayan range, which are around 50 km to the north of the BMG rocks. The younger phase is represented by tourmaline bearing leucogranites (TLG), which occur as minor phase generally along the marginal parts of the BMG rocks.

The BMG rocks in comparison to TLG rocks have relatively high concentrations of CaO, MgO, TiO₂, Fe₂O₃(T), Nb, Zr, Sr, Pb and Th and low concentrations of SiO₂, Na₂O, P₂O₅, Rb,Ga and U. They distinctly plot as two geochemical populations on different variation diagrams. The BMG rocks are characterised by enriched REE concentrations and show fractionating bends, while the TLG rocks have low REE concentrations and show less fractionated trends and depleted HREE pattern. The field, petrographic and geochemical studies suggest that the two phases of Jaspa pluton, the BMG and the TLG rocks were generated from the partial melting of the continental crust in which two chemically different source types were involved.