Petrogenetic evolution of pegmatites of the Shigar Valley, Skardu, Gilgit-Baltistan, Pakistan

Abstract

Pegmatites of granitic composition intrude the Dassu orthogneiss of the Asian plate in the Shigar Valley of Pakistan. On the basis of field and petrographic studies, these pegmatites are distinguished into evolved and simple pegmatites. The evolved pegmatites are further distinguished as muscovite-tourmaline-beryl-garnet and muscovite-tourmaline pegmatites whereas the simple pegmatites as biotite ± garnet ± muscovite and muscovite-biotite ± garnet pegmatites. In the evolved pegmatites, the predominant minerals are albite and muscovite while in the simple pegmatites orthoclase and biotite are distinctive. Both the pegmatite types seem to be associated genetically to one common magma source and the variation within the mineral constituents are due to fractional crystallization. In spider diagrams, all the pegmatite types illustrate similar negative sloping trends from LILEs to HFSEs with depletion in Ba, Sr, and Ti concentration. In simple pegmatites, P is strongly depleted as compared to the evolved pegmatites. REEs illustrate nearly smooth patterns in all the pegmatite types with contrasting negative Eu anomaly signifying plagioclase fractionation. Both the evolved and simple pegmatites contain aluminous mineral assemblages and show mainly peraluminous geochemistry, which suggest their derivation from pelitic rocks of the Asian continental crust in syn-collision tectonic setting, ensued possibly by the collision of the Indian plate with Asian plate.