Middle Miocene denudation of the Higher Himalayan Crystallines revealed by chemical composition of detrital garnets from the Siwalik foreland basin sediments, western and central Nepal

Abstract

The Middle Miocene to Early Pleistocene Siwalik Group is composed of sediments deposited in the Himalayan foreland basin. In this study, the heavy mineral assemblage and chemical composition of detrital garnets from the sandstones in the Lower Siwalik sub-group, which are exposed along the Karnali River in western Nepal and the Tinau Khola in central Nepal, are studied and the provenance shift during the Middle Miocene is discussed. The heavy mineral assemblage in the lower section of the Lower Siwalik sub-group is characterized as predominantly zircon-tourmaline-rutile. Chemical analysis of the detrital garnets showed a monomodal almandine-rich composition with low pyrope (Prp<10) and moderate grossular (Grs10-20) content. The composition pattern subsequently changed to a bimodal distribution with an increase in pyrope (Prp10-25) and low grossular (Grs<10) content with the appearance of detrital staurolite. These results indicate that the Middle Miocene erosion of the Higher Himalayan Crystallines (HHC) are the source of the deposits in the Siwalik Group. Changes in the heavy mineral assemblage and garnet composition are recognized in the Karnali River section at 14–12 Ma and in the Tinau Khola section at 11–10 Ma, which suggests that the denudation of the HHC progressed from west to east in the western and central Nepal Himalayas.