Provenance of the Late Neogene Siwalik sandstone, Kumaun Himalayan Foreland Basin: Constraints from the metamorphic rank and index of detrital rock fragments

摘要

An understanding about lithology, tectonics and unroofing history of provenance is mostly drawn from the compositional and textural parameters of the detrital fragments. We here use different metamorphic ranks (Rm) and metamorphic index (MI) values of rock fragments present in Late Neogene Siwalik sandstone of the Ramganga sub-basin to infer the provenance history. The study indicates maximum contribution from metamorphic ranks 1 and 2 (Rm1 and Rm2; meta-sedimentary and very low grade metamorphic rocks) and minimum from metamorphic rank 4 (Rm4; high metamorphic grade rocks). The metamorphic index (MI) values range from 118 to 224, with an average of 186. The meta-sedimentary and very low-grade metamorphic rock fragments are derived from the Lesser Himalayan domain. The medium-to-high grade metamorphic fragments are derived from the Lesser Himalayan Crystalline bodies. The abundance of Rm2 and Rm3 detrital modes suggest the exhumation of crystalline bodies of the Ramgarh and Almora most likely occurred prior to 7 Ma and subsequently the source area shifted and resulted abundant supply of Rm1 fragments due to the upliftment along Main Boundary Thrust around 5.55 Ma.