Petrography and whole-rock geochemistry of Oligocene Barail Sandstones of Surma basin: Implications for tectono-provenance and paleoclimatic condition

The Oligocene Barail Sandstones of Surma basin from parts of Champhai District of Mizoram, northeast India has been studied for their tectono-provenance setting using petrography and geochemistry. The studied sandstones are poor to moderately sorted, subarkosic to sub-litharenite and show dominance of quartz (avg. 54.46%) followed by feldspars (avg. 7.22%), rock fragments (avg. 4.98%), mica (avg. 5.89%), matrix (avg. 14.47%) and cement (avg. 12.98%). Dominance of polycrystalline and undulatory monocrystalline quartz indicates contribution from medium grade metamorphic sources, primarily of granite gneisses. High concentration of SiO2 with moderate to low concentration of Al2O3, Fe2O3, MgO, Na2O and K2O resembles the composition of upper continental crust. Moreover, moderate to high concentration of Th and V, depleted value of Ni along with TiO2/Zr suggests derivation of sediments from felsic sources. The ratios of Eu/Eu* (avg. 0.63) and LaN/LuN bears resemblance with the upper continental crust. Overall analyses show that the sediments were derived from the felsic terrain of neighbouring orogens probably represented by granite-gneisses and have undergone a moderate degree of weathering (avg. values of CIA: 70.20, CIW: 81.03, PIA: 77.63, WIP: 38.45, ICV: 1.06, Iw = 2) in a semi-humid to humid climatic conditions [Qp/(F + RF):Qt/(F + RF) = 0.20].