Petrography of Barail sandstone of Champhai-Mualkawi section in Champhai district, Mizoram: Implication on provenance and tectonic setting

Abstract

The area around Champhai district contains exposed Barail Group of rocks (Oligocene) which comprises of buff-coloured sandstones with alternating beds of shale and siltstone. This study focussed on the provenance and tectonic setting of the Barail sandstones with the help of detailed petrographic studies and quantitative and qualitative modal analysis by point counting technique. The general lithological units consist of sandstone, shale, crumpled shale, silty-shale and siltstone. A modal analysis of the 25 sandstone samples was carried out on selected detrital grains like quartz, feldspar and lithic fragments which indicated that quartz ranges between 69.75 to 82.25%, feldspar ranges between 1.80 to 6.91% and lithic fragments ranges between 13.82% to 29.43%. The sandstones belong to sublitharenite, litharenite and feldspathic litharenite indicating predominant presence of quartz in the sandstones with lesser components of feldspar and lithic fragments. Different variants of quartz such as undulatory monocrystalline quartz and polycrystalline quartz and other lithic fragments like schist and gneiss indicate metamorphic source rock. The common occurrence of micas in almost all the sandstone samples may also indicate the common occurrence of mica-bearing metamorphic schists or gneiss in the provenance. Various characteristics of feldspar like strong alteration in orthoclase, deformed twinning of plagioclase and presence of micas may be indicative of low to high grade metamorphic rocks and acidic to basic igneous rocks. On the basis of different tectonic discrimination diagram using QFL and QmFL, the tectono-provenance of the sandstones was found to be Quartzose Recycled Orogen. It is concluded that the source area might be the tectonically active continent-continent collision zone of eastern Himalaya and the Indo-Burmese belt and that the associated extrusive and intrusive igneous rocks are prone to weathering and erosion by various denudation processes.