Mineralogy and whole-rock geochemistry of the Oligocene Barail Group of rocks of Belt of Schuppen, Northeast India: Implications for tectono-provenance and paleo-weathering

Abstract

The petrographic and geochemical attributes of the Oligocene Barail Group of rocks are used to decipher the likely source area(s) or tectonic domains, as this sequence of rocks was deposited in a foreland basin governed by orogenic domain, namely the North-east Arunachal Himalayas. The river system that gave rise to the Brahmaputra River (Yarlung-Tsangpo), which flowed through several tectonic domains of the Himalayan ranges, primarily from Bomi-Chayu, Gangadese Granitoid, Higher Himalayan Leucogranites, and Namche Barwa into the proto Bengal Basin now a part of Assam Arakan Basin and Naga Schuppen Belt, was the main source of the sandstone formation of the Barail Group. The purpose of sandstone petrography, which combines modal analysis with XRF (Major Oxides) and HR-ICP-MS (Trace & Rare Earth Elements) research, is to identify the type of source rock(s), their weathering pattern, and its paleo-environmental circumstances. These sandstones were formed from recycled orogen and include lithic and sublithic arenite variants with advanced texture and chemical maturity. The sediments were felsic (Th/Co: 1.38, Cr/Th: 9.78, La/Lu: 11.58, Th/Sc: 0.99, Eu/Eu*: 0.66, La/Sc: 3.05, La/Co: 4.18), with contributions from intermediate source rocks and low-rank metamorphics deposited in an active continental margin to a continental island arc setting. Climatic conditions impacted the sediments of Barails, characterised by being warm and semi-humid to humid which resulted in moderate to a high degree of chemical weathering, as shown by weathering indices like CIA (79.14), PIA (85.47), CIW (86.9), WIP (32.50), ICV (0.71), and Th/U (6.03), which were further additionally supported by C-Value (1.01), PF (1.20), Sr/Cu (2.04), and Rb/Sr (0.97).