Geochemistry of Mesoproterozoic Bijaigarh Shale, Upper Vindhyan Group, Son Valley, India: Implications for source area weathering, provenance and tectonic setting

The similar to 1.2 Ga Bijaigarh Shale (BS) is a significant geological formation in the Vindhyan Supergroup of Son Valley and is widely targeted for understanding Mesoproterozoic oceanic redox conditions and crustal evolution. In the present study, we have analysed the major, trace and rare earth elements compositions of the BS and attempted to infer the composition, weathering history, and tectonic setting of the Mesoproterozoic continental crust. The BS is comprised of quartz, smectite, kaolinite and illite, with occasional framboidal pyrites. High chemical index of alteration (CIA) and plagioclase index of alteration (PIA) values, and depletion of Na2O, CaO, Sr and Ba imply that the source region underwent a high degree of chemical weathering under warm and humid climatic conditions. Various immobile element ratios in the BS suggest felsic source rocks and insignificant recycling. Fractionated chondrite normalised REE patterns (La/Yb-N = 6.7-12.3, avg. 9.5) and negative Eu anomalies (Eu/Eu* = 0.50-0.60, avg. 0.53) also indicate felsic provenance. REE mixing modelling implies that the source area was comprised of 40% Mahakoshal Granite, 25% Bundelkhand Granite, 30% Porcellanite and 5% Chhotanagpur Granite Gneiss Complex (CGGC). The sedimentation in Kaimur basin is thought to have begun at similar to 1.2 Ga, accompanied with the crustal extension in the aftermath of Central Indian Tectonic Zone (CITZ) orogeny (1.6-1.5 Ga) and domal uplift of Bundelkhand craton. The positive areas were created by orogenic movements, which exposed the older crustal blocks containing granites and metamorphic rocks in the CITZ (Chhotanagpur Gneisses and Mahakoshal belt) and Bundelkhand granite complex (BGC). The Mahakoshal Group and CGGC located on the southern side, the BGC on the northern side of the basin, and the uplifted Lower Vindhyans were the likely sources of the BS. Such source areas are also indicated by the palaeoflow directions.