Permo-Carboniferous Climate Change: Geochemical Evidences from Lower Gondwana Glacial Sediments,Rangit Valley,Sikkim Lesser Himalaya,India

The Rangit Gondwana Basin of Sikkim in the lesser Himalaya witnessed a wide span of climate change during the Permo-Carboniferous period. The principal objective of the present study is to document this Permo-Carboniferous climate change in the form of a geochemical signature preserved in the siliciclastic facies of the Rangit Pebble Slate Formation. The stratigraphic sequences of the Rangit Gondwana Basin are categorically well defined and subdivided into upper and lower sequences on the basis of their depositional environment. The lower sequences of the Rangit Gondwana basin are comprised of massive diamictite with large stromatolitic dolomite boulders and dropstone embedded in the coarser sandstone which indicate the cold glaciomarine environment of deposition, whereas upper sequences consist of repeated alternate beds of sandstone, black shale and coal seam with a regular interval depicting the fluvial and deltaic environment of deposition. The enrichment of SiO₂, Al₂O₃, TiO₂, MnO, MgO, and K₂O indicates that these sediments were mostly derived from felsic rock source areas. Chemical Index of Alteration (CIA) and Index of Compositional Variability (ICV) CaO + Na₂O + K₂O/Al₂O₃, and SiO₂ vs. (Al₂O₃ + K₂O + Na₂O) values suggest that the sediments maturity and paleoclimatic environment deposition of the sediments of lower sequences was cold and semi-humid whereas the deposition of sediments of upper sequences was warm and humid. The A-CN-K ternary plot and CIA vs ICV binary plot also indicate and verify that the source areas were subjected to prolonged intense chemical weathering from low to high grade due to shifting of cold to warm humid paleo-climatic condition.