Geochemical characteristics of Neogene sandstones of the East and West Siang Districts of Arunachal Pradesh, NE India: implications for source-area weathering, provenance, and tectonic setting

Sandstones from the Neogene Siwalik successions of the East and West Siang Districts of Arunachal Pradesh were analyzed to evaluate source-area weathering, provenance, and tectonic setting by using major, trace, and rare earth elements (REEs) as proxies. The sandstones are classified as lithic arenite and wacke arenite based on their mineralogical compositions. The values of different weathering indices such as Chemical Index of Alteration (CIA; 60.93-89.86) and Chemical Index of Weathering (CIW; 40-96.8) suggest moderate to intense weathering in the source area. The plot of Th/Sc versus Zr/Sc indicates enrichment of zircon by sediment sorting and/or recycling from a weathered source. The high positive correlation between Al2O3 and K2O points towards a strong influence of the constituent clay minerals on the major oxide composition of the sandstones. Petrographic analysis together with enriched LREE, flat HREE, negative Eu anomalies (Eu/Eu* = 0.47 to 0.90) in the chondrite-normalized diagrams, and the ratios of La/Sc, La/Co, Th/Sc, Th/Co, Cr/Th collectively suggests that the Neogene sediments were derived from felsic igneous and/or reworked sedimentary/metasedimentary sources in an upper continental crustal setting. The geochemical characteristics of the studied Neogene Siwalik sandstones indicate that the sediments were sourced from pre-Himalayan gneisses and granitoids together with metabasic rocks, which had formed in a passive margin tectonic setup.