Chemical weathering, provenance, and tectonic setting inferred from recently deposited sediments of Dharla River, Bangladesh

The petrographic and geochemical composition of the Dharla River sediments has been examined to infer their sediment type, degree of weathering, provenance, and tectonic settings. Petrographically the sediments are characterized by the high quartz content (64.97 to 74.24 wt%), followed by feldspar (7.04 to 15.20 wt%), mica (5.38 to 19.92 wt%), lithic fragment (3.46 to 8.14 wt%), and heavy minerals (1.98 to 6.94 wt%). Geochemical composition shows marked enrichment of SiO2 (mean similar to 74.16%) and a strong negative correlation with the other major oxides because of quartz dilution. The Chemical index of alteration (CIA, 45.52 to 63.51); Plagioclase index of alteration (PIA, 43.13 to 66.55); W index (20.15 to 32.86) and Rb/Sr ratios (0.35 to 0.98) suggest low to moderate intensity of chemical weathering in the source area. Geochemical classifications of the studied samples show mostly litharenitic immature type of sediments also reflects high index of compositional variability (ICV, 0.96 to 1.72). The ternary diagrams of Al2O3-(CaO + Na2O)-K2O (or A-CN-K) and of mafic rocks, felsic rocks and degree of weathering of the source rocks (or MFW) and several immobile trace element ratios (e.g. light rare earth element/Light rare earth element or LREE /HREE, Eu/Eu*, La-N/Lu-N, La/Sc, La/Co, Th/Sc, and Th/Co) reflect the contribution of common felsic source rock, with the composition close to average rhyolite, granodiorite, and granite. Chondrite-normalized rare earth element (REE) pattern shows high LREE enrichments and almost flat HREE pattern with a sharp negative Eu anomaly suggesting a felsic source provenance and characteristically supports the Himalayan source rocks compositions seemingly observed in both active and passive continental margins. The geochemical nature of weathering patterns manifests the Dharla somewhat of mass distributor en route the Brahmaputra to the global ocean.