Provenance, tectonics and source weathering of modern fluvial sediments of the Brahmaputra-Jamuna River, Bangladesh: Inference from geochemistry

摘要

This present study describes the elemental geochemistry of fluvial sediments in the Kurigram (upstream) to Sirajganj-Tangail (downstream) section of the Brahmaputra-Jamuna River, Bangladesh, with the aim of evaluating their provenance, weathering and tectonic setting. Petrographically, the sediments are rich in quartz (68%), followed by feldspars (8.5%) and lithic grains (7%). The bulk sediment chemistry is influenced by grain size. Concentrations of TiO2, Fe2O3, MgO, K2O, P2O5, Rb, Nb, Cr, V, Y, and, Ce, Th and Ga slightly decrease with increasing SiO2/Al2O3 and grain size, suggesting clay matrix control. In contrast, concentrations of CaO, Na2O, Sr and Pb increase with increasing SiO2/Al2O3 and grain size, suggesting residence of these substances in feldspar. Decrease in Zr as grain size increases is likely controlled both by clay matrix and heavy minerals. In addition, heavy minerals' sorting also influences Ce, Th, Y and Cr abundances in some samples. The sediments are predominantly quartzose in composition with abundant low-grade metamorphic and sedimentary lithics, low feldspars and trace volcanic detritus, indicating a quartzose recycled orogen province as a source of the sediments. Discriminant diagrams together with immobile element ratio plots show that, the Brahmaputra-Jamuna River sediments are mostly derived from rocks formed in an active continental margin. Moreover, the rare earth element ratios as well as chondrite-normalized REE patterns with flat HREE, LREE enrichment, and negative Eu anomalies indicate derivation of the sediments of Brahmaputra-Jamuna River from felsic rock sources of upper continental crust (UCC). The chemical indices of alteration suggest that Brahmaputra-Jamuna River sediments are chemically immature and experienced low chemical weathering effects. In the A-CN-K ternary diagram, most of the samples close to the plagioclase-K-feldspar join line and to the UCC plot, and in the field of various lithologies of Higher Himalayan Crystalline Series, suggesting that rocks in these series are likely source rocks. Therefore, the elemental geochemistry of the Brahmaputra-Jamuna River sediments is controlled mostly by mechanical breakdown of lithic fragments and subsequent preferential attrition of muscovite>albite>quartz.