Abstract
Sediments from stable sand bars along a 40 km section of the Brahmaputra River in northern Bangladesh were analyzed for their major, trace and rare earth element contents to determine their provenance, compositional maturity, source area weathering and tectonic setting. Geochemically, the sediments were classified as litharenites and the Index of Compositional Variability (ICV) varied between 1.4 and 2.0, indicating low compositional and mineralogical maturity. A high mean SiO2 concentration (72.9 wt.%) and low Al2O3 (11.1 wt.%) were consistent with a low abundance of shale and clay components. The depletion of the oxide components Na2O, CaO and K2O relative to average upper crustal compositions (UCC) reflected loss of feldspar during chemical weathering in the source region. Average TiO2 values for most samples were higher than average crustal levels, consistent with the northern section of the Brahmaputra River being a potential resource for valuable Fe-Ti oxide heavy minerals. Major and trace element ratios indicated the sediments represented erosional products from typical felsic upper continental crustal materials with contamination (30%–40%) from more intermediate/mafic compositions. The rare earth element patterns showed negative Eu anomalies (0.57–0.71), indicating they were derived mainly from fractionated felsic rocks. Resemblance of the sediment compositions to mean compositions from Higher Himalaya crystalline rocks pointed to these being potential source rocks but with components from a mafic source also present. Major element chemistries and low to intermediate weathering indices for all sediments indicated a lack of substantial chemical weathering. Evidence from tectonic discrimination diagrams suggested the Brahmaputra River sediments were derived from rock types that formed in a transitional tectonic setting ranging from an ancient passive margin to an active continental margin. Deposition occurred under cool to semi-arid climatic conditions in an oxic environment.
Highlights
Provenance studies of terrigenous clastic sediments are conducted to disclose the composition and the geological evolution of the sediment source areas and to limit the tectonic setting of the depositional basin (e.g., Morton et al [1] and references therein)
Minerals 2020, 10, 813 and typically include a determination of the location and nature of the sediment source areas, the pathways by which sediment is transferred from the source to the depositional basin, and the factors that influence the composition of the sedimentary rocks
The current study focuses on northern Brahmaputra river sediments using a new set of geochemical data to decipher information regarding provenance, tectonic setting, weathering and depositional environment with respect to Himalayan source region tectonics
Summary
Provenance studies of terrigenous clastic sediments are conducted to disclose the composition and the geological evolution of the sediment source areas and to limit the tectonic setting of the depositional basin (e.g., Morton et al [1] and references therein). The geochemistry of clastic sediments is the product of interacting factors including provenance, sorting, weathering and tectonism In this context, a number of geochemical studies based on clastic sediments derived from regions of the Himalaya have been carried out and have provided valuable information on the depositional history of sedimentary basins within the region Sand concentrates [15,16] and a detailed analysis of the composition and sources for the HMs will provide key information regarding the future feasibility of northern Bangladesh river systems for the recovery of potentially valuable HM resources
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