Major, trace, and REE geochemistry of the Meghna River sediments, Bangladesh: Constraints on weathering and provenance

Abstract

The major, trace, and rare earth element (REE) compositions of sediments from the Meghna River in Bangladesh have been examined to infer their sediment type, compositional maturity, chemical weathering intensity, provenance, and tectonic setting.Geochemically, the sediments are classified as litharenites, shale, and wacke. The ICV (Index of Compositional Variability) values (0.92 to 1.10) indicate that the sediments have low compositional and mineralogical maturity. Major and trace element abundances display marked depletion of Na 2 O, CaO, MgO, K 2 O, Fe 2 O 3 T, Sr, and Ba relative to UCC (upper continental crust), suggesting loss of feldspars during chemical weathering in the source. Zirconium, Th, Ce, and Y are enriched relative to UCC, indicating that these elements are primarily controlled by resistant heavy minerals. The CIA (Chemical Index of Alteration), PIA (Plagioclase Index of Alteration), and Rb/Sr and K 2 O/Rb ratios of the river sediments suggest low to moderate intensity of chemical weathering in the source area. Chondrite-normalized REE patterns show that high LREE enrichment, nearly flat HREE fractionation (La N /Yb N = 7.62 to 8.73) and marked negative Eu anomalies (Eu/Eu* = 0.49 to 0.69) suggesting a felsic source provenance. Several discriminant function diagrams and immobile trace element ratios (parameters indicate that the river sediments were primarily derived from a common felsic source rock, with composition close to the average rhyolite, granodiorite, granite, and UCC. Tectonic setting signatures of the Meghna River sediments are both active continental margin and passive margin environments.

Geochemistry of Recent Brahmaputra River Sediments: Provenance, Tectonics, Source Area Weathering and Depositional Environment

“…Recycled sources typically represent quartzose sediments from a mature continental provenance, with the derivation of the sediments likely from a highly weathered granite-gneiss terrain and/or from a pre-existing sedimentary terrain such as in the southern Himalayas [13,37]. Similar results were obtained for Miocene Surma Group sandstones in the Bengal Basin by Rahman and Suzuki [37], for Brahmaputra-Jamuna River (BJR) sediments by Bhuiyan et al [7] and for Ganges-Brahmaputra (GBR) sediments (data reported in Hossain [9]).…”

“…This is consistent with previous compositional and mineralogical results reported by Rahman et al [16] who determined the Brahmaputra sediments contained a significant amount of quartz (60%-65%), along with feldspar (5%-8%) and rock fragments and/or mafic minerals (30%-40%). Similar geochemical studies of clastic sediments from the Meghna [9] and Brahmaputra [7] rivers also indicated the sediments were primarily lithic arenites with SiO2 contents ranging from 58-80 wt.% SiO2 and with quartz contents of around 65%. The classifications arising from the major element ratio plots emphasizes the relative chemical and mineralogical immaturity of the Brahmaputra sediments, with none being classified as mature quartz arenite by either scheme.…”

“…Minerals 2020, 10, x; doi: FOR PEER REVIEW www.mdpi.com/journal/minerals the Bengal Basin by Rahman and Suzuki [37], for Brahmaputra-Jamuna River (BJR) sediments by Bhuiyan et al [7] and for Ganges-Brahmaputra (GBR) sediments (data reported in Hossain [9]). Figure 6.…”

“…Major element provenance discriminant plot for the Brahmaputra River sediments. Fields for Siwalik Group sediments in NW Himalaya (India) from data in Ranjan and Banerjee [39] and the Ganges-Brahmaputra Rivers (GBR) from Hossain [9]. Stars: BA, AN, DA, RD, RH-average basalt, andesite, dacite, rhyodacite and rhyolite, as plotted by Roser and Korsch [38].…”

“…The geochemistry of clastic sediments therefore 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 (e.g., Dalai et al [4]; Najman [5]; Hossain et al [6]; Bhuiyan et al [7]; Roy and Roser [8]; Hossain [9]).…”