Geochemical characteristics of Gondwana shales from the Barapukuria basin, Bangladesh: implications for source-area weathering and provenance

Abstract

The major, trace, and rare earth element (REE) compositions of shales from the drill hole GDH-40, Barapukuria basin, northwestern Bangladesh have been examined to evaluate their compositional variations, source rock weathering, and provenance. The shale samples are characterized by very low to moderate abundance of SiO2 (38.84–62.91 wt.%) and high Al2O3 (19.08–30.36 wt.%). The Gondwana shales are mainly enriched in Al2O3, TiO2, Th, Ce, Zr, Nb, Pb, Sc, Y, Ni, Cr, and V relative to average upper continental crust (UCC) and marked depleted in CaO, Na2O, K2O, MgO, and Ba. Depletion of these labile elements relative to UCC indicates destruction of feldspar during fluvial transportation. The Index of Compositional Variability (ICV) values range from 0.08 to 0.21, indicating high compositional maturity. Chondrite-normalized REE patterns for the shales are very similar to the average UCC, with significant LREE enrichment (LaN/YbN 10.03–13.58), flat HREE, and negative Eu anomalies (Eu/Eu* 0.36–0.73) representing a felsic source-rock provenance. The Chemical Index of Alteration (CIA), Plagioclase Index of Alteration (PIA), K2O/Rb, and Rb/Sr ratios indicate that the Gondwana shales were largely derived from deeply weathered source rocks. Discriminant function diagrams, immobile trace element ratios (Th/Sc, Zr/Sc, Ce/Sc, and Ti/Zr), and REE parameters (∑LREE/HREE and GdN/YbN) in the Gondwana shales further support felsic detritus input, with compositions close to the average rhyolites, UCC, and I- and S-type granites.