Geochemistry and sedimentology of sediments in a short fluvial system, NW China: implications to the provenance and tectonic setting

Six outcrop sections in Fenghe River, Northwestern China, were sampled and analyzed. This study aims to determine provenance, tectonic setting, and source-area paleoweathering of the sediments of Fenghe River in combination of lithofacies analysis and bulk-rock geochemical data. The lithofacies in the studied area were classified as gravel, sand, and fine-grained clastic lithofacies, reflecting generally channel fill deposits, channel bar deposits, and over-bank deposits, respectively. The Chemical Index of Alteration (CIA), Plagioclase Index of Alteration (PIA), and Chemical Index of Weathering (CIW) values ranged 50.10–62.29, 50.13–66.35, 56.52–71.12, respectively, together with element ratios such as Rb/Sr, K/Na, Rb/K, Th/K, Rb/Ti, and Cs/Ti indicate that the source area was under a low to moderate chemical weathering condition probably in cold and semi-arid climates. Moreover, plot of SiO2 vs. (Al2O3+K2O+Na2O) suggests that the sediments were deposited in a semi-arid climate. Plots of Cr/Th vs. Th/Sc, TiO2 vs. Zr, La/Yb vs. rare earth element (REE), and La-Th-Sc ternary diagrams, along with the lithology, indicate that the sediments in Fenghe River were mainly originated from felsic igneous rocks. Major elements-based discrimination diagrams and Th-Sc-Zr/10 and La-Th-Sc ternary diagram of the samples indicates that the source rocks of Fenghe River developed in a composite active continental margin and continental island arc field.