Geochemistry of the Eocene clastic sediments (Suonahu Formation) in the North Qiangtang Basin, Tibet: implications for paleoclimate conditions, provenance and tectonic setting

Abstract

The geochemistry of the clastic sedimentary units of the Suonahu Formation from the QD17 well in the northern Qiangtang basin, Tibet, was studied using various chemical analyses. SiO2/Al2O3 ratios indicated that compositional maturity and recycling of the sediments were low to moderate. The total rare earth element (∑REE) contents of the clastic sediments ranged from 29.28 to 191.92 ppm. Element abundances suggest that the studied clastic sediments in the northern Qiangtang Basin were mainly sourced from felsic rocks mixed with small amounts of intermediate rocks, and that they were mostly developed in a continental setting. The REE geochemistry of the clastic sediments suggests that these different lithological samples were derived from a similar terrigenous source and that the Eu anomaly was inherited from the source rocks. The paleoclimate index (C-value) varied from 0.01 to 0.36, reflecting generally arid to semiarid conditions. In addition, Rb/Sr (about 0.41) and Sr/Cu (about 37.02) ratios support the idea that arid conditions prevailed during the deposition of the Suonahu Formation. Sr/Ba ratios (0.48–3.23) suggest a paleoenvironment with variable salinity. The covariation among this factor and paleoclimate indicators suggests that variations in climatic conditions exerted a primary control on salinity. The chemical index of alteration (CIA), A–CN–K ternary diagram, and low Th/U ratios indicate that the parent rocks of the clastic sediments experienced weak chemical weathering. Two multidimensional tectonic discrimination diagrams based on major elements show that the Suonahu Formation was deposited in a rift-related basin. The TiO2 versus Zr, La/Th versus Hf, and Co/Th versus La/Sc bivariate diagrams and multi-major elements discrimination diagram indicate that the detritus was primarily derived from felsic igneous rocks with less contribution from intermediate igneous rocks.