Geochemical characteristics of Late Triassic sandstones in the western part of Bayan Har Basin, Northern Tibetan Plateau, Western China: Constraints on provenance, source weathering, tectonic setting, and palaeoenvironment

Abstract

The Bayan Har Group in Quanshuigou area of the northern Tibetan Plateau is regionally subdivided into three litho‐stratigraphic units, namely, the upper, middle, and lower formations. In this study, we report geochemical data for the fine‐grained sandstones from the upper formation aiming to constrain their source area weathering, provenance, palaeoenvironment, and tectonic setting. The fine‐grained sandstones exhibit similar contents of SiO2 and Al2O3 relative to upper continental crust (UCC) and Post‐Archean Australian Shale and classified as wacke on log (SiO2/Al2O3)–log (Fe2O3/K2O) diagram. The trace elements are characterized by the depletion of the transition elements compared with the UCC. The chondrite‐normalized rare earth element (REE) patterns demonstrate enrichment of light REE (LREE) and depletion of heavy REE (HREE) with LREE/HREE varying from 7.99 to 10.28, as well as distinctive negative Eu anomalies (δEu = 0.63–0.73). The high index of compositional variability values indicates the fine‐grained sandstones are compositionally immature and first cycled. The low chemical index of alteration and plagioclase index of alteration values of the samples indicate the weak source weathering condition. Additionally, the characteristics of A–CN–K plot reveal that K‐metasomatism occurred after burial. A series of palaeoenvironment indexes imply that the Bayan Har Group was deposited in a oxic/suboxic marine–terrestrial transitional environment. On the basis of the newly established DF1–DF2 discrimination functions as well as other tectonic setting discrimination plots, we speculate that the Bayan Har Basin was a back‐arc basin during the Late Triassic.