Geochemical characteristics, depositional environment, and provenance attitude of the Middle Jurassic Yangye Formation lacustrine mudstones in Kashi Sag, south-western Tarim Basin

Middle Jurassic mudstones from the Yangye Formation are widely exposed in the Kashi Sag and regarded as significant potential source rocks. Geochemical characteristics of mudstones in the Keziletao Depression have been analysed to decipher their source-area weathering, provenance, tectonic setting, and depositional environment that have received significantly little attention. Mudstones are characterized by high Ca, U, Th, and REE contents when compared to those of PAAS. Their total REE abundances range from 155.2 to 234.2 ppm, with the enrichment of LREEs and distinct negative Eu anomalies, implying a principal felsic provenance. Various indicators (e.g., CIA, Th/Sc, ICV, and Rb/Sr) and A-CN-K ternary diagram suggest parental rocks experienced a moderate to intense chemical weathering degree and simple recycling and sorting. Furthermore, paleoenvironmental indexes (e.g., B, "S," Sr/Ba, "C," Sr/Cu, and V/(V + Ni)) indicate that they were likely deposited in a suboxic to anoxic freshwater setting, with semi-arid to semi-humid paleoclimatic conditions during the deposition of Yangye Formation. The geochemical data also demonstrate that the provenances were derived predominantly from the unroofing of felsic rocks with a minor intermediate supply. These parental rocks were commonly formed in the continental island arc, active continental margin, and/or collision tectonic settings. The results of this study, combined with published data, suggest the West Kunlun Orogen and thrusting zone to the west and south-west of the basin were probably arrested to provide substantial detritus for sediments, stimulating a better understanding for the formation mechanism of the Yangye Formation sediments (especially for potential source rocks) in the Kashi Sag.