Element geochemical characteristics, provenance attributes, and paleosedimentary environment of the Paleogene strata in the Lenghu area, northwestern Qaidam Basin

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The Lenghu area is a key exploration block in the northwestern Qaidam Basin. Due to the low exploration level, the provenance and paleosedimentary environment of the Paleogene strata is not clear. Based on the major and trace element contents and the carbon and oxygen isotopes of 547 samples collected from the Paleogene strata (E32, E31, and E1+2), this study investigated the provenance attributes and paleosedimentary environment of the entire Paleogene period. The samples from the Paleogene strata have relatively low contents of SiO2 (39.75%–61.81%) and Al2O3 (6.23%–20.48%) and high contents of La, Sc, and Co (223.07 ppm, 273.84 ppm, and 325.45, respectively). Moreover, the La–Th-Sc, Ti/Zr–La/Sc and SiO2–K2O/Na2O discrimination diagrams suggest that the tectonic environment of the provenance is an oceanic island arc. The Co/Th–La/Sc and Zr/TiO2-(V + Ni + Cr)/Al2O3 diagrams indicate that the provenance lithology of the study area is mafic rocks with a high degree of weathering. These results imply that the source rocks of the Paleogene strata are the mafic submarine volcanic rocks of the Tanjianshan group in the Saishiteng Mountains. The Sr/Cu, Sr/Rb, and Mg/Ca indexes indicate that the paleosedimentary environment had a humid climate with a trend of slight aridity. The Sr/Ba and Rb/K ratios and the isotope coefficient Z (calculated by carbon and oxygen isotopes) indicate that the paleosalinity of the water body was predominantly fresh water with a salty trend from the Early to the Late Paleogene. The paleoredox environment during the Paleogene sedimentation was relatively oxidized with a reduction trend based on the V/(V + Ni) and V/Cr values. This combined with the trend of increasing paleosalinity suggests that there was a relatively large lake transgression in the Late Paleogene.