Late Eocene to early Oligocene geochemical characteristics and paleoclimatic significance of the second member of Niubao Formation in the Lunpola Basin，Tibet

The Lunpola Basin, which is located in the hinterlands of the Tibetan Plateau, is the most sensitive territory for uplifting processes and related responses of environmental changes.It not only records the continental collision process and the deformation history of the lithosphere and crust, but it is also considered the most preferred study site to investigate the paleotopography, paleogeomorphology, and paleoclimatic evolution. In order to reveal the late Eocene to early Oligocene paleoclimate in the Lunpola Basin, a total of 67 rock samples from the second member of the Niubao Formation at 382 Daoban section in the southwest margin of the Lunpola Basin were selected. Based on major and trace elements, a variety of chemical weathering ratios and indices, including the combination of elements (C value), Rb/Sr, Sr/Cu, Sr/Ba, chemical index of alteration (CIA), elemental weathering index (α), and Ln(Al₂O₃/Na₂O) were used to assess the intensity of weathering regimes and paleoclimatic evolution in the source area. The results showed that arid paleoclimate was prevalent during deposition of the second member in the Niubao Formation. However, there was an apparent cooling event at the boundary transition of the Eocene-Oligocene (EOT), which was not only confined to the Lunpola basin but also widely documented in the Tibetan Plateau and other regions along the circumferential margin. This Eocene-Oligocene cooling event is considered a well-developed continental response to the first global Cenozoic cooling event in the Lunpola lake basin.