Oligocene-Miocene (28–13 Ma) climato-tectonic evolution of the northeastern Qinghai-Tibetan Plateau evidenced by mineralogical and geochemical records of the Xunhua Basin
Abstract Changes in sediment composition in the Xunhua Basin provide insights into the climatic and tectonic evolution of the northeastern Qinghai-Tibetan Plateau during the Oligocene and Miocene. Here, we analyzed the clay mineralogy (i.e., the proportions of smectite, illite, and chlorite and clay indicators), bulk mineral composition (i.e., the amounts of calcite, quartz, gypsum, K-feldspar, plagioclase, dolomite, and halite), and trace-element indicators (i.e., Rb/Sr and Ba/Sr ratios) of sediments deposited from ~28 to 13 Ma in the Xunhua Basin to reveal the climate and/or provenance changes. The results suggest that significant changes in sediment provenance occurred at 25.1 Ma and 21.6 Ma, caused respectively by initial uplift and accelerated uplift of the Laji Shan. The Xunhua Basin experienced warm and humid conditions at 28.0–25.1 Ma, cool and dry conditions at 25.1–21.6 Ma, and somewhat warmer and wetter conditions at 21.6–19.2 Ma, followed by aridification in two steps at 19.2 Ma and 13.9 Ma. The climate cooling event at 25.1 Ma corresponds temporally to a major phase of uplift of central Tibet. Relatively warmer and wetter conditions during the early Miocene (21.6–19.2 Ma) are considered to record the influence of the Asian summer monsoon and accelerated uplift of the Laji Shan. The shift toward more arid conditions on the northeastern Qinghai-Tibetan Plateau at 19.2–13.9 Ma was probably driven by uplift of the Tibetan Plateau, and further aridification since 13.9 Ma may reflect a weakening of the East Asian summer monsoon and global climatic cooling.