Recognition of tuffs in the middle-upper Dingqinghu Fm., Lunpola Basin, central Tibetan Plateau: Constraints on stratigraphic age and implications for paleoclimate

摘要

The Lunpola Basin plays a key role in determining the elevation history of the central Tibetan Plateau, which is closely related to uplift dynamics and paleoclimatic evolution. However, accurate age constraints on the basin stratigraphy reflecting tectonic, paleo-elevation and climate information have long been vigorously debated, mostly due to a lack of absolute age controls. Here, we found two altered tuff layers in the middle-upper Dingqinghu Formation (Fm.) in the basin stratigraphic sequence. Detailed lithological, mineralogical and geochemical analyses of the tuffs indicate that they came from trachyandesite and identify analcime as the typical alteration mineral, suggesting primary deposition. LA-ICP-MS zircon U-Pb dating of the tuffs yields concordant ages of 20.6 +/- 0.1 Ma and 20.7 +/- 0.1 Ma, indicating that the end of the Dingqinghu Fm., i.e., the Lunpola stratigraphic sequence, and the uplift of the plateau to its current high elevations occurred much later than previously estimated. The contemporaneous thick black oil shales in the Lunpola deep lake and biocoenosis together suggest that the climate was humid and warm during this period.