Provenance of the Upper Cretaceous-Eocene Deep-Water Sandstones in Sangdanlin, Southern Tibet: Constraints on the Timing of Initial India-Asia Collision

The first arrival of sedimentary material from Asia onto the Indian continental margin provides a minimum constraint on the timing of initial India-Asia collision. A combination of petrology, detrital Cr-spinel geochemistry, and zircon U-Pb dating of the Upper Cretaceous to Eocene deep-water succession at the Sangdanlin section, southern Tibet, provides evidence for major provenance change within the strata. The Upper Cretaceous-Paleocene Denggang Formation quartzarenites contain zircons with dominant Proterozoic-Ordovician U-Pb ages, with an additional age peak of Early Cretaceous, which we interpret to be derived from the northern Indian margin. By contrast, the lithic sandstones of the Early to Middle Eocene Sangdanlin and Zheya formations are dominated by zircons younger than 200 Ma, showing one major peak at similar to 80-125 Ma and two subdominant peaks at similar to 54-70 and similar to 180-196 Ma, comparable to those from the Gangdese magmatic arc. Cr-spinels in the Sangdanlin and Zheya formations are abundant and characterized by extremely low TiO2 wt%, indicating an ophiolitic source. We consider the Sangdanlin and Zheya formations syncollisional, deposited in a foredeep basin, with the provenance being the Gangdese arc and the Yarlung Zangbo suture zone. The abrupt sedimentary provenance change between the Denggang and Sangdanlin formations denotes the onset of India-Asia continental collision occurring before the late Ypresian (similar to 50 Ma). Comparison of our data with those from coeval strata along the Himalaya suggests limited diachroneity in the India-Asia continental collision process.