The determination of red beds in Lajiu area of Lhorong, Tibet and its its constraints on the evolution of the Eastern Bangong-Nujiang Ocean

The study on the Bangong-Nujiang suture zone in eastern Tibet has been relatively weak. In this paper, we take the purple-red terrestrial clastic rocks exposed intermittently in Lajiu area of Lhorong, Tibet on the south side of the Eastern Bangong-Nujiang suture zone as the object of study, and reports the studies on the sedimentary environment and depositional age of the purple-red terrestrial clastic rocks. We find that it is mainly composed of purple-red and reddish-brown polymict conglomerate interbedded with pebbly sandstone, which indicates that the formation is near source rapid accumulation with gravity-flow depositional characteristics. Indicate that the stratigraphic units in this study are mainly deposited in alluvial-fan, and a small amount is deposited in braided-river. The U-Pb dating results of detrital zircons in the study object show that the latest age is 109.6 ± 1.8 Ma, and the zircon U-Pb age of the diorite-porphyrite vein intruded into the research object is ~70 Ma. So, the stratigraphic age of this study is the Late Cretaceous, and its deposition time is not earlier than 109.6 Ma and not later than 70 Ma. Therefore, we believe that the the continental red beds previously classified as the Eocene Zongbai Group (E₂Z) in the Lajiu area of Luolong should be determined as the Upper Cretaceous Jingzhushan Formation (K₂j), and the Eastern Bangong-Nujiang Ocean around Lhorong to Baxoi, Tibet was completely closed and entered the stage of continental basin evolution during the late Cretaceous.