The discovery of Qushenla Formation argillaceous cherts in the western part of the Bangong Co-Nujiang suture zone, Tibet and its significance

The Qushenla Formation argillaceous cherts, which occur along two horizons, were found for the first time in the western part of the Bangong Co-Nujiang suture. In order to discuss depositional environment and origin of argillaceous cherts and infer the closure of the western part of Bangong Co-Nujiang Tethys Ocean, the authors made a systematic analysis of petrology and geochemistry of argillaceous cherts in this study. The ratios of Al₂O₃/(Al₂O₃+Fe₂O₃), Ce/Ce^*, (La/Ce)_N, V/(Ni+V), Ce/La, Ce_anom, Eu_anom of argillaceous cherts in the first horizon are 0.60, 0.80, 1.24, 0.72, 1.84, -0.08, 0.01 respectively, while their ratios in the second horizon are 0.65, 0.83, 1.16, 0.77, 1.97, -0.07, 0.02 respectively. Considering the diagrams of Fe₂O₃/TiO₂ vs. Al₂O₃/(Al₂O₃+ Fe₂O₃), (La/Ce)_N vs. Al₂O₃/(Al₂O₃ + Fe₂O₃) and Hf/3-Th-Ta, the authors hold that these characteristics indicate that all of argillaceous cherts tended to occur in an active continental margin setting and an anoxic environment. The diagrams of U-Th, ZnNi-Co, La-Ce and La/Yb-REE and the characteristics of REE demonstrates that the argillaceous cherts of the two horizons originated from hydrothermal sediments, closely related to basalt magmatism event, and the activities of hot water might have been more pronounced in the second horizon. These data provide petrologic and geochemical evidence for the evolution of Bangong CoNujiang Tethys Ocean. In Shiquanhe area there might have existed a certain scale ocean basin in the Early Cretaceous, which was closed at a time later than 109Ma.