The evolution of the Bangong–Nujiang Neo-Tethys ocean: Evidence from zircon U–Pb and Lu–Hf isotopic analyses of Early Cretaceous oceanic islands and ophiolites

• The Bangong–Nujiang Neo-Tethys ocean opened initially in the late Permian (254–269 Ma).
• The Bangong–Nujiang Neo-Tethys ocean continued to open extensively between the late Permian and Early Triassic (247–255 Ma).
• The subduction of the Bangong–Nujiang Neo-Tethys ocean started during the Middle Jurassic (about 162–168 Ma).
• The Bangong–Nujiang Neo-Tethys ocean did not close completely until after the late Early Cretaceous (117–120 Ma).

We conducted in situ U–Pb analyses of zircons from three basalts and one gabbro from the Zhonggang oceanic island, one basalt from the Zhaga oceanic island, and one gabbro from the Kangqiong ophiolite (all located in the middle segments of the Bangong–Nujiang suture zone of Tibetan Plateau), as well as in situ Hf isotope analyses of zircons from one gabbro from the Zhonggang oceanic island to constrain the tectonic evolution of the Bangong–Nujiang Neo-Tethys ocean. All samples contain numerous inherited zircons, and all the zircons contain magmatic oscillatory zoning and have Th/U ratios exceeding 0.4. Moreover, the average ΣREE content of these zircons is less than 2000 ppm, and they display clear negative Eu and variable positive Ce anomalies, indicating a magmatic origin. LA–ICP–MS U–Pb dating of the zircons revealed three clear peaks in the age distribution, at 248–255, 162–168, and 117–120 Ma; Lu–Hf isotopic analyses of zircons from the gabbro of the Zhonggang oceanic island yielded a 269 Ma crust–mantle separation age. Taking into account the regional geology, previous data, and our new analyses, we infer that the middle and western segments of the Bangong–Nujiang Neo-Tethys ocean had initially opened in the late Permian (254–269 Ma) and that the ocean opened substantially between the late Permian and the Early Triassic (248–255 Ma). In addition, we infer that the initiation of subduction of the Bangong–Nujiang Neo-Tethys ocean took place at ~ 162–168 Ma, which is Middle Jurassic. The 117–120 Ma age is the time when the oceanic islands and ophiolites were formed, indicating that the Bangong–Nujiang Neo-Tethys ocean was, to some extent, still open at that time.