Timing of closure of the Meso-Tethys Ocean: Constraints from remnants of a 141−135 Ma ocean island within the Bangong−Nujiang Suture Zone, Tibetan Plateau

Knowledge of the timing of the closure of the Meso-Tethys Ocean as represented by the Bangong−Nujiang Suture Zone, i.e., the timing of the Lhasa-Qiangtang collision, is critical for understanding the Mesozoic tectonics of the Tibetan Plateau. But this timing is hotly debated; existing suggestions vary from the Middle Jurassic (ca. 166 Ma) to Late Cretaceous (ca. 100 Ma). In this study, we describe the petrology of the Zhonggang igneous−sedimentary rocks in the middle segment of the Bangong−Nujiang Suture Zone and present results of zircon U−Pb geochronology, whole-rock geochemistry, and Sr−Nd isotope analysis of the Zhonggang igneous rocks. The Zhonggang igneous−sedimentary rocks have a thick basaltic basement (> 2 km thick) covered by limestone with interbedded basalt and tuff, trachyandesite, chert, and poorly sorted conglomerate comprising limestone and basalt debris. There is an absence of terrigenous detritus (e.g., quartz) within the sedimentary and pyroclastic rocks. These observations, together with the typical exotic blocks-in-matrix structure between the Zhonggang igneous−sedimentary rocks and the surrounding flysch deposits, lead to the conclusion that the Zhonggang igneous−sedimentary rocks are remnants of an ocean island within the Meso-Tethys Ocean. This conclusion is consistent with the ocean island basalt-type geochemistry of the Zhonggang basalts and trachyandesites, which are enriched in light rare earth elements (La_N/Yb_N = 4.72−18.1 and 5.61−13.7, respectively) and have positive Nb−Ta anomalies (Nb_PM/Th_PM > 1, Ta_PM/U_PM > 1), low initial ⁸⁷Sr/⁸⁶Sr ratios (0.703992−0.705428), and positive mantle ε_Nd(t) values (3.88−5.99). Zircon U−Pb dates indicate that the Zhonggang ocean island formed at 141−135 Ma; therefore, closure of the Meso-Tethys Ocean and collision of the Lhasa and Qiangtang terranes must have happened after ca. 135 Ma.