Identification and Tectonic Significance of Ocean Island-Seamount Systems in the Middle Segment of the Yarlung Zangbo Suture Zone

Abstract

Mesozoic exotic tectonic fragments in the Yarlung Zangbo suture zone (YZSZ) preserve key records for reconstructing Neo-Tethyan evolution. This study investigates basalts and limestones from the Buma area in the central segment through petrology, calcite U-Pb geochronology, and whole‑rock geochemistry. Calcite U-Pb dating of two recrystallized limestone samples yields ages of ca. 244 Ma and ca. 246 Ma. The Buma basalts are subalkaline, with SiO₂ contents of 37.95–53.61 wt.% and high TiO₂ (1.63–3.43 wt.%). Their rare earth elements (REEs) and trace element patterns resemble Hawaiian ocean island basalts (OIBs), and they exhibit moderately depleted Nd isotopic compositions (ε_Nd(t) = +4.20 to +4.93). The pronounced negative Ce anomalies in limestones and the absence of terrigenous clastic components in collapse conglomerates indicate deposition in an open-ocean setting. Integrated evidence indicates that the basalt–limestone association in the Buma area represents a Neo-Tethyan ocean island-seamount system. Combined with the 253 Ma continental rift basalts from the Zhongba area, this result suggests that the opening of the Neo-Tethys Ocean in the central segment of the YZSZ occurred no later than the Early Triassic, providing key empirical evidence for the spatiotemporal framework of the early evolution of the Neo-Tethys Ocean.