The closure time of the Bangong Co-Nujiang Tethys Ocean (BNTO) between Lhasa terrane and Qiangtang terrane directly constrains the early tectonic evolution of the Tibetan Plateau. Recently, alkaline volcanic rocks have been recognized from the Kaimeng ophiolitic mélange, a southern sector of Bangong Co-Nujiang suture zone. These rocks are mainly fresh mugearite with trachytic structure. The phenocrysts are mainly oligoclase with minor augite, and the groundmass is mainly composed of oligoclase, pyroxene and trace alkaline feldspar filling other minerals. The chemical compositions of the mugearite are fairly uniform, with the abundances of SiO₂ and TiO₂ ranging from 51.34% to 53.91% and from 1.02% to 1.55%, respectively, suggesting alkaline series volcanic rocks with high Na₂O content (4.90%~6.36%), low K₂O content (0.05%~0.88%) and intermediat Rittman index (σ=3.65~4.47). The most of Mg^# values are higher than 60, even up to 68.62. The trace elements of the mugearite are characterized by relative enrichment of large ion lithophile elements (LILE) Sr, Rb, Ba, Th and U and depletion of high field strength elements (HFSE), with negative Nb and Ta anomalies. The stable Nb/U, Zr/Nb and La/Yb ratios (7.45~8.51, 15.92~17.26 and 7.26~8.06, respectively), the initial ⁸⁷Sr/⁸⁶Sr ratios(0.706~0.707)and the (¹⁴³Nd/¹⁴⁴Nd)_t ratios (0.512 368~0.512 548) as well as the Ce/Pb-SiO₂ diagram suggest that the source of the mugearite was probably mixed primitive mantle (PM) and lower continental crust and abyssal sediments. The zircon U-Pb isotopic dating shows that the age of the mugearite is 101.8±1.1 Ma, indicating that it was likely produced by the partial melting of asthenospheric mantle upwelled by the slab break-off at the end of oceanic crust subduction, which implies that the BNTO was closed at the late stage of early Cretaceous period.