Abstract: The nearly EW-trending mafic dyke swarms are widely distributed in South Xizang, and the layered pyroxenite occurs in the gabbro.Two weighted average ages of gabbro and pyroxenite were obtained by SHRIMP zircon U-Pb dating, i.e., (131.71±2.7)Ma(2σ, MSDW=2.8) and(135.0±2.4)Ma(2σ, MSDW=2.1) respectively.The crystallization of the ultramafic rocks took place 4 Ma earlier than that of the mafic ones.Element geochemical study shows that, in gabbro, SiO₂ is 50.85%, TiO₂ is 1.45%, MgO is10.05%, TFe₂O₃ is 8.57%, and Mg^# is 71.33, whereas in pyroxenite, SiO₂ is 48.28%, TiO₂ is 2.02%, MgO is 18.68%, TFe₂O₃ is13.08%, and Mg^# is 78.76.Their K₂O content is high(0.68 and 0.81), Eu loss is not obvious, Nb, Ta and other elements are depleted, and the patterns of REE distribution are similar to those of OIB.The formation of the mafic-ultramafic rocks was related to the mantle thermal events occurring in the same period, and might have been contaminated by crustal materials in the rising process.