Abstract: The high-Mg diorite newly discovered in Sumdo area in the eastern part of the Gangdise magmatic belt is of importance for investigating the evolution history of the subduction of Neo-Tethyan oceanic slab of Tibet. This paper reports LA-ICP-MS zircon U-Pb ages, zircon Hf isotopic and whole-rock major and trace element compositions of the high-Mg diorite. LA-ICP-MS zircon U-Pb isotopic dating indicates that the diorite was emplaced at 186.8±1.3Ma. It is characterized by high content of SiO₂ (54.17%~58.81%), MgO (6.26%~7.29%), Mg^# (64~67), Cr (294×10^-6~373×10^-6) and Ni (42×10^-6~50×10^-6). It is also characterized by smooth gently right-inclined REE patterns with slight enrichment of LREE and a little negative Eu anomaly. The large ion lithophile elements (LILEs) such as Rb, Ba, Pb, K are enriched, and the high field strength elements (HFSEs) such as Nb, Ta, Ti are depleted. LuHf isotopic study demonstrates that its ε_Hf(t) values vary between -5 and -14.3, and the ancient zircon Hf crustal modal ages vary greatly from 1543Ma to 2129Ma, which indicates that the formation of Sumdo diorites resulted from partial melting of ancient crustal rocks. The discrimination diagrams of Sr/Y-Y and (La/Yb)_N-Yb_N show that the high-Mg diorites were likely formed in a mantle wedge setting similar to the magmatic process of sanukitie. The comprehensive studies show that the Sumdo area was located in an active continental margin setting and affected by the subduction of the Yarlung Zangbo Neo-Tethyan oceanic slab towards Eurasia plate before Early Jurassic.