The high-Mg diorites occur in the Shibanjing area of central Beishan orogenic belt and are dated at 456±2 Ma by the LA-ICP-MS zircon U-Pb method, which indicates that they were generated in the Late Ordovician. According to geochemical data, the high-Mg diorites belong to the sodic calc-alkaline series, the content of SiO₂ is 51.94%~55.30%. They are geochemically characterized by high MgO (4.12%~6.19%, avg. 5.17%), Mg^# (0.45~0.69, avg. 0.53), Cr (35.82×10^-6~985.50×10^-6, avg. 284.84×10^-6), and Ni (20.93×10^-6~148.59×10^-6, avg. 70.23×10^-6), and also exhibit nearly flat right-declined REE patterns with slight enrichment of LREE. The high-Mg diorites are strongly enriched in large-on-lithophile elements such as Rb, Ba, Th, U and K, and are characterized by depletion of high-field-strength elements such as Nb, Ta, P and Ti. These geochemical features are analogous to those of Cenozoic Setouchi volcanic belt in southwestern Japan. Petrogenesis analysis shows that the rock mass was probably formed by the interaction between the aqueous fluid produced by partial melting of subducted oceanic crust and sediments and the mantle peridotite, suggesting that it was formed in an island arc environment. It is thus concluded that the Hongliuhe-Niuquanzi-Xichangjing ancient ocean plate was presumably in the process of subduction during the late Ordovician, and the collision between Kazakhstan plate and the Tarim plate might have occurred after Ordovician.