This paper provided petrographic, zircon U- Pb age and geochemical characteristics of intermediate—basic complex in the upper reaches of Pangu River in the northern Great Hinggan Mountains. The formation age, genesis and geological implications are discussed.Methods:Based on the field work, through the microscopic observation, the whole rock chemical analysis of different rock types and the LA- ICP- MS zircon U- Pb isotopic chronology of hornblende gabbro.Results:The rock types of the composite pluton are mainly hornblende gabbro and gabbro, rare quartz diorite, diorite, pyroxenite, olivine gabbro and other intermediate rocks or ultrabasic rocks. The dating results show that the formation age of the hornblende gabbro is 479.2±2.1 Ma, formed in the Early Ordovician. The intermediate—basic composite pluton in the upper reaches of the Pangu River have SiO2 contents ranging from 40.74% to 54.95%, Fe2O3T contents ranging from 3.99% to 14.66%, MgO contents ranging from 3.36% to 17.78%, Mg# values ranging from 47 to 79, which are (high potassium) calc- alkaline series rocks, relatively enriched in light rare earth elements and depleted in heavy rare earth elements with δEu from 0.77 to 2.06. They are relatively enriched in large ion lithophile and radioactive elements such as Rb, Ba, U, K, Pb and Sr, and depleted in high field strength elements such as Nb, Ta and Ti, which is similar to the geochemical characteristics of arc magmatic rocks in subduction zone.Conclusions:The magma was originated from partial melting of mantle dominated by subduction fluid metasomatism supplemented by sediment melt metasomatism. The source area is dominated by spinel bearing garnet lherzolite, and a small amount of spinel lherzolite, which experienced different degrees of crystallization differentiation in the rising process. Comprehensive analysis shows that the Early Paleozoic intermediate—basic complex in the upper reaches of Pangu River was formed in the tectonic environment of continental extensional zone (or initial rift), which is probably related to the lithosphere extension background after the collision between the Siberian plate and the Erguna massif.