The accretionary history of the Central Asian Orogenic Belt (CAOB) is critical for understanding the assembly of Pangea. However, the timing of final amalgamation among the blocks in the eastern CAOB remains contentious, leading to uncertainty regarding their integration into Pangea. This study presents a detailed investigation of the Fudiyingzi ultramafic-mafic complex (FUMC), located in the northern segment of the Hegenshan-Heihe suture (HHS). The FUMC comprises plagioclase-bearing lherzolite, troctolite, and gabbro and crystallized between 299 Ma and 277 Ma. Integrated mineral chemistry, zircon Hf isotope, whole-rock geochemistry, and Sr-Nd-Pb isotopic data indicate that the parental magma underwent fractional crystallization and was derived from a depleted asthenospheric mantle source metasomatized by subduction-related fluid. The FUMC does not represent an Alaskan-type, ophiolitic, or stratiform ultramafic-mafic complex; instead, it shows strong affinities with ultramafic-mafic complexes formed in a postcollisional extensional setting. This interpretation constrains the timing of HHS formation to the latest Carboniferous (ca. 300 Ma). A comparative analysis of Paleozoic magmatism, sedimentary strata, and ophiolites across the Xing’an (XAT), Songliao (SAT), and Zhangguangcai (ZGCAT) accretionary terranes indicates that the XAT and ZGCAT constituted a coherent accretionary belt along the margin of the Erguna-Bureya-Jiamusi-Khanka block chain. This belt was bent into a southward-opened orocline, likely due to subduction of the Mongolia-Okhotsk Ocean and Panthalassa Ocean since the Late Carboniferous. Furthermore, we propose that the Hegenshan-Nenjiang Ocean and Longfengshan Ocean originally formed a unified oceanic domain, which experienced early Paleozoic single-directional subduction, late Paleozoic bidirectional subduction, and final closure by the latest Carboniferous. This closure records the final amalgamation of microcontinental blocks in the eastern CAOB.