The assembly of Pangea is usually thought to be finally completed by the accretion of the Siberian continent to Laurussia, but the exact timing of the accretion is poorly constrained in global paleogeographic reconstructions, which lagged behind the Kazakhstan-Laurussian amalgamation in the latest Mississippian and was possibly coeval with, or after, the amalgamation of the Tarim and Kazakhstan-Junggar continents in the Pennsylvanian. In this respect, the geological features, such as the medium-pressure facies series metamorphism in the Irtysh Complex, NW China, from the junction between the Siberian and Kazakhstan-Junggar continents are expected to provide crucial constraints on the timing of the accretion. The metamorphic rocks in the Irtysh Complex record the high-pressure granulite-facies metamorphism with the assemblage of kyanite + K-feldspar + garnet and show a complete clockwise metamorphic P-T-t path, involving rapid burial to the peak P-T conditions of ∼11 Kbar and 800 °C at ∼300 Ma and complicated exhumation to the surface during the Permian. The Pennsylvanian (∼322–300 Ma) rapid burial was coeval with the collision of Siberian and Kazakhstan-Junggar continents, whereas the Permian exhumation was triggered by post-collisional extension and sinistral shearing and can be divided into three substages: isothermal decompression at ∼300 Ma, near-isobaric cooling (∼300–268 Ma), and decompression-cooling to the surface prior to ∼253 Ma. The high-pressure granulite-facies metamorphism suggests that the Siberian continent was amalgamated with the Kazakhstan-Junggar continent in the Pennsylvanian, almost coeval with the amalgamation of the Tarim and Kazakhstan-Junggar continents. Therefore, the assembly of Pangea was completed in the latest Carboniferous at least.