Located in the compound part of the EW-trending Paleozoic Central Asia-Mongolia orogenic belt and the eastern margin of the NNE-trending Asian continent orogenic belt of littoral West Pacific, the southwestern slope of the Da Hinggan Mountains in Inner Mongolia is a newly discovered nonferrous metallogenic belt with ideal mineralization potential. The unmetamorphosed magmatic rocks emplaced at the middle-late stage of Neopaleozoic are widely distributed in this area, and the lithologic association includes not only intermediate-acidic to alkaline rocks but also bimodal volcanic rocks. Although there exists great difference in their emplacement ages, they were all formed later than the metamorphic time of the Xilin Gol complex and the peak orogenic stage of the eastern segment of the Central Asia-Mongolia orogenic belt. Spatially, most of these magmatic rocks, with A-type or bimodal properties in geochemistry, are related to the extensional tectonic setting after the closure of the main suture. In this study, the authors acquired zircon SHRIMP U-Pb data of late Variscan magmatic rocks outcropped in southeastern Inner Mongolia, tectonically situated at the area between Erlian-Hegen suture and Solon-Xarmoron suture, with the purpose of making constraint on the closure time of the main suture between the Siberian plate and the Sino-Korean plate. Zircon SHRIMP U-Pb analyses yield emplacement ages of 310±2 and 311±2 Ma for quartz-diorite and diorite from the Weilasituo silver-polymetallic deposit, 319±3 Ma for granodiorite from the Bairendaba deposit, and 300±5 Ma for dacitic tuff from the Daolundaba deposit, indicating that they all belong to late Carboniferous. The emplacement ages of these magmatic rocks are about 20～40 Ma later than the metamorphic age (337±6 Ma) of the Xilin Gol complex formed at the peak stage of orogeny. These magmatic rocks were not affected by deformation and metamorphism, implying that they belong to late-/post-orogenic magmatic rocks. The authors hold that, although the Central Asia-Mongolia orogenic belt is characterized by poly-island arc accretions and poly-sutures, the orogenic process must have undergone many times of collision and convergence between palaeo-ocean crust and palaeocontinent, island arc and island arc, island arc and continent, palaeo-ocean crust and micro-continental block, micro-continental block and micro-continental block, and micro-continental block and palaeocontinent, and the main suture between the Siberian plate and the Sino-Korean plate is situated near the Erenhot-Hegenshan belt.