Objective: Mount Langshan is the western section of Yinshan Mountain in the southern Inner Mongolia, north of China. Yinshan Mountain, being treated as a typical intraplate orogenic belt, has been regarded as the northern margin within the North China Craton by former geologists. However, our new discovery in Mount Langshan has attributed it to collisional orogeny. Our study indicates that Yinshan Mountain should be the Late Cretacous collision margin between North China Craton (NCC) and Central Asian Orogenic Belt (CAOB).Methods: We found the outcrop of ophiolitic mélanges in Mount Langshan, west of Inner Mongolia, in a geologic expedition in 2015. To confirm our discovery we conducted more field investigation in 2016 and collected ample rock samples for laboratory tests. We examined the textures and mineral compositions of major ophiolitic mélanges through microscopic petrology, analyzed the chemical compositions (major oxides and trace elements) of key rock types, dated the geological times of important igneous rocks through UPb zircon ages.Results: Geological fieldwork reveals that there are a series of thrust sheets in the southern flank of Mount Langshan. All the thrust sheets incline northward, and the ophiolitic mélanges are enclosed within one thrust sheet. The high Aluminum basalts and related igneous rocks (Al2O3>17%) indicate that the major ophiolitic mélanges were originated in the subduction setting, representing the relics of the oceanic crust. The youngest ages of basalts and diabases imply their cooling time in the Late Cretaceous.Discussion and Conclusion: The authors of this article found the outcrops of ophiolitic mélanges in the southern piedmont of Mount Langshan, western part of Inner Mongolia. Our study reveals the following achievements: The ophiolitic mélanges are enclosed in a large thrust sheet of more than 500m in width and 3km in length. The thrust sheet extended in east—west direction and inclined northward, and there are many thrust sheets of other rock types connected to the south with the same inclination toward north. The ophiolitic mélanges are composed of two parts: matrix and tectonic blocks. The matrix is mainly chlorite schists and metamorphic basalts in dark gray to green colors. The tectonic rock blocks include red cherts, quartzkeratophyres, metadiabases, basalt and metamorphic gabbros etc. Geochemical analysisand UPb zircon dating show that chlorite schists, basalts, and metadiabases should have been originated from a high aluminum basaltic arc magma closely related to subduction setting during the Late Cretaceous. The ophiolitic mélangesare the remains of the fossil oceanic crust connected to the North China Craton trapped in the orogenic belt during the northward subduction of Ordos Block beneath the southern margin of Central Asian Orogenic Belt. Thus, the ophiolitic mélanges imply the latest collision between CAOB and NCC in the Late Cretaceous. Mount Langshan is the western section of Yinshan Mountain.According to the similarity principle of tectonic landforms that similar landforms are the results of similar tectonic events, we can infer that the whole scope of Yinshan Mountain and Yanshan Mountain should be the result of the same tectonic event——the latest collision between NCC and CAOB during the Late Cretaceous. We suggest that the northern margin of NCC should be removed from Bayan Obo—Xar Moron River to Yinshan—Yanshan Mountains. Because we have found the Cretaceous ophiolitic mélanges in Mount Langshan (the western section of Yinshan Mountain), Yinshan Mountain is no longer the “intraplate orogenic belt” but a collisional orogenic belt between the North China Craton and the southern margin of Central Asian Orogenic Belt in which its southern margin, Yinshan Mountain, was uplifted by the end of the Late Cretaceous. The discovery of ophiolitic mélanges can also provide new tectonic regime clues for the exploration of Cu—Pb—Zn sulfide minerals in the Mount Langshan and the other similar places alongYinshan Mountain, Yanshan Mountain and even the Great Khingan Ranges. Furthermore, our researchalso suggests that the tectonic process of North China Plate during the whole Mesozoic time might not be a process of “North China Craton breakup” but a series of collision events between many small terranes in northern Asia and Siberia.