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The Xing’an-Inner Mongolia accretionary belt in the eastern Central Asian Orogenic Belt (CAOB) was produced by the subduction of three oceanic plates: the Paleo-Asian, Mongol-Okhotsk and Paleo-Pacific oceanic plates. The interactions between these plates remain unclear. Here we report an Alaskan-type ultramafic-mafic intrusion in the Kedanshan area, central Inner Mongolia, China. The main lithologies of this intrusion include cumulate dunite, pyroxene peridotite, olivine pyroxenite and cumulate gabbro, with late gabbroic/anorthositic dykes. Minerals and whole-rock compositional variations display characteristics of an arc cumulate trend (Alaskan type), through fractional crystallization of Mg-rich and hydrous basaltic magma associated with oceanic subduction. Zircons from two gabbro samples yield Early Jurassic ages of 193 +/- 6 Ma and 179 +/- 4 Ma, respectively. We conclude that this ultramafic-mafic complex is an accumulated intrusion from an arc-related, high-Mg magma chamber above a supra-subduction zone. Considering the ages, location and tectonic setting of the complex, we suggest that it was most likely generated by melting of a large and triangle-shaped mantle wedge during superimposed subduction between the Mongol-Okhotsk and the Paleo-Pacific oceanic plates in the Jurassic.