Abstract: The Xiaozhangjiakou mafic-ultramafic complex, north Hebei, is composed mainly of pyroxenites and subordinate dunite. Lenses of harzburgites can be seen in the complex. Pertological and geochemical data suggest a cumulate origin for the pyroxenites and dunites, while harzburgites represent mantle xenoliths (mantle residues after extraction of basaltic melts). The pyroxenites and dunites show significantly enriched Nd-Sr isotopic compositions, with εNd (t )=-2~-10 and I Sr=0.7045~0.7081,suggesting that their parental magma was derived from partial melting of enriched mantle portions. Geochemical and isotopic data consistently demonstrate that variable proportions of lower continental crustal components were incorporated in the maficultramafic complex during the processes of olivine- and pyroxene-dominated fractional crystallization. The pyroxenites experienced the most significant crustal contamination, and cannot be representative of genuine isotopic features of the mantle source. The dunites, however, was the first cumulate crystallized from magma and was contaminated only slightly by crustal components.Therefore, the Nd isotopic composition (εNd(t )=-1.9) of the dunites could be close to that of the mantle source. Formation of theXiozhangjiakou complex was probably related to the closure of the Ancient Asian Ocean at about 240~250 Ma.