The southwestern margin of Alxa massif in northern China, one of the major provinces of mafic-ultramafi rocks, can be divided into Beidashan, Longshoushan and Beihaizi rock belts. Generally speaking, the Beidashan rock belt is represented by Yejili feldspar-bearing pyroxene peridotite, whose Sm-Nd rock-mineral isochron age is 773.1±10.8 Ma,εNd(t) values are +0.8～+10.8, and ΣREE are 7.59×10-6～20.34×10-6. The Longshoushan rock belt is represented by Jinchuan lherzolite and Zangbutai pyroxene peridotite, whose Sm-Nd rock-mineral isochron ages are 1 508±31 Ma and 1 511±168 Ma respectively,implying mantle products formed almost at the same time. Jinchuan intrusion has ΣREE of 3.33×10-6～115.52×10×10-6 and εNd(t) of -1.9～-4.3, but Zangbutai has εNd(t) of +2.6～+2.9, showing obvious differences. The Beihaizi rock belt is spread limitedly, whose gabbro Sm_Nd rock-mineral isochron age is 1 440±220 Ma,εNd(t)>0, and ΣREE are 179.64×10-6～207.76×10-6 . The Longshoushan and Beihaizi rock belts were both probably formed by the early Middle Proterozoic magma derived from the lower mantle due to the action of superplume. Their εNd(t) difference indicates that every rock body has its unique petrogenetic process. Yeliji rock body is the product of Later Proterozoic depleted mantle, whose formation environment and origin are remarkably different from those of Jinchuan rock body.