Diagenetic Epoch and Geochemical Characteristics of Mafic-ultramafic Rocks on the Southern Margin of Alashan Massive in North China

Abstract: The Southwestern margin of Alashan massive in North China is one of the major distributive provinces of mafic-ultramafi rocks, which is divided into Beidashan, Longshoushan and Beihaizi rock zones. Generally, Beidashan rock zone is representative of Yejili feldsprar-bearing pyroxene peridotite, Sm-Nd rock-mineral isochron date is 77.3 ±10.8Ma.εNd(t)=+0.8~10.8, and ΣREE is 7.59~20.34×10-6; Longshoushan rock zone is representative of Jinchuan lherzolite and Zangbutai pyroxene peridotite, of which Sm-Nd rock-mineral isochron date, in turn, are 1508±31Ma and 1511±168Ma, and almost are the products of mantle origin on same time. Jinchuan intrusionΣREE is 3033~11.52×10-6, but theεNd(t)=-1.9~-4.3, ZangbutaiεNd(t)=+2.6~+2.9, difference appear directly; Beihaizi rock zone spread limited, and the gabbro Sm-Nd rock-minera isochron date is 1440 ± 220Ma, εNd(t)>0, ΣREE 179064~207.76×10-6. Longshoushan and Beihaizi rock zones all probably belong to the products of the early stage of Middle Proterozoic magma induced from lower mantle because of the funcation of superplume. Their εNd(t) difference indicates that every rock body has respectively diagenetic process. Yeliji rock body is the product of Late