Characteristics and tectonic environment of Mesoproterozoic granitoid rocks in western section of northern margin of the North China Craton.

The Mesoproterozoic granitoid rocks in western section of northern margin o f the North China craton are only distributed in Yabulai-Bayinlergong Tecto nic Belt in the south of the Caganchulu-Houershen Tectonic Belt, being cont rolled generally by NEE striking long-living linear structure, and can be c lassified into two types by rock facies. plagiogranite and quartz diorite. Plagiogranite is S-type granite, having higher Cr, Ni, SiO2 and Al2O3 of 67 2 mug/g, 405 mug/g, 71.40% and 14.93% respectively on the average and large variation of delta Eu (0.34 similar to 3.95), with some character of I-typ e. These characteristics of the rocks mean that they originated from mixing of mantle-genesis magma and crust-genesis material. Quartz diorite is typi cal I-type granite, having higher Cr, Ni contents, of 220 mug/g and 162 mug /g respectively on the average, and similar to co-melting granites of South ern China. The upper intercept of U-Pb zircon isochron dating yielded an ag e of 1256Ma for plagiogranite representing an important thermal-tectonic ev ent in western section of northern margin of North China craton. Accompanie d by this thermal-tectonic event, there were extending and spreading in the area, and developed into the evolving stage of extensional type of transit ional crust. Mesoproterozoic granitoid rocks were the important products of the thermal-tectonic event. It is speculated that the crust of the Caganch ulu-Huoershen Tectonic Belt were thinned greatly during that event. There w as no magmatism of the era in the Zhunleishan-Shalazhashan Tectonic Belt, n orth of the linear structure, so it is called nonv-olcanic passive continen tal margin. Because there were granitiod rocks, mafic, ultramafic intrusion rocks, and volcanic-sedimental assemblage of Mesoproterozoic in the Yabula i-Bayinlergong Tectonic Belt, the belt is called volcanic passive continent al margin. Both of the margins constituted the passive continental margin " couple".