Abstract: Petrography, SHRIMP U-Pb chronology of the zircon and geochemistry of the Hercynian biotite monzonitic granites in Deersi area of Urad Zhongqi are studied to constrain the genesis of the rock and regional tectonic evolution in the study area. The emplacement of the biotite monzonitic granite in this area can be divided into two periods, the earlier is medium-coarse prophyritic biotite monzonitic granite and the latter is medium-fine grain biotite monzonitic granite. Most of zircons from biotite monzonitic granites show core-mantle ring structure, fine-scale oscillatory zoning and Th/U ratios of 0.16-0.50, implying their magmatic origin. Whereas most of cores of rouded zircons have fine-scale oscillatory zoning and Th/U ratios of 0.06-0.44, which indicates their magmatic genesis and residual zircons. While, there are some zircons with weak zoning or non-zoning, indicating their metamorphic genesis. The weighted mean ages of the edge of zircons from the biotite monzonitic granites indicate that the granite formed in the EarlyMiddle Permian ((279±3) Ma and (266±3) Ma), but some results of residual zircons are (1 972±63) Ma，(1 962±43) Ma, implying the formation time of source rock. The biotite monzonitic granites belong to sub-alkaline series. Their rare earth elements (REE) patterns are right diping with LREE/HREE ratios range from 5.86 to 22.81. The biotite monzonitic granites are depleted in high field strength element Nb, and enriched in large ion lithophile elements Rb and Ba. These characters are similar to the igneous rocks from active continental margin setting. Taken all together, geochemical character of the Early-Middle Permian biotite monzonitic granites reflects the tectonic evolution including the subduction of the paleo-Asian oceanic plate beneath the North China craton and the dying out of paleo-Asian Ocean.

Key words: I-type granite, Early-Middle Permian, geochemistry, active continental margin, Urad Zhongqi