Zircon U-Pb age, geochemical characteristics and geo-logical significance of granitoids in the Maozangsi deposit, Northern Qilian Mountain

The Maozangsi Cu-Mo deposit in Gansu is a porphyry type deposit related to granitoids. Maozangsi granitoids are com-posed of porphyritoid monzogranite and granodiorite. In this paper, zircon U-Pb dating and geochemical study of the Maozangsi granitoids were conducted to constrain its geochronology and discuss petrogenesis and its relationship with mineralization. Zircon LA-ICP-MS dating yielded concordant ages of 455.8±3.1Ma and 425.0±2.8Ma respectively, indicating that the two plutons were formed in Late Ordovician and late Silurian respectively. Geochemical data show that porphyritoid monzogranite is a peraluminous granite and belongs to the calc-alkaline series, whereas granodiorite is a aluminous granite and belongs to the high-K calc-alkaline series. They are characterized by enrichment of LILEs and depletion of HFSEs, with REE patterns exhibiting the right-deviation type and strong fractionation with LREE enrichment. The porphyritoid monzogranite show weak positive Eu anomalies (δEu=1.18~1.24) and geochemical affinity to adakite, probably resulting from the slab melting (including marine sediments) of the subduction of North Qilian Ocean with the residual minerals of garnet in the source. The granodiorite shows weak negative Eu anomalies and was generated in a post-collisional extension setting and derived from partial melting of basaltic rocks in the lower crust induced by asthe-nosphere after the breakup of previously subducted North Qilian oceanic slab. The features of porphyritoid monzogranite are in ac-cordance with the characteristics of ore-forming period adakitic rocks and thus suggest good mineralization conditions. In combina-tion with data obtained from previous studies, the authors hold that it is possible to find Cu-Mo-Au deposits related to adakites in the eastern section of the Northern Qilian Mountain.