Geochronological and Geochemical Characteristics of the Zhazigou Adakite in South Qilian and the Geological Significance

Zhazigou batholith, which is composed of quartz diorite and granodiorite, is located in Danghenanshan of South Qilian.The LA-ICP-MS and zircon U-Pb age of the granodiorite is (450±12)Ma (MSWD=0.23, n=24), and the emplacement age is late Ordovician. The major elements show that SiO₂ is 58.46%~69.14%, Al₂O₃ is 13.70%~16.6%, and MgO is 1.43%~4.31%, belonging to High-K calc-alkaline quasi-aluminous granite. Trace elements are rich in light rare earth and large ion lithophile elements such as Rb, Ba, Th, U, Sr, and depleted in heavy rare earth and high field strength elements such as Nb, Ta, Ti, P. The content of Sr is 130.0×10^-6~460.0×10^-6 with an average content of 310.93×10^-6; the content of Yb is 0.7×10^-6~1.81×10^-6 with an average content of 1.26×10^-6. The content of Y is 6.44×10^-6~17.68×10^-6 with an average content of 12.96×10^-6, weak to positive Eu anomaly (δEu=0.77~1.55) and higher Sr/Y ratio(20.2~37.1), having adakite characteristics geochemically. Combined with the regional geological background, it is believed that Zhazigou batholith formed in the continental collision environment, which was related to the close of south Qilian ocean in the Late Ordovician.The collision of the Oulongbuluke block and the middle Qilian block caused the thickening of the crust and the delamination. Then the hot mantle material upwelled leading to the partial melting of the lower crust.