Zircon U-Pb ages, geochemical characteristics and petrogenesis of ringed pluton in the Hancaohu area, eastern Tianshan Mountains of Xinjiang

Choosing the Hancaohu intermediate-acid annular pluton distributed in East Tianshan Mountains of Xinjiang as the study object, the authors used LA-ICP-MS zircon U-Pb isotope chronology and geochemical characteristics to study the petrogenesis and geological significance of the pluton. The results indicate that the age of the earliest intrusive tonalite pluton is 275.0 ±2.9Ma (MSWD=4.8), suggesting that its emplacement epoch is Permian. The Al₂O₃ of the ringed pluton is 14.46%~17.05%, and A/CNK is 0.93~1.09, suggesting metaluminous and weak peraluminous series. The rocks are enriched in K₂O, and their MgO content is 0.71%~2.84% and Mg^# value is 33.3~48.6. Their trace elements are high in Sr and low in Y, the content of Sr is 217×10^-6~740×10^-6, that of Y is 4.26×10^-6~21.4×10^-6, and Sr/Y ratio is 16.87~145.07. They are enriched in LILEs(Rb, Sr, Ba) and depleted in HFSEs(Nb, Ta, Ti). They are characterized by LREE enrichment and HREE depletion, implying that they were formed in an area with stable garnet, rutile and unstable plagioclase, belonging to the transitional stage from amphibolite facies to eclogite facies; they were probably formed by underplating in the same period. The variation of main elements and race elements content and discontinuous LREE/HREE show that the pluton was not formed by crystallization evolution of one magmatic event, and the plutons of different lithologies are not separately crystallized from each other. Comprehensive analyses show that the lithospheric extension after Permian collision in Huangshan-Jingerquan area caused asthenosphere mantle upwelling and lithosphere delamination. The lithosphere vertical accretion at the boundary of the underplating crust and mantle induced the partial melting and produced high K calc-alkaline magma. With the instant intrusion of the magma, the difference of partial melting and newly formed mantle source components led to the formation of Hancaohu granitic pluton. It is shown that there was violent intermediate-acid magmatism in the Hancaohu area in Permian, and it was the response to tectonic-magmatism evolution in Permian in East Tianshan Mountains.