Petrogenesis of the Gneissic Granite in Xiaosaishiteng Mountain, Northern Qaidam： Constraint from Geochemistry, Zircon U-Pb Geochronology and Hf Isotopes

The Xiaosaishiteng Moutain is located in the Northwest Qaidam tectonic belt. A integrated study of petrography, zircon U-Pb chronology, geochemistry and zircon Hf isotopes in gneissic granite of the Xiaosaishiteng Moutain were conducted in this paper. The LA-ICP-MS ziron U-Pb data results indicate that the gneissic granites, which have high SiO₂(70.16%～72.43%), total alkali(Na₂O+K₂O=7.20%～9.04%) with K₂O/Na₂O＞1, and low MgO(0.82%～1.29%), P₂O₅(0.09%～0.13%) and TiO₂(0.3%～0.49%), were formed in the late Silurian(422.1±1.6)Ma. Their A/CNK ratios are larger than 1, the Ritman index mainly are less than 3.3, therefore these gneissic granites are belong to peraluminous and high-K calc-alkaline series. They have relatively high REEs contents, enrichment in light REE with depletion in heavy REE, and exhibit weak negative Eu anomalies(δEu=0.73～0.96). The chondrite normalized REE patterns show dipped-right pattern, the primitive mantle Normalized trace elements pider diagram have the similar pattern. The gneissic granites strongly enriched in LILE and depletion of HFSE setting, such as Nb and Ti. The zircon saturation temperature is 721～725℃, the average is 739℃, belongs to I-type granite. The results of zircon Hf isotope shows that the ε_Hf(t) of gneissic granite are negative(-6.47～12.85), and the crustal model age(2030Ma) indicates that the source rocks of the Xiaosaishiteng moutain gneissic granite was mixed with the granitic magma and the ancient crustal material of the Middle Proterozoic. The granite was formed when fluid was added, which from the remelting of crust. Comprehensive analysis shows that the granites was formed in the post-collision environment.