The Zircon U-Pb geochronology and dynamics mechanism for the formation of monzonitic granite in the Qiongbola area, south of Yili basin

Located in the south of Yili basin, the Western Tianshan Orogen belt played an essential provenance role in the formation of Yili basin. This paper focused on the dominant monzonitic granites of Qiongbola area, which is situated on the northern margin of Western Tianshan belt. The authors investigated their U-Pb geochronology by using zircon and geochemistry. These granites are Early Carboniferous intrusions, which mainly include three stages, which are (351.2±3.0) Ma, (339.7±2.2) Ma, and (330.5±2.2) Ma, respectively, according to the LA-ICP-MS U-Pb dating results of zircons. The whole-rock U-Pb dating of ore-bearing sandstone of 511 U deposit yielded an isochron age of 308±26Ma, suggesting that these granites were probably the main provenance for orebearing sandstone. The SiO₂ content of these granites varies from 70.15% to 73.38%, the K₂O+Na₂O values vary from 6.32% to 7.88%, and the A/CNK ratios vary from 0.82 to 1.03, suggesting that these intrusions are high-K calc-alkaline granites. The LREE values are from 50.19×10^-6 to 87.92×10^-6, suggesting relatively more enriched than HREE which are 9.44×10^-6 to 12.08×10^-6, with δEu being commonly 0.71 to 0.97. In addition, these granites are commonly rich in Rb, Th, K, Pb, and Sr and depleted in Nb, Ta, Zr, P, and Ti. The Sr isotope ratios range from 0.7050 to 0.7082, the ¹⁴³Nd/¹⁴⁴Nd ratios vary from 0.512217 to 0.512254, and the ε_Nd(t) vary from 0.3 to 1.0, with model ages being from 1010 to 1098 Ma. These geochemical results highly suggest that these intrusions were the mixture of mantle-derived basaltic and crustal molten magma, and were formed in the syn-collision tectonic environment.