Zircon U–Pb Dating and Geochemical Features of the Jiaozishan Granite in the South Margin of the North China Craton and Its Tectonic Implications

The Jiaozishan batholith was a Yanshanian intrusive rock with the second largest area in the stretching branch of the Funiu Mountains, eastern of the Qinling orogenic belts. Its petrogenetic model was significant to understand the regular pattern of the early Cretaceous acid magma and deep tectonic evolution in the stretching branch of the Funiu Mountains. Among the total 30 zircon spots from dating sample JZS07 by LA–ICP–MS dating method, the U–Pb isotopic results of 18 zircon spots were available, and they were divided into two groups with (129.5±1.4) Ma (1 spot) and (116.1±1.4)～(122.2±1.3) Ma (17 spots), which had formed the zircon age spectrum. The weighted mean age of the second group was (119.3±0.9) Ma. Combined with the previous zircon U–Pb age data of 10 available zircon spots, the weighted average age of 27 zircons was (118.2±1.2) Ma, which represented the formation age of the Jiaozishan granite. The samples of the Jiaozishan granite were characterized by higher SiO₂ and alkali, and lower MgO and CaO, and compositional spots fell into high–K calc-alkaline series in the SiO₂–K₂O diagram. The positive anomalies of Rb, Th, Zr and Hf, and negative anomalies of Sr, P and Ti were shown in spider diagram. Their total REEs contents were from 20.9×10⁻⁶ to 204×10⁻⁶ with (La/Yb)_N ratios of 4.24～21.0. They had shown the characteristics of LREE–enrichment, MREE and HREE depletion with Eu negative anomalies (δEu values were 0.53～0.78). In the La_N– (La/Yb)_N diagram, the sample points of the Jiaozishan granite had shown a positive correlation, indicating that the compositional variation was controlled by partial melting of source rock. The residual phases of its partial melting source included mainly hornblende, plagioclase and rutile, and no garnet. The Jiaozishan granite was formed in the crust with normal thickness, and there are two partial melting sources with depths of roughly 40～50 km and less than 30 km, respectively. The Jiaozishan granite experienced magma/fluid mixing and reactivation, and its formation tectonic setting was intraplate orogenic environment. The Jiaozishan granite was one of the products of the lithosphere delamination in～120 Ma.