Zircon U-Pb ages of Fengdie and Sangrika granitic plutons in West Qinling Orogen and their geological significance

In this paper, the authors petrographically studied both Fengdie and Sanrika granitic plutons in Zhouqu County and Xiahe County of West Qinling Orogen and determined their zircon U-P ages. These two plutons which used to be recognized as Yanshanian (Middle-Late Mesozoic) intrusions yielded zircon U-Pb ages of 201.3±0.9Ma and 232.6±2.2Ma, respectively, suggesting that they should be products of the Indosinian (Early Mesozoic) magmatism and thus clarifying their timing in certain geological maps (1:250000). Combining previous age data with the authors' study, the authors argue that there is basically no Yanshanian granite exposed in West Qinling Orogen and the majority of granites of West Qinling Mountain are Indosinian in age and spatially distributed in the northern part of West Qinling Orogen. Therefore, West Qinling can be tectonically compared with South Qinling tectonic unit of East Qinling, and the former is the western extension of the latter. It can be seen from both the age and the geochemical data that Indosinian granites of West Qinling and South Qinling were derived from the similar sources and possessed characteristics of the basement of the Yangtz block. The reason for lacking Yanshanian granite in West Qinling can be attributed to obvious difference of the tectonic location between West Qinling and East Qinling, especially the Xiaoqinling where Yanshanian granites were well developed. The formation of Yanshanina granites in the Xiaoqinling was probably related to the destruction of the North China cratonic lithosphere or to the intracontinental subduction of the North China block.