The U-Pb zircon dating of the granite dike in Nanzhao pluton and it's constraints on tectonic setting in Yanshanian

It is usually believed that plutons and dikes resulted from the same magmatic activity. Deeply evolved magma invaded into the pluton from cracks. Nanzhao pluton is located in the east of North Qinling Mountain, in which a 5m wide granite dike was discovered recently. The dike has purple, porphyritic texture and the phenocrysts are quartz.The U-Pb zircon LA-MC-ICP-MS dating shows the age of the dike is (119.1±0.8) Ma, which is quite different from the age of the pluton (452.3±6.2) Ma. The Hf isotope analysis shows that the source magma maybe a mixture of juvenile crust. This result and the geochemical study indicate that the granitic dike exhibits the characteristics of Ⅰ-A type granite. The similar age data, Hf isotope analysis and geochemical characteristics suggest that the dike in Nazhao pluton may have the same source magma as adjacent Funiu Mountain. According to the previous results and the analysis of regional geological background, the authors hold that the subduction of the Pacific plate occurred in the Yanshanian period, whereas the Yangtze plate and the North China plate experienced pull-apart along Qinling orogenic belt. A tectonic environment of the pull-apart was formed and, as a result, a series of faults were formed between the plate. After that, the granite magma invaded onto the surface along the faults. Most of the magma formed Funiu Mountain, while a little magma formed a dike in Nanzhao pluton, with the exchange of elements during the invasion. Therefore, the granite dike in Nanzhao pluton has tectonic implications.