Zircon U-Pb ages of alkaline dykes from the Qinling Group in the Tongbai area, central China: Constraints on the timing of granulite-facies metamorphism

Since the age data obtained for metamorphic zircons from high-grade rocks in the Qinling Group, the Tongbai area, are scattered, there has been a debate on the timing of granulite-facies metamorphism. In this paper a weakly deformed and undeformed alkaline dykes intruded in the Qinling Group were dated using LA-MC-ICPMS zircon U-Pb dating method. The emplacement ages obtained for these two dykes are 429. 9 +/- 1. 5Ma and 430. 3 +/- 1. 3Ma, respectively. Coupled with the published geochronological data in the area, we infer that granulite-facies metamorphism in the Qinling Group took place at ca. 440 similar to 430Ma, which is triggered by the arc-continent collision in the southern margin of the North China Block. Post-collisional arc magmatism occurred mainly in ca. 430Ma. This widespread intrusive event may have resulted in the slow cooling of the Qinling Group, and therefore caused scattered age data of granulite-facies metamorphic zircons.