Petrogenesis and Tectonic Significance of Late Yanshanian Granites in Yunkai Area, Southeast China: Evidence from Zircon U-Pb Ages and Hf Isotopes

Compared with abundant Precambrian and Early Paleozoic geological records in the Yunkai area, southeast China, Late Yanshanian granitoids are sparsely distributed in the area and are thus reported less. In this study, we present LA-MC-ICP-MS U-Pb age and Hf isotope data of zircons from three representative granites in the Yunkai area, southeastern Guangxi, each from the Michang area, Sanchachong W and Songwang W-Sn-Mo deposits respectively. Zircon U-Pb results indicate that all the granites were formed in Late Yanshanian period, with emplacement ages of 113±1 Ma (Michang biotite granite), 103±1 Ma (Sanchachong biotite granite) and 88±1 Ma (Songwang granite) respectively. The Sanchachong biotite granite and Songwang granite have consistent zircon ε_Hf(t) values (-5.2 to -2.7 and -5.2 to -3.6 respectively) and two-stage Hf modal ages (T_DM2) (1.3-1.5 Ga and 1.4-1.5 Ga respectively), indicating that they were formed by reworking of Mesoproterozoic juvenile crust; whereas the Michang biotite granite has relatively higher zircon ε_Hf(t) values of -2.3 to 1.4 and corresponding T_DM2 of 1.1-1.3 Ga, which, combined with widespread microgranular mafic enclaves (MMEs) in the granite, suggests that it was formed by mixing of crust- and mantle-derived magmas. In combination of Mesozoic tectono-magmatic evolution in southeast China, the Late Yanshanian magmatism in the Yunkai area was likely related to the rollback and break-off of the subducted Pacific plate during the Cretaceous.