Geochronology of Youshan and Pingtian Granites in Nanling Range and Its Geological Implication

In previous studies,the Youshan granite and Pingtian granite located to the east of the Zhuguangshan composite granite body in Nanling Range were considered being formed in the Yanshanian period.In this study,we present LA-ICP-MS U-Pb zircon dating results which shows that the ages of the Youshan granite and the northwestern part of the Pingtian granite are(213.4±3.0)Ma and(238.8±2.2)Ma,respectively.These results indicate that the Youshan granite was formed by the Indosinian magmatism and the Pingtian granite is probably a composite granite batholith formed in the Indosinian and Yanshanian periods.In addition,it is revealed that both granites contain zircons having inherited cores with ages of Archean and 541 Ma~1 642 Ma,implying existence of Archean crust as well as Proterozoic crust containing rocks formed by several periods of magmatism in the source.Some of the zircons from the Youshan granite display dark rims(in CL images)with Yanshanian ages,which suggests that the granite was probably affected by hydrothermal fluids during the Yanshanian period.Narrow dark rims of zircons are also observed in the Pingtian granite,suggesting effect of hydrothermal fluids.