Formation epoch and origin of the Yizhang andesite, diabase and granite-porphyry in Yizhang County, southern Hu′nan Province: Zircon U-Pb age and Hf isotopes

Early Yanshanian (Jurassic) rock masses dominated by andesite, diabase and granite-porphyry are widely exposed in Yizhang County, Hunan Province. However, the formation age and petrogenesis of these rock masses remain controversial. The U-Pb ages and Hf isotopes of zircons have been discussed in this paper by using in situ zircon analysis of the SHRIMP and LA-ICP-MS. Zircon SHRIMP ages for Changchengling diabase are 153.7Ma±3.1Ma and 227.0Ma±4.2Ma; LA-ICP-MS ages for Changchengling granite-porphyry are 153Ma±14Ma and 231.58Ma±0.67Ma. SHRIMP and LA-ICP-MS dating of Pinghe andesite yielded ages of 159Ma±14Ma and 229.3Ma±7.6Ma as well as 161Ma±1Ma, which are consistent with the major formation time of the "Nanling-series" rock bodies in neighboring areas, suggesting Indo-Chinese underplating of mantle-source mafic magma and emplacement of early Yanshanian magma. The εHf(t) values vary from slightly positive to negative, suggesting that the rock masses were derived at least from two sources instead of being formed by the magmas derived from a single source. Based on several age data of inherited zircons from granites in the neighboring Nanling Range, it can be concluded that the granitites were produced mainly by remelting of the Mesoproterozoic basement rocks. They were probably derived from partial melting of the Proterozoic igneous protoliths with minor addition of the juvenile crust or newly mantle-derived magmas, accompanied by assimilation and fractional crystallization.