New chronology of the Ningyuan alkali basalt in southern Hunan, China: Evidence from LA-ICP-MS zircon U-Pb dating.

The zircons from Ningyuan alkali basalt show euhedral-subhedral shapes and striped absorption on their cathodoluminescence (CL) images. Most zircons ring with a clear plate, which is the typical structure of basic igneous rocks zircon. The Th/U ratios of Baoanxu Zhongxinpu zircons range from 0. 37 to 0.95 , and Baoanxu Lizhaixiang zircons range from 0. 43 to 2.53, indicating that they were formed by normal crystallization of magma. U-Pb dating of 20 zircons from Baoanxu Zhongxinpu , and Baoanxu Lizhaixiang was made on LA-ICP-MS. The results show that the data of Zhongxinpu 34 zircons cluster around the concordia of 204～221 Ma , yielding a weighted mean 206Pb/238U age of 212.3 Ma±1.7 Ma , and that the data of Lizhaixiang 23 zircons yield a weighted mean 206Pb/238U age of 205.5 Ma±3.0 Ma , which implied that the crystallization of the Ningyuan alkali basalt occurred in Late Triassic.