湘南宁远地区碱性玄武岩形成时代的新证据:锆石LA-ICP-MS U-Pb 定年

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

    • 摘要: 采自宁远保安圩中心铺和李宅湘的碱性玄武岩中锆石的阴极发光图像显示出自形-半自形的晶形和均匀条带状吸收的特征。大部分锆石具有明显的板状环带,这是中基性岩中岩浆结晶锆石的典型结构。保安圩中心铺锆石的Th/ U 比值介于0.37~0.95 之间, 保安圩李宅湘锆石的Th/ U 比值介于0. 43~2.53 之间,表明它们都具有岩浆成因的特征。对保安圩中心铺玄武岩中20 粒锆石和保安圩李宅湘玄武岩中13 粒锆石进行了LA-ICP-MS U-Pb 年龄测定,结果显示,保安圩中心铺34个测定点的206Pb/238U 年龄主要集中在204~221Ma 之间,加权平均值为212.3Ma±1.7Ma ,保安圩李宅湘23个测定点的206Pb/238U年龄主要集中在196~226Ma之间,其加权平均值为205.5Ma±3.0Ma,表明保安圩玄武岩的结晶年龄为晚三叠世。

       

      Abstract: 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.

       

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