广东新丰稀土花岗岩中褐帘石LA-ICP-MS的U-Th-Pb定年研究

LA-ICP-MS Allanite U-Th-Pb Geochronology Study on Guangdong Xinfeng REE-Rich Granite

褐帘石是一种常见的副矿物，是岩石中LREE、Th、U和sr等微量元素的重要载体。由于Th、U可以在褐帘石中以类质同象形式进入并富集于其晶格，因而褐帘石是一种良好的年代学研究对象。但是，由于褐帘石中含有不同含量的普通铅，分析结果中的普通铅扣除成为难题，这就使得近年来褐帘石U—Th～Pb定年的应用受到很大的限制。本文通过激光熔样一电感耦合等离子质谱（LA-ICP—MS）原位分析技术，对广东新丰稀土花岗岩中褐帘石单颗粒矿物进行了原位分析，运用232Th／206Pbc-208Pb／205Pbc等时线（206Pbc为普通206Pb的含量）方法，再根据等时线年龄利用铅同位索演化两阶段模型扣除普通铅的方式，对建立单颗粒褐帘石的U—Th_Pb激光定年方法进行了探讨。运用该方法对广东新丰稀土花岗岩中单颗粒褐帘石进行分析得出的年龄约为160Ma，与早期通过岩浆锆石U—Pb定年法得出的年龄（159～165Ma）十分相似。褐帘石的微量元素、8Eu、Th／U比值和普通铅含量均显示其为特征的岩浆成因褐帘石。褐帘石定年这一定年方法具有简单、方便、准确的特点，对于确定含此矿物的岩体年龄，尤其是对成矿花岗岩的岩体形成时代将发挥重大作用，具有非常良好和广泛的应用前景。

Allanite is a common accessory mineral, which is an important carrier of LREE, Th, U, Sr and other trace elements in rocks. Th, U can be enriched in allanite lattice in the form of isomorphism, so allanite is a good geochronological tool. However, the variable incorporation of common lead in allanite makes it difficult to subtract the common Pb,which limits the application of allanite U-Th-Pb dating. In this paper, procedures have been developed to analyze the allanite from Guangdong Xinfeng rare earth element granite by laser ablation ICP-MS. 232 Th/206 pbc_208 pb/206 Pbc isochron（206 Pbc is common206 Pb） assumes age, and then common Pb correction is used according to the lead isotopic evolution of the two-stage mode. Using this method, the allanite Th-Pb age of Guangdong Xinfeng rare earth-rich granite has been dated to be of 160Ma,which is very similar to 159Ma -165Ma as dated by magmatic zircon U-Pb dating method. Trace elements,SEu, Th/U ratio and common lead content of allanite indicate the magmatic origin. Allanite dating is the first time used in China. The dating method is simple, convenient and accurate for determination the age of the rocks containing allanite. The method may play an important role in geochronology,and has a very good and broad application prospect.