四川冕宁牦牛坪碱性花岗岩锆石铀-铅同位素年龄及其地质意义

U-Pb ISOTOPIC AGE OF ZIRCON FROM THE MAONIUPING ALKALI GRANITE,SICHUAN PROVINCE AND ITS GEOLOGICAL SIGNIFICANCE

对四川冕宁牦牛坪富稀土碱性花岗岩锆石的铀-铅同位素年龄研究,得出一致线上交点年龄为(249±43)×10~6a,下交点年龄为(12±2)×10~6a。前者相当于岩体中残余锆石的形成年龄,可能代表碱性花岗岩母体岩石的年龄,后者应是碱性花岗岩及新生锆石的形成年龄。(12±2)×10~6a的年龄值是攀西裂谷带内,甚至整个扬子准地台内最年轻的花岗岩年龄值。新的喜马拉雅期成岩成矿年龄数据的得出,对于认识区域构造-岩浆活动历史,了解稀土元素矿床的时间-空间分布,有重要意义。

A study of the U-Pb isotopic age of zircon from the Maoniuning alkali granite, Mianning County, Sichuan Province, shows that the age of upper concordia interept(T_1)is 249±43 Ma and the age of lower concordia intercept (T) is 12±2 Ma. The former is the crystallization age of residual zircon in the granite, probably representing the formation age of the mother rock of the alkali granite, whereas the latter should be considered as the formation age of the alkali granite and the neocrystic zircon. 12±2 Ma is the youngest age for granitoids in the Panxi rift zone and even in the whole Yangtze Paraplatform. This age is of great significance in the understanding of regional tectonomagmatic activities and time-space distribution of the REE ore deposits in the region.