新疆阿尔泰额尔齐斯构造带片麻岩的锆石U-Pb SHRIMP定年及其地质意义

SHRIMP U-Pb Ages of Zircon in the Gneiss of Erqisi Tectonic Belt in Altay,Xinjiang,and Their Geological Significances

对包括额尔齐斯构造带中片麻岩在内的新疆阿尔泰地区中深变质岩石的时代一直存在争议,李天德等将阿尔泰山造山带中大部分中深变质岩划分为古—中元古界克木齐群和新元古代富蕴群,引起了广泛的关注和争议,特别是对位于额尔齐斯构造带中片麻岩的时代问题争议颇多。笔者对该片麻岩采样进行主量元素、微量元素分析及锆石U-PbSHRIMP年龄测定,表明该处片麻岩以正变质岩为主,成分与闪长质、石英闪长质岩石相当,为后碰撞阶段岩浆活动的产物。锆石U-Pb SHRIMP定年结果表明其锆石主要为岩浆锆石,^206Pb/^238U年龄主要为350～295 Ma,其中10个点构成的谐和年龄为（326±6）Ma,为石炭纪构造—岩浆活动的产物。该片麻岩中没有前寒武纪古老锆石的年龄信息,基本上可以否定该片麻岩形成于前寒武纪的观点,应为晚古生代的产物。结合前人研究成果,认为阿尔泰造山带中深变质岩可能为不同时代的产物。

It is a long debate on the age of the middle-and high-metamorphic rocks of Erqisi tectonic belt in Altay,Xinjiang.The middle-and high-metamorphic rocks have been regarded as the Kemuqi Group,Lower-and Middle-Proterozoic erathem,and the Fuyun Group,Upper-Proterozoic,yet other research disagree.The age of the gneiss in Erqisi tectonic belt especially follows much interesting.According to the studies of the petrography,major-and trace-element chemistry,and zircon SHRIMP U-Pb ages of the gneiss in this paper,we suggest that the gneiss is similar to diorite and/or quartz-diorite in chemical compositions,and should be ortho-gneiss and the magma products in post-collision.Zircon U-Pb ages range from 350 Ma to 295 Ma,and ten spots yield a concordant mean U-Pb age of（326±6）Ma,indicating that they are Carboniferous intrusions.Furthermore,no Precambrian zircon has been documented in our measurements,which strongly argue against that the gneiss in Erqisi is Precambrian.Combining previous studies and our own data,we suggest that the middle-and high-metamorphic rocks in Altay could be of different ages.