新疆天格尔糜棱岩化花岗岩的岩石学及其SHRIMP年代学研究：兼论花岗岩中热液锆石边的定年

Petrology and SHRIMP chronology of mylonitized Tianger granite,Xinjiang:Also about the dating on hydrothermal zircon rim in granite.

出露在新疆中天山天格尔山脉一带的片麻状花岗岩是早志留世花岗岩体被糜棱岩化的结果。呈NWW向展布的天格尔韧性剪切带切割天格尔花岗岩体。本文沿乌库公路完成的实测地质剖面、糜棱岩化花岗岩岩石学以及锆石SHRIMP年代学研究结果表明,糜棱岩化花岗岩中的锆石具有复杂的核-幔-边结构,锆石核形成于470～1788 Ma年前,岩浆锆石幔的SHRIMP表观年龄主要集中在432～453 Ma。糜棱岩化过程中流体交代花岗岩中的岩浆锆石,形成了热液锆石边,这种热液锆石在CL图像中呈暗一深灰色。花岗岩岩体侵位时代依据岩浆锆石幔的SHRIMP测定为441．6±3．8 Ma(MSWD=1．06)。热液锆石边的SHRIMP年龄相对年轻,6个分析结果得到一个加权平均年龄为408．6±1．7 Ma(有可能代表热液锆石边与岩浆锆石幔的混合年龄)。因此,天格尔一带出露的糜棱岩化花岗岩是早志留世早期岩浆侵位的产物,并不是中天山基底的组成部分。本文报道的锆石数据还表明,花岗质岩石在韧性剪切变形过程中能够结晶出热液锆石。对这种热液锆石的深入研究为限定韧性剪切变形的时间提供了一种新的手段。

The gneissic granite occurring along the Tianger mountain （Bingdaban） of the Central Tianshan Mountains was formed during the mylonitization process of the Early Silurian granite. The NWW-trending Tianger shear zone cut through the Tianger granite. This so-called gneissic granite was believed to be a component of the Precambrian basement of the Yili-Central Tianshan terrane. Here we present a geological section through the Tianger shear zone along Wuku road, petrography of the mylonitized granite and its zircon SHRIMP geochronology data. Zircons in the mylonitized granite show complex core-mantle-rim texture. The zircon cores have apparent SHRIMP ages of 470 - 1788Ma. The zircon mantles with clear magmatic zonation show apparent SHRIMP ages of 432 - 453Ma. The hydrothermal zircon rims with dark cathodoluminescence images （non-luminescent） were formed during the mylonitization by fluid replacement of the magmatic zircon mantles at a late period probably 〈421Ma based on SHRIMP dating results. The Tianger granite probably intruded during 441.6 ± 3.8Ma （ MSWD = 1.06） based on the SHRIMP data on the zircon mantles. The hydrothermal zircon rim was dated at 408.6 ± 1.7Ma. The formation of the hydrothermal zircon rim probably was related with the mylonitizing process in the studied Early Silurian granite.