内蒙古阿拉善地区阿拉善群(狭义)的形成时代

Formation Age of Alxa Group-complex(Special)in Alxa Area, Inner Mongolia

阿拉善群（狭义）主要分布于内蒙古雅布赖山、巴彦乌拉山、阿拉坦敖包一带,是一套陆源碎屑-碳酸盐建造,岩石组合为黑云片岩、石榴黑云片岩、含十字石二云片岩、石英岩和大理岩。对其中的石英岩中的碎屑锆石进行LA-MC-ICP-MS测年,得到的年龄谱范围为1206~3132 Ma,以1400~1800 Ma占主体,峰值为1737 Ma,表明该样品中的碎屑锆石主要来源于中元古代早期形成的岩石,最小谐和年龄为1206 Ma,暗示阿拉善群石英岩的形成时代应小于1206 Ma。同时,测定出与阿拉善群呈侵入接触关系产出的眼球状黑云母二长花岗片麻岩的锆石U-Pb年龄为893 Ma。综上考虑,笔者认为阿拉腾敖包一带的阿拉善群应形成于1206~893 Ma之间,属中元古代晚期-新元古代早期的沉积建造。

The Alxa Group-complex（Special） is mainly located in Yabrai Hill, Bayan Urals and Altan Obo in In- ner Mongolia, which consists of biotite schist, garnet biotite schist, staurolite two-mica schist, quartzite and mar- ble. By LA-MC-ICP-MS U-Pb Dating, this study gets the age of 1206-3132 Ma from detrital zircons of the quartzite. The youngest ages （ - 1206 Ma） indicate that the formation age of the quartzite is younger than 1206 Ma. Simultaneously, the Zircon U-Pb age of augen biotite monzonite granite gneiss intruding into Alxa Group-complex is - 893 Ma. In summary, it is suggested that The Alxa Group-complex （Special） should form be- tween 1206 - 893 Ma, belonging to the late Proterozoic-Early Neoproterozoic sedimentary formation.