内蒙古宝音图岩群变质基性火山岩锆石U-Pb年龄及意义

U-Pb Single Zircon Ages of Metamorphic Basic Volcanic Rocks of Baoyintu Rock Group in Inner Mongolia

内蒙古达茂旗宝音图岩群是在达茂旗、查干呼绍地区 1:5万区调和白云鄂博幅 1:2 5万区调过程中从过去的巴特敖包群和白云鄂博群未分岩组解体出来的中级变质岩系。通过对宝音图岩群中阳起钠长片岩、二云片岩 (原岩均为基性火山岩 )进行单颗粒锆石U -Pb法年龄测定 ,获得了两条不一致线上交点年龄 (2 486± 42 )Ma、(2 496± 2 6 )Ma ,代表了宝音图岩群基性火山岩及宝音图岩群的形成时代 ;在不一致线之外不同成因的锆石的2 0 7Pb/ 2 0 6Pb表面年龄 (2 2 2 7± 15 )Ma ,寓示着 2 2 0 0Ma左右发生了一次区域性变质热事件 ;下交点年龄 (319±486 )Ma和 (40 6± 46 )Ma ,代表了 2 5 0～ 480Ma时期该区大面积岩浆侵位及宝音图岩群后期变形变质改造的时期 ,两个年龄样的获得为研究古元古代古陆的裂解及白云鄂博海槽的形成提供了直接依据。

Baoyintu Rock Group, loucated in Damaoqi Conty and Chaganhushao district, is of a middle-grade metamorphic rock series which was separated from Bate'aobao and Baiyun'ebe Groups during the geological survey of 1:50000 and 1:250000. Resently, two U-Pb ages have been got by texting the single zircons from the actinolite-albite-schist and the two-mica-schist(the protolith of both schists are basic vocanic rocks) in Guanintu Rock Group. They are up-intercept ages, one is (2 486±42) Ma, the other one is (2 496±26) Ma, suggesting the forming age of the basic vocanic rocks and Baoyintu Rock Group. The 207Pb/ 206Pb apparent age of a zircon which isn't on the concordia line is (2 227±15) Ma, implying there was a regional metamorphic event at that time. The two down-intercept ages are (319±486) Ma and (406±46) Ma, maybe presenting the extensive magma intruding and the reformingof Baoyintu Rock Group then.