内蒙古北山地区北山岩群斜长角闪岩LA-ICP-MS锆石U-Pb定年及其地质意义

内蒙古北山地区标山一带北山岩群出露相对齐全,基于区域地质、岩石学及地球化学等特征研究,在北山岩群中解体出斜长角闪岩,其原岩为辉长岩、辉绿岩等基性岩脉。采用LA-ICP-MS方法对斜长角闪岩样品开展锆石U-Pb同位素测年分析,获得U-Pb加权平均年龄为1623±21Ma,207Pb/206Pb表面年龄为1547~1691Ma。样品中锆石多为半自形双锥柱状,粒径介于0.03~0.21mm,发育较明显的振荡环带,具有核-幔-边结构,30个锆石分析点Th/U比值为0.38~0.92,绝大多数大于0.4,表明这些锆石为岩浆成因而非变质成因锆石。因此,这些锆石的同位素年龄为斜长角闪岩的结晶年龄,从而说明北山岩群的成岩年龄应不晚于1623±21Ma。综合研究表明,斜长角闪岩形成于古元古代大陆扩张环境,这些认识对北山岩群的时代归属及其北山地区构造发展演化历史的研究均具有重要意义。

LA-ICP-MS zircon U-Pb dating of amphibolites of the Beishan group in the Beishan area, Inner Mongolia and its geological significance

The exposure of Beishan group in the Biaoshan area of Beishan, Inner Mongolia is relatively complete. According to the regional geology, petrological and geochemical characteristics, amphibolites from the Beishan group were separated in this work, with protolith of basic dykes of gabbro and diabase. We used LA-ICP-MS method to conduct zircon U-Pb isotopic dating on the amphibolites to obtain weighted average age of 1623 ± 21 Ma and the apparent 207 Pb/ 206 Pb ages of 1547~1691 Ma. The zircons are mostly hypautomorphic dipyramidal-prismatic crystals, with grain size of 0. 03 ~0. 21 mm and showing oscillatory zoning, and obvious core-mantle-rim textures. The Th/U ratios are 0. 38 ~0. 92 obtained from 30 zircon analysis points, and mostly are >0. 4. The above features indicate that zircons are of a magmatic origin, not a metamorphic origin. Therefore, the isotope age of zircons is the crystallization age of amphibolites, indicating that the diagenetic age of Beishan group is earlier than 1623 ± 21 Ma. Comprehensive research shows that the amphibolite was formed in the Paleoproterozoic extensional continental environment. This result has important significance for the research of the age of Beishan group and the tectonic evolution history of the Beishan area.