内蒙古锡林浩特跃进公社地区温都尔庙群年代学、地球化学特征及构造环境

温都尔庙群为一套绢云绿泥石英片岩、绢云石英片岩、二云石英片岩、黑云斜长片麻岩、变粒岩、石英岩等的变质岩组合,形成于新元古代。LA-ICP-MS锆石U-Pb同位素定年结果显示,温都尔庙群中的岩浆热事件年龄为459.0±1.8Ma,变质年龄为898.9±5.2Ma,即锡林浩特地区存在900Ma左右的变质基底岩系。温都尔庙群片岩样品显示轻稀土元素富集的中等分异特征,并有一定的负Eu异常,其稀土元素分布模式接近于安第斯型;在微量元素原始地幔标准化图解中,出现明显的高场强元素Nb负异常,表明原岩受地壳混染作用影响;Sr/Ba平均值为0.30,小于1,基本反映了副变质岩的特点。通过岩石地球化学图解分析,认为本区温都尔庙群哈尔哈达组的原岩为沉积岩,岩性主要为砂岩和泥岩。地球化学特征及形成构造环境判别结果表明,温都尔庙群哈尔哈达组片岩原岩沉积环境为活动大陆边缘体系。

Geochronological and geochemical features and tectonic environment of Ondor Sum Group in Yuejin commune area of Xilinhot, Inner Mongolia

The Ondor Sum Group is a set of combined metamorphic rocks, which consist mainly of sericite chlorite schist, quartz sericite schist, two mica quartz schist, biotite plagioclase gneiss, leptynite and quartzite, and was formed in Neoproterozoic. LA-ICPMS zircon U- Pb dating results show that the age of magmatic event in the Ondor Sum Group is 459.0 ±1.8Ma, and the metamorphic age is 898.9±5.2Ma, suggesting that there exists a metamorphic basement rock series of 900Ma in Xilinhot area.The Ondor Sum Group schist samples show moderate differentiation characteristics of LREE enrichment, and have some negative anomalies of Eu, whereas the REE distribution pattern is close to the Andean type. In the normalized multi- element diagrams of primitive mantle, the high field strength element Nb has clear negative anomalies, indicating that the original rock was affected by the action of crustal contamination. The mean Sr/Ba ratio is 0.30, which is less than 1, basically reflecting the characteristics of parametamorphism. Based on the lithogeochemical analysis, it is considered that the original rocks of the Harhada Formation in the Ondor Sum Group in this area were sedimentary rocks, with the lithology being mainly sandstone and mudstone. The comprehensive geochemical characteristics and the discrimination of the tectonic environment indicate that the sedimentary environment of the Harhada Formation schist rock in the Ondor Sum Group is of an active continental margin system.