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-ICP-MS 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.