Geochronology And Geochemistry Of Xiong'er Group In The Southwestern Ordos Basin

Classic Xiong'er Group are exposed dominantly in the the Southern borderland of north China Block (Xionger mountain, Xiao mountain and Xiaoqinling region). In the southwestern margin of Ordos Basin, Shanxi Geological Survey Institute according to stratigraphic correlation delineated a small scale Xiong'er Group in 2003, which located in the southern part of Fengjiashan reservoir, Qianyang County, Baoji City. Available data suggest that the Xiong'er Group in the southwestern margin of the NCB are significantly different from the classic Xiong'er Group(located in Xionger mountain, Xiao mountain and Xiaoqinling region). However, the study of this region is relatively weak, lacked reliable age data and geochemical data. Therefore, this paper selected the Xiong'er Group in the southern part of Fengjiashan reservoir as the main research object. By using rock slices identification, zircon LA-ICP-MS u-pb dating, whole-rock major and trace element geochemical analysis, this paper studied the petrology, geochronology and geochemistry characteristics of original Xiong'er Group green schist and meta-sedimentary, discussed the stratigraphic horizor, forming environment and regional tectonic background of Xiong'er Group.Xiong'er Group in the southern part of Fengjiashan reservoir is composed of meta-sedimentary rocks and green schist. The contact relation between them is fault contact. The green schist had the characteristics of low Si, high Ti, low Mg, low K, high Na and poor alkaline. The protolith of green schist is basalt and it belongs to continental tholeiite. The rocks are enriched in large ion lithophile elements(LILE) and faintly fractionated in high field strength elements. It also has non-obvious Eu anomalies. Primitive-mantle normalized spider diagram and Chondrite-normalized REE-patterns diagram showed that the geochemical characteristics are similar to E-MORB and the magma may come from enriched mantle.The U-Pb ages of the meta-sedimentary were 1091±46 to 2854±170Ma, which belongs to Mesoarchean to Mesoproterozoic. The stratum was consistent with Guandaokou Group of Jixian System in study area.The peak age 1.96Ga corresponded to the geological events of cratonization, activation and reconstruction of the North China Block. It shows that the North China block could be a main provenance; The U-Pb ages of the interbedded Siltstone in green schist were 206±3Ma?2251±19Ma, and it shows that the forming age of green schist was not older than 206Ma. It corresponded to West Qinling post-collisional crustal extension events in indo-chinese epoch. The formation mechanism of green schist could be:In the Late Triassic West Qinling post-collisional crustal extension background, the southern margin of North China Block occurred continental rifting. Mantle materials upwelled and formed Basalts, it encountered metamorphism by the late tectonic movement and turned to green schist.The above result of this research shows that the XIong'er Group in the southern part of Fengjiashan reservoir is non-existent, the Xiong'er Group in this area should be disintegrated...