Provenance and Tectonic Settings Analysis of Yingtaogou Formation of Middle Ordovician in Helan Mountain Area

Key words: 
• Ordos Basin /
• Helan Mountain area /
• Yingtaogou Formation /
• tectonic setting /
• provenance

Abstract: Yingtaogou Formation in the Helan Mountain area, located in the western margin of  Ordos basin, consists of a set of terrigenous detrital rocks, with carbonate slump gravity flow deposits. The Early Paleozoic tectonic setting of Ordos basin is still in debate due to the lack of reliable material source data. In this paper, the petrologic and geochemical characteristics have been researched systematically. The characteristics of debris composition of sandstone show that Yingtaogou Formation comes from recycling orogeny area. The geochemical signatures of major and trace elements of sandstone from Yingtaogou Formation indicate its double provenance supply feature. More specifically, comparison study on REE distribution patterns shows clastic rocks from Yingtaogou Formation derive from Alashan and/or North Qilian orogenic belt. The geochemical signatures of major and trace elements and REEs, in Combination  with the discrimination diagrams for tectonic settings have mirrored that the study area when Yingtaogou Formation deposited was surrounded by the active and passive continental margin, which means it is affected by both Helan aulacogen and North Qilian Early Paleozoic orogenic belt. REE patterns between Yingtaogou Formation and its adjacent areas display the former is in accordance with Alashan paleolandmass to the northwest and granite from North Qilian Early Paleozoic orogenic belt, and the REE distribution pattern show they are rich in LREE and lack of HREE and exhibit slightly Eu anomalies and marked negative Tm anomalies. After comprehensive consideration of the middle Ordovician palaeocurrents in the western Ordos and detrital zircon age features, this paper suggests that the provenance of Yingtaogou Formation was mainly the North Qilian orogenic belt and the Alashan paleolandmass as a minor one.