The Characteristics and Tectonic Significance of Ordovician Siliceous Rocks in the Yushigou Area, North Qilian

The Yushigou area in North Qilian, which is a research hotspot of thr paleo-oceanic basin, is located at the tectonic zone between the Alashane block and the Qaidam block at the north margin of the Qinghai-Tibet Plateau. The tectonic setting and the tectonic evolution of early Caledon at North Qilian is controversial, between rifted basin and oceanic basin. Some scholars believe that the north Qilian formed the matured ocean and the trench-arc-basin system in the early Ordovician period, while others insist that it just continued its rift valley evolution. In this study, the Axiosmax X-ray Fluorescence Spectrometer was used to measure the content of major elements and the X-Series 2 Inductively Coupled Plasma-Mass Spectrometry to measure the content of the trace elements and REE of the siliceous rocks collected from North Qilian. The results show that the mean ratio of Al/(Al+Fe+Mn) is 0.56 and 0.60, Al/(Al+Fe) is 0.57 and 0.61, (La/Yb)_SN is 0.89 and 1.2, δCe_SN is 0.88 and 0.93 and δEu_SN is 1.15 and 1.08, for samples collected from two sites at Yushigou, respectively. Chondritic Standardized REEs distribution plot is a LREEs enrichment pattern which shows an outstanding rightward decreasing tendency. This evidence indicates that the tectonic settings of the siliceous rocks associated with subduction complex in Yushigou have a tectonic background of archipelagic ocean, of which some parts are close or far from the continental margin or the surrounding sea basin.