The Characteristics And Tectonic Significance Of Ordovician Siliceous Rocks In The Yushigou Area Of The North Qilian Orogen

The Yushigou area of the North Qilian is located in the northernmargin of the Qinghai-Tibet Plateau. The tectonic position is sited on the Alashaneblock between the Qaidam block, which is a place of research focus of ancient oceanicbasinThis paper gave a division of Yingou Group of early Ordovician strata: O1yaandO1yb. At the time, Ordovician strata of this area has an angular unconformity contactwith Carboniferous strata; an overthrust fault contact with ophiolitic mélange andunderlying Tuolai Rock group. It grew many ruptures, silicify, limonitization andpyritization in Yingou Group, which provides a promising guidance for mineralexploration.Meanwhile， this paper provides results by analyzing the geochemistrycharacteristics of siliceous rocks within this area: the Al/（Al+Fe+Mn）average of4samples in Yushigou（west） is0.56, Al/（Al+Fe）is0.57,（La/Yb）_SNis0.89, δCe_SNis0.88, and δEu_SNis1.15. The North American Shale Standardized curve pattern of REEsis flat while that of Chondritic Standardized is a relatively-concentrated pattern ofLREEs which has an clearly rightward developing tendency. About other7samples inYushigou, the average ratio of Al/（Al+Fe+Mn）is0.60（excluding the sample YSG-7that will easily be influenced by heated fluid）, that of Al/（Al+Fe）is0.61, that of （La/Yb）_SNis1.2, that of δCe_SNis0.93, and that of δEu_SNis1.08. The North AmericanShale Standardized curve pattern of REEs is flat while that of Chondritic Standardizedis a relatively-concentrated pattern of LREEs which has an clearly rightward developingtendency. Those statistics reveal that those siliceous rocks associated with subductioncomplex in Yushigou have a tectonic background of archipelagic ocean, of which someparts are close to the continental margin or the surrounding sea basin and some are farfrom.