Tectonic Setting And Prospecting Significance Of The Volcanic Rocks From Ordovician To Silurian In The Western Of South Qilian,NW China

The South Qilian Mountain, connecting the North Qaidam and Qilian Mountain, was located in the most important position in geological evolution process. The study for its tectonic features and history of evolution has attracted more attention. Paleozoic volcanic rocks in South Qilian Moutain is becoming the key to understand its tectonic environment. A set of Silurian volcanic rocks have be found in Dexiqieergande and Dangjinshan by predecessor, but haven’t be researched detailedly, which gratly restrict the restrcution of geological evolution progress of the South Qilian Mountain. On the basis of previous research, we focus on the this set volcanic rocks by the means of studying its peotrology, geochemistry and geochronology.In order to resolve the problems above, we carry out a study for the Silurian volcanic rocks distributed in Danghenanshan and Dangjinshan, including the research of field geological investigation, petrology, geochemistry and accurate LA-ICP-MS zircon U-Pb dating. Besides, we comprehensively analyzed the geochemical data for Ordovician volcanic rocks,collected by previous study, to identify the types of rocks and discuss its genesis, source and forming environment. And we finally focus on the forming age and tectonic environment of Silurian rocks, which is crucial to restrcut geological evolution progress of the South Qilian Mountain.The main conclusions are as follows:(1) The intermediate-basic volcanic rocks mainly consist of block basalt and almond-basalt, which distributed in the Balonggonggaerzu strata for Danghenanshan and Dangjinshan area, western of South Qilian Mountain. The result of LA-ICP-MS zircon U-Pb dating is 424.5±6.6Ma, indicating the rocks formed in middle Silurian, provide significant chronological evidence for the strata.(2) The rocks in Danghenanshan and Dangjinshan show the same feature in petrology and geochemistry, by comprehensively analyzing. We carry out a conclution that the source of volcanic rocks is less fertile mantle through fliud metasomatic, and formed in post orogenic extensive environment.(3) We have collected more latest Ordovician volcanic rocks’ geochemical data in Danghenanshan, and then got a clear understanding of its forming tectonic environment suggesting a Andest continental margin arc by comprehensively analyzing its petrological characteristics, genesis, source and tectonic environment.(4) We conclude a primary geological tectonic evolution mode:(a) The South Mountain experienced continental decomposition and becomes a ocean crust in middle Cambrian to early Ordovician;(b) midle-late Ordovician: South Qilian Moutain develop a set of continental arc volcanic rocks and subduct-like granitic intrusion during subductio stage;(c) The South Qilian Ocean was becoming closed and the bilateral plate starts to collided in early Silurian;(d) In middle Silurian, South Qilian was in a environment of post orogenic extention.