Early Palaeozoic Tectonic Framework And Evolution In The Junction Of Western Qinling Orogenic Belt And Qilian Orogenic Belt

This dissertation focuses on detailed petrology,petrogeochemistry and isotopicchronology studies on the early Palaeozoic ophiolite,volcanics and intrusive rocks from thejunction (the eastern part) of Qilian Orogenic belt-Western Qinling on the basic of regionalgeological mapping and several related scientific projects.While studying thepetrogeochemistry characters of volcanics and intrusive rocks in the objective areas,weacquire a set of advanced-technique-testing and reliable isotopic ages,on which we establish apreliminary tectonic framework of two early Palaeozoic"Ocean-land"with two sides,thenorth side and the south side,belonging to two different tectonic units and with the large-scaleXinyang-Yuanlong shear zone as boundary.We also discuss the forming and evolution historyfor the two early Palaeozoic"Ocean-land".In areas to the south of the large-scale Xinyang-Yuanlong shear zone belongs to thenorthern margin of Western Qinling orogenic belt,in which bears the fragments of oceaniccrust(corresponding to eastern Qinling Shangdan Ocean) that represented by late CambrianGuanzizhen-Wushan ophiolite.In the north side of Guanzizhen-Wushan ophiolite,there arelate Cambrian-early Ordovician Liziyuan group volcanic-sedimentary rocks which wereformed in island arc-forearc basin,Ordovician setting a typical represent by Caotangouvolcanics and Liushuigou-Baihua intermediate-basaltic magma complex.In areas to the southof Guanzizhen-Wushan ophiolite,there are early Paleozoic metamorphic detrital rock ofTaiyangsi formation which formed from passive continental margin and represented by quartzsandstone.The Ocean basin,represented by Guanzizhen-Wushan ophiolite,subducted fromsouth to north in early Ordovician.The Xinyang-Yuanlong large-scale shear zone belongs to the eastern part of Qilianorogenic belt,which is mainly known for a intracontinental rifting-small ocean basin coveredby Hongtubao-Huluhe metamorphism basaltic rock.In the north of the shear zone,detritalrock of Chenjiahe group sedimented in forearc basin and island-arc volcanics of Chenjiahegroup in late Ordovician and island-arc granites of Yanjiadian formation and Huangmenchuanformation outcrop in order.This small ocean basin started to dive to the north in middleOrdovician,and collided with orogenic movement in late Ordovician-early Silurian,after then which deposited the Silurian Huluhe group represented by a set of typical flyschconstrction.Through detailed studies on the physical components,structural deformationcharacteristics,Geometry characteristics,kinematics characteristics and different isotopegeochronology research of the large-scale Xinyang-Yuanlong shear zone,the predominateboundary of diving Western Qinling belt and Qilian orogenic belt,we conclude that thejunction is a large-scale dextro-rotational ductile strike-slip shear zone based on the detailedmeasurement of section,macro-micro structural observation and analysis of structural statisticdata.The La-ICP-Ms U-Pb zircon isotopic dating of granitic mylonites from that shear zoneprovides a magma crystallization age of 914.2-981Ma and indicates they are theNeoproterozoic products of tectonic-magmatic thermal events and the responses to theaggregation of Rodinia Super-continent in this area.Through the biotite single mineral40Ar/39Ar isotopic dating,we obtained a plateau age of 351.70Ma,which shows that thisshear zone had a event of deformation with metamorphism in Early Carboniferous.In otherwords,this shear zone had crossed through the orogenic belts and the Qinling-Qilianorogenic belt strike-slip collage was occurred in Early Carboniferous period.