The Early Paleozoic Tectonic Evolution In The Leng Longling Of North Qilian

The North Qilian suture orogenic zone locates between the Alashan massif and the central Qilian massif,It is northwest trending,cutting by Arkin left strike slip fault in the northwest,And adjacent to the central Qilian massif in the southwest,which was bounded by Daban mountain deep fault zone.In the northeast,it is adjacent to Alashan massif which is bounded by Lenglongling deep fault zone.The article will choose Lenglongling zone which locates in middle of the North Qilian suture orogenic zone as study area,Based on the comprehensive study of the sedimentary characteristics,magmatic activity and tectonic characteristics of the early Paleozoic,Theorize about the tectonic evolution history of the ocean and continent transformation in the early Paleozoic.The study area mainly located in the middle of the North Qilian,Early Paleozoic strata include Heicigou formation of middle Cambrian,Yinggou formation of early Ordovician,Daliang formation of middle Ordovician,Koumenzi formation of late Ordovician and Angzanggou formation of early Silurian.Mainly along the Lenglongling,Daban mountain and Tuolai mountain which trending in NW or NWW direction.In the Heicigou formation of middle Cambrian of the first member is a shallow tidal flat sedimentary period,the second member is a set of marine fissure volcano eruptions accumulation;In the first member of the early Ordovician,the first member of the group is a marine volcanic lava,The second section is a neritic shelf facies clastic sediments.The first member of Daliang formation of middle Ordovicianis is a set of deep-sea clastic sediments,The second section is a set of marine flysch sediments,The third section is a set of shallow marine carbonate platform sediments.The first member of the Koumenzi formation of late Ordovician is a set of marine volcano lava accumulation,the second member is a set of shallow shelf facies clastic sediments,The third section is a set of intermediate-basic volcanic rocks in a marine.The first member of the Angzanggou formation of early Silurian is a set of marine clastic sediments,and the second member is a marine and continental molasse formation accumulated.The intrusive rocks in the study area are more developed,mainly along the Lenglongling suture zone,Tuolai mountain suture zone and Daban mountain suture zone,which trending in NW-NWW direction.Late Ordovician diorite,quartz diorite,tonalite,quartz monzonite belong to island arc granitoids or continental arc granitoids.Silurian syenite granite belongs to the CCG type granite and is formed in the platecollision tectonic environment.The ophiolite in the study area is a structural fault block and appears in each structural suture zone.Lenglongling ophiolite formed in Ordovician period,and may be formed in the back-arc basin in the back of paleostructure.The Tuolaishan ophiolite is formed in the paleo tectonic environment of back arc spreading.The Dabanshan ophiolite was formed in Ordovician,The basalt in southwest of Menyuan County and south of Heixiakou formed in expanding environment over subduction related consumering zone.Volcanic rocks are part of the SSZ ophiolite,The volcanic rocks in Ertang area are formed on the active continental margin(island arc)environment,and the volcanic rocks belong to the island arc.The study area across North Qilian suture orogenic zone,has experienced a series of tectonic movements evolution from Paleoproterozoic Era to now,formation a subduction and collision-intracontinental composite orogenic zone,The folds,faults and ductile shear are developed.In the study area the folds of Caledonian are well developed,mainly dominated by closed linear asymmetric inclined folds,The axis direction and orogenic zone towards the same,and the length is long,the folds axial plane are inclined.In the study area the faults of Caledonian also well developed,and distribution wide,mainly trending in NW-NWW direction.The ductile shear zones in the study area are distributed along the major faults,and tend to be NWW or NW.Showing a wide or narrow banding.The tectonic evolution of the study area is very complicated,Through the comprehensive study of sedimentary characteristics,magmatic activity and tectonic characteristics to deduced the tectonic evolution history of the study area.Early Paleozoic tectonic evolution can be divided into three stages: first is rift expansion and stretch into the ocean period in Cambrian,second is the complex oceanic crust subduction and collision during the Ordovician,the third is continental collision orogenic period during the Silurian.In the middle Cambrian,Due to the stretching mechanism of western plate,The Lenglongling ocean and Dabanshan Ocean which respectively located in north and Southern in the study area has large scale oceanic basins,The Tuolai mountain area which located in the central part of the study area has evoluted from rift to small oceanic basin.In early Ordovician,the small ocean basin of Tuolai mountain squeezed and closed,oceanic crust in the early small oceanic has metamorphic to ophiolite fragment by extrusion.The northern Lenglongling ocean and southern Daban mountain ocean crust downthrust to continental crust,and begin subduction.In the middle Ordovician,The oceanic crust of Lenglongling ocean and Daban mountain ocean subduction was weaken,the central part of the study area has in the back arc basin deposition,From the early deep-sea sediment to late shallow sea sediments.In late Ordovician,the Lenglongling ocean and Daban mountain ocean subduction are rintensified,the study area is in the volcanic island arc,With the development of arc granite and arc volcanic rock belt,the island arc is mixedwith the oceanic crust ophiolite,Shows an arc continent collision process.In early Silurian,each landmass in study area collision,Daban mountain ocean and Lenglongling ocean end development.With the subduction and collision type granite intrusion,the study area from the sea gradually evolved into land.In late Silurian,the study area continued to be squeezed from NNE to SSW.The whole crust is uplifted and eroded.So far,the tectonic evolution of the study area changed from the active plate margin to the craton type.