The Material Composition, Structure And Tectonic Evolution Of The North Qinling Belt In North Margin Of The West Qinling, Tianshui, Gansu Province

Qinling Orogenic belt is a composite continental, which is superimposed by differentsatges and different types of orogenesis and has undergone a long and complicatedevolutionary history. Based on the field geological investigation and comprehensive analysis,this thesis focuses on detailed petrology, petrogeochemistry and isotopic chronology studiesof the Neoproterozoic metamorphic volcanic rocks, early Palaeozoic ophiolite, volcanics,intrusive rocks and metaporphic sedimentary rocks from northern margin of the West Qinling.The petrographic and geochemical characteristics of the volcanics and intrusive rocks in theobjective areas was well known, soon afterwards a set of advanced test technique and highreliable isotopic age data was acquired, on which a preliminary tectonic framework ofNeoproterozoic-early Palaeozoic “Ocean-land” of the northern margin of West Qinling wasestablished, and the forming and evolution history of which was also discussed. The importantimprovements achieved are as follows:1. There is a set of deep metamorphic rocks in Liziyuan-Muqitan-Huamiaohe-Xiajiaping,south of Tianshui, and the protolith may be basic volcanic rocks. The Muqitan rock group hasa character of mid-ocean ridge rocks between E-MORB and N-MORB through thepetrological features, geochemical characteristics with reference to previous research results.The age of Muqitan group is Nanhua Period, which may be the product of the initialexpansion stage of the Tianshui-Wushan ocean. The Liushuigou magmatic complex has thetypical characteristic of the arc magmatic rocks, the age of which is (548±1.8) Ma. There is arock association with characteristics of mid-ocean ridge in Guanzizhen area: thecharacteristics of the meta-ultrabasic rocks are similar to the most of the ophiolites mantleperidotite around the world; the characteristics of the basic volcanic rocks are similar to themid-ocean ridge basalt. The age of gabbro in Guanzizhen Ophiolite is Late Cambrian. TheMuqitan group and the Guanzizheng Ophiolite prove that there are complete evidences ofocean expansion in the northern margin of West Qinling.2. According to the research of magmatic complex in the area, the Baihuaintermediate-basic magmatic complex have the typical characteristics of arc magmatic rocks,which is formed when the oceanic crust dive under the continent, and the age is (444.0±2.8)Ma. The Caotangou group is a meta-sedimentary-volcanic rocks formed in the background ofisland arc through the research of meta-sedimentary-volcanic rocks in the south area, and theage is Late Ordovician. The Liziyuan group is a meta-sedimentary-volcanic rocks, part of thevolcanic rocks has the characteristics of boninite, which prove that the Liziyuan group formed in the island arc environment or fore-arc basin environment and the formation age is theOrdovician in combination with the age and deformation characteristics of the volcanic rocks.The existence of the Baihua intermediate-basic magmatic complex, the Caotangou group andthe Liziyuan group prove that there were large-scale subduction in Late Ordovician．3. The research results of the rock assemblage and geochemistry characteristics of theTaiyangsi rock group indicate the original rock is a set of stable sedimentary assemblage. Thedetrital zircon age spectra show that the Taiyangsi rock group formed in Late Silurian, the ageis413~434Ma, and whose provenance come from the northern basically while the Qinlingblock and the Qilian block have lift up already, and the sediments began to fill into thefore-arc basin from north to south．4. According to the comprehensive research of the deformation mechanisms of thedeformed geological body in the research area, the deformation history was divided into sixstages: the first stage occurred in the Late Ordovician, characterized by regionalmetamorphism; the second stage occurred in the Late Ordovician-Early Silurian,characterized by regional ductile shear deformation; the third stage by regional ductilestrile-slip; the fourth stage by the deformation of regional foliation; the fifth stage by regionalthrust nappe and the last stage occurred in MesoCenozoic, characterized by the developmentof brittle fault.5. In association with the geochemical characteristics, chronology and deformationstages of main geological bodys in the north margin of the West Qinling, the tectonicevolution in the study area can be divided into four stages：the forming stage of the basementin Palae-Mesoproterozoic, plate tectonic evolution stage in Neoproterozoic-Early Paleozoic,plate tectonic evolution and full collision orogeny stage in Neopaleozoic-Triassic,intracontinental orogenic evolution stage in Meso-Cenozoic.