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Composition, Structural Characteristics And Geological Evolution Of North Qilian Orogen (the Eastern) In Early Paleozoic

Posted on:2014-04-18Degree:MasterType:Thesis
Country:ChinaCandidate:F H WeiFull Text:PDF
GTID:2250330422961113Subject:Structural geology
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The North Qilian orogenic belt is located in the junction of Qinling Mountains structure and theQilian orogenic belt. It is not only the interface between the North China continents and South Chinacontinents but also the geological transitions of east and west parts of China. The two sets of tectonic lineexhibit bevelled relationship under the effects of Tianshui-Baoji fault for long terms. The geologicalstructures in study area are complex and the structural deformations are extremely strong. The thrust andstrike-slip movements distribute widely. As the junction of Qinling Mountains structure and the Qilianorogenic belt, the studying of material compositions and structural deformations in North Qilian orogenicbelt is an important part on the study of the whole Central Orogenic System. Via synthetically using of themethods, such as structural geology, petrology, geochemistry and isotopic geochronology, this thesiscarries out the studies mainly on the material composition, formation age, formation environment,deformation characteristics and deformation sequence of the Qilian orogenic belt. Basing on these studyingoutcomes, the ocean-continental tectonic pattern in study area of early Paleozoic is reconstructed, and thetectonic evolution of eastern North Qilian orogenic belt is also discussed. Main progress achieved andseveral preliminary understandings are as follows:1. Study on the material compositions of North Qilian orogenic beltThe accurate formation ages and tectonic attributes of the active continental magmatic rocks inmiddle-late Ordovician, the continental margin arc plutonic intrusive rocks formed by subduction ofback-arc basin in middle-late Ordovician and collision type granite in early Silurian are ascertained. Twosets of clastic rocks which formed in continental margin rift process and contraction process of back-arcbasin also collated and stipulated.Because of the subduction of oceanic crust in south, active continental magmatic rocks formed ineastern Qilian orogenic belt in middle-late Ordovician. The Middle Ordovician Chenjiahe groupintermediate-acid volcano rocks are mainly composed of rhyolite, dacite, porphyritic rhyolite, mixedbreccia and tuff and a bit of grayish green metabasalt. Through its geological characteristics, consideringthat volcanic rocks in Chenjiahe group formed in initial cracking environment,of continental marginalarc,,which is the production of the norward subduction of ocean crust in south. The LA-ICP-MS zirconU-Pb ages of Chenjiahe Group intermediate-acid volcanic rock in Lijiahe and Houchuanxia are461.2±3.1Ma(MSWD=0.35) and462.4±3.4Ma(MSWD=0.17); The upper Ordovician Hongtubu basic volcano rocks are a low greenschist metamorphic rock system, and the protolith are basic rocks(basalt).The geochemical characteristics show that the magma derived from the mantle wedge metasomatized bysubduction fluids, and experienced a certain degree of fractional crystallization. The basic rocks formed inthe active continental margin back-arc oceanic basin tectonic environment, and associated siliceous rocksand pillow basalts show that back-arc basin has begun to take shape; Huangmenchuan granodiorite mainlyconsists of a set of granodiorite, and the enclaves in them are dioritic enclaves of different shapes anddifferent sizes. Geochemical characteristics of host rocks show the influence of subduction fluids, and havethe features of continental marginal arc magmatic rocks. The LA-ICP-MS zircon U-Pb ages of host rocksand dark dioritic enclaves in Huangmenchuan area are457.0±3.2Ma(MSWD=0.71) and458.8±2.9Ma(MSWD=0.18).Late Ordovician Yanjiadian quartz diorite are the formation of northward subduction of back-arc basin.The main rocks are quartz diorite, and there are gabbro and gabbro diorite locally. Geochemicalcharacteristics showed that it was a set of continental margin arc intrusive rock influenced by subduction.The Early Silurian Caochuanpu collision type granite are monzonitic granite as a whole. It exhibitsthe characteristics of Adakite such as low Mg content. Caochuanpu granite are the production of themelting of the lower crust in the collision process of plates (blocks).The Middle Ordovician Chenjiahe group clastic rock formation is dominated by a set of biotite quartzschist, biotite plagioclase leptynite rocks. And the protolith is composed mainly of clastic rocks(sandstones, greywackes, argillaceous, sandstone and etc); The Lower Silurian Huluhe group are a set oflow amphibolite facies and greenschist facies metamorphic clastic rocks. Geochemical features indicatethat the tectonic environment of Chenjiahe group sandstone provenance is active continental margin, andthe parent rocks are mainly felsic rocks. The depositional environment may be active continental margininitial rifting basin; The tectonic setting of Huluhe group sandstones is active continental margin to passivecontinental margin conversion environment, and the source rocks are mainly felsic rocks. Depositionalenvironment is probably foreland basin formed in the contraction process of back-arc basin.2. Study on the structural characteristics of eastern North Qilian orogenic beltEastern North Qilian orogenic belt has experienced many times and different types of structuraldeformation and tectonic superposition since Paleoproterozoic. Based on the study of the geometry,kinematics and dynamics of geologic bodies and tectonic boundaries, the deformation could be dividedinto six stages: tectonic deformation in Precambrian, plate subduction and collision deformation in Middle Ordovician to early Devonian, plate oblique collision deformation in late Devonian to Early Carboniferous,intracontinental extensional tectonic deformation in Late Carboniferous to Early Permian, intracontinentaltectonic deformation in Indosinian and intracontinental deformation Yanshan to Himalayan.3.Study on the evolution of eastern North Qilian orogenic beltThe North Eastern Qilian orogenic belt experienced a long and complex evolution process. Itgenerally experienced five stages of evolution: formation of Precambrian continental margin basement,ocean-continental evolution in Early Cambrian to Early Devonian, intracontinental extension-compressionisostatic adjustment in Middle Devonian to Late Permian, intracontinental compression and thrusting stagein Triassic, intracontinental tectonic evolution in Cenozoic. Early Cambrian to Early Devonianocean-continental evolution stage can be subdivided into four stages: formation and initial growth stage ofthe main oceanic basin (534Ma~472Ma), subduction of the main oceanic basin and the formation ofcontinental margin (472Ma~440Ma), continent-continent or continental arc collision orogenic stage(440Ma~424Ma) and rear collision stage(424Ma~411Ma).
Keywords/Search Tags:Eastern North Qilian orogenic belt, Early Paleozoic, Material composition, Geochemistry, LA-ICP-MS zircon U-Pb dating, Tectonic deformation, Tectonic evolution
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