Study On Material Composition, Tectonic Evolution And Transformation At The Conjunction Of Qinling And North Qilian Orogen

The conjunction of Qinling and Qilian orogens, situated in the middle segment of CentralOrogenic Belt, are the tectonic transformation of North Qinling tectonic zone and NorthQilian orogenic belt, and have experienced the convergence and break-up of Rodinia in theearly Neoproterozoic, and have developed a complete ocean-arc-basin system related toProto-Tethys ocean in the Early Paleozoic. But there still are some controversies on thetectonic evolution of Tianshui-Wushan ocean （a branch of Proto-Tethys ocean） and theconjunction and transformation of North Qinling and North Qilian. Taking the principalgeological bodies within or beside the Xinyang-Yuanlong ductile shear belt as object, bycomprehensive research on the attributes and formation ages of those geological bodies andthe deformation characteristics of the ductile shear zone, the thesis has reconstructed theocean-continent framework and the conjunction and transformation of Qinling and Qilianorogens. The following cognitions are acquired in the thesis.1. The Neoproterozoic granitic mylonites in Xinyang-Yuanlong ductile shear zone havethe features of typical strongly peraluminous S-type granites, and features of crust-derivedand typical continent-continent collisional granite. The Paleoproterozoic Qinling Groupwhose protoliths were immature psammites containing higher pelitic compositions probablyserved as the source rocks. The generation of the granitic mylonites resulted from dehydrationmelting due to the breakdown of micaceous minerals. The granitic mylonites suggest acollision between the North Qinling micro-block and Longshan block during（914.7±7.6）^981±5Ma，as a response to the assembly of Rodinia.The newly discovered Xiweizigou mafic-ultramafic rocks mainly consists ofserpentinized olivine pyroxenite, diopsidite, meta-gabbro and meta-gabbro-diorite. The majorelements and rare elements characteristics show that the generation of the meta-gabbro-dioriteresulted from high-level partial melting of enriched lithospheric mantle which had sufferedthe metasomatism and reformation of crust materials, and suggests an island-arc setting, andfurther indicates the Xiweizigou mafic-ultramafic containing the SSZ-type ophiolite （formedat the supra subduction zone） and island-arc intrusion which is not the member of theophiolite. The meta-gabbro-diorite yields a zircon U-Pb age of （443.6±1.8）Ma, belonging to the late Late Ordovician, which indirectly restricts the formation upper-limit of the SSZ-typeophiolite it intruded in.The newly discovered Moshigou meta-basalt is conformable with the associated silicarocks. The abundance and variation of high field strength elements and ratios of associatedelements suggest that Moshigou meta-basalts are E-type MORB, derived from weakly tostrongly enriched mantle. The geochemistry characteristics indicate that the generation ofMoshigou meta-basalt results from the late stage of a back-arc basin. Moshigou meta-basaltyields a zircon U-Pb age of （432.6±3.8）Ma，belonging to the Wenlock of Middle Silurian.Both the Xiweizigou mafic-ultramafic rocks and Moshigou meta-basalt are the results at thesubduction zone due to the northward subduction of Tianshui-Wushan ocean.The Yuanyangzhen meta-gabbro-diorite directly intruded in Wushan ophiolite mélange.The major elements and rare elements characteristics show that the generation of theYuanyangzhen meta-gabbro-diorite resulted from small-scale partial melting of enrichedlithospheric mantle, which had suffered the metasomatism and reformation of oceansediments, with high-degree of fractional crystallization. The Yuanyangzhen meta-gabbro-diorite suggests a late post-collision stage with a thinning setting of lithospheric mantle, andits generation came to an end of the Proto-Tethys tectonic cycle. Yuanyangzhen meta-gabbro-diorite yields a zircon U-Pb age of （408±2）Ma，belonging to the Early Devonian.2. Through detailed analyses and preliminarily discussed on the geometry characteristics,kinematics characteristics and deformational environment of Xinyang-Yuanlong ductileshear zone, and by the exploratory studies on its geodynamics, this thesis has differentiatedtwo structural deformational records responding to the tectonic evolution in the Paleozoic.The early record is the sinistral shear deformation that is the adjustment to the thrusting in adirection of NNE to SSW, caused by the continent-arc or continent-continent collision relatedto the closure of the Tianshui-Wushan ocean （corresponding to the Shangnan-Danfeng oceanin East Qinling） in the Silurian. The later record is the dextral shear deformation and theinversional S-type tectonic style caused by the intense oblique thrusting with concurrentdextral strike-slip at the conjunction of Qinling and Qilian orogen in the Late Devonian toEarly Carboniferous. According to analyses on the macroscopic structures, microscopicstructures and petrofabric using EBSD （Electron Backscatter Diffraction）, the Xinyang- Yuanlong ductile shear zone has experienced deformational temperatures of high tomoderate-temperature and low to low-temperature with deformation environments from lowgreenschist facies to high greenschist facies, then to low amphibolite facies. The calculatedprincipal stress value is about47.532Mpa.3. The North Qinling tectonic zone and North Qilian orogenic belt are two differenttectonic units with different Precambrian basements rock series. As a response to theconverging of Rodinia, the two cohered as a whole representing the active continental marginof Tianshui-Wushan ocean, then lasted and developed the Post-Cambrian evolution at theconjunction, including the four stages in the Early Cambrian to Early Devonian, namely theformation and initial spreading stage of Tianshui-Wushan ocean （534⁴89Ma）, thesubduction of the ocean and the formation of arc and backarc-basin system （489⁴38Ma）, thearc-continent or continent-continent collision stage （438⁴24Ma） and the post-collision stage（424⁴08Ma）.