The Tectonic Characteristics And Geological Formation Of North Qinling Tectonic Belt In Feng-liangdang County Region

The North Qinling tectonic belt is important in the study on the formation and evolution of the Qinling Orogenic belt, The Tangzang region of Fengxian County in Shaanxi Province and the Zhangjiazhuang region of Liangdang county in Gansu province are selected for this thesis work. The research area located within West segment of North Qinling and was very little involved by previous studies. The thesis make approaches of field geological observations, geological section investigations, petrology, geochemistry and isotope chronology, attempting to constrain the formation and evolution on western segment of North Qinling tectonic belt.The Caotangou group in Tangzhang comprise basaltic to intermediate volcanic and pyroclastic rocks, distinct from the Erlangping group located to its east. Basaltic of the Caotangou group are chosen for the major and trace element geochemical studies. Analitical results display it is similar to E-MORB, and have a characteristics of the island-arc volcanic rocks.Granite to the north of Caotangou group, includes two distinct types of granites, which are light brown coarse-grain granite and grey-green fine-grain amphibole-biotite granite. Zircon U-Pb age dating yield magmatic ages of (220±2) Ma and (210±1) Ma, indicating they are related to Indosinian Baoji intrusions. Their geochemical characteristics indicate the granites emplaced in within-plate environment, and the North Qinling orogenic belt has tranfered into the intra- continental stage.The Qinling group in research area consist of aluminous parametamorphic genisses (sillinanite-garnet-biotite-plagioclasegneiss), of which protoliths are predominantly pelitic clasts. The typical mineral assemblage are sillimanite + garnet + biotite + plagioclase + quarz. A large amount of detrital zircons was separated from the parametamorphic rocks, and dated by LA-ICP-MS. The result show two age peaks of 0.4-0.5Ga and 1.8-2.0Ga, with the0.4-0.5Ga population representing the Caledonian metamorphism event, and the 1.8-2.0Ga population represent the major magmatic event. The 1.8-2.0Ga event is apparently related to the luliang movement.According to the above age data, and combined with geological evolution of adjacent area, evolution of North Qinling could be divide four main stages: 1) the basement of NCC formed by the end of the early Paleozoic(1.8-1.9Ga), the continental shelf received the sediment from the north (the formation stage of Qinling group); 2) the continental shelf along southern margin of NCC breakup , Qinling group break away from the continental margin, and Kuanping rift formed; 3) in Neopaleozoic, oceanic basins developed to the South and north side of the qinling group;. 4) in Paleozoic(Caledonian), the Southern and northern oceanic basin closed up, by the collision of NCC, North Qinling and Yangtze Block.