Component Features And Tectonic Evolution Of Mianlue Tectonic Zone In Kangxian-Lueyang Area,South Margin Of Qinling Orogen

Mianlue tectonic zone, located in southen margin of South Qinling Orogen, is an important junctional zone of North and South China masses. Its component features, structure attribute and evolution have important significance both for the study of the formation and evolution of Qinling Orogen and for the collision orogenic process of North China Plate and Yangtze Plate. Based on the main academic controversies on the Mianlue Tectonic Zone, this doctoral thesis carried on a systematic study on the field geology, petrology, geochemistry and isotope chronology in Kangxian-Lueyang area, and obtained the following main research results and understanding.1. The volcanic rocks of Mianlue Tectonic could be divided into four types through petrology, geochemistry and chronology study, and they are respectively oceanic crust, Adakite, continental marginal arc and rift type volcanic rocks. Oceanic crust type volcanic rocks have some differences between Kangxian area and Lueyang area. The former is a combination of N-MORB, E-MORB and OIB type volcanic rocks, and the latter is mainly N-MORB type volcanic rocks. The age of new found Zhangergou Adakite which is the product of subduction of Mianlue Ocean is about 840 Ma. The age of continental marginal arc volcanic rocks, which is a combination of andesite, basalt, rhyolite and dacite, is between 837 Ma to 797 Ma. Rift type volcanic rocks were mainly formed about from 783 Ma to 728 Ma. In combination with predecessors’ achievements, we consider that the main age of volcanic rocks in Mianlue Tectonic Zone is Neoproterozoic.2. The metamorphic sedimentary rocks in Mianlue Tectonic Zone coule be divided into three systems, and they are respectively cracking-stable sedimentary series in Nanhua-Sinian period, stretch-rift sedimentary series in Cambrian-Silurian period, stable inter-platform basin-platform type sedimentary series in Devonian-Carboniferous period. The age of sandstone detrital zircons from Guanjiagou Formation of Upper Nanhua is 877 Ma to 718 Ma and 861 Ma to 678 Ma, and the provenance is mainly from adjacent magmatic rocks. Primary Linjiang Fromation(Z2~?1) is disintegrated into Xianggongshan dolomite(Z2?1x) and new Linjiang Fromation(?1l). The geochemical characteristics of continental margin setting from the siliceous rocks in new Linjiang Formation imply the inexistence of Early Paleozoic ocean in Mianlue area.3. In combination with predecessors’ achievements, the tectonic evolution of Mianlue Tectonic Zone and adjacent regions is also discussed. The existence time of Mianlue Ocean is maily early to middle Neoproterozoic(~800Ma), and its extinction is mainly because of the bidirectional subduction of Mianlue Ocean into North China Plate and Yangtze Plate. The extensive continental marginal arc volcanic rocks in Mianlue area are exactly the product of this period. The rift type volcanic rocks that display a planar or dotted spatial distribution in Mianlue Tectonic Zone and adjacent regions e are mainly formed in middle to late Neoproterozoic(800Ma~), and they are likely to be the magmatic response to the breakup of Rodinia supercontinent. The splitting action which started from middle to late Neoproterozoic(800Ma~) continued to Silurian had a brief pause in Sinian, and eventually made Mianlue downfold. Devonian-Carboniferous period was stable inter-platform basin-platform type sedimentary series. The Mianlue downfold probably died out in Early-Middle Triassic, and new round intracontinental evolution followed.