Genesis And Tectonic Evolution Of Indosinian Intermediate-acidic Rocks In Lueyang Area Of South Qinling Structural Belt

The Qinling orogenic belt,as the backbone of continent China,traverses the center of Chinese Continent and plays a vital role in the formation and evolution of Chinese Continental.As a composite intracontinental orogenic belt,it was formed by the Paleo-Tethyan Ocean closure and the collision between the Yangtze and the Qinling microblocks.As the direct part of the collision between the Yangtze and the North China Blocks,the South Qinling Structural Belt has undergone a long-lived tectonic evolution process.Igneous rocks are widespread in the South Qinling Structural Belt,because of its unique tectonic location and the complex and diverse magmatic evolutions.The igneous rocks in southern Qinling Structural Belt are represented by large-scale exposed granitoid rocks,which are effective carriers for further research of the formation mechanism of South Qinling Structural Belt and deeply dynamic processes and tectonic-magmatic events of the Qinling orogeny.Previous studies have shown that the tectonic evolution of the Qinling Orogen Belt can be traced back to the two stages of plate convergence and collision,especially the clossure of the Paleo-Tethyan Ocean from the Late Paleozoic to Early Mesozoic.Therefore,this thesis selected the Xujiaping syenite and the Miaoshuiba granite veins in Lueyang area of the South Qinling Structural Belt as research objects.Combing field geological research with testing and analysis,as well as multi-disciplinary comprehensive research,it has carried out systematic research on petrology,petrogeochemistry,and isotopic geochronology.The discussion is focused on the characteristics of the source area,their genetic relationship,tectonic-magmatic activity,tectonic environment and geological significance of two types of intrusive rocks.The rocks in Xujiaping area mainly include fine-grained syenite and fine-grained syenite porphyry.These syenites are alkali-rich,smialr to the affinity of A-typegranites.They are also enriched in LREE,flat in HREE pattern,enriched in large ion lithophile element(LILE),and extremely depleted in high field strength elements(HFSE),but Ta and Nb with weak positive anomalies.The types of the Miaoshuiba granites mainly include fine-grained granodiorite and fine-grained and porphyric quartz diorite.They belong to high-potassium calcium-alkaline I-type granites.They are enriched in LREE,but relatively flat in HREE,enriched in large ion lithophile element(LILE),and relatively deficient in high field strength elements(HFSE).Ta and Nb have significant depletion.Isotopic geochronology studies show that the crystalline age of syenite is 216?215Ma,and the crystal age of the porphyric granodiorite is 210.4±0.5Ma.These show that the formation period of the medium-acid magmatic rocks in the Lueyang area of South Qinling Structural Belt is the Late Triassic(late Indosinian).The ptreogenesis of granitic rocks are relation to the Mianlue finite ocean basin northward subducting and finally disappearing,the Yangtze Blocks and the Qinling microblocks subsequently land-land collision stage in the Middle-Late Triassic.The thickened lower crust delamination due to heat disturbance by the mantle-derived magma.A large number of mafic magma underplating induced partially melting of the lower continental crust meta-basite with a small amount of meta-sandstones,and the different types of medium-acid magma formed at the extension system.As granite magma underwent complex evolved processes,such as ascending and emplacement,separation and crystallization,the different types of granites with arc magmatic characteristics were formed.The magma source area is dominated by crustal source,but mantle-derived magma is essential to contribute.Combining field geological characteristics,petrology,petrogeochemistry,and isotope chronology,it is confirmed that the medium-acid magmatic rocks in the Lueyang area of the South Qinling Structural Belt are generally formed in a post-collision to extension environment,and it is the product of the thickened lower crust delamination phase of Indosinian thickening in the southern Qinling region.Based on previous research results,this thesis subdivided the Indosinian tectonic evolution of the South Qinling Structural Belt into two main stages:(1)Subduction-syncollision phase(250?225Ma);(2)Post-collision phase(225?200Ma).However,the transition of the Indosinian compression system to the extension system in the South Qinling structural belt should start at about 220 Ma.