The Analysis On The Genesis Mechanism Of Laohuangfen Granite In Westren Fengqing,Yunnan

Nujiang-Langcangjiang-Jinshajiang region located where the confluence of Circum Pacific tectonic domain and eastern Himalayas Tethyan tectonic domain in western Yunnan, it is by the combined effect of Pacific Plate, Indian Plate and Eurasian Plate, experienced the complex process of the formation and evolution of Tethys, India Eurasia collision and plateau uplift. The Lincang composite granite base located in this region, has the largest outcrop area. And it is an important part of Paleo-Tethys tectonic domain. Based on Field investigation, the Linhuangfen granite which located northern of Lincang granite base is systematically researched in this study, in which the subjects of petrology, elemental geochemistry. Correlated with the known, the purpose of this study endeavors to bring up the geological constraints for its petrogenesis, age and regional magmatic evolution under the tectonic setting.The main rock is biotite quartz diorite in the Lao Huangfen granite which located the Damenglong zone of metamorphism sandwiched granodiorite which show irregular, island or banded shapes in the rock mass, the late Yanshanian granite emplacement as a small stock. Heavy rock surface weathering, mainly as chlorite, silicification, potassic, sodium zoisite petrochemical, sericite. Metamorphism affected by ductile shear some sections had been mylonitized early mylonitized, cataclasticed. Mineral chemical analysis showed that plagioclase is intermediate-acidic plagioclase. Biotite is Mg-biotite with the trend of transition to Fe-biotite. Hornblende are from the metamorphic metasomatism, are mainly actinolite magnesium and sodium amphibole. The geochemical characteristics reveal that, the SiO2 of the rocks range 64.2%~69.56%, averagely 66.83% respectively. The K2O/Na2 O ratio is ranging 1.51~27.42, with average of 9.25. A/CNK varies from 1.025 to 2.906, averagely 1.87. From this we can see that the SiO2 of the rocks is high. Lao Huangfen granites are high-Kalc-alkaline peraluminous series. Rocks are also enriched in LILEs, but relatively depleted in transition element. Enrichment of light rare earth is an important symbol of the effects of deep fluid in the process of rock metasomatic alteration. These characteristics indicate that the the deep fluid metasomatism related magmatic processes and plate subduction. The Zr-TiO2, TFeO/(TFeO+MgO)-MgO 、CaO/Na2O-Al2O3/TiO2、 Rb/Ba-Rb/Sr and La/Sm-La indicated that the Laohuangfen granit is not produced by crystallization differentiation of crust-derived magma, but by complex tectonic magmatism, Separation of crystallization is obvious. Speculated that the rock mass is formed by fractional crystallization and low degree of crystallization differentiation of calcium alkaline granite magma which is from partial melting of crust. In the process of crystallization is a degree of mantle fluid metasomatism.Compared with the Lingcang granit, Laohuangfen granit has the different petrographic characteristics, but similar geochemical characteristics, suppose that may be the same or similar products of greywacke, Formed in the post collision.The comprehensive research shows, Laohuangfen granite in wertern Fengqing belong to evolutionary transition of Lincang composite granite base, formed between P-T2, the production of Lanping- Simao block and Baoshan block in the collision process, and the production of Changning-Menglian oceannic plate during the closure to Yangtze block subduction. The main formation mechanism is continental crust transformation products of partial melting of the upper crust, because of the subduction of oceanic plates leading to continental collision.