Researches On Metamorphism And Deformation Characteristics Of The Ximeng Group In Changning Area, The West Yunnan

The Ximeng Group is sandwiched by carboniferous Nujinshan ophiolite melange(CNop) and Yanshanian granite. The metamorphism and deformation and their relationship about the Ximeng Group has seldom be discussed. This paper based on the geological field work and rock slices identification, has exactly confirmed the types of metamorphism rocks and mineral symbiosis. In order to determine the protolith of the Ximeng Group and its properties, by the text of petrogeochemistry(Whole rock, REE, Trace) united the field geologic characteristic. By the analysis of electron microprobe, we gained the temperature and pressure. By the analysis of zircon U-Pb geochronology test analysis, accurately define the role metamorphism and deformation of times.The Ximeng Group divided into three lithologic in the study area: section A is mainly dominated by(garnet) felsic mylonite, leptite, mica(quartz) schist, biotite plagioclase gneiss, clip a small amount of marble, amphibole-biotite schist. Section B is mainly dominated by some light metamorphic rocks, such as felsic mylonite, biotite plagioclase gneiss, leucoleptite, garnet two-mica plagioclase schist. Section C is interbedded by gneiss and felsic gneisses, clip a small amount of biotite schist, granitic gneiss.Combine with geological characteristics of field outcrops and geochemical characteristic of lab analysis, it shows that the protolith of biotite schists and quartz schists should be parametamorphic rock, may be some slity claystones or shale rocks. The protolith of hornblende biotite schists may be the intermediate and mafic volcanic rocks. The protolith of genisses mostly are feldspathic sandstone, a few are intermediate-acid igneous rocks. The felsic mylonithes may come from intermediate-acid igneous rocks, suffered metamorphic and ductile deformation.Two zircon U-Pb geochronology samples of the Ximeng Group proves that the former Ximeng Group should be disintegrated of Yanshanian granite and metamorphic rocks. The metamorphic rocks only include schists, gneisses, leptite class, marble rocks, and some felsic mylonites.The Ximeng Group has three stages of metamorphism: main metamorphism(M1) is an amphibolite facies of regional metamorphism in medium-temperature and medium-pressure, its temperature and pressure conditions of 540±10°C and 0.4±0.03 GPa, its modification time is Jinningian. The second metamorphism(M2) is a greenschist facies of regional dynamic metamorphism, the time of metamorphism is Himalayan between 20 Ma to 14 Ma. The third metamorphism(M3) is a greenschist facies retrograde metamorphism, after which time of 14 Ma.The Ximeng Group has three stages of deformation: the first stage of deformation(D1) based metamorphism(M1) with the deterioration of the deformation, the formation of a regional foliation and gneissosity. The second stage of deformation(D2) after the first metamorphism(M1),which maked the foliation and gneissosity bending and kinking. The third stage of deformation is almost the north-south sinistral strike-slip shear deformation.