Pyrometamorphism And Ultrahigh-Temperature Metamorphism Deformation And Tectonic Implication Of The Day Nui Con Voi In The Northwestern Vietnam And Yaoshan Group In Western Yunnan

The Yaoshan-Day Nui Con Voi metamorphic belt in western Yunnan and northern Vietnam is the southernmost one of the four metamorphic belts in Red River Shear Zone.It is mainly comprised of several kinds of rock and mineral,which are in amphibolite facies,such as garnet sillimanite biotiteschist,gneiss,graphite schist,marble,amphibolite,migmatized granite and so on.The main objective of study in my thesis is Yaoshan-Day Nui Con Voi metamorphic belt.On the basis of summarizing pervious researchers'work,the article finds there are three stages of the metamorphism and deformation of Yaoshan-Daxiangshan group,through outcrops observation,microstructure analysis,EBSD rock construction analysis and deformation-metamorphism temperature-pressure testing.These three stages are:the ultra-high temperature metamorphism during the initial plate impact before 60Ma;the granulite facies metamorphism and sinistral strike-slip related to high greenschist-high amphibolite metamorphism during 60-32Ma;and sinistral slip accompanied with north-south differences uplift and east-west tilting uplift from 28Ma.Newly discovered rock masses composed of corundum+ akerite+ ganet+ sillimanite representing ultrahigh-temperature metamorphism.The coexistence of spinel and quartz indicates that the metamorphic temperature is higher than 900?.Meanwhile,the biotite-garnet thermometer and biotite-plagioclase-garnet-quartz association thermobarometer for the mineral associations during retrogressive metamorphism gave temperature and pressure ranges of 879?917 ? and 0.90?0.94GPa,respectively.The early stage metamorphism is featured by the occurrence of corundum+fibrolite assemblage,which marks a shift of metamorphic conditions from upper amphibolite facies to granulite facies.Meanwhile,the peak metamorphism is characterized by the spinel+quartz assemblage,which indicates an ultrahigh-temperature metamorphic condition(temperature higher than 900 ?).On the other hand,the formation of ilmenite during retrogressive metamorphism suggests that the rocks experienced a drastic decrease in pressure after the peak metamorphic conditions.The metamorphic P-T path shows that there is an early coeval increase in temperature and pressure and a subsequent rapid increase in temperature before reaching the peak conditions.In addition,a fast isothermal decrease in pressure characterizes temperature and pressure evolution after the peak metamorphism.Such a P-T path is in accordance with the temperature and pressure evolution of anomalously heated slab due to upwelling the asthenosphere by the break-off of subducting oceanic plate during plate convergence.SIMS U-Pb dating of zircons from the ultrahigh-temperature metamorphic rocks gave a minimum age of 58Ma.Therefore,it is suggested that the ultrahigh-temperature metamorphism along the Day Nui Con Voi is related to the convergence between the Indian and the Eurasian plates.