Metamorphic Characteristics And Zircon U-pb Geochronology Of Xuanfuyu Khondalite In Shangyi, Northern Hebei

Xuanfuyao is located at the junction of the western block and the Trans-North China Orogen.Khondalite and felsic veins are mainly developed in the area.Khondalite is mainly distributed in Bakou,Xuanfuyao,Dongshangcun,Wanghuayao,Xiabaiyao,etc.The felsic vein is small in scale,but widely distributed.It gradually changes and transits with khondalite and closely coexists.Some khondalite and felsic vein have clear boundaries and relict of khondalite can be seen in felsic veins.Khondalite in the region is mainly alumina-rich gneisses,gray,medium-coarse grain texture,gneissic structure.Mineral components are mainly feldspar?30-35%?,quartz?15-35%?,garnet?10-25%?,biotite?5-15%?and sillimanite?0-15%?.Among them,garnet is transformed into phenocryst,and the transformed matrix is mainly feldspar,quartz and biotite.Alumina-rich gneisses can be further divided into biotite-sillimanite-garnet gneisses,biotite-garnet-plagioclase gneisses and sillimanite-biotite-garnet K-feldspar gneisses.In terms of mineral characteristics,the typical mineral combination in the early stage of alumina-rich gneisses formation is K-feldspar+quartz+garnet+biotite+kyanite±sillimanite.With the progress of metamorphic reaction,sillimanite appeared in large quantities in the metamorphic peak stage and cordierite appeared in the retrogressive metamorphic stage.In mineral chemistry,garnet in alumina-rich gneisses belongs to almandine?54.93-67.55%?-pyrope?29.31-42.34%?.It's relatively rich in magnesium and its edge is relatively rich in iron.Plagioclase is oligoclase-andesine?An=24.76-34.60?;The biotite developed at the edge of garnet has a lower Ti value?0.22-0.30?and a higher X_Mg value?0.72-0.76?,which indicates that Fe-Mg exchange occurs between biotite and garnet and undergoes a rebalancing process.There are two types of felsic vein in the area:?1?Sillimanite-garnet felsic veins,gray-white,euhedral-subhedral texture,which can have gneissic structure.The mineral components are mainly feldspar?⁴0%?,quartz?30-35%?,garnet?¹0%?,minor minerals are a small amount of biotite?²%?and sillimanite?²%?.It is in gradual contact with khondalite,and relict of khondalite can be seen in this vein.Garnet is almandine?57.75-63.02%?-pyrope?33.34-38.59%?.Garnet has uniform composition in stone and edge.Plagioclase is oligoclase?An=21.91-27.97?;?2?Garnet felsic veins has a relatively large scale.Some of the vein bodies are produced as lenticular bodies in khondalite.They are porcelain white,euhedral-subhedral texture and massive structure.The mineral components are mainly plagioclase?25-30%?,K-feldspar?20-25%?,quartz?25-30%?and garnet?10-15%?.Garnet is also a series of almandine?59.27-63.77%?-pyrope?31.49-36.38%?.Garnet has more uniform composition in stone and edge.Plagioclase is oligoclase?An=18.35-22.74?.The plagioclase in felsic vein and plagioclase in khondalite have a gradual change in composition.At the same time,based on the close spatial relationship between khondalite and felsic and cliff physiognomy,felsic vein is considered as the product of partial melting of khondalite.Zircon U-Pb dating shows that the metamorphic age of khondalite is 1919±52Ma,which is related to the"tectonic-thermal"event formed in the khondalite belt.The formation age of garnet felsic vein containing evening line is 1926±19Ma,while that of garnet felsic vein containing evening line is 1858±28Ma,which means that at least two partial melting of khondalite.Based on the above research and the analysis of regional data,the khondalite belt should have been formed by collision between Yinshan and Erdos block before¹.92Ga.Under the background of collision khondalite underwent partial melting,forming the area sillimanite-garnet felsic veins.Affected by the collision and assemblage of the Western and Eastern block of the North China Craton at¹.85Ga,another stage of partial melting of khondalite occurred in the area,resulting in partial melting and formation of garnet felsic veins.Therefore,the khondalite and felsic veins in the area record two important tectonic events formed by the collision and assemblage of paleoproterozoic in North China Craton.