Metamorphic Evolution And Geochronology Of The Al-Rich Gneiss In Wulashan Area Inner Mongllia

The Wulashan area is located in the Khondalite Belt,North China Craton,and the high-grade metamorphic rocks in the region are very well developed,which is one of the important investigate the evolution of the North China Craton.We selects typically Al–rich gneiss from Garnet-Biotite association of Wulashan rock group as the main research object.based on the field geological work,a systematic study carried out on the Al–rich gneiss by integrating petrology,geochemistry,mineral chemistry,traditional geological thermometer,phase equilibrium mode,zircons LA–ICP–MS U–Pb age dating.Combining previous research results,the main conclusion include:?1?The Al-rich gneiss are mainly distributed in the central part of Wulashan area: Xiaomiaozigou-Deersutai,undergoing the highly-amphibolite facies-granulite facies metamorphism,The main lithology of Al-rich gneiss is Crd–Grt–Bt gneiss,Sil-Crd–Bt–Grt gneiss,Bt-Grt gneiss,Sil–Bt–Grt gneiss,Sil–Bt–Grt gneiss,Bearing Hy–Bt–Sp-Grt gneiss,Bear Bt–Grt–Hy granulite.?2?The Al-rich gneiss in Wulashan area divided into four-period of metamorphic stages,and each stage of the metamorphic mineral assemblage and metamorphic conditions respectively: Early mineral assemblage of metamorphic stage?M₁?are as follows Grt+ Bt+Q+Pl+Sil,the temperature condition is: 637-650 ?;Peak mineral assemblage of metamorphic stage?M₂?are as follows: Grt+Sil+Bt+Pl+Q+Kfs+Ilm±Ru±Hy,the temperature and pressure condition is:p =9.0-12.1 kbar,T = 830-918 ?;the post metamorphic stage?M₃?are as follows: Grt+Crd+Bt+Kfs+Pl+Q+Sil ± Sp ± Opx,the temperature and pressure condition is: p = 6.1-7.5 kbar,T = 845-870 ?,if spinel observed in this stage,its temperature may reach at 910 ?±;the late coiling stage?M₄?are as follows: Grt+Bt+Pl+Q+Chl ± Ms ± Mag ± Kfs,the temperature and pressure condition is : T = 650-659 ?,P = 4.3 4.8 Kbar.?3?According to each stage of the metamorphic mineral assemblage and metamorphic conditions defines nearly isothermal decompression clockwise?ITD?p-T path,suggesting that they experienced the evolution stahe of early collision between continent–continent and later rapid uplifting.?4?In Neoarchean,the protolith of the Al–rich gneiss was formed.At this stage,the North China Craton may have formed as a unified craton,and experienced the late paleoproterozoic metamorphic event,the Yinshan block in the North and the Ordos block in the sourth has the subduction and collision,eventually formed the uniform western block of the North China Craton.