Dynamics and genetic grids of sapphirine-bearing spinel gneiss in Daqing Mountain orogen zone, Inner Mongolia.

A special kind of rock which is sapphirine-bearing spinel gneiss from Daqin g Mountain orogen tenet Inner Mongolia has been reported. This kind of rock is characterised by abundant AI-rich minerals and its chemical composition is alumina-rich and silica-undersatuated. The study of metamorphic mineral s and crystalloblastic sequences indicates that evolution of crystalloblast ic sequences in typical mineral assemblages of sapphirine-bearing spinel gn eiss is Sap + Sil + Gt + Pl + Bi --> Sp + Gt + Sil + Pl + Perth + Bi --> Co rd + Coru + Mt + Pl + Mic + Bi. By means of genetic grids,the crystalloblas tic sequence of the main minerals assemblages in rock is inferred theoretic ally. The result is well consistent to the actual assemblages. According to crystalloblastic sequence and characteristics of assemblages formation of different stages of mineral development in rock, a clockwise P-T-t path is set up by studying genetic grids. The model of metamorphism is interpreted as follows: in the early stage of orogenics, the crust was thickened rapidl y leading to pressure increase to peak and then crust was heated near isoda r and temperature reached to peak condition, at last. Crust was lifted up a nd P-T fell down. In the course of oregenics sapphirine-bearing spinel gnei ss wes formed in orogen root zone. The dynamic conditions are P>0. 85GPa, T >850 degrees C.