Metamorphism And Geological Significance Of The Sillimanite-cordierite-garnet-biotite Plagiogneiss In Liaodong Sanjiazi Area

Liaohe group is a unique Paleo-proterozoic metamorphic series distributed in Liao-ji Orogenic Belt,which has been transformed by multiple tectonic stages and metamorphic events,and the metamorphic evolution is of great significance to the tectonic evolution of the Liao-Ji Orogenic Belt.The high grade metamorphic aluminum-rich gneiss of South Liaohe group found in Wangjiapuzi,the northern Xiuyan in the east of Liaoning remains the typical reduced pressure reaction structure.In this thesis,we focus on the systematical petrography,mineral chemistry and metamorphism,and discuss the factor of high grade metamorphic aluminum-rich gneiss formation and the tectonic evolution of South Liaohe group,and provide new evidence and data for the metamorphism study of South Liaohe group.The type of aluminum-rich gneiss is sillimanite-garnet-cordierite-biotite-plagioclase-gneiss,and the irregular rock mass is in fault contact with marble of Dashiqiao Fm.and mica schist of Gaixian Fm.Based on the preliminary study on the petrography,mineral chemistry,metamorphism of aluminum-rich gneiss,it can be concluded that aluminum-rich gneiss is mainly composed of minerals such as sillimanite,garnet,cordierite,biotite,plagioclase,potassium feldspar and quartz.The protolith may be volcano-sedimentary rocks,and 4 mineral symbiotic assemblages of different metamorphic evolutionary stages can be identified:?1?the early M₁ stage is a greenschist facies mineral assemblage represented by Chl,Ms,Bt,Pl/Ab and Qz,etc.;?2?the mineral assemblage of M₂ in the progressive metamorphic stage,Grt+St+Ky+Bt+Ms+Pl+Qz,represents the highest pressure conditions and the maximum depth of the rock at that time.?3?Mineral assemblage in M₃ stage of metamorphic peak is Sil+Grt+Crd+Bt+Pl+Kfs+Qz.?4?The mineral combination of M_4-1 stage after the peak near isothermal depressurization is Grt+Crd+Sil+Pl+Qz,and mineral combination of M_4-2 stage during cooling is Grt+Bt+Pl+Qz.According to the estimation of traditional geothermobarometry,the metamorphic temperature-pressure condition of metamorphic peak?M₃?is T=640⁶90?,P=5.5⁷.0 kbar,and the metamorphic temperature-pressure condition of the peak after nearly isothermal decompression(M_4-1)is T=560⁶60?,P=4.7⁴.9 kbar,and metamorphic temperature pressure conditions of cooling stage(M_4-2)is T=520⁵60?,P=2.9³.0kbar.The characteristics of mineral assemblage,mineral phase transition structure and P-T conditions of aluminium-rich gneiss define a clockwise P-T path with near isothermal deccompression and cooling following the peak stage,mineral phase transition structure and P-T conditions of aluminium-rich gneiss define a clockwise P-T path with near isothermal decompression and cooling following the peak stage,which is different from the counterclockwise P-T path obtained by predecessors.In conclusion,this thesis believes that the metamorphic age of aluminum-rich gneiss is 1.90¹.95 Ga.The sedimentary environment of the South Liaohe group may be back-arc basin of the active continental margin.The Jiao-liao-ji Tectonic Belt experienced the early extension and opening of the continental blocks to form the ocean basin receiving sedimentation,then the basin contracted and the oceanic crust subducted,and finally the orogenic metamorphism occurred in the collision.The formation of the clockwise P-T path is related to the rapid exhumation after collision.