Sedimentation,Metamorphism And Tectonic Significance Of The South Liaohe Group In Liaodong Area

The Jiao-Liao-Ji Belt is one of the representative Paleoproterozoic orogenic belts in the North China Craton and can provide key information for understanding the early formation and evolution of the North China Craton.However,since the orogenic belt has undergone the transformation of multistage tectonic thermal events in the Phanerozoic,its tectonic evolution process is controversial.The proposed tectonic evolution models include（1）intracontinental rift opening and closing,（2）arc-continent collision,and（3）rift opening-ocean basin opening-plate subduction-collision orogeny.Therefore,it is very important to determine the material composition,petrogenesis,metamorphic evolution,magmatic-metamorphic event chronology framework and related tectonic background in the area to deeply understand the complex tectonic evolution history of the Paleoproterozoic Jiao-Liao-Ji Belt.Focusing on the above key geological problems,this paper carried out a corridor-through mapping study on the South Liaohe Group in eastern Liaoning,aiming to reflect the configuration of the South Liaohe Group.In this paper,the typical metasedimentary rocks of each formation in the South Liaohe Group are taken as the research object,and systematic petrological,detrital zircon U-Pb chronology and geochemical studies are carried out.Combined with the genesis and tectonic setting of Paleoproterozoic magmatic rocks in the area,the material source,sedimentary environment and sedimentary age of the metasedimentary rocks of the South Liaohe Group are discussed.At the same time,this paper also takes the（diopside）amphibolite and the new discovered and widely distributed aluminum-rich gneiss in the study area as the research objects,and carries out systematic petrology,mineral chemistry,phase equilibrium simulation and metamorphic zircon U-Pb geochronology research.The metamorphic facies map is compiled,the spatial distribution characteristics of metamorphic intensity of the South Liaohe Group are identified,and the metamorphic evolution process,metamorphic age and related structural deformation characteristics of the South Liaohe Group are discussed.Combined with the previous research,the main achievements are as follows:1.Identified the material source of the South Liaohe Group.Determined the sedimentary environment and sedimentary age of the South Liaohe Group.The metasedimentary rocks of the South Liaohe Group have two groups of detrital zircon U-Pb age peaks of 2490 Ma and 2177 Ma.These metamorphic clastic rocks have undergone moderate weathering,and the sediment maturity is low.They are near-source sedimentary products.Their clastic sources are mainly upper crustal felsic rocks,including Neoarchean metamorphic basement rocks and Paleoproterozoic Liaoji granite.Their protoliths are a set of aluminum-rich clay rocks,shales or graywackes deposited in the back-arc basin environment of the active continental margin,and the related sedimentation occurred at 2.18-1.93 Ga.As one of the detrital sources of metasedimentary rocks in the South Liaohe Group,the Paleoproterozoic granitic rocks in the region are A-type granites formed at 2.18 Ga,which may be related to the partial melting of the crust caused by the upwelling of the mantle under the back-arc extensional setting.2.Complied the metamorphic facies map of the South Liaohe Group in the study area.Divided the deformation period of the South Liaohe Group.Based on detailed field geological mapping and indoor petrographic studies,the spatial distribution and interrelationship of different geological units of the South Liaohe Group were determined.Combined with the relevant metamorphic studies,the metamorphic facies map is compiled,and the spatial distribution characteristics of metamorphic intensity of the South Liaohe Group are ascertained.It is found that they are composed of metasedimentary rock slices with different metamorphic degrees,mainly a set of metamorphic clastic-carbonate rock formations.Therefore,it is proposed in this paper that the South Liaohe Group is the product of the original strata being involved in different crustal levels and uplifted out of the land surface through different degrees of metamorphism during the orogeny.These rock slices often appear as structural collage,without continuity and integrity of the original stratigraphic sequence.In addition,this paper also finds that the South Liaohe Group experienced at least three episodes of tectonic deformation in the Paleoproterozoic,which are formed in the early subduction stage,the middle compression and contraction stage and the post-orogenic extension and strike-slip stage,and determines the characteristics of tectonic deformation in each stage.They are the results of plate subduction,collisional orogeny and post-orogenic extension.3.Determined the metamorphic evolution history the South Liaohe Group.In this study,the high-grade metamorphic aluminum-rich gneisses are new discovered and widely distributed in the South Liaohe Group.They records abundant metamorphic evolution information and are good research objects of metamorphic evolution.Based on detailed petrographic studies,the clockwise P-T path of metamorphic evolution of the South Liaohe Group with near-isothermal decompression is determined by using the traditional thermometer and the phase equilibrium simulation methods,rather than the counterclockwise pattern previously research.Metamorphic evolution history of aluminum-rich gneiss in the South Liaohe Group includes 6 metamorphic stages:（1）the M₁ mineral assemblage in the early metamorphic stage is Grt+Bt+Ms+Pl+Qz+Chl;（2）the M₂ mineral assemblage in the prograde metamorphic stage is Grt+St+Bt+Ms+Pl+Qz+Chl,and the estimated temperature and pressure conditions are600-650°C/0.65-0.70 GPa;and（3）the mineral assemblage of the pressure peak metamorphic stage M₃ is Grt+Ky+Bt+Ms+Pl+Qz,and its estimated temperature and pressure conditions are 670-710°C/0.95-1.10 GPa,which records amphibolite-facies metamorphism.（4）The mineral assemblage of M₄ is Sil+Crd+Grt+Bt+Pl+Qz+Ilm at the temperature peak stage after heating and decompression,and the estimated temperature and pressure conditions are 785-820°C/0.77-0.92 GPa,which records the metamorphism of medium-pressure granulite facies.（5）Subsequently,the rock underwent near-isothermal decompression metamorphism(M_5-1)and cooling decompression metamorphism(M_5-2),and the temperature and pressure conditions were760-785°C/0.61-0.64GPa and 690-715°C/0.56-0.60GPa,respectively.The mineral assemblages are Sil+Grt+Crd+Bt+Pl+Qz+Ilm and Grt+Bt+Pl+Qz.（6）Late greenschist facies retrograde metamorphic stage M₆ shows the mineral transformation structure of biotite and garnet into chlorite.In addition,this paper uses the metamorphic zircon formed by high-grade metamorphic aluminum-rich gneisses to accurately limit the metamorphic age of the South Liaohe Group.The metamorphic zircon U-Pb dating results show that the metamorphism of the South Liaohe Group occurred at 1.93-1.82Ga and peaked at about 1.89 Ga.4.Remodelled the tectonic evolution history of the South Liaohe Group in the Jiao-Liao-Ji belt.Based on the reaearch of this paper and regional,this paper believes that the tectonic evolution of the southern Liaohe rock group in the Jiao-Liao-Ji belt can be divided into four stages:（1）oceanic slab subduction,resulting in back-arc intracontinental extension,ental extension,and the upwelling of the mantle causes the ancient crustal material to melt to form 2.19-2.16 Ga granitic rocks.（2）Along with the extension and contraction of the back-arc basin,a large amount of clastic material was deposited in the basin.The sedimentation occurred at 2.18-1.93 Ga,and a large range of2.16-2.06 Ga basic magmatism developed during the continuous extension of this period.（3）Collision orogeny occurred with the closure of the back-arc basin.At approximately2.0-1.89 Ga,adakitic granites related to crustal thickening during continental collision were formed.Continuous orogeny caused a large number of sedimentary strata to undergo greenschist-medium pressure granulite-facies metamorphism at about 1.93-1.89Ga.（4）Subsequently,the orogenic belt collapsed and entered the postorogenic extension stage.The tectonic thickening stopped,and the metamorphic rocks of different crustal levels began to turn back,accompanied by postcollisional granitic and basic magmatic activities of 1875-1850 Ma.