Metamorphism Of Archeansupracrustal Rocks Of The Jiapigou Area In South Jilin Province

The Jiapigou area in southern Jilin Province is an important Archean outcroppin g area in the northeast margin of the North China Craton.The metamorphic crust rocks of the Neoarchean amphibolite facies to granulite facies are developed in the area,and the study of metamorphism in the area provides a favorable condition for determining the tectonic evolution environment of the late Neoarchean North China Craton,Research on gold mine and other mineral resources provides the neoozoic metamorphic rocks in the area.In this paper,the Jiapigou area in the south of Jilin Province is selected as the key research area,and the Neoarchean metamorphic crust rocks are taken as the research object.Through detailed petrology,petrography,mineral chemistry characteristics and zircon U-Pb chronology of metamorphic crust rocks,the rock composition,metamorphic degree and metamorphic stage of metamorphic crust rocks in this area are revealed.The tectonic evolution pattern of this area is further discussed by means of metamorphic age,Pt locus pattern,protolites type and geodynamic process reflected.New understanding has been gained in the following aspects:The metamorphic crust rocks in the study area mainly include metamorphic argillaceous gneiss and metamorphic basic granulite.Metamorphic argillaceous gneiss,the rock type is SPAR garnet black cloud feldspar gneiss;The metamorphic basic granulite rock types include garnet amphibolic plagioclase pyroxenite,garnet erxenite granulite and Perilla garnet garnet garnet.Based on detailed petrographic observation,combined with metamorphic reaction structure and mineral chemical characteristics,the metamorphic argillaceous gneiss was divided into three stages of metamorphic mineral assemblages,and the temperature and pressure conditions of each stage were estimated by using traditional geological thermobarometer.Early progressive metamorphic stage(M₁):The core of garnet contains some early progressive metamorphic minerals,and the metamorphic mineral assemblage is Grt(core)+Sil+Bi+Pl+Q,and the temperature and pressure range is 533.0?581.0?and 4.0?4.3kbar.Peak metamorphic stage(M₂):the metamorphic mineral combination is Grt+Bi+Sil+Kfs+Pl+Q,and the temperature and pressure range is 619.1?647.0?and 5.42?5.92kbar.Late retrometamorphic stage(M₃):the metamorphic mineral assemblage is Grt(edge)+Bi+Pl,and the temperature and pressure range is 540.8?599.8?and 4.50?4.98kbar.The high pressure granulite metamorphic stage(M₁)of metamorphic basic granulite:the metamorphic mineral assemblage is Grt+Cpx+Pl+Mag+Q,with the characteristics of typical basic high pressure granulite facies,and the temperature and pressure range is 842?911?and9.49?9.86kbar.In the metamorphic stage(M₂),the metamorphic mineral composition is Opx+Cpx+Grt+Q+Mag,and the typical cooling structure--"red eye"structure appears.The temperature and pressure range is 754?819?and 7.73?8.75kbar.Late amphibole retrograde metamorphic stage(M₃):the metamorphic mineral assemblage is Grt+Hb+Pl,and the temperature and pressure range is 681?711?and 6.86?7.40kbar.It has the characteristics of counterclockwise near isobaric cooling(IBC)P-T trajectory.The petrogeochemistry of the metamorphic supercrust rocks in Jiapigou area is studied.Combined with the discriminant diagram of the protolith and the discriminant diagram of the structure,the protolith of the metamorphic supercrust rocks in the study area is concluded to be basaltic volcanic rocks and a few sedimentary rocks,which formed in the tectonic environment of the island arc or the continental margin.LA-ICP-MS zircon U-Pb dating shows that the metamorphic crust rocks in the Jiapigou area are 2506?2544Ma,which indicates that metamorphic events occurred in the late Neoarchean period.The metamorphic events in this period may have been caused by large-scale mantle derived magma under mantle plume environment.The zircon U-Pb age of 1949±13Ma indicates that the metamorphic crust rocks in the Jiapigou area were metamorphic.The metamorphic crust rocks in the study area also underwent a metamorphic event during the Paleoproterozoic,which may be related to the subduction collision between continental blocks and the later upwelling of the mantle.