The Significance Of The Jiapigou Complex To The Archean Evolution In The Southern Area Of Jilin Province

The Jiapigou complex in the southern Jilin province,located in the northern margin of North China Craton(NCC),is one of the crucial area of TTG gneisses in Late Archean.In order to study the tectono-magmatic thermal event of the NCC during Late Archean and the formation and evolution of the Early Precambrian continental crust,we choose the TTG gneisses of Jiapigou complex as the research object.The result shows that the acid component of Jiapigou gneisses are mainly plagioclase gneiss,the protolith of which is the TTG rocks.The rocks are characterized by right-slop chondrite-normalized rare earth element(REE)patterns and are rich in light rare earth elements(LREE)and depleted in high rare earth elements(HREE).In addition,Eu abnormality is not evident,and it is enriched in LILE such as Sr,Ba and depleted in HFSE like Nb and Ta,low contents of Yb and Y with high Sr/Y radio(36.0~57.7).These geochemical features request melting of plagioclase component and relicts of garnet,amphibole and some Ti-bearing in the source,indicating that the gneiss is classified into high-pressure TTG series,which generally formed in the environment related to the thickening of subduction.The basic components of Jiapigou gneiss are mainly amphibolites,the protolith of which is tholeiitic basalt.The amphibolite are characterized by nearly-flat chondrite-normalized rare earth element(REE)patterns and showed obvious HREEs depletion and LREEs enrichment.In addtion,the samples' ratio of(La/Yb)N are range from 2.03~3.75,and the Eu anomalies are not obvious(0.94~1.16).Furthermore,the amphibolites possess(Nb/La)N value range from 0.68~0.72 and have negligible negative Nb-Ta anomalies in the primitive mantle normalized trace elements diagram.It projects into the upper side of the mantle trend in the La/Yb – Nb/ Yb diagrams and shows that its petrogenesis is related to reformation of subduction fluid metamorphism in the(Hf/Sm)N-(Nb/La)N diagram.All of the geochemical characteristics are similar to the IAT(island arc tholeiite)-type,classified by Wang wei in 2015,which general represent the product metasomatized by slab-derived fluids in the initial stage of subduction.The mafic rocks with calcium alkaline evolution characteristics are similar to the CAB(calcium alkaline basalt)-type in the study area,which may represent the further mature product of island arc subduction system.The geochemical characteristics of TTG gneiss show a consecutive sequence of chemical composition from the initial subduction of oceanic crust-mature-collision thickening.Combined with the intense deformation and anatexis in the study area,it suggests a horizontal tectonic regime which is similar to the subduction of plate may have already started in the southern Jilin province in Late Archean.The Neoarchean event(~2509Ma),which high grade metamorphic event and the magmas event are simultaneous happen,the clockwise PTt in the granulite facies rocks,and metamorphic facies belt distribution both indicated that the vertical tectonic regime similar to the mantle plume still plays an important role.