Neoarchean Genesis Of Granitic Rocks And Geological Significance In Banshigou Area,northeast Of North China Craton

Banshigou is located in the north 9km of BaiShan city,Jilin Province.The geotectonic position belongs to the northeast edge of the North China Carton.There are a large number of Archean granitic rocks,which make it be an ideal regions to discuss the origin and evolution of the north China craton.In order to study the nature of the tectono-magmatic thermal event of the North China Carton during Neoarchean and evolution of the early Precambrian continental crust,we choose Banshigou granitic rocks as the research object.Based on the geological characteristics and mineral assemblage of rocks,the Neoarchean granitic rocks are mainly TTG rock series in the Banshigou area.It is composed of two rock units,tonalitie gneisses and trondhjemite.Because of the strong metamorphism and deformation,tonalitie gneisses are belong to a typical gneisse structure and showing a striped in the most parts of the area.However,the metamorphism and deformation of trondhjemite is weak,only some of the rocks show the gneisse structure.Geochemical characteristics of the TTG rock series indicate that the rocks are high Si?SiO₂>65%?,high Na and low K?K/Na=0.1⁰.62?,high Al?Al₂O₃>15%?and low Mg.The rocks are characterized by right-slop chondrite-normalized rare earth element?REE?patterns and the sample 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.These geochemical features request that the residual minerals are garnet,amphibole and rutile in the source,indicating that the rocks are classified into high-pressure TTG rock series.Combined with the Hf iostope characteristics of the samples,we presumed that there might be a modern plate subduction,the continental crust produced TTG magma through subduction and melting along the oceanic crust.The U-Pb dating results of LA-ICP-MS zircon show that the age of trondhjemite is2598±29Ma,the age of tonalitie gneisses is 2619±28Ma.The crystallization age of them is slightly different,they are very close,it belongs to the same magmatic thermal event in².6Ga.It suggests that the tectonic setting of the Neoarchean granitic rocks in this area is similar to the island arc environment of plate subduction.Combined with the latest research results of the crust rocks in this area,the granitic rocks and the crust rocks may have reflected a crust thickening which from subduction to arc-continent collision.