| The Baishan area that located in the northeastern North China Craton?NCC?,is a typical exposure of the Archean basement,where Neoarchean supracrustal rocks,TTG?tonalite–trondhjemite–granodiorite?gneisses,metamorphic mafic intrusions and potassic granitoids are widespread,which provide important basis to recognize the material constituent,formation and evolution history of the early continental crust.On the basis of detailed field geological investigations,this paper carried out LA-ICP-MS and SHRIMP zircon U-Pb dating,zircon Lu-Hf isotopic analysis and whole-rock geochemical study on the Archean basement rocks in Baishan area.The main conclusions are as follows.1)The supracrustal rocks in Baishan area mainly comprise amphibolite,hornblende schist,plagiogneiss and magnetite quartzite.LA-ICP-MS zircon U–Pb isotopic dating reveals that the hornblende schist and amphibolite were formed during2554–2548 Ma.Zircons from the hornblende schists mainly show high positive?Hf?t?values close to the contemporaneous depleted mantle line,suggesting that their magmatic precursors were most likely generated from depleted mantle and representing a episode of crustal growth.Nevertheless,zircons from the amphibolite have highly variable?Hf?t?values and TDM1M1 ages,indicating their magma was derived from depleted mantle and contaminated by crustal materials.These tholeiitic basaltic volcanic rocks were probably formed in an island arc setting,representing the initial subduction of the ocean crust.2)The TTG gneisses in Baishan area are dominated by tonalitic and trondhjemitic gneisses,whereas granodiorites are relatively few.Tonalitic gneisses and trondhjemitic gneisses were emplaced at 2661–2602 Ma and 2608–2533 Ma,respectively,while the granodiorite was emplaced at 2550 Ma.Most TTG gneisses subjected to metamorphism or anatexis at 2514–2465 Ma.Based on the zircon Hf isotopic and geochemical compositions,we speculate that the magmatic precursors of the 2.66–2.58 Ga tonalitic–trondhjemitic gneisses were generated by partial melting of dominantly juvenile basaltic materials with different contents of sediments and may be related to mantle plume activities.The magmatic precursors of the 2.55–2.53 Ga trondhjemitic–granodioritic gneisses were produced by partial melting of subducted oceanic slabs and minor sediments,and might formed in a subduction-related active continental margin setting.Some samples with high positive?Hf?t?values indicate crustal growth during 2.66–2.60 Ga.3)Metamorphic mafic intrusions in Baishan area are mainly of metagabbros.LA-ICP-MS zircon U–Pb isotopic dating reveals that the metagabbros were emplaced at2534–2527 Ma.They show highly variable positive?Hf?t?and TDM1,with the highest?Hf?t?values are close to the contemporaneous depleted mantle line,representing crustal growth during2.53 Ga.The parental magmas of the metagabbros were possibly generated by partial melting of a depleted mantle wedge that was metasomatized by slab-derived melts or fluids.They also formed in a subduction-related active continental margin setting.4)The potassic granitoids in Baishan area are mainly monzogranites with minor moyites.Zircon SHRIMP U–Pb isotopic analysis indicates that the monzogranite formed at 2490 Ma.Based on the previous studies on the potassic granites in sourthern Jilin Province,we suggest that they were derived from partial melting of Neoarchean potassic basalt,tonalitic gneisses and sediments,representing a crustal recycling process and might formed under post-orogenic environment.5)Neoarchean–early Paleoproterozoic?2.66–2.49 Ga?magmatism was almost continuous exist in the Baishan area,and underwent juvenile crustal growth at2.66–2.60Ga,2.55Ga and 2.53Ga,suggesting that the Neoarchean was a significant period of crustal growth in the Baishan area.6)Combined with previous investigations,we concluded that the Neoarchean crustal evolution history of the southern Jilin Province is as follows.From2.80 Ga to2.70 Ga,abundant juvenile basaltic crust differentiated from the mantle,which might occurred under the mantle plume regime.From 2.70 Ga to 2.57 Ga,the partial melting of juvenile basaltic crust generates the tonalitic–trondhjemitic magmas.2.56 Ga,the initiation subduction of the ocean slab towards the Eastern Block resulting in the eruption of the tholeiitic basaltic supracrustal rocks,which may representing the operate of plate tectonic processes since2.56 Ga.From 2.56 Ga to 2.52 Ga,the mantle thermal flows and subduction-derived fluids triggered the partial melting of the subducted slab,the lower crust and the mantle wedge to generated TTG,supracrustal rocks and gabbroic magmas,which were most likely emplaced within an active continental margin setting.From2.50 Ga to 2.47 Ga,the arc-continent collision resulted in the braek-off of the subducted ocean slab and mantle underplating.The pre-existing Neoarchean rocks subjected to metamorphism or anatexis and voluminous potassic granitoids emplacement at the same time.These processes led to the final stabilization and the primary cratonization of the NCC. |
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