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Neoarchean Tectono-Magmatic Events And Crustal Evolution Of Southern Jilin Province,Longgang Block,North China Craton

Posted on:2024-07-25Degree:DoctorType:Dissertation
Country:ChinaCandidate:Z S LiFull Text:PDF
GTID:1520307064990779Subject:Energy and environmental protection
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The evolution of early continental crust is one of the hotest issues in Earth science.It is meaningful for the study of the Archean tectonic system transformation and mineralization,as well as the evolution of habitable Earth environment.Neoarchean granitoids and metavolcanic rocks are distributed throughout southern Jilin Province(SJP),which is located at the northeastern segment of the Longgang Block.Based on field investigation and sampling,this paper systematically studies the spatial distribution,geological,geochronological and geochemical characteristics of the Neoarchean metamorphic complex,as well as the characteristics of zircon Lu-Hf and whole rock Sm-Nd isotopes.The magmatic source characteristics,magma evolution process,thermal state,and tectonic environment of the Neoarchean TTG,potassic granite and quartz diorite are further discussed.This paper summarized the tectonic evolution history of the study area in the late Neoarchean and established a new tectonic model.The main innovative understandings were obtained as following:(1)Classification of Neoarchean felsic magmatism types and activity stages.The results of the Neoarchean petrology and zircon U-Pb chronology in SJP indicate that the Neoarchean magmatism is composed of sodium-rich TTG gneisses,potassium-rich granites,and quartz diorites.It can be concluded that the earliest TTG magmatic activity occurred at~2.78 Ga,and then developed at 2.70–2.60 Ga,2.58–2.53 Ga and 2.53–2.50 Ga.Potassic granite mainly consist of 2.56–2.49 Ga monzogranite and 2.53–2.47Ga K-feldspar granite.The occurrence of quartz diorite rocks is rare,and the magmatic activity is mainly concentrated during 2.54–2.48 Ga.Combined with previous data,it is shown that the Neoarchean felsic magmatism occurred in SJP has two peaks at~2.7Ga and~2.5 Ga.The first peak is dominated by TTG,whereas the second peak is characterized by earlier large-scale TTG and subsequent potassium-rich granites and magnesium-rich diorites.(2)Determination of the thickened lower crust environment of SJP in the late Neoarchean.The TTGs of this study have relatively low MgO(0.66–2.74 wt.%)contents,and Mg#values range from 33 to 50.Most of them have high Sr content,low Y and Yb content,and high Sr/Y and(La/Yb)N ratio.TTG gneiss has variableεHf(t)(+0.7 to+8)andεNd(t)(-2.77 to+2.79)values.These characteristics indicate that the TTG gneissis belongs to the middle and low pressure type,and may be formed by the partial melting of 2.9–2.7 Ga juvenile thickened lower basaltic crust.The monzogranites contain high Si O2(66.1–76.0 wt.%)and K2O(4.03–5.06 wt.%)contents,while low TFe2O3(0.99–4.97 wt.%)and MgO(0.21–1.27 wt.%)contents.They also display high Sr/Y and(La/Yb)N ratios,and variable epsilon Hf(t)(+0.6 to+5.4)and epsilon Nd(t)values(+3.47 to+0.15).The geochemical and Hf-Nd isotopic compositions of the monzogranites indicate that their parental magmas may have been formed by partial melting of the thickened lower crust,and the source rocks are not uniform.(3)Thermodynamic phase equilibrium modelling reveals that of the SJP was under a high geothermal state during the late Neoarchean.The 2.55 Ga tholeiite basaltic rocks in SJP and 2.8 Ga tholeiite basaltic rocks in west Shandong terrane were selected as potential source rocks.Based on the reasonable determination of initial water content,thermodynamic phase equilibrium modelling and trace element modelling are conducted.The modelling reveals that the Neoarchean TTGs of this study were formed by the partial melting of amphibolite breakdown at a P-T condition of 1.0±0.1 GPa and860–880±50℃.Moreover,the geothermal gradient in SJP was exceeded 730℃GPa-1,suggesting a high geothermal state.(4)Revealing the late Neoarchean cratonization in SJP.The late Neoarchean K-feldspar granite has relatively low MgO contents(0.11–0.84 wt.%)and Mg#values(15-51),with high Sr/Y and(La/Yb)N ratios.The negativeεHf(t)values(-17.5–-1.5)indicate that the K-feldspar granite might be generated from partial melting of ancient crust.The geochemical compositions of quartz diorites suggest that they were derived from partial melting of metasomatized mantle wedge,which was modified by melts released by subducted plates.Therefore,the transformation from dominant TTGs to potassic K-feldspar granite and quartz diorite at the end of Neoarchean indicates considerable crustal maturity and cratonization of the SJP at the end of Neoarchean.(5)Establishing a new tectonic model that the SJP was under a jointly controlled by mantle plume and subduction at the late Neoarchean.Combined with previous researches,we propose that prior to~2.70 Ga,the SJP was dominated by mantle plume regime,the 2.78 Ga Jiabigou trondhjemite is the product of this stage.2.70–2.60 Ga:continuous mantle plume upwelling induced partial melting of the early mafic lower crust to form 2.70–2.60 Ga TTG rocks in the interior of the Longgang Block(e.g.,Tonghua,Liuhe and Baishan,etc.,),simultaneously,the margin of the Longgang Bolck(e.g.,Helong region)might be under an intra-oceanic subduction environment and formed large-scale 2.68–2.65 Ga arc-like volcanic rocks.2.60–2.53 Ga:the internal domain of the Longgang Block was still dominated by the activity of mantle plume,and under a such regime,the thickened lower mafic crust partial melted to produce large volume of TTG magmas,and the early existed TTG rocks partial melted to generate the magmas of monzogranites.Meanwhile,the margin regions(e.g.,Helong,Jiabigou)were effected by subduction and formed 2.60–2.53 Ga island arc tholeiite,calc-alkaline basalt,andesite and dacite.During 2.53–2.47 Ga period,accompanying with decreasing of mantle potential temperature,the plate tectonics dominated the evolution of the SJP.The SJP experienced arc-continental collision and subsequent post-collisional extension,and generated K-feldspar granites,quartz diorite as well as mafic dikes,indicating the process of crustal mature and cratonization.
Keywords/Search Tags:North China Craton, Longgang Block, Crutal growth, Neoarchean tectonic regime, TTG rocks
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