延边和龙地区早侏罗世花岗岩LA-ICP-MS锆石U-Pb年龄、地球化学特征及成因意义

LA-ICP-MS zircon U-Pb ages, geochemical characteristics and petrogenesis of the Early Jurassic granites in Helong area, Yanbian, Jilin

吉林省延边朝鲜族自治州和龙地区早侏罗世花岗岩广泛出露,LA-ICP-MS锆石U-Pb定年结果显示其形成于195.2±1.3 Ma。地球化学特征表明花岗岩具有高硅(SiO2=73.83%~75.55%)、富碱(Na2O+K2O=7.63%~8.37%)、富铝(Al2O3=13.15%~14.25%)、贫镁、贫钙(MgO=0.22%~0.43%,CaO=0.87%~1.41%)的特征,属于高钾钙碱性系列,岩石稀土总量不高(ΣREE=106.60×10^-6~127.8×10^-6),轻稀土分馏系数(La/Sm)N=5.71~11.25,重稀土分馏系数(Gd/Yb)N=1.48~2.37,Eu为中等负异常(δEu=0.42~0.76),微量元素以富集LREE、Rb、Th、U、K,强烈亏损Sr、P、Ti为特征,指示为壳源岩浆,在构造图解几乎全部落入火山弧花岗岩区,显示可能形成于活动大陆边缘环境。结合延边太平岭地区出现的早侏罗世花岗岩组合到小兴安岭—张广才岭所报道早侏罗世花岗岩及双峰式火山岩组合,共同构成了平行洋壳的俯冲方向的成分极性,表明了俯冲方向,反映了在早侏罗世期间古太平洋板块对我国东北地区北北西方向俯冲的渐进过程。

Objectives:The Early Jurassic granites in the Helong area,Yanbian Korean Autonomous Prefecture,Jilin Province,are widely exposed.Although a large number of previous studies have been conducted on granites in Yanbian area by means of chronology and geochemistry,these research results are mainly focused on large-scale studies on the surface.However,due to the complex tectonic evolution in this area,the isotopic system of the granites was damaged,the forming ages,geochemistry and petrogenesis of the granites are not so clear.In view of this,based on the study of the chronology and geochemistry of monzogranite,this paper discusses the magmatic activity period,the genesis of rocks and the geodynamic background of magmatic formation in the study area,provides a new data basis for the research project on magmatic sequence and geological metallogenic events in the mineralization concentration area of the eastern Jilin-Heilongjiang Provinces.Methods:Detailed geological survey,LA-ICP-MS zircon U-Pb dating of monzogranite,and whole rock geochemical analysis of monzogranite in Helong area.The zircon U-Pb isotope analysis and element geochemical data testing was completed in the State Key Laboratory of Continental Dynamics of Northwest University.Principal component analysis was performed on XRF;trace and rare earth elements analyses were on an inductively coupled plasma mass spectrometer(ICP-MS).Results:The zircon U-Pb dating results show that it is formed in 195.2 ± 1.3 Ma.According to their geochemical characteristics,the granites are high-silicon(SiO2= 73.83%~ 75.55%),alkali-rich(Na2O+K2O =7.63%~ 8.37%),aluminum-rich(Al2O3= 13.15%~ 14.25%),magnesium-and calcium-depleted(MgO =0.22%~ 0.43%,CaO = 0.87%~ 1.41%),belong to the high-K calc-alkaline(HKCA) series;the total rare earth elements in the granites is not high(ΣREE = 106.60 × 10^-6~ 127.8 × 10^-6) with a light rare earth elements fractionation factor(La/Sm)N= 5.71 ~ 11.25 and heavy rare earth elements fractionation factor(Gd/Yb)N= 1.48~ 2.37;the granites have a medium negative Eu anomaly(δEu = 0.42 ~ 0.76) and contain such trace elements characterized by enrichment of LREE,Rb,Th,U and K and severe depletion of Sr,P and Ti,indicating that such granites are formed by crust source magma and almost fall into the volcanic arc granite area in the structural diagram,so they may form in the active continental margin environment.Conclusions:The main lithology of the granites in Helong area,Yanbian Korean Autonomous Prefecture,Jilin Province,are composed of monzonitic granite.The dating results of zircon U-Pb ages show that the zircon formed in the Early Jurassic of 195 ± 1.3 Ma,not the Indosinian period as previous results.The geochemical characteristics of granites elements in Helong area are crust-derived magma and I-type granite.The magma source area is partial melting of basic crustal materials newly generated from the depleted mantle during the Meso-Neoproterozoic.Combined with the Early Jurassic granites and the bimodal volcanic rocks appeared in the Taiping Mountains,Yanbian area,to the Zhangguangcai Mountains and Xiao Hinggan Mountains(Lesser Hinggan Mountains) indicate the subduction side,which together constituted the polarity of the subduction direction of the parallel oceanic crust,showed that the subduction side,reflecting the gradual process of subduction of the Paleo-Pacific plate to the north-northwest of northeast China during the Early Jurassic period.