早古生代古亚洲洋向北俯冲记录:来自吉中地区早志留世岛弧型火山岩的年代学、地球化学证据

Early Paleozoic northward subduction records of Paleo-Asian Ocean:zircon U-Pb geochronological and geochemical evidence from Early Silurian island-arc volcanic rocks in central Jilin Province

对吉中地区庆丰火山岩进行了LA-ICP-MS锆石U-Pb定年、岩石地球化学研究以制约该区的区域构造演化。定年结果表明,庆丰火山岩形成于早志留世晚期(434±2.3 Ma)。火山岩的岩石学、地球化学研究表明,岩石组合为安山岩和流纹岩,没有发现英安岩,SiO_(2)含量分别为59.2%~62.5%和74.91%~75.53%,具有明显的断面,属于典型的双峰式火山岩。Al_(2)O_(3)(12.83%~17.32%)、Na_(2)O(1.84%~3.75%)、K_(2)O(1.48%~4.76%),低的MgO(0.18%~3.53%)、CaO(0.73%~5.78%),过铝指数A/CNK(0.91~1.13),TiO_(2)(<2%),里特曼指数σ均<3.3,微量元素具富集大离子亲石元素(Rb、K)、高场强元素(Th、U)及LREE等,亏损高场强元素(Ta、Nb、Ti),为准铝质-过铝质中-高钾钙碱性中酸性火山岩,具有岛弧火山岩地球化学特征。结合区域地质资料,认为早志留世松嫩地块南东缘应处于成熟岛弧构造环境,庆丰火山岩的形成与岛弧演化过程中拉张阶段有关。

In order to restrict the regional tectonic evolution of the central part of Jilin Province,the authors report LA-ICP-MS zircon U-Pb dating and lithogeochemistry of Qingfeng volcanic rocks.Zircon U-Pb dating results show that Qingfeng volcanic rocks were formed in Late Early Silurian(434±2.3 Ma).The petrological and geochemical studies of volcanic rocks show that the rock assemblages are made up of andesite and rhyolite.The SiO_(2)contents are 59.2%~62.5%and 74.91%~75.53%,respectively,which has the characteristics of bimodal volcanic rocks.The Qingfeng volcanic rocks have Al_(2)O_(3)(12.83%~17.32%),Na_(2)O(1.84%~3.75%),K_(2)O(1.48%~4.76%),low MgO(0.18%~3.53%),CaO(0.73%~5.78%),A/CNK(0.91~1.13)and TiO_(2)(<2%).In addition,the Rittmann index(σ)is less than 3.3.The trace elements are enriched in large ion lithophile elements(Rb,Th,U,K)and light rare earth elements(LREE),however,depleted in high field strength elements(Ta,Nb,Ti).They belong to metaluminous to peraluminous,medium to high-K calc alkaline,intermediate-acid volcanic rocks with geochemical characteristics of island arc volcanic rocks.Combined with regional geological data,the authors consider that the southeast margin of the Songnen block should be in a mature island arc tectonic environment in Early Silurian,and the generation of Qingfeng volcanic rocks are related to the extensional stage during the evolution of the island arc.