北祁连东段白银岩群双峰式火山岩锆石U-Pb年龄、Hf同位素及地球化学特征  

Zircon U⁃Pb Ages,Hf Isotopes and Geochemistry of Bimodal Volcanic Rocks in Baiyin Group,Eastern North Qilian

作　　者：计波 李向民[2] 时超[1,2] 余吉远 王国强[2] Ji Bo;Li Xiangmin;Shi Chao;Yu Jiyuan;Wang Guoqiang(State Key Laboratory of Continental Dynamics,Department of Geology,Northwest University,Xi’an 710069,China;MNR Key Laboratory for the Study of Focused Magmatism and Giant Ore Deposits,Xi’an Center of Geological Survey,China Geological Survey,Xi’an 710054,China)

机构地区：[1]西北大学大陆动力学国家重点实验室,陕西西安710069 [2]中国地质调查局西安地质调查中心,自然资源部岩浆作用成矿与找矿重点实验室,陕西西安710054

出　　处：《地球科学》2024年第7期2490-2507,共18页Earth Science

基　　金：陕西省自然科学面上基金项目(No.2023-JC-YB-274);国家自然科学青年基金项目(No.41802133);中国地质调查局项目(No.DD20221636)。

摘　　要：白银厂矿田的含矿岩系白银岩群位于北祁连造山带东南缘,为了揭示该火山岩系的形成时代、成因及其构造环境,运用岩石学、岩石地球化学、同位素年代学等分析测试方法对其进行了系统的研究.白银岩群中石英角斑岩和流纹岩的LA-ICP-MS锆石U-Pb年龄分别为473.0±1.7 Ma和473.9±2.1 Ma,表明白银岩群火山岩形成于早奥陶世.玄武岩具有高Al2O3、低K_(2)O、TiO_(2)和P2O5特征,属亚碱性低钾拉斑系列;流纹岩SiO_(2)含量(>70%)较高,FeO^(T)/MgO比值(2.44~2.80)与稀土元素含量较低(32.1×10^(-6)~44.3×10^(-6)),显示湿冷氧化性流纹岩特征.玄武岩与流纹岩不相容元素Ba、Th、U等相对富集,高场强元素Nb、Ta、Ti明显亏损,Eu负异常不明显(δEu=0.76~0.92),显示岛弧岩浆特征.流纹岩锆石ε_(Hf)(t)值变化于-4.14~14.78,二阶段模式年龄为1707~505 Ma.上述结果表明白银岩群玄武岩是受俯冲流体改造的亏损地幔部分熔融的产物,并在岩浆上升过程中受到地壳混染;流纹岩与玄武岩具有不同的岩浆来源,其主要来自地壳物质的部分熔融,并混入了少量幔源物质.白银岩群双峰式火山岩形成于岛弧向弧后盆地过渡的环境,是早奥陶世北祁连洋向北俯冲的产物.The Baiyin Group,which is the ore-hosted strata in Baiyin ore field,is located at the southeast of North Qilian.The research of isotope chronology,petrology and whole-rock geochemistry was carried out on the bimodal volcanic rocks to reveal their ages,petrogenesis and forming environment.LA-ICP-MS zircon U-Pb dating results show that the group age of the rhyolites are 473.0±1.7 Ma and 473.9±2.1 Ma(206Pb/238U ages),and it can be confirmed that the rhyolite suite formed during the Early Ordovician.The basalt samples,which are regarded as the low-K sub-alkaline tholeiitic series,have geochemical characteristics including high Al2O3,low K_(2)O,TiO_(2) and P2O5.The rhyolitic rocks(SiO_(2)>70%),which belong to cold-wet-oxidized rhyolite,exhibit low FeO^(T)/MgO values(2.44-2.80)and total REE content.With enrichment in incompatible element(Ba,Th,U),obvious negative anomalies of Nb,Ta,Ti and no obvious Eu anomalies(δEu=0.76-0.92),the bimodal volcanic rocks show the geochemical features of island arc magma.The rhyolite zirconε_(Hf)(t)values range from-4.14 to 14.78 with Hf Model ages(tDM2)varying from 1707 Ma to 505 Ma.These features can be concluded that the basalt derived from partial melting of depleted mantle under the effect of subduction fluids and crustal contamination.Meanwhile,the rhyolite may be derived from partial melting of crustal material with a small amount of mantle-derived magma,therefore they are not comagma.The petrology and geochemical information of bimodal volcanic rocks from Baiyin Group indicate they formed in transitional environment from island arc to back-arc basin when northward subduction happened in Early Ordovician.

关 键 词：白银岩群 双峰式火山岩 锆石U-Pb定年 LU-HF同位素 北祁连 岩石学 

分 类 号：P581[天文地球—岩石学]