| 闽西南早侏罗世火山岩的锆石U-Pb年龄和Sr-Nd同位素特征及其地质意义 |
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| 引用本文： | 许中杰,程日辉,何奕言,王嘹亮,蓝艺植.闽西南早侏罗世火山岩的锆石U-Pb年龄和Sr-Nd同位素特征及其地质意义[J].地球科学,2019,44(4):1371-1388. |
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| 作者姓名： | 许中杰 程日辉 何奕言 王嘹亮 蓝艺植 |
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| 作者单位： | 1.吉林大学地球科学学院, 吉林 长春 130061 |
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| 基金项目： | 国家自然科学基金项目41402087中国博士后科学基金面上项目2013M530976国家自然科学基金项目41872101 |
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| 摘 要： | 华南陆缘在湘南、湘东南、赣南及闽西南地区早中生代发育一条呈近EW向展布的火山岩带，闽西南永定堂堡和五湖藩坑组火山岩是其中的典型代表之一，利用锆石U-Pb测定其形成年龄分别为（189±6）~（195±2）Ma和184±2 Ma，是早侏罗世火山喷发形成的.为了解闽西南早侏罗世火山岩岩浆来源及形成构造背景等信息，对火山岩进行了岩石学鉴定、主量、微量元素地球化学特征和Sr-Nd同位素测试分析.藩坑组火山岩基性和酸性端员戴里间断达20左右，是典型的双峰式火山岩.基性端员玄武岩（碱玄岩）主量元素高TiO2、高TFe2O3（全铁），（Rb/Sr）N、（La/Nb）N、（Ba/Nb）N和（Rb/Yb）N的比值均高于原始地幔值，具有∑REE较高、轻稀土富集，δEu有微弱异常的特征，Isr值在0.704 4~0.707 5之间，εNd（t）介于-1.19~4.30，说明岩浆源可能为较深的软流圈地幔，同时存在部分来自岩石圈地幔的组分.酸性端员流纹岩（英安岩）（Rb/Sr）N和（Rb/Yb）N比值均高于原始地幔值，（Ba/Nb）N比值全部低于原始地幔值，（La/Nb）N既有高值也有低值.具有∑REE较高、轻稀土富集、δEu存在异常的特征.Isr值在0.701 2~0.705 3之间，εNd（t）介于-5.40~-1.95，说明岩浆物质来源为壳幔混熔型，主要为壳源成因，但有幔源组分参与.闽西南早侏罗世双峰式火山岩形成于板内靠近大陆边缘的特殊构造位置，受到古太平洋板块的低角度俯冲-弧后伸展-玄武岩浆底侵作用影响.
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| 关 键 词： | 火山岩 早侏罗世 闽西南 锆石U-Pb测年 Sr-Nd同位素 地球化学 构造背景 |
| 收稿时间： | 2018-06-01 |
Zircon U-Pb Ages,Sr-Nd Isotopes and Geological Significance of Early Jurassic Volcanic Rocks from Southwest Fujian |
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| Abstract: | An Early Mesozoic volcanic belt with nearly EW distribution develops in South Hunan, Southeast Hunan, South Jiangxi and Southwest Fujian of the South China continental margin, which is represented by the volcanic rocks of Fankeng Formation from Tangpu and Wuhu villages of Yongding County in the Southwest Fujian. The zircon U-Pb dating results indicate that the volcanic rocks of Fankeng Formation formed in 189±6 Ma-195±2 Ma and 184±2 Ma, i.e., the Early Jurassic. In order to reveal the magma source and tectonic setting of the Early Jurassic volcanic rocks in the Southwest Fujian, the lithology identification, major and trace elements, and Sr-Nd isotope data are presented in this paper. The Daly gap of the basic end-member and the acid end-member of volcanic rocks of Fankeng Formation reaches to about 20, and they therefore constitute a typical bimodal association of volcanic rocks. The basic end-member basalt (tephrite) has high TiO2 and TFe2O3 (all Fe) values, and high ratios of (Rb/Sr)N, (La/Nb)N, (Ba/Nb)N and (Rb/Yb)N ratios, which are all higher than the original mantle values. They are also characterized by the higher ∑REE, enrichment in light rare earth elements, and negative anomalies of Eu. Their Isr values are between 0.704 4 and 0.707 5 and the εNd(t) are between -1.19 and 4.30, respectively, indicating that the magma source was derived from the deeper asthenosphere mantle and some components were derived from the lithosphere mantle. The (Rb/Sr)N and (Rb/Yb)N ratios of acid end-member rhyolite (dacite) are all higher than the initial mantle values. However, the (Ba/Nb)N ratios are all less than the initial mantle values and the (La/Nb)N ratios are both high and low. They are characterized by the high ∑REE, enrichment in light rare earth elements, and obvious anomalies of Eu. Their Isr values are between 0.701 2 and 0.705 3 and the εNd(t) are between -5.40 and -1.95, respectively, indicating that the magma source is the crust-mantle misciblemagma, which is mainly composed of the crust-derived materials with involvement of the mantle-derived components. The Early Jurassic bimodal volcanic rocks in Southwest Fujian were formed in the intraplate with a special tectonic position near the continental margin. They were influenced by the effect of low angle subduction of paleo-Pacific plate-back arc extension-basaltic magma underplating. |
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