内蒙古克什克腾旗五道石门基性火山岩锆石U-Pb年龄及其地质意义
Zircon U-Pb dating of the mafic lava from Wudaoshimen, Hexigten, Inner Mongolia and its geological significance
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文摘
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内蒙古克什克腾旗五道石门水库出露一套由灰绿色玄武岩、枕状玄武岩和硅质粉砂岩组成的火山-沉积岩系。枕状玄武岩主要出露于水库大坝南岸，枕体呈不规则椭球状，长轴约0.4~1.0m，边部具有冷凝边，内部气孔和杏仁状构造较发育。镜下观察其矿物组成主要为细长条状斜长石和少量辉石，间粒结构。前人研究认为它们属于早古生代蛇绿岩，本次研究对枕状玄武岩样品进行了LA-ICP-MS锆石U-Pb年龄测试。锆石测年结果大致可分为四组，依次为262~290Ma（A组，15颗)、301~345Ma（B组，5颗)、446~572Ma（C组，6颗)和763~948Ma（D组，5颗)。其中A组锆石呈长柱状，自形，长宽比约2:1~3:1,Th/U值介于0.22~0.98；振荡环带较宽，体现了基性岩浆锆石的特征。15个点的谐和年龄为277±3Ma，应代表岩石的形成年龄，表明五道石门枕状玄武岩形成于早二叠世晚期，应属大石寨组。其余三组锆石多呈自形-半自形，粒状或短柱状，振荡环带较窄，且没有270~280Ma的生长边，表明可能是基性岩浆上升过程中从围岩捕获的锆石，表明五道石门玄武岩可能发育在具有晚元古-古生代基底的陆壳内，暗示早二叠世晚期本区处于板内拉张环境。五道石门枕状玄武岩曾被认为是早古生带大洋蛇绿岩，但综合我们的年代学结果和对锆石类型的分析，它应属早二叠世晚期伸展环境中基性岩浆活动的产物。 |
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其他语种文摘
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A series of volcanic-sedimentary rocks, composed of gray-green basalts, pillow basalts in lower part and siliceous siltstone in upper part, was recognized in Wudaoshimen area, Hexigten Banner, Inner Mongolia. Previous studies considered them as part of the Early Paleozoic ophiolite suite. In this paper, a pillow basalt sample was collected to date U-Pb ages of zircons by using LA-ICP-MS analytical techniques. Existed south of the Wudaoshimen dam,the pillows are irregular ellipsoidal with long axis ranging from 0. 4 ~ 1. 0m, and have obvious condensation rims and little pores inside. The pillow basalt sample display typical intergranular texture with plenty of narrow-strip feldspar and some tiny pyroxene under microscope. Most zircons are colorless,transparent,and euhedral to subhedral in shape. Cathodoluminescene (CL) imaging shows most of them have oscillatory zones. Combined with their high Th/U ratios (> 0. 1, except for one spot of 0. 08) , it is suggested that these zircons are of igneous origin. The 31 zircon analyses are concordant, and show four groups: 261 ~290Ma (group A, n = 15) , 301 ~345Ma (group B, n = 5),446 ~572Ma (group C, n =6) and 763 ~948Ma (group D, n =5) , respectively. Group A zircons are euhedral and columnar with length:width ratios of 2 : 1 ~3 : 1,and display typical widely-spaced oscillatory zoning, which indicates they crystallized at relatively high temperature. Fifteen zircon analyses yield a concordant age of 277 ±3Ma (MSWD = 0. 02) , representing that the Wudaoshimen pillow basalts formed in the late Early-Permian and belong to the Dashizhai Formation. Group B zircons are euhedral and columnar, while group C and D zircons are subhedral-euhedral and granular, respectively. Furthermore, most of these three group zircons display closely-spaced concentric oscillatory zoning. The remarkable differences between group A and the other three groups suggest different source. Combined with the lack of magmatic rims with age of 270 ~280Ma in older zircons,they may be more likely captured from the wall rocks when mafic magma intruding. Thus the scattered ages of 301 ~ 345Ma, 446 ~ 572Ma and 763 ~ 948Ma reflect information of the wall rocks. The Wudaoshimen pillow basalts were regarded as part of the Early Paleozoic ophiolites in previous research. However, based on the concordant age of 277 ±3Ma,we consider that they formed in the late Early-Permian. Because no ultramafic rocks have been found in research area,we suggest they are not part of ophiolite suite. The three group older captured zircons in the Wudaoshimen pillow basalts display that they formed in a continental crust with the Neoproterozoic to Paleozoic basement during an intraplate extension. |
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来源
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岩石学报
,2014,30(7):2055-2062 【核心库】
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关键词
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大石寨组
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枕状玄武岩
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锆石U-Pb年龄
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兴蒙造山带
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地址
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1.
北京大学地球与空间科学学院, 造山带与地壳演化教育部重点实验室, 北京, 100871
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沈阳地质矿产研究所, 沈阳, 110034
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语种
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中文 |
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文献类型
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研究性论文 |
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ISSN
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1000-0569 |
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学科
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地质学 |
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基金
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国家973计划
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国家自然科学基金项目
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国土资源部中国地质调查局项目
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文献收藏号
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CSCD:5188635
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