| 大兴安岭南段林西地区中生代酸性岩类岩浆的混染作用 |
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| 引用本文： | 张永北,孙世华,本间弘次,毛骞.大兴安岭南段林西地区中生代酸性岩类岩浆的混染作用[J].岩石学报,2003,19(3):369-384. |
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| 作者姓名： | 张永北 孙世华 本间弘次 毛骞 |
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| 作者单位： | 1. 中国科学院地质与地球物理研究所,中国矿物资源重点实验室,北京,100029;中国地质大学,武汉,地球科学学院,武汉,430074
2. 中国科学院地质与地球物理研究所,中国矿物资源重点实验室,北京,100029 |
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| 基金项目： | 中国科学院资源与生态环境重大项目(项目号:KZ951-B1-404)资助 |
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| 摘 要： | 大兴安岭南段林西地区广泛出露以海相玄武安山岩、安山岩为主的晚古生代火山．沉积岩。中生代火山-侵入杂岩也广泛分布其中。花岗岩体的边部普遍二长花岗岩化和花岗闪长岩化(甚至出现闪长岩)，并含有丰富的以玄武质-安山质为主的岩石包体。侵入体-围岩接触带附近为围岩被岩浆侵蚀的港湾状交代反应过渡带。英安岩-粗面英安岩也含有相同的岩石包体，并且与流纹岩密切共生。岩石包体周缘的寄主岩常见淬冷边，聚集针状磷灰石，表明它被捕获时已是冷却的岩石，而非热的玄武岩浆团；它们的Nb／Ta、Zr／Hf比值、弱交代者的化学成分与晚古生代火山-沉积岩一致，Rb-Sr同位素比值散布在晚古生代火山-沉积岩等时线附近，其含量、未交代程度、尖棱状程度、刚性程度在侵入体中从内部到边部增多(强)，且较新鲜者结构构造与围岩相同，表明它们来自晚古生代火山-沉积岩组成的围岩；其含量与岩体边部的位置及二长花岗岩、花岗闪长岩、(粗面)英安岩的成分变化有关，表明包体成分已加入到这些岩石当中。这些岩石的环带状斜长石有两种内核：一种较富钙；另一种较贫钙。两种内核的斜长石外环成分趋同，强交代岩石包体成分和富含岩石包体的酸性岩成分趋同。两者皆记录了两种来源的物质成分同化的过程。同一岩体边部(或火山岩中)酸度较低的岩石化学成分、高场强元素在Harker图解和Nb／Ta(和Zr／Hf)-SiO2及Nb／Ta-Zr／Hf图解中具有中生代花岗或流纹岩浆与晚古生代火山-沉积岩物质混合的特征。这些事实证明林西地区中生代酸性岩浆被较基性的晚古生代火山物质混染，并生成酸度较低的混染岩浆。较基性的火山物质是在低绿片岩相变质的基础上，经钠化交代反应被酸性岩浆同化的。
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| 关 键 词： | 大兴安岭南段 岩浆成因 混染作用 同化作用 岩石包体 |
| 文章编号： | 1000-0569/2003/019(03)-0369-84 |
| 修稿时间： | 2002年9月29日 |
Magma contamination of acidic rocks in Linxi area in southern Da Hinggan Mountains, northeast China. |
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| ZHANG YongBei,SUN ShiHu,HONGMA Hiroji and MAO Qian Key Laboratory of Minerals and Resource.Magma contamination of acidic rocks in Linxi area in southern Da Hinggan Mountains, northeast China.[J].Acta Petrologica Sinica,2003,19(3):369-384. |
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| Authors: | ZHANG YongBei SUN ShiHu HONGMA Hiroji and MAO Qian Key Laboratory of Minerals and Resource |
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| Institution: | ZHANG YongBei,SUN ShiHu,HONGMA Hiroji and MAO Qian Key Laboratory of Minerals and Resource,Institute of Geology and Geophysics,Chinese Academy of Sciences,Beijing,China Earth Sciences Faculty,China University of Geosciences,Wuhan,China |
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| Abstract: | The late Paleozoic (Rb-Sr, 275Ma, (87Sr/86Sr)i=0.70463) volcanic and pyroclastic sedimentary rocks that are dominated by submarine basalt and andesite are widely outcreped in Linxi area in the southern part of Dahinggan mountains. In the same area, the Mesozoic intrusive rocks and volcanic rocks also widely distributed there. In the margin of granite pluton, the rocks commonly transfer into adamellite and granodiotite gradually, and include plentiful enclaves dominated by mafic and intermediate rocks. The contact zone with wall rocks is a transitional metasomatic reaction zone, the boundary like a gulf shape is actually formed by magma eroding the wall rocks on the contact zone. Dacite, rachydacite with the acidity lower than rhyolite also includes plentiful mafic-intermediate enclaves and closely co-exist with rhyolite. The chilled margins with plentiful acicular apatites are frequently observed in the contact zone surrounding those enclaves in the rocks. Most of the enclaves have various metasomatic reaction boundaries, and low grade metasomatic enclaves are similar to wall rocks in the features of texture and structure, minerals. The HFSE ratios (Nb/Ta, Zr/Hf) of the enclaves are close to the ones of late-Paleozoic volcanic rocks, and Sr isotope ratios data are scattered along the isochron of late-Paleozoic volcanic rocks. The content of the enclaves is related to position in the margin of a granite pluton and related to composition variation of adamellite, granodiorite and dacite, trachydacite. These evidences suggest that the enclaves are the xenoliths actually from late-Paleozoic volcanic rocks, which were cool before they were captured by hot magma, and the acid rocks with low acidity had been added in the materials of late-Paleozoic volcanic rocks. There are two kinds of the kernel of zoning plagioclase in adamellite, granodiorite and dacite, trachydacite: one was rich in Ca and the other was low in Ca. But the compositions of outer zone of the zoning plagioclase only change into the same. The composition of highly metasomated enclaves and the margin of granite pluton including those enclaves also change the same. Those composition variation trends recorded the assimilation process of two kinds of different materials. The petro-chemical features in Harker diagram and Nb/Ta (and Zr/Hf) vs SiO2 and Zr/Hf vs Nb/Ta reveal that the compositions of the acid rocks with low acidity of granite pluton or rhyolite lava flow have the mixing features of granite or rhyolite with late-Paleozoic volcanic rocks. These facts suggest that the low acidity rocks were produced while an acid magma contaminated by the materials from late-Paleozoic volcanic rocks. The mafic and intermediate volcanic rocks have been assimilated by metasomation on the basis of greenschist phase metamorphism. |
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| Keywords: | Da Hinggan Petrigenesis Contamination Assimilation Enclaves |
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