| 内蒙古中部四子王旗大庙岩体时代及成因 |
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| 引用本文： | 章永梅,张华锋,刘文灿,周志广.内蒙古中部四子王旗大庙岩体时代及成因[J].岩石学报,2009,25(12):3165-3181. |
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| 作者姓名： | 章永梅 张华锋 刘文灿 周志广 |
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| 作者单位： | 1. 中国地质大学(北京)地质过程与矿产资源重点实验室,北京,100083
2. 中国地质大学(北京)土地科学技术学院,北京,100083 |
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| 基金项目： | 国家自然基金项目，国土资源部地调局华北地区基础地质调查及数据更新项目，高等学校创新引智计划 |
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| 摘 要： | 华北北缘的内蒙古中部地区出露大量晚古生代-早中生代花岗岩类,在空间上构成一条巨大的东西向花岗岩带.四子王旗大庙岩体作为一个典型的代表,以花岗闪长岩为主,其内部普遍发育暗色微粒包体(MMEs),是认识花岗岩岩石成因和演化的关键.本文对包体及寄主岩进行了同位素测年、岩相学、矿物化学、全岩主量元素和微量元素分析.寄主岩石中的锆石LA-ICPMS U-Pb年龄平均为265±7Ma(2σ),包体中单颗粒黑云母Rb-Sr年龄为253±5Ma(MSWD=0.85),属晚二叠世-早三叠世岩浆活动的产物.包体具塑性外形及岩浆结构,存在多种不平衡矿物组合;MME中的斜长石An组分及黑云母斑晶中MgO成分呈多期震荡,同时总体上均显示出幔部高于核、边部的特征,暗示斑晶可能为围岩捕虏晶,这种相似的成分变化指示包体与寄主岩相互作用引起的结晶环境改变,标志着岩浆成分的变化,是岩浆混合的标志之一;主量和微量数据进一步证明岩体的岩浆混合成因.Rb/Sr-K/Rb变化关系反映包体非结晶分异或黑云母堆晶的产物,而Ce/Pb-Ce、Ba-δEu和P_2O_5-δEu图及其他微量元素比值图等均表明花岗闪长岩体发生了岩浆混合作用,这也得到岩浆物理化学条件的支持.岩浆底侵和岩浆混合作用是该区岩体形成的主要机制和方式.岩石地球化学特征表明该岩体不同于加厚地壳和俯冲洋壳熔融的TTG和埃达克质岩石,而黑云母矿物化学和岩石地球化学显示其构造背景很可能为同碰撞环境.
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| 关 键 词： | 暗色微粒包体 花岗闪长岩 岩浆混合作用 四子王旗 内蒙古 |
Timing and petrogenesis of the Damiao granodiorite, Siziwangqi, Inner Mongolia |
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| ZHANG YongMei,ZHANG HuaFeng,LiU WenCan,ZHOU ZhiGuang.Timing and petrogenesis of the Damiao granodiorite, Siziwangqi, Inner Mongolia[J].Acta Petrologica Sinica,2009,25(12):3165-3181. |
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| Authors: | ZHANG YongMei ZHANG HuaFeng LiU WenCan ZHOU ZhiGuang |
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| Abstract: | A huge volume of Late Paleozoic to Early Mesozoic granitoids is located in the middle part of the Inner Mongolia, which constitutes a giant granitic belt trending EW. As a representative of those intrusions, the Damiao granitoid occurs in the northern portion of the Siziwangqi area, mainly composed of mid-coarse-grained granodiorite with abundant maric microgranular enclaves (MMEs) occurring. The MMEs can provide important clues for genesis and evolvement of granites. Petrological, geochronological, mineralogical results about the MMEs and host granite are presented in this paper. Zircons from the granite give a LA-ICPMS mean U-Pb age of 265±TMa (2σ), and biotite grains from MME yield a Rb-Sr isochron age of 253±5Ma (MSWD =0.85). MMEs generally show ellipsoidal shape, and minerals in microscope are characterized by igneous and disequilibrium texture, which are considered as important evidences for magma mixing. The plagioclase phenocrysts have low composition end member of the anorthite (An) in the coreand rim, compared to that of the mantle, while the contents of MgO in biotite phenocryst core and rim are lower than that of the mantle, both of them show oscillatory zone in the mantle. The resembled oscillatory variations in composition of the plagioclase and biotite phenocrysts from MME indicate that the crystallization environment was changed, due to magma mixing. The major and trace element compositions of the host granite and MEEs also show evidence on their genesis of magma mixing. The Rb/Sr-K/Rb diagram indicates that the MEEs are neither products by mineral fractional crystallization nor biotite accumulation. The Ce/Pb versus Ce and Ba, P_2O_5 versus δEu diagrams together with other trace elements bivariate ratio diagrams, imply that MMEs were generated by magma mixing, evidently consistent with their physical and chemical conditions. Geochemistry of the Damiao granodiorite is different from TTG or subduction-related Adakites. Underplating and magma mixing is a dominated mechanism for generation of the granite. In addition, the mineral chemistry and geochemistry indicate that the Damiao granodiorite is likely to form in syn-collisional environment. |
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| Keywords: | Mafic microgranular enclave Granodiorite Magma mixing Siziwangqi Inner Mongolia |
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