Multi-stage serpentinization of ultramafic rocks in the Manlay Ophiolite, southern Mongolia
Multi-stage serpentinization of ultramafic rocks in the Manlay Ophiolite, southern Mongolia
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DOI：
10.5564/mgs.v26i53.1787
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发表时间：
2021-12
影响因子：
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通讯作者：
Nomuulin Amarbayar;N. Tsuchiya;Otgonbayar Dandar;A. Okamoto;M. Uno;Undarmaa Batsaikhan;Jiajie Wang
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作者：
Nomuulin Amarbayar;N. Tsuchiya;Otgonbayar Dandar;A. Okamoto;M. Uno;Undarmaa Batsaikhan;Jiajie Wang
蛇绿岩中超镁铁质岩的蛇纹石化作用是理解全球元素循环和岩石圈地幔物理性质变化的关键。蒙古是中亚造山带的中部，蛇绿混杂岩众多，但对其超镁铁岩的变质作用研究甚少。本文介绍了蒙古南部曼来蛇绿岩中超镁铁质岩体蛇纹石化作用的研究结果。蛇纹岩的Cr尖晶石成分[Mg# = Mg/（Mg + Fe ~（2+））= 0.54，Cr# = Cr/（Cr + Al）= 0.56]和岩石化学组成（Mg/Si = 1.21-1.24，Al/Si < 0.018）表明其形成于弧前环境。利蛇纹石产于网状结构的核心和边缘（Mg# = 0.97），温石棉见于各种矿点，包括碧云母（Mg# = 0.95）、网状结构的核心（Mg# = 0.92）、网状结构的边缘（Mg# = 0.96）和晚期大脉（Mg# = 0.94）。利蛇纹石和温石棉的存在和叶蛇纹石的缺乏表明低温蛇纹石化（<300 °C）。蛇纹岩中水镁石的缺乏意味着富硅流体对曼莱蛇绿岩超镁铁质岩的渗透。根据显微结构和矿物化学特征，曼雷蛇绿岩中超镁铁质岩的蛇纹石化作用经历了三个阶段：（1）利蛇纹石取代橄榄石，（2）斜方辉石之后形成纤蛇纹石（碧云母）取代橄榄石残体，（3）纤蛇纹石脉的发育贯穿了所有以前的结构。Manlay蛇绿岩中蛇纹岩的复杂结构表明，在蒙古南部和CAOB，这些蛇绿岩的超镁铁质部分经历了多个阶段的流体渗透。
Serpentinization of ultramafic rocks in ophiolites is key to understanding the global cycle of elements and changes in the physical properties of lithospheric mantle. Mongolia, a central part of the Central Asian Orogenic Belt (CAOB), contains numerous ophiolite complexes, but the metamorphism of ultramafic rocks in these ophiolites has been little studied. Here we present the results of our study of the serpentinization of an ultramafic body in the Manlay Ophiolite, southern Mongolia. The ultramafic rocks were completely serpentinized, and no relics of olivine or orthopyroxene were found. The composition of Cr-spinels [Mg# = Mg/(Mg + Fe2+) = 0.54 and Cr# = Cr/(Cr + Al) = 0.56] and the bulk rock chemistry (Mg/Si = 1.21–1.24 and Al/Si < 0.018) of the serpentinites indicate their origin from a fore-arc setting. Lizardite occurs in the cores and rims of mesh texture (Mg# = 0.97) and chrysotile is found in various occurrences, including in bastite (Mg# = 0.95), mesh cores (Mg# = 0.92), mesh rims (Mg# = 0.96), and later-stage large veins (Mg# = 0.94). The presence of lizardite and chrysotile and the absence of antigorite suggests low-temperature serpentinization (<300 °C). The lack of brucite in the serpentinites implies infiltration of the ultramafic rocks of the Manlay Ophiolite by Si-rich fluids. Based on microtextures and mineral chemistry, the serpentinization of the ultramafic rocks in the Manlay Ophiolite took place in three stages: (1) replacement of olivine by lizardite, (2) chrysotile formation (bastite) after orthopyroxene and as a replacement of relics of olivine, and (3) the development of veins of chrysotile that cut across all previous textures. The complex texture of the serpentinites in the Manlay Ophiolite indicates multiple stages of fluid infiltration into the ultramafic parts of these ophiolites in southern Mongolia and the CAOB.