Abstract In this work, we examine the tectono-metamorphic evolution of gabbroic rocks of the Atlantis Bank oceanic core complex, South West Indian Ridge, focusing on multistage strain localisation associated with exhumation. We study a sample from the core complex footwall below the depth of seawater hydrothermal fluid-rock interaction. We identify a succession of increasing strain localisation where early solid-state deformation of an olivine gabbro is succeeded by two episodes of melt present deformation resulting in increasing localisation of both strain and melt flux into a narrowing zone. The early melt-absent solid-state deformation occurs dominantly in the dislocation creep regime, and localises strain at the tens of metre scale. This evolved into melt present deformation characterised by (i) grain size reduction by replacement reactions, and (ii) dislocation creep and incipient grain boundary sliding in coarse- and fine-grained parts, respectively. This is based on (1) microstructures indicative of the former presence of melt, (2) a shift to lower calcium plagioclase, and (3) reaction textures involving partial replacement of olivine and diopside by fine grained enstatite and hornblende exhibiting only minor internal deformation features. A narrow (2–3 mm) high strain zone which cuts the earlier shear zone foliation at ~30°, exhibits strong and near complete grain size reduction by melt-rock reaction and tightly spaced foliation within a millimetre-wide high strain zone. Here, strain localisation is contemporaneous with a second melt influx shown by (1) complete olivine replacement by fine grained hornblende and enstatite exhibiting no crystallographic preferred orientation, (2) microstructures indicative of the former presence of melt, and (3) changes in the mineral chemistry of diopside, plagioclase, enstatite and hornblende. We suggest deformation was dominated by melt assisted grain boundary sliding allowing higher volumes of melt flux, a high degree of weakening and deformation focused in the narrow shear zone than during the porous melt flow in the wider, early shear zone. This combined microstructural and microchemical study highlights that the recognition of changes in deformation regime, their time relationships and potential melt related deformation are critical for understanding, and therefore modelling, progressive strain localisation, melt flux and the rheological evolution of oceanic detachment faults.

中文翻译：

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显微结构揭示了大洋中辉长岩中多级熔体存在应变局部化

摘要 在这项工作中，我们研究了西南印度洋脊亚特兰蒂斯银行海洋核心复合体辉长岩的构造变质演化，重点是与折返相关的多阶段应变定位。我们研究了来自海水热液流体-岩石相互作用深度以下的核心复杂下盘的样本。我们确定了一系列增加的应变局部化，其中橄榄石辉长岩的早期固态变形继而发生两次熔体变形，导致应变和熔体通量局部化增加到狭窄区域。早期无熔体的固态变形主要发生在位错蠕变状态，并在几十米尺度上局部化应变。这演变成熔体变形，其特征在于（i）通过置换反应使晶粒尺寸减小，(ii) 分别在粗晶和细晶部分中的位错蠕变和初始晶界滑动。这是基于 (1) 表明先前存在熔体的微观结构，(2) 向低钙斜长石的转变，以及 (3) 涉及橄榄石和透辉石被细粒顽火石和角闪石部分替代的反应结构，仅表现出轻微的内部变形特征。一个狭窄的（2-3 毫米）高应变区在 ~30° 处切割早期的剪切带叶理，通过熔岩反应和毫米宽的高应变区内紧密间隔的叶理表现出强烈且近乎完全的晶粒尺寸减小。在这里，应变局部化与第二次熔体流入同时发生，表现为 (1) 橄榄石被细粒角闪石和顽火石完全取代，没有晶体择优取向，(2) 表明熔体以前存在的微观结构，以及 (3) 透辉石、斜长石、顽辉石和角闪石的矿物化学变化。我们认为变形由熔体辅助晶界滑动主导，允许更高体积的熔体通量，与更宽的早期剪切区中的多孔熔体流动相比，窄剪切区中的高度弱化和变形集中。这种结合微观结构和微化学的研究强调，识别变形状态的变化、它们的时间关系和潜在的熔体相关变形对于理解和建模、渐进应变局部化、熔体通量和海洋分离断层的流变演化至关重要。(3)透辉石、斜长石、顽辉石和角闪石的矿物化学变化。我们认为变形由熔体辅助晶界滑动主导，允许更高体积的熔体通量，与更宽的早期剪切区中的多孔熔体流动相比，窄剪切区中的高度弱化和变形集中。这种结合微观结构和微化学的研究强调，识别变形状态的变化、它们的时间关系和潜在的熔体相关变形对于理解和建模、渐进应变局部化、熔体通量和海洋分离断层的流变演化至关重要。(3)透辉石、斜长石、顽辉石和角闪石的矿物化学变化。我们认为变形由熔体辅助晶界滑动主导，允许更高体积的熔体通量，与更宽的早期剪切区中的多孔熔体流动相比，窄剪切区中的高度弱化和变形集中。这种结合微观结构和微化学的研究强调，识别变形状态的变化、它们的时间关系和潜在的熔体相关变形对于理解和建模、渐进应变局部化、熔体通量和海洋分离断层的流变演化至关重要。与在较宽的早期剪切区中的多孔熔体流动期间相比，在狭窄的剪切区中出现高度的弱化和变形。这种结合微观结构和微化学的研究强调，识别变形状态的变化、它们的时间关系和潜在的熔体相关变形对于理解和建模、渐进应变局部化、熔体通量和海洋分离断层的流变演化至关重要。与在较宽的早期剪切区中的多孔熔体流动期间相比，在狭窄的剪切区中出现高度的弱化和变形。这种结合微观结构和微化学的研究强调，识别变形状态的变化、它们的时间关系和潜在的熔体相关变形对于理解和建模、渐进应变局部化、熔体通量和海洋分离断层的流变演化至关重要。