Abstract Although experimental studies have shown dislocation creep to be an important deformation mechanism in magnetite at medium to high temperature, evidence of intracrystalline deformation in magnetite remains to be established in natural tectonically deformed rocks. In this study we investigate intracrystalline deformation features and nanostructures in elongated magnetite from a naturally deformed rock (mylonitized mica schist deformed in a large-scale shear zone of the Seve nappe, Swedish Caledonides). The magnetite grains have very high aspect ratios (up to 10.40) that result in very high degree of magnetic anisotropy in the rock. We show low and high angle grain boundaries (LAGB and HAGB) in magnetite using a combination of electron backscatter diffraction and high-resolution transmission electron microscopy (HRTEM) analysis. HRTEM studies on lamellae excavated perpendicular to the LAGB and HAGB reveal translational and rotational Moire fringes, respectively. Dislocations, slip bands, stacking faults, twins and recrystallized domains are observed in the vicinity of the grain boundaries, thus providing unequivocal evidence of intracrystalline deformation of magnetite. Our study also reveals the presence of biotite inclusions intergrown epitaxially with magnetite that show no evidence of lattice defects, thus suggesting that the intracrystalline deformation of magnetite took place under wet conditions. The movement at the grain boundaries is interpreted as a response to regional tectonics with a top-to-NW transport direction. It is established that at the nanoscale, the LAGB and HAGB were favourably oriented to accommodate strain dominantly by translation and rotation, respectively. Thus, the nanotectonic processes are consistent with the regional tectonic reference frame. The importance of evaluating ductile behaviour of magnetite from deformed polymineralic rocks in petrofabric analysis and modeling the relation between strain and rock magnetic anisotropy is discussed.

中文翻译：

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天然变形岩石磁铁矿的晶内变形和纳米构造过程

摘要 尽管实验研究表明位错蠕变是磁铁矿在中高温下的重要变形机制，但磁铁矿晶内变形的证据仍有待在天然构造变形岩石中建立。在这项研究中，我们研究了自然变形岩石（瑞典 Caledonides Seve 推覆岩的大规模剪切带变形的糜棱化云母片岩）中拉长磁铁矿的晶内变形特征和纳米结构。磁铁矿颗粒具有非常高的纵横比（高达 10.40），从而导致岩石具有非常高的磁各向异性。我们结合使用电子背散射衍射和高分辨率透射电子显微镜 (HRTEM) 分析显示了磁铁矿中的低角度和高角度晶界（LAGB 和 HAGB）。垂直于 LAGB 和 HAGB 挖掘的薄片的 HRTEM 研究分别揭示了平移和旋转莫尔条纹。在晶界附近观察到位错、滑移带、堆垛层错、孪晶和再结晶畴，从而提供了磁铁矿晶内变形的明确证据。我们的研究还揭示了与磁铁矿共生的黑云母包裹体的存在，没有显示出晶格缺陷的证据，因此表明磁铁矿的晶内变形发生在潮湿条件下。晶界处的运动被解释为对具有顶部到 NW 传输方向的区域构造的响应。已经确定，在纳米尺度上，LAGB 和 HAGB 有利于通过平移和旋转来适应应变，分别。因此，纳米构造过程与区域构造参考系是一致的。讨论了在岩石织物分析中评估变形多矿物岩石磁铁矿的延展性行为以及模拟应变与岩石磁各向异性之间的关系的重要性。