云母之路：Raphaël Gottardi等 Geology (2024)-层状硅酸盐对自然变形岩石中石英变形组构的影响

云母属于铝硅酸盐矿物，具有连续层状硅氧四面体构造。分为三个亚类：白云母、黑云母和锂云母。白云母包括白云母及其亚种（绢云母）和较少见的钠云母；黑云母包括金云母、黑云母、铁黑云母和锰黑云母；锂云母是富含氧化锂的各种云母的细小鳞片。工业上尤其是电气工业中常用的是白云母和金云母。特性是绝缘、耐高温。工业上应用最多的是白云母和金云母。因其独特的晶体结构和物理化学性质，使它在电工电子、绝缘材料、化妆品、涂料、新能源等领域有广泛应用。

长期以来，人们对云母的结构、构造形态和地球化学成分开展了大量的研究，无论是云母的结构、构造形态和地球化学成分对岩浆-热液演化作用的响应，还是与成矿事件的密切联系，尤其是云母的形变和变形能够反应受力变形、区域变质热事件等。近期，云母的变形与形变作用反应也引起了人们的极大关注，尤其是美国阿拉巴马州奥本市奥本大学，Raphaël Gottardi和Gabriele Casale等在地球科学领域顶级期刊《Geology》发表了A little mica goes a long way: Impact of phyllosilicates on quartz deformation fabrics in naturally deformed rocks，针对糜棱岩中石英和云母的力学变形对矿物流动规律、地壳中的剪切作用和应变局部化机制提出了卓有成效的约束。

石英变形组构反映了糜棱岩中的应力应变条件，其解释已成为运动学和构造分析的支柱。晶粒尺寸和形状的量化以及反映变形机制的纹理的解释可以提供流动应力、应变率、运动涡度和变形温度的估计。石英流动规律的经验校准和测定是基于纯样品的实验室实验；然而，纯石英岩糜棱岩相对少见。尤其是，层状硅酸盐可定位和分配可抑制或增强的菌株不同的变形机制。实验结果表明，即使少量的层状硅酸盐含量（<15vol%）也能显著改变石英的应变行为；然而，很少有实地研究在自然环境中证明这些影响。

为了研究层状硅酸盐对石英应变组构的作用，我们量化了中新世Raft河拆离剪切带（美国犹他州西北部）石英岩糜棱岩中层状硅酸盐的含量和分布。我们使用显微结构分析和电子背散射衍射来量化石英变形组构和白云母的空间分布，并使用X射线计算机断层扫描来量化样品中白云母的含量，样品中白云母的含量在滑脱剪切带上采集。层状硅酸盐含量对石英变形机制有直接的控制作用，基于石英变形组构的压力计和流动定律的应用产生应变率和流动应力，在我们的样品中变化高达两个数量级。这些研究结果对石英岩糜棱岩流动规律的应用和中地壳剪切带的应变局部化机制具有重要意义。

图1：（A） 筏河滑脱带的综合垂直剖面。（B） 石英c轴和a轴的电子背散射衍射（EBSD）极图，取向如c.CPO晶体优先取向所示的薄片；M、 U.D.-均匀分布的倍数。（C） 样品的代表性微观结构（偏振器朝向叶理∨45°拍摄的交叉偏振光薄片照片，以最大限度地提高白云母照明度）。白云母；以下叶理。（D） EBSD图显示了重结晶（蓝色）和残留（红色）石英颗粒以及两个子集区域：一个包含纯石英（1区）和一个包含白云母（2区）。

Figure 1. (A) Synthetic vertical profile of the Raft River detachment zone. (B) Electron backscatter diffraction (EBSD) pole figures of quartz c-axis and a-axes oriented like the thin sections shown in C. CPO—crystallographic preferred orientation; M.U.D.—multiples of uniform distribution. (C) Representative microstructures of the samples (cross-polarized thin section photos taken with polarizer oriented at ∼45° to foliation to maximize muscovite illumination). musc—muscovite; Fol—foliation. (D) EBSD maps showing recrystallized (blue) and relict (red) quartz grains and two subset areas: one consisting of pure quartz (zone 1) and one containing muscovite (zone 2).

图2：（A–D）Raft河滑脱剪切带石英岩糜棱岩代表性显微结构的正交偏光薄片显微照片。残余和再结晶晶粒用黑色箭头表示。黄色箭头表示白云母钉状界线。（E–G）残余颗粒显示波状消光（ue）并包含变形片层（dl）。薄片垂直切割于叶理（Fol）且平行于线理；照片是用偏振器拍摄的，偏振器朝向叶理45°，以最大限度地提高白云母亮度。

igure 2. (A–D) Cross polarized thin section photomicrographs of representative microstructures of the Raft River detachment shear zone quartzite mylonite. Relict and recrystallized grains are shown by black arrows. Yellow arrows indicate grain boundary pinning by muscovite. (E–G) Relict grains show undulose extinction (ue) and contain deformation lamellae (dl). Thin sections are cut perpendicular to foliation (Fol) and parallel to lineation; photos were taken with polarizer oriented at 45° to foliation to maximize muscovite illumination.

图3：（A） 筏河滑脱剪切带的综合垂直剖面。符号说明见图1A。（B） 通过X射线计算机断层扫描获得白云母（Ms）含量。（C） 通过电子背散射衍射分析获得的再结晶石英颗粒（RxGS）平均直径的均方根。（D） 使用Cross等人（2017）压力计估算流动应力。（E） 基于Hirth et al.（2001）石英岩流动定律的应变率估计。半透明的蓝色和绿色框分别表示筏板河滑脱剪切带的独立流动应力和应变率估计（见讨论文本）

Figure 3. (A) Synthetic vertical profile of the Raft River detachment shear zone. See Figure 1A for symbol explanations. (B) Muscovite (Ms) content obtained from X-ray computed tomography. (C) Root mean square of mean diameter of recrystallized quartz grain (RxGS) obtained by electron backscatter diffraction analysis. (D) Flow stress estimates using the Cross et al. (2017) piezometer. (E) Strain rate estimates based on the Hirth et al. (2001) quartzite flow law. Semi-transparent blue and green boxes indicate independent flow stress and strain rate estimates, respectively, for the Raft River detachment shear zone (see text for discussion)

图4：流动应力-应变率-分析样品的高度。灰色阴影表面表示使用Hirth et al.（2001）石英岩流动定律在400°C下计算的应变率。红色阴影区域指Gottardi和Teyssier（2013）的独立应变率估计值。

Figure 4. Flow stress versus strain rate versus elevation of the analyzed samples. Grey shaded surface represents strain rate calculated using the Hirth et al. (2001) quartz ite flow law at 400 °C. Red shaded area refers to independent strain rate estimates of Gottardi and Teyssier (2013).

In this project, we have investigated the effect of muscovite on quartz deformation in the RRDSZ. Our results demonstrate a strong inverse relationship between quartz RxGS and muscovite content both between samples and within sub-millimeter-scale subregions within individual samples. The range in grain sizes between our proximal samples translates into variation in calculated strain rate of greater than one order of magnitude (∼1.1×10−14to 2.4×10−13s−1) and more than doubling of the calculated flow stress from most- to least pure quartzite (32–70 MPa). These estimates are incompatible with previous field and ther mochronology integrated strain rate estimates of the RRDSZ. Elsewhere, similar textural observations have been interpreted as evidence for polyphase deformation, reactivation, and superposition. We offer a simpler explanation that relatively small amounts of mica exert a disproportionate influence on quartz deforma tion fabrics, which has important implications for the application of flow laws in quartzite mylonites and the interpretation of strain local ization mechanisms.

在本项目中，我们研究了白云母对RRDSZ中石英变形的影响。我们的结果表明，石英RxGS和白云母含量之间存在强烈的反比关系，无论是在样品之间还是在单个样品的亚毫米尺度分区内。我们的近端样品之间的晶粒尺寸范围转化为计算应变率的变化大于一个数量级（∼1.1×10−14至2.4×10−13s−1），以及从最纯石英岩到最纯石英岩（32–70MPa）的计算流动应力的两倍以上。这些估计值和之前的RRDSZ的现场和热年代学综合应变率估计值不一致。在其他地方，类似的织构观察被解释为多相变形、再活化和叠加的证据。我们提供了一个简单的解释，即相对少量的云母对石英变形组构产生了不相称的影响，这对石英岩糜棱岩流动规律的应用和应变局部化机制的解释具有重要意义。

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