Competence inversion in high-grade metamorphic rocks: Insights from mullions

片麻岩寄主岩中的基性片层一般表现为赋存层，受褶皱和束缚作用而变形。然而，在某些情况下，基层是不称职的，并显示向外指向的尖端，这些尖端与在短接触和在扩展接触中的反向褶皱有关。这种能力的倒置很难理解。已发表的研究结果表明，如果变形发生在上部角闪岩-麻粒岩相条件下，基性薄片中的千粒状体尤其常见。然而，可以通过变形机制的变化来解释能力反演的薄片和主体成分矿物的微组构数据在很大程度上是缺乏的。为了更好地解释基性片麻岩/片麻岩储集岩组合中横棱纹和逆褶皱的生长，并解释能力反演，我们研究了Weschnitz岩体（Odenwald）的一个自然例子，并进行了首次模拟实验。微组构和计算机断层扫描（CT）研究表明，片麻质寄主岩中的基性薄片是在约650℃的收缩环境中形成的。薄片和基体均表现出固态扩散蠕变，这不仅是高温的支持，也是小晶粒尺寸的支持。由于薄片的晶粒尺寸比基体的晶粒尺寸小一个数量级，因此薄片中的应变速率必然更快，从而导致能力反演。本提案将通过进一步研究南波西米亚地块和黑山地区暴露的基性片麻岩宿主实例来检验这一工作假设。显微镜、EBSD和CT分析将揭示变形微结构以及组成矿物的晶格和形状偏好取向。我们的初步实验研究表明，在体积收缩的情况下，mullion生长得特别快。这些研究将继续使用聚二甲基硅氧烷（PDMS）作为岩石模拟物，以确保线性粘性行为，这是固态扩散蠕变所固有的。利用一种新的变形装置，将具有不同厚度的失效单层嵌入在有效基质中，以不同的粘度比进行变形。3D扫描系统和透明PDMS作为基质的使用，将允许进行透明实验，并对具有渐进应变的生长mullions进行连续监测。实验竖片岩的几何数据可以作为天然竖片岩的古粘度计，它不仅可以在基性片麻岩主组合中观察到，而且可以在不同地球动力学背景下在各种变质条件下变形的许多其他岩石中观察到。具有经济意义的例子包括嵌套在岩盐中的不称职的钾盐层，以及具有乳石边界的块状硫化物体和尖端的矿石经济浓度。

Mafic sheets in gneissose host rocks behave generally as competent layers, which are deformed by folding and boudinage. In some cases, however, the mafic sheets are incompetent and show outward pointing cusps related to mullions in shortened and inverse folds in extended contacts. This inversion in competence is hardly understood. Published results indicate that mullions in mafic sheets are particularly common if deformation occurred under upper amphibolite to granulite facies conditions. However, microfabric data of the constituent minerals of sheet and host, which could explain the competence inversion by changes in deformation mechanisms, are largely lacking. To shed more light on the growth of mullions and inverse folds in mafic sheet/gneissose host rock associations, and to explain competence inversion, we studied a natural example from the Weschnitz pluton (Odenwald) and carried out first analogue experiments. Microfabric and computertomographic (CT) studies of the natural example revealed that the mafic sheet in gneissose host rock was mullioned in a constrictional environment at T > 650°C. Both the sheet and the host show evidence for solid-state diffusion creep, which is supported not only by high temperature, but also by small grain size. As the grain size of the sheet is one order of magnitude smaller than that of the host, the strain rate in the sheet must have been faster resulting in competence inversion. This working hypothesis will be tested in the present proposal by studying further mafic sheet/gneissose host examples exposed in the southern Bohemian Massif and in the Montagne Noire. Microscopic, EBSD and CT analyses will reveal the deformation microfabrics as well as lattice- and shape-preferred orientation of the constituent minerals. Our preliminary experimental studies suggest that mullions grow particularly fast under bulk constriction. These studies will be continued using Polydimethylsiloxan (PDMS) as rock analogue to ensure linear viscous behavior, which is inherent to solid-state diffusion creep. Incompetent single layers with varying thickness, embedded in a competent matrix, will be deformed at varying viscosity ratios using a new deformation apparatus. A 3D scanning system and the use of transparent PDMS as matrix will allow see-through experiments and a continuous monitoring of the growing mullions with progressive strain. The geometric data of the experimental mullions can be used as a paleoviscosimeter to natural mullions, which can be observed not only in mafic sheet/gneissose host associations, but also in many other rocks deformed under various metamorphic conditions in different geodynamic settings. Examples of economic significance of mullions include incompetent layers of potash salt embedded in rock salt, and massive sulfide bodies with mullioned boundaries and economic concentrations of ore in the cusps.