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

片麻岩寄主岩石中的基性岩席一般表现为强层状，它们通过褶皱和石香肠作用变形。然而，在某些情况下，镁铁质岩片是不合格的，并显示出向外指向的尖点相关的竖框缩短和反向褶皱在延长接触。这种能力的倒置很难理解。已发表的结果表明，如果变形发生在上角闪岩麻粒岩相条件下，镁铁质岩席中的竖框是特别常见的。然而，显微组构数据的组成矿物片和主机，可以解释能力反转的变形机制的变化，在很大程度上是缺乏的。为了更好地阐明镁铁质岩片/片麻岩寄主岩石组合中竖框和逆褶皱的生长，并解释能力反转，我们研究了Weschnitz岩体（Odenwald）的自然实例，并进行了第一次模拟实验。对该自然样品的显微组构和计算机断层扫描（CT）研究表明，片麻岩寄主岩石中的镁铁质岩席在T> 650 ° C的收缩环境中被竖框化。片材和主体都显示固态扩散蠕变的证据，这不仅由高温支持，而且由小晶粒尺寸支持。由于片晶的晶粒尺寸比基体的晶粒尺寸小一个数量级，片晶中的应变速率一定更快，从而导致能力反转。在本提案中，将通过进一步研究南波希米亚地块和黑山暴露的镁铁质岩片/片麻岩宿主实例来检验这一工作假设。显微镜，EBSD和CT分析将揭示变形显微组构以及晶格和形状的组成矿物的优选取向。我们的初步实验研究表明，竖框增长特别快，下批量收缩。这些研究将继续使用聚二甲基硅氧烷（PDMS）作为岩石模拟，以确保线性粘性行为，这是固有的固态扩散蠕变。用一种新的变形装置，将不同厚度的非主管单层嵌入主管基质中，在不同的粘度比下进行变形。3D扫描系统和透明PDMS作为基质的使用将允许透视实验和连续监测具有渐进应变的生长竖框。实验中的几何数据可以作为一个古粘度计，以自然的竖框，这不仅可以观察到在镁铁质片/片麻岩主机组合，但也在许多其他岩石变形的各种变质条件下，在不同的地球动力学设置。竖框的经济意义的例子包括嵌在岩盐中的钾盐层，以及具有竖框边界的块状硫化物体和尖点中的矿石经济浓度。

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.