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

高品位变质岩的能力反演：来自竖框的见解

• 批准号: 424098293
• 负责人: Professor Dr. Gernold Zulauf
• 金额: --
• 依托单位: Ústav geologie a paleontologie
• 依托单位国家: 德国
• 项目类别: Research Grants
• 财政年份: 2019
• 资助国家: 德国
• 起止时间: 2018-12-31 至 2022-12-31
• 项目状态: 已结题
• 来源: https://gepris.dfg.de/gepris/projekt/424098293?language=en
• 关键词: Competence; inversion; grade; metamorphic; rocks

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.

片麻岩主岩中的镁铁质片层通常表现为有能力的层，它们通过折叠和布丁变形而变形，然而，在某些情况下，镁铁质片层是无能力的，并在这种反转中显示出与缩短和反向褶皱相关的向外指向的尖端。已发表的结果表明，如果变形发生在上部角闪岩到麻粒岩相条件下，则镁铁质片中的竖框特别常见。片状和主体成分矿物的微结构数据在很大程度上缺乏，这些数据可以解释变形机制变化引起的能力反转，以进一步阐明镁铁质片状/片麻岩主体岩石组合中竖框和反向褶皱的生长。为了解释能力倒置，我们研究了来自韦施尼茨岩体（Odenwald）的自然例子，并对自然例子进行了首次模拟实验，结果表明：片麻岩主岩中的镁铁质片在 T > 650°C 的收缩环境中形成竖框，片和主岩都显示出固态扩散蠕变的证据，这不仅得到高温的支持，而且得到了小晶粒尺寸的支持。由于片材的晶粒尺寸比主体的晶粒尺寸小一个数量级，因此片材中的应变速率一定更快，从而导致能力反转，该工作假设将在本提案中通过进一步研究镁铁质片材/进行测试。片麻岩在波希米亚地块南部和黑山中暴露的主要例子，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方向，我们的初步实验研究表明，竖框生长得特别快。这些研究将继续使用聚二甲基硅氧烷（PDMS）作为岩石类似物，以确保线性粘性行为，这是固态扩散蠕变所固有的。具有不同厚度的不合格单层嵌入有能力的基质中，将使用新的变形装置以不同的粘度比变形，并且使用透明的PDMS作为基质将允许透明实验和连续监测。具有渐进应变的生长竖框的几何数据可以用作天然竖框的古粘度计，其不仅可以在镁铁质片/片麻岩主体组合中观察到，还可以在许多其他岩石在不同的地球动力学环境下的各种变质条件下变形，竖框的经济意义的例子包括嵌入岩盐中的不完整的钾盐层，以及具有竖框边界和尖点处矿石的经济集中度的块状硫化物体。