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岩片一般表现为能力层，通过褶皱和缠绕作用而变形。然而，在某些情况下，镁铁质岩片是不合格的，并在延长接触中显示出与竖挺相关的向外指向的尖端，呈缩短和反向褶皱。这种能力倒置很难理解。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，镁铁质岩片中的竖隆尤其常见。然而，能够解释变形机制变化导致的能级反转的片状和寄主成分矿物的显微组构数据很少。为了更好地解释基性岩席/片麻质寄主岩石组合中的竖刺和反向褶皱的生长，并解释能量性反转，我们研究了韦施尼茨岩体(Odenwald)的一个天然例子，并进行了第一次模拟实验。对天然岩样的显微组构和CT研究表明，片麻状围岩中的镁铁质岩席是在650℃的收缩环境中形成的，岩片和主岩都显示出固态扩散蠕变的证据，这种蠕变不仅受到高温的支持，而且受到小颗粒的支持。由于板材的晶粒尺寸比主体的小一个数量级，板材中的应变速率一定更快，从而导致能级反转。这一工作假说将在本提案中通过进一步研究在南部波西米亚地块和蒙塔涅黑岩中暴露的镁铁质岩片/片麻岩寄主实例来检验。微观、EBSD和CT分析将揭示组成矿物的变形微组构以及晶格和形状择优取向。我们的初步实验研究表明，在体积收缩的情况下，竖琴生长得特别快。这些研究将继续使用聚二甲基硅氧烷(PDMS)作为岩石模拟物，以确保固体扩散蠕变固有的线性粘性行为。嵌入在合适的基质中的不合格的厚度不同的单层将使用新的变形装置以不同的粘度比变形。3D扫描系统和使用透明的PDMS作为基质，将允许进行透明实验，并连续监测不断生长的具有渐进应变的骡子。实验测得的竹竿几何数据可以作为天然竹竿的古粘度计，不仅可以在镁铁质岩片/片麻岩寄主组合中观察到，而且还可以在许多其他岩石中观察到，这些岩石在不同的地球动力学环境下在不同的变质条件下变形。竖杠的经济意义的例子包括嵌在岩盐中的不合格的钾盐层，以及尖端具有多层边界和经济集中矿石的大量硫化物矿体。