The 3D quantitative lattice and shape preferred orientation of a mylonitised metagranite from Monte Rosa (Western Alps): Combining neutron diffraction texture analysis and synchrotron X-ray microtomography

摘要

Two complementary 3D techniques, neutron diffraction and synchrotron X-ray microtomography (SXR-μCT), were used to compare the Shape and Lattice Preferred Orientations of a mylonitised metagranite from the Monte Rosa unit (Western Alps, Italy). The goal of using these techniques was to obtain two different orientation distribution functions. Although the two functions describe relatively independent characteristics of the rock fabric, nonetheless they also exhibit close relationships to macroscopic fabrics and may be complementarily used to quantify rock fabrics and microstructures, thereby highlighting 3D features that cannot be obtained with either technique, if used independently. We describe an approach that can be potentially useful in various disciplines, e.g., structural geology, rock mechanics, tectonics and geophysics, when a complete data set of preferred orientations and size distribution is needed. Micas display a strong orthorhombic symmetry between mesoscopic lineation and microscopic SPO and LPO, whereas quartz and feldspars are characterised by a monoclinic symmetry between mesoscopic lineation and LPO. These observations suggest a Theological decoupling between the weak phase mica layers and the stronger quartz + feldspar layers. This mechanical decoupling occurred during the Alpine subduction-collision, when the Monte Rosa unit was part of the Insubric Line system and accommodated large vertical strain.