Control of magnetic rock fabrics by mica preferred orientation: a quantitative approach

Abstract

Magnetic anisotropy analysis represents a well-established and frequently used method in structural geology. Several attempts were made to relate the anisotropy of magnetic susceptibility (AMS) to rock fabrics and to discover the sources of the AMS. In gneissic rocks, paramagnetic phyllosilicates with lattice-dependent magnetic properties are assumed to control the whole rock AMS i.e. it is dominated by mica lattice preferred orientation (texture). In this study, AMS was modelled on the basis of the mica texture, the rock composition and compared to the experimentally determined AMS. The orientations of the modelled and experimental tensors agree quite well, whereas all the other characteristic parameters (shape, anisotropy, mean susceptibility) display large differences. It has been concluded that accessory high susceptible phases, as well as the diamagnetic rock constituents, may have significant influence on AMS. Consequently, mica pole figure measurements cannot be replaced by AMS measurements without control of the sources of AMS. It was also concluded, that the March model to evaluate strain from mica-preferred orientations is not valid for highly strained rocks. Beside the fact that the obtained strains are too low, the deformation is generally inhomogeneous, which leads to an overprint of the mica texture. Since the microstructural fabric elements which originate in different strain regimes (prolate or oblate) may create similar modifications of the mica orientation patterns, it is obvious that the relationship between AMS and deformation is rather complex.