CONTROL OF MAGNETIC ROCK FABRICS BY MICA PREFERRED ORIENTATION - A QUANTITATIVE APPROACH

Magnetic anisotropy analysis represents a well-established and frequen tly used method in structural geology. Several attempts were made to r elate the anisotropy of magnetic susceptibility (AMS) to rock fabrics and to discover the sources of the AMS. In gneissic rocks, paramagneti c phyllosilicates with lattice-dependent magnetic properties are assum ed to control the whole rock AMS, i.e. it is dominated by mica lattice preferred orientation (texture). In this study, AMS was modelled on t he basis of the mica texture, the rock composition and compared to the experimentally determined AMS. The orientations of the modelled and e xperimental tensors agree quite well, whereas all the other characteri stic parameters (shape, anisotropy, mean susceptibility) display large differences. It has been concluded that accessory high susceptible ph ases, as well as the diamagnetic rock constituents, may have significa nt influence on AMS. Consequently, mica pole figure measurements canno t be replaced by AMS measurements without control of the sources of AM S. It was also concluded, that the March model to evaluate strain from mica-preferred orientations is not valid for highly strained rocks. B eside the fact that the obtained strains are too low, the deformation is generally inhomogeneous, which leads to an overprint of the mica te xture. Since the microstructural fabric elements which originate in di fferent strain regimes (prolate or oblate) may create similar modifica tions of the mica orientation patterns, it is obvious that the relatio nship between AMS and deformation is rather complex.