Magnetic susceptibility anisotropy and fabrics in granites

The principle of anisotropy of magnetic susceptibility measurement, or AMS, and elements of magnetic mineralogy applied to granitic rocks, are presented. AMS measurement allows the study of the magnetic fabric of magnetite-free, paramagnetic granites in which the iron-bearing silicates carry the signal, and ferromagnetic granites dominated by magnetitie. The case of mixed magenetic mineralogies is also evoked. Several examples demonstrate that crystal fabrics in granites, as deduced from AMS studies, are remarkably homogenous from the sample scale to the scale of a whole pluton. This was totally unforeseen up to now. Origin of crystal fabrics in granites is briefly discussed in terms of preferred orientation of crystals acquired in the deforming magma before its full crystallisation. Kinematic and rheologic consequences of these fabrics are evoked and their perspective leading to a renewal of geological studies in basement rocks dominated by granitic rocks is stressed. (C) 2000 Academie des/Sciences/Editions scientifiques et medicales Elsevier SAS.