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 present ed. AMS measurement allows the study of the magnetic fabric of magnetite-fr ee, paramagnetic granites in which the iron-bearing silicates carry the sig nal, and ferromagnetic granites dominated by magnetitie. The case of mixed magenetic mineralogies is also evoked. Several examples demonstrate that cr ystal fabrics in granites, as deduced from AMS studies, are remarkably homo genous from the sample scale to the scale of a whole pluton. This was total ly unforeseen up to now. Origin of crystal fabrics in granites is briefly d iscussed in terms of preferred orientation of crystals acquired in the defo rming magma before its full crystallisation. Kinematic and rheologic conseq uences of these fabrics are evoked and their perspective leading to a renew al of geological studies in basement rocks dominated by granitic rocks is s tressed. (C) 2000 Academie des/Sciences/Editions scientifiques et medicales Elsevier SAS.