Granite is Never Isotropic: An Introduction to AMS Studies of Granitic Rocks

Abstract

This paper develops the author’s and co-workers’ findings that well-organized crystalline fabrics are ubiquitous in granitic rocks. Traditional structural and microstructural measurements of foliation and lineation in granites, performed directly in the field, in the laboratory from macroscopic oriented samples, and applied to entire plutons, reveal that magmatic structures may be homogeneous over huge areas. Hence structural mapping of granitoid massifs is of great interest in the understanding of magma emplacement and deformation in the crust. The low-field magnetic fabric measurement (AMS), a low cost, quick and easy technique, gives a quantitative description of the crystalline fabric and is now used systematically for structural mapping of granites. The present state of the « art » on the magnetic fabrics in granites is reviewed, based on the distinction between the paramagnetic, or magnetite-free, and the ferromagnetic, or magnetite-bearing granites. Magnetic fabrics in granitic rocks are discussed in relation to their variability at various scales and their kinematic and rheological significance. Finally, the relationship between fabrics and modes of emplacement is discussed.