Fábricas magnéticas secundarias originadas por anisotropía de distribución en el plutón sincinemático de Brealito (Salta, Argentina)

This work deals with the structural interpretation of the anisotropy of magnetic susceptibility fabrics of samples from the magnetite-bearing Brealito granite, a small synkinematic pluton emplaced 472 ± 2 Ma ago in the Puncoviscana Formation at Salta, in the NW corner of Argentina. Most ot the fabrics are of primary, magmatic origin and can be correlated with the crystallographic orientation ot biotite crystals. However, some samples have developed secondary magnetic fabrics due to the imprint ot a weak deformation, which is associated to the development of ductile shear zones coeval with the pluton emplacement. In these cases, the magnetic fabrics reflect a distribution anisotropy due to magnetic interactions between neighbour magnetite grains. From the microstructural point of view, such a distribution anisotropy can be related with the opening of tension microfractures that were produced during the deformation of biotite crystals by intracrytalline slip. These microfractures display a preferred orientation at 60°° to the (001 ) biotite-planes. They are filled in by magnetite grains that are elongate at right angle to the fracture-walls and rest in close contact with the nearest grains, which could explain the high anisotropy of the magnetic susceptibility provided by these samples.