Anisotropy of magnetic susceptibility and magmatic structures in the Guérande Granite Massif (France)

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Abstract
A multidisciplinary approach has been used in order to map out the syntectonic magmatic structures of the Guérande leucogranite in southern Brittany (Western France): on one hand, extensive field work and microscopic observations, and on the other, a detailed magnetofabric investigation, have been carried out. The former showed a magmatic flow plane trending generally to the ENE‐WSW with a moderate dip to the north and a flow direction slightly dipping toward NNE. These observations suggest a blade shaped body which is not deeply rooted. The magnetofabric as defined by the anisotropy of magnetic susceptibility is mainly related to hematite‐ilmenite grains with a mean rock susceptibility of about 3.10−6 G/Oe. There is an excellent agreement between the magnetic lineation (Kmax) and the magmatic flow direction; the agreement between the magnetic foliation (Kmax‐Kint plane) and the magmatic flow plane is not as good. The quantitative analyses of the anisotropy allows one to group the data into two domains: the one with planolinear ellipsoids corresponding to the central part of the granitic body where flow was mainly achieved on the viscous state and the other in the flattening domain related to the northern border area which exhibits incipient orthogneissification, that is, some deformation on the solid state. Such a favorable comparison with the classical structural method is encouraging for using the anisotropy of magnetic susceptibility technique more routinely for structural mapping of granitic bodies, provided that the microstructural state of the rock is already well known.