Anisotropy of magnetic susceptibility as the key to understand the ascending and emplacement mechanisms of granitic plutons: a review

The present study includes a multidisciplinary approach applied to seven ilmenite-type granite intrusions outcropping in Northern and Central Portugal, inside the so-called Galícia-Trás-os-Montes (GTMZ) and Central Iberian (CIZ) geotectonic zones. The main goals are to understand the ascent, emplacement mechanisms and deformation patterns evolution during the late stages of Variscan orogeny. To this end, different granites were studied, which currently exemplify distinct levels of the intrusion suites, as they intrude metasedimentary rocks of low metamorphic grade (namely, Freixo de Numão granite (FNG)), or the contact of metasedimentary-granites rocks (such as, Capinha granite (CaG) and Valpaços granite (ValG)), or other granites (e.g., Caria-Vila da Ponte pluton (CVPP) and Esmolfe-Matança pluton (EMP)). Anisotropy of magnetic susceptibility (AMS), microstructural and bulk magnetic susceptibility (K_m) studies were performed. The correlation between AMS and microstructures pointed out that the studied granites exhibit magnetic fabric that mimics the magmatic fabric inherited during a passive ascent and emplacement under an extensional regime. On the other hand, ValG magnetic fabric resulted from the combination of magmatic and tectonic fabrics, being the last one less expressive. AMS data allowed to infer the trajectories drawn by the magnetic foliations providing important clues about the granite intrusions shape. Therefore, distinct 3D shapes were suggested: bell-shape intrusion for CVPP; tongue-shape intrusion for CaG; asymmetric laccolith for EMG and FNG; and, inverted flat drop-shape intrusion for ValG. The fieldwork relationships between the granite intrusions, host rocks and local/regional structures together with AMS results allowed to verify that the ascent and emplacement of studied intrusions were structural controlled using structures like shear zones (ValG, EMG and FNG) and deep strike-slip faults (CaG and CVPP).