Magnetic fabrics and emplacement mechanisms of Valpacos and Freixo de Numao Variscan granites (Northern Portugal)

The Valpacos (ValG) and Freixo de Numao (FNG) ilmenite-type granites, were studied to understanding ascent and emplacement mechanisms and deformation patterns evolution during the end of Variscan orogeny in Northeastern Portugal. Anisotropy of magnetic susceptibility, microstructural and microfracturing studies were performed in both granites. The correlation between magnetic anisotropy and deformation pointed out that ValG magnetic fabric resulted from the combination of magmatic and tectonic fabrics, the last one less expressive. The magnetic lineations (K-1) WNW-ESE trajectories suggesting an ascending and emplacement assisted by pre-existing Variscan structures with similar orientation. The ascent was enabled by the presence of these structures, even so the spreading of magma to south was forceful since the metasedimentary foliations are adapted to the granite. The circular trajectories defined by the vertical magnetic foliations suggest a symmetric laccolith shape intrusion for ValG. The FNG magnetic fabric mimics the magmatic fabric inherited during the passive ascent and emplacement. The magnetic fabric suggests an ascent and emplacement assisted by NW-SE structures. The magnetic lineations describe trajectories to NE and SW consistent with a NW-SE dyke feeder zone located near the SW part. The magnetic trajectories defined by the directional parameters suggest an asymmetric shape for the FNG, with its dome located in the NE part. The deformation patterns evolution showed that ValG displays ductile to brittle-ductile deformation and FNG shows ductile deformation, both developed during the crystallization. The brittle structures recorded in both granites allow to reconstruct the maximum compressive stress (sigma(1)) rotation from NE-SW to N-S and, later from NW-SE to WNW-ESE.

Automated summary

The Valpacos and Freixo de Numao granites in Northeastern Portugal were studied to understand the ascent and emplacement mechanisms and deformation patterns evolution. The magnetic fabrics of ValG were influenced by a combination of magmatic and tectonic fabrics, while FNG inherited a magmatic fabric from passive ascent and emplacement. Brittle structures recorded in both granites help reconstruct the rotation of maximum compressive stress.