Magnetic fabrics in Portuguese Variscan granites: structural markers of the Variscan orogeny

This work focuses on the magnetic fabric of 20 variscan granitic massifs from northern and central Portugal and considers the Anisotropy of Magnetic Susceptibility (AMS) results obtained in about 750 sampling sites. In the northern and central Portugal, three main ductile deformation phases were recognized and described: D₁, D₂ and D₃, being the variscan magmatism events mainly related to D₃ phase. D₃ produced wide amplitude folds with NW-SE subhorizontal axial plane and subvertical dextral and sinistral ductile shear zones, forming obtuse angles with the maximum compression direction, σ1, NE-SW oriented. The post-D₃ brittle phase was responsible for the development of conjugate faults (NNW-SSE, NNE-SSW and ENE-WSW), related to a N-S maximum compression. The studied granites were subdivided according to U-Pb dating, field observations and considering the chronology of their emplacement relative to the D₃ phase of Variscan orogeny. Therefore, the studied granites are subdivided into: (1) syn-D₃ two-mica granites, ca. 311 Ma; (2) late-D₃ monzogranites, biotite-rich and two-mica granites, ca. 300 Ma; (3) post-D₃ monzogranites and biotite-rich granites, ca. 299 – 297 Ma. Magnetic fabric gives two types of directional data, magnetic foliations and magnetic lineations, which provide important information regarding the orientation of the magmatic flow, feeder zone location, relationship between the magma emplacement and tectonics and, also, the stress field. The data obtained for the magnetic fabric, based on AMS technique, allowed concluding: (i) syn-D₃ granites show magnetic foliations and lineations consistent with the syn-D₃ variscan structures ca. N110°-120°E, related to a NE-SW maximum stress field . The foliations are, mainly, subvertical (> 60º), which may indicate a high thickness of the granitic body and deep rooting; on the other hand, the magnetic lineations exhibit variables plunges. (ii) Late-D₃ granites are characterized by foliations and lineations, dominantly NNW-SSE to NNE-SSW oriented. The foliations are subvertical dips (> 60º) and the lineations have, generally, soft plunges. (iii) Post-D₃ granites have, in general, magnetic foliations and lineations associated with important regional post-D₃ brittle structures, which display NNE-SSW and ENE-WSW trending. The subhorizontal fabric may suggest a small thickness of the granitic bodies. In all granite sets under study there is a dominance of weakly dipping lineations (slope <60º), indicating that the feeding zones are deep, which supports the idea of an emplacement at high structural levels.

Acknowledgments: The authors thank Department of Geosciences, Environment and Spatial Planning at Faculty of Sciences of the University of Porto and the Earth Sciences Institute (Porto Pole, Project COMPETE 2020 (UID/GEO/04683/2013), reference POCI-01-0145-FEDER-007690).