Variscan tectonic evolution, magnetic anomalies and metallogenetic potential in the western Central Iberian Zone (Iberian Massif)

The Iberian Massif presents a complex Late Paleozoic evolution, with intense compressional tectonics followed by gravitational collapse of the thickened crust and orocline development. In NW Iberia, extensional detachments and associated shear zones developed during high temperature-low pressure metamorphism in relation to partial melting in gneiss domes. These structures also feature a conspicuous relationship with magnetic anomalies that define a curvature, delineating the geometry of the internal part of the Central Iberian Arc. Regardless of the geometry of these anomalies and their relationship to extensional tectonics, their source probably differs from northern to central and western Iberia. While in northern Iberia extensional tectonics triggered oxidation and development of magnetite in migmatites and S-type granites, in central Iberia basic rocks associated with I-type granites seem to be the carriers of the magnetization. This study aims to describe the western branch of the Central Iberian Arc magnetic anomaly: the Porto-Viseu-Guarda Magnetic Anomaly (PVGMA) and its metallogenetic potential previously related with magnetite-type granites.

Polyphase deformation within the Porto-Viseu metamorphic belt later affected by the Douro-Beira shear zone and Porto-Tomar fault presents syn-tectonic staurolite and sillimanite-bearing schists and migmatites (Mindelo Migmatite Complex), great abundance of syn and late S-type two mica-granites, and a post-orogenic porphyritic biotite I-type granite with uncommon high values of magnetic susceptibility (Lavadores granite). These rocks crop out at the northwestern tip of the PVGMA and are thought to be related to it. We sampled migmatites, calc-silicate resisters embedded on them and Lavadores granite for its mineralogical and magnetic characterization.

Anisotropy of the magnetic susceptibility sometimes show stable N-S to N90°E, 0°-20° E to NE plunge magnetic lineations and a WNW-ESE magnetic foliation subparallel to the shearing in the area. In migmatites, thin sections feature the expected high temperature metamorphism manifested by sillimanite and ptygmatic folding. Here, rock magnetism studies show Curie temperatures (Tc) around 300°C and low coercivities indicative of titanomagnetite or some sort of multidomain pyrrhotite. Low to moderate magnetic susceptibilities contrast with very high magnetic remanences leading to Königsberger ratios (Qn) of up to 22 in resisters and 10 in the migmatites. Contrarily, the Lavadores granite has high magnetic susceptibilities and moderate Qn (0.1-2). These rocks feature higher Tc=550° and low coercivities indicative of magnetite. Paleomagnetic results show heterogenous directions for both lithologies implying complicated thermal evolutions and possibly late tilting. Despite their proximity, no relationship seems to exist between the Lavadores granite and the Mindelo Migmatite complex protolith. Contrarily to what it is found in northern Iberia, no relationship has been found between extensional features and magnetic mineralization, so if these rocks are the source of the PVGMA, it is most probably related to the characteristics of the protoliths.

Despite the PVGMA lies on top of the Sn belt across Portugal, geochemical results do not support Lavadores as a potential Sn metallogenetic granite, further indicating the lack of relationship between the formation of magnetite and that of Sn mineralizations.

Acknowledgements: Project SA084P20 (regional CYL government); Grants PID2020-117332GB-C21 funded by MCIN/AEI/10.13039/501100011033 and TED2021-130440B-I00; Projects UIDB/04683/2020 and UIDP/04683/2020 (Portugal).