Heterogeneous origin of the magnetic anomalies of the Central-Iberian Arc. Constraints on the source of the Central System Magnetic Anomaly

The Iberian Massif presents a geometry characterized by two oroclines: the conspicuous and tight Ibero-armorican arc to the north and the older and less pronounced Central Iberian Arc (CIA) to the south. The latter is clearly depicted by low amplitude, short wavelength magnetic anomalies in its external part and long wavelength, higher amplitude magnetic anomalies in its internal part. The origin of these is under study as they seem to be indicators of the deep evolution of this orogen in the Iberian Peninsula.

Three magnetic anomalies stand out in the internal part of the CIA. To the north, the Eastern Galicia Magnetic Anomaly overlaps the Lugo Gneiss Dome, a structure delineated by extensional detachments. Here, the exhumation at high temperatures during late Variscan gravitational collapse triggered the formation of magnetite in metasediments, migmatites and S-type syn-orogenic granites, thus stablishing a clear relationship between tectonics and magnetization. To the west, the Porto-Viseu-Guarda Magnetic Anomaly has still an unclear origin, but it also overlaps an area characterized by extensional tectonics, gneiss domes development and granite intrusion. However, the most magnetic outcropping rocks are late Variscan I-type granites (Lavadores granite) and the Mindelo Migmatitic Complex. None of them shows a relationship between magnetization and extension. Contrarily, magnetic minerals seem to be related to the composition of the resisters in migmatites and to the formation of the Lavadores granite itself. Finally, the Spanish Central System Magnetic Anomaly, at the core of the CIA, also overlaps the exhumed products of late Variscan extension (granites and migmatites). The magnetic anomaly associated to this part of the arc has been studied in the Castellanos Antiform, a discrete extensional dome located in its northern part, where the interaction between high degree metasediments, extension, and migmatization can be revised. New high resolution magnetic and gravity data indicate that the magnetic anomaly coincides with a high Bouguer gravity anomaly, and overlaps an outcrop of granitoids with tonalitic xenoliths and gabbros. The relationship between gravity and magnetic anomalies, together with the lack of outcropping magnetic granitoids and/or migmatites in the Central System, and the high amount of heterogeneous xenoliths, including basic rocks, suggest that in central Iberia, late Variscan extension might have involved deeper levels of the crust and maybe the mantle. Considering the location of this area, in the core of the CIA, and the simultaneity between late Variscan extension and the CIA formation, we suggest that the development of the latter might have played an important role in the supply of mantle material.

Funding:  grant PID2020-117332GB-C21 and projects SA084P20,  MCIN/AEI/10.13039/501100011033 and TED2021-130440B-I00