Paleomagnetic evolution of the Armorican Massif in Late Paleozoic times, in the light of overprints recorded in Cadomian and Paleozoic units

Abstract

Listen

Translate article

Previous investigations have shown that in Paleozoic outcrops of the Variscides, the major part of the paleomagnetic record consists of overprints. After identification and dating of these overprints, the magnetic record can be used to constrain the geotectonic evolution of the investigated area after the major tectonic event. In order to test this hypothesis, a paleomagnetic investigation was carried out on various volcanic and plutonic units from Northern Brittany (48.5°N/3°W), emplaced and metamorphosed in the Late Proterozoic, during the Cadomian orogeny. To calibrate the overprints in terms of their ages, doleritic dykes intruded into the Cadomian basement in the Early Paleozoic, as well as Early Carboniferous intrusive and extrusive rocks were also sampled. Thermal demagnetization often shows multivectorial behaviour of most NRMs and indicates a wide range of magnetic components, characterized by various unblocking temperatures. A large majority of the components isolated in Cadomian units display directions that are consistent whith the results from the younger, Paleozoic units. With exception of the 〈D〉 components (mean VGP: 7°S/152°E) which are interpreted as being Ordovician in age, all other mean directions/poles agree with results obtained from Visean volcanic, plutonic and metamorphic rocks of the central part of the Variscan belt. This means that no Cadomian magnetization has survived the tectonic, magmatic and metamorphic phases that have affected the Cadomian basement during the Paleozoic. The major part of the magnetic overprints were acquired during the Variscan orogeny, particularly in relation to Visean and Westphalian magmatism.The Early Carboniferous (340–328Ma) 〈Cn〉 (7°S/53°E), 〈Co〉 (29°S/60°E) and 〈Cp〉 (51°S/62°E) components, the Middle Carboniferous (328–310Ma) 〈B〉 components (11°N/124°E), the Late Carboniferous (310–300Ma) 〈A1〉 (30°N/151°E), the Late Permian 〈A〉 (51°N/154°E) and the Jurassic 〈J1〉 overprints (72°N/107°E) delineate an apparent polar wander path that can be interpreted in terms of geotectonic evolution. Since the Late Visean, this evolution was the same as for the Massif Central, the Vosges, the Black Forest and the Odenwald-Spessart. The variable inclinations of the 〈C〉 components suggest a northwestward tilting of the area north of Saint-Brieuc and a possible latitudinal drift of the whole massif in Late Visean times. The 〈C–B〉 and 〈B–A1〉 segments of the APWP reveal two successive clockwise rotations by 70–80° in the latest Visean and by 30° in the Late Namurian–Early Westphalian. The first rotation closed the Rhenohercynian basin. During the last rotation the Variscan belt was tied to northern Europe. The 〈A〉 and 〈J1〉 overprints give evidence of hydrothermal activity in Northern Brittany in the Late Permian and the Early Jurassic.