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

Previous investigations have shown that in Paleozoic outcrops of the Varisc ides, the major part of the paleomagnetic record consists of overprints. Af ter identification and dating of these overprints, the magnetic record can be used to constrain the geotectonic evolution of the investigated area aft er the major tectonic event. In order to test this hypothesis, a paleomagne tic investigation was carried out on various volcanic and plutonic units fr om Northern Brittany (48.5 degreesN/3 degreesW), emplaced and metamorphosed in the Late Proterozoic, during the Cadomian orogeny. To calibrate the ove rprints in terms of their ages, doleritic dykes intruded into the Cadomian basement in the Early Paleozoic, as well as Early Carboniferous intrusive a nd extrusive rocks were also sampled. Thermal demagnetization often shows m ultivectorial behaviour of most NRMs and indicates a wide range of magnetic components, characterized by various unblocking temperatures. A large majo rity of the components isolated in Cadomian units display directions that a re consistent with the results from the younger, Paleozoic units. With exce ption of the (D) components (mean VGP: 7 degreesS/152 degreesE) which are i nterpreted as being Ordovician in age, all other mean directions/poles agre e with results obtained from Visean volcanic, plutonic and metamorphic rock s of the central part of the Variscan belt. This means that no Cadomian mag netization 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, particu larly in relation to Visean and Westphalian magmatism. The Early Carboniferous (340-328 Ma) (Cn) (7 degreesS/53 degreesE), (Co) (2 9 degreesS/60 degreesE) and (Cp) (51 degreesS/62 degreesE) components, the Middle Carboniferous (328-310 Ma) (B) components (11 degreesN/124 degreesE) , the Late Carboniferous (310-300 Ma) (A1) (30 degreesN/151 degreesE), the Late Permian (A) (51 degreesN/154 degreesE) and the Jurassic (J1) overprint s (72 degreesN/107 degreesE) delineate an apparent polar wander path that c an be interpreted in terms of geotectonic evolution. Since the Late Visean, this evolution was the same as for the Massif Central, the Vosges, the Bla ck Forest and the Odenwald-Spessart. The variable inclinations of the (C) c omponents 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 degrees in the latest Visean and by 30 degrees in the La te Namurian-Early Westphalian. The first rotation closed the Rhenohercynian basin. During the last rotation the Variscan belt was tied to northern Eur ope. The (A) and (J1) overprints give evidence of hydrothermal activity in Northern Brittany in the Late Permian and the Early Jurassic. (C) 2001 Else vier Science B.V. All rights reserved.