A palaeomagnetic study of two Precambrian-Cambrian dyke swarms from the Armorican Massif

Abstract The palaeomagnetic analysis of two dolerite dyke swarms, metamorphosed during the late stages of the Cadomian Orogeny, is reported. The Vazon dyke swarm of Guernsey, Channel Islands, has yielded a characteristic component of magnetization (D = 199°, I = 48°, a95 = 12°, n = 8 dykes) which is dual polarity, while six other sampled dykes record intermediate field directions. The magnetization is interpreted as being acquired shortly after dyke emplacement and the corresponding palaeomagnetic pole (lat, 9°S; long, 340°E) is in good agreement with previously determined Cambrian poles from Armorica. An emplacement age for the dykes of 560–540 Ma is therefore suggested on the basis of the radiometric ages assigned to these previously published results. The Dahouet dyke swarm of the Baie de St Brieuc was also sampled and has yielded two remanence directions (A and B) common to most sites. The A remanence (D = 198°, I = 2°, a95 = 7°, n = 7 dykes) yields a pole (lat, 37°S; long, 334°) which is interpreted as being an overprint acquired during the Variscan Orogeny and is consistent with Carboniferous results. The B remanence (D = 202°, I = 34°, A95 = 8°, N = 6 dykes) and its corresponding pole (lat, 20°S; long, 335°E) is more ambiguous in its interpretation as it lies between the known Cambrian APW path and a diffuse area of poles associated with overprinting during the Variscan orogeny. A tentative interpretation of this remanence as being Cambrian in origin is consistent with the suggestion that both dyke swarms were emplaced relatively closely in time, as suggested by their similar geological settings.