Palaeomagnetism of late Precambrian-Cambrian volcanics and intrusives from the Armorican Massif, France

Samples of the keratophyre tuffs of Tréguier (640 ± 12 Myr) of the Upper Brioverian series, and the microgranite dykes of Loguivy (548 ± 24 Myr) have been collected from the north coast of Brittany, France. The samples were progressively demagnetized with AF and, predominantly, thermal methods. Blocking temperatures, coercivities and acquisition of Isothermal Remanent Magnetization (IRM) indicate that the keratophyre remanence is variably carried in magnetite, hematite, or occasionally in both. After removal of a soft viscous component, two stable palaeomagnetic directions were found, one in magnetite, the other in hematite. The characteristic magnetite direction (Tb=300-580°C) is a pre-folding primary Thermoremanent Magnetization (TRM) which after structural correction has a mean declination/inclination of 217.9°/-30.4°. The younger hematite component has a mean in situ direction of 320.4°/+20.3°.