Palaeomagnetism of late precambrian-cambrian volcanics and intrusives from the armorican massif, France

Russell Perigo (1) , Rob van Der Voo (1) , Bernard Auvray (2) , Norbert Bonhommet (2)

1 Department of Geological Sciences [Ann Arbor]
2 CAESS - Centre Armoricain d'Etude Structurale des Socles

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°. The microgranite Natural Remanent Magnetization (NRM) intensity distribution is distinctly bimodal, forming high and low (≤ 1.6 × A m-1) intensity groups. Upon demagnetization, a single stable remanence (in magnetite) was removed between 300 and 580°C or 20 and 100 mT. Two characteristic directions were found to be statistically related to the intensity groups at the 99 per cent confidence level. Low-intensity samples have a mean direction of 201.9°/+2.3° and probably resulted from a Late Devonian - Early Carboniferous local magnetization. High intensity microgranites have a mean direction of 200°/+50.8° and this remanence is shown to be a primary TRM. Palaeopoles obtained from the two keratophyre magnetizations (magnetite 44.7°N, 116.4°E; hematite 39.5°N, 233.6°E), as well as the high intensity microgranite remanence (5.8°S, 339.9°E), show good agreement with similar age poles from the apparent polar wander path for the Armorican Massif. Comparison of these data to recent palaeomagnetic data from southern England and Wales gives additional evidence to link that region with the Armorica plate as proposed by Van der Voo and Hagstrum et al.

Mots clés: GEOPHYSIQUE; PALEOMAGNETISME; PRECAMBRIEN; CAMBRIEN; MASSIF ARMORICAIN; INTRUSION; DOMAINE NORD ARMORICAIN; ROCHE VOLCANIQUE

Domaines: Géophysique [physics.geo-ph]

Russell Perigo, Rob van Der Voo, Bernard Auvray, Norbert Bonhommet. Palaeomagnetism of late precambrian-cambrian volcanics and intrusives from the armorican massif, France. Geophysical Journal International, 1983, 75 (75), pp.235-260. ⟨10.1111/j.1365-246X.1983.tb01922.x⟩. ⟨insu-01537985⟩