Eocambrian—Cambrian palaeomagnetism of the Armorican Massif, France

Abstract

In an attempt to clarify pre‐Hercynian continental configurations, palaeomagnetic data were collected from the spilites de Paimpol (640 ± 12 Myr), the diorite de St Quay (583 ± 40 Myr), the gabbro de Keralain (undated), the granite de Port‐Scarff (557 ± 16 Myr), the rhyolitic ignimbrites of Lézardrieux (546 ± 8 Myr) and the rhyolites de St Germain‐le‐Gaillard (Undated) from the unmetamorphosed to slightly metamorphosed Eocambrian–Cambrian terranes of the Domnonean Domain of northern Brittany and north‐western Normandy. Upon stepwise thermal and/or alternating field demagnetization and using vector subtraction, the spilites de Paimpol yield a characteristic direction with declination (D) = 226.4°, inclination (I) =– 15.7°. Local remagnetization by microgranite dykes (approximately 560 Myr old) produced a dircction of D= 235.1°, I=+ 63.4°. The diorite de St Quay exhibits multivectorial behaviour revealing a characteristic component of D= 31.2°, I=– 2.3°, and a secondary component of D= 299.9°, I =+ 38.2°. The gabbro de Keralain shows a characteristic component (D= 290.9°, I= 41.4°) similar to the diorite's secondary component, and gives a secondary component of D= 221.0°, I=+ 55.8°. This secondary component of the gabbro appears again as the characteristic (although somewhat scattered) directions of the granite de Porz‐Scarff. Thus, the gabbro may be relatively dated as having been magnetized before the intrusion and cooling of the granite (557 ± 16 Myr) and after that of the diorite (583 ± 40 Myr). The data from the rhyolites are the least certain because of ambiguities in the radiometric dating and uncertain bedding corrections, but they contain directions near to those of the granite and the microgranite dykes, all believed to be of the same episode of acidic volcanism. The trend in palaeomagnetic poles from the Armorican Massif for the Eocambrian–Cambrian corresponds well with coeval data from the southern United Kingdom and Czechoslovakia (Bohemian Massif), suggesting that a single continental mass, Armorica, contained all three areas. A comparison with poles from Gondwanaland also shows a remarkable agreement and suggests a period of coherent movement during the Latest Precambrian and the Cambrian for Armorica and Gondwana, followed by later separation. This leaves the Armorica plate as a separate continental unit at the outset of the Caledonian orogeny later in the Palaeozoic. Copyright © 1980, Wiley Blackwell. All rights reserved