Magmatisme fissural triasico-liasique dans l'ouest du Massif armoricain (France): pétrologie, géochimie, âge, et modalités de la mise en place

Abstract

The dolerite dykes at the western end of Britanny (France), which is located on the North Atlantic margin, were emplaced in a NW–SE field of faults at the Trias–Lias time limit, between 210 and 195 Ma. These rocks are quartz tholeiites enriched in TiO2, with a trace element composition close to that of the Deccan basalts. The initial concentrations of Ba, K, and Rb, though slightly modified by the alteration, and of Nb seem to have been affected by crustal contamination during the magma transfer. These dykes are comparable, in age and composition, to the tholeiitic dykes exposed in the Iberian Peninsula, in the Moroccan Atlas and in the Northern Appalachians; they however have a different orientation. Furthermore, these various domains differ in their postintrusion history: the rifting leading to the opening of the Central Atlantic occurred soon after the intrusions of the dykes, whereas the opening of the North Atlantic began several tens of millions of years later, following the direction of the Armorican array of dykes. The study of the mode of emplacement of the intrusions shows evidence of two sets of dykes: "simple dikes", 5 to 10 m thick, resulting from a single magmatic injection and "multiple dykes", about 30 m thick, composed of five magmatic units that are individually 5 to 10 m wide. The western Armorican dyke field is the result of the intrusion of successive sets of dolerite having a uniform thickness and composition, suggesting a cyclic magmatic supply from homogeneous source. [Journal Translation]

Basement Control on Shaping and Infilling of Valleys Incised at the Southern Coast of Brittany, France

“…2), that lie between the coast and the horst. The faults were reactivated episodically during the Cenozoic (Vigneresse, 1988;Le Corre et al, 1991;Caroff et al, 1995), mainly in response to NW-SE-oriented Alpine compression (Bevan and Hancock, 1986;Muller et al, 1992;Hibsch et al, 1993). Some of them still have a seismogenic signature (Lenôtre et al, 1999;Perrot et al, 2005).…”

Origin of widespread Cretaceous alkaline magmatism in the Central Atlantic: A single melting anomaly?

“…Our new plateau age of 76.0 ± 0.7 Ma obtained on JAR basaltic sample has to be interpreted with care since it was obtained on groundmass (see discussion above). Since in basaltic samples, in particular MORB, potassium is mainly incorporated into the groundmass, slight pervasive alteration could have biased the age even if the rock displays low LOI (Caroff et al, 1995;Merle et al, 2009). The sample shows a LOI slightly high for a fresh MORB (2.6%) and the petrographic evidence for seawater alteration (presence of clay minerals, chlorite and epidote, Houghton, 1979) suggests that our new age does not represent the crystallisation of the rock.…”

Pour une géoarchéologie du patrimoine : pierres, carrières et constructions en Bretagne

“…La roche peut être rattachée provisoirement à la famille gabbroïque. (Caroff et al, 1995). La ténacité des dolérites et le rapprochement des diaclases qui les parcourent peuvent constituer de graves handicaps à leur utilisation.…”