Any news from the Icartian in the Armorican Massif in Brittany?
Résumé
The Neoproterozoic terranes throughout the circum-Atlantic (Carolina, Cadomia and Avalon) are considered as the remnants of subduction related magmatic arcs and associated basins developed off the margins of Gondwana. As they are found in very distant regions, establishing correlations between the tectonothermal evolutions for these terranes remains a challenge. One key to unravel these correlations lies in the study of their basement components. Although lim- ited in size and geographically distant, several localities (Tregor, Normandy and the Channel Islands of Guernsey and Sark) are known to expose fragments of this basement in the Cadomia terrane. All these Paleoproterozoic fragments have been described as belonging to the Icartian orogeny, named after the Icart Point in Guernsey where the reference outcrops have been first described. Although some of these Paleoproterozoic outcrops have been recently studied using modern geochronology (Guernsey, Normandy), little is known about the exposures found in Brittany. In order to shed some light on these basement rocks, we collected some samples from the Port-Beni beach (Tregor, Côtes d’Armor). Indeed, this outcrop is well exposed at low tide and contains Paleoproterozoic orthogneiss rafts embedded in granodiorite and crossed cut by E-W orientated doleritic dykes. This orthogneiss is characterized by the presence of large K-feldspath porphyroblasts in a mineral assemblage comprising quartz, chlorite after biotite, K-feldspar and plagioclase, amphibole, apatite, zircon and opaque minerals. In-situ U-Pb dating of the zircon unravel a complex history for this gneiss with core yielding a date of 2019 ± 6 Ma interpreted as the age of the granitic protolith surrounded by metamorphic rims at 641 ± 4 Ma interpreted as the age of the metamorphism responsible for the orthogneissification. Zircon from the host granodiorite yields an age of 600 ± 3 Ma identical within error with a second generation of overgrowth on the gneiss zircon at 603 ± 3 Ma and with the metamorphic age of 604 ± 3 Ma obtained on apatite grains from the orthogneiss. Last, the dolerite apatite U-Pb dating returns a date of 599 6 Ma interpreted as the emplacement age for these E-W doleritic dykes. The Paleoproterozoic age is therefore slightly younger than the most recent ages proposed for the Icartian in Guernsey and Normandy. These data also evidence for the first time the Neoproterozoic age of the Tregor doleritic dyke swarm.