Résumé
Cinq unités lithologiques sont distinguées au sein des terrains volcaniques cadomiens de l'est du Trégor (Bretagne Nord) : 1) les rhyolites ignimbritiques de Lézardrieux -faciès rhyolitique principalement ignimbritique -, 2) les tufs de Tréguier -laves et pyroclastites intermédiaires (andésites et dacites) -, 3) les rhyolites de Loguivy - roches acides vitreuses, parfois litées, plus ou moins porphyriques présentant un faciès granophyrique subordonné (microgranites de Loguivy ss) -, 4) les microgranodiorites de Launay-Pleubian - roches porphyriques microgrenues surtout intermé- diaires, rarement acides, à mésostase granophyrique à sphérolitique -, 5) le granite de Pomelin-Bréhat - roche microgrenue à grenue comparable à certaines des microgranodiorites précédentes. En Baie de Saint-Brieuc, les rhyolites ignimbritiques de Lézardrieux, présentent un fort pendage vers le sud et passent en continuité vers le nord aux tufs de Tréguier. Au nord de cet ensemble lavique, les tufs alternent en bancs décamétriques avec les microgranodiorites de Launay-Pleubian. Ces dernières recoupent les rhyolites de Loguivy, en filons et masses intrusives superficielles, et passent en continuité vers le nord au granite de Pomelin-Bréhat. Ces observations, comme les similitudes pétrographiques entre les différentes unités nord-trégorroises, nous conduisent à réinterpréter l'ensemble comme un complexe "volcano-plutonique" unique, mis en place au cours d'épisodes successifs vers 615 Ma. Au sud, cet ensemble est en contact tectonique avec les spilites de Paimpol, l'accident pouvant être observé dans la vallée du Trieux, au niveau de Lézardrieux. Ces résultats posent le problème de la signification des âges cambriens (~ 550 Ma), obtenus par la méthode Rb/Sr (roche totale), sur les rhyolites de Lézardrieux et Loguivy.
Abstract
Data collected from three sections (Baie de Saint-Brieuc, Trieux river valley and Jaudy river valley) across the eastern part of the Trégor region resulted in a new interpretation of the mode of emplacement and stratigraphy of the Cadomian volcanic rocks in this part of North Brittany. Following previous work (Barrois, 1908; Auvray et al., 1976; Auvray, 1979), five types of eruptive or intrusive rocks are grouped into coherent formations: (1) the Lézardrieux Ignimbritic Rhyolite, composed of strictly acidic rocks of mainly pyroclastic origin; (2) the Tréguier Tuff, composed of intermediate to acidic lavas (andesites and dacites) and pyroclastites; (3) the Loguivy Rhyolite which includes slightly to strongly acidic lavas with a glassy matrix and subordinate granophyre (Loguivy Microgranite ss,); (4) the Launay-Pleubian Microgranodiorite, composed of fine-grained and porphyritic rocks of intermediate to acidic composition with a granophyric to spherulitic matrix; (5) the Pomelin-Bréhat Granite, a fine- to medium-grained granitoid similar to some of the microgranodiorites of the Launay-Pleubian Formation. In the Baie de Saint-Brieuc, the Lézardrieux Ignimbritic Rhyolite is steeply inclined toward the south and grades into the Tréguier Tuff, to the north. Acidic rocks similar to the Lézardrieux Ignimbritic Rhyolite also occur within the Tréguier Tuff. These observations are not consistent with the existence of a major break between the two formations. Based on petrographic and field evidence a shallow level intrusive origin may be deduced for the Loguivy Rhyolite; such evidence includes the recognition of autobrecciated rocks at the top and bottom of units, flow folds in bedded rhyolites, pyroclastic and even ignimbritic horizons. In the Baie de Saint-Brieuc and in the Trieux river valley, the Loguivy Rhyolite is cut by shallow intrusive bodies several metres to tens of metres in size attributed to the Launay-Pleubian Microgranodiorite. To the north, this unit grades into the more homogeneous Pomelin-Bréhat Granite, and to the south it is in contact with the Tréguier Tuff: this contact is visible in the Jaudy river valley, where alternating bands of tuff and microgranodiorite crop out over a distance of more than 500 m, suggesting that the latter were emplaced as sills within the former. The intermediate to acidic volcanic and plutonic formations of the Trégor are interpreted as different episodes of a single magmatic cycle, dated around 615 Ma (Graviou, 1984). On its southern side, this "volcano-plutonic" complex is in contact with the mainly basaltic Paimpol Spilite Formation. This contact may be observed in the Trieux river valley and is clearly tectonic. Finally, the Cambrian ages (550 Ma) obtained by Auvray (1975) from the Lézardrieux and Loguivy units (Rb/Sr method) is inconsistent with the field data. Do these ages reflect a resetting of the Rb/Sr system long after the eruption of the lavas ?
Dernière mise à jour le 29.07.2015