The solid geology of the area between Binic and Brehec, Cotes du Nord, France

Abstract

The area mapped is situated on the west side of the Baie de Saint Brieuc, Cates du Nord, France. It is bounded by the towns of Binic, Brehec, Lantic and Plehedel. Rocks of Pentevrian, Brioverian and post Brioverian age were recorded. The Pentevrian, which was previously unreported, is composed of the pelitic and semipelitic Port Goret gneisses, the metasediments and metavolcanics of the Plouha Series and the late Pentevrian intrusion, the Port Moguer tonalite. The Port Goret gneisses and the Plouha Series had undergone at least 4 major fold phases and 4 regional metamorphic events prior to the Brioverian sedimentation. Locally the structural and metamorphic histories are found to be more complex. However the Port Moguer tonalite has a relatively simple structural history.
Two sequences of Brioverian age are recorded, the Binic Brehec Series and the Palus plage metasediments and metavolcanics. The sedimentology of the former has been studie~ and it is conluded that they are a sequence of distal turbidites. The latter are composed of pelites, psammites and acid volcanics and they display an unconformable relationship with the Port Goret gneisses. The original surface of unconformity may not be preserved. The Brioverian and locally the Pentevrian have been affected by the late Precambrian Cadomian orogeny, which has produced 3 main phases of folding and up to five phases of metamorphism. The grade of anyone metamorphic event varies throughout the area. Late Cadomian brittle structures are recorded.
The Port Goret gneisses are intruded by a post-Brioverian, synCadomian composite norite, hornblende ferropypersthene gabbro and quartz hornblende gabbro intrusion, the Saint Quay intrusion. Petrological and petrochemical studies indicate that this body was formed by two intrusive phases, the second of which was more differentiated than the first.
The regional significance of this research has been discussed.