Structure and tectonic evolution of the Armorican Massif

Abstract

Report of a meeting of the Tectonic Studies Group held at Burlington House, 14 March 1979. The meeting was organized by Dr A. J. Barber and Dr P. R. Cobbold. The papers presented provided a timely review of the major advances during the last decade in understanding the structure and tectonic evolution of the Armorican Massif. Thestablishment of special research groups at the Universities of Rennes, Brest, Caen and Nantes, supported by the CNRS, has been the prime factor in this development. As a result, the geological evolution of this region is probably better understood than that of other Nercynian Massifs in central and western Europe. The geological evolution of the Armorican Massif can be traced back more than 2000 Ma, beginning with the deposition of a Lower Proterozoic or late Archaean supracrustal sequence which was polyphase deformed, metamorphosed, and intruded by granitic and quartz-dioritic magmas between 2500 Ma and 1900Ma to produce the Pentevrian basement of the northern part of the Massif. Professor CognC portrayed the Pentevrian as a fragment of continental crust originally closely related to the W African Craton, which similarly exhibits magmatism around 2200 Ma to 1900 Ma (Eburnian event). It was upon this basement and within adjacent oceanic domains that the Upper Proterozoic Brioverian Supergroup was deposited. The closure a portion of this ocean, the segment lying NW of Capdela Hague-Tregor Pentevrian axis and S continental crust represented by the Precambrian southern Britain, was, according to the model proposed by Auvray & Lefort, the result