Geochemical Study of the Brioverian (late Proterozoic) Volcanic Rocks in the Northern Armorican Massif (France). Implications for Geodynamic Evolution During the Cadomian

Abstract

Summary A major and trace element geochemical study has been made of the Brioverian volcanic rocks in the NW of Brittany. They are compared with recent and modern volcanic rocks to identify their magmatic characteristics and thus to relate them to different tectonic contexts and integrate them into an overall geodynamic interpretation. Three complexes in different locations across the Cadomian orogenic belt have been studied: The volcanic rocks in Lannion Bay are typical of calcalkaline rocks of frontal volcanic arcs associated with subduction. The volcanic rocks of St Brieuc Bay are similar to tholeiitic within-arc or back-arc volcanic rocks, associated with internal extension at a convergent plate margin. The Fresnaye Bay volcanic rocks, however, resemble those from extensional within-plate settings and are quite different from the other two volcanic associations. These various volcanic settings are integrated into a general model of geodynamic evolution in the Armorican Massif during the Cadomian, to which features of the regional geology lend support.