The tectonothermal evolution of the Cadomian belt of northern Brittany, France: a Neoproterozoic volcanic arc

Abstract

The Neoproterozoic Cadomian belt (northern Brittany, France) results from the accretion of crustal blocks that record distinctive P–T-deformation paths. In addition, abundant gabbro-dioritic to granitic intrusions during and after deformation influence the thermal pattern and are used as time markers. As a whole, the deformation history of the Cadomian belt is characterised by a migration in time of the active zones, which become younger towards the Southeast.Relics of Eocadomian rocks reveal that arc development took place at about 660–640Ma. Abundant calc-alkaline magmatism intruding strongly deformed Palaeoproterozoic orthogneisses characterise the northernmost unit of the Cadomian belt, i.e. the Trégor Unit, which cooled before 600Ma. The Lanvollon basin, a short-lived intra-arc rift containing abundant mafic and felsic volcanics followed by greywackes, opened at about 600–580Ma.The subsequent tectonothermal history of the Cadomian belt is divided into two major episodes. The first episode is related to the shortening of the intra-arc rift, and its southwestward thrusting over the Guingamp Unit. This led to crustal thickening, inducing partial melting in the Guingamp Unit. Exhumation of the latter rocks is accommodated by late extensional shearing along the boundary between the Saint-Brieuc and Guingamp Units. Cooling of the Saint-Brieuc Unit is dated at about 570–560Ma. The second episode is dominated by sinistral wrenching mainly localised in the eastern part of the Cadomian belt, at the boundary between the Domnonean (i.e. the arc domain) and Mancellian (possibly a fore-arc basin) Domains. Several shear zones developed during this episode, some of them being associated with minor granitic intrusions dated around 550Ma. The sinistral shear located Southeast of the arc domain is reworking or crosscutting earlier high-temperature fabrics within the Saint-Brieuc Unit. In addition, the vertical component of this shearing episode contributes significantly to the exhumation of the Saint-Malo migmatites.Near the Proterozoic/Cambrian boundary, cordierite–biotite granitoids intruded at shallow depths the Fougères Unit, their emplacement outlasting deformation of the sedimentary sequences. Minor stocks of muscovite leucogranites of Cambrian age are possible equivalents of the abundant volcanism in an adjacent NW–SE-trending rift. This marks a major change in the kinematic framework, with predominance of extension. Finally, minor reactivation of the Cadomian structures occurred during the Carboniferous, associated with the Variscan orogeny.