The Cadomian active margin (North Armorican Massif, France): a segment of the North Atlantic Panafrican belt

Abstract

Within the European Variscan belt, the Armorican Massif (northwestern France) contains an isolated fragment of the North Atlantic Panafrican belt — the Cadomian block. Mostly unaffected by Variscan deformation, this block is composed of Neoproterozoic rocks, ranging in age from 620 to 540Ma (Brioverian), locally overlying a composite basement of Icartian (2000Ma) or Pentevrian (750Ma) age.The Cadomian tectonics, the last magmatic manifestations of which are dated at around 540Ma, were sealed by a cover of Early Cambrian rocks in Normandy and Early Ordovician rocks in Brittany.Five units are distinguished in this Cadomian block — Trégor, Saint-Brieuc, Guingamp, Saint-Malo and Fougères. Although the units are bound by major faults, the overall coherence of their different protolithic characteristics suggests that they can be fitted into a simple geodynamic model.The Trégor Unit, represented by the North Trégor Batholith of crustal affinity and its Icartian host rock, probably reflects an early Cadomian evolution.The Saint-Brieuc Unit, essentially made up of magmatic rocks of juvenile affinity, includes two assemblages juxtaposed by the Cadomian tectonics: (1) a volcanosedimentary succession formed in a typical island-arc setting reflecting subduction around 610Ma; and (2) a volcanic–plutonic complex emplaced around 590Ma, in an extensional active continental margin of Pentevrian age.The Guingamp Unit, predominantly migmatitic, includes a tectonic slice of deep origin, the extrusion of which accompanied the exhumation of the migmatites; it may represent the trace of a possible suture.The Saint-Malo Unit is composed of Brioverian detrital rocks, the significance of which is uncertain — passive continental margin or accretionary prism; it is structured around a late migmatitic dome (540Ma).The Fougères Unit corresponds to the outer domain of the Cadomian orogeny; it was only weakly deformed by Cadomian tectonics, and is intruded by the large Mancellian Batholith of crustal origin, dated at 540Ma.Together the structural units are interpreted in the framework of an evolving active margin juxtaposing continental margin, marginal basin and volcanic arc, before being shortened during the Cadomian convergent tectonics.