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Armorican Massif |
Laboratoire de géologie de Normandie occidentale, Université de Caen 14032 CAEN Cedex, France
The oldest Precambrian rocks of the northeastern Armorican Massif are represented by the migmatites and gneisses of La Hague which make up the Lower Proterozoic Pentevrian basement. This is overlain by the low-grade Upper Proterozoic rocks of the Brioverian succession which were affected by Cadomian deformation and plutonism.
The Lower Brioverian Group consists of four tholeiitic volcanic formations overlain by terrigenous units which contain some black chert horizons. In the Coutances area this assemblage was affected by a local tectonometamorphic event which led to the formation of the Constantian arc (585 Ma). The uplift and erosion of this arc yielded the Upper Brioverian flysch which includes spilitic rocks at Vassy and sulphide deposits at Rouez.
A plate tectonic model is proposed for the Cadomian orogeny in the northeastern Armorican Massif. Major and trace element geochemical features of the Brioverian igneous rocks allow us to assign it to a ‘Pacific’ tectonic setting. An island-arc system was established on Pentevrian crust remnants and led to magmatic arcs separated by marginal basins floored by MORB. Strain analysis shows that Cadomian thrusts and folds verge northwestwards, synthetic to southeasterly-directed subduction. A second southeastward-dipping subduction zone developed and was responsible for the closure of the back-arc basin and the generation of the Mancellian Batholith. The folded Upper Brioverian rocks were then intruded by these late-tectonic granodiorites at c. 540 Ma. The Brioverian rocks and Cadomian plutons were then uplifted, eroded and unconformably overlain by the Lower Cambrian cover.
In the northeastern Armorican Massif, this Cadomian evolution corresponds to a major period of crustal growth and represents progressive cratonisation of the Brioverian basin and arc system.
