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.
Detailed structural analysis of the northern part of the Cotentin Peninsula provides evidence for major Variscan reworking of Proterozoic structures. Two stages in the regional evolution of the Variscan orogeny are distinguished: the Palaeozoic Siouville Syncline was formed before the development of a south-facing ductile overthrust. This latter stage is dated as Carboniferous in age from its relationship with the synkinematic intrusion of the Barfleur granitic pluton.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.