This occurred on the margin of the Gondwana continent, involving one or more collisions of island arcs and accretion of other material at a subduction zone.
The precise events, and geographical position, are uncertain, but are thought to involve the terranes of Avalonia, Armorica and Iberia.
The interpretation is that the belt was formed as oceanic crust subducted below the Armorica land mass in a similar way to the Andes.
Sediments deposited on the continental margin were pushed up onto the continent, at the same time as intrusions of calc-alkaline magmas occurred.
The pre Cadomian basement rocks consist of Orosirian or Paleoproterozoic Icart gneiss dated at close to 2,018 million years ago.
Foliated quartz diorite occur at Baie de St Brieuc, at Coutances, La Hague, Guernsey, Alderney and Sark.
The Rance valley has a metamorphic sequence including phyllites, amphibolite, metatexites, and diatexites which have melted into granite.
At the Baie de St Brieuc and near Coutances in Manche and Trégor region there are pillow lavas, basalt erupted under water.
A later stage of acid volcanics in the form of andesite and rhyolite have been erupted on Brioverian sediments on Jersey at 533 million years ago, and at St Germain-le-Gaillard in Lower Normandy.
The southern edge of the Gulf of Saint-Malo between Tregor and Cancale shows the deformational structures of the Cadomian Orogeny.
Secondly P. J. Treloar considers that the North Armorican Massif was put together from a series of terranes 540 million years ago, their joins are the shear zones.