The Cadomian Orogeny is characterized by two major tec-tonometamorphic events, one that pre-dates 570 Ma and another at c. 540 Ma. Until recently, the majority of migmatites occurring within the Cadomian belt of northern Brittany were interpreted as having been formed during the second event. The discovery of the Jouguet migmatites, which have been hornfelsed by the St-Brieuc diorite, emplaced at 533 ± l2 Ma, suggests that a migmatitic event occurred before 540 Ma. These data demand a reinterpretation of the structural sketch of the Cadomian Orogeny.

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