The late Proterozoic rocks of Central Brittany, represented essentially by Upper Brioverian sediments, are considered to be a late to post-tectonic molasse to the Cadomian chain. The poly-orogenic character (Cadomian and Hercynian) of the deformation structures affecting these rocks is noted from the Baie de Douarnenez to the Rennes region. However, the main syn-metamorphic deformation event is clearly of Hercynian age. Furthermore, we show that this event and the associated metamorphism are directly related to the emplacement of the Hercynian leucogranites. The syntectonic nature of the leucogranites, with respect to the deformation of their envelope, is therefore affirmed and the concept of a ‘Brioverian basement’, in the mechanical sense of the term and contrasted with a ‘Palaeozoic cover’, must be abandoned.