A well-constrained clockwise P-T path has been established for the Southern Brittany Migmatite Belt. The prograde P-T path is characterized by a large dT/dP and is abruptly interrupted by a retrograde stage consisting of near isothermal decompression followed by near isobaric cooling. The migmatites are attributed to anatexis, which was the result of thermal relaxation in slightly overthickened crust, followed by decompression during the ascent of an anatectic granite diapir which dragged its thermal envelope, the migmatites, to shallower crustal levels. An evaluation of data from the migmatite belt and several other possibly allochthonous key elements of southern Armorican geology has been incorporated into a coherent tectonic model which accounts for the origin and subsequent involvement of micaschists, blueschists, eclogites, orthogneisses and migmatites juxtaposed by later Variscan strike-slip tectonics in the Palaeozoic evolution of southern Armorica. The tectonic model involved subduction, the creation of a volcanic arc and its associated back-are basin and their eventual destruction during continent-continent collision.