This study presents new structural and monazite chemical U-Th/Pb geochronological constraints for the magmatic rocks of the Golfe du Morbihan area, in southern Brittany, south of the South Armorican shear zone (SASZ). A major extensional shear zone, defined here as the “Sarzeau shear zone” (SSZ), separates Carboniferous migmatites and the Ste-Anne d’Auray type anatectic granite from highly retrogressed micaschists in its footwall and hangingwall, respectively. Late Carboniferous leucogranite dykes, called the Sarzeau granite that intrude the Lower Unit are progressively sheared and mylonitised within the SSZ. The SSZ is characterised by a low to moderately SE-dipping foliation and a NW-SE trending stretching lineation. Kinematic criteria indicate a top-to-the-SE sense of shear. Below the SSZ, NNE-SSW-trending, leucogranitic dykes sometimes present a wall-parallel magmatic layering. These dykes that intrude into vertical NW-SE trending migmatites are interpreted here as emplaced as tension gashes, whose opening direction is consistent with the NW-SE regional stretching. The 316-321 Ma U-Th/Pb ages yielded by the monazite in the dykes comply with the interpretation of a synkinematic magmatism. In the Golfe du Morbihan, geometric relationships between the SSZ and the migmatitic host rocks do not support a previous interpretation as a metamorphic core complex. Regionally, the SSZ kinematics is consistent with the Late Carboniferous orogen-parallel extension, already recognised in other areas of southern Armorica, but does not support the 200 km-long flat detachment fault model.