The Southern Brittany Migmatite Belt developed during the Ligerian Orogeny within a narrow time interval 403 Ma to 372 Ma. Detailed field mapping of several key localities within the belt (Port Navalo, Port Blanc and Roguedas, around the Golfe du Morbihan, and Ville-es-Martin at St. Nazaire) has revealed a heterogeneous suite of high grade gneisses, Al-silicate rich gneisses (morbihannites), low to high melt fraction metatexites and diatexites. Careful petrography and mineralogy has allowed the establishment of reaction histories for each rock type which have been utilised in constructing segments along a P-T path. Reverse zoned rims and the replacement of garnet by Crd and/or Bt + PI suggests the following generalised reactions have occurred: Grt + Bt + S11 + Qtz~ Crd + He + Um + Kfs + H 0 and Grt --=;>Bt + PI + Qtz. tarnet-cordierite gneisses record an early prograde event. Growth zoned garnet cores and a sequence of inclusions, from the garnet core to the garnet rim, of Qtz + 11m + Ky, PI + St + Rt + Ky + Bt and PI + St + Rt + Sil + Bt constrain the prograde evolution and suggest the crossing of the simplified reactions Ms + Chl~ St + Bt + Qtz and St + Qtz ~Grt + Ky + vap. A detailed evaluation of the available thermometers and barometers, equilibrium considerations, the stability relations of biotite, and petrographic analysis have enabled the construction of a tightly constrained P-T path. It is suggested that the prograde P-T path is the result of a series of sediments deposited in an ensialic marginal basin with a higher than normal geothermal gradient which has been tectonically buried by overthrusting during basin closure. The retrograde near isothermal decompression path is interpreted to be the result of the development of a rising anatectic granite diapir which has dragged its thermal envelope, the migmatites, to shallow crustal levels during its ascent.