P-T-T-DEFORMATION PATHS RECORDED BY KINZIGITES DURING DIAPIRISM IN THE WESTERN VARISCAN BELT (GOLFE DU MORBIHAN, SOUTHERN BRITTANY, FRANCE)

The high-grade metamorphic rocks of southern Brittany underwent a complex tectonic evolution under various P-T conditions (high-P, high-T), related to stacking of nappes during Palaeozoic continent-continent collision. The east to west thrusting observed in the whole belt is strongly perturbed by vertical movements attributed to the ascent of anatectic granites in the high-T area. The field reconstruction of subvertical, closed elliptical structures in gneisses and migmatites, associated with the subhorizontal, doubly radial pattern of stretching lineation in the mica schists, suggests the existence of an elliptical diapiric body buried at depth beneath the present erosion level. Deformation is associated with a complex P-T evolution partly recorded in aluminous gneisses (kinzigites, e.g. morbihanites). A chronology of successive episodes of mineral growth at different compositions is established by detailed studies of the mineral-microstructure relationships in X-Z sections, using the deformation-partitioning concept (low- and high-strain zones). Several thermometric and barometric calibrations are applied to mineral pairs either in contact or not in contact but in equivalent microstructural positions with respect to the deformation history. This methodology provides a continuous microstructural control of P-T variations through time and leads to three P-T-t-d paths constructed from numerous successive P-T estimations. Path 1 is a clockwise retrograde path preserved in low-strain zones, which records general exhumation movements after crustal thickening. Paths 2 and 3 are clockwise prograde/retrograde paths from high-strain zones; they are interpreted and discussed in the light of models of crustal anatexis and upward movement of magma (diapirism). Deformation and P-T effects induced by diapirism can be distinguished from the general deformation-metamorphic history of a belt, and would seem to be produced during a late stage of its history. The present microstructural-petrological approach to defining successive mineral equilibria in relation to progressive deformation steps provides a far more accurate evaluation of the metamorphic evolution than is possible by 'standard' thermobarometry.