A two-stage exhumation in Western French Massif Central: New geochronological evidences of syn-collisional extension
Résumé
Systematic 40Ar/39Ar dating of granitoids and surrounding metamorphic rocks has been performed in the Limousin area (W of French Massif Central) in order to provide new constrains concerning its late-orogenic thermal and tectonic evolution. The same 40Ar/39Ar age of ca. 335-338 Ma is found on the present-day outcrop surface for most of the Limousin metamorphic rocks, hence indicating that the entire massif was uniformly cooled down at this period. These ages are very close to the 40Ar/39Ar ages proposed in the Sioule and Brevenne areas in the literature, but also to the coupled tectonic and magmatic activity in the south Limousin. These results help to document the existence of two exhumation stages, each one associated with an extensional stage. A first exhumation stage, has been active at ca. 335-340 Ma. Available data indicate that this extension and associated exhumation and cooling occurred over a distance of more than 200 km in the internal zones of the belt, whereas compression was still active in the northern and southern parts of the belt. A second exhumation stage may be related to the late-orogenic extension and collapse of the belt, at ca. 310-305 Ma, and is characterized by the uplift of the St. Sylvestre complex by unroofing of the overlying metamorphic and granitoidic formations.