The Variscan French Massif Central - a new addition to the ultrahigh pressure metamorphic 'club': exhumation processes and geodynamic consequences

This paper presents the first documentation of coesite-bearing eclogites in the eastern French Massif Central (Monts du Lyonnais unit) and discusses the exhumation processes for these very high-pressure rocks. A combination of mineralogical and geochronological datasets allows us to quantify a depth-time path and related exhumation rates. High-pressure metamorphism is constrained to 400-420 Ma (minimum 28 kbar or ca. 90 km). By 360-380 Ma, the rocks were exhumed to 30 km depth. These kinetic results conform to the geological constraints extracted from the tectonic and sedimentary record of the eastern French Massif Central. These multidisciplinary approaches provide new information on Paleozoic orogeny and allow us to discuss the relative roles of subduction and collision in exhumation of very high-pressure rocks. We suggest that a significant amount of exhumation of these rocks occurred during subduction, prior to continental collision; continental collision itself was responsible only for the final stages of exhumation under a transpressive regime. (C) 2001 Elsevier Science B.V. All rights reserved.