EAST-WEST DIACHRONISM OF THE COLLISIONAL STAGE IN THE FRENCH MASSIF-CENTRAL - IMPLICATIONS FOR THE EUROPEAN VARISCAN OROGEN

This paper presents a review of radiometric data relevant to the collisional history of the french Massif Central, located in the southern branch of the European Variscan Belt. Previous U-Pb. Rb-Sr and K-Ar data are discussed together with Ar-39/Ar-39 mineral ages newly acquired on the high-grade metamorphic nappes of the eastern part of the massif. The petrological and tectonic setting of the dated rocks and minerals is discussed in order to provide a successful understanding of the ages. The radiometric data now available in the french Massif Central show that the main tectono-metamorphic evolution took place at least 360-340 Ma ago in the eastern part and at least 390-370 Ma ago in the western part. This diachronism is closely correlated with (i) a difference in the nature and number of units involved in the collisional process and (ii) the establishment of contrasted thermal regimes in the eastern and western parts during collision. Similar chronological, lithological and thermal evolutions are observed along the southern branch of the European Variscan Belt. These fundamental contrasts are debated in order to point out their possible consequences on the evolution through time and space of the Variscan orogen.