New geochronological, geochemical and isotopical data on the Forez volcanism: relationship with the Cenozoic evolution of the French Massif Central upper mantle.

In the Forez district, two volcanic events are documented. The earlier one, of Paleocene age, predates the Oligocene rifting. The magmas (melilitite) are of carbonatitic affinity and they would correspond to small volume melt s responsible for the chemical and mineralogical modifications previously i dentified in the lithospheric mantle of this region. The later lavas are Lo wer Miocene and of basanitic composition. Their isotopic signatures (Sr, Nd , Pb) suggest that they partly originated from partial melting of this modi fied lithosphere. The Paleocene magmatism would indicate the initiation of asthenospheric upwelling beneath the French Massif Central. (C) 2000 Academ ie des sciences / Editions scientifiques et medicales Elsevier SAS.