Late Visean thermal event in the northern part of the French Massif Central. New 40Ar/39Ar and Rb-Sr isotopic constraints on the Hercynian syn-orogenic extension.

Abstract : he French Massif Central belongs to the central metamorphic domain of the European Hercynian Belt. 40Ar/39Ar mica and amphibole dates have been obtained on magmatic and metamorphic rocks from the northern part of the French Massif Central (Sioule and Brévenne-Violay series). The 336- to 337-Ma dates recorded by a granodiorite (the St. Gervais pluton) and its mylonitic border along the Ste Christine fault and the 337-5-Ma muscovite ages from the Brévenne series are consistent with the age of wrenching related to the late stage of the Hercynian compression. The remaining age spectra from the Sioule area provide ages ranging from 331 to 335 Ma that are interpreted as cooling ages related to rock exhumation after the main stages of metamorphism and deformation in the studied area. Late Visean dates widespread in North Massif Central correspond to a huge magmatic event characterized by the emplacement of "red granites", granophyres and sub-aerial acidic volcanics known as the "Tufs anthracifères" formation. Rb-Sr isotopic data from volcanic and hypovolcanic rocks from the Sioule and Montagne Bourbonnaise areas place constraints on the source of the magmas. Simple metapelite crustal melting or a major contribution from depleted mantle sources are ruled out, and instead, mantle-derived magmas were probably involved, at least as a heat source, during the Late Visean magmatism. Exhumation and cooling of metamorphic rocks below 300°C occurred contemporaneously with this Late Visean magmatism. In the geodynamic evolution of the Hercynian Belt, the Late Visean (ca. 335-330 Ma) magmatism corresponds to a major break marking the onset of post-collisional extension in the inner zone which had been thickened by collision in Latest Devonian-Earliest Carboniferous. This Late Visean extension in the inner zone which is synchronous to compression in the northern and southern forelands of the belt appears as a syn-orogenic process.