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

The French Massif Central belongs to the central metamorphic domain of the European Hercynian Belt. 40 Ar/ 39 Ar mica and amphibole dates have been obtained on magmatic and metamorphic rocks from the northern part of the French Massif Central (Sioule and BrØvenne-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 3375-Ma muscovite ages from the BrØvenne 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 ªred granitesº, granophyres and subaerial acidic volcanics known as the ªTufs anthracifresº 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 synorogenic process.

Electron microprobe monazite geochronology of magmatic events: Examples from Variscan migmatites and granitoids, Massif Central, France

“…Post-thickening crustal melting responsible for Variscan magmatism and hightemperature regional metamorphism in the Cévennes area is geologically well documented (Duthou et al, 1984, Ledru et al, 1989, Pin and Duthou, 1990, Montel et al, 1992, Faure et al, 2001and Faure et al, 2002.…”

“…The latter Late Visean-Early Namurian thermal event, described from the northern part of Massif Central (e.g., Faure et al, 2002), remains poorly understood in the southern part. Forthcoming work will provide new data on the post-orogenic thermal processes in this part of the Variscan Belt.…”

Elevator tectonics and orogenic collapse of a Tibetan-style plateau in the European Variscides: the role of the Bohemian shear zone

“…In view of the equatorial paleogeographic position of the Variscan belt, such a cold climate should exist only at altitudes [4.5 km (Becq-Giraudon et al 1996). However, synorogenic extension in the internal part of the French Massif Central is recorded from 335-330 Ma (Burg et al 1994;Faure et al 2002), meaning that the orogenic collapse in the French Massif Central started somewhat later than the collapse in the Bohemian Massif.…”

“…Extensional shear zones of collapsing orogens commonly show a low to moderate dip (e.g., South Tibetan Detachment System in the Himalayan orogen, Burchfiel et al 1992; low-angle normal faults in the Basin and Range province of the western US, Coney and Harms 1984). Such types of shear zones have also been described from the Variscan belt of Europe, particularly from the French part (e.g., Echtler and Malavielle 1990;van den Driessche and Brun 1992;Faure et al 1993Faure et al , 2002Burg et al 1994). The style of orogenic collapse of the French Massif Central has been compared with that of the Tibetan plateau and the Basin and Range province (Mattauer et al 1988; Ménard and Molnar 1988).…”

Middle Carboniferous crustal melting in the Variscan Belt: New insights from U–Th–Pbtot. monazite and U–Pb zircon ages of the Montagne Noire Axial Zone (southern French Massif Central)

“…As a whole, the Carboniferous tectonic evolution is controlled by a south-directed subduction (e.g. [Matte, 1991], [Matte, 2001], [Franke, 2000], [Faure et al, 2002], [Faure et al, 2009a] and [Faure et al, 2009b]). In this framework, the migmatitic, plutonic and volcanic complexes presented above are located in the upper plate, i.e.…”