New 40 Ar/ 39 Ar constraints for the “Grande Nappe”: The largest rhyolitic eruption from the Mont-Dore Massif (French Massif Central)

Abstract Since the 1960s, an early explosive activity in the Mont-Dore Massif is associated with a major pyroclastic rhyolitic eruption (5–7 km 3 ) known as the “Grande Nappe” (GN). This event, linked to the formation of a 6-km-diameter cryptic caldera named “Haute Dordogne”, was before our investigation dated by 40 Ar/ 39 Ar at 3.07 ± 0.04 Ma. Our new single-crystal laser fusion 40 Ar/ 39 Ar dates obtained on two outcrops of the GN (Rochefort-Montagne and Ludieres) questioned several hypotheses made concerning this “landmark” event of the Mont-Dore Massif history. We demonstrate that: (1) the GN rhyolitic eruption has occurred much later than previously estimated ( i.e. 2.77 ± 0.02–0.07 Ma full external uncertainties); (2) the correlation made between the Vendeix rhyolitic complexes (intra-caldera position) dated back to 2.74 ± 0.04 Ma and the GN is proposed; (3) xenocryst contamination could be very high ( i.e. 70% for the Rochefort-Montagne GN outcrop) and explains the noticeable older age obtained previously; (4) a link between the GN eruption and the formation of a caldera is questionable; the hypothesis of a northward-oriented blast channeled eastward toward the paleo-Allier River is thus proposed.