LATE VISEAN HIDDEN BASINS IN THE INTERNAL ZONES OF THE VARISCAN BELT - U-PB ZIRCON EVIDENCE FROM THE FRENCH MASSIF-CENTRAL

U-Pb zircon ages from volcanic rocks interbedded in two Upper Carbonif erous basins of the Variscan Massif Central (France) document the pres ence of an unexpected late Visean sedimentary sequence. Zircons extrac ted from a fireclay at the base of the sedimentary pile in the Bosmore au basin provide an age of 332 +/- 4 Ma, identical to the 333 +/- 2 Ma age obtained from a rhyolitic lava sampled in the Decazeville basin. In addition, both volcanic rocks contain inherited or xenocrystic zirc ons, reflecting the ages of the basement and/or source components. The Bosmoreau fireclay contains a limited amount of xenocrystic zircons h aving ages of ca, 390-400 Ma, whereas inheritance is much more abundan t in the Decazeville rhyolite, which includes a Mesoproterozoic source component (>1.0 Ga). The ca, 330 Ma ages demonstrate that these two b asins with mainly Stephanian sedimentary infill were actually initiate d during the late Visean and are the first known upper crustal evidenc e of the Visean-Westphalian extensional event, recognized previously o nly in midcrustal plutonic bodies. Moreover, the location of these bas ins in the hanging-wall blocks of two major fault systems (i,e,, the A rgentat and Sillon Houiller faults) suggests a genetic link between fa ult motion and basin opening as early as the late Visean, These result s indicate that magmatism and associated volcanism, block movement alo ng major crustal-scale faults, erosion, and basin opening were contemp oraneous and shed new light on the Visean-Westphalian synconvergent ex tension that was previously understood only as a deep-seated event.