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1 Laboratoire de Géochimie Isotopique, CNRS-UMR 5567, Université de Montpellier II, C. C. 066, Place E. Bataillon, 34095 Montpellier, France
2 BRGM, Département Métallogénie et Géodynamique, 3 Avenue C. Guillemin, BP 6009, 45060 Orléans, France
U-Pb zircon ages from volcanic rocks interbedded in two Upper Carboniferous basins of the Variscan Massif Central (France) document the presence of an unexpected late Visean sedimentary sequence. Zircons extracted from a fireclay at the base of the sedimentary pile in the Bosmoreau 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 zircons, reflecting the ages of the basement and/or source components. The Bosmoreau fireclay contains a limited amount of xenocrystic zircons having ages of ca. 390–400 Ma, whereas inheritance is much more abundant in the Decazeville rhyolite, which includes a Mesoproterozoic source component (>1.0 Ga). The ca. 330 Ma ages demonstrate that these two basins with mainly Stephanian sedimentary infill were actually initiated during the late Visean and are the first known upper crustal evidence of the Visean-Westphalian extensional event, recognized previously only in midcrustal plutonic bodies. Moreover, the location of these basins in the hanging-wall blocks of two major fault systems (i.e., the Argentat and Sillon Houiller faults) suggests a genetic link between fault motion and basin opening as early as the late Visean. These results indicate that magmatism and associated volcanism, block movement along major crustal-scale faults, erosion, and basin opening were contemporaneous and shed new light on the Visean-Westphalian synconvergent extension that was previously understood only as a deep-seated event.
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