High-precision U-Pb zircon ages for explosive volcanism calibrating the NW European continental Autunian stratotype.
Résumé
Volcanism was intense in western European basins during the Late Carboniferous–Early Permian transition, related to the late Variscan extension and associated magmatism. In the Autun Basin, explosive eruptions, expressed as highly weathered distal ashfall deposits (tonsteins), were preserved in lacustrine to swamp environments. The analysis of eleven tonsteins, confirmed as non-reworked volcanic horizons, indicates a highly explosive acidic calc-alkaline volcanism, with active volcanic centres probably linked to Massif Central or Vosges–Black Forest magmatism. New accurate CA-ID-TIMS U–Pb ages are obtained from four of these tonsteins, recorded within the lower Autunian (Igornay and Muse Fms), which defines the lower part of the Autunian stratotype in the Autun Basin. The middle part of the Igornay Fm is dated at 299.9 ± 0.38 Ma, the Lally oil-shale bed is dated at 298.91 ± 0.08 Ma and the upper part of the Muse oil-shale bed is dated between 298.05 ± 0.19 and 298.57 ± 0.16 Ma. For the first time, a numerical calibration of the regional continental Autunian stage in its stratotype is proposed, thus allowing correlation with global marine stages defined in the Urals, and highlighting a late Gzhelian/early Asselian age for the lower Autunian. The tonstein in the Lally oil-shale appears to demarcate the Carboniferous–Permian boundary within the ± 100 kyr resolution of the U-Pb ages in both the Autun and southern Uralian basins. Comparison with other European domains confirms that these new ages are consistent with well-known macrofloral assemblages, characteristic of the Autunian.