Was there a super-eruption on the Gondwanan coast 477 Ma ago?

Abstract

Precise zircon and monazite ID-TIMS U–Pb dating of three Lower Ordovician altered ash-fall tuff beds (K-bentonites) in the Cantabrian Zone of NW Iberia yielded coeval ages together with an equivalent previously studied sample (477.5±1 (Gutierrez-Alonso et al., 2007)), of 477±1.3Ma, 477.2±1.1Ma and 477.3±1Ma, with a pooled concordia age (all analyses in the four samples) of 477.2±0.74Ma. A conservative estimation of the volume and mass of the studied K-bentonite beds (using exclusively the CZ data) yields a volume for the preserved deposits of ca. 37.5km3 (Volcanic Explosivity Index — VEI=6, Colossal). When considering other putative equivalent beds in Iberia and neighboring realms (i.e. Armorica, Sardinia) the volume of ejecta associated to this event would make it reach the Supervolcanic–Apocalyptic status (VEI=8, >1000km3). At variance with most known cases of this kind of gigantic eruption events, geological observations indicate that the studied magmatic event was related to continental margin extension and thinning and not to plate convergence. We speculate that a geochronologically equivalent large caldera event recognized in the geological record of NW Iberia could be ground zero of this super-eruption.