| Popis: |
The detrital volcanic horizons in Vendee, called “porphyroides” correspond to an important period or rhyolitic volcanism in the Armorican part of the Western European Hercynian Belt. Their stratigraphical assignment is still debated. Using the UPb zircon dating method, acid volcanic and volcanic clastic rocks from the Mareuil-sur-Lay Formation in Vendee are found to be 405 ± 5 Ma old. Interbedded black cherts yield tubular elements of unknown organic origin, up to now only previously recorded in the Upper Silurian and the Early Devonian of Bohemia (Czechoslovakia) and Artois (northern France). On the other hand, closely related carbonaceous remains have been discovered in other south Armorican “porphyroodes” formations. Therefore, the age of 405 ± 5 Ma is an additional datum for the understanding of the calibration of the Silurian-Devonian boundary. Thereby, there exist at least two major episodes of acidic volcanism with porphyritic characteristics. One, well represented in the Iberian Peninsula (Ollo de Sapo), is situated at the base of the Paleozoic; it results from a period of crustal melting at the end of the Ca-domian Orogeny. The other, of Siluro-Devonian age, is studied in this paper. It has a calc-alkaline trend and is marked by ancient crustal contamination. It is linked to an acidic volcanism related to an active continental margin, contemporaneous with eo-Hercynian metamorphic events. |
