Geochronology and geochemistry of Ordovician felsic volcanism in the Southern Armorican Massif (Variscan belt, France): Implications for the breakup of Gondwana
Résumé
Low-grade, porphyritic felsic metavolcanics make up a significant volume of the South Armorican domain (Variscan belt, France). Their age and geochemistry are the first focus of this paper. These rocks have long been considered to be Silurian in age because they are either thrust over Silurian sediments (the so-called "Porphyroid Nappe") or insertedwithin the Silurian sediments, acting as a decollement level (Para-autochthon), and because of a previous zircon age. Five new U-Pb SHRIMP zircon dates for the felsic volcanics from the Porphyroid Nappe and its Para-autochthon define Ordovician ages between 473±5 and 494±4 Ma. These Ordovician metavolcanics are mainly of calc-alkaline affinity, probably in response to the reworking of the Ediacaran sediments derived from the Cadomian belt (Brioverian), variably mixed with a Paleoproterozoic (Icartian) component in metatuffs. A second focus of this paper is the integration of this volcanismat the scale of the Variscan belt. Because of its Early Ordovician age and calk-alkaline chemistry, the felsic volcanismfromthe Porphyroid Nappe is similar to thewellknownOllo de Sapo Formation in the IberianMassif.Hence, the volcanismallows a better understanding of (i) the Early Ordovician rifting along the northern Gondwana margin, and (ii) the large-scale structure of the Variscan belt from the Iberian Massif to the French Massif Central (i.e. along the Ibero-Armorican Arc).