The Crozon peninsula and Buçaco syncline belong respectively to the Central Armorican Domain and the Central Iberian Zone, located to either side of the Variscan ‘Ibero-Armorican Arc’ structure at present. The Ordovician sedimentary successions at both sites are known to display identical lithofacies. The paired Upper Katian formations also exhibit comparable volcanic successions of anorogenic affinity, including volcaniclastic beds and pillow lavas. The tephra deposits are inferred to have resulted from the activity of subaqueous monogenetic basaltic tuff cones above a <100 m deep calcareous soft substrate, in the context of a passive margin. The Surtseyan-style hydrovolcanic eruptions were followed by effusion of pillow lavas. Both sites also display a large range of similar peperite facies, sometimes related to magmatic segregation structures likely to derive from vapour differentiation. The deeper doleritic dykes and sills, comparable in terms of mineralogy and geochemistry, contain identical metasedimentary xenoliths, whose high-temperature phyllosilicates display a very restricted range of compositions. In the Late Ordovician, Crozon and Buçaco were probably nearby volcanic edifices belonging to a single monogenetic field inside the Armorica microplate. The sudden appearance of high-latitude Katian carbonates, associated with volcanism only at these two peri-Gondwanan sites, just preceded the Gondwanan Hirnantian glaciation.

Supplementary material: Analytical data for minerals and whole-rock samples and an M+–4Si–R2+ diagram are available at

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