A late-Ordovician phreatomagmatic complex in marine soft-substrate environment: The Crozon volcanic system, Armorican Massif (France)
Résumé
The mafic lavas and the diabases of Crozon (Armorican Massif, France), belong to an anorogenic Ordovician volcanic complex, emplaced on a rifted passive margin in North Gondwana. Magma passed through syn-volcanic soft sedimentary substrate, which is today mostly composed of alternating sandstones and mudstones, from Llanvirn to Ashgill in age. Field observations together with microscopic studies and geochemical analyses of magmatic rocks lead us to propose a model of volcano formation which combines hydromagmatic processes, peperitic intrusions, a shallow submarine tephra settling, eruption-fed turbidity currents, and a pillow lava effusion. The Crozon outcrops can be used to reconstruct a complete cross-section from the root of the volcanic complex to the lavas and breccias emplaced on the sea floor. The sites expose: (i) a hypabyssal breccia containing mud chunks and coarse-grained diabase clasts with amoeboidal fine-grained magmatic material; (ii) bulbous peperitic sills and pillow-like lobes bearing a great quantity of sediment-derived enclaves of fluidal morphology; (iii) volcaniclastic breccias containing near-spherical magmatic clasts that resulted from the complete fragmentation of sills in the ductile regime; (iv) a rhythmic peperitic breccia interpreted as the product of mingling between thin lava flows and soft calcareous sediment. The Crozon volcanic form, resulting from explosive interaction with subsurface/surface water, was probably a subaqueous collapsed tuff cone. This upper part of the system is synchronous with an Ashgill carbonate sedimentation, which overlies an Ordovician siliciclastic succession deposited in shelf environments.
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