Magma–sediment mingling in an Ordovician rift basin: The Plouézec–Plourivo half-graben, Armorican Massif, France

The Plouézec–Plourivo Basin (PPB), Armorican Massif, is an Ordovician half-graben filled with red-bed deposits lying on a Cadomian metamorphic basement. Peperites crop out along its northeast coastal sector, in connection with diverse magmatic and hydrothermal products. Combining field structural observations and geochemical analyses of magmatic rocks leads us to propose a model of emplacement of peperites in an extensional setting. The PPB peperites result from mingling between magma and initially wet sediment. The main corresponding facies consist of rounded magmatic bodies, corrugated lava–sediment contacts, and a remarkable thin sheet of fluidized sediment extending over about 100 m, parallel and c. 10 cm above the base of a porphyritic lava flow. A few PPB peperites formed under a dominant brittle regime are autoclastic breccias, at the base of a lava flow, and polymict breccias, including both fluidal and blocky volcanic clasts. Sediment fluidization, formation of vapor films, magma–sediment density contrasts, and non-explosive fragmentation are the main mechanisms invoked to generate the PPB peperites. The c. 100 m extended sediment-derived sheet lying near the base of a lava flow likely formed by diapiric rise of fluidized sediment through molten lava. We show that silicic contamination accompanied these peperitic processes.