The whole-rock Rb-Sr isochron date of 472 ±5 Ma obtained on the Plouézec volcanics, N Brittany, is interpreted as an emplacement age and thus indicates that the 'Séries rouges' in the Bay of St Brieuc (including probably the Erquy-Fréhel Group as well), generally considered to be the equivalent of the Old Red Sandstone, is in fact of Lower Palaeozoic age.

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