B. Auvray, J. Mace, Ph. Vidal, R. Vander Voo; Rb-Sr dating of the Plouézec volcanics, N Brittany: implications for the age of red beds(‘Series rouges’) in the northern Armorican Massif. Journal of the Geological Society ; 137 (2): 207–210. doi: https://doi.org/10.1144/gsjgs.137.2.0207
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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.