Geochronological investigations have been carried out for the Mancellia granitoïdes by the Rb–Sr whole rock isochron method (λ 87Rb = 1.47 10−11 y−1). It appears that three periods of magmatism have occurred: 596 ± 12 m.y. for the main massif, 507 ± m.y. for the leucogranites, and 487 ± 15 m.y. for the Lanhélin blue granodiorite. These data, very similar to those obtained in eastern North America, should prove useful in the attempts to correlate circumatlantic basement rocks.