Île Dumet (Armorican Massif, France) and its glaucophane eclogites: the little sister of Île de Groix

Abstract

Blueschist-facies rocks are scarce within the Variscan orogen. Two main occurrences are known in the Armorican Massif (NW France), at Île de Groix and Bois-de-Céné. Another glaucophane occurrence was discovered in 1988 but went unnoticed; it is located on Île Dumet, an uninhabited island off the coast of southern Brittany, in the estuary of the river Vilaine. Orthogneiss occurs on the SW half of the island; the original granitoid magma had intruded mica schists on the NE half, where numerous 1 to 10 m long boudins of mafic rocks occur. These lenses are typically retrogressed into plagioclase-bearing amphibolite, but a few contain remnants of glaucophane-bearing eclogite, which also occurs as numerous loose blocks along the NE coast of the island, suggesting that the best-preserved eclogites lie in situ offshore in that direction. The glaucophane eclogites contain garnet, omphacite, quartz, amphibole, clinozoisite/epidote, minor phengite, paragonite, rutile and rare apatite. Prograde metamorphic evolution is indicated by garnet crystals zoned from Mn-rich cores to Mg-richer rims (typically (core→rim): Alm44→58 Prp1→12 Grs33→29 Sps22→1) and amphibole grains with glaucophane nuclei and Ca–Na-amphibole overgrowths that show sharp transitions, supporting evolution through a solvus, as predicted by the thermodynamic modelling. Modelling of the P–T conditions using the P–T pseudosection technique indicates a peak of metamorphism at about 620 ∘C and 16 kbar. The retrograde evolution of the metabasites is evidenced by the late formation of albite, titanite and ferro-actinolite. The surrounding mica schists, composed of quartz, garnet, phengite, paragonite and chlorite, were also largely retrogressed during exhumation. The orthogneiss of the SW part of the island does not show clear evidence of high-pressure metamorphism, since the magmatic feldspars are still preserved, similarly to the orthogneiss of Les Sables Rouges on the island of Groix. Île Dumet and the western part of the Vilaine estuary represent a blueschist-facies equivalent to Île de Groix (Brittany) and Bois-de-Céné (Vendée) on the mainland. All three occurrences occupy the centres of wide synforms whose concentric units are, from rims to core (i.e. from base to top), (a) a high-T migmatitic basement; (b) Cambro-Ordovician metasediments and acid metavolcanites (“porphyroids”); and (c) blueschist-facies mica schists and metabasites, serpentinites, and minor orthogneisses derived from a pre-Variscan oceanic accretionary prism. There are about 10 similar occurrences within the Ibero-Armorican Arc, forming a discontinuous high-pressure belt, but most of them have remained unnoticed due to a high degree of retrogression.