Geochemical and isotopic characteristics of blueschist facies rocks from the Île de Groix, Armorican Massif (northwest France)

Abstract

Nine samples of blueschist facies metabasic rocks and four samples of associated metasedimentary lithologies were analysed for REE contents and Nd isotopic composition. Sampling includes the main rock types exposed on Groix, concentrating on those metabasic rocks with least dispersed RbSr systems. The REE abundance patterns appear not to have suffered significant alteration since emplacement of the protolith magma despite subsequent blueschist facies metamorphism. The data, when integrated into a wider geochemical data-base, allow the identification and characterization of both tholeiitic and alkaline magma-types in the original igneous suite which was differentiated from depleted mantle sources. The basalts probably originated in an oceanic context. Some sediments are predominantly volcaniclastic, in which a continental crustal component may be recognized. Other sediments are closer in composition to continental crust with variable additions of tholeiitic or alkali-basalt debris. The volcanic-sedimentary assemblage on Groix is suggestive of ocean-island magmatic activity associated with a sedimentary sequence derived from a nearby continent.