Ile de Groix: retrogression and structural developments in an extensional régime

The Paleozoic metamorphic rocks of Ile de Groix, France, record a retrogressive metamorphism from eclogite facies through blueschist to greenschist facies, produced during progressive uplift and exhumation, and accompanied throughout by high extensional strain. Extension was accompanied by formation of a generally flat-lying foliation, the result of either pure shear or conjugate simple shearing, with top to the northwest and southeast shear senses equally important. The foliation is either wrapped around pods of relatively competent and/or higher grade rock, elongate in the direction of stretching, or folded on axes parallel to the stretching direction; these structures represent the effects of the intermediate strain axis which transiently and locally fluctuated between being one of extension or shortening. Large-scale open folds of foliation may represent a wrapping of foliation about two giant pods which define the entire island. We suggest the so-called lawsonite pseudomorphs might possibly be altered calcium-bearing plagioclase which indicates a simpler path of retrogression than would lawsonite. The plate tectonic setting of Groix might be solved by applying the concept of suspect tectonostratigraphic terranes to Gondwanan France and Iberia, and invoking large-scale displacements.