Granulite facies metamorphism and retrogressive evolution of the Monts du Lyonnais metabasites (Massif Central, France)

Abstract

Garnet-bearing pyriclasites have been found within the Monts du Lyonnais formation near St. Andréla-Côte, in the eastern part of the Massif Central. The rocks appear to have differentiated from a tholeiitic magma of continental affinity and they may be genetically related to quartzofeldspathic granulites considered to be the products of crustal melts. By means of electron microprobe analyses, the consistent pressure and temperature estimates based essentially on pyroxene-garnet equilibria show that the peak of metamorphism was at about 860°C and 10 kbar. Furthermore this study tends to confirm the affiliation of Monts du Lyonnais granulites to the Early Variscan (granulite-eclogite facies) metamorphism which extends from Bohemia to Spain through central Europe. This event may be expected in the vicinity of an ancient suture zone where a continental collision followed a subduction.