Precisions nouvelles sur les terrains metamorphiques du mont Pilat (Massif central) et leur histoire cristallogenetique

Abstract

Abstract Metamorphic rocks of the Pilat mountain area of the Central Massif near St. Etienne exhibit three periods of metamorphic effects involving anomalies. Period I, the oldest, is represented by mesozonal gneiss and schist (amphibolite-almandine facies) cropping out along the NW side of the area. They are part of the pre-Hercynian metamorphics of the south flank of the Lyonnais mountains. Period II involved epizonal retromorphism and recrystallization. These rocks crop out in a belt along the SE margin of the Period II series. Period III involved thermal, and probably contact metamorphism, resulting in zonation of a belt with chlorite, biotite, cordierite, andalusite, and sillimanite zones in succession SE to the Pilat mountain granite. Period III probably is younger than II. The chemical composition of the granite is anomalous. This may be the metamorphism under deficient shearing stress described by Harker, or Buchan type metamorphism, as described by Read and used by Forestier to explain the schists of the upper Allier river area of the massif.