Ductile thrusting recorded by the garnet isograd from blueschist-facies metapelites of the Ile de Groix, Armorican Massif, France

Mineral assemblages in the blueschist-facies metapelites from the Ile de Groix (Amorican Massif, France) permit the distinction of two main units. The Upper Unit is characterized by: (1) high modal proportions of garnet; (2) larger grain size; (3) the rarity of graphite-bearing layers; (4) a single, although composite, foliation S-1. A Lower Unit is defined by: (1) low modal proportions of garnet; (2) smaller grain size; (3) an abundance of graphite-bearing layers; (4) a pervasive crenulation cleavage S-2. In the Upper Unit, coexisting garnet and chloritoid are more magnesian and less manganiferous than in the Lower Unit. The differences in modal Proportions and chemistry of coexist g minerals reflect different P-T conditions. The P-T history of the blueschist-facies metapelites estimated using a simplified petrogenetic grid in the NFMASH system and thermodynamic calculations, which suggest peak P-T conditions tit about P = 16=18 kbar, T = 450-500degreesC and P = 14-16 kbar, T = 400-450degreesC in the Upper and Lower Units, respectively. Peak P-T conditions were followed by a nearly isothermal decompression for both units at slightly different temperatures (of the order of 50degreesC). The contact between the two units, i.e. the garnet isograd, is interpreted as a greenschist-facies ductile thrust. Thrusting of the higher-grade unit, i.e. the Upper Unit, over the Lower Unit occurred after the high-pressure event, i.e. during the exhumation of both units. The observed superposition of higher-grade rocks over lower-grade rocks argues against models where the exhumation history is entirely controlled by crustal-scale vertical shortening (i.e. extension).