Ductile Thrusting Recorded by the Garnet Isograd from Blueschist-Facies Metapelites of the Ile de Groix, Armorican Massif, France

Abstract

Mineral assemblages in the blueschist-facies metapelites from the Ile de Groix (Armorican 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 S1. 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 S2. 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 coexisting minerals reflect different P–T conditions. The P–T history of the blueschist-facies metapelites is estimated using a simplified petrogenetic grid in the NFMASH system and thermodynamic calculations, which suggest peak P–T conditions at about P = 16–18 kbar, T = 450–500°C and P = 14–16 kbar, T = 400–450°C 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 50°C). 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).