Ductile Thrusting Recorded by the Garnet Isograd from Blueschist-Facies Metapelites of the Ile de Groix, Armorican Massif, France |
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Authors: | BOSSE, V. BALLEVRE, M. VIDAL, O. |
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Affiliation: | 1LABORATOIRE DE GÉOCHRONOLOGIEGÉOCHIMIE, GÉOSCIENCES AZUR (UMR-CNRS 6526), UNIVERSITÉ DE NICESOPHIA ANTIPOLIS, PARC VALROSE, 06102 CEDEX NICE, FRANCE 2GÉOSCIENCES RENNES (UMR-CNRS 6118), UNIVERSITÉ DE RENNES 1, 35042 RENNES CEDEX, FRANCE 3LABORATOIRE DE GÉODYNAMIQUE DES CHAÎNES ALPINES (UMR-CNRS 5025), UNIVERSITÉ J. FOURIER, BP 53, 38041 GRENOBLE CEDEX 9, FRANCE |
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Abstract: | Mineral assemblages in the blueschist-facies metapelites fromthe Ile de Groix (Armorican Massif, France) permit the distinctionof two main units. The Upper Unit is characterized by: (1) highmodal proportions of garnet; (2) larger grain size; (3) therarity of graphite-bearing layers; (4) a single, although composite,foliation S1. A Lower Unit is defined by: (1) low modal proportionsof garnet; (2) smaller grain size; (3) an abundance of graphite-bearinglayers; (4) a pervasive crenulation cleavage S2. In the UpperUnit, coexisting garnet and chloritoid are more magnesian andless manganiferous than in the Lower Unit. The differences inmodal proportions and chemistry of coexisting minerals reflectdifferent PT conditions. The PT history of theblueschist-facies metapelites is estimated using a simplifiedpetrogenetic grid in the NFMASH system and thermodynamic calculations,which suggest peak PT conditions at about P = 1618kbar, T = 450500°C and P = 1416 kbar, T =400450°C in the Upper and Lower Units, respectively.Peak PT conditions were followed by a nearly isothermaldecompression 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-faciesductile thrust. Thrusting of the higher-grade unit, i.e. theUpper Unit, over the Lower Unit occurred after the high-pressureevent, i.e. during the exhumation of both units. The observedsuperposition of higher-grade rocks over lower-grade rocks arguesagainst models where the exhumation history is entirely controlledby crustal-scale vertical shortening (i.e. extension). KEY WORDS: Armorican Massif; blueschist facies; Ile de Groix; metapelites; PT path; garnet isograd |
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