REGIONAL VS CONTACT-METAMORPHISM OF GARNET METAPELITES IN THE VICINITY OF LATE VARISCAN GRANITES (CENTRAL ARMORICAN DOMAIN, BRITTANY, FRANCE)

Late Variscan granites intruded Brioverian (Upper Proterozoic) and Low er Paleozoic pelitic sequences to the north of the South Armorican she ar zone. In the vicinity of the granites, Brioverian garnet micaschist s contain pre/syn-S-2 assemblages with garnet +/- staurolite and post- S-2 assemblages with staurolite +/- andalusite. Andalusite appeared pr e/syn- and post-S-2 in garnet-free micaschists. The garnets in the Bri overian micaschists are zoned with increasing Mg and decreasing Mn and Ca from core to inner rim. Only poor garnet zonations occur in Paleoz oic hornfelses of enclaves in the Rostrenen granite. The results of a microstructurally controlled application of garnet-biotite geothermome ters and garnet-plagioclase geobarometers are similar to P-T trends ob tained by the Gibbs method of garnet zonation modelling in the system NCFMnMASH. The P-T paths of a pre/syn-S-2 regional metamorphism are cl ockwise between 500-550 degrees C/8 kbar and 700 degrees C/5 kbar, fol lowed by cooling decompression. They contrast with isobaric contact me tamorphism between 500 and 700 degrees C at 2.5-3 kbar in Paleozoic ho rnfelses. This points to a two-stage Variscan metamorphism with a pre- granitic pressure-dominated event in the Brioverian micaschists, follo wed by Late Variscan contact metamorphism, and suggests the existence of a pre-granitic tectonic boundary between the micaschists and overly ing low-grade sequences.