spe423-12 page 267 267 Geological Society of America Special Paper 423 2007 Cadomian and Variscan metamorphic events in the Léon domain (Armorican Massif, France): P-T data and EMP monazite dating Bernhard Schulz* Institut für Mineralogie, Brennhausgasse 14, D-09596 Freiberg, Germany Erwin Krenn Fritz Finger Abteilung für Mineralogie der Universität, Hellbrunner Strasse 34, A-5020 Salzburg, Austria Helene Brätz Reiner Klemd Institut für Mineralogie und Kristallstrukturlehre, Am Hubland, D-97074 Würzburg, Germany ABSTRACT The Léon domain adjacent to the Cadomian realm in the North Armorican domain appears to be a displaced crustal block, as its metamorphism and rock types bear a resemblance to the South Armorican domain of the internal Variscan belt. The amphibolite-facies Conquet-Penze Micaschist unit overlies the high-grade Lesneven Gneiss unit in the central part of the Léon. Timing and conditions of the metamorphic evolution have been evaluated. At the base of the Lesneven Gneiss unit, a high-pres- sure eclogite-facies stage (700 °C at >13 kbar) was followed by a high-temperature event (800 °C at 8 kbar), which is characterized by the crystallization of garnet-cor- dierite assemblages in aluminous paragneisses. Maximal temperatures in the upper parts of the Lesneven Gneiss unit were 630 °C at 6 kbar. Zoned garnet in assem- blages with staurolite recorded prograde P-T paths from 490–610 °C at 5–8 kbar in the upper and at 6–9 kbar in the lower parts of the Conquet-Penze Micaschist unit. Garnet Y, heavy rare earth elements, and Li are low in high-grade gneisses and dis- play strong zonations in the micaschists. A younger population of monazite with a broad range of Y contents displays Th-U-Pb ages between 340 and 300 Ma. It crys- tallized subsequent to formation of foliations S 1 -S 2 and Variscan peak metamorphic assemblages. In contrast, an older population of Cadomian monazite at 552–517 Ma is uniformly rich in Y, suggesting an earlier crystallization than garnet, however, at elevated temperatures. The findings do not support a South Armorican provenance of the Léon domain. The Léon units appear as part of a Cadomian crust at the northern margin of the former Armorican microplate. During a Variscan collision, this crust was strongly overprinted by underthrusting toward the southeast or east beneath *E-mail: bschulz@mineral.tu-freiberg.de. Schulz, B., Krenn, E., Finger, F., Brätz, H., and Klemd, R., 2007, Cadomian and Variscan metamorphic events in the Léon domain (Armorican Massif, France): P-T data and EMP monazite dating, in Linnemann, U., Nance, R.D., Kraft, P., and Zulauf, G., eds., The evolution of the Rheic Ocean: From Avalonian-Cadomian active margin to Alleghenian-Variscan collision: Geological Society of America Special Paper 423, p. 267–285, doi: 10.1130/2007.2423(12). For permission to copy, contact editing@geosociety.org. ©2007 Geological Society of America. All rights reserved.