TSK 11 Göttingen 2006 Schulz et al. Cadomian and Variscan metamorphic events in the Léon Domain (Armori- canMassif) resolved by trace element analysis in monazite and garnet Poster Bernhard Schulz1 Erwin Krenn2 Fritz Finger2 Helene Brätz3 Reiner Klemd3 The question, whether crustal domains are allochthonous terranes or not is crucial for plate tectonic models of the Ibero-Armorican segment of the Variscan belt. The Léon Domain in the Armorican Massif appears as a dis- placed crustal block as it bears a resem- blance to the South Armorican Domain of the internal Variscan belt (Le Corre et al. 1989). In the central part of the Léon, the amphibolite-facies Conquet- Penze Micaschist Unit (CPMU) overlies the high-grade Lesneven Gneiss Unit (LGU). At the base of the LGU, a high-pressure stage at 700°C/>13 kbar, recorded by garnet-clinopyroxene as- semblages in eclogites was followed by a high-temperature event at 800°C/8 kbar with garnet and cordierite in aluminous paragneisses. Maximal temperatures in the upper parts of the LGU were 630°C/6 kbar. In the micaschists of the Conquet-Penze Unit, microstruc- tures indicate a crystallization of garnet and then staurolite during the develop- ment of S1 and S2 foliations. Zoned garnet in assemblages 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 CPMU (Fig. 1A, B). The foliation S2 was over- 1 Institut für Mineralogie, Brennhausgasse 14, 09596 Freiberg/Sachsen 2 Abteilung für Min- eralogie, Hellbrunner Str. 34, A-5020 Salzburg 3 Institut für Mineralogie, Am Hubland, 97074 Würzburg printed by shear bands with a top-to- SW directed normal sense of shear, cor- responding to a dextral strike-slip move- ment (Balé & Brun 1986). A younger population of monazite with variable Y contents displays Variscan Th-U-Pb ages (EMP dating method) between 340 and 300Ma (Fig. 1C). In contrast, an older population of Cado- mian monazite at 552–517Ma is uni- formly rich in Y and was observed in samples with only few or even no gar- net. As the 330–340Ma Saint Renan- Kersaint granite postdates the folia- tions S1 and S2 with peak metamor- phic assemblages one can conclude that 340–300Ma Variscan monazites should postdate garnet crystallization. In metapelites, the crystallization of gar- net and accessory xenotime and mon- azite are linked by reactions with net transfer of Y. Trace-elements in garnet were analyzed by LA-ICPMS. Garnet Y, HREE and Li are low in high-grade gneisses. In amphibolite-facies micas- chists strong zonations of Y and HREE are observed. Like Mn, both Y and HREE decrease from garnet core to rim at increasing temperature when garnet appears with xenotime and monazite, which are consumed. At low tempera- tures, xenotime is supposed to be a sta- ble phase. In some samples, the Y is very low in the Mn-rich cores of garnet which crystallized at low temperatures. The Y contents strongly increase toward the inner rims, indicating that Y was initially bound in xenotime and/or mon- azite and then deliberated by the break- down of these phases under prograde metamorphic conditions. This suggests that high-Y Cadomian monazite crys- tallized previous to the garnet and at high temperatures. Presumably, gar- net did not crystallize at the Cadomian 1 Schulz et al. TSK 11 Göttingen 2006 event due to coeval low-pressure con- ditions, but at Variscan medium pres- sures. Whatever, Cadomian monazite dates a distinct thermal event. One could speculate on a contact metamor- phism in the vicinity of intrusions like the Pointe des Renards metagranitoid or a Cadomian regional low-pressure meta- morphism as it was described from the St. Malo and Fougères units of the ad- jacent North Armorican Cadomian Do- main (Ballève et al. 2001). P-T paths in combination with the mon- azite ages underline that the central Léon units represent a normal crustal pile which was underthrusted towards the SE or E beneath the Central Ar- morican Domain during a Variscan colli- sion, as proposed by Rolet et al. (1986). Then, the range of Variscan monazite ages is linking this event to a Late- Carboniferous stage with overprinting of the S1–S2-structures by dextral shear- ing. The finding of Cadomian remnants does not support a South Armorican provenance. The Léon units were rather parts of a suture zone along the northern boundary of the Armorican microplate, Figure 1: (A, B) P-T paths from the Léon. Garnet-clinopyroxene equilibria (Grt-Cpx) and relative evolution derived from garnet zonation trend. Data from green amphiboles (gAm). Data from garnet-sillimanite-plagioclase-quartz (GASP) and garnet-cordierite (Grt-Cd) equilibria, calculated with garnet core (c) compositions. Data from garnet- muscovite-biotite-plagioclase-quartz equilibria in micaschists (GMBP); prograde P–T paths derived from garnet core-to-rim (c, r) zonations. (C, D) Th-U-Pb monazite ages. 2 TSK 11 Göttingen 2006 Schulz et al. hence related to the margin of a for- mer Rheic Ocean between Armorica and Avalonia. References Balé P, Brun, JP (1986) Les complexes méta- morphiques du Léon (NW Bretagne): un seg- ment du domaine éo-hercynien sud armori- cain translaté au Dévonien. Bulletin Société Géologique de France 1986/3, 471–477 Ballèvre M, Le Goff E, Hébert R (2001) The tectonothermal evolution of the Cadomian belt of northern Brittany, France: a Neo- proterozoic volcanic arc. Tectonophysics 331,19–43 Le Corre C, Bale P, Georget Y (1989) Le Léon: un domaine exotique au Nord-Ouest de la chaine varisque armoricaine (France). Geod- inamica Acta 3/2, 57–71 Rolet J, Le Gall B, Darboux JR, Thonon P, Gravelle M (1986) L’ évolution géody- namique dévono-carbonifère de lťextrémité occidentale de la chaîne hercynienne dťEu- rope sur le transect Armorique - Cornwall. Bull Soc Géol France 1986-8/1, 43–54 3