Cadomian and Variscan metamorphic events in the Léon Domain (Armorican Massif) resolved by trace element analysis in monazite and garnet
11. Symposium "Tektonik, Struktur- und Kristallingeologie"
Link zum Zitieren/Bookmarken: http://hdl.handle.net/11858/00-1735-0000-0001-3473-6
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 displaced crustal block as it bears a resemblance 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 assemblages 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, microstructures indicate a crystallization of garnet and then staurolite during the development 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. The foliation S2 was overprinted by shear bands with a top-to- SW directed normal sense of shear, corresponding to a dextral strike-slip movement (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. In contrast, an older population of Cadomian monazite at 552–517Ma is uniformly rich in Y and was observed in samples with only few or even no garnet. As the 330–340Ma Saint Renan- Kersaint granite postdates the foliations S1 and S2 with peak metamorphic assemblages one can conclude that 340–300Ma Variscan monazites should postdate garnet crystallization.
- Geologie