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-pressure 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 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 Conquet-Penze Micaschist unit. Garnet Y, heavy rare earth elements, and Li are low in high-grade gneisses and display 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 crystallized subsequent to formation of foliations S₁-S₂ 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 the Central Armorican domain and by later uplift accompanied by Late Carboniferous dextral shear tectonics. The features are typical of the Variscan Saxo-Thuringian zone, which faced the Rheic Ocean to the north.