The Léon Domain (French Massif Armoricain): a westward extension of the Mid-German Crystalline Rise? Structural and geochronological insights

Abstract

The Léon Domain in the NW part of the French Massif Armoricain is a stack of synmetamorphic nappes displaced from south to north in ductile conditions. From bottom to top, an orthogneissic basement is overthrusted successively by (1) a Lower Nappe of gneiss including mafic eclogites, (2) an Intermediate Nappe of biotite-garnet-staurolite micaschists with mafic blocks, and (3) an upper nappe made up of Neoproterozoic phyllites covered by unmetamorphosed Paleozoic sedimentary series. This microstructural study documents a polyphase evolution with firstly a top-to-the-N shearing, secondly followed by upright folding of the stack of nappes coeval with migmatization, and lastly, a dextral wrenching along the North Armorican Shear Zone associated with emplacement of synkinematic plutons. New UTh/Pb chemical dating of monazites from biotite-garnet-staurolite micaschists, migmatites, and granitoids argue for 340-335 Ma, 335-327 Ma, and about 320 Ma ages for synthrusting metamorphism, anatexis, and wrenching, respectively. A metagabbro from Le Conquet yields a zircon LA-ICP-MS age of 478 ± 4 Ma, which corresponds to magma emplacement time. The Léon Domain is interpreted as a microcontinent separated from Armorica by the Le Conquet-Penzé suture to the south and east, and from Laurussia by the Rheic suture to the north. A possible correlation with the Mid-German Crystalline Rise of Central Europe is discussed.

LiDAR offshore structural mapping and U/Pb zircon/monazite dating of Variscan strain in the Leon metamorphic domain, NW Brittany

“…Our dating of the St-Renan granite is compatible within error limits with the 321 ± 5 Ma age on monazites previously obtained by the EMPA technique (Faure et al, 2010).…”

“…Available U-Th/Pb dating of monazites yielded ages of 321 ± 5 Ma for the syntectonic emplacement of the St-Renan granite along the NASZ (Faure et al, 2010), and ages of 338 ± 5 Ma (Faure et al, 2010), and 315-305 Ma (Schulz, 2013) for the amphibolite-type metamorphism Paquette and Tiepolo (2007). Detailed analytical procedures and conditions used in this study can be found in Hurai et al (2010Hurai et al ( , 2012.…”

“…downsection, from greenschist facies to partial-melting conditions. The upper allochthonous units in the southern flank of the antiform comprise, from top to bottom, the Brest orthogneiss, the protolith of which yielded ages of 504 ± 15 Ma (Marcoux et al, 2009), the Le Conquet amphibolite-facies micaschists and metagabbros, dated at 478 ± 4 Ma (Faure et al, 2010) and the Lesneven gneisses that pass northwards into sillimanite-rich facies with pockets of anatectic granites. The basal unit is thrust over the para-autochthonous Treglonou orthogneiss in the core of the eroded antiform.…”

“…Its protolith has been dated at 385-391 Ma by U/Pb method on zircons (Marcoux et al, 2009). Published U-Th/Pb monazite ages of synthrusting metamorphism range from 340-335 Ma (Faure et al, 2010) to 315 Ma (Schulz, 2013). Relics of an earlier (439 ± 12 Ma) HP eclogitic event (Paquette et al, 1987) are found locally in mafic lenses within the high-grade paragneiss exposed on the northern flank of the antiform (Cabanis and Godard, 1987).…”

“…1b and c). Early dextral shearing occurred to the south along (1) the PNSZ that extends as a 10-15 kmlong corridor throughout a narrow band of undated (Pierres Noires) granites and part of the Brest gneisses (Chauris, 1994), and (2) the NASZ that cuts though the southern margin of the 321 ± 5 Ma StRenan-Kersaint synkinematic granite (U-Th/Pb monazite ages; Faure et al, 2010), in turn, secant to the south with respect to the strained Lesneven metamorphic terrains (Goré and Le Corre, 1987). Younger sinistral shearing took place further north, along the N70°E-trending GPSZ, which also acted as the root zone of the 301 ± 3 Ma Aber-Ildut southerly-directed granitic sheet (Le Corre et al, 1989;Marcoux et al, 2004).…”