Late orogenic exhumation of the Variscan high‐grade units (South Armorican Domain, western France), combined structural and40Ar/39Ar constraints

Abstract

The current structure of the South Armorican Domain in the French Variscan Belt, provides an excellent record of the late orogenic evolution, as documented by the complex interactions between extensional structures and partial melting. In the Golfe du Morbihan, the South Brittany Migmatite Belt (SBMB), tectonically overlain by non‐melted metamorphic units, was studied by combined structural and40Ar/39Ar methods. The recognition of successive deformation stages points to a strain continuum initiated at depth at the end of a partial melting event, and continued to near‐surface brittle faulting, encompassing ductile strain localization over a major top‐to‐the‐ESE extensional shear zone (SSZ) consistent with an overall WNW‐ESE orogen‐parallel stretching. This multidisciplinary study making the link between structure, texture, microchemistry and40Ar/39Ar geochronology allow distinguishing between cooling, recrystallization and syn‐tectonic crystallization ages. The first extensional insights, witnessed by dyke intrusion were dated at ∼319 ± 6 Ma immediately after the SBMB partial melting. Strain localization along the SSZ, occurred until ∼302–298 Ma when SBMB crossed the ductile‐brittle transition. Moreover, in situ40Ar/39Ar analysis, of syn‐tectonic white micas sampled in the ductile and brittle structures yield similar ∼300 Ma. Formation or reactivation of the SSZ may have occurred during the rapid cooling of the SBMB settled at ∼305 Ma by conventional40Ar/39Ar analysis. Large‐scale late orogenic extensional event of the previously overthickened Variscan crust thus lasted over ∼20 ± 3 Myr in the same extensional tectonic regime guided by the South Armorican Shear Zone.