Kersantite is the international designation for hypabyssal lamprophyres essentially made up of dark micas and plagioclase. Kersantites from the type locality (Kersanton hamlet) and the W-Armorican surrounding area are Serpukhovian/Bashkirian in age. They are systematically associated in the field with hypabyssal microgranodiorites, both types being intruded synchronously as syn-compressional magmatic bodies that postdated pre-tectonic Tournaisian/Visean dolerites. The kersantitic amphibole compositions can be used to constrain the crystallization pressures and oxygen fugacities (redox conditions much more oxidizing than the Ni-NiO buffer). The oscillatory zoning observed in clinopyroxene phenocysts is consistent with a magma chamber repeatedly refilled by mafic liquid. The melting source of the W-Armorican kersantites corresponded to a lithospheric mantle located in the spinel-garnet transition zone (75–85 km depth), highly metasomatized by fluids probably released by a subducted sediment-bearing slab. Microgranodiorites could result from intimate mixing between mantle- and crust-derived magmas, with cordierite as possibly melting phase. The emplacement of dolerites with anorogenic geochemical signature prior to kersantites/microgranodiorites implies upwelling of the asthenospheric mantle up to a relatively high level, probably as the result of an extension induced by either roll back of the subducted lithospheric slab or delamination of the overriding plate. The heat input associated to the long-lasted asthenosphere rise could have favored (1) the formation of early dolerites between 347 and 330 Ma; and then, (2) the genesis of syn-compressional kersantites and microgranodiorites between 330 and 310 Ma, once the overlying lithospheric mantle was metasomatized by crust-derived fluids. This magmatic sequence marked the beginning of the Avalonia-Armorica collision, just prior to the main Variscan compressional event in this zone at c. 315 Ma.