The South Brenterc’h 9.5 m-thick diabase dyke belongs to the intermediate-Ti quartz-normative tholeiitic magmatism, which has intruded the western end of Britanny (France) near the Trias–Lias boundary. It has been previously identified as a “simple” dyke, resulting from a single magmatic injection. The present study is based on ten samples collected along a half transverse section of this dyke. Geochemical analyses and some textural parameters have been used to point out two flow-related differentiation processes, partially masked by alteration. These late magmatic mechanisms have induced subtle geochemical variations among samples, which sometime counteract each other. The processes pointed out here are (i) expulsion of phenocrysts from the borders during flow differentiation, likely by the way of an hydrodynamic grain dispersive pressure, and (ii) emplacement of a more evolved magma in the central part of the intrusion, probably due to a viscosity segregation mechanism during the flow.