ABSTRACT We collected and investigated mylonitic gabbro nodules from Stromboli’s oldest pyroclastic rocks, and report on them here for the first time. The petrography, bulk-rock geochemistry, and mineral chemistry of these mylonitic gabbros show that they are cognates with undeformed cumulate gabbros also found in the same host rocks. Both types of gabbro nodules are inferred to have crystallized within the lowercrustal levels of this active volcano. The mylonitic gabbros are characterized by discontinuous layers of plagioclase aggregates, interspersed with amphibole, pyroxene, and olivine crystals, and they are enveloped by a mylonitic foliation consisting of <5- to <100-µm-sized, dynamically recrystallized plagioclase grains. The deformed rims of the plagioclase porphyroclasts disaggregate by fracturing into rectangular grains along the main cleavage systems {010} and {001}. In higher-strain zones, the grain size becomes smaller, and the shape is controlled by grain boundary migration driven by melt-assisted diffusion creep that accommodates grain boundary sliding. This is consistent with the dispersed crystallographic orientations of the mylonitic grains and their complex compositional zoning patterns. We invoke a switch from cataclastic flow to melt-assisted, grain boundary sliding as the main deformation mechanisms for these mylonitic gabbro nodules. The cataclastic flow was triggered by the hydrostatic pressure of a magma column acting at the base of the lowermost Stromboli crustal magma chamber (7–11 km). These intrusive processes, which occur along the transtensional shear zones of the Stromboli NESW–striking fault systems, should have finally driven the mylonitic deformation in a brittle-ductile transition zone that is inferred to have been active during the evolution of the magmatic crustal roots of the volcano.