Continental gabbros in the Dent Blanche Tectonic System (Western Alps): from the pre-Alpine crustal structure of the Adriatic palaeomargin to the geometry of an alleged subduction interface

Several Permian gabbro bodies are known in the continental-derived Dent Blanche Tectonic System (Western Alps). A new gabbroic complex, the Berrio Gabbro, is described in this paper, and compared with the other gabbro bodies of the Adriatic palaeomargin, as well as of the Tsatè Unit, a remnant of the Piemonte–Liguria ocean, immediately underlying the Dent Blanche Tectonic System. The Berrio Gabbro comprises ultramafic cumulates (clinopyroxenite, wehrlite and hornblendite) and mafic rocks (leucocratic and mesocratic gabbros). Ultramafic (serpentinite) and mafic rocks (gabbro) are also present in the oceanic Tsatè Unit of the Combin Zone. The ultramafic–mafic rocks of the Dent Blanche and the Tsatè are clearly distinguished by their mineralogy and their geochemistry. In addition, their behaviour during the Alpine deformation is distinct. Detailed mapping associated with geochemical characterization of the protoliths allows us to infer the structure of the Adriatic crust during the Permian extension and refute a model of mixing between oceanic and continental material along the hypothesized subduction interface.