Geochemistry of mafic rocks in the Sesia zone (western Alps): New data and interpretations

Twenty-nine whole rock geochemical analyses of basic rocks were carried out in the Sesia zone in order to characterise the provenance and evolution of both basement and monometamorphic cover mafic lithologies of this Alpine unit. We are able to subdivide the basic rocks into the following groups: a) monometamorphic metabasalts with a tholeiitic mid oceanic ridge (MORB) signature; b) eclogitised basement amphiboiites, derived from transitional to alkaline within-plate (WPB) basalts; c) mafic sheets and boudins within the leucocratic gneisses of the monometamorphic covers, which are tholeiitic WPB Fe-basalts to trachyandesites in origin; d) mafic metabreccias related to the monometamorphic basalts, which are spilitized tholeiitic trachybasalts/trachyandesites in origin; and e) mylonitic metagabbros probably derived from cumulitic Mg-gabbros. Glaucophane-bearing gabbroic rocks in the southern Sesia zone (Corio and Monastero regions) show a geochemical signature similar to the eclogitised basement amphibolites and do not have any chemical resemblance to the analysed Mg-gabbros. The monometamorphic tholeiitic MORB basalts have a geochemical pattern similar to the extrusive basic rocks of the ophiolitic units of the Western Alps, while they differ from the Middle Triassic mafic rocks of the Eastern Alps in both chemical composition and geotectonic signature. Because of their lithostratigraphic position and their relationships with the other monometamorphic lithologies they have been interpreted as emplaced in a distal edge of the continental margin position during the early opening phases of the Alpine Tethys basin, in the Early Jurassic time. The analysed Mg-gabbros are geochemically equivalent to the other presumably Early Permian intrusive mafic stocks of the Western Austroalpine system (e.g. Matterhorn-Mont Collon, M. Nery etc.).