GEOCHEMISTRY OF MAFIC ROCKS IN THE SESIA ZONE (WESTERN ALPS) - NEW DATA AND INTERPRETATIONS

Twenty-nine whole rock geochemical analyses of basic rocks were carrie d out in the Sesia zone in order to characterise the provenance and ev olution of both basement and monometamorphic cover mafic lithologies o f 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 monome tamorphic covers, which are tholeiitic WPB Fe-basalts to trachyandesit es in origin; d) mafic metabreccias related to the monometamorphic bas alts, 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 z one (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 tholeiiti c MORB basalts have a geochemical pattern similar to the extrusive bas ic rocks of the ophiolitic units of the Western Alps, while they diffe r from the Middle Triassic mafic rocks of the Eastern Alps in both che mical composition and geotectonic signature. Because of their lithostr atigraphic position and their relationships with the other monometamor phic lithologies they have been interpreted as emplaced in a distal ed ge of the continental margin position during the early opening phases of the Alpine Tethys basin, in the Early Jurassic time. The analysed M g-gabbros are geochemically equivalent to the other presumably Early P ermian intrusive mafic stocks of the Western Austroalpine system (e.g. Matterhorn-Mont Collon, M. Nery etc.).