Jurassic U-Pb ELA-ICP-MS zircon ages for segregation of huge chromitite layers in the Finero mantle body: new insights into the geodynamic evolution of the Southern Alps.

The mafic-ultramafic Finero complex represents the northernmost sectorof the Ivrea-Verbano Zone (IVZ), Southern Alps. It consists of a mantlebody, surrounded by a mafic-ultramafic intrusive sequence [1],documenting an association of mantle and crustal rocks inferred to beplaced at the bottom of the continental crust of the Adria plate before theopening of Ligurian-Piedmontese branch of the Jurassic neo-Tethys.Nevertheless, the Finero complex shows several tectonic andpetrochemical differences with respect to those of the central IVZ. Forinstance, published geochronological U-Pb zircon data constrain theemplacement of the Mafic Complex in the central sector of the IVZ at280-to-295 Ma ([2] and references therein). Conversely, the intrusion ageof almost part of the Mafic Complex in the Finero area (SHRIMP U-Pbzircon dating) indicate that the External Gabbro, the largest unit formingthe Finero Mafic Complex, intruded the bottom of the Kinzigite Formationduring Ladinian [3]. A Triassic ages have been also determined for themultiple metasomatic and intrusive events affecting the associated mantlebody ([4] and references therein). This is mainly formed by harzburgitemodally enriched in phlogopite and amphibole and with large LILEcontents. Based on the geochemical and isotopic composition, severalauthors proposed that the growth of phlogopite-bearing assemblages wasrelated to the migration of melts containing significant crustal-derivedcomponents. With the aim to place constraints on the timing of meltmigration through the Finero mantle body, zircons were separated fromthree, dm-thick chromitite bands enclosed in huge dunite bodiesoutcropping along the Cannobino River and on the Mt. Sasso Rosso. U-PbELA-ICP-MS data result in concordant ages comprised in the range of188-186 Ma, which is believed to date the segregation of the chromititebands during channeled porous-flow melt migration. These ages aresignificantly younger than that determined through conventionalmultigrain dating by [4], whose data define an intercept Triassic age of208±2 Ma. Although the studied chromitite bands do not show primaryphlogopite, the major and trace element mineral chemistry point to acognate origin with the migrating melts forming the phlogopiteharzburgite. The finding that the porous-flow ascent of LILE-enrichedmelts through the Finero mantle unit postdates the emplacement of theFinero Mafic Complex, and the absence in the latter of the record of thismagmatic stage, suggest that the Finero mantle unit might have beenemplaced tectonically in contact with the lower crust at some time afterlower Jurassic. This hypothesis is supported by the observation that thecontact between mantle unit and Mafic Complex is always tectonic. As awhole these data confirm that the northernmost part of the IVZ,represented by the Finero complex, cannot be longer considered as a partof the central IVZ, thus suggesting that the entire geological setting of thearea should be revised.