Sources, migration mechanisms and geodynamic environment of K-LILE-Mg-rich melts: Evidence from the Finero Complex (Southern Alps)

The Finero Complex is located in the northern sector of the Ivrea-Verbano Zone (IVZ, Southern Alps). It outcrops with an antiformal structure showing a mantle unit at the core and a layered mafic-ultramafic intrusion on the flanks, i.e. the Finero Mafic Complex. The youngest unit of the latter (i.e. the External Gabbro) emplaced at the bottom of the continental basement at 232±3 Ma (Zanetti et al., 2013). The Finero Complex records the unique worldwide example of subcontinental lithospheric mantle column in offcraton setting extensively metasomatised with segregation of phlogopite-bearing mineral assemblages. Presently, there is a large consensus in considering that the mantle unit experienced a multi-stage melt migration in a supra-subduction environment (Zanetti et al., 1999). However, the sequence of metasomatic stages and the mechanisms of melt migration are poorly constrained. Besides, the nature of the crustal component(s) (oceanic vs. continental crust) occurring in the ascending melts is still controversial, rendering doubtful the geodynamic reconstruction. This contribution is aimed at providing new data about field relationships, petrographic features, major and trace elements mineral chemistry of the main lithologies of the mantle unit (e.g. phlogopite-amphibole-bearing harzburgites, dunites with chromitite and pyroxenite bands, phlogopite-bearing websterite, orthopyroxenites), as well as about the O isotope mineral composition and in-situ U-Pb and Lu-Hf isotope data for zircons from chromitite layers. Our investigation points out that the mantle unit experienced a virtually complete metasomatic recrystallization as a results of a discrete number of episodes of pervasive-to-channelled porous flow migration of hydrous melts, alternated with stages of melt migration in open fractures. The latter mechanism formed pyroxenites usually containing Opx, Cpx, Amph and Phl. Both peridotites and pyroxenites display a similar geochemical signature, characterized by low contents in Al, Ti, Nb, Ta, HREE and Y, associated to large content in Mg, K, Th, U, Sr, Pb, Ba and LREE. The finding of δ18OOpx vs. SMOW‰ up to +8 and of negative εHf in chromitite zircons suggest that large volumes of crustal component were present in the migrating melts. New U-Pb zircon data for the chromitite layers provide Lower Jurassic ages. The sources of the migrating liquids, the age and the geodynamic environment of the mantle metasomatism, as well as the age of the crustal accretion of the mantle unit will be addressed. In particular, it will be stressed out the possibility that the K-LILE-Mg-enriched melt migration took place in a late-orogenic environment, similarly to the high-MgO ultrapotassic, lamproitic magmatism widespread in different Mediterranean areas from Oligocene to Pleistocene in association with shoshonitic and calk-alkaline rocks. Zanetti A., Mazzucchelli M., Rivalenti G., Vannucci R. 1999. The Finero phlogopite-peridotite massif: an example of subduction-related metasomatism. Contrib. Mineral. Petrol., 134, 107-122. Zanetti A., Mazzucchelli M., Sinigoi S., Giovanardi T., Peressini G., Fanning M. 2013. SHRIMP U-Pb Zircon Triassic Intrusion Age of the Finero Mafic Complex (Ivrea-Verbano Zone, Western Alps) and its Geodynamic Implications. J. Petrol., 54, 2225-2265.