SHRIMP U^Pb ZirconTriassic Intrusion Age of the Finero Mafic Complex (Ivrea-Verbano Zone, Western Alps) and its Geodynamic Implications

Interstitial, subhedral, slightly pink zircons have been separated from a diorite of the External Gabbro unit of the Finero Mafic Complex, outcropping in the northernmost tip of the Ivrea-Verbano Zone (IVZ, Southern Alps). They are characterised by oscillatory zoning at cathodoluminescence (CL), high Th/U ratio (0.9-0.4), associated to high U, Th, Pb, REE, Ta, Nb, Li and P contents, consistent with a magmatic nature. The mean of the single concordant ages is 232±3 Ma, which is here proposed to constrain the timing of intrusion of the Finero Mafic Complex. Most of these magmatic zircons exhibit an outer rim, up to 50-μm thick, with a bright, “white pest”-like CL emission, with transgressive and/or sub-concordant contacts to the internal, oscillatory-zoned domains, in which the Th/U ratio is decreased, as well as the U, Th, Pb, REE, Ta and Nb contents. They result in younger, slightly discordant 206Pb/238U ages, between 219±3 Ma to 205±3 Ma, which are interpreted to highlight a fluid-assisted recrystallisation event. Gabbros also showed an interstitial zircons population consisting of rounded, small, colourless, bright-to-milky crystals with blurred patterns in CL. These zircons show low U, Th, Pb, Nb and Ta contents, and low Th/U ratio (down to 0.08). The overall metamorphic character, together with the field evidence of the presence of blocks of paragneiss belonging to the crystalline basement at the roof of the External Gabbro unit, indicate that these zircons are inherited from country crustal rocks. They have U and Pb isotopic compositions strongly altered and discordant, showing upper limit of the recrystallization consistent with the Middle Triassic emplacement age obtained from the magmatic zircons. It is concluded that the Finero Mafic Complex cannot be longer considered as a part of the huge Permian Mafic Complex of the central IVZ. It follows that the intrusive record preserved in the IVZ allows to look at the interplay between the continental crust and mantle melts through time (i.e. Permian through Triassic). In this scenario, the modest crustal assimilation shown by the Finero Mafic Complex can be interpreted in first instance as the consequence of the limited volume of intruded melt (less than 2 km in thickness). However, the possible role played by the decreased fertility of the metamorphic country rocks due to a inferred Permian magmatic event must be better constrained. The intrusion of the Finero Mafic Complex may represent the deep-crustal counterparts of the Ladinian volcanism widespread throughout the Southern Alps. The geodynamic setting of this magmatism is still strongly debated. The results presented in this study, along with several other structural, petrochemical and age heterogeneities documented in literature, suggest the occurrence of a “Finero-type IVZ” (i.e. northern IVZ) and a “Balmuccia-type IVZ” (i.e. central IVZ), which underwent different magmatic and tectonic evolutions.