Origin and age of zircon-bearing chromitite layers from the Finero phlogopite peridotite (Ivrea-Verbano Zone, Western Alps) and geodynamic consequences

An investigation has been performed on three chromitite layers segregated in dunite bodies of the PhlogopitePeridotite mantle unit in the Finero complex (FPP, Ivrea-Verbano Zone, Southern Alps) aimed at providingnew constraints to their origin and evolution.Field relationships, the sub-chondritic Hf isotopic composition of the zircons (?Hf(188) as lowas-5.4), the heavyO isotopic composition of zircons and pyroxenes (?18Oup to 6.9?), the strict similarity of the trace element compositionbetween the clinopyroxenes and amphiboles from the chromitites and those from the phlogopiteharzburgites and pyroxenites forming the typical FPP association, as well as the REE composition of zircons,which approaches equilibrium with the associate clinopyroxene, suggest that the studied chromitites weresegregated frommelts, highly contaminated fromcontinental crust, during the pervasive cycle of metasomatismrecorded by the FPP. An LA-ICP-HRMS survey of chromitite zircon grains has provided Early Jurassic U-Pb agesmostly between 199 ± 3 Ma and 178 ± 2 Ma, with a pronounced peak at 187 Ma. Relevant exceptions areinherited domains of two grains giving Triassic ages of 242 ± 7 Ma and 229 ± 7 Ma, and a third homogeneouszircon giving 208 ± 3 Ma. Our geochronological data and those reported in the literature show that the FPPchromitites have zircon populations with different internal CL textures, but the same sub-chondritic Hf isotopiccomposition, which define an overall U-Pb age span from ~290 Ma to 180. The segregation of the chromititelayers and the main pervasive metasomatism likely occurred in the Early Permian (in a post-collisional,transtensional setting) or before (possibly, in a subduction-related setting). The rejuvenation of the zircon ageswas accompanied by a progressive disappearance of the internal zoning, interpreted as the result of a prolongedresidence at mantle depths with progressive re-equilibration of the U-Pb system due to thermal perturbations.The age peak at ~187 Ma is argued to constrain the timing of FPP exhumation at shallower, crustal levels. Thisprocess was characterised by an important reheating event, possibly due to lithospheric hyperextension. Theevolution of the FPP appears completely different than that of mantle bodies of the central IVZ (i.e., the ValSesia-type bodies),whichwere emplacedwithin the continental crust, as part of accretionary prisms, at or beforethe end of the Variscan orogeny.