U-Pb and Hf isotopes in zircons from mantle chromitites of the Finero Peridotite (Ivrea Verbano Zone)

The Finero Phlogopite Peridotite unit (hereafter Ph-Pd, Ivrea-Verbano Zone, IVZ: Southern Alps) is the unique worldwide example of orogenic mantle massif completely constituted by phlogopite-bearing ultramafics (mainly harzburgites, websterites and dunites) after pervasive to channelled melt migration events. A precious opportunity to provide further constraints on the petrologic and geodynamic evolution of such a mantle sequence is given by the occurrence of zircons in chromitites usually composed by Chromite+Orthopyroxene\ub1Clinopyroxene\ub1Amphibole. Zircons show low CL emission and are generally homogeneous. Few grains display cores slightly darker than the rims. Zircons have up to 1420 and 800 ppm of U and Th, respectively, with Th/U ratio up to 1.6. U-Pb LA-ICP-HRMS analyses yield most concordant Lower Jurassic dates with a weighted average 206Pb/238U age at 187\ub12 Ma. A few darker cores yield Middle Triassic concordant 206Pb/238U ages from 242\ub17 Ma to 229\ub17 Ma. Zircons also show 176Hf/177Hf ratios in the range of 0.282486-0.282610, which give subchondrititic epsilon Hf(188) (-6.0 to -1.6). The relatively high U and Th contents and the large Th/U ratios are the evidence that the chromitite zircons crystallised from a melt, which, according to the low epsilon Hf(188) values, had a marked crustal signature. This melt is analogous to that at the origin of the phlogopite harzburgites and websterites. The absence of internal zoning in zircon is interpreted as the result of homogenisation after a prolonged residence at high temperature mantle conditions. In this frame, the Lower Jurassic ages are proposed to date the cooling of the mantle sequence during exhumation for the opening of the Alpine Tethys. Conversely, the few Middle Triassic dates could represent the age of mantle metasomatism. As a whole, our data strongly support that the Finero area experienced a different geodynamic evolution with respect to the rest of IVZ