Are the refractory mantle peridotites from Monte Civrari (W Alps) of subcontinental or abyssal origin?

Here we present mineralogical, geochemical and Nd-Pb isotope data for the ophiolitic mantle body of Monte Civrari, which is exposed west of the Lanzo Massif. The outcrops of Monte Civrari are mainly composed of antigorite serpentine schists with decameter-sized lenses of exceptionally fresh mantle peridotites locally intruded by small veins of rodingitized metagabbros. The samples of this study are medium- to coarse-grained spinel harzburgites to cpx-poor lherzolites (Cpx < 5 vol %). They generally show a protogranular texture. The spinels frequently form clusters with orthopyroxene ± clinopyroxene, possibly as products of garnet breakdown. The clinopyroxene (Mg# 90-91) show very low TiO2 (0.05-0.15 wt. %) and Na2O (< 0.1 wt. %) coupled with high Cr2O3 contents (1.2-1.5 wt%). Spinels have compositions similar to those of abyssal peridotites, with Cr# and Mg# varying between 0.32-0.34 and 0.68-0.71, respectively, and low TiO2 contents (0.05-0.10 wt%). Most samples do not show any evidence of interaction with melts and preserve highly refractory chemical compositions. The clinopyroxenes display strongly fractionated REE spectra (CeN/YbN ~ 0.001), with prominent LREE depletions (CeN/SmN = 0.004-0.005) and low HREE abundances (YbN ~ 5-6) associated with HREE fractionation (GdN/YbN = 0.4-0.5). Clinopyroxene REE compositions may be reproduced by small amounts (~5-6%) of fractional melting of a garnet lherzolite precursor followed by 10% melting in the spinel peridotite field. As a whole, Pb isotope compositions of bulk rocks fall in the fields of global MORB and plot close to the 4.53 Ga reference isochron. Nd isotopes of clinopyroxenes confirm the presence of refractory mantle domains with old ages of depletion in the Ligurian Tethys oceanic lithosphere. The origin of these mantle domains has been previously attributed to incorporation of SCLM derived from a late Variscan DMM melting event (see Mc Carthy & Muntener, 2015 and quoted references). However, geothermometry based on trivalent REE+Y exchange between clino- and orthopyroxene, yields high T estimates (TREE) of 1170-1300°C, associated with high T values obtained applying Ca-in Opx (1200-1280°C) and pyroxene solvus methods (TBKN ~ 1100°C). Thermal evolution of the Monte Civrari mantle body is thus consistent with rapid cooling and exhumation from asthenospheric conditions, similar to modern abyssal type peridotites, which apparently argues against a long residence time in the SCLM after the Permian event. McCarthy, A. & Müntener, O. (2015): Ancient depletion and mantle heterogeneity: Revisiting the Permian-Jurassic paradox of Alpine peridotites. Geology, 43, 255-258.