New constraints on the Mesozoic emplacement of dioritic dykes in the Baldissero mantle peridotite massif (Ivrea-Verbano Zone)

Dioritic dykes in the Baldissero mantle peridotite have high Mg# values in bulk-rock and mafic phases which positively correlate with incompatile element concentrations. The peridotite at the dyke contact is enriched in orthopyroxene, iron and incompatible trace element with respect to the LREE-depleted lherzolites typical of the Baldissero massif. The geochemical characteristics of the dykes are explained by flow, accompanied by crystallisation and reaction with the ambient peridotite, of a hydrous silica-saturated basaltic melt under crustal condition. The least evolved melts in this process are those that have the lowest incompatible trace element concentration and Mg#. New Internal Sm-Nd isochrons on two dyke samples (198±29, with Ndi = +5.6 and 204±31 Ma with Ndi = +6.0) and Re-Os data on the peridotite at the contact indicate late Triassic - lower Jurassic ages. Recently, Sm-Nd isochron ages published for the Mafic Complex in the Val Sesia and Val Sessera area (from 274±11 to 244±4 Ma, Mayer et al., 2000; Voshage et al. 1987) has been interpreted to date the cooling of mineral assemblages below the blocking temperature for the Sm-Nd system (Peressini et Al., 2007). However, the samples wich give the youngest ages, show negative εNdi values (from -3.8 to -5.3), suggesting substantial crustal contamination and, therefore, open system processes. The sample with the oldest age has, on the contrary, positive εNdi (+9.7) indicating the absence of remarkable crustal contamination. The span between this age (274±11 Ma) and the U/Pb zircon ages reported in Peressini et al. (2007) is around 10 Ma. Moreover, differences of about 10 Ma between Sm-Nd internal isochron ages (isochrons with 3 minerals showing positive εNdi from +2.7 to + 5.3, Lu et al, 1997) and U/Pb zircon ages (Peressini et al. (2005) are reported for the Mafic Complex in the Finero area. Therefore, even if the ages obtained for the diorite dykes in Baldissero may be slightly younger with respect to the age of emplacement, which should be, anyway, not older than the late Triassic. At this estimated age of dyke intrusion, the Ivrea mantle peridotites were already emplaced in the crust. Thus, the low Mg# of the melt intruding the peridotite is explained by previous fractionation in the crust. Mesozoic igneous events are unknown in the southern Ivrea region, but only in the northernmost Finero region. Furthermore, the mantle phlogopite-bearing peridotite of Finero, show a metasomatic imprint of late Triassic age, which was attributed to a hydrous silica-saturated component of crustal origin, deriving from a subducting slab (Hartmann & Wedephol, 1993, Zanetti et al. 1999). The evidence of a Mesozoic magmatic activity in the westernmost sector of the Southern Alps, presently confined by tectonic lineaments to the southernmost and northernmost portion of the Ivrea-Verbano Zone, respectively, and the nature of the involved parent melts are a potentially important mark in the geodynamic evolution of the Southern Alps.