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

The Baldissero peridotite is the main mantle massif occurring in the south-western part of the Ivrea zone. It is in contact at the East with the gabbros of the Ivrea-Verbano Mafic Complex and at the West it is sheared by the faults of the Insubric system and in tectonic contact with the terrains of the Austroalpine Domain (Canavese one). Another Alpine shear zone, the Cremosina line, limits at the North-West the Baldissero terrains from the rest of the Ivrea-Verbano Mafic Complex. 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 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 Mafic Complex in the Finero area show differences of about 10 Ma between Sm-Nd internal isochron ages (isochrons with 3 minerals, ranging from 223 to 231 Ma and having positive εNdi from +2.7 to + 5.3, Lu et al, 1997) and U/Pb zircon ages (232 ± 2 Ma; Peressini et al., 2005). Therefore, even if the ages obtained for the diorite dykes in Baldissero may be slightly younger with respect to the age of emplacement, this last should be, anyway, not older than the late Triassic. At this estimated age of dyke intrusion, the Ivrea-Verbano 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-Verbano region, but only in the northernmost Finero region. The Baldissero dykes, besides for their age, exhibit geochemical and isotopic similarities with the External Gabbro Unit of the Finero Mafic Complex, constituted by amphibole-bearing diorites. The Finero Mafic Complex is in tectonic contact, marked by a high-temperature ENE shear-zone, with the Permian relatively anhydrous mafic-ultramafic sequences occurring in Val Sesia and on the right side of the Val d'Ossola. Therefore, the evidences of Mesozoic magmatism in the westernmost sector of the Southern Alps are, presently, confined by tectonic lineaments to the southernmost and northernmost portion, respectively. 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, its particular location within the Ivrea-Verbano Zone and the nature of the involved parent melts are a potentially important mark in the geodynamic evolution of the Southern Alps.