Age and geochemistry of mantle peridotites and diorite dykes from the Baldissero body: Insights into the Paleozoic–Mesozoic evolution of the Southern Alps

Trace element and isotopic data obtained for mantle spinel lherzolites and diorite dykes from the Baldisseromassif (Ivrea–Verbano Zone, Western Italy) provide new, valuable constraints on the petrologic and geodynamic evolution of the Southern Alps in Paleozoic to Mesozoic times. Whole rock and mineral chemistry indicates that Baldissero lherzolites can be regarded as refractory mantle residues following limited melt extraction. In particular, the Light Rare Earth Elements (LREE)-depleted and fractionated compositions of whole rock and clinopyroxene closely match modelling results for refractory residues after low degrees (~4–5%) of near-fractional melting of depleted mantle, possibly under garnet-facies conditions.Following this, the peridotite sequence experienced subsolidus re-equilibration at lithospheric spinel-faciesconditions and intrusion of several generations of dykes. However, lherzolites far from dykes show verymodest metasomatic changes, as evidenced by the crystallisation of accessory titanian pargasite and theoccurrence of very slight enrichments in highly incompatible trace elements (e.g. Nb). The Re–Os data for lherzolites far from the dykes yield a 376 Ma (Upper Devonian) model age that is considered to record apartial melting event related to the Variscan orogenic cycle s.l.Dioritic dykes cutting the mantle sequence have whole rock, clinopyroxene and plagioclase characterised byhigh radiogenic Nd and low radiogenic Sr, which point to a depleted to slightly enriched mantle source.Whole rock and mafic phases of diorites have high Mg# values that positively correlate with the incompatible trace element concentrations. The peridotite at the dyke contact is enriched in orthopyroxene, iron and incompatible trace elements with respect to the lherzolites far from dykes. Numerical simulationsindicate that the geochemical characteristics of the diorites can be explained by flow of a hydrous, silicasaturated melt accompanied by reaction with the ambient peridotite and fractional crystallisation. Thecomposition of the more primitive melts calculated in equilibrium with the diorite minerals show tholeiiticto transitional affinity.Internal Sm–Nd, three-point isochrons obtained for two dykes suggest an Upper Triassic-Lower Jurassicemplacement age (from 204±31 to 198±29 Ma). Mesozoic igneous events are unknown in the southernIvrea–Verbano Zone (IVZ), but the intrusion of hydrous melts, mostly silica-saturated, have been welldocumented in the Finero region, i.e. the northernmost part of IVZ and Triassic magmatism with calc-alkalineto shoshonitic affinity is abundant throughout the Central–Eastern Alps. The geochemical and chronological features of the Baldissero diorites shed new light on the geodynamic evolution of the Southern Alps before the opening of the Jurassic Tethys.