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 Baldissero massif (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-facies conditions and intrusion of several generations of dykes. However, lherzolites far from dykes show very modest metasomatic changes, as evidenced by the crystallisation of accessory titanian pargasite and the occurrence 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 a partial melting event related to the Variscan orogenic cycle s.l.Dioritic dykes cutting the mantle sequence have whole rock, clinopyroxene and plagioclase characterised by high 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 simulations indicate that the geochemical characteristics of the diorites can be explained by flow of a hydrous, silica-saturated melt accompanied by reaction with the ambient peridotite and fractional crystallisation. The composition of the more primitive melts calculated in equilibrium with the diorite minerals show tholeiitic to transitional affinity.Internal Sm–Nd, three-point isochrons obtained for two dykes suggest an Upper Triassic-Lower Jurassic emplacement age (from 204 ± 31 to 198 ± 29 Ma). Mesozoic igneous events are unknown in the southern Ivrea–Verbano Zone (IVZ), but the intrusion of hydrous melts, mostly silica-saturated, have been well documented in the Finero region, i.e. the northernmost part of IVZ and Triassic magmatism with calc-alkaline to 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.

Research Highlights
► Mesozoic diorite dykes in the Baldissero mantle peridotite (Ivrea–Verbano Zone).
► Unusual geochemistry and emplacement age of diorite dykes.
► Implications of diorite dykes for the Southern Alps evolution.
► Mantle peridotite: refractory residue after 4–5% near-fractional melting of DM.
► Devonian Re–Os model age for mantle peridotite: Variscan partial melting event.