New insights into the evolution of the Finero Mafic Complex, north-eastern Ivrea-Verbano Zone

The Finero Mafic Complex outcrops in the northern sector of the Ivrea-Verbano Zone (IVZ, Southern Alps). It occurs at the flank of an antiform which core is constituted by mantle peridotites, and it consists of mafic/ultramafic rocks subdivided in three units: a) the Layered Internal Zone (LIZ), in tectonic contact with the mantle unit; b) the Amphibole Peridotite (Amph-Pd); c) the External Gabbro (EG), which is in tectonic contact with the Variscan crystalline basement (Rivalenti et al., 1984; Siena & Coltorti, 1989). Recent studies point to a Middle Triassic emplacement age for the EG unit (Zanetti et al., 2013), suggesting that it is unrelated to the Permian Mafic Complex of the central IVZ. Owing to the lack of a detailed petrochemical characterisation of the FMC, we performed new major (EMP) and trace element (LA-ICPMS) analyses on representative samples from the LIZ and Amph-Pd. The LIZ mainly consists of hornblende-gabbros; anorthosites and garnet hornblendites with minor pyroxenites. The Amph-Pd is mostly made up of Amph-bearing harzburgites and dunites with minor piroxenites. Locally, Amph-rich veins with a variable thickness from a few cm to about 1 m crosscut the magmatic layering. Olivine (Fo87-82) only occurs in the perfrom Amph-Pd, whereas amphibole and clinopyroxene are common throughout the entire sequence. The Mg# of Cpx and Amph tends to increase from the LIZ towards the upper part of the Amph-Pd whereas the Al2O3 content in Cpx and Amph is up to 11 and 18 wt.%, respectively and show an opposite trend. In garnet-free pyroxenites and hornblendites from LIZ, Amph and Cpx have slightly LREE-depleted patterns with flat HREE (at 2xCI in Cpx) and marked positive Eu, Sr, Pb and U anomalies. Similar features are shown by the Cpx and Amph from the associated gabbros, but they are strongly depleted in HREE indicating chemical equilibration with garnet. Cpx and Amph from the Amph-bearing peridotites (Amph-Pd) have instead LREE-enriched spoon-shape patterns with HREE contents comparable with those of the LIZ lithologies, being also characterised by Eu, Sr and U enrichments. The LILE enrichments and fractionation can be reconciled by an interaction dominated by ion exchange chromatographic-type process with strongly LILEenriched melts: the composition of the latter is recorded by the amphibole-dominated lithologies. The new data suggest that the LIZ with a clear crustal signature. Instead, the trace element variations in the Amph-Pd cannot be explained via a closed-system evolution, pointing to the presence of significant changes in the composition of the uprising mantle melts.