New insights into the evolution of the Finero Mafic Complex

The Finero Complex outcrops as an antiform in the northern sector of the Ivrea-Verbano Zone (Southern Alps). The antiform core is constituted by a mantle unit surrounded by a cumulitic sequence, i.e. the Finero Mafic Complex (FMC) [1,2]. The complex is divided 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. Owing to the lack of a detailed petrochemical characterisation of the FMC, we performed new major and trace element (LA-ICPMS) analyses on representative samples from the LIZ and Amph-Pd. The LIZ mainly consists of Grt-hornblendites and Hbl-gabbros, with minor anorthosites and pyroxenites. The Amph-Pd is mostly made up of Amph-bearing harburgites and dunites (Ol: Fo87-82), with recrystallisation fronts along which the peridotites become modally-dominated by Amph. The Al2O3 content is up to 11 and 18 wt% in Cpx and Amph, respectively: it increases from the peridotites (Amph-Pd) through gabbros to the hornblendites and pyroxenites (LIZ). In the garnet-free pyroxenites and hornblendites from LIZ, Amph and Cpx have slightly LREE-depleted patterns with flat HREE (at 2 CI in Cpx) and marked positive Eu, Sr, Pb and U anomalies. Similar features are shown by the Cpx and Amph from the associated gabbros, they differ in having HREE-depleted patterns, thereby indicating chemical equilibration with garnet. Cpx and Amph from the Amph-Pd have variable LREE-enriched spoon-shaped patterns, with nearly flat HREE-pattern and positive Eu, Sr and U anomalies. The LREE gradient can be explained by interaction with percolating LREE-enriched melts, dominated by ion exchange processes. Amph-enriched peridotites, which contain the highest LREE contents are a proxy for the composition of the percolating melts. The new data suggest that the LIZ and Amph-Pd units may have crystallised from melts of cognate origin with a clear crustal component. However, the recrystallisation of the Amph-Pd cannot be explained by a closed-system evolution, pointing to significant changes in the composition of the uprising mantle melts.