Nature of the mantle-crust transition in the Finero Complex (Ivrea-Verbano Zone, Southern Alps)

The Finero Complex (northern Ivrea-Verbano Zone; Southern Alps) has well-exposed contacts between rocks of the upper mantle and lower continental crust. The core of the Complex is composed by the Finero Phlogopite Peridotite (FPP) mantle unit, which is wrapped out by an intercalation of mafic ultramafic crustal rocks (Zanetti et al., 1999; Ogunyele et al. 2024). The first crustal unit in contact with the FPP is the Layered Internal Zone (LIZ), overlaid by the Amphibole Peridotite and the External Gabbro units. In order to characterize the nature of this transition, a detailed investigation has been performed on the outcrop at the confluence between Rio Cannobino and Rio Creves (46° 6'17.00"N, 8°32'39.88"E).
The mantle unit mostly comprises secondary medium-granular harzburgite, including phlogopite and amphibole, showing a pervasive foliation parallel to the mantle/crust contact. This latter is apparently magmatic. As the contact is approached, the size of the olivine grains decreases, and the peridotite composition gets enriched in orthopyroxene, phlogopite and amphibole. The mantle side of the contact is predominantly characterized by a layer, up to 1 m thick, of weakly deformed coarse-granular amphibole-biotite-bearing orthopyroxenite, with the strike roughly parallel to the foliation. Subordinately, the phlogopite harzburgite is directly in contact with the LIZ. The crustal side consists of a layered series of garnet-amphibole-bearing gabbroic rocks, which are intruded by concordant to discordant pegmatoidal hornblendite layers and pockets, sometimes with interstitial plagioclase. In places, hornblendite layers reach the contact with the FPP, reacting with the orthopyroxenite layer.
In-situ geochemical characterization was performed on various mineral phases, including amphibole, olivine, pyroxene, garnet, plagioclase, and phlogopite, from rocks of both mantle and crustal sections. A total of seven samples were analyzed. The contact between the mantle and crustal sections is marked by an orthopyroxenite layer, represented by the sample amphibole-bearing orthopyroxenite (15FI33D). Four samples were collected from the mantle unit at varying distances from this contact: amphibole-phlogopite-bearing harzburgite (15FI33A, 3 m; 15FI33B, 1.5 m; 15FI33C, 0.5 m). The crustal unit comprised three samples, also taken at different distances from the contact: hornblendite (15FI33E, 0.5 m), plagioclase-bearing hornblendite (15FI33F, 1.5 m), and amphibole gabbro with garnet (15FI33G, 1.7 m).