Evolution of mantle melts intruding the lowermost continental crust: constraints from the Monte Capio–Alpe Cevia mafic–ultramafic sequences (Ivrea–Verbano Zone, northern Italy)

This study presents a new petrological–geochemical data set for the Monte Capio and Alpe Cevia mafic–ultramafic sequences, which are exposed in the deepest levels of the Ivrea–Verbano Zone. These sequences are composed of a peridotite core, with dunite in the center, mantled by minor orthopyroxene-dominated pyroxenites and subordinate hornblende gabbronorites. Amphibole is ubiquitous in the peridotites and the pyroxenites (≤ 15 vol % and 10–40 vol %, respectively), and the peridotite–pyroxenite associations are frequently crosscut by amphibole-rich (45–90 vol %) veins/dykes showing sinuous-to-sharp planar boundaries towards host rocks. The whole-rock Mg# [100 × Mg/(Mg + Fetot2+)] decreases from the peridotites to the pyroxenites and the crosscutting amphibole-rich dykes (84–81, 80–77, and 73–66, respectively), consistently with the Mg# variations shown by included orthopyroxene, clinopyroxene, and amphibole. Olivine has relatively low forsterite and NiO amounts (84–78 mol % and ≤ 0.14 wt%), and spinel is characterized by low Cr# [100 × Cr/(Cr + Al)] of 7–24. The anorthite content of plagioclase varies from 91 to 88 mol% in plagioclase-bearing pyroxenites to 91–75 mol% in amphibole-rich dykes. The chondrite-normalized REE patterns of amphibole from peridotites and pyroxenites show nearly flat MREE–HREE, no evident Eu anomaly, and LREE that are slightly depleted to slightly enriched with respect to MREE. Amphibole from the amphibole-rich veins/dykes exhibits slight LREE depletion. Whole-rock and amphibole separates show substantial variations in initial Nd–Sr isotopic compositions (e.g., whole-rock εNd calculated at 290 Ma ranges from − 0.3 to − 4.7), irrespective of the rock-type and of incompatible element amphibole compositions. We propose that the Monte Capio–Alpe Cevia dunites formed by cooling of magma lenses that intruded the lowermost continental crust of the Ivrea–Verbano Zone. The chemically evolved signature of the dunites documents earlier crystallization of chemically primitive dunites at lower levels, or olivine fractionation within the dunites during melt ascent. Associated pyroxene-bearing peridotites show a magmatic evolution ruled by reaction of a melt-poor crystal mush with migrating melts relatively rich in SiO2 and H2O, which developed orthopyroxene and amphibole at the expenses of olivine ± clinopyroxene. These migrating melts may be reconciled with those feeding the crosscutting amphibole-rich veins/dykes, whose compositions suggest formation by chemically evolved H2O-rich basalts with an arc-type incompatible trace-element fingerprint. Unraveling the origin of the Monte Capio–Alpe Cevia pyroxenites is hampered by the complex open-system magmatic evolution, which also included assimilation of material released by basement metasediments and/or involvement of primary melt batches with different compositions.