Gabbroic dykes in the Finero Phlogopite-Peridotite Massif: evidence for meltperidotite interactions at mantle conditions and igneous sapphirine formation by autometasomatism

The Finero Phlogopite Peridotite (FPP: Ivrea-Verbano Zone, western Alps, Italy) is a mantle sequence completely recrystallized by several events of melt migration. Literature studies provide evidence that a main metasomatic event induced the pervasive crystallisation of Amphibole-Phlogopite-bearing mineral assemblages (harzburgites and pyroxenites; Amph, Phl), as well as the formation of dunite bodies. These lithologies have similar geochemical compositions characterised by strong crustal components, as testified by enrichment in K, Mg, H2O, LREE and LILE and depletion in HREE and HSFE, radiogenic Sr and Pb, and unradiogenic Nd. Besides, the FPP shows bands, veins or pockets with variable mineral assemblages, but usually rich in Apatite (Ap) and Carbonates (Crb). Late gabbroic dyke swarms, different from all the other lithologies and containing Sapphirine (Spr), were recently described (Giovanardi et al., 2013). These dykes were formed by multi-stage magma intrusions via hydraulic-fracturing, characterised by early crystallisation of Amph ± Ap ± Phl (i.e the Early Amph Zone) followed by segregation at the vein core of Plagioclase ± Amph ± Crb (the Leucocratic Zone). Spr occurred in a reaction zone placed between them. Another reaction zone occurs at the contact between the host peridotite and the vein, being characterised by the complete replacement of peridotite Olivine by secondary Orthopyroxene (the Opx Zone). The occurrence of Spr only in the Late Amph Zone was possibly interpreted as i) direct magmatic crystallization from an evolved Al-rich melt or ii) crystallization by auto-metasomatic process with interaction between the Early Amph Zone with the most evolved melt segregating the Leucocratic Zone. A new study on the Spr-bearing gabbroic dykes of the FPP allowed us: i) to constrain the occurrence of local (up to 8 cm from the veins) melt interaction with the host harzburgite, which provides high δ18O, Al, Mg, K, H2O, locally associated to pronounced enrichments in U, Th, LILE and LREE, which unravels different degrees of contamination by host rock; ii) to provide new evidence supporting the Spr formation by auto-metasomatic process, after reaction of the first cumulates (i.e. the Early Amph Zone) with the most evolved melt which crystallized the Leucocratic Zone; iii) to give new constraints on the nature of the gabbroic dykes parent melts. The formation of the Opx Zone within the host peridotite and the cumulus crystallization of hydrous phases (i.e. Amph and Phl) suggest to a silica-saturated, hydrous evolved melt as parent melt of the gabbroic dykes. The L/MREE-enriched convex-upward patterns of the Amph, point to an alkaline geochemical affinity of the parent melts.