Igneous sapphirine as a product of melt-peridotite interactions in the Finero Phlogopite-Peridotite Massif, Western Italian Alps

Sapphirine is generally interpreted to be of metamorphic origin in high-MgO-Al2O3 rocks. Igneous sapphirine, i.e. sapphirine crystallised from melt, is very rare. We examined sapphirine-bearing magmatic veins in the Finero Phlogopite- Peridotite Massif, Western Italian Alps, to investigate a possible igneous origin for sapphirine by crystallisation from a melt, acquiring particularly high Al2O3 content via melt-rock reaction and fractional crystallisation. Sapphirine locally occurs in melanocratic zones placed between leucogabbroic veins and the host peridotite. The leucogabbroic veins cut at high angle the mantle foliation and the lithological layering of the peridotite massif, which is defined by alternating phlogopite-rich harzburgites and pyroxenites. This observation, along with their peculiar major-element mineral chemistry, indicates that leucogabbroic veins were unrelated to the pervasive metasomatic recrystallisation of the host mantle sequence, recording a later, distinct event of melt injection. Melanocratic seams are observed on both sides of the leucogabbroic veins. They show a marked zoning of the mode: an orthopyroxene-rich zone overgrows upon the host peridotite (OPX zone), whereas an amphibole-rich zone occurs towards the leucogabbroic vein (AMPH zone). Sapphirine commonly mantles spinel or occurs as discrete grains, located either (1) within large light-brown pargasite crystals in the AMPH zone or (2) interstitially, between light-brown pargasite in both AMPH zone and OPX zone. Light-brown pargasite can also enclose spinel that does not have sapphirine envelope. To explain the petrochemical features of the sapphirine-bearing veins and the host peridotite a four-stage process involving melt–rock reactions and fractional crystallisation is here proposed. During the first stage (Stage A), the interaction between the uprising SiO2-saturated melt and the host peridotite caused the replacement of peridotite olivine, amphibole and phlogopite by newly formed orthopyroxene close to the contact and Al2O3, TiO2, FeO enrichments in the host peridotite beyond the recrystallisation front. This mineralogical reaction resulted in high Al2O3/SiO2 and MgO/FeO ratios in the migrating melt. The modified melt crystallised Al2O3-rich dark-brown pargasite (16.5 wt% Al2O3) and apatite in the open conduit (Stage B). Sapphirine/spinel saturation was actually achieved later, in the Stage C, in presence of a more differentiated melt, which reacted with the early dark-brown pargasite locally producing pseudo-symplectite textures made by lightbrown pargasite and spinel/sapphirine. A peculiar Al2O3-enriched composition for the parent melt segregating sapphirine is indicated by the composition of the associated light-brown pargasite (17.5 wt% Al2O3), phlogopite and spinel. Locally, this melt percolated also the OPX zone, segregating sapphirine-bearing mineral assemblages. The sapphirine-free leucogabbroic vein was finally segregated during the Stage D, after splitting of the AMPH zone likely due to hydraulic fracturing.