High-pressure metamorphism of the Lanzo peridotite and its oceanic cover, and some consequences for the Sesia–Lanzo zone (northwestern Italian Alps)

This paper presents new data on the Alpine high-pressure evolution of the Lanzo peridotite massif and its metasedimentary cover (Western Alps). Field relations in the northwestern Lanzo massif show that the serpentinized part is covered by a metasedimentary sequence including local occurrences of metaophicarbonate, metabasalt, calcschist, metaquartzite and gneiss. Remnants of granulite-facies mafic gneiss associated with metaophicarbonate are found within the serpentinite. Major element garnet zonation profiles in metaquartzite and mafic gneiss were used to explore the contrasting origin of metasediments. Bell-shaped Mn zoning in garnet from metaquartzite indicates a one-stage Alpine metamorphic evolution, while inverse zoning in garnet of the mafic gneiss shows that the protolith equilibrated under high-grade metamorphic conditions in pre-Alpine times, similar to kinzigites within the neighbouring Sesia zone. Phase equilibria in metaquartzite and mafic gneiss point to 500 ± 50 °C at pressures of 0.9 to 1.3 GPa for the Alpine metamorphic equilibration that contrasts with the 550–620 °C and pressures in excess of 2.0 GPa obtained for kyanite + talc + chloritoid and talc + chloritoid + garnet domains in a metagabbro dike from the internal part of the Lanzo massif. These different P–T trajectories can be explained by a much stronger retrograde overprint at the basement–cover contact than in the internal part of the Lanzo massif.