Per. Mineral. (2007), 76, 2-3, 199-221 doi:10.2451/2007PM0025 SPECIAL ISSUE: In honour of Ezio Callegari on his retirement http://go.to/permin l PERIODICO di MINERALOGIA established in 1930 An International Journal of MINERALOGY, CRYSTALLOGRAPHY, GEOCHEMISTRY, ORE DEPOSITS, PETROLOGY, VOLCANOLOGY and applied topics on Environment, Archaeometry and Cultural Heritage ABSTRACT. — The North Lanzo peridotite body is mostly composed of fertile spinel lherzolites showing diffuse pyroxenite bands. These lithologies typically preserve older (lithospheric) spinel- facies assemblages, but most of them experienced a widespread recrystallization at plagioclase-facies conditions. In particular, spinel pyroxenites are deeply recrystallized to plagioclase+olivine-rich assemblages. The high pyroxenes modal contents of the lherzolites and the diffuse occurrence of pyroxenite banding indicate that this mantle section escaped important modifications by reactive interaction (i.e. pyroxene dissolution and olivine precipitation) of undersaturated MORB-type melts at spinel-facies conditions, prior to the subsolidus partial equilibration, as widespread in the South Lanzo peridotite body. According to the petrographic evidence, the early plagioclase-bearing assemblages originated during subsolidus partial recrystallization, likely during the emplacement of the mantle sequence at relatively shallow levels. Field, structural and compositional features suggest that hectometre-scale areas of the North Lanzo peridotites were successively enriched in plagioclase + olivine + pyroxenes as a result of large diffuse percolation and impregnation mostly operated by aggregate MOR-type melts. This indicates that asthenospheric melts migrated via porous flow through the North Lanzo peridotites when they had been already exhumed to plagioclase- facies conditions, i.e. to relatively shallow levels in the mantle lithosphere. Replacive, strongly pyroxene-depleted, spinel harzburgites cut in the form of channels through both spinel-plagioclase peridotites and impregnated plagioclase peridotites. They have small interstitial magmatic pyroxenes showing clear alkaline affinity. This indicates that, after diffuse percolation and impregnation by MOR-type melts, the North Lanzo peridotites underwent focused percolation along strongly pyroxene-depleted channels of alkaline melts, deriving from garnet-bearing fertile mantle sources. Available data indicate that the North Lanzo spinel- facies lherzolite protolith represent the ancient sub- continental lithospheric mantle, which was isolated from the convective mantle, as previously argued on the basis of Sr-Nd isotopic data. These peridotites record a composite tectonic- metamorphic and melt-related evolution that indicates that this piece of sub-continental lithospheric mantle has been exhumed to shallow levels, it has been subsequently pervasively percolated and impregnated by asthenospheric MOR-type melts, and finally it has The North Lanzo peridotite body (NW Italy): lithospheric mantle percolated by MORB and alkaline melts GIOVANNI B. PICCARDO 1 * , ALBERTO ZANETTI 2 , AZZURRA PRUZZO 1 and MATTEO PADOVANO 1 1 Dipartimento per lo Studio del Territorio e delle sue Risorse, Università degli Studi di Genova, Corso Europa, 26, 16132 Genova, Italy 2 CNR, Istituto di Geoscienze e Georisorse, Unità di Pavia, Via Ferrata, 1, 27100 Pavia, Italy * Corresponding author, E-mail: piccardo@dipteris.unige.it