The plagioclase-lherzolite body of Lanzo (Italian Alps) is considered to be a residual piece of upper mantle, after MORB extraction. The mantle source had a composition very similar to primitive mantle estimates, except for a variable REE fractionation (Ce N Yb N< 1) defining a kilometre-scale heterogeneity. Melting occurred mainly in the spinel stability field, up to the transition between spinel and plagioclase peridotite facies. The percentage of melt extracted varied from 6 to 12% in the main southern part of the massif, where the expelled magmas had T to N-MORB compositions with REE patterns similar to those of ophiolitic basalts in Western Alps, while it was lower than 6% in the north, where the peridotites equilibrated with T-MORB. The dunites are considered to be preferential zones for magma tapping and circulation into the partially molten region, while the layered pyroxenites are crystal segregates separated from magmas rising to the surface into feeder dykes. The peculiarities of the northern part of the massif are attributed to a transform fault activity. The rest of the massif and the associated mafic rocks record a multi-stage evolution leading to the development of a reduced and discontinuous oceanic crust. Judging by the number of peridotites which have been dredged or drilled on the oceanic bottoms, the generation of a Lanzo-type uppermost lithosphere may not be an uncommon feature in the evolution of ocean crust.