The Lanzo peridotite plays a key role in exploring the effects of
deformation and magmatism in a system evolving from rifting to sea
floor spreading. In this field guide we present new field observations,
structural and petrological data on the exhumation evolution of the
northern Lanzo peridotites during the opening of the Piemonte Ligurian
ocean. Gabbroic dikes ranging from troctolite to gabbronorite to oxide
gabbros and plagiogranites indicate fractionation of mafic magmas in a
relatively thick thermal lithosphere. This lithosphere was thinned by a
large shear zone separating different mantle domains. The thermal
evolution of the shear zone is recorded by porphyroclastic to mylonitic
plagioclase peridotites. Later, at least parts of the mantle rocks were
exhumed to the ocean floor, as indicated by widespread
serpentinization, rodingitization of mafic dikes, local occurrences of
ophicarbonates; and metabasalts. Possible origins of crustal remnants
(pre-alpine granulite) within the serpentinites are discussed.