EXHUMATION OF MANTLE LITHOSPHERE: FIELD RELATIONS, AND INTERACTION PROCESSES BETWEEN MAGMATISM AND DEFORMATION (FIELD TRIP TO THE NORTHERN LANZO PERIDOTITE)

Abstract

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.