Emplacement of mantle peridotite in the lower continental crust, Ivrea-Verbano zone, northwest Italy

Abstract

The Ivrea-Verbano zone has been cited repeatedly as an exposed pre-Alpine crust-mantle transition on the basis, in part, of the presence of lenses of mantle peridotite and a possible petrologic Moho. A 10-km-thick mafic complex has been described as an intrusion at the contact between the mantle and a thick crustal sequence of metamorphosed wacke and pelitic rocks. The significance of these features is now challenged by new mapping that demonstrates that mantle peridotites in the southern Ivrea-Verbano zone were lenses tectonically interfingered with metasedimentary rocks prior to intrusion of the gabbroic complex. Although it remains possible that the Ivrea-Verbano section exposes part of a complex crust-mantle interface within which crustal and mantle rocks were interleaved, the association of peridotite and metasedimentary rocks is also consistent with assembly in an accretionary wedge. In either case, present exposures are from an unknown distance above the pre-Alpine contiguous mantle and reference to the section as a complete crust-mantle transition could be misleading.