Reappraisal of the geodynamic evolution of the mantle massifs of the Ivrea-Verbano Zone based on new field, petrochemical and geochronological data

The Ivrea-Verbano Zone (IVZ) is the westernmost sector of the Southern Alps. It is constituted by granulite-to-amphibolite-facies continental crust representing the basement of the Adria plate. The IVZ contains many orogenic mantle peridotites. The largest mantle bodies are aligned along the Insubric Line at the lowest stratigraphic units, in contact with mafic-ultramafic crustal intrusives. Mantle bodies in the central and southern sectors of IVZ are spinel lherzolites with spinel dunites and variable amount of clinopyroxenite, websterite and subordinate anhydrous/hydrous gabbroic bodies (e.g. the Baldissero, Balmuccia, Premosello peridotites). Conversely, modally-metasomatised spinel harzburgites with large dunite bodies and phlogopite-and-amphibole-bearing websterites (e.g. the Finero peridotite) crop out in the northern IVZ.

The constant association of the IVZ mantle peridotites with High-T shear zones suggests that none of them was emplaced into the crust by mantle diapirism. Alternative hypotheses involve emplacement at the crustal level at the onset of the Mesozoic extensional regime or tectonic addition to accretionary wedges of Paleozoic subduction zones. Recent gravimetric and seismic data converge in indicating that high-density rocks are very close to the surface near the Insubric Line, thus supporting the possibility that the largest mantle peridotites may be a direct expression of the underlying subcontinental mantle.

This contribution focuses on new field, petrographic, geochemical and geochronological data, to address some relevant issues, such as the nature of the spinel lherzolite (refractory residue vs. refertilised mantle), the origin of pyroxenites and gabbros, the relationships with the associated crustal intrusives and the record of Mesozoic tectono-magmatic events.

The final goal is to provide new insights into the geodynamic evolution of the mantle bodies and the mantle-crust systems at the Laurasia-Gondwana margin, defining in particular how the mantle heterogeneity acquired during Paleozoic may have governed the rifting process of the Adria margin in Jurassic times.