The Ivrea-Verbano Zone (Southern Alps, Northern Italy) is regarded as a typical example of a section of the lower continental crust. A considerable magnetic anomaly runs along its outcrop area. A study has been made of 153 samples collected along three transverse sections (the Cannobina, Strona and Sessera valleys) and from the southernmost outcrops near Ivrea to determine the relationships between this anomaly and the magnetic properties of the rocks, and to gain an insight into the nature of magnetism of the lower crust. Most rocks displayed magnetic susceptibility values ranging from 100 to 10,000 × 10-6 SI units. Serpentinized peridotites from the Finero and Baldissero massifs, the main gabbro and kindred units, and a few amphibolites and mafic granulites of the Kinzigite Series are the only rocks with values (29,000-200,000 × 10-6 SI) comparable with those required to produce the observed anomaly. In addition, the intensity of their remanent magnetization is of the same order of magnitude as the induced magnetization, and the directions of remanence are mostly scattered around the direction of the present field. Their total magnetization is enough to explain the anomaly of the Ivrea-Verbano Zone. The results of these measurements and the surface geology suggest that a set of distinct slab-like bodies formed of serpentinized peridotite and main gabbro can be proposed as a model for interpretation of the Ivrea-Verbano magnetic anomaly. Amphibolites and mafic granulites can be regarded as responsible for the short-wavelength anomalies only. The main magnetic minerals are pyrrhotite, which is widely distributed but always in low concentrations, and magnetite, which is confined to a few lithotypes but can reach high concentrations. Magnetite must thus be held responsible for the anomaly, as in other sections of the lower crust investigated so far. The reason for its presence in individual lithotypes, however, is not always the same and no systematic regional correlation with mineralogical and metamorphic characteristics can be established. In conclusion, the phenomena observed in the Ivrea-Verbano Zone suggest that the magnetism of rocks regarded as typical of the lower crust is highly variable, and that there is no ready explanation for the high magnetization values postulated in the light of the satellite data. © 1990.
