The Ivrea Zone and lower crustal magnetization

Abstract

The Ivrea Zone in northern Italy is an arcuate belt of rocks whose geological and geophysical properties indicate that they were once deep in the crust. It is one of several presumed lower crustal cross sections whose magnetic properties have been well studied in a systematic way; the others are the Lofoten-Vesterålen terrane in Norway, the Pikwitonei-Cross Lake subprovince in Manitoba, the Kapuskasing uplift in Ontario and (to a lesser extent) the Lewisian terrane in Scotland. Rocks from the Ivrea Zone have highly variable magnetic properties. Consistently strong magnetization is associated with serpentinized ultramafics and the main metagabbro sequence. Many amphibolites, and a few metapelites, are also strongly magnetic. Natural remanent magnetization, generally with a strong present field overprint, has intensities comparable to induced magnetization in the more highly magnetic rocks. Although the strongly magnetic units have magnetizations high enough (about 4 A/m) to account for the magnetic anomaly associated with the zone, they would be required to make up a considerably larger proportion of the subsurface body than they represent in the surface outcrop—a common observation in the other crustal cross sections as well. Like the other cross sections, magnetization in the Ivrea Zone is carried by magnetite, with some pyrrhotite in less magnetic metapelitic samples. The Ivrea Zone contains more ultramafic rocks than most of the other sections, but shows no direct correlation between magnetization and composition; degree of serpentinization has a far greater influence on the magnetization of the ultramafic rocks.