Magnetic petrology of deep crustal rocks—Ivrea Zone, Italy

Abstract

Magnetic petrology is an extension of petrology, integrating magnetic property studies with conventional petrology for the purpose of understanding the development and modification of the magnetization in rocks. The magnetic properties of a suite of samples from the Ivrea Zone, northern Italy, are described with the aim of developing a magnetic petrology for deep crustal rocks. Samples studied include a variety of metamorphosed mafic (granulites, granofelses, plagioklasfelse, amphibolite, pyriclasite) and pelitic/quartzo-feldspathic rocks (acid granulites, stronalite and kinzigite gneisses, schists), and representative ultramafic rocks (phlogopite peridotite, pyroxenite, hornblendite). Ilmenite is the chief oxide mineral in a majority of the samples, but in a few mafic rocks, magnetite is the principal oxide constituent. Cr,Al-spinel is common in ultramafic samples and rutile is present in some metapelites. Sulfides are generally subordinate in abundance to oxide minerals. The mafic rocks in the Ivrea section have the broadest range of magnetic properties and include the most strongly magnetic samples. The Ivrea ultramafic rocks are moderately magnetic—the magnetite present appears to be secondary and is associated with crustal alterations—while pelitic and quartzo-feldspathic lithologies are dominantly paramagnetic or only weakly magnetic. The main ferromagnetic mineral in all strongly magnetic samples is magnetite that is nearly pure Fe 3O 4 in composition. Consequently, Curie temperatures are 565–580°C for these rocks. Pyrrhotite contributes to the magnetism in several ultramafic and mafic rocks, and it is the sole ferromagnetic mineral in the pelitic and quartzo-feldspathic samples. The results of our study indicate that certain mafic rock types (amphibolites, mafic granulites, pyriclasites) are the most likely source of the Ivrea Zone's regional scale magnetic anomalies. If the Ivrea Zone represents a tectonically exposed cross section of continental crust, the geographic distribution of these rock types suggests the presence of multiple and variably thick, strongly magnetic layers, with magnetite Curie temperatures, in the deep crust. Petrographic evidence suggests, however, that the magnetic mineralogy of the sampled Ivrea rocks might have been changed subsequent to peak metamorphic conditions. Therefore, considerable care must be taken in evaluating the magnetic record in these crustal cross sections, particularly if they are to be used to model the magnetic characteristics of the lower crust.