Magnetic and Paleomagnetic Characterization of the Ivrea-Verbano Lower Crust Body (NW Italy): Assessing the Magnetization of Variscan-Age Lower Crust

Abstract

The source of high-intensity magnetic anomalies from (mostly Precambrian) lower crust of continent interiors has long been debated, as it requires speculative rocks yielding 2–6 A/m magnetization. We report on the magnetic and paleomagnetic investigation of the Ivrea-Verbano Zone (IVZ), Western Alps, where metamorphic and intrusive lower crust rocks of Late Variscan-Permian ages are exposed. We sampled 39 oriented sites along the Cannobina, Ossola, Strona, and Sesia valleys/sections. Low (0.27–2.1·10⁻³ SI) magnetic susceptibility (k) values were measured in metapelite-metabasite metamorphic rocks from the Ossola and Strona valleys. There only two metabasite (one amphibolite and one granulite) out of 25 metamorphic sites containing pseudo-single domain (PSD) magnetite yield 0.48–1.1·10⁻¹ SI k values that remain constant until 550°C heating. K of gabbros-granodiorites from Sesia valley mimic low values from metamorphic rocks, whereas at Cannobina valley one gabbro and one mafic granulite display values comparable to the two strongly magnetic sites from Ossola/Strona valleys. Peridotite lenses at Balmuccia and Finero similarly yielded low (0.24–5.5·10⁻³ SI) k values, consistently with their low (<20%) serpentinization. Remanence contribution is negligible, as (a) Q < 1 values imply remanent magnetization subordinate to induced magnetization, (b) paleomagnetic directions from most magnetic sites are scattered, and (c) remanence is unstable at lower crust temperatures. We conclude that IVZ lower crust rocks could not yield magnetic anomalies generated by Precambrian lower crust from continent interiors, and similar conclusions might stand for other Variscan-age lower crust sections. Scattered high-intensity metabasites could be candidates, if their PSD magnetite-rich mineralogy dominated Precambrian lower crust.