Magnetic and paleomagnetic characterization of the Ivrea-Verbano lower crust body (NW Italy): Assessing the magnetization of Variscan-age lower crust

The source of high-intensity magnetic anomalies from cratons has long been debated, as it requires speculative rocks yielding 2-6 A/m magnetization. Magnetic properties of the lower crust lithology are generally poorly constrained, considering their low exposure at surface. Here we report on the magnetic and paleomagnetic investigation of the Ivrea-Verbano Zone (IVZ), Western Alps, where both metamorphic and intrusive lower crust rocks of Late Variscan-Permian ages are spectacularly exposed. We sampled 312 oriented cores at 39 sites along the Cannobina, Ossola, Strona, and Sesia valleys/sections. Low (0.27-2.1·10^-3) magnetic susceptibility (k) values were routinely measured in metamorphic rocks from the Ossola and Strona valleys. There, only two metabasite (one in amphibolite and one in granulite metamorphic grade) out of 25 metamorphic sites containing pseudo single domain (PSD) magnetite yield 0.48-1.1·10^-1 k values that remain strikingly constant until 550°C heating. K values of gabbros and granodiorites from Sesia valley mimic low values from metamorphic rock, whereas at Cannobina valley one gabbro and one mafic granulite display k values comparable to the two strongly magnetic sites from Ossola/Strona valleys. Peridotite lenses embedded in gabbros at Balmuccia and Finero similarly yielded low (0.24-5.5·10^-3) k values, consistently with their low (<20%) serpentinization degree. Results indicate that remanence contribution is negligible, as 1) Q <1 values imply remanent magnetization subordinate to induced magnetization, 2) paleomagnetic directions are generally scattered consistently with PSD magnetite grain size, and 3) remanence is notoriously unstable at lower crust temperatures. We conclude that IVZ lower crust could not yield cratonic magnetic anomalies, and similar conclusions might stand for other Variscan-age lower crust sections. Scattered high-intensity metabasites could be candidates, if their PSD-MD magnetite-rich mineralogy dominates pre-Cambrian lower crust.