Unravelling magma emplacement mechanism in the lower crust: A forensic investigation of the Mafic Complex, Ivrea-Verbano Zone (Italy)

The Ivrea-Verbano Zone (Southern Alps, Italy) is a unique target for assembling data on magma emplacement mechanism in the Earth's lower crust. In its southern portion cross-cut by the Mastallone (north), Sesia (central), and Sessera river (south), the 13-km thick Permian Mafic Complex consists of hornblende-gabbro (lower part), gabbro-norite (intermediate part) and diorite (upper part), emplaced into a pre-Permian metasedimentary sequence, and represents the deep roots of the Permian Sesia Magmatic System. New field data (foliation and lineation measurements), three-dimensional analysis of oriented rock microstructures by X-ray tomography (50 μm spatial resolution), and calculations of physical properties (rock density, P- and S-wave velocity) based upon mineral chemistry and modal proportions indicate a magma body constructed by the repetitive emplacement of sills. We observe from bottom to top: a) a 2-km thick, cumulate region with an upward increase of pyroxene/amphibole (up to 75%) and oxide content (up to 6%); b) a 10-km main region preserving two 1- to 2-km thick pyroxenite intrusions interlayered with two 4-km thick plagioclase-dominated (up to 80%) zones in the body core, with its northern margin also characterised by presence of garnet (up to 77%); c) a 1.5-km portion composed of a hornblendite embedded in a biotite- (up to 20%) and plagioclase-dominated (up to 60%) region. Respective 20% and 13% rock density and seismic velocity variations between plagioclase-rich regions and pyroxenite intrusions suggest an internal stratification of physically distinct portions. Indeed, P- to S-wave velocity ratios (V_P/V_S) show a body constructed by mafic pulses (V_P/V_S = 1.75-1.79), denser than the basal cumulate region, overlain by less dense melt-enriched regions (V_P/V_S = 1.84-1.87), which are able to intermingle with and assimilate metapelite septa of the pre-existing crust and, potentially, evolve into a K-rich hydrous body at the roof portion. The Mafic Complex architecture is therefore not the result of a catastrophic gravitational collapse during crustal thinning, as previously proposed upon observation of upwardly concave layering and foliation in ophiolitic gabbros, but rather an incremental emplacement of pulses adapted to the geometry of the pre-existing crust.