Petrological investigation of garnet- and olivine-bearing mafic crustal rocks in the Sesia Valley (Ivrea-Verbano Zone, Italy)

The factors and mechanisms related to magmatic underplating in continental settings remain a topic of active scientific debate. The Ivrea-Verbano Zone (IVZ), western Southern Alps, provides a rare opportunity to contribute to this debate, being an exhumed, nearly complete section of the continental crust that also include the roots of a completely exposed Permian magmatic system. Some parts of these roots are represented by the so-called Layered Series, a sequence of ultramafic and mafic rocks located between the localities of Balmuccia and Vocca in the Sesia valley. They have been interpreted as the first stages of magmatic underplating that later formed an 8-km thick sequence of gabbroic cumulates formed in the lower continental crust.

This work focuses on an 80 m wide outcrop within the Upper Zone (UZ) of the Layered Series, which is located between the villages of Isola and Sassiglioni. It includes a lithological sequence primarily composed of partially foliated gabbros with variable amounts of garnet, olivine, amphibole, and/or hercynite, along with layers of anorthosites and pyroxenites, pseudotachylites, and mafic pegmatites.

The objective of this work is to use this outcrop to constrain the P-T conditions of magmatic crystallization and subsequent metamorphic re-equilibration of the UZ and combine this to the existing data on the lower crustal rocks from IVZ. To reach this objective, a comprehensive characterization of the outcrop is ongoing, which include virtual outcrop modelling, structural and petrological field characterization and petrographic and geochemical analyses. These data are being used to decrypt the spatial relationships between the original magmatic units, which were possibly modified by post-Permian metamorphism and tectonics.

In this first stage, we are focusing on the garnet-, amphibole- and olivine-bearing gabbros of this outcrop. They have been characterized in detail through petrography and electron probe microanalysis. Their main petrological features can be summarized as follows:

• Granoblastic to polygonal textures formed by pyroxenes, plagioclase, olivine, amphibole and hercynite-magnetite assemblages;
• Significant variability in the presence of garnet (Alm_50–0.52, Py_0.31–0.34, Gr_0.15–0.17) among the analyzed samples, which mostly occur as coronitic textures, sometimes associate with vermicular clinopyroxene, suggesting a metamorphic origin;
• Plagioclase (An_50-70) shows slight optical zoning and presence of exsolution lamellae of hercynite-rich spinel within the cores;
• Olivine with composition from Fo₅ to Fo_0.6, clino- and orthopyroxene with mg# in the range 0.68–0.81 and 0.65–0.81, and Al₂O₃ in the range 4.5–8.1 wt% and 2.2–3.8 wt%, respectively.

The presence of olivine-bearing gabbros allows to apply the recently developed geobarometer for the assemblage olivine + clinopyroxene + plagioclase + spinel. An initial application attempt indicates a pressure of 6.7 ± 1.8 kbar, consistent or slightly lower with respect to existing estimates from the metapelitic lithologies of nearby outcrops. Further calculations are ongoing, which are part of an approach that include multiple-reaction thermobarometry, pseudosection modelling and petrographic constraints to decrypt the P-T path underwent by the gabbros.