Characterization of mafic rocks at the crust-mantle transition zone: insights from the ICDP-DIVE Project (5071_1_A, Val d’Ossola, Ivrea-Verbano Zone, Italy).

The composition and origin of the lower continental crust (LCC) is difficult toassess due to the diversity of granulite facies rock types, compositionalcontrasts between terranes and xenoliths, and the nonunique interpretation ofseismic data. Seismic data alone cannot distinguish between differentgranulites, leading to uncertainties in LCC models. The Ivrea-Verbano Zone(IVZ) in the Alps provides key insights into lithological variability, preserving aPaleozoic felsic lower crust modified by Lower Permian mafic underplating. Wepresent whole-rock data from the 909.5 m borehole 5071_1_A (Megolo) of theICDP-funded DIVE project. This borehole sampled dominantly granulite faciesmafic and metasedimentary rocks.A systematic sampling strategy was pursued to produce representativecoverage of compositional variability, with 6–12 cm long drill core samplescollected at ~10 m intervals. The main mafic lithologies identified on-site includegabbronorites (opx + pl ± grt), gabbros (cpx + pl + grt ± amp ± opx), garnet-granulites (grt + pl + cpx + opx), intrusive gabbros (amp + cpx + pl ± opx),pyroxenites (opx + cpx ± ol), and garnetite (grt ± pl). Subsequent petrographicobservations reveal strong lithological variability, with mineral modal proportionvarying at the centimeter scale. Geochemical data show no clear trends withinlithologies or with depth. Mafic rocks are depleted in radiogenic elements (K,Th, U) compared to felsic units, correlating strongly with gamma ray loggingdata. To further distinguish the different rock types, a classification based onmineral modes and supported by whole-rock geochemistry (REE abundances,XMg, La/Sm ratio, etc.) was established. Gabbros and gabbronorites aredistinguished by XMg (gabbros <0.55; gabbronorites >0.55). Both show positiveEu anomalies and have elevated La/Sm ratios (>1) compared to garnet-granulites and intrusive gabbros (<1).The weighted bulk composition matches the more mafic estimates for bulk LCCvalues, but such averages obscure the complex geodynamic history of the Ivrealower crust. Preliminary interpretations highlight the role of Permian magmatismin shaping its present composition.