Geochemical characterisation of Permian lower continental crust: case study from Ivrea-Verbano Zone (NW Italy)

Investigating the main geochemical characteristics of the lower continental crust is essential to understand its formation and evolution, identifying crustal differentiation processes and possible crust-mantle interactions. We performed bulk rock (major and trace elements), noble gases isotopes (He, Ne, Ar), and fluid inclusions (Raman spectroscopy) analyses on metamorphic rocks from Ivrea-Verbano Zone (Southern Italian Alps). Specifically, we studied various lithologies (metapelite, metagabbro, mafic and felsic granulite, amphibolite, and gneiss) to analyse the continuous metamorphic gradient from amphibolite- to granulite-facies.Bulk rock analyses confirm the mafic nature of the protoliths for metagabbros (MgO = 5.36-10.25 wt.%), mafic granulites (MgO = 8.32-25.80 wt.%) and amphibolite (MgO = 7.98 wt.%) plotting in the metabasite field of the ACF chemographic diagram. Felsic granulite and sillimanite-gneiss fall within metamorphosed quartz-feldspar rocks, except for metapelite, which approaches the metacarbonate field, due to the presence of secondary carbonates. Metagabbros, mafic granulites and amphibolite show low REE concentrations (∑REE between 3 and 25 ppm) and high Cr and Ni contents (up to 1865 and 265 ppm respectively in mafic granulite), reflecting the mafic/ultramafic nature of the protoliths, whereas felsic granulite, sillimanite-gneiss and metapelite show higher REE contents (∑REE between 48 and 197 ppm).3He/4He isotope ratios in metamorphosed quartz-feldspar rocks (0.06-0.30 Ra) and metabasites (0.15 and 0.45 Ra) are significantly radiogenic, although the metabasites show slightly higher values, corroborating a more primitive component in their source. Most samples plot near the air component in the 20Ne/22Ne vs 21Ne/22Ne diagram, except for mafic granulites which show a crustal-air mixing trend. As regards the Ar isotope ratios, all samples appear rich in radiogenic component (40Ar/36Ar up to 2645 in metagabbros).Raman spectroscopy analyses on fluid inclusions in orthopyroxene from mafic granulites show the coexistence of talc, graphite and magnesite with methane, providing direct evidence of a complex history in terms of post-metamorphic reactions and P-T-fO2 conditions.Our preliminary results show the compositional diversity and evolution of the lower continental crust, highlighting the interplay between mafic and sedimentary sources and the importance of fluid interactions and post-metamorphic processes.