Probing continental roots: geochronology, petrology and rock physics of a lower crustal section, Ivrea–Verbano Zone

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ID: 330886
2026
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Abstract
Abstract Exposed lower crustal terranes provide direct constraints on the composition, structure and evolution of the continental crust at depths that are otherwise inaccessible. This study presents a multi-method investigation of a currently ~8.5 km thick lower crustal section exposed along Val d'Ossola in the Ivrea–Verbano Zone, Southern Alps, based on surface samples and ICDP–DIVE project drill cores. The investigated section exposes felsic metasedimentary and metamafic rocks with minor calcsilicates, marbles, ultramafic rocks and leucocratic dykes. The Anzola Shear Zone separates the amphibolite- from the granulite-facies section, the latter containing abundant metamafic rocks, minor felsic granulites, pyroxenites and spinel peridotites. Bulk rock geochemistry reveals a chemically stratified terrane with two distinct endmembers: the amphibolite-facies sector, which has a composition similar to that of the upper crust, and the granulite-facies sector, whose composition more closely resembles that of the lower crust on a global and regional scale. This compositional dichotomy reflects the volumetric dominance of felsic metasedimentary rocks in the shallower section and mafic lithologies at greater palaeodepth, with the deeper sector being modified by partial melting, melt transfer and mafic intrusions during high-temperature metamorphism. Zircon U–Pb geochronology from felsic and mafic lithologies as well as mobilised leucosomes suggests that high-temperature metamorphism and partial melting occurred between 285–260 Ma. Thermobarometry using multiple methods indicates that the preserved Permian metamorphic field gradient ranges from ~0.5 GPa, 650 °C to ultra-high temperature conditions of ~1.0 GPa, 950 °C. Furthermore, geobarometry results point to a steep preserved apparent (post-deformation) geobaric gradient of 0.79 kbar/km, corresponding to a significant thinning factor β of ~2.75 (2σ range 1.95–3.51). We propose that this lower crustal section experienced thinning by the combined effect of diffuse, pervasive thinning and localised deformation along shear zones during Permian–Triassic extension and Jurassic rifting. Rock physical properties change with palaeodepth as a function of lithological proportions and metamorphic grade. Density increases from ~2740 to ~3160 kg/m3, and P-wave velocity (Vp) increases from ~6.5 to ~8.0 km/s across the section, with significant local variability. Radiogenic heat production is lithologically controlled, with systematically higher values in the felsic metasedimentary rocks (~1.5–2.0 μW/m3) and lower values for the metamafic rocks (~0.1–0.2 μW/m3). This work provides quantitative constraints on the nature of the lower crust and the timing and conditions of lower crustal metamorphism.
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Authors Luca Pacchiega, Sarah Degen, Kim Lemke, Alexia Secrétan, Ankan Bhattacharyya, Zheng Luo, A. Hofer, Sarah Kurzen, György Hetényi, Othmar Müntener, Jörg Hermann, Daniela Rubatto
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Journal journal of petrology
Year 2026
DOI
10.1093/petrology/egag085
URL
https://doi.org/10.1093/petrology/egag085
Keywords Keywords not found

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