Geochemical characteristics of lower continental crust metasediments: insights from the DIVE Project (5071_1_B, Val d’Ossola, Ivrea-Verbano Zone, Italy)

Our understanding of the Earth's interior - its physical structure, geochemical composition, and dynamic evolution - largely relies on seismic observations, particularly seismic wave velocities. Evidence derived from seismic P-wave velocities and heat flow measurements suggests that the chemical composition of the lower continental crust (LCC) ranges from predominantly mafic to felsic. More recent models, however, suggest an intermediate to felsic compositional range, raising the question of the significance of felsic components. Therefore, metasediments play a critical role in deciphering the LCC’s composition and evolution.

The Ivrea-Verbano Zone in the Alps offers insights into the lithological variability from a pre-Permian felsic lower crust then modified by Lower Permian mafic underplating. This study presents initial whole-rock data from the ICDP-funded DIVE project (Drilling the Ivrea-Verbano zonE), with drill cores from the first drilling target 5071_1_B (Ornavasso). The whole sequence of drill cores (578 m) is representative of the upper Ivrea LCC and consists of amphibolite facies rocks.

To estimate the bulk rock composition and volatile budget (e.g. Degen et al.) of the lower crust, a systematic sampling strategy was employed. Results presented are from a broad sampling approach, with 6–12 cm long samples collected from each lithology at approximately 10-meter intervals along the entire borehole. In order of lithological abundance, the retrieved lithologies include metasediments (kinzigites, ~73 vol-%), metamafic rocks (~13 vol-%), and calcsilicates (~11 vol-%):

Kinzigites (Qz + Pl + Bt ± Gt ± Kfs ± Sil), local name for felsic gneisses characterized by biotite, range from pelites to psammites and are predominantly peraluminous. They exhibit LREE enrichment, slight HREE depletion, and a negative Eu anomaly.
Metamafic rocks, primarily amphibolites (Amp + Pl + Qz ± Px ± Bt ± Gt), are locally interlayered with kinzigites. They present a flat REE pattern with a subtle negative Eu anomaly. A distinct subgroup, enriched in K2O and CaO, occurs at contacts/transitional zone between kinzigites, amphibolites and calc-silicates, reflecting increasing modal biotite and Ca-rich minerals. This subgroup has a REE pattern similar to
Calcsilicate rocks occur as heterogenous layers of cm to dm scale aggregates of Ca-rich minerals (i.e. grossular-rich Gt, Pl, Scp, Ttn) ± Cpx ± Amp, and up to 14% carbonate minerals. These rocks are metaluminous and exhibit significant variability in their chemistry as a result of the mineral modal proportion, leading to highly variable major oxide and trace elements.

No distinct chemical trends are observed with increasing depth along the borehole. However, elements such as K, Th, and U differ between units and align with gamma-ray logging data. Intensities are notably higher in the kinzigite units compared to the more mafic units reflecting higher amounts of biotite and accessory phases (monazite, zircon). The weighted calculated bulk composition of 5071_1_B aligns with the upper end of LCC literature estimates.

Bulk trace element ratios (Th/La, Sm/La, Sm/Nd) suggest that the metasediments likely originated from (Paleozoic?) turbidites. Subduction and accretion processes may explain the dominance of metasediments in this section of the Ivrea-Verbano Zone LCC.