Variability of lower continental crust constrained by drill core data of the Ivrea Zone (DIVE project DT-1B, Ornavasso, Val d’Ossola, Italy)

The estimates of the chemical composition of the lower continental crust ranges from predominantly mafic to felsic. The Ivrea Zone in the Southern European Alps provides insight into this variability, featuring a pre-Permian mostly felsic lower crust modulated by additions of mafic rocks during Permian underplating. The Ivrea zone is an ideal location to examine major, trace, and volatile elements over the full range of proposed lower crustal compositions. Our study presents whole-rock data derived from a drill core of the first hole (DT-1B) of the ICDP-funded project DIVE (Drilling the Ivrea-Verbano Zone). The drilled section spans nearly 600 m, representing an upper part of the Ivrea lower continental crust. Logging of the drill core showed that biotite-gneisses (Qtz + Pl + Bt ± Gt ± Kfs ± Sil – 75 vol%) and metamafic rocks (Amp + Pl + Qtz ± Px ± Bt ± Gt – 21 vol%) are the main rock types with minor calcsilicate rocks (Cc + Gt + Px + Ttn + Qtz + Pl ± Amp – 2 vol%), and some minor pegmatites (2 vol%). Both targeted and grid sampling strategies aimed to minimize sampling bias, providing a reliable basis for understanding the Ivrea lower continental crustal composition and extrapolating the results toward a realistic assessments of the LCC composition in general.

Amphibolite facies metasediments (34 samples) range from calc-silicates to pelites and psammites, exhibiting a wide range of major element compositions (32 - 89 wt.% SiO₂; 0.5 - 5.8 wt.% K₂O; 0.35 - 0.54 Mg#). Metamafic rocks (16 samples) cover a more restricted compositional range (43 - 57 wt.% SiO₂; 0.1 - 5 wt.% K₂O; 0.3 - 3 wt.%; 0.36 - 0.61 Mg#). Most mafic rocks are LREE enriched, but a few resemble MORB-like compositions. A preliminary comparison of the bulk rock estimate of the entire drill core relative to the integrated composition derived from geological maps indicates that deviations between the two approaches are considerable, ranging from <10% up to 30% difference for major elements (calculated bulk vs compiled data from the literature: 62.8 vs 57.6 wt.% SiO₂; 2.3 vs 1.95 wt.% K₂O; 0.47 vs 0.50 Mg#). Results also indicate that fluid-mobile elements are mostly conservative with respect to potential protoliths. The chemical variability points to a possible origin of sediments derived from an accretionary wedge. The evaluation of bulk trace element ratios (i.e., Th/La, Sm/La, Nb/K₂O) suggests that the drilled sequence likely originated from (Paleozoic?) turbidites. Subduction of sediments and accretion to the lower continental crust are the most likely processes to explain the predominance of metasediments in this part of the Ivrea lower continental crust.