The geochemistry of leucite-bearing lavas from early stages of the Somma-Vesuvius volcanic complex: Feeder systems and mantle enrichment processes in the Neapolitan district of the Roman Magmatic Province

Abstract

The lavas of the Mt. Somma volcanic epoch were erupted during the early stage of the Somma-Vesuvius volcanic complex. These lavas are mildly differentiated with the presence of plagioclase-clinopyroxene-olivine- ± leucite-bearing rocks (leucite tephrites, leucite-bearing shoshonites, latites), also characterized by low in MgO, Cr and Ni, with a Sr-Nd-isotope range (⁸⁷Sr/⁸⁶Sr = 0.706865–0.707861; ¹⁴³Nd/¹⁴⁴Nd = 0.51244–0.51258) that overlaps with lavas of the late stage Vesuvius erupted after 1631 CE (late stage of the Somma-Vesuvius volcanic complex). Differentiation is dominated by closed-system processes, with fractional crystallization of clinopyroxene, calcic plagioclase, olivine, magnetite, and leucite. Open-system differentiation processes are subordinate and associated with limited interaction with crustal rocks. Oxygen isotopes on clinopyroxene and olivine phenocrysts (δ¹⁸O = 6.5–7.9 ‰) are higher than typical uncontaminated mantle magmas, suggesting a crustal contribution to the melt. Although open-system assimilation + fractional crystallization certainly took place, this process alone does not adequately reproduce the chemical and isotopic composition of the Mt. Somma ultrapotassic magmas. Therefore, a contribution from a recycled crustal component in the mantle source is required, but probably dominated by sediment-derived fluids and melts. The Mt. Somma lavas are characterized by distinctly different geochemical features compared to the mafic products of the neighboring volcanic areas (i.e., Phlegrean, Procida and Ischia volcanic fields), where the recycled crustal component is less pronounced.