The problem of rock assimilation by Somma-Vesuvius magma

New chemical and modal analyses are presented of magmatic rocks and pyroxenes nodules from the Somma-Vesuvius volcano and of trachytes from the surrounding area. Geochemical evidence is employed in checking the generally accepted suggestion that magma composition develops from that of trachyte into phonolitic tephrite and finally into tephritic leucitite (vesuvite) by assimilation of dolomite. At the Somma stage of the volcano, phonolitic tephrites (tephritic phonolites) alone have been produced. Vesuvius lavas (tephritic leucitites) are comparatively high in K₂O, CaO, F, Cl, Mn, Cu, Sr and Ba and low in SiO₂ and Nb. The formation of vesuvites by assimilation of dolomitic sedimentary rocks, combined with gravity-separation of early-formed pyroxenes from a trachytic or tephritic magma, is improbable as shown by chemical balances between the respective igneous and sedimentary rocks.