Tholeiitic basalt magmatism of Mount Etna and its relations with the alkaline series

Mount Etna is composed for the most part of intermediate alkaline products, most of them porphyritic-the etnaïtes-, that may be defined as sodic trachybasalts or trachyandesites. The strato-volcanio itself overlies tholeiitic basalts (usually aphyric, except for olivine) belonging to three major types: olivine tholeiites (normative Ol+Hy; modal olivine and augite, titanomagnetite and ilmenite), pigeonite tholeiites (normative Hy+minor Ol or Qz; modal pigeonite and augite with minor olivine, ilmenite and titanomagnetite), transitional tholeiites, i.e. transitional between pigeonite tholeiites (aphyric) and alkali basaltic etnaïtes (porphyritic, with normative Ol+Ne or minor Hy; modal augite and olivine, titanomagnetite alone). An analcite basalt, chemically close to alkali basaltic etnaïtes, forms the small Cyclopean Islands, SE of Etna, and an alkali olivine basalt composes a neck at Paterno, SW foot of Etna.Both pigeonite tholeiites and alkali basaltic etnaïtes may be derived from a primitive olivine tholeiite magma by subtraction or addition of phases crystallized at moderate and low pressure (kaersutite±olivine, calcic plagioclase and clinopyroxene). The differentiation process implies crystal fractionation of the primitive olivine tholeiite magma at varying levels of the crust. The speed of ascent of the magma is thought to be the factor controlling the level at which differentiation may take place: in low velocity regimes, fractionation takes place at deeper levels of the crust. Slow ascent speeds would be the consequence of a developing crustal extension episode, induced by mantle diapirism that generated the olivine tholeiite magma below the Mount Etna area.