In this paper are presented petrographic and mineralogical features of some base-surge and pyroclastic flow deposits outcropping at Vulcanello and La Fossa crater in the Vulcano Island. Lithics, scoriae, pumices and crystals were examined. Cognate lithics are predominant. Glassy clasts show a very variable composition from leucittephriphonolites to rhyolites with predominant trachytes. Among the mineral phases, clinopyroxene and plagioclase phenocrysts are both prevalent and olivine is subordinate. The chemical composition of the main mineral phases is rather constant in the basic and in the trachytic up to rhyolitic deposits, so there are many disequilibrium phenocrysts. The chemical variations of glasses are consistent with a crystal-liquid fractionation process from tephriphonolitic to silica undersaturated trachytic melts and with a subordinate evolution from silica oversaturated to alkaline rhyolitic ones. Furthermore, temperatures and PH <sub>2</sub> O conditions during the crystallization of the melts of all pyroclastic deposits were calculated according to various methods. The PH <sub>2</sub> O values suggest a model of a deep-seated magma chamber and a shallower other, within which the upward migrating magmas evolved by fractionation. At Vulcanello and Fossa there are textural and chemical evidences for magma mixing as the coexistence, in the same deposit, of chemically very different glasses (rhyolites associated with silica undersatured trachytes and, in one sample, even with tephriphonolites) and the presence of corroded phenocrysts. In the Commenda Cycle (Vulcano Fossa) deposits, the chemical and mineralogical variations and the vertical distribution of the glassy clasts are consistent with the interpretation that, prior to eruption, the magma body, in the magma chamber, was compositionally and thermally layered, with a rhyolitic magma at the top (at about 3-4 Km below the active crater La Fossa), underlain by a trachytic layer. The magma chamber was fed by more basic and hotter tephriphonolitic magma which gave rise to magma commingling and limited hybridization and triggered very powerful hydromagmatic eruptions.

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