The Rare Trachyandesitic Lavas at Mount Etna: A Case Study to Investigate Eruptive Process and Propose a New Interpretation for Magma Genesis

The growth of Mount Etna volcano reflects the superimposition of various eruptive centers, the most voluminous of which is the Ellittico, whose stratigraphic sequence is well exposed on the steep walls of Valle del Bove. The uppermost levels of the sequence have been sampled and investigated through a new set of geochemical data on mineral phases and bulk rock. Sampled rocks display a marked bimodality with aphyric banded trachyandesites, which are some of the most evolved and rare products of the entire Etnean succession (SiO2 58-60 wt.%), intercalated in plagioclase rich porphyritic mugearites (SiO2 49-50 wt.%, P.I. 35-40). In this paper, we provide a detailed textural, mineralogical, and chemical characterization of these products, providing a new interpretative model for their genesis and significance in the context of the Etnean system. Our approach discusses, in a critical way, the "classic" fractional crystallization model of magmas, not supported by field evidence, and proposes a novel hypothesis in which the aphyric-banded trachyandesites represent be the primary products of a gas-induced partial melting of hypabyssal sills and dykes. This hypothesis represents a step towards a comprehensive description of igneous systems that takes into account not exclusively the evolution of basaltic melts, but also the role of volatile contributions in governing volcanic behavior.