Evidence of crustal melting events below the island of Salina (Aeolian arc, southern Italy)

Abstract

Quartz-rich xenoliths hosted in the basaltic–andesitic and andesitic lavas of Monte dei Porri, in the island of Salina, contain fluid and melt inclusions whose study makes it possible to trace back the magma ascent path of this volcano; moreover, they are a source of information about the anatectic processes that occurred under this island. The texture, mineralogy and whole-rock chemistry of these rocks exhibit the typical characteristics of a residual rock which went through several melt extraction events. The chemistry of melt inclusions hosted inside quartz grains (SiO₂>72.7%, Al₂O₃=9.0–13.5%, CaO=0.2–4.3%, K₂O>3.1%, MgO, FeO_t≤1.5%) supports the hypothesis that a progressive and continuous anatectic processes affected a peraluminous high-grade metamorphic rock (gneiss or micaschist), that constituted the Calabro–Peloritani basement and generated a quartz-bearing residue. The fluids trapped inside these inclusions are rarely composed of high-density CO₂, with a homogenization temperature to liquid phase (Th_L) within the range 26.2–31.0°C, corresponding to densities of 520 to 690 kg/m³. More commonly, they are made up of low-density CO₂, with a homogenization temperature to vapour phase (Th_V) within the range 13.2–31.0°C, corresponding to density values of 150 to 460 kg/m³. It is proposed that, during ascent, batches of magma rising from the mantle sampled various fragments of a residual rock from a depleted metamorphic basal layer and carried them to the surface. Magmas probably stopped inside at least two distinct reservoirs located at different depths (≤12.7 km and between 1.7 and 4.7 km), in which they underwent fractional crystallization. The ascent speed of the xenolith-bearing magmas from the deeper storage area to the shallower reservoir was fairly high, as confirmed by the low number of high-density fluid inclusions preserved that show re-equilibration features, and by the contemporaneous presence of denser cumulitic Ol+Cpx±Opx xenoliths in the host lava.