SUBDUCTION-RELATED MAGMATISM IN THE AEOLIAN ISLANDS: RECHARGE, ASSIMILATION AND FRACTIONAL CRYSTALLIZATION RECORDED IN WHOLE ROCK AND IN SITU PLAGIOCLASE DATA FROM ALICUDI, FILICUDI AND SALINA

The Aeolian arc is the product of subduction of the Ionian microplate beneath the European plate. Alicudi, Filicudi and Salina are among the western-most islands and are dominantly calc-alkaline basalts to andesites; rare dacites are observed. Whole rock (WR) and plagioclase in situ studies of selected samples from these islands highlight the dominance of recharge, assimilation, and fractional crystallization (RAFC) as primary mechanisms that generated compositional diversity. At Alicudi, variable plagioclase textural, An and trace element signatures indicate recharge occurred, and WR data implicate gabbro assimilation. Basalts have more radiogenic and diverse Sr isotope signatures than andesites. These data support mixing, assimilation, storage, and homogenization of magmas in a shoaling, coalescing plumbing system. Basalt to dacite samples from Fossa delle Felci, Salina, are characterized by systematic WR trends, suggesting FC as a dominant differentiation mechanism. Plagioclase characteristics vary widely. Of particular note are high An (>90), variably zoned crystals, and andesitic and dacitic crystals that have higher Sr concentrations than basaltic crystals. These data and MELTS models suggest that Fe-Mg minerals fractionated at deeper crustal levels (~12 km) from relatively H₂O-rich magma. At ~3-4 km, plagioclase crystallization began, producing more silicic magma. Basalt intruded the shallow storage region, shared some of its crystals with host magmas, and erupted. A dacite to basaltic andesite sequence of eruptions followed, with basaltic andesites as mixing products. Filicudi is characterized by eruption cycles that became more silicic with time. High An, unzoned plagioclase provides support for polybaric crystallization, with plagioclase formation suppressed until ~4 km. Recharge is recorded by plagioclase rim characteristics. With time, a deeper storage zone evolved to more silicic composition, and continued FC at shallower levels yielded an array of compositions that erupted within each cycle. While RAFC are operative in each of the 3 plumbing systems, WR and in situ plagioclase data emphasize that the balance of these processes differs, as a consequence of differences in thermal structure, efficacy of wallrock assimilation and mass of recharge magma.