Magma Evolution at Vulsini Volcanoes (Roman Province) in the Light of Phenocryst Melt Inclusions, Thermochemical Modeling and HP-HT Laboratory Experiments

Volcanic terrains south of Bolsena lake (Vulsini Volcanoes, central Italy) include a large variety of lava and pyroclastic rock-types of potassic affinity from Campi Vulsini, Latera and Montefiascone volcanic complexes in the 0.5-0.1 Ma span time. By tradition, the erupted compositions, ranging continuously from trachybasalts and basanites-tephrites to trachytes and phonolites in the TAS diagram, have been related to different magmatic series - i.e: KS, HKS -, originated from different primary magma sources. Here we propose, instead, a common parental magma for the typical compositional trends, based on several lines of evidence: i) the close association in time and space, even at the scale of individual eruptive centers, of rock-types attributed to different series; ii) the occurrence of trachybasalt (Al2O3=12 wt%, MgO=9 wt%, CaO=16 wt%) as the most primitive composition among the erupted products, as also found from melt inclusions in early olivine phenocrysts through the different volcanic complexes. The eruption of this parental (possibly primary) magma was favoured by the peculiar volcano-tectonic setting, related to major faults bordering Bolsena, Latera and Montefiascone caldera structures. Phase relations at different P, T, %H2O(melt) and fO2, from MELTS code thermochemical calculations, as also supported by HP-HT laboratory experiments performed on the most primitive rock-types, may explain consistently the observed variety of erupted magma compositions.