GNGTS 2019 - Atti del 38° Convegno Nazionale

198 GNGTS 2019 S essione 1.3 compositional analyses of the erupted deposits, allowed to interpret the stratigraphic succession as being related to two distinct eruptive episodes (Baia and Fondi di Baia) separated by a short time interval, and each characterized by different eruptive phases (Pistolesi et al. , 2017). The Baia eruptive episode started in a shallow-water environment with an explosive vent-opening phase that formed a breccia deposit (Unit I), rapidly followed by alternating fallout activity and dense, pyroclastic density current deposits generation (Unit II). Sedimentological features and pumice textural analyses suggest that deposition of Unit II coincided with the intensity peak of the eruption, with the fallout deposit being characterized by a volume of 0.06 ± 0.008 km 3 , a column height of 17 km, and a corresponding mass flow rate of 1.8 × 10 7 kg s −1 . The associated tephra also shows the highest vesicularity, vesicle number density and decompression rate. This peak phase waned to turbulent, surge-like activity possibly associated with Vulcanian explosions and characterized by progressively lower intensity (Unit III). This first eruptive episode was followed by a short quiescence, interrupted by the onset of a second eruptive episode (Fondi di Baia) whose vent opening deposited a breccia bed (Unit IV) which at some key outcrops directly overlies the fallout deposit of Unit II. The final phase of the Fondi di Baia episode strongly resembles Unit II, although sedimentological and textural features, together with a more limited dispersal, suggest that this phase of the eruption had a lower intensity. Mineralogical, geochemical (major, and trace elements on whole rocks, major and volatile elements on matrix glasses, and melt inclusions), and Sr isotope characterization of the tephra material sampled along the entire sequence was carried out in order to constrain magmatic evolution and dynamics of the feeding system (Voloschina et al. , 2018). The large range of groundmass glass compositions, associated with variable proportions of highly (phonolitic– trachytic) and mildly mildly (tephriphonolitic–latitic) evolved end-members in the erupted products, suggests that these eruptive episodes were fed by three different magma batches that interacted during the different phases. The three compositional groups identified in matrix glasses and interpreted as representative of different magma bodies are: (i) a trachyte (SiO 2 60.3– 64.7 wt.%), which is volumetrically predominant; (ii) a tephriphonolite-latite (SiO 2 :55.1–57.9 wt.%); and (iii) an intermediate magma group between phonolite and trachyte compositions. This wide compositional heterogeneity contrasts with the narrow variability recognized in the bulk-rock compositions, which are all trachytic. Mineral, melt inclusions, and Sr isotope data suggest that the trachytic magma possibly derived from the Campanian Ignimbrite reservoir located at 6–9 km depth. Volatile content in matrix glass indicates a storage depth of at least 6 km for the tephriphonolite-latitic magma. The intermediate magma is interpreted as being derived from a remelting and assimilation process of a partially crystallized trachytic body (crystal mush) by the hotter tephriphonolite-latitic magma. As the tephriphonolite-latite was erupted together with the trachyte from the beginning of the eruption, the ascent of this magma possibly played a fundamental role in triggering the eruption. Upwards through the tephra sequence, a progressive increase of the tephriphonolite-latitic and intermediate phonolite- trachytic components can be observed. The presence of banded clasts characterized by different compositions is also indicative of syn-eruptive mingling during the final phases of the eruption. The reconstructed magma dynamics suggests that, after a period of rest, the ascent of a less evolved magma reactivated magma batches residing at the depth of the Campanian Ignimbrite reservoir, which in turn interacted with a complex polybaric plumbing system. This is particularly relevant for the western sector of the caldera, where eruptions are less frequent and generally characterized by more evolved compositions, and where structural features possibly play a major role. This detailed physical volcanological and geochemical study carried out with variable analytical techniques shed light on the complexity of the Phelgrean Fields plumbing system. This, coupled with further investigations (e.g., diffusion profiles in matrix glasses and crystals), could also help estimating more precisely the timescales of its dynamics, providing an important tool for volcanic emergency plans at Phelgrean Fields and more generally at calderas.