Geochemistry, isotopes and mineral chemistry of the Colli Albani volcanic rocks: constraints on magma genesis and evolution

The Colli Albani volcano belongs to the Roman Magmatic Province and is characterized by strongly silica undersaturated leucite-bearing ultrapotassic rocks. Melilite-bearing leucititic lavas, beside tephritic to tephritic phonolitic ignimbrites, were erupted during the pre-caldera (Vulcano Laziale) period. The post-caldera phase opened with magmas erupted from different feeding systems, with melilite-bearing leucitites in the early phases followed by tephritic and phonolitic tephritic lavas. The late-stage activity (i.e. the Via dei Laghi period) is characterized by hydromagmatic tuffs with small juvenile fragments that prevent a clear compositional definition of the magma triggering the eruptions. Despite their mineralogical and compositional similarities, the Vulcano Laziale period (pre-caldera) has significantly higher levels of incompatible trace elements and 87Sr/86Sr isotopes than found in magmatic rocks erupted after the caldera formation. Pre- and post-caldera parental magmas are considered to be significantly different from each other and generated within a metasomatized upper mantle under different degrees of partial melting. Crustal-derived carbonate-rich metasomatism is thought to have affected the mantle wedge of the Italian peninsula. Melting of pelitic sediments with different amounts of CaCO3 is considered the source of the metasomatic agents, which are able to re-fertilize the lithospheric upper mantle. Partial melting of this modally metasomatized lithospheric mantle under high XCO2 produced the strongly silica undersaturated ultrapotassic magmas observed at the Colli Albani volcano. A second-order differentiation process occurs at shallow levels, with fractional crystallization and crustal assimilation of wall rock (AFC), locally changing the compositions of magmas and producing several differentiation pathways that have given rise to the geochemical and petrological complexity of the Colli Albani volcano. Assimilation of carbonate sediments and silicoclastic sedimentary lithologies also occurred coevally, suggesting the existence of several separate magmatic reservoirs at shallow levels, possibly at different depths and surrounded by different sedimentary formations.