A large K-foiditic hydromagmatic eruption from the early activity of the Alban Hills Volcanic District, Italy

In this paper we discuss the uncommon case of an energetic, pyroclastic-flow-forming eruption with a SiO2-poor (42-45 wt.%), K-foiditic magma composition. The Trigoria-Tor de' Cenci Tuff (TTC; 561 ka) is the product of the first large-scale explosive event (of the order of 1-10 km(3) of erupted products) in the Alban Hills Volcanic District, near the city of Rome, Italy. After an initial Plinian phase that produced a scoria fall horizon, pyroclastic current activity emplaced ash deposits with leucite-bearing juvenile scoria lapilli. The abundance of accretionary lapilli, the most distinctive feature of these deposits, together with the high degree of fragmentation, the abundance of minute lithic inclusions and the morphology of ash particles, indicates a hydromagmatic character for the most part of the eruption. The absence of vent-derived carbonate lithic clasts from the deep regional aquifer and the abundance of cognate lithic fragments suggest that the interaction with external water involved a surficial aquifer in the older Alban Hills volcanic terrains. Perhaps the most striking aspect of the TTC is the K-foiditic composition of the pre-eruptive melt, which, to our knowledge, is unique among explosive events of comparable size elsewhere in the world. The pre-eruptive magma system feeding the TTC was controlled mainly by leucite+clinopyroxene fractionation under a(H2O)