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-flo w-forming eruption with a SiO2-poor (42-45 wt.%), K-foiditic magma composit ion. The Trigoria-Tor de' Cenci Tuff (TTC; 561 ka) is the product of the fi rst large-scale explosive event (of the order of 1-10 km(3) of erupted prod ucts) in the Alban Hills Volcanic District, near the city of Rome, Italy. A fter an initial Plinian phase that produced a scoria fall horizon, pyroclas tic current activity emplaced ash deposits with leucite-bearing juvenile sc oria lapilli. The abundance of accretionary lapilli, the most distinctive f eature of these deposits, together with the high degree of fragmentation, t he abundance of minute lithic inclusions and the morphology of ash particle s, indicates a hydromagmatic character for the most part of the eruption. T he absence of vent-derived carbonate lithic clasts from the deep regional a quifer and the abundance of cognate lithic fragments suggest that the inter action 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 leu cite+clinopyroxene fractionation under a(H2O)<1 conditions. The low SiO2 ac tivity prevented plagioclase and K-feldspar crystallization. The depth of t he magma chamber can be estimated at 3-6 km within the carbonate substrate. In contrast to the other major pyroclastic-flow-forming eruptions of the A lban Hills, the juvenile volatile exsolution due to magma crystallization i s not seen as the main mechanism driving the TTC eruption. We suggest that the explosive behaviour of the TTC magma in the early magmatic phase result ed from a rapid decompression due to a regional seismic event and from magm a-water interaction in the succeeding phase.