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Carmela Freda, Istituto Nazionale di Geofisica e Vulcanologia (Italy)
Mario Gaeta, Sapienza Università (Italy)
Biagio Giaccio, CNR-Istituto di Geologia Ambientale e Geoingegneria (Italy)
Fabrizio Marra, Istituto Nazionale di Geofisica e Vulcanologia (Italy)
Danilo Palladino, Sapienza Università (Italy)
Piergiorgio Scarlato, Istituto Nazionale di Geofisica e Vulcanologia (Italy)
Gianluca Sottili, Sapienza Università (Italy)
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Conventionally, the degree of explosivity of volcanic activity increases with increasing SiO2 content, in terms of both eruption intensities (mass eruption rates) and magnitudes (erupted volumes): i.e., relatively low-viscosity mafic magmas feed effusive or mildly explosive eruptions (e.g., hawaiian and strombolian), whereas high-viscosity silicic magmas feed large explosive eruptions such as plinian and pyroclastic-flow forming ones. The products of major explosive eruptions from the Colli Albani ultra-potassic volcanic district (Central Italy), however, represent a striking exception. The juvenile fraction in pyroclastic flow deposits, which attain individual volumes in the order of tens of km3, is K-foiditic in composition, with SiO2 contents as low as 42 wt%, i.e. even much lower than those typical of basalts. In particular, we will discuss the driving mechanisms of the ∼456 ka Pozzolane Rosse (PR) large mafic explosive event based on a reconstruction of the pre-eruptive scenario in the light of event dynamics and petrological modeling. The PR eruptive succession begins with the effusion of a wide lava plateau from a peripheral vent and continues with a subplinian to moderate plinian eruption, producing a well-sorted, up to decimeter-thick fine scoria lapilli fallout horizon. The main PR unit consists of a single massive, poorly sorted pyroclastic flow body made up of poorly to moderately vesicular scoria lapilli and blocks, abundant lava and thermally metamorphosed sedimentary lithic lapilli and blocks and scarce granular inclusions, all embedded in a poorly consolidated coarse ash matrix. Finally, decimeter-thick layered scoria lapilli fallout deposits overlie incipiently pedogenised PR co-ignimbrite ash fall deposits. Of note is the widespread occurrence of scoria clasts with carbonate lithic cores, which record a significant degree of magma-country rock interaction. The CO2 increase in the magma, due to carbonate assimilation from country rocks, is considered as the major factor driving magma differentiation through extensive leucite crystallisation. This, will in turn, result in dramatic volatile pressurisation and a magma viscosity increase, leading to a change in the eruptive dynamics from early lava effusion to sustained column phase and pyroclastic flow activity.
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