Evolution of the Sabatinian Volcanic District (central Italy) as inferred by stratigraphic successions of its northern sector and geochronological data.

The Sabatinian Volcanic District (central Italy) consists of a system of several vents clustered around the lake-filled Bracciano volcano- tectonic depression. Its activity spans between 0.8 My and 0.04 My and overlaps that of the neighbouring Vico and Alban Hills Volcanoes in time and space. A field survey of the northern sector together with new geochronological and geochemical data allows us to reconstruct the stratigraphy and volcanic evolution of the Sabatinian Volcanic District. Six main phases of activity have been recognised on the basis of geological data, following the UBSU criteria. The presence of three unconformities of regional significance also enables us to distinguish three different synthems, representing the main stages in the volcanic evolution of the Sabatinian District. The initial phases of activity (from I to IV) are within the Barca di Parma Synthem (800-200 ky) and are dominated by pyroclastic fall and flow deposits and subordinate phonolitic tephritic lava flows. The phase V is related to the Archi di Pontecchio Synthem (200-155 ky) and characterised by prevailing strombolian deposits with a composition ranging from phonolitic tephrite to tephrite. During the phase VI (Casalone Synthem) both magmatic and phreatomagmatic eruptive cycles occurred in the northern sector. The magmatic cycle (154-134 ky) is dominated by pyroclastic fall and lava flows with a variable composition from phonolitic tephrite to potassic trachibasalt. The phreatomagmatic cycle marked the end of the volcanic activity (<134ky) in the northern sector of the Sabatinian District and produced small, single eruptive centres (maars) or coalescent craters which are aligned in a NW-SE regional fracture system at a right angle to the tectonic orientation of the initial Sabatinian Volcanic activity. This fracture system could be connected to the main regional tectonic line that crosses the Vico Volcano and the eastern Sabatinian District.