Volcanological and petrological evolution of the Eastern Vulsini District, Central Italy

On the basis of recent field mapping data, stratigraphic reconstruction and K/Ar datings, a volcanological and petrological study of the Bolsena and Montefiascone Volcanic Zones (Eastern Vulsini District) has been carried out. Ten main eruptive phases, with both effusive and explosive activity, have been recognized; these are characterized by a large range of magma compositions belonging to the leucitite, basanite and shoshonite magmatic series. Within each series, near-primary magmas and slightly to strongly differentiated products are products are present, although in different proportions. The great majority of differentiated products (mostly trachytic) outcrops in the northern sectors of the Eastern Vulsini District (Bolsena Volcanic Zone), where lavas or pyroclastics were erupted during moderate- to large-volume Plinian-type and ignimbrite-forming explosive activity. By contrast, abundant, rather undifferentiated magmas are more frequently found in the southern sectors of the Eastern Vulsini District (Montefiascone Volcanic Zone). The structural setting of the sedimentary substratum, as well as its lithological characteristics, may have played an important role in producing this magma distribution. The very shallow position (a few hundred meters below the topographic surface) of the top of the fracturated carbonate basement would have facilitated the uprising of abundant, less evolved magmas in the Montefiascone Volcanic Zone, whereas relatively large magma bodies, a few kilometer deep, can only be inferred in the Bolsena Volcanic Zone (Bolsena caldera), where a front of overthrust sheets of the Tuscan and Umbrian sedimentology sequences is most probably present. In this area, source vents of explosive activity show a tendency to shift northeastward with time. Feeding systems were also differently oriented: E-W and WNW-ESE in the southern sectors and NW-SE and NNW-SSE in the northern sectors, while a NS system was related to a late-stage activity of the whole Eastern Vulsini District.