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 M ontefiascone 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 composi tions belonging to the leucitite, basanite and shoshonite magmatic series. Within each series, near-primary magmas and slightly to strongly differenti ated products are present, although in different proportions. The great maj ority of differentiated products (mostly trachytic) outcrops in the norther n sectors of the Eastern Vulsini District (Bolsena Volcanic Zone), where la vas or pyroclastics were erupted during moderate- to large-volume Plinian-t ype and ignimbrite-forming explosive activity. By contrast, abundant, rathe r undifferentiated magmas are more frequently found in the southern sectors of the Eastern Vulsini District (Montefiascone Volcanic Zone). The structu ral setting of the sedimentary substratum, as well as its lithological char acteristics, may have played an important role in producing this magma dist ribution. The very shallow position (a few hundred meters below the topogra phic surface) of the top of the fracturated carbonate basement would have f acilitated the uprising of abundant, less evolved magmas in the Montefiasco ne Volcanic Zone, whereas relatively large magma bodies, a few kilometer de ep, can only be inferred in the Bolsena Volcanic Zone (Bolsena caldera), wh ere a front of overthrust sheets of the Tuscan and Umbrian sedimentology se quences is most probably present. In this area, source vents of explosive a ctivity show a tendency to shift northeastward with time. Feeding systems w ere 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. (C) 1998 Else vier Science B.V. All rights reserved.