Montefiascone, Viterbo Province, Lazio, Italyi

Regional Level Types
Montefiascone	Commune
Viterbo Province	Province
Lazio	Administrative Region
Italy	- not defined -

Volcanic rocks (Montefiascone volcanic complex) occurring near Montefiascone, at the southeastern corner of Lake Bolsena.

The Montefiascone volcanic complex (MVC) belongs to the Vulsini volcanic district, northern part of the Roman Magmatic Province. The Montefiascone activity occurred in a time span of about 50 ka (286–227 ka; Brocchini et al., 2000). The MVC is a large cone whose summit rises to 400 m above its surroundings. It consists of several coalescing eruptive centres and a 2.5 km wide caldera. The volcanic sequence is made of pyroclastic deposits and lava flows ranging in composition from leucitites to slightly undersaturated trachybasalts (Civetta et al., 1984; Rogers et al., 1985; Coltorti et al., 1991; Di Battistini et al., 1998). The pyroclastic rocks include two main phreatomagmatic units (lower and upper), surge deposits and scoria cones. Small melilitite lava flows have been found in the Forcinella and Feltricci areas, located respectively west and east of Montefiascone. The rocks underlying the MVC consist of a thick sequence up to 5–6 km of sedimentary rocks of Mesozoic–Tertiary age limestones, marls and sandstones belonging to the Liguride and Tuscan nappes of the Northern Apennines underlain by low-grade metamorphic rocks of the Tuscan basement.

The Forcinella flow outcrops along the southeastern part of Lake Bolsena, reaching a maximum length of 250 m and a thickness of 1–2 m. It consists of blue-gray massive rocks characterized by small and rare leucite phenocrysts visible in hand specimen. In the stratigraphic succession of the volcanic products in this area, the melilitites are covered by ca. 70 cm of reworked pyroclastic deposits and by a thin (ca. 10 cm) dark-brown palaeosol. The overlying volcanic products are scorias, wet surge deposits and the Montefiascone ignimbrite. In a nearby locality, the same succession is capped by the Orto Piatto basanitic lava, which is one of the youngest Montefiascone eruptive products (226 ka, Nappi et al., 1995).
Another melilititic lava is exposed at the Feltricci locality, about 2 km E–NE of Montefiascone, along the strem Fosso Feltricci. It is interstratified with the intermediate pyroclastic deposits and covered by the Montefiascone ignimbrite. The flow has a thickness of 2–3 m and a length of about 300 m.

The Montefiascone ignimbrite represents a low temperature–high density phreatomagmatic product (Cioni et al., 1991) characterised by the occurrence of plutonic ejecta of variable size and composition. It occurs both inside the Montefiascone caldera and in the surrounding areas up to 4 km from the caldera. In the eastern part of Montefiascone ignimbrite (e.g. Poggio della Rena quarry), it reaches the highest thickness (up to 40 m); considerably lower thickness has been found in the southern sector (ca. 20 m) and inside the caldera (ca. 15 m). For instance, in the La Berlina area (eastern side of the Montefiascone caldera), the ignimbrite overlies a wide scoria cone and a basal pyroclastic breccia and is in turn covered by a surge deposit. The ignimbrite consists of a coarse-grained cineritic matrix including phlogopite crystals, and abundant cognate and accidental ejecta (plutonic clasts, older lavas and sedimentary rocks); at Poggio della Rena, the ignimbrite has similar characteristics, but it is more massive and lithified. The outcrops of Mentuccia (to the north of those of La Berlina) exhibit the same volcanological feature and the same stratigraphic position as the La Berlina outcrops: they consist of low-temperature high-density phreatomagmatic products covered by the Montefiascone late surge deposits and overlying the Strombolian products of the Poggio dell’Asino scoria cone.

Plutonic ejecta found in Montefiascone ignimbrite can be divided into two main groups: clinopyroxene-bearing and clinopyroxene-free. The clinopyroxene-bearing rocks may contain one or more of the following phases: olivine, melilite, phlogopite, feldspars, leucite, sodalite, kalsilite, Fe–Ti oxides. The clinopyroxene-free rocks include melilite + kaliophilite and K-feldspar + plagioclase + haüyne.

Fossils

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References

Other Databases

Wikipedia:	https://en.wikipedia.org/wiki/Montefiascone
Wikidata ID:	Q176142
GeoNames ID:	6538635

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