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Roccamonfina

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Aeolian Arc

other sections will be added soon

Venanzite

Venanzite: Melanocratic variety of leucite olivine melilitite largely composed of melilite, leucite, kalsilite and olivine. Rosenbusch (1899) called this rock euktolite unaware that it had already been named venanzite. A kamafugitic rock and now regarded as a kalsilite-phlogopite-olivine-leucite melilitite.

The San Venanzo volcanic center
The post-Pleistocene volcanoes of San Venanzo (Fig. 1) are located on the western margin of a graben forming the Tiber valley, about 30 Km SSW the city of Perugia, and represents the most prominent among several Umbria-Latium Ultra-Alkaline District (ULUD), which consist of several scattered volcanic centers in the inner part of the Appenine chain. This area began to undergo extension during the Pliocene, which led to the formation of graben structures and small basins filled with continental sediments. Most of the igneous centers are located along graben faults and are characterized by short-lived volcanism with diatreme formations and tuff aprons. Lava flows and dykes are less frequent. Rock types include phonolite, kamafugite melilitite, melilitolite and calcite carbonatite.

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Fig.1: Right - Detailed location of San Venanzo and related structural geology. Left - Geological sketch map of the San Venanzo formations. After Stoppa (1998).

Fig.2: Detailed geological map of the San Venanzo volcanos. After Stoppa (1996).

Fig.3: Geological section of the San Venanzo volcanoes. After Stoppa (1995).

The San Venanzo maar
The San Venanzo maar (Fig.2) lies near the village of San Venanzo (Terni). The prevolcanic terrain consists of Plio-Pleistocene continental clays and sands which lie unconformably on early Miocene marls (Marnosa-Umbra formation). The San Venanzo volcano lies on a subaerial erosion surface, slightly inclined eastward in an area occupied by a small arm of a lake basin during early Pleistocene times. Lacustrine clay and sand 2 km NE of the San Venanzo volcano attain a thickness of approximately 100 m, decreasing to only a few metres beneath the volcano.
The San Venanzo maar is formed, from top to bottom, by three pyroclastic facies: a lithic tuff, a lapilli tuff and a lithic breccia. The igneous components of the lithic breccia are dominated by highly vesiculated glassy lapilli and coarse-grained ash. Green-grey euhedral to subhedral forsterite and light to dark green subhedral diopside crystals, are abundant in the finer-grained layers and lowermost 2 m of the deposit. Potassium feldspar crystals are common in the matrix and abundant in crystal-rich layers. In addition to highly vesiculated lapilli and coarse-grained ash, the lapilli tuff contains microvesiculated carbonatitic bombs and lithics in a fine-grained calcite matrix.

The Pian di Celle tuff ring
The Pian di Celle tuff ring (Fig.2), 450 m in diameter, is located 500 m S of the San Venanzo maar and consists of a scoria accumulation which is approximately 30 m thick in the area SE of the eruptive vents. The pyroclastics of Pian di Celle cover an area of approximately 1.5 km² at an average thickness of about 20 m and a total volume of 3x10⁶ m³.
From stratigraphic top to bottom, the Pian di Celle tuff ring, consists of two lava flow (Le Selvarelle and Belvedere), intruded by a sill, a scoria-tuff cone, a chaotic breccia and a lapilli ash-tuff. The crystal component of the lapilli ash-tuff is mainly discrete euhedral and glass-mantled crystals of forsterite and diopside. Carbonatite ash layers 2-3 cm thick occur. The chaotic breccia, composed mainly of vesicular carbonatite, melilitite lapilli and bombs, forms the dominant part of the sequence. Lapilli typically consist of near-spherical, concentric shells around lithic fragments and dunite microliths. A typical carbonatite assemblage consisting of calcite, apatite, perovskite and magnetite alternates with shells containing phenocrystal forsterite, melilite, kalsilite and leucite in a glassy matrix.

The Pian di Celle lava flows
The Pian di Celle lava was originally called euktolith by Rosenbusch (1898) and venanzite by Sabatini (1899). It was assigned to the kamafugite series from South-West Uganda by Sahama (1974) and, according to Le Maitre (1989), the rock is an olivine-leucite-kalsilite melilitite. Le Selvarelle and Belvedere lava flows (Fig.2) cover the area around the northern, western and southern flanks of Pian di Celle pyroclastic crater. The Le Selvarelle lava flow (51.000 m²) attains a maximum thickness of about 20 m and a maximum width of 200 m, whereas the Belvedere lava flow (12.000 m²) attains a maximum thickness of about 6 m.
A feeding-dyke crops out in the southern rim of the main crater; this dyke fed the Le Selvarelle lava flow and continues up to a small scoria cone; a second dyke is located at the western crater rim and link the Belvedere lava flow to the Le Selvarelle lava flow. Lava and dykes rocks have a poorly porphyritic holocrystalline texture and contain microphenocrysts of olivine (Fo90-65) set in a groundmass composed of melilite, leucite, phlogopite, Al-poor clinopyroxene, kalsilite, spinel, monticellite and calcite. Spinel, perovskite and apatite are present as accessory phases. The lava flow contains coarse-grained pegmatoid veins formed of leucite, melilite, spinel, olivine, clinopyroxene, phlogopite, kalsilite, calcite and apatite.

The Celli lapilli cone
The Celli lapilli cone (Fig.2) is a small, separate vent of approximately 40m diameter located approximately 500 m east of the Pian di Celle tuff ring. It consists of a diatreme whose cross-cutting sharp contact with the country rocks displays clear signs of thermal effects. The facies which fills the conduit is a subvolcanic tuff, called tuffisite, and it made up of concentric shelled lapilli and lithics (5%) of marly fragments.

Evolution of the San Venanzo maar
The volcanic activity of San Venanzo maar resulted in the formation of a pyroclastic ring around a maar crater. Collapse and reworking of the pyroclastic rim and vent substratum rocks into the diatreme produced a wide range of vesicularity, juvenile components of widely differing composition and lithic fragments with variable degrees of rounding and heating.
After the relatively high-energy, vent-opening event that produced both pyroclastic flow and surge and chaotic accessory-lithic breccia debris flow deposits, the regular layering of a mainly juvenile, cross-laminated deposit and the decrease in accessory lithics indicate an almost continuously sustained eruption from an open conduit which produced a tractive surge deposit. The presence of heterogeneous lithics and pyroclasts suggest reworking of previously erupted material caused by convective movements in the upper part of the diatreme. A decrease of eruption energy and magma volume at the vent are inferred by the formation of thin ash falls which mantle the inward crater wall. Similar ash falls are found at the base of Pian di Celle lava flow, which possibly indicates that this event took place immediately before the activity at Pian di Celle.

Evolution of the Pian di Celle tuff ring
The Pian di Celle crater area is 50-60 m above the level of the maar structure of San Venanzo, although the pre-eruptive base is at the same level. The volcanic sequence at Pian di Celle lacks an initial high-energy event, and the lowest strata comprise relatively wellbedded tephra associated with fallout and/or surge and subjected to grain-flow mechanism. The climactic phase was characterised by a low, sustained pyroclastic fountain whose collapse produced a relatively large pyroclastic flow deposit. This event occurred in the middle of the sequence and was followed by activity which produced a steep accumulation of scoriae forming a tephra slope.
The main activity at Pian di Celle was strombolian in the sense that it was associated with predominantly explosive events which did not produce a sustained eruptive column and widely dispersed air falls. The Pian di Celle lavas represent the last erupted volatilepoor magma portion.

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Fig.4: Paleogeographyc reconstruction of the San Venanzo volcanos several hundred year ofter the eruption. Rigth = San Venanzo maar (with internal lake), Top = Pian di celle tuff ring and melilitite lava flow (gray), Left = Celli lapilli cone.

Panoramic view of coarse-grained pegmatitic Venanzinte thin section. Image by Andy Tindle (Virtual Microscope). PPL image, field of view = 2.5cm.

Panoramic view of coarse-grained pegmatitic Venanzinte thin section. Image by Andy Tindle (Virtual Microscope). XPL image, field of view = 2.5cm.

Pyroclastic deposits; San Venanzo, Italy.

Pyroclastic deposits; San Venanzo, Italy.

Contact between coarse-grained pegmatitic venanzinte (melilitolite) and venanzite aphanitic lava at Pian di Celle lava flow, San Venanzo, Italy.

Contact between coarse-grained pegmatitic venanzinte (melilitolite) and venanzite aphanitic lava at Pian di Celle lava flow, San Venanzo, Italy.

Coarse-grained pegmatitic venanzinte (melilitolite) at Pian di Celle lava flow, San Venanzo, Italy.

Bibliography

• Stoppa, F., & Sforna, S. (1995). Geological map of the San Venanzo volcano (Central Italy): explanatory notes. Acta Vulcanologica, 7(1), 85-91.
• Stoppa, F. (1996). The San Venanzo maar and tuff ring, Umbria, Italy: eruptive behaviour of a carbonatite-melilitite volcano. Bulletin of Volcanology, 57(7), 563-577.
• Stoppa, F., & Cundari, A. (1998). Origin and multiple crystallization of the kamafugite-carbonatite association: The San Venanzo-Pian di Celle occurrence (Umbria, Italy). Mineralogical Magazine, 62(2), 273-289.
• Peccerillo.A (2005): Plio-Quaternary Volcanism in Italy.

Photo
Olivine crystals embedded in a groundmass made by melilite (the small rectangular crystals) and olivine. PPL image, 2x (Field of view = 7mm)	Olivine crystals embedded in a groundmass made by melilite and olivine. XPL image, 2x (Field of view = 7mm)	Olivine crystals embedded in a groundmass made by melilite (the small rectangular crystals) and olivine. PPL image, 2x (Field of view = 7mm)
Olivine crystals embedded in a groundmass made by melilite (the small rectangular crystals) and olivine. PPL image, 2x (Field of view = 7mm)	Olivine crystal embedded in a groundmass made by melilite (the small rectangular crystals) and olivine. PPL image, 10x (Field of view = 2mm)	Olivine crystal embedded in a groundmass made by melilite (the small crystals with blue interfence colors) and olivine. XPL image, 10x (Field of view = 2mm)
Olivine crystals embedded in a groundmass made by melilite and olivine. XPL image, 2x (Field of view = 7mm)	Olivine crystals embedded in a groundmass made by melilite (the small rectangular crystals) and olivine. PPL image, 2x (Field of view = 7mm)	Olivine crystal embedded in a groundmass made by melilite (the small crystals with blue interfence colors) and olivine. XPL image, 10x (Field of view = 2mm)
Groundmass made by melilite (the small rectangular crystals) and olivine. PPL image, 10x (Field of view = 2mm)	Groundmass made by melilite (the small crystals with blue interfence colors) and olivine. XPL image, 10x (Field of view = 2mm)	Groundmass made by melilite (the small crystals with blue interfence colors) and olivine. XPL image, 10x (Field of view = 2mm)
Groundmass made by melilite (the small rectangular crystals) and olivine. PPL image, 10x (Field of view = 2mm)	Groundmass made by melilite (the small crystals with blue-gray interfence colors) and olivine. XPL image, 10x (Field of view = 2mm)	Groundmass made by melilite (the small crystals with blue-gray interfence colors) and olivine. XPL image, 10x (Field of view = 2mm)
Groundmass made by melilite (the small crystals with blue-gray interfence colors) and olivine. XPL image, 10x (Field of view = 2mm)	Groundmass made by melilite (the small rectangular crystals) and olivine. PPL image, 10x (Field of view = 2mm)	Groundmass made by melilite (the small crystals with blue-gray interfence colors) and olivine. XPL image, 10x (Field of view = 2mm)
Olivine crystal embedded in a groundmass made by melilite (the small crystals with blue interfence colors) and olivine. XPL image, 2x (Field of view = 7mm)	Olivine crystal embedded in a groundmass made by melilite (the small crystals with blue interfence colors) and olivine. XPL image, 2x (Field of view = 7mm)	Groundmass made by melilite (the small rectangular crystals) and olivine. PPL image, 10x (Field of view = 2mm)
Groundmass made by melilite (the small crystals with blue-gray interfence colors) and olivine. XPL image, 10x (Field of view = 2mm)	Groundmass made by melilite (the small crystals with blue-gray interfence colors) and olivine. XPL image, 10x (Field of view = 2mm)	Groundmass made by melilite (the small crystals with blue-gray interfence colors) and olivine. XPL image, 10x (Field of view = 2mm)
Groundmass made by melilite (the small crystals with blue-gray interfence colors) and olivine. XPL image, 10x (Field of view = 2mm)	Groundmass made by melilite (the small crystals with blue-gray interfence colors) and olivine. XPL image, 10x (Field of view = 2mm)	Groundmass made by melilite (the small crystals with blue-gray interfence colors) and olivine. XPL image, 10x (Field of view = 2mm)
Groundmass made by melilite (the small crystals with blue-gray interfence colors) and olivine. XPL image, 10x (Field of view = 2mm)	Groundmass made by melilite (the small crystals with blue-gray interfence colors) and olivine. XPL image, 10x (Field of view = 2mm)