Mount Vulture

Volcanology and Stratigraphy
Mount Vulture (Fig.1-2) is a 1326 m high isolated composite cone with a few eccentric domes and craters. It is constructed at the intersection between NE-SW and NW-SE trending faults, on the eastern side of the Apennine chain, where Apennine thrust front overlaps the Apulia carbonate platform. The contact between the Apulia carbonate platform and the overlying sediments occurs at about 5 km depth, where the magma chamber of Vulture volcano probably developed.

Fig.1: (a) Sketch map of central-southern Italy with the main occurrences of the Quaternary volcanism and the front of the Apennine prism. (b) Geotectonic sketch map of the Mt. Vulture area, the arrow points to the position of the carbonatite volcanic center of Vallone Toppo di Lupo.

The volcanic sequence (Fig.4) consists of dominant pyroclastic deposits and minor lava flows, which cover an area of about 150 km². Activity took place between about 0.8 and 0.1 Ma.
The oldest activity (0.8-0.7 Ma) was characterised by phonolitic to trachytic ignimbritic eruptions and emplacement of some lava domes. The following activity was dominated by pyroclastic eruptions and minor lava flows that formed the main cone and summit caldera collapses; rock compositions of this phase range from tephrite and basanite to phonotephrites, and include a few melilitites, hauynophyres (at Melfi, 0.56 Ma) and melafoidites.
The most recent activity (0.13 Ma) occurred after a quiescence period of about 200 ka and produced two intra-caldera maars occupied by the Monticchio Lakes (Fig.3), and a tuff sequence reported to have a carbonatite-melilitite composition. Ultramafic nodules and megacrysts of clinopyroxene, amphibole, and phlogopite are found in these deposits. Some nodules are believed to represent fragments of upper mantle rocks and have shown subduction-related geochemical signatures.

Fig.2: Mount Vulture.

Fig.3: Monticchio Lakes.

Fig.4: The three major eruptive phases: 1) Phonolitic to trachytic ignimbritic eruptions and emplacement of some lava domes (Toppo San Paolo). 2) Formation of the the main cone. 3) Final stage and intra-caldera maars formation. From Boenzi et al., 1987.

Petrography and Mineral Chemistry
The Vulture rocks range from foidite, tephrite and basanite to tephriphonolites and phonolite, with a few melafoidites and melilitites. Rock textures are generally porphyritic.

• Basanites: are observed as dykes at Fontana Giumentari (locality n. 8 in Fig. 5), and Cimitero di Foggiano (locality n. 11 in Fig. 5), lava fows and scoria cones e.g. At Ciaulino (locality n. 19 in Fig. 5), Barile (locality n. 5 in Fig. 5), and the to top of Mt. Vulture; they are porphyritic, with phenocrysts of olivine with Cr-spinel inclusions), diopsidic-salitic clinopyroxene, and sometimes hauyne. The groundmass is made up of the same phases together with magnetite, rare phlogopite, apatite, and interstitial or microlitic plagioclase and/or alkali feldspar.

• Tephrites and phonotephrites: represent most of the lava fows of the stratovolcano. The phenocryst assemblage consists of abundant zoned green salitic clinopyroxene, sparse plagioclase laths, magnetite, leucite very often analcimized,hauyne and resorbed amphibole and biotite. Plagioclase, anorthoclase, clinopyroxene, magnetite, and feldspathoids make up the microcrystalline groundmass; apatite is the main accessory phase.

• Foidites: are petrographically similar to tephrites and phonotephrites, the only difference being a higher abundance of hauyne and absence of feldspar phenocrysts, with the latter restricted to tiny microlites in the groundmass. Clinopyroxene is always the most abundant phenocryst phase.

• Phonolites: have been found as lava-domes and blocks in the lowermost pyroclastics at Braide, (locality n. 2 in Fig. 5). They are characterized by phenocrysts of alkali feldspar, hauyne, deep-green Fe-salitic clinopyroxene, Ti-magnetite, melanite, sphene, and sporadic leucite and plagioclase, set in a groundmass made up of the same phases plus nepheline.

• Melilitite dyke of Prete della Scimmia (locality n. 12 in Fig. 5) shows phenocrysts of a Akermanitic melilite, Ti-rich salitic clinopyroxene and opaque oxide in a holocrystalline groundmass also containing nepheline, leucite and hauyne together with apatite, perovskite and Ti-rich garnet. Ocellar textures composed of calcite, magnetite, clinopyroxene and nepheline are also observed.

• Melfi hauynophyre (locality n. 1 in Fig. 5) is a holocrystalline rock with phenocrysts of hauyne with sodalite rims), zoned salitic to ferrosalitic clinopyroxene, subordinate leucite and apatite in a groundmass made up of the same phases plus nepheline, Na+Fe-rich melilite and magnetite.

• Melafoidite: The Santa Caterina melilite-bearing melafoidite lava fow is porphyritic with dominant Ca-Ti-rich salitic clinopyroxenes and extremely subordinate hauyne; the fine-grained groundmass is composed of Ti-rich salitic clinopyroxene, Ca-rich nepheline, hauyne, leucite, apatite, opaque oxides, and gehlenite-rich melilite.

• Cumulate xenoliths: are occasionally included in pyroclastic products and sometimes in basanitic dykes; they range from phlogopite-bearing wehrlites, with cumulus olivine, clinopyroxene crystals and poikilitic brown mica, to apatite-biotite clinopyroxenites and amphibole clinopyroxenites. Textures vary from meso- to orthocumulitic, being characterised by 20-50 vol% of intercumulus minerals.

Fig.5: (a) Sketch map Mt. Vulture area with the most important localities.

Genesis
The genesis and evolution of Mt. Vulture magmatism have been the object of several recent studies (Melluso et al., 1996; Stoppa and Principe, 1998; Beccaluva et al., 2002; Downes et al., 2002; De Astis et al., 2006). In referring to Mt. Vulture, some of these authors used the adjectives anomalous or hybrid to underline that the products of this volcano, as well as its geological setting, are different from those of the other Quaternary alkaline volcanoes from the Italian peninsula. In particular, though still characterized by negative anomalies of Nb, Ta, P and Ti, as typically observed for magmas derived from orogenic sources, Mt. Vulture rocks show an attenuation of these anomalies, and, in addition, they are characterized by high relative contents of Th, U, Pb and LREE and negative anomalies of Rb and K.
Taken as a whole, the Sr-Nd-Pb isotope compositions of Mt. Vulture rocks are intermediate between those observed for the intra-plate-type, Plio-Quaternary magmas from southern Italy (e.g. Mt. Etna, Hyblean Mts.) and the enriched values of the Roman Province volcanoes.

According to Beccaluva et al. (2002), Mt. Vulture magmas were produced by low-degree melting of a deep lithospheric mantle enriched in Na-alkaline/carbonatite melts and furtherly modified by the addition of a subduction-related component. De Astis et al. (2006), proposed that Mt. Vulture is sourced by an African-type mantle (FOZO-HIMU-type), flowing westward in response to the detachment of the downgoing Adria Plate, and modified by melts released from the sedimentary cover of this latter.
The slab detachment could have occurred at about 0.8 Ma, when compression phases in the Vulture area ended and distension tectonics generated fractures along which Vulture magmas ascended to the surface. A general sketch in Fig. 6 shows such a geodynamic evolution model.

Fig.6: Geodynamic evolution model for the southern Italian Peninsula. A) A continuous subduction zone of the Apulian-Ionian plate was active until about 0.8 Ma. B) Slab breakoff in the Apulian sector generated distension regime at the contact between Apulia and the southern Apennines, mantle contamination by subduction components beneath the edge of the Apulian plate and the formation of Vulture volcano; active subduction continued in the Ionian sector. C) Sinking and rollback of the narrow Ionian slab generated suctioning of intraplate asthenosphere from the Apulia foreland; this was contaminated by sedimentary material and fluids from the subduction zone and generated a hybrid OIB-arc mantle wedge, whose melting gave the Campanian volcanoes and Stromboli.

According to slab break-off model, the arc signatures of Mount Vulture would derive from melts and fluids released by the detached and sinking slab, which contaminated the mantle beneath the edge of the Apulia plate, where Mount Vulture magmas were formed.

Bibliography

• De Fino, M., La Volpe, L., Peccerillo, A., Piccarreta, G., & Poli, G. (1986). Petrogenesis of Monte Vulture volcano (Italy): inferences from mineral chemistry, major and trace element data. Contributions to Mineralogy and Petrology, 92(2), 135-145.
• D'Orazio, Massimo, et al.(2007). "Carbonatites in a subduction system: the Pleistocene alvikites from Mt. Vulture (southern Italy)." Lithos 98.1: 313-334.
• Jones, A. P., Kostoula, T., Stoppa, F., & Woolley, A. R. (2000). Petrography and mineral chemistry of mantle xenoliths in a carbonate-rich melilititic tuff from Mt. Vulture volcano, southern Italy. Mineralogical Magazine, 64(4), 593-613.
• Beccaluva, L., Coltorti, M., Di Girolamo, P., Melluso, L., Milani, L., Morra, V., & Siena, F. (2002). Petrogenesis and evolution of Mt. Vulture alkaline volcanism (Southern Italy). Mineralogy and Petrology, 74(2-4), 277-297.
• Peccerillo, A. (2005). Plio-quaternary volcanism in Italy (Vol. 365). Springer-Verlag Berlin Heidelberg.