Alberto Renzulli

Evidence of incremental growth in the Vulsinian calderas (central Italy)

Abstract A comparative work on the Vulsini calderas has been carried out, fundamentally from volcanological field study, drilling and gravity anomalies data. On the basis of a detailed stratigraphy, four volcanic complexes have been recognized in the Vulsini Volcanic District from the oldest to the youngest: Paleo-Bolsena, Bolsena, Montefiascone and Latera, the latter two complexes having a contemporaneous evolution. In their activity, each complex gave rise to several little calderas caused by piecemeal collapses due to moderate-size explosive eruptions or coherent subsidence of a superficial piston-like block. Nevertheless, the extensional volcano-tectonic regime affecting this area could have accounted for as much of the total subsidence as the caldera-forming eruptions. Bolsena and Latera calderas, respectively 16 km and 8 km in average diameter, represent the main depressions of the Vulsini District and their topographic features are still well preserved. Several collapses forming the main poligenetic calderas of Bolsena and Latera are masked at present by the Bolsena Lake, the younger volcanic covering, the lacustrine deposits and erosion in a minor amount. Only the little Montefiascone and Vepe calderas, which represent the youngest collapses of the Vulsini area, have not been masked. Bolsena caldera could be considered a depression caused mostly by a subsidence hinged to the southwest, by down-sagging and by collapses which occurred after the most important explosive eruptions of the Paleo-Bolsena, Bolsena and Montefiascone volcanic complexes. Meanwhile, Latera caldera is mostly due to a piecemeal collapse linked to several moderate-size ignimbrite units which were emplaced throughout the explosive activity of the Latera complex. Therefore, the Vulsinian calderas did not form by catastrophic eruptions, because several and frequent moderate-size eruptions, downsagging and tectonic hinged subsidence concur, indicating an incremental growth.

Volcanological and petrological evolution of Marsili Seamount (southern Tyrrhenian Sea)

Abstract A swath bathymetric survey was conducted on Marsili Volcano, the biggest seamount in the Tyrrhenian Sea. It stands 3000 m above the surrounding oceanic crust of the 3500 m-deep Marsili back-arc basin and is axially located within the basin. The seamount has an elongated shape and presents distinctive morphology, with narrow ( 2400 m) on the edifice is governed by H2O and, probably, CO2 exsolution and is not a feature indicative of shallow water depth eruption.

Iron-bearing chlor-fluorapatites in crustal xenoliths from the Stromboli volcano (Aeolian Islands, Southern Italy): an indicator of fluid processes during contact metamorphism

Chlor-fluorapatite crystals, with an unusually high Fe content (up to 0.715 apfu), have been found in cordierite- and feldspar-bearing hornfels metapelitic xenoliths included in calcalkaline basaltic andesite lavas from the Stromboli volcano. These xenoliths originated in a mid-crustal (200–350 MPa) contact aureole (800°C) surrounding the magmatic plumbing system of the volcano. The high and somewhat variable Cl, Fe, Na, and Mn content in the chlor-fluorapatite can be interpreted as CaCl2, FeCl2, FeCl3, NaCl, and MnCl2 hydrous chloride melts present in the calcalkaline basaltic andesite magma responsible for the metamorphism of the metapelites to hornfels facies in the contact aureole. During the partial melting and dehydration of the metapelites, H2O was strongly partitioned into the partial melt and subsequently into the calcalkaline basaltic andesite magma. This H2O-rich partial melt was partly replaced by hydrous chloride melts, which metasomatised the original fluorapatite to chlor-fluorapatite with variable amounts of Cl, Fe, Na, and Mn. This scenario is supported by the patchy enrichment of Cl, Fe, Na, and Mn in the chlor-fluorapatite grains and the correspondingly low water activities present during the formation of the hornfels as confirmed by the absence of hydrous phases in the modal mineralogy of the xenoliths.

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 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.

Provenance and trade of volcanic rock millstones from Etruscan-Celtic and Roman archaeological sites in Central Italy

Rectangular hopper-rubber (Olynthian-style) and rotary (Pompeian-style) millstones were found, respectively, in the archaeological sites of Monte Bibele (4th–3th century B.C., Etruscan-Celtic Age) and Fossombrone (2nd century B.C.-6th century A.D., Roman Period). All the hopper-rubber millstones from Monte Bibele and three rotary millstones from Fossombrone were made using the leucite phonolite lavas from the same quarries which are located near Orvieto (Vulsini Volcanic District, Roman Volcanic Province; Central Italy). Since these leucite phonolites were also used in the Iron Age (pre-6th century B.C.) to produce small oval saddle-querns, a long continuous period of quarrying and production of millstones is therefore established for the Orvieto centre. Euganean Hills (Na-trachyte), Etna (hawaiite) and Iblean Mounts (tholeiitic basalt) were also identified as additional volcanic source areas for some millstones, mortars and/or worked lavas found at Fossombrone. This strongly supports the existence of a well organized millstone trade from different volcanic areas in the Roman Period.

Cordierite-anorthoclase hornfels xenoliths in Stromboli lavas (Aeolian Islands, Sicily): an example of a fast cooled contact aureole

High-grade hornfels xenoliths in the 60 ka old calcalkaline basaltic andesite lava flows of Stromboli (Omo lavas; Paleostromboli II period) consist of fine- to medium-grained holocrystalline rocks. Feldspar (mainly anorthoclase) and cordierite represent ca . 85 vol% of the modal mineralogy. Aluminiferous spinel (hercynite), sillimanite, ± corundum, ± ilmenite, ± chlorapatite are also present. Major and trace element compositions, together with Sr, Nd and Pb isotopic data, indicate that the source rocks are metapelites compatible with upper continental crust-derived materials. According to common petrogenetic grids for pelitic rocks ( e.g. NaKFMASH), a pressure range of 2–3.5 kbar and a peak temperature close to 800° C can be inferred for the mineral assemblage. A honeycomb texture and wide, high-T compositional range of the ternary feldspars (Ab 35–70 Or 3–63 An 2–49 ) suggest incipient melting followed by quenching. Accordingly, the xenoliths may well represent contact metamorphosed rocks (contact aureole), compatible with metapelites belonging to the continental crust basement of Stromboli. TEM (Transmission Electron Microscopy) observations of alkali-feldspar showed the presence of a cross-hatched pattern oriented parallel to (010) and (001), and of albite twinning in the orthoclase-enriched and -depleted samples, respectively. Both textures are related to a monoclinic to triclinic phase transition in a highly disordered feldspar. A highly disordered Al-Si configuration is also corroborated by cell parameters. Incipient decomposition textures, but no exsolution lamellae, are present in the intermediate alkali-feldspars. These results support a fast cooling of the hornfels xenoliths, of the order of degrees per hour. The high-T feldspar textures were preserved because the xenoliths, stoped into the uprising basaltic andesite host magma, were rapidly transported to the surface shortly after they reached the contact aureole peak temperature.

The Paroxysmal event and its deposits

The 5 April 2003 eruption of Stromboli volcano (Italy) was the most violent of the past 50 years. It was also the best documented due to the accurate geophysical monitoring of the ongoing effusive eruption. Detailed field studies carried out a few hours to a few months after the event provided further information that were coupled with visual documentation to reconstruct the explosive dynamics. The eruption consisted of an 8-min-long explosive event preceded by a short-lived precursory activity that evolved into the impulsive ejection of gas and pyroclasts. Meter-sized ballistic blocks were launched to altitudes of up to 1400 m above the craters falling on the volcano flanks and on the village of Ginostra, about 2 km far from the vent. The vertical jet of gas and pyroclasts above the craters fed a convective plume that reached a height of 4 km. The calculated erupted mass yielded values of 1.1―1.4 × 10 8 kg. Later explosions generated a scoria flow deposit, with an estimated mass of 1.0―1.3 × 10 7 kg. Final, waning ash explosions closed the event. The juvenile fraction consisted of an almost aphyric, highly vesicular pumice mingled with a shallow-derived, crystal-rich, moderately vesicular scoria. Resuming of the lava emission a few hours after the paroxysm indicate that the shallow magmatic system was not significantly modified during the explosions. Combination of volume data with duration of eruptive phases allowed us to estimate the eruptive intensity: during the climactic explosive event, the mass discharge rate was between 10 6 and 10 7 kg/s, whereas during the pyroclastic flow activity, it was 2.8―3.6 ― 10 5 kg/s. Strong similarities with other historical paroxysms at Stromboli suggest similar explosion dynamics.

Evolutionary stages of crystallization of weakly peralkaline syenites: evidence from ejecta in the plinian deposits of Agua de Pau volcano (São Miguel, Azores Islands)

Abstract Weakly peralkaline syenite ejecta within the plinian fall deposits of Agua de Pau volcano (São Miguel, Azores Islands) are divisible into (1) silica-undersaturated and (2) silica-saturated/oversaturated groups. The first group occurs in the pre-Fogo-A pyroclastic falls (15.2 ka) whereas the second occurs in the Fogo-A pumices (5 ka). The petrography indicates that the syenites of both groups crystallized at shallow depths and whole-rock geochemistry shows that they have compositions reflecting those of the Agua de Pau trachyte magmas. The syenites are inferred to be derived from solidification zones around the margins of the parental magma chamber. Mineral parageneses in both syenite groups permit distinction of three magmatic stages of crystallization: (1) an early crystallization stage; (2) a main crystallization stage during which most of the feldspar growth occurred; and (3) a late interstitial stage. Crystallization of the trachytic magmas yielded peralkaline residuals, highly enriched in volatiles, alkalis, HFSE and REE. In the silica-undersaturated syenites, the complex Zr-Ti silicate minerals, eudialyte and a låvenite-like mineral, crystallized interstitially from the residual magmas. In contrast, the zircono-silicate dalyite, and REE-rich minerals (chevkinite-(Ce), britholite-(Ce) and pyrochlore), together with thorite, were late-stage interstitial products of the silica-saturated/oversaturated syenites.

Sub-volcanic crystallization at Sete Cidades volcano, São Miguel, Azores, inferred from mafic and ultramafic plutonic nodules

SummarySuites of coarse-grained clasts (“plutonic nodules“) within the scoria of two separate mafic post-caldera parasitic vents, on Sete Cidades volcano, are described. These involve dunites, wehrlites, olivine clinopyroxenites, clinopyroxenites and olivine gabbros in the Eguas ankaramitic cone and pyroxene hornblendites, kaersutite gabbros and diorites in the Pico das Camarinhas basaltic-hawaiitic cone. The plutonic nodules are inferred to be cumulates, crystallised within the crust from magmas co-genetic with the post-caldera mafic lavas erupted on the flanks of Sete Cidades.The dunites, wehrlites, pyroxenites and olivine gabbros have assemblages and mineral composition consistent with growth as early, high-temperature cumulates, whereas the pyroxene hornblendites, kaersutite gabbros and diorites represent lower temperature products. However, whereas the kaersutite gabbros were not wholly crystallised at the time of their disruption and contain up to 4% intercumulus trachytic and/or tephriphonolitic glass, the pyroxene hornblendites and the diorites were fully crystallised. Compositions of the intercumulus glasses are similar to those of postcaldera trachytic pumices erupted on Sete Cidades.Although the two suites of plutonic clasts can be considered as complementary, they probably derive from different intrusions. We envisage a lensoid (laccolithic) intrusion at very shallow depth as the source for the pyroxene hornblendite - kaersutite gabbro -diorite suite whereas the dunite - wehrlite - olivine clinopyroxenite - clinopyroxenite -olivine gabbro nodules probably originate from intrusive bodies at greater depth.ZusammenfassungWir beschreiben Gruppen von grobkörnigen Klasten (‘Plutonic nodules’) in der Scoria von zwei separaten mafischen post-caldera parasitischen Kratern am Sete Cidades Vulkan: Diese umfassen Dunite, Wehrlite, Olivin-Klinopyroxenite und Olivin-Gabbros in dem ankaramitischen Kegel von Eguas. In den basaltisch-hawaiitischen Kegeln von Pico das Camarinhas, kommen Pyroxen-Hornblendite, Kaersutit-Gabbros und Diorite vor. Wir nehmen an, daß die plutonischen nodules Kumulate repräsentieren, die in der Kruste aus Magmen kristallisierten, die mit den post-caldera mafischen Laven, die an den Flanken von Sete Cidades ausgetreten sind kogenetisch sind.Die Dunite, Wehrlite, Pyroxenite und Olivin-Gabbros führen Paragenesen und Mineral-Zusammensetzungen, die im Einklang mit einer Entwicklung als frühe Hoch Temperatur-Kumulate sind. Hingegen stellen die Pyroxen-Hornblendite, Kaersutit-Gabbros und Diorite niedriger temperierte Produkte dar. Die Kaersutit-Gabbros waren zur Zeit /:ihrer Umlagerung noch nicht ganz auskristallisiert und führen bis zu 4% intercumulus trachytisches oder tephriphonolitisches Glas, die Pyroxenhornblendite und die Diorite sind jedoch ganz kristallisiert. Die Zusammensetzungen der Interkumulusgläser sind denen von post-caldera trachytischen Bimssteinen von Sete Cidades ähnlich.Obwohl die zwei Gruppen plutonischer Klasten als komplementär betrachtet werden können, stammen sie wahrscheinlich von verschiedenen Intrusionen. Es dürfte sich um eine lensoide (lakkolithische) Intrusion in geringer Tiefe als Quelle für die Pyroxenhomblendit-Kärsutitgabbro-Diorit-Abfolge handeln, während die Gruppe der Dunite, Wehrlite, Olivinklinopyroxenite und Olivingabbro-Nodules wahrscheinlich aus größerer Tiefe stammt.

Two-stage fractionation history of the alkali basalt-trachyte series of Sete Cidades volcano (São Miguel Island, Azores).

A comprehensive data set of mineral and whole-rock major element compositions of the Na-alkaline lavas and pumices erupted by the Sete Cidades volcano (Sao Miguel Island, Azores) is here presented. Mineralogy and geochemistry of the Sete Cidades extrusives indicate a low-pressure fractional crystallization, at relatively high fO2, as the main magmatic process generating the volcanic suite, which is mostly made up of basaltic-hawaiitic and trachytic rocks; very few mugearites are known on Sete Cidades emphasizing the possibility that residual liquids were able to leave the hawaiitic crystal mush only when the density had been lowered by compositional change or volatile exsolution ( i.e. at the trachytic stage). We mainly show how compositional variations within each group of silicates (and non-silicate) and the major-trace element covariances in the mineral structures through the alkali basalt-trachyte series are highly dependent on the sequence of crystallization and the correspondent magmatic environment (temperature, pressure, fO 2 ). Relatively primitive, slightly ne - normative basalts with MgO up to 14 % were tapped through flank eruptions indicating they were not held at any crustal levels, with rapid ascent through the plumbing system. Some of these basalts were held close to the Moho where they evolved by wehrlite fractionation to produce slightly hy - normative hawaiites with less than 7 % MgO. Slightly evolved basalts or hawaiitic magmas leaving the deep crustal reservoir were either erupted or were trapped in high level magma chambers to fractionate through towards metaluminous, q - + an - normative two-feldspar trachytes and thence to slightly peralkaline, one-feldspar, q - + ac - normative trachytes. Evidence of this liquid line of descent is also constrained by the hornblendite-kaersutite gabbro-diorite shallow plutonic roots of the volcano. Geothermobarometric and fO 2 calculations, performed on equilibrated pair of minerals (or mineral/liquid) generally agree with the proposed two-stage fractionation history of the Sete Cidades Na-alkaline magmas occurred within the relatively thick Azorean oceanic crust.