Journal of Volcanology and Geothermal Research 208 (2011) 23–38

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Journal of Volcanology and Geothermal Research

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Rock magnetism and compositional investigation of Brown Tuffs deposits at Lipariand Vulcano (Aeolian Islands — Italy)

Alex M.P. Cicchino a,1, Elena Zanella a,b,⁎, Gianfilippo De Astis c, Roberto Lanza a,b, Federico Lucchi d,Claudio A. Tranne d, Giulia Airoldi a,2, Sara Mana a,3

a Dipartimento di Scienze della Terra, Università di Torino, Italyb ALP — Alpine Laboratory of Paleomagnetism, Peveragno, Italyc Istituto Nazionale di Geofisica e Vulcanologia, Sezione di Roma 1, Italyd Dipartimento di Scienze della Terra e Geologico-Ambientali, Università di Bologna, Italy

⁎ Corresponding author at: Dipartimento di Scienze d35, 10125 Torino, Italy.

E-mail address: elena.zanella@unito.it (E. Zanella).1 Now at Department of Earth Sciences, Durham Univ2 Now at SEAconsulting, Torino, Italy.3 Now at Department of Earth and Planetary Sciences,

USA.

0377-0273/$ – see front matter © 2011 Elsevier B.V. Alldoi:10.1016/j.jvolgeores.2011.08.007

a b s t r a c t

a r t i c l e i n f o

Article history:Received 15 February 2011Accepted 23 August 2011Available online 08 September 2011

A rock-magnetic investigation was carried out on the nonwelded ash deposits of the Brown Tuffs (AeolianIslands, southern Tyrrhenian Sea) to improve the stratigraphic correlation between the deposits croppingout on Lipari and Vulcano islands and locate their source area. The study was supplemented by petrograph-ical and geochemical analyses on selected strata, with the intent to compare the Brown Tuffs to other rocksemplaced at Vulcano in the same time span. More than 30 levels were sampled in the intermediate (56±4 kaN IBTN21–22 ka) and upper (21–22 kaNUBT) parts of the Brown Tuffs sequences on the two islands.Their characteristic remanent magnetization (ChRM) directions were derived from stepwise thermal demag-netization, and the magnetic fabric from measurements of the anisotropy of magnetic susceptibility. Thelevels with indistinguishable ChRM directions were regarded as coeval and to form an individual stratigraph-ic unit. The units were referred to the Brown Tuffs sequence of Lucchi et al. (2008) on the grounds of theiremplacement age, provided by comparison of their mean paleomagnetic direction with the paleosecular var-iation curves of the southern Tyrrhenian region, as well as the field constraints. The closer correlation be-tween the sequences of Lipari and Vulcano contributes to a better understanding of the volcanic activitythat produced the Brown Tuffs, and shows that most of the IBT and the oldest UBT levels were emplaced ina short time span, between ≈24 and 20–17 ka. The magnetic fabric is typically well developed, but at mostsites the magnetic foliation is very close to horizontal and no imbrication is defined. The source area of theBrown Tuffs parent pyroclastic flows, as constrained from the intersection of the magnetic lineations, fallsin the northeastern part of La Fossa Caldera structure. Although limited to major elements, compositionaldata provide further indication about the parent plumbing system and its behaviour. Magma batch(es) in-volved in the IBT eruptions have homogeneous features and underwent frequent refilling and tapping pro-cesses. Conversely, those involved in the early UBT eruptions are compositionally more variable. Thissuggests more complex evolution and plumbing system activity: the UBT eruptions represent either residualmafic magmas from the previous eruptions or the arrival of new, fresh shoshonitic magma in the system.

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Rutgers University, Piscataway,

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1. Introduction

The Brown Tuffs (BT) are nonwelded ash deposits emplaced onmost of the Aeolian archipelago by recurrent, large-scale hydromag-matic eruptions that occurred during the last 80 kyr. The large volumeand the wide geographic and time distribution of the deposits make

their study important to both a deeper understanding of the recentvolcanic activity in the islands and a better assessment of the volcanichazard. Lucchi et al. (2008) have identified three main successions:the Lower (LBT), Intermediate (IBT) and Upper (UBT) Brown Tuffs.They are separated by two regional stratigraphic markers called Ischiatephra (56±4 ka) and Monte Guardia pyroclastics (22–21 ka). Thelithological features of BT are rather homogeneous, regardless of ageand location. Fine subdivision of the main successions can only bemade upon minor interbedded markers, such as characteristic volca-nic products, paleosols and erosive unconformities. However, not allsuch markers are evenly distributed on all islands, and in places theyare missing. Their stratigraphic correlation is therefore difficult andchronology of the eruptive cycles that produced the various BT levelsnot fully defined, since only few levels have been dated by 14C

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analysis. Moreover, definite evidence for the source area is still lack-ing, even if stratigraphic and volcanological data suggest that the BTmost likely erupted from the Vulcano area presently occupied by LaFossa Caldera (De Astis et al., 1997; Lucchi et al., 2010).

The present paper reports on rock magnetic, petrographic andgeochemical investigations carried out on the IBT and UBT at Lipariand Vulcano (Fig. 1). Rock magnetism was designed to integrate thestratigraphic constraints of the BT, and provide an estimate of theiremplacement ages and vents location. Petrographic and geochemicalanalyses were performed on selected levels, with the dual aim tocompare the features of the magma generating the BT deposits withcoeval magmas erupted on Vulcano island, and strengthen their pa-leomagnetic correlation.

2. Geological setting

The Aeolian volcanism is younger than 1.3–1 Ma, and responsiblefor submarine and subaerial activities that formed several seamountsand seven large stratovolcanoes rising from sea floor (1000–2000 mb.s.l.) to heights up to 900–1000 m a.s.l. (Favalli et al., 2005). The vol-canic activity was mainly effusive, or explosive with low to mediumenergy. Chronostratigraphic data, however, indicate that some highenergy explosive activity occurred in more recent periods of the Aeo-lian volcanic history. These events involved intermediate to evolved

Fig. 1. Geological sketch map of Lipari and Vulcano (simpl

magmas from shallow reservoirs, e.g. on Salina island at 24 ka (thePollara I event — Calanchi et al., 1993) and on Lipari island, at 22–21 ka (the Monte Guardia eruptions — Crisci et al., 1981) and 8 ka(Vallone del Gabellotto eruption — Cortese et al., 1986). Conversely,the Vulcano island system seems to have evolved differently. Itsvolcanic activitywas characterised by recurrentmedium to high energyexplosive eruptions, at times associatedwith sector collapses (Pasquarèet al., 1993). A process of this type has been proposed for the emplace-ment of the Tufidi Grotte dei Rossi deposits (TGR—DeAstis et al., 1997).Such deposits consist of massive, poorly coherent to coherent, brown-reddish ash beds. The Brown Tuffs (BT) exposed on Lipari, Filicudi andSalina islands show lithological features very similar to those of theTGR (Keller, 1967, 1980a, 1980b; Crisci et al., 1983; Manetti et al,1988; Losito, 1989). A systematic investigation of BT features, and thepossible correlation of their outcrops on a regional scale were accord-ingly addressed in a paper by Lucchi et al. (2008).

The BT ash deposits are widespread throughout the Aeolian archi-pelago and as far as to Capo Milazzo in Sicily. They were generated bymajor eruptions from the central–southern sector of the archipelago.A process of magma–water interaction largely controlled magmafragmentation, as well as dispersal and emplacement of the ash de-posits (Lucchi et al., 2008). Based on thickness and grain-size varia-tions of pyroclastic beds exposed along the Vulcano–Lipari–Salinaaxis, IBT and UBT seem to have originated from a source area within

ified after Lucchi et al., 2008). Dot=sampled section.

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or close to the La Fossa Caldera multi-stage structure, in the northernpart of Vulcano island. Rock components, grain-size, morphology andcompositional data show that IBT and UBT deposits form overlappedbeds that may be related to the propagation of multiple, dilute andturbulent pyroclastic density currents (PDCs) (De Astis et al., 1997).IBT and UBT depositional units are associated with thin layers offine ash, especially in distal outcrops. Such layers are interpreted asthe result of fallout from low-concentration, co-ignimbritic ash cloudsand their associated eruption columns (Lucchi et al., 2008). Lithological,textural and structural features together concur to prove the aboveinterpretation and are summarised below.

The BT deposits consist of millimetric glass shards (65–90%) andvariable, but subordinate amounts of crystals (18–30%) and lithicgrains, sometimes in the lapilli class and rarely exceeding 10%. Poorly-vesiculated glass shards, with hydration cracks and adhering particles,are the abundant evidence of hydro-magmatism (Lucchi et al., 2008).The BT deposits generally form massive beds, which vary from a fewdecimetres to several metres thick in the paleo-valley. Interbedding ofthin ash layers, with tractional structures such as planar to low-anglecross lamination and thin layers of scoriaceous lapilli, is common; indu-rated laminae of fine ash are also documented. Due to distinctive colourand/or slight grain-size variations, few of the massive beds show inter-nal banding. Others beds display small amounts of randomly dis-seminated scoriaceous lapilli, and mm-sized, euhedral clinopyroxenecrystals. Lapilli and pyroxene crystals are either co-aligned parallel tobedding, or concentrated in pockets. The basal portion of thesemassivebeds can be enriched in scoriaceous lapilli or lithic grains embeddedfrom underlying layers for erosional action on the substrate. Locally,carbonised wood fragments are found within BT layers. They indicateemplacement temperatures higher than 300–350 °C (Jones and Chaloner,1991). Compositionally, the IBT range from basaltic trachy-andesite totrachyte and the UBT from basaltic trachy-andesite to rhyolite. The UBTbeds, however, locally have homogeneous composition, with the inter-mediate, trachy-andesitic type more common. The mineral assemblageof the BT comprises euhedral clinopyroxene (dominant), plagioclase,olivine and K-feldspar (De Astis et al., 1997). The composition of thejuvenile glass shards is another peculiar characteristic and supportsthe correlations established among the different units.

3. Sampling

Samples have been collected in the sections reported in Lucchi et al.(2008) andDeAstis et al. (1997), when possible at different stratigraphicheights within each individual layer. Sampling suffered two main dif-ficulties: 1) some sections either have disappeared or are no longeraccessible because of the many buildings and facilities due to tourismdevelopment; 2) some layers are too thin or loose to be sampled; thisis unfortunately the case for the two main stratigraphic markers, theIschia tephra and the Monte Guardia pyroclastics.

The BT deposits are commonly loose, thus collection of orientedsamples in the field, as well as their subsequent laboratory prepara-tion as specimens with regular shape and standard dimensions, wasquite difficult. Different techniques were devised in the course ofthe various sampling campaigns according to the rock cohesion:

(1) Coherent: cores were drilled using a battery powered drillerand oriented in-situ;

(2) Poorly coherent: a large rectangular block (≈10×15×20 cm)was shaped using trowel and paddle, smoothed and orientedon its front side and then gently taken off from the outcrop.In the laboratory, the block was bathed in ethyl-silicate, leftto consolidate and finally cored;

(3) Loose: a block was shaped and oriented as above, yet it was leftin-situ, sprayed with abundant ethyl-silicate and collectedmore than 2 weeks later, once consolidated enough to proceedwith coring in the laboratory.

Twenty-two layers were sampled in the UBT and IBT at Lipari and12 at Vulcano for a total of more than 450 standard cylindrical speci-mens (Φ=25 mm, h=23 mm). Within the text, the BT layers wesampled are referred to as “levels”, whereas “units” refers to the strat-igraphic units of Lucchi et al. (2008), referred to with the acronyms“btl” (Lipari sections) and “grt” (Vulcano sections) (Fig. 2).

4. Paleomagnetism

Paleomagnetic correlation of stratigraphic units within a volcanicregion relies on the basic assumption that deposits with the same/different paleomagnetic direction(s) were emplaced by the same/distinct eruption(s). This is not always true, because the Earth's mag-netic field in a given region changes with time around the directionof the geocentric axial dipole (GAD) and the same paleomagneticdirection may occur at different times and be recorded within de-posits emplaced during distinct eruptions. Such limitation appearsimportant, yet it may become negligible when paleomagnetic dataare constrained by further information such as field stratigraphy, iso-topic dating, etc. The soundness of chronological data derived frompaleomagnetism therefore depends on the whole of available informa-tion. A time resolution as high as 100 years may be achieved, for in-stance, when paleomagnetism is applied to historical lava flows,because written sources and archaeological findings contribute to drawdetailed reference curves (Speranza et al., 2006; Tanguy et al., 2007). Pa-leomagnetic directions of Pleistocene volcanic rocks dating back to a fewhundred ka are compared to paleosecular variation (PSV) curves mainlyderived from lake sediments. The PSV curves are far less accurate thanarchaeomagnetic ones because the locking time of remanence in sedi-ments smooths the record of the Earth's field direction (Butler, 1992).Yet they represent the variation trend of the Earth's field and, supple-mented by stratigraphic information, may be used to infer the age of vol-canic rocks.

4.1. Measurements

Measurements were carried out at the ALP laboratory (Peveragno,Italy). The natural remanent magnetization (NRM) was measuredusing spinner magnetometers AGICO JR-5 and JR-6. Stepwise thermaldemagnetization was carried out using a Schonstedt furnace, in orderto isolate the NRM's components. The NRM intensity is on the orderof 0.1 to 1 A/m. All specimens were demagnetized in 8 to 12 steps,at intervals of 40–60 °C. The blocking temperature spectrum (Fig. 3)shows that 70% to 80% of the initial remanence is erased below≈400 °C. The maximum blocking temperatures typically fall within500–560 °C, but a few values are as low as 420 °C or as high as 610 °C.Such values point at low-Ti titanomagnetite, which has been shown inthe literature to be themain, and inmany cases the only, ferromagneticmineral within BT (Losito, 1989; Zanella et al., 1999; Cicchino, 2007).Thermal demagnetization diagrams were interpreted using PaleoMacprogram (Cogné, 2003). Besides a small viscous component usuallyerasedwithin 100–150 °C, the NRM consists of two components (Fig. 3):

(1) A low-T component, stable up to ≈400 °C. This is the mainNRM fraction, which we regard as the characteristic remanentmagnetization (ChRM) acquired by BT during cooling followingdeposition. The ChRM direction is consistent with the PSVrange around the GAD direction at the Aeolian Islands (D=0°,I=59°);

(2) A small high-T component (N400 °C) characterised by varyingdirections. This might be related to magmatic grains withhigh blocking temperature cooled during ash transportation(Zanella et al., 2011).

Analysis of Zijderveld (1967) diagrams and equal-area projections(Fig. 3) shows that while the low- and high-T components are clearlyisolated in some specimens, they overlap in others. Accordingly,

Fig. 2. Synthetic stratigraphical sections of the Brown Tuffs (BT) successions at Lipari and Vulcano. Acronyms: UBT, IBT, LBT=upper, intermediate, lower BT; btl=Lipari BT units;grt=Vulcano UBT units. Stratigraphic markers (acronyms in italics) and isotopic ages after Lucchi et al. (2008).

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individual specimens are characterised either by a stable end-pointsdirection or a best-fitting great circle (McFadden and McElhinny,1988) and data derived by both methods are used to calculate themean ChRM direction of individual levels (Fig. 4; Table 1).

4.2. Results

The paleomagnetic data are analysed and interpreted in threesteps:

(1) ChRManalysis. The ChRMdirections of individual levels are com-pared within the UBT and IBT sequences. In cases in which theyare statistically indistinguishable, the corresponding levels areregarded as emplaced by the same eruption and thus belongingto the same stratigraphic unit. The unit mean direction is calcu-lated as the mean of the ChRM directions of all the specimensof the unit.

(2) Paleomagnetic interpretation. The directions of the units arerelocated via pole (Noel and Batt, 1990) to the same referencesite (Viterbo, λ=42° 27′ N, ϕ=12° 02′ E) in order to comparethem to the paleomagnetic directions of known age from Aeolianvolcanoes (Lanza and Zanella, 2003) and the PSV curves from theMezzano (back to 30 ka) and LagoGrande diMonticchio (N30 ka)sediment cores (Brandt et al., 1999).

(3) Stratigraphic interpretation. The chronological informationfrom point (2), field constraints and available isotopic agesare used to infer the stratigraphic position of the various levelswithin the units of Lucchi et al. (2008) BT sequence.

In the following sections, the analysis and discussion of the resultsfollow the Brown Tuffs main division in UBT and IBT. The stratigraphicconstraints and isotopic ages (Lucchi et al. 2008, and references therein)

are discussed in the text. Substantial paleomagnetic data and chrono-logical information are summarised in Tables 1, 2 and Figs. 5 to 8.

4.2.1. Upper Brown Tuffs (UBT) — Lipari and VulcanoThe UBT deposits at Lipari overlie the Monte Guardia marker (gu,

20–22 ka — Fig. 2) and are in turn overlaid by the Monte Pilato pyro-clastics (mp, 1.4 ka). At Vulcano, mp overlie gu in some sections, andeither the Quadrara or Spiaggia Lunga scoriae of similar age (sq, 21±3 ka and sl, 24±5 ka, respectively) in others; they are overlain bythe Punte Nere pyroclastics (pnl, 5.5±1.3 ka). On the grounds ofthese and other minor markers, Lucchi et al. (2008) defined threestratigraphical units at Lipari – btl14, btl13 and btl12 – and four atVulcano — grt4, grt3, grt2 and grt1 (Fig. 2).

Four levels were sampled at Lipari (acronym L). Here, field rela-tions did not allow the assessment of their stratigraphic positionwithin the UBT succession. At Vulcano, seven levels (TGRi) were sam-pled below the Cugni di Molinello scoriae (cm — Fig. 2), and thus inthe grt1 unit, and four (TGRs) above, within the grt4, grt3 and grt2units that could not be differentiated in the sampled outcrops. TheChRM directions of the L and TGRs levels gather into three distinctgroups (Tables 1, 2 and Fig. 5):

(1) Group 1: L12+L27+TGRs3. The mean direction of this group ischaracterised by a low inclination value and a westward declina-tion and only consistent with the PSV curves at about 15 ka(Fig. 6). This result suggests that the L12, L27 levels belong tothe btl13 unit, whose 14C age is 16.8±2 ka, and the TGRs3 levelbelongs to the grt2 unit, which lies below the Monte Saracenomarker (sal, 8.3 ka — Fig. 2).

(2) Group 2: L6+L22+TGRs6. The mean direction of the secondgroup is consistent with the PSV curves at 5–6 ka, 10–11 ka(Fig. 6), and other ages older than 18 ka, which, however, can

Fig. 3. Thermal demagnetization of representative specimens. Left: normalised intensity decay curve; middle: Zijderveld (1967) diagram (full/open dot=declination/apparent in-clination); right: equal-area projection (symbols: full/open dot=positive/negative inclination).

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be discarded because TGRs6 is younger than grt1 unit (datedaround 17–20 ka, as discussed below). Levels L6 and L22 aretherefore younger than those of Group 1 and may be referredto the btl14 unit, whose stratigraphic position above the Gabel-lotto marker (vg — Fig. 2) points to an age younger than 8.6±1.5 ka. Level TGRs6 thus corresponds to grt4, the youngest unitat Vulcano that overlies the same Gabellotto marker and hasbeen dated at 7.7±0.1 ka.

(3) Group 3: TGRs4+TGRs5. The levels of the third group are as-cribed to the same grt4 unit, because their mean direction is con-sistent to the PSV curves at about 4.5–5.5 ka (Fig. 6). Thisdirection is distinct from that of TGRs6 and close to that of thePunte Nere lava (pnl, 5.5±1.3 ka) (Fig. 5). Occurrence of distinctpaleomagnetic directions within unit grt4 suggests that the levelTGRs6 corresponds to the blt14 level in Lipari. Conversely, levelsTGRs4 and TGRs5 are younger and without corresponding UBT

deposits on the Lipari island, because such deposits are eithermissing or have not been sampled/recognised.

As for the levels sampled in the grt1 unit at Vulcano (TGRi), themean paleomagnetic direction of TGRi5 and TGRi9 (Table 2) is consis-tent with the PSV curves at around 20.5 and 17.5–18 ka (Fig. 6) andthat of TGRi4, TGRi7 and TGRi10 at around 17.5–16.5 ka. The grt1unit overlies the Monte Guardia marker and is further constrainedto be older than 15 ka, which is the age of the grt2 unit, as discussedabove. There are thus two hypotheses to account for the age of thelevels included within grt1:

(1) Should the age of TGRi5 and TGRi9 be around 20.5 ka, theselevels would be coeval with btl12 unit at Lipari, whose 14Cage is 20.5 ka. The TGRi deposits would have been emplacedduring two distinct eruptive episodes: an older one, around20 ka – TGRi5, TGRi9 –which would have also interested Lipari

Fig. 4. Equal-area projection of paleomagnetic directions of representative levels. Symbols: dot=ChRM direction; great circle=remagnetization circle (Halls, 1976, 1978); star=levelmean paleomagnetic direction with α95 ellipse of confidence (Fisher, 1953; McFadden and McElhinny, 1988).

28 A.M.P. Cicchino et al. / Journal of Volcanology and Geothermal Research 208 (2011) 23–38

(btl12 unit), and a younger one, around 17 ka – TGRi4, TGRi7,TGRi10 – missing or not recognised at Lipari.

(2) In the case the age is around 17 ka, all such levels would havebeen emplaced during an eruptive cycle that occurred over atime span on the order of 1 kyr.

Lastly, no correlation to the PSV curve is possible for levels TGRi1and TGRi8, because the very high inclination of their paleomagneticdirection (I≈65°, Table 1), does not match any part of the PSV curve.

4.2.2. Intermediate Brown Tuffs (IBT)Correlation of the BT levels below the Monte Guardia marker

(Fig. 2) is far more complex, for at least two main reasons. First, theIschia tephra which separates the IBT from the LBT, was not recognisedin the sections sampled at Lipari. Therefore, the BT levels stratigraphicallybelow the Punta del Perciato marker (pe2, 40 kabageb56 ka — Fig. 2)and above the Grey Porri Tuffs (l1, 67–70 ka), a widespread markerwithin the LBT, can a priori belong to the IBT as well as the LBT. Second,none of the stratigraphic markers occurring in the other islands is inter-bedded within the IBT sequence at Vulcano. Since the Ischia tephra ismissing, the IBT cannot be distinguished from the LBT.

4.2.2.1. Lipari. Lucchi et al. (2008) defined three units in between theMonte Guardia and Monte Falcone (fal, 40 ka) markers: one (btl11)above and two (btl10, btl9) below the Lower Pollara Tuffs (lpt,b22.9 ka). Their 14C ages are 22.6±0.3, 22.4±1.1 and 23.5±0.9, re-spectively. Two levels (L5, L26) were sampled in the btl11 deposits,five (L4, 11, 25, 29a, 29b) in the undifferentiated deposits of unitsbtl10 and btl9, and a last one (L21) in a section where the LowerPollara Tuffs marker is missing and which can potentially belong to

any one of the three units. The ChRM directions of these 8 levels fallwithin three groups (Fig. 7, Tables 1 and 2):

(1) Group 1: L5, L26.(2) Group 2: L25, L29a, L29b.(3) Group 3: L4, L11+L21.

The mean directions of the first two groups are close to each other,implying their emplacement in a short time span; consequently, thelevels of the second group should belong to the btl10 unit and beolder than those of the first group. The directions of levels L4 andL11 are different from those of Groups 1 and 2, and similar to thatof L21. These three layers can be regarded as coeval and older thanthe others, and thus to belong to the btl9 unit. Comparison with thePSV curves (Fig. 8) shows that the paleomagnetic directions of Groups1 and 2 are consistent with the PSV curve at 21–22 ka, even thoughtheir inclination value is a little higher. However, it is worth notingthat their directions are similar to that of the 22.0±4.0 ka Gelso-Petrulla lava at Vulcano (D=5°, I=60°,α95=2.9°— Lanza and Zanella,2003). Themean direction of Group 3 is in turn close to the PSV curve ataround 24 ka, which is consistent with the 14C age of btl9 (23.5±0.9 ka). In summary, paleomagnetic data of units btl11, btl10 and btl9substantiate their 14C ages showing that the three units were emplacedduring a time span no greater than 2–3 kyr.

Three levels (L3, L24, L31) were sampled in unit btl8, comprisedbetween the Monte Falcone (fal) and the underlying Punta del Perciato(pe2) markers. Their ChRM directions (Table 1) are indistinguishable;their mean direction is consistent with an age of around 42 ka or be-tween 52 and 56 ka on the PSV curves (Fig. 8). Below Punta del Perciatoand above the Grey Porri Tuffs (gpt, 67±8 ka), three levels (L19, L23,L32) were sampled in sections where the Ischia tephra is missing.They may therefore belong to btl7 unit, which is the lowest unit of the

Table 1Mean ChRM directions of BT levels from Lipari and Vulcano. Symbols: n=number ofspecimens, D, I=declination, inclination; k, α95=precision, semi-angle of confidenceof Fisher's (1953) statistics.

Section Level n D I k α95

UBTLipari

LS1 L6 9 14.5 57.5 235 3.4LS3 L12 10 336.1 32.1 77 5.8LS4 L22 9 11.3 57.6 168 4.1LS5 L27 9 346.6 30.7 73 6.1

VulcanoVS8 TGRi8 19 354.0 64.9 276 2.1VS9 TGRs3 16 350.8 32.0 132 3.2

TGRi9 6 12.6 54.8 66 9.8TGRi10 19 6.8 46.8 55 4.7

VS11 TGRs4 15 14.9 43.4 82 4.2VS12 TGRs5 23 17.2 44.9 128 2.7VS13 TGRs6 12 12.5 60.0 122 4.0VS12 TGRi7c 10 1.9 43.0 88 5.6VS16 TGRi1 10 2.3 61.3 // 7.4VS17 TGRi4 15 355.2 43.6 146 3.2VS18 TGRi5 11 10.1 50.3 256 3.0

IBTLipari

LS1 L5 12 4.4 53.9 145 3.8L4 14 353.8 48.3 59 5.3L3 11 6.8 52.6 162 3.8

LS3 L11 9 359.4 53.8 52 7.8LS4 L21 14 350.5 51.8 43 6.1

L20 11 18.7 56.2 67 5.7L19 16 4.1 56.9 92 3.9

LS5 L26 19 357.0 56.8 288 2.0L25 10 2.5 57.1 85 5.5L24 17 358.2 58.6 220 2.5L23 7 354.1 58.3 67 7.8

LS6 L29a 8 351.2 60.5 53 10.4L29b 6 6.2 63.3 51 10.1L31 11 356.2 54.1 95 4.8L32 6 358.5 60.7 69 8.3

VulcanoVS10 V14 13 346.6 52.9 62 5.3

V13 7 346.2 69.0 105 6.9VS14 V16 11 354.1 57.1 180 3.4

V15 10 333.3 56.4 140 4.1VS15 V18 7 328.7 64.4 238 4.1

V17 10 351.3 59.6 169 3.6

Fig. 5. Equal-area projection of UBT paleomagnetic directions with α95 ellipse of confi-dence. (a) level mean directions; (b) dots=unitmean directions; other symbols=paleo-magnetic directions of Aeolian volcanic rocks younger thanMonte Guardiamarker (Lanzaand Zanella, 2003).

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IBT sequence (Fig. 2), but also to btl6, btl5 and btl4, which are the upperunits of the LBT sequence. They have similar ChRM directions and canthus be referred to a same unit. Their mean direction (Table 2) is close

Table 2Mean paleomagnetic directions of coeval BT levels and inferred stratigraphic units(btl=Lipari; grt and Grotta dei Pisani=Vulcano). Symbols: D, I=declination, inclina-tion; α95=semi-angle of confidence.

Level Unit D I α95

UBTTGRs4,TGRs5 Upper grt4 16.3 44.4 2.3L6, L22, TGRs6 btl14 grt4 12.9 58.5 2.1L12, L27, TGRs3 btl13 grt2 345.8 32.3 2.9TGRi5,TGRi9 grt1 10.1 52.0 3.2TGRi4,TGRi7,TGRi10 grt1 0.9 45.6 2.6

IBTL5, L26 btl11 356.9 56.8 1.4V14,16,17 Grotta dei Pisani 350.6 57.5 3.8L25, L29a, L29b btl10 0.2 59.8 4.3L4, L11, L21 btl9 353.3 50.5 3.4L3, L24, L31 btl8 358.7 56.1 2.1L19, L23, L32 btl7 359.6 58.3 3.1

to that of the levels sampled in btl8 unit, and consistent with an agebetween 52 and 56 ka. As the age of the Ischia tephra is 56±4 ka, it isreasonable to assume that they correspond to the btl7 unit.

4.2.2.2. Vulcano. Each of the three sections sampled in the neighbour-hood of Monte Saraceno consists of two distinct levels stratigraphi-cally below Spiaggia Lunga formation (sl, 24±5 ka): V13–V14, V15–V16, V17–V18. The upper levels of the sections (V14, V16, V17),which belong to the Grotta dei Pisani formation, have statisticallyindistinguishable paleomagnetic directions (Fig. 7, Table 1). Further-more, their mean direction is indistinguishable from that of the btl11unit at Lipari and close to that of btl10 (Table 2). The 14C ages of thesetwo units are similar to each other and fully consistent with the upper(Monte Guardia, gu 21–20 ka) and lower (Lower Pollara Tuffs, lpt,b22.9 ka) interbedded markers. All these isotopic ages fall within theuncertainty range of the Scoriae di Spiaggia Lunga age (24±5 ka),whose chronological relations with the Lower Pollara Tuffs can not bedefined. Paleomagnetic data point to an age of around 22 ka for theemplacement of levels V14, V16 and V17, which would be coeval withboth btl11 and btl10 units of Lipari. Such an attribution implies thatthe actual age of the Scoriae di Spiaggia Lunga falls in the youngerpart of the uncertainty range of their isotopic age.

The paleomagnetic directions of the levels V13, V15 and V18, low-ermost in their sections, do not match any part of the PSV curves.These levels can be assigned to IBT on the grounds of their strati-graphic position, but their age cannot be better defined using paleo-magnetic data.

5. Compositional data

Major element analyses of glasses and crystals were obtained from80 mm-thick polished thin sections using a JEOL JXA-8200 WD-EDcombined microprobe at the INGV (Istituto Nazionale di Geofisica eVulcanologia — Rome, Italy). The following operating conditions forWavelength Dispersive Spectroscopy (WDS) analysis were adopted:10 nA probe current, 15 kV accelerating voltage. A probe diameterof 5 μm, with a final spot size of about 7 μm, was used for the analysison glass, in order to limit the degree of alkali volatilization duringprobing. Counting time was 10 s on peak and 5 s on backgroundand data reduction was carried out using a ZAF correction method

Fig. 7. Equal-area projection of IBT paleomagnetic directions with α95 ellipse of confi-dence. (a) Lipari and (b) Vulcano level mean directions; (c) dots=unit mean directions;other symbols=paleomagnetic directions of Aeolian volcanic rocks emplaced at about22–25 ka (Lanza and Zanella, 2003).

Fig. 6. Paleomagnetic directions of UBT units plotted against Lago di Mezzano (Brandtet al., 1999) and Aeolian volcanoes (Lanza and Zanella, 2003) PSV curves. The two stip-pled stripes encompasse the UBT age interval. The grey boxes show possible correlationbetween UBT paleomagnetic directions (vertical line segments correspond to the relo-cated unit mean D and I) and the PSV curves. Symbols: dot=paleomagnetic directionsfrom Aeolian islands; vertical/horizontal bar=age error/ΔD, ΔI.

30 A.M.P. Cicchino et al. / Journal of Volcanology and Geothermal Research 208 (2011) 23–38

included in the JEOL microprobe software package. Relative standarderror is: b5% for Si, Al, Mg, Fe, Ca, K and Na, and b10% for Cl and P. Mn,F, Cr and Ni contents were usually nearby the detection limit witherrors reaching also N30%.

The initial petrographic study of the BT ashes provided an estimateof the components, which mostly include juvenile glass fragments(~65 up to 90% in volume) and crystals (5–35%); lithic grains are pre-sent in low amounts (≤5%) as small fragments of various porphyriticlavas, commonly altered. The UBT at Vulcano have higher content ofcrystals and lithic grains (locally up to 11% — De Astis et al., 1997)than at Lipari. In agreement with observations by previous authors(De Rosa et al., 1992; De Astis et al., 1997; De Rosa et al., 2005), theglass shards are generally blocky and aphyric, subordinately vesicularand micro-crystalline. Their surfaces are commonly slightly alteredand colours vary from colourless to dark brown.

Data reported by Lucchi et al. (2008) indicate that the BT glass com-position ranges frombasaltic trachy-andesite to rhyolite, with a cloud ofanalytic points plotting in the trachy-andesite and trachy-dacite fieldsof the TAS diagram. These data also reveal compositional differences be-tween IBT and UBT units: i) IBT glass shards have a fairly homogeneoustrachy-andesitic composition, with SiO2 content below 62 wt.% and

alkali content ranging from 4.5 wt.% to more than 13 wt.% (althoughthe latter is limited to just a few samples); ii) when compared tothose of the IBT, major element compositions of the UBT glass shardsfrom both Lipari and Vulcano cluster over a common but more wide-spread compositional range, with an almost continuous variationfrom the trachy-andesite to rhyolite fields, and a narrow alkali range(Alk=7–12%).

Three IBT levels (L5, V14, V16) and four UBT levels (L6, TGRs4,TGRs6, TGRi8) were selected for geochemical analyses. TAS diagrams(Fig. 9) confirm the literature data, as they show IBT samples concen-trated in the trachy-andesite field and SiO2 content below 59 wt.%. Con-versely, the UBT samples are mainly concentrated among the trachy-andesite, trachy-dacite and trachyte fields, with a few in the rhyoliticfield. Since the finer fraction of TGRi ash (b0.125 mm) is almost entirelycomposed of glass fragments (De Astis, 1995), XRF whole rock analysesperformed on samples coming from various TGRi beds are also reportedin the UBT diagram (Fig. 9). TGRi data are characterised by a decreasein the alkali content for similar SiO2 values, which can be reliably attrib-uted to leaching effects on very small fragments. The IBT glass composi-tions display a limited but comparable range of major elements in thevariation diagrams (Fig. 10). No well defined trend can be visualisedwith increasing silica content; however, rough negative trends forMgO, CaO, FeO, and to some extent for K2O are observed disregardingthe V14 shards.

Both TAS and variation diagrams (Fig. 11) show that the TGRi depositsrelated to the early UBT eruptions are characterised by a wide range ofsilica and other major elements content. In contrast, the samples fromyounger Vulcano UBT beds – TGRs6 and TGRs4 – display less evolvedcompositions and seem to mirror the features of a rather homogeneous

Fig. 8. Paleomagnetic directions of IBT units plotted against Lago Grande di Monticchio(Brandt et al., 1999) and Aeolian volcanoes (Lanza and Zanella, 2003) PSV curves. Thetwo stippled stripes encompasse the IBT age interval. Symbols as in Fig. 6.

31A.M.P. Cicchino et al. / Journal of Volcanology and Geothermal Research 208 (2011) 23–38

batch of magma. If compared with shards from earlier UBT with similarSiO2 content, they show lower content of MgO, CaO and higher contentof K2O and P2O5. As a whole, the UBT samples display decreasing trendsof the major elements (MgO, CaO, FeO, P2O5) vs. SiO2, that are typicallyinterpreted as due to mafic mineral fractionation; K2O and Al2O3 trendsare consistent with this interpretation and with late feldspars fraction-ation. It is worth noting that the paleomagnetic direction of TGRs6 levelhas been related to that of the Lipari L6 level (Section 4.2.1), whereasthe compositions of the two levels are different (Figs. 9, 11). This discrep-ancy is likely due to the fact that the same direction of the Earth's fieldmay recur at different times (Section 4).

The crystals occurring within the BT ashes as loose particles arecommonly rimmed by glass and mainly represented by mm-sizedclinopyroxene phenocrysts of fairly homogeneous salitic compositionand minor percentages of plagioclase (An=48–52% in the UBT, andAn=57–78% in the IBT), K-feldspar, olivine, Fe–Ti oxides and amphi-bole. Olivines found in the IBT levels have Fo%=61–68, whereasthose from UBT have Fo%=70–73. Amphiboles are found only inthe IBT while they are absent in the UBT. Their occurrence is thus adistinctive characteristic of older BT and suggests magmatic systemswith different features for the generations of the different BT groups.

The limited trace element data allow some speculation about themagma source. The immobile High Field Strength Elements (HFSE)contents and their ratios, within both IBT and UBT deposits, are com-parable to those of the Vulcano products (De Astis, 1995) erupted inthe time span between 30 and 6 ka (Fig. 12) and quite differentfrom those of the Lipari magmas (Crisci et al., 1991). They point to theVulcano system as the BT magma source and discard the possibility ofa genetic relation with Lipari.

6. Magnetic fabric

Anisotropy of magnetic susceptibility (AMS) is a standard tool todetect the flow-induced fabric in PDCs. The fabric is characterisedby preferential alignment of elongated grains, which tend to lie withinthe bedding plane, imbricated up-flow toward the source area, and tobe aligned parallel to the flow direction (Tarling and Hrouda, 1993).When Ti-magnetite is the main primary ferromagnetic mineral, themaximum magnetic susceptibility occurs along the longest directionof the grains and their preferential alignment results in the magneticlineation, which lies within the magnetic foliation plane. The foliationplunges upstream and the lineation corresponds to the flow direction.Since the PDCs proceed radiating from the vent, the source area maybe located by finding the intersection of the magnetic lineationsand/or the foliation plunge from individual sites (Hillhouse and Wells,1991; LaBerge et al., 2009). This method ignores the possible deviatingeffects of the uneven topographical surface the PDC travels over, andshould therefore be based on sufficient mutually consistent data.

The magnetic susceptibility of all specimens and its anisotropywere measured before remanence analyses using a KLY-3 bridge.Bulk susceptibility of most levels shows limited variations between9,000 and 20,000 μSI (Table 3) and no systematic difference occursbetween UBT and IBT, whose mean values are 13,603 and 14,239 μSIrespectively. The degree of anisotropy (the ratio between the maxi-mum and minimum susceptibility values, P=k1/k3) varies between1.002 and 1.024, with an exceptional value of 1.043 observed inlevel V18. Susceptibility, therefore, varies little within the BT deposits,as typical for their lithological features. Notwithstanding the low Pvalues, the magnetic foliation is typically well defined, as shown bythe limited within-level dispersion of its poles, which correspond tothe anisotropy minimum axes k3. The maximum k1 and intermediatek2 axes are either well grouped or more or less dispersed within thefoliation plane (Fig. 13a, b). The magnetic lineation of a site, whichis given by the mean of the k1 axes, is accordingly defined with vari-able precision from level to level. In a few cases the principal axes aredispersed and the fabric is chaotic (Fig. 13c). At most sites, the mag-netic foliation is close to horizontal and the 95% ellipse of confidenceof its pole encompasses the vertical (Fig. 13). No plunge, and thus noimbrication, can be significantly defined and the magnetic foliationcannot be used to infer the flow direction except in a few favourablecases. We instead chiefly rely on triangulation of the magnetic linea-tions, which is based on the assumption that the flow directionsshould converge toward the source area from which the PDCs radiat-ed. Since lineation is bidirectional, at each site one can draw two op-posite fans, which contain the flow direction. They are centred on thelineation azimuth and their spread equals the 95% confidence angleE1,2 (Table 3).

In the case of IBT, the imbrication of the foliation plane is signifi-cantly defined at two sites and, since it plunges upstream, it givesthe absolute sense of flow (Fig. 14) and roughly indicates the sourceregion. This information assists in determining which of the two op-posite fans at other sites points toward the source. The various fansintersect each other and envelope a region where the maximumnumber of intersection occurs (Fig. 14), which may therefore be as-sumed as source area of the PDCs that laid down the IBT deposits.The source area derived from the geometry of the magnetic fabric ofUBT roughly coincides with that of IBT. Its extent is larger, mainly be-cause of the few data from Lipari and the wide spread of the fans fromVulcano.

7. Discussion and conclusions

The paleomagnetic correlation of the BT sequences of Lipari andVulcano is summarised in Fig. 15 together with the 14C isotopic agesavailable in the literature (Lucchi et al., 2008). The upper levels ofthe IBT sequence of Vulcano are related to the youngest units of the

Fig. 9. TAS classification diagrams for IBT (a) and UBT (b).

32 A.M.P. Cicchino et al. / Journal of Volcanology and Geothermal Research 208 (2011) 23–38

Lipari IBT. Within the UBT sequence, a correspondence between thestratigraphic units of the two islands is found, with the only exceptionbeing the gtr3 unit at Vulcano, which has no corresponding unit atLipari. Besides a closer definition of the stratigraphic correlation, themain outcome is the evidence for the uneven time distribution ofthe volcanic activity that produced the BT deposits. Major pyroclasticeruptions emplacedmost of the IBT and part of the UBT between≈24and 20–17 ka, and were preceded by a long quiescence of the BT-typeexplosive activity, which started around 56–52 ka and was possiblyinterrupted by the emplacement of unit btl8, whose age might be≈42 ka (Fig. 15).

The wide range of the volcanism that occurred at Lipari and Vulcanoin the time span between 24 and 20 ka involved different types ofmagma in both islands and produced explosive eruptions with quitedifferent energy. Ascent and eruption of shoshonitic to latitic magmas,

represented by Spiaggia Lunga scoriae and the BT, alternated withthose of intermediate-evolved magmas (Monte Guardia in southernLipari, Quadrara and Monte Lentia products in Vulcano). This testifiesfor a complex plumbing systembeneath a large area roughly located be-tween the central–northern part of Vulcano and southern Lipari. It isthen reasonable to suggest that the arrival of the BT-type mafic-to-intermediate magmas was the trigger (or one of the triggers) mecha-nism of the more evolved and viscous magmas.

With regard to the BT source area, we first remark that many geo-volcanological features of the IBT erupted around 24–22 ka at Vulcanoindicate a correlation with Il Piano caldera-filling pyroclastic deposits.In terms of composition and provenance, Lucchi et al. (2008) associatethese IBT to the M. Molineddo 3 formation (mo3 — Fig. 15) due to thedistinctive occurrence of scattered well vesiculated yellow to black sco-riae observed in some corresponding outcrops on Vulcano and southern

Fig. 10. IBT variation diagrams for selected major elements.

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Lipari. These authors identify the source of M. Molineddo deposits aswell as those of the coeval Grotta dei Pisani formation in the now disap-peared Cardo volcanic centre, which should have been located withinthe present La Fossa Caldera area. This hypothesis explains the originand attitudes of the various pyroclastic products that have beenemplaced in Il Piano area and the Primordial Vulcano flanks.

Based on the AMS data, the source area of the IBT units is locatedslightly to the northeast of the present morphologic depression corre-sponding to La Fossa Caldera (Fig. 14). However, the distribution ofthe sampling sites is far from uniform and most sites are locatedalong a line roughly oriented N–S. This geometry entails that the azi-muth of the fans' axes are not far from each other and the definition oftheir intersection is accordingly low. The situation is the same for theUBT, whose source area derived from AMS data has the same locationand a wider extent. The most recent morpho-structural data frommultibeam bathymetrical surveys (Lucchi et al., 2010) have allowedthe recognition of the submarine continuation of the north-easterncollapse rims of La Fossa Caldera, which is close to the source areaof BT indicated by the AMS data. Both methods therefore agree in

locating the BT vents in the northern sector of Vulcano island, witha reliable connection to the explosive eruptions from La Fossa Caldera.Nonetheless, it is not yet possible to give the precise location of thevarious individual eruptive vents.

Compositional data, although limited to major elements, providesome indications about the plumbing system evolution and behaviour.The evidence of restricted variation in the glass compositions of thelevels from IBT units (i.e. btl9, btl10, btl11 and Grotta dei Pisani,~22–24 ka) could indicate that magma batch(es) involved in thiseruptive period were relatively homogeneous and underwent fre-quent refilling and tapping processes, incurring only a slight modifica-tion of their composition. The whole composition of this magma couldbe assimilated to Spiaggia Lunga Scoriae (Fig. 9). Conversely, themagma batches involved in the BT eruptions later than the MonteGuardia rhyolitic phase (i.e. grt1, btl 12) reveal different, and probablymore complex processes that produced both trachy-andesitic andrhyolitic melts. Geochemical data and variations indicate that evolu-tionary processes drove the magmas to higher compositional degrees,due to two possible causes: a) shallow and individual magma batches

Fig. 12. Variation of Zr/Nb vs. SiO2 (a) and Nb (b) in rocks from Lipari (after Crisci et al., 1991), Vulcano (after De Astis, 1995), IBT and UBT (this paper).

Fig. 11. UBT variation diagrams for selected major elements.

34 A.M.P. Cicchino et al. / Journal of Volcanology and Geothermal Research 208 (2011) 23–38

Table 3Magnetic susceptibility and AMS data of the BT levels from Lipari and Vulcano. Symbols: n=number of specimens; km=bulk susceptibility, P=degree of anisotropy; k1=magneticlineation (D, I=declination, inclination; E1–2, E1–3=angles of 95% ellipse of confidence); k3=magnetic foliation pole (D, I=declination, inclination; E3–1, E3–2=angles of 95% ellipseof confidence).

Section Level n km(μSI)

P k1 k3

D I E1–2 E1–3 D I E3–2 E3–1

UBTLipari

LS1 L6 17 10180 1.008 285 7 38.2 21.5 167 76 30.3 14.5LS3 L12 14 10920 1.003 72 14 28.9 13.9 328 43 31.7 13.9LS4 L22 9 15050 1.007 350 10 53.0 10.2 125 76 18.1 4.1LS5 L27 13 9054 1.006 168 78 19.9 2.5 74 1 5.7 1.5

VulcanoVS7 TGRs1 11 15560 1.012 12 2 43.5 3.5 256 85 5.6 3.7VS8 TGRs2 11 16310 1.007 229 4 44.0 14.1 339 78 19.2 13.7VS9 TGRs3 16 15270 1.007 24 6 60.2 16.0 132 73 24.9 15.4VS11 TGRs4 12 14300 1.009 209 3 66.6 7.0 84 84 13.9 7.0VS12 TGRs5 20 14600 1.012 266 6 68.5 8.8 112 83 9.7 8.6VS13 TGRs6 11 14790 1.007 209 9 59.4 7.1 53 81 12.5 5.8

IBTLipari

LS1 L5 10 16620 1.013 112 1 27.7 16.1 335 89 24.5 12.5L4 11 17100 1.011 89 12 11.0 5.2 230 74 7.0 5.2L3 11 15180 1.010 227 53 15.5 13.9 119 13 54.6 15.0

LS3 L11 13 14550 1.010 224 10 26.1 9.7 338 66 25.7 10.6L10 6 9330 1.008 92 8 54.5 9.4 353 48 20.9 16.8L9b 14 12680 1.003 – – – – – – – –

LS4 L21 14 12970 1.005 350 14 49.9 13.4 231 62 63.6 12.8L20 12 10830 1.003 – – – – – – – –

L19 17 12720 1.002 103 18 58.0 30.3 215 50 41.5 30.6LS5 L26 20 18270 1.018 115 8 36.0 6.4 328 81 22.6 6.9

L25 14 20050 1.007 112 1 18.4 10.9 205 75 23.0 10.9L24 14 14270 1.004 117 2 21.6 18.9 324 88 31.6 20.1L23 7 13150 1.004 323 5 28.3 11.4 122 85 14.6 5.6

LS6 L30 3 14980 1.021 341 15 29.9 4.9 135 74 16.3 3.0L29b 8 13310 1.006 326 15 8.5 7.7 137 75 10.3 4.5L29a 13 14930 1.005 317 4 21.2 8.0 152 86 12.1 8.0L31 24 10710 1.005 341 4 15.6 6.1 74 43 56.3 7.3L32 7 14340 1.010 242 33 41.5 29.5 356 32 31.3 26.2

VulcanoVS10 V14 18 17630 1.016 107 1 68.3 9.0 248 89 10.6 8.1

V13 9 13240 1.020 246 29 44.7 28.4 94 58 33.9 58.4VS14 V16 15 18440 1.018 29 9 34.6 11.9 280 62 44.2 11.1

V15 16 12720 1.003 136 23 53.0 19.6 287 64 22.1 19.8VS15 V17 13 12000 1.024 11 5 44.3 8.2 238 83 13.1 7.5

V18 16 11720 1.043 38 5 15.2 8.9 197 85 10.4 2.9

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below the present La Fossa Caldera area, containing melts of rhyoliticliquids residual from early Monte Lentia activity (27.9–25 ka), werereactivated throughmafic-intermediatemagma arrival and originatedthe UBT explosive activities, enhanced by magma–water interaction;b) a single zonedmagma chamberwas spilled at depth, withmajor in-volvement of mafic-intermediate melts and limited trachy-rhyoliticones, with partial mingling-mixing processes. By contrast, the final

Fig. 13. Equal-area projection of AMS data of representative levels. Symbols: square, trianvalues with 95% ellipse of confidence; great circle=magnetic foliation.

UBT eruptions (grt2, grt3, grt4, btl14) are representative of the resid-ualmaficmagmas of the previous eruptions or the arrival of new, freshshoshonitic magma in the plumbing system.

Lastly, the correlations between the Lipari and Vulcano UBT se-quences derived from the paleomagnetic results provide a new in-sight in the recent Vulcano activity. The TGRi5 and TGRi9 levels,both around 20.5 ka, crop out in the western sector of Vulcano and

gle, dot=maximum, intermediate, minimum axis (k1, k2, k3); large symbols=mean

Fig. 14. Location of the IBT source area. Symbols: dot=sampling site; dashed line=absolute sense of flow from imbrication of magnetic foliation; bow-tie=95% confidence limit oflineation azimuth; stippled area=inferred source area.

36 A.M.P. Cicchino et al. / Journal of Volcanology and Geothermal Research 208 (2011) 23–38

are associated with the btl12 unit at Lipari (Fig. 15). They representthe early and strongest episode of UBT able to override the topo-graphic high of Mt. Saraceno (440–450 m at that time) and to expandup to the Lipari central sector. They could represent a high-energyopening of the UBT succession, with prevailing direction west andnorth. The following deposits – TGRi4, TGRi7, TGRi10 – crop out inthe western and central sector of Il Piano and would have beenemplaced during a younger eruptive episode, ~17 ka, whose cloudsmainly expanded to south and did not reach Lipari. The correlationbetween the youngest UBT deposits, grt4 at Vulcano and btl14 atLipari, suggests that the whole area of Il Piano was invaded by PDCsbetween ~7.7 ka and 5 ka, a time span that undoubtedly is in part

related to La Fossa Cone activity (Frazzetta et al., 1983; Dellino andLa Volpe, 1997). This correlation is the first evidence that PDCs gener-ated from recent Vulcano vents were able to reach the southern partof Lipari, with obvious implications for the volcanic risk related to thepresent-day activity.

Acknowledgements

We are grateful to M. Ort and an anonymous reviewer for manyvaluable comments. Research funded by the 192 INGV-DPC 2004–2006 project, sub-Project V3_5 — Vulcano (G. De Astis, M. Piochi).

Fig. 15. Correlation and chronology of the BT successions at Lipari and Vulcano. 14C ages refer to BT levels, isotopic ages to interbedded markers. All ages in ka, acronyms as in Fig. 2.The grey areas show the inferred correlation; the box shows the correspondence between levels and stratigraphic units.

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