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

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

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Rock magnetism and compositional investigation of Brown Tuffs deposits at Lipari and 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, Italy ALP — Alpine Laboratory of Paleomagnetism, Peveragno, Italy Istituto Nazionale di Geofisica e Vulcanologia, Sezione di Roma 1, Italy d Dipartimento di Scienze della Terra e Geologico-Ambientali, Università di Bologna, Italy b c

a r t i c l e

i n f o

Article history: Received 15 February 2011 Accepted 23 August 2011 Available online 8 September 2011

a b s t r a c t A rock-magnetic investigation was carried out on the nonwelded ash deposits of the Brown Tuffs (Aeolian Islands, southern Tyrrhenian Sea) to improve the stratigraphic correlation between the deposits cropping out on Lipari and Vulcano islands and locate their source area. The study was supplemented by petrographical and geochemical analyses on selected strata, with the intent to compare the Brown Tuffs to other rocks emplaced at Vulcano in the same time span. More than 30 levels were sampled in the intermediate (56 ± 4 ka N IBT N 21–22 ka) and upper (21–22 ka N UBT) parts of the Brown Tuffs sequences on the two islands. Their characteristic remanent magnetization (ChRM) directions were derived from stepwise thermal demagnetization, and the magnetic fabric from measurements of the anisotropy of magnetic susceptibility. The levels with indistinguishable ChRM directions were regarded as coeval and to form an individual stratigraphic unit. The units were referred to the Brown Tuffs sequence of Lucchi et al. (2008) on the grounds of their emplacement age, provided by comparison of their mean paleomagnetic direction with the paleosecular variation curves of the southern Tyrrhenian region, as well as the field constraints. The closer correlation between the sequences of Lipari and Vulcano contributes to a better understanding of the volcanic activity that produced the Brown Tuffs, and shows that most of the IBT and the oldest UBT levels were emplaced in a short time span, between ≈24 and 20–17 ka. The magnetic fabric is typically well developed, but at most sites the magnetic foliation is very close to horizontal and no imbrication is defined. The source area of the Brown Tuffs parent pyroclastic flows, as constrained from the intersection of the magnetic lineations, falls in the northeastern part of La Fossa Caldera structure. Although limited to major elements, compositional data provide further indication about the parent plumbing system and its behaviour. Magma batch(es) involved in the IBT eruptions have homogeneous features and underwent frequent refilling and tapping processes. Conversely, those involved in the early UBT eruptions are compositionally more variable. This suggests more complex evolution and plumbing system activity: the UBT eruptions represent either residual mafic magmas from the previous eruptions or the arrival of new, fresh shoshonitic magma in the system. © 2011 Elsevier B.V. All rights reserved.

1. Introduction The Brown Tuffs (BT) are nonwelded ash deposits emplaced on most of the Aeolian archipelago by recurrent, large-scale hydromagmatic eruptions that occurred during the last 80 kyr. The large volume and the wide geographic and time distribution of the deposits make

⁎ Corresponding author at: Dipartimento di Scienze della Terra, Via Valperga Caluso 35, 10125 Torino, Italy. E-mail address: [email protected] (E. Zanella). 1 Now at Department of Earth Sciences, Durham University, UK. 2 Now at SEAconsulting, Torino, Italy. 3 Now at Department of Earth and Planetary Sciences, Rutgers University, Piscataway, USA. 0377-0273/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.jvolgeores.2011.08.007

their study important to both a deeper understanding of the recent volcanic activity in the islands and a better assessment of the volcanic hazard. 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 Ischia tephra (56 ± 4 ka) and Monte Guardia pyroclastics (22–21 ka). The lithological features of BT are rather homogeneous, regardless of age and location. Fine subdivision of the main successions can only be made upon minor interbedded markers, such as characteristic volcanic products, paleosols and erosive unconformities. However, not all such markers are evenly distributed on all islands, and in places they are missing. Their stratigraphic correlation is therefore difficult and chronology of the eruptive cycles that produced the various BT levels not 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 lacking, even if stratigraphic and volcanological data suggest that the BT most likely erupted from the Vulcano area presently occupied by La Fossa Caldera (De Astis et al., 1997; Lucchi et al., 2010). The present paper reports on rock magnetic, petrographic and geochemical investigations carried out on the IBT and UBT at Lipari and Vulcano (Fig. 1). Rock magnetism was designed to integrate the stratigraphic constraints of the BT, and provide an estimate of their emplacement ages and vents location. Petrographic and geochemical analyses were performed on selected levels, with the dual aim to compare the features of the magma generating the BT deposits with coeval magmas erupted on Vulcano island, and strengthen their paleomagnetic correlation. 2. Geological setting The Aeolian volcanism is younger than 1.3–1 Ma, and responsible for submarine and subaerial activities that formed several seamounts and seven large stratovolcanoes rising from sea floor (1000–2000 m b.s.l.) to heights up to 900–1000 m a.s.l. (Favalli et al., 2005). The volcanic activity was mainly effusive, or explosive with low to medium energy. Chronostratigraphic data, however, indicate that some high energy explosive activity occurred in more recent periods of the Aeolian volcanic history. These events involved intermediate to evolved

magmas from shallow reservoirs, e.g. on Salina island at 24 ka (the Pollara 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. Its volcanic activity was characterised by recurrent medium to high energy explosive eruptions, at times associated with sector collapses (Pasquarè et al., 1993). A process of this type has been proposed for the emplacement of the Tufi di Grotte dei Rossi deposits (TGR — De Astis et al., 1997). Such deposits consist of massive, poorly coherent to coherent, brownreddish ash beds. The Brown Tuffs (BT) exposed on Lipari, Filicudi and Salina islands show lithological features very similar to those of the TGR (Keller, 1967, 1980a, 1980b; Crisci et al., 1983; Manetti et al, 1988; Losito, 1989). A systematic investigation of BT features, and the possible correlation of their outcrops on a regional scale were accordingly addressed in a paper by Lucchi et al. (2008). The BT ash deposits are widespread throughout the Aeolian archipelago and as far as to Capo Milazzo in Sicily. They were generated by major eruptions from the central–southern sector of the archipelago. A process of magma–water interaction largely controlled magma fragmentation, as well as dispersal and emplacement of the ash deposits (Lucchi et al., 2008). Based on thickness and grain-size variations of pyroclastic beds exposed along the Vulcano–Lipari–Salina axis, IBT and UBT seem to have originated from a source area within

Fig. 1. Geological sketch map of Lipari and Vulcano (simplified after Lucchi et al., 2008). Dot = sampled section.

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or close to the La Fossa Caldera multi-stage structure, in the northern part of Vulcano island. Rock components, grain-size, morphology and compositional data show that IBT and UBT deposits form overlapped beds that may be related to the propagation of multiple, dilute and turbulent pyroclastic density currents (PDCs) (De Astis et al., 1997). IBT and UBT depositional units are associated with thin layers of fine ash, especially in distal outcrops. Such layers are interpreted as the result of fallout from low-concentration, co-ignimbritic ash clouds and their associated eruption columns (Lucchi et al., 2008). Lithological, textural and structural features together concur to prove the above interpretation and are summarised below. The BT deposits consist of millimetric glass shards (65–90%) and variable, but subordinate amounts of crystals (18–30%) and lithic grains, sometimes in the lapilli class and rarely exceeding 10%. Poorlyvesiculated 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 few decimetres to several metres thick in the paleo-valley. Interbedding of thin ash layers, with tractional structures such as planar to low-angle cross lamination and thin layers of scoriaceous lapilli, is common; indurated laminae of fine ash are also documented. Due to distinctive colour and/or slight grain-size variations, few of the massive beds show internal banding. Others beds display small amounts of randomly disseminated scoriaceous lapilli, and mm-sized, euhedral clinopyroxene crystals. Lapilli and pyroxene crystals are either co-aligned parallel to bedding, or concentrated in pockets. The basal portion of these massive beds can be enriched in scoriaceous lapilli or lithic grains embedded from underlying layers for erosional action on the substrate. Locally, carbonised wood fragments are found within BT layers. They indicate emplacement temperatures higher than 300–350 °C (Jones and Chaloner, 1991). Compositionally, the IBT range from basaltic trachy-andesite to trachyte and the UBT from basaltic trachy-andesite to rhyolite. The UBT beds, however, locally have homogeneous composition, with the intermediate, trachy-andesitic type more common. The mineral assemblage of the BT comprises euhedral clinopyroxene (dominant), plagioclase, olivine and K-feldspar (De Astis et al., 1997). The composition of the juvenile glass shards is another peculiar characteristic and supports the correlations established among the different units. 3. Sampling Samples have been collected in the sections reported in Lucchi et al. (2008) and De Astis et al. (1997), when possible at different stratigraphic heights within each individual layer. Sampling suffered two main difficulties: 1) some sections either have disappeared or are no longer accessible because of the many buildings and facilities due to tourism development; 2) some layers are too thin or loose to be sampled; this is unfortunately the case for the two main stratigraphic markers, the Ischia tephra and the Monte Guardia pyroclastics. The BT deposits are commonly loose, thus collection of oriented samples in the field, as well as their subsequent laboratory preparation as specimens with regular shape and standard dimensions, was quite difficult. Different techniques were devised in the course of the various sampling campaigns according to the rock cohesion: (1) Coherent: cores were drilled using a battery powered driller and oriented in-situ; (2) Poorly coherent: a large rectangular block (≈10 × 15 × 20 cm) was shaped using trowel and paddle, smoothed and oriented on its front side and then gently taken off from the outcrop. In the laboratory, the block was bathed in ethyl-silicate, left to consolidate and finally cored; (3) Loose: a block was shaped and oriented as above, yet it was left in-situ, sprayed with abundant ethyl-silicate and collected more than 2 weeks later, once consolidated enough to proceed with coring in the laboratory.

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Twenty-two layers were sampled in the UBT and IBT at Lipari and 12 at Vulcano for a total of more than 450 standard cylindrical specimens (Φ = 25 mm, h = 23 mm). Within the text, the BT layers we sampled are referred to as “levels”, whereas “units” refers to the stratigraphic 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 volcanic region 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 magnetic field in a given region changes with time around the direction of the geocentric axial dipole (GAD) and the same paleomagnetic direction may occur at different times and be recorded within deposits emplaced during distinct eruptions. Such limitation appears important, yet it may become negligible when paleomagnetic data are constrained by further information such as field stratigraphy, isotopic dating, etc. The soundness of chronological data derived from paleomagnetism therefore depends on the whole of available information. A time resolution as high as 100 years may be achieved, for instance, when paleomagnetism is applied to historical lava flows, because written sources and archaeological findings contribute to draw detailed reference curves (Speranza et al., 2006; Tanguy et al., 2007). Paleomagnetic directions of Pleistocene volcanic rocks dating back to a few hundred ka are compared to paleosecular variation (PSV) curves mainly derived from lake sediments. The PSV curves are far less accurate than archaeomagnetic ones because the locking time of remanence in sediments smooths the record of the Earth's field direction (Butler, 1992). Yet they represent the variation trend of the Earth's field and, supplemented by stratigraphic information, may be used to infer the age of volcanic rocks. 4.1. Measurements Measurements were carried out at the ALP laboratory (Peveragno, Italy). The natural remanent magnetization (NRM) was measured using spinner magnetometers AGICO JR-5 and JR-6. Stepwise thermal demagnetization was carried out using a Schonstedt furnace, in order to isolate the NRM's components. The NRM intensity is on the order of 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 within 500–560 °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 in the literature to be the main, and in many cases the only, ferromagnetic mineral within BT (Losito, 1989; Zanella et al., 1999; Cicchino, 2007). Thermal demagnetization diagrams were interpreted using PaleoMac program (Cogné, 2003). Besides a small viscous component usually erased within 100–150 °C, the NRM consists of two components (Fig. 3): (1) A low-T component, stable up to ≈400 °C. This is the main NRM fraction, which we regard as the characteristic remanent magnetization (ChRM) acquired by BT during cooling following deposition. The ChRM direction is consistent with the PSV range around the GAD direction at the Aeolian Islands (D = 0°, I = 59°); (2) A small high-T component (N400 °C) characterised by varying directions. This might be related to magmatic grains with high 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 clearly isolated in some specimens, they overlap in others. Accordingly,

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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).

individual specimens are characterised either by a stable end-points direction or a best-fitting great circle (McFadden and McElhinny, 1988) and data derived by both methods are used to calculate the mean ChRM direction of individual levels (Fig. 4; Table 1). 4.2. Results The paleomagnetic data are analysed and interpreted in three steps: (1) ChRM analysis. The ChRM directions of individual levels are compared within the UBT and IBT sequences. In cases in which they are statistically indistinguishable, the corresponding levels are regarded as emplaced by the same eruption and thus belonging to the same stratigraphic unit. The unit mean direction is calculated as the mean of the ChRM directions of all the specimens of the unit. (2) Paleomagnetic interpretation. The directions of the units are relocated via pole (Noel and Batt, 1990) to the same reference site (Viterbo, λ = 42° 27′ N, ϕ = 12° 02′ E) in order to compare them to the paleomagnetic directions of known age from Aeolian volcanoes (Lanza and Zanella, 2003) and the PSV curves from the Mezzano (back to 30 ka) and Lago Grande di Monticchio (N30 ka) sediment cores (Brandt et al., 1999). (3) Stratigraphic interpretation. The chronological information from point (2), field constraints and available isotopic ages are used to infer the stratigraphic position of the various levels within the units of Lucchi et al. (2008) BT sequence. In the following sections, the analysis and discussion of the results follow the Brown Tuffs main division in UBT and IBT. The stratigraphic constraints and isotopic ages (Lucchi et al. 2008, and references therein)

are discussed in the text. Substantial paleomagnetic data and chronological information are summarised in Tables 1, 2 and Figs. 5 to 8. 4.2.1. Upper Brown Tuffs (UBT) — Lipari and Vulcano The UBT deposits at Lipari overlie the Monte Guardia marker (gu, 20–22 ka — Fig. 2) and are in turn overlaid by the Monte Pilato pyroclastics (mp, 1.4 ka). At Vulcano, mp overlie gu in some sections, and either the Quadrara or Spiaggia Lunga scoriae of similar age (sq, 21 ± 3 ka and sl, 24 ± 5 ka, respectively) in others; they are overlain by the Punte Nere pyroclastics (pnl, 5.5 ± 1.3 ka). On the grounds of these and other minor markers, Lucchi et al. (2008) defined three stratigraphical units at Lipari – btl14, btl13 and btl12 – and four at Vulcano — grt4, grt3, grt2 and grt1 (Fig. 2). Four levels were sampled at Lipari (acronym L). Here, field relations did not allow the assessment of their stratigraphic position within the UBT succession. At Vulcano, seven levels (TGRi) were sampled below the Cugni di Molinello scoriae (cm — Fig. 2), and thus in the grt1 unit, and four (TGRs) above, within the grt4, grt3 and grt2 units that could not be differentiated in the sampled outcrops. The ChRM directions of the L and TGRs levels gather into three distinct groups (Tables 1, 2 and Fig. 5): (1) Group 1: L12+ L27+ TGRs3. The mean direction of this group is characterised by a low inclination value and a westward declination and only consistent with the PSV curves at about 15 ka (Fig. 6). This result suggests that the L12, L27 levels belong to the btl13 unit, whose 14C age is 16.8 ±2 ka, and the TGRs3 level belongs to the grt2 unit, which lies below the Monte Saraceno marker (sal, 8.3 ka — Fig. 2). (2) Group 2: L6 + L22 + TGRs6. The mean direction of the second group 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

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

be discarded because TGRs6 is younger than grt1 unit (dated around 17–20 ka, as discussed below). Levels L6 and L22 are therefore younger than those of Group 1 and may be referred to the btl14 unit, whose stratigraphic position above the Gabellotto marker (vg — Fig. 2) points to an age younger than 8.6 ± 1.5 ka. Level TGRs6 thus corresponds to grt4, the youngest unit at Vulcano that overlies the same Gabellotto marker and has been dated at 7.7 ± 0.1 ka. (3) Group 3: TGRs4 + TGRs5. The levels of the third group are ascribed to the same grt4 unit, because their mean direction is consistent to the PSV curves at about 4.5–5.5 ka (Fig. 6). This direction is distinct from that of TGRs6 and close to that of the Punte Nere lava (pnl, 5.5 ± 1.3 ka) (Fig. 5). Occurrence of distinct paleomagnetic directions within unit grt4 suggests that the level TGRs6 corresponds to the blt14 level in Lipari. Conversely, levels TGRs4 and TGRs5 are younger and without corresponding UBT

deposits on the Lipari island, because such deposits are either missing or have not been sampled/recognised. As for the levels sampled in the grt1 unit at Vulcano (TGRi), the mean paleomagnetic direction of TGRi5 and TGRi9 (Table 2) is consistent with the PSV curves at around 20.5 and 17.5–18 ka (Fig. 6) and that of TGRi4, TGRi7 and TGRi10 at around 17.5–16.5 ka. The grt1 unit overlies the Monte Guardia marker and is further constrained to be older than 15 ka, which is the age of the grt2 unit, as discussed above. There are thus two hypotheses to account for the age of the levels included within grt1: (1) Should the age of TGRi5 and TGRi9 be around 20.5 ka, these levels would be coeval with btl12 unit at Lipari, whose 14C age is 20.5 ka. The TGRi deposits would have been emplaced during two distinct eruptive episodes: an older one, around 20 ka – TGRi5, TGRi9 – which would have also interested Lipari

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

(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 have been emplaced during an eruptive cycle that occurred over a time span on the order of 1 kyr.

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

Lastly, no correlation to the PSV curve is possible for levels TGRi1 and TGRi8, because the very high inclination of their paleomagnetic direction (I ≈ 65°, Table 1), does not match any part of the PSV curve.

The mean directions of the first two groups are close to each other, implying their emplacement in a short time span; consequently, the levels of the second group should belong to the btl10 unit and be older than those of the first group. The directions of levels L4 and L11 are different from those of Groups 1 and 2, and similar to that of L21. These three layers can be regarded as coeval and older than the others, and thus to belong to the btl9 unit. Comparison with the PSV curves (Fig. 8) shows that the paleomagnetic directions of Groups 1 and 2 are consistent with the PSV curve at 21–22 ka, even though their inclination value is a little higher. However, it is worth noting that their directions are similar to that of the 22.0 ± 4.0 ka GelsoPetrulla lava at Vulcano (D= 5°, I = 60°, α95 = 2.9° — Lanza and Zanella, 2003). The mean direction of Group 3 is in turn close to the PSV curve at around 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 btl9 substantiate their 14C ages showing that the three units were emplaced during a time span no greater than 2–3 kyr. Three levels (L3, L24, L31) were sampled in unit btl8, comprised between 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 between 52 and 56 ka on the PSV curves (Fig. 8). Below Punta del Perciato and 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

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, the Ischia tephra which separates the IBT from the LBT, was not recognised in the sections sampled at Lipari. Therefore, the BT levels stratigraphically below the Punta del Perciato marker (pe2, 40 kab age b 56 ka — Fig. 2) and above the Grey Porri Tuffs (l1, 67–70 ka), a widespread marker within 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 interbedded within the IBT sequence at Vulcano. Since the Ischia tephra is missing, the IBT cannot be distinguished from the LBT. 4.2.2.1. Lipari. Lucchi et al. (2008) defined three units in between the Monte 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, respectively. Two levels (L5, L26) were sampled in the btl11 deposits, five (L4, 11, 25, 29a, 29b) in the undifferentiated deposits of units btl10 and btl9, and a last one (L21) in a section where the Lower Pollara Tuffs marker is missing and which can potentially belong to

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

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Table 1 Mean ChRM directions of BT levels from Lipari and Vulcano. Symbols: n = number of specimens, D, I = declination, inclination; k, α95 = precision, semi-angle of confidence of Fisher's (1953) statistics. Section UBT Lipari LS1 LS3 LS4 LS5 Vulcano VS8 VS9

VS11 VS12 VS13 VS12 VS16 VS17 VS18 IBT Lipari LS1

LS3 LS4

LS5

LS6

Vulcano VS10 VS14 VS15

α95

Level

n

D

I

k

L6 L12 L22 L27

9 10 9 9

14.5 336.1 11.3 346.6

57.5 32.1 57.6 30.7

235 77 168 73

3.4 5.8 4.1 6.1

TGRi8 TGRs3 TGRi9 TGRi10 TGRs4 TGRs5 TGRs6 TGRi7c TGRi1 TGRi4 TGRi5

19 16 6 19 15 23 12 10 10 15 11

354.0 350.8 12.6 6.8 14.9 17.2 12.5 1.9 2.3 355.2 10.1

64.9 32.0 54.8 46.8 43.4 44.9 60.0 43.0 61.3 43.6 50.3

276 132 66 55 82 128 122 88 // 146 256

2.1 3.2 9.8 4.7 4.2 2.7 4.0 5.6 7.4 3.2 3.0

L5 L4 L3 L11 L21 L20 L19 L26 L25 L24 L23 L29a L29b L31 L32

12 14 11 9 14 11 16 19 10 17 7 8 6 11 6

4.4 353.8 6.8 359.4 350.5 18.7 4.1 357.0 2.5 358.2 354.1 351.2 6.2 356.2 358.5

53.9 48.3 52.6 53.8 51.8 56.2 56.9 56.8 57.1 58.6 58.3 60.5 63.3 54.1 60.7

145 59 162 52 43 67 92 288 85 220 67 53 51 95 69

3.8 5.3 3.8 7.8 6.1 5.7 3.9 2.0 5.5 2.5 7.8 10.4 10.1 4.8 8.3

V14 V13 V16 V15 V18 V17

13 7 11 10 7 10

346.6 346.2 354.1 333.3 328.7 351.3

52.9 69.0 57.1 56.4 64.4 59.6

62 105 180 140 238 169

5.3 6.9 3.4 4.1 4.1 3.6

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

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

Unit

D

I

α95

UBT TGRs4,TGRs5 L6, L22, TGRs6 L12, L27, TGRs3 TGRi5,TGRi9 TGRi4,TGRi7,TGRi10

Upper grt4 btl14 grt4 btl13 grt2 grt1 grt1

16.3 12.9 345.8 10.1 0.9

44.4 58.5 32.3 52.0 45.6

2.3 2.1 2.9 3.2 2.6

IBT L5, L26 V14,16,17 L25, L29a, L29b L4, L11, L21 L3, L24, L31 L19, L23, L32

btl11 Grotta dei Pisani btl10 btl9 btl8 btl7

356.9 350.6 0.2 353.3 358.7 359.6

56.8 57.5 59.8 50.5 56.1 58.3

1.4 3.8 4.3 3.4 2.1 3.1

Fig. 5. Equal-area projection of UBT paleomagnetic directions with α95 ellipse of confidence. (a) level mean directions; (b) dots = unit mean directions; other symbols = paleomagnetic directions of Aeolian volcanic rocks younger than Monte Guardia marker (Lanza and Zanella, 2003).

to that of the levels sampled in btl8 unit, and consistent with an age between 52 and 56 ka. As the age of the Ischia tephra is 56 ± 4 ka, it is reasonable to assume that they correspond to the btl7 unit. 4.2.2.2. Vulcano. Each of the three sections sampled in the neighbourhood of Monte Saraceno consists of two distinct levels stratigraphically 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 statistically indistinguishable paleomagnetic directions (Fig. 7, Table 1). Furthermore, their mean direction is indistinguishable from that of the btl11 unit at Lipari and close to that of btl10 (Table 2). The 14C ages of these two 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 the uncertainty range of the Scoriae di Spiaggia Lunga age (24 ± 5 ka), whose chronological relations with the Lower Pollara Tuffs can not be defined. Paleomagnetic data point to an age of around 22 ka for the emplacement of levels V14, V16 and V17, which would be coeval with both btl11 and btl10 units of Lipari. Such an attribution implies that the actual age of the Scoriae di Spiaggia Lunga falls in the younger part of the uncertainty range of their isotopic age. The paleomagnetic directions of the levels V13, V15 and V18, lowermost in their sections, do not match any part of the PSV curves. These levels can be assigned to IBT on the grounds of their stratigraphic position, but their age cannot be better defined using paleomagnetic data. 5. Compositional data Major element analyses of glasses and crystals were obtained from 80 mm-thick polished thin sections using a JEOL JXA-8200 WD-ED combined microprobe at the INGV (Istituto Nazionale di Geofisica e Vulcanologia — Rome, Italy). The following operating conditions for Wavelength Dispersive Spectroscopy (WDS) analysis were adopted: 10 nA probe current, 15 kV accelerating voltage. A probe diameter of 5 μm, with a final spot size of about 7 μm, was used for the analysis on glass, in order to limit the degree of alkali volatilization during probing. Counting time was 10 s on peak and 5 s on background and data reduction was carried out using a ZAF correction method

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Fig. 7. Equal-area projection of IBT paleomagnetic directions with α95 ellipse of confidence. (a) Lipari and (b) Vulcano level mean directions; (c) dots= unit mean directions; other symbols= paleomagnetic directions of Aeolian volcanic rocks emplaced at about 22–25 ka (Lanza and Zanella, 2003).

Fig. 6. Paleomagnetic directions of UBT units plotted against Lago di Mezzano (Brandt et al., 1999) and Aeolian volcanoes (Lanza and Zanella, 2003) PSV curves. The two stippled stripes encompasse the UBT age interval. The grey boxes show possible correlation between UBT paleomagnetic directions (vertical line segments correspond to the relocated unit mean D and I) and the PSV curves. Symbols: dot = paleomagnetic directions from Aeolian islands; vertical/horizontal bar = age error/ΔD, ΔI.

included in the JEOL microprobe software package. Relative standard error 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 with errors reaching also N30%. The initial petrographic study of the BT ashes provided an estimate of the components, which mostly include juvenile glass fragments (~65 up to 90% in volume) and crystals (5–35%); lithic grains are present in low amounts (≤5%) as small fragments of various porphyritic lavas, commonly altered. The UBT at Vulcano have higher content of crystals 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), the glass shards are generally blocky and aphyric, subordinately vesicular and micro-crystalline. Their surfaces are commonly slightly altered and colours vary from colourless to dark brown. Data reported by Lucchi et al. (2008) indicate that the BT glass composition ranges from basaltic trachy-andesite to rhyolite, with a cloud of analytic points plotting in the trachy-andesite and trachy-dacite fields of the TAS diagram. These data also reveal compositional differences between IBT and UBT units: i) IBT glass shards have a fairly homogeneous trachy-andesitic composition, with SiO2 content below 62 wt.% and

alkali content ranging from 4.5 wt.% to more than 13 wt.% (although the latter is limited to just a few samples); ii) when compared to those of the IBT, major element compositions of the UBT glass shards from both Lipari and Vulcano cluster over a common but more widespread compositional range, with an almost continuous variation from 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 concentrated in the trachy-andesite field and SiO2 content below 59 wt.%. Conversely, the UBT samples are mainly concentrated among the trachyandesite, trachy-dacite and trachyte fields, with a few in the rhyolitic field. Since the finer fraction of TGRi ash (b0.125 mm) is almost entirely composed of glass fragments (De Astis, 1995), XRF whole rock analyses performed on samples coming from various TGRi beds are also reported in the UBT diagram (Fig. 9). TGRi data are characterised by a decrease in the alkali content for similar SiO2 values, which can be reliably attributed to leaching effects on very small fragments. The IBT glass compositions display a limited but comparable range of major elements in the variation diagrams (Fig. 10). No well defined trend can be visualised with increasing silica content; however, rough negative trends for MgO, CaO, FeO, and to some extent for K2O are observed disregarding the V14 shards. Both TAS and variation diagrams (Fig. 11) show that the TGRi deposits related to the early UBT eruptions are characterised by a wide range of silica and other major elements content. In contrast, the samples from younger Vulcano UBT beds – TGRs6 and TGRs4 – display less evolved compositions and seem to mirror the features of a rather homogeneous

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6. Magnetic fabric

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. The two stippled stripes encompasse the IBT age interval. Symbols as in Fig. 6.

batch of magma. If compared with shards from earlier UBT with similar SiO2 content, they show lower content of MgO, CaO and higher content of K2O and P2O5. As a whole, the UBT samples display decreasing trends of the major elements (MgO, CaO, FeO, P2O5) vs. SiO2, that are typically interpreted as due to mafic mineral fractionation; K2O and Al2O3 trends are consistent with this interpretation and with late feldspars fractionation. It is worth noting that the paleomagnetic direction of TGRs6 level has been related to that of the Lipari L6 level (Section 4.2.1), whereas the compositions of the two levels are different (Figs. 9, 11). This discrepancy is likely due to the fact that the same direction of the Earth's field may recur at different times (Section 4). The crystals occurring within the BT ashes as loose particles are commonly rimmed by glass and mainly represented by mm-sized clinopyroxene phenocrysts of fairly homogeneous salitic composition and minor percentages of plagioclase (An = 48–52% in the UBT, and An = 57–78% in the IBT), K-feldspar, olivine, Fe–Ti oxides and amphibole. Olivines found in the IBT levels have Fo% = 61–68, whereas those from UBT have Fo% = 70–73. Amphiboles are found only in the IBT while they are absent in the UBT. Their occurrence is thus a distinctive characteristic of older BT and suggests magmatic systems with different features for the generations of the different BT groups. The limited trace element data allow some speculation about the magma source. The immobile High Field Strength Elements (HFSE) contents and their ratios, within both IBT and UBT deposits, are comparable to those of the Vulcano products (De Astis, 1995) erupted in the time span between 30 and 6 ka (Fig. 12) and quite different from those of the Lipari magmas (Crisci et al., 1991). They point to the Vulcano system as the BT magma source and discard the possibility of a genetic relation with Lipari.

Anisotropy of magnetic susceptibility (AMS) is a standard tool to detect the flow-induced fabric in PDCs. The fabric is characterised by preferential alignment of elongated grains, which tend to lie within the bedding plane, imbricated up-flow toward the source area, and to be aligned parallel to the flow direction (Tarling and Hrouda, 1993). When Ti-magnetite is the main primary ferromagnetic mineral, the maximum magnetic susceptibility occurs along the longest direction of the grains and their preferential alignment results in the magnetic lineation, which lies within the magnetic foliation plane. The foliation plunges upstream and the lineation corresponds to the flow direction. Since the PDCs proceed radiating from the vent, the source area may be located by finding the intersection of the magnetic lineations and/or the foliation plunge from individual sites (Hillhouse and Wells, 1991; LaBerge et al., 2009). This method ignores the possible deviating effects of the uneven topographical surface the PDC travels over, and should therefore be based on sufficient mutually consistent data. The magnetic susceptibility of all specimens and its anisotropy were measured before remanence analyses using a KLY-3 bridge. Bulk susceptibility of most levels shows limited variations between 9,000 and 20,000 μSI (Table 3) and no systematic difference occurs between UBT and IBT, whose mean values are 13,603 and 14,239 μSI respectively. The degree of anisotropy (the ratio between the maximum and minimum susceptibility values, P = k1/k3) varies between 1.002 and 1.024, with an exceptional value of 1.043 observed in level V18. Susceptibility, therefore, varies little within the BT deposits, as typical for their lithological features. Notwithstanding the low P values, the magnetic foliation is typically well defined, as shown by the limited within-level dispersion of its poles, which correspond to the anisotropy minimum axes k3. The maximum k1 and intermediate k2 axes are either well grouped or more or less dispersed within the foliation plane (Fig. 13a, b). The magnetic lineation of a site, which is given by the mean of the k1 axes, is accordingly defined with variable precision from level to level. In a few cases the principal axes are dispersed and the fabric is chaotic (Fig. 13c). At most sites, the magnetic foliation is close to horizontal and the 95% ellipse of confidence of its pole encompasses the vertical (Fig. 13). No plunge, and thus no imbrication, can be significantly defined and the magnetic foliation cannot be used to infer the flow direction except in a few favourable cases. We instead chiefly rely on triangulation of the magnetic lineations, which is based on the assumption that the flow directions should converge toward the source area from which the PDCs radiated. Since lineation is bidirectional, at each site one can draw two opposite fans, which contain the flow direction. They are centred on the lineation azimuth and their spread equals the 95% confidence angle E1,2 (Table 3). In the case of IBT, the imbrication of the foliation plane is significantly defined at two sites and, since it plunges upstream, it gives the absolute sense of flow (Fig. 14) and roughly indicates the source region. This information assists in determining which of the two opposite fans at other sites points toward the source. The various fans intersect each other and envelope a region where the maximum number of intersection occurs (Fig. 14), which may therefore be assumed as source area of the PDCs that laid down the IBT deposits. The source area derived from the geometry of the magnetic fabric of UBT roughly coincides with that of IBT. Its extent is larger, mainly because of the few data from Lipari and the wide spread of the fans from Vulcano. 7. Discussion and conclusions The paleomagnetic correlation of the BT sequences of Lipari and Vulcano is summarised in Fig. 15 together with the 14C isotopic ages available in the literature (Lucchi et al., 2008). The upper levels of the IBT sequence of Vulcano are related to the youngest units of the

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

Lipari IBT. Within the UBT sequence, a correspondence between the stratigraphic units of the two islands is found, with the only exception being the gtr3 unit at Vulcano, which has no corresponding unit at Lipari. Besides a closer definition of the stratigraphic correlation, the main outcome is the evidence for the uneven time distribution of the volcanic activity that produced the BT deposits. Major pyroclastic eruptions emplaced most of the IBT and part of the UBT between ≈24 and 20–17 ka, and were preceded by a long quiescence of the BT-type explosive activity, which started around 56–52 ka and was possibly interrupted 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 Vulcano in the time span between 24 and 20 ka involved different types of magma in both islands and produced explosive eruptions with quite different energy. Ascent and eruption of shoshonitic to latitic magmas,

represented by Spiaggia Lunga scoriae and the BT, alternated with those of intermediate-evolved magmas (Monte Guardia in southern Lipari, Quadrara and Monte Lentia products in Vulcano). This testifies for a complex plumbing system beneath a large area roughly located between the central–northern part of Vulcano and southern Lipari. It is then reasonable to suggest that the arrival of the BT-type mafic-tointermediate magmas was the trigger (or one of the triggers) mechanism of the more evolved and viscous magmas. With regard to the BT source area, we first remark that many geovolcanological features of the IBT erupted around 24–22 ka at Vulcano indicate a correlation with Il Piano caldera-filling pyroclastic deposits. In terms of composition and provenance, Lucchi et al. (2008) associate these IBT to the M. Molineddo 3 formation (mo3 — Fig. 15) due to the distinctive occurrence of scattered well vesiculated yellow to black scoriae observed in some corresponding outcrops on Vulcano and southern

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

Lipari. These authors identify the source of M. Molineddo deposits as well as those of the coeval Grotta dei Pisani formation in the now disappeared Cardo volcanic centre, which should have been located within the present La Fossa Caldera area. This hypothesis explains the origin and attitudes of the various pyroclastic products that have been emplaced in Il Piano area and the Primordial Vulcano flanks. Based on the AMS data, the source area of the IBT units is located slightly to the northeast of the present morphologic depression corresponding to La Fossa Caldera (Fig. 14). However, the distribution of the sampling sites is far from uniform and most sites are located along a line roughly oriented N–S. This geometry entails that the azimuth of the fans' axes are not far from each other and the definition of their intersection is accordingly low. The situation is the same for the UBT, whose source area derived from AMS data has the same location and a wider extent. The most recent morpho-structural data from multibeam bathymetrical surveys (Lucchi et al., 2010) have allowed the recognition of the submarine continuation of the north-eastern collapse rims of La Fossa Caldera, which is close to the source area of BT indicated by the AMS data. Both methods therefore agree in

locating the BT vents in the northern sector of Vulcano island, with a reliable connection to the explosive eruptions from La Fossa Caldera. Nonetheless, it is not yet possible to give the precise location of the various individual eruptive vents. Compositional data, although limited to major elements, provide some indications about the plumbing system evolution and behaviour. The evidence of restricted variation in the glass compositions of the levels from IBT units (i.e. btl9, btl10, btl11 and Grotta dei Pisani, ~22–24 ka) could indicate that magma batch(es) involved in this eruptive period were relatively homogeneous and underwent frequent refilling and tapping processes, incurring only a slight modification of their composition. The whole composition of this magma could be assimilated to Spiaggia Lunga Scoriae (Fig. 9). Conversely, the magma batches involved in the BT eruptions later than the Monte Guardia rhyolitic phase (i.e. grt1, btl 12) reveal different, and probably more complex processes that produced both trachy-andesitic and rhyolitic melts. Geochemical data and variations indicate that evolutionary processes drove the magmas to higher compositional degrees, due to two possible causes: a) shallow and individual magma batches

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

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).

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Table 3 Magnetic 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 = magnetic lineation (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% ellipse of confidence). Section

UBT Lipari LS1 LS3 LS4 LS5 Vulcano VS7 VS8 VS9 VS11 VS12 VS13 IBT Lipari LS1

LS3

LS4

LS5

LS6

Vulcano VS10 VS14 VS15

Level

n

km (μSI)

P

k1

k3

D

I

E1–2

E1–3

D

I

E3–2

E3–1

L6 L12 L22 L27

17 14 9 13

10180 10920 15050 9054

1.008 1.003 1.007 1.006

285 72 350 168

7 14 10 78

38.2 28.9 53.0 19.9

21.5 13.9 10.2 2.5

167 328 125 74

76 43 76 1

30.3 31.7 18.1 5.7

14.5 13.9 4.1 1.5

TGRs1 TGRs2 TGRs3 TGRs4 TGRs5 TGRs6

11 11 16 12 20 11

15560 16310 15270 14300 14600 14790

1.012 1.007 1.007 1.009 1.012 1.007

12 229 24 209 266 209

2 4 6 3 6 9

43.5 44.0 60.2 66.6 68.5 59.4

3.5 14.1 16.0 7.0 8.8 7.1

256 339 132 84 112 53

85 78 73 84 83 81

5.6 19.2 24.9 13.9 9.7 12.5

3.7 13.7 15.4 7.0 8.6 5.8

L5 L4 L3 L11 L10 L9b L21 L20 L19 L26 L25 L24 L23 L30 L29b L29a L31 L32

10 11 11 13 6 14 14 12 17 20 14 14 7 3 8 13 24 7

16620 17100 15180 14550 9330 12680 12970 10830 12720 18270 20050 14270 13150 14980 13310 14930 10710 14340

1.013 1.011 1.010 1.010 1.008 1.003 1.005 1.003 1.002 1.018 1.007 1.004 1.004 1.021 1.006 1.005 1.005 1.010

112 89 227 224 92 – 350 – 103 115 112 117 323 341 326 317 341 242

1 12 53 10 8 – 14 – 18 8 1 2 5 15 15 4 4 33

27.7 11.0 15.5 26.1 54.5 – 49.9 – 58.0 36.0 18.4 21.6 28.3 29.9 8.5 21.2 15.6 41.5

16.1 5.2 13.9 9.7 9.4 – 13.4 – 30.3 6.4 10.9 18.9 11.4 4.9 7.7 8.0 6.1 29.5

335 230 119 338 353 – 231 – 215 328 205 324 122 135 137 152 74 356

89 74 13 66 48 – 62 – 50 81 75 88 85 74 75 86 43 32

24.5 7.0 54.6 25.7 20.9 – 63.6 – 41.5 22.6 23.0 31.6 14.6 16.3 10.3 12.1 56.3 31.3

12.5 5.2 15.0 10.6 16.8 – 12.8 – 30.6 6.9 10.9 20.1 5.6 3.0 4.5 8.0 7.3 26.2

V14 V13 V16 V15 V17 V18

18 9 15 16 13 16

17630 13240 18440 12720 12000 11720

1.016 1.020 1.018 1.003 1.024 1.043

107 246 29 136 11 38

1 29 9 23 5 5

68.3 44.7 34.6 53.0 44.3 15.2

9.0 28.4 11.9 19.6 8.2 8.9

248 94 280 287 238 197

89 58 62 64 83 85

10.6 33.9 44.2 22.1 13.1 10.4

8.1 58.4 11.1 19.8 7.5 2.9

below the present La Fossa Caldera area, containing melts of rhyolitic liquids residual from early Monte Lentia activity (27.9–25 ka), were reactivated through mafic-intermediate magma arrival and originated the UBT explosive activities, enhanced by magma–water interaction; b) a single zoned magma chamber was spilled at depth, with major involvement of mafic-intermediate melts and limited trachy-rhyolitic ones, with partial mingling-mixing processes. By contrast, the final

UBT eruptions (grt2, grt3, grt4, btl14) are representative of the residual mafic magmas of the previous eruptions or the arrival of new, fresh shoshonitic magma in the plumbing system. Lastly, the correlations between the Lipari and Vulcano UBT sequences derived from the paleomagnetic results provide a new insight in the recent Vulcano activity. The TGRi5 and TGRi9 levels, both around 20.5 ka, crop out in the western sector of Vulcano and

Fig. 13. Equal-area projection of AMS data of representative levels. Symbols: square, triangle, dot = maximum, intermediate, minimum axis (k1, k2, k3); large symbols = mean values with 95% ellipse of confidence; great circle = magnetic foliation.

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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 of lineation azimuth; stippled area = inferred source area.

are associated with the btl12 unit at Lipari (Fig. 15). They represent the early and strongest episode of UBT able to override the topographic high of Mt. Saraceno (440–450 m at that time) and to expand up to the Lipari central sector. They could represent a high-energy opening of the UBT succession, with prevailing direction west and north. The following deposits – TGRi4, TGRi7, TGRi10 – crop out in the western and central sector of Il Piano and would have been emplaced during a younger eruptive episode, ~17 ka, whose clouds mainly expanded to south and did not reach Lipari. The correlation between the youngest UBT deposits, grt4 at Vulcano and btl14 at Lipari, suggests that the whole area of Il Piano was invaded by PDCs between ~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 and La Volpe, 1997). This correlation is the first evidence that PDCs generated from recent Vulcano vents were able to reach the southern part of Lipari, with obvious implications for the volcanic risk related to the present-day activity.

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

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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.

References Brandt, U., Nowaczyk, N.R., Ramrath, A., Brauer, A., Mingram, J., Wulf, S., Negendank, J.F.W., 1999. Palaeomagnetism of Holocene and Late Pleistocene sediments from Lago di Mezzano and Lago Grande di Monticchio (Italy): initial results. Quaternary Science Reviews 18, 961–976. Butler, R.F., 1992. Paleomagnetism: Magnetic Domains to Geologic Terranes. Blackwell Sci, Malden, Mass, p. 238. Calanchi, N., De Rosa, R., Mazzuoli, R., Rossi, P.L., Santacroce, R., Ventura, G., 1993. Silicic magma entering a basaltic magma chamber: eruptive dynamics and magma mixing — an example from Salina (Aeolian Islands, southern Tyrrhenian Sea). Bulletin of Volcanology 55, 504–522. Cicchino, A., 2007. Studio paleomagnetico dei depositi di Brown Tuff dell'isola di Lipari. M.Sc thesis, University of Torino, pp 1–144. Cogné, J.P., 2003. PaleoMac: a Macintosh™ application for treating paleomagnetic data and making plate reconstructions. Geochemistry Geophysics Geosystems 4, 1007. doi:10.1029/2001GC000227. Cortese, M., Frazzetta, G., La Volpe, L., 1986. Volcanic history of Lipari (Aeolian Islands, Italy) during the last 10,000 years. Journal of Volcanology and Geothermal Research 27, 117–133. Crisci, G.M., De Rosa, R., Lanzafame, G., Mazzuoli, R., Sheridan, M.F., Zuffa, G.G., 1981. Monte Guardia Sequence: a late-Pleistocene Eruptive Cycle on Lipari (Italy). Bulletin of Volcanology 44, 241–255. Crisci, G.M., Delibrias, G., De Rosa, R., Mazzuoli, R., Sheridan, M.F., 1983. Age and petrology of the Late-Pleistocene Brown Tuffs on Lipari, Italy. Bulletin of Volcanology 46, 381–391. Crisci, G.M., De Rosa, R., Esperanca, S., Mazzuoli, R., Sonnino, M., 1991. Temporal evolution of a three component system: the Island of Lipari (Aeolian arc, southern Italy). Bulletin of Volcanology 53, 207–221. De Astis, G., 1995. Evoluzione vulcanologica e magmatologica dell'isola di Vulcano (Isole Eolie). Unpublished Ph.D. Thesis, University of Bari, pp. 1–308. De Astis, G., Dellino, P., De Rosa, R., La Volpe, L., 1997. Eruptive and emplacement mechanism of widespread fine-grained pyroclastic deposits on Vulcano island (Italy). Bulletin of Volcanology 59, 87–102. Dellino, P., La Volpe, L., 1997. Stratigrafia, dinamiche eruttive e deposizionali, scenario eruttivo e valutazioni di pericolosità a La Fossa di Vulcano. In: La Volpe, L., Dellino, P., Nuccio, M., Privitera, E., Sbrana, A. (Eds.), “Progetto Vulcano”: Risultati dell'attività di Ricerca 1993–1995. CNR - Gruppo Nazionale per la Vulcanologia, Felici Editore Pisa, pp. 214–237.

De Rosa, R., Frazzetta, G., La Volpe, L., 1992. An approach for investigating the depositional mechanism of fine-grained surge deposits. The example of the dry surge deposits at “La Fossa di Vulcano”. Journal of Volcanology and Geothermal Research 43, 215–233. De Rosa, R., Scarciglia, F., Donato, P., Lucchi, F., Tranne, C.A., 2005. Insights on the genesis of Brown Tuffs (Lipari island, Aeolian Archipelago). GeoItalia 2005, Spoleto, 21–23 Settembre. Favalli, M., Karàtson, D., Mazzuoli, R., Pareschi, M.T., Ventura, G., 2005. Volcanic geomorphology and tectonics of the Aeolian archipelago (Southern Italy) based on integrated DEM data. Bulletin of Volcanology 68, 157–170. doi:10.1007/s00445-005-0429-3. Fisher, R.A., 1953. Dispersion on a sphere. Proceedings of the Royal Society London 217, 295–305. Frazzetta, G., La Volpe, L., Sheridan, M.F., 1983. Evolution of the Fossa cone, Vulcano. In: Sheridan, M.F., Barberi, F. (Eds.), Explosive Volcanism. Journal of Volcanology and Geothermal Research 17, 329–360. Halls, H.C., 1976. A least-squares method to find a remanence direction from converging remagnetization circles. Geophysical Journal of the Royal astronomical Society 45, 297–304. Halls, H.C., 1978. The use of converging remagnetization circles in paleomagnetism. Physics of the Earth and Planetary Interiors 16, 1–11. Hillhouse, J.W., Wells, R.E., 1991. Magnetic fabric, flow directions, and source area of the Lower Miocene Peach Spring Tuff in Arizona, California, and Nevada. Journal of Geophysical Research 96, 12443–12460. Jones, T.P., Chaloner, W.G., 1991. Fossil charcoal, its recognition and palaeoatmospheric significance. Global and Planetary Change 5, 39–50. Keller, J., 1967. Alter und Abfolge der vulcanischen Ereignisse auf den Äolischen Inseln. Bericht Naturforschenden Gesellschaft Freiburg 57, 33–67. Keller, J., 1980a. The Island of Salina. Rendiconti della Società Italiana di Mineralogia e Petrografia 36, 489–524. Keller, J., 1980b. The Island of Vulcano. Rendiconti della Società Italiana di Mineralogia e Petrografia 36, 369–414. LaBerge, R.D., Porreca, M., Mattei, M., Giordano, G., Cas, R.A.F., 2009. Meandering flow of a pyroclastic density current documented by the anisotropy of magnetic susceptibility (AMS) in the quartz latite ignimbrite of the Pleistocene Monte Cimino volcanic centre (central Italy). Tectonophysics 644, 64–78. doi:10.1016/j.tecto.2008.09.009. Lanza, R., Zanella, E., 2003. Paleomagnetic secular variation at Vulcano (Aeolian Islands) during the last 135 kyr. Earth and Planetary Science Letters 213, 321–336. Losito, R., 1989. Stratigrafia, caratteri deposizionali e aree sorgente dei Tufi bruni delle isole Eolie. Dissertation, University of Bari.

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Lucchi, F., Tranne, C.A., De Astis, G., Keller, J., Losito, R., Morche, W., 2008. Stratigraphy and significance of Brown Tuffs of the Aeolian Islands (southern Italy). Journal of Volcanology and Geothermal Research 117, 49–70. doi:10.1016/j.jvoplgeores.2007.11.006. Lucchi, F., De Astis, G., Tranne, C.A., Romagnoli, C., Braga, R., Casalbore, D., Chiocci, F.L., Bosman, A., Forni, F., Rossi, P.L., 2010. Polycyclic formation of the La Fossa Caldera by merging volcanological and morpho-structural features from subaerial and submerged portions of Vulcano Island (Aeolian archipelago, Italy). 3rd workshop on collapse calderas, La Réunion Island, October 3–9, 2010, abstract volume, pp. 100–101. Manetti, P., Pasquaré, G., Tibaldi, A., Tsegaye, A., 1988. Geology of the island of Filicudi (Aeolian arc): Bollettino GNV, IV, pp. 368–382. McFadden, P.L., McElhinny, M.W., 1988. The combined analysis of remagnetization circles and direct observations in palaeomagnetism. Earth and Planetary Science Letters 87, 161–172. Noel, M., Batt, C.M., 1990. A method for correcting geographically separated remanence directions for the purpose of archaeomagnetic dating. Geophysical Journal International 102, 753–756. Pasquarè, G., Francalanci, L., Gardunom, V.H., Tibaldi, A., 1993. Structure and geologic evolution of the Stromboli Vulcano, Aeolian Islands, Italy. Acta Vulcanologica 3, 79–89.

Speranza, F., Branca, S., Coltelli, M., D'Ajello Caracciolo, F., Vigliotti, L., 2006. How accurate is “paleomagnetic dating”? New evidence from historical lavas from Mount Etna. Journal of Geophysical Research 111, B12S33. doi:10.1029/2006JB004496. Tanguy, J.C., Condomines, M., LeGoff, M., Chillemi, V., La Delfa, S., Patanè, G., 2007. Mount Etna eruptions of the last 2,750 years: revised chronology and location through archeomagnetic and 226Ra–230Th dating. Bulletin of Volcanology. doi:10.1007/s00445-007-0121-x. Tarling, H.D., Hrouda, F., 1993. The Magnetic Anisotropy of Rocks. Chapman and Hall, London, pp. 1–232. Zanella, E., De Astis, G., Dellino, P., Lanza, R., La Volpe, L., 1999. Magnetic fabric and remanent magnetization of pyroclastic surge deposits from Vulcano (Aeolian Islands, Italy). Journal of Volcanology and Geothermal Research 93, 217–236. Zanella, E., Cicchino, A., Lanza, R., 2011. Composite detrital and thermal remanent magnetization in tuffs from Aeolian Islands (southern Tyrrhenian Sea) revealed by magnetic anisotropy. International Journal of Earth Sciences. doi:10.1007/ s00531-011-0651-5. Zijderveld, J.D.A., 1967. AC demagnetization of rocks: analysis of results. In: Runcorn, S.K., Creer, K.M., Collinson, D.W. (Eds.), Methods in Paleomagnetism. Elsevier, Amsterdam, pp. 254–286.