Petrology of volcanic products younger than 42 ka on the Lipari–Vulcano complex (Aeolian Islands, Italy): an example of volcanism controlled by tectonics
Introduction
Volcanoes the activity of which is linked to regional tectonic structures may present particular difficulties in the reconstruction of their shallow magmatic systems, since active tectonics may strongly influence the formation, depth and size of the magma chambers, their refilling episodes and the triggering of eruptions independently from magmatic evolution. Silicic activity in these areas is generally characterised by small volume eruptions with linearly arranged vents (Bacon, 1985). An example of this type of activity can be found in the recent volcanism on Lipari and Vulcano (Aeolian Islands, Italy; Fig. 1, Fig. 2). The two islands are aligned with Salina along the regional fault Tindari–Letojanni, trending NNW–SSE (Fig. 3a,b). On Lipari, silicic dome activity from several vents close in space and presumably clustered in time occurred from 42 to 22 ka ago in the southern part of the island (Crisci et al., 1991); successively, rhyolitic lava flows and pyroclastic deposits emplaced in the northern sector. The lava domes are associated with pyroclastic products that in some cases show a wide compositional range indicating magma mixing processes (Crisci et al., 1981, De Rosa and Sheridan, 1983). The eruptive vents are aligned along two different directions (NNW–SSE, N–S), corresponding to the main structural trends active in the area (Mazzuoli et al., 1995). The volume of magma erupted in each eruptive phase is from 0.5 km3 to much lower. Silicic lava domes emplaced around 13 ka (Soligo et al., 1999) on the northern sector of Vulcano, were preceded between 28 and 25 ka by latitic–trachytic effusive and explosive eruptions. The composition of these products is very similar to that of the products erupted on Lipari between 42 and 22 ka and the volume of magma erupted is comparable. Differently from Lipari, at Vulcano intense shoshonitic–latitic volcanism occurred in the last 50 ka, with sporadic eruptions of basalts and trachytes.
Several detailed petrological studies exist on the products of the Aeolian Arc (e.g. Ellam et al., 1989, Ellam and Harmon, 1990, Francalanci et al., 1993) and on the single Lipari or Vulcano islands (Crisci et al., 1991, Esperança et al., 1992, De Astis et al., 1997a, Del Moro et al., 1998, De Astis et al., 2000). Recent works evidenced that, from a structural point of view, the two islands are parts of one volcanic complex (Mazzuoli et al., 1995, Ventura et al., 1999). As already suggested by Crisci et al. (1983), we retain that the volcanic activity of the two islands after 42 ka should be investigated considering southern Lipari and northern Vulcano as one volcanic complex. To this end we present in this paper new geochemical and petrological data, concerning trace elements and Sr, Nd and Pb isotopes, on the rhyolitic and latitic products erupted on Lipari and Vulcano in the last 42 ka. The work was mainly directed to the identification of the different eruptive phases, the assessment of geochemical differences among them and the reconstruction of the processes leading to the final composition of the erupted products. Finally, we relate the geochemical features to the main tectonic trends of the region, and make inferences on the tectonic control on magma storage and ascent in the crust.
Section snippets
Geological background
The Quaternary Aeolian island arc (southern Italy) is located at the southern border of the Tyrrhenian Basin (Fig. 3). Magmatism in this area was first interpreted in the frame of the subduction of the Ionian domain beneath the Calabrian Arc (Barberi et al., 1973, Keller, 1980, Ellam et al., 1989, Ellam and Harmon, 1990), on the basis of the geochemical features and of the presence of a NW-dipping Benioff zone under the arc (Gasparini et al., 1982). More recently, the seismicity has been
Analytical methods
Samples of lava flows, domes and of the juvenile fraction of pyroclastic deposits were selected. Enclaves were carefully isolated. In the pyroclastic deposits, white and grey pumice lumps were crushed separately, and macroscopically banded clasts were avoided.
Modal analysis was carried out by point counting using an optical microscope of 1–3 thin sections for each sample rock.
X-ray fluorescence (XRF) analysis for major and trace elements were performed on pyroclastic deposits at the DST,
Petrography and mineral chemistry
The volcanic products erupted on Lipari during the last 42 ka show higher K2O contents than the older Lipari ones (Table 1). In the classification diagram of Peccerillo and Taylor (1976), these rocks vary from latite to rhyolite (Fig. 5). The rhyolites are the dominant products in this period, and occur both as lava domes and flows and pyroclastic deposits. The majority of the intermediate products, representing juvenile clasts in pyroclastic deposits, fall in the high-K dacite field. The most
Major and trace elements
Major and trace elements variation diagrams are shown in Fig. 8a,b. In general, Na2O and K2O present rather flat trends from the mafic to the felsic rocks. Negative trends can be observed for MgO, Fe2O3(t), CaO, TiO2, P2O5 (not shown), Al2O3; between 56 and 66 wt% of SiO2, the samples show variable Al2O3, MgO and CaO contents, reflecting their porphyritic nature and variable plagioclase/clinopyroxene abundance. Among the trace elements, Rb and Nb increase with increasing SiO2, while La remains
Rhyolites
The cycles VII and VIII-W rhyolites show the same mineral assemblage and composition, and similar phenocryst abundance (<5 vol%). The composition of the K-feldspar and oligoclase phenocrysts was used to calculate the temperature of equilibrium with the equations of Fuhrman and Lindsley (1988), using the software SOLVCALC (Wen and Nekvasil, 1994). Few feldspar pairs were suitable for application of the geothermometer, because most rhyolitic products show evidence of overheating, due to mixing of
Concluding remarks
Around 40–50 ka, an important pulse of the tectonic activity linked to the dextral Tindari–Letojanni lithospheric strike-slip structure occurred, generating intense volcanism in the area comprising southern Lipari and northern Vulcano. Although the volcanism on Lipari was characterised by emission of rhyolitic products and on Vulcano by the eruption of prevalently mafic magmas, in the last 42 ka the products show geochemical variations with time, which could be correlated between the two
Acknowledgements
The authors gratefully acknowledge C. Cristiani for carrying out the Pb isotopic analyses and M. Bertoli for atomic absorption spectrometry and LOI determinations. M. D’Orazio is thanked for the technical assistance during the ICP–MS trace element determinations, M. Menichini for the XRF analysis, and F. Colarieti for the preparation of polished rock sections. The authors are grateful to Michelle Coombs and Gianfilippo De Astis for their constructive and critical reviews that improved the
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