This paper summarizes the presently available geological and geophysical data on the Tyrrhenian area to give an up-to-date geodynamic setting for the peculiar volcanism of this region. In the Tyrrhenian two small oceanic basins (Vavilov and Marsili) are separated from the Sardinia hinterland to the west by a well developed passive margin, while along the eastern margin they are bound by the Appeninic-Maghrebian orogenic belt. The rifting processes started in Late Tortonian times at the Sardinian margin and propagated eastward during Pliocene and Pleistocene with the formation, respectively, of the Vavilov and Marsili basins and the corresponding eastward migration of the Calabrian Arc away from the Sardinia hinterland. Volcanic activity in the Tyrrhenian area appears consistent with this pattern of evolution. Subduction-related volcanism took place in Sardinia in the Oligocene and Early Miocene, in the Aeolian Arc and in the Roman Magmatic Province during the Quaternary and, probably, during Pliocene times along a submerged arc located between the Vavilov and the Marsili basins. Extension-related volcanism occurred during the main phases of basin formation at the rear of the arc. The geodynamic mechanism for the evolution of the Tyrrhenian which better account for all these features is represented by the passive subduction of the ronian lithosphere. According to this model the migration of the arc and the related back-arc extension is driven by the gravitational sinking of the downgoing slab which was initiated at the site of a former orogenic belt.
Marine investigations (electroacoustic and high resolution seismic profiles), carried out on the submerged portions of the Panarea and Stromboli volcanic complexes, have enabled a reconstruction of the main morphologic and structural features of the volcanic edificies. Limited petrochemical data give some information on composition of eruptive products. The data demonstrate a number of features of the Late Pleistocene-Holocene volcanism, volcano-tectonism and eustatic effects. Both complexes show a preferential development along NE-SW lineaments, which coincides with the regional structural trend of this sector of the Aeolian structure. This is apparent from the morphology of the complexes and from the distribution of numerous parasitic and flanking submarine eruptive centers. Faulting, caldera collapse and tectonic tilting were interrelated and fundamentally influenced by activation of the NE-SW fractures. Some NE-SW faults are active at Dresent.
The geologic evolution of Stromboli volcano is recorded in its subaerial part for a time span from about 100,000 years B.P. to the present strombolian activity of the Recent Sciara volcano. Four major periods, Paleostromboli, Vancori cycles, Neostromboli and Recent Stromboli. have been recognized and further subdivided into 30 volcanostratigraphic, geochemically controlled mapping units. Paleostromboli (PST) I, II and III are in temporal sequence and represent the main units of the oldest period. They crop out to the southern and eastern part of the island and are constituted by lavas and pyroclastic rocks (mainly fall, flow and lahar). Lavas predominate over pyroclastites during the Vancori cycle, forming the summit of the volcano and flowing along the NE and E flanks. Neostromboli period is characterized by an effusive activity whose lavas form the western and northwestern sectors of the island. Recent Stromboli rocks are mainly erupted by the present-day strombolian activity filling the Sciara del Fuoco depression. Pizzo Sopra la Fossa pvroclastites are part of the older Sciara activity- S- Bartolo lavas also belong to the period of the Recent Stromboli activity although differing sharply in their chemical composition from the Sciara products. The volcanologically defined periods of Stromboli generally have constant and distinct petrochemical characteristics, which vary from calc-alkaline (CA) to leucite-bearing (Lc-) shoshonitic or Potassic (KS) through high-K calc-alkaline (HKCA) and shoshonitic (SHO), showing a general trend of K- enrichment with time. In detail, however, this trend is not regular and displays several fluctuations: PST I and PST III rocks belong to the HKCA series whereas PST II volcanics are calcalkaline; Vancori cycles form a shoshonitic association and Recent Stromboli magmas have again HKCA to SHO compositions, after the more potassic Lc-shoshonites of Neostromboli. Strombolicchio neck is distinctly calcalkaline and is related to a pre-Stromboli volcanic center formed about 200,000 years B.P.
K-Ar age determinations of the major volcanic units of Stromboli cover the age range from <100 ka and 85 ka until present. The entire evolution of the Stromboli cone occurred within the last 100,000 years. The isolated neck of Strombolicchio has given a distinctly older age of 204±25 ka and belongs to a pre-Stromboli stage. It is obvious from the age data that activity in the Upper Quaternary is almost continuous without repose times longer than about 10,000-20,000 years. Particularly well constrained is the age of the calc-alkaline interlude in the Paleostromboli (PST) cycle. This PST II stage is dated by three K-Ar determinations to about 60 ka (±5) which confirm an earlier Th/U dating of 61.5 ka. The shoshonitic volcanism of the three Vancori stages, Lower, Middle and Upper Vancori, is dated between 26 and 13 ka B.P. Upper Vancori is followed by the leucite-shoshonites of the Neostromboli with ages of 13 and 5 ka B.P. With time there is a general trend of increasing potassic nature of the lavas. Radiometric dating provides the time scale for the evolution in distinct magmatological series from HKCA in the early Paleostromboli (PST I) to strictly calc-alkaline in PST II, again HKCA in the upper Paleostromboli, shoshonitic in the Vancori series and finally to the potassic leucite-bearing shoshonites of Neostromboli. The change and alternation between different magma series occurred thus in geologically very short terms, suggesting the independent origin of calc-alkaline and shoshonitic magma series. The present shoshonitic activity of the Sciara center started less than 5,000 years B.P.
The island of Stromboli together with Strombolicchio and another probable
off-shore vent, represents the summit of a large composite volcano. The
Stromboli volcano was affected by several collapses which took place in
two main stages: an older one characterized by several pyroclastic eruptions
related to a flank collapse toward SE and three concentric caldera collapses-
a younger stage characterized by two sector collapses, one flank collapse
towards NW and predominant lava effusions. The youngest flank eollapse
formed Ihe Sciara del Fuoco. Dlking, fissure eruptions and parasitic vents
are concentrated in a NE swarm located in the axial part on the island.
Statistically, fractures and some rare small faults strike in the same
predominant NE angular sector with a main NW dip, even if some disperslon
Is present. Dike chronology shows that the NE swarm is stable in the course
of time. In the NW side of the island youngest rock failures and dikes
show a horseshoe arrangement following the amphitheatre of the oldest NW
sector collapse. It is here suggested that diking and fracturing of the
volcano is influenced by expansion towards NW along NE trending regional
structure and/or by lower buttress forces caused by the NW deepening of
the sea floor. Slope instability of the upper part of
the volcano is in turn influeneed by magma injection along the NE striking
planes. Diking is also locally linked to pre-existing amphitheatre collapse
depressions.
In this paper we present the first results of a survey conducted in 1991 on the offshore continuation of Sciara del Fuoco, the steep trough located on the central-western side of Stromboli Bathymetric and seismoacoustic profilmg enabled us to obtain a first detailed reconstruction of this submerged area. A brief description of the submarine setting is given, together with a first interpretations in terms of processes. Particularly, a collapsed sector as extended as the subaerial Sciara del Fuoco has been observed in the submarine portion, prolonging in an active canyon and faced by a wide accumulation area. This latter appeared clearly related to the Sciara ( and probably previous) sliding events. This new setting maybe contribute to better understand the morphologie evolubon of Stromboli and other volcanoes; some similarities with the evolution of other volcanic edifices in terms of slope-instability processes have been as well evidenced.
Microprobe analyses have been carried out on 13 samples pertaining to the different magmatic series of the Stromboli volcano. Olivines (olv) have contents in forsterite component ranging from 58 to 86, and generally display small zonings. The FeO/MgO partition coefficient (Kolv/liq) of 0.3-0.4 is larger than the value reported for anhydrous conditions and is interpreted as depending on the high P PH20 Of these magmas. The clinopyroxene (cpx) composition falls at the salite-augite boundary of the Wo-En-Fs diagram for potassic series (KS) and in the augite field for the other series. A few diopsidic compositions have also been found. The higher Al content of KS-cpx´s is explained as due to the higher crystallization temperature and to the lower SiO2 content of the KS magma. On the basis of MgO/FeO ratios, a higher PH20 is suggested for the host magma of the cpx erupted as lapilli by the present-day activity, in respect to the host magma of the cpx of the STR90 lava belonging to the same activity. The opaques are mainly Ti-magnetite, with an ulvospinel (usp) content of 22-55%, and show compositional variations mainly due to the processes of ion substitutions and to the different compositions of the host magmas. Plagioclases (plg) display contents in anorthite component between 90 and 45, as well as variable zonings. Based on the compositional variations observed in the plg, cpx and olv of a same rock, different evolutionary processes of the magmas have been recognized. They are mainly represented by simple fractional crystallization in solid/liquid disequilibrium conditions, even if mixing events between variously evolved magmas have been also recognized. Moreover, the complex zonings found in the lapilli-cpx´s clearly testify the variable physico-chemical conditions of the magma erupted by the present-day strombolian activity.
Silicate-melt and CO2 inclusions in phenocrysts from the products of shoshonitic and calc-alkaline magmas erupted from Stromboli volcano (Italy), have been analyzed in order to characterize the physico-chemical conditions of the host magmas. Petrographic and microthermometric investigations have been performed together with SEM-EDS and electron microprobe analyses. Primary and secondary (pseudosecondary?) silicate-melt inclusions were found in olivine, clinopyroxene and plagioclase and are the result of trapping a homogeneous silicate melt. Rare CO2 inclusions were found in minerals only from calc-alkaline rocks. Based on the preliminary microthermometric data on CO2 and silicate-melt inclusions in calc-alkaline mineral phases, the trapping pressure is estimated to be from 0.7 and 2.0 kbar with a clinopyroxene crystallization temperature in basaltic rocks between 1150 and 1250°C. Microprobe analyses on the silicate-melt inclusions in shoshonitic rocks show compositional variations between the inclusions hosted by olivine and clinopyroxene phenocrysts as well as between the inclusions in the core and the rim of the host phase. Pearce element ratio diagrams have been used to distinguish the phases involved in pre- and post-trapping processes. Such features suggest different evolutionary processes operating in the Stromboli magmatic environment for shoshonites, including simple fractional crystallization and single or multiple episodes of mixing with more basic silicate melts.
A petrological study on the Stromboli volcano is presented, together with a general review and discussion of the different theories published on the subject. The Stromboli volcano is characterized by a mildly explosive strombolian activity, which has been persistent since historical time. The composition of the products is variable, ranging from calc-alkaline (CA), to high-K calc-alkaline (HKCA) and shoshonitic (SHO), up to potassic (KS). The different products were repeatedly erupted, with a general increase of potassium with time. All the subaerial volcanics were extruded during the last 100,000 a. CA rocks are mainly basaltic-andesites, which show the highest MgO, FeO. Ni, Cr, Co, Sc contents and Nd isotope ratios and the lowest incompatible element abundances and Sr isotope ratios found at Stromboli. HKCA and SHO series consist of basic to intermediate rocks which display good correlations of major and several trace elements against silica, and poorly variable Sr and Nd isotope ratios. KS rocks are basic in composition and show the highest incompatible element contents and Sr isotope ratios and the lowest 143Nd/144Nd and 206Pb/ 204Pb ratios. Petrological and geochemical data suggest that the HKCA and SHO series were generated by simple fractional crystallization processes, starting from mafic parental magmas. Instead, KS magmas underwent moderate deerees of evolution. which appears to be dominated by selective assimilation of upper crustal material by various batches of potassic magmas (Assimilation plus Equilibrium Crvstallization, AEC). Mixing processes between SHO latites and basic KS liquids and between high-K andesites and mafic CA magmas are additional processes which also plaved a role in the evolution of the Stromboli magmatism. The occurrence of mafic magmas with different petrological and geochemical characteristics. erupted from a single volcano over a relativeiy short time span, is the most important feature of Stromboli. Such a complexity can be accounted for by two hypotheses, one envisaging a genesis of mafic magmas from a heterogeneous. vertically zoned mantle source: the other is that mafic magmas with different enrichments in potassium and incompatible elements were generated by repeated cycles of RTFA (Refillin_ Tapping Fractionation Assimilation)processes, occurring in a deep-sited magma chamber. starting from a primary, mantle-derived CA liquid. The inferences of these hypotheses on the volcanological evolution of the Stromboli volcano are discussed.
A review is given of geophysical investigations that have been carried out on the island of Stromboli, based on gravity, magnetism, magnetotellurics, self-potential measurements, tectonic- and volcanic seismology. Different models which are proposed with the explanation of the structure and eruptive dynamics of the volcano are discussed. The results so far collected point out both some peculiar features of this volcanic system and at the same time the problems involved in the understanding of the eruptive mechanism for a "simple" volcano like Stromboli.
The explosive activity of strombolian type, as a particular eruptive phase of a number of volcanoes, is recognized after Stromboli as persistently mild and cyclically occuring during long lasting time intervals. In order to quantify the cyclic behavior by its representative parameters, the explosivity at the summit crater has been evaluated by means of image analysis precedures and its seismic equivalent recorded by the local network. Photographic sequences of the explosive events are processed and the kinetic energy content computed by determining the mass flux of the ejecta and their velocity. The corresponding explosion quakes mark the associated seismic energy. The number of explosion quakes and seismic energy release both follow a cyclic pattern over long time series as driven by steady conditions at a moderate level of activity. Therefore, a correlation with the periodicities of the locally recorded earth and ocean tides is intended to infer the modulating effects of external forces over the system. Detailed analysis of the seismic record is expected to focus the basic characters by which the explosive mechanism hypotheses could be implied, also in accordance with the prospects from the experiments that are presently carried out and are going to be performed more extensively.
Stromboli volcano is characterized by a peculiar state of permanent activity consisting of mild intermittent explosions and continuous gas streaming. The quiete character of the normal activity is broken from time to time by eruptive crises either represented by lava emission or by more violent explosions. Over the last three centuries the largest explosions (paroxysms) have periodically threatened the island´s population claiming collectively a few dozens of lives and causing some damages to the settled areas. A critical re-examination of reports and documents of the last three centuries of activity of the volcano was undertaken with the aim of assessing the types of hazard, evaluating the areas potentially exposed to the eruptive phenomena and estimating their probability of occurrence. Lava flow emissions from central craters occur at an average rate of one event every 3.7 years; the lava flow hazard is considered to be restricted to the depression of Sciara del Fuoco as neither historical nor geological evidence exists of recent lava flow outside the Sciara. The largest explosions have been arbitrarily classified into two categories: major explosions and paroxysms. The former have the potential of affecting only the summit of the island (average occurrence of 2,1 events per year), whereas the latter can also hit the settled areas (average occurrence of one event every 5-15 years). Fallout of large blocks and incandescent bombs up to about 1.5 km from craters is the main hazard caused by major explosions. In comparison paroxysms are much more violent involving a larger volume of ejected material and a broader spectrum of phenomena. The two villages of Stromboli and Ginostra are situated in the seafest parts of the island; nevertheless they can be affected during paroxysms, by the fallout of bombs and blocks, by tephra accumulation, the formation of hot avalanches and small tsunamis. In this century the main damages occurred during the paroxysms of 1919 and 1930.
Investigations were carried out on the CO2 soil degassing in some areas of Mt Etna. Data collected from 1987 to 1989 indicate that tectonic lineaments mainly contribute to anomalous soil emanations, although the volcanic ones are probably the responsible for the high background values. The isotopic compositions (d13C(CO2) = -2 ÷ -3 %o) suggest that the CO2 is mainly released by magma. Repeated measurements on the central part of the eastern flank of Etna show that the anomalies are connected to active faults and are subject to large variations both in space and in time. This behaviour seems related to the volcanic activity, thus making this area very interesting as a site for systematic measurements of the CO2 soil emanations in order to monitor the volcanic and seismic phenomena.
Fluid inclusions were studied in quartz and feldspar crystals of granitoid xenoliths in recent (<10 ka) lavas and ignimbrites at Pantelleria to constrain the fluid phases evolution and also try to evaluate the geothermal potential of the island. Four main types of aqueous fluid inclusions are distinguished: Type I, three-phase (L+S+V), halite-bearing inclusions with ThL = 450 - 620°C and salinity = 35 - 76 equiv. wt % NaCl; Type II, biphase (V + L), vapour-rich aqueous inclusions with Thv = 378 - >600°C and salinity = 0.2 -18 equiv. wt % NaCl; Type III, biphase (L + V) aqueous inclusions with ThL = 314 - 20°C; salinity = 2 -19 equiv. wt % NaCl, which occur in association with Type I and II and sometimes as isolated inclusions; Type IV, biphase, silicate melt inclusions. Early brines (Type I) and vapour-rich (Type II) inclusions are interpreted to be the result of fluid boiling at T > 600°C and P > 750 bars, alternatively they are postulated to be magmatic fluids resulting directly, through an aqueous phase separation, from the continuous crystallization of a subvolcanic intrusive. The inclusions are indicative of "hydrostatic" pressure regimes. Trapping of boiling fluids is recorded down to about 350°C and 200 bars, with salinity up to about 40 equiv. wt % NaCl. Type III inclusions may represent the original homogeneous magmatic fluid which underwent boiling at a shallow depth of emplacement of these intrusive rocks, that were later erupted as xenoliths. Comparison with previous studies shows that the thermobaric evolution of the fluids in the inclusions remained stable in the age range 5O ka to < 10 ka (with the assumption that the xenoliths,or at least the trapped fluids, are coeval with the host lavas) suggesting that present temperatures at shallow depths may be close to measured values in the granitoid xenoliths (< 350°C). As far as the geothermal potential of Pantelleria island is concerned, the studied fluid inclusions indicate the presence of fluids circulating in the system at depths of 2800-750 m (assuming a lithostatic pressure regime of a magma column with density of 2.7 g/cm 3), at least before 10 ka. No information on geothermal fluid circulation at pressures lower than 200 bars is recorded by the inclusions. Possibly the fast ascent of xenoliths to the surface during explosive eruptions prevented interaction with (and hence the trapping of) any fluids circulating at shallower depths.
Filicudi volcanic complex, formed in the last 0.2 Ma, is constituted by six main volcanic centers (Banco di Filicudi, La Canna, Zucco Grande, Fili di Sciacca, Chiumento and Fossa Felci) and several minor eccentric vents. Lavas and pyroclastics of the different volcanoes define a general magmatic series from basalts to basaltic-andesites, up to high-K andesites. Geological and geochemical evidence indicates that the different volcanic centers represent magmatic systems which evolved separately in time and/or space. Compositional gaps are found at different evolution degree within the rocks of Zucco Grande. Fili di Sciacca and Chiumento volcanoes. Fractionation at higher pressure of prevailing femic phases, followed by shallow level crvstallization and accumulation of plagioclase, had lead to the formation of the Filicudi basalts with high alumina and low MgO contents. All magmas are shown to have evolved by fractional crystallization associated to mixing processes between evolved and more basic magmas and to crustal contamination. In particular, among the Fili di Sciacca rocks, the AEC mechanism was recognized to have played an important role, with the more mafic rocks being more contaminated due to their higher temperature.
Four eruptive phases have been recognized in the field for the pliocenic activity of Monte Arci volcanic complex: (1) felsie lavas (two, distinct, rhyolitic episodes), (2) dacites and andesites, (3) trachytes, (4) basic lavas. 40Ar/39Ar ages have been obtained on separated feldspars from lavas belonging to each eruptive phase. The results achieve a very good internal precision, of a few ka. Plateau ages indicate that three of the four activity phases of the volcanic system were concentrated in a very narrow time-span: the basal Phase 1 rhyolites are younger than 3.24 Ma, the Phase 3 trachytes are 3.16 ± 0.3 Ma old. The 40Ar/39Ar data show the occurrence of large amounts of excess Ar which causes discrepancies between field stratigraphy and literature K/Ar ages, emphasizing that the latter cannot be used for a chronostratigraphic reconstruction.
New petrological, trace element and isotopic (Sr, Nd, O) data are reported for volcanic rocks from the island of Alicudi, a volcano consisting of basaltic, basaltic andesitic and high-potassium andesitic lavas and pyroclastics. All rocks are porphyritic with a phenocryst mineralogy dominated by plagioclase + clinopyroxene + olivine in the basaltic rocks and by plagioglase + clinopyroxene + orthopyroxene + brown amphibole in the andesites. Basalts with Mg value up to 73, and contents of Ni up to about 50 ppm represent the most primitive compositions known within the entire Aeolian arc. FeO total MgO, CaO, Ni, Co, Sc and Cr decrease with increasing silica, whereas incompatible elements such as Rb, Ba, Th, and the LREE display the opposite trend. Significant positive correlations of incompatible elements are observed on interelemental variation diagrams. Sr abundances are scattered and show a large range of values in both basic and intermediate rocks. Sr isotope ratios range between 0.70342 to 0.70410 and exhibit the usual negative correlation with 143Nd/144Nd (0.51289-0.51279). Basalts and basaltic andesites have higher Sr- and lower Nd-isotopic ratios than high-K andesites; a negative trend of Sr vs. 87Sr/86Sr is defined by basaltic volcanics. Negative trends are also observed between 87Sr/86Sr and absolute abundances of incompatible trace elements and some ratios of elements with different degrees of incompatibility such as La/Yb, Th/La, Th/Hf, Nb/Zr. Magmatic d180 values for both basalts and andesites, as inferred from plagioclase and pyroxene separates, are around + 6 . I %o. Geochemical and isotopic data are discussed in terms of two contrasting models. One assumes that the mafic magmas were generated in a heterogeneous source which gave a range of isotopically distinct liquids; in such a case, the negative correlation of Sr isotopic ratios with many incompatible trace element abundances and ratios imply that evolutionary processes affected at various degrees the magmas with different isotopic signatures. The second hypothesis suggests that isotopically homogeneous calcalkaline mafic liquids evolved within magma chambers to give a range of derivate liquids, which interacted with wall rocks. This model implies that the more mafic (and hotter) basaltic and basaltic andesitic liquids assimilated higher proportions of metamorphic country rocks than did the cooler high-K andesitic liquids. This hypothesis conflicts with the low and poorly variable d180 values observed in the analyzed mineral separates, unless it is assumed that assimilation occurred after phenocryst crystallization. When compared with mafic rocks from other Aeolian islands, the Alicudi basalts are geochemically and isotopically more primitive. Going eastward along the arc, there is a decrease in Ni and Cr abundances, Mg value and Nd isotope ratios parallel to an increase in Sr isotope ratios in basaltic rocks. These compositional variations are typical of volcanic series which have evolved by interaction with upper crustal material and may indicate that this process contributed significantly to the regional geochemical and isotopic trends observed along the Aeolian arc.
The paper describes a procedure to draw with computer a petrogenetic grid from a general mixing equation considering the effects of fractional crystallization and mixing. The program allows to plot rock samples and to calculate mixing and fractionation degree of the samples plotted.