ABSTRACT LXXIII, August 2004 n. 1:

Massimo Mattei, Nicola D’Agostino, Claudio Faccenna, Claudia Piromallo and Federico Rossetti - Some remarks on the geodynamics of the Italian region

Abstract - In this paper we present geological and geophysical data which constrain the Tertiary and Quaternary evolution of the Italian region, relevant to the interpretation of the genesis of magmatism in the frame of the geodynamic processes. GPS results show that, at the longitude of Sicily, the African approaches the Eurasian plate at a velocity of about 5 mm/yr in a NW direction. Furthermore, data show that the Adriatic foreland presently moves independently from the African plate. Geodetic and seismic data show that NW-SE oriented extension is the main active tectonic process in the Apennine chain.

Relative and absolute motions of the Africa and the Eurasia plates indicate several hundred kilometers of convergence since the early Tertiary, in the central Mediterranean. This convergence has been achieved by northwestward dipping subduction of the African plate. The main present-day geodynamic feature of the Italian region is represented by seismicity on a well defined Benioff zone, which reveals a still active process of subduction from the Ionian foreland below the Calabrian Arc and Tyrrhenian Sea. This slab is the result of a NW directed long running subduction process active since the Tertiary, which consumed the Ligure oceanic basin first, then a small fragment of the Apulian continental lithosphere, and finally most of the present-day Ionian lithosphere, whose subduction is still ongoing.

We also suggest that the lateral break-off of the Ionian subducting lithosphere could allow lateral astenospheric flow above the subducted plate either from the Apulian plate and from the Sicily Channel-western Sicily.

Pietro Armienti, Sonia Tonarini, Massimo D’Orazio and Fabrizio Innocenti - Genesis and evolution of Mt. Etna alkaline lavas: petrological and Sr-Nd-B isotope constraints

Abstract - Mt. Etna lies at the northern margin of the African plate, on the accretionary prism of the Africa-Europe subduction system. Differential roll-back of the Ionian oceanic lithosphere has created a vertical slab window through which the passive rise of asthenosphere causes partial melting and magma genesis. In this rapidly evolving geodynamic context, the increase of alkalinity in time, accompanied by variations of Sr and Nd isotopes, is a first order feature. Fluid mobile elements and B systematics reveal the fundamental role played by fluids released by the dehydrating oceanic lithosphere, even if they are added to the mantle source in amounts no larger than 1 wt.%. This induces relatively high H₂O contents in Etna magmas that exert a strong control on phase relationships. Compositions of alkaline primary melts have been reconstructed and used to constrain the depth of origin and fractionation sequences at various pressures. It was found that phase relations at the crust-mantle boundary can only produce basaltic compositions akin to the less evolved sub-aphyric lavas, whereas trachybasaltic compositions are generated in the depth range 12-3 km together with cumulate bodies detected through geophysical investigations and here related to massive pyroxene and plagioclase fractionation. Detailed monitoring of Sr-isotope equilibrium between pyroxene and host rock revealed both the increase of ⁸⁷Sr/⁸⁶Sr over time and the occurrence of mixing processes between distinct batches of magma in the plumbing system. Sr, Nd and B systematics also reveal contamination effects due to interactions with lower crust and sporadic interaction with the sedimentary basement.

Teresa Trua, Giancarlo Serri, Michael P. Marani, Piermaria L. Rossi, Fabiano Gamberi and Alberto Renzulli - Mantle domains beneath the southern Tyrrhenian: constraints from recent seafloor sampling and dynamic implications

Abstract - Southern Tyrrhenian Quaternary magmatism represents one example of an active arc/back-arc system where IAB- and OIB-type magmas coexist. IAB-type magmatism is the most common, in both arc and back-arc settings, whereas OIB-type magmas are restricted to few areas. Geochemical and isotopic characteristics of southern Tyrrhenian submarine volcanic samples are here summarized, with special attention to those samples recovered during recent seamounts/seafloor sampling. Petrological data of the most basic lavas have been evaluated with the aim to characterize the mantle sources of IAB and OIB magmas. These data provide important insights into the petrogenesis of the southern Tyrrhenian submarine magmatism and on the possible geodynamic scenario able to explain the coexistence of IAB and OIB magmas in this area.

Lorella Francalanci, Riccardo Avanzinelli, Chiara M. Petrone and Alba P. Santo - Petrochemical and magmatological characteristics of the Aeolian Arc volcanoes, southern Tyrrhenian Sea, Italy: inferences on shallow level processes and magma source variations

Abstract - The Aeolian volcanic arc, constituted by seven islands and several seamounts, is emplaced on continental lithosphere. The islands are mainly formed by lava flows, domes and pyroclastic deposits, and emerged from the sea in a short time span, from around 200 ka ago at Filicudi, Lipari and Stromboli (Strombolicchio neck), to about 100 ka at Alicudi and Stromboli. At Panarea, an intense fumarolic activity is still present, the last eruptions at Lipari took place on 580 A.D., whereas Vulcano and Stromboli are still active. The rock compositions belong to different magmatic series and show a large silica range (48-76 wt%). Calc-alkaline (CA) and high-K calc-alkaline (HKCA) volcanics are present in all the islands, except for CA rocks at Vulcano. Shoshonitic (SHO) products are only lacking at Alicudi, Filicudi and Salina. Potassic (KS) volcanics have been erupted at Vulcano and Stromboli. Basalts are not found at Lipari, whereas a large amount of rhyolites are present in the central arc islands (Lipari, Vulcano, Salina, Panarea), having different petrochemical characteristics. ⁸⁷Sr/⁸⁶Sr increases from the western to the eastern sectors of the arc (0.70342-0.70757), whereas ¹⁴³Nd/¹⁴⁴Nd decreases (0.51289-0.51243). Pb isotope ratios show a large similar range in the western and central arc islands, but decrease at Panarea and Stromboli (e.g., ²⁰⁶Pb/²⁰⁴Pb: 18.93-19.77). Among CA and HKCA rocks, incompatible trace element contents and ratios change passing from the central part of the arc to the external sectors. The isotopic and geochemical compositions of SHO and KS volcanics from Stromboli and Vulcano are distinct, with the Vulcano compositional characteristics resembling those of the CA and HKCA magmas from the central arc. Significant rock compositional variations are also observed within the single volcanoes.

Aeolian magmas underwent multiple differentiation processes during the ascent to the surface from their mantle source. Fractional crystallisation is often associated to crustal contamination which affected at higher extent either the most evolved magmas at Vulcano, Salina and Lipari or the most mafic magmas at Alicudi, Filicudi and Stromboli. Multiple mixing also played an important role often associated with the other differentiation processes. These evolutionary processes occurred at polybaric conditions, with higher crystallisation pressure for Filicudi and Salina.

The different parental magmas were originated in an heterogeneous mantle wedge, metasomatised by subduction-related components at increasing extent, going from west to east (= variation of Sr and Nd isotope ratios). It is also suggested that the mantle source of CA and HKCA magmas from central arc (Salina, Lipari and CA Panarea) is a MORB-like asthenospheric source, contaminated by aqueous fluids released by subducted oceanic crust + pelagic sediments. The same type of source can be envisaged for Vulcano SHO and KS parental magmas. In the external sectors of the arc, mass transfer from the subducted slab to mantle seems to be occurred by higher melt/fluid proportions. At west (Alicudi and Filicudi), the other components involved in the magma genesis remain similar to those proposed for the central arc source. On the contrary, the compositional characteristics of eastern magmas (HKCA Panarea and Stromboli) suggest a different pre-contamination mantle source (continental lithosphere?) and/or a different crustal contaminant (with low Pb isotopes) of the mantle wedge. A decreasing partial melting degree of distinct mantle sources is considered to generate magmas with an increasing potassic character at Vulcano and Stromboli.

Michele Lustrino, Vincenzo Morra, Leone Melluso, Pietro Brotzu, Fosco d’Amelio, Lorenzo Fedele, Luigi Franciosi, Roberto Lonis and Alfredo Massimo Petteruti Liebercknecht - The Cenozoic igneous activity of Sardinia

Abstract - During the Cenozoic, the island of Sardinia was the location of two different magmatic episodes: 1) a Oligocene to Miocene (hereafter OM) cycle ~32-15 Ma and 2) a Pliocene to Quaternary (hereafter PQ) cycle (~5-0.1 Ma). These two volcanic cycles differ in many aspects: 1) geographic occurrence [the OM rocks occur almost exclusively in a graben structure called the Fossa Sarda (Sardinian Trough) that cuts the entire island from north to south, whereas the PQ rocks are scattered throughout the island]; 2) petrography (the OM rocks are mostly porphyritic, whereas the PQ rocks are mostly aphyric); 3) geochemical affinity (the OM rocks are mostly subalkaline with a tholeiitic to calcalkaline character, whereas the PQ rocks are mostly sodic alkaline with fewer tholeiitic types); 4) major element compositions [the OM rocks are mostly dacites to rhyolites with fewer basaltic andesites, andesites and rare basalts while the PQ rocks are mostly hawaiites, mugearites and basaltic andesites, with both SiO₂-oversaturated and SiO₂-undersaturated evolved types (rhyolites and phonolites); moreover, for a given SiO₂, OM rocks have higher CaO, lower TiO₂ and lower Na₂O compared to the PQ rocks]; 5) trace element abundances and ratios (the OM rocks have lower HFSE and REE contents and higher La/Nb and Zr/Nb ratios compared to the PQ samples); 6) Sr isotopic composition (the OM rocks have ⁸⁷Sr/⁸⁶Sr generally > 0.7047, whereas the PQ rocks have ⁸⁷Sr/⁸⁶Sr generally < 0.7050). ¹⁴³Nd/¹⁴⁴Nd and ²⁰⁶Pb/²⁰⁴Pb ratios of the OM rocks (from 0.5127 to 0.5122 and from 18.52 to 18.71, respectively) fall within the range of the PQ samples (from 0.5129 to 0.5122 and from ~17.5 to 19.42, respectively).

The OM rocks show geochemical features typical of magmas emplaced in subduction-related settings. They are believed to have been generated within the mantle wedge developed above a west-directed subduction of (Mesogean?) oceanic lithosphere below the southern continental margin of Europe. On the other hand, the PQ volcanic rocks were emplaced concurrent with the formation of the Tyrrhenian Sea and share some geochemical similarities with magmas emplaced in within-plate (anorogenic) tectonic settings, although they exhibit peculiar characteristics.

The PQ rocks can be divided into two groups: one group (Unradiogenic Pb Volcanics = UPV) has relatively high ⁸⁷Sr/⁸⁶Sr (0.7043-0.7051), low ¹⁴³Nd/¹⁴⁴Nd (0.5124-0.5126), and is characterised by the least radiogenic Pb isotopic composition so far recorded in Italian (and Circum-Mediterranean) Cenozoic igneous rocks (²⁰⁶Pb/²⁰⁴Pb = 17.36-18.07); these are the most widespread volcanic rocks and crop out in central and northern Sardinia. The other group (Radiogenic Pb Volcanics = RPV) has chemical and Sr-Nd-Pb isotopic ratios indicative of a markedly different source (⁸⁷Sr/⁸⁶Sr = 0.7031-0.7040; ¹⁴³Nd/¹⁴⁴Nd = 0.5127-0.5129; ²⁰⁶Pb/²⁰⁴Pb = 18.8-19.4), and crop out only in the southern part of the island. The less differentiated rocks of the two groups also show distinct trace element contents and ratios (e.g. Ba/Nb>14, Nb/U <38 and Ce/Pb <20 for the UPV; Ba/Nb<9, Nb/U >45 and Ce/Pb >24 for the RPV).

The transition from igneous cycles with orogenic (s.l.) to anorogenic (s.l.) geochemical features is relatively common throughout the entire circum-Mediterranean area; in other places this shift of chemical compositions has been related to «slab detachment» and/or the development of «slab window» processes. In this paper, to explain the geochemical differences between the OM and PQ volcanic products of Sardinia, we propose the involvement of different mantle sources: an asthenospheric mantle source slightly modified by subduction-related metasomatism for the OM rocks and a lithospheric mantle source strongly modified during ancient times (possibly during Hercynian orogenesis) for the great majority of the PQ volcanic rocks.

Sandro Conticelli, Leone Melluso, Giulia Perini, Riccardo Avanzinelli and Elena Boari - Petrologic, geochemical and isotopic characteristics of potassic and ultrapotassic magmatism in central-southern Italy: inferences on its genesis and on the nature of mantle sources

Abstract - Miocene to Quaternary magmatic rocks, found along the Tyrrhenian border of peninsular Italy, mostly belong to potassic and ultrapotassic suites. They can be divided into three different petrographic provinces, where magmatism is confined in terms of space, time and petrographic characteristics.

The Tuscan Magmatic Province is the northernmost province, in which mantle-derived potassic and ultrapotassic, leucite-free volcanic rocks occur, prevailing over high potassium calc-alkaline rocks, and covering a time span of activity between 14.2 and 0.19 Ma. These rocks are silica-saturated to silica-oversaturated and range from high-potassium calc-alkaline to ultrapotassic lamproite, through potassic and ultrapotassic shoshonitic series.

The Roman Magmatic Province extends from Northern Latium to the Umbrian and Campanian areas, arranged in a volcanic belt along the Tyrrhenian border of the Apennine chain. It is made up of rare shoshonitic rocks (KS) and leucite-bearing rocks (HKS). Some HKS may contain melilite, and therefore belong to the kamafugitic group (KAM). Minor high potassium calc-alkaline rocks are also found in the Pontine archipelago and in drill in the Campanian plain. Volcanism has been active from 0.7 Ma to 0.1 Ma in the northern districts of the Latian area (i.e., Vulsinian, Vico, Sabatinian, Alban, Hernican, Auruncan) whereas in the southernmost portion of the province, the Neapolitan district, shoshonitic and ultrapotassic magmatism are consistently younger than the Latian ones, ranging from 0.3 Ma to present. Historical eruptions in the Neapolitan district are indeed recorded at Phlegrean Fields, Procida, Ischia and Vesuvius volcanoes.

The Lucanian Magmatic Province is the easternmost volcanic region characterized by rocks rich in both Na and K. Most of the rocks are haüyne- and leucite-bearing, and were erupted at Monte Vulture volcano between 0.6 and 0.1 Ma. Carbonatites have been described in the last phase of activity.

Minor amounts of K-rich rocks are also found in the Aeolian Arc, in the southern Tyrrhenian Sea. These rocks are intimately associated with calc-alkaline rocks at Vulcano, Vulcanello, and Stromboli.

Enrichment in K₂O and related incompatible trace elements is accompanied by strong to mildly fractionation of high field strength elements with respect to large ion lithophile elements. This can be attributed to the input of a crustal component into the mantle source of the magma prior to partial melting. Variations in trace element enrichment and isotope characteristics of the three magmatic provinces are thought to be the result of different metasomatic events and complex processes of partial melting of the mantle sources. Peculiar geochemical and isotopic characteristics of the Lucanian and Neapolitan regions are the result of different channelling of withinplate material through lateral inflow from foreland, during the roll-back of the Ionian subduction. Metasomatism affected lithospheric mantle sources characterised by variable degrees of depletion.

The peculiar petrologic, geochemical and isotopic features of the mafic magmas are consistent with a post-orogenic subduction-related geodynamic setting for the production of their parental magmas.

Angelo Peccerillo - Carbonate-rich pyroclastic rocks from central Apennines: carbonatites or carbonated rocks? A commentary

Abstract - The Pleistocene association of melilititic ultrapotassic kamafugites and carbonate-rich pyroclastic rocks from Apennine has been interpreted to represent a melilitite-carbonatite province, such as those typically found in intra-continental rift settings. Kamafugitic lavas have incompatible element and REE patterns that closely match those of leucite tephrites from the nearby Roman Province. Carbonate-rich pyroclasts have similar trace element patterns as kamafugites and Roman rocks, but, when single carbonate-rich pyroclastic rocks are compared with the associated kamafugitic lavas from the same volcano, they show a depletion for all the incompatible elements, which becomes stronger as the amount of carbonate fraction increases. This shows that the carbonate fraction is geochemically barren and its presence in the carbonate-rich rocks generates a dilution for almost all the elements. Oxygen isotope ratios of carbonates in these volcanic rocks are invariably high (d¹⁸O in the range +21 to + 28). Phenocrysts from kamafugitic lavas also have high concentrations of heavy oxygen (d¹⁸O = +11 to + 14).

These data cast serious doubts on the hypothesis that the carbonate-rich pyroclastics from central Italy represent carbonatitic magmas. Geochemical and isotopic data strongly suggest that these rocks represent fragmented kamafugitic magmas, which have suffered addition of geochemically barren carbonate material from wall rocks. Geological data make this hypothesis likely, inasmuch as the volcanoes from the internal Apennines are monogenetic centres, which cut through some thousand meters of carbonate rocks of the Apennine sedimentary sequences.

Gianluca Bianchini, Luigi Beccaluva and Franca Siena - A reappraisal of ultra-alkaline Intra-Apennine volcanism in central-southern Italy: evidence for subduction-modified mantle sources

Abstract - The ultra-alkaline Intra-Apennine Volcanism (IAV), which includes kamafugites (central Italy) and melilitites (Vulture) generally associated with carbonatitic rocks, is reviewed in order to assess its tectono-magmatic significance. Primitive mantle normalised incompatible element distributions and Sr-Nd isotope systematics of IAV products are completely different from the Cenozoic anorogenic magmas of the Mediterranean area. Similar geochemical differences are observed between mantle xenoliths associated to anorogenic magmas and those from IAV, which are characterised by the widespread presence of phlogopite. Moreover, the IAV ultra-alkaline magmas (including carbonatites) totally differ, in terms of Sr-Nd isotopes, from those of typical continental intra-plate rift systems, such as the western branch of the African rift and the northern border of the Paranà basin. By contrast, the high Sr and low Nd isotopic ratios recorded in the IAV kamafugites of central Italy, are strictly comparable to those observed in potassic volcanics of the Roman Magmatic Province (RMP). This implies a common generation of IAV and RMP magmas from subduction-modified mantle sources with the possible recycling of continental crust material during the Apenninic orogenic events, as already suggested by several authors. The location of the IAV products above the verticalised relict-subducted Adriatic lithosphere suggests that it is affected by important tectonic discontinuities, or real slab break-off, which allowed access of subduction components to the IAV mantle sources. Extremely low partial melting degrees of highly metasomatized and hybridised mantle sources, deep in the lithosphere, could account for the generation of IAV ultra-alkaline/carbonated melts.

Giampiero Poli - Genesis and evolution of Miocene-Quaternary intermediate-acid rocks from the Tuscan Magmatic Province

Abstract - Occurrence of a large variety of rock types, intrusive and effusive, closely associated in space and time reveal a complex magmatic setting for the Tuscan Magmatic Province. Extensive petrological and geochemical investigations carried out over the later years indicate that main rock associations are represented by three groups of rocks at different degrees of evolution: i) mafic ultrapotassic rocks with lamproitic affinity, and high-potassium calc-alkaline and shoshonitic rocks; ii) intermediate-acid rocks bearing strong petrographic and geochemical evidence of magma interaction processes; iii) acid volcanics and intrusives showing petrological and geochemical characteristics of both extremely evolved and pure anatectic melts. Literature and new data suggest that a process of interaction between basic and acid end-members is responsible for the evolution of the Tuscan Magmatic Province magmatism. Major and trace elements and isotopic systematic help to recognize the basic end-members as compositionally akin to three basic-intermediate magmas belonging to Capraia shoshonites, Capraia high potassium calc-alkaline rocks rich in Sr, and lamproites from Tuscan area that acted together even in a single intrusive or effusive complex. The acid end-members in the interaction process are crustal anatectic melts derived by partial melting at ca 4-6 kbar of gneiss and garnet micaschists of the Tuscany basement having a sedimentary protolith. Residual assemblages of the partial melting process calculated by geochemical models agree with experimental petrological data, and help to reconstruct levels of melting and emplacement for intrusive complexes, and level of crystallization of phenocrysts for the effusive ones. The petrological model reported in this work fits well with geophysical data indicating a superposition of upper crust of both the European and Adriatic plates in westernmost Tuscany.

Elisabetta Rampone - Mantle dynamics during Permo-Mesozoic extension of the Europe-Adria lithosphere: insights from the Ligurian ophiolites

Abstract - Petrologic and isotope investigations on the Ligurian ophiolites have provided evidence that they do not resemble oceanic lithosphere formed at a mid-ocean ridge setting, rather, they represent peculiar and atypical sectors of oceanic lithosphere composed by older lithospheric mantle peridotites (Proterozoic and Permian) intruded by younger MORB-type magmas (Jurassic and late-Jurassic). The Ligurian ophiolites thus reflect a lithologic association which develops in response to passive lithosphere extension and slow-spreading oceanization, and which characterizes embryonic stages of evolution of an oceanic basin. Mantle peridotites from the External Liguride (EL, Northern Apennines) and the Erro-Tobbio (ET, Ligurian Alps) ophiolitic units both record a tectono-metamorphic subsolidus evolution characterized by progressive recrystallization from spinel- to plagioclase- to amphibole-bearing assemblages and deformation from granular to tectonite- to mylonite- types. Sr, Nd and Os isotope investigations have indicated that the EL lherzolites were accreted to the subcontinental lithospheric mantle since Proterozoic times. Sm-Nd dating on the plagioclase-facies recrystallization stage have yielded 273-313 Ma in the ET peridotites and 165 Ma in the EL lherzolites. The ET and EL peridotites thus represent different pieces of subcontinental lithospheric mantle which experienced tectonic exhumation during distinct stages of extension of the Europe-Adria continental lithosphere, leading to the formation of the Jurassic Ligurian Tethys ocean. Results on the ET peridotites point that the decompressional evolution of lithospheric mantle was already active since Late-Carboniferous-Permian times. A striking feature of oceanic basins developed by passive lithosphere extension is therefore the tectonic sea-floor exposure of large sectors of subcontinental lithospheric mantle. This is consistent with the results of petrologic and structural investigations on mantle peridotites from modern oceanic analogues (embryonic oceans and passive continental margins). Another peculiar feature of the Ligurian ophiolites is the predominant lack of a mantle-crust cogenetic link, as it would be expected in mid-ocean ridge type oceanic lithosphere. Both the EL and ET peridotites represent subcontinental lithospheric mantle whose tectonic exhumation was even completely unrelated to mantle melting and melt production. Sm/Nd isotope studies on the depleted mantle peridotites from the Internal Liguride (Northern Apennine) ophiolitic units provided Permian (275 Ma) DM model age of depletion. Associated gabbroic rocks have been dated to 164 ± 14 Ma. Thus, even in the IL ophiolitic sequences, residual mantle and associated crustal rocks are not cogenetic and coheval. Sm/Nd isotope data on the depleted ophiolitic peridotites from Mt. Maggiore (Corsica) have furnished Jurassic (165 Ma) DM model age of depletion. Associated gabbroic rocks have been dated to 162 + 10 Ma. The Mt. Maggiore gabbro-peridotite association thus constitutes the first record of the attainment of a mature oceanic stage of the Ligurian Tethys ocean, where residual peridotites and associated magmatic rocks are in isotopic equilibrium.

Angelo Peccerillo and Eugenio Turco - Petrological and geochemical variations of Plio-Quaternary volcanism in the Tyrrhenian Sea area: regional distribution of magma types, petrogenesis and geodynamic implications

Abstract - The Plio-Quaternary magmatism in the Tyrrhenian Sea area shows a strong regionality in the distribution of magma types, which allows distinguishing several magmatic provinces or zones. These are: 1) Tuscany, 2) Latium, 3) Intra-Apennine region, 4) Neapolitan area, 5) Ernici-Roccamonfina, 6) Mt. Vulture, 7) Aeolian Arc, 8) Sicily, 9) Tyrrhenian Sea floor, and 10) Sardinia. The different provinces are sometimes divided by important tectonic lines and show distinct mechanical characteristics of the lithosphere (i.e., Moho depth, lid thickness, occurrence of deep seismicity, etc.).

In Tuscany, mafic calc-alkaline to ultrapotassic magmas coexist with silicic intrusive and extrusive rock of crustal anatectic origin. Magmatism in central Italy ranges from calc-alkaline to ultrapotassic, but each province displays peculiar major and/or trace element and/or isotopic composition. Calc-alkaline to shoshonitic magmatism dominates in the Aeolian Arc. Igneous activity in the Sicily and Sardinia provinces consist of tholeiitic to Na-alkaline products, which show low LILE/HFSE ratios, typical of intraplate volcanics. Intraplate and arc-type rocks coexist on the Tyrrhenian Sea floor.

Radiogenic isotope compositions (Sr, Nd, Pb) of mafic rocks show moderate within-province variations. However, when the whole Plio-Quaternary magmatism in considered, continuous trends are observed, which connect various mantle compositions. Rocks from the Sicily province have HIMU-type signatures composition trending toward DMM-EM1. Rocks from Sardinia and the Tyrrhenian Sea plot between EM1 and HIMU composition. Rocks from the Aeolian Arc to Tuscany define a trend connecting the HIMU-like rocks of Sicily with a Tuscany mafic ultrapotassic end-member characterised by high ⁸⁷Sr/⁸⁶Sr, low ¹⁴³Nd/¹⁴⁴Nd and moderately low ²⁰⁶Pb/²⁰⁴Pb. This end-member is more enriched in radiogenic Sr than EM2 mantle component, and resembles closely the upper crust for both trace element ratios and radiogenic isotope ratios.

The anorogenic trace element signatures of magmatism in Sicily and Sardinia suggest a genesis in a mantle which suffered little or no contamination by subduction processes, at least in recent times. Radiogenic isotopic trends reveal an interaction between various mantle components (HIMU, DMM, EM1). In contrast, the magmatism in the Aeolian Arc and peninsular Italy highlights interaction between HIMU-type and upper crustal reservoirs. Geochemical modelling demonstrates that such an interaction cannot have occurred during magma ascent to the surface (magma contamination), but took place in the upper mantle by addition of upper crustal material (source contamination). Such a conclusion implies a subduction-related origin for the magmatism in the Aeolian Arc and the Italian peninsula. The coexistence of anorogenic-type and arc-type rocks in the Tyrrhenian Sea calls for a role of both intraplate and subduction-modified mantle sources.

The hypothesis that best explains the regionality of magma types in Italy and their age, geochemical and isotopic characteristics is that subduction of Ionian-Adria plate affected the Sardinia block during Oligo-Miocene and successively shifted south-eastward towards its present position in the southern Tyrrhenian Sea. This caused a migration of subduction-related magmatism from Sardinia, through the Tyrrhenian Sea to southern Italy and the Aeolian Arc. Contemporaneously, the back-arc opening of the Tyrrhenian Sea basin generated passive asthenospheric upwelling and the formation of anorogenic-type magmas, which coexist with subduction-related rocks. The Apennine chain underwent anticlockwise rotation and segmentation during Tyrrhenian Sea opening, breaking into various arc sectors along which variable types of sedimentary material were subducted. These induced strong heterogeneities in the mantle wedge, which are reflected by regional variations of geochemical and isotopic composition of the magmatism along the Italian peninsula. The hypothesis of the presence of deep mantle plumes in the Tyrrhenian Sea area is unable to explain most of the compositional characteristics of the magmatism. However, the HIMU-like isotopic signatures of rocks in Sicily (e.g. Etna, Ustica, Iblei) resemble closely those of several intraplate volcanics in central and eastern Europe (e.g. Massif Central in France, Eifel region in Germany). This calls for a wide European mantle reservoir, may be well explained by a deep mantle plume, but does not necessarily require one.