Figure 3: Th/Yb vs. Ta/Yb diagram for Plio-Quaternary mafic rocks from Italy, discriminating between intraplate and arc basalts.

Regional distribution of magma types

There is a strong correlation between the petrological characteristics of Plio-Quaternary magmas in Italy and their regional distribution (Figure 1). Tholeiitic, transitional and Na-alkaline rocks with typical intraplate geochemical signatures (e.g., low LILE/HFSE ratios, Figure 3) occur in western Sicily and Sicily Channel (Etna, Iblei, Linosa, Pantelleria), in Sardinia (Lustrino et al., 2000), and in the Tyrrhenian Sea (e.g. Ustica). Calcalkaline, shoshonitic and K-alkaline rocks with island-arc geochemical signatures (high LILE/HFSE ratios, Figure 3) are concentrated in the Aeolian arc, and along the Tyrrhenian side of the Italian peninsula, but also occur as seamounts on the Tyrrhenian Sea floor. Orogenic magmas display an overall decrease in age from west to south-east (Figure 1, inset).

Regional variation of trace element and Sr-Nd-Pb isotope compositions of mafic rocks

Radiogenic isotope compositions (Sr, Nd, Pb, Hf) define smooth trends (Figure 5). There is an overall increase of ⁸⁷Sr/⁸⁶Sr and a decrease of ¹⁴³Nd/¹⁴⁴Nd and Pb isotopes from south to the north, with the various regions displaying distinct isotopic signatures.

Figure 5: Sr-Nd isotope diagram for Plio-Quaternary mafic volcanic rocks from Italy. Note the strong regional variation.

The ⁸⁷Sr/⁸⁶Sr vs. ²⁰⁶Pb/²⁰⁴Pb diagram (Figure 6) reveals that all magmatism in Italy may be described in terms of interaction between three end-members:

Petrogenesis

The variable petrological characteristics of Italian Plio-Quaternary magmatism call for a wide variety of mantle compositions and petrogenetic processes, including degrees and pressure of partial melting, mantle mineral compositions, and fluid pressure. The Na-alkaline rocks are believed to be generated in mantle sources similar to those of OIBs. Calcalkaline, shoshonitic and potassic rocks show many geochemical characteristics in common (e.g., high LILE/HFSE ratios), although they display different enrichments in incompatible elements. These rocks are probably derived from anomalous mantle sources which were enriched in incompatible elements by the subduction processes. There is a debate about the age of mantle enrichment. It has been suggested to have occurred during the Tertiary evolution of the Alpine-Apennine chains (Peccerillo, 1999), or at some 1-2 Ga (e.g., Castorina et al., 2002). Establishing the age of mantle contamination has profound implications for understanding the geodynamic evolution of the Tyrrhenian Sea and surrounding areas (see Peccerillo, 1999, 2002 for discussion).

Geodynamic significance: Plume vs. shallow mantle processes

Much of the discussion on the geodynamic significance of Italian Plio-Quaternary magmatism has addressed the problem of whether it relates to subduction processes or represents intraplate magmatism possibly connected with the emplacement of deep mantle plumes (e.g., Ayuso et al., 1997; Peccerillo, 1999, 2002, 2003; Gasperini et al., 2002; Bell, 2002). Some authors suggest that only the Etna-Iblei-Ustica (i.e., HIMU-FOZO) and Sardinia (i.e., EM2) rocks derive from mantle plumes, whereas the rest of the magmatism represents mixtures of plume material and subducted upper crustal components (Gasperini et al., 2002). Other authors suggest the more extreme hypothesis that all the magmatism reflects interaction among various plume reservoirs (Bell, 2002).

Evidence invoked in favour of the plume hypothesis includes:

1. The ultrapotassic nature of a large portion of the magmatism, which is typical of settings such as East Africa, classically believed to have developed above mantle plumes,
2. The occurrence of carbonate-rich rocks in Central Italy, which have been suggested represent carbonatites, an alleged “pathfinder” of mantle plumes,
3. The smooth trends of Sr-Nd-Pb isotopes connecting discrete mantle reservoirs. FOZO, HIMU and EM1 are universally believed to represent plumes. ITEM displays very radiogenic Sr signatures, which has been suggested to require recycling and aging of subduction-contaminated mantle for at least 2 Ga (Hawkesworth & Vollmer, 1979; Vollmer, 1989),
4. The very similar radiogenic isotope compositions of some Italian rocks (e.g. Etna, Iblei) to other Na-alkaline rocks from central and western Europe, which would derive from a wide European Asthenospheric Reservoir (EAR) representing a plume head (Wilson & Bianchini, 1999).

Objections to these points are:

1. Italian ultrapotassic rocks have incompatible element distributions, which are totally different from East Africa. The latter show enrichment in Ta and Nb and depletion in Rb, Cs, Th and other LILE. In contrast, Italian potassic rocks show high enrichment in LILE and depletion in HFSE.
2. Carbonate rich rocks in Central Italy represent ultrapotassic volcanics that have experienced secondary addition of carbonate material from carbonate wall rocks. This is demonstrated by the high delta-¹⁸O (around +20 to +25) of the carbonate material, amongst other things (see discussion in Peccerillo, 1998),
3. Points 3. and 4. above are the strongest arguments in favour of the plume hypothesis. However, the smooth trends of radiogenic isotopes may be generated by other processes than mantle plumes. For instance, the main trend connecting HIMU-FOZO to ITEM could be simply related to mantle contamination by upper crustal material brought into the upper mantle by subduction processes, which affected the Apennines during the Tertiary and are still active in the southern Tyrrhenian Sea. Moreover, the mantle xenoliths occurring in Italian and other European rocks have similar isotopic signatures to the host lavas (Downes, 2001), which suggests a lithospheric origin for the magmas.

Evidence in favour of shallow-mantle processes includes:

1. The strong regionality in the distribution of magma types and the relations between the geochemistry of the volcanic rocks and the mechanical characteristics of the lithosphere in the various areas.
2. The geochemical features of the ultrapotassic rocks, which resemble very closely the upper crust (Peccerillo, 1999) and require recently subducted crustal material rather than long periods of recycling into the mantle.
3. Na-alkaline rocks with HIMU-FOZO characteristics occur along normal or transcurrent lithospheric faults. In the plume hypothesis, this requires that plumes were emplaced just, and only along, these faults. This is an ad hoc explanation. A simpler hypothesis is to assume that magmatism is triggered by faulting and that the composition of magma erupted at the surface depends on the particular nature of the upper mantle in this zones: simplex sigillum veri !

A good example of fault-related volcanism is in the Etna-Aeolian arc zone, where a lithospheric fault (the Tindari-Letojanni fault) cuts both the Aeolian arc and its foreland, separating the actively subducting Ionian oceanic crust from Sicily (Figure 1). The active volcanoes of Etna, Vulcano and Lipari occur along this fault. However, while Etna is Na-alkaline and has a HIMU-FOZO signature, Lipari and Vulcano are calcalkaline to shoshonitic and have island-arc signatures.

Conclusions

Plio-Quaternary volcanism in Italy shows strong compositional variations, which reveal heterogeneous compositions and complex evolution processes of mantle sources. Both subduction-related and intraplate signatures are observed.

The hypothesis that best explains this complex magmatic province is continent-continent convergence in which the leading edge of the African plate is subducted beneath the Italian peninsula to generate heterogeneous mantle sources that then produce the wide variety of volcanic rocks observed (from calcalkaline to ultrapotassic) with subduction-related geochemical signatures (Doglioni et al., 1999). The large amount of potassic rocks requires extensive and repeated episodes of mantle metasomatism (Peccerillo, 1999; 2002). Mantle end-members (HIMU, FOZO and EM1) could be plume-related, but they could also represent resident mantle material, which exhibits similar isotopic compositions all over Europe.

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