Journal of the Virtual Explorer

During the Cenozoic, the island of Sardinia was the site of intense orogenic igneous activity, with a late Eocene-Middle Miocene phase of arc-tholeiitic to shoshonitic magmatism. Similar magmatism also took place during Eocene-Miocene in Corsica, in the Liguro-Provençal Sea basin, in Provence and in the Valencia Trough, and is apparently genetically related to the orogenic magmatism of Sardinia. Moreover, Oligo-Miocene volcaniclastic levels, linked to orogenic-type volcanism, occur in clastic formations of the Apennines, although their relation with the contemporaneous Sardinian volcanic activity is ambiguous (e.g., Mattioli et al., 2002). The orogenic magmatism in Sardinia was followed by a middle Miocene-Pleistocene phase of anorogenic volcanism, showing a tholeiitic to Na-alkaline affinity (Lustrino et al., 2004).

Sardinia and Corsica form a continental microplate, sited in the central Mediterranean Sea, between the Tyrrhenian Sea and the Liguro-Provençal-Balearic basin. This block was part of the southern European continental margin until late Oligocene, when backarc extension generated continental breackup, formation of the Liguro-Provençal- Balearic basin, separation of the Sardinia-Corsica microblock, and its eastward migration (e.g., Rollet et al., 2002; Schettino & Turco, 2006; Lustrino et al., 2009 with references). Rifting in the southern European paleomargin possibly started close to the Eocene/Oligocene boundary (~34 Ma), after the Eocene Pyrenean compressional phase (Cherchi et al., 2008 with references). Spreading of the Liguro-Provençal basin and eastward drifting of the Sardina-Corsica microplate seems to have started around the Aquitanian/Burdigalian boundary (~20.5 Ma) and ceased not later than ~ 15 Ma (Gattacceca et al., 2007; Cherchi et al., 2008). According to Gattacceca et al. (2007), between 20.5 and 15 Ma the Sardinia-Corsica block rotated ~45° counterclockwise with respect to stable Europe, around a pole located north of Corsica. Most of the drifting occurred during the period 20.5–18 Ma, when ~ 30° of rotation was completed (Montigny et al., 1981; Gattacceca et al., 2007). In a recent paper, Lustrino et al. (2009) inferred the beginning of the westward Apennine subduction to have started between ~ 49–42 Ma.

Several authors (e.g., Malinverno & Ryan, 1986; Doglioni et al., 1997; Carminati et al., 1998; Gueguen et al., 1998; Faccenna et al., 2001) relate the opening of the Liguro-Provençal basin and drifting of the Sardinia-Corsica block to the south-eastward retreat and roll-back of an Adriatic/Ionian slab, subducting north-westward under the southern European paleomargin.

Orogenic magmatism occurred in Sardinia during a relative long period, from ~ 38 to ~12 Ma (K/Ar: Coulon et al., 1974; Bellon et al., 1977; Giraud et al., 1979; Savelli et al., 1979; Montigny et al., 1981; Beccaluva et al., 1985 for a review; Rb/Sr: Morra et al., 1994; Ar/Ar: Deino et al., 2001; Edel et al., 2001; Speranza et al., 2002; Gattacceca et al., 2007; Lustrino et al., 2009), even though most of the activity concentrated around ~ 21–18 Ma, almost contemporaneously with the main phase of rotation of the Sardinia-Corsica block.

Igneous activity occurred exclusively along and within the Sardinia Trough (Fossa Sarda), the Oligocene-Miocene rift system that crosses the western part of the island from north to south (Lustrino et al., 2009 with references). The volcanic products were emplaced as pyroclastic flows and minor lava flows and domes, both in a subaerial and submarine environment. Rock compositions are dominated by dacites and rhyolites, with minor andesites and very scarce basalts. Plutonic rocks, mainly represented by gabbros and diorites, are very rare. In the Sulcis area of south-western Sardinia, peralkaline trachytes and rhyolites were also erupted (Morra et al., 1994).

Igneous products with ages and petrological characteristics similar to the Oligocene-Miocene Sardinian magmatism, are also found 1) along the western Provençal margin of southern France, where ~ 34-18.7 Ma old microdiorites, basalts, andesites and dacites are found (Ivaldi et al., 2003; Beccaluva et al., 2004), 2) in southern Corsica, where rhyolitic and dacitic tuffs occur with ages comprised between ~17.8-21.2 Ma (Ottaviani-Spella et al., 2001; Ferrandini et al., 2003) and 3) south-west of the Corsican margin, where porphyric clinopyroxene basalt, amphibole-biotite andesite and pyroclastic breccia were dredged within the submarine prolongation of the Sardinia Trough (~16.0-17.2 Ma; Rossi et al., 1998). Calcalkaline to shoshonitic basalts, andesites and dacites have also been recovered in other areas of the Ligurian-Provençal Sea, with K-Ar ages in the range 30-12 Ma (Beccaluva et al., 2004).

Abundant Upper Oligocene volcanic material, including porphyritic to aphyric clasts and glass shards, ranging in composition from basalt to rhyolite, as well as isolated crystals, including mainly plagioclase with minor pyroxene, amphibole, biotite and opaque minerals, is found in the Sub-Ligurian sandstone formations of the Northern Apennines. These include the Ranzano Formation where the volcanic material is ~ 32-30 Ma old (Cibin et al., 1998) and the Aveto-Petrignaccola Formation (Ar/Ar ages ~ 29-32.1 Ma; Mattioli et al., 2002). Oligo-Miocene volcanoclastic levels are also found in the Bisciaro Formation of the Umbria-Marche sector of the Apennines (~ 26.8-17.1 Ma; Balogh et al., 1993) and in several other localities of southern Italy (Critelli, 1993; De Capoa et al., 2002). These volcanic clasts may have originated from Sardinia or also from volcanic centres located along the western part of the Alpine Periadriatic line and from volcanic edifices presently buried under a thick pile of the Po plane sediments (e.g., Mortara volcanic body) (e.g., Anelli et al., 1994; Cibin et al., 1998; Fantoni et al., 1999; Mattioli et al., 2002; Garzanti & Malusà, 2008).

The Eocene-Miocene volcanic and minor sub-volcanic rocks of Sardinia range in composition from basaltic to rhyolitic on the TAS diagram (Fig. 17a) and mainly have a CA and HKCA affinity (Fig. 17b). Some evolved rocks reach peralkaline, trachyte and rhyolite compositions. Although dacites and rhyolites are by far the most abundant lithologies, most of the whole-rock geochemical and isotopic studies have concentrated on basalt and andesites.

Major and trace elements show decrease in MgO, CaO, FeO_tot, Ni and Cr, and an increase in alkalies with SiO₂. TiO₂ and Al₂O₃ show more or less bell-shaped trends with much scattering among basalts (TiO₂ ~ 0.3-1.7% and Al₂O₃ ~ 12.3-21.0 %). Some mafic rocks are rich in MgO (~ 9.0-13.4 %), Ni and Cr and have lower SiO₂, Al₂O₃ and FeO_tot compared to the high-Al basalts, and are considered to represent primitive mantle compositions (e.g., Morra et al., 1997; Mattioli et al., 2000; Franciosi et al., 2003). Major element variation from the high-Mg basalts to high-Al basalts are consistent with crystal/liquid fractionation dominated by olivine and clinopyroxene (Morra et al., 1997).

K₂O/Na₂O ratios for samples with MgO > 3% are generally less than unity ad show positive correlations with SiO₂. Most of the incompatible trace elements (e.g., Rb, Th, U, HFSE, LREE) increase continuously with increasing SiO₂, whereas Sr generally decreases with magma evolution. Ba increases constantly up to SiO₂ ~ 70% and then drops abruptly passing to the peralkaline rhyolites. The last show also extreme enrichments in Nb (up to 140 ppm) and Zr (up to 1120 ppm).

Figure 17. Alkali-silica diagrams of Oligo-Miocene rocks, Sardinia

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A - TAS and B - K₂O vs. SiO₂ classification diagrams for Sardinian orogenic igneous rocks. Some data for southern France (Provence) are also reported. Dashed line is the divide between subalkaline and alkaline rocks (Irvine & Baragar, 1971).

Trace element features of the least differentiated samples (MgO > 3%) are typical of subduction-related magmas. Mantle-normalized trace element patterns (Fig. 18) exhibit enrichments of LILE over HFSE, with pronounced K, Pb and Sr positive anomalies. REE patterns of mafic rocks show variable and generally low LREE/HREE fractionation, with La_NYb_N mostly < 8 and La generally below 100 times chondrite. Eu negative anomalies are generally absent and when present are small.

Figure 18. Spider-diagrams of Oligo-Miocene rocks, Sardinia

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Incompatible element patterns normalized to primordial mantle composition for mafic rocks (MgO > 3%) from Sardinia and Provence (Southern France).

Sr isotope compositions are close to mantle values for the most basic rocks (⁸⁷Sr/⁸⁶Sr ~ 0.704) and become progressively more radiogenic in the intermediate to felsic compositions. In particular, in a ⁸⁷Sr/⁸⁶Sr versus SiO₂ diagram (not shown) two distinct trends are evident, stemming from common relatively low ⁸⁷Sr/⁸⁶Sr (< 0.705) mafic compositions (SiO₂ < 50%). One trend is characterized by a steep increase of ⁸⁷Sr/⁸⁶Sr up to ~ 0.7113, whereas the other trend shows a sharp increase in the mafic to intermediate composition, and becomes flat in the acidic rocks, in which values of ⁸⁷Sr/⁸⁶Sr, ~ 0.7065-0.7075 are observed. The peralkaline felsic samples plot at the high-SiO₂ end of this second trend while the steep section of the trend is defined by basaltic to andesitic lavas from the Mt. Arcuentu area in south-western Sardinia. The Mt. Arcuentu lavas have also high whole-rock δ¹⁸O values, ranging from +6.4‰ in basalts to +12.4‰ in andesites. Clinopyroxene phenocrysts δ¹⁸O values for the same lavas are generally lower and in the range +6.17 to +7.46‰ (Downes et al., 2001). The differences between whole-rock and clinopyroxene δ¹⁸O values are probably do to late stage alteration of the ground mass (Downes et al., 2001).

Nd isotope ratios (= 0.51271-0.51217) exhibit the usual negative correlation with ⁸⁷Sr/⁸⁶Sr (Fig. 19). Pb isotopes for Sardinian orogenic magmas are moderately variable with ²⁰⁶Pb/²⁰⁴Pb ~ 18.53-18.75, ²⁰⁷Pb/²⁰⁴Pb ~ 15.62-15.68 and ²⁰⁸Pb/²⁰⁴Pb ~ 38.39-39.11.

Figure 19. Sr-Nd isotopes of Oligo-Miocene rocks, Sardinia

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Sr-Nd isotopic compositions of Sardinian orogenic igneous rocks. Data for southern France (Provence) are also reported (bleu squares).

Modest fractionation of REE have been suggested to reflect a magma origin from a MORB-type spinel-bearing lherzolite mantle source metasomatized by fluids/melts released from subducted oceanic crust (Morra et al., 1997). Franciosi et al. (2003) modelled trace element and isotopic compositions of the basaltic magmas with about 15% partial melting of MORB-like mantle source metasomatized by 0.1-0.5% fluid derived from altered MORB and less than 0.1% fluid derived from sediment. Fluid-induced rather than melt-induced mantle metasomatism was inferred mainly on the basis of the low Th/Pb and Th/Nd ratios displayed by the basaltic magmas (Franciosi et al., 2003). On the other hand, Downes et al. (2001) attributed the strong correlation of radiogenic isotope ratios with SiO₂ displayed by Mt. Arcuentu lavas, to addition of variable large amounts (2-10%) of subducted siliceous sediment to a depleted MORB-source. However, other authors (e.g., Franciosi et al., 2003) favour high degrees of crustal assimilation by ascending magmas.

In general, assimilation-fractional crystallization is considered to be the main process controlling the evolution of the Sardinian tertiary orogenic magmatism and responsible for the geochemical and isotopic characteristics (e.g., high ⁸⁷Sr/⁸⁶Sr) of the prevailing dacitic to rhyolitic lavas, including the peralkaline trachytic-rhyolitic rocks (e.g., Morra et al., 1994; Caron & Orgeval, 1996; Morra et al., 1997; Franciosi et al., 2003; Lustrino et al., 2004). Regarding the peralkaline felsic lavas from the Sulcis region, these can be derived from the same parental subalkaline magma that gave the calcalkaline lavas. Crystal/liquid fractionation processes involved first separation of plagioclase-pyroxene and then separation of plagioclase plus K-feldspar, promoting the jump from subalkaline to peralkaline compositions (Morra et al., 1994).

The geochemical and isotopic characteristics displayed by the Eocene-Miocene magmatism found in Sardinia are best explained by origin from a MORB-type mantle source variably metasomatized by fluids released from subducting oceanic lithosphere, with a minor contributions from sedimentary material. Magma differentiation was dominated by fractional crystallization and crustal assimilation processes.

The limited geochemical and isotopic data available for the volcanic and subvolcanic bodies outcropping in the neighbouring areas of Sardinia (e.g., southern France, Liguro-Provençal Sea and Corsica) do not allow to comprehensively infer source characteristics and magma differentiation processes for these igneous products. However, the close spatial distribution, the overlapping ages and the similar petrographic characteristics and calcalkaline affinities strongly suggests a common origin with the orogenic magmatism occurring in Sardinia.

The subduction-related geochemical characteristics of the Eocene-Miocene igneous rocks occurring in the areas surrounding and within the Liguro-Provençal basin are generally believed to be related to concomitant north-westward Apennine subduction (Lustrino et al., 2009 and references therein). However, some authors (e.g., Schettino & Turco, 2006) have also proposed that the Tertiary orogenic magmatism of Sardinia and surrounding areas, including volcaniclastic deposits of the Apennines, was related to south-eastward subduction of the Valais ocean along the westernmost continuation of the Alpine orogenic system.