Intraplate Cenozoic volcanism in Italy
The petrogenesis and tectonomagmatic significance of the Veneto, Iblei and Sardinia volcanic provinces is discussed taking first into consideration the origin of primary basic melts (MgO>7), namely the P-T-X (pressure, temperature and composition) conditions of mantle sources from which they were generated. Mantle xenoliths, often exhumed by basic alkaline volcanics in the three provinces, were also considered since they represent direct evidence of the lithosphere overlying the magma sources. In fact, the integrated study of near-primary melts and associated mantle xenoliths, and comparison with petrological experimental petrology results (Green and Falloon, 2005 and references therein) can provide important quantitative constraints on P-T-X conditions of magma generation, and extent of partial melting processes (Frey et al., 1978; Beccaluva et al., 1998; 2005b; 2007a).
Geochemical characteristics of basic magmas and associated xenoliths are discussed in terms of the notional mantle components defined for Ocean Island Basalts (OIB) (Zindler and Hart, 1986; Weaver, 1991; Carlson, 1995; Hoffmann, 1997).
In the following sections we discuss the distinctive geochemical features of each volcanic provinces focussing attention on the mantle sources characteristics and P-T conditions of magma generation.
Paleogene volcanism of the Veneto volcanic province
The Eocene-Oligocene Veneto Volcanic Province consists of a number of NNW-SSE oriented eruptive centres related to tensional tectonics of the Adria foreland in response to the Alpine collisional events (Beccaluva et al., 2005b and references therein). The lavas of the Veneto Volcanic Province (< 20 km3) are represented by relatively undifferentiated magmas which include (mela)-nephelinites, basanites, alkaline basalts, transitional basalts and tholeiites (Beccaluva et al., 2007a). This volcanism bears close analogies with those of the “low volcanicity” impactogenic rifts, strong transtensional systems (Barberi et al., 1982) and similarly characterized by small volumes of scarcely fractionated lavas covering a wide alkalinity range.
Southward, along the Adriatic plate, similar anorogenic magmatic occurrences are represented by the Paleogene Mt. Queglia lamprophyric dykes and Na-alkaline Pietre Nere subvolcanic body (Conticelli et al., 2007; Bianchini et al., 2008; Avanzinelli et al., 2011).
The petrological characteristics of the least differentiated basic magmas are compatible with their segregation from lithospheric mantle sources located between 30 and 90 km depth (Fig. 2). The entire P-T segregation trend plots from ca. 9 kb/900°C to ca. 28 kb/1400°C between the experimental dry and hydrated-carbonated mantle solidi, approaching the inferred regional geotherm to depth. This suggests that partial melting processes could be easily triggered by local decompression effects, in turn related to a limited intraplate tensional regime.
The lavas of the Veneto Volcanic Province show incompatible element patterns and Sr-Nd isotope ratios similar to those of OIB-type magmas, particularly with HIMU-like affinity (Fig. 3). Pb isotopes confirm that Veneto magmas have a composition intermediate between DM and HIMU components, showing analogies with the magmatic rocks from Pietre Nere and Mt. Queglia (Fig. 4). Note that the extremely high Pb composition of Mt. Queglia lamprophyres could also results by time integrated radiogenic ingrowth (Avanzinelli et al., 2011).
Figure 2. P-T conditions of magmagenesis
Calculated P-T conditions of magma segregation for Veneto, Iblei and Sardinia intraplate primary melts, according to Albarede (1992). Results are consistent with phase equilibria constraints (Falloon and Green, 1988; Falloon et al., 1988; Hirose and Kushiro, 1993; Hirose and Kawamoto, 1995; Olaffson and Eggler, 1983; White and Wyllie, 1992; Green and Falloon, 2005): tholeiitic basalts, 10-16 kb, 1150-1250°C; alkali-basalts, 14-22 kb, 1200-1280°C; basanites and nephelinites, > 22 kb, 1250-1350°C. Experimental mantle peridotite solidi for dry and H2O-CO2 conditions after Green et al. (1987) and Wyllie (1987). MBL (Mechanical Boundary Layer), TBL (Thermal Boundary Layer) and MOHO, conductive geotherm and equilibration conditions of mantle xenoliths from Beccaluva et al., 2005b and references therein.
Therefore, geochemical data coherently indicate that the pristine depleted lithospheric mantle of the Adriatic microplate was pervasively enriched by metasomatizing agents with prevalent HIMU-like isotopic and elemental fingerprint, usually referred to as EAR (European Asthenospheric Reservoir; Cebria and Wilson 1995) or LVC (Low Velocity Component; Hoernle et al., 1995). The influence of these metasomatic components is widespread from eastern Atlantic to Europe and the Mediterranean area at least since Late Cretaceous (Wilson and Bianchini, 1999; Beccaluva et al., 2005b; Bianchini et al., 2008).
The most alkaline lavas of the Veneto Volcanic Province often include spinel-peridotite xenoliths that, according to thermobarometric estimates, come from the mechanical boundary layer of the underlying lithosphere at depths not exceeding 50-60 km (P<20kb; Siena and Coltorti, 1993; Beccaluva et al., 2001a; Fig. 2). These mantle xenoliths consist of predominant spinel lherzolites and minor harzburgites characterised by widespread metasomatic (pyrometamorphic) textures consisting of spongy clinopyroxene, and variably recrystallized glassy patches (Siena and Coltorti, 1989 and 1993; Coltorti et al., 2000; Beccaluva et al., 2001a).
These peridotites are variably enriched in Light (L) Rare Earth Elements (REE), showing LaN/YbN up to 19.2, related to metasomatic processes (Beccaluva et al., 2001a). Metasomatic effects are confirmed by parallel LREE enrichment of the constituent clinopyroxenes (LaN/YbN up to 5.7). The metasomatic processes can be accounted for by the addition of 1-6% alkaline basic melt/s to the pristine peridotite matrix (Beccaluva et al., 2001a).
The Sr-Nd isotope compositions of mantle xenoliths from the Veneto Volcanic Province (whole rocks and clinopyroxene separates, Fig. 5) plot between the depleted mantle (DM) and the HIMU components, confirming that the latter represent the isotopic signature of the metasomatizing agent.
Figure 3. Sr-Nd isotopic composition of anorogenic lavas from the central Mediterranean area
Sr-Nd isotopic composition of intraplate lavas from (a) volcanic districts on the African and Adriatic lithosphere and (b) volcanic districts on the European lithosphere. Data for the Veneto Volcanic Province are from Macera et al. (2003) and Beccaluva et al. (2007a). Data for the neighbouring Euganean occurrence are from Milani et al. (1999). Composition of the Calceranica dykes is from Galassi et al. (1994). Data on other magmatic occurrences of the Adria plate, such as Pietre Nere and Mt. Queglia are from Hawkesworth and Vollmer (1979); Conticelli et al. (2007), Bianchini et al. (2008), Avanzinelli et al. (2011).
Data for the Iblean Volcanic Province lavas are from Tonarini et al. (1996), Trua et al. (1998) and Bianchini et al (1999). Data for the Sicily Channel from Esperança and Crisci (1995) and Civetta et al. (1998). Data for Mt. Etna from Armienti et al. (2004). Compositional field for Hoggar alkaline lavas is from Allegre et al. (1981) and Dupuy et al. (1993). Data for the Sardinian Volcanic Province from Lustrino et al. (2000; 2002), and Gasperini et al. (2000) are integrated with Authors unpublished data. Further compositional fields: Massif Central (Wilson and Downes, 1991; Wilson and Patterson, 2001) and Languedoc (Dautria et al., 2010) in France, Olot in NE Spain (Cebrià et al., 2000) and the western Germany volcanic districts (Wörner et al., 1986; Kramm and Wedephol, 1990; Wedephol et al., 1994). Geochemical components DM, HIMU, EM1, and EM2 after Zindler and Hart (1986).
Figure 4. Pb isotopic composition of anorogenic lavas from the central Mediterranean area
Pb isotopic composition of of intraplate lavas from Central-Western Mediterranean.
Data on lavas from the Veneto Volcanic Province from Macera et al. (2003) and Beccaluva et al. (2007a). Data for the Iblean Volcanic Province lavas are from Trua et al. (1998) and Bianchini et al. (1999). Data for Sardinian lavas are from Gasperini et al. (2000) and Lustrino et al. (2000; 2002). The compositional fields of other within-plate magmatic occurrences from the Adria/North Africa domain are reported for comparison: Pietre Nere (Vollmer, 1976; Conticelli et al., 2007; Avanzinelli et al., 2011); Mt Queglia dikes (Bianchini et al., 2008); Mt. Etna (Carter and Civetta, 1977); Sicily Channel (Esperança and Crisci, 1995; Civetta et al., 1998). Also reported compositional fields of some European volcanic districts: Massif Central (Wilson and Downes, 1991) and Languedoc (Dautria et al., 2010) in France; Olot in NE Spain (Cebrià et al., 2000) and western Germany volcanic fields (Wedepohl and Baumann, 1999). Geochemical components DM, HIMU, EM1, and EM2 after Zindler and Hart (1986).
Figure 5. Sr-Nd isotopic composition of mantle xenoliths from the central Mediterranean area
Sr-Nd isotopic composition of mantle xenoliths from (a) volcanic districts on the African and Adriatic lithosphere; data on peridotite xenoliths from the Veneto Volcanic Province from Gasperini et al. (2006) and Beccaluva et al. (2007a); data for Iblean xenoliths are from Tonarini et al. (1996) and Bianchini et al. (2010a); Other data for African xenoliths occurrences are from (Beccaluva et al., 2007b; 2008; Lucassen et al., 2008; Wittig et al., 2010). (b) volcanic districts on the European lithosphere; data on Sardinian peridotite xenoliths from Beccaluva et al., 2001b; peridotite xenoliths from the Massif Central and Languedoc from Wilson and Downes (1991), Zangana et al. (1997), Downes (2001) and Dautria et al. (2010); peridotite xenoliths from Olot are from Bianchini et al., 2007. Data on orogenic peridotite massifs are also reported for comparison (Bodinier et al. 1991; Downes et al. 1991). Mantle end-member compositions from Zindler and Hart (1986) are also shown.
Neogene-Quaternary volcanism of the Iblean volcanic province
The Miocene and Pliocene-Pleistocene Iblean volcanism developed with subaerial and submarine eruptions, ranging in composition from tholeiitic to nephelinitic lavas along a regional NE-SW lithospheric wrench fault system oblique to the Maghrebian chain in Sicily (Bianchini et al, 1998; Beccaluva et al., 1998; Di Grande et al., 2002).
The Miocene (mainly Tortonian) volcanic phase was dominantly represented by alkaline diatremes; volcanism resumed in the Pliocene with volumetrically predominant tholeiitic lavas, followed in order of decreasing abundance by basanites, alkali-basalts + hawaiites, transitional basalts and nephelinites. This volcanism is therefore similar to that of the Veneto Volcanic Province, and it can be similarly ascribed to a “low volcanicity” transtensional rift system.
Accordingly, incompatible element patterns for the Iblean basic lavas mainly display HIMU OIB affinity, and are closely comparable to those of analogous lavas from the Veneto Volcanic Province. Sr-Nd isotope data (Fig. 3) range from the depleted mantle (DM) to HIMU-like signatures (Beccaluva et al., 1998; Bianchini et al., 1998; 1999; Trua et al., 1998). In particular, subalkaline lavas approach the DM composition (87Sr/86Sr 0.70271 – 0.70303 and 143Nd/144Nd 0.51325 – 0.51299), whereas alkaline products show a more marked HIMU-like affinity (87Sr/86Sr 0.70287 – 0.70328 and 143Nd/144Nd 0.51302 – 0.51291).
The HIMU signature is confirmed by the high 206Pb/204Pb ratios (Fig. 4) which range from 19 (in tholeiites) to 19.9 in alkaline lavas (Carter and Civetta 1977; Trua et al., 1998; Bianchini et al., 1999).
Petrogenetic and thermobarometric estimates for Iblean magmas indicate spinel peridotite lithospheric mantle sources (30 to ca. 110 km depth). These are progressively deeper, from tholeiites to nephelinites, with a parallel decrease in the degree of melting (≈ 30 to ≈ 3%; Beccaluva et al., 1998; 2005b).
It should be noted that incompatible element and Sr-Nd-Pb isotopic compositions of the Iblean lavas show remarkable analogies with those from the Sicily Channel (Linosa and Pantelleria islands; Esperança and Crisci, 1995; Civetta et al., 1998; Di Bella et al., 2008), consistently indicating the prevalence of the HIMU metasomatic components in magma sources of the Adriatic and north African plates.
Northward, the Etnean lavas while sharing petrological characteristics with the Iblean products, display a comparatively distinct enrichment of Low Field Strength Elements (LFSE) suggesting a possible influence of the neighbouring Ionian subduction on the Etna volcanism (Beccaluva et al., 1982; Cristofolini et al., 1987). Detailed Sr-Nd-B isotopic data confirmed that the Etnean mantle sources have been influenced by the Ionian subduction (Tonarini et al., 2001).
Spinel-peridotite mantle xenoliths, ranging from lherzolites to harzburgites, are commonly included in the Iblean alkaline lavas, with largest-sized and most abundant samples occurring in nephelinitic Miocene diatremes (Di Grande et al., 2002 and references therein). Thermobarometric estimates from CO2 fluid inclusion data (Bergamini, 1992; Siena and Coltorti, 1993) and crystallochemistry of clinopyroxenes (Nimis, 1998) indicate a pressure range of equilibration conditions between 9 and 15 kb (≤ 50 km, Fig. 2).
Modal metasomatism is widespread and testified by secondary phases including phlogopite and feldspar, spongy borders in clinopyroxene and glassy patches.
REE distribution of peridotites and constituent clinopyroxene confirms interactions between alkaline metasomatic agent(s) and the mantle peridotite matrix inducing general LREE enrichment, with LaN/YbN up to 26 in bulk rock (Sapienza and Scribano, 2000; Beccaluva et al., 2005b) and up to 20 in clinopyroxene (Perinelli et al., 2008; Beccaluva et al., 2005b).
Isotopic data by Bianchini et al. (2010a) on separated clinopyroxene cluster around the HIMU component (87Sr/86Sr from 0.70271 to 0.70330, and 143Nd/144Nd from 0.51291 to 0.51325, Fig. 5), thus suggesting that the latter represents the geochemical signature of the metasomatizing agents, as already observed for xenoliths from the Veneto Volcanic Province. The Sr and Nd isotopes of pyroxenite samples included in the xenolith population (0.70305-0.70326; 0.51292-0.51299) perfectly overlap the field of the Iblean alkaline lavas.
Neogene-Quaternary volcanism in Sardinia
Within-plate Neogene-Quaternary fissural volcanism in Sardinia took place in concomitance with the Late Miocene rifting phase which ultimately lead to the opening of the Tyrrhenian basin.
Alkaline, transitional and subalkaline differentiation series can be observed in three representative volcanic complexes: alkali basalts/trachybasalts and basanites, to trachyphonolites and phonolites at Montiferro; transitional basalts to quartz-trachytes at Capo Ferrato and Mt. Arci; subalkaline basalts to rhyolites at Mt. Arci (Beccaluva et al., 2005b). Although some sporadic events occurred since 12 Ma (Lustrino et al., 2009 and references therein), most of the volcanic activity took place in the time span from 4 to < 0.2 Ma, with the climax of subalkaline activity slightly earlier (3.5-3.0 Ma) than that of alkaline volcanism (Beccaluva et al., 1985).
The incompatible element abundances of the least differentiated mafic lavas of the various volcanic series exhibit a close correspondence to typical EMI OIB patterns (Weaver, 1991). Petrological modelling (Beccaluva et al., 2005b and references therein) and P-T estimates (Fig. 2) suggests that subalkaline basalts to basanites could be generated at increasing depth (30-80 km), by decreasing partial melting degrees (from 25 to 6%) from lherzolitic sources.
Sr-Nd-Pb isotopic data cover a wide compositional range (87Sr/86Sr: 0.70315 - 0.70534; 143Nd/144Nd: 0.51289 - 0.51235; 206Pb/204Pb: 17.5 - 18.0, Fig. 3 and 4) generally corresponding to EMI geochemical signature except for a few samples showing HIMU-like affinity (Lustrino et al., 2000 and 2002; Gasperini et al., 2000). It should be remarked that although partially overlapping the field of other Cenozoic European lavas, most of the Sardinian volcanic rocks exhibit extreme EM1 composition. This metasomatic component is more marked in subalkaline basalts (87Sr/86Sr 0.70453 - 0.70534, 143Nd/144Nd 0.51254 - 0.51235), whereas alkali-basalts and basanites are displaced to lower Sr and higher Nd isotopic ratios (0.70315-0.70514, 0.51289-0.51251). This suggests that the EMI metasomatic component was preferentially preserved in the shallow lithospheric mantle (<50-60 km depth) where subalkaline basalts were generated.
Mantle xenoliths, ranging in composition from spinel lherzolites to spinel harzburgites were entrained by alkaline basic lavas from type localities in eastern (Dorgali) and western (Scano) Sardinia. According to CO2 fluid inclusion data and thermobarometric estimates (Beccaluva et al., 1989; Siena and Coltorti, 1993), they represent the uppermost lithospheric mantle (ca. 40 km depth). Xenoliths from Dorgali are lherzolites (cpx content varying from 6% up to 16%), whereas those from Scano are mostly harzburgites with subordinate cpx-poor lherzolites (clinopyroxene content never exceeding 10%). Evidence of metasomatic processes is provided by pyrometamorphic textures with feldspar, phlogopite and glassy blebs.
Metasomatic effects, attributable to alkaline agents, are confirmed by LREE-enriched patterns in both whole rock (LaN/YbN up to 40.7) and constituent clinopyroxene (up to LaN/YbN 45.0).
The Sr-Nd isotope composition of the Sardinian xenoliths (whole rock and clinopyroxene separates, Fig. 5) shows a large range of variation, extending from the DM toward the EMI end-member (87Sr/86Sr 0.70262-0.70461 and 143Nd/144Nd 0.51323-0.51254), conforming to the geochemical fingerprint of the associated lavas.