The Miocene Magmatic events of the Western Tyrrhenian Magmatic Province (Corsican)
Miocene ultrapotassic, shoshonitic and high-K calc-alkaline igneous rocks are found in close temporal association distributed along the Eastern margin of the Sardinia-Corsica micro-plate, although a small monogenetic volcano at Zenobito volcano was produced during the Pliocene (Fig. 9). Ultrapotassic rocks, with lamproitic affinity, are found in the Northeastern portion of the Corsica Island, 15 km north of Bastia, in the form of a sill intruded into the Alpine terranes belonging to the “Schistes Lustrés” (i.e., Serie of the Castagniccia; e.g., Velde, 1967; Wagner & Velde, 1986; Peccerillo et al., 1988); shoshonitic to high-K calc-alkaline sub-volcanic to volcanic rocks are found few kms offshore from Sardinia and Corsica islands, at Sarcya seamount (Cornacya) and Capraia Island, respectively (e.g., Mascle et al., 2001; Chelazzi et al., 2006; Conticelli et al., 2007, 2009a, 2011a; Gasparon et al., 2008; Avanzinelli et al., 2009). The Punta dello Zenobito volcano is a small monogenetic center lying on the southwestern edge of the Capraia Island, and it is made up by a cinder cone and few lava flows.
Figure 9. Distribution of Miocene ultrapotassic igneous rocks (dykes and volcanic rocks) and associated shoshonites and calc-alkaline rocks from western Tyrrhenian Sea (Corsica Magmatic Province)
Ultrapotassic rocks are found exclusively along the northeastern edge of the Corsica island, whereas shoshonites are from submarine centers offshore of the southeastern edge of Sardinia Island. High-K calc-alkaline are from the Capraia volcano, located within the northen Tyrrhenian Sea (Conticelli et al., 2011a).
The oldest sampled product of the Corsica Magmatic Province is the Sisco lamproite: one whole rock, one K-feldspar and two phlogopites of different grain-size, for a total of six analyses, have been used to calculate a K/Ar isochron plot of 14.58 ± 0.2 Ma (Civetta et al., 1978). Mascle et al. (2001) report a 40Ar-39Ar total age of 12.6 ± 0.3 Ma (± 1s) for a dredged andesitic pebble from Cornacya. This age was calculated from 15 in-situ 40Ar-39Ar analyses performed on a single automorph crystal of biotite; a weighted average calculation on the same data points, carried out in this paper, gives an age of 13.07 ± 0.5 Ma (95% conf. lev., MSWD=1.5), slightly older but equal within error to the total age. It is worth to note that both Sisco and Cornacya age data are related to a unique sample, hence there aren’t data that allow to infer the time span of magmatic activity, if any.
The dating of volcanic products of Capraia Island displays a noticeable disagreement among K/Ar, Ar-Ar and Rb/Sr data. Whereas K/Ar (9.8-5.0 Ma: Borsi, 1967; Pierattini, 1978) and Rb/Sr biotite-whole rock data (6.9-3.5 Ma: Barberi et al., 1986, abstract only) evidence a quite long volcanic history, Ar-Ar data (Gasparon et al., 2009) show that the majority of the actually subaerial samples are comprised in a narrow interval of time (7.8-7.2 Ma). Age data related to Punta dello Zenobito monogenetic volcano, whose orogenic affinity has been questioned (Chelazzi et al., 2006; Conticelli et al., 2007, 2009a, 2011a) disagree as well. K/Ar and Ar-Ar data are quite concordant within error (4.93 Ma, Borsi, 1967, and 4.76 Ma, Gasparon et al., 2009, respectively), while either the 2.72 Ma K/Ar datum of Pierattini (1978) or the 3.92 Ma Rb/Sr datum of Barberi et al. (1986) are younger. Ar-Ar ages are on average preferred relative to K/Ar model ages, but the spread registered also by Rb/Sr biotite ages might be indicative of hidden problems in dating Capraia samples.
The Sisco lamproite is a leucite- and plagioclase-free ultrapotassic rocks with intersertal texture and a parageneses made of phlogopite, clinopyroxene, olivine, sanidine, and K-richterite associated to subordinate abundance of chromian spinel, ilmenite, pseudobroockite and priderite. Rare roedderite has also been found. Si+Al tetrahedral deficiency has been also observed in the chain silicate minerals (Wagner & Velde, 1986). Shoshonitic (Cornacya) and high-K calc-alkaline rocks (Capraia) range from shoshonites, to olivine latites, trachytes, high-K andesites, trachy-dacites, and rhyolites (Fig. 10a), and they are characterised by the occurrence of modal plagioclase, with sanidine and horneblende restricted to the most differentiated terms (Mascle et al., 2001; Gagnevin et al., 2007; Conticelli et al., 2011a). The Punta dello Zenobito volcanic rocks are made up by porphyritic trachy-andesites with olivine and clinopyroxene phenocrysts set in a groundmass made up by abundant plagioclase.
The Sisco lamproite has a peralkaline index > 1, with high MgO (6.4-7.1 wt.%, Peccerillo et al., 1988). Sisco lamproite has the highest K2O and the lowest Al2O3 among the whole Central Mediterranean lamproites (Prelevic et al., 2008; Conticelli et al., 2009a). Shoshonitic rocks from Cornacya show a wide compositional range from mafic to felsic commons (Mascle et al., 2001). High-K calc-alkaline rocks from Capraia volcano are intermediate to felsic in compositions, with MgO between 1.38 and 3.91 wt. % (Table 3), overlapping the trend of high-K calc-alkaline rocks from Western Alps and Murcia-Almeria (Conticelli et al., 2009a, 2011a). The Punta dello Zenobito volcanic rocks, which are youngest ones of this region show peculiar compositional characteristics with mild enrichment in TiO2, a characteristic not observed in the older Cornacya and Capraia rocks (Table 3; Chelazzi et al., 2006).
Similarly to the the Western Alps ultrapotassic and related rocks, all mafic rocks of this association are enriched in incompatible trace elements, at different levels for the different magmatic series, with a clear positive correlation with K2O contents (Conticelli et al., 2009a). Throughs at Ba Ta, Nb, P and Ti are observed, although with some characteristic difference, with peaks at Th, U, Pb, and Sm (Fig. 10c). Sisco lamproite are distinguished by all other ultrapotassic Mediterranean rocks for their high Hf and Zr contents, and for their lowest enrichments in Cs, Pb, and U (Fig. 10c). The youngest volcanic episode of Punta dello Zenobito volcano is also characterised by the smallest LILE/HFSE fractionation with an almost flat pattern for incompatible trace elements (Fig. 10c), a characteristic that speak for the occurrence of an important within plate signature in its mantle source.
Figure 10. Classification and incompatible trace element characteristics of Western Tyrrhenian Miocene ultrapotassic and related rocks
Classification and geochemical characteristics of Western Tyrrhenian (Corsican) Miocene ultrapotassic (lamproites) and related rocks. A) Total Alkali-Silica (TAS) classification diagram (Le Maitre, 2002). B) K2O wt.% vs. SiO2 wt.% classification diagram with reported the grid for orogenic volcanic rock suites (Peccerillo & Taylor, 1976). C) Incompatible trace element patterns for mafic volcanic rocks normalised to the primordial mantle values of Sun & McDonough (1989). Data from Peccerillo et al. (1988); Conticelli and Peccerillo (1992); Mascle et al. (2001); Conticelli et al. (2002, 2007, 2009a, 2011a); Prelevic et al. (2008).
