Seminari di Scienze della Terra
Universita’ di Genova
30 Gennaio 2007

 Multidisciplinar Approach to Natural Triggers of Lithosphere Evolution

The Sr-Nd-Pb isotopic composition of the Sardinian RPV group, resemble closely the so called EAR (European Asthenospheric Reservoir), a very common signature throughout the European, North African and Middle Eastern igneous districts. Recent data seem to move back the beginning of the volcanic activity in Sardinia to middle Miocene (Serravalian; ~11.8 Ma). During middle Pliocene to upper Pleistocene (~3.9-0.1 Ma), a new volcanic activity (the UPV group) developed in the central-northern sector of the island, with totally different geochemical signatures. The UPV (Unradiogenic Pb Volcanics) group is by far the most abundant rock type of the island (>99% of the outcrops) and shows less alkaline (transitional) to subalkaline (tholeiitic) compositions. The most important geochemical characteristics of the UPV group are: a) relatively SiO2-rich compositions (SiO2 mostly >50 wt%); b) low CaO and CaO/Al2O3 ratios; c) slightly Cr- and Ni-rich compositions; d) relatively low HFSE (Zr, Nb, Ta, Hf) and HREE (e.g., Yb, Lu) content; e) high Ba/Nb, Ba/La and La/Nb but “normal” Ba content; f) relatively high 87Sr/86Sr and low 143Nd/144Nd; g) low 206Pb/204Pb and 208Pb/204Pb and high 207Pb/206Pb and 208Pb/206Pb. The major and trace elements, as well as Sr-Nd-Pb isotopic peculiarities of the Sardinian UPV group have been explained in term of interaction with lower crustal lithologies at mantle depths. SICILY CHANNEL: The volcanic rocks of Pantelleria (~118-4 ka) are entirely alkaline with sodic affinity; evolved rocks reach peralkaline compositions. Basalt and hawaiite flows are volumetrically scarce with respect to more evolved types such as trachytes and rhyolites. Variable trace element ratios and elevated 207Pb/204Pb are believed to be indicators of the involvement of ancient enriched mantle source components (which resemble the enriched lithosphere sampled by circum-Tyrrhenian potassic magmas) in addition to DMM-HIMU (±EMI) mantle sources. The total range of 87Sr/86Sr and 143Nd/144Nd is 0.7030-0.7033 and 0.51300-0.51292, respectively. Compared to the limited range of Sr-Nd isotope compositions, the Pb isotopic ratios show a much wider range (206Pb/204Pb = 18.30-19.94, 208Pb/204Pb = 38.04-39.62). No substantial differences in terms of Sr-Nd-Pb isotopic ratios are evident between trachytes/pantellerites and mafic rocks.

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