Petrogenesis of Italian alkaline lavas deduced from Pb-Sr-Nd relationships.

New Pb isotope data on 36 young (less than 2 Ma) alkaline volcanics from volcanic centers of Central and South Italy are integrated with available 87Sr/86Sr and 143Nd/144Nd data on the same rocks. The investigated samples are representative of both the potassic and high-potassium series exposed widely in the area. Most are mafic (MgO = 4.5 to 9.6 wt. %) and were selected in order to minimize the effects of shallow level AFC-type processes on the isotope systematics. Although some samples display covariations with MgO, indicative of these processes, the most primitive have widely variable 87Sr/86Sr, 143Nd/144Nd, 206Pb/204Pb, 207Pb/204Pb and 208Pb/204Pb ratios that are independent of MgO concentrations. This isotopic variability is attributed to enrichment processes which affected the source regions of magmas to different extents from NW to SE. In combined Sr-Nd-Pb diagrams, comparison of data for Italian potassic and sodic Volcanics shows that the isotopic variability is gradual from Latium (NW), where the most radiogenic Sr values occur, to Campania (SE) and the Aeolian Arc. Further southward, in sodic volcanics of Sicily, and at Pietre Nere (Adriatic coast), the lowest Sr ratios occur, while Pb and Nd ratios increase. These rocks appear to have been derived from source regions virtually unaffected by the enrichment processes and thus their isotopic composition should closely reflect that of the mantle source for the potassic alkaline magmatism of Italy prior to the enrichment. The potassic lavas result from mixtures of crustal and mantle components. The mantle component is represented by the sodic lavas of Etna and Pietre Nere, which are predominantly mixtures of depleted and enriched mantle components. The crustal component is accounted for by sediments similar to those observed in the floor of the Mediterranean Sea. Given the highly mafic character of most of the potassic rocks, bulk assimilation of sediment is unlikely, rather selective addition of crustal material by partial melting or fluid processes is more likely. This enrichment process is thought to be the result of a past subduction of lithosphere that occurred during the African-Eurasian collision. The sunken lithospheric body is still recognizable seismically under the Apennine Mountain Chain, constituting the backbone of the Italian peninsula, and is thought to be responsible for the deep seismic activity in the Western Tyrrhenian Sea. Seismic tomography indicates that a low velocity zone occurs at depths of ca. 100–200 km, which could represent the location of interaction between crustal and mantle components and thus of primary magma generation.