The within-plate Na-alkaline magmatism of the Pelagian Block (Southern Italy): Sr-Nd-Pb and U-series isotope constraints from Pantelleria, Linosa and Pachino-Capo Passero basalts

The origin of the mantle sources of the Na-alkaline magmas erupted within the Pelagian Block (southern Italy) are here investigated through the determination of Sr, Nd and Pb isotope ratios on basaltic volcanic from Linosa, Pantelleria and Pachino-Capo Passero. Despite their geographical proximity these rocks were erupted over a large time span (Upper Cretaceous to Quaternary). U-series disequilibria have been measured on the recent products of Pantelleria (120-9 ka) volcano in order to constrain the mechanism of mantle melting in relationship to the geodynamic setting of the studied area. The isotope data, along with trace element ratios are used to assess the possible role of the interaction with the continental crust and/or the Sub-Continental Lithospheric Mantle (SCLM). The data show little variation in Sr and Nd isotopes and a continuous trend towards more radiogenic Pb isotope composition from Linosa to the oldest mafic activity of Pantelleria (i.e. Paleo-Pantelleria) and Pachino-Capo Passero, with intermediate values measured in the youngest Pantelleria lavas (Neo-Pantelleria). The Sr-Nd-Pb isotope compositions of the studied rocks are also compared with literature data from the Canary Islands, as representative of a putative plume component contaminating the mantle region, but some clear distinction, especially in terms of Pb isotopes are observed. The increasing FOZO-like character of the studied magmas and the variation of some key trace element ratios is interpreted as a feature of the ambient asthenospheric mantle, due to the extremely widespread process of prolonged subduction and recycling of mafic oceanic crust of variable age and composition. Pantelleria basalts have ubiquitous 230Th-excess ranging from 7% to 20%. These data suggest the magmas are originated within the asthenospheric mantle, with little or no interaction with either the continental crust or the SCLM. The Coupled 230Th-238U and 231Pa-235U data in a Pantelleria sample were used to perform quantitative dynamic melting models in order to constrain physical parameters of mantle melting in the Sicily Channel. The combined modelling yielded positive solutions only for high DU/DTh (≥ 2.5) and low melting rates (-4 kg/m3/a). These data argue against any important role for amphibole in the genesis of these magmas and are consistent with a peridotite source possibly well mixed with recycled components. The modelled values of melting rate can also be converted into estimates of the upwelling rate of the mantle that are compatible with slow passive upwelling along the Sicily Channel rift.