Mantle melting and the origin of the bimodal magmatism of Pantelleria volcano, Sicily Channel: Contraints from Combined U-Series and Sr-Nd-Pb isotopes

In the southernmost sector of Italy continental within-plate volcanism (e.g. Sicily Channel, Etna, Ustica) occurs in close geographical and geodynamic association with both subduction-related magmatism (e.g. Aeolian Islands) and ocean spreading (Tyrrhenian seafloor). Magmatism in the Sicily Channel (Pantelleria and Linosa) is due to the passive up-welling of asthenosopheric mantle in response to the development of pull-apart basins related to south-eastward roll-back of the subduction of the Ionian plate with respect to the rest of the subducting African plate. Within the Sicily Channel, Pantelleria, the type locality of pantellerite, is a Late-Pleistocene magmatic system characterised by a bimodal association of alkali basalts and peralkaline rocks (trachytes to pantellerites), separated by a large compositional gap in SiO2 (50–67 wt.%). The relationships between basaltic and differentiated magmas at Pantelleria, and in bimodal magmatic suite in general, have been the focus of debate in the last three decades. In particular the evolved per-alkaline products are interpreted either as derived from basaltic parental magmas by fractional crystallisation or by re-melting of a gabbroic cumulate. This study presents new 238U-230Th isotope data along with Sr, Nd and Pb isotope ratios and trace element contents of both the mafic and the felsic peralkaline products of Pantelleria volcano. 235U-231Pa disequilibria have also been measured on two samples with basaltic and hawaiitic composition. The aim of this contribution is to investigate i) the melting regimes responsible for the generation of the least differentiated alkali basaltic products, and ii) the relationships existing between mafic and differentiated peralkaline magmas. Pantelleria mafic products have ubiquitous 230Th excesses ranging from 7% to 21%. This range is due to a rather wide variation of (238U/232Th) at an almost constant (230Th/232Th). A notable exception is constituted by two hawaiitic samples that presents comparable 230Th-238U disequilibria with alkali basalts, but at significantly lower (230Th/232Th) and (238U/232Th). Hawaiites also display similar (231Pa/235U), but different Sr, Nd and Pb isotopes with respect to alkali basalts suggesting these two types of magma derive from two distinct mantle sources, but similar melting regimes. Trachytes and pantellerites also have (238U/232Th) similar to those of alkali basalts, but slightly lower (230Th/232Th). This feature argues against the possibility of generating the peralkaline products by re-melting of a gabbroic cumulate, favouring an origin through fractional crystallisation in timescales comparable to the half-life of 230Th.