Triassic Tholeiitic and alkaline basaltic magmatism in western central Sicily (southern Italy): geochemical and isotopic clues to mantle sources involved during early continental rifting.

During Early Mesozoic, a diffuse magmatic activity developed in western-central Sicily in relation to the extensional tectonics associated with the opening of the Neotethys Ocean. In this work, we report and discuss geochemical and Sr-Nd isotopic data for Sicilian Triassic tholeiitic and alkaline volcanic rocks. Tholeiitic basalts from western Sicily are characterized by relative HFSE enrichment, high HFSE/LILE ratios, flat REE patterns (LaN/YbN = 2.27-2.48), Ti, Sr and K negative anomalies and Pb peaks in primitive mantle-normalized diagrams. The Sr (87Sr/86Sri = 0.707-0.708) and Nd isotopic compositions (εNdi = from -1.7 to -2.2) are compatible with an origin from enriched mantle sources. Additionally, the relatively high Sr isotopic ratios and diagnostic trace elements ratios, such as La/Nb (~ 17), LaN/SmN (~ 1.20), Nb/U (~ 20) and Pb/Nd (~ 0.3), could reflect involvement of the lower crust in the magmatic evolution of the tholeiitic rocks. Alkali basalts from central Sicily preserve a clear geochemical signature despite the intense alteration. The behaviour of immobile elements such as the low Y/Nb and Zr/Nb and high Th/Yb and Ta/Yb ratios, positive Nb-Ta-Ti anomalies, and the radiogenic Nd isotope ratio (εNdi = +3.4 to +3.7) suggests an OIB-like source. The LREE/HREE ratio (LaN/YbN = 7.43-8.76) is higher than in the tholeiitic samples, suggesting a possible role of residual garnet in the magma source, or a lower degree of partial melting, or both. The behaviour of mobile elements, such as the Sr isotopic composition (87Sr/86Sri = 0.706-0.707) and the K depletion, does not contradict this. On the basis of trace elements and Sr-Nd isotopic composition, the Sicilian Triassic dykes could reflect the early stages of rifting in a continental setting, including initial lithospheric thinning, passive upwelling of hot asthenosphere, and related processes of partial melting involving differently enriched mantle reservoirs.