Magmatic evidence for African mantle propagation into the southern Tyrrhenian backarc region.

Major, trace element and Sr-Nd-Pb isotope compositions are presented for an extensive dataset of samples (44) recovered along the active ridge axis of the southern Tyrrhenian back-arc basin (BAB) represented by the Marsili volcano (< 0.7 Ma). In addition, major and trace elements are presented for the few lavas sampled from the summit area (active at 0.4 to 0.1 Ma) of the Vavilov volcano, located in the oceanic subbasin (i.e., Vavilov Basin) formed at the first stage of the southern Tyrrhenian BAB formation. Overall, these data confirm that the southern Tyrrhenian BAB magmatism is chemically heterogeneous ranging from an island arc basalt (IAB) type, prevalent throughout the back-arc evolution, to an ocean island basalt (OIB)-like magmatism that occurred later in the development of the basin. Since the lavas sampled from the Vavilov volcano resulted strongly altered, their isotope composition was not acquired in this study. Thus, our attempt to identify the mantle domains beneath the southern Tyrrhenian system was restricted to its youngest and active part, i.e., the Marsili BAB and the Aeolian Arc. These new data together with previously published trace element and Sr-Nd-Pb isotope data for Marsili volcano lavas reveal that two mantle domains are involved in their petrogenesis: one represents the southern Tyrrhenian ambient mantle from which the mid-oceanic ridge basalts (MORB) flooring the Vavilov BAB derived, and the other of HIMU OIB-like character, resembling the mantle source of the nearby Ustica Na-alkaline lavas. Subduction-related signatures characterise both, although to a lesser extent in the OIB-like domain respect to the MORB-like mantle. The new data provide a much needed insight into the evolution of the southern Tyrrhenian BAB system, confirming that the HIMU OIB-like component results from the propagation of deep northern African mantle inflow around the southern tear of the subducting Ionian slab, rather than being a component that was present in the mantle wedge prior to the Ionian subduction process. Furthermore, a comparison between the Marsili back-arc and Aeolian Arc lavas permits interpretation of the trajectories followed by the African OIB-like mantle inflow within the southern Tyrrhenian mantle wedge. In particular, mantle trajectories involve upward-directed flow from the slab edge beneath Ustica Island and Prometeo submarine lava field, slab-parallel flow beneath the Marsili back-arc volcano and arcward-directed flow affecting the western margin of the Aeolian Arc, thus compositionally influencing the Alicudi basic lavas.