Major- and trace-element and Sr-Nd-Pb isotope compositions are presented for an extensive data set of samples (44) recovered along the active ridge axis of the southern Tyrrhenian backarc basin 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 0.4–0.1 Ma) of Vavilov volcano, located in the oceanic subbasin (i.e., Vavilov Basin) formed during the first stage of the southern Tyrrhenian backarc basin formation. Overall, these data confirm that southern Tyrrhenian backarc basin magmatism is chemically heterogeneous, ranging from an island-arc basalt (IAB) type, prevalent throughout the backarc evolution, to an ocean-island basalt (OIB)–like magmatism that occurred later in the development of the basin. Since the lavas sampled from Vavilov volcano have been 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 backarc basin 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-ocean-ridge basalts (MORB) flooring the Vavilov backarc basin are derived, and the other has HIMU (high U/Pb) OIB-like character, resembling the mantle source of the nearby Ustica Na-alkaline lavas. Subduction-related signatures characterize both, although to a lesser extent in the OIB-like domain with respect to the MORB-like mantle. The new data provide a much needed insight into the evolution of the southern Tyrrhenian backarc basin 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 backarc 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 backarc volcano, and arcward-directed flow affecting the western margin of the Aeolian arc, thus compositionally influencing the Alicudi basic lavas.

In this work, unconformity-bounded stratigraphic (UBS) units are applied to a complex volcanic terrain. UBS units allow us to summarize the spatial-temporal relationships between the cartographic units of a volcanic district or complex, and identify individual successions of volcanic deposits and their probable connection with morphogenetic and/or tectonic phases. The UBS unit approach for the interpretation of significant preserved unconformity surfaces is supported by data from geomorphology, sedimentology, petrology, tectonics, and volcano-tectonics. The result is a stratigraphic scheme that combines the recognized lithostratigraphic units into a series of synthems, in which several phases of geological evolution of Ustica Island are recognized. The emplacement in the early Pleistocene of the basaltic Monte Guardia dei Turchi lithosome, which represents the emerged part of the volcanic seamount, was followed by hydromagmatic activity and construction of a tuff cone that is part of the Monte Costa del Fallo synthem. Following a period of quiescence, volcanism on the island resumed with a Plinian eruption that deposited a thick sequence of trachytic pumice breccias and was accompanied by a caldera collapse. Subsequently, marine terracing occurred in the southwest, while extrusive volcanics filled the caldera in the northern part of the island. Lava flows associated with this volcanism are present on the western flank of the volcano, whereas along the sublittoral zones, dike swarms provide evidence of phreatomagmatic activity. This volcanic activity was followed by repeated alluvial deposition and marine erosion. Finally, recent subaerial monogenetic centers produced shallow subvolcanic bodies, scoria and tuff cones, and lava flows.