Petrology and K/Ar ages of volcanics dredged from the Eolian seamounts: implications for geodynamic evolution of the southern Tyrrhenian basin

Abstract

Systematic marine investigations carried out in the last decade indicate that the Eolian island orogenic volcanism extends to the seamounts located on the western (Sisifo, Enarete, Eolo Seamounts) and the northeastern (Alcione, Lametini Seamounts) sides of the emerged Eolian Island arc, as well as on the upper part of Palinuro and Marsili Seamounts, constituting on the whole a ring-like structure. Basaltic to rhyolitic lava samples dredged from these localities mostly belong to calc-alkaline and shoshonitic associations and are strictly comparable, both in petrographical and geochemical characteristics, to subaerial products outcropping on the Eolian islands. Moreover a few tholeiitic basalts, with island arc affinity, have been recovered for the first time from north Lametini and lowermost Eolian slope. The calc-alkaline magmatic activity appears to date as far back as1.3–0.9 ± 0.2m.y. to the west (Sisifo Seamount) and probably postdates (or is synchronous with) the tholeiitic episodes, whereas the oldest shoshonitic volcanism so far found at Eolo and Enarete Seamounts has an age of 0.85–0.64 ± 0.06m.y. The geochronological data indicate a general trend of within-serial rejuvenation of the volcanism moving counterclockwise from the Sisifo area, as well as a chronological zonation of magmatic products characterized by a rapid transition, within the time span of about 0.1 m.y., to more abundant shoshonitic and leucite-tephritic lavas in limited portions of the structure (Lipari, Vulcano and Stromboli). Model calculations based on a large spectrum of incompatible elements indicate that the parental melts of the various magma series could be derived by different partial melting degrees of spinel- to garnet-peridotite mantle sources heterogeneously enriched through the influx of distinct metasomatizing fluids driven off the subduction zone. Subduction reactivation and the related Eolian volcanism appear to be diachronous with respect to the oceanic spreading in the Tyrrhenian marginal basin and characteristically analogous, in timing, to the tectonic and magmatic evolution of the western Pacific island arc and back arc basin systems, where an earlier opening of the marginal basin was followed by an arc volcanism on the rifted-off migrating plate. The chronological zonation inside limited sectors of the structure coupled with the widespread age of volcanism of different serial affinity along the whole structure and the counterclockwise rejuvenation of the within-serial magmatic activity, could be related to deformation (via torsion, segmentation and lateral stretching) and progressive steepening of the subducted slab resulting in the present concavity of the Benioff zone which corresponds to a maximal oroclinal distortion of the Apenninic-Maghrebian chain.