Origin and evolution of Pontine Islands magmatism (Tyrrenian Sea, Italy): geochemical constraints from submarine volcanics

Abstract

The spatial-temporal distribution and compositional variability of Plio-Quaternary magmatic products emplaced in the Tyrrhenian and surrounding regions is related to subduction processes and extensional phases, which acted in different ways in the northern and the southern part of the basin. In this framework, the magmatism of the Pontine Islands is very interesting as 1) emplaced at the boundary between the Tuscan and the Roman Magmatic Provinces; 2) strictly related to geodynamic processes involved in the opening of the Tyrrhenian oceanic domain. Previous studies defined that: 1) Pleistocene volcanic episodes were represented by potassic products, with basic to intermediate rocks in the islands of Ventotene and Santo Stefano and with intermediate to highly evolved rocks (trachytes up to peralkaline rhyolites) in the south-eastern part of Ponza and Palmarola island; 2) Pliocene volcanic episodes are represented by calcalkaline products, with silica-rich volcanic units constituting the dominant products in Ponza and Zannone islands; the latter were characterized by lack of primitive rocks raising questions on the petrological processes as well as on the tectonic mechanisms that triggered magma genesis. In this contribution we present new data including the investigation of the Pontine submarine portions carried out in four oceanographic cruises (aboard R\V Urania), during which seafloor samples were dredged. An important result is that off-shore sampling also recovered less-evolved terms, i.e. basalts and andesites, of the calcalkaline series. The new sample collection has been characterized through major/trace element data and Sr-Nd isotopic analyses, implementing the data-set already available in the literature to provide better constraints on the genesis of both calcalkaline and potassic series. Previous studies (Conte and Dolfi; 2002; Cadoux et al., 2005) interpreted the Pontine magmatism in the framework of a transition from subduction-related to intraplate geodynamic settings. The new data suggest that such transition has not been fully accomplished, pointing to ubiquitous and persisting subduction-related signatures in the Pliocene-Pleistocene mantle sources. In particular, geochemical and isotopic characters of the mafic products indicate highly heterogeneous mantle sources, in turn related to crustal components recycled in the mantle via subduction. Further assimilation of crustal materials possibly occurred as result of shallow level processes in the plumbing system, contributing to define the observed isotopic fingerprint