Petrological and geochemical characteristics of Plio-Pleistocene Volcanics from Ponza Island (Tyrrhenian Sea, Italy).

Volcanic rocks from Ponza Island (Tyrrhenian Sea, central Italy) consist of Pliocene submarine rhyolites and Pleistocene subaerial trachyte and comendite lavas. Chemical variations and the homogeneous Sr and Nd isotopic signatures within the analyzed Pliocene rocks are ascribed to crystal fractionation. The absolute isotopic values, however, indicate the important role of a crustal component in the origin of these magmas. The very high-silica rocks were probably derived from a superimposed mechanism which may have been connected to the ascent of hydrothermal magmatic ftuids. Compositional and 87Sr/86Sr variations at constant 143Nd/144 Nd values in the Pleis- tocene rocks are likely due to fractionation of the observed phenocryst assemblage, possibly coupled with minor crustal interaction. These processes, however, cannot account for the extreme enrichment of many incompatible trace elements in the co- mendites. Some evidence suggests the inftuence of a halogen- and/or C02-rich volatile phase.