Volcanological and geochemical features of young pyroclastic levels (< 12 ka) in the urban area of Naples (S. Italy)

Two stratigraphic sections (Palazzo Reale and Castel Nuovo), found in Naples during archaeological excavations, were investigated from the geochemical and volcanological points of view. These sections consist of Phlegrean and Vesuvian pyroclastic rocks, emitted after the emplacement of the Neapolitan Yellow Tuff (NYT; 12 ka). Pyroclastic products belonging to Pomici Principali (trachyphonolitic), Soccavo 4 (trachytic) and Minopoli 2 (shoshonitic) levels were identified in the Palazzo Reale section. These eruptions belong to the first epoch of post-NYT magmatic activity. The Pomici Principali deposit, more than 2 m thick, shows no appreciable change in chemical composition upwards, due to the tapping of a chemically homogeneous magma chamber. The other two pyroclastic layers reach a total thickness of about 30 em. Trachytic rocks with higher contents of incompatible elements (Nb 52-67 ppm, Zr 374-500 ppm), correlated to the last epoch of the Phlegrean magmatic activity (4.8-3.8 ka BP), are found in the Castel Nuovo section. The correlation between trachytic rocks at Castel Nuovo and the last-epoch products of the Phlegrean magmatic activity is confirmed by the lack of the Pomici Principali deposit and by their chemical affinity; these rocks generally display contents of incompatible elements higher than the products of the first and second postNYT epochs. This is related to the long quiescence period between the two last phases of magmatic activity (about 3.5 ka), during which the Phlegrean magmatic system mainly evolved by fractional crystallization processes.