STUDIO PETROGRAFICO E CRONOLOGICO DI UN LIVELLO TUFITICO INTERCALATO NELLA SUCCESSIONE CONTINENTALE PRE-EVAPORITICA DELL'AREA DI SASSA (PROVINCIA DI LIVORNO)

Abstract

A crystal tuff layer about 5 cm thick, was found at Fai­tona (comune di Bibbona, Livorno) interlayered in day sedi­ments belonging to the Miocene pre-evaporitic fluvio-lacu­strine succession of the Volterra - Radicondoli - Chiusdino Basin. The crystal tuff is fairly well lithified and is made up of predominant arenitic-grained plagioclase and biotite crys­tals. The biotite K/ Ar dating yelded an apparent age of 8.07 ± 0.11 Ma. According to the most recent literature, no mag­matic activity was present at this time in the Tuscan area; nevertheless, the crystal tuff minerai assemblage and its grain size suggest a parentage with the volcanic activity of the near­by Island of Capraia. The finding of a volcanic deposit in the Tortonian fluvio-lacustrine succession outcropping west of the ''Dorsale Medio-toscana'' perrnitted to draw some con­siderations about the age of the whole succession and its sedimentation rate.