Biostratigraphical correlation and geochronology of volcaniclastic horizons across the Tortonian/Messinian boundary in the Apennine foreland basin system

In northern and central Apennines several volcanogenic horizons, interbedded in different stratigraphic intervals within Neogene deposits, have been detected. This study deals with the correlability of some of this horizons deposited during Late Miocene in different sites of the Apennine foreland basin system. Two volcaniclastic levels outcropping respectively in the Peglio (northern Apennines) and the Pietrasecca (central Apennines) sections have been studied. Both the horizons have been analysed from a petrographic and biostratigraphical point of view. Moreover, a geochronological study has been performed on the Peglio layer, comparing the results with the data already obtained for the Pietrasecca layer. 40Ar-39Ar analyses failed to provide unequivocal chronological informations, due to disturbance of the isotopic system: obtained dates on plagioclases (</= 7.2 Ma and </= 6.7 Ma for Pietrasecca and Peglio samples, respectively) are only ondicative while some informations rely on biotite (about 7.1 Ma on Peglio section). On the other hand, the biostratigraphy of the calcareous nannofossils has evidenced the Tortonian/Messinian boundary in both the sections in corrispondence of the volcaniclastic horizons. These layers are interpreted as syn-eruptive deposits affected by penecontemporaneous mass-flow processes shortly after their fall into the sedimentary basin. Theur petrography indicates a relationship with high-K andesites-dacite magmas of the Phase II (7.3-6.0 Ma) of the Neogene-Quaternary magmatism of Central Italy which could be tentatively considered a potential source (high-K andesites-dacites of the Capraia Island).