40Ar/39Ar chronostratigraphy of Vico ignimbrites

40Ar-39Ar dating experiments on 5 sanidines and 2 leucites from the Vico Volcanic Complex (Northern Latium) allow us to put timemarks on the volcanological evolution. 6 out of 7 Ar-Ar spectra yields plateaus; isochron plots yield atmospheric intercepts and thus substantiate the plateau approach. The ages obtained for three successive explosive events (Ignimbrite B, C, D) are 157 ± 3ky, 151 ± 3ky and 138 ± 2 ky, respectely. The paleosoil between ignimbrites B and C thus formed at a rate of about 0.1 m/ky. The time resolution of our experiment allows us to discriminate between pyroclastites as close as 6 ky at the 2 sigma level. The lava immediately underlying the ignimbrits yields a plateau age of 258 ± 2 ky for the sanidine and a pertubated spectrum with excess 40Ar for the coexisting leucite; the latter does not, however, contain any the peculiar "high temperature excess Ar" observed in the southern Roman volcanic Province. The best age estimate for the basal event (Tufi Stratificati Varicolori) is 419 ± 6 ky. The activity of the Vico Volcanic Complex can be subdivided into 4 phases (explosive-effusive-explosive-effusive); the present work shows that each phase consists of a rather short climax (20-30 ky) followed by a stasis which is substantially longer (60-100 ky)