Nuovi dati geocronologici sulle vulcaniti incontrate dal pozzo David Lazzeretti

David Lazzaretti Well (DLW) was drilled in the southern part of Monte Amiata, close to the Poggio Trauzzolo lava dome at an altitude of 1086 m a.s.l.; it reached a depth of 545 m below ground level (b.g.l.) and was almost entirely continuous cored. DLW crossed the volcanic complex and, at 530 m b.g.l., reached the sedimentary substratum. In the DLW rocks correlated to the oldest outcropping units (BTC Auct.) were observed from top of the core until 317 m b.g.l. (about 250 m thick). From 317 m b.g.l. further volcanic units were found almost continuously until 530 m b.g.l., where they rest directly on top of the sedimentary substratum without any palaeosoil interposition. These "oldest" units are composed mainly of porphyritic trachydacite with distinctive presence of mega K-feldspars (up to 5 cm), set in a highly vesicular and microcrystalline groundmass; they show a typical abundance of magmatic enclaves and xenoliths. These characteristics are interpreted as lava dome lithofacies by the close similarity with typical lava dome lithofacies at Monte Amiata. The core sample TRZ 51 (326-327 m b.g.l.) is sited stratigraphically below the oldest outcropping units. 40Ar-39Ar dating on this sample of the pair sanidine megacryst-ground mass displays a slight difference in the ages, 301.6 ± 3.3 and 291.1 ± 3.5 ka, respectively. The average and standard deviation of the four 40Ar-39Ar ages in literatures on sanidines of the oldest outcropping units is 299.8 ± 6.2 ka, a value that overlaps within error with both new ages.