Volcanic stratigraphy and radiometric age constraints at the northern margin of a mega-caldera system: Athesian Volcanic Group (Southern Alps, Italy)

The Athesian Volcanic Group (AG) constitutes a major part of the Permian magmatism in the central-eastern Southern Alps. We applied a volcanic stratigraphy model in order to constrain the volcano-tectonic evolution of the investigated area, which represents the northern sector of a caldera margin. In-situ zircon geochronology was also obtained by combined methods (i.e. LA ICP-MS and SHRIMP U/Pb dating) on volcanic, plutonic and metamorphic rocks. The volcano-tectonic collapse involved the rising of a huge dome, which uplifted the central portion of the just-filled caldera with simultaneous collapse of some sectors. The volcanic succession of the north-western sector of the AG is comprised between 279-274 Ma which is about half the time interval (i.e.12-10 Ma) determined for the area immediately to the south. The Ivigna pluton is confirmed to be coeval (i.e. 293.3 ± 1.5 Ma) to the Monte Croce pluton (285.4 ± 1.6 Ma and 284.3 ± 0.7 Ma; cf. Marocchi et al. 2008). Related rhyodacitic dykes (276 ± 1 Ma) are also connected to the Permian magmatism. The age of the protolith of the acid meta-volcanites from the South-Alpine metamorphic basement is constrained by SHRIMP U/Pb geochronology as Ordovician (472 ± 4 Ma), with possibly even older (Proterozoic) sedimentary protoliths. Finally, the radiometric ages confirm an Alpine age (i.e. 34.0 ± 1.0 and 28.6 ± 1.5) for the tonalitic lamellae intruded along the Periadriatric Line.