Evolution of large silicic magma systems: new U/Pb zircon data on the NW Permian Athesian Volcanic Group (Southern Alps, Italy)

During the Permian, the Southern Alps were affected by prominent igneous activity that produced voluminous basic to acidic volcanic and plutonic rocks. The magmatism originated during a period in which postorogenic extension and transtensional faulting controlled magma ascent and emplacement. Within this scenario, the Athesian Volcanic Group (AG), with lithologies ranging from basaltic andesites to rhyolites, constitutes a major part of the Permian magmatism in the central and eastern Southern Alps. We present an up-to-date stratigraphic reconstruction of the northwestern sector of the AG volcanic sequence combining detailed field mapping and new geochronological data. Single-zircon U-Pb age data suggest that the volcanism in the AG lasted ca. 10 m.yr., between 285 and 274 Ma, with a deposition rate that increased throughout the eruptive cycle. The processes of magma deposition and basin formation were strongly influenced by tectonic activity over the entire history of the AG. At least two major caldera margins have been identified. The age of the Tregiovo Basin, formerly based on palynostratigraphic constraints, is now bracketed between radioisotope ages of the underlying and overlying volcanics at 276:5 ± 1:1 to 274:1 ± 1:6 Ma. All the determined singlezircon U-Pb ages fit well into the general age pattern found for the Permian magmatism in the Southern Alps, suggesting that the process of magma generation and emplacement in the lower crust, hybridization, and upper-crustal emplacement occurred contemporaneously, that is, within a short time span between ca. 285 and 274 Ma, along the entire transect of the Southern Alps.