A hidden Oligocene pluton linked to the Periadriatic Fault System beneath the Permian Bressanone pluton, eastern Southern Alps

The Bressanone (Brixen) pluton, cropping out at the culmination of the Southalpine indenter between the North Giudicarie and the Pustertal-Gailtal faults, is mainly composed of Permian granodiorite to granite, with minor gabbros and diorites in its southern part. New U–Pb SHRIMP zircon ages reveal two distinct crystallization episodes at 289.7 ± 3.2 and 280 ± 2.2 Ma, respectively. The pluton is affected by a hydrothermal potassic to sodic + Cu metasomatic alteration, which has long been ascribed to a late phase of the Permian magmatism. In contrast with this hypothesis, we report new 39Ar–40Ar data for different generations of metasomatic K-feldspar, which indicate formation ages between 35.3 ± 0.3 and 27.8 ± 0.5 Ma. This interval overlaps with the ages of the widespread ‘Periadriatic’ calc-alkaline magmatism, which extends from the Western to the Eastern Alps straddling the Periadriatic Fault System. The observed hydrothermalism has geochemical characteristics compatible with those of the coeval calc-alkaline Periadriatic magmatism. These data altogether suggest the release of fluids from a hidden intrusion during the main stage of the Alpine orogenic magmatism. Our results provide the first evidence of Oligocene magmatic activity in the Southern Alps east of the Giudicarie Line, bridging the gap between the western-central and eastern Alpine magmatic plutons. A fairly continuous Oligocene magmatic belt straddling the Periadriatic Fault System is consistent with a triggering of the magmatism by slab steepening or slab break-off, either of which are considered to be an essential driver for the Miocene lithospheric rearrangement in the Eastern Alps. Our finding is particularly relevant given that the outcropping area of the Bressanone pluton is centred above the imaged subducting lithosphere gap that separates the Western-Central and the Eastern Alps, hence at a location where mantle upwelling should have been easier although no relevant magmatism was found to date.