Cooling history of the Biella pluton and implication for Oligocene to Miocene tectonics of the Sesia-Lanzo Zone, Austroalpine, Western Alps

We present new emplacement constrains for the Biella pluton within the eclogitic Sesia-Lanzo Zone, western Austroalpine domain. This zone, with the autochthon andesitic cover, is separated from the Southalpine Ivrea Zone by the western Periadriatic Line. Thermobarometry indicates 710 and 750°C, and 0.9 and 2.3 kbar for the final pluton emplacement that accomplished at depths between 3 and 8.5 km. Soon after the emplacement, the pluton was affected by hydrothermal circulations at a progressively lower temperature. By 3D modelling, surface and volume distribution of emplacement pressure and temperature are retrieved, and isobaric surfaces are interpolated. The orientation of isobaric surfaces indicates a post-intrusive southeastward rotation of 69 ± 5° about an N 35° trending axis. These results match palaeomagnetic studies on andesites and are consistent with the pluton feeding part of the andesite lava flows. According to the updated pluton emplacement conditions and recent fission track data, we suggest that: (i) the crustal section above the pluton consisted mostly of the andesites; (ii) before 26 Ma the andesites experienced first a depositional and then a tectonic burial during rotation; (iii) between 24 and 19 Ma uplift by back-thrusting with on-going rotation started; (iv) the rotation lasted at least until 16–12 Ma.