The timescale of solid-state deformation in the Northern Adamello igneous intrusive suite

The Cenozoic Adamello batholith in the Southern Alps records solid-state deformation structures including, in order of decreasing relative age, cooling joints, shear zones, and faults. In the present study, we constrained age and duration of each phase with 40Ar/39Ar geochronology. The Avio granodiorite, cooling joints, mylonites, pseudotachylytes, and cataclasites were characterized through microstructural, mineralogical, µCT and EPMA analyses. The dated K-bearing phases are: (i) magmatic biotite; (ii) biotite and K-feldspar in joints and mylonites; (iii) pseudotachylytes; and (iv) hydrothermal K-feldspar in cataclasites. The wall-rock biotite is 33.2 ± 0.2 Ma old, independent of grainsize, overlapping with the age of the cooling joints. Bulk biotite-rich mylonites have ages between 32.4 ± 0.5 and 30.8 ± 0.08 Ma. The K-feldspar cementing cataclasite is 25.5 ± 1.1 Ma old. Pseudotachylytes cluster between 29.8 ± 0.3 and 31.7 ± 3.1 Ma, one is 25.6 ± 0.3 Ma old. The resolvable difference in age between magmatic biotite and mylonites indicate that biotite is not a thermochronometer; as its age is mostly controlled by deformation and fluid-rock interactions. 40Ar/39Ar dates mostly confirm the relative ages determined from field relations, with mylonites active within a time window of 1.6 Ma and subsequent seismic faulting protracting for almost 6 Ma.