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 host granodiorite, cooling joints, mylonites, pseudotachylytes and cataclasites were characterized through microstructural and mineralogical analysis, micro-computerized tomography and electron probe microanalysis. The dated K-bearing phases are (1) magmatic biotite, (2) biotite and K-feldspar in joints and mylonites, (3) pseudotachylytes and (4) hydrothermal K-feldspar in cataclasites. The wall-rock biotite has an age of 33.2 ± 0.2 Ma, independent of grain size, 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 has an age of 25.5 ± 1.1 Ma. Pseudotachylyte ages cluster between 29.8 ± 0.3 and 31.7 ± 3.1 Ma, with one of 25.6 ± 0.3 Ma. The resolvable difference in age between magmatic biotite and mylonites indicates 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 relationships, with mylonites active within a time window of 1.6 myr and subsequent seismic faulting occurring for almost 6 myr.

Supplementary material: Full analytical data are available at https://doi.org/10.6084/m9.figshare.c.5838146

Thematic collection: This article is part of the Isotopic Dating of Deformation collection available at: https://www.lyellcollection.org/cc/isotopic-dating-of-deformation

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