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

Abstract

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 40 Ar/ 39 Ar 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. 40 Ar/ 39 Ar 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