Abstract.
Muscovite and biotite from a crustal-scale mylonite zone (Pogallo Shear Zone, southern Alps) were investigated using furnace step-heating and in-situ UV-laser ablation 40Ar/39Ar geochronology. Undeformed muscovite porphyroclasts yield 40Ar/39Ar plateau ages of 182.0±1.6 Ma, whereas in-situ UV-laser ablation 40Ar/39Ar dating and furnace step-heating of strongly deformed muscovite and biotite grains display a range of apparent ages that are systematically younger. The range of 40Ar/39Ar ages measured in the deformed muscovite and biotite is consistent with protracted cooling through argon closure in minerals that exhibit variably developed segmentation on the intra-grain scale. These microstructurally controlled segments are bound by either first-order lattice discontinuities, sub-microscopic structural defects and/or zones of high defect density, which create variable length-scales for intragranular argon diffusion. The observed deformational microstructures within muscovite and biotite acted as intra-grain fast diffusion pathways in the slowly cooled mylonitic rocks. Therefore, the high-spatial resolution 40Ar/39Ar data record the initial and final closure to argon diffusion over a time span of about 60 Ma.
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Mulch, A., Cosca, M. & Handy, M. In-situ UV-laser 40Ar/39Ar geochronology of a micaceous mylonite : an example of defect-enhanced argon loss. Contrib Mineral Petrol 142, 738–752 (2002). https://doi.org/10.1007/s00410-001-0325-6
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DOI: https://doi.org/10.1007/s00410-001-0325-6