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Journal of the Geological Society

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Recrystallization or cooling ages: in situ UV-laser ⁴⁰Ar/³⁹Ar geochronology of muscovite in mylonitic rocks

Institut de Minéralogie et Géochimie, Université de Lausanne, BFSH-2, CH-1015 Lausanne, Switzerland (e-mail: andreas.mulch@img.unil.ch)

The intra-grain ⁴⁰Ar/³⁹Ar age distributions of muscovite from greenschist-facies mylonite zones are shown to be sensors of metamorphic history. Textures, compositions and in situ UV-laser ablation ⁴⁰Ar/³⁹Ar data from two extensional shear zones demonstrate that cooling ages can be distinguished from neo- or recrystallization ages. Deformed muscovite porphyroclasts and recrystallized shear band muscovite from the Pogallo Shear Zone (Ivrea Zone, southern Alps) reveal variable intra-grain ⁴⁰Ar/³⁹Ar ages with internal age variations of more than 65 Ma. The wide range of ⁴⁰Ar/³⁹Ar ages within compositionally homogeneous grains is consistent with diffusion-dominated argon loss controlled by observable intra-grain microstructures. In contrast, intra-grain ⁴⁰Ar/³⁹Ar ages of muscovite fish from Proterozoic mylonite of the Porsgrunn–Kristiansand Shear Zone (Southern Norway) display only minor age dispersion, and age variations are correlated with changes in muscovite composition. The sympathetic age–composition correlations in different muscovite microstructures directly relate to sequential neo- and recrystallization of muscovite and ultimately provide insight into the protracted recrystallization history during extensional deformation along the Porsgrunn–Kristiansand Shear Zone.

KEYWORDS: absolute age, argon, muscovite, diffusion, recrystallization

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