The formation of a friction melt or pseudotachylyte in the footwall of the Outer Isles thrust, Scotland, during thrust movement in the early Paleozoic Caledonian orogeny caused argon loss from biotites in the host gneiss. Mean 40Ar/39Ar ages for biotite grains decrease from 1450 to 923 Ma in a zone <2 mm wide, approaching the pseudotachylyte boundary. However, the laser-microprobe age traverses of the individual grains do not exhibit typical diffusive argon loss profiles. The beating event probably lasted less than 10 s, leading to temperatures >730°C in the host gneiss; these high temperatures may have caused catastrophic argon loss rather than loss by volume diffusion. An 40Ar/39Ar age traverse across the pseudotachylyte vein revealed old ages adjacent to the margin, reflecting the incorporation of partially outgassed host-rock clasts. Apparent ages for the pseudotachylyte decrease unevenly toward the center of the vein. A weighted mean age of 430 ±6 Ma (2σ) obtained in the center corresponds closely to movement ages derived for the associated Moine thrust zone.

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