LASER-PROBE 40AR 39AR INVESTIGATION OF A PSEUDOTACHYLYTE AND ITS HOSTROCK FROM THE OUTER ISLES THRUST, SCOTLAND/

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 h ost gneiss. Mean Ar-40/Ar-39 ages for biotite grains decrease from 145 0 to 923 Ma in a zone <2 mm wide, approaching the pseudotachylyte boun dary. However, the laser-microprobe age traverses of the individual gr ains do not exhibit typical diffusive argon loss profiles. The heating event probably lasted less than 10 s, leading to temperatures >730-de grees-C in the host gneiss; these high temperatures may have caused ca tastrophic argon loss rather than loss by volume diffusion. An Ar-40/A r-39 age traverse across the pseudotachylyte vein revealed old ages ad jacent to the margin, reflecting the incorporation of partially outgas sed host-rock clasts. Apparent ages for the pseudotachylyte decrease u nevenly toward the center of the vein. A weighted mean age of 430 +/-6 Ma (2sigma) obtained in the center corresponds closely to movement ag es derived for the associated Moine thrust zone.