PT - JOURNAL ARTICLE AU - Sherlock, Sarah C. AU - Strachan, Rob A. AU - Jones, Kevin A. TI - High spatial resolution <sup>40</sup>Ar/<sup>39</sup>Ar dating of pseudotachylites: geochronological evidence for multiple phases of faulting within basement gneisses of the Outer Hebrides (UK) AID - 10.1144/0016-76492008-125 DP - 2009 Dec 01 TA - Journal of the Geological Society PG - 1049--1059 VI - 166 IP - 6 4099 - http://jgs.lyellcollection.org/content/166/6/1049.short 4100 - http://jgs.lyellcollection.org/content/166/6/1049.full SO - Journal of the Geological Society2009 Dec 01; 166 AB - Pseudotachylites within the Neoarchaean to Palaeoproterozoic basement of the Lewisian Gneiss Complex have been analysed using the high spatial resolution 40Ar/39Ar UV laserprobe technique. It has been possible to measure 40Ar/39Ar age and ‘apparent' age variations that would be beyond resolution by lower spatial resolution or bulk 40Ar/39Ar dating methods. We report a range of complexities including significantly younger narrow (&lt;1 mm) margins on much older pseudotachylite veins, and heterogeneous excess argon contamination. We also report pseudotachylite veins of varying magnitudes that have been subject to later thermal reheating, whereby there is a relationship between vein thickness and extent of argon loss. In spite of these it is still possible to determine geologically meaningful 40Ar/39Ar ages for pseudotachylite-forming events. In Lewis the c. 1900 Ma event may relate to early stages of Laxfordian reworking. In Barra and South Uist a 1300–1200 Ma event is recorded, whereas in Barra, South and North Uist a second event is recorded at c. 700 Ma, which is likely to be associated with NW- and west-directed thrusting, and could represent far-field effects of tectonic events recorded at this time east of the Moine Thrust in NW Scotland.Supplementary material: 40Ar/39Ar data are available at http://www.geolsoc.org.uk/SUP18372.