High spatial resolution 40Ar/39Ar dating of pseudotachylites: geochronological evidence for multiple phases of faulting within basement gneisses of the Outer Hebrides (UK)
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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 (<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.