Abstract:

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.