High spatial resolution 40Ar/39Ar dating of pseudotachylites: geochronologicalevidence for multiple phases of faulting within basement gneisses of the OuterHebrides (UK)

摘要

Pseudotachylites within the Neoarchaean to Palaeoproterozoic basement of the Lewisian GneissComplex have been analysed using the high spatial resolution 40Ar/39Ar UV laserprobe technique. It has beenpossible to measure 40Ar/39Ar age and `apparent' age variations that would be beyond resolution by lowerspatial resolution or bulk 40Ar/39Ar dating methods. We report a range of complexities including significantlyyounger narrow (<1 mm) margins on much older pseudotachylite veins, and heterogeneous excess argoncontamination. We also report pseudotachylite veins of varying magnitudes that have been subject to laterthermal reheating, whereby there is a relationship between vein thickness and extent of argon loss. In spite ofthese it is still asible to determine geologically meaningful 40Ar/39Ar ages for pseudotachylite-formingevents. In Lewis the c. 1900 Ma event may relate to early stages of Laxfordian reworking. In Barra and SouthUist a 1300-1200 Ma event is recorded, whereas in Barra, South and North Uist a second event is recorded atc. 700 Ma, which is likely to be associated with NW- and west-directed thrusting, and could represent far-fieldeffects of tectonic events recorded at this time east of the Moine Thrust in NW Scotland.