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Journal of the Geological Society

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

¹ Department of Earth and Environmental Sciences, Centre for Earth, Planetary, Space & Astronomical Research (CEPSAR), The Open University, Walton Hall, Milton Keynes MK7 6AA, UK
² School of Earth and Environmental Sciences, University of Portsmouth, Portsmouth PO1 3QL, UK
³ Hills Road Sixth Form College, Hills Road, Cambridge CB2 8PE, UK

*Corresponding author (e-mail: s.sherlock{at}open.ac.uk)

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.

Supplementary material: ⁴⁰Ar/³⁹Ar data are available at http://www.geolsoc.org.uk/SUP18372.