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Department of Geological Sciences, University of Liverpool, PO Box 147, Liverpool L69 3BX, UK
Palaeomagnetic investigations of the Lewisian rocks identify a diverse remanence record which varies from zone to zone. The characteristic (A) remanence in the central mainland zone is a strong and stable NW downward-directed and magnetite-held component. This occurs sporadically elsewhere but is largely displaced in the northern mainland zone and in Lewis and Harris by lower blocking temperature/coercivity components with more variable directions; in addition hematite overprints occur sporadically in the central zone and in the Loch Torridon area of the southern mainland zone. Geological relationships and palaeomagnetic field tests suggest that all of these components were acquired during post-Laxfordian uplift and cooling of this terrain but a steep E downward-directed component is a feature of pre-Laxfordian belts in the south and may be considerably older. The collective data define migration of the geomagnetic field axis during uplift and cooling of the terrain and are linked in general terms to mineral ages in the range c. 1720–1500 Ma; further rock magnetic and 39Ar/40Ar investigations are in progress to characterize the areal significance of the components of remanence and to date them more precisely.
