A preliminary fission track dating survey of Caledonian “newer and last granites” from the Highlands of Scotland

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Abstract
Synopsis: The published radiometric age determinations for Scottish Caledonian Newer and Last Granites are reviewed briefly. A short description is given of the theory and techniques of fission track dating. The results of 66 fission track determinations on sphene, zircon and apatite from 17 Newer and Last Granites from the Highlands are presented and their interpretation discussed in relation to previously published ages and the geology of the intrusions. The mean sphene date is 389 ± 8 Myr and the mean zircon date 387 ± 18 Myr; these dates are interpreted as the mean age of initial post-intrusion cooling. Dates from co-existing apatite are substantially lower, showing maxima of ∼ 230 Myr and ∼ 280 Myr. Various interpretations of the apatite data are suggested: the discrepant ages may result from either the slow cooling of the granites at a rate of ∼ 0.8°C/Myr, or from the partial or complete resetting of the dates by subsequent thermal events, or from a combination of slow cooling and resetting.