POTASSIUM-ARGON AGE PATTERN OF THE BRITISH CALEDONIDES

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Abstract
Summary: The results of over 160 potassium–argon age determinations on Moine and Dalradian rocks are summarized. Histograms reveal a strong Moine peak at about 425 million years and a spread down to 570 million years; in the Dalradian the peak is at 435 million years (possibly influenced by geographical sampling bias) and the spread to 490 million years. Interpretation of the age pattern is discussed on the basis of (a), a rapid cooling hypothesis in which the Moine and Dalradian areas are considered to have been affected by an orogenic-metamorphic event about 425 million years ago and (b), a slow cooling hypothesis in which the spread of ages younger than about 460 million years in the combined Moine and Dalradian pattern is considered to reflect late-orogenic cooling. The merits of both hypotheses are considered in relation to the known polyphase orogenic-metamorphic history of the Caledonides. An assessment is made of the geological and radiometric evidence bearing upon the putative Pre-Cambrian metamorphisms of the Moine and Dalradian areas.