Isotopic Ages of Late Caledonian Granitic Intrusions in the British Isles

Summary Published isotopic ages, together with new determinations on sixteen intrusions, show that the majority of the Newer Granites were emplaced 400 ± 10 million years ago. Potassium-argon ages reveal a peak in the Moine rocks at about 420 to 430 million years and a somewhat earlier peak in the Dalradian. Although the majority of the Newer Granites were therefore emplaced in country rocks which had already become closed systems to radiogenic argon, several important exceptions to this rule have been recognized and present results indicate that some intrusions yield potassium-argon ages concordant with those of their country rocks. It is suggested that variations in intrusive style of the granitic masses may be related to their age and hence to the temperature of their country rocks and depth of cover.