Revised 40 AR– 39 AR age for granites of the Mourne Mountains, Ireland

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Abstract
The British Tertiary Igneous Province (BTIP) consists of many geographically separate areas throughout the British Isles and the question arises whether activity occurred contemporaneously throughout the province. In the absence of palaeontological evidence, this is best answered using radiometric dating, and many potassium-argon analyses have been carried out. However, experience has shown that conventional K–Ar dating in the BTIP is prone to error, often giving spuriously high or low dates. For this reason, Rb–Sr isochron and 40Ar–39Ar step–heating plateau dates are preferred, for internal acceptance criteria can be applied to them. The Mourne Mountain granites consist of five successive granite intrusions, G1 to G5, and lie to the NE of the Carlingford Complex and Slieve Gullion central intrusive complexes; together these form the only major masses of Tertiary intrusive rocks in Ireland. They are described in Meighan et al. (1984). Table 1 lists available dates. The most reliable are the 40Ar–39Ar plateau dates of 59.6 ± 1.6 Ma for G2 and 59.5 ± 1.6 Ma for G5; though acceptance criteria were not applied to establish the validity of the plateaus chosen, the steps selected for each plateau, although increasing in apparent age, are concordant at the 1σ level and contain the bulk of released 39Ar. Thus they appear to establish the age of the Mournes as about 59.5 Ma. However, three new 40Ar–39Ar step-heating analyses are reported here that indicate that the age is significantly younger. An analysis has been performed for a biotite from a mafic-rich facies within G2 (Meighan . . .