Significance of in situ SIMS chronometry of zoned monazite from the Lewisian granulites, northwest Scotland

In situ studies of Lewisian metamorphic monazites show them to be strongly zones in Th content, ThU ratio and 207Pb206Pb age. Traverses across compositional zones of individual monazites by SIMS and EMPA show that there are no resolvable shifts between 232Th238U and 208Pb206Pb ratios and between Th and Pb contents in any of the profiles obtained. Furthermore, the difference in 208Pb232Th ages between two zones of monazites in the same thin section is indistinguishable from the difference in 207Pb206Pb ages determined by SIMS. These observations together with simple models suggest that transport of Pb relative to Th and U did not occur significantly during younger (e.g., the Laxfordian) events in the Lewisian and the ages preserved in these zoned monazites are close to concordant. Moreover, monazites having different compositions yield indistinguishable 207Pb206Pb ages, which suggests that the closure temperature of the UPb system in these monazites is not significantly dependent upon their composition. Consequently, the pattern of age distribution observed in the monazites is taken to reflect the metamorphic history of the Lewisian. The results are consistent with the recent interpretations of the Lewisian thermal history which suggest two high-grade metamorphic events separated by ∼ 200 Ma.