Volcanic Group, English Lake District Sm-Nd garnet age for the Ordovician Borrowdale

SUMMARY: Garnets separated from andesites and dacites of the calc-alkaline Borrowdale Volcanic Group are highly enriched in heavy rare earth elements (HREE) relative to their host rocks (D$& from 9 in andesite to 37 in dacite). Sm/Nd ratios and D?;,,, show a wide range, probably resulting from small quantities of an REE-rich impurity in the separates. The four garnet-host rock pairs analysed yield Sm-Nd ages identical within analytical error of a weighted mean of 457 f 4 Ma. The Borrowdale. Group overlies upper Llanvirn sediments and is unconformably overlain by mid-Caradoc. The age of 457 Ma is considered to be the best estimate available for the age of the Llandeilo, as the samples analysed are from the lower part of the Group. The age suggests that a Lower Palaeozoic timescale intermediate between those of Gale et al. (1979) and McKerrow et al. (1980) might be the most appropriate. A number of attempts have recently been made to assign absolute ages to Lower Palaeozoic stratigraphic divisions. Timescales published for the Silurian and Ordovician differ by more than 30 Ma for some divisions (Gale