Contrasting zircon U‐Pb and model Sm‐Nd ages for the Archaean Logue Brook Granite

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Abstract
Zircon U‐Pb age determinations made by ion microprobe show conclusively that the crystallization age of the Logue Brook Granite is 2612 ± 5 Ma, not 2905 ± 35 Ma as has been inferred from its chondritic uniform reservoir (CHUR) Sm‐Nd model age. It follows that the initial 143Nd/144Nd of the Logue Brook Granite was appreciably lower than that predicted by the CHUR model for 2612 Ma (i.e. higher than CHUR for 2905 Ma). The dispersion of Sm‐Nd model ages in the Darling Range Batholith probably reflects not variation in emplacement age, but variation in 143Nd/144Nd arising from components of the batholith's source rocks that experienced one or more prior episodes of rare‐earth element fractionation. It is re‐emphasized that Sm‐Nd model ages do not provide direct estimates of crystallization ages.