The REEs abundances and neodymium isotope compositions of sandstones and siltstones from the Early Proterozoic Chelmsford Formation reveal that the REEs in some samples were redistributed during the Early Proterozoic by surficial processes and that prior to redistribution, the Nd isotope system was partially reset. The Chelmsford Formation is a coarse-grained turbidite sequence, the youngest of three elastic units that fill the Sudbury Basin in Ontario. It is constrained to have been derived from nearby Late Archean Superior Province rocks by petrography, major- and trace-element chemistry, and Pb isotopic compositions of whole-rocks and feldspar. Although REE patterns and Nd model ages for samples from around the base of the formation reflect their derivation from undisturbed Superior Province rocks (T-DM similar to 2800 Ma), five samples from higher in the stratigraphic section show evidence that up to 60% of their La has been removed, and Sm/Nd has increased by up to 60%. These samples also have anomalously high Nd model ages (T-DM = 3400 to 4400 Ma). The presence of pronounced positive Ce anomalies in the samples with anomalous T-DM indicates the influence of surficial processes on the REE composition. The data scatter about a vertical array on a plot of f(Sm/Nd) vs. epsilon(Nd)(t=1800) (epsilon(Nd) calculated at 1800 Ma, approximately the depositional age), with values of f(Sm/Nd) between -0.45 and -0.14. Mixing of Nd from other sources is difficult to reconcile with the petrography, major- and trace-element geochemistry, and Nd and Pb isotopes. Removal of an LREE-enriched phase from the sedimentary system provides a viable mechanism to produce the REE and Nd isotopic characteristics of the Chelmsford Formation samples. Neodymium isotopes require that this phase was partially isotopically equilibrated with the matrix sediment near the time of sedimentation. This study shows the dangers of indiscriminate use of Nd model ages in provenance studies and demonstrates that the simultaneous examination of Sm/Nd and Nd isotopes on f(Sm/Nd) vs. epsilon(Nd) (or isochron) plots is required for the recognition of sedimentary redistribution of the REE.
