RESETTING OF NEODYMIUM ISOTOPES AND REDISTRIBUTION OF REES DURING SEDIMENTARY PROCESSES - THE EARLY PROTEROZOIC CHELMSFORD FORMATION, SUDBURY BASIN, ONTARIO, CANADA

The REEs abundances and neodymium isotope compositions of sandstones a nd siltstones from the Early Proterozoic Chelmsford Formation reveal t hat the REEs in some samples were redistributed during the Early Prote rozoic by surficial processes and that prior to redistribution, the Nd isotope system was partially reset. The Chelmsford Formation is a coa rse-grained turbidite sequence, the youngest of three elastic units th at fill the Sudbury Basin in Ontario. It is constrained to have been d erived 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), f ive samples from higher in the stratigraphic section show evidence tha t up to 60% of their La has been removed, and Sm/Nd has increased by u p 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 surficia l 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) calcu lated 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 d ifficult to reconcile with the petrography, major- and trace-element g eochemistry, and Nd and Pb isotopes. Removal of an LREE-enriched phase from the sedimentary system provides a viable mechanism to produce th e REE and Nd isotopic characteristics of the Chelmsford Formation samp les. Neodymium isotopes require that this phase was partially isotopic ally equilibrated with the matrix sediment near the time of sedimentat ion. This study shows the dangers of indiscriminate use of Nd model ag es in provenance studies and demonstrates that the simultaneous examin ation of Sm/Nd and Nd isotopes on f(Sm/Nd) vs. epsilon(Nd) (or isochro n) plots is required for the recognition of sedimentary redistribution of the REE.