REE CHEMISTRY AND SM-ND SYSTEMATICS OF LATE ARCHEAN WEATHERING PROFILES IN THE FORTESCUE-GROUP, WESTERN-AUSTRALIA

Two weathering profiles, each consisting of an upper, sericite-rich zo ne and a lower, chlorite-rich zone, are preserved between flows of the Mt. Roe Basalt in the Fortescue Group, Hamersley Basin, Western Austr alia. REE concentrations in samples from these two profiles, which ori ginally developed ca 2,760 Ma, show large variations depending on stra tigraphic position. LREE abundances and (La/Yb)n are greatest at depth s of 3-6 m below the paleosurface of the Mt. Roe #1 profile and are so mewhat lower in samples above this level. The LREEs reach concentratio ns 6-9 times greater than in the underlying basalt, and thus appear to have been mobilized downward in the paleosol and concentrated in its middle part. LREE concentrations in the #2 profile show a similar dist ribution but with a sharp increase in all REE concentrations within 50 cm of the paleosurface. The distinction between the REE profiles in t he two paleosols may be related to the difference in the overlying mat erial. The #1 paleosol is overlain by a few meters of sediments and th en by basalt, whereas the #2 paleosol is directly overlain by basalt. The LREEs appear to have been mobilized both during chemical weatherin g of the parental basalt and during later lower-greenschist-facies met amorphism and metasomatism of the paleosols. Remobilization of the REE s during the regional metamorphism of the Fortescue Group is confirmed by a whole-rock Sm-Nd reference isochron of Mt. Roe #1 samples with a n age of 2,151 +/- 360 Ma. Variable initial Nd-143/Nd-144 values of un weathered basalt samples which may represent the paleosol protolith pr events a confident determination of the magnitude of LREE mobility. Bo th the initial mobilization of the REEs during weathering and the meta somatic remobilization appear to have taken place under redox conditio ns where Ce was present dominantly as Ce3+, because Ce anomalies are n ot developed within the sericite zone samples regardless of concentrat ion. Europium anomalies in the paleoweathering profile are somewhat va riable and were probably modified by mobilization of Eu2+ at metamorph ic conditions. In all samples, the HREEs appear to have been relativel y immobile and correlate with Al, Ti, Cr, V, Zr, and Nb. Sm-Nd systema tics and REE patterns of four unweathered basalt samples indicate deri vation of the Mt. Roe Basalts from a heterogeneous and enriched source having epsilon(Nd) between -4.0 and -7.4. Initial Nd-143/Nd-144 value s of these basalts are even lower than those reported by Nelson et al. (1992) for Fortescue Group basalts and indicate a substantial crustal component in the generation of Mt. Roe Basalts.