A new look at the Archaean-Proterozoic boundary sediments and the tectonic setting constraint

To identify meaningful geochemical changes in sediments across the Archaean-Proterozoic (A-P) boundary (at 2500 Ma), it is necessary to compare sediments from similar lithologic associa- tions to minimize the effect of tectonic setting. This study evaluates data for pelites from quartzite-pelite (QP) and greenstone (GR) associations. Of the compositional differences believed to occur in GR and QP pelites across the A-P boundary, decreases in Ni and Cr are the only we11-documented examples. Higher Cr and Ni in Archaean pelites may be due to 1) intense chemical weathering of komatiite In the sediment source or 2) scavenging of Ni-Cr by clay-size particles from seawater that is enriched in these elements by hydrothermal leaching of komatiite at ocean ridges. Relatively small decreases in Eu/Eu^* and increases in La/Sc and Th/Sc in GR pelites occur across the A-P boundary and similar changes are allowed but not demanded by results from QP pelites. La/Yb ratios in QP pelites are relatively constant and Th/U ratios are variable through time. Based on limited data, early Archaean QP pelites are more like post-Archaean than late-Archaean QP pelites. Changes in element ratios in GR pelites at the A-P boundary probably reflect a greater proportion of granite in arc systems after 2500 Ma, which in turn, may be caused by a greater importance of fractional crystallization. Variable and high CIA values of some Archaean QP pelites may reflect variable and locally intense chemical weathering. Pelites from Precambrian cratonic basins typically exhibit increases in light REE, HFSE, La/Sc and Th/Sc and decreases in Eu/Eu^* and Ti/Zr with increasng stratigraphic level; these changes probably reflect progressive erosion of deeper crustal levels containing greater proportions of granite. Compared to GR associations, QP associations may be under-represented in the Archaean geologic record due to selective burial or erosion at collisional plate boundaries and selective preservation of GR associations as a result of widespread tonalitic underplating. Continental growth rates should not be inferred from trace-element distributions in pelites.