Geochemical changes in baslts and andesites across the Archean-Proterozoic boundary: Identification and significance

To identify accurately changes in rock composition across the Archean-Proterozoic boundary, it is necessary to compare rocks from similar lithologic associations to constrain the effect of tectonic setting. Most basalts and andesites from the greenstone association (volcanic-dominated submarine supracrustal rocks) possess a geochemical subduction-zone component similar to their counterparts from modern arc systems. Basalts with island arc geochemical affinities dominate in Archean greenstones while those with calc-alkaline affinities are most abundant in Proterozoic greenstones. Basalts with MORB or oceanic within-plate geochemical characteristics are rare in Precambrian greenstones of all ages. Preserved early Archean greenstone basalts (≥3500 Ma) reflect mantle sources less depleted than late Archean greenstone basalts (2500-3500 Ma). Proterozoic greenstone basalts are derived from relatively enriched mantle sources compared to all Archean sources, a feature which may be due to recycling of continental sediments into the mantle following rapid late Archean continental growth. Precambrian andesites are geochemically similar to andesites from modern arcs, but Archean andesites are unique in that they are depleted in heavy REE and Y. Results are consistent with Archean andesite production by partial melting of descending mafic crust with amphibole/garnet remaining in the residue, while Proterozoic (and younger) andesites are produced by fractional crystallization of basalts.