Lithos, 23 (1989) 1-18 1 Elsevier Science Publishers B.V., Amsterdam -- Printed in The Netherlands Geochemical changes in basalts and andesites across the Archean-Proterozoic boundary: Identification and significance KENT C. CONDIE Department of Geoscience, New Mexico Institute of Mining and Technology, Socorro, NM 87801 (U.S.A.) LITHOS Condie, K.C., 1989. Geochemical changes in basalts and andesites across the Archean-Proterozoic bound- ary: Identification and significance. In: R. Gorbatschev (Editor), Proterozoic Geochemistry. Lithos, 23: 1-18. To identify accurately changes in rock composition across the Archean-Proterozoic boundary, it is nec- essary 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. (Accepted February 10, 1989) Introduction Numerous geochemical changes are reported to occur across the Archean-Proterozoic (A-P) boundary at 2.5 Ga and these are interpreted to re- flect changes in mantle or crustal composition (R.C.D. Gill, 1979; Condie, 1985; Taylor and McLennan, 1985). Whether these differences are real is significant in terms of evolution of the crust- mantle system and in terms of continental growth rates. Identification of geochemical changes gener- ally relies on comparison of similar rocks on both sides of the A-P boundary. Considering the varia- tion in composition of specific rock types forming in different tectonic settings on the Earth today, however, one might question the validity of such an approach in comparing ancient rocks. Because of the large number of chemical analyses available, it is convenient to compare averages of specific litholo- gies. However, such averages are critically depen- dent upon the number of analyses available from various tectonic settings. Modern basalts, for in- stance, reflect a variety of mantle sources depend- ing on their tectonic setting. Shales are deposited in different tectonic settings and their compositions reflect provenance characteristics of these settings. To compare mean compositions of such rocks as basalt or shale without consideration of tectonic setting may mask some geochemical differences and enhance others. Some of the compositional differ- ences proposed to occur in pelites across the A-P boundary that ignore the tectonic-setting effect ap- pear to be incorrect (Condie and Wronkiewicz, 1988). Because it is not possible, due to preservational and sampling biases, to weight mean chemical com- 0024-4937/89/$03.50 © 1989 Elsevier Science Publishers B.V.