The Archean-Proterozoic transition: Evidence from the geochemistry of metasedimentary rocks of Guyana and Montana

Abstract

Chemical and isotopic compositions of ancient sedimentary rocks have in some cases provided evidence of first-order changes yin the composition and extent of continental crust, from the Archean to the Proterozoic. We reversed the typical sampling pattern, comparing metapelites and carbonates from Proterozoic greenstone belts with those from an Archean continental margin sequence, and found the reverse of the typical age-composition relations. Metapelites and volcaniclastic metagreywackes from the Early Proterozoic greenstone belts of northern Guyana resemble those of many Archean greenstone belts in REE, trace, and major element compositions (mean ratios: La Sm 5.0, La Th 5.1, Eu Eu ∗ 0.80, Th U 3.4; median K 2O Na 2O 0.8). Schists from an Archean continental marginal sequence in Montana have compositions like many post-Archean metapelites derived from continental sources (mean ratios: La Sm 6.8, La Th 2.7, Eu Eu ∗ 0.67, Th U 5.2; median K 2O Na 2O 1.7). Other Archean continental basin sediments also resemble their post-Archean counterparts, implying that upper continental crusts of these eras may not have differed significantly in these respects. However, temporal change may have occurred in upper crustal contents of Ni and Cr many Archean clastic metasedimentary rocks of both continental and greenstone belt provenance have contents of these compatible elements higher than in their post-Archean equivalents. Dolomitic limestone from a greenstone belt of northern Guyana has a very low 87Sr 86Sr ratio of 0.7007. Such low values may reflect the local influence of mantle-derived volcanic rocks during carbonate deposition, diagenesis, or metamorphism and they may not be reliable indicators of the evolution of Sr in seawater. Archean dolomitic marbles from the continental marginal sequence of Montana have 87Sr 86Sr ratios of 0.7065 and higher, significantly more radiogenic than many accepted values for carbonates of Archean greenstone belts.