A craton-wide geochemical study of Neoarchaean carbonate rocks of Zimbabwe

Abstract

Neoarchaean greenstone belts of the Zimbabwe craton host microbialite-bearing limestone successions that range in age from ~2.82 to 2.68 Ga. The best-preserved successions, according to Raman geothermometry, are situated in the Bulawayo and Belingwe greenstone belts where they have been subjected to lower greenschist facies metamorphism with a peak temperature of ~350 °C. Elsewhere, such as in the Masvingo belt, peak temperatures reached ~540 °C, giving rise to marbles. The carbonate rocks consist mainly of calcite and, to a lesser extent, dolomite and contain variably admixtures of siliciclastic detritus. Samples with the least amount of siliciclastic detritus have shale-normalized rare earth element and Y distribution patterns that indicate open marine conditions, with positive La, Eu, and Gd anomalies, superchondritic Y/Ho ratios, and depleted light rare earth elements relative to the heavy rare earth elements. The δ13СVPDB values are −2.0 to 2.1 ‰, and δ18OVPDB values cluster in the range of −20 to −10 ‰. Strontium isotope ratios vary in concert with siliciclastic detritus. The lowest initial 87Sr/86Sr ratio of 0.70155 is close to the depleted mantle value at the time of deposition, suggesting minimal contribution from evolved felsic crust to the Neoarchean ocean. Only samples from the Masvingo belt show radiogenic Sr isotope ratios (0.70202–0.70340) indicative of an epicratonic setting with exposed felsic basement. For the rest, open marine conditions prevailed in a proto-cratonic setting characterized by mantle-derived greenstone volcanism. Carbonate sedimentation on the Zimbabwe craton thus predate the fundamental rise in the marine 87Sr/86Sr Sr isotope signal in the late Neoarchaean.