Geochemistry of Precambrian carbonates: 3-shelf seas and non-marine environments of the Archean

Abstract

A comprehensive whole-rock study of mineralogical, chemical, and isotopic attributes of Archean carbonates suggests that their lithologies and facies have been controlled by tectonic setting. In the first two papers of this series we have shown that the dominant lithology of sedimentary carbonates in greenstone belt settings is limestone ( Fe-rich dolostone of ankeritic composition in older examples ). In this paper we suggest that the Archean shelf sequences are mostly dolostone, and the contemporaneous lacustrine playa lakes are characterized by limestone facies. The present study is of the shelf environments of the Archean, represented by the Pongola Supergroup of South Africa and the Hamersley Group (Wittenoom and Carawine Dolomite ) of Australia. The lacustrine playa examples have been sampled from the Ventersdorp Supergroup of South Africa and the Fortescue Group of Australia. Geological, trace element, and oxygen isotope considerations of the shelf carbonates suggest that their original mineralogy may have been aragonite and that the Pongola dolostones probably represent a direct dolomitization product of this precursor. In contrast, the stabilization of the Hamersley carbonates may have involved an additional step of transformation of a metastable precursor into limestone prior to dolomitization. Subsequently, both sequences, but the Pongola Supergroup much more so, were subjected to pervasive thermal events which overprinted their chemical and isotopic signatures. Consideration of the “best preserved” samples yields +0.9 ± 2.1 and −6 ± l%. PDB for the δ 13C and δ 18O, respectively, for the initial Archean signature. If compared to Phanerozoic carbonates, the Archean δ 13C values are comparable, but the oxygen isotope measurements again support the primary nature of 18O depletion in ancient marine sediments, regardless of its ultimate cause. The 87Sr 86Sr of ~2.5 Ga old sea water was not more than 0.7033. If this value reflects the contemporaneous sea water, it would indicate the commencement of a significant 87Sr input from the growing and aging continents. It is, however, entirely possible that the slight enrichment in radiogenic 87Sr—relative to the contemporaneous mantle—represents only an unresolved artefact of post-depositional alteration. If so, the 87Sr 86Sr of the 2.5 Ga old sea water may still have been within a mantle range. The lacustrine limestones retain a strong facies control in their trace element attributes. They are also ~4%. depleted in 13C compared to marine carbonates and strongly depleted in 18O (−19 ± l%. PDB). These light δ 18O values are somewhat enigmatic, unless the lakes were of the high altitude/latitude variety. The 87Sr 86Sr of 0.711 for the Ventersdorp lakes suggests that their drainage basin incorporated not only volcanic rocks but also aged continental crust. Archean carbonates of all lithologies and facies are enriched in Mn and Fe compared to their Phanerozoic counterparts.