Abstract Products of geological processes, such as rock formations, unconformities, structures, minerals, fossils and landforms, represent unique records of the evolution of the Earth. These form a coherent picture showing how the Earth evolved, but the picture becomes blurred with antiquity. Consequently, there are challenges in gathering information from the Archean, the period during which the foundations of the Earth were laid down. The 2.7 Ga Belingwe Greenstone Belt in Zimbabwe has proved to be valuable because it has some of the best-preserved Archean stratigraphy in the world. An unconformity between sialic basement and supracrustal rocks of the greenstone belt, and exotic rocks, such as komatiites and stromatolites, contributes immensely toward our knowledge of the evolution of the young Earth and the beginning of life. The frequent use of the Belingwe Greenstone Belt examples to explain geotectonic processes of the early Earth gives testimony to the importance of this structure. Interpretation of some of the features of the Greenstone Belt is sometimes controversial, which forms areas of endless research to better understand the Archean Era. It is for these reasons that arguments are presented for consideration of the Belingwe Greenstone Belt as a national geoheritage site.

Abstract Tectonic histories and structural settings of the Diavik, Murowa, Argyle, and Bunder deposits—two kimberlitic- and two lamproitic-hosted diamond resources, respectively—are described to bring attention to similarities and differences that may help to better understand their tectonic and structural controls, and to derive some general principles about the evolution of primary diamond deposits that may be applicable to diamond exploration. As the evolution of all four deposits was multistage and took place over billions of years, possible tectonic and structural controls were assessed for the entire history of their host cratons. To facilitate comparison, each craton is discussed in five stages: (1) Mesoarchean lithosphere formation, (2) Neoarchean overprint of Early Archean protocontinental nuclei leading to cratonization, (3) Proterozoic breakup of Archean cratons and postbreakup modifications until kimberlite/lamproite emplacement, (4) tectonic and structural controls of emplacement, and (5) postemplacement preservation and erosion of diamondiferous host rocks. Whether kimberlite or lamproite, the formation and survival of cool Early Archean P-type roots, or remnants thereof, were essential requirements for deposit formation. Beginning with the breakup of the Archean protocratons, the tectonic settings of the kimberlitic and lamproitic deposits diverged significantly. The Murowa and Diavik deposit sites remained well within the Mesoarchean nuclei of their respective cratons while passive rifting occurred at craton margins, whereas the Argyle and Bunder deposits are located above or near the rifted Proterozoic craton margins. Higher P-type diamond grades survived in the roots sampled by the kimberlites than in those sampled by the lamproites. Additions of Proterozoic eclogites with subduction signatures to preexisting, relatively cool craton roots significantly improved the diamond grade of the Diavik kimberlite and raised the grade of the Argyle lamproite from uneconomic to one of the highest-grade deposits (by carats) in the world. As to kimberlite and lamproite emplacement, no definitive correlations with plume events can be made for any of the deposits, though a case can be made for some that plate margin processes were involved in metasomatic enrichment at depth as well as triggering the melting process. Emplacement sites for all four deposits were controlled by local structures.

ABSTRACT Large crystal pseudomorphs, composed of limestone and dolomite, that radiate upward to form centimeter- to meter-tall fans are known from every well-preserved Late Archean carbonate platform on earth. In many cases these crystal fans are an important facies, constituting as much as 50% of the observed volume of carbonate rock. Texturally, the fans are composed of elongate blades consisting of a mosaic of crystals with randomly oriented optic axes. In some pseudomorphs, trains of inclusions define the fibrous character of the precursor mineral, and the blades exhibit blunt terminations when draped by micrilic sediment. Some of the pseudomorphs contain strontium concentrations of up to 3700 ppm. Associated facies include strongly elongate giant stromatolites, hummocky cross-stratified sandstones, ooid-intraclast packstone to grainstone, small domal stromatolites, and several thinly laminated micritic facies that may display desiccation cracks. Previously, some of these crystal fans have been interpreted as calcite-and dolomite-replaced pseudomorphs after gypsum, formed under restricted conditions resulting from evaporative concentration of seawater. However, replacement textures and elevated strontium concentrations suggest that the crystal fans are more likely the result of neomorphism of large botryoids of aragonite that formed thick crusts directly on the sea floor. Furthermore, occurrence of the crystal fans in direct association with strongly elongate giant stromatolites and hummocky cross-stratified sediments suggests precipitation of the fans in open marine, wave- and current-swept environments. Although evaporation of seawater may have contributed to the growth of fans in some peritidal environments, most occurrences are not associated with any other indicators of evaporitic conditions such as halite or gypsum pseudomorphs. The reinterpretation of most reported occurrences of Late Archean gypsum pseudomorphs as aragonite pseudomorphs indicates that calcium sulfate precipitation from Late Archean seawater was rare, and that precipitation of aragonite as thick crusts on the sea floor was significantly more abundant than during any subsequent time in earth history. Rapid aragonite precipitation rates and the paucity of calcium sulfate precipitation can be accounted for in a model for Late Archean seawater featuring, relative to present-day seawater, higher supersaturation with respect to calcium carbonate and high HC0 3 concentrations.

Abstract An overview is presented of the field relations, age data and geochemical characteristics of the Neoarchaean granites and greenstones of the Zimbabwe Craton, southern Africa. A major tectono-magmatic event at c. 2.7 Ga produced two distinct greenstone successions. One succession is reminiscent of rift- or back-arc environments and is associated with an old continental fragment. A second succession is indicative of arc magmatism and is associated with juvenile crust. Both were affected by a major accretionary event that, in an apparent sense, swept across the craton between 2.68 and 2.60 Ga. During this 80 Ma time period, concomitant late volcanism, regional deformation, the development of syntectonic sedimentary successions in foreland-type basins, and late syntectonic plutonism took place in selected shear-zone-bounded tectonic domains over limited periods of time (<10–20 Ma). Deformation led to isostatically stable, 30–40 km thick continental crust, without significant exhumation of high-pressure rocks, suggesting that lithospheric shortening was accommodated independently in a rheologically strong upper and weak lower crust. Deformation was followed by pan-cratonic crustal melting and strike-slip shear motions, and led to stabilization of the crust at 2575 Ma, heralded by the emplacement of the Great Dyke.