Allan H. Wilson

The early Archaean nondweni greenstone belt, southern Kaapvaal Craton, South Africa, Part II. Characteristics of the volcanic rocks and constraints on magma genesis

The early Archaean Nondweni greenstone belt is dominated by mafic volcanic rocks which have a wide range in compositions. Primary volcanic structures and compositional characteristics point to this succession being unique in Archaean geology and an important indicator of controls of magma formation and environment of deposition. Compositionally the Nondweni lavas are broadly similar to those of the Barberton Sequence (high values for CaO/Al2O3, Ti/Y and low Al2O3/TiO2). They are also distinctly different in that SiO2 content is markedly higher and incompatible trace element ratios are also different. The high silica content has promoted the subdivision of komatiitic and basaltic rocks into the following categories: komatiite, komatiitic andesite, komatiitic basalt, basalt and basaltic andesite. The recognition of the distinct suite of komatiitic andesites is aided by the lower Zr/Ti ratios than those lithologies of equivalent MgO content reported from other greenstone belts, and SiO2 contents as high as 58%. These characteristics suggest similarity to Recent high-Mg andesites. None of the lava compositions exceed 23% MgO (anhydrous) and pyroxene, rather than olivine, dominates the crystallization assemblage. Olivine spinifex textures are entirely absent in the Nondweni greenstone belt and instead, pyroxene (or amphibole after pyroxene) spinifex is abundantly developed and is characterised by spectacular radial conical structures which have their apices towards the tops of the flow units. Some of the spinifex cone structures attain heights of up to 6 m. Branching pyroxene crystals and feathery textured plagioclase typify the spinifex textures of the komatiitic andesites. The common occurrence of coned and branching spinifex textures in a wide range of magma compositions, indicates that these were controlled by thermal characteristics and depositional environment for the flow units rather than solely by composition. This may have been facilitated by emplacement in shallow water rather than in deep water, as envisaged for the Onverwacht Group of the Barberton Sequence. Shallow-water depositional environments in the Nondweni Group are supported by sedimentary lithofacies. Well preserved pillow structures also show a wide range in textural forms. Sizes of pillows, degree of welding of pillow margins, development of hyaloclastite and formation of shelf structures, formed by repeated filling of lava tubes, are related to both compositional type as well as physical controls such as extrusion rate and nature of the substrate. Komatiites seldom form pillows and very large pillow formation is restricted to the komatiitic andesite lava type. Komatiite flow successions are typified by spinifex textured units, as well as by massive and highly brecciated aphyric flow units. Unaltered pyroxene and plagioclase characterise some of the massive flow units and pillowed basalts of the Witkop Formation. The present paper is the second part of a series on the Nondweni greenstone belt. Part I (Wilson and Versfeld, 1994) describes the stratigraphy, sedimentology, mineralization and depositional environment. There are strong of Kaapvaal Craton may have been different to that of the Barberton greenstone belt. A present day analogue to the Nondweni tectonic setting may be an ensialic back-arc basin adjacent to a continental margin.

The Nsuze Group, Pongola Sequence, South Africa: Geochemical evidence for Archaean volcanism in a continental setting

Abstract The Archaean Nsuze Group in southeast South Africa represents an important volcano-sedimentary succession that is markedly different compositionally and lithologically from older and contemporaneous sequences elsewhere in southern Africa. The Nsuze volcanic rocks cropping out in the vicinity of the Pongola River in northern Natal display a complete spectrum of chemical compositions from basalt to rhyolite, with lavas of intermediate compositions predominant but ultramafic lavas absent. Flows of different compositions are complexly interdigitated. The uppermost rocks of the Nsuze Group reflect a gradual decrease in volcanic activity accompanied by an increase in sedimentation. The Nsuze lavas are tholeiitic with total Fe (as Fe 2 O 3 ) contents approaching 17% in the basaltic andesites. All lithologies are characterized by moderate to strong light rare earth element (REE) enrichment. Heavy REE slopes range from moderate in the basalts and basaltic andesites to flat in the rhyolites, with the exception of two basaltic andesites and a dacite which have flat heavy REE slopes. Basalts have small or no negative Eu anomalies but increasingly larger negative anomalies are a feature of the intermediate and acid lavas. Variations in Cr content appear to be related to stratigraphic position. Basalts and basaltic andesites in the lower part of the volcanic sequence have higher Cr contents (by a factor of six to eight times) than lavas with similar MgO abundances in the upper part of the sequence. Provisional modelling of the available chemical data favours low pressure, crystal fractionation from and evolved basaltic parent, that could be derived by initial non-modal melting of a garnet-lherzolite source. Variable degrees of crustal contamination of the evolved magmas are considered probable.

The early Archaean Nondweni greenstone belt, southern Kaapvaal Craton, South Africa, Part I. Stratigraphy, sedimentology, mineralization and depositional environment

Abstract The Nondweni greenstone belt is an exceptionally well preserved ∼3.4 Ga succession of predominantly volcanic rocks near the southern margin of the Kaapvaal Craton. Nondweni Group rocks show both similarities and differences to the Barberton Sequence, of similar age, 300 km to the north. In contrast with the latter, none of the Nondweni lavas exceed 23% MgO and spinifex textures are exclusively of pyroxene type. The characteristics of the lavas and magma genesis are discussed in Part II of this series of papers. Three Formations are recognized in the Nondweni Group: the lowermost Mogongolozi Formation comprises basaltic andesites, komatiitic andesites and basalts with intercalated coarser-grained pyroxenites and gabbros. The Toggekry Formation is a felsic unit of quartz-sericite schists with small CuZn massive sulphide ore bodies of volcanogenic origin. The uppermost Witkop Formation comprises mainly basalts, komatiites and komatiitic basalts intercalated with chert layers and capped by a clastic sedimentary succession. The total exposed thickness of the Nondweni Group is 16,500 m is the result of duplication of stratigraphy and tectonic thickening during early D1 thrusting in the bedding plane of much thinner lithological units. Tuffaceous sediments comprise a relatively minor component of the stratigraphy but are indicative of depositional environment. Banded chert layers are largely derived from accumulations of accretionary lapilli tuffs, although a diverse origin is indicated for some varieties. At the base of the Witkop Formation a number of undeformed, banded, coarse-grained graphitic quartzite bodies, entirely devoid of sedimentary structures, are considered to have originated as hydrothermal deposits resulting from fumarolic activity. Selective brecciation of layers resulted from explosive release of fluids and gases. The textures of these bodies are in marked contrast to the highly rounded pseudo-conglomerates from the clastic sedimentary succession at the top of the Witkop Formation. This ∼500 m thick succession passes upwards through reworked tuffs and alluvial fan deposits, into a zone of evaporites and stromatolites. Swallow-tail twin crystals of barite, and an evaluation of crystal interfacial angles, are indicative f replacement of primary evaporative gypsum. Most primary features of the Nondweni greenstone belt suggest a shallow-water or subaerial depositional environment, and together with characteristics of the magma composition, indicate a tectonic setting in a subduction-related continental margin setting.

Stratigraphy and petrology of the Archaean Nsuze Group, northern Natal and southeastern Transvaal, South Africa

Abstract The Nsuze Group is the lower, dominantly volcanic, division of the Pongola Supergroup that accumulated on a sialic basement between 3.1 and 2.9 Ga. The Nsuze Group is subdivided into a lower sedimentary unit (800 m thick), a middle volcanic unit (± 7500 m thick) and an upper volcaniclastic-sedimentary unit (5–600 m thick). The predominant sediments in the lower unit are immature, medium- to very coarse-grained quartz wackes with thin intercalated lenses of quartz and arkosic arenites, and minor conglomeratic wackes. These sediments were deposited in a distal braided stream environment. There followed a major period of volcanism during which lavas showing a continuous spectrum of compositions from basalt to rhyolite were extruded subaerially. Flows of both different and similar compositions are complexly interfingered on both regional and local scales. As volcanism waned, pyroclastic and sedimentary rocks became dominant in the upper unit. The Nsuze Group is gently dipping and is metamorphosed to low greenschist grade. The Nsuze Group is significant in that it provides evidence for the existence of high-standing continental sialic crust in the southeastern part of the Kaapvaal province at ca. 3.0 Ga. Volcanism and sedimentation in the Pongola Supergroup are more typical of Proterozoic basins than of Archaean environments, despite their age. Komatiitic and high-Mg basalts were, however, being extruded in Zimbabwe contemporaneously with the Nsuze lavas.

AN OXYGEN ISOTOPE STUDY OF THE LOWER MAFIC SUCCESSION OF THE DARWENDALE SUBCHAMBER OF THE GREAT DYKE, ZIMBABWE

Abstract Oxygen isotope values for plagioclase, clinopyroxene and orthopyroxene from the upper part of the Great Dyke are presented. The variation in δ 18 O values in these silicates is narrow and the average values are: plagioclase, δ 18 O = +6.90‰ ; orthopyroxene, δ 18 O = +6.‰ ; clinopyroxene, δ 18 O = +6.91‰ ; and melt, δ 18 O = +6.65‰ . An estimated equilibration isotopic temperature of 1091°C was calculated for this Lower Mafic Succession of the Great Dyke. The oxygen isotope compositions of the silicate minerals are not very close to their magmatic values, but they are roughly similar to those from the Bushveld Complex, the Stillwater Complex and the Kiglapait intrusion. The Δ clinopyroxene-orthopyroxene values can be as high as 0.63‰ with an average value of 0.33‰. Since the fractionation between these pyroxenes at magmatic temperatures should be approximately zero (Kyser et al., 1981), the high Δ clinopyroxene-orthopyroxene values are likely to have been caused by lower-temperature crystallisation of both pyroxenes. The low Δ clinopyroxene-orthopyroxene values in the cumulate-rich samples are interpreted as products of high-temperature crystallisation while the high Δ clinopyroxene-orthopyroxene values are products of low-temperature crystallisation associated with postcumulus textures and abundant low-temperature phases. The slightly 18 O-rich silicates from the Great Dyke acquired additional oxygen (up to 59%) from the Archaean rocks of the Zimbabwe craton which has whole-rock δ 18 O values going up to +7.3‰ (Barker et al., 1976) which occur as roof pendants in the Darwendale Subchamber of the Great Dyke.