Layered mafic intrusions are important in understanding the generation of continental crust in oceanic arcs. The Mesoproterozoic (c. 1200 Ma) Tugela Terrane in southeast South Africa is made up of a series of thrust slices of varied rock-types purported to have been derived in oceanic arcs prior to accretion onto the southern margin of the Archean Kaapvaal Craton. They are not ophiolites. Mafic intrusions in two adjacent thrust slices are known as the Tugela Rand and Mambula Complexes. Both intrusions are intensely layered on scales of centimetres to several tens of metres but clear cyclic units are not apparent and crystal fractionation is limited indicating these were open systems with magma chamber through-flow. Tugela Rand is made up of dominantly olivine-bearing rocks ranging from dunite and pyroxenite to gabbro. In contrast, Mambula is dominantly gabbroic with only rare olivine-bearing rocks and is more evolved with layers of titaniferous-magnetite. Primary magmatic structures in both complexes include graded bedding, slumping and erosion features. Relatively high pressure of formation is indicated by the aluminous nature of the pyroxenes and corona textures by reaction between plagioclase and olivine. Chromitites in Tugela Rand range from massive to podiform with the rare orbicular variety indicating complex controls on chromite accretion. They include the high-Al compositional variety. There are no other similar chromitite occurrences in South Africa. The complexes, together with their enclosing rock-types, draw striking parallels with the lower arc crust observed in the late Cretaceous Kohistan arc complex in NE Pakistan. The Tugela Rand Complex shares many similarities with the Chilas Complex in that terrane, while the Mambula Complex is considered to be a more evolved derivative of the same magma. This study shows that generation of juvenile continental crust formation in mature island arc systems may have been firmly established by the Mesoproterozoic.

The Base Metal zone at Sandsloot in the Northern limb of the Bushveld Complex, South Africa, is a highly unusual and high-grade Os-Ir-Ru-Rh, Fe-Ni sulfide-rich horizon hosted within the deep Platreef, below the main platinum group element (PGE) horizon. The Base Metal zone ranges from 5 to 100 meters in thickness and is located up to 150 meters beneath the PGE reef. Base metal sulfide mineralization occurs as disseminated/blebby to semimassive/massive sulfides, with a typical assemblage of similar to 60/25/15 pyrrhotite/pentlandite/chalcopyrite modal %. The Base Metal zone is characterized by high (Os + Ir + Ru + Rh)/(Pt + Pd) ratios that reflect monosulfide solid solution, primitive mantle-normalized PGE profiles. The PGM assemblage is dominated by laurite (RuS2) (62% by area) and iridium-group platinum group element (IPGE) + Pt arsenosulfides (21% by area). The PGE tenors of the sulfides vary between different textural styles, either reflecting R-factor variations or dilution of tenors by addition of crustal S. Disseminated/blebby sulfides have the highest tenors (up to 153 ppm Pd, 249 ppm Rh, 818 ppm Ru), whereas semimassive/massive sulfides have lower tenors (up to 2.8 ppm Pd, 1.8 ppm Pt, 11 ppm Rh, 17 ppm Ru, 2.2 ppm Os, 3.5 ppm Ir). The PGE geochemistry, IPGE-dominant platinum group metal (PGM) assemblage, abundance of Fe sulfides, and high Ni/Cu ratios are consistent with the Base Metal zone representing the monsulfide solid solution portion of a sulfide liquid formed by fractional crystallization. Furthermore, the Cu + Pt + Pd + Au-poor nature of the Base Metal zone suggests that these metals were removed from the Base Metal zone, and some Cu-rich veins and sections are present around the margins of Ni-Fe sulfide to support this. Increasing Pd/Ir and decreasing Rh/Cu ratios downhole indicate the sulfide liquid fractionated downward. Therefore, a residual Curich liquid, with associated Pt + Pd + Au, likely separated from monosulfide solid solution and was mobilized downward and away from the Base Metal zone. Significantly, the mobilization of a Cu-rich liquid leaves the possibility that an undiscovered Cu + Pt + Pd + Au orebody may exist at depth.

Archaean palaeosols are a rare record of surface processes on the early Earth. We report on the discovery of a palaeoweathering profile developed at the contact between Mesoarchaean basement and the Neoarchaean to Palaeoproterozoic Transvaal Supergroup along the northern margin of the Johannesburg Dome, Kaapvaal Craton. Granodiorite -3.1 Ga old was subjected to weathering just prior to the deposition of the -2.6 Ga Black Reef Formation. The -25 m-thick profile is dominated by a palaeosaprolite and organic matter-rich palaeosol remnant at the top that experienced erosion upon marine transgression on the Kaapvaal Craton. The Al concentration of the profile, sericite content and chemical index of alteration (CIA) values progressively increase from the parent granodiorite towards the nonconformity as Fe, Mg, Na and Ca concentrations decrease. The decrease of Co and Zn together with Fe and the retention of U, V and Cr collectively suggest a reducing and acidic environment at the time of weathering. Carbonaceous matter with negative delta C-13 isotopic values (-27 to -23 parts per thousand V-PDB) in the palaeosol is compatible with the former presence of photoautotrophic microbes. Copper depletion and mobility of P may indicate the presence of organic acids that enhanced chemical weathering of the bedrock. The palaeoweathering profile was subjected to diagenetic-hydrothermal K-metasomatism, regional metamorphism and deformation, which collectively transformed the regolith into a quartz-sericite schist, as it presently occurs. Despite these modifications, the original record of palaeoweathering and microbial colonisation is well preserved. Colonisation of a large swath of the Kaapvaal Craton by terrestrial soil-dwelling microbiota at 2.60 Ga is recorded in abundant soil-derived carbonaceous matter in transgressive marine conglomerates that erosively overlie the palaeosol.

The Mesoarchaean Pongola Supergroup of South Africa hosts the oldest succession of stromatolitic carbonates deposited in an intracontinental setting. In this study, carbonates within two (tectono-) stratigraphic units in the Buffalo River inlier were investigated. Stratiform stromatolite and wave-ripple-laminated dolostone facies are the most common and are interpreted to have formed in a tide-dominated shallow-marine environment. Dolostones typically contain a large amount of siliciclastic detritus, and small domal stromatolites are commonly found scattered in subtidal sandstone facies. The carbonates experienced deformation and greenschist facies metamorphic conditions at peak temperatures of similar to 450 +/- 50 degrees C. Analyses for major, trace and rare earth elements (REEs) along with carbon, oxygen and strontium isotopes have been undertaken on the best-preserved samples. Their REE + Y distribution patterns reflect their marine origin, with positive La-SN, Gd-SN and Y-SN anomalies, super-chondritic Y/Ho ratios, and depleted light REEs relative to heavy REEs. The delta(13) C-VPDB and delta(18) O-VPDB values least affected by diagenetic and metamorphic overprints are 2.2 parts per thousand and -16.1 parts per thousand, respectively. The least radiogenic Sr-87/Sr-86 ratio is 0.704 and reflects isotopic exchange with a siliciclastic component. Despite their alteration, the carbonates of the Buffalo River inlier provide additional constraints on microbial carbonate deposition on the Earth oldest preserved craton. They can be traced laterally for several tens of kilometres and point to environmental conditions suitable for the deposition and preservation of marine carbonate rocks 3.0 Ga ago.

An integrated approach embracing field studies, petrographic and geochemical investigations together with zircon U-Pb-Hf data was used to investigate the petrogenesis of potassic granite suites along the southern margin of the Zimbabwe Craton. Zircon U-Pb geochronology identifies age relationships, revealing coeval magmatism of the ca. 2 635 ± 5 to 2 625 ± 3 Ma Chilimanzi Suite, and the ca. 2 627 ± 7 Ma Razi Suite. Both suites represent syn- to late-tectonic, high-K, calc-alkaline, and metaluminous to weakly peraluminous granites and granodiorites with I-type affinity. The granite suites contain xenocrystic zircons, with the Chikwanda Pluton of the Chilimanzi Suite yielding a grain of up to 3 206 Ma old. Both granite suites exhibit eHf values of between -5.6 ± 1.3 and -7.3 ± 1.6 and TDM model ages of ca. 3.4 to 3.5 Ga which suggests a similar crustal source. The unradiogenic zircon Hf isotopic compositions are consistent with formation of the granite suites through partial melting of pre-existing crustal protoliths, including Palaeoarchaean tonalite-trondhjemite-granodiorites (TTGs) of the Zimbabwe proto-craton. Partial melting of lower crust gave rise to granitic melts that became emplaced over a relatively short time interval from 2 635 to 2 625 Ma and heralded the stabilisation of the Zimbabwe Craton. In addition to virtually identical ages, the Razi and Chilimanzi suites have similar geochemistry. Small geochemical differences between the Chilimanzi and the Razi suites are attributed to the crustal level at which they are preserved, the modal mineralogy and the extent to which the melts are evolved. The Razi Suite melts were generated from lower crust partial melting of thickened charnockite-enderbite source rocks rich in heat producing elements. The partial melting occurred under fluid-absent conditions and magmas were emplaced at lower to mid crustal levels. The Chilimanzi Suite magmas were similarly derived by the partial melting of TTG lower crust and were emplaced at upper crustal levels. Accordingly, the Chilimanzi Suite exhibits more evolved magmatic fractionation indices indicated by high Rb/Sr, as well as low K/Rb ratios relative to the Razi Suite. Both suites reveal varying degrees of enrichment in incompatible elements including Rb, Th, and U, as well simultaneous depletions in Ba, Sr, and Hf which underscores the role of fractional crystallisation in the evolution of the granitic magmas.

Several Archean granitoid-greenstone terranes are exposed on the southeastern Kaapvaal craton in South Africa, but they received little scientific attention compared to the archetypal greenstone belt successions of the Bar-berton Mountain Land at the eastern craton margin. This study reports on a detailed field and geochemical survey of the Buffalo River Greenstone Belt at the southern Kaapvaal craton margin in KwaZulu-Natal, with focus on hitherto unstudied komatiites and basaltic rocks from this volcanic succession. Cross-cutting relationships and new U-Pb zircon age determinations for several granitoid units establish a minimum age of 3.26 Ga for komatiitic volcanism, possibly as old as ca. 3.5 Ga if a 3.47 Ga granodiorite sheet is interpreted as 'intrusive' into the greenstone succession.Geochemical data reveal three types of Paleoarchean komatiites at Buffalo River. Spinifex textured lava flows represent Al-depleted komatiites, with subchondritic Al2O3/TiO2 ratios and enrichment of LREE over HREE. The second type comprises Al-undepleted komatiites that have chondritic Al2O3/TiO2 and flat REE patterns. The third type identified comprises Al-enriched komatiites that display suprachondritic Al2O3/TiO2 ratios, with significant LREE depletion. The Al-depleted and Al-undepleted komatiites from Buffalo River are geochemically similar to komatiites from the 3.48 Ga Komati and 3.26 Ga Weltevreden formations of the Barberton Supergroup respec-tively, whereas the Al-enriched komatiites resemble the 3.33 Ga Commondale komatiites on the southeastern Kaapvaal craton.To explain the co-occurrence of three discrete komatiite types within a single volcanic succession at Buffalo River, we suggest that each major komatiite magmatic pulse originated from the same upwelling mantle source, from which melt was extracted at different pressure but similarly hot temperature conditions. 187Os/188Os data for the Al-depleted komatiites suggest an ultimate magma origin from a primitive mantle reservoir. The con-trasting & gamma;Os values for Kaapvaal craton komatiites (zero to positive) and peridotitic mantle xenoliths (zero to negative) support a complementary nature of these lithologies as high-degree melts and depleted residues linked by vigorous mantle plume activity at around 3.5 Ga. Such a relationship can explain the contrasting Re/Os systematics of komatiites and lithospheric mantle peridotites, which creates the contrasting & gamma;Os over time. The highly siderophile element patterns of the Al-depleted komatiites from Buffalo River are similar to those of Barberton-type komatiites, for which an origin from the deepest upper mantle with high melt retention in an upwelling plume source was suggested. We confirm that this ca. 3.5 Ga mantle source had only 60-80 % of the platinum-group element budget of the modern ambient mantle, which points indirectly to a location at great depth in the aftermath of the meteoritic late accretion. Progressive melting of such an upwelling mantle source, to the point of majoritic garnet exhaustion, may explain the Al-undepleted and Al-enriched komatiites at Buffalo River. The presence of all three major komatiite types within a single volcanic succession may be linked to deep critical melting of a large mantle plume associated with growth of the Kaapvaal 'continent' at 3.5 Ga.

The Stolzburg Complex is a prominent and well-preserved Palaeo- to Mesoarchaean layered ultramafic-mafic complex in the vicinity of the Barberton Greenstone Belt. Whole rock Sm-Nd, Lu-Hf and 142Nd isotopic data obtained from a variety of lithologies, and augmented by major and trace element geochemistry, are presented to examine petrogenesis, magma source composition, and mantle differentiation as well as the geodynamic setting of emplacement. Geochemically, all samples are characterized by unfractionated normalized trace element patterns (La/Luchond. = 0.7-2.3, La/Gdchond. = 0.6-1.7, Sm/Luchond. = 0.9-1.4) associated with weak HFSE anomalies (Nb/[0.5Th + 0.5La]PRIMA = 0.5-2.3). Trace element concentrations vary as a function of mineralogy, but differences in trace element compositions are unsystematic across lithologies. The coherence of trace element characteristics is consistent with a common mantle source that underwent moderately large degrees of melting at mantle pressures within the spinel stability field. Whole rock Sm-Nd and Lu-Hf data yield well-defined apparent isochrons corresponding to ages of 3367 +/- 62 (n = 12, MSWD = 1.5) and 3396 +/- 36 (n = 11, MSWD = 2.1), respectively. Initial Hf and Nd isotopic compositions vary from +3.3 to +5.7 and + 0.9 to +1.7, respectively, indicative of derivation from slightly to moderately depleted mantle source(s). Isochron ages and a regressed initial Hf isotopic composition of +4.0 +/- 0.9 (at 3.4 Ga) contrast with an established U-Pb emplacement age (zircon, titanite) of 3.25 Ga and epsilon Hf values for zircon of -2.5 to +3.0. Hydrothermal alteration or weathering, incorporation of evolved pre-existing (continental) crust and fractionation by high-pressure phases prior to melt extraction can all be ruled out to have significantly affected isotope systematics. Protracted magma chamber activity and non-synchronous emplacement of unrelated gabbroic magmas also fail to fully account for the discrepancy between whole rock and mineral age and isotopic data. Instead, the coherence in apparent isochron ages and variability of initial Hf and Nd isotopic composition is adequately explained by reworking (partial or near-complete remelting) of aged mafic-ultramafic (likely oceanic) crust in staging chambers with compositions indistinguishable from newly formed magmas. Thus, apparent isochrons represent mixing lines and do not record timing of crystallization. A subset of samples was also analyzed for 142Nd isotopic compositions to trace preserved mantle heterogeneities caused by early Archaean crust-mantle differentiation. The results (mu 142Nd = -2.3 +/- 2.2 to +1.7 +/- 2.1) are not resolvable from the modern mantle value, indicating that 142Nd-enriched or depleted mantle reservoirs had not remained isolated from convective homogenization in the asthenosphere, or the volume of mantle underlying the layered complex was too small to sample mantle heterogeneities. The Stolzburg Complex was possibly emplaced in ancient oceanic lithosphere, followed by pervasive but inhomogeneous Ca-metasomatism as documented by exposed rodingites.

The Singhbhum Craton in India contains well-preserved Palaeoarchaean greenstone belts that rival their Australian and South African counterparts, but their stratigraphic make-up remains to be resolved. Here we provide new SHRIMP-and LA-ICP-MS-based U-Pb and Lu-Hf data on magmatic and detrital zircons from stratigraphically resolved samples of the Daitari Greenstone Belt (DGB) in the south of the Singhbhum Craton. A minimum age for the Kalisagar Formation, the lowermost mafic-ultramafic volcanic suite of the DGB, is provided by a felsic sill dated at 3505 +/- 6 Ma. Felsic volcaniclastic rocks from the overlying Talpada Formation provided a weighted mean age of 3507 +/- 5 Ma. Zircon epsilon Hf(t) indicate a juvenile source for 3.5 Ga felsic volcanic rocks. Erosion of the volcanic edifice of the Talpada Formation acted as a predominant source for the overlying turbidite sequence of the Sindurimundi Formation, having identical U-Pb detrital zircon ages of 3505 Ma. A maximum age of deposition of 3502 +/- 2 Ma is proposed for the Sindurimundi Formation. Magmatic zircon ages obtained from different intrusive felsic rocks in the Daitari belt ranges from c. 3.37 to 3.35 Ga and support the notion of widespread felsic magmatism of the Singhbhum Suite in the southern part of the craton. Xenocrystic zircon crystals dated at c. 3.55 to 3.52 Ga may correspond to an earlier phase of magmatism. 3.5 Ga marks an important period in the Earth's history, when extensive felsic magmatism facilitated continental crustal growth as recorded in the Singhbhum, Kaapvaal and Pilbara cratons.

This review paper reports on the discovery of komatiites and the impact this has had on the geosciences internationally. Komatiites were first discovered by Richard and Morris Viljoen in the Barberton greenstone belt of South Africa and proposed as a new class of ultramafic volcanic rock with >18 wt% MgO. Komatiites are in places associated with spinifex textures (elongated bladed and plumose olivine crystals). The discovery and petrological study of komatiites and understanding of the field controls have given rise to intense debate about their mantle origin, water content and geodynamic setting of eruption. The observation that komatiites are found predominantly in the Archean and to some extent in the Palaeoproterozoic, has led to the proposition that the early Earth was much hotter (by c. 400 degrees C) than the present day. However, some workers have challenged this model involving secular cooling of the Earth since Archean times. These workers argue that komatiites formed in a cooler Archean subduction zone, rather than in a mantle plume. Komatiites of the mid-to late-Archean have also proven to be important economically (e.g. Yilgarn Craton, Australia; Superior Craton, Canada) as they host important nickel-sulphide deposits. Today, novel petrological techniques (e.g. secondary ion microprobe analyses of olivine melt inclusions) are being applied to komatiites to address important outstanding questions in the geosciences, including the question of whether or not plate tectonics was operating in the early Archean.

Derelict asbestos mine sites in South Africa pose a considerable risk to human, environmental and socio-economic health. Comprehensive mineralogical and geochemical datasets for the existing hazardous geological materials still exposed in Southern African derelict asbestos mines remain largely non-existent, as very little published and up-to-date literature is available. In this study, three representative types of asbestos mineral fibres from derelict asbestos mines in Southern Africa, namely chrysotile from Havelock mine, amosite from Penge mine and crocidolite from Prieska mine, are characterized mineralogically and geochemically to critically evaluate actual hazards in rural and asbestos-fibre-contaminated regions. The samples were examined using polarising light microscopy, X-ray fluorescence (major and trace elemental analysis), X-ray diffraction (including Rietveld refinement), specific surface area analysis and bio-durability testing. Data are discussed in view of their potential toxicities on both human health and the environment in the context of developing countries. Finally, information on the mineralogical and geochemical status of asbestos mine waste and its importance as baseline data for rehabilitation considerations is also evaluated.

Oxygen is the most abundant element in the mantle.Character-izing the oxygen isotopic compositions of the early Earth's mantle is of importance as it is the starting point of the differentiation of the bulk silicate Earth[1-3].However,the original oxygen isotopic signature of the early Earth's mantle is poorly constrained,because it is rarely preserved and sampled at the Earth's surface[4].Olivine from Archean komatiite can provide insight into the early mantle with three key advantages:Archean komatiite is the highest-MgO volcanic rock-type on Earth and was derived from a deep mantle source that experienced minimal disturbance by subse-quent convection of the upper mantle[5,6];olivine is the earliest crystallized phase in ultramafic magmas so that the effect from assimilation of supracrustal materials during magmatic evolution is potentially avoided[7,8];and isotopic fractionation between mantle and komatiite olivine is minimal and therefore δ18O values obtained from such olivine would closely reflect the mantle source[1,3].

Ultramafic-mafic layered complexes are important but not-well studied components of Archaean granitoid-greenstone terranes. In the vicinity of the Barberton Greenstone Belt (BGB), at least 27 such complexes are intimately associated with the supracrustal succession. The petrogenesis of one of these layered bodies, the Stolzburg Complex (SC), is explored, together with its relationship to the surrounding Barberton volcanic succession. Previous models for the origin of Barberton layered complexes proposed a variety of mechanisms, such as single chamber subvolcanic sills, ponded lavas, and alpine-type tectonites. In contrast, the present work suggests that emplacement mostly occurred as sheeted sills of crystal slurries into the country rocks. Unlike the subvolcanic sills model, whereby each complex grew through repetitive magma injection and differentiation in a single chamber, the preferred model regards the layered bodies as ???stacks??? of discrete intrusions, where each magmatic unit represents a distinct sill. Through comparison of trace element geochemistry (i.e., trace element ratios and patterns), the Lower and Upper divisions of the SC are inferred to be petrogenetically related, but compositionally distinct from the enveloping Nelshoogte volcanic rocks. The trace element geochemistry of the Lower and Upper divisions of the complex is indistinguishable. While the SC ultramafic rocks display an Al-undepleted character, Nelshoogte metavolcanics can be classified as Al-depleted komatiites and komatiitic basalts.

The Mozaan Group represents the youngest unit of the c. 2.9 Ga Pongola Supergroup located along the southeastern margin of the Kaapvaal Craton. It comprises a ca. 4800 m thick succession of clastic sedimentary rocks intercalated by minor chemical and volcano-sedimentary rocks deposited in shallow marine to fluvial environments, and is stratigraphically correlated with the auriferous Witwatersrand Supergroup. This correlation, however, is speculative, in particular as systematic information about depositional ages and sediment provenances are absent. To address these problems, we present new combined sets of U-Pb ages, Hf isotopes, and shape parameters (width, length, aspect ratios and roundness) of > 700 detrital zircon grains from seven samples of the Mozaan type profile in the Hartland area. These data reveal a switch in provenance between the lower and upper Mozaan Group. Zircons in sandstones of the lower Mozaan Group (Sinqeni to Ntombe formations) were supplied from surrounding proto-Kaapvaal Craton, and those in upper Mozaan Group rocks (Delfkom to Ntanyana formations) predominately from a juvenile hinterland, comprising sources as far as the Pietersburg and/or Kimberley blocks, which became amalgamated to the proto-Kaapvaal Craton at 2.97-2.87 Ga. Significant overlap of zircon age spectra, Hf isotope data, and maximum depositional ages (2908 +/- 8 Ma to 2866 +/- 7 Ma) suggest similar sources for upper Mozaan Group and Central Rand Group sediments of the Witwatersrand Basin. In contrast, sedimentary rocks of the West Rand Group have no counterparts in the Pongola Basin, except for the Orange Grove Formation, which shows good agreement with the Sinqeni Formation. The provenance switch indicated by the age-Hf isotope data is not identified by zircon shape parameters. These rather reflect differences in depositional environment (littoral, fluvial, volcanogenic), related to the duration and energy of sediment transport and reworking, as indicated by specific patterns in grain size vs. roundness diagrams.

The deep upper mantle is the main source of high-temperature magmatism, but the only known naturally occurring samples of high-pressure mantle constituents are mineral inclusions in diamonds. Trace elements in olivine crystals from the 3.33 Ga Commondale Greenstone Belt in South Africa reveal that these crystals formed in the deep upper mantle as high-pressure phenocrysts, and some perhaps even formed in the mantle transition zone (410-600 km) where they began as wadsleyite. The crystals were entrained within ascending komatiite magma and conveyed to the surface. The olivine crystals have the highest contents of Al2O3 (0.3 wt%) recorded in any terrestrial olivine, which is indicative of formation at high pressure. The deep mantle gave rise to Archean komatiites, extraordinarily hot magmas (up to 1700 degrees C), which provide insight into Earth's early mantle evolution and the formation of most ancient continental and oceanic crust. In spite of extensive research since their discovery over 50 years ago, the origins of komatiites have remained contentious. Plumes-thermochemical instabilities originating at the core-mantle boundary-are the most likely source, but no direct evidence of a deep mantle origin of komatiite has yet been recognized.

The nature of the early Archean ocean floor remains a topic of important debate. There are relatively few well-preserved occurrences worldwide where such terrains may be studied in detail because of structural dismemberment, metamorphic overprinting and pervasive early stage hydrothermal alteration to recent weathering. The 3.41-Ga dominantly mafic formations of the Nondweni Greenstone Belt (NGB) covering 270 km(2) in the south-eastern Kaapvaal Craton comprise submarine volcanics that exhibit a wide range of textural features, including pillows, chill zones and brecciated flow tops, and various spinifex textures, including the rare platy pyroxene type, cumulate layers, and tuffs. Channelized subaqueous lava lakes that underwent fractionation are capped by thick spinifex-textured units and pillows. Early stage seafloor alteration is regionally variable, ranging from intense to minimal, with preservation of original mineralogy in many areas. Mafic volcanic rocks of the NGB contrast with those of the Barberton Greenstone Belt both in the style of volcanism and in the associated compositional range of komatiitic basalt to basalt with a complete absence of high-Mg komatiites. Olivine-phyric rocks, or derivatives thereof, are largely absent and pyroxene is the main controlling phase with orthopyroxene in the most primitive komatiitic basalts and clinopyroxene in the evolved lava lake sequences. The abundance of orthopyroxene typifies the long-standing silica-enriched character of the Kaapvaal Craton. Three exceptionally well-preserved and well-exposed sequences were studied utilizing hand-drilled samples and deep coring providing unprecedented stratigraphic and textural detail and field controls for more than 400 samples. A unifying feature of the mafic volcanics of the NGB is the range of compositions and ratios of incompatible elements most clearly illustrated by a series of high- and low-Ti compositional lineages reflecting differing sources or degrees of mantle partial melting. Sharp boundaries between high- and low-Ti flow successions indicate sudden changes in the melting regimes or the interaction of flow sequences from different volcanic centres. Th/Nb ranges from 0.1 to 0.2 and reveals crustal contamination of primitive lavas. The primary magma that gave rise to the most primitive komatiitic basalts with 19.5% MgO was derived from partial melting of a mantle plume source in the garnet stability field. Trace element modeling shows that the sequences studied in detail have been modified by fractionation and crustal contamination with the most likely contaminant being the Ancient Gneiss Complex (3.43-3.66 Ga), which is extensively exposed in Eswatini and probably underlies the Paleoarchean terrains in the southern Kaapvaal Craton. The geotectonic setting was likely that of a submerged felsic crustal platform as enclaves within an oceanic plateau.

SignificanceDue to active plate tectonics, there are no direct rock archives covering the first ca. 500 million y of Earth's history. Therefore, insights into Hadean geodynamics rely on indirect observations from geochemistry. We present a high-precision 182W dataset for rocks from the Kaapvaal Craton, southern Africa, revealing the presence of Hadean protocrustal remnants in Earth's mantle. This has broad implications for geochemists, geophysicists, and modelers, as it bridges contrasting 182W isotope patterns in Archean and modern mantle-derived rocks. The data reveal the origin of seismically and isotopically anomalous domains in the deep mantle and also provide firm evidence for the operation of silicate differentiation processes during the first 60 million y of Earth's history.

Abstract Orangeites are a significant source of diamonds, yet ambiguity surrounds their status among groups of mantle-derived potassic rocks. This study reports mineralogical and geochemical data for a c. 140 Ma orangeite dyke swarm that intersects the Bushveld Complex on the Kaapvaal craton in South Africa. The dykes comprise distinctive petrographic varieties that are linked principally by olivine fractionation, with the most evolved members containing minor amounts of primary carbonate, sanidine and andradite garnet in the groundmass. Although abundant groundmass phlogopite and clinopyroxene have compositions that are similar to those of cratonic lamproites, these phases show notable Ti-depletion, which we consider a hallmark feature of type orangeites from the Kaapvaal craton. Ti-depletion is also characteristic of bulk rock compositions and is associated with strongly depleted Th–U–Nb–Ta contents at high Cs–Rb–Ba–K concentrations. The resultant high large ion lithophile element/high field strength element ratios of orangeites suggest that mantle source enrichment occurred by metasomatic processes in the proximity of ancient subduction zones. The Bushveld-intersecting orangeite dykes have strongly enriched Sr–Nd–Hf isotopic compositions (initial 87Sr/86Sr = 0.70701–0.70741; εNd = −10.6 to −5.8; εHf = −14.4 to −2.5), similar to those of other orangeites from across South Africa. Combined with the strong Ti–Nb–Ta depletion, this ubiquitous isotopic feature points to the involvement of ancient metasomatized mantle lithosphere in the origin of Kaapvaal craton orangeites, where K-rich metasomes imparted a ‘fossil’ subduction geochemical signature. Previous geochronology studies identified ancient K-enrichment events within the Kaapvaal cratonic mantle lithosphere, possibly associated with collisional tectonics during the 1.2–1.1 Ga Namaqua–Natal orogeny of the Rodinia supercontinent cycle. It therefore seems permissible that the cratonic mantle root was preconditioned for ultrapotassic magma production by tectonomagmatic events that occurred along convergent plate margins during the Proterozoic. However, reactivation of the K-rich metasomes had to await establishment of an extensional tectonic regime, such as that during the Mesozoic breakup of Gondwana, which was accompanied by widespread (1000 × 750 km) small-volume orangeite volcanism between 200 and 110 Ma. Although similarities exist between orangeites and lamproites, these and other potassic rocks are sufficiently distinct in their compositions such that different magma formation processes must be considered. In addition to new investigations of the geodynamic triggers of K-rich ultramafic magmatism, future research should more stringently evaluate the relative roles of redox effects and volatile components such as H2O–CO2–F in the petrogeneses of these potentially diamondiferous alkaline rocks.

The Stolzburg Complex is one of several early Archaean ultramafic-mafic layered complexes situated along the northern edge of the Barberton Greenstone Belt on the eastern Kaapvaal Craton. In order to constrain timing and mode of emplacement, as well as post-emplacement processes affecting this igneous complex, we present whole rock trace element compositions as well as isotopic (U-Pb, O, Hf) and REE-Ti concentration data for zircon and titanite from three representative meta-gabbroic samples from the central and western portions of the complex. Zircon (SHRIMP) and titanite (LA-MC-ICPMS) analyses yield precise and identical U-Pb ages of 3247 ± 3 Ma, 3252 ± 16 Ma and 3259 ± 6 Ma (2SE), respectively. Cathodo-luminescence imaging of zircon reveals remnant, faint oscillatory and convolute zoning that is cross-cut by luminescent, irregular fracture fillings. Zircon crystals from one meta-gabbro sample yield oxygen isotopic compositions (δ18O) of <1 to +6‰, which are uncorrelated with the initial Hf isotopic compositions (εHf) of 0 to +3 epsilon units. Zircon REE systematics are highly variable across crystal domains and characterized by enriched light and medium REE. Micro-textural evidence of post-emplacement modification, along with zircon δ18O and trace element systematics, are indicative of pervasive but heterogeneous/irregular overprinting involving high-T hydrothermal fluids (>250 °C) soon after magma emplacement. Some recent metamictization may have also contributed to the low δ18O component identified in the magmatic zircon, but is difficult to disentangle from hydrothermal effects. Based on zircon trace element systematics, the gabbros appear to have formed in an oceanic setting, consistent with the lack of whole rock geochemical evidence for assimilation of more evolved felsic crustal material. Considering analytical uncertainty, zircon εHf values are relatively uniform and point to derivation of the mafic magma from a Depleted Mantle source. Titanium-in-zircon temperatures (assuming αSiO2 = 1.0, αTiO2 = 1.0) range from 840 to 1020 °C. Although such apparent crystallization temperatures do neither constrain magma petrogenesis nor tectonic setting uniquely, they match temperatures recorded by modern mid-oceanic gabbros that experienced extensive hydrothermal alteration. Thus, the meta-gabbros of the ca. 3.25 Ga Stolzburg Complex provide a rare record of fluid-rock interaction at high temperatures within mafic crust on an ancient seafloor.

Our experimental thermobarogeochemical study of the primary melt inclusions in olivine (Fo91) from poikilitic harzburgite to dunite of the Uitkomst Complex in the Bushveld Igneous Province revealed a series of melt compositions that are the result of combined fractional crystallization and assimilation. The primary melt inclusions up to 50 mu m in size are fully crystallized and composed of predominantly olivine and orthopyroxene with highly subordinate chromite, sulfide, amphibole and plagioclase accompanied by a fluid phase in shrinkage voids. The most primitive melt within the series has a re-homogenization temperature of >1450 degrees C and contains 22.47 wt% MgO, 12.57 wt% FeO, 51.90 wt% SiO2, 2.50 wt% CaO, 8.27 wt% Al2O3 and 1.69 wt% Na2O recalculated to equilibrium with the host olivine. This is the first direct measurement of the komatiitic parental magma of the Bushveld magmas, inferred in previous studies. The estimated maximum PT parameters of the melt corresponds to 2.5 GPa and 1590 degrees C implying that the primitive melt inclusions in the olivine were trapped while still in the upper mantle and before further crustal assimilation. This primitive melt shows the steepest slope (La/Yb)PM, the highest Ni and is depleted in volatile components (168 ppm H2O, 264 ppm Cl, 258 ppm F and 114 ppm S according to secondary ion mass spectrometry measurements) compared to more evolved compositions, which were progressively enriched in volatiles up to 7665 ppm H2O, 1728 ppm Cl, 2288 ppm S, and 549 ppm F as the rehomogenization temperature fell to 1335 degrees C. The progressive assimilation of crustal material responsible for magma modification is also manifested by the occurrence of Ca-Na volatile-rich minerals (such as amphibole, phlogopite, albite, sodalite, natrolite, pectolite, Cl-apatite and carbonate) in the interstitial assemblage and in "exotic" polyphase inclusions in cumulus olivine. The concentrations and ratios of the incompatible elements in the melt inclusions in Uitkomst olivine are similar to those in the model compositions of the ultramafic Bushveld parentalmagma and support a common source of ultramafic melts in the Bushveld Province. The main components of the melts, however, are not in accord with either boninitic or komatiitic lineages indicating a unique nature for the Bushveld magmas. (C) 2021 Published by Elsevier B.V.

The Transvaal Basin in South Africa hosts a 15 km thick pile of sedimentary successions deposited over a period of > 600 Ma during the Neoarchean to Paleoproterozoic. Presently, little is known about the source of these sediments, as well as about the tectono-magmatic evolution in the hinterland of the Transvaal Basin, preventing detailed geotectonic correlations of the Kaapvaal Craton (KC) with other cratons worldwide. To solve this problem, we present the first systematic study of combined U-Pb and Lu-Hf isotope data of > 2000 detrital zircons from fourteen formations of the Transvaal Supergroup. These reveal that clastic sedimentary rocks were supplied from sources on and off the present-day KC. Detrital zircons in conglomerates of the Wolkberg and Black Reef formations, maximum deposition ages at 2769 +/- 8 and 2618 +/- 11 Ma respectively, were mainly supplied from surrounding KC, either from Pietersburg Block Basement (PBB), and/or from eroded sedimentary successions of the Witwatersrand, Pongola and/or Ventersdorp Supergroups. In contrast, clastic sedimentary rocks of the Rooihoogte, Duitschland and Timeball Hill formations (maximum deposition ages at 2353 +/- 18 Ma, 2342 +/- 18 and 2290 +/- 8 Ma, respectively) were predominately supplied from a juvenile Neoarchean terrane (JUNAT) formed at 2570-2500 Ma (eHf(2500 Ma) = +2 to + 9) and intensely reworked at 2400 Ma, and to a minor amount from a composite Archean terrane (CAT) emplaced by granitoids between 3540 and 2680 Ma, and affected by crust reworking at 2570-2430 Ma (eHf(2.5Ga) = -3 to -12) in a Neoarchean to Paleoproterozoic continental arc terrane (NPCAT). Subsequent periodic reworking of JUNAT at 2250-2220 Ma and 2120 Ma is recorded by detrital zircons in sandstones of the overlying Boshoek, Dwaalheuwel, Daspoort, Magaliesberg and post-Magaliesberg formations, having maximum deposition ages at 2243 +/- 7, 2242 +/- 7, 2240 +/- 7, 2080 +/- 7, and 2068 +/- 7 Ma, respectively. The Archean zircons (age > 2650 Ma) in all these formations were mainly supplied from PBB. The new data sets also suggest that the KC was connected to CAT, NPCAT and JUNAT at < 2350 Ma. The nearly absence of detrital zircons with ages of 2570-2500 Ma in all formations younger than Boshoek perhaps results from intense reworking of JUNAT during magmatic events at 2400, 2340, 2220, and 2120 Ma, causing loss of the original juvenile character. Paleoproterozoic zircons with ages of 2220 and 2120 Ma in Dullstroom sandstones most likely result from re-deposition of post-Magalisberg sedimentary rocks, and Archean zircons from sources similar to Moodies and Fig Tree sandstones of the Barberton greenstone belt. Comparison of our new data from the Transvaal Basin with such from the Turee Creek and Horseshoe basins in NW-Australia provides no evidence for Kaapvaal-Pilbara Craton connection during the Neoarchean to Paleoproterozoic.