Fig Tree Group Research Articles

Siderophile element patterns, PGE nuggets and vapour condensation effects in Ni-rich quench chromite-bearing microkrystite spherules, ∼ 3.24 Ga S3 impact unit, Barberton greenstone belt, Kaapvaal Craton, South Africa

Energy dispersive spectrometry (EDS), laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS) track analyses of chlorite-dominated quench-textured microkrystite spherules and LA-ICPMS spot analyses of intra-spherule Ni-rich skeletal quench chromites from the 3243 ± 4 Ma Barberton S3 impact fallout unit (lower part of the Mapepe Formation, Fig Tree Group, Barberton greenstone belt, Kaapvaal Craton, South Africa) reveal fractionated siderophile and PGE trace element patterns corresponding to chondrite-contaminated komatiite/basalt compositions. The chlorites, interpreted as altered glass, contain sharp siderophile elements and PGE spikes inherited from decomposed metal and Ni-rich chromite particles. LA-ICPMS spot analysis identifies PGE-rich micronuggets in Ni–chromites (Ir ∼ 12–100 ppm, Os ∼ 9–86 ppm, Ru ∼ 5–43 ppm) and lower levels of the volatile PGEs (Rh ∼ 1–11 ppm, Pd ∼ 0.68–0.96 ppm). Previously reported PGE anomalies in the order of hundreds of ppb in some Barberton microkrystite spherules are accounted for in terms of disintegration of PGE-rich micronuggets. Replacement of the Ni-chromites by sulphide masks primary chondritic patterns and condensation element distribution effects. High refractory/volatile PGE ratios pertain to both the chlorites and the Ni-rich chromites, consistent with similar compositional relations in microkrystite spherules from other impact fallout units in the Barberton greenstone belt and the Pilbara Craton, Western Australia. The near-consistent low Pt/Re and high V/Cr and V/Sc ratios in chlorite of the spherules, relative to komatiites, are suggestive of selective atmospheric condensation of the spherules which favored the relatively more refractory Re and V. Selective condensation may also be supported by depletion in the volatile Yb relative to Sm. Ni–Cr relationships allow estimates of the proportion of precursor crustal and meteoritic components of the spherules. Mass balance calculations based on the iridium flux allow estimates of the order of magnitude of the diameter of the chondritic projectile.

Geochemistry of shales from the late-Archean Chitradurga Greenstone Belt, Karnataka, Southern India

Archean shales have been collected from three different stratigraphic horizons namely Vanivilas, Ingaldhal and Hiriyur Formations representing the lower and upper levels of Chitradurga greenstone belt (CGB). These shales/pelites are characterized by mica-quartz-chlorite+graphite mineralogy. The high Al2O3/TiO2 values, enriched incompatible and HFSE indicate a greater abundance of detrital and residual heavy minerals. REE patterns of the three different formations are similar, absence of negative Eu anomalies, LREE enrichment and flat HREE (Fig. 1). One sample belonging to the Vanivilas Formation shows strong negative Ce anomaly indicating perhaps prevalence of localized oxygenated environment as evidenced by the occurrence of stromatolites. These patterns are also similar to the Fig Tree Groups in South Africa [1] and indicate contribution of detritus from source rock types to the basin. La/ Th, Th/Sc, Co/Th and Cr and Ni distributions in these pelites show a mixed provenance. High Chemical Index of Alteration (CIA) and depletion of Sr and CaO in the multi-element plot normalized against average Archean Upper Crust (AUC) indicate intense chemical weathering of the source terrain. The Chitradurga pelites on an average are more mafic in composition than the post-Archean sediments indicating strong similarities to that of Archean turbidite deposits.

The geochemistry of sedimentary rocks from the Fig Tree Group, Barberton greenstone belt: Implications for tectonic, hydrothermal and surface processes during mid-Archaean times

The ca. 3.25Ga old Fig Tree Group in the southern part of the Barberton greenstone belt consists of deep- to shallow-water shale, greywacke, jaspilitic banded iron formation (BIF), and carbonaceous chert. The sequence is more than 1200m thick and crops out as tectonically duplicated, but stratigraphically distinct tectono-stratigraphic units. Chemical weathering of the source terrain of Fig Tree strata was minor. Instead, hydrothermal-metasomatic events affected the sedimentary rocks in the study area, resulting in the depletion of alkaline earth elements and K metasomatism. Provenance modelling using REEs and trace element ratios indicate varying contributions from ultramafic to mafic greenstones, TTGs and HREE-undepleted granites. A clear stratigraphic control on the composition, in the form of increasing trace metal contents and decreasing LaN/YbN ratios, reflects progressive increase of ultramafic and mafic volcanic rocks in the source. Banded iron formation shows REE-Y patterns consistent with its precipitation in a marine environment from Eu-enriched seawater. Carbonaceous cherts at the base of the Fig Tree Group do not represent chemical precipitates out of a hot Archaean ocean, but represent carbonaceous sediments that were silicified during low-temperature hydrothermal fluid emanations on the seafloor.

PGE, Re-Os, and Mo isotope systematics in Archean and early Proterozoic sedimentary systems as proxies for redox conditions of the early Earth

Re-Os data and PGE concentrations as well as Mo concentrations and isotope data are reported for suites of fine clastic sediments and black shales from the Barberton Greenstone Belt, South Africa (Fig Tree and Moodies Groups, 3.25–3.15 Ga), the Belingwe Greenstone Belt, Zimbabwe (Manjeri Formation, ca. 2.7 Ga) and shales from the Witwatersrand, Ventersdorp and Transvaal Supergroups, South Africa ranging from 2.95 to 2.2 Ga. Moderately oxidizing conditions are required to mobilize Re and Mo in the environment, Mo fractionation only occurs in solution, and these parameters thus have potential use as paleoredox proxies for the early Earth. PGE + Re abundance patterns of Barberton Greenstone Belt sediments are uniform and very similar in shape to those of komatiites. This indicates (1) that the PGE came from a source of predominantly ultramafic composition and, (2) that PGE were transported and deposited essentially in particulate form. Sediments from the younger Belingwe Greenstone Belt show more fractionated PGE + Re patterns and have Re/Os ratios 10 to 100× higher than those of Barberton sediments. Their PGE abundance patterns and Re/Os ratios are intermediate between those of the mid-Archean shales and Neoproterozoic to Recent black shales. They reflect scavenging of Re from solution in the sedimentary environment. δ 98/95Mo values of black shales of all ages correlate with their concentrations. The Barberton Greenstone Belt samples have ∼1–3 ppm Mo, similar to a granitoid-basaltic source. This Mo has δ 98/95Mo between −1.9 and −2.4‰ relative to present day mean ocean water molybdenum, MOMO and is thus not isotopically fractionated relative to such a source. Similar to the PGE this indicates transport in solid form. Sediments from the Belingwe Greenstone Belt show in part enhanced Mo concentrations (up to 6 ppm) and Mo isotope fractionation (δ 98/95Mo up to −1.4‰ relative to MOMO). The combined PGE + Re and Mo data show mainly reducing conditions in the mid-Archean and suggest that by 2.7 Ga, the atmosphere and oceans had become more oxidizing. Substantially younger samples from the Transvaal Supergroup (to ca. 2.2 Ga) surprisingly have mainly low Mo concentrations (around 1 ppm) and show no significant Mo isotope fractionation relative to the continental source. Among possible explanations for this are a return to reducing atmospheric conditions after 2.7 Ga, reservoir effects, or Mo removal by sulfide precipitation following sulfate reduction in early Proterozoic oceans.

Thermal history of the 3.5–3.2 Ga Onverwacht and Fig Tree Groups, Barberton greenstone belt, South Africa, inferred by Raman microspectroscopy of carbonaceous material

Research Article| January 01, 2004 Thermal history of the 3.5–3.2 Ga Onverwacht and Fig Tree Groups, Barberton greenstone belt, South Africa, inferred by Raman microspectroscopy of carbonaceous material Michael M. Tice; Michael M. Tice 1Department of Geological and Environmental Sciences, Stanford University, Stanford, California 94305, USA Search for other works by this author on: GSW Google Scholar Benjamin C. Bostick; Benjamin C. Bostick 1Department of Geological and Environmental Sciences, Stanford University, Stanford, California 94305, USA Search for other works by this author on: GSW Google Scholar Donald R. Lowe Donald R. Lowe 1Department of Geological and Environmental Sciences, Stanford University, Stanford, California 94305, USA Search for other works by this author on: GSW Google Scholar Geology (2004) 32 (1): 37–40. https://doi.org/10.1130/G19915.1 Article history received: 09 Jun 2003 rev-recd: 09 Sep 2003 accepted: 11 Sep 2003 first online: 09 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Michael M. Tice, Benjamin C. Bostick, Donald R. Lowe; Thermal history of the 3.5–3.2 Ga Onverwacht and Fig Tree Groups, Barberton greenstone belt, South Africa, inferred by Raman microspectroscopy of carbonaceous material. Geology 2004;; 32 (1): 37–40. doi: https://doi.org/10.1130/G19915.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract Raman spectra of carbonaceous material were collected in situ from samples of cherts of the Onverwacht and Fig Tree Groups in the central Barberton greenstone belt. The spectra feature two dominant peaks characteristic of disordered carbon: the D peak at ∼1310 cm−1 and the O peak at 1580–1600 cm−1. D peak positions and relative peak intensities and areas indicate that all samples have been altered to lower greenschist facies or above. No correlation was observed between maximum temperature and stratigraphic position or degree of hydrothermal alteration, implying that metamorphism in the central Barberton greenstone belt was regional and unaccompanied by the flow of large quantities of hydrothermal fluids. Samples from the Marble Bar Chert of the Pilbara block, Western Australia, have been heated to the same extent as samples from Barberton. This study demonstrates the use of Raman spectra of carbonaceous material as a sensitive geothermometer for low-temperature metamorphic facies. This application could also be used to establish the antiquity of putative microfossils from metamorphic terranes. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

High Archean climatic temperature inferred from oxygen isotope geochemistry of cherts in the 3.5 Ga Swaziland Supergroup, South Africa

New and compiled oxygen isotope data combined with the results of geological and sedimentological studies demonstrate that enclaves of synsedimentary to very early diagenetic cherts are widely preserved in the 3.5-3.2 Ga Swaziland Supergroup, Barberton greenstone belt, South Africa. The low δ 1 8 O values of these cherts indicate extremely high ocean temperatures of 55-85 °C. Previously, the large depletion in 1 8 O shown by all Barberton cherts relative to their Phanerozoic counterparts has been attributed to low 1 8 O in Archean oceans, chert formation during late diagenesis, wholesale loss of 1 8 O during alteration, and/ or regional silicification of sediments around hot springs. These alternative explanations are not compatible with the new results. Cherts in the Onverwacht Group display an isotopic stratigraphy that is inversely repeated in conglomerates in the overlying Fig Tree and Moodies Groups, demonstrating that the chert δ 8 0 O values were fixed prior to Archean uplift and erosion, which started at 3.26 Ga. The maximum δ 1 8 O value in Barberton cherts (+22‰) is lower than the minimum values (+23‰) in Phanerozoic bedded cherts, precluding late diagenesis as the explanation of the overall low δ 1 8 O values. Regional metamorphic, hydrothermal, or long-term resetting of original δ 1 8 O values is also precluded by preservation of δ 1 8 O values across different metamorphic grades and by systematic δ 1 8 O differences among interbedded chert types, stratigraphic units, and conglomerate clasts. The 7‰ δ 1 8 O variation of these Archean cherts is similar to that of Phanerozoic deep-sea cherts-formed when opal converted to microquartz during burial-but the actual Archean values are ∼10‰ lower. Marine opal was apparently converted to microquartz during burial to depths of <1 km. Cherts with δ 1 8 O < 15‰ reflect conversion during deepest burial or in local areas of enhanced geothermal gradient and/ or hydrothermal activity. Cherts with higher δ 1 8 O values formed during early diagenesis and indicate an extremely hot Archean ocean and surface environment.

Amphibolite facies metamorphism in the Schapenburg schist belt: A record of the mid-crustal response to 3.23 Ga terrane accretion in the Barberton greenstone belt

The Schapenburg Schist Belt is one of several large greenstone remnants exposed in the granitoid-dominated terrane to the south of the Barberton greenstone belt and is unique in that it contains a well-developed metasedimetary sequence in addition to the typical mafic-ultramafic volcanic rocks. Detrital zircons within the metasediments have ages as young as ~3.24 Ga and consequently, these rocks correlate with Fig Tree Group sediments exposed in the central portions of the Barberton greenstone belt some 60km to the north, where they are metamorphosed to greenschist facies conditions. The Schapenburg metasediments are relatively K2O-poor, and are commonly characterised by the peak metamorphic assemblage garnet + cordierite + gedrite + biotite + quartz ± plagioclase. Other assemblages recorded are garnet + cummingtonite + biotite + quartz, cordierite + biotite + sillimanite + quartz and cordierite + biotite + anthophyllite. In all cases the peak assemblages are texturally very well equilibrated and the predominantly almandine garnets from all rock types show almost flat zonation patterns for Fe, Mg, Mn and Ca. Analysis using FMASH reaction relations, as well as a variety of geothermometers and barometers, has constrained the peak metamorphic pressure-temperature conditions to 640 ± 40°C and 4.8 ± 1.0 kbar. The maximum age of metamorphism is defined by the ~3.23 Ga age of a syntectonic tonalite intrusion into the central portion of the schist belt. Thus, sedimentation, burial to mid-crustal depths, and amphibolite facies equilibration were achieved in a time span similar to ~15 Ma. The strong bedding-parallel foliation in the metasediments dips to the east at an angle of 75 to 85° In this foliation plane elongated cordierite rods produced during prograde metamorphism define a close to vertical mineral lineation. The metasedimentary succession youngs to the east and in this direction is overlain by older Onverwacht Group rocks. Thus, the cordierite lineation might represent a transposed lineation developed during thrust stacking. Post-kinematic static recrystallization has largely obscured the prograde history of the rocks. However, the pressure-temperature conditions of peak metamorphism, the age of metamorphism, the rapidity of metamorphism following sedimentation, the presence of cordierite on the prograde path, as well as the timing of metamorphism relative to deformation strongly suggest that metamorphism occurred in an arc-style collisional setting. Thus, the rocks of this study most likely represent an exhumed mid-crustal terrane equilibrated during the proposed ~3230 Ma terrane accretion event in the Barberton Greenstone Belt, and record an apparent geothermal gradient typical for portions of modern orogenic belts of this type.

Geology and geodynamic setting of Archaean silicic metavolcaniclastic rocks of the Bien Venue Formation, Fig Tree Group, northeast Barberton greenstone belt, South Africa

Mapping of the northeastern part of the Barberton greenstone belt (BGB) has delineated a new lithostratigraphic unit, the Bien Venue Formation, within the Fig Tree Group, Swaziland Supergroup, South Africa. The formation is composed mainly of quartz–muscovite (±andalusite±chlorite±chloritoid±pyrophyllite) schists derived from a sequence of quartz-phyric, dacitic-to-rhyodacitic volcaniclastic protoliths dated at 3256±1 and 3259±5 Ma. Subordinate rock types include banded chert, phyllite, biotite–plagioclase (±chlorite±carbonate) schists derived from plagioclase-phyric dacitic volcaniclastic assemblages, and chlorite (±carbonate±amphibole) schists representing altered basalt or basalt-andesite. These rocks were deposited on deep-water shales, turbiditic greywackes, banded iron-formations (BIFs) and silicic metavolcaniclastic assemblages correlated with the Belvue Road Formation, and are, in turn, overlain by alluvial to shallow-marine sandstones and conglomerates of the Moodies Group along a regional unconformity. In places, the quartz–muscovite schists host subeconomic massive sulphide (Zn–Cu–Pb–Ag) and barite deposits of probable volcanogenic origin. The sedimentary structures, lithologies, stratigraphic associations, and mineral deposits suggest that the Bien Venue Formation represents a proximal volcaniclastic succession formed in a largely subaqueous depositional environment that may have varied from deep (>1 km depth) to shallow water. The protoliths of the Bien Venue schists were geochemically similar to calc-alkaline and transitional tholeiitic-calc-alkaline suites formed at Phanerozoic mature active continental margins. Variably fractionated rare earth element (REE) profiles, moderate abundances of Yb and Y, and relative depletions at Ta–Nb, P and Ti on normalised immobile multi-element diagrams are consistent with hydrous fusion of upper mantle material above steeply subducted oceanic crust undergoing dehydration. The features suggest that the Bien Venue Formation records Early Archaean continental arc volcanism along the north-east flank of the BGB. The lithochemical signatures of the Bien Venue rocks are distinctly different from those of silicic volcanic suites hosting major syngenetic polymetallic massive sulphide deposits in Canada. The host rocks of the Bien Venue massive sulphide deposit do not appear to have undergone significant fractionation in a high-level magma repository which could have supplied heat for the development of a major mineralising hydrothermal fluid system. These observations suggest low potential for the discovery of economic base-metal massive sulphide deposits in the rocks of the Bien Venue Formation. The Bien Venue rocks were previously included in the Theespruit Formation of the Onverwacht Group, however, the isotopic age data indicate that they are some 290 million years younger than the oldest assemblages within the lower Onverwacht. The schists are also 30 million years older than plagioclase-phyric dacitic rocks within the Schoongezicht Formation (ca. 3226 Ma), uppermost formation of the Fig Tree Group along the northwest flank of the BGB, precluding direct correlation with this unit. Published ages indicate that the Bien Venue Formation is a temporal correlative of proximal volcaniclastic rocks making up the Auber Villiers Formation (3256 Ma), and possibly also some distal dacitic tuffaceous sequences within the Mapepe Formation (3258–3227 Ma), both of which form part of the southern Fig Tree facies.

An Arch˦an arc-arc collisional event: a short-lived (ca 3 Myr) episode, Weltevreden area, Barberton greenstone belt, South Africa

Detailed mapping, structural analysis and high precision UPb zircon ages from hypabyssal porphyries and tonalites has enabled the timing of the major collisional episode of the Barberton greenstone belt to be determined. This episode, known as the D2 deformation, can be divided into D2-early and D2-late.D2-early deformation, bracketed as having occurred between 3229 and 3227 Ma, was typified by isoclinal folding (F2 of shales and associated cherts (Fig Tree Group), but not of the overlying quartzose clastic rocks (Moodies Group).D2-late deformation was at 3226 Ma, or later, with large F3 open-tight folds dominating the map pattern in the Weltevreden study area. Late D2 faults, such as the Moodies Fault, that define the boundaries of the major structural domains converge near the Saddleback Fault, a major boundary within the Barberton greenstone belt. Strain calculations suggest that plane strain was dominant, with at least 60% shortening occurring during the D2 deformational events.Although an early and a late stage of D2 deformation can be distinguished in the field, precise UPb zircon ages indicate that these events represent a continuum, ranging from the onset of an arc-arc assembly event (D2-early), through to major collisional and post-amalgamation events (D2-late), all of which spanned ∼3 million years.

Sulfur isotope evidence for the origin of 3.4 to 3.1 Ga pyrite at the Princeton gold mine, Barberton Greenstone Belt, South Africa

The Princeton mine is one of several mesothermal gold deposits found in the Barberton Greenstone Belt. The host rocks of the ore bodies are carbonaceous and pyritic shales of the Fig Tree Group (∼3.4 to 3.2 Ga in age). The methothermal gold mineralization has occurred during the latest thrusting and deformation events at ∼3.1 Ga. Micro-analyses of sulfur isotopes of pyrite were performed on four representative samples collected from the Princeton mine. The δ 34S values were determined in situ on individual pyrite crystals or aggregates of fine-grained pyrite by the laser microprobe method (58 analyses), and also on aggregates of separated pyrite crystals by the conventional Cu 2O combustion method (11 analyses). Trace element concentrations of pyrite were also determined using an electron microprobe. Based on their grain sizes and chemical compositions, the pyrite samples examined are divided into two groups: (1) fine-grained pyrite and (2) coarse-grained pyrite. The fine-grained group includes disseminated pyrite and pyrite laminae concordant to sedimentary bedding. Their textures suggest that the fine-grained pyrite was formed during sedimentation of the carbonaceous shale at 3.4 to 3.2 Ga. The δ 34S values of the fine-grained pyrite range from −0.8 to +4.4‰ within a micro-scale area (1 mm×2 mm) (29 laser microprobe analyses), suggesting that the microbial reduction of seawater sulfate was responsible for the formation of fine-grained pyrite. This further suggests that the 3.4 to 3.2 Ga ocean already contained an appreciable amount of sulfate, which is in contrast to the previously popular theory that the Archean ocean was H 2S-rich and pyrite in Archean sedimentary rocks was formed by an inorganic process. Coarse-grained pyrite is associated with auriferous quartz veins and often contains gold grains. Trace element concentrations of coarse-grained pyrite, such as As concentrations, are different from these of fine-grained pyrite. The petrographic features suggest that the most coarse-grained pyrite was directly precipitated from high temperature hydrothermal fluids. Coarse-grained pyrite has homogeneous δ 34S values, ranging from +1.1 to +3.6‰ (29 laser microprobe analyses), suggesting that source of sulfur for the coarse-grained pyrite was reduced sulfur species in hydrothermal fluids.

Characterization, provenance, and tectonic setting of Fig Tree greywackes from the Archaean Barberton Greenstone Belt, South Africa

In this study, mineralogical and geochemical data, as well as Sr and Nd isotopic compositions of early Archaean greywackes from the Fig Tree Group are presented to provide further information about the evolution of the Barberton Greenstone Belt (BGB) of the Kaapvaal Craton, South Africa. The chemical data on the collected greywackes from the BGB suggest limited chemical modification of the source rocks. The Eu/Eu * anomaly is negative with an average value of 0.76 which is slightly higher than that of Eu-depleted crustal rocks (0.65). High Cr and Ni concentrations and a Cr/Ni ratio of about 1.6 indicate derivation from ultramafic sources without fractionation. Although rounded zircons occur, the combination of CIW/CIA (Chemical Index of Weathering/Chemical Index of Alteration) values and Eu/Eu * ratios suggests little transportation until final deposition, whereas the juvenile crustal material comes from local sources. The siliciclastic material seems to be of mixed felsic and mafic-to-ultramafic origin. The tectonic setting of the source areas appears to reflect a continental island arc of Andean type, supporting the existence of an active plate-tectonic geodynamic system in early Archaean time. Sr isotopic data indicate an alteration during a post-depositional event, most probably at 2.6–2.7 Ga, which disturbed the Sr isotopic system. The Sm–Nd isotopic system must also have been altered since deposition of the rocks, as suggested by ε Nd values both lower and higher than expected. The calculated depleted-mantle model ages of 3.0 to 3.8 Ga are in the range of BGB sedimentary rocks. The higher model ages appear to reflect the underlying belt, which is represented by the Onverwacht Group and the surrounding intrusives, as being the major source of the greywackes.

Oceanic mega-impacts and crustal evolution

Lunar mare crater counts, the terrestrial impact flux, and astronomical observations of asteroids and comets define a consistent impact rate of 4–6 ṁ 10 −15 km −2 ṁ yr −1 within the inner solar system since the end of the late heavy bombardment ∼3.8 Ga. Coupled with the observed crater size vs. cumulative crater size frequency relationship of N ∝ D c −1.8 ( N = cumulative number of craters of diameter > D c ), these rates imply formation on Earth of more than 450 D c ≥ 100-km-diameter craters, more than 50 D c ≥ 300-km-diameter craters, and more than 20 D c ≥ 500-km-diameter craters. Geochemical and isotopic constraints require that more than 80% of the projectiles impacted on time-integrated oceanic crust since the late heavy bombardment. The injection of shock energies calculated at >10 8 Mt TNT equivalent by a D p >10-km-diameter projectile may result in propagating fractures and rift networks, thermal perturbations, and ensuing magmatic activity. Examinations of the geologic record for correlated impact and magmatic fingerprints of such events remain inconclusive in view of isotopic age uncertainties. Potential but unproven connections may be represented by the (1) Cretaceous-Tertiary boundary (ca. 65 Ma) impact(s), onset of the Carlsberg Ridge spreading, Deccan volcanism, and onset of the mantle plume of the Emperor-Hawaii chain; (2) Jurassic-Cretaceous boundary (ca. 145 Ma) impacts, onset of Gondwana breakup, including precursors of the East African rift structures; (3) Permian-Triassic boundary (ca. 251 Ma) impact(s), Siberian Norilsk traps, and Early Triassic rifting; and (4) the 3.26 Ga basal Fig Tree Group (east Transvaal) Ir-rich and Ni-rich quench spinel-bearing impact spherules and contemporaneous igneous-tectonic activity. Tests of the theory require further identification and isotopic dating of distal ejecta, impact spherule condensates, and meteoritic geochemical anomalies.

Sm–Nd, Rb–Sr and Pb–Pb dating of silicic carbonates from the early Archaean Barberton Greenstone Belt, South Africa: Evidence for post-depositional isotopic resetting at low temperature

Two silicic carbonate sequences occurring in the Onverwacht Group (OG) and the overlying Fig Tree Group (FTG) of the early Archaean Barberton Greenstone Belt, South Africa, contain silicified dolomites with primary structures and textures. They occur with volcanic rocks dated by single zircons. The associated Onverwacht felsic volcanics have an age of ∼3440–3416 Ma, whereas the Fig Tree volcanics are ∼3225 Ma old. Therefore, the depositional age of both sequences is between 3440 and 3225 Ma. Pb-isotopic data for different parts of a single OG whole rock sample form a linear array corresponding to a Pb–Pb age of 2632±99 Ma. Sm–Nd and Rb–Sr isotopes were analysed in four whole rock samples of the FTG silicified carbonates, and form an Sm–Nd array corresponding to an age of 2612±84 Ma with initial εNd=−8.1±0.5, and an Rb–Sr array of 2743±36 Ma with initial 87Sr/ 86Sr=0.7013±0.0003 . These whole rock samples have Rb/Sr ratios of ∼1 and unusually high Rb concentrations for carbonates (∼100 ppm) which derive from processes accompanying silicification. All three isotopic systems of whole rocks indicate ages of ∼2.7 Ga, much younger than the depositional age of the successions. The agreement provides strong evidence that an alteration event reset all of the isotopic systems (Sm–Nd as well as Rb–Sr and U–Pb). We suggest that the ages represent the timing of silicification and sericitization of the carbonates by large-scale fluid movement at an apparent temperature of ∼200°C.

Chemical variations in clay minerals of the Archaean Barberton Greenstone Belt (South Africa)

Four rock profiles of shales, carbonates and greywacke-type sediments were sampled from the mafic compositional influenced sedimentary Fig Tree Group and from overlying, more felsic related, sedimentary Moodies Group of the Archaean Barberton Greenstone Belt in South Africa. Clay separates of forty sedimentary rocks were examined by XRD and SEM to study the mineralogical, chemical and morphological variations of the constitutive illite and chlorite. The clay minerals showed a significant K-enrichment in the illites and Mg- and Fe-depletions in the chlorites upwards the stratigraphic sequence. Three independent parameters influenced these variations: (1) a change in the origin of the source rocks; (2) the host-rock lithologies; and (3) a tectono-thermal recrystallization process.

Prolonged magmatism and time constraints for sediment deposition in the early Archean Barberton greenstone belt: evidence from the Upper Onverwacht and Fig Tree groups

The single zircon evaporation, SHRIMP ion-microprobe and conventional dissolution techniques were used to determine 207Pb/ 206Pb and UPb ages on samples from the Upper Onverwacht and Fig Tree groups of the early Archean Barberton greenstone belt, South Africa. Zircons from dacitic rocks of the upper Hooggenoeg Formation yield ages of ∼ 3445–3452 Ma. A tuff in the basal Kromberg Formation has a mean age of 3416 ± 5 Ma. A tuffaceous band, 5 cm thick, in the uppermost Kromberg Formation contains igneous zircons with a mean age of 3334 ± 3 Ma. The 1700 m section of Kromberg Formation between these two samples is composed of basaltic lavas, minor komatiites and cherty metasediments. The overlying Mendon Formation is composed of interbedded komatiitic lavas and metasediments with a minimum thickness of 600 m. A cherty, stromatolitic metasediment 300 m above the base contains several thin ash layers with a mean zircon age of 3298 ± 3 Ma. The basal Fig Tree Group has units as old as 3259 ± 3 Ma, and upper units in the Fig Tree are as young as 3225 ± 3 Ma. Xenocrystic zircons in the Upper Onverwacht and overlying Fig Tree Group samples suggest that successive igneous units inherited zircons from underlying units and that, over several hundred million years, episodes of intermediate to felsic igneous activity took place at 20–40 Ma intervals. Structural repetition by isoclinal folding and thrust faulting are important components of late greenstone belt evolution, but should not obscure the importance of the prolonged interval of magmatic evolution represented by the thick pile of volcanic rocks observed in the Barberton greenstone belt.

The oldest part of the Barberton granitoid-greenstone terrain, South Africa: evidence for crust formation between 3.5 and 3.7 Ga

Many stratigraphic and age relationships in the southern part of the Barberton greenstone belt (BGB) remain unresolved due to strong deformation including thrusting, nappe stacking and strike-slip faulting. A relatively undisturbed sequence from the lower Onverwacht Group (∼ 3.48-3.45 Ga), followed by the upper Onverwacht (∼ 3.42-3.3 Ga), the Fig Tree Group (∼ 3.26-3.23 Ga) and the Moodies Group (> 3.22 Ga) was established by single zircon dating using various techniques, but the position of the Theespruit Formation is still uncertain. Using the single zircon evaporation and the vapour digestion techniques we obtained remarkably uniform 207Pb/ 206Pb and UPb ages of 3544 ± 3 to 3547 ± 3 Ma for felsic rocks mapped as Theespruit in the Steynsdorp Anticline of the southeastern BGB, some 100 Ma older than all other dated greenstone units. These rocks were intruded by the 3502–3511 Ma old Steynsdorp TTG pluton containing zircon xenocrysts as old as 3553 ± 4 Ma. A 3.5 Ga granodiorite plug intrusive into metavolcanics of the Komati Formation (lower Onverwacht Group) contained two 3702 ± 2 Ma zircon xenocrysts, the oldest so far measured in the Barberton-Swaziland area and testifying to the presence of very ancient crust in the region. The Theespruit felsic metavolcanics have ϵ Nd(t) values between + 1.1 and − 1.1 and Nd T DM model ages between 3.5 and 3.7 Ga, suggesting variable contamination of their protoliths with older continental crust. The above zircon ages extend the history of the BGB back to ∼ 3.55 Ga. We suggest that the area of the Steynsdorp Anticline constitutes the oldest nucleus of the BGB onto which successively younger units were tectonically and magmatically accreted. We also speculate that the mafic-felsic volcanic units of the southern BGB may perhaps represent distinct oceanic plateaux, rather than ocean floor material, which amalgamated between 3.55 and 3.42 Ga ago. Our data support the concept that the BGB consists of a number of discrete, fault-bounded terranes, and that large-scale lithological correlations are therefore not justified.

Geology and metallogeny of the Barberton greenstone belt: a survey

The principal episodes of the geotectonic history of the Barberton greenstone belt span some 400 Ma from the initial submarine eruption of the Onverwacht laves (at about 3500 Ma) to the compression and inversion of the depository as a fold-and-thrust belt between 3230 and 3215 Ma, followed by late-tectonic hydrothermal Au mineralization at about 3100 Ma. The crust beneath the greenstone belt was fortified by sialic underplating due to episodic felsic plutonism at about 3445 Ma, 3227 Ma and 3105 Ma. The main stratigraphic units of the Barberton Supergroup comprise the Onverwacht Group, a thick assortment of ultramafic and mafic volcaulics including a number of sill-like layered ultramafic complexes, which is overlain by the Fig Tree Group of turbiditic greywacke sandstones and associated mudstones and banded ferruginous shales. These strata are, in turn, paraconformably overlain by continentally-derived shallow-water arenites of the Moodies Group. The broader aspects of the deformation of the Barberton depository can be considered in terms of one of two models: either the structures represent a single compressional episode of upright folding accompanied by high-angle thrusting between 3230 and 3215 Ma, or they resulted from an early episode of tectonic stacking, followed by a later event involving the rotation of early thrust slices into upright folds and accompanied by high-angle thrusting. Substantial deposits of chrysotile asbestos, magnesite and talc were developed in the ultramafic host rocks of the Onverwacht Group and important mesothermal deposits of Au were formed in the reactive host rocks belonging to all three Groups in splays adjacent to regionally-developed shear zones. There is still uncertainty as to the primary metallogenic setting of the belt during Onverwacht times and what part accretionary or collage tectonics may or may not have played in the evolution of the Barberton greenstone belt.

Depositional and tectonic setting of the Archean Moodies Group, Barberton Greenstone Belt, South Africa.

The 3.22-3.10 Ga old Moodies Group, uppermost unit of the Swaziland Supergroup in the Barberton Greenstone Belt (BGB), is the oldest exposed, well-preserved quartz-rich sedimentary sequence on earth. It is preserved in structurally separate blocks in a heavily deformed fold-and-thrust belt. North of the Inyoka Fault, Moodies strata reach up to 3700 m in thickness. Detailed mapping, correlation of measured sections, and systematic analysis of paleocurrents show that the lower Moodies Group north of the Inyoka Fault forms a deepening- and fining-upward sequence from a basal alluvial conglomerate through braided fluvial, tidal, and deltaic sandstones to offshore sandy shelf deposits. The basal conglomerate and overlying fluvial facies were derived from the north and include abundant detritus eroded from underlying Fig Tree Group dacitic volcanic rocks. Shoreline-parallel transport and extensive reworking dominate overlying deltaic, tidal, and marine facies. The lithologies and arrangement of Moodies Group facies, sandstone petrology, the unconformable relationship between Moodies strata and older deformed rocks, presence of at least one syndepositional normal fault, and presence of basaltic flow rocks and airfall fall tuffs interbedded with the terrestrial strata collectively suggest that the lower Moodies Group was deposited in one or more intramontane basins in an extensional setting. Thinner Moodies sections south of the Inyoka Fault, generally less than 1000 m thick, may be correlative with the basal Moodies Group north of the Inyoka Fault and were probably deposited in separate basins. A northerly derived, southward-thinning fan-delta conglomerate in the upper part of the Moodies Group in the central BGB overlies lower strata with an angular unconformity. This and associated upper Moodies conglomerates mark the beginning of basin shortening by south- to southeast-directed thrust faulting along the northern margin of the BGB and suggest that the upper Moodies Group was deposited in a foreland basin. Timing, orientation, and style of shortening suggest that this deformation eventually incorporated most of the BGB into a major fold-and-thrust belt.

Contrasting low- and high-Ca granites in the Archean Barberton Mountain Land, Southern Africa

Two distinct groups of granites can be recognized among the late plutons of the Barberton Mountain Land. Discrimination between the two groups is provided by geochemical and mineralogical parameters which are considered to reflect the source material from which the magmas were derived. A high-Ca suite has geochemical parameters consistent with derivation from an igneous source (I-type), and an accessory mineral assemblage comprising zircon, apatite, allanite and sphene. A low-Ca suite contains distinctly different accessory minerals, comprising zircon, apatite, zirconosilicates, CaTh-phosphates, very rare xenotime and early monazite, and has a chemical signature consistent with derivation from a metasedimentary precursor (S-type). Major and trace element trends on Harker diagrams support the existence of two distinct granite suites. The I-type suite exhibits inverse relationships between SiO 2 and CaO, TiO 2, Fe 2O 3, MgO and P 2O 5, while Al 2O 3, K 2O and Na 2O remain relatively constant. These characteristics are consistent with magma formation from an intermediate parent composition, and in equilibrium with a hornblendite restite. The S-type granites formed by vapour-absent melting of siliceous metapelite and exhibit inverse correlations between SiO 2 and Al 2O 3, Na 2O and K 2O, with the other major elements remaining relatively unchanged. This interpretation of restite unmixing is not unambiguous since RbSr trends could be viewed as reflecting crystal fractionation and partial melting processes. High field strength elements tend to reflect the character of accessory mineral phases and are not amenable to modelling in terms of crystal-melt equilibria. The recognition of possible S- and I-type suites among the late granite plutons of the Barberton Mountain Land supports recent models which propose plate tectonic analogues in the region. The two granite suites are distributed along two subparallel linear arrays which suggests scenarios invoking subduction or hot-spot related magmatism, with the I-types forming in the time span 2740-2690 Ma along a western array. The putative S-types occurring entirely along an easterly belt were emplaced at 3074 and 2820 Ma. The presence of S-type plutons as old as 3074 Ma implies that sediments (such as the Moodies and Fig Tree Groups, or their equivalents) had been buried to depths of at least 15 km, and that by this time, at least in the Barberton area, granites had evolved beyond the TTG mode that typifies many early Archaean shield areas.

Reassessment of Archean crustal development in the Barberton Mountain Land, South Africa, based on U‐Pb dating

This paper presents U‐Pb ages for 23 samples from the Barberton Mountain Land, an Archean granite‐greenstone terrain in the Kaapvaal craton of South Africa. A reexamination of the geology of this area in the light of previous U‐Pb dating and the new ages indicates that much of its deformation can probably be accounted for by a single short‐lived compressional event within the time span 3230–3215 Ma which was synchronous with deposition of the Moodies Group and the upper part of the Fig Tree Group sediments. This event may have been responsible for thrusting of much older sequences in the Onverwacht Group, including slivers of 3.54 Ga old gneiss, circa 3.47 Ga old komatiitic and tholeiitic sequences, and circa 3.45 Ga old felsic sequences. Key constraints on the timing of deformation are previously determined 3227–3225 Ma ages for detrital zircons in thrusted units of the Fig Tree Group; a 3229 +4/−3 Ma age for a deformed porphyry which is pre‐ or synthrusting; a previously determined 3227 ± 3 Ma age for an undeformed porphyry in an inferred thrust fault; 3227 ± 1 Ma ages for both the Kaap Valley pluton and an ignimbrite in the Stolzburg syncline, both of which predate regional upright folding; and a 3216 ± 2 Ma age for the Dalmein pluton which appears to cut regional, upright NE‐SW trending folds in the Kromberg syncline, part of the final phase of deformation associated with regional compression. Areally extensive granitoid sheets and syenogranitic plutons were subsequently emplaced into the Barberton region at about 3105 Ma. The linear configuration of the syenogranitic plutons, which are also aligned with the gabbroic Usushwana complex as well as the geometry of the granitoid sheets, suggests that the circa 3105 Ma magmatism was caused by incipient rifting around a NW‐SE trending crustal fracture. This episode was probably coeval with a late regional transtensional phase of deformation recorded in the greenstone belt. Some of the above interpretations are in conflict with previous models for early development of the Barberton greenstone belt. A 3352 ± 6 Ma age from a metagabbro which is comagmatic with a proposed ultramafic sheeted dike complex is over 100 m.y. younger than the probable age of volcanic rocks which host the dikes. This conflicts with the suggestion that much of the Onverwacht Group comprises an ophiolitic section, obducted in an intraoceanic setting at circa 3.45 Ga, that partially melted to form trondhjemitic magmas which intruded along active thrust faults. Present data suggest that, despite its antiquity and the diversity of ages and lithologies found within it, the Barberton greenstone belt can generally be understood in terms of actualistic tectonic processes at convergent margins.