Fig Tree Group Research Articles

Sm-Nd dating of Fig Tree clay minerals of the Barberton greenstone belt, South Africa.

Sm-Nd isotopic data from carbonate-derived clay minerals of the 3.22-3.25 Ga Fig Tree Group, Barberton greenstone belt, South Africa, form a linear array corresponding to an age of 3102 +/- 64 Ma, making these minerals the oldest dated clays on Earth. The obtained age is 120-160 m.y. younger than the depositional age determined by zircon geochronology. Nd model ages for the clays range from approximately 3.39 to 3.44 Ga and almost cover the age variation of the Barberton greenstone belt rocks, consistent with independent evidence that the clay minerals are derived from material of the belt. The combined isotopic and mineralogical data provide evidence for a cryptic thermal overprint in the sediments of the belt. However, the highest temperature reached by the samples since the time of clay-mineral formation was <300 degrees C, lower than virtually any known early Archean supracrustal sequence.

Archaean lode gold deposits in Africa: Crustal setting, metallogenesis and cratonization

Archaean rocks in Africa are preserved in seven important tectonic and physiographic terranes: the Zimbabwe craton; the Kaapvaal craton of South Africa; the Tanzanian Shield; the North Zaire Massif which extends westwards into the Central African Republic; the Kasai-Angola Massif of southern Zaire; the Atlantic Rise of Cameroun, Gabon, and Congo extending southwards to the Malanje “Strip” of northern Angola; the Guinea Rise of Guinea, Sierra Leone, and Liberia; and the Reguibat Rise of Mauritania, Morocco, and northwest Algeria. Dilational vein and brittle-to-ductile shear zone mineralization is common in the Zimbabwe craton, distribution of the gold being controlled by the permeability characteristics of the hydrothermal conduits and by the competence and chemistry of the wallrocks. Within the Kaapvaal craton, the Barberton greenstone belt has the highest productivity with a marked concentration of lode-type mineralization at or near the tectonized contact between the basalt-dominated Onverwacht Group and the metasediments of the Fig Tree Group. Smaller lode gold deposits are also present in the greenstone belts of the central and northern Kaapvaal craton, the more important being the stibnite-gold mineralization that accompanied late brittle deformation of carbonatized high-Mg basalts in the Murchison greenstone belt. The Archaean basement and supracrustal sequences of the Guinea Rise, the Reguibat Rise, the Atlantic Rise, and the Kasai-Angola Massif are dominated by medium- and high-grade metamorphic rocks, and, with the exception of small greenschist-facies sequences in central Sierra Leone (the Guinea Rise), are mostly devoid of gold mineralization. However, vein-type gold lodes are more numerous in the low-metamorphic grade greenstone belts of the North Zaire Massif and the Tanzanian Shield. The important gold-producing terranes are characterized by a complex tectonic history of strike-slip and reverse dip-slip shearing, the productive lodes commonly being hosted by second- or higher-order shear zones. These shear zones demonstrate an important component of late reverse, dip-slip activation or reactivation which provided optimum conditions for crustal de-watering during prograde metamorphism and emplacement of tonalitic magmas. The stratigraphic sequences and tectonic evolution are generally compatible with, but do not unequivocally demonstrate, Cordilleran-type accretionary regimes. A link between gold metallogeny and the tectonothermal events that preceded and ultimately culminated in cratonization is commonly evident. However, cratonization in Africa, in a broad sense, was diachronous, ranging from ca. 3.0 Ga in the Barberton greenstone belt, through ca. 2.6-2.5 Ga in the Zimbabwe and Tanzanian cratons, to the ca. 2.0 Ga Eburnean event of West Africa, the latter contributing to the important gold mineralization of southwest Ghana. This multiplicity of tectonothermal events and the evidence for higher and more persistent crustal heat fluxes in the early Precambrian terranes of West and Equatorial Africa have important ramifications for crustal evolution, metallogeny, and gold exploration.

Noble metal abundances in an Early Archean impact deposit

We report detailed analyses on the concentrations of the noble metals Pd, Os, Ir, Pt, and Au in an early Archean spherule bed (S4) of probable impact origin from the lower Fig Tree Group, Barberton Greenstone Belt, South Africa. Compared to other sedimentary deposits of known or suspected impact origin, some noble metals are present in exceptionally high concentrations. Noble metal abundances are fractionated relative to abundances in chondrites with ratios of Os Ir , Pt Ir , Pd Ir , and Au Ir at only 80, 80, 41, and 2% of these values in CI chondrites. Although an extraterrestrial source is favored for the noble metal enrichment, the most plausible cause of the fractionation is by regional hydrothermal/ metasomatic alteration.

LASER step-heating [formula omitted] age spectra from early Archean (~3.5 Ga) Barberton greenstone belt sediments: A technique for detecting cryptic tectono-thermal events

Samples from sediments of the Fig Tree Group located in the central part of the circa 3.2 to 3.5 Ga Barberton greenstone belt (BGB) have been analyzed by the 40Ar 39Ar laser step-heating technique. This technique has enabled previously cryptic thermal overprints to be detected in various sedimentary units which include: reworked chemical sediments (barite), clastic sediments (sandstone/shale), and one stromatolite. In most cases the apparent age plateaux can be identified by corresponding Ca K and Cl K ratio plots as belonging to separate mineral phases. Most of the samples exhibit simple Ar diffusion loss during the low temperature part of the experiments while occasionally showing excess Ar, or possible 39Ar recoil, effects. The various sediments analyzed show evidence for distinct overprinting between 2025 and 2090 Ma. One barite sample gave T 0 (original blocking age) = 2673 ± 3 Ma (1σ) which is close to a calculated model Ar diffusion-loss age for the same sample of 2688 ± 3 Ma (1σ). This age is interpreted as representing final granitoid activity adjacent to the BGB, and/or craton-scale tectonism associated with the Limpopo Orogeny. Two samples gave T 0 ages of ∼2350–2400 Ma which may reflect increased thermal gradients associated with the formation of the thick Transvaal sedimentary basin that may once have covered the BGB. The dominant apparent age plateaux together with modelled diffusive Ar loss ages of 2025–2090 Ma, are thought to represent regional thermal anomalies related to large-scale tectono-thermal activity in the Kaapvaal craton, of which the Bushveld Complex (which covers a surface area of ~ 60,000 km 2) and formation of the Vredefort Structure are obvious manifestations. The strong thermal overprinting recorded by the sediments has effectively removed any ancient atmosphere (i.e., 40Ar 36Ar ratios < 295.5) signatures.

Chronology of early Archaean granite-greenstone evolution in the Barberton Mountain Land, South Africa, based on precise dating by single zircon evaporation

We report precise 207Pb 206Pb single zircon evaporation ages for low-grade felsic metavolcanic rocks within the Onverwacht and Fig Tree Groups of the Barberton Greenstone Belt (BGB), South Africa, and from granitoid plutons bordering the belt. Dacitic tuffs of the Hooggenoeg Formation in the upper part of the Onverwacht Group yield ages between 3445 ± 3 and 3416 ± 5 Ma and contain older crustal components represented by a 3504 ± 4 Ma old zircon xenocryst. Fig Tree dacitic tuffs and agglomerates have euhedral zircons between 3259 ± 5 and 3225 ± 3 Ma in age which we interpret to reflect the time of crystallization. A surprisingly complex xenocryst population in one sample documents ages from 3323 ± 4 to 3522 ± 4 Ma. We suspect that these xenocrysts were inherited, during the passage of the felsic melts to the surface, from various sources such as greenstones and granitoid rocks now exposed in the form of tonalite-trondhjemite plutons along the southern and western margins of the BGB, and units predating any of the exposed greenstone or intrusive rocks. Several of the granitoids along the southern margin of the belt have zircon populations with ages between 3490 and 3440 Ma, coeval with or slightly older than Onverwacht felsic volcanism, while the Kaap Valley pluton along the northwestern margin of the belt is coeval with Fig Tree dacitic volcanism. These results emphasize the comagmatic relationships between greenstone felsic volcanic units and the surrounding plutonic suites. Some of the volcanic and plutonic units contain zircon xenocrysts older than any exposed rocks. These indicate the existence of still older units, possibly stratigraphically lower and older portions of the greenstone sequence itself, older granitoid intrusive rocks, or bodies of older, unrelated crustal material. Our data show that the Onverwacht and Fig Tree felsic units have distinctly different ages and therefore do not represent a single, tectonically repeated unit as proposed by others. Unlike the late Archaean Abitibi greenstone belt in Canada, which formed over about 30 Ma, exposed rocks in the BGB formed over a period of at least 220 Ma. The complex zircon populations encountered in this study imply that conventional multigrain zircon dating may not accurately identify the time of felsic volcanic activity in ancient greenstones. A surprising similarity in rock types, tectonic evolution, and ages of the BGB in the Kaapvaal craton of southern Africa and greenstones in the Pilbara Block of Western Australia suggests that these two terrains may have been part of a larger crustal unit in early Archaean times.

Fan-delta sequence in the Archean Fig Tree Group, Barberton Greenstone Belt, South Africa

The Fig Tree Group of the Barberton Greenstone Belt in South Africa records the onset of tectonism and significant terrigenous and volcaniclastic sedimentation following a period of predominantly effusive volcanism. In the southern part of the greenstone belt the lower Fig Tree conformably overlies the Onverwacht Group and can be grouped into either: (1) volcaniclastic deposits consisting of pyroclastic and reworked volcanic detritus, or; (2) non-volcanogenic, terrigenous deposits of sandstone and conglomerate. These petrologic groups are further divided into six lithofacies. The lithofacies within the lower Fig Tree Formation can be grouped into two facies associations: (1) a subaqueous, delta-front association made up of fine-grained airfall pyroclastic and slightly reworked pyroclastic debris along with resedimented, land-derived sandstone and conglomerate, and (2) a subaerial association that is interpreted as forming from deposition of sandstone and conglomerate by flashy, ephemeral-stream processes on an alluvial-fan surface. The stratigraphic sequence coarsens upward from predominantly siltstone and fine-grained sandstone at the base to pebble and cobble conglomerate at the top. This sequence is interpreted as forming from the progradation of a fan delta into a body of relatively shallow, quiet water. Composition of clasts in the conglomerates indicates that immediately underlying greenstone-belt rocks or coeval volcanic rocks were the source for these coarse sedimentary accumulations. Erosion of uplifted greenstone-belt terrain was apparently only to shallow levels. Both tectonic and volcanic activity played a role in producing several cycles of fan-delta progradation.

The stratigraphy of the 3.5-3.2 Ga Barberton Greenstone Belt revisited: A single zircon ion microprobe study

Recent field and geochemical studies indicate a need to test the stratigraphy of the ca. 3.5 Ga Barberton Greenstone Belt as it is presently adopted [1,2]. This work uses the ion microprobe SHRIMP, to attempt such a test. Results show that: (1) Volcaniclastic sediments of the Theespruit Formation (< 3453 ± 6Ma) could be younger than the (structurally) overlying mafic and ultramafic volcanics of the Komati Formation (3482 ± 5Ma). A major structural discontinuity may therefore exist between the two formations. (2) An age of 3538 ± 6Ma established for a tectonic wedge of tonalitic gneiss within the Theespruit Formation confirms the presence of a sialic basement and deformed unconformity below that unit. The tonalitic gneiss is the oldest unit yet recorded within the greenstone belt, equal in age to the older components of the adjacent Ancient Gneiss Complex. (3) The interpreted ages of felsic volcanic rocks from both the Hooggenoeg (3445 ± 8Ma) and Theespruit Formations and the nearby Theespruit Pluton (3437 ± 6Ma) are essentially the same, and corroborate field and geochemical evidence that the felsic units were probably cogenetic and emplaced simultaneously as high-level equivalents of trondjhemite-tonalite plutons that intrude the greenstone belt at its southwestern extremity. (4) Felsic-intermediate volcanic-volcaniclastic rocks locally separating the two major groups (the Fig Tree and Moodies Groups) which overlie the Onverwacht Group record a second major peak of tonalitic magmatism in the Barberton terrain at about 3250 Ma. This is close to the age of the Kaap Valley tonalite pluton which intrudes the Barberton Greenstone Belt at ca. 3226 Ma along its northwestern margin. The present results indicate the Barberton Greenstone Belt is part of an allochthonous sequence containing major tectonic and stratigraphic breaks, with a protracted history; of which the last 200 million years, at least, evolved within a tectonically active convergent environment.

Geological and Geochemical Record of 3400-Million-Year-Old Terrestrial Meteorite Impacts

Beds of sand-sized spherules in the 3400-million-year-old Fig Tree Group, Barberton Greenstone belt, South Africa, formed by the fall of quenched liquid silicate droplets into a range of shallow-to deep-water depositional environments. The regional extent of the layers, their compositional complexity, and lack of included volcanic debris suggest that they are not products of volcanic activity. The layers are greatly enriched in iridium and other platinum group elements in roughly chondritic proportions. Geochemical modeling based on immobile element abundances suggests that the original average spherule composition can be approximated by a mixture of fractionated tholeiitic basalt, komatiite, and CI carbonaceous chondrite. The spherules are thought to be the products of large meteorite impacts on the Archean earth.

Sandstone petrology of the Archean Fig Tree Group, Barberton greenstone belt, South Africa: Tectonic implications

It has become clear that simple vertical tectonics do not adequately describe the tectonic evolution of many greenstone belts and that horizontal shortening played an important role in the development of these unique terranes from Earth9s early history. The Fig Tree Group of the Barberton greenstone belt in South Africa has recently been interpreted, mainly by structural and stratigraphic analyses, as having been deposited in a foredeep associated with thrusting. Sandstone modal mineralogy has been shown to primarily reflect tectonic setting and prove-nance terrane. Fig Tree sandstones have framework mineralogies that fall mainly in the recycled orogen field and show an evolution consistent with interpretations of the greenstone belt that utilize petrographic and other evidence.

Discrimination of acidic magmatic rocks using a Rb-Tb-Ta diagram. Petrogenetic implications

The Ulundi Formation of the lowermost Fig Tree Group in the type area in the Barberton Mountain Land consists of a sequence of quartz—sericite schists, banded cherts and tuffaceous rocks up to 40 m thick. It overlies talc—carbonate schists assigned to the upper part of the Onverwacht Group. In the cherts, which in parts represent the sulfide facies of a banded-iron formation, silicified spheroids have been observed which appear to be diagenetically altered, locally zeolitized accretionary lapilli. Portions of the cherts formed through silicification of fine-grained felsic tuffs and contain abundant stylolite seams along which ∼ 4% of the rock thickness has been removed by intrastratal solution during deformation.A supposedly organogenic spheroidal structure, Ramsaysphera, has been previously described from the upper part of the Ulundi Formation. The spheroids, with diameters of up to 8 mm, occur in a distinct bed of up to at least 1.6 m thickness and can be traced in outcrop over an area which originally covered ∼ 17 × 30 km. The bed was deposited in fairly still water of several 100 m depth. The spheroids accumulated with only little finer-grained detrital material, the interstices between them being filled by cherty silica and minor amounts of dolomite. Up to ∼ 65% of the spheroids consist of tectonically elongated, originally round fragments of a micro-felsitic groundmass with remnants of phenocrysts. The majority of the remaining spheroids consists of concentrically banded bodies of siliceous material with minor amounts of chlorite and sericite, as well as small dolomite crystals. These spheroids represent highly altered granules of volcanic glass deposited from volcanic ash clouds through a body of water. The prototype of Ramsay-sphera also belongs to this group of spheroids, and there is little microscopic or macroscopic evidence for its explanation as an organic structure.

Origin and timing of the metasomatic silicification of an early archean komatiite sequence, barberton mountain land, South Africa

Stratiform units of pervasively silicified early Archean ultramafic rocks occur near the boundary between the Onverwacht and Fig Tree groups in the Barberton Mountain Land, Republic of South Africa. The origin of these units has been variously ascribed to early Archean subaerial weathering or submarine alteration, to cataclastic metamorphism, and to the alteration of silicic tuffs. An exceptionally well-exposed example of such an alteration sequence along the Skokohla River valley in the central Barberton Mountain Land was studied to determine the nature of the precursor rocks and the timing and nature of the alteration processes.

Early Archean foredeep sedimentation related to crustal shortening: a reinterpretation of the Barberton Sequence, Southern Africa

The upper, dominantly terrigenous sedimentary successions of the Early Archean Barberton Sequence, Kaapvaal Province, record an evolution similar to that of Phanerozoic foredeeps and may represent the oldest known foredeep sequence, rather than a passive continental-margin sequence as previously proposed. Pre-foredeep ocean crust, the 3.5 Ga-old Onverwacht Group, consists of bimodal volcanics with intercalated shallow-water sediments. Gravity modeling and boundary shear zones indicate that deformed Onverwacht crust was obducted northward onto sialic continental crust immediately before, during or after burial by the synorogenic terrigenous clastic fill. Foredeep basin deepening due to crustal loading from the south is recorded by basinal iron-formation, chert, and mudstone of the basal Fig Tree Group which accumulated paraconformably on the Onverwacht. Overlying sediments of the Fig Tree Group consist of ~ 2 km of submarine-fan greywackes and mudstones. Shoaling of the basin is indicated by the transition to the overlying braided-alluvial and shallow-marine sediments of the ca. 3.3 Ga-old Moodies Group (~ 3 km thick). Changing REE patterns and relative abundances of Ni, Cr, Sr, Th and Ur in the sediments of the Fig Tree and Moodies Groups indicate a significant ultramafic-mafic volcanic component diluted upward in the stratigraphic sequence by an increasing granitic contribution. A similar trend is reflected in sandstone petrography, in which the ratio of extrusive to intrusive rock fragments decreases upwards in the Fig Tree Group whereas the Moodies Group sediments are enriched in quartz and potassium feldspar. A persistent chert component in both units suggests that the tectonic setting of the basin is analogous to that of Phanerozoic recycled orogens. The distribution of facies in the Fig Tree and Moodies Groups, and paleocurrent, petrographic and geochemical data, indicate progressive unroofing of a southerly source terrane consisting of the Onverwacht volcanics and intercalated sediments, and the 3.5-3.2 Ga-old Ancient Gneiss Complex (AGC). In addition, older components of the Fig Tree Group were recycled into overlying stratigraphie units. The AGC consists of mantle-derived 3.5 Ga-old bimodal metavolcanics and metasediments, a tonalite-gneiss batholith, metabasite dikes, and meta-anorthosites. Its structures record progressive northwestward uplift, beginning around 3.4 Ga ago. Intense homogeneous ductile strains were overprinted by northwest verging major folds rotated by large-scale northwestward simple shear, followed by imbricate faulting and pseudotachylite formation. By 3.3 Ga ago the Fig Tree and Moodies Groups had also shortened, with similar kinematics to those of the AGC, by (partly syndepositional) folding and volume loss during the formation of transecting cleavage. Further folding and slide formation followed during subsolidus diapiric rise of the initially tabular Kaap Valley tonalite by 3.2 Ga and emplacement of trondhjemite plutons from 3.3 to 3.0 Ga ago.

Upb zircon ages bearing on the nature of early archaean greenstone belt evolution, barberton mountainland, southern africa

Abstract UPb zircon systematics for granitic and gneissic clasts from the Moodies conglomerate of the Barberton Greenstone Belt, South Africa, exhibit discordancy patterns that suggest variable and multi-episodic lead loss and indicate crystallisation ages for the parent granitoids between 3.3 and 3.47 Ga. Metamict zircons in some of these clasts indicate complete resetting of the older material or new growth at about 2.8–2.9 Ga ago that is ascribed to a regional thermal event as also shown by published RbSr data for a nearby granitoid pluton. Since the Moodies clasts consist largely of potassic granites our ages suggest the presence of highly evolved continental crust in the source region of the greenstone belt sediments, a feature that contradicts the commonly accepted model for the primitive nature of the granitoid crust in early Archaean granite-greenstone terrains. The maximum age of 3.3 Ga for the Moodies Group also shows that greenstone belt evolution in the Barberton region took place over a period of at least 150 Ma from about 3450 to 3300 Ma ago. The Kaap Valley and Stentor plutons intrude the lower part of the greenstone succession, and their diapiric emplacement caused large-scale deformation of earlier structures in the entire greenstone belt sequence at about 3.23–3.25 Ga ago. Detrital zircons in the sediments of the Fig Tree Group date the mean age of their source terrain at c. 3520 Ma which is also the maximum age of deposition for these strata. This age is indistinguishable from the oldest age reported from the Barberton greenstones and suggests the possibility that pre-greenstone granitoid crust may exist in the eastern Kaapvaal craton.

Comments on the Eu and Th geochemistry of the archaean continental crust of the Kaapvaal Craton, South Africa

Chemical data from sedimentary and igneous rocks of the Kaapvaal Craton have been used to trace the occurrence of highly fractionated K2O-rich granites with pronounced negative Eu-anomalies back to the early Archaean. From the sediments of the Fig Tree Group (about 3.4 Ga-old) and granite pebbles in the overlying Moodies Group it was estimated that the igneous and metamorphic part of the continental crust at that time already contained up to about 35% of such granites. Their formation requires the presence of a thick continental crust to allow intracrustal partial melting with plagioclase as a residual phase, and the intrusion of the resulting granites at high levels within the crust. Save for some small remnants these Eu-depleted Archaean granites have since been removed by erosion. Shales of the 3.0 Ga-old Pongola Supergroup reflect only small source areas and contain considerable amounts of material derived from residual soils. The Eu-anomalies of the Pongola shales suggest up to 30% Eu-depleted granites in the source area. The shales of the Witwatersrand Supergroup (about 2.6 Ga-old) — with Eu-anomalies as low as 0.45 — reflect a source area already having typical post-Archaean crustal compositions. The Th-concentrations in South African shales support this conclusion and typical post-Archaean concentrations of above 10 ppm are found already in the early Archaean Sheba shales. Uranium and thorium mineralization has also occurred on the Kaapvaal Craton since the early Archaean. The South African late Proterozoic and Phanerozoic sedimentary record of Eu-anomalies and Th-concentrations confirms the trend observed on other cratonic areas and supports the concept of worldwide uniformity in crustal composition since the end of the Archaean. On the Kaapvaal Craton crustal growth and consolidation began 500–600 Ma earlier than in other parts of the world. The Kaapvaal Craton thus appears to represent a ‘cold spot’ on the face of the Earth.

Stromatolites from the 3,300–3,500-Myr Swaziland Supergroup, Barberton Mountain Land, South Africa

A morphologically variable assemblage of stromatolites has been discovered in thin chert layers within the Fig Tree Group of the Swaziland Supergroup, South Africa. They are commonly low-relief, nearly stratiform, laterally linked domes. Rarer forms include pseudocolumns and crinkly stratiform stromatolites. The stromatolites grew on a substrate of altered komatiitic lava and sediments deposited on the lava surface, and in most places are covered by later komatiitic flows. Abundant fine-grained tourmaline included within the stromatolite laminae suggests that stromatolites formed in an environment dominated by boron-rich hot-spring emissions and evaporitic brines.

Rare-earth concentrations in 3.5-billion-year-old Onverwacht cherts: An indicator for early Precambrian crustal environments

3.5-billion-year-old cherts from Onverwacht Group, Swaziland System in the Barberton area, South Africa were analyzed for rare earth elements (REE), Sc, Hf, Fe, Cr, Co and Na by means of instrumental neutron activation analysis (INAA). The Onverwacht cherts showed less fractionated REE abundances compared with those in younger cherts and other sedimentary rocks. The chondrite-normalized La/Yb ratios observed for the samples from the older Theespruit Formation and younger Hooggenoeg Formation of the Onverwacht Group are 1.2∼2.5, and 4.6, respectively. These values are smaller than the La/Yb values observed for the Fig Tree Group (5.5) and Moodies Group (7.8), which overlie the Onverwacht Group in the Barberton area, indicating an increasing trend for La/Yb ratio in the sedimentary rocks with time in the Swaziland System. The results appear to indicate that the relative fractionation between light and heavy REE in the crustal region which had supplied REE to the sedimentary rocks increased with time in the Archaean era from nearly chondritic La/Yb ratio to about the present-day value in a very short time span of a few hundred million years. Absence of Ce anomaly, together with variable Eu anomalies in the REE patterns, possibly shows reducing conditions in the sedimentary environments. It is also suggested that the negative Eu anomalies commonly observed for the post Archaean sedimentary rocks were derived by accumulation of plagioclase in the lower continental crust by sinking of plagioclase in magmas in the upper crust.

The microbial community at Laguna Figueroa, Baja California Mexico: from miles to microns.

Laguna Figueroa is a lagoonal complex on the Pacific coast of the Baja California penisula 200 km south of the Mexican-United States border. The hypersaline lagoon is 16 km long and 2-3 km wide with a salt marsh and evaporite flat and is separated from the ocean by a barrier dune and beach. At the salt marsh-evaporite flat interface a stratified microbial community dominated by Microcoleus chthonoplastes is depositing laminated sediments. Similar stratiform deposits with associated microbial mat communities have been found in cherts of the Fig Tree Group, South Africa which are 3.4 GE in age. Heavy rains in the winters of 1978-1979 and 1979-1980 flooded the evaporite flat with 1-3 meters of meteoric water and buried the laminated sediment under 5-10 cm of siliciclastic and clay sediment. These flooding events had a dramatic effect on the composition of the mat community. The Microcoleus dominated community, with species of Chloroflexus sp. and an Ectothiorhodospira-like filamentous purple phototroph, disappeared leaving a community dominated by the purple phototrophs Chromatium sp. and Thiocapsa sp. Recolonization of the surface by species of the cyanobacteria Oscillatoria sp. and Spirulina sp. preceded the return of the Microcoleus community. Field conditions were monitored by ground based observations and supplemented with LandSat and Skylab imagery. The microbial community was studied with light microscopy and transmission electron microscopy. The change in dominating microbial species was correlated with the episodes of flooding.

Stratigraphic and sedimentological evidence bearing on structural repetition in early Archean rocks of the Barberton greenstone belt, South Africa

The 3.5–3.3 Ga-old Swaziland Supergroup in the Barberton greenstone belt, South Africa, includes a lower, predominantly volcanic unit, the Onverwacht Group, and an upper, largely sedimentary section, the Fig Tree and Moodies Groups. Although Viljoen and Viljoen concluded that the Onverwacht Group exceeds 15 km in stratigraphic thicknesses, more recent workers have suggested that it represents a much thinner section of rocks repeated by folding and faulting. Our mapping indicates that the upper half of the Group includes at least 8 km of unrepeated stratigraphic section. Inferences of structural repetition have arisen in part because a major shear zone is present in the upper half of the Onverwacht Group on the west limb of the Onverwacht anticline. Shearing and the overturning of large blocks of the igneous sequence have combined with the later intrusion of felsic magma to produce a generally stratiform, highly complex zone of deformation up to 2 km wide. The displacement across the zone is unknown, but the presence of unfaulted sections of this part of the stratigraphic sequence indicates that the gross stratigraphic arrangement of units within the Group has not been significantly altered. Structures previously mapped as major fold hinges within the upper part of the Onverwacht Group can be shown to have formed by local deformation and reflect neither large-scale folding nor structural repetition of the rocks.Several kilometers higher in the section, rocks of the uppermost Onverwacht and Fig Tree Groups are stacked and repeated in a series of thrust nappes. Individual nappes extend for up to 15 km along strike and include as much as 900 m of allochthonous volcanic and sedimentary rocks. They were apparently emplaced by faulting or gravity sliding at or near the Earth's surface. Their formation involved the movement of sheets made largely up of sedimentary rocks along shear surfaces within underlying, much weaker, serpentinized ultramafic komatiites. Locally, the sheets were deformed to form fold nappes. The lower limbs of the fold nappes commonly show extreme attenuation and brittle deformation whereas the upper limbs are largely intact. The thrust faults are the loci of small hypabyssal igneous bodies that appear to be co-magmatic with a suite of upper Fig Tree volcanic rocks. This style of faulting has not affected rocks of the overlying Moodies Group. Although estimates of the timing of deformation are as yet imprecise, these features suggest that it occurred during a late Fig Tree phase of tectonism and volcanism.

Geochemical evolution of Archean shales from South Africa. I. The Swaziland and Pongola Supergroups

Abstract The Fig Tree Group and Moodies Group, part of the Swaziland Supergroup, are exposed in the Barberton Mountain Land and are ∼ 3.3–3.5 Ga old. Shales from the Fig Tree Group are characterized by variable rare earth element (REE) patterns with LaN/YbN = 5.5 ± 1.8 (95% confidence) and Eu/Eu ∗ = 0.97 ± 0.18 . Shales from the Moodies Group have more fractionated REE patterns (LaN/YbN = 8.7 ± 2.0) and less variable Eu-anomalies ( Eu/Eu ∗ = 0.92 ± 0.02 ). Both groups are characterized by very high abundances of Cr and Ni. Modelling of those trace elements, which are relatively unfractionated during sedimentary processes (REE, Th, Sc), is consistent with the derivation of these sedimentary rocks from a mixture of the Archean bimodal igneous suite (mafic volcanics — tonalites, trondhjemites, felsic volcanics), with the Moodies Group requiring a larger percentage of the felsic end member. The data for shales from the Swaziland Supergroup support models suggesting that the Archean exposed crust was of more mafic composition than the post-Archean upper crust, and that intracrustal melting was relatively unimportant in differentiating the Archean crust. The Pongola Supergroup is younger than the Swaziland Supergroup, but older than ∼ 2.9 Ga. Shales and mudstones from the Nsuze and Mozaan Groups show REE characteristics intermediate between the older shales of the Swaziland Supergroup and typical post-Archean shales. One quartzite sample from the Witwatersrand Supergroup (ca. 2.6 Ga) has a REE pattern virtually parallel to average post-Archean shales, but with much lower abundances. The change from typical Archean to post-Archean sedimentary trace element patterns has been documented in early Proterozoic successions from Canada and Australia. This change has been related to a short interval of intense growth and chemical differentiation of the continental crust during the late Archean (∼ 2.7-2.5 Ga). In southern Africa, this change appears to have taken place much earlier (∼ 3.2-2.9 Ga), but can be related to similar processes. Thus, the period of crustal evolution related to the Archean-Proterozoic boundary, while virtually episodic regionally, occurred over a protracted period world-wide during the period ca. 3.2-2.5 Ga.

Pseudo-oolites in rocks of the Ulundi Formation, lower part of the Archaean Fig Tree Group (South Africa)

Abstract The Ulundi Formation of the lowermost Fig Tree Group in the type area in the Barberton Mountain Land consists of a sequence of quartz—sericite schists, banded cherts and tuffaceous rocks up to 40 m thick. It overlies talc—carbonate schists assigned to the upper part of the Onverwacht Group. In the cherts, which in parts represent the sulfide facies of a banded-iron formation, silicified spheroids have been observed which appear to be diagenetically altered, locally zeolitized accretionary lapilli. Portions of the cherts formed through silicification of fine-grained felsic tuffs and contain abundant stylolite seams along which ∼ 4% of the rock thickness has been removed by intrastratal solution during deformation. A supposedly organogenic spheroidal structure, Ramsaysphera, has been previously described from the upper part of the Ulundi Formation. The spheroids, with diameters of up to 8 mm, occur in a distinct bed of up to at least 1.6 m thickness and can be traced in outcrop over an area which originally covered ∼ 17 × 30 km. The bed was deposited in fairly still water of several 100 m depth. The spheroids accumulated with only little finer-grained detrital material, the interstices between them being filled by cherty silica and minor amounts of dolomite. Up to ∼ 65% of the spheroids consist of tectonically elongated, originally round fragments of a micro-felsitic groundmass with remnants of phenocrysts. The majority of the remaining spheroids consists of concentrically banded bodies of siliceous material with minor amounts of chlorite and sericite, as well as small dolomite crystals. These spheroids represent highly altered granules of volcanic glass deposited from volcanic ash clouds through a body of water. The prototype of Ramsay-sphera also belongs to this group of spheroids, and there is little microscopic or macroscopic evidence for its explanation as an organic structure.