A possible Mesoarchaean impact structure at Setlagole, North West Province, South Africa : aeromagnetic and field evidence

A 25 to 30 km wide magnetic anomaly within the >2.79 Ga granite-greenstone rocks of the northwestern Kaapvaal Craton is spatially associated with megabreccia outcrops near the village of Setlagole in the North West Province, South Africa. The breccia comprises angular to rounded ciasts of TTG gneisses, granites and granodiorites, with les.ser amounts of amphilxjlite, calc-silicate rock and banded iron-formation as well as unusual dark grey to black, irregular, centimetre- to decimetre-sized clasts that show evidence of fluidal behaviour and plastic deformation during incorporation into the breccia. The largest cla.sts reach up to several metres in size. Evidence of fluvial transport is found in rare thin sandy to gritty layers that show crude bedding and upward-fining with layers dipping gently to the northeast. The breccia matrix is highly variable but is dominated by angular mineral clasls (mainly quartz and feldspar, with subsidiary biotite, amphibole and epidote) with interstitial chlorite. The clasts show variable amounts of alteration (saussuritization, sericitization, chloritization of biotite and amphibole). The dark clasts contain angular quartz and feldspar and small biotite fragments in a cryptocrystalline chlorite-dominant matrix. Textures indicate a lower greenschist faciès metamorphic overprint. The absence of lava, dolomite or quartzite cla.sts suggests that the breccia formed prior to the deposition of the Neoarchaean Ventersdorp and Eoproterozoic Transvaal Supergroups, whereas the metamorphic grade indicates that it postdates the ca. 2.79 Ga amphibolite-facies metamorphic peak in the region. This suggests a late Mesoarchaean or early Neoarchaean (ca. 2.79 to 2.71 Ga) age for the breccia. A similar age is inferred for the magnetic anomaly based on postulated crosscutting dyke ages. Despite a comprehensive search, unequivocal shock-diagnostic microdeformation features have not yet Ixïen found in either the breccia or the highly-weathered granitic gneiss outcrops in the central parts of the anomaly. The unusual plastically-deformed dark clasts may represent chloritized mud clasts or impact melt clasts. Geochemical data on these clasts and other components of the megabreccia provide no conclusive support for a meteoritic origin, but the unparalleled comptjsition of the clasts and their high trace element abundances of Ni, Cr, V, Zn and Co relative to rocks of the Kraaipan granite-greenstone basement, sugge.sts an unusual origin for this matrix material. Given the distinctive nature of the breccia and its proximity to a large circular magnetic anomaly, it is postulated that the megabreccia could represent a mass or debris flow in a marine .setting triggered by an impact tsunami or resurge. Subsequent faulting may have led to the preferential preservation of these deposits. This interpretation of the Setlagole megabreccia and geophysical anomaly is evaluated in terms of other possible modes of origin and it is concluded that a meteoritic source best fits the available data

U-Pb SHRIMP data for the Madibe greenstone belt : implications for crustal growth on the western margin of the Kaapvaal Craton, South Africa

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Episodic granitoid emplacement in the Archaean Amalia-Kraaipan terrane, South Africa: confirmation from single zircon U-Pb geochronology

The Amalia-Kraaipan granite-greenstone terrane, located in the western part of the Kaapvaal Craton, South Africa consists of metamorphosed mafic volcanic rocks and interlayered ferruginous and siliceous metasediments (mainly banded iron formations), intruded by a variety of granitoid rocks comprising tonalitic and trondhjemitic gneisses, granodiorites and adamellites. This study presents new single zircon dating for several granitoid rocks in order to define the chronology of magmatic events influencing this terrane. New TIMS and SHRIMP U-Pb data demonstrate episodic granitoid emplacement events, which occurred over a time-span of approximately 250 Ma. The oldest granitoid rocks so far recognized are dated at ca. 3008 Ma and are represented by biotitetrondhjemite gneisses located southwest of the town of Amalia. Homogeneous leuco-trondhjemite dykes intruding these gneisses yield an age of ca. 2940 Ma. Two granodiorite samples from the central part of the Amalia-Kraaipan terrane were dated at 2913 ± 17 and 2915 ± 12 Ma, respectively, while a further two granodiorite samples from the central and northern parts of the study area yielded ages of 2879 ± 9 and 2879 ± 11 Ma. The youngest granitoid body, the Mosita adamellite, which is exposed in the northwest sector of the Amalia-Kraaipan terrane, was dated at 2791 ± 8 Ma. Since the 3008 Ma biotite-trondhjemite gneisses contain conformably interlayered amphibolite xenoliths, at least some of the Kraaipan Group volcano-sedimentary rocks are probably older than 3008 Ma. Based on available age data the 3010-2920 Ma Amalia-Kraaipan granitoids described in this paper could represent a possible source for some of the Witwatersrand sediments 100 km to the east and their contained gold. The ca. 2791 Ma age obtained for the Mosita adamellite permits a possible genetic link between this granitoid body and the ca. 2781 Ma Gaborone Granite Complex exposed approximately 120 km north in neighbouring Botswana

Re–Os isotope systematics and HSE abundances of the 3.5 Ga Schapenburg komatiites, South Africa: Hydrous melting or prolonged survival of primordial heterogeneities in the mantle?

International audienceWe report Re­Os isotope and highly siderophile element (HSE) abundance data for komatiites from the Schapenburg Greenstone Remnant (SGR), South Africa, an equivalent to the lowermost formations of the Onverwacht Group in the Barberton Greenstone Belt (BGB). The Re­Os isotopic data for 13 whole-rock samples define a regression line with an age of 3549 ± 99 Ma, consistent with the ~ 3.5 Ga age of the Onverwacht Group. The immobility of Os during alteration and the correct Re­Os age provide evidence that the initial ?187Os = + 3.7 ± 0.3 derived from the regression reflects a time-integrated suprachondritic 187Re/188Os in the source of the SGR komatiites. The HSE abundances in the emplaced SGR komatiite lavas are 2— to 3— lower than those in other well-studied komatiites. Compared to the Primitive Upper Mantle (PUM) estimate, the calculated mantle source of the SGR komatiites was moderately depleted in HSE and was characterized by a fractionated HSE pattern. The enrichment in 187Os, HSE depletion, and fractionated HSE pattern in the SGR komatiite source could be the result of fluid transport of radiogenic Os from the subducting slab, incorporation in the overlying mantle, and hydrous melting of the modified mantle to produce the komatiites. This would imply that plate tectonic processes operated as early as 3.5 Ga and that at least some komatiites formed via wet melting in island arc settings at relatively shallow depths and temperatures not exceeding 1400 °C. An alternative model would include dry melting of a chemically distinct, majorite-enriched mantle domain formed very early in Earth's history as a result of an initial stratification of the mantle during crystallization of a magma ocean. Preferential partitioning of Re into the majorite-rich domain would have resulted in its acquiring suprachondritic Re/Os. In order to grow the radiogenic Os isotopic composition, the majorite-rich domain would have to have remained isolated from the rest of the mantle for a billion years. If this model is correct, the data would require a deep plume origin for the SGR komatiites at temperatures ~ 300 °C higher than the ambient mantle temperatures. Both models have their shortcomings in reconciling the available geochemical and petrologic data for the SGR komatiites. Because BGB komatiites are geochemically closer to island arc tholeiites and boninites than the SGR komatiites, it will be especially important to determine the Re­Os isotope and HSE systematics of the BGB komatiites and of typical boninites

Archaean granitoid intrusions of the Kaapvaal Craton

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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 EMASH 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-remperature 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 terrine accretion event in the Barberton Greenstone Belt, and record an apparent geothermal gradient tyical for portions of modern orogenic belts of this type.Articl

U-Pb SHRIMP data for the Madiba greenstone belt: implications for crustal growth on the western margin of the Kaapvaal Craton, South Africa

The Madibe greenstone belt is the easternmost belt of the northern Kraaipan terrane, located in the Kimberley Block of the Kaapvaal Craton. It comprises a mature Middle to Late Archaean volcanic island arc succession that was welded onto the evolving western margin of the Kaapvaal Shield. The lithologies strike in a north-south direction and represent an assemblage of metamorphosed and strongly deformed volcanic and minor sedimentary rocks, now represented by phyllites, quartz-sericite schists, quartz-chlorite schists, amphibolites, talc-carbonate schists and banded iron formations. We report a SHRIMP U-Pb age of 3082.5 ± 5.9 Ma for an intermediate metavolcanic rock. Ages of zircon grains within a volcano-sedimentary rock define ages around 3.43, -3.18 and, ∼3.09 Ga and are as young as ∼3.04 Ga, suggesting the presence of older, mature continental crust. Age relationships of rocks from the Madibe greenstone belt and the Murchison greenstone belt suggest that the Kaapvaal Shield apparently experienced an episode of rapid crustal growth at around 3090 Ma along both the northern and western margins

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 th

Metamorphism of the granite-greenstone terrane south of the Barberton greenstone belt, South Africa: an insight into the tectono-thermal evolution of the lower portions of the Onverwacht Group

This paper reports new geochronological and metamorphic data from Onverwacht Group greenstone remnants exposed in the granitoid-dominated terrain to the south of the Barberton greenstone belt. The greenstone remnants consist of metamorphosed mafic and ultramafic metavolcanic sequences, with minor sedimentary units that comprise thin chert and banded iron formation layers, interbanded with the (ultra) mafic volcanic units, as well as an up to 8 m-thick clastic sedimentary unit that contains well-preserved primary sedimentary features such as trough cross-bedding. Coarse-grained portions of the clastic sediments contain up to 4.5 wt.% K2O and represent metamorphosed impure arkoses. SHRIMP and conventional U-Pb dating of detrital zircons reveal dates ranging between ca. 3521 and 3540 Ma, indicating that at least two protoliths for these sediments predate the formation of the bulk of the Barberton greenstone belt. A minimum age of 3431 ± 11 Ma for the formation of the metasediments is given by a trondhjemite gneiss that intrudes the greenstone remnant. Thus, these metasediments were deposited between ca. 3521 and 3431 Ma, contemporaneously with the erosion of spacially associated older, and presumably potassium-rich, granitoid rocks. Other portions of the clastic metasediments have a mafic affinity and are characterised by the peak-metamorphic mineral assemblage diopside + andesine + garnet + quartz. This assemblage, and garnet in particular, is extensively replaced by epidote. Peak-metamorphic mineral assemblages of magnesio - hornblende + andesine + quartz, and quartz + ferrosilite + magnetite + grunerite have been recorded from adjacent amphibolites and interlayered iron formation units, respectively. In these rocks, retrogression is marked by actinolitic rims around peak metamorphic magnesio-hornblende cores in the metamafic rocks, and by a second generation of grunerite that occurs as fibrous aggregates rimming orthopyroxene in the iron formation. PT calculations, using a variety of geothermometers and barometers, for the peak-metamorphic mineral assemblages in all these rock types vary between 650 and 700 °C and 8 and 11 kbar. This implies a tectonic setting comparable to some modern orogenic belts and that the granite-greenstone terrane investigated in this study possibly represents an exhumed mid- to lower-crustal terrane that formed a 'basement' to the Barberton greenstone belt at the time of the peak metamorphic event at ca. 3230 Ma