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Validating the existence of Vaalbara in the Neoarchean
Abstract
An interesting aspect of Precambrian geology is the similarities between successions of the Kaapvaal and Pilbara cratons of southern Africa and Australia. Coeval trends in these successions are commonly used to reconstruct global atmospheric and oceanic conditions during the Archean-Proterozoic transition. The similarities, however, could also suggest their paleogeographic proximity in the form of a supercraton, or even Earth's oldest assembled continent, named Vaalbara. If these cratons indeed were nearest neighbours in a supercraton, the parallel trends preserved in supracrustal sequences may reflect local effects in a single basin instead of global paleoenvironmental conditions. Here we report a paleomagnetic pole from the Neoarchean Ventersdorp Supergroup of South Africa, which provide quantitative support for Vaalbara's existence. Our reconstruction differs greatly from earlier suggestions and contests those that place the cratons far apart. It provides the oldest example, and the only Archean instance, of paleomagnetic reconstruction between continental blocks in terms of paleolatitude and relative longitude. If correct, our reconstruction implies that previous paleoenvironmental conclusions may need reconsideration.
... Unlike Proterozoic supercontinents (Rodinia and Columbia), the paleogeography for the late Archean cratons is not so clear, owing to the general paucity of paleomagnetic data from most cratons (Buchan et al., 2000, Pesonen et al., 2003. Nevertheless, particularly during the Neoarchean Era, the relative positions of cratons are becoming tractable by the increasing numbers of refined paleomagnetic data and geochronologic studies (e.g., de Kock et al., 2009;Denyszyn et al. 2013;Smirnov et al., 2013;Salminen et al., 2019;;Liu et al., 2021). ...
... We report new paleomagnetic data for basaltic lava flows from two well-preserved deep drill cores sampled from the Carajás Basin in the northern Carajás Province. Our goal is to provide the first paleogeographic constraints for this Archean block, yielding a paleolatitude estimate for the block and discussing its affiliation to previously proposed Archean cratonic assemblies (e.g., Williams et al., 1991;Bleeker, 2003;Bleeker and Ernst 2006;de Kock et al., 2009;Gumsley et al., 2017;Salminen et al., 2019;Liu et al., 2021). Furthermore, the thick sequence of basalts in the Carajás Basin has a good potential to provide evidence for geomagnetic reversals across the succession. ...
... One of the earliest known potential Archean crustal configurations is that of Vaalbara (e.g., Cheney, 1996;de Kock et al., 2009), which incorporates ancient crust in southern Africa (Kaapvaal) and Western Australia (Pilbara). The connection between the Pilbara and Kaapvaal cratons begins with a comparison between the geology and geochronology of both cratons, and the parallel development of the Neoarchean-Paleoproterozoic stratigraphy that is the core of the Vaalbara hypothesis. ...
Establishing the positions of continents during the initial stages of Earth's evolution is one of the most important challenges in geosciences today. This challenge is mainly due to the severe limitations in obtaining geological and/or geophysical data from early Earth time, particularly robust paleomagnetic data. Here, we report the first paleomagnetic data from an Archean block in the Amazonian craton, the Carajás Province, for ∼2.76–2.74 billion years ago (Ga), when extensive dominantly mafic volcanism (Parauapebas Formation) covered an area of ∼18,000 km². The paleomagnetic investigation was conducted on fresh drill cores drilled into the Carajás iron ore mine and cutting across the Parauapebas Formation. After rotating the drill core segments to geographic coordinates using the viscous magnetic component, two characteristic components, Carajás 1 and 2 (C1 and C2) were identified and further used to calculate paleomagnetic poles: C1 (∼2759 Ma; 40.5°E, −44.6°S, N = 5 A95 = 6.5°, K = 18.5) and C2 (∼2749 Ma; 342.4°E, −54.3°S, N = 28, A95 = 14.8°, K = 27.8). Pole C2 is based on a bigger number of sites, passes a reversal test and is considered robust. A baked contact test was attempted for this component, but it is not conclusive. Our results, integrated with geological evidence reveals that the Carajás block occupied low latitudes at the time, and could have been part of the Superia supercraton during the Neoarchean (∼2.75 Ga) at equatorial latitudes. Finally, a consistent succession of six magnetic reversal events was identified in the lava flow sequence from the Parauapebas Formation, pointing to an already dynamic geodynamo pre-2.7 Ga.
... Alternatively, the Zimbabwe and Kaapvaal cratons are regarded by some investigators as distinct crustal fragments in pre-2.0 Ga continent configurations (e.g., Kenorland, Vaalbara, and Zimgarn;Cheney, 1996;Bleeker, 2003;De Kock et al., 2009a;Smirnov et al., 2013). p0030 The Paleoproterozoic crust of the proto-Kalahari Craton is composed of distinct structural terranes that include the Rehoboth Block, the Kheis Belt, the Okwa Block and the Magondi Belt (Figs. 12.1 and 12.2B). ...
... The Neoarchean is represented by seventeen data entries in PALEOMAGIA for the Kaapvaal Craton and six entries from the Zimbabwe Craton, but some studies are superseded, represent dual polarity magnetizations or magnetic overprints, or are from units belonging to the same magmatic event. Data from the Kaapvaal Craton are mostly from units of the c.2.78À2.65 Ga Ventersdorp LIP(s), of which the younger magmatic pulses span the Colesberg Magnetic Anomaly between the western and eastern parts of the craton (Evans and McElhinny, 1966;Evans, 1967;Wingate, 1998;De Kock et al., 2009a;Lubnina et al., 2010;Maré and Fourie, 2012;Denyszyn et al., 2013). Additional data come from a Neoarchean overprint on older rock units (Biggin et al., 2011), the c.2.69 Ga Mbabane Pluton that was emplaced during final cratonization (Layer et al., 1989), as well as from the first volcano-sedimentary supracrustal succession that developed subsequently (De Kock et al., 2009b). ...
... p0525 Neoarchean volcanic rocks also constrain the Kaapvaal Craton to mid-high latitudes. Paleomagnetic data from intraplate magmatic events at B2.78 and 2.70 Ga have been used to reconstruct the Kaapvaal Craton with the Pilbara Craton of Western Australia to form Vaalbara (Wingate, 1998;Strik et al., 2007;De Kock et al., 2009a). However, more recent paleomagnetic and geochronologic constraints from the Pilbara Craton have cast doubt on Vaalbara's existence (see Section 12.5.2.2). p0530 Piercing points for Neoarchean continental reconstruction are provided by the radiating, but dominantly eastÀwest-trending c.2.69À2.66 ...
Available paleomagnetic data from Precambrian southern Africa are reviewed and evaluated. Reliable paleopoles are used to define Precambrian apparent polar wander paths and constrain the paleolatitudinal drift and evolution of the Kalahari Craton and its constituents. Available paleomagnetic data do not support the reconstruction of the Kaapvaal and Pilbara cratons in a contiguous Vaalbara configuration, and incorporation of Vaalbara as a crustal element in Kenorland and Columbia is questioned. The Zimbabwe and Kaapvaal cratons were far apart between ~2.63 and 2.43 Ga, and a modern relative configuration is supported only after ~1.85 Ga. Much of Paleoproterozoic and parts of the Mesoproterozoic drifts are relatively well-constrained, but a dearth of reliable ~1.64–1.40 Ga data obscures the position of Proto-Kalahari within Columbia. Also, the Neoproterozoic paleomagnetic database from the Kalahari Craton is poorly populated. The database of well-dated reliable Precambrian poles in general, however, has grown significantly in recent years.
... To do so we examine and characterize the morphology and chemistry of the Au at Purdy's Reward, determine if there is any ultrafine Au in the sequence, and investigate the conglomerate sedimentology and clast lithogeochemistry. We also consider possibilities of a link between the Pilbara and Witwatersrand deposits as part of a postulated Vaalbara continent (Cheney, 1996;Zegers et al., 1998;de Kock et al., 2009;Rajesh et al., 2020) in the context of their respective Au mineral systems. ...
... However, given the similarities in geology and age between the Comet Well Conglomerate Au in the Pilbara craton and the conglomerate Au of the Ventersdorp contact reef in the Kaapvaal craton, it is useful to consider the possibility that the Fortescue basin and the depositional basin of the Ventersdorp Supergroup were connected as part of the postulated Vaalbara continent reconstruction (Cheney, 1996;Zegers et al., 1998;de Kock et al., 2009;Rajesh et al., 2020), thus potentially being part of the same mineral system. ...
... These commonalities may be argued to be consistent with a model of lateral continuity of depositional settings between the Venterspost Conglomerate Formation and the Comet Well Conglomerate within a Vaalbara continent; however, this is unlikely given the lateral distance of several hundred kilometers. Paleomagnetic reconstructions of the Fortescue Group and the Ventersdorp Supergroup have placed the cratons proximal to one another (de Kock et al., 2009), further supporting this, although the evidence of different tectonic settings and histories suggests there must have been a separation of tens or, more likely, hundreds of kilometers at the time of deposition (cf. fig. ...
Recently discovered Au in boulder conglomerate between the Mesoarchean West Pilbara superterrane basement and the overlying volcano-sedimentary stratigraphy of the Neoarchean Fortescue Group in Western Australia has renewed comparisons with the Witwatersrand conglomerate Au deposits in South Africa. As such, this has reignited the question of the Pilbara and Kaapvaal cratons being linked as part of the postulated Vaalbara continent during the Archean. However, little is known about the origin of the Pilbara conglomerate Au and its host conglomerates, as they are hitherto unstudied, and their formation and/or source is uncertain.
Here we present a detailed study on the textures, composition, and sedimentology of one newly discovered Pilbara conglomerate Au deposit at the base of the Neoarchean Fortescue Group in the northwestern Pilbara craton. The Pilbara conglomerate Au occurrences are characteristically Ag-bearing but Hg-poor polycrystalline discoid masses that are overgrown by Au-poor chloritic halos, which are further enveloped by a hydrothermal alteration halo of disseminated Au within chlorite. Both the discoids and the auriferous chlorite halo are Ag bearing, with up to ~9 wt % Ag, consistent with a hydrothermal (orogenic) origin. The discoids do not display any physical or chemical evidence for sedimentary transport; thus, their formation (placer versus hydrothermal) remains unclear. However, the position of the Au in the conglomerate, limited to the basal section of the conglomerate, is difficult to account for in a purely hydrothermal deposit model.
We argue the Pilbara conglomerate Au represents a modified placer deposit from a primary orogenic Au source, with surface evidence for sedimentation removed by partial dissolution during later hydrothermal alteration in the host conglomerate and the crystalline basement. While the basal Fortescue Group conglomerate Au shares commonalities with the time equivalent (>~2.7 Ga) Venterspost Conglomerate Formation, which overlies the Witwatersrand Supergroup, inconsistencies remain, with different Au chemistries and tectonic, magmatic, sedimentary, and metamorphic-metallogenic histories of the Pilbara and Kaapvaal cratons prior to deposition of the >2.7 Ga conglomerate sequences. This collectively indicates the drivers of Au metallogenesis and ultimate Au deposition in conglomerate facies were fundamentally different in the Pilbara and Kaapvaal cratons.
... Nevertheless, the strong overlap between our Hf isotope data and those of the Yilgarn Craton together with the occurrence of a Hadean zircon crystal in the Boolgeeda diamictite, similar in age and Hf isotopic signature to the Jack Hills zircons (Whitehouse et al., 2017 and references therein) makes the Yilgarn Craton a possible source for the Boolgeeda glacial horizon. This conclusion is in apparent contradiction with the cratonisation history and paleogeographic reconstruction of the Yilgarn and Pilbara cratons, which are considered to be part of two different supercratons during the early Paleoproterozoic (Cheney, 1996;de Kock et al., 2009;Smirnov et al., 2013), and to have accreted during the~1.96-2.0 Ga Glenburgh orogeny (Johnson et al., 2013), that is about 200 Ma later than the upper age limit of the TCG (Müller et al., 2005; see chapter 7.4 below for further discussion). ...
... Magmatic zircons from the Kaapvaal Craton mainly crystallized from material that is much older than the one recorded for the Boolgeeda and TCG detrital zircons (Fig. 5e, Zeh et al., 2009). Assuming the 'Vaalbara' hypothesis (Cheney, 1996;de Kock et al., 2009;see Eriksson et al., 2011 for an alternative hypothesis), the post-2080 Ma Magaliesberg Quartzite, which is stratigraphically located at the top of the Transvaal Supergroup, could represent an analog of the TCG sediments. Two populations of zircons can be distinguished on the basis of U-Pb ages and initial ε Hf (t) values (Fig. 5d, Zeh et al., 2016). ...
... The significant overlap between our Hf isotope data and those of the Yilgarn Craton, together with the occurrence of an Hadean zircon crystal preserved in the Boolgeeda diamictite with similar age and Hf isotopic signature than the Jack Hills zircons (Whitehouse et al., 2017 and references therein) makes the Yilgarn Craton a possible source material for the Boolgeeda glacial horizon. Available paleogeographic reconstructions based on paleomagnetic data and cratonization history suggest that during the Paleoproterozoic, the Yilgarn and Pilbara cratons were part of two different supercratons, the Zimgarn comprising the Zimbabwe and Yilgarn cratons (Smirnov et al., 2013), and the Vaalbara composed of the Kaapvaal and Pilbara cratons (Cheney, 1996;de Kock et al., 2009). This argues against Yilgarn and Zimbabwe cratons as a source for the Turee Creek and Boolgeeda zircons ( Fig. 5c and d). ...
We investigated the Hf isotopic systematics of detrital zircons in a succession of siliciclastic sediments and glacial diamictites from the early Paleoproterozoic sequence of the Boolgeeda Iron Formation (Hamersley Group) and overlying Turee Creek Group of the Pilbara Craton, Western Australia. About 400 detrital zircons yielding > 95% concordant U-Pb ages were analyzed for Hf isotopes to constrain their magmatic sources. 70% of the analyzed zircons display super-chondritic initial Hf isotopic compositions, demonstrating crystallization in mantle-derived magmas. Most of the data are comprised between model age lines at ∼2.5 and 3.2 Ga, which suggests a sub-continuous crust generation by extraction from the depleted mantle during this time period. A single grain yields a 4.0 Ga age, which represents the first Hadean age for a zircon from the Pilbara Craton. Our results are significantly different from zircon Hf isotope data of the Glenburgh Terrane, in the southern border of the Turee Creek Group, or older successions of the Pilbara, Kaapvaal and Superior cratons, but show overlap with some of the Yilgarn Craton. This together with the occurrence of a Hadean zircon crystal preserved in the Boolgeeda glacial diamictite with similar Hf isotopic signature than the Jack Hills zircons makes the Yilgarn Craton a possible source material for the Boolgeeda glacial horizon. Alternatively, the majority of the zircons analyzed show ages which are consistent with those of the underlying 2.45 – 2.78 Ga Hamersley and Fortescue groups, formed by sedimentary successions interleaved with thick subaerial volcanic sequences associated with the emplacement of Large Igneous Provinces. Such subaerial volcanic rocks could account for the relatively juvenile character of the zircon analyzed. A local provenance of the siliciclastic material delivered to the Turee Creek Basin would support the role of large subaerial magmatic provinces as triggers of the rise of atmospheric oxygen and the onset of glaciations at the beginning of the Proterozoic.
... In particular, meter-to decimeter-scale cyclicity between iron(III) oxide-rich 'BIF' and iron(II) silicate-and carbonate-rich 'shale' observed in both the Brockman and Kuruman IFs suggest Milankovitch forcing at the scale of Earth's orbital eccentricity vari-ations (Lantink et al., 2019;de Oliveira Rodrigues et al., 2019), while decimeter-to centimeter-scale Knox cyclothem alternations in the Joffre Member were recently linked to climatic precession (Lantink et al., 2022). Paleomagnetic reconstructions (de Kock et al., 2009;Gumsley et al., 2017) and lithostratigraphic similarities between the Brockman and Kuruman IF (Trendall, 1969;Cheney, 1996;Martin et al., 1998;Beukes and Gutzmer, 2008) suggest that the two IF units may have been deposited in a single large basin or along a contiguous margin (spanning >1000 km) of the ancient Vaalbara supercraton, situated at low to equatorial latitude and in likely connection to open ocean. As such, cyclostratigraphic and geochemical investigation of the Brockman IF cyclostratigraphy offers the potential of acquiring insights into regional-scale marine redox processes in relation to (low-latitude) astronomical climate forcing acting at the global scale. ...
... In this conceptual model, we link the inferred variations in continental nutrient input, marine primary productivity and iron chemocline oscillations to precession-driven changes in (summer) monsoon intensity. Because monsoonal systems are highly sensitive to variations in summer insolation and seasonality, which is dominated by precession especially at low latitudes (Kutzbach et al., 2008;Bosmans et al., 2015), we consider a monsoonal control the most logical scenario given the evidence for a precession origin of the Knox cyclothems (as opposed to obliquity), and the low paleolatitude position of the Hamersley-Transvaal basin indicated by paleomagnetic reconstructions (de Kock et al., 2009;Gumsley et al., 2017). Moreover, monsoonal activity would have been intensified with the emergence of relatively large, elevated landmasses around the Archean-Proterozoic transition (Gumsley et al., 2017;Bindeman et al., 2018). ...
... In particular, meter-to decimeter-scale cyclicity between iron(III) oxide-rich 'BIF' and iron(II) silicate-and carbonate-rich 'shale' observed in both the Brockman and Kuruman IFs suggest Milankovitch forcing at the scale of Earth's orbital eccentricity vari-ations (Lantink et al., 2019;de Oliveira Rodrigues et al., 2019), while decimeter-to centimeter-scale Knox cyclothem alternations in the Joffre Member were recently linked to climatic precession (Lantink et al., 2022). Paleomagnetic reconstructions (de Kock et al., 2009;Gumsley et al., 2017) and lithostratigraphic similarities between the Brockman and Kuruman IF (Trendall, 1969;Cheney, 1996;Martin et al., 1998;Beukes and Gutzmer, 2008) suggest that the two IF units may have been deposited in a single large basin or along a contiguous margin (spanning >1000 km) of the ancient Vaalbara supercraton, situated at low to equatorial latitude and in likely connection to open ocean. As such, cyclostratigraphic and geochemical investigation of the Brockman IF cyclostratigraphy offers the potential of acquiring insights into regional-scale marine redox processes in relation to (low-latitude) astronomical climate forcing acting at the global scale. ...
... In this conceptual model, we link the inferred variations in continental nutrient input, marine primary productivity and iron chemocline oscillations to precession-driven changes in (summer) monsoon intensity. Because monsoonal systems are highly sensitive to variations in summer insolation and seasonality, which is dominated by precession especially at low latitudes (Kutzbach et al., 2008;Bosmans et al., 2015), we consider a monsoonal control the most logical scenario given the evidence for a precession origin of the Knox cyclothems (as opposed to obliquity), and the low paleolatitude position of the Hamersley-Transvaal basin indicated by paleomagnetic reconstructions (de Kock et al., 2009;Gumsley et al., 2017). Moreover, monsoonal activity would have been intensified with the emergence of relatively large, elevated landmasses around the Archean-Proterozoic transition (Gumsley et al., 2017;Bindeman et al., 2018). ...
This article shows how redox cycles within BIF of the Hamersley Group in Western Australia were controlled by Milankovitch cyclicity
... The Pilbara and Kaapvaal cratons have long been considered as part of the Vaalbara Supercraton (Beukes & Gutzmer, 2008;Cheney, 1996;de Kock, Beukes, & Armstrong, 2012;de Kock, Evans, & Beukes, 2009;Nelson, Trendall, & Altermann, 1999). Later, there were abundant new data that can both support and discredit the concept Vaalbara (e.g., Eriksson & Condie, 2014;Evans & Muxworthy, 2019;Evans, Smirnov, & Gumsley, 2017;Gumsley et al., 2017Gumsley et al., , 2020Kampmann, Gumsley, de Kock, & Söderlund, 2015;Zeh, Wilson, & Gerdes, 2020). ...
... Note that the last Neoarchean Mammona magmatic event of ( (Shchegolev, 1985). These data also provide strong support for the intercontinental correlation of the (Barley, Bekker, & Krapez, 2005;de Kock et al., 2009de Kock et al., , 2012Gumsley et al., 2020;Nelson et al., 1999). ...
... Data supporting small-scale D 33 S variation before 2.7 Ga are scarce and mainly developed from a limited number of formations in sedimentary sequences from South Africa (Kaapvaal Craton; Farquhar et al., 2007) and Western Australia (Pilbara Craton; Ohmoto et al., 2006;Farquhar et al., 2007;Kaufman et al., 2007;Thomazo et al., 2009a;Marin-Carbonne et al., 2018). Spatially, these observations could represent part of the same contiguous basin of the Vaalbara or Supervaalbara Archean supercontinents (Cheney, 1996;de Kock et al., 2009;Gumsley et al., 2017). If true, other supercratons, such as Superia (North America), Dharwar (India), Zimbabwe (Southern Africa), North China (China), Amazon (Brazil), and São Francisco (Brazil), may never have shared a common history in a single Archean supercontinent (Bleeker, 2003). ...
... Since anaerobic methanotrophy and an early organic haze were contemporaneous (Havig et al., 2017), both processes may have contributed to small S-MIF values in the RDV BIF. However, RDV represents the São Francisco Craton, which at 2.7 Ga is thought to be separated from the cratons forming the Vaalbara or Supervaalbara supercontinents (Cheney, 1996;Wit, 1998;de Kock et al., 2009;Gumsley et al., 2017), and therefore minimizes the potential for sampling bias. Diagenetic reactions associated with active sulfur cycling triggered by nascent oxygen production (and thus sulfate production) have been identified as processes that may mix and unmix S-MIF signatures (Farquhar and Wing, 2003;Halevy, 2013). ...
Sulfur mass-independent fractionation (S-MIF) preserved in Archean sedimentary pyrite is interpreted to reflect atmospheric chemistry. Small ranges in Δ³³S that expanded into larger fractionations leading up to the Great Oxygenation Event (GOE) 2.45 to 2.2 Ga are disproportionately represented by sequences from the Kaapvaal and Pilbara Cratons. These patterns of S-MIF attenuation and enhancement may differ from the timing and magnitude of minor sulfur isotope fractionations reported from other cratons, thus obscuring local for global sulfur cycling dynamics. By expanding the Δ³³S record to include the relatively underrepresented São Francisco Craton in Brazil, we suggest that marine biogeochemistry affected S-MIF preservation prior to the GOE. In an early Neoarchean sequence (2763–2730 Ma) from the Rio das Velhas Greenstone Belt, we propose that low δ¹³Corg (< −30‰) and dampened Δ³³S (0.4‰ to −0.7‰) in banded iron formation reflect the marine diagenetic process of anaerobic methane oxidation. The overlying black shale (TOC up to 7.8%) with higher δ¹³Corg (−33.4‰ to −19.2‰) and expanded Δ³³S (2.3‰ ± 0.8‰), recorded oxidative sulfur cycling that resulted in enhance preservation of S-MIF input from atmospheric sources of elemental sulfur. The sequence culminates in a metasandstone, where concomitant changes to more uniform δ¹³Corg (−30‰ to −25‰), potentially associated with the RuBisCO I enzyme, and near-zero Δ³³S (−0.04‰ to 0.38‰) is mainly interpreted as evidence for local oxygen production. When placed in the context of other sequences worldwide, the Rio das Velhas helps differentiate the influences of global atmospheric chemistry and local marine diagenesis in Archean biogeochemical processes. Our data suggest that prokaryotic sulfur, iron, and methane cycles might have an underestimated role in pre-GOE sulfur minor isotope records.
... The Archean geological record is highly fragmented in several cratons and small basement inliers that are thought to be part of ancient supercontinents (Bleeker, 2003). Fitting the puzzle pieces is crucial for understanding the early earth dynamics and their bearing on the evolution of the continental crust (e.g., Kock et al., 2009;Liu et al., 2021;Smirnov et al., 2013). ...
Supracrustal rocks offer a window into tectonic processes of the early Earth, since they are common in the Archean lithosphere. However, these rocks are usually affected by several episodes of metamorphism that can compromise their Usingle bondPb systematics, leading to equivocal interpretations of depositional ages and sources. In northeast Brazil, supracrustal rocks are frequent within the Archean basement of the São José do Campestre Massif. These metapelitic migmatites show a high-temperature mineral assemblage, with garnet + sillimanite ± spinel and retrograde cordierite, with abundant anatectic melt migration at conditions of upper amphibolite to granulite facies. Zircon Usingle bondPb dating coupled to trace elements analysis through LA-ICP-MS, as well as zircon internal zoning patterns suggest a maximum depositional age of 3305 ± 16 Ma followed by high-temperature metamorphism in the Mesoarchean, Paleoproterozoic and Neoproterozoic. Mesoarchean high temperature metamorphism occurred between 3084 ± 4 and 3006 ± 6 Ma and generated a wide range of textures that could be grouped in, at least, three stages of zircon growth. The first, during prograde heating, led to dissolution of detrital cores through the process of Ostwald Ripening and reprecipitation in oscillatory zoned rims. The second, probably at peak conditions, occurred above the stability of monazite, as evidenced by high Th/U ratios within zircon grains, rounded shape and sector-zoned cores. The third, during retrograde cooling, is mostly driven by garnet breakdown, and resulted in the crystallization of convoluted rims. Ti-in-zircon temperatures indicate minimum temperatures of 712 ± 21 °C for prograde/retrograde stages and 881 ± 50 °C for peak conditions. The Paleoarchean sedimentation and Mesoarchean metamorphism are coeval with similar events in Kaapvaal and Dharwar Cratons (South Africa and South India, respectively), but show no correlation to any Archean domains in South America to date. Solid-state recrystallization during the Paleoproterozoic (ca. 2.0 Ga) and the Neoproterozoic (ca. 0.6 Ga) correlates with orogenic events in both the Borborema Province and São Francisco Craton, suggesting a common evolution since the Rhyacian.
... Zircon of such ages is in general scarce in the Pretoria Group, except in the Leeuwpoort Formation. Zeh et al. (2016) interpreted detrital zircon data from the Magaliesberg Formation in terms of sources in the Pilbara Craton in present-day western Australia, assuming that the Late Archaean Vaalbaara continental assembly (Cheney 1996;de Kock et al., 2009) was stable at the time of deposition of the upper Pretoria Group. Although there are arguments supporting parallel tectonometamorphic histories in the two provinces as late as ca. 2 150 Ma (Rasmussen et al., 2019), detrital zircon data from Western Australia (e.g., Krapez et al., 2017) do not support a common provenance for Palaeoproterozoic strata in the two provinces (Zeh et al., 2020). ...
The Palaeoproterozoic sandstones and quartzites of the Pretoria Group (Transvaal Supergroup) in the Transvaal Basin of South Africa are important markers for regional correlations and dating of events of global importance (e.g., the Great Oxidation Event). The succession has few independent age markers, and much of the discussion about the time of deposition and the source of material of these rocks has been based on data from detrital zircon suites.
The clastic sedimentary rocks of the Pretoria Group contain detrital zircon grains ranging from the Mesoarchaean to ages that are near-contemporaneous to, and even younger than the overlying and crosscutting igneous rocks of the Bushveld Complex. We show that the U-Pb age and Lu-Hf isotope distributions of the detrital zircon population in the Pretoria Group are the result of three different types of processes, acting successively: (1) Crystallisation in the igneous or metamorphic protosource rock (i.e., the rock where the zircon originally crystallised), (2) Metamorphic and hydrothermal resetting of the U-Pb chronometer induced by emplacement and crystallisation of the 2 055 Ma Bushveld Complex, and (3) Late, low-temperature processes (e.g., weathering).
Critical age markers of maximum ages of deposition obtained after excluding effects of (2) and (3) are the 2 200 Ma Magaliesberg Formation (outside of the Bushveld aureole) and the 2 080 to 2 100 Ma Lakenvalei Formation. The Leeuwpoort Formation is a worst-case example, containing both young (<2 200 Ma) unmodified detrital zircon and hydrothermally altered zircon in the same age range. The two can only be distinguished from trace element analyses.
Age distributions of Archaean and early Palaeoproterozoic zircon age fractions overlap with detrital zircon age suites in lower (i.e., pre-Timeball Hill Formation) parts of the Transvaal Supergroup, suggesting recycling within the basin or from the basin margin. Overlaps in 2 200 to 2 350 Ma zircon ages with those of volcanogenic zircon in the Timeball Hill Formation again suggest recycling. The origin of 2 080 to 2 150 Ma zircon is uncertain, but neither poorly constrained sources in the Kaapvaal Craton (e.g., Okwa Basement Complex) nor recycling of volcanogenic material from post-Magaliesberg formations can be ruled out.
... Furthermore, at 2000 Ma, southern Africa was likely close to several other landmasses, including parts of western Australia. The precise configuration and paleolatitude of the Yilgarn and Kaapvaal crustal components in early Paleoproterozoic reconstruction models is debated (e.g., Bleeker, 2003, Letts et al., 2009, Liu et al., 2019b, Smirnov et al., 2013, Stark et al., 2019, Wabo et al., 2020, Zhao et al., 2002a, Zhao et al., 2002b; however, they are considered to have been proximal as the Pilbara and Kaapvaal components of the Vaalbara supercontinent disassociated, traded cratonic partners, and reassembled into the Kalahari (Zimbabwe-Kaapvaal) and West Australian (Pilbara-Yilgarn) cratons by 1950-1800 Ma (Cheney, 1996, de Kock et al., 2009, Smirnov et al., 2013. ...
... Our starting point at 2000 Ma (Fig. 9a) takes into account palaeogeographic considerations for earlier times, e.g., the Archaean "Vaalbara" (Kaapvaal together with Pilbara; Cheney, 1996;de Kock et al., 2009;Zegers et al., 1998) and "Zimgarn" (Zimbabwe together with Yilgarn; Smirnov et al., 2013) connections as well as global reconstructions . We also have the Amazonia, West Africa and Congo-São Francisco cratons together by ca. ...
Establishing how tectonic plates have moved since deep time is essential for understanding how Earth’s geodynamic system has evolved and operates, thus answering longstanding questions such as what “drives” plate tectonics. Such knowledge is a key component of Earth System science, and has implications for wide ranging fields from core-mantle-crust interaction and evolution, geotectonic phenomena such as mountain building and magmatic and basin histories, the episodic formation and preservation of Earth resources, to global sea-level changes, climatic evolution, atmospheric oxygenation, and even the evolution of life. In this paper, we take advantage of the rapidly improving database and knowledge about the Precambrian world, and the conceptual breakthroughs, both regarding the presence of a supercontinent cycle and possible dynamic coupling between the supercontinent cycle and mantle dynamics, in order to establish a full-plate global reconstruction from 540 Ma back to 2000 Ma. We utilise a variety of global geotectonic databases to constrain our reconstruction, and use palaeomagnetically recorded true polar wander events and global plume records to help evaluate competing geodynamic models and also provide new constraints on the absolute longitude of continents and supercontinents. After revising the configuration and life span of both supercontinents Nuna (1600—1300 Ma) and Rodinia (900—720 Ma), we present a 2000—540 Ma animation, starting from the rapid assembly of large cratons and supercratons (or megacontinents) between 2000 Ma and 1800 Ma. This occurred after a billion years of dominance by small cratons, and kick-started the ensuing Nuna and Rodinia supercontinent cycles and the emergence of stable, hemisphere-scale (long-wavelength) degree-1/degree-2 mantle structures. We further use the geodynamicly-defined type-1 and type-2 inertia interchange true polar wander (IITPW) events, which likely occurred during Nuna (type-1) and Rodinia (type-2) times as shown by the palaeomagnetic record, to argue that Nuna assembled at about the same longitude as the latest supercontinent Pangaea (320—170 Ma), whereas Rodinia formed through introversion assembly over the legacy Nuna subduction girdle either ca. 90◦ to the west (our slightly preferred model) or to the east before the migrated subduction girdle surrounding it generated its own degree-2 mantle structure by ca. 780 Ma. Our interpretation is broadly consistent with the global LIP record. Using TPW and LIP observations and geodynamic model predictions, we further argue that the Phanerozoic supercontinent Pangaea assembled through extroversion on a legacy Rodinia subduction girdle with a geographic centre at around 0◦E longitude before the formation of its own degree-2 mantle structure by ca. 250 Ma, the legacy of which is still present in present-day mantle.
(the paper is of OPEN ACCESS at http://dx.doi.org/10.1016/j.earscirev.2023.104336)
... Our precession interpretation for the Knox cyclothems is consistent with a low paleolatitude at the time of deposition of these sediments suggested by paleomagnetic reconstructions (40)(41)(42) and the absence of a clear obliquity signal. We also suggest that the characteristic double whitish chert bands that we observe per precession-related cycle are most likely of diagenetic origin, considering the internal build-up of the alternations (Figs. 1 and 3) and the resemblance to diagenetically controlled quadruplet cycles from the Pliocene (43). ...
The long-term history of the Earth–Moon system as reconstructed from the geological record remains unclear when based on fossil growth bands and tidal laminations. A possibly more robust method is provided by the sedimentary record of Milankovitch cycles (climatic precession, obliquity, and orbital eccentricity), whose relative ratios in periodicity change over time as a function of a decreasing Earth spin rate and increasing lunar distance. However, for the critical older portion of Earth’s history where information on Earth–Moon dynamics is sparse, suitable sedimentary successions in which these cycles are recorded remain largely unknown, leaving this method unexplored. Here we present results of cyclostratigraphic analysis and high-precision U–Pb zircon dating of the lower Paleoproterozoic Joffre Member of the Brockman Iron Formation, NW Australia, providing evidence for Milankovitch forcing of regular lithological alternations related to Earth’s climatic precession and orbital eccentricity cycles. Combining visual and statistical tools to determine their hierarchical relation, we estimate an astronomical precession frequency of 108.6 ± 8.5 arcsec/y, corresponding to an Earth–Moon distance of 321,800 ± 6,500 km and a daylength of 16.9 ± 0.2 h at 2.46 Ga. With this robust cyclostratigraphic approach, we extend the oldest reliable datum for the lunar recession history by more than 1 billion years and provide a critical reference point for future modeling and geological investigation of Precambrian Earth–Moon system evolution.
... There is also a debate on the extent of the Archean continental crust, although it seems certain that a few continental fragments started to emerge during this period (see e.g., Stern, 2018). Some authors even argue that the first supercontinents might have formed during the Archean: Vaalbara at 3700 Ma (de Kock et al., 2009;Mahapatro et al., 2011), Ur at 2600 Ma (Rogers, 1996), and Kernoland at 2300 Ma (Bradley, 2011;Nance et al., 2014). ...
Earth is ~ 4600 million years old. An immense time for human times scales. Since its formation, Earth has undergone many changes, including the formation of oceans, kick start of plate tectonics, climate changes, and the emergence of life. To study these past events, earth scientists have to rely on the observation of the geological record. Over the years, we have been able to organize geological time. This was not a trivial endeavor, and it was only possible due to the advances in stratigraphy and dating techniques. Today, we recognize that the history of the Earth can be divided into four major Eons: the Hadean, the Archean, the Proterozoic, and the Phanerozoic. This chapter introduces the story of geological time and chronostratigraphy and briefly describes the major events in the Earth's history.
... Several studies have suggested that the Kimberley Block was itself part of an even larger continental mass that formed either before or during the 2.90 Ga event and that included the Pilbara craton (Cheney, 1996;De Kock et al., 2009;Nelson et al., 1999). The granitoid magmatism evidenced in the M4 drillcore may thus be linked to amalgamation of a considerably larger continental mass than just the current Kaapvaal craton. ...
One of the most enigmatic aspects of the geology of the Kaapvaal craton of southern Africa concerns the age and nature of the rocks involved in the formation of its western parts, which are almost completely obscured beneath extensive Neoarchaean to Cenozoic supracrustal sequences. A 369-m-deep drillhole near the centre of the ~70 km wide Morokweng impact structure intersects a suite of Neoarchaean, calc-alkaline, granitic-granodioritic, trondhjemitic and monzonitic gneisses. The gneisses are LREE-enriched, but most display primitive mantle-like to slightly depleted HREE concentrations suggestive of a mantle source with garnet retention in the restite. The monzonitic gneiss is distinctly more enriched in trace and REE, and particularly LREE, which is attributed to differentiation processes in parent magmas to the granitoids. Microbeam zircon UPb geochronology indicates an emplacement age of 2922 ± 5 Ma for the oldest granite, with the remaining granitic, granodioritic and monzonitic rocks being emplaced coevally at 2906 ± 6 Ma. Rare xenocrystic cores preserve an inherited >3.3 Ga crustal component. These crystallization ages coincide with the proposed culmination of westward-directed subduction beneath a significant continental fragment – the Kimberley Block - that led to its collision with the proto-Kaapvaal craton (Witwatersrand Block) between 2.93 and 2.88 Ga, and support a magmatic arc setting. Based on the age data, the granitoid gneisses intersected in the M4 drillhole predate ca. 2.88 Ga unfoliated granitoids found in outcrops and other drillcores in the vicinity that have been interpreted as post-orogenic intrusions but which nonetheless show similar geochemical characteristics to the M4 core granitoids. The distinct age of the M4 granitoid gneisses relative to other granitoid rocks in the Morokweng region may reflect the greater exhumation that occurred in the central parts of the 146 Ma Morokweng impact structure relative to its margins.
... Attachment with other Archaean terranes to the south is speculative. Some authors suggest a connection with the Pilbara Craton during the Archaean, but the geometry of this connection remains controversial (Cheney, 1996;de Kock et al., 2009de Kock et al., , 2012Zeh et al., 2016;Wilson & Zeh, 2018, and discussion therein). The latest studies on detrital zircon populations in Transvaal Basin sandstones excludes such a correlation (for details see Zeh et al., 2020). ...
The Mozaan Group represents the youngest unit of the c. 2.9 Ga Pongola Supergroup located along the south-eastern margin of the Kaapvaal Craton. It comprises a ca. 4800 m thick succession of clastic sedimentary rocks intercalated by minor chemical and volcano-sedimentary rocks deposited in shallow marine to fluvial environments, and is stratigraphically correlated with the auriferous Witwatersrand Supergroup. This correlation, however, is speculative, in particular as systematic information about depositional ages and sediment provenances are absent. To address these problems, we present new combined sets of U-Pb ages, Hf isotopes, and shape parameters (width, length, aspect ratios and roundness) of >700 detrital zircon grains from seven samples of the Mozaan type profile in the Hartland area. These data reveal a switch in provenance between the lower and upper Mozaan Group. Zircons in sandstones of the lower Mozaan Group (Sinqeni to Ntombe formations) were supplied from surrounding proto-Kaapvaal Craton, and those in upper Mozaan Group rocks (Delfkom to Ntanyana formations) predominately from a juvenile hinterland, comprising sources as far as the Pietersburg and/or Kimberley blocks, which became amalgamated to the proto-Kaapvaal Craton at 2.97–2.87 Ga. Significant overlap of zircon age spectra, Hf isotope data, and maximum depositional ages (2908 ± 8 Ma to 2866 ± 7 Ma) suggest similar sources for upper Mozaan Group and Central Rand Group sediments of the Witwatersrand Basin. In contrast, sedimentary rocks of the West Rand Group have no counterparts in the Pongola Basin, except for the Orange Grove Formation, which shows good agreement with the Sinqeni Formation. The provenance switch indicated by the age-Hf isotope data is not identified by zircon shape parameters. These rather reflect differences in depositional environment (littoral, fluvial, volcanogenic), related to the duration and energy of sediment transport and reworking, as indicated by specific patterns in grain size vs. roundness diagrams.
... Several studies have suggested that the Kimberley Block was itself part of an even larger continental mass that formed either before or during the 2.90 Ga event and that included the Pilbara craton (Cheney, 1996;De Kock et al., 2009;Nelson et al., 1999). The granitoid magmatism evidenced in the M4 drillcore may thus be linked to amalgamation of a considerably larger continental mass than just the current Kaapvaal craton. ...
... Because Schröder et al. (2016) did not find exposures of equivalent-age volcanic rocks on the Kaapvaal craton, they invoked the Woongarra Rhyolite (c. 2.45 Ga) in the Pilbara craton of Western Australia, which was allegedly joined to the Kaapvaal craton at the time (Cheney, 1996;de Kock et al. 2009;Schröder et al. 2016 and references therein). More recently, however, mafic volcanism and dyking of the c. 2.4 Ga Ongeluk event has been recognized in the Kaapvaal and Zimbabwe cores of the composite proto-Kalahari craton (de Kock et al. 2018). ...
The Maz Metasedimentary Series is part of the Maz Complex that crops out in the sierras of Maz and Espinal (Western Sierras Pampeanas) and in the Sierra de Umango (Andean Frontal Cordillera), northwestern Argentina. The Maz Complex is found within a thrust stack of Silurian age, which later underwent open folding. The Maz Metasedimentary Series mainly consists of medium-grade garnet–staurolite–kyanite–sillimanite schists and quartzites, with minor amounts of marble and calc-silicate rocks. Transposed metadacite dykes have been recognized along with amphibolites, metagabbros, metadiorites and orthogneisses. Schist, quartzite and metadacite samples were analysed for SHRIMP U–Pb zircon dating. The Maz Metasedimentary Series is polymetamorphic and records probably three metamorphic events during the Grenvillian orogeny, at c . 1235, 1155 and 1035 Ma, and a younger metamorphism at c . 440–420 Ma resulting from reactivation during the Famatinian orogeny. The sedimentary protoliths were deposited between 1.86 and 1.33–1.26 Ga (the age of the Andean-type Grenvillian magmatism recorded in the Maz Complex), and probably before 1.75 Ga. The main source areas correspond to Palaeoproterozoic and, to a lesser magnitude, Meso-Neoarchaean rocks. The probable depositional age and the detrital zircon age pattern suggest that the Maz Metasedimentary Series was laid down in a basin of the Columbia supercontinent, mainly accreted between 2.1 and 1.8 Ga. The sedimentary sources were diverse, and we hypothesize that deposition took place before Columbia broke up. The Rio Apa block, and the Río de la Plata, Amazonia and proto-Kalahari cratons, which have nearby locations in the palaeogeographic reconstructions, were probably the main blocks that supplied sediments to this basin.
... In the EPT and BGGB, the Pilbara and Kaapvaal Cratons contain perhaps the best-preserved granite-greenstone belts on Earth, which has inevitably led to comparisons between the two, and the postulated existence of an Archaean-Palaeoproterozoic supercraton termed 'Vaalbara' (Cheney et al., 1988), which has been variously supported and contested on the basis of palaeomagnetic reconstruction (de Kock et al., 2009;Evans and Muxworthy, 2019;Wingate, 1998). However, there remain strong similarities between the two cratons in terms of their Palaeo-to Neoarchaean stratigraphy (e.g., Van Kranendonk et al., 2015;Zegers et al., 1998), and at least superficially in terms of their TTG domes, which means the concept of Vaalbara has persisted. ...
The continental crust that dominates Earth’s oldest cratons comprises Eoarchaean to Palaeoarchaean (4.0 to 3.2 Ga) felsic intrusive rocks of the tonalite-trondhjemite-granodiorite (TTG) series. These are found either within high-grade gneiss terranes, which represent Archaean mid-continental crust, or low-grade granite-greenstone belts, which represent relic Archaean upper continental crust. The Palaeoarchaean East Pilbara Terrane (EPT), Pilbara Craton, Western Australia, and the Barberton Granite-Greenstone Belt (BGGB), Kaapvaal Craton, southern Africa, are two of the best exposed granite-greenstone belts. Their striking geological similarities has led to the postulated existence of Vaalbara, a Neoarchaean-Palaeoproterozoic supercraton. Although their respective TTG domes have been compared in terms of a common petrogenetic origin reflecting a volcanic plateau setting, there are important differences in their age, geochemistry, and isotopic profiles. We present new zircon Hf isotope data from five granite domes of the EPT and compare the geochemical and isotopic record of the Palaeoarchaean TTGs from both cratons. Rare >3.5 Ga EPT evolved rocks have juvenile εHf(t) requiring a chondritic source. In contrast, younger TTG domes developed via 3.5 to 3.4 and 3.3 to 3.2 Ga magmatic supersuites with a greater range of εHf(t) towards more depleted and enriched values, trace element signatures requiring an enriched source, and xenocrystic zircons that reflects a mixed source to the TTGs, which variously assimilates packages of older felsic crust and a more juvenile mafic source. EPT TTG domes are composite and record multiple pulses of magmatism. In comparison, BGGB TTGs are less geochemically enriched than those of the EPT and have different age profiles, hosting coeval magmatic units. Hafnium isotopes suggest a predominantly juvenile source to 3.2 Ga northern Barberton TTGs, limited assimilation of older evolved crust in 3.4 Ga southern Barberton TTGs, but significant assimilation of older (Hadean-Eoarchaean) crust in the ca. 3.6 Ga TTGs of the Ancient Gneiss Complex. The foundation of the EPT is younger than that for the oldest components of the Eastern Kaapvaal. Although the broader prevailing Palaeoarchaean geologic framework in which these two cratons formed may reflect similar a geodynamic regime, the superficial similarities in dome structures and stratigraphy of both cratonic terranes is not reflected in their geochemical and age profiles. Both the similarities and the differences between the crustal histories of the two cratons highlights that they are formed from distinct terranes with different ages and individual evolutionary histories. Vaalbara sensu lato represents typical Palaeoarchaean cratonic crust, not in the sense of a single homogeneous craton, but one as diverse as the continents are today.
... In the EPT and BGGB, the Pilbara and Kaapvaal Cratons contain perhaps the best-preserved granite-greenstone belts on Earth, which has inevitably led to comparisons between the two, and the postulated existence of an Archaean-Palaeoproterozoic supercraton termed 'Vaalbara' (Cheney et al., 1988), which has been variously supported and contested on the basis of palaeomagnetic reconstruction (de Kock et al., 2009;Evans and Muxworthy, 2019;Wingate, 1998). However, there remain strong similarities between the two cratons in terms of their Palaeo-to Neoarchaean stratigraphy (e.g., Van Kranendonk et al., 2015;Zegers et al., 1998), and at least superficially in terms of their TTG domes, which means the concept of Vaalbara has persisted. ...
The Barberton region of South Africa is characterized by a broad variety of granite types that range in age from ca. 3.5 Ga to 2.7 Ga and reflect the processes involved in the formation of Archaean continental crust on the Kaapvaal Craton. These granites are subdivided into three groups, as follows:
A tonalite-trondhjemite-granodiorite (TTG) suite diapirically emplaced at 3 450 Ma and 3 250 Ma into pre-existing metamorphosed greenstone belt material. TTG melts were derived from melting amphibolite in the lower crust, with individual plutons being emplaced at various crustal levels. The dome-and-keel geometry that characterizes the TTG-greenstone dominated crust at this time is inconsistent with a plate tectonic domain and reworking was likely controlled by gravity inversion or ‘sagduction’;
Regionally extensive potassic batholiths (the GMS suite) were emplaced at 3 110 Ma during a period of crustal thickening and melting of a TTG-dominated lower crust. Subsequent to emplacement of the voluminous GMS granites, the thickened continental crust had stabilized sufficiently for large sedimentary basins to form;
Late granite plutons were emplaced along two distinct linear and sub-parallel arrays close to what might have been the edge of a Kaapvaal continent at 2 800 to 2 700 Ma. They are subdivided into high-Ca and low-Ca granites that resemble the I- and S-type granites of younger orogenic episodes. The high-Ca granites are consistent with derivation from older granitoids in the lower crust, whereas the low-Ca granites may have been derived by melting metasedimentary precursors in the lower-mid crust. Granites with similar characteristics are associated with a subduction zone in younger terranes, although the recognition of such a feature at Barberton remains unclear.
The petrogenesis of granites in the Barberton region between 3.5 Ga and 2.7 Ga provides a record of the processes of Archaean crustal evolution and contributes to discussions related to the onset of plate tectonics.
... The position of Wyoming in Superia based on matching geology has been confirmed and refined with paleomagnetism . Recent geochronologic studies defining the magmatic barcode and stratigraphy of the Kaapvaal craton of southern Africa (Gumsley, 2017;Gumsley et al., 2017) have led to the proposal of adding "Vaalbara," a supercraton consisting of the Kaapvaal and Pilbara cratons (de Kock et al., 2009;Gumsley et al., 2017), to the larger Superia supercraton , which we adopted with slight modifications (Fig. 3; Table S3). ...
Many Archean cratons exhibit Paleoproterozoic rifted margins, implying they were pieces of some ancestral landmass(es). The idea that such an ancient continental assembly represents an Archean supercontinent has been proposed but remains to be justified. Starkly contrasting geological records between different clans of cratons have inspired an alternative hypothesis where cratons were clustered in multiple, separate “supercratons.” A new ca. 2.62 Ga paleomagnetic pole from the Yilgarn craton of Australia is compatible with either two successive but ephemeral supercontinents or two long-lived supercratons across the Archean-Proterozoic transition. Neither interpretation supports the existence of a single, long-lived supercontinent, suggesting that Archean geodynamics were fundamentally different from subsequent times (Proterozoic to present), which were influenced largely by supercontinent cycles.
... The proposed connection between Kaapvaal and Pilbara cratons (known as the Vaalbara connection) could have produced a small composite craton that was possibly long-lasting (ca. 2.8-2.1 Ga) 159 , but its existence has been called into question on palaeomagnetic grounds 160 . Without contiguity with other cratons (if any), the relatively small size of continental area would have likely been insufficient to steer mantle convection towards dominance of the very large scales, such as degree 1 and degree 2 flow. ...
Supercontinents signify self-organization in plate tectonics. Over the past ~2 63 billion years, 3 major supercontinents have been identified, with increasing age: Pangaea, 64 Rodinia, and Columbia. In a prototypal form, a cyclic pattern of continental assembly and 65 breakup likely extends back to ~3 billion years ago, albeit on the smaller scale of Archaean 66 supercratons which, unlike global supercontinents, were tectonically segregated. The 67 emergence of supercontinents provides a firm minimum age for the onset of the modern 68 global plate tectonic network, whereas supercratons might reflect an earlier geodynamic and 69 nascent tectonic regime. The assembly and breakup of Pangaea attests that the supercontinent 70 cycle is intimately linked with whole mantle convection. In this Review, the supercontinent 71 cycle is interpreted both as an effect and a cause of mantle convection, emphasizing the 72 importance of both top-down and bottom-up geodynamics and the coupling between them. 73 However, the nature of this coupling and how it has evolved remains highly controversial, 74 resulting in strikingly contrasting models of supercontinent formation. Conceptual models 75 can be tested by quantitative geodynamic modeling and geochemical proxies. Specifically, 76 which oceans close to create a supercontinent, and how such predictions are linked to mantle 77 convection, are directions for future research. 78 79
... Shales from the Klein Naute Formation are thought to have been deposited ∼1,000 km away from those in the Mt. McRae Shale, either along the same ocean margin or at least connected to the same ocean (de Kock et al. 2009). Thus, it is possible using both sets of shales to track 2.5 Ga environmental conditions on an at least regional scale. ...
The rise of molecular oxygen (O 2 ) in the atmosphere and oceans was one of the most consequential changes in Earth's history. While most research focuses on the Great Oxidation Event (GOE) near the start of the Proterozoic Eon—after which O 2 became irreversibly greater than 0.1% of the atmosphere—many lines of evidence indicate a smaller oxygenation event before this, at the end of the Archean Eon (2.5 billion years ago). Additional evidence of mild environmental oxidation—probably by O 2 —is found throughout the Archean. This emerging evidence suggests that the GOE might be best regarded as the climax of a broader First Redox Revolution (FRR) of the Earth system characterized by two or more earlier Archean Oxidation Events (AOEs. Understanding the timing and tempo of this revolution is key to unraveling the drivers of Earth's evolution as an inhabited world—and has implications for the search for life on worlds beyond our own. ▪ Many inorganic geochemical proxies suggest that biological O 2 production preceded Earth's GOE by perhaps more than 1 billion years. ▪ Early O 2 accumulation may have been dynamic, with at least two AOEs predating the GOE. If so, the GOE was the climax of an extended period of environmental redox instability. ▪ We should broaden our focus to examine and understand the entirety of Earth's FRR.
Expected final online publication date for the Annual Review of Earth and Planetary Sciences, Volume 49 is May 2021. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.
... These dyke swarms have the potential to provide valuable constraints on the secular changes in Archean and Proterozoic mantle/lithosphere chemistry, and the paleogeographic position of the Archean nuclei of India in reconstructions of Precambrian supercontinents such as the Neoarchean Vaalbaara supercontinent (e.g. Wingate, 1998;de Kock et al., 2009;Kumar et al., 2017), the Paleoproterozoic Columbia supercontinent (e.g. Rogers, 1996;Meert, 2002;Zhao et al., 2004;Srivastava and Gautam, 2015;Srivastava et al., 2019), and the Meso-Neoproterozoic Rodinia supercontinent (e.g. ...
Mafic dykes and dyke swarms in continental settings provide information on the evolution of the subcontinental mantle and can be key elements in the reconstruction of paleo-geographic settings of now separated crustal terranes. This study focuses on the petrogenesis and geochronology of mafic dykes of the WNW (∼125°) trending Bhanupratappur swarm in the central Bastar Craton, central India. Dykes of the Bhanupratappur swarm yield an average U-Pb (ID-TIMS) baddeleyite age of 2360±4 Ma, which is interpreted as their emplacement age. The compositions of the dykes range from tholeiitic basalt to basaltic-andesite. Their rare earth element and multi-element patterns indicate the involvement of a crustal component in their petrogenesis. The whole rock initial ⁸⁷Sr/⁸⁶Sr2360 Ma ranges 0.70097 to 0.70506 with most being more radiogenic than the contemporaneous undifferentiated mantle reservoir (i.e. ⁸⁷Sr/⁸⁶Sr2360 Ma = 0.70173). The initial εNd 2360 Ma (+0.85 to -2.7) is chondritic to sub-chondritic. The Sr-Nd Isotope composition and major- and trace element chemistry suggest an enriched-heterogeneous mantle source. The closely matching ages and chemistry of the Bhanupratappur swarm (2360 Ma) and the Karimnagar-Bangalore swarms (2363-2369 Ma) of the Dharwar Craton indicate affinities to a common Large Igneous Province, which further implies that the Bastar and Dharwar cratons were already juxtaposed at 2.37-2.36 Ga. The dykes of the Bhanupratappur (WNW-trending) and Bangalore (E-W trending) swarms converge towards the east indicating a plume center in the east. If the Karimnagar swarm was also linked (and was converging) to the same plume, the present-day mismatch in the orientations of the Karimnagar dykes (NE- to ENE-trending) with the Bangalore and Bhanupratappur dykes may indicate a ∼55° counterclockwise rotation of the northern block of the Eastern Dharwar Craton with respect to the southern block after 2.37-2.36 Ga.
... Published ages are from: (1) Martin et al. (1998), Nelson et al. (1999), (3) Hannah et al. (2004); (4) Rasmussen et al. (2013); (5) Cornell et al. (1996), (6) on the KC (Griqualand West and Kanye basins, Fig. 1) and on other cratons worldwide. Based on available information from stratigraphy, paleomagnetic studies and detrital zircons, it is suggested that the KC was attached to the Pilbara Craton during the Archean to Paleoproterozoic, assembled in the continent Vaalbara (e.g., Cheney, 1996;de Kock et al., 2009de Kock et al., , 2102Kampmann et al., 2015;Zeh et al., 2016;Wilson and Zeh, 2018), whereas other authors see no clear indication for such a connection (e.g., Nelson et al., 1999;Wingate, 1998;Strik et al. 2007;Nelson, 2008;Eriksson et al. 2011). Zeh et al. (2016) furthermore suggested that clastic sedimentary rocks of the Magaliesberg Formation of the uppermost Pretoria Group were derived from two major sources on and off the KC; from the Archean Pietersburg Block, which limits the Transvaal Basin to the north, and possibly also from the Ophthalmia orogen, which was formed by collision between the southern margin of the Pilbara Craton in W-Australia (present day coordinates) with the Glenburgh terrane (to the south of the Pilbara craton) between 2.215 Ga and 2.145 Ga (Rasmussen et al., 2005;Martin and Morris, 2010;Johnson et al., 2011). ...
The Transvaal Basin in South Africa hosts a 15 km thick pile of sedimentary successions deposited over a period of more than 600 Ma during the Neoarchean to Paleoproterozoic. Presently, little is known about the source of these sediments, as well as about the tectono-magmatic evolution in the hinterland of the Transvaal Basin, preventing detailed geotectonic correlations of the Kaapvaal Craton (KC) with other cratons worldwide. To solve this problem, we present the first systematic study of combined U-Pb and Lu-Hf isotope data of more than 2000 detrital zircons from fourteen formations of the Transvaal Supergroup. These reveal that clastic sedimentary rocks were supplied from sources on and off the present-day KC. Detrital zircons in conglomerates of the Wolkberg and Black Reef formations, maximum deposition ages at 2769±8 and 2618±11 Ma respectively, were mainly supplied from surrounding KC, either from Pietersburg Block Basement (PBB), and/or from eroded sedimentary successions of the Witwatersrand, Pongola and/or Ventersdorp Supergroups. In contrast, clastic sedimentary rocks of the Rooihoogte, Duitschland and Timeball Hill formations (maximum deposition ages at 2353±18 Ma, 2342±18 and 2290±8 Ma, respectively) were predominately supplied from a juvenile Neoarchean terrane (JUNAT) formed at 2570-2500 Ma (εHf2500 Ma = +2 to +9) and intensely reworked at 2400, and to a minor amount from a composite Archean terrane (CAT) emplaced by granitoids between 3540 and 2680 Ma, and affected by crust reworking at 2570-2430 Ma (εHf2.5Ga = -3 to -12) in a Neoarchean to Paleoproterozoic continental arc terrane (NPCAT). Subsequent periodic reworking of JUNAT at 2250-2220 Ma and 2120 Ma is recorded by detrital zircons in sandstones of the overlying Boshoek, Dwaalheuwel, Daspoort, Magaliesberg and post-Magaliesberg formations, having maximum deposition ages at 2243±7, 2242±7, 2240±7, 2080±7, and 2068±7 Ma, respectively. The Archean zircons (age >2650 Ma) in all these formations were mainly supplied from PBB. The new data sets also suggest that the KC was connected to CAT, NPCAT and JUNAT at <2350 Ma. The nearly absence of detrital zircons with ages of 2570-2500 Ma in all formations younger than Boshoek perhaps results from intense reworking of JUNAT during magmatic events at 2400, 2340, 2220, and 2120 Ma, causing loss of the original juvenile character. Paleoproterozoic zircons with ages of 2220 and 2120 Ma in Dullstroom sandstones most likely result from re-deposition of post-Magalisberg sedimentary rocks, and Archean zircons from sources similar to Moodies and Fig Tree sandstones of the Barberton greenstone belt. Comparison of our new data from the Transvaal Basin with such from the Turee Creek and Horseshoe basins in NW-Australia provides no evidence for Kaapvaal-Pilbara Craton connection during the Neoarchean to Paleoproterozoic.
... Ga depositional age of the volcano-sedimentary successions of the Sargur Group, Dharwar craton (Jayananda et al. 2008(Jayananda et al. , 2018, the Sonakhan Greenstone belt of Bastar craton (Das et al. 2009;Saha et al. 2000), and the Iron Ore Group of Gorumahisani-Badampahar area of Singhbhum craton Mukhopadhyay et al. 2008). The stratigraphic history and the paleomagnetic data, the latter as yet unknown, of the Neoarchean-Paleoproterozoic greenstone belts from peninsular India might throw some light on India's participation during the formation of the Vaalbara supercontinent (de Kock et al. 2009). The recently reported 3.2-3.5 ...
The term supercontinent generally implies grouping of formerly dispersed continents and/or their fragments in a close packing accounting for about 75% of earth’s landmass in a given interval of geologic time. The assembly and disruption of supercontinents rely on plate tectonic processes, and therefore, much speculation is involved particularly considering the debates surrounding the applicability of differential plate motion, the key to plate tectonics during the early Precambrian. The presence of Precambrian orogenic belts in all major continents is often considered as the marker of ancient collisional or accretionary sutures, which provide us clues to the history of periodic assembly of ancient supercontinents. Testing of any model assembly/breakup depends on precise age data and paleomagnetic pole reconstruction. The record of dispersal of the continents and release of enormous stress lie in extensional geological features, such as rift valleys, regionally extensive flood basalts, granite-rhyolite terrane, anorthosite complexes, mafic dyke swarms, and remnants of ancient mid-oceanic ridges.
Although Hadean rocks are missing, detrital zircons from Australia and India suggest that Earth’s continental crust and water existed as early as 4.4 Ga. Molecular phylogeny and the record of biogenic carbon indicate that life might have existed in the Hadean during the heavy bombardment period. The oldest crusts are exclusively Eoarchean, including the Nuvvuagittuq Craton of Canada, the Isua Craton of Greenland, the Kaapvaal Craton of South Africa, the Pilbara Craton of Australia, and the Singhbhum Craton of India. The volcano-sedimentary rocks of the greenstone facies of these five cratons have yielded a rich record of early life in hydrothermal vent environments in chemofossils, microfossils, and stromatolites. During the Eoarchean, Earth had oceans, continents, and an anoxic atmosphere. The oldest microfossils (≥4 Ga) are known from the Nuvvuagittuq Craton of Canada in the form of delicate tiny filaments and tubes in jasper-carbonate banded iron formations (BIFs) in the submarine hydrothermal vent environment, indicating an affinity toward hyperthermophilic bacteria. Isua rocks of Greenland have yielded chemofossils in the form of biogenic carbon (~3.8 Ga) and stromatolites (3.7 Ga) of possible bacterial origin. The close stratigraphic correlation between the Onverwacht Group of the Kaapvaal Craton in South Africa and the Warrawoona Group of the Pilbara Craton in Western Australia suggests that they were once part of the ancient supercontinent Vaalbara (3.6–2.8 Ga). The hydrothermal volcano-sedimentary rocks from Vaalbara have yielded the oldest and best-preserved early Archean microfossils in chert beds. The Pilbara Supergroup of Australia consists of the Warrawoona and Kelly Groups. Three sequences within the Warrawoona volcaniclastic rocks may host the evidence for Earth’s earliest life. From the oldest to the youngest, these formations represent the Dresser Formation (~3.49 Ga) at the bottom, the Apex Chert (~3.46 Ga) in the middle, and the Strelley Pool Formation (~3.43 Ga) at the top. Hyperthermophilic bacteria and archaea were the major components of ancient microbial activity, as evidenced by carbonaceous remains and fragmentary remains of cell walls from the hydrothermal black cherts of the Warrawoona. The fossil record suggests that two domains of life were already split from the last universal common ancestor (LUCA) during this time. The chemofossils from the Jack Hills of Western Australia reinforce the view that life originated on Earth at least 300 million years earlier in the Hadean Eon. The Kaapvaal Craton of South Africa is another extraordinary storehouse for the earliest record of life, such as primitive hyperthermophilic bacteria and archaea from the hydrothermal cherts sandwiched between pillow lavas. The Barberton greenstone belt of the Kaapvaal Craton of South Africa represents the oldest known volcano-sedimentary sequences that have provided critical evidence of early life. The Swaziland Supergroup is divided into three distinct units: the basal volcano-sedimentary Onverwacht Group, the middle sedimentary Fig Tree Group, and the top Moodie Group, all containing rare archives of early life (~3.5 Ga). The Iron Ore Group (IOG) of the Singhbhum Craton of eastern India has yielded spheroidal and filamentous microfossils of hyperthermophilic affinity (~3.4 Ga). Fossil records from these ancient cratons suggest that hyperthermophilic bacteria and archaea appeared in the early Archean about four billion years ago. This was followed by the evolution of anoxygenic photosynthetic bacteria and then oxygenic bacteria. The arrival of oxygenic photosynthesis allowed life to escape the hydrothermal setting and invade an utterly new environment—the broad continental shelves of the global ocean. Cyanobacteria contributed to the geological processes by providing vast amounts of carbonate sediments and stromatolitic structures in the shallow seas; they formed oxygen as a byproduct, transforming the ocean and the atmosphere around 3.2 billion years ago and triggering an explosive evolution. This development led to the origin of eukaryotes.
Recent evidence for astronomical-induced cycles in banded iron formations (BIFs) hints at the intriguing possibility of developing astrochronological, i.e. precise time-stratigraphic, frameworks for the earliest Proterozoic as also reconstructed for parts of the Mesozoic and Paleozoic. The ca 2.47-Ga Kuruman Iron Formation (Griqualand West Basin, South Africa) and Dales Gorge Member of the Brockman Iron Formation (Hamersley Basin, Western Australia) are of special interest in this regard, given their inferred temporal overlap and similar long-period eccentricity imprint. This suggests that these two BIFs may be correlated on the basis of their large-scale cycle patterns and using additional radio-isotopic age constraints.To examine the possibility of establishing such a framework, we generated and analysed several high-resolution proxy records from both drill-core and outcrop, combined with chemical abrasion ID-TIMS U–Pb dating of presumed volcanically sourced zircon. Time-series analysis of these records yields a variety of spectral peaks, of which a prominent ~5 m and ~16 m cycle can be linked to the basic stratigraphic alternations and bundling. New and improved U–Pb ages of the Dales Gorge Member and Kuruman Iron Formation, respectively, indicate a comparable average sedimentation rate of 10–12 m/Myr for both BIF units. Based on this rate, we attribute the ~5 m cycle to the long 405-kyr eccentricity cycle. More tentatively, we interpret the ~16 m cycle as the very long 2.4-Myr eccentricity cycle, having a reduced period of ~1.3 Myr due to chaotic behaviour in the solar system. Other identified cycles (~580 kyr, ~700 kyr and ~1.8 Myr) might be explained in terms of weaker eccentricity components and/or as harmonics and combination tones of these cycles.An initial attempt to establish cyclostratigraphic correlations between the Kuruman Iron Formation and Dales Gorge Member solely based on their characteristic cycle patterns proved unsuccessful, which may be due to a difference in stratigraphic recording of the astronomical signal between their different depositional environments. Next, we used the U–Pb ages to first constrain correlations at the scale of the ~16 m cycle, followed by a correlation of the basic ~5 m cycles. The resultant framework remains problematic and debatable at the individual 405 kyr cycle-level, and should merely be considered as a starting point for future studies. Particularly, our findings highlight the need for further investigations into how Milankovitch forcing influenced BIF sedimentation and paleoenvironmental conditions at a time when the Earth and solar system behaved fundamentally different from today.
Here, we discuss the role of the main South American cratonic units in the Columbia and Rodinia supercontinents, and Gondwana megacontinent. According to paleomagnetic and geological data Amazonia and West Africa were linked to Baltica, Laurentia and Siberia forming West Columbia at ca. 1.78-1.75 Ga. The 1.78 to 1.42 Ga paleomagnetic data for Amazonia, Baltica and Laurentia suggest either, that West Columbia preserved its integrity, at least, up to 1.42 Ga, or Amazonia/West Africa broke-up from West Columbia at some time between 1.53 and 1.42 Ga.. On the other hand, the Congo/São Francisco, North China, Rio de la Plata, India and proto-Australia formed the East Columbia at ca. 1.78 Ga. However, the presently available Paleo to Mesoproterozoic paleomagnetic data for these cratonic blocks suggest that East Columbia was short-lived. At 1.1 Ga ago, Amazonia/West Africa, Congo-São Francisco, Kalahari and India probably formed a megacontinent that later collided with Laurentia and Baltica forming Rodinia at ca. 1.0 Ga. Most probably, Rodinia broke-up at ca. 750 Ma, when Congo/São Francisco, Kalahari and other smaller blocks rotated ca. 90° counterclockwise, closing the Brasiliano/Clymene ocean and docked against Amazonia/West Africa and Rio de la Plata at ca. 600-570 Ma ago forming West Gondwana.
Plate tectonics is a fundamental factor in the sustained habitability of Earth, but its time of onset is unknown, with ages ranging from the Hadaean to Proterozoic eons 1–3 . Plate motion is a key diagnostic to distinguish between plate and stagnant-lid tectonics, but palaeomagnetic tests have been thwarted because the planet’s oldest extant rocks have been metamorphosed and/or deformed ⁴ . Herein, we report palaeointensity data from Hadaean-age to Mesoarchaean-age single detrital zircons bearing primary magnetite inclusions from the Barberton Greenstone Belt of South Africa ⁵ . These reveal a pattern of palaeointensities from the Eoarchaean (about 3.9 billion years ago (Ga)) to Mesoarchaean (about 3.3 Ga) eras that is nearly identical to that defined by primary magnetizations from the Jack Hills (JH; Western Australia) 6,7 , further demonstrating the recording fidelity of select detrital zircons. Moreover, palaeofield values are nearly constant between about 3.9 Ga and about 3.4 Ga. This indicates unvarying latitudes, an observation distinct from plate tectonics of the past 600 million years (Myr) but predicted by stagnant-lid convection. If life originated by the Eoarchaean ⁸ , and persisted to the occurrence of stromatolites half a billion years later ⁹ , it did so when Earth was in a stagnant-lid regime, without plate-tectonics-driven geochemical cycling.
This study explored the relationships between organizational justice perception and commitment among shipyard employees in a maritime organization in Türkiye. Data were collected using two-scale and sociodemographic questionnaires that were answered via email or hand by 290 participants who were shipyard workers in Antalya, Türkiye. The questionnaire included the organizational justice scale consisting of 20 questions, the organizational commitment scale consisting of 18 questions, and 7 questions about demographic characteristics. To analyze the data, IBM SPSS 26 was used. A simple linear regression analysis technique was used to determine the effects between scales. Moreover, interviews were conducted with the employees using the semistructured interview method. It was determined that there was a positive, high-level relationship between organizational justice scale subdimension scores and total scores and this relationship was statistically significant. It was also shown that there was a positive, low-level relationship among the organizational commitment subdimensions' scores of the employees and that the relationship was statistically significant. Regression analysis identified that the organizational justice scale total scores of the employees had a statistically remarkable impact on the organizational commitment scale total scores.
Previous investigations of the northern Pilbara Craton are briefly summarized, followed by an outline of the region’s lithostratigraphy and major tectonic units. Previous interpretations of its tectonic evolution have not taken account evidence that the presently preserved 500,000 km2 Pilbara Craton is composed of fragments of much larger Paleoarchean and Mesoarchean continents. This consideration provides important new insights on the original scales of the processes and tectonic units that existed before two major events of continental breakup.
Following deformation and magmatic activity of the 3325–3290 Ma Emu Pool Event (Chap. 5), deposition of the Sulphur Springs Group marked the beginning of crustal extension and rifting that led to the continental breakup of the Pilbara Craton. The extension and rifting are attributed to the arrival of the last major mantle plume to impact the Pilbara Craton. Melting of the mantle and crust resulted in an ultramafic–mafic–felsic volcanic cycle in the Sulphur Springs Group and the intrusion of granitic rocks of the 3274–3223 Ma Cleland Supersuite. The Sulphur Springs Group and the Fig Tree Group of the eastern Kaapvaal Craton are transitional successions from Paleoarchean large igneous provinces to Mesoarchean sedimentary basins. Deposition of the Sulphur Springs Group ended with breakup of the Pilbara Craton at c. 3220 Ma. The breakup was followed by the separation of at least three plates of continental crust and the evolution of intervening basaltic basins. It marked the beginning of plate tectonic processes in which Paleoarchean vertical deformation and crustal recycling were replaced by Mesoarchean horizontal deformation and melts derived from plate separation, collision, and subduction.
At 2775 Ma, the Neoarchean crust of the Pilbara Craton began to be extended and rifted resulting in the widespread eruption of basaltic lavas. Between c. 2775 and 2710 Ma, mafic–felsic volcanic and intrusive activity continued in stages that were separated by periods of uplift, folding, erosion, and sedimentation. The first basaltic formation deposited across the eroded surface of the craton was the Mount Roe Basalt, up to 2.44 km thick and fed from dolerite dykes intruded into extensional fractures; this was the first regionally extensive formation of the Fortescue Group. Deformation and erosion of the Mount Roe Basalt were followed by clastic deposition and felsic volcanism and intrusion of the 2766–2749 Ma Hardey Formation. The stratigraphic nomenclature of the Fortescue Group from 2749 Ma onwards differs between the North and South Pilbara. Even so, the same magmatic events affected both areas. Almost all volcanic activity ended at c. 2710 Ma following eruption of the Maddina Formation of the North Pilbara (correlated with the Bunjinah Formation in the south). Between c. 2710 and 2630 Ma, mainly clastic sedimentary rocks of the Jeerinah Formation, the upper formation of the Fortescue Group, were deposited in both areas. Because the stratigraphy and sedimentology of the Jeerinah Formation indicates passive margin basin deposition, it is interpreted that extension and rifting of the Pilbara Craton culminated in continental breakup and plate separation at c. 2710 Ma. Most workers have interpreted the mainly volcanic 2775–2710 Ma lower Fortescue Group as a large igneous province formed by one or more mantle plumes. A mantle plume origin is consistent with the crustal extension and rifting of the Pilbara Craton, the continental breakup, and the stratigraphy of the volcanic succession that includes ultramafic–mafic–felsic volcanic cycles.
Unconformably overlying thick continental crust, the Kelly Group comprises three formations, in ascending stratigraphic order: the 3350–3335 Ma Euro Basalt, up to 9 km thick, and composed of komatiite, basaltic komatiite, and tholeiite; the 3325–3315 Ma Wyman Formation, up to 2 km thick, and composed of rhyolite flows and subvolcanic rhyolite intrusions; and the undated Charteris Basalt, up to 2 km thick, and containing komatiite, basaltic komatiite, and tholeiite. With an average stratigraphic thickness of 4 km, and erupted across at least 100,000 km2 of the Pilbara Craton, the Euro Basalt forms the main part of a Paleoarchean large igneous province, the Kelly LIP. The plume-related ultramafic–mafic–felsic volcanic cycle that commenced with eruption of the Euro Basalt ended with eruption of the felsic volcanics of the Wyman Formation. However, unlike the Euro Basalt the Wyman Formation is restricted to the eastern half of the East Pilbara Terrane and was derived from partial melting of older felsic crust. Eruption of the Wyman Formation was accompanied by numerous granodiorite and monzogranite intrusions of the Emu Pool Supersuite. Geochronology indicates that in some of the East Pilbara domes there was a ten-million-year interval between eruption of the Euro Basalt and Wyman Formation, during which time some parts of the Euro Basalt were folded and eroded. The undated Charteris Basalt is lithologically similar to the Euro Basalt and might form part of a second volcanic cycle.
—The time span between 3 and 2 Ga in the geologic history encompassed a number of key events on the cooling Earth. The cooling interrupted heat transfer within and across the mantle, which caused changes in Earth’s major spheres and in the mechanisms of their interaction. The great thermal divergence at 2.5 Ga and differentiation into the depleted upper asthenospheric and primitive lower mantle affected the compositions of oceanic basalts. The lower mantle cooling recorded by a systematic decrease in the temperature of komatiite magma generation at the respective depths began at 2.5 Ga and was accompanied by increasing abundance of arc basalts and by changes in the behavior of the Sr, Nd, and O isotope systems. It was the time when the continental lithosphere consisting of subcontinental lithospheric mantle and crust began its rapid growth, while the crust became enriched in felsic material with high contents of lithophile elements. Magmatism of the 3–2 Ga time span acquired more diverse major-element chemistry, with calc-alkaline and alkaline lithologies like carbonatite and kimberlite. The dramatic changes were driven by subduction processes, whereby the crust became recycled in the mantle and the double layer (D”) formed at the core–mantle boundary. The events of the 3–2 Ga interval created prerequisites for redox changes on the surface and release of free oxygen into the atmosphere. In terms of global geodynamics, it was transition from stagnantlid tectonics to plate tectonic regime, which approached the present-day style about 2.0–1.8 Ga.
The existence of Vaalbara, the combined Neoarchaean to Palaeoproterozoic Kaapvaal-Pilbara supercraton, is questionable during the early Neoarchaean when scrutinised through the lens of recent Australian and South African palaeomagnetic data. Remarkably similar ~2.7 to 2.5 Ga geological successions (with near bed-for-bed correlatability) support a coherent Vaalbara at the end of the Neoarchaean. Here we report palaeomagnetic and rock magnetic results from the Klipriviersberg Group of South Africa, which is the oldest rock sequences used to define Vaalbara originally. A positive syn-fold test illustrated a high-temperature remanence component acquired during the formation of the Witwatersrand syncline. This fold structure predates the Vredefort Impact Structure and its formation is synchronous with the deposition of the Mesoarchaean Central Rand Group and extrusion of the Klipriviersberg Group. The studied rocks of the Klipriviersberg Group are not directly dated, but most are likley younger than 2 780 to 2 789 Ma, based on detrital zircon ages from the lowermost Ventersdorp Supergroup and U-Pb baddeleyite ages for mafic sills that intrude the Witwatersrand Supergroup that are regarded as feeders of the Kliprivierberg Group lavas, but older than the overlying 2 720 to 2 750 Ma Platberg Group. The Klipriviersberg Group pole is at 27.7°S, 32.7°E with an A95 of 11°. A comparison of Meso- to Neoarchaean palaeopoles from the Kaapvaal and Pilbara cratons suggests their shared drift path traversing the polar circle and thus supports the existence of Vaalbara across the 2.78 to 2.70 Ga interval.
The inclination of Planet Earth’s axis of rotation by 23½° resulted in extreme climatic changes. Weak, solar radiation upon the Polar Circles during half-a-year alternated with half-a-year of darkness, turning them into freezing terrains for nearly the whole Precambrian. Exhalant hydrothermal solutions formed huge lakes over Polar Regions, undergoing intensive evaporation and condensation. Chemical interactions incidentally created primitive live forms, surviving as chemoautotrophic bacteria under the weakest UV rays. Daily changing solar radiation emitting UV rays over low latitudes prevented any life form. Some of these polar bacteria developed photosynthesis, improving their nourishment simultaneously releasing oxygen. The high content of ferrous iron in the lakes absorbed toxic oxygen forming iron oxides as banded iron formation (BIF). Excess photosynthetic oxygen molecules escaped into the anoxic atmosphere. At ~ 1.8 Ga oxygenated meteoric water infiltrated the continental subsurface oxidizing hydrothermal fluids, precipitating underground layered iron-oxides followed by silica only during the dry summers. The dilution of the rising solutions terminated biologically induced BIF precipitation. Consequently, intensive evaporation cemented sililiciclasts into stratified ferruginous formations, becoming abundant in the Late Neoproterozoic as NIF. The co-occurrence of Paleoproterozoic BIF and Neoproterozoic NIF sites evidence the tectonic and climatic stability of the Polar Circles. Magmatic convection currents split them ~ 750 Ma ago, but only after 580 Ma shifted the individual plates radially to low latitudes with advance of ‘Plate Tectonics’. The polar bacteria connected with the open sterile sea for the first time had to adapt to daily changing ecosystems by combining into mobile primitive eukaryotes (Ediacaran Biota) and further diversify, erroneously referred to the ‘Cambrian Explosion’. Geological evidence corroborated the activity of convection currents since Earth’s consolidation controlling its inner heat budget and supplying hydrothermal solutions forming lakes on the Polar Circles where life originated and iron ores accumulated.
Mineral systems with their core of ore deposits require a rare conjunction of geodynamic settings, crustal or lithospheric fertility, crustal architecture and suitable host rocks, and presentation potential. They are thus an integral component of Earth’s thermal and tectonic evolution which also control the supercontinent cycles with progressive assembly and breakup such as those of Ur, Kenorland, Columbia, Rodinia, Gondwana, and Pangea. Despite the ongoing debate, some form of plate tectonics has operated on Earth since the Eoarchean. However, the hotter Archean mantle generated a long-term double-layered convection system which was disrupted by episodic mantle overturns, with the largest in the early Neoarchean potentially enriching the mantle in metals that form the Earth’s core. Cratons with thick subcontinental mantle lithosphere (SCLM) or tectosphere keels commenced to form in the Mesoarchean as small continents amalgamated. The conjunction of pre-4.0Ga crust, giant Ti, Cr, Fe, Ni and PGE-enriched layered mafic intrusions and major diamond fields provide strong evidence that the Kaapvaal and Zimbabwe Cratons and Wyoming Craton formed part of Ur with its early potentially core-metal-fertilized SCLM. Orogenic gold deposits and VMS Cu-Zn-Pb deposits with their high preservation potential were deposited in subduction-related convergent margins that activated the assembly of all supercontinents with giant provinces related to assembly of Kenorland, Columbia and Gondwana-Pangea. Erosion-susceptible porphyry Cu-Au and epithermal Au-Ag deposits were most abundant at the time of Gondwana and Pangea and in Cenozoic convergent margins and collisional orogens, although there are rare examples associated with assembly of all supercontinents. Magmatic intrusion-related Ni-Cu-PGE, and magmatic-hydrothermal IOCG Cu-Au and Kiruna-type Fe-P deposits formed near craton margins. However, although giant Ni-Cu-PGE deposits formed during the breakup of all supercontinents, giant IOCG deposits were largely restricted to extensional episodes related to Kenorland and Columbia and Kiruna-type deposits to those involved in Columbia. The evolution of the Earth’s atmosphere-hydrosphere-biosphere was an additional influence on that of the supercontinent cycle in terms of the evolution of metallogenic provinces. The Great Oxidation Event (GOE) at ca. 2.4-2.0 Ga witnessed the end of the great era of deposition of BIFs that became the hosts to high-grade Fe and Mn deposits which formed under more oxidizing conditions, with Oligocene sediment-hosted Mn deposits and late-Cenozoic Mn nodules becoming the dominant Mn resources and potential resource, respectively. The GOE was also responsible for the evolution of U deposits from the Mesoarchean paleoplacer uraninite deposits of the Witwatersrand, through Mesoproterozoic unconformity-related deposits to Phanerozoic sandstone roll deposits. The Cambrian ‘explosion of life’, following a second GOE event, magnified the importance of organisms, particularly those secreting Ca and Mg, carbonate in the formation or ore deposits in sedimentary basins. Late Paleoproterozoic-Mesoproterozoic shale-hosted SEDEX Zn-Pb-Cu deposits were progressively replaced by Phanerozoic carbonate-hosted MVT Pb-Zn deposits and Neoproterozoic-Cambrian Zambian-type Cu-Co deposits hosted in calcareous sedimentary sequences. Carlin-type Au-Ag deposits hosted by calcareous and carbonaceous sequences appeared in the Cretaceous to Paleogene epochs to rival the more ubiquitous orogenic gold deposits in terms of global importance. It is evident that the evolution of the great metallogenic belts of the Earth was intrinsically linked to the thermal and tectonic evolution of the Earth and particularly to plate tectonics and the supercontinent cycles. The nature of contained mineral deposits of elements with multiple valency states and those requiring particularly reactive host rocks was strongly influenced by the evolution of the atmosphere-hydrosphere-biosphere system.
Sedimentary rocks and minerals formed during the final two-hundred million years of the Archean Eon (2.7 to 2.5 billion years ago, or Ga) are more depleted in ⁵⁶Fe than at any other time in Earth’s past. Three hypotheses are proposed to explain these ⁵⁶Fe depletions: (1) a very negative late-Archean seawater δ⁵⁶Fe value, (2) “shuttling” of isotopically light Fe across the chemocline in redox-stratified settings, and (3) pyrite formation in an Fe(II)-rich ocean. Each of these scenarios has different implications for the initial oxidation of Earth’s surface, the climax of which – the Great Oxidation Event – immediately post-dates the appearance of these ⁵⁶Fe depletions in the rock record. To help inform this debate, we measured the Fe isotope ratios of 120 shale and pyrite samples from Western Australia (Mt. McRae Shale and Jeerinah Formation) and South Africa (Klein Naute Formation) deposited between ∼2.65 Ga and ∼2.50 Ga. As in previous studies, we also find very strong sedimentary ⁵⁶Fe depletions, to as low as δ⁵⁶Fe = −2.06 ± 0.08‰ in bulk shales and δ⁵⁶Fe = −2.31 ± 0.08‰ in pyrite. Some, but not all, of the severest ⁵⁶Fe depletions appear alongside evidence of an Fe shuttle and local pyrite formation. These processes need not be mutually exclusive, and some combination of them likely played a partial, probably faciliatory role in driving some strong ⁵⁶Fe depletions in our dataset. Most interestingly, and with little exception, the severest ⁵⁶Fe depletions appear in samples deposited farther from shore under H2S-rich and anoxic (“euxinic”) conditions. We find it difficult to explain this connection without invoking the persistent presence of a very negative global seawater δ⁵⁶Fe value during the latest Archean, one that was most consistently captured in sediments formed in distal euxinic settings. In order to impart this isotopic effect on seawater, the global seawater Fe(II) reservoir needed to have been partially oxidized during at least the final few hundreds of millions of years leading up to the Great Oxidation Event. Our new data add support to the idea that Earth’s initial oxidation was a long and protracted process rather than a rapid event.
We propose a Precambrian megacraton (consisting of two or more ancient cratons), DHABASI in the Indian Shield, which includes the Dharwar, Bastar and Singhbhum cratons. This interpretation is mainly based on seven large igneous provinces (LIPs) that are identified in these three cratons over the age range of c. 3.35–1.77 Ga, a period of at least 1.6 Ga. In addition to their use in recognizing. We suggest that the megacraton DHABASI was an integral part of supercontinents/supercratons through Earth’s history, and that it should be utilized as a distinct building block for palaeocontinental reconstructions
rather than using the individual Dharwar, Bastar and Singhbhum cratons.
Several different Neoarchean–Paleoproterozoic supercontinents or supercratons have been proposed, including Kenorland, Protopangea, Vaalbara, Superia, Supervaalbara, Sclavia, and Nunavutia. We used high-quality paleomagnetic data and an updated magmatic record to test these various cratonic reconstructions. Based on these analyses, we suggest that a Vaalbara configuration might be possible through at least part of the Neoarchean–Paleoproterozoic transition, contradicting recent suggestions. We also propose a modified Superia reconstruction with a looser fit of the Karelia–Kola and Superior cratons than the original Superia configuration. Disagreement between the paleomagnetic poles, different drift velocities, and the latitudinal positions of Superia and Kaapvaal at 2.7–2.2 Ga and Superia and Nunavutia at 2.4–2.2 Ga indicate that these supercratons were separate, negating a single Neoarchean–Paleoproterozoic supercontinent and the proposed Supervaalbara configuration, and thus also arguing against the existence of a full-fledged Kenorland landmass during that interval of time. This also argues against stagnant-lid tectonics during the Archean–Paleoproterozoic transition. In addition, drift velocities at 2.4–2.2 Ga that are in the range of current plate motions contradict the proposed tectono-magmatic shutdown or a tectono-magmatic lull in the Paleoproterozoic.
We present a compilation of reliable Precambrian paleomagnetic poles from three successive international workshops (in years 2009, 2014, 2017), comprising paleomagnetists specializing in Precambrian tectonic reconstructions. The working groups compiled lists of two global classes of poles, published through the end of 2017.
“Grade-A” results are judged to provide essential constraints on tectonic reconstructions; “Grade-B” poles are judged to be suggestive of high-quality, but not yet demonstrated to be primary, or perhaps lacking precise geochronologic or other constraints. Our catalog documents a resurgence of high-quality data acquisition in recent
years, and highlights specific cratons and time intervals that are most lacking in the data needed to reconstruct those blocks through supercontinental cycles.
The East European Craton is a collage of Early Precambrian crustal fragments including Fennoscandia, Sarmatia, and Volga-Uralia, which are welded by Palaeoproterozoic collisional orogens. Here, we present a detailed overview of the sedimentary basins in Sarmatia that incorporate giant belts of banded iron formations (BIFs) and are therefore important in understanding the geological history and global correlations during the Archean-Proterozoic transition. Among the two sedimentary basins in Sarmatia (Mikhailovsky and Tim-Kryvyi Rih), the Mikhailovsky Basin is characterized by the presence of a carbonate platform underlying BIFs. The BIFs are locally overlain by thin clastic deposits. Thick-bedded dolomites occur with BIF in the Tim Kryvyi Rih Basin. In the Mikhailovsky Basin, after their deposition there was a long-lasting hiatus. In the Mikhailovsky Basin, there are no sedimentary rocks after the regional hiatus except for glacial deposits. Sedimentation resumed with the development of continental rift-related structures, where the accumulation of terrigenous sediments was accompanied by, and culminated with, outflows of basalts at 2.1 Ga. A detailed evaluation of the history of sedimentary basins in Sarmatia record transgression (~2.6–2.4 Ga) with the accumulation of giant BIFs (~2.50–2.45 Ga), regression (~2.4–2.2 Ga), hiatus and glaciations (~2.4–2.2 Ga), and rift-related volcanism (~2.2–2.1 Ga). We attempt a correlation of the sedimentary sequences in Sarmatia with those of Pilbara, Kaapvaal, and São Francisco cratons which show that the geological events on all these cratons were similar during 2.6–2.4 Ga. We thus propose that the Sarmatia Craton may serve as a link in the palaeocontinental correlations of the Vaalbara Supercraton and the São Francisco Craton, based on the striking similarity in the Neoarchean-Early Palaeoproterozoic sedimentary basins.
The mode and rates of tectonic processes and lithospheric growth during the Archean [4.0 to 2.5 billion years (Ga) ago] are subjects of considerable debate. Paleomagnetism may contribute to the discussion by quantifying past plate velocities. We report a paleomagnetic pole for the ~3180 million year (Ma) old Honeyeater Basalt of the East Pilbara Craton, Western Australia, supported by a positive fold test and micromagnetic imaging. Comparison of the 44°±15° Honeyeater Basalt paleolatitude with previously reported paleolatitudes requires that the average latitudinal drift rate of the East Pilbara was ≥2.5 cm/year during the ~170 Ma preceding 3180 Ma ago, a velocity comparable with those of modern plates. This result is the earliest unambiguous evidence yet uncovered for long-range lithospheric motion. Assuming this motion is due primarily to plate motion instead of true polar wander, the result is consistent with uniformitarian or episodic tectonic processes in place by 3.2 Ga ago.
Banded Iron Formations (BIFs) are Precambrian sedimentary rocks interpreted to have been precipitated from anoxic seawater prior to the first permanent rise in atmospheric oxygen in the Great Oxidation Event at ca. 2.45–2.32 Ga. BIFs hold the key to understanding the chemistry of the oceans and atmosphere, and how these interacted with microbial life, prior to and during the evolution of oxygenic photosynthesis. To unlock this information, it is essential to know how BIFs formed, that is, what minerals were precipitated, by what mechanisms and in which environments. BIFs are found in almost all depositional settings at times when input of clastic detritus was lacking, for example, submarine proximal volcanic environments, basin floor, slope, deep marine shelf and shallow shelf settings. High-resolution electron microscopy of finely laminated BIFs and ferruginous cherts that range in age from 3.45 to ca. 2.4 Ga and that preserve depositional features indicates that the original sediment was a very fine mud composed of nanoparticles of the iron-silicate greenalite. In places, the greenalite mud experienced very early silicification on the sea floor enabling preservation of the mineral and its depositional textures. Very fine grained siderite and hematite in laminated BIFs post-date dehydration of the early silica cement and are not primary minerals. Greenalite nanoparticles are found in BIFs from all depositional settings indicating a common origin, likely precipitation resulting from mixing of plume water from hydrothermal vents with ambient seawater. The nanoparticles were carried throughout the oceans and were deposited on the seafloor and on continental margins to form the primary sediments of BIFs.
The stratigraphy within the Wittenoom Formation’s Paraburdoo Member of the Hamersley Basin in Western Australia has lacked a significant and identifiable marker bed until the recent discovery of the Paraburdoo Spherule Layer (PSL). In this paper, we correlate this stratigraphy residing at the three known exposed locations of the PSL within the Hamersley Basin and use these correlations in conjunction with detailed observations and analyses to assess the type of sedimentation recorded by these alternating ‘sand’ and ‘mud’ beds. We also characterize in detail these same several meters of stratigraphy using the PSL as a distinctive marker bed and add to the existing depositional model of the Hamersley Basin. We find that (1) the strata surrounding the PSL was not deposited by sediment gravity flows, but rather long-acting deep-marine bottom current(s), (2) the rhythmicity recorded as alternating ‘sand’ and ‘mud’ beds is the result of increasing and decreasing current velocities, (3) grain size decreases while mud content increases to the east within the basin, (4) bed thinning occurs to the west within the basin, (5) microbial activity may be recorded in laminations in the finer-grained mud beds present within these strata, and (6) the diagenetic history changes across the Hamersley Basin.
Banded iron formations of the ca. 2770-2405 Ma Hamersley province of Western Aus-tralia were locally upgraded to high-grade hematite ores during the Early Paleoprotero-zoic by a combination of hypogene and supergene processes after the initial rise of atmospheric oxygen. Ore genesis was associated with the stratigraphic break between the Lower and Upper Wyloo Groups of the Ashburton province, and has been variously linked to the Ophthalmian orogeny, late-orogenic extensional collapse, and anorogenic continental extension. Small-spot in situ Pb/Pb dating of baddeleyite by sensitive high-resolution ion microprobe (SHRIMP) has resolved the ages of two key suites of mafic intrusions, constraining for the first time the tectonic evolution of the Ashburton province and the age and setting of iron-ore formation. Mafic sills dated as ca. 2208 Ma were folded during the Ophthalmian orogeny and then cut by the unconformity at the base of the Lower Wyloo Group. A mafic dike swarm that intrudes the Lower Wyloo Group and has a close genetic relationship to iron ore is ca. 2008 Ma, slightly younger than a new syneruptive 2031 6 Ma zircon age for the Lower Wyloo Group. These new ages constrain the Ophthalmian orogeny to the period between ca. 2208 and 2031 Ma, before Lower Wyloo Group extension, sedimentation, and flood-basalt volcanism. The ca. 2008 Ma dikes present a new maximum age for iron-ore genesis and deposition of the Upper Wyloo Group, thereby linking ore genesis to a ca. 2050-2000 Ma period of continental extension similarly recorded by Paleoproterozoic terrains worldwide well after the initial oxidation of the atmosphere by ca. 2320 Ma.
[1] A late Archaean (circa 2775–2715 Ma) succession of terrestrial continental flood
basalts, mafic tuffs, felsic volcanic rocks, and clastic sedimentary rocks in the Nullagine
Synclinorium (and Meentheena Centrocline) of the East Pilbara Basin, Western Australia,
has been sampled for a palaeomagnetic study. Over 500 oriented, mostly basalt, drill cores
were collected from the supracrustal succession and associated dykes. Thermal and
alternating field demagnetization revealed two distinct components. Positive fold,
conglomerate, and reversal tests confirm that the primary natural remanent magnetization
(NRM) is still preserved. The secondary component is interpreted as the record of
remagnetization during a major thermal event, possibly in the Early Proterozoic. Analysis
of the primary NRM directions results in a magnetostratigraphy and an apparent polar
wander path (APWP) for the 60 Myr interval covered by the sampled succession.
Assuming a geocentric axial dipole during this time interval, the APWP shows that the
Pilbara Craton was drifting during the late Archaean and that drift rates probably varied
significantly. In particular, a mean 27.2� shift in palaeolatitude is recorded across an
unconformity that represents a relatively short time period and that marks a significant
change in basalt geochemistry. This study suggests that continents moved horizontally
during the late Archaean and that the rates of movement were significantly faster than in
the Phanerozoic. In addition, a reversed polarity interval, with a positive reversal test, is
recorded. We argue that it documents the oldest known geomagnetic reversals of the
geomagnetic field.
The early geological development of the Pilbara and Kaapvaal cratons has many features in common. Attempts have been made to correlate geologically similar features of the two cratons, and it has been postulated that they originated as contiguous components of a single continent, ‘Vaalbara’, during this time. The early geological histories of the Pilbara and Kaapvaal cratons are here compared in detail and the evidence that they were initially contiguous is assessed. These comparisons indicate significant differences in the chronologies of magmatic events within the granite–greenstone crusts of the Pilbara and Kaapvaal cratons. In addition, igneous correlatives emplaced during ca 2985 and 2782 Ma magmatic events on the Kaapvaal Craton have not been identified on the Pilbara Craton, and a well-defined 2760 Ma magmatic event, manifest as widespread emplacement of granitic rocks into the Pilbara granite–greenstone basement and eruption of flood basalts of the lower part of the Fortescue Group, is absent from the Kaapvaal Craton. Furthermore, similarities in first- and second-order transgression–regression cycles within the sedimentary supracrustal sequences may be attributable to global sea-level fluctuations, and thus may be irrevelant to the question of former contiguity. However, similarities in some aspects of the geological development of the Pilbara and Kaapvaal cratons imply that there were periods, extending for between 60 and 200 Ma, of the Archaean era during which the style of crust formation, intensity of volcanism and subaerial erosion, and magnitude of sea-level fluctuations may have varied on a global scale. Such similarities include the overall duration of formation of the granite–greenstone crusts from ca 3650 to 3100 Ma, the onset of craton-wide erosion in the interval ca 3125 to 3000 Ma, the major episodes of flood basaltic volcanism between 2760 and 2680 Ma, the predominance of chemical (carbonate and banded iron-formation) sedimentation between ca 2630 and 2440 Ma and the transition to widespread clastic sedimentation within the interval 2440 to 2200 Ma.
PbPb age determinations of zircons, using the evaporation technique, have been undertaken on three phases of the Kgale Granite (granite, granophyric granite and spherulitic microgranophyre) and on rhyolite of the Kanye Volcanic Formation in the area north of Mafikeng, western Transvaal, South Africa. The lithologies analysed are considered to be eastern extensions of the Gaborone Granite Complex, the bulk of which is located farther west, in Botswana.The zircon data indicate indistinguishable crystallisation ages close to the weighted mean date of 2780.6 ± 1.8 Ma for each of the units analysed, although metamict zircon populations in the granite phase of the Kgale Granite provide younger dates, suggesting Pb loss in its early geological history. The 2.78-Ga age is slightly lower than an previously published UPb zircon age of 2.83 Ga for the Gaborone Granite Complex in the Kubung area, Botswana.The zircon PbPb data emphasise the coeval nature of the various phases of the Kgale Granite and the Kanye Volcanic Formation and support cogenetic models for the origin of these rocks.
Paleomagnetic data from more than 250 samples of the lower part of the Waterberg Group (Nylstroom Subgroup) are reported in
order to reevaluate the apparent polar wander path (APWP) of the Kaapvaal craton during the Paleoproterozoic Era. Our study
broadly confirms the established APWP, but reveals some previously unidentified complexities in the time interval ~2.05 Ga
to ~1.87 Ga. A primary remanence direction from the lower part of the Swaershoek Formation provides a paleomagnetic pole of
(36.5° north, 051.3° east, K = 23.4, A95 = 10.9) for the Waterberg unconformity-bounded sequence I (WUBS-I) at ~2.05 Ga. A large shift in pole position takes place
from the lower Swaershoek into the upper Swaershoek Formation and overlying Alma Formation of Waterberg unconformity-bounded
sequence II (WUBS-II; pole at −10.5° north, 330.4° east, K = 25.0, A95 = 9.8), confirming the existence of a major unconformity and/or rapid continental motion within the span of Swaershoek deposition.
Banded iron formations of the ca. 2770 2405 Ma Hamersley province of
Western Australia were locally upgraded to high-grade hematite ores
during the Early Paleoproterozoic by a combination of hypogene and
supergene processes after the initial rise of atmospheric oxygen. Ore
genesis was associated with the stratigraphic break between the Lower
and Upper Wyloo Groups of the Ashburton province, and has been variously
linked to the Ophthalmian orogeny, late-orogenic extensional collapse,
and anorogenic continental extension. Small-spot in situ Pb/Pb dating of
baddeleyite by sensitive high-resolution ion microprobe (SHRIMP) has
resolved the ages of two key suites of mafic intrusions, constraining
for the first time the tectonic evolution of the Ashburton province and
the age and setting of iron-ore formation. Mafic sills dated as ca. 2208
Ma were folded during the Ophthalmian orogeny and then cut by the
unconformity at the base of the Lower Wyloo Group. A mafic dike swarm
that intrudes the Lower Wyloo Group and has a close genetic relationship
to iron ore is ca. 2008 Ma, slightly younger than a new syneruptive 2031
± 6 Ma zircon age for the Lower Wyloo Group. These new ages
constrain the Ophthalmian orogeny to the period between ca. 2208 and
2031 Ma, before Lower Wyloo Group extension, sedimentation, and
flood-basalt volcanism. The ca. 2008 Ma dikes pre s ent a new maximum
age for iron-ore genesis and deposition of the Upper Wyloo Group,
thereby linking ore genesis to a ca. 2050 2000 Ma period of continental
extension similarly recorded by Paleoproterozoic terrains worldwide well
after the initial oxidation of the atmosphere by ca. 2320 Ma.
The Gaborone granite suite and the Kanye Formation formed during a
single magmatic event in the late Archean evolution of the Kaapvaal
craton, southern Africa, and may be the oldest rapakivi
granite-anorthosite-rhyolite suite in the world. The Gaborone granite
suite underlies an area of >6000 km2 in the northwestern
part of the craton and comprises A-type rapakivi granite, leucogranite,
granophyric microgranite, and minor anorthosite. It is partly surrounded
by the Kanye Formation, a 1000-m-thick pile of pyroclastic and
flow-banded rhyolitic lava. Precise U-Pb dating of granitic and
granophyric components of the Gaborone suite and rhyolite of the Kanye
Formation shows that all three rock types are the same age within error
(2783-2785 Ma). Emplaced in the source area during
upper Witwatersrand Supergroup sedimentation, the Gaborone-Kanye event
has important implications for the tectonic and magmatic evolution of
the northern Kaapvaal craton and possibly also played an important role
in the development of the adjacent auriferous sediments.
The Olifantshoek Group in southern Africa contains Paleoproterozoic red beds that are exceptionally well preserved, lying unconformably atop a regionally extensive lateritic paleoweathering profile. We studied the basal unit of this succession, known as the Gamagara or Mapedi Formation, and the lateritized substrate (so-called ``Drakenstein'' or ``Wolhaarkop'' paleosol) on which it developed. Two ancient magnetic components are observed. One (INT), usually with a distributed unblocking spectrum between 300° and 600°C but occasionally persisting to >675°C, is directed shallowly southward or northward. A mesoscale fold test at South Sishen Mine indicates that this component was acquired during deformation; similarity of the direction to previous results suggests that it was acquired at ~1240 Ma, during early Namaqua orogenesis. Combining our INT results with existing data from the Namaqua eastern zone (NEZ), we calculate the NEZ pole at (44.9°N, 021.5°E, K = 23.2, A95 = 12.8°, Q = 5). The most stable component from our data set (HIG), always persisting as a nonzero endpoint to demagnetization at >665°-680°C, is observed in 32 samples from South Sishen and Beeshoek Mines. Directed moderately east-downward (Sishen) or west-upward (Beeshoek), this component predates the mesoscale fold at Sishen. More importantly, a conglomerate test at Beeshoek indicates that HIG was acquired prior to Paleoproterozoic deposition of the Gamagara/Mapedi Formation. The concordance between directions from the paleoweathering zone and immediately overlying red beds indicates that HIG is a primary magnetic remanence for the basal Gamagara/Mapedi (BGM) Formation. The dual-polarity BGM paleomagnetic pole (02.2°N, 081.9°E, dp = 7.2°, dm = 11.5°, Q = 6) lies neatly between previous Kaapvaal poles with ages of ~2220 (Ongeluk lavas) and 2060 Ma (Bushveld complex). Our data thus support recent correlations of the Gamagara/Mapedi Formation with pre-Bushveld sediments of the Pretoria Group. A pre-Bushveld age for BGM is also consistent with its substantial distance from a new, albeit less reliable, paleomagnetic pole from the ~1930-Ma Hartley lavas, higher within the Olifantshoek succession (12.5°N, 332.8°E, K = 18.6, A95 = 16.0°, Q = 3). Our conglomerate test at Beeshoek confirms previous allegations that the intense hematitization observed in the Drakenstein-Wolhaarkop paleosol occurred during Paleoproterozoic weathering under a highly oxygenated atmosphere.
Shales from the late Archean Witwatersrand Supergroup of South Africa yield stable, well-grouped remanent magnetizations after thermal and alternating field demagnetization. Virtual geomagnetic poles from 17 sites located throughout the central and western parts of the basin indicate that rocks of the basin were remagnetized after folding. A paleomagnetic fold test shows that the FisherJan precision parameter, K, decreases from 22 to 3 after restoring the beds to paleohorizontal. Illite crystallinity indices show that the remagnetized samples were subjected to at least middle greenschlst facies temperatures, whereas magnetic blocking temperature estimates indicate upper greenschist facies temperatures. A K-Ar age obtained from clays of 1945 :!: 40 Ma (2s) is assigned to this metamorphic event. The new paleomagnetic pole obtained from the Witwatersrand Basin sites (19°N, 46°E, K = 22, Ao5 = 8 ø) is consistent with other 1950-2050 Ma poles from Southern Africa, confirming independently the K-Ar age. The distinctive overprint direction seen in shale specimens may be useful as a directional marker for orienting deep bore cores.
The conglomerate test is widely used in palaeomagnetism to date components of natural remanent magnetization with respect to deposition of conglomerates. It has been demonstrated, however, that this test may be positive even if the data are strongly contaminated by a secondary remanence, especially for the commonly used small number of clasts Starkey & Palmer 1970). Here we show with the aid of numerical simulations that different statistical procedures employed in this test have similar low sensitivities to remagnetization. We suggest a new conglomerate test which incorporates additional information on the direction of a secondary palaeomagnetic component which is isolated from either clasts themselves or their host rocks. Numerical simulations show that this new test is about twice as sensitive to remagnetization as the previous procedures and is robust with respect to small errors in the direction of a secondary component.
Palaeomagnetic and magnetomineralogical results are reported from charnockites in basement terrane at the eastern sector of the WSW–ENE granulite belt of South India. Magnetite is the dominant ferromagnet identified by rock magnetic and optical study; it is present in several phases including large homogeneous titanomagnetites and disseminated magnetite in microfractures linked to growth stages ranging from primary charnockite formation to uplift decompression and exhumation within the interval ∼2500–2100 Ma. Several components of magnetization are resolved by thermal demagnetization and summarized by four pole positions; in the northern (Pallavaram) sector these are P1 (33°N, 99°E, dp/dm = 8/9°) and P2 (79°N, 170°E, dp/dm = 3/6°), and in the southern (Vandallur) sector they are V1 (23°N, 116°E, dp/dm = 8/9°) and V2 (26°S, 136°E, dp/dm = 5/10°). These magnetizations are linked to uplift cooling of the basement and unblocking temperature spectra suggest acquisition sequences P1 → P2 and V1 → V2 in each case implying movement of the shield from higher to lower palaeolatitudes sometime between 2500 and 2100 Ma. Palaeomagnetic poles from the cratonic nuclei of Africa, Australia and India all identify motion from higher to lower palaeolatitudes in Early Palaeoproterozoic times, and this is dated ∼2400 and ∼2200 Ma in the former two shields. The corresponding apparent polar wander (APW) segments match the magnetization record within the charnockite basement terranes of southern India to yield a preliminary reconstruction of the ‘Ur’ protocontinent, the oldest surviving continental protolith with origins prior to 3000 Ma. Although subject to later relative movements these nuclei seem to have remained in proximity until the Mesozoic break-up of Gondwana.
Paleomagnetism is the only quantitative method available to test for lateral motions by tectonic plates across the surface of ancient Earth. Here, we present several analyses of such motions using strict quality criteria from the global paleomagnetic database of pre–800 Ma rocks. Extensive surface motion of cratons can be documented confi dently to older than ca. 2775 Ma, but considering only the most reliable Archean data, we cannot discern differential motion from true polar wander (which can also generate surface motions relative to the geomagnetic reference frame). In order to fi nd evidence for differential motions between pairs of Precambrian cratons, we compared distances between paleomagnetic poles through precisely isochronous intervals for pairs of cra-tons. The existing database yields several such comparisons with ages ranging from ca. 1110 to ca. 2775 Ma. Only one pair of these ages, 1110–1880 Ma, brackets signifi cantly different apparent polar wander path lengths between the same two cratons and thus demonstrates differential surface motions. If slightly less reliable paleomagnetic results are considered, however, the number of comparisons increases dramatically, and an example is illustrated for which a single additional pole could constrain differential cratonic motion into the earliest Paleoproterozoic and late Neoarchean (in the interval 2445–2680 Ma). In a separate analysis based in part upon moderately reliable paleo-magnetic poles, if a specifi c reconstruction is chosen for Laurentia and Baltica between ca. 1265 and 1750 Ma, then those cratons' rotated apparent polar wander paths show convergence and divergence patterns that accord with regional tectonics and appear to be remarkably similar to predictions from a plate-tectonic conceptual model. Care-fully targeted and executed future paleomagnetic studies of the increasingly well-dated Precambrian rock record can imminently extend these tests to ca. 2700 Ma, and with substantially more effort, to perhaps as old as ca. 3500 Ma.
1] This brief note provides an overview of a new Macintosh 2 application, PaleoMac, (MacOS 8.0 or later, 15Mb RAM required) which permits rapid processing of paleomagnetic data, from the demagnetization data acquired in the laboratory, to the treatment of paleomagnetic poles, plate reconstructions, finite rotation computations on a sphere, and characterization of relative plate motions. Capabilities of PaleoMac include (1) high interactivity between the user and data displayed on screen which provides a fast and easy way to handle, add and remove data or contours, perform computations on subsets of points, change projections, sizes, etc.; (2) performance of all standard principal component analysis and statistical processing on a sphere [Fisher, 1953] etc.); (3) output of high quality plots, compatible with graphic programs such as Adobe Illustrator, and output of numerical results as ASCII files. Beyond its usefulness in treating paleomagnetic data, its ability to handle plate motion computations should be of large interest to the Earth science community. Components: 2847 words, 5 figures. Index Terms: 1525 Geomagnetism and Paleomagnetism: Paleomagnetism applied to tectonics (regional, global); 8155 Tectonophysics: Evolution of the Earth: Plate motions—general; 0910 Exploration Geophysics: Data processing; 1594 Geomagnetism and Paleomagnetism: Instruments and techniques.. Cogné, J. P., PaleoMac: A Macintosh TM application for treating paleomagnetic data and making plate reconstructions, Geochem. Geophys. Geosyst., 4(1), 1007, doi:10.1029/2001GC000227, 2003.
The only remaining areas of pristine 3.6-2.7 Ga crust on Earth are parts of the Kaapvaal and Pillbara cratons. General similarities of their rock records, especially of the overlying late Archean sequences, suggest that they were once part of a larger Vaalbara supercontinent. Here we show that the present geochronological, structural and palaeomagnetic data support such a Vaalbara model at least as far back as 3.1 Ga, and possibly further back to 3.6 Ga, Vaalbara fragmented prior to 2.1 Ga, and possibly as early as 2.7 Ga, suggesting supercontinent stability of at least 400 Myr, consistent with Neoproterozoic and Phanerozoic analogues.
Geochronological comparisons of large datasets are facilitated by the use of structured databases. Data for the Precambrian
of South Africa, Swaziland, Lesotho and Botswana have been compiled in a DateView database and linked to chronostratigraphy
and GIS databases to produce a series of ‘time-slice’ maps illustrating the development of the Kaapvaal Craton. Linking geochronological
data to GIS coverages provides a valuable visual perspective on the development of the southern African lithosphere.
The oldest preserved rock formation dates occur south of the Barberton Greenstone Belt in South Africa and Swaziland. Subsequent
scattered development of new crust occurred in the south eastern, eastern and northern Kaapvaal Craton before being ‘stitched’
together by extensive granitoid intrusions at ~3.25 Ga and ~3.1 Ga. Coeval development of new crust occurred in what would
later become the central zone of the Limpopo Belt. The patterns of igneous activity from ~3.1 Ga to ~2.8 Ga, outboard of major
cratonic lineaments (Colesberg lineament in the west and Thabazimbi-Murchison lineament in the north) may indicate that these
lineaments represent suture zones along which the younger domains were accreted during formation of the Kaapvaal Craton.
By ~3 Ga the lithosphere was sufficiently rigid to support development of the Dominion, Witwatersrand and Pongola sedimentary
basins, followed by extensive volcanism during the Ventersdorp and concomitant granitoid activity throughout the Craton. Subsequent
geological activity, not necessarily evident in the available geochronological record, was concentrated on craton with the
development of the widespread Transvaal Supergroup followed by essentially coeval extrusion of the Rooiberg felsites and intrusion
of the Bushveld Complex at ~2.06 Ga. Deposition of sediments comprising the Waterberg and Soutpansberg Groups followed.
Igneous activity along the south-western edge of the Kaapvaal Craton terminated at ~1.93 Ga with formation of the Hartley
basalts, Olifantshoek Supergroup. Post-Olifantshoek Supergroup and pre-Volop Group tectonism has been reported from the western
margin of the Kaapvaal Craton. There is currently no geochronological evidence for major igneous or metamorphic activity post-dating formation of the Olifantshoek Supergroup until the early stages of the Namaqua-Natal
Belt subsequent to ~1.4 Ga i.e. there is no geochronological evidence for a major late-Palaeoproterozoic ‘Kheisian orogeny’. Off-craton, new crust formed in the Richtersveld Sub-province
at ~1.8 Ga but was presumably only accreted to the Kaapvaal Craton some 700 million years later during the Namaqua-Natal orogenesis.
Integrated U-Pb geochronology and palaeomagnetic study of mafic to felsic volcanic rocks of the Derdepoort Belt of South Africa are employed to test the hypothesis that the Pilbara and Kaapvaal Cratons were joined as part of a Late Archaean 'Vaalbara' supercontinent. An age of 2782 ± 5 Ma is deduced for eruption of the Derdepoort basalts, bracketed by a concordant SHRIMP zircon age of 2781 ± 5 Ma for overlying felsic volcanics and a concordant isotope dilution zircon age of 2783 ± 2 Ma for underlying granite of the Gabarone Complex. Based on the low (subgreenschist) metamorphic grade of the basalts, the presence of highly stable single domain magnetite, and a positive conglomerate test, the magnetization of the Derdepoort basalts is inferred to date from the time of their emplacement and cooling at 2782 Ma. Results yield a primary palaeopole at 005°E, 40°S (A95 = 18°), and indicate a palaeolatitude of 64.5 ± 17.5°for the Kaapvaal Craton at 2782 Ma. Published palaeomagnetic data for the Mount Roe Basalts of the Pilbara Craton indicate a palaeolatitude of 34.3 ± 6.4°at 2772 ± 2 Ma. The latitudinal separation of 30°implies that the cratons were not contiguous at 2.77 to 2.78 Ga, although the possibilities that the cratons could have been joined during other intervals of time, or that they were non-contiguous parts of a larger continent, are not ruled out.
Field relationships and chemical and geochronological data are presented that are most consistent with the Hampton Formation of the Buffelsfontein Group being correlative with the Kameeldoorns Formation of the Ventersdorp Supergroup. Correlations of the Hampton Formation with the Ventersdorp Contact Reef and Central Rand Group are less likely but cannot be excluded entirely. The Witfonteinrand and Tygerkloof Formations of the Buffelsfontein Group are probably younger than the Pniel Unconformity but are older than the black Reef Quartzite Formation of the Transvaal Supergroup. As such they may correlate with the Bothaville or Allenridge Formations (Pniel Sequence) of the Ventersdorp Supergroup. -from Authors
The Transvaal Basin (in Southern Africa) and the Hamersley-Nabberu basins (in Western Australia) show a number of remarkable parallels. They are of roughly the same age. They have the same geotectonic setting. Each is divided into a basal volcanic and clastic unit, a chemical sedimentary unit, and an upper clastic unit. Stratigraphic relations, lithologies, and depositional environments within each of these divisions are similar. The basins have a number of important mineral deposits in common, including iron, manganese, and crocidolite asbestos. Some mineral deposits, now known in only one of the basins, may well be found in the other basin as exploration proceeds. The parallel behaviour of the basins suggests a uniform tectonic history over a large portion of the Gondwana Supercontinent. Continental reconstructions that place the western margin of Australia against the African-Madagascan unit are favoured on geological grounds.
Integrative models of crust and mantle structure, age, and growth of the oldest continental nuclei—the Archean cratons—are critical to understanding the processes that stabilize continental lithosphere. For the Kaapvaal craton of southern Africa, conflicting ages of stabilization have been derived from studies of its crust and underlying mantle. New U–Pb zircon geochronological data from the western Kaapvaal craton reveal that two older (3.7 to 3.1 billion year old) continental masses, the Kimberley and Witwatersrand blocks, were juxtaposed by a significantly younger, previously unresolved episode of subduction and terrane collision between 2.93 and 2.88 billion years ago. Geological evidence indicates that convergence was accommodated by subduction beneath the Kimberley block, culminating in collisional suturing in the vicinity of the present-day Colesberg magnetic lineament. The timing of these convergent margin processes is further shown to correlate with the strong peak in Re–Os age distributions of Kimberley block mantle peridotites, eclogites, and eclogite-hosted diamonds. These data thus support the petrogenetic coupling of continental crust and lithospheric mantle through a model of continental arc magmatism, subduction zone mantle wedge processing and terminal collisional advective thickening to form Archean continental tectosphere.
The Transvaal succession in the northeastern part of the Kaapvaal Province of southern Africa is the erosional remnant of four, 2.7 to 2.1 Ga unconformity-bounded stratigraphic units, or sequences, each of which is 102 to 103 m thick and spans 107 to 108 years. Three of these Phanerozoic-like cratonic sequences occur in the western part of the Kaapvaal Province and in the Pilbara Province of Western Australia; thus, the two provinces once were part of the same continent, Vaalbara.The stratigraphically lowest sequence historically assigned to the Transvaal succession is part of the Ventersdorp Supergroup in Kaapvaal and the upper part of the Fortesque Group in Pilbara. It consists predominantly of arkosic sandstone and basaltic andesite; it probably is ⩽ 2687 ± 2 Ma. The second sequence contains the quartz arenites, dolomites, and banded-iron formations of the Transvaal and Griqualand West successions in the Kaapvaal and the Hamersley Group in the Pilbara. The span of ages (2684 ± 6 to 2432 ± 31 Ma), lithologies, and geographic extent of this sequence imply that the Limpopo orogeny, which welded the Zimbabwe Province to Vaalbara, is ⩽ 2.47 Ga (not 2.67 ± 0.05 Ga). The third sequence (the Pretoria and Postmasburg groups in the Kaapvaal and the lower part of the Wyloo Group in the Pilbara probably is the clastic wedge generated by the Limpopo orogeny. The fourth sequence (dominated by the felsites of the Rooiberg Group) is restricted to northeastern Kaapvaal.Sequence stratigraphy, lithostratigraphy, and lithofacies indicate that the Pilbara rifted from the southern edge of western Kaapvaal. The Penge Iron Formation in northeastern Kaapvaal, the Asbesheuwels Iron Formation in southwestern Kaapvaal and the Brockman Iron Formation in the Pilbara are the same formation, in which cyclothemic units 0.6 to 15.0 m thick extended ⩾ 1200 km.
Samples have been collected from eleven sites in the Modipe gabbro which crops out on the border of Bechuanaland and South Africa. After partial demagnetization in alternating magnetic fields, ten of these sites give almost vertical directions of magnetization, forming a close group which corresponds to a paleomagnetic pole at 33°S, 31°E. The gabbro is intruded by the Gaberones granite, and samples from two sites in the granite show almost horizontal magnetization. On initial measurement gabbro samples collected near the contact yield directions close to those of the granite, which suggests that the gabbro was reheated at the time of emplacement of the granite (2350 m.y. ago). After magnetic cleaning, some of these samples give directions which agree with those at the other gabbro sites. Together with detailed alternating field and thermal demagnetization studies, this agreement provides strong evidence that the magnetically cleaned directions are those of the TRM acquired when the gabbro originally cooled. The gabbro has an age of approximately 2600 m.y. and is to date the oldest rock unit that has been studied paleomagnetically; the results imply the existence of a geomagnetic field in this early stage of the earth's history. Alternating field demagnetization experiments indicate that some of the magnetic grains have very high coercivities. Thermal demagnetization and IRM studies exclude the presence of hematite and show that the magnetic mineral present is magnetite. Microscopic observations and theoretical considerations suggest that the high coercivities observed are due to the shape anisotropy of small elongated single-domain grains of magnetite.
With only minor exceptions, the 1.5 km thick sediments of the 2.5 Ga Hamersley Group are either chemical/biological (iron-formation, chert and carbonates) or pyroclastic/chemical (“shales”). Terrigenous clastics are sparse or absent. Palinspastic reconstructions indicate that the sediments were deposited on a submarine, essentially volcanogenic platform or bank (the Fortescue Group) built on an older Archaean, sialic, northwest-trending shelf protruding into, or marginal to, an ocean. A deep ocean basin is precluded by the geologic setting. Deposition in a barred basin is considered unlikely in the combined absence of terrigenous clastics, a defined shoreline or lateral facies changes.Upwelling of marine bottom currents resulted in precipitation of iron, silica and other components derived under anoxic conditions, largely from the pulsed output of a large oceanic rift or hot spot, possibly supplemented by normal continental drainage. The currents generally persisted during sedimentation of the Hamersley Group, temporarily interrupted or perhaps diverted by eustatic changes, growth of barrier reefs or the oscillating emergence and submergence of intervening volcanic chains. Ash emissions from the latter, combined with chemical precipitates, were largely responsible for the “shales” in the succession.
Samples have been collected in the vicinity of the De Beers diamond mine from at least two flows of the Ventersdorp lavas. The specimens responded equally well to both thermal and magnetic cleaning methods and a preliminary paleomagnetic pole position is presented for Ventersdorp times. The samples studied are shown not to have been influenced either magnetically or petrologically by the emplacement of the De Beers mine kimberlite in cretaceous times.
The c. 2.06 Ga Bushveld Complex intrudes the Transvaal Supergroup, South Africa. Calcsilicate xenoliths within the mafic phase of the Bushveld Complex (Rustenburg Layered Suite, or RLS) preserve calcsilicate xenoliths with high-temperature (c. 1200°C) mineralogies that were later metasomatized by hydrous, retrograde (c. 600-700°C) fluids whose timing has been unconstrained. New U-Pb isotope data from newly grown titanite within a completely retrogressed xenolith indicates that retogression occurred at 2058.9 ± 0.8 Ma. This suggests that retrogression was due to hydrothermal circulation associated with the cooling RLS, and not slightly later Bushveld-related granites. The new data place a tight constraint on the minimum age of emplacement of the RLS.
The origin of Precambrian banded iron formations is controversial. One type of evidence is a comparison of different occurrences. Three of the best-preserved basins of Precambrian iron formation deposition are here denned and compared systematically: the Hamersley Basin of Western Australia, the Animikie Basin of North America, and the Transvaal System Basin of South Africa. After a brief summary of the broad structural and stratigraphic evolution of each basin, their differences and similarities are reviewed. They differ mainly in: major stratigraphic sequence, thickness of iron formation, thickness variation, stratigraphic continuity, lithology of iron formation, nature of sedimentary structures, conspicuousness of varves (micro-banding), clastic association, and abundance of diagenetic riebeckite. With some reservations, they resemble each other in: age, basin size, chemistry and mineralogy, broad time-structural evolution of basin, precedence of chert, volcanic association, and in the restricted, medial and transitional stratigraphic status of the main iron formation. It is concluded that the Western Australian and South African iron formations are very closely similar, but that both differ markedly from the Animikie iron formations of the Lake Superior area. However, all three are more closely similar in chemistry than is required by the fact that their definition is chemical. Quantitative geochemical argument indicates a direct volcanic contribution of iron to the Hamersley Basin; it is possible that such vulcanicity is restricted to a limited medial part of basin development in a particular kind of basin-forming process.
The East African Antarctic orogen is one of the largest orogenic belts on the planet. It resulted from the collision of various parts of proto East and West Gondwana during late Neoproterozoic early Paleozoic time (between 650 and 500 Ma). We propose that the southern part of this Himalayan-type orogen can be interpreted in terms of a lateral-escape tectonic model. Modern Gondwana reconstructions show that the southern part of the East African Antarctic orogen can best be reassembled when a number of microplates (the Falkland, Ellsworth-Haag, and Filchner blocks) are positioned between southern Africa and East Antarctica. This microplate assemblage is unusual. The microplates probably represent shear-zone bounded blocks, produced by tectonic translation during lateral escape, similar to those currently evolving in Southeast Asia. One of the escape-related shear zones is exposed as the 20-km-wide Heimefront transpression zone in western Dronning Maud Land. Coats Land, a crustal block within the orogen, probably represents a block of older crust that was not subjected to tectonometamorphic reworking ca. 500 Ma by lateral tectonic escape. The southern part of the orogen is also typified by very large volumes of late-tectonic A2-type granitoids, intruded ca. 530 490 Ma, probably as a consequence of delamination of the orogenic root and the subsequent influx of hot asthenospheric mantle during tectonic escape. Erosional unroofing of the orogen is documented by the remnants of originally massive areas covered by Cambrian Ordovician molasse-type sedimentary rocks throughout Africa, Arabia, and Antarctica, testifying to the past extent and size of this largest of orogens.
The ca. 2.2 Ga Hekpoort paleosol of the Transvaal Supergroup in southern Africa has been considered a type example and the youngest iron-depleted paleosol formed under a reducing atmosphere in the early Precambrian. However, the mineralogical and geochemical data on recently acquired deep drill core intersections indicate that the Hekpoort paleosol represents part of an ancient lateritic weathering profile with an iron-depleted pallid lower zone and an iron-enriched lateritic upper zone. Previous studies of the paleosol took place in areas where only the lower pallid zone was preserved from erosion prior to deposition of cover beds. The laterite profile is comparable to that of modern tropical laterites formed under an oxic atmosphere in the presence of abundant terrestrial biomass. Revised stratigraphic correlation indicates that the Hekpoort laterite profile is a correlative to highly ferruginous laterite profiles of Wolhaarkop in Griqualand West. This information indicates that the oxygen-evolution curve, based on loss or retention of iron in paleosols, should be reexamined.
New U-Pb SHRIMP ages from East Antarctica point to the existence of a laterally continuous orogenic belt that bisects the East Antarctic craton. This orogenic belt juxtaposes Archean crust to the south and east against Neoproterozoic metamorphic rocks to the north and west. It defines the margin of a separate lithospheric block that consists of a large section of East Antarctica and India that did not form part of east Gondwana or Rodinia as they are currently reconstructed. Instead, this Indo-Antarctic continent accreted with west Gondwana along the Mozambique suture shortly before collision and suturing along a second "Pan-African" suture now cropping out in the southern Prince Charles Mountains and Prydz Bay regions of Antarctica. This scenario is consistent with (1) the abrupt termination of ca. 990-900 Ma tectonism recognized in the northern Prince Charles Mountains-Rayner Complex-Eastern Ghats against Paleozoic orogenic belts, (2) the lack of terranes of equivalent age found elsewhere in either Antarctica or other previously adjacent continents, and (3) the distinct detrital-zircon populations obtained from either side of this proposed suture.
Eleven high precision (±2–5 million years) SHRIMP zircon U–Pb ages have been obtained from felsic rocks within a single stratigraphic section of late Archaean volcanic and sedimentary rocks in the east Pilbara of Western Australia. The stratigraphic succession (Nullagine and Mount Jope Supersequences in sequence-stratigraphic terminology, Fortescue Group in lithostratigraphic terminology) is interpreted to be the rock record of three major geotectonic cycles that formed in an extensional, rift-related environment between about 2772 and 2715Ma. The geochronology is constrained by a detailed stratigraphic framework based on unconformities and supported by a preliminary magnetostratigraphy. Field mapping, geochemical and petrographic studies have shown that previously unrecognised thin felsic tuff bands are interbedded in subaerial flood basalt piles and mafic tuffs. While flood basalts and proximal felsic volcanic rocks comprise by volume most of the volcanogenic components of the succession, felsic volcanism is now known to have been active periodically through each geotectonic cycle. The succession covers a time period of about 57 million years. The lower ∼1400m of a thick (∼1700m) clastic sedimentary succession from the oldest geotectonic cycle was deposited at a rate of about 100m per million years over a mean time period of 14 million years. In contrast, a younger ∼150m thick cogenetic tuff-basalt unit accumulated in less than 3 million years, and others probably accumulated at similar rates, comparable to those of Phanerozoic flood basalts. Unconformities in the succession are shown to be of variable duration and one unconformity marking the boundary between the first and second geotectonic cycles may represent a time-gap of more than 10 million years. The unconformity-based stratigraphic framework, the new geochronology and palaeomagnetic studies [J. Geophys. Res. 108 (2003) B12, 2551, EMP 2-1 to 2-21] have been combined to determine a possible late Archaean continental drift rate for one part of the succession, implying a period of motion as fast as or up to five times faster than any known from the Phanerozoic.
Geochemical and SmNd isotopic data are reported for igneous rocks of the 2.76 to 2.69 Ga Fortescue Group of the Pilbara Craton, Western Australia, and the ∼ 2.70 Ga Ventersdorp Supergroup of the Kaapvaal Craton, southern Africa. The metamorphic history of the Fortescue Group has also been investigated using whole-rock RbSr and PbPb isochron techniques. Igneous of both sequences have mixed tholeiitic and calc-alkaline affinities and show immobile element correlations indicating derivation from chemically heterogeneous, LREE-enriched sources. Both sequences have similar negative ϵNd values (between −1.5 and −4.4 for the Fortescue Group and 0 and −3.4 for the Ventersdorp Supergroup) which are not correlated with Mg/(Mg + Fe2+). The negative ϵNd values of the Fortescue Group rocks probably do not result from crustal assimilation within crustal-level magma chambers during or following differentiation as the Fortescue Group felsic differentiates have ϵNd values within the ranges of values found in associated mafic rocks and zircon xenocrysts are absent in the felsic differentiates of the Fortescue Group. Instead, these results are consistent with either extensive crustal contamination of primitive (komatiitic or picritic) Fortescue Group parent magmas prior to their differentiation or derivation from negative ϵNd (or enriched) mantle sources. The presence of zircon xenocrysts in some Ventersdorp samples provides compelling evidence of some crustal contamination, although correlations between immobile element ratios (such as La/Yb, Ti/Zr, Ti/Sc and V/Zr) and ϵNd indicate derivation from chemically heterogeneous, enriched mantle sources. A whole-rock SmNd isochron date of 3308 ± 138 Ma, ∼ 600 Ma older than the time of eruption indicated by UPb zircon data and within, ∼ 300 Ma of the formation of the Kaapvaal granite-greenstone terrane, may date an SmNd fractionation event within the subcontinental lithospheric mantle sources of the Ventersdorp rocks. RbSr and PbPb isochrons obtained for several flows from the Fortescue Group register hydrothermal events occurring between ∼ 2.45 and 2.0 Ga and probably associated with burial metamorphism. Similarities in the field geology, geochemistry and isotopic characteristics of the Fortescue and Ventersdorp sequences and Phanerozoic examples of continental flood basalt volcanism suggest a common mode of origin, possibly involving the interaction of asthenospheric mantle plumes with subcontinental lithospheric mantle sources which have been modified by subduction processes.
The Ventersdorp Supergroup (2700–2750 Ma) is comprised chiefly of mafic to intermediate subaerially erupted volcanics with smaller amounts of clastic sediments. The lower unit, the Klipriviersberg Group, consists of basal basaltic komatiites overlain by basalts which show a progressive increase in Mg number and Ni content and a decrease in incompatible element contents with increasing stratigraphic height. Volcanics of the overlying Platberg and Pniel Groups consist of basaltic andesites and andesites, and some felsic volcanics.All Ventersdorp volcanics exhibit a subduction zone geochemical component and are similar in incompatible element distributions to volcanics from continental-margin arc systems. Both tholeiitic and calc-alkaline trends occur in the Ventersdorp succession, with the former dominating. Crustal contamination appears to have played a minor role in the evolution of Ventersdorp magmas. With the exception of basaltic komatiite, the Klipriviersberg lavas can be related by progressive melting of a garnet lherzolite source (containing a subduction zone component) followed by up to 30% of shallow fractional crystallization. The basaltic komatiite can be produced from the same source at higher temperatures and the Pniel lavas by shallow fractional crystallization of basaltic komatiite. The Platberg volcanics must come from a within-plate enriched mantle source, and mafic and felsic members of this group can be related by shallow fractional crystallization.Geochemical and geologic data are consistent with a model for Ventersdorp magmas involving a subduction zone in which Klipriviersberg and Pniel magmas are produced from the mantle wedge and Platberg magmas come from an enriched mantle lithosphere with a superimposed subduction zone component. Secular trends in Klipriviersberg lavas can be explained by progressive adiabatic melting of an ascending plume rooted in the mantle wedge.
The Trompsburg intrusion of South Africa is a large layered intrusion, measuring approximately 2500 km2. Little is known about its age and composition. Boreholes drilled in the 1940s to constrain a strong gravimetric and magnetic anomaly intersected up to 2 km of gabbro–troctolite–anorthosite containing up to 19 massive magnetite layers. Sr isotopic analyses performed in 1970 indicated an age of 1372±142 Ma for the intrusion, suggesting no direct genetic link to the 2054 Ma Bushveld Complex. No further work was conducted on the Trompsburg intrusion during the last decades. Our results for a secondary ion mass spectrometry U/Pb isotope study on zircons from two gabbroic samples of the Trompsburg intrusion indicate a crystallisation age of 1915±6 Ma, supporting the occurrence of a global 1.9 Ga superplume event. Using the new age, we recalculated available Sr isotope data. The results suggest that the Trompsburg intrusion has a lower crustal component than the Bushveld Complex, with Sri approximately 0.704. A genetic relationship between the Trompsburg and Bushveld intrusions remains, therefore, unlikely.
UPb isotope systematics of shock metamorphosed zircon grains from pseudotachylitic breccias and Granophyre from the controversial Vredefort Structure, South Africa, provide new and compelling evidence for an impact origin for this structure. Zircon grains from these rocks exhibit planar microstructures and polycrystalline textures similar to those from the Chicxulub crater breccia, K/T boundary ejecta, and rocks from the Sudbury Structure. A concordant 2023 ± 4 Ma (2σ) age for newly crystallized, unshocked zircon grains from recrystallized pseudotachylitic breccia from the central part of the Vredefort Structure provides a good approximation of the time of impact. This age indicates that the impact post-dates Bushveld magmatism by at least 30 m.y. UPb isotopic results for individual, pre-impact zircon grains indicate crystallization ages from about 3060 to 3300 Ma and Pb loss at ca. 2000 Ma. Data for high U grains plot below a discordia line from 3060 to 2023 Ma and indicate both impact- and post-impact related Pb loss. The data and granular morphology of a zircon grain from the Granophyre indicates probable ca. 2.0 Ga and ca. 1.0 Ga Pb loss. Although planar microstructures in zircon are ubiquitous, there are also some unshocked, low-U grains, and these record a ca. 3.1 Ga primary age. The older ca. 3.1–3.3 Ga ages for shocked zircons reflect formation and modification of granitoid crust in the region of the Vredefort Structure prior to and during a metamorphic event at about 3080 Ma.The resilience of zircon shock features to post-impact alteration and annealing, in combination with precise UPb dating of individual shocked grains provide a valuable method for indentifying ancient, metamorphosed and tectonically modified impact structures.
Detailed palaeomagnetic and rock magnetic analyses provide improved palaeomagnetic results from 23 sites in the Borgmassivet intrusions in the Ahlmannryggen region of Dronning Maud Land, East Antarctica. These intrusions are of similar age to their host, the ca. 1130 Ma Ritscherflya Supergroup (RSG). A mean direction of D=235.4°, I=−7.6° with k=45.9 and α95=4.5° was obtained from this study. When combined with previously reported results from 11 sites in the same region, including sites from the Ritscherflya Supergroup, it gives an overall mean direction for 34 sites from the igneous suite with D=236.5°, I=−3.6°, k=27.9 and α95=4.8°. Isothermal remanent magnetization (IRM) experiments on several specimens suggest magnetite or titanomagnetite as the primary remanence carrier, while high temperature magnetic susceptibility experiments indicate the presence of single domain particles. These observations, together with field evidence and the high coercivities and unblocking temperatures, support a primary origin for the observed characteristic remanence. The Borgmassivet palaeomagnetic pole lies at 54.5°E, 8.3°N with A95=3.3°. If Antarctica is moved to its Gondwanan position adjacent to southeast Africa, the Borgmassivet pole (BM) coincides with that of the African well-established, well-dated (1100 Ma) Umkondo Large Igneous Province pole, supporting the hypothesis that the Grunehogna craton of Dronning Maud Land was part of the Kalahari craton of southern Africa at ca. 1100 Ma.
The end-Paleozoic Pangea appears to have contained three continents that had grown in the Precambrian and remained intact until Mesozoic rifting: Ur, formed at ∼3 Ga and accreted to most of East Antarctica in the middle Proterozoic to form East Gondwana; Arctica, an approximately 2.5-2 Ga continent that contained Archean terranes of the Canadian and Siberian shields and Greenland; and Atlantica, formed at ∼2 Ga of cratons of ∼2 Ga age that now occur in West Africa and eastern South America. Arctica grew at ∼1.5 Ga by accretion of most of East Antarctica plus Baltica to form the continent of Nena. Collision of Nena, Ur, and Atlantica, plus minor plates, formed the supercontinent of Rodina at ∼1 Ga. Rifting of Rodinia between 1 and 0.5 Ga formed three continents: East Gondwana; Atlantica (which became the nucleus for West Gondwana); and Laurasia (which contained North America, Greenland, Baltica, and Siberia). Gondwana formed at ∼0.5 Ga by amalgamation of its eastern and western parts. Various plates accreted to Laurasia during the Paleozoic, and collision of Gondwana with Laurasia created Pangea at ∼0.3 Ga.
It is generally accepted that several criteria must be met before a paleomagnetic result is deemed reliable, meaning that a paleomagnetic direction faithfully represents the ancient geomagnetic field at the time the rock formed. A set of seven such criteria has often been used in paleopole compilations, resulting in a Q factor that equals the number of criteria satisfied. However, results with Q = 7 are very rare, and usually a minimum Q of 3 has been used as a threshold. In this presentation, we take this type of analysis one step further and examine whether one of the best datasets for any geological interval and any continent is sufficiently robust to believe that the very small cones of confidence are representative of the uncertainty associated with the results. This dataset is for extra-Alpine Europe for the interval 300 - 220 Ma and comprises more than 60 individual pole positions with Q greater than or equal to 3. We classified results according to characteristics that may produce a systematic bias in pole positions, including lithology (volcanics vs. sedimentary rocks), demagnetization code, and quality of age control. We found that Early Permian (280+10 Ma) mean poles based on different subsets can differ by more than 10 degrees, suggesting that a combination of (1) inclination shallowing in sediments, (2) unrecognized present-day field overprints that have not been adequately removed by demagnetization, and (3) underestimation of rock ages, may introduce a bias. All three causes, as well as possible octupole contributions to the total geomagnetic field, would conspire to yield more southerly paleolatitudes for Europe than warranted. This matters greatly for Pangea reconstructions based on paleomagnetic data. With the highest paleolatitude (23 degrees N at Oslo), a Pangea-A type fit is easily possible without any continental overlap, whereas the lowest (12 degrees N) produces a large Gondwana-Laurussia overlap in a Pangea-A type fit, which could lead to the conclusion that a Pangea-B type reconstruction is preferable for the Early Permian. Thus the uncertainty in paleolatitude (more than 10 degrees) far exceeds the small alpha-95 value about the mean pole.
Three Grenville-age provinces can be distinguished in East Antarctica with U-Pb zircon data. The Maud, Rayner, and Wilkes provinces each have a distinctive age signature for late Mesoproterozoic early Neoproterozoic magmatism and high-grade metamorphism and are correlated with similar rocks in the Namaqua-Natal (Africa), Eastern Ghats (India), and Albany-Fraser (Australia) provinces, respectively. These crustal segments represent three separate collisional orogens. They are separated by regions of intense late Neoproterozoic Early Cambrian tectonism, consistent with their juxtaposition during the final assembly of Gondwana and indicating that previous models for a single, continuous, Grenville-age mobile belt around the East Antarctic coastline should be discarded.
“PaleoMag,” a paleomagnetics analysis package originally developed for the Macintosh operating system in 1988, allows examination of demagnetization of individual samples and analysis of directional data from collections of samples. Prior to recent reinvigorated development of the software for both Macintosh and Windows, it was widely used despite not running properly on machines and operating systems sold after 1995. This somewhat surprising situation demonstrates that there is a continued need for integrated analysis software within the earth sciences, in addition to well-developed scripting and batch-mode software. One distinct advantage of software like PaleoMag is in the ability to combine quality control with analysis within a unique graphical environment. Because such demands are frequent within the earth sciences, means of nurturing the development of similar software should be found.
Evidence for the existence of Neoarchatan greenstone rocks in the western part of the Kaapvaal Craton is provided by ID-TIMS and SHRIMP U-Pb age data on zircons from supracrustal rocks of the Amalia greenstone belt. Although all the Units described from the Amalia greenstone belt might not be coeval, an age of 2750.1 +/- 4.6 Ma found for two samples appears to be the best estimate for the deposition of this part of the Amalia greenstone belt sequence. These samples also contain younger zircons, the majority of them yielding a 1099 +/- 32 Ma Upper intercept age. This date of -1.1 Ga might indicate that these rocks have been affected by the crustal thickening and magmatism event coeval with the peak of metamorphism during the Namaqua orogeny -1.1 Ga. These data provide the first evidence for the development of a Neoarchacan greenstone sequence in the western part of the Kaapvaal Craton. They also demonstrate that the Namaqua orogeny affected rocks within the craton itself.
Ion microprobe U–Th–Pb analyses of baddeleyite and zircon yield precise ages for several mafic intrusions in the Pilbara Craton of Western Australia. Baddeleyite was dated from four dolerite dykes of the north‐northeast‐trending Black Range swarm intruded into granitoid‐greenstone basement in the northern part of the craton. The mean Pb*/Pb* age of 2772 ± 2 Ma, interpreted as an unambiguous age of emplacement for the dykes, is within error of previous ion microprobe U–Pb zircon ages for the Mt Roe flood basalts and confirms that the dykes acted as feeders to the volcanic rocks. The Sylvania Inlier, in the southeastern Pilbara Craton, also contains north‐northeast‐trending dykes that were correlated previously with the Black Range swarm. Based on concordant and discordant zircon analyses from samples of two dykes, the best estimate of the age of the Sylvania dykes is 2747 ± 4 Ma. The Sylvania dykes thus appear to be significantly younger than, and hence unrelated to, the Black Range swarm, but may have acted as feeders to younger volcanic units in the Fortescue Group such as the Kylena Formation.
SHRIMP dating of xenotime overgrowths on detrital zircon grains can constrain maximum durations since diagenesis and therefore provide minimum dates of sediment deposition. Thus, xenotime dating has significant economic application to Precambrian sediment-hosted ore deposits, such as Witwatersrand Au–U, for which there are no precise depositional ages. The growth history of xenotime in the Witwatersrand Supergroup is texturally complex, with several phases evident. The oldest authigenic xenotime 207Pb/206Pb age obtained in sandstone underlying the Vaal Reef is 2764 ± 5 Myr (1 σ), and most likely represents a mixture of diagenetic and hydrothermal growth. Nevertheless, this represents the oldest authigenic mineral age yet recorded in the sequence and provides a minimum age of deposition. Other xenotime data record a spread of ages that correspond to numerous post-diagenetic thermotectonic events (including a Ventersdorp event at ≈ 2720 Ma) up to the ≈2020 Ma Vredefort event.
The Archean Pilbara Craton contains five geologically distinct terranes – the East Pilbara, Karratha, Sholl, Regal and Kurrana Terranes – all of which are unconformably overlain by the 3.02- to 2.93-Ga De Grey Superbasin. The 3.53–3.17 Ga East Pilbara Terrane (EP) represents the ancient nucleus of the craton that formed through three distinct mantle plume events at 3.53–3.43, 3.35–3.29 and 3.27–3.24 Ga. Each plume event resulted in eruption of thick dominantly basaltic volcanic successions on older crust to 3.72 Ga, and melting of crust to generate first tonalite-trondhjemite-granodiorite (TTG), and then progressively more evolved granitic magmas. In each case, plume magmatism was accompanied by uplift and crustal extension. The combination of conductive heating from below, thermal blanketing from above, and internal heating of buried granitoids during these events led to episodes of partial convective overturn of upper and middle crust. These mantle melting events caused severe depletion of the subcontinental lithospheric mantle, making the EP a stable, buoyant, unsubductable continent by c. 3.2 Ga. Extension accompanying the latest event led to rifting of the protocontinent margins at between 3.2 and 3.17 Ga. After 3.2 Ga, horizontal tectonic forces dominated over vertical forces, as revealed by the geology of the three terranes (Karratha, Sholl and Regal) of the West Pilbara Superterrane. The c. 3.12-Ga Whundo Group of the Sholl Terrane is a fault bounded, 10-km-thick volcanic succession with geochemical characteristics of modern oceanic arcs (including boninites and evidence for flux melting) that indicate steep Archean subduction. At 3.07 Ga, the 3.12-Ga Sholl Terrane, 3.27-Ga Karratha Terrane and c. 3.2-Ga Regal Terrane accreted together and onto the EP during the Prinsep Orogeny. This was followed by development of the De Grey Superbasin – an intracontinental sag basin and widespread plutonism (2.99–2.93 Ga) as a result of orogenic relaxation and slab break off. Craton-wide compressional deformation at 2.95–2.93 Ga culminated with 2.91-Ga accretion of the 3.18 Ga Kurrana Terrane with the EP. This compression caused amplification of the dome-and-keel structure in the EP. Final cratonization was effected by emplacement of 2.89–2.83 Ga post-tectonic granites.
The pattern of occurrence of the massif-type anorthosites of the Proterozoic on a Rodinia reconstruction suggests that the geneses of the anorthosites, the associated granulites, and their chief repository- the Grenville age mobile belt - may be interrelated. This relationship has been examined within the broad geodynamic and spatial-temporal framework of the Proterozoic supercontinent. All over the Grenville age mobile belt, approximately within 1500 - 1000 Ma, two successive, back-to-back episodes can be recognized. A continent-continent collisional episode with massive sedimentation, great crustal shortening and thickening with large folds and nappes, metamorphism and calcalkaline magmatism, and accretion of juvenile crust was followed within little over a hundred million years or so by an extensional episode, beginning with large scale mantle-derived basic magma invasion, ponding and differentiation at the base of the thickened orogen, anorthosite diapirism and pervasive thermal overprinting of the lower and middle crust producing granulite belts. Cooling, unroofing and erosion of the orogen coincided with the later stages of the extension, which ended with splitting of the supercontinent. We have argued that the first episode marks the phase of closing in and amalgamation of the converging continental lithospheric plates and the second episode represents the phase of reversal, rifting, plate separation and drifting away of the post-split continental blocks. We believe, the supercontinental closing and opening cycles provide (a) realistic force fields and appropriate spatial-temporal and thermal-tectonic framework for the origin of the Proterozoic massif-type anorthosites and the associated granulites and (b) an explanation for the spatially overlapping but apparently conflicting evidence of both collisional and extensional tectonic signatures in the Grenville age orogenic belts.