Kaapvaal craton

The Kaapvaal craton (Limpopo province of South Africa), along with the Pilbara craton of Western Australia, are the only remaining areas of pristine 3.6-2.5 Ga crust on Earth. Similarities of rock records from both these cratons, especially of the overlying late Archean sequences, suggest that they were once part of the Vaalbara supercontinent (Zegers "et al.", 1998).

The Kaapvaal craton covers an area of approximately 1.2 million square kilometers and is joined to the Zimbabwean craton to the north by the Limpopo Belt. To the south and west, the Kaapvaal craton is flanked by Proterozoic orogens, and to the east by the Lebombo monocline that contains Jurassic igneous rocks associated with the breakup of Gondwana. The Kaapvaal craton formed and stabilized between 3.7 and 2.6 billion years ago by the emplacement of major granitoid batholiths that thickened and stabilised the continental crust during the early stages of an arc-related magmatism and sedimentation cycle. The craton is a mixture of early Archean (3.0-3.5 Ga) granite greenstone terranes and older tonalitic gneisses (ca. 3.6-3.7 Ga), intruded by a variety of granitic plutons (3.3-3.0 Ga). Subsequent evolution of the Kaapvaal craton (3.0-2.7 Ga) is thought to be associated with continent–arc collision that caused an overlaying successions of basins filled with thick sequences of both volcanic and sedimentary rocks. This was then followed by episodic extension and rifting when the Gaborone–Kanye and Ventersdorp sequences were developed. Early Archean crust is well exposed only on the east side of the craton and comprises a collage of subdomains and crustal blocks characterized by distinctive igneous rocks and deformations. Late Archean metamorphism joined the Southern Marginal Zone of the Kaapvaal craton to the Northern Marginal Zone of the Zimbabwe craton ca. 2.8-2.5 Ga by the 250 km wide orogenic Limpopo Belt. The belt is an east-northeast trending zone of granulite facies tectonites that separates the granitoid-greenstone terranes of the Kaapvaal and Zimbabwe cratons.

Limpopo Central Zone

The crustal evolution of the Limpopo Central Zone can be summarized into three main periods: 3.2-2.9 Ga, ~2.6 Ga, and ~2.0 Ga. The first two periods are characterized by magmatic activity leading to the formation of Archaean tonalite-trondhjemite-granodiorite] (TTG) such as the Sand River Gneisses and the Bulai Granite intrusion. Early Proterozoic high-grade metamorphic conditions produced partial melting that formed large amounts of granitic melt. (Chavagnac "et al.", 1999).

There is no indication that the Neoarchean to early Paleoproterozoic succession on the craton were sourced from the 2.65-2.70 Ga orogenic event preserved in the Limpopo Metamorphic Complex. However, younger late Paleoproterozoic red bed successions contain zircons of this time interval as well as many ~2.0 Ga detrital zircons. This implies that the Limpopo Complex together with the Zimbabwe craton only became attached to the Kaapvaal craton at ~2.0 Ga during formation of the Magondi Mobile Belt which in turn sourced the voluminous late Paleoproterozoic red beds of southern Africa. (Beukes "et al.", 2004). Evidence of the horizontal layering and riverine erosion can be found throughout the Waterberg Massif within the Limpopo Central Zone.

Barberton greenstone belt

The Barberton greenstone belt is situated in the eastern age of Kaapvaal Craton. It is well known for its gold mineralisation. Some of the oldest exposed rocks on Earth (>3.6 Ga) are located in the Barberton greenstone belt of the Swaziland–Barberton areas. Only the rocks found in the Isua greenstone belt of Western Greenland are older. The Barberton greenstone belt consists of a sequence of mafic to ultramafic lavas and metasedimentary rocks emplaced and deposited between 3.5 and 3.2 Ga. The granitoid rocks were emplaced over a 500 Myr time-span and can be divided into two suites: The tonalite-trondhjemite-granodiorite (TTG) suite (emplaced ca 3.5 - 3.2 Ga), and the granite-monzogranite-syenitegranite (GMS) suite (emplaced ca 3.2 - 3.1 Ga). The GMS suite are found over large parts of the Kaapvaal Craton and their emplacement coincides with the first stabilization of the central parts of the craton. "The GMS suite in the Barberton granite-greenstone terrane shows very different internal and external characteristics from the earlier TTG suite. Individual plutons may cover several thousand square kilometres and these composite granitoid bodies have traditionally been referred to as batholiths, alluding to their compositionally and texturally heterogeneous nature and enormous areal extent. For the most part, the plutons appear undeformed." (Westraat "et al.", 2005).

The Barberton area underwent two tectonic episodes of terrane accretion at ∼3.5 and 3.2 Ga. Early stages of shield development are exposed in the Barberton Mountains where the continent formation first took place by magmatic accretion and tectonic of small protocontinental blocks. Several small diachronous blocks (3.6-3.2 Ga) have been found in the area. Apparently each block represents a cycle of arc-related magmatism and sedimentation. The Hooggenoeg Formation of the Barberton greenstone belt is dated at 3.45 Ga. and evolved through magmatism. This crustal development phase was followed by a period of Mesoarchaean cratonic magmatism (3.1-3.0 Ga) and is marked by the formation of a large crescent-shaped, juvenile arc that was accreted onto the northern and western margins of the evolving Kaapvaal shield. Archaean greenstone belts are hypothesized to have been formed from passive margin oceanic crust that became part of an extensive subduction-undercut margin. The TTG intrusions are thought to have been formed by post-subduction magmatism when subduction was halted, perhaps by arrival of a micro-craton.

The 3.1 Ga Mpuluzi batholith in the Barberton granite–gneiss terrane is made up of granite sheets. The structurally higher parts are underlain by an anastomosing network of steeply dipping, variably deformed dykes and sheets. According to a study done by Westraat "et al." (2005): "Multiple intrusive relationships and geochronological evidence suggests that granite sheeting and the assembly of the pluton occurred over a period of 3–13 Ma. The spatial and temporal relationship between deformation and magma emplacement reflects episodes of incremental dilation related to deformation along the bounding shear zones and granite sheeting. The transition to the mainly subhorizontal granite sheets at higher structural levels of the tabular Mpuluzi batholith indicates the intrusion of the granites during subhorizontal regional shortening, where the reorientation of the minimum normal stress to vertical attitudes at the shallow levels of emplacement allowed for vertical dilation and subhorizontal emplacement of the granite sheets."

Barberton greenstone belt TTG and GMS suites

The Barberton Mountain is a well preserved pre-3.0 Ga granite-greenstone terrane. The greenstone belt consists of a sequence of mafic to ultramafic lavas and metasedimentary rocks emplaced and deposited between 3.5 and 3.2 Ga. The granitoid rocks were emplaced over a 500 Myr time-span and can be divided into two suites. The TTG suite (emplaced ca 3.5-3.2 Ga) contains tonalites, trondhjemites and granodiorites; and the GMS suite (emplaced ca 3.2 - 3.1 Ga) includes granites, monzogranites and a small syenite-granite complex. Geochemically, the TTGs are typically low- to medium-K (potassium), metaluminous I-type granites, Their chondrite-normalized rare-earth-element (REE) patterns show two trends. The majority of plutons are LREE-enriched (low in rare-earth-elements), HREE-depleted (high in rare-earth-elements) and with small or no Eu anomalies, while the Steynsdorp and Doornhoek plutons are relatively HREE-undepleted with significant Eu anomalies. Nd (neodymium) isotope analyses show that the 3.4 Ga TTGs have positive εNd (epsilon neodymium) values of (0 to +3.7), indicative of depleted-mantle sources, similar to the oldest greenstone belt formations (the Onverwacht series). In contrast, the 3.2 Ga TTGs have negative εNd, suggesting crustal or enriched-mantle input into the magmas. The GMS rocks, on the other hand, are medium- and high-K metaluminous I-type rocks. They display two dominant REE patterns. Medium-K GMS rocks (the Dalmeinand portions of Heerenveen) are LREE-enriched, HREE-depleted and have no Eu-

According to a study by Yearron "et al." (2003): "The TTGs are typically low- to medium-K , metaluminous I-type granites, Their chondrite-normalised REE patterns show two trends. The majority of plutons are LREE-enriched, HREE-depleted and with small or no Eu anomalies, whilst the Steynsdorp and Doornhoek plutons are relatively HREE-undepleted with significant Eu anomalies. Nd isotope analyses show that the 3.4 Ga TTGs have positive εNd values (0 to +3.7), indicative of depleted-mantle sources, similar to the oldest greenstone belt formations (the Onverwacht). In contrast, the 3.2 Ga TTGs have negative εNd, suggesting crustal or enriched-mantle input into the magmas.

Extensive granite plutons of a subsequent magmatic episode are associated with the intrusion of vast amounts of granodiorite-monzogranite-syenite GMS suites. The GMS rocks are medium- and high-K metaluminous I-typerocks. They display two dominant REE patterns. Medium-K GMS rocks (the Dalmeinand portions of Heerenveen) are LREE-enriched, HREE-depleted and have no Eu-anomalies, whereas, the high-K GMSs (Heerenveen, Mpuluzi and Boesmanskop) are relatively HREE-enriched with negative Eu anomalies. Positive and negative εNd val-ues (-4.4 to +4.8) for the Boesmanskop Syenite suggests depleted-mantle and crystal signatures. The εNd and REE patterns, in particular, provide insights into the compositions of potential source rocks and restites for the TTG and GMS suites. Since HREEs and Eu are readily accommodated in garnet and plagioclase, respectively, their depletion suggests the presence of these minerals in the restite. For the TTG suite, we therefore suggest a garnet-rich amphibolitic or eclogitic depleted-mantle source at a depth >40 km. This has been confirmed by experimental work constraining the stability of garnet in the trondhjemite compositions, and at magmatic temperatures, to a pressure of 15.24 ± 0.5 kbar corresponding to a depth of 54.9 ± 1.8 km. In contrast, the GMS suite most probably had a plagioclase-rich, garnet-poor source that may be a mixture of depleted-mantle and crustal materials.

The two episodes of terrane accretion at ∼3.5 and 3.2 Ga correspond to ages of TTG magmatism. This compressional tectonic regime, and the partial melting of greenstone-type material, suggest that basaltic amphibolites of the greenstone sequences are the source materials for the TTG suites. The likely source rocks for the GMS suite are not easily deduced, but chemistry and εNd values of the Boesmanskop syenite suggest a hybrid mantle-crustal source. This type of hybrid source might also explain the features of the monzogranitic batholiths. Close associations between syenite and monzogranites are common, particularly in post-orogenic extensional/transtensional settings. Although extensional activity has not been documented in Barberton, ∼3.1 Ga strike-slip activity has. A post-orogenic thinning of the crust might explain the production of large voluminous monzogranite batholiths and the passive nature of their intrusion dynamics." (Yearron "et al.", 2003).

Hooggenoeg Formation of the Barberton greenstone belt

Some controversy exists pertaining to the origin and emplacement of Archaean felsic suites. According to a dissertation by Louzada (2003): "The upper part of the Hooggenoeg Formation is characterized by ultramafic massive and pillow lavas, a trondhjemitic suite of silicified felsic intrusive and flow banded rocks, and sedimentary chert beds. Veins of felsic, chert and ultramafic material intrude the belt. The depositional environment is thought to be a shoaling shallow sea in which the Hooggenoeg Formation has been deposited in a west-block down, listric faulted, synsedimentary setting."

The Hooggenoeg Formation felsic rocks can be divided into two groups: an intrusive group of interlocking and shallow intrusive rocks, and a porphyritic group of rocks from the veins. Lavas from the upper part of the felsic unit are too altered to be assigned to one of these groups. The intrusive group is related to the tonalite-trondhjemite-granodiorite TTG-suite Stolzburg Pluton, which intruded along the southern margin of the Barberton greenstone belt. Melting of an amphibolite quartz eclogite has been suggested as a probable origin for these high-Al₂O₃ felsic magmas. Ultramafic rocks of the Hooggenoeg Formation were most likely not parental for the felsic rocks. Subduction processes may have played a role in the generation of the felsic rocks, but a tectonic setting for the ultramafic rocks remains uncertain. The felsic units of the Hooggenoeg Formation are very similar to those of the Panorama Formation of the Early Archaean Coppin Gap greenstone belt of Western Australia (See Yilgarn craton). Similarities in geological setting, petrography, and geochemical (trace element in particular) characteristics suggest a possible genetic relation between the two formations and support the theory that a combined continent Vaalbara existed ~3.45 Ga. (Louzada, 2003).

Johannesburg Dome

The Archaean Johannesburg Dome is located in the central part of the Kaapvaal craton and consists of trondhjemitic and tonalitic granitic rocks intruded into mafic-ultramafic greenstone. Studies using U-Pb single zircon dating for granitoid samples yield an age of 3340 +/- 3 Ma and represents the oldest granitoid phase recognized so far. "Following the trondhjemite-tonalite gneiss emplacement a further period of magmatism took place on the dome, which resulted in the intrusion of mafic dykes that are manifest as hornblende amphibolites. The age of these dykes has yet to be determined quantitatively, but they fall within the time constraints imposed by the age of the trondhjemitic gneisses (3340-3200 Ma) and later, crosscutting, potassic granitoids. These rocks. consisting dominantly of granodiorites constitute the third magmatic event and occupy an area of batholithic dimensions extending across most of the southern portion of the dome. The southern and southeastern parts of the batholith consist mainly of medium-grained, homogeneous, grey granodiorites dated at 3121 +/- 5 Ma....The data, combined with that from other parts of the Kaapvaal craton, further supports the view that the evolution of the craton was long-lived and episodic, and that it grew by accretionary processes, becoming generally younger to the north and west of the ca. 3.5 Ga Barberton-Swaziland granite-greenstone terrane situated in the southeastern part of the craton." (Poujol and Anheusser, 2001).

∼3.25 Ga Barberton greenstone belt asteroid impact

The ∼3.26–3.24 Ga asteroid impact on the Barberton greenstone belt represents the first documented example of a "possible" cause–effect relation between extraterrestrial bombardment and major tectonic and igneous events. This study is by Glikson and Vickers(2006). The asteroid ejecta are located immediately above a sharp break between a >12 km-thick mafic–ultramafic volcanic crust and turbidite–felsic volcanic rift-facies association. A boundary there is correlated with break and peak magmatic and faulting events, represented by the truncation of a 3.255–3.235 Ga volcanic sequence at the Sulphur Springs Group (SSG) by a turbidite-banded iron formation–felsic volcanic association. These events are accompanied by ∼3.252–3.235 Ga granitoids. The top of the komatiite–tholeiite–rhyolite sequence of the SSG is associated with a marker chert defined at 3.238–3.235 Ga, abruptly overlain by an olistostrome consisting of mega-clasts of felsic volcanics, chert and siltstone up to 250×150 m-large, intercalated with siliciclastic sedimentary rocks and felsic volcanics.

The structure and scale of the olistostrome is interpreted as intense faulting and rifting, supported by topographic relief represented by deep incision of overlying arenites (the Corboy Formation) into underlying units. The age of these fault and rifts overlaps the 3.255–3.235 Ga peak igneous activity represented by the SSG and the Cleland plutonic suite and the 3.258 Ga Barberton impact unit. Should correlations between the Barberton impact units and magmatic and tectonic events in the Pilbara Craton be confirmed, this would imply impact-triggered reactivation of mantle convection, crustal anatexis, faulting and strong vertical movements in Archaean granite–greenstone terrains associated with a large asteroid impact, culminating in transformation from sima-dominated crust to continental rift environments.

ee also

*Zimbabwe craton
*Pilbara craton
*Vaalbara

References

* Beukes, N.J., Dorland, H.C., Gutzmer, J., Evans, D.A.D. and Armstrong, R.A. (2004) [http://gsa.confex.com/gsa/2004AM/finalprogram/abstract_78610.htm "Timing and Provenance of Neoarchean-Paleoproterozoic Unconformity Bounded Sequences on the Kaapval Craton"] , Geological Society of America "Abstracts with Programs", 36 (5), 255
* Chavagnac, V., Kramers, J.D. and Naegler, T.F. (1999) [http://www.the-conference.com/JConfAbs/4/140.html "Can we Still Trust Nd Model Ages on Migmatized Proterozoic Rocks?"] , Early Evolution of the Continental Crust, "Journal of Conference Abstracts", 4 (1), A08:4A:13:G2
* Glikson, A. and Vickers, J. (2006) [http://www.chemweb.com/journals?type=issue&jid=0012821X&iid=02410001 "The 3.26–3.24 Ga Barberton asteroid impact cluster: Tests of tectonic and magmatic consequences, Pilbara Craton, Western Australia"] , "Earth and Planetary Science Letters", 241 (1-2), 11–20, doi:10.1016/j.epsl.2005.10.022
* Louzda, K.L. (2003) [http://www.people.fas.harvard.edu/%7elouzada/research.htm "The magmatic evolution of the upper ~3450 Ma Hooggenoeg Formation, Barberton greenstone belt, Kaapvaal Craton, South Africa"] , Utrecht University : unpubl. MSc project abstr.
* Poujol, M. and Anhaeusser, C.R. (2001) [http://serials.cib.unibo.it/cgi-ser/start/it/spogli/df-s.tcl?prog_art=3844649&language=ITALIANO&view=articoli "The Johannesburg Dome, South Africa: new single zircon U-Pb isotopic evidence for early Archaean granite-greenstone development within the central Kaapvaal Craton"] , "Precambrian Research", 108 (1-2), 139–157, doi:10.1016/S0301-9268(00)00161-3
* Poujol, M., Robb, L.J., Anhaeusser, C.R. and Gericke, B. (2003) "A review of the geochronological constraints on the evolution of the Kaapvaal Craton, South Africa", "Precambrian Research", 127 (1-3), 181–213, doi:10.1016/S0301-9268(03)00187-6
* Westraat, J.D, Kisters, A.F.M., Poujo, M. and Stevens, G. (2005) [http://www.ingentaconnect.com/content/geol/jgs/2005/00000162/00000002/art00012 "Transcurrent shearing, granite sheeting and the incremental construction of the tabular 3.1 Ga Mpuluzi batholith, Barberton granite–greenstone terrane, South Africa"] , "Journal of the Geological Society", 162 (2), 373-388, doi:10.1144/0016-764904-026
* Yearron, L.M., Clemens, J.D., Stevens, G. and Anhaeusser, C.R.(2003) [http://www.cosis.net/abstracts/EAE03/02639/EAE03-J-02639.pdf "Geochemistry and Petrogenesis of the Granitoids of the Barberton Mountainlan, South Africa"] , "Geophysical Research Abstracts", 5, 02639
* Zegers, T.E., de Wit, M.J., Dann, J. and White, S.H. (1998) [http://www.geo.uu.nl/Research/Paleomagnetism/publications/Zegers98b.pdf "Vaalbara, Earth's oldest assembled continent? A combined. structural, geochronological, and palaeomagnetic test"] , "Terra Nova", 10, 250-259