Kaapvaal craton
Encyclopedia
The Kaapvaal craton along with the Pilbara craton
of Western Australia
, are the only remaining areas of pristine 3.6-2.5 Ga (billion years ago) crust on Earth. Similarities of rock records from both these craton
s, 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 1200000 km² (463,322.6 sq mi) and is joined to the Zimbabwe craton
to the north by the Limpopo Belt
. To the south and west, the Kaapvaal craton is flanked by Proterozoic
orogen
s, 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 Ga by the emplacement of major granitoid
batholith
s that thickened and stabilized 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
terrane
s and older tonalitic
gneiss
es (ca. 3.6-3.7 Ga), intruded by a variety of granitic pluton
s (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 rift
ing 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
approximately 2.8-2.5 Ga by the 250 kilometres (155.3 mi) wide orogenic
Limpopo Belt. The belt is an east-northeast trending zone of granulite
facies
tectonite
s that separates the granitoid-greenstone terranes of the Kaapvaal and Zimbabwe cratons.
(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 zircon
s of this time interval as well as many ~2.0 Ga detrital
zircon
s. This implies that the Limpopo Complex together with the Zimbabwe craton only became attached to the Kaapvaal craton at approximately 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.
greenstone belt of the Swaziland–Barberton areas and these contain some of the oldest traces of life on earth. 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-million-year time span and can be divided into two suites: The tonalite-trondhjemite-granodiorite (TTG) suite (emplaced approximately 3.5–3.2 Ga), and the granite-monzogranite-syenite
granite (GMS) suite (emplaced approximately 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 about 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 amalgamation 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 belt
s 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 dike
s 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 million years. 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."
s, trondhjemite
s and granodiorite
s; and the GMS suite (emplaced approximately 3.2–3.1 Ga) includes granite
s, 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 Europium anomalies
, while the Steynsdorp and Doornhoek pluton
s are relatively HREE-undepleted with significant Eu
anomalies. Nd (neodymium
) isotope analyses show that the 3.4 Ga TTGs have positive εNd (epsilon Samarium-neodymium dating
) 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 europium.
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 amarium-neodymium_dating#Epsilon_notationεNd 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 values (-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).
suites. According to a dissertation by Louzada (2003): "The upper part of the Hooggenoeg Formation is characterized by ultramafic massive and pillow lava
s, 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).
These rocks consisting mainly 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).
-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 arenite
s (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.
Pilbara craton
The Pilbara craton , along with the Kaapvaal craton are the only remaining areas of pristine Archaean 3.6-2.7 Ga crust on Earth...
of Western Australia
Western Australia
Western Australia is a state of Australia, occupying the entire western third of the Australian continent. It is bounded by the Indian Ocean to the north and west, the Great Australian Bight and Indian Ocean to the south, the Northern Territory to the north-east and South Australia to the south-east...
, are the only remaining areas of pristine 3.6-2.5 Ga (billion years ago) crust on Earth. Similarities of rock records from both these craton
Craton
A craton is an old and stable part of the continental lithosphere. Having often survived cycles of merging and rifting of continents, cratons are generally found in the interiors of tectonic plates. They are characteristically composed of ancient crystalline basement rock, which may be covered by...
s, especially of the overlying late Archean
Archean
The Archean , also spelled Archeozoic or Archæozoic) is a geologic eon before the Paleoproterozoic Era of the Proterozoic Eon, before 2.5 Ga ago. Instead of being based on stratigraphy, this date is defined chronometrically...
sequences, suggest that they were once part of the Vaalbara
Vaalbara
Vaalbara is theorized to be Earth's first supercontinent, beginning its formation about , completing its formation by about and breaking up by . The name Vaalbara is derived from the South African Kaapvaal craton and the West Australian Pilbara craton...
supercontinent
Supercontinent
In geology, a supercontinent is a landmass comprising more than one continental core, or craton. The assembly of cratons and accreted terranes that form Eurasia qualifies as a supercontinent today.-History:...
(Zegers et al., 1998).
The Kaapvaal craton covers an area of approximately 1200000 km² (463,322.6 sq mi) and is joined to the Zimbabwe craton
Zimbabwe craton
The Zimbabwe craton is an area in Southern Africa of ancient continental crust and an example of Early Archaean lithology dating back to 3.5 billion years ago in the southern African nation of Zimbabwe. Late Archean metamorphism joined the Southern Marginal Zone of the Kaapvaal craton to the...
to the north by the Limpopo Belt
Limpopo Belt
The Limpopo Belt is located in South Africa and Zimbabwe, runs E-NE, and joins the Kaapvaal craton to the south with the Zimbabwe craton to the north. The belt is of high-grade metamorphic rocks that have undergone a long cycle of metamorphism and deformation that ended 2.0 billion years ago, after...
. To the south and west, the Kaapvaal craton is flanked by Proterozoic
Proterozoic
The Proterozoic is a geological eon representing a period before the first abundant complex life on Earth. The name Proterozoic comes from the Greek "earlier life"...
orogen
Orogeny
Orogeny refers to forces and events leading to a severe structural deformation of the Earth's crust due to the engagement of tectonic plates. Response to such engagement results in the formation of long tracts of highly deformed rock called orogens or orogenic belts...
s, and to the east by the Lebombo
Lebombo Mountains
The Lebombo Mountains, also called Lubombo Mountains, are an 800km long, narrow range of mountains in Southern Africa stretching from Hluhluwe in KwaZulu-Natal in the south to Punda Maria in the Limpopo Province in South Africa in the north. Part of the mountains are found in South Africa,...
monocline
Monocline
A monocline is a step-like fold in rock strata consisting of a zone of steeper dip within an otherwise horizontal or gently-dipping sequence.-Formation:Monoclines may be formed in several different ways...
that contains Jurassic
Jurassic
The Jurassic is a geologic period and system that extends from about Mya to Mya, that is, from the end of the Triassic to the beginning of the Cretaceous. The Jurassic constitutes the middle period of the Mesozoic era, also known as the age of reptiles. The start of the period is marked by...
igneous rocks associated with the breakup of Gondwana
Gondwana
In paleogeography, Gondwana , originally Gondwanaland, was the southernmost of two supercontinents that later became parts of the Pangaea supercontinent. It existed from approximately 510 to 180 million years ago . Gondwana is believed to have sutured between ca. 570 and 510 Mya,...
.
The Kaapvaal craton formed and stabilized between 3.7 and 2.6 Ga by the emplacement of major granitoid
Granite
Granite is a common and widely occurring type of intrusive, felsic, igneous rock. Granite usually has a medium- to coarse-grained texture. Occasionally some individual crystals are larger than the groundmass, in which case the texture is known as porphyritic. A granitic rock with a porphyritic...
batholith
Batholith
A batholith is a large emplacement of igneous intrusive rock that forms from cooled magma deep in the Earth's crust...
s that thickened and stabilized the continental crust
Crust (geology)
In geology, the crust is the outermost solid shell of a rocky planet or natural satellite, which is chemically distinct from the underlying mantle...
during the early stages of an arc
Volcanic arc
A volcanic arc is a chain of volcanoes positioned in an arc shape as seen from above. Offshore volcanoes form islands, resulting in a volcanic island arc. Generally they result from the subduction of an oceanic tectonic plate under another tectonic plate, and often parallel an oceanic trench...
-related magmatism
Magmatism
The formation of igneous rocks from magma is known as Magmatism.Magmatism is a process responsible for mountain formation. The process of magmatism produces an additional mass and volume to the Earth’s surface. For example the formation of volcanoes or island arcs at convergent plate boundaries...
and sedimentation
Sedimentation
Sedimentation is the tendency for particles in suspension to settle out of the fluid in which they are entrained, and come to rest against a barrier. This is due to their motion through the fluid in response to the forces acting on them: these forces can be due to gravity, centrifugal acceleration...
cycle. The craton is a mixture of early Archean (3.0-3.5 Ga) granite greenstone
Greenstone belt
Greenstone belts are zones of variably metamorphosed mafic to ultramafic volcanic sequences with associated sedimentary rocks that occur within Archaean and Proterozoic cratons between granite and gneiss bodies....
terrane
Terrane
A terrane in geology is short-hand term for a tectonostratigraphic terrane, which is a fragment of crustal material formed on, or broken off from, one tectonic plate and accreted or "sutured" to crust lying on another plate...
s and older tonalitic
Tonalite
Tonalite is an igneous, plutonic rock, of felsic composition, with phaneritic texture. Feldspar is present as plagioclase with 10% or less alkali feldspar. Quartz is present as more than 20% of the rock. Amphiboles and pyroxenes are common accessory minerals.In older references tonalite is...
gneiss
Gneiss
Gneiss is a common and widely distributed type of rock formed by high-grade regional metamorphic processes from pre-existing formations that were originally either igneous or sedimentary rocks.-Etymology:...
es (ca. 3.6-3.7 Ga), intruded by a variety of granitic pluton
Pluton
A pluton in geology is a body of intrusive igneous rock that crystallized from magma slowly cooling below the surface of the Earth. Plutons include batholiths, dikes, sills, laccoliths, lopoliths, and other igneous bodies...
s (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
Succession (geology)
A succession, in geology, is a group of rock units or strata that succeed one another in chronological order. Rock successions are often easily seen on exposed sections of a geological column or wall.Based on uncomfortable or comfortable succession....
of basins filled with thick sequences of both volcanic and sedimentary rocks. This was then followed by episodic extension and rift
Rift
In geology, a rift or chasm is a place where the Earth's crust and lithosphere are being pulled apart and is an example of extensional tectonics....
ing 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
Metamorphism
Metamorphism is the solid-state recrystallization of pre-existing rocks due to changes in physical and chemical conditions, primarily heat, pressure, and the introduction of chemically active fluids. Mineralogical, chemical and crystallographic changes can occur during this process...
joined the Southern Marginal Zone of the Kaapvaal craton
Craton
A craton is an old and stable part of the continental lithosphere. Having often survived cycles of merging and rifting of continents, cratons are generally found in the interiors of tectonic plates. They are characteristically composed of ancient crystalline basement rock, which may be covered by...
to the Northern Marginal Zone of the Zimbabwe craton
Zimbabwe craton
The Zimbabwe craton is an area in Southern Africa of ancient continental crust and an example of Early Archaean lithology dating back to 3.5 billion years ago in the southern African nation of Zimbabwe. Late Archean metamorphism joined the Southern Marginal Zone of the Kaapvaal craton to the...
approximately 2.8-2.5 Ga by the 250 kilometres (155.3 mi) wide orogenic
Orogeny
Orogeny refers to forces and events leading to a severe structural deformation of the Earth's crust due to the engagement of tectonic plates. Response to such engagement results in the formation of long tracts of highly deformed rock called orogens or orogenic belts...
Limpopo Belt. The belt is an east-northeast trending zone of granulite
Granulite
Granulites are medium to coarse–grained metamorphic rocks that have experienced high temperature metamorphism, composed mainly of feldspars sometimes associated with quartz and anhydrous ferromagnesian minerals, with granoblastic texture and gneissose to massive structure...
facies
Facies
In geology, facies are a body of rock with specified characteristics. Ideally, a facies is a distinctive rock unit that forms under certain conditions of sedimentation, reflecting a particular process or environment....
tectonite
Tectonite
Tectonites are metamorphic or tectonically deformed rocks whose fabric reflects the history of their deformation, or rocks with fabric that clearly displays coordinated geometric features that indicate continuous solid flow during formation. Planar foliation results from a parallel orientation of...
s 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-granodioriteTonalite-Trondhjemite-Granodiorite
Tonalite–trondhjemite–granodiorite series are an aggregation of rocks that are formed by melting of hydrous mafic crust at high pressure. It is widely accepted that most Archaean granite–greenstones are dominated by TTG, although Late Archaean terranes, such as in the Yilgarn Craton, are dominated...
(TTG) such as the Sand River Gneisses and the Bulai Granite intrusion. Early Proterozoic
Proterozoic
The Proterozoic is a geological eon representing a period before the first abundant complex life on Earth. The name Proterozoic comes from the Greek "earlier life"...
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
Neoarchean
The Neoarchean is a geologic era within the Archaean. It spans the period of time from —the period being defined chronometrically and not referenced to a specific level in a rock section on Earth...
to early Paleoproterozoic
Paleoproterozoic
The Paleoproterozoic is the first of the three sub-divisions of the Proterozoic occurring between . This is when the continents first stabilized...
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
Paleoproterozoic
The Paleoproterozoic is the first of the three sub-divisions of the Proterozoic occurring between . This is when the continents first stabilized...
red bed successions
Succession (geology)
A succession, in geology, is a group of rock units or strata that succeed one another in chronological order. Rock successions are often easily seen on exposed sections of a geological column or wall.Based on uncomfortable or comfortable succession....
contain zircon
Zircon
Zircon is a mineral belonging to the group of nesosilicates. Its chemical name is zirconium silicate and its corresponding chemical formula is ZrSiO4. A common empirical formula showing some of the range of substitution in zircon is 1–x4x–y...
s of this time interval as well as many ~2.0 Ga detrital
Detrital
Detritus is a geological term used to describe particles of rock derived from pre-existing rock through processes of weathering and erosion. Detrital particles can consist of lithic fragments , or of monomineralic fragments...
zircon
Zircon
Zircon is a mineral belonging to the group of nesosilicates. Its chemical name is zirconium silicate and its corresponding chemical formula is ZrSiO4. A common empirical formula showing some of the range of substitution in zircon is 1–x4x–y...
s. This implies that the Limpopo Complex together with the Zimbabwe craton only became attached to the Kaapvaal craton at approximately 2.0 Ga during formation of the Magondi Mobile Belt which in turn sourced the voluminous late Paleoproterozoic
Paleoproterozoic
The Paleoproterozoic is the first of the three sub-divisions of the Proterozoic occurring between . This is when the continents first stabilized...
red beds of southern Africa
Southern Africa
Southern Africa is the southernmost region of the African continent, variably defined by geography or geopolitics. Within the region are numerous territories, including the Republic of South Africa ; nowadays, the simpler term South Africa is generally reserved for the country in English.-UN...
. (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 edge of Kaapvaal Craton. It is well known for its gold mineralisation and for its komatiites, an unusual type of ultramafic volcanic rock named after the Komati River that flows through the belt. Some of the oldest exposed rocks on Earth (greater than 3.6 Ga) are located in the BarbertonBarberton, Mpumalanga
Barberton is a town in the Mpumalanga province of South Africa, which has its origin in the 1880s gold rush in the region. It is situated in the De Kaap Valley and is fringed by the Mkhonjwa Mountains...
greenstone belt of the Swaziland–Barberton areas and these contain some of the oldest traces of life on earth. Only the rocks found in the Isua greenstone belt
Isua greenstone belt
The Isua Greenstone Belt is an Archean greenstone belt in southwestern Greenland. The belt is aged between 3.7 and 3.8 Ga, making it among the oldest rock in the world. The belt contains variably metamorphosed mafic volcanic and sedimentary rocks...
of Western Greenland
Greenland
Greenland is an autonomous country within the Kingdom of Denmark, located between the Arctic and Atlantic Oceans, east of the Canadian Arctic Archipelago. Though physiographically a part of the continent of North America, Greenland has been politically and culturally associated with Europe for...
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-million-year time span and can be divided into two suites: The tonalite-trondhjemite-granodiorite (TTG) suite (emplaced approximately 3.5–3.2 Ga), and the granite-monzogranite-syenite
Syenite
Syenite is a coarse-grained intrusive igneous rock of the same general composition as granite but with the quartz either absent or present in relatively small amounts Syenite is a coarse-grained intrusive igneous rock of the same general composition as granite but with the quartz either absent or...
granite (GMS) suite (emplaced approximately 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
Accretion (geology)
Accretion is a process by which material is added to a tectonic plate or a landmass. This material may be sediment, volcanic arcs, seamounts or other igneous features.-Description:...
at about 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 amalgamation of small protocontinental
Craton
A craton is an old and stable part of the continental lithosphere. Having often survived cycles of merging and rifting of continents, cratons are generally found in the interiors of tectonic plates. They are characteristically composed of ancient crystalline basement rock, which may be covered by...
blocks. Several small diachronous
Diachronic
Diachronic or Diachronous,from the Greek word Διαχρονικός , is a term for something happening over time. It is used in several fields of research.*Diachronic linguistics : see Historical linguistics...
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 belt
Greenstone belt
Greenstone belts are zones of variably metamorphosed mafic to ultramafic volcanic sequences with associated sedimentary rocks that occur within Archaean and Proterozoic cratons between granite and gneiss bodies....
s 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
Batholith
A batholith is a large emplacement of igneous intrusive rock that forms from cooled magma deep in the Earth's crust...
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 dike
Dike (geology)
A dike or dyke in geology is a type of sheet intrusion referring to any geologic body that cuts discordantly across* planar wall rock structures, such as bedding or foliation...
s and sheets. According to a study done by Westraat et al. (2005): "Multiple intrusive relationships and geochronological
Geochronology
Geochronology is the science of determining the age of rocks, fossils, and sediments, within a certain degree of uncertainty inherent to the method used. A variety of dating methods are used by geologists to achieve this, and schemes of classification and terminology have been proposed...
evidence suggests that granite sheeting and the assembly of the pluton occurred over a period of 3–13 million years. 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 million year time span and can be divided into two suites. The TTG suite (emplaced approximately 3.5–3.2 Ga) contains tonaliteTonalite
Tonalite is an igneous, plutonic rock, of felsic composition, with phaneritic texture. Feldspar is present as plagioclase with 10% or less alkali feldspar. Quartz is present as more than 20% of the rock. Amphiboles and pyroxenes are common accessory minerals.In older references tonalite is...
s, trondhjemite
Trondhjemite
Trondhjemite is a leucocratic intrusive igneous rock. It is a variety of tonalite in which the plagioclase is mostly in the form of oligoclase. Trondhjemites are sometimes known as plagiogranites....
s and granodiorite
Granodiorite
Granodiorite is an intrusive igneous rock similar to granite, but containing more plagioclase than orthoclase-type feldspar. Officially, it is defined as a phaneritic igneous rock with greater than 20% quartz by volume where at least 65% of the feldspar is plagioclase. It usually contains abundant...
s; and the GMS suite (emplaced approximately 3.2–3.1 Ga) includes granite
Granite
Granite is a common and widely occurring type of intrusive, felsic, igneous rock. Granite usually has a medium- to coarse-grained texture. Occasionally some individual crystals are larger than the groundmass, in which case the texture is known as porphyritic. A granitic rock with a porphyritic...
s, monzogranites and a small syenite
Syenite
Syenite is a coarse-grained intrusive igneous rock of the same general composition as granite but with the quartz either absent or present in relatively small amounts Syenite is a coarse-grained intrusive igneous rock of the same general composition as granite but with the quartz either absent or...
-granite complex.
Geochemically, the TTGs are typically low- to medium-K (potassium
Potassium
Potassium is the chemical element with the symbol K and atomic number 19. Elemental potassium is a soft silvery-white alkali metal that oxidizes rapidly in air and is very reactive with water, generating sufficient heat to ignite the hydrogen emitted in the reaction.Potassium and sodium are...
), metaluminous I-type granites. Their chondrite
Chondrite
Chondrites are stony meteorites that have not been modified due to melting or differentiation of the parent body. They formed when various types of dust and small grains that were present in the early solar system accreted to form primitive asteroids...
-normalized
Annealing (metallurgy)
Annealing, in metallurgy and materials science, is a heat treatment wherein a material is altered, causing changes in its properties such as strength and hardness. It is a process that produces conditions by heating to above the recrystallization temperature, maintaining a suitable temperature, and...
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 Europium anomalies
Europium Anomaly
The Europium anomaly, in geochemistry, is the phenomenon whereby Europium concentration is either depleted or enriched in a rock relative to the other rare earth elements ....
, while the Steynsdorp and Doornhoek pluton
Pluton
A pluton in geology is a body of intrusive igneous rock that crystallized from magma slowly cooling below the surface of the Earth. Plutons include batholiths, dikes, sills, laccoliths, lopoliths, and other igneous bodies...
s are relatively HREE-undepleted with significant Eu
Europium
Europium is a chemical element with the symbol Eu and atomic number 63. It is named after the continent of Europe. It is a moderately hard silvery metal which readily oxidizes in air and water...
anomalies. Nd (neodymium
Neodymium
Neodymium is a chemical element with the symbol Nd and atomic number 60. It is a soft silvery metal that tarnishes in air. Neodymium was discovered in 1885 by the Austrian chemist Carl Auer von Welsbach. It is present in significant quantities in the ore minerals monazite and bastnäsite...
) isotope analyses show that the 3.4 Ga TTGs have positive εNd (epsilon Samarium-neodymium dating
Samarium-neodymium dating
Samarium-neodymium dating is useful for determining the age relationships of rocks and meteorites, based on decay of a long-lived samarium isotope to a radiogenic neodymium isotope. Nd isotope ratios are used to provide information on the source of igneous melts as well as to provide age data...
) values of (0 to +3.7), indicative of depleted-mantle sources, similar to the oldest greenstone belt formations (the Onverwacht series
Onverwacht series
The Onverwacht series or Onverwacht group is a greenstone mountain formation from the Archean eon in the Kaapvaal craton in South Africa and Swaziland....
). 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 europium.
According to a study by Yearron et al. (2003): "The TTGs are typically low- to medium-K , metaluminous I-type granites, Their chondrite
Chondrite
Chondrites are stony meteorites that have not been modified due to melting or differentiation of the parent body. They formed when various types of dust and small grains that were present in the early solar system accreted to form primitive asteroids...
-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 amarium-neodymium_dating#Epsilon_notationεNd 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 values (-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
Garnet
The garnet group includes a group of minerals that have been used since the Bronze Age as gemstones and abrasives. The name "garnet" may come from either the Middle English word gernet meaning 'dark red', or the Latin granatus , possibly a reference to the Punica granatum , a plant with red seeds...
and plagioclase
Plagioclase
Plagioclase is an important series of tectosilicate minerals within the feldspar family. Rather than referring to a particular mineral with a specific chemical composition, plagioclase is a solid solution series, more properly known as the plagioclase feldspar series...
, respectively, their depletion suggests the presence of these minerals in the restite
Restite
Restite is the residual material left at the site of melting during the in place production of granite through intense metamorphism.Generally, restite is composed of a predominance of mafic minerals because these are harder to melt . Typical minerals are amphibole, biotite, pyroxene, ilmenite or...
. For the TTG suite, we therefore suggest a garnet-rich amphibolitic or eclogitic
Eclogite
Eclogite is a mafic metamorphic rock. Eclogite is of special interest for at least two reasons. First, it forms at pressures greater than those typical of the crust of the Earth...
depleted-mantle source at a depth >40 km. This has been confirmed by experimental work constraining the stability of garnet in the trondhjemite
Trondhjemite
Trondhjemite is a leucocratic intrusive igneous rock. It is a variety of tonalite in which the plagioclase is mostly in the form of oligoclase. Trondhjemites are sometimes known as plagiogranites....
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
Plagioclase
Plagioclase is an important series of tectosilicate minerals within the feldspar family. Rather than referring to a particular mineral with a specific chemical composition, plagioclase is a solid solution series, more properly known as the plagioclase feldspar series...
-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 felsicFelsic
The word "felsic" is a term used in geology to refer to silicate minerals, magma, and rocks which are enriched in the lighter elements such as silicon, oxygen, aluminium, sodium, and potassium....
suites. According to a dissertation by Louzada (2003): "The upper part of the Hooggenoeg Formation is characterized by ultramafic massive and pillow lava
Pillow lava
Pillow lavas are lavas that contain characteristic pillow-shaped structures that are attributed to the extrusion of the lava under water, or subaqueous extrusion. Pillow lavas in volcanic rock are characterized by thick sequences of discontinuous pillow-shaped masses, commonly up to one metre in...
s, a trondhjemitic
Trondhjemite
Trondhjemite is a leucocratic intrusive igneous rock. It is a variety of tonalite in which the plagioclase is mostly in the form of oligoclase. Trondhjemites are sometimes known as plagiogranites....
suite of silicified felsic intrusive and flow banded rocks, and sedimentary chert
Chert
Chert is a fine-grained silica-rich microcrystalline, cryptocrystalline or microfibrous sedimentary rock that may contain small fossils. It varies greatly in color , but most often manifests as gray, brown, grayish brown and light green to rusty red; its color is an expression of trace elements...
beds. Veins of felsic, chert and ultramafic material intrude the belt. The depositional environment
Sedimentary depositional environment
In geology, sedimentary depositional environment describes the combination of physical, chemical and biological processes associated with the deposition of a particular type of sediment and, therefore, the rock types that will be formed after lithification, if the sediment is preserved in the rock...
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
Porphyritic
Porphyritic is an adjective used in geology, specifically for igneous rocks, for a rock that has a distinct difference in the size of the crystals, with at least one group of crystals obviously larger than another group...
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
Eclogite
Eclogite is a mafic metamorphic rock. Eclogite is of special interest for at least two reasons. First, it forms at pressures greater than those typical of the crust of the Earth...
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
Western Australia
Western Australia is a state of Australia, occupying the entire western third of the Australian continent. It is bounded by the Indian Ocean to the north and west, the Great Australian Bight and Indian Ocean to the south, the Northern Territory to the north-east and South Australia to the south-east...
(See Yilgarn craton
Yilgarn craton
The Yilgarn Craton is a large craton which constitutes the bulk of the Western Australian land mass. It is bounded by a mixture of sedimentary basins and Proterozoic fold and thrust belts...
). Similarities in geological setting, petrography
Petrography
Petrography is a branch of petrology that focuses on detailed descriptions of rocks. Someone who studies petrography is called a petrographer. The mineral content and the textural relationships within the rock are described in detail. Petrographic descriptions start with the field notes at the...
, 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
Vaalbara
Vaalbara is theorized to be Earth's first supercontinent, beginning its formation about , completing its formation by about and breaking up by . The name Vaalbara is derived from the South African Kaapvaal craton and the West Australian Pilbara craton...
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 mainly 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 turbiditeTurbidite
Turbidite geological formations have their origins in turbidity current deposits, which are deposits from a form of underwater avalanche that are responsible for distributing vast amounts of clastic sediment into the deep ocean.-The ideal turbidite sequence:...
-banded iron formation
Banded iron formation
Banded iron formations are distinctive units of sedimentary rock that are almost always of Precambrian age. A typical BIF consists of repeated, thin layers of iron oxides, either magnetite or hematite , alternating with bands of iron-poor shale and chert...
–felsic volcanic association. These events are accompanied by ∼3.252–3.235 Ga granitoids. The top of the komatiite
Komatiite
Komatiite is a type of ultramafic mantle-derived volcanic rock. Komatiites have low silicon, potassium and aluminium, and high to extremely high magnesium content...
–tholeiite
Tholeiite
The tholeiitic magma series is one of two main magma series in igneous rocks, the other magma series being the calc–alkaline. A magma series is a series of compositions that describes the evolution of a mafic magma, which is high in magnesium and iron and produces basalt or gabbro, as it...
–rhyolite
Rhyolite
This page is about a volcanic rock. For the ghost town see Rhyolite, Nevada, and for the satellite system, see Rhyolite/Aquacade.Rhyolite is an igneous, volcanic rock, of felsic composition . It may have any texture from glassy to aphanitic to porphyritic...
sequence of the SSG is associated with a marker chert defined at 3.238–3.235 Ga, abruptly overlain by an olistostrome
Olistostrome
The term olistostrome is derived from olio - a dish of many ingredients or stew; a mixture of heterogeneous elements; hodgepodge. and stroma - a bed covering...
consisting of mega-clasts of felsic volcanics, chert and siltstone
Siltstone
Siltstone is a sedimentary rock which has a grain size in the silt range, finer than sandstone and coarser than claystones.- Description :As its name implies, it is primarily composed of silt sized particles, defined as grains 1/16 - 1/256 mm or 4 to 8 on the Krumbein phi scale...
up to 250×150 m-large, intercalated
Intercalation
Intercalation is the insertion of a leap day, week or month into some calendar years to make the calendar follow the seasons or moon phases. Lunisolar calendars may require intercalations of both days and months.- Solar calendars :...
with siliciclastic
Siliciclastic
Siliciclastic rocks are clastic noncarbonate sedimentary rocks that are almost exclusively silica-bearing, either as forms of quartz or other silicate minerals. All siliciclastic rocks are formed by inorganic processes, or deposited through some mechanical process, such as stream deposits that are...
sedimentary rocks and felsic volcanics.
The structure and scale of the olistostrome
Olistostrome
The term olistostrome is derived from olio - a dish of many ingredients or stew; a mixture of heterogeneous elements; hodgepodge. and stroma - a bed covering...
is interpreted as intense faulting and rifting, supported by topographic
Topology
Topology is a major area of mathematics concerned with properties that are preserved under continuous deformations of objects, such as deformations that involve stretching, but no tearing or gluing...
relief represented by deep incision
Incised
Incised means cut, particularly with a "V" shape. It is a term found in a number of disciplines.-Geology: In geomorphology, the term Incised refers to when a river has cut downward through its riverbed. The river may have been incising through sediment or bedrock. The river begins at one elevation...
of overlying arenite
Arenite
Arenite is a sedimentary clastic rock with sand grain size between 0.0625 mm and 2 mm and contain less than 15% matrix. The related adjective is arenaceous...
s (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
Mantle convection
Mantle convection is the slow creeping motion of Earth's rocky mantle caused by convection currents carrying heat from the interior of the Earth to the surface. The Earth's surface lithosphere, which rides atop the asthenosphere , is divided into a number of plates that are continuously being...
, crustal anatexis
Anatexis
Anatexis in geology, refers to the differential, or partial, melting of rocks, especially in the forming of metamorphic rocks such as migmatites.-Optimum Temperature Conditions for Crustal Melting:...
, faulting and strong vertical movements in Archaean granite–greenstone terrains associated with a large asteroid impact, culminating in transformation from sima
Sima (geology)
Sima is the name for the lower layer of the Earth's crust. This layer is made of rocks rich in magnesium silicate minerals. Typically when the sima comes to the surface it is basalt, so sometimes this layer is called the 'basalt layer' of the crust. The sima layer is also called the 'basal crust'...
-dominated crust to continental rift environments.