Zimbabwe craton
Encyclopedia
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 (Ga.) in the southern Africa
n nation of Zimbabwe
. 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. The 250 km wide Limpopo belt
is an east-northeast
trending zone of granulite
facies tectonites that separates the granitoid-greenstone
terrane
s of the Kaapvaal and Zimbabwe cratons.
, Zvishavane and Masvingo
. The much smaller northern fragment is known as the Rhodesdale Gneiss. Granitoids of the same and greater age as the 3.46 Ga Tokwe granitoid gneisses in the south central portion of the Zimbabwe craton are recognised in the central Midlands region, strongly suggesting synchronous formation of the two areas. This crustal block, also known as the Sebakwe Proto-craton, was fully stabilised by around 3.35 Ga with a later granitoid emplacement event around 3.2 Ga and provided the basement for the 3.0-2.6 Ga Late Archaean granite-greenstone magma
tism. This second stage of evolution therefore resulted in the present predominance of these rocks, and occurred ca. 400 Ma after the initial stabilisation of the craton. The synchroneity and extent of the Tokwe Segment is considered strong evidence supporting a predominantly intra-cratonic origin for the Late Archaean greenstone belt
s of Zimbabwe and refuting an arc accretion origin for the craton.
and volcanism
probably occurred here as a result of intracontinental extension associated with late Archaean plume
.
The Ngezi Group consists of a basal sedimentary sequence (Manjeri Formation), overlain by komatiitic and tholeiitic volcanic rocks (Reliance and Zeedebergs Formations), and a second sedimentary sequence (Cheshire Formation). The late Archaean (~2.7 Ga) Ngezi Group greenstones in the Belingwe Greenstone Belt, south central Zimbabwe, show uplift and increased erosion during deposition of alluvial fan
s, local derivation of sedimentary material containing no arc derived component and eruption of komatiitic lava. Sedimentation and volcanism probably occurred as a result of intracontinental extension associated with an active, late Archaean plume.
The Manjeri Formation was deposited in a fluviatile and shallow marine setting, with subsequent alluvial fans and fan-deltas during active tectonism. Changes in the degree of chemical weathering
of the provenance area during deposition of the formation, as measured by CIA values, reflect uplift and increased rates of erosion. Detrital mineralogy and rare-earth element
patterns, are consistent with derivation from very local sources. Palæogeographic variation in the measured Sm-Nd depleted-mantle model ages between 2.9 and 3.7 Ga are consistent with deposition over basement varying in age from ca. 2.9 Ga - 3.5 Ga, again suggesting local derivation. The facies and geochemical association imply sedimentation in an extensional continental setting.
Samples by the NERCMAR drill hole through the 2.7 Ga Manjeri Formation in the Belingwe Greenstone Belt compared to data on the metamorphosed and deformed iron formations from the 3.7 Ga Isua Greenstone belt
. Carbon and sulphur isotopic fractionations in the Belingwe samples may be interpreted in terms of a complex bacteria/archaea eclogical community. REE and Nd-isotopic variations may be modelled by contributions from a reduced hydrothermal component and a component surprisingly similar in REE pattern to modern seawater. Isua rocks are less well preserved but the overall similarity of the REE compositions implies deposition from a broadly similar ocean to that in the late Archaean.
The model ages mark a discrete and well established Archean crust forming event in Africa. Small observed variations may indicate minor contribution of juvenile crust. U/Pb isotopic multigrain analyses of distinctly zoned zircons revealed highly discordant ages of 2.507-2.585 Ga for the Wedza and 2.402-2.448 Ga for the Chilimanzi suite. These ages confirm the intrusion age of about 2.6 Ga for the Wedza suite. However, the age of 2.4 Ga for the posttectonic Chilimanzi suite conflicts with the timing of the Great Dyke
emplacement into an already consolidated crust. The geochemical and radiometric investigations suggest a dynamic crust forming process initialised at ca. 3.2-2.9 Ga with the formation of a crustal protolith. The final stage of this process is marked by emplacement of large volumes of monzogranitic/granodioritc material at 2.6 to 2.4 Ga.
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...
of ancient continental crust and an example of Early Archaean lithology
Lithology
The lithology of a rock unit is a description of its physical characteristics visible at outcrop, in hand or core samples or with low magnification microscopy, such as colour, texture, grain size, or composition. It may be either a detailed description of these characteristics or be a summary of...
dating back to 3.5 billion years ago (Ga.) in the 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...
n nation of Zimbabwe
Zimbabwe
Zimbabwe is a landlocked country located in the southern part of the African continent, between the Zambezi and Limpopo rivers. It is bordered by South Africa to the south, Botswana to the southwest, Zambia and a tip of Namibia to the northwest and Mozambique to the east. Zimbabwe has three...
. 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
Marginal zone
The marginal zone is the region at the interface between the non-lymphoid red pulp and the lymphoid white-pulp of the spleen. A marginal zone also exists in lymph nodes.-Composition and markers:It is composed of cells derived...
of the Kaapvaal craton
Kaapvaal craton
The Kaapvaal craton , along with the Pilbara craton of Western Australia, are the only remaining areas of pristine 3.6-2.5 Ga crust on Earth...
to the Northern Marginal Zone of the Zimbabwe craton ca. 2.8-2.5 Ga. The 250 km wide 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...
is an east-northeast
Boxing the compass
Boxing the compass is the action of naming all thirty-two points of the compass in clockwise order. Such names are formed by the initials of the cardinal directions and their intermediate ordinal directions, and are very handy to refer to a heading in a general or colloquial fashion, without...
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 tectonites that separates the granitoid-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 of the Kaapvaal and Zimbabwe cratons.
Tokwe Segment
This is the larger of two Early Archaean continental crustal fragments that stabilised in Zimbabwe before 3.3 Ga, extending for over 450 km between ShurugwiShurugwi
Shurugwi, formerly Selukwe, a town and administrative centre in Midlands Province, southern Zimbabwe, located about 350 km south of Harare; population 16,138...
, Zvishavane and Masvingo
Masvingo
Masvingo is a town in south-eastern Zimbabwe and the capital of Masvingo Province. The town is close to Great Zimbabwe, the national monument from which the country takes its name.- History :...
. The much smaller northern fragment is known as the Rhodesdale Gneiss. Granitoids of the same and greater age as the 3.46 Ga Tokwe granitoid gneisses in the south central portion of the Zimbabwe craton are recognised in the central Midlands region, strongly suggesting synchronous formation of the two areas. This crustal block, also known as the Sebakwe Proto-craton, was fully stabilised by around 3.35 Ga with a later granitoid emplacement event around 3.2 Ga and provided the basement for the 3.0-2.6 Ga Late Archaean granite-greenstone magma
Magma
Magma is a mixture of molten rock, volatiles and solids that is found beneath the surface of the Earth, and is expected to exist on other terrestrial planets. Besides molten rock, magma may also contain suspended crystals and dissolved gas and sometimes also gas bubbles. Magma often collects in...
tism. This second stage of evolution therefore resulted in the present predominance of these rocks, and occurred ca. 400 Ma after the initial stabilisation of the craton. The synchroneity and extent of the Tokwe Segment is considered strong evidence supporting a predominantly intra-cratonic origin for the Late 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 of Zimbabwe and refuting an arc accretion origin for the craton.
Ca. 2.7 Ngezi Group
The Ngezi Group, ca. 2.7 Ga, of the Bulawayan Supergroup is found throughout the Zimbabwean Province. In the Belingwe Greenstone Belt of south central Zimbabwe, the Ngezi Group has been uplifted and eroded. SedimentationSedimentation
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...
and volcanism
Volcanism
Volcanism is the phenomenon connected with volcanoes and volcanic activity. It includes all phenomena resulting from and causing magma within the crust or mantle of a planet to rise through the crust and form volcanic rocks on the surface....
probably occurred here as a result of intracontinental extension associated with late Archaean plume
Mantle plume
A mantle plume is a hypothetical thermal diapir of abnormally hot rock that nucleates at the core-mantle boundary and rises through the Earth's mantle. Such plumes were invoked in 1971 to explain volcanic regions that were not thought to be explicable by the then-new theory of plate tectonics. Some...
.
The Ngezi Group consists of a basal sedimentary sequence (Manjeri Formation), overlain by komatiitic and tholeiitic volcanic rocks (Reliance and Zeedebergs Formations), and a second sedimentary sequence (Cheshire Formation). The late Archaean (~2.7 Ga) Ngezi Group greenstones in the Belingwe Greenstone Belt, south central Zimbabwe, show uplift and increased erosion during deposition of alluvial fan
Alluvial fan
An alluvial fan is a fan-shaped deposit formed where a fast flowing stream flattens, slows, and spreads typically at the exit of a canyon onto a flatter plain. A convergence of neighboring alluvial fans into a single apron of deposits against a slope is called a bajada, or compound alluvial...
s, local derivation of sedimentary material containing no arc derived component and eruption of komatiitic lava. Sedimentation and volcanism probably occurred as a result of intracontinental extension associated with an active, late Archaean plume.
The Manjeri Formation was deposited in a fluviatile and shallow marine setting, with subsequent alluvial fans and fan-deltas during active tectonism. Changes in the degree of chemical weathering
Weathering
Weathering is the breaking down of rocks, soils and minerals as well as artificial materials through contact with the Earth's atmosphere, biota and waters...
of the provenance area during deposition of the formation, as measured by CIA values, reflect uplift and increased rates of erosion. Detrital mineralogy and rare-earth element
Rare earth element
As defined by IUPAC, rare earth elements or rare earth metals are a set of seventeen chemical elements in the periodic table, specifically the fifteen lanthanides plus scandium and yttrium...
patterns, are consistent with derivation from very local sources. Palæogeographic variation in the measured Sm-Nd depleted-mantle model ages between 2.9 and 3.7 Ga are consistent with deposition over basement varying in age from ca. 2.9 Ga - 3.5 Ga, again suggesting local derivation. The facies and geochemical association imply sedimentation in an extensional continental setting.
Samples by the NERCMAR drill hole through the 2.7 Ga Manjeri Formation in the Belingwe Greenstone Belt compared to data on the metamorphosed and deformed iron formations from the 3.7 Ga 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...
. Carbon and sulphur isotopic fractionations in the Belingwe samples may be interpreted in terms of a complex bacteria/archaea eclogical community. REE and Nd-isotopic variations may be modelled by contributions from a reduced hydrothermal component and a component surprisingly similar in REE pattern to modern seawater. Isua rocks are less well preserved but the overall similarity of the REE compositions implies deposition from a broadly similar ocean to that in the late Archaean.
Ca. 2.5 Ga Wedza and Chilimanzi suites, south-eastern Zimbabwe craton
The final stage of the cratonization process of the Zimbabwe craton is marked by emplacement of large volumes of monzogranitic and granodioritc material at 2.6-2.4 Ga. Monzogranitic and granodioritic granitoids and gneisses of the Wedza and Chilimanzi suites form large intrusive complexes in the south-eastern part of the Zimbabwe craton, between the Mutare-Odzi greenstone belt and the Northern Marginal Zone of the Limpopo Belt. The older units of the Wedza suite are of syn-kinematic origin, while the younger Chilimanzi suite was emplaced late- to post-kinematically. Internal differentiation of the major elements indicates similar paths of magmatic evolution for both suites. Comparison with the major element distribution of older granodioritic/tonalitic intrusives within the Mutare greenstone belt suggests an overall magmatic evolution from primitive, tonalitic towards monzogranitic compositions.The model ages mark a discrete and well established Archean crust forming event in Africa. Small observed variations may indicate minor contribution of juvenile crust. U/Pb isotopic multigrain analyses of distinctly zoned zircons revealed highly discordant ages of 2.507-2.585 Ga for the Wedza and 2.402-2.448 Ga for the Chilimanzi suite. These ages confirm the intrusion age of about 2.6 Ga for the Wedza suite. However, the age of 2.4 Ga for the posttectonic Chilimanzi suite conflicts with the timing of the Great Dyke
Great Dyke
The Great Dyke is a linear geological feature that trends nearly north-south through the centre of Zimbabwe passing just to the west of the capital, Harare. It consists of a band of short, narrow ridges and hills spanning for approximately . The hills become taller as the range goes north, and...
emplacement into an already consolidated crust. The geochemical and radiometric investigations suggest a dynamic crust forming process initialised at ca. 3.2-2.9 Ga with the formation of a crustal protolith. The final stage of this process is marked by emplacement of large volumes of monzogranitic/granodioritc material at 2.6 to 2.4 Ga.