Limpopo Belt
Encyclopedia
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 the stabilisation of the adjacent massifs. The belt comprises 3 components: the Central Zone, the North Marginal Zone and the South Marginal Zone.
s separating the granitoid-greenstone terranes of the Kaapvaal and Zimbabwe cratons. Large scale ductile shear zone
s are an integral part of Limpopo belt architecture. They define the boundaries between the belt and the adjacent cratons and separate internal zones within the belt. The shear zones forming the external (northern, southern and western) margins of the belt are interpreted as uplift structures of the overthickened crust.
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 two first periods are mainly characterized by magma
tic activity leading to the formation of Archaean Tonalite
-Trondhjemite
-Granodiorite
(TTG) such as the Sand River Gneisses or the Bulai Granite intrusion. The Early Proterozoic
event took place under high-grade metamorphic
conditions during which partial melting formed large amount of granitic melt.
The Limpopo Central Zone shows relics of late Archean high grade metamorphism. In the Northern (NMZ) and Southern Marginal Zones (SMZ) that adjoin the Zimbabwe and Kaapvaal cratons, respectively, the last high grade metamorphic episodes were late Archean. The relics in the Central Zone are characterised by an anticlockwise p-T-evolution, at ca. 2.55 Ga. In the NMZ repeated crustal remelting and intrusion of charnoenderbitic magmas continued to 2.58 Ga, producing counterclockwise p-T paths. In contrast the SMZ consists of medium to high pressure
granulites, which underwent a clockwise p-T-evolution at ca. 2.69 Ga, followed by decompression and isobaric cooling. In the Northern Kaapvaal Craton (Renosterkoppies Greenstone Belt, Pietersburg area) tectonism
took place under amphibolite facies conditions at ca. 2.75 Ga and can therefore not be related to any events in the Limpopo belt. Thus the different tectonic units have different late Archean tectonometamorphic histories. Trace element
geochemistry as well as Pb + Nd isotope characteristics of the SMZ are similar to those of the Kaapvaal Craton, with low Th and U concentrations around 2 and 0.7 ppm. Low U concentrations in the SMZ are not a consequence of high grade metamorphism. The NMZ, with high Th, U concentrations (10.8 and 2.5 ppm) and radiogenic Pb, resembles the adjoining Zimbabwe craton more. The differences in late Archean tectonic styles between NMZ and SMZ+KC is a possible consequence of the differences in Th and U content of these provinces.
"The Mahalapye granite in the extreme western part of the Central Zone is a post-tectonic intrusion that crystallized at 2.023 Ga. This suggests that mineral ages of ca 2.0 Ga from the eastern part of the Central Zone date metamorphism during reworking of Archaean age shear zones rather than a collision between the Kaapvaal and Zimbabwe cratons as has been earlier suggested.
South Africa
The Republic of South Africa is a country in southern Africa. Located at the southern tip of Africa, it is divided into nine provinces, with of coastline on the Atlantic and Indian oceans...
and 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...
, runs E-NE, and joins 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 south with 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. 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 the stabilisation of the adjacent massifs. The belt comprises 3 components: the Central Zone, the North Marginal Zone and the South Marginal Zone.
Ca. 3.2-2.9 Ga Limpopo Central Zone
The 250 km wide Limpopo belt of southern Africa is an east-northeast trending zone of granulite facies tectoniteTectonite
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 separating the granitoid-greenstone terranes of the Kaapvaal and Zimbabwe cratons. Large scale ductile shear zone
Shear zone
A shear zone is a very important structural discontinuity surface in the Earth's crust and upper mantle. It forms as a response to inhomogeneous deformation partitioning strain into planar or curviplanar high-strain zones. Intervening blocks stay relatively unaffected by the deformation...
s are an integral part of Limpopo belt architecture. They define the boundaries between the belt and the adjacent cratons and separate internal zones within the belt. The shear zones forming the external (northern, southern and western) margins of the belt are interpreted as uplift structures of the overthickened crust.
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 two first periods are mainly characterized by 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...
tic activity leading to the formation of Archaean Tonalite
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...
-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....
-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...
(TTG) such as the Sand River Gneisses or the Bulai Granite intrusion. The 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"...
event took place under high-grade metamorphic
Metamorphic rock
Metamorphic rock is the transformation of an existing rock type, the protolith, in a process called metamorphism, which means "change in form". The protolith is subjected to heat and pressure causing profound physical and/or chemical change...
conditions during which partial melting formed large amount of granitic melt.
The Limpopo Central Zone shows relics of late Archean high grade metamorphism. In the Northern (NMZ) and Southern Marginal Zones (SMZ) that adjoin the Zimbabwe and Kaapvaal cratons, respectively, the last high grade metamorphic episodes were late Archean. The relics in the Central Zone are characterised by an anticlockwise p-T-evolution, at ca. 2.55 Ga. In the NMZ repeated crustal remelting and intrusion of charnoenderbitic magmas continued to 2.58 Ga, producing counterclockwise p-T paths. In contrast the SMZ consists of medium to high pressure
High pressure
High pressure in science and engineering is studying the effects of high pressure on materials and the design and construction of devices, such as a diamond anvil cell, which can create high pressure...
granulites, which underwent a clockwise p-T-evolution at ca. 2.69 Ga, followed by decompression and isobaric cooling. In the Northern Kaapvaal Craton (Renosterkoppies Greenstone Belt, Pietersburg area) tectonism
Plate tectonics
Plate tectonics is a scientific theory that describes the large scale motions of Earth's lithosphere...
took place under amphibolite facies conditions at ca. 2.75 Ga and can therefore not be related to any events in the Limpopo belt. Thus the different tectonic units have different late Archean tectonometamorphic histories. Trace element
Trace element
In analytical chemistry, a trace element is an element in a sample that has an average concentration of less than 100 parts per million measured in atomic count, or less than 100 micrograms per gram....
geochemistry as well as Pb + Nd isotope characteristics of the SMZ are similar to those of the Kaapvaal Craton, with low Th and U concentrations around 2 and 0.7 ppm. Low U concentrations in the SMZ are not a consequence of high grade metamorphism. The NMZ, with high Th, U concentrations (10.8 and 2.5 ppm) and radiogenic Pb, resembles the adjoining Zimbabwe craton more. The differences in late Archean tectonic styles between NMZ and SMZ+KC is a possible consequence of the differences in Th and U content of these provinces.
"The Mahalapye granite in the extreme western part of the Central Zone is a post-tectonic intrusion that crystallized at 2.023 Ga. This suggests that mineral ages of ca 2.0 Ga from the eastern part of the Central Zone date metamorphism during reworking of Archaean age shear zones rather than a collision between the Kaapvaal and Zimbabwe cratons as has been earlier suggested.