Zambezi belt
The Zambezi belt is a fold and thrust belt. It is located in central southern Africa and extends over parts of Zambia and Zimbabwe .
In rift fractures and sedimentary basins , igneous components and sediments of Neoproterozoic age were deposited , beginning around 880 million years ago (in the following text abbreviated as mya ).
During the Pan-African Orogeny , 620 to 500 mya, these rocks were subject to compression and deformation, mainly as a result of the interaction of the Congo-São Francisco (Congo-SF) and Kalahari cratons , which closed the rift fractures and sedimentary basins again and formed folds and thrust complexes .
Geographical extension
The Zambezi Belt (abbreviated as SG in the following text) is bounded in the north by the then still connected Congo-São Francisco craton and in the south by the Zimbabwe craton, the northern part of the Kalahari craton. In northern Zimbabwe , the SG branches off from a local triple junction (triple point) in the southwestern Mozambique belt and continues west to southern Zambia , where the Mwembeshi shear zone forms the border to the northwest lying Lufilian arc . To the west the SG merges into the Damara belt.
The SG is part of a larger orogen complex that stretches across southern Africa. This connects to the east of the Mozambique Belt and, in addition to the Zambezi Belt, also includes the Lufilian Arch and the Damara Belt. All are assigned to the broader Pan-African orogen complex that arose during the formation of the greater continent of Gondwana .
Geological evolution
Basement
The basement of the SB is made up of archaic and mesoproteroziosic rocks from middle and lower crustal areas. These contain high proportions of silicon and aluminum (SIAL) as well as felsic and mafic intrusions ( igneous rock penetrations ).
As a result of the voluminous intrusions, the rocks in the basement were severely deformed and metamorphically shaped. They exist as gneiss and orthogneiss, the age of which is dated around 1,100 mya. Presumably these processes are related to the Kibara orogeny, which occurred during the formation of the supercontinent Rodinia in (today's central and southern) Africa.
Rift and sedimentary basin formation, rock deposits
The formation of the SG began with tectonic plate movements and thermal events around 880 to 820 mya when the supercontinent Rodinia began to disintegrate. It created one or more relatively narrow intracontinental aulakogene not to oceanic spreads leading grave breaches , which were flooded with sea water.
On the basis of bimodal volcanic rocks from felsic and mafic lavas , thick supracrustal layers ( exposed on the surface of the earth's crust) made up of fine and medium-sized clasts (rock fragments) and sediments consisting of pelites and psammites were deposited . These alternated with extensive layers of carbonates and calcium silicate . Frequently occurring in metamorphic areas scapolite - framework silicates are as originally present evaporites interpreted (precipitation and evaporation rocks).
Terrane within the Zambezi Belt
In relation to the different rock formations and their developments, the SG is structured into several terranes.
The Migmatitic Gneiss Terrain corresponds to an area with a reshaped archaic basement in the contact zone between the SG and the Zimbabwe craton. It consists mainly of archaic tonalitic gneisses , mica schist and various metasedimentary (metamorphically transformed) inclusions. The rocks show a predominantly amphibolite - facies (rock property).
The Marginal Gneiss Terrain is structurally on top of the Migmatitic Gneiss Terrain and contains metasedimentary sequences from the Rushinga Metamorphic Suite and various gneiss from the Chimanda Metamorphic Suite.
The Basal Rushinga Intrusive Complex is a flat granite iodine body that represents the transition between the Migmatitic Gneiss Terrain and the Marginal Gneiss Terrain.
The Rushinga Metamorphic Suite is interpreted as a severely deformed sequence of shallow water sediments that have been deposited along the transition zone to the Zimbabwe craton. The rocks were subject to various metamorphic overprints and show amphibolite to granulite facia.
The Zambezi Allochthonous Terrain shows the highest structure in the north of the SG. It corresponds to a thrust pile of felsic and mafic intrusive rocks from the Masoso Metamorphic Suite and the Mavuradonha Metamorphic Suite that were pushed onto the marginal gneiss terrain. The rocks have a very high metamorphic grade and consist of granulite of various admixtures.
The Masoso Metamorphic Suite forms the basal unit of the Zambezi Allochthonous Terrain and consists of bimodal gneiss of different types and metamorphosed plutons (magma bodies crystallized in the earth's crust) with amphibolite facies.
The Mavuradonha Metamorphic Suite is the most highly structured unit in the northeast of the SG. It was postponed to the Masoso Metamorphic Suite and represents the highest metamorphic grade in this area. The basis of this suite is formed by large gneisses of various forms with amphibolite facies. Above it lie gabbros with granulite facies.
Deformations, compressions, metamorphoses
As a result of the approach or collision of the cratons Kongo-SF and Kalahari, the intracontinental basins closed again in several phases. Both emerged transform faults (lateral Erdplattenverschiebungen) as well as from South-prone Southwest thrusts with intense mylonitic deformations and wrinkling of rocks in these zones. The Mwembeshi shear zone formed.
The rocks mostly show a metamorphic amphibolite to granulite facies. The occurrence of eclogites and other high pressure overprinted rocks, such as gabbro accumulations, in some SG areas that were exhumed from greater depths indicates that the crust has been greatly thickened.
In the most important mountain-forming phase around 820 mya, voluminous batholith plutons made of granites and gneiss intruded into a transform fault in the central SG. These batholites created large and thick layers of magma on the surface .
Three main deformation events (DMZ 1 to 3) can be represented in the SG.
- In the DMZ 1 phase, deep crusts were probably thickened as a result of the accumulation of melts when ocean plates were submerged (underplating). These events are limited to the Masoso Metamorphic Suite and Mavuradonha Metamorphic Suite.
- The DMZ 2 phase is associated with the formation of the Zambezi Allochthonous Terrain with metamorphic overprinting of the Marginal Gneiss Terrain under high pressure conditions and the exhumation of deep granulite crusts.
- The DMZ 3 phase corresponds to the formation of folds and northward thrusts. It was the result of tectonic compressions during the closure of the fractures and basins.
Chewore Inliers
The Chewore Inliers are centrally located in the SG on the northern zone of the Zimbabwe craton. They are of particular importance for SG research. With the help of structures, metamorphoses and geochronology , the protracted and diverse processes from the Mesoproterozoic to the early Paleozoic can be traced.
literature
- Richard E. Hanson and others: Geologic evolution of the neoproterozoic Zambezi orogenic belt in Zambia. In: ScienceDirect, Journal of African Earth Sciences, Volume 18, Issue 2, February 1994, Pages 135–150 doi: 10.1016 / 0899-5362 (94) 90026-4 , [2]
- Ben Goscombe and others: Geology of the Chewore Inliers, Zimbabwe: Constraining the Mesoproterozoic to Palaeozoic evolution of the Zambezi Belt. In: Research School of Earth Science, Australian National University, Canberra, Australia, Journal of African Earth Sciences (Impact Factor: 1.38). 04/2000; 30 (3): 589-627. DOI: 10.1016 / S0899-5362 (00) 00041-5 , [3]
Individual evidence
- ↑ Mario Müller: bm.opus.hbz-nrw.de The Mavuradonha Layered Complex: Neoproterozoic emplacement and Pan-African granulite-facies metamorphism in the Zambezi Allochthonous Terrane of the Mt. Darwin Area, Zambezi belt, NE-Zimbabwe. ( Memento of the original from March 29, 2016 in the Internet Archive ) Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice. , Dissertation, Department of Geosciences at Johannes Gutenberg University in Mainz
- ↑ Fold and thrust belt. In: Spektrum.de , Lexicon of Geosciences. Spektrum Akademischer Verlag, Heidelberg, accessed on January 3, 2019 .
- ↑ Fernandez-Alonso and others: The proterozoic history of the proto-congo craton of Central Africa , Department of Earth Sciences, Royal Museum for Central Africa, Tervuren
- ↑ Armin Zeh and others: Archean Accretion and Crustal Evolution of the Kalahari Craton — the Zircon Age and Hf Isotope Record of Granitic Rocks from Barberton / Swaziland to the Francistown Arc. In: Journal of Petrology 5/2009, pp. 933-966 doi: 10.1093 / petrology / egp027
- ↑ TM Kusky: Neoarchaean tectonic evolution of the Zimbabwe Craton. In: Geology, February, 1998, v. 26, p. 163–166 doi: 10.1144 / GSL.SP.2002.199.01.10 , [1]
