The Archean Belingwe
greenstone belt in southern Zimbabwe has proved to be
one of the most important Archean terranes for testing models
for the early evolution of the Earth and the formation of continents.
It has been variously interpreted to contain a continental
rift, arc, flood basalt, and structurally emplaced ophiolitic or
oceanic plateau rocks. It is a typical Archean greenstone belt,
being an elongate belt with abundant metamorphosed mafic
rocks and metasediments, deformed and metamorphosed at
greenschist to amphibolite grade. The basic structure of the
belt is a refolded syncline, although debate has focused on the
significance of early folded thrust faults.
The 3.5-billion-year-old Shabani-Tokwe gneiss complex
forms most of the terrain east of the belt and underlies part
of the greenstone belt. The 2.8–2.9-billion-year-old Mashaba
tonalite and Chingezi gneiss are located west of the belt.
These gneissic rocks are overlain unconformably by a 2.8-billion-
year-old group of volcanic and sedimentary rocks known
as the Lower Greenstones or Mtshingwe Group, including
the Hokonui, Bend, Brooklands, and Koodoovale Formations.
These rocks, and the eastern Shabani-Tokwe gneiss are
overlain unconformably by a shallow water sedimentary
sequence known as the Manjeri Formation, consisting of
quartzites, banded iron formation, graywacke, and shale. A
major fault is located at the top of the Manjeri Formation,
and the Upper Greenstones structurally overlie the lower
rocks being everywhere separated from them by this fault.
The significance of this fault, whether a major tectonic contact
or a fold accommodation related structure, has been the
focus of considerable scientific debate. The 2.7-billion-yearold
Upper Greenstones, or the Ngezi Group, includes the
ultramafic-komatiitic Reliance Formation, the 6-kilometerthick
tholeiitic pillow lava-dominated Zeederbergs Formation,
and the sedimentary Cheshire Formation. All of the units are
intruded by the 2.6-billion-year-old Chibi granitic suite.
The Lower Greenstones have been almost universally
interpreted to be deposits of a continental rift or rifted arc
sequence. However, the tectonic significance of the Manjeri
Formation and Upper Greenstones has been debated. The
Manjeri Formation is certainly a shallow water sedimentary
sequence that rests unconformably over older greenstones
and gneisses. Correlated with other shallow-water sedimentary
rocks across the southern craton, it may represent the remnants
of a passive-margin type of sedimentary sequence.
The top of the Manjeri Formation is marked by a fault,
the significance of which has been disputed. Some scientists
have suggested that it may be a fault related to the formation
of the regional syncline, formed in response to the rocks in
the center of the belt being compressed and moving up and
out of the syncline. Work on the sense of movement on the
fault zone, however, shows that the movement sense is
incompatible with such an interpretation, and that the fault is
a folded thrust fault that placed the Upper Greenstones over
the Manjeri Formation. Therefore, the tectonic setting of the
Upper Greenstones is unrelated to the rocks under the thrust
fault, and the Upper Greenstones likely were emplaced from
a distant location. The overall sequence of rocks in the Upper
Greenstones, including several kilometers of mafic and ultramafic
lavas, is very much like rock sequences found in contemporary
oceanic plateaus or thick oceanic crust, and such
an environment seems most likely for the Upper Greenstones
in Belingwe and other nearby greenstone belts of the Zimbabwe
craton.
A tectonic model for the evolution of the Zimbabwe
craton has been proposed by Timothy Kusky in his 1998
article “Tectonic Setting and Terrane Accretion of the
Archean Zimbabwe Craton.” An ancient gneiss complex
(The Tokwe Terrain) forms the core of the craton and
Further Reading
Kusky, Timothy M. “Tectonic Setting and Terrane Accretion of the
Archean Zimbabwe Craton.” Geology 26 (1998): 163–166.
Kusky, Timothy M., and Pamela A. Winsky. “Structural Relationships
along a Greenstone/Shallow Water Shelf Contact, Belingwe
Greenstone Belt, Zimbabwe.” Tectonics 14 (1995): 448–471.
Kusky, Timothy M., and William S. F. Kidd. “Remnants of an
Archean Oceanic Plateau, Belingwe Greenstone Belt, Zimbabwe.”
Geology 20 (1992): 43–46.
greenstone belt in southern Zimbabwe has proved to be
one of the most important Archean terranes for testing models
for the early evolution of the Earth and the formation of continents.
It has been variously interpreted to contain a continental
rift, arc, flood basalt, and structurally emplaced ophiolitic or
oceanic plateau rocks. It is a typical Archean greenstone belt,
being an elongate belt with abundant metamorphosed mafic
rocks and metasediments, deformed and metamorphosed at
greenschist to amphibolite grade. The basic structure of the
belt is a refolded syncline, although debate has focused on the
significance of early folded thrust faults.
The 3.5-billion-year-old Shabani-Tokwe gneiss complex
forms most of the terrain east of the belt and underlies part
of the greenstone belt. The 2.8–2.9-billion-year-old Mashaba
tonalite and Chingezi gneiss are located west of the belt.
These gneissic rocks are overlain unconformably by a 2.8-billion-
year-old group of volcanic and sedimentary rocks known
as the Lower Greenstones or Mtshingwe Group, including
the Hokonui, Bend, Brooklands, and Koodoovale Formations.
These rocks, and the eastern Shabani-Tokwe gneiss are
overlain unconformably by a shallow water sedimentary
sequence known as the Manjeri Formation, consisting of
quartzites, banded iron formation, graywacke, and shale. A
major fault is located at the top of the Manjeri Formation,
and the Upper Greenstones structurally overlie the lower
rocks being everywhere separated from them by this fault.
The significance of this fault, whether a major tectonic contact
or a fold accommodation related structure, has been the
focus of considerable scientific debate. The 2.7-billion-yearold
Upper Greenstones, or the Ngezi Group, includes the
ultramafic-komatiitic Reliance Formation, the 6-kilometerthick
tholeiitic pillow lava-dominated Zeederbergs Formation,
and the sedimentary Cheshire Formation. All of the units are
intruded by the 2.6-billion-year-old Chibi granitic suite.
The Lower Greenstones have been almost universally
interpreted to be deposits of a continental rift or rifted arc
sequence. However, the tectonic significance of the Manjeri
Formation and Upper Greenstones has been debated. The
Manjeri Formation is certainly a shallow water sedimentary
sequence that rests unconformably over older greenstones
and gneisses. Correlated with other shallow-water sedimentary
rocks across the southern craton, it may represent the remnants
of a passive-margin type of sedimentary sequence.
The top of the Manjeri Formation is marked by a fault,
the significance of which has been disputed. Some scientists
have suggested that it may be a fault related to the formation
of the regional syncline, formed in response to the rocks in
the center of the belt being compressed and moving up and
out of the syncline. Work on the sense of movement on the
fault zone, however, shows that the movement sense is
incompatible with such an interpretation, and that the fault is
a folded thrust fault that placed the Upper Greenstones over
the Manjeri Formation. Therefore, the tectonic setting of the
Upper Greenstones is unrelated to the rocks under the thrust
fault, and the Upper Greenstones likely were emplaced from
a distant location. The overall sequence of rocks in the Upper
Greenstones, including several kilometers of mafic and ultramafic
lavas, is very much like rock sequences found in contemporary
oceanic plateaus or thick oceanic crust, and such
an environment seems most likely for the Upper Greenstones
in Belingwe and other nearby greenstone belts of the Zimbabwe
craton.
A tectonic model for the evolution of the Zimbabwe
craton has been proposed by Timothy Kusky in his 1998
article “Tectonic Setting and Terrane Accretion of the
Archean Zimbabwe Craton.” An ancient gneiss complex
(The Tokwe Terrain) forms the core of the craton and
Further Reading
Kusky, Timothy M. “Tectonic Setting and Terrane Accretion of the
Archean Zimbabwe Craton.” Geology 26 (1998): 163–166.
Kusky, Timothy M., and Pamela A. Winsky. “Structural Relationships
along a Greenstone/Shallow Water Shelf Contact, Belingwe
Greenstone Belt, Zimbabwe.” Tectonics 14 (1995): 448–471.
Kusky, Timothy M., and William S. F. Kidd. “Remnants of an
Archean Oceanic Plateau, Belingwe Greenstone Belt, Zimbabwe.”
Geology 20 (1992): 43–46.
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