DEFINITION OF BELINGWE GREENSTONE BELT, ZIMBABWE

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

extended at 2.9–2.8 billion years ago, forming the Lower

Greenstones and related rocks. At 2.7 billion years ago, a

continental margin volcanic arc was built on the northwestern

side of the craton, while an ocean basin closure on the

southeastern margin emplaced the Upper Greenstones over

the continental shelf/passive margin sequence preserved in

the Manjeri Formation. Folding, intrusion of the Chibi intrusives,

and other deformation is related to the closure of the

basin in the southeast, and later collision of the Zimbabwe

and Kaapvaal cratons by 2.5 billion years ago. The tectonic

history of the Belingwe greenstone belt and Zimbabwe craton

is therefore very much like the evolution of modern convergent

margins, showing that Precambrian plate tectonics

was similar to modern style plate tectonics.

See also ARCHEAN; CRATONS; GREENSTONE BELTS;

STRUCTURAL GEOLOGY.

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