Central Gneissic Unit Tokwe Terrane

Three and a half billion-year-old gneissic and greenstone rocks are well exposed in the area between Masvingo (Fort Victoria), Zvishavane (Shabani), and Shurugwi (Selukwe), in the Tokwe segment. The circa 3.5-3.6 billion-year-old Mashaba tonalite forms a relatively central part of this early gneissic terrane,

The Great Dike, a 2.5 billion-year-old magmatic intrusion in Zimbabwe, seen from the space shuttle Endeavour during mission STS-54, January 13-19, 1993 (NASA/Photo Researchers, Inc.)

and other rocks include mainly tonalitic to granodio-ritic, locally migmatitic gneissic units such as the circa 3.475 billion-year-old Tokwe River gneiss, Mushan-dike granitodiorite (2.95 billion years old), 3.0 billion-year-old Shabani gneiss, and 3.5 billion-year-old Mount d'Or tonalite. Similar rocks extend in both the northeast and southwest directions, but they are less well exposed and intruded by younger rocks in these directions. The Tokwe terrane probably extends to the northeast to include the area of circa 3.5 Ga greenstones and older gneissic rocks southeast of Harare. The Tokwe segment represents the oldest known portion of the Tokwe terrane, which was acting as a coherent terrane made up of 3.5-2.95 billion-year-old tectonic elements by circa 2.9 Ga.

The 3.500-2.950 billion-year-old Tokwe ter-rane also contains numerous narrow greenstone belt remnants, which are typically strongly deformed and multiply folded along with interlayered gneiss. The area in the northeasternmost part of the central gneissic terrane southeast of Harare best exhibits this style of deformation, although it continues southwest through Shurugwi. In the Mashava area west of Masvingo, ultramafic rocks, iron formations, quartz-ites, and mica schist are interpreted as 3.5 billion-year-old greenstone remnants tightly infolded with the ancient gneissic rocks. The 3.5 billion-year-old Shurugwi (Selukwe) greenstone belt was the focus of Clive W. Stowe's classic studies in the late 1960s, in which he identified Alpine-type inverted nappe structures and proposed that the greenstone belt was thrust over older gneissic basement rocks, forming an imbricated and inverted mafic/ultramafic allochthon. This was subsequently folded and intruded by granitoids during younger tectonic events.

The Tokwe terrane is in many places uncon-formably overlain by a heterogeneous assemblage of volcanic and sedimentary rocks known as the Lower Greenstones. In the Belingwe greenstone belt this Lower Greenstone assemblage is called the Mtshin-gwe Group, composed of mafic, ultramafic, intermediate and felsic volcanic rocks, pyroclastic deposits, and a wide variety of sedimentary rocks. Isotopic ages on these rocks range from 2.9 to 2.83 billion years, and the rocks are intruded by the 2.83 billion-year-old Chingezi tonalite. The Lower Greenstones are also well developed in the Midlands (Silobela), Filabusi, Antelope-Lower Gwanda, Shangani, Bubi, and Gweru-Mvuma greenstone belts. The upper part of the Lower Greenstones has yielded U-Pb ages of 2.8 billion years in the Gweru greenstone belt, and 2.79 billion years in the Filabusi belt.

The Buhwa and Mweza greenstone belts contain the thickest section of 3 billion-year-old shallow-water sedimentary rocks in the Zimbabwe craton. The Buhwa belt contains a western shelf succession and an eastern deeper water basinal facies association. The shelf sequence is up to 2.5 miles (4 km) thick and includes units of quartzite and quartz sandstone, shale, and iron formation, whereas the eastern deep-water association consists of strongly deformed shales, mafic-ultramafic lavas, chert, iron formation, and possible carbonate rocks. The Buhwa greenstone belt is intruded by the Chipinda batholith, which has an estimated age of 2.9 billion years. Shelf-facies rocks may have originally extended along the southeastern margin of the Tokwe terrane into Botswana, where a similar assemblage is preserved in the Matsitama greenstone belt. Rocks of the Matsitama belt include interlayered quartzites, iron formations, marbles and metacarbonates, and quartzofeldspathic gneisses in a 6-12 mile (10-20 km) thick structurally imbricated succession. The strong penetrative fabric in this belt may be related to deformation associated with the formation of the Limpopo belt to the south, but early nappes and structural imbrication that predate the regional cleavage-forming event are also recognized. The Matsitama belt (Mosetse Complex) is separated from the Tati belt to the east by an accretionary gneiss terrane (Motloutse Complex) formed during convergence of the two crustal fragments. The Tati and Vumba greenstone belts (Francistown granite-greenstone complex) were overturned before penetrative deformation, possibly indicating that they represent lower limbs of large regional nappe structures. The mafic, oceanic-affinity basalts of the Tati belt are overlain by andesites and other silicic igneous rocks, and intruded by syn-tectonic granitoids, typical of magmatic arc deposits. Similar arc-type rocks occur in the lower Gwanda greenstone belts to the east. The Lower Gwanda and Antelope greenstone belts are allochthonously overlain by basement gneisses thrust over the greenstones prior to granite emplacement.

A second sequence of sedimentary rocks lies unconformably over the Lower Greenstone assemblage and overlaps onto basement gneisses in several greenstone belts, most notably in the Belingwe belt, where the younger sequence is known as the Manjeri Formation. The Manjeri Formation contains conglomerates and shallow-water sandstones and locally carbonates at the base, and ranges stratigraphically up into cherts, argillaceous beds, graywacke, and iron formation. The top of the Manjeri Formation is marked by a regional fault. The Manjeri Formation is between 800 and 2,000 feet (250-600 m) thick along most of the eastern side of the Belingwe belt, except where it is cut out by faulting, and it thins northward to zero meters north of Zvishavane. It is considerably thinner on the western edge of the belt. on the scale of the Belingwe belt, the Manjeri Formation thickens toward the southeast, with some variation in structural thickness attributed to either sedimentary or tectonic ramping. The age of the Manjeri Formation is poorly constrained and may be diachronous across strike. However, the Manjeri Formation must be younger than the unconformably underlying circa 2.8 billion-year-old Ga Lower Greenstones, and it must be older than or in part contemporaneous with the thrusting event that emplaced circa 2.7 billion-year-old magmatic rocks of the Upper Greenstones over the Manjeri Formation. The Manjeri Formation overlaps onto the gneissic basement of the Tokwe terrane on the eastern side of the Belingwe belt, and at Masvingo, and rests on older (3.5 billion-year-old Sebakwian Group) greenstones at Shurugwi. Regional stratigraphic relationships suggest that the Manjeri Formation forms a southeast-thickening sedimentary wedge that prograded onto the Tokwe terrane.

Was this article helpful?

0 0
The Basic Survival Guide

The Basic Survival Guide

Disasters: Why No ones Really 100 Safe. This is common knowledgethat disaster is everywhere. Its in the streets, its inside your campuses, and it can even be found inside your home. The question is not whether we are safe because no one is really THAT secure anymore but whether we can do something to lessen the odds of ever becoming a victim.

Get My Free Ebook


Post a comment