C. A. Smit

Tectonic model for the evolution of the Limpopo Belt

Abstract Constraints on which any model for the tectonic evolution of the Limpopo Belt must be based include the following data: crustal thickening to at least 65 km between ∼2700 and 2650 Ma ago was responsible for the formation of the granulite terrane exposed in the Limpopo Belt today. This crustal thickening probably resulted from the thrusting of the Kaapvaal Craton over the Zimbabwe Craton along the south dipping, Triangle-Tuli-Sabi Shear Zone. Other northwardverging thrusts related to this event occur in the Kaapvaal Craton south of the Limpopo Belt. In the Central and Southern Marginal Zones and the Kaapvaal Craton, this shortening is directed to the southwest while in the Northern Marginal Zone, it was directed to the north-northwest. Peak metamorphism was superimposed on the thickened crust and then nearly isothermal decompression of several kbars occurred throughout the belt. During this decompression, rocks moved upward and spread outward onto the adjacent cratons from the zone of thickened crust along several inward directed shear zones, creating a regional “pop up”. High-grade rocks were thrust over low-grade rocks on the cratons, producing along the terrane boundary retrograde metamorphism in the Limpopo Belt and prograde metamorphism on the cratons.

Geochronology of the Hout River Shear Zone and the metamorphism in the Southern Marginal Zone of the Limpopo Belt, Southern Africa

Abstract In this paper monazite U–Pb and zircon evaporation dates, stepleaching Pb/Pb results on garnet, staurolite and kyanite, and hornblende Ar/Ar data are presented which constrain the timing of granulite facies metamorphism in the Southern Marginal Zone of the Limpopo Belt and its thrusting onto the Kaapvaal Craton. The Southern Marginal Zone of the Limpopo Belt is considered to be a lower crustal equivalent of the northern Kaapvaal Craton. Granulite exhumation is associated with southward thrusting along the Hout River Shear Zone which is a set of thrust and strike slip shear zones. Zircon ages for the Matok Intrusive Complex which was emplaced within the zone during this thrusting (charno-enderbites: 2671±4 Ma; granodiorites: between 2667 and 2664 Ma) have previously been interpreted as evidence for rapid exhumation of the Southern Marginal Zone within only ∼7 Ma. We have obtained a U/Pb date of 2691±7 Ma for monazite from the Bandelierkop Quarry in the Southern Marginal Zone, interpreted as the age of high grade metamorphism. A single zircon evaporation Pb/Pb date of 2643±1 Ma from a leucosome band at the same locality may indicate longer lasting metamorphism or decompression melting during exhumation. Anatexis of metapelitic xenoliths within the Matok Intrusive Complex was dated at 2663±4 Ma by U/Pb on monazite, indistinguishable from existing zircon ages for this complex. Pb/Pb step leaching dates obtained on synkinematically grown garnet (2691±20 Ma), staurolite (2712±37 Ma) and kyanite (2672±51 Ma) from the Khavagari Hills in the Giyani Greenstone Belt, in the immediate footwall of the Hout River Shear Zone, indicate that early thrusting was contemporaneous with peak metamorphism in the Southern Marginal Zone. Ar/Ar dating and geochemistry on syntectonic hornblende separates from the same shear zone system yielded disturbed spectra and indicated multiple populations, probably reflecting repeated or continuous tectonic activity of the Hout River Shear Zone up to about 2600 Ma.

The Granulite-Facies Rocks of the Limpopo Belt, Southern Africa

The granulite-facies Limpopo Belt (LB) is subdivided into three zones, i.e. a Central Zone (CZ), with a Southern (SMZ) and a Normern Marginal Zone (NMZ). Each zone has its own distinctive geological signature, and is separated from the other zones, and also from the surrounding cratons, by prominent east-north-east trending terrane boundaries. The Central Zone is characterized by an unique shelf-type supracrustal sequence — the Beit Bridge Complex — which is made up of quartzo-feldspathic gneiss, quartzite, marble, calc-silicate rocks, metapelitic gneiss, and mafic and ultramafic gneiss, which all possibly overlay the Sand River Gneiss. Both Marginal Zones are typified by tectonically dismembered greenstone slivers (ultramafic, mafic and metapelitic gneisses with banded iron formation), which are intimately mixed with tonalitic and trondhjemitic gneisses. The lithologies of the three zones were subjected to granulite facies metamorphism in response to the collision of the Kaapvaal and Zimbabwe Cratons during the Limpopo orogeny at approximately 2700 Ma. The clockwise P-T-time evolution of the Central Zone and Southern Marginal Zone, recording a continuous single loop, suggests that the high-grade terrane of the LB was subjected to burial to a depth of about 35km. Re-equilibration of the isotherms led to peak metamorphic conditions (T ~ 800°C)(M1) being superimposed on the D1–structures and fabrics, which were caused by the collision. This episode was followed by rapid, nearly isothermal decompression (M2) to about 6 kbar during uplift, as indicated by the presence of numerous corona textures in metapelites. At the margins of the high-grade terrane, the granulites of the Southern Marginal Zone and Northern Marginal Zone were respectively thrust onto the adjacent low-grade granite- greenstone terranes of the Kaapvaal and Zimbabwe Cratons. The associated D2–shear zones acted as conduits for the migration of metamorphic fluids, derived probably from the dehydration of the underthrusted granite-greenstone crust. This resulted in the establishment of a retrograde orthoamphibole isograd and associated zones of rehydrated granulites (M3) (P=6 kbar, T=600°C) in the Southern Marginal Zone, and probably also in the Northern Marginal Zone. The most important conclusion is that the Limpopo Belt offers us the chance to examine the deep roots of a mountain chain which was caused by continental collision in the late Archaean.

The structural framework of the southern margin of the Limpopo Belt, South Africa

Abstract The Southern Marginal Zone is characterized by two prominent regional structural features: (i) massive crustal wedges (20 km × 50 km), containing large oval-shaped closed-fold structures, which are related to an early (D 1 ) phase of deformation, and which are bounded by, (ii) major ductile shear zones that strike in a general E-W direction. The Hout River Shear Zone forms the southern boundary of the Southern Marginal Zone where granulites are juxtaposed against lower grade granite-greenstones of the Kaapvaal Craton. This fault system also acted as the sole thrust for the other high-grade shear zones in the Southern Marginal Zone and played a major role in the development of the zone of retrogression and the establishment of the orthoamphibole isograd in the Southern Marginal Zone. The Hout River Shear Zone and related faults within the Southern Marginal Zone clearly post-date D 1 and a period of peak metamorphism (M 1 ). They are composite structures showing dominantly reverse-sense displacements and are responsible for the emplacement of the granulite terrane over and onto the Kaapvaal Craton. In contrast to these older structures are the reactivated shear zones. These structures clearly cross-cut and displace the high-grade shear zones in the Southern Marginal Zone as well as the retrograde orthoamphibole isograd. The fact that the Hout River, and related shear zones are in metamorphic equilibrium with the surrounding wall rocks suggests their age is that of the metamorphism, i.e., ∼ 2670 Ma (Barton and Van Reenen, 1992).

Evolution of the Northern High-Grade Margin of the Kaapvaal Craton, South Africa

The northern Kaapvaal craton is divided into three distinctive crustal zones: (1) the high-grade Southern Marginal Zone (SMZ) of the Limpopo belt in the north; (2) a transitional zone of technically stacked metamorphic rocks which includes the area from the Sutherland belt in the north to the Pietersburg and Murchison belts in the south; and (3) a low-grade granite-greenstone terrane farther to the south. The change across the SMZ boundary is expressed by an overall increase in the grade of metamorphism, a strip of retrogression related to a zone of thrusting, abrupt changes in intensity of deformation, rock densities, magnetic lineament patterns, and isotopic ages in compositionally similar rocks. Regional metamorphism in the northern Kaapvaal craton is demonstrably not a continuum in the sense that the highest-grade rocks underwent all earlier stages of metamorphism; rather, that rapid local tectonic processes must have occurred. A proposed evolutionary path for the SMZ was initiated with crustal thickening during the collision of the two continents in the Late Archean. Tectonic burial and superimposition of granulite facies metamorphism was followed by upward movement of deeper crustal rocks and thrusting onto the adjacent transition zone. The original collisional record in the granulite terrane was, however, obliterated by the later mainly upward response of the thickened crust. Response related zones of thrusting acted as channelways for the migration of

Structural geological and metamorphic significance of the Kaapvaal Craton-Limpopo Belt contact

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Structural and P–T Evolution of a Major Cross Fold in the Central Zone of the Limpopo High-Grade Terrain, South Africa

-rich fluids, which were responsible for the establishment of a retrograde orthoam-phibole isograd at lower temperatures. These fluids were also responsible for the local transformation of quartzo-feldspathic gneisses to charaockites in the hanging wall blocks of the thrust faults at higher grade. Late-tectonic granitic bodies intruded along some of these structures. The present reconstruction of the evolution of the northern margin of the Kaapvaal craton supports suggestions that plate tectonic processes were active in the Archean. The transition zone is made up of relatively smaller zones of higher-grade rocks thrust onto lower-grade rocks during the collision of the Zimbabwe and Kaapvaal cratons. It is suggested that the former possibly moved southwards over the latter and depressed the Limpopo belt to granulite conditions of metamorphism. The Limpopo belt then responded to this burial by subsequent regional uplift.

Deep Crustal Shear Zones, High-Grade Tectonites, and Associated Metasomatic Alteration in the Limpopo Belt, South Africa: Implications for Deep Crustal Processes

Abstract The Hout River Shear Zone forms the terrane boundary between the granite-greenstone terrane of the Kaapvaal Craton (Cratonic terrane) and the high-grade granulite terrane (high-grade gneiss province) of the Limpopo Belt. This structure is superimposed onto a thin-skinned northward verging thrust system which exercises a significant control on the distribution of the ENE-WSW trending greenstone belts. The Hout River Shear Zone, however, dips steeply to the north and flattens out toward the south. This structure was responsible for the emplacement of hot granulites against and over the cooler granite-greenstone terrane and strongly influenced the metamorphic evolution of the area. The shear zone acted as a conduit for infiltrating CO2-rich, H2O-bearing fluids into still hot crust producing a retrograde isograd and associated zone of rehydration in the granulitic hanging wall of the structure. Retrogressed granulites in the hanging wall are characterized by fine-grained kyanite as part of the retrograde assemblages, while the greenstones in the footwall are characterized by coarse-grained prograde kyanite. The terrane boundary appears to be in thermal equlibrium with the lithologies in both the footwall and hanging wall so that tectonically mixed prograde greenstones and retrogressed granulites in and adjacent to the shear zone are apparently at a similar metamorphic grade. The granulites appear to have had a “hot iron” effect on the cooler footwall rocks over which they have been displaced.