J.M. Barton

Unraveling the record of successive high grade events in the Central Zone of the Limpopo Belt using Pb single phase dating of metamorphic minerals

Abstract Dating of relic metamorphic assemblages can provide important information about the timing and character (metamorphic grade and/or PT-evolution) of early high grade episodes in polymetamorphic provinces. Using Pb stepwise leaching of metamorphic silicates, we have dated multiple granulite facies metamorphic episodes in the Central Zone (CZ) of the Limpopo Belt. Ages of 2.52 Ga were obtained from sillimanite and cogenetic garnet and ages of about 2.01 Ga from titanite, garnet and clinopyroxene. Together with new and published conventional age data from accessory phases and in the context of combined petrological and structural data, these results lead us to a reinterpretation of the tectono-metamorphic history of the CZ. Three distinct high grade events at about 3.2−3.1 Ga, 2.65−2.52 Ga and 2.0−0.05 Ga are recognized. Each of these is suggested to correspond to a tectonic episode of distinct character: (a) for the 3.2 Ga event magmatic activity can mainly be identified (best represented, for example, by the Sand River Gneisses or the Messina Layered Intrusion). The field relationships concerning the tectonometamorphic history of this Early-Archean event are largely erased by at least two high grade metamorphic overprints. (b) Late-Archean (∼2.6-2.52 Ga) low pressure granulite facies metamorphism was associated with voluminous granitic and charnockitic plutonism. The anticlockwise P-T evolution of these granulites probably reflects deep crustal processes, associated with magmatic underplating (or in-plating), contemporaneous with vertical crustal growth of the Zimbabwe craton around 2.6 Ga. (c) During the Proterozoic event (∼2.05-1.95 Ga) tectonic thickening was caused by the collision of the Kaapvaal and Zimbabwe cratons. The CZ was squeezed between these two cratons and as a consequence underwent high pressure granulite facies metamorphism with a clockwise P-T evolution. The structural, metamorphic and geochronological data can be best explained with a tectonic model that describes this final event as a dextral transpressive orogeny.

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.

When was the Limpopo Orogeny

Abstract The term “Limpopo Orogeny” refers to the common high-grade metamorphic and tectonic event that affected the Archaean rocks of the Central and Southern Marginal Zones of the Limpopo Belt and its influence on the adjacent portions of the Kaapvaal Craton. Available geochronological data from muscovite-kyanite prograde metamorphic assemblages, zircon in igneous charnoenderbite and enderbite, muscovite in syntectonic pegmatite bodies formed in shear zones, partial melts and syn- and post-tectonic plutons of granodiorite and granite indicate that this orogeny occurred during the time interval from ∼ 2700 Ma ago to ∼ 2650 Ma ago. The retrograde phase of this orogeny, from the onset of the response phase (M 2 ) until the rocks were subjected to rehydration (M 3 ), was very rapid, probably lasting ∼ 7 Ma from ∼ 2671 Ma to ∼ 2664 Ma ago. Similar but limited data from the Northern Marginal Zone and adjacent portions of the Zimbabwe Craton suggest that rocks there were subjected to a more recent interval of high-grade metamorphism and tectonism, extending from ∼ 2600 Ma until ∼ 2550 Ma.

Tectonothermal history of the western part of the Limpopo Belt: tectonic models and new perspectives

Abstract The westernmost part of the Central Zone of the Limpopo Belt in Botswana is commonly interpreted as a frontal ramp of a westward extruding block or nappe sheet, related to a late Archaean Limpopo Orogeny. In this paper the structural observations from this area are reviewed and new geochronological results presented, in order to test the existing tectonic models. Two tectonometamorphic events can be distinguished. Neoarchaean granulite metamorphism (M 1 ) is associated with the generation of voluminous granitic bodies at ca 2.6 Ga. The nature of the Archaean tectonometamorphism is difficult to interpret because of a major high-grade metamorphic overprint at 2.0 Ga, which is characterised by a complicated succession of ductile deformational phases. The structural patterns indicate that during the 2.0 Ga evolution of the Limpopo Belt convergence directions changed from north to northwest and west. During the same period metamorphic conditions gradually decreased from upper amphibolite-facies to greenschist-facies. The structural features in the western part of the Central Zone are not compatible with a frontal ramp geometry. Models proposing a single Neoarchaean Limpopo Orogeny do not account for the polyphase tectonometamorphic evolution in the Central Zone and are also rejected. This study suggests that a Proterozoic orogeny involving transpression best explains the geometries encountered in the western Limpopo Belt.

Isotopic and REE characteristics of the intrusive charnoenderbite and enderbite geographically associated with the Matok Pluton, Limpopo Belt, southern Africa

Abstract Intrusive charnoenderbite and enderbite in the Southern Marginal Zone of the Limpopo Belt are genetically linked with peak metamorphism (M1) during the 2700 Ma to 2650 Ma Limpopo Orogeny. They occur geographically associated with and are intruded by the granodiorite and granite of the Matok Pluton which were emplaced between the response phase (M2) and the rehydration phase (M3) of that Orogeny. Rb-Sr, Pb, REE and Sm-Nd elemental and isotopic data for whole rock samples and U-Pb data for populations of zircon suggest that the charnoenderbite and enderbite were intruded ∼ 2671 Ma ago and were derived from a crustal precursor. The granodiorite and granite of the Matok Pluton were intruded at approximately the same time, between ∼ 2667 Ma and ∼ 2664 Ma ago; they also were derived from a crustal precursor. Nd model ages and REE patterns for the charnoenderbite and enderbite and the granodiorite are distinct. This relationship may imply that they are not cogenetic. Alternatively, these rock types could be cogenetic if the magma from which they were derived was assimilating older crustal material as it evolved. Nd isotopic data indicate that the magma giving rise to the charnoenderbite and enderbite formed by partial melting of rocks compositionally similar to some portions of the Baviaanskloof Gneiss. Data for the granodiorite are consistent with the possibility that this unit formed in a similar manner from rocks compositionally similar to other portions of the Baviaanskloof Gneiss. The close temporal relationship between the emplacement of the charnoenderbite and enderbite and the granitic rocks of the Matok Pluton indicates that the retrograde portion of the Limpopo Orogeny, beginning ∼ 2670 Ma ago, occurred rapidly, perhaps over ∼ 7 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.

Evidence for metamorphic and igneous charnockites in the Southern Marginal Zone of the Limpopo Belt

Abstract Metamorphic and igneous charnoenderbite and enderbite can be distinguished on the basis of field relations and bulk rock geochemistry in the Southern Marginal Zone of the Limpopo Belt. Termed here metamorphic charnockite and igneous charnockite respectively, both varieties are petrographically similar. They are massive, homogeneous rocks which formed after the D1, fabric forming event associated with the Limpopo Orogeny and they at least experienced the entire retrograde portion of that Orogeny. The metamorphic charnockite probably developed first during peak metamorphism (M1) and is often closely associated with a banded, orthopyroxene-bearing quartzo-feldspathic gneiss (the Baviaanskloof Gneiss). The metamorphic charnockite-Baviaanskloof Gneiss relations in the field are those of quiescent obliteration of foliation, and the charnockite is often characterized by the presence of relict banding. The chemical composition of the metamorphic charnockite is identical with that of the associated Baviaanskloof Gneiss. The igneous charnockite, locally retaining igneous textures, is characterized by intrusive relationships with the Baviaanskloof Gneiss and in most cases by chemical compositions that are distinct from those of the older unit. Available geochronological data indicate that the metamorphic charnockite, or its protolith, formed at or after ∼ 2715 Ma ago, significantly before emplacement of the igneous charnockite of the Matok Complex at ∼ 2671 Ma. Emplacement of igneous charnoenderbite and enderbite in the rest of the Southern Marginal Zone and in the Matok Complex was not synchronous but occurred respectively during and after peak metamorphism (M1). Metamorphic charnockite with preserved relict banding probably formed by in situ biotite dehydration to orthopyroxene in the presence of a fluid phase with locally different activity of CO2. H2O dilution through extraction of a granitic melt is considered to be unfeasable because of similar bulk chemistry. The igneous charnockite crystallized from calc-alkaline magmas derived by partial melting of lower crustal rocks, similar to some phases of the Baviaanskloof Gneiss.

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

Geology of the Chewore Inliers, Zimbabwe: constraining the Mesoproterozoic to Palæozoic evolution of the Zambezi Belt

CO_{2}

The significance of Rb-Sr ages of biotite and phlogopite for the thermal history of the Central and Southern Marginal Zones of the Limpopo Belt of southern Africa and the adjacent portions of the Kaapvaal Craton

-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.