D.D. van Reenen

Deep crystal response to continental collision: The Limpopo belt of southern Africa

The Limpopo belt is an example of an Archean, 2700 Ma, continental collisional zone in which the thickened crust (±70 km) has virtually attained equilibrium with the adjacent non-thickened plates by a process of upward movement and lateral spreading of the underthrust, high-grade metamorphosed plates.

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.

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.

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.

Response to comments by Nicoli et al. on paper by Belyanin et al. (2012)

The basis of the Nicoli’s et al. (this issue) comment is a critique of the temperature of *1,100 C estimated by Belyanin et al. (2012) from rare Al–Mg-rich mineral assemblages in sample DR19 from the Southern Marginal Zone (SMZ) Limpopo Complex (LC). However, these authors used this critique to promote the argument that UHT conditions are impossible (their terminology) in the SMZ and that Tmax could not have been much higher than 850 C. In the reply, we address major issues highlighted by Nicoli’s et al. (this issue), namely that of biotite stability and dehydration melting of metapelite, and provide new P–T data based on combination of PERPLE_X pseudosection modeling and TWQ conventional thermobarometry for sample DR19. We provide unequivocal evidence that TMax in the SMZ must have been at least 900 C in accordance with the requirement for UHT conditions (e.g., Harley 2008). In fact, higher temperatures could easily have been attained (e.g., Tsunogae et al. 2004). Discussion of the Nicoli’s et al. comments

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).

Regional geological setting of the Limpopo Belt

Abstract The ∼ 2.7 Ga Limpopo Belt granulite terrane is situated between two lower-grade granite-greenstone cratons, the Kaapvaal Craton in the south and the Zimbabwe Craton in the north. This granulite-facies terrane is subdivided into three zones, each with a distinctive geological signature and separated from each other, and from the adjacent cratons, by well defined shear zones: (1) the Southern Marginal Zone, composed of granite-greenstone material very similar to that of the Kaapvaal Craton, has retrogressed granulites on its southern margin in the hanging wall of the northward-dipping terrane boundary (Hout River Shear Zone) separating the granulite terrane from the adjacent Kaapvaal Craton; (2) the Northern Marginal Zone, a granite-greenstone terrane at granulite grade, which is separated from the Zimbabwe Craton by a southward-dipping terrane boundary; and (3) the Central Zone, a unique crustal element, with distinctive carbonate platform lithologies and a metagabbroic layered complex which intruded before or at ∼ 3250 Ma. This zone cannot be related to the Kaapvaal Craton or to the Zimbabwe Craton. It is separated from the two Marginal Zones by mylonitic shear zones with horizontal stretching lineations. The entire Limpopo Belt is characterized by a distinctive deformational and metamorphic style which contrasts markedly from that of the adjacent granite-greenstone terranes of the Kaapvaal and Zimbabwe Cratons. This distinctive pattern developed during the Limpopo Orogeny, probably as a consequence of continental collision during 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.