Central Regions Unit

Pretoria, South Africa

Central Regions Unit

Pretoria, South Africa
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Macey P.H.,University of the Western Cape | Miller J.A.,Stellenbosch University | Rowe C.D.,University of Cape Town | Rowe C.D.,McGill University | And 5 more authors.
Precambrian Research | Year: 2013

The Monapo Klippe in north-east Mozambique is an ovoid-shaped outcrop measuring approximately 35. ×. 40. km and is clearly visible on satellite and geophysical images. Based on recent field mapping, geochemical studies and new geochronological data, we present a revision of the lithostratigraphy of the klippe and offer a model for its origin and emplacement in the framework of regional tectonics. There are three main groups of rocks within the klippe: (1) the Metachéria Metamorphic Complex; (2) the Mazerapane Intrusive Suite; and (3) the Ramiane Intrusive Suite. The Metachéria Metamorphic Complex consists of a mélange of granulite gneiss, including mafic, felsic, pelitic and carbonate rocks, characterised by a strong penetrative shear fabric. The largely undeformed Mazerapane and Ramiane Suites have intruded into the Metachéria Metamorphic Complex. The Mazerapane Suite consists of foid-bearing ultramafic and mafic gneisses and intrudes into the western half of the complex, whereas the Ramiane Suite is dominated by alkaline granitic rocks, contains no foid-bearing units and intrudes into the eastern half of the complex. In addition to these three main units, there are a number of minor but structurally important units, the main ones of which include amphibolite-facies tonalitic gneisses and the Evate calcite carbonatite. Underlying all of these units is a narrow, high strain mylonite zone. Undeformed pegmatite bodies and dykes cross-cut all rock types of the Monapo Klippe including the marginal mylonite. Near identical dates for the intrusion of the Ramiane Suite at 637. ±. 5. Ma and metamorphism of the Metachéria Complex at 634. ±. 8. Ma indicates a major episode of granulite-facies metamorphism and crust generation at this time. The ~635. Ma age for the granulite-facies metamorphism is comparable to granulite-facies events identified in other parts of the East African Orogen in Tanzania, Madagascar and other parts of northern Mozambique to the north of the Lúrio Belt. The absence of granulite-facies rocks in the underlying Nampula Block is consistent with structural arguments that the Monapo Klippe is the remnant of an allochthonous thrust sheet. In this context, the Monapo Complex is very similar to other granulite-facies "klippe" in East Africa, Antarctica and Sri Lanka, lending support to the idea of a Pan-African mega-nappe formerly existing across greater East Gondwanaland. © 2013 Elsevier B.V.

Grantham G.H.,Central Regions Unit | Mendonidis P.,Vaal University of Technology | Thomas R.J.,British Geological Survey | Satish-Kumar M.,University of Shizuoka
Geoscience Frontiers | Year: 2012

Four different varieties of charnockitic rocks, with different modes of formation, from the Mesoproterozoic Natal belt are described and new C isotope data presented. Excellent coastal exposures in a number of quarries and river sections make this part of the Natal belt a good location for observing charnockitic field relationships. Whereas there has been much debate on genesis of charnockites and the use of the term charnockite, it is generally recognized that the stabilization of orthopyroxene relative to biotite in granitoid rocks is a function of low aH 2O (± high CO 2), high temperature, and composition (especially Fe/(Fe +Mg)). From the Natal belt exposures, it is evident that syn-emplacement, magmatic crystallization of charnockite can arise from mantle-derived differentiated melts that are inherently hot and dry (as in the Oribi Gorge granites and Munster enderbite), as well as from wet granitic melts that have been affected through interaction with dry country rock to produce localized charnockitic marginal facies in plutons (as in the Portobello Granite). Two varieties of post-emplacement sub-solidus charnockites are also evident. These include charnockitic aureoles developed in leucocratic, biotite, garnet granite adjacent to cross-cutting enderbitic veins that are attributed to metamorphic-metasomatic processes (as in the Nicholson's Point granite, a part of the Margate Granite Suite), as well as nebulous, patchy charnockitic veins in the Margate Granite that are attributed to anatectic metamorphic processes under low-aH 2O fluid conditions during a metamorphic event. These varieties of charnockite show that the required physical conditions of their genesis can be achieved through a number of geological processes, providing some important implications for the classification of charnockites, and for the interpretation of charnockite genesis in areas where poor exposure obscures field relationships. © 2012, China University of Geosciences (Beijing) and Peking University. Production and hosting by Elsevier B.V. All rights reserved.

Grantham G.H.,Central Regions Unit | Manhica A.D.S.T.,University of Pretoria | Armstrong R.A.,Australian National University | Kruger F.J.,University of Witwatersrand | Loubser M.,University of Pretoria
Journal of African Earth Sciences | Year: 2011

Whole rock major and trace element data from granitoids adjacent to the Kalahari Craton-Mozambique-Maud Belt boundary are described. The data from ∼1140Ma old granodioritic and ∼1110Ma old granitic bodies in the Mozambique Belt show that they are typical of calc-alkaline and A-type granitoids respectively. Radiogenic Rb/Sr and Sm/Nd isotope data from the two granitoid bodies suggest significant older crustal contributions during their genesis. The granodioritic gneisses show TDM model ages of ∼2100-3500Ma whereas megacrystic granitic gneisses have TDM model ages of ∼1600-3100Ma. Granite from the Archaean-age Kalahari Craton has TDM model ages of ∼3000-3500Ma.The data from Mozambique are compared with whole rock major and trace element chemistry and U/Pb zircon SHRIMP data from the Maud Belt in western Dronning Maud Land. These show that ∼1140. Ma old granodioritic gneisses in Sverdrupfjella and Kirwanveggan have similar ages and chemical compositions to similar rocks in central Mozambique. Radiogenic isotope characteristics of the gneisses from central Mozambique and Sverdrupfjella are similar and suggest older crustal contributions in contrast to the juvenile nature of the gneisses from Kirwanveggan.Similarly, ∼1090. Ma old granitic gneisses from central Mozambique, Sverdrupfjella and Kirwanveggan have similar ages and A-type chemical compositions. In contrast the radiogenic isotope compositions from Kirwanveggan are juvenile whereas those from central Mozambique show a significant older crustal contribution.The whole rock radiogenic isotope data can be interpreted to suggest that the Mesoproterozoic Mozambique Belt rocks were generated by partial melting which probably involved mixing of Archaean/Paleoproterozoic crust and younger Mesoproterozoic juvenile magma at ∼1100. Ma and suggest that the Kalahari Craton probably extends eastwards at depths for more than 30. km from its exposure at surface.The data support correlations between the Mozambique Belt and the Maud Belt in Antarctica in general and more specifically show similarities between the Kalahari Craton boundary and the Mozambique-Maud Belt in lithologies immediately adjacent to that boundary.Two episodes of anatectic migmatisation are recognized in rocks from the Mozambique Belt in central Mozambique. These show an earlier migmatitic vein phase oriented parallel to the planar foliation in the granitic and tonalitic gneisses and a later discordant vein phase which is oriented parallel to localized but intense N-S oriented shearing along the Kalahari Craton/Mozambique Belt boundary zone. SHRIMP zircon data from the younger migmatitic vein phase suggests a crystallization age of 997 ± 4. Ma. Small numbers of inherited zircons have ages of ∼2700. Ma and ∼1100-1200. Ma. Younger discordant analyses suggesting metamorphic disturbance between ∼400. Ma and 550. Ma are seen. The data imply the high strain along the eastern margin of the Kalahari Craton in the Manica area, occurred at ∼1000. Ma and not at ∼450. Ma as was previously thought. The data suggest the Pan African deformation and metamorphism in the area involved minor reworking. The undeformed to weakly deformed Tchinadzandze Granodiorite intruded into the Kalahari Craton has an age of 2617 ± 16. Ma. © 2010 Elsevier Ltd.

Moabi N.G.,Central Regions Unit | Grantham G.H.,Central Regions Unit | Roberts J.,University of Pretoria | le Roux P.,University of Cape Town | Matola R.,Direccao Nacional de Geologia
Journal of African Earth Sciences | Year: 2014

Whole rock major and trace element chemistry as well as radiogenic isotope data from the Espungabera Formation of central Mozambique are compared with published data from the Umkondo Formation lavas in SE Zimbabwe and Straumsnutane Formation lavas in western Dronning Maud Land, Antarctica. These formations form part of the ~1100. Ma Umkondo Igneous Province in southern Africa and are now preserved on the Grunehogna (in Antarctica) and Zimbabwe (in Zimbabwe) Cratons.The chemical data indicate that the Espungabera Formation lavas are dominantly tholeiitic and basaltic to basaltic andesitic in composition. The Espungabera lavas are dominated by plagioclase, clinopyroxene and Fe-Ti oxides. Metamorphic mineral assemblages indicate the lavas have been metamorphosed under mid-greenschist facies on a retrograde path to prehnite-pumpellyite facies conditions. The decrease in FeOt with increasing MgO content in the Espungabera lavas and the slight decrease in TiO2 with increasing MgO indicates fractionation of Fe-Ti oxides. The lavas are characterised by negative Nb anomalies; enriched LILE's and high 87Sr/86Sr isotopic ratios. The 87Sr/86Sr data calculated at 1100Ma suggest contamination by continental crust during the petrogenesis of the lavas. The Espungabera volcanics have negative εNd values (-2.83 to -3.49) also suggesting that the magma was contaminated by older crust.Comparison of the chemical data from the Espungabera Formation with data from the Umkondo Group basalts from SE Zimbabwe and the Straumsnutane Formation lavas from Dronning Maud Land, Antarctica shows that they are similar. These similarities, along with similarities in the available geochronological data suggest that these rocks are comagmatic. Both units are also geochemically similar to some rock units that form part of the Umkondo Large Igneous Province (i.e. Zimbabwe basalts that were regarded as Umkondo basalts by Munyanyiwa (1999), Waterberg sills, Umkondo sills and Type III Mutare and Guruve dykes identified by Ward (2002)), and therefore we conclude that the Espungabera lavas in Mozambique also form part of the Umkondo Igneous Province.The craton-based tholeiitic Umkondo Igneous Province is broadly coeval with tonalitic calc-alkaline and granitic gneisses in the Nampula and Maud Terranes in Mozambique and Antarctica respectively, immediately east of the Kalahari Craton in a reconstructed Gondwana. These data can be interpreted to indicate that the Espungabera and Straumsnutane lavas form part of a back-arc complex, west of a volcanic arc/subduction zone along the eastern margin of the Kalahari Craton at ~1100. Ma. © 2014 Elsevier Ltd.

Midzi V.,Seismology Unit | Bommer J.J.,Imperial College London | Strasser F.O.,Seismology Unit | Albini P.,Italian National Institute of Geophysics and Volcanology | And 3 more authors.
Journal of Seismology | Year: 2013

A database of intensity observations from instrumentally recorded earthquakes in South Africa has been compiled as a contribution to the characterisation of seismic hazard. The database contains about 1,000 intensity data points (IDPs) that have been assigned from macroseismic observations retrieved from newspaper reports and questionnaires, and also digitised from previously published isoseismal maps. The database includes IDPs from 57 earthquakes with magnitudes in the range of M w 2.2 to 6.4, at epicentral distances up to 1,000 km. Sixteen events have 20 or more IDPs, with half of these events having more than 80 IDPs. The database is dominated by relatively low intensity values, mostly determined from human perception of shaking rather than structural damage. However, 19 IDPs correspond to intensity values greater than VI MMI-56. Using geological maps of South Africa, the sites of 60 % the IDPs were geologically classified as either 'rock' or 'soil', the uncertainty in locations precluding such a classification for the remaining data points. A few of the IDPs identified as being from soil sites appear to be strongly influenced by site effects, and these were removed from the trimmed database created for exploring ground-motion levels. The trimmed database includes 15 earthquakes which have a minimum of five useful IDPs, excluding those with intensity MMI = I and those based on a single observation. After removing such points, and those identified as clear 'outliers', a total of 436 useful IDPs were selected. © 2013 Springer Science+Business Media Dordrecht.

Mendonidis P.,Vaal University of Technology | Thomas R.J.,Council for Geoscience | Grantham G.H.,Central Regions Unit | Armstrong R.A.,Australian National University
Precambrian Research | Year: 2015

The Margate Granite Suite underlies much of the Margate Terrane of the Natal Metamorphic Province, SE South Africa. It consists of foliated granites grouped into four main lithotypes: garnet leucogranite, garnet-free leucogranite, charnockite and garnet-biotite augen gneiss. In this study we present new U-Pb (SHRIMP) zircon geochronological data on each of the four lithotypes to constrain the timing of emplacement of the various granite phases and of charnockite formation. Magmatic zircon ages span a period of about 125 Ma, indicating that the Margate Suite does not comprise a single coeval group of plutons. The oldest crystallisation age of 1169 ± 14 Ma, obtained from the garnet-biotite augen gneiss phase, is statistically similar to that of the Sikombe Granite which is exposed to the south of the Margate Terrane, with which a correlation is made. This implies that the magmatic history of the Margate Terrane is longer and more complex than previously thought. The original granite protolith of a sample from the charnockitised granite in the thermal aureole of the (ca. 1040 Ma) Oribi Gorge granite yielded an age of 1135 ± 11 Ma, which is statistically similar to the published age of the gneissic Mzimilo Granite in the Mzumbe Terrane. Zircon overgrowths in this sample, dated at 1037 ± 13Ma are coeval with the age of the Oribi Gorge granite and are interpreted to date the secondary charnockitisation of the Margate granite. A sample of a partly charnockitised garnet leucogranite provided an age of 1088±9Ma. This granite contains a pervasive foliation (S2), partly obliterated in charnockitic patches, indicating that both the D2 event and the charnockitisation are younger. This confirms previous work in which the maximum age of the main fabric-forming deformation was constrained by the 1091±9Ma age of the Glenmore Granite. A sample from the garnet leucogranite in the type area of the Margate Granite Suite yielded an age of 1043±4Ma, which is statistically similar to that of the previously dated, garnet-free leucogranite of the Portobello granite, and zircon metamorphic dates of the secondary charnockitisation event associated with emplacement of the Oribi Gorge Suite. Our new data show that the Margate Terrane was subjected to at least four magmatic/thermal events, at ~1170Ma, ~1135-1140Ma, ~1082-1093Ma, and 1025-1050Ma. These events can be correlated with coeval magmatic and thermal episodes in surrounding crustal blocks within Rodinia and Gondwana. In particular the Margate Terrane appears to correlate well with the Vardeklettane Terrane of Dronning Maud Land, East Antarctica and furthermore that the Natal belt may be up to 80Ma older than the Maud belt east of the Heimefront Shear Zone, which is made up of younger crust which was accreted westwards against the Natal belt. In this scenario, the Cape Merdith Complex, West Falkland, which shows no zircon evidence of crust older than ca. 1135 Ma, forms part of the Maud belt, not the older Natal belt. © 2015 Elsevier B.V.

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