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Provenance ages and timing of sedimentation of selected Neoarchean and Paleoproterozoic successions on the Kaapvaal Craton

Granitic and rhyolitic magmatism: constraints on continental reconstruction from geochemistry, geochronology and palaeomagnetism

Formation of major fold types during distinct geological events in the central zone of the Limpopo Belt, South Africa: new structural, metamorphic and geochronologic

  • Authors: Boshoff, Rene
  • Date: 2009-01-27T07:18:07Z
  • Subjects: Geology , Structural geology , Metamorphism (Geology) , Folds (Geology) , Geological time , Limpopo Belt (South Africa)
  • Type: Thesis
  • Identifier: uj:14827 , http://hdl.handle.net/10210/1951
  • Description: M.Sc. , The Limpopo Complex (LC) of southern Africa is one of the best-studied Precambrian granulite facies terrains in the world, yet workers still disagree on fundamental aspects of the geological evolution of this complexly deformed high-grade terrain. Most workers agree that the two marginal zones were exhumed in the late-Archaean, but disagree on the timing of major tectono-metamorphic events that affected the Central Zone (CZ) of Limpopo Belt, and the mechanism/s of its formation. There are currently two main schools of thought: The first school regards the LC as a late-Archaean orogenic zone that resulted from a north-south collision of the Zimbabwe and Kaapvaal cratons. Granitic plutons throughout the entire LC are considered to be accurate time-markers for this orogeny. The second school suggests that the CZ evolved as a result of a major Paleoproterozoic tectono-metamorphic event based mainly on the interpretation of metamorphic mineral ages. The present study focuses on two aims, namely (i) to provide a synthesis of published data as a basis to understand the ongoing age controversy concerning the evolution of the CZ, and (ii) to show that specific fold types in the CZ can be related to either the late-Archaean or the Paleoproterozoic event. New age, structural, metamorphic, and petrographic data are presented to show that (i) major sheath folds reflect the peak tectono-metamorphic event that affected the CZ in the late-Archaean, while (ii) major cross folds developed as a result of a transpressive event in the Paleoproterozoic. The age of formation of the Avoca sheath fold located about 40 km west of Alldays is accurately constrained by the age of emplacement of different structural varieties of precursors to the Singelele Gneiss: penetratively deformed syn- to late-tectonic Singelele gneisses with a zircon SHRIMP age of 2651 ± 8 Ma, date the time of formation of the sheath fold that is characterized by a single population of linear elements that define the central fold axis. The Avoca sheath fold documents top-to-the-NNE movement of material during the exhumation of the high-grade CZ rocks. Weakly foliated late-tectonic L-tectonites with a zircon SHRIMP age of 2626.8 ± 5.4 Ma, outcrop near the centre of the sheath fold, and provide a minimum age for the shear deformation event. An almost undeformed (post-tectonic) variety of the Singelele Gneiss was emplaced after the shear event. A detailed metamorphic study of metapelitic gneisses from the large Baklykraal cross fold, located about 20 km east of the Avoca sheath fold, documents a single decompression-cooling (DC) P-T path for the evolution of this structure. Three studied metapelitic samples characterized by a single generation of garnet provide a Pb-Pb age of 2023 ± 11 Ma, that accurately constrain the time of formation of this major fold to the Paleoproterozoic. A metapelitic sample characterized by two generations of garnet provide a slightly older Pb-Pb age of 2173 ± 79 Ma, that is interpreted to also reflect the late-Archaean event. The Baklykraal cross fold is characterized by two populations of linear elements: the one population defines the shallow N-S oriented fold axes, while the second population is associated with top-to-the-NNE movement of material during exhumation, resulting in folds with a nappe-like geometry. A DC P-T path for the Campbell cross fold (Van Kal, 2004) located just west of Musina, suggests that cross folds developed under significantly lower P-T conditions than is the case with sheath folds, providing an explanation for the lack of significant anatexis associated with the Paleoproterozoic event. The late-Archaean orogeny in contrast, was accompanied by widespread anatexis during a major magmatic event that is characterized by an abnormal high radiogenic signature. This study, for the first time, provides evidence that link specific fold types, and thus deformational events, to different tectono-metamorphic events. The main conclusion is that the CZ was exhumed as the result of two distinct orogenies, one in the late-Archaean, and the other in the Paleoproterozoic.
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Provenance of the Neoproterozoic to early Palaeozoic successions of the Kango Inlier, Saldania Belt, South Africa

  • Authors: Naidoo, Thanusha
  • Date: 2009-04-28T06:57:55Z
  • Subjects: Geology , Petrology , Geochemistry , Geological time , Cape of Good Hope (South Africa)
  • Type: Thesis
  • Identifier: uj:8308 , http://hdl.handle.net/10210/2437
  • Description: M.Sc. , The configuration of the supercontinent Rodinia, at the end of the Mesoproterozoic to the beginning of the Neoproterozoic (1100-750 Ma), and its subsequent break up into cratonic fragments that would later result in the formation of Gondwana (Early Palaeozoic), is still not completely understood. This is largely due to ambiguity surrounding relationships between cratons, craton evolution and timing of significant tectonic or sedimentary events. Particular to this study is the evolution and palaeogeographic history of the Kalahari Craton and a comprehensive provenance analysis of Neoproterozoic to early Palaeozoic clastic sedimentary rocks from the Kango Inlier (Saldania Belt, South Africa). This includes the Cango Caves and Kansa Groups as well as the Schoemanspoort and the adjacent Peninsula Formation (Table Mountain Group, Cape Supergroup). A well established lithostratigraphy, in addition to recent establishment of age constraints by UPb zircon dating and microfossil evidence, allowed for strategic sampling with the objective of gaining insight to the crustal evolution of SW Gondwana. In this study, a progression from immature, moderately altered rocks in the Cango Caves Group (Upper Neoproterozoic) to mature, strongly altered rocks in the Lower Palaeozoic Kansa Group and overlying formations is observed. Thus, rapid sedimentation of the former is anticipated, while the subsequent formations developed at a passive/rifted margin culminating in the laterally extensive deposition of the Peninsula Formation. Ongoing extensional movement is evident due to chronologically deeper-water facies and the progressive influence of a less fractionated component in the Cango Caves Group, particularly in the Huis Rivier Formation. The association of these rocks with an active margin is not certain since index trace element concentrations are too high for typical arc terranes. Thus, the mixing of a younger (570-600 Ma) magmatic source (close to an active margin) with mafic and felsic rocks of the older Mesoproterozoic Natal- Namaqua Mobile Belt (NMB) is the most likely possibility. A maximum, pre-Cape Granite age of 571 Ma can be assigned to the Huis Rivier Formation (Cango Caves Group) by detrital zircon dating, and thus correlation with the Malmesbury Group can be made. Ediacaran age zircons might be related to the active continental margin (Trans Antarctic Orogen) surrounding southern Gondwana, but this is still hypothetical. The post-Cape Granite Kansa Group and overlying Schoemanspoort Formation were most likely deposited as basin infill subsequent to folding and transtensional tectonics affecting the underlying Cango Caves Group. The Kansa Group may be comparable with the Klipheuwel Formation (southwest South Africa) in terms of its stratigraphic position beneath the Table Mountain Group. Deposition of the Table Mountain Group is much younger than previously believed in light of Ordovician zircon ages (471, 485, 499 Ma) obtained from the underlying Kansa Group. However, the provenance of these thus far unheard of ages for magmatic events in South Africa is a matter of contention. The proximal Ordovician Ross-Delamerian Orogenic event in Antarctica is the most likely source. Peninsula Formation deposition represents a cover sequence i.e. the culmination of small isolated basins (e.g. the Kansa Group and lower Table Mountain Group) into a larger, laterally extensive basin where reworking played a dominant role. This basin is likely to be a rift-related. However, it is not clear which crustal entity rifted away from vi South Africa and if, during the Ordovician an, active continental margin further to the south - bridging the South American Famatina Orogen with the Ross-Delamerian arc in Antarctica - existed. The Natal-Namaqua Mobile Belt appears to be the predominant source throughout the succession as indicated by Nd-isotope data and zircon populations. This implies that simple crustal recycling of Natal-Namaqua basement (or rocks with similar Nd-isotope characteristics) led to the genesis of the magmatic material younger than 1 Ga, observed in this study.
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Distribution and geochronology of unconformity-bound sequences in paleoproterozoic Elim-Olifantshoek red beds: implications for timing of formation of Sishen-type iron ore and heavy carbonate carbon isotope excursion

  • Authors: Da Silva, Richard
  • Date: 2012-08-16
  • Subjects: Iron ores , Geochronometry , Paleontology, Proterozoic , Carbon isotopes , Geological time , Griqualand West (South Africa)
  • Type: Thesis
  • Identifier: uj:9517 , http://hdl.handle.net/10210/5946
  • Description: M.Sc. , Bracketing the depositional age of the Gamagara/Mapedi to Lucknow and Olifantshoek succession in Griqualand West is important because it not only represents one of the oldest known red bed successions in the world but also hosts some of the first well preserved lateritic soil profiles and carbonates with heavy 13C values traditionally correlated with the so-called Lomagundi carbonate carbon excursion. In addition the ancient supergene very large high-grade hematite iron ore deposits of the Sishen-Postmasburg area on the Maremane dome are associated with the erosional unconformity at the base of the Gamagara Formation (a lateral equivalent of the Mapedi Formation). However, the depositional age of especially the Gamagara/Mapedi to Lucknow succession is under dispute because it has been considered a) correlative to the lower part of the Waterberg Group in the Transvaal area, with the implication that it is younger than the Bushveld Complex with an age of ~2,054 Ga, and b) correlative to the Dwaalheuwel-Magaliesberg succession of the pre-Bushveld Pretoria Group of the Transvaal Supergroup in the Transvaal area. The upper age limit of the Gamagara/Mapedi to Lucknow succession is defined by 1,92 Ga felsic volcanics in the overlying Neylan-Hartley succession of the Olifantshoek Group. The Hartley Lava Formation is overlain by Volop quartzites. This study involves age determinations of detrital zircon populations extracted from the basal Doornfontein conglomerate member of the Gamagara/Mapedi succession, and quartzites of the Gamagara/Mapedi, Lucknow, Neylan, Hartley and Volop Formations at various localities in Griqualand West. Based on field work, three unconformity-bound sequences are defined, namely the Gamagara/Mapedi-Lucknow, Neylan-Hartley and Volop sequences. Most interestingly quartzites of the Gamagara/Mapedi-Lucknow sequence contain abundant zircons with ages similar to that of the Bushveld Complex at ~2,054-2,06 Ga in addition to zircons as young as ~1,98-2,01 Ga. An exception is results on one sample of the Doornfontein Member analyzed so far (it is from the Rooinekke iron ore mine south of Postmasburg) that contains only zircons that are older than the Bushveld Complex with a rather prominent youngest population bracketed between 2,2 Ga and 2,32 Ga. The youngest detrital zircon populations in the Neylan-Hartley sequence are either slightly older than the Hartley lava or contain zircons with similar age to Hartley felsic lavas at 1,92 Ga. This sequence thus appears to have developed immediately prior to and coeval with Hartley volcanism. The overlying Volop sequence contains abundant zircons as young as ~1,89 Ga. The results clearly illustrate that the Gamagara/Mapedi to Lucknow succession is certainly not a lateral correlative of the pre-Bushveld Dwaalheuwel-Magaliesberg succession of the Pretoria Group. Rather it should be considered time-equivalent lower parts of the Waterberg Group in the Transvaal area. This implies that the heavy carbonate carbon excursion known from the Lucknow Formation is at least 100 my. younger than the one known from the upper part of the Silverton Formation along the contact with the overlying Magaliesberg Quartzite. There are thus at least three heavy carbonate carbon excursions, known from Paleoproterozoic cover successions of the Kaapvaal Craton in southern Africa, namely one in the ~2.35 Ga Duitschland Formation, a second in the ~2,1 Ga Silverton Formation of the Pretoria Group of the Transvaal Supergroup and the third in the ~1,98-1,92 Ga Lucknow Formation. It is further known that carbonates with normal open marine 13C values of close to zero occur in stratigraphic intervals between each of the heavy carbonate carbon excursions.
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Mafic, ultramafic and anorthositic rocks of the Tete complex, Mozambique : petrology, age and significance

  • Authors: Evans, Richard John
  • Date: 2012-09-11
  • Subjects: Geology - Mozambique , Ultrabasic rocks - Mozambique , Petrology - Mozambique , Mineralogical chemistry , Mineralogy - Mozambique , Geochronometry - Mozambique , Geological time
  • Type: Thesis
  • Identifier: uj:10074 , http://hdl.handle.net/10210/7460
  • Description: M.Sc. , The ca. 800 km2 Tete Complex of NW Mozambique is located at the eastern end of the 830 ±30 Ma Zambezi Belt, near the transition zone into the Neoproterozoic Mozambique Belt. The Complex is located just south of the Sanangoe Shear Zone where Mesozoic and Late Palaeozoic cover rocks obscure much of the region. Country rocks immediately in contact with the Tete Complex include amphibolitic gneiss, graphite-bearing marble, calcsilicate gneiss, muscovite and biotite schist and quartzite of the Chidue Group. The Tete Complex may have been intrusive into the Chidue Group, although there is evidence inferring tectonic emplacement. Those few contact exposures that exist are equivocal. Some of the rocks within the Tete Complex have been affected by metamorphism up to amphibolite grade, although large proportions of the rocks retain pristine magmatic mineralogy and texture. The Tete Complex contains mafic, ultramafic and anorthositic rocks, dolerite dykes and minor Fe-Ti oxide-rich rocks that occur as rubble. Pyroxenite occurs as thin (<1-2 m), cumulate layers within gabbroic rocks. Most exposed anorthositic rocks occur in the Nyangoma area in the eastern part of the Tete Complex. The anorthosites and leucotroctolites are massive, coarse grained (2-3 cm), and contain plagioclase (An47-An57) megacrysts up to 10 cm in length, interstitial olivine (Fo59-Fobs) and orthopyroxene (En59- En75, mean A1203 = 1.84 wt.%) rimmed by clinopyroxene (mean = Wo 46En38Fs i6), pyrite and Fe-Ti oxides. Secondary biotite, iddingsite, epidote and green spinet are present. The stable coexistence of olivine and plagioclase limits the depth of emplacement to <7-8 kbar, or <20- 25 km; a relatively shallow level of emplacement is favored by the generally fine grain size of the gabbroic and doleritic rocks. Compositions of coexisting plagioclase and mafic silicates (orthopyroxene and olivine) are similar to those of massif-type anorthosites. Previously unmapped meta-anorthosite occurs along the western and northern margin (within the Sanangoe Shear Zone) of the Tete Complex and has been metamorphosed to amphibolite grade. The rock contains plagioclase (An38-An39), with the more Ab-rich compositions related to the formation of garnet (mean = A1m67GrotsPYI6Sp2). Metamorphic orthopyroxene (Enso-En53), clinopyroxene (mean = Wo37En38Fs25), mizzonitic scapolite (Me63), amphibole, biotite and apatite are present. High Cl contents in amphibole, scapolite and biotite (e.g., up to 4.7 wt. % in amphibole), suggest that a Cl-rich metamorphic fluid infiltrated the western margin of the Tete Complex. Olivine melagabbro from the north-central part of the Tete Complex contains plagioclase (An70-An26), olivine (Fo82-Fos4) and clinopyroxene (mean = WanEn1Fs0.2, mean A1203 = 2.56 wt. %), with primitive compositions compared to those in Nyangoma anorthositic rocks and pyroxenites. Pyroxenites are modally dominated by clinopyroxene (mean = Wo46-48En36-39Fsi3-18) with accessory interstitial plagioclases (Ano-An45) and discrete and exsolved orthopyroxenes (En 56-En75). Clinopyroxenes with high A1203 contents up to 9 wt. % are similar to high-Al pyroxene megacrysts. One sample of pyroxenite contains orthopyroxene (En56-En60) and plagioclase (An40-An45) with more evolved compositions compared to those in Nyangoma anorthositic rocks and olivine melagabbro. Normal Fe4- and Na-enrichment trends accompanying fractionation from magmas that may be common to the Nyangoma anorthositic rocks, pyroxenites and olivine melagabbro, are associated with an increase in Al relative to Cr along a line of nearly constant relative Ti content. Gabbro contains olivine and plagioclase crystals that are commonly zoned, thus ranging widely in composition (Fool -Fos°, Anss-Ans2)• Clinopyroxene (mean = Wo36En47Fsi6) constitutes ca. 34 modal % of gabbro. New whole-rock (Nyangoma anorthosite and leucotroctolite) and mineral (plagioclase, clinopyroxene and orthopyroxene) Sm-Nd isotopic data yields ages between 975 ±33 Ma and 1041 ±131 Ma. The igneous crystallization age of the anorthositic rocks is estimated at 1025 ±79 Ma (9-point whole-rock regression). Rb-Sr isotopic compositions for whole-rock samples reveal no meaningful age relationships. Initial Nd isotopic compositions (calculated at 1.0 Ga) correspond to E Nd values between +3.5 and +4.5 (mean = +4.1) with Is, = 0.70276 — 0.70288 (mean = 0.70282), both inferring magmatic derivation from a depleted mantle source, possibly with little or no contamination by Archaean crustal components. TDM model ages range between 1074 and 1280 Ma (mean = 1148 Ma). There is a striking similarity between the Tete Complex anorthosites and those of SW Madagascar in terms of Nd isotopic compositions and the nature of country rocks; in both regions the anorthosites were emplaced either magmatically or tectonically into shelf-type supracrustal metasediments (marbles, quartzites, graphitic schists, etc.). Anorthosites intruded similar country rocks in Draining Maud Land, eastern Antarctica. Although anorthosites from Mozambique and Madagascar share a common depleted mantle signature with little or no contamination by Archaean crustal components, a direct stratigraphic correlation between these two areas (and possibly eastern Antarctica), awaits further geological and geochronological data.
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⁴⁰ Ar/³⁹Ar and (U-Th)/He dating attempts on the fossil-bearing cave deposits of the Malapa and Sterkfontein hominin sites of the Cradle of Humankind, South Africa

  • Authors: Makhubela, Tebogo Vincent
  • Date: 2015-04-22
  • Subjects: Argon-argon dating - South Africa - Cradle of Humankind World Heritage Site , Geological time , Fossil hominids - South Africa - Cradle of Humankind World Heritage Site
  • Type: Thesis
  • Identifier: uj:13556 , http://hdl.handle.net/10210/13697
  • Description: M.Sc. (Geology) , The Cradle of Humankind is a 47 000 hectare demarcated area with over three dozen fossil-bearing cave sites well known for the preservation of fossil evidence of early hominin taxa such as Australopithecus Africanus, Australopithecus Sediba, Paranthropus Robustus and Early Homo. As a result, a database of precise and accurate chronological data for fossil-bearing cave deposits of the Cradle of Humankind (similar to that for East African fossil sites) is very important, but developing one has proven extremely challenging. The main challenge is that the fossil-bearing deposits at the cradle are mainly complex breccias with a chaotic, localized stratigraphy and no association to any volcanic ash beds, unlike the East African deposits which are lacustrine and fluviatile deposits interbedded with volcanic ash layers. However, substantial success has been obtained recently through the combination of U-Pb dating of CaCO₃ speleothems and palaeomagnetic dating (magnetostratigraphy) after many attempts and unconvincing results from techniques such as biostratigraphic correlations, electron spin resonance on teeth and cosmogenic burial dating of the sediments. The problem with U-Pb dating of CaCO₃ speleothems is that this requires samples that are extremely clean (i.e. detrital-free) and have an appreciable U content (close to 1 ppm), and such material is at many sites not available...
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