Includes abstract. / Includes bibliographical references. / The False Bay dyke swarm is the southern-most set of Early Cretaceous dolerite dyke intrusions along the western margin of southern Africa associated with rifting of Gondwana and opening of the South Atlantic Ocean. Previous work highlighted the evolution of a single magma type from olivine-tholeiite basalt to ferro-tholeiite andesite. This thesis presents detailed field observations of each dyke in the False Bay - Cape Peninsula region and the focus of the study is on the finer details of the differentiation process to define combined assimilation and fractional crystallisation models.

Includes bibliography. / The Rietfontein kimberlite is an off-craton kimberlite pipe, located west of the Kaapvaal Craton at 26.75°, 20.04°E and hosts a range of xenocryst lithologies, including peridotite, eclogite and a suite of megacryst minerals. This study focuses on a suite of eclogite xenoliths, which were subject to a detailed petrographical and geochemical study, aimed at their characterisation and comparison to eclogites from on-craton and other off-craton localities. Garnet, clinopyroxene, accessory and secondary minerals were analysed for major element compositions using electron microprobe techniques and garnet and clinopyroxene trace element compositions were determined by Laser Ablation Inductively-Coupled-Plasma Mass Spectrometry (LA-ICP-MS) techniques. Oxygen isotopic compositions of five garnet samples were obtained using laser flourination techniques, followed by analysis by gas source mass spectrometry.

Bibliography: leaves 139-146. / The Swartruggens (156 Ma) and Star (128 Ma) kimberlites are two Group II, diamondiferous, hypabyssal kimerlite dyke swarms, situated in the Northern Province and the Free State respectively, South Africa. Representative samples from all dykes exposed in the mining operations, the Main and Changhouse Dykes, South Fissure and the barren Muil Dyke at Swartruggens, and the Wynandsfontein, East Star, Clewer, Byrnes and Barren dykes at Star, have been analysed for their major and trace element contents and Sr, Nd and Hf isotope compositions. Primary kimberlite magma chemistry is subjected to considerable modification due to the incorporation of both mantle and crustal material during ascent to the surface, crystal fractionation, and post-emplacement alteration by deuteric fluids. This study aims to constrain the effects of these processes, and thus to identify least-modified, close-to-primary, parental magma compositions, with the view to understanding the source region characteristics of, and the petrogenetic processes giving rise to, these kimberlites.

Includes abstract. / Includes bibliographical references (leaves 88-102). / Nephelinite-basanite intrusions found on the Tsurib farm near Aus in southern Namibia are described for the first time. The petrography is very similar to each other wih texture ranging from porphyritic to microporphyritic where olivine is dominant, and sometimes the only, phenocryst/microphenochryst phase, and less commonly clinopyroxen; the groundmass consists of clinopyroxine, olivine, Fe-Ti oxides and secondary generation amphibole. Rare felsic xenoliths comprising alkali feldspar (orthoclase) and clinopyroxene (diopside and aegerine-augite) occur in the two major intrusive bodies.

Variations in enrichment of mineralization, expressed in ore deposits, in the continental crust may be one way to test for secular changes in crustal genesis. This study collates and analyses fundamental information about mineral deposits with which to 'fingerprint' the metal endowment of African crust of different age. Three areas of juvenile African crust (e.g. mantle derived over similar lengths of time of ~500 million years, and excluding recycled older crust) of different ages with similar geology are compared. The areas range in age from 0.5 to 3.0 Ga, [e.g. the Zimbabwe Craton (2.5-3.0 Ga), the Birimian Shield (1.8-2.3 Ga), and the Arabian-Nubian Shield (0.5-1.0 Ga)]. The three areas have a total of 2671 mineral deposits, which are divided into six groups according to their geochemical affinities. Using these known deposits, mineral potential maps are created through a data driven approach, using weights of evidence (WotE). The layers/themes used in Woffi are (1) lithology, (2) structures (faults and shear zones), and (3) lithological contacts. The analysis shows that there is strong lithology control on mineralization in all three areas. Archean crust has high predictive values compared to the younger crust. A measure of spatial association (spatial coefficient), based on the WotE approach, is also used to 'fingerprint' the met I endowment in the three selected regions of African crust. The patterns of the mineral deposits distribution within all regions shows that each region has a unique metal endowment, and that there is a greater concentration of mineral deposits in the crust of the Archean Zimbabwe Craton relative to the younger crust of the Birimian and Arabian-Nubian Shields. The analysis of this study therefore quantitatively corroborates studies that suggest older crust is more mineral diverse and more enriched in mineral deposits than younger crust. Thus, secular changes in mineralization or rates of tectonic processes, or both, are implicated, and mineral endowment in the African crust has undergone major evolutionary changes from Archean to Neoproterozoic time.

Bibliography: pages 183-191. / The Mesozoic basalts and dolerites of Botswana underlie an estimated area of 150 000km2 and form part of the Karoo Igneous Province of southern Africa. The distribution of Karoo basalts in Botswana is limited essentially to three main sub-basins, the Central Kalahari Subbasin, northern Botswana and the Tuli Syncline and a major dyke swarm, with a WNW strike, extends across Botswana from the Namibian to Zimbabwean borders. This dissertation is a reconnaissance study which concentrates on the recognition and definition of distinct geocheinical sub-groups within the Karoo volcanic and intrusive rocks of · Botswana. 128 new whole rock samples were analyzed for major and trace element concentrations, in addition to the 70 whole rock analyses from Botswana which were available in the UCT database.· Mineral analyses and rare earth element compositions for selected samples are also presented. The basalts and dolerites of Botswana are assigned to one of the three geochemical lineages, i.e. the low-K20, the high-K20 and the felsite lineages on the basis of Si02, MgO and K20 concentrations. A number of distinct geochemical sub-groups· are recognised within these lineages according to whole rock compositions, normative mineralogy, petrography and outcrop character. The low-K20 lineage is subdivided into two main sub-groups on the basis of the Ti02 and Zr concentrations, i.e. the LTZ- and HTZ-type basalt and dolerite sub-groups. The LTZtype basalt sub-group (with ~2% Ti02 and ~250ppm Zr) represents the bulk of the Botswana dataset where the LTZ basalts of Botswana are shown to be lateral equivalents to the Lesotho Formation basalts of the Central Karoo area, considerably expanding the known outcrop area of this basalt type. Two dolerites are the only samples of intrusive equivalents of this voluminous LTZ basalt type in Botswana.,

There is some controversy in terms of the basement geology of Malawi which ultimately stems from the overall lack of metamorphic studies conducted in the area. The geological complexity of Malawi comes from that fact that it sits at the intersection of three major orogenic belts: The Palaeoproterozoic Ubendian Belt, Mesoproterozoic Kibaran/Irumide Belt, and Pan African Mozambique Belt. Its complexity makes it difficult to unravel, especially in terms of identifying features of older orogenic events which have already experienced multiple metamorphic overprinting from subsequent events. This thesis provides a more detailed pre-rift evolution of the Malawian basement rocks by reporting ages and P-T conditions from four localities surrounding Lake Malawi, namely Chilumba, Mlowe, Maganga, and Mangochi. Results reveal that at 1985-1974 Ma, garnet-cordierite granulites were equilibrated under conditions of 760°C at 4.5-5 kbar possibly as a result of subduction-related magmatism. Subsequently, at 1100 Ma, charnockites were emplaced and metamorphosed under peak conditions of 770-780°C at 4.3-6 kbar due to Kibaran-age magmatic underplating. Remnants of the Irumide/Kibaran Orogeny is relatively scarce throughout Malawi and although the Mangochi charnockites were emplaced during Kibaran-age tectonism, it also experienced at least two different metamorphic events thereafter. The first occurred either during early stages of the East African orogen or Rodinia break-up at 900-800 Ma while the second occurred during the late stages of the East African orogen at 650-600 Ma. Possible remnants of the Kuunga Orogeny are recorded in Chilumba and Maganga as an amphibolite facies metamorphic event which took place around 570 Ma under peak conditions of roughly 660-670°C at 6-8 kbar. Findings of this study have not only provided a more detailed metamorphic history of Malawi but also paved way for future studies in the area to further explore why similar rocks found in such close proximity to each other preserve vastly different tectonic environments.

Includes bibliographical references. / Changes in circulation and productivity in the southeastern South Atlantic Ocean over the last 850 kyr are investigated through the multiproxy study of two giant piston cores, MD962080 and MD962084, retrieved from the Agulhas Bank and Olifants River continental slopes of South Africa. The stable oxygen isotope record of the benthic foraminifer, Cibicidoides wuellerstorfi, and the planktonic taxon, Globorotalia inflata, provide the stratigraphic framework from which the age models were created. The results indicate that biotic responses to surface hydrological changes in the study area are complex and involve both high- and low-frequency variations.

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