The most extensive evidence for the evolution of the primitive crust is preserved at the base of the continental roots (140 to 200 km depth). However, this is completely inaccessible and hard to evaluate, except through the study of mantle xenoliths brought to the surface by kimberlite magmas. Most mantle xenoliths undergo kimberlitic/carbonaceous metasomatism prior to their entrapment, altering their primary composition. Despite a very complex history and a very low abundance (180 nodules), sampled in four main localities from the Siberian (Obnazhennaya, Udachnaya) and South African (Jagersfontein, Roberts Victor) cratons. Major and trace element compositions have been analysed in representative bimineralic (garnet (gt)-omphacite (cpx)), coesite-, kyaniteand corundum-bearing eclogites. Metasomatized eclogites (Type I) have a higher alkali- (Na2O in garnet and K2O in omphacite) and LREE, LILE-content. Non-metasomatized (Type II) eclogites include two main compositional groups, derived from different protoliths. The first group (Type IIA) is typically more magnesian, enriched in LREE and has δ18O values from 3.73 to 7.50 ‰, with positive Sr anomalies and corresponding to a low pressure-temperature layer. The whole-rock composition is consistent with a basaltic protolith. The second group (Type IIB) is more sodic and aluminous, depleted in LREE and has δ18O values of 2.35 to 3.59 ‰, corresponding to equilibrium at high pressure and temperature. The whole-rock trace element composition is consistent with a pyroxenitic protolith. Eclogites that contain coesite, kyanite and corundum (coe-ky-cor) are typically characterized by jadeite-rich clinopyroxenes with positive Eu and Sr anomalies and grossular-rich garnets with corresponding positive Eu and negative Sr anomalies. Additionally, corundum-bearing samples are overall LREE-depleted. Pressure-temperature estimates indicate coe-ky-cor-bearing eclogites equilibrated in the lowermost part of the cratonic keel, and the reconstructed whole-rock trace element composition corresponds to a very depleted gabbroic protolith. Corundum-bearing eclogites often show topotactic exsolution textures hosted in omphacite, consisting of a more calcic garnet ± kyanite/zoisite. Based on Si, Al and Mg diffusion profiles it can be inferred that exsolution was controlled by chemical exchange in an isochemical system. Similarly, an interpretation can be made for the variable HREE fractionation of intermediate composition between the exsolutions and the host omphacite, as evidence for intermineral partitioning. The formation of zoisite (1.7 % H2O) from a precursor clinopyroxene requires a significant amount of hydrogen (as H+ , OH- or H2O) incorporated in omphacite at mantle conditions. Calculated water content of omphacite, based on Fourier transform infrared spectrometry (FTIR) analyses, varies from ~930—1410 to ~1100—1680 ppm by weight H2O, according to different absorption coefficients. Primary garnets are typically anhydrous (<7 ppm H2O), whereas coarse exsolutions contain between 165—1950 ppm H2O. Reconstructed estimates for whole-rock water content (~310—890 ppm H2O) for the Obnazhennaya eclogites are significantly higher than those of the surrounding peridotites. The variability of δ18O in garnet among 41 xenoliths, shows a bi-modal distribution with median values at 3.57 ‰ and 5.68 ‰ and strong correlation (r = 0.96) between garnet and omphacite. The δ18O values and the reconstructed whole-rock trace element compositions indicate an oceanic crustal protolith for mantle eclogite xenoliths. This is consistent with the subduction of a hydrothermally altered, basaltic to websteritic sequence of an incompatible-element-depleted oceanic crust. Furthermore, although eclogites represent a small fraction of the upper mantle, they may be an important water reservoir at the base of the cratonic root. High water content in the lithospheric mantle would have major consequences for the longevity of the cratonic keel, forphysical and chemical properties in minerals, partial melting, mantle rheology and electrical conductivity and the global water cycle.

This thesis explores the nature of the subcontinental lithosphere underlying the southwestern margin of the São Francisco craton and the relation of variations in the petrochemistry of kimberlites and related alkali igneous rocks to variations in age, thickness and thermodynamic history of their continental lithospheric hosts. The São Francisco craton is a mid- to late-Archean basement granite-greenstone terrain flanked to the west by the Proterozoic Tocantins Province (Almeida, 1977, Almeida et al., 1981). New Rb-Sr and Sm-Nd data are presented for both onand off-craton crustal rock sequences. The ultramafic greenstone association of the Rio das Velhas Supergroup yields 3.2 Ga Rb-Sr and Sm-Nd ages, in agreement with widespread 3.2 Ga old zircons from the area. Granitic gneiss and juvenile granitoids associated with the greenstones in the Congonhas area give a Transamazonian 2128 Ma Rb-Sr age, which is in agreement with a published 2124 Ma zircon age. Funher west, syntectonic granitoids and metabasalts from the Araxa Group define a 711 Ma Rb-Sr isochron. This latter age is interpreted as a Sr-isotope rehomogenization related to the development of the Brasilia orogenic and foreland thrust belt. A 823 Ma Sm-Nd errorchron indicate that these rocks may be coeval to the felsic volcanism of the Araxa Group which was recently dated at 794 Ma by zircon work (Pimentel et al., 1991). Funher to the west still, combined samples from the Nique!andia mafic-ultramafic igneous complex and associated granitic basement rocks yield a 1.26 Ga Rb-Sr isochron, which is best interpreted as a metamorphic age. Crystallization ages of the crustal sequences decrease and ENd values increase with increasing distance westward from the Archean craton margin. The isotopic characteristics are consistent with a model which requires that large volumes of crust, derived in the Proterozoic from mantle reservoirs similar to the sources for modern oceanic basalts, were accreted onto the pre-existing Archean nucleus during the Brasiliano orogenic event.

Bibliography: leaves 238-251. / Worldwide most Neoproterozoic sequences are characterised by an intimate association of glaciogenic diamictite with marine carbonate deposits, indicating possibly rapid climatic changes from icehouse to greenhouse conditions. Such sequences also occur in the Pan-African Gariep Belt and in the Kango inlier of the Saldania Belt in Namibia and South Africa. Correlation of these sequences with others has been problematic because of poor age control, structural complications and poor exposure in the Saldania Belt.

Word processed copy. / Includes bibliographical references.

Includes bibliographical references. / Cenozoic sedimentary successions have a restricted distribution and are largely incomplete due to erosion and non-deposition on the western margin of southern Africa. For this reason, much controversy and uncertainty remains on the geological evolution of the western margin. The wide western margin is largely devoid of bathymetric features, except for the deeply incised Cape Canyon that crosscuts the continental slope and shelf ~150 km to the northwest of Cape Town. The Head of the Cape Canyon forms a well-developeed trough landwards of the Western Ridge, which separates the middle and outer shelf. More than 50 cores, up to 6 m in length, at water depths between 190 and 450 m were recovered from the Head of the Cape Canyon region. Siliclastic, authigenic and biogenic sediments, varying in age from Cretaceous to Holocene provide the basis of a detailed sedimentary analysis. The diversity of lithostratigraphic units recovered from the condensed sedimentary record provides a unique opportunity to define in detail, for the first time, a late Cenozoic stratigraphic record for the western outer continental shelf.

Includes bibliographical references.

Bibliography: p. 277-308. / This thesis is divided into 2 parts. The first part (Chapters 1-5) concentrates on unravelling the . Precambrian geological history of the high-grade poly-metamorphic and poly-deformed basement of southernmost India, using field geology and U-Pb geochronology. Because Peninsular India occupied a central position in Gondwana, this work bears on reconstruction models of central Gondwana between circa 600 Ma and 200 :f1,a. The second part of the thesis (Chapter 6) tests the use of organic carbon isotopes as a chemostratigraphic tool to correlate between the lower part of terrestrial Gondwana (Upper Carboniferous to Triassic) sequences in India. This work contributes to (i) intra-basinal and inter-basinal stratigraphic correlation and carbon isotope variations during the Pennian-Triassic extinction event of Gondwana; and (ii) refines the inter-continental correlations of the Gondwana type sequences in India with those in South Africa and Madagascar. This century much attention has been focused on the reconstruction of Gondwana, and as a result, the broad relative positions of different Gondwana fragments and their later dispersal histories is now well understood. Finer-scale reconstructions, based on modern geochronological and isotope studies integrated with field studies of continental rocks, are still needed to resolve the detailed kinematic histories of continental break-up and the evolution of continental lithosphere in general. Shear zones that cross the boundaries of continental fragments provide 'piercing points' in fitting the fragments back together. To realize this, the kinematics and the timing of major shear zones need to be determined. The first part of this thesis, therefore, primarily focuses on understanding the kinematics and geochronology of a number of subvertical continental scale shear zones in the Southern Granulite Terrain of India (SGT).

Includes bibliogaphic references. / This thesis is concerned with the Mesozoic geological history of the Agulhas Bank on the continental shelf off the south coast of South Africa. It incorporates the results of oil exploration activities in the area since 1967: this involved the drilling of 17 boreholes (43 000 m) and the recording of about 24 000 line km of seismic reflection profiles. A structural investigation was facilitated by regional mapping of two key seismic reflectors; these are acoustic basement (horizon D at tile base of the Mesozoic) and a prominent reflector and unconformable horizon (horizon C) within the Lower Cretaceous. An additional two horizons were mapped over part of the Agulhas Bank; they are horizons A (base of the Uppper Cretaceous) and 11 (within the Lower Cretaceous). These reflector horizons were tied to stratigraphic control points in the boreholes. A stratigraphic framework was erected and correlations were established by means of a study of borehole cuttings samples, cores and sidewall cores and a variety of wireline logs. The existing Mesozoic litho-stratrigraphic framework was supplemented by defining reference stratotypes for the Swart¬kops end Sundays River formations. Subsurface stratotypes are defined for two new units; they are termed the Infanta and Alphard Formations. A subsurface reference stratotype for the Alexandria Formation is presented. A chronostratigrapic framework and correlations, which became available through a study of microfauna and microflora were intergrated with structural and lithostratigraphic information. As a consequence four litho-tectonic units are recognised; their evolution is discussed also in terms of varying rates of sediment accumulation. The Mesozoic geologic history of the Agulhas bank is discussed in reference to the plate tectonic history.

The Grootegeluk Formation in the Waterberg Coalfield consists of coal and mudstone layers that were deposited during the Late Permian. In the south-central part of the Waterberg Basin rapid subsidence resulted in the formation of strata (-70 m thick) which consist of relatively thin coal beds interbedded with a multitude of mudstone and carbonaceous mudstone layers. Mudstones from the Grootegeluk Formation, the underlying upper Vryheid Formation and the base of the overlying Beaufort Group were obtained predominantly from borehole cores. The thesis integrates thin-section petrography, mineralogy, maceral composition, maximum vitrinite reflectivity and geochemical analyses to investigate: (1) the source of the sediments and the palaeo-environment of formation of the coal and mudstone layers; (2) the stratigraphical correlations of some South African coal formations and the formation of coal, on a continental scale, during the Late Permian and Triassic times; (3) the possibility of contamination to the environment by the waste products of coal-mining ( carbonaceous mudstones) and coal-burning in a local power-station (fly ash and bottom ash). The lower portion of the Grootegeluk Formation is dominated by kaolinite, quartz and minor amounts of anatase, and the upper parts of by quartz, kaolinite and minor amounts of montmorillonite-illite and microcline. These minerals are predominantly allogenic. Mineralogical evidence from the mudstones suggests that the base of the Grootegeluk Formation was relatively more distal and the upper portions more proximal to the source of the sediments. The maceral variation of the coal seams and the organic matter in the mudstones reveal that conditions for the preservation of organic matter were more suitable during the deposition of the sediments in the upper (vitrinite-rich) rather than the lower (inertinite-rich) Grootegeluk Formation. At the base of the Grootegeluk Formation a 2 m thick carbo-tonstein occurs that is dominated by kaolinite, organic matter (-40 weight %), siderite, calcite and minor proportions of apatite. The carbo- tonstein has mineralogical, maceral and chemical characteristics distinctive from the other mudstones. Syn-depositional calcite lenses occur predominantly in the upper-half of the Grootegeluk Formation. Early diagenetic globular pyrite and spherulitic siderite occur in the coals and organic rich-mudstones of the Grootegeluk Formation. Globular pyrite and granular siderite are present predominantly in the organic-poor mudstones of the Grootegeluk Formation and Beaufort Group. Marcasite sometimes occurs along bedding planes and calcite is generally present as deat-filling in the coal seams. Based on their geochemistry, the Grootegeluk Formation and Beaufort Group mudstones have the same provenance, dominantly "granitic", possibly granodioritic in composition (Late Proterozoic Sm-Nd model ages). The provenance of the Vryheid Formation was relatively more mafic (Early Proterozoic Sm-Nd model ages). Smooth, systematic changes in the mineralogy and major element chemistry make major element whole-rod chemistry an ideal stratigraphic-indicator tool for the Grootegeluk Formation. The mudstones had very low concentrations of the alkali and alkaline-earth elements suggesting that the rocks had undergone very high degrees of chemical alteration. Trace elements in the carbonaceous mudstones are predominantly hosted in the mineral fraction. The carbo-tonstein had anomalously high concentrations of all the trace elements except S, Co, As and Zn, which, in addition to its mineralogical character, make the carbo-tonstein an important ( chrono-) stratigraphic marker for local and intra-basinal correlation. Anomalous concentrations of the chalcophile elements in the mudstones from the overlying lower Beaufort Group mudstones are considered to have been concentrated as a result of diagenesis and(or) reduction-oxidation reactions. The concentrations of trace elements and stable isotope data suggest that the sediments were deposited in fresh rather than in marine waters. Evidence from organic 13 C analyses and palynological studies indicates that ~ 13 C values can be used in stratigraphical correlations and as palaeo-ecological indicators. The ~ 13 C values of the Permian and Triassic terrestrial organic matter suggest a change in the atmospheric 13 C/1 2 C ratio during this time. Leachable concentrations of trace elements which may be toxic to plants, animals or humans are too low in coal mining discards to be of any concern. Mudstones that immediately overlie the Grootegeluk Formation coal-mudstones have leachable concentrations of elements that exceed the critical concentrations believed to be harmful to the environment. Experiments on fly ash and bottom ash samples from the Matimba power station revealed that the fly ash had unacceptably high concentrations of leachable elements, such as As, Cd and Mo. Leachates of bottom ash samples had very low concentrations of potentially harmful elements.

Bibliography: pages 195-212. / Epigenetic, rare earth element, gold, tin and tungsten, mineralisation in the NE-trending, intracratonic branch of the deeply eroded, Late Proterozoic/Early Palaeozoic, Pan-African Damara Orogen is hosted by meta-arkoses, marbles and metaturbidites in the magmatic arc (Central Zone; CZ) of the orogen, a tectonostratigraphic entity that is characterised by multiple deformation, greenschist/amphibolite-facies, low-pressure/high-temperature metamorphism and numerous granitic and pegmatitic intrusions. This thesis integrates regional and detailed geological mapping with petrographic studies, whole rock geochemistry (obtained by X-ray fluorescence spectrometry, atomic absorption spectrometry, fire assay and infra-red spectroscopy), mineral chemistry studies (electron microprobe and X-ray diffraction) and geochronological work (Rb-Sr whole-rock and mineral age determinations and Pb-Pb model ages) to examine epigenetic mineralisation in the Usakos-Karibib-Omaruru district, central Namibia. The investigated area straddles the magnetically defined Omaruru Lineament, which divides the CZ into southern (SCZ) and northern (NCZ) portions. Mapping and lithostratigraphic work support the geophysical evidence for this subdivision: in the SCZ, predominantly continental and shallow marine Damaran metasediments are floored by a 1.7-2.0 Ga granite gneiss basement inlier, that is transected by numerous ENE-trending aurif erous megashears. These structures controlled late Proterozoic mafic dyke emplacement, Damaran rifting and sedimentation, alkaline volcanism and the localisation of hydrothermal fluids in the overlying cover rocks. In the NCZ, where basement inliers are not exposed, the metamorphosed equivalents of Damaran shallow marine carbonates are overlain by deeper water sediments and turbidites. Sedimentation was controlled by rifting, facies belts being oriented parallel to the axis of the orogen. Felsic and mafic volcanic rocks comprise only a fraction (< 5 % ) of the Damara Sequence.