M.Sc. / Known outcrops of the supracrustal Mesoarchean Mozaan Group of the Pongola Supergroup occur in north-eastern Kwazulu-Natal and southern Mpumalanga in South Africa, and southern Swaziland. Outcrops of the Mozaan succession in Swaziland are preserved in the Ntungulu-Mahlangatsha and Kubuta-Mooihoek areas. The succession is composed of polymictic conglomerate, poorly sorted scour based quartzite, orthoquartzite, shale, iron-formation, polymictic diamictite and lava. In the Kubuta- Mooihoek area a 3000m thick succession is preserved and correlates almost bed for bed with that in the Hartland area in South Africa. The succession is preserved from the Dipka member of the Sinqeni Formation at the base to the Tobolsk lava at the top. The depositional environment ranges essentially between fluvial and marine with two distinct glaciogenic diamictite units and one unit of lava near the top of the succession. Seven unconformity bounded sequences are recognised in the succession and from these a relative sea-level curve could be constructed. Trace element geochemistry of the shale reveals that the source area was predominantly felsic with a mafic component probably derived from the uplifted pre-Pongola granitoids and Nsuze Group. The petrography of the quartzite in the succession suggests a change in provenance from a low-lying deeply weathered to uplifted moderately weathered source area higher up in the stratigraphy. Part of the tectonic uplift may have been associated with isostatic rebound related to melting of continental glaciers. The Tobolsk lava is a continental flood basalt also possibly related to a tectonic uplift event. There are indications of sediment recycling in the upper part of the succession where conglomerates are predominantly composed of chert clasts A pretectonic quartz porphyry sill, folded with the strata, provides an upper age limit of 2837±5 Ma for the deposition of the Mozaan Group. The Mooihoek granite (2824±6 Ma) that intrudes and deforms the synclinal structure along its eastern flank, provides an upper age limit of the folding event. This suggests that the deformation of the Mozaan succession took place in the intervening 13 Ma period between 2824 and 2837 Ma ago.

M.Sc.(Geology) / The Mesoarchean Witwatersrand Supergroup is a remarkably well preserved siliciclastic dominated cratonic platform succession located on the Kaapvaal Craton in South Africa. The vast gold resources which have been mined since 1886 make it relevant for study. The study aimed to identify significant provenance shifts throughout the depositional life of the basin which should be reflected in the in heavy mineral populations and the geochemical composition of the siliciclastic rocks. The study identified major changes in the source rock compositions through the basin lifespan and inferred major tectonic events during the life of the basin. It was found that the mechanical effects of sorting in different depositional environments tended to obscure provenance shifts, but with careful evaluation of the various factors in play significant provenance shifts could be identified. It was found that these provenance shifts corresponded closely with major unconformity sequence boundaries identified by Beukes (1995). These major provenance shifts are a record of a major tectonic event during the development of the basin. The Hospital Subgroup records a passive trailing margin, fed by a combination of felsic and ultra-mafic source rocks. Within the Hospital Hill Subgroup, there is a trend of increasing ultramafic components in the source area with increasing stratigraphic height. This trend is believed to reflect progressive unroofing of tonalite and greenstone belt complexes over the life of the Hospital Hill Subgroup. At the base of the Promise Formation a basin wide unconformity is present, which marks a shift from mature shallow marine and outer shelf sediments of the Hospital Hill Subgroup to immature fluvial quartzites for the Government and Jeppestown Subgroups (Beukes, 1995). In addition to the major change in depofacies that was recognised by Beukes (1995), this study found evidence for a shift in provenance to generally more fractionated source rocks, that were heterogeneous, but well mixed. The presence of lithoclasts indicates a possible metamorphic component was also present in the source area. This is consistent with a source area containing granitoid batholiths, and granite plutonism which is associated with early subduction tectonics and volcanic arc formation during the deposition of the Government and Jeppestown Subgroups (Wronkiewicz and Condie, 1987 and Poujol, et al., 2003, Kositcin and Krapez, 2004). Another important basin wide unconformity is present at the base of the Johannesburg Subgroup, and marks another major provenance change. These rocks are chemically more mature than the Government and Jeppestown Subgroups and represent a shift to an immature fluvial depositional setting related to basin closure (Beukes, 1995). A shift to moderate Th:Sc and La:Sc suggests a less fractionated mix of source rocks. The disappearance of the lithoclasts indicates that the metamorphic source rocks no longer supplied material to the basin. A small increase in the chromite to zircon ratio also suggests that some unfractionated source rocks were present. The narrow range in Th:Sc, La:Sc, Nb:Y ratios suggests that a homogeneous source area is present, but this is contradicted by the highly variable zircon ages measured by Kositcin and Krapez (2004), so the narrow spread might indicate that the rocks are very well mixed. Zircon populations measured by Kositcin and Krapez (2004) suggest that source terrain of the Johannesburg Subgroup probably consisted of a mixture of the granitoid batholiths from which the Government and Jeppestown Subgroups are a derived as well as some intermediate igneous material with ages of 3000-2870 ma. This would reflect incorporation of syntectonic granitoid plutons into the source areas, Kositcin and Krapez, (2004). The Turffontein Subgroup rocks are very coarse and chemically mature, but they display poor to moderate sorting and rounding. The rocks were deposited in a fluvial environment but marine quartzites are not uncommon. It is believed that these rocks were transported in a high energy environment, but the duration of transportation was short. This allows for effective winnowing but insufficient time for physically mature rocks with well-rounded grains to develop, explaining the mature chemical composition but immature physical composition. The source rocks of the Turffontein Subgroup were probably the same as the Johannesburg Subgroup with the higher energy mode of transportation responsible for the observed increase in Zr:Ti ratio. It would also explain the scarcity of feldspars and chlorite in the Turffontein Subgroup. Th:Sc and Nb:Y ratios suggest highly fractionated source rocks, but care must be taken because the mature nature and coarse grainsize of these rocks make trace element analyses unreliable. The zircon population indicates the presence of 3090-3060ma (Kositcin and Krapez, 2004) granite batholiths, as well as 3000-2870 Ma (Kositcin and Krapez, 2004) syntectonic granite plutons, as well as ancient granitoid gneiss (Kositcin and Krapez, 2004) in the source area. This study has provided new support for a foreland basin origin of the Witwatersrand Supergroup, proposed by Beukes (1995), Beukes and Nelson (1995) and Nhleko (2003), resulting from orogenic collision of the Witwatersrand and Kimberley blocks along the western margin of the Witwatersrand block. The Amalia, Kraaipan and Madibe greenstone belts and Colesberg Magnetic Anomaly are probably the only remaining remnants of this orogeny today.

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M.Sc. (Geology) / Please refer to full text to view abstract

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The Archean (~3.0 Ga) Buhwa Greenstone Belt, and surrounding granite-gneiss terrane, is the least understood major greenstone belt in the Archean Zimbabwe Craton, despite occupying a critical position between an early Archean continental nucleus and the Limpopo Belt. The cover succession in the Buhwa Greenstone Belt, which was probably deposited on the margin of this nucleus, is divisible into shelfal and basinal facies associations separated by a transitional facies association. The shelfal association consists mostly of quartzarenite and shale, but also contains a thick succession of iron-formation. Geochemical characteristics of the shales indicate that the source terrane consisted of several lithologies including tonalite, mafic-ultramafic volcanic rocks, and granite that underwent intense chemical weathering. Basinal deposits consist dominantly of greenstones, with less abundant chert and ironformation. The cover succession, which was deposited on a stable shelf transitional to deep water, has no stratigraphic equivalents elsewhere on the Archean Zimbabwe Craton. However, time and lithologic correlatives in the central zone of the Limpopo The Archean (-3.0 Ga) Buhwa Greenstone Belt, and surrounding granite-gneiss terrane, is the least understood major greenstone belt in the Archean Zimbabwe Craton, despite occupying a critical position between an early Archean continental nucleus and the Limpopo Belt. The cover succession in the Buhwa Greenstone Belt, which was probably deposited on the margin of this nucleus, is divisible into shelfal and basinal facies associations separated by a transitional facies association. The shelfal association consists mostly of quartzarenite and shale, but also contains a thick succession of iron-formation. Geochemical characteristics of the shales indicate that the source terrane consisted of several lithologies including tonalite, mafic-ultramafic volcanic rocks, and granite that underwent intense chemical weathering. Basinal deposits consist dominantly of greenstones, with less abundant chert and ironformation. The cover succession, which was deposited on a stable shelf transitional to deep water, has no stratigraphic equivalents elsewhere on the Archean Zimbabwe Craton. However, time and lithologic correlatives in the central zone of the Limpopo ~2.9 Ga in southern Africa. At ~2.9 Ga, the northern margin of the greenstone belt experienced kilometerscale, oblique-slip dextral shearing. This shear zone and the surrounding margins of the greenstone belt were later intruded by the ~2.9 Ga Chipinda batholith, which ranges from granitic to tonalitic in composition. A number of events occurred during the time period spanning 2.9-2.5 Ga and current geochronology cannot separate their order; some are known to be coeval. Crustal shortening to the northwest, which resulted in map-scale folding of the cover succession (and surrounding batholith) and greenschist-facies metamorphism, occurred along a set of discrete high-angle reverse-sense shear zones in response to uplift the Northern Marginal Zone of the Limpopo Belt over the Zimbabwe Craton. Two suites of potassic granites were intruded into the area near the end of reverse shearing. Analysis of a conjugate fault pair that is developed within one of the potassic granite suites, yields a principal compressive stress consistent with continued northwest-directed crustal shortening. The region was stabilized by ~2.5 Ga, with intrusion of the Great Dyke of Zimbabwe. It is possible that the last events to affect the area, which include sinistral shearing, transecting cleavage development, and northwest-striking open folding, took place during the 2.9-2.5 Ga time intervaL These structures post-date regional folding and metamorphism, but because of limited magnitude and extent, do not show obvious cross-cutting relationships with other rocks or structures. A tenable alternative is that these late structures formed at ~2.0 Ga. an age that is proving to be of great significance in the evolution of the Limpopo Belt and along parts of the southern margin of the Zimbabwe Craton. / Ph. D.

Thesis (PhD)--Stellenbosch University, 2013. / ENGLISH ABSTRACT: The sodic and leucocratic Tonalite, Trondhjemite and Granodiorite (TTG) granitoid series of rocks characterise Paleo- to Meso- Archaean felsic continental crust, yet are uncommon in the post-Archaean rock record. Consequently, petrogenetic studies on these rocks provide valuable insight into the creation and evolution of Earth’s early continental crust. The highpressure (HP)-type of Archaean TTG magmas are particularly important in this regard as their geochemistry requires that they are formed by high-pressure melting of a garnet-rich eclogitic source. This has been interpreted as evidence for the formation of these magmas by anatexis of the upper portions of slabs within Archaean subduction zones. In general, TTG magmas have been assumed to arise through fluid-absent partial melting of metamafic source rocks. Therefore, very little experimental data on fluid-present eclogite melting to produce Archaean TTG exist, despite the fact that water drives magmatism in modern arcs. Consequently, this study experimentally investigates the role of fluid-present partial melting of eclogite-facies metabasaltic rock in the production of Paleo- to Meso-Archaean HP-type TTG melts. Experiments are conducted between 1.6 GPa and 3.0 GPa and 700 ºC and 900 ºC using natural and synthetic eclogite, and gel starting materials of low-K2O basaltic composition. Partial melting of the natural and synthetic eclogite occurred between 850 ºC and 870 ºC at pressures above 1.8 GPa, and the melting reaction is characterised by the breakdown of sodic clinopyroxene, quartz and water: Qtz + Cpx1 + H2O ± Grt1 = Melt + Cpx2 ± Grt2. The experimental melts have the compositions of sodic peraluminous trondhjemites and have compositions that are similar to the major, trace and rare earth element composition of HPtype Archaean TTG. This study suggests that fluid-present eclogite melting is a viable petrogenetic model for this component of Paleo- to Meso-Archaean TTG crust. The nature of the wet low-K2O eclogite-facies metamafic rock solidus has been experimentally defined and inflects towards higher temperatures at the position of the plagioclase-out reaction. Therefore, the results indicate that a crystalline starting material is necessary to define this solidus to avoid metastable melting beyond temperatures of the Pl + H2O + Qtz solidus at pressures above plagioclase stability. Furthermore, this study uses numerical and metamorphic models to demonstrate that for reasonable Archaean mantle wedge temperatures within a potential Archaean subduction zone, the bulk of the water produced by metamorphic reactions within the slabs is captured by an anatectic zone near the slab surface. Therefore, this geodynamic model may account for HP-type Archaean TTG production and additionally provides constraints for likely Archaean subduction. The shape of the relevant fluid-present solidus is similar to the shape of the pressure-temperature paths followed by upper levels of the proposed Archaean subducting slab, which makes water-fluxed slab anatexis is very dependant on the temperature in the mantle wedge. I propose that cooling of the upper mantle by only a small amount during the late Archaean ended fluid-present melting of the slab. This allowed slab water to migrate into the wedge and produce intermediate composition magmatism which has since been associated with subduction zones. / AFRIKAANSE OPSOMMING: Die reeks natruimhoudende en leukokraties Tonaliet, Trondhjemiet en Granodioriet (TTG) felsiese stollingsgesteentes is kenmerkend in die Paleo- tot Meso-Argeïkum felsiese kontinentale kors, maar is ongewoon in die post-Argeïese rots rekord. Gevolglik, petrogenetiese studies op hierdie rotse verskaf waardevolle insig in die skepping en evolusie van die aarde se vroeë kontinentale kors. Die hoë-druk (HD)-tipe van die Argeïkum TTG magmas is veral belangrik in hierdie verband as hulle geochemie vereis dat hulle gevorm word deur hoë druk smelting van 'n granaat-ryk eklogitiese bron. Dit word interpreteer as bewys vir die vorming van hierdie magmas deur smelting van die boonste gedeeltes van die blaaie in Argeïese subduksie sones. TTG magmas in die algemeen, is veronderstel om op te staan deur middel van water-afwesig gedeeltelike smelting van metamafiese bron rotse. Daarom bestaan baie min eksperimentele data op water-teenwoordig eklogiet smelting om Argeïkum TTG te produseer, ten spyte van die feit dat water magmatisme dryf in moderne boë. Gevolglik is hierdie studie ‘n eksperimentele ondersoek in die rol van water-teenwoordig gedeeltelike smelting van eklogiet-fasies metamafiese rots in die produksie van Paleo- tot Meso-Argeïkum HD-tipe TTG smelte. Eksperimente word uitgevoer tussen 1.6 GPa en 3.0 GPa en 700 ºC en 900 ºC met behulp van natuurlike en sintetiese eklogiet, en gel begin materiaal van lae-K2O basaltiese samestelling. Gedeeltelike smelting van die natuurlike en sintetiese eklogiet het plaasgevind tussen 850 ºC en 870 ºC te druk bo 1.8 GPa, en die smeltings reaksie is gekenmerk deur die afbreek van natruimhoudende klinopirokseen, kwarts en water: Qtz + Cpx1 + H2O ± Grt1 = Smelt + Cpx2 ± Grt2. Die eksperimentele smelte het die komposisies van natruimhoudende trondhjemites en is soortgelyk aan die hoof-, spoor- en seldsame aard element samestelling van HD-tipe Argeïkum TTG. Hierdie studie dui daarop dat water-teenwoordig eklogiet smelting 'n lewensvatbare petrogenetiese model is vir hierdie komponent van Paleo- tot Meso-Argeïkum TTG kors. Die aard van die nat lae-K2O eklogietfasies metamafiese rock solidus is eksperimenteel gedefinieër en beweeg na hoër temperature by die posisie van die plagioklaas-out reaksie. Daarom dui die resultate daarop dat 'n kristallyne materiaal nodig is om hierdie solidus te definieër en metastabiele smelting buite temperature van die Pl + H2O + Qtz solidus druk bo plagioklaas stabiliteit te vermy. Verder maak hierdie studie gebruik van numeriese en metamorfiese modelle om aan te dui dat die grootste deel van die water geproduseer deur metamorfiese reaksies binne die blaaie bestaan vir redelike Argeïkum mantel wig temperature binne 'n potensiële Argeïkum subduksie sone, en word opgevang deur 'n smelting sone naby die blad oppervlak. Daarom kan hierdie geodinamies model rekenskap gee vir HD-tipe Argeïkum TTG produksie en dit bied ook die beperkinge vir waarskynlik Argeïese subduksie. Die vorm van die betrokke waterteenwoordig solidus is soortgelyk aan die vorm van die druk-temperatuur paaie gevolg deur die boonste vlakke van die voorgestelde Argeïkum subderende blad, wat water-vloeiing blad smeltingbaie afhanklik maak van die temperatuur in die mantel wig. Ons stel voor dat afkoeling van die boonste mantel met slegs 'n klein hoeveelheid gedurende die laat Argeïese, die water-vloeiing smelting van die blad beëindig. Dit het toegelaat dat die blad water in die wig migreer en intermediêre samestelling magmatisme produseer wat sedert geassosieer word met subduksie sones.