Thesis (DSc)--Stellenbosch University, 2015. / ENGLISH ABSTRACT: Anatexis is the first step in granite genesis. Partial melting in the lower crust may produce leucoratic features of unusual chemical compositions, very different from the final products of crustal differentiation. Therefore, the links that exists between some migmatites and crustal-derived granites can be ambiguous. This study is an investigation of the anatectic history of a high-grade terrain: the Southern Marginal Zone of the Limpopo Belt (SMZ), north to the Kaapvaal Craton in South Africa. The work involved an integrated field, metamorphic, geochemical and geochronogical study of the metasedimentary granulites from two separate quarries in the northern zone of the Southern Marginal Zone, the Bandelierkop quarry and the Brakspruit quarry, where Neoarchean high-grade partial melting features can be observed. The project has aimed to address two main issues: (1) to accurately constrain the pressuretemperature conditions and the age of the metamorphic episode in the SMZ, with implication for the geodynamic processes near the end of the Archean, (2) to investigate the fluid-absent partial melting reactions that control formation of K2O-poor leucosomes and to understand the chemical relationships in the system source-leucosome-melt–S-type granite. The P-T-t record retained in the Bandelierkop Formation metapelites, constrained by phase equilibria modelling as well as zircon LA-ICP-MS geochronology, gives an insight into crustal differentiation processes in the lower crust. Rocks in both quarries indicate high-temperature metamorphism episodes with peak conditions of 840-860 oC and 9-11 kbar at c. 2.71 Ga with formation of leucosomes (L1) during the prograde path. Minor leucocratic features (L2) were produced during decompression to 6-7 kbar. The end of the metamorphic event is marked by the granulites/amphibolites facies transition (< 640 oC) at c. 2.68 Ga. The maximum deposit age for the detrital zircons in the metapelites (c. 2.73 Ga) indicates a rapid burial process ( 0.17 cm.y1). Those evidences strongly support that the Southern Marginal Zone contains sediments deposited in an active margin during convergence, and that the metapelites were metamorphosed and partially melted as a consequence of continental collision along the northern margin of the Kaapvaal Craton at c. 2.7 Ga. The leucocratic features generated along this P-T-t path display an unusual chemistry with low K2O and FeO+MgO content and high CaO content. The combination of field observations, chemical mapping and geochemical analyses leads to the conclusion the major part of the leucosomes (L1) crystallized prior to syn-peak of metamorphism concurrent with melt extraction from the source. This study documents the details of leucosomes formation using field observations in the Southern Marginal Zone and numerical modelling. This work demonstrates that the formation of K2O-poor leucosome in the metasedimentary lower crust is controlled by the difference in volume of equilibration and heterogeneities within the migmatites. The partial melting of the source coupled with melt loss and water diffusivity within the melt transfer site is a potential mechanism to explain the chemical link in the sytem residuum– melt–S-type granite. / AFRIKAANSE OPSOMMING: Anateksis is die eerste stap in granietgenese. Meganismes wat in die onderste kors aan die werk is, is verantwoordelik vir korsdifferensiasie en bepaal die chemiese samestelling van die graniet. Hierdie studie het’n ondersoek behels van die anatektiese geskiedenis van ’n ho egraadse terrein: die suidelike randstreek van die Limpopo-gordel, noord van die Kaapvaal-kraton in Suid-Afrika. Die werk het ’n ge integreerde veld- , metamorfiese, geochemiese en geochronologiese studie van die metasedimentêre granuliete van twee afsonderlike groewe in die noordelike sone van die suidelike randstreek (SRS), die Bandelierkop-groef en die Brakspruit-groef, waar Neoarge iese ho egraadse gedeeltelike smeltkenmerke waargeneem kan word, ingesluit. Die projek was gerig op die ondersoek van twee belangrike kwessies: (1) om die drukâtemperatuurtoestande en die ouderdom van die metamorfiese episode in die SRS akkuraat te beheer, met implikasie vir die geodinamiese prosesse naby die einde van die Arge ikum, en (2) om die reaksies onder gedeeltelik gesmelte toestande wat die vorming van migmatiete beheer, te ondersoek en die chemiese verwantskappe in die stelsel bron - leukosoom - smelt - S-tipe graniet te begryp. Die P-T-t-rekord wat in die Bandelierkop-formasie metapeliete behoue is, ingeperk deur modellering van fase-ekwilibria asook sirkoon LA-ICP-MS-geochronologie, gee insig in korsdifferensiasieprosesse in die onderste kors. Rotse in albei groewe dui op metamorfismeepisodes teen hoë temperature met piektoestande van 840â860 oC en 9â11 kbar teen ongeveer 2.71 Ga met vorming van leukosome (L1) gedurende die progradeerpad. Geringe leukokratiese eienskappe (L2) het tydens dekompressie tot 6â7 kbar ontstaan. Die einde van die metamorfiese voorval word gekenmerk deur die fasiesoorgang van granuliete / amfiboliete (<640 oC) by ongeveer 2.68 Ga. Die maksimum afsettingsouderdom vir die detitrale sirkone in die metapeliete (ongeveer 2.73 Ga) dui op Å snelle begrawingsproses ( 0.17 cm.y1). Daardie bewyse bied sterk ondersteuning daarvoor dat die SRS sedimente bevat wat gedurende konvergensie in Å aktiewe rand afgeset is, en dat die metapeliete gemetamorfoseer en gedeeltelik gesmelt het as gevolg van kontinentbotsing langs die noordelike rand van die Kaapvaal-kraton teen ongeveer 2.7 Ga. Die leukokratiese eienskappe wat langs hierdie P-T-t-pad opgewek word, toon Å ongewone chemiese samestelling met lae K2O en FeO+MgO-inhoud en ho e CaO-inhoud. Die kombinasie van veldwaarnemings, chemiese kartering en geochemiese ontledings lei tot die gevolgtrekking dat die grootste deel van die leukosome (L1) gekristalliseer het voor die syn-piek van metamorfisme tesame met smeltekstraksie van die bron. Hierdie studie het die besonderhede van leukosoomformasie met behulp van veldwaarnemings in die SRS en numeriese modellering opgeteken. Hierdie werk toon aan dat korsdifferensiasie in die metasedimentêre onderste kors deur Å ander volume van ekwilibrasie en heterogeniteite in die migmatiete beheer word. Die gedeeltelike smelting van die bron gepaard met smeltverlies en waterdiffusiwiteit tot in die smeltoordragterrein is ’n potensiele meganisme om die chemiese skakel in die stelsel residuum-smelt-S-tipe graniet te verklaar.

M.Sc. / A fluid inclusion study was performed on the following rock types: ● Metapelitic granulites that occur as xenoliths in the Bulai Pluton. ● Metapelitic granulites that occur around the Bulai Pluton. ● Granite from the Bulai Pluton. These rocks outcrop on the farm Boston near Messina in the so called Three Sisters area. The main aims of this study are the following: ● Which fluids are associated with granulite metamorphism? ● Do the fluid inclusions record more than one metamorphic event, if so, what are the P-T conditions of this/these event(s)? ● How do the fluids compare to fluids in the Southern Marginal Zone in terms of composition and density? The metapelites are typically characterised by a peak metamorphic mineral assemblages of (1) quartz, K-feldspar, plagioclase, garnet, biotite, cordierite and sillimanite and (2) quartz, K-feldspar, plagioclase, garnet, orthopyroxene and biotite. The first assemblage have been used by other workers (Van Reenen et al., in prep.) to derive a P-T path for the metapelitic xenoliths and host rock, which is characterised by decompression-cooling. This assemblage also shows typical high temperature metasomatic formation of feldspar around quartz that is in contact with quartz. The peak metamorphic conditions were estimated to be ~850°C and ~7.5 kbar. Fluid inclusions were studied in garnet, quartz inclusions in garnet, and matrix quartz. The following principle fluid types were identified: ● High salinity aqueous fluids. ● CO2-rich (±CH4) carbonic fluids. ii Petrographic evidence indicates that both fluids were present at peak metamorphic conditions under conditions of fluid-fluid immiscibility. The high-salinity aqueous fluid is most likely responsible for the high temperature metasomatic textures. The density of the fluids trapped at peak metamorphic conditions have been reset to lower values at pressures below that of the peak metamorphic conditions. This is probably the result of the emplacement of the Bulai Pluton at shallow crustal levels (2- 3 kbar). Later fluids are dominated by low-density carbonic fluids with significant amounts of CH4. The CH4 is the result of retrograde hydration reactions at relatively low oxygen fugacities.

Thesis (MSc)--Stellenbosch University, 2011. / ENGLISH ABSTRACT: The ~2.67 Ga Matok pluton comprises calc‐alkaline pyroxene (px)‐bearing and px‐free granitoids. The pluton was constructed by means of two episodes of intrusion each of which had co‐magmatic px‐bearing and px‐free granitoid groups. All the granitoid groups (px‐bearing and px‐free) are characterised by non‐porphyritic and porphyritic varieties. The phenocrysts in both episodes of intrusion are plagioclase ± alkali feldspar and are aligned parallel to the trend of the Limpopo Belt, attesting to a syntectonic emplacement. The time gap between the first and second intrusion is insignificant and magma was most likely stored in the chamber after the first intrusion. Petrography and geochemical signature of both px‐bearing and px‐free granitoid samples have been studied and a petrogenetic model which accounts for the coeval px‐bearing and px‐free granitoids is proposed. The relevance of the syntectonic emplacement of the Matok pluton ie n the Limpopo Belt is also addrssed. Px‐bearing granitoids always have clinopyroxene but orthopyroxene is not always present. Magnetite and ilmenite are present in both px‐bearing and px‐free granitoids but are more abundant in the px‐bearing granitoids and subordinate in the px‐free granitoids. Plagioclase in both px‐bearing and px‐free granitoids is of oligoclase (An12‐30) composition but is relatively more calcic and increases in modal abundance in the px‐bearing granitoids. Alkali feldspar is more dominant in the px‐free granitoids. Hornblende is present in all the px‐bearing granitoids and the px‐free granitoids with ≤71 wt.% SiO2 but is absent in the px‐free granites with >71 wt.% SiO2. Both magmatic epidote and titanite occur exclusively in the px‐free granitoids with ≤71 wt.% SiO2 and are absent in all the px‐bearing granitoids as well as the px‐free gra nites with >71 wt.% SiO2. Px‐bearing granitoids are mainly of dioritic and granodioritic and have subordinate granitic composition while px‐free granitoids are mainly of granitic and granodioritic and have subordinate dioritic composition. All the rocks define well correlated variation of SiO2 with the rest of the major elements. However, there is always a hiatus between the granites with >71 wt.% SiO2 and all other rocks. Px‐bearing and px‐free granitoids at the same SiO2 concentrations tend to have approximately equal concentrations of MgO, CaO and TiO2, whereas K2O concentration is distinctively higher for the px‐free granitoids. The distribution of the high field strength elements (HFSE; Nb, Ta, Zr and Hf) and rare earth elements (REE) is similar in both px‐bearing and px‐free granitoids. On contrary, Th, U, Cs and Rb are characteristically higher in the px‐free granitoids. All granitoids are characterised by negative anomalies of the HFSE (Nb, Ta and Ti) and the LILE (Th, U and Sr) on primitive mantle normalised diagrams. On the one hand, concentrations of compatible elements (Cr, Ni and Mg) in the Matok pluton granitoids are rather low for a mantle source. On the other hand, all the granitoids have superchondtritic Nb/Ta ratios that overlap with those of the Ventersdorp continental flood basalts which extruded in the Kaapvaal Craton at ~2.7 Ga. The continental crust typically has subchondritic Nb/Ta ratio, and superchondtritic Nb/Ta ratios are widely accepted to resemble a mantle source. The implication is that the Matok pluton granitoids had inherited the superchondtritic Nb/Ta ratio from their source; juvenile underplated mafic magmas that had ponded owing to the impact of the Ventersdorp mantle plume. The large volumes of ponded magma s probably induced the high grade metamorphism in the Limpopo Belt. All the granitoids of the Matok pluton are probably products of one partial melting event. One possible way to account for the co‐existence of px‐bearing and px‐free granitoids in the Matok pluton is by means of, at least, two magma chambers; one which was filled with anhydrous magma and the other which was filled with hydrous magma. An alternative model would be that in which there was only one chamber. In the one chamber scenario, the magma was hydrodynamically sorted into zones that differed mostly in fH2O and concentrations of highly fluid‐mobile elements but conserved the uniformity in fluid immobile elements. Regardless of the number of chambers, magma batches intruded in the form of feeder dikes which minimally interacted, thus avoiding the hydration of pyroxene in the px‐bearing granitoids. / SELELEKELA: Plutone ya Matok e fumanehang profinsing ya Limpopo sebakeng seo ho digeologist se tsebahalang ka hore ke Lebanta la Limpopo e ile ya aheya dilemong tse 2.67 biliyone tse fetileng. Plutone ena eile ya aheya ka mekgahlelo e mmeli, mme mokgahlelo ka mong o ne o bopilwe ka majwe a nang le pyroxene le a senang yona. Majwe kaofela ke a mofuta wa calc‐alkaline. Phapang e kgolo dipakeng tsa mefuta ena e mmedi ya majwe ke boteng ba pyroxene le boteng ba epidote le titanite majweng a nang le pyroxene le a senang pyroxene ka ho latellana. Ha e le diminerale tse ding kaofela tsona ha likgethe mofuta wa lejwe; liteng mefuteng ya majwe ka bobedi. Kgonahalo ya hore plutone ya Matok e ahwe ka mefuta ena e mmedi (px‐bearing and px‐free) e tlile ka mekgoa e mmedi kapa o mong wa mekgwa ena yo ka bobedi e ka etsahalang. (1)Tlaase semelong sa lesheleshele moralla (magma) hone ho ena le didiba tse pedi, seseng se tshetse lesheleshele le chesang haholo ebile le le metsi a fokolang (anhydrous magma) ha se seng se ne se tshetse lesheleshele le metsi a mangata (hydrous magma). Ho tloheng moo didibeng tse pedi ho tla moo plutone ea Matok eleng teng kajeno masheleshele ana a ne a tla ka mokgwa wa di‐dike tseo kaofela phello ya tsona e neng e le sebakeng se le seng‐plutone ya Matok. (2) Mokgwa wa bobedi ke haeba ho ne ho ena le sediba se le seng sa lesheleshele moralla, mme ka sedibeng ka moo ho ne ho ena le maqulwana (zones) a neng a fapane ka bongata ba metsi. Ho tloha sedibeng moo masheleshele ana a ne a tloha ka bona boqulwana boo entse ele ka mokhwa wa di‐dike, mme kaofela phello ya di‐dike ene ele plutone ya Matok. Kaofela majwe a plutone ya Matok a na le feldspar eo boholo ba nako e patlameng ho ya nqa bophirimela‐bochabela (W‐E), e leng nqa eo Lebanta la Limpopo le phatlaletseng ka teng. Hona ho tiisa hore plutone ya Matok e aheile nakong yo Lebanta la Limpopo le neng le ntse le aheya le lona. Ke dilemong tse kabang 2.7 biliyone tse fetileng ha dikarolong tse ding tsa Cratone ya Kaapvaal ho ne ho aheya majwe a moralla a Ventersdorp. Majwe ana ke a hlahang tlaase botebong ba lefatshe (mantle), mme a susumeditswe ke plumo. Karolo boholo ya lesheleshele moralla hae ya ka ya nyoloha ho fihla hodimo lefatsheng. Empa mofuthu o mongata ho nyoloha leshelesheleng moo ke ona oileng wa 'pheha' majwe ho phatlalla le Lebanta la Limpopo. Ho nyoloha hona ha plumo ho etsahetse ka nako e lengwe le ho tsukutleha ho hoholo ho potapota le Cratone ya Kalahari, mme bobedi diketsahalo tsena diile tsa tswala Lebanta la Limpopo. Hobane plutone ya Matok e aheile hanghang ka mora hore lesheleshele la moralla le dule tlaase ho lekgapetla la lefatshe (crust), dielemente tse ratang haholo diminerale tsa ditemperetjha tse hodimo diile tsa feela jwalo di nkile lefa hotswa lesheleshele moralleng la Ventersdorp.

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.

D.Phil. (Geology) / Please refer to full text to view abstract