Igneous and metamorphic charnockitic rocks in the Southern Marginal Zone of the Limpopo Belt with special emphasis on the Matok Enderbitic - Granatic Suite.
- Authors: Bohlender, Frank
- Date: 2014-06-04
- Subjects: Charnockite - South Africa - Limpopo Belt. , Metamorphic rocks - South Africa - Limpopo Belt. , Igneous rocks - South Africa - Limpopo Belt. , Limpopo Belt (South Africa)
- Type: Thesis
- Identifier: uj:11406 , http://hdl.handle.net/10210/11044
- Description: D.Phil. (Geology) , Please refer to full text to view abstract
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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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Fluids in metapelitic granulites and Bulai granitoids of the Messina area, central zone of the Limpopo Belt, South Africa
- Authors: Flattery, Yvonne
- Date: 2009-01-28T09:39:06Z
- Subjects: Fluid inclusions , Granulite , Geology , Limpopo Belt (South Africa)
- Type: Thesis
- Identifier: http://ujcontent.uj.ac.za8080/10210/390367 , uj:14835 , http://hdl.handle.net/10210/1959
- Description: 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.
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