Laser ablation ICP-MS age determination of detrital zircon populations in the Phanerozoic Cape and Lower Karoo Supergroups (South Africa) and correlatives in Argentina.
- Authors: Vorster, Clarisa
- Date: 2014-01-14
- Subjects: Karoo Supergroup , Cape Supergroup , Zircon - South Africa , Zircon - Argentinia , Inductively coupled plasma mass spectrometry , Geochronometry
- Type: Thesis
- Identifier: uj:7897 , http://hdl.handle.net/10210/8789
- Description: Ph.D. (Geology) , The successions of the Cape- and Karoo Supergroups preserve an integrated history of sedimentation along the paleo-Pacific margin of Gondwana from the Paleozoic to the Early Mesozoic. The Cape- and Karoo Supergroups have been well studied with regard to stratigraphy, sedimentary facies and depositional environment. However, the nature and location of their source regions, especially for the changeover from deposition within an Atlantic-type continental margin basin for the successions of the Cape Supergroup to an Andean-type continental foreland basin for some of the units of the Karoo Supergroup, remains poorly understood. In order to shed light on the nature of these source regions, a comprehensive U-Pb detrital zircon study of the successions of the Cape- and lower Karoo Supergroups was launched. A representative number of samples from the upper and lower successions of the Table Mountain- Bokkeveld- and Witteberg Groups of the Cape Supergroup as well as the Dwyka and Ecca Groups of the Karoo Supergroup were collected throughout the western, southwestern and southern Cape region. A few samples of the Dwyka Group were also collected within the more eastern outcrop regions of the succession located in Kwazulu-Natal. The sedimentary rocks of the Natal Group and Msikaba Formation have long been regarded as coeval with the Cape Supergroup. Similar to the successions of the Cape- and Karoo Supergroups, very little is known about their sedimentary source regions. Also, their relative age of sedimentation remains poorly constrained. The U-Pb detrital zircon study of the successions of the Cape- and lower Karoo Supergroups was thus extended so as to include the successions of the Natal Group and Msikaba Formation. The detrital zircon age populations of the successions of the Natal Group and Msikaba Formation would not only improve the present understanding with regards to the sedimentary source regions to these units but would also facilitate the evaluation of possible correlations between these units and the stratigraphic units of the Cape Supergroup. Samples of both the lower Durban Formation and the upper Mariannhill Formation of the Natal Group and the Msikaba Formation (which is presently regarded as being part of the Cape Supergroup) were therefore collected within their respective outcrop regions in the Kwazulu-Natal area. The similarities in litho- and bio-stratigraphy between the successions of the Cape- and Karoo Supergroups and those of the Ordovician to Early Permian successions of the Ventania System and the Ordovician to Silurian successions of the Tandilia System in Argentina have long been recognized. Although the detrital zircon populations of some of the formations within these Systems have been evaluated in the past, it is yet to be determined whether these successions and those of the Cape- and lower Karoo Supergroups have certain source regions in common. In order to facilitate such a comparison, samples of selected units of the Ventania System were therefore collected near Sierra de la Ventania, while a sample of the Balcarce Formation of the Tandilia System was obtained near Mar del Plata. The detrital zircon age populations of the successions of the Ventania and Tandilia Systems were also further evaluated in the light of establishing or confirming a time-correlation between these formations and those of the Cape- and lower Karoo Supergroups. U-Pb age determination of the detrital zircons population of the samples was conducted by means of Laser Ablation-Inductively Coupled Plasma-Mass Spectrometry (LA-ICP-MS). Although LA-ICP-MS is a routine, well-established technique where the U-Pb age determination of detrital zircons is concerned, it was yet to be established at the centralized analytical facility of the University of Johannesburg, SPECTRUM, using the instrumentation currently available (i.e. 213nm Nd:YAG laser coupled to Quadrupole-based ICP-MS). The U-Pb age determination of detrital zircons was therefore preceded by a fair amount of instrument optimization and method development. Well studied shortcomings of U-Pb detrital zircon dating by LA-ICP-MS such as laser induced elemental fractionation, mass discrimination effects and as well as the possible occurrence of minor common-Pb needs were addressed and corrected for. The detrital zircon populations of successions in the Cape Supergroup have a distinct major Neoproterozoic to Early Cambrian age component, which can be attributed to an input of detritus from successions related to the Pan-African Orogeny in South Africa, such as the Gariep- and Saldania Belts located towards the north of the Cape Basin. A substantial amount of Mesoproterozoic detrital zircon grains is also present in all the samples from the successions of the Cape Supergroup. These grains of Mesoproterozoic age were probably derived from the Namaqua-Natal Metamorphic Province, which is also regarded as the source of some minor amounts of Paleoproterozoic detrital zircon grains. The near absence of Archean grains from the detrital zircon populations of the successions of the Cape Supergroup is notable, and is thought to be due to the Namaqua-Natal Metamorphic Province acting as a geomorphological barrier at the time of their deposition. The minor Paleozoic (Ordovician to Carboniferous) detrital zircon populations in the samples from the formations of the Cape Supergroup increase progressively upwards through the succession. ....
- Full Text:
- Authors: Vorster, Clarisa
- Date: 2014-01-14
- Subjects: Karoo Supergroup , Cape Supergroup , Zircon - South Africa , Zircon - Argentinia , Inductively coupled plasma mass spectrometry , Geochronometry
- Type: Thesis
- Identifier: uj:7897 , http://hdl.handle.net/10210/8789
- Description: Ph.D. (Geology) , The successions of the Cape- and Karoo Supergroups preserve an integrated history of sedimentation along the paleo-Pacific margin of Gondwana from the Paleozoic to the Early Mesozoic. The Cape- and Karoo Supergroups have been well studied with regard to stratigraphy, sedimentary facies and depositional environment. However, the nature and location of their source regions, especially for the changeover from deposition within an Atlantic-type continental margin basin for the successions of the Cape Supergroup to an Andean-type continental foreland basin for some of the units of the Karoo Supergroup, remains poorly understood. In order to shed light on the nature of these source regions, a comprehensive U-Pb detrital zircon study of the successions of the Cape- and lower Karoo Supergroups was launched. A representative number of samples from the upper and lower successions of the Table Mountain- Bokkeveld- and Witteberg Groups of the Cape Supergroup as well as the Dwyka and Ecca Groups of the Karoo Supergroup were collected throughout the western, southwestern and southern Cape region. A few samples of the Dwyka Group were also collected within the more eastern outcrop regions of the succession located in Kwazulu-Natal. The sedimentary rocks of the Natal Group and Msikaba Formation have long been regarded as coeval with the Cape Supergroup. Similar to the successions of the Cape- and Karoo Supergroups, very little is known about their sedimentary source regions. Also, their relative age of sedimentation remains poorly constrained. The U-Pb detrital zircon study of the successions of the Cape- and lower Karoo Supergroups was thus extended so as to include the successions of the Natal Group and Msikaba Formation. The detrital zircon age populations of the successions of the Natal Group and Msikaba Formation would not only improve the present understanding with regards to the sedimentary source regions to these units but would also facilitate the evaluation of possible correlations between these units and the stratigraphic units of the Cape Supergroup. Samples of both the lower Durban Formation and the upper Mariannhill Formation of the Natal Group and the Msikaba Formation (which is presently regarded as being part of the Cape Supergroup) were therefore collected within their respective outcrop regions in the Kwazulu-Natal area. The similarities in litho- and bio-stratigraphy between the successions of the Cape- and Karoo Supergroups and those of the Ordovician to Early Permian successions of the Ventania System and the Ordovician to Silurian successions of the Tandilia System in Argentina have long been recognized. Although the detrital zircon populations of some of the formations within these Systems have been evaluated in the past, it is yet to be determined whether these successions and those of the Cape- and lower Karoo Supergroups have certain source regions in common. In order to facilitate such a comparison, samples of selected units of the Ventania System were therefore collected near Sierra de la Ventania, while a sample of the Balcarce Formation of the Tandilia System was obtained near Mar del Plata. The detrital zircon age populations of the successions of the Ventania and Tandilia Systems were also further evaluated in the light of establishing or confirming a time-correlation between these formations and those of the Cape- and lower Karoo Supergroups. U-Pb age determination of the detrital zircons population of the samples was conducted by means of Laser Ablation-Inductively Coupled Plasma-Mass Spectrometry (LA-ICP-MS). Although LA-ICP-MS is a routine, well-established technique where the U-Pb age determination of detrital zircons is concerned, it was yet to be established at the centralized analytical facility of the University of Johannesburg, SPECTRUM, using the instrumentation currently available (i.e. 213nm Nd:YAG laser coupled to Quadrupole-based ICP-MS). The U-Pb age determination of detrital zircons was therefore preceded by a fair amount of instrument optimization and method development. Well studied shortcomings of U-Pb detrital zircon dating by LA-ICP-MS such as laser induced elemental fractionation, mass discrimination effects and as well as the possible occurrence of minor common-Pb needs were addressed and corrected for. The detrital zircon populations of successions in the Cape Supergroup have a distinct major Neoproterozoic to Early Cambrian age component, which can be attributed to an input of detritus from successions related to the Pan-African Orogeny in South Africa, such as the Gariep- and Saldania Belts located towards the north of the Cape Basin. A substantial amount of Mesoproterozoic detrital zircon grains is also present in all the samples from the successions of the Cape Supergroup. These grains of Mesoproterozoic age were probably derived from the Namaqua-Natal Metamorphic Province, which is also regarded as the source of some minor amounts of Paleoproterozoic detrital zircon grains. The near absence of Archean grains from the detrital zircon populations of the successions of the Cape Supergroup is notable, and is thought to be due to the Namaqua-Natal Metamorphic Province acting as a geomorphological barrier at the time of their deposition. The minor Paleozoic (Ordovician to Carboniferous) detrital zircon populations in the samples from the formations of the Cape Supergroup increase progressively upwards through the succession. ....
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Distribution and geochronology of unconformity-bound sequences in paleoproterozoic Elim-Olifantshoek red beds: implications for timing of formation of Sishen-type iron ore and heavy carbonate carbon isotope excursion
- Authors: Da Silva, Richard
- Date: 2012-08-16
- Subjects: Iron ores , Geochronometry , Paleontology, Proterozoic , Carbon isotopes , Geological time , Griqualand West (South Africa)
- Type: Thesis
- Identifier: uj:9517 , http://hdl.handle.net/10210/5946
- Description: M.Sc. , Bracketing the depositional age of the Gamagara/Mapedi to Lucknow and Olifantshoek succession in Griqualand West is important because it not only represents one of the oldest known red bed successions in the world but also hosts some of the first well preserved lateritic soil profiles and carbonates with heavy 13C values traditionally correlated with the so-called Lomagundi carbonate carbon excursion. In addition the ancient supergene very large high-grade hematite iron ore deposits of the Sishen-Postmasburg area on the Maremane dome are associated with the erosional unconformity at the base of the Gamagara Formation (a lateral equivalent of the Mapedi Formation). However, the depositional age of especially the Gamagara/Mapedi to Lucknow succession is under dispute because it has been considered a) correlative to the lower part of the Waterberg Group in the Transvaal area, with the implication that it is younger than the Bushveld Complex with an age of ~2,054 Ga, and b) correlative to the Dwaalheuwel-Magaliesberg succession of the pre-Bushveld Pretoria Group of the Transvaal Supergroup in the Transvaal area. The upper age limit of the Gamagara/Mapedi to Lucknow succession is defined by 1,92 Ga felsic volcanics in the overlying Neylan-Hartley succession of the Olifantshoek Group. The Hartley Lava Formation is overlain by Volop quartzites. This study involves age determinations of detrital zircon populations extracted from the basal Doornfontein conglomerate member of the Gamagara/Mapedi succession, and quartzites of the Gamagara/Mapedi, Lucknow, Neylan, Hartley and Volop Formations at various localities in Griqualand West. Based on field work, three unconformity-bound sequences are defined, namely the Gamagara/Mapedi-Lucknow, Neylan-Hartley and Volop sequences. Most interestingly quartzites of the Gamagara/Mapedi-Lucknow sequence contain abundant zircons with ages similar to that of the Bushveld Complex at ~2,054-2,06 Ga in addition to zircons as young as ~1,98-2,01 Ga. An exception is results on one sample of the Doornfontein Member analyzed so far (it is from the Rooinekke iron ore mine south of Postmasburg) that contains only zircons that are older than the Bushveld Complex with a rather prominent youngest population bracketed between 2,2 Ga and 2,32 Ga. The youngest detrital zircon populations in the Neylan-Hartley sequence are either slightly older than the Hartley lava or contain zircons with similar age to Hartley felsic lavas at 1,92 Ga. This sequence thus appears to have developed immediately prior to and coeval with Hartley volcanism. The overlying Volop sequence contains abundant zircons as young as ~1,89 Ga. The results clearly illustrate that the Gamagara/Mapedi to Lucknow succession is certainly not a lateral correlative of the pre-Bushveld Dwaalheuwel-Magaliesberg succession of the Pretoria Group. Rather it should be considered time-equivalent lower parts of the Waterberg Group in the Transvaal area. This implies that the heavy carbonate carbon excursion known from the Lucknow Formation is at least 100 my. younger than the one known from the upper part of the Silverton Formation along the contact with the overlying Magaliesberg Quartzite. There are thus at least three heavy carbonate carbon excursions, known from Paleoproterozoic cover successions of the Kaapvaal Craton in southern Africa, namely one in the ~2.35 Ga Duitschland Formation, a second in the ~2,1 Ga Silverton Formation of the Pretoria Group of the Transvaal Supergroup and the third in the ~1,98-1,92 Ga Lucknow Formation. It is further known that carbonates with normal open marine 13C values of close to zero occur in stratigraphic intervals between each of the heavy carbonate carbon excursions.
- Full Text:
- Authors: Da Silva, Richard
- Date: 2012-08-16
- Subjects: Iron ores , Geochronometry , Paleontology, Proterozoic , Carbon isotopes , Geological time , Griqualand West (South Africa)
- Type: Thesis
- Identifier: uj:9517 , http://hdl.handle.net/10210/5946
- Description: M.Sc. , Bracketing the depositional age of the Gamagara/Mapedi to Lucknow and Olifantshoek succession in Griqualand West is important because it not only represents one of the oldest known red bed successions in the world but also hosts some of the first well preserved lateritic soil profiles and carbonates with heavy 13C values traditionally correlated with the so-called Lomagundi carbonate carbon excursion. In addition the ancient supergene very large high-grade hematite iron ore deposits of the Sishen-Postmasburg area on the Maremane dome are associated with the erosional unconformity at the base of the Gamagara Formation (a lateral equivalent of the Mapedi Formation). However, the depositional age of especially the Gamagara/Mapedi to Lucknow succession is under dispute because it has been considered a) correlative to the lower part of the Waterberg Group in the Transvaal area, with the implication that it is younger than the Bushveld Complex with an age of ~2,054 Ga, and b) correlative to the Dwaalheuwel-Magaliesberg succession of the pre-Bushveld Pretoria Group of the Transvaal Supergroup in the Transvaal area. The upper age limit of the Gamagara/Mapedi to Lucknow succession is defined by 1,92 Ga felsic volcanics in the overlying Neylan-Hartley succession of the Olifantshoek Group. The Hartley Lava Formation is overlain by Volop quartzites. This study involves age determinations of detrital zircon populations extracted from the basal Doornfontein conglomerate member of the Gamagara/Mapedi succession, and quartzites of the Gamagara/Mapedi, Lucknow, Neylan, Hartley and Volop Formations at various localities in Griqualand West. Based on field work, three unconformity-bound sequences are defined, namely the Gamagara/Mapedi-Lucknow, Neylan-Hartley and Volop sequences. Most interestingly quartzites of the Gamagara/Mapedi-Lucknow sequence contain abundant zircons with ages similar to that of the Bushveld Complex at ~2,054-2,06 Ga in addition to zircons as young as ~1,98-2,01 Ga. An exception is results on one sample of the Doornfontein Member analyzed so far (it is from the Rooinekke iron ore mine south of Postmasburg) that contains only zircons that are older than the Bushveld Complex with a rather prominent youngest population bracketed between 2,2 Ga and 2,32 Ga. The youngest detrital zircon populations in the Neylan-Hartley sequence are either slightly older than the Hartley lava or contain zircons with similar age to Hartley felsic lavas at 1,92 Ga. This sequence thus appears to have developed immediately prior to and coeval with Hartley volcanism. The overlying Volop sequence contains abundant zircons as young as ~1,89 Ga. The results clearly illustrate that the Gamagara/Mapedi to Lucknow succession is certainly not a lateral correlative of the pre-Bushveld Dwaalheuwel-Magaliesberg succession of the Pretoria Group. Rather it should be considered time-equivalent lower parts of the Waterberg Group in the Transvaal area. This implies that the heavy carbonate carbon excursion known from the Lucknow Formation is at least 100 my. younger than the one known from the upper part of the Silverton Formation along the contact with the overlying Magaliesberg Quartzite. There are thus at least three heavy carbonate carbon excursions, known from Paleoproterozoic cover successions of the Kaapvaal Craton in southern Africa, namely one in the ~2.35 Ga Duitschland Formation, a second in the ~2,1 Ga Silverton Formation of the Pretoria Group of the Transvaal Supergroup and the third in the ~1,98-1,92 Ga Lucknow Formation. It is further known that carbonates with normal open marine 13C values of close to zero occur in stratigraphic intervals between each of the heavy carbonate carbon excursions.
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