Granitic and rhyolitic magmatism: constraints on continental reconstruction from geochemistry, geochronology and palaeomagnetism
- Authors: Carter, Lisa
- Date: 2009-01-27T07:17:25Z
- Subjects: Continental drift , Paleomagnetism , Geochemistry , Geological time , Rajasthan (India) , Seychelles , Madagascar
- Type: Thesis
- Identifier: uj:14822 , http://hdl.handle.net/10210/1947
- Description: M.Sc. , Please refer to full text to view abstract
- Full Text:
- Authors: Carter, Lisa
- Date: 2009-01-27T07:17:25Z
- Subjects: Continental drift , Paleomagnetism , Geochemistry , Geological time , Rajasthan (India) , Seychelles , Madagascar
- Type: Thesis
- Identifier: uj:14822 , http://hdl.handle.net/10210/1947
- Description: M.Sc. , Please refer to full text to view abstract
- Full Text:
Paleomagnetism of selected neoarchean-paleoproterozoic cover sequences on the Kaapvaal Craton and implications for Vaalbara
- Authors: De Kock, Michiel Olivier
- Date: 2008-08-25T06:27:36Z
- Subjects: Paleomagnetism , Stratigraphic geology , Cratons , Paleogeography
- Type: Thesis
- Identifier: uj:3676 , http://hdl.handle.net/10210/905
- Description: The Kaapvaal craton of southern Africa and the Pilbara craton of Western Australia, two of the best-preserved Archean cratons in the world, are covered by remarkably similar early Precambrian cover sequences. This has led to the proposal of the so-called Vaalbara hypothesis, which promotes the existence of the two cratons as a single crustal entity, and possibly, Earth’s oldest assembled continent in Neoarchean-early Paleoproterozoic times. Previous studies have failed to prove the existence of Vaalbara conclusively, principally due to a lack of reliable ages or because of uncertainty and gaps in the paleomagnetic record from the Kaapvaal craton. During the present study paleomagnetic samples were collected from selected Neoarchean- Paleoproterozoic cover sequences of the Kaapvaal craton for the establishment of well-defined paleomagnetic poles. In addition, the Hartswater Group of the Ventersdorp Supergroup was sampled for zircon SHRIMP analyses in order to constrain the ages of poles defined from that succession. The paleopoles established here, together with existing paleopoles from the Kaapvaal craton, are used to evaluate the apparent polar wander path of the craton during the Neoarchean-Paleoproterozoic and are compared with poles of similar age from the Pilbara craton as a test of the Vaalbara hypothesis. Regarding the age of the Hartswater Group, zircon SHRIMP ages of 2735 ± 3 Ma and 2724 ± 6 Ma cast doubt on younger ages from the Klipriviersberg Formation, which comprise the base of the Ventersdorp Supergroup. Traditional (younger) age constraints from the Ventersdorp Supergroup do not support the original Vaalbara correlation. A new correlation is suggested here, taking the new ages into account, showing that the Ventersdorp Supergroup overlaps in time with the Fortescue Group of the Pilbara craton. Most importantly, the new ages also provide constraints on the magnetization within the Platberg Group and the Allanridge Formation. Six new paleopoles, of various quality, are added to the existing database from that craton. These poles from the ~2.73 Ga Platberg Group and ~2.7Ga Allanridge Formation of the Ventersdorp Supergroup, the ~2.5Ga lower Transvaal Supergroup, the lower two unconformitybounded sequences of the Waterberg Group (2.05 Ga and ~1.99 Ga) and the upper Soutpansberg Group (~1.76 Ga) have, together with existing poles from the Kaapvaal craton, led to the definition of an APWP for that craton for a period ~2.78 to ~1.76 Ga. Particularly the poles from the Waterberg and Soutpansberg Groups provided the information to identify complexities (looping) in the APWP that have gone unrecognized in the past. The paleomagnetic data gathered and the newly defined APWP could be used in conjunction with geological evidence from the Kaapvaal and Pilbara cratons to evaluate, and validate, the Vaalbara hypothesis. A good match between the APWP’s of the two cratons for the period ~2.78 to ~2.70 Ga and the geological features (lithology and structure) of the two cratons provide the best evidence that Vaalbara existed as a cratonic unit in the late Archean. Paleomagnetic data constrain the position of the Pilbara craton in immediate proximity to the northwest of the Kaapvaal craton (in a Kaapvaal reference frame). The position of the Zimbabwe craton relative to the Pilbara and Kaapvaal cratons is still unresolved, but indications are that it was most likely in a proximal position to the Kaapvaal craton at 2.7 Ga in a configuration not much different from its present day configuration. This would imply that Vaalbara was most probably the Earth’s oldest assembled continent as proposed by earlier workers. The new paleomagnetic data further suggest that Vaalbara did not exist anymore at ~2.0Ga. When evaluated in conjunction with geological evidence a strong argument can be made for the existence of the Vaalbaran continent up until ~2.22 Ga and that the Pilbara and Kaapvaal cratons became separate entities from about ~2.05 Ga. , Prof. NJ Beukes Prof. DAD Evans
- Full Text:
- Authors: De Kock, Michiel Olivier
- Date: 2008-08-25T06:27:36Z
- Subjects: Paleomagnetism , Stratigraphic geology , Cratons , Paleogeography
- Type: Thesis
- Identifier: uj:3676 , http://hdl.handle.net/10210/905
- Description: The Kaapvaal craton of southern Africa and the Pilbara craton of Western Australia, two of the best-preserved Archean cratons in the world, are covered by remarkably similar early Precambrian cover sequences. This has led to the proposal of the so-called Vaalbara hypothesis, which promotes the existence of the two cratons as a single crustal entity, and possibly, Earth’s oldest assembled continent in Neoarchean-early Paleoproterozoic times. Previous studies have failed to prove the existence of Vaalbara conclusively, principally due to a lack of reliable ages or because of uncertainty and gaps in the paleomagnetic record from the Kaapvaal craton. During the present study paleomagnetic samples were collected from selected Neoarchean- Paleoproterozoic cover sequences of the Kaapvaal craton for the establishment of well-defined paleomagnetic poles. In addition, the Hartswater Group of the Ventersdorp Supergroup was sampled for zircon SHRIMP analyses in order to constrain the ages of poles defined from that succession. The paleopoles established here, together with existing paleopoles from the Kaapvaal craton, are used to evaluate the apparent polar wander path of the craton during the Neoarchean-Paleoproterozoic and are compared with poles of similar age from the Pilbara craton as a test of the Vaalbara hypothesis. Regarding the age of the Hartswater Group, zircon SHRIMP ages of 2735 ± 3 Ma and 2724 ± 6 Ma cast doubt on younger ages from the Klipriviersberg Formation, which comprise the base of the Ventersdorp Supergroup. Traditional (younger) age constraints from the Ventersdorp Supergroup do not support the original Vaalbara correlation. A new correlation is suggested here, taking the new ages into account, showing that the Ventersdorp Supergroup overlaps in time with the Fortescue Group of the Pilbara craton. Most importantly, the new ages also provide constraints on the magnetization within the Platberg Group and the Allanridge Formation. Six new paleopoles, of various quality, are added to the existing database from that craton. These poles from the ~2.73 Ga Platberg Group and ~2.7Ga Allanridge Formation of the Ventersdorp Supergroup, the ~2.5Ga lower Transvaal Supergroup, the lower two unconformitybounded sequences of the Waterberg Group (2.05 Ga and ~1.99 Ga) and the upper Soutpansberg Group (~1.76 Ga) have, together with existing poles from the Kaapvaal craton, led to the definition of an APWP for that craton for a period ~2.78 to ~1.76 Ga. Particularly the poles from the Waterberg and Soutpansberg Groups provided the information to identify complexities (looping) in the APWP that have gone unrecognized in the past. The paleomagnetic data gathered and the newly defined APWP could be used in conjunction with geological evidence from the Kaapvaal and Pilbara cratons to evaluate, and validate, the Vaalbara hypothesis. A good match between the APWP’s of the two cratons for the period ~2.78 to ~2.70 Ga and the geological features (lithology and structure) of the two cratons provide the best evidence that Vaalbara existed as a cratonic unit in the late Archean. Paleomagnetic data constrain the position of the Pilbara craton in immediate proximity to the northwest of the Kaapvaal craton (in a Kaapvaal reference frame). The position of the Zimbabwe craton relative to the Pilbara and Kaapvaal cratons is still unresolved, but indications are that it was most likely in a proximal position to the Kaapvaal craton at 2.7 Ga in a configuration not much different from its present day configuration. This would imply that Vaalbara was most probably the Earth’s oldest assembled continent as proposed by earlier workers. The new paleomagnetic data further suggest that Vaalbara did not exist anymore at ~2.0Ga. When evaluated in conjunction with geological evidence a strong argument can be made for the existence of the Vaalbaran continent up until ~2.22 Ga and that the Pilbara and Kaapvaal cratons became separate entities from about ~2.05 Ga. , Prof. NJ Beukes Prof. DAD Evans
- Full Text:
Selected magnetostratigraphic studies in the main Karoo Basin (South Africa): implications for mass extinction events and the supercontinent of Pangea
- Authors: De Kock, Michiel Olivier
- Date: 2009-01-27T07:18:31Z
- Subjects: Stratigraphic geology , Paleomagnetism , Paleoclimatology , Pangaea (Geology) , Karoo Basin (South Africa)
- Type: Thesis
- Identifier: uj:14829 , http://hdl.handle.net/10210/1953
- Description: M.Sc. , The Late Carboniferous to early Jurassic Karoo Supergroup of South Africa witnessed two of the “big five” Phanerozoic mass extinction events, and the formation and subsequent break-up of the supercontinent Pangea. The closure of the Permian Period witnessed the greatest biotic crisis in the history of life. What is known about the Permian-Triassic boundary (hereafter referred to as the PTB) comes almost exclusively from marine successions in Europe and Asia. Although a major extinction event has been recognized in terrestrial successions, surprisingly little is known about its effects and timing. The exact placement of the PTB in the Karoo basin is not well constrained due to shortcomings of stratigraphic methods employed to date. This has made it extremely difficult to correlate the mass extinction events in the marine and non-marine environments; however, paleomagnetic studies could provide answers to both problems of absolute placement and correlation of the PTB in non-marine and marine successions. The PTB appears to lie within an interval of reversed polarity in many marine successions. A detailed magnetostratigraphic survey across the presumed PTB in the Karoo succession at localities in the north and south of the main Karoo Bain reveal two magnetic chrons, reversed followed by normal (with the boundary close to the reversal), which extends to slightly younger results from a previous study that identified an N/R pattern, thereby identifying a R/N/R pattern. The normal chron might correlate with the long basal Triassic normal polarity interval and the reversed polarity zones above and below it known from marine successions in the Alps, Russia, Pakistan and China. The PTB is thought to be situated coincident with the LAD of Dicynodon and the event bed of Ward et al. (2000), apparently above but not necessarily diachronous with a lithology change from predominantly green- to predominantly red mudstone. This placement falls within a normal polarity interval, but could conceivably have taken place at a time of reverse polarity due to delayed acquisition of magnetic remanence. The idea of an extraterrestrial impact as the cause of the end-Permian mass extinctions is strongly enhanced by a synchronous relationship between them. The configuration of the supercontinent Pangea during this time of earth history has been the matter of debate for the last three decades, with numerous alternative reconstructions to the classic Pangea A1 having been proposed for the time preceding the Jurassic. Paleomagnetic data from the Karoo allow for the definition of a new paleopole for West Gondwanaland, which prove a valuable tool for evaluating these various reconstructions. It is neither consistent with a Pangea B-type not C reconstruction for Pangea during this time interval, because of geological ambiguities. The most likely solution to the problem is that of a persistent non-dipole field contribution to the geomagnetic field during this time. Approximately 50 million years later Pangea was unambiguously in a classic Pangea A1 configuration, and life on earth suffered yet another set back. The end-Triassic mass extinction, which marks the sequence boundary between the Triassic and the Jurassic, has not received as much attention as the other four big Phanerozoic biotic disasters. In the Karoo a pronounced turnover in faunal assemblages from typical Triassic fauna to Jurassic Fauna (dinosaurs) is seen in the Elliot Formation. Magnetostratigraphic study of localities in the north and south of the Karoo Basin provided a magnetic zonation pattern for the Elliot Formation, a tool that has led to the constraining of the sequence boundary to the transition from the lower Elliot Formation to the middle Elliot and added to the hypothesis that the faunal turnover is globally synchronous. The determination of a paleolatitude for the Elliot Formation in combination with characteristically arid lithologies (eolian sandstones) provided the base for the evaluation of the paleoclimate that characterized Pangea during the Late Triassic to Early Jurassic. Key words: Karoo Basin, Magnetostratigraphy, Mass Extinction, Paleoclimate, Paleogeography, Paleomagnetism, Pangea, Permian-Triassic, Triassic-Jurassic
- Full Text:
- Authors: De Kock, Michiel Olivier
- Date: 2009-01-27T07:18:31Z
- Subjects: Stratigraphic geology , Paleomagnetism , Paleoclimatology , Pangaea (Geology) , Karoo Basin (South Africa)
- Type: Thesis
- Identifier: uj:14829 , http://hdl.handle.net/10210/1953
- Description: M.Sc. , The Late Carboniferous to early Jurassic Karoo Supergroup of South Africa witnessed two of the “big five” Phanerozoic mass extinction events, and the formation and subsequent break-up of the supercontinent Pangea. The closure of the Permian Period witnessed the greatest biotic crisis in the history of life. What is known about the Permian-Triassic boundary (hereafter referred to as the PTB) comes almost exclusively from marine successions in Europe and Asia. Although a major extinction event has been recognized in terrestrial successions, surprisingly little is known about its effects and timing. The exact placement of the PTB in the Karoo basin is not well constrained due to shortcomings of stratigraphic methods employed to date. This has made it extremely difficult to correlate the mass extinction events in the marine and non-marine environments; however, paleomagnetic studies could provide answers to both problems of absolute placement and correlation of the PTB in non-marine and marine successions. The PTB appears to lie within an interval of reversed polarity in many marine successions. A detailed magnetostratigraphic survey across the presumed PTB in the Karoo succession at localities in the north and south of the main Karoo Bain reveal two magnetic chrons, reversed followed by normal (with the boundary close to the reversal), which extends to slightly younger results from a previous study that identified an N/R pattern, thereby identifying a R/N/R pattern. The normal chron might correlate with the long basal Triassic normal polarity interval and the reversed polarity zones above and below it known from marine successions in the Alps, Russia, Pakistan and China. The PTB is thought to be situated coincident with the LAD of Dicynodon and the event bed of Ward et al. (2000), apparently above but not necessarily diachronous with a lithology change from predominantly green- to predominantly red mudstone. This placement falls within a normal polarity interval, but could conceivably have taken place at a time of reverse polarity due to delayed acquisition of magnetic remanence. The idea of an extraterrestrial impact as the cause of the end-Permian mass extinctions is strongly enhanced by a synchronous relationship between them. The configuration of the supercontinent Pangea during this time of earth history has been the matter of debate for the last three decades, with numerous alternative reconstructions to the classic Pangea A1 having been proposed for the time preceding the Jurassic. Paleomagnetic data from the Karoo allow for the definition of a new paleopole for West Gondwanaland, which prove a valuable tool for evaluating these various reconstructions. It is neither consistent with a Pangea B-type not C reconstruction for Pangea during this time interval, because of geological ambiguities. The most likely solution to the problem is that of a persistent non-dipole field contribution to the geomagnetic field during this time. Approximately 50 million years later Pangea was unambiguously in a classic Pangea A1 configuration, and life on earth suffered yet another set back. The end-Triassic mass extinction, which marks the sequence boundary between the Triassic and the Jurassic, has not received as much attention as the other four big Phanerozoic biotic disasters. In the Karoo a pronounced turnover in faunal assemblages from typical Triassic fauna to Jurassic Fauna (dinosaurs) is seen in the Elliot Formation. Magnetostratigraphic study of localities in the north and south of the Karoo Basin provided a magnetic zonation pattern for the Elliot Formation, a tool that has led to the constraining of the sequence boundary to the transition from the lower Elliot Formation to the middle Elliot and added to the hypothesis that the faunal turnover is globally synchronous. The determination of a paleolatitude for the Elliot Formation in combination with characteristically arid lithologies (eolian sandstones) provided the base for the evaluation of the paleoclimate that characterized Pangea during the Late Triassic to Early Jurassic. Key words: Karoo Basin, Magnetostratigraphy, Mass Extinction, Paleoclimate, Paleogeography, Paleomagnetism, Pangea, Permian-Triassic, Triassic-Jurassic
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The Pongola Supergroup in Swaziland
- Authors: Nhleko, Noah
- Date: 2009-01-28T09:42:41Z
- Subjects: Geology , Stratigraphic geology , Geochemistry , Paleomagnetism
- Type: Thesis
- Identifier: uj:14843 , http://hdl.handle.net/10210/1966
- Description: D.Phil. , The Mesoarchean Pongola Supergroup, cropping out in the southeastern region of the Kaapvaal craton, is one of the oldest known supracrustal successions in the world. It represents an erosional remnant of a once extensive cratonic cover sequence. The succession is subdivided into the lower Nsuze Group, a volcano sedimentary succession, and the Mozaan Group composed mainly of siliciclastics and minor volcanics. The Mozaan Group is host to the world’s oldest known glacial deposit. The Pongola Supergroup is well preserved both geochemically and structurally. Outcrops extend from the northern part of KwaZulu-Natal and Mpumalanga Province in South Africa into Swaziland. This study presents results of an integrated stratigraphic, sedimentological, geochemical, geochronological, and paleomagnetic investigation of the Pongola Supergroup in Swaziland and drill cores from the Nongoma Graben in KwaZulu-Natal. The Nsuze Group displays marked cyclicity between volcanic and siliciclastic rocks that were probably deposited in an intracratonic ‘sag and dome’ basin. The volcanicsedimentary cycles are thought to represent periodic heat loss from a hot regional mantle beneath the Kaapvaal craton. In the sag basins the rate of subsidence was gradual and sedimentation marked by near shore deposition. Volcanism in the Agatha Formation was episodic and displays a cyclicity of 2-14Myr in duration. Magmatic eruption was marked by the development of a low crustal level magma chamber. Crustal contamination trough assimilation and fractional crystallization at these low crustal level magma storages is recorded by compositional bimodal volcanism of basaltic andesite-rhyodacite and andesite-rhyolite association. After cessation of the volcanism of the Nsuze Group, subsequent development of the Pongola basin was marked by thermal subsidence and marine transgression in an epicratonic basin, at the time of deposition of the Mozaan Group. The Mozaan Group overlies the Nsuze Group with an erosive unconformity developed over an in situ weathering profile, i.e. a paleosol. The Mozaan Group consists of alternating quartzite, shale, conglomerate, iron-formation. Three units of contemporaneous flood basalt, namely the Tobolsk, Gabela and Ntanyana formations are interbedded with the siliciclastic deposits in the upper part of the succession. Unimodal paleocurrent directions based on fluvial quartzites indicate initial provenances to the south and north. This indicates that the basin morphology was in a form of a trough. Eventually paleocurrent distribution patterns turn southeast, and marine flooding extended further towards the west, to form the Greater Witwatersrand Basin, which was then modified by development of a foreland basin towards the west and northwest. This suggests that the present outcrops of the Mozaan Group represent mere remnants of an extensive basin. Siliciclastic deposits of the Mozaan succession harbours a wealth of information on the crust-forming events that affected the Kaapvaal craton. Detrital zircons from quartzites and diamictite samples yield ages that record magmatic events that extend from the early (ca. 3.6Ga) to middle (2.89Ga) Archean. Late magmatic events were coeval with the development of the Mozaan basin. The Mozaan succession also is host to the oldest known glacial deposits, namely the Klipwal and the Mpatheni Member diamictites. The absence of deformation, erosional contacts and the presence of incorporated underlying material suggest that grounded glaciers were not the mode of deposition of these diamictites, but that the glacial deposits can be explained as gravity flows from collapse of oversteepened slopes of low relief glaciomarine setting that degenerated in more distal part of the basin into clast poor turbidity flows. Analysis of shale and matrix composition in diamictites show that mechanical erosion processes dominated the source area. CIA values range between 70 and 81, suggesting negligible chemical weathering in the sediment source area. CIA values increase only in the upper part of the stratigraphy, i.e. Ntanyana Formation. Incompatible to compatible trace element ratios are low and suggest that mafic and ultramafic rock dominated the source area. This, perhaps, demonstrates that the greenstone belts were a major source of detritus to the Mozaan basin. Well-constrained paleomagnetic data sets acquired from the Klipwal Member diamictite and Tobolsk lava give a very good estimate of the paleogeographic setting of the Kaapvaal craton during the deposition of the Mozaan Group. The Klipwal diamictite was apparently deposited at high-paleolatitude setting of 48°. The craton then moved slightly to the north to latitude ~43° at the time of eruption of the Tobolsk lavas. From the results it appears that available geochemical classification schemes based on the composition of Phanerozoic volcanic rocks are not suitable to classify the lavas of the Agatha Formation unequivocally. To arrive at any tectonic model for these igneous rocks it is necessary to consider stratigraphic relationships, physical volcanology and geochemical characteristics of the rock succession. On the other hand, the wellconstrained paleomagnetic data indicate that the global climate system in the Mesoarchean was similar to modern day earth where glacial deposits are constrained largely to Polar Regions.
- Full Text:
- Authors: Nhleko, Noah
- Date: 2009-01-28T09:42:41Z
- Subjects: Geology , Stratigraphic geology , Geochemistry , Paleomagnetism
- Type: Thesis
- Identifier: uj:14843 , http://hdl.handle.net/10210/1966
- Description: D.Phil. , The Mesoarchean Pongola Supergroup, cropping out in the southeastern region of the Kaapvaal craton, is one of the oldest known supracrustal successions in the world. It represents an erosional remnant of a once extensive cratonic cover sequence. The succession is subdivided into the lower Nsuze Group, a volcano sedimentary succession, and the Mozaan Group composed mainly of siliciclastics and minor volcanics. The Mozaan Group is host to the world’s oldest known glacial deposit. The Pongola Supergroup is well preserved both geochemically and structurally. Outcrops extend from the northern part of KwaZulu-Natal and Mpumalanga Province in South Africa into Swaziland. This study presents results of an integrated stratigraphic, sedimentological, geochemical, geochronological, and paleomagnetic investigation of the Pongola Supergroup in Swaziland and drill cores from the Nongoma Graben in KwaZulu-Natal. The Nsuze Group displays marked cyclicity between volcanic and siliciclastic rocks that were probably deposited in an intracratonic ‘sag and dome’ basin. The volcanicsedimentary cycles are thought to represent periodic heat loss from a hot regional mantle beneath the Kaapvaal craton. In the sag basins the rate of subsidence was gradual and sedimentation marked by near shore deposition. Volcanism in the Agatha Formation was episodic and displays a cyclicity of 2-14Myr in duration. Magmatic eruption was marked by the development of a low crustal level magma chamber. Crustal contamination trough assimilation and fractional crystallization at these low crustal level magma storages is recorded by compositional bimodal volcanism of basaltic andesite-rhyodacite and andesite-rhyolite association. After cessation of the volcanism of the Nsuze Group, subsequent development of the Pongola basin was marked by thermal subsidence and marine transgression in an epicratonic basin, at the time of deposition of the Mozaan Group. The Mozaan Group overlies the Nsuze Group with an erosive unconformity developed over an in situ weathering profile, i.e. a paleosol. The Mozaan Group consists of alternating quartzite, shale, conglomerate, iron-formation. Three units of contemporaneous flood basalt, namely the Tobolsk, Gabela and Ntanyana formations are interbedded with the siliciclastic deposits in the upper part of the succession. Unimodal paleocurrent directions based on fluvial quartzites indicate initial provenances to the south and north. This indicates that the basin morphology was in a form of a trough. Eventually paleocurrent distribution patterns turn southeast, and marine flooding extended further towards the west, to form the Greater Witwatersrand Basin, which was then modified by development of a foreland basin towards the west and northwest. This suggests that the present outcrops of the Mozaan Group represent mere remnants of an extensive basin. Siliciclastic deposits of the Mozaan succession harbours a wealth of information on the crust-forming events that affected the Kaapvaal craton. Detrital zircons from quartzites and diamictite samples yield ages that record magmatic events that extend from the early (ca. 3.6Ga) to middle (2.89Ga) Archean. Late magmatic events were coeval with the development of the Mozaan basin. The Mozaan succession also is host to the oldest known glacial deposits, namely the Klipwal and the Mpatheni Member diamictites. The absence of deformation, erosional contacts and the presence of incorporated underlying material suggest that grounded glaciers were not the mode of deposition of these diamictites, but that the glacial deposits can be explained as gravity flows from collapse of oversteepened slopes of low relief glaciomarine setting that degenerated in more distal part of the basin into clast poor turbidity flows. Analysis of shale and matrix composition in diamictites show that mechanical erosion processes dominated the source area. CIA values range between 70 and 81, suggesting negligible chemical weathering in the sediment source area. CIA values increase only in the upper part of the stratigraphy, i.e. Ntanyana Formation. Incompatible to compatible trace element ratios are low and suggest that mafic and ultramafic rock dominated the source area. This, perhaps, demonstrates that the greenstone belts were a major source of detritus to the Mozaan basin. Well-constrained paleomagnetic data sets acquired from the Klipwal Member diamictite and Tobolsk lava give a very good estimate of the paleogeographic setting of the Kaapvaal craton during the deposition of the Mozaan Group. The Klipwal diamictite was apparently deposited at high-paleolatitude setting of 48°. The craton then moved slightly to the north to latitude ~43° at the time of eruption of the Tobolsk lavas. From the results it appears that available geochemical classification schemes based on the composition of Phanerozoic volcanic rocks are not suitable to classify the lavas of the Agatha Formation unequivocally. To arrive at any tectonic model for these igneous rocks it is necessary to consider stratigraphic relationships, physical volcanology and geochemical characteristics of the rock succession. On the other hand, the wellconstrained paleomagnetic data indicate that the global climate system in the Mesoarchean was similar to modern day earth where glacial deposits are constrained largely to Polar Regions.
- Full Text:
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