An extrusive rock unit in the Mesoproterozoic Pilanesberg alkaline complex that has been referred to as the “Beacon Heights Tinguaite” since the work of Shand (1928) has been reexamined in the field and studied by petrographic microscopy, scanning electron microscopy, and whole-rock major and trace element analysis. The new observations indicate that the unit (here informally renamed the “Beacon Heights Phonolite” to comply with current petrographical nomenclature) is a laminated, volcanic rock in which mafic layers rich in sodic pyroxene alternate on any scale with discontinous, felsic layers consisting of alkali feldspar, nepheline, sodalite and their alteration products natrolite and analcime. Highly strontian apatite, fluorite, a manganiferous pectolite-group mineral, sphalerite and pyrrhotite are minor to accessory minerals. In addition, the rock carries indicator minerals typical of agpaitic rocks (including lamprophyllite and eudialyte-group minerals and their replacement products). The rock contains abundant crystal fragments of felsic minerals (alkali feldspar, nepheline and sodalite), fragments and elongated lenses of nepheline syenite, some of which contain primary magmatic lamprophyllite, and rare armoured lapilli. The overall structure of the rock is that of a welded ashflow tuff (igmimbrite) affected by post-magmatic recrystallisation processes. Whole-rock major and trace element compositions suggest a compositional affinity with members of the agpaitic, intrusive Green Foyaite Suite of the Pilanesberg Complex, and probably also a genetic relationship. The Beacon Heights Phonolite is a rare, possibly unique, example of an ashflow deposit of agpaitic, highly silica-undersaturated composition.
{"title":"The Beacon Heights “Tinguaite” (Phonolite): A recrystallised agpaitic ignimbrite in the Mesoproterozoic Pilanesberg alkaline complex, South Africa","authors":"J. U. Akoh, M. Elburg, T. Andersen","doi":"10.25131/sajg.126.0017","DOIUrl":"https://doi.org/10.25131/sajg.126.0017","url":null,"abstract":"\u0000 An extrusive rock unit in the Mesoproterozoic Pilanesberg alkaline complex that has been referred to as the “Beacon Heights Tinguaite” since the work of Shand (1928) has been reexamined in the field and studied by petrographic microscopy, scanning electron microscopy, and whole-rock major and trace element analysis. The new observations indicate that the unit (here informally renamed the “Beacon Heights Phonolite” to comply with current petrographical nomenclature) is a laminated, volcanic rock in which mafic layers rich in sodic pyroxene alternate on any scale with discontinous, felsic layers consisting of alkali feldspar, nepheline, sodalite and their alteration products natrolite and analcime. Highly strontian apatite, fluorite, a manganiferous pectolite-group mineral, sphalerite and pyrrhotite are minor to accessory minerals. In addition, the rock carries indicator minerals typical of agpaitic rocks (including lamprophyllite and eudialyte-group minerals and their replacement products). The rock contains abundant crystal fragments of felsic minerals (alkali feldspar, nepheline and sodalite), fragments and elongated lenses of nepheline syenite, some of which contain primary magmatic lamprophyllite, and rare armoured lapilli. The overall structure of the rock is that of a welded ashflow tuff (igmimbrite) affected by post-magmatic recrystallisation processes. Whole-rock major and trace element compositions suggest a compositional affinity with members of the agpaitic, intrusive Green Foyaite Suite of the Pilanesberg Complex, and probably also a genetic relationship. The Beacon Heights Phonolite is a rare, possibly unique, example of an ashflow deposit of agpaitic, highly silica-undersaturated composition.","PeriodicalId":49494,"journal":{"name":"South African Journal of Geology","volume":" ","pages":""},"PeriodicalIF":1.8,"publicationDate":"2023-08-26","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"49097858","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
The Okorusu carbonatite complex and the mine’s tailings dump constitute a potential resource of rare earth elements meaning lanthanides and yttrium (REY). Small (≤100 μm), hydrothermal REE fluorcarbonates (bastnäsite, parisite, röntgenite and synchysite) are the principal light REE carriers while the heavy REE and Y are primarily hosted in magmatic apatite, magmatic-hydrothermal carbonates (calcite, dolomite, ankerite) and hydrothermal fluorite. Bastnäsite and röntgenite occur as individual, liberated crystals in the tailings albeit syntaxial intergrowths of acicular parisite-synchysite-röntgenite are more common. There is a clear trend of decreasing REY contents in the magmatic carbonates over the early hydrothermal carbonates to the late hydrothermal carbonates. The lower total REY abundance in the hydrothermal carbonates is interpreted to be mainly a consequence of their limited redistribution during hydrothermal activity following the breakdown of the primary magmatic minerals and precipitation of REE fluorcarbonates. During this process hydrothermal fluids changed the isotopic composition of the Okorusu carbonatites by shifting δ13C-δ18O pairs of magmatic carbonatite (δ13C: -6.5‰ to -5.4‰; δ18O: +6.4‰ to +11.2‰) to higher values of hydrothermally modified carbonatite (δ13C: -4.2 to -2.2; δ18O: +12.0‰ to +17.5‰). The preferential presence of REY in the magmatic carbonates and to a minor degree in the hydrothermal carbonates in the Okorusu carbonatite and tailings is an important exploration tool for primary magmatic REY mineralisations. The recovery of REE fluorcarbonates can be considered in the development of the flowsheet for the planned recycling of the tailings material as a potential REY by-product.
{"title":"Okorusu tailings – Part II: REE mineralisation, and oxygen and carbon isotope variations","authors":"S. Lohmeier, J. Machleidt, R. Klemd, R. Ellmies","doi":"10.25131/sajg.126.0015","DOIUrl":"https://doi.org/10.25131/sajg.126.0015","url":null,"abstract":"\u0000 The Okorusu carbonatite complex and the mine’s tailings dump constitute a potential resource of rare earth elements meaning lanthanides and yttrium (REY). Small (≤100 μm), hydrothermal REE fluorcarbonates (bastnäsite, parisite, röntgenite and synchysite) are the principal light REE carriers while the heavy REE and Y are primarily hosted in magmatic apatite, magmatic-hydrothermal carbonates (calcite, dolomite, ankerite) and hydrothermal fluorite. Bastnäsite and röntgenite occur as individual, liberated crystals in the tailings albeit syntaxial intergrowths of acicular parisite-synchysite-röntgenite are more common. There is a clear trend of decreasing REY contents in the magmatic carbonates over the early hydrothermal carbonates to the late hydrothermal carbonates. The lower total REY abundance in the hydrothermal carbonates is interpreted to be mainly a consequence of their limited redistribution during hydrothermal activity following the breakdown of the primary magmatic minerals and precipitation of REE fluorcarbonates. During this process hydrothermal fluids changed the isotopic composition of the Okorusu carbonatites by shifting δ13C-δ18O pairs of magmatic carbonatite (δ13C: -6.5‰ to -5.4‰; δ18O: +6.4‰ to +11.2‰) to higher values of hydrothermally modified carbonatite (δ13C: -4.2 to -2.2; δ18O: +12.0‰ to +17.5‰). The preferential presence of REY in the magmatic carbonates and to a minor degree in the hydrothermal carbonates in the Okorusu carbonatite and tailings is an important exploration tool for primary magmatic REY mineralisations. The recovery of REE fluorcarbonates can be considered in the development of the flowsheet for the planned recycling of the tailings material as a potential REY by-product.","PeriodicalId":49494,"journal":{"name":"South African Journal of Geology","volume":" ","pages":""},"PeriodicalIF":1.8,"publicationDate":"2023-08-18","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"49446053","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Former open-pit fluorite mining and processing at the Okorusu carbonatite deposit, Namibia, resulted in a large tailings dump comprising an old (~553 kt) and a new tailings part (~3.8 Mt). We characterise dump tailings mineralogically and geochemically in order to evaluate the potential for processing of relict fluorite and recovery of other commodities of interest. The tailings comprise largely quartz, K-feldspar, different carbonates (ankerite, calcite, dolomite, siderite and manganese carbonates), fluorite, apatite, pyroxenes, hornblende and Fe (hydro)oxides, whereas all other mineral phases such as baryte, pyrite, pyrochlore and REE fluorcarbonates occur in traces. The largest proportional difference between samples from the surface of the new and old tailings pertains to fluorite (average old dump: ~12 vol.%; average new dump: ~5 vol.%) and apatite proportions (average old dump: ~8 vol.%; average new dump: ~5 vol.%). Mineralogical contrasts between both tailings parts result largely from fluorite, apatite, quartz and Fe (hydro)oxides being more abundant in old tailings and carbonates being more abundant in new tailings. Geochemically, these contrasts are reflected in the major element composition, while variances in trace element compositions are mostly small. The mineral proportions clearly point out considerable fluorite (≥804 kt) and apatite (≥742 kt) resources, with potential by-products of Nb and REE based on drilling data. The readily available material in combination with a high proportion (about 80%) of liberated fluorite are favourable for re-processing by locally available flotation technology. Therefore, the tailings dump can be considered a valuable resource for the critical raw material fluorite and the fertiliser raw material apatite, which are wanted materials on international and national markets.
{"title":"Okorusu tailings – Part I: General characterisation of superficial tailings","authors":"S. Lohmeier, R. Ellmies, T. Adolffs, S. Sindern","doi":"10.25131/sajg.126.0014","DOIUrl":"https://doi.org/10.25131/sajg.126.0014","url":null,"abstract":"\u0000 Former open-pit fluorite mining and processing at the Okorusu carbonatite deposit, Namibia, resulted in a large tailings dump comprising an old (~553 kt) and a new tailings part (~3.8 Mt). We characterise dump tailings mineralogically and geochemically in order to evaluate the potential for processing of relict fluorite and recovery of other commodities of interest. The tailings comprise largely quartz, K-feldspar, different carbonates (ankerite, calcite, dolomite, siderite and manganese carbonates), fluorite, apatite, pyroxenes, hornblende and Fe (hydro)oxides, whereas all other mineral phases such as baryte, pyrite, pyrochlore and REE fluorcarbonates occur in traces. The largest proportional difference between samples from the surface of the new and old tailings pertains to fluorite (average old dump: ~12 vol.%; average new dump: ~5 vol.%) and apatite proportions (average old dump: ~8 vol.%; average new dump: ~5 vol.%). Mineralogical contrasts between both tailings parts result largely from fluorite, apatite, quartz and Fe (hydro)oxides being more abundant in old tailings and carbonates being more abundant in new tailings. Geochemically, these contrasts are reflected in the major element composition, while variances in trace element compositions are mostly small. The mineral proportions clearly point out considerable fluorite (≥804 kt) and apatite (≥742 kt) resources, with potential by-products of Nb and REE based on drilling data. The readily available material in combination with a high proportion (about 80%) of liberated fluorite are favourable for re-processing by locally available flotation technology. Therefore, the tailings dump can be considered a valuable resource for the critical raw material fluorite and the fertiliser raw material apatite, which are wanted materials on international and national markets.","PeriodicalId":49494,"journal":{"name":"South African Journal of Geology","volume":" ","pages":""},"PeriodicalIF":1.8,"publicationDate":"2023-08-18","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"43638598","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
D. van REENEN, M. Clark, C. Smit, T. Tsunogae, O. Safonov
This paper addresses the credibility of published data utilised to underpin conflicting models recently proposed for the geodynamic evolution of the Limpopo Complex (LC), Southern Africa, in the Neoarchaean and Palaeoproterozoic. We are mainly focused on the tectonic and metamorphic processes that affected the Central Zone (CZ) of the LC, but also consider the significance of the timing of the thermo-tectonic interaction of the Southern Marginal Zone (SMZ) of the LC with the granite-greenstone terrane of the Kaapvaal Craton (KVC) at the position of the steep north-dipping Hout River Shear Zone (HRSZ). HRSZ-linked tectonism at the contact with the KVC is expressed as a narrow “hot-iron zone” directly dated at 2.72 to 2.69 Ga and developed in the footwall of the north-dipping HRSZ. HRSZ-linked tectonic activity intermittently continued up to 2.65 to 2.62 Ga with no structural-metamorphic or geochronological evidence that the SMZ and the rest of the KVC were affected by regional thermo-tectonic (orogenic) activity after ca. 2.68 Ga. The complementary evolution of the CZ at 2.72 to 2.62 Ga prior to emplacement at 2.612 Ga of the Bulai granitic pluton is expressed by two thermo-tectonic events, at 2.72 to 2.66 Ga and 2.65 to 2.62 Ga, respectively. The early 2.72 to 2.66 Ga event was associated with near-vertical exhumation of the CZ from a rising crustal-scale granulite diapir, accompanied by emplacement of steeply-dipping isoclinal folded and granoblastic-textured CZ granulites at the mid-crustal level (20 km depth). A moderately (~45°) northeast-directed shear deformational event, accompanied by extensive granitic diapirism, controlled the final emplacement of the CZ in the Neoarchaean at 2.65 to 2.62 Ga prior to intrusion of the 2.612 Ga Bulai pluton. This second tectono-thermal event is expressed by major sheared structural features that include mega-closed folds, mega-north-south trending folds, and the 29 km-wide southwest-northeast-trending and moderately southeast-dipping and northeast-verging Tshipise Straightening Zone (TSZ) that bounds the CZ in the south. The CZ was finally exhumed and emplaced at the upper crustal level 600 Myr later (at ca. 2.02 Ga) during a regional high-temperature Palaeoproterozoic thermal event associated with major near-vertical strike-slip shear zones that overprint Neoarchaean oblique slip shear zones that bound the CZ. Thermo-tectonic activity in the CZ that is associated with this mainly thermal event is recognised as discrete steeply-dipping narrow fabric-parallel shear zones dated at ca. 2.02 Ga that overprint older structures. A gravity-driven crustal-scale diapiric model is utilised to explain the evolution of the SMZ and CZ of the LC at 2.72 to 2.62 Ga during the Limpopo Orogeny. Data presented and discussed contradict alternative published models that propose a continent-continent collisional orogeny at 2.65 to 2.62 Ga involving the SMZ and KVC, followed 600 Myr later at 2.02 Ga by a transpressional orog
{"title":"Review of the thermo-tectonic evolution of the Central Zone of the Limpopo Complex with implications for conflicting published geodynamic models","authors":"D. van REENEN, M. Clark, C. Smit, T. Tsunogae, O. Safonov","doi":"10.25131/sajg.126.0010","DOIUrl":"https://doi.org/10.25131/sajg.126.0010","url":null,"abstract":"\u0000 This paper addresses the credibility of published data utilised to underpin conflicting models recently proposed for the geodynamic evolution of the Limpopo Complex (LC), Southern Africa, in the Neoarchaean and Palaeoproterozoic. We are mainly focused on the tectonic and metamorphic processes that affected the Central Zone (CZ) of the LC, but also consider the significance of the timing of the thermo-tectonic interaction of the Southern Marginal Zone (SMZ) of the LC with the granite-greenstone terrane of the Kaapvaal Craton (KVC) at the position of the steep north-dipping Hout River Shear Zone (HRSZ). HRSZ-linked tectonism at the contact with the KVC is expressed as a narrow “hot-iron zone” directly dated at 2.72 to 2.69 Ga and developed in the footwall of the north-dipping HRSZ. HRSZ-linked tectonic activity intermittently continued up to 2.65 to 2.62 Ga with no structural-metamorphic or geochronological evidence that the SMZ and the rest of the KVC were affected by regional thermo-tectonic (orogenic) activity after ca. 2.68 Ga. The complementary evolution of the CZ at 2.72 to 2.62 Ga prior to emplacement at 2.612 Ga of the Bulai granitic pluton is expressed by two thermo-tectonic events, at 2.72 to 2.66 Ga and 2.65 to 2.62 Ga, respectively. The early 2.72 to 2.66 Ga event was associated with near-vertical exhumation of the CZ from a rising crustal-scale granulite diapir, accompanied by emplacement of steeply-dipping isoclinal folded and granoblastic-textured CZ granulites at the mid-crustal level (20 km depth). A moderately (~45°) northeast-directed shear deformational event, accompanied by extensive granitic diapirism, controlled the final emplacement of the CZ in the Neoarchaean at 2.65 to 2.62 Ga prior to intrusion of the 2.612 Ga Bulai pluton. This second tectono-thermal event is expressed by major sheared structural features that include mega-closed folds, mega-north-south trending folds, and the 29 km-wide southwest-northeast-trending and moderately southeast-dipping and northeast-verging Tshipise Straightening Zone (TSZ) that bounds the CZ in the south. The CZ was finally exhumed and emplaced at the upper crustal level 600 Myr later (at ca. 2.02 Ga) during a regional high-temperature Palaeoproterozoic thermal event associated with major near-vertical strike-slip shear zones that overprint Neoarchaean oblique slip shear zones that bound the CZ. Thermo-tectonic activity in the CZ that is associated with this mainly thermal event is recognised as discrete steeply-dipping narrow fabric-parallel shear zones dated at ca. 2.02 Ga that overprint older structures. A gravity-driven crustal-scale diapiric model is utilised to explain the evolution of the SMZ and CZ of the LC at 2.72 to 2.62 Ga during the Limpopo Orogeny. Data presented and discussed contradict alternative published models that propose a continent-continent collisional orogeny at 2.65 to 2.62 Ga involving the SMZ and KVC, followed 600 Myr later at 2.02 Ga by a transpressional orog","PeriodicalId":49494,"journal":{"name":"South African Journal of Geology","volume":" ","pages":""},"PeriodicalIF":1.8,"publicationDate":"2023-06-14","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"46331958","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
B. N. Mvile, E. B. Kiswaka, O. O. Osinowo, E. Mshiu, V. E. Mboya
The offshore Tanga Basin of north-eastern Tanzania contains Cenozoic sedimentary successions that have been poorly studied. The basin development was due to an interplay of multiple factors including periods of active fault movement linked to the East African Rift system (EARs) which influenced the Cenozoic development of the offshore Tanga Basin. The EARs recorded several discrete tectonic episodes that were associated with magmatic activities and massive volcanism. However, there is no report on the possible presence of magmatic intrusions indicative of magmatic activities and volcanism in the history of the Tanga Basin. Timing of occurrence and distribution of magmatic intrusions are among the key components needed to evaluate the petroleum potential of the basin. A detailed 2D qualitative seismic interpretation, coupled with core logging data and analysis of elemental proxies, has been employed to evaluate the petroleum potential of the Cenozoic successions of the offshore Tanga Basin considering the presence and timing of occurrence of the magmatic intrusions. These used data and the associated interpretation techniques have not been used before to meet similar objectives. Both core logging and elemental proxies are newly collected information used in this study. Results suggest that the Tanga Basin has been variedly intruded by magmatic sills and dikes. Seismic well tie and correlation to age-equivalent deposits across the onshore successions in the EWB revealed that the volcanic events occurred during tectonic episodes that influenced the development of the East African Rift basins. Seismic interpretation also suggests that these tectonic episodes occurred possibly during the Miocene, Pleistocene and Holocene periods when magmatic intrusions are believed to have promoted source rock maturation and facilitated the formation of structural elements for petroleum preservation.
{"title":"Timing of the Cenozoic magmatic intrusions in the offshore Tanga Basin, Tanzania: correlation to age equivalent deposits in the Eyasi-Wembere Basin and their implications for petroleum potential","authors":"B. N. Mvile, E. B. Kiswaka, O. O. Osinowo, E. Mshiu, V. E. Mboya","doi":"10.25131/sajg.126.0013","DOIUrl":"https://doi.org/10.25131/sajg.126.0013","url":null,"abstract":"\u0000 The offshore Tanga Basin of north-eastern Tanzania contains Cenozoic sedimentary successions that have been poorly studied. The basin development was due to an interplay of multiple factors including periods of active fault movement linked to the East African Rift system (EARs) which influenced the Cenozoic development of the offshore Tanga Basin. The EARs recorded several discrete tectonic episodes that were associated with magmatic activities and massive volcanism. However, there is no report on the possible presence of magmatic intrusions indicative of magmatic activities and volcanism in the history of the Tanga Basin. Timing of occurrence and distribution of magmatic intrusions are among the key components needed to evaluate the petroleum potential of the basin. A detailed 2D qualitative seismic interpretation, coupled with core logging data and analysis of elemental proxies, has been employed to evaluate the petroleum potential of the Cenozoic successions of the offshore Tanga Basin considering the presence and timing of occurrence of the magmatic intrusions. These used data and the associated interpretation techniques have not been used before to meet similar objectives. Both core logging and elemental proxies are newly collected information used in this study. Results suggest that the Tanga Basin has been variedly intruded by magmatic sills and dikes. Seismic well tie and correlation to age-equivalent deposits across the onshore successions in the EWB revealed that the volcanic events occurred during tectonic episodes that influenced the development of the East African Rift basins. Seismic interpretation also suggests that these tectonic episodes occurred possibly during the Miocene, Pleistocene and Holocene periods when magmatic intrusions are believed to have promoted source rock maturation and facilitated the formation of structural elements for petroleum preservation.","PeriodicalId":49494,"journal":{"name":"South African Journal of Geology","volume":" ","pages":""},"PeriodicalIF":1.8,"publicationDate":"2023-06-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"49427376","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
R. Mapeo, R.M. Key (MBE), A. Moore, J. Mulder, N. Gardiner, L. Robb
LA-ICPMS U-Pb isotope analyses are presented for zircons from a thin granite sheet intersected in a borehole drilled into the upper Transvaal Supergroup wall rocks to the Molopo Farms Complex in southern Botswana. Many of the zircons have irregular or angular grain margins, and some have rounded cores. Approximately half of the analysed grains yielded concordant 207Pb/206Pb ages ranging between 2 282 ± 29 and 2 113 ± 16 Ma. Assuming that these grains were inherited from the surrounding upper Transvaal Supergroup sedimentary strata, the youngest zircon age provides a maximum depositional age for these sediments. Importantly, these zircon ages coincide with dates from detrital zircons in upper Transvaal Supergroup strata in the eastern Kaapvaal Craton and in the Magondi Supergroup in western Zimbabwe. Hf isotope analyses of the rounded zircons suggest that these grains were sourced from a variable mixture of an old basement and more juvenile material. A younger age of 2 060 ± 12 Ma was obtained from a single subhedral zircon grain from the granite sheet. This is interpreted as the granite’s emplacement age, making it coeval with the adjacent Molopo Farms Complex that forms part of the Bushveld Large Igneous Province.
{"title":"Syn-Bushveld “granite sheets” associated with the Molopo Farms Complex intruding into Transvaal Supergroup strata in southern Botswana","authors":"R. Mapeo, R.M. Key (MBE), A. Moore, J. Mulder, N. Gardiner, L. Robb","doi":"10.25131/sajg.126.0012","DOIUrl":"https://doi.org/10.25131/sajg.126.0012","url":null,"abstract":"\u0000 LA-ICPMS U-Pb isotope analyses are presented for zircons from a thin granite sheet intersected in a borehole drilled into the upper Transvaal Supergroup wall rocks to the Molopo Farms Complex in southern Botswana. Many of the zircons have irregular or angular grain margins, and some have rounded cores. Approximately half of the analysed grains yielded concordant 207Pb/206Pb ages ranging between 2 282 ± 29 and 2 113 ± 16 Ma. Assuming that these grains were inherited from the surrounding upper Transvaal Supergroup sedimentary strata, the youngest zircon age provides a maximum depositional age for these sediments. Importantly, these zircon ages coincide with dates from detrital zircons in upper Transvaal Supergroup strata in the eastern Kaapvaal Craton and in the Magondi Supergroup in western Zimbabwe. Hf isotope analyses of the rounded zircons suggest that these grains were sourced from a variable mixture of an old basement and more juvenile material. A younger age of 2 060 ± 12 Ma was obtained from a single subhedral zircon grain from the granite sheet. This is interpreted as the granite’s emplacement age, making it coeval with the adjacent Molopo Farms Complex that forms part of the Bushveld Large Igneous Province.","PeriodicalId":49494,"journal":{"name":"South African Journal of Geology","volume":" ","pages":""},"PeriodicalIF":1.8,"publicationDate":"2023-06-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"43844334","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
L. Cabri, A. McDonald, T. Oberthür, N. Tamura, A. Vymazalová, K. C. Ross, F. Melcher
A hundred years after the discovery of the Merensky Reef in 1924, it is appropriate to present the new mineral andrieslombaardite in honour of Andries Frederik Lombaard who was instrumental in its discovery. Andrieslombaardite, RhSbS, was first described as an unknown mineral from placer deposits associated with the Tulameen Alaskan-Uralian type complex, British Colombia, Canada (Raicevic and Cabri, 1976) but has since been reported from several other deposits including the platiniferous Driekop, Mooihoek, and Onverwacht pipes in the eastern Bushveld Complex, South Africa. The mineral and the name were approved by the Commission on New Minerals Nomenclature and Classification (CNMNC) of the International Mineralogical Association (IMA no. 2022-076) based on data in the co-type samples from Onverwacht and a co-type sample from the Yubdo stream, Birbir River, Ethiopia. Andrieslombaardite in the Onverwacht sample is a single 8 x 20 μm grain attached to laurite in a matrix of altered silicate and Fe-oxyhydroxide minerals. In the Yubdo samples, there are many grains of pale brownish gray andrieslombaardite up to 25 x 55 μm in size, included in Pt-Fe alloys, some associated with erlichmanite, and others attached to bornite and chalcopyrite. The reflectance values (R%) measured in air and in oil at the COM wavelengths are 48.3 and 33.0 (470 nm), 49.3 and 34.0 (546 nm), 51.0 and 35.9 (589 nm), and 51.8 and 36.7 (650 nm). The colour values x, y, Y, λd, and Pe in air are 0.317, 0.322, 50.3, 580, and 3.2, and in oil are 0.319, 0.324, 35.6, 579, and 4.5. The composition of andrieslombaardite is ideally RhSbS, but it contains variable amounts of Fe, Pt, Pd, and Ir that may substitute for Rh. The mineral is cubic with unit-cell dimensions of a = 6.0278(4) Å, V = 219.01(6) Å3 and Z = 4. It was synthesised at 400 and 550°C using stoichiometric elemental amounts. It is a member of the cobaltite group. The mineralisation of the intrusive dunite pipes was probably introduced at high temperatures, under magmatic conditions. The primary assemblages were to a certain degree overprinted and redistributed by low-temperature hydrothermal fluids. The Pt-Fe alloys from Yubdo containing PGM inclusions such as andrieslombaardite in the Yubdo-Alaskan-type complex were formed at some post-magmatic stage owing to PGE remobilisation during hydrothermal or metamorphic episodes.
1924年梅伦斯基礁被发现一百年后，为了纪念在其发现过程中发挥重要作用的安德里斯·弗雷德里克·隆巴德，我们可以将新矿物安德里斯·隆巴德（andrieslombaardite）赠送给大家。Andrieslombaardite，RhSbS，最初被描述为一种未知矿物，来自加拿大不列颠哥伦比亚省Tulameen Alaskan Uralian型杂岩的砂矿（Raicevic和Cabri，1976），但后来又被报道来自其他几个矿床，包括南非Bushveld杂岩东部的含铂Driekop、Mooihoek和Onverwacht管道。该矿物和名称由国际矿物学协会新矿物命名和分类委员会（CNMNC）（IMA编号：2022-076）根据Onverwacht的同型样品和埃塞俄比亚Birbir河Yubdo河的同型样本中的数据批准。Onverwacht样品中的Andrieslombaardite是一个8 x 20μm的颗粒，附着在蚀变硅酸盐和氧化铁矿物基质中的月桂石上。在Yubdo样品中，Pt-Fe合金中含有许多大小达25 x 55μm的浅棕灰色安德里斯洛姆巴rdite颗粒，其中一些与伊利石有关，另一些与斑铜矿和黄铜矿有关。在空气和油中测得的COM波长的反射率值（R%）分别为48.3和33.0（470 nm）、49.3和34.0（546 nm）、51.0和35.9（589 nm）以及51.8和36.7（650 nm）。在空气中的色值x、y、y、λd和Pe分别为0.317、0.322、50.3、580和3.2，在油中分别为0.319、0.324、35.6、579和4.5。andrieslombaardite的组成理想地为RhSbS，但它含有可替代Rh的可变量的Fe、Pt、Pd和Ir。该矿物为立方体，晶胞尺寸为a=6.0278（4）Å，V=219.01（6）Å3和Z=4。它是在400和550°C下使用化学计量元素量合成的。它是钴矿群的一员。侵入性纯橄榄岩管道的矿化作用可能是在高温、岩浆条件下引入的。初级组合在一定程度上被低温水热流体叠加和重新分布。Yubdo-Alaskan型杂岩中来自Yubdo的含有PGM包裹体的Pt-Fe合金，如andrieslombaardite，是在热液或变质过程中由于PGE的再活化而在岩浆后期形成的。
{"title":"Andrieslombaardite, RhSbS, a new platinum-group mineral from the platiniferous Onverwacht Pipe, Republic of South Africa","authors":"L. Cabri, A. McDonald, T. Oberthür, N. Tamura, A. Vymazalová, K. C. Ross, F. Melcher","doi":"10.25131/sajg.126.0011","DOIUrl":"https://doi.org/10.25131/sajg.126.0011","url":null,"abstract":"\u0000 A hundred years after the discovery of the Merensky Reef in 1924, it is appropriate to present the new mineral andrieslombaardite in honour of Andries Frederik Lombaard who was instrumental in its discovery. Andrieslombaardite, RhSbS, was first described as an unknown mineral from placer deposits associated with the Tulameen Alaskan-Uralian type complex, British Colombia, Canada (Raicevic and Cabri, 1976) but has since been reported from several other deposits including the platiniferous Driekop, Mooihoek, and Onverwacht pipes in the eastern Bushveld Complex, South Africa.\u0000 The mineral and the name were approved by the Commission on New Minerals Nomenclature and Classification (CNMNC) of the International Mineralogical Association (IMA no. 2022-076) based on data in the co-type samples from Onverwacht and a co-type sample from the Yubdo stream, Birbir River, Ethiopia. Andrieslombaardite in the Onverwacht sample is a single 8 x 20 μm grain attached to laurite in a matrix of altered silicate and Fe-oxyhydroxide minerals. In the Yubdo samples, there are many grains of pale brownish gray andrieslombaardite up to 25 x 55 μm in size, included in Pt-Fe alloys, some associated with erlichmanite, and others attached to bornite and chalcopyrite.\u0000 The reflectance values (R%) measured in air and in oil at the COM wavelengths are 48.3 and 33.0 (470 nm), 49.3 and 34.0 (546 nm), 51.0 and 35.9 (589 nm), and 51.8 and 36.7 (650 nm). The colour values x, y, Y, λd, and Pe in air are 0.317, 0.322, 50.3, 580, and 3.2, and in oil are 0.319, 0.324, 35.6, 579, and 4.5.\u0000 The composition of andrieslombaardite is ideally RhSbS, but it contains variable amounts of Fe, Pt, Pd, and Ir that may substitute for Rh. The mineral is cubic with unit-cell dimensions of a = 6.0278(4) Å, V = 219.01(6) Å3 and Z = 4. It was synthesised at 400 and 550°C using stoichiometric elemental amounts. It is a member of the cobaltite group.\u0000 The mineralisation of the intrusive dunite pipes was probably introduced at high temperatures, under magmatic conditions. The primary assemblages were to a certain degree overprinted and redistributed by low-temperature hydrothermal fluids. The Pt-Fe alloys from Yubdo containing PGM inclusions such as andrieslombaardite in the Yubdo-Alaskan-type complex were formed at some post-magmatic stage owing to PGE remobilisation during hydrothermal or metamorphic episodes.","PeriodicalId":49494,"journal":{"name":"South African Journal of Geology","volume":" ","pages":""},"PeriodicalIF":1.8,"publicationDate":"2023-05-09","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"48159841","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Underground workings at Rowland Shaft, Lonmin Platinum, near Marikana have exposed a large (>7 x 5 m) calc-silicate xenolith in mottled anorthosite ~20 m below the UG 1 chromitite of the upper Critical Zone. The xenolith comprises a monticellite + forsterite + spinel assemblage that is consistent with a metamorphic Tmax of ≥975°C being reached at P ~1 kbar and XCO2 ~1. Internal variations in the modal proportions of these phases are interpreted as an artefact of sedimentary layering in the xenolith. The xenolith is flanked by a 20 to 30 cm wide magmatic skarn that contains several small, cm- to dm-scale, calc-silicate fragments displaying the same peak assemblage as the main xenolith, as well as a dm-scale chromitite autolith and mm- to dm-scale lenses and stringer-like masses of fine-grained uvarovite. The skarn matrix is mineralogically zoned, with an up to 10 cm wide clinopyroxenite layer proximal to the xenolith contact and calc-silicate fragments grading outwards into a gabbroic, plagioclase-clinopyroxene assemblage in which plagioclase becomes progressively more dominant distally. Notwithstanding its pseudo-ophitic texture, the distal skarn matrix displays a range of features consistent with crystallisation of magma contaminated by Ca from the decomposing xenolith under elevated fO2 conditions, including: (a) the unusual mineral compositions (zoned aluminian-ferrian diopside and anorthite, An>99), (b) the presence of calcite as inclusions in plagioclase and as a minor interstitial phase, (c) corroded wollastonite inclusions in clinopyroxene, and (d) thin grossular reaction rims between clinopyroxene and plagioclase. This chemical evolution is further substantiated by zoned spinel grains comprising Cr-rich cores and Al-enriched rims, which are, in turn, enclosed in garnet aggregates that grade outwards from uvarovite to grossular. Local development of grossular-vesuvianite symplectite indicates limited retrograde hydrous (XCO2 <0.1) fluid influx under subsolidus conditions (T <750°C). Bedding in the xenolith displays a steep westerly dip, approximately orthogonal to the magmatic layering. This, together with evidence in the skarn of both plastic shear strain and out-of-sequence magmatic autoliths, suggests that the xenolith underwent density-driven foundering within the RLS magma from an initial location above the UG 1 chromitite layer.
{"title":"Petrographic and geochemical evidence of calc-silicate xenolith – magma interaction in the western Bushveld Complex, South Africa","authors":"B. Koovarjee, R. Gibson, P. Nex","doi":"10.25131/sajg.126.0008","DOIUrl":"https://doi.org/10.25131/sajg.126.0008","url":null,"abstract":"\u0000 Underground workings at Rowland Shaft, Lonmin Platinum, near Marikana have exposed a large (>7 x 5 m) calc-silicate xenolith in mottled anorthosite ~20 m below the UG 1 chromitite of the upper Critical Zone. The xenolith comprises a monticellite + forsterite + spinel assemblage that is consistent with a metamorphic Tmax of ≥975°C being reached at P ~1 kbar and XCO2 ~1. Internal variations in the modal proportions of these phases are interpreted as an artefact of sedimentary layering in the xenolith. The xenolith is flanked by a 20 to 30 cm wide magmatic skarn that contains several small, cm- to dm-scale, calc-silicate fragments displaying the same peak assemblage as the main xenolith, as well as a dm-scale chromitite autolith and mm- to dm-scale lenses and stringer-like masses of fine-grained uvarovite. The skarn matrix is mineralogically zoned, with an up to 10 cm wide clinopyroxenite layer proximal to the xenolith contact and calc-silicate fragments grading outwards into a gabbroic, plagioclase-clinopyroxene assemblage in which plagioclase becomes progressively more dominant distally. Notwithstanding its pseudo-ophitic texture, the distal skarn matrix displays a range of features consistent with crystallisation of magma contaminated by Ca from the decomposing xenolith under elevated fO2 conditions, including: (a) the unusual mineral compositions (zoned aluminian-ferrian diopside and anorthite, An>99), (b) the presence of calcite as inclusions in plagioclase and as a minor interstitial phase, (c) corroded wollastonite inclusions in clinopyroxene, and (d) thin grossular reaction rims between clinopyroxene and plagioclase. This chemical evolution is further substantiated by zoned spinel grains comprising Cr-rich cores and Al-enriched rims, which are, in turn, enclosed in garnet aggregates that grade outwards from uvarovite to grossular. Local development of grossular-vesuvianite symplectite indicates limited retrograde hydrous (XCO2 <0.1) fluid influx under subsolidus conditions (T <750°C). Bedding in the xenolith displays a steep westerly dip, approximately orthogonal to the magmatic layering. This, together with evidence in the skarn of both plastic shear strain and out-of-sequence magmatic autoliths, suggests that the xenolith underwent density-driven foundering within the RLS magma from an initial location above the UG 1 chromitite layer.","PeriodicalId":49494,"journal":{"name":"South African Journal of Geology","volume":" ","pages":""},"PeriodicalIF":1.8,"publicationDate":"2023-04-21","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"43009810","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
D. Cornell, M. Harris, D. Frei, B. Mapani, T. Malobela, A. Jonsson, C. Lundell, M. Kristoffersen
The Koras Group is a bimodal volcanosedimentary group located in post-tectonic grabens in a foreland thrust complex in the Kaaien Terrane of the Mesoproterozoic Namaqua-Natal Province of southern Africa. It contains two sequences of mafic and felsic volcanic rocks with an unconformity between them, only the lower sequence being slightly folded. The Koras Group was long regarded as having formed at the end of the 1 210 to 1 000 Ma Namaqua Orogeny, because it lacks the severe deformation and metamorphism of the underlying rocks, with igneous minerals preserved in many samples. Following years of unsuccessful attempts to precisely date the volcanic rocks, the first two ion probe U-Pb zircon studies both reported ages of ~1 172 Ma for the Swartkopsleegte Formation felsic lava in the slightly folded lower sequence (based on relatively few dated zircons) and ~1 100 Ma for the Leeuwdraai Formation rhyolite in the undeformed upper sequence. Thus a major 70 m.y. hiatus seemed apparent between the lower and upper sequences despite their similar geochemistry and rift-related setting. This gave rise to models which envisaged the Kaaien Terrane being unaffected by the syn- to late-tectonic deformation, migmatisation and granite intrusions, documented between 1 200 and 1 150 Ma in the adjoining Namaqua-Natal terranes to the west. A high-pressure (10 kbar) metamorphic event, recognised in the Kaaien Terrane basement just south of hardly-deformed Koras Group exposures and dated at 1 150 Ma, is inconsistent with such models. A re-investigation and microbeam dating campaign on the Koras Group confirms the 1 101 ± 2 Ma (n = 6) age for felsic volcanic rocks of the upper sequence, but establishes a new reliable age of 1 114 ± 4 Ma for the lower one (n = 2). The 1 170 ages obtained in the earlier two studies were revisited and are now considered to reflect the age of zircon xenocrysts from the source rocks, which dominate the zircon population of some Swartkopsleegte Formation samples. Several criteria to distinguish autocrystic (magmatic) from antecrystic (age-overlappping xenocrystic) data points were investigated. One sample had high Th levels in only the younger zircons, but histograms of sufficiently precise 207Pb/206Pb ages provided the main criterion. Calculations of zircon crystallisation temperature intervals were not useful in predicting the abundance or proportions of magmatic and antecryst zircons. A multi-episode model of magmatic generation and crystallisation events is probably appropriate. In cases when felsic volcanic samples yield few zircons, care must be taken to avoid the problem exposed in this study. The Koras Group sediments have similar detrital zircon U/Pb age distributions to those of the Rehoboth Basement Inlier. This supports the concept that the Kaaien Terrane originated as the southern part of the Rehoboth Province.
Koras群是一个双峰火山沉积群，位于非洲南部纳马夸-纳塔尔省中元古代Kaaien Terrane的前陆冲断复合体中的构造后地堑中。它包含两个镁铁质和长英质火山岩序列，它们之间不整合，只有较低的序列轻微褶皱。长期以来，Koras群被认为是在1210至1000 Ma Namaqua造山运动末期形成的，因为它缺乏下伏岩石的严重变形和变质作用，许多样品中保存着火成矿物。经过多年来对火山岩精确测年的失败尝试，前两次离子探针U-Pb锆石研究都报告了轻微褶皱的下部序列中Swartkopsleegte组长英质熔岩的年龄为~1172Ma（基于相对较少的测年锆石），未变形的上部序列中Leeuwdrai组流纹岩的年龄为~100Ma。因此，尽管它们的地球化学和裂谷相关背景相似，但下部和上部层序之间似乎明显存在70 m.y.的主要间断。这产生了一些模型，这些模型设想Kaaien Terrane不受同期至晚期构造变形、混合岩化和花岗岩侵入的影响，在西面毗邻的Namaqua-Natal地体中记录了1200至150 Ma之间的情况。在几乎不变形的Koras群暴露以南的Kaaien Terrane基底中发现的高压（10kbar）变质事件，其年代为1150 Ma，与此类模型不一致。对Koras群的重新调查和微束测年活动证实了上部序列长英质火山岩的年龄为1011±2Ma（n=6），但为下部序列（n=2）确定了新的可靠年龄为1144±4Ma。对前两项研究中获得的1170个年龄进行了重新研究，现在认为这些年龄反映了源岩中锆石捕虏晶的年龄，这些捕虏晶在一些Swartkopsleegte组样品的锆石种群中占主导地位。研究了区分自成岩（岩浆）和前成岩（年龄覆盖的捕虏晶）数据点的几个标准。一个样本仅在较年轻的锆石中具有高Th水平，但足够精确的207Pb/206Pb年龄的直方图提供了主要标准。锆石结晶温度区间的计算对于预测岩浆锆石和前锆石的丰度或比例没有用处。岩浆生成和结晶事件的多期模型可能是合适的。如果长英质火山样品产生的锆石很少，则必须小心避免本研究中暴露的问题。Koras群沉积物具有与Rehoboth基底Inlier相似的碎屑锆石U/Pb年龄分布。这支持了Kaaien Terrane起源于Rehoboth省南部的概念。
{"title":"Zircon xenocrysts obscured the zircon date for the lower Koras Group, southern Africa","authors":"D. Cornell, M. Harris, D. Frei, B. Mapani, T. Malobela, A. Jonsson, C. Lundell, M. Kristoffersen","doi":"10.25131/sajg.126.0009","DOIUrl":"https://doi.org/10.25131/sajg.126.0009","url":null,"abstract":"\u0000 The Koras Group is a bimodal volcanosedimentary group located in post-tectonic grabens in a foreland thrust complex in the Kaaien Terrane of the Mesoproterozoic Namaqua-Natal Province of southern Africa. It contains two sequences of mafic and felsic volcanic rocks with an unconformity between them, only the lower sequence being slightly folded. The Koras Group was long regarded as having formed at the end of the 1 210 to 1 000 Ma Namaqua Orogeny, because it lacks the severe deformation and metamorphism of the underlying rocks, with igneous minerals preserved in many samples. Following years of unsuccessful attempts to precisely date the volcanic rocks, the first two ion probe U-Pb zircon studies both reported ages of ~1 172 Ma for the Swartkopsleegte Formation felsic lava in the slightly folded lower sequence (based on relatively few dated zircons) and ~1 100 Ma for the Leeuwdraai Formation rhyolite in the undeformed upper sequence. Thus a major 70 m.y. hiatus seemed apparent between the lower and upper sequences despite their similar geochemistry and rift-related setting. This gave rise to models which envisaged the Kaaien Terrane being unaffected by the syn- to late-tectonic deformation, migmatisation and granite intrusions, documented between 1 200 and 1 150 Ma in the adjoining Namaqua-Natal terranes to the west.\u0000 A high-pressure (10 kbar) metamorphic event, recognised in the Kaaien Terrane basement just south of hardly-deformed Koras Group exposures and dated at 1 150 Ma, is inconsistent with such models. A re-investigation and microbeam dating campaign on the Koras Group confirms the 1 101 ± 2 Ma (n = 6) age for felsic volcanic rocks of the upper sequence, but establishes a new reliable age of 1 114 ± 4 Ma for the lower one (n = 2). The 1 170 ages obtained in the earlier two studies were revisited and are now considered to reflect the age of zircon xenocrysts from the source rocks, which dominate the zircon population of some Swartkopsleegte Formation samples.\u0000 Several criteria to distinguish autocrystic (magmatic) from antecrystic (age-overlappping xenocrystic) data points were investigated. One sample had high Th levels in only the younger zircons, but histograms of sufficiently precise 207Pb/206Pb ages provided the main criterion. Calculations of zircon crystallisation temperature intervals were not useful in predicting the abundance or proportions of magmatic and antecryst zircons. A multi-episode model of magmatic generation and crystallisation events is probably appropriate. In cases when felsic volcanic samples yield few zircons, care must be taken to avoid the problem exposed in this study.\u0000 The Koras Group sediments have similar detrital zircon U/Pb age distributions to those of the Rehoboth Basement Inlier. This supports the concept that the Kaaien Terrane originated as the southern part of the Rehoboth Province.","PeriodicalId":49494,"journal":{"name":"South African Journal of Geology","volume":" ","pages":""},"PeriodicalIF":1.8,"publicationDate":"2023-04-10","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"47188816","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
The Windhoek Graben is a north-south trending rift in central Namibia that forms a prominent topographic feature bisecting an area of plateau uplift. It occupies a potentially crucial role in the propagation of the Late Cenozoic Southwest African Rift system regarding a possible continuation to the west of the Eiseb Rift. It is an unusual example of intra-continental rifting because it has no significant sediment fill associated with the period of active rifting, and hence the timing of rift activity and its tectonic relevance has not hitherto been established. To constrain the age of the Windhoek Graben we examine its regional geomorphic context and its relationship to four sites of igneous activity in the central Namibian Highlands. Two of these consist of clusters of eroded phonolitic tholoid bodies that have yielded 40Ar/39Ar dates of 32 Ma and 52 Ma, respectively, that we use to bracket the age of formation of a prominent remnant land surface, termed here the P52 Surface. From previous mapping of older intrusive igneous bodies, we argue that an even older land surface is partially preserved on the highest features in the area, and this surface (termed PRS) defines an initial domally uplifted surface from which initial drainage radiated, and onto which the earliest volcanic products associated with the Graben were erupted. In particular, the strong similarity in dyke and fault orientations is used to argue for a causal connection between the earliest magmatic activity and the onset of rifting. Long range correlation of PRS into the adjacent Aranos Basin strongly suggests a Late Cretaceous age for this earliest magmatic activity and the onset of rifting, but we cannot exclude a younger origin, any time up to the Early Eocene.
{"title":"Late Cretaceous to Early Cenozoic initiation of rifting of the Windhoek Graben, Namibia","authors":"R. Waren, J. Cartwright, M. Daly, R. Swart","doi":"10.25131/sajg.126.0007","DOIUrl":"https://doi.org/10.25131/sajg.126.0007","url":null,"abstract":"\u0000 The Windhoek Graben is a north-south trending rift in central Namibia that forms a prominent topographic feature bisecting an area of plateau uplift. It occupies a potentially crucial role in the propagation of the Late Cenozoic Southwest African Rift system regarding a possible continuation to the west of the Eiseb Rift. It is an unusual example of intra-continental rifting because it has no significant sediment fill associated with the period of active rifting, and hence the timing of rift activity and its tectonic relevance has not hitherto been established. To constrain the age of the Windhoek Graben we examine its regional geomorphic context and its relationship to four sites of igneous activity in the central Namibian Highlands. Two of these consist of clusters of eroded phonolitic tholoid bodies that have yielded 40Ar/39Ar dates of 32 Ma and 52 Ma, respectively, that we use to bracket the age of formation of a prominent remnant land surface, termed here the P52 Surface. From previous mapping of older intrusive igneous bodies, we argue that an even older land surface is partially preserved on the highest features in the area, and this surface (termed PRS) defines an initial domally uplifted surface from which initial drainage radiated, and onto which the earliest volcanic products associated with the Graben were erupted. In particular, the strong similarity in dyke and fault orientations is used to argue for a causal connection between the earliest magmatic activity and the onset of rifting. Long range correlation of PRS into the adjacent Aranos Basin strongly suggests a Late Cretaceous age for this earliest magmatic activity and the onset of rifting, but we cannot exclude a younger origin, any time up to the Early Eocene.","PeriodicalId":49494,"journal":{"name":"South African Journal of Geology","volume":" ","pages":""},"PeriodicalIF":1.8,"publicationDate":"2023-03-20","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"48390476","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
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