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New geochronological constraints on the Klipriviersberg Group: defining a new Neoarchean large igneous province on the Kaapvaal Craton, South Africa

Stamsnijder, Joaen LU (2017) In Dissertations in Geology at Lund University GEOR02 20162
Department of Geology
Abstract
In recent years, there has been a significant improvement in our ability to determine the ages of extensive short-lived magmatic events (large igneous provinces) dominated by rocks of mafic (silica-poor) compositions. This has been aided by targeting the magmatic feeders to these large igneous provinces (e.g., dykes and sills), which often host trace amounts of datable zirconium minerals such as baddeleyite and zircon. These age determinations are of great importance for unravelling the geological history and paleogeography of our planet. The Kaapvaal Craton in southern Africa hosts a rich and pristine geological history with many geological units and events in need of better age constraints. One is the Neoarchean Ventersdorp Supergroup,... (More)
In recent years, there has been a significant improvement in our ability to determine the ages of extensive short-lived magmatic events (large igneous provinces) dominated by rocks of mafic (silica-poor) compositions. This has been aided by targeting the magmatic feeders to these large igneous provinces (e.g., dykes and sills), which often host trace amounts of datable zirconium minerals such as baddeleyite and zircon. These age determinations are of great importance for unravelling the geological history and paleogeography of our planet. The Kaapvaal Craton in southern Africa hosts a rich and pristine geological history with many geological units and events in need of better age constraints. One is the Neoarchean Ventersdorp Supergroup, with the Klipriviersberg Group, and underlying successions of the Meso- to Neoarchean Witwatersrand Supergroup, of which the latter is world-wide known for its association with gold findings. By extracting baddeleyite from intrusive mafic sills from the Witwatersrand sediments, we can obtain critical age constraints for these successions using U–Pb geochronology on baddeleyite. Here we show that feeders to the Klipriviersberg Group are ca. 2790 Ma using ID-TIMS and complimentary LA-ICP-MS on baddeleyite from two mafic sills. This age makes the Klipriviersberg Group magmatic event almost 80 Myr older than previously thought, and is in agreement with several indirect studies which have suggested a similar age in Ventersdorp Supergroup temporally equivalent basins. Our results imply that there is a significant time gap in the Ventersdorp Supergroup between the deposition of the Klipriviersberg Group and the Platberg Group, and is associated with craton-wide mafic and felsic magmatism. This conclusion implies that the deposition of the Witwatersrand Supergroup ceased before 2790 Ma, which is when the eruption of the Klipriviersberg Group basalts commenced. Our results define a new large igneous province on the Kaapvaal Craton, and redefine the timing of both the beginning of Ventersdorp and termination of Witwatersrand supergroups. This new timing indicates that world class gold-bearing conglomerates within the Witwatersrand succession were deposited before 2790 Ma. (Less)
Popular Abstract (Swedish)
Åldersdatering innebär att man åldersbestämmer geologiska enheter och processer. Detta görs genom att mäta förhållandet mellan moder- och dotterisotop för radioaktiva grundämnen. Geokronologin är viktig för att kunna återskapa den geologiska historien för en kontinent. Man kan t.ex. erhålla tidsinformation kring när större landmassor bröts upp, genom att t.ex. identifiera och studera stora vulkaniska provinser, s.k. ”Large Igneous Provinces”, vilka anser kunna initiera uppsprickning av superkontinenter. Genom jordens historia har endast en begränsad mängd kontinental jordskorpa bevarats. En av dessa gamla och stabila delar av kontinental jordskorpa återfinns i södra Afrika och kallas Kaapvaalkratonen. Denna kraton har en omfattande och rik... (More)
Åldersdatering innebär att man åldersbestämmer geologiska enheter och processer. Detta görs genom att mäta förhållandet mellan moder- och dotterisotop för radioaktiva grundämnen. Geokronologin är viktig för att kunna återskapa den geologiska historien för en kontinent. Man kan t.ex. erhålla tidsinformation kring när större landmassor bröts upp, genom att t.ex. identifiera och studera stora vulkaniska provinser, s.k. ”Large Igneous Provinces”, vilka anser kunna initiera uppsprickning av superkontinenter. Genom jordens historia har endast en begränsad mängd kontinental jordskorpa bevarats. En av dessa gamla och stabila delar av kontinental jordskorpa återfinns i södra Afrika och kallas Kaapvaalkratonen. Denna kraton har en omfattande och rik geologisk historia, och ett stort antal geologiska enheter och händelser som ännu inte åldersbestämts. En av dessa utgörs av en ca 2800–2500 miljoner år gammal vulkanisk provins som kallas Ventersdorp Supergruppen vilken överlagrar guldbärande sedimentära lager tillhörande Witwatersrand Supergruppen. Ventersdorp Supergrupp-provinsen innefattar bl.a. Klipriviersberg Gruppen, som domineras av basaltlavor. Dessa basalter avsattes under några miljoner år när den underliggande berggrunden sprack upp och magma transporterades uppåt via vertikala sprickor. Åldersdatering av dessa kortlivade magmatiska händelser, så som avsättningen av just flodbasalter, är möjlig genom att man analyserar U–Pb förhållandet i uran-bärande mineral, t.ex. baddeleyit. Genom att extrahera och analysera baddeleyit från diabas-sillar i Witwatersrand-sedimenten, kan vi med hjälp av U–Pb geokronologi bedöma åldern på de sedimentära successionerna, samt på de överliggande Klipriviersberg flodbasalterna. Med hjälp av ID-TIMS samt kompletterande LA-ICP-MS visar detta arbete att baddeleyit från diabas tillhörande Klipriviersberg Gruppen att magmatism och vulkanism skedde för ca 2790 miljoner år. Detta innebär att Klipriviersberg Gruppen är uppemot 80 Ma äldre vad man tidigare trott. Resultaten antyder att det finns en hiatus på ca 50 miljoner år i Ventersdorp Supergruppen, mellan depositionen av Klipriviersberg Gruppen och de övriga grupperna. Detta innebär att depositionen av Witwatersrand Supergruppen upphörde innan ca 2790 Ma, vid tidpunkten då eruption av Klipriviersberg vulkaniterna inleddes. Dessa nya resultat definierar en ny vulkanisk provins på Kaapvaalkratonen, och omdefinierar tiden för såväl början av Ventersdorp och avslutningen på Witwatersrand Supergrupperna. Den nya åldern innebär att guldbärande sedimentära bergarter från Witwatersrand successionerna deponerades innan 2790 Ma. (Less)
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author
Stamsnijder, Joaen LU
supervisor
organization
course
GEOR02 20162
year
type
H2 - Master's Degree (Two Years)
subject
keywords
LA-ICP-MS, U–Pb geochronology, ID-TIMS, baddeleyite, Klipriviersberg Group, LIP, South Africa
publication/series
Dissertations in Geology at Lund University
report number
524
language
English
id
8927417
date added to LUP
2017-10-16 15:31:00
date last changed
2018-10-02 03:45:00
@misc{8927417,
  abstract     = {In recent years, there has been a significant improvement in our ability to determine the ages of extensive short-lived magmatic events (large igneous provinces) dominated by rocks of mafic (silica-poor) compositions. This has been aided by targeting the magmatic feeders to these large igneous provinces (e.g., dykes and sills), which often host trace amounts of datable zirconium minerals such as baddeleyite and zircon. These age determinations are of great importance for unravelling the geological history and paleogeography of our planet. The Kaapvaal Craton in southern Africa hosts a rich and pristine geological history with many geological units and events in need of better age constraints. One is the Neoarchean Ventersdorp Supergroup, with the Klipriviersberg Group, and underlying successions of the Meso- to Neoarchean Witwatersrand Supergroup, of which the latter is world-wide known for its association with gold findings. By extracting baddeleyite from intrusive mafic sills from the Witwatersrand sediments, we can obtain critical age constraints for these successions using U–Pb geochronology on baddeleyite. Here we show that feeders to the Klipriviersberg Group are ca. 2790 Ma using ID-TIMS and complimentary LA-ICP-MS on baddeleyite from two mafic sills. This age makes the Klipriviersberg Group magmatic event almost 80 Myr older than previously thought, and is in agreement with several indirect studies which have suggested a similar age in Ventersdorp Supergroup temporally equivalent basins. Our results imply that there is a significant time gap in the Ventersdorp Supergroup between the deposition of the Klipriviersberg Group and the Platberg Group, and is associated with craton-wide mafic and felsic magmatism. This conclusion implies that the deposition of the Witwatersrand Supergroup ceased before 2790 Ma, which is when the eruption of the Klipriviersberg Group basalts commenced. Our results define a new large igneous province on the Kaapvaal Craton, and redefine the timing of both the beginning of Ventersdorp and termination of Witwatersrand supergroups. This new timing indicates that world class gold-bearing conglomerates within the Witwatersrand succession were deposited before 2790 Ma.},
  author       = {Stamsnijder, Joaen},
  keyword      = {LA-ICP-MS,U–Pb geochronology,ID-TIMS,baddeleyite,Klipriviersberg Group,LIP,South Africa},
  language     = {eng},
  note         = {Student Paper},
  series       = {Dissertations in Geology at Lund University},
  title        = {New geochronological constraints on the Klipriviersberg Group: defining a new Neoarchean large igneous province on the Kaapvaal Craton, South Africa},
  year         = {2017},
}