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When did the Kalahari craton form? : constraints from baddeleyite U-Pb geochronology and geo-chemistry of mafic intrusions in the Kaapvaal and Zimbabwe cratons

März, Nadine (2010) In Dissertations in Geology at Lund University
Department of Geology
Abstract (Swedish)
Nordost om Bushveldkomplexen i Kaapvaal kratonen finns en mäktig gångsvärm av nordost-orienterade diabasgångar tillhörande ”the Black Hills dyke swarm”. Med hjälp av U- Pb metoden av mineral baddeleyit daterades fem gångar inom ålderspannet 1.87 - 1.85 Ga, där två av dem ger exakta åldrar på 1852 ± 5 Ma och 1863 ± 7 Ma. Denna gångsvärm är alltså lika gammal som lagergångar tillhörande ”post- Waterberg sills” (1.88 - 1.87 Ga), basalter tillhörande ”Soutpansberg” i norra Kaapvaal och Mashonaland lagergångar (1.89- 1.86 Ga) i Zimbabwe och Botswana. Kaapvaal- och Zimbabwe-kratonerna har således ”minnen” av tre gemensamma och regionala magmatiska händelser vid: 1.99-1-85 Ga (ovan nämnda enheter), ca. 1.1 Ga (Umkondo) samt ca. 0.18 Ga (Karoo).... (More)
Nordost om Bushveldkomplexen i Kaapvaal kratonen finns en mäktig gångsvärm av nordost-orienterade diabasgångar tillhörande ”the Black Hills dyke swarm”. Med hjälp av U- Pb metoden av mineral baddeleyit daterades fem gångar inom ålderspannet 1.87 - 1.85 Ga, där två av dem ger exakta åldrar på 1852 ± 5 Ma och 1863 ± 7 Ma. Denna gångsvärm är alltså lika gammal som lagergångar tillhörande ”post- Waterberg sills” (1.88 - 1.87 Ga), basalter tillhörande ”Soutpansberg” i norra Kaapvaal och Mashonaland lagergångar (1.89- 1.86 Ga) i Zimbabwe och Botswana. Kaapvaal- och Zimbabwe-kratonerna har således ”minnen” av tre gemensamma och regionala magmatiska händelser vid: 1.99-1-85 Ga (ovan nämnda enheter), ca. 1.1 Ga (Umkondo) samt ca. 0.18 Ga (Karoo). Ålderspassningar äldre än 1.9 Ga förekommer däremot inte. Dessa resultat indikerar Zimbabwe och Kaapvaal tillhörde ett och samma block från 1.9 Ga till idag, och att Kalaharikratonen inte existerade förrän 2.0 Ga.

Kemidata av 28 gångar från Black Hills svärmen och från 2 Mashonaland lagergångar i Zimbabwe analyserades med avseende på huvud- och spårelement. Tidigare dateringar (Johan Olsson, in prep.) visar att vissa gångar tillhörande Black Hills svärmen är betydligt äldre, cirka 2.7 Ga, än de som daterats i dennta studie. Geokemidata för dessa olika generationer av diabas är sammantaget snarlika, vilket skulle kunna betyda att de har bildats genom uppsmältning av liknande ursprungsmaterial (mantel). Datan indikerar också att båda generationerna bildades genom uppsmältning från relativt grunda djup (spinell-förande mantel) och att de kemiska sammansättningarna pekar på fraktionering av plagioklas, Mg-rika mineral (olivin och pyroxen) och apatit. Proverna innehåller höga halter inkompatibla spårelement, och härstammar därför troligen från en mantelkälla som är mer anrikad än en MORB källa. Anrikning av LILE och negativa Nb- Ta avvikelser visar kontaminering och/eller partiell smältning av litosfärsfäriska manteln tillhörande Kaapvaalkratonen. (Less)
Abstract
The NE-trending Black Hills dolerite dykes make up a prominent swarm northeast of the Bushveld Igneous Complex in the Kaapvaal craton. Baddeleyite U-Pb dates of five dykes suggest emplacement ages between ca. 1.87 Ga and 1.85 Ga, with two samples yielding robust ages of 1852 ± 5 Ma and 1863 ± 7 Ma. The Black Hills swarm is thus largely coeval with the post-Waterberg dolerite sills (1.88-1.87 Ga) and basalts of the Soutpansberg Group in northern Kaapvaal as well as with the extensive Mashonaland sill complex (1.89-1.86 Ga) that is abundant across Zimbabwe and Botswana. Together, these intrusions and extrusions manifest a regional-scale extensional event that is common in both the Kaapvaal and Zimbabwe cratons. Additional, younger events... (More)
The NE-trending Black Hills dolerite dykes make up a prominent swarm northeast of the Bushveld Igneous Complex in the Kaapvaal craton. Baddeleyite U-Pb dates of five dykes suggest emplacement ages between ca. 1.87 Ga and 1.85 Ga, with two samples yielding robust ages of 1852 ± 5 Ma and 1863 ± 7 Ma. The Black Hills swarm is thus largely coeval with the post-Waterberg dolerite sills (1.88-1.87 Ga) and basalts of the Soutpansberg Group in northern Kaapvaal as well as with the extensive Mashonaland sill complex (1.89-1.86 Ga) that is abundant across Zimbabwe and Botswana. Together, these intrusions and extrusions manifest a regional-scale extensional event that is common in both the Kaapvaal and Zimbabwe cratons. Additional, younger events common in both cratons are the ca. 1.1 Ga Umkondo and ca. 0.18 Ga Karoo large igneous provinces, suggesting that the Kaapvaal and Zimbabwe cratons have been nearest neighbours from at least 1.9 Ga to present time. In contrast, not a single common event older than 1.9 Ga has been recorded suggesting that the Kalahari craton was not formed until ca. 2.0 Ga.

Recent U-Pb dating has revealed the presence of older dykes, approximately 2.7 Ga in age (Johan Olsson, unpublished data), intermixed with the ca. 1.87– 1.85 Ga dykes of the Black Hills swarm. Geochemistry of 28 dykes of the Black Hills swarms and of 2 Mashonaland sills in Zimbabwe were analysed with respect to both major and trace elements. Geochemical data indicate that each generation of dykes can be petrogenetically related. There are no significant differences between the 2.7 Ga and ca. 1.87– 1.85 Ga dykes, but more so between more evolved and primitive dykes within each group. It is possible that primary melts were generated at relatively shallow (from the spinel stability field) mantle depths and that this primary melts subsequently experienced shallow crustal fractionation of, at least, plagioclase, some Mg-rich phase(s) and apatite. Relatively high concentrations of most incompatible elements suggest that the mantle source was more enriched than a normal MORB source. Any additional enrichment in large-ionic lithophile elements and negative Nb-Ta anomalies can be ascribed to contamination and/or partial melting of the Kaapvaal craton lithosphere. (Less)
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author
März, Nadine
supervisor
organization
alternative title
Baddeleyit U-Pb dateringar och geokemi av mafiska intrusioner i Kaapvaal (Sydafrika) och Zimbabwe med implikationer för när Kalahari bildades
year
type
H2 - Master's Degree (Two Years)
subject
keywords
geography, geology, Kalahari craton, baddeylite U-Pb geochronology, mafic intrusions, Kaapvaal craton, Zimbabwe craton
publication/series
Dissertations in Geology at Lund University
report number
268
language
English
additional info
Martin Klausen, University of Stellenbosch, South Africa.
id
2278657
date added to LUP
2012-01-10 15:10:12
date last changed
2012-01-10 15:10:12
@misc{2278657,
  abstract     = {The NE-trending Black Hills dolerite dykes make up a prominent swarm northeast of the Bushveld Igneous Complex in the Kaapvaal craton. Baddeleyite U-Pb dates of five dykes suggest emplacement ages between ca. 1.87 Ga and 1.85 Ga, with two samples yielding robust ages of 1852 ± 5 Ma and 1863 ± 7 Ma. The Black Hills swarm is thus largely coeval with the post-Waterberg dolerite sills (1.88-1.87 Ga) and basalts of the Soutpansberg Group in northern Kaapvaal as well as with the extensive Mashonaland sill complex (1.89-1.86 Ga) that is abundant across Zimbabwe and Botswana. Together, these intrusions and extrusions manifest a regional-scale extensional event that is common in both the Kaapvaal and Zimbabwe cratons. Additional, younger events common in both cratons are the ca. 1.1 Ga Umkondo and ca. 0.18 Ga Karoo large igneous provinces, suggesting that the Kaapvaal and Zimbabwe cratons have been nearest neighbours from at least 1.9 Ga to present time. In contrast, not a single common event older than 1.9 Ga has been recorded suggesting that the Kalahari craton was not formed until ca. 2.0 Ga. 

Recent U-Pb dating has revealed the presence of older dykes, approximately 2.7 Ga in age (Johan Olsson, unpublished data), intermixed with the ca. 1.87– 1.85 Ga dykes of the Black Hills swarm. Geochemistry of 28 dykes of the Black Hills swarms and of 2 Mashonaland sills in Zimbabwe were analysed with respect to both major and trace elements. Geochemical data indicate that each generation of dykes can be petrogenetically related. There are no significant differences between the 2.7 Ga and ca. 1.87– 1.85 Ga dykes, but more so between more evolved and primitive dykes within each group. It is possible that primary melts were generated at relatively shallow (from the spinel stability field) mantle depths and that this primary melts subsequently experienced shallow crustal fractionation of, at least, plagioclase, some Mg-rich phase(s) and apatite. Relatively high concentrations of most incompatible elements suggest that the mantle source was more enriched than a normal MORB source. Any additional enrichment in large-ionic lithophile elements and negative Nb-Ta anomalies can be ascribed to contamination and/or partial melting of the Kaapvaal craton lithosphere.},
  author       = {März, Nadine},
  keyword      = {geography,geology,Kalahari craton,baddeylite U-Pb geochronology,mafic intrusions,Kaapvaal craton,Zimbabwe craton},
  language     = {eng},
  note         = {Student Paper},
  series       = {Dissertations in Geology at Lund University},
  title        = {When did the Kalahari craton form? : constraints from baddeleyite U-Pb geochronology and geo-chemistry of mafic intrusions in the Kaapvaal and Zimbabwe cratons},
  year         = {2010},
}