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SHRIMP baddeleyite and zircon ages for an Umkondo dolerite sill, Nyanga Mountains, Eastern Zimbabwe

2001; Geological Society of South Africa; Volume: 104; Issue: 1 Linguagem: Inglês

10.2113/104.1.13

1996-8590

M.T.D. Wingate,

Geochemistry and Geologic Mapping

Research Article| March 01, 2001 SHRIMP baddeleyite and zircon ages for an Umkondo dolerite sill, Nyanga Mountains, Eastern Zimbabwe M.T.D. Wingate M.T.D. Wingate Tectonics Special Research Centre, Department of Geology and Geophysics, The University of Western Australia, 35 Stirling Highway, Crawley, WA 6009, Australia., e-mail: mwingate@tsrc.uwa.edu.au Search for other works by this author on: GSW Google Scholar Author and Article Information M.T.D. Wingate Tectonics Special Research Centre, Department of Geology and Geophysics, The University of Western Australia, 35 Stirling Highway, Crawley, WA 6009, Australia., e-mail: mwingate@tsrc.uwa.edu.au Publisher: Geological Society of South Africa First Online: 07 Mar 2017 Online ISSN: 1996-8590 Print ISSN: 1012-0750 © 2001 Geological Society of South Africa South African Journal of Geology (2001) 104 (1): 13–22. https://doi.org/10.2113/104.1.13 Article history First Online: 07 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn Email Permissions Search Site Citation M.T.D. Wingate; SHRIMP baddeleyite and zircon ages for an Umkondo dolerite sill, Nyanga Mountains, Eastern Zimbabwe. South African Journal of Geology 2001;; 104 (1): 13–22. doi: https://doi.org/10.2113/104.1.13 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietySouth African Journal of Geology Search Advanced Search Abstract Extensive mafic sills known as the Umkondo dolerites intrude sedimentary strata of the Umkondo Group, which overlie the eastern margin of the Archaean Zimbabwe craton. Zircons and composite baddeleyite-zircon crystals were recovered from a sample of coarse-grained felsic gabbro collected near the upper contact of a thick sill in the Nyanga area. Ion microprobe analyses of baddeleyite yield a mean 207Pb/2°Pb age of 1098 ±10 Ma. Analyses of single zircons and zircon overgrowths on baddeleyite are concordant to strongly discordant, and their dispersion suggests that they have• experienced more than one episode of radiogenic Pb loss. The four least discordant zircon analyses indicate a mean 207Pb/2°Pb age of 1103 ±32 Ma. The best estimate of the age of crystallisation of the sill, based on data for baddeleyite and the four most concordant zircons, is 1099 ±9 Ma. Zircon overgrowths exhibit concentric euhedral zoning, and high and variable U and Th contents and Th/U ratios. These characteristics are similar to those of single zircons from the same sample, and are typical of late-crystallising zircon in dolerite intrusions, indicating that the overgrowths formed at the same time as the single zircons, during initial cooling of the sill. A re-interpretation of discordant U-Pb data obtained previously for zircons from a similar dolerite sill in the Chimanimani area, taking into account the likelihood of recent Pb loss, indicates an age of 1107 ±6 Ma. Combining isotopic ages from both samples yields an estimate of 1105 ±5 Ma for the Umkondo magmatic event in eastern Zimbabwe. The sill dated in this study has yielded characteristic Umkondo palaeomagnetic directions, therefore the Umkondo palaeopole is dated precisely at 1105 Ma, and can be used with increased confidence in defining the late Mesoproterozoic position of the Kalahari Craton. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.