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.

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