SHRIMP baddeleyite and zircon ages for an Umkondo dolerite sill, Nyanga Mountains, Eastern Zimbabwe
SHRIMP baddeleyite and zircon ages for an Umkondo dolerite sill, Nyanga Mountains, Eastern Zimbabwe
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DOI：
10.2113/104.1.13
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发表时间：
2001-03
影响因子：
1.8
通讯作者：
M. Wingate
中科院分区：
文献类型：
--
作者：
M. Wingate
被称为Umkondo粗粒岩的广泛的镁铁质岩床侵入Umkondo群的沉积地层，该地层覆盖在津巴布韦克拉通的东缘。从尼扬加地区厚岩床上接触面附近采集的粗粒长英质辉长岩样品中回收了锆石和复合斜锆石晶体。斜锆石的~（207）Pb/2°Pb年龄为1098 ± 10 Ma。斜锆石上的单颗粒锆石和锆石增生的分析是一致的，强烈不一致的，他们的分散性表明，他们经历了一个以上的事件的放射成因铅损失。四个最小不一致锆石的207 Pb/2°Pb年龄为1103 ± 32 Ma。根据斜锆石和四个最协调的锆石的数据，岩床结晶年龄的最佳估计为1099 ± 9 Ma。锆石增生具有同心自形环带，高和可变的U和Th含量和Th/U比值。这些特征与来自同一样品的单锆石的特征相似，并且是粗玄岩侵入体中晚结晶锆石的典型特征，表明在岩床初始冷却期间，过生长物与单锆石同时形成。考虑到最近铅损失的可能性，对以前从Chimanimani地区类似的粗玄岩岩床中获得的锆石的不一致U-Pb数据进行重新解释，表明年龄为1107 ± 6 Ma。结合两个样品的同位素年龄，估计津巴布韦东部乌姆孔多岩浆事件的年龄为1105 ± 5 Ma。在这项研究中的窗台已经产生了独特的Umkondo古地磁方向，因此Umkondo古磁极的年代精确地在1105马，并可以使用增加的信心，在确定晚中元古代的卡拉哈里盆地的位置。
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.