U-Pb Ages on Single Detrital Zircon Grains from the Witwatersrand Basin, South Africa: Constraints on the Age of Sedimentation and on the Evolution of Granites Adjacent to the Basin
U-Pb Ages on Single Detrital Zircon Grains from the Witwatersrand Basin, South Africa: Constraints on the Age of Sedimentation and on the Evolution of Granites Adjacent to the Basin
复制标题
南非威特沃特斯兰德盆地单个碎屑锆石颗粒的 U-Pb 年龄：对沉积年龄和盆地附近花岗岩演化的限制
DOI：
10.1086/629407
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发表时间：
1990
期刊：
影响因子：
--
通讯作者：
S. Kamo
中科院分区：
文献类型：
--
作者：
L. Robb;D. Davis;S. Kamo
Dominion 和 Witwatersrand 序列中不同地层层位的单个碎屑锆石颗粒的 U-Pb 年龄提供了对沉积最大年龄的限制，并表明了向盆地提供碎屑的（花岗岩）源区的年龄分布模式。 Dominion 沉积物中的锆石年龄范围为 3191-3105 Ma，几何平均值 (\documentclass{aastex}
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\begin{document}$${\mathrm{\overline{X}}}$$\end{document}) 在 3153 Ma。来自 Witwatersrand 下部沉积物（West Rand Group）的那些范围为 3305-3044 Ma，\documentclass{aastex}
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\begin{document}$${\mathrm{\overline{X} = 3097\ Ma}}$$\end{document}，Witwatersrand 上部沉积物（中央兰德群）中的锆石年龄在 3207-2894 Ma 之间，年代为 \documentclass{aastex}
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\begin{文档}$${\mathrm{\overline{X} = 3053\ Ma}}$$\end{文档}。碎屑锆石的年龄在地层记录中普遍向上递减，<3000 Ma的老锆石仅见于中央兰德群。这一趋势意味着较年轻的花岗岩可能是在威特沃特斯兰德下部沉积之后的某个时间形成的，或者腹地的持续侵蚀导致较年轻的花岗岩相继剥落。数据集中明显的锆石年龄 (411 Ma) 分布广泛，表明 Witwatersrand 源区的花岗岩在大约 3300-2900 Ma 之间几乎连续形成。 45% 的锆石年龄在 30 m.y 以内。总数据集的几何平均值，表明主要的结壳事件发生在 $$3073 \pm 30 Ma$$。源区花岗岩类可分为（i）前Dominion基底； (ii) Dominion 花岗岩，其侵位与 Dominion 火山岩的挤压同时发生，(iii) Randian 花岗岩，其侵位与 Witwatersrand 沉积同步。这一系列事件支持了最近的构造模型，该模型将威特沃特斯兰德序列视为沉积在前陆盆地中。 Witwatersrand 序列中的不整合面沉积包被认为代表了对源区正在进行的构造岩浆事件的沉积响应。
U-Pb ages of single detrital zircon grains from various stratigraphic horizons in the Dominion and Witwatersrand sequences provide constraints on the maximum age of sedimentation as well as indicating the pattern of age distribution in the (granitoid) source area providing detritus into the basin. Zircon ages in the Dominion sediments range from 3191-3105 Ma with a geometric mean (\documentclass{aastex}
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\begin{document}$${\mathrm{\overline{X}}}$$\end{document}) at 3153 Ma. Those from the lower Witwatersrand sediments (West Rand Group) range from 3305-3044 Ma with \documentclass{aastex}
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\begin{document}$${\mathrm{\overline{X} = 3097\ Ma}}$$\end{document}, and zircons in the upper Witwatersrand sediments (Central Rand Group) are between 3207-2894 Ma old with \documentclass{aastex}
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\begin{document}$${\mathrm{\overline{X} = 3053\ Ma}}$$\end{document}. Ages of detrital zircons generally decrease upward in the stratigraphic record, and <3000 Ma old zircons are only found in the Central Rand Group. This trend implies that younger granites may have formed at some time subsequent to lower Witwatersrand deposition, or that continued erosion of the hinterland resulted in the unroofing of successively younger granites. The wide spread of zircon ages (411 Ma) evident in the data set indicates that granites formed virtually continuously between circa 3300-2900 Ma in the Witwatersrand source area. Of the zircon ages 45% fall within 30 m.y. of the geometric mean of the total data set, suggesting that a major crust-forming event occurred at $$3073 \pm 30 Ma$$. Granitoids in the source area can be divided into (i) pre-Dominion basement; (ii) Dominion granites, whose emplacement coincided with the extrusion of Dominion volcanics, and (iii) Randian granites, which were emplaced synchronously with Witwatersrand deposition. This sequence of events supports recent tectonic models that view the Witwatersrand sequence as having been deposited in a foreland basin. Unconformity-bound sedimentary packages in the Witwatersrand sequence are considered to represent the depositional responses to on-going tectono-magmatic events in the source area.