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首页> 外文期刊>South African Journal of Geology: Being the Transaction of the Geological Society of South African: Syndie die Verhandelinge van die Geologiese Vereniging van Suid-Afrika >Microbeam U-Pb Zircon dating of the Makwassie Formation and underlying units in the Ventersdorp Supergroup of South Africa
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Microbeam U-Pb Zircon dating of the Makwassie Formation and underlying units in the Ventersdorp Supergroup of South Africa

机译：Microbeam U-PB Zircon在南非Ventersdorp Supergroup中的Makwassie形成和基础单位约会

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摘要

The Archaean Ventersdorp Supergroup is the most widespread volcanosedimentary cover sequence on the Kaapvaal Craton. It unconformably overlies the Witwatersrand Supergroup and is unconformably overlain by the Transvaal Supergroup. Due to exploration drilling for Witwatersrand gold, the Ventersdorp Supergroup is the globally most extensively drilled sequence. Early efforts to date its volcanic units have commonly been thwarted by pervasive 2.3 to 2.0 Ga alteration. In situ U-Pb zircon dating promises a panacea, but the effects of lead mobility and high common lead remain potential pitfalls. The ages of the 'type' formations of the Ventersdorp Supergroup sensu stricto are still not well established and much of the published data is on units only considered to be correlated with the 'type' formations. Quartz porphyry rhyolites of the Makwassie Formation in the Platberg Group are reliably dated by the laser ablation ICPMS U-Pb zircon method on four samples with a combined age of 2720 ± 2 Ma. This supercedes the first ion probe date of 2709 ± 4 Ma which is now regarded as a minimum age due to probable ancient lead loss and associated high common lead. The published 2714 ± 16 Ma age for the stratigraphically lower Klipriviersberg Group was affected by similar problems. The Makwassie Formation is the only formation of the Ventersdorp Supergroup sensu stricto for which an accurate and reliable date has now been established by more than one precise age determination. Two samples considered to be from the Goedgenoeg Formation, which underlies the Makwassie Formation, were investigated. The formation is best dated by a 2746 ± 9 Ma age on a feldspar porphyry borehole sample from near Wolmaransstad. A sample from a similar rock type also assigned to the Goedgenoeg Formation, which underlies the Makwassie Formation in Wesselton Mine at Kimberley, gave an age of 2781 ± 5 Ma. A 30 Ma period of Goedgenoeg volcanism is considered to be unlikely. The older sample may rather be a
机译：Archaean Ventersdorp Supergroup是Kaapvaal Craton上最广泛的Volcanateary覆盖序列。它不规则地覆盖Witwatersrand Supergroup，并且由TransVaal Supergroup不合格地覆盖。由于探索钻探Witwatersrand Gold，Ventersdorp超级组是全球最广泛的钻井顺序。迄今为止其火山组的早期努力通常被普遍存在2.3到2.0 GA更改挫败。原位U-PB Zircon约会承诺是一个灵丹妙药，但铅迁移率和高常见铅的影响仍然是潜在的陷阱。 Ventersdorp Supergroup Sensureco的“类型”的年龄仍然没有很好地建立，并且许多公布的数据仅被视为与“类型”的形成相关。普拉伯格组中Makwassie形成的石英斑岩流脉络受激光烧蚀ICPMS U-PB锆石型在四个样品中可靠地日期，组合年龄为2720±2 mA。这取代了2709±4 mA的第一离子探测日，现在被视为最低年龄，因为古代铅损失和相关的高普通铅。地层较低的Klipriviersberg组出版的2714±16 mA年龄受到类似问题的影响。 Makwassie形成是Ventersdorp超级Sensu Sticho的唯一形成，即现在通过一个精确的年龄决定确定了一个准确和可靠的日期。考虑到从Goedgenoeg形成的两种样品进行了调查基础，这是麦克斯维亚地区的形成。在Wolmaransstad附近的长石斑岩钻孔样品上，该形成最佳日期为2746±9 mA时代。来自类似岩石类型的样品，也分配给Goedgenoeg形成，这是在金伯利的Wesselton Mine中的Makwassie形成下来，达到了2781±5 mA。据认为，Goedgenoeg Volcanism的30 mA期间不太可能。较旧的样本可能宁愿是一个

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