Abstract We present 40Ar/39Ar ages and boron isotopes of tourmalines from a quartz-tourmaline intrusion cross-cutting the Hadean detrital-zircon-bearing metasedimentary rocks from the Mt. Alfred and Brooking Hills localities of the Illaara Greenstone Belt, Western Australia. Concordant 40Ar/39Ar plateau ages on the tourmalines at Mt. Alfred give a weighted mean age of 2935 ± 9 Ma (2σ). These results are the first reported Archean 40Ar/39Ar ages directly obtained on tourmaline, and provide a minimum depositional age for this sequence, placing constraints on the depositional timing of detrital Hadean zircons to between 2935 ± 9 Ma and c.a. 3250 Ma (youngest detrital zircon age in sequence). This is the last known occurrence of abundant detrital Hadean zircons having been directly deposited in a sedimentary sequence, and thus suggests that most Hadean zircon sources have been exhausted by this time. The 2935 ± 9 Ma tourmalines occur within a quartz-tourmaline intrusion with associated stratiform layers and veins, inferred to have grown during a hydrothermal fluid circulation event. A younger generation of tourmalines from the Brooking Hills, 30 km south of Mt. Alfred yielded concordant 40Ar/39Ar plateau ages with a weighted mean age of 2624 ± 16 Ma. These tourmalines occur within and along the margins of post-kinematic quartz veins, parallel to the foliation fabric and marks the end stage of high temperature tectono-thermal events within the Illaara Greenstone Belt. The retention of 2935 ± 9 Ma tourmaline ages within the Illaara Greenstone Belt shows that the K/Ar system of tourmaline remained closed throughout repeated upper-greenschist metamorphic events (∼450 °C) between the ages of c.a. 2930 to 2630 Ma in contrast to muscovite recording plateau ages of about 2600 Ma. This is in agreement with an approximate Ar closure temperature between ∼534 and 628 °C experimentally determined in this study.

中文翻译：

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电气石 40Ar/39Ar 年代学和热年代学：来自伊尔加恩克拉通最年轻的含锆石的硅质碎屑变沉积岩示例

摘要 我们展示了石英电气石侵入体中电气石的 40Ar/39Ar 年龄和硼同位素，该侵入体横切了来自 Mt. 的 Hadean 碎屑-锆石变质沉积岩。西澳大利亚州 Illaara 绿岩带的阿尔弗雷德和布鲁金山地区。山上碧玺的一致 40Ar/39Ar 高原年龄。阿尔弗雷德给出的加权平均年龄为 2935 ± 9 Ma (2σ)。这些结果是首次报道的直接在电气石上获得的太古代 40Ar/39Ar 年龄，并提供了该序列的最小沉积年龄，将碎屑冥河锆石的沉积时间限制在 2935 ± 9 Ma 和约 3250 Ma（最年轻的碎屑锆石年龄顺序）。这是已知的最后一次在沉积层序中直接沉积的大量碎屑冥河锆石，从而表明此时大部分冥界锆石资源已经枯竭。2935 ± 9 Ma 的碧玺出现在石英-碧玺侵入体内，并带有相关的层状层和脉，推测是在热液循环事件中生长的。布鲁金山以南 30 公里的年轻一代碧玺。阿尔弗雷德得出一致的 40Ar/39Ar 高原年龄，加权平均年龄为 2624±16 Ma。这些碧玺出现在后运动石英脉的内部和边缘，平行于叶理结构，标志着 Illaara 绿岩带内高温构造热事件的结束阶段。Illaara 绿岩带内 2935 ± 9 Ma 电气石年龄的保留表明，在大约 2930 至 2630 Ma 年龄之间的重复上绿片岩变质事件（~450 °C）中，电气石的 K/Ar 系统保持封闭白云母记录的高原年龄约为 2600 Ma。这与本研究中通过实验确定的大约 534 至 628°C 之间的近似 Ar 闭合温度一致。