Abstract The timing of the emergence of modern-style plate tectonics on Earth is of fundamental importance in understanding the thermal and compositional history of the planet. Although the magmatism and metamorphism in the eastern Kaapvaal Craton was thought by some as geological records for mid-Archean subduction at 3.2 Ga, their petrogenesis was fiercely debated. To reveal the nature of the 3.2 Ga magmatism and metamorphism in the eastern Kaapvaal Craton, here we provide a comprehensive zircon and monazite U-Pb geochronological investigation, coupled with in situ trace element and Hf-O isotopic analyses, for the magmatic and metamorphic rocks from the Barberton granitoid-greenstone terrane in South Africa and Ancient Gneiss Complex in Swaziland. In spite of deriving from potentially different sources as revealed by different lithology, nine magmatic rocks sampled from different plutons have consistent zircon/monazite SIMS 207Pb/206Pb ages of 3240–3220 Ma. Distinct zircon grains, including inherited magmatic and metamorphic, are identified in the metamorphic rocks by detailed CL images and U-Pb geochronological, trace elemental and Hf-O isotopic analyses. The metamorphic zircons give consistent 207Pb/206Pb ages of 3240–3220 Ma. One granitic gneiss and a metapelite give monazite SIMS 207Pb/206Pb dates of 3230.6 ± 2.7 Ma and 3223.1 ± 2.4 Ma, respectively, which are consistent with the results of metamorphic zircon and interpreted as the age of metamorphism. Most metamorphic samples give Ti-in-zircon temperatures of 650–740 °C and one sample yields Y-in-monazite temperatures of 918–966 °C, which imply that the rocks of AGC were universally metamorphosed to upper amphibolite facies, and locally reaching high to ultrahigh temperature (HT-UHT) granulite facies (>900 °C) at 3240–3220 Ma. This study, in combination with previous results, suggests widespread distribution for the 3.2 Ga magmatism and metamorphism in the eastern Kaapvaal Craton without asymmetry in the thermal structure, which is quite different from those of the modern subduction zones. The synchronous occurrence of the metamorphism and magmatism in the eastern Kaapvaal Craton at 3.2 Ga was interpreted as the results of partial convective overturn of the crust.

中文翻译：

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广泛的岩浆作用和变质作用在大约。3.2 Kaapvaal Craton 东部的 Ga

摘要 地球上现代板块构造出现的时间对于理解地球的热和成分历史至关重要。尽管卡普瓦尔克拉通东部的岩浆作用和变质作用被一些人认为是 3.2 Ga 中太古代俯冲的地质记录，但它们的岩石成因引起了激烈的争论。为了揭示 Kaapvaal Craton 东部 3.2 Ga 岩浆作用和变质作用的性质，我们在此对岩浆岩和变质岩进行了全面的锆石和独居石 U-Pb 年代学调查，并结合了原位微量元素和 Hf-O 同位素分析来自南非的巴伯顿花岗岩-绿岩地体和斯威士兰的古片麻岩复合体。尽管来自不同岩性所揭示的潜在不同来源，从不同岩体中取样的九个岩浆岩具有一致的锆石/独居石 SIMS 207Pb/206Pb 年龄为 3240-3220 Ma。通过详细的 CL 图像和 U-Pb 年代学、痕量元素和 Hf-O 同位素分析，在变质岩中识别出不同的锆石颗粒，包括继承的岩浆和变质岩。变质锆石给出一致的 207Pb/206Pb 年龄为 3240-3220 Ma。一块花岗片麻岩和一块变泥质岩给出的独居石 SIMS 207Pb/206Pb 日期分别为 3230.6 ± 2.7 Ma 和 3223.1 ± 2.4 Ma，这与变质锆石的结果一致，并被解释为变质作用的年龄。大多数变质样品的锆石中钛温度为 650–740 °C，一个样品中独居石中的 Y 温度为 918–966 °C，这意味着 AGC 的岩石普遍变质为上角闪岩相，并在 3240-3220 Ma 局部达到高温到超高温 (HT-UHT) 麻粒岩相 (>900 °C)。本研究结合以往的研究结果表明，卡普瓦尔克拉通东部的 3.2 Ga 岩浆作用和变质作用广泛分布，热结构不对称，与现代俯冲带有很大不同。Kaapvaal Craton 东部 3.2 Ga 变质作用和岩浆作用的同步发生被解释为地壳部分对流翻转的结果。这与现代俯冲带完全不同。Kaapvaal Craton 东部 3.2 Ga 变质作用和岩浆作用的同步发生被解释为地壳部分对流翻转的结果。这与现代俯冲带完全不同。Kaapvaal Craton 东部 3.2 Ga 变质作用和岩浆作用的同步发生被解释为地壳部分对流翻转的结果。