Abstract Epidosites are suggested to form in up-flow zones located at the base of ore-forming oceanic hydrothermal systems, and are commonly preserved in ophiolitic sequences. However, occurrences of epidosites in Archean tectonic melanges are rarely documented. In this study, we report the presence of late Neoarchean epidosites in the Zanhuang ophiolitic melange in the North China Craton using meter-scale litho-structural and petrologic analysis, zircon U-Pb dating, major and trace element, and oxygen and strontium isotopic geochemistry. Field studies show that the epidosites are mainly preserved as the cores of meta-pillow lavas, and are distributed as blocks in different matrices within the tectonic melange. The pillow rims are used as protolith to estimate the element mobility during the alteration process. The epidosites display strong depletion in Na2O (0.01–0.27 wt%), MgO (0.43–3.01 wt%), depletion in large ion lithophile elements (LILE) (e.g. Rb, Ba, Cs, U) and transition metals (e.g., Co, Sc, Cu, Ni, Zn), and show enrichment of CaO (13.35–21.0 wt%), Sr and Pb. The high field strength elements (HFSE) (e.g. Zr, Hf, Ta, Th) of epidosites are varied but rare earth elements (REE) are unchanged. The amphibolitic pillow rims are characterized by enrichment of LILE, flat heavy rare earth elements (HREE) patterns, and depletion in HFSE with negative Ti anomalies and positive Pb and Ba anomalies. Oxygen and strontium isotopic ratios of epidosites are 5.0–8.6‰ and 0.70284–0.70551, respectively, both of which are greater than those of fresh basalts. The major and trace elements, oxygen and strontium isotopic values and extensive element migration indicate that the protolith was altered by a submarine hydrothermal system in the absence of continental influence. Magmatic zircons from two samples of epidosites yield 207Pb/206Pb ages of 2512 ± 13 Ma (MSWD = 0.51) and 2542 ± 13 Ma (MSWD = 1.4), confirming that the epidosites were generated at ca. 2.51–2.54 Ga. Based on geochemical data and field observations, we suggest that the protolith of the epidosites formed in an arc-related tectonic setting during the late Neoarchean. This study shows that hydrothermal activity operating in the late Neoarchean was similar to that of the modern ocean floor.

中文翻译：

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华北克拉通赞皇蛇绿混杂岩玄武岩新太古代海底热液变质作用

摘要 绿帘石形成于成矿海洋热液系统底部的上升流带，通常保存在蛇绿岩层序中。然而，在太古代构造混杂岩中出现绿帘石的记录很少。在这项研究中，我们使用米级岩石结构和岩石学分析、锆石 U-Pb 定年、主要和微量元素以及氧和锶同位素地球化学报告了华北克拉通赞黄蛇绿混杂岩中晚新太古代绿泥石的存在. 野外研究表明，绿帘岩主要保存为变枕状熔岩的核心，并在构造混杂岩内的不同基质中呈块状分布。枕边用作原石来估计蚀变过程中的元素迁移率。绿帘石显示出 Na2O (0.01–0.27 wt%)、MgO (0.43–3.01 wt%)、大离子亲石元素 (LILE)（例如 Rb、Ba、Cs、U）和过渡金属（例如 Co 、Sc、Cu、Ni、Zn），并显示出 CaO（13.35–21.0 wt%）、Sr 和 Pb 的富集。绿帘石的高场强元素（HFSE）（例如Zr、Hf、Ta、Th）变化多端，但稀土元素（REE）不变。角闪石枕缘的特征是 LILE 富集、平坦的重稀土元素 (HREE) 模式和 HFSE 的耗尽与负 Ti 异常和正 Pb 和 Ba 异常。绿帘石的氧和锶同位素比分别为5.0-8.6‰和0.70284-0.70551，均大于新鲜玄武岩。主要和微量元素，氧和锶同位素值以及广泛的元素迁移表明，原岩在没有大陆影响的情况下被海底热液系统改变。来自两个绿帘石样品的岩浆锆石产生 207Pb/206Pb 年龄为 2512 ± 13 Ma (MSWD = 0.51) 和 2542 ± 13 Ma (MSWD = 1.4)，证实绿帘石产于约 2.51-2.54 Ga。根据地球化学数据和野外观测，我们认为绿帘岩的原岩形成于新太古代晚期与弧相关的构造环境中。这项研究表明，新太古代晚期的热液活动与现代海底的热液活动相似。来自两个绿帘石样品的岩浆锆石产生 207Pb/206Pb 年龄为 2512 ± 13 Ma (MSWD = 0.51) 和 2542 ± 13 Ma (MSWD = 1.4)，证实绿帘石产于约 2.51-2.54 Ga。根据地球化学数据和野外观测，我们认为绿帘岩的原岩形成于新太古代晚期与弧相关的构造环境中。这项研究表明，新太古代晚期的热液活动与现代海底的热液活动相似。来自两个绿帘石样品的岩浆锆石产生 207Pb/206Pb 年龄为 2512 ± 13 Ma (MSWD = 0.51) 和 2542 ± 13 Ma (MSWD = 1.4)，证实绿帘石产于约 2.51-2.54 Ga。根据地球化学数据和野外观测，我们认为绿帘岩的原岩形成于新太古代晚期与弧相关的构造环境中。这项研究表明，新太古代晚期的热液活动与现代海底的热液活动相似。