Abstract Middle Permian limestone and marl of the Maokou Formation deposited along the eastern Paleo-Tethys margin (South China) host abundant chert nodules that formed in association with the global Permian Chert Event (PCE). An enhanced understanding of the origin of the chert may help elucidate Middle Permian oceanic and diagenetic conditions. However, the mechanisms responsible for the formation of chert, especially the source of silica, remain controversial. Maokou chert nodules, which are made up of quartz and subordinate calcite, hematite, pyrite, and fluorite, and rimmed by F-bearing sepiolite laminae, appear to have formed mainly by the replacement of precursor nodular limestone by silica. The nodular limestone may have formed by differential compaction of interbedded limestone and marl, after which carbonate was replaced gradually outward from nodule centers by dissolved silica released from the dissolution of siliceous skeletal grains (including spicules and radiolarians) in host marl during early diagenesis. The introduction of high-T hydrothermal fluids into ocean water is evidenced by the presence of F-bearing minerals (fluorite and sepiolite), a hybrid rare earth element (REE) + Y pattern, and a wide range of δ30Si values (−0.4‰ to +0.7‰) documented from the Maokou nodular chert. Conservative estimation indicated that Middle Permian seawater may have had a δ30Si composition (+0.7‰ to +1.8‰) similar to that of modern open ocean water, suggesting that seawater was the dominant silica source of nodular chert. Mixing calculations based on REE + Y geochemistry suggest additional contributions of ca. 0.05–0.2% high-T hydrothermal fluid and ca. 1–10% of continental-derived freshwater to ocean water. Such increased influx of silica to seawater may have been related to the early rifting induced by Emeishan volcanism and intensified chemical weathering during an interglacial period, which played an important role in promoting the flourishing of siliceous organisms and subsequent chert enrichment during the Middle Permian.

中文翻译：

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东古特提斯边缘（华南）中二叠世灰岩中结节状燧石的海水和热液混合源

摘要 沿东古特提斯边缘（华南）沉积的茅口组中二叠世灰岩和泥灰岩含有丰富的燧石结核，这些燧石结核与全球二叠纪燧石事件（PCE）有关。对燧石起源的深入了解可能有助于阐明中二叠纪海洋和成岩条件。然而，导致燧石形成的机制，尤其是二氧化硅的来源，仍然存在争议。茅口燧石结核由石英和次要方解石、赤铁矿、黄铁矿和萤石组成，边缘是含 F 的海泡石薄片，似乎主要是由硅石取代前驱球状石灰岩形成的。球状灰岩可能是由灰岩和泥灰岩互层压实作用形成的，此后，碳酸盐从结核中心向外逐渐被早期成岩过程中寄主泥灰岩中硅质骨骼颗粒（包括针状体和放射虫）溶解释放的溶解二氧化硅所取代。将高 T 热液引入海水的证据是含 F 矿物（萤石和海泡石）、混合稀土元素 (REE) + Y 模式和大范围的 δ30Si 值 (-0.4‰)至 +0.7‰) 记录于茅口结节状燧石。保守估计表明，中二叠世海水可能具有与现代开阔海水相似的δ30Si成分（+0.7‰至+1.8‰），表明海水是结节状燧石的主要二氧化硅来源。基于 REE + Y 地球化学的混合计算表明大约有额外的贡献。0.05–0。2% 高 T 热液和约。1-10% 来自大陆的淡水变成了海水。海水中二氧化硅的大量涌入可能与峨眉山火山作用引起的早期裂谷作用和间冰期化学风化作用加剧有关，对中二叠世硅质生物的繁盛和随后的燧石富集起到了重要的促进作用。