Deglaciation following the Sturtian Ice Age led to widespread manganese carbonate precipitates within marine mudrocks, reflecting unique climatic and oceanographic changes. The Mn carbonates hold economic importance in the Datangpo Formation, Yangtze Block, South China. Although their genesis in the Nanhua Rift Basin is well understood, little is known regarding their formation mechanism in the North Margin Rift Basin (NMRB). With growing interest in Mn ore exploration in this region, this study examines the weathering trend, climate patterns and watermass conditions using a variety of geochemical proxies. The results reveal two phases of climate variation during the deglaciation. Climate during the early phase (Phase I) was unstable, characterized by fluctuating values of Chemical Index of Alteration (CIA = 65 ± 4) and elevated Li isotope (8.8 ± 3.0‰) patterns. The degree of chemical weathering during the later stage (Phase II) was relatively stable (CIA = 65 ± 1, δLi = 5.4 ± 2.7‰), but the weathering intensity increased due to the reduced denudation rate, possibly related to a shift in the basin development. Fe speciation data indicate euxinia prevailing much of the section. However, Mo and U enrichment (> 1) and fluctuating ratios of Mo versus total organic content (0–40) suggest that pulses of freshwater influx associated with climate variation during Phase I caused the fluctuation of the chemocline along the continental shelf, facilitating the formation of Mn carbonates through the particulate Fe-Mn-hydroxide shuttle. With the establishment of stabilized climate, basin tectonics, and watermass circulation during Phase II, the accumulation of Mn carbonates in the sediments became less significant. The accumulation of Mn carbonates in the NMRB conforms to the “bathtub ring model”, distinct from the metallogenic model in the NRB, providing valuable guidance for Mn ore prospecting within this area and elsewhere with similar settings.

中文翻译：

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扬子地块北缘裂谷盆地大塘坡组泥岩中碳酸盐的形成控制

斯图尔蒂安冰河时代之后的冰川消退导致海洋泥岩中广泛存在碳酸锰沉淀物，反映了独特的气候和海洋变化。碳酸盐锰矿在华南扬子地块大塘坡组具有重要的经济价值。尽管它们在南华裂谷盆地的成因已广为人知，但对其在北缘裂谷盆地（NMRB）的形成机制却知之甚少。随着人们对该地区锰矿勘探的兴趣日益浓厚，本研究利用各种地球化学指标研究了风化趋势、气候模式和水体条件。结果揭示了冰消期气候变化的两个阶段。早期阶段（第一阶段）的气候不稳定，其特征是化学蚀变指数值波动（CIA=65±4）和锂同位素升高（8.8±3.0‰）模式。后期（第二阶段）化学风化程度相对稳定（CIA=65±1，δLi=5.4±2.7‰），但由于剥蚀率降低，风化强度增加，可能与盆地开发。铁形态数据表明该剖面的大部分地区普遍存在euxinia。然而，Mo 和 U 富集度 (> 1) 以及 Mo 与总有机物含量的波动比率 (0-40) 表明，第一阶段期间与气候变化相关的淡水流入脉冲引起了大陆架趋化素的波动，促进了通过颗粒状 F​​e-Mn-氢氧化物梭形成锰碳酸盐。随着第二阶段气候、盆地构造和水团循环的稳定建立，沉积物中碳酸锰的积累变得不那么重要。 NMRB中锰碳酸盐的富集符合“浴盆环模型”，与NRB的成矿模型不同，为该地区和其他类似环境的锰矿找矿提供了有价值的指导。