Abstract The present Yangtze Block was proposed to be formed as a unified block in the Mesoproterozoic-Neoproterozoic transition, but the detailed timing and process are poorly constrained. Here we report integrated in situ U–Pb and Hf–O isotopic analyses of detrital zircons from the Xiaoxikou metamorphosed sedimentary rocks of the Miaowan Complex in the Yangtze Block, as well as U–Th–Pb and Nd–O isotopic analyses of magmatic monazites from an undeformed and unmetamorphosed leucocratic vein intruding these metasedimentary rocks. U–Pb dating results for detrital zircons from the metasedimentary rocks display two main age peaks at ca 970 and 900 Ma. The 970 Ma zircons are dominantly characterized by mantle-like δ18Ozir values (4.5–5.8‰) and highly depleted eHf(t) values (+5 to +14). The 900 Ma zircons mostly have O and Hf isotopic features similar to the 970 Ma zircons, except relative small proportion of grains with high δ18O values (6.0–9.2‰) and variable eHf(t) values (–33 to +7). Monazites from the vein are dated at ca 850 Ma and have uniform δ18O values (5.4–6.6‰), but significantly enriched eNd(t) values (–14.5 to –12.6). Based on the new isotopic results and immature lithology, the Xiaoxikou metasedimentary rocks are interpreted to be deposited between 900 Ma and 850 Ma in a fore-arc basin, and clastic materials derived from 970–900 Ma arc rocks were related to the northward subduction (present-day coordinates) of the Miaowan oceanic plate. The 850 Ma leucocratic vein shows geological and isotopic characteristics consistent with local 860–840 Ma post-tectonic granitic plutons, and may represent a fractionated product of local coeval granitic magmas. We propose that the closure of the Miaowan oceanic basin and formation of the current unified Yangtze Block occurred between 900 Ma and 860 Ma, i.e., later than the deposition of the fore-arc metasedimentary rocks while earlier than the emplacement of the post-tectonic plutons.

中文翻译：

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从庙湾杂岩变质沉积岩解码扬子地块早新元古代组合

摘要 目前扬子地块被提出形成为中元古代—新元古代过渡期的一个统一地块，但具体的时间和过程约束较差。在这里，我们报告了扬子地块庙湾杂岩小溪口变质沉积岩碎屑锆石的原位 U-Pb 和 Hf-O 同位素综合分析，以及岩浆独居石的 U-Th-Pb 和 Nd-O 同位素分析来自未变形和未变质的白斑静脉侵入这些变质沉积岩。来自变沉积岩的碎屑锆石的 U-Pb 测年结果显示在大约 970 和 900 Ma 的两个主要年龄峰值。970 Ma 锆石的主要特征是类似地幔的 δ18Ozir 值 (4.5–5.8‰) 和高度耗尽的 eHf(t) 值 (+5 到 +14)。900 Ma 锆石大多具有与 970 Ma 锆石相似的 O 和 Hf 同位素特征，但具有较高 δ18O 值（6.0–9.2‰）和可变 eHf（t）值（–33 到 +7）的颗粒比例相对较小。来自矿脉的独居石的年代为大约 850 Ma，具有均匀的 δ18O 值 (5.4–6.6‰)，但 eNd(t) 值显着丰富（–14.5 到 –12.6）。根据同位素新结果和未成熟岩性，小溪口变质沉积岩被解释为弧前盆地900~850 Ma沉积，970~900 Ma的弧岩碎屑物质与向北俯冲有关。庙湾大洋板块的现代坐标。850 Ma 白质脉显示出与当地 860-840 Ma 后构造花岗岩岩体一致的地质和同位素特征，并且可能代表当地同时期花岗岩岩浆的分馏产物。我们认为庙湾洋盆的闭合和目前统一的扬子地块的形成发生在900-860 Ma之间，即晚于前弧变质沉积岩的沉积，早于后构造岩体的就位。 .