Abstract

The Imnyakh Formation (1500–1800 m thick), which is developed east of the Mama–Bodaibo Synclinorium (inner zone of the Patom Upland), comprises crystalline limestones (marbles) and carbonate shales (metamorphic marls). The carbon and strontium isotope compositions (δ¹³C_av = –9.0 ± 0.4‰, ⁸⁷Sr /⁸⁶Sr = 0.70810–0.70845) in these rocks are completely identical to those in unmetamorphosed limestones of the Zhuya Group (outer zone) and the Torgo Formation (adjacent part of the Siberian Platform), which are comparable with the global Ediacaran C isotope anomaly (Shuram–Wonoka event). Metamorphism was not accompanied by the removal of Sr whose content in marbles of the Imnyakh Formation are as high (up to 6290 μg/g) as in limestones of the Zhuya Group in the deepest parts of the Patom paleobasin. Thus, metamorphism was only manifested in the depletion of rocks with the heavy oxygen isotope by 3–4‰ (δ¹⁸O_av = 17.2 ± 1‰), relative to unmetamorphosed limestones. Carbonates of the pre-Imnyakh black carbonate-bearing shales and deep black crystalline limestones in the Khomolkha and Ugakhan formations are characterized by wide variations of δ¹³C (–4.0 to 7‰) and δ¹⁸O (15.2 to 23.4‰), indicating strong postsedimentary alterations. Nevertheless, their Early Vendian (Early Ediacaran) age is confirmed by such features as position in the section, abnormally high δ¹³C values, and the minimum value ⁸⁷Sr /⁸⁶Sr = 0.70753 recorded in a sample with the Sr content of 11700 μg/g. Although the isotope systems of carbonates in the Sogdiondon and Vitim formations (δ¹³C –14.2 to –3.7‰, δ¹⁸O 12.2 to 13.4‰, and 8⁸⁷Sr/⁸⁶Sr 0.71120 to 0.71337) occurring at the Neoproterozoic sequence base in the Zhuya River underwent even stronger modification, it is possible that δ¹³C values in these formations were initially negative. In terms of the C and O isotope compositions (δ¹³C_av = –9.5 ± 0.7‰, δ¹⁸O_av = 17.3 ± 1.8‰), crystalline limestones and carbonate shales in the metamorphosed analogs of the Zhuya Group from the Mama River mouth (west of the synclinorium) are similar to the Imnyakh Formation. Current problems of the correlation, age, duration, and origin of the largest in geological history negative C isotope anomaly are discussed.

中文翻译：

---

帕托姆高地内部（中西伯利亚）文迪安 Shuram-Wonoka 异常区和相关变质沉积岩的 C、O 和 Sr 同位素地球化学

摘要

Imnyakh 组（1500-1800 m 厚）发育在 Mama-Bodaibo Synclinorium（Patom Upland 内带）以东，包括结晶灰岩（大理石）和碳酸盐页岩（变质泥灰岩）。碳和锶同位素组成 (δ ¹³ C _av = –9.0 ± 0.4‰, ⁸⁷ Sr / ⁸⁶这些岩石中的 Sr = 0.70810–0.70845) 与朱亚群（外带）和 Torgo 组（西伯利亚地台的相邻部分）未变质灰岩中的完全相同，可与全球埃迪卡拉纪 C 同位素异常（舒拉姆）相媲美。 –Wonoka 事件）。变质作用并没有伴随着 Sr 的去除，Sr 在 Imnyakh 组大理岩中的含量与 Patom 古盆地最深处的朱亚群石灰岩中的含量一样高（高达 6290 μg/g）。因此，变质作用仅表现为重氧同位素损耗3-4‰(δ ¹⁸ O _av= 17.2 ± 1‰)，相对于未变质的石灰岩。Khomolkha 和 Ugakhan 地层中的前伊姆尼亚克黑色碳酸盐页岩和深黑色结晶灰岩的碳酸盐的特征是 δ ¹³ C（–4.0 至 7‰）和 δ ¹⁸ O（15.2 至 23.4‰）变化很大，表明强烈的后沉积变化。尽管如此，他们的早期文代（Ediacaran）年龄仍被断面位置、异常高的δ ¹³ C 值以及在 Sr 含量为 11700 μg 的样品中记录的最小值⁸⁷ Sr / ⁸⁶ Sr = 0.70753等特征所证实/G。尽管 Sogdiondon 和 Vitim 地层中碳酸盐的同位素系统 (δ ¹³ C –14.2 至 –3.7‰, δ ¹⁸O 12.2～13.4‰和8 ⁸⁷ Sr/ ⁸⁶ Sr 0.71120～0.71337)发生在珠牙河新元古代层序基部发生了更大的变化，可能这些地层中的δ ¹³ C值最初为负值。C、O同位素组成（δ ¹³ C _av = –9.5±0.7‰，δ ¹⁸ O _av = 17.3±1.8‰），玛玛河口珠崖群变质类似物中结晶灰岩和碳酸盐页岩（向斜层以西）类似于 Imnyakh 组。讨论了地质历史上最大的负C同位素异常的相关性、年龄、持续时间和起源等问题。