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C, O, and Sr Isotope Geochemistry of the Vendian Shuram–Wonoka Anomaly and Associated Metasedimentary Rocks in the Inner Part of the Patom Upland (Central Siberia)

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 (δ 13 C av = –9.0 ± 0.4‰, 87 Sr / 8...

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Published in:Lithology and Mineral Resources 2021-09, Vol.56 (5), p.390-417
Main Authors: Pokrovsky, B. G., Bujakaite, M. I., Kolesnikova, A. A., Petrov, O. L., Khlebnikov, M. S.
Format: Article
Language:English
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Summary: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 (δ 13 C av = –9.0 ± 0.4‰, 87 Sr / 86 Sr = 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‰ (δ 18 O 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 δ 13 C (–4.0 to 7‰) and δ 18 O (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 δ 13 C values, and the minimum value 87 Sr / 86 Sr = 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 (δ 13 C –14.2 to –3.7‰, δ 18 O 12.2 to 13.4‰, and 8 87 Sr/ 86 Sr 0.71120 to 0.71337) occurring at the Neoproterozoic sequence base in the Zhuya River underwent even stronger modification, it is possible that δ 13 C values in these formations were initially negative. In terms of the C and O isotope compositions (δ 13 C av = –9.5 ± 0.7‰, δ 18 O 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.
ISSN:0024-4902
1608-3229
1573-8892
DOI:10.1134/S0024490221050047