1. 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)
- Author
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M. S. Khlebnikov, Boris G. Pokrovsky, A. A. Kolesnikova, O. L. Petrov, and M. I. Bujakaite
- Subjects
Metamorphic rock ,Geochemistry ,Metamorphism ,Isotopes of strontium ,Isotopes of oxygen ,chemistry.chemical_compound ,chemistry ,Geochemistry and Petrology ,Isotope geochemistry ,Marl ,Carbonate ,Economic Geology ,Syncline ,Geology - 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 (δ13Cav = –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‰ (δ18Oav = 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 (δ13Cav = –9.5 ± 0.7‰, δ18Oav = 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. more...
- Published
- 2021
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