Comprehensive petrological and isotope-geochemical study of marble was carried out in the upper part of the Sortavala Group of the northeastern lense at the Ruskeala deposit in the Northern Ladoga area. The petrological study showed that the carbonate rocks of the Sortavala Group underwent medium-temperature low-pressure amphibolite-facies metamorphism. The mineral assemblages of Ruskeala marbles were formed at temperature of 550–600°C and pressure of ~3–5 kbar in equilibrium with a mixed water-carbon dioxide fluid with \({{X}_{{{\text{C}}{{{\text{O}}}_{2}}}}}\) ~ 0.5–0.8. Dolomite marbles contain admixture (up to 2%) of finely disseminated carbonaceous matter and about 8–15% of calcite. Dolomites contain small amounts of Mn (70–110 ppm) and Fe (1600–3600 ppm) and high content of Sr (122–256 ppm). The initial ⁸⁷Sr/⁸⁶Sr ratio in the dolomites is within 0.70465–0.70522, δ¹³C values vary between +0.6…+1.9 ‰, and δ¹⁸О within –13.2 … –10.2‰ (V-PDB). Calcite marbles are free of carbonaceous matter, have very low contents of Mg (0.2–0.8%), Mn (10–90 ppm) and Fe (160–640 ppm) and very high Sr concentration (850–2750 ppm). The initial ⁸⁷Sr/⁸⁶Sr ratios in the calcite marbles range from 0.70482 to 0.70489, δ¹³С from 1.5 to 2.1‰, and δ¹⁸О from –10.9 to ‒8.1 ‰ (V-PDB). Tremolite-bearing marbles have higher ⁸⁷Sr/⁸⁶Sr ratio (up to 0.70522), while their δ¹³С and δ¹⁸О values decrease to 0.1‰ and –12.2‰, respectively. The metamorphism of Ruskeala carbonate even under medium-temperature amphibolite-facies conditions was essentially isochemical process, which retained the unique Sr and C chemostratigraphic potential of calcite and dolomite marbles for the reconstruction of ⁸⁷Sr/⁸⁶Sr and δ¹³C in the Paleoproterozoic seawater. The ⁸⁷Sr/⁸⁶Sr ratio in the 1.9–2.0 Ga Svecofennian ocean was 0.70463–0.70492, and δ¹³С value +1.5 ± 1‰. New Sr-isotope data record an increase in radiogenic Sr input in the ocean about 2 billion years ago, which was probably related to the growth of continental crust and more intense weathering. The δ¹³С in Ludicovian carbonates mark the beginning of C-isotope stasis in ocean after the Lomagundi-Jatulian anomaly of ¹³C_carb.

在拉多加北部地区鲁斯卡拉的矿床中，对东北透镜的Sortavala群上部进行了大理石的岩石学和同位素地球化学综合研究。岩石学研究表明，Sortavala组的碳酸盐岩经历了中温低压闪石岩相的变质作用。Ruskeala大理石的矿物组合是在550-600°C的温度和约3-5 kbar的压力下与具有（（{ } {{{\ text {O}}} _ {2}}}} \）〜0.5–0.8。白云石大理石包含精细散布的碳质混合物和方解石约8-15％的混合物（最多2％）。白云岩含有少量的锰（70-110 ppm）和铁（1600-3600 ppm）以及高含量的锶（122-256 ppm）。最初的⁸⁷ SR / ⁸⁶在白云岩Sr比值是内0.70465-0.70522，δ ^13个C值之间变化+ 0.6 ... + 1.9‰，δ ¹⁸ О内-13.2 -10.2 ...‰（V-PDB）。方解石大理石不含碳质物质，Mg（0.2–0.8％），Mn（10–90 ppm）和Fe（160–640 ppm）的含量非常低，而Sr浓度很高（850–2750 ppm）。初始⁸⁷ SR / ⁸⁶ Sr组成的方解石弹珠比例范围为0.70482至0.70489，δ ¹³ С为1.5〜2.1‰，δ ¹⁸ О从-10.9至-8.1‰（V-PDB）。透闪石轴承大理石具有较高的⁸⁷ SR / ⁸⁶ Sr比值（高达0.70522），而它们的δ ¹³ С和δ ¹⁸О值分别降至0.1‰和–12.2‰。即使在中温角闪岩相条件Ruskeala碳酸盐的变质基本上isochemical过程，其中保留方解石和白云石的大理石的独特的Sr和C化学地层潜力的重建⁸⁷ SR / ⁸⁶ Sr和δ ¹³在古元海水Ç 。在⁸⁷ SR / ⁸⁶在1.9-2.0嘎Svecofennian海洋Sr比值为0.70463-0.70492，δ ¹³ С值1.5±1‰。新的Sr同位素数据记录了大约20亿年前海洋中放射性Sr的输入量增加，这可能与大陆壳的生长和更强烈的风化有关。该δ ¹³Ludicovian碳酸盐中的С标志着在¹³ C_碳的Lomagundi-Jatulian异常之后海洋中C同位素停滞的开始。