Sr- and C-Chemostratigraphy Potential of the Paleoproterozoic Sedimentary Carbonates under Medium-Temperature Metamorphism: the Ruskeala Marble, Karelia

摘要

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 degrees C and pressure of similar to 3-5 kbar in equilibrium with a mixed water-carbon dioxide fluid with chi(CO2) similar to 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 Sr-87/Sr-86 ratio in the dolomites is within 0.70465-0.70522, delta C-13 values vary between +0.6 horizontal ellipsis +1.9 parts per thousand, and delta O-18 within -13.2 horizontal ellipsis -10.2 parts per thousand (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 Sr-87/Sr-86 ratios in the calcite marbles range from 0.70482 to 0.70489, delta C-13 from 1.5 to 2.1 parts per thousand, and delta O-18 from -10.9 to -8.1 parts per thousand (V-PDB). Tremolite-bearing marbles have higher Sr-87/Sr-86 ratio (up to 0.70522), while their delta C-13 and delta O-18 values decrease to 0.1 parts per thousand and -12.2 parts per thousand, 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 Sr-87/Sr-86 and delta C-13 in the Paleoproterozoic seawater. The Sr-87/Sr-86 ratio in the 1.9-2.0 Ga Svecofennian ocean was 0.70463-0.70492, and delta C-13 value +1.5 +/- 1 parts per thousand. 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 delta C-13 in Ludicovian carbonates mark the beginning of C-isotope stasis in ocean after the Lomagundi-Jatulian anomaly of C-13(carb).