Sr isotopic composition of Paleoproterozoic ~(13)C-rich carbonate rocks: The Tulomozero Formation, SE Fennoscandian Shield

摘要

Rb-Sr systematics has been studied in ~(13)C-rich carbonate rocks of the Paleoproterozoic (2.09±0.07 Ga) Tulomozero Formation in the northern Onega Lake area, the SE Fennoscandian Shield. The formation is divided into eight members (A-F) consisting of greenschist-facies-grade, variegated sandstones, siltstones, mudstones, stromatolitic dolostones and subordinate crystalline limestones. Samples of carbonate rocks were obtained from two overlapping drillholes intersecting the entire thickness of the Tulomozero Formation. Prior to isotope analysis, the rocks powders were treated with 1N ammonium acetate for partial removal of the late epigenetic carbonate phases. Major resetting of the Rb-Sr systems in the Tulomozero carbonate rocks appears to take place during the Svecofennian regional metamorphic event, and it was screened by using Mn/Sr, Fe/Sr, Mg/Ca, and ~(18)O/~(16)O ratios. High Sr content (up to 2080μg/g in limestones, and 530μg/g in dolostones) coupled with low Fe/Mn (<0.40) ratios in the Tulomozero carbonate rocks of Members A, B (the lower part), D, F, and E are consistent with accumulation of original carbonate sediments in evaporitic lacustrine, playa, and sabkha environments. A decrease in the Sr content with concurrent increase in the Fe/Mn ratio (>0.40) in dolostones of the upper part of Member B, and of Members G and H is indicative of seawater influxes (sea transgression) into the Tulomozero basin. The ~(87)Sr/~(86)Sr values in the least altered (Mn/Sr<2.0) marine dolostones are 0.70418-0.70442 and 0.70343-0.70409 for the earlier and late phases of the marine transgression, respectively. The decrease in the ~(87)Sr/~(86)Sr ratio in ca. 2.1 Ga seawater is attributable to an increase in hydrothermal flux Sr into the Palaeoproterozoic ocean.