Stratigraphical position and Pb—Pb age of Lower Proterozoic carbonate rocks from the Kalix Greenstone Belt, northern Sweden

Abstract

Abstract In the Kalix area, a more than 3600 m thick sequence of continental rift-associated, well-preserved metasedimentary and mafic metavolcanic rocks occurs in a Lower Proterozoic greenstone belt which has no connection to other areas of mafic volcanites. These supracrustals are probably resting on an Archaean basement, and overlain by at least 2000 m of schists of marine origin. By analysing for Pb isotopes, an attempt was made to date intertidal dolomites with evenly laminated stromatolites and supratidal, pisolitic dolomite calcretes. The resultant age was 2.1±0.2 Ga. The large uncertainty is partly due to a narrow range of Pb isotopic ratios. Small-scale postdepositional redistribution of lead in the stromatolites is probable. The sample sites are stratigraphically situated just below a 400 m thick sequence of mafic metalavas, which can be correlated with c. 2.1 Ga old mafic metalavas in the Peräpohja Schist Belt in adjacent parts of northern Finland. Despite the large uncertainty of the age obtained, the lithostrati-graphical correlations with the Peräpohja Belt suggest that 2.1 Ga is a good approximation of the depositional age of the rocks studied. Later transgression with subsequent deposition of marine sediments is older than the peak of Svecofennian igneous activity at c. 1.9 Ga.