Abstract

Carbon and strontium isotope stratigraphy has been applied to constrain the depositional ages of high-grade marble sequences in the Ofoten district of the North-Central Norwegian Caledonides. Two marble formations hosted by diverse schists from the Bogen Group, all previously correlated over long distances with a Late Ordovician–Early Silurian, low-grade, fossiliferous succession, have been studied for carbon, oxygen and strontium isotopes. The least altered 87Sr/86Sr ratios ranging between 0.7062 and 0.7068, and the best preserved δ13C values falling between +5.0 and +6.5 ‰ obtained from two marble formations, are consistent with a seawater composition in the time interval 700–600 Ma. The results obtained do not support the previously proposed correlation of the Bogen Group with an Ordovician–Silurian lithostratigraphic succession further north. The apparent depositional ages suggest that the tectonostratigraphic succession studied is inverted and that the tectonostratigraphy of the region requires revision. The Neoproterozoic depositional ages combined with the palaeogeographic position of Baltica imply that carbonates were initially accumulated in seas on a continental shelf, probably Laurentia, and were tectonically transported onto Baltica during Early Silurian, Scandian collision, at c. 425 Ma. Prospecting for new dolomite marble deposits of the Hekkelstrand type and carbonate-hosted manganese–iron ores should be restricted to 700–600 Ma sequences in the Uppermost Allochthon of the Norwegian Caledonides.

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