Pre-Sturtian (800–730Ma) depositional age of carbonates in sedimentary sequences hosting stratiform iron ores in the Uppermost Allochthon of the Norwegian Caledonides: a chemostratigraphic approach

Abstract Carbon and strontium isotope chemostratigraphy (52 δ 13 C carb and δ 18 O, and 50 87 Sr/ 86 Sr analyses of carbonate components in whole-rock samples) was applied for constraining an apparent depositional age of the carbonate protolith to amphibolite-grade, calcite marbles occurring in siliciclastic sedimentary sequences hosting iron formations (the Dunderlandsdalen type iron ores) of previously unknown age in the Rodingsfjallet Nappe Complex of the Rana region, Nordland, Norway. The least altered 87 Sr/ 86 Sr (0.70676) and δ 13 C (+2.5 to +5.6‰) values of the marbles (Dunderland Marble 2a) in the hanging wall sequence of the Stensundtjern iron formation in the Rana region are consistent with seawater composition in the time interval 800–730 Ma, hence the Middle Cryogenian (pre-Sturtian). The least altered Sr- and C-isotopic values obtained from two other marbles units (Marble 3 and 4) of the Rodningsfjallet Nappe Complex are consistent with the Late Cryogenian (c. 660 and 670–700 Ma, respectively). The ages obtained provide the first insight into the depositional time of sediment-hosted iron formations of the Uppermost Allochthon in the North-Central Norwegian Caledonides. The Middle Cryogenian-age of marbles associated with iron formations in the Rana region and those located c. 250 km to the north (the Hafjellet iron ore horizon) share similar 87 Sr/ 86 Sr and δ 13 C ratios and hence similar chemostratigraphic ages. These iron formations were originally accumulated outside of Baltica, on a glacially influenced carbonate-siliciclastic shelf, apparently on a margin of an unknown microcontinent. The Scandinavian Dunderlandsdalen and the Hafjellet iron ores of Middle Cryogenian age (800–730 Ma) were accumulated in an open marine environment distant from volcanic centres, and hence represent an outstanding exception to other reported Neoproterozoic iron formations which were all accumulated in volcanically active continental rift settings.