Carbon and strontium isotope chemostratigraphy (178 δ13Ccarb and δ18O, and 81 87Sr/86Sr analyses of carbonate components in whole-rock samples) was applied to constrain apparent depositional ages of the carbonate protoliths of amphibolite-grade, calcite marbles occurring in siliciclastic sedimentary sequences within the Upper and Uppermost Allochthons in the North–Central Norwegian Caledonides. The Sr-rich marbles hosting banded iron formations occur only in the Uppermost Allochthon. The marbles show, over a distance of 350 km, rather similar least-altered 87Sr/86Sr (0.70645–0.70665) and δ13C (+6 to +8‰) values which are all consistent with a late Tonian (800–735 Ma) age. This sets up a maximum depositional age for the overlying iron formations and somewhat younger diamictites. The apparent maximum ages of the Scandinavian iron formations suggest their contemporaneous deposition with the oldest known Neoproterozoic iron formations reported from China (Shilu Formation) and Namibia (Chuos Formation). However, these maximum ages do not rule out the iron deposition and the diamictite accumulation in the early Cryogenian within a presumed Tonian–Cryogenian transition. Three other studied marble units in schistmarble sequences, spatially unrelated to iron formations, show different 87Sr/86Sr and δ13C values matching younger apparent depositional ages of 685–600 Ma (the Uppermost Allochthon), and 550 or 425–410 Ma (the Upper Allochthon). The schist-marble-iron formations sequences in several areas contain extrusive meta-igneous rocks, and rare glacial diamictites. In places, all are intruded by intermediate and mafic sills. The iron formations were originally formed outside Baltica and were subsequently thrust upon the Baltoscandian margin during the Scandian orogeny. The provenance of these iron formations represents an enigma, hinting towards a passive continental margin of an unknown, apparently missing microcontinent. The accumulation of the Scandinavian iron formations within a passive continental margin or a large back-arc basin, in places glacially influenced, represents an exception to other reported clastic, sediment-dominated, Neoproterozoic (Cryogenian) iron formations which all were formed in volcanically active continental rift settings.