Isotopic evidence for a complex Neoproterozoic to Silurian rock assemblage in the North-Central Norwegian Caledonides

Abstract

Depositional ages of high-grade marble sequences in the Ofoten district of the North-Central Norwegian Caledonides have been constrained by the application of carbon and strontium isotope stratigraphy. Several marble units intercalated with various schists of the Evenes Group, previously correlated over long distances farther to the north with a late Ordovician–early Silurian, low-grade, fossiliferous succession (Balsfjord Group), have been studied for carbon, oxygen and strontium isotopes. The least altered 87Sr/ 86Sr ratios (0.7066 and 0.7077), and the best preserved δ 13C values (+5.0 and +8.0‰) obtained from the lowermost and uppermost marble formations of the Evenes Group are consistent with a seawater composition in the Neoproterozoic (650–600 and 620–610 Ma, respectively). The isotopic data indicate that only one formation ( 87Sr/ 86Sr=0.7083, δ 13C=+5.1‰), forming the middle Evenes Group, is consistent with the previously suggested early Silurian correlation. This is supported by new isotopic data obtained from coral and brachiopod-bearing, Llandovery (443–428 Ma) metalimestones ( 87Sr/ 86Sr=0.7083, δ 13C=+4.3‰) of the Balsfjord Group. The remaining, fourth formation of the Evenes Group shows isotope data ( 87Sr/ 86Sr=0.7088, δ 13C=+2.1‰) which are consistent with a Cambrian seawater composition. The results obtained have several implications: (i) the late Ordovician–early Silurian Elvenes Conglomerate/ophiolite assemblage has a tectonic contact with the structurally overlying marble formation dated to 650–600 Ma; (ii) the isotopic and geological data do not support the previously proposed correlation of the entire Evenes Group with a late Ordovician–early Silurian, lithostratigraphic succession farther north in the Balsford area; (ii) the Evenes Group is not a lithostratigraphic unit and should be abandoned, as it is composed of a number of marble formations of different age, tectonically emplaced in a non-chronostratigraphic order. The Neoproterozoic depositional ages combined with the palaeogeographic position of Baltica suggest that a large part of the carbonate rock succession forming the Evenes Group was initially accumulated in warm seas on a continental shelf, probably Laurentia, and was tectonically transported onto Baltica during the early Silurian, Scandian collision, at ca. 425 Ma. The complex tectonic imbrication of the polymetamorphosed and polydeformed Neoproterozoic, Cambrian and early Silurian carbonate formations also suggests that more than one orogenic episode should be invoked to explain the tectonic juxtaposition of these assemblages. Some of the fault contacts juxtaposing rocks of Neoproterozoic and Cambrian age and the obduction of the ophiolite complex might have been associated with a mid to late Ordovician, Taconian event.