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

• Published in: Precambrian Research
• Main Authors: Melezhik, V.A., Roberts, D., Gorokhov, I.M., Fallick, A.E., Zwaan, K.B., Kuznetsov, A.B., Pokrovsky, B.G.
• Format: Article in Journal/Newspaper
• Language: unknown
• Published: 2002
• Subjects: QE Geology; Balsfjord; Elvenes; Evenes; Ofoten
• Online Access: https://eprints.gla.ac.uk/462/; https://doi.org/10.1016/S0301-9268(01)00218-2

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 Sr-87/Sr-86 ratios (0.7066 and 0.7077), and the best preserved delta(13)C values ( + 5.0 and + 8.0parts per thousand) 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 (Sr-87/Sr-86 = 0.7083, delta(13)C = + 5.1parts per thousand), 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 (Sr- 87/Sr-86 = 0.7083, delta(13)C = + 4.3parts per thousand) of the Balsfjord Group. The remaining, fourth formation of the Evenes Group shows isotope data (Sr-87/Sr-86 = 0.7088, delta(13)C = + 2.1parts per thousand) 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 ...