Provenance characteristics of Scandinavian basement terrains: constraints from detrital zircon ages in modern river sediments

A study of modern river sediments from northern Scandinavia has shown that detrital zircon populations derived from the Fennoscandian Shield are fundamentally different to those derived from the Caledonian Nappe Domain. The zircon age spectra derived from the Fennoscandian Shield are essentially unimodal or bimodal, whereas the Caledonian Nappe Domain spectra are much more diverse, showing multiple peaks and a wider range of ages. The greater diversity of the Caledonian Domain zircon populations compared with those from the Fennoscandian Shield reflects the fundamentally different nature of the source regions. The Caledonian Nappe Domain largely comprises metasedimentary rocks, which were themselves derived from the variety of precursor sources available at the time, mainly situated in Baltica but with Laurentian influences in the Uppermost Allochthon. By contrast, the Fennoscandian Shield zircons were largely derived from granitoid rocks formed during specific crust-forming events. Thus, the Fennoscandian Shield spectra are typical of first-cycle, basement-derived detritus, whereas the Caledonian Nappe Domain spectra are typical of a multicyclic source. The study suggests that zircon age spectra, when combined with mineralogical or petrographic data, can help to distinguish first-cycle from polycyclic sediment, with complex zircon age spectra being likely to diagnose polycyclic sources, and simple spectra more likely to indicate first-cycle sediment. Comparison between the zircons in Scandinavian rivers and Cretaceous sandstones in the Norwegian Sea support previous interpretations of provenance data that indicate a non-Scandinavian source (East Greenland) for some of the Cretaceous sandstones.