Tectonic reconstruction and sediment provenance of a far-traveled nappe, west-central Norway

The Helgeland Nappe Complex (HNC) is located in the Uppermost Allochthon of the north-central Norwegian Caledonides. The Uppermost Allochthon originated from the Laurentian margin and was transferred to Baltica during the closing of the Iapetus Ocean in Late Silurian to Early Devonian time. The islands of Rodoy, Bolvaer, and Leka are located in the western potion of the HNC and each are composed of ultramafic and mafic basement rocks unconformably overlain by metaconglomerates and fine-grained metasedimentary rocks. Petrological, geochemical, isotopic, and zircon geochronological data indicate that Rodoy, Bolvaer, and Leka shared similar tectonic and depositional histories in the Late Cambrian to Early Ordovician and are accordingly assigned to the Sauren-Torghatten (S-T) Nappe of the HNC. The meta-igneous rocks on the three islands have geochemical and Sm-Nd and Rb-Sr isotopic compositions consistent with formation in a supra-subduction zone setting. Metasedimentary rocks contain locally-derived detritus from the oceanic basement rocks, as well as isotopically evolved, continentally-derived detritus. Isotopic data indicate that the proportion of continentally-derived sediment increases up-section. Detrital zircon U-Pb age spectra from metasandstone cobbles and sandy detritus indicate that Bolvaer and Leka received sediment with comparable Archean, Paleoproterozoic, and Mesoproterozoic age elements that are consistent with a southeast Greenland/Labrador provenance. A primitive Cambrian-Ordovician age arc contributed detritus to Leka. Final sedimentation in the HNC nappes, nappe thrusting, and nappe stacking occurred in rapid succession at approximately 480 to 475 Ma.