Geochronology and structural geology of the Kjerringøy Peninsula, Nordland, Norway

Abstract

Field mapping, structural and petrographic analysis, ID-TIMS U-Pb geochronology, 40Ar/39Ar thermochronology, and chemical dating of minerals were employed to decipher the geologic history of the Salten area of north-central Norway. The “Heggmovatn Dome” was examined where it is overlain by rocks of the Bodø Nappe. Geologic mapping indicates that the Landegode, Bratten, and Tårnvika megacrystic granites are one structurally continuous unit, likely a composite batholith comprising numerous plutons. An ID-TIMS U-Pb age determination of ~950 Ma for the Tårnvika pluton and ~428 Ma for the Fjærhesten granite is evidence that the “Heggmovatn Dome” is not a Baltic basement gneiss dome but rather is a Scandian thrust nappe that evolved during the Neoproterozoic Valhalla orogeny. This juxtaposition of Tonian and Ordovician-Silurian plutons gives a distinctive fingerprint unique to the Laurentian Caledonides of the North Atlantic, clearly linking the Heggmo Nappe to Laurentia. Geologic findings point to a pre-Scandian kyanite-grade metamorphic event with U-Pb ages of zircon (~460 Ma) and rutile (~460-470 Ma) and a chemical monazite age (~445-475 Ma) supporting Taconian/Grampian deformation, likely along the Laurentian margin. Apart from regional folding and tonalitic and pegmatitic intrusions, evidence for Silurian-Devonian Scandian orogenesis is not abundant in the Heggmo Nappe. The greenschist-facies, tops-down-to-the-west Fjær-Osvika\Steigtinden shear zone (FOSZ) down-dropped rocks of the Bodø Nappe upon rocks of the Heggmo Nappe. Devonian extension along the FOSZ helps to clarify the geometry and distribution of the extensional detachment system in its northernmost extent in Norway. Brittle Mesozoic to Tertiary normal faults trend northeast/southwest throughout the study area and record rifting that eventually led to continental separation in the Eocene.