Early Silurian magmatism and the Scandian evolution of the Kalak Nappe Complex, Finnmark, Arctic Norway

Abstract

Three orogenic events, Porsanger (pre-800 Ma), Finnmarkian (490–540 Ma) and Scandian (400–425 Ma) have been proposed to explain the evolution of the Kalak Nappe Complex, a major component of the Caledonian orogen in Norway. The Kalak Nappe Complex has been considered as Baltoscandian passive margin metasediments telescoped eastwards onto the Baltic Shield. Two granitoid bodies with a weighted mean U–Pb zircon age of 438 ± 2 Ma cut the Hellefjord Schist, regarded as the highest stratigraphic unit in the Kalak Nappe Complex. A volcaniclastic unit within the Hellefjord Schist yields an identical U–Pb zircon age of 438 ± 4 Ma. Magmatism and sedimentation are synchronous within analytical error. The Hellefjord Schist cannot have been affected by Finnmarkian or Porsanger orogenesis and cannot be part of a continuous stratigraphic package. A Laurentian affinity for these sediments is suggested and radical revision of the tectonostratigraphy is required. The Hellefjord Schist is reassigned to the Magerøy Nappe of the Upper–Uppermost Allochthon. As the entire stratigraphy is infolded with the Hellefjord Schist a major deformation phase of Scandian age affected the Kalak Nappe Complex. The Hellefjord Schist represents a Laurentian flysch sequence that was juxtaposed with Baltica during the Scandian Orogeny.