The Bratten–Landegode gneiss complex: a fragment of Laurentian continental crust in the Uppermost Allochthon of the Scandinavian Caledonides

Abstract

AbstractThis report presents new field observations and geochronology (isotope-dilution thermal ionization mass spectrometry/ID-TIMS and secondary-ion mass spectrometry/SIMS) on an igneous complex and its country rocks in the Bodø area, northern Norway, traditionally interpreted to represent the (par)autochthonous crust of Baltica (c. 1.8 Ga). Field observations however indicate that the rocks are allochthonous and comprise the uppermost tectonostratigraphic level in the area. The presence of a migmatitic megacrystic granite with an emplacement age of 946 Ma strongly supports such an interpretation and indicates that the Bratten–Landegode gneiss complex is exotic with respect to Baltica. The 946 Ma granite intrudes metasedimentary rocks. The rocks were metamorphosed in the Late Ordovician and intruded by granitic pegmatites and diorites at 430 and 427 Ma, respectively. The Bratten–Landegode gneiss complex shows a close correlation with Mesoproterozoic–Neoproterozoic rock complexes in the East Greenland Caledonides and we interpret it to be a Laurentian pre-Caledonian continental fragment. The discovery of Laurentian pre-Caledonian continental crust in the Uppermost Allochthon calls for a revision of the tectonostratigraphy of this part of the Caledonides and provides important constraints on the sequence of events on the Laurentian margin prior to continent–continent collision between Baltica and Laurentia as well as on intercontinental interactions during the Caledonian orogeny.