Age and origin of thin discontinuous gneiss sheets in the distal domain of the magma-poor hyperextended pre-Caledonian margin of Baltica, southern Norway

Abstract

The vestiges of the pre-Caledonian magma-poor hyperextended margin of Baltica are preserved in a mélange zone in southern Norway. The rock assemblage in the distal domains of magma-poor hyperextended margins includes extensional allochthons and basement slivers, which share a pre-rift tectonometamorphic history with the non-rifted basement. After incorporation into an orogenic belt, these distinct fingerprints may be used to test whether such heterogeneous mélange units were derived from the basement units below or have been transported to the orogen from elsewhere. Later magmatic additions and tectonometamorphic events may provide additional information on the geological evolution of the margin. We present U–Pb results from 12 gneisses and meta-igneous rocks from the mélange zone. We find Proterozoic gneisses of Telemarkian affinity over the length of the mélange zone that support the formation of the mélange along the ancient Baltican margin. We also find latest Cambrian–early Ordovician meta-igneous rocks that may be linked either to shortening of transitional crust formed in the Ediacaran or, alternatively, to an episode of extension in the Ordovician. Scandian thrusting initiated between 438 and 427 Ma and was followed by the emplacement of syntectonic ‘Scandian’ granitoids at 421 Ma; that emplacement coincides with peak metamorphism. Supplementary material: A high-resolution photomicrograph of the chlorite schist from Stølsheimen is available at https://doi.org/10.6084/m9.figshare.c.3592064 .