Abstract
Abstract The Scandinavian Caledonides consist of disparate nappes of Baltican and exotic heritage, thrust southeastwards onto Baltica during the Mid-Silurian Scandian continent–continent collision, with structurally higher nappes inferred to have originated at increasingly distal positions to Baltica. New U–Pb zircon geochronological and whole-rock geochemical and Sm–Nd isotopic data from the Rödingsfjället Nappe Complex reveal 623 Ma high-grade metamorphism followed by continental rifting and emplacement of the Umbukta gabbro at 578 Ma, followed by intermittent magmatic activity at 541, 510, 501, 484 and 465 Ma. Geochemical data from the 501 Ma Mofjellet Group is indicative of arc magmatism at this time. Syntectonic pegmatites document pre-Scandian thrusting at 515 and 475 Ma, and Scandian thrusting at 429 Ma. These results document a tectonic history that is compatible with correlation with peri-Laurentian and/or peri-Gondwanan terranes. The data allow correlation with nappes at higher and lower tectonostratigraphic levels, including at least parts of the Helgeland, Kalak and Seve nappe complexes, implying that they too may be exotic to Baltica. Neoproterozoic fragmentation of the hypothesized Rodinia supercontinent probably resulted in numerous coeval, active margins, producing a variety of peri-continental terranes that can only be distinguished through further combined geological, palaeomagnetic and palaeontological investigations.
Highlights
We present new U–Pb zircon geochronological and whole-rock geochemical and Sm–Nd isotopic data from the central parts of the Scandinavian Caledonides currently assigned to the Rödingsfjället Nappe Complex (RNC) of the Uppermost Allochthon (Fig. 1a)
The youngest identifiable population consists of four analyses that yield a concordia age of 1030 + 17 Ma (MSWD = 0.13), considered the best estimate of the maximum age of deposition
The detrital zircon dataset presented here is rather small and limited to the Kjerringfjell Group, the host rock to the Umbukta gabbro, but seems to be similar to other metasedimentary units in the Caledonides, including previously published detrital zircon data from the Plura Nappe (Slagstad and Kirkland 2017)
Summary
Zircon crystals were separated using standard techniques (Wilfley or Rogers water table, heavy liquid, Frantz magnetic separation). Zircons from the non-magnetic fraction were picked under alcohol, mounted in 1 inch-diameter epoxy resin mounts and polished to expose an equatorial section through the grains. The laser was set to ablate single, up to 60 μm-long lines, using a spot size of 20 or 15 μm, a repetition rate of 10 Hz and an energy corresponding to a fluence of 4– 5 J cm−2. Each analysis included 30 s of background measurement followed by 30 s of ablation. The reference material GJ-1 (Jackson et al 2004) was used for fractionation correction of isotopic ratios, whereas 91500 (Wiedenbeck et al 1995) and an in-house standard (OS-99-14, 1797 + 3 Ma: Skår 2002) were used to check precision and accuracy. The data were reduced using GLITTER® (Van Achterbergh et al 2001) and plots were made using Isoplot (Ludwig 2003)
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