Abstract
AbstractThe Helgeland Nappe Complex (HNC), part of the Uppermost Allochthon of the north-central Norwegian Caledonides, originated near the Laurentian margin and was transferred to Baltica during the closure of Iapetus in Late Silurian–Early Devonian time. The islands of Rødøy, Bolvær and Leka, located in the Sauren–Torghatten (S–T) nappe of the HNC, are composed of ultramafic and mafic basement rocks unconformably overlain by metaconglomerates and fine-grained metasedimentary rocks. Geochemical and isotopic characteristics of the basement rocks are consistent with formation in a supra-subduction zone setting. Overlying metasedimentary rocks record an increasing proportion of continental detritus supplied to the basins through time. Precambrian cratonic source regions supplied cobbles and other detritus. This source area may have been located in modern SE Greenland/Labrador or in the Lower Nappe of the HNC. The second alternative best accounts for the short transport distances required by the coarse-grained conglomerates. The maximum age of deposition is constrained by the age of the youngest zircon grain dated at 471±8 Ma. Final sedimentation, nappe thrusting and nappe stacking occurred in rapid succession during c. 480–475 Ma.Supplementary material:Geochemical analyses and Nd isotopic data are available at http://www.geolsoc.org.uk/SUP18654.
Highlights
Detrital zircon from a metasandstone located in the Sauren– Torghatten (S– T) Nappe near Vevelstad (AND-60), like the Havna metagreywacke studied by McArthur (2007), are dominated by 500– 480 Ma ages (Barnes et al 2007)
This study proposes that the Rødøy, Bolvær and Leka ultramafic-mafic rocks are correlative units that may represent segmented pieces of a oncecontinuous section of peri-Laurentian oceanic island-arc/back-arc system (e.g. Pedersen et al 1988; Slagstad 2003) that formed in the Late Cambrian Iapetus Ocean
We assign Leka to the S– T Nappe together with Rødøy and Bolvær based on these similarities
Summary
Major and trace element geochemical data on meta-igneous and metasedimentary rocks were obtained by inductively coupled plasma atomic emission spectroscopy (ICP-AES) at Texas Tech University. Most meta-igneous cobbles have the same lower REE contents and patterns that characterize most of the basement rocks (Fig. 4). The high and scattered concentrations of CaO, MgO, FeO(t), Al2O3, Sc, Ni and Cr for most of the basement rocks and a large proportion of the cobbles are suggestive that these samples are cumulates of plagioclase + pyroxene or olivine (Fig. 3) This conclusion is consistent with the low abundances of TiO2, P2O5, Zr and the REE for all but one of the basement rocks: a metadiabase (Figs 3 & 4). Meta-igneous cobbles with 1Nd values lower than the basement rocks (21.1 to +4.1) were sampled from polymict metaconglomerates at Rødøy and Leka. This pattern of increasing influence of continental input upsection is exhibited on all three islands (Fig. 7)
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