We present new structural, geochemical, and U-Pb zircon data from syn-to lateorogenic sedimentary-volcanic basins in the southwestern part of the Trondheim Nappe Complex (TNC), central Norwegian Caledonides. In this area, a succession of E-MORB type metabasalt, jasper, ribbon chert with associated sandstone and conglomerate, and green siltstone is interpreted to represent volcanism and sedimentation in a hitherto little known spreading-dominated tectonic environment. This environment is different from the suprasubduction zone ophiolite setting dominating the Iapetus rock record elsewhere in the Scandinavian Caledonides. This volcanic and sedimentary succession was overturned and isoclinally folded in a pre-427 Ma orogenic phase. Post-427 Ma cross-bedded sandstones were deposited on the eroded surface of the previously deformed rocks, representing a rare example of a late Silurian or younger sedimentary basin within the Scandinavian Caledonides. The crossbedded sandstones are intercalated and/or overlain by post-427 Ma intermediate volcanic/subvolcanic rocks of calc-alkaline composition, representing a hitherto unknown volcanic phase within the TNC and elsewhere within the Scandinavian Caledonides. Their particular geochemical signature could be the result of late-stage subduction zone volcanism just prior to the onset of continent-continent collision between Baltica and Laurentia, or much younger post-collisional extensional melting with inherited subduction signatures.

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The late Neoproterozoic to Palaeozoic Iapetus Ocean developed between Laurentia, Baltica, Siberia and Gondwana. Its Palaeozoic closure history is recorded by volcano-sedimentary successions within the Caledonian orogen of Scandinavia, the British Isles and Newfoundland. We present new lithological, geochemical and geochronological data relevant for the Iapetan closure history from the hitherto poorly known Trollhøtta-Kinna basin (central Norwegian Caledonides). This basin consists of alternating siliciclastic rocks, MORB, and felsic volcanic rocks highly enriched in e.g. Th, U and LREE. Rhyolites from the stratigraphically upper part are dated by zircon U-Pb TIMS to 473.3 ± 1.0 and 472.4 ± 0.7 Ma. Detrital zircon spectra indicate deposition after ~480 Ma, with sediments derived from composite Cambro-Ordovician and Archean to Neoproterozoic landmass(es), possibly the Laurentian margin or a related microcontinent. The peculiar bimodal volcanic association is interpreted as an intermittent phase of marginal basin rifting, derived from a heterogeneous mantle source previously metasomatized by continental material. The tectonic mechanisms behind rifting could be slab retreat and/or break-off, or far-field tectonic forces within the Iapetan realm. Comparison of this basin with other Iapetus-related, similarly-aged volcanosedimentary successions along the Caledonian-Appalachian orogen indicate that the bimodal MORB and highly enriched rocks reflect a palaeotectonic setting hitherto unknown in the orogen. Supplementary material: Analytical results for detrital zircon analyses.

We present field observations, geochemical data and detrital zircon U-Pb ages from the newly defined Skuggliberga unit of the Oppdal area, central Norwegian Caledonides. The unit occurs in two separate areas, where it unconformably overlies the c. 475-470 Ma Trollhøtta and Skarvatnet units. The Skuggliberga unit consists of a local basal conglomerate and a cross-stratified to massive sandstone, overlain by bedded, fragment-bearing volcanic rocks, probably representing pyroclastic units erupted in a terrestrial or shallow-marine setting. Geochemical data from nine samples of volcanic rocks show calc-alkaline basaltic andesitic and andesitic compositions, with negative Nb-Ta anomalies indicating a continental arc-related signature, which is distinctly different from the underlying Trollhøtta and Kinna volcanic rocks. One sandstone sample from the Skaret succession, below the unconformity, is dominated by Palaeozoic detrital zircons, with a major peak at c. 490 Ma. Two sandstone samples from the Skuggliberga unit contain Archaean, Palaeo-to Neoproterozoic and Palaeozoic detrital zircons. Different methods to calculate the maximum depositional age for theseA c c e p t e d m a n u s c r i p t samples vary by up to 50 Myr (from c. 427 to c. 475 Ma), some estimates being younger than the c.435 Ma plutonic Innset massif that intrudes the Skuggliberga unit, indicating that the youngest detrital zircons must have experienced concealed lead loss. This lead loss could have occurred during regional metamorphism of the unit; an Ar-Ar plateau age of 416 ± 3 Ma was derived for biotite in one Skuggliberga volcanic sample representing cooling after regional greenschist-facies metamorphism. The Skuggliberga unit has no known direct correlative in the Trondheim Nappe Complex but overlaps in age and geochemical characteristics with arc-related plutonic rocks along the central Norwegian coast, indicating that it represents the surface expression of the latter. The orogenic phase predating Skuggliberga deposition is probably related to Taconian accretionary events along the Laurentian margin.

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