Early­­­­–Middle Ordovician sedimentation and bimodal volcanism at the margin of Iapetus: the Trollhøtta–Kinna Basin of the central Norwegian Caledonides

Abstract

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.

The Skuggliberga unit of the Oppdal area, central Scandinavian Caledonides: calc-alkaline pyroclastic volcanism in a fluvial to shallow-marine basin following a mid-Ordovician orogenic event

“…There is no systematic difference between samples from the southern and northern deposits. The highly enriched mafic to intermediate volcanic rocks of the Kinna volcanic succession ( Dalslåen et al, 2020a) and the MORBlike basalts from the Trollhøtta unit (Dalslåen et al, 2020b) Unfortunately, the volcanic rocks we separated (with up to 137 ppm of Zr) did not contain zircon, monazite or other heavy minerals that could have provided an age of volcanism. To obtain at least a maximum depositional age of the Skuggliberga unit, two sandstone samples were selected for detrital zircon analysis.…”

“…The mapping by Dalslåen et al (2020b) showed that large parts of what was previously assigned to the Hovin Group, particularly in the Gisnadalen and Skjørstadhovden-Stølen areas (Fig. 2A), are turbiditic rocks intercalated with Trollhøtta metavolcanic rocks.…”

“…2A). Dalslåen et al (2020b) also presented a different interpretation of the Rohr-Torp profile (compare Fig. 1D with Fig.…”

“…This geochemical composition is different from that of the SSZ-type LVB ophiolites with which the Støren Group traditionally has been correlated. These MORB-type metabasalts, together with intercalated turbiditic silt-and sandstones and minor rhyolites dated at 473 ± 1 and 472 ± 1 Ma, were referred to as the Trollhøtta unit by Dalslåen et al (2020b;Fig. 2A).…”

“…1A). The Oppdal area within the southern TNC has recently received considerable attention due to the discovery of a peculiar volcano-sedimentary succession consisting of highly enriched, largely pyroclastic, mafic to felsic volcanic rocks and MORB-like pillow basalts intercalated with turbiditic sedimentary rocks, indicating a hitherto little-known spreading environment within a marginal basin of Iapetus at c. 475-470 Ma (Stokke et al, 2018;Dalslåen et al, 2020a;Dalslåen et al, 2020b). This succession was shown by Stokke et al (2018) to have been deformed and eroded prior to the deposition of cross-stratified sandstones and andesitic volcanic rocks, highlighting the existence of a major angular unconformity in the area.…”