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The Scandian Sætra Nappe: Stratigraphic and Geochemical Correlation Into Highly Deformed Amphibolite and Eclogite Facies Rocks in Western Norway

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The Scandian Sætra Nappe: Stratigraphic and Geochemical Correlation Into Highly Deformed Amphibolite and Eclogite Facies Rocks in Western Norway The Scandian Sætra Nappe: stratigraphic and geochemical correlation into highly deformed amphibolite and eclogite facies rocks in western Norway Kurt Hollocher Peter Robinson Emily Walsh Michael P. Terry Abstract The Sætra Nappe (=Särv) is one of many thrust sheets transported eastward from the Baltica margin during the Silurian – Early Devonian Scandian Orogeny. The Nappe is mostly metamorphosed Neoproterozoic sandstone up to 2 km thick that was intruded by diabase dikes during rifting of Rodinia from Baltica to produce the Iapetus Ocean. Sampling from 15 localities was done to extend correlation of the Sætra Nappe from lower-grade rocks in west-central Sweden and eastern Norway to highly deformed amphibolite- and eclogite-facies rocks in the Western Gneiss Region, W and SW of Trondheim, where the Nappe is as thin as 1 m. Mafic rocks were also collected from two units underlying the Sætra: augen gneisses of the Risberget (=Tännäs) Nappe, and nearby dike-rich basement gneisses ("sausage rock"). The dikes at most suspected Sætra sites are indistinguishable from those at the classic Oppdal locality, though there are subtle variations. They are somewhat LREE-enriched, with most having Lan/Smn ratios 1.0-1.8 (Oppdal Group), but with more alkaline varieties having ratios up to 2.4 (Ystland Group). All have Nb/La ratios of 0.8-1.4. Negative Eu anomalies are small to none, except large for obviously evolved compositions. Spider patterns show a consistent lack of Nb-Ta anomalies, with common slight negative Zr-Hf anomalies, and common small positive P and U anomalies. These characteristics indicate a continental-rift or flood-basalt eruptive setting. Many dikes cutting Risberget augen gneiss are compositionally indistinguishable from those in the Sætra, as are dikes in basement "sausage rock", and are interpreted to represent the same dike swarms. This suggests that the basement "sausage rock" areas are part of the Middle Allochthon with Sætra and Risberget. Other, generally coarser mafic rocks in the Risberget are different from Sætra and probably Mid-Proterozoic, having large negative Nb-Ta anomalies and other characteristics indicative of arcs or continental crust contamination. Data from this and other studies from the Sætra Nappe show geographic compositional variations, with MORB-like compositions to the north and south and more alkaline compositions at about the latitude of northern Trondheimsfjord. We interpret the latter to have been influenced by enriched mantle of a nearby hot-spot during opening of Iapetus. Base map after Tucker et al., 2004, AJS Blåhø Sætra Sausage Risberget Basement Sætra, Oppdal quarries Sætra, Orkanger Sætra, Moldefjord syncline Sætra, Skør Sætra quartzite Sætra quartzite Sætra quartzite Sætra dikes: bottom line: 1.9 areas have chemically similar dikes cutting quartzite. 2.This supports correlations from tectonic stratigraphy. 3.Correlation of these quartzites as parts of the Sætra Nappe is supported. Other amphibolites in quartzite Åsbøen ≅Sætra Ura ≠Sætra Other quartzites: bottom line: 1.The dikes in quartzite at Ura are not like the Sætra. Odd tectonostratigraphic position. Correlation with Sætra is not supported. 2.The dikes in quartzite at Åsbøen are like the Sætra. Mapped as basement, more work probably needed. Basement sausage rock, Midsund Basement sausage rock, Midsund Basement sausage rock, Lepsøy Basement rock, Geita Basement sausage rock Geita Kjora Midsund Lepsøya Basement sausage rock: bottom line: 1.Sausages are chemically like Sætra dikes. 2.Composition range includes Oppdal and Ystland chemical types. 3.Suggests the sausage rock was proximal to Sætra quartzite in Late Proterozoic, and was injected by the same dike swarms (sausages). 4.Sausage rock belongs in the Middle Allochthon, based on apparent outboard source. Risberget augen gneiss at Brattvåg Risberget augen gneiss at Lauvøya Risberget augen gneiss at Skår Risberget: ≅Sætra and ≠ Sætra Årnes Kjora Ura Moldefjord syncline Oppdal Risberget augen gneiss: bottom line: 1.Some Risberget amphibolite compositions do resemble Sætra dikes, others don’t. 2.Do: Probably the same as Late Proterozoic Sætra dikes. 3.Don’t: Generally in large, coarse, gabbroic-looking bodies that are probably Middle Proterozoic intrusions (but no dates). Color scheme Dot color represents the median value for each location Sætra dike composition variation with location Modern Hypothetical Late Proterozoic hot spots hot spot influence during breakup, same scale ?? Conclusions 1. The compositions of Sætra Nappe dikes helps confirm long-distance tectonostratigraphic correlations. 2. Dikes span a composition range from N-MORB-like to alkaline. 3. MORB-normalized spider patterns are relatively featureless. 4. Basement sausage rock and Sætra dikes are indistinguishable in composition. 5. Basement sausage rock is probably from an outboard region proximal to the Sætra during dike injection, and so belongs in the Middle Allochthon rather than the Lower Allochthon. 6. The Risberget Nappe augen gneiss also contains rocks indistinguishable from Sætra dikes. 7. Other mafic rocks in the Risberget are different, probably Middle Proterozoic intrusions unrelated to the opening of Iapetus. 8. The distribution of Sætra dike compositions has a pattern: MORB-like to the north and south, and more alkaline at the latitude of northern Trondheimsfjord. 9. This suggests a Late Proterozoic hotspot locally enriched the rift melting region. 10.Differential movement of thrust slices during thrust emplacement was insufficient to destroy the geographic pattern of Sætra dike compositions..
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