Provenance of Neoproterozoic sediments in the Särv nappes (Middle Allochthon) of the Scandinavian Caledonides: LA-ICP-MS and SIMS U–Pb dating of detrital zircons
Yaron Be’eri-ShlevinDavid G. GeeStefan ClaessonAnna LadenbergerJarosław MajkaChristopher L. KirklandPeter RobinsonDirk Frei
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Abstract Palaeomagnetic data from mid to late Palaeozoic rocks are potentially valuable for testing models of the assembly of Spitsbergen and for determining the palaeo-position of parts of Spitsbergen relative to the major tectonic elements that comprised the Old Red Continent. Fine-grained red sandstones from the Tournaisian to Namurian Billefjorden Group, collected by members of Cambridge Spitsbergen expeditions, were subjected to stepwise chemical and thermal demagnetization. The characteristic magnetization is found in normal and reverse polarities and corresponds to a pole position at 23° S, 332 °E, dp = 5.7°, dm = 10.5°, which is near an early Carboniferous pole computed for the Baltic Shield-Russian Platform (Baltica). When compared to Laurentia (North America) in the context of the revised Mauch Chunk palaeomagnetic study, Spitsbergen falls a few degrees south of the Bullard reconstruction but the error of the determination of the palaeolatitude overlaps with this position. Therefore relative motion between any part of Spitsbergen and Laurentia and Baltica during the Carboniferous is not resolved.
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ABSTRACT Close geochronological correspondence of Mesoproterozoic extension‐related magmatism and comparable tectonic and geochronological evolution of the Grenville and Svecononvegian orogens suggests that the Laurentian and Baltic Shields could have been joined during the Mesoproterozoic and did not separate until after the Grenvillian‐Sveconorwegian orogeny. The combined Grenville‐Svecononvegian orogen is here interpreted to have formed during the collision of coherent Laurentia‐Baltica with an unknown craton, which is in contrast to earlier explanations that favour a late‐Mesoproterozoic rotation between Laurentia and Baltica followed by collision of these two cratons with each other.
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Northeastern and northwestern margins of Baltica in Neoproterozoic time: evidence from the Timanian and Caledonian Orogens
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ABSTRACT Ordovician faunal data from the Scandinavian Caledonides is tested with new geochemical information from zircons to give U/Pb ages and source origins of volcanic arc and ophiolite sequences. Early Ordovician (Arenig‐Llanvirn), low latitude, Toquima‐Table Head faunas from the upper Upper Allochthon are associated with an island arc system formed adjacent to Laurentia. Contemporaneous mafic magmas were contaminated by crustal material during subduction and associated granites contain inherited zircons of Archaean age. The nearest source for such rocks is on the Laurentian rather than the Baltic side. Higher latitude Celtic province faunas from the upper Upper Allochthon are from one insular site accessible to forms from both Laurentia and Baltica. The late Ordovician low‐latitude Holorhynchus and subtropical Hirnantia faunas occur in overstep sequences above deeply eroded early Ordovician arc complexes. The transgression appears to be coeval with a second generation of spreading‐related complexes. Single detrital zircons from sediments show sources from Archaean, Proterozoic and early Ordovician terranes. This suggests that deposition was in a basin situated along the same continental margin (Laurentia) to which the early Ordovician ophiolite/arc sequences had already become accreted. The late Ordovician faunas link both Laurentia and Baltica at a time of narrowing of lapetus. The new geochemical data together with the faunal information is supported by recent palaeomagnetic studies.
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Underthrusting of Laurentia by the continental margin of Baltica during Caledonian orogeny resulted in the lateral emplacement of Iapetus Ocean-related terranes of the Upper Allochthon at least 500 km onto Baltica. The underlying Lower and Middle allochthons of the Baltoscandian margin mostly comprise Cryogenian, Ediacaran and Cambro-Silurian sedimentary successions; basement to these formations are present only as minor, isolated fragments, except at the base of the Middle Allochthon and within the underlying windows. The upper parts of the Middle Allochthon are notable for the presence of early Ediacaran dyke-swarms and other components of the Baltoscandian continent–ocean transition zone (COT). New data are presented here on the c. 610 Ma age of the COT-related dolerites in the Kalak Nappe Complex in Northern Norway and also on detrital zircons in the underlying Laksefjord and Gaissa nappes. The former confirms that the Baltoscandian COT has a similar age along the length of the orogen; the latter shows that the detrital zircon signatures in the Lower and Middle allochthons are comparable throughout the orogen. These sedimentary rocks have dominating populations of Mesoproterozoic to latest Palaeoproterozoic zircons similar to those from southern parts of the orogen, where Sveconorwegian complexes comprise the basement to the Caledonides. Thus, they help define the probable character and age of the crystalline basement that existed along this outer margin of Baltica during the Neoproterozoic, continental lower crust that was partly subducted during Ordovician continent-arc collision and subsequently lost beneath Laurentia during the 50 million years of Scandian collisional orogeny.
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Abstract Abstract Metamorphism in the nappe terrains of the Central Scandinavian Caledonides is characterized by inverted gradients and other indications of disequilibrium caused by the superposition of nappes of various metamorphic grade and the translation of the nappe pile onto the margin of the Baltic Shield. In the lower nappes there is a gradual westward increase of grade, but some inverted gradients persist. In the upper allochthon, the variation of metamorphism is essentially vertical. It is possible to identify and evaluate parageneses formed prior to, during, and after nappe emplacement. Their interpretation in terms of separate Caledonian or Precambrian events is often ambiguous. Early Caledonian metamorphism apparently represented a range of P-T-conditions. The build-up and translation of the nappe pile was characterized by transient high pressure conditions. A metamorphic pulse, manifest particularly in the west, succeeded nappe emplacement onto the Baltic Shield. Strain-induced metamorphism in the vicinity of thrusts contributes to the overall metamorphic pattern. A variety of P/T-ratios characterized metamorphism in the Scandinavian Caledonides. This conclusion modifies current concepts about an essentially Barrovian metamorphic evolution of the orogen. Key Words: Metamorphismisogradsstrainnappesthrust faultsgeothermal gradientsfadesorogenyP-T conditions pangenesisautochthonparautochthonallochthonJämtland SupergroupLower AllochthonMiddle AllochthonOfferdal NappeUpper AllochthonSärv NappeSeve NappeGula GroupKöli NappeTrondheim NappePrecambrianLower PalaeozoicCaledonideswestern JämtlandSør-TrøndelagNord-Trøndelagwest central Swedeneast central NorwayN6200 N6500 E1500 E1000
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(1993). Tectonostratigraphy and Caledonian structure of the Singis-Tjuoltajaure area, central Norrbotten Caledonides, Sweden. Geologiska Föreningen i Stockholm Förhandlingar: Vol. 115, No. 2, pp. 165-180.
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