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  • 1. Andersson, U. B.
    et al.
    Begg, G. C.
    Griffin, W. L.
    Högdahl, Karin
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Solid Earth Geology.
    Ancient and juvenile components in the continental crust and mantle: Hf isotopes in zircon from Svecofennian magmatic rocks and rapakivi granites in Sweden2011In: Lithosphere, ISSN 1941-8264, E-ISSN 1947-4253, Vol. 3, no 6, p. 409-419Article in journal (Refereed)
    Abstract [en]

    The sources of igneous rocks in the continental crust are elusive, but they may be traced by radiogenic isotopes, which convey a message about the age and composition of the concealed parts of the continent. We investigated the Hf-isotope composition of zircon in ten rocks from central and southern Sweden. Two felsic metavolcanic rocks and two metagabbros (ca. 1.89 Ga) from Bergslagen, southern Sweden, show epsilon(Hf)(t) ranges of -1.8 to +5.1 and +2.6 to +6.8, respectively, suggesting that juvenile sources have contributed to both. A 1.85 Ga granite from southern Bergslagen shows a epsilon(Hf)(t) range of -2.6 to +4.6 for magmatic zircons, but both highly negative and positive values for inherited grains, providing evidence for both Archean and juvenile crustal sources. These and previous data confirm the existence of juvenile proto-Svecofennian crust (<2.2-1.9 Ga) with a minor Archean component, from which later crustal magmas were generated. The Hf-isotope evolution curve for this crust can be approximated by epsilon(Hf)(1.90) = 3 +/- 3 and (176)Lu/(177)Hf = 0.018. Similarly, the present data, together with data for younger mafic intrusions, can be used to infer the presence of a "mildly depleted" sub-Svecofennian mantle evolution curve with epsilon(Hf)(1.90) = 4.5 +/- 2.5 and (176)Lu/(177)Hf = 0.0315. Zircons from four out of five rapakivi intrusions (1.53-1.50 Ga) in central Sweden yield negative epsilon(Hf)(t) in the range -9.8 to -4.6, suggesting mixed Archean and juvenile Svecofennian sources. One intrusion farther south ranges between epsilon(Hf)(t) of -4.1 and -1.6, and has a larger contribution from Svecofennian crust. The data suggest that the crust in Bergslagen, southern Sweden, is dominantly Paleoproterozoic, while higher proportions of Archean material are present below central Sweden.

  • 2.
    Gee, David G.
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Janák, Marian
    Geological Institute, Slovak Academy of Sciences, Bratislava Slovak Republic .
    Majka, Jaroslaw
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Solid Earth Geology.
    Robinson, Peter
    Geological Survey of Norway, Leiv Eirikssons vei 39, 7040 Trondheim, Norway .
    van Roermund, Herman
    Structural Geology and Tectonics, Department of Earth Sciences, Utrecht University, The Netherlands .
    Subduction along and within the Baltoscandian margin during closing of the Iapetus Ocean and Baltica-Laurentia collision2013In: Lithosphere, ISSN 1941-8264, E-ISSN 1947-4253, Vol. 5, no 2, p. 169-178Article in journal (Refereed)
    Abstract [en]

    The recent discovery of ultrahigh-pressure (UHP) mineral parageneses in the far-transported (greater than 400 km) Seve Nappe Complex of the Swedish Caledonides sheds new light on the subduction system that dominated the contracting Baltoscandian margin of continental Baltica during the Ordovician and culminated in collision with Laurentia in the Silurian to Early Devonian. High-grade metamorphism of this Neoproterozoic to Cambrian rifted, extended, dike-intruded outer-margin assemblage started in the Early Ordovician and may have continued, perhaps episodically, until collision of the continents at the end of this period. The recent discovery of UHP kyanite eclogite in northern Jämtland (west-central Sweden) yields evidence of metamorphism at depths of 100 km. Although UHP rocks are only locally preserved from retrogression during the long-distance transport onto the Baltoscandian platform, these high-pressure parageneses indicate that deep subduction played an important role in the tectonothermal history of the complex. Based on existing isotopic age data, this UHP metamorphism occurred in the Late Ordovician, shortly before, or during, the initial collision between the continents (Scandian orogeny). In some central parts of the complex, migmatization and hot extrusion occurred in the Early Silurian, giving way to thrust emplacement across the Baltoscandian foreland basin and platform that continued into the Early Devonian. Identification of HP/UHP metamorphism at different levels within the Scandian allochthons, definition of their pressure-temperature-time paths, and recognition of their vast transport distances are essential for an understanding of the deeper structural levels of the orogen in the hinterland (e.g., the Western Gneiss Region), where the attenuated units were reworked together during the Early Devonian.

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