International Geologiical Congress - Oslo 2008


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HPP-06 The evolving Earth system through Archaean?Palaeoproterozoic transition


Linking Archaean crustal segments in the North Atlantic realm: The basement of western Troms, north Norway and the Lewisian of Scotland


Steffen Bergh, University of Tromsø (Norway)
Kåre Kullerud, University of Tromsø (Norway)
Per Inge Myhre, University of Tromsø (Norway)
Erling Ravna, University of Tromsø (Norway)
Fernando Corfu, University of Oslo (Norway)
Paul Armitage, Natural History Museum, London (United Kingdom)
Robert Holdsworth, Durham University (United Kingdom)
Holger Stunitz, University of Tromsø (Norway)


Temporal and spatial linkage of Archaean crustal provinces in the North Atlantic realm requires a well-established geological and geodynamic framework. New paleomagnetically-based Palaeoproterozoic reconstructions indicate that three major Archaean plates collided: The North Atlantic (western Greenland), Central Greenland (Nagssugtoqidian belt) and the Fennoscandian (Lapland-Kola) plates. In this scenario, the Lewisian block of NW Scotland lay somewhere in between these plates. Along-strike correlation of the Central Greenland and the Lapland-Kola plates in northern Fennoscandia would restore the basement complex in western Troms and the Lewisian of Scotland along an interconnected line of these belts.
The Archaean-Palaeoproterozoic West Troms Basement Complex (WTBC), North Norway and the Lewisian Complex of Scotland are very similar crustal regions in terms of lithology, age, igneous, structural and metamorphic features and tend to share a similar tectonic history. In both regions, Archaean (2.9 Ga-2.6 Ga) tonalitic-granitoid gneisses make up a significant portion. These gneisses were deformed and metamorphosed in granulite facies (Badcallian-Inverian vs. Lopian; 2.7-2.6 Ga), then metamorphically reset in amphibolite facies prior to intrusion of a huge mafic dyke swarm (Scourie and Ringvassøya dykes; 2.4 Ga-2.2 Ga), and followed by deposition of widespread continental margin-like supracrustal cover units (e.g. Loch Maree and Vanna Groups). The mafic dykes were coeval with a global rifting event initiated when an Archaean supercontinent likely broke up into smaller continental blocks and where overlain by platform sediments.
Huge suites of felsic granitoids and mafic plutonic rocks (1.8-1.7Ga) intruded the gneisses, and during the Laxfordian and Svecofennian tectonic events (c. 1.9-1.57Ga), the gneisses in both regions were amalgamated in between NW-SE trending metasupracrustal belts creating an internally complex, lensoidal-shaped structural architecture. The block-bounding supracrustal units suffered high-strain continental contraction and oblique, transpressive sinistral shearing, and were accompanied by amphibolite to greenschist facies metamorphic reworking.
The same controversy exists in the WTBC and the Lewisian regarding timing and mechanisms of Archean-Palaeoproterozoic terrain amalgamation. A valid terrain model requires: (i) crustal scale sutures/shear zones, (ii) assembly by collision of svereal microplates or crustal rejuvenation of a single terrain, and (iii) assessment of different crustal depth portions rather than spatially separated terrains. By comparison, both the Laxfordian and Svecofennian juxtaposition involved arc-accretion into at least two major stable Archean continental terrains (Fennoscandia or Laurentia?), followed by differential uplift, metamorphic retrogression and reactivation during the Neoproterozoic.


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