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Marginal Basin Development in the Svecofennian orogenic province 2.30 to 1.85 billion years ago

R.W. Roye Rutland 1 and Ian S. Williams 1

1 Research School of Earth Sciences, The Australian National University, Canberra, ACT 0200, Australia

We continue to test the evidence for an alternative marginal basin accretion model for the development of the main part of the Svecofennian orogenic province in Sweden and Finland. From structural and aeromagnetic evidence in several districts we have concluded that, contrary to the prevailing arc-accretion model, the main episode of deformation in the underlying metamorphic complexes occurred before, not after, the overlying volcanic sequences were deposited (Rutland et al., 2001a, b). We have focused on dating this early tectonothermal episode and distinguishing it from a younger episode that affected the volcanic sequences. Structural field evidence has been used to select samples of metasediments (e.g. from the Robertsfors Grp., N. Sweden, Fig. 1) that might preserve evidence of the early episode as metamorphic overgrowths on detrital zircons. This episode has now been dated at ~1.92–1.91 Ga in the metamorphic complexes on both sides of the Gulf of Bothnia, and in the complex south of the Central Finland Granitoid Complex. Ages of ~1.94 Ga have also been obtained for granitic rocks that were deformed in the early deformation episode. The sedimentary protoliths of all these metamorphic complexes were deposited before ~1.94 Ga and detrital zircons indicate derivation from 2.10–1.98 Ga sources (Rutland et al., 2004; Skiöld & Rutland, 2006).

Crustal scale belts of high electrical conductivity in the Svecofennian province lie within the metamorphic complexes. The continuity of these belts provides further evidence for the continuity of the sedimentary basin in which the protolith sediments were deposited . We have interpreted this as a large back-arc marginal basin which developed after ~1.98 Ga, and which was accreted during a major tectonothermal episode at ~1.92–1.91 Ga.

We consider that the main episode of igneous activity in the Svecofennian province, between 1.90 and 1.87 Ga, was extensional, and was developed in and above the basement formed by the accreted marginal basin. This magmatism may still be regarded as arc-related magmatism in the sense that an active margin probably lay to the west or southwest, with the subducting slab dipping beneath the accreted marginal basin. We suggest, however, that the concept of crustal growth by accretion of a number of arcs within the province after 1.9 Ga can no longer be sustained.

Work in progress in collaboration with Swedish and Finnish colleagues is extending our studies to the Kiruna district in the boundary zone between the Svecofennian province and the Archaean basement of the Karelian province. The synthesis of these various studies will provide a new interpretation of the tectonic evolution of the Svecofennian province in northern Sweden and Finland, and a new framework for the setting of the numerous ore deposits.

References: Rutland, R.W.R., Kero, L., Nilsson, G. and Stølen, L.K. (2001) Nature of a major tectonic discontinuity in the Svecofennian province of northern Sweden. Precambrian Research 112 , 211-237.

Rutland, R.W.R., Skiöld, T. and Page, R.W. (2001) Age of deformation episodes in the Palaeoproterozoic domain of northern Sweden, and evidence of a pre-1.9 Ga crustal layer. Precambrian Research 112 , 239-259.

Rutland, R.W.R., Williams, I.S. and Korsman, K. (2004) Pre-1.91 Ga deformation and metamorphism in the Palaeoproterozoic Vammala Migmatite Belt, southern Finland, and implications for Svecofennian tectonics. Bulletin of the geological Society of Finland 76, 93-140.

Skiöld, T. and Rutland, R.W.R. (2006) Successive ~1.94 Ga plutonism and ~1.92 Ga deformation and metamorphism south of the Skellefte district, northern Sweden: Substantiation of the marginal basin accretion hypothesis of Svecofennian evolution. Precambrian Research 148, 181-204.