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

In contrast to the prevailing arc-accretion hypothesis, it has recently been argued that the most extensive deformation and metamorphism (D1) in the Skellefte region of north-central Sweden took place in older metasedimentary sequences before the eruption of the ∼1.90–1.87 Ga volcanic sequences. The new data presented here provide a basis for a refined hypothesis involving accretion of a pre-1.92 Ga marginal basin during an Early Svecofennian orogenic episode between ∼1.92–1.91 Ga.To the south of the Skellefte district, in a part of the metasedimentary Robertsfors Group dominated by NE (D1) structural trends, 40 SIMS analyses of zircon from a metasediment (Robertsfors-1) have been made. Fourteen analyses of overgrowths and structureless zircon grains provide strong evidence for a post-depositional metamorphic event dated to 1916 ± 5 Ma, which we correlate with the observed D1 metamorphism and migmatisation. This age is indistinguishable from that of a corresponding event in the Vammala Migmatite Belt (VMB) in Finland. This is the most important crustal thickening episode in the evolution of the Svecofennian province, here distinguished as the early Svecofennian orogenic episode. Nine Archaean and 15 older Palaeoproterozoic (OP) detrital zircon grains also yielded data comparable to that from the VMB. The age of 1980 ± 6 Ma obtained from the total OP population possibly indicates derivation from a single magmatic source area, with similarly aged metamorphic overgrowths on some of the grains.Two sampled localities to the NE show D1 schistosity overprinted by D2 shears and granitic veining. Sample Rönnskär-3, from a relatively homogeneous granitoid, and sample Rönnskär-2, from a garnetiferous felsic layer in well-banded layered gneisses, both contain igneous zircon populations. These have been affected by D1 and D2 events but the SIMS data allow the age of magmatism in both rocks to be placed between 1940 and 1950 Ma, thus reinforcing the links to the Knaften/Storuman areas to the WNW. In the granitoid, six analyses from highly luminescent rims, yielding an imprecise age of 1856 ± 25 Ma, are consistent with the earlier more precise 207Pb/206Pb dating of the D2 event at 1858 ± 2 Ma. The D2 event is also recorded by ID-TIMS analyses of monazite from five widely spaced localities in the Skellefte structural zone with ages ranging from 1849 ± 3 to 1866 ± 2 Ma. The D2 deformation has similar characteristics in the Skellefte/Robertsfors and Tampere/Vammala areas. We suggest that it can be allocated to a second and Middle Svecofennian orogenic episode between 1.88 and 1.85 Ga, affecting a wide area.It has been demonstrated that the protoliths of the Robertsfors Group were deposited before ∼1.95 Ga; the group was intruded by granitoids between ∼1.94 and 1.95 Ga; and it was strongly deformed and metamorphosed between ∼1.92 and 1.91 Ga. Proximity to the post-1.90 Ga Skellefte Group during the deposition of the latter is indicated for both Robertsfors and Knaften/Storuman areas by the presence of ∼1.90 Ga granitoids and ∼1.88 Ga conglomerates like those in the Skellefte district. The regional evidence for the continuity of metamorphic complexes over wide areas, and the similarity in pre- and post-depositional histories of the Robertsfors Group and the Vammala Migmatite Belt 300 km apart, support the hypothesis that accretion of a large (Svionian) marginal basin at ∼1.92–1.91 Ga provided the basement for the subsequent volcanism.