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The Palaeoproterozoic Västervik quartzites in southeastern Sweden are commonly included in the Svecofennian Domain of the Baltic Shield. In spite of later deformation and metamorphism the metasediments are in parts remarkably well preserved. Sedimentary facies analysis demonstrates depositional environments ranging from fluviatile to tidal and shallow-water marine. This indicates a setting at a continental margin. The sedimentary material has been derived from at least two different sources, a fluvial drainage area and a marine source.
Previous U-Pb studies of detrital zircon have given both Archaean ages of 2.7-3.0 Ga with single crystals as old as 3.6 Ga, and a main Proterozoic age group at 1.9-2.1 Ga. Here we present new detrital zircon U-Pb ages for quartzite samples with various, well characterized depositional settings. Objectives of the study are to constrain the source regions of the sediments, the timing of deposition, and implications for the regional Archaean and Svecofennian crustal evolution.
Our new results confirm and add more detail to the provenance age pattern. 30% of the analysed crystals give ages in the range 2.2-3.1 Ga, with a concentration in the interval 2.4-2.8 Ga. The majority are c 1.8-2.1 Ga with a maximum at 1.99 Ga. Ages from fluviatile sediments are without exception 1.87 Ga or older, while samples from tidal-influenced and marine environments carry a small but distinct component of younger, 1.80-1.85 Ga material.
1.9-2.1 Ga material is common in Svecofennian metasediments and in Palaeoproterozoic metasediments in the North Atlantic region in general, but exposed possible Svecofennian source rocks older than 1.9 Ga are not common. However, recent results from various parts of the Baltic Shield indicate that an early, pre-1.9 Ga phase of Svecofennian accretionary evolution has been more widespread than previously recognized. Our results from fluviatile metasediments, which demonstrate this material is derived from a mainly Palaeoproterozoic, 1.87-2.05 Ga crustal segment with an Archaean component, provide strong support for such a model. A possible source of the 1.80-1.85 Ga material from tidal and marine environments is volcanic arc rocks of the 1.83 Ga Oskarshamn-Jönköping Belt. It should also be noted that magmatic 1.9-2.1 Ga rocks are common in the Ukrainian Shield, in Amazonia and elsewhere.
Our results provide maximum deposition ages for the sedimentation. At the same time, the 1859 Ma Loftahammar granite which is intrusive into the northern part of the Västervik basin shows much of the sedimentation there is older. This raises the possibility that the Västervik sedimentation took place over an extended period of time, and was not synchronous in all parts of the basin.
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