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Jenny Andersson, Geological Survey of Sweden (Sweden)
Charlotte Moller, Geological Survey of Sweden (Sweden)
Leif Johansson, Lund University (Sweden)
Ulf Soderlund, Lund University (Sweden)
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Electron microscope images and U-Pb-Th geochronological data for zircon have been compiled for more than thirty samples of high-grade felsic orthogneisses in the polymetamorphic eastern part of the Sveconorwegian orogen, SW Baltic Shield. The zircon data have been integrated with petrological data, field data and the geologic and aeromagnetic map pattern to constrain the timing for metamorphism and deformation.. The orthogneisses have been deformed and metamorphosed in the upper amphibolite to high-P granulite facies conditions at about 0.97 Ga during the Sveconorwegian orogeny. An earlier high-grade metamorphic event included gneissification and partial melting of the same crust at about 1.45 Ga. The intensity of veining of the studied orthogneisses varies.
Sparsely veined to unveined samples essentially lack secondary zircon, while it is abundant in strongly migmatised samples. Progressive veining is thus intimately associated with increased formation of secondary zircon. The direct correlation between vein intensity and formation of secondary zircon suggest that the age of the secondary zircon dates vein formation. Three principal types of secondary zircon are identified; (1) in situ recrystallised domains with remnants of oscillatory igneous zoning (ghost zoning), commonly rich in U (>1000 ppm), and with transitional contacts to protolith zircon. (2) Unzoned or irregularly zoned (fir tree zoned) overgrowths or embayments with distinct discordant contacts to the protolith zircon. (3) Newly crystallised euhedral unzoned or irregularly zoned (fir tree zoned) simple crystals. Type 2 and 3 are more commonly low in U (<50 ppm) and are interpreted to represent newly crystallised zircon formed from resorption and subsequent re-precipitation of protolith zircon or from Zr released by other protolith mineral phases. In places where both type 1 and 2 occur in the same sample, type 1 is texturally older. Despite the high-grade polymetamorphic nature of the rocks, the investigated samples, as a rule, contain only one age generation of secondary zircon, dated at either 1.45 or 0.97 Ga. Unequivocal presence of both phases is confirmed in one sample only, in which the younger phase makes up less than 5% of the volume of secondary zircon. Furthermore, the older 1.45 Ga metamorphic event is more commonly represented by type 1 zircon whereas the 0.97 Ga old Sveconorwegian zircon almost exclusively occurs as type 2 and 3.
Zircon data for structurally different generations of melt structures have been combined with mapping of regional scale tectonic banding and fold patterns. This data has allowed identification and characterization the Sveconorwegian tectonic evolution, as well as recognition and characterization of transposed 1.45 Ga gneissic layering and vein structures.
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