Charlotte Möller

Zircon geochronology of migmatite gneisses along the Mylonite Zone (S Sweden): a major Sveconorwegian terrane boundary in the Baltic Shield

The southern section of the Mylonite Zone (MZ) is a major lithotectonic terrane boundary in the SW Baltic Shield. It separates the parautochthonous Eastern Segment of the Sveconorwegian orogen from the allochthonous Western Segment. Complex zircons in migmatised and banded orthogneisses along the southern MZ were investigated by ion microprobe analyses guided by backscattered electron imaging to directly date the partial melting and associated penetrative ductile deformation in this zone. In the eastern part of the MZ (Eastern Segment), resorbed igneous zircon cores in stromatic orthogneiss are 1686 +/- 11 Ma old, whereas extensive overgrowths and abundant newly formed simple zircons are 969 +/- 13 Ma old. Migmatised K-feldspar megacrystic granite has 1359 +/- 26 Ma igneous protolith zircon cores and abundant 968 +/- 13 Ma overgrowths and simple grains. Both rock units contain amphibolitised mafic pods with remnants of garnet-clinopyroxene-bearing, high-pressure granulite facies parageneses. In the western part of the MZ (Western Segment), igneous protolith zircon cores in stromatic orthogneiss, are dated at 1585 +/- 11 Ma and abundant new growth of zircon at 980 +/- 13 Ma. Enclosed mafic rocks have middle amphibolite facies parageneses. Secondary zircon in all three samples occurs as distinct, sub-idiomorphic overgrowths or as simple crystals with low Th/U ratios ( < 0.07). The morphology and high modal abundance of secondary zircon (25-50% of the total volume of zircon), the absence of early- or pre-Sveconorwegian secondary zircon, and field relations provide evidence for that anatexis and associated penetrative ductile deformation in the southern MZ took place between 980 +/- 13 and 968 +/- 13 Ma. These late-Sveconorwegian ages contradict previous interpretations of a pre-1.55 Ga age for the principal penetrative ductile deformation and stromatic layering of orthogneisses along the southern MZ. As a consequence, the role of the MZ as an important amalgamation zone for crustal growth in the Baltic Shield during the 1.70-1.55 Ga Gothian orogeny is questioned. Instead, the new data suggest that the MZ is a major Sveconorwegian lithologic and metamorphic terrane boundary along which middle-amphibolite facies supracrustals and orthogneisses of the Western Segment (protolith ages: 1.64-1.58 Ga) were juxtaposed against high-pressure granulite to upper-amphibolite facies orthogneisses of the Eastern Segment (protolith ages: 1.70-1.66 Ga) in late Sveconorwegian time. By implication, the pre-Sveconorwegian relation between crustal units west of the MZ and the margin of continent Baltica remains to be established. Understanding of the late Sveconorwegian tectonic evolution, particularly the amount of displacement along the MZ, is therefore, a pre-requisite for modelling the early and pre-Sveconorwegian tectonic evolution and crustal block configurations in the SW Baltic Shield.

Zircon geochronology in polymetamorphic gneisses in the Sveconorwegian orogen, SW Sweden: ion microprobe evidence for 1.46–1.42 and 0.98–0.96 Ga reworking

Ion microprobe U-Th-Pb analyses of zircons in variably metamorphosed and veined orthogneisses in the southern part of the parautochthonous Eastern Segment of the Sveconorwegian (1.20-0.90 Ga) orogen, SW Sweden, broadly define two age groups, oscillatory and sector zoned magmatic zircon cores yield 1.70-1.68 Ga while overgrowths, homogeneous crystals, and recrystallized domains in primary zircon yield 1.46-1.42 Ga. In addition, a late-kinematic pegmatite was dated at 0.96 Ga, while a penetratively deformed granite dyke contains both 1444 +/- 8 Ma magmatic and 982 +/- 15 Ma metamorphic zircons. The 1.70-1.68 Ga ages date the orthogneiss protoliths and fall in the same age range as well-preserved rocks of the Transscandinavian Igneous Belt that forms a major part of the crust east of the Sveconorwegian orogen. Despite Sveconorwegian penetrative deformation under granulite to upper amphibolite conditions, secondary zircons yielding Sveconorwegian ages are virtually absent in the 1.70-1.68 Ga orthogneisses but are abundant in rocks younger than ca. 1.45 Ga. It is suggested that Zr hosted in magmatic phases was redistributed to form new zircon during the 1.46-1.42 Ga event, resulting in a mineralogy in which the main minerals were depleted in Zr. These data, therefore, imply that high-grade metamorphism may occur without associated growth of new zircon. Furthermore, the absence of secondary zircons with ages > ca. 1.46 Ga suggests a re-assessment of models calling for extensive Gothian deformation and metamorphism in the Eastern Segment

Geochronology of eclogite facies metamorphism in the Sveconorwegian Province of SW Sweden

Abstract Decompressed eclogites in the Sveconorwegian Province, SW Sweden, have been dated using U-Pb geochronology. Zircons are common as inclusions in garnet and kyanite, and other minerals in the decompressed eclogites. Titanite inclusions are found exclusively in the core of garnets. The mineral inclusions and the chemical zoning of the garnets suggest inital growth under prograde amphibolite facies conditions followed by eclogite facies metamorphism and subsequent decompression through the high-pressure granulite and upper amphibolite facies. The zircon and titanite thus formed prior to the eclogite stage of the P-T path. The age of the eclogite forming event was determined by ion probe dating of zircon inclusions in garnets. The obtained age of 972±14 Ma is the maximum age of the eclogitisation. The age of the titanite inclusions in garnet is 945±4 Ma. This age is similar to other U-Pb ages of titanite in the region which suggest that the titanite has been isotopically reset and that the age reflects cooling. The mode of occurrence, textural relationships and the chemical homogeneity suggest that the zircons formed from Zr released from magmatitic Fe–Ti oxides and possibly amphiboles during breakdown of magmatic minerals at the onset of the Sveconorwegian metamorphism. Spot analyses of complex zircons from a granitic dyke in the eclogite yielded an age of 1403±15 Ma for magmatic cores and an age of 963±22 Ma for metamorphic rims. The older age is a minimum age of the eclogite protolith and correspond to the age of a generation of granites in the region. The rim age is within error identical to the age of eclogite metamorphism. The eclogite metamorphism in SW Sweden is younger than its Grenvillian counterparts in Scotland, Canada and USA.

40Ar/39Ar geochronology across the Mylonite Zone and the Southwester Granulite Province in the Sveconorwegian Orogen of S Sweden

Abstract The crustal segment between the Mylonite Zone (MZ) and the Protogine Zone in southern Sweden (the Southwestern Granulite Province, SGP) is characterized by numerous occurrences of granulite and upper amphibolite-facies rocks including migmatitic gneisses, mafic granulites, garnet amphibolites and charnockites. The southwest part of the Mylonite Zone (north of Varberg) marks a tectonic upper boundary of the SGP. A 40 Ar 39 Ar study was initiated across the Mylonite Zone and within the SGP in order to constrain and compare the ages obtained from within the SGP with those obtained north of and within the Mylonite Zone. 23 homblendes and one muscovite have been analyzed. The results of these analyses indicate the following. (1) Hornblendes from within and near the MZ yield 40 Ar 39 Ar hornblende ages of ∼ 915 Ma. These ages are interpreted to date the dynamic crystallization or cooling below 500°C of the deformation associated with the MZ. (2) Away from the MZ there is a distribution of ages between those which yield younger Sveconorwegian ages of 931–934 Ma, and those which yield older Sveconorwegian ages generally between 960 and 1007 Ma. The younger age may be associated with orogenic uplift, while the older one may be associated with an earlier Sveconorwegian crustal thickening event. (3) The one muscovite age of 904 Ma would indicate a late cooling rate of at least 6–15°C/Ma. Cooling rates of this magnitude may suggest that the final uplift of the Sveconorwegian orogen was related to large-scale crustal extension

Formation of sapphirine during retrogression of a basic high-pressure granulite, Roan, Western Gneiss Region, Norway

Sapphirine-bearing rocks occur in the northern part of the Western Gneiss Region, Vestranden, central Norway. The sapphirine-bearing rocks are characterized by a high MgO/(MgO + FeO) ratio, high Al2O3, MgO and CaO, and low SiO2 contents. These rocks form layers within larger complexes which originated as layered magmatic rocks. High PT-metamorphism produced a cpx+ky+gt assemblage. The P and T estimates are P = 14.5±2 kbar and T= 870±50° C. During retrogression, the high-P granulite assemblage broke down to form an intermediate-P granulite mineralogy comprising orthopyroxene, spinel, anorthite, andesine, sapphirine and corundum. Textural relationships suggest that sapphirine formed by the reaction: spinel+kyanite → sapphirine+corundum, and probably also by a reaction between corundum, spinel and orthopyroxene. All reactions took place within the stability field of kyanite. Textural and micro-chemical relationships indicate equilibrium, conditions during the peak metamorphism, whereas pronounced disequilibrium characterizes the mineral associations formed during the early retrogression at low PH2O. The investigation shows that parts of the northern segment of the Western Gneiss Region underwent a metamorphic evolution similar to the Caledonian one recorded from eclogite/granulite terrains further south.

Age constraints on the regional deformation within the Eastern Segment, S. Sweden: Late Sveconorwegian granite dyke intrusion and metamorphic-deformational relations

Abstract A post‐deformational granitic dyke and the metamorphic‐deformational relations of the rocks in the Gallared area place important constraints on the timing of regional deformation and metamorphism within the southern, internal part of the Eastern Segment of the SW Swedish Gneiss Province. The petrologic data suggest a Sveconorwegian clockwise P‐T evolution for the rocks in the Ullared‐Gallared area, characteristic for tectonically thickened continental crust. Metamorphic‐deformational relations show that the deformation of the rocks in the Ullared‐Gallared area took place under Sveconorwegian high‐ to medium‐grade metamorphic conditions. The intrusion of the Gallared granite dyke sets a clear lower limit for the deformation and the high‐grade metamorphism. Five Pb‐evaporation datings of single idiomorphic zircons from the dyke have provided a very well defined plateau age of 956±7 Ma. The granite dyke is unmetamorphosed except for very low‐grade local alterations, and truncates a high‐grade metamo...

Petrology and ion microprobe U-Pb chronology applied to a metabasic intrusion in southern Sweden: A study on zircon formation during metamorphism and deformation

The Aker metabasite occupies a key position in a major tectonic lineament in southernmost Sweden, the Protogine Zone, which coincides closely with the eastern boundary of the late Mesoproterozoic Sveconorwegian orogen of southwest Scandinavia. Metamorphic reactions, associated with the transformation from isotropic gabbro to foliated garnet amphibolite, were identified from disequilibrium textures of which some involved release of zirconium (Zr) and growth of metamorphic zircon. Ion microprobe dating of igneous zircon gave 1562 +/- 6 Ma, whereas metamorphic zircons yielded ages of 1437 +/- 21, 1217 +/- 75, and 1006 +/- 68 Ma. The presence of baddeleyite pseudomorphs made up of saccharoidal zircon and a higher abundance of older rather than younger metamorphic zircons suggest redistribution of Zr into new zircon, first by the breakdown of baddeleyite (ZrO2) and later by the consumption of igneous phases containing trace amounts of Zr. Several generations of metamorphic zircon and the presence of 1.56 and 1.22 Ga mafic intrusions along the Protogine Zone call for a complex tectonic history probably reaching back to at least similar to1.56 Ga. Growth of metamorphic zircon at similar to1.44 Ga may relate to a regional, compressional event. The WNW trending deformational structures on both sides of the Protogine Zone may possibly relate to that event. The similar to1.22 Ga metamorphic zircons are coeval with the emplacement of numerous granitic, syenitic, and mafic intrusions along and parallel to the Protogine Zone. The age around 1.0 Ga, finally, marks Sveconorwegian metamorphism for which thermobarometry of the Aker garnet-amphibolite suggests 1000-1200 MPa at 600degreesC-630degreesC. Thereafter, significant relative uplift of the rocks to the west of the Protogine Zone occurred on nearly vertical, north-south trending deformation zones. (Less)

Zircon U-Pb and Hf - isotopes from the eastern part of the Sveconorwegian Orogen, SW Sweden : Implications for the growth of Fennoscandia

Abstract Current models for the growth of Fennoscandia, including the eastern part of the Sveconorwegian Province, are largely based on U–Pb data and do not discriminate between juvenile and reworked crust. Here we present new combined U–Pb and Hf isotopic data, from the Eastern Segment and the Idefjorden terrane of the Sveconorwegian Province, and suggest a revised model of crustal growth. Most of the crystalline basement in this part of the shield formed by mixing of a 2.1–1.9 Ga juvenile component and Archaean crust. Archaean reworking decreases between 1.9 and 1.7 Ga and a mixed Svecofennian crustal reservoir is generated. Succeeding magmatism between 1.7 and 1.4 Ga indicates reworking of this reservoir with little or no crust generation. At c. 1.2 Ga, an influx of juvenile magma is recorded by granite to quartz-syenite magmatism with mildly depleted (ϵHf 1.18 Ga of c. 3) signatures. The amount of recycled crust in the 1.9–1.7 Ga arc system is in contrast to previously proposed models for the growth of the southwestern part of the Fennoscandian Shield. This model agrees with long-term subduction along the western margin of Fennoscandia, but suggests substantial reworking of existing crust and decreasing amounts of <1.9 Ga crustal growth. Supplementary material: The analytical method, U–Pb SIMS table, U–Pb LA-SF-ICP-MS table and Lu–Hf table are available at www.geolsoc.org.uk/SUP18648

Metamorphic zircon formation at the transition from gabbro to eclogite in Trollheimen-Surnadalen, Norwegian Caledonides

Abstract A transition from gabbro to eclogite has been investigated at Vinddøldalen in south-central Norway, with the aim to link reaction textures to metamorphic zircon growth and to obtain a direct U–Pb zircon age of the metamorphic process. In the different rocks of the transition zone zircon occurs as (I) igneous prismatic grains, (II) metamorphic polycrystalline rims and pseudomorphs after baddeleyite, and (III) as tiny (<10 µm) bead-like zircon grains. Textural relations suggest that type II zircon formed by breakdown of baddeleyite in the presence of silica, whereas Fe–Ti oxides were the main Zr source for the type III zircon. Subsolidus liberation of Zr and formation of bead zircon took place by oxyexsolution of titanomagnetite during fluid-assisted metamorphism, and by resorption of Fe–Ti oxide in rock domains that were completely recrystallized to eclogite. SIMS (secondary ion mass spectrometry) and TIMS (thermal ionization mass spectrometry) dating provides comparable U–Pb ages of magmatic zircon and baddeleyite. Baddeleyite (TIMS) yielded an age of 1457±11 Ma for the gabbro emplacement. Bead-type metamorphic zircon from eclogite gave 425±10 Ma (TIMS) dating the metamorphic transition from gabbro to eclogite in the upper basement of the Lower Allochthon in the south-central Scandinavian Caledonides.

A Sveconorwegian zone (system?) within the Eastern Segment, Sveconorwegian orogen of SW Sweden - a first report

Abstract A recently recognized, more than 10 km wide deformation zone (the Ullared Deformation Zone, UDZ) can be followed for at least 30 km from the Mylonite Zone southeastwards into the Eastern Segment of the Sveconorwegian orogen of SW Sweden. The deformation affects a heterogeneous, high‐grade metamorphic gneiss complex, with locally preserved eclogite relics. The deformation is characterized by a strong, locally mylonitic, gneissosity, with a subhorizontal or moderately E‐ or ESE‐plunging stretching lineation. Deformational fabrics are related to metamorphic decompression and retrogression, and kinematic indicators show a top‐to‐the‐east and dextral sense of shear. Protolith ages (zircon) of pre‐ and post‐tectonic rocks bracket the age of deformation in the eastern part of the UDZ between c. 1380 and 955 Ma, but metamorphic mineral ages suggest an upper age limit of c. 1000 Ma. The deformation in the western part of the UDZ is contemporaneous with or younger than that in the eastern part. The deforma...