Ulf Söderlund

The decay constant of 176Lu determined from Lu-Hf and U-Pb isotope systematics of terrestrial Precambrian high-temperature mafic intrusions

Abstract At present, there is large uncertainty in the decay constant of 176Lu needed for Lu–Hf isotopic studies. We have determined λ176Lu by cross-calibration of U–Pb and Lu–Hf isotopic systems on mineral fractions from the Proterozoic Karlshamn and Sorkka dolerites in Sweden and Finland. The dolerites crystallized at shallow depths from homogeneous, high-temperature magmas, carry olivine that is nearly 100% unaltered, and show no signs of post-magmatic isotopic disturbance. The Lu and Hf isotopic compositions of plagioclase, olivine, pyroxene, apatite, ilmenite and baddeleyite were determined by multicollector-inductively coupled plasma mass spectrometry (MC-ICPMS). Calibrating the Lu–Hf results against baddeleyite U–Pb dates of 954.1±1.2 and 1256.2±1.4 Ma for the dolerites yields a mean λ176Lu of 1.867±0.008×10−11 year−1. The pristine character of the rocks and the agreement of the λ176Lu values with those from other terrestrial data sets [E. Scherer et al., Science 293 (2001) 683–687] suggest that the true value of λ176Lu lies between 1.86 and 1.87×10−11 year−1. Calibration experiments on extraterrestrial samples give significantly higher (4–6%) values, a discrepancy that may be due to plotting of non-cogenetic samples on the same Lu–Hf isochron diagram, or may have other, as yet undetermined, causes. The result of this study also indicates that the Lu–Hf method is capable for dating the crystallization of mafic rocks. The high 176Lu/177Hf ratio in apatite suggests that intrusive ages can be determined at a precision of a few million years or better.

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.

Timing and tempo of the Great Oxidation Event

Significance We present U-Pb ages for the extensive Ongeluk large igneous province, a large-scale magmatic event that took place near the equator in the Paleoproterozoic Transvaal basin of southern Africa at ca. 2,426 Ma. This magmatism also dates the oldest Paleoproterozoic global glaciation and the onset of significant atmospheric oxygenation. This result forces a significant reinterpretation of the iconic Transvaal basin stratigraphy and implies that the oxygenation involved several oscillations in oxygen levels across 10−5 present atmospheric levels before the irreversible oxygenation of the atmosphere. Data also indicate that the Paleoproterozoic glaciations and oxygenation were ushered in by assembly of a large continental mass, extensive magmatism, and continental migration to near-equatorial latitudes, mirroring a similar chain of events in the Neoproterozoic. The first significant buildup in atmospheric oxygen, the Great Oxidation Event (GOE), began in the early Paleoproterozoic in association with global glaciations and continued until the end of the Lomagundi carbon isotope excursion ca. 2,060 Ma. The exact timing of and relationships among these events are debated because of poor age constraints and contradictory stratigraphic correlations. Here, we show that the first Paleoproterozoic global glaciation and the onset of the GOE occurred between ca. 2,460 and 2,426 Ma, ∼100 My earlier than previously estimated, based on an age of 2,426 ± 3 Ma for Ongeluk Formation magmatism from the Kaapvaal Craton of southern Africa. This age helps define a key paleomagnetic pole that positions the Kaapvaal Craton at equatorial latitudes of 11° ± 6° at this time. Furthermore, the rise of atmospheric oxygen was not monotonic, but was instead characterized by oscillations, which together with climatic instabilities may have continued over the next ∼200 My until ≤2,250–2,240 Ma. Ongeluk Formation volcanism at ca. 2,426 Ma was part of a large igneous province (LIP) and represents a waning stage in the emplacement of several temporally discrete LIPs across a large low-latitude continental landmass. These LIPs played critical, albeit complex, roles in the rise of oxygen and in both initiating and terminating global glaciations. This series of events invites comparison with the Neoproterozoic oxygen increase and Sturtian Snowball Earth glaciation, which accompanied emplacement of LIPs across supercontinent Rodinia, also positioned at low latitude.

Conventional U-Pb multigrain dating vs. single zircon evaporation dating of complex zircons from a pegmatite in the high-grade gneisses of southwestern Sweden

U-Pb multi-grain dating of zircons from a pegmatite dyke in southwestern Sweden yield analyses which define a discordia with upper and lower intercept ages of 1510 ± 32 Ma and 631 ± 64 Ma respectively. Dating by the single-zircon evaporation method on prismatic zircons from the same dyke indicates a primary crystallization age of 1409 ± 20 Ma. In addition, four oval zircon grains yield a 207Pb206Pb age of 969 ± 27 Ma. The older single-zircon evaporation age is interpreted as representative of pegmatite emplacement, whereas the younger age is suggested to correspond to isotopic disturbance and new zircon growth during a late Sveconorwegian metamorphic event. To explain the false intercept ages in the U-Pb concordia diagram, this study suggests a three-stage model including (1) crystallization of zircons during pegmatite intrusion, (2) episodic lead-loss and new zircon growth during high-grade metamorphism and (3) a recent lead-loss event. As the dykes are strongly deformed, the protolith age of the pegmatite provides an upper age limit for the gneiss-forming event. The single-zircon age of 969 ± 27 Ma is interpreted to be associated with this event, which took place during high-grade metamorphic conditions.

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)

High-precision dating of the Kalkarindji large igneous province, Australia, and synchrony with the Early-Middle Cambrian (Stage 4-5) extinction

The voluminous Kalkarindji flood basalts erupted in Australia during the Cambrian and covered >2 x 10(6) km(2). New U-Pb and Ar-40/Ar-39 age data from intrusive rocks and lava flows yielded statistically indistinguishable ages at ca. 511 Ma, suggesting a relatively brief emplacement for this province. A zircon age of 510.7 +/- 0.6 Ma shows that this province is temporally indistinguishable at the few-hundred-thousand-year level from the Early-Middle Cambrian (Stage 4-5) boundary age of 510 +/- 1 Ma, which marks the first severe extinction of the Phanerozoic and an extended marine anoxia period. Sulfur concentration measurements ranging from < 50 to 1900 mu g/g, and fractal analysis of extensive explosive volcanic breccias, suggest that blasts and phreatomagmatic explosions have contributed to injection of large amounts of sulfur into the stratosphere. In addition, magma intrusions in oil, gas, and sulfate deposits may have generated significant emission of CH4 and SO2 which, along with volcanic gases, would have combined to cause an oscillation of the climate and led to the Cambrian extinction. (Less)

Mesoproterozoic bimodal magmatism along the Protogine Zone, S Sweden: three magmatic pulses at 1.56, 1.22 and 1.205 Ga, and regional implications

Abstract The Protogine Zone is a major tectonic zone situated at the eastern boundary of the Sveconorwegian orogenic belt in southern Sweden. This structure hosts Mesoproterozoic dolerite dykes and plutonic rocks including ultramafic cumulate, syenite, monzonorite and granite. Pb–Pb zircon evaporation ages are reported for seven samples of rocks along the Protogine Zone, including the Vaggeryd syenite pluton. Two samples of the main syenite facies of the Vaggeryd pluton yield ages of 1220±3 and 1219±3 Ma. Three samples of anorthosite gabbro, aplite and pegmatite, all of which form dykes in the Vaggeryd pluton, yield equivalent ages of 1220±3, 1221±3 and1218±3 Ma. They demonstrate that the different facies recorded in the pluton belong to a single magmatic pulse at 1220 Ma, significantly older than previous estimate of 1204±4 Ma (Jarl 2002). Samples of the Gumlösa-Glimåkra granite in NE Scania and the Åker metabasite, located immediately west of the Vaggeryd syenite, were dated at 1204±3 and 1567±3 Ma, respectively, hence reproducing earlier reported ages for these intrusions determined by conventional TIMS and SIMS U–Pb techniques (1204−15 +16 Ma, Johansson 1990 and 1562±6 Ma, Söderlund et al. 2004a). The 1575–1562 Ma suite of mafic intrusions along the Protogine Zone shows temporal correspondence with rapakivi magmatism in SW Finland and suggests that the Protogine Zone first may have originated as a zone of crustal weakness, which later was reactivated (e.g. at 1224–1215 and ca. 1204 Ma) in an extensional, possibly back-arc, tectonic setting prior to the 1.13–0.95 Ga Grenvillian-Sveconorwegian orogeny.

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.