Proterozoic Crustal Building in the Sveconorwegian Province: Evidences from Zircon Ages

Geochronology of the Kautokeino Greenstone Belt, Finnmark

Electronic Supplement 1: Analytical methods

Major and trace element analyses

Samples were analysed for major and trace elements by X-ray fluoresence at the Geological Survey of Norway (Electronic Supplement2). Geochemical data are plotted using the GCDKit software (Janoušek et al., 2006). Granitoids are named following the normative albite–anorthite–orthoclase ternary classification diagram of O'Connor (1965; Fig. 7).

U–Pb zircon SIMS analyses

Zircon was separated from crushed rock, using a water table, heavy liquids and magnetic separation. Zircon crystals were selected by hand picking in alcohol under a binocular microscope. Selected crystals were mounted in epoxy together with the reference zircon, and polished with 6, 3, and 1 µm diamond paste. The grains were imaged individually with a backscattered electron detector and a panchromatic cathodoluminescence (CL) detector in a variable pressure Scanning Electron Microscope at the Geological Survey of Norway.

Zircon U–Pb analyses were performed on 16 of the samples by Secondary Ion Mass Spectrometry (SIMS) with the Cameca IMS 1280 instrument at the NORDSIM laboratory in Stockholm, with a primary oxygen beam. Analytical protocols and data reduction follow Whitehouse et al. (1999) and Whitehouse and Kamber (2005). Analyses were calibrated using the91500 Geostandard zircon with an age of 1065 Ma(Wiedenbeck et al., 1995), measured regularly after 5 unknown analyses. The diameter of the analytical pit ranges from 12 to 20 μm. The analyses are corrected for common Pb using the 204Pb signal, if this signal is above background (Electronic Supplement3).

Zircon U–Pb data are reported in a Tera–Wasserburg (inverse) concordia diagram, prepared with the ISOPLOT macro for Microsoft Excel(Ludwig, 2001). The weighted average 207Pb/206Pb age is selected as best age estimate for clusters of concordant analyses. Alternatively, the upper intercept age is selected for partly discordant sets of analyses. Errors are quoted at a 2 level (decay constant uncertainties and systematic uncertainties resulting from interlaboratory experiments are not propagated).

U–Pb zircon LA–ICP–MS analyses

For 3 samples, zircon U–Pb analyses were performed instead by laser ablation inductively coupled plasma mass spectrometry (LA–ICP–MS)after a similar sample preparation. Analyses were done at the Geological Survey of Norway with a double-focusing sector field, single collector, mass spectrometer, model ELEMENT XR from Thermo Scientific, fed by a UP193–FX 193 nm short-pulse excimer laser ablation system from New Wave Research.The laser is programmed to ablate up to 60 µm-long lines, using a spot size of 15 µm, a repetition rate of 10 Hz and an energy corresponding to a flux of 4–5 J/cm2. The ablated material is transported in He, and mixed with Ar via a Y-piece for introduction in the mass spectrometer. Each analysis includes 30 s of background measurement followed by 30 s of ablation. Masses 202Hg, 204(Hg + Pb), 206Pb, 207Pb, 208Pb, 232Th and 238U are measured. The reference material GJ–1 (Jackson et al., 2004) is used for calibration of isotopic ratios. Geostandard zircon 91500 (Wiedenbeck et al., 1995) and reference zircons OS–99–14 (1797 ± 3 Ma; Skår, 2002) and Kaap Valley Tonalite (3227 ± 1 Ma; Kamo & Davis, 1994) are analysed at regular intervals to monitor drift, precision and accuracy. The data are not corrected for common Pb, but monitoring of the signal from mass 204 allows exclusion of analyses containing common Pb. LA–ICP–MS data (Electronic Supplement 4) are reduced using the GLITTER® software (Van Achterbergh et al., 2001).

40Ar–39Ar biotite geochronology

The sample was crushed, ground and subsequently sieved to obtain 180–250 μm fractions. The mineral separates were washed in acetone and deionised water several times before handpicking under a stereomicroscope. Mineral grains with coatings or inclusions were avoided. The samples were packed in aluminum capsules together with the Hb3gr amphibole flux monitor standard along with pure (zero age) K2SO4 and CaF2 salts. The samples were irradiated together at IFE (Institutt for Energiteknikk, Kjeller, Norway) for 157 hours with a nominal neutron flux of 1.3×1013 n*(cm-2 *s-1). The correction factors for the production of isotopes from Ca were determined to be (39Ar/37Ar)Ca = (7.907 ± 0.0653)*10-4, (36Ar/37Ar)Ca = (3.1122 ± 0.0473)*10-4 and (40Ar/39Ar)K = (1.614698 ± 0.12631)*10-2 for the production of K (errors quoted at 1σ). The samples were step heated using a defocused Merchantek CO2 laser. The extracted gases were passed over SAES GP–50 getters for the first 2 minutes, then for 9 minutes in a separate part of the extraction line with SAES AP–10 getters. They were analyzed with anautomated MAP 215–50 mass spectrometer in static mode, installed at the Geological Survey of Norway. The peaks were determined during peak hopping for 10 cycles (15 integrations per cycle) on the different masses (41–35Ar) on a Balzers electron multiplier and regressed back to zero inlet time. Blanks were analyzed every third measurement. After blank correction, a correction for mass fractionation, 37Ar and 39Ar decay and neutron-induced interference reactions produced in the reactor was undertaken using in-house software. It implements the equations of McDougall & Harrison (1999) and the newly proposed decay constant for 40K after Renne et al. (2010). A 40Ar/36Ar ratio of 298.56 ± 0.31 from Lee et al. (2006)[w1], was used for the atmospheric argon correction and mass fractionation calculation (power law). We calculated J-values relative to an age of 1080.4 ± 1.1 Ma for the Hb3gr sanidine flux monitor (Renne et al., 2010). Weighted mean ages are calculated by weighting on the inverse of the variance (analytical uncertainties). All ages are reported at the 1.96σ confidence level (Electronic Supplement5).

Reference list

Jackson, S.E., Pearson, N.J., Griffin, W.L. & Belousova, E.A. 2004: The application of laser ablation-inductively coupled plasma-mass spectrometry to in situ U–Pb zircon geochronology. Chemical Geology211, 47–69.

Janoušek, V., Farrow, C.M. & Erban, V. 2006: Interpretation of whole-rock geochemical data in igneous geochemistry: introducing Geochemical Data Toolkit (GCDkit). Journal of Petrology47, 1255–1259.

Kamo, S.L. & Davis, D.W. 1994: Reassessment of Archean crustal development in the Barberton Mountain Land, South Africa, based on U–Pb dating. Tectonics13, 167–192.

Ludwig, K.R. 2001: Users manual for Isoplot/Ex version 2.49, a geochronological toolkit for Microsoft Excel. Berkeley Geochronology Center Special Pubication 1a, Berkley, 53 pp.

McDougall, I. & Harrison, M. 1999: Geochronology and thermochronology by the 40Ar/39Ar method. Oxford Univerity Press, Oxford, XX pp.[w2]

O'Connor, J.T. 1965: A classification for quartz-rich igneous rocks based on feldspar ratios. US Geological Survey Professional PaperB525, 79–84.

Renne, P.R., Mundil, R., Balco, G., Min, K. & Ludwig, K.R. 2010: Joint determination of 40K decay constants and 40Ar/40K for the Fish Canyon sanidine standard, and improved accuracy for 40Ar/39Ar geochronology. Geochimica et Cosmochimica Acta74, 5349–5367.

Skår, Ø. 2002: U-Pb geochronology and geochemistry of early-Proterozoic rocks of the tectonic basement windows in central Nordland, Caledonides of north-central Norway. Precambrian Research116, 265–283.

Van Achterbergh, E., Ryan, C.G., Jackson, S.E. & Griffin, W.L. 2001: Appendix 3. Data reduction software for LA-ICP-MS. In Sylvester, P.J. (ed.): Laser-Ablation-ICPMS in the earth sciences-principles and applications, Mineralogical Association of Canada Short Course Series29, pp. 239–243.

Whitehouse, M.J. & Kamber, B.S. 2005: Assigning dates to thin gneissic veins in high-grade metamorphic terranes: a cautionary tale from Akilia, Southwest Greenland. Journal of Petrology46, 291–318.

Whitehouse, M.J., Kamber, B.S. & Moorbath, S. 1999: Age significance of U-Th-Pb zircon data from early Archaean rocks of west Greenland – a reassessment based on combined ion-microprobe and imaging studies. Chemical Geology160, 201–224.

Wiedenbeck, M., Allé, P., Corfu, F., Griffin, W.L., Meier, M., Oberli, F., Von Quadt, A., Roddick, J.C. & Spiegel, W. 1995: Three natural zircon standards for U-Th-Pb, Lu-Hf, trace element and REE analyses. Geostandards Newsletter19, 1–23.

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