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Supplementary Document 1. Analytical methods
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1) Whole-rock geochemical analyses
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X-Ray Fluorescence (XRF)
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The major elements and four trace elements (Ba, Ga, Rb and Sr) were analyzed by XRF at
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the Petrotectonics Analytical Facility, Department of Geological Sciences, Stockholm
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University. Rock powder was weighed into ceramic crucibles then roasted at 105 °C for 12
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hours and subsequently at 1000 °C for a further 12 hours to calculate loss on ignition (LOI).
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The roasted powder was reground with an agate mortar and pestle, weighed and fused with a
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flux. 2 g of the powder was mixed with 5 g of 66 % lithium tetraborate: 34 % lithium
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metaborate flux, weighed to a precision of ±0.0002 g, then fused in platinum for 10 minutes
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at 1100 °C using a Phoenix VFD automated fusion machine. This produces a homogeneous
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32 mm glass bead with a lower surface that is of sufficient quality to be analyzed directly
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without further polishing. The automated fusion machine allows an identical fusion routine to
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be used for all standards and samples thereby eliminating any potential errors generated by
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subtle differences introduced during preparation. The fused glass beads are measured by
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comparison of X-ray intensity for each element with calibration lines generated from 24
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international standards of known concentrations. Operating conditions are unremarkable and
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comparable to other facilities (e.g. Johnson et al., 1999). As an internal monitor of quality,
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two standards (USGS standards AGV 2 and RGM 1) covering the compositional range were
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analyzed as an unknown every 6th sample.
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Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS)
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After XRF analysis, representative samples were selected for further analyses. The glass
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disks were drilled using diamond bits, rinsed with deionized water, mounted in epoxy casts
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and polished for trace element analysis. Laser ablation inductively coupled plasma mass
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spectrometry (LA-ICP-MS) was performed at the Petrotectonics Analytical Facility,
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Department of Geological Sciences, Stockholm University, utilizing an UP-213 frequency-
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quintupled solid state Nd:YAG laser system from ESI/New Wave Research coupled to a
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Thermo-Scientific Xseries-2 quadrupole ICP-MS. To ensure robust plasma conditions,
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fractionation effects from incomplete ionization were minimized by tuning the ICP-MS on
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NIST SRM-610 for maximum signal-to-noise ratios across the entire mass range of interest
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(i.e. from 29Si to
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by 156CeO/140Ce and 248ThO/232Th ratios). To correct for the effects of instrumental mass bias
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and drift, a standard-sample-standard procedure (following Longerich et al., 1996) was used,
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bracketing a maximum of 16 analyses by 8 measurements of the NIST SRM 612 glass
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standard, viz. 4 before and 4 after the sample analysis. To control the accuracy the analysis
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NIST SRM 612 as an “unknown” was analyzed during the analytical session. Prior to loading,
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samples and standard were carefully cleaned with ethanol to remove surface contamination.
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After insertion, the ablation cell was purged with the carrier gas for 15 minutes to minimize
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gas blank level. To ensure stable laser output energy, a laser warm-up time of 15 minutes was
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applied prior to operation. Data were acquired from single spot analysis of 80 μm in diameter,
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using nominal laser fluency of 10-12 J/cm2 and a pulse rate of 10 Hz. The same beam size
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was used for the standard and sample analyses to avoid fractionation resulting from the use of
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different laser beam diameters. Total acquisition time for single analysis was ≤ 2 minutes,
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including a 50 seconds’ gas blank measurement followed by laser ablation for 30 seconds.
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Data reduction and determination of ratios and elemental concentrations were calculated off-
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line using the public domain software Iolite (Hellstrom et al., 2008; Paton et al., 2010).
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2) U–Pb zircon geochronology
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Handpicked zircon grains were mounted in a 25 mm diameter epoxy puck and polished to
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reveal a cross section. Cathodoluminescence (CL) and secondary electron images were made
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U) while optimizing for low element-oxide production levels (monitored
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using a Hitachi S4300 SEM fitted with a Gatan mini-CL detector in Swedish Museum of
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Natural History.
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LA-ICP-MS
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Zircons were analyzed for U, Th and Pb isotopes by LA-ICP-MS using a New Wave NWR-
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193 excimer laser coupled to a Thermo Xseries2 quadrupole ICP-MS at the Petrotectonics
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Analytical Facility, Department of Geological Sciences, Stockholm University. Each analysis
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consisted of 30 seconds’ background acquisition followed by 40 seconds’ data acquisition
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and 15 seconds’ wash-out using a 20 µm spot size. Analytical conditions included a laser
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pulse frequency of 10 Hz and a laser energy density of 8.5 J/cm2. The signal was tuned for
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maximum sensitivity for Pb and U while keeping the ThO/Th ratio below 0.5 %. External
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standardization was performed using Plešovice zircon (Sláma et al., 2008), analyzed at the
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start and end of the analytical sequence and bracketing a maximum of 10 analyses by two
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measurements of standard zircon (Plešovice). Data reduction and calculation of ratios and
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ages were performed off-line through the software Iolite (Hellstrom et al., 2008; Paton et al.,
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2011), using the Iolite-integral VizualAge DRS (data reduction scheme) routine of Petrus and
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Kamber (2012). The Iolite includes a correction routine for down-hole isotopic fractionation
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(Paton et al., 2010) and also provides routines for data that may require correction for
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common Pb, calculated through measured mass 204 (204Pb +
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described in Andersen (2002).
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Secondary Ion Mass Spectrometry (SIMS)
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High spatial resolution SIMS U-Th-Pb analyses were performed using a Cameca IMS1270
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ion-microprobe (NordSIMS facility) at the Swedish Museum of Natural History. The
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automated procedure outlined by Whitehouse and Kamber (2005) was followed: a defocused
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primary O2-ion beam resulted in elliptical analysis spots 15 µm in diameter. Pb/U ratios,
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Hg) or the approach
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elemental concentrations and Th/U ratios were calibrated relative to the zircon 91500
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reference material which has an age of 1065 Ma (Wiedenbeck et al., 1995). Common Pb was
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monitored using the
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composition from the model of Stacey and Kramers (1975) presuming that the common Pb is
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largely surface contamination introduced during sample preparation. Ages were calculated
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using Isoplot v 3.0 (Ludwig, 2003).
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3) Nd and Sr isotopic analyses
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Analyses for Sr and Nd isotopes and Sm, Nd, Rb, and Sr concentrations were performed at
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the Laboratory for Isotope Geology, Swedish Museum of Natural History. Sm and Nd
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analyses were performed on a Thermo Scientific TRITON Thermal Ionization Mass
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Spectrometry (TIMS) using the total spike method with a mixed
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Concentrations and ratios were reduced assuming exponential fractionation. Samarium
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concentrations were determined in multicollector static mode on rhenium double filaments.
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Samarium ratios were normalized to
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mode on double rhenium filaments using rotating gain compensation. Calculated ratios were
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normalized to
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values for La Jolla standard was 9.0 ppm. Accuracy correction was not necessary since the
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mean 143Nd/144Nd ratio was 0.5118484±46 (n=32).
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An aliquot of each sample was spiked for Rb and Sr. For Sr determination, an
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spike was added and after ion exchange analyzed on a Thermo Scientific TRITON TIMS
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using a load of purified sample mixed with tantalum activator on a single rhenium filament.
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Two/one hundred 8 seconds’ integrations were recorded in multicollector static mode,
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applying rotating gain compensation. Measured
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interference assuming
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204
Pb signal and corrections were made using the modern terrestrial Pb
Sm/150Nd spike.
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Sm/152Sm = 0.560667. Neodymium was run in static
Nd/144Nd = 0.7219. The external precision for
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87
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Nd/144Nd as judged from
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Sr enriched
Sr intensities were corrected for Rb
Rb/85Rb = 0.38600 and ratios were reduced using the exponential
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Sr/86Sr = 8.375209. The external precision for
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Sr/86Sr as judged
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fractionation law and
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from running 987 standard was 8 ppm (n=12) , while repeated measurements of prepared CIT
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#39 sea water gave a reproducibility of ±0.0000083 or 12 ppm (n=14), which is taken to be
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the best estimate of the external precision. An accuracy correction made as the 87Sr/86Sr ratio
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for the NBS 987 standard was 0.710217±06 (n=12). For Rb determination, the same aliquot
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was spiked with an
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sample was dissolved in water and loaded on a tantalum double filament and analyzed on a
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MAT261 TIMS. Eighty 8 sec. integrations were recorded in multicollector static mode.
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Measuring a Rb-standard determined the average fractionation to 0.39 %/AMU, which was
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the correction applied to the samples.
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Rb enriched spike. After ion exchange purification a fraction of the
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References
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Andersen, T., 2002, Correction of common lead in U-Pb analyses that do not report 204Pb:
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Chemical Geology, v. 192, p. 59-79.
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Hellstrom, J., Paton, C., Woodhead, J., and Hergt, J., 2008, Iolite: software for spatially
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resolved LA-(quad and MC) ICPMS analysis: Mineralogical association of Canada
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short course series, v. 40, p. 343-348.
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Johnson, D., Hooper, P., and Conrey, R., 1999, XRF analysis of rocks and minerals for major
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and trace elements on a single low dilution Li-tetraborate fused bead: Advances in X-
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ray Analysis, v. 41, p. 843-867.
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Longerich, H. P., Jackson, S. E., and Günther, D., 1996, Inter-laboratory note. Laser ablation
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inductively coupled plasma mass spectrometric transient signal data acquisition and
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analyte concentration calculation: Journal of Analytical Atomic Spectrometry, v. 11,
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Ludwig, K. R., 2003, Isoplot 3.0: a geochronological toolkit for microsoft excel: Berkeley
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Stacey, J. S., and Kramers, J. D., 1975, Approximation of terrestrial lead isotope evolution by
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and Spiegel, W., 1995, Three natural zircon standards for U‐Th‐Pb, Lu‐Hf, trace
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