Analytical methods

Zircon U-Pb LA-SF-ICP-MS analyses
Heavy minerals were separated using a Wilfley shaking-table and the heavy residue was transferred to disposable plastic Petri dishes and magnetic minerals were removed using a hand magnet. Zircon grains were hand picked directly from the final heavy mineral separate in the Petri dish. The hand picked zircon grains were moulded into epoxy and polished to expose a central cross-section of each grain. Cathodoluminescence imaging of zircon crystals was performed on polished, gold-coated samples using a Gatan CL3 detector installed on a Hitachi S-4300 electron microscope at the Swedish Museum of Natural History. Contrast and brightness were adjusted individually for each grain to enhance visualisation of internal structures. The mount was cleaned in an ultrasonic bath and with ethanol prior to loading it into the laser ablation sample cell. 
Zircon ages were obtained using a Laser-Ablation Sector Field Inductively Coupled Plasma Mass Spectrometer (LA-SF-ICP-MS) at the Department of Geological Mapping, Geological Survey of Denmark and Greenland (GEUS). The laser ablation microprobe uses a focused laser beam to ablate a small amount of a sample contained in an airtight sample cell. Detailed analytical protocols are described by Gerdes and Zeh (2006) and Frei and Gerdes (2009), but a brief summary is given here. Samples and standards were mounted in a low-volume ablation cell specially developed for U-Pb-dating (Horstwood et al. 2003). Helium was used to flush the sample cell and was mixed downstream with the Ar sample gas of the mass-spectrometer. We used a NewWave Research®/Merchantek® UP213 laser ablation unit, which emits a beam wavelength of 213 nm and a 10 Hz repetition rate. For the spot diameter (20 μm) and ablation times (30 s) used in this study, the ablated mass of zircon was typically between 150-300 ng. The ablated material was transferred to the mass-spectrometer in an Ar-He carrier gas via Tygon® tubing into an Element2 (ThermoFinnigan®, Bremen) single-collector double focusing magnetic sector ICPMS. The total acquisition time for each analysis was 60 seconds of which the first 30 seconds were used to determine the gas blank. The instrument was tuned to give large, stable signals for the 206Pb and 238U peaks, low background count rates (typically around 150 counts per second for 207Pb) and low oxide production rates (238U16O/238U generally below 2.5 %). 202Hg, 204(Pb + Hg), 206Pb, 207Pb, 208Pb, 232Th and 238U intensities were determined through peak jumping using electrostatic scanning in low resolution mode and with the magnet resting at 202Hg. Each peak was determined at four slightly different masses and integrated sampling and a settling time of 1 ms for each isotope. Mass 202Hg was measured to monitor the 204Hg interference on 204Pb where the 202Hg/204Hg ≡ 4.36, which can be used to correct significant common Pb contributions using the model by Stacey and Kramers (1975). 207Pb/235U was calculated from the 207Pb/206Pb and 206Pb/238U assuming 238U/235U ≡ 137.88. The laser induced elemental fractionation and the instrumental mass bias on measured isotopic ratios was corrected through standard-sample bracketing using the GJ-1 zircon (Jackson et al. 2004). Samples were analysed in sequences were three standards bracket each set of ten samples. The Plesovice zircon standard (Aftalion et al. 1989) has been used as an external reproducibility check, and yield long-term 2σ RSD precisions (n=109) of 2%, 2.3% and 1.1% for the 206Pb/238U, 207Pb/235U and 207Pb/206Pb ratios respectively (Frei et al. 2006). The raw data is corrected for instrumental mass bias and laser-induced U-Pb fractionation through normalisation to the GJ-1 zircon using in-house data reduction software. 
All isotope data was plotted and evaluated using Isoplot/EX 3.06 (Ludwig 2003). Model age calculation and error propagation follows Sambridge and Lambert (1997).

Zircon U-Pb SIMS analyses
Secondary ion mass spectrometer (SIMS) U-Th-Pb analyses were carried out using a large geometry Cameca IMS1270 instrument at the Swedish Museum of Natural History.  Instrument set up broadly follows that described by Whitehouse and Kamber (2005) and references therein. An O2- primary beam with 23 kV incident energy (-13kV primary, +10 kV secondary) was used to sputter zircon.  For this study, the primary beam was operated in aperture illumination (Köhler) mode yielding a c. 15-20 μm spot.  Presputtering with a 25 μm raster for 120 seconds, centering of the secondary ion beam in the 3000 μm field aperture (FA), mass calibration optimisation, and optimisation of the secondary beam energy distribution were performed automatically for each run, FA and energy adjustment using the 90Zr216O+ species at nominal mass 196.  Mass calibration of all peaks in the monocollection sequence was performed at the start of each session; within run mass calibration optimisation scanned only those peaks that yield consistently high signals from the zircon matrix, namely 90Zr216O+, 94Zr216O+ (nominal mass 204), 177HfO2+ (nominal mass 209), 238U+ and 238U16O2+, with intermediate peaks adjusted by interpolation. A mass resolution (M/ΔM) of c. 5400 was used to ensure adequate separation of Pb isotope peaks from nearby HfSi+ species. Ion signals were detected using the axial ion-counting electron multiplier.  All analyses were run in fully automated chain sequences.  
Data reduction assumes a power law relationship between Pb+/U+ and UO2+/U+ ratios with an empirically derived slope in order to calculate actual Pb/U ratios based on those in the 91500 standard. U concentrations and Th/U ratio are also referenced to the 91500 standard. Common Pb corrections are made only when 204Pb counts statistically exceed average background and assume a 207Pb/206Pb ratio of 0.83 (equivalent to present day Stacey and Kramers (1975) model terrestrial Pb). Age interpretations use the routines of Isoplot/Ex (Ludwig 2003). Decay constants follow the recommendations of Steiger and Jäger (1977).

Zircon Hf MC-LA-ICP-MS analyses
Following U-Pb isotope work, Hf-analyses were made in situ at the Memorial University, St. Johns, Newfoundland, Canada, using a Finnigan Neptune Multicollector ICP-MS connected to a GeoLas Laser ablation system. The Multi Collector LA-ICP-MS was equipped with nine Faraday detectors and 171Yb, 173Yb, 174Hf+Yb, 175Lu, 176Hf+Yb+Lu, 177Hf, 178Hf and 179Hf data was collected during 600 pulses, c. 60 seconds. Laser repetition rate of 10 Hz was applied and laser flux was maintained at 5 J/cm2. Ablation was conducted with He carrier gas in the ablation cell and with Ar makeup carrier gas added afterwards via T-piece just before the torch. The gas background was measured for 30 s in the beginning of each run and used for blank corrections. Blank corrected signal intensities were corrected for isobaric interferences of Yb and Lu on 176Hf. 179Hf/177Hf = 0.7325 (Patchett & Tatsumoto 1980) and the exponential law, was used for mass bias correction of Hf. 173Yb/171Yb = 1.1301 and 176Yb/171Yb = 0.7938 (Segal et al. 2003), with the exponential law, was used for mass bias correction of Yb. No invariant ratio for Lu is available as Lu only has two naturally occurring isotopes (175Lu and 176Lu). Therefore it was assumed that the mass bias constant for Lu is the same as for Yb, which β-value was used to correct for Lu with 176Lu/175Lu = 0.2656 (Chu et al. 2002).
Similar zircon phases as dated by U-Pb analyses were used for Hf-analysis. A spot size between 49 and 69 µm was used, and cracks, inclusions and texturally complex BSE zones were avoided. Data quality was controlled using standards Plesovice (Sláma et al. 2008), R-33 (Black et al. 2004), FC-1 and Temora (Woodhead and Hergt 2005). 
We use ΔεHft as a measure of the magnitude in parts per 10 000 of the age corrected 176Hf/177Hf for each sample where

ΔεHft = [(176Hf/177Hfmeas./176Hf/177Hft)-1] × 10 000

176Hf/177Hfmeas. is the measured zircon Hf-isotope composition, corrected for mass bias, 176Lu and 176Yb interference. 176Hf/177Hft is the age corrected zircon Hf-isotope composition corrected t using the in situ measured 176Lu/177Hf. Ideally for low Lu/Hf zircon, the correction is effectively zero, while high Lu/Hf zircon have a correction magnitute <3, i.e. similar or slightly greater than in-run analytical precision.
All results are presented as 2σ errors, and current depleted mantle values 176Hf/177Hf = 0.28325 and 176Lu/177Hf = 0.0384 from Griffin et al. (2002) and 176Lu decay constant of Scherer et al. (2001) and Söderlund et al. (2004) were used in calculations. Values for CHUR are from Bouvier et al. (2008).

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Chu, N.C., Taylor, R.N., Chavagnac, V., Nesbitt, R.W., Boella, R.M. Milton, J.A., German, C.R., Bayon, G. & Burton, K. 2002. Hf isotope ratio analysis using multi-collector inductively coupled plasma mass spectrometry: an evaluation of isobaric interference corrections. Journal of Analytical Atomic Spectrometry, 17 (12), 1567–1574.

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Gerdes, A. & Zeh, A. 2009. Zircon formation versus zircon alteration — New insights from combined U–Pb and Lu–Hf in-situ LA-ICP-MS analyses, and consequences for the interpretation of Archean zircon from the Central Zone of the Limpopo Belt. Chemical Geology, 261, 230–243. doi:10.1016/j.chemgeo.2008.03.005

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Sláma, J., Kosler, J., Condon, D.J., Crowley, J.L., Gerdes, A., Hanchar, J.M., Horstwood, M.S.A., Morris, G.A., Nasdala, L., Norberg, N., Schaltegger, U., Schoene, B., Tubrett, M.N. & Whitehouse, M.J. 2008. Plesovice zircon - A new natural reference material or U-Pb and Hf isotopic microanalysis. Chemical Geology, 249, 1–35.

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Söderlund, U,. Patchett, P.J., Vervoort, J.D. & Isachsen, C.E. 2004. The 176Lu decay constant determined by Lu–Hf and U–Pb isotope systematics of Perecambrian mafic intrusions, Earth and Planetary Science Letters, 219, 311–324.

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