Appendix 
Analytical Methods
SIMS U-Pb 
Two 15 kg samples of Galwolie Lamprophyre were selected for zircon extraction. Each was split into c. 2-4 cm cubes and crushed in a tungsten carbide Tema mill. A fraction passing a 3 mm mesh sieve was panned with a warm aqueous detergent solution using a Garret Gravity Trap gold pan to produce a heavy concentrate, from which zircons were directly hand-picked. Grains were mounted in epoxy resin and polished to reveal grain interiors. Representatives of the full grain size spectrum were analyzed. U-Th-Pb zircon analyses were performed on a Cameca IMS 1270 ion-microprobe using following the methodology of Whitehouse and Kamber (2005) and references therein.  A spot size of c. 20 µm, over two analytical sessions was used. U/Pb ratio calibration was based on repeat analyses of the Geostandards zircon 91500 and used a power law fit between Pb/U and UO2/U. External spot-to-spot (reproducibility) uncertainties were 1.6 % for the first session and 1.1 % for the second session. 238U/206Pb* calibration uncertainties (external error) are propagated in quadrature and included in the errors of 238U/206Pb* ratios and dates. Uncertainty on 207Pb/206Pb ratios are the greater of either the observed analytical uncertainty or the counting statistics uncertainty. All age errors quoted in the text are at the 95% confidence level. Common lead corrections were applied using a modern-day average terrestrial common Pb composition, i.e., 207Pb/206Pb = 0.83 (Stacey and Kramers, 1975), where significant 204Pb counts were recorded and is assumed to represent surface contamination.

LA-MC-ICPMS Lu-Hf 
Hafnium isotope analyses were conducted on previously dated zircon grains using a Geolas laser ablation system attached to a Thermo Finnigan Neptune multiple collector inductively coupled plasma mass spectrometer (LA-MC-ICPMS). The analytical methods and data reduction techniques are fully documented by Valley et al. (2010).  The analyses employed a beam diameter of ~50 µm and a 10 Hz repetition rate which produced typical ablation pits 40–60 µm deep. 176Yb and 176Lu were removed from the 176 mass signal using 176Yb/173Yb = 0.7938 (Segal et al, 2003) and 176Lu/175Lu = 0.2656 (Chu et al., 2002), with an exponential law mass bias correction using 173Yb/171Yb = 1.1301 (Segal et al., 2003). The interference corrected 176Hf/177Hf was normalized to 179Hf/177Hf = 0.7325 (Patchett and Tatsumoto, 1980) for mass bias correction. 176Hf/177Hf measurement outliers were removed by rejection using a two standard deviation limit. 176Lu/177Hf and 176Yb/177Hf ratios were also corrected for mass bias using 179Lu/177Hf, but without outlier rejection.

The corrected 178Hf/177Hf ratio was calculated to monitor the accuracy of the mass bias correction and yielded an average value of 1.467200 ± 0.000044 (2SD, n=16), which is within the range of values reported by Thirlwall and Anczkiewicz (2004). Plešovice zircon was analyzed as a standard and yielded an average 176Lu/177Hf ratio = 0.282474 ± 0.000035 (2SD, n=26), within uncertainty of the reported value of 0.282482 ± 0.000013 (Slama et al., 2008).

References
Chu, N. C., Taylor, R. N., Chavagnac, V., Nesbitt, R. W., Boella, R. M., Milton, J. A., German, C. R., Bayon, G. and 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.
Patchett, P. J. and Tatsumoto, M. 1980. A routine high-precision method for Lu-Hf isotope geochemistry and chronology. Contributions to Mineralogy and Petrology 75, 263-267. Thirlwall, M. F. and Anczkiewicz, R. 2004. Multidynamic isotope ratio analysis using MC-ICP-MS and the causes of secular drift in Hf, Nd and Pb isotope ratios. International Journal of Mass Spectrometry 235, 59–81.
Segal, I., Halicz, L. and Platzner, I. T. 2003. Accurate isotope ratio measurements of ytterbium by multiple collection inductively coupled plasma mass spectrometry applying erbium and hafnium in an improved double external normalization procedure. Journal of Analytical Atomic Spectrometry 18(10), 1217-1223.
Slama, 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.  Plešovice zircon – A new natural reference material for U-Pb and Hf isotopic microanalysis, Chemical Geology 249, 1-35.
Stacey, J. S. and Kramers, J. D. 1975. Approximation of terrestrial lead isotope evolution by a 2-stage model. Earth and Planetary Science Letters 26(2), 207-221.
Thirlwall, M. F. and Anczkiewicz, R. 2004. Multidynamic isotope ratio analysis using MC-ICP-MS and the causes of secular drift in Hf, Nd and Pb isotope ratios. International Journal of Mass Spectrometry 235(1), 59-81.
Valley, P., Fisher, C.M., Hanchar, J.M., Lam, R. and Tubrett, M.  2010.  Hafnium isotopes in zircon: A tracer of fluid-rock interaction during magnetite-apatite (Kiruna-type) mineralization.  Chemical Geology 275, 208-220.
Whitehouse, M.J. and Kamber, B.S. 2005. Assigning dates to thin gneissic veins in high-grade metamorphic terranes - a cautionary tale from Akilia, southwest Greenland. Journal of Petrology 46, 291-318.