Author: hmrajesh
Last saved by: Tamzin Anderson
Company: UJ

Appendix 1 – Details of CL-imaging and Ion microprobe U-Pb dating techniques
Handpicked zircons from the studied meta-anorthosite were mounted in epoxy with the reference standard, polished and coated with gold for CL imaging and ion microprobe dating. CL imaging was performed using a Hitachi S4300 SEM and Gatan mini-CL detector at the Swedish Museum of Natural History, Stockholm. 
After careful examination of the polished mounts of grains, selected domains of the zircons were analysed for U, Th and Pb isotopic compositions using a high sensitivity, high mass resolution CAMECA IMS 1270 ion microprobe hosted at the Swedish Museum of Natural History.
The operating techniques are similar to those described by Whitehouse et al. (1999), with the exception that in the present study, a Gaussian primary beam was used yielding a ca. 3nA signal in a 10 micron spot. The instrument was operated at a mass resolution of ca. 5300 (M/ΔM) to resolve Pb isotopes from HfSi species.  In order to counter increased analytical uncertainty resulting from the use of small primary beam and consequently lower secondary ion intensities, the number of mass scans was increased from 16 to 24. This resulted in a total analytical time of ca. 23 minutes, including automated pre-sputtering, beam and mass centring and energy adjustment.  Common Pb corrections were assumed to result largely from surface contamination (e.g. Zeck & Whitehouse 1999) and were corrected using the measured ^204Pb signal (where this was statistically distinguishable from background on the detector) using the present day terrestrial Pb isotope composition given by the Stacey & Kramers (1975) model. Pb/U ratios were normalised to the Geostandards zircon 91500 reference (Wiedenbeck et al. 1995), which has an age of 1065 Ma, using a power law relationship between Pb/U and UO_2/U ratios; data reduction routines were developed in house by one of the authors (MJW). All age calculations assume the decay constant recommendations of Steiger & Jäger (1977) and plots were constructed using Isoplot/Ex (Ludwig, 1998).  
References.
Ludwig, K.R. 1998. ISOPLOT for MS-DOS, a plotting and regression program for radiogenic isotope data for IBM-PC compatible computers, version 1.00. In USGS Open-file report, 82-820.
Stacey, J.S. & Kramers, J.D. 1975.  Approximation of terrestrial lead isotope evolution by a two-stage model. Earth and Planetary Science Letters, 26, 207-221.

Steiger, R.H. & Jäger, E. 1977. Subcommission on geochronology: convention on the use of decay constants in geo and cosmochronology. Earth Planetary Science Letters, 36, 359-362.
Whitehouse, M. J., Kamber, B. & 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 Geology, 160, 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 Newsletter, 19, 1-23.
Zeck, H.P. & Whitehouse, M.J. 1999.  Hercynian, Pan-African, Proterozoic and Archean ion-microprobe zircon ages for a Betic-Rif core complex, Alpine belt, W. Mediterranean - consequences for its P-T-t path. Contributions to Mineralogy and Petrology, 134, 134-149.
Supplementary online material
Appendix 1. Details of Ion microprobe U-Pb dating.
Table S1: Analytical data and derived parameters.