Methods
Samples 11-SH-10 and 11-SH-12 were crushed and zircons separated by standard techniques (Rogers water table, Frantz magnetic separation and methylene iodide heavy-liquid separation). Several hundred zircons from each sample were hand-picked in ethanol under a binocular microscope, in order to ensure representative populations.
Cathodoluminescence (CL) images of zircon grains (Fig. S1) were acquired at the Centre for Microscopy and Analysis at Trinity College, Dublin (TCD) using a Tescan MiraXMU Scanning Electron Microscope with a KE Developments Centaurus Cathodoluminescence Detector. CL maps were made of the entire grain mounts and were used to select target spots for analysis. CL imaging of zircon show sector and fir-tree zoning patterns and large parts of grains which are homogeneous in CL, all of which consistent with a single phase of metamorphic growth (Corfu et al., 2003). No inherited cores were evident.
Zircons were analyzed by Quadrupole (Q-) LA-ICP-MS (Thermo Scientific iCAP Qs) at the Department of Geology, TCD. The LA-ICP-MS system was tuned while ablating a line raster on NIST SRM 612 synthetic glass in order to obtain optimal sensitivity for Pb, Th and U, while keeping ThO+ ⁄ Th low and Th ⁄U close to 1. Full running conditions are given in Crowley et al. (2014). A 35µm spot was used for ablations, which were performed using a Photon Machines 193 nm ArF Excimer laser equipped with a HeLex ablation cell. 91500 zircon (Wiedenbeck et al., 1995) was used as a primary reference material and Plešovice zircon (Sláma et al., 2008) as a secondary reference material. A standard-sample bracketing approach was utilized using 91500 as the primary reference material after every c. 10 analyses of unknowns. Plešovice gave a mean 206Pb/238U age of 341.4 ± 5 Ma (MSWD = 1.01, n=17) within error of the accepted age of 337.13 ± 0.37 Ma (Sláma et al., 2008).
All data reduction was performed using the VizualAge data reduction scheme (Petrus and Kamber, 2012) of Iolite (Paton et al, 2011). Once raw data have been imported from the mass spectrometer software, Iolite uses several steps to carry out data processing. These include baseline corrections, defining integrations for U–Pb primary reference materials, calculating a down-hole U-Th-Pb fractionation correction, defining integrations for secondary reference materials and unknowns and applying a session-wide fractionation and instrumental drift corrections. Once processed, U-Pb data were plotted using Isoplot v4.11 (Ludwig, 1993).
U-Th-Pb data are provided in Table S1. Errors and uncertainties are presented at a 2σ confidence level. All reported data are non-common Pb corrected.
References
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