Results and interpretation Analytical methods

Fig. 2. Cathodoluminescence images of selected grains from Butt of Lewis diorite sample DB96-S117. enhance secondary ion yields of 206 Pb to ca. 15 – 20 cps nA − 1 ppm − 1 Whitehouse et al., 1997a. PbU ratios were calibrated relative to the 1065 Ma Geostandards zircon 91500 Wiedenbeck et al., 1995 using a power law relationship between PbU and UO 2 U ratios. Corrections for common Pb assume that this is largely derived from surface contamination during polishing of grain mounts e.g. introduction into micro-cracks; this is moni- tored using 204 Pb, with the correction assuming a present day terrestrial Pb-isotopic composition Stacey and Kramers, 1975. During the course of this study, 69 analyses of the 91500 standard zircon yielded a weighted average 207 Pb 206 Pb age of 1063 9 9 Ma MSWD = 3.2, no rejections and 1059 9 6 Ma MSWD = 0.5, n = 60. Analytical data and derived ages are presented in Table 1 all ages are calculated with the decay constants of Steiger and Ja¨ger 1977. All weighted average and regression ages have been calculated using IsoplotEx Ludwig, 1998, with errors reported at the 95 ca. 2s confidence level.

5. Results and interpretation

5 . 1 . Butt of Lewis diorite, Lewis, DB 96 -S 117 Zircons from the Butt of Lewis diorite sample range in size from ca. 150 – 500 mm and in mor- phology from anhedral, almost spherical, to euhe- dral bi-pyramidal grains. Some rounding of the crystal faces and vertices of all grains probably results from cataclasis andor high-grade meta- morphism. Cathodoluminesence CL imaging of these grains Fig. 2 reveals complex internal structures reflecting a number of growth andor reworking phases. Most grains exhibit two or three growth phases, within which CL images identify CL dark, zoned ‘cores,’ which are par- tially resorbedreworked rounded margin and truncation of zoning by a structureless CL bright phase, itself rounded ‘core overgrowth’. Three phase grains show both CL dark and CL bright pyramidal terminations ‘rims’, some of which show zoning. These CL bright terminations are developed on a thin too small to analyse CL dark phase. ning through Berneray, precludes unambiguous assignment to Archean or Proterozoic crust gener- ation episodes. Sample MJW97-Ber43 locality, NF 927819 is a tonalitic rock, which has been isoclinally folded together with flaggy diorites, but clearly cuts an earlier foliation in the diorite, and thus, represents a later phase cf. Ba`gh Steinigidh ?co-magmatic rocks. This sample was collected in order to test whether Paleoproterozoic calc-alka- line magmatism, similar to that seen in the South Harris igneous complex, extends south into the Sound of Harris.

4. Analytical methods

Ion microprobe U – Th – Pb analyses were per- formed using the Cameca IMS 1270 high mass- resolution instrument in Stockholm Nordsim facility. Details of instrument parameters and basic analytical techniques and data reduction have been given by Whitehouse et al. 1997b. Analyses presented in this study were all obtained using an oxygen-bleed into the sample chamber to M .J . Whitehouse , D . Bridgwater Precambrian Research 105 2001 227 – 245 Table 1 U–Th–Pb analytical data and derived parameters a 207 Pb 206 Pb [U] ppm 206 Pb 238 U Discordance d 207 Pb 206 Pb [Pb] ppm 206 Pb 238 U ThU meas. f 206 c Samplespot b Ma 9 s Ma 9 s Butt of Lewis diorite, Lewis, DB 96 -S 117 i Rims 0.1932 9 0.0015 -34 1676 9 5 1138 9 8 0.1028 9 0.0003 378 25b 0.49 0.005 81 0.1092 9 0.0031 29 0.3258 9 0.0035 1787 9 52 1818 9 17 11 0.014 3.28 15a 7 0.1117 9 0.0027 0.3127 9 0.0029 -3 1827 9 44 1754 9 14 3 0.013 29e 3.73 0.2684 9 0.0030 -17 1839 9 57 1533 9 15 0.1124 9 0.0035 0.015 5.97 29e rpt. 43 15 0.1125 9 0.0006 345 0.3246 9 0.0035 1840 9 10 1812 9 17 125 0.006 0.32 31a 0.1128 9 0.0008 140 0.3335 9 0.0039 1845 9 13 1855 9 19 52 0.008 0.16 10b 0.3283 9 0.0030 1846 9 5 1830 9 14 0.1129 9 0.0003 4a 0.24 0.001 462 1258 0.1132 9 0.0005 677 0.3296 9 0.0075 1851 9 7 1836 9 37 250 0.001 0.26 22 0.3298 9 0.0041 1854 9 14 1838 9 20 0.1134 9 0.0009 134 31a rpt. 0.15 0.006 125 0.1137 9 0.0011 169 0.3345 9 0.0039 1860 9 18 1860 9 19 63 0.007 0.11 7b 0.3219 9 0.0044 -1 1862 9 15 1799 9 21 7b rpt. 70 25 0.006 0.02 0.1138 9 0.0010 0.3263 9 0.0032 -1 1864 9 14 1820 9 16 0.1140 9 0.0009 115 2 0.04 0.008 42 0.1145 9 0.0010 67 0.3424 9 0.0042 1873 9 15 1898 9 20 26 0.008 0.06 10b rpt. 0.06 0.1149 9 0.0004 0.3407 9 0.0032 1879 9 7 1890 9 15 11a 557 212 0.004 0.3479 9 0.0033 1 1883 9 7 1924 9 16 0.1152 9 0.0005 0.002 53 19a 137 0.09 0.1176 9 0.0053 9 0.3257 9 0.0025 -5 1920 9 81 1818 9 12 4 0.011 5.50 26c 0.43 0.1304 9 0.0017 0.3666 9 0.0037 -3 2103 9 23 2013 9 17 13 115 57 0.644 ii Cores, two and three phase grains 0.2233 9 0.0114 -34 2040 9 36 1299 9 60 0.1258 9 0.0026 0.121 1.63 32 270 73 0.1389 9 0.0005 921 0.3509 9 0.0039 -12 2213 9 6 1939 9 18 397 0.301 0.06 31b 0.3863 9 0.00028 -6 2256 9 11 2106 9 13 17b 21 11 0.664 0.50 0.1424 9 0.0009 0.3487 9 0.0045 -15 2275 9 8 1928 9 22 0.1439 9 0.0006 398 31b rpt. 0.07 0.318 172 0.1527 9 0.0009 18 0.4015 9 0.0030 -9 2376 9 10 2176 9 14 9 0.586 0.44 28a 0.1571 9 0.0037 806 0.4082 9 0.0044 -9 2425 9 40 2207 9 20 459 0.687 2.12 10d 0.4344 9 0.0034 -5 2450 9 9 2326 9 15 0.1595 9 0.0008 0.550 0.12 4b 25 14 0.1623 9 0.0010 261 0.4393 9 0.0055 -4 2479 9 10 2347 9 25 160 0.680 2.03 10d rpt. 0.1635 9 0.0006 26 0.4900 9 0.0041 2 2492 9 7 2571 9 18 16 0.219 0.06 29b 0.4499 9 0.0033 -4 2507 9 5 2395 9 15 0.1650 9 0.0005 27a 0.89 0.154 47 85 0.06 0.1651 9 0.0008 0.4719 9 0.0109 2509 9 8 2492 9 48 24 237 156 0.740 0.4551 9 0.0042 -3 2517 9 8 2418 9 19 0.1660 9 0.0008 201 14 0.05 0.589 121 0.1671 9 0.0008 526 0.4103 9 0.0029 -13 2528 9 8 2216 9 13 301 0.749 0.08 30a rpt. 0.1672 9 0.0014 1770 0.4446 9 0.0053 -5 2530 9 15 2371 9 24 933 0.042 0.33 30b rpt. 0.4876 9 0.0112 2538 9 8 2560 9 48 0.1680 9 0.0008 20 0.04 1.214 150 200 0.1689 9 0.0015 1033 0.4490 9 0.0031 -6 2547 9 15 2391 9 14 548 0.026 0.12 30b rpt. 0.4387 9 0.0034 -8 2548 9 8 2345 9 15 0.1691 9 0.0008 0.203 153 29b rpt. 283 0.05 0.1695 9 0.0012 88 0.4472 9 0.0057 -6 2552 9 12 2383 9 26 51 0.337 0.62 15b rpt. 0.17 0.1713 9 0.0015 0.4935 9 0.0055 2570 9 15 2586 9 24 30a 1124 784 0.857 M .J . Whitehouse , D . Bridgwater Precambrian Research 105 2001 227 – 245 233 Table 1 Continued [U] ppm 207 Pb 206 Pb 206 Pb 238 U Discordance d 207 Pb 206 Pb [Pb] ppm 206 Pb 238 U ThU meas. Samplespot b f 206 c Ma 9 s Ma 9 s 21b 0.4859 9 0.0357 128 2603 9 14 2553 9 155 125 3.074 0.14 0.1747 9 0.0014 0.5070 9 0.0055 2608 9 15 2644 9 24 0.1752 9 0.0015 228 15b 0.24 0.370 148 0.1763 9 0.0015 116 0.5061 9 0.0389 2618 9 15 2640 9 167 74 0.340 0.23 6b 0.24 0.1772 9 0.0022 0.4526 9 0.0036 -9 2627 9 20 2407 9 16 82 12b 60 1.481 0.4648 9 0.0042 -6 2634 9 8 2461 9 18 0.1780 9 0.0009 0.754 0.29 5 141 92 0.1799 9 0.0010 239 0.5034 9 0.0046 2652 9 9 2628 9 20 166 0.674 0.04 7c rpt. 0.1828 9 0.0022 27 0.4844 9 0.0037 -5 2678 9 20 2546 9 16 16 0.260 0.28 26b 0.5053 9 0.0055 2679 9 14 2636 9 24 0.1828 9 0.0015 8 0.07 0.821 166 232 0.03 0.1830 9 0.0005 0.4961 9 0.0062 -1 2680 9 5 2597 9 27 11b rpt. 145 179 1.660 0.4937 9 0.0036 -3 2692 9 9 2587 9 15 0.1843 9 0.0010 21 11b 0.07 1.796 18 0.1871 9 0.0012 860 0.5159 9 0.0058 2717 9 10 2682 9 25 641 0.950 0.04 7c 19b 0.5409 9 0.0069 68 2822 9 22 2787 9 29 49 0.451 0.80 0.1995 9 0.0026 0.5580 9 0.0069 2843 9 14 2858 9 29 0.2021 9 0.0017 20 27 19b rpt. 0.76 0.460 iii Core o6ergrowths, two and three phase grains 0.3675 9 0.0054 -5 2162 9 33 10c 2018 9 25 87 42 0.614 0.37 0.1348 9 0.0026 0.3682 9 0.0063 -8 2226 9 21 2021 9 30 0.1399 9 0.0017 0.202 0.39 16 12 5 0.1439 9 0.0018 3 0.4255 9 0.0036 2275 9 22 2286 9 16 1 0.102 0.65 27b 0.1471 9 0.0025 54 0.3966 9 0.0043 -6 2312 9 30 2153 9 20 26 0.146 0.42 30c 0.3835 9 0.0075 -8 2329 9 27 2093 9 35 0.1485 9 0.0024 30c rpt. 0.32 0.137 9 20 0.86 0.1540 9 0.0050 0.3773 9 0.0055 -14 2391 9 56 2064 9 26 50 10c rpt. 26 0.569 0.4478 9 0.0047 2392 9 19 2386 9 21 0.1541 9 0.0018 0.056 0.25 26a 2 1 0.1562 9 0.0041 2 0.4922 9 0.0038 7 2415 9 44 2580 9 16 1 0.149 3.11 29c 0.4840 9 0.0061 3 2426 9 12 2545 9 26 15c rpt. 53 30 0.078 0.05 0.1572 9 0.0011 0.4529 9 0.0045 2434 9 41 2408 9 20 0.1580 9 0.0038 3 29a 1.52 0.165 2 0.1622 9 0.0010 120 0.4547 9 0.0105 2479 9 11 2416 9 47 70 0.310 1.68 25a 0.1640 9 0.0046 7 0.4202 9 0.0084 -8 2497 9 48 2261 9 38 3 0.030 0.29 3 0.4739 9 0.0054 2530 9 11 2500 9 24 0.1672 9 0.0011 0.364 0.19 15c 120 73 0.1675 9 0.0015 51 0.4846 9 0.0045 2533 9 15 2547 9 20 38 1.134 0.11 17a 0.1678 9 0.0015 27 0.4890 9 0.0124 2536 9 15 2566 9 54 21 1.347 0.25 21a 0.4690 9 0.0044 -4 2597 9 13 2479 9 19 0.1740 9 0.0013 18 0.02 0.711 96 146 0.28 0.1754 9 0.0026 0.4428 9 0.0040 -10 2609 9 24 2363 9 18 29 9 19 0.961 0.4810 9 0.0048 -3 2627 9 7 2531 9 21 0.1772 9 0.0008 0.17 1 56 34 0.261 23 0.4892 9 0.0113 67 2662 9 14 2567 9 49 48 0.954 0.12 0.1810 9 0.0015 Ba`gh Steinigie tonalite, South Harris, DB 96 -S 144 0.02 0.1119 9 0.0012 0.3426 9 0.0064 1831 9 19 1899 9 31 227 16a 0.359 550 0.3475 9 0.0026 3 1850 9 8 1922 9 13 0.1131 9 0.0005 5a 0.04 0.096 111 281 59 0.1137 9 0.0012 0.3218 9 0.0024 -2 1859 9 19 1799 9 12 22 0.127 11a 0.23 0.3331 9 0.0066 1863 9 11 1853 9 32 0.1140 9 0.0007 0.145 215 20a 551 0.56 0.1140 9 0.0004 1088 0.2701 9 0.0024 -18 1863 9 6 1541 9 12 350 0.233 0.38 12a 0.01 0.1142 9 0.0004 0.3341 9 0.0061 1868 9 6 1858 9 30 22a 1644 666 0.355 M .J . Whitehouse , D . Bridgwater Precambrian Research 105 2001 227 – 245 Table 1 Continued [U] ppm 207 Pb 206 Pb 206 Pb 238 U Discordance d 207 Pb 206 Pb [Pb] ppm 206 Pb 238 U ThU meas. Samplespot b f 206 c Ma 9 s Ma 9 s 0.3332 9 0.0062 14a 591 1869 9 7 1854 9 30 236 0.299 0.02 0.1143 9 0.0005 0.3430 9 0.0026 1869 9 5 1901 9 12 0.1143 9 0.0003 624 10a 0.03 0.410 264 0.1144 9 0.0006 653 0.3422 9 0.0064 1870 9 9 1897 9 31 272 0.378 0.04 23a 0.01 0.1145 9 0.0004 0.3453 9 0.0061 1871 9 7 1912 9 29 510 19a 212 0.312 0.3430 9 0.0031 1874 9 7 1901 9 15 0.1146 9 0.0004 0.286 0.05 9a 443 182 0.1147 9 0.0005 506 0.3310 9 0.0061 1875 9 8 1843 9 30 203 0.355 0.04 13a 0.1147 9 0.0004 521 0.3532 9 0.0065 1 1876 9 7 1950 9 31 219 0.283 0.01 21a 0.3385 9 0.0061 1876 9 6 1879 9 29 0.1148 9 0.0004 18a 0.00 0.326 264 650 0.02 0.1149 9 0.0007 0.3395 9 0.0026 1879 9 11 1884 9 12 232 1a 91 0.134 0.3453 9 0.0061 1879 9 5 1912 9 29 0.1150 9 0.0003 976 17a 0.02 0.367 410 0.1150 9 0.0002 1661 0.3417 9 0.0065 1880 9 4 1895 9 31 685 0.335 0.01 25a 0.1151 9 0.0005 685 0.3206 9 0.0057 -2 1882 9 8 1793 9 28 265 0.314 0.04 24a 0.3330 9 0.0061 1883 9 7 1853 9 29 0.1152 9 0.0004 15a 0.01 0.344 260 646 0.3346 9 0.0026 1887 9 16 1861 9 13 1b 92 36 0.139 0.28 0.1155 9 0.0010 0.3237 9 0.0024 -4 1892 9 8 1808 9 12 0.1158 9 0.0005 0.090 0.22 7a 315 117 0.1160 9 0.0005 305 0.3410 9 0.0025 1896 9 8 1892 9 12 119 0.095 0.11 2a 0.1163 9 0.0005 388 0.3132 9 0.0024 -7 1899 9 7 1757 9 12 138 0.067 0.12 8a 0.3483 9 0.0026 1910 9 6 1926 9 12 0.1169 9 0.0004 3b 0.02 0.085 206 518 0.4282 9 0.0032 -8 2498 9 10 6a 2297 9 14 476 268 0.514 0.10 0.1641 9 0.0010 0.4648 9 0.0046 2505 9 11 2461 9 20 0.1648 9 0.0011 4a 0.05 0.422 159 267 3a 0.4541 9 0.0034 877 -21 2955 9 15 2414 9 15 508 0.215 0.17 0.2166 9 0.0020 Tra`igh na Cleabhaig psammite, South Harris, MJW 97 -SH 30 0.3174 9 0.0035 11 1586 9 62 13a 1777 9 17 286 106 0.175 2.45 0.0980 9 0.0032 0.3233 9 0.0037 1824 9 10 1806 9 18 0.1115 9 0.0006 363 28a 0.76 0.137 137 0.1119 9 0.0007 281 0.3363 9 0.0041 1831 9 12 1869 9 20 112 0.189 1.09 10a 0.56 0.1121 9 0.0005 0.3529 9 0.0042 5 1834 9 9 1949 9 20 156 27a 0.297 369 0.3548 9 0.0043 4 1854 9 10 1958 9 20 0.1134 9 0.0006 0.244 0.16 26a 398 167 0.1138 9 0.0004 528 0.3134 9 0.0232 1860 9 7 1757 9 114 188 0.070 0.10 21a 0.1142 9 0.0003 426 0.3471 9 0.0039 1 1868 9 5 1921 9 19 174 0.225 0.06 26b 0.3353 9 0.0075 1870 9 9 1864 9 36 0.1144 9 0.0006 7a 0.22 0.207 131 332 292 0.1144 9 0.0005 0.3478 9 0.0045 1 1870 9 9 1924 9 22 118 0.188 9a 0.04 0.3601 9 0.0046 4 1871 9 7 1983 9 22 0.1144 9 0.0004 283 16a 0.11 0.209 119 0.1145 9 0.0003 454 0.3667 9 0.0047 6 1872 9 4 2014 9 22 198 0.262 0.07 12a 0.3377 9 0.0039 1875 9 6 1876 9 19 5a 308 121 0.174 0.22 0.1147 9 0.0004 0.3511 9 0.0040 2 1876 9 10 1940 9 19 0.1148 9 0.0007 711 1a 0.41 0.014 280 0.1150 9 0.0002 1146 0.3334 9 0.0038 1879 9 3 1855 9 18 448 0.201 0.02 3a 0.86 0.1150 9 0.0011 0.3319 9 0.0071 1880 9 17 1848 9 35 20a 182 73 0.337 0.3591 9 0.0034 3 1892 9 5 1978 9 16 0.1158 9 0.0003 0.256 418 27b 972 0.32 0.1164 9 0.0006 767 0.3605 9 0.0037 3 1901 9 9 1984 9 17 317 0.052 0.60 12b 0.12 0.1183 9 0.0003 0.3465 9 0.0040 1930 9 5 1918 9 19 25a 877 347 0.067 M .J . Whitehouse , D . Bridgwater Precambrian Research 105 2001 227 – 245 235 Table 1 Continued [U] ppm 207 Pb 206 Pb 206 Pb 238 U Discordance d 207 Pb 206 Pb [Pb] ppm 206 Pb 238 U ThU meas. Samplespot b f 206 c Ma 9 s Ma 9 s 6a 0.3319 9 0.0230 683 1974 9 6 1847 9 111 283 0.497 0.06 0.1212 9 0.0004 0.3648 9 0.0079 1976 9 5 2005 9 37 0.1213 9 0.0004 637 16b 0.53 0.321 284 0.1231 9 0.0004 643 0.3847 9 0.0043 3 2002 9 5 2098 9 20 311 0.488 0.02 19a 0.3899 9 0.0053 4 10b 2012 9 5 337 2122 9 24 160 0.338 0.04 0.1238 9 0.0003 0.3772 9 0.00035 2038 9 4 2063 9 17 0.1257 9 0.0003 11a 0.13 1.050 850 1588 0.3832 9 0.0044 2077 9 6 2091 9 21 22a 248 123 0.597 0.02 0.1284 9 0.0004 0.3608 9 0.0085 -4 2131 9 13 1986 9 40 0.1325 9 0.0010 28b 0.32 0.655 106 218 0.03 0.1430 9 0.0015 0.4191 9 0.0089 2263 9 18 2256 9 40 1252 1b 613 0.096 Loch a Bha`igh tonalite, Berneray, MJW 97 -Ber 43 0.4745 9 0.0044 -9 2744 9 8 183 2503 9 19 0.051 315 3a 1.04 0.1902 9 0.0009 0.5003 9 0.0049 -4 6a 2754 9 7 283 2615 9 21 171 0.053 0.16 0.1914 9 0.0008 0.5376 9 0.0052 2797 9 4 2773 9 22 0.1965 9 0.0005 0.13 8a 335 218 0.052 0.4977 9 0.0046 -7 2816 9 6 2604 9 20 1a 308 235 1.102 0.66 0.1987 9 0.0007 0.4964 9 0.0047 -8 2825 9 5 2598 9 20 0.1999 9 0.0006 9a 0.14 0.847 232 325 0.2001 9 0.0012 134 0.4885 9 0.0048 -10 2827 9 9 2564 9 21 90 0.612 0.13 1b 0.5249 9 0.0049 -3 2828 9 5 2720 9 21 0.2002 9 0.0007 206 8b 0.12 0.599 148 0.2004 9 0.0013 101 0.4815 9 0.0047 -11 2829 9 11 2534 9 21 66 0.471 1.14 2a 0.09 0.2011 9 0.0012 0.5235 9 0.0049 -3 2835 9 10 2714 9 21 7b 71 102 0.452 0.4732 9 0.0045 -13 2844 9 8 2498 9 20 0.2021 9 0.0009 0.643 0.31 7a 173 114 0.2023 9 0.0006 203 0.5430 9 0.0051 2845 9 5 2796 9 21 153 0.659 0.05 4a 0.5398 9 0.0056 -1 5a 2847 9 7 95 2783 9 23 69 0.523 0.33 0.2025 9 0.0009 0.5023 9 0.0047 -8 2859 9 9 2624 9 20 0.2041 9 0.0012 103 0.10 0.491 10a 152 a All errors quoted in this table are 1s. Data are presented in order of increasing 207 Pb 206 Pb age within each sample group. b Asterix indicates that CL image of grain is illustrated in Figs. 2 and 5 or Fig. 6. c Percentage of 206 Pb contributed by common Pb, assuming Stacey and Kramers 1975 approximation of present day terrestrial Pb isotopic composition. d Discordance of data if\2s error on analysis. Seventy-two spot analyses were performed on 32 grains from the Butt of Lewis diorite zircons; these include 15 replicate analyses made in order to improve the quality of data obtained from an analytical session with low sensitivity. Four analy- ses represent mixed phases and are not considered further in this discussion. Data are presented in Table 1 and Figs. 3 and 4a. The rim analyses with 207 Pb 206 Pb ages in the range ca. 1.8 – 1.9 Ga provide the most consistent group. A weighted average of 15 of these analyses yields an age of 1859 9 10 Ma MSWD = 2.7, n = 15; Fig. 6a. Together with an additional, highly discordant point, these analyses define a regression line with an upper intercept concordia age of 1861 9 7 Ma MWSD = 1.3; Figs. 3 and 4a. Although the lower intercept age of 427 9 24 Ma is dependent largely upon a single discordant analysis, and so cannot be treated with the highest confidence, it is interesting to note that it overlaps the ca. 430 9 6 Ma estimate for the latest reactivation of the Fig. 4. U – Pb concordia diagram 207 Pb 206 Pb vs. 238 U 206 Pb plotting analyses from a Butt of Lewis diorite zircons DB96- S117; b Ba`gh Steinigidh tonalite DB96-S144; and c Tra`igh na Cleabhaig psammite MJW97-SH30 in the same ca. 1.79 – 1.94 Ga concordia age range a number of older ages for DB96-S144 and MJW97-SH30 are not plotted — refer to Table 1 for details. Error bars are plotted at 1s. Inset diagrams along right-hand axes are combined cumulative probability curves and histograms of the 207 Pb 206 Pb ratios. Horizontal dashed lines and shaded area under the cumulative probability curves represent the weighted average 207 Pb 206 Pb age 9 2s for each sample — analyses included in this weighted average are shaded grey. Dash-dot-dash line in a is the same regression plotted in Fig. 3. Fig. 3. U – Pb concordia diagram 207 Pb 206 Pb vs. 238 U 206 Pb plotting analyses from Butt of Lewis diorite zircons DB96- S117; error bars are plotted at 1s, in some cases obscured by the symbol. Classification of analyses follows that used in Table 1 and the text. Dashed outlined box shows the area expanded in Fig. 4a, and the dash-dot-dash line represents the 1861 9 7 Ma regression line through all rims with 207 Pb 206 Pb age B 2 Ga. The thick grey dashed line represents a hypothet- ical ancient Pb-loss trajectory from the oldest cores at ca. 2.83 Ga to the ca. 1.86 Ga age defined by the rim analyses. Present day Pb-loss on this concordia representation is a horizontal trajectory. Outer Isles fault Kelley et al., 1994, probably responsible for development of the cataclastic fab- ric in these rocks. ThU ratios from these rim Fig. 5. Cathodoluminescence images of selected grains from a Ba`gh Steinigidh tonalite sample DB96-S144; b Tra`igh na Cleabhaig psammite sample MJW97-SH30. analyses are uniformly low B 0.02, Table 1, consistent with development of this phase of zir- con in a metamorphic environment e.g. Williams and Claesson, 1987. One slightly older rim analysis 13, 207 Pb 206 Pb age ca. 2.1 Ga dis- cordant has a much higher ThU ratio ca. 0.64 and it is probable that this analysis is a mixed phase. Analyses of zircon phases categorised as cores or core overgrowths occupy a broad range of the concordia diagram Fig. 3, scattering about an ancient Pb-loss trajectory with its lower intercept at the ca. 1.86 Ga event defined by the rims. The oldest of the core analyses grain 19 may define a minimum protolith age of ca. 2.83 Ga, al- though this is ca. 100 Ma older than the next youngest concordant ages and the possibility of an inherited older grain cannot be ruled out. An interesting feature of these data is that the ap- parent age range of the core overgrowths, and their upper age limit, is almost identical to that of the cores Fig. 3. The pattern of analyses is difficult to interpret with a high degree of confi- dence, given the possibility of Pb-loss during tec- tonothermal events at ca. 1.86 Ga rim overgrowths, ca. 2.2 Ga Ness anorthosite em- placement, Whitehouse, 1990b, and ?earlier, as well as during the Caledonian ca. 430 Ma and at present day. Most of the analyses define a fan array from a ca. 2.7 – 2.8 Ga protolith towards both ca. 2.3 and 1.9 Ga. A single concordant analysis of a core overgrowth at ca. 2.3 Ga analysis 27b suggests that development of this phase might have occurred during events associ- ated with emplacement of the Ness anorthosite, with this particular analysis perhaps recording complete Pb-loss at this time, hence resetting to concordia. Development of core overgrowths at ca. 2.3 Ga without complete Pb-loss would result in an array of analyses both cores and over- growths between 2.3 and 2.7 – 2.8 Ga. Later par- tial Pb-loss, in particular at ca. 1.86 Ga, would then result in analyses plotting within the trian- gle defined by the protolith age, and events at ca. 2.3 Ga and 1.86 Ga, which is generally seen in Fig. 3. Given the broad spread of data and the polyphase Pb-loss history of these rocks, it is not possible to define the protolith age to better than ca. 2.7 – 2.8 Ga, or to rule out the possibil- ity of early events cf. the ca. 2.5 Ga Inverian event seen in the mainland central region, Kinny and Friend, 1997. 5 . 2 . Ba`gh Steinigidh tonalite, South Harris, DB 96 -S 144 Zircons from the Ba`gh Steinigidh tonalite, DB96-S144, show a range of morphologies, from anhedral, rounded grains B 200 mm to euhedral prisms B 500 mm. Prismatic grains are mostly clear or pale-brown in colour, while anhedral grains are mostly pale to dark-brown. CL imag- ing Fig. 5a shows that the near- prismatic grains display finely growth-banded internal structure, occasionally with a rounded core and some minor recrystallisation of the external part of the grain. Some of the more anhedral grains are uniformly CL-dark and no internal structure can be discerned, while a few others show CL- bright, structureless reworking of older cores. The range of zircon morphologies present is con- sidered to be consistent with a possible mixed magma origin suggested by field relationships. Analyses of zircons from this sample are mostly concordant, yielding 207 Pb 206 Pb ages in the range 1.91 – 1.83 Ga, with a few older ages Table 1, Fig. 4b. A weighted average age of 1876 9 5 Ma MSWD = 2.2, n = 20 obtained from these analyses is interpreted as the proba- ble igneous crystallisation age of the tonalite. This age agrees with the lower limit for magma- tism in the South Harris complex derived from the 2.06 – 1.87 Ga range of Sm – Nd t DM model ages for the main diorite body Cliff et al., 1983, and an unpublished ca. 1.88 Ga zircon age R.T. Pidgeon and M. Aftalion, cited in Cliff et al., 1983. Three older ages have been ob- tained, two as cores in grains with ca. 1.9 Ga rims. These yield 207 Pb 206 Pb ages of ca. 2.5 Ga one concordant, one discordant and 2.95 Ga discordant, indicating a relatively minor input of older material into the tonalite, consistent with previous reports of Sm – Nd t DM model ages of ca. 2.1 – 2.5 Ga from calc-alkaline intrusives of the South Harris igneous complex Cliff et al., 1983, 1998. 5 . 3 . Tra`igh na Cleabhaig psammite, South Harris, MJW 97 -SH 30 Zircons from the Tra`igh na Cleabhaig psam- mite sample, MJW97-SH30, range in size from ca. 100 – 200 mm, with the characteristic roundedan- hedral morphology of detrital grains. Some grains show elongation up to 2.5:1, also with rounded grain surfaces, although rare facets may be pre- served. Irregular pitting of some of the grain surfaces may be seen in transmitted light. CL imaging of the grains shows a range of internal structure, with several polyphase grains in which the cores are rounded previous early sedimentary cycle or resorption in a magma? and overgrown by zoned, sometimes finely growth-banded zircon. A sedimentary cycle, and hence detrital origin, is indicated by rounding of this latest phase, with truncation of banding against grain margins and off-centre cores e.g. grains 25 and 27, Fig. 5b. Many grains display a thin B 5 mm CL bright rim, which probably results from the post-deposi- tion high-grade metamorphism of these rocks; this final growth phase cannot be dated. Near-concordant to concordant analyses of zir- cons from the Tra`igh na Cleabhaig psammite yield 207 Pb 206 Pb ages ranging from ca. 2.3 – 1.8 Ga Table 1, Fig. 4c. A small degree of reverse discordance is observed in these analyses, of un- known cause, but probably arising from an error in the PbU calibration for this particular analyti- cal session which will not affect 207 Pb 206 Pb ratios and derived ages. On a combined cumulative probabilityhistogram diagram, there is a pro- nounced peak at ca. 1.87 Ga made up by 12 analyses, which yield a weighted average of 1873 9 5 Ma MSWD = 1.3, n = 11, one age rejected. Three analyses yield 207 Pb 206 Pb ages slightly younger than ca. 1.87 Ga, and one considerably younger but strongly reverse discordant 13a. These four analyses indicate a relatively high level of apparent common Pb as determined by 204 Pb counts; f 206 ca. 0.6 – 2.5, Table 1 and the accu- racy of the corrected 207 Pb 206 Pb ages is, thus, dependent upon the validity of assumptions that 1 counts at mass 204 represent only 204 Pb, and 2 the exact composition of this common Pb can be predicted. Since neither of these assumptions can be given a particularly high degree of confi- dence, we prefer to interpret these younger ages as analytical artefacts, rather than assigning any geochronological significance to them. Ten analyses yield 207 Pb 206 Pb ages greater than ca. 1.87 Ga, ranging from ca. 1.9 – 2.3 Ga Table 1. Six of these, spanning the complete range of ages, are analyses of distinct rounded cores in grains whose outer regions yield ca. 1.87 Ga ages. Interpretation of these ages is complicated by the lack of any grouping and the possibility of modifi- cation by Pb-loss during the ca. 1.87 Ga event recorded by the outer regions of many of these grains. Pb-loss along a short chord close to con- cordia would be undetectable given the analytical errors associated with these data in this case, exacerbated by reverse discordance. An early Proterozoic source, perhaps related to ca. 2.3 Ga components within the South Harris igneous com- plex is considered most likely, but inheritance of late-Archean zircon which experienced later Pb- loss cannot be ruled out. The similarity of the ca. 1.87 Ga weighted average ages obtained from this metasupracrustal rock and the Ba`gh Steinigidh tonalite suggests an origin during the same tectonothermal event. The detrital zircons were probably derived from a 1.87 Ga calc-alkaline igneous rock, probably the high- level volcanic equivalent of the presently exposed South Harris plutonic complex, of which the Ba`gh Steinigidh tonalite is a part. In this scenario, the zircons would be eroded from a magmatic ?volcanic edifice, deposited as a clastic sedimen- tary precursor to the psammite, and raised to their present granulite-facies metamorphic grade within a few tens of millions of years, given the minimum age estimate of 1.827 9 0.016 Ga for this event Cliff et al., 1998. This available timescale is more than adequate given typical rates of burial of ca. 10 – 30 mm a − 1 during collisional tectonics e.g. Alps and Himalayas, but requires a major heat source to attain neces- sary temperatures. This heat could perhaps be provided by continued magmatism documented by the granulite-facies fabric-cutting net-veined gabbro, which provides the lower age constraint on this metamorphism Cliff et al., 1998. Fig. 6. Cathodoluminescence images of selected grains from Loch a Bha`igh tonalite sample MJW97-Ber43. ca. 200 – 500 mm in size. CL imaging reveals a very simple population of zircons dominated by fine- scale oscillatory zoning typical of igneous zircons. Obvious cores are absent, or very small, and there are no overgrowths, although some of the grains exhibit embayment by CL-dark zircon, which is probably related to a post-igneous crystallisation metamorphic reworking e.g. grain 3, Fig. 6. All zircons from the Loch a Bha`igh tonalite are late-Archean, with 207 Pb 206 Pb ages ranging from 2.74 – 2.86 Ga Table 1, Fig. 7. Three analyses, corresponding to reworked parts of the grains 3a, 6a, 8a have the youngest 207 Pb 206 Pb ages, ac- companied by very low ThU ratios ca. 0.05. These three grains define a Pb-loss trajectory to ca. 1 Ga, with the concordant analysis 8a indicat- ing a probable age for the reworking event of ca. 2.80 Ga. The possibility of a ca. 1 Ga Pb-loss event may be supported by evidence for a Grenville age thermal event in the Outer Hebrides Cliff and Rex, 1989. Although these documented ages all occur to the north of the Langavat belt, it remains possible that a regional Greenville event might have caused Pb-loss in damagedmetamict older zircons in rocks, which did not experience 5 . 4 . Loch a Bha`igh tonalite, Berneray, MJW 97 -Ber 43 Zircons from this sample are mostly euhedral, Fig. 7. U – Pb concordia diagram 207 Pb 206 Pb vs. 238 U 206 Pb plotting analyses from the Loch a Bha`igh tonalite MJW97-Ber43. Error bars are plotted at 1s. conditions that would reset Ar – Ar systematics in biotite. Analyses of the main, finely banded zircon phase define a present-day Pb-loss trajectory, with a weighted average 207 Pb 206 Pb age of 2834 9 9 Ma MSWD = 3.5, n = 10. This age is interpreted as the igneous protolith age for the tonalite, clearly placing it in the early complex. Signifi- cantly, there is no record either of Paleoproterozic South Harris equivalent age events, or late- Archeanearly-Proterozoic events corresponding to the ca. 2.5 Ga Inverian event of the mainland central region.

6. Discussion