Data presentation and U – Pb results

Pb at 30 – 50 micron sampling resolution. Abla- tion of the zircons is accompanied by some ele- mental fractionation so to minimize this we: a use a defocused laser beam to reduce thermal edge effects; b direct a jet of cooling Ar at the abla- tion point; and c use identical ablation condi- tions on the sample and standard including the interval integrated for each analysis. Calibration is made using an in-house concordant zircon stan- dard Oslo Rift 02123 295 9 1 Ma by thermal ionization mass spectrometry. Three measure- ments on the zircon standard and one on NBS 612 glass are made at the beginning and end of each run of 20 measurements. High instrumental Hg background prohibits accurate measurement of 204 Pb, so no common Pb correction is made. Pb background is minimised by careful attention to system cleanliness, and monitored throughout a run. Data are acquired in peak jumping mode using time-resolved software. Each analysis re- quires about 60 s of background, followed by 10 – 40 s of ablation depending on the sample size. The time-resolved signal data is reduced off-line, where it is possible to see the change in isotopic ratio profiles to detect zoning, andor heterogenei- ties. Appropriate signal intervals are then inte- grated accordingly. 235 U counts are calculated from 238 U. The strategies described allow the ra- tios of 206 Pb 238 U, 207 Pb 235 U, and 208 Pb 232 Th to be measured with precision 1 r.s.d. of about 9 5 and 207 Pb 206 Pb to 9 1. Two sigma uncer- tainties on the ages determined using the individ- ual ratios are given in Table 1. Accuracy using these procedures is estimated to be 9 10 – 20 Ma based on comparing TIMS analyses of concor- dant zircons ranging from ca. 290 – 2700 Ma, with those obtained by LAM-ICP-MS on zircons from the same sample sometimes the same grain Jackson and Dunning, unpublished.

5. Data presentation and U – Pb results

LAM-ICP-MS is a relatively new technique and there is little background concerning the interpre- tation, discussion and presentation of analytical data, in comparison with the large amount of studies based on ion microprobe SHRIMP, SIMS U – Pb single zircon dating. In the latter case, results of U – Pb analyses are currently re- ported in different ways such as Tera-Wasserburg 207 Pb 206 Pb vs 238 U 206 Pb concordia diagrams, conventional 206 Pb 238 U vs 207 Pb 235 U concordia plots, age histograms and cumulative probability plots. A common feature to these techniques, in contrast to single grain TIMS U – Pb analyses, are the data reduction strategies used cf. Hirata and Nesbitt, 1995 and the assumptions made for iso- topic ratio corrections. The above, combined with a lower capacity for isotope mass separation and measurement, results in less precise data and sometimes yields discordant data points that are not interpretable in a straightforward manner. In the present study, which reports a set of 83 zircon U – Th – Pb LAM-ICP-MS analyses with varying degrees of concordancy and precision, our major aim has been to identify the modes in the age spectra. We have been able to use the geological constraints on the data as a means for testing the meaningfulness of actual ages outside of the purely analytical interpretation. In addition, data sets were acquired on different selections of zir- cons in a number of samples, over a period of time, by three different analysts. These data sets all yielded consistent results. These analytical re- sults are discussed in the following sections. The analytical data containing isotope ratios, ages and errors are reported in Table 1. We report Pb 238 U 207 Pb 235 U concordant analyses on con- ventional concordia plots Fig. 4. These plots were made using an unpublished program from the Royal Ontario Museum. In addition, a fre- quency histogram of 207 Pb 206 Pb ages Fig. 5 is presented, that includes data from concordant and non-concordant analyses. We now briefly comment upon the criteria followed to select the best age estimate when all analyses are not con- cordant within error limits. For younger zircons in the age range from about 500 to 800 Ma, the 206 Pb 238 U ages are preferred because 207 Pb 206 Pb ages are compromised by low 207 Pb count rates. In these younger zircons, 208 Pb 232 Th ages Table 1 are often consistent with the 206 Pb 238 U age. In discordant zircons older than ca. 1.5 Ga, we use 207 Pb 206 Pb ages because they have comparatively J . Ferna ´ndez -Sua ´rez et al . Precambrian Research 102 2000 185 – 206 Table 1 LAM-ICP-MS U–Pb results 206 Pb 238 U Analysis 207 Pb 206 Pb 207 Pb 206 Pb 208 Pb 232 Th 208 Pb 232 Th 207 Pb 235 U 206 Pb 238 U 207 Pb 235 U ratio Age Ma 2s Age Ma 2s Age Ma 2s Age Ma 2s ratio ratio ratio Sample JG 1 42 802 18 748 0.04784 106 1.1732 945 93 0.06422 Z1 0.1320 788 Z2 84 0.06666 765 12 810 210 807 150 0.06150 1.1587 0.1261 781 36 734 15 810 120 820 754 35 1.1005 0.1206 Z3 0.06616 0.04297 730 0.06187 50 750 24 670 144 752 90 0.03790 1.0523 0.1234 Z4 971 0.07133 96 973 29 966 248 984 71 0.04990 1.6029 0.1630 Z5 67 1024 24 1060 108 1072 1036 71 Z6 0.1721 1.7730 0.05445 0.07472 55 943 38 870 90 1175 55 Z7 0.06781 0.05985 1.4735 0.1576 920 81 1924 98 2772 152 2238 2367 200 0.19366 Z8 0.3478 9.2877 0.11707 864 0.07124 49 826 44 964 170 904 83 0.04059 1.3420 0.1366 Z9 81 1089 61 920 218 1168 Z10 78 0.06973 0.05943 1.7695 0.1841 1034 55 911 42 1096 198 885 967 117 0.07608 Z11 0.1518 1.5921 0.04474 613 0.06392 50 579 52 738 134 685 62 0.02684 0.8283 0.0940 Z12 1025 0.07005 44 1070 48 928 90 986 53 0.04998 1.7446 0.1806 Z13 50 957 55 856 126 955 927 40 1.4920 0.1600 Z14 0.06762 0.04838 761 0.06310 36 778 20 710 160 716 14 0.03606 1.1159 0.1283 Z15 911 0.06765 47 934 57 856 112 931 59 0.04712 1.4536 0.1558 Z16 59 700 63 758 154 618 714 53 Z17 0.1147 1.0206 0.03107 0.06453 0.1889 1307 29 1116 7 1634 64 1397 49 0.10063 Z18 0.07156 2.6214 Sample SJ 1 30 968 24 1020 0.05062 84 Z1 998 22 0.07324 1.6371 0.1621 985 0.1206 794 67 734 26 966 292 686 37 0.03451 Z2 1.1868 0.07135 0.1110 728 47 679 13 884 178 667 32 0.02844 Z3 1.0489 0.06853 22 1110 13 1268 70 1245 1165 55 Z4 0.1879 2.1493 0.06352 0.08298 21 875 16 1074 Z5 72 0.07521 926 18 0.04585 1.5077 0.1454 934 39 639 19 684 176 627 649 33 0.8950 0.1041 Z6 0.06233 0.03150 2504 0.18392 26 2283 61 2688 50 2477 44 0.13036 10.7787 0.4250 Z7 2372 0.18117 33 2048 62 2662 46 2235 51 0.11694 9.3416 0.3740 Z8 34 1724 71 2462 40 1824 2086 137 Z9 0.3066 6.7988 0.09445 0.16074 37 1551 29 1990 94 Z10 1589 0.12238 30 0.08180 4.5926 0.2720 1748 44 561 36 1050 146 607 669 55 0.07437 Z11 0.0910 0.9333 0.03050 539 0.06495 26 485 9 772 140 753 36 0.02268 0.6997 0.0781 Z12 525 0.06378 21 478 8 734 110 789 27 0.02448 0.6776 0.0770 Z13 60 899 31 1410 140 957 1061 63 Z14 0.1496 1.8428 0.04848 0.08931 26 759 12 854 92 660 Z15 40 0.06759 0.03317 1.1645 0.1249 784 59 1044 77 1042 82 1030 1044 127 1.7962 0.1758 Z16 0.07406 0.05227 1431 0.13953 86 960 46 2220 190 1855 243 0.09612 3.0919 0.1607 Z17 1210 0.07752 45 1253 57 1134 82 1314 72 0.06718 2.2928 0.2145 Z18 40 727 36 842 122 782 756 61 Z19 0.06715 0.1194 1.1057 0.03946 J . Ferna ´ndez -Sua ´rez et al . Precambrian Research 102 2000 185 – 206 193 Table 1 Continued 206 Pb 238 U Analysis 207 Pb 206 Pb 207 Pb 206 Pb 208 Pb 232 Th 208 Pb 232 Th 207 Pb 235 U 206 Pb 238 U 207 Pb 235 U ratio Age Ma 2s Age Ma 2s Age Ma 2s Age Ma 2s ratio ratio ratio Sample JG 12 55 662 32 Z1 634 0.06086 146 722 49 0.03637 0.9072 0.1081 656 698 0.06326 19 692 17 716 112 705 14 0.03552 0.9887 0.1133 Z2 56 931 22 1064 62 994 971 68 1.6027 0.1553 Z3 0.07485 0.04627 972 0.07859 97 899 80 1160 192 1097 74 0.05579 1.6059 0.1482 Z4 722 0.06443 61 711 27 754 236 816 121 0.04122 1.0360 0.1166 Z5 37 1688 35 1880 90 1690 1776 22 Z6 0.2994 4.7499 0.08049 0.11505 Z7 91 0.06463 788 14 650 290 781 33 0.03994 1.1580 0.1300 758 65 1832 55 1840 146 1945 1836 111 0.11257 Z8 0.3287 5.1013 0.12303 1067 0.07376 83 1082 32 1034 230 1046 88 0.05312 1.8595 0.1828 Z9 1281 0.09780 87 1109 52 1582 198 1481 119 0.07265 2.5323 0.1878 Z10 90 1003 74 984 152 964 997 145 Z11 0.1684 1.6706 0.04886 0.07196 35 828 23 814 86 838 39 Z12 0.06624 0.04231 1.2514 0.1370 824 73 1260 56 1358 224 1228 120 1300 2.5964 0.07322 0.08740 Z13 0.2154 Sample JG 16 0.0886 524 60 547 17 484 228 512 32 0.02567 Z1 0.6758 0.05531 0.0931 663 96 574 29 976 206 555 74 0.02782 Z2 0.9208 0.07170 73 623 22 718 270 632 644 63 Z3 0.1014 0.8854 0.03176 0.06329 2224 0.16592 101 1920 86 2516 208 1964 211 0.10206 7.9378 0.3470 Z4 26 2027 22 2382 48 2017 2209 51 7.8107 0.3695 Z5 0.15332 0.10491 612 0.06604 68 560 28 806 294 573 45 0.02785 0.8266 0.09098 Z6 62 819 28 1014 100 972 Z7 83 0.07302 0.04098 1.3646 0.1355 874 46 2136 52 2580 60 1942 2372 54 0.17240 Z8 0.3928 9.3368 0.10083 569 0.06020 34 559 16 610 186 536 31 0.02688 0.7513 0.0905 Z9 611 0.06545 20 600 8 620 94 576 54 0.02891 0.8248 0.0985 Z10 31 753 21 856 114 737 780 41 1.1561 0.1240 Z11 0.06764 0.03715 907 0.07296 81 865 30 1012 258 959 38 0.04341 1.4440 0.1435 Z12 810 0.07613 52 709 23 1098 174 995 34 0.03344 1.2212 0.1163 Z13 43 605 15 638 206 567 612 34 Z14 0.0983 0.8266 0.02640 0.06097 0.0864 579 19 534 16 756 106 522 18 Z15 0.06445 0.02111 0.7679 Sample JG 2 19 1952 24 1938 0.10200 32 5.0798 1963 30 0.3537 0.11890 Z1 1946 0.1832 1089 20 1084 14 1098 60 1014 92 0.05145 Z2 1.9224 0.07611 0.1867 1090 16 1104 15 1062 54 1017 35 0.05106 Z3 1.9251 0.07478 29 569 18 620 120 534 579 32 Z4 0.0923 0.7694 0.02678 0.06047 0.05605 445 29 444 16 454 140 400 30 0.01992 0.5507 Z5 0.0713 28 443 8 380 190 374 433 30 0.0711 Z6 0.05418 0.01870 0.5315 18 2302 26 2840 24 1978 Z7 32 0.20557 0.010283 12.1646 0.4292 2617 24 1690 23 1920 52 1475 1796 39 Z8 0.11769 0.2998 4.8652 0.07568 J . Ferna ´ndez -Sua ´rez et al . Precambrian Research 102 2000 185 – 206 Table 1 Continued 206 Pb 238 U 207 Pb 235 U 206 Pb 238 U 207 Pb 206 Pb 208 Pb 232 Th Analysis 208 Pb 232 Th 207 Pb 206 Pb 207 Pb 235 U 2s Age Ma 2s Age Ma 2s Age Ma Age Ma 2s ratio ratio ratio ratio 0.1410 942 48 850 47 1162 114 1301 107 Z9 0.06650 0.07862 1.5280 Sample JG 3 35 1967 37 1984 0.08705 68 5.9961 1687 67 0.3568 0.12189 Z1 1975 0.3353 1933 37 1864 31 2006 80 1836 58 0.09507 Z2 5.7901 0.12349 19 Z3 1183 0.07760 12 1136 58 1152 48 0.05865 2.1556 0.2015 1167 21 595 9 650 98 562 607 16 Z4 0.0967 0.8174 0.02819 0.06133 12 616 7 522 56 581 Z5 11 0.058782 0.02916 0.7992 0.1002 596 36 594 30 576 100 546 590 40 Z6 0.0965 0.7886 0.03633 0.05925 Z7 16 0.05440 460 6 386 92 425 11 0.02123 0.5546 0.0739 448 32 1754 34 1832 54 2005 1790 41 4.8299 Z8 0.11200 0.10426 0.31270 Z9 24 0.07669 1281 20 1112 64 1265 45 0.06984 2.3241 0.2198 1220 smaller uncertainties owing to higher amounts of 207 Pb. In the case of zircons of intermediate age 1 – 1.5 Ga 207 Pb 206 Pb age is preferred when it is consistent with the 208 Pb 238 Th age. The 208 Pb 238 U age is otherwise preferred. Similar weighting of young ages to 206 Pb 238 U and older ages to Fig. 4. Concordia plots of U – Pb analytical data. Ellipses represent 2s uncertainties. Fig. 5. Histograms of 207 Pb 206 Pb ages. a Neoproterozoic samples; b lower Palaeozoic samples. Five zircons from sample SJ-1 Upper Villalba Series yielded Late Proterozoic subconcordant ages Table 1, Fig. 4b with 206 Pb 238 U ages rang- ing from 639 to 759 Ma. Z12 and Z13 are discor- dant and yielded Neoproterozoic 207 Pb 206 Pb ages of 772 and 734 Ma, respectively. Note that Z12 is a rim to Z19. Zircons Z1, Z16 and Z18 are subconcordant and yielded Middle Proterozoic 206 Pb 238 U ages of 968, 1044 and 1253 Ma, respec- tively. Zircons Z4, Z5 and Z11 are discordant and yielded Mesoproterozoic 207 Pb 206 Pb ages of 1268, 1074 and 1050 Ma, respectively. Zircons Z10 and Z17 are discordant and yielded Palaeoproterozoic 207 Pb 206 Pb ages of 1990 and 2220 Ma, respec- tively. Discordant analysis Z14 yielded a late Mesoproterozoic 207 Pb 206 Pb age 1410 Ma. Fi- nally, discordant analyses Z7, Z8 and Z9 yielded Archaean 207 Pb 206 Pb ages between 2462 and 2688 Ma. Five zircons from sample JG-12 Tineo Series yielded Late Proterozoic ages Fig. 4c, Table 1, with 206 Pb 238 U ages ranging from 662 to 828 Ma. Three zircons yielded concordant Mesoprotero- zoic ages with 206 Pb 238 U ages of 1082, 1003 and 1260 Ma, respectively. Z3 and Z4 yielded sub- concordant data points with 206 Pb 238 U ages of 931 and 900 Ma and 207 Pb 206 Pb ages of 1064 and 1164 Ma, respectively. In this case, the 207 Pb 206 Pb ages are preferred as they are more consistent with the ThPb ages Table 1. Zircon Z8 yielded a concordant Palaeoproterozoic age of ca. 1.83 Ga 206 Pb 238 U age not shown in Fig. 4. Zircons Z6 and Z10 are discordant and yielded Palaeoproterozoic 207 Pb 206 Pb ages of ca. 1.88 and 1.58 Ga, respectively. 5 . 2 . Cambrian sandstone Ca´ndana Formation, JG 16 Eight zircons separated from the Lower Cam- brian arkose yielded Late Proterozoic ages younger than those of zircons found in the Neoproterozoic greywackes, the 206 Pb 238 U ages ranging from ca. 540 to ca. 623 Ma. Sub-concor- dant Z7, Z12 and discordant Z13 zircons yielded 207 Pb 206 Pb Mesoproterozoic ages of 1014, 1012 and 1098 Ma, respectively. Although the uncertainties associated to the 207 Pb 206 Pb ages are 207 Pb 206 Pb is used in ion probe work cf. Ireland et al., 1998. 5 . 1 . Proterozoic metagreywackes JG 1 , JG 12 and SJ 1 Fig. 4 a, b, c Zircons in sample JG-1 Lower Villalba Series yielded a number of sub-concordant analyses with Neoproterozoic 206 Pb 238 U ages ranging from ca. 800 to ca. 700 Ma and Middle Proterozoic 206 Pb 238 U ages ranging from 911 to 1089 Ma. Zircons Z8 and Z18 are discordant and have Achaean ca. 2.77 Ga and Palaeoproterozoic ca. 1.63 Ga 207 Pb 206 Pb ages. Z12 has a somewhat ambiguous age around 600 Ma, while Z9 is best described by an age of ca. 800 – 850 Ma. quite large, these ages are in this case preferred to the U – Pb ages since they are more consistent with the ThPb ages Table 1. In addition, three discordant zircons Z4, Z5 and Z8 have 207 Pb 206 Pb ages of ca. 2.4 – 2.6 Ga Archaean-Early Proterozoic boundary. 5 . 3 . Ordo6ician 6olcanosedimentary metagreywackes Ollo de Sapo complex, JG 2 and JG 3 Zircons from these samples yielded mostly con- cordant U – Pb ages that fall into four age group- ings: Ordovician ca. 440 – 460 Ma, Late Neoproterozoic ca. 570 – 620, Mesoproterozoic ca. 1 – 1.2 Ga and Palaeoproterozoic ca. 1.9 – 2 Ga. The youngest U – Pb age grouping ca. 440 – 460, Table 1 is considered to represent the age of the volcanic component of the OS rocks. This age is younger than ages reported for other magmatic units of this complex in areas situated further to the S-SE along the OS antiform ca. 465 – 490 Ma, Lancelot et al., 1985; Gebauer, 1993; Valverde- Vaquero and Dunning, 1997. One zircon from sample JG2 Z4 and three from sample JG-3 Z4, Z5, Z6 yielded Late Proterozoic concordant ages Fig. 4e. The 206 Pb 238 U ages range from ca. 569 to ca. 616 Ma. The third group, of which three concordant ages are reported in Fig. 4e, repre- sents the recycling of a mid-Proterozoic crustal component of ca. 1 – 1.2 Ga. Two zircons from sample JG-2 Z2 and Z3 yielded overlapping concordant ages of ca. 1080 – 1105 Ma and a zircon from sample JG3 Z3 yielded an older concordant age of ca. 1183 Ma. Zircons Z9 JG2 and Z9 JG3 yielded discordant data points with 207 Pb 206 Pb ages of 1162 and 1112 Ma, respec- tively. The fourth group of recycled zircons is represented by two concordant and partially over- lapping U – Pb ages of ca. 1952 Ma Z1, sample JG2 and ca. 1967 Ma Z1, sample JG3. A second zircon from JG3 Z2, although not con- cordant, yielded a 207 Pb 206 Pb age of 2 Ga, within error limits of the concordant zircon from the same sample. Discordant analyses Z8 JG2 and Z8 JG3 yielded 207 Pb 206 Pb ages of 1920 and 1832 Ma, respectively. Finally, a discordant zir- con from sample JG-2 Z6 has a 207 Pb 206 Pb age of ca. 2.8 Ga, the oldest Archaean age found in this study.

6. Discussion