U – Pb results Directory UMM :Data Elmu:jurnal:P:Precambrian Research:Vol105.Issue2-4.2001:

age of 295 9 1 Ma Fig. 5; Table 3. Typically, outliers were those analyses that did not overlap in 206 Pb 238 U age with the TIMS-determined age Fig. 5. Analyses of the 02123 standard that were selected for use in the age calculations yielded an average 206 Pb 238 U age and uncertainty of 294.5 9 9.4 Ma 2s uncertainty; n = 36 over the duration of the analytical work. Because of the low count rates on 207 Pb for the standard, precise calibration for 207 Pb 235 U and 207 Pb 206 Pb di- rectly from the 207 Pb and calculated 235 U count rates is problematic. However, since the fractiona- tion correction is the same for both 207 Pb 235 U and 206 Pb 238 U and the 02123 standard is concor- dant, an excellent calibration for 207 Pb 235 U and consequently 207 Pb 206 Pb can be derived directly from the 206 Pb 238 U ratio. This employs a mass discrimination correction for the 207 Pb 235 U ratio derived from the 206 Pb 238 U ratio, assuming that mass discrimination is linear over the mass range 206 Pb to 238 U. Uncertainties on the ratios were calculated directly from the integrated repeats. The corrected ratios and 2s uncertainties were then plotted and processed exactly as standard TIMS data at Memorial University.

5. U – Pb results

5 . 1 . Basement gneiss beneath the Lower Aillik group sample 1 Along the south shore of Kaipokok Bay, migmatitic rocks correlated with Archaean base- ment gneiss north of Kaipokok Bay Marten, 1977 are exposed in a region northeast of Post Hill Fig. 2. This area is mainly underlain by variably deformed, migmatitic granitoid or- thogneiss, but bodies and dykes of younger foli- ated granite and granodiorite are also observed. Toward Post Hill, all these rock types become increasingly deformed, and are mylonitized in a 700 m thick zone beneath the Lower Aillik Group. Although resembling the mixture of Ar- chaean and Proterozoic plutonic rocks north of Kaipokok Bay, this gneissic-plutonic package is distinct in that thin generally B 2 m wide, folia- tion-parallel layers of feldspathic quartzite, quartz-muscovite paragneiss, and biotite amphi- bolite are observed in several locations. The Table 2 LAM-ICP-MS operating conditions and data acquistion parameters ICP-MS Model VG PQII+‘S’ Forward power 1350 kW Gas flows: Plasma 14 Lmin Auxilliary 1 Lmin Carrier ca. 1.1 Lmin Sampler cone Custom made, 0.7 mm aperture Expansion 4.5×10 − 1 Torr chamber pressure Standard with flared bonnet ICP torch LAM 266 nm Wavelength 10 Hz Repetition rate 9 ns Pulse duration FWHM ca. 4 mm Apertured beam diameter 10×, UV laser achromat, f.l. = 20 mm Focusing objective 200 mm above sample Degree of defocusing 0.25–0.43 mJ Incident pulse energy Cell design Spot cooling Data acquition parameters Data acquisition Time resolved analysis protocol Peak hopping, 1 point per peak Scanning mode Isotopes 206 Pb, 207 Pb, 208 Pb, 232 Th, 238 U determined 8.5 ms Dwell time per isotope 1.5 ms Quad. settling time Timescan 50 ms Number of Max. ca. 2400 ca.120 s scans Samples and standards Samples Hand picked, best quality grains, non-magnetic fraction Mounts 25 mm diameter polished grain mounts Standard Pegmatitic gem zircon, ‘02123’, Norway; 295 9 1 Ma Table 3 supracrustal nature of the quartz-rich rocks is certain, but whether the biotite amphibolite layers represent transposed dykes, mafic volcanic hori- zons, or some other lithology could not be deter- mined. The quartz-rich and amphibolite layers could not be traced over more than a few tens of metres due to poor exposure, and their original relationship with the host gneisses is masked by the generally high state of strain. Younger amphi- bolite dykes found throughout the Postville region cross-cut all of these units, but are themselves moderately to strongly deformed. The overall character of this gneissic-plutonic complex compared to basement rocks north of Kaipokok Bay which generally lack supracrustal intercalations led us to speculate that the com- plex might consist of Paleoproterozoic rocks com- prising a high-grade thrust sheet beneath the Post Hill klippe. In order to test this possibility, a sample of migmatitic tonalite gneiss representing one of the oldest components and possibly the oldest component in the complex was collected for U – Pb dating. Leucosome-rich bands were removed from more restitic portions of the sample prior to crushing in order to minimize metamor- phic zircons, if present, in the heavy mineral separate. The sample yielded abundant small colourless zircon needles mainly 3:1 length to breadth ratio and less abundant larger equant prisms and ellip- soidal grains. Grain quality ranges from transpar- ent to slightly turbid for all zircon types, and a few larger grains have evidence of very thin rim and tip overgrowths. Grains of glassy and turbid anhedral monazite were also recovered from the sample. Five multigrain fractions of strongly abraded zircon and three of abraded monazite were analysed; results are presented in Fig. 6a and Table 3. The five zircon analyses are moderately to strongly discordant and do not fit a single regression line within analytical uncertainty. However, using the error expansion routine of Davis 1982, which enlarges individual analysis uncertainties weighted according to degree of discordance until a \ 50 probability of fit to a single, best-fit line is achieved, the five analyses yield an upper intercept age of 2878 + 27 − 16 Ma and an imprecise lower intercept age of 1247 Ma. We consider the upper intercept age to date igneous crystallization of the gneissic precursor, which demonstrates an Archaean rather than Pa- leoproterozoic age for this component of the gneiss package beneath Post Hill. The gneiss is similar in age to the 2883 9 3 Ma Knee Lake intrusion, a unit of the Kanairiktok intrusive suite in the southern Nain Province James et al., 1997. Correlation of southern Nain Province rocks with reworked Archaean gneiss in the Kaipokok domain has already been suggested on lithologic grounds Ermanovics, 1993 and is sup- ported by this result. The three monazite analyses are moderately discordant and cluster together with 207 Pb 206 Pb ages of 2799 – 2792 Ma. Although these analyses also confirm an Archaean age for the gneiss, their significance is uncertain. They could either indi- cate that partial isotopic resetting of Archaean monazite occurred during the Makkovikian orogeny, or that the monazite fractions consist of mixtures of Archaean and Paleoproterozoic grains. The former interpretation is preferred here due to the close clustering of analyses suggesting a uniform degree of Pb loss rather than uniform mixing, and suggests that Archaean monazite was not completely reset during Makkovikian metamorphism and deformation. This result is identical to an earlier finding of incomplete mon- azite resetting in this region Ketchum et al., 1997, and supports the argument that peak meta- morphic temperatures in the Kaipokok domain remained below the 725°C blocking tempera- ture of monazite Parrish, 1990 during the Makkovikian orogeny. 5 . 2 . Drunken Harbour quartzite sample 2 A sample of quartzite was collected from the Drunken Harbour supracrustal belt in order to evaluate detrital zircon populations, to determine if Paleoproterozoic grains are present, and to constrain maximum deposition age. The sample was collected from the easternmost supracrustal package in this belt Fig. 2 and consists of ‘clean’ metaquartzite with minor feldspar, biotite, mus- covite, and opaque minerals. Although this unit is strongly deformed and recrystallized, relict bed- J .W .F . Ketchum et al . Precambrian Research 105 2001 331 – 356 Table 3 U–Pb isotopic data TMS Concentration Measured Atomic Ratios d Age [Ma] Fraction Weight U [ppm] Pb rad b Total common Pb 206 Pb 204 Pb c 208 Pb 206 Pb 206 Pb 238 U 207 Pb 235 U 207 Pb 206 Pb 206 Pb 238 U 207 Pb 235 U 207 Pb 206 Pb 02123 Zircon standard for LAM-ICP-MS analyses by G. Dunning, Memorial Uni6ersity clr sm frags 0.431 173 8.7 5 43101 0.2028 0.04668 14 0.3362 12 0.05223 6 294 294 295 Z1 clr sm frags 0.629 168 8.3 18 17133 Z2 0.1772 0.04687 18 0.3374 12 0.05222 8 295 295 295 Z3 lrg pale br frags 0.508 145 7.2 8 27963 0.1716 0.04676 16 0.3370 12 0.05226 6 295 295 297 pale br frags 0.596 155 7.7 27 10130 0.1789 Z4 0.04681 14 0.3372 12 0.05224 6 295 295 296 lrg clr frags 0.559 185 9.2 6 54107 0.1802 Z5 0.04697 14 0.3383 10 0.05224 4 296 296 296 1 Migmatitic tonalite gneiss basement to Lower Aillik Group ; 9 5 MKN- 82 sm needles 0.015 178 109.5 92 989 Z1 0.1293 0.5368 32 15.046 90 0.20328 22 2770 2818 2853 Z2 sm needles 0.005 168 97.5 9 2889 0.1185 0.5103 27 14.078 70 0.20008 42 2658 2755 2827 turbid lrg needles 0.011 214 121.8 19 3931 0.1153 0.5037 20 13.646 53 0.19647 24 2630 2725 2797 Z3 sm clr pr 0.006 153 91.5 7 4712 Z4 0.1021 0.5326 33 14.809 90 0.20165 30 2753 2803 2840 Z5 sm clr needles 0.002 151 95.0 28 381 0.1480 0.5412 32 15.166 86 0.20326 54 2788 2826 2853 yel anh 0.006 901 3848.5 26 6909 8.2015 M1 0.5244 25 14.190 71 0.19623 18 2718 2762 2795 yel anh 0.010 1395 5219.3 282 1646 M2 7.0011 0.5268 18 14.227 53 0.19588 18 2728 2765 2792 M3 sm glassy anh 0.008 1629 4855.1 24 17431 5.3880 0.5226 13 14.175 38 0.19674 10 2710 2761 2799 2 Drunken Harbour quartzite 94 MKJ- 27 a single sharp pk pr 0.013 145 85.6 257 260 Z1 0.0734 0.5366 30 15.192 92 0.20533 32 2769 2827 2869 Z2 single 2:1 pr 0.011 79 46.0 100 277 0.2192 0.4826 24 12.116 62 0.18208 48 2539 2613 2672 single br euh 0.010 355 237.1 40 3148 Z3 0.1582 0.5586 22 17.873 73 0.23208 20 2861 2983 3066 single clr eq 0.018 151 86.7 40 2216 Z4 0.1062 0.5145 18 13.812 52 0.19471 22 2676 2737 2782 Z5 single clr eq 0.007 96 50.6 9 2183 0.1356 0.4652 31 11.230 60 0.17507 76 2463 2542 2607 single clr eq 0.009 117 68.3 19 1852 0.1222 Z6 0.5142 25 13.703 66 0.19329 38 2674 2729 2770 single clr eq 0.015 76 45.0 21 1731 Z7 0.1632 0.5079 27 13.305 67 0.19000 48 2647 2702 2742 Z8 single clr eq 0.010 61 37.7 39 536 0.1435 0.5323 23 14.879 65 0.20273 46 2751 2808 2848 single sharp pk pr 0.012 491 240.0 81 1978 Z9 0.1488 0.4305 13 10.118 32 0.17047 16 2308 2446 2562 single clr rnd 0.006 233 155.9 20 2575 Z10 0.0706 0.5919 24 19.892 82 0.24375 22 2997 3086 3145 M1 glassy euh-anh 0.023 1349 3331.5 526 1232 7.5499 0.3290 14 5.056 23 0.11146 12 1833 1829 1823 two glassy grs M2 0.003 685 2700.7 18 2328 12.7598 0.3277 21 5.044 27 0.11163 42 1827 1827 1826 br frags best 0.116 134 43.5 399 770 T1 0.1158 0.3095 9 4.541 16 0.10642 14 1738 1739 1739 T2 br frags 2nd best 0.327 141 45.2 1022 893 0.0975 0.3095 6 4.537 10 0.10634 8 1738 1738 1738 lyel frags T3 0.274 110 35.4 2327 267 0.1138 0.3080 7 4.486 13 0.10565 16 1731 1728 1726 3 Post Hill quartzite 94 MKJ- 61 a single br rnd 0.006 511 299.4 Z1 6 17748 0.1177 0.5217 20 13.409 54 0.18643 24 2706 2709 2711 Z2 single clr rnd 0.008 54 35.7 5 2886 0.1372 0.5743 29 16.834 80 0.21259 48 2925 2925 2925 single pk rnd 0.006 151 96.9 6 5170 Z3 0.0969 0.5683 27 16.922 82 0.21597 24 2901 2930 2951 Z4 single clr pr 0.009 34 23.2 13 921 0.0907 0.5983 38 18.758 99 0.22737 92 3023 3029 3034 single br rnd Z5 0.006 595 361.8 8 14806 0.2048 0.5096 15 12.738 38 0.18128 12 2655 2660 2665 single lpk rnd 0.008 129 90.6 7 5467 0.1914 0.5786 15 Z6 17.320 46 0.21711 16 2943 2953 2959 4 Intermediate tuff layer in Post Hill amphibolite 95 MKJ- 118 clr pr 0.042 43 20.0 106 441 Z1 0.2210 0.4001 30 7.506 65 0.13607 58 2169 2174 2178 clr pr 0.048 39 17.8 117 415 Z2 0.1936 0.3978 26 7.477 43 0.13632 66 2159 2170 2181 Z3 2:1 clr pr 0.050 44 19.8 50 1103 0.1985 0.3953 14 7.364 28 0.13510 22 2147 2157 2165 Z4 clr sm pr 0.016 39 18.1 124 146 0.2001 0.4018 23 7.538 57 0.13608 64 2177 2178 2178 clr pr 0.028 45 20.0 192 178 0.1908 0.3922 19 Z5 7.243 43 0.13393 52 2133 2142 2150 J .W .F . Ketchum et al . Precambrian Research 105 2001 331 – 356 345 Table 3 Continued Concentration Measured Atomic Ratios d Age [Ma] Fraction Weight U [ppm] Pb rad b Total common Pb 206 Pb 204 Pb c 208 Pb 206 Pb 206 Pb 238 U 207 Pb 235 U 207 Pb 206 Pb 206 Pb 238 U 207 Pb 235 U 207 Pb 206 Pb [16 ] 5 Psammite, metasedimentary formation 94 MKN- 74 f single pitted clr 0.023 60 32.0 23 1775 Z1 0.1198 0.4741 15 12.510 42 0.19139 22 2501 2644 2754 Z2 single sharp clr pr 0.007 212 96.8 24 1441 0.3277 0.3660 14 6.253 25 0.12389 22 2011 2012 2013 2:1 sharp pr Z3 0.011 185 77.2 28 1788 0.1016 0.3922 20 7.284 37 0.13470 24 2133 2147 2160 single lrg lpk eq 0.006 172 99.9 5 6228 Z4 0.3114 0.4582 21 10.187 42 0.16125 34 2431 2452 2469 single br pr 0.003 724 332.9 4 15607 0.0414 0.4457 17 9.737 38 0.15843 12 Z5 2376 2410 2439 single clr euh pr 0.003 157 61.8 5 1986 0.1263 0.3653 29 6.320 48 0.12546 36 Z6 2007 2021 2035 a All fractions were abraded following the method of Krogh 1982. Z, zircon; M, monazite; T, titanite; eq, equant; pr, prismatic; grs, grains; lrg, large; sm, small; euh, euhedral; anh, anhedral; frags, fragments; rnd, rounded; clr, colourless; br, brown; l yel, light yellow; l pk, light pink; 2:1, length:breadth ratio. Needles are generally \4:1. Single grain fractions are indicated-all others are multigrain fractions. b Total radiogenic Pb after correction for blank, common Pb, and spike. c Measured, uncorrected ratio. d Ratios corrected for fractionation, spike, 5–10 pg laboratory blank, initial commom Pb calculated with the model of Stacey and Kramers 1975 for the age of the sample, and 1 pg U blank. Uncertainties 2s on the isotopic ratios refer to the final digits. ding is preserved in the form of 1 – 2 cm wide bands of varying feldspar content. The quartzite unit is roughly 30 m wide at this location and contains several intervals of more feldspathic and or calcareous quartzite. The sample yielded a large number of detrital zircons ranging from rounded and strongly pitted, colourless, pink, and brown grains, to numerous euhedral prisms of similar colour but with sharp facets and terminations. The latter population consists of equant to 3:1 grains, most which have eight-sided cross-sections and show very little evi- dence of detrital abrasion. An original igneous source and minimal detrital transport of this pop- ulation is inferred from these characteristics. The sample also contained glassy yellow, idioblastic to xenoblastic monazite, and fragments of brown and light yellow titanite. Ten single, mainly sharp euhedral zircons dated by the TIMS method yield moderately to strongly discordant results Fig. 7a, with 207 Pb 206 Pb ages of 3145 – 2562 Ma indi- cating that the original source rocks for these grains were Archaean. Two monazite analyses are concordant at 1828 9 4 Ma and are presumed to indicate metamorphic growth of monazite at this time. Using the assumption that 1828 Ma also represents the time of Pb loss from detrital zir- cons, this suggests that the analysed zircons range in age from ca. 3240 to 2800 Ma. TIMS titanite analyses yield an age of 1738 9 3 Ma for two fractions of brown fragments, and a younger age of 1726 9 3 Ma for a single fraction of light yellow fragments Fig. 7a. These ages are relatively young for Makkovik Province titanite Kerr et al., 1992; Ketchum et al., 1997 and have been previously attributed to late structural reacti- vation or fluid infiltration rather than to regional metamorphism Ketchum et al., 1997. Growing evidence for structural reactivation of the Makkovik Province at 1740 – 1600 Ma 40 Ar 39 Ar age data of Wilton, 1996; Brown, 1997; Sinclair, 1999, which is likely linked both to post- Makkovikian plutonism and the far-field effects of Labradorian orogenesis Gower et al., 1992, provides one mechanism for titanite resetting and or new growth. Laser ablation ICP-MS analyses of 25 zircon grains representing the entire range of morpholo- gies in the sample provide a distribution of ages similar to that for TIMS data Table 4. Results are shown with 2s uncertainties in Fig. 7b. 207 Pb 206 Pb ages of 3322 – 2438 Ma are obtained, and a population consisting entirely of Archaean zircons is indicated assuming that Pb loss is predomi- nantly Paleoproterozoic. The majority of analyses plot below concordia with 207 Pb 206 Pb ages of 2.9 – 2.7 Ga. Two analyses plot above concordia but also yield 207 Pb 206 Pb ages within this range. Their negative discordance is potentially due ei- ther to high common Pb or to an incorrect correc- tion for U – Pb fractionation i.e. fractionation in the sample varied from that in the standard. Although we cannot entirely rule out a Pale- oproterozoic contribution, the combined TIMS and LAM-ICP-MS data suggest that the Drunken Harbour quartzite contains detritus solely of Ar- chaean age. Potential source rocks in the adjacent Nain Province include units of the Maggo gneiss 3142 Ma; James et al., 1997 and ca. 2885 – 2840 Ma plutons of the Kanairiktok intrusive suite Loveridge et al., 1987; James et al., 1997. Ar- chaean rocks underlying the Kaipokok domain, which are derived from the Nain craton, also represent potential source lithologies. 5 . 3 . Post Hill quartzite sample 3 Southeast of the town of Postville, the Post Hill amphibolite is underlain by an up to 100 m-thick package of fine-grained quartzite, micaceous quartzite, psammite, and feldspathic paragneiss. This metasedimentary package is best observed on the northeast flank of Post Hill where a transition from dominantly psammitic to dominantly mafic metavolcanic layers occurs over several tens of metres at the top of the succession Fig. 8a. The psammitic layers are distinct from thin intermedi- ate tuff horizons that occur at a higher structural level within the Post Hill amphibolite see below. This interlayered zone is interpreted here as a normal stratigraphic transition from the sedimen- tary package to the Post Hill amphibolite, and is unlikely to represent a region of structural inter- leaving as originally proposed by Marten 1977. However, this observation does not rule out the existence of structural boundaries elsewhere within this volcanic-sedimentary succession. Table 4 U–Pb isotopic data LAM-ICP-MS Atomic Ratios Grain Age Ma 206 Pb 238 U R.S.D. 207 Pb 235 U R.S.D. R.S.D. 206 Pb 238 U 207 Pb 206 Pb 207 Pb 235 U 207 Pb 206 Pb 2 Drunken Harbour quartzite 94MKJ-27a 0.7900 8.734 1.030 1k 0.1700 0.3727 1.1200 2042 2311 2556 9 36 1I 0.4954 1.3000 12.400 1.420 0.1815 0.7300 2594 2635 2666 9 24 1.0000 11.239 1.540 0.1781 1.5700 2430 1h 2543 0.4577 2634 9 52 1.8800 16.488 2.010 0.2016 0.5932 2.1900 1g 3002 2906 2838 9 72 1.0300 14.461 1.640 0.2091 1f 1.6000 0.5017 2621 2780 2898 9 52 1.3600 12.994 2.370 0.1934 0.4872 2.1400 1e 2559 2679 2770 9 72 0.3285 1c 0.8300 7.174 3.030 0.1584 2.3200 1831 2133 2438 9 78 0.8700 13.363 1.570 0.1916 0.5060 1.2800 1b 2639 2706 2754 9 42 0.5447 1a 1.0200 18.075 1.850 0.2407 1.7600 2803 2994 3124 9 54 2k 1.1500 0.4872 12.417 1.540 0.1849 1.2000 2558 2636 2696 9 40 0.7800 15.162 1.370 0.2107 0.5220 1.1500 2I 2708 2825 2910 9 38 0.8800 16.073 1.520 0.2159 2h 1.3300 0.5400 2783 2881 2950 9 42 0.9800 11.110 1.210 0.1817 0.4435 1.1400 2g 2366 2532 2668 9 38 0.5822 2f 1.0300 19.978 1.310 0.2489 1.6600 2958 3090 3176 9 52 0.9300 13.922 1.310 0.2028 0.4978 1.1400 2e 2604 2744 2848 9 36 0.5384 2d 0.9500 16.901 1.350 0.2277 1.2100 2777 2929 3034 9 38 0.5508 2c 1.1700 16.220 1.870 0.2136 2.0000 2829 2890 2932 9 64 0.9900 19.886 1.510 0.2462 0.5858 1.3600 2b 2972 3086 3160 9 42 0.5075 2a 1.4200 13.766 1.860 0.1967 1.5400 2646 2734 2798 9 52 0.8100 12.217 1.690 0.1872 3k 1.2600 0.4733 2498 2621 2716 9 42 1.2500 13.571 2.150 0.1952 0.5041 2.2200 3I 2631 2720 2786 9 74 0.6228 3h 1.8700 17.610 1.710 0.2050 2.0700 3121 2969 2866 9 66 3g 1.1000 0.4817 11.469 2.050 0.1726 2.4900 2535 2562 2582 9 82 1.0000 13.287 1.410 0.1903 0.5062 1.4700 3f 2640 2700 2744 9 48 Ld 0.6900 0.6136 23.095 2.040 0.2730 1.7100 3085 3231 3322 9 52 3 Post Hill quartzite 94MKJ-61a 0.6200 11.738 1.560 4k 0.1700 0.5006 1.3700 2616 2584 2556 9 46 0.8100 11.080 1.420 0.1646 0.4880 1.4300 4I 2562 2530 2502 9 48 0.4298 4h 0.8100 9.863 0.950 0.1664 0.7300 2305 2422 2520 9 24 1.0600 11.868 2.060 0.1789 1.9100 2531 2594 2642 9 64 4g 0.4809 0.8200 11.523 1.500 0.1821 0.4588 1.3200 4f 2434 2566 2672 9 42 1.0500 17.164 1.880 0.2304 4e 1.9600 0.5402 2784 2944 3054 9 62 0.7600 9.601 0.940 0.1674 0.4160 0.8000 4d 2242 2397 2530 9 26 0.4376 4c 0.9000 10.429 1.000 0.1728 1.3700 2340 2474 2584 9 46 0.9600 12.497 1.140 0.1856 0.4884 1.2300 4b 2564 2643 2702 9 40 0.4789 4a 0.9500 12.194 1.940 0.1847 2.0700 2522 2619 2694 9 68 0.8800 5k 14.609 0.5142 1.710 0.2061 1.6800 2674 2790 2874 9 54 1.0300 11.322 1.590 0.1983 0.4141 1.4400 5i 2234 2550 2812 9 46 0.5010 5h 1.3000 12.697 1.910 0.1838 2.3200 2618 2657 2686 9 76 0.6100 11.102 1.120 0.1740 5g 1.0400 0.4628 2452 2532 2596 9 34 2.1500 13.270 3.870 0.1850 0.5201 4.3800 5b 2700 2699 2698 9 144 0.4553 5a 1.2300 11.584 2.450 0.1845 2.5900 2419 2571 2692 9 86 1.1500 11.977 2.110 0.1876 2.2400 2453 2603 2720 9 74 3e 0.4631 0.7900 13.022 2.900 0.1901 0.4969 2.9400 3d 2601 2681 2742 9 98 0.6064 3c 1.5500 20.210 1.790 0.2417 2.1600 3056 3101 3130 9 68 0.6500 4.591 1.110 0.1201 0.8600 1577 1748 1958 9 30 3b 0.2772 0.8200 13.505 1.580 0.2000 0.4899 1.3900 3a 2570 2716 2824 9 46 0.8900 12.496 5d 0.980 0.5125 0.1768 1.3000 2667 2642 2622 9 44 Table 4 Continued Grain Age Ma Atomic Ratios R.S.D. 207 Pb 235 U R.S.D. 207 Pb 206 Pb 206 Pb 238 U R.S.D. 206 Pb 238 U 207 Pb 235 U 207 Pb 206 Pb 5 Psammite, Metasedimentary formation94MKN-74f 0.5200 5a 30.585 0.6187 1.100 0.3585 1.0800 3105 3506 3742 9 34 0.7800 5.420 1.290 0.1323 0.2971 1.3800 5d 1677 1888 2128 9 48 0.9600 6.149 1.790 0.1268 5e 1.5800 0.3517 1943 1997 2054 9 54 0.9200 4.736 1.110 0.1326 0.2590 0.9900 5j 1485 1774 2132 9 36 0.9400 5b 8.254 0.3831 1.620 0.1563 1.9900 2091 2259 2414 9 68 0.6200 5.861 2.230 0.1272 0.3342 2.1200 5f 1859 1956 2058 9 76 1.2300 8.815 1.270 0.1636 5g 0.7500 0.3907 2126 2319 2492 9 24 2.1200 12.474 2.490 0.1883 0.4805 1.8700 4b 2529 2641 2726 9 60 0.9900 5.120 1.220 0.1317 4c 1.4000 0.2819 1601 1839 2120 9 48 0.6200 7.963 1.980 0.1483 0.3895 1.8300 4d 2120 2227 2326 9 62 2.4300 6.297 2.660 0.1329 4j 1.1600 0.3437 1905 2018 2136 9 42 2.1000 9.721 3.140 0.1691 0.4169 3.6400 3b 2247 2409 2548 9 122 1.1300 3c 4.269 0.2434 1.320 0.1272 0.7000 1404 1687 2060 9 26 3.4800 6.859 3.910 0.1289 0.3858 2.0100 3f 2103 2093 2082 9 72 0.7600 5.735 2.280 0.1207 3h 2.0200 0.3446 1909 1937 1966 9 72 0.8100 4.405 0.940 0.1284 0.2487 0.8700 4e 1432 1713 2076 9 30 0.6700 8.535 1.150 0.1635 4f 1.1600 0.3786 2070 2290 2492 9 38 0.5900 9.421 0.810 0.1719 0.3974 0.7100 4k 2157 2380 2576 9 24 1.1100 4.865 2.380 0.1258 3e 2.3300 0.2805 1594 1796 2040 9 82 0.4500 6.758 1.460 0.1424 0.3443 1.3700 3g 1907 2080 2256 9 46 0.7000 5.893 1.220 0.1288 3i 1.3300 0.3319 1848 1960 2080 9 48 0.5800 11.522 1.360 0.1813 0.4608 1.6000 1a 2443 2566 2664 9 52 1.0200 5.414 2.810 0.1291 3.3800 1712 1887 2d 2084 9 118 0.3043 0.9600 9.034 1.310 0.1683 0.8900 2120 2341 0.3894 2540 9 30 2f A sample was collected from the metasedimen- tary package near the shore of Kaipokok Bay. The sample is a banded mylonitic quartzite with a cherty appearance and minor feldspar and mica- ceous mineral contents. Like the Drunken Har- bour quartzite, this unit also varies to feldspathic quartzite, but unlike the Drunken Harbour quartzite, calcareous minerals e.g. diopside, acti- nolite were not observed. Zircons from the quartzite sample consist of strongly rounded and pitted, colourless, brown, and pink grains. A subset of this population preserves relict crystal faces, and some grains have a euhedral form, but all show evidence of abra- sion during detrital transport. Conventional U – Pb dating of six single grains of varying colour and degree of rounding yields concordant and near-concordant 207 Pb 206 Pb ages of 3034 – 2665 Ma Fig. 7c, similar to the range of ages from the Drunken Harbour quartzite. Twenty-two individ- ual grains analysed by laser ablation provide a similar but more discordant range of ages Fig. 7d. Excluding one anomalously young, strongly turbid grain grain 3b, with a 207 Pb 206 Pb age of 1958 Ma; Table 4, 207 Pb 206 Pb ages span the interval 3130 – 2502 Ma. Somewhat older Ar- chaean crystallization ages can be assumed given that that discordance is likely due largely to 1.9 – 1.8 Ga Pb loss. The younger apparent age for grain 3b is not considered indicative of Pale- oproterozoic detritus due to the poor quality of this grain and a probable large degree of Pb loss. The analysed zircons are considered therefore to be entirely Archaean, and except in degree of discordance, isotopically resemble the detrital zir- con population in the Drunken Harbour quartz- ite. We suggest on this basis that the Drunken Harbour and Post Hill quartzites, although differ- Fig. 8. a Transition zone between metasedimentary rocks and overlying mafic metavolcanic rocks on the northeast flank of Post Hill. Metasedimentary layers light coloured layers become increasingly sparse above the field of view, with the sequence grading into massive Post Hill amphibolite several metres above this section. The transition is regarded here as a gradational stratigraphic contact between Lower Aillik Group metasedimentary rocks and the Post Hill amphibolite. b Contact between the Post Hill amphibolite and the overlying Metasedimentary formation near the top of Post Hill. This contact does not appear to be tectonic and may represent an erosional unconformity based on U – Pb data discussed in the text. ing in location, appearance, mineralogy, and zircon morphology, contain detritus that was shed from the same source regions. The implications of this result are discussed below. 5 . 4 . Post Hill intermediate tuff sample 4 A grey, homogeneous, highly-deformed, amphi- bole-bearing quartzofeldspathic gneiss occurs as distinctive 5 – 30 cm wide horizons interlayered with mafic schist near the base of the Post Hill amphibolite. The intermediate bulk composition of this lithology and its distribution within the Post Hill amphibolite suggests that it may have originated as tuff horizons within the volcanic pile B. Chadwick, personal communication, 1995. A horizon of this gneiss exposed on the shore of Kaipokok Bay was sampled for U – Pb geochronology Fig. 2. The sample yielded a single morphology of small, colourless, doubly- terminated euhedral zircon prisms with square cross-sections and numerous fluid inclusions. No distinctive grains that could be reasonably inter- preted to represent inherited older zircon were observed. Five multigrain fractions of euhedral prisms were analysed and are concordant to slightly discordant, with concordant analyses Z1 and Z4 yielding identical ages of 2178 9 4 Ma Fig. 6b. The remaining fractions appear to have suffered minor but variable degrees of Pb loss. The presence of a single zircon population with a high-temperature morphology Pupin, 1980 is consistent with a tuffaceous origin for the quart- zofeldspathic gneiss. We therefore suggest that the grey gneiss horizons were deposited during vol- canic activity at 2178 9 4 Ma. Given that these rocks are intimately associated with lowermost rocks of the Post Hill amphibolite, the 2178 Ma age is considered to date the onset of mafic vol- canism that formed the protolith to the amphibolite. 5 . 5 . Lower Aillik Group psammite sample 5 Sample 5 was collected from an 800 m thick unit of interlayered semipelitic and psammitic rocks that crops out along the south shore of Kaipokok Bay Metasedimentary formation of Marten, 1977. This unit is correlated with a similar rock package overlying the Post Hill am- phibolite Fig. 2 on the basis of lithological and stratigraphic similarity, the presence of a distinc- tive sulphide- and graphite-bearing marker hori- zon, and similar contact relationships Marten, 1977. The sample of biotite – muscovite psammite was collected to compare the range of detrital zircon ages in this unit with those from the Post Hill and Drunken Harbour quartzites. Zircons consist mainly of colourless and brown prismatic grains of variable size. Both colour types range from pristine to strongly pitted, and several grains have visible core and overgrowth components. Six single grains were selected for conventional U – Pb dating. The youngest grain, a sharp, colourless prism with minor detrital pit- ting, is concordant at 2013 Ma Fig. 7e. A mor- phologically similar but much smaller zircon with fluid inclusions yields a slightly discordant 207 Pb 206 Pb age of 2160 Ma. This grain overlaps fraction Z5 from the intermediate tuff sample 4 within uncertainly and may be derived from this struc- turally lower unit. The oldest grain is a pitted, large, colourless prism with a discordant age of 2754 Ma, clearly indicating the presence of Ar- chaean detritus. Twenty-four grains representing the range of zircon morphologies in the sample were analysed by laser ablation ICP-MS. As is apparent in Fig. 7f, most grains have discordant 207 Pb 206 Pb ages between 1.9 – 2.2 Ga, with less abundant older discordant analyses clustering around 2.3 – 2.7 Ga. The oldest grain not shown in Fig. 7f is strongly discordant with a 207 Pb 206 Pb age of 3742 Ma Table 4. The distribution of analyses indicates that the sample contains both Proterozoic and Archaean detritus, with the former apparently more abundant than the latter. In conjunction with the TIMS data, this suggests that Nain Province basement rocks and Lower Aillik Group lithologies underlying the psammite-semipelite unit are possible source-rock candidates. The youngest TIMS analysis demonstrates that this unit was deposited after 2013 Ma. A LAM-ICP- MS analysis that touches concordia at ca. 1.9 Ga Fig. 7f is not considered here to provide a younger maximum deposition age due to the rela- tively large uncertainties on the laser ablation analyses.

6. Discussion