U – Pb analytical techniques

Culshaw et al., 1998. The Post Hill amphibolite and its basal mylonite occupy a thrust klippe that largely escaped the effects of the later deformation Marten, 1977.

3. Petrochemistry of the Post Hill Amphibolite

Field relations and limited geochemical data two samples with major element analyses from White, 1976 suggested to Gower et al. 1982 that volcanic activity represented by the Post Hill am- phibolite may have been related to continental rifting, although Wilton 1996 refuted this sug- gestion based on geochemical data from a single additional sample. Samples from the structurally higher Metasedimentary formation have bulk rock chemistry indicating a minimal component of Archaean detritus Gower et al., 1982, consis- tent with o Nd of + 2 for anatectic granite within this formation Kerr, 1989. To better constrain the tectonic setting of mafic volcanism, 14 samples were collected from the Post Hill amphibolite in the Post Hill area, from which eight typical samples a – h, Table 1 were selected for major and trace element analyses. In addition, two samples collected from the thinner extensions of the amphibolite to the southwest and northeast were also analysed i and j, respec- tively, Table 1. Sample locations are shown in Fig. 2. The samples consist mainly of pleochroic green or green-blue amphibole 50 – 80, with less abundant, mainly untwinned plagioclase and minor quartz, epidote, and opaque minerals. Bi- otite is present rarely. Most samples are fine- grained with a moderate to strong foliation, and preserve no relict igneous textures. The analysed samples show a narrow range in SiO 2 content from 48.5 – 51.4 Table 1. They are tholeiitic, with FeO t MgO ranging from 1.5 to 3.0 Fig. 3a. The sample suite does not have the high TiO 2 and FeO t of the two samples from White 1976 that led Gower et al. 1982 to conclude that the rocks formed in a continental rift. The data are similar to the analysis presented by Wilton 1996 for a sample from the northeast- ern extension of the amphibolite on Kaipokok Bay D. Wilton, written communication 1998. The latter sample is included for comparison in Fig. 3 and Fig. 4. The samples of Post Hill amphibolite have a range in composition similar to that shown by Mesozoic mafic volcanic and intrusive rocks of eastern North America, a classic rifted continental margin assemblage e.g. Wang and Glover, 1992. Wang and Glover 1992 showed that such mafic rocks have a range in compositions due to diverse factors such as variations in source rocks and degree of crustal contamination. Like the Meso- zoic suite, the Post Hill amphibolite ranges from compositions typical of island arc tholeiites through to typical within-plate basalt Fig. 3b, c. However, some features, such as low Y Fig. 3d, Fig. 4 are more consistent with island arc suites than with continental rifting suites. In general, the analyses span the range between the average low- potassium tholeiite island arc tholeiite and the average within-plate tholeiite Fig. 4. Given the demonstrably wide chemical range possible in continental rifting suites such as the Mesozoic rocks of eastern North America, and a close relationship between the Post Hill amphibo- lite and underlying and overlying sedimentary se- quences that both contain Archaean detritus see below, we suggest that geochemical data for the Post Hill amphibolite indicate a rifted continental margin setting rather than an island arc setting.

4. U – Pb analytical techniques

TIMS and LAM-ICP-MS analyses were carried out at the Memorial University of Newfoundland. Mineral separation, dissolution, ion exchange, and mass spectrometric techniques for TIMS analyses of zircon, monazite, and titanite are briefly described below see Ketchum et al., 1997 for a more detailed version. All fractions to be analysed were air abraded Krogh, 1982, and a mixed 205 Pb 235 U isotopic tracer solution was added to the sample prior to dissolution. Extrac- tion of Pb and U by ion exchange column chem- istry followed modified versions of the procedure of Krogh 1973 for zircon and Corfu and Stott 1986 for titanite and monazite. Fractions were loaded along with silica gel onto outgassed Re Table 1 Geochemical data from the Post Hill amphibolite, Lower Aillik Group a Sample Wt. b c d a e f g h i j 48.46 49.64 51.40 48.48 SiO 2 51.39 51.17 50.76 49.70 49.67 48.94 0.81 1.10 0.75 1.78 0.98 1.19 0.85 TiO 2 0.81 1.68 1.10 16.64 13.85 12.98 13.52 13.47 12.77 12.99 Al 2 O 3 13.84 13.33 13.61 15.73 Fe 2 O 3t 11.79 14.27 13.80 15.18 12.87 14.21 12.38 12.58 13.79 0.23 MnO 0.18 0.24 0.22 0.22 0.22 0.22 0.20 0.19 0.21 6.74 6.86 7.15 5.99 6.62 4.83 5.85 MgO 6.78 8.17 7.06 11.39 8.88 9.95 9.59 10.82 9.60 9.97 9.82 CaO 12.08 9.34 2.16 2.66 1.78 2.29 1.49 1.76 2.29 Na 2 O 2.80 3.19 1.28 0.40 K 2 O 0.13 0.11 0.26 0.34 0.18 0.54 0.58 0.14 0.20 0.19 P 2 O 5 0.06 0.07 0.05 0.21 0.07 0.11 0.06 0.05 0.08 0.73 0.63 0.65 0.68 0.75 0.19 0.60 LOI 0.70 0.66 0.65 99.09 98.31 99.00 98.29 98.87 Total 98.36 98.29 97.86 98.62 98.99 ppm Ba 124 242 259 217 244 128 181 317 74 276 Rb 16 11 11 17 16 12 18 17 10 25 200 126 92 180 155 134 143 Sr 276 122 110 13 Y 18 32 15 32 14 21 15 16 33 51 59 42 142 68 132 95 Zr 66 49 64 8 Nb 5 5 5 9 6 7 5 5 29 1 Th 1 B 1 B 1 1 B 1 B 1 1 1 3 4 5 5 4 4 7 4 Pb 8 10 5 14 11 11 14 14 Ga 13 12 13 12 17 79 95 88 113 83 112 105 Zn 88 90 85 107 Cu 89 109 125 57 98 38 110 83 81 22 Ni 175 48 66 49 47 36 63 79 83 260 328 318 382 305 374 319 V 330 331 276 Cr 209 38 150 77 107 109 77 134 205 147 3 10 12 5 9 9 5 Sc 5 11 12 62 Co 67 60 75 55 72 61 52 54 81 3 U 2 2 3 2 B 1 2 1 2 2 1 B 1 B 1 7 2 3 1 Sn 2 2 nd b La 26 22 15 12 17 13 19 27 13 5 12 19 8 26 14 21 21 12 nd b 19 Nd a Analyses by standard X-Ray Fluorescence techniques using a Philips PW2400 X-ray spectrometer at the Regional Geochemical Centre, Saint Mary’s University, Halifax, NS, Canada; analyst Dr D. Slauenwhite. Uncertainties are 9 1 on major elements and 9 5 on trace elements. b not determined. filaments and analysed on a Finnigan-MAT 262 mass spectrometer in multicollection mode. Small samples were measured in peak-jumping mode using a secondary electron multiplier-ion counter system. Uncertainties 2s on the measured atomic ratios are reported in Table 3 and shown as ellipses in Figs. 5 – 7. Uranium decay constants used in the age calculations are those of Jaffey et al. 1971. Additional details of the technique can be found in the footnotes to Table 3. The technique for laser ablation analysis of single zircon grains at Memorial University is described in detail below. Zircons selected for LAM-ICP-MS analysis were, like their TIMS counterparts, hand-picked in alcohol under a binocular microscope, and were chosen from the least magnetic split obtained with a Frantz isody- namic separator. Approximately 20 – 30 grains were selected for LAM-ICP-MS from each of three samples with a detrital zircon population, with these grains representing the entire range of observed morphological and colour types. Al- though the laser ablation technique is rapid and highly amenable to analysing a larger number of grains, we selected only 20 – 30 grains in order to obtain a first-order understanding of the range of detrital zircon ages. This number of analyses was sufficient to include all observed zircon morpholo- gies, to determine whether samples are dominated by Archaean or Paleoproterozoic detritus a key goal of this study, and to allow speculation on possible source regions. The number was not sufficient, however, for a statistical treatment of detrital zircon age populations, which was not considered an appropriate task due to the early developmental stage of this technique. Zircons were mounted in epoxy in 2.5 cm di- ameter circular grain mounts and polished until a portion of each grain not less than 60 mm in diameter was exposed. In general, this procedure removed only a small amount of material from each grain but more than obtained by conven- Fig. 3. a Plot of TiO 2 against FeO t MgO for mafic samples from the Post Hill amphibolite data in Table 1. Typical tholeiitic and calc-alkalic trends are after Miyashiro 1974. Data for the average within-plate tholeiitic WPT basalt and low-potassium volcanic arc tholeiite LKT are from Pearce 1982. Dashed line encloses most Mesozoic mafic volcanic and plutonic data from eastern North America Wang and Glover, 1992. b Plot of TiO 2 against Zr for samples of Post Hill amphibolite and other units as in a. Fields for arc, within-plate and mid-ocean ridge basalts MORB are from Pearce 1982. c Plot of V against Ti for Post Hill amphibolite and other units as in a. Fields are from Shervais 1982. d Plot of Cr against Y for samples of Post Hill amphibolite and other units as in a. Fields for MORB mid-ocean ridge basalts, WPB within-plate basalt, and IAT island-arc tholeiite are from Pearce 1982. Fig. 4. Multi-element variation diagram for samples from the Post Hill amphibolite and other units as in Fig. 3a. Data are normalized to average normal mid-ocean ridge basalt N- MORB from Sun and McDonough 1989, except V, Ni, and Cr which are from Taylor and McLennan 1985. Fig. 5. U – Pb analyses of 02123 zircon standard used in LAM-ICP-MS geochronology. Upper diagram shows 36 laser ablation analyses of the standard that overlap the TIMS-deter- mined age 295 9 1 Ma; bottom diagram and were used in age calculations for detrital zircons. Middle diagram shows eight analyses of the standard that were not used in the age calcula- tions. TIMS data provided by Greg Dunning Memorial Uni- versity. tional abrasion techniques, thereby leaving suffi- cient material for analysis. Grain mounts contain- ing the samples and the standardisation materials ‘02123’ zircon standard and NIST SRM 612 glass were cleaned in 2N nitric acid for approxi- mately 1 h prior to analysis. LAM-ICP-MS analyses were performed using a custom-built ultraviolet LAM coupled to a VG PQII + ‘S’ ICP-MS. The current configuration of the system has been described recently by Taylor et al. 1997. The laser and ICP-MS operating conditions used in this study are presented in Table 2. ICP-MS operating conditions were opti- mised, using continuous ablation of NIST 612 glass, to provide maximum sensitivity for the heavy mass range Lu – U while maintaining low oxide formation ThO + Th + B 1. The sample mount and standardisation materials were ablated in a custom-built sample chamber with a nozzle for introducing the Ar carrier gas in a focused jet directly onto the ablation site. This sample-cool- ing technique has been demonstrated to reduce thermal effects contributing to the elemental frac- tionation of U relative to Pb during ablation Jackson et al., 1997. The sample was transferred to the ICP-MS though acid-washed plastic tubing. Data acquisition parameters are listed in Table 2. Data were acquired on five isotopes using the instrument’s time-resolved analysis data acquisi- tion software. Useful data could not be acquired Fig. 6. U – Pb TIMS geochronological data for a migmatitic tonalite gneiss beneath the Lower Aillik Group and b an intermediate tuff horizon within the Post Hill amphibolite Lower Aillik Group. Sample locations are given in Fig. 2. Fraction labels: Z, zircon; M, monazite. for 204 Pb due to the overpowering isobaric inter- ference from 204 Hg, a significant contaminant in the Ar supply gas, which could not be reduced sufficiently using either activated charcoal or gold filters. The time-resolved analysis software reports signal intensity data counts per second for each mass sweep performed by the mass spectrometer. This data acquisition protocol allows acquisition of signals as a function of time ablation depth, and subsequent recognition of isotopic hetero- geneity within the ablation volume e.g. zones of Pb loss or common Pb related to fractures or areas of radiation damage; also inclusions, inher- ited cores, etc.. The signals can then be selectively integrated. Background and ablation data for each analysis were collected over single runs last- ing 60 – 120 s, with background measurements obtained over the first 30 s, prior to initiation of ablation. For each measurement session 20 spot analyses, the order was: four analyses of 02123, one NIST 612, 10 unknowns, one NIST 612, and four 02123. To minimise U – Pb fraction- ation related to the relative change in focus of the laser as it penetrates into the sample, all analyses were performed with the laser focused 200 mm above the sample, which yielded an ablation pit 40 mm in diameter. One spot per grain was analysed using a laser beam operated at a fixed energy and focusing condition thoughout each run to maintain constant U – Pb fractionation. Ablation of unknowns was carried out until be- tween 30 – 90 s of data were obtained or until grains were completely penetrated by the laser. Under the operating conditions given in Table 2, the penetration rate of the laser was between 1 – 2 m ms. The raw data were downloaded to a PC for processing. Raw count rates were pre-integrated by averaging consecutive groups of 15 mass sweeps into single readings. The data were then processed using LAMTRACE, an in-house data reduction program. 207 Pb 206 Pb, 208 Pb 206 Pb, 208 Pb 232 U, 206 Pb 238 U and 207 Pb 235 U 235 U = 238 U137.88 ratios were calculated for each read- ing and the time-resolved ratios for each analysis were then carefully examined. Signal intervals for the background and ablation were selected for each sample and matched with similar intervals for the standards. Net background-corrected count rates for each isotope were used for calcula- tion of sample ages. Analyses of the 02123 zircon standard were used to correct the effects of U – Pb fractionation and mass discrimination of the mass spectrome- ter. Of the eight analyses of the standard acquired during each measurement session, typically one to three were rejected due to low precision or dis- agreement with the precisely determined TIMS Fig. 7. U – Pb TIMS and LAM-ICP-MS data for metasedimentary rocks. Both the Drunken Harbour and Post Hill quartzites contain detrital zircons only of Archaean age, whereas the psammite contains both Archaean and Paleoproterozoic grains. See text for additional details. Sample locations are given in Fig. 2. Fraction labels: Z, zircon; M, monazite; T, titanite. 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