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169062 Porphyritic Syenogranite, Contessis Bore

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169062 Porphyritic Syenogranite, Contessis Bore 169062.1.pdf - Geochronology dataset 136 GSWA Record 2002/2 169062: porphyritic syenogranite, Contessis Bore Location and sampling EDMUND (SF 50-14), EDMUND (2158) MGA Zone 50, 414280E 7367460N Sampled on 15 October 2000. The sample was taken from a 3 m-high whaleback on the western side of a prominent hill of rocky pavements, whalebacks and tors, located 3 km south-southwest of Contessis Bore. Tectonic unit/relations This sample is a medium to dark grey, medium-grained, porphyritic syenogranite of the Dingo Creek Granite (Pearson et al., 1996; Martin et al., 2002), Gascoyne Complex. The granite contains abundant cubic and tabular feldspar phenocrysts up to 10 mm long that define a flow foliation, and contains inclusions of medium-grained leucocratic, tourmaline-bearing granite and porphyritic biotite granite. At the sampling site, the granite has been intruded by irregular, leucocratic tourmaline-bearing pegmatite dykes up to 20 cm in thickness. The sample taken was free of any obvious dykes and veins. Petrographic description This sample consists principally of K-feldspar (40 vol.%), quartz (30 vol.%), plagioclase (15–20 vol.%), muscovite (7–8 vol.%), biotite (5 vol.%), and apatite (≤1 vol.%), with accessory opaque oxide (trace), fluorite (trace), and zircon (trace). This is a porphyritic, muscovite- and biotite-bearing microgranite or syenogranite. The thin section has phenocrysts of K-feldspar (orthoclase and/or microcline) to 5 mm long as well as ragged but clearly bipyramidal quartz phenocrysts to 4 mm in diameter. Ragged aggregates of coarse-grained quartz, to 7 mm in diameter, may partly represent recrystallized phenocrysts, but lack any obvious bipyramidal outline. Relatively rare plagioclase phenocrysts occur, to 4 mm long, with irregular alteration to sericite and fine-grained muscovite. There are scattered larger flakes of muscovite and biotite, to 3 mm long, with some of the muscovite elongate at a high angle to its cleavage. Much of the host rock is fine-grained, 0.2 to 1 mm in grain size, with microcline and quartz dominant over plagioclase, biotite, and muscovite. Granular apatite and opaque oxide are disseminated, but are mostly about 0.1 mm in grain size, with traces of fine-grained fluorite and rare zircon crystals. Microcrystalline radioactive grains, enclosed in biotite and muscovite, have pleochroic haloes, and microcrystalline titanite indicates retrograde modification of the biotite, although chloritization is rare. Zircon morphology The zircons isolated from this sample are light brown, dark brown or black, generally between 40 × 80 µm and 100 × 230 µm in size, and are commonly irregular fragments or subrounded and equant to slightly elongate in shape. Many grains are internally structureless or have faint internal zonation. Thin rims could be distinguished on a small minority of grains. Mineral inclusions are common. Many grains have irregular zones of dark alteration, and many are metamict. 59 60 Compilation of geochronology data, 2001 Table 14. Ion microprobe analytical results for sample 169062: porphyritic syenogranite, Contessis Bore Grain U Th Pb ƒ206% 207Pb/206Pb ±1σ 208Pb/206Pb ±1σ 206Pb/238U±1σ 207Pb/235U±1σ % 207Pb/206Pb ±1σ .spot (ppm) (ppm) (ppm) concordance age 1.1 134 199 60 1.358 0.11536 0.00212 0.47607 0.00541 0.3143 0.0035 4.999 0.113 93 1 886 33 2.1 304 136 96 0.236 0.10196 0.00092 0.12641 0.00180 0.2961 0.0031 4.163 0.061 101 1 660 17 3.1 125 201 50 0.571 0.10042 0.00164 0.46005 0.00454 0.2925 0.0033 4.050 0.085 101 1 632 30 4.1 1 120 252 338 0.081 0.10345 0.00039 0.06580 0.00055 0.2988 0.0030 4.262 0.048 100 1 687 7 5.1 298 60 118 0.630 0.14595 0.00102 0.07475 0.00183 0.3700 0.0039 7.446 0.100 88 2 299 12 6.1 81 202 38 1.174 0.09258 0.00274 0.71022 0.00818 0.2962 0.0036 3.781 0.126 113 1 479 56 7.1 230 37 72 0.150 0.10836 0.00096 0.04667 0.00149 0.3114 0.0033 4.653 0.068 99 1 772 16 8.1 308 386 118 0.293 0.10183 0.00086 0.36722 0.00228 0.2993 0.0032 4.203 0.060 102 1 658 16 9.1 211 25 62 0.804 0.09920 0.00153 0.03059 0.00299 0.2914 0.0054 3.986 0.102 102 1 609 29 10.1 78 174 35 2.101 0.10029 0.00368 0.61995 0.00995 0.2838 0.0057 3.925 0.172 99 1 630 68 11.1 221 67 73 0.619 0.10880 0.00133 0.08310 0.00254 0.3171 0.0059 4.757 0.112 100 1 779 22 12.1 1 061 237 354 0.349 0.10996 0.00051 0.06733 0.00089 0.3252 0.0059 4.931 0.095 101 1 799 8 13.1 186 176 64 0.570 0.10808 0.00142 0.15914 0.00294 0.3096 0.0058 4.613 0.111 98 1 767 24 14.1 622 135 187 0.208 0.10378 0.00063 0.06191 0.00104 0.2966 0.0054 4.245 0.084 99 1 693 11 15.1 53 57 20 5.504 0.09174 0.00804 0.26387 0.01873 0.2802 0.0062 3.545 0.330 109 1 462 167 16.1 135 239 54 1.116 0.10195 0.00229 0.49992 0.00620 0.2842 0.0054 3.995 0.125 97 1 660 42 15.1 599 399 194 0.373 0.10188 0.00075 0.17435 0.00157 0.2909 0.0053 4.087 0.084 99 1 659 14 17.1 333 198 117 0.634 0.10901 0.00116 0.16589 0.00243 0.3131 0.0058 4.706 0.105 98 1 783 19 18.1 584 903 226 0.641 0.10308 0.00061 0.45119 0.00172 0.2848 0.0036 4.047 0.059 96 1 680 11 19.1 386 349 128 0.321 0.10243 0.00064 0.19990 0.00138 0.2934 0.0037 4.143 0.061 99 1 669 12 20.1 372 476 139 0.217 0.10138 0.00058 0.35695 0.00154 0.2961 0.0038 4.140 0.060 101 1 650 11 21.1 121 200 50 0.523 0.10173 0.00130 0.47687 0.00370 0.2992 0.0040 4.196 0.082 102 1 656 24 22.1 199 452 90 0.153 0.10341 0.00077 0.65855 0.00289 0.2954 0.0038 4.211 0.066 99 1 686 14 23.1 122 204 49 0.445 0.10342 0.00124 0.48264 0.00357 0.2912 0.0039 4.153 0.079 98 1 686 22 24.1 130 386 65 0.473 0.10034 0.00117 0.83722 0.00452 0.2940 0.0039 4.067 0.076 102 1 630 22 25.1 1 113 170 327 0.061 0.10301 0.00030 0.04359 0.00034 0.2974 0.0037 4.224 0.056 100 1 679 5 26.1 552 628 205 0.099 0.10299 0.00044 0.32925 0.00115 0.3002 0.0038 4.263 0.059 101 1 679 8 27.1 176 107 61 0.342 0.10887 0.00095 0.17079 0.00190 0.3110 0.0040 4.669 0.077 98 1 781 16 GSWA Record 2002/2 Analytical details This sample was analysed on 29 July, and 6 and 15 August 2001. The counter deadtime during all three analysis sessions was 32 ns. During the first analysis session, four analyses of the CZ3 standard indicated a Pb*/U calibration error of 0.204 (1σ%). A calibration error of 1.0 (1σ%) was applied to analyses of unknowns obtained during the first analysis session. Analyses 1.1 to 8.1 were obtained during the first analysis session. During the second analysis session, four analyses of the CZ3 standard indicated a Pb*/U calibration error of 1.79 (1σ%). Analyses 9.1 to 17.1 were obtained during the second analysis session. During the third analysis session, four analyses of the CZ3 standard indicated a Pb*/U calibration error of 1.24 (1σ%). Common-Pb corrections were applied assuming Broken Hill common-Pb isotopic compositions for all analyses, with the exception of analysis 18.1, for which isotopic compositions determined using the method of Cumming and Richards (1975) were assumed. Results Twenty-seven analyses were obtained from 27 zircons. Results are given in Table 14 and shown on a concordia plot in Figure 19. Interpretation Most analyses are concordant to slightly discordant with the discordance patterns consistent with one dominant recent episode of radiogenic-Pb redistribution. On the basis of their 207Pb/206Pb ratios, many analyses may be assigned to one of two groups. Nineteen concordant to slightly discordant analyses of 19 zircons, assigned to Group 1, have 207Pb/206Pb ratios defining a single population and indicating a weighted mean 207Pb/206Pb date of 1674 ± 8 Ma (chi-squared = 1.70). Six concordant analyses of six 0.40 2100 2050 Analysis 5.1 2000 1950 0.35 1900 1850 Group 2: 6 analyses of 6 zircons 1788 ± 15 Ma (95% confidence) 1800 Pb/ U 206 238 Analysis 1.1 0.30 Analysis 6.1 1700 1600 Group 1: 19 analyses of 19 zircons 1674 ± 8 Ma (95% confidence) 1550 1500 169062 0.25 3 4 5 6 78 DN127 207Pb/ 235 U 03.05.02 Figure 19.
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