169060.1.pdfCompilation - ofGeochronology geochronology datasetdata, 2001 134

169060: porphyritic syenogranite, Yangibana Bore

Location and sampling

EDMUND (SF 50-14), EDMUND (2158) MGA Zone 50, 419130E 7355190N Sampled on 15 October 2000. The sample was taken from the edge of an undulating pavement located 5 km southeast of Yangibana Bore.

Tectonic unit/relations This sample is a light to medium grey, undeformed seriate porphyritic syenogranite of the Pimbyana Granite (Pearson et al., 1996; Martin et al., 2002), Gascoyne Complex. The syenogranite contains scattered tabular, feldspar phenocrysts up to 5 cm long, within a dark, fine-grained groundmass. Feldspar phenocrysts are aligned and range from very abundant to sparse over distances of several metres.

Petrographic description This sample consists principally of microcline and orthoclase (60–65 vol.%), quartz (20 vol.%), plagioclase (10 vol.%), biotite (4 vol.%), and muscovite (3 vol.%), with accessory apatite (trace) opaque oxide (trace), possible monazite (trace), and zircon (trace). This is a porphyritic syenogranite with partly altered biotite and muscovite, very minor opaque oxide and zircon, and possible monazite. Feldspar phenocrysts to 30 mm long are visible in hand specimen. In thin section these are perthitic K-feldspar, varying from orthoclase to microcline, with inclusions of quartz, altered plagioclase, fresh to altered biotite, and muscovite. The largest grain within the area of the thin section is only about 10 mm long, but larger crystals are more widely dispersed. Separate plagioclase grains are up to 4 mm long, with irregular flooding by sericite. Quartz occurs as lensoidal to rounded grains to 7 mm long, deformed with undulose extinction and subgrains. Biotite occurs as flakes to 3 mm long and is partly altered to chlorite or clay, with or without epidote. Muscovite is also common, passing into aggregates of fibrous muscovite and sericite of secondary origin. Inclusions of plagioclase and quartz in K-feldspar are locally rimmed by muscovite, some of which have myrmekite-like quartz inclusions, suggesting a subsolidus origin for the muscovite. Granular apatite occurs, to 0.5 mm in grain size, with rare possible monazite 0.2 mm in diameter. Very minor fine-grained opaque oxide is also present.

Zircon morphology The zircons isolated from this sample are colourless, pale pinkish brown, dark brown or black, generally between 40 × 80 µm and 100 × 230 µm in size, and are commonly irregular fragments, or subhedral and elongate in shape. Many grains have faint internal zonation, and mineral inclusions are common. Many have irregular zones of dark alteration, and many are metamict.

Analytical details This sample was analysed on 3 June 2001. The counter deadtime was 32 ns. Thirteen analyses of the CZ3 standard obtained during the analysis session indicated a Pb*/U calibration error of 1.82 (1σ%). Common-Pb corrections were applied assuming Broken Hill common-Pb isotopic compositions for all analyses, with the exception of analysis

56 GSWA Record 2002/2 σ Pb ±1 206 Pb/ 207 % concordance age σ U±1 235 Pb/ 207 σ U±1 238 Pb/ 206 σ Pb ±1 206 Pb/ 208 σ Pb ±1 206 Pb/ 207 Table 13. Ion microprobe analytical results for sample 169060: porphyritic syenogranite, Yangibana Bore Yangibana syenogranite, sample 169060: porphyritic for analytical results microprobe 13. Ion Table 1.12.13.1 2124.1 1795.1 1676.1 188 2737.1 213 748.1 165 2769.1 297 75 77 188 163 71 145 73 231 0.513 109 47 0.349 81 97 0.438 214 0.10119 0.240 92 0.12132 0.459 0.11950 28 0.00093 1.467 0.10870 59 145 0.00089 0.10970 0.517 0.00098 0.324 0.00064 0.24561 0.10299 0.188 0.00089 0.11644 0.10996 0.27375 0.00212 0.00116 0.11002 0.31393 0.18080 0.00162 0.25369 0.00159 0.00221 0.00098 0.00155 0.2978 0.00066 0.15783 0.00201 0.3663 0.17725 0.3581 0.00254 0.0055 0.17801 0.3237 0.25629 0.0068 0.3213 0.00338 0.0066 0.00203 0.00118 0.2921 0.0059 4.155 0.0059 6.128 0.3180 5.900 0.3202 0.090 0.0054 0.5060 4.851 0.127 4.860 0.0060 0.125 0.0059 0.097 0.0093 4.148 0.102 102 4.822 12.615 0.094 4.858 102 101 102 0.122 0.243 0.105 100 646 1 98 976 1 949 1 778 1 99 100 99 17 794 1 13 15 679 1 11 799 1 15 800 1 660 2 21 26 16 6 19.1 30 9 19 0.985 0.19317 0.00268 0.07693 0.00491 0.5699 0.0117 15.179 0.396 105 769 2 23 10.111.112.1 23413.1 53514.115.1 52 152 17516.1 242 79017.1 15318.1 49 212 78 168 171 150 20 117 0.375 84 0.543 34 65 75 224 45 0.10132 57 0.703 0.315 0.10237 67 0.088 59 0.00079 1.819 53 0.18115 0.00056 0.10311 0.353 0.10313 37 4.888 0.18213 0.12313 0.00180 0.00090 0.12674 1.595 0.00033 0.10888 0.00167 0.00154 0.11292 0.25961 0.00113 0.35255 0.00091 0.11854 0.00752 0.00251 0.2984 0.20276 0.00359 0.00229 0.2962 0.00205 0.00037 0.12463 0.00340 0.0055 0.10302 0.5165 0.2920 0.0054 0.00174 0.22012 0.2955 0.00594 0.3063 4.168 0.0100 0.0054 4.181 0.00458 0.3266 0.0054 0.2871 0.087 0.0057 12.901 0.082 4.151 0.3634 0.0060 4.202 0.295 0.0054 5.200 0.089 0.0069 0.079 102 4.903 100 0.123 4.471 0.104 101 5.940 0.138 98 99 648 1 0.161 668 1 86 102 663 2 88 14 681 1 10 103 681 1 002 2 16 781 1 16 847 1 6 22 934 1 15 40 31 Grain.spot U (ppm) (ppm) Th (ppm) Pb ƒ206%

57 Compilation of geochronology data, 2001

0.60

Analysis 19.1

0.55 2800

2700 0.50 2600 Analyses 9.1 & 12.1

2500

2400 0.45

2300

Group 2: 6 analyses of 6 zircons Pb/ U 2200

206 238 1788 ± 17 Ma (95% confidence) 0.40 2100

2000 0.35 1900

Group 3: 4 analyses of 4 zircons 1967 ± 39 Ma (95% confidence) 1700 0.30 Group 1: 6 analyses of 6 zircons 1673 ± 15 Ma (95% confidence)

169060 0.25 345678910 11 12 13 14 15 16 DN126 207Pb/ 235 U 10.05.02

Figure 18. Concordia plot for sample 169060: porphyritic syenogranite, Yangibana Bore

17.1, for which isotopic compositions determined using the method of Cumming and Richards (1975) were assumed.

Results Nineteen analyses were obtained from 19 zircons. Results are given in Table 13 and shown on a concordia plot in Figure 18.

Interpretation Most analyses are concordant to slightly discordant with the discordance patterns consistent with at least one recent episode of radiogenic-Pb redistribution. A range of 207Pb/206Pb dates of from c. 1670 to 2770 Ma is indicated. On the basis of their 207Pb/ 206Pb ratios, many analyses may be assigned to one of three groups. Six concordant analyses of six zircons (1.1, 6.1, 10.1, 11.1, 13.1 and 14.1), assigned to Group 1, have 207Pb/206Pb ratios defining a single population and indicating a weighted mean 207Pb/ 206Pb date of 1673 ± 15 Ma (chi-squared = 1.38). Six concordant analyses of six zircons (4.1, 5.1, 7.1, 8.1, 16.1 and 17.1), assigned to Group 2, have 207Pb/206Pb ratios defining a single population and indicating a weighted mean 207Pb/206Pb date of 1788 ± 17 Ma (chi-squared = 0.73). Concordant analyses 2.1, 3.1, 15.1 and 18.1, assigned to Group 3, have 207Pb/206Pb ratios defining a single population and indicating a weighted mean 207Pb/206Pb date of 1967 ± 39 Ma (chi-squared = 1.55). The remaining analyses (9.1, 12.1 and 19.1) cannot be confidently grouped. The weighted mean 207Pb/206Pb date of 1673 ± 15 Ma indicated by the six analyses of Group 1 is interpreted as corresponding to the age of igneous crystallization of the syenogranite. The remaining analyses interpreted to be of xenocryst zircons.

58 GSWA Record 2002/2

STRATIGRAPHIC REFERENCE: MARTIN, D. Mc., THORNE, A. M., and OCCHIPINTI, S. A., 2002, Edmund, W.A. Sheet 2158: Western Australia Geological Survey, 1:100 000 Geological Series. PEARSON, J. M., TAYLOR, W. R., and BARLEY, M. E., 1996, Geology of the alkaline Gifford Creek Complex, Gascoyne Complex, Western Australia: Australian Journal of Earth Sciences, v. 43, p. 299309.

Recommended reference for this publication:

NELSON, D. R., 2002, 169060: porphyritic syenogranite, Yangibana Bore; in Compilation of geochronology data, 2001: Western Australia Geological Survey, Record 2002/2, p. 56–58. OR NELSON, D. R., 2002, 169060: porphyritic syenogranite, Yangibana Bore; Geochronology dataset 134; in Compilation of geochronology data, June 2006 update: Western Australia Geological Survey.

Data obtained: 03/06/2001; Data released: 26/06/2002