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 Compilation of geochronology data, 2001 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 14. Ion microprobe analytical results for sample 169062: porphyritic syenogranite, Contessis Bore syenogranite, porphyritic sample 169062: for analytical results 14. Ion microprobe Table 1.12.13.1 1344.1 3045.1 125 120 1 6.1 1997.1 136 2988.1 201 2529.1 81 60 230 96 308 60 338 50 211 202 1.358 37 0.236 386 118 0.081 0.571 0.11536 25 38 0.10196 0.630 72 118 0.10345 0.10042 0.00212 1.174 0.00092 62 0.150 0.14595 0.293 0.00039 0.00164 0.47607 0.09258 0.804 0.12641 0.00102 0.10836 0.10183 0.06580 0.46005 0.00541 0.00274 0.00180 0.09920 0.00096 0.00086 0.07475 0.00055 0.00454 0.3143 0.71022 0.00153 0.2961 0.00183 0.04667 0.36722 0.2988 0.2925 0.0035 0.00818 0.0031 0.03059 0.00149 0.00228 0.3700 0.0030 0.0033 4.999 0.2962 0.00299 4.163 0.3114 0.0039 0.2993 4.262 4.050 0.113 0.0036 0.2914 0.061 0.0033 0.0032 7.446 0.048 0.085 3.781 0.0054 0.100 4.653 4.203 93 101 0.126 100 101 3.986 0.068 0.060 0.102 88 886 1 113 660 1 687 1 102 632 1 99 33 102 17 299 2 30 479 1 7 658 1 772 1 12 609 1 56 16 16 29 27.1 176 107 61 0.342 0.10887 0.00095 0.17079 0.00190 0.3110 0.0040 4.669 0.077 98 781 1 16 10.111.112.113.1 78 221 061 1 14.115.1 186 17416.1 237 622 6715.117.1 53 176 135 3518.1 135 354 599 7319.1 33320.1 57 2.101 239 64 584 0.34921.1 187 0.619 399 38622.1 198 372 0.10029 0.57023.1 903 0.10996 0.208 20 54 121 0.1088024.1 194 349 199 0.0036825.1 117 476 0.10808 0.00051 5.504 122 1.116 0.10378 0.0013326.1 226 200 0.373 130 113 1 128 452 0.634 0.00142 0.61995 139 0.09174 0.06733 0.00063 204 0.10195 0.641 552 0.08310 0.10188 386 0.321 50 170 0.10901 0.00995 0.15914 0.217 0.00804 0.00089 90 0.00229 0.06191 0.10308 0.00254 0.00075 628 49 0.523 0.10243 0.00116 327 65 0.00294 0.2838 0.153 0.10138 0.26387 0.3252 0.00104 0.00061 0.49992 0.3171 0.445 0.17435 205 0.00064 0.10173 0.061 0.473 0.16589 0.0057 0.3096 0.00058 0.10341 0.01873 0.0059 0.00620 0.2966 0.45119 0.0059 0.00157 0.10342 0.099 0.00130 0.19990 0.00243 0.10301 0.10034 0.0058 0.00077 3.925 0.35695 0.2802 0.00172 0.2842 0.0054 4.931 0.00124 4.757 0.2909 0.10299 0.00138 0.47687 0.00030 0.00117 0.3131 0.172 0.00154 0.65855 4.613 0.0062 0.095 0.0054 0.2848 4.245 0.112 0.0053 0.00044 0.48264 0.00370 0.2934 0.0058 0.04359 0.83722 0.00289 0.111 0.2961 3.545 0.084 0.0036 3.995 0.00357 0.32925 4.087 0.2992 0.0037 0.00034 99 0.00452 101 4.706 100 0.2954 0.0038 0.330 0.125 4.047 0.00115 0.2912 0.084 0.0040 4.143 98 0.2974 0.2940 0.105 0.0038 99 4.140 0.059 0.0039 0.3002 630 1 0.061 799 1 4.196 109 0.0037 0.0039 779 1 97 0.060 4.211 99 0.0038 4.153 0.082 98 767 1 4.224 693 1 4.067 68 0.066 96 8 22 0.079 99 4.263 462 1 101 0.056 0.076 660 1 24 659 1 11 102 0.059 783 1 167 99 680 1 42 98 669 1 14 100 102 650 1 19 101 656 1 11 12 686 1 11 686 1 679 1 630 1 24 679 1 14 22 22 5 8 Grain.spot U (ppm) (ppm) Th (ppm) Pb ƒ206%

60 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. Concordia plot for sample 169062: porphyritic syenogranite, Contessis Bore

61 Compilation of geochronology data, 2001

zircons (7.1, 11.1, 12.1, 13.1, 17.1 and 27.1), assigned to Group 2, have 207Pb/206Pb ratios defining a single population and indicating a weighted mean 207Pb/206Pb date of 1788 ± 15 Ma (chi-squared = 0.61). Discordant analyses 1.1 and 5.1 indicate substantially older 207Pb/206Pb dates, whereas analysis 6.1 is slightly reversely discordant and indicates a slightly younger 207Pb/206Pb date than the analyses of Group 1. The weighted mean 207Pb/206Pb date of 1674 ± 8 Ma indicated by the 19 concordant to slightly discordant analyses of Group 1 is interpreted as the age of igneous crystallization of the syenogranite. The reverse discordance of analysis 6.1 may be due either to gain of radiogenic Pb at this analysis site, incorrect common-Pb composition for this analysis, or to incorrect centring on mass peak 206Pb during acquisition of this analysis, with consequent undercounting at the 207Pb mass station. The remaining analyses are interpreted to be of xenocryst zircons.

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, 169062: porphyritic syenogranite, Contessis Bore; in Compilation of geochronology data, 2001: Western Australia Geological Survey, Record 2002/2, p. 59–62. OR NELSON, D. R., 2002, 169062: porphyritic syenogranite, Contessis Bore; Geochronology dataset 136; in Compilation of geochronology data, June 2006 update: Western Australia Geological Survey.

Data obtained: 15/08/2001; Data released: 26/06/2002