168902.1.pdf - Geochronology dataset 58 Geochronology data obtained in 2002 168902: biotite syenogranite, Hospital Rocks
Location and sampling Zircon morphology
MENZIES (SH 51-5), MULLINE (2938) MGA Zone 51, The zircons isolated from this sample are pale yellowish- 221396E 6696008N brown, or dark yellow-brown to black, generally between 30 × 60 µm and 150 × 300 µm in size, and euhedral and Sampled on 11 May 1999 elongate. Many are black and metamict but most show traces of internal growth zoning. Many grains have a The sample was taken from the edge of a pavement, about speckled appearance in transmitted light. In reflected light, one third of the way up a rock slope, and 200 m northwest the surfaces of many grains have irregular, dark patches of the rock cairn at Hospital Rocks. and zones. Cathodoluminescence images of representative zircons are given in Figure 1. Tectonic unit/relations The sample is from a coarse-grained, porphyritic biotite Analytical details syenogranite of the Southern Cross Granite–Greenstone This sample was analysed on 22 September 2002. The Terrane. The syenogranite contains tabular and irregular- counter deadtime during the analysis session was 24 ns. shaped feldspar phenocrysts up to 2 cm long that are Eight analyses of the CZ3 standard obtained during the weakly aligned in what is probably a flow foliation. The first analysis session indicated a Pb*/U calibration sample contains epidotized fractures. It was collected as uncertainty of 2.78% (1σ). Common-Pb corrections were a representative sample of the broad area of granite that applied assuming Broken Hill common-Pb isotopic lies between greenstone belts in this region (Wyche, 1999). compositions for all analyses, with the exception of analyses 1.1, 2.1, 4.1, 5.1, 8.1, 10.1, 11.1, 12.1 and 3.2, Petrographic description for which isotopic compositions determined using the method of Cumming and Richards (1975) were assumed. The principal minerals in this sample are alkali feldspar (60 vol.%), quartz (20 vol.%), plagioclase (15–20 vol.%), and biotite (3 vol.%), with accessory chlorite (trace), Results muscovite (trace), epidote (trace), opaque oxide (trace), Twenty analyses were obtained from 13 zircons. Results and zircon (trace). It is an altered biotite syenogranite with are given in Table 1 and shown on a concordia plot in chloritized biotite and minor sericite. There is a primary Figure 2. igneous foliation, defined by megacrysts of K-feldspar up to 15 mm long, with lenses of quartz and granular to bladed plagioclase. The alkali feldspar is orthoclase or Interpretation microcline, with microcline twinning in places. It is weakly perthitic and has inclusions of plagioclase to All analyses are concordant or slightly to highly 1.5 mm in grain size, locally with lobate masses of discordant, with the discordance pattern consistent with myrmekite. Plagioclase is up to 4 mm in grain size, a dominant recent episode of radiogenic-Pb loss. All 20 typically with weak sericitization, and rarely with fine- analyses of 13 zircons have 207Pb/206Pb ratios defining a grained muscovite and epidote. Rims of myrmekite occur single population and indicating a weighted mean date of locally. Quartz is not abundant in the thin section but 2621 ± 5 Ma (chi-squared = 1.20). occurs as ragged deformed grains in lenses to 8 × 5 mm in size. Biotite, to 2 mm in grain size and partly contorted, The date of 2621 ± 5 Ma indicated by the weighted is completely altered to chlorite and muscovite or epidote, mean 207Pb/206Pb ratio of all 20 analyses of 13 zircons is or both. Opaque oxide occurs as grains to 0.3 mm in interpreted as the age of igneous crystallization of the diameter, in lenses to 0.5 mm in diameter, or enclosed in syenogranite. altered biotite. Some of the biotite has pleochroic haloes. Recommended reference for this publication: Geochronologist: D. R. Nelson NELSON,Data obtained: D. R., 2004, 2002 168902: biotite syenogranite, Hospital Rocks;Data released: Geochronology 2004 dataset 58; in Compilation of geochronology data, June 2006 update: Western Australia Geological Survey.
Data obtained: 22/09/2002; Data released: 06/12/2004
1 D. R. Nelson, 2004
a) b) 3.2 4.1 2.1
3.1 X
20 µm 50 µm
c) d) 10.2
10.1
20 µm 8.1 50 µm
e) f) 11.1 13.3 13.2
11.2
20 µm 100 µm 13.1
DN349 30.04.04
Figure 1. Cathodoluminescence images of representative zircons from sample 168902: biotite syenogranite, Hospital Rocks
2 Geochronology data obtained in 2002
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 1. Ion microprobe analytical results for sample 168902: biotite syenogranite, Hospital Rocks 1.12.13.1 2594.1 2405.1 2616.1 322 3837.1 362 6568.1 582 176 759.1 968 181 192 391 163 207 1.984 318 106 91 7.143 316 422 0.048 120 0.17588 1.074 48 130 0.17199 208 1.710 0.17614 66 0.00118 0.17613 1.364 0.1417.2 0.00211 74 0.175533.2 0.00065 3.0176.2 0.36845 0.00083 0.17821 213 0.17640 0.258 0.51144 0.00084 411 0.17443 0.61262 0.00261 69 0.00235 0.70736 0.00082 389 0.00486 0.17671 0.17998 320 0.00184 0.00262 0.4830 0.00224 158 107 0.4543 0.46973 0.00129 0.48306 0.00169 243 0.5151 0.41448 0.4977 0.0135 0.102 0.00551 50 0.00200 0.5209 0.0127 0.63453 1.444 0.0144 11.713 0.00592 0.4390 0.17720 0.5003 0.0139 10.774 0.352 0.00361 0.17597 0.0145 12.508 0.2724 0.346 12.086 0.00078 0.17423 0.5197 0.0124 0.344 0.0140 12.606 0.00085 0.358 0.0076 10.787 0.349 12.169 0.00155 0.50002 97 0.0146 0.363 0.19929 94 6.552 102 0.00193 0.352 12.663 0.352 100 0.42399 0.00172 104 0.219 0.5121 0.00354 2 614 0.380 0.4757 100 89 2 577 2 617 0.5210 0.0143 2 617 60 11 103 0.0133 2 611 20 12.513 6 0.0147 11.541 2 619 2 636 8 8 12.515 0.361 2 601 0.333 2 622 22 8 0.384 101 25 12 96 104 2 627 2 615 2 599 7 8 15 13.3 168 403 139 0.137 0.17691 0.00090 0.66118 0.00262 0.5207 0.0146 12.702 0.369 103 2 624 8 10.111.112.1 14313.1 30813.2 45511.2 128 11010.2 216 165 264 76 92 212 162 151 426 256 209 0.968 82 0.641 160 139 0.550 158 0.17627 0.189 105 0.17621 0.091 0.17676 0.273 0.00127 0.17847 0.472 0.00086 0.17740 0.17732 0.00069 0.25552 0.17926 0.00117 0.25244 0.00090 0.00074 0.05266 0.00257 0.00105 0.00167 0.53646 0.70770 0.00108 0.22488 0.5072 0.3915 0.00295 0.27591 0.00272 0.5214 0.00132 0.5045 0.0142 0.00205 0.5156 0.0109 0.4921 0.0145 12.326 0.5086 0.0142 9.511 0.0144 12.708 0.0137 0.367 12.414 0.0142 12.611 0.275 12.031 0.364 12.572 0.368 101 0.367 0.345 81 103 0.368 100 102 98 2 618 100 2 618 2 623 2 639 12 2 629 8 2 628 6 2 646 11 8 7 10 Grain.spot U (ppm) (ppm) Th (ppm) Pb ƒ206%
3 D. R. Nelson, 2004 28.01.04 168902 2700 2600 20 analyses of 13 zircons 2621 ± 5 Ma (95% confidence) 2500 Pb/ U 207 235 2400 2300 8 9 11 12 14 2200 71013 Figure 2. Concordia plot for sample 168902: biotite syenogranite, Hospital Rocks 2100
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0.60 U Pb/ 0.50 0.45 0.25 0.55 0.40 0.35 0.30 0 238 206 DN318
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