185486: porphyritic microgranite, mount clarke · figure 2. u–pb analytical data, not corrected...
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185486.1.pdf Geochronology Record 1401
185486: porphyritic microgranite, Mount Clarke (Warakurna Supersuite, Musgrave Province)
Location and sampling
Talbot (SG 52-9), Warburton Range (4245)MGA Zone 52, 265657E 7118740N
Sampled on 2 July 2012
This sample was collected from an outcrop about 4.8 km northwest of Pussy Cat Hill, 3.2 km east-northeast of Wururu Rockhole, and 0.9 km southeast of Mount Clarke.
Tectonic unit/relations
The unit sampled is a microgranite assigned to the Warakurna Supersuite of the Musgrave Province (Evins et al., 2010). The Warakurna Supersuite groups all igneous intrusive and extrusive rocks related to the c. 1085 to 1040 Ma Giles Event in the Musgrave Province (Howard et al., 2015; Smithies et al., 2015), and includes the c. 1078 to 1070 Ma mafic pulse of the Warakurna large igneous province, which extended over at least 2 × 106 km2 of western and central Australia (Wingate et al., 2004; Wingate, 2017). At this locality, the microgranite intrudes, at a low angle, a sequence of basaltic and siliciclastic rocks of the Glyde Formation of the Pussy Cat Group (Daniels, 1974). A syenogranite of the Warakurna Supersuite, collected about 13.7 km to the northeast, yielded an igneous crystallization age of 1063 ± 8 Ma (GSWA 208453, Kirkland et al., 2014).
Petrographic description
The sample is a fine-grained, porphyritic microgranite, consisting of about 65% quartz–feldspar groundmass, 25% plagioclase and K-feldspar phenocrysts, and 7% iron–titanium oxide minerals, epidote, and chlorite. The groundmass is cryptocrystalline to microcrystalline, and contains phenocrysts of plagioclase (oligoclase, An
23)
and K-feldspar (microcline and perthite), which are up to 1.2 mm in size, subhedral, corroded, and clouded, with relatively clear rims. Clots of iron–titanium oxide minerals, epidote, and chlorite form pseudomorphs after an unknown mineral.
Zircon morphologyZircons isolated from this sample are mainly colourless, and subhedral to euhedral. The crystals are up to 300 µm long, and equant to elongate, with aspect ratios up to 7:1.
In cathodoluminescence (CL) images, concentric zoning is ubiquitous. Many crystals contain high-uranium cores that are variably metamict. A CL image of representative zircons is shown in Figure 1.
Analytical detailsThis sample was analysed on 15–16 August 2013, using SHRIMP-B. Twelve analyses of the BR266 standard were obtained during the session, of which 11 analyses indicated an external spot-to-spot (reproducibility) uncertainty of 0.80% (1σ), and a 238U/206Pb* calibration uncertainty of 0.27% (1σ). Calibration uncertainties are included in the errors of 238U/206Pb* ratios and dates listed in Table 1. Common-Pb corrections were applied to all analyses using contemporaneous isotopic compositions determined according to the model of Stacey and Kramers (1975).
ResultsTwenty-nine analyses were obtained from 29 zircons. Results are listed in Table 1, and shown in a concordia diagram (Fig. 2).
InterpretationThe analyses are concordant to strongly discordant (Fig. 2). The analyses yield 207Pb*/206Pb* dates that correlate with their common-Pb corrections (ƒ204, Table 1), indicating that corrections using 204Pb are inaccurate for some or all of these analyses. The date for this sample is therefore determined from the intersection with the concordia curve of a regression through uncorrected data (Fig. 2), anchored at contemporaneous initial Pb (207Pb/206Pb = 0.9146 at c. 1071 Ma; Stacey and Kramers, 1975). One analysis indicates very high uranium and thorium content and is interpreted to be affected by different sputtering characteristics to the lower-concentration standard (i.e. matrix effects). The date obtained from this analysis (Group D; Table 1) is imprecise or unreliable, and considered not to be geologically significant. The remaining 28 analyses can be divided into two groups, based on their 207Pb/206Pb and 238U/206Pb ratios.
Group I comprises 26 analyses (Table 1), which yield a regression that intersects the concordia curve at 1072 ± 5 Ma (MSWD = 0.98).
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185486.1.pdf Geochronology Record 1401
Figure 1. Cathodoluminescence image of representative zircons from sample 185486: porphyritic microgranite, Mount Clarke. Numbered circles indicate the approximate locations of analysis sites
Figure 2. U–Pb analytical data, not corrected for common Pb, for zircons from sample 185486: porphyritic microgranite, Mount Clarke. Yellow squares indicate Group I (magmatic zircons); black squares indicate Group P (radiogenic Pb-loss); crossed square indicates Group D (matrix effects). The dashed line indicates a regression from initial Pb through data in Group I
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185486.1.pdf Geochronology Record 1401
Gro
up ID
Sp
ot
no.
Gra
in.
spot
238 U
(pp
m)
232 T
h(p
pm
)
232 T
h23
8 Uƒ2
04(%
)
238 U
/206 P
b±
1σ
207 P
b/20
6 Pb
± 1σ
238 U
/206 P
b*
± 1σ
207 P
b*/
206 P
b*
± 1σ
238 U
/206 P
b*
dat
e (M
a) ±
1σ20
7 Pb
*/20
6 Pb
*d
ate
(Ma)
±1σ
Dis
c.(%
)
I8
8.1
5562
1.15
0.70
65.611
0.39
70.07
468
0.00
147
5.65
10.40
00.06
881
0.00
283
1050
7389
385
-17.6
I13
13.1
7661
0.82
0.31
65.41
50.08
50.07
394
0.00
119
5.43
20.08
60.07
131
0.00
178
1089
1696
651
-12.7
I19
19.1
107
104
1.00
0.27
15.56
00.07
60.07
459
0.00
100
5.57
50.07
70.07
232
0.00
143
1063
1499
540
-6.9
I15
15.1
4240
1.00
0.71
05.43
40.10
50.07
891
0.00
168
5.47
30.10
70.07
297
0.00
317
1082
201013
88-6.8
I5
5.1
7377
1.09
0.38
25.63
10.08
80.07
653
0.00
121
5.65
30.08
90.07
333
0.00
188
1050
1510
2352
-2.6
I1
1.1
159
131
0.85
0.27
25.65
10.06
90.07
572
0.00
080
5.66
70.07
00.07
344
0.00
114
1048
1210
2631
-2.1
I14
14.1
6973
1.09
0.22
45.59
80.08
60.07
542
0.00
119
5.611
0.08
70.07
354
0.00
161
1057
1510
2944
-2.8
I17
17.1
9395
1.06
0.37
55.48
00.07
90.07
702
0.00
110
5.50
10.08
00.07
388
0.00
170
1077
1510
3846
-3.7
I11
11.1
160
168
1.08
0.27
75.62
00.06
80.07
643
0.00
081
5.63
50.06
90.07
411
0.00
116
1053
1210
4431
-0.8
I4
4.1
142
198
1.44
0.23
45.53
90.07
00.07
611
0.00
086
5.55
20.07
10.07
415
0.00
118
1068
1310
4632
-2.1
I27
27.1
828
624
0.78
0.03
05.62
40.05
30.07
458
0.00
037
5.62
50.05
30.07
433
0.00
039
1055
910
5011
-0.4
I16
16.1
7368
0.96
0.22
55.52
40.08
50.07
624
0.00
119
5.53
60.08
50.07
436
0.00
162
1070
1510
5144
-1.8
I28
28.1
9789
0.95
-0.055
5.51
30.07
70.07
391
0.00
100
5.51
00.07
70.07
437
0.00
110
1075
1410
5130
-2.3
I26
26.1
1084
1262
1.20
0.06
15.49
50.05
10.07
498
0.00
030
5.49
80.05
10.07
447
0.00
033
1077
910
549
-2.2
I29
29.1
541
446
0.85
0.04
95.53
40.05
70.07
514
0.00
055
5.53
70.05
70.07
473
0.00
060
1070
1010
6116
-0.8
I23
23.1
1226
1302
1.10
0.017
5.52
30.05
10.07
490
0.00
029
5.52
40.05
10.07
476
0.00
030
1073
910
628
-1.0
I20
20.1
511
501
1.01
0.07
35.52
40.05
50.07
564
0.00
044
5.52
80.05
50.07
503
0.00
050
1072
1010
6913
-0.2
I12
12.1
8270
0.88
0.00
05.49
60.08
10.07
506
0.00
111
5.49
60.08
10.07
506
0.00
111
1078
1510
7030
-0.7
I22
22.1
1412
2044
1.50
0.012
5.41
50.04
90.07
519
0.00
027
5.41
50.04
90.07
509
0.00
028
1092
910
717
-2.0
I7
7.1
6567
1.07
-0.089
5.60
00.09
00.07
528
0.00
130
5.59
50.09
00.07
603
0.00
150
1060
1610
9639
3.3
I3
3.1
7076
1.13
0.08
25.61
60.09
10.07
750
0.00
128
5.62
10.09
10.07
681
0.00
146
1056
161116
385.4
I6
6.1
9010
81.24
0.17
85.44
20.07
90.07
834
0.00
108
5.45
20.07
90.07
684
0.00
139
1086
151117
362.8
I24
24.1
6272
1.19
-0.100
5.46
20.09
10.07
618
0.00
138
5.45
60.09
10.07
702
0.00
161
1085
171122
423.3
I9
9.1
8164
0.82
-0.198
5.51
80.08
20.07
631
0.00
113
5.50
80.08
20.07
797
0.00
148
1076
151146
386.2
I10
10.1
7072
1.06
-0.293
5.51
00.30
90.07
587
0.00
119
5.49
40.30
80.07
834
0.00
171
1078
591156
436.7
I21
21.1
6770
1.07
2.58
15.24
20.08
70.10
028
0.00
164
5.38
10.09
40.07
851
0.00
472
1099
181160
119
5.3
P18
18.1
4033
0.84
0.56
55.87
70.10
60.07
799
0.00
165
5.911
0.10
80.07
326
0.00
290
1008
1710
2180
1.3
P2
2.1
169
198
1.21
-0.237
6.26
00.101
0.07
460
0.00
150
6.24
50.101
0.07
659
0.00
206
957
151111
5413
.8
D25
25.1
2311
2452
1.10
0.00
05.28
80.04
70.07
494
0.00
020
5.28
80.04
70.07
494
0.00
020
1117
910
675
-4.6
Tabl
e 1.
Io
n m
icro
prob
e an
alyt
ical
res
ults
for
zirc
ons
from
sam
ple
1854
86:
porp
hyri
tic
mic
rogr
anit
e, M
ount
Cla
rke
4
185486.1.pdf Geochronology Record 1401
Group P comprises two analyses (Table 1), which yield 207-corrected 238U/206Pb* dates of 1007 and 951 Ma.
The date of 1072 ± 5 Ma for the 26 analyses in Group I is interpreted as the magmatic crystallization age of the microgranite. This date also serves as a minimum depositional age for the Glyde Formation of the Pussy Cat Group, which is intruded by the microgranite. The dates of 1007 and 951 Ma for the two analyses in Group P are interpreted to reflect minor loss of radiogenic Pb.
ReferencesDaniels, JL 1974, The geology of the Blackstone region,
Western Australia: Geological Survey of Western Australia, Bulletin 123, 257p.
Evins, PM, Smithies, RH, Howard, HM, Kirkland, CL, Wingate, MTD and Bodorkos, S 2010, Devil in the detail; the 1150–1000 Ma magmatic and structural evolution of the Ngaanyatjarra Rift, west Musgrave Province, Central Australia: Precambrian Research, v. 183, p. 572–588.
Howard, HM, Smithies, RH, Kirkland, CL, Kelsey, DE, Aitken, A, Wingate, MTD, Quentin De Gromard, R, Spaggiari, CV and Maier, WD 2015, The burning heart — the Proterozoic geology and geological evolution of the west Musgrave Region, central Australia: Gondwana Research, v. 27, p. 64–94.
Kirkland, CL, Wingate, MTD, Quentin De Gromard, R, Howard, HM and Smithies, RH 2014, 208453: syenogranite, Windarra Waterhole; Geochronology Record 1225: Geological Survey of Western Australia, 4p.
Smithies, RH, Kirkland, CL, Korhonen, FJ, Aitken, ARA, Howard, HM, Maier, WD, Wingate, MTD, Quentin de Gromard, R and Gessner, K 2015, The Mesoproterozoic thermal evolution of the Musgrave Province in central Australia — plume vs. the geological record: Gondwana Research, v. 27, p. 1419–1429.
Stacey, JS and Kramers, JD 1975, Approximation of terrestrial lead isotope evolution by a two-stage model: Earth and Planetary Science Letters, v. 26, p. 207–221.
Wingate, MTD 2017, Mafic dyke swarms and large igneous provinces in Western Australia get a digital makeover: Geological Survey of Western Australia, Record 2017/2, p. 4–8.
Wingate, MTD, Pirajno, F and Morris, PA 2004, The Warakurna large igneous province: a new Mesoproterozoic large igneous province in west-central Australia: Geology, v. 32, p. 105–108.
Recommended reference for this publicationLu, Y, Wingate, MTD, Kirkland, CL, Howard, HM, and Quentin
de Gromard, R 2017, 185486: porphyritic microgranite, Mount Clarke; Geochronology Record 1401: Geological Survey of Western Australia, 4p.
Data obtained: 16 August 2013
Data released: 12 May 2017
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