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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