Beneath the Perth BasinNew U–Pb SHRIMP zircon ages from the Pinjarra Orogen, Western Australia, 2016

GEOSCIENCE AUSTRALIARECORD 2016/31

S. Bodorkos1, I. C. W. Fitzsimons2, L. S. Hall1, K. N. Sircombe1, and C. J. Lewis1

1. Geoscience Australia, GPO Box 378, Canberra ACT 2601.2. Department of Applied Geology, Western Australian School of Mines, Curtin University, GPO Box U1987, Perth WA 6845.

Department of Industry, Innovation and ScienceMinister for Resources and Northern Australia: Senator the Hon. Matthew CanavanAssistant Minister for Industry, Innovation and Science: The Hon. Craig Laundy MPSecretary: Ms Glenys Beauchamp PSM

Geoscience AustraliaChief Executive Officer: Dr Chris PigramThis paper is published with the permission of the CEO, Geoscience Australia

© Commonwealth of Australia (Geoscience Australia) 2016

With the exception of the Commonwealth Coat of Arms and where otherwise noted, this product is provided under a Creative Commons Attribution 4.0 International Licence. (http://creativecommons.org/licenses/by/4.0/legalcode)

Geoscience Australia has tried to make the information in this product as accurate as possible. However, it does not guarantee that the information is totally accurate or complete. Therefore, you should not solely rely on this information when making a commercial decision.

Geoscience Australia is committed to providing web accessible content wherever possible. If you are having difficulties with accessing this document please email clientservices@ga.gov.au.

ISSN 2201-702X (PDF)ISBN 978-1-925297-32-4 (PDF)eCat 102042

Bibliographic reference: Bodorkos, S., Fitzsimons, I. C. W., Hall, L. S., Sircombe, K. N. and Lewis, C. J., 2016. Beneath the Perth Basin: New U–Pb SHRIMP zircon ages from the Pinjarra Orogen, Western Australia, 2016. Record 2016/31. Geoscience Australia, Canberra. http://dx.doi.org/10.11636/Record.2016.031

Contents

Executive Summary................................................................................................................................ 6

1 Introduction.......................................................................................................................................... 7

2 Samples Analysed............................................................................................................................... 92.1 Unnamed felsic orthogneiss, WAPET Sue 1..................................................................................9

2.1.1 Sampling details....................................................................................................................... 92.1.2 Petrography.............................................................................................................................. 92.1.3 Zircon description................................................................................................................... 102.1.4 U–Pb isotopic results..............................................................................................................102.1.5 Geochronological interpretation..............................................................................................14

2.2 Unnamed leucocratic orthogneiss, BMR Beagle Ridge 10A........................................................172.2.1 Sampling details.....................................................................................................................172.2.2 Petrography............................................................................................................................ 172.2.3 Zircon description................................................................................................................... 182.2.4 U–Pb isotopic results..............................................................................................................182.2.5 Geochronological interpretation..............................................................................................19

3 Discussion......................................................................................................................................... 213.1 Significance of the new U–Pb zircon dates..................................................................................213.2 Implications for crustal evolution of the Pinjarra Orogen..............................................................22

Acknowledgements............................................................................................................................... 24

References........................................................................................................................................... 25

Appendix A SHRIMP Data Acquisition, Reduction and Presentation....................................................28A.1 Analytical procedures...................................................................................................................28

A.1.1 Sample acquisition and crushing............................................................................................28A.1.2 Mineral separation..................................................................................................................29A.1.3 Mount preparation..................................................................................................................29A.1.4 Instrument setup and data acquisition....................................................................................30

A.2 Data reduction and presentation..................................................................................................31A.2.1 Calibration procedures...........................................................................................................31A.2.2 Propagation of uncertainties...................................................................................................32A.2.3 Discordance........................................................................................................................... 32

Beneath the Perth Basin: New U–Pb SHRIMP zircon ages from the Pinjarra Orogen, Western Australia, 2016 iii

List of Figures

Figure 1.1 Interpreted bedrock geology of southwestern Western Australia, showing the locations of the two samples analysed and documented in this Record (red circles, labelled with GA SampleNo), and the locations of isotopic age determinations by the Geological Survey of Western Australia (yellow squares, predominantly U–Pb SHRIMP zircon analyses, labelled with interpreted igneous crystallisation ages, in Ma). Map data: 1:500 000 State interpreted bedrock geology of Western Australia, 2016 (Geological Survey of Western Australia, 2016a) and compilation of geochronology information, 2016 update (Geological Survey of Western Australia, 2016b)..................................................................................................................................... 8

Figure 2.1 Representative zircons from the unnamed felsic orthogneiss, WAPET Sue 1 (3073.2–3073.7 m, GA 2124222). Transmitted-light image is shown in the upper half; cathodoluminescence image in the lower half. SHRIMP analysis sites are indicated, and labelled ‘grain.area’............................................................................................................................... 10

Figure 2.2 SHRIMP U-Pb data for zircons from the unnamed felsic orthogneiss, WAPET Sue 1 (3073.2–3073.7 m; GA 2124222). (a) All data (n = 64); (b) ‘accepted’ analyses (n = 44). In (a), red ellipses denote analyses with high 206Pbc (>1%); black ellipses denote analyses with very high U (2849–5947 ppm) and UO/U values significantly higher than those measured in the reference zircons, interpreted to reflect loss of crystallinity in the analysed domains; bright yellow ellipses denote analyses with very large uncertainties in 207Pb/206Pb (8–18% [1σ], despite having U > 150 ppm and Mesoproterozoic apparent ages), interpreted to reflect ‘unsupported’ or redistributed radiogenic Pb; white ellipses denote analyses that are grossly discordant (>30%); grey ellipses denote analyses ‘accepted’ for further isotopic interpretation (see (b) and Figure 2.4). In (b), pale yellow ellipses denote magmatic crystallisation affected by Neoproterozoic isotopic resetting; purple ellipses denote Paleoproterozoic to Mesoproterozoic inheritance; green ellipses denote high-Th/U outliers to the Mesoproterozoic magmatic population; blue ellipse denotes an analysis interpreted as affected by recent loss of radiogenic Pb.............................12

Figure 2.3 Th/U versus 207Pb/206Pb date for zircons from the unnamed felsic orthogneiss, WAPET Sue 1 (3073.2–3073.7 m; GA 2124222). Pale yellow squares denote Mesoproterozoic magmatic crystallisation affected by Neoproterozoic isotopic resetting; purple squares denote Paleoproterozoic to Mesoproterozoic inheritance; green squares denote high-Th/U outliers to the Mesoproterozoic magmatic population; blue square denotes an analysis interpreted as affected by recent loss of radiogenic Pb...............................................................................................13

Figure 2.4 SHRIMP U-Pb data for the unnamed felsic orthogneiss, WAPET Sue 1 (3073.2–3073.7 m; GA 2124222), with ellipses coloured by calculated Th/U. There is a weak correlation between Th/U and 207Pb/206Pb. Heavy black dashed line represents the Model 1 Discordia regression, with its 95% confidence envelope......................................................................................14

Figure 2.5 Zircons from the unnamed leucocratic orthogneiss, BMR Beagle Ridge 10A (1464.0–1467.0 m, GA 2124223). Transmitted-light image is shown in the upper half; cathodoluminescence image in the lower half. SHRIMP analysis sites are indicated, and labelled ‘grain.area’. Most of the mounted crystals are monazite.........................................................18

Figure 2.6 SHRIMP U-Pb data for zircons from the unnamed leucocratic orthogneiss, BMR Beagle Ridge 10A (1464.0–1467.0 m, GA 2124223). Pale yellow ellipses denote magmatic crystallisation affected by recent loss of radiogenic Pb; purple ellipses denote Mesoproterozoic inheritance; green ellipse denotes an analysis of a Paleozoic zircon likely to represent a contaminant.......................................................................................................................................... 19

iv Beneath the Perth Basin: New U–Pb SHRIMP zircon ages from the Pinjarra Orogen, Western Australia, 2016

List of Tables

Table 1.1 Summary of results: New U–Pb SHRIMP zircon ages from crystalline basement to the Perth Basin............................................................................................................................................. 7

Table 2.1 Summary of results: unnamed felsic orthogneiss, WAPET Sue 1 (3073.2–3073.7 m; GA 2124222).......................................................................................................................................... 9

Table 2.2 SHRIMP U-Pb zircon data from the unnamed felsic orthogneiss, WAPET Sue 1 (3073.2–3073.7 m; GA 2124222). All dates are 207Pb/206Pb..................................................................15

Table 2.3 Summary of results: unnamed leucocratic orthogneiss, BMR Beagle Ridge 10A (1464.0–1467.0 m; GA 2124223).........................................................................................................17

Table 2.4 SHRIMP U-Pb zircon data from the unnamed leucocratic orthogneiss, BMR Beagle Ridge 10A (1464.0–1467.0 m, GA 2124223). All dates are 207Pb/206Pb unless otherwise indicated in the group heading..............................................................................................................20

Table 3.1 Summary of published whole-rock Sm–Nd data from the Pinjarra Orogen (Fletcher et al., 1985; McCulloch, 1987; Black et al., 1992; Fletcher and Libby, 1993), normalised and recalculated by Fitzsimons (2003)........................................................................................................23

Beneath the Perth Basin: New U–Pb SHRIMP zircon ages from the Pinjarra Orogen, Western Australia, 2016 v

vi Beneath the Perth Basin: New U–Pb SHRIMP zircon ages from the Pinjarra Orogen, Western Australia, 2016

Executive Summary

This Record contains new zircon U-Pb geochronological data, obtained via Sensitive High-Resolution Ion Micro Probe (SHRIMP), from two samples of metamorphosed felsic igneous rocks from the Proterozoic Pinjarra Orogen (Western Australia), intersected in diamond drillcore at the base of deep petroleum exploration wells penetrating the Paleozoic sedimentary successions of the Perth Basin.

In the southern Perth Basin, petroleum exploration well Sue 1 was terminated at depth 3074.2 m, in crystalline basement rocks of the southern Pinjarra Orogen. Abundant zircon from a biotite-bearing felsic orthogneiss at depth 3073.2–3073.7 m yielded a complex array of U–Pb isotopic data, indicative of significant post-crystallisation disturbance of the isotopic system. A Discordia regression fitted to the array yielded an upper intercept date of 1076 ± 35 Ma (all quoted uncertainties are 95% confidence intervals unless specified otherwise) interpreted to represent magmatic crystallisation of the igneous precursor to the orthogneiss, and a lower intercept date of 680 ± 110 Ma which is our best estimate of the age of the tectonothermal event responsible for post-crystallisation disturbance of the U–Pb system. Crust of known Mesoproterozoic age is rare in the southern Pinjarra Orogen: pre-1000 Ma igneous crystallisation ages in the Leeuwin Complex were previously known only from two c. 1090 Ma garnet-bearing orthogneisses at Redgate Beach (Nelson, 1999), 30 km west of Sue 1. All other dated outcrops have revealed Neoproterozoic (780–680 Ma) granitic protoliths reworked by Early Cambrian (540–520 Ma) magmatism, deformation and metamorphism (Nelson, 1996, 2002; Collins, 2003).

In the northern Perth Basin, petroleum exploration well Beagle Ridge 10A was terminated at depth 1482 m, in crystalline rocks of the northern Pinjarra Orogen. A leucocratic orthogneiss sampled within the interval 1464.0–1467.0 m yielded only sparse zircon, but four of the seven grains analysed yielded a weighted mean 207Pb/206Pb date of 1092 ± 27 Ma, interpreted to represent magmatic crystallisation of the precursor to the orthogneiss. Our data show no evidence for Neoproterozoic U–Pb resetting of the c. 1090 Ma zircons: where present, isotopic disturbance is predominantly geologically recent.

The two newly dated samples are located at opposite ends of the Perth Basin (about 470 km apart), and although the two magmatic crystallisation ages are imprecise, the date of 1092 ± 27 Ma from the Beagle Ridge 10A leucocratic orthogneiss is indistinguishable from the date of 1076 ± 35 Ma from the Sue 1 felsic orthogneiss. Furthermore, both rocks contain inherited zircon (1620–1180 Ma in Sue 1; 1290–1210 Ma in Beagle Ridge 10A), indicating the presence of pre-1100 Ma crustal components in their parent magmas. The possibility of an extended crustal prehistory is supported by Sm–Nd analyses of buried Pinjarra Orogen orthogneisses (Fletcher et al., 1985; Fletcher and Libby, 1993), which yielded depleted-mantle model ages spanning 2234–2078 Ma in the north (near Beagle Ridge 10A) and 2040–1996 Ma in the south (including one analysis of granitic gneiss obtained from Sue 1).

Our new U–Pb zircon data expand the known extent of 1100–1050 Ma felsic magmatism, and in Sue 1, we have established the first direct link between 1100–1050 Ma protolith ages and Sm–Nd depleted-mantle model ages of 2200–2000 Ma in the southern Pinjarra Orogen. This Sm–Nd signature appears to be characteristic of (1) granitic basement beneath the Perth Basin, and (2) granitic and metasedimentary outcrop in the Northampton and Mullingarra complexes of the northern Pinjarra Orogen, where 1100–1050 Ma protoliths are widespread. It is therefore possible that a belt of isotopically coherent Paleoproterozoic crust spans the full strike length of the orogen (Fletcher and Libby, 1993). This belt is distinct from more juvenile components of the Leeuwin Complex to the west, where 780–680 Ma protoliths are paired with Sm–Nd depleted-mantle model ages of 1600–1100 Ma.

Beneath the Perth Basin: New U–Pb SHRIMP zircon ages from the Pinjarra Orogen, Western Australia, 2016 1

1 Introduction

This Record contains new zircon U-Pb geochronological data, obtained via Sensitive High-Resolution Ion Micro Probe (SHRIMP), from two samples of metamorphosed felsic igneous rocks of the Proterozoic to Early Cambrian Pinjarra Orogen (Western Australia), intersected in diamond drillcore at the base of deep petroleum exploration wells penetrating the Paleozoic sedimentary successions of the Perth Basin.

The two samples were obtained from discrete geographic regions (Figure 1.1):

In the southern Perth Basin (Augusta 1:250 000 sheet), Sue 1 was drilled by West Australian Petroleum Pty Ltd (WAPET) in 1966, and intersected crystalline basement of the southern Pinjarra Orogen from 3054.1 m to the total depth of the hole at 3074.2 m (Williams and Nicholls, 1966). Felsic orthogneiss from the interval 3073.2–3073.7 m was selected for zircon separation and U–Pb SHRIMP analysis.

In the northern Perth Basin (Dongara 1:250 000 sheet), Beagle Ridge 10A was drilled by the Australian Bureau of Mineral Resources (BMR) in 1960, and intersected crystalline basement of the northern Pinjarra Orogen from 1461.2 m to the total depth of the hole at 1482 m (MacTavish, 1965). Leucocratic orthogneiss from the interval 1464.0–1467.0 m was selected for zircon separation and U–Pb SHRIMP analysis.

This Record documents detailed results for each sample individually, encompassing sample location, geological context, petrography, zircon descriptions, an evaluation of the relevant analytical data, and a brief geochronological interpretation. The results are summarised in Table 1.1, and are also available via Geoscience Australia’s Geochron Delivery system (http://www.ga.gov.au/geochron-sapub-web/). A comprehensive description of sample acquisition and processing procedures, preparation and analysis of SHRIMP mounts, and data reduction and presentation methods are included in Appendix A, along with analytical session-specific details of the calibration data collected on the reference 238U/206Pb and 207Pb/206Pb materials.

Table 1.1 Summary of results: New U–Pb SHRIMP zircon ages from crystalline basement to the Perth Basin.

Drillhole and Depth Interval

SampleNo1

GDA94Latitude

GDA94Longitude

GeologicalProvince

RockType2

Interpreted Dates(±95% confidence)

WAPET Sue 1, 3073.2–3073.7 m

2124222 -34.06464 115.31930 Pinjarra Orogen(southern)

Felsic orthogneiss [n = 34/64]

1076 ± 35 Ma(igneous crystallisation)

680 ± 110 Ma(isotopic resetting)

BMR Beagle Ridge 10A, 1464.0–1467.0 m

2124223 -29.82667 114.97500 Pinjarra Orogen(northern)

Leucocratic orthogneiss [n = 4/7]

1092 ± 27 Ma(igneous crystallisation)

1 SampleNo is the sample number from Geoscience Australia’s FIELDSITES database.2 n is the number of zircon analyses included in the calculated date(s), as a fraction of the total number of analyses collected.

2 Beneath the Perth Basin: New U–Pb SHRIMP zircon ages from the Pinjarra Orogen, Western Australia, 2016

Figure 1.1 Interpreted bedrock geology of southwestern Western Australia, showing the locations of the two samples analysed and documented in this Record (red circles, labelled with GA SampleNo), and the locations of isotopic age determinations by the Geological Survey of Western Australia (yellow squares, predominantly U–Pb SHRIMP zircon analyses, labelled with interpreted igneous crystallisation ages, in Ma). Map data: 1:500 000 State interpreted bedrock geology of Western Australia, 2016 (Geological Survey of Western Australia, 2016a) and compilation of geochronology information, 2016 update (Geological Survey of Western Australia, 2016b).

Beneath the Perth Basin: New U–Pb SHRIMP zircon ages from the Pinjarra Orogen, Western Australia, 2016 3

2 Samples Analysed

2.1 Unnamed felsic orthogneiss, WAPET Sue 1

Table 2.2 Summary of results: unnamed felsic orthogneiss, WAPET Sue 1 (3073.2–3073.7 m; GA 2124222).

GA SampleNo 2124222

Drillhole and Depth Interval WAPET Sue 1 (3073.2–3073.7 m)

Lithology Felsic orthogneiss

Stratigraphic Unit —

Parent Unit —

Informal Identifier Unnamed felsic orthogneiss, WAPET Sue 1

Province Pinjarra Orogen

1:250 000 Sheet Augusta (SI50-09)

1:100 000 Sheet Leeuwin (1929)

Location (GDA94) 34.06464°S, 115.31930°E

Analytical Session(s) 110130 (see Appendix Table A.1 for parameters derived from measurements of 238U/206Pb and 207Pb/206Pb reference zircons)

Interpreted Age(s) Magmatic crystallisation: 1076 ± 35 Ma (upper intercept)Isotopic resetting: 680 ± 110 Ma (lower intercept)(both 95% confidence; 34 analyses of 34 zircons)

Calculation Method Upper and lower Concordia intercepts of Model 1 Discordia regression

Isotopic Ratio(s) Used 238U/206Pb and 207Pb/206Pb (204Pb-corrected)

2.1.1 Sampling details

Sue 1 is a petroleum exploration well, drilled in the southern Perth Basin by West Australian Petroleum Pty Ltd (WAPET) in 1966 (Williams and Nicholls, 1966). Crystalline basement rocks of the southern Pinjarra Orogen (comprising garnet-bearing granulite) were first intersected at depth 3051.4 m, before biotite-bearing granitic gneiss was encountered between depth 3064.2 m and the total depth of the hole at 3074.2 m. A segment of drillcore (3073.2–3073.7 m), described as ‘gneissic, medium- to coarse-grained biotite granite’ (J.E. Glover in Williams and Nicholls, 1966; see also Peers and Trendall, 1968), was sampled for zircon separation and U–Pb SHRIMP analysis.

2.1.2 Petrography

This rock is a felsic orthogneiss comprising 50–55% K-feldspar (predominantly microcline, and showing local perthitic or myrmekitic texture), 20% quartz, 10–15% plagioclase (partly sericitised and/or saussuritised following hydrothermal alteration), 5–8% biotite (partially altered to chlorite), and minor muscovite. Accessory minerals include apatite, and euhedral to rounded zircon. Some prismatic zircons feature pyramidal terminations with well-developed facets, consistent with a magmatic origin.

4 Beneath the Perth Basin: New U–Pb SHRIMP zircon ages from the Pinjarra Orogen, Western Australia, 2016

Aligned micas define a moderate to strong foliation, and this rock probably represents a deformed granite. The high feldspar content and presence of two micas probably indicates a peraluminous composition, possibly reflecting partial melting of a sedimentary protolith.

2.1.3 Zircon description

Zircons separated from this rock are mostly between 70 µm and 100 µm in length, and range from elongate prismatic crystals with pyramidal terminations, to embayed, ovoid, and rounded grains (Figure 2.2). Many grains are cracked, and some host pale mineral inclusions. In transmitted light, the grains are predominantly colourless, although cracked grains feature occasional brown discolouration.

Cathodoluminescence (CL) images reveal prominent concentric zoning and long-axis parallel banding. In most grains, central domains with lower CL emission intensity appear to be semi-conformably overgrown by rims with brighter CL, although there are grains in which cores with very bright CL are disconformably overgrown by darker CL mantles and brighter CL rims. Some core-rim interfaces show complex embayments (Figure 2.2).

Figure 2.2 Representative zircons from the unnamed felsic orthogneiss, WAPET Sue 1 (3073.2–3073.7 m, GA 2124222). Transmitted-light image is shown in the upper half; cathodoluminescence image in the lower half. SHRIMP analysis sites are indicated, and labelled ‘grain.area’.

2.1.4 U–Pb isotopic results

Sixty-four analyses were collected from 54 zircons (Figure 2.3a, Table 2.3), with 10 zircons (5, 44–47, and 50–54) each analysed twice, primarily to investigate core-rim relationships. A total of 20 analyses are considered unreliable, and were excluded from further consideration on the following grounds:

Nine (red in Figure 2.2a) are characterised by elevated common Pb (206Pbc > 1%), indicating either leakage of radiogenic Pb, ingress of ambient Pb, or both.

Four (black in Figure 2.2a) are characterised by very high U contents (2849–5947 ppm), as well as relatively high UO/U values (6.25–7.13, relative to a total range of 5.68–5.95 for 66 measurements of the reference zircons TEMORA2 and OG1). This combination of parameters indicates probable

Beneath the Perth Basin: New U–Pb SHRIMP zircon ages from the Pinjarra Orogen, Western Australia, 2016 5

loss of crystallinity in the analysed domains as a consequence of radiation damage to the zircon lattice. Whilst such domains are capable of retaining syn-crystallisation 207Pb/206Pb values, their calibrated 238U/206Pb values are inevitably compromised by the enhanced sputtering of Pb relative to U from zones of amorphous material (e.g. White and Ireland, 2012), and their plotted positions on Concordia diagrams are unreliable.

Three (yellow in Figure 2.3a) are characterised by very large uncertainties in 207Pb/206Pb (8–18% [1σ]) despite U contents exceeding 150 ppm and Mesoproterozoic apparent ages. These large uncertainties reflect large variations in measured Pb-isotope abundances as each analysis progressed, despite the absence of concomitant variation in parent U and Th species. Such patterns have previously been interpreted to reflect ‘unsupported’ or redistributed radiogenic Pb (e.g. Williams et al., 1984; Kusiak et al., 2013), but Kusiak et al. (2015) demonstrated that in at least some zircons subjected to a complex geological history involving ultra-high temperature metamorphism, these isotopic patterns are attributable to the presence of nanospheres of native Pb, most commonly developed post-crystallisation within zircons that have experienced a polyphase high-grade metamorphic history.

Four (white in Figure 2.3a) are characterised by a large degree of discordance (>30%), indicating significant recent loss of radiogenic Pb from these domains.

The remaining 44 accepted analyses (grey on Figure 2.3a) are shown in more detail on Figure 2.3b and Figure 2.4. They are subdivided into two broad clusters:

Six analyses (purple in Figure 2.3b and Figure 2.4) of inherited cores, identified both texturally (as distinct, usually bright-CL domains) and chemically (Figure 2.4). Their U contents are varied (116–1470 ppm) but mostly low (median 191 ppm), and their Th/U is relatively high (0.66–1.43, median 1.00). Most of the analyses are near-concordant, and their 207Pb/206Pb dates are dispersed between c. 1620 Ma and c. 1180 Ma.

Thirty-eight analyses of (i) discrete crystals with broad banded CL zoning parallel to the long axes, and (ii) broad banded rims that have disconformably overgrown inherited cores in rounded grains. Their U contents are varied (359–2009 ppm) but mostly moderate (median 698 ppm), their Th/U is relatively low (0.02–0.48, median 0.08), and their 207Pb/206Pb dates are dispersed between c. 1160 Ma and c. 850 Ma. Within this cluster of 38 analyses, four potential outliers are discerned:

Three analyses (green in Figure 2.3b and Figure 2.4) with slightly elevated Th/U (0.28–0.48), which are excluded on the possibility that they represent pre-1170 Ma zircon components that have undergone substantial isotopic resetting during the event responsible for crystallisation of the other zircons with similar 207Pb/206Pb dates but lower Th/U (0.02–0.23) in this cluster.

One analysis (pale blue in Figure 2.3b and Figure 2.4) characterised by a slightly higher degree of normal discordance (12%) than the other analyses in this cluster, and which is interpreted to be affected by recent loss of radiogenic Pb.

The weighted mean 207Pb/206Pb date calculated from the remaining 34 analyses (pale yellow in Figure 2.3b and Figure 2.4) displays significant excess scatter, well beyond that attributable to analytical uncertainty (MSWD = 7.9), but younger 207Pb/206Pb dates are correlated with younger 238U/206Pb dates, which suggests that the cluster has been affected by post-crystallisation loss of radiogenic Pb and/or partial isotopic resetting during a non-recent event.

6 Beneath the Perth Basin: New U–Pb SHRIMP zircon ages from the Pinjarra Orogen, Western Australia, 2016

Figure 2.3 SHRIMP U-Pb data for zircons from the unnamed felsic orthogneiss, WAPET Sue 1 (3073.2–3073.7 m; GA 2124222). (a) All data (n = 64); (b) ‘accepted’ analyses (n = 44). In (a), red ellipses denote analyses with high 206Pbc (>1%); black ellipses denote analyses with very high U (2849–5947 ppm) and UO/U values significantly higher than those measured in the reference zircons, interpreted to reflect loss of crystallinity in the analysed domains; bright yellow ellipses denote analyses with very large uncertainties in 207Pb/206Pb (8–18% [1σ], despite having U > 150 ppm and Mesoproterozoic apparent ages), interpreted to reflect ‘unsupported’ or redistributed radiogenic Pb; white ellipses denote analyses that are grossly discordant (>30%); grey ellipses denote analyses ‘accepted’ for further isotopic interpretation (see (b) and Figure 2.4). In (b), pale yellow ellipses denote magmatic crystallisation affected by Neoproterozoic isotopic resetting; purple ellipses denote Paleoproterozoic to Mesoproterozoic inheritance; green ellipses denote high-Th/U outliers to the Mesoproterozoic magmatic population; blue ellipse denotes an analysis interpreted as affected by recent loss of radiogenic Pb.

Beneath the Perth Basin: New U–Pb SHRIMP zircon ages from the Pinjarra Orogen, Western Australia, 2016 7

Figure 2.4 Th/U versus 207Pb/206Pb date for zircons from the unnamed felsic orthogneiss, WAPET Sue 1 (3073.2–3073.7 m; GA 2124222). Pale yellow squares denote Mesoproterozoic magmatic crystallisation affected by Neoproterozoic isotopic resetting; purple squares denote Paleoproterozoic to Mesoproterozoic inheritance; green squares denote high-Th/U outliers to the Mesoproterozoic magmatic population; blue square denotes an analysis interpreted as affected by recent loss of radiogenic Pb.

Figure 2.5 shows a Model 1 Discordia regression fitted to these 34 analyses, which significantly reduces the degree of excess scatter (MSWD = 2.0, probability of fit = 0.001), and defines upper and lower Concordia intercepts of 1076 ± 35 Ma and 680 ± 110 Ma respectively (both 95% confidence, with decay-constant uncertainties neglected).

Within these 34 analyses (pale yellow squares in Figure 2.3), there is a tendency towards marginally higher calculated Th/U in marginally older crystals (pre-1120 Ma 207Pb/206Pb dates; Figure 2.4, Figure 2.5). However, this trend is confined to a narrow range of low Th/U values, and its geological significance is difficult to assess in the context of the analytical uncertainties. It is possible that the pre-1120 Ma ‘minority’ represents subtle inheritance within the population; alternatively, these analyses may reflect least-disturbed isotopic compositions within a population pervasively affected by Neoproterozoic overprinting. The available data do not permit one of these interpretations to be favoured over the other.

8 Beneath the Perth Basin: New U–Pb SHRIMP zircon ages from the Pinjarra Orogen, Western Australia, 2016

Figure 2.5 SHRIMP U-Pb data for the unnamed felsic orthogneiss, WAPET Sue 1 (3073.2–3073.7 m; GA 2124222), with ellipses coloured by calculated Th/U. There is a weak correlation between Th/U and 207Pb/206Pb. Heavy black dashed line represents the Model 1 Discordia regression, with its 95% confidence envelope.

2.1.5 Geochronological interpretation

Notwithstanding the residual excess scatter in the Discordia regression, the upper intercept of 1076 ± 35 Ma represents our best estimate of the crystallisation age of the low-Th/U zircon, with the lower intercept indicating a Late Neoproterozoic resetting event, the age of which (680 ± 110 Ma) is poorly constrained. The magmatic crystallisation age of 1076 ± 35 Ma is indistinguishable from those of two c. 1090 Ma orthogneisses exposed at Redgate Beach (Nelson, 1999) some 30 km to the west of WAPET Sue 1; however, pre-1000 Ma protoliths are otherwise unknown in the Leeuwin Complex.

The isotopic resetting event implied by the lower intercept of the Discordia regression is broadly coeval with mid-Neoproterozoic (780–680 Ma) felsic plutonism in the Leeuwin Complex (Nelson, 1996, 2002; Collins, 2003). However, it is possible that some of the dispersion in the Discordia regression reflects divergent histories of radiogenic Pb loss from crystal to crystal: the event responsible for loss of radiogenic Pb loss in analyses such as 10.1, 29.1 and 39.1 appears to be predominantly mid-Neoproterozoic, whereas the corresponding event in analyses such as 3.1, 41.1 and 42.1 appears to be predominantly more recent. Collins (2003) documented a pervasive Early Cambrian (540–520 Ma) tectonothermal event in the coastal exposures of the Leeuwin Complex, and this event may be responsible for some of the complexity in the observed Pb loss patterns (see Discussion).

Beneath the Perth Basin: New U–Pb SHRIMP zircon ages from the Pinjarra Orogen, Western Australia, 2016 9

Table 2.3 SHRIMP U-Pb zircon data from the unnamed felsic orthogneiss, WAPET Sue 1 (3073.2–3073.7 m; GA 2124222). All dates are 207Pb/206Pb.

Sample-grain.area

206Pbc

(%)U

(ppm)Th

(ppm)232Th/238U

238U/206Pb

±1σ(%)

207Pb/206Pb

±1σ(%)

Date(Ma)

±1σ(Ma)

Disc(%)

Pre-1170 Ma, high-Th/U inherited cores (n = 6)

222-53.1C 0.20 116 147 1.30 3.167 1.39 0.09995 1.61 1623 30 -10

222-44.1C 0.06 208 225 1.12 3.918 1.12 0.09247 0.82 1477 16 1

222-49.1C 0.09 139 90 0.66 4.146 2.86 0.08640 2.15 1347 42 -4

222-5.2C -0.02 436 279 0.66 4.654 1.36 0.08402 0.63 1293 12 3

222-45.1C -0.11 173 238 1.43 4.975 1.16 0.07970 1.17 1190 23 1

222-50.1C 0.08 1470 1257 0.88 5.828 1.41 0.07925 1.05 1178 21 14

Post-1170 Ma, low-Th/U (<0.25) analyses included in Discordia regression (n = 34)

222-16.1 0.03 522 55 0.11 5.334 2.41 0.07825 2.06 1153 41 4

222-19.1 0.10 470 52 0.11 5.108 3.10 0.07816 2.00 1151 40 0

222-28.1 0.06 755 132 0.18 5.566 1.05 0.07787 1.44 1144 29 7

222-15.1 0.49 1087 241 0.23 5.532 1.12 0.07750 1.24 1134 25 6

222-13.1 0.01 359 53 0.15 5.553 1.72 0.07729 0.76 1129 15 6

222-8.1 -0.02 474 60 0.13 5.542 2.68 0.07706 3.81 1123 76 5

222-12.1 -0.01 1240 44 0.04 5.110 3.32 0.07673 3.81 1114 76 -4

222-25.1 0.02 887 67 0.08 5.436 1.83 0.07672 1.86 1114 37 2

222-1.1 0.12 579 52 0.09 5.177 3.25 0.07589 2.32 1092 46 -5

222-48.1 0.04 600 34 0.06 5.812 2.12 0.07581 2.22 1090 44 7

222-26.1 0.07 641 63 0.10 5.656 2.32 0.07566 2.12 1086 43 4

222-31.1 0.05 1656 50 0.03 5.477 0.88 0.07511 0.58 1072 12 -1

222-7.1 0.01 821 31 0.04 5.309 1.66 0.07508 1.27 1071 26 -4

222-54.2R 0.04 688 14 0.02 5.517 1.54 0.07499 1.21 1068 24 -1

222-27.1 0.10 788 41 0.05 5.747 0.92 0.07490 2.70 1066 54 3

222-18.1 0.04 680 49 0.08 5.716 2.32 0.07488 2.80 1065 56 3

222-43.1 0.06 2009 165 0.08 5.535 1.06 0.07483 0.67 1064 13 -1

222-34.1 0.02 1215 60 0.05 5.478 2.85 0.07479 2.12 1063 43 -2

222-9.1 0.01 1205 26 0.02 5.443 1.65 0.07462 1.51 1058 30 -3

222-11.1 0.13 496 51 0.11 5.677 3.16 0.07428 3.47 1049 70 0

222-32.1 0.02 1097 55 0.05 5.839 2.02 0.07420 1.77 1047 36 3

222-24.1 0.02 508 36 0.07 5.907 2.43 0.07402 0.67 1042 14 4

222-22.1 0.00 747 68 0.09 6.092 0.91 0.07384 1.48 1037 30 6

222-4.1 0.05 514 44 0.09 5.515 0.96 0.07372 1.48 1034 30 -4

222-5.1R 0.24 490 50 0.11 6.109 2.41 0.07351 2.50 1028 50 5

222-38.1 0.09 1085 226 0.21 5.991 0.97 0.07327 1.35 1021 27 3

222-37.1 0.05 600 36 0.06 5.327 0.96 0.07326 2.12 1021 43 -9

10 Beneath the Perth Basin: New U–Pb SHRIMP zircon ages from the Pinjarra Orogen, Western Australia, 2016

Sample-grain.area

206Pbc

(%)U

(ppm)Th

(ppm)232Th/238U

238U/206Pb

±1σ(%)

207Pb/206Pb

±1σ(%)

Date(Ma)

±1σ(Ma)

Disc(%)

222-17.1 0.03 626 34 0.06 6.266 0.96 0.07321 0.65 1020 13 7

222-40.1 0.01 787 49 0.06 5.510 1.92 0.07261 0.50 1003 10 -8

222-53.2R -0.02 582 32 0.06 5.972 1.60 0.07215 1.98 990 40 -1

222-6.1 0.00 766 38 0.05 5.807 4.34 0.07159 3.83 974 78 -6

222-10.1 0.03 559 40 0.07 6.350 0.94 0.07051 2.90 943 59 0

222-39.1 0.07 708 40 0.06 6.910 2.18 0.07041 0.73 940 15 8

222-29.1 0.05 582 58 0.10 7.500 0.95 0.06798 0.86 868 18 8

Post-1170 Ma analyses excluded from Discordia regression owing to elevated Th/U (n = 3)

222-35.1 0.26 1020 472 0.48 5.933 3.08 0.07699 1.75 1121 35 11

222-33.1 0.06 596 159 0.28 5.814 0.93 0.07566 0.60 1086 12 6

222-30.1 0.74 1618 476 0.30 6.325 4.53 0.07103 4.64 958 95 1

Post-1170 Ma analysis excluded from Discordia regression owing to probable recent Pb loss (n = 1)

222-41.1 0.32 1897 150 0.08 6.415 0.92 0.07441 0.86 1053 17 12

Not considered: Domains with common 206Pb > 1% (n = 9)

222-47.1C 7.86 274 259 0.98 4.814 3.38 0.14065 5.12 2235 89 50

222-52.2R 3.29 1424 779 0.57 5.882 1.33 0.07836 4.31 1156 86 13

222-46.2R 10.67 2396 1563 0.67 8.310 2.37 0.07743 15.24 1132 303 37

222-47.2R 8.70 3354 1835 0.57 13.716 1.54 0.07621 3.49 1101 70 61

222-52.1C 7.40 349 447 1.32 6.426 1.05 0.07619 4.02 1100 80 16

222-20.1 2.96 540 304 0.58 6.434 1.90 0.07565 5.96 1086 119 15

222-50.2R 1.12 478 365 0.79 5.760 2.05 0.07529 3.68 1076 74 4

222-36.1 2.05 921 113 0.13 6.311 0.91 0.07350 2.83 1028 57 8

222-23.1 5.27 666 194 0.30 4.961 11.43 0.06621 18.62 813 389 -50

Not considered: High-U domains with poor crystallinity (U > 2500 ppm and UO/U > 6.2; n = 4)

222-51.2R 0.04 4799 259 0.06 6.619 7.98 0.07414 2.04 1045 41 14

222-45.2R 0.04 4912 248 0.05 6.972 1.44 0.07356 0.25 1030 5 17

222-21.1 0.07 2849 89 0.03 17.439 12.40 0.07023 1.77 935 36 63

222-44.2R 0.15 5946 686 0.12 18.389 4.13 0.06333 1.58 719 34 54

Not considered: Domains with very large 207Pb/206Pb uncertainties (n = 3)

222-51.1C 0.02 150 74 0.51 3.875 3.76 0.09669 15.48 1561 290 6

222-54.1C 0.04 152 39 0.26 4.259 9.00 0.09111 17.46 1449 332 7

222-14.1 0.05 555 42 0.08 5.400 3.11 0.07925 8.09 1178 160 8

Not considered: Analyses significantly affected by recent Pb loss (discordance > 30%; n = 4)

222-46.1C 0.25 611 109 0.19 5.017 2.55 0.10633 2.37 1737 43 36

222-3.1 0.09 1524 304 0.21 14.818 1.38 0.07500 3.14 1068 63 63

222-42.1 0.52 1527 407 0.28 8.159 2.14 0.07455 2.92 1056 59 31

222-2.1 0.11 2052 203 0.10 10.072 2.49 0.07043 2.92 941 60 37

Beneath the Perth Basin: New U–Pb SHRIMP zircon ages from the Pinjarra Orogen, Western Australia, 2016 11

2.2 Unnamed leucocratic orthogneiss, BMR Beagle Ridge 10A

Table 2.4 Summary of results: unnamed leucocratic orthogneiss, BMR Beagle Ridge 10A (1464.0–1467.0 m; GA 2124223).

GA SampleNo 2124223

Drillhole and Depth Interval BMR Beagle Ridge 10A, 1464.0–1467.0 m

Lithology leucocratic orthogneiss

Stratigraphic Unit —

Parent Unit —

Informal Identifier Unnamed leucocratic orthogneiss, BMR Beagle Ridge 10A

Province Pinjarra Orogen

1:250 000 Sheet Dongara (SH50-05)

1:100 000 Sheet Beagle Islands (1838)

Location (GDA94) 29.82667°S, 114.97500°E

Analytical Session(s) 110130 (see Appendix Table A.1 for parameters derived from measurements of 238U/206Pb and 207Pb/206Pb reference zircons)

Interpreted Age(s) Magmatic crystallisation : 1092 ± 27 Ma(95% confidence; 4 analyses of 4 zircons)

Calculation Method Weighted mean

Isotopic Ratio(s) Used 207Pb/206Pb (204Pb-corrected)

2.2.1 Sampling details

Beagle Ridge 10A (also known as Dongara 10A) is a petroleum exploration well, drilled in the northern Perth Basin by the Australian Bureau of Mineral Resources (BMR) in 1960 (MacTavish, 1965). Crystalline basement rocks of the northern Pinjarra Orogen were first intersected at depth 1461 m (MacTavish, 1965), and drilling was terminated at 1482 m. A segment of drillcore within the interval 1464.0–1467.0 m, described as ‘granitic augen gneiss’ and associated with garnet-bearing gneiss and quartz-feldspar-biotite-garnet schist (MacTavish, 1965; Peers and Trendall, 1968), was sampled for zircon separation and U-Pb SHRIMP analysis.

2.2.2 Petrography

This rock is a leucocratic orthogneiss comprising 45–50% quartz, 40% feldspar (possibly 30% plagioclase, 10% K-feldspar, but distinguishing the two is difficult owing to the overprinting effects of hydrothermal alteration), and 8–10% biotite, most of which has been altered to chlorite. Accessory minerals include very minor muscovite (as a late stage alteration product), apatite, opaque oxide minerals, small rounded zircon grains, and possible titanite. Aligned biotite (mostly altered to chlorite) defines a gneissic fabric within this leucocratic rock, which may represent a severely deformed aplitic component of the igneous protolith.

12 Beneath the Perth Basin: New U–Pb SHRIMP zircon ages from the Pinjarra Orogen, Western Australia, 2016

2.2.3 Zircon description

The mounted heavy mineral concentrate contained only seven zircons. These grains are between 80 µm and 120 µm in length, and range from subhedral crystals to rounded grains and fragments (Figure 2.6). Most are pale brown in transmitted light. Cathodoluminescence (CL) images reveal prominent concentric zoning and, in some cases, bright-CL cores disconformably overgrown by zoned rims.

Figure 2.6 Zircons from the unnamed leucocratic orthogneiss, BMR Beagle Ridge 10A (1464.0–1467.0 m, GA 2124223). Transmitted-light image is shown in the upper half; cathodoluminescence image in the lower half. SHRIMP analysis sites are indicated, and labelled ‘grain.area’. Most of the mounted crystals are monazite.

2.2.4 U–Pb isotopic results

Seven analyses were obtained from seven zircon crystals (Figure 2.7, Table 2.5). These can be divided into three groups:

Four analyses (pale yellow in Figure 2.7) are characterised by relatively low Th/U (0.05–0.23) and 207Pb/206Pb dates that are indistinguishable within their analytical uncertainties. Even though two of the analyses are near-concordant, have low common 206Pb (<0.1%) and moderate U (500–842 ppm), and the other two are grossly (>40%) discordant, have much higher common 206Pb (>1%) and significantly higher U (1787–2505 ppm), their geochemical and isotopic similarities support the possibility of a cogenetic origin, with the higher-U domains affected by recent (rather than ancient) loss of radiogenic Pb. Their individual 207Pb/206Pb dates range between c. 1134 Ma and c. 1075 Ma, and yield a statistically coherent weighted mean 207Pb/206Pb date of 1092 ± 27 Ma (95% confidence, MSWD = 0.57, P = 0.64).

Two analyses (purple in Figure 2.7) are characterised by higher Th/U (0.43–0.63), and older, concordant 207Pb/206Pb dates of c. 1212 Ma and c. 1293 Ma.

Beneath the Perth Basin: New U–Pb SHRIMP zircon ages from the Pinjarra Orogen, Western Australia, 2016 13

One analysis (green in Figure 2.7) is characterised by significantly younger (Paleozoic) 207Pb/206Pb and 238U/206Pb dates, despite a relatively low U content (171 ppm) and pristine-looking concentric oscillatory CL zoning.

Figure 2.7 SHRIMP U-Pb data for zircons from the unnamed leucocratic orthogneiss, BMR Beagle Ridge 10A (1464.0–1467.0 m, GA 2124223). Pale yellow ellipses denote magmatic crystallisation affected by recent loss of radiogenic Pb; purple ellipses denote Mesoproterozoic inheritance; green ellipse denotes an analysis of a Paleozoic zircon likely to represent a contaminant. Heavy black dashed line represents the weighted mean 207Pb/206Pb date, with its 95% confidence envelope.

2.2.5 Geochronological interpretation

Based on the circumstantial evidence outlined above, the weighted mean 207Pb/206Pb date of 1092 ± 27 Ma (95% confidence) defined by the four lowest-Th/U analyses is tentatively interpreted as the best estimate of the age of magmatic crystallisation of the igneous precursor to the leucocratic orthogneiss. This date is similar to those previously obtained from the Northampton Complex, which is characterised by U–Pb data indicating 1090–1020 Ma plutonism and high-grade metamorphism (Bruguier et al., 1999; Ksienzyk et al., 2012; and see Discussion).

The two pre-1200 Ma, higher-Th/U analyses are interpreted to reflect xenocrystic components within the igneous precursor. In the absence of supporting chemical or textural evidence, the single Paleozoic analysis is unlikely to reflect Phanerozoic (but non-recent) isotopic resetting of a Proterozoic crystal: we consider it more likely that this grain is extraneous to the targeted sample, and represents contamination.

14 Beneath the Perth Basin: New U–Pb SHRIMP zircon ages from the Pinjarra Orogen, Western Australia, 2016

Table 2.5 SHRIMP U-Pb zircon data from the unnamed leucocratic orthogneiss, BMR Beagle Ridge 10A (1464.0–1467.0 m, GA 2124223). All dates are 207Pb/206Pb unless otherwise indicated in the group heading.

Sample-grain.area

206Pbc

(%)U

(ppm)Th

(ppm)232Th/238U

238U/206Pb

±1σ(%)

207Pb/206Pb

±1σ(%)

Date(Ma)

±1σ(Ma)

Disc(%)

Pre-1200 Ma, high-Th/U xenocrysts (n = 2)

223-5.1 0.03 750 458 0.63 4.549 1.20 0.08392 0.46 1291 9 1

223-3.1 0.00 203 83 0.42 4.922 1.11 0.08061 0.91 1212 18 2

Post-1200 Ma, lower-Th/U population interpreted as magmatic crystallisation (n = 4)

223-6.1 1.01 1784 166 0.10 9.498 1.15 0.07750 1.90 1134 38 45

223-2.1 0.09 842 43 0.05 5.934 0.90 0.07599 0.56 1095 11 9

223-1.1 0.09 500 59 0.12 5.541 0.95 0.07555 0.69 1083 14 1

223-4.1 2.26 2505 550 0.23 17.798 0.96 0.07524 4.84 1075 97 69

Probable contaminant (n = 1; 238U/206Pb date tabulated)

223-7.1 -0.20 171 51 0.31 14.891 1.17 0.05853 2.48 419 5 25

Beneath the Perth Basin: New U–Pb SHRIMP zircon ages from the Pinjarra Orogen, Western Australia, 2016 15

3 Discussion

3.1 Significance of the new U–Pb zircon dates

The two newly dated samples are located at opposite ends of the Perth Basin, about 470 km apart. Both magmatic crystallisation ages are relatively imprecise, but the weighted mean 207Pb/206Pb date of 1092 ± 27 Ma from the leucocratic orthogneiss (GA 2124223) in BMR Beagle Ridge 10A (northern Pinjarra Orogen) is indistinguishable from the upper intercept date of 1076 ± 35 Ma defined by the Model 1 Discordia regression obtained from the felsic orthogneiss (GA 2124222) in WAPET Sue 1.

The discovery of additional Mesoproterozoic igneous protoliths in the southern Pinjarra Orogen is particularly significant, as pre-1000 Ma magmatic crystallisation ages in the coastal Leeuwin Complex were previously known only from two garnet-bearing orthogneisses sampled at Redgate Beach, which yielded SHRIMP 207Pb/206Pb dates of 1091 ± 8 Ma and 1091 ± 17 Ma (Nelson, 1999). All other dated outcrops have revealed Neoproterozoic granitic protoliths (780–680 Ma) reworked by Early Cambrian (540–520 Ma) magmatism, deformation and high-grade metamorphism (Nelson, 1996, 2002; Collins, 2003). Although the regional geometry of Meso- and Neoproterozoic orthogneiss protoliths is unclear (Janssen et al., 2003), our data from WAPET Sue 1 some 30 km east of Redgate Beach (Figure 1.1) extend the geographic range of Mesoproterozoic protoliths in the southern Pinjarra Orogen.

The lower intercept date of 680 ± 110 Ma defined by the Discordia regression in the felsic orthogneiss from WAPET Sue 1 suggests that at least some of the post-magmatic disturbance of the zircon U–Pb isotopic system was coeval with Neoproterozoic (780–680 Ma; Nelson, 1996, 2002) magmatism in the Leeuwin Complex, rather than the Early Cambrian tectonothermal overprint that pervades coastal exposures west of WAPET Sue 1 (Collins, 2003). Our data do not, however, preclude a degree of additional Early Cambrian isotopic resetting in the WAPET Sue 1 felsic orthogneiss, as our Model 1 Discordia regression retains scatter beyond that attributable to analytical uncertainty (MSWD = 2.0 for 34 analyses). In particular, if the zircon domains most affected by loss of radiogenic Pb during the inferred c. 680 Ma event were also those most susceptible to isotopic resetting during a subsequent c. 530 Ma event, the net effect would be anticlockwise rotation of the calculated Discordia regression. In this scenario, the upper and lower intercept dates calculated for the WAPET Sue 1 felsic orthogneiss would both represent minimum ages for the events they are interpreted to constrain.

In the northern Pinjarra Orogen, the data from the BMR Beagle Ridge 10A leucocratic orthogneiss (GA 2124223) are much sparser, and two of the four analyses defining the weighted mean 207Pb/206Pb date of 1092 ± 27 Ma constraining magmatic crystallisation are highly discordant (>40%). The fact that the two discordant individual 207Pb/206Pb dates are within uncertainty of those obtained from the two more concordant analyses (MSWD = 0.57) suggests that post-crystallisation isotopic disturbance of this zircon population was predominantly a geologically recent event. These four zircons do not preserve any evidence for loss of radiogenic Pb during Neoproterozoic and/or Early Cambrian tectonothermal events of the kind observed in the southern Pinjarra Orogen. Rather, the c. 1090 Ma magmatic crystallisation age interpreted for the protolith of the BMR Beagle Ridge 10A leucocratic orthogneiss is consistent with the existing framework of 1090–1020 Ma plutonism and high-grade metamorphism in the Northampton Complex (Bruguier et al., 1999; Ksienzyk et al., 2012), where granitic rocks as young as 1068 ± 13 Ma are affected by regional deformation, and a minimum age for tectonothermal activity is established by undeformed (‘post-tectonic’) pegmatite at 989 ± 2 Ma (Bruguier et al., 1999). Our leucocratic orthogneiss is likely to differ compositionally from dated granite in the Northampton Complex, which is sillimanite-bearing with xenoliths of garnet granulite (Bruguier et al., 1999), but

16 Beneath the Perth Basin: New U–Pb SHRIMP zircon ages from the Pinjarra Orogen, Western Australia, 2016

other rocks from BMR Beagle Ridge 10A are garnet-bearing and have been correlated with the exposed Northampton Complex (MacTavish, 1965; Peers and Trendall, 1968).

3.2 Implications for crustal evolution of the Pinjarra Orogen

Notwithstanding the very small number of zircon analyses, the presence of 1290–1210 Ma inheritance in the leucocratic gneiss from BMR Beagle Ridge 10A indicates an older crustal component in the parent magma. This is consistent with regional evidence for an extended crustal prehistory revealed by whole-rock Sm–Nd analyses of granitic rocks from the northern Pinjarra Orogen (Fletcher et al., 1985), intersected by petroleum exploration wells in the northern Perth Basin. No analyses were conducted on material from BMR Beagle Ridge 10A, but Fletcher et al. (1985) did report Sm–Nd data for granitic rock in WAPET Jurien 1 and gneissic granite from WAPET North Yardarino 1 (about 35 km south and 70 km north of BMR Beagle Ridge 10A, respectively) that indicate Paleoproterozoic depleted-mantle model ages (TDM = 2234 Ma and 2078 Ma respectively; Table 3.6). In the exposed northern Pinjarra Orogen, Sm–Nd data from four meta-igneous and metasedimentary rocks of the exposed Northampton Complex and Mullingarra Complex (Fletcher et al, 1985) yielded remarkably similar TDM ages (2094–2060 Ma; Table 3.6). These Sm–Nd data, in combination with the similarities between our new U–Pb zircon age from BMR Beagle Ridge 10A and the predominantly 1090–1020 Ma U–Pb zircon and monazite dates from the Northampton Complex and Mullingarra Complex (e.g. Bruguier et al., 1999; Ksienzyk et al., 2012), suggest that Pinjarra Orogen crust beneath the northern Perth Basin is isotopically similar to that exposed in the Northampton and Mullingarra complexes.

In the southern Pinjarra Orogen, the zircon U–Pb and whole-rock Sm–Nd isotopic systems are directly linked by two samples of the same felsic orthogneiss in WAPET Sue 1 (Fletcher et al., 1985; this study). Six inherited zircon cores in our U–Pb SHRIMP sample (GA 2124222; depth 3073 m) yielded 1620–1180 Ma dates, and this direct evidence for an older crustal component is supported by the depleted-mantle model age of 2040 Ma (Table 3.6) calculated from data reported by Fletcher et al (1985) from a sample at depth 3055 m. In addition, two granitic gneisses underlying the Perth Basin some 20–25 km to the northwest of WAPET Sue 1 (drillholes CRA CRCH 1 and Treeton DDH 2; Fletcher and Libby, 1993) yielded TDM ages of 2002 Ma and 1996 Ma (Table 3.6): there is thus little isotopic variation apparent within granitic crust of the Pinjarra Orogen beneath the southern Perth Basin. Furthermore, despite the sparsity of the data, there are remarkable similarities between the (associated) U–Pb and Sm–Nd signatures of rocks from the Northampton Complex, the Mullingarra Complex, and the basement gneisses flooring both the northern and southern Perth Basin, and it is possible that a belt of isotopically coherent crust spans the full strike length of the Pinjarra Orogen.

The east-west extent of this belt is debatable, as the TDM ages of 2200–2000 Ma characteristic of ‘Northampton–Mullingarra–Perth Basin basement’ contrast strongly with more juvenile values obtained from five meta-igneous samples collected from the Leeuwin Complex (McCulloch, 1987; Black et al., 1992; Fletcher and Libby, 1993) some 30 km to the west of WAPET Sue 1. All five analyses yielded Mesoproterozoic TDM ages (1556–1141 Ma; Table 3.6), and Fletcher and Libby (1993) argued for an isotopic discontinuity between Mesoproterozoic crust of the Leeuwin Complex and Paleoproterozoic crust to its east and north. However, it is likely that all published Sm–Nd data for the Leeuwin Complex are from gneisses with 780–680 Ma (or younger) protoliths, as those rocks appear to be prevalent in the coastal exposures (Figure 1.1), including the Sugarloaf Rock and Augusta localities that provided at least three of the five TDM ages (Table 3.6). The available data do not preclude an isotopic link between protoliths with U–Pb zircon ages of 1100–1050 Ma at Redgate Beach and in WAPET Sue 1; Sm–Nd analysis of the Redgate Beach orthogneisses would allow this hypothesis to be tested.

Beneath the Perth Basin: New U–Pb SHRIMP zircon ages from the Pinjarra Orogen, Western Australia, 2016 17

Table 3.6 Summary of published whole-rock Sm–Nd data from the Pinjarra Orogen (Fletcher et al., 1985; McCulloch, 1987; Black et al., 1992; Fletcher and Libby, 1993), normalised and recalculated by Fitzsimons (2003).

SampleID

Biblio.ref.1

Geologicaldomain/complex2 Rock type and locality

143Nd/144Ndmeasured

147Sm/144Ndmeasured

143Nd/144NdCHUR(0)3 εNd(0)4

143Nd/144Ndnormalised5 TDM (Ma)6

Northern Pinjarra Orogen (n = 6)

1757 A (Perth Basin basement) granitic rock, WAPET Jurien 1 0.511525 0.11393 0.512638 -21.71 — 2234

1758 A (Perth Basin basement) granite gneiss, WAPET North Yardarino 1 0.511708 0.11889 0.512638 -18.14 — 2078

20593 A Northampton Complex paragneiss, Mary Springs 0.511608 0.11244 0.512638 -20.09 — 2094

57420 A Northampton Complex granite gneiss, Riverside 0.511633 0.11242 0.512638 -19.60 — 2060

1797 A Mullingarra Complex garnet-biotite granofels, Yandanooka Hills 0.511637 0.11445 0.512638 -19.53 — 2092

56433B A Mullingarra Complex psammite gneiss, Ikewah Range 0.511645 0.11348 0.512638 -19.37 — 2063

Southern Pinjarra Orogen (n = 8)

1672 A (Perth Basin basement) granite gneiss, WAPET Sue 1 0.511447 0.09745 0.512638 -23.23 — 2040

93867 D (Perth Basin basement) garnet-biotite granite gneiss, CRA CRCH 1 0.511667 0.11169 0.512638 -18.94 — 2002

75497 D (Perth Basin basement) garnet-biotite granite gneiss, Treeton DDH 2 0.511548 0.10234 0.512638 -21.26 — 1996

82/426 B Leeuwin Complex mafic granulite, Sugarloaf Rock 0.51153 0.1409 0.511836 -5.98 0.512332 1499

82/425 C Leeuwin Complex granite gneiss, locality not specified 0.511944 0.1061 0.512638 -13.54 — 1556

82/427 C Leeuwin Complex granodiorite gneiss, locality not specified 0.512268 0.1191 0.512638 -7.22 — 1283

G3778 D Leeuwin Complex garnet gneiss, Augusta 0.512072 0.08596 0.512638 -11.04 — 1192

82/427a B Leeuwin Complex granulite, Sugarloaf Rock 0.51150 0.1104 0.511836 -6.56 0.512301 1141

1 Bibliographic reference: A = Fletcher et al. (1985), B = McCulloch (1987), C = Black et al. (1992), D = Fletcher and Libby (1993).2 Parentheses indicate drillcore samples of uncertain tectonic affinity; the remainder are outcrop samples.3 Present-day 143Nd/144Nd value assumed for chondritic uniform reservoir (CHUR). 147Sm/144Nd CHUR(0) = 0.1967 throughout.4 Present-day εNd, calculated using measured 143Nd/144Nd and specified 143Nd/144Nd CHUR(0).5 Normalisation of measured 143Nd/144Nd (if required) using reference ratio 146Nd/144Nd = 0.7219, to enable comparison of different Sm–Nd datasets (following Fitzsimons, 2003).6 Nd model ages recalculated using the normalised 143Nd/144Nd (if required; after Fitzsimons, 2003), relative to a depleted mantle (TDM) with present-day 143Nd/144Nd = 0.51316 and 147Sm/144Nd = 0.225, using λ(147Sm) = 6.54 x 10-12 a-1, and employing the linear εNd growth model of Goldstein et al. (1984).

18 Beneath the Perth Basin: New U–Pb SHRIMP zircon ages from the Pinjarra Orogen, Western Australia, 2016

Acknowledgements

We thank Michael Wingate of the Geological Survey of Western Australia (GSWA) for encouraging our efforts, and Richard O’Brien (GSWA) for facilitating this project by co-ordinating access to drillcore and permission to sample. The U–Pb SHRIMP analytical program was conducted using high quality zircon separates, mounts, and images skilfully prepared by David DiBugnara, Benjamin Linehan and Simon Webber (Mineral Separation Laboratory, GA). Patrick Burke (SHRIMP Laboratory, GA) provided valuable technical support in optimising analytical conditions during data acquisition. Kathryn Waltenberg assisted with the preparation of Figure 1.1. Natalie Kositcin and Kathryn Waltenberg (Geochronology and Stratigraphy Section, GA) provided reviews which improved the manuscript.

Beneath the Perth Basin: New U–Pb SHRIMP zircon ages from the Pinjarra Orogen, Western Australia, 2016 19

References

Black, L.P., Kamo, S.L., Allen, C.M., Davis, D.W., Aleinikoff, J.N., Valley, J.W., Mundil, R., Campbell, I.H., Korsch, R.J., Williams, I.S. and Foudoulis, C., 2004. Improved 206Pb/238U microprobe geochronology by the monitoring of a trace-element-related matrix effect; SHRIMP, ID-TIMS, ELA-ICP-MS and oxygen isotope documentation for a series of zircon standards. Chemical Geology 205, 115–140. http://dx.doi.org/10.1016/j.chemgeo.2004.01.003

Black, L.P., Sheraton, J.W., Tingey, R.J. and McCulloch, M.T., 1992. New U-Pb zircon ages from the Denman Glacier area, East Antarctica, and their significance for Gondwana reconstruction. Antarctic Science 4, 447–460. http://dx.doi.org/10.1017/S095410209200066X

Bodorkos, S., Blevin, P.L., Simpson, C.J., Gilmore, P.J., Glen, R.A., Greenfield, J.E., Hegarty, R. and Quinn, C.D., 2013. New SHRIMP U-Pb zircon ages from the Lachlan, Thomson and Delamerian orogens, New South Wales: July 2009–June 2010. Geoscience Australia, Record 2013/29; Geological Survey of New South Wales, Report GS2013/427. http://dx.doi.org/10.11636/Record.2013.029

Bruguier, O., Bosch, D., Pidgeon, R.T., Byrne, D.I. and Harris, L.B., 1999. U-Pb chronology of the Northampton Complex, Western Australia – evidence for Grenvillian sedimentation, metamorphism and deformation and geodynamic implications. Contributions to Mineralogy and Petrology 136, 258–272. http://dx.doi.org/10.1007/s004100050537

Carson, C.J., Hollis, J.A., Glass, L.M., Close, D.F., Whelan, J.A. and Wygralak, A., 2010. Summary of results. Joint NTGS-GA geochronology project: Arunta, Pine Creek regions July 2007–July 2009. Northern Territory Geological Survey, Record 2010-004. http://www.nt.gov.au/d/Minerals_Energy/Geoscience/Content/File/Pubs/Record/NTGSRec2010-004.pdf

Chisholm, E.I., Sircombe, K.N. and DiBugnara, D.L., 2014. Handbook of Geochronology Mineral Separation Laboratory Techniques. Geoscience Australia, Record 2014/46. http://dx.doi.org/10.11636/Record.2014.046

Claoué-Long, J.C., Compston, W., Roberts, J. and Fanning, C.M., 1995. Two Carboniferous ages: a comparison of SHRIMP zircon dating with conventional zircon ages and 40Ar/39Ar analysis. In Berggren, W.A., Kent, D.V., Aubry, M.-P. and Hardenbol, J., eds., Geochronology, Time Scales and Global Stratigraphic Correlation. SEPM Special Publication 54, pp. 3–21. Society for Sedimentary Geology, Tulsa, Oklahoma, USA. http://dx.doi.org/10.2110/pec.95.04.0003

Collins, A.S., 2003. Structure and age of the northern Leeuwin Complex, Western Australia: Constraints from field mapping and U–Pb isotopic analysis. Australian Journal of Earth Sciences 50, 585–599. http://dx.doi.org/10.1046/j.1440-0952.2003.01014.x

Compston, W., Williams, I.S. and Meyer, C., 1984. U-Pb geochronology of zircons from lunar breccia 73217 using a sensitive high mass-resolution ion microprobe. Journal of Geophysical Research 89 (Supplement), B525–B534. http://dx.doi.org/10.1029/JB089iS02p0B525

DiBugnara, D.L., 2016. Standard operating procedure for preparation of grain mounts for SHRIMP analysis: Mineral Separation Laboratory. Geoscience Australia, Record 2016/19. http://dx.doi.org/10.11636/Record.2016.019

Fitzsimons, I.C.W., 2003. Proterozoic basement provinces of southern and southwestern Australia, and their correlation with Antarctica. In Yoshida, M., Windley, B.F. and Dasgupta, S., eds., Proterozoic East Gondwana: Supercontinent Assembly and Breakup. Geological Society of London, Special Publication 206, pp. 93–130. http://dx.doi.org/10.1144/GSL.SP.2003.206.01.07

Fletcher, I.R. and Libby, W.G., 1993. Further isotopic evidence for the existence of two distinct terranes in the southern Pinjarra Orogen, Western Australia. Geological Survey of Western Australia, Report 34, 81–83.

Fletcher, I.R., Wilde, S.A. and Rosman, K.J.R., 1985. Sm–Nd model ages across the margins of the Archaean Yilgarn Block, Western Australia — III. The western margin. Australian Journal of Earth Sciences 32, 73–82. http://dx.doi.org/10.1080/08120098508729314

20 Beneath the Perth Basin: New U–Pb SHRIMP zircon ages from the Pinjarra Orogen, Western Australia, 2016

Geological Survey of Western Australia, 2016a. 1:500 000 State interpreted bedrock geology of Western Australia, 2016 (digital dataset). http://www.dmp.wa.gov.au/geoview

Geological Survey of Western Australia, 2016b. Compilation of geochronology information, 2016 update (digital dataset). http://www.dmp.wa.gov.au/geoview

Goldstein, S.L., O’Nions, R.K. and Hamilton, P.J., 1984. A Sm-Nd study of atmospheric dust and particulates from major river systems. Earth and Planetary Science Letters 70, 221–236. http://dx.doi.org/10.1016/0012-821X(84)90007-4

Jaffey, A.H., Flynn, K.F., Glendenin, L.E., Bentley, W.C. and Essling, A.M., 1971. Precision measurement of half-lives and specific activities of 235U and 238U. Physical Review C 4, 1889–1906. http://dx.doi.org/10.1103/PhysRevC.4.1889

Janssen, D.P., Collins, A.S. and Fitzsimons, I.C.W., 2003. Structure and tectonics of the Leeuwin Complex and Darling Fault Zone, southern Pinjarra Orogen, Western Australia — a field guide. Geological Survey of Western Australia, Record 2003/15.

Ksienzyk, A.K., Jacobs, J., Boger, S.D., Kosler, J., Sircombe, K.N. and Whitehouse, M.J., 2012. U–Pb ages of metamorphic monazite and detrital zircon from the Northampton Complex: evidence of two orogenic cycles in Western Australia. Precambrian Research 198–199, 37–50. http://dx.doi.org/10.1016/j.precamres.2011.12.011

Kusiak, M.A., Dunkley, D.J., Wirth, R., Whitehouse, M.J., Wilde, S.A. and Marquadt, K., 2015. Metallic lead nanospheres discovered in ancient zircons. Proceedings of the National Academy of Sciences 112, 4958–4963. http://dx.doi.org/10.1073/pnas.1415264112

Kusiak, M.A., Whitehouse, M.J., Wilde, S.A., Nemchin, A.A. and Clark, C., 2013. Mobilization of radiogenic Pb in zircon revealed by ion imaging: Implications for early Earth geochronology. Geology 41, 291–294. http://dx.doi.org/10.1130/G33920.1

Ludwig, K.R., 2003. User’s Manual for Isoplot 3.6 (April 2008 revision). Berkeley Geochronology Center, Special Publication 4. http://sourceforge.net/projects/isoplot/

Ludwig, K.R., 2009. SQUID 2 Rev. 2.50: A User’s Manual. Berkeley Geochronology Center, Special Publication 5. http://sourceforge.net/projects/squid2/

MacTavish, R.A., 1965. Completion report B.M.R. 10 and 10A, Beagle Ridge, Western Australia. Bureau of Mineral Resources, Report 80. http://www.ga.gov.au/metadata-gateway/metadata/record/gcat_10714

McCulloch, M.T., 1987. Sm-Nd isotopic constraints on the evolution of Precambrian crust in the Australian continent. In Kröner, A. ed., Proterozoic Lithospheric Evolution. Geodynamics Series 17, pp. 115–130. American Geophysical Union, Washington DC, USA. http://dx.doi.org/10.1029/GD017p0115

McIntyre, G.A., Brooks, C., Compston, W. and Turek, A., 1966. The statistical assessment of Rb–Sr isochrons. Journal of Geophysical Research 71, 5459–5468. http://dx.doi.org/10.1029/JZ071i022p05459

Nasdala, L., Hofmeister, W., Norberg, N., Mattinson, J.M., Corfu, F., Dörr, W., Kamo, S.L., Kennedy, A.K., Kronz, A., Reiners, P.W., Frei, D., Kosler, J., Wan, Y., Götze, J., Häger, T., Kröner, A. and Valley, J.W., 2008. Zircon M257 – a homogeneous natural reference material for the ion microprobe U–Pb analysis of zircon. Geostandards and Geoanalytical Research 32, 247–265. http://dx.doi.org/10.1111/j.1751-908X.2008.00914.x

Nelson, D.R., 1996. Compilation of SHRIMP U-Pb zircon geochronology data, 1995. Geological Survey of Western Australia, Record 1996/5.

Nelson, D.R., 1997. Compilation of SHRIMP U-Pb zircon geochronology data, 1996. Geological Survey of Western Australia, Record 1997/2.

Nelson, D.R., 1999. Compilation of geochronology data, 1998. Geological Survey of Western Australia, Record 1999/2.

Nelson, D.R., 2002. Compilation of geochronology data, 2001. Geological Survey of Western Australia, Record 2002/2.

Peers, R. and Trendall, A.F., 1968. Precambrian rocks encountered during drilling in the main Phanerozoic sedimentary basins of Western Australia. Western Australia Department of Mines, Annual Report 1967, 107–115.

Beneath the Perth Basin: New U–Pb SHRIMP zircon ages from the Pinjarra Orogen, Western Australia, 2016 21

Sircombe, K.N., Cassidy, K.F., Champion, D.C. and Tripp, G., 2007. Compilation of SHRIMP U-Pb geochronological data: Yilgarn Craton, Western Australia, 2004–2006. Geoscience Australia, Record 2007/01. http://www.ga.gov.au/metadata-gateway/metadata/record/gcat_64996

Stacey, J.S. and Kramers, J.D., 1975. Approximation of terrestrial lead isotope evolution using a two-stage model. Earth and Planetary Science Letters 26, 207–221. http://dx.doi.org/10.1016/0012-821X(75)90088-6

Steiger, R.H. and Jäger, E., 1977. Subcommission on geochronology: Convention on the use of decay constants in geo- and cosmochronology. Earth and Planetary Science Letters 36, 359–362. http://dx.doi.org/10.1016/0012-821X(77)90060-7

Stern, R.A., Bodorkos, S., Kamo, S.L., Hickman, A.H. and Corfu, F., 2009. Measurement of SIMS instrumental mass fractionation of Pb isotopes during zircon dating. Geostandards and Geoanalytical Research 33, 145–168. http://dx.doi.org/10.1111/j.1751-908X.2009.00023.x

White, L.T. and Ireland, T.R., 2012. High-uranium matrix effect in zircon and its implications for SHRIMP U–Pb age determinations. Chemical Geology 306–307, 78–91. http://dx.doi.org/10.1016/j.chemgeo.2012.02.025

Williams, C.T. and Nicholls, J., 1966. Sue No. 1 well completion report. West Australian Petroleum Pty Limited (unpublished). https://wapims.dmp.wa.gov.au/WAPIMS/

Williams, I.S., 1998. U-Th-Pb geochronology by ion microprobe. In McKibben, M.A., Shanks III, W.C. and Ridley, W.I., eds., Applications of Microanalytical Techniques to Understanding Mineralizing Processes. Reviews in Economic Geology 7, pp. 1–35. Society of Economic Geologists, Littleton, Colorado, USA.

Williams, I.S., Buick, I.S. and Cartwright, I., 1996. An extended episode of early Mesoproterozoic metamorphic fluid flow in the Reynolds Range, central Australia. Journal of Metamorphic Geology 14, 29–47. http://dx.doi.org/10.1111/j.1525-1314.1996.00029.x

Williams, I.S., Compston, W., Black, L.P., Ireland, T.R. and Foster, J.J., 1984. Unsupported radiogenic Pb in zircon: a cause of anomalously high Pb–Pb, U–Pb and Th–Pb ages. Contributions to Mineralogy and Petrology 88, 322–327. http://dx.doi.org/10.1007/BF00376756

22 Beneath the Perth Basin: New U–Pb SHRIMP zircon ages from the Pinjarra Orogen, Western Australia, 2016

Appendix A SHRIMP Data Acquisition, Reduction and Presentation

A.1 Analytical procedures

Isotopic analyses reported herein were undertaken using the SHRIMP IIe at Geoscience Australia (GA), Canberra. A summary of key parameters from the analytical session is shown in Appendix Table. Analytical procedures for zircon follow those published by Compston et al. (1984), Claoué-Long et al. (1995), Nelson (1997), and Williams (1998). Procedures and parameters relevant to the GA instrument are detailed in previous Records (e.g. Bodorkos et al., 2013), and are updated below.

Appendix Table A.1 Summary of session-specific metadata, parameters obtained from 238U/206Pb and 207Pb/206Pb reference materials, and samples analysed.

Session 110130

MountID GA6179

Session dates 11–14 November 2011238U/206Pb reference material TEMORA2 (416.8 Ma)

Analyses used 36 of 36238U/206Pb session-to-session error (2σ) 0.418%238U/206Pb spot-to-spot error (1σ) 0.81%207Pb/206Pb reference material OG1 (3465.4 Ma)

Analyses used 30 of 30

Mean 207Pb/206Pb date (95% confidence) 3467.5 ± 1.9 Ma

Number of samples co-analysed 2

Drillhole and depth interval (GA SampleNo) WAPET Sue 1, 3073.2–3073.7 m (2124222)

Drillhole and depth interval (GA SampleNo) BMR Beagle Ridge 10A, 1464.0–1467.0 m (2124223)

A.1.1 Sample acquisition and crushing

Drillhole locations were obtained from the relevant well completion reports, and are referred to the Geocentric Datum of Australia 1994 (GDA94). Co-ordinates are reported as latitude and longitude in decimal degrees. Both samples were obtained from diamond drillcore, and weighed about 180 g each. These were submitted to the Mineral Separation Laboratory at GA, where they were pulverised using a pre-cleaned hydraulic splitter, ultrasonically washed in water, and dried under heat lamps. This material was then crushed using a Rocklabs Boyd crusher, and milled using a Rocklabs continuous ring mill (Chisholm et al., 2014).

A.1.2 Mineral separation

Mineral density separation was undertaken using a Wilfley table, with multiple iterations employed to successively deslime the rock flours and decant bulk low-density minerals such as quartz and

Beneath the Perth Basin: New U–Pb SHRIMP zircon ages from the Pinjarra Orogen, Western Australia, 2016 23

feldspar, thereby reducing the sample to about 5% of its post-milling weight. Strongly paramagnetic grains were successively removed from this heavy fraction using a ferrous hand-magnet and a rare-earth element hand-magnet, before the remainder underwent a series of magnetic separations using a Frantz barrier separator. This typically involved 10–12 separations in total (Chisholm et al., 2014): the first 5–6 iterations increased the magnet current from 0.4 A to 1.5 A whilst maintaining a constant transverse ramp tilt of 15°, and the second 5–6 iterations decreased the transverse ramp tilt from 15° to 1° whilst maintaining a constant magnet current of 1.5 A. This resulted in the sequential removal of minerals with progressively weaker paramagnetism. Zircon was usually separated from the fraction with the weakest paramagnetism (non-magnetic at magnet current 1.5 A and transverse ramp tilt usually less than 5°, and frequently less than 2°).

In cases where the initial Frantz separation (aimed at removing the most strongly paramagnetic material) demonstrated that the bulk of the heavy fraction was only weakly paramagnetic, the heavy fraction was immersed in liquid diiodomethane (specific gravity 3.3), in order to further reduce the low-density mineral content before the remaining Frantz separations were carried out. Where available, 200–300 crystals of the target mineral were hand-picked for each sample, commencing with grains in the least magnetic fraction, and progressing to successively more magnetic fractions in samples with low yield, without any discrimination based on the external morphologies of the grains (DiBugnara, 2016).

A.1.3 Mount preparation

Hand-picked zircon separates were placed in sample-specific rows (with both samples on the same mount) on a single strip of adhesive tape, along with one row of the Devonian 238U/206Pb reference zircon TEMORA2 (Black et al., 2004), one row of the Archean 207Pb/206Pb reference zircon OG1 (Stern et al., 2009), and one fragment of the Neoproterozoic gem zircon M257 from Sri Lanka (Nasdala et al., 2008) as a uranium concentration reference material. The grains were then mounted in a 25 mm diameter epoxy disc, and once cured, the mount surface was polished using a series of successively finer diamond suspensions, in order to expose longitudinal sections of the crystals (DiBugnara, 2016). The grain-rows were photographed in transmitted light and reflected light using a Leica DM6000M microscope with a mounted Leica DFC310 FX 1.3 Mp camera. Polished surfaces were cleaned with ethanol and Milli-Q water.

The mount (GA6179) was then coated with a 2 nm thickness of gold, in preparation for imaging via the JEOL JSM-6490LV scanning electron microscope housed at GA. Cathodoluminescence (CL) images were acquired using an accelerating voltage of 15 keV and a Robinson CL detector. The gold coat applied for SEM imaging was removed using a Bemcot cotton wipe saturated with ethanol. Non-radiogenic Pb contamination on the polished mount surface was minimised by ultrasonic cleaning using (in sequence) 99.95% ethanol, propanol, petroleum spirit, and a detergent solution comprising 10% RBS35 in deionised water detergent, prior to quadruple rinsing in Milli-Q water (DiBugnara, 2016). The clean mount was dried overnight in a 30°C oven, and recoated with a 15 nm thickness of high-purity (99.999%) gold to facilitate the establishment of a homogeneous electric potential across the surface during analysis. Finally, the coated mount was loaded into the high-vacuum SHRIMP sample lock and stored there for 48 hours to enable complete outgassing of the newly-cured epoxy, before being lowered into the SHRIMP source chamber immediately prior to analysis.

A.1.4 Instrument setup and data acquisition

A 15–20 µm-diameter primary beam of O2– ions at 10 keV, purified by means of a Wien filter to

minimise the presence of OH– species, was employed to sputter secondary ions from the surface of

24 Beneath the Perth Basin: New U–Pb SHRIMP zircon ages from the Pinjarra Orogen, Western Australia, 2016

the target zircons. Before each analysis, the surface of the analysis site was pre-cleaned by rastering of the primary beam for 4 minutes, in order to reduce the amount of common Pb on the mount surface. The total ion current measured at the mount surface was about 1 nA, and uniform ion flux across each spot area was achieved by Kohler focusing of the primary beam, which resulted in even, flat-bottomed pits less than 1 μm deep. Secondary ions were extracted through a low gradient electrostatic field (~0.4 keV/mm), accelerated to 10 keV, and steered firstly through a double-focusing cylindrical 85° electrostatic analyser with a turning radius of 1.27 m, and secondly through a 72.5° magnet sector with a turning radius of 1 m. Ion currents of the relevant secondary species were then determined by switching the magnetic field to direct the secondary ion beam into a single electron multiplier with a deadtime of 25 ns.

Data acquisition involved cycling the magnetic field through a run table comprising 10 mass stations (Appendix Table A.2). The measurement positions of 204Pb+ and background at mass 204.1 were both fixed relative to the position of 196Zr2O+, and the measurement positions of 207Pb+ and 208Pb+ were both fixed relative to 206Pb+. A full cycle through the mass stations is termed a scan, and each mass peak was re-centred once per scan. Each analysis comprised six sequential scans and typically took about 25 minutes to complete.

Appendix Table A.2 Species, nominal masses and counting times for zircon analyses reported herein.

Species Nominal mass (amu) Count time per scan (seconds)

[90Zr216O]+ 196 2

[204Pb]+ 204 20

background 204.05 20

[206Pb]+ 206 15

[207Pb]+ 207 40

[208Pb]+ 208 5

[238U]+ 238 5

[232Th16O]+ 248 2

[238U16O]+ 254 2

[238U16O2]+ 270 2

Mass resolution (M/ΔM at 1% peak height, as measured on 208Pb+ on Pb-rich feldspar from Broken Hill) was 5400, and total Pb+-ion sensitivity was 26 cps/ppm/nA. Analyses were collected sequentially, with one measurement of the TEMORA2 reference zircon after every third or fourth sample analysis, and one measurement of the OG1 reference zircon after every second or third TEMORA2 analysis. Labels for individual analyses (as shown in the sample-specific data tables) take the form ABC-X.Y, where ABC are the final three digits of the GA SampleNo, X is the ‘grain number’ (usually assigned sequentially within a sample, at the time of analysis) and Y is the ‘area number’ within grain X (used to distinguish between analyses in different areas of the same grain). Some analyses bear the suffix ‘C’ or ‘R’, denoting core and rim analyses.

A.2 Data reduction and presentation

Data from the SHRIMP were reduced, calculated and portrayed using Microsoft Excel 2003, and the add-ins SQUID 2.50.11.02.03 (February 2011 revision of Ludwig, 2009) and Isoplot 3.71.09.05.23 (May 2009 revision of Ludwig, 2003). The decay constants used are those of Jaffey et al. (1971),

Beneath the Perth Basin: New U–Pb SHRIMP zircon ages from the Pinjarra Orogen, Western Australia, 2016 25

together with present-day 238U/235U = 137.88, following Steiger and Jäger (1977). All unknown analyses were corrected for common Pb, with common 206Pb (as a percentage of total measured 206Pb) estimated using the 204Pb-correction, which uses the measured 204Pb/206Pb and a 204Pb/206Pb value approximated by the Stacey and Kramers (1975) model bulk-Earth value of equivalent age.

Dates derived from the pooling of multiple analyses are inverse variance-weighted means unless otherwise specified, and their uncertainties are quoted at the 95% confidence level unless otherwise indicated. Each weighted mean has an associated Mean Square of Weighted Deviates (MSWD) value, which is a measure of the degree of scatter of the constituent analyses relative to the assigned uncertainties (McIntyre et al., 1966; Ludwig, 2003), and a ‘probability of equivalence’ (P) value, which is the probability that the constituent analyses are equivalent within their uncertainties. By convention, scatter beyond uncertainty is assumed to be present when P is less than 0.05. In cases where P is equal to or greater than 0.05, but the MSWD value exceeds 1, the implied dispersion of the data-points beyond their analytical uncertainties is acknowledged by expanding the 95% confidence interval of the mean, via multiplication of its 1σ uncertainty firstly by Student’s t for n – 1 degrees of freedom (where n is the number of analyses), and secondly by the square root of the MSWD (Ludwig, 2003).

A.2.1 Calibration procedures

Elemental U concentrations in the unknowns were calibrated using the M257 reference zircon (840 ppm U; Nasdala et al., 2008), and the power-law relationship of Claoué-Long et al. (1995):

[ Zr2❑196 O+ / U+

❑238 ] = A × [ UO+

❑254 / U+

❑238 ]0.66

(1)

where A is a session-dependent constant determined from measurements on M257. All uranium concentration data tabulated for unknowns have uncertainties of the order of 15–20%, based on the extent of known variations in U abundance in M257.

The values of 232Th/238U were calculated using the relationship proposed by Williams et al. (1996):

Th❑232 / U❑

238 = [ ThO+❑

248 / UO+❑

254 ]× {(0.03446 × [ UO+❑

254 / U+❑

238 ] )+0.868 } (2)

The values of 238U/206Pb were calibrated using TEMORA2 (206Pb/238U = 0.0668, corresponding to an age of 416.8 Ma; Black et al., 2004), and a power-law relationship (Claoué-Long et al., 1995):

[ Pb+❑

206 / U+❑

238 ] = B × [ UO+❑

254 / U+❑

238 ]2 (3)

where B is a session-dependent constant determined from measurements on TEMORA2.

The values of 207Pb/206Pb in the unknowns were monitored using the OG1 reference zircon (207Pb/206Pb = 0.29907 ± 0.00011, corresponding to an age of 3465.4 ± 0.6 Ma; Stern et al., 2009); however, the error-weighted mean 207Pb/206Pb for OG1 (Appendix Table A.1) was within uncertainty of the reference value. In the absence of any evidence for instrumental mass fractionation of Pb-isotopes, the 204Pb-corrected 207Pb/206Pb values for each sample are tabulated as measured.

26 Beneath the Perth Basin: New U–Pb SHRIMP zircon ages from the Pinjarra Orogen, Western Australia, 2016

A.2.2 Propagation of uncertainties

A ‘calibration constant’ value was determined for each individual analysis of the 238U/206Pb reference material (i.e. bi = [206Pb+/238U+]i/([254UO+/238U+]i

2 for analyses of TEMORA2 zircon, following equation (3)). Uncertainties associated with each of these individual ‘calibration constants’ (i.e. ± bi) were governed primarily by the counting statistics associated with the constituent isotopic ratio(s). The value of the session ‘calibration constant’ (B) was calculated as the error-weighted mean of the session-specific population of individual calibration constants. However, this population displayed significant excess scatter, manifested as an MSWD value for B that far exceeded unity, despite the fact that most reference materials are (by definition) characterised by 238U/206Pb homogeneity at a range of scales. This indicates that the values of ± bi were underestimated from analysis to analysis. Consequently, SQUID calculates the constant additional uncertainty per spot (expressed as a percentage) that must be added in quadrature to each ± bi value, in order to produce MSWD ~ 1 for the population of bi values used to calculate B (Ludwig, 2009). This constant additional uncertainty is termed the ‘spot-to-spot uncertainty’ (or ‘repeatability’), and its 1σ value is presented in Appendix Table A.1. The spot-to-spot uncertainty is added in quadrature to the other sources of error (principally related to counting statistics and the common Pb correction) for each value of 238U/206Pb in the unknowns, and thus is incorporated in the uncertainties for all individual 238U/206Pb values presented in the analytical data-tables.

SQUID also calculates an uncertainty for the session-specific calibration constant (i.e. ± B). This uncertainty is termed the ‘session-to-session uncertainty’ (or ‘calibration uncertainty’), and its 2σ value is presented in Appendix Table A.1. The session-to-session uncertainty is not included in the uncertainties for individual 238U/206Pb values presented in the analytical data-tables, and should be neglected when comparing error-weighted mean 238U/206Pb dates for unknowns co-analysed in a single analytical session. However, it must be accounted for when seeking to compare 238U/206Pb datasets more widely (e.g. between different analytical sessions), especially when calculating error-weighted mean 238U/206Pb dates for unknowns.

A.2.3 Discordance

Discordance is a measure of the internal agreement of the dates derived from the independent 207Pb/206Pb and 238U/206Pb isotopic systems, within a single analysis. In 204Pb-corrected analyses of Mesoproterozoic and older age, discordance values for single analyses (and discordance patterns within a population of analyses) can provide important information about the timing and extent of post-crystallisation loss of radiogenic Pb (see Sircombe et al. (2007) and Carson et al. (2010) for more detail), and are often a valuable indicator of ‘geological’ data quality. The comparison has been performed throughout this Record, using the equation:

Disc (%) = 100 × {1 – ( U❑238 / Pb date❑

206 ) / (exp [ λ238 × { Pb❑207 / Pb❑

206 date } ] – 1 )} (4)

where λ238 is the 238U decay constant (Jaffey et al., 1971). The analysis-specific results of equation (4) are reported in the analytical data-tables.

Beneath the Perth Basin: New U–Pb SHRIMP zircon ages from the Pinjarra Orogen, Western Australia, 2016 27