Mid- to lower-crustal architecture of the northern Lachlan and southern Thomson orogens: evidence from O–Hf isotopes

The nature of the substrate below the northern Lachlan Orogen and the southern Thomson Orogen is poorly understood. We investigate the nature of the midto lower-crust using O and Lu–Hf isotope analyses of zircons from magmatic rocks that intrude these regions, and focus on the 440–410 Ma time window to minimise temporal effects while focussing on spatial differences. Over the entire region, weighted mean dO values range from 5.5 to 9.8‰ (relative to VSMOW, Vienna Standard Mean Oceanic Water), and weighted mean eHft range from 8.8 to þ8.5. In the northern Lachlan Orogen and much of the southern Thomson Orogen, magmatic rocks with unradiogenic eHft ( 7 to 4) and elevated dO values ( 9 to 10‰) reflect a supracrustal source component that may be common to both orogens. Magmatic rocks intruding the Warratta Group in the western part of the Thomson Orogen also have unradiogenic eHft ( 9 to 6) but more subdued dO values ( 7‰), indicating a distinct supracrustal source component in this region. Some regions record radiogenic eHf and mantle-like dO values, indicative of either a contribution from arc-derived rocks or a direct mantle input. In the northeast Lachlan Orogen Hermidale Terrane, magmatic rocks record mixing of the supracrustal source component with input from a infracrustal or mantle source component (eHft as high as þ8.5, dO values as low as 5.5‰), possibly of Macquarie Arc affinity. Samples in the west-southwestern Thomson Orogen also record some evidence of radiogenic input (eHft as high as 0.5, dO values as low as 6.4‰), possibly from the Mount Wright Arc of the Koonenberry Belt. Overall, our results demonstrate a strong spatial control on isotopic compositions. We find no isotopic differences between the bulk of the Lachlan Orogen and the bulk of the Thomson Orogen, and some indication of similarities between the two. ARTICLE HISTORY Received 23 August 2017 Accepted 3 April 2018


Introduction
The Tasmanides constitute the eastern third of the Australian continent, and broadly record early Paleozoic (ca 500 Ma) to early Mesozoic (ca 250 Ma) crustal addition to the eastern margin of Precambrian Australia, via arc-related accretion and associated sedimentation (e.g. Cawood & Buchan, 2007;Champion, 2016;Collins, 2002;Glen, 2005;Gray & Foster, 2004).
There exist numerous, but variable, models to explain the current geological configuration of the Tasmanides. Common elements include successive orogenic cycles on an east-facing continental margin (e.g. Burton, 2010;Fergusson & Henderson, 2013;Glen et al., 2013;Gray & Foster, 2004;Kirkegaard, 1974;Moresi, Betts, Miller, & Cayley, 2014;Murray & Kirkegaard, 1978;Scheibner, 1973). Current understanding of the Tasmanides is based largely on relatively well-exposed geological relationships in the southern and eastern Tasmanides: the Delamerian Orogen in South Australia, the Lachlan Orogen in Victoria and New South Wales (NSW) and the New England Orogen in northern NSW and southern Queensland ( Figure 1).
A major blind spot in understanding the Tasmanides is the Thomson Orogen: a very large region of Queensland and northern NSW. Its location largely coincides with the region now covered by the Mesozoic Eromanga Basin. As such, exposure is scarce, and basement intersections in drill core are sparse. Limited information indicates that Paleozoic metasedimentary and igneous rocks comprise the bulk of geology beneath the Eromanga Basin (Brown, Carr, & Purdy, 2012;Murray, 1994).
Earliest proponents of the term 'Thomson Orogen' used it to distinguish the region from the Lachlan Orogen to the south because so little was known about the geology below the Mesozoic cover (Kirkegaard, 1974;Murray & Kirkegaard, Orogen. This remains to be fully tested. Indeed, there are multiple interpretations of the relationship between the Lachlan Orogen and the Thomson Orogen (e.g. Burton, 2010;Glen, 2013;Moresi et al., 2014). Just as the Lachlan Orogen is subdivided into multiple subdomains (e.g. Glen et al., 2013;Glen, Percival, & Quinn, 2009;Seymour & Calver, 1995;VandenBerg et al., 2000), the size of the Thomson Orogen suggests that it is likely to consist of multiple subdomains with contrasting origins.
The widespread usage of the provincial entities Lachlan Orogen and Thomson Orogen in NSW implies the existence of a boundary between the two. Kirkegaard (1974) proposed the boundary between the Thomson Orogen and the Lachlan Orogen to be the 'Darling Lineament' because it marked a change in structural trend. Wellman (1976) first described a boundary between two distinct gravity domains, which occupy the current locations of the Thomson and Lachlan orogens (Murray & Kirkegaard, 1978). The location of this proposed boundary, now named the Olepoloko Fault and extensions, has largely been upheld in more recent investigations (e.g. Glen et al., 2013), although its status as the boundary between the Thomson Orogen and Lachlan Orogen remains debated (e.g. Burton, 2010;Glen et al., 2013;Moresi et al., 2014).
The north-dipping Olepoloko Fault occurs along the southwestern margin of the southern Thomson Orogen, from north of the Koonenberry Belt, trending south-southeast to the Cuttaburra region ( Figure 2; Glen et al., 2013;Greenfield, Gilmore, & Mills, 2010). It separates the Warratta Group of the Thomson Orogen from the Teltawongee Group in the Koonenberry Belt of the Delamerian Orogen .
Farther to the east, geological and geophysical evidence for continuation of the Olepoloko Fault becomes scarce, in part due to cover by the Siluro-Devonian Cobar Supergroup . Glen et al. (2009) inferred that the Olepoloko Fault continues to the east, south around the Louth Volcanics, then along the northern faults of the Louth-Eumarra shear zone, namely the Little Mountain Fault and Mount Oxley Fault (Figure 2). Other authors dispute the Olepoloko Fault as the boundary between the Lachlan Orogen and the Thomson Orogen, and even the existence of the Thomson Orogen as a separate provincial entity (e.g. Burton, 2010). For clarity, we refer to the Olepoloko Fault and its inferred extensions, which run along the boundary between the two orogens as shown in Figure 1, as the Lachlan-Thomson Boundary Structures (LTBS).
The contiguity or otherwise of the northern Lachlan Orogen-southern Thomson Orogen has implications for mineral systems and prospectivity. The exposed northern Lachlan Orogen is exceptionally well endowed with mineral deposits. Contiguity of crust across the Olepoloko Fault raises the possibility that similar mineral systems might exist under cover in the Thomson Orogen. Alternatively, if the Thomson Orogen represents fundamentally different crust, this has different implications for mineral prospectivity.
Here, we use the oxygen and lutetium-hafnium isotopic systems in zircon to interrogate the nature of the mid-to lower-crustal substrate in the northern Lachlan and southern Thomson orogens, in particular, whether there is a fundamental difference between substrate to the Lachlan and Thomson orogens, and to determine whether the LTBS defines the boundary between two isotopically distinct crustal substrates. We confine our investigation to a narrow time window  to limit the effects of crustal evolution on the isotopic results.

Geological setting
The Lachlan and Thomson orogens are broad (>600 km east-west, Figure 1), and outcrop is poor. Over most of both orogens, the oldest rocks exposed are Ordovician turbiditic rocks (although Cambrian greenstone belts outcrop in Victoria; Crawford & Keays, 1978), and the nature of their substrate is poorly understood. The eastern margin of both orogens is obscured by the predominantly Permian Bowen, Gunnedah and Sydney basins. Glen et al. (2009) divided the Lachlan Orogen into several tectonostratigraphic terranes (Figure 1). The Albury-Bega Terrane is the largest of these by area, spanning the entire north-south extent of the Lachlan Orogen, and is $500 km east-west in places; this terrane has been divided into west and east components. The Macquarie Arc, an extensive volcanic arc system (although alternate models exist, e.g. Quinn, Percival, Glen, & Xiao, 2014), active during the Ordovician and earliest Silurian (ca 489-437 Ma, Percival & Glen, 2007), occurs in belts from the eastern margin of the western Albury-Bega Terrane across the northern part of much of the eastern Albury-Bega Terrane (Figure 2). These belts are thought to represent a single magmatic arc that has subsequently been disjointed by accretion and rifting in the Silurian and Devonian (Percival & Glen, 2007). In the north, abutting the southeastern Thomson Orogen, the Hermidale Terrane is distinguished from the Albury-Bega Terrane by the absence of Upper Ordovician units and the presence of mafic volcanic rocks .
Subdivision of the Thomson Orogen has proven to be more difficult owing to thick Mesozoic-Cenozoic cover. Most outcropping rock in the Thomson Orogen occurs in the far north, in the Greenvale, Charters Towers and Anakie provinces. Sparse exposure of igneous and metasedimentary rocks outcrop around the Eulo Ridge region in southern Queensland (Bultitude & Cross, 2013), the Tibooburra region in northwest NSW , and in the Louth region near the boundary with the Lachlan Orogen . Elsewhere, direct information about pre-Mesozoic geology comes from drill core (Brown et al., 2012;Murray, 1994). Carr, Purdy, and Brown (2014) and Purdy, Cross, Brown, Carr, and Armstrong (2016) made use of the geological information available from drill core to map the distribution of units, and U-Pb zircon dating to determine their provenance.
Magmatic rocks are found close to the surface in the southern Thomson Orogen near the Culgoa lineament: an elongate, narrow aeromagnetic anomaly that represents a significant, deep zone of faulting that is associated with a basement high (Figure 2; Doublier, Purdy, Hegarty, Nicoll & Zwingmann, 2018;Hegarty & Doublier, 2015). Drilling intersected intrusive rock at 184 m (Burtt & Lockheed, 2008).
The upper Cambrian to Lower Ordovician metasedimentary Warratta Group ( Figure 2) has very restricted outcrop in the southwest Thomson Orogen, but it does have a distinct, relatively high magnetic character, which distinguishes it triangle, I-Type; circle, S-type; square, A-type. Samples outlined in red and with red sample numbers are from other publications-refer to text. Major crustal boundaries and faults are marked with black lines, following those illustrated in Glen et al. (2009Glen et al. ( , 2013 and Korsch and Doublier (2015). The Lachlan-Thomson Boundary Structures (LTBS) consist of the Olepoloko Fault, lmf, Little Mountain Fault and mof, Mount Oxley Fault. bf, Boothergandra Fault; rf, Rookery Fault System. Towns are marked by small yellow spots, and the QLD-NSW border is marked by a dashed line. from surrounding units . This distinction suggests that the Warratta Group has a much more extensive sub-surface extent, as interpreted by Purdy, Hegarty, and Doublier (2018). Magmatic rocks of the Tibooburra Suite and Evelyn Creek volcanics intrude the outcropping Warratta Group .
The southwestern Thomson Orogen abuts the upper Precambrian to middle Cambrian sedimentary-volcanic Koonenberry Belt, an exposed region of the Delamerian Orogen . The Koonenberry Belt records development of calc-alkaline volcanism of the Mount Wright Volcanics, which are interpreted to represent a volcanic arc (Greenfield, Musgrave, Bruce, Gilmore, & Mills, 2011). The lower-middle Cambrian Mount Wright Volcanics are only exposed in the Mount Wright region of the Koonenberry Belt . Greenfield et al. (2011) estimated the overall volume, based on aeromagnetic and gravity data and supported by drilling in the Bancannia Trough, to be much more extensive. Gatehouse (1986) suggested that the lower Cambrian Mooracoochie Volcanics in the Warburton Basin to the north may be a northward extension of the Mount Wright Arc.

Scientific approach
The mid-lower crust is not directly observable, although it can be imaged in seismic-reflection studies. The approach taken here is to use felsic magmatic rocks as 'deep crustal probes.' These rocks are interpreted to form by melting within the mid to lower crust before crystallising in the upper crust.
Granite formation is a chemically and isotopically complex process. Mantle-derived mafic magmas can be involved both as a primary isotopic source in an assimilation and fractionation crystallisation sequence, or can behave primarily as a thermal source, driving partial melting of mid-lower-crustal rocks of potentially diverse isotopic composition (Chappell & Stephens, 1988;Gray, 1984;Kemp, Wormald, Whitehouse, & Price, 2005b).
Nevertheless, commonly the only component of these processes that we can physically interact with is the granite itself. Granites should have isotopic signatures that reflect the processes and components that contributed to their formation, and measuring the isotopic composition of accessory minerals in these rocks is one of few insights into their source regions (Keay, Collins, & McCulloch, 1997;Kemp, Whitehouse, Hawkesworth, & Alarcon, 2005a;Kemp et al., 2005b, Kemp et al., 2008. However, final isotopic compositions preserved in these minerals do not necessarily uniquely prescribe the processes and inputs into granite generation (Chappell, 1979). Notwithstanding the possible complexity of petrogenetic processes, isotopic characterisation is particularly valuable in our study area because exposure is so poor that there is no realistic opportunity to collect suites (sensu stricto Chappell & White, 1992) of chemically and isotopically related rocks.
Pioneering studies (e.g. Bennett & DePaolo, 1987) and more recent applications (e.g. Champion, 2013;Gardiner et al., 2017;Mole et al., 2013;Mole et al., 2014) show the potential to use the isotopic character of granites to 'map' the deep crust. Here, we use the O and Hf isotopic signature preserved in zircons from magmatic rocks to investigate character and possible variability of the deep crust beneath the northern Lachlan-southern Thomson Orogens. This study complements other isotopic studies focussed on the Thomson Orogen Purdy et al., 2016).
As indicated above, various tectonic models exist for the Tasmanides, and for the relationship between the Lachlan Orogen and the Thomson Orogen. Some of these models have different implications for the nature of the deep crust. Results from this study may, therefore, provide some testing of these models.

Analytical methods
The results of SHRIMP U-Pb, SHRIMP oxygen isotope and MC-LA-ICP-MS Lu-Hf isotope analyses of zircons from 22 samples are presented here. All U-Pb isotope analyses were performed by other researchers, as cited on a sample-by-sample basis (see Table 1). U-Pb isotope analyses of all zircons used in this study were collected as part of the Geoscience Australia (GA) geochronology program, and the relevant epoxy zircon mounts and grain images were available from the GA mount archive.
Oxygen, then Lu-Hf isotope analyses were performed sequentially on spots from the U-Pb magmatic age population. These isotopic analyses were performed through the SHRIMP and LA-ICP-MS facilities at the Research School of Earth Sciences (RSES) at The Australian National University. The number of O and Lu-Hf isotopic data points retrieved from each sample was directly limited by the number of precursor U-Pb isotope analyses made on zircon from that sample, and by the quality of those zircons. Not all U-Pb isotope analytical spots are suitable for follow-up O and Lu-Hf isotope analyses owing to the larger sampling volume required (see below). This requirement, together with the fact that, in many cases, followup O and Lu-Hf isotope analyses were not envisaged by the analysts at the time of original U-Pb isotope analyses, means that for some samples the number of suitable targets for O and Lu-Hf isotope analyses was limited. Figure 3 illustrates the geometric difference between U-Pb, O and Lu-Hf isotope analytical spots. Analytical O and Lu-Hf session data and full isotopic data tables are provided in the Supplementary Papers (Appendix A).
Oxygen isotope analyses were performed on either of the SHRIMP II or SHRIMP SI instruments at RSES. Ickert et al. (2008) describe the general procedures for SHRIMP oxygen isotope analyses at the RSES facilities. Co-mounted TEMORA2 zircons are used as the primary reference material in all sessions, and other reference zircons were analysed if they were present on the sample mounts. The ablation pit size was on average 25 mm, which excavated pits less than 1 mm deep over the course of the analysis. d 18 O values are reported relative to VSMOW (Vienna Standard Mean Oceanic Water; Baertschi, 1976).  Armistead and Fraser (2015).
Lu-Hf results in the literature are commonly presented as individual spot analyses, with the sole source of uncertainty originating from the uncertainty on the analytical measurement. To properly assess a null-hypothesis that zircon from magmatic rocks represent a single population, we have expanded the Hf spot uncertainty by also incorporating the uncertainty on co-analysed reference materials, as is typically done with SHRIMP U-Pb isotope analyses.
Hf isotope analyses of some samples grouped within statistical uncertainty, and so Hf results from these samples were treated as a single population. We rejected any Hf isotope analyses with 2SE( 176 Hf/ 177 Hf m ) > 0.000200. This mostly arose owing to insufficient integration time during analysis, but where analysis time was sufficient, the high uncertainty was most likely due to isotopic heterogeneity or analysis of multiple domains.
O isotope analyses are more statistically dispersed than Hfanalyses in all samples. The significance of dispersion in the oxygen isotopes from these samples cannot be conclusively determined from this dataset. No images were recorded of the grains post O isotope and prior to Hf isotope analysis so it is not possible to determine whether outliers in the O isotope data could be due to suboptimal spot placement during SHRIMP O isotope analysis. The quality of SHRIMP O isotope analysis is also highly sensitive to mount surface topography (Ickert et al., 2008). Mount degradation may affect O-isotope systematics. The epoxy mounts originally prepared for U-Pb SHRIMP analysis were revisited for O SHRIMP isotope analysis from 16 months (mount GA6273) to 10 years (mount Z4804) after initial preparation.
Diffusion experiments demonstrate that oxygen  diffuses much more readily from zircon than Pb (Cherniak & Watson, 2001), U, Th and Hf (Cherniak, Hanchar, & Watson, 1997). It is therefore possible for geological processes to disturb oxygen isotope distribution in zircon post-crystallisation, with little effect on Pb, U, Th or Hf systematics . For this reason, it is not possible to rule out post-crystallisation factors as a source of the dispersion in the d 18 O values.
Owing to the difficulties in assessing whether the scatter in d 18 O values was due to geological (syn-or post-crystallisation) or analytical causes, only gross outliers or those that were also outliers in Hf isotope analysis for the corresponding spot were excluded from consideration.
A detailed description of the full statistical treatment of O and Hf isotope analyses is provided in the Supplementary Papers (Appendix A).

Results
Magmatic rocks were chosen to represent regions both proximal and distal to the LTBS ( Figure 2). We analysed samples from both the Thomson and the Lachlan orogens to determine whether their isotopic signatures are different. We also analysed samples east-west along the interpreted extent of the LTBS. A combination of 13 outcrop and 12 drill-core samples were utilised in this study. We focussed solely on samples previously dated by SHRIMP, which provided a library of samples for which non-destructive U-Pb isotope analyses were performed. This also allowed us to select samples largely within a 440-410 Ma time window, to minimise temporal effects and maximise the ability to interpret results within a spatial framework. A complete list of samples and their locations is provided in Table 1, and the O and Hf results are plotted in Figure 4. Below, we provide an overview of the isotopic results by spatial domain.

Albury-Bega Terrane samples
The most southerly of the sample groups, these magmatic rocks outcrop on the Nymagee 1:250 000 map sheet, in the Albury-Bega Terrane ) of the central Lachlan Orogen (Figure 2). The Albury-Bega Terrane is characterised by extensive and monotonous Lower to Middle Ordovician turbidite sequences overlain by Upper Ordovician black shale-dominated sequences . The northern section of the western Albury-Bega Terrane is faulted against the northwest component of the Lachlan Orogen to the west along the Boothergandra Fault, and against the Hermidale Terrane  to the east along the Rookery Fault System ( Figure 2). The northwest Albury-Bega Terrane was extensively intruded at 430-410 Ma (e.g. Black, 2007;Bodorkos et al., 2015;Chisholm, Blevin, Downes, & Simpson, 2014;Downes et al., 2016;Isaacs, 2000;Norman, 2004;Spandler, 1998).

Erimeran Granite
Black (2007) documented SHRIMP U-Pb isotope analyses of this sample of Erimeran Granite (1925341). Twenty-one analyses were performed on 21 zircons. Fifteen of these analyses group to form a magmatic crystallisation age of 427.2 ± 2.4 Ma. The remainder were excluded owing to Pb loss (four analyses) and instrumental instability (two analyses).
Thirteen Hf isotope analyses were performed on nine magmatic zircons ( Figure 4a). Two analyses were rejected: one analysis owing to large analytical uncertainty and one because it targeted a possible inherited core. The remaining 11 analyses range from eHf t ¼ 0.282266 to 0.282374 and form a statistically coherent weighted mean 176 Hf/ 177 Hf 422.6 Ma of 0.282313 ± 0.000018 (95% CI, eHf 422.6 Ma ¼ -6.9 ± 0.6, MSWD ¼ 1.6, p ¼ 0.100).

Boolahbone Granite
Black (2007) documented SHRIMP U-Pb isotope analyses of this sample of Boolahbone Granite (1925339). Twenty-six analyses were performed on 26 zircons. Fourteen of these analyses group to form a magmatic crystallisation age of 420.0 ± 2.5 Ma. Six analyses are interpreted to represent Cambrian to earliest Silurian inheritance, five are affected by Pb loss and one contains high common Pb.
Twelve Hf isotope analyses were performed on 12 magmatic zircons (Figure 4a). Rejecting one spot owing to insufficient integration time, and excluding two obvious outliers,   the remaining nine analyses range from eHf t ¼ 0.282234 to 0.282359 and form a population with an arithmetic mean 176 Hf/ 177 Hf 420.0 Ma of 0.282318 ± 0.000092 (2SD, eHf ¼ -7.2 ± 3.2), a weighted mean 176 Hf/ 177 Hf 420.0 Ma of 0.282308 ± 0.000034 and scatter beyond analytical uncertainty (MSWD ¼ 3.4, p ¼ 0.000). Chisholm et al. (2014) documented SHRIMP U-Pb isotope analyses of this sample of Thule Granite (2128514). Thirty-one analyses were performed on 29 zircons. Twenty-eight analyses group to form a magmatic crystallisation age of 425.7 ± 2.4 Ma. The remaining three analyses are attributed to inheritance.
Twenty-five Hf isotope analyses were performed on 25 magmatic zircons (Figure 4a). Rejecting eight analyses owing to high analytical uncertainty, the remaining 17 analyses range from eHf t ¼ 0.282291 to 0.282444 and form a statistically coherent population with a weighted mean 176 Hf/ 177 Hf 425.7 Ma of 0.282346 ± 0.000021 (95% CI, eHf 425.7 Ma ¼ -5.7 ± 0.7, MSWD ¼ 0.66, p ¼ 0.83). Chisholm et al. (2014) documented SHRIMP U-Pb isotope analyses of this sample of the Mount Halfway Volcanics (2128512). Thirty-three analyses were performed on 28 zircons. Twenty-three analyses group to form a magmatic crystallisation age of 422.8 ± 2.6 Ma. Seven analyses recorded inheritance and three were not considered due to instrument instability.
Twenty-three Hf isotope analyses were performed on 22 magmatic zircon grains ( Figure 4a). The mounted zircons were thin and it was difficult to obtain sufficient integration times on these grains, resulting in rejecting one analysis that did not obtain any usable data and nine other grains owing to high uncertainty. Excluding one obvious outlier, the remaining 12 analyses range from eHf t ¼ 0.282203 to 0.282452 and form a statistically coherent weighted mean 176 Hf/ 177 Hf 422.8

Hermidale Terrane samples
This region corresponds to the Hermidale Terrane as described in Glen et al. (2009). Like the Albury-Bega Terrane to the southwest, this domain comprises Lower to Middle Ordovician turbidite and chert sequences, but differs in the absence of Upper Ordovician units and the presence of mafic volcanic rocks ). This terrane is bounded by the Rookery Fault Zone to the southwest, which separates it from the Albury-Bega Terrane, the Tullamore Fault Zone to the east, which separates it from the Macquarie Arc, and the Little Mountain Fault-Mount Oxley Fault of the Louth-Eumarra Shear Zone to the north, which separate it from the Thomson Orogen ( Figure 2). The overlying upper Silurian-Devonian Cobar Basin is intruded by multiple igneous units of mostly I-type composition.
We analysed 10 samples from this domain, all I-type in character. In the south of the Hermidale Terrane, we analysed the 420.6 ± 2.8 Ma Yellow Mountain Granite (Black, 2007). From the northern half of the Hermidale Terrane: the 422.6 ± 2.4 Ma Glenariff Granite (Black, 2007) from 'DDH1' from the Glenariff D2000 program drilled by the Department of Mineral Resources in 1996; a 421.9 ± 3.5 Ma unnamed granite (Fraser et al., in prep.) 21 km to the southwest of the hamlet of Gongolgon in the Glen Idyll Prospect from diamond drill core TDBTRCD002 drilled by Straits Resources Limited; the outcropping 416.8 ± 2.5 Ma Byrock Granite (Black, 2007); the 430.4 ± 2.6 Ma Knightvale Granite (Black, 2007) from diamond drill hole drilled by CRA Exploration Pty Ltd in 1978; and the 417.8 ± 3.2 Ma Galambo Granite (Fraser et al., in prep.) from drill hole 5064/ CAC-05.5, drilled by Croesus Mining NL in 1997. Two younger granites were analysed, the 235.2 ± 1.5 Ma Midway Granite (Black, 2006), and a 230.8 ± 1.4 Ma unnamed quartz porphyry dyke (Black, 2006) to the southwest of the Midway Granite. The Midway Granite is an extremely fractionated I-type granite and is associated with Sn mineralisation at the Doradilla prospect (Blevin, 2011;Burton, Trigg, & Black, 2007). The Midway Granite and associated dyke were analysed to understand the relationship between these younger, spatially restricted magmatic rocks and the more common and widespread 430-410 Ma occurrences. We also analysed the outcropping 408.9 ± 2.4 Ma Mount Kelly Granite (Black, 2006) and the 442.1 ± 2.1 Ma Honeybugle Granite (Fraser et al., 2014). The Honeybugle Granite is older than the bulk of the samples in this study, but its inclusion aids with investigation of the influence of the proximal Macquarie Arc.

Yellow Mountain Granite
Black (2007) documented SHRIMP U-Pb isotope analyses of this sample of Yellow Mountain Granite (1925340). Twentythree analyses were performed on 23 zircons. Only eight of these analyses are interpreted to represent magmatic crystallisation at 420.6 ± 2.8 Ma. The remaining 15 analyses are interpreted to represent inheritance.
Six Hf isotope analyses were performed on six magmatic zircons (Figure 4b). Excluding one outlier, the remaining five analyses range from eHf t ¼ 0.282372 to 0.082451 and form a statistically coherent population with a weighted mean 176 Hf/ 177 Hf 420.6 Ma of 0.282414 ± 0.000039 (eHf 420.6 Ma ¼ -3.4 Glenariff Granite Black (2007) documented SHRIMP U-Pb isotope analyses of this sample of Glenariff Granite (1925854). Twenty-three analyses were performed on 22 zircons. Fourteen group to form a magmatic crystallisation age of 422.6 ± 2.4 Ma. The remainder were affected by Pb loss (8) or analysed an area of corerim overlap.
Twelve Hf isotope analyses were performed on 12 magmatic zircons grains (Figure 4b). Rejecting one analysis owing to high analytical uncertainty, the remaining 11 analyses range from eHf t ¼ 0.282402 to 0.282565 and form an arithmetic mean 176 Hf/ 177 Hf 422.6 Ma of 0.282494 ± 0.000133 (2SD, eHf 422.6 Ma ¼ -0.5 ± 4.7), a weighted mean 176 Hf/ 177 Hf 422.6 Ma of 0.282504 ± 0.000042 (95% CI), and scatter beyond analytical uncertainty (MSWD ¼ 7.2, p ¼ 0.000). Fraser et al. (in prep.) documented SHRIMP U-Pb isotope analyses of this sample of unnamed biotite granite at the Glen Idyll Prospect (2169065). Twenty-eight analyses were performed on 28 zircons. Twenty-one analyses group to form a magmatic crystallisation age of 421.9 ± 3.5 Ma. The remaining seven analyses had high common Pb.

Byrock Granite
Black (2007) documented SHRIMP U-Pb isotope analyses of this sample of Byrock Granite (1925852), where it was referred to as 'Westella Granite.' Twenty-three analyses were performed on 23 zircons. Thirteen analyses group to form a magmatic crystallisation age of 416.8 ± 2.5 Ma. The remaining analyses were attributed to inheritance (5 spots) and radiogenic Pb loss (5 spots).
Nine Hf isotope analyses were performed on nine magmatic zircon grains (Figure 4b). Rejecting three analyses owing to high analytical uncertainty, the remaining six analyses range from eHf t ¼ 0.282528 to 0.282673 and form a Knightvale Granite Black (2007) documented SHRIMP U-Pb isotope analyses of this sample of Knightvale Granite (1925853). Twenty-two analyses were performed on 22 zircons. Ten group to form a magmatic crystallisation age of 430.4 ± 2.6 Ma. Eleven targeted inheritance, and one was rejected because it was situated on a crack.
Eight Hf isotope analyses were performed on eight magmatic zircon grains (Figure 4b). We rejected one analysis because it did not produce any usable data, and another because of high analytical uncertainty. The remaining six analyses range from eHf t ¼ 0.282490 to 0.282605 and form an arithmetic mean 176 Hf/ 177 Hf m of 0.282577 ± 0.000087 (2SD, eHf 430.4 Ma ¼ þ1.2 ± 3.4), a weighted mean 176 Hf/ 177 Hf m of 0.282554 ± 0.000052 and scatter beyond analytical uncertainty (MSWD ¼ 2.4, p ¼ 0.037). Fraser et al. (in prep.) documented SHRIMP U-Pb isotope analyses of this sample of Galambo Granite (2169078). Thirty-two analyses were performed on 32 zircons. Twenty-six analyses grouped to form a magmatic crystallisation age of 417.8 ± 3.2 Ma. Five analyses had high common Pb, and one was affected by Pb loss.
Unnamed quartz porphyry dyke, Doradilla Black (2006) documented SHRIMP U-Pb isotope analyses of this sample of unnamed quartz porphyry dyke (1681319), which occurs as part of a dyke swarm near the Midway Granite. Thirty-eight analyses were performed on 38 zircons. Thirty-four of these spots have an age range of 236-224 Ma, and form a magmatic crystallisation age of 230.8 ± 1.4 Ma. One spot recorded earliest Triassic inheritance, and three spots are interpreted to have been affected by loss of radiogenic Pb.
Twenty-five Hf isotope analyses were performed on 25 magmatic zircon grains (Figure 4c). Rejecting two analyses because of high analytical uncertainty, in addition to the removal of one outlier (also an outlier in O isotope analysis), the remaining analyses range from eHf t ¼ 0.282590 to 0.282736 and form a statistically coherent weighted mean 176 Hf/ 177 Hf 230.7 Ma of 0.282687 ± 0.000015 (eHf 230.7 Ma ¼ þ2.1 ± 0.5, MSWD ¼ 1.4, p ¼ 0.102). Black (2006) documented SHRIMP U-Pb isotope analyses of this sample of Mount Kelly Granite (1681321). Thirty-eight analyses were performed on 38 zircons. Thirty-seven of these analyses range in age from 421 to 398 Ma and group to form a magmatic crystallisation age of 408.9 ± 2.4 Ma. The remaining spot was affected by Pb loss.
Twenty-five Hf isotope analyses were performed on 25 magmatic zircon grains (Figure 4c). Rejecting the final three analyses owing to possible analytical instability, the remaining 22 analyses range from eHf t ¼ 0.282722 to 0.282828 and form a statistically coherent group with a weighted mean 176 Hf/ 177 Hf 408.9 Ma of 0.282759 ± 0.000014 (95% CI, eHf 408.9 Ma ¼ þ8.5 ± 0.5, MSWD ¼ 0.90, p ¼ 0.59). Fraser et al. (2014) documented SHRIMP U-Pb isotope analyses of this sample of Honeybugle Complex diorite (2129191). Twenty-eight analyses were performed on 27 zircon grains. Twenty-seven analyses group to form a magmatic crystallisation age of 442.1 ± 2.1 Ma. The remaining analysis was not considered owing to high common Pb.
Twenty-eight Hf isotope analyses were performed on 27 magmatic grains (Figure 4c). Rejecting four analyses owing to high analytical uncertainty and one that targeted a region identified to have high common Pb, the remaining 23 analyses range from eHf t ¼ 0.282633 to 0.282783 and form a statistically coherent population with a weighted mean 176 Hf/ 177 Hf 442.1 Ma of 0.282723 ± 0.000014 (95% CI, eHf 442.1 Ma ¼ þ8.0 ± 0.5, MSWD ¼ 1.4, p ¼ 0.09).

Southwest Thomson Orogen samples
The surface of the southwest Thomson Orogen is largely covered by unconsolidated sediments, however there are three exposures of upper Cambrian to Lower Ordovician sediments of the Warratta Group on or near the boundary of the Thomson Orogen, separated from the Koonenberry Belt by the Olepoloko Fault (Figure 2; Greenfield et al., 2010). The Warratta Group is interpreted to have an extensive sub-surface extent . A gravity high is observed under the Warratta Group, interpreted to be magmatic underplating, and the surface expression of this geophysical feature is interpreted to be the Tibooburra Suite, which intrudes parts of the Warratta Group . Geochemistry of rocks from the Tibooburra Suite suggests they have a heterogeneous mantle source, with variable amounts of crustal contamination . The Koonenberry Belt hosts the lower-middle Cambrian Mount Wright Arc, interpreted to be volcanic arc system, oriented with the back-arc to the southwest (Greenfield et al., 2011).
We analysed two samples from the Tibooburra Suite: the 420.5 ± 3.2 Ma I-type Tibooburra Granodiorite (Black, 2007), and a 420.2 ± 3.3 Ma I-type hornblende diorite of the Dynamite Tank Granodiorite (Black, 2007). Si egel (2015) reports the U-Pb, O and Hf isotopic data from the 419.0 ± 2.7 Ma S-type Wolgolla Granite from the base of petroleum exploration well WOLGOLLA 1 (drilled by Delhi in 1973), which is also relevant to this study. Black (2007) documented SHRIMP U-Pb isotope analyses of this sample of the Tibooburra Granodiorite (1925330). Eightythree analyses were performed over several analytical sessions. Fifty-eight analyses group to form a magmatic crystallisation age of 420.5 ± 3.2 Ma. A further 19 analyses are interpreted to also represent magmatic crystallisation but are from an aberrant SHRIMP session. A single analysis was attributed to inheritance, four were interpreted to have been affected by Pb loss, and a single analysis contained high common Pb.

Dynamite Tank Granodiorite
Black (2007) documented SHRIMP U-Pb isotope analyses of this sample of hornblende diorite (1925331), interpreted to be a marginal phase of the Dynamite Tank Granodiorite  near Tibooburra. Eighty-two analyses were performed over several analytical sessions. Fifty-eight analyses group to form a magmatic crystallisation age of 420.2 ± 3.3 Ma. A further 18 analyses are interpreted to represent magmatic crystallisation but are from an aberrant SHRIMP session. Four analyses were interpreted to represent inheritance, and two analyses were young outliers to the magmatic population.

Southeast Thomson Orogen samples
Surface exposure of granite is sparse in the southeast Thomson Orogen. We investigated several samples from within a few kilometres to the north of the eastern Olepoloko Fault, the Little Mountain Fault and the Mount Oxley Fault (Figure 2).
We analysed four plutonic rocks from drill core in the Cuttaburra region: a 428.2 ± 3.1 Ma medium-grained, biotiterich I-type granite (Armistead & Fraser, 2015) from drill hole F1DD02 (Thomson Resources, drilled in 2013), a 424.5 ± 4.9 Ma leucocratic granitic I-type dyke (Armistead & Fraser, 2015) from drill hole F1DD01 (Thomson Resources, drilled in 2013), a 428.3 ± 2.8 Ma I-type granodiorite named 'Cuttaburra granite' (Chisholm et al., 2014) from drill hole CUTAD01 (Thomson Resources, drilled in 2011), and a 429.0 ± 8.5 Ma biotite-rich S-type quartz diorite (Armistead & Fraser, 2015) from drill hole CUTACD02 (Thomson Resources, drilled in 2011). Armistead et al. (2017) performed petrographic and geochemical classification of these rocks. Although these rocks were described as geochemically transitional between S-and I-type, the researchers acknowledged that some samples were intensely hydrothermally altered. Petrography of the S-type diorite from CUT1CD02 revealed 2-3 mm domains suggestive of xenoliths of sedimentary rock, suggesting that the bulk-rock geochemistry of this rock may be affected by country-rock contamination (Armistead et al., 2017). The d 18 O values from these samples are more suggestive of all of these samples being I-type (see below).
We also analysed the S-type 420.9 ± 2.3 Ma Brewarrina Granite (Bodorkos et al., 2013) farther to the east, to the north of the Mount Bendemeer Fault.
Unnamed granite, F1 prospect Armistead and Fraser (2015) documented SHRIMP U-Pb isotope analyses of this sample of unnamed granite (2167675) from the F1 prospect. Forty-two analyses were performed on 40 zircon grains. Thirty-one analyses group to form a magmatic crystallisation age of 428.2 ± 3.1 Ma. Ten other analyses targeted inheritance, and one was interpreted to have been affected by Pb loss.
Thirty O isotope analyses were performed on 30 magmatic zircons (Figure 4e). Excluding one spot as an outlier that may have been situated on an inclusion, the remainder range from d 18 O ¼ 6.02 to 8.35‰, with a weighted mean of 6.94 ± 0.23‰ (95% CI), an arithmetic mean of 6.94 ± 1.13‰ (2SD) and scatter beyond analytical uncertainty (MSWD ¼ 6.0, p ¼ 0.000).
Unnamed granitic dyke, F1 prospect Armistead and Fraser (2015) documented SHRIMP U-Pb isotope analyses of this sample of unnamed granitic dyke (2167701) from the F1 prospect. Twenty-seven analyses were performed on 22 zircon grains. Thirteen of the analyses group to form a magmatic crystallisation age of 424.5 ± 4.9 Ma. The remaining analyses represent inheritance.
Unnamed granite, Cuttaburra Chisholm et al. (2014) documented SHRIMP U-Pb isotope analyses of this sample of unnamed granite (2128516) near Cuttaburra. Twenty-three analyses were performed on 21 zircons. Nineteen analyses group to form a magmatic crystallisation age of 428.3 ± 2.8 Ma. The remaining four analyses are attributed to inheritance.
Twenty-three Hf isotope analyses were performed on 21 zircons (Figure 4e). Many of the mounted zircons were thin, and it was difficult to obtain sufficient integration time. Rejecting 10 grains owing to high analytical uncertainty, and four because they targeted inherited regions of grains, the remaining nine analyses range from eHf t ¼ 0.282198 to 0.282380 and form a statistically coherent population with a weighted mean 176 Hf/ 177 Hf 428.3 Ma of 0.282314 ± 0.000029 (95% CI, eHf 428.3 Ma ¼ -6.8 ± 1.6, MSWD ¼ 1.8, p ¼ 0.067).
Unnamed quartz diorite, Cuttaburra Armistead and Fraser (2015) documented SHRIMP U-Pb isotope analyses of this sample of unnamed quartz diorite (2167668) from drill hole CUTACD02. Forty-two analyses were performed on 41 zircon grains. Six of these analyses group to form a magmatic crystallisation age of 429.0 ± 8.5 Ma. Twenty-five of the remaining analyses represent inheritance, and 11 others were not considered owing to high common Pb.
Six Hf isotope analyses were performed on six magmatic zircon grains (Figure 4e). The analyses range from eHf t ¼ 0.282168 to 0.282307 and give an arithmetic mean 176 Hf/ 177 Hf 429.0 Ma of 0.282262 ± 0.0000102 (2SD, eHf 429.0 Ma ¼ -8.6 ± 3.6), a weighted mean 176 Hf/ 177 Hf 429.0 Ma of 0.282258 ± 0.000060, and scatter beyond analytical uncertainty (MSWD ¼ 2.8, p ¼ 0.017). Bodorkos et al. (2013) documented SHRIMP U-Pb isotope analyses of this sample of the Brewarrina Granite (2002535). Forty analyses were conducted on 34 zircon grains. Thirty-two group to give a magmatic crystallisation age of 420.9 ± 2.3 Ma. Seven analyses in the range 1729-466 Ma were interpreted as inherited, and one analysis was interpreted to have been affected by Pb loss.

Inboard Thomson Orogen samples
The samples in this group were emplaced at least 100 km from the LTBS in the Thomson Orogen (Figure 2). The Thomson Orogen is generally composed of covered Neoproterozoic to Ordovician metasedimentary and metavolcanic rocks (Purdy et al., 2016). Owing to sparse outcrop and thick cover by sediments and sedimentary rocks in this region, samples of igneous rock are not common. Granite exposures outcrop along the Eulo Ridge, a series of basement inliers with sparse surface expression (Bultitude & Cross, 2013). Elsewhere, basement drill core provides the only means of sampling in situ igneous material.
The scarcity of samples meant that we did not have any available samples from our chosen time window, but other researchers have analysed samples relevant to this study. Cross et al. (2018) report U-Pb, O and Hf isotope data from the 425.4 ± 6.6 Ma S-type Ella Granite from the base of the ELLA 1 petroleum well drilled by Delhi in 1986 and the 419.1 ± 2.5 Ma S-type Hungerford Granite. In addition, the 401.8 ± 3.1 Ma Itype Tinchelooka Diorite (Bodorkos et al., 2013) was analysed to aid in coverage. This sample is slightly younger, and it is from a SW-NE-trending region of high magnetic intensity associated with the Culgoa Lineament . Bodorkos et al. (2013) documented SHRIMP U-Pb isotope analyses of this sample of the Tinchelooka Diorite (2002537). Twenty-eight analyses were performed on 20 zircons. Twenty of these analyses group to give a magmatic crystallisation age of 401.8 ± 3.1 Ma. Seven analyses between 2989 Ma and 424 Ma were interpreted as inherited, and one grain was interpreted to be affected by loss of radiogenic Pb.
Nineteen Hf isotope analyses were performed on 12 magmatic zircons (Figure 4f). Rejecting two spots owing to high analytical uncertainty, the remainder fall into two groups: the first group consists of 14 analyses that range from eHf t ¼ 0.282628 to 0.282712 and form a statistically coherent weighted mean 176 Hf/ 177 Hf 401.8 Ma of 0.282661 ± 0.000016 (95% CI, eHf 401.8 Ma ¼ þ4.9 ± 0.5; MSWD ¼ 1.2, p ¼ 0.30). The second group (three analyses) range from eHf t ¼ 0.282465 to 0.282472 and give a weighted mean 176 Hf/ 177 Hf 401.8 Ma of 0.282469 ± 0.000051 (95% CI, eHf 401.8 Ma ¼ -1.9 ± 1.8; MSWD ¼ 0.03, p ¼ 0.97). The significance of this second group is unknown; it may represent a different but approximately coeval batch of magma that assimilated a greater amount of (meta)sedimentary material.

Statistical treatment and depiction of data
When we apply statistical grouping to our data, we find that most of our samples have Hf isotopic results that form statistically coherent groupings with MSWDs that approach 1. This suggests that in many cases the Hf isotopic compositions of these rocks are homogeneous within the limits of analytical uncertainty, either because these granites originate from a single homogeneous source or because mixing of multiple sources was efficient enough to homogenise the Hf isotopic compositions, at least at sample-scale.
Oxygen-isotope values are more scattered, but it is unclear whether this is due to analytical, syn-or post-crystallisation effects. Oxygen is more susceptible to disturbance than U, Th, Pb or Hf in zircon (Cherniak & Watson, 2001;Cherniak et al., 1997; so samples with evidence for post-crystallisation disturbance of U-Pb systematics (i.e. Pb loss) are also more likely to have experienced disturbance of their d 18 O values. We reduce, but do not eliminate, the impact of post-crystallisation disturbance by analysing zircon spots that do not appear to be disturbed in U-Pb isotope systematics. For example, the high number of U-Pb spots attributed to Pb loss in samples indicates a higher probability of oxygen isotope disturbance across all zircons from the sample.
Where data scatter beyond the limits of analytical uncertainty, it is less statistically valid to apply a weighted mean calculation to a population that may have two or more distinguishable components. However, in the interests of applying a consistent statistical treatment to all data, and also to enable the use of MSWD to assess statistical coherence and test for the presence or absence of such complexity, we report both weighted and arithmetic means in Table 1. Arc-like' crust is shown as a yellow band on the lower plot, with a 176 Lu/ 177 Hf composition of 0.023 (estimated from bulk-rock geochemical data in Kemp et al., 2009). All plotted data are weighted mean ± 95% confidence interval, except for Kemp et al. (2009) data, which is plotted as arithmetic mean ± 2 standard deviations. Where the age uncertainty was not provided by Kemp et al. (2009), data points were assigned an arbitrary value of ± 5 Ma (95% CI).
Twenty-five samples are plotted in Figure 5 using weighted means ± 95% confidence intervals. The uncertainties plotted are smaller than ±2SD uncertainties, but the size of the error ellipses in Figure 5 reflects both scatter in the population and the number of analyses of the population. Larger error ellipses reflect more scattered data, fewer data points in the population or both, giving a visual indication of the reliability of the calculated weighted mean.

Isotopic trends
Ignoring the Triassic Midway granites in the Hermidale Terrane, the range of U-Pb ages of granites analysed for O and Lu-Hf isotopes in this study spans approximately 40 million years (440-400 Ma; Figure 5b). Despite this relatively narrow time window (which precludes significant secular variation in the isotopic nature of the source regions), Figure 5a illustrates the wide range of eHf t and d 18 O values measured across the area of interest, a range that cannot be explained by simple two-component mixing between a single radiogenic source and a single unradiogenic source.
Rather, the data in Figure 5a appear to define two distinct mixing lines. Both have an endmember with radiogenic Hf and mantle-like d 18 O values; one has an unradiogenic-Hf endmember with elevated d 18 O values (Trend A), and the other has an unradiogenic-Hf endmember with more subdued d 18 O values (Trend B).
The common, radiogenic-Hf and mantle-like d 18 O value endmember is represented most strongly in granites from the Hermidale Terrane, and in the Tinchelooka Diorite. These granites are more radiogenic (eHf t up to þ8.5) and mantlelike/infracrustal (d 18 O as low as 5.52‰).
Other samples appear to be mixtures of two of these three components, but only between radiogenic-unradiogenic eHf pairs, and not between the two unradiogenic endmembers. Given the complexity of granite formation, it is unlikely that any samples represent pure endmember compositions. The following sections discuss possible explanations for these isotopic patterns, with particular emphasis on spatial trends that can be related to previously proposed tectonostratigraphic subdivision of the Lachlan and Thomson orogens. These major spatial trends are illustrated in Figure 6.

High d 18 O-unradiogenic eHf endmember of Trend A
Middle to upper Silurian S-type magmatic rocks of the northern Albury-Bega Terrane display unradiogenic eHf t values and elevated d 18 O values. Our results are consistent with measured Hf isotope ratios from granites from further south in the Lachlan Orogen (Kemp, Hawkesworth, Collins, Gray, & Blevin, 2009; Figure 5b). The limited range of eHf t and d 18 O mean values (relative to the entire range across the 22 samples reported here) identified across much of the Lachlan Orogen indicates remarkable uniformity in the source region over a large spatial extent (hundreds of kilometres).
The source of S-type Lachlan Orogen granites has long been attributed to a metasedimentary origin (e.g. Chappell & White, 1992), commonly attributed to the Ordovician turbiditic metasediments that exist across much of the Lachlan Orogen (e.g. McCulloch & Chappell, 1982). The bulk-rock eNd signature of Ordovician Lachlan Orogen metasediments has been measured by McCulloch and Chappell (1982; State Circle Shale eNd ¼ -9.7; Cooma Gneiss eNd ¼ -8.3) and McCulloch and Woodhead (1993; country rock surrounding the Bega Batholith, eNd ¼ À10.5, À9.9, À9.7, À9.5). We use the terrestrial trend of Vervoort, Patchett, Blichert-Toft, and Albar ede (1999; eHf ¼ 1.36eNd þ 2.95) to convert these values to estimates of eHf (we assume no isotopic decoupling), giving an estimate of eHf ¼ À11.3 to À8.3 for bulk Lachlan Orogen Ordovician metasediments. This range is plotted on Figure 5a, with d 18 O >10‰ as per the Eiler (2001) range for clastic sediments. The Albury-Bega Terrane samples from this study plot in more radiogenic and lower d 18 O space, suggesting that even the most supracrustal and unradiogenic granites in this study are not formed purely from melting of this supracrustal source, and do not represent true endmember composition.
Furthermore, upper Silurian S-type granites with very similar eHf t and d 18 O values appear to be widespread on the northern side of the LTBS, across the breadth of the southern Thomson Orogen from Ella Granite in southwestern Queensland, through the Hungerford Granite in the interior, to the Brewarrina Granite in the east, proximal to the Macquarie Arc. This suggests that supracrustal rocks of broadly similar isotopic character floor much of both the Albury-Bega Terrane and the southern Thomson Orogen. This may indicate some geological continuity of the Lachlan Orogen into the Thomson Orogen. Alternatively, there may be another isotopically similar supracrustal source in the Thomson Orogen with no genetic relationship to the source in the Lachlan Orogen.

Low d 18 O-unradiogenic eHf endmember of Trend B
On the northern side of the central (east-west-trending) region of the LTBS, mid-Silurian granites in the Cuttaburra region display unradiogenic eHf t values spanning a range similar to that observed in the Albury-Bega Terrane (eHf t ¼ -8.8 to -6.7). They are, however, distinct in terms of their d 18 O values, which are only slightly elevated above the mantle-like window (d 18 O ¼ 6.9-7.3‰). The Cuttaburra granites are interpreted to intrude the upper Cambrian to Lower Ordovician Warratta Group , a unit with little surface expression but significant sub-surface extent .
Similarly subdued d 18 O values were measured in the upper Silurian I-type rocks of the Tibooburra Suite (ca 420 Ma, eHf t ¼ -1.8 to -0.5, d 18 O ¼ 6.4 to 6.5‰) in the southwestern Thomson Orogen where they intrude outcropping rocks of the Warratta Group ( Figure 6). These Tibooburra rocks have a stronger isotopic influence from a radiogenic component (Figure 5a). The combined Cuttaburra and Tibooburra data indicate that granites intruding the domain interpreted as Warratta Group (Figure 6) have an isotopically distinct source component different to the supracrustal source component flooring much of the northern Albury-Bega Terrane and southern Thomson Orogen. The muted d 18 O values of the region suggests that this source component may be infracrustal rather than supracrustal, for example a meta-igneous source.

Low d 18 O-radiogenic eHf endmembers of Trend A and B
Across much of the study region, granites seem to have formed from the isotopic combination of the unradiogenic sources outlined above with one or more radiogenic sources with mantle-like/infracrustal isotopic character. The potential identity of this component varies with region, and so we will discuss options region-by-region, for (1) the Hermidale Terrane, (2) the southwest Thomson Orogen and (3) the inboard southern Thomson Orogen.

Hermidale Terrane
In the northeastern Lachlan Orogen, the Hermidale Terrane represents a crustal segment isotopically distinct from both the Albury-Bega Terrane to the west, and the Thomson Orogen to the north. In general, the samples analysed are Lower Devonian I-type granites (although the Honeybugle Complex is lower Silurian and the Midway granites are Triassic), their eHf t values are significantly more radiogenic (eHf t ¼ -3.4 to þ8.5) than elsewhere, and d 18 O values (d 18 O ¼ 5.5 to 8.0‰) are much more mantle-like/infracrustal. These measured Hf-O values are broadly consistent with twocomponent mixing between a radiogenic-eHf t , mantle-liked 18 O source and an unradiogenic-eHf t , elevated-d 18 O values supracrustal source (Trend A; Figure 5a).
Ordovician arc rocks, possibly related to the Macquarie Arc, are one potential identity of the radiogenic source component. The Macquarie Arc outcrops to the east of the Hermidale Terrane and to the east of the Albury-Bega Terrane further south of this study area. Seismic-reflection profiling in the Temora region provides evidence that Ordovician volcanic rocks underlie the eastern part of the Wagga Belt in the Albury-Bega Terrane, representing a westerly continuation of Macquarie Arc rocks undercover (Glen et al., 2002). Although this is not direct evidence that the same geometry exists under the Hermidale Terrane, the seismic-reflection profiling evidence does permit Ordovician arc rocks to extend west of the outcropping Macquarie Arc and underlie the Hermidale Terrane. Kemp et al. (2009) analysed Hf isotopes on zircons from Macquarie Arc rocks, and they are plotted relative to our samples in Figure 5b. The Honeybugle Complex (2129191) is the most radiogenic sample in this study, and overlaps the Macquarie Arc field in both age and Hf isotopic composition. The isotopic similarity between these rocks, and their close spatial proximity, suggests that the Honeybugle Complex may be closely genetically related to the Macquarie Arc. The Mount Kelly Granite (1681321) is younger than the youngest known Macquarie Arc rocks, but falls within the Hf evolution window for the Macquarie Arc, consistent with being sourced from Macquarie Arc-like material at depth, with little mixing with other sources. All other samples in the Hermidale Terrane fall between the supracrustal source component composition and the Macquarie Arc evolution window (Figure 5b), and all isotopic variations in this region can be explained by variable mixing between the two components.
There is a loose correlation between radiogenic Hf signature and proximity to the Macquarie Arc, which may suggest thickening of this component towards the surface expression of the arc, but may also be explained by a range of other geological processes not easily evaluated here, such as depth of melting.
Others (Keay et al., 1997;Kemp et al., 2005b) have considered Ordovician arc rocks as an end member source component for granites in other locations in the Lachlan Orogen, however these researchers favoured direct mantle input at the time of granite generation rather than remelting of emplaced arc rocks. Our isotopic study cannot resolve between the two possibilities (or whether the composition may result from a combination of both options), partly because all rocks in this study incorporate some degree of supracrustal influence, and partly owing to the isotopic similarity of the potential radiogenic sources.
The younger Midway granites have mantle-like O and Hf isotope signatures that can be derived from the same Trend A mixed source as the older granites in the region. Additional Triassic mantle input is not required to explain the O-Hf isotopic signature of these granites, but, as with the other samples, we cannot exclude this possibility.

Southwest Thomson Orogen
Farther to the west and within the interpreted extent of the Warratta Group (Figure 6), the Tibooburra samples have an Hf isotope signature that is also suggestive of contribution from a radiogenic source component. One viable source is the Mount Wright Arc, which occurs nearby to the southwest. The radiogenic component expressed in the Tibooburra granites may reflect Mount Wright Arc-type material at depth in this region, beneath the Warratta Group. This would be consistent with a northeasterly dip for the Olepoloko Fault in this region, juxtaposing Warratta Group in the hanging wall with Mount Wright Arc rocks in the footwall.
The geometry of the Mount Wright Arc proposed by Greenfield et al. (2011) is consistent with radiogenic material occurring along the Tibooburra region and possibly north (i.e. inclusive of the Wolgolla Granite for which eHf and d 18 O also indicate a radiogenic source component), but not in the Cuttaburra region. Keay et al. (1997) proposed a three-component model for granite genesis in the Lachlan Orogen. Their components were an Ordovician turbidite component, a depleted mantle component and a mafic crustal component. Their mafic crustal component was attributed to Cambrian greenstones, the eNd signature of which (Nelson, Crawford, & McCulloch, 1984) spans the equivalent of our entire eHf range for Trend B (Figure 5a). Given geochemical indications that the Cambrian greenstones, preserved in the Lachlan Orogen in Victoria, are likely to have assimilated variable amounts of sedimentary material during formation (Whelan, Hergt, & Woodhead, 2007), it is unsurprising that the greenstones span a wide range of eNd (and presumably eHf).
Trend B (Figure 5a) is consistent with Keay's model, where radiogenic mafic greenstone-type rocks are emplaced in the crust and are represented by the radiogenic endmember of Trend B. These mafic rocks may have assimilated variable amounts of sedimentary material, so emplaced mafic rocks could be more unradiogenic than the pure mantle-derived melt. Subsequent melting of these greenstone belts during granite generation is also likely to melt spatially associated sedimentary units, which will also drive the isotopic signature to a more unradiogenic isotopic character. One potential issue with this model comes from the more muted d 18 O values than might be expected if the unradiogenic component was contaminated by sediment.
If greenstone rocks were the source of granites in the southwest Thomson Orogen, this would suggest that there may be some genetic relationship between the Mount Wright Arc and the greenstone belts in Victoria. If correct, this in turn demonstrates another commonality between the Lachlan Orogen and the southern Thomson Orogen.
Analysis of geochemical data led to the interpretation by Greenfield et al. (2010) that the Tibooburra Suite granites formed by partial melting of a mantle source that was contaminated by crustal sources. A direct mantle source is also consistent with our isotopic data.

Tinchelooka Diorite
The marginally younger (ca 402 Ma) I-type Tinchelooka Diorite is spatially associated with the highly magnetic Culgoa lineament in the central southern Thomson Orogen, and is characterised by significantly more radiogenic (albeit bimodal) eHf t and d 18 O values elevated only marginally above the 'mantle-like' window. The geology of this region is poorly known, so the identity of the radiogenic source component of the Tinchelooka Diorite is much more cryptic.
Mafic/ultramafic samples are identified along the Culgoa lineament . The age of these mafic/ultramafic Culgoa lineament rocks (at depth) is unknown, but if they were the right age (i.e. slightly but not significantly older than the diorite) they might have influenced the isotopic signature of the Tinchelooka Diorite. Alternatively, the radiogenic source for the Tinchelooka Diorite may be caused by direct mantle input.

Nature of the LTBS-testing regional models
The nature of the LTBS appears to change in character across the southern margin of the Thomson Orogen. To the west, previous authors have outlined the evidence for the boundary between the Delamerian Orogen and the Thomson Orogen occurring at or near the Olepoloko Fault in the southwest of the Thomson Orogen (e.g. Glen et al., 2013;Greenfield et al., 2010). The isotopic results for the Tibooburra Suite provide evidence that the Thomson Orogen and Delamerian Orogen must have been in their current configuration by 420 Ma, if these granites record the influence of arc material in their source regions. This in turn suggests that subsequent late Paleozoic SSE-directed movement on the eastern segment of the Olepoloko Fault (which folded ca 385 Ma Tabberabberan structures in the Cobar Basin into E-W orientations; Downes et al., 2016) did not generate significant strike-slip movement on the western segment of the Olepoloko Fault (which separates the Delamerian Orogen from the southwest Thomson Orogen).
Our results also show an isotopic distinction between the southeastern Thomson Orogen and the Hermidale Terrane ( Figure 6). The Hermidale Terrane has a distinct mixture of radiogenic and unradiogenic source components that appears to terminate at major fault boundaries. The Little Mountain-Mount Oxley Fault and the Rookery Fault appear to define the limits of underlying radiogenic material (potentially Macquarie Arc-related).
There is no distinguishable Hf or O isotopic difference between the bulk of the southern Thomson Orogen and the bulk of the Lachlan Orogen, as represented by the Albury-Bega Terrane. In the south-central region of this study area, these two regions adjoin each other, and orogen-bounding faults have not been identified in this area. The faulting to the west (Olepoloko Fault) and the faulting to the east (Little Mountain Fault and Mount Oxley Fault) are possibly unrelated structures separated by a narrow region that remains an unbroken connection between the stratigraphically coherent Lachlan Orogen and southern Thomson Orogen.
There is no unique solution that resolves the available isotopic evidence; however, some inferences can be drawn. Although data coverage remains sparse, there is a lack of radiogenic Hf and mantle-like d 18 O values along the southern margin of the Thomson Orogen that would support the presence of an intact east-west-trending volcanic arc related to north-dipping subduction along the Olepoloko Fault as inferred by Glen (2013) and Gray and Foster (2004).
Evidence that the Little Mountain-Mount Oxley Fault system to the east represents the boundary between the Thomson Orogen and the Lachlan Orogen may be confounded by the presence of the Hermidale Terrane, an isotopically unique section of crust more strongly influenced by a radiogenic source component than the surrounding regions to the north, west or south.
The relationship between the Thomson Orogen and the Lachlan Orogen could be further tested by finding distinctions between sections of crust, which are truly representative of the bulk of these orogens, namely the bulk of the southern Thomson Orogen excluding the far-western component and the magnetic high associated with the Culgoa lineament, and the Lachlan Orogen as represented by the Albury-Bega Terrane. Despite the limited data coverage caused by sparse sample availability, it is clear that the Thomson Orogen is more internally complex than current mapping reveals.

Conclusions
The isotopic character of all samples in this study can be best described by three-component mixing of variable amounts of high d 18 O-unradiogenic eHf source component (across Lachlan Orogen and most of the Thomson Orogen; Trend A), a high d 18 O-unradiogenic eHf source component (confined to the Warrata Group in the Thomson Orogen; Trend B) and one or more low d 18 O-radiogenic eHf source components (arc-related, or direct mantle input; Trend A and B).
Data coverage remains sparse owing to the paucity of appropriate available samples, especially within the Thomson Orogen; however, we have not yet identified any contrast in Lu-Hf or O-isotopic character between the northern Lachlan Orogen Albury-Bega Terrane and the southern Thomson Orogen.
We detect potential arc influences from the Macquarie Arc in intrusions in the Hermidale Terrane into the northwest Lachlan Orogen, and from the Mount Wright Arc in intrusions into the Warratta Group in the southwest Thomson Orogen; however, our isotopic data cannot distinguish between these sources and granite formation as a result of direct mantle input into the granite source region.
The status of the southern Thomson boundary structure is not yet clear; however, our results demonstrate that isotopic variations within both the Thomson Orogen and the Lachlan Orogen appear to be larger than any north-to-south variation across the central section of the LTBS.
This study has highlighted that the Hf and O isotopic character of the bulk of the northern Lachlan Orogen and the bulk of the southern Thomson Orogen are indistinguishable, and has also highlighted regions of different isotopic character, specifically in the Hermidale Terrane, and the crust underlying the Warratta Group. paid for by the Geological Survey of New South Wales. This project would not have been possible without the foresight and diligence of Lance Black, Chris Foudoulis and David DiBugnara for creating highquality SHRIMP mounts, and for systematic archival of mounts, mount maps and zircon photographs. We thank the attendees of the southern Thomson Orogen workshops for their encouragements and insights, and Geoff Fraser for his significant feedback and discussions during the early phases of this paper. Thank you to the GA internal reviewers Geoff Fraser and Natalie Kositcin. We thank Bill Collins and an anonymous reviewer for comments that greatly improved this paper. This paper is published with the permission of the CEO of Geoscience Australia.

Disclosure statement
No potential conflict of interest was reported by the authors.

Supplementary papers
Appendix A. Sample selection and preparation, analytical methods and data processing. Table A1. Lu-Hf session information (Excel file). Table A2. Oxygen isotope session information (Excel file). Table A3. Lu-Hf reference results (Excel file). Table A4. Oxygen isotope reference results (Excel file). Table A5. Lu-Hf and oxygen isotope sample results (Excel file).