A correlation of Aileron Province stratigraphy in central Australia

Precambrian Research 166 (2008) 230–245

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A correlation of Aileron Province stratigraphy in central Australia a,∗ ´ Jonathan Claoue-Long , Christine Edgoose b , Kurt Worden a a b

Geoscience Australia, GPO Box 378, Canberra, ACT 2601, Australia Northern Territory Geological Survey, PO Box 8760, Alice Springs, NT 0871, Australia

a r t i c l e

i n f o

Article history: Received 15 May 2006 Received in revised form 13 June 2007 Accepted 30 June 2007 Keywords: Aileron Province North Australian Craton Palaeoproterozoic Provenance

a b s t r a c t In the Palaeoproterozoic Aileron Province of central Australia, the ages of detrital zircons in sedimentary rocks help to establish the superposition of major basin packages, and the correlation with similar sediment packages elsewhere in northern Australia. The earliest known sedimentary sequence across the west and north Aileron Province, the Lander Package, contains detritus from a terrane dominated by 1880–1840 Ma crystalline rocks and was intruded by 1820–1800 Ma magmatism. The Killi Killi Formation of the Tanami Region and the Ooradidgee Group of the Tennant Region have similar provenance and depositional timing. Deposition of the Ongeva Package of metamorphosed sedimentary and volcanic rocks in the east Arunta Region was coeval with the 1810–1800 Ma Stafford Event, the Ware Group of the Tanami Region, and the Hatches Creek Group of the Tennant Region. A previously unrecognised sandstone sequence with maximum age ca. 1805 Ma underlies the regional angular unconformity below the Reynolds Range Group and the first thermal event seen to affect the overlying Reynolds Package was the ca. 1590 Ma Chewings Event, 200 Myr later. This permits a range of correlation possibilities for the Reynolds Package and its basal unconformity. The detrital zircon age basis for these correlations is preserved in rocks that were metamorphosed to granulite facies and offers a means of identifying the protoliths of high-grade metamorphic units in a complex polymetamorphic terrane. Crown Copyright © 2008 Published by Elsevier B.V. All rights reserved.

1. Introduction This contribution describes the application of a relatively new tool in Proterozoic stratigraphy: dating of detrital zircons in sedimentary rocks. Although widely deployed in Phanerozoic basins where provenance constraints add value in basins where timing and correlation frameworks already exist (e.g. Morton et al., 1996), the purpose here is to establish basic stratigraphic superposition and correlation in the absence of a more direct dating approach. The outcomes demonstrate that, though expensive and data-intensive, the approach can assist with primary stratigraphic constraints in a complex Proterozoic region. 1.1. The geological context of the Aileron Province The Arunta Region (Fig. 1) is one of the most geologically complex areas in Australia (Shaw et al., 1984). It comprises a 200,000 km2 exposure of Proterozoic rocks which record serial overprinting of geological events over a 1500 million year period from before 1800 Ma through to the Palaeozoic (Collins and Shaw, 1995). The Arunta encompasses more than one separately evolved

∗ Corresponding author. Fax: +61 2 6249 9111. ´ E-mail address: [email protected] (J. Claoue-Long).

and geologically distinct terrane, and so is referred to as a geological Region, in preference to terms such as Province, Block or Inlier which connote a degree of geological continuity. The major part has an evolution similar to that observed to the north, in the Tanami and Tennant Regions, and is known as the Aileron Province (Scrimgeour, 2003). This contribution focuses on stratigraphic developments within the Aileron Province. In restricted areas of the south and southeast of the Arunta Region, the Warumpi and Irindina provinces are terranes which express separate evolutions. Pietsch (2001), Claoué-Long (2003) and Scrimgeour (2003) have recently revised the event framework for the Aileron Province from newly dated chronologies of thermal events which generated magmatic and metamorphic rocks amenable to U–Pb isotopic dating. The earliest of these was the ca. 1810–1800 Ma Stafford Event in which bimodal magmatism intruded and metamorphosed preexisting Lander Package sedimentary sequences (cf. Claoué-Long and Edgoose, 2008). Dating of bimodal magmatism has also enabled constraint of the ca. 1790–1770 Ma Yambah Event. The Strangways Event (ca. 1740–1690 Ma) and Chewings Event (ca. 1600–1580 Ma) were magmatic, metamorphic and deformational episodes which affected parts of the Aileron Province. A variety of later geological systems extended into the Palaeozoic (cf. Scrimgeour, 2003; Claoué-Long and Hoatson, 2005). Much of the present configuration of Aileron Province geology, in which major regional-scale faults juxtapose low (up to greenschist facies) and high (granulite facies)

0301-9268/$ – see front matter. Crown Copyright © 2008 Published by Elsevier B.V. All rights reserved. doi:10.1016/j.precamres.2007.06.022

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Fig. 1. Map showing the Arunta, Tennant and Tanami Regions in central Australia, with the location of the Reynolds, Anmatjira and Wabudali Ranges in the west-central Aileron Province, and the locations of dispersed Aileron Province samples.

metamorphic grade rocks, is an expression of the 400–300 Ma Alice Springs Orogeny. The majority of the Aileron Province is interpreted to comprise sedimentary rocks or their metamorphosed equivalents. Owing to the paucity of dateable volcanic horizons and the complexities of deformation and metamorphism, a detailed stratigraphy is not available for the sedimentary successions. The detailed available dating of magmatic and metamorphic events contrasts with a relative lack of timing constraints for the major phases of basin development. However, Pietsch (2001) and Scrimgeour (2003) have proposed, as a starting point, broad province-scale packages of sedimentary rocks of like characteristics. The earliest extant rocks of the Aileron Province are widespread clastic sediments, now at various metamorphic grades, collectively known as the Lander Package (Pietsch, 2001). It is often divided into mappable units on the basis of metamorphic grade rather than identifiable stratigraphy (e.g. Young et al., 1995a,b). The term Lander Rock Formation (formerly Beds) is widely used for exposures at low metamorphic grade, but other locally named units include the Mount Stafford Beds and the granulite grade Weldon Metamorphics and Aileron Metamorphics. Collectively, the metasedimentary rocks assigned to this package occupy more than 60% of the exposure of the Aileron Province. The thickness of the sedimentary pile is unknown because neither the base, nor the basement onto which it was deposited, have been identified. Throughout the region the Lander Rock Formation consists of interbedded pelitic and psammitic rocks interpreted to represent dominantly turbiditic sedimentation. Locally interlayered within the sequence, and folded with it, are basaltic or doleritic units representing either lavas or sills. Original sedimentary structures, apart from gross bedding, are usually obscured by early schistosity and later cleavages. The Lander Package is directly intruded by major plutons in the age range ca. 1770–1795 Ma and older bodies belonging to the ca. 1810–1800 Ma Stafford Event, including the 1803 ± 5 Ma Mount Hay mafic granulite (Claoué-Long and Hoatson, 2005) and the 1803 ± 6 Ma Ngadarunga Granite (Claoué-Long and Edgoose, 2008). These intrusions impose a minimum age on the sedimentary sequence but the deposition age of the Lander Package is otherwise unconstrained. The correlation of Lander Package

units at both low and high metamorphic grades is the subject of this contribution. The observed top of the Lander Package is a major regional angular unconformity. Above this unconformity lies the Reynolds Package, named after the Reynolds Range Group in the Reynolds Range northwest of Alice Springs. The Reynolds Range Group is a distinctive package of quartzite, pelitic and psammitic schist, minor calc-silicate rock and rare basalt interpreted to have been deposited in a shallow-marine environment. Metamorphic grade in the Reynolds Range type area varies from greenschist facies in the northwest to granulite facies in the southeast where there is a ca. 1600–1570 Ma Chewings Event overprint (Williams et al., 1996). The Reynolds Package is preserved mainly as discontinuous erosional remnants in the keels of major synclines in the north and west of the Aileron Province. It is not seen in direct contact with intrusions or metamorphic effects of the Stafford (ca. 1810–1800 Ma), Yambah (ca. 1790–1770 Ma) or Strangways (ca. 1740–1690 Ma) events, all of which affect the stratigraphic units underlying the unconformity. Available age evidence for granitoids which intrude the Reynolds Package in the Reynolds and Wabudali Ranges is discussed below. The stratigraphic age and correlation of the Reynolds Package are also considered in this contribution. In the east of the Aileron Province, variably metamorphosed sedimentary and volcanic units are assigned as the Ongeva Package (Scrimgeour, 2003). These units are the protoliths of variably metamorphosed units including the Strangways Metamorphic Complex, the Kanandra Granulite, and the Bonya Schist. The package is dominated by pelites and psammites but also includes mafic and felsic gneisses interpreted as having volcanic protoliths. The nature of the contact between the Ongeva Package and the Lander Package is unclear because it has been obscured by tectonism and younger cover, but the deposition timing has been constrained by Hussey et al. (2005), Claoué-Long and Hoatson (2005) and Claoué-Long et al. (2008-b). Granulite grade lithologies which host base metal deposits in the Strangways Metamorphic Complex have yielded unimodal zircon ages interpreted to represent primary volcanic crystallisation at Edwards Creek (1803 ± 5 Ma), Utnalanama (1810 ± 4 Ma) and Harry Creek (1801 ± 3 Ma); further east, a unit within the Bonya Schist has an age of 1807 ± 17 Ma. These all

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appear to be stratigraphic ages and therefore place Ongeva Package units coeval with the ca. 1810–1800 Ma Stafford Event bimodal magmatism that intrudes the Lander Package. This important constraint requires the Ongeva Package basin phase to post-date the Lander Package. Also defined locally within, and to the north of, the Strangways Metamorphic Complex are apparently younger metasedimentary protoliths collectively known as the Cadney and Ledan packages. This contribution focuses on the stratigraphic age and correlation of the three areally most important Aileron Province sediment associations: the Lander, Reynolds and Ongeva packages, their timing relationships with each other, and wider correlations northwards with other Proterozoic regions of northern Australia. 1.2. Correlation alternatives to the north An important reference for comparing stratigraphic constructions of the Aileron Province is the superposition and timing observed in other north Australian Proterozoic terranes, especially the Tennant and Tanami Regions immediately to the north, and more distally the Pine Creek and Halls Creek Orogens. In the ‘Barramundi’ model for the development of Proterozoic northern Australia (Etheridge et al., 1987) the evolution was modelled as the termination of an early basin phase by widespread synchronous folding, metamorphism and syntectonic magmatism, followed by deposition of a later cover sequence of sedimentary rocks. This was based on commonalities in the style of deformation in the Halls Creek, Pine Creek and Mount Isa regions of far north Australia. The time span of the orogeny was estimated at ca. 1880–1850 Ma from geochronology reported by Page (1988). The Aileron Province is separated from most of these observations by younger cover, but termination of the Lander Package turbidite-dominated basin phase by deformation and metamor-

phism, followed by a later cover of Reynolds Package sediments, was inferred to correlate with the same event sequence (Etheridge et al., 1987). The unconformity below the Aileron Province Reynolds Package was interpreted to be the same as the unconformity in the Tennant Region that separates the Warramunga Group from the overlying Ooradidgee and Hatches Creek Groups (e.g. Dirks and Wilson, 1990), and Blake et al. (1987) placed the main early folding of the Tennant Region Warramunga Group in the Barramundi Orogeny. Thus, the Lander and Reynolds Packages were interpreted as, respectively, the pre- and post-orogenic sedimentary sequences, with the unconformity separating them representing the time of the Barramundi Orogeny. This model of correlation is illustrated in Fig. 2a. Compston (1995) has subsequently documented a narrow range of ages ca. 1850 Ma for the Tennant Region granites which intrude Warramunga Group sedimentary rocks, and Smith (2001) dated volcanic units within the Warramunga Group and correlatives at ca. 1860 Ma. The stratigraphic age of volcanic and sedimentary stratigraphy in the overlying Ooradidgee and Hatches Creek Groups is documented by Claoué-Long et al. (2008-a) as two deposition phases at ca. 1840 Ma and ca. 1815–1800 Ma. This documented stratigraphic timing offers a basis for testing the correlation of Aileron Province sedimentary sequences. An important first-order constraint, established by the dating of intercalated volcanics, is the time-equivalence of the Tennant Region Hatches Creek Group (ca. 1815–1800 Ma) with the Aileron Province Ongeva Package (ca. 1810–1800 Ma) and the ca. 1810–1800 Ma Stafford Event magmatism. Another link to the north is to the Tanami Region, which has been a focus of recent geochronological study (Cross and Crispe, 2007). In this region, the earliest Stubbins Formation is dated ca. 1860 Ma, and the overlying Tanami Group is dated at ca. 1840 Ma. A third phase of deposition, the Ware Group, is dated at ca. 1825–1810 Ma.

Fig. 2. Alternative correlations of Aileron Province stratigraphy with adjacent Proterozoic regions. (a) ‘Barramundi’ model of Etheridge et al. (1987): unconformity between Lander and Reynolds Packages attributed to 1880–1850 Ma Barramundi Orogeny. (b) Correlations permitted by current dating. Tennant Region from Compston (1995), ClaouéLong et al. (2008-a). Tanami Region from Crispe et al. (2007), Cross and Crispe (2007). Aileron Province from this paper and Claoué-Long and Hoatson (2005), Claoué-Long and Edgoose (2008), Claoué-Long et al. (2008-b). Lander Package deposition ca. 1840–1810 Ma at same time as Ooradidgee and Tanami Groups. Ongeva Package ca. 1810–1800 Ma coeval with Stafford Event, Ware Group, Hatches Creek Group. Two alternatives for correlating the Reynolds Package. In [I] all Aileron Province units are pre-Yambah Event. In [II] all Aileron Province units are pre-Chewings Event: unnamed sandstones immediately below unconformity correlate with Pargee Group, unconformity attributed to 1740–1690 Ma Strangways Event, Reynolds Package (RRG) correlates with Birrindudu Group. See text for discussion.

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Fig. 3. SHRIMP U–Pb isotopic data for detrital zircons in basal Mount Thomas Quartzite of Reynolds Range Group in the type area. (a) Concordia diagram of 94 random analyses of unsorted zircons shows that less than 55% are concordant. (b) 128 random analyses of non-magnetic zircons of which 95% are concordant. (c) The combined concordant zircon ages are a continuum of ages between <1800 Ma and >2500 Ma. (d) Detail of the youngest end of the detrital age spectrum. Grain 26 is the youngest replicated age (1785 ± 16 Ma) and the youngest statistically defined group of eight analyses has mean age 1798 ± 12 Ma.

Although sedimentary lithologies vary, the timing of this sequence of three dated basin phases is similar to the Tennant Region. The Tanami Region Ware Group and Tennant Region Hatches Creek Group (both ca. 1825–1800 Ma) were deposited broadly coeval with the Aileron Province Ongeva Package. Later cover sequences in the Tanami Region include the Pargee Group, a sequence of shallowmarine or fluvial conglomerate and sandstone, and, above a major regional unconformity, the basal Gardiner Sandstone of the Birrindudu Group. Detrital zircon ages require the Pargee Group to have been deposited after ca. 1775 Ma (Cross and Crispe, 2007) (Fig. 2). 1.3. Provenance signatures in the Tanami Region The extended succession of basin phases in the Tanami Region is documented with an important database of detrital zircon provenance patterns and maximum deposition ages characteristic of each major sequence (Cross and Crispe, 2007). Representative Tanami Region provenance signatures are plotted in Fig. 4 and briefly reviewed here as a reference for comparing the data reported here for the Aileron Province. Claoué-Long et al. (2008-a) show that the more restricted range of basin phases present in the Tennant Region shares the same progression of provenance. This important commonality provides a basis for comparisons with the undated metasedimentary packages of the Aileron Province. In combination with the constraints imposed by dated magmatic and metamorphic events within the Aileron Region itself, this provides constraints on alternative correlations. The Tanami Group is represented by the provenance pattern of the Killi Killi Formation, here obtained from a new sample in drillcore CYDD114/227.15-228.1 m at the Coyote gold prospect in

the northwest Tanami Region (Fig. 4a). Its detrital zircon age pattern is the same as that in other Killi Killi Formation samples across the region (Cross and Crispe, 2007) with a dominance of detrital zircons in the age range 1880–1830 Ma, and subordinate older populations up to 2900 Ma. The Killi Killi Formation overlies the Dead Bullock Formation within which an interpreted volcaniclastic unit is dated at 1838 ± 6 Ma (Cross and Crispe, 2007). The Tanami Group is intruded by 1820–1800 Ma granites coeval with the Aileron Province Stafford Event (Smith, 2001). The overlying Ware Group has been dated at ca. 1820–1800 Ma by intercalated volcanics, time-equivalent with the granite magmatism and with the Aileron Province Ongeva Package. Ware Group clastic sediments have detrital zircon provenance patterns similar to (and probably recycled from) the underlying Killi Killi Formation, with the addition of younger detrital zircons derived from the proximal volcanism (Fig. 4b). The zircon age spectrum in the Pargee Formation marks the incoming of a new provenance. The dominance of ca. 1840–1880 Ma detritus that is distinctive of Killi Killi Formation and Ware Group sedimentary rocks is replaced by a wider distribution of detrital ages across the range 1840–2700 Ma with prominent groupings at ca. 2500 Ma and ca. 1880 Ma (Fig. 4c). The detritus also includes a distinctively young component of zircons in the age range ca. 1780–1730 Ma which has a mean age of 1768 ± 14 Ma (2). Detrital zircons in the basal Gardiner Sandstone of the Birrindudu Group are dominated by ages in the range ca. 1840–1800 Ma (Fig. 4d) with subordinate populations as old as ca. 2700 Ma, and a youngest component at 1812 ± 8 Ma. The zircon age spectrum probably reflects sediment recycling because the deposition age was significantly younger: volcanics in the overlying Limbunya Group within the Victoria River Basin sequence have

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Fig. 4. Cumulative frequency diagrams of detrital zircon age spectra in representative Tanami Region sedimentary units: (a) Killi Killi Formation (Coyote Prospect, Table 9); (b, c and d) Ware Group, Pargee Formation and Gardiner Sandstone spectra plotted from data in Cross and Crispe (2007). Grey stipple denotes zone of ages <1800 Ma. Detail to right of each spectrum shows youngest individual analyses in each sample.

zircon U–Pb ages of 1636 ± 5 Ma and 1639 ± 7 Ma ages (Cutovinos et al., 2002). 2. Analytical procedures Zircons were obtained by crushing and conventional heavy liquid separation, mounted for probing in epoxy discs and imaged by light microscopy and cathodoluminescence. Analysis for U–Pb isotopic compositions used the SHRIMP ion microprobes at the Australian National University using procedures described by ´ Claoue-Long et al. (1995). Decay constants used are those recommended by Steiger and Jaeger (1977) and corrections for common Pb used the measured abundances of 204 Pb and compositions of crustal common Pb modelled by Stacey and Kramers (1975). Calibration of Pb/U ratios assumed an age of 1850 Ma for reference zircon QGNG, whose 207 Pb/206 Pb measurement was also used as a monitor of instrumental mass fractionation. Detrital zircons were analysed at random with no analyst selection of grains, to obtain representative age spectra. Analytical data are listed in Tables 5–20. 2.1. Rationale of detrital zircon dating The Lander and Reynolds packages in the Aileron Province have no known, dateable volcanic horizons. Recourse has therefore been made to the only other timing information contained in the sedimentary rocks: detrital zircon grains which preserve crystallisation ages relating to their provenance. Three types of information can come from dating detrital zircons. One is a possible fingerprinting of the provenance from which the sediment was ultimately derived, because the age spectrum of the eroded zircon grains will reflect the magmatic and metamorphic events in that provenance. A derivative of this is inferences of correlation of like sediment pack-

ages, because sediments derived from a common provenance, and deposited at about the same time, may share a common provenance age pattern that is distinct from that of other sediment transport systems. A third type of information is a maximum age of sediment deposition from the measured age of the youngest detrital component. None of these types of information is direct. Any inferences can be complicated by sediment recycling, and by the fact that a provenance may continue to shed detritus over a very long period of time. Sedimentary rocks with a shared provenance may have been derived from the same provenance at different times. Maximum deposition ages are not a direct age constraint because a sediment can be deposited hundreds of millions of years after the crystallisation age of its youngest provenance component. A potential additional complication is different ways of calculating a maximum deposition age and a consistent approach described by Claoué-Long and Edgoose (2008) is applied here so that all maximum ages are stated on the same basis. The reader is referred to that paper for the detailed methodology. While all these issues must be considered when dealing with detrital zircon age data, Hallsworth et al. (2000) have highlighted the primary importance of achieving randomness in sampling of detrital zircons. Operator choice can introduce significant risk of biasing the provenance spectrum or even eliminating age populations. Sircombe and Stern (2002) have shown that laboratory grading of detrital zircons can lead to biases in the measured age spectra. There is debate about the optimum quantity of data to achieve statistical significance (e.g. Vermeesch, 2004): in practice this is always a compromise between the mathematical criteria and the logistic and expense implications of acquiring immense quantities of data. This study has aimed at acquiring a minimum of 60 concordant analyses for each sedimentary sample, with this figure

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Table 1 Maximum deposition ages in Lander Rock Formation and correlatives Youngest individual ±2

Unit name and region

97.5% confidence maximum (Ma)

Youngest coherent group ±2

97.5% confidence maximum (Ma)

Mount Stafford Beds 2001082054

1846 ± 12

1858

1866 ± 3 (N = 20; MSWD = 1.6)

1869

Lander Rock Fm Reynolds Range area

1828 ± 38

1856

1858 ± 5 (N = 26, MSWD = 1.6)

1863

Lander Rock Fm Mount Peak area 99096013

1822 ± 56

1878

1863 ± 5 (N = 18; MSWD = 1.6)

1868

Lander Rock Fm Mount Doreen area 99096014

1847 ± 16

1863

1862 ± 6 (N = 16; MSWD = 1.5)

1868

Lander Rock Fm Mount Solitaire area 2001082554

1834 ± 42

1876

1869 ± 4 (N = 34, MSWD = 1.0)

1873

doubled or trebled in some cases; in doing so it reports ca. 1200 individual analyses of detrital zircons in 15 samples. The need for randomness can be compromised by the tendency of weathered Proterozoic rocks to yield discordant zircon compositions. This problem is well illustrated by data for the Mount Thomas Quartzite (Fig. 3). Completely random dating of unsorted zircon grains yielded the dataset in Fig. 3a: 94 zircon analyses of which more than 40 are discordant and therefore ambiguous ages. This is a very high proportion of redundancy. Magnetic sorting for zircon quality yielded the dataset in Fig. 3b: 128 zircon analyses of which less than 10 are discordant. The concordant analyses in the unsorted and magnetically graded datasets are subtly different, but they do give broadly similar age spectra and similar maximum deposition ages. Magnetically graded zircons make the acquisition of large concordant datasets achievable, so this approach has been applied to the other samples. It must be borne in mind that this immediately compromises any inferences made on the basis of the apparent absence of age populations, which could be due to sampling bias. This caveat particularly applies to the meaning of maximum deposition ages based on the presence or absence of certain detrital age groupings: the presence of an age population in a dataset has definite geological meaning, but the absence of the same grouping from other datasets could be a meaningless artefact. Fig. 3c and d illustrates considerations involved in the calculation of a maximum deposition age from the youngest zircon component in a provenance spectrum. In the Mount Thomas Quartzite the data form a seamless age continuum from <1800 Ma to >2500 Ma, in which the identification of any apparent grouping would be illusory. Definition of a youngest ‘group’ of grains must therefore be arbitary or based on a statistical criterion. To help with this issue, and to establish the isotopic integrity of the youngest measured grains, analyses in this sample were replicated.

The youngest apparent age is not supported by duplicate analysis within the same grain and is not considered reliable. The youngest replicated age is for grain 26, whose mean age is 1785 ± 16 Ma (2). Applying the maximum age methodology of Claoué-Long and Edgoose (2008), this means 97.5% confidence that the sediment was deposited after 1801 Ma. The youngest statistically defined grouping of analyses has a mean age of 1798 ± 12 Ma, and so a 97.5% confidence maximum deposition age of 1810 Ma. Maximum deposition ages are stated on the same calculation basis for every sample in this study and listed in Tables 1–4. This conservative approach produces consistency and comparable data by stating maximum ages at the oldest end of available possibilities. 3. The Lander Rock Formation Five samples of Lander Rock Formation in different areas of the Aileron Province (Fig. 1), but all at greenschist facies or lower metamorphic grade, have been analysed to characterise the provenance of Lander Package in samples uncomplicated by thermal overprinting. The detrital age spectra are illustrated in cumulative age frequency diagrams in Fig. 5, together with a detailed view of the youngest analyses from which maximum deposition ages are calculated. 3.1. Mount Stafford beds 2001082054; AGD66 location 250169/7562433 This sandstone is from the northern Anmatjira Range, ca. 10 km from granulite-facies equivalents on Mount Stafford. U–Pb ages were measured using longer count times than applied to other samples, to maximise the precision of individual ages. The result is a highly resolved age spectrum (Fig. 5a) with a clear dominance of zircons in the age range ca. 1890–1845 Ma (44% of the sample),

Table 2 Maximum deposition ages in the Reynolds Range and Wabudali Range Unit name and region

Youngest individual ±2

97.5% confidence maximum (Ma)

Youngest coherent group ±2

97.5% confidence maximum (Ma)

Reynolds Range transect Pine Hill Formation 2001082052

1832 ± 20

1852

1846 ± 10 (N = 6; MSWD = 1.05)

1856

Mt. Thomas Quartzite 2001082050 (a) Unsorted zircons (b) Non-magnetic zircons (c) Combined data Sandstone 2001082048 Sandstone 2001082047 Sandstone 2001082044 Sandstone 2001082043

1797 ± 18 1785 ± 16 duplicated 1785 ± 16 duplicated 1795 ± 12 duplicated 1820 ± 23 1819 ± 18 1814 ± 36 duplicated

1815 1801 1801 1807 1843 1837 1854

N/a 1798 ± 12 (N = 7; MSWD = 1.9) 1798 ± 10 (N = 8; MSWD = 1.6) 1808 ± 5 (N = 13; MSWD = 1.8) 1859 ± 9 (N = 6; MSWD = 1.9) 1832 ± 14 (N = 3; MSWD = 1.1) 1821 ± 14 (N = 7; MSWD = 2.1)

N/a 1810 1808 1813 1868 1846 1835

1811 ± 42

1853

1850 ± 7 (N = 12; MSWD 1.6)

1857

1826 ± 22

1848

1855 ± 9 (N = 10; MSWD = 2)

1864

Wabudali Rangea Mount Thomas Quartzite 2002082517 Sandstone below unconformity 2002082518 a

Recalculated from data in Worden et al. (2004).

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Table 3 Maximum age calculations in high-grade metamorphosed sediments Unit name and region

Youngest individual ±2

97.5% confidence maximum (Ma)

Youngest coherent group ±2

97.5% confidence maximum (Ma)

Weldon metamorphics Anmatjira range 2001082055

1836 ± 32

1868

1871 ± 8 (N = 11; MSWD = 1.7)

1879

Metapsammite Cordierite Creek 2001082057

1798 ± 8

1806

1799 ± 8 (N = 3; MSWD = 0.4)

1807

Metaquartzite Cordierite Creek 2001082056

1810 ± 18

1828

1816 ± 8 (N = 6; MSWD = 1.6)

1824

Metasandstone near Cordierite Ck 2001082058

(a) 1763 ± 12 (b) 1787 ± 10 (c) 1837 ± 10

1775 1797 1847

N/a 1790 ± 7 (N = 3; MSWD = 0.6 1845 ± 9 (N = 6; MSWD = 2)

1797 1854

Table 4 Maximum deposition ages in Tanami Region sedimentary units Unit name and region

Youngest individual ±2

Gardiner Sandstonea Tanami Region Pargee Formationa Tanami Region Ware Groupa Tanami Region Killi Killi Formation Tanami Region 2001082009

1742 1734 1764 1830

a

± ± ± ±

50 38 98 36

97.5% confidence maximum (Ma)

Youngest coherent group ±2

97.5% confidence maximum (Ma)

1792 1772 1862 1866

1814 ± 8 (N = 37; MSWD = 1.5) 1765 ± 14 (N = 9, MSWD = 1.4) 1834 ± 8 (N = 39, MSWD = 1.3) 1862 ± 6 (N = 20; MSWD = 1.4)

1822 1779 1842 1868

Recalculated from data in Cross and Crispe (2007).

Fig. 5. Cumulative frequency diagrams of detrital zircon age spectra from Lander Rock Formation across the Aileron Province, showing the common pattern of dominance by ca. 1840–1880 Ma detritus and a subordinate presence of older populations to ca. 2700 Ma. Grey stipple denotes zone of ages <1800 Ma. Detail to right of each spectrum shows youngest individual analyses in each sample.

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and a discontinuous presence of older grains to ca. 2700 Ma with no prominent groupings. The youngest individual grain has a 97.5% confidence maximum age of 1858 Ma, and the youngest statistically defined group (of 20 grains) gives a 97.5% confidence maximum age of 1869 Ma (Table 1). The per-spot precision is sufficient to resolve the dominant 1890–1845 Ma detrital group as dispersed beyond measurement errors indicating that it comes from a diversity of sources formed over a 40 Ma period. This highlights the inevitably arbitary nature of selecting a youngest group within a continuum for calculating a maximum deposition age. 3.2. Lander Rock Formation 99096006; AGD66 location 270708/7539107 This sample is from a medium-grained sandstone mass flow unit within a sequence of interbedded shales and sandstones in the Reynolds Range. The detrital age spectrum (Fig. 5b) is very similar to that in the Mount Stafford Beds with a dominant population of grains ca. 1880–1830 Ma, and subordinate presence of older grains to ca. 2750 Ma. Maximum deposition age

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calculations are indistinguishable from the Mount Stafford Beds (Table 1). 3.3. Lander Rock Formation 99096014; AGD66 location 712950/7561550 The sample is from drillcore of Bouma sequences of interbedded shales, and fining-upwards sandstones with sharp bases, near Clark Mine in the Mt. Doreen area. The detrital zircon age pattern (Fig. 5c) and maximum age constraints (Table 1) are indistinguishable from the samples in the Reynolds and Anmatjira ranges. The detrital age spectrum in another sample of Lander Rock Formation in the Mount Doreen area is described by Claoué-Long and Edgoose (2008) and is identical in character. 3.4. Lander Rock Formation 99096006; AGD66 location 302600/7599800 This drillcore sample from the Anningie tin field northeast of Reynolds Range is from a sand-dominated sequence interpreted

Fig. 6. Cumulative frequency diagrams showing the progression of detrital zircon age spectra in a Reynolds Range stratigraphic transect. The lower four sandstone samples underly the unconformity and have age spectra different from regional Lander Rock Formation in Fig. 4. Above the unconformity, ages in the basal Mount Thomas Quartzite indicate detrital zircons recycled from underlying sediments. Grey stipple denotes zone of ages <1800 Ma. Detail to right of each spectrum shows youngest individual analyses in each sample.

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to represent shallower water deposition than turbidite sequences elsewhere. The precision of individual analyses is not as high as that in other samples and, as a consequence, the detail of the spectrum is less well resolved (Fig. 5d). However, it is dominated by 1830–1890 Ma grains (more than 40% of the zircons in the sediment) with subordinate populations as old as ca. 2700 Ma and cannot be distinguished from the other samples. 3.5. Lander Rock Formation AGD66 location 784659/7683849 This sample is from the Walkeley Hills ca. 150 km north of the Reynolds Range. The detrital age spectrum is indistinguishable from the other samples (Fig. 5e) with 1880–1840 Ma detritus dominating, and a subordinate presence of older detritus up to ca. 2700 Ma. 4. A stratigraphic transect in the Reynolds Range To test the progression of provenance patterns towards the top of the Lander Rock Formation and across the unconformity into the Reynolds Range Group, a transect of samples was studied in the type area for the Reynolds Package in the northwest Reynolds Range. Here, the angular unconformity between the two sediment packages is well displayed in rocks of low metamorphic grade (greenschist facies or below). The sedimentary units below the unconformity at this location are not the regionally typical turbidites; instead they are a series of sand-dominated, coarseningupwards cycles recording a prograding sedimentary sequence deposited in shallower water. Six such coarsening-upwards cycles can be counted from the base of the ridge up to the unconformity and detrital zircons were extracted from the coarsest sand layer towards the top of four cycles, including the first and last. Above the unconformity, Reynolds Range Group samples were studied from the basal Mount Thomas Quartzite and from a sandstone horizon within the overlying pelite-dominated Pine Hill Formation. The progression of detrital zircon spectra is illustrated in Fig. 6, together with detail of the youngest analyses from which maximum deposition ages are calculated. 4.1. Sandstone 2001082043; AGD66 location 269763/7540330 This sample is the lowest observable sandstone horizon in the transect. The detrital zircon age spectrum in Fig. 6a differs significantly from the regional Lander Rock Formation described above. The proportions of the populations are distinctly different, and they extend to younger ages. Instead of a single dominant 1880–1840 Ma population and subordinate presence of older grains, this provenance has ages distributed across several populations, with the youngest group no larger than others, and with a prominent age group at ca. 2500 Ma. The age of the youngest individual grain was duplicated and is 1814 ± 36 Ma (2). There are five grains younger than ca. 1820 Ma, and the statistical definition of the youngest group with acceptable MSWD extends to seven grains with a mean age of 1821 ± 14 Ma. This is a distinctly younger maximum age than that found in the regional Lander Rock Formation. It is evident that this unit has a provenance with similar age components to that yielding the Lander Rock Formation, but in different proportions, and that it contains an important younger component in the age range ca. 1820–1800 Ma. 4.2. Sandstone 2001082044; AGD66 location 269759/7540315 This is the second coarsening-upwards sandstone cycle in the transect and its provenance is similar to the unit below, with detrital ages distributed in populations across the range ca. 2700–1800 Ma

and a prominent group at ca. 2500 Ma (Fig. 6b). The younger age component ca. 1800–1820 Ma is present, if sparsely represented by just two dated grains, and the maximum deposition age is similar to that of the underlying sample (Table 2). 4.3. Sandstone 2001082047; AGD66 location 269727/7540242 This is the fifth coarsening-upwards sandstone cycle in the transect and its provenance is similar to that of underlying units, with ages distributed across the range ca. 2700–1820 Ma and a prominent ca. 2500 Ma population (Fig. 6c). The youngest age component is represented by a single dated grain at 1820 ± 23 Ma. 4.4. Sandstone 2001082048; AGD66 location 269657/7540211 This sample is of ferruginised medium-grained sandstone a few metres below the unconformity with the overlying Reynolds Range Group. It contains a similar range of detrital ages to underlying units, but in slightly different proportions (Fig. 6d). The youngest component comprises 13% of the analysed grains in the narrow range ca. 1830–1800 Ma. Other detrital ages are distributed across several populations ca. 2700–1840 Ma with a prominent ca. 2500 Ma group. The age of the youngest individual grain was duplicated and has a mean of 1795 ± 12 Ma (2), giving a 97.5% confidence maximum deposition age of 1807 Ma, while the 13 grains in the youngest statistically defined population have a mean age of 1808 ± 5 Ma and maximum age of 1813 Ma. 4.5. Reynolds Range Group, Mount Thomas Quartzite 2001082050; AGD66 location 269570/7540213 Immediately above the unconformity the basal Mount Thomas Quartzite of the Reynolds Range Group is a cross-bedded sandstone with quartz cement, in which heavy-mineral bands are preserved. It is interpreted as an intertidal-zone shallow-water sediment. A very large quantity of data was obtained for this sample as described in Section 2 and illustrated in Fig. 3. The spectrum (Fig. 6e) is very similar to that in the sandstone immediately below the unconformity, with a presence of eight analyses in the age range 1820–1785 Ma (some of these analyses were duplicated to test the isotopic integrity of the youngest grains). The youngest replicated individual grain is 1785 ± 16 Ma (2) and the youngest statistically defined group of grains is 1798 ± 12 Ma. The remaining data are distributed across several groupings in the age range 2700–1840 Ma. 4.6. Pine Hill Formation 2001082052; AGD66 location 269483/7539988 Stratigraphically above the Mount Thomas Quartzite is pelite of the Pine Hill Formation, which includes rare, thin sandstone horizons. One of these was sampled and its detrital zircon age spectrum differs significantly from that in the underlying quartzite (Fig. 6f). A ca. 1820–1790 Ma age component is not present. Instead, the youngest individual zircon is 1832 ± 20 Ma (2) and the youngest statistically defined group of six grains has a mean age of 1846 ± 10 Ma. The distribution of the age spectrum across groupings ca. 2700–1830 Ma is similar to that of sandstones underlying the unconformity (compare Figs. 6a–c), with a prominent presence of ca. 2500 Ma grains. 5. The Reynolds Range Group in the western Aileron Province Approximately 200 km west of the Reynolds Range is the Wabudali Range (Fig. 1), where a prominent angular unconfor-

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Fig. 7. Cumulative frequency diagrams of detrital zircon age spectra: (a and b) immediately below and above the unconformity in the Wabudali Range, compared with (c) Mt. Thomas Quartzite spectrum in the Reynolds Range (from Fig. 5) and (d) the Lake Mackay area (Worden et al., 2004). Note presence of a single 1628 ± 26 Ma grain in the Lake Mackay sample. Grey stipple denotes zone of ages <1800 Ma. Detail to right of each spectrum shows youngest individual analyses in each sample.

mity separates underlying psammite from a prominent overlying quartzite unit. On lithological grounds this succession and unconformity is believed to be the same as that observed in the Reynolds Range, with the quartzite assigned as the Mount Thomas Quartzite. The provenance character of the two sediment packages (Worden et al., 2004) is compared in Fig. 7 with the type examples from the Reynolds Range. Approximately 200 km further to the west, Mount Thomas Quartzite outcrops in the Lake Mackay area in the far west Aileron Province, and the provenance spectrum at this location (Worden et al., 2006) is also plotted in Fig. 7. 5.1. Wabudali Range psammite underlying the unconformity 2002082518; AGD66 location 675248/7566318 Detrital ages are distributed across a range of populations ca. 2700–1840 Ma, with a prominent grouping at ca. 2500 Ma (Fig. 7a). The spectrum lacks the signature dominance of ca. 1880–1840 Ma detritus found in regional examples of Lander Rock Formation, and matches that of the sandstones underlying the unconformity in the Reynolds Range. Ca. 1820–1800 Ma detrital ages are not clearly evident in this sample, but their presence could be masked within the analytical uncertainties. The youngest individual grain is 1826 ± 22 Ma, and the youngest statistically coherent group of 10 grains has a mean age of 1855 ± 9 Ma. 5.2. Wabudali Range Mount Thomas Quartzite 2002082517; AGD66 location 675460/7566765 Provenance information in this sediment is similar to that in psammite underlying the unconformity, with detrital ages distributed across a range of populations in the age range ca. 2700–1840 Ma (Fig. 7b). The youngest population is represented by 15% of analysed grains which have a mean age of 1850 ± 7 Ma, not distinguishably younger than that in psammite underlying the

unconformity. Detritus in the age range ca. 1820–1800 Ma is not detected in this dataset. 5.3. Western Aileron Province Mount Thomas Quartzite 2003089509; AGD66 location 598178/7507480 This is the westernmost analysed sample of Mount Thomas Quartzite, approx. 400 km west of the Reynolds Range near the Western Australia border. The provenance pattern is similar to that in the other Mount Thomas Quartzite samples (Fig. 7d) but with the younger components present at higher proportions and so defining the maximum deposition age with greater confidence. There is a prominent mode of 23 detrital ages (30% of the sample, MSWD 1.5) in the range ca. 1820–1750 Ma with a mean age of 1800 ± 6 Ma. This imposes a clearly defined maximum deposition age of 1806 Ma, which compares with 1785 ± 16 Ma for the sample in the Reynolds Range type area. In addition, this sample includes a single measurement of a detrital zircon with a concordant age of 1628 ± 26 Ma (2). This individual is significantly younger than other zircons in the sample and further work is needed to establish if other such young grains can be found; however, appraisal of the grain, the placement of the analytical spot, and the analytical quality, suggest that it cannot be excluded as a valid detrital age. 6. Granulite facies Aileron Province metasediments In areas of the Aileron Province affected by later tectono-thermal events, sedimentary protoliths have been metamorphosed and, in places, migmatised. The effects at high temperature and low pressure have generally been accompanied by little deformation, and original sedimentary features such as gross bedding are commonly preserved, even at granulite facies. Lithologies have been correlated to protolith sedimentary packages on the basis of broad lithological similarities. For example, in the zone of the southeast Reynolds Range metamorphosed to granulite facies by the

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Chewings Event (Rubatto et al., 2001), pelite-dominated units are mapped as Reynolds Package metasediments, and psammitedominated units are assigned as Lander Package protoliths. Detrital zircons preserved in four examples of high-grade metamorphic rocks have been dated here, to test the utility of the provenance tool in identifying the protoliths of these rocks. 6.1. Weldon Metamorphics 2001082055; AGD66 location 292077/7534681 The Weldon Metamorphics on the west side of the Anmatjira Range are granulite facies metasediments in fault contact with Lander Rock Formation at lower metamorphic grade (greenschist facies or below). The unit comprises metabasic and metasedimentary rocks with felsic gneisses, interpreted as possible Lander equivalents, intruded by granites. These rocks have experienced two high-grade thermal events: metamorphism and anatexis at ca. 1780–1770 Ma and subsequent metamorphism to granulite facies during the 1600–1570 Ma Chewings Event (Collins and Williams, 1995). A psammitic lithology was sampled from mixed psammitic and pelitic lithologies containing melt segregations. Some analysis overlaps with metamorphic zircon are evident in apparent ages as young as ca. 1590 Ma (Chewings Event metamorphism) and low Th/U ratios, and are excluded from the protolith spectrum (Table 17). The remaining age spectrum comprises a dominant distribution of ages in the range ca. 1900–1840 Ma, with subordinate older groupings up to ca. 2700 Ma. The youngest individual zircon is 1836 ± 32 Ma and the mean age of the youngest coherent group of 11 grains is 1871 ± 8 Ma. Comparison of this protolith signature with the detrital age pattern in unmetamorphosed equivalents supports the assignment of the Weldon Metamorphics as a Lander Rock Formation protolith.

6.2. Cordierite Creek metasandstone 2001082058; AGD66 location 306705/7507635 A granulite facies metapsammite at Cordierite Creek in the southeast Reynolds Range, mapped as metamorphosed Lander Package, was sampled where psammitic and pelitic granulites contain garnet- and cordierite-bearing partial melt segregations. The age spectrum for zircon cores within metamorphic overgrowths is illustrated in Fig. 8b. The spectrum is dominated by detrital ages in the range 1880–1840 Ma, with a subordinate presence of older populations to ca. 2700 Ma. It is broadly similar to the age spectrum in unmetamorphosed Lander Rock Formation and supports the lithological correlation previously based on the dominant psammitic lithology. While the overall spectrum may be a reasonable representation of the original provenance character of the protolith, calculation of a maximum deposition age is compromised by the difficulty of completely resolving protolith from metamorphic zircon zones in a granulite facies metasediment. Four analyses targeted within zircon cores, and with Th/U ratios within the range defined by the other zircon cores, have distinctly young apparent ages in the range from 1763 ± 6 Ma () to 1800 ± 11 Ma () (Table 17, Fig. 8b). The possibility that these analyses include a cryptic component of metamorphic Pb loss cannot be ruled out. If these four outlier ages are disregarded, the maximum deposition age of the sample is consistent with a correlation to the Lander Rock Formation (Fig. 8b detail). 6.3. Cordierite Creek metapsammite 2001082057; AGD66 location 304420/7507931 Within pelite-dominated lithologies interpreted as metamorphosed Reynolds Range Group, a thin local psammite layer was

Fig. 8. Cumulative frequency diagrams of age spectra preserved in zircon cores in granulite grade metasedimentary rocks: (a) Weldon Metamorphics in Anmatjira Range; (b, c and d) three units metamorphosed by the 1600–1580 Ma Chewings Event at Cordierite Creek, southeast Reynolds Range. Grey stipple denotes zone of ages <1800 Ma. Detail to right of each spectrum shows youngest individual analyses in each sample.

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sampled. Gross bedding is preserved within the psammite even though it adjoins large melt bodies formed in the surrounding pelites. The lithological association is similar to the presence of rare thin sandstone horizons within unmetamorphosed Reynolds Range Group Pelite Unit. The detrital zircon age pattern is illustrated in Fig. 8c and has ages across a series of populations in the range ca. 2700–1840 Ma, with prominent groupings at ca. 1860 Ma, ca. 2200 Ma, and ca. 2500 Ma. The 1840–1880 Ma age population, while prominent, does not dominate the spectrum as is found in regional samples of Lander Rock Formation. Instead, the distribution of ages is similar to that described above in the unmetamorphosed Mount Thomas Quartzite, and in the unnamed sandstones immediately underlying the unconformity in the Reynolds Range. The youngest measured ages (three grains with a mean age of 1799 ± 8 Ma) are consistent with this. 6.4. Cordierite Creek metaquartzite 2001082056; AGD66 location 304413/7507934 A sample of quartzite 4 m from the above psammite was sampled to check the consistency of provenance pattern found in these metamorphic rocks. The detrital age pattern is illustrated in Fig. 8d, and compares closely with that found in the adjacent psammite and elsewhere in unmetamorphosed Mount Thomas Quartzite: distribution of ages across a series of populations in the range ca. 2700–1840 Ma, and a youngest group of grains having a mean age of 1816 ± 8 Ma. The overall pattern and the maximum age both support the interpretation of these units as Reynolds Package metasediments. 6.5. Identification of granulite facies metasedimentary protoliths With some limitations, the cores of metamorphic zircons in granulite facies metasediments do preserve the detrital age patterns seen in unmetamorphosed equivalents. Two examples preserve age spectra consistent with a Lander Rock Formation protolith, even though metamorphism has compromised the calculation of maximum deposition ages. Two others, attributed as Reynolds Range Group correlatives on lithological grounds, preserve the distinctive detrital age pattern appropriate to that protolith. This encouraging result is a basis for provenance identification more widely in high-grade metasediments in the Aileron Province and elsewhere. The approach depends on there being sufficiently distinctive variation in original protolith signatures, and requires caution in the interpretation of the youngest ages found, which could be artefacts of metamorphic Pb loss. The primary importance of these data for high grade metasediments is support for the view that both high-grade and low-grade metasediments across the Aileron Province relate to the same broadly identifiable sedimentary sequences: the Lander and Reynolds Packages. 7. Correlation of Aileron Province sedimentary packages The patterns of detrital zircon provenance documented above describe Aileron Province sedimentary packages which lack direct isotopic dates. Basic superposition of units is clear only in the least-metamorphosed transects studied in the Reynolds and Wabudali Ranges; elsewhere, the stratigraphic framework, including the nature of the contacts between major packages, is not established. Potentially, the detrital zircon provenance tool offers constraints on superposition and correlation in a deformed and metamorphosed region. Constraints must be interpreted from maximum deposition ages, consistencies in the progression of sediment provenance

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signatures, relationships with dated magmatic intrusions, and commonalities with directly dated provenance signatures in the adjacent Tanami and Tennant Regions. The extent to which stratigraphic timings and correlations can be constrained by current evidence is explored below. 7.1. The Lander Package The Lander Package of metasedimentary rocks is the earliest known rock association in the Aileron Province. The leastmetamorphosed examples show a strongly consistent provenance wherever they are sampled and the spectrum is distinctive enough to be recognisable when preserved in the cores of metamorphic zircons in granulite facies metasediments. The distinctive feature is dominance by ca. 1880–1840 Ma detrital zircon ages, usually contributing more than 40% of the detritus, with a subordinate presence of older Proterozoic components and only rare material older than ca. 2700 Ma. There is the implication that a large common sediment transport system delivered a homogeneous sediment provenance over a wide area, and that the provenance was dominated by 1880–1840 Ma magmatic and metamorphic rocks. Maximum deposition ages are relatively constant, with the main source of variation being analytical precision rather than real geological variation in the ages of the youngest grains. The detail of individual analyses in Fig. 5 shows that the spectra which appear to stray younger than ca. 1800 Ma are an artefact of one or two analyses with wide error bars overlapping younger ages: paradoxically, the older ends of the same error bars extend to older maximum age calculations. This analytical source of variability is partly overcome if maximum ages are calculated from the youngest statistically defined groups of analyses; Table 1 shows these to be in the narrow range from 1863 Ma to 1873 Ma. It should be noted that the highly conservative approach to maximum age calculation inevitably masks the whole age range of the youngest component in the provenance, by focussing solely on the oldest permitted number. The spectra in Fig. 4 make clear that this provenance was dominated by 1880–1840 magmatic and metamorphic rocks. The Lander sedimentary package was intruded by ´ 1810–1800 Ma plutons of the Stafford Event (cf. Claoue-Long and ´ Hoatson, 2005; Claoue-Long and Edgoose, this volume), so the basin phase represents the short time span between ca. 1840 Ma and 1810 Ma. The same evidence requires the Lander Package to predate the ca. 1810–1800 Ma Ongeva package in the eastern Aileron Province. The nature of the contact between the Lander and Ongeva packages is obscured by deformation and later cover, so the isotopic age evidence is currently the only means to establishing stratigraphic superposition. As shown in Fig. 2b, provenance spectra and the timing constraints of later intrusions are a basis for correlating the Aileron Province Lander Package in time with the Tanami Region Killi Killi Formation (cf. Cross and Crispe, 2007) and the Tennant Region Ooradidgee Group (as described by Claoué-Long et al., 2008-a). These units in the three regions all postdate a crustal event in their provenance ca. 1880–1840 Ma, have similar provenance spectra, and were intruded by 1820–1800 Ma plutons indicating that deposition took place in a relatively narrow time period. 7.2. The Ongeva Package The stratigraphic timing of the Ongeva Package in the eastern Aileron Province is documented by Hussey et al. (2005) within the narrow age range ca. 1810–1800 Ma. Deposition of this package of sedimentary and volcanic protoliths (subsequently metamorphosed up to granulite facies) was coeval with the ca. 1810–1800 Ma

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Stafford Event of bimodal magmatism which intruded the earlier Lander Package. In the Tanami Region, this time period is equivalent with deposition of the Ware Group (Cross and Crispe, 2007). In the Tennant Region, the Hatches Creek Group is also broadly the same age (Claoué-Long et al., 2008-a). These three sequences have in common that they comprise sedimentary and bimodal volcanic successions coeval with bimodal intrusions emplaced into the underlying basin phases in their respective regions. It is important to note that detrital zircon data in this succession reflect the same sedimentary provenance as that in the underlying basin phase, with the addition of some detrital zircons whose ages indicate local derivation from the proximal volcanism. This reflects derivation of sediment from the same ultimate source, perhaps via sediment recycling. 7.3. The unconformity above the Lander Package The Aileron Province presents a deformed, sporadically exposed and variably metamorphosed stratigraphy. In this complexity, the most prominent stratigraphic marker is the regionally extensive angular unconformity recognised at the base of the Reynolds Package. Two well preserved transects through this succession are studied here: the Wabudali Range and the type area in the Reynolds Range. At both locations the sandstones below the unconformity, although mapped as Lander Package sedimentary rocks, do not share the provenance of the regional Lander Rock Formation. The signature dominance of 1880–1840 Ma detritus is not present and instead there is distribution of detrital ages across the range 1800–2700 Ma with a prominent population of ca. 2500 Ma detrital zircons. A younger component ca. 1820–1800 Ma is also present, albeit at low frequency in some samples, with the result that maximum deposition ages are distinctly younger than those in the Lander Rock Formation. Replicated analysis of the youngest zircon in one of these sandstones gives an age of 1795 ± 12 Ma (2) and the youngest statistically defined group of grains is 1808 ± 5 Ma, imposing conservative maximum ages of, respectively, 1807 Ma and 1813 Ma (Table 2). These data indicate that a previously unrecognised sandstone succession underlies the regional unconformity and that it represents a distinct depositional phase, younger than the Lander Rock Formation. The presence within it of ca. 1820–1800 Ma detritus suggests deposition after the Stafford Event, whereas the regional Lander Rock Formation is intruded by plutons of the Stafford Event. Thus, the provenance data provide a clear sense of superposition even though the contact between Lander Rock Formation and the younger sequence is not known. The detrital zircons in this unit point to the incoming of a new source of sediment supply. This distinguishes the succession from the Ongeva Package and time-equivalent sequences in the Tanami and Tennant regions (respectively: Ware Group, and Hatches Creek Group), which do share the characteristic provenance of the underlying basin phase. While it is possible that the Lander Rock Formation and this younger sequence represent lower and upper phases of one basin system, they could represent distinct basin phases separated by the Stafford Event. This opens the possibility that the overlying basal unconformity of the Reynolds Package relates to a younger crustal episode. 7.4. The Reynolds Package Provenance in the basal Reynolds Range Group does not help to resolve age of the underlying unconformity because the sediment was derived from the same ultimate source which supplied the underlying sandstones, perhaps via sediment recycling. In the

type area of the Reynolds Range, the youngest detritus in the Mount Thomas Quartzite is the duplicated analysis of a 1785 ± 16 Ma detrital zircon, supported by the mean age of 1798 ± 10 Ma for the youngest statistically defined group of eight grains. In the Wabudali Range this youngest detrital age group is not detected, but in the far west of the Aileron Province a quartzite mapped as Mount Thomas Quartzite contains a high frequency of the youngest detrital zircon population with 23 grains contributing a mean age of 1800 ± 6 Ma, and there is one grain dated at 1628 ± 26 Ma. It is unclear how much confidence should be placed in this single grain which implies a maximum deposition age of 1654 Ma. These findings highlight the sometimes low probability of finding the youngest detritus in a sedimentary rock if it is present as just one or two among more than 60 dated zircons. Any failure to find these sparse representatives due to chance, or bias introduced by sampling procedures, means that inferences cannot be based securely on the apparent absence of a particular ‘youngest’ component in any one sample. Only the discovered presence of a population has definite geological significance. The provenance data supply only a maximum age for deposition of the Reynolds Package. Minimum age constraints can potentially come from dated intrusions and metamorphism affecting these sedimentary rocks. It is notable that the Reynolds Range Group is not seen in direct contact with intrusions or metamorphic effects of the Stafford (1810–1800 Ma), Yambah (ca. 1790–1770 Ma) or Strangways (ca. 1740–1690 Ma) events, which do affect the underlying Lander Rock Formation. Collins and Williams (1995) have previously proposed that the Reynolds Package predates the ca. 1790–1770 Ma Yambah Event based on study of the Warimbi Schist, which is a deformed and metamorphosed granitoid in contact with the Reynolds Package in the Reynolds Range area. They concluded that the granite emplacement was synchronous with peak (M2) metamorphism, then attributed to the ca. 1775 Ma ‘early Strangways’ Event (now renamed the Yambah Event: Scrimgeour, 2003). However, the metamorphism of this area has since been dated to the much younger ca. 1590 Ma Chewings Event (Vry et al., 1996). Collins and Williams (1995) reported that the Warimbi Schist is strongly contaminated with variably assimilated metasediment from the Reynolds Package country rock it intrudes, and that zircon ages within it are a continuum in which the youngest grains have ca. 1785 Ma ages identical to detrital zircons in the contaminating Mount Thomas Quartzite. If the interpreted relationship of the Warimbi Schist to peak metamorphism is correct, then it probably intruded at ca. 1590 Ma and further work is required to measure its magmatic age. Similar considerations apply to the Coniston Schist, which is another foliated felsic intrusion in contact with Mount Thomas Quartzite in the northwest Reynolds Range. Smith (2001) reported zircon ages in this unit as young as ca. 1780 Ma but this may not be the date of intrusion because the rock is a strongly metasedimentary S-type composition and contains enclaves of metaquartzite country rocks; further work is required to establish the magmatic age. The interpretation of zircon ages in a metasedimentary melt is addressed further by Claoué-Long and Edgoose (2008). The earliest crustal event demonstrated to have affected Reynolds Package stratigraphy is the ca. 1600–1580 Ma Chewings Event which has metamorphosed Reynolds Package units in the southeast Reynolds Range up to granulite facies (Vry et al., 1996; Williams et al., 1996; Rubatto et al., 2001). According to Shaw (1994), Chewings Event deformation, which controls the preserved regional distribution of the Reynolds Range Group in tight, upright boat-like macroscopic synclines developed on major shear zones, postdated the 1779 ± 9 Ma (Yambah Event) Carrington-suite granites in the western Aileron Province, and predates a granite intruded at 1567 ± 6 Ma (Young et al., 1995a,b).

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The combined limits imposed by maximum and minimum ages permit the correlation of this part of the Aileron Province stratigraphy within a 200 Myr age range. Detrital zircon ages require the Reynolds Package to have been deposited after the ca. 1805 Ma maximum age of underlying sandstones, possibly significantly after this date if the time for the intervening unconformity is considered. If sandstones underlying the unconformity were deposited after the Stafford Event, then the unconformity itself may represent the time of a later Aileron Province crustal episode and the overlying Reynolds Package would then represent a younger basin phase. A single detrital zircon found in one Reynolds Package sample could indicate deposition after ca. 1650 Ma, but this requires substantiation. The package was deformed and metamorphosed during the ca. 1590 Ma Chewings Event. 7.5. Tanami Region correlation of the Reynolds Package In the Tennant Region to the north, stratigraphic levels equivalent to the Reynolds Package are obscured by later cover. However, the Tanami Region record does extend to younger successions and potentially offers a reference for comparing the succession observed in the Aileron Province. In the Tanami Region (Cross and Crispe, 2007) the basin phases represented by the Tanami and Ware Groups share the distinctive provenance that is found also in the Aileron Province Lander Package. It is in the overlying Pargee Group where there is the first arrival of a new provenance similar to that of the unnamed sandstones under the Reynolds Range unconformity (compare Figs. 4c, 5a and b, 7a). It has detrital ages distributed across the range 1800–2700 Ma with a prominent grouping at ca. 2500 Ma, and characteristically lacks the single dominant 1880–1840 Ma group which is the primary feature of the underlying succession. Maximum deposition ages in the sandstones sharing this provenance are variable from place to place: 1768 ± 14 Ma (Tanami Region Pargee Formation), 1808 ± 5 Ma (Aileron Province Reynolds Range) and 1855 ± 9 Ma (Aileron Province Wabudali Range). As noted above, the absence of a particular youngest component from a measured spectrum may be an unreliable indicator. In both the Aileron and Tanami regions, the sandstones with this new provenance underlie a prominent regionwide unconformity: in the Aileron Province, the basal unconformity below the Reynolds Range Group; overlying the Pargee Group in the Tanami Region is the major unconformity recognised across northern Australia as the base of the Birrindudu Group. These observations invite comparison of the Aileron Province Reynolds Package with the Birrindudu Group. The lithological character of the Reynolds Package has been described in detail by Dirks (1990a,b). The basal Mount Thomas Quartzite is a mature orthoquartzite in which sedimentary structures indicate intertidal high-energy sedimentation. Stratigraphically overlying it, and replacing it as the basal unit in some areas, is a Lower Calc-silicate Unit comprising finely layered carbonate-poor calc-silicate rocks, quartzites and rare marbles. The overlying Pelite Unit (equivalent to the Pine Hill Formation of Stewart et al. (1980)) is at least 500m thick and comprises shales with rare interlayered sheets of fine siltstone and sandstone which are interpreted by Dirks (1990a) as storm deposits (one of which supplied the detrital zircons reported in this study). A 20–300-m thick Upper Calcsilicate Unit occurs as a series of lenses within the Pelite Unit and is dominated by limestone and dolomites which locally preserve stromatolites and sedimentary structures such as climbing ripples. This compares with the stratigraphy of the Birrindudu and Victoria River Basins in northwestern Australia, which has recently been updated by Cutovinos et al. (2002). The Birrindudu Group is a sandstonedominated succession with some carbonate and mudstone rocks in the upper part. Where it unconformably overlies the Tanami

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Region, the basal unit is the Gardiner Sandstone. Overlying it in the Birrindudu Basin is the Limbunya Group of interlayered siliciclastic and hypersaline carbonate rocks which include stromatolites and other features interpreted as shallow to very shallow marine deposition. Timing control is available in the Limbunya Group from the U–Pb ages of volcaniclastic units in the 1636 ± 5 Ma Bluehole Formation and the 1639 ± 7 Ma Campbell Springs Dolostone (Smith, 2001). These stratigraphic ages compare with the single ca. 1630 Ma detrital zircon found in one sample of Aileron Province Mount Thomas Quartzite. The distinctive succession of shallowmarine sedimentation, from basal sand-dominated sequences to higher levels containing carbonate units, has similarities with the Reynolds Package. On the basis of these observations, two possible correlations of the Reynolds Package and its basal unconformity are shown in Fig. 2. In one, the Reynolds Range Group is assumed to have been deposited before the 1790–1770 Ma Yambah Event, as would be required if the protoliths of the Warimbi Schist and Coniston Schist (which intrude the Reynolds Package) represent ca. 1780 Ma magmatism (cf. Collins and Williams, 1995). This construction can be accommodated within the available maximum and minimum age constraints. The resulting time compression of the underlying succession permits very little time for the basal unconformity, and it is not clear how the regional angular unconformity can be related to a known period of uplift and deformation in the Aileron Province. In this model the unconformity does not represent a major hiatus, and its timing is correlated within the early part of the Yambah Event, which was dominantly a magmatic episode (cf. Scrimgeour, 2003). An alternative construction in Fig. 2 is based on the possibility that similar provenance in the Tanami Region Pargee Group, and in the unnamed Aileron Province sandstones underlying the Reynolds Range unconformity, indicates broadly time-equivalent deposition. Detrital zircons in the Pargee Group require sediment with this provenance to have been deposited after the 1790–1770 Ma Yambah Event and correlation would therefore place the overlying Aileron Province unconformity within the ca. 1740–1690 Ma Strangways Event (Claoué-Long et al., 2008-b), which was a major period of deformation and metamorphism. This construction implies that the Birrindudu Group platform cover of northern Australia is represented in the Aileron Province by the Reynolds Package, which was later deformed, metamorphosed and intruded by the ca. 1590 Ma Chewings Event. These two scenarios represent the older and younger limits of correlation permitted by current evidence. 8. Conclusions This contribution focuses on the usage of detrital zircon age spectra as a tool in stratigraphic reconstructions, in a complex Palaeoproterozoic region which lacks a stratigraphic framework and whose correlation has been uncertain. Important limitations are inherent in the detrital zircon approach, but integration of provenance data with other sources of age control permits interpretation of some primary stratigraphic relationships. 8.1. The detrital zircon approach to correlation Obtaining detrital zircon age data in sufficient quantity, and at sufficient precision to be useful, is an intensive use of expensive SHRIMP instrument time. Through the indirect measure of provenance timing it does provide correlation constraints in a Proterozoic terrane largely devoid of dateable volcanic horizons. This study shows that there is value in designing data collection for precision as well as quantity, because highly resolved age spectra are more useful than those which identify age populations only in terms of

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generalities. This becomes especially important when calculating the age of the youngest detrital component in a sedimentary rock as a constraint on the maximum deposition age. Examples in this study show that the presence, and frequency, of a youngest detrital age component can be highly variable from place to place within the same sedimentary unit, and later metamorphism can compromise the interpretation of maximum deposition ages. Depending on the source terrane, sediment deposition might be 10 s or 100 s of millions of years after the youngest detrital age. Where sediment is recycled from underlying sequences, then the whole provenance spectrum is anyway a second-hand constraint on deposition timing. The overall distributions of shared detrital zircon age patterns are a subjectively assessed constraint. However, their consistency across the Aileron Province, and adjacent regions, permits interpretations of the correlation of sedimentary rocks of like provenance across a wide area. Through integrating these data with other structural, stratigraphic and intrusion-timing constraints, reasonable inferences can be made about major basin packages. 8.2. Correlation of the major Aileron Province sedimentary packages The sedimentary provenance data documented here are a basis for beginning to interpret the stratigraphic superposition and correlation of Aileron Province basin phases. Provenance information preserved in zircon cores suggests that the same evidence can be used in granulite facies metasedimentary rocks, which is an important consideration in a region that has experienced several high-grade metamorphic episodes over a protracted period. The indirect timing information from provenance can be placed in context with dated magmatic and metamorphic episodes which provide the crustal event framework for the region. The earliest known sedimentary rocks in the Aileron Province are the Lander Rock Formation and these sandstones have a distinctive and consistent detrital zircon age pattern throughout the region. Their provenance was dominated by 1880–1840 Ma crystalline rocks, and they were intruded by 1810–1800 Ma Stafford Event magmatism, so deposition timing is constrained within a short time period. This sandstone provenance and its deposition timing compares closely with the Tanami Group in the Tanami Region and the Ooradidgee Group in the Tennant Region. The Ongeva Package of sedimentary and volcanic rocks in the east Aileron Province is dated within the narrow range ca. 1810–1800 Ma. It was coeval with the Stafford Event and broadly time-equivalent with sedimentary and bimodal volcanic successions in the Ware Group of the Tanami Region and the Hatches Creek Group of the Tennant Region. Directly underlying the regional angular unconformity below the Reynolds Package, some Aileron Province sandstones previously mapped as Lander Rock Formation have a distinct and younger provenance, deposited after 1805 Ma. Maximum deposition ages in the overlying Reynolds Package are similar. Available evidence permits wide younger limits to the correlation of the Reynolds Package and its basal unconformity. The underlying Lander Package was deformed, metamorphosed and intruded during the 1810–1800 Ma Stafford Event, but the first crustal episode demonstrated to have affected the Reynolds Package was the ca. 1590 Ma Chewings Event. If the formation of the Reynolds Package, its basal unconformity, and the underlying sandstones, was all completed before the 1790–1770 Ma Yambah Event, then the regional unconformity may not represent a significant hiatus. Alternatively, current evidence permits the unconformity to be as young as the major hiatus corresponding to the Strangways Event, and the Reynolds Package could be as young as the Birrindudu Group which forms platform cover over much of northern Australia.

Establishing the stratigraphic framework of the Aileron Province is at an early stage. This contribution constrains some primary relationships but major issues remain unresolved. The nature of the basement onto which the Lander Rock Formation was deposited is not known, and stratigraphy equivalent to an earlier basin phase known in the Tennant Region (ca. 1860 Ma Warramunga Formation) has not yet been identified in the Aileron Province. The younger sandstone sequence underlying the Reynolds Range unconformity is currently distinguished only by the cryptic evidence of detrital zircons and its relationship with the Lander Package is unknown. The age and significance of the regional unconformity at the base of the Reynolds Package remains a question of primary importance for correlation, both within the Aileron Province with the other Palaeoproterozoic regions of northern Australia. Acknowledgements JCL and KW conduct SHRIMP research as Visitors of the Research School of Earth Sciences, Australian National University, and publish with permission of the Chief Executive Officer of Geoscience Australia. CJE publishes with the permission of the Director, NTGS. We thank Dennis Gee and Barry Pietsch for promoting this study, and Ian Scrimgeour, Nigel Donnellan, David Huston, Andrew Cross, Andrew Crispe and Kelvin Hussey for many discussions of Proterozoic geology. W. Collins and W. McLelland reviewed an earlier version of the manuscript. Lisa Heard is acknowledged for assistance with diagrams. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.precamres.2007.06.022. References Blake, D.H., Stewart, A.J., Sweet, I.P., Hone, I.G., 1987. Geology of the proterozoic Davenport Province, central Australia. Bur. Miner. Resour. Bull. 226, 70. Claoue´ -Long, J.C., 2003. Event chronology in the Arunta Region. Annual Geoscience Exploration Seminar (AGES) 2003 Record of abstracts. North. Territory Geol. Surv. Rec., 2003–001. ´ Claoue-Long, J.C., Compston, W., Roberts, J., Fanning, C.M., 1995. Two Carboniferous ages: a comparison of SHRIMP zircon dating with conventional zircon ages and 40 Ar/39 Ar analysis. In: Bergrenn, W.A., Kent, D.V., Aubry, M.P., Hardenbol, J. (Eds.), Geochronology, Time Scales and Global Stratigraphic Correlation, vol. 54. SEPM Special Publication, pp. 1–21. ´ Claoue-Long, J.C., Hoatson, D.M., 2005. Proterozoic mafic-ultramafic intrusions in the Arunta Region, central Australia. Part 2. Event chronology and regional correlations. Precambrian Res. 142, 134–158. Claoué-Long, J.C., Edgoose, C., 2008. The age and significance of the Ngadarunga Granite in central Australia. Precambrian Res. 166, 219–229. Claoué-Long, J.C., Maidment, D., Donnellan, N., 2008-a. Stratigraphic timing constraints in the Davenport Region, central Australia: a basis for Palaeoproterozoic correlations. Precambrian Res. 166, 204–218. Claoué-Long, J.C., Maidment, D., Hussey, K., Huston, D., 2008-b. The duration of the Strangways Event in central Australia. Precambrian Res. 166, 246–262. Collins, W.J., Shaw, R.D., 1995. Geochronological constraints on orogenic events in the Arunta Inlier: a review. Precambrian Res. 71, 315–346. Collins, W.J., Williams, I.S., 1995. SHRIMP ionprobe dating of short-lived Proterozoic tectonic cycles in the northern Arunta Inlier, central Australia. Precambrian Res. 71, 69–89. Compston, D.M., 1995. Time constraints on the evolution of the Tennant Creek Block, northern Australia. Precambrian Res. 71, 107–129. Crispe, A.J., Vandenberg, L.C., Scrimgeour, I.R., 2007. Geological framework of the Archean and Palaeoproterozoic Tanami Region, Northern Territory. Mineralium Deposita 42, 3–26. Cross, A.J., Crispe, A.J., 2007. SHRIMP U-Pb analysis of detrital zircon: a window to understanding the early Palaeoproterozoic development of the Tanami Basin, Northern Territory. Mineralium Deposita 42, 27–50. Cutovinos, A., Beier, P.R., Kruse, P.D., Abbot, S.T., Dunster, J.N., Brescianini, R.F., 2002. Limbunya, Northern Territory (second edition). 1:250,000 geological map series explanatory notes, SE 52-07. North. Territory Geol. Surv., Darwin, 36 p. Dirks, P.H.G.M., 1990a. Intertidal and subtidal sedimentation during a midProterozoic marine transgression, Reynolds Range Group, Arunta Block, central Australia. Aust. J. Earth Sci. 37, 409–422.

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