Proterozoic mafic–ultramafic intrusions in the Arunta Region, central Australia

Precambrian Research 142 (2005) 134–158

Proterozoic mafic–ultramafic intrusions in the Arunta Region, central Australia Part 2: Event chronology and regional correlations Jonathan C. Claou´e-Long ∗ , Dean M. Hoatson Geoscience Australia, GPO Box 378, Canberra, ACT 2601, Australia Received 29 July 2004; received in revised form 30 June 2005; accepted 23 August 2005

Abstract The Arunta Region in central Australia records multiple Proterozoic crustal processes over a 1500-million-year period. The timing of mafic–ultramafic magmatism in this evolution is constrained by SHRIMP U–Pb dating of zircon, which is a primary phase in mafic–ultramafic intrusions across the region. The earliest mafic magmatism was part of the 1810–1800 Ma Stafford Event, and a second episode is recognised during the 1790–1770 Ma Yambah Event. Zircon overgrowths in some of these early intrusions record metamorphism during the Strangways Event, a regionally pervasive metamorphic episode whose termination at ca. 1690 Ma coincided with intrusion of dolerite dykes. Gabbro intrusion during the ca. 1640 Ma Liebig Event is related to the geologically distinct Warumpi Province in the south of the region. There is no record of mafic magmatism during the ca. 1590 Ma Chewings Event. Later systems include the ca. 1130 Ma alkaline-ultramafic Mordor Complex; and basaltic bodies are components of the Irindina Province which experienced high-grade metamorphism during the Palaeozoic. The dating associates mafic–ultramafic magmatism with each of these events, which suggests that repeated extensional systems are an important aspect of the tectonic evolution. The example of routinely obtaining zircon for dating from mafic and even ultramafic intrusions, using an integrated approach of field and geochemical criteria to guide sampling, opens the possiblility of dating mafic–ultramafic intrusions elsewhere. Direct dating of mafic–ultramafic magmatism permits its significance in Proterozoic event systems to be evaluated directly, free of assumed or conjectural timing correlations with the felsic plutons that are more often studied for age control. Crown Copyright © 2005 Published by Elsevier B.V. All rights reserved. Keywords: North Australian Craton; Proterozoic mafic magmatism; Zircon; U–Pb isotopic dating; Arunta Region; Crustal evolution

1. Introduction Dating mafic igneous rocks is one of the more difficult challenges in geochronology, especially in the Precambrian where reliance is placed on zircon U–Pb isotopic studies for unravelling complex timing information. Zircon is a common component of felsic plutons and high-grade (granulite) metamorphic rocks, but is rarely

∗

Corresponding author. Tel.: +61 2 62499418; fax: +61 2 62499971. E-mail address: [email protected] (J.C. Claou´e-Long).

sought as an igneous phase in mafic magmatic systems. As a consequence, the timing of mafic igneous systems is often constrained only by inferred associations with dated felsic units, even though the mafic magmas may reflect event systems—especially extension—that are distinct, or offset in time, from those that generate felsic magmas. The effort should be made to date mafic intrusions independently, so that all components in the evolution of a terrane can be understood (cf. Gebauer and Grunenfelder, 1979). Fractionated mafic magmas can contain igneous zircon (ZrSiO4 ), and, in low silica environments, its oxide

0301-9268/$ – see front matter. Crown Copyright © 2005 Published by Elsevier B.V. All rights reserved. doi:10.1016/j.precamres.2005.08.006

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equivalent baddeleyite (ZrO2 ), but their origin has often been regarded as xenocryst ingestion from country rocks, unrelated to the primary crystallisation of the mafic magmas. However, magmatic crystallisation ages have been measured in some Precambrian units using both baddeleyite and zircon (e.g. Paces and Miller, 1993; Heaman and LeCheminant, 1993; Wingate et al., 1998; Page and Hoatson, 2000). This contribution addresses timing relationships of mafic intrusions in the Arunta Region of central Australia, by applying zircon U–Pb dating in combination with careful petrological and geochemical appraisal. A companion paper (Hoatson et al., this issue) documents the field, geochemical and isotopic nature of the same intrusions. The outcomes demonstrate complexity in the occurrence of Zr minerals in mafic intrusions. The crystallisation conditions, chemical fractionation and field expressions of mafic magmatic systems all need to be understood thoroughly—both to predict where the U-bearing trace minerals are to be found, and to determine whether they are magmatic crystals or xenocrysts. In favourable situations, credible timing information may be obtained that imposes unique constraints on event evolution and correlations. 1.1. The geological context of the Arunta Region 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

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that record serial overprinting of geological events over a 1500-million-year period from before 1800 Ma through to the Palaeozoic (Collins and Shaw, 1995). Faced with this complexity, and the inherent difficulty of establishing original geological relationships, the basic geological framework, on which models of tectonic environments might be built, is still being established. The exposed area encompasses more than one separately evolved 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 (Scrimgeour, 2003). There exists no comprehensive documentation of the mafic–ultramafic intrusions and the purpose of this contribution is a first attempt to assess the significance of mafic–ultramafic magmatism in the development of the region. In other Proterozoic fold belts worldwide such as Antarctica (Moores, 1991), the Baltic Shield (Gorbatschev and Gaal, 1987) and the Canadian Shield (Hofmann, 1980) the geological evidence has been described in terms consistent with plate-tectonic processes. In northern Australia, however, an alternative to the plate tectonic explanation has been proposed in which much of Proterozoic northern Australia, including the Arunta Region, was seen as expressing an isochronous ‘Barramundi’ orogenic event between about 1880 and 1850 Ma, and later systems were interpreted to have developed on or within this earlier crust (Etheridge et al., 1987). Subsequently, there has been debate about

Fig. 1. Outline map of the Arunta Region showing the geographical locations of the mafic intrusions dated in this study, its broad division into geological provinces, and major regional faults referred to in the text.

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whether geological continuity exists between the Arunta and regions to its north, and the concept of a North Australian Craton developed by Myers et al. (1996) has the Arunta Region as part of a separate ‘Central Australian Terrane’ accreted to the craton at a later time. Myers et al. (1996), Scott et al. (2000) and Betts et al. (2002) have sought to develop a dynamic evolution for the Australian continent, invoking the horizontal accretion of modern plate-boundary processes, but in doing so they accepted that a stable North Australian Craton had been formed prior to 1830 Ma and they confined plate-tectonic modelling to developments at its margins after 1800 Ma. In these treatments, rock types and processes observed in the Arunta Region are seen as expressions of an interpreted location at, or outboard from, the southern margin of a craton. Clearly, the existence of mafic–ultramafic magmatism in the Arunta Region, the timing of this activity and the geochemical character over time are primary constraints that can be used to develop these fundamental constructions. The earliest extant rocks of the Arunta Region are widespread clastic sediments at various metamorphic grades, collectively known as the Lander Package (Pietsch, 2001), which occupy more than 60% of the exposed geology. In the period 1840–1800 Ma when these were being deposited, Claou´e-Long (2003), Scrimgeour (2003) and Claou´e-Long et al. (2005) have now demonstrated geological continuity between most of the Arunta Region and both the Tennant and Tanami inliers to the north where related sediments were also accumulated. Many of the mafic–ultramafic bodies documented here intrude this sediment package or its correlatives. The major part of the Arunta Region which expresses this early evolution and northward geological continuity is now referred to as the Aileron Province (Fig. 1; Scrimgeour, 2003). Overprinting the early Proterozoic rocks of the Aileron Province in some areas are two distinctive high-grade metamorphic events referred to as the Strangways Event (ca. 1740–1690 Ma) and Chewings Event (ca. 1600–1580 Ma), and a variety of later geological systems. The present configuration of Aileron Province geology, in which major regional-scale faults juxtapose both shallow (up to greenschist facies) and deep crust (granulite grade) Proterozoic rocks, is an expression of the Alice Springs Event which terminated during the late Palaeozoic. In the south and southeast of the region, two discrete, fault-bound terranes are now regarded as separate from this northward continuity on the basis of distinct event histories. The Warumpi Province (Fig. 1; Close et al., 2003) is an east–west trending terrane extending along the southern margin of the Arunta Region west of

Alice Springs. It has protolith ages of 1690–1630 Ma, postdating the Strangways metamorphism that is a distinctive overprinting feature of the Aileron Province. Mafic–ultramafic intrusions of the Warumpi Province are documented in this contribution, and their origin and tectonic context may be separate from those in the Aileron Province to the north. A second distinct terrane in the east is the Irindina Province (Fig. 1; Hand et al., 1999) which comprises a sedimentary and volcanic sequence deposited in the late Neoproterozoic to Cambrian, subsequently metamorphosed to granulite facies during the Ordovician Larapinta Event. Mafic magmatism is a distinctive component of this terrane also, and reflects a younger, Phanerozoic, tectonic context completely separate from the Palaeo- and Meso-Proterozoic systems that comprise most of this study. 2. Field and analytical procedures The field expression of many Arunta Region intrusions is complicated or obscured by deformation and high-grade metamorphism, and some large units have been dismembered by tectonism. Thus, outcrop exposures are not always sufficient to identify the upper parts of the magma chamber or the most fractionated zones which are prospective for magmatic zircon or baddeleyite. For this reason, characterisation of the mafic systems for this study used chemical data directly as a mapping tool by sending samples for rapid analysis of Zr concentration. Zirconium is incompatible in the major phases of mafic magmas, so its concentration is a direct measure of the degree of fractional crystallisation. Its abundance also indicates the potential for Zr-bearing minerals to be present. Analysis for a single element facilitated very rapid return of the chemical data, and this permitted us to select geochronology samples while still mapping in the field. The data produced surprises, the main one being that this study obtained a 100% success rate in finding zircon in mafic samples, giving us confidence that the combined approach of geochemical and field characterisation will yield mafic samples amenable to U–Pb isotopic study. Abundant zircon was found in a simple dolerite dyke less than 2 m thick, a rock type which would normally be disregarded as a zircon dating target. Zircon was also found in two ultramafic bodies: plagioclase pyroxenite in the Mordor Complex, and a gabbroic zone within the Papunya ultramafic; in both cases the zircon is considered to be a primary magmatic phase. Two anorthosite intrusions were impor-

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tant targets for dating and on field criteria alone it was thought that these evolved plagioclase-rich rocks would be prospective for Zr minerals. However, they were found to contain <5 ppm Zr, eliminating them as candidates for dating. We speculate that zircon precipitation requires a threshold concentration of Zr to be reached in a fractionating mafic magma. Significant crystallisation of zircon will then strip Zr from the remaining magma and more evolved compositions, such as anorthosite, may then have very low Zr concentrations and no ability to precipitate Zr minerals. In this study, mafic–ultramafic rocks with more than 60 ppm Zr all contained zircon. Samples were taken in the field and stripped of surface weathering at the outcrop. Mineral separation used conventional gravity and magnetic techniques; no separate search for baddeleyite was conducted because zircon was found in all samples. Grains were hand picked and mounted together with a standard zircon in epoxy discs, sectioned approximately in half by polishing, and then photographed at high magnification and imaged by cathodoluminescence (CL) to reveal interior zoning. Analysis for U–Pb isotopic compositions used the SHRIMP II ion microprobe at the Research School of Earth Sciences, Australian National University. A primary ion beam of O2 was focussed to approximately 20–30 ␮m diameter and used to probe selected zones within target zircons. Mass resolution >6500 ensured that significant spectral interferences were eliminated. Each analysis comprises seven scans through the masses of interest, and analyses of the reference zircon were interspersed between each four analyses of unknowns to track instrumental behaviour. Data processing used the software of Ludwig (2001, 2003) in which the approach to peak switching is double interpolation, and which applies the calibration protocols described by Claou´eLong et al. (1995). Decay constants used are those recommended by Steiger and Jaeger (1977). Corrections for common Pb used the measured abundances of 204 Pb and assumed the 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. Data obtained for the QGNG zircon also offer a measure of mass fractionation, although the reference 207 Pb/206 Pb ratio of QGNG is not well constrained (Black et al., 2003). Throughout this study SHRIMP returned 207 Pb/206 Pb ages in the range 1840–1845 Ma for QGNG, consistent with mass fractionation in the range 0.25–0.5%, and this amount of correction has been applied. All mean ages are quoted with 95% confidence uncertainties.

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3. Zircon age constraints Isotopic age results are described here in geographic groups of related event chronologies. The reader is referred to Hoatson et al. (this issue) and to Hoatson and Stewart (2001) for field, petrological and geochemical descriptions of the intrusions, together with outcrop maps and sample locations. Supplementary Tables 1–17 with isotopic data for the samples described here are in Appendix A and available online in OZCHRON, the geochronological database of Geoscience Australia, at www.ga.gov.au. Summary details of each sample are listed in Table 1. 3.1. Irindina province 3.1.1. Riddock Amphibolite The Riddock Amphibolite is a large composite belt of amphibolitic metamorphosed igneous and sedimentary units within the Harts Range Group (Maidment et al., 2004). Metamorphic grade ranges from upper amphibolite to local areas of granulite grade. Lithologies include both compositionally layered and massive amphibolites, interlayered with metasedimentary gneisses, and a minor component of intrusive ultramafic and gabbroic bodies. Layer-parallel fabrics and penetrative foliations frustrate confident identification of original lithologies, but the sequence is considered to represent basaltic flows and subvolcanic equivalents interlayered with sediments (Hoatson and Stewart, 2001). The geochronology sample is a compositionally layered amphibolite with quartz–garnet segregations. Although sampled for its field appearance as possibly a metamorphosed igneous lithology, the zircons contained in it indicate a more likely metasedimentary origin. Zircons in the sample all have the ovoid, mutifaceted form associated with metamorphic zircon and have extremely high optical clarity, indicating low degrees of metamictisation and trace-element substitution. Most grains are internally homogeneous, but a few have small discordant cores which were probed separately (Fig. 2l). Their high optical clarity is a reflection of anomalously low abundances of uranium ranging as low as 5 ppm and this posed a challenge to SHRIMP analysis requiring extremely long count times and careful checks of the mass spectrum before and after each analysis for possible beam drift and small mass interferences. Despite this difficulty, isotopic data for the metamorphic zircons are coherent and clearly indicate a Phanerozoic crystallisation age. On the Concordia diagram (Fig. 3) raw isotopic compositions without correction for common Pb array as a simple mixing line

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Table 1 Zircon characteristics in Arunta Region mafic intrusions Unit name

Lithology

GDA94 East

North

Zircon morphologies

Comments

Ages

Event

Probably para-amphibolite (metasediment)

<734 ± 44 461 ± 6

Maximum deposition age Metamorphism

1786.4 ± 4.2

Igneous crystallisation

Dyke cuts Attutra Metagabbro Intruded by Attutra Metagabbro

1775.2 ± 4.6

Igneous crystallisation

1807 ± 17 478 ± 13

Igneous crystallisation Metamorphism

Intrudes adjacent syenite Coeval with granite melt

1133 ± 5

Igneous crystallisation

1632.9 ± 2.8

Igneous crystallisation

1636.5 ± 2.4 1639.2 ± 2.0 1634.6 ± 4.8 1710–1660

Igneous crystallisation Igneous crystallisation Metamorphic zircon Inherited ages

Cuts Johannsen Metagabbro and Harry Anorthositic Gabbro Intrudes Johannsen Metagabbro, cut by dolerite dyke

1689 ± 8

Igneous crystallisation

1786.9 ± 3.3 1685 ± 20

Igneous crystallisation Metamorphism

Riddock Amphibolite

Amphibolite

473902

7448845

N/a

Ovoid, multifaceted metamorphic zircons with small cores

Attutra Metagabbro

Gabbro

635228

7499425

153

Tonalite dyke

Tonalite

635228

7499425

N/a

Bonya Schist

Psammitic schist

635184

7499424

N/a

Euhedral zircons with broad euhedral zoning Euhedral zircons with broad euhedral zoning Euhedral prismatic igneous zircons

Mordor Complex

Plagioclase pyroxenite

448651

7408367

112

Andrew Young Hills

Gabbronorite

697394

7474000

115

Papunya gabbro Papunya ultramafic South Papunya gabbro

Gabbro Gabbronorite Gabbro

775879 776315 789617

7423495 7421751 7420123

221 43 35

Part-resorbed euhedral zoning Part-resorbed euhedral zoning Range of igneous morphologies and rare ovoid multifaceted grains

Dolerite dyke

Dolerite

404377

7429359

85

Broad internal growth zones

Un-named tonalite

Tonalite

404449

7429436

93

Metamorphic rims enclosing complex recrystallised cores

Johannsen Metagabbro

Mafic granulite

405982

7429714

58

Metamorphic rims enclosing complex recrystallised cores

Cut by Harry Anorthositic Gabbro, tonalite, and dolerite dyke

1805.4 ± 3.4 1697 ± 7

Igneous crystallisation Metamorphism

Enbra Granulite

Mafic granulite

383477

7433235

93

Metamorphic rims enclosing complex recrystallised cores

Coeval with granite melt

1810.7 ± 2.6 1685 ± 11

Igneous crystallisation Metamorphism

Euhedral zircons with broad euhedral zoning Part-resorbed euhedral zoning

Contaminated with country rock, age may be contact metamorphism

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Zr (ppm)

Igneous crystallisation Metamorphism Metamorphism

Youngest inheritance

1771 + 10/−6 1725 ± 11 1591 ± 6

1882 ± 6 Intrudes Dead Bullock Formation Broken and abraded crystals 7729354 597929 Dolerite Coora Dolerite (Tanami Region)

273600 Felsic granulite

7418972

N/a

Igneous crystallisation 1774.0 ± 1.9

Coeval with felsic melt Coeval with mafic melt Metamorphic rims enclosing complex recrystallised cores Metamorphic rims enclosing complex recrystallised cores 273600 Mafic granulite

Mount Chapple Metamorphics Mount Chapple Metamorphics

7418972

N/a

Igneous crystallisation Metamorphism 1803 ± 5 1700 ± 17 Metamorphic rims enclosing complex recrystallised cores 118 300975 Mafic granulite

East

Mount Hay Granulite

7407259

North GDA94 Lithology Unit name

Table 1 (Continued )

182

Event Zr (ppm)

Zircon morphologies

Comments

Ages

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between radiogenic Pb on the Concordia and the composition of common Pb on the Y-axis. Projected onto Concordia, the common-Pb corrected compositions of 20 analyses agree at a weighted mean 206 Pb/238 U age of 461 ± 6 Ma (95% confidence, MSWD = 0.83). Two compositions have lower 206 Pb/238 U ratios than the main group and as outliers are not included in this mean. On the basis of the metamorphic character of the zircons, the 461 ± 6 Ma age is considered to record upper amphibolite facies metamorphism of the Riddock Amphibolite. Fourteen cores were probed within these metamorphic zircons. They have more normal U contents between 20 and 700 ppm U, and most have slightly discordant compositions. Their 207 Pb/206 Pb ages are spread across the range ca. 735–1920 Ma. There are four cores older than 1750 Ma, six in the range ca. 1640–1150 Ma, and three ca. 950–1050 Ma. The two youngest grains agree at a mean 207 Pb/206 Pb age of 734 ± 44 Ma (95% confidence). The dispersed ages of these zircon cores are consistent with the sample including a metasedimentary component of detrital grains. The Mesoproterozoic age components indicate a provenance distinctly younger than known Arunta Region crust. Taken together, this evidence brackets deposition of the protolith of the Riddock Amphibolite to the period between 734 ± 44 Ma (the youngest inherited zircon cores) and 461 ± 6 Ma (the age of metamorphism). 3.2. Warumpi Province mafic intrusions Four mafic intrusions relating to the Warumpi Province, in the south of the Arunta Region, were selected for geochronology to complement research into this terrane. The northern boundary of the Warumpi Province as defined by Close et al. (2003) encompasses the three ‘Papunya’ mafic–ultramafic intrusions. The large Andrew Young Hills mafic intrusion lies north of this line, within the Aileron Province, but its age associates it with the same magmatic event as the Papunya intrusions to its south. 3.2.1. Andrew Young Hills intrusion On the basis of geophysical interpretation, the Andrew Young Hills mafic intrusion has a very wide regional extent, but its only outcrop expression is five prominent hills of gabbro south of the Ngalia Basin (see Fig. 7 in Hoatson et al., this issue). Young et al. (1995) reported a zircon U–Pb age of 1635 ± 9 Ma for a granitic enclave within it and suggested that the mafic magma could be younger. To date the mafic magma directly, a

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Fig. 2. Cathodoluminescence (CL) images showing the range of zircon zoning expressions in Arunta Region mafic–ultramafic intrusions. Scale bar at top for reference. (a) Andrew Young Hills intrusion; (b) Papunya gabbro; (c) Papunya ultramafic: the amoeboid partly resorbed forms with truncated euhedral internal growth-zoning are typical of igneous zircon in mafic magmas. (d) Johannsen Metagabbro: dark-CL remnants in the centre of the grain preserve the original ca. 1805 Ma crystallisation age; bright-CL overgrowth formed at ca. 1690 Ma during granulite-grade metamorphism; grey-CL recrystallisation throughout the grain interior has dispersion of ages 1800–1700 Ma. (e) Tonalite associated with Harry Anorthositic Gabbro: thin bright-CL overgrowth and bright-CL recrystallisation of grain interior (ca. 1685 Ma) overprinting ghosts of original ca. 1785 Ma igneous zoning in the grain interior. (f) Dolerite dyke intruding Harry Anorthositic Gabbro: resorbed zircon shape similar in form to grains in (a–c). (g) Mount Hay Granulite; (h) Enbra Granulite: similar to (d–e) with dark-CL remnants of original igneous zircon (ca. 1810 Ma) pervasively recrystallised and overgrown by metamorphic zircon at ca. 1690 Ma. (i) Mount Chapple Metamorphics mafic lithology: broad internal zoning and partly resorbed form similar to (a–e). (j) Mordor Complex pyroxenite: euhedral igneous zircon with broad internal growth zoning; (k) Attutra Metagabbro: indistinct wispy zoning and partly resorbed zircon shape; (l) Riddock Amphibolite: ovoid multifaceted metamorphic zircon with small remnant cores of detrital zircon.

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3.2.3. Papunya ultramafic A sample of the Papunya ultramafic with 43 ppm Zr similarly contains a uniform population of partly resorbed igneous zircons similar to those in the Papunya gabbro (Fig. 2c). Most analysed grains lie within error of Concordia (Fig. 4c) with only three showing evidence of slight Pb loss. All analyses, including the three slightly discordant ones, have the same 207 Pb/206 Pb age within error giving a weighted mean age of 1639.2 ± 2.0 Ma (95% confidence, MSWD 1.02) which is interpreted to record the igneous crystallisation of the ultramafic body.

Fig. 3. Concordia diagram of zircon isotopic data in the Riddock Amphibolite. The major mixing trend between common and radiogenic Pb is for the total Pb compositions of metamorphic zircons before common Pb correction; core compositions are plotted as radiogenic compositions.

sample of gabbronorite with 115 ppm Zr was selected for geochronology. Zircons in the sample are of high optical clarity but have anhedral, partly resorbed shapes (Fig. 2a). Internal growth zoning is truncated by the ovoid or amoeboid crystal outlines, consistent with late partial resorption of igneous zircon. The zircons have very uniform isotopic compositions. In the Concordia diagram (Fig. 4a) all 44 analyses are within error of Concordia and the 207 Pb/206 Pb ages agree at a weighted mean of 1632.9 ± 2.8 Ma (95% confidence, MSWD 1.01). This simple age result is interpreted to record the crystallisation age of the gabbronorite and is consistent with the 1635 ± 9 Ma granite sample of Young et al. (1995) being coeval. 3.2.2. Papunya gabbro Further south near Papunya, a series of small gabbroic and ultramafic intrusions includes the Papunya gabbro and the Papunya ultramafic bodies (see Fig. 6 in Hoatson et al., this issue). The metamorphically recrystallised gabbro is massive and not compositionally layered. A sample with 221 ppm Zr was chosen for geochronology and found to contain abundant clear, subhedral zircons (Fig. 2b). Like the Andrew Young Hills example, internal euhedral oscillatory growth zoning is truncated by partly resorbed crystal outlines (Fig. 2b). All the isotopic compositions lie within error of Concordia (Fig. 4b) and agree at a weighted mean 207 Pb/206 Pb age of 1636.5 ± 2.4 Ma (95% confidence, MSWD 1.2) which is interpreted to record the igneous crystallisation of the gabbro.

3.2.4. South Papunya gabbro Southeast of Papunya is a third mafic unit named the South Papunya gabbro. At the lower contact of the gabbro with underlying felsic country rocks, minor contact melting is observed, with back-intrusion of felsic melt into the gabbro. A sample was collected approximately 8 m above the lower contact of the intrusion with the country rocks. Zircons in this sample are abundant but heterogeneous in appearance. The dominant form (90%) is euhedral prismatic grains with oscillatory growth zoning characteristic of igneous crystallisation. A subordinate quantity of grains comprises multifaceted euhedral crystals lacking growth zoning. In the Concordia diagram (Fig. 4d), all the zircon compositions are concordant or near-concordant, indicating minimal isotopic disturbance. The dominant prismatic zircons have an array of 207 Pb/206 Pb ages between 1700 and 1670 Ma in which the dispersion of ages is well beyond measurement error (MSWD > 4), indicating the presence of multiple geological age populations. In contrast, the rare multifaceted grains have a uniform age: only 11 were found, but all agree within error at a weighted mean 207 Pb/206 Pb age of 1634.6 ± 4.8 Ma (95% confidence, MSWD 1.7), distinctly younger than the other grains. It is likely that the dispersed age range of the older group reflects an origin as xenocrysts ingested at the gabbro margin by the observed backmelting of the country rocks. The age significance of the rare multifaceted grains is more problematic. Their age association with the other ca. 1635 Ma intrusions can be used to support a view that they record igneous crystallisation of the gabbro. However, their ovoid multifaceted form and low Th/U compositions are normally associated with metamorphic zircon crystallisation, so it is possible that they record the severe contact metamorphism of country rock by the intruding gabbro and were ingested from the country rock orthogneiss together with the other grains. Either view leads to interpretation that the 1634.6 ± 4.8 Ma age records the gabbro emplacement.

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Fig. 4. Concordia diagrams of zircon compositions in Warumpi Province mafic intrusions: (a) Andrew Young Hills gabbro, (b) Papunya gabbro, (c) Papunya ultramafic, and (d) South Papunya gabbro contains inheritance of 1710–1660 Ma zircons, and a crystallisation age of ca. 1635 Ma for rare grains of distinctive morphology (see text).

3.3. Mafic granulites in the central Aileron Province 3.3.1. Field relations of the mafic granulites The central part of the Aileron Province, north and west of Alice Springs, includes the large mafic granulites bodies at Mount Hay and Mount Chapple, and smaller units at Enbra Hills and Johannsen. They have been deformed and metamorphosed by multiple events and are interfolded with intermediate and felsic units which are also at granulite grade. Relationships between these mafic granulite bodies are not clear because they are separated by later structures. However they are linked by proximity to the Harry Creek Thrust which separates Mount Hay and Mount Chapple in the east of the region, bounds the south and north margins

of the Enbra Granulite, and passes immediately south of the Johannsen Metagabbro. There are no published geochronological constraints, although a field guide (Bonnay et al., 2000) refers to unpublished SHRIMP zircon data from Mount Hay, indicating 1819 ± 9 Ma as the crystallisation age of charnockite, 1790 ± 5 Ma and 1774 ± 7 Ma in two samples of tonalitic leucosomes, and two ages of 1783 ± 5 and 1756 ± 17 Ma in a granitic dyke. Together, these unsubstantiated ages were used to suggest igneous emplacement at Mount Hay at 1819 ± 9 Ma with complex subsequent metamorphism in the period 1790–1750 Ma; a younger metamorphism at <1600 Ma was suggested for Mount Chapple. This geochronology study has focussed on the Johannsen Metagabbro, which is a series of 50–250 m

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Fig. 5. Concordia diagrams of zircon in Aileron Province rocks associated with the Johannsen Metagabbro mafic granulite. (a) Zircons in a late vertical dolerite dyke. (b) Zircons in tonalite associated with the Harry Anorthositic Gabbro have overgrowths with low-U and an imprecise age ca. 1685 Ma, and cores with a primary age ca. 1787 Ma. Isotopically disturbed data within the cores omitted for clarity. (c) Zircons in the Johannsen Metagabbro have rims formed at ca. 1696 Ma and cores with a primary age of 1805 Ma. Disturbed ages within the cores omitted for clarity. (d) Histogram of apparent ages in the variably recrystallised zircon cores of zircons in the Johannsen Metagabbro. Compositions range continuously between the primary ages of cores and overgrowths.

wide mafic granulite sheets separated by felsic granulite, exposed in the Utnalanama Range. At this important location, field relationships establish the relative timing of three distinct generations of mafic igneous rock and these can be used to constrain the isotope geochronology (see Fig. 5 in Hoatson et al., this issue). The southeastern extent of the mafic granulite sheets is truncated by intrusion of the large east–west trending body of the Harry Anorthositic Gabbro. This anorthosite has been metamorphosed to upper amphibolite or possibly granulite facies, consistent with it having shared the metamorphic history of the earlier Johannsen mafic sheets. The margin of the anorthosite is intimately associated with large volumes of tonalite, with mutually intruding contacts and

trace amounts of quartz present in the anorthosite where it is in contact with tonalite; on this basis it is interpreted that the tonalite and anorthosite magmas may have been coeval, or the tonalite may be slightly younger. A swarm of thin (1–3 m), north–northeast trending dolerite dykes of much lower metamorphic grade (sub-amphibolite facies) cuts all the high-grade metamorphic rocks. 3.3.2. Dolerite dyke The late dolerite dyke has 85 ppm Zr and yielded abundant zircon crystals. The zircons are all colourless with high optical clarity, and have remnants of original crystal faces but are mostly amoeboid, partly resorbed shapes (Fig. 2f); interior zoning has the broad zones asso-

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ciated with zircon crystallised from mafic-intermediate magmas. These features are similar to the igneous zircons in the Warumpi Province gabbros and are consistent with igneous zircons formed in, and then partly resorbed by, their host mafic magma. The U–Pb isotope systematics of the dyke zircons are relatively simple. The Concordia diagram (Fig. 5a) shows the compositions to be concordant, and they all agree within error at a weighted mean 207 Pb/206 Pb age of 1689 ± 8 Ma (95% confidence, MSWD 1.3). This is interpreted to record the igneous crystallisation age of the dolerite dyke. 3.3.3. Tonalite associated with Harry Anorthositic Gabbro Geochemical sampling found the Harry Anorthositic Gabbro to contain less than 5 ppm Zr throughout and, therefore, not prospective for zircon dating. A sample of the adjacent, and possibly coeval, tonalite was taken and this yielded abundant zircons. These zircons have twostage growth revealed in CL (Fig. 2e). Internally they have relatively low luminescence and euhedral oscillatory growth zoning, and this zoning is truncated by an ovoid resorption front. Overgrowing the resorption surface on about half of the grains is a thin and distinctive brightly luminescent zone which forms ovoid multifaceted outer crystal faces, typical of metamorphic zircon growth. Only a few of the overgrowths are thick enough for the 20–30 ␮m SHRIMP microbeam to sample, but 10 were targeted without overlapping any core material. Consistent with their bright luminescence they have low U (30–70 ppm) but they have higher thorium abundances, with Th/U ratios between 3 and 8. The U–Pb isotopic compositions are all concordant (Fig. 5b), and they agree within error at a weighted mean age of 1685 ± 20 Ma (95% confidence, MSWD 0.6). The low precision of this result reflects the low-U, low-Pb compositions and the small number of analyses; however, it is comparable with the 1689 ± 8 Ma magmatic age of the dolerite dyke. The compositions of the zircon cores are completely different, with 300–2000 ppm U and Th/U ratios less than 1. Twenty-one analyses of the cores agree at a concordant mean 207 Pb/206 Pb age of 1786.9 ± 3.3 Ma (95% confidence, MSWD 1.3). A number of analyses have apparent ages intermediate between these two end-member ages, which is attributed to partial metamorphic recrystallisation within the igneous cores (see Fig. 2e). The evidence leads to interpretation that the 1786.9 ± 3.3 Ma age records the igneous crystallisation of the tonalite, and 1685 ± 20 Ma records a metamorphic event that generated both new zircon overgrowths and

partial recrystallisation of some of the original igneous zircon. 3.3.4. Johannsen Metagabbro Most of the mafic granulite sheets that comprise the Johannsen Metagabbro have less than 45 ppm Zr, but a sample with 58 ppm Zr yielded abundant zircon crystals. There is preservation of the original sheet-like intrusion form of the mafic units indicating minimal deformation and low strain during the granulite-grade metamorphism. The zircons are internally very complex (Fig. 2d). The majority have thin brightly luminescent overgrowths defining their present crystal shape, which has the ovoid and multifaceted form of metamorphic zircon. These rims surround complex cores in which a grey-luminescent phase appears to be a recrystallisation front replacing an earlier dark-CL phase which is preserved as isolated remnants. Also present are patches of bright-luminescent zircon similar in appearance to the overgrowths; this is also interpreted as partial recrystallisation in situ because wispy ghost-remnants of original oscillatory zoning are retained. More than 100 analyses were made of these complex zircons in an effort to unravel the age complexity. The outermost overgrowths have simple compositions. They have relatively low uranium contents between 40 and 140 ppm U and constant Th/U ratios of ca. 0.2. The 18 analyses are all concordant (Fig. 5c) and although some are of low precision, they agree within error at a mean 207 Pb/206 Pb age of 1696 ± 8 Ma (95% confidence, MSWD 0.8), which is interpreted to record metamorphic zircon growth. Within the zircon cores, 17 analyses of the black CL phase form a group of concordant compositions with moderate uranium (300–1500 ppm U) and Th/U ratios in the range 0.1–1. The replicate analyses agree at a weighted mean 207 Pb/206 Pb age of 1805.4 ± 3.4 Ma (95% confidence, MSWD 1.5). This is the best available estimate of the primary crystallisation age of the cores; there is evidence that many of the core interiors have been partly recrystallised, so it is not possible to exclude the potential that the group includes some disturbed isotopic compositions. A single analysis is distinctly older at 1844 ± 11 Ma (σ). Between these two ages for cores and overgrowths there are more than 60 measured compositions in the zircon cores with individual apparent ages in a continuum between ca. 1790 and 1700 Ma (Fig. 5d). There is complete overlap of data between isotopically altered black-CL cores and grey or bright-luminescent recrystallised patches: black-CL cores have apparent ages as young as 1697 ± 7 Ma; conversely, bright-luminescent

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recrystallisation patches range as old as ca. 1780 Ma. This continuum is interpreted as the result of variable partial recrystallisation of the ca. 1805 Ma zircon cores in response to the thermal event that produced the ca. 1696 Ma overgrowths. It is possible that the continuum masks the presence of one or more discrete thermal events but it is not possible to relate any age systematically to zircon zoning. Thus, the zircons in the Johannsen Metagabbro contain evidence for primary zircon crystallisation at 1805.4 ± 3.4 Ma. Tonalite and, by association, the Harry Anorthositic Gabbro, were emplaced at 1786.9 ± 3.3 Ma. Metamorphic zircon in the tonalite at 1685 ± 20 Ma and in the Johannsen Metagabbro at 1696 ± 8 Ma compare with the 1689 ± 8 Ma age of the dolerite dyke cutting them both. This interpreted sequence of events is completely consistent with the relative timing imposed by field relations between the various mafic units. 3.3.5. Enbra Granulite The Enbra Granulite is a fault-bounded wedge-shaped body of granulite west of the Johannsen Metagabbro at the western end of the Strangways ranges (Fig. 1). Its outcrop covers an area of 8 km × 15 km and aeromagnetics indicate an overall size of about 10 km × 28 km, with boundaries defined by fault splays from the Harry Creek Thrust. This body has a uniform composition of mafic granulite throughout, although interlayering with felsic granulites is present locally. In areas of low strain, flattened mafic pillows chilled against felsic lithology, and intricate net-vein textures, support an intrusive origin for the mafic magma coeval with the felsic magmatic host. More evolved, plagioclase-rich and anorthositic rocks are present in the north. Preservation of original intrusion relationships, and net-veining textures, indicate minimal deformation and low strain during granulite-grade metamorphism. Zircons in the Enbra Granulite sample (Fig. 2h) are very similar in form to those in the nearby Johannsen Metagabbro, comprising dark-CL cores with variable patches of grey or bright-CL recrystallisation. Distinctive bright-CL overgrowths are also present but most are too thin to probe with the ca. 20–30 ␮m SHRIMP primary beam. The pattern of isotopic compositions is also similar. Data from the dark-CL cores are dominated by a group of concordant analyses close to 1810 Ma, and there is continuity of progressively younger compositions down to ca. 1700 Ma (Fig. 6a). As in the Johannsen Metagabbro, this is interpreted to reflect an original crystallisation age overprinted by partial recrystallisation within the zir-

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con cores. Statistical discrimination of the primary group indicates that 39 of the core compositions are within error of a mean 207 Pb/206 Pb age of 1810.7 ± 2.6 Ma (95% confidence; MSWD 1.6); the possibility that this group includes one or more slightly disturbed isotopic compositions cannot be excluded, however. Only six of the bright-CL overgrowths were probed successfully. Four have a mean 207 Pb/206 Pb age of 1685 ± 11 Ma, while two have slightly younger apparent ages near 1650 Ma; this is sparse evidence but 1685 ± 11 Ma is an estimate of the overgrowth crystallisation age, and the two younger measurements could reflect subsequent isotopic disturbance. 3.3.6. Mount Hay Granulite Mount Hay is a prominent massif of granulites rising more than 600 m above the alluvial Burt Plain west of Alice Springs (see Fig. 2 in Hoatson et al., this issue). It is a fault-bound wedge-shaped body between the Harry Creek Deformed Zone to the north (which separates it from Mount Chapple) and the Redbank Thrust to the south. Mount Hay is dominated by variably deformed mafic granulites interlayered with subordinate felsic and intermediate lithologies. To the northeast it appears to be faulted against a smaller granulite body which is dominated by anorthosite (Anburla Anorthosite). In rare less deformed zones there are exposures, similar to those in the Enbra Granulite, of field relations that indicate interaction of coeval mafic and felsic magmas. Near the northeast margin of the main mafic granulite body are small exposures of metasediments and gneisses in different stages of migmatisation, which may represent country rocks intruded by the mafic magma. The mafic granulite was sampled where the highest abundance of Zr (118 ppm) was found along a geochemical traverse. The internal zoning of these zircons is similar to that in the Johannsen Metagabbro and comprises variably recrystallised grey-CL cores preserving in the interiors rare small embayed zones of black-CL (Fig. 2g). Many grains have a distinctive bright-CL overgrowth. In some grains the bright-CL overprint takes the form of variable recrystallisation rather than overgrowth and penetrates into grain interiors with variable preservation of wispy ghosts of the earlier zoning pattern. These zoning expressions resolve into the same age components as found in the Johannsen Metagabbro (Fig. 6b), but with the complication that very few remnant cores or overgrowths are large enough to probe. Of the analyses that successfully targeted the black-CL cores without overlap onto recrystallisation, 12 agree within error at a weighted mean 207 Pb/206 Pb age of 1803 ± 5 Ma (MSWD 1.6),

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Fig. 6. Concordia diagram of zircon compositions in Aileron Province mafic granulites similar to the Johannsen Metagabbro. (a) Enbra Granulite showing primary crystallisation age (shaded) and disturbed (unshaded) compositions; (b) Mount Hay Granulite with disturbed compositions omitted for clarity; (c) histogram of the continuum of partly recrystallised core compositions in Mount Hay Granulite zircons, showing their similarity to the range found in the Johannsen Metagabbro.

and this is an estimate of the original crystallisation age. Two cores have distinctly older ages near 1850 Ma and may represent inheritance similar to the single ca. 1840 Ma core found in the Johannsen Metagabbro. Of the targeted zones of bright-CL overgrowth or recrystallisation, 10 analyses agree at a mean 207 Pb age of 1700 ± 17 Ma (MSWSD 1.3); the imprecision of this age reflects the sparse data and relatively low-U compositions. Four analyses have younger apparent ages in the range 1640–1560 Ma representing isotopic disturbance at an unconstrained later date. The remaining 50 analyses (65% of the data for the sample) are a continuum of ages for internal zones with various tones of grey in CL, interpreted to represent variable degrees of recrystallisation of the original black-CL cores.

3.3.7. Mount Chapple Metamorphics Mount Chapple is an east-trending 62-km long body of granulite grade metamorphic rocks in the central Aileron Province, separated from Mount Hay to the southeast by the Harry Creek Thrust. Compared with Mount Hay, which is dominated by mafic lithologies, Mount Chapple contains a higher proportion of felsic and intermediate rock types, and metasediments. Mafic granulites form large irregular bodies, pods, or layer-parallel lenses within felsic granulite. In rare areas of low strain, there is preservation of mafic pillows with cuspate margins chilled against felsic rock, indicating coexisting mafic and felsic magmas at the time of emplacement. Separate samples of mafic and felsic lithology were taken in one such low-strain zone where

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Fig. 7. Concordia diagram of zircons compositions in the Mount Chapple Metamorphics. (a) Mafic lithology zircons have a primary age at ca. 1774 Ma with incoherent data for metamorphically recrystallised areas. (b) Felsic lithology zircons have a similar primary age and two distinct metamorphic ages.

the coeval felsic–mafic magma relationship could be observed. Zircons in the mafic lithology have forms similar to those of the Warumpi Province mafic intrusions, but with added complexity. They have amoeboid outer shapes that truncate internal broad euhedral growth zoning with an overall dark-CL response, consistent with partial resorption of original igneous zircons (Fig. 2i). There are no metamorphic overgrowths, but overprinting the internal zoning in some grains are areas of metamorphic recrystallisation evident as grey or bright-CL patches of irregular shape embaying and replacing the earlier growth zoning. Isotopic data for the different zoning expressions are shown in Fig. 7a and are dominated by a coherent cluster

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of analyses of the euhedral and dark-CL zones with a mean 207 Pb/206 Pb age of 1774.0 ± 1.9 Ma (MSWD 1.1). This is interpreted as the primary crystallisation age. Isotopic data for the recrystallisation patches are difficult to interpret. There is a continuum of individual ages over a 400 million year range from ca. 1760 Ma to a single nearconcordant analysis at ca. 1320 Ma. Most are older than 1650 Ma, but there are five distinctively low Th/U compositions with apparent ages between 1500 and 1580 Ma. Together these data are evidence of a complex and protracted metamorphic history but define no clear overprint age. In an effort to unravel this complexity, zircons were extracted from the adjacent felsic lithology which has contact features suggesting coeval liquid interaction with the mafic magma. These zircons differ from grains in the mafic unit in having distinct darkluminescence overgrowths of metamorphic appearance. However, the oscillatory-zoned cores mantled by this later growth are similar to the zircons in the adjacent mafic lithology, overprinted to variable degrees by irregular patches of internal recrystallisation. Isotopic data for the primary oscillatory-zoned phase are concordant but the 20 measured ages are dispersed beyond error with an MSWD of 3 (Fig. 7b). The median 207 Pb/206 Pb age of the group is 1771 + 10/−6 Ma, similar to the igneous crystallisation age of the mafic unit. It is interpreted that this dispersion reflects a range of primary ages inherited from the country rocks that were melted to form the felsic lithology; the data may also include cryptic areas of partial metamorphic recrystallisation. Five analyses wholly within recrystallisation patches are concordant and define a mean 207 Pb/206 Pb age of 1725 ± 11 Ma (MSWD 1.8). Most of the dark-CL overgrowths are too thin to probe, but six were analysed successfully and five agree at a mean 207 Pb/206 Pb age of 1591 ± 6 Ma (MSWD 0.8). Together, these data for adjacent mafic and felsic units constrain the emplacement age of the Mount Chapple lithologies and a complex evolution of highgrade metamorphic events. Zircon cores in both lithologies record a primary crystallisation age best estimated at 1774.0 ± 1.9 Ma in the mafic unit. The most systematic record of metamorphic effects is in the felsic unit zircons, in which internal recrystallisation of zircons took place at 1725 ± 11 Ma and a second high-grade event at 1591 ± 6 Ma produced distinctive overgrowths. Zircon compositions in the mafic unit, while consistent with this history, have the definition of metamorphic ages blurred by subsequent isotopic leakage.

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3.4. Eastern Aileron Province 3.4.1. Attutra Metagabbro The Attutra Metagabbro intrudes metasediments of the Bonya Schist in the east Arunta Region (see Fig. 4 in Hoatson et al., this issue). It is a relatively homogeneous, weakly recrystallised metagabbro body with good preservation of original igneous textures. At its western contact the gabbro has a ca. 15-m wide chilled zone that contains enclaves of variably metamorphosed Bonya Schist. The country rock is locally melted by contact with the gabbro, and felsic melts back-intrude the gabbro, resulting in complex relative timing relationships around the gabbro margin. Minor tonalite and granite dykes have mutually crosscutting relationships with each other, the gabbro and the country rock; and net-vein features indicate local hybridisation of coexisting felsic and mafic magmas. The geochronology sample was taken from the chilled margin of the gabbro on the basis of its high Zr content (153 ppm Zr) and was found to contain abundant zircons. Samples were also obtained from a thin dyke of tonalite intruding the gabbro margin, and from the Bonya Schist country rock ca. 40 m from the gabbro contact. Zircons in the gabbro are of uniform appearance, with well-defined euhedral crystal shapes and indistinct growth zoning consistent with igneous zircon crystallisation (Fig. 2k). The Concordia diagram (Fig. 8a) shows a homogeneous and concordant population of zircon compositions with only four discordant grains (off-scale in the diagram). The 34 concordant grains agree at a weighted mean 207 Pb/206 Pb age of 1786.4 ± 4.2 Ma (95% confidence, MSWD 1.4). The origin of the zircons in the gabbro must be taken into consideration before interpreting the significance of this age. Contact melting of country rocks, and gabbro–granite–tonalite magma hybridisation at the gabbro margin, indicate that zircons in the gabbro near its margin could be xenocrysts ingested from country rock interaction—as with the South Papunya Gabbro described above. In an effort to constrain this issue, zircons in the country rock were also studied. 3.4.2. Bonya Schist The Bonya Schist, where it is intruded by the gabbro, is a strongly foliated metasediment. Zircons in it are of uniform appearance with euhedral prismatic crystal shapes, pyramidal terminations, and oscillatory growth zoning consistent with igneous crystallisation. Many of the grains are fragile, elongate shapes. This morphological type and consistency, and the coherent age composition of the grains (see below) lead to interpretation that

the lithology is a volcaniclastic unit. The zircons have an extreme range of U contents, ranging from a few hundred to more than 5000 ppm U, and they have responded differentially to post-crystallisation processes in proportion to the very different radiation doses they each have experienced. Nearly every grain is discordant to some degree (Fig. 8b), and common Pb contents range from below 1% to extreme values of up to 50%. It is clear that many of the highly metamict grains have leaked radiogenic Pb and exchanged common Pb with the environment subsequent to crystallisation. The compositions of zircons with high common Pb have large uncertainties and ambiguous calculated ages, so those with more than 1.5% common Pb (about half of the analyses) have been excluded from further consideration. The 26 analyses with less than 1.5% common Pb form a distinctive discordia trend (Fig. 8b) with a Proterozoic upper intercept and Phanerozoic lower intercept. There is a strong correlation with U content: low U compositions are clustered towards the older end of the discordia, and grains progressively higher in U track towards the lower intercept. Regression of the data yields an MSWD of 1.7 with an upper intercept on Concordia at 1807 ± 17 Ma, and a lower intercept at 478 ± 13 Ma. The observed MSWD of 1.7 is slightly higher than the upper value of 1.6 expected of 26 observations at 95% confidence, but the degree of coherence indicates only slight secondary perturbation of the original discordia trend. The primary crystallisation age of these igneous zircons is estimated by the upper Concordia intercept at 1807 ± 17 Ma. The 478 ± 13 Ma lower intercept provides an age estimate for a thermal event that variably reset isotopic compositions in the zircons in proportion to the radiation dose and metamictisation they had accumulated. The metamorphism was sufficient to generate open system Pb diffusion in high-U (metamict) zircons, but not to generate new zircon crystallisation as overgrowths: this points to a metamorphic grade below the upper amphibolite or granulite grade that promotes metamorphic zircon growth. The timing of this event compares with 461 ± 6 Ma metamorphism recorded in the Riddock Amphibolite 150 km to the west. On this evidence the age relationships of the Attutra Metagabbro and its country rock pose an apparent conundrum. The gabbro can be interpreted as a Palaeoproterozoic body emplaced ca. 20 Ma after the country rocks it intrudes. But zircons in the Bonya Schist register strong effects from a Phanerozoic metamorphism not found in the intrusion a few metres away. Is the Phanerozoic disturbance in the country rock a contact effect from intrusion of the gabbro itself, meaning that the gabbro

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Fig. 8. Concordia diagrams for zircons in mafic–ultramafic units of the eastern Aileron Province. (a) Attutra Metagabbro zircons have a primary concordant age at ca. 1786 Ma and Pb loss (offscale) in a few grains. (b) Zircons in the Bonya Schist country rock a few metres from the gabbro contact have a primary age of ca. 1807 Ma and a discordia towards Palaeozoic isotopic disturbance. (c) Zircons in a tonalite dyke intruding the Attutra Metagabbro have a primary crystallisation age ca. 1775 Ma and minor inheritance, perturbed by Pb loss in a number of grains. (d) Mordor Complex zircons.

intruded at 461 ± 6 Ma and the Proterozoic zircons found within it are ingested xenocrysts? To resolve this ambiguity, zircons were dated from a small tonalite sheet that intrudes the gabbro chilled margin. 3.4.3. Tonalite associated with Attutra Metagabbro As a more felsic magma, the tonalite dyke intruding the Attutra Metagabbro contains undoubtedly primary igneous zircon; and relative timing relations require its crystallisation age to be the same as, or slightly younger than, the gabbro. Many of the igneous zircons in the tonalite have leaked radiogenic Pb, indicating disturbance at an unconstrained age (Fig. 8c), but the majority are concordant and have a weighted mean 207 Pb206 Pb age of 1775.2 ± 4.5 Ma (95% confidence). Three optically distinct cores and one whole grain record older

ages in the range ca. 1810–1870 Ma that identify them as xenocrysts. This Palaeoproterozoic crystallisation age for the tonalite supports interpretation that the zircon age in the gabbro is also its magmatic crystallisation age. The balance of evidence, therefore, favours intrusion of both the metagabbro and its associated tonalite at their indicated zircon crystallisation ages of, respectively, 1786.4 ± 4.2 million years and 1775.2 ± 4.5 million years , ca. 20 million years after deposition of the 1807 ± 17 Ma Bonya Schist that they both intrude. The country rock, and to a lesser extent the tonalite, both record isotopic disturbance at a later date estimated as 478 ± 13 Ma. The metamorphic effect appears to be restricted to the very high-U (and therefore more metamict) zircons in the Bonya Schist.

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3.4.4. Mordor Complex The Mordor Complex is an undeformed composite plug-like body of alkaline and ultramafic igneous rocks outcropping in an inlier partly surrounded by Amadeus Basin sediments (see Fig. 8 in Hoatson et al., this issue). Both syenitic and ultramafic rocks (peridotite and minor pyroxenite) are present. Previous attempts to date the complex include a Rb–Sr whole-rock isochron age of 1128 ± 20 Ma and Rb–Sr mineral isochron age of 1118 ± 17 Ma (Langworthy and Black, 1978, recalculated by Nelson et al., 1989), and a Sm–Nd whole-rock isochron age of 1100 ± 280 Ma (Nelson et al., 1989). To refine this age control and specifically date the ultramafic component, a sample of pyroxenite was selected for its high Zr content from the northeast corner of the complex. Twenty metres east of the sample site, an apophysis of the sampled pyroxenite body intruded and was chilled against adjacent syenite, establishing the relative age relationships at that location. Remarkably, the plagioclase-pyroxenite unit has 112 ppm Zr and yielded abundant large and distinctive zircon crystals (Fig. 2j). They have well-defined crystal faces, some of which are embayed by sharply defined shapes corresponding to intergrown phases, and they contain inclusions of the major minerals of the host pyroxenite. This evidence of igneous intergrowth is consistent with very late zircon crystallisation in the interstices between previously crystallised phases and indicates that the zircon content is a primary igneous feature. The zircons have moderate U contents between 150 and 550 ppm U, higher abundances of thorium (Th/U ratios between 1 and 3) and very low contents of common Pb (less than 0.1%), making their ages insensitive to common Pb correction. In the Concordia diagram (Fig. 8d) their isotopic compositions are uniformly concordant, and 38 analyses agree at a weighted mean 207 Pb/206 Pb age of 1133 ± 5 Ma (95% confidence, MSWD 0.87). On the basis of the intergrowth morphologies of the zircons, this age is interpreted to record igneous crystallisation of the pyroxenite. 4. Zircon in mafic–ultramafic intrusions Obtaining credible age information from Proterozoic mafic–ultramafic intrusions is not a simple matter, but U–Pb isotopic dating of zircons within them can be attempted provided that careful attention is paid to the field, petrological, and geochemical nature of the intrusions. This study has shown that analysis of Zr content is an excellent indicator for the presence of trace zircon in a mafic or even an ultramafic igneous rock. Use of

this simple tool, in combination with thorough field and petrological study, led us to sample rock types—such as a dolerite dyke and ultramafic bodies—that would not otherwise have been considered for zircon dating. Table 1 summarises the zircon characteristics and age data compiled in this study. The origin of zircons in a mafic–ultramafic intrusion cannot be assumed to be magmatic crystallisation: it is a question that must explicitly be answered on a case-by-case basis. In the examples discussed here, the nature of the crystals themselves sometimes provides the evidence. For example, those in the Mordor Complex pyroxenite are intergrown with the major mafic igneous phases. Zircons in the Warumpi Province intrusions and a dolerite dyke of the central Aileron Province have euhedral oscillatory growth zoning characteristic of igneous crystallisation, with partly resorbed amoeboid outer shapes that appear to be a common characteristic of zircons in gabbros. However, the South Papunya gabbro introduces a cautionary note to these apparently simple situations. This gabbro, sampled a few metres from its contact with country rocks, contains an abundance of ingested xenocrysts; the high Zr content that led to sample selection correctly identified that zircon is present in the rock, but few (perhaps none) of them are from in situ magmatic crystallisation. A challenge to confident interpretation was provided by data from the Attutra Metagabbro: a Palaeoproterozoic age for zircons in the intrusion appears to be belied by the Ordovician age of metamorphism within its contact aureole. The ambiguity was only resolved by devising, from field appraisal, detailed tests of the alternative interpretations. This is a good example of how it can be easy to measure an isotopic age, but more difficult to ascribe a correct geological meaning to that age. The most difficult dating problem attempted here is the event timing of the Aileron Province intrusions which are now granulite-grade metamorphic complexes. These rocks have experienced multiple high-grade metamorphic events and it is remarkable that any of them preserve a remnant from their original crystallisation. In the case of the Mount Hay Granulite the overprinted isotopic compositions are so dominant, and the preserved remnants are so sparse (amongst 80 analyses, just a dozen small areas within complex zircon cores) that a confident timing solution could not be proposed from dating of this unit alone. Again, the solution to this difficulty came from observation in the field: recognition that the sequential intrusion elsewhere of the Johannsen Metagabbro, Harry Anorthositic Gabbro, tonalite, and a late dolerite dyke, could be used to constrain the interpretations. The critical property of the mutual relationships between these rock units is that they are observable within the

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same outcrop, and not reliant on inferred correlations across a long distance. Finally, we note the differential response to metamorphism of zircons in mafic–ultramafic rocks and their counterparts in felsic lithologies. The ca. 1640 Ma igneous zircons in the Warumpi Province mafic–ultramafic bodies have formed no overgrowths or internal isotopic disturbance from the Chewings (ca. 1590 Ma) high-grade metamorphism that produced well developed zircon overgrowths in their metasedimentary and meta-granitic country rocks (Close et al., 2003). Similarly, metamorphism of the ca. 1775 Ma Mount Chapple Metamorphics is best recorded in zircons from a felsic lithology. We speculate that this reflects a compositional control, with felsic mineralogies more readily releasing zirconium and silicon to new zircon growth during metamorphic breakdown. At a lower metamorphic grade, zircons in the ca. 1785 Ma Attutra Metagabbro of the eastern Arunta Region record none of the open-system isotopic behaviour found in country rock zircons; this could reflect partitioning of strain, fluid and thermal effects into country rocks, implying rock competency rather than composition as the control in this case. 5. Arunta Region event chronology The first attempt to produce a framework for the Proterozoic systems of the Arunta Region was documented by Shaw et al. (1984) and Stewart et al. (1984) from previous decades of geological mapping. These authors established the primary field relationships of outcropping Proterozoic units and younger parts of the evolution were addressed by Rb–Sr dating. Subsequently, the availability of U–Pb zircon analysis began the process of unravelling the original Palaeoproterozoic evolution, and led Collins and Shaw (1995) to propose a large number of stratigraphic packages and metamorphic events which they attempted to correlate across the region. They drew attention to the complexity of the event systems being investigated, and the sparsity of the isotopic data then available in which ages for some samples were being inferred from less than 10 analyses. SHRIMP data published up to about 1995 did not have the guidance of CL imaging of the internal character of the zircons being probed. The ages reported here for Arunta Region mafic– ultramafic intrusions are listed in Table 2. They are based on large quantities of data acquired with CL imaging, and a remarkably consistent pattern emerges. Across the region there are distinct episodes when a great deal (geologically) is happening, and long periods between

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when no magmatic or high-grade metamorphic process is recorded. Age uncertainties of better than 5 Ma for the primary magmatic ages highlight the very close age correspondence of magmatism within any one association. Metamorphic ages have lower precision, reflecting the lower U contents in metamorphic zircons and the rarity of overgrowths thick enough for the SHRIMP microbeam to probe, but these also are correlated in time—both with other metamorphic ages and with new magmatism coincident with the metamorphic episodes. There is no simple correspondence between the age of a rock and the degree of metamorphism and deformation that it has experienced. For example, the undeformed and little-metamorphosed Attutra Metagabbro in the eastern part of the region is similar in age to the Mount Chapple Metamorphics of the central region which have been metamorphosed to granulite grade at least twice. Instead there is geographic restriction to the effects of certain events—contradicting the attempt by Collins and Shaw (1995) to build an evolution from inferred metamorphic correlations. On the basis of these data, and concurrent studies of other Arunta Region rocks, Scrimgeour (2003) has proposed a revised nomenclature for the Palaeoproterozoic evolution of the Arunta Region. The events in which the Arunta Region mafic–ultramafic intrusions play a role are outlined below. 5.1. Stafford Event (1810–1800 Ma) The Stafford Event is now the earliest thermal event recorded in the Arunta Region. In the compilation of Collins and Shaw (1995) an earlier ‘Yuendemu’ Event was inferred, corresponding to the geographically widespread ‘Barramundi’ orogeny of Etheridge et al. (1987). This was based on the interpretation by Young et al. (1995) of 1880 ± 5 Ma as the age of the Ngadarunga Granite in the western Aileron Province; but Claou´e-Long (2003) has shown this to be, instead, the age of inherited zircons in that granite, whose emplacement age is now measured as 1803 ± 5 Ma. The earliest mafic intrusions dated here were emplaced at the same time: Enbra Granulite, 1810.7 ± 2.6 Ma; Johannsen Metagabbro, 1805.4 ± 3.4 Ma; and Mount Hay Granulite, 1803 ± 5 Ma. The three mafic granulite bodies are now geographically separate but they share distinctive zircon responses to metamorphism at ca. 1700 Ma which implies a common crustal history during their first 100 million years. All three bodies are linked by the crustal-scale dislocation of the Harry Creek Thrust, and, as discussed by Hoatson et al. (this issue), the shared characteristics make it likely that they are tectonically dismembered representatives of one intrusion system.

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Table 2 Relationship of mafic–ultramafic intrusions to Arunta Region crustal events Stafford (1810–1800 Ma)

Yambah (1790–1770 Ma)

Strangways (1740–1690 Ma)

Liebig (1640–1630 Ma)

Chewings (1600–1570 Ma)

Teapot (1150–1130 Ma)

<734 ± 44; 461 ± 6

Warumpi province Andrew Young Hills gabbro South Papunya gabbro Papunya gabbro Papunya ultramafic Eastern aileron province Mordor Complex pyroxenite Attutra Metagabbro Associated tonalite Bonya Schist metavolcanic country rock Central aileron province Dolerite dyke Tonalite—Harry Anorthositic Gabbro Johannsen Metagabbro Mount Hay Granulite metagabbro Enbra Granulite metagabbro Mount Chapple Metamorphics felsic unit Mount Chapple Metamorphics mafic unit

1632.9 ± 2.8 1634.6 ± 4.8 1636.5 ± 2.4 1639.2 ± 2.0

1807 ± 17

1133 ± 5

1786.4 ± 4.2 1775.2 ± 4.6

1786.9 ± 3.3 1805.4 ± 3.4 1803 ± 5

478 ± 13 1689 ± 8 1685 ± 20 1697 ± 7 1700 ± 17

1810.7 ± 2.6 1771 + 10/−6

1685 ± 11 1725 ± 11

1591 ± 6

1774.0 ± 1.9

Bold type denotes magmatic zircon crystallisation ages; normal type denotes zircon isotopic disturbance, recrystallisation or overgrowth during metamorphism.

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Irindina province Riddock Amphibolite

Larapinta (500–460 Ma)

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The Stafford Event intrusions were emplaced into metasedimentary country rocks whose youngest detrital zircons indicate deposition after ca. 1840 Ma (Claou´eLong et al., 2005). In the eastern Aileron Province the Ongeva Package country rocks include felsic volcaniclastics dated in the range 1810–1800 Ma (Hussey et al., 2005) of which the Bonya Schist, dated here at 1807 ± 17 Ma, is an example. It is unfortunate that the Stafford Event is named after the Mount Stafford Granite in the northwest Reynolds Range, because the available age control on that intrusion is uncertain; Collins and Williams (1995) report that it is an S-type granite with dominantly inherited zircons, and the interpreted 1818 ± 15 Ma age is from a relatively small quantity of data. The Stafford Event is constrained by the data described here for coeval mafic and felsic magmatism in the age range 1800–1810 Ma. It appears to have been dominantly a magmatic episode. According to Scrimgeour (2003) evidence for metamorphism associated with this magmatism is localised high-temperature and low-pressure conditions with rapid lateral changes in grade. This is consistent with local direct heating of country rocks by the intruding magmas rather than a pervasive regional metamorphism. There is evidence that the Stafford Event mafic magmatism was geographically and volumetrically significant. The representatives dated here are three mafic intrusions in the central Aileron Province. Further east, protoliths in the Strangways Metamorphic Complex have now been dated to the same 1800–1810 Ma age range (Hussey et al., 2005); this package includes a high proportion of mafic rocks, much of it now mafic granulite, associated with metasediments and felsic volcanic protoliths, and this indicates shallow level intrusions or erupted basaltic equivalents of the intrusions dated here. In the north of the Aileron Province, dolerite sheets intrude at Mount Stafford (Fig. 1) and the form of these stacked dolerite sheets is similar to the Johannsen Metagabbro (Hoatson and Stewart, 2001). A correlation outside the Arunta Region is in the Davenport Ranges, 200 km north of the Arunta Region, where a little-metamorphosed exposure of sediments and felsic volcanics includes the regionally extensive Kudinga Basalt horizon in the upper part of the stratigraphy (Blake and Page, 1988). Felsic units underlying this basalt have been dated at 1815 ± 3 Ma (Claou´eLong et al., 2005) so this is an erupted equivalent of the Stafford Event mafic intrusions that has escaped the high-grade metamorphism of the Arunta Region. A more tentative correlation may exist 500 km to the northwest in the Tanami Region where the stratigraphy is considered to be continuous with the Aileron Province and

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Fig. 9. Concordia diagram of zircon compositions in the Tanami Region Coora Dolerite which intrudes the Dead Bullock Formation.

includes a sequence of basalts, the Mount Charles Formation (Crispe and Vandenburg, 2005). The sediments are intruded by numerous dolerite sills which have not been dated directly, but they could correspond with the Stafford Event because the Killi Killi Formation which hosts some of the sills was deposited after ca. 1840 Ma. In an attempt to test this connection, zircons have been dated from the Coora Dolerite which is intruded into sediments that underlie the Killi Killi Formation. The dolerite yielded a range of zircon ages from the Archaean to ca. 1882 ± 8 Ma (Fig. 9) indicating contamination of the magma by country rocks, so it is only possible to determine that 1882 ± 8 Ma is a maximum estimate for the emplacement age. Further research is required to constrain the timing of mafic magmatism in the Tanami Region. The correlation and nature of mafic magmatism in the Stafford Event provide significant constraints on the earliest evolution of the Arunta Region. The 1810–1800 Ma magmatism is the first Arunta Region thermal event and there is no evidence for correlation of the earlier 1880–1850 Ma ‘Barramundi’ tectonism or magmatism into the Arunta Region. Hoatson et al. (this issue) discuss the chemistry of the magmas, which is consistent with the mantle processes of a subduction-related or back-arc setting. Much of the Stafford-age mafic magmatism is now granulite-grade metamorphic complexes. This has led to perceptions that they represent intrusions at deep crustal levels, subsequently exhumed, in turn implying that thick continental crust existed at Stafford time. The evidence reviewed here shows that, instead, the granulite-grade overprint was imposed 100 Ma after intrusion, in the later Strangways Event, and there is strong inference

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that magmas were emplaced at shallow depths. As discussed by Hoatson et al. (this issue), the phase relations of the Mount Hay mafic intrusion indicate crystallisation at 1 kbar; the Johannsen Metagabbro preserves the field and relative timing expression of a subvolcanic sheet complex intruding a coeval felsic volcanic pile; and the regional metamorphic expression of the Stafford Event, as localised high-temperature and lowpressure conditions with rapid lateral changes in grade, indicates the arrival of hot magma into unmetamorphosed shallow-level sediments. Collectively, the shallow intrusion of mafic magma into basin sediments, the evidence of bimodal subduction-related or back-arc magmatism at 1810–1800 Ma, and the absence of evidence for a pre-existing thick substrate of continental crust at that time, suggest that the Aileron Province developed in a continental margin setting at Stafford time. 5.2. Yambah Event (1790–1770 Ma) In the eastern Aileron Province the Stafford-age Bonya Schist was intruded by the Attutra Metagabbro at 1786.4 ± 4.2 Ma; in the central part of the region the Harry Anorthositic Gabbro and an associated tonalite intruded Stafford-age country rocks at 1786.9 ± 3.3 Ma, and in the west the coeval mafic and felsic magmas of the Mount Chapple Metamorphics were emplaced at 1774.0 ± 1.9 Ma. Felsic plutons of similar age are documented by Zhao and Bennett (1995). The age range 1790–1770 Ma was included by Collins and Shaw (1995) as an early part to their definition of the Strangways Orogeny, but is here distinguished as an earlier magmatic episode; the time distinction from the 1810 to 1800 Ma Stafford magmatism is brief and it is not clear whether it is a continuation of the earlier system or a separate event. Evidence for metamorphism associated with the Yambah-age magmatism is equivocal. For example, the Stafford-age mafic plutons described here have numerous U–Pb apparent ages in the 1790–1770 Ma range which could be used to interpret a Yambah thermal ‘event’ overprinting those rocks; however, those apparent ages are interpreted here as variable partial recrystallisation of Stafford-age zircons by a much later ca. 1700 Ma metamorphism. Further research is required to test whether regional metamorphism accompanied the intrusion of Yambah Event magmas. Correlatives of the Yambah Event mafic magmatism are unknown. In the central and eastern parts of the Aileron Province, the Reynolds, Cadney, and Ledan metasediment packages are believed to represent the

time period 1780–1740 Ma, but they include no basaltic volcanism to match the Yambah Event intrusions. 5.3. Strangways (1740–1690 Ma) and Chewings (1600–1570 Ma) Events Over much of the Arunta Region a pervasive event converted many of the early rock associations to upper amphibolite or granulite-grade metamorphic complexes. Collins and Shaw (1995) termed the responsible event the Strangways Orogeny extending from 1780 Ma to termination at ca. 1730 Ma, but the earlier part of this time is now distinguished as the magmatic Yambah Event. High-grade metamorphism is recorded in mafic intrusions of the central Aileron Province by distinctive zircon overgrowths in the age range 1725 ± 11 Ma to 1685 ± 11 Ma. This range overlaps, but extends younger than, the ca. 1730–1710 Ma age range for metamorphic leucosomes reported in the Strangways Range by M¨oller et al. (2003). In addition to promoting new zircon growth, this metamorphism caused invasive recrystallisation of Stafford-age and Yambah-age magmatic zircons in central Aileron Province mafic intrusions. The severity of the Strangways metamorphism is less evident in the far east of the Arunta Region where zircons in the Attutra Metagabbro and its country rocks record no isotopic disturbance at this time. A new finding in this study is that the ending of the high-grade metamorphism coincides with intrusion of a local dolerite dyke swarm in the Strangways Range at 1689 ± 8 Ma. The dykes have not experienced the high-grade metamorphic conditions, and their presence implies that the termination of the Strangways thermal system was an extensional process. The second major metamorphic system affecting the central Arunta Region is the Chewings Event, recognised as a high-grade metamorphism locally in the southeast of the Reynolds Range Province (Rubatto et al., 2001) and within the Warumpi Province (Close et al., 2003). No known mafic magmatism is associated with this event. The only record in this study is the Yambah-age Mount Chapple Metamorphics, which were affected by the earlier Strangways metamorphism, and have a second generation of metamorphic zircon overgrowths at 1591 ± 6 Ma. 5.4. Liebig Event (1640–1630 Ma) South of the Redbank Thrust is a separate terrane—the Warumpi Province—in which recent geochronology has established an evolution distinct from the Aileron Province to the north (Close et al., 2003). The later part of this evolution included the Liebig Event,

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which is unique to the Warumpi Province and resulted in granulite-grade metamorphism with voluminous granite and charnockite intrusion. Coeval with the felsic magmatism was intrusion of mafic–ultramafic bodies in the age range 1640–1635 Ma. The mafic–ultramafic intrusions imply an extensional component to the Liebig Event, whose geodynamic setting and relationship to the cratonic evolution north of the Redbank Thrust require further investigation. The large Andrew Young Hills mafic intrusion was emplaced at the same time, but the boundary of the Warumpi Province defined by Close et al. (2003) passes south of it, placing this intrusion within the Aileron Province. 5.5. Teapot Event (1150–1130 Ma) The 1150–1130 Ma Teapot Event is represented by local intrusions and thermal effects in restricted locations across the southern part of the Arunta Region. Among them is the alkaline and ultramafic Mordor Complex in which a pyroxenite is dated here at 1133 ± 5 Ma. 5.6. Larapinta Event (500–460 Ma) The latest mafic magmatism in the region is associated with the recently recognised Larapinta Event metamorphism in the fault-bounded Irindina Province to the east (Maidment et al., 2004). This province includes abundant tholeiitic mafic dykes, and the geographically extensive Riddock Amphibolite which is a metamorphosed package of interlayered basaltic and sedimentary material. The metamorphism of the Riddock Amphibolite is constrained here at 461 ± 6 Ma. The stratigraphic age is not constrained directly but it includes Mesoproterozoic clastic detritus with a maximum depositional age of 734 ± 44 Ma, which is consistent with other evidence that the Ordovician metamorphism affected the distinctive sedimentary protoliths of the Centralian Superbasin, as young as the Mesoproterozoic or the Cambrian and laterally equivalent to the Amadeus and Georgina Basins (Maidment et al., 2004). This study provides the first evidence that the Larapinta metamorphism affected Palaeoproterozoic basement rocks, in the form of open-system isotopic behaviour in zircons of the Stafford-age Bonya Schist, 150 km east of the Riddock Amphibolite, at 478 ± 13 Ma. This timing suggests a possible correlation of Larapinta mafic magmatism with the Antrim Plateau Volcanics, which is the major flood-basalt province marking the termination of the Proterozoic in Australia (Bultitude, 1976; Buick et al., 2001). Only erosional remnants of this province remain and the Riddock Amphi-

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bolite is 500 km south of the nearest documented occurrence. Hanley and Wingate (2000) placed the flood basalt eruption between the latest Proterozoic (ca. 580 Ma) and early mid-Cambrian (ca. 515 Ma) and showed that the 250-km long Milliwindi dolerite dyke, 300 km west of the basalt outcrops, is part of the same magmatic system. This northwest-trending dyke borders the Canning Basin whose initiation coincides with the rifting denoted by the dyke (Purcell, 1984). The Riddock Amphibolite is along strike from the Milliwindi dolerite, within what may have been a continent-wide extensional zone whose northwest end is now the Canning Basin and southeast end is the Irindina Province of the Arunta Region. If this association is correct, it extends the geographic range of the Antrim Plateau Volcanics southwards by 500 km and increases the area of the floodbasalt province by a half to at least 1,000,000 km2 ; on this basis the Antrim Plateau Volcanics may have been one of the world’s largest flood-basalt provinces. A single dated basalt occurrence 800 km southwest could extend the range even further: the Table Hill Volcanics in the Officer Basin overlie Ediacara fauna and have a Rb–Sr mineral isochron age of 563 ± 40 Ma (Compston, 1974; recalculated to new decay constants). Like the Riddock Amphibolite, the Table Hill Volcanics were emplaced as part of the Centralian Superbasin, of which the Officer Basin is a structural remnant (Walter et al., 1995). These possible correlations draw attention to the continent-wide extension linking the Milliwindi dolerite in the northwest, and the Riddock Amphibolite in the southeast, as a candidate for the main flood basalt eruption site and as the likely extensional driver for the magmatism. The world’s largest Ni–Cu–PGE deposits, such as the Noril’sk region in Siberia (Naldrett, 1997), are similarly located within the major trans-crustal structures that focussed the passage of huge quantities of magma through sediment piles during later flood-basalt episodes. The Riddock Amphibolite belt may, therefore, be prospective for similar mineralisation systems. 6. Conclusions Mafic and even ultramafic intrusions are excellent targets for U–Pb dating. They commonly contain zircon or baddeleyite and the presence of these trace minerals can be detected by field, petrological, and geochemical appraisal of the magmatic systems. This study has paid particular attention to the origin of zircon in mafic intrusions: are they from igneous crystallisation, metamorphism, or magma contamination? The evidence suggests

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that mafic–ultramafic magmas are less prone to carrying inherited zircons than are many granitic magmas, which simplifies the interpretation of measured ages. There is also evidence that zircons in mafic lithologies respond to metamorphism in different ways, and at different times, from surrounding felsic lithologies, probably reflecting the fact that Zr availability draws from the metamorphic reaction of different minerals in different host lithologies. Exploiting this differential response to event systems could be a promising avenue for future research. With the exception of the ca. 1590 Ma Chewings Event, the dating reported here shows that nearly all the event systems known in the Arunta Region include a significant mafic magmatic component. In turn, this requires the consideration of extensional tectonic environments during the region’s development. In the earliest evolution, Stafford- and Yambah-Event mafic intrusions in the Aileron Province have chemical and other characteristics that indicate a back-arc continental margin tectonic setting in the period 1810 Ma to 1770 Ma. Mafic magmatism at ca. 1640 Ma, related to the Warumpi Province in the south, similarly indicates an extensional or at least stable tectonic setting at ca. 1640 Ma, especially for the development of the very large Andrew Young Hills layered intrusion. Within the Aileron Province, the Strangways Event was a major regional metamorphic episode at ca. 1730–1680 Ma for which an extensional setting could be implied by the coincident intrusion of a local dolerite dyke swarm. The alkaline-ultramafic Mordor Complex, part of the Teapot Event at ca. 1130 Ma, may represent a transition to within-plate magmatism; and the Palaeozoic Larapinta Event in the eastern Arunta Region appears to be part of a major within-plate extension that was accompanied by large volumes of mafic magmatism. Acknowledgments J.C.L. conducts SHRIMP research as a Visiting Fellow at the Research School of Earth Sciences, Australian National University. We are grateful for logistic assistance from the Northern Territory Geological Survey and guidance in the field from Alastair Stewart. David Maidment generously shared his understanding of the Larapinta Event and Ian Scrimgeour is thanked for numerous discussions of Arunta evolution. John Pyke’s rapid geochemical analyses allowed us to characterise the intrusions and target geochronology samples while still mapping in the field. Chris Foudoulis is thanked for his sampling technique and long hours characterising and imaging zircon samples. Tas Armstrong,

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