High grade metamorphism of sedimentary rocks during Palaeozoic rift basin formation in central...

Gondwana Research 24 (2013) 865–885

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High grade metamorphism of sedimentary rocks during Palaeozoic rift basinformation in central Australia

D.W. Maidment a,b,1, M. Hand c,⁎, I.S. Williams a

a Research School of Earth Sciences, Australian National University, Canberra ACT 0200, Australiab Geoscience Australia, GPO Box 378, Canberra, ACT 2601, Australiac Centre for Tectonics, Resources and Exploration, University of Adelaide, Adelaide, SA 5005, Australia

⁎ Corresponding author.E-mail address: [email protected] (M. H

1 Present Address: St Barbara Ltd, PO Box 1161, West

1342-937X/$ – see front matter © 2013 International Ahttp://dx.doi.org/10.1016/j.gr.2012.12.020

a b s t r a c t
a r t i c l e i n f o
Article history:Received 29 April 2012Received in revised form 7 December 2012Accepted 11 December 2012Available online 20 January 2013

Keywords:Central AustraliaRift basinMetamorphismDetrital zirconGeochronologyIntraplate

Exhumation of middle and lower crustal rocks during the 450–320 Ma intraplate Alice Springs Orogeny in centralAustralia provides an opportunity to examine the deep burial of sedimentary successions leading to regionalhigh-grademetamorphism. SIMSzirconU–Pbgeochronology shows that high-grademetasedimentaryunits record-ing lower crustal pressures share a depositional history with unmetamorphosed sedimentary successions insurrounding sedimentary basins. These surrounding basins constitute parts of a large and formerly contiguous intra-plate basin that covered much of Neoproterozoic to early Palaeozoic Australia. Within the highly metamorphosedHarts RangeGroup,metamorphic zircon growth at 480–460 Ma recordsmid-to-lower crustal (~0.9–1.0 GPa)meta-morphism. Similarities in detrital zircon age spectra between the Harts Range Group and Late Neoproterozoic–Cambrian sequences in the surrounding Amadeus and Georgina basins imply that the Harts Range Group is a highlymetamorphosed equivalent of the same successions.Maximumdepositional ages for parts of theHarts RangeGroupare as low as ~520–500 Ma indicating that burial to depths approaching 30 km occurred ~20–40 Ma after deposi-tion. Palaeogeographic reconstructions based on well-preserved sedimentary records indicate that throughout theCambro–Ordovician central Australia was covered by a shallow, gently subsiding epicratonicmarine basin, and pro-vide a context for the deep burial of the Harts Range Group. Sedimentation and burial coincided with voluminousmafic magmatism that is absent from the surrounding unmetamorphosed basinal successions, suggesting that theHarts RangeGroup accumulated in a localised sub-basin associatedwith sufficient lithospheric extension to generatemantle partial melting. The presently preserved axial extent of this sub-basin is >200 km. Its width has beenmod-ified by subsequent shortening associatedwith the Alice Springs Orogeny, butmust have been>80 km. Seismic re-flection data suggest that the Harts Range Group is preservedwithin an inverted crustal-scale half graben structure,lending further support to the notion that it accumulated in a discrete sub-basin. Based on palaeogeographic con-straints we suggest that burial of the Harts Range Group to lower crustal depths occurred primarily via sedimentloading in an exceptionally deep Late Cambrian to Early Ordovician intraplate rift basin. High-temperature Ordovi-cian deformationwithin theHarts RangeGroup formed a regional low angle foliation associatedwith ongoingmaficmagmatism that was coeval with deepening of the overlying marine basin, suggesting that metamorphism of theHarts Range Group was associated with ongoing extension. The resulting lower crustal metamorphic terrain istherefore interpreted to represent high-temperature deformation in the lower levels of a deep sedimentary basinduring continued basin development. If this model is correct, it indicates that regional-scale moderate- tohigh-pressure metamorphism of supracrustal rocks need not necessarily reflect compressional thickening of thecrust, an assumption commonly made in studies of many metamorphic terrains that lack a palaeogeographiccontext.

© 2013 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved.

1. Introduction

The burial of sedimentary successions to lower crustal depths(>25–30 km) and their subsequent regional high-grade metamor-phism are generally regarded as reflecting compressional tectonic

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regimes (e.g. Vance et al., 1998; Brown, 2001; Lee and Cho, 2003;Scrimgeour et al., 2005). This notion is consistent with contractionalstyle structures that are observed in many metamorphic terrains, aswell as thermomechanical modelling which shows that large-scalecontractional deformation is an efficient way to bury supracrustalrocks (e.g. Jamieson et al., 2002). In metamorphic terrains that lacka well defined palaeogeographic or palaeotectonic context, metamor-phic style is often used as a primary constraint on the potential tec-tonic setting (e.g. Vance et al., 1998; Scrimgeour et al., 2005). This is

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866 D.W. Maidment et al. / Gondwana Research 24 (2013) 865–885

particularly the case in terrains that are poorly preserved due tooverprinting by later events (e.g. Cutts et al., 2010). Despite the generalnotion that deep burial of sedimentary successions reflects compres-sional processes, however, there are a number of exampleswhere burialto depths in excess of 20 km is associated with the formation ofcrustal-scale rift basins (Allen et al., 1995; Ojakangas et al., 2001;Baldwin et al., 2003; Maystrenko et al., 2003). The preservation ofthese basins, allowing their thickness to be directly measured, meansthat their keels are still deeply buried, precluding direct examinationof the thermal character of the basal rift successions. Deep rift basins,however, are commonly associated with mafic magmatism and highbasal heat flows (e.g. Ruppel, 1995), making them a logical setting forregional high-grade metamorphism if rifting and magmatism weremaintained for a sufficient period of time (e.g. Wickham and Oxburgh,1985; Aitken et al., 2012). Therefore we should expect that deeplyinverted crustal-scale rift basins might expose highly metamorphosedsequences that were deposited, buried and metamorphosed withinthe evolving rift.

Central Australia contains a remarkable record of intraplate deforma-tion expressed by the formation of a Neoproterozoic to mid-Palaeozoiccontinental-scale intraplate basin, the Centralian Superbasin (Fig. 1),and its modification during rifting events and intraplate orogeny(Lindsay and Korsch, 1991; Walter et al., 1995; Camacho et al., 1997;Hand and Sandiford, 1999; Mawby et al., 1999; Scrimgeour and Close,1999; Camacho and McDougall, 2000; Haines et al., 2001; Wade et al.,2008a; Raimondo et al., 2009). The superbasin was initiated at about

Fig. 1. Map of Australia showing the major basement and basin domains. The outline of thenow a series of structural remnants (Officer-Savory, Amadeus, Georgina and Ngalia) whichintracratonic orogenies which dismembered the basin (Sandiford and Hand, 1998).

830 Ma as a broad, shallow sag coeval with rifting along the now easternmargin of Proterozoic Australia (Lindsay et al., 1987; Lindsay, 2002). Basindevelopment for about the next 250 Mawasmaintained by small rift sys-tems linked to on-going extension along the eastern Australian margin(Lindsay et al., 1987; Walter et al., 1995). The basin underwent two pe-riods of orogenic-scale intraplate inversion (Sandiford and Hand, 1998;Hand and Sandiford, 1999; Sandiford et al., 2001) that divided it into a se-ries of structural remnants, the Amadeus, Georgina, Officer andNgalia ba-sins (Fig. 1). The first inversion event was the Petermann Orogenybetween 580 and 530 Ma, which exhumed Mesoproterozoic basementfrom depths of up to 50 km (Scrimgeour and Close, 1999; Camacho etal., 1997; Wade et al., 2005, 2006; Raimondo et al., 2009). The secondwas theAlice SpringsOrogenybetween450 and320 Ma,which exhumedmiddle and lower crustal rocks from depths of up to 30 km (Collins andTeyssier, 1989a,b; Dunlap and Teyssier, 1995; Dunlap et al., 1995;Mawby et al., 1999). Between these two intraplate orogenic events, shal-lowmarine epicontinental sedimentationwasmaintained acrossmuch ofcentral Australia.

Until recently, all models for the intraplate evolution of central Aus-tralia considered that the mid- and lower-crustal rocks exhumed duringthe Alice Springs Orogeny formed part of the Palaeoproterozoic base-ment to the Centralian Superbasin (Stewart et al., 1984; Ding andJames, 1985; James and Ding, 1988; Collins and Teyssier, 1989a,b;Dunlap and Teyssier, 1995; Braun and Shaw, 1998). However, SIMS (sen-sitive high resolution ion microprobe (SHRIMP)) zircon U–Pb dating ofmetasedimentary rocks from the Harts Range Group in the Harts Range

formerly contiguous Centralian Superbasin (Walter et al., 1995) is shown. The basin iswere formed during the 580–530 Ma Petermann and the 450–320 Ma Alice Springs

867D.W. Maidment et al. / Gondwana Research 24 (2013) 865–885

Metamorphic Complex in the eastern part of the Alice Springs Orogen(Fig. 2) indicates that their protoliths were deposited as recently as theCambrian, more than 1.2 Ga later than previously thought (Buick et al.,2001, 2005). Zircon/monazite U–Pb and garnet Sm–Nd ages, coupledwith metamorphic petrological constraints, indicate that the HartsRange Group underwent peakmetamorphism and regional deformationbetween ~480 and ~460 Ma, at about 0.9–1.0 GPa and 800 °C (Hand etal., 1999a; Mawby et al., 1999; Buick et al., 2001, 2005). The youngprotolith ages for rocks belonging to the Harts Range Group means thatthe burial and metamorphism of these sequences need to be evaluatedin the context of the Cambro–Ordovician palaeogeography of centralAustralia.

In this paper we report SHRIMP detrital and metamorphic zirconU–Pb dating of metasedimentary rocks belonging to the Harts RangeGroup. In conjunction with existing data (Buick et al., 2001, 2005),the new detrital zircon data encompass the entire metasedimentarysuccession to constrain: 1) the extent of the Harts Range Group incentral Australia, 2) the depositional ages and provenance of the

A

Fig. 2. Geological setting of the Harts Range Metamorphic Complex in the eastern Arunta Regoutcrop level and is enclosed and underlain by Palaeoproterozoic metamorphic and igneous rocsequences of the southern Georgina and eastern Amadeus basins and is overlain in its eastern esect across the region (Korsch et al., 2011). The line of transect is marked A–B in (A). ThPalaeoproterozoic crust. The current structural architecture is largely the consequence of the 45and lower crust, and converted the formerly contiguous Amadeus and Georgina basins (Shaw ewere pieced by thick skinned, basement-involved deformation. MF=Milly Fault, AIFZ=AtnarEntia Point Shear Zone, DSZ=Delny Shear Zone.

protoliths to these highly metamorphosed supracrustal rocks, and 3)the stratigraphic relationships between the Harts Range Group andthe sedimentary units of the adjacent Amadeus and Georgina basins.An earlier detrital zircon study of these unmetamorphosed basin suc-cessions (Maidment et al., 2007) allows stratigraphic correlations be-tween the Harts Range Group and sedimentary units to be more fullyevaluated, and provides a context for understanding the geodynamicsetting that accompanied deep burial of the Harts Range Group.

2. Geological overview

The Harts Range Metamorphic Complex is located in the easternArunta region, ~150 km ENE of Alice Springs in central Australia(Figs. 2, 3). It consists of Palaeoproterozoic (~1.80–1.73 Ga) rocks thatbroadly appear to be correlatives of the extensive system ofPalaeoproterozoic rocks that lie to the west of the Harts Range region(Wade et al., 2008b; Figs. 2, 3). The Palaeoproterozoic rocks in theHarts Range Metamorphic Complex are structurally overlain by a

B

ion of central Australia. (A) The Harts Range Group comprises the bulk of the complex atks. The entire region is surrounded by unmetamorphosed Neoproterozoic–mid Palaeozoicxtent by the Cretaceous to recent Eromanga Basin. (B) Deep crustal seismic reflection tran-e Harts Range Group forms a thin package that is underlain by thick (up to ~60 km)0–320 MaAlice Springs Orogeny, which exhumed theHarts Range Group from themiddlet al., 1991; Maidment et al., 2007) into structurally separated foreland basins that in placesta Imbricate Fault Zone, ISZ=Ilogwa Shear Zone, BDZ=Bruna Detachment Zone, EPSZ=

image of Fig.�2

Fig. 3. Geological map of the Harts Range region showing the location of samples discussed in this study. The mapping is based on Shaw et al. (1990), Shaw and Wells (1983), Shawand Freeman (1985), Ding and James (1985), and Freeman (1986). Sample number grid references and zircon morphological summaries are shown in Table 1.

Fig. 4. Generalised lithostratigraphy of the Harts Range Group Joklik (1955) and Shawet al. (1979, 1982). Thicknesses of individual packages are not to scale and are variabledue to the affects of deformation. The total current structural thickness of the HartsRange Group is approximately 6 km.


package of upper amphibolite- to granulite-facies supracrustal rocks thatcomprise the Harts Range Group (Joklik, 1955; Shaw et al., 1979, 1982;Ding and James, 1985; Mawby et al., 1999; Wade et al., 2008b). Thepresent tectonic architecture of the region is largely the consequence ofthe 450–320 Ma intraplate Alice Springs Orogeny which exhumed theHarts Range Metamorphic Complex and the adjoining Palaeoproterozoicrocks from mid- to lower-crustal depths (Ballèvre et al., 2000; Hand etal., 1999a,b; Mawby et al., 1999). This event inverted the northeasternmargin of the intraplate Centralian Superbasin (Sandiford and Hand,1998), which is now represented by the structurally remnant andunmetamorphosed Amadeus and Georgina basins (Hand and Sandiford,1999; Haines et al., 2001; Maidment et al., 2007). Recently acquireddeep crustal seismic reflection data show that the Harts Range Groupsits within an array of crustal-scale faults controlled by a north-dippingmaster fault that connects to the Moho (Fig. 2; Korsch et al., 2011). Thethickest crust (~60 km thick) in the region underlies the Harts RangeGroup and reflects significant thickening during the Alice Springs Oroge-ny (Korsch et al., 2011). This thickening was responsible for exhumationof the Harts Range Metamorphic Complex along the crustal-scale faults.

The Harts Range Group consists of a sequence of metapelite,metabasite, quartzite, calc-silicate rock, quartzofeldspathic gneissand marble. It has been subdivided into five lithostratigraphic units(Fig. 4) which are now typically in structural contact, but are consid-ered to be in essentially their original stratigraphic order (Hand et al.,1999b). Ding and James (1985) recognised that the Harts RangeGroup formed a structural cover sequence, naming it the Irindina

Supracrustal Assemblage, because the Harts Range Group as original-ly defined by Joklik (1955) and Shaw et al. (1979, 1982) also incorpo-rated rocks assigned to the Palaeoproterozoic basement succession. In

image of Fig.�4


this study, we refer to the cover sequence as the Harts Range Group,redefining the term to exclude the Palaeoproterozoic rocks.

The Harts Range Group consists of the Irindina Gneiss (including theNaringa Calcareous Member, Stanovos Gneiss Member and the HartsRange Meta-Igneous Complex), and the overlying Brady Gneiss (Fig. 4).The Irindina Gneiss consists of a relatively uniform sequence of Grt–Sil–Bt–Pl–Qtz±Kfs migmatitic schist and gneiss with minor marbleand quartzite (all mineral abbreviations except for clinopyroxene (Cpx)after Kretz, 1983). The Naringa Calcareous Member crops out in thewestern Harts Range (Fig. 3), and was considered by Shaw et al. (1979,1984) to bewithin the lower part of the Irindina Gneiss, but it is in struc-tural contact with overlying metapelites of the Irindina Gneiss, andmight instead represent an older unit. The Stanovos Gneiss Member inthe southeastern Harts Range is situated towards the structural base ofthe sequence, and is possibly a correlative of the Naringa CalcareousMember (Hand et al., 1999b). Within the Stanovos Gneiss, Cambrian-aged (~520 Ma) layer-parallel granites (Maidment, 2005) are up to sev-eral kilometres in strike length. They show mingling relationships withmafic intrusives (Burdett, 2004; Lawley, 2005), indicating that maficmagmatism is also Cambrian in age. Numerous layers of migmatitic am-phibolite, as well as anorthosite, occur within the Irindina Gneiss andhave been collectively termed the Harts Range Meta-Igneous Complex(Sivell and Foden, 1985), with individual units ranging in thicknessfrom a few centimetres to ~2 km. The Brady Gneiss is the structurallyhighest member of the Harts Range Group, consisting of a lower unitof Grt–Ms–Bt–Qtz–Plag±Sil migmatitic gneiss and an upper unit ofQtz–Czo–Hbl–Cpx–Scp calc-silicate rock.

The succession has been highly deformed and metamorphosed. Par-tial melting associated with a low angle, layer-parallel foliation resultedin the formation of coarse-grained migmatitic assemblages which inmetabasite contain in Grt±Cpx (crystal size up to 20 cm)±Tit-bearingleucosomes (Miller et al., 1997; Mawby et al., 1999; Storkey et al., 2005).Similarly coarse-grained garnet-bearing assemblages define the peak as-semblage in the metapelitic Irindina Gneiss. P–T calculations on peakmetamorphic assemblages yield estimates of 0.9–1.0 GPa and ~800 °C(Miller et al., 1997; Mawby et al., 1999), implying burial to depths of~30 km. The peak metamorphic mineral assemblages were overprintedby a pervasive, gently-dipping retrograde foliation consisting of Hbl–Pl–Grt–Qtz in metabasites and Grt–Sil–Bt–Pl–Qtz in metapelites, whichyield P–T estimates of 0.6–0.7 GPa and 700 °C (Lawrence et al., 1987;Oliver et al., 1988; Mawby et al., 1999). The peak and retrograde assem-blages define a clockwise P–T evolution and the associated fabricscontain a generally N–S-trending mineral lineation with locally well de-veloped top-to-the-north kinematic indicators. These are overprinted byregional-scale mylonite and fold systems recording the effects of the450–320 Ma Alice Springs Orogeny (Collins and Teyssier, 1989a,b;Hand et al., 1999a,b; Mawby et al., 1999; Buick et al., 2008). Due to theintensity of deformation, the original stratigraphic thickness of theHarts Range Group is difficult to determine, but its current structuralthickness is about 6–7 km (Hand et al., 1999b).

Sm–Nd dating of peak metamorphic garnet-bearing assemblagesand U–Pb dating of metamorphic zircon and monazite indicate thatpeak and retrograde assemblages formed during the Early Ordovician,between ~480 and ~460 Ma (Hand et al., 1999a,b; Mawby et al.,1999; Buick et al., 2001, 2005). The timing of peakmetamorphism coin-cides with the deposition of shallowmarine sediments in the intraplateGeorgina and Amadeus basins to the north and south of the Harts Rangeregion respectively (Fig. 2; Hand et al., 1999a,b; Mawby et al., 1999;Buick et al., 2005; Maidment et al., 2007). This relationship and thestructural/metamorphic evolution of the Harts Range Group led theseworkers to suggest that metamorphism and deformation took placewithin an extensional setting, coincident with the formation of theepicratonic Larapintine Sea (Webby, 1978). The extensional historywas terminated at 450 Ma when exhumation of the Harts RangeGroup from the middle and lower crust was initiated during the earlystages of the Alice Springs Orogeny (Mawby et al., 1999) by thrusting

of the Harts Range Group across the underlying Palaeoproterozoicrocks. Final exhumation occurred during the mid-late Carboniferousand early Permian during the terminal stages of the Alice Springs Orog-eny (Haines et al., 2001).

Until relatively recently, the protoliths of the Harts Range Groupwere considered to have been deposited in the Palaeoproterozoic(Ding and James, 1985; Mortimer et al., 1987; Cooper et al., 1988;James and Ding, 1988; Ding and James, 1989; Collins and Shaw,1995). The Harts Range Group contains populations of detrital zirconas young as Cambrian, however, thus constraining deposition to be-tween the Early Cambrian and the high grade metamorphism in theEarly Ordovician (Buick et al., 2001, 2005). Buick et al. (2005)measureddetrital zircon ages from the Irindina Gneiss, Naringa Calcareous Mem-ber and Brady Gneiss in the Harts Range and compared them with zir-con ages from Cambrian sedimentary rocks from the Amadeus Basin.They found similar age components in both successions, and suggestedthat bothmight have originally formed as part of the same sedimentarysystem. If this is the case, then thewell-documented depositional histo-ry of the Amadeus and Georgina basins (e.g. Lindsay and Korsch, 1991;Walter et al., 1995) should place important constraints on the tectonicsetting associated with burial of the Harts Range Group to lower crustaldepths.

In this study detrital zircon ages have been measured from theStanovos Gneiss, Irindina Gneiss, Harts Range Meta-Igneous Complexand the Brady Gneiss to more fully characterise the provenance of theHarts Range Group and its age of deposition. Additionally, detrital zirconfrommetasedimentary rocks at localities up to 125 km east of the HartsRange that in aeromagnetic images appear similar to the Harts RangeGroup has also been analysed to determine the extent of the HartsRange Group and Ordovician metamorphism. These data, in conjunctionwith those of Buick et al. (2001, 2005), provide a template for correlatingunits in the Harts Range Group with those in the adjacent Amadeus andGeorgina basins (Maidment et al., 2007). This provides a framework forunderstanding the relationship between the highly metamorphosedHarts Range Group and the surrounding unmetamorphosed basinsuccessions.

3. Methodology

Zircon concentrates were prepared from ~3 kg rock samples usingclean heavy liquid and magnetic mineral separation techniques.Hand-picked zircon was mounted in epoxy resin with zircon stan-dards SL13, QGNG and Temora 2. Mounts were documented prior toanalysis by reflected and transmitted light photomicroscopy andcathodoluminescence (CL) imaging on a Hitachi S-2250N SEM at theANU Electron Microscopy Unit. Summary descriptions of zircon char-acteristics are listed in Table 1, and representative CL images for eachsample are presented in Fig. 5. U–Th–Pb isotopic analyses were car-ried out on SHRIMPs I and II at the Research School of Earth Sciences,Australian National University using procedures similar to those de-scribed by Williams and Claesson (1987) and Claoué-Long et al.(1995). The diameter of the analysis spot was ~20–40 μm with pri-mary ion currents of ~2–4 nA O2

−. Positive secondary ions wereextracted at 10 kV, mass analysed at a resolution of ~5000 and theZr, Pb, U and Th species of interest measured on a single electron mul-tiplier by cyclic stepping of the magnetic field. Fractionation of Pbrelative to U was corrected using the relationship Pb+/U+=A(UO+/U+)2 (Claoué-Long et al., 1995). The compositions as-sumed for standards were SL13: U=238 ppm; Temora 2: radio-genic 206Pb/238U=0.06683; and QGNG: radiogenic 206Pb/238U=0.33076. Common Pb corrections weremade using the present day lab-oratory common Pb isotopic composition (Broken Hill Pb) because anobserved decrease in 204Pb count rates during most analyses indicatedthat the common Pb was a surface contaminant. Dates were calculatedusing the decay constants recommended by the IUGS Subcommissionon Geochronology (Steiger and Jäger, 1977). Analytical uncertainties

Table 1Locations of samples and characteristics of separated zircons.

Unit Samplenumber

Eastinga Northinga Typical zircongrain size

Zircon morphology Zircon internal structure in CL imagery

Lower Stanovos Gneiss 2001080196 539728 7422671 200–300 μm Subhedral to euhedral multifacetedgrains.

Irregular cores with broadly zoned totextureless dark overgrowths.

Upper Stanovos Gneiss 2002080188 544844 7415912 Up to 250 μm Anhedral to subhedral. Cores with moderate-brightness rims≤10 μm thick.

Lower Irindina Gneiss 2001080230 540412 7446577 100–250 μm Mostly rounded to multifacetedequant grains; a few with aspectratios up to 3:1.

Many grains consist of an irregular core withgenerally thin, moderate brightness rim thatis up to 100 μm thick in a few grains.

Upper Irindina Gneiss 2002080003 506955 7425079 150–200 μm; a fewto 500 μm

Rounded to subhedral. Rounded or irregular cores; a few grainswith overgrowths of differing intensity.Late-stage moderate-intensity rim, typically≤10 μm thick.

HRMIC 2001080226 532894 7436070 Up to 200 μm Stubby, subhedral. Rounded and irregular cores with weaklyconcentrically zoned,moderate intensity rims.

Upper Brady Gneiss 2001080254 540929 7439848 100–200 μm Stubby, subhedral to euhedral. Irregular cores with dark, irregularlytextured overgrowths that formembayments in core. Late-stage bright rims5–10 μm thick.

Mt Bird metapelite 2001080038 549673 7444076 100–200 μm Rounded or irregular to subhedral. Rounded and irregular cores with relativelythick dark to moderate intensity overgrowth.Bright, faceted rims ≤5 μm thick.

Crossing Bore schist 2001080104 564073 7443740 50–300 μm Rounded to subhedral. Irregular core with discontinuous rims up tp10 μm thick.

Badens Camp quartzite 2001080028 586786 7432486 100 μm Euhedral to subhedral. Bright, irregular cores with dark faceted rimsto 50 μm thick.

Jervois quartzite 2002080134 616839 7461442 Up to 250 μm Rounded to subhedral. Cores with moderate-intensity rims typically≤10 μm thick; a few thicker rims.

Atula calc-silicate 2002080141 613924 7444273 40–50, up to 150 μm Rounded to subhedral. Cores with dark to moderate intensity rimstypically ≤10 μm thick.

Atula quartzite 2001080168 634182 7419872 Up to 250 μm Stubby subhedral. Irregular core with overgrowths having arange of brightness; bright rims ≤5 μm thick.

a GDA94, Zone 53.


listed in the tables and plotted in the figures are one standard error pre-cision estimates, as are those for individual analyses quoted in the text.Uncertainties in pooled ages are 95% confidence limits (tσ) and in thecase of 206Pb/238U ages, include the uncertainty in the Pb/U analysis ofthe appropriate standard. A minimum of 60 detrital zircon grains wasanalysed for most samples, giving a 95% probability that no populationwith a proportion >8.5% of the total was missed (Vermeesch, 2004).Probability density diagrams were constructed using data thatoverlapped Concordia within 2σ uncertainty and used 206Pb/238Udates for grains ≤1.3 Ga and 207Pb/206Pb dates for grains >1.3 Ga.

Sample locations are shown in Fig. 3. U–Pb results are illustrated inFigs. 6 and 7, and the data are available in Supplementary files fromthe online version of the manuscript.

4. Metasediments in the Harts Range

4.1. Stanovos Gneiss Member

The Stanovos GneissMember of the IrindinaGneiss in the southeast-ern Harts Range (Fig. 3) consists of two lithological associations: (1) astructurally lower sequence consisting of quartzite, calc-silicate andmarble, and (2) an upper sequence comprising metapelitic gneiss,quartzofeldspathic gneiss and metabasite.

4.1.1. Lower Stanovos Gneiss (sample 2001080196)The lower Stanovos Gneiss was sampled from a ~20 m thick quartz-

ite in the Stanovos Valley (Fig. 3). Sixty-nine cores and 19 overgrowthswere analysed (Fig. 6A). The dates from the cores lie within two maingroupings: a broad group between ~1.90 and ~1.35 Ga, with a peak at~1.58 Ga, and a second group with dates between ~1.20 and ~1.04 Gaand a peak at ~1.07 Ga. Zircon overgrowths have moderate U contents(260–600 ppm),moderately low Th/U (0.12–0.28) and forma cluster at~450 Ma. Three analyses with lower 206Pb/238U are interpreted to have

been affected by a small amount of radiogenic Pb loss, leaving 16analyses with a weighted mean age of 452.8±4.3 Ma (MSWD=1.46).

4.1.2. Upper Stanovos Gneiss (sample 2002080188)The upper Stanovos Gneiss was sampled from a 1 m thick quartzite

within a sequence of migmatitic biotite-rich quartzofeldspathic gneissand amphibolite. One hundred and twenty five cores were analysed(Fig. 6B). None of the ≤10 μm thin overgrowths were accessible toanalysis. The age spectrum for the cores is dominated by dates between1.24 and 0.96 Ga, including peaks at ~1.16 and ~1.06 Ga,with a scatter-ing of older dates and a few younger dates down to 830±12 Ma. A sin-gle concordant grain at ~630 Ma suggests that the sedimentaryprotolith was deposited after this time, although the fact that this isonly a single isolated date means that this interpreted maximum depo-sitional age should be treated with caution.

4.2. Irindina Gneiss

The Irindina Gneiss is dominated by garnet- and sillimanite-bearingmetapelite and quartzofeldspathic gneiss with minor quartzite and mar-ble. Despite the apparent lithological uniformity of the Irindina Gneiss,airborne magnetic and gamma-ray spectrometric data indicate that itconsists of two distinct units: 1) a structurally lower K- and Th-rich unitwith an irregular, low to moderate magnetisation and 2) an upper unitwith low-K and very low magnetisation.

4.2.1. Lower Irindina Gneiss (sample 2001080230)The unit was sampled from migmatitic garnet–sillimanite-bearing

metapelite in the central Harts Range (Fig. 3). Fifty analyses wereobtained from cores and 23 analyses from overgrowths (Fig. 6C). Mostcores form a concordant to near concordant group with dates between~1.80 and 1.60 Ga, with two older grains at ~2.39 and ~2.35 Ga.Although the main population forms a near-symmetric distributionaround ~1.73 Ga, the relatively high MSWD of this group indicates

Fig. 5. Cathodoluminescence images of representative zircon grains from: A) lower Stanovos Gneiss; B) upper Stanovos Gneiss; C) lower Irindina Gneiss; D) upper Irindina Gneiss; E) HartsRange Meta-Igneous Complex; F) upper Brady Gneiss; G) Mount Bird metapelite; H) Crossing Bore metapelite; I) Badens Camp quartzite; J) Jervois quartzite; K) Atula calc-silicate rock; andL) Atula quartzite. Scale is identical for all images.


that it is not a single population. Zircon overgrowths have low tomoderate U (110–390 ppm), low Th/U (0.03–0.13) and form acluster at ~460 Ma. Two analyses (12.1 and 20.1) have significantlylower 206Pb/238U and are interpreted to have lost radiogenic Pb. Theremaining 16 analyses have equal 206Pb/238U within analytical uncer-tainty and yield a weighted mean age of 461.5±3.5 Ma (MSWD=1.59). Five rounded overgrowths with moderate uranium contents(77–416 ppm) and higher Th/U (0.11–0.94) are much older, withdates ranging between 1.62 and 1.73 Ga, and are interpreted to bepart of the detrital component.

Four concordant to near-concordant analyses of three grains gavedates considerably younger than the other cores and overgrowths.These grains are euhedral to subhedral, with no overgrowths, andhave moderate uranium contents (403–836 ppm) and lower Th/U(0.05–0.07) than the remnant detrital cores, and may have a metamor-phic origin. Their dates range between ~343 and ~372 Ma, with aspread beyond analytical uncertainty. If the youngest analysis is omitted(interpreted to have undergone slight radiogenic Pb loss), the remaining3 analyses have equal 206Pb/238Uwithin uncertainty, yielding a weightedmean age of 365±10 Ma. Although there are insufficient analyses to

image of Fig.�5

Fig. 6. Tera–Wasserburg concordia plots and probability density histograms for: A) lower Stanovos Gneiss; B) upper Stanovos Gneiss; C) lower Irindina Gneiss; D) upperIrindina Gneiss; E) Harts Range Meta-Igneous Complex; and F) upper Brady Gneiss. Zircon core analyses are dark grey; rim analyses are light grey. Analyses with verylow precision are unfilled for clarity.


image of Fig.�6

Fig. 7. Tera–Wasserburg concordia plots and probability density histograms for: A) Mount Bird metapelite; B) Crossing Bore metapelite; C) Badens Camp quartzite; D) Jervoisquartzite; E) Atula calc-silicate rock; and F) Atula quartzite. Zircon core analyses are dark grey; rim analyses are light grey. Analyses with very low precision are unfilled for clarity.


image of Fig.�7


define a robust mean, this age suggests that high-grade metamorphismtook place during the Alice Springs Orogeny, coincident with graniteemplacement in the northeastern Harts Range (Maidment, 2005; Buicket al., 2008).

4.2.2. Upper Irindina Gneiss (sample 2001080003)The upper IrindinaGneisswas sampled from the central Harts Range

(Fig. 3), where it consists of migmatitic Qz–Bt–Pl–Grt–Sil schist.Seventy-four analyses were made of cores and six analyses of over-growths (Fig. 6D). A large proportion of grains have dates between~1.30 and ~1.00 Ga, with peaks at ~1.23, ~1.11 and ~1.02 Ga. There isalso a range of younger dates, with a secondary peak at ~860 Ma anda spread between ~750 and ~500 Ma. The two youngest grains (8 and31) were analysed multiple times to obtain better precision, yieldingweighted mean ages of 517±13 and 504±21 Ma, respectively. Theschist has a similar detrital age spectrum to that of the Irindina Gneissfrom north of the Harts Range (Buick et al., 2001), which haspopulations at ~1.80–1.60 Ga, ~1.30–1.10 Ga and ~750–500 Ma, witha youngest grain giving a date of ~511 Ma.

Most of the overgrowths were too thin to be analysed, although sixanalyses were made, yielding a range of dates. One relatively thick,rounded overgrowth with high U (>1500 ppm) and Th/U of 0.12 wasanalysed twice, yielding a mean age of ~630 Ma (analyses 27.1, 27.2).An irregular overgrowth with a U content of 735 ppm and Th/U of0.03 gave a date of ~1.05 Ga (analysis 41.2). Three euhedral over-growths with moderate U contents (280–386 ppm) and low Th/U(0.01–0.02) have equal 206Pb/238Uwithin uncertainty, yielding a pooledage of 478±15 Ma. These younger overgrowths have a similar appear-ance in CL imagery to other thin overgrowths in the sample and areinterpreted to date high grade metamorphism. The older, more round-ed overgrowths are interpreted to be remnant overgrowths on detritalgrains.

4.3. Harts Range Meta-Igneous Complex (sample 2001080226)

The Harts Range Meta-Igneous Complex consists of numerous layer-parallel metabasite and minor anorthosite units within the IrindinaGneiss. Some of the mafic units have thicknesses in excess of 1 km andcan be traced for over 100 km along strike. The high abundance of detri-tal zircon in the amphibolites (Buick et al., 2001) and the intimateinterlayering of metasediments and metabasite in many areas impliesthat at least some of the units are volcanic or volcaniclastic, rather thanintrusive. A quartz-rich layer of amphibolite gneiss was sampled fromthe southern margin of a thick amphibolite body in the southern HartsRange (Fig. 3). Sixty analyses of zircon cores and 14 analyses of over-growths are plotted in Fig. 6E. There is a relatively large proportion of1.20–1.00 Ga dates, with peaks at ~1.15 and ~1.02 Ga, and a smallergroup of zircons with dates between 1.78 and 1.51 Ga. Secondarypeaks occur at ~1.80, ~1.59 and ~0.85 Ga, with a scattering of dates be-tween ~700 and ~560 Ma. The zircon overgrowths have low to moder-ate U contents (~40–415 ppm) and generally low Th/U (0.02–0.46).Two analyses (15.2, 18.2) have significantly lower 206Pb/238U and thosegrains are interpreted to have lost some radiogenic Pb. The remaining12 analyses have equal 206Pb/238U within analytical uncertainty andyield a weighted mean age of 462.2±5.4 Ma (MSWD=0.58).

4.4. Upper Brady Gneiss (sample 200180254)

The Brady Gneiss is the structurally highest unit of the Harts RangeGroup and crops out in an arcuate belt along the northern and easternedge of the Harts Range (Fig. 3). It consists of two units: a lowermetapelitic and an upper calc-silicate unit. Calc-silicate of the upperBrady Gneiss was sampled in the eastern Harts Range (Fig. 3). Thecalc-silicate rock consists of Qtz–Pl–Cpx–Act–Czo–Ttn–Cal–Scp. Fifty-eight analyses were made of cores and 22 of overgrowths (Fig. 6F). Thecores have a much larger component of dates between ~700 and

~500 Ma than structurally lower units, with secondary populations at1.89–1.78, 1.20–1.05 and ~0.87 Ga. The youngest three cores yielddates of ~499, ~531 and ~549 Ma, suggesting that the protolith was de-posited during the Cambrian, possibly as recently as the Late Cambrian.These data are consistent with detrital zircon data from the lower unitof the Brady Gneiss which has similar age groupings and detrital grainsas young as ~510 Ma (Buick et al., 2005).

Nineteen analyses were made of the relatively thick, dark over-growths, which have moderate to high U contents (430–1050 ppm)and low Th/U (0.03–0.11). The data are spread in 206Pb/238U beyond an-alytical uncertainty and can be divided into two groups at ~460 and~480 Ma. If one analysis with low 206Pb/238U is omitted from the youn-ger group, the remaining 13 analyses form a population with a weight-ed mean age of 458.6±4.4 Ma (MSWD=1.61). The older group of 5analyses has equal 206Pb/238U within uncertainty, and yields an age of478.7±6.9 Ma (MSWD=1.32). Three of the thin, bright-CL over-growthswere thick enough to be analysed. Theyhave lowuraniumcon-tents (27–102 ppm), low Th/U (0.04–0.17) and yield dates of 359±6,361±5 and 379±5 Ma respectively, indicating that a second phase ofhigh-grade metamorphism took place during the Middle to Late Devo-nian, at a similar time to granite and pegmatite emplacement elsewherein the Harts Range (Maidment, 2005; Buick et al., 2008).

5. Metasedimentary rocks east of the Harts Range

5.1. Mount Bird metapelite (sample 2001080038)

Garnet-bearing metapelitic gneiss with lithological similarities tothe Irindina Gneiss is exposed in the Mount Bird area, ~15 km east ofthe main topographic extent of the Harts Range (Fig. 3). Sixty onecores and 15 overgrowths were analysed (Fig. 7A). The cores yield anarrow range of Palaeoproterozoic dates, ranging from 1.85 to1.70 Ga, with a peak at ~1.74 Ga, and a few grains at ~2.6 Ga. TheMSWD of this population indicates that the range in 207Pb/206Pb ofthe main group is greater than expected for a single population andthe cores are interpreted to represent a population of remnant detritalgrains. Analyses of the zircon overgrowths form a near-concordant clus-ter at ~1.70 Ga. Discarding two significantly discordant analyses leavesa group of 13 analyses with equal 207Pb/206Pb within analytical uncer-tainty, yielding an age of 1704±9 Ma (MSWD=1.48). The lithologicalsimilarities between the Mt Bird metapelite and the Irindina Gneiss,coupled with the low magnetisation of this unit, suggest that themetapelite is part of the Harts Range Group rather than of thePalaeoproterozoic basement. The rounded and irregular appearanceof most of the overgrowths might thus be a result of abrasion duringsediment transportation.

5.2. Crossing Bore schist (sample 2001080104)

Isolated outcrops of tightly folded Qtz–Bt–Ms schist occur in associ-ation with scattered outcrops of calc-silicate rock ~5 km east of Cross-ing Bore (Fig. 3). Most of the 65 analysed cores are concordant tonear-concordant and are dominated by dates between ~1.25 and1.05 Ga (Fig. 7B). There is also a scattering of older dates and threelower dates at ~716, ~662 and ~627 Ma. The only overgrowth thickenough to be analysed yielded a date of ~484 Ma. Collectively thedata constrain the deposition of the protolith to the Late Neoproterozoicor Cambrian, indicating that it was metamorphosed during the highgrade event that affected rocks in the Harts Range to the west.

5.3. Badens Camp quartzite (sample 2001080028)

A sequence of quartzite, calc-silicate rock and calcareous Bt–Msschist in the Badens Camp area, ~50 km E of the Harts Range com-prises the largest group of outcrops east of the Harts Range (Fig. 3).A thickly bedded quartzite was sampled from ~4 km NE of Badens


Bore. Eighty-five cores and 18 overgrowths were analysed (Fig. 7C).The resultant age spectrum is complex, with most dates between~1.87 and 0.85 Ga, and peaks at ~1.72, ~1.58, ~1.28, 1.08–1.04 and~0.91 Ga. Seventeen of the 18 overgrowths analysed form a clusterabout 460 Ma with moderate U (485–745 ppm) and low Th/U(0.11–0.14). A single discordant overgrowth with higher Th/U(0.37) and a 207Pb/206Pb date of 1.64 Ga is interpreted to be part ofthe detrital component. The remaining 16 analyses form a groupwith equal 206Pb/238U within analytical uncertainty, yielding an ageof 460.5±5.3 Ma (MSWD=0.95).

5.4. Jervois quartzite (sample 2002080134)

Low ridges of feldspathic quartzite and Bt–Ms–Grt–Sil schist occur inthe vicinity of Jervois Homestead, ~100 km east of the Harts Range(Fig. 3). Thesemetasedimentary rocks have not been confidently corre-lated with other units of the eastern Arunta region, although Freeman(1986) noted the lithological similarities between them and those ofthe Harts Range Group. A muscovite-bearing, feldspathic quartzitewas sampled from a sequence of interlayered quartzite and arkose atJervois Homestead. Analyses of 73 cores give age groups of ~1.27 and~1.04 Ga, with secondary groups at ~1.73 and ~1.59 Ga (Fig. 7D). Theyoungest core gives a date of ~985 Ma, indicating that the quartzite ispart of the Harts Range Group, rather than the Palaeoproterozoic base-ment. Two overgrowths were thick enough to be analysed. They havemoderate U contents (446 and 704 ppm), low Th/U (0.09 and 0.13),and yield 206Pb/238U dates of ~465 and ~477 Ma.

5.5. Atula calc-silicate rock (sample 2002080141)

Qtz–Cpx–Ep–Czo–Ttn–Bt–Cal calc-silicate rock with a weak layer-parallel foliation was sampled ~17 km SSW of Jervois Homestead(Fig. 3). The metamorphic grade in this area is lower than that in theHarts Range, with cross-bedding preserved in some layers. Sixty-twoanalyses of zircon cores are dominated by dates between ~1.25 and~0.95 Ga, with secondary peaks at ~1.76, ~1.38 and ~0.63 Ga, and ascattering of dates up to ~2.66 Ga (Fig. 7E). The youngest populationat 630 Ma places an upper limit on the depositional age of the protolith.

5.6. Atula quartzite (sample 2001080168)

The most easterly metasedimentary unit sampled in this study is athick quartzite unit exposed ~11 km SW of Atula Homestead, and~120 km ESE of the Harts Range (Fig. 3). Sixty-six cores and 31overgrowths were analysed (Fig. 7F). The zircon cores yield significantage groups between ~1.90 and ~1.60 Ga, ~1.30 and ~1.05 Ga and800–550 Ma, with smaller clusters at ~2.05 and ~2.50 Ga. The over-growths give a range of dates, with groups at ~1.90–1.60 and~1.07 Ga, and ~460 Ma. There is significant recent radiogenic Pb lossevident in many of the Palaeoproterozoic and Mesoproterozoic over-growths, which have ages similar to those of the remnant detritalcores, and are interpreted to be part of the detrital component. Theyoungest concordant overgrowths have moderate to high U concentra-tions (490–1000 ppm) and very low Th/U (0.004–0.075). Two analyses(2.1 and 21.1) marginally overlapped cores, resulting in slightly older,mixed ages. Omitting these analyses leaves a group of 12 analyseswith equal 206Pb/238U within uncertainty, yielding an age of 461.1±6.9 Ma (MSWD=1.08).

6. Depositional age and provenance of the Harts Range Group

The zircon cores from metasedimentary rocks of the Harts RangeGroup are interpreted to represent remnant detrital grains that haveretained their isotopic compositions through high-grade metamor-phism. Such preservation is common in zircon due to the extremelylow diffusion rates of Pb, Th and U in the crystal lattice, even at high

temperature and pressure (Lee et al., 1997; Cherniak and Watson,2001, 2003). Although small amounts of radiogenic Pb loss cannotbe ruled out for individual analyses, the high proportion of concor-dant analyses indicates that, in general, isotopic disturbance is mini-mal since disturbance during Palaeozoic metamorphism woulddrive the data away from Concordia. The youngest concordant datesobtained from these cores thus provide maximum depositional agesfor the sedimentary protoliths, while minimum age constraints areprovided by the Early Ordovician metamorphic zircon overgrowths.Together, these data confirm the results of previous studies by Buicket al. (2001, 2005) which indicated that the sedimentary protolithsof the Harts Range Group were deposited during the Neoproterozoicto Cambrian and therefore represent a lithostratigraphic successiondistinct from the underlying Palaeoproterozoic rocks.

Three main groupings of detrital zircon ages were obtained fromthe Harts Range Group: (1) ~1.90–1.70 Ga; (2) ~1.20–1.00 Ga; and(3) ~700–500 Ma (Figs. 8, 9). Smaller peaks occur at ~1.65–1.55 Ga and~2.5 Ga. The populations at 1.90–1.70 Ga, ~1.65–1.55 Ga and ~2.5 Gaare consistentwith derivation fromPalaeoproterozoic rocks of the AruntaRegion, where significant felsic magmatism occurred at ~1.80–1.70 Ga,~1.68–1.63 Ga (Liebig Event) and 1.59–1.57 Ga (Chewings Event), withmetasedimentary rocks from the region containingdominant populationsof detrital zircon grains at ~2.5 and ~1.90–1.85 Ga (Collins and Shaw,1995; Cross et al., 2003; Maidment et al., 2005; Scrimgeour et al., 2005;Worden et al., 2006a,b; Claoué-Long et al., 2008; Carson et al., 2009,2011).

Zircon grains in the Harts Range Group with ages of 1.20–1.00 Gahave a potential source in the Musgrave Province ~500 km to thesouthwest of the Harts Range area (Fig. 1), where there are voluminousfelsic intrusive rocks of this age (Edgoose et al., 2004; Wade et al.,2008a; Smithies et al., 2011). These were exhumed during the580–530 Ma PetermannOrogeny (Raimondo et al., 2009) and shed sed-iment northward across the Amadeus Basin (Maidment et al., 2007).Limited magmatism occurred along the southern margin of the Aruntaregion at ~1.13 Ga (Black and Shaw, 1992; 1995; Claoué-Long andHoatson, 2005; Morrissey et al., 2011), which might also have contrib-uted zircon to the Harts Range Group. Smaller detrital components inthe Harts Range Group aged between ~1.60 and ~1.40 Ga are presentin metasedimentary and igneous rocks of the Musgrave Province (e.g.Wade et al., 2008a), and in the central and southern parts of the Aruntaregion (Budd et al., 2001).

Zircon with ages between 700 and 500 Ma does not have an obvi-ous central Australian source, with most magmatism of this age beingrestricted to the margin of the continent. However, zircon in this agerange is abundant in Cambro–Ordovician sedimentary rocks of theLachlan Fold Belt ~1500 km to the southeast of the Harts Range re-gion (e.g. Williams et al., 2002). The ultimate source of this zircon isthought to have been the Pacific margin of Gondwana, probably theMozambique Orogenic Belt of east Africa (Williams et al., 2002;Goodge et al., 2004). Relatively limited felsic igneous rocks of thisage have also been identified in Western Australia in the PatersonProvince, ~1300 km to the west (Nelson, 1996; Maidment et al.,2010) and the Leeuwin Complex (Nelson, 1996) ~2300 km to thesouthwest. Although it is possible that sources in Western Australiacontributed to the 700–500 Ma age grouping in the Harts RangeGroup, an evaluation of the Early Ordovician palaeogeography of theregion by Haines and Wingate (2007) suggested that there was littleor no connectivity between the Amadeus Basin in central Australiaand the Canning Basin in Western Australia, based on significant dif-ferences in stratigraphy, depositional style, detrital zircon age spectra,petroleum systems and a high degree of endemism in faunal species.Haines and Wingate (2007) instead favoured a linkage between theAmadeus Basin and the eastern Australian margin during the Ordovi-cian, consistent with the striking similarities in detrital zircon age sig-natures of Cambro–Ordovician sedimentary rocks in the two areas(Maidment et al., 2007).

Fig. 8. Detrital zircon data for metasedimentary rocks of the Harts Range Group.Data are from this study and Buick et al. (2001, 2005).


We thus consider there to be three main sediment source regionsfor the Harts Range Group: (1) the Arunta Region and broader NorthAustralian Craton; (2) the Musgrave Inlier; and (3) the Pacific marginof Gondwana.

7. Provenance changes in the Harts Range Group

When the detrital zircon data from this study are combined withthose obtained from previous studies (Buick et al., 2001, 2005), it isapparent that there is a systematic change in the provenance of theHarts Range Group between the base and the top of the succession(Fig. 8). The lowermost units of the Harts Range Group, the NaringaCalcareous Member (Buick et al., 2005) and the lower StanovosGneiss Member, contain no analysed zircon grains younger than~1.0 Ga. Both contain significant 1.20–1.00 Ga populations and zirconwith a range of dates up to ~1.9 Ga. The similar age signatures of

these two units support their proposed correlation based on lithologicalsimilarities (Hand et al., 1999b).

The upper Stanovos Gneiss, which is lithologically distinct from thelower unit, has a very different detrital zircon age signature, being dom-inated by dates between ~1.20 and 1.00 Ga, with some as young as~630 Ma. Moving stratigraphically upwards through the overlyingIrindina Gneiss, Harts Range Meta-Igneous Complex and Brady Gneissthere is a progressively larger proportion of zircon grainswith dates be-tween 700 and 500 Ma, and a corresponding decrease in the proportionof 1.20 to 1.00 Ga zircon. In contrast with the lowest parts of the HartsRange Group, 1.90–1.70 Ga zircon forms a relatively insignificant com-ponent of these units, suggesting that the 1.85–1.70 Ma Arunta regionwas covered by either water or sediment during their deposition.

The lower Irindina Gneiss forms a striking exception to this pat-tern, being dominated by zircon dates between 1.80 and 1.60 Ga,with no analysed Meso- to Neoproterozoic grains. The restrictedrange of dates obtained from both dated samples of this unit is

image of Fig.�8

Fig. 9. Combined detrital zircon age data formetasedimentary rocks of the Harts Range Group, and Neoproterozoic to early Palaeozoic units the northeastern Amadeus and southwesternGeorgina basins, showing the close similarity in provenance between the metamorphosed and unmetamorphosed successions.Data are from this study, Zhao et al. (1992), Buick et al. (2001, 2005), and Maidment et al. (2007).


consistent with a local source in the Arunta region basement, whichcontains felsic magmatism in this age range (Cooper et al., 1988;Zhao and Bennett, 1995; Maidment et al., 2005; Claoué-Long et al.,2008; Wade et al., 2008b). The lack of diversity in the dates is unusualfor a pelitic rock, and indicates that the depositional system becamemore restricted during the deposition of the protoliths of the lowerIrindina Gneiss, possibly as a result of localised subsidence or uplift.Although the sedimentary protoliths of this unit appear to havebeen locally derived, the pelitic character of the unit indicates thatthe depocentre was still of a sufficient size to allow for the depositionof fine-grained facies.

8. Correlations within the Harts Range Metamorphic Complex

The systematic variations in provenance described above provide aframework within which previously unassigned metasedimentaryrocks of the eastern parts of the Harts Range Metamorphic Complexcan be compared. In most of the areas examined in the eastern part ofthe complex, only the most resistant lithologies such as quartzite andcalc-silicate rock are exposed, and it is difficult to establish lithologicalcorrelations between rock packages. The detrital zircon age spectracan therefore be used to provide a complementary dataset to assist cor-relation.Many of the analysed samples have age spectra that are similar

to those of units in the western part of the complex, and these similar-ities have been used to place these samples in inferred stratigraphic po-sitions (Fig. 9). In particular, striking matches exist in distinctive agespectra from the lower Irindina Gneiss and metapelite at Mt Bird, andthe upper Stanovos Gneiss and metapelite from Crossing Bore.

Although correlations based on detrital zircon age signatures areinherently non-unique, the systematic shifts in the provenance ofthe Harts Range Group over time mean that general correlations canbe made where age spectra do not show an exact match to units sam-pled from the stratigraphic record preserved in the Harts Range. Theprogressive increase in the proportion of 700–500 Ma zircon throughthe Cambrian in particular provides a means by which units may beplaced in their approximate stratigraphic positions.

The similarity in the detrital zircon age spectra between meta-sedimentary units in the Harts Range area and those up to 120 km tothe east indicates that the Harts Range Group was deposited within adepocentre of significant size. A zone of generally low magnetisation co-incident with the Harts Range Group extends for at least 250 km to thesoutheast of the Harts Range (Fig. 10), suggesting that the Harts RangeGroup also underlies Mesozoic sedimentary rocks of the EromangaBasin and possibly grades into unmetamorphosed Cambro–Ordoviciansedimentary rocks and basalts of the Warburton Basin (Sun and Purvis,2002). Overall the distribution of the Harts Range Group delineates a

image of Fig.�9

Fig. 10. ‘0.5 vertical derivative’ (0.5 VD) image of total magnetic intensity for the Harts Range area, produced by P.Milligan (Geoscience Australia), using the Australian nationalmagneticdataset (Milligan and Franklin, 2004). Harts RangeGroup:Harts RangeGroup, EPSZ: Entire Point Shear Zone, FMSZ: Florence-Muller Shear Zone, SMC: StrangwaysMetamorphic Complex.White circles indicate the locations of samples described in this study. White squares are the locations of samples described by Buick et al. (2005).


wedge-shaped block that widens to the east, although this pattern wasmodified to some extent by deformation during the Alice Springs Oroge-ny (Collins and Teyssier, 1989a,b).

9. Relationship of Harts Range Group to the Amadeus and Georginabasins (Centralian Superbasin)

The ages of the youngest detrital grains in the Harts Range Groupshow that its protoliths were deposited at the same time as sedimentsin the adjacent Amadeus and Georgina basins (Figs. 9, 10), which arethe structural remnants of the former Centralian Superbasin that wasbroken up during the intraplate Alice Springs Orogeny. This indicatesthat either the metasediments of the Harts Range Group are thehigh-grade equivalents of the basin successions, or that the distinctiveOrdovician metamorphism in the Harts Range Group means that thepackages are allochthonous with respect to the eastern Arunta region.If the Harts Range Group was allochthonous, however, it would havehad to be transported to approximately its current location by450 Ma, which is the onset of the Alice Springs Orogeny, the affects ofwhich are recorded in the Harts Range Group and the enclosing rocks,including the Amadeus and Georgina basins (Mawby et al., 1999;Haines et al., 2001). It would seem improbable, however, that theHarts Range Group is an allochthonous terrane, given that during theOrdovician the eastern Arunta region was at least 500 km from thenearest continental margin and was covered by a shallow epicratonicmarine basin. Therefore there is no orogenic belt from which to havesourced the Harts Range Group, nor are there any obvious structuresthat could have transported the terrain to its current location. Insteadthe most likely alternative is that the Harts Range Group is the highlymetamorphosed correlative of the surrounding basins, as suggestedby Buick et al. (2001, 2005). If this is the case, there should be similari-ties between the detrital zircon age populations in the Harts RangeGroup and the Amadeus and Georgina basins.

Detrital zircon ages from the Amadeus and Georgina basins (Zhao etal., 1992; Buick et al., 2005; Maidment et al., 2007) provide a referenceagainst which the detrital age spectra of the Harts Range Group can be

compared. Figs. 9 and 11 show the combined detrital zircon ages fromNeoproterozoic to early Ordovician sedimentary units of the northeasternAmadeus Basin and southwestern Georgina basins; and samples from theHarts RangeGroup and its extension east of theHarts Range. Ages fromallthree successions show three dominant groupings: 1.85–1.72, 1.20–1.00and 0.70–0.50 Ga, with minor populations at ~2.50, ~1.58 and ~1.35 Ga.The striking overall similarity between the age spectra from the HartsRange Group and adjacent unmetamorphosed basins (Fig. 11) suggeststhat these successions were sourced from similar regions and have a sim-ilar spread of depositional ages. Within the Palaeoproterozoic age peak,the Harts Range Group has a lower proportion of 1.85–1.78 Ga grainsand a higher proportion of ~1.73 Ga grains than the adjacent basins, al-though both age groups are found in the eastern Arunta region. This dif-ference is probably not significant. The fact that Cambrian units areover-represented in the Harts Range Group dataset relative to theAmadeus and Georgina basins tends to reduce the proportion ofPalaeoproterozoic and Archaean zircon grains, which are more commonin the early to middle Neoproterozoic sedimentary rocks. Thus, the rela-tive proportions of the age componentsmeasured in the different regionsare considered to be of less significance than the coincidence between theages of the peaks themselves.

In addition to having provenances of similar age, the Harts RangeGroup and the sedimentary basin successions share a similar evolutionof provenance over time (Fig. 9), suggesting that both shared a similarhistory of basin development. The oldest analysed units of the AmadeusBasin, the ~830 Ma Heavitree Quartzite and the ~660–650 Ma LimblaMember of the Aralka Formation, are dominated by zircon with agescharacteristic of the Arunta region (~1.85–1.72 Ga), with a significant1.20–1.00 Ga component and no grains younger than 1.00 Ga (Zhaoet al., 1992;Maidment et al., 2007). The late Neoproterozoic sedimenta-ry rocks in the Amadeus and Georgina basins have almost identical de-trital age spectra, suggesting that those basins were linked at that time,and include a small proportion of grains between 1.00 and 0.80 Ga(Maidment et al., 2007). The evidence that they were linked as part ofa single basin is consistent with the conclusion reached by previousworkers (e.g. Freeman, 1986; Shaw et al., 1991; Walter et al., 1995;

image of Fig.�10

Fig. 11. Probability density plots of detrital zircon data from the Harts Range Group, theAmadeusBasin and theGeorginaBasin,withmetasedimentaryunits of the eastern exposuresof the Harts Range Group placed in their inferred stratigraphic positions.Data are from this study, Zhao et al. (1992), Buick et al. (2001, 2005), andMaidment et al.(2007).


Maidment et al., 2007) who correlated the Neoproterozoic history ofthe basins on the basis of sequence development and lithotype.

The detrital zircon signature of the Early Cambrian Arumbera Sand-stone in the Amadeus Basin is significantly different from that of olderunits, being dominated by 1.20–1.00 Ga zircon derived from erosionof the Musgrave Inlier during the 580–530 Ma Petermann Orogeny(Zhao et al., 1992; Buick et al., 2005; Maidment et al., 2007). As sedi-mentation progressed during the Cambrian, the relative proportion of1.20–1.00 Ga zircon decreased broadly in concert with a progressive in-crease in the proportion of zircon between 700 and 500 Ma old. By thelatest Cambrian–early Ordovician, 700–500 Ma ages dominate the de-trital signatures, with smaller proportions of 1.20–1.00 Ga ages andvery few Palaeoproterozoic grains. The Arrinthrunga Formation in theGeorgina Basin stands out as an exception to this pattern, with adominantly Arunta-age signature interpreted to reflect depositionon a sediment-starved, carbonate-rich platform rather than in thedeeper-water environment that existed to the south (Maidment etal., 2007).

The temporal changes in provenance for the Harts Range Group,summarised above, closely match those in the surrounding sedimen-tary basins, supporting the correlation between the high-grademetasedimentary units and the adjacent unmetamorphosed succes-sions. Similar up-sequence shifts in provenance in the Harts RangeGroup, and the Amadeus and Georgina basins, support the interpreta-tion that the Harts Range Group is regionally right-way-up and pre-served in its original stratigraphic order, as suggested by Hand et al.(1999b).

Some of the metasedimentary rocks in the Harts Range Group havedetrital zircon age signatures that closely match those of specific sedi-mentary units in the adjacent basins, raising the possibility that theseunits might be direct correlatives. The lower Stanovos Gneiss, NaringaCalcareous Member (Buick et al., 2005), Jervois quartzite (Harts RangeGroup) and Heavitree Quartzite (Amadeus Basin — Maidment et al.,2007) contain no zircon with ages younger than ~1.0 Ga and have sim-ilar age populations at 1.90–1.40 and 1.25–1.00 Ga, although the rela-tive proportions of these populations differ somewhat. The lithologicalassociation of quartzite with overlying carbonate-rich units in thelower Stanovos Gneiss and Naringa Calcareous Member is similar tothe association between the Heavitree Quartzite and the overlying cal-careous Bitter Springs Formation in the Amadeus Basin, and it is possi-ble that these units are equivalents.

The upper Stanovos Gneiss and Crossing Bore schist (Harts RangeGroup) have similar detrital zircon signatures to the Arumbera Sand-stone in the Amadeus Basin. The characteristic detrital zircon age signa-ture of the Arumbera Sandstone is a result of erosion of the MusgraveProvince during the 580–530 Ma Petermann Orogeny (Zhao et al.,1992; Buick et al., 2005; Maidment et al., 2007). The Arumbera Sand-stone was deposited between ~550 and 535 Ma (Kennard and Lindsay,1991), and the similarities in the detrital age spectra between theArumbera Sandstone and the upper Stanovos Gneiss and Crossing Boreschist imply that the protoliths of these high-grade metasedimentaryunits were deposited at a similar time. A minimum depositional age forthe protolith of the upper Stanovos Gneiss is provided by granitoidswithin the unit which have a zircon age of ~520 Ma (Maidment, 2005).

The detrital zircon signature of the lower Brady Gneiss (Buick et al.,2005) is similar to that of the Late Cambrian Goyder Formation in theAmadeus Basin, whereas the upper Brady Gneiss has a similar signatureto the Cambro–Ordovician Pacoota and Stairway sandstones in theAmadeus Basin and the Cambro–Ordovician Tomahawk Formation ofthe Georgina Basin (Fig. 9). The Cambro–Ordovician units in the sedi-mentary basins were deposited within the epicratonic Larapintine Seathat reached its maximum extent in the early- to mid-Ordovician(Cook and Totterdell, 1991; Walley et al., 1991; Haines, 2005). Theprotoliths of the Brady Gneiss thus appear to have been deposited dur-ing the development of this epicratonic sea in the Late Cambrian(~500 Ma).

10. Metamorphism of the Harts Range Group

Previous metamorphic studies have shown that the Harts RangeGroup experienced peak metamorphic conditions of up to 800 °C and1.0 GPa (Miller et al., 1997; Mawby et al., 1999; Storkey et al., 2005),leading to the formation of coarse-grained garnet-bearing anatecticassemblages produced by partial melting in both metabasic andmetapelitic lithologies. Zircon overgrowths from the Harts RangeGroup have ages of ~480–460 Ma, are only weakly zoned in CL imagery,and have generally low Th/U (typically 0.01–0.15), consistent withgrowth of metamorphic zircon in metasedimentary rocks coeval withthe growth of metamorphic monazite (e.g. Williams and Claesson,1987; Rubatto and Gebauer, 2000). These ages are similar to garnetSm–Nd and monazite U–Pb ages (Hand et al., 1999a; Mawby et al.,1999), which are almost certainly metamorphic, and are thereforeinterpreted to record the general timing of high-grade metamorphism(Hand et al., 1999a,b).

Although the metamorphic grade and intensity of deformation de-crease eastwards, the ~480–460 Ma ages obtained from zircon over-growths in most analysed samples indicate that relatively high-grademetamorphism took place during the Early Ordovician across thewhole of the exposed Harts Range Group. A collation of all U–Pb meta-morphic zircon ages from theHarts Range Group forms a spread greaterthan that expected for a single population (Fig. 12), with a distributionthat is distinctly bimodal, withmodes at ~480 and ~460 Ma. Some sam-ples contain both modes in their metamorphic zircon components, for

image of Fig.�11

Fig. 12. Probability density histogramofOrdovicianmetamorphic zircon ages from theHartsRange Group, defining a bimodal distribution with modes at ~477 Ma and ~459 Ma thatmight reflect peak and retrograde phases of the Larapinta Event respectively.


example the upper Brady Gneiss (this study) and the Naringa Gneiss(Buick et al., 2005). Two distinct generations of zircon growth havealso been observed in small-scale tonalite bodies derived from partialmelting of metabasite near Mount Ruby in the southern Harts Range(Storkey et al., 2005), which yielded a mean core age of 483±6 Maand a rim age of 457±7 Ma (Maidment, 2005). Given the ~480 and~460 Ma ages obtained from single samples, it seems possible that theapparent bimodality of the zircon ages reflects the prograde and retro-grade phases of the high-grade Ordovicianmetamorphism identified byearlier workers (Miller et al., 1997; Mawby et al., 1999), rather than asingle protracted phase of regional metamorphism. More detailedwork is required, however, to distinguish between these alternatives.

11. Burial of the Harts Range Group

In many high-grade metamorphic terrains that record conditionssimilar to those in theHarts RangeGroup, the burial of sedimentary suc-cessions to lower crustal depths is commonly ascribed to orogenicthickening (e.g. Vance et al., 1998; Brown, 2001; Lee and Cho, 2003;Scrimgeour et al., 2005). This is especially the case where palaeo-geographic constraints are lacking and the sedimentary units had notpreviously been metamorphosed. However, the zircon age datapresented above place tight constraints on the time interval overwhich burial of the Harts Range Group must have occurred. Further-more, the striking similarity in detrital zircon age spectra between thehighly metamorphosed Harts Range Group, and the well preservedunmetamorphosed sequences in the adjacent Amadeus and Georginabasins (Fig. 11), suggests that they were deposited in the same basinsystem. This means that the deep burial of the Harts Range Group se-quences can be placed into a palaeogeographic context.

Fig. 13 shows the evolving palaeogeography of the central Austra-lian region during the Cambro–Ordovician, which encompasses theperiod in which burial occurred. In the Amadeus and Georgina basins,the Cambrian and Early Ordovician were characterised by shallowepicratonic marine conditions and generally slow rates of sedimenta-tion, with ≤2 km of subsidence over ~70 Ma (Wells et al., 1970;Lindsay et al., 1987; Lindsay and Korsch, 1989, 1991; Bradshaw, 1991;

Fig. 13. (A–D) Palaeogeographic maps of the eastern Arunta region during the Cambrian andCook and Totterdell (1991), and Veevers (2001). Burial of sedimentary units to lower crustnental transgression and marine sedimentation, pointing to an extensional rather than comlocation and present distribution of the Harts Range Group as well as Cambrian and OrdovicOrdovician palaeogeography (from Veevers, 2001). During the early Palaeozoic, Australia wern cratonic Australia boarded a complex long-lived back arc system that expanded as thedeposition, burial and metamorphism of the Harts Range Group coincided with extensiona

Gorter, 1991; Shaw, et al., 1991; Shergold et al., 1991; Walter et al.,1995). The sedimentary rocks deposited during this period are mature,well-sorted and characterised by a relative paucity of zircon of Aruntaregion age, suggesting that there was no significant uplift in the Aruntaregion at this time (Maidment et al., 2007). In the Amadeus Basin thesesedimentary rocks were deposited within sub-basins which formedalong the northernmargin of the basin during the latest Neoproterozoicto Early Cambrian (Lindsay, 1987; Shawet al., 1991;Walter et al., 1995).In the Georgina Basin, shallow marine siliciclastic and carbonate sedi-mentary rocks were deposited in NW-trending depocentres adjacentto theHarts Range region, andwere associatedwith amarine transgres-sion that expanded northwards from the southern margin of the basinduring the Middle to Late Cambrian (Smith, 1972; Southgate andShergold, 1991) (Fig. 13). Isopachs of Cambrian to Early Ordovician suc-cessions in both the Amadeus and Georgina basins thicken towards theHarts Range area (Smith, 1972; Lindsay, 1987; Gorter, 1991; Shaw et al.,1991; Lindsay, 1993), which led Mawby et al. (1999) to conclude thatthe region was a depocentre during this period. The Cambro–Ordovicianmarine transgression in central Australia culminated in the formation ofthe epicratonic Larapintine Seaway, which attained its maximum extentin the Early Ordovician (Fig. 13) (Webby, 1978; Walley et al., 1991).Therefore the deep burial and subsequent metamorphism of the HartsRange Group did not coincide with crustal thickening. Instead, burialwas associated with intraplate subsidence, leading to the establishmentof a long-lived epicontinental marine system.

Burial of theHarts RangeGroupwas associatedwith voluminous tho-leiitic mafic magmatism of the Harts Range Meta-Igneous Complex.These mafic rocks have geochemical characteristics typical of low-pressure mantle melting driven by lithospheric thinning (Sivell andFoden, 1985; Sivell, 1988), and were coeval with granites emplacedinto the lowermost parts of the Harts Range Group at ~520 Ma(Maidment, 2005). Mafic magmatism has not been recorded in the sur-rounding Amadeus and Georgina basins, and the dominantly peliticcharacter of the metasedimentary units interlayered with the HartsRange Meta-Igneous Complex contrasts with the shallow marine faciesdeposits in the adjacent basins. These observations, coupled with thesimilarity in detrital zircon age spectra between the Harts Range Groupand the surrounding unmetamorphosed basins, support the contentionthat the Harts Range Group occupied a rift-related depocentre within abroader shallow epicratonic marine basin. Furthermore, the palaeo-geographic constraints indicate that the entire burial andmetamorphismof the Harts Range Group occurredwhile this epicontinental marine sys-tem was maintained. The implication is that deep burial of the HartsRange Groupwas not caused by crustal thickening, but insteadwas driv-en by sediment loading in a deep rift-controlled depocentre (Fig. 14).This suggestion is consistent with the detrital zircon ages, which implythat the Harts Range Group is internally right-way up and overliesPalaeoproterozoic basement rocks. Therefore we interpret the HartsRange Group to be the keel of a now deeply inverted depocentre. Basininversion was driven by shortening associated with the Alice SpringsOrogeny, which juxtaposed the Harts Range Group against less deeplyburied parts of the same overall basin system now represented by theAmadeus and Georgina basins. Although the Alice Springs Orogenyresulted in structural reworking, the overall regional distribution of theHarts Range Group suggests that the rift basin trended WNW–ESE.

Deep seismic imaging indicates that exhumation of the HartsRange Group was accommodated by a bivergent orogenic architec-ture (Korsch et al., 2011) that geometrically resembles an invertedcrustal-scale graben structure containing the remnants of the deeply

Early Ordovician, when burial of the Harts Range Group took place, after Cook (1988),al depths in association with mafic magmatism took place during a period of epiconti-pressional setting. (E) Gravity image of Australia (Nakamura et al., 2011) showing theian structural features (from Cook and Totterdell, 1991), and Middle Cambrian to Earlyas part of the large East Gondwana landmass that faced an open ocean to the east. East-convergent margin migrated eastward (Collins, 2002; Collins and Richards, 2008). Thel basin development elsewhere in cratonic Australia.

image of Fig.�12

A B

C

E

D


image of Fig.�13


buried Harts Range Group. Thus, the crustal-scale structural architec-ture is also consistent with the notion that the Harts Range Group wasvery deeply buried in a rift controlled depocentre. The developmentof regional high-temperature, medium-to-high pressure Ordovicianmetamorphism was probably thermally controlled by the combina-tion of heat advection into the pile associated with mafic magmatism,coupled with a high basal heat flow arising as a consequence of rifting(e.g. Wickham and Oxburgh, 1985). Deformation associated withhigh-grade metamorphism produced a stratigraphy-parallel fabricwith approximately north–south extension (Hand et al., 1999a) thattrends at a high angle to the inferred overall orientation of the riftbasin. This Early Ordovician structural fabric formed during ongoingmarine sedimentation in the overlying Larapintine Seaway (Webby,1978; Walley et al., 1991), and is interpreted to reflect syn-rift plasticdeformation in the thermally perturbed keel of the basin.

The interpretation that burial of sedimentary units to depthsapproaching 30 km occurred via sediment loading challenges the con-ventional paradigm linking deep supracrustal burial and high-grademetamorphism to crustal thickening (e.g. Brown, 2001). Howeverthere are several examples of rift-driven basins which contain packages>20 km in thickness, indicating that sediment loading can lead to deepburial on a regional scale. Three of the deepest examples, the lateMesoproterozoic Midcontinent Rift System of North America (Allen etal., 1995; Hinze et al., 1997; Ojakangas et al., 2001), the PalaeozoicDnieper–Donets Basin in eastern Europe (Chekunov et al., 1992;Maystrenko et al., 2003) and the Devonian to Holocene Petrel Sub-basinin northwestern Australia (Baldwin et al., 2003) all formed within

A

B

Fig. 14. (A) Schematic interpretation of the tectonic setting for burial of the Harts Range Groupbasins resulted in accumulation of thick Cambrian and early Ordovician sequences in associatioGeorgina basin successions and units in the Harts Range Group suggest that specific sequencesBasin and the Crossing Bore Schist and Stanovos Gneiss in the Harts Range Group. 2=Late CamBasin) and Tomahawk Formation (Georgina Basin) have very similar b2000 Ma detrital ages tohigh-grade deformation in the early Ordovician occurred in the deep crust beneath the Larinterpreted to record the lower crustal response to ongoing rifting. (B) Present day structural a(Korsch et al., 2011). The Harts Range Group is preserved as a structural remnant marking the

intraplate rift settings, and each currently preserves more than 25 km ofbasin fill. For example the Midcontinent Rift System was formed at~1.10 Ga by rifting within central North America (Allen et al., 1995;Hinze et al., 1997; Ojakangas et al., 2001). The rift packages consist of alower igneous sequence dominated by basalt and minor felsic volca-nic rocks, and a clastic dominated upper sequence (Allen et al.,1995; Ojakangas et al., 2001). Seismic reflection data indicate thatin places the total succession has a maximum thickness of >30 km(e.g. Behrendt et al., 1988; Cannon et al., 1989). Age constraints suggestthat these sequencesweer deposited in less than50 Ma (Allen et al., 1995).

Baldwin et al. (2003) noted several characteristics common to mostdeep sedimentary basins. These include (i) proximity to, and trend nor-mal to, ocean basins; (ii) V-shape in plan view; (iii) an extensionalorigin; and (v) large amounts of extension. Additionally, deep intraplaterifts generally appear to form within much older continental crust thatwas to a large extent thermally stable prior to rift initiation (e.g.Cloetingh et al., 1995). The basin that appears to have accommodatedthe Harts Range Group possesses many of these features. The roughlyeast–west trending sub-basin apparently formed at a high angle to theCambrian continental margin of Australia (Fig. 13), and in its currentform, albeit modified, is broadly V-shaped in plan view, narrowing tothe west. Due to the obscuring effects of later sedimentary basins, it isnot certain that the sub-basin opened eastward directly into an oceanicbasin, however the presence of detrital zircon in the Harts RangeGroup with a signature similar to that of sediment from the Pacific mar-gin of Gondwana implies a connection with the eastern margin of Aus-tralia. As outlined above, there is also evidence for extension during

. During the Cambrian, rifting of the region between the preserved Amadeus and Georginan withmafic magmatism. Similarities in detrital zircon age spectra between Amadeus andmay be correlatives. 1=Early Cambrian sequences: Arumbera Sandstone in the Amadeusbrian–early Ordovician sequences: Goyder Formation and Pacoota Sandstone (Amadeusthe Upper Irindina Gneiss, Lower and Upper Brady Gneiss (Harts Range Group). Regional

apintine Sea and produced a layer-parallel fabric associated with N–S stretching that isrchitecture following compressional inversion associated with the Alice Springs Orogenylocation of the earlier rift controlled depocentre.


deposition of the Harts Range Group in the form of voluminous maficmagmatism which points to significant thinning of the continentallithospheric mantle. More regionally, the low rates of subsidence inthe broader Amadeus–Georgina basin system, which overliesPalaeoproterozoic crust 35–40 km thick (Korsch et al., 1998), suggestthat the rift basinwhich accommodated the Harts Range Groupwas ini-tiated within a comparatively thermally stable continental lithosphere,and throughout its development was enclosed by comparatively stablelithosphere.

If the above interpretation is correct, it is possible that the inferredIrindina Sub-basin represents an aulacogen developed at a high angleto the eastern margin of Australia during the Cambro–Ordovician. Alter-natively, the Harts Range Group could have been deposited within a pullapart rift basin associated with large-scale strike-slip deformation (e.g.Jolivet et al., 2012). Deposition of the protoliths to the Irindina Gneissand HRMIC in the Early to Middle Cambrian (~520–500 Ma) and meta-morphism of the Harts Range Group at 480–460 Ma each coincide withepisodes of interpreted regional extension at the eastern margin of thecontinent, as evidenced by the development of similar age oceanicback-arc basins and by the eastward retreat of the 490–440 Ma Mac-quarie Arc (Collins and Richards, 2008). The fundamental driver forthis regional extension is considered by Collins and Richards (2008) tobe subduction zone rollback, periodically interrupted by subduction ofmore buoyant crust and hinge advance.

The recognition that burial leading to regional high-grade meta-morphism of rocks within the Harts Range Group occurred within abroad intraplate basinal system has important implications for as-sumptions that are commonly applied to the tectonic drivers leadingto high grade metamorphism of supracrustal packages. For example,Vance et al. (1998) and Scrimgeour et al. (2005) used the burial ofsupracrustal rocks to pressures similar to those recorded in theHarts Range Group to argue for tectonic settings involving collision.In those cases, the metamorphic terrains were largely obliterated bylater reworking, and the metamorphic character was used as a prima-ry tool for the interpretation of the tectonic setting. However theburial of the Harts Range Group to depths of around 30 km, and itssubsequent metamorphism, shows that regional high-grade meta-morphism can occur in the keels of deep intraplate rift basins, indicat-ing that unless palaeogeographic or other boundary constraints exist,the tectonic interpretation of deeply buried supracrustal rocks shouldbe treated with caution.

12. Conclusions

Intraplate orogeny in central Australia has exposed high grademetasedimentary rocks that underwent peak regional metamorphismat conditions of 0.9–1.0 GPa and 800 °C in the early- to mid-Ordovician (480–460 Ma). U–Pb dating of detrital and metamorphiczircon from these high-grade metasedimentary rocks indicates thattheir protoliths were deposited in the Neoproterozoic to Cambrian.Comparison with detrital zircon age spectra for the adjacent un-metamorphosed Amadeus and Georgina basins indicates that themetasedimentary rocks are the highly-metamorphosed equivalents ofthe basin successions. Palaeogeographic reconstructions indicate thatburial of the metamorphic precursors to depths approaching 30 kmwas not associated with crustal thickening. Instead, based on the welldescribed sedimentology of the unmetamorphosed Amadeus andGeorgina basins, thepalaeogeography indicates that burial occurredwith-in an epicratonic setting characterised by long-lived shallowmarine sed-imentation, and was associated with voluminous mafic magmatismgenerated by shallow melting of thinned continental lithosphere. Theseobservations are interpreted to indicate that deep burial occurred withina discrete rift-controlled depocentre in a broader intraplate basin that de-veloped on comparatively thermo-mechanically stable lithosphere. De-spite later compression-driven inversion, deep seismic data shows thatthe highly metamorphosed rocks are located in a crustal-scale graben

architecture, consistent with the notion that burial occurred within a riftbasin, and not as a consequence of crustal thickening. The high lower-crustal temperatures are likely to have arisen through the combinationof heat advection associated with mafic magmatism and high basal heatflow arising from rifting. The existence of very deep rift-related sedimen-tary basins in the geological record suggests that regional-scale high-T,moderate-P, supracrustal-dominatedmetamorphic terrains can be gener-ated largely through basin-forming processes, and need not necessarilyreflect tectonic burial driven by compression.

Acknowledgements

Work presented here was completed as part of a PhD study byDWM carried out at the Research School of Earth Sciences, The Aus-tralian National University. Geoscience Australia is kindly acknowl-edged for granting permission for DWM to undertake postgraduateresearch. C. Kirkland and an anonymous reviewer provided valuablecomments that improved the quality of the manuscript.

Appendix A. Supplementary data

Supplementary data to this article can be found online at http://dx.doi.org/10.1016/j.gr.2012.12.020.

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