Tectonic setting and provenance of the Paleoproterozoic Willyama Supergroup, Curnamona Province, Australia: Geochemical and Nd isotopic constraints on contrasting source terrain components

Precambrian Research 166 (2008) 318–337

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Tectonic setting and provenance of the Paleoproterozoic Willyama Supergroup, Curnamona Province, Australia: Geochemical and Nd isotopic constraints on contrasting source terrain components K. Barovich ∗ , M. Hand Continental Evolution Research Group, School of Earth and Environmental Sciences, University of Adelaide, Adelaide, SA 5005 Australia

a r t i c l e

i n f o

Article history: Received 19 June 2006 Received in revised form 30 April 2007 Accepted 25 June 2007 Keywords: Geochemical provenance REE Nd isotopes Proterozoic Australia Continental reconstruction

a b s t r a c t The Curnamona Province Paleoproterozoic Willyama Supergroup was deposited in one of a series of basins developed along the eastern margin of the Proterozoic Australian continent. Linked tectonic evolutionary paths have previously been drawn between the northern and southern Australian cratons, based in part on similar depositional ages and stratigraphy, and tectonic evolutions of the sedimentary basins of this region. Detailed geochemical and Nd isotopic analyses from the Paleoproterozoic Willyama Supergroup suggest an intracratonic source for the lower parts of the sequence, with introduction of a significantly more juvenile source region for the ca. ≤1650 Ma uppermost part, marking a significant change in tectonic regime. The geochemical and Nd isotopic signature of both mature quartz-rich and pelitic sedimentary rocks of the lower part of the Willyama Supergroup support a dominant intracratonic central and northern Australian provenance, marked by a recycled and felsic upper crustal composition unusually enriched in REE and incompatible elements. Sedimentary REE abundance patterns double those of the Post-Archean Australian Shale average. Initial εNd values between −7 and −4 suggest old upper crustal recycled source basement. In contrast, the uppermost part of the Willyama sedimentary section has REE patterns much more like Post-Archean upper crustal shale average, and is characterised by more primitive initial εNd values of around −3 to 0. Significant ca. 1650 Ma felsic source terranes with such a juvenile Nd isotope signature are unknown in Australia. The data suggest proximity of a relatively juvenile as yet unidentified source to the present-day east of the basin. Some pre-Rodinian reconstruction models place eastern Australia proximal to southwestern Laurentia, which contains rocks of appropriate geochemical and isotopic signatures and could have delivered juvenile felsic sediment into the Willyama basin. © 2008 Elsevier B.V. All rights reserved.

1. Introduction Paleo- to Mesoproterozoic crystalline and metasedimentary rocks in the northern Australian Mt. Isa, Coen, and Georgetown Inliers and the southern Australian Curnamona Province, along the eastern margin of Australia, represent the truncated present-day eastern boundary of Proterozoic Australia (Fig. 1). As such, these rocks may provide an important constraint in models that reconstruct pre-Rodinian plate configurations. Numerous Paleo- to Mesoproterozoic reconstructions place either northern Canada (SWEAT model) (Moores, 1991), southwestern USA (AUSWUS) (Brookfield, 1993; Karlstrom et al., 2001), Mexico (AUSMEX) (Wingate et al., 2002), or south China (Li et al., 1995; Evans et al., 2000) along the eastern Australian boundary. These models incorporate a variety of evidence, including coincident magmatic

∗ Corresponding author. Tel.: +61 8 8303 3879; fax: +61 8 8303 4347. E-mail address: [email protected] (K. Barovich). 0301-9268/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.precamres.2007.06.024

timelines, similarities in tectonic belts and structural lineaments, and paleomagnetic constraints. In recent Proterozoic Australia tectonic evolution models, correlations of the north and south Australian cratons during the Paleoproterozoic have been drawn by a number of workers, based largely on lithostratigraphic, metamorphic and metallogenic similarities (Laing and Beardsmore, 1986; Laing, 1996a,b; Giles et al., 2002, 2004). Laing and Beardsmore (1986) and Laing (1996a,b) linked the Willyama and Mt Isa basins in the Diamantina orogen across their present-day 1500 km separation. A proposed geometric reconstruction rotates the South Australian Craton about 52◦ counter-clockwise, placing the Curnamona Province immediately southeast of the Mount Isa terrain, thereby correlating the Curnamona Province Paleoproterozoic Willyama Supergroup basin with basins of similar age in the Mt Isa Inlier (Fig. 4 of Giles et al., 2004). Giles et al. (2002) suggested that these Paleoproterozoic basins evolved in an intracontinental setting linked to far-field (ca. 1000 km distant) subduction processes along the southern margin of the northern Australian craton.

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Fig. 1. Location of the Curnamona Province and other Proterozoic terrains in Australia (after Myers et al., 1996; Wade et al., 2006).

Sedimentary basin fill presents a detailed record of tectonic processes and paleogeography of potential source regions. In multiply deformed, tectonically displaced, and metamorphosed metasedimentary sequences, traditional techniques such as field correlations and detrital mineralogical analyses may be severely hampered. This is a particular problem in the Curnamona Province Willyama Supergroup, which was metamorphosed to granulite facies during the ca. 1600 Ma Olarian Orogeny (Marjoribanks et al., 1980; Clarke et al., 1987, 1995; Page et al., 2000a). Detrital zircon U–Pb geochronology, geochemistry and Nd isotope signatures are independent tools that may be used to discriminate sediment provenance and tectonic setting (e.g., McLennan et al., 1993; McLennan et al., 1995; Yamashita et al., 2000; Lahtinen et al., 2002; Tran et al., 2003; Li et al., 2005; Gonzalez-Alvarez et al., 2006). Detrital zircon ages provide an age spectrum of zircon-bearing sources, and youngest detrital zircon ages define the maximum depositional age of the sediment. The major disadvantages of detrital zircon geochronology are: (1) the loss of small zircons in separation; (2) the lack of contribution from fine-grained volcanic sources; and (3) the lack of contribution from less felsic sources that may be relatively zircon-poor, thus biasing data toward felsic rock types.

Sm–Nd isotope studies are often combined with rare earth and trace element geochemistry. For example, once weathering and sorting processes are accounted for (e.g., Cullers and Podkovyrov, 2002; Lopez et al., 2005), the geochemistry of the sediment carries the record of the input of source material, and allows distinction between evolved crustal material and mantle-derived igneous rocks. It is assumed that Nd isotope data provide the average crustal residence age of all contributing protoliths, but the data are unable to distinguish ages of individual protoliths (McLennan et al., 1993). The evolution and provenance of the eastern Proterozoic Australian margin basins may be studied through the use of detrital zircon geochronology, geochemistry and Nd isotopic data from basin infill. Such data can provide important constraints for preRodinian reconstruction models. The chronostratigraphy of the Curnamona Province Willyama basin is now well-constrained by previous workers (Page et al., 1998; Conor and Page, 2003; Page et al., 2005a,b). Their chronological data provide firm temporal correlations within the basin across its exposed 200 km width (Fig. 2). One previous geochemical provenance study of solely the Broken Hill Group from the eastern part of the Willyama basin relied only on major and trace element geochemical data (Slack and Stevens, 1994).

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Fig. 2. Simplified geologic map of the southern Curnamona Province (after Page et al., 2005b), showing the locations of the Olary and Broken Hill Domains in South Australia and New South Wales, respectively, and the distribution of the Willyama Supergroup. The inset shows the full extent of the outcropping Curnamona Province.

In this study we compare new geochemical and Sm–Nd isotopic datasets for coeval sedimentary rocks from across the Willyama basin. By utilising Page’s et al. (2005b) sample suite, with published detrital zircon geochronology from the eastern side of the Willyama basin (Page et al., 2005a,b), we may directly compare Nd isotope data with detrital zircon age U–Pb information. We are also able to compare trace element and Sm–Nd isotope variations between correlative samples of different lithologies from west to east across the basin. The combined set of geochemistry and Nd isotopes provides us with a method to examine the relative contributions of intracontinental recycled crustal input vs. juvenile crustal input. The goal is to further constrain the provenance of the sedimentary rocks, particularly the relative importance of intra- vs. inter-continental sources, and examine current models of the tectonic evolution of the basin during development of Proterozoic Australia.

2. Geological setting 2.1. Willyama Supergroup, Curnamona Province, South Australia The Curnamona Province is a roughly circular feature of Paleo-to Mesoproterozoic metasedimentary and igneous rocks in southern Australia (Figs. 1 and 2) (Robertson et al., 1998). The supracrustal rocks of the Paleoproterozoic Willyama Supergroup, which contains the world’s largest known Pb–Zn–Ag deposit at Broken Hill, are basement to the region, cropping out largely only in the Olary Domain (OD) and Broken Hill Domain (BHD) in the south. The accepted domain boundary between Olary and Broken Hill Domains (Fig. 2) is a strong southwesterly magnetic trend (Stevens, 1986). There is both a metamorphic grade change and substantial lithologic differences across this boundary (Stevens, 1986; Ashley

et al., 1997). The rocks are a sequence of generally shallow water metamorphosed quartzofeldspathic and pelitic sedimentary rocks, calc-silicates and minor volcanic and subvolcanic rocks. Neither the base nor the top is identifiable, but estimated thickness of the known sequence is around 7 km in the BHD (Stevens et al., 1983). Multiple tectonism during the ca. 1600 Ma Olarian Orogeny and the ca. 500 Ma Delamerian Orogeny has dismembered and inverted the stratigraphy and metamorphosed the rocks up to granulite facies (e.g., Page and Laing, 1992; Dutch et al., 2005; Rutherford et al., 2007). Despite the lack of continuous stratigraphy, detailed lithological studies and geochronological work on tuffaceous metasedimentary rocks across the domains has established a lithostratigraphic and chronostratigraphic framework (Stevens et al., 1983; Willis et al., 1983; Page et al., 1998, 2000a, 2005a,b). The Willyama Supergroup has been divided into formal stratigraphy on each side of the basin (Willis et al., 1983; Stevens et al., 1988; Conor, 2000; Page et al., 2005a,b). Based on U–Pb zircon geochronological constraints, Page et al. (2005a,b) correlate the stratigraphy across the terrain as follows (Fig. 3): (1) the older OD Curnamona Group, a variably albitised volcano-sedimentary sequence, generally equivalent to all units below the Ettlewood Calcsilicate Member of the Broken Hill Group in the BHD, overlain by (2) The OD Strathearn Group, a sequence of psammopelites and pelites, equated with the Broken Hill, Sundown and Paragon Groups of the BHD. Conor (2006) suggests a redefinition of the Strathearn Group to exclude the Saltbush Subgroup, resulting in an older Saltbush Group and a younger Strathearn Group. The older part of the stratigraphy is characterised by now highly albitised quartzofeldspathic sediments deposited in a shallow marine or lacustrine setting. 1715–1710 Ma volcanic units of the Basso Suite at various stratigraphic levels within the Curnamona Group constrain its age (Page and Laing, 1992; Ashley et al.,

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Fig. 3. Relationship of major Willyama Supergroup major stratigraphic units from west to east across the Curnamona Province (after Willis et al., 1983; Stevens et al., 1988; Conor, 2000; Page et al., 2005b). Unless otherwise indicated, SHRIMP U–Pb zircon ages taken from Page et al. (2005a,b) represent maximum depositional ages of sedimentary units. MS: Metasediments.

1996). The Curnamona Group has historically been correlated with the Thackaringa Group, but Page et al. (2005a) and Conor (2006) suggest that the Thackaringa Group may be younger than the Curnamona Group, on the basis that 1715–1710 Ma volcanic units are absent from the former. Youngest detrital zircons in the Curnamona and Thackaringa Groups are from 1730 to 1710 Ma (Donaghy et al., 1998; Page et al., 2005a,b). The younger part of the stratigraphy is characterised by psammopelitic lithologies, interpreted as a deepening upwards marine environment relative to the older parts (e.g., Conor, 2000). Zircon ages of ca. 1693 Ma from the OD tuffaceous Plumbago Formation are correlated with 1693 Ma depositional age of tuffaceous rocks from the BHD basal Ettlewood Calc-silicate member of the Broken Hill Group (Page et al., 2005b). The 1695–1685 Ma Broken Hill Group

has been viewed as largely absent from the OD (Conor, 2000; Page et al., 2005a,b), although it is not recognized if the absence is depositional, erosional or structural. More recently, based on lithologic similarities, Conor (2006) suggests Broken Hill Group equivalent units may be present in OD, albeit substantially thinner. A nearly unimodal 1651 Ma zircon population from tuffaceous sandstones of the Mt. Howden Subgroup in the uppermost part of the OD Strathearn Group correlate with tuffaceous metasiltstone ages in the BHD Bijerkerno and Dalnit Bore Metasediments of the middle to upper Paragon Group of ca. 1657–1642 Ma (Page et al., 2005b). Conor and Page (2003) estimate the stratigraphic thickness between the well-constrained OD 1693 Ma Plumbago Formation at the base of the Strathearn Group and the maximum depositional age of ca. 1651 Ma in the upper Mt. Howden Group is only 50–100 m,

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while the equivalent interval in the BHD is 1–3 km. Thus, a depositional break of up to 40 Ma is defined within the upper part of the OD stratigraphy (Page et al., 2005a). Lithostratigraphic evidence, including the generally shallow water nature of the sediments (Willis et al., 1983) and geochemical characteristics of syndepositional bimodal magmatism that are indicative of a continental rift environment (Ashley et al., 1996; Rutherford et al., 2006), has led workers to suggest Willyama Supergroup deposition in an intracontinental basin setting, with only restricted ocean access (e.g., Willis et al., 1983; Wright et al., 1987; Raetz et al., 2002). Based largely on the ca. 1710–1600 Ma rock interval, Laing and Beardsmore (1986) and Giles et al. (2002) have suggested the Willyama basin was part of a larger tectonic system of intracratonic rift basins that include the Mt Isa and Georgetown Inliers. The ca. 1600 Ma Olarian Orogeny provides a minimum depositional age. Multiple ductile and brittle deformation, up to granulite facies metamorphism, and Na–Ca–K metasomatism is documented (Clark et al., 2006; Rutherford et al., 2007). This event was accompanied by syn- to post-orogenic voluminous peraluminous to metaluminous felsic to dioritic magmatism, far more voluminous in the west. The timing of deformation in the Curnamona Province is highly debated. Some workers have linked D1 structural fabrics to an early (1690–1640 Ma) metamorphic event (Gibson et al., 2004). These workers suggest that parts of the OD Mt Howden and BHD Paragon Groups lack evidence of the earliest recognized S1–D1 fabric, and may represent a younger metasedimentary packaged deposited after D1. Other workers (Page et al., 2005a,b; Rutherford et al., 2007) find no evidence for a high-grade metamorphic event older than about 1600 Ma.

3. Sampling and analytical methods 3.1. Sampling Two factors complicate the sampling of the Willyama Supergroup rocks: (1) grain size variation; and (2) post-depositional metamorphism and metasomatism. It is preferable to use finegrained sedimentary rocks for REE geochemical provenance studies, as REE are concentrated in these sediments, and a finegrained sediment is suggested to be more likely to represent a homogeneous source mixture (e.g., Condie et al., 1992; Cullers and Podkovyrov, 2000; Tran et al., 2003). Variations and anomalies in the REE abundance patterns of coarse-grained sedimentary rocks may result from sedimentary sorting processes rather than source variation (e.g., McLennan et al., 1993), although other workers (e.g., Crichton and Condie, 1993; Yamashita et al., 2000) have shown that in some cases even diamictites and conglomerates can yield valid trace and rare earth element provenance information. Conversely, detrital zircon U–Pb studies must target coarser grained rocks such as quartzites and psammites, which offer the best opportunity of obtaining a zircon population sufficient in size to be analytically valid (Andersen, 2005). REE patterns and Sm–Nd isotope data from the BHD geochronological samples used in the Page et al. (2005b) study can be compared with their finer-grained correlatives from the OD to evaluate factors that may hamper provenance information, including the possible effects of quartz dilution and mineral sedimentary sorting on the trace and rare earth element patterns. Zones of migmatisation and partial melting are evident throughout the Curnamona Province (Stevens et al., 1983; Clarke et al., 1987). In the detailed detrital zircon geochronological studies undertaken in the Curnamona Province Willyama Supergroup by Page et al. (2005a,b), all sedimentary samples contain meta-

morphic zircon ages of ca. 1600 Ma. Additionally, portions of the lower Willyama Supergroup stratigraphy have undergone variable degrees of Na-metasomatism at least two different times during their history, an early diagenetic alteration and a syn-Olarian metasomatic event, with albitisation the most common product of the fluid modification (Ashley et al., 1996; Clark et al., 2006). The level of trace element mobility in sedimentary rocks due to diagenesis, weathering and post-depositional metamorphism remains controversial (Zhao et al., 1992; Bock et al., 1994; Ohr et al., 1994; Nesbitt et al., 1996; Tran et al., 2003; Swain et al., 2005). Trace element redistribution on a mineralogical scale during these processes is generally not a problem for whole-rock studies (Ohr et al., 1994; Bock et al., 2004) and a number of papers have documented the robustness of whole-rock Sm–Nd systematics at the hand-sample scale after post-depositional processes (e.g., Barovich and Patchett, 1992; Swain et al., 2005). Of bigger concern is melt and fluid mobility, and large-scale Sm–Nd disturbances have been documented in such cases (e.g., Gruau et al., 1996; Moorbath et al., 1997). Multiple Na-metasomatic events in the lower parts of the Willyama stratigraphy render oxides such as K2 O, Na2 O and CaO invalid in interpreting original sedimentary protolith compositions. While the most severely metasomatised lithologies were avoided, within the OD, such alteration is nearly ubiquitous in the lower parts of the stratigraphy. With respect to trace element mobility during albitisation, Bierlein (1995) has shown that even in the most severely albitised sequences in the OD, the REE have remained immobile, with REE patterns unaffected. Most fine-grained clastic sedimentary rocks have been shown to exhibit fairly constant REE normalized patterns and isotopic Sm/Nd ratios from about 0.10 to 0.12 (e.g., McLennan and Hemming, 1992). Careful examination for unusual REE patterns and unusually fractionated Sm/Nd ratios can identify geochemically disturbed samples (e.g., Zhao et al., 1992; Lev et al., 1999; Bock et al., 2004). In all cases zones of migmatisation and partial melting were avoided. Whole-rock aliquots of most of the BHD sedimentary samples from the Page et al. (2005b) study were obtained for Sm–Nd isotope analyses. Geochemical analyses were previously undertaken by Geoscience Australia (Budd et al., 2002). Locations are provided in Table 1. A sample suite spanning the OD Willyama stratigraphy was collected from well-documented localities (Table 1). Only one OD sample, 98185016, also has detrital zircon U–Pb data (Page et al., 2005b). 3.2. Analytical methods Where required, samples were cut to remove weathered surfaces, crushed, then powdered in a tungsten carbide mill. OD whole-rock powders were sent to Amdel Laboratories for major and trace element geochemical analyses. In the case of the BHD samples from the Page et al. (2005b) study (samples starting with a ‘9’), whole-rock powders were prepared by the Geoscience Australia analytical laboratory, and analysed at their facility in Canberra. Data for those samples were compiled from the Geoscience Australia OZCHEM database (Budd et al., 2002). The Geoscience Australia whole-rock open-file geochemical data presented in this paper can also be downloaded from http://www.ga.gov.au/oracle. Major elements were analysed by X-ray fluorescence spectrometry (XRF), and some trace elements (Sc, V, Cr, Ni, Cu, Zn, As, Rb, Sr, Zr, and Ba) were analysed by XRF on pressed powder pellets. Inductively coupled plasma-mass spectrometry (ICP-MS) was used to determine the remainder. The precision (1 level) for major elements was better than ±1% for concentrations greater than 5%, and for trace-element analyses was typically ±3% at the 30–100 ppm level.

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Table 1 Rock units of the Willyama Supergroup, sample number, stratigraphic unit, lithology and location Sample

Stratigraphic unit

Olary Domain Upper Strathearn Group, Mt. Howden Subgroup Dayana Fm. 98185020a Mooleulooloo Fm. 98185018a 98185019 Mooleulooloo Fm. Alconie Fm. 98185016a 98185017 Alconie Fm. 98185021 Alconie Fm. R632637 Alconie Fm. (upper) R632631 Alconie Fm. (middle) R632630 Alconie Fm. (lower)

Lithology

Easting

Northing

Pelitic schist Pammopelite Psammite Andalusite schist Pelite Calc-albite ms Psammite Psammopelite Chiastolite schist

424256 424025 424053 423966 423966 424310 424168 424183 424197

6467011 6466900 6466531 6467223 6467223 6466506 6466702 6466683 6466659

Lower Strathearn Group R350483a R350482 R350481a R350485a

Walparuta Fm. Walparuta Fm. Plumbago Fm. Bimba Fm.

Quartz-biotite-schist Quartz-mica schist Qtz-muscovite schist Calc-silicate ms

409723 409723 409723 408991

6433280 6433280 6432280 6432852

Curnamona Group R350486a R350487 R350488 R350489 R350490 R350491a R350493a R350494a R350495a R350496 R350497 R350480a R350479 R350478 R350477a R350476a R350475 R350474a

Upper Ethiudna SG Middle Ethiudna SG Middle Ethiudna SG Middle Ethiudna SG Middle Ethiudna SG Lower Ethiudna SG Lower Ethiudna SG Wiperaminga SG Wiperaminga SG Wiperaminga SG Wiperaminga SG Wiperaminga SG Wiperaminga SG Wiperaminga SG Wiperaminga SG Wiperaminga SG Wiperaminga SG Wiperaminga SG

Qtz-plag-mica schist Qtz-plag-mica schist Layered quartzite Qtz-plag-mica schist Qtz-plag-biotite schist Calc-albitite Calc-albitite Quartzite Plag qtz mica schist Plag qtz mica schist Albitic psammite Albitic psammite Albitic psammite Albitic psammite Albitic psammite Albitite Albitite Albitite

408797 408715 408690 408750 408750 408711 408727 408682 408682 408682 408631 409322 409471 409568 409695 409795 408876 408948

6432680 6432691 6432693 6432666 6432666 6432677 6432671 6432691 6432691 6432691 6432739 6432282 6432125 6432083 6431970 6431880 6430550 6432762

Dalnit Bore MS Bijerkerno MS Bijerkerno MS Bijerkerno MS Bijerkerno MS Cartwrights Creek MS Cartwrights Creek MS Cartwrights Creek MS Cartwrights Creek MS Cartwrights Creek MS King Gunnia CS Member

Pelite Psammite Psammite Psammopelite Pulite Psammite Pelitic schist Pelitic schist Psammite Psammite Calc-silicate

537019 558000 538369 536817 555532 555428 555420 555435 533847 524119 558770

6515512 6522580 6508081 6506946 6530608 6530589 6530731 6530712 6507262 6485829 6523543

Sundown Group 99185506a 99185510a 99185511a 99185520 99185521a

Sundown Group Sundown Group Sundown Group Sundown Group Sundown Group

Psammite Psammite Psammite Psammite Calc-silicate

535046 556047 557132 541342 541342

6505544 6522697 6522725 6472625 6472625

Broken Hill Group 99185517a 99185518 99185519a 99185527a

Freyers MS Ettlewood CS Member Allendale MS Allendale MS

Psammite Calc-silicate Albitic psammite Pelite

545340 546536 546811 526400

6504520 6505203 6505689 6482250

Thackaringa Group 99185516a 99185525a

Himalaya Formation Thorndale Comp. Gneiss

Albitic psammite Sillimanite schist

547825 556644

6504634 6463343

Broken Hill Domain Paragon Group 99185501a 99185528Aa 99185504 99185505a KB04-23a KB04-20a KB04-21a Kb04-22a 99185507 99185512 99185509a

Broken Hill Domain Eastings and Northings are AGD66 reference, Olary Domain values in italics are GDA94, others are AMG54. a Sample used for Sm–Nd analysis; SG: subgroup; MS: Metasediment; CS: calc-silicate.

Details of the Sm–Nd isotope methods are given in Wade et al. (2006). Sample preparation and isotopic analyses were undertaken at the University of Adelaide isotope laboratory. Powdered wholerock samples were spiked with a mixed 149 Sm–150 Nd spike before

oven digestion in sealed Teflon steel bombs. Two-stage exchange columns were used for separation of Sm and Nd. Nd analyses were carried out on a Finnigan MAT 262 multi-collector mass spectrometer in dynamic mode, and on a Finnegan MAT 261 single-collector

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mass spectrometer for Sm. Analyses of the La Jolla standard yielded 0.511839 ± 0.000005 (1 S.D.) on 12 runs during the course of the study.

4. Geochemical and Sm–Nd isotope results 4.1. Major and trace elements Results of major and trace element analyses are presented in Table 2. The Willyama Supergroup samples show considerable lithologic variability from psammites to pelites, highlighted geochemically by their roughly linear inverse SiO2 –Al2 O3 variation (Fig. 4A). Seven SiO2 -poor samples with anomalously low Al2 O3 are highly altered (albitites) of the lower Wiperaminga Formation or various calc-silicate members of the stratigraphy (Fig. 3, Table 2). The transition from pelite to psammite in the sample suite, as defined by the SiO2 –Al2 O3 relationship, is continuous, and for purposes of discussion, an arbitrary division is made at SiO2 = 75% (Fig. 4A). A higher proportion of the BHD samples are psammitic, reflecting the targeted sampling for detrital zircon U–Pb geochronology by Page et al. (2005b). The OD samples, with the exception of the variably albitised quartzofeldspathic Wiperaminga Group, are finer-grained lithologies including pelites and psammopelites, as evidenced by lower SiO2 and higher Al2 O3 . There is less lithological variation between pelites and psammites of the upper part of the stratigraphy, and SiO2 ranges only from 62 to 81% and Al2 O3 down to 10%, while there is a much greater difference between pelites and psammites from the lower parts, where SiO2 varies from 42 to 89% and Al2 O3 down to 4%. A combination of the quartz dilution effect and erratic patterns introduced by heavy mineral sorting may be responsible for the greater variation in the lower parts. Variable and low K2 O/Na2 O ratios (0.01–14) (Fig. 4B) reflect variable Na-metasomatic alteration during the ca. 1600 Ma Olarian Orogeny. This metasomatism is highlighted particularly in the lower parts of the OD stratigraphy, where all but one of the nine Wiperaminga Supgroup K2 O/Na2 O ratios is less than 0.20. Discounting the highly altered Wiperaminga samples and the calcsilicates, the average K2 O/Na2 O ratio for all samples, psammitic and pelitic, is about 3, similar to the Post-Archean Australian Shale (PAAS) value of 3.1 (Taylor and McLennan, 1985). Generally, there is a reverse correlation with SiO2 and trace elements in the Willyama Supergroup pelites and psammites (Fig. 4C and D, Table 2) probably reflecting a quartz dilution effect. Slack and Stevens (1994) highlighted unusual trace element signatures of BHD Broken Hill Group sedimentary rocks, and our results show that this pattern is reflected in the whole of the Willyama Supergroup. Average pelite Cr, Ni and Sc concentrations are significantly and uniformly less than PAAS values (59 ppm vs. 110 ppm, 16 ppm vs. 55 ppm, and 12 ppm vs. 16 ppm, respectively). Psammitic Cr, Ni and Sc averages are even lower at 22.3, 7.7 and 6.4 ppm, respectively (Table 2). Other trace elements in the samples show larger variations (Table 2), e.g., in the pelites, Th (14.2–25.4 ppm, average = 18), Zr  (145–320 ppm, average = 204) and REE (123.4–612.5 ppm, average = 243; excluding Pr and Tm, for which data sets are incomplete). The PAAS values for these trace elements are 14.6, 210, 175.5 ppm,  respectively. The average Th, Zr and REE values for the psammites are 15, 404 and 187 ppm, respectively. This Zr average of 404 is significantly higher in the psammites vs. the pelites, but this is due to extreme enrichment in two OD psammite samples, R350477 and R350480, which contain 1035 and 1978 ppm Zr, respectively. The Th, Zr and REE variability may indicate heavy mineral sorting, as minerals such as allanite, monazite, apatite and zircon may influ-

Fig. 4. Selected Harker-type variation diagrams for Willyama Supergroup metasedimentary rocks. PAAS: the post-Archean Australian shale (McLennan, 1989); BHD: Broken Hill sedimentary rocks; OD: Olary Domain sedimentary rocks. Calc-albitites are from the OD. (A) Al2 O3 –SiO2 graph showing the distribution of the sedimentary rocks, and the division between psammite and pelite. (B) K2 O/Na2 O ratio-SiO2 graph showing the effects of Na-metasomatism on the calc-albitite rocks and many of the Wiperaminga subgroup samples from the lower parts of the OD stratigraphy. (C and D) Th vs. SiO2 and Cr vs. SiO2 for psammitic and pelitic samples from the Willyama Supergroup. General trend is decreasing trace element content with increasing SiO2 , probably reflecting a quartz dilution effect in the sedimentary material.

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325

Table 2 Major and trace element analyses for Willyama Supergroup sedimentary samples Olary Domain Upper Strathearn Group, Mt. Howden Subgroup 98185020a (pelite) wt.% SiO2 TiO2 Al2 O3 Fe2 O3 MnO MgO CaO Na2 O K2 O P2 O5 LOI Total ppm Rb Sr Ba Th U Pb Zr Nb La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Yb Lu Y Cr Ni V Sc Cu Zn Ga (La/Yb)n b (Gd/Yb)n Eu/Eu*c La/Th Th/Sc

wt.% SiO2 TiO2 Al2 O3 Fe2 O3 MnO MgO CaO Na2 O K2 O P2 O5 LOI Total

98185018a (psammite)

98185019 (psammite)

Lower Strathearn Group, Saltbush Subgroup 98185016 (pelite)

98185017 (pelite)

98185021 (albitic pelite)

R632637a (psammite)

R632631a (pelite)

R632630a (pelite)

R350483a (pelite)

R350482 (pelite)

68.25 0.57 17.48 3.19 0.01 0.74 0.44 2.90 2.92 0.07 3.28

74.51 0.38 14.08 1.71 0.02 0.77 0.47 4.17 1.96 0.05 1.81

80.81 0.24 11.18 0.40 0.10 0.28 0.69 3.49 1.17 0.08 1.21

64.68 0.69 16.10 2.07 0.07 1.50 0.53 2.21 3.77 0.06 7.24

62.57 0.71 19.52 1.83 0.16 1.57 0.50 0.91 3.93 0.13 3.90

63.44 0.73 14.75 3.38 0.05 2.34 3.34 7.45 1.58 0.19 n.d.

80.40 0.29 11.30 0.70 0.21 0.21 1.49 3.84 0.66 0.03 n.d.

69.50 0.45 13.50 2.32 0.06 3.00 1.71 2.48 2.37 0.04 n.d.

64.70 0.65 20.20 5.57 0.02 0.88 0.34 0.88 2.93 0.07 n.d.

71.95 0.55 13.16 5.03 0.16 1.52 0.95 1.69 2.57 0.06 2.81

65.68 0.62 17.26 4.40 0.07 1.80 0.29 0.74 4.70 0.10 4.47

99.85

99.93

99.65

98.92

95.73

97.25

99.13

95.43

96.24

100.45

100.13

176 110 627 17.9 4.4 39.0 247 16.8 58.1 121.4 13.8 51.2 10.1 1.5 8.6 1.4 7.2 1.5 4.4 4.4 0.7 42.4 25 8 64 17 32 59 18.9 8.92 1.58 0.49 3.25 1.05

80 84 434 14.2 3.5 11.0 242 15.6 36.8 76.6 9.1 34.3 6.7 1.0 6.1 1.1 5.9 1.3 3.9 4.0 0.6 39.0 23 19 21 9 4 20 15.3 6.22 1.24 0.48 2.59 1.58

44 106 479 10.9 3.2 16.0 227 12.7 49.6 106.0 n.d. 46.6 9.1 1.5 8.3 1.4 7.3 1.5 4.5 4.5 0.7 48.1 5 2 7 6 5 33 8.9 7.45 1.49 0.53 4.55 1.82

201 163 1023 15.3 9.1 8.5 182 15.0 42.3 92.0 10.2 35.5 5.8 0.9 4.2 0.6 3.0 0.6 1.7 1.8 0.3 17.2 77 8 261 15 57 37 22.6 15.88 1.89 0.56 2.76 1.02

231 154 801 22.1 8.3 29.5 273 17.6 58.1 116.1 12.9 46.3 8.3 1.4 7.4 1.1 6.1 1.2 3.7 3.5 0.5 38.4 80 37 85 15 9 88 25.7 11.22 1.71 0.55 2.63 1.47

43 32 519 18.2 3.6 1.5 210 14.7 21.8 70.5 9.9 40.3 7.9 1.5 6.6 1.1 5.4 1.1 3.1 3.0 0.4 33.8 64 18 105 14 7 16 19.4 4.91 1.78 0.63 1.20 1.30

22 170 170 12.5 2.9 22.0 140

125 220 850 16.0 4.5 45.5 140

145 105 650 26.5 4.5 30.5 190

42.0 80.0 10.5 38.0 6.5 1.1 5.0 0.7 4.1 0.8 2.5 2.8 0.4 20.0 20 2 20 5 22 22 9.5 10.14 1.45 0.59 3.36 2.50

45.5 82.0 10.5 40.0 7.0 1.8 6.0 0.9 5.5 1.0 3.0 3.1 0.5 26.0 40 8 50 8 35 62 25.5 9.92 1.57 0.85 2.84 2.00

66.0 120.0 15.5 58.0 10.5 1.9 7.5 0.9 5.0 0.8 2.3 2.3 0.4 20.0 50 24 70 14 32 86 33.0 19.39 2.64 0.65 2.49 1.89

196 88 370 16.7 6.6 47.5 172 14.2 35.0 71.2 8.2 29.8 6.0 1.1 5.5 1.0 5.3 1.1 3.2 3.2 0.5 31.1 59 13 65 10 96 145 15.6 7.39 1.39 0.59 2.10 1.67

297 64 745 19.5 15.1 42.0 190 17.5 53.8 108.1 12.2 43.9 8.1 1.2 7.0 1.1 5.7 1.1 3.3 3.1 0.5 34.3 82 14 270 11 24 157 22.1 11.73 1.83 0.49 2.76 1.77

Saltbush Subgroup

Curnamona Group, Ethiudna Subgroup

R350481a (pelite)

R350486a (pelite)

R350485a (calc-silicate)

R350487 (pelite)

R350488 (psammite)

Curnamona Group, Wiperaminga Subgroup R350489 (pelite)

R350490 (pelite)

R350491a (albitic pelite)

R350493a (albitic pelite)

R350494a (psammite)

R350495a (pelite)

68.52 0.65 14.64 0.82 0.01 0.84 0.08 0.33 4.63 0.06 9.06

50.72 0.27 8.17 2.24 0.13 3.07 21.08 0.56 2.30 0.32 10.98

63.07 0.81 17.00 5.99 0.08 2.07 0.19 1.15 5.17 0.10 3.95

62.53 0.73 17.00 5.96 0.14 2.27 0.29 0.90 5.67 0.12 3.11

81.66 0.44 8.30 3.40 0.11 1.19 0.38 1.56 1.95 0.07 1.26

62.23 0.82 17.94 5.68 0.10 2.55 0.71 1.83 5.05 0.21 3.26

65.24 0.71 16.21 5.08 0.12 2.44 2.05 2.99 3.41 0.09 2.07

61.68 0.62 15.87 6.93 0.18 2.23 0.81 2.01 7.53 0.21 1.77

63.36 0.80 17.84 4.88 0.10 2.32 0.58 1.59 5.39 0.16 3.06

85.23 0.19 6.76 1.73 0.02 0.19 0.07 0.99 3.91 0.06 0.58

63.44 0.59 15.66 7.23 0.13 2.81 0.46 1.47 5.78 0.16 2.65

99.64

99.84

99.58

98.72

100.32

100.38

100.41

99.84

100.08

99.73

100.38

326

K. Barovich, M. Hand / Precambrian Research 166 (2008) 318–337

Table 2 (Continued )

ppm Rb Sr Ba Th U Pb Zr Nb La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Yb Lu Y Cr Ni V Sc Cu Zn Ga (La/Yb)n b (Gd/Yb)n Eu/Eu*c La/Th Th/Sc

Saltbush Subgroup

Curnamona Group, Ethiudna Subgroup

Curnamona Group, Wiperaminga Subgroup

R350481a (pelite)

R350485a (calc-silicate)

R350486a (pelite)

R350487 (pelite)

R350488 (psammite)

R350489 (pelite)

R350490 (pelite)

R350491a (albitic pelite)

R350493a (albitic pelite)

R350494a (psammite)

R350495a (pelite)

185 58 1242 18.4 8.6 99.5 186 16.1 31.2 54.9 6.0 21.2 3.8 0.6 3.2 0.5 3.0 0.7 2.1 2.3 0.3 22.0 62 0 155 12 15 28 21.3 9.17 1.13 0.53 1.70 1.53

90 242 3573 8.1 7.1 22.5 97 7.7 25.6 52.2 5.8 21.2 3.9 0.7 3.6 0.6 3.0 0.6 1.7 1.6 0.2 19.0 30 16 49 19 54 92 10.3 10.81 1.82 0.57 3.16 0.43

251 69 721 17.1 6.9 75.5 172 15.1 56.9 108.4 12.3 45.1 8.0 1.5 6.5 0.9 4.3 0.8 2.2 2.0 0.3 23.8 78 8 141 13 75 226 23.4 19.23 2.63 0.64 3.33 1.32

254 50 698 17.6 10.0 118.5 145 14.8 158.3 277.5 32.2 117.8 20.5 2.8 16.1 2.1 8.8 1.5 3.8 2.9 0.4 40.9 83 24 113 14 79 416 26.5 36.89 4.50 0.47 8.99 1.26

120 62 212 12.8 5.0 163.5 218 11.5 31.4 59.0 7.2 26.7 5.2 0.9 4.8 0.8 3.9 0.8 2.2 2.3 0.3 21.7 43 8 38 7 20 294 9.1 9.23 1.69 0.55 2.45 1.83

224 94 808 17.7 7.5 94.5 178 15.1 70.2 136.4 15.3 56.2 9.7 1.7 8.2 1.2 5.4 1.0 2.6 2.2 0.3 27.1 81 25 163 14 146 604 23.6 21.56 3.02 0.58 3.97 1.26

192 160 488 14.6 5.8 88.5 158 13.5 45.4 92.3 10.5 38.6 7.1 1.4 6.1 0.9 4.7 0.9 2.4 2.2 0.3 24.4 73 10 177 15 130 279 20.3 13.95 2.25 0.65 3.11 0.97

299 107 2991 18.0 4.8 15.0 165 13.6 62.3 124.3 14.1 51.3 9.6 1.7 8.6 1.3 6.2 1.1 3.1 2.7 0.4 31.7 71 28 85 13 14 56 20.3 15.59 2.58 0.57 3.46 1.38

230 105 1098 18.5 7.5 89.0 207 16.4 49.1 103.3 11.8 43.2 8.1 1.4 6.9 1.1 5.2 1.0 2.8 2.6 0.4 28.0 81 15 159 14 93 149 25.7 12.76 2.15 0.57 2.65 1.32

90 110 1546 7.2 1.9 18.0 355 2.9 17.8 38.9 4.0 14.6 2.9 0.6 2.6 0.4 2.1 0.4 1.3 1.2 0.2 12.9 17 0 16 2 14 8 5.1 10.02 1.76 0.67 2.47 3.60

322 40 1361 18.1 2.1 6.5 181 13.4 44.2 96.8 11.1 40.6 8.3 1.3 7.4 1.2 5.9 1.2 3.7 4.0 0.6 33.1 68 21 72 13 31 45 22.3 7.47 1.50 0.51 2.44 1.39

Wiperaminga Subgroup R350496 (pelite) wt.% SiO2 TiO2 Al2 O3 Fe2 O3 MnO MgO CaO Na2 O K2 O P2 O5 LOI Total ppm Rb Sr Ba Th U Pb Zr Nb La Ce Pr Nd Sm Eu Gd Tb

R350497 (pelite)

Broken Hill Domain Paragon Group

R350480a (albitic R350479 (albitic psammite) psammite)

R350478 (albitic psammite)

R350477a (albitic R350476a R350475 (albitite) (albitite) psammite)

62.72 0.74 17.40 6.58 0.07 1.64 0.50 1.76 6.11 0.17 2.46

69.86 0.64 16.40 2.45 0.02 0.25 0.41 7.67 1.28 0.16 0.99

88.67 0.71 4.58 2.85 n.d. 0.06 0.05 2.42 0.21 0.04 0.38

85.77 0.22 6.86 2.68 n.d. 0.06 0.10 3.09 0.46 0.04 0.64

84.91 0.28 7.80 2.09 n.d. 0.09 0.07 3.93 0.44 0.03 0.53

88.30 0.41 4.89 3.09 n.d. 0.06 0.06 2.61 0.24 0.04 0.33

100.15

100.13

99.97

99.92

100.17

100.03

40 81 212 20.7 7.9 16.5 208 11.1 66.3 142.6 15.8 56.6 9.6 1.6 7.4 1.0

5 16 22 34.8 5.7 13.0 1978 13.2 72.7 140.4 15.7 56.7 9.4 1.4 7.5 1.1

12 19 82 8.5 2.9 6.0 282 4.0 33.4 68.2 8.0 29.8 4.8 0.7 3.9 0.5

11 20 81 12.0 1.2 8.0 437 6.3 33.0 67.9 7.9 29.1 5.2 0.8 3.8 0.5

9 10 40 18.2 4.1 5.5 1035 15.5 23.7 48.7 5.7 21.2 4.4 0.8 5.7 1.1

262 50 1687 21.9 2.7 14.0 225 15.3 61.4 115.8 13.8 49.8 9.2 1.9 8.4 1.2

63.20 0.52 12.38 15.21 0.01 0.07 0.05 6.25 1.75 0.07 0.69

R350474a (albitite)

99185501a (pelite)

99185528Aa (psammite)

63.25 0.56 12.46 14.74 0.01 0.06 0.04 5.34 2.96 0.09 0.71

57.31 0.64 15.30 13.62 0.01 0.08 0.03 9.15 3.10 0.10 0.82

72.24 0.45 14.33 2.40 0.02 1.05 0.40 3.33 2.42 0.04 3.27

77.82 0.36 10.72 1.67 0.03 0.89 0.30 2.57 1.86 0.09 3.53

100.2

100.22

100.16

99.95

99.84

31 25 311 10.8 1.9 4.0 224 13.2 42.5 83.4 9.6 35.5 6.5 1.2 6.0 0.9

53 28 509 11.6 2.3 4.5 216 13.8 45.6 89.6 10.3 37.9 6.9 1.3 7.0 1.2

124 140 527 14.9 3.99 77 268 14.4 49.1 107.0

109 115 789 14.4 4.85 106 339 11.5 45.3 96.1

42.4 7.7 1.4 6.3 1.0

38.1 7.0 1.2 6.2 1.0

203 28 1338 19.7 5.2 6.0 198 10.9 40.6 79.2 9.1 34.0 6.7 1.2 7.4 1.3

K. Barovich, M. Hand / Precambrian Research 166 (2008) 318–337

327

Table 2 (Continued ) Wiperaminga Subgroup R350496 (pelite) Dy Ho Er Yb Lu Y Cr Ni V Sc Cu Zn Ga (La/Yb)n b (Gd/Yb)n Eu/Eu*c La/Th Th/Sc

5.7 1.1 3.0 2.5 0.4 32.4 77 24 90 12 85 26 25.4 16.60 2.72 0.66 2.80 1.83

Broken Hill Domain Paragon Group

R350497 (pelite)

R350480a (albitic R350479 (albitic psammite) psammite)

R350478 (albitic psammite)

R350477a (albitic R350476a R350475 (albitite) (albitite) psammite)

4.5 0.8 2.1 1.8 0.3 23.0 63 3 50 6 106 12 17.7 24.89 3.33 0.58 3.20 3.45

5.4 1.1 3.3 3.5 0.6 35.6 21 n.d. 33 3 94 12 6.2 14.04 1.74 0.51 2.09 11.60

2.4 0.4 1.3 1.1 0.2 13.7 18 n.d. 30 2 49 3 9.1 20.27 2.80 0.55 2.75 6.00

6.6 1.4 4.0 3.7 0.6 45.2 17 n.d. 31 4 10 n.d. 8.1 4.33 1.25 0.49 1.30 4.55

2.8 0.6 1.6 1.4 0.2 18.1 22 n.d. 51 1 15 n.d. 9.1 16.12 2.26 0.49 3.93 8.50

4.7 0.9 2.6 2.1 0.3 30.8 49 10 63 5 7 2 17.3 13.68 2.32 0.59 3.94 2.16

R350474a 99185501a (albitite) (pelite)

6.3 1.3 3.5 2.8 0.4 43.0 53 11 68 4 8 2 15.8 11.01 2.03 0.57 3.93 2.90

7.3 1.5 4.3 3.6 0.5 49.7 71 10 86 6 21 2 16.4 7.62 1.67 0.52 2.06 3.28

5.2 1.1 3.2 3.4 0.5 69.2 30 11 40 9 15 38 14.8 9.82 1.51 0.62 3.30 1.66

99185528Aa (psammite) 5.0 1.0 3.0 3.1 0.5 62.7 20 8 26 8 11 36 8.7 10.04 1.64 0.56 3.15 1.80

Sundown Group 99185504 (psammite) wt.% SiO2 TiO2 Al2 O3 Fe2 O3 MnO MgO CaO Na2 O K2 O P2 O5 LOI Total ppm Rb Sr Ba Th U Pb Zr Nb La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Yb Lu Y Cr Ni V Sc Cu Zn Ga (La/Yb)n b (Gd/Yb)n Eu/Eu*c La/Th Th/Sc

a

99185505 (pelite)

a

a

a

a

KB04-23 (pelite)

KB04-20 (psammite)

KB04-21 (pelite)

KB04-22 (pelite)

99185507 (pelite)

99185512 (psammite)

a

99185509 (calc-silicate)

99185506a (psammite)

99185510a (psammite)

78.92 0.39 11.28 0.71 0.02 0.53 0.14 1.00 4.73 0.07 n.d.

69.96 0.44 17.12 1.42 0.02 1.16 0.26 3.79 3.81 0.06 1.82

65.20 0.61 18.60 2.69 0.11 1.78 0.29 2.77 5.39 0.03 n.d.

80.40 0.28 10.10 2.17 0.03 0.47 0.17 1.66 2.27 0.02 n.d.

72.50 0.63 13.70 1.77 0.03 1.16 0.36 2.62 2.96 0.05 n.d.

68.30 0.50 14.70 5.38 0.08 1.99 0.14 1.61 4.01 0.07 n.d.

68.19 0.49 13.99 3.62 0.08 2.00 0.10 0.61 2.51 0.08 n.d.

75.75 0.39 12.12 0.49 0.07 1.26 2.92 1.80 1.45 0.06 n.d.

61.19 0.36 11.89 3.99 0.20 8.62 9.10 0.87 1.54 0.10 2.20

74.07 0.40 10.90 7.99 0.09 1.54 0.05 0.15 1.77 0.04 3.57

77.16 0.43 11.01 4.36 0.05 0.84 0.38 0.65 3.35 0.06 1.84

97.79

99.86

97.47

97.57

95.78

96.78

91.67

96.31

100.06

100.57

100.13

240 130 1150 20.0 4.3 38.0 250

74 88 1200 11.5 3.8 19.0 110

110 190 850 18.5 6.0 54.0 180

210 120 1250 17.0 5.5 33.0 170

64.0 120.0 15.0 58.0 10.0 1.9 7.5 0.9 4.6 0.8 2.4 2.8 0.5 20.0 40 4 40 8 17 45 41.5 15.45 2.17 0.67 3.20 2.50

24.5 44.0 7.0 26.5 4.7 1.1 3.8 0.6 3.2 0.5 1.7 1.8 0.3 14.0 30 4 n.d. 5 15 56 25.0 9.46 1.76 0.80 2.13 2.30

50.0 82.0 9.5 33.5 6.0 1.3 4.5 0.7 4.2 0.8 2.7 3.2 0.5 21.0 30 n.d. 60 8 29 21 27.0 10.56 1.14 0.74 2.70 2.31

41.0 72.0 9.5 37.0 6.5 1.5 5.0 0.7 3.7 0.6 2.1 2.4 0.4 17.0 40 12 50 8 41 115 33.5 11.54 1.69 0.80 2.41 2.13

122 66 655 15.6 5.17 318 218 14.0 51.5 109.0

100 135 421 12.9 4.79 84.6 145 12.3 131.0 254.0

93 153 1251 12 2.89 67.4 110 13.1 36.5 75.4

112 77 364 12.4 5.51 45.1 168 12.5 25.6 66.2

209 77 478 17.7 4.32 46.7 266 11.5 37.5 82.3

46.5 9.3 1.3 7.5 1.1 5.3 1.0 2.9 2.6 0.4 73.8 43 24 50 14 n.d. 436 18.6 13.18 2.30 0.46 3.30 1.11

106.0 17.3 1.2 12.8 1.7 7.0 1.1 2.5 2.0 0.3 61.5 38 11 57 9 n.d. 43 13.4 44.71 5.24 0.24 10.16 1.43

30.0 5.6 0.8 5.0 0.8 4.2 0.9 2.5 2.5 0.4 53.8 33 24 54 8 18 336 15.6 9.83 1.61 0.46 3.04 1.50

23.3 4.7 0.8 4.5 0.7 3.3 0.6 1.7 1.5 0.2 55.2 35 22 44 10 127 108 16.4 11.77 2.48 0.50 2.06 1.24

32.7 6.1 1.1 5.1 0.8 3.8 0.8 2.1 2.1 0.3 48.9 36 17 49 10 20 61 15.1 12.18 2.00 0.59 2.12 1.77

149 104 1139 11.4 4.6 24.5 173 11.1 27.0 57.9 24.9 4.6 0.8 4.2 0.7 3.5 0.7 2.1 2.1 0.3 23.5 11 8 40 10 17 139 11.8 8.69 1.62 0.56 2.37 1.14

155 76 902 19.5 4.2 6.5 320 21.2 63.7 136.6 56.6 9.9 1.4 8.7 1.4 7.5 1.6 4.9 5.2 0.8 52.1 15 9 23 12 6 22 26.4 8.28 1.36 0.46 3.27 1.63

328

K. Barovich, M. Hand / Precambrian Research 166 (2008) 318–337

Table 2 (Continued ) Broken Hill Group 99185511a (psammite) wt.% SiO2 TiO2 Al2 O3 Fe2 O3 MnO MgO CaO Na2 O K2 O P2 O5 LOI Total ppm Rb Sr Ba Th U Pb Zr Nb La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Yb Lu Y Cr Ni V Sc Cu Zn Ga (La/Yb)n b (Gd/Yb)n Eu/Eu*c La/Th Th/Sc

99185520 (psammite)

Thackaringa Group

99185521a (calc-silicate)

99185517a (psammite)

99185518 (calc-silicate)

99185519a (albitic psammite)

99185527a (pelite)

99185516a (albitic psammite)

99185525a (pelite)

85.53 0.29 7.96 1.56 0.03 0.38 0.24 0.31 1.88 0.04 1.72

78.80 0.37 9.34 4.15 0.11 0.80 1.91 1.62 1.01 0.06 2.11

54.69 0.31 9.77 4.67 1.03 0.21 17.40 0.02 0.01 0.08 1.88

82.58 0.31 7.39 3.68 0.14 0.85 1.79 0.89 0.92 0.04 1.54

61.78 0.27 7.89 2.34 0.81 5.39 14.16 0.19 0.46 0.30 n.d.

84.86 0.29 7.77 1.13 n.d 0.19 0.13 2.68 0.80 0.04 2.06

65.73 0.64 15.78 3.83 0.05 1.50 0.53 1.90 3.94 0.06 5.84

84.16 0.33 9.05 0.08 n.d 0.08 0.14 5.17 0.07 0.08 0.78

64.24 0.83 17.84 6.16 0.02 1.55 0.26 1.49 4.05 0.09 3.57

99.94

100.28

90.07

100.13

93.59

99.95

99.80

99.94

100.10

98 62 386 17.3 3.59 46.1 426 7.5 32.7 69.3

86 111 121 17.5 3.45 64.5 257 9.5 34.3 72.4

5 94 32 8.3 2.52 62.5 109 7.6 24.7 54.2

74 45 201 18.8 3.14 482 362 8.5 49.0 102.0

34 155 676 7 13.1 98.5 80 8.8 21.3 43.2

48 30 66 11.4 2.68 17.3 263 6.9 34.9 77.3

5 21 13 11.2 2.3 1.5 257 7.1 38.9 72.3

135 66 650 25.4 2.78 10.1 266 16.0 68.1 130.0

27.5 4.9 1.0 4.1 0.6 3.3 0.7 2.1 2.3 0.4 44.5 16 7 18 6 2 23 9.4 9.82 1.48 0.68 1.89 2.88

31.0 6.4 1.1 5.9 0.9 5.0 1.0 3.0 2.9 0.4 62.0 31 16 31 8 5 59 11.5 8.05 1.66 0.53 1.96 2.19

24.6 6.0 1.1 7.8 1.3 7.2 1.5 4.3 4.0 0.6 102.0 26 n.d. 82 9 29 44 33.3 4.19 1.59 0.48 2.98 0.92

37.7 6.1 1.0 5.5 1.0 5.3 1.1 3.2 3.0 0.4 65.7 21 11 32 6 6 87 9.1 11.04 1.50 0.53 2.61 3.13

17.2 3.0 0.6 2.8 0.4 2.0 0.4 1.1 1.1 0.2 12.1 26 15 60 5 18 410 11.1 13.08 2.06 0.63 3.04 1.40

30.6 5.9 1.1 4.8 0.6 2.8 0.5 1.4 1.3 0.2 39.1 20 8 13 3 33 5 6.2 18.14 2.98 0.64 3.06 3.80

246 103 1631 16.6 11.7 82.5 165 15.1 47.1 92.7 37.9 6.8 1.0 5.9 0.9 4.6 0.9 2.6 2.6 0.4 54.2 78 14 189 15 79 197 19.5 12.48 1.88 0.50 2.84 1.11

27.4 4.6 0.9 3.7 0.5 2.2 0.4 1.1 1.1 0.2 12.2 17 5 9 20 n.d. 4 10.2 23.90 2.73 0.67 3.47 0.56

53.7 9.3 1.4 7.6 1.1 5.0 0.9 2.3 1.8 0.3 58.1 84 40 99 19 17 20 25.3 25.01 3.35 0.51 2.68 1.34

Data for samples beginning with ‘9’ are from the Geoscience Australia OZCHEM database (Budd et al., 2002). a Sample used for Sm–Nd analysis. b N: normalized, using chrondrite values of Taylor and McLennan (1985). c Eu/Eu* = EuN/(SmN × GdN)0.5 .

ence Th, Zr and rare earth element contents (e.g., McLennan, 1989; McLennan et al., 1993; Bea, 1996). This possibility will be addressed later by careful examination of compositional variation trends of trace elements that may be influenced by the accumulation of zircon, apatite or monazite, including Th, Zr and the REE. Th and Sc are relatively incompatible and compatible elements, respectively, during igneous processes. Because they are relatively insoluble during sedimentary processes, together they can provide a sensitive indicator of provenance (McLennan et al., 1989; McLennan et al., 1993; Cullers and Podkovyrov, 2002). Th/Sc ratios in fine-grained Post-Archaean sedimentary rocks are generally around 1.0 (Taylor and McLennan, 1985). The Zr/Sc vs. Th/Sc ratio plot for Willyama Supergroup pelites and psammites is shown in Fig. 5. Provenance differences would result in scatter about the

compositional variation line, which was defined from active margin samples, interpreted to be little affected by sedimentary sorting and recycling (McLennan et al., 1993). Zircon concentration through sediment recycling and sorting would result in Zr enrichment relative to Th. The pelites and most of the psammite samples exhibit the relationship expected for provenance-dependent trace element variation. For a few psammitic samples, spectacular enrichments are seen in Zr/Sc ratios, especially in the lower OD quartzofeldspathic sequences of the Curnamona Group. This relationship, and its implications for REE enrichment, will be discussed later. The REE are generally insoluble, so the REE abundance patterns of fine-grained sediments should reflect the pattern of their source(s) (e.g., Nesbitt, 1979; McLennan et al., 1993; Cullers and Podkovyrov, 2000). The presence of more quartz in the sediment

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Fig. 5. Th/Sc vs. Zr/Sc (McLennan et al., 1993) for Willyama Supergroup metasedimentary rocks. PAAS: the post-Archean Australian shale (McLennan, 1989). The compositional variation line was defined by McLennan et al. (1993) for active margin turbidites, interpreted to be least affected by sorting and recycling. The zircon concentration line was defined by trailing edge turbidites, more likely to be enriched in heavy minerals such as zircon. Some Willyama Supergroup psammites show enrichments in zircon, possibly from sedimentary sorting.

should serve only as a dilution effect on the REE abundances, although the presence of heavy minerals in coarser sands may cause variations in the REE pattern itself (McLennan et al., 1993; Cullers and Podkovyrov, 2002). Some notable differences between REE abundances and patterns between these sequences are noted in the results below. Despite the variations in total REE abundances, the overall REE patterns for pelites and psammites are broadly similar to each other (Fig. 6), particularly for the upper part of the sequence (Fig. 6A). Exceptions to these patterns are samples R350487 and 99185512, from the lower Ethiudna Group and the upper

Fig. 7. PAAS-normalised REE plots of Willyama Supergroup metasedimentary rocks. The heavy dashed line represents average Paleoproterozoic Gawler Craton finegrained sedimentary rocks (Schwarz et al., 2002). The heavy solid line represents average Paleoproterozoic Mt Isa Western Succession fine-grained sedimentary rocks (Eriksson et al., 1992). (A) Upper sequence and (B) lower sequence.

Cartwrights Creek Metasediment, which are unusually enriched in REE. When normalized to PAAS (Fig. 7) the overall REE enrichment in the Willyama Supergroup sedimentary rocks is evident. Pelites from the lower sequences have an average (La/Yb)n ratio of 17.2, nearly twice the PAAS value of 9.2. The average (La/Yb)n ratio of 11.6 from pelites in the upper part of the sequence is much closer to the PAAS value (9.2). For all pelites, the LREE are significantly more fractionated than the HREE. Pelites from the lower and upper sequences have average (La/Sm)n ratios of 14.46 and 15.71, respectively, about 3.5× the PAAS value of 4.27. Average (Gd/Yb)n ratios are 1.99 and 1.78, just less than 1.5× the PAAS value of 1.36. Lower sequence pelite Eu anomalies are generally greater than for upper pelites (average 0.55 vs. 0.62). 4.2. Sm–Nd isotope data

Fig. 6. Chondrite-normalised REE plots of Willyama Supergroup metasedimentary rocks. In general, the psammites have slightly lower total REE abundances, but pelite and psammite patterns are similar in shape. (A) Upper sequence and (B) lower sequence. Normalising values after Taylor and McLennan (1985).

Sm–Nd isotope results for 35 of the Willyama Supergroup samples are presented in Table 3 and Fig. 8. Initial εNd values are calculated at the estimated stratigraphic age (Page et al., 2005a,b). The 147 Sm/144 Nd ratios range between 0.0966 and 0.1236, except for one calc-silicate sample, 98185521, at 0.1472. This range is typical for fine-grained sedimentary rocks. εNd (0) values range between −26.1 and −14.8. Excluding two albitites from the lowest part of the OD Wiperaminga Subgroup, initial εNd values for the lower parts of the stratigraphy vary only from −6.8 to −4.0. Samples R350476 and R350474 have slightly less negative initial values of −2.8 and −3.4, respectively. The upper OD Mt Howden Subgroup and the BHD Paragon Group have initial εNd values ranging from −3.2 to 0.0. The more negative values occur only in the lowermost part of the Cartwrights Creek

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Table 3 Sm–Nd isotope data for selected Willyama Supergroup samples 2S.E.

εNd(0) c

εNd(T)

0.511673 0.511765 0.511663 0.511605 0.511665

7 7 9 9 9

−18.8 −17.0 −19.0 −20.1 −19.0

−1.4 −0.5 0.0 −1.8 −0.7

0.1193 0.1083 0.1057 0.1094

0.511523 0.511428 0.511422 0.511449

9 8 9 11

−21.8 −23.6 −23.7 −23.2

−4.9 −4.4 −3.9 −4.2

48.9 47.1 55.4 51.0 12.5 37.9 56.7 21.5 39.2 34.0

0.1050 0.1046 0.1105 0.1097 0.1217 0.1236 0.0992 0.1294 0.1095 0.1209

0.511349 0.511354 0.511398 0.511401 0.511452 0.511550 0.511335 0.511668 0.511523 0.511617

9 11 7 10 8 9 11 9 9 9

−25.1 −25.1 −24.2 −24.1 −23.1 −21.2 −25.4 −18.9 −21.8 −19.9

−5.2 −5.0 −5.4 −5.2 −6.8 −5.3 −4.2 −4.3 −2.8 −3.4

10.0 6.8 10.4 9.6 4.5 5.8 6.4 5.9

56.4 37.8 57.7 54.2 24.7 32.1 33.9 32.1

0.1074 0.1095 0.1092 0.1074 0.1108 0.1099 0.1145 0.1109

0.511664 0.511617 0.511681 0.511648 0.511642 0.511579 0.511589 0.511549

7 9 8 8 8 12 8 9

−19.0 −19.9 −18.7 −19.3 −19.4 −20.7 −20.5 −21.3

−0.2 −0.9 −0.3 −0.6 −1.4 −2.4 −3.2 −2.3

1670 1670 1670 1670

4.9 6.3 4.9 6.0

23.7 33.8 27.5 24.7

0.1245 0.1121 0.1077 0.1472

0.511562 0.511462 0.511410 0.511880

10 11 9 8

−21.0 −22.9 −24.0 −14.8

−5.9 −5.2 −5.3 −4.0

Broken Hill Group 99185517 99185519 99185527

1690 1690 1690

5.9 6.0 6.8

37.2 31.4 38.0

0.0966 0.1148 0.1087

0.511322 0.511433 0.511342

8 9 8

−25.7 −23.5 −25.3

−4.1 −5.7 −6.1

Thackaringa Group 99185516 99185525

1710 1710

5.0 9.8

29.5 56.6

0.1018 0.1045

0.511323 0.511300

8 9

−25.6 −26.1

−5.0 −6.1

Sm (ppm)

Nd (ppm)

147

Olary Domain Upper Strathearn Group 5020 1650 5018 1650 R632637 1650 R632631 1650 R632630 1650

7.8 6.7 6.4 7.0 10.2

40.3 34.2 36.3 38.6 56.2

0.1166 0.1179 0.1064 0.1095 0.1095

Lower Strathearn Group R350483 1690 R350492 1690 R350481 1690 R350485 1690

6.0 18.1 3.6 4.1

30.5 100.9 20.9 22.4

Curnamona Group R350486 R350486 R350491 R350493 R350494 R350495 R350480 R350477 R350476 R350474

1710 1710 1710 1710 1710 1710 1710 1710 1710 1710

8.5 8.1 10.1 9.3 2.5 7.7 9.3 4.6 7.1 6.8

Broken Hill Domain Paragon Group 99185501 99185528A 99185505 KB04-23 KB04-20 KB04-21 Kb04-22 99185509

1640 1655 1655 1655 1655 1655 1655 1655

Sundown Group 99185506 99185510 99185511 99185521

Sample

Strat age (Ma)a

Sm/144 Nd

143

Nd/144 Ndb

Samples in italics are duplicate runs from separate dissolutions. a Stratigraphic ages from Page et al. (2005a,b). b 143 Nd/144 Nd ratios are normalised to 146 Nd/144 Nd = 0.7219. c Present day 143 Nd/144 Nd(CHUR) = 0.512638 and 147 Sm/144 Nd(CHUR) = 0.1967 (DePaolo and Wasserburg, 1976).

Formation of the Paragon Group. Initial εNd values for the Willyama sequences which contain the ca. 1650 Ma detrital zircon population (Page et al., 2005a,b) range from −1.8 to 0. The Nd isotope signature of the Cartwrights Creek Formation is transitional between the signatures of the lower stratigraphy, rocks which are constrained to a maximum depositional age of about 1670 Ma, and the uppermost units, which contain ca. 1650 Ma detrital zircons.

5. Discussion 5.1. Geochemical patterns: Sediment recycling vs. source character The large trace element and REE variations, anomalously high Zr/Sc ratios and unusual enriched REE abundance patterns in some

Willyama Supergroup sedimentary rocks require that we examine the possibility that the trace element signature of the sediments does not properly reflect their source region, but instead is a function of heavy mineral sorting. This issue is particularly important in terms of using the Sm–Nd isotope data to constrain average provenance ages of basin fill, as heavy mineral concentrations may erratically affect REE patterns, and therefore Sm/Nd ratios. Common heavy minerals that must be considered are zircon and monazite, due to their high REE distribution coefficients. The Zr/Sc vs. Th/Sc plot (Fig. 5) suggests that zircon concentration in the lower Willyama psammitic sediment material through mineral sorting or sediment recycling is likely. The broad Th/Sc and Zr/Sc correlation for the pelites and most of the psammitic samples is consistent with effects due to compositional variation in the source. The pelites and most of the psammites are clustered, suggesting a relatively uniform and highly felsic source. Five psammitic

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Fig. 8. εNd vs. time for Willyama Supergroup metasedimentary rocks and possible surrounding regions. Willyama rocks are plotted at estimated time of deposition. Open squares are samples from the lower sequences of the Willyama Supergroup, filled triangles are from the upper sequences (upper Strathearn Group and Paragon Group, Table 3). Data for Gawler Craton, Arunta block, northern Australia-Mt Isa and Cochise block SW USA from: Schaefer (1998, unpublished thesis), Stewart and Foden (2001), Schwarz et al. (2002), Swain et al. (2005), Zhao and Bennett (1995) and Eisele and Isachsen (2001). Depleted mantle curve is from DePaolo (1981).

samples from the OD quartzofeldspathic Wiperaminga Subgroup, two from the BHD correlative Thackaringa Group, and one sample from the BHD Sundown Group show an increase in Zr/Sc ratios, consistent with concentration of zircon. All these samples contain over 83% SiO2 . Sedimentary recycling, especially in the lower parts of exposed sequence, is not unlikely, given that sedimentological models for the Willyama basin suggest shallow water or non-marine sedimentation in an intracratonic setting (Stevens et al., 1983; Willis et al., 1983; Conor and Page, 2003). A relatively larger degree of recycling is generally expected in cratonic settings (Crichton and Condie, 1993; Ugidos et al., 1997). A simple analysis of the effect of zircon accumulation on the REE abundances can be demonstrated by calculating the amount of Yb and Sm taken up by zircon in the samples, assuming that all the whole-rock Zr in the sample is in zircon. Zircon has particularly high distribution coefficients for the HREE (e.g., Rollinson, 1993). Yb abundances in zircon in the literature vary widely, and we do not have zircon REE abundances for the different detrital populations present in the samples. From a survey of REE composition of zircons, (e.g., Bea, 1996; Schaltegger et al., 1999; Hoskin et al., 2000; Hermann and Rubatto, 2003; Belousova et al., 2006) we assume a relatively high average zircon Yb concentration of 400 ppm, and Sm of 5 ppm. A plot of the whole-rock Yb ppm for all samples vs. the % Yb in zircon (Fig. 9A) shows for the pelites and most of the psammites, even if all the Zr in the whole rock is in zircon, < 11% of the Yb is contained in that mineral. Only in the eight psammitic samples discussed above with anomalously high Zr/Sc ratios is the potential zircon control of HREE abundances significant. Because zircon has a significantly greater affinity for the HREE over the LREE, in the case of Sm, the effective contribution of Sm from the zircon in the samples to the whole-rock Sm budget is less than 0.5% in all cases, even the psammites (Fig. 9B). This is less than the uncertainties on the isotope dilution measurement of whole-rock Sm concentrations (±0.5%). Even if zircon is a significant control on the HREE budget in some of the psammites, the whole-rock Sm concentrations and hence Sm/Nd ratios and isotope values are unaffected. The assumption that all the Zr is concentrated in zircon, and thus affecting HREE abundances in the whole-rock psammites, can also be tested by examining the relationship between Zr and the HREE relationship in the Willyama Supergroup sedimentary rocks. If zircon is controlling the HREE abundance, we should see a corre-

Fig. 9. (A) Plot of %Yb contained in zircon vs. whole-rock concentration of Yb in ppm in Willyama Supergroup metasedimentary rocks. Yb in zircon calculated assuming 400 ppm Yb in zircon, and all Zr in rock is in zircon. (B) Plot of %Sm in zircon vs. concentration of Sm in ppm in Willyama Supergroup metasedimentary rocks. Sm in zircon calculated assuming 5 ppm Sm in zircon, and all Zr in the rock is in zircon.

lation between Zr and the (La/Yb)n ratio. Instead, there is significant scatter (Fig. 10A). In the case of the LREE enrichments in sedimentary samples, monazite has been suggested as a controlling detrital phase, as monazite is strongly enriched in the LREE (e.g., Gromet and Silver, 1983). There is no correlation between Th and the (La/Yb)n ratio. Additionally, relationships between Al2 O3 and Zr, Th, HREE and (Gd/Yb)n all show moderate positive correlations for all pelite and psammite samples, suggesting that the trace element abundances are in part controlled by original clay minerals in the samples. 5.2. Composition of source areas and provenance implications Detrital zircon patterns from all Willyama sedimentary rocks include a wide and recurring range of Paleoproterozoic to Archean ages at ca. 1780 Ma, 1820 Ma, 1850–1870 Ma, 2000–2300 Ma and 2400–2700 Ma (Page et al., 2005b). The combined detrital zircon data for ten samples from the lower sequences in the BHD are represented in the cumulative age histogram plot constructed from their work (Fig. 11). As Page et al. (2005b) point out, this prevalence of common ages throughout the lower Willyama sequence supports sedimentological models that suggest widespread averaging and sediment recycling of a stable cratonic source area. The detrital zircon patterns, with dominant peaks around 1770–1790 Ma and 1850–1870 Ma, allow the following Australian intracratonic sources; Late Archean to Paleoproterozoic Gawler Craton to the west, and the north Australian craton to the north, including the granite-dominated Paleoproterozoic Arunta region and the Late Archean to Paleoproterozoic rocks of the Tanami, Mt Isa and McArthur region. The influx of ca. 1640–1670 Ma detrital zircons that appears in the uppermost Willyama sequences (Page et al.,

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Fig. 11. Histogram showing U–Pb detrital zircon ages for Willyama Supergroup sedimentary rocks from the Curnamona Province. Data is from Page et al. (2005b).

Fig. 10. Geochemical provenance indicators (Zr, Eu/Eu* and Th/Sc) vs. (La/Yb)n ratio for Willyama Supergroup metasedimentary rocks. Arunta granite data from Zhao and McCulloch (1995). Gawler Craton Paleoproterozoic sedimentary data from Schwarz et al. (2002). Mt Isa Paleoproterozoic Western Succession sedimentary data from Eriksson et al. (1992). (A) Zr (ppm) vs. (La/Yb)n for Willyama Supergroup psammites and pelites. Lack of correlation illustrates zircon is probably not controlling the REE pattern of the rocks. (B) Eu/Eu* vs. (La/Yb)n for Willyama Supergroup pelites, distinguished between lower and upper sequence. Note that the upper sequences have significantly lower (La/Yb)n ratios, and higher Eu/Eu* ratios, suggesting a crustal source less mature than the recycled upper crustal sources suggested for the lower part of the sequence, with higher (La/Yb)n ratios and larger Eu/Eu* anomalies. (C) Th/Sc vs. (La/Yb)n ratios for the Willyama pelites (average) and average Paleoproterozoic sedimentary units in the Gawler Craton and Mt Isa Western Succession.

2005b) (Fig. 11), combined with the change in geochemical and Nd isotopic characteristics of those sediments, requires a new source component of an age and Nd isotopic character that is not known in these Australian intracratonic terrains. Possible sources will be discussed later. LREE-enriched patterns relative to PAAS (Figs. 7B and 10B) suggest a highly intracrustally recycled enriched upper crustal source region for the lower part of the Willyama Supergroup, up through the OD Saltbush Subgroup and the BHD Sundown Group. The presence of large Eu anomalies in the lower sequence pelites (Fig. 10B) indicates the source region was strongly affected by intracrustal melting processes. While there are large variations in absolute trace element abundances between psammites and pelites that we have

attributed in part to heavy mineral sorting and quartz dilution, the overall geochemical homogeneity of the pelites (e.g., REE patterns) supports an extensive stable source region, albeit one with unusually high Th, Zr, Y and REE abundances. The relatively restricted range of initial εNd values and the evolved Nd isotope signature for the lower Willyama Supergroup samples requires an evolved homogeneous source. Fig. 8 illustrates possible source areas based on the Nd isotope composition of currently exposed rocks. εNd values for the Willyama sedimentary rocks are shown at their stratigraphic age. The Gawler Craton, central Australian Arunta Block and northern Australian crustal evolution trend is shown. The voluminous ca. 1850 Ma Donington granitoid suite on the Gawler Craton was available for erosion and supply of 1850 Ma detrital zircons into the Willyama basin, as evidenced by the fact that there are abundant ca. 1850 Ma detrital zircons in only slightly older to coeval metasedimentary rocks on the eastern Gawler Craton (Jagodzinski, 2005). However, the evolution of the Gawler Craton upper crust, with voluminous Late Archean basement, is isotopically too evolved to have solely provided the Willyama sediment signature (Fig. 8). Additionally, the Gawler upper crust available for erosion at that time was not highly enriched in incompatible and trace elements as required by the Willyama sediment geochemistry. This point is illustrated by examining Paleoproterozoic sediments deposited directly on the Gawler Craton (Schwarz et al., 2002). These units do not show marked enrichments, and their REE patterns, abundances and (La/Yb)n ratios (PIRSA database) are similar to PAAS (Figs. 7 and 10A–C). In contrast, the lower Willyama pelites, and to a lesser extent, the upper Willyama pelites, exhibit significant (La/Yb)n enrichments. Also, there is no known extensive 1770–1790 Ma zircon source in the Gawler Craton to explain the dominant peak in the Willyama Supergroup sediments, a point that is also supported by the paucity of this age range in Mesoproterozoic metasedimentary rocks along the eastern Gawler Craton (Jagodzinski, 2005). Northern Australian sources of 1770–1780 Ma in the central Mt Isa Inlier (e.g., the Argylla Volcanics and Bottletree Formation), and ca. 1850–1880 Ma (Barramundi suites) (Page et al., 2000b) are present in that region. Thus, the abundant detrital ages of 1770–1780 Ma in the WIllyama sediments may have a source in northern Australia. However, although data are sparse, the Nd isotope evolution of the northern Australian craton (Fig. 8; McDonald et al., 1997; Page and Sun, 1998) is still somewhat too evolved to explain the isotope signature of the Willyama sediments. The geochemical composition of proximal Paleoproterozoic sedimentary rocks in the upper Calvert superbasin of the Mt Isa Western Suc-

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cession (Eriksson et al., 1992), which are coeval with the Willyama sedimentary package can be compared (Figs. 7 and 10A–C). The Mt Isa sedimentary rocks have detrital zircon patterns similar to that of the lower Willyama sequences (Page et al., 2000b; Southgate et al., 2000; Jackson et al., 2005), but are more like PAAS in their trace element and rare earth abundances. Eriksson et al. (1992) have demonstrated that a mixture of Archean and Paleoproterozoic rocks from the proximal northern Australian craton are the likely sources for the Mt. Isa rocks. Thus, while the age of felsic igneous sources in northern Australia are more suitable for the detrital ages of the Willyama sediments, again, the upper crustal geochemical and Nd isotope compositions are not appropriate. Based on our geochemical and Nd isotope data, we favor a dominant central Australian Arunta source (Fig. 1) for the lower Willyama sedimentary rocks. This suggestion has been made previously based on the enriched geochemical nature of the Willyama sediments and the detrital zircon populations (Slack and Stevens, 1994; Page et al., 2005b). Granites are a dominant part of the Arunta Inlier, with magmatic populations from ca. 1880 to 1600 Ma (Black and Shaw, 1992; Zhao and Bennett, 1995; Collins and Williams, 1995). Zhao and McCulloch (1995) identified three granite suites from 1880 to 1710 Ma along the southern Arunta margin, a calc-alkaline suite, a high heat-producing suite and a main group. The latter two are highly enriched in incompatible elements, including Zr, Th and the rare earths. This source region has the extreme Th/Sc ratios (Arunta granite average = 6) and steep highly fractionated LREE enriched patterns (Zhao and McCulloch, 1995) required to explain the enrichments in the Willyama sedimentary rocks (Fig. 10A–C). The ca. 1880–1710 magmatic rocks are isotopically evolved, with average initial εNd values from −5 to 0, reflecting substantial intracrustal melting and reprocessing. The Arunta Block upper crustal evolution yields an appropriate Nd isotope signature for the lower Willyama sediments (Fig. 8). The preponderance of a 1770–1790 Ma detrital zircon peak in all Willyama units (Page et al., 2005a,b) is well matched by the abundance of granites of that age in the Arunta. Suggestions of a continental back-arc basin setting for the Willyama basin (Giles et al., 2002, 2004) are not supported by the geochemistry and Nd isotope signature. The Nd isotopic and geochemical signature of the ca. 1680 Ma basaltic magmatism in the Curnamona Province is interpreted to suggest that the Broken Hill area was in an axial position in the forming rift (Raveggi et al., 2006; Rutherford et al., 2006, 2007). There is no evidence of two sources for the sediments in the lower part of the sequence, as would be expected in a back-arc tectonic setting, i.e., a volcanic arc sediment source and a cratonic source, and further they show none of the geochemical characteristics expected from the input of arc material (e.g., Condie et al., 1992; McLennan et al., 1993), such as variable Eu anomalies and geochemical immaturity. The variations in the trace and rare earth element data and Nd isotope character between the lower and upper parts of the Willyama sequence highlight a substantial change in source region for the upper part. Page et al. (2005b) demonstrated a previously unseen ca. 1657–1642 Ma felsic source for the OD Mt. Howden Subgroup and the BHD Paragon Group through the appearance of detrital zircon populations of that age range in tuffaceous sediments. This is a relatively rare magmatic age in Proterozoic Australia. There are a few tuffs of this age from the upper parts of the northern Australian Mt Isa and McArthur sedimentary sequences (Page et al., 2000b), but magmatic rock terrains of this age within Proterozoic eastern Australia are rare. Close et al. (2004) have identified a 1690–1660 Ma magmatic suite in the Warumpi Province, along the southern edge of the Arunta Block. However, the Nd isotopic signature of this province is still too evolved to explain the Nd isotope values of around zero for the upper Willyama sequence.

333

The presence of ca. 1650 Ma zircons and the lack of increase of mafic elemental indicators such as Cr or Sc in the upper sequence suggests the new source is felsic crust, but less differentiated than the dominant Arunta source we have suggested for the lower part of the sequence. The REE element patterns for pelites from the upper part of the sequence are still slightly enriched relative to PAAS (Fig. 7A), but relatively flat. The Arunta Block enriched upper granite-dominated crustal source region contributing to the lower part of the sequence must still be a sediment source contributor. But in comparing REE patterns of pelites throughout the Willyama Supergroup, for which we can be confident that other trace element abundances reflect their source region, there is a substantial decrease in average (La/Yb)n ratios to values much more like PAAS, from lower to upper pelites. Geochemical evidence of the entry of a new source region for the upper Willyama sediments is well illustrated in a comparison of Eu anomalies and REE ratios (Fig. 10B). The patterns for each group of pelites are only broadly correlated, and could not be interpreted to represent mixing trends. There is however a clear difference in the two groups, with the lower pelites trending toward a highly LREE-enriched Eu-depleted granitic source such as the Arunta Block, and the upper pelites trending toward a more typical less differentiated upper crustal granitic source. Initial εNd values calculated from the Sm–Nd isotope data require a significant input from a juvenile felsic source for the OD Mt. Howden Subgroup and the BHD Paragon Group (Table 3, Fig. 8). This abrupt change in isotope character across the basin is at the same point as the first appearance of ca. 1650 Ma detrital zircons in the sediments (Page et al., 2005a,b). Positioned at the easternmost edge of the late ArcheanMesoproterozoic Australian craton (Fig. 1), the Curnamona Province should contain geologic elements that can be identified in the continent that was attached to Australia in pre-Rodinia times. The provenance of the lower Willyama basin sedimentary rocks as determined from the geochemical and Nd isotope data in our work is explained by derivation from Australian intracratonic Late Archean to Paleoproterozoic upper crustal recycled sources, albeit particularly enriched in incompatible and rare earth elements compared to other Paleoproterozoic sedimentary sequences in the world. The significantly less evolved Nd isotope signature of the uppermost Willyama sedimentary rocks, and the lesser geochemical enrichments, combined with ca. 1650 Ma detrital zircon input (Page et al., 2005a) require a source not easily matched within Proterozoic Australia. This ca. 1650 Ma zircon age is also identified in tuffaceous layers of the northern Australian Late Paleoproterozoic Isa-McArthur region (Fig. 1). Various workers have reconstructed a pre-Rodinian plate configuration involving Australia and another continent, including Laurentia (Moores, 1991; Burrett and Berry, 2000; Karlstrom et al., 2001), Siberia (Sears and Price, 2000) and South China (Li et al., 1995). Proposing a unique match between any of the continental fits suggested in these models is beyond the constraints provided by geochemical and isotopic provenance data. However, the Nd isotope character of the uppermost Willyama sedimentary rocks and its ca. 1650 Ma detrital volcaniclastic zircons (Page et al., 2005a) allows us to examine the Laurentia connection in some detail, as geochronological and Nd isotope data exist for source rocks along the western Laurentian margin. Alternative sources for the upper Willyama sediments in these pre-Rodinian reconstruction models are South China and Siberia, but U–Pb geochronology and Nd isotope data for pre-Mesoproterozoic rocks along postulated proximal margins are sparse. The broad occurrence of ca. 1670–1590 Ma tuffaceous layers in Paleoproterozoic sequences throughout the Mount Isa-McArthur region of northern Australia has led previous workers to suggest

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that a distal magmatic arc must have been peripheral to these basins throughout their depositional history (Scott et al., 2000). Major bends and inflections in the Australian Proterozoic apparent polar wander path between 1730–1575 Ma have also been linked to the evolution of these basins in response to either local or farfield tectonism (Idnurm, 2000; Southgate et al., 2000). The abrupt change in the geochemical and Nd isotope character of the upper Willyama basin sequences to less enriched and more juvenile sediment material requires clastic input from a previously undetected source not known in intracratonic Australia. A previous study of the geochemistry of shales from the western Fold Belt of the Mount Isa region did not detect any substantive changes in provenance stratigraphically (Eriksson et al., 1992), as we have highlighted for the upper Willyama basin fill. Limited Sm–Nd whole rock isotope data from the Western Fold Belt Isan sequence do not show any change in provenance at around 1650 Ma depositional times, and initial εNd values are restricted and evolved, ranging between −7 and −3 (Barovich et al., 2005). Intriguingly, two initial εNd values for the Solders Cap Group of the Mount Isan Eastern Fold Belt, coeval with the Mt Isa Group and the uppermost sequences of the Willyama Supergroup, are +0.8 (Page and Sun, 1998). These data were anomalous with respect to the dataset for older potential source rocks, and were dismissed as due to alteration effects. It may be that the Eastern Fold Belt of the Mount Isa Inlier also received detritus from a more juvenile source at around 1650 Ma. Along the truncated western margin of Precambrian North America, the Mazatzal crustal province contains crystallization events around 1650 Ma (Karlstrom and Bowring, 1988; Wooden and Miller, 1990; Eisele and Isachsen, 2001). Within the southeastern most exposures of the Mazatzal province (the Cochise Block, Eisele and Isachsen, 2001), felsic volcanic activity and accompanying shallow water sedimentation is documented between 1674 and 1630 Ma (Erickson and Bowring, 1990; Eisele and Isachsen, 2001). Volcanics and contemporaneous sedimentary rocks have initial εNd values from 2.9 to 3.8, representing juvenile crust (Eisele and Isachsen, 2001). It is feasible that in an AUSWUS or AUSMEX-style reconstruction, the felsic volcanic activity within this province could have supplied air fall tuff material in the northern Australian basins and contributed clastic detritus to the upper Willyama sequences that resulted in the elevation of the initial εNd values. A geochronological and whole-rock Sm–Nd isotope study 300 km west of the Cochise Block and closer to the Laurentia truncated margin delineates a region of significantly foliated and deformed metasedimentary units with restricted provenance (1655–1650 Ma) and felsic orthogneisses with emplacement ages from ca. 1650 to 1640 Ma (Nourse et al., 2005). Initial εNd values of magmatic rocks are depleted, ranging from +2 to +4. Interestingly these workers have also documented a cryptic zircon recrystallization event from ca. 1600 to 1575 Ma. This is an uncommon age in terms of Laurentian tectonics and metamorphism, but has been recently identified in other Paleoproterozoic terranes of the southwest United States and attributed to a previously unrecognized contractional event in southwestern Laurentia (Nyman et al., 1994; Duebendorfer et al., 2006). Nourse et al. (2005) and Duebendorfer et al. (2006) have pointed out important geochronologic overlaps with eastern Australian zircon ages, including felsic tuffs within the Isa superbasin between ca. 1690 and 1585 Ma (Page et al., 2000b), a ca. 1590–1570 Ma metamorphic event in the Isan region (Page and Sun, 1998; Page et al., 2000b), the ca. 1600 Ma Chewings orogeny in the southern Arunta Inlier (Collins and Shaw, 1995) and the ca. 1590 Ma Olarian orogeny in the Curnamona Province (Page et al., 2000a). Due to ice cover, little U–Pb geochronologic or Nd isotope data are available from East Antarctica to assist in constraints for the fit of East Antarctica-Australia to western Laurentia. Goodge et al.

(2004) suggest that abundant 1400 Ma detrital zircons in the Neoproterozoic Beardmore and Cambrian lower Byrd Group units in the Transantarctic Mountains, combined with evidence of that sediment transport from East Antarctica are evidence for extension of the Proterozoic ca. 1.4 Ga southwestern Laurentian anorogenic province into the East Antarctic shield, currently hidden by the ice cover. This conclusion highlights a SWEAT reconstruction placing East Antarctica next to southwestern Laurentia rather than the AUSWUS or AUSMEX reconstruction favored by the present study. However, the constraints provided by ca. 1.4 Ga zircon detritus suggest Laurentia and East Antarctica-Australia were proximal, and argue against a Siberia– or South China–southwestern Laurentia connection, at least around 1.4 Ga. Sears and Price (1978, 2000, 2003) and Sears et al. (2004) propose a pre-Rodinian plate reconstruction that places the northeastern margin of the Siberian craton along southwestern Laurentia, with northern Australia connected to the southern margin of the Siberian craton. Such a configuration juxtaposes the Laurentian Mesoproterozoic Belt-Purcell basin against the Siberian Udzha basin of the northern Siberian craton. ca. 1510–1610 Ma zircons dominate the detrital pattern in the lower Belt basin sedimentary units (Ross and Villenueve, 2003). Such ages are rare in Laurentia, but abundant along the entire eastern margin of Precambrian Australia (e.g., Curnamona region (Page et al., 2000a,b); Mt Isa region (Blewett et al., 1998; Southgate et al., 2000; Page et al., 2000a,b). Sears et al. (2004) therefore proposed a continental scale ca. 1400 km long drainage system from northern Australia across the Siberian craton into the Udzha Trough, and ultimately filling the lower Belt basin. Such a reconstruction model dictates that we consider the southern Siberian craton as a possible source for isotopically juvenile detritus and ca. 1650 Ma zircons into the upper Willyama basin. The southern part of the Siberian craton is composed of poorly outcropping Archean to Paleoproterozoic basement (Rosen et al., 1994). In an overview of Siberian craton geochronology and Nd isotope data, Kovach et al. (2001) highlight Archean to Paleoproterozoic crust-forming events, including collision of the Aldan shield and the Stanovoy belt at ca. 1.93 Ga. These workers suggest that the significant portion of 2.4–2.0 Ga Siberian continental crustal material is difficult to reconcile with the relative lack of 2.4–2.0 Ga continental crust in western Laurentia within the context of any reconstruction models that juxtapose the two. In a more detailed study, Poller et al. (2004) found evidence of a multistage history from ca. 3.4 Ga magmatism to granite emplacement at 1.85 Ga in the southern Siberian Craton. This latter magmatic event is considered to mark the final stabilization event of the craton, and no younger Proterozoic geologic event is recorded. To date, there appears to be little evidence for juvenile ca. 1650 Ma magmatism along the southern Siberian margin that could supply detritus to the upper Willyama. Initial εNd values of the Paleoproterozoic ca. 1880–1850 Ma magmatic rocks range from −2.2 to −6.8 (Poller et al., 2005), indicating reworking of Archean crustal material, with little juvenile crustal production at that time. Nd model ages from 3.0 to 2.3 Ga for Siberian continental crust referred to by Kovach et al. (2001) are also much too old to explain the juvenile isotopic nature of the upper Willyama sediments. While the Siberian craton may have been in a pre-Rodinian reconstruction next to Laurentia with northern Australia to the present day southwest, existing geochronological and Nd isotope data do not favor Siberia as a source region for the upper Willyama sediments. The pre-Rodinian configuration that places South China between Australia and Laurentia was originally based on correlations between Neoproterozoic rift systems and alignment of Grenvillian tectonic belts (Li et al., 1995). In this configuration, the Yangtze block is directly adjacent to central eastern Australia. More

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recently, workers have concentrated on the correlation of Neoproterozoic ca. 825 Ma mafic–ultramafic magmatic activity in South China and central and southeastern Australia (e.g., Li et al., 1999, 2002, 2003) to constrain this reconstruction. Unfortunately, these studies do not provide data that can be used to constrain source region for the upper Willyama sediments. Limited data indicate basement to the Yangtze craton is as old as 2.9 Ga (Zhang et al., 2006), and detrital zircons are up to 3.2 Ga (Qiu et al., 2000). A recent compilation of zircon U–Pb analyses from the South China Block (Green et al., 2006) shows Archean ages in the Yangtze Block are dominant, suggesting that the Nd crustal evolution of the South China Block would be quite evolved by ca. 1650 Ma. Additionally, there is no documented ca. 1650 Ma magmatism in South China to account for the influx of this detrital age into the upper Willyama sediments. Hence, based on available data, it is unlikely that the South China Block can account for the marked change in the geochemical and Nd isotope character of the upper Willyama sediments. 6. Conclusions On the basis of the geochemical and Sm–Nd whole rock isotope data, the following conclusions regarding the provenance of Paleoproterozoic Willyama basin fill along the eastern margin of Australia are supported: (1) Based on geochemical and isotopic provenance indicators, the Willyama Supergroup metasedimentary rocks can be divided into two groups: lower and upper. The major and trace element characteristics for the lower part of the sedimentary rocks, including high Th, low Sc and Cr, high REE and high(La/Yb)n ratios, and the evolved and restricted initial εNd isotope values of around −5 ± 1, support a highly evolved and highly enriched intracratonic felsic source. Sedimentary recycling and heavy mineral sorting expected in an intracontinental sedimentary environment can be documented in the geochemical signature of the psammitic units of the sequence. In combination with previous detrital zircon constraints (Page et al., 2005a,b), we suggest the Paleoproterozoic Arunta province of central/northern Australia, with its voluminous evolved and highly enriched upper crustal granitoids, is the dominant source region. (2) The major, and trace element characteristics for the uppermost parts of the Willyama Supergroup are more like PAAS, and with initial εNd isotope values from −3 to 0. These features require input from a significantly geochemically less enriched and isotopically more juvenile source region. Detrital zircon data from Page et al. (2005b) indicate a felsic source with ages around 1670–1640 Ma. Crystalline rocks of this age are not found in any abundance within the Australian Proterozoic. Major crustal felsic regions of such an isotopically juvenile nature are undocumented in the Australian Proterozoic. (3) Efforts to constrain the Laurentian western continental partner in pre-Rodinian reconstructions have led to a number of different configurations involving Australia. In considering previous pre-Rodinian reconstruction models, the AUSWUS and AUSMEX-style reconstructions that place the southwestern Laurentian Mazatzal-Yavapai terranes in proximity to the Willyama basin are supported by our evidence of the influx of significantly less differentiated but more isotopically juvenile felsic material into the basin. These terranes also have appropriate magmatic ages to match the appearance of the ca. 1650 Ma zircon populations in the uppermost Willyama sequences. Magmatic units of ca. 1650 Ma are not documented

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in the Siberian Craton or the South China Block. Additionally, Nd isotope data from those regions do not suggest a source for the juvenile sedimentary material that filled the upper Willyama basin. Our results cannot absolutely constrain preRodinian Laurentian/Australian configurations, but the influx of source material into the uppermost sedimentary sequences of the Willyama basin between ca. 1650 and 1600 Ma requires a source region not present within intracontinental Proterozoic Australia. Acknowledgements Supported by Australian Research Council Linkage Project LP0347342, and Primary Industries and Resources South Australia, Minerals Group. References Andersen, T., 2005. Detrital zircons as tracers of sedimentary provenance: limiting conditions from statistics and numerical simulation. Chem. Geol. 216, 249–270. Ashley, P.M., Cook, N.D.J., Fanning, C.M., 1996. Geochemistry and age of metamorphosed felsic igneous rocks with A-type affinities in the Willyama Supergroup, Olary block, South Australia, and implications for mineral exploration. Lithos 38, 167–184. Ashley, P.M., Lawrie, D.C., Conor, C.H.H., Plimer, I.R., 1997. Geology of the Olary Domain, Curnamona Province, South Australia and Field Guide to 1997 Excursion Stops. South Australia Dept. of Mines and Energy. Report Book, 97/17. Barovich, K.M., Patchett, P.J., 1992. Behavior of isotopic systematics during deformation and metamorphism; a Hf, Nd and Sr isotopic study of mylonitized granite. Contrib. Mineral. Petrol. 109, 386–393. Barovich, K.M., Neumann, N., Hand, M., 2005. Links between Proterozoic Australia: Geochemical Provenance of Paleoproterozoic metasedimentary rocks from the Mount Isa Inlier and the Curnamona Province. In: Supercontinents and Earth Evolution Symposium, Perth. Bea, F., 1996. Residence of REE, Y, Th and U in granites and crustal protoliths; implications for the chemistry of crustal melts. J. Petrol. 37, 521–552. Belousova, E.A., Griffin, W.L., O’Reilly, S.Y., 2006. Zircon crystal morphology, trace element signatures and Hf isotope composition as a tool for petrogenetic modelling: examples from eastern Australian granitoids. J. Petrol. 47, 329–353. Bierlein, F.P., 1995. Rare-earth element geochemistry of clastic and chemical metasedimentary rocks associated with hydrothermal sulphide mineralisation in the Olary Block, South Australia. Chem. Geol. 122, 77–98. Black, L.P., Shaw, R.D., 1992. U–Pb zircon chronology of prograde Proterozoic events in the Central and Southern Provinces of the Arunta Block, central Australia. Aust. J. Earth Sci. 39, 153–171. Blewett, R.S., Black, L.P., Sun, S.S., Knutson, J., Hutton, L.J., Bain, J.H.C., 1998. U–Pb zircon and Sm–Nd geochronology of the Mesoproterozoic of North Queensland; implications for a Rodinian connection with the Belt Supergroup of North America. Precamb. Res. 89, 101–127. Bock, B., McLennan, S.M., Hanson, G.N., 1994. Rare earth element redistribution and its effects on the neodymium isotope system in the Austin Glen Member of the Normanskill Formation, New York. USA Geochim. Cosmochim. Acta 58, 5245–5253. Bock, B., Hurowitz, J.A., McLennan, S.M., Hanson, G.N., 2004. Scale and timing of rare earth element redistribution in the Taconian foreland of New England. Sedimentology 51, 885–897. Brookfield, M.E., 1993. Neoproterozoic Laurentia–Australia fit. Geology 21, 683–686. Budd, A.R., Hazell, M., Sedgmen, A., Segmen, L., 2002. OZCHEM National Whole Rock Geochemistry Database [Digital Datasets]. Geoscience Australia, Canberra. Burrett, C., Berry, R., 2000. Proterozoic Australia–Western United States (AUSWUS) fit between Laurentia and Australia. Geology 28, 103–106. Clark, C., Schmidt Mumm, A., Hand, M., Faure, K., 2006. Episodic shear zone associated fluid flow in the Curnamona Province, South Australia’. J. Geochem. Explor. 89, 69–72. Clarke, G.L., Guiraud, M., Powell, R., Burg, J.P., 1987. Metamorphism in the Olary Block, South Australia: compression with cooling in a Proterozoic fold belt. J. Metam. Geol. 5, 291–306. Clarke, G.L., Powell, R., Vernon, R.H., 1995. Reaction relationships during retrograde metamorphism at Olary, South Australia. J. Metam. Geol. 13, 715–726. Close, D., Scrimgeour, I., Edgoose, C., Cross, A., 2004. Late Palaeoproterozoic development of the SW margin of the North Australian Craton. Geol. Soc. Aust. 73, 149. Collins, W.J., Shaw, R.D., 1995. Geochronological constraints on orogenic events in the Arunta Inlier: a review. In: W.J. Collins, R.D. Shaw (Eds.), Time Limits on Tectonic Events and Crustal Evolution Using Geochronology: Some Australian Examples. Precamb. Res. 71, 315–346. Collins, W.J., Williams, I.S., 1995. Short-lived Proterozoic tectonic cycles in the northern Arunta Inlier, central Australia. Precamb. Res. 71, 69–89.

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