Decoding Mesoproterozoic and Cambrian metamorphic events in Willyama Complex metapelites through the application of Sm–Nd garnet geochronology and P–T pseudosection analysis

Gondwana Research 17 (2010) 59–74

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Decoding Mesoproterozoic and Cambrian metamorphic events in Willyama Complexmetapelites through the application of Sm–Nd garnet geochronology and P–Tpseudosection analysis

Chris Clark a,⁎, Martin Hand b

a The Institute for Geoscience Research (TIGeR), Department of Applied Geology, Curtin University, GPO Box U1987, Perth, WA, 6845, Australiab Tectonics, Resources and Exploration (TRaX), School of Earth and Environmental Sciences, University of Adelaide, Adelaide SA 5005, Australia

⁎ Corresponding author. Tel.: +61 08 9266 2446.E-mail address: [email protected] (C. Clark).

1342-937X/$ – see front matter © 2009 International Adoi:10.1016/j.gr.2009.09.002

a b s t r a c t
a r t i c l e i n f o
Article history:Received 3 April 2009Received in revised form 3 September 2009Accepted 8 September 2009Available online 23 September 2009

Keywords:THERMOCALCPolymetamorphismDelamerian OrogenyOlarian OrogenyLP–HT metamorphism

The pressure–temperature (P–T) path for the Palaeoproterozoic Willyama Supergroup rocks in the OlaryDomain, South Australia has been reconstructed through detailed petrographic observations, in conjunctionwith calculation of compositionally specific P–T pseudosections of metapelitic rock units and Sm–Nd garnetgeochronology. The P–T path for the Willyama Complex has historically been interpreted to follow a singleanticlockwise path, however the results of this study demonstrate that this path can be better described bytwo metamorphic events (M1 and M2) separated by 1100 Ma. The M1 event occurred at c. 1600 Ma and wasassociated with high temperature–low pressure metamorphism. Sm–Nd garnet geochronology constrainsthe timing of garnet growth at c. 1585 Ma. The growth of large andalusite porphroblasts during M1 exerted afirst order control on the bulk composition during the subsequent M2 event. Mineral chemistries coupledwith quantitative phase diagrams constrain peak conditions to be in the order of c. 550°C and 5.5 kbar duringM2. The identification of two metamorphic events calls into question interpretations of metamorphic corecomplex formation and the single anticlockwise P–T paths being associated with the early stages of thetectonic evolution of the terrain.

© 2009 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved.

1. Introduction

Recent advances in the application of in-situ geochronologicaltechniques and the forward modelling of metamorphic mineralassemblages have allowed the evolution of complex metamorphicterranes to be re-examined and in some cases re-interpreted (Clark et al.,2009; Clark et al., 2007; Huang et al., 2001; Kelsey et al., 2008; Kelseyet al., 2007; Liu et al., 2009; Santosh et al., 2003). The metamorphicevolution of the Willyama Complex in the southern AustralianProterozoic (Fig. 1) is a classic example of this problem as it has longbeen established in the literature that it followed a single anticlockwiseP–T evolution (Clarkeet al., 1986; Clarke et al., 1987;Corbett andPhillips,1981; Forbes et al., 2005; Phillips and Wall, 1981; Stuwe and Ehlers,1997; White et al., 2005). In part this P–T evolution was based on themineral assemblages in the retrograde shear zones that dissect theterrain. However recent geochronological investigations (Clark et al.,2006a; Dutch et al., 2005) of retrograde shear zones, which hadpreviously been attributed to the waning stages of the MesoproterozoicOlarian Orogeny (e.g. Clarke et al., 1987), have consistently yielded

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Cambrian ages, for the crystallisation ofmetamorphic assemblages in theshear zones. These studies have also found that mineral assemblageslinked to shear zone activity formed at mid-amphibolite faciesconditions. In combination these findings call into question theapplicability of the single P–T evolutionary path inferred for the terraneby earlier workers.

To understand the metamorphic imprint of the separate metamor-phic events experienced by the Willyama Complex rocks metamorphicforward models have been calculated for a range of rock compositions.One key observation is that the original rock volume metamorphosedduring the Olarian Orogeny developed large andalusite porphyroblasts.These porphyroblasts have the potential to exert a strong control on thesubsequentmetamorphic events experienced by the rock volume due tothe formation of domains of different chemical compositions. A majorfocus of this paper is to understand how the domainal nature of the rockimparted during the Olarian Orogeny influences the subsequentmetamorphic evolution of the rock systemduring the Cambrian orogenicevent. This study aims to provide detailed constraints on the thermo-barometric history of the c. 1600 Ma Olarian Orogeny through theapplication of petrography, mineral chemistry, calculated metamorphicphase diagrams and Sm–Nd garnet geochronology. The temperature–composition (T–X) and pressure–temperature (P–T) pseudosectionspresented in this study will be used to model the metamorphic imprint

Published by Elsevier B.V. All rights reserved.

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Fig. 1. Location of the southern Curnamona Province within Australia and showing sample locations used in this study for petrology and geochronology.

60 C. Clark, M. Hand / Gondwana Research 17 (2010) 59–74

of the Cambrian metamorphic event and should allow its effects to bedisentangled from the Olarian metamorphic event, something thatprevious studies, due primarily to the lack of appropriate geochronolog-ical data, have not to date achieved.

2. Regional geology

The rocks of the Palaeoproterozoic Curnamona Province in north-eastern South Australia and western New South Wales outcrop as asequence of Palaeoproterozoic (c. 1715–1640 Ma) metasediments,metavolcanics and intrusive igneous rocks known as the WillyamaSupergroup (Fig. 1). The CurnamonaProvince has been sub-divided intothe Olary and Broken Hill Domains, the main difference being that ofmetamorphic grade. There are well established correlations betweenthe stratigraphic and tectonic framework of the two domains (Pageet al., 2005). The geology of the Olary Domain has been summarised byClarke et al., (1986, 1987), Cook and Ashley (1992), Flint and Parker(1993), Robertson et al. (1998), Clark and James (2003), Clark et al.(2004) and Gibson & Nutman (2004). The Broken Hill Domaingeological evolution has been discussed in studies by Hobbs (1966),Vernon (1969), Laing et al. (1978), Marjoribanks et al. (1980), Williset al. (1983), Hobbs et al. (1985) Stevens (1986), Raetz et al. (2002),Gibson et al. (2004), Williams et al. (2009). While there are stillimportant aspects of the terrain evolution to be resolved (e.g. Conor

et al., 2005; Gibson and Nutman, 2004; Gibson et al., 2004) there is ageneral consensus that the structure and metamorphic style of theWillyama Complex is dominated by the effects of the c. 1610–1580 MaOlarian Orogeny. This period of compressional deformation is char-acterised by LP–HT amphibolite to granulite facies metamorphismranging from550 to 750 °C at between 3 and 5 kbar (Clark et al., 2006b;Clarke et al., 1987). The Olarian aged structures (D1–D3) and anassociated metamorphic assemblage (M1) is overprinted by a series ofductile retrograde shear zones. This terrain scale network of shear zonesrange in grade from greenschist to amphibolite facies (450–550 °C andc.4–6 kbar) andhave been attributed to thewaning stages of theOlarianOrogeny (Clarke et al., 1987; Corbett andPhillips, 1981; Flint and Parker,1993). The attribution of these shear zones and their associatedmineralassemblages to the end of the Olarian Orogeny has in part led to thedefinition of an anticlockwise P–T path for the evolution of the OlaryDomain (Corbett & Phillips, 1981; Clarke et al., 1987).

Clarke et al. (1987) defined the presence of the anticlockwise P–Tpath on the basis of the timing relationships of the alumosilicateporphyroblasts and overprintingmineral assemblages. The petrologicalcase presented by Clarke et al. (1987) suggested that andalusite formedduring peakmetamorphism and recumbent foldingwas overprinted bysillimanite during upright folding followed by the growth of chlorite,chloritoid and paragonite. The conclusion of the Clarke et al. (1987)study was that the assemblages formed during a single metamorphic

61C. Clark, M. Hand / Gondwana Research 17 (2010) 59–74

cycle and the nature of the overprinting relationships indicated therewas a post peak increase in pressure associatedwith cooling resulting inan inferred anticlockwise P–T path for the Olarian Orogeny. Clarke et al.(1987) attributed the formation of the retrograde shear zones to the endof the Olarian event as the shear zones overprint all the D1–D3 Olarianstructures. However recent Sm–Nd garnet and EPMA monazite dating(Clark et al., 2006a;Dutch et al., 2005;Hand et al., 2003), on a number ofshear zones indicates that the equilibrium metamorphic assemblages(M2) contained within these shear zones were formed during thec. 500 Ma Delamerian Orogeny and not in the waning stages of theOlarian Orogeny (Williams and Betts, 2009; Williams et al., 2009).Delamerian ages for shear zone formation and elevated metamorphicgrades bring into question the applicability of a single anticlockwise P–Tevolution of the terrain.More recently Rutherford et al. (2006) obtainedSm–Nd garnet and U–Th–Pb monazite ages from a number of differentmineral assemblages and demonstrated that there is a distinctmetamorphic imprint at c. 500 Ma outside the shear zones. However,no attempt was made in this study to constrain the nature andconditions of the individual metamorphic events. This paper seeks toestablish the conditions associated with both the Olarian andDelamerian metamorphic events with the aim of critically assessingthe validity of previously proposed models for the metamorphicevolution of the Willyama Complex rocks in the Olary Domain.

This study is focused on rocks of the Curnamona Group thatoutcrop in the Tommie Wattie Bore and Northern Walparuta areas(Fig. 1). Within these areas mineral growth is best preserved in thealuminous mica schists of the Wiperaminga Subgroup, which formspart of the lower most stratigraphic units in the terrain. Within bothareas an early layer parallel fabric (S1) is present the formation of thisfabric is attributed to the early stages of the Olarian Orogeny (Flint andParker, 1993). An S2 fabric is inferred to occur throughout the regionand represents the axial surface of large scale recumbant folds(Gibson and Nutman, 2004) and in most areas is transposed intoparallelismwith the S1 fabric (Flint and Parker, 1993). The S1/2 fabric isoverprinted by a northeast trending S3 fabric that is axial planar to theregional scale F3 folding that dominates the structural framework ofthe terrane. The development of the upright northeast trending F3folds is related to the D3 event of the Olarian Orogeny. The rocks usedfor this study contain muscovite, biotite, garnet, staurolite, quartz,plagioclase, ilmenite, paragonite and andalusite. Chlorite is found insome rocks as is sillimanite. In the Tommie Wattie bore area, gahnitelocally occurs in Zn-rich bulk compositions, and in the NorthernWalparuta area kyanite is present.

3. Metamorphic petrology

The two samples (TWB-1 and TWB-2) from the TommieWattie Borearea described in this study are compositionally similar (Table 1). TWB-1

Table 1Whole rock compositions from the NorthernWalparuta and TommieWattie Bore areasused for pseudosection calculations.

Compositions in mol%

NW4 TWB1 TWB2 NW5 Fig. 10

SiO2 70.29 71.37 70.64 53.64 61.97Al2O3 14.70 13.24 12.83 31.26 22.98FeO 5.86 6.47 7.04 5.05 5.46MgO 3.13 3.47 3.59 1.92 2.53MnO 0.24 0.18 0.25 0.07 0.16CaO 0.63 0.66 0.65 0.8 0.72Na2O 1.42 1.03 1.46 1.57 1.50K2O 3.73 3.58 3.55 5.68 4.71Total 100.00 100.00 100.00 100.00 100.00

Fig. 10 composition intermediate between that of NW4 and NW5 (Table 1) andindicated by an arrow in Fig. 8a and b.

and TWB-2 aremuscovite–biotite–garnet–staurolite–paragonite bearingschists (Fig. 2a), both contain minor sillimanite and andalusite. TWB-2contains minor amounts of chloritoid, chlorite and gahnite. Microstruc-turally these rocks can be divided into two main domains. The firstdomain is contained within early porphyroblasts of andalusite that havebeen subsequently retrogressed to fine-grained muscovite and para-gonite (Fig. 3a). These pseudomorphed andalusites exhibit the mosttexturally varied mineral assemblages, which in addition to muscoviteand paragonite include garnet, staurolite and sillimanite (Fig. 2b) (and inTWB-2 chlorite and gahnite). The second domain consists of alignedbiotite, muscovite and quartz that define the regional S3 fabric thatenclose the retrogressed andalusite porphyroblasts (Fig. 2c). Garnetis found in both the S3 fabric and within the pseudomorphed andalusitedomains. Two texturally distinct garnet generations can be identified,the first is an inclusion rich variety where elongate quartz inclusionsdefine a foliation that is inmany places at a high angle to the dominant S3foliation (Fig. 3b). Inclusion rich garnet located outside andalusiteporphyroblasts exhibit curvilinear inclusion trails that are continuouswith the external schistosity near the edges, though the S3 fabric isdeflected around the garnet (Fig. 3c). The second garnet generationoccurs as both small euhedral to subhedral crystals up to 5 mm indiameter containing small randomly oriented inclusions of quartzand biotite (Fig. 3d.) and as rims on the inclusion rich garnet (Fig. 3c).Where the second-generation garnet is observed in the matrix, itovergrows the S3 fabric, consistent with post-kinematic development.

Sample NW-4 from theNorthernWalparuta Inlier is compositionallysimilar to samples TWB-1 and TWB-2 (Table 1), but preserves a morecomplete textural record of mineral growth. NW-4 contains theassemblage muscovite–biotite–chlorite–paragonite–garnet–staurolite–andalusite–quartz with minor kyanite and ilmenite. Muscovite andbiotite againdefine thedominant foliation,which is interpreted to be theregional S3 fabric. In contrast to TWB-1 and -2, andalusite, poikiloblasticstaurolite (Fig. 4a) and garnet all formporphyroblastswrapped by the S3foliation, most porphyroblasts containing inclusion trails of quartz thatare at a high angle to the matrix foliation (Fig. 4b). As in TWB-1 and -2,andalusite is replaced by muscovite and paragonite but relict andalusiteis locally preserved. A second generation of staurolite is randomlyorientedandovergrowsboth the retrogressedandalusite porphyroblastsand the S3 foliation (Fig. 4c and d). Two types of garnet are observed, theearliest variety is quartz inclusion rich. The inclusion rich garnet occursboth as inclusions in poikiloblastic staurolite (Fig. 4a), and as euhedralcrystals that arewrappedby theS3 foliation. Linear inclusion trailswithinthe garnet show some degree of rotation with respect to the externalfoliation (Fig. 4b). A second generation of garnet occurs both as small(<5 mm) euhedral crystals both inside and outside the retrogressedandalusite porphyroblasts and as rims on the first generation garnet(Fig. 4b). This generation of garnet overgrows thematrix schistosity andcan also be found as inclusions in the randomly oriented inclusion freestaurolite (Fig. 4c). Rare laths of kyanite overgrow andalusite (Fig. 4e)and the second-generation garnet. Early staurolite has been pseudo-morphed by paragonite and muscovite within the pseudomorphedandalusite porphyroblasts. This relationship can be observed in Fig. 4fwhere the remaining staurolite is optically continuous even though ithas nearly been completely replaced by muscovite and paragonite.Randomly oriented chloritoid is also observed overgrowing muscoviteand paragonite within pseudomorphed andalusite porphyroblasts(Fig. 4g).

4. Mineral chemistry

Mineral compositions were analysed in two metapelitic samplesfromtheTommieWattie Bore area (TWB-1and -2) andon three samplesfrom the Northern Walparuta area (NW-1, -4 and R504843). Sampleswere analysed using a Cameca SX-51 electron microprobe at AdelaideMicroscopy, University of Adelaide. The analyses were obtained usingwavelength dispersive spectrometers with an accelerating voltage of

Fig. 2. (a) Relict bedding (S0/S1) at an angle to the later dominant S3 fabric in the TommieWattie Bore area. (b) Retrogressed andalusite porphyroblast enclosed by S3 fabric and containinglate garnet and staurolite grown during the M2 event, Northern Walparuta. (c) Strongly flattened andalusite porphyroblasts enclosed by the S3 fabric, Northern Walparuta.


15 kV, a current of 20 nA and a beam diameter of 2–3 μm and a ZAFmatrix correction protocol was used. A summary of the analyses can befound in Tables 2 and 3.

4.1. Garnet

Two distinct types of garnet have been identified in this study andtheir compositional attributes will be described separately. First-generation garnet from the Tommie Wattie Bore and the NorthernWalparuta areas are generally almandine-richwith Xalm [Fe/(Fe+Mg+Mn+Ca)] value ranges of 0.71–0.75 and 0.69–0.71 respectively. Thisvariation occurs between individual grains as opposed to withinindividual garnets (Table 2). Similar intra-grain variations can beobserved for other end-members and these are shown in the X-rayelemental maps and in the cation zoning profiles in Fig. 5. First-generation garnet has similar Xsps compositions that are Mn-rich(Xsps>0.15) in both areas (Table 3). The Fe:Mg ratio [XFe=Fe/(Fe+Mg)] is relatively constant in garnet from both areas although slightlylower at Northern Walparuta. Second-generation garnet, which occursas both euhedral garnets and overgrowths on inclusion-rich garnet, hasmore elevated Xalm (Xalm=0.74–0.81) than the inclusion-rich garnet.This difference is most likely to reflect the decrease in Xsps which islower in the inclusion-free garnet (Xsps<0.11). The XFe 0.90–0.92 isrelatively constant in garnet from both areas (Table 3).

A striking feature of the garnet from the Tommie Wattie Borearea is the development of a Ca-rich Mn-poor rim surrounding theearly generation of inclusion-rich garnet. This relationship is bestseen in the Ca and Mn X-ray maps presented in Fig. 5. Rutherfordet al. (2006) reported that similar high-Ca low-Mn rim from thenearby Cathedral Rock area (their sample CR) yielded an Sm–Nd ageof 513±12 Ma and attributed the growth of this rim to the

Delamerian Orogeny. However, they did not report the age of thegarnet mantled by the rim.

4.2. Staurolite

Aswith garnet, twodistinct types of staurolite can beobserved in theone sample, a poikiloblastic–staurolite and other randomly orientaleuheral crystals. Poikiloblastic–staurolite was only observed in thesamples from the Northern Walparuta area. Staurolite shows nocompositional variation in XFe (XFe: 0.82–0.84) between the twotexturally distinct generations. There is, however, a marked variationin theXZn [XZn=Zn/(Fe+Zn+Mn+Mg)with poikiloblastic–staurolitehave significantly higher XZn values (XZn=0.21–0.24) than therandomly oriented euhedral variety, with XZn ranging from 0.09 to0.10 in the Tommie Wattie Bore and 0.01–0.03 in Northern Walparuta.

4.3. Muscovite

Muscovite compositions are consistent across both areas (Table 2)and are in general phengitic with Si typically constituting c. 6.1cations per formula unit. Na contents in muscovite were typicallyaround 0.28 cations per formula unit. Fe and Mg constituteapproximately 0.25 cations per formula unit with Fe always in excessof Mg (XFe=0.58–0.64), Ti is present in minor amounts (0.03–0.04cations per formula unit).

4.4. Paragonite

Retrogressed andalusite porphyroblasts in the Northern Walparutaand Tommie Wattie bore area contain paragonite with Na constituting.

Fig. 3. Photomicrographs of textural relationships observed in the schists of the Tommie Wattie Bore area. (a) Retrogressed (to muscovite (mu) and paragonite (par)) andalusiteporphyroblasts enclosed by S3 fabric with two-stage garnet (grt1, grt2) and staurolite (st2). (b) Two stage garnet (grt1, grt2) with curvilinear inclusion trails enclosed by S3 fabric.Second generation garnet (grt2) statically overgrows S3 fabric defined by muscovite. (c) Garnet (grt1) with curvilinear inclusion trails continuous with the external schistosityenclosed by S3 fabric. (d) Second generation euhedral garnet with small randomly oriented quartz inclusions overgrowing muscovite+paragonite.


2.45 cations per formula units. XFe (XFe 0.52) of the paragonite wasconsistently lower than that measured in the muscovite.

4.5. Biotite

XFe of biotite is similar in the two sample localities with valuesaround 0.57 consistentlymeasured. Biotite has Ti contentswith valuesup to 1.25 wt.% in some samples.

5. Sm–Nd geochronology

Garnet Sm–Nd geochronology was undertaken on samples fromNorthern Walparuta and Ameroo Hill (Fig. 1). Samples were alsoselected on the criteria that they contained minimal oxide (ilmenite),and no epidote inclusions, which can lead to significant analyticalcontamination. Individual garnets were cut from selected samples,crushed in amortar and pestle and sieved. Impurities were removed viamagnetic and heavy liquid separation techniques. Surface contamina-tion on the handpickedmineral separateswas removed by anultrasoniccleaner in 1 M HCl solution. A whole-rock fraction of the matrixsurrounding the garnets was crushed and milled. Between 125 and300 mg and 100–150 mg of sample was used for mineral separates andwhole rock samples, respectively. The mineral separates were milledunder ethanol in an agate mortar to a grain size, c. 2 μm. To minimizecontamination of mineral fractions by REE-rich inclusions, the milledfractions were leached in hot HF for 1 hour. The leachate was pipettedfrom the residual solid material and the solid material washed threetimes in cold 6 M HCl separately to remove any trace of the leachate

fraction. For thewhole-rockcomponent, around150 mgofmilledwholerockwasdissolved inHNO3–HFacidmixtures for periods between1 and10 days. All samples were spiked with amixed 147Sm–150Nd spike priorto dissolution. Nd and Sm isotopic ratios were measured by thermalionization mass spectrometry (TIMS) on a Finnigan MAT 262 system instatic mode. The isotopic ratios were corrected for fractionation to146Nd/144Nd=0.721903 and to a 152Sm/149Sm ratio of 1.9347. Spikedsamples of BCR-1 yielded a 143Nd/144Nd ratio of 0.512598±17 afterspike unmixing. Reported errors (Table 4) on themeasured 143Nd/144Ndare 2 S.E. analytical uncertainties. The 143Nd/144Nd reproducibility of theinternal standard over the course of the study (n=10)was 0.511602±0.00001. For age calculations Sm/Nd errors were estimated to be±0.3%.Isochron calculations were carried out using Isoplot v. 3.0 (Ludwig,2003)with ages (reported at 95% confidence) based on a decay constantfor 147Sm of 6.54×10−12 a−1. The total procedural Sm and Nd blankswere 100 pg.

Two samples from northern Walparuta (Fig. 1), samples OL2 andWW9C (Fig. 6), were analysed (Fig. 6; Table 4). In samples OL2 andWW9C the garnet iswrapped by the S0/S1 fabric (Fig. 6a and b),which isfolded around the upright regional ENE-trending F3 folds (Fig. 6a). Boththe OL2 andWW9C samples come from theWiperaminga Subgroup asthe regional structural facing places it below the nearby EthiudnaSubgroup units. Analysis of the whole rock–garnet 1–garnet 2 forsample WW9C gave an apparent age of 1584±5 Ma (Fig. 7a) andanalysis of whole rock–garnet–garnet leachate for sample OL2 gave anear identical apparent age of 1583±5 Ma (Fig. 7b). Sample AH 5B isfrom the Strathearn Group immediately east of Ameroo Hill (Fig. 1) thedated garnet is wrapped by the S1 fabric (Fig. 6c) and analysis of whole

Fig. 4. Photomicrographs of textural relationships observed in the NorthernWalparuta schists. (a) Retrogressed (to muscovite (mu) and paragonite (par) andalusite porphyroblastsenclosed by S

3

fabric with early poikiloblastic staurolite (st1) with early garnet inclusion (g1). (b) Two stage garnet (grt1, grt2) with curvilinear inclusion trails enclosed by S3

fabric.Second generation garnet (grt2) statically overgrows S

3

fabric defined by muscovite. (c) Second generation garnet (grt2) inclusion in late staurolite (st2) and late chlorite (chl) allstatically overgrowing S

3

fabric. (d) Retrogressed andalusite porphyroblasts overgrown by staurolite (st2). (e) late kyanite (ky) overprinting andalusite (and) and retrogressionassemblage. (f) Muscovite and paragonite replacing early staurolite (st1). Staurolite is optically continuous. (g) Randomly oriented chloritoid (ctd) overprinting muscovite andparagonite that is pseuomorphing andalusite.


Table 2Representative mineral compositions.

mu mu par par bi bi chl chl st1 st1 st2 st2 g1 g1 g2 g2

SiO2 47.39 47.02 46.49 46.41 36.21 36.54 25.18 25.13 27.02 27.34 27.49 27.00 36.19 36.23 36.02 37.14TiO2 0.34 0.31 0.57 0.56 1.22 1.25 0.07 0.08 0.00 0.07 0.36 0.32 0.05 0.06 1.74 0.00Al2O3 35.18 35.73 37.08 36.58 19.21 19.50 23.23 23.74 53.94 53.36 53.65 53.78 20.96 20.89 20.21 21.00FeO 2.82 3.00 0.84 0.92 20.63 20.33 23.90 24.28 10.71 11.00 12.59 13.98 30.50 30.61 34.73 34.21MnO 0.70 0.77 0.04 0.04 0.00 0.00 0.09 0.00 0.13 0.16 0.10 0.07 8.07 9.77 3.77 4.32MgO 0.03 0.02 0.43 0.48 8.75 8.65 15.01 15.14 1.35 1.35 1.29 1.31 1.87 1.48 1.69 1.91CaO 0.00 0.01 0.01 0.00 0.04 0.06 0.01 0.01 0.00 0.00 0.02 0.01 0.75 0.81 1.84 2.09Na2O 1.29 1.25 9.84 9.69 0.11 0.08 0.04 0.00 0.00 0.01 0.08 0.06 0.05 0.02 0.07 0.02K2O 9.23 9.70 0.76 0.72 8.36 8.10 0.04 0.00 0.20 0.14 0.02 0.00 0.00 0.04 0.03 0.03ZnO 0.00 0.00 0.09 0.00 0.00 0.17 0.01 0.09 5.79 5.04 1.84 1.88 0.00 0.13 0.00 0.00Total 96.98 97.81 96.15 95.40 94.54 94.70 87.57 88.47 99.15 98.46 97.43 98.40 98.45 100.05 100.10 100.72Num oxSi 6.11 6.11 5.98 6.01 5.47 5.49 5.24 5.19 7.54 7.65 7.69 7.54 2.98 2.96 2.94 2.99Al 5.75 5.71 5.62 5.59 3.42 3.45 5.72 5.79 17.73 17.60 17.69 17.70 2.04 0.00 1.94 2.00Ti 0.03 0.04 0.06 0.05 0.14 0.14 0.01 0.01 0.00 0.01 0.08 0.07 0.00 0.00 0.11 0.00Fe 0.14 0.16 0.09 0.10 2.61 2.55 4.16 4.19 2.50 2.57 2.95 3.27 2.13 2.09 2.37 2.31Mn 0.00 0.01 0.00 0.00 0.00 0.00 0.02 0.00 0.03 0.04 0.02 0.02 0.56 0.68 0.26 0.29Mg 0.10 0.09 0.08 0.09 1.97 1.94 4.66 4.66 0.56 0.56 0.54 0.54 0.23 0.18 0.21 0.23Zn 0.00 0.00 0.01 0.00 0.00 0.00 0.00 0.01 1.19 1.04 0.38 0.39 0.00 0.01 0.00 0.00Ca 0.00 0.00 0.00 0.00 0.01 0.01 0.00 0.00 0.00 0.00 0.01 0.00 0.07 0.07 0.16 0.18Na 0.28 0.29 2.45 2.44 0.03 0.02 0.03 0.00 0.00 0.01 0.04 0.03 0.00 0.00 0.00 0.00K 1.46 1.45 0.13 0.12 1.61 1.55 0.02 0.00 0.07 0.05 0.01 0.00 0.00 0.00 0.00 0.00Total 12.41 12.12 11.84 11.85 13.60 13.57 19.81 19.85 29.55 29.48 29.34 29.52 7.94 5.92 7.98 8.01XFe 0.58 0.64 0.52 0.52 0.57 0.57 0.47 0.47 0.82 0.82 0.85 0.86 0.90 0.92 0.92 0.91


rock–garnet core–garnet core leachate gave an age of 1563±29 Ma(Fig. 7c).

6. Phase diagram calculations and the effect of an evolving bulkcomposition on mineral equilibria

The sequence of mineral growth in the Northern Walparuta areaallows some constraints to be placed on themetamorphic evolution ofthe southern Curnamona Province. To allow more rigorous inter-pretations about the metamorphic (P–T–t) path within the studiedsequences, quantitative phase diagrams (P–T pseudosections) havebeen constructed in the MnO–Na2O–CaO–K2O–FeO–MgO–Al2O3–

SiO2–H2O (MnNCKFMASH) model system have been constructed.P–T pseudosections allow textural observations to be compared withforward models of the specific chemical bulk composition from theNorthern Walparuta area. The phase were constructed using theTHERMOCALC (v3.31) software of Powell and Holland (1988) andPowell et al. (1998), using the internally consistent thermodynamicdataset (November 2003 update) updated from that of Holland andPowell (1998). The a–x relationships of Holland et al. (1998) forchlorite (chl); Powell & Holland (1999) for biotite (bi); Stowell et al.

Table 3Summary of mineral chemistries of garnet and staurolite from Table 2.

Tommie Wattie Bore Northern Walparuta

Garnet 1Xalm 0.71–0.75 0.69–0.71Xpyr 0.07–0.08 0.08–0.09Xgrs 0.01–0.02 0.03–0.04Xsps 0.15–0.20 0.17–0.19XFe 0.91–0.92 0.89–0.90

Garnet 2Xalm 0.77–0.81 0.74–0.76Xpyr 0.06–0.08 0.07Xgrs 0.05–0.07 0.07–0.08Xsps 0.07–0.10 0.09–0.11XFe 0.90–0.92 0.91–0.92

Staurolite 1XFe – 0.83–0.84XZn – 0.21–0.24

Staurolite 2XFe 0.84–0.87 0.82–0.84XZn 0.09–0.11 0.01–0.03

(2001) for garnet (g), cordierite (cd), staurolite (st) and potassiumfeldspa (ksp)r; White et al. (2001) for silicate melt (liq); Coggon &Holland (2002) for paragonite (par) and muscovite (mu); andHolland & Powell (2003) for plagioclase (pl) were used. Bulk rockcompositions were taken directly from the whole rock data presented(analysed by XRF) in Table 1 and normalized to 100 to account forcomponents either treated as in excess (H2O) or ignored in the modelsystem (TiO2 and ZnO).

A combination of T–X and P–T pseudosections (Figs. 8a–b, 9 and10) are used to establish the extent to which the domainal nature ofthe rock, imposed by the pseudomorphs after andalusite, affect thestability of the mineral assemblages in the calculated systems. Thecomponents TiO2 and ZnO are not considered as they constitute asmall proportion of the total rock (Table 1) and form a minorcomponent in one or two of the major phases; TiO2 in biotite andilmenite, and ZnO in staurolite (Boger and Hansen, 2004). At theinferred pressures and temperatures, TiO2 does not significantly affectthe stability of the dominant minerals observed (Boger and Hansen,2004; White et al., 2001, 2000), although ZnO does increase thestability of staurolite particularly to higher pressures. Early formedstaurolite in both the Tommie Wattie Bore and Northern Walparutaareas contains appreciable Zn (up to 5wt.%). The effect of this willexpand the staurolite bearing assemblages to lower temperaturecompared to their zinc-free model equivalents in Fig. 8. However, theformation of early staurolite would sequester the ZnO from the bulkcomposition. Therefore the subsequent effective bulk composition islikely to approach the modeled composition.

The extent to which the andalusite pseudomorph domains commu-nicate with the matrix are illustrated in Fig. 8a and b. Fig. 8a and b aretemperature (T) composition (X) pseudosections calculated at a fixedpressure (3 and 5kbar respectively). These pressures were used as theywithin the range of pressures inferred from previous studies ofmetamorphism in the region (e.g. Clarke et al., 1995; Dutch et al.,2005). The range in X varies from, on the right, the whole rockcomposition of NW4, which is the equivalent to the whole rockcomposition to the composition of NW5 on the left, which is thecomposition of a pseudomorphed andalusite porphyroblast. These twosamples represent the range of compositions that the equilibrationvolumes could be expected to lie within. The T–X pseudosections areused to determine the most appropriate compositions for thecalculation of the pressure–temperature pseudosections in Figs. 9 and

Fig. 5. Compositional garnet maps from and garnet profiles from Tommie Wattie Bore (top) and Northern Walparuta (bottom). Note the differing composition on the rim of theTommie Wattie Bore sample associated with the growth of the second generation of garnet.


10. These pseudosections are then used to infer the P–T history of therecorded by the rocks from the Northern Walparuta area. The mostsignificant differences between the two end member chemicalcompositions are the increase in Al2O3 (NW4=17.40mol%,NW5=31.26mol%) and decreases in SiO2 (NW4=70.29mol%,NW5=53.64mol%) and MnO (NW4=0.24mol%, NW5=0.07mol%)

Table 4Garnet Sm–Nd isotopic data.

Sample Sm(ppm)

Nd(ppm)

147Sm/144Nd Error(±)

143Nd/144Nd Error(±)

WW9CWR 5.06 9.76 0.313 0.001 0.513196 0.000009gt1 7.38 1.51 2.951 0.010 0.540642 0.000035gt2 9.63 4.08 1.524 0.005 0.525822 0.000068

OL2WR 8.17 25.33 0.195 0.001 0.512282 0.000008gt 6.76 1.14 3.564 0.012 0.547311 0.000032gt leachate 2.00 1.56 0.775 0.003 0.518334 0.000028

AH 5BWR 2.53 14.16 0.108 0.000 0.511508 0.000013gt core 2.03 9.76 0.126 0.000 0.511686 0.000020gt core leachate 0.30 0.83 0.217 0.001 0.512629 0.000015

from thewhole rock composition to the composition of the retrogressedandalusite porphyroblasts.

Fig. 8a illustrates the effect that at 5kbar the progressive changein composition, reflecting an increased degree of communicationbetween an effectively isolated retrogressed andalusite porphyroblast(right hand side of the diagram) and the whole rock composition (lefthand side of the diagram), will have on the equilibrium mineralassemblage. As can be seen there is little variability in the equilibriummineral assemblages across a range of bulk composition contents athigher temperatures (>550°C). The most notable change being alowering in temperature of the sillimanite-in phase boundary as thebulkcomposition becomes more aluminous. At temperatures below 550°Cthe equilibrium assemblage of a completely isolated retrogressedandalusite porphyroblast is staurolite+chloritoid+plagioclase+quartz, as greater interaction between the compositions of theporphyroblast (X=0.76 NW5) and the whole rock (X=0.24 NW4)staurolite becomes unstable. As the chemical composition approachesthat of the NW4 biotite then garnet become stable and finally as thecomposition is totally dominated by that of NW4 chloritoid becomesunstable. These changes are dominated by the decreasing Al2O3 content(loss of staurolite andchloritoid) and the increase in theMnO(expandedstability of garnet) content of the rock. The decrease inMnObetween theinitial bulk rock and the isolated retrogressed andalusite porphyroblasts,would indicate that a proportion ofMnOwas isolated from the bulk rock

Fig. 6. (a) Hand specimen of the structural relationships of garnetWW9C from the NorthernWalparuta area used for geochronology. The garnet is wrapped by the S0/S1 fabric whichis then folded around an F3 fold, (b) enlargement of the garnet shown in Fig. 5a illustrating the fabric being deflected around the dated garnet. (c) Sample AH 5B from the Ameroo Hilllocation used for geochronology. The garnet in this location is also enclosed by the S0/S1 fabric.


by the early growth of garnet on the initial prograde metamorphic path(e.g. Boger and Hansen, 2004; Marmo et al., 2002). Sequestered MnOwould not be easily reincorporated back into the bulk rock as anydecrease in the mode of garnet via garnet breakdown reactions willresult in immediate regrowth of garnet. This is due to the immediateexpansion of garnet stability field as the MnO is liberated by garnetbreakdown thereby effectively buffering garnet stability along theretrograde metamorphic path (Boger and Hansen, 2004).

The T–X section calculated at a pressure of 3kbar (Fig. 8b) againhighlights the importance of the growth of andalusite porphyroblastsduring the Olarian Orogeny will have on themetamorphic assemblagesdevelopedduring subsequentmetamorphismrelated to theDelamerianOrogeny. Fig. 8b predicts that at lower temperatures (<530°C) thestability of garnet is restricted to bulk rock compositions dominated bythe NW4 whole rock composition. This suggests that the sequestrationofMnO into Olarian garnet will have amajor influence on the growth ofgarnet during the Delamerian. If there is relatively little communicationbetween the retrogressed andalusite porphyroblasts and thematrix it ispredicted that more aluminous phases such as staurolite and chloritoidwill dominate the assemblage at the expense of biotite and garnet. Attemperatures greater than 530°C the more aluminous composition ofNW5 expands the stability of andalusite from ca. 545°C to less than530°C. However the equilibrium assemblages at higher temperaturesare relatively unchanged. The most significant difference is that at thenear pure end member composition of NW5 the equilibrium assem-blages are quartz absent.

7. Discussion

Fig. 8 a and b illustrates the mineralogical effect of partitioning thebulk chemical composition of the equilibrium volume at the P–T

conditions in the southwestern Curnamona Province due growth ofporphyroblastic minerals. The importance of identifying the possiblepartitioning of the bulk composition is vital when reconstructing themetamorphic evolution of a polymetamorphic terrane such as theOlary Domain (e.g. Clarke et al., 1995). This is because the growth ofporphyroblasts during the Olarian Orogeny will exert a major controlover the effective bulk composition available for metamorphismduring the subsequent Delamerian Orogeny. Johnson et al. (2004)have previously highlighted the role the development of andalusiteporphyroblasts can impose on the textural evolution of rocks underhigher metamorphic grades than achieved in the Olary Domain. In therocks of the Olary Domain previously reported Sm–Nd garnet ageshave demonstrated that somemineral growthwithin the retrogressedandalusites is related to the c.500 Ma Delamerian Orogeny and not thewaning stages of the Olarian Orogeny (Rutherford et al., 2006). Themost significant variable to consider when identifying realistic bulkcompositions to explain the observed sequence of mineral growth inthe Northern Walparuta area during metamorphism is the degree towhich the pseudomorphed andalusite domains communicate withthe rest of the rock. To represent the complex petrographic relation-ships developed as a consequence of the differing equilibrium bulkcompositions two pseudosections have been calculated at differingbulk compositions as discussed above. The initial Olarian P–Tevolution is best described by the original whole rock composition(Fig. 9; Table 1). In contrast, due to imposition of a domainal naturethrough the growth and subsequent pseudomorphing of andalusiteporphyroblasts during the Olarian orogeny, the Delamerian P–Tevolution is better described by the more aluminous compositions,consisting of dominantly the pseudomorphed andalusite porphyro-blasts in partial communication with the surrounding matrix, thathave been used to calculate the pseduosections in Fig. 10. It is difficult

Fig. 7. Sm–Nd isochron plots of the Olarian Northern Walparuta garnets (a) sampleWW9c, (b) sample OL-2 and (c) Amerroo Hill garnets sample AH 5B.


to constrain the exact bulk composition that is appropriate for theDelamerian metamorphic evolution however we have chosen acomposition based on the petrographic observation that garnetgrowth occurred prior to staurolite growth and the stability ofchloritoid in the Delamerian assemblage. On Fig. 8a this field is quiteextensive and ranges from 0.02 to 0.67 we have chosen anintermediate composition at 0.50 (Table 1) as an appropriate bulkcomposition to reflect partial communication of the retrogressedandalusite porphyroblast with the remaining bulk rock. Although this

is a relatively arbitrary choice of composition the changes in theposition of phase boundaries does no vary significantly over the rangeof 0.02 to 0.67 therefore the topology of the resulting pseudosectionsand the position of the compositional isolpleths would not beexpected to vary significantly within this range.

8.1. Olarian P–T–t evolution

Considering the petrographic observations described in the previoussection, it can be concluded that the series ofmineral growth during theOlarian Orogeny was garnet→staurolite+andalusite. Garnet crystalscontain inclusion trails (Fig. 3b,c) suggesting that the onset of garnetgrowth post-dated the initial deformation (D1–D2). Inclusion trailswithin the garnets are commonly curvilinear and are rotated relative tothe preserved S3matrix foliation (Figs. 3b,c and 4d) implying that fabricdevelopmentwas continuous during initial garnet growth right throughto the development of the S3 fabric consistent with the presence ofgarnet along all parts of the P–T evolution shown in the pseudosection(Fig. 10). Sm–Nd data obtained from the first generation of garnetgrowth effectively constrain the timing of the event at ~1585 Ma(Fig. 7a–c).

A striking feature of garnet growth associated with the OlarianOrogeny is the lack of compositional zoning exhibited by garnetcrystals (Fig. 5). There are two possible mechanisms to explain theabsence of compositional zoning in Olarian garnet crystals. The firstinvolves an initial stage of garnet porphyroblast growth in equilib-rium with the bulk composition along a prograde P–T path controlledby Raleigh fractionation and the development of smoothly curvedvariations in cation compositions across the grain (e.g. Loomis, 1983).The curved compositional profiles developed during Raleigh fraction-ation would then be modified by intracrystalline diffusional processeswhich occur at or near the peak metamorphic conditions experiencedby the rocks (e.g. Chakraborty and Ganguly, 1991; Chakraborty andGanguly, 1992). The second mechanism that could result in theabsence of compositional zoning in garnet involves the oversteppingof the garnet-in reaction and the nucleation of the majority of thegarnet at constant P–T–X conditions (e.g. Zeh and Holness, 2003).Theoretical calculations and experimental results (Brearley and Rubie,1990; Cygan and Lasaga, 1982; Ridley, 1986; Rubie and Thompson,1985) have demonstrated that mineral nucleation commonly requiresa significant overstep of the P–T conditions, that in effect allows thepersistence of metastable mineral assemblages (Zeh and Holness,2003). A consequence of this reaction overstep is that the composi-tional variation in garnets becomes flattened with increasing reactionoverstep. This is due to the fact that the garnet growth rate increaseswith the degree of reaction overstep due to the increasing potentialenergy of the reaction. As a result abundant garnet of uniformcomposition can be formed rapidly (Zeh and Holness, 2003). Onecaveat in this scenario is that the material supply during garnetgrowth is not modified by fractionation processes or refractoryreactant phases (Zeh and Holness, 2003).

The reaction overstep scenario is considered to be an unlikelymechanism for the generation of compositionally homogeneous garnetsas the curved inclusion trails in the garnets suggest a progressive growthcoupled with foliation development rather than garnet formed rapidlyby reaction overstep. In the case of the Olary Domain rocks thermally-driven volume diffusional homogenisation is the preferred mechanismto account for the observed profiles. Although at temperatures <600 °Cthis process is quite slow. For example if a peak temperature of ~580 °C isassumed, and using the diffusion equation of Cygan and Lasaga (1985),a garnet of 1000 μm diameter would take in the order of 40–50 Ma tohomogenise and develop a flat compositional profile. In the OlaryDomain, specifically the Northern Walparuta area, there is someevidence that the Olarian Orogeny was prolonged. Rutherford et al.(2007) report monazite ages from within unretrogressed andalusite


in the Northern Walparuta Inlier of ~1610Ma and 1560–1570 formonazite in retrograde mica.

Overgrowing the early garnet is a generation of large poikiloblasticstaurolite crystals (Fig. 3d) that are rimmed by coarse-grained biotite.The inclusions within the staurolite are consistent with the early D1–D2

associated fabrics, and staurolite also deflects the S3 foliation (Fig. 3d).The absence of chloritoid (predicted by the pseudosection) from therocks can be explained by the crossing of the discontinuous KFMASHreaction:

Fe−chloritoid→garnet + staurolite: ð1Þ

For the inferred bulk rock composition of NorthernWalparuta, thisreaction is represented by a narrow trivariant field that extends from2.5 to 4.2 kbar at around 520 to 530 °C (Fig. 9).

The growth of andalusite at the expense of staurolite is most likelyrelated to the up-temperature crossing of the discontinuous KFMASHreactions:

staurolite + bioitite→garnet + andalusite ð2Þ

Or

staurolite→garnet + andalusite + biotite: ð3Þ

In Fig. 9 these reactions are consistent with the crossing of thetrivariant garnet–plagioclase–staurolite–andalusite–biotite field attemperatures between 530 and 570 °C. In Fig. 9 the quadrivariantfield garnet–plagioclase–andalusite–biotite has been contoured (iso-pleths) for the molar proportions of grossular (Xgrs) and spessartine(Xsps) and the XFe content of the garnet. The intersection of theseisopleths, when compared to the garnet compositions in Table 3, canbe used to deduce the approximate conditions of garnet formation. Ascan be seen in Fig. 9 the intersection of the appropriate isopleths isconsistent with equilibrium garnet compositions at pressures ofc. 2.6–3.7 kbar and temperatures of 550–600 °C (area shaded red inFig. 9). We interpret the assemblage muscovite–biotite–andalusite–garnet±plagioclase to reflect the peakmetamorphic conditions in theTommie Wattie Bore and Northern Walparuta areas during theOlarian Orogeny. An upper temperature limit of between 600 °C isinferred due to the absence of sillimanite in the rocks (Fig. 10).

Clarke et al. (1995) describe the contemporaneous growth ofstaurolite and andalusite. Clarke et al. (1987) also describe thereplacement of andalusite by sillimanite suggesting higher tempera-tures were achieved in different parts of the Olary Domain than wereobserved in the samples of this study. This observation is consistentwith the replacement of andalusite by sillimanite and the occurrenceof migmatites in the Broken Hilll Domain (W. Collins, pers comm.).

8.2. Timing of retrogression of andalusite porphyroblasts

Andalusite porphyroblasts are interpreted to have grown prior tothe D3 event as they deflect the S3 fabric. Retrogression of theandalusite porphyroblasts tomuscovite–paragonite is thought to haveoccurred post D3 (Crooks and Webb, 2003) and may be associatedwith a late-stage fluid flow event (e.g. Clark et al., 2005). Position Amarked on the pseudosection in Fig. 9 represents the peak Tassemblage defined by the compositional isopleths of garnet. If wefollow the logic of Clarke et al. (1987) that following the peaktemperature the Olary Domain underwent a period of crustalthickening and burial (D3 event) the rocks in the Tommie WattieBore and Northern Walparuta areas would have experienced an uppressure evolution during the Olarian D3 event. Clark et al. (2005)suggested that the Olarian D3 event was accompanied by a fluidflow event that led to widespread retrogression of mineral assem-blages throughout the Olary Domain. Position B on Fig. 9 is represents

the approximate conditions of fluid infiltration as determined by Clarket al. (2005). The addition of fluid to the NW4 bulk composition couldpotentially lead to the retrogression of the andalusite porphyroblaststo muscovite and paragonite at the end of the Olarian Orogeny.

8.3. Delamerian P–T–t evolution

Recent work on the conditions of shear zone formation in thesouthern Curnamona Province have demonstrated peakmetamorphicassemblages which formed at conditions of between 530 and 600 °Cduring the c. 500 Ma Delamerian Orogeny (Clark et al., 2006a, b;Dutch et al., 2005). This prograde metamorphic regime is inferred tohave been in part linked to the thick sedimentation associated withthe development of the Adelaidean rift system between c. 700 and530 Ma and minor shortening during the Delamerian Orogeny (Dutchet al., 2005). To date the metamorphic expression of the DelamerianOrogeny has not been described in thewall rocks to these shear zones.The Sm–Nd garnet data for garnets growing within the retrogressedandalusite porphyroblasts tie metamorphic mineral growth to theDelamerian Orogeny (Rutherford et al., 2006).

In the Northern Walparuta area, the imposition of a domainalchemical structure and the resulting change of equilibriumvolume fromthewhole rock compositionused todescribe theOlarianmetamorphismto a more aluminous and less manganiferous chemistry (Fig. 10) is themajor controlling factor in the metamorphic mineral assemblagesassociated with Delamerian aged metamorphism. Delamerian meta-morphism in these areas is dominated by the post S3 overgrowth of thelate-Olarian andalusite porphyroblasts by the assemblage garnet–paragonite–staurolite–muscovite–chlorite±kyanite±chloritoid. Thesequence of mineral growth observed in the Northern Walparuta areais garnet→staurolite→kyanite whilst in the Tommie Wattie Bore areathe sequence is garnet→staurolite→fibrolite. The peak metamorphicassemblage (M2) for both areas is garnet–staurolite–muscovite–biotite–paragonite–quartz. The peak metamorphic conditions associated withthis event can be effectively constrained by garnet compositions(Table 3) and the corresponding isopleth (lines of equal chemicalcomposition) intersections on the associated P–T pseudosection(Fig. 10). The intersection of XFe and Xsps isopleths (area shadedred in Fig. 10) in the quadrivariant assemblage garnet–paragonite–chloritoid–biotite–muscovite–quartz indicating peak conditions ofbetween 5.2 and 5.8 kbar and 510 to 530 °C during the DelamerianOrogeny is consistent with estimates from the late stage shearzones (Dutch et al., 2005). As previously mentioned the presence ofZn in the rock may expand the stability field of stauroliteexplaining the growth of staurolite at slightly lower temperaturesthan predicted by the calculated pseudosection in Fig. 10. The lategrowth of kyanite, which was used to infer the anticlockwise P–Tpath for the Olarian Orogeny by Clarke et al. (1997), is onlyobserved in the Northern Walparuta area (Fig. 3) and is interpretedto reflect a more aluminous bulk composition as represented in thepseudosection in Fig. 8a. The presence of late kyanite in the OlaryDomain has been tied to Delamerian aged metamorphism in thestudy of Dutch et al. (2005). The mostly post-kinematic over-growth of the Delamerian aged metamorphic assemblages suggeststhat burial metamorphism played a key role in generating thethermal energy required to initiate the onset of metamorphism.These findings are in agreement with previous structural studies(Paul, 1998; Paul et al., 2000) that found the bulk of shortening/deformation in the Proterozoic basement of Olary Domain duringthe Delamerian Orogeny was accommodated in pre-existingstructural weaknesses such as shear zones.

8.4. Implications for existing tectonic models

Both the textural and geochronological evidence preserved in themetapelitic rocks from the Tommie Wattie Bore and Northern


Fig. 9. P–T pseudosection calculated using the composition of the whole rock NW4 (Table 1). White circles A and B represent peak temperature conditions defined by theintersection of garnet isopleths as defined in this study and conditions of retrograde fluid infiltration and andalusite porphyroblast retrogression after Clark et al. (2005) respectively.


Walparuta areas point to the occurrence of two distinct and unrelatedmetamorphic events separated by a fluid flow event. These findingsrepresent a departure from the traditional view of the Olary Domainevolving through a classical anticlockwise P–T path linked tocompression with cooling (e.g. Clarke et al., 1987). The difficulties inseparating the two distinct metamorphic events present in the OlaryDomain are now beginning to be resolved through the application ofin-situ dating techniques allowing metamorphic assemblages to betemporally constrained (e.g. Dutch et al., 2005). The observation thattwo separate tectonothermal events were experienced by the rocks ofthe Olary Domain and the key finding that overprinting chloritoid,chlorite and paragonite assemblages, used by Clarke et al. (1987) todefine the anticlockwise evolution, most likely occurred at the end ofthe Olarian Orogeny. This observation suggests that anticlockwise P–Tpath invoked by Clarke et al. (1987) is still applicable. However, themetamorphic evolution of the southwestern Curnamona Provinceshould be viewed in the context of two distinct P–T paths, separatedby 1.0 Ga. The initial LP–HT evolution that is attributed to the OlarianOrogeny remains the same as that proposed by Clarke et al. (1987)followed by a period of retrogression most likely linked to crustalshortening, thickening and fluid infiltration contemporaneous withthe D3 event (e.g. Clark et al., 2005).

In a more recent and perhaps more controversial study on thetectonothermal evolution of the Olary Domain by Gibson and Nutman(2004) it was proposed that an early (1690–1670 Ma) metamorphic

Fig. 8. T–X pseudosections calculated for varying degrees of interaction between the compchemical composition (NW4; Table 1) calculated at pressures of (a) 5 kbar and, (b) 3 kbapseudosection in Fig. 10 is indicated by an arrow.

event associated with the development of a low angle extensionaldetachment surface and the development of a metamorphic corecomplex system. The core complex system was subsequently over-printed at 1600 Ma by shortening associated with the OlarianOrogeny. One of the key aspects of this model is that the peakmetamorphic conditions in the hypothesised lower plate occurred at1690–1670 Ma, whereas the upper plate remained essentiallyunmetamorphosed. The upper and lower plates are defined strati-graphically with the Curnamona Group comprising lower plate andthe rocks of the Strathearn Group the upper plate.

During the c. 1600 Ma Olarian Orogeny rocks of the lower platewould have been re-metamorphosed, while rocks belonging to theupper plate underwent theirfirst substantivemetamorphic event. Oneof the difficulties of applying a metamorphic test of this extensionalmodel and its subsequent re-working during the Olarian is that inmuch of the Curnamona Province the metamorphic intensity of theOlarian is such that any early metamorphism would be difficult toclearly identify. However in the southwestern Curnamona Province inthe TommieWattie Bore and NorthernWalparuta areas, metamorphicgrade at no stage exceeded temperatures of greater than 600 °C, andthere is essentially complete preservation of mineral texturesdeveloped through the entire metamorphic history of the rocks.

This model is currently the subject of vigorous debate on the basis ofexisting geochronological and stratigraphic datasets (see Conor et al.,2005; Gibson and Nutman, 2004). Gibson and Nutman (2004) contend

osition of a retrogressed andalusite porphyroblast (NW5; Table 1) and the whole rockr. The position representing the bulk rock chemical composition used to calculate the

Fig. 10. P–T pseudosection calculated using a composition intermediate between that of NW4 and NW5 (Table 1) and indicated by an arrow on Fig. 8a and b. Red shaded areaindicates peak conditions defined by the intersection of garnet compositional isopleths. (For interpretation of the references to color in this figure legend, the reader is referred to theweb version of this article.)


that there are two sequences, an upper and lower plate, separated by aregional scale detachment surface. Each plate has its own associatedmetamorphicmineral assemblagewith the lower plate being andalusitefree and the upper plate containing andalusite. The observations madein this study cast some doubt on the validity of the metamorphicarguments proposed by Gibson and Nutman (2004) and thereby on themodel for early extension and core complex formation proposed inthese studies.

Firstly, rocks forming both the upper (Tommie Wattie Bore) andlower (NorthernWalparuta) plateswere investigated in this study. Bothareas show a near identical metamorphic history recorded by thegrowth of successive porphyroblasts. Secondly, the earliest metamor-phic relationships preserved at each locality indicate that the earliestmetamorphic event, resulting in the growth of garnet and andalusiteporphyroblasts, was associated with compression and not extension assuggested byGibson andNutman (2004). This coupledwith our Sm–Ndgarnet geochronology indicating only one major metamorphic event atc.1580 Ma is in direct contrast to the conclusions drawn by Gibson andNutman (2004). Gibson and Nutman (2004) require two separateProterozoic P–T evolutions separated by 70–90 Ma, whereas both themetamorphic and geochronological evidence presented in this studycombined with the many geochronological studies (Page et al., 2005;Rutherford et al., 2007; Rutherford et al., 2006) performed throughoutthe terrane strongly suggest only one cycle ofmetamorphismduring theProterozoic.

9. Conclusions

The recognition that bulk rock compositions available for meta-morphic equilibration have evolved through time requires that the

metamorphic evolution of the Palaeoproterozoic rocks of the OlaryDomain to be revisited. This realisation, coupled with new studies(Dutch et al., 2005; Rutherford et al., 2007; Rutherford et al., 2006) inthe region highlighting the widespread effects of the DelamerianOrogeny suggests that the previous models for the tectonothermalevolution may require rethinking. In the light of these new findingsthe assertion that the P–T–t evolution of the Olary Domain follows asingle anticlockwise trajectory (Clarke et al., 1987) is shown to becorrect however this interpretation does not to take into account thepolymetamorphic nature of the terrane. Results from this study showthat the P–T–t history is better characterised by two separate P–Tpaths, separated by 1.0 Ga.

The conclusions from this study also call into question the validity ofmore recentmodels (Gibson and Nutman, 2004; Gibson et al., 2004) forthe style and nature of the terranes early history. The textural evidencepresented in this study indicate no evidence for: (1) differingmetamorphic histories and the timing of metamorphism between theupper and lower plates of the Gibson and Nutman (2004) model; and,(2) early metamorphism being associated with extension but ratherfinds that that the earliest metamorphic events are up-pressure and canbe better explained by compressional orogenic processes.

Acknowledgements

CC and MH would like to acknowledge the support of the AustralianResearch Council Linkage grant scheme in association with PrimaryIndustries and Resources, South Australia (PIRSA) for providing analyticaland field support for this project through grant LP0347584 andLP0560887. We are very grateful to comments on various versions ofthis manuscript from Geoff Clarke, Tim Johnson, Chris McFarlane and


Krishnan Sajeev. CC would like to thank The Institute for GeoscienceResearch (TIGeR) and Curtin University for a Targeted Research Fellow-ship. This is TIGeR publication number 275 and TRAX publication 19.

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Documents

Decoding Mesoproterozoic and Cambrian metamorphic events in Willyama Complex metapelites through the application of Sm–Nd garnet geochronology and P–T pseudosection analysis