Age constraints on terrane-scale shear zones in the Gawler Craton, southern Australia

Precambrian Research 139 (2005) 164–180

Age constraints on terrane-scale shear zones inthe Gawler Craton, southern Australia

G.M. Swaina,∗, M. Handa, J. Teasdaleb, L. Rutherforda, C. Clarka

a Continental Evolution Research Group, School of Earth and Environmental Sciences, University of Adelaide, SA 5005, Australiab FrOG Tech Pty Ltd., 6/50 Geils Court, Deakin West, ACT 2600, Australia

Received 28 October 2004; received in revised form 14 June 2005; accepted 16 June 2005

Abstract

Electron Probe Micro Analysis (EPMA) chemical dating of monazite from crustal scale shear zones in the late Archaeanto Mesoproterozoic Gawler Craton, southern Australia, highlights a long history of shear zone development associated withassembly and reworking of the craton. In the southeast Gawler Craton, dextral transpression was focussed into the >250 kmlong Kalinjala Shear Zone, which dominates the structural character of that part of the craton. Monazite in upper amphibolitefacies shear fabrics within the Kalinjala Shear Zone give a mean age of 1682± 10 Ma. This is indistinguishable from an age of1693± 16 Ma obtained from monazite inclusions within garnet porphyroclasts within the shear fabric. In the western GawlerCraton, monazite populations from two separate locations along the >400 km long Tallacootra Shear Zone give ages of 1680± 37and 1679± 39 Ma, respectively, suggesting that both the Kalinjala and Tallacootra Shear Zone systems record regional-scale

ri Faultprise the

ne haver Suitee NawacentralHiltaba

soungerr tosociated-scale

points to

strain partitioning toward the end of the ca. 1730–1700 Ma Kimban Orogeny. In the northwest Gawler Craton, the KaraZone can be traced for >700 km, and separates late Archaean rocks from an assembly of tectonic terranes that comnorthern and northwest Gawler Craton. Monazites from amphibolite-grade mylonites associated with the Karari Fault Zoa mean age of 1631± 12 Ma. This age is identical to the timing of regional arc-like magmatism represented by the St. Petein the central Gawler Craton, and suggests that exhumation of the ca. 1650–1640 Ma ultra high-grade granulites of thDomain in the northwest Gawler Craton was linked to the development of the St. Peter Suite magmatic margin. In theGawler Craton, monazite in the >200 km long Yerda Shear Zone, which sheared granites belonging to the ca. 1580 MaSuite, have a mean age of 1503± 16 Ma. Similar ages of 1516± 18 and 1508± 14 Ma are obtained from monazite inclusionin garnet porphyroclasts within mylonites along the >300 km long Coorabie Fault Zone in the western Gawler Craton. Yages of 1468± 12 and 1457± 22 Ma from the amphibolite-grade mylonitic fabrics in the Coorabie Fault Zone are simila40Ar–39Ar ages from the western Gawler Craton, and suggest that reactivation of the Coorabie Shear Zone in part was aswith regional cooling and stabilisation of the Gawler Craton at ca. 1450 Ma. The history of deformation on the terraneshear zones in the Gawler Craton therefore does not reflect a single period of terrane assembly or reworking, rather ita complex pattern of strain partitioning during tectonic events in the late Palaeoproterozoic to early Mesoproterozoic.© 2005 Elsevier B.V. All rights reserved.

Keywords: EPMA; Gawler Craton; Monazite; Proterozoic; Shear zones

∗ Corresponding author. Tel.: +61 8 83034971; fax: +61 8 83034347.E-mail address: [email protected] (G.M. Swain).

0301-9268/$ – see front matter © 2005 Elsevier B.V. All rights reserved.doi:10.1016/j.precamres.2005.06.007

G.M. Swain et al. / Precambrian Research 139 (2005) 164–180 165

1. Introduction

Unravelling the timing of tectonothermal events inmultiply reworked terranes is critically important indeveloping models that describe the kinematic andthermal history of the terrane (e.g.Foster and Ehlers,1998; Johnson and Kattan, 2001; Spikings et al., 2002;Goscombe et al., 2003). This is particularly impor-tant in terranes that contain regional-scale systems ofshear zones, since the linking of individual shear zonescan appear to form a continuous shear zone array thatmay erroneously invoke a regionally coherent defor-mation system (e.g.Tavarnelli and Holdsworth, 1999,and references therein). In regions of extensive out-crop, direct observations of overprinting relationshipsconstrain distinct structural events, however in poorlyexposed terranes, determination of the absolute tim-ing of deformation within shear zone systems is one ofthe few tools available to constrain the tectonic history.In addition, there has been a growing awareness that insitu age dating is a powerful approach that preserves themicrostructural and metamorphic relationships. In par-ticular in situ chemical U–Th–Pb dating of monazite bythe EPMA method (e.g.Montel et al., 1996) has beenestablished as a valuable chronometer for many geolog-ical processes (e.g.Cocherie et al., 1998; Williams etal., 1999; Shaw et al., 2001). Monazite is characterisedby comparatively high U–Th contents and low Pb dif-fusivities, leading to in-growth of readily measurableamounts of radiogenic Pb that largely dilute any inher-i ;P ica

ataf fab-r thad eant us-t di-v ingt og-i ,1 001F lyc gthst bieF ne;F ter-

ranes, such as the Tallacootra Shear Zone (Teasdale,1997), highlighting their importance in the tectonichistory of individual domains. Despite the obviousscale and significance of the shear zones within theGawler Craton, there are few direct constraints onthe timing of deformation of the tectonic regimesthat controlled their evolution (e.g.Teasdale, 1997;Ferris et al., 2002). The best understood is the Kalin-jala Shear Zone system in the southeast Gawler Craton,which evolved during dextral transpression at around1730–1700 Ma (Hand et al., 1995; Vassallo and Wilson,2002). The strike extent and scale of the shear zones inthe Gawler Craton is analogous to modern orogenicsystems involving plate-scale processes (Jones andStrachan, 2000; Johnson and Kattan, 2001; Vassalloand Wilson, 2002). Therefore, the tectonic frameworkof the Gawler Craton may hold important clues forunderstanding the larger-scale evolution of ProterozoicAustralia.

2. Geological setting

2.1. Regional Geological framework of theGawler Craton

The Gawler Craton (Fig. 1a) is a geologicallycomplex late Archaean to Mesoproterozoic terrane(Thomson, 1975; Parker, 1990, 1993; Teasdale, 1997;

inend

jor-the

nicro-

aalas-

een

dichre-. Aast

ted common Pb (Williams et al., 1983; Corfu, 1988arrish, 1990), providing a powerful tool for tectonnalysis.

In this paper, we present EPMA monazite drom mid to upper amphibolite facies shear zoneics for the system of terrane-scale shear zonesominate the structural character of the late Archa

o Mesoproterozoic Gawler Craton in southern Aralia (Fig. 1a). The Gawler Craton has been subided into a number of tectonic domains accordo varying magnetic, gravity, structural, geochronolcal, isotopic and geochemical character (Thomson980; Teasdale, 1997; Stewart and Foden, 2erris et al., 2002), the boundaries to which commonorrespond to major shear zones with strike lenypically >200 km (e.g. Karari Fault Zone, Cooraault Zone, Yerda Shear Zone, Kalinjala Shear Zoig. 1a). Many shear zones also crosscut older

t

;

Daly et al., 1998; Ferris et al., 2002; Swain et al.,preparation) which is relatively poorly understood duto extensive sedimentary cover. Tectonothermal acrust forming events can be grouped into two matime intervals of tectonic activity and are briefly summarised below. The shear zones systems that arefocus of this paper formed during the second tectocycle, in the interval spanning the late Palaeoprotezoic to early Mesoproterozoic.

The first period of tectonism began at ca. 2560 Mwith arc-like magmatism and deposition of basinsequences (including greenstone packages) on anyet unidentified basement, interpreted to have bclose to an evolving convergent margin (Swain et al.,in preparation). Basin development was terminateby the ca. 2500–2400 Ma Sleafordian Orogeny, whconsolidated the proto-Gawler Craton, and is repsented by the Sleaford and Mulgathing Complexesperiod of tectonic quiescence then ensued for at le

166 G.M. Swain et al. / Precambrian Research 139 (2005) 164–180

Fig. 1. (a) Interpreted subsurface geology of the Gawler Craton, South Australia (modified from:Daly et al., 1998; Ferris et al., 2002; Skirrowet al., 2002), highlighting major lithologies, crustal scale shear zones and sample localities. (b) Greyscale first vertical derivative of totalmagnetic intensity (TMI) from the western Gawler Craton, highlighting shear zone relationships and the four blocks of the ca. 1540 Ma FowlerDomain (modified from:Teasdale, 1997). Lower crustal granulites of the Nundroo Block and adjacent high-grade shear zone-bounded blocksare juxtaposed against comparatively un-metamorphosed ca. 1630–1610 St. Peter Suite along the Coorabie Fault Zone.


400 Ma (Parker, 1993; Daly et al., 1998; Ferris et al.,2002; Swain et al., in preparation).

The second major period of tectonism began ataround 1900 Ma, marking the onset of around 400 Maof granitic magmatism, basin formation and medium tohigh-grade regional deformation. The first major eventin this interval was at ca. 1850 Ma along the easternmargin of the craton, encompassing batholithic-scalefelsic magmatism and amphibolite to granulite-grademetamorphism (Mortimer et al., 1988; Parker et al.,1993; Daly et al., 1998). This 1850 Ma tectonic sys-tem was reworked during the Kimban Orogeny overthe interval ca. 1730–1700 Ma. On the eastern marginof the Gawler Craton, the Kimban Orogeny recordsbulk non-coaxial deformation associated with dex-tral transpression (Hand et al., 1995; Vassallo andWilson, 2002), and was largely responsible for shap-ing the current crustal architecture. Elsewhere in theGawler Craton, the effects of the Kimban Orogeny aremanifest by deformation and mafic to felsic magmatismof the ca. 1690–1680 Ma Tunkillia Suite (Teasdale,1997; Fanning, 1997; Daly et al., 1998; Ferris et al.,2002). However, aside from the southeast Gawler Cra-ton, the evolution of the Kimban Orogeny is poorlyunderstood.

In the western Gawler Craton, the poorly understoodKararan Orogeny (ca. 1650–1540 Ma) is marked byultra high-T metamorphism (>950◦C and >9.5 kbar)in the Nawa Domain (Fig. 1a) at around 1653± 8 Ma(Teasdale, 1997; Daly et al., 1998). The KararanO ec-t t oft c tog ra-t2 r-w enya bytn ranO

ajort Mar sic,a g toHI wasa 0 Ma

(e.g.Parker, 1990; Haynes, 2002), whereas in the cen-tral part of the craton, slightly younger compressionaldeformation occurred along crustal-scale shear zonesincluding the Yarlbrinda Shear Zone (e.g.Ferris, 2001)during the latter stages of the Hiltaba Event. In the cen-tral Gawler Craton, the Hiltaba Suite also constrains themaximum possible age of deformation on the YerdaShear Zone (Fig. 1a), since it deforms plutons belong-ing to the Hiltaba Suite (e.g.McLean and Betts, 2003).

The compressional regime that marked the end ofthe Hiltaba Suite appears to have focussed into atranspressional belt that reworked parts of the north-ern and western Gawler Craton in the interval ca.1565–1540 Ma (Daly et al., 1998; Ferris et al., 2002).In the Cooper Pedy Domain and the Fowler Domain,this transpressional event was associated with regionalhigh-grade metamorphism (Fanning, 1997; Teasdale,1997; Daly et al., 1998; Ferris et al., 2002). Teasdale(1997)andDaly et al., (1998)suggested that many ofthe major shear zone systems (e.g. Tallacootra, Coora-bie and Karari) that dominate the western GawlerCraton were active during this time; however the dis-tribution of deformation during the ca. 1565–1540 Mainterval is still poorly understood, and there have beenno age constraints obtained from the shear zone sys-tems.

2.2. Structural and metamorphic character of theGawler Craton shear zones

2

b stt ng at geo-p tos otra( artz-p -lL byi romc liticm an-u aeo-pb bricc

rogeny may have formed part of an evolving tonic system that culminated with the emplacemenhe voluminous, syn-deformational, arc-like tonalitiranodioritic St. Peter Suite in the central Gawler C

on (Fig. 1a) between ca. 1630 and 1610 Ma (Ferris,001; Ferris et al., 2002). However, regions that undeent high-grade tectonism during the Kararan Orogre separated from the bulk of the Gawler Craton

he craton-scale Karari Fault Zone (Fig. 1a) and it isot clear if there is a wider expression of the Kararogeny within the craton.In the eastern and central Gawler Craton, a m

hermal event over the interval ca. 1595–1575esulted in the emplacement of voluminous felnd subordinate mafic, magmatic rocks belonginiltaba Suite and Gawler Range Volcanics (Fig. 1a).

n the eastern Gawler Craton, the Hiltaba Eventssociated with extensional deformation at ca. 159

.2.1. Western Gawler CratonThe Tallacootra Shear Zone (TSZ;Fig. 1a and

, Teasdale, 1997) is defined by a set of northeao eastwest trending, anastomosing fabrics definierrane 5–10 km wide zone that can be tracedhysically for over 700 km from the northeastouthwest parts of the craton. At Lake TallacoFig. 1a), the shear zone contains fine-grained qulagioclase-biotite± garnet± epidote mylonite inter

ayered with mylonitic granitic gneisses (Fig. 2). Atake Ifould (Fig. 1a) the shear zone is defined

ntensely deformed granitic gneisses derived fa. 1690–1680 Ma Tunkillia Suite, and metapeylonites consisting of reworked metapelitic grlites belonging to the late Archaean to early Palroterozoic Mulgathing Complex (Teasdale, 1997). Inoth locations the northeast-trending mylonitic faontains a steeply plunging stretching lineation (Fig. 2),


Fig. 2. Representative outcrop of the Tallacootra Shear Zone, LakeTallacootra.

with kinematic indicators (S–C fabrics and porphyro-clasts) indicating east side up dip-slip movement. How-ever, regional geophysical datasets show dextral trans-port into the shear zone, suggesting that the TallacootraShear Zone is a macroscopic dextral transpressionalstructure. Mineral assemblages within the shear zoneindicate deformation occurred at medium-pressureamphibolite-grade conditions of around 600◦C and6 kbar (Teasdale, 1997).

The Coorabie Fault Zone (CFZ;Fig. 1a,Teasdale, 1997) delineates the boundary betweenthe Fowler/Nuyts Domains and the Christie/WilgenaDomains and can be traced for >300 km into thenortheast Gawler Craton. To the south, the shearzone can be traced geophysically into the currentcontinental shelf, suggesting its continuation wouldbe present on the northern coast of Terra Adelie Landin Antarctica (e.g.Oliver and Fanning, 1997, 1999;

Peucat et al., 1999, 2002; Teasdale et al., 2003). In thesouthwest part of the craton, the shear zone is definedby a set of anastomosing terranes with a total width of∼10 km. South of Mt. Christie near Wynbring Siding(Fig. 1a), the Coorabie Fault Zone is dominated byfelsic lithologies ranging from protomylonitic granitegneiss to slatey ultra-mylonite. The mylonites are fineto very fine grained, with a dynamically recrystallisedquartzo-feldspathic groundmass, and relict garnetand feldspar porphyroclasts. In the southwesternGawler Craton, where the Coorabie Fault Zone crossesthe coast, metapelitic mylonitic and ultra-myloniteformed at lower amphibolite grade conditions ofaround 500◦C and 4 kbar (Teasdale, 1997). Thenortheast-trending mylonitic fabric contains a steeplyplunging elongation lineation, with kinematic indi-cators indicating west side up dip-slip movement.However, geophysical data shows large-scale sinistraltranslations into the shear zone, implying it is amacroscopic transpressional structure. The magnitudeof displacement is difficult to estimate, however themovement on the Coorabie Fault Zone exhumed lowercrustal (8–10 kbar) granulites of the Nundroo Block(Teasdale, 1997), and juxtaposed them with midcrustal rocks belonging to the Nuyts Domain (Daly etal., 1998), implying at least 10–15 km of differentialuplift (Teasdale, 1997).

The Karari Fault Zone (KFZ;Fig. 1a) has beendescribed in detail byRankin et al. (1989). It isassociated with a 450 km long linear aeromagnetica thec d byqm earz aringg est( 02f ockst r zonesme enm malya rlay-e tites,i ationw ord

nomaly that separates Nawa Domain, fromentral parts of the craton. The shear zone is defineuartz-feldspar-biotite-magnetite-garnet± sillimaniteylonitised gneiss. At its southwest tip, the sh

one separates 1650 Ma quartz + sapphirine-beranulites in the Nawa Domain to the northwTeasdale, 1997; Daly et al., 1998; Ferris et al., 20),rom late Archaean and early Palaeoproterozoic ro the southeast. Farther to the northeast, the sheaeparates high-grade Mesoproterozoic (1565± 8 Ma)etamorphic rocks of the Cooper Pedy Domain (Dalyt al., 1998; Ferris et al., 2002) from terranes in thorthern Gawler Craton (Fig. 1a). Magnetite-richetasediments produce the aeromagnetic anossociated with the Karari Fault Zone, and are intered with psammopelitic gneisses and thin pegma

nterpreted to have been emplaced during deformithin the shear zone. Kinematic indicators rec


sinistral strike-slip movement (Rankin et al., 1989),consistent with aeromagnetic data, which showssinistral translations into the shear zone.

2.2.2. Eastern Gawler CratonThe Kalinjala Shear Zone (KSZ;Fig. 1a) is a

>250 km long north to northeast trending high gradedextral transpressional shear zone that formed duringthe ca. 1730–1700 Ma Kimban Orogeny (Parker,1980; Hand et al., 1995; Vassallo and Wilson, 2002).The southern extension of the shear zone can also betraced geophysically into the current continental shelf,suggesting continuation on the northern coast of TerraAdelie Land in Antarctica (e.g.Oliver and Fanning,1997, 1999; Peucat et al., 1999, 2002; Teasdale et al.,2003). At its northern end, it is unconformably over-lain by felsic volcanics belonging to the ca. 1590 MaGawler Range Volcanics. The shear zone is a steeply-dipping structure up to 4 km wide that separatesthe 1850 Ma Donington Suite granitic terrane from1900–1850 Ma metasedimentary rocks belonging tothe Hutchison Group, and metasediments belongingto the late Archaean to early PalaeoproterozoicSleaford Complex (Parker et al., 1993; Swain et al., inpreparation). In the core of the shear zone, migmatiticgranulite-grade mylonites derived from the DoningtonSuite and pre-deformation mafic dyke swarms, recorddeformational conditions of 800◦C and 10 kbar (Handet al., 1995) during the formation of a gently northor south-plunging mineral elongation lineation. Thew rnet-s icr omt e dram ew .

2s

r zoicM kil-l f theN earz ani( aH jor( for-

mation interpreted as a major zone of syn-magmaticdextral strike-slip movement that facilitated ascentof Hiltaba Suite granite, bound to the south by theOolabinnia Shear Zone (Fig. 1a; McLean and Betts,2003). At its western end, the shear zone is truncatedby the Coorabie Fault Zone (Fig. 1a and b).

3. Methods

3.1. Scanning electron microscopy (SEM)

Monazites were identified in thin section both pet-rographically, and on a Philips XL30 FEGSEM atAdelaide Microscopy, University of Adelaide. Mul-tiple texturally suitable monazites were selected andscanned at operating conditions of 15 kV to producebackscatter electron (BSE) images, which detect inter-nal zonation or compositional heterogeneity.

3.2. Electron Probe Micro Analysis (EPMA)

Electron Probe Micro Analysis (EPMA) of mon-azites was undertaken on a CAMECA SX51 ElectronProbe Micro Analyser at Adelaide Microscopy, Uni-versity of Adelaide. Th–U–Pb including an additionalsuite of elements (Ca, P, Y, La, Ce, Pr, Nd, Sm, Gd,Dy, Er, Si, Al) were analysed under standard operat-ing conditions using an accelerating voltage of 20 kVand beam current of 60 nA. A PAP correction pro-g dP Uw inesPN h, Ua posi-t witho sec-o yr ta redb anda aly-s AD)( us-t ei .K

estern margin of the shear zone contains gaillimanite-biotite-quartz± spinel-bearing metapelitocks belonging to the Hutchison Group. Away frhe shear zone, metamorphic pressures decreasatically (Hand et al., 1995), indicating the shear zonas a focussed region of lower crustal exhumation

.2.3. Central Gawler CratonThe Yerda Shear Zone (YrSZ;Fig. 1a) separate

ocks of the late Archaean to early Palaeoproteroulgathing Complex from ca. 1690–1680 Ma Tun

ia Suite and ca. 1630–1610 Ma St. Peter Suite ouyts Domain in the central Gawler Craton. The shone is characterized by foliated granodiorite, withnterpreted U–Pb crystallisation age of 1592± 11 MaFerris, 2001), belonging to the ca. 1595–1575 Miltaba Suite. It forms the northern margin of a ma

∼250 km long) east–west trending corridor of de

-

ram was used to correct matrix effects (Pouchou anichoir, 1985). Total counting times for Pb, Th andere 320, 160 and 80 s, respectively using X-ray lbMβ, ThMα and UMβ. Huttonite (ThSiO4), UO2 andBS824 Pb standards were used for calibrating Tnd Pb, respectively. Background measurement

ions were selected so as to minimise overlapsther elements, and the spectral interference of thend order Ce escape peak on PbMβ was corrected for beducing Pb concentration by 100 ppm (c.f.Kelsey el., 2003). Assessment of the reliability was monitoy routine analysis of an in-house standard prior tofter analysis of the unknown sample. Repeat anes of in-house standard Madagascan monazite (MSHRIMP standard at Curtin University, Western Aralia) yielded an age of 512± 10 Ma (95% confidencnterval) compared to a SHRIMP age of∼511 Ma (Pinny, personal communication).


3.3. Age calculation

Age calculations were made using the techniqueof Montel (1996). Based on the assumptions that: (1)essentially all measured Pb is a product of the radio-genic breakdown of Th and U; and (2) there is noalteration of Th, U and Pb ratios by subsequent leadloss or thermal resetting (Parrish, 1990; Montel et al.,1996), an age can be calculated for each analysis fromthe equation:

Pb=(

Th

232

)(eλ232τ − 1)208+

(U

238.04

)0.9928

× (eλ238τ − 1)206+(

U

238.04

)0.0072

× (eλ235τ − 1)207

(Pb, Th and U in ppm;�232, �238 and�235 are decayconstants of Th232, U238 and U235, respectively). Theage equation is solved iteratively by entering ageestimates into the equation with the known concen-trations of Th, U, until the calculated Pb matchesthe measured Pb (Williams and Jercinovic, 2002).Individual spot age uncertainties were calculated bypropagating counting errors through the age equa-tion. Once ages and errors were calculated for eachindividual spot analysis, population ages were calcu-lated using the weighed average function in Isoplotversion 3.0 (Ludwig, 2003). Weighted average agesq htedl orc

4

ticalc nol-o ralr ,t di-vr nce(

zitez ho-r

Thorium contents can vary significantly, and because itis a heavy element, has a significant effect on the bright-ness of BSE imagery (e.g.Zhu and O’Nions, 1999).However, it should be noted that compositional zoningneed not signify age zoning (e.g.Zhu and O’Nions,1999).

4.1. Tallacootra Shear Zone (TSZ)

Sample WGC101 from Lake Tallacootra (Fig. 1a)is a reworked granulite derived from the ca.2560–2400 Ma Mulgathing Complex (Teasdale, 1997).It contains a primary garnet-cordierite-sillimanite-K-feldspar-quartz assemblage that has been partlyreplaced by a foliated syn-shear zone assemblageconsisting of orthoamphibole-kyanite-garnet-quartz(Fig. 3a) that formed at around 600◦C and 6 kbarduring near isothermal compression (Teasdale, 1997).Monazite is distributed throughout both the primaryand secondary mineral assemblages, and, in places,acted as a nucleation site for orthoamphibole replace-ment of cordierite (Fig. 3a). Monazite inclusions withincordierite characteristically display reverse concen-tric zoning with light cores and dark rims (Fig. 4a),reflecting Th contents of∼4–5 and∼2–3 wt.%, respec-tively, while monazite in the foliated secondary min-eral assemblage generally has light patchy zoning, andsimilar ranges in Th concentration (Fig. 4b). Zoningin these monazites gives rise to a distinctive bimodalage population. In total, 181 analyses were obtainedf a-c earm com-b kz binet

c lasem 0a e-d them yc plei ys dala1 s,r

uoted in this study are at 95% confidence. Weigeast squares± 2� (MSWD) ages are reported fomparison.

. Results

This section presents a summary of the three criomponents of EPMA chemical monazite geochrogy. Firstly, the petrographic and microstructuelationships involving monazite (Fig. 3). Secondlyhe textural and chemical characteristics of inidual monazites (Fig. 4, Table 1). Thirdly, theespective EPMA ages reported at 95% confideTable 1, Fig. 6).

For further reference, bright and dark monaones are generally a reflection of high and low Tium contents, respectively (Zhu and O’Nions, 1999).

rom Sample WGC101. The light cores of both intrordierite monazite and monazite within the syn-shineral assemblage have comparable ages andined give 2342± 21 Ma (n = 121), while the darones from monazite in both textural settings como give 1680± 37 Ma (n = 60;Table 1).

Sample WGC98 from Lake Ifould (Fig. 1a)ontains a garnet-biotite-muscovite-quartz-plagiocylonitic assemblage that formed at around 60◦Cnd 6 kbar (Teasdale, 1997). Monazite grains are prominantly located within biotite, which definesineral elongation lineation (Fig. 3b), and displa

oncentric zoning as well as complex and simntergrowth-like zoning (Fig. 4b). Th contents varimilarly to WGC101, as does the comparable bimoge distribution giving 2327± 37 Ma (n = 34) and679± 39 Ma (n = 38;Table 1) for light and dark zoneespectively.


Fig. 3. Backscattered electron (BSE) images of textural setting of representative monazite. (a) WGC101 TSZ, monazite distributed bothwithin the primary garnet-cordierite-sillimanite-K-feldspar-quartz assemblage, and within the secondary shear zone assemblage consisting oforthoamphibole-kyanite-garnet-quartz. In places, monazite within primary mineral assemblage acted as a nucleation site for orthoamphibolereplacement of cordierite. (b) WGC98 TSZ, monazite in biotite, which defines the mineral elongation lineation, within a garnet-biotite-muscovite-quartz-plagioclase mylonitic mineral assemblage. (c) 102832 KSZ, monazite included in garnet. (d) 1028108 KSZ, monazite inbiotite, which defines the foliation adjacent to feldspar and garnet porphyroblasts. (e) Ool3 KFZ, monazite located in mylonitic matrix of biotiteand dynamically recrystallised quartz and feldspar. (f) Ool1 Nawa Domain granulite, monazite adjacent to garnet and as isolated inclusionswithin perthite. (g) NDR5 CFZ, monazite within garnet. (h) NDR5a CFZ, monazite within mylonitic biotite-sillimanite foliation. (i) WGC24CFZ, monazite within garnet. (j) WGC24a CFZ, monazite within biotite-muscovite-plagioclase-quartz mylonitic matrix. (k) GCT1 YrSZ,monazite located in a quartz-plagioclase-biotite-muscovite bearing granitic gneiss. (l) GCT1a YrSZ, monazite within the foliated granite. (mnz,monazite; cd, cordierite; oamp, orthoamphibole; ky, kyanite; plag, plagioclase; qtz, quartz; bi, biotite; mus, muscovite; sill, sillimanite; k-spar,K-feldspar; mag, magnetite).


Fig. 4. Backscattered electron (BSE) images of representative monazites. Bright and dark monazite zones reflect high and low Thorium contents,respectively (Zhu and O’Nions, 1999). (a) WGC101 TSZ, reverse concentric zoning with light core and dark rims, reflecting distinctive bimodalage population. (b) WGC98 TSZ, complex and simple intergrowth zoning reflective of bimodal age population. (c) 102832 KSZ, weak patchyzoning reflective of minor chemical variation. (d) 1028108 KSZ and (e) Ool3 KFZ, unzoned chemically homogeneous monazite. (f) Ool1Nawa Domain granulite, distinct complex concentric zoning. (g) NDR5 CFZ, comparatively chemically homogeneous monazite with weakreverse concentric zoning. (h) NDR5a CFZ, chemically homogenous monazite elongate with the mylonitic foliation. (i) WGC24 CFZ, unzonedmonazite within garnet. (j) WGC24a CFZ, simple concentric zoned monazite with unimodal age population. (k) GCT1 and (l) GCT1a YrSZ,patchy zoned monazite.

4.2. Kalinjala Shear Zone (KSZ)

Samples 102832 and 1028108 are from MineCreek on southern Eyre Peninsula (Fig. 1a). Bothsamples are garnet-biotite-plagioclase-quartz-K-

feldspar mylonites from the western central part of theKalinjala Shear Zone. Alternating layers, which definethe foliation are rich in quartz and feldspar, adjacentto garnet porphyroclasts up to 1.5 cm in diameter thatare wrapped by a biotite-sillimanite-bearing fabric.


Table 1Summary of EPMA chemical monazite age data for shear zones of the Gawler Craton

Sample Location n Wt.% range Age± error (Ma)

Pb Th U 95% conf. Wlsq± 2σ (MSWD)

WGC 101 Lake Tallacootra, TSZ,bimodal

60 0.19–0.87 2.53-6.79 0.03–0.23 1680± 37 1678± 34 (1.14)

121 0.27–1.05 2.41–5.99 0.14–1.3 2342± 21 2342± 19 (1.02)WGC 98 Lake Ifould, TSZ, bimodal 38 0.27–1.14 3.32–7.2 0.13–1.51 1679± 39 1668± 38 (1.4)

34 0.27–1.1 1.68–7.21 0.17–1.41 2327± 37 2316± 36 (1.01)102832 Mine Creek, KMZ, mnz in gt 60 0.43–0.75 3.36–8.15 0.37–0.81 1693± 16 1689± 15 (1.3)1028108 Mine Creek, KMZ, mnz in

foliation170 0.55–0.70 4.61–6.18 0.36–1.09 1682± 10 1679± 9 (1.3)

Ool 3 Ooldea DDH3 220.2m, KFZ,mnz in foliation

201 0.34–0.69 3.18–4.69 0.23–1.26 1631± 12 1628± 12 (0.79)

Ool 1 Ooldea DDH1 288.7m,granulite

64 0.55–1.17 5.73–13.96 0.21–1.29 1640± 12 1638± 12 (0.89)

NDR 5 Nundroo DDH5 45.5m, CFZ,mnz in gt

96 0.24–0.50 3.20–5.32 0.19–0.38 1516± 18 1513± 21 (0.64)

NDR 5a Nundroo DDH5 45.5m, CFZ,mnz in foliation

144 0.32–0.52 4.26-7.27 0.14-0.43 1468± 12 1466± 16 (0.64)

WGC 24 Cape Adieu, CFZ, mnz in gt 110 0.31–0.46 2.22-3.90 0.42-1.02 1508± 14 1508± 17 (0.57)WGC 24a Cape Adieu, CFZ, mnz in

foliation39 0.27–0.73 2.85-5.27 0.18-1.50 1457± 22 1466± 28 (0.5)

GCT 1 Foliated Hiltaba Suite, YrSZ 107 0.15–0.55 2.07-6.24 0.02-0.47 1567± 24 1555± 22 (0.88)GCT 1a Foliated Hiltaba Suite, YrSZ 160 0.29–0.46 3.61-5.92 0.11-0.24 1503± 16 1493± 16 (0.88)

n = number of spot analysis. Wlsq± 2σ (MSWD) ages (Ma) tabulated for comparison.

Monazite occurs as rare inclusions in garnet (Fig. 3c),but predominantly occurs in biotite in the myloniticmatrix (Fig. 3d). Monazite displays minor chemicalvariation, reflected in the BSE images that showlittle zonation to weak patchy zonation (Fig. 4c andd). The age of monazite from garnet inclusions is1693± 16 Ma (n = 60; 102832, Table 1), while theage of monazite in biotite aligned with the foliation is1681± 9 Ma (n = 170; 1028108,Table 1).

4.3. Karari Fault Zone (KFZ)

Sample Ool3 is from drill hole Ooldea DDH 3which intersects the Karari Fault Zone (Rankin et al.,1989) towards its western end (Fig. 1a). Ool 3 is a lay-ered garnet-biotite-muscovite-magnetite-plagioclase-K-feldspar-quartz-bearing mylonitic gneiss. Garnet,plagioclase and K-feldspar occur as porphyroclasts,and are enclosed by a matrix of biotite and dynam-ically recrystallised quartz and feldspar. Monazite inthe mylonitic matrix (Fig. 3e) displays homogeneouschemistry with Th contents of∼3–5 wt.% (Fig. 4e)and gives a combined age of 1631± 12 Ma (n = 201;

Table 1). Sample Ool1 from drill hole Ooldea DDH 1(∼30 km northwest of the Karari Fault Zone and drillhole Ooldea DDH 3) is a garnet-sillimanite-cordierite-magnetite-biotite-perthite-bearing granulite. Monaziteoccurs adjacent to garnet and as isolated inclu-sions within perthite (Fig. 3f). Geochemically, Ool1monazites have distinct ranges in Th content from∼5.7–7.5 wt.% for dark cores and∼10.5–14 wt.%for light coloured rims, reflected in the BSE imageswhich are characterised by complex concentric zon-ing (Fig. 4f). Calculated ages from geochemically dis-tinct domains are indistinguishable, and combined give1640± 12 Ma (n = 64;Table 1).

4.4. Coorabie Fault Zone (CFZ)

Sample NDR5 consists of a mylonitic garnet-biotite-sillimanite-K-feldspar-quartz schist from drillhole Nundroo DDH 5, which intersects the CoorabieFault Zone in the western Gawler Craton (Fig. 1a).Monazite occurs as inclusions within porphyroclasticgarnet, and also as grains within the mylonitic matrix.A large (∼600�m) diameter, comparatively chemi-


cally homogeneous monazite inclusion within garnet(NDR 5;Figs. 3g and 4g) gave an age of 1516± 18 Ma(n = 96;Table 1). Monazite grains within the myloniticbiotite-sillimanite foliation (NDR5a) are individuallychemically homogeneous (Figs. 3h and 4h), but rangein Th content between∼4.3–7.3 wt.% across differ-ent grains. The age of monazite in the foliation is1468± 12 Ma (n = 144;Table 1).

Sample WGC24 is a garnet-sillimanite-biotite-muscovite-plagioclase-K-feldspar-quartz mylonitefrom Cape Adieu (ca. 40 km south of Sample NDR5),where the Coorabie Fault Zone crosses the coastlineof the western Gawler Craton (Fig. 1a). As withSample NDR5, monazite occurs as inclusions ingarnet porphyroclasts, and also within the myloniticmatrix. A total of 149 analyses were obtained from thesample. Monazite within garnet (WGC24, Fig. 3i)is chemically homogenous (Fig. 4i) and gives an ageof 1508± 14 Ma (n = 110; Table 1). Monazite withinthe mylonitic matrix (WGC24a, Fig. 3j) displayssimple concentric zoning in the BSE images (Fig. 4j),reflecting the range of Th between∼2.8 and 5.3 wt.%.The age of monazite in the foliation is 1457± 22 Ma(n = 39;Table 1).

4.5. Yerda Shear Zone (YrSZ)

Sample GCT1 consists of a foliated quartz-plagio-clase-biotite-muscovite± magnetite-bearing graniticgneiss from the eastern part of the Yerda Shear Zone.A elyf f1 efT rz ac

5

5

Cra-t stst evo-l ngs truc-

tures have a demonstrably large vertical component(up to 20 km,Teasdale, 1997) resulting in the juxtapo-sition of contrasting metamorphic terranes, the strikelength (>300 km), steep dip and linear surface expres-sion of the shear zones suggests that they were activein strike-slip or transpressional regimes (e.g.Rankin etal., 1989; Teasdale, 1997; Vassallo and Wilson, 2002).However, the age data obtained in this study stronglyimplies that the shear zone systems formed, or werereactivated during a number of different events overthe interval ca. 1700–1450 Ma, rather than reflectinga single transpressional event. The episodic nature ofdeformation will be discussed in the context of tec-tonic events, including the ca. 1730–1700 Ma KimbanOrogeny and the ca. 1650–1540 Ma Kararan Orogeny.Fig. 5presents a time-space plot which depicts the threemain Orogenic events, magmatic events and frequencydistribution plots for analysed monazites from shearzones of the Gawler Craton.

5.1.1. ca. 1730–1700 Ma Kimban OrogenyMonazite ages (ca. 1690–1680 Ma) from the Kalin-

jala and Tallacootra Shear Zones, and are within error ofthe accepted duration of the ca. 1730–1700 Ma KimbanOrogeny (Parker and Lemon, 1982; Hand et al., 1995;Fanning, 1997). The Kimban Orogeny largely shapedthe tectonic architecture of the southeast margin of theGawler Craton (Vassallo and Wilson, 2002), howeverits affects within the broader context of the GawlerCraton are less well understood. The ca. 1680 Ma ageso tudyr enyw wlerC ran-u th-i phica rdE onesoW gh-g to forer witht radef tedo atelyt bric

monazite grain analysed from a less intensoliated sample GCT1 (Fig. 3k) gave an age o567± 24 Ma (n = 107;Table 1). Monazites within th

oliated granitic gneiss (GCT1a; Fig. 3l), range inh content between∼3.6–6 wt.% and display minooning (Fig. 4l). A total of 160 analyses producedombined age of 1503± 16 Ma (Table 1).

. Discussion

.1. Deformation in the Gawler Craton

The scale of the shear zones in the Gawleron (Fig. 1a), both in width and strike length, suggehat they played a fundamental role in the crustalution of the terrane. Although crustal offsets alohear zones such as the Karari and Coorabie s

btained from the Tallacootra Shear Zone in this sepresent further evidence that the Kimban Orogas an important tectonic event in the western Garaton. The Tallacootra Shear Zone reworked glites belonging to the ca. 2560–2400 Ma Mulga

ng Complex, which preserve prograde metamorssemblages that formed at around 630◦C and 7 kbauring the Sleafordian Orogeny (Teasdale, 1997),PMA ages between ca. 2340 and 2330 Ma from zf high Th content within these monazites (WGC101,GC 98) are interpreted to be related to this hi

rade metamorphic event (Fig. 5). The developmenf the Tallacootra Shear Zone at ca. 1680 Ma thereepresents a primary orogenic structure associatedhe Kimban Orogeny, as opposed to being a retrogeature. Although the mineral lineations in the limiutcrop of the Tallacootra Shear Zone are moder

o steeply plunging, the steep dip of the shear fa


Fig. 5. Time-space diagram with frequency distribution plots for monazites analysed from shear zones in the Gawler Craton. Shaded barsrepresent orogenic events and solid boxes represent magmatic events. Samples WGC98 and WGC101 have bimodal age populations, whileOol 1 and GCT1 are interpreted as metamorphic and magmatic ages, respectively.

coupled with the strike extent and the regional dex-tral translations into the shear zone are suggestive of astrike-slip dominated regime (Teasdale, 1997).

The age of deformation on the Tallacootra ShearZone is identical to the emplacement age of I-typegranites belonging to the ca. 1690–1680 Ma Tunkil-lia Suite (Fig. 5), which are deformed by the shearfabrics (Teasdale, 1997; Ferris et al., 2002). Thesegranites have geochemical characteristics suggestingderivation from thickened crust (Teasdale, 1997; Ferriset al., 2002), consistent with the transpressional settinginferred for the development of the Tallacootra ShearZone.

In the eastern Gawler Craton, the Kalinjala ShearZone is the dominant regional-scale structure. Itis inferred to have accommodated significant dis-placement during dextral transpression (Vassallo andWilson, 2002). The core of the shear zone con-tains lower crustal granulites, which form a narrowzone of deeply exhumed rocks, flanked by mid andupper crustal rocks (Hand et al., 1995) that containcomparatively well-preserved pre-Kimban Orogenyrecords. Monazite inclusions within garnet (Fig. 3c)

at 1693± 16 Ma preserve the age of the metamor-phic mineral assemblage, which provides a maximumage constraint on shearing. Monazite elongate withina micro-shear between two plagioclase porphyroclasts(Fig. 3d) at 1682± 10 Ma is interpreted to record theage of shearing. This implies that the Kalinjala ShearZone was the focus of a comparatively narrow, high-strain orogenic belt that reworked older crust. Thesimilarity in the timing of dextral transpressional alongboth the Kalinjala and Tallacootra Shear Zones impliesthat existence of a Gawler Craton-wide dextral trans-pressional regime between ca. 1690 and 1680 Ma.

5.1.2. ca. 1650–1540 Ma Kararan OrogenyEPMA monazite age data obtained from the Karari

Fault Zone suggests that sinistral strike-slip deforma-tion (e.g.Rankin et al., 1989) along the shear zoneoccurred at 1631± 12 Ma during the early stages of theKararan Orogeny. This age is essentially identical to thetiming of the St. Peter Suite magmatism (Fig. 5; Ferris,2001; Ferris et al., 2002), which volumetrically domi-nates the central Gawler Craton (Fig. 1a). The St. PeterSuite is a comparatively juvenile syn-deformational


gabbroic to granitic igneous complex, with geochem-ical characteristics distinctive of arc-related magmas.It plots within the volcanic arc field on thePearce etal., (1984) discrimination diagram (Ferris, 2001;Ferris et al., 2002). Interpretation of regional magneticdatasets (Daly et al., 1998) suggests that the St. PeterSuite occurs in a southwest-trending zone, bound to thenorthwest by the Coorabie Fault Zone, and to the eastby the Yarlbrinda Shear Zone, displaying an overallparallelism with the western extent of the Karari FaultZone. Although it has been long recognised that theSt. Peter Suite was emplaced during regional deforma-tion (Flint et al., 1990; Daly et al., 1998; Ferris et al.,2002), there has been no evaluation of the structuralor kinematic framework that accompanied the magma-tism. However, the similarity in timing of sinistral strikemovement along the western Karari Fault Zone and St.Peter Suite magmatism suggests that the apparent arc-related rocks were emplaced along an active marginwith an important component of oblique convergence(e.g.Daly et al., 1998).

The 1631± 12 Ma age obtained from the KarariFault Zone (Ool3) is statistically younger than the1653± 8 Ma U–Pb SHRIMP zircon age for quartz-sapphirine-bearing granulites of the Nawa Domain(Fanning, 1997; Teasdale, 1997) immediately north-west of the Karari Fault Zone (Fig. 1a). This1653± 8 Ma age is interpreted to represent peak meta-morphism (Teasdale, 1997) and is within error of the1640± 12 Ma monazite age from Ool1 granulite min-em s oft andptF ula-t ost-h thea ne.T 0a oft us-t so-c elyu ug-g arariF ateA red

soon after the ultra-high temperature metamorphicevent.

EPMA monazite ages from inclusions in porphyrob-lastic garnet at two locations along the Coorabie FaultZone are 1516± 18 Ma (NDR5) and 1508± 14 Ma(WGC 24), which provides a maximum age constrainton shearing. These are within error of the 1503± 16 Maage obtained from monazite in the foliation fabricwithin the Yerda Shear Zone, and also the timing oflate monazite growth at ca. 1510 Ma in the YarlbrindaShear Zone in the central Gawler Craton (Ferris andBerry, 2003). The similarity of monazite EPMA agesfrom major shear zones in the central and westernGawler Craton points to an important phase of ter-rane reactivation at around 1510 Ma, which possiblylink to the waning stages of the late Kararan Orogenyrecorded by ca. 1550–1540 Ma U–Pb zircon ages foramphibolite to granulite grade metamorphism in theCooper Pedy, Nawa and Fowler Domains (Fanning,1997; Daly et al., 1998; Ferris et al., 2002). The youngerages of 1468± 12 Ma (NDR5a) and 1457± 22 Ma(WGC 24a) from the mylonitic fabric in the CoorabieFault Zone suggest a further phase of reactivation of theshear zone array in the western Gawler Craton, consis-tent with regional aeromagnetic data, which shows thatthe Yerda Shear Zone is truncated by the Coorabie FaultZone (e.g.Fig. 1b).

5.2. Shear zone and monazite age systematics

onea s tob mon-a sen-t arlye rela-t ne.A as tiono hearz awaD zitea , thes f theC ye ion-s eda ated

ral assemblage obtained in this study (Table 1). Ool 1onazites from granulite grade metamorphic rock

he Nawa Domain are temporally, geochemicallyetrographically distinct from Ool3 monazites from

he Karari Fault Zone (Table 1;Fig. 3e and f andig. 4e and f). This evidence suggests that pop

ions of monazite within the shear zone grew pigh-grade metamorphism, and conceivably datege of fabric development in the Karari Fault Zohe Nawa Domain granulites formed at around 95◦Cnd 10 kbar (Teasdale, 1997), and represent some

he highest grade metamorphic rocks on the Aralian continent. Although the tectonic setting asiated with the Nawa Domain granulites is largnknown, the EPMA data obtained in this study sests that exhumation of the granulites along the Kault Zone, and their juxtaposition against the Lrchaean rocks of the Mulgathing Complex occur

Given the general lack of outcrop of the shear zrray in the Gawler Craton, a question that neede addressed is whether the samples used andzite analysed in this study are regionally repre

ative of the shear zone events. This is particulxemplified by the age data and apparent timingionships along the 450 km long Karari Fault Zot its western end, the EPMA monazite age froministral mylonitic shear fabric suggests deformaccurred at 1630 Ma. This is consistent with the sone crosscutting ca. 1650 Ma granulites of the Nomain to the northwest, which yielded a monage of 1640 Ma. However, towards its eastern endhear zone cross cuts ca. 1565 Ma granulites oooper Pedy Domain (Fig. 1a; Fanning, 1997; Dalt al., 1998). These relative and absolute age relathips suggest that either: (1) the Karari Fault formt 1630 Ma along its entire length, and was reactiv


sometime after ca. 1565 Ma toward its northeast end,or; (2) the Karari Fault formed incrementally with east-ward propagation via at least two phases of growth. Atpresent there is little to distinguish between these possi-bilities. However, despite this cautionary note, the agedata obtained from this study, coupled with existingdata, is largely consistent with regional scale temporaland structural overprinting relationships. For example,(1) a monazite age of 1567± 24 Ma from a slightly foli-ated Hiltaba Suite granite within the Yerda Shear Zone(GCT 1; Table 1;Figs. 3k and 4k, and 5) is younger, butwithin error of a U–Pb SHRIMP zircon crystallisationage of 1592± 11 Ma (Ferris, 2001). This 1567± 24 Maage, attributed here to magma genesis, is systematicallyolder than the 1503± 16 Ma age of monazite that ismicrostructurally aligned with the foliated granitic fab-ric (GCT 1a; Table 1;Figs. 3l, 4l, and 5), interpretedas the age of shearing, and; (2) in the central GawlerCraton, the Yarlbrinda Shear Zone (Fig. 1a) forms theboundary between the western margin of the 1590 MaGawler Range Volcanics from ca. 1690–1610 Ma gran-ites of the Tunkillia and St. Peter Suites in the NuytsDomain. The shear zone both truncates, and is stitchedby, granites belonging to the 1595–1575 Ma HiltabaSuite (Ferris, 2001), and is therefore interpreted tohave its major phase of movement at ca. 1580 Ma. TheYarlbrinda Shear Zone is truncated by the Yerda andOolabinnia Shear Zones (e.g.Fig. 1a) which have aninferred age of ca. 1510 Ma (this study). In turn, theYerda Shear Zone is truncated by the Coorabie FaultZ tedag ardt tec-t l ofP lerC

an-u ader artonB in(t osedc ora-b a.1 ostt otraS one

of comparatively young high-grade rocks, boundedby shear zones with strike-slip geometries that abutolder rocks, strongly resembles a transpressional sys-tem (Vassallo and Wilson, 2002; Goscombe et al.,2003). If this is the case for the western Gawler Craton,it implies that the current arrangement of crustal blocksreflect large-scale transpressional reactivation of theentire craton. Whether exhumation of the lower crustalrocks occurred at ca. 1510 Ma, or ca. 1460 Ma (EPMAmonazite ages from the Coorabie Fault Zone), or acombination of both is unclear at this stage. However,regional cooling patterns obtained from Ar–Ar micaand K-feldspar data suggest two periods of cooling inthe western Gawler Craton, one at around 1540 Ma, andone around 1450 Ma (Fraser et al., 2002). This suggeststhat there may have been multiple, distinct phases oftectonic activity that formed the current configurationof crustal blocks in the western Gawler Craton.

5.3. Regional correlations in Proterozoic Australia

The timing of deformation on the terrane-scale shearzones in the Gawler Craton has temporal similari-ties with several important tectonic events elsewherein the Australian Proterozoic. In the Gawler Cra-ton, the dextral transpressional deformation recordedby the Kalinjala and Tallacootra Shear Zones at ca.1690–1680 Ma occurred during the latter stages of theKimban Orogeny, synchronous with emplacement ageof I-type granites belonging to the Tunkillia Suite. Thise rvala ousE alia(a ng-w belt,w cre-t n. Int andS entso truc-t 5t ionals opicd eiv-a sticsm thes ural

one (e.g.Fig. 1b), interpreted to have been reactivat ca. 1460 Ma (this study;Fraser et al., 2002). This pro-ressive pattern of structural truncation moving tow

he western Gawler Craton points to a focusing ofonic activity toward the end of the 400 Ma intervaalaeo-Mesoproterozoic tectonic activity in the Gawraton.In the western Gawler Craton, lower crustal gr

lites of the Nundroo Block and adjacent high-grocks in shear zone-bounded packages (e.g. Block) form part of the ca. 1540 Ma Fowler Doma

Daly et al., 1998; Ferris et al., 2002), which is jux-aposed against the comparatively un-metamorpha. 1630–1610 Ma St. Peter Suite along the Coie Fault Zone (Fig. 1a and b). Further west, the c540 Ma Fowler Domain is bounded by rocks that h

he ca. 1690–1680 Ma amphibolite-grade Tallacohear Zone. This macroscopic pattern with a z

vent occurred over approximately the same intes the Late Strangways Orogeny and Argilke Ignevent in the southern Arunta Block, in central Austr

Collins and Shaw, 1995; Close et al., 2003). Giles etl. (2004)suggest that both the Kimban and Straays system represented a continuous orogenichich was an artefact of protracted southward ac

ion and arc-magmatism along a convergent margiheir model, the now-spatially separated Kimbantrangways belts may represent dispersed fragmf a single orogen. However, based on available s

ural data (e.g.Goscombe, 1991; Lafrance et al., 199),he Strangways Orogeny was a sinistral transpressystem, which strongly contrasts with the megascextral character of the Kimban Orogeny. Concbly, these contrasting bulk structural characteriight reflect different phases in the evolution of

ame orogenic belt. However, until detailed struct


geochronology is undertaken in both belts, the signifi-cance of these bulk structural differences is unclear. Analternative possibility is that the belts are mechanicallyunrelated.

Although there has been debate about the assemblyof the Australian Proterozoic (e.g.Zhao, 1994; Zhaoand McCulloch, 1995; Myers et al., 1996; Betts et al.,2002; Giles et al., 2002, 2004), there appears little doubtthat by around 1550 Ma, the Australian Proterozoic wasessentially a contiguous continental terrane with thebulk of the tectonic activity restricted to the Mt Isa Inlierin northern Australia and the Gawler Craton (e.g.Gileset al., 2004). The ca. 1510 Ma ages from the sinistralYerda and Coorabie Fault zones in the central and west-ern Gawler Craton correspond with the latter stagesof the Isan Orogeny (e.g.Connors and Page, 1995;MacCready et al., 1998). Similarly, the ca. 1460 Mareactivation of the Coorabie Fault Zone coincides withregional cooling in the Mt. Isa region and developmentof large-scale strike-slip fault systems (e.g.Spikings etal., 2002).

6. Conclusion

EPMA dating of monazite from terrane-scale shearzones in the Gawler Craton reveals a protracted recordof transpressional deformation and terrane reactivationover the interval ca. 1700–1450 Ma (Fig. 5).

( 90–lla-g ofandof athe

s of

( ter-atedero-byikeidesag-onental

t.

(3) The Yerda Shear Zone and Coorabie Fault Zonein the central and western Gawler Craton under-went deformation at ca. 1510 Ma, with reactivationof the Coorabie Fault Zone at ca. 1460 Ma. TheCoorabie Fault Zone forms part of an early Meso-proterozoic transpressional system that resulted inthe exhumation of discrete lower crustal granuliteblocks in the Fowler Domain.

(4) The current crustal architecture of the GawlerCraton has been largely controlled by defor-mation along the terrane-scale system of shearzones. However, the history of shear zone devel-opment and/or reactivation over the interval ca.1700–1450 Ma implies that the current arrange-ment of crustal blocks does not reflect a sin-gle tectonothermal event, but rather a series ofevents in the late Palaeoproterozoic to earlyMesoproterozoic.

Acknowledgements

We would like to thank John Terlet and AngusNetting from Adelaide Microscopy, The Universityof Adelaide, for their assistance with the EPMA.Thankyou to Michael Schwarz, Gary Ferris, Neil Gray,Mark Flintoft and Daniel Gray from Primary Industriesand Resources of South Australia, for assistance withfieldwork and discussion on the geology of the GawlerC fort

R

B of95.

C L.J.,ce:03.ey,

C 1998.situ

ionsa 62

C s onRes.

1) Dextral transpressional deformation at ca.161680 Ma is recorded in the Kalinjala and Tacootra Shear Zone systems. The coeval timindeformation on the widely separated KalinjalaTallacootra Shear Zones implies the existenceterrane-scale dextral transpressional regime inGawler Craton, associated with the latter stagethe Kimban Orogeny.

2) The late Archaean to early Palaeoproterozoicranes in the central Gawler Craton are separfrom ultra-high temperature, late Palaeoprotzoic granulites in the northwest Gawler Cratonthe Karari Fault Zone. This records sinistral strslip movement at ca. 1630 Ma. The age coincwith the emplacement of voluminous arc-like mmatic rocks, suggesting that the Karari Fault Zrecords deformation associated with a continemargin with a significant strike-slip componen

raton. Finally, we thank the reviewers and editorheir valued comments and useful suggestions.

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F fso-ana(5),

F cton-,, V.P.f the

ust.,

G karcralia.

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Documents

Age constraints on terrane-scale shear zones in the Gawler Craton, southern Australia