Australian Journal of Earth Sciences (1999) 46, 377–389

INTRODUCTION

The southern Adelaide Fold-Thrust Belt is part of theDelamerian Orogen formed by convergence along the east-ern and southeastern margin of the Gawler Craton duringLate Cambrian and Early Ordovician times (Offler &Fleming 1968; Daily & Milnes 1973; Preiss 1987; Coney et al.1990; Jenkins 1990; Jenkins & Sandiford 1992; Flöttmann etal. 1994). Most stages of the development of the southernAdelaide Fold-Thrust Belt were accompanied by igneousactivity (Figure 1) and the associated magmatic rocks pro-vide a useful chronological framework for the developmentof the orogen as well as important insights into the role ofdeeper crustal and mantle processes (J. Foden unpubl.data). In the southern Adelaide Fold-Thrust Belt deform-ation was immediately preceded by the deposition of thecarbonate-rich Normanville Group and the clastic tur-bidites of the Kanmantoo Group. This Early Cambriansedimentation was accompanied by small volumes of maficvolcanism which has been interpreted by some workers toreflect an extensional (Preiss 1987) or transpressional(Flöttmann et al.1995) tectonic regime. The character of themagmas changed from mafic to felsic (dominantly granitic)by the time of development of the first identifiable tectonicfabrics (Sandiford et al. 1992) in the core of the orogen. Theimmediate post-convergent phase of the orogen was associ-ated with high-level, bimodal igneous activity (Foden et al.1990; Turner & Foden 1990; Turner et al. 1992).

The Rathjen Gneiss (Milnes et al. 1975; Fleming & White1984) forms one of a number of variably deformed graniticintrusions in the core of the southern Adelaide Fold-ThrustBelt, some 60 km east of Adelaide (Figures 1, 2). Because ofthe important role ‘granitic’ rocks play in the orogenicprocesses, these felsic orthogneisses and associated gran-ites have received considerable attention in recent discus-sions of the evolution of the southern Adelaide Fold-ThrustBelt (Milnes et al. 1975; Fleming & White 1984; Foden et al.1990; Sandiford et al. 1992, 1995; Oliver & Zakowski 1995). Allworkers agree that the Rathjen Gneiss is particularlyimportant as structural criteria (see below) clearly indicatethat it represents the oldest of the felsic igneous suites inthis part of the belt. Because of its importance in definingthe structural evolution of the belt its absolute age iscritical, but has hitherto not been determined. Moreover,opinions also have differed on the nature of its contactrelations with surrounding Kanmantoo metasediments(Fleming & White 1984; Sandiford et al. 1992) and on theregional significance of the deformational fabrics withinthe gneiss (Oliver & Zakowski 1995; Sandiford et al. 1995).The purpose of this paper is to summarise results of fieldobservations that clarify some of this uncertainty and topresent new isotopic and geochemical data pertaining tothe age and origin of the Rathjen Gneiss. These new dataprovide an important framework for discussing the evolu-tion of the early stages of the development of the southernAdelaide Fold-Thrust Belt.

Geochemistry and geochronology of the Rathjen Gneiss:implications for the early tectonic evolution of theDelamerian OrogenJ. FODEN1, M. SANDIFORD1, J. DOUGHERTY-PAGE1 AND I. WILLIAMS2

1Department of Geology, University of Adelaide, SA 5005, Australia. 2Research School of Earth Sciences, Australian National University, ACT 0200, Australia.

The Rathjen Gneiss is the oldest and structurally most complex of the granitic intrusives in the southernAdelaide Fold-Thrust Belt and therefore provides an important constraint on the timing of theDelamerian Orogen. Zircons in the Rathjen Gneiss show a complex growth history, reflecting inheri-tance, magmatic crystallisation and metamorphism. Both single zircon evaporation (‘Kober’technique) and SHRIMP analysis yield best estimates of igneous crystallisation of 514 6 5 Ma,substantially older than other known felsic intrusive ages in the southern Adelaide Fold-Thrust Belt.This age places an older limit on the start of the Delamerian metamorphism and is compatible withknown stratigraphic constraints suggesting the Early Cambrian Kanmantoo Group was deposited,buried and heated in less than 20 million years. High-U overgrowths on zircons were formed duringsubsequent metamorphism and yield a 206Pb/238U age of 503 6 7 Ma. The Delamerian Orogeny lastedno more than 35 million years. The emplacement of the Rathjen Gneiss as a pre- or early syntectonicgranite is emphasised by its geochemical characteristics, which show affiliations with within-plate oranorogenic granites. In contrast, younger syntectonic granites in the southern Adelaide Fold-ThrustBelt have geochemical characteristics more typical of granites in convergent orogens. The EarlyOrdovician post-tectonic granites then mark a return to anorogenic compositions. The sensitivity ofgranite chemistry to changes in tectonic processes is remarkable and clearly reflects changes in thecontribution of crust and mantle sources.

Key words: Adelaide Fold-Thrust Belt, Delamerian Orogeny, granite, radiometric dating, Rathjen Gneiss.

378 J. Foden et al.

REGIONAL SETTING AND GEOCHEMISTRY OFTHE GRANITES

Granitic rocks in the southern Adelaide Fold-Thrust Beltfall into two broad groups defined on both field and geo-chemical grounds. An older, syntectonic suite is charac-terised by tectonic foliations, and clearly pre-dates anundeformed post-tectonic suite intruded after the cessationof Delamerian folding. The syntectonic granites are largelyconfined to the central, highest metamorphic grade, coreof the orogen (Figure 1) in the eastern parts of the Mt LoftyRanges, the Fleurieu Peninsula and along the south coastof Kangaroo Island. On the basis of field relationships theRathjen Gneiss is the oldest known Cambrian granite in thebelt (see discussion below). Other members of this syntec-tonic suite include the Palmer, Victor Harbor and TanundaCreek granites, the Reedy Creek granodiorite and a numberof unnamed granites exposed along the south coast ofKangaroo Island near Vivonne Bay.

The post-tectonic granites and associated igneous rocksare largely confined to isolated outliers in the Murray Basinimmediately east of the Mt Lofty Ranges between Sedanand Murray Bridge and along the Padthaway Ridge. Theseinclude the Murray Bridge, Mannum, Sedan, Padthaway,Christmas Rocks and Mt Monster granites (Turner &Foden 1996). These granites form part of a bimodal suitethat includes mafic intrusions at Reedy Creek and in theBlack Hill, Sedan, Cambrai area (Turner 1996). The age ofthe post-tectonic suite is constrained by a number ofstudies (Milnes et al. 1975; Turner 1996; Turner et al. 1996)to the range 490–480 Ma and provides a minimum age forthe cessation of pervasive Delamerian deformation.

The geochemical distinction between the two suites in the southern Adelaide Fold-Thrust Belt (Table 1) hasbeen highlighted by several authors (Foden et al. 1990;Mancktelow 1990; Sandiford et al. 1992). The time-dependanttransitions of magma types in the Delamerian orogen rep-resent changing proportions of crust and mantle sourcematerials coupled with systematic variation in the extentof fractionation (J. D. Foden unpubl. data). The post-tectonic suite consists of highly fractionated, siliceous A-type granites with low CaO, MgO and Sr abundances,high K2O and Rb abundances and high K2O/Na2O ratios(Figure 4). They have very low Mg and are rich in F, rare-earth elements, Y and Zr (Figures 4, 5). They have charac-teristics (Turner et al. 1992) which imply high-temperaturefractionation at low aH2O, with very late onset of zircon andaccessory phase fractionation. In contrast, the syntectonicgranites are compositionally variable with a spectrumbetween locally derived peraluminous, S-types (e.g. theVictor Harbor granite) through to obvious I-types, such asthe Reedy Creek granodiorite, which are relatively calcic,hornblende-rich, and K2O-poor (Figure 4). In comparisonwith the post-tectonic suite, they are Sr-rich with low Rb/Srratios, poorer in Y, Nb and Zr and have lower Nb/Zr ratios.They have low F abundances, and feldspar and biotite com-positions which imply relatively high aH2O. The apparentonset of the fractionation of zircon and accessory phasesat SiO2 contents < 70% implies evolution at temperatureslower than the post-tectonic series. These geochemicaldistinctions between the two suites are highlighted by thestandard discrimination diagrams (Figure 5). For exampleon the Pearce et al. (1984) discrimination diagrams the earlyseries with lower Rb and Y and Nb, fall in the volcanic-arc

Figure 1 Distribution of igneousrocks in the southern AdelaideFold-Thrust Belt (after Foden etal. 1990).

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granites field, while the later series are all in the within-plate granites field (Figure 5).

The two granite groups also have distinctive Nd and Sr isotopic compositions (Figures 6, 7). The younger suite has limited variation of isotopic compositions andquite primitive (non-crustal) compositions. Initial eNd

values are in the range -2 to 12 and 87Sr/86Sr(I) values are in the range 0.7045–0.7065 (Turner et al. 1992).By comparison the older suite shows much more variable isotopic composition with eNd(I) ranging from 213 to 11 and 87Sr/86Sr(I) from 0.7050 to 0.7250. We interpretthese geochemical signatures to indicate that the post-tectonic suite is dominated by mantle sources (Turner

et al. 1992; Turner 1996; Turner & Foden 1996) whereas the syntectonic suite has a more significant crustalcomponent.

FIELD SETTING AND GEOCHEMISTRY OF THERATHJEN GNEISS

The Rathjen Gneiss is a coarse- to medium-grainedorthogneiss of granitic to granodioritic composition with‘I-type’ mineralogy and chemistry. It is composed of quartz,partly zoned plagioclase (An25–45) and mildly perthiticmicrocline. The main ferromagnesian mineral is biotite,

Table 1 Analyses of Rathjen Gneiss samples and means of analyses of syntectonic (45 samples) and post-tectonic (36 samples) gran-ites from the southern Adelaide Fold-Thrust Belt.

88-RG1 898-309 Syntectonic SD Post-tectonic SD Syntectonic SDRathjen Rathjen S-type

SiO2 73.67 73.97 68.8 4.3 72.0 7.5 72.1 1.2TiO2 0.55 0.44 0.6 0.2 0.4 0.5 0.5 0.1Al2O3 12.26 12.45 14.8 1.7 13.5 1.9 13.4 0.9Fe2O3* 3.39 2.81 3.7 1.2 2.7 2.3 3.4 0.3MnO 0.04 0.03 0.0 0.0 0.0 0.0 0.1 0.0MgO 0.9 0.81 1.3 0.7 0.6 1.2 1.5 0.2CaO 1.7 1.66 2.6 1.1 1.6 2.2 1.6 0.2Na2O 3.41 3.55 3.7 0.7 3.8 0.4 2.6 0.2K2O 3.23 3.17 3.4 0.9 4.4 1.5 3.8 0.5P2O5 0.11 0.44 0.1 0.1 0.1 0.1 0.1 0.1Total 99.52 99.33 99.6 0.3 99.6 0.3 99.5 0.3

Al index 1.01 1.01 1.03 0.04 0.98 0.06 1.18 0.05Mg# 0.34 0.37 0.39 0.07 0.21 0.14 0.47 0.01K2O/Na2O 0.95 0.89 1.02 0.53 1.19 0.45 1.48 0.19

Cr 4 4 26 13 5 1 63 17Ni 10 7 9 6 6 8 15 11Sc 9.5 8.7 9 4 7 6 12 2V 47 40 77 61 29 62 55 9Rb 132.19 123 138 49 146 73 190 40Sr 110.34 115 303 200 158 259 147 9Ba 664 697 935 545 371 189 547 48Ga 18 16 19 2 19 2 17 3Y 62 41 33 23 58 35 38 16Zr 314 260 246 69 231 89 167 20Nb 16.4 13 15 2 23 6 15 1La 68 48 55 20 72 45 29 3Ce 137 102 98 40 134 82 59 10Nd 56.89 42 39.4 18.6 51.8 32.7 24.3 13.3Sm 10.99 – 7.0 3.8 9.7 6.1 5.2 2.6

Zr/Nb 19.1 20.0 16.0 3.4 10.6 4.4 11.6 2.2Y/Nb 3.8 3.2 2.1 1.3 2.6 1.4 2.9 1.2Zr/Y 5.1 6.3 0.4 0.4 4.9 2.7 1.6 2.5La/Y 1.1 1.2 3.3 4.2 1.6 1.3 0.9 0.3

143Nd/144Nd (t 5 0) 0.512052 6 25 – 0.512128 0.000140 0.512319 0.000127 0.511829 0.00013143Nd/144Nd(t 5 500) 0.511657 – 0.511792 0.000189 0.511949 0.000085 0.511384 0.00011147Sm/144Nd 0.1169 – 0.1055 0.0192 0.1161 0.0192 0.1310 0.0073e Nd(t 5 0) –11.43 – –9.9 2.7 –6.2 2.5 –15.7 2.7e Nd(I) –6.2 –4.1 3.5 –1.2 1.7 –11.4 2.1Model Age (DM) 1.6 – 1.4 0.4 1.2 0.2 2.3 0.187Sr/86Sr (t 5 0) 0.73701 – 0.72784 0.02058 0.76842 0.05730 0.74435 0.0032387Sr/86Sr(I) 0.71144 0.70921 0.00409 0.70386 0.00097 0.71873 0.0010887Rb/86Sr 3.48 2.6 2.4 9.3 8.4 3.5 0.6

All data quoted to 2s.

380 J. Foden et al.

but hornblende is also common. Minor or accessory phasesinclude apatite, zircon, sphene, magnetite and rare allanite.In outcrop it forms a sheet-like structure in the core of aregional north-northwest-trending anticline, exposed inelevated country to the south of Springton. The main bodyof the Rathjen Gneiss forms a narrow canoe-shaped body,up to 3 km wide and 12 km long between Springton and theTungkillo–Palmer road (Figure 2). A thin ‘tail’ about 100 mwide extends further southwards for another 8 km. Both themain body and the tail are essentially concordant with theregional stratigraphy, hosted by migmatitic metasedimentsof the Backstairs Passage Formation of the KanmantooGroup. The outcrop pattern is consistent with the RathjenGneiss forming a sheet-like body several hundred metresthick. This concordance has led a number of authors (e.g.Fleming & White 1984) to argue that the Rathjen Gneiss isderived from a volcanic succession. However, recent gully-ing near the northern end of the granite (Adelaide 1:250 000map sheet, GR 210000E, 710500N), has exposed clearintrusive contacts between narrow granite apophyses andthe surrounding metasediments (Figures 2, 3). Likewise,

near the southern ‘tail’, thin layers of felsic magmatic rockoutcrop as discontinuous horizons parallel to the beddingof host Kanmantoo Group metasediments beneath theRathjen Gneiss. In the past these have been taken asevidence for a synsedimentary volcanic origin; however,it is clear that this interpretation is not justified as mildly discordant apophyses, which contain metasedi-mentary xenoliths, also intrude from the upper surface of the sill into the overlying metasedimentary rocks(Figure 3).

Although the overall concordant nature and sheet-likegeometry of the body in part reflect an original sill-likeemplacement, this shape also reflects strain associated withthe development of the prominent gneissosity, which isessentially parallel to the surrounding stratigraphy and tothe migmatitic layering developed in the surroundingmetasediments. The tectonic significance of this foliationremains obscure. It is largely confined to the immediateenvironment of the syntectonic granites between MurrayBridge and Springton becoming much less prominent in thelower grade rocks to the north. The foliation is associated

Figure 2 Southern AdelaideGeosyncline area showing prin-cipal granites of the syntectonicsuite, including the RathjenGneiss. The sample site and thelocation of Figure 3 are indicated.

Rathjen Gneiss, SA 381

with north–south-trending lineations in part formed by itsintersection by younger axial-plane fabrics to uprightsecond-generation folds. However, as discussed by Oliverand Zakowski (1995) this north–south lineation is, in someplaces, demonstrably a mineral-elongation lineation andtherefore implies a strain increment with a principalnorth–south stretch direction during the associated defor-mation. This direction is orthogonal to the principal con-vergence direction associated with development of younger(second generation) regional-scale folds in this part of thebelt. Oliver and Zakowski (1995) attributed this north–southstretching to a period of previously unrecognised regionalextensional deformation in the orogenic belt.

In the context of the regional geochemical variation ofthe granites, the Rathjen Gneiss is distinctive (Table 1).Although clearly the earliest of the syntectonic suites it hasgeochemical and isotopic characteristics transitionalbetween the syntectonic and post-tectonic suites, as indi-cated by relatively high rare-earth elements, high Zr, Nb

and Y and high Nb/Zr ratios. On the Rb vs Nb1Y and Y vsNb discrimination diagrams it lies directly on the trans-ition between the volcanic-arc granites and within-plategranites fields (Figure 5) and on all other geochemical vari-ation diagrams falls between the syn- and post-tectonicfields (Figures 4–7). Ce or Zr vs SiO2 variation typifies thecontrasting evolution of the syn- and post-tectonic series.Whereas the post-tectonic suite shows enrichment of REEand Zr to SiO2 levels > 70%, the syntectonic suite showsdepletion of these elements when SiO2 exceeds 70%. Thisbehaviour is controlled by the later onset of accessoryphase (apatite, zircon, allanite) saturation in the post-tectonic suites. In keeping with its transitional behaviourfor other elements, on Harker diagrams the Rathjen Gneissplots directly where the two series diverge.

As with other geochemical parameters, the RathjenGneiss also has transitional isotopic compositions, with eNd 26 and 87Sr/86Sr(I) of 0.7100 (Figures 6, 7; Table 1). Itshould also be noted that the initial Nd and Sr isotopic com-positions of the Rathjen Gneiss are significantly differentfrom either the host Kanmantoo Group metasediments (eNd] 5 213, 87Sr/86Sr(I) 5 0.7250) or possible Proterozoiccrystalline basement at depth (eNd < 220, 87Sr/86Sr > 0.7550)at 514 Ma. The isotopic composition of the Rathjen Gneisslies on a continuum of Nd and Sr isotopic variationbetween mantle and crustal sources defined by the granitesthat form the syntectonic suite.

AGE OF THE RATHJEN GNEISS

In order to assess the age of emplacement of the RathjenGneiss and thus a maximum age of initiation of theDelamerian Orogeny we have separated zircons and carriedout U–Pb isotopic analysis both by ion probe (SHRIMP I andII) at the Research School of Earth Sciences, AustralianNational University and the emitter bedding Pb–Pbtechnique (Kober 1987) at the University of Adelaide(Dougherty-Page & Foden 1996). In both cases we have used

Figure 3 Intrusive contacts between apophyses of the RathjenGneiss and the surrounding metasediments at location GR210000E, 710500N. See Figure 2 for location.

Figure 4 Major element com-position of Cambro-Ordoviciangranites from the Adelaide Fold-Thrust Belt. h, syn-Delameriangranites; r, post-Delameriangranites; m, Rathjen Gneiss.

382 J. Foden et al.

zircons from the same population that were separated from10 kg of crushed sample from the southern end of the mainRathjen Gneiss body (Figure 2: Adelaide 1:250 000 mapsheet, GR 214550E, 698550N). The zircons were examinedoptically and using cathodoluminescence (Koschek 1993)and were found to have a complex structure (Figure 8). Thegrains were typically medium sized, (150–200 mm) and wereshort, prismatic, doubly terminated euhedra, unlike theelongate forms commonly associated with precipitationfrom felsic volcanic magmas. Under cathodoluminescencea large proportion of grains have a three-fold structure con-sisting of a variable sized rounded core, surrounded by abroad layer of euhedrally zoned zircon and, finally, a thin(< 10 mm thick) structureless outer zone. SHRIMP ion probeanalyses revealed that the cores were older (inherited) andin general relatively U-rich. The euhedrally zoned layer hasmoderate to low U content and high Th/U ratios, clearlyimplying a magmatic origin. The thin outermost zones are

U-rich and most have very low Th/U ratios and are the prob-able products of growth from a metamorphic fluid or froma hydrous minimum melt.

Zircon evaporation (‘Kober’ technique) data

In the ‘Kober’ technique individual zircon crystals werecrimped into Re filaments, and heated in a Finnigan Mat261 mass spectrometer. Pb-isotope ratios were determineddynamically, using a Secondary Electron Multiplier set atconstant amplification, scanning in the order 206 – 207 – 208 – 207 – 206 – 204. Heating the zircon results in thebreakdown of zircon to baddeleyite (ZrO2) (Chapman &Roddick 1994), releasing radiogenic Pb, from which an ageis determined. Within a high-quality crystal, the break-down takes place along a sharply defined reaction front, which progresses into the grain with continuedheating. Depending on age and U content an individualzircon crystal may contain sufficient Pb for severalanalyses in which case it may be analysed in a series ofdiscrete heating steps.

The 207Pb/206Pb age given by each heating step representsthe average age of radiogenic lead released by the passageof the reaction front through the crystal during that step.Thus the 207Pb/206Pb age only represents a true crystallis-ation age when this material is a single isotopically concor-

Figure 5 Geochemical discrimination diagrams of Pearce et al.(1984). h, syn-Delamerian granites; e, post-Delamerian granites;m, Rathjen Gneiss. Arrow indicates the general temporal shift of granite composition from earliest Delamerian to post-Delamerian. VAG, volcanic-arc granites; syn-COLG, syn-colli-sional granites; WPG, within-plate granites; ORG, oceanicgranites.

Figure 6 Initial (500 Ma) Nd–Sr isotopic composition of Cambro-Ordovician magmatic rocks from the Adelaide Fold-Thrust Belt.h, mafic rocks; j, syn-Delamerian granites. Rathjen Gneiss indi-cated. The inset shows the post-Delamerian granites (e<) and theDelamerian mafic rocks (h) and the upper part of the syntectonicfield shaded. DM, depleted mantle at 500 Ma. The Gawler Cratonmean is that of Palaeoproterozoic to Mesoproterozoic basementrocks from the craton to the west of the Adelaide Fold-ThrustBelt. The shaded Kanmantoo field is that of Cambrian clasticsedimentary rocks which are intruded by the Cambrian granites(data source Turner et al. 1992 and authors’ unpubl. data).

Rathjen Gneiss, SA 383

dant phase of zircon. The passage of the reaction frontthrough several isotopically distinct domains results in a

geologically meaningless ‘mixed age’ proportional to therelative quantities of Pb supplied by each of the end-member components. Thus incorporation of materialderived from a later rim results in an age younger than the true crystallisation age, while the incorporation ofmaterial derived from an inherited core results in an ageolder than the true crystallisation age for that heating step.A distinct age plateau, reproduced by several heatingsteps, strongly indicates a true concordant crystallisationage as it is considered unlikely that several distinct agecomponents will repeatedly mix in the same proportions.In this study, zircons with radiation damage, large cracksor obvious cores were avoided as the reaction front propa-gates rapidly along cracks and through radiation damagedareas. Such damage to a crystal increases total area of thereaction front within the zircon crystal and thus decreasesthe possible resolution of zonation.

Ages given by step-heating data for Rathjen Gneiss zir-cons (Table 2) were sorted in order of ascending age andeach data point was then assigned a rank within this sortedsequence (with the youngest analysis given a rank of 1).Heating steps are then plotted as their age against rank(Figure 9). In this graph, mixing between various age com-ponents results in inter-step age variation while true agestend to be more frequently repeated, producing distinct ageplateaux. This type of graph has advantages over the fre-quency histograms traditionally used to present zirconevaporation data (Kober 1987; Dougherty-Page & Foden1996) in that the degree of complexity within the populationis immediately apparent, the visual interpretation of ageplateau does not depend on arbitrary bin sizes, and eachindividual data point may be identified and its associatedanalytical error displayed.

The addition of non-radiogenic 207Pb derived fromcommon Pb results in an overestimation of the 207Pb/206Pbage. The effect of common Pb addition is greater in

Figure 7 SiO2 and Mg# vs initial Nd isotopic composition. h,Delamerian mafic rocks; j, syn-Delamerian granites; e, post-Delamerian granites. Rathjen Gneiss indicated. S-types field isthat of samples with Al index (see Figure 4) > 1.1. Arrow illus-trates a possible contemporary mantle–crust mixing trend toexplain the source of the syntectonic granites.

Figure 8 Cathodoluminescence–SEM images of zircons from theRathjen Gneiss. Circled numbersrefer to analysis spots as given inTable 3. The grains show the 3-foldgrowth structure common to most zircon from the gneiss: (i) arounded non-luminescent corethat isotopic analysis reveal to beinherited; (ii) a sequence of mod-erately luminescent euhedralgrowth zones interpreted as mag-matic in origin; and (iii) an irregu-lar structureless non-luminescentrim interpreted as subsolidus ornear-solidus growth during subse-quent high-grade metamorphism(note the irregular overgrowthabutting the grey feldspar grainadhering to the upper right cornerof grain 14). In grain 14 theembayed boundary between thezoned zircon and overgrowthsuggests a period of zircon dissol-ution between the magma crystal-lisation and the subsequentmetamorphism.

comparatively recent rocks such as the Rathjen Gneiss thanin more ancient samples, due to the decrease in the pro-duction of radiogenic 207Pb relative to radiogenic 206Pbthrough time. Common Pb contents are determined from204Pb/206Pb ratios and corrected by application of the equa-tion suggested by Cocherie et al. (1992). The appropriatecommon Pb isotopic composition for the age of the zirconwas determined from the Stacey and Kramers (1975) two-stage Pb-evolution curve. As a consequence of a dynamiccollection routine in which all Pb-isotope beams were mea-sured at constant amplification, the smaller 204Pb beam wasfrequently below detection. Quoted crystallisation ages arecalculated from the average age of the plateau heating stepswhich record a single zircon growth stage. Errors quotedon age estimations are 2s (excluding individual analyticalerrors). Where age estimations are based on assumedvalues of common Pb, the effect of the assumed correction(63 Ma at 500 Ma) is added to the quoted error. During theanalyses of zircons from the Rathjen Gneiss, zircons fromthe Palaeoproterozoic Coulta Granodiorite, used as astandard at the University of Adelaide, were also analysed.They returned an age of 2512 6 2 Ma (2s, weighted average),against an internally accepted value of 2514 6 7 Ma, and anexternal (SHRIMP) determination of 2520 6 9 Ma (C. M.Fanning pers. comm. 1997).

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Figure 9 Common Pb corrected age data computed from207Pb/206Pb measurements on 32 heating steps carried out onzircons from the Rathjen Gneiss (data in Table 2). The ages givenby these heating steps have been sorted into ascending order andare plotted against their ‘rank’ within this age sequence. Themain diagram shows a clear lower age plateau at ca 500 Ma, witholder ages due to inheritance becoming predominant afterheating step rank number 17. The inset shows the plateau on anexpanded scale, with 2s error bars on the individual heatingsteps. The 14 steps indicated between the arrows yield an age of514 6 4 Ma.

Table 2 Pb isotope data for analysis of 16 zircons from the Rathjen Gneiss in a total of 32 heating steps (plotted on Figure 9).

Zircon Step 208Pb/206Pb 207Pb/206Pb 204Pb/206Pb Age Rank

1 1 0.121327 6 0.011082 0.057819 6 0.000227 bd 520 6 8 131 2 0.26708 6 0.003405 0.058352 6 0.000505 bd 540 6 6 191 3 0.410837 6 0.025158 0.057863 6 0.000638 bd 521 6 15 151 4 0.488448 6 0.002832 0.057767 6 0.000123 bd 518 6 6 112 1 0.368267 6 0.003109 0.057784 6 0.000202 bd 518 6 7 122 2 0.406225 6 0.001355 0.057762 6 0.000495 bd 517 6 13 103 1 0.110689 6 0.005675 0.057598 6 0.000257 bd 511 6 8 73 2 0.161385 6 0.005008 0.059198 6 0.000156 bd 571 6 6 213 3 0.047041 6 0.000001 0.066455 6 0.000185 bd 819 6 6 265 1 0.132161 6 0.00048 0.059172 6 0.000354 bd 570 6 10 205 2 0.133235 6 0.002513 0.06539 6 0.000057 bd 785 6 4 245 3 0.113863 6 0.000706 0.069226 6 0.000408 bd 904 6 9 276 1 0.156016 6 0.001634 0.079439 6 0.003441 bd 1182 6 45 296 2 0.117308 6 0.256358 0.129834 6 0.001246 bd 2095 6 10 306 3 0.067176 6 0.000032 0.147628 6 0.00026 bd 2318 6 3 316 4 0.066245 6 0.000158 0.154921 6 0.00011 bd 2401 6 2 327 1 0.141948 6 0.005028 0.057663 6 0.000036 bd 514 6 4 87 2 0.24483 6 0.00056 0.057909 6 0.000287 bd 523 6 9 178 1 0.198362 6 0.00192 0.061899 6 0.000399 0.000043 6 0.00002 649 6 9 229 1 0.222855 6 0.125824 0.057861 6 0.00055 bd 521 6 14 14

10 1 0.381454 6 0.002066 0.057435 6 0.000029 bd 505 6 4 210 2 0.43371 6 0.000774 0.057538 6 0.000113 bd 509 6 6 511 1 0.12652 6 0.017104 0.063655 6 0.001678 0.000067 6 0.000028 698 6 9 2311 2 0.19477 6 0.000764 0.070656 6 0.000028 bd 946 6 3 2812 1 0.15467 6 0.003154 0.057672 6 0.000086 bd 514 6 5 912 2 0.303685 6 0.002811 0.058398 6 0.000043 0.000067 6 0.000053 507 6 0 314 1 0.128751 6 0.010232 0.057506 6 0.000124 bd 508 6 6 414 2 0.243935 6 0.006716 0.057409 6 0.000034 bd 504 6 4 114 3 0.189724 6 0.005222 0.057578 6 0.000647 bd 510 6 16 615 1 0.126444 6 0.000617 0.065926 6 0.003711 bd 802 6 61 2516 1 0.135434 6 0.002 0.057866 6 0.000677 bd 521 6 16 1616 2 0.039772 6 0.094947 0.058016 6 0.000119 bd 527 6 6 18

bd, below detection.Isotope ratios are uncorrected for common Pb, all errors are quoted to 2s. Where 204Pb/206Pb was below detection, a ratio of0.000005 6 0.000005 (2s) was applied in the common Pb age correction (see text for discussion). The rank of the heating steps refers totheir relative positions when tabulated in ascending order of common Pb corrected age, with the youngest ranked 1 and the oldest 32.

Fourteen zircon crystals were analysed, in a total of 32heating steps. Eight of these zircons contained inheritedcores. No distinct age plateau were achieved for theseinherited zircon heating steps and oldest ages recordedfrom individual inherited zircon grains are regarded asminimum ages for that core, as the final analysis may stillhave contained a component of zircon of Rathjen Gneissmagmatic age. Excluding the heating steps of very variableage from rank 17 onwards (taken to be inherited cores) andthe first two ranked steps, the age of magmatic zircongrowth is interpreted as given by the 14 heating steps fromranks 3 to 16 inclusive (Figure 9). This plateau is formedby a continuous suite of ages from 507 to 521 Ma. and yieldsa crystallisation age of the Rathjen Gneiss of 514 6 4 Ma(2s, including error due to estimated common Pbcorrection). This age is identical to that achieved with the SHRIMP by analysis of the broad, euhedrally zonedareas of the zircons (see below). As it was clear from thecathodoluminescence images that the zircons had a

complex internal history, ion microprobe analyses wereundertaken to assess to what extent the Pb–Pb evaporationages were influenced by older Pb from the inherited cores, or from possible younger Pb in the thin outer-most rims.

Ion microprobe (SHRIMP) data

Preliminary ion probe results were initially obtained usingSHRIMP I and were subsequently repeated using SHRIMPII. Analysis techniques were similar to those used byWilliams and Claesson (1987). Although the two SHRIMPanalysis sessions yielded consistent results, the earliersession was completed before the zonal structure of thezircons was recognised. Therefore SHRIMP results inTable 3 include data from inherited cores from both ses-sions, but only that from the SHRIMP II sessions for mag-matic euhedrally zoned zircon and the thin overgrowths(Figure 10).

Rathjen Gneiss, SA 385

Figure 10 (a.) Concordia plot ofSHRIMP U–Pb isotopic analyses(data in Table 3). The enlargementhighlights magmatic zircon popu-lations and Late Neoproterozoicinherited cores with ages rangingfrom 650 to 550 Ma. (b.) 207Pb/206Pb vs238U/206Pb (Tera-Wasserburg) dia-gram. Crosses indicate analysesbefore common Pb correction.Squares with diagonal lines repre-sent analyses of the broad euhe-drally zoned magmatic rims(luminescent zone in the cathodo-luminescence image in Figure 8),yielding an age of 514.6 6 5.9 Ma(2s). Open circles are thin meta-morphic overgrowths [503.3 6

7.4 Ma (2s)]. Filled circles areanalyses from cores.

386 J. Foden et al.

Tab

le3

Pb

and

U i

soto

pic

data

an

d ag

e ca

lcu

lati

ons

from

th

e S

HR

IMP

an

alys

es o

fzi

rcon

s fr

om t

he

Rat

hje

n G

nei

ss (

plot

ted

in F

igu

re 1

0).

UT

hP

b*%

206

208 P

b/20

8 Pb/

206 P

b/20

7 Pb/

207 P

b/A

geA

geA

geA

gepp

mpp

mT

h/U

ppm

204 P

b/20

6 Pb

com

.20

6 Pb

232 T

h23

8 U23

5 U20

6 Pb

8/32

6/38

7/35

7/6

Ove

rgro

wth

20.1

1648

660.

0412

40.

0000

160.

030.

0105

60.

0005

0.02

156

0.00

10.

0818

60.

0006

0.65

16

0.00

70.

0577

60.

0004

430

620

507

63

509

64

518

614

21.1

1044

769

0.74

780.

0001

210.

190.

0205

60.

001

0.00

226

0.00

10.

0806

60.

0006

0.63

36

0.00

80.

057

60.

0006

456

250

06

449

86

549

26

2214

.195

091

50.

9670

0.00

0103

0.16

0.02

726

0.00

120.

0022

60.

001

0.07

936

0.00

050.

644

60.

008

0.05

896

0.00

0645

62

492

63

505

65

564

623

2.3

930

520.

0671

0.00

0003

00.

0177

60.

0005

0.02

596

0.00

080.

0821

60.

0009

0.65

76

0.00

90.

058

60.

0004

517

615

509

65

513

66

530

616

15.1

911

370.

0467

0.00

0089

0.14

0.09

16

0.00

110.

018

60.

0022

0.08

086

0.00

060.

635

60.

009

0.05

76

0.00

0636

06

4450

16

349

96

649

16

25Z

oned

17.1

334

394

1.18

350.

0000

830.

130.

3555

60.

0032

0.02

566

0.00

040.

0849

60.

0009

0.67

16

0.01

60.

0573

60.

0012

511

67

525

65

521

610

503

645

19.1

242

223

0.92

230.

0001

150.

190.

2804

60.

0057

0.02

526

0.00

060.

0829

60.

001

0.65

36

0.02

60.

0572

60.

0021

503

612

513

66

511

616

499

682

16.1

169

148

0.87

160.

0001

560.

250.

2528

60.

0056

0.02

46

0.00

070.

083

60.

0013

0.63

56

0.02

70.

0555

60.

0021

479

613

514

68

499

617

434

686

25.1

158

167

1.06

150.

0001

70.

270.

3267

60.

0061

0.02

496

0.00

060.

0806

60.

0013

0.63

56

0.02

60.

0571

60.

0021

497

613

499

68

499

616

497

681

18.1

129

119

0.92

120.

0000

10.

020.

2826

60.

0043

0.02

556

0.00

050.

0834

60.

0011

0.67

56

0.01

70.

0587

60.

0012

509

611

516

66

524

610

558

643

23.1

129

101

0.78

120.

0001

260.

20.

2432

60.

0049

0.02

616

0.00

070.

0838

60.

0011

0.64

46

0.02

30.

0558

60.

0018

520

613

518

66

505

614

444

672

15.2

122

262

2.14

110.

0003

080.

490.

4051

60.

0103

0.01

386

0.00

040.

0729

60.

0011

0.58

36

0.04

30.

0579

60.

0041

277

68

454

67

466

628

527

616

232

.111

013

01.

1811

0.00

0217

0.35

0.35

566

0.00

490.

0249

60.

0005

0.08

316

0.00

110.

634

60.

022

0.05

546

0.00

1749

86

1051

56

649

96

1442

76

718.

291

740.

828

0.00

001

0.02

0.26

166

0.00

490.

0259

60.

0008

0.08

096

0.00

170.

636

0.02

50.

0565

60.

0018

516

616

501

610

496

616

473

671

24.1

7876

0.98

70.

0000

10.

020.

3123

60.

0058

0.02

556

0.00

090.

0797

60.

0019

0.65

86

0.02

50.

0599

60.

0016

508

617

494

612

514

616

601

659

22.1

5854

0.93

60.

0000

10.

020.

296

60.

0076

0.02

66

0.00

090.

0816

60.

0018

0.69

46

0.03

70.

0617

60.

0028

519

618

506

611

535

622

664

610

1C

ore

28.1

2073

1803

0.87

217

0.00

0001

00.

2624

60.

0011

0.02

776

0.00

020.

0919

60.

0005

0.75

26

0.00

60.

0593

60.

0003

553

64

567

63

569

64

577

612

40.1

1159

364

0.31

106

0.00

0072

0.12

0.00

646

0.00

160.

0021

60.

0005

0.1

60.

0011

0.84

56

0.01

60.

0613

60.

0009

416

1061

56

762

26

964

86

3011

.211

3682

0.07

103

0.00

0013

0.02

0.00

736

0.00

060.

0099

60.

0008

0.09

846

0.00

080.

817

60.

012

0.06

026

0.00

0719

96

1660

56

460

66

661

26

2431

.110

8952

0.05

890.

0000

080.

010.

0137

60.

0005

0.02

566

0.00

090.

0888

60.

0008

0.71

56

0.00

90.

0584

60.

0004

511

618

549

65

548

65

545

617

19.2

720

517

0.72

269

0.00

0043

0.07

0.05

466

0.00

110.

0276

60.

0006

0.36

286

0.00

337.

051

60.

090.

1409

60.

0011

551

612

1996

616

2118

611

2239

614

35.1

698

860.

1210

90.

0000

380.

060.

0353

60.

001

0.04

686

0.00

140.

1644

60.

0015

1.55

96

0.02

60.

0687

60.

0009

925

628

981

68

954

610

891

627

22.2

619

302

0.49

540.

0000

620.

10.

0279

60.

0013

0.00

536

0.00

030.

093

60.

0018

0.77

60.

022

0.06

60.

0011

107

67

573

611

580

613

605

639

41.1

565

335

0.59

272

0.00

001

0.02

0.14

336

0.00

110.

1037

60.

0013

0.42

816

0.00

379.

473

60.

099

0.16

056

0.00

0819

946

2322

976

1723

856

1024

616

816

.252

643

0.08

430.

0000

780.

120.

0155

60.

0027

0.01

686

0.00

290.

0891

60.

0013

0.73

46

0.03

0.05

976

0.00

2133

76

5855

06

855

96

1859

36

8036

.148

181

01.

6953

0.00

0128

0.2

0.17

966

0.00

30.

011

60.

0011

0.10

276

0.00

140.

868

60.

021

0.06

136

0.00

1122

06

2363

06

863

46

1164

96

3939

.144

434

90.

7945

0.00

0082

0.13

0.24

286

0.00

270.

0279

60.

0005

0.09

046

0.00

10.

736

60.

016

0.05

96

0.00

155

76

955

86

656

06

956

96

3810

.235

740

91.

1544

0.00

0131

0.21

0.10

536

0.00

30.

0112

60.

0003

0.12

146

0.00

151.

112

60.

028

0.06

646

0.00

1422

46

773

96

875

96

1482

06

452.

432

187

0.27

320.

0000

10.

020.

095

60.

0113

0.03

566

0.00

450.

1015

60.

0046

0.85

66

0.09

0.06

116

0.00

5570

76

8962

36

2762

86

5064

46

206

38.1

282

584

2.07

480.

0000

940.

150.

1601

60.

002

0.01

236

0.00

020.

1592

60.

0016

1.63

96

0.03

20.

0747

60.

0012

248

64

953

69

985

613

1059

632

34.1

273

319

1.17

320.

0000

420.

070.

3569

60.

0038

0.02

896

0.00

040.

0946

60.

0009

0.78

46

0.01

90.

0601

60.

0013

576

69

583

66

588

611

606

646

42.1

270

175

0.65

100

0.00

0032

0.05

0.14

516

0.00

180.

0755

60.

0013

0.33

836

0.00

365.

847

60.

081

0.12

536

0.00

0914

726

2518

796

1819

536

1220

346

138.

324

344

51.

8330

0.00

005

0.08

0.48

366

0.00

680.

0244

60.

0007

0.09

256

0.00

150.

752

60.

018

0.05

896

0.00

0948

86

1457

06

956

96

1056

46

3537

.117

913

20.

7418

0.00

0015

0.02

0.20

346

0.00

480.

0253

60.

0007

0.09

186

0.00

130.

771

60.

025

0.06

096

0.00

1650

46

1456

66

858

06

1463

56

6033

.117

124

21.

4236

0.00

0092

0.15

0.19

146

0.00

320.

0263

60.

0005

0.19

446

0.00

212.

156

60.

046

0.08

046

0.00

1452

46

1111

456

1111

676

1512

076

3326

.182

931.

139

0.00

001

0.02

0.37

016

0.00

710.

0293

60.

0009

0.08

926

0.00

20.

737

60.

029

0.06

60.

0017

583

618

551

612

561

617

602

664

27.1

7944

0.56

160.

0000

10.

020.

1752

60.

0042

0.05

986

0.00

220.

1921

60.

0049

2.15

86

0.07

10.

0815

60.

0015

1174

642

1133

626

1167

623

1233

637

9.1

a83

971.

168

90.

0000

10.

180.

3659

60.

009

0.02

766

0.00

10.

0882

60.

002

0.73

26

0.04

20.

0602

60.

003

551

619

545

612

558

625

609

611

211

.1a

1177

300.

025

105

0.00

0089

0.16

0.00

746

0.00

10.

0289

60.

0043

0.09

756

0.00

220.

86

0.02

0.05

956

0.00

0657

66

8560

06

1359

76

1158

66

2010

.1a

137

860.

622

180.

0001

620.

290.

2308

60.

007

0.04

376

0.00

160.

1179

60.

0026

1.02

16

0.05

0.06

286

0.00

2586

56

3271

86

1571

46

2570

26

8813

.1a

354

975

2.75

711

00.

0001

190.

220.

6035

60.

003

0.04

696

0.00

110.

2141

60.

0048

2.45

26

0.06

0.08

36

0.00

0792

66

2112

516

2512

586

1812

706

16

ade

not

es S

HR

IMP

I a

nal

yses

;all

oth

ers

SH

RIM

P I

I.F

our

anal

ysed

spo

ts f

rom

th

e S

HR

IMP

II

sess

ion

hav

e be

en o

mit

ted

from

th

e ta

ble

(2.2

,3.2

,29.

1,30

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due

to h

igh

com

mon

Pb

or c

lear

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ss.A

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ata

quot

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.

The broad euhedrally zoned component of the zirconsis interpreted as magmatic crystallisation at the time ofintrusion of the Rathjen Gneiss protolith. This zircon haslow to moderate U contents and quite high Th/U ratios(mean about 0.95). From 11 spot analyses of this material,the ion microprobe data yielded a 206Pb/238U age of514.6 6 5.9 Ma (95% confidence level) which is identical tothe result from the Kober method.

The thin outermost rims were difficult to analyse asthey were mostly narrower than the ion beam spot. Thesehave consistently very high U (900–1600 ppm) and some (butnot all) have very low Th. These rims probably crystallisedduring post-intrusive metamorphism or even incipientpartial melting of the Rathjen Gneiss. It seems probablethat these rims represent growth during subsequent upperamphibolite facies metamorphism (D2 and D3) (Offler &Fleming 1968; Fleming & White 1984; Sandiford et al. 1992;Chen & Lui 1996). The SHRIMP II data yield a weightedmean 206Pb/238U age of 503 6 7 Ma (95% confidence level),consistent with constraints provided by the ages of youngersyntectonic granites in the belt.

In retrospect, it is clear that our evaporation Pb–Pbanalyses must represent Pb largely derived from the broadzoned magmatic growths and the two datasets are stronglycorroborative. The young ‘Kober’ ages (step ranks 1 and 2,Table 2) excluded from the evaporation plateau (Figure 9)probably incorporate some Pb from the these thin outerzones. Likewise, when the SHRIMP data (Table 3) are con-sidered, the indistinct upper limit of the magmatic zirconplateau (at step ranks 15–18) is likely to reflect the effectsof mixed evaporation of magmatic and minor amounts ofinherited zircon whose age is < 50 million years older thanthe age of crystallisation of the Rathjen Gneiss magma.

Age of inherited zircon cores

The combined age frequency spectrum of inherited coreanalyses from both SHRIMP I and II (Table 3) and fromPb–Pb evaporation results are plotted in Figure 11. Theseresults show ages to be broadly clustered in theNeoproterozoic and in the Late Archaean and earliestPalaeoproterozoic. In detail there are suggestions of peaksin the range 675–550 Ma, ca 1100–900 Ma, ca 1250 Ma, ca 2200–1950 Ma and ca 2500 Ma. These frequencies are quite unlikethose of nearby Proterozoic crystalline basement (GawlerCraton, Olary, Broken Hill) which are dominated by agesin the range 1900–1580 Ma (Foden 1996). The inheritance pat-tern corresponds well with detrital zircons from Kanman-too Group sedimentary rocks (Figure 11) and is unlike theAdelaidean sedimentary rocks whose detrital zircon popu-lation is dominated by Palaeoproterozoic and Mesopro-terozoic (1850–1550 Ma) zircons (Foden 1996; Ireland et al.1998). Along with the Nd isotopic composition the inheritedzircon component implies that the Rathjen Gneiss protolithwas not sourced from typical Palaeoproterozoic toMesoproterozoic South Australian cratonic basement.

ORIGIN OF THE RATHJEN GNEISS

The composition of the Rathjen Gneiss is transitionalbetween that of the granites of the post-tectonic suite

whose ages are in the range 490–480 Ma (Turner et al. 1992;Turner & Foden 1996) and that of the syntectonic suite (e.g.Cape Willoughby Granite, Kangaroo Island, 50867 Ma:Fanning 1990). Because it has high Nb/Y and Nb/Rb ratios,the Rathjen Gneiss plots in the within-plate granites fieldon geochemical discrimination diagrams for granites(Pearce et al. 1984). In the context of the southern AdelaideFold-Thrust Belt, it is the oldest of a series of granites with compositions showing progressive transition to thevolcanic-arc granites field during the ensuing compressivephase of the Delamerian Orogeny. This temporal trend isfollowed by migration back to the within-plate granites fieldby the post-tectonic granites.

Although the syntectonic granites fall into the volcanic-arc granites field (Pearce et al. 1984), there is no othercompelling evidence for subduction-related activity in ornear the Adelaide Fold-Thrust Belt (although contempor-aneous arc activity is recorded ~300 km east in westernVictoria). As we have articulated based on the evidence thatNd and Sr isotopic compositions vary with geochemicalconcentration (Figure 7), the granitic rocks of the AdelaideFold-Thrust Belt reflect the product of crust–mantle inter-action (assimilation–fractional crystallisation processes:DePaolo et al. 1992). Since their composition reflects vari-able crustal contamination of contemporary mafic magmasthey may be expected to bear similarities to granites

Rathjen Gneiss, SA 387

Figure 11 Plot of the age frequency of detrital zircon ages fromthe Kanmantoo Group (Ireland et al. 1998) compared with thecombined (Kober and SHRIMP) inherited zircon age populationfrom the Rathjen Gneiss.

from active arc settings, where mantle-derived magmasclearly interact with continental crust. In fact we may con-clude that the granitic discrimination diagrams devised byPearce et al. (1984 ) may be incapable of discerning graniteswhich are assimilation–fractional crystallisation-drivencontinental crust-contaminated mantle magmas from thoseof truly subduction-related volcanic-arc origin. On theother hand, as illustrated by this study from the southernAdelaide Fold-Thrust Belt, these discriminations distin-guish very accurately between granites of orogenic originand the anorogenic (Figure 5) post-tectonic granites, whichare much closer to the mantle isotope field and are mainlyfractionation products of mantle melts (Figures 6, 7).

The crustal end-member may be identified using Nd andSr isotope data (Figure 6) and the ages of inherited zircons.Their Nd–Sr isotope composition indicates that even themost S-type crust-dominated granites have Nd or Sr isotopecompositions that are respectively no lower or higher thanthe Kanmantoo Group sedimentary rocks, having signifi-cantly lower Sr and higher Nd isotopic ratios than theGawler Craton basement (Figure 6). In common with othersyntectonic granites from the belt, the Rathjen Gneiss hasinherited zircons whose ages are in the range 1200–550 Ma(Figure 11) and which are the same as detrital zircons inthe Kanmantoo Group clastic sedimentary rocks. They areunlike zircons from the Gawler Craton or other adjacentexposed Proterozoic cratonic regions, where zircons haveage frequency maxima between 2400 Ma and 1590 Ma.(Foden 1996).

DISCUSSION

The combination of new geochronological, structural andgeochemical data for the Rathjen Gneiss provide importantnew constraints on the tectonic development of thesouthern Adelaide Fold-Thrust Belt, and imply a morecomplex evolution than hitherto understood.

The new dates are consistent with previous age con-straints, which place the deposition of the NormanvilleGroup at 526 6 4 Ma (Cooper et al. 1992), and with an esti-mate of 510 6 2 Ma for the age of the D2 deformation (Chen& Lui 1996). The relationship between the intrusion of theRathjen Gneiss and initiation of deformation (D1) in theorogen remains problematic. Field relationships allow theRathjen Gneiss to pre-date the development of the subhor-izontal foliation and associated north–south-trendingstretching lineation, although most recent workers haveregarded it more likely that the Rathjen Gneiss is a trulysyntectonic granite (Sandiford et al. 1992, 1995; Oliver &Zakowski 1995). The arguments for this remain essentiallycircumstantial. For example, Sandiford et al. (1992, 1995)have argued that the elevated P–T conditions associatedwith the D1 deformation fabrics (600–650°C at ~400 MPa)require input of heat by magmas (see also Sandiford et al.1991). Furthermore, Sandiford et al. (1992) argued that theinitiation of crustal thickening in the orogen was coinci-dent with and responsible for the transition in the compo-sition of contemporary high-level crustal magmatism frommafic to felsic. In this scenario the emplacement of theRathjen Gneiss effectively marks the beginning ofDelamerian orogenic activity. This interpretation and the

new data imply that deposition, burial and heating ofthe Kanmantoo Group occurred in a relatively short inter-val (between 526 6 4 and 514 6 4 Ma). Together with con-straints on age of emplacement of the post-tectonicgranites at 490–480 Ma (Turner et al. 1996), it also providesa maximum duration for the Delamerian orogeny of~35 million years.

The S1 subhorizontal foliation and associated L1

north–south-trending stretching lineations found in theRathjen Gneiss and in the enclosing migmatitic metasedi-ments, provide interesting insight into the tectonic com-plexity of the belt. As discussed earlier, this fabric iskinematically distinct from fabrics resulting from sub-sequent upright folding during east–west shortening andwhich are probably associated with the development ofwestward-verging thrust-stacking in the more externallower grade parts of the belt (Jenkins & Sandiford 1992;Flöttmann et al. 1994, 1995). The possibility that thesefabrics are due to an early phase of north–south extensionaldeformation has been argued by Oliver and Zakowski(1995). We also note that the association of relatively deep-water sedimentation and mafic magmatism of the typeobserved in the Kanmantoo basin prior to ca 520 Ma prob-ably indicates lithospheric extension and rifting.

Emerging from this study is an interpretation of a morecomplex structural evolution than the traditional view ofeast–west convergence and shortening (Flöttmann et al.1994, 1995). Both structural and magmatic histories implythe juxtaposition of extensional and compressional tec-tonic regimes over relatively short time intervals. Zircongeochronology and Nd–Sr isotope studies each imply thatthe deep crustal component of these granite sources doesnot include Palaeoproterozoic to Mesoproterozoic rocks.This suggests that the Kanmantoo basin conceals animportant lithospheric structure characterised by younger mean crustal ages to the east. As simple closure of a pre-Delamerian rift from direct reversal and convergence onthe original extensional vectors would be expected toreturn South Australian autochthonous basement from theeast, the change in detrital zircon ages of Kanmantoo basinfill must support the translation of a new younger crustalsource terrain from the southeast. A combination of someelement of oblique convergence on this boundary, coupledwith variable geometry from north to south, may lead toperiodic local transition from extension to compressionperhaps indicative of a transpressional regime as suggestedby Flöttmann et al. (1995). Indeed, such a scenario wouldprovide an ideal environment to produce variations in theinteraction of mantle-derived magmas with, and fraction-ation in, the crust required to yield the variety of granitechemistries observed in the southern Adelaide Fold-ThrustBelt.

ACKNOWLEDGEMENTS

We thank W. Preiss, T. Flöttmann, S. Turner, S. F. Lui andAndy Burtt for their constructive and helpful discussionover the years. We acknowledge informed and constructivereviews by Alec Trendall and Nick Oliver. We also thank J. Stanley, S. Proferes and D. Bruce in the Department ofGeology and Geophysics, University of Adelaide for their

388 J. Foden et al.

assistance, and S. Stowe of the Australian NationalUniversity Electron Microscopy Unit and N. Gabbitas forassistance with cathodoluminescence imaging.

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Received 18 November 1997; accepted 3 December 1998

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