neoproterozoic deformation in the radok lake region of the northern prince charles mountains, east...

Precambrian Research 104 (2000) 1–24

Neoproterozoic deformation in the Radok Lake region ofthe northern Prince Charles Mountains, east Antarctica;

evidence for a single protracted orogenic event

S.D. Boger a,*, C.J. Carson b, C.J.L. Wilson a, C.M. Fanning c

a School of Earth Sciences, The Uni6ersity of Melbourne, Park6ille, Vic. 3010, Australiab Department of Geology and Geophysics, Yale Uni6ersity, New Ha6en, CT 06511, USA

c Research School of Earth Sciences, The Australian National Uni6ersity, Canberra, ACT. 0200, Australia

Received 7 May 1999; accepted 14 April 2000

Abstract

Ion microprobe dating of structurally constrained felsic intrusives indicate that the rocks of the northern PrinceCharles Mountains (nPCMs) were deformed during a single, long-lived Neoproterozoic tectonic event. Deformationevolved through four progressively more discrete phases in response to continuous north–south directed compression.In the study area (Radok Lake), voluminous granite intrusion occurred at �990 Ma, contemporaneous withregionally extensive magmatism, peak metamorphism, and sub-horizontal shearing and recumbent folding. Subse-quent upright folding and shear zone development occurred at �940 Ma, while new zircon growth at �900 Maconstrains a final phase of deformation that was accommodated along low-angle mylonites and pseudotachylites. Thisfinal period of deformation was responsible for the allochthonous emplacement of granulites over mid-amphibolitefacies rocks in the nPCMs. The age constraints placed on the timing of deformation by this study preclude thehigh-grade reworking of the nPCMs as is postulated in some of the recent literature. Furthermore, 990–900 Maorogenesis in the nPCMs is at least 50 Myr younger than that recognised in other previously correlated Grenville agedorogenic belts found in Australia, east Africa and other parts of the Antarctic. This distinct age difference implies thatthese belts are probably not correlatable, as has been previously suggested in reconstructions of the supercontinentRodinia. © 2000 Elsevier Science B.V. All rights reserved.

Keywords: Northern Prince Charles Mountains; East Antarctica; Granulites; Rodinia; Gondwana; Orogenesis

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1. Introduction

The margin of the east Antarctic craton, includ-ing the northern Prince Charles Mountains

(nPCMs), has traditionally been considered partof an extensive Neoproterozoic orogenic belt(1300–900 Ma) that has been correlated withmetamorphic belts of similar age in India, parts ofeast Africa, Sri Lanka, and Australia (Fig. 1a).These belts were thought to represent a majoraccretionary system that led to the formation of

* Corresponding author.E-mail address: s–[email protected] (S.D.

Boger).

0301-9268/00/$ - see front matter © 2000 Elsevier Science B.V. All rights reserved.

PII: S 0301 -9268 (00 )00079 -6

S.D. Boger et al. / Precambrian Research 104 (2000) 1–242

east Gondwana during the growth and consolida-tion of Rodinia (Grew and Manton, 1986; Katz,1989; Moores, 1991; Clarke et al., 1995; Rogers,1996). East Gondwana was thought to have thenremained intact and generally internally unde-formed until rifting in the Mesozoic (Yoshida etal., 1992). However, the more recent recognitionof extensive Palaeozoic tectonism within east

Antarctica (Zhao et al., 1992; Shiraishi et al.,1994; Hensen and Zhou, 1995; Carson et al.,1996; Fitzsimons et al., 1997) has lead to a num-ber of authors questioning the validity of thismodel. Instead, it has been suggested that eastGondwana may represent a collage of continentalfragments that accreted during the Palaeozoic(Hensen and Zhou, 1997).

Fig. 1. (a) Traditional reconstruction of Rodinia at �1000 Ma showing the location of East Gondwana within this reconstruction(after Hoffman, 1991; Unrug, 1997). In these models, east Gondwana is inferred to have formed though the accretion of parts ofAustralia, India and east Africa along a single laterally extensive Meso-Neoproterozoic orogenic belt thought to have rimmed theeast Antarctic coastline. (b) Gondwana at 500 Ma with the continents of east and west Gondwana illustrated. The position of thenPCMs is highlighted and enlarged in (c). Traditional models for the construction of Gondwana suggest that it remained intact fromRodinian times and formed a keystone onto which west Gondwana accreted. (c) Expanded section shows the gross geology of theregion of interest. NC, Napier Complex; VH, Vestfold Hills; sPCMs, southern Prince Charles Mountains; RC, Rayner Complex;nPCMs, northern Prince Charles Mountains; LHB, Lutzow-Holm Bay; PB, Prydz Bay. The more complicated tectonic frame workarising from the dissection of the Proterozoic mobile belt exposed in the nPCMs by Palaeozoic terrains recognised in Prydz andLutzow Holm Bays are highlighted.

S.D. Boger et al. / Precambrian Research 104 (2000) 1–24 3

The nPCMs, together with the Mawson Coastand the Rayner Complex, separate Prydz andLutzow-Holm Bays (Fig. 1c). With the recogni-tion of high-grade Palaeozoic tectonism withinthese terrains, the nPCMs has received consider-able attention regarding the extent of possiblePalaeozoic reworking. A number of authors havepostulated that a late Proterozoic to earlyPalaeozoic accretionary belt may have linkedPrydz and Lutzow-Holm Bays (Kriegsman, 1995;Hensen and Zhou, 1997) effectively crossing thenPCMs–Mawson Coast–Rayner Complex re-gion. Within the nPCMs, this inference has beensupported by Sm–Nd age data presented byHensen et al. (1997) from which they infer twosignificant tectonothermal events overprinting thewidely recognised �1000 Ma orogen; one at�800 Ma and a second at � 630–500 Ma.Similarly, Scrimgeour and Hand (1997) suggestthat the complex pressure–temperature paths ob-served along the eastern edge of the nPCMsreflect thermal interference between two unrelatedtectonic events. They infer that �1000 Ma tec-tonism is overprinted in the east by the affectsof 550–500 Ma orogenesis recognised to thenortheast in Prydz Bay. These studies contrastwith that of Kinny et al. (1997), who argue thatthe lack of new zircon growth or Pb-loss discon-cordia post-dating �1000 Ma indicate thatlate Proterozoic to early Palaeozoic tectonism inthe nPCMs was of relatively minor importance.This interpretation is more consistent with earlierstudies from the area (Tingey, 1982, 1991; Man-ton et al., 1992). These different hypotheses ariseprimarily due to a paucity of structurally well-constrained geochronologic data from thenPCMs, an issue that we have aimed to address inthis study.

In this paper, we refine the temporal frameworkof high-grade deformation and metamorphism inthe nPCMs. We describe the sequence of high-grade structural events recognised, and couple ourgeometric observations with structurally con-strained geochronological data obtained from fel-sic intrusives and locally derived leucosomes. NewSHRIMP age data from four structurally con-strained samples collected in the vicinity of RadokLake are presented, and the relative contributions

of Neoproterozoic and possible post-Proterozoicorogenesis in the nPCMs are assessed.

2. Regional geologic setting

The nPCMs are exposed as a series of isolatedinland ranges and massifs located on the westernmargin of the Amery Ice Self (Fig. 2). They formpart of an east–west trending orogenic belt, dom-inated by granulite facies felsic and mafic gneisses,interleaved with subordinate metasedimentaryand calc-silicate units (Crohn, 1959; Tingey, 1982,1991; McKelvey and Stephenson, 1990; Fitzsi-mons and Thost, 1992; Thost and Hensen, 1992;Kamenev et al., 1993; Hand et al., 1994b). Thesequence as a whole was intruded episodically bysignificant volumes of granitic and charnockiticmagma, as well as by locally derived partial melts(Munksgaard et al., 1992; Sheraton et al., 1996;Kinny et al., 1997; Zhao et al., 1997). At BeaverLake (Fig. 2), the high-grade gneisses are overlainby relatively undeformed Permo-Triassic sedi-ments (Crohn, 1959; Mond, 1972; Webb andFielding, 1993; Fielding and Webb, 1995, 1996;McLaughlin and Drinnan, 1997a,b). These arethought to lie in a sub-basin on the western sideof the Lambert Graben, an inferred rift systemthat separates the nPCMs from the Palaeozoic(ca. 550–500 Ma) granulite facies terrain of PrydzBay (Ren et al., 1992; Zhao et al., 1992; Carson etal., 1995; Dirks and Wilson, 1995; Harley andFitzsimons, 1995; Hensen and Zhou, 1995; Car-son et al., 1996; Fitzsimons, 1997; Fitzsimons etal., 1997). To the north and west of the nPCMs,the extent of the terrain is unconstrained. How-ever, it probably extends to at least the MawsonCoast (Fig. 3), where rocks of similar age andgrade are exposed (Young and Black, 1991;Young et al., 1997), and has been tentativelycorrelated with the Rayner Complex still furtherto the west (Black et al., 1987). The terrain isbounded in the south by exposures of olderMeoproterozoic volcanics at Fisher Massif(Kamenev et al., 1993; Beliatsky et al., 1994;Mikhalsky et al., 1996; Kinny et al., 1997; Laibaand Mikhalsky, 1999), and by granitic Archaeanbasement complex overlain by two or more super-


Fig. 2. Schematic map of the northern Prince Charles Mountains showing the study area, extent of outcrop and the distribution ofthe Proterozoic basement and Permo-Triassic strata. Localities of existing U–Pb zircon geochronolgical data are also illustrated.Data from Mt McCarthy, Loewe Massif, Mt Collins and the Fisher Massif are after Kinny et al. (1997); data from Jetty Peninsulaare after Manton et al. (1992). Locality of cross-section illustrated in Fig. 3 is also shown. Insert shows the geographic position ofthe northern Prince Charles Mountains along the western margin of the Amery Ice Self. Mawson and Davis refer to AustralianAntarctic Stations.

crustal sequences in the southern Prince CharlesMountains (Grew, 1982; Tingey, 1982, 1991;Kamenev et al., 1993).

Previous studies have established that thenPCMs attained granulite facies metamorphicconditions of approximately 800°C and 6–7 kbar(Fitzsimons and Thost, 1992; Fitzsimons andHarley, 1994a,b; Hand et al., 1994a; Scrimgeourand Hand, 1997), and followed a retrograde pathdominated by cooling (Fitzsimons and Harley,1992; Thost and Hensen, 1992; Fitzsimons andHarley, 1994a,b; Stephenson and Cook, 1997). Inthe southern and eastern parts of the nPCMs,these cooling trajectories are thought to be over-printed by a subsequent phase of decompression

(Hand et al., 1994a; Nichols 1995; Scrimgeour andHand, 1997).

The earliest geochronological data from thePrince Charles Mountains were reconnaissanceRb–Sr ages obtained by Arriens (1975). Wholerock isochrons presented by Arriens (1975) yieldages of �1000 Ma, whereas mineral dates (biotiteand muscovite) produced ages that clusteredaround 500 Ma. On the basis of these results,Tingey (1982, 1991) suggested that high-grademetamorphism in the nPCMs occurred at �1000Ma, overprinted by a widespread but relativelylow-grade thermal event recorded by mica systemsat �500 Ma.


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More recent conventional and SHRIMP (Sensi-tive High Resolution Ion Microprobe) dating ofzircon has yielded results consistent with the ini-tial ages obtained by Arriens (1975). Manton etal. (1992) reported upper intercept ages of 1000+14/−11 Ma (orthogneiss) and 940+27/−17 Ma(leucogranite) from rocks outcropping at JettyPeninsula (Fig. 3). The former age is interpretedas dating granulite facies metamorphism (Mantonet al., 1992). U–Pb SHRIMP dating by Kinny etal. (1997) has produced ages for felsic intrusives atLoewe Massif (980921 Ma), Mt Collins (976925, 98497 and 984912 Ma) and Mt McCarthy(990930 Ma). The intrusive ages from all locali-ties are statistically identical, and are interpretedto suggest that the nPCMs experienced a wide-spread magmatic event that occurred concurrentlywith regional high-grade metamorphism (Kinny etal., 1997).

Younger zircon and monazite ages of about550–500 Ma were obtained by Manton et al.(1992) from minor pegmatites and granites crop-ping out at Jetty Peninsula. These are the onlyreported U–Pb ages younger than �940 Mafrom the nPCMs. They are similar to Rb–Srmineral isochron ages of about 480 Ma and theprojected lower intercept ages of some zircondiscordia obtained by Manton et al. (1992), bothof which were interpreted as reset ages associatedwith granite and pegmatite emplacement. Else-where in the nPCMs, two-point Sm–Nd garnet-whole rock and garnet-matrix ages from a varietyof rock types form two age groupings at �800and 630–550 Ma (Hensen et al., 1997). The �800 Ma ages are more prevalent in the west of thenPCMs, while the younger �630–550 Ma agescome predominantly from the east. Hensen et al.(1997) interpreted the Sm–Nd ages as datinghigh-temperature thermal events post-dating the�980 Ma magmatism recorded by zircon.

3. Structure

Detailed structural studies within the nPCMshave previously centred upon the Aramis, Porthosand Athos ranges approximately 100 km to thenorthwest of Radok Lake (Fitzsimons and

Harley, 1992; Thost and Hensen, 1992), and atElse Platform located approximately 75 km to thenortheast (Hand et al., 1994b). An outline of thestructure at Radok Lake was also presented byMcKelvey and Stephenson (1990). All previousstudies describe a pervasive layer-parallel folia-tion, folded initially into isoclinal layer-parallelfolds, which were subsequently reoriented aboutupright east–west trending folds, and then trans-posed into east–west trending steeply dippinghigh-strain zones. The different fold and foliationnomenclature used by previous authors is summa-rized in Table 1. An identical sequence of struc-tures observed during this study is described inthe following. In addition, a later phase of dis-crete mylonitisation was recognised, and is de-scribed in this paper. All structural relationshipsand orientation data are illustrated in Fig. 3, aschematic cross-section through the basement ex-posures at Radok Lake.

3.1. D1–2 deformation

The earliest recognised fabric element is a com-posite S0/S1 fabric defined by an intense andpervasive preferred mineral orientation, which isalways concordant with lithological layering (S0).This fabric defines the dominant foliation surfacein the nPCMs (Fitzsimons and Thost, 1992; Thostand Hensen, 1992; Hand et al., 1994b), and char-acteristically contains a well-developed east–northeast (ENE) trending L1 lineation (Fig. 3).Although overprinted by two episodes of folding(D2 and D3), S1 is well preserved and only weaklyoverprinted by the development of new fabricsassociated with subsequent folding.

D2 resulted in the reorientation of the com-posite S0/S1 fabric about recumbent, isoclinal F2

folds. Although folding S0/S1 isoclinally, there islittle development of an axial planar fabric.Rather, S1 remains the predominant foliation,potentially intensified on the limbs of F2 folds,where its orientation is parallel to the F2 axialplane. Large-scale F2 folds were not recognised,but were inferred, as mesoscale F2 isoclinal foldsare common with type-three F2 and F3 fold inter-ference patterns (Ramsay, 1967) recognised in D3

low-strain zones. F2 folds form about ENE trend-


ing axes that parallel the L1 lineation (Fig. 3). Theformation of an L2 lineation is not recognised,although the development of such a lineationcannot be precluded. If present, L2 was of thesame grade and formed parallel to and potentiallyintensified L1 so that the two were indistinguish-able. The formation of both a flat-lying foliation(S1) and recumbent fold closures (F2) is poten-tially consistent with ongoing dominantly sub-horizontal simple shear. Indeed granulite faciesassemblages define both the S1 foliation and,where developed, the subtle S2 fabric. Addition-ally, F2 folding has formed coaxially with thelineation (L1) (Fig. 3). Although the rotation andtransposition of lithological layering (S0) into par-allelism with S1 may represent the preservation ofan otherwise overprinted prograde history, thesimilarity in metamorphic grade and the coaxialnature of the structures suggests that a progres-sive evolution from D1 to D2 is more likely. Asimilar conclusion was drawn by a number ofprevious structural studies from the region (Fitzsi-mons and Harley, 1992; Thost and Hensen, 1992;Hand et al. 1994b).

Tectonic transport during D1–2 is inferred tohave been north–south directed, perpendicular tothe lineation orientation and the F2 fold axis.Although lineations are commonly inferred toparallel the transport direction, there is no evi-

dence of this recognised at Radok Lake. Kine-matic indicators such as asymmetric F2 folds werefound on outcrop surfaces normal to the lin-eation, while F2 and F3 (described below) are allcoaxial, the latter event clearly having developedin response to north–south directed shortening.Furthermore, the recent work of Passchier (1997)describes the development of orthogonal lin-eation–transport direction relationships undernon-ideal simple shear conditions, consistent withthe overall deformational environment envisagedfor D1–2.

3.2. D3 deformation

D3 was responsible for the major east–westtrending structural grain and the gross geometryobserved in the nPCMs. D3 folded the compositeS0/S1 surface (and D2 structures) into a series ofmeso- to macroscale upright F3 fold closures (Fig.4a). Folding occurred in response to broadlynorth–south directed compression, producing aseries of ENE trending folds that are parallel toboth the F2 fold axes and the L1 stretching lin-eation (Fig. 3). F3 folds plunge moderately toshallowly, to either the ENE or WSW (Fig. 3).However, with ongoing shortening, the increase inD3 strain is accompanied by a localised decreasein fold wavelength and an increase in D3 foldplunge. A weak axial planar S3 foliation is dis-

Table 1Summary of the nomenclature used to describe the deformational features observed throughout the northern Prince CharlesMountains

Formation of Steeply dippingUpright foldingAuthor Isoclinal Low-angle discreteregional gneissic shear zone mylonite andrecumbent

formationlayering folding pseudotachyliteformation

D3 D4D3D2D1This studyD1 D2Scrimgeour and Hand (1997) D3 D4D1/MY1 D2Nichols (1995) D3 MY2 MY3D1 D1Hand et al. (1994b) D2 D2 D3

Thost and Hensen (1992) D5D1–D2–D3 D6D4 D5D4 D5 D6 D7–D8Fitzsimons and Thost (1992) D1–D2–D3

D1 D2 D3McKelvey and Stephenson(1990)


Fig. 4. Structural and intrusive features from Radok Lake: (a) upright west-plunging F3 antiform; (b) D3 upright high-strain zone;(c) photomicrograph of coplanar D4 pseudotachylite and mylonite; (d) reverse offset-D4 mylonite with subtle drag folding on uppersurface; (e) concordant intrusive contact between syn-D2 granite (distinctive pale unit) and host lithologies (sample 9628-141); (f)orthopyroxene bearing leucosome localised within D3 boudin neck (sample 9628-73); (g) clinopyroxene bearing leucosome formedconcordant with, but locally cross-cutting S0/S1 foliation within amphibolite-facies intermediate and felsic orthogneisses.

cernible in some F3 closures. However, the devel-opment of S3 is generally restricted to the limbs ofF3 folds where discrete D3 high-strain zones aredeveloped.

D3 high-strain zones (Fig. 4b) are tens to hun-dreds of metres in width. Within these zones, S3 isgenerally indistinguishable in appearance from S0/

S1. However, continued localisation of strainwithin D3 shear zones has locally overprinted thegneissic S3 foliation with a mylonitic to ultramy-lonitic fabric. The mylonitic fabric is defined by adeformation-induced grain size reduction and bythe growth of new, lower grade metamorphicassemblages (Hand et al., 1994a; Nichols, 1995;


Scrimgeour and Hand, 1997). In the vicinity ofRadok Lake, D3 high-strain zones are orientatedparallel to the S3 axial surface, trending ENE anddipping steeply to the north. Elsewhere in thenPCMs, some D3 high-strain zones can dipsteeply to the south (for example, Hand et al.,1994b). The S3 foliation contains a steeply plung-ing L3 stretching lineation (Fig. 3) while S–Cfabric relationships and offset leucosomes showlineation-parallel, reverse kinematics. Althoughlarge and regionally significant features, it is un-likely that D3 high-strain zones accommodate sig-nificant movement as there are no observed orreported changes in metamorphic grade acrossthese structures.

The accommodation of D3 strain by uprightfolding, then shearing along steeply dipping high-strain zones is consistent with a transition duringD3 from dominantly pure shear flattening to dom-inantly vertically oriented simple shear. This tran-sition is manifested by the development of anintermediate non-coaxial pure shear regime inwhich the F3 fold axes were rotated toward avertical stretching axis, reflected by the observedincrease in F3 fold plunge (Fig. 6).

3.3. D4 deformation

Deformation post-dating D3 folding and up-right shear zone development is characterised bythe formation of low- to moderate-angle my-lonites and coplanar pseudotachylites (Fig. 4c,d).With the exception of minor drag folds, D4 my-lonites do not reorient earlier high-grade fabricelements. They form discrete localised zones ofdeformation up to 100 mm in width, typicallydefined by biotite and quartz. The presence ofcoexisting garnet and sillimanite, and/or greenhornblende within some D4 mylonites, is consis-tent with their formation under amphibolite faciesconditions, at temperatures, at least initially, inexcess of 520°C (i.e. within the stability field ofsillimanite). The development of coplanar pseudo-tachylites was probably a function of strain rate(Hobbs et al., 1986), and the anhydrous nature ofthe granulite facies host rocks (Camacho et al.,1995). The overprinting of earlier textural rela-tionships by D4 is limited to areas within, or

immediately adjacent to, the mylonite zones.Thus, the textural evolution of the nPCMsfinished with the cessation of D3 and resulted inthe preservation of granulite facies metamorphictextures formed during D1–3.

D4 mylonites generally trend northwest, andform a moderately northeast and southwest dip-ping conjugate set (Fig. 5). Offset marker hori-zons and parasitic drag folds show reverse offset(Fig. 4d), while palaeostress analysis suggest thatD4 formed in response to NNE–SSW-directedcompression (Fig. 5). Offset on most mylonites istypically minor (B2 m). However, a major flat-lying D4 mylonite zone exposed along the westernshore of Radok lake juxtaposes granulite faciesgneisses over amphibolite facies interlayered felsicand intermediate orthogneisses (Fig. 4). The felsicorthogneiss consists of equigranular quartz, K-feldspar and plagioclase. The intermediate or-thogneisses are composed of weak to randomlyorientated biotite and fine-grained quartz inter-grown with coarse grained plagioclase. Euhedralhornblende, green in hand specimen and thinsection, may occur in the matrix, although it morecommonly forms a reaction rim separating thefelsic and mafic bands. Sphene typically occursalong grain boundaries between biotite and il-menite, while chlorite partially to completelypseudomorphs biotite. Clinopyroxene is also lo-cally observed in the leucosomes, where it is gen-erally rimmed by green hornblende. In contrast tothe overriding units, these rocks do not containgarnet, orthopyroxene or brown hornblende, orrelics thereof, minerals that are ubiquitous in thegranulites typical of the nPCMs. They contain noevidence of ever undergoing granulite facies meta-morphism and are considered possible equivalentsof the lower grade rocks exposed at Fisher Massifto the south. Shear sense indicators suggestthrusting involved emplacement of the granulitesto the south, consistent with this interpretation,whilst the juxtaposition of granulites over lowergrade rocks at Radok lake implies that at least inthe southern portion of the nPCMs may be al-lochthonous. This is an interpretation first for-warded by Manton et al. (1992) to explain thepresence of As, Mo, Be and B in post-orogenichydrous pegmatites outcropping at JettyPeninsula.


Fig. 5. Equal area stereographic projections of mylonite planeand contained lineation data (top), and palaeostress recon-structions after the technique of Oncken (1988) (bottom),relating to D3 and D4.

vidual sectioned zircons. Zr2O+, 206Pb+, 207Pb+,208Pb+, 238U+, 232ThO+, and 238UO+ were mea-sured in cycles by magnetic field switching, sevencycles per data set. Analysis of unknowns wasinterspersed with analyses of the standard SL13(which has a radiogenic 206Pb/238U ratio of0.0928) in order to monitor the differential frac-tionation between U and Pb. Radiogenic Pb com-positions were determined after subtractingcontemporaneous common Pb as modelled byCumming and Richards (1975). All reported agesare based on 207Pb/ 206Pb ratios corrected forcommon Pb by the 204Pb technique (Compston etal., 1984, 1992). Ages presented in the text arestated with 2s confidence limits.

Fig. 6. Interpretive cartoon illustrating the evolution of thestrain regime during deformation in the Radok Lake area ofthe nPCMs.

4. Analytical procedure

Zircons for SHRIMP analysis were separatedby standard heavy liquid and magnetic proce-dures, and then by hand picking. They were thenmounted in epoxy resin discs along with frag-ments of zircon standard SL13. The discs werepolished and Au coated before being analysed oneither the SHRIMP I or SHRIMP II (sample9628-142) ion-microprobe at the Australian Na-tional University, Canberra. Cathodoluminescent(CL) imaging was conducted to assess the internalstructure of the unknown zircons from whichselected zircon domains were analysed for U, Thand Pb isotopic composition. A primary beam ofO− ions was used to sputter positive secondaryions from areas �25 mm in diameter from indi-


5. Geochronological results

5.1. K-Feldspar granite — west wall of Radoklake (sample 9628-141)

Sample 9628-141 represents one of a numberof granitic sheets containing quartz, pink K-feldspar, minor garnet and biotite, as well asaccessory apatite and zircon. Equivalent graniticbodies at Radok lake vary in width from one toseveral hundred metres, and are part of a suiteof K-feldspar granites that form the most volu-minous intrusive bodies observed in the vicinityof Radok Lake. They occur on either side of theBattye Glacier, at Fox Ridge and at ManningMassif (Fig. 2), and vary from coarse grainedand megacrystic, to finer grained and equigranu-lar.

The sampled granite (sample 9628-141) formsa sheet that has concordant boundaries with thesurrounding host lithologies and the S0/1 foliation(Fig. 5e). It contains a well-developed layer-par-allel foliation reoriented by upright folding (F3).The foliation within the granite does not defineF2 fold closures, nor were recumbent folds (F2)recognised within, or defined by, any of thegranites of this generation. We therefore con-clude that granite emplacement did not precededeformation as the granites do not preserve theearliest structures recognised in their host litholo-gies, but clearly pre-date D3 as they are foldedby this event. Thus, these granites are interpretedto have intruded synchronously with D1–2.

Zircons from sample 9628-141 are orange andtranslucent, and form a subhedral to anhedralpopulation of uniform size. Average elongationratios of 1:1–2:1 are observed for grain lengthsbetween 150 and 200 mm. The zircons typicallycontain 200–500 ppm U, with a Th/U ratio be-tween 0.5 and 0.9. Zircons can show some inter-nal sector zoning that occasionally mantles smalldetrital cores (Fig. 7a), which were not analysedduring this study. The zircons generally lackovergrowths but, where rims do exist, they arediscontinuous, highly luminescent, and too nar-row for analysis (Fig. 7b). A simple igneousorigin is inferred for these zircons, with the

rounded terminations inferred to reflect partialmetamorphic resorption.

The isotopic data for the zircons from sample9628-141 are presented in Table 2. Twenty zircongrains were analysed, of which all but two analy-ses produce a concordant mean 207Pb/206Pb ageof 990918 Ma (mean square of weighted devi-ates (MSWD=1.13)) (Fig. 8a). The discrepantanalyses, 3.1 and 20.1, were both highly discor-dant (61 and 50%, respectively) and are inter-preted as the result of partial Pb loss. Theconcordant age given by the remaining 18 analy-ses is interpreted as the crystallisation age of thegranite and is considered, to constrain D2, tohave occurred at �990 Ma.

5.2. K-Feldspar granite — west wall of Radoklake (sample 9628-142)

Sample 9628-142 was collected from a 1–2 mwide, coarse-grained, sub-vertically orientatedENE-trending granitic dyke located along thewest wall of Radok Lake (Fig. 3). The dykeintruded along the axial surface of an F3 fold, isunfoliated, cross-cuts structures developed duringF3 folding, and is offset by later D4 mylonites.The dyke contains quartz, pink K-feldspar, mi-nor garnet and biotite, and accessory apatite andzircon. It is very similar in both colour and min-eralogy to the more volumetrically significantpre-D3 sills (sample 9628-141) already described.The intrusion of sample 9628-142 is inferred tohave occurred late-syn- to post-D3 and is consid-ered to place a minimum age on the timing ofD3 fold development.

Zircons from sample 9628-142 are orangeand translucent, and are similar in appearanceto those from sample 9628-141. They form aeuhe-dral to subhedral population of varyinggrain size. Zircons vary from 200 to 400 mm inlength, and have length:width ratios of approxi-mately 2:1. However, longer grains with elonga-tion ratios up to 4:1 do occur. The zircons fromthis sample can show internal sector zoning aswell as planar growth bands (Fig. 7c), althoughthey mostly do not show much internal structure.Rare rounded detrital cores are found in some


Tab

le2

U–T

h–P

bis

otop

icco

mpo

siti

ons

ofzi

rcon

sfr

omth

eno

rthe

rnP

rinc

eC

harl

esM

ount

ains

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Pb

(ppm

)20

4 Pb/

205 P

bT

h(p

pm)

f206

(%)

Rad

ioge

nic

rati

osA

ges

(Ma)

Con

cord

ance

(%)

Gra

in.s

pot

Th/

UU

(ppm

)

206 P

b/23

8 U20

7 Pb/

206 P

b9

207 P

b/23

5 U20

7 Pb/

235 U

920

7 Pb/

206 P

b9

920

6 Pb/

238 U

9

Sam

ple

9628

-141

( K-f

elds

par

gran

ite

—w

est

wal

lof

Rac

lok

Lak

e)0.

590.

1468

00.

0046

1.37

720.

063

1.1

0.08

830

10.

0032

883

2887

987

010

210

233

51.

1154

0.00

034

480.

0001

30.

220.

1578

30.

0081

1.67

740.

098

0.07

20.

0021

945

4596

110

0061

9526

42.

122

20.

841.

360.

1184

43.

10.

0150

285

1.30

260.

202

0.08

00.

0060

722

8784

711

9115

761

189

0.66

390.

0008

00.

080.

1538

90.

0030

1.58

110.

058

0.07

50.

0021

922

170.

0000

596

30.

8810

5859

8738

4.1

212

286

0.00

012

494

0.20

0.16

355

0.00

321.

5943

0.04

40.

071

0.00

1297

618

9094

936

103

239

0.48

845.

1B

0.01

0.14

086

0.01

191.

5316

0.14

10.

075

0.00

2789

067

6.1

943

189

1069

7483

158

0.84

320.

0000

00.

190.

1521

00.

0081

1.51

370.

094

0.07

2O

.001

891

346

0.00

011

906

342

991

5292

7.1

550.

5318

10.

0000

133

90.

020.

1001

40.

0083

1.65

070.

094

0.07

50.

0013

958

4699

010

8236

9016

90.

5058

8.1

0.15

0.15

806

0.00

641.

5824

0.07

90.

073

9.1

0.00

1823

194

638

963

1003

5094

205

0.89

420.

0000

90.

540.

1613

60.

0031

1.50

800.

090

0.06

80.

0022

964

170.

0003

193

434

086

289

112

10.1

640.

9632

80.

0002

050

60.

350.

1658

90.

0058

0.55

400.

072

0.06

80.

0019

989

3195

286

758

114

290

0.57

8911

.10.

200.

1530

40.

0090

1.45

960.

109

0.06

90.

0027

915

5012

.191

440

490

483

102

233

0.58

660.

0001

2B

0.01

0.16

220

0.00

411.

6295

0.06

00.

073

0.00

1899

8922

0.00

0090

225

110

1050

9813

.143

0.49

124

0.00

011

178

0.20

0.15

105

0.00

641.

5048

0.09

30.

072

0.00

2990

736

932

993

8491

149

0.84

3114

.10.

380.

1533

90.

0027

1.44

660.

049

0.06

80.

0019

920

1590

888

158

105

15.1

489

405

0.83

850.

0002

20.

230.

1488

40.

0074

1.50

160.

099

0.07

30.

0027

894

420.

0001

493

10.

8110

1977

8869

16.1

407

330

0.00

023

1216

0.39

0.16

874

0.00

671.

6880

0.07

80.

072

0.00

1410

0537

1003

999

3910

174

0.06

194

17.1

0.21

0.16

187

0.00

611.

5833

0.07

40.

071

0.00

1796

734

964

956

4910

118

.170

938

10.

5412

10.

0001

20.

020.

1572

40.

0048

1.56

340.

050

0.07

20.

0005

941

270.

0000

195

639

498

913

9519

.126

8411

80.

042.

490.

0856

10.

0016

20.1

0.87

9544

340.

038

0.07

50.

0029

530

964

110

5579

5029

90.

0736

60.

0014

5

Sam

ple

9628

-142

( K-f

elds

par

gran

ite

—w

est

wal

lof

Rad

okL

ake)

0.02

0.15

581.

10.

0034

196

0.52

50.

039

0.07

110.

0008

932

1994

095

922

9715

70.

7935

0.00

001

0.06

0.15

270.

0034

1.44

50.

038

0.06

870.

0008

916

190.

0000

290

80.

8088

825

103

492.

128

422

60.

0000

123

30.

020.

1532

0.00

331.

497

0.03

60.

0709

0.00

0691

919

929

955

1896

189

0.81

413.

10.

020.

1627

0.00

361.

582

0.03

90.

0705

0.00

0697

220

4.1

963

331

943

1610

334

11.

0364

0.00

001

0.06

0.15

520.

0033

1.49

50.

034

0.06

990.

0005

930

180.

0000

392

832

492

414

101

5.1

601.

0333

30.

0000

129

70.

020.

1573

0.00

331.

512

0.03

60.

0697

0.00

0694

219

935

920

1710

229

10.

9855

6.1

0.02

0.15

090.

0035

1.48

60.

0338

0.07

140.

0006

906

2092

597

016

937.

120

818

20.

8736

0.00

001

0.01

0.16

140.

0033

1.56

90.

034

0.07

050.

0004

985

190.

0000

195

80.

8894

310

102

978.

152

146

00.

0000

527

30.

090.

1546

0.00

351.

483

0.03

80.

0696

0.00

0792

619

923

916

2110

124

90.

9149

9.1

0.02

0.15

750.

0034

1.55

80.

036

0.07

180.

0004

943

1995

498

013

9810

.132

214

30.

4553

0.00

001

0.07

0.14

770.

0032

1.41

70.

035

0.06

960.

0008

888

180.

0000

489

649

916

1897

11.1

284

262

0.92

0.02

0.15

350.

0032

12.1

1.47

734

00.

036

0.06

960.

0008

921

1892

192

222

100

673

1.98

760.

0000

1

Sam

ple

9628

-73

( Opx

-bea

ring

leuc

osom

e—

nort

hw

all

ofB

atty

eG

laci

er)

0.08

0.15

720.

0021

1.52

70.

041

1.1

0.07

0437

10.

0015

941

1294

194

145

100

229

0.62

580.

0000

30.

020.

1535

0.00

361.

497

0.04

10.

0707

0.00

0992

120

0.00

001

929

339

949

2597

2.1

500.

4715

90.

0000

115

80.

020.

1509

0.00

361.

500

0.05

00.

0072

10.

0014

906

2193

098

939

9220

81.

3228

3.1

B0.

0l0.

1637

0.00

471.

631

0.05

50.

0723

0.00

1197

726

4.1

982

675

994

3198

300

0.44

106

0.00

000

0.19

0.15

960.

0033

1.54

30.

046

0.07

010.

0014

954

180.

0001

194

834

093

341

102

6.1

540.

6120

60.

0000

735

90.

120.

1621

0.00

501.

548

0.05

30.

0693

0.00

0696

828

949

906

2510

722

50.

6358

7.1

0.22

0.15

420.

0052

1.52

20.

057

0.07

160.

0009

925

298.

193

950

997

425

9535

90.

7180

0.00

013

B0.

010.

1696

0.00

311.

676

0.04

10.

0717

0.00

1010

1017

0.00

000

999

0.76

976

2810

484

9.1

481

365

0.00

002

507

0.03

0.15

350.

0054

1.51

00.

057

0.07

130.

0007

921

3093

496

720

9538

90.

7781

10.1

0.02

0.17

020.

0121

1.79

00.

181

0.07

630.

0048

1013

6711

.110

4263

911

0213

292

449

0.70

110

0.00

001

0.02

0.16

320.

0046

1.56

10.

047

0.08

940.

0006

975

260.

0000

195

536

391

017

107

12.1

600.

7226

00.

0000

432

10.

070.

1545

0.00

561.

456

0.05

90.

0683

0.00

0992

632

912

878

2710

620

40.

6450

13.1


Tab

le2

(Con

tinu

ed)

Pb

(ppm

)20

4 Pb/

205 P

bf2

06(%

)R

adio

geni

cra

tios

Age

s(M

a)C

onco

rdan

ce(%

)T

h(p

pm)

Gra

in.s

pot

Th/

UU

(ppm

)

206 P

b/23

8 U9

207 P

b/23

5 U9

207 P

b/20

6 Pb9

206 P

b/23

8 U9

207 P

b/23

5 U20

7 Pb/

206 P

b9

14.1

0.07

461

0.15

580.

0090

1.50

50.

090

0.07

010.

0008

933

5093

283

023

100

323

0.70

730.

0000

40.

220.

1671

0.00

721.

590

0.08

40.

0890

0.00

1899

640

0.00

013

966

277

899

5411

115

.145

0.57

157

0.00

000

443

B0.

010.

1509

0.00

411.

562

0.05

30.

0709

0.00

1495

823

955

953

4010

032

80.

7473

16.1

0.05

0.15

700.

0040

1.52

317

.10.

044

553

0.07

040.

0008

940

2294

093

922

100

422

0.76

900.

0000

3B

0.01

0.16

020.

0043

1.58

40.

049

0.07

170.

0009

958

2496

497

827

980.

0000

00.

6829

142

718

.189

Sam

ple

9628

-196

( Cpx

bear

ing

leuc

osom

e—

wes

tw

all

ofR

adok

Lak

e)11

0.00

000

B0.

010.

1389

0.00

791.

517

0.13

60.

0792

0.00

4983

945

937

1177

128

7162

1.1

340.

550.

100.

0879

0.00

401.

516

0.13

12.

10.

1620

970.

0091

424

2493

724

7698

1758

0.59

110.

0000

6B

0.01

0.14

610.

0026

1.49

50.

062

0.07

420.

0028

879

150.

0000

092

914

010

4874

843.

127

0.77

108

0.00

005

305

0.06

0.13

960.

0072

1.31

20.

077

0.06

820.

0015

842

4185

187

447

9613

90.

4650

4.1

670.

0002

00.

340.

1615

0.00

311.

522

0.05

60.

0684

0.00

2096

517

939

879

6111

039

0.59

5.1

13B

0.0l

0.15

530.

0031

1.60

70.

065

0.07

500.

0025

931

170.

0000

097

380

1069

6787

6.1

170.

4742

0.00

012

497

0.21

0.16

870.

0057

1.89

80.

074

0.07

300.

0018

1005

3110

0810

1350

9913

50.

2797

7.1

0.32

0.16

150.

0036

1.49

60.

054

0.06

720.

0017

968

208.

192

979

843

5511

556

1.70

170.

0001

90.

040.

1699

0.00

471.

642

0.05

30.

0701

0.00

2610

1126

0.00

002

987

256

932

2910

99.

156

0.50

132

0.00

046

3915

0.78

0.09

710.

0032

0.90

90.

048

0.08

790.

0026

597

1986

596

580

8948

0.01

405

10.1

0.58

0.19

710.

0091

1.90

30.

123

0.07

000.

0028

1160

4910

8292

911

.184

242

125

124

0.51

600.

0003

4B

0.01

0.17

430.

0104

1.74

30.

107

0.07

250.

0007

1038

570.

0000

010

250.

5510

0120

103

101

12.1

485

254

0.00

021

235

0.35

0.18

370.

0102

1.73

20.

102

0.08

840.

0010

1087

5610

2188

131

123

185

0.79

5713

.1B

0.01

0.15

250.

0059

1.55

80.

071

0.07

410.

0015

915

3395

410

4414

.140

495

8852

81.

0710

70.

0000

0B

0.01

0.15

000.

0044

1.43

30.

050

0.06

930.

0011

901

250.

0000

090

321

690

732

9915

.112

4240

50.

33B

0.01

16.1

0.16

7430

90.

0029

1.63

30.

084

0.07

070.

0024

996

1698

395

070

105

135

0.45

600.

0002

0

aU

ncer

tain

ties

give

nat

the

1sle

vel;

f206

%de

note

sth

epe

rcen

tage

of20

6 Pb

that

isco

mm

onP

b;co

rrec

tion

for

com

mon

Pb

was

mad

eus

ing

the

mea

sure

d20

4 Pb/

206 P

bra

tio;

for

%C

onco

rdan

ce,

100%

deno

tes

aco

ncor

dant

anal

ysis

.V

alue

s\

100

are

reve

rse

disc

orda

nt.


grains and have not been analysed in this study.Overgrowths are generally lacking. Zircons fromthis sample have U contents of 200–5000 ppmwith a Th/U ratio of between 0.5 and 2.0. Mostgrains having a ratio of �1.0.

Twelve zircons were analysed from sample9628-142. All 12 analyses form a statistically sim-ple concordant population yielding a mean 207Pb/206Pb age of 936914 Ma (MSWD=1.5) (Fig.8b). This age is interpreted to date the timing of

Fig. 7. Cathodoluminescence images of representative zircon morphologies: (a) sample 9628-141, analysis points 10.1 and 18.1; (b)sample 9628-141, analysis points 7.1, 8.1 and 9.1; (c) sample 9628-142, analysis points 5.1 and 12.1; (d) sample 9628-73, analysispoints 10.1 and 11.1; (e) sample 9628-73, analysis point 12.1; (f) sample 9628-196, analyses points 13.1 and 14.1; (g) sample 9628-196,analysis points 3.1 and 4.1; (h) sample 9628-196, analysis point 11.1.


Fig. 8. U–Pb concordia diagrams showing SHRIMP data for samples from Radok Lake. 206Pb/207Pb ages are stated to 2s (95%)confidence limits, while the illustrated error ellipses reflect 1s confidence limits (68%). MSWD, Mean square of weighted deviates.Histograms show the distribution of individual analyses and highlight the single zircon population found in samples 9628-141,9628-142 and 9628-73 compared with the two populations found in sample 9628-196.

crystallisation of the granite, and suggests D3

folding occurred at, or prior to, �940 Ma.

5.3. Orthopyroxene-bearing leucosome — northwall of Battye Glacier (sample 9628-73)

Sample 9628-73 is a medium-grained leucosomeconsisting of quartz, K-feldspar, subordinate pla-gioclase and orthopyroxene. The sample was col-lected from within a steeply north-dipping D3

shear zone on the north side of the Battye Glacier(Fig. 3). The leucosome is unfoliated and is lo-calised within the neck of a boudin formed as aresult of D3 shearing (Fig. 4f). We infer that theleucosome formed syn-D3, concurrent with forma-tion of the high-strain zone. Leucosome forma-tion at this time is consistent with regionalobservations that suggest that extensive partialmelting occurred during D3, particularly withinmetapelitic units.


Zircons from sample 9628-73 are generally tur-bid and pale brown to pale reddish brown incolour. They form a subhedral population thatvaries in length from 100 to 500 mm, with averagelength/width ratios of approximately 3:1. A Th/Uratio of 0.7 is relatively consistent for all grainsanalysed. Likewise, the U contents of the zirconslie in a narrow range typically between 300 and600 ppm. Most zircons show growth zoning,marked by subtle concentric bands of varyingluminescence (Fig. 7d). Many contain highly lu-minescent inclusions of apatite (Fig. 7e). Somegrains are overgrown by an unzoned more lu-minescent rim. However, most zircons have asimple igneous appearance and are inferred tohave formed at the time of leucosome formation.

The isotopic data for the zircons from sample9628-73 are presented in Table 2. All 18 analysesform a single concordant mean 207Pb/206Pb age of942917 Ma (MSWD=1.44) (Fig. 8c). The indi-cated age for this sample is taken as the crystalli-sation age of the leucosome, and constrains thedevelopment of the upright high-strain zone at�940 Ma.

5.4. Clinopyroxene-bearing leucosome — westwall of Radok lake (sample 9628-196)

Sample 9628-196 was taken from medium- tocoarse-grained leucosome consisting of sericitisedalkali and plagioclase feldspars, quartz, clinopy-roxene and green hornblende. The leucosome oc-curs within the amphibolite facies felsic andintermediate orthogneisses, exposed along thelower cliff faces at Radok Lake (Fig. 3). Theamphibolite facies rocks at this locality tectoni-cally underlie the granulites that make up the bulkof the nPCMs. Leucosomes within these rocks(including sample 9628-196) form elongate layersthat parallel S0/1, but which also locally formspurs and accumulations that cross-cut the folia-tion at high angles (Fig. 4g). We interpret leuco-some formation to have post-dated deformation,probably coincident with peak metamorphism,which appears to post-date deformation given therandom to weakly orientated assemblages. Wesuggest that peak metamorphic conditions were

attained as a result of the emplacement of thegranulites over the amphibolites, the granuliteseither advectively heating the underlying units, ortheir emplacement resulting in the net burial andsubsequent heating of the underthrust units. Wetherefore propose a syn-D4 timing of leucosomeformation. No equivalent leucosome developmentoccurred as a result of D4 deformation within theoverlying granulites.

Zircons from sample 9628-196 are reddishbrown, euhedral to subhedral, and vary in lengthfrom 150 to 450 mm. Zircons from this samplehave length/width ratios of approximately 2:1–3:1, and are generally more euhedral than thosefrom the previous samples. CL imaging showsthat they also have more complicated internalmorphologies. Most grains contain cores that arecommonly dark and can be either homogeneousor concentrically zoned. The internal structure ofthese cores is often cross-cut by a rounded resorp-tion surface (Fig. 7f–h), which is then overgrownby euhedral, generally more luminescent rims.However, examples of poorly luminescent euhe-dral rims were also observed (Fig. 7h). Highlyluminescent unzoned euhedral zircons are alsopresent, and may represent the same period ofzircon growth as that which formed the rims onother zircons. The Th/U ratio of both cores andrims lies in the range 0.3 to 1.0, with most analy-ses �0.5. There is no consistent contrast in Ucontent between core and rim analyses, with con-siderable overlap occurring between individualgrains.

Sixteen zircon grains from sample 9628-196were analysed, of which 15 (excluding 2.1) lie onor near concordia and produce a weighted mean207Pb/206Pb age of 954938 Ma (MSWD=2.71)(Fig. 8d). However, the large MSWD indicatesexcess statistical scatter about the mean. Mod-elling suggests that there are two distinct sub-pop-ulations that are separated by a distinct age gapof �50 Myr (Fig. 8d). This reflects a subtledifference in age obtained from core and rimanalyses. Core analyses 1.1, 3.1, 6.1, 7.1, 12.1 and14.1 definite an older grouping that yields a mean207Pb/206Pb age of 1017931 Ma (MSWD=0.685), while rim analyses 4.1, 5.1, 8.1, 9.1, 10.1,11.1, 13.1 and 15.1 define a younger grouping and


give a mean 207Pb/206Pb age of 900928 Ma(MSWD=0.498). We suggest that the older pop-ulation is inherited from the orthogneiss, andrecord an age reflecting orthogneiss emplacement,whereas the rims are considered to have formed atthe time of leucosome development and are con-sidered to constrain the timing of D4. If thisinterpretation is correct, felsic orthogneiss intru-sion occurred at �1020 Ma, and D4 occurred at�900 Ma. Alternatively, the zircons may repre-sent a single, somewhat scattered populationsourced from the leucosome, which would implythat the time interval between D3 and D4 was veryshort, as the ages of samples 9628-142 (D3), 9628-73 (D3) and 9628-196 (D4) are all statisticallyidentical. We prefer the first alternative, althoughcannot conclusively preclude the latter.

6. Discussion

On the basis of our geochronological results, weconclude that deformation and high-grade meta-morphism in the Radok Lake area occurred overa period spanning approximately 90 Myr. D1 andD2 are considered progressive, a conclusion alsodraw by a number of previous studies (Fitzsimonsand Harley, 1992; Thost and Hensen, 1992; Handet al. 1994b), and occurred concurrently withregionally extensive magmatism and peak meta-morphism at �990–980 Ma. The subsequent de-velopment of upright folds (F3) and steeplydipping high-strain zones occurred at �940 Ma.These pervasive features were then overprinted bydiscrete mylonites and pseudotachylites that de-veloped at �900 Ma.

Our structural observations show that both foldgenerations (F2 and F3) formed coaxially with L1

(Fig. 3), and that the resolved palaeo-transportdirections from D3 high-strain zones and D4 my-lonites are also subparallel (Fig. 5). All fourevents show evidence of having formed in re-sponse to north–south-directed compression.Given the consistency in orientation of thepalaeostress field and the relative proximity in ageof the four deformational events (Fig. 9), wesuggest that the recognised sequence of deforma-

tional episodes (D1–D4) all developed during asingle evolving north–south compressive orogenicevent (Fig. 10). During this time, north–south-di-rected compression shortened the terrain, throughgradually more discrete phases of deformation.The accumulation of strain occurred in responseto the same compressive stress field. However, thestyle of deformation changed in response tochanges in the orientation and magnitude of theintermediate and stretching axes, and the relativecontribution of the components of pure and sim-ple shear (Fig. 6).

The constraints on deformation presented areconsistent with published structural and age dataavailable from throughout the nPCMs. The intru-sion ages presented by Kinny et al. (1997) fromfelsic bodies at Loewe Massif (charnockite, 980921 Ma), Mt Collins (granites, 976925 and 98497 Ma; quartz syenite, 984912 Ma) and MtMcCarthy (leucogneisses, 990930 Ma) are allstatistically identical to the 990918 Ma intrusionage obtained in this study (Fig. 9). Equivalentintrusive ages of 985929 and 954912 Ma werealso recorded from charnockites outcroppingalong the Mawson Coast (Young and Black,1991). Structurally, all of these �980 Ma intru-sives are inferred to predate upright folding, con-sistent with the conclusion drawn from this study.Furthermore, the 940+27/−17 Ma age obtainedby Manton et al. (1992) from Jetty Peninsula hasbeen interpreted by Hand et al. (1994b) to datethe emplacement of a pre- to syn-F3 leucogneiss.This interpretation concurs with the results of thisstudy as it also suggests that F3 folding occurredat about 940 Ma (Fig. 9). Finally, SHRIMP datafrom Mt Kirkby suggests F3 folding and shearzone formation occurred at �910 Ma (Carson etal., 2000), an age that is within error of theestimates on the timing of D3 and D4 obtainedfrom this study. This consistency in published agedata is mirrored by a remarkable consistency inthe sequence and orientation of structures ob-served throughout the nPCMs (compare Fig. 6 ofFitzsimons and Harley (1992) with Fig. 10). Thus,it is considered likely that the conclusions drawnin this study are broadly applicable over much ofthe nPCMs.


As well as pervasively deforming the terrain,990–900 Ma orogenesis in the nPCMs has em-placed granulite facies gneisses, which form thebulk of the exposed rock in the nPCMs, overamphibolite facies intermediate and felsic unitsthat crop out in a window exposed at RadokLake. This relationship suggests that the gran-ulites at the southern end of the nPCMs are likelyto be allochthonous, a scenario already proposedto explain the geochemistry of post-tectonic peg-matite dykes on Jetty Peninsula (Manton et al.,1992). If correct, this could imply that the amphi-bolite facies rocks exposed at Radok Lake areequivalent to the amphibolite facies metavolcanicsequence exposed further to the south at FisherMassif (Kamenev et al., 1993; Beliatsky et al.,

1994; Mikhalsky et al., 1996; Laiba and Mikhal-sky, 1999). Although this can not be shown con-clusively, it is noteworthy that the Fisher Massifmetavolcanics are intruded by a biotite granitethat yielded an age of �1020 Ma (Kinny et al.,1997), identical to that obtained from the inher-ited zircon population obtained from sample9628-196. This relationship may well be coinci-dental. However, it supports the inference that thegranulites of the nPCMs may tectonically overliethe Fisher terrain.

The geochronological data presented here fromthe nPCMs is readily correlated with publisheddata from the Mawson Coast (Young and Black,1991; Young et al., 1997) and Rayner Complex(Black et al., 1987) in the east Antarctic, and the

Fig. 9. Summary of U–Pb zircon ages (both SHRIMP and conventional) from the nPCMs and Mawson Coast, superimposed withconstraints on deformation in the nPCMs.


Fig. 10. Schematic block diagram illustrating the structural evolution of the Radok Lake region in the northern Prince CharlesMountains.

Eastern Ghats of India (Grew and Manton, 1986;Paul et al., 1990; Shaw et al., 1997). All yieldsyn-orogenic ages of between 990 and 900 Ma. In

many east Gondwana reconstructions (for exam-ple, Moores, 1991; Rogers 1996), these belts havebeen correlated with other Meso-Neoproterozoic


belts exposed in Africa, Australia and the Antarc-tic to form a single laterally extensive orogenicbelt, thought to have extended along the coast ofeast Antarctica from Coats Land at the edge ofthe Weddell Sea, through to the Albany–FraserBelt of southwest Australia and into the Mus-grave block of central Australia. However, 990–900 Ma orogenesis recognised in thenPCMs–Mawson Coast–Rayner Complex–East-ern Ghats provinces in east Antarctica and Indiais significantly younger than that recognised bothto the east and the west (Fig. 11). The orogenicbelts exposed in the Musgrave block of centralAustralia (Clarke et al., 1995; White et al., 1999),the Albany–Fraser belt of southwest Australia(Pidgeon, 1990) and the Windmill Islands (Tingey,

1991; Post et al., 1997), and the Bunger Hills(Sheraton et al., 1990, 1993; Black et al., 1992) ofeast Antarctica, can be correlated on the basisthat all experienced high-grade metamorphism,magmatism and deformation between 1300 and1050 Ma (White et al., 1999). The youngest eventrecognised in these terrains is �60 Myr olderthan the onset of deformation and metamorphismin the nPCMs. Likewise, the metamorphic beltsexposed in the Maud Province of east Antarctica(Arndt et al., 1991; Jacobs et al., 1995, 1998) andthe Namaqua-Natal Province of east Africa (Cor-nell et al., 1996; Thomas et al., 1996; Jacobs et al.,1997; Hanson et al., 1998) are correlatable, butolder than that recognised in the nPCMs. In theMaud and Namaqua-Natal Provinces, felsic vol-

Fig. 11. Tectonic map of East Antarctica and adjacent parts of Gondwana showing the Archaean-Palaeoproterozoic cratonic blocks,and Meso-Neoproterozoic and Palaeozoic orogenic belts. The disparate ages of the Meso-Neoproterozoic orogenic belts, which havebeen previously correlated, and the two recently recognised intervening Palaeozoic belts in Lutzow-Holm Bay and Prydz Bay areillustrated. G, Gawler craton; K, Kalahari craton; sPCMs, southern Prince Charles Mountains; V, Vestford Hills; LHB,Lutzow-Holm Bay; P, Prydz Bay; nPCMs, northern Prince Charles Mountains.


canism and plutonism at �1140 Ma was fol-lowed by high-grade deformation and metamor-phism at 1060–1040 Ma (Arndt et al., 1991;Jacobs et al., 1995, 1998). Again, tectonism inthese terrains ceased �50 Ma prior to the onsetof deformation in the nPCMs. Given the signifi-cant differences in age of these previously corre-lated terrains, it seems unlikely that these beltsrepresent a single continuous suture. Instead,these three terrains probably represent separatefragments of disparate Meso- to Neoproterozoicorogenic belts. This is consistent with the recentlyrecognised Palaeozoic tectonism in Lutzow-HolmBay and Prydz Bay, two younger orogenic beltsthat separate each of these different Meso- toNeoproterozoic terrains (Fig. 11).

Finally, our results impact on the debate as tothe extent of Palaeozoic reworking experienced bythe nPCMs. The data presented in this paperconstrains all high-grade deformation and meta-morphism to have occurred during the Neopro-terozoic, precluding the possibility of subsequenthigh-grade events post-dating �900 Ma. We donot rule out the possibility of later stages ofdeformation, but suggest that they were of arelatively low grade and of a discrete nature.Whereas, the adjacent terrain of Prydz Bay hasbeen pervasively reworked by an early Palaeozoicgranulite facies event, equivalent high-grade de-formation is not recognised in the nPCMs.

7. Conclusions

The structural and geochronological results pre-sented in this paper constrain the granulite faciesdeformation and metamorphism observed in thenPCMs to have occurred between �990 and�900 Ma. During this period, north–south-di-rected compression deformed the terrain throughfour progressively more discrete phases of defor-mation. Implicitly, our results suggest that oroge-nesis can be long lived, in the case of the nPCMs,lasting up to 90 Myr. Our results also suggest thatorogenesis in the nPCMs–Mawson Coast–Rayner Complex–Eastern Ghats is temporallyquite distinct from other Meso-Neoroterozoicmetamorphic belts thought to be involved in the

formation of east Gondwana and Rodinia. In-stead, we suggest that these Meso-Neoroterozoicmetamorphic belts are of separate origin and wereprobably accreted together during the Palaeozoicalong orogenic belts recognised in Lutzow-HolmBay and Prydz Bay. With respect to Palaeozoicreworking of the nPCMs, we do not rule out thepossibility of later discrete stages of deformation.However, our results suggest that all high-gradepenetrative fabrics observed in the nPCMs formedduring the Neoproterozoic.

Acknowledgements

The authors would like to thank the AustralianAntarctic Division for logistical support over the1996–1997 summer. The cost of field expensesand analytical time on SHRIMP I and SHRIMPII was met from an ASAC grant to C.J.L.W. TheAustralian Geological Survey Organisation arethanked for providing air-photos, while DougThost is also thanked for his assistance andfriendship in the field. We would also like tothank Pete Kinny and Ian Fitzsimons for thor-ough and constructive reviews that greatly im-proved the quality of this manuscript.

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neoproterozoic deformation in the radok lake region of the northern prince charles mountains, east...

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