Directory UMM :Data Elmu:jurnal:P:Precambrian Research:Vol104.Issue1-2.2000:

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Neoproterozoic deformation in the Radok Lake region of

the northern Prince Charles Mountains, east Antarctica;

evidence for a single protracted orogenic event

S.D. Boger

_{*, C.J. Carson}

_{, C.J.L. Wilson}

_{, C.M. Fanning}

a_{School of Earth Sciences}_,_{The Uni}

6ersity of Melbourne,Park6ille,Vic. 3010, Australia b_{Department of Geology and Geophysics}_,_{Yale Uni}

6ersity,New Ha6en,CT 06511, USA c_{Research School of Earth Sciences}_,_{The Australian National Uni}

6ersity,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 Prince Charles Mountains (nPCMs) were deformed during a single, long-lived Neoproterozoic tectonic event. Deformation evolved 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 with regionally 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 Ma constrains a final phase of deformation that was accommodated along low-angle mylonites and pseudotachylites. This final period of deformation was responsible for the allochthonous emplacement of granulites over mid-amphibolite facies rocks in the nPCMs. The age constraints placed on the timing of deformation by this study preclude the high-grade reworking of the nPCMs as is postulated in some of the recent literature. Furthermore, 990 – 900 Ma orogenesis in the nPCMs is at least 50 Myr younger than that recognised in other previously correlated Grenville aged orogenic belts found in Australia, east Africa and other parts of the Antarctic. This distinct age difference implies that these belts are probably not correlatable, as has been previously suggested in reconstructions of the supercontinent Rodinia. © 2000 Elsevier Science B.V. All rights reserved.

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

www.elsevier.com/locate/precamres

1. Introduction

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

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

* Corresponding author.

E-mail address:_{s –[email protected] (S.D.} Boger).

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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 then remained intact and generally internally unde-formed until rifting in the Mesozoic (Yoshida et al., 1992). However, the more recent recognition of 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 this model. Instead, it has been suggested that east Gondwana may represent a collage of continental fragments that accreted during the Palaeozoic (Hensen and Zhou, 1997).

Fig. 1. (a) Traditional reconstruction of Rodinia at1000 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 of Australia, India and east Africa along a single laterally extensive Meso-Neoproterozoic orogenic belt thought to have rimmed the east Antarctic coastline. (b) Gondwana at 500 Ma with the continents of east and west Gondwana illustrated. The position of the nPCMs is highlighted and enlarged in (c). Traditional models for the construction of Gondwana suggest that it remained intact from Rodinian times and formed a keystone onto which west Gondwana accreted. (c) Expanded section shows the gross geology of the region of interest. NC, Napier Complex; VH, Vestfold Hills; sPCMs, southern Prince Charles Mountains; RC, Rayner Complex; nPCMs, northern Prince Charles Mountains; LHB, Lu¨tzow-Holm Bay; PB, Prydz Bay. The more complicated tectonic frame work arising from the dissection of the Proterozoic mobile belt exposed in the nPCMs by Palaeozoic terrains recognised in Prydz and Lu¨tzow Holm Bays are highlighted.

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The nPCMs, together with the Mawson Coast and the Rayner Complex, separate Prydz and Lu¨tzow-Holm Bays (Fig. 1c). With the recogni-tion of high-grade Palaeozoic tectonism within these terrains, the nPCMs has received consider-able attention regarding the extent of possible Palaeozoic reworking. A number of authors have postulated that a late Proterozoic to early Palaeozoic accretionary belt may have linked Prydz and Lu¨tzow-Holm Bays (Kriegsman, 1995; Hensen and Zhou, 1997) effectively crossing the nPCMs – Mawson Coast – Rayner Complex re-gion. Within the nPCMs, this inference has been supported by Sm – Nd age data presented by Hensen et al. (1997) from which they infer two significant tectonothermal events overprinting the

widely recognised 1000 Ma orogen; one at

800 Ma and a second at 630 – 500 Ma.

Similarly, Scrimgeour and Hand (1997) suggest that the complex pressure – temperature paths ob-served along the eastern edge of the nPCMs reflect thermal interference between two unrelated

tectonic events. They infer that 1000 Ma

tec-tonism is overprinted in the east by the affects of 550 – 500 Ma orogenesis recognised to the northeast in Prydz Bay. These studies contrast with that of Kinny et al. (1997), who argue that the lack of new zircon growth or Pb-loss

discon-cordia post-dating 1000 Ma indicate that

late Proterozoic to early Palaeozoic tectonism in the nPCMs was of relatively minor importance. This interpretation is more consistent with earlier studies from the area (Tingey, 1982, 1991; Man-ton et al., 1992). These different hypotheses arise primarily due to a paucity of structurally

well-constrained geochronologic data from the

nPCMs, an issue that we have aimed to address in this study.

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

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

2. Regional geologic setting

The nPCMs are exposed as a series of isolated inland ranges and massifs located on the western margin of the Amery Ice Self (Fig. 2). They form part of an east – west trending orogenic belt, dom-inated by granulite facies felsic and mafic gneisses, interleaved with subordinate metasedimentary and 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). The sequence as a whole was intruded episodically by significant volumes of granitic and charnockitic magma, 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 Beaver Lake (Fig. 2), the high-grade gneisses are overlain by relatively undeformed Permo-Triassic sedi-ments (Crohn, 1959; Mond, 1972; Webb and Fielding, 1993; Fielding and Webb, 1995, 1996; McLaughlin and Drinnan, 1997a,b). These are thought to lie in a sub-basin on the western side of the Lambert Graben, an inferred rift system that separates the nPCMs from the Palaeozoic (ca. 550 – 500 Ma) granulite facies terrain of Prydz Bay (Ren et al., 1992; Zhao et al., 1992; Carson et al., 1995; Dirks and Wilson, 1995; Harley and Fitzsimons, 1995; Hensen and Zhou, 1995; Car-son et al., 1996; Fitzsimons, 1997; Fitzsimons et al., 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 Mawson Coast (Fig. 3), where rocks of similar age and grade are exposed (Young and Black, 1991; Young et al., 1997), and has been tentatively correlated with the Rayner Complex still further to the west (Black et al., 1987). The terrain is bounded in the south by exposures of older

Meoproterozoic volcanics at Fisher Massif

(Kamenev et al., 1993; Beliatsky et al., 1994; Mikhalsky et al., 1996; Kinny et al., 1997; Laiba and Mikhalsky, 1999), and by granitic Archaean basement complex overlain by two or more

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super-Fig. 2. Schematic map of the northern Prince Charles Mountains showing the study area, extent of outcrop and the distribution of the 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 Peninsula are after Manton et al. (1992). Locality of cross-section illustrated in Fig. 3 is also shown. Insert shows the geographic position of the northern Prince Charles Mountains along the western margin of the Amery Ice Self. Mawson and Davis refer to Australian Antarctic Stations.

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

Previous studies have established that the nPCMs attained granulite facies metamorphic conditions of approximately 800°C and 6 – 7 kbar (Fitzsimons and Thost, 1992; Fitzsimons and Harley, 1994a,b; Hand et al., 1994a; Scrimgeour and Hand, 1997), and followed a retrograde path dominated by cooling (Fitzsimons and Harley, 1992; Thost and Hensen, 1992; Fitzsimons and Harley, 1994a,b; Stephenson and Cook, 1997). In the 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 and Hand, 1997).

The earliest geochronological data from the Prince Charles Mountains were reconnaissance Rb – Sr ages obtained by Arriens (1975). Whole rock isochrons presented by Arriens (1975) yield

ages of 1000 Ma, whereas mineral dates (biotite

and muscovite) produced ages that clustered around 500 Ma. On the basis of these results, Tingey (1982, 1991) suggested that high-grade

metamorphism in the nPCMs occurred at 1000

Ma, overprinted by a widespread but relatively low-grade thermal event recorded by mica systems

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Boger

Precambrian

Research

104

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–

Fig. 3. Cross-section through the outcrop exposure at Radok Lake showing the extent of outcrop (top) and the distribution of rock types and the observed structure (bottom). S1is the dominant form surface illustrated, except in D3shear zones, where S3is dominant. Equal area stereographic projections summarise structural data and illustrate lineation (L1), fold axis (F2&3) and combined foliation and lineation data (D3&4) for all observed structures.

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More recent conventional and SHRIMP (Sensi-tive High Resolution Ion Microprobe) dating of zircon has yielded results consistent with the ini-tial ages obtained by Arriens (1975). Manton et

al. (1992) reported upper intercept ages of 1000+

14/ −11 Ma (orthogneiss) and 940+27/ −17 Ma

(leucogranite) from rocks outcropping at Jetty Peninsula (Fig. 3). The former age is interpreted as dating granulite facies metamorphism (Manton et al., 1992). U – Pb SHRIMP dating by Kinny et al. (1997) has produced ages for felsic intrusives at

Loewe Massif (980921 Ma), Mt Collins (9769

25, 98497 and 984912 Ma) and Mt McCarthy

(990930 Ma). The intrusive ages from all

locali-ties are statistically identical, and are interpreted to suggest that the nPCMs experienced a wide-spread magmatic event that occurred concurrently with regional high-grade metamorphism (Kinny et al., 1997).

Younger zircon and monazite ages of about 550 – 500 Ma were obtained by Manton et al. (1992) from minor pegmatites and granites crop-ping out at Jetty Peninsula. These are the only

reported U – Pb ages younger than 940 Ma

from the nPCMs. They are similar to Rb – Sr mineral isochron ages of about 480 Ma and the projected lower intercept ages of some zircon discordia obtained by Manton et al. (1992), both of which were interpreted as reset ages associated with granite and pegmatite emplacement. Else-where in the nPCMs, two-point Sm – Nd garnet-whole rock and garnet-matrix ages from a variety

of rock types form two age groupings at 800

and 630 – 550 Ma (Hensen et al., 1997). The

800 Ma ages are more prevalent in the west of the

nPCMs, while the younger 630 – 550 Ma ages

come predominantly from the east. Hensen et al. (1997) interpreted the Sm – Nd ages as dating high-temperature thermal events post-dating the

980 Ma magmatism recorded by zircon.

3. Structure

Detailed structural studies within the nPCMs have previously centred upon the Aramis, Porthos and Athos ranges approximately 100 km to the

northwest of Radok Lake (Fitzsimons and

Harley, 1992; Thost and Hensen, 1992), and at Else Platform located approximately 75 km to the northeast (Hand et al., 1994b). An outline of the structure at Radok Lake was also presented by McKelvey and Stephenson (1990). All previous studies describe a pervasive layer-parallel folia-tion, folded initially into isoclinal layer-parallel folds, which were subsequently reoriented about upright east – west trending folds, and then trans-posed into east – west trending steeply dipping high-strain zones. The different fold and foliation nomenclature used by previous authors is summa-rized in Table 1. An identical sequence of struc-tures observed during this study is described in the following. In addition, a later phase of dis-crete mylonitisation was recognised, and is de-scribed in this paper. All structural relationships and orientation data are illustrated in Fig. 3, a schematic 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 and

pervasive preferred mineral orientation, which is

always concordant with lithological layering (S0).

This fabric defines the dominant foliation surface in the nPCMs (Fitzsimons and Thost, 1992; Thost and 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

(D2and D3), S1is well preserved and only weakly

overprinted by the development of new fabrics associated 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 is

little 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 axial

plane. Large-scale F2 folds were not recognised,

but were inferred, as mesoscale F2 isoclinal folds

are common with type-three F2 and F3fold

inter-ference patterns (Ramsay, 1967) recognised in D3

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trend-ing axes that parallel the L1lineation (Fig. 3). The

formation of an L2 lineation is not recognised,

although the development of such a lineation

cannot be precluded. If present, L2 was of the

same grade and formed parallel to and potentially

intensified 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 facies

assemblages define both the S1 foliation and,

where developed, the subtle S2 fabric.

Addition-ally, F2 folding has formed coaxially with the

lineation (L1) (Fig. 3). Although the rotation and

transposition of lithological layering (S0) into

par-allelism with S1 may represent the preservation of

an otherwise overprinted prograde history, the similarity in metamorphic grade and the coaxial nature of the structures suggests that a

progres-sive evolution from D1 to D2 is more likely. A

similar conclusion was drawn by a number of previous 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 to

have been north – south directed, perpendicular to

the lineation orientation and the F2 fold axis.

Although lineations are commonly inferred to parallel the transport direction, there is no

evi-dence of this recognised at Radok Lake.

Kine-matic indicators such as asymmetric F2folds were

found on outcrop surfaces normal to the

lin-eation, while F2 and F3 (described below) are all

coaxial, the latter event clearly having developed in response to north – south directed shortening. Furthermore, the recent work of Passchier (1997) describes the development of orthogonal lin-eation – transport direction relationships under non-ideal simple shear conditions, consistent with the overall deformational environment envisaged for D1 – 2.

3.2. D3 deformation

D3 was responsible for the major east – west

trending structural grain and the gross geometry

observed in the nPCMs. D3 folded the composite

S0/S1 surface (and D2 structures) into a series of

meso- to macroscale upright F3fold closures (Fig.

4a). Folding occurred in response to broadly north – south directed compression, producing a series of ENE trending folds that are parallel to

both the F2 fold axes and the L1 stretching

lin-eation (Fig. 3). F3 folds plunge moderately to

shallowly, to either the ENE or WSW (Fig. 3). However, with ongoing shortening, the increase in

D3 strain is accompanied by a localised decrease

in fold wavelength and an increase in D3 fold

plunge. A weak axial planar S3 foliation is

dis-Table 1

Summary of the nomenclature used to describe the deformational features observed throughout the northern Prince Charles Mountains

Formation of Upright folding Steeply dipping

Author Isoclinal Low-angle discrete

regional gneissic recumbent shear zone mylonite and formation

layering folding pseudotachylite

formation

D3 D4

D3 D2

D1 This study

D1 D2

Scrimgeour and Hand (1997) D3 D4

D1/MY1 D2

Nichols (1995) D3 MY2 MY3

D1 D1

Hand et al. (1994b) D2 D2 D3

Thost and Hensen (1992) D1–D2–D3 D4 D5 D5 D6

D4 D5 D6 D7–D8

Fitzsimons and Thost (1992) D1–D2–D3

D1 D2 D3

McKelvey and Stephenson (1990)

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Fig. 4. Structural and intrusive features from Radok Lake: (a) upright west-plunging F3antiform; (b) D3upright high-strain zone; (c) photomicrograph of coplanar D4pseudotachylite and mylonite; (d) reverse offset-D4mylonite with subtle drag folding on upper surface; (e) concordant intrusive contact between syn-D2granite (distinctive pale unit) and host lithologies (sample 9628-141); (f) orthopyroxene bearing leucosome localised within D3boudin neck (sample 9628-73); (g) clinopyroxene bearing leucosome formed concordant 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 S3is generally restricted to the limbs of

F3 folds where discrete D3 high-strain zones are

developed.

D3 high-strain zones (Fig. 4b) are tens to

hun-dreds of metres in width. Within these zones, S3is

generally indistinguishable in appearance from S0/

S1. However, continued localisation of strain

within D3shear zones has locally overprinted the

gneissic S3 foliation with a mylonitic to

ultramy-lonitic fabric. The myultramy-lonitic fabric is defined by a deformation-induced grain size reduction and by the growth of new, lower grade metamorphic assemblages (Hand et al., 1994a; Nichols, 1995;

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Scrimgeour and Hand, 1997). In the vicinity of

Radok Lake, D3 high-strain zones are orientated

parallel to the S3axial surface, trending ENE and

dipping steeply to the north. Elsewhere in the

nPCMs, some D3 high-strain zones can dip

steeply to the south (for example, Hand et al.,

1994b). The S3foliation contains a steeply

plung-ing L3 stretching lineation (Fig. 3) while S – C

fabric relationships and offset leucosomes show lineation-parallel, reverse kinematics. Although large and regionally significant features, it is

un-likely that D3high-strain zones accommodate

sig-nificant movement as there are no observed or reported changes in metamorphic grade across these structures.

The accommodation of D3 strain by upright

folding, then shearing along steeply dipping high-strain zones is consistent with a transition during

D3from dominantly pure shear flattening to

dom-inantly vertically oriented simple shear. This tran-sition is manifested by the development of an intermediate non-coaxial pure shear regime in

which the F3 fold axes were rotated toward a

vertical stretching axis, reflected by the observed

increase in F3 fold plunge (Fig. 6).

3.3. D4 deformation

Deformation post-dating D3 folding and

up-right shear zone development is characterised by the 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 fabric elements. They form discrete localised zones of deformation up to 100 mm in width, typically defined by biotite and quartz. The presence of

coexisting garnet and sillimanite, and/or green

hornblende within some D4 mylonites, is

consis-tent with their formation under amphibolite facies conditions, at temperatures, at least initially, in excess of 520°C (i.e. within the stability field of sillimanite). The development of coplanar pseudo-tachylites was probably a function of strain rate (Hobbs et al., 1986), and the anhydrous nature of the 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 nPCMs

finished with the cessation of D3 and resulted in

the preservation of granulite facies metamorphic

textures formed during D1 – 3.

D4 mylonites generally trend northwest, and

form 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 that

D4 formed in response to NNE – SSW-directed

compression (Fig. 5). Offset on most mylonites is

typically minor (B2 m). However, a major

flat-lying D4mylonite zone exposed along the western

shore of Radok lake juxtaposes granulite facies gneisses over amphibolite facies interlayered felsic and intermediate orthogneisses (Fig. 4). The felsic orthogneiss consists of equigranular quartz, K-feldspar and plagioclase. The intermediate or-thogneisses are composed of weak to randomly orientated biotite and fine-grained quartz inter-grown with coarse grained plagioclase. Euhedral hornblende, green in hand specimen and thin section, may occur in the matrix, although it more commonly forms a reaction rim separating the felsic and mafic bands. Sphene typically occurs along grain boundaries between biotite and il-menite, while chlorite partially to completely pseudomorphs biotite. Clinopyroxene is also lo-cally observed in the leucosomes, where it is gen-erally rimmed by green hornblende. In contrast to the overriding units, these rocks do not contain garnet, orthopyroxene or brown hornblende, or relics thereof, minerals that are ubiquitous in the granulites typical of the nPCMs. They contain no evidence of ever undergoing granulite facies meta-morphism and are considered possible equivalents of the lower grade rocks exposed at Fisher Massif to the south. Shear sense indicators suggest thrusting involved emplacement of the granulites to the south, consistent with this interpretation, whilst the juxtaposition of granulites over lower grade rocks at Radok lake implies that at least in the southern portion of the nPCMs may be al-lochthonous. This is an interpretation first for-warded by Manton et al. (1992) to explain the presence of As, Mo, Be and B in post-orogenic

hydrous pegmatites outcropping at Jetty

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

vidual sectioned zircons. Zr2O

+_, 206_Pb+_, 207_Pb+_,

208_Pb+_, 238_U+_, 232_ThO+_{, and} 238_UO+ _were

mea-sured in cycles by magnetic field switching, seven cycles per data set. Analysis of unknowns was interspersed with analyses of the standard SL13

(which has a radiogenic 206_Pb_/238_{U ratio of}

0.0928) in order to monitor the differential frac-tionation between U and Pb. Radiogenic Pb

com-positions were determined after subtracting

contemporaneous common Pb as modelled by Cumming and Richards (1975). All reported ages

are based on 207

Pb/ 206

Pb ratios corrected for

common Pb by the 204_{Pb technique (Compston et}

al., 1984, 1992). Ages presented in the text are

stated with 2sconfidence limits.

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

4. Analytical procedure

Zircons for SHRIMP analysis were separated by standard heavy liquid and magnetic proce-dures, and then by hand picking. They were then mounted in epoxy resin discs along with frag-ments of zircon standard SL13. The discs were polished and Au coated before being analysed on either the SHRIMP I or SHRIMP II (sample 9628-142) ion-microprobe at the Australian Na-tional University, Canberra. Cathodoluminescent (CL) imaging was conducted to assess the internal structure of the unknown zircons from which selected zircon domains were analysed for U, Th and Pb isotopic composition. A primary beam of

O− _{ions was used to sputter positive secondary}

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indi-5. Geochronological results

5.1. K-Feldspar granite — west wall of Radok lake (sample9628-141)

Sample 9628-141 represents one of a number of granitic sheets containing quartz, pink K-feldspar, minor garnet and biotite, as well as accessory apatite and zircon. Equivalent granitic bodies at Radok lake vary in width from one to several hundred metres, and are part of a suite of K-feldspar granites that form the most volu-minous intrusive bodies observed in the vicinity of Radok Lake. They occur on either side of the Battye Glacier, at Fox Ridge and at Manning Massif (Fig. 2), and vary from coarse grained and megacrystic, to finer grained and equigranu-lar.

The sampled granite (sample 9628-141) forms a sheet that has concordant boundaries with the

surrounding host lithologies and the S0/1foliation

(Fig. 5e). It contains a well-developed

layer-par-allel foliation reoriented by upright folding (F3).

The foliation within the granite does not define

F2 fold closures, nor were recumbent folds (F2)

recognised within, or defined by, any of the granites of this generation. We therefore con-clude that granite emplacement did not precede deformation as the granites do not preserve the earliest structures recognised in their host

litholo-gies, but clearly pre-date D3 as they are folded

by this event. Thus, these granites are interpreted

to have intruded synchronously with D1 – 2.

Zircons from sample 9628-141 are orange and translucent, and form a subhedral to anhedral population of uniform size. Average elongation ratios of 1:1 – 2:1 are observed for grain lengths

between 150 and 200 mm. The zircons typically

contain 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 occasiointer-nally mantles small detrital cores (Fig. 7a), which were not analysed during this study. The zircons generally lack overgrowths but, where rims do exist, they are discontinuous, highly luminescent, and too nar-row for analysis (Fig. 7b). A simple igneous origin is inferred for these zircons, with the

rounded terminations inferred to reflect partial metamorphic resorption.

The isotopic data for the zircons from sample 9628-141 are presented in Table 2. Twenty zircon grains were analysed, of which all but two

analy-ses produce a concordant mean 207_Pb_/206_{Pb age}

of 990918 Ma (mean square of weighted

devi-ates (MSWD=1.13)) (Fig. 8a). The discrepant

analyses, 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. The concordant age given by the remaining 18 analy-ses is interpreted as the crystallisation age of the

granite and is considered, to constrain D2, to

have occurred at 990 Ma.

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

Sample 9628-142 was collected from a 1 – 2 m wide, coarse-grained, sub-vertically orientated ENE-trending granitic dyke located along the west wall of Radok Lake (Fig. 3). The dyke

intruded along the axial surface of an F3 fold, is

unfoliated, cross-cuts structures developed during

F3 folding, and is offset by later D4 mylonites.

The dyke contains quartz, pink K-feldspar, mi-nor garnet and biotite, and accessory apatite and zircon. It is very similar in both colour and min-eralogy to the more volumetrically significant

pre-D3 sills (sample 9628-141) already described.

The intrusion of sample 9628-142 is inferred to

have occurred late-syn- to post-D3 and is

consid-ered to place a minimum age on the timing of

D3 fold development.

Zircons from sample 9628-142 are orange and translucent, and are similar in appearance to those from sample 9628-141. They form a euhe-dral to subhedral population of varying

grain size. Zircons vary from 200 to 400 mm in

length, 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 from this sample can show internal sector zoning as well as planar growth bands (Fig. 7c), although they mostly do not show much internal structure. Rare rounded detrital cores are found in some

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

U–Th–Pb isotopic compositions of zircons from the northern Prince Charles Mountainsa

Pb (ppm) 204_Pb_/205_Pb

Th (ppm) f206 (%) Radiogenic ratios Ages (Ma) Concordance (%) Grain.spot U (ppm) Th/U

206_Pb_/238_U ₉ 207_Pb_/235_U₉ 207_Pb_/206_Pb ₉ 206_Pb_/238_U₉ 207_Pb_/235_U207_Pb_/206_Pb ₉

Sample9628-141 (

K-feldspar granite—west wall of Raclok Lake)

0.59 0.14680 0.0046 1.3772 0.063

1.1 301 335 1.11 54 0.00034 0.088 0.0032 883 28 879 870 102 102

48 0.00013 0.22 0.15783 0.0081 1.6774 0.098 0.072 0.0021 945 45 961 1000 61 95 264

2.1 222 0.84

1.36 0.11844

3.1 285 189 0.66 39 0.00080 0.0150 1.3026 0.202 0.080 0.0060 722 87 847 1191 157 61 0.08 0.15389 0.0030 1.5811 0.058 0.075 0.0021 922 17

0.00005 963

0.88 38 1058 59 87

4.1 212 286

0.00012

494 239 0.48 84 0.20 0.16355 0.0032 1.5943 0.044 0.071 0.0012 976 18 90 949 36 103 5.1

B0.01 0.14086 0.0119 1.5316 0.141 0.075 0.0027 890 67

6. 1 189 158 0.84 32 0.00000 943 1069 74 83

0.19 0.15210 0.0081 1.5137 0.094 0.072 O.0018 913 46

0.00011 906

342 991 52 92

7.1 181 0.53 55

0.00001

339 169 0.50 58 0.02 0.10014 0.0083 1.6507 0.094 0.075 0.0013 958 46 990 1082 36 90 8. 1

0.15 0.15806 0.0064 1.5824 0.079 0.073

9.1 231 205 0.89 42 0.00009 0.0018 946 38 963 1003 50 94

0.54 0.16136 0.0031 1.5080 0.090 0.068 0.0022 964 17

0.00031 934

340 862 89 112

10.1 328 0.96 64

0.00020

506 290 0.57 89 0.35 0.16589 0.0058 0.5540 0.072 0.068 0.0019 989 31 952 867 58 114 11.1

0.20 0.15304 0.0090 1.4596 0.109 0.069 0.0027 915 50

12.1 404 233 0.58 66 0.00012 914 904 83 102

B0.01 0.16220 0.0041 1.6295 0.060 0.073 0.0018 9989 22

0.0000 902

251 1010 50 98

13.1 124 0.49 43

0.00011

178 149 0.84 31 0.20 0.15105 0.0064 1.5048 0.093 0.072 0.0029 907 36 932 993 84 91 14.1

0.38 0.15339 0.0027 1.4466 0.049 0.068 0.0019 920 15 908 881 58 105 15.1 489 405 0.83 85 0.00022

0.23 0.14884 0.0074 1.5016 0.099 0.073 0.0027 894 42

0.00014 931

0.81 69 1019 77 88

16.1 407 330

0.00023

1216 74 0.06 194 0.39 0.16874 0.0067 1.6880 0.078 0.072 0.0014 1005 37 1003 999 39 101 17.1

0.21 0.16187 0.0061 1.5833 0.074 0.071 0.0017 967 34 964 956 49 101 18.1 709 381 0.54 121 0.00012

0.02 0.15724 0.0048 1.5634 0.050 0.072 0.0005 941 27

0.00001 956

394 989 13 95

19.1 2684 118 0.04

2.49 0.08561 0.0016

20.1 4434 299 0.07 366 0.00145 0.8795 0.038 0.075 0.0029 530 9 641 1055 79 50 Sample9628-142

(

K-feldspar granite—west wall of Radok Lake)

0.02 0.1558

1.1 196 157 0.79 35 0.00001 0.0034 0.525 0.039 0.0711 0.0008 932 19 940 959 22 97 0.06 0.1527 0.0034 1.445 0.038 0.0687 0.0008 916 19

0.00002 908

0.80 49 888 25 103

2.1 284 226

0.00001

233 189 0.81 41 0.02 0.1532 0.0033 1.497 0.036 0.0709 0.0006 919 19 929 955 18 96 3.1

0.02 0.1627 0.0036 1.582 0.039 0.0705 0.0006 972 20

4.1 331 341 1.03 64 0.00001 963 943 16 103

0.06 0.1552 0.0033 1.495 0.034 0.0699 0.0005 930 18

0.00003 928

324 924 14 101

5.1 333 1.03 60

0.00001

297 291 0.98 55 0.02 0.1573 0.0033 1.512 0.036 0.0697 0.0006 942 19 935 920 17 102 6.1

0.02 0.1509 0.0035 1.486 0.0338 0.0714 0.0006 906 20 925 970 16 93 7.1 208 182 0.87 36 0.00001

0.01 0.1614 0.0033 1.569 0.034 0.0705 0.0004 985 19

0.00001 958

0.88 97 943 10 102

8.1 521 460

0.00005

273 249 0.91 49 0.09 0.1546 0.0035 1.483 0.038 0.0696 0.0007 926 19 923 916 21 101 9.1

0.02 0.1575 0.0034 1.558 0.036 0.0718 0.0004 943 19 954 980 13 98 10.1 322 143 0.45 53 0.00001

0.07 0.1477 0.0032 1.417 0.035 0.0696 0.0008 888 18

0.00004 896

49 916 18 97

11.1 284 262 0.92

0.02 0.1535 0.0032

12.1 340 673 1.98 76 0.00001 1.477 0.036 0.0696 0.0008 921 18 921 922 22 100 Sample9628-73

(

Opx-bearing leucosome—north wall of Battye Glacier)

0.08 0.1572 0.0021 1.527 0.041

1.1 371 229 0.62 58 0.00003 0.0704 0.0015 941 12 941 941 45 100

0.02 0.1535 0.0036 1.497 0.041 0.0707 0.0009 921 20

0.00001 929

339 949 25 97

2.1 159 0.47 50

0.00001

158 208 1.32 28 0.02 0.1509 0.0036 1.500 0.050 0.00721 0.0014 906 21 930 989 39 92 3.1

B0.0l 0.1637 0.0047 1.631 0.055 0.0723 0.0011 977 26

4.1 675 300 0.44 106 0.00000 982 994 31 98

0.19 0.1596 0.0033 1.543 0.046 0.0701 0.0014 954 18

0.00011 948

340 933 41 102

6.1 206 0.61 54

0.00007

359 225 0.63 58 0.12 0.1621 0.0050 1.548 0.053 0.0693 0.0006 968 28 949 906 25 107 7.1

0.22 0.1542 0.0052 1.522 0.057 0.0716 0.0009 925 29

8.1 509 359 0.71 80 0.00013 939 974 25 95

B0.01 0.1696 0.0031 1.676 0.041 0.0717 0.0010 1010 17

0.00000 999

0.76 84 976 28 104

9.1 481 365

0.00002

507 389 0.77 81 0.03 0.1535 0.0054 1.510 0.057 0.0713 0.0007 921 30 934 967 20 95 10.1

0.02 0.1702 0.0121 1.790 0.181 0.0763 0.0048 1013 67

11.1 639 449 0.70 110 0.00001 1042 1102 132 92

0.02 0.1632 0.0046 1.561 0.047 0.0894 0.0006 975 26

0.00001 955

363 910 17 107

12.1 260 0.72 60

0.00004

321 204 0.64 50 0.07 0.1545 0.0056 1.456 0.059 0.0683 0.0009 926 32 912 878 27 106 13.1

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Boger

Precambrian

Research

104

(2000)

–

Pb (ppm) 204_Pb_/205_{Pb f206 (%)} _{Radiogenic ratios} _{Ages (Ma)} _{Concordance (%)}

Th (ppm) Grain.spot U (ppm) Th/U

206_Pb_/238_U ₉ 207_Pb_/235_U₉ 207_Pb_/206_Pb ₉ 206_Pb_/238_U₉ 207_Pb_/235_U207_Pb_/206_Pb ₉

14.1 461 323 0.70 73 0.00004 0.07 0.1558 0.0090 1.505 0.090 0.0701 0.0008 933 50 932 830 23 100 0.22 0.1671 0.0072 1.590 0.084 0.0890 0.0018 996 40

0.00013 966

277 899 54 111

15.1 157 0.57 45

0.00000

443 328 0.74 73 B0.01 0.1509 0.0041 1.562 0.053 0.0709 0.0014 958 23 955 953 40 100 16.1

0.05 0.1570 0.0040 1.523

17.1 553 422 0.76 90 0.00003 0.044 0.0704 0.0008 940 22 940 939 22 100

B0.01 0.1602 0.0043 1.584 0.049 0.0717 0.0009 958 24 964 978 27 98 0.00000

0.68 291

427

18.1 89

Sample9628-196 (

Cpx bearing leucosome—west wall of Radok Lake)

11 0.00000 B0.01 0.1389 0.0079 1.517 0.136 0.0792 0.0049 839 45 937 1177 128 71 62

1.1 34 0.55

0.10 0.0879 0.0040 1.516 0.131

2.1 97 58 0.59 11 0.00006 0.1620 0.0091 424 24 937 2476 98 17

B0.01 0.1461 0.0026 1.495 0.062 0.0742 0.0028 879 15

0.00000 929

140 1048 74 84

3.1 108 0.77 27

0.00005

305 139 0.46 50 0.06 0.1396 0.0072 1.312 0.077 0.0682 0.0015 842 41 851 874 47 96 4.1

67 39 0.59 0.00020 0.34 0.1615 0.0031 1.522 0.056 0.0684 0.0020 965 17 939 879 61 110

5.1 13

B0.0l 0.1553 0.0031 1.607 0.065 0.0750 0.0025 931 17

0.00000 973

80 1069 67 87

6.1 42 0.47 17

0.00012

497 135 0.27 97 0.21 0.1687 0.0057 1.898 0.074 0.0730 0.0018 1005 31 1008 1013 50 99 7.1

0.32 0.1615 0.0036 1.496 0.054 0.0672 0.0017 968 20

8.1 79 56 1.70 17 0.00019 929 843 55 115

0.04 0.1699 0.0047 1.642 0.053 0.0701 0.0026 1011 26

0.00002 987

256 932 29 109

9.1 132 0.50 56

0.00046

3915 48 0.01 405 0.78 0.0971 0.0032 0.909 0.048 0.0879 0.0026 597 19 865 965 80 89 10.1

0.58 0.1971 0.0091 1.903 0.123 0.0700 0.0028 1160 49 1082 929

11.1 242 124 0.51 60 0.00034 84 125

B0.01 0.1743 0.0104 1.743 0.107 0.0725 0.0007 1038 57

0.00000 1025

0.55 101 1001 20 103

12.1 485 254

0.00021

235 185 0.79 57 0.35 0.1837 0.0102 1.732 0.102 0.0884 0.0010 1087 56 1021 881 31 123 13.1

B0.01 0.1525 0.0059 1.558 0.071 0.0741 0.0015 915 33 954 1044

14.1 495 528 1.07 107 0.00000 40 88

B0.01 0.1500 0.0044 1.433 0.050 0.0693 0.0011 901 25

0.00000 903

216 907 32 99

15.1 1242 405 0.33

B0.01

16.1 309 135 0.45 60 0.00020 0.1674 0.0029 1.633 0.084 0.0707 0.0024 996 16 983 950 70 105

a_{Uncertainties given at the 1}slevel; f206% denotes the percentage of206_{Pb that is common Pb; correction for common Pb was made using the measured}204_Pb_/206_{Pb ratio; for % Concordance, 100% denotes a concordant}

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grains and have not been analysed in this study. Overgrowths are generally lacking. Zircons from this sample have U contents of 200 – 5000 ppm

with a Th/U ratio of between 0.5 and 2.0. Most

grains having a ratio of 1.0.

Twelve zircons were analysed from sample 9628-142. All 12 analyses form a statistically

sim-ple concordant population yielding a mean207_Pb

206_{Pb age of 936}₉_{14 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, analysis points 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.

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Fig. 8. U – Pb concordia diagrams showing SHRIMP data for samples from Radok Lake.206_Pb/207_{Pb ages are stated to 2}_s_(95%) confidence limits, while the illustrated error ellipses reflect 1sconfidence 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 — north wall of Battye Glacier (sample9628-73)

Sample 9628-73 is a medium-grained leucosome consisting 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 a

result of D3 shearing (Fig. 4f). We infer that the

leucosome formed syn-D3, concurrent with

tion of the high-strain zone. Leucosome forma-tion at this time is consistent with regional observations that suggest that extensive partial

melting occurred during D3, particularly within

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Zircons from sample 9628-73 are generally tur-bid and pale brown to pale reddish brown in colour. They form a subhedral population that

varies in length from 100 to 500mm, with average

length/width ratios of approximately 3:1. A Th/U

ratio of 0.7 is relatively consistent for all grains analysed. Likewise, the U contents of the zircons lie in a narrow range typically between 300 and 600 ppm. Most zircons show growth zoning, marked by subtle concentric bands of varying luminescence (Fig. 7d). Many contain highly lu-minescent inclusions of apatite (Fig. 7e). Some grains are overgrown by an unzoned more lu-minescent rim. However, most zircons have a simple igneous appearance and are inferred to have formed at the time of leucosome formation. The isotopic data for the zircons from sample 9628-73 are presented in Table 2. All 18 analyses

form a single concordant mean207_Pb_/206_{Pb age of}

942917 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 the development of the upright high-strain zone at

940 Ma.

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

Sample 9628-196 was taken from medium- to coarse-grained leucosome consisting of sericitised alkali and plagioclase feldspars, quartz, clinopy-roxene and green hornblende. The leucosome oc-curs within the amphibolite facies felsic and intermediate orthogneisses, exposed along the lower cliff faces at Radok Lake (Fig. 3). The amphibolite facies rocks at this locality tectoni-cally underlie the granulites that make up the bulk of the nPCMs. Leucosomes within these rocks (including sample 9628-196) form elongate layers

that parallel S0/1, but which also locally form

spurs 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 the random to weakly orientated assemblages. We suggest that peak metamorphic conditions were

attained as a result of the emplacement of the granulites over the amphibolites, the granulites either advectively heating the underlying units, or their emplacement resulting in the net burial and subsequent heating of the underthrust units. We

therefore propose a syn-D4 timing of leucosome

formation. No equivalent leucosome development

occurred as a result of D4 deformation within the

overlying granulites.

Zircons from sample 9628-196 are reddish brown, euhedral to subhedral, and vary in length

from 150 to 450 mm. Zircons from this sample

have length/width ratios of approximately 2:1 –

3:1, and are generally more euhedral than those from the previous samples. CL imaging shows that they also have more complicated internal morphologies. Most grains contain cores that are commonly dark and can be either homogeneous or concentrically zoned. The internal structure of these cores is often cross-cut by a rounded resorp-tion surface (Fig. 7f – h), which is then overgrown by euhedral, generally more luminescent rims. However, examples of poorly luminescent euhe-dral rims were also observed (Fig. 7h). Highly luminescent unzoned euhedral zircons are also present, and may represent the same period of zircon growth as that which formed the rims on

other zircons. The Th/U ratio of both cores and

rims lies in the range 0.3 to 1.0, with most

analy-ses 0.5. There is no consistent contrast in U

content between core and rim analyses, with con-siderable overlap occurring between individual grains.

Sixteen zircon grains from sample 9628-196 were analysed, of which 15 (excluding 2.1) lie on or near concordia and produce a weighted mean

207_Pb_/206_{Pb age of 954}₉_{38 Ma (MSWD}₌_2.71)

(Fig. 8d). However, the large MSWD indicates excess statistical scatter about the mean. Mod-elling suggests that there are two distinct sub-pop-ulations that are separated by a distinct age gap

of 50 Myr (Fig. 8d). This reflects a subtle

difference in age obtained from core and rim analyses. Core analyses 1.1, 3.1, 6.1, 7.1, 12.1 and 14.1 definite an older grouping that yields a mean

207_Pb_/206_{Pb age of 1017}₉_{31 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

(17)

give a mean 207_Pb_/206_{Pb age of 900}₉_{28 Ma}

(MSWD=0.498). We suggest that the older

pop-ulation is inherited from the orthogneiss, and record an age reflecting orthogneiss emplacement, whereas the rims are considered to have formed at the time of leucosome development and are

con-sidered to constrain the timing of D4. If this

interpretation 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 population sourced from the leucosome, which would imply

that the time interval between D3and D4was very

short, as the ages of samples 9628-142 (D3),

9628-73 (D3) and 9628-196 (D4) are all statistically

identical. We prefer the first alternative, although cannot conclusively preclude the latter.

6. Discussion

On the basis of our geochronological results, we conclude that deformation and high-grade meta-morphism in the Radok Lake area occurred over

a period spanning approximately 90 Myr. D1 and

D2 are considered progressive, a conclusion also

draw by a number of previous studies (Fitzsimons and Harley, 1992; Thost and Hensen, 1992; Hand et al. 1994b), and occurred concurrently with regionally extensive magmatism and peak

meta-morphism at 990 – 980 Ma. The subsequent

de-velopment of upright folds (F3) and steeply

dipping high-strain zones occurred at 940 Ma.

These pervasive features were then overprinted by discrete mylonites and pseudotachylites that

de-veloped at 900 Ma.

Our structural observations show that both fold

generations (F2and F3) formed coaxially with L1

(Fig. 3), and that the resolved palaeo-transport

directions from D3 high-strain zones and D4

my-lonites are also subparallel (Fig. 5). All four events show evidence of having formed in

re-sponse to north – south-directed compression.

Given the consistency in orientation of the palaeostress field and the relative proximity in age of the four deformational events (Fig. 9), we suggest that the recognised sequence of

deforma-tional episodes (D1– D4) all developed during a

single evolving north – south compressive orogenic event (Fig. 10). During this time, north – south-di-rected compression shortened the terrain, through gradually more discrete phases of deformation. The accumulation of strain occurred in response to the same compressive stress field. However, the style of deformation changed in response to changes in the orientation and magnitude of the intermediate and stretching axes, and the relative contribution of the components of pure and sim-ple shear (Fig. 6).

The constraints on deformation presented are consistent with published structural and age data available from throughout the nPCMs. The intru-sion ages presented by Kinny et al. (1997) from

felsic bodies at Loewe Massif (charnockite, 9809

21 Ma), Mt Collins (granites, 976925 and 9849

7 Ma; quartz syenite, 984912 Ma) and Mt

McCarthy (leucogneisses, 990930 Ma) are all

statistically identical to the 990918 Ma intrusion

age obtained in this study (Fig. 9). Equivalent

intrusive ages of 985929 and 954912 Ma were

also recorded from charnockites outcropping along 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 obtained

by Manton et al. (1992) from Jetty Peninsula has been interpreted by Hand et al. (1994b) to date

the emplacement of a pre- to syn-F3 leucogneiss.

This interpretation concurs with the results of this

study as it also suggests that F3 folding occurred

at about 940 Ma (Fig. 9). Finally, SHRIMP data

from Mt Kirkby suggests F3 folding and shear

zone formation occurred at 910 Ma (Carson et

al., 2000), an age that is within error of the

estimates on the timing of D3 and D4 obtained

from this study. This consistency in published age data is mirrored by a remarkable consistency in the sequence and orientation of structures ob-served throughout the nPCMs (compare Fig. 6 of Fitzsimons and Harley (1992) with Fig. 10). Thus, it is considered likely that the conclusions drawn in this study are broadly applicable over much of the nPCMs.

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As well as pervasively deforming the terrain, 990 – 900 Ma orogenesis in the nPCMs has em-placed granulite facies gneisses, which form the bulk of the exposed rock in the nPCMs, over amphibolite facies intermediate and felsic units that crop out in a window exposed at Radok Lake. This relationship suggests that the gran-ulites at the southern end of the nPCMs are likely to be allochthonous, a scenario already proposed to 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 are equivalent to the amphibolite facies metavolcanic sequence exposed further to the south at Fisher Massif (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 Massif metavolcanics are intruded by a biotite granite

that yielded an age of 1020 Ma (Kinny et al.,

1997), identical to that obtained from the inher-ited zircon population obtained from sample 9628-196. This relationship may well be coinci-dental. However, it supports the inference that the granulites of the nPCMs may tectonically overlie the Fisher terrain.

The geochronological data presented here from the nPCMs is readily correlated with published data 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 with constraints on deformation in the nPCMs.

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

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

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

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belts exposed in Africa, Australia and the Antarc-tic to form a single laterally extensive orogenic belt, thought to have extended along the coast of east Antarctica from Coats Land at the edge of the Weddell Sea, through to the Albany – Fraser Belt of southwest Australia and into the Mus-grave block of central Australia. However, 990 –

900 Ma orogenesis recognised in the

nPCMs – Mawson Coast – Rayner Complex – East-ern Ghats provinces in east Antarctica and India is significantly younger than that recognised both to the east and the west (Fig. 11). The orogenic belts exposed in the Musgrave block of central Australia (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) of east Antarctica, can be correlated on the basis that all experienced high-grade metamorphism, magmatism and deformation between 1300 and 1050 Ma (White et al., 1999). The youngest event

recognised in these terrains is 60 Myr older

than the onset of deformation and metamorphism in the nPCMs. Likewise, the metamorphic belts exposed in the Maud Province of east Antarctica (Arndt et al., 1991; Jacobs et al., 1995, 1998) and the 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, but older than that recognised in the nPCMs. In the Maud 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 have been previously correlated, and the two recently recognised intervening Palaeozoic belts in Lu¨tzow-Holm Bay and Prydz Bay are illustrated. G, Gawler craton; K, Kalahari craton; sPCMs, southern Prince Charles Mountains; V, Vestford Hills; LHB, Lu¨tzow-Holm Bay; P, Prydz Bay; nPCMs, northern Prince Charles Mountains.

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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 in

these terrains ceased 50 Ma prior to the onset

of deformation in the nPCMs. Given the signifi-cant differences in age of these previously corre-lated terrains, it seems unlikely that these belts represent a single continuous suture. Instead, these three terrains probably represent separate fragments of disparate Meso- to Neoproterozoic orogenic belts. This is consistent with the recently recognised Palaeozoic tectonism in Lu¨tzow-Holm Bay and Prydz Bay, two younger orogenic belts that separate each of these different Meso- to Neoproterozoic terrains (Fig. 11).

Finally, our results impact on the debate as to the extent of Palaeozoic reworking experienced by the nPCMs. The data presented in this paper constrains all high-grade deformation and meta-morphism to have occurred during the Neopro-terozoic, precluding the possibility of subsequent

high-grade events post-dating 900 Ma. We do

not rule out the possibility of later stages of deformation, but suggest that they were of a relatively low grade and of a discrete nature. Whereas, the adjacent terrain of Prydz Bay has been pervasively reworked by an early Palaeozoic granulite 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 facies deformation and metamorphism observed in the

nPCMs to have occurred between 990 and

900 Ma. During this period, north –

south-di-rected compression deformed the terrain through four 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 that

orogenesis in the nPCMs – Mawson Coast –

Rayner Complex – Eastern Ghats is temporally quite distinct from other Meso-Neoroterozoic metamorphic belts thought to be involved in the

formation of east Gondwana and Rodinia. In-stead, we suggest that these Meso-Neoroterozoic metamorphic belts are of separate origin and were probably accreted together during the Palaeozoic along orogenic belts recognised in Lu¨tzow-Holm Bay and Prydz Bay. With respect to Palaeozoic reworking of the nPCMs, we do not rule out the possibility of later discrete stages of deformation. However, our results suggest that all high-grade penetrative fabrics observed in the nPCMs formed during the Neoproterozoic.

Acknowledgements

The authors would like to thank the Australian Antarctic Division for logistical support over the 1996 – 1997 summer. The cost of field expenses and analytical time on SHRIMP I and SHRIMP II was met from an ASAC grant to C.J.L.W. The Australian Geological Survey Organisation are thanked for providing air-photos, while Doug Thost is also thanked for his assistance and friendship in the field. We would also like to thank Pete Kinny and Ian Fitzsimons for thor-ough and constructive reviews that greatly im-proved the quality of this manuscript.

References

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Arriens, P., 1975. The Precambrian geochronology of Antarc-tica. First Australian Geological Convention, Adelaide, Geology Society of Australia, Abstracts, pp. 97 – 98. Beliatsky, B.V., Laiba, A.A., Mikhalsky, E.V., 1994. U – Pb

zircon age of metavolcanic rocks of Fisher Massif (Prince Charles Mountains, East Antarctica). Antarct. Sci. 6, 355 – 358.

Black, L.P., Harley, S.L., Sun, S.S., McCulloch, M.T., 1987. The Rayner Complex of east Antarctica, complex isotopic systematics within a Proterozoic mobile belt. J. Metamor-phic Geol. 5, 1 – 26.

Black, L.P., Sheraton, J.W., Tingey, R.J., McCulloch, M.T., 1992. New U – Pb zircon ages from the Denman Glacier area, East Antarctic, and their significance for Gondwana reconstruction. Antarct. Sci. 4, 447 – 460.

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Camacho, A., Vernon, R.H., Fitzgerald, J.D., 1995. Large volumes of anhydrous pseudotachylite in the Woodroffe Thrust, eastern Musgrave Ranges. Aust. J. Struct. Geol. 17, 371 – 383.

Carson, C.J., Dirks, P.H.G.M., Hand, M., Sims, J.P., Wilson, C.J.L., 1995. Compressional and extensional tectonics in low-medium pressure granulites from the Larsemann Hills, East Antarctica. Geol. Mag. 132, 151 – 170.

Carson, C.J., Fanning, C.M., Wilson, C.J.L., 1996. Timing of the Progress Granite, Larsemann Hills, evidence for Early Palaeozoic orogenesis within the East Antarctic Shield and implications for Gondwana assembly. Aust. J. Earth Sci. 43, 539 – 553.

Carson, C.J., Boger, S.D., Fanning, C.M., Wilson, C.J.L., Thost, D. 2000. SHRIMP U – Pb geochronology from Mt Kirkby, northern Prince Charles Mountains, East Antarc-tica. Antarctic Sci. (in press).

Clarke, G.L., Sun, S.S., White, R.W., 1995. Grenville-age belts and associated older terrains in Australia and Antarctica. AGSO J. Aust. Geol. Geophys. 16, 25 – 39.

Compston, W., Williams, I.S., Meyer, C., 1984. U – Pb geo-chonology of zircons from lunar breccia 73217 using a sensitive high mass-resolution ion microprobe. J. Geophys. Res. 89, B525 – B534.

Compston, W., Williams, I.S., Kirschvink, J.L., Zichao, Z., Guogan, M., 1992. Zircon U – Pb ages for the early Cam-brian time-scale. J. Geol. Soc. Lond. 149, 171 – 184. Cornell, D.H., Thomas, R.J., Bowring, S.A., Armstrong,

R.A., Grantham, G.H., 1996. Protolith interpretation in metamorphic terranes: a back-arc environment with Besshi-type base metal potential for the Quha Formation, Natal Province, South Africa. Precam. Res. 77, 243 – 271. Crohn, P.P., 1959. A contribution to the geology and

glaciol-ogy of the western part of Australian Antarctic Territory. Aust. Bur. Miner. Res., Geol. Geophys. Bull. 52, 1 – 103. Cumming, G.L., Richards, J.R., 1975. Ore lead isotopes in a

continuously changing Earth. Earth Planet. Sci. Lett. 28, 155 – 171.

Dirks, P.H.G.M., Wilson, C.J.L., 1995. Crustal Evolution of the East Antarctic mobile belt in Prydz Bay, continental collison at 500 Ma? Precam. Res. 75, 189 – 207.

Fielding, C.R., Webb, J.A., 1995. Sedimentology of the Per-mian Radok Conglomerate in the Beaver Lake area of Mac Robertson Land, East Antarctica. Geol. Mag. 132, 51 – 63.

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pres-sure – temperature paths in the Proterozoic Complex of east Antarctica. In: Yoshida, Y., Kaminuma, K., Shiraishi, K. (Eds.), Recent Progress in Antarctic Earth Science. Terra Scientific Publishing Company, Tokyo, pp. 103 – 111. Fitzsimons, I.C.W., Harley, S.L., 1994. The influence of

retro-grade cation exchange on granuliteP–T estimates and a convergence technique for the recovery of peak metamor-phic conditions. J. Petrol. 35, 543 – 576.

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Fitzsimons, I.C.W., Kinny, P.D., Harley, S.L., 1997. Two stages of zircon and monazite growth in anatectic leucogneiss, SHRIMP constraints on the duration and intensity of Pan-African metamorphism in Prydz Bay, East Antarctica. Terra Nova 9, 47 – 51.

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C.J.L., 1994. Geological observations in high grade mid-Proterozoic rocks from Else Platform, northern Prince Charles Mountains region, East Antarctica. Aust. J. Earth Sci. 41, 311 – 329.

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Harley, S.L., Fitzsimons, I.C.W., 1995. High grade metamor-phism and deformation in the Prydz Bay region, East Antarctica: events and regional correlations. In: Yoshida, Y., Santosh, M. (Eds.), India and Antarctica during the Precambrian. Geology Society of India Memoir, vol. 34. pp. 73 – 100.

Hensen, B.J., Zhou, B., 1995. A Pan-African granulite facies metamorphic episode in Prydz Bay, Antarctica, evidence from Sm – Nd garnet dating. Aust. J. Earth Sci. 42, 249 – 258.

Hensen, B.J., Zhou, B., 1997. East Gondwanaamalgamation by Pan-African collision? Evidence in the Prydz Bay re-gion, east Antarctica. In: Ricci, C.A. (Ed.), The Antarctic Region, Geological Evolution and Processes. Terra Antarctica Publication, Siena, pp. 115 – 119.

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

Eastern Ghats of India (Grew and Manton, 1986;

Paul et al., 1990; Shaw et al., 1997). All yield

syn-orogenic ages of between 990 and 900 Ma. In

many east Gondwana reconstructions (for

exam-ple, Moores, 1991; Rogers 1996), these belts have

been correlated with other Meso-Neoproterozoic

(2)

belts exposed in Africa, Australia and the

Antarc-tic to form a single laterally extensive orogenic

belt, thought to have extended along the coast of

east Antarctica from Coats Land at the edge of

the Weddell Sea, through to the Albany – Fraser

Belt of southwest Australia and into the

Mus-grave block of central Australia. However, 990 –

900 Ma

orogenesis

recognised

in

the

nPCMs – Mawson Coast – Rayner Complex –

East-ern Ghats provinces in east Antarctica and India

is significantly younger than that recognised both

to the east and the west (Fig. 11). The orogenic

belts exposed in the Musgrave block of central

Australia (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) of

east Antarctica, can be correlated on the basis

that all experienced high-grade metamorphism,

magmatism and deformation between 1300 and

1050 Ma (White et al., 1999). The youngest event

recognised in these terrains is

60 Myr older

than the onset of deformation and metamorphism

in the nPCMs. Likewise, the metamorphic belts

exposed in the Maud Province of east Antarctica

(Arndt et al., 1991; Jacobs et al., 1995, 1998) and

the 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, but

older than that recognised in the nPCMs. In the

Maud and Namaqua-Natal Provinces, felsic

(3)

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 in

these terrains ceased

50 Ma prior to the onset

of deformation in the nPCMs. Given the

signifi-cant differences in age of these previously

corre-lated terrains, it seems unlikely that these belts

represent a single continuous suture. Instead,

these three terrains probably represent separate

fragments of disparate Meso- to Neoproterozoic

orogenic belts. This is consistent with the recently

recognised Palaeozoic tectonism in Lu¨tzow-Holm

Bay and Prydz Bay, two younger orogenic belts

that separate each of these different Meso- to

Neoproterozoic terrains (Fig. 11).

Finally, our results impact on the debate as to

the extent of Palaeozoic reworking experienced by

the nPCMs. The data presented in this paper

constrains all high-grade deformation and

meta-morphism to have occurred during the

Neopro-terozoic, precluding the possibility of subsequent

high-grade events post-dating

900 Ma. We do

not rule out the possibility of later stages of

deformation, but suggest that they were of a

relatively low grade and of a discrete nature.

Whereas, the adjacent terrain of Prydz Bay has

been pervasively reworked by an early Palaeozoic

granulite 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 facies

deformation and metamorphism observed in the

nPCMs to have occurred between

990 and

900 Ma. During this period, north –

south-di-rected compression deformed the terrain through

four 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 that

orogenesis

in

the

nPCMs – Mawson

Coast –

Rayner Complex – Eastern Ghats is temporally

quite distinct from other Meso-Neoroterozoic

metamorphic belts thought to be involved in the

formation of east Gondwana and Rodinia.

In-stead, we suggest that these Meso-Neoroterozoic

metamorphic belts are of separate origin and were

probably accreted together during the Palaeozoic

along orogenic belts recognised in Lu¨tzow-Holm

Bay and Prydz Bay. With respect to Palaeozoic

reworking of the nPCMs, we do not rule out the

possibility of later discrete stages of deformation.

However, our results suggest that all high-grade

penetrative fabrics observed in the nPCMs formed

during the Neoproterozoic.

Acknowledgements

The authors would like to thank the Australian

Antarctic Division for logistical support over the

1996 – 1997 summer. The cost of field expenses

and analytical time on SHRIMP I and SHRIMP

II was met from an ASAC grant to C.J.L.W. The

Australian Geological Survey Organisation are

thanked for providing air-photos, while Doug

Thost is also thanked for his assistance and

friendship in the field. We would also like to

thank Pete Kinny and Ian Fitzsimons for

thor-ough and constructive reviews that greatly

im-proved the quality of this manuscript.

References