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 D
4
. If this interpretation is correct, felsic orthogneiss intru-
sion occurred at 1020 Ma, and D
4
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 D
3
and D
4
was very short, as the ages of samples 9628-142 D
3
, 9628- 73 D
3
and 9628-196 D
4
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. D
1
and D
2
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 F
3
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 F
2
and F
3
formed coaxially with L
1
Fig. 3, and that the resolved palaeo-transport directions from D
3
high-strain zones and D
4
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 D
1
– D
4
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, 980 9 21 Ma, Mt Collins granites, 976 9 25 and 984 9
7 Ma; quartz syenite, 984 9 12 Ma and Mt McCarthy leucogneisses, 990 9 30 Ma are all
statistically identical to the 990 9 18 Ma intrusion age obtained in this study Fig. 9. Equivalent
intrusive ages of 985 9 29 and 954 9 12 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-F
3
leucogneiss. This interpretation concurs with the results of this
study as it also suggests that F
3
folding occurred at about 940 Ma Fig. 9. Finally, SHRIMP data
from Mt Kirkby suggests F
3
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 D
3
and D
4
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.
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.
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
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.
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
