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 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
at 500 Ma.
S .D
. Boger
et al
. Precambrian
Research
104 2000
1 –
24
5 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. S
1
is the dominant form surface illustrated, except in D
3
shear zones, where S
3
is dominant. Equal area stereographic projections summarise structural data and illustrate lineation L
1
, fold axis F
23
and combined foliation and lineation data D
34
for all observed structures.
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 980 9 21 Ma, Mt Collins 976 9
25, 984 9 7 and 984 9 12 Ma and Mt McCarthy 990 9 30 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
