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
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
. D
1
–
2
deformation The earliest recognised fabric element is a com-
posite S S
1
fabric defined by an intense and pervasive preferred mineral orientation, which is
always concordant with lithological layering S .
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 L
1
lineation Fig. 3. Although overprinted by two episodes of folding
D
2
and D
3
, S
1
is well preserved and only weakly overprinted by the development of new fabrics
associated with subsequent folding. D
2
resulted in the reorientation of the com- posite S
S
1
fabric about recumbent, isoclinal F
2
folds. Although folding S S
1
isoclinally, there is little development of an axial planar fabric.
Rather, S
1
remains the predominant foliation, potentially intensified on the limbs of F
2
folds, where its orientation is parallel to the F
2
axial plane. Large-scale F
2
folds were not recognised, but were inferred, as mesoscale F
2
isoclinal folds are common with type-three F
2
and F
3
fold inter- ference patterns Ramsay, 1967 recognised in D
3
low-strain zones. F
2
folds form about ENE trend-
ing axes that parallel the L
1
lineation Fig. 3. The formation of an L
2
lineation is not recognised, although the development of such a lineation
cannot be precluded. If present, L
2
was of the same grade and formed parallel to and potentially
intensified L
1
so that the two were indistinguish- able. The formation of both a flat-lying foliation
S
1
and recumbent fold closures F
2
is poten- tially consistent with ongoing dominantly sub-
horizontal simple shear. Indeed granulite facies assemblages define both the S
1
foliation and, where developed, the subtle S
2
fabric. Addition- ally, F
2
folding has formed coaxially with the lineation L
1
Fig. 3. Although the rotation and transposition of lithological layering S
into par- allelism with S
1
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 D
1
to D
2
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 D
1 – 2
is inferred to have been north – south directed, perpendicular to
the lineation orientation and the F
2
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 F
2
folds were found on outcrop surfaces normal to the lin-
eation, while F
2
and F
3
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 D
1 – 2
.
3
.
2
. D
3
deformation D
3
was responsible for the major east – west trending structural grain and the gross geometry
observed in the nPCMs. D
3
folded the composite S
S
1
surface and D
2
structures into a series of meso- to macroscale upright F
3
fold 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 F
2
fold axes and the L
1
stretching lin- eation Fig. 3. F
3
folds plunge moderately to shallowly, to either the ENE or WSW Fig. 3.
However, with ongoing shortening, the increase in D
3
strain is accompanied by a localised decrease in fold wavelength and an increase in D
3
fold plunge. A weak axial planar S
3
foliation is dis-
Table 1 Summary of the nomenclature used to describe the deformational features observed throughout the northern Prince Charles
Mountains Formation of
Steeply dipping Upright folding
Author Isoclinal
Low-angle discrete regional gneissic
shear zone mylonite and
recumbent formation
layering folding
pseudotachylite formation
D3 D4
D3 D2
D1 This study
D1 D2
Scrimgeour and Hand 1997 D3
D4 D1MY1
D2 Nichols 1995
D3 MY2
MY3 D1
D1 Hand et al. 1994b
D2 D2
D3 Thost and Hensen 1992
D5 D1–D2–D3
D6 D4
D5 D4
D5 D6
D7–D8 Fitzsimons and Thost 1992
D1–D2–D3 D1
D2 D3
McKelvey and Stephenson 1990
Fig. 4. Structural and intrusive features from Radok Lake: a upright west-plunging F
3
antiform; b D
3
upright high-strain zone; c photomicrograph of coplanar D
4
pseudotachylite and mylonite; d reverse offset-D
4
mylonite with subtle drag folding on upper surface; e concordant intrusive contact between syn-D
2
granite distinctive pale unit and host lithologies sample 9628-141; f orthopyroxene bearing leucosome localised within D
3
boudin neck sample 9628-73; g clinopyroxene bearing leucosome formed concordant with, but locally cross-cutting S
S
1
foliation within amphibolite-facies intermediate and felsic orthogneisses.
cernible in some F
3
closures. However, the devel- opment of S
3
is generally restricted to the limbs of F
3
folds where discrete D
3
high-strain zones are developed.
D
3
high-strain zones Fig. 4b are tens to hun- dreds of metres in width. Within these zones, S
3
is generally indistinguishable in appearance from S
S
1
. However, continued localisation of strain within D
3
shear zones has locally overprinted the gneissic S
3
foliation with a mylonitic to ultramy- lonitic fabric. The mylonitic 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;
Scrimgeour and Hand, 1997. In the vicinity of Radok Lake, D
3
high-strain zones are orientated parallel to the S
3
axial surface, trending ENE and dipping steeply to the north. Elsewhere in the
nPCMs, some D
3
high-strain zones can dip steeply to the south for example, Hand et al.,
1994b. The S
3
foliation contains a steeply plung- ing L
3
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 D
3
high-strain zones accommodate sig- nificant movement as there are no observed or
reported changes in metamorphic grade across these structures.
The accommodation of D
3
strain by upright folding, then shearing along steeply dipping high-
strain zones is consistent with a transition during D
3
from 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 F
3
fold axes were rotated toward a vertical stretching axis, reflected by the observed
increase in F
3
fold plunge Fig. 6.
3
.
3
. D
4
deformation Deformation post-dating D
3
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, D
4
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, andor green
hornblende within some D
4
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 D
4
is limited to areas within, or immediately adjacent to, the mylonite zones.
Thus, the textural evolution of the nPCMs finished with the cessation of D
3
and resulted in the preservation of granulite facies metamorphic
textures formed during D
1 – 3
. D
4
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 D
4
formed in response to NNE – SSW-directed compression Fig. 5. Offset on most mylonites is
typically minor B 2 m. However, a major flat- lying D
4
mylonite 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
Peninsula.
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 D
3
and D
4
.
vidual sectioned zircons. Zr
2
O
+
,
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 2s confidence 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
