Analytical procedure Geochronological results

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

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 ions from areas 25 mm in diameter from indi-

5. Geochronological results

5 . 1 . K-Feldspar granite — west wall of Radok lake sample 9628 - 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 S 01 foliation Fig. 5e. It contains a well-developed layer-par- allel foliation reoriented by upright folding F 3 . The foliation within the granite does not define F 2 fold closures, nor were recumbent folds F 2 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 D 3 as they are folded by this event. Thus, these granites are interpreted to have intruded synchronously with D 1 – 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 ThU ratio be- tween 0.5 and 0.9. Zircons can show some inter- nal sector zoning that occasionally 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 990 9 18 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 D 2 , 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 F 3 fold, is unfoliated, cross-cuts structures developed during F 3 folding, and is offset by later D 4 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-D 3 sills sample 9628-141 already described. The intrusion of sample 9628-142 is inferred to have occurred late-syn- to post-D 3 and is consid- ered to place a minimum age on the timing of D 3 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 S .D . Boger et al . Precambrian Research 104 2000 1 – 24 Table 2 U–Th–Pb isotopic compositions of zircons from the northern Prince Charles Mountains a Pb ppm 204 Pb 205 Pb Th ppm f206 Radiogenic ratios Ages Ma Concordance Grain.spot ThU U ppm 206 Pb 238 U 207 Pb 206 Pb 9 207 Pb 235 U 207 Pb 235 U 9 207 Pb 206 Pb 9 9 206 Pb 238 U 9 Sample 9628 - 141 K-feldspar granite — west wall of Raclok Lake 0.59 0.14680 0.0046 1.3772 0.063 1.1 0.088 301 0.0032 883 28 879 870 102 102 335 1.11 54 0.00034 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 0.0150 285 1.3026 0.202 0.080 0.0060 722 87 847 1191 157 61 189 0.66 39 0.00080 0.08 0.15389 0.0030 1.5811 0.058 0.075 0.0021 922 17 0.00005 963 0.88 1058 59 87 38 4.1 212 286 0.00012 494 0.20 0.16355 0.0032 1.5943 0.044 0.071 0.0012 976 18 90 949 36 103 239 0.48 84 5.1 B 0.01 0.14086 0.0119 1.5316 0.141 0.075 0.0027 890 67 6. 1 943 189 1069 74 83 158 0.84 32 0.00000 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 55 0.53 181 0.00001 339 0.02 0.10014 0.0083 1.6507 0.094 0.075 0.0013 958 46 990 1082 36 90 169 0.50 58 8. 1 0.15 0.15806 0.0064 1.5824 0.079 0.073 9.1 0.0018 231 946 38 963 1003 50 94 205 0.89 42 0.00009 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 64 0.96 328 0.00020 506 0.35 0.16589 0.0058 0.5540 0.072 0.068 0.0019 989 31 952 867 58 114 290 0.57 89 11.1 0.20 0.15304 0.0090 1.4596 0.109 0.069 0.0027 915 50 12.1 914 404 904 83 102 233 0.58 66 0.00012 B 0.01 0.16220 0.0041 1.6295 0.060 0.073 0.0018 9989 22 0.0000 902 251 1010 50 98 13.1 43 0.49 124 0.00011 178 0.20 0.15105 0.0064 1.5048 0.093 0.072 0.0029 907 36 932 993 84 91 149 0.84 31 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 1019 77 88 69 16.1 407 330 0.00023 1216 0.39 0.16874 0.0067 1.6880 0.078 0.072 0.0014 1005 37 1003 999 39 101 74 0.06 194 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 0.8795 4434 0.038 0.075 0.0029 530 9 641 1055 79 50 299 0.07 366 0.00145 Sample 9628 - 142 K-feldspar granite — west wall of Radok Lake 0.02 0.1558 1.1 0.0034 196 0.525 0.039 0.0711 0.0008 932 19 940 959 22 97 157 0.79 35 0.00001 0.06 0.1527 0.0034 1.445 0.038 0.0687 0.0008 916 19 0.00002 908 0.80 888 25 103 49 2.1 284 226 0.00001 233 0.02 0.1532 0.0033 1.497 0.036 0.0709 0.0006 919 19 929 955 18 96 189 0.81 41 3.1 0.02 0.1627 0.0036 1.582 0.039 0.0705 0.0006 972 20 4.1 963 331 943 16 103 341 1.03 64 0.00001 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 60 1.03 333 0.00001 297 0.02 0.1573 0.0033 1.512 0.036 0.0697 0.0006 942 19 935 920 17 102 291 0.98 55 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 0.88 943 10 102 97 8.1 521 460 0.00005 273 0.09 0.1546 0.0035 1.483 0.038 0.0696 0.0007 926 19 923 916 21 101 249 0.91 49 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 1.477 340 0.036 0.0696 0.0008 921 18 921 922 22 100 673 1.98 76 0.00001 Sample 9628 - 73 Opx-bearing leucosome — north wall of Battye Glacier 0.08 0.1572 0.0021 1.527 0.041 1.1 0.0704 371 0.0015 941 12 941 941 45 100 229 0.62 58 0.00003 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 50 0.47 159 0.00001 158 0.02 0.1509 0.0036 1.500 0.050 0.00721 0.0014 906 21 930 989 39 92 208 1.32 28 3.1 B 0.0l 0.1637 0.0047 1.631 0.055 0.0723 0.0011 977 26 4.1 982 675 994 31 98 300 0.44 106 0.00000 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 54 0.61 206 0.00007 359 0.12 0.1621 0.0050 1.548 0.053 0.0693 0.0006 968 28 949 906 25 107 225 0.63 58 7.1 0.22 0.1542 0.0052 1.522 0.057 0.0716 0.0009 925 29 8.1 939 509 974 25 95 359 0.71 80 0.00013 B 0.01 0.1696 0.0031 1.676 0.041 0.0717 0.0010 1010 17 0.00000 999 0.76 976 28 104 84 9.1 481 365 0.00002 507 0.03 0.1535 0.0054 1.510 0.057 0.0713 0.0007 921 30 934 967 20 95 389 0.77 81 10.1 0.02 0.1702 0.0121 1.790 0.181 0.0763 0.0048 1013 67 11.1 1042 639 1102 132 92 449 0.70 110 0.00001 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 60 0.72 260 0.00004 321 0.07 0.1545 0.0056 1.456 0.059 0.0683 0.0009 926 32 912 878 27 106 204 0.64 50 13.1 S .D . Boger et al . Precambrian Research 104 2000 1 – 24 13 Table 2 Continued Pb ppm 204 Pb 205 Pb f206 Radiogenic ratios Ages Ma Concordance Th ppm Grain.spot ThU U ppm 206 Pb 238 U 9 207 Pb 235 U 9 207 Pb 206 Pb 9 206 Pb 238 U 9 207 Pb 235 U 207 Pb 206 Pb 9 14.1 0.07 461 0.1558 0.0090 1.505 0.090 0.0701 0.0008 933 50 932 830 23 100 323 0.70 73 0.00004 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 45 0.57 157 0.00000 443 B 0.01 0.1509 0.0041 1.562 0.053 0.0709 0.0014 958 23 955 953 40 100 328 0.74 73 16.1 0.05 0.1570 0.0040 1.523 17.1 0.044 553 0.0704 0.0008 940 22 940 939 22 100 422 0.76 90 0.00003 B 0.01 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 Sample 9628 - 196 Cpx bearing leucosome — west wall of Radok Lake 11 0.00000 B 0.01 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 0.1620 97 0.0091 424 24 937 2476 98 17 58 0.59 11 0.00006 B 0.01 0.1461 0.0026 1.495 0.062 0.0742 0.0028 879 15 0.00000 929 140 1048 74 84 3.1 27 0.77 108 0.00005 305 0.06 0.1396 0.0072 1.312 0.077 0.0682 0.0015 842 41 851 874 47 96 139 0.46 50 4.1 67 0.00020 0.34 0.1615 0.0031 1.522 0.056 0.0684 0.0020 965 17 939 879 61 110 39 0.59 5.1 13 B 0.0l 0.1553 0.0031 1.607 0.065 0.0750 0.0025 931 17 0.00000 973 80 1069 67 87 6.1 17 0.47 42 0.00012 497 0.21 0.1687 0.0057 1.898 0.074 0.0730 0.0018 1005 31 1008 1013 50 99 135 0.27 97 7.1 0.32 0.1615 0.0036 1.496 0.054 0.0672 0.0017 968 20 8.1 929 79 843 55 115 56 1.70 17 0.00019 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 56 0.50 132 0.00046 3915 0.78 0.0971 0.0032 0.909 0.048 0.0879 0.0026 597 19 865 965 80 89 48 0.01 405 10.1 0.58 0.1971 0.0091 1.903 0.123 0.0700 0.0028 1160 49 1082 929 11.1 84 242 125 124 0.51 60 0.00034 B 0.01 0.1743 0.0104 1.743 0.107 0.0725 0.0007 1038 57 0.00000 1025 0.55 1001 20 103 101 12.1 485 254 0.00021 235 0.35 0.1837 0.0102 1.732 0.102 0.0884 0.0010 1087 56 1021 881 31 123 185 0.79 57 13.1 B 0.01 0.1525 0.0059 1.558 0.071 0.0741 0.0015 915 33 954 1044 14.1 40 495 88 528 1.07 107 0.00000 B 0.01 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 B 0.01 16.1 0.1674 309 0.0029 1.633 0.084 0.0707 0.0024 996 16 983 950 70 105 135 0.45 60 0.00020 a Uncertainties given at the 1s level; f206 denotes the percentage of 206 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 analysis. Values \100 are reverse discordant. 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 ThU 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 mean 207 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. Fig. 8. U – Pb concordia diagrams showing SHRIMP data for samples from Radok Lake. 206 Pb 207 Pb ages are stated to 2s 95 confidence limits, while the illustrated error ellipses reflect 1s confidence limits 68. MSWD, Mean square of weighted deviates. Histograms show the distribution of individual analyses and highlight the single zircon population found in samples 9628-141, 9628-142 and 9628-73 compared with the two populations found in sample 9628-196. crystallisation of the granite, and suggests D 3 folding occurred at, or prior to, 940 Ma. 5 . 3 . Orthopyroxene-bearing leucosome — north wall of Battye Glacier sample 9628 - 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 D 3 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 D 3 shearing Fig. 4f. We infer that the leucosome formed syn-D 3 , concurrent with forma- 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 D 3 , particularly within metapelitic units. 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 500 mm, with average lengthwidth ratios of approximately 3:1. A ThU 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 mean 207 Pb 206 Pb age of 942 9 17 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 S 01 , 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-D 4 timing of leucosome formation. No equivalent leucosome development occurred as a result of D 4 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 lengthwidth 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 ThU 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 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