to the southwest, suggesting a dominant horizon- tal component of movement.
2
.
1
.
8
. D
5
Fold axes
vary systematically
in plunge
throughout the mainland exposures of the Nor- nalup Complex, consistent with folding by a later
generation of regional-scale folds F
5
with half- wavelengths in the order of 20 km. F
5
folds plunge shallowly to the northwest and reflect
moderate NE-SW horizontal shortening. A crenu- lation cleavage S
5
related to this deformation is developed in amphibolites in the Malcolm Gneiss.
3. Geochronology
3
.
1
. Pre6ious geochronology A comprehensive review of geochronological
investigation in the Albany – Fraser Orogen is pre- sented in Nelson et al. 1995. A summary of the
major studies and their implications, including recent work, is presented below.
3
.
1
.
1
. Eastern Albany – Fraser Orogen With the exception of several studies conducted
in the Fraser Complex region Bunting et al., 1976; Baksi and Wilson, 1980; Fletcher et al.,
1991, the age structure of the eastern part of the Albany – Fraser Orogen was unknown prior to a
reconnaissance SHRIMP U-Pb zircon study by the Geological Survey of Western Australia Nel-
son et al., 1995. The Biranup Complex was found to comprise Late Archaean c. 2595 – 2640 Ma
basement intruded by c. 1600 – 1700 Ma and c. 1300 Ma felsic plutonic rocks. In the Nornalup
Complex, pre-orogenic basement rocks outcrop only in the Malcolm Gneiss. Zircons from a
metasedimentary gneiss from near Point Malcolm yielded a wide spectrum of detrital ages. Two
distinct populations at 1560 9 40 and 1807 9 35 Ma, and single grain analyses ranging in age from
2033 to 2734 Ma, suggest that the sedimentary precursors to these rocks were not derived from
the vicinity of the Albany – Fraser Orogen Nelson et al., 1995. The 1560 9 40 Ma population pro-
vides a maximum estimate for the age of deposi- tion of the precursor sediments.
Two major felsic intrusive events relating to the Albany – Fraser Orogeny were identified by Nel-
son et al. 1995. Six samples of granitic gneiss representative of the widely-distributed Recherche
Granite Myers, 1990; Fig. 2 yielded crystallisa- tion ages of between c. 1330 and 1283 Ma. These
rocks intruded during a period of high-grade metamorphism and intense deformation recog-
nised throughout the eastern Albany – Fraser Oro- gen Myers, 1995a; Nelson et al., 1995. The
layered basic rocks of the Fraser Complex crys- tallised under granulite facies conditions during
this event, as constrained by an Sm-Nd isochron age of 1291 9 21 Ma Fletcher et al., 1991. On
the basis of this age, and Rb-Sr and Ar-Ar cool- ing ages of between 1285 and 1262 Ma Bunting
et al., 1976; Baksi and Wilson, 1980; Fletcher et al., 1991, Fletcher et al. 1991 argued that the
Fraser Complex intruded, was metamorphosed, and was subsequently tectonically emplaced into
the upper crust all in a period of 30 Ma. The cooling ages for the Fraser Complex have also
been interpreted to date the termination of high- grade metamorphism throughout the eastern part
of the orogen Nelson et al., 1995.
Two outcrops of undeformed granite Esper- ance Granite, Myers, 1995a gave imprecise U-Pb
zircon crystallisation ages of 1138 9 38 and 1135 9 56 Ma Nelson et al., 1995. These ages
were interpreted by Myers 1995a as dating a second period of tectonism and metamorphism
correlating to the major c. 1190 – 1170-Ma oro- genic episode identified in the western part of the
orogen. Although Esperance Granite plutons have not been recognised in the Biranup Complex, a
folded 1187 9 12 Ma pegmatite dyke intruding Palaeoproterozoic gneisses at Lake Gidong Nel-
son et al., 1995 may be related to this second thermo-tectonic event.
3
.
1
.
2
. Western Albany – Fraser Orogen The western part of the Albany – Fraser Orogen
is dominated by late to post-kinematic granite plutons. These rocks occur mainly within the
Nornalup Complex, but locally occur across the Nornalup – Biranup Complex boundary Myers,
1995b; Fig. 1b. U-Pb zircon crystallisation ages of six representative plutons range between c.
1170 and 1190 Ma Pidgeon, 1990; Black et al., 1992a. Granite intrusion was preceded by high-
grade metamorphism and deformation, dated by U-Pb in zircon at about 1190 Ma Black et al.,
1992a.
On the basis of inherited Archaean zircons c. 3100 Ma within felsic orthogneiss, Black et al.
1992a interpreted much of the Biranup Complex to represent reworked Yilgarn Craton crust.
These authors found no evidence for thermo-tec- tonic activity relating to c. 1300 Ma events in the
eastern part of the orogen Fletcher et al., 1991, and therefore interpreted the c. 1190 – 1170-Ma
event to be the principal period of orogenesis in the western part of the orogen. However, a
1289 9 10 Ma crystallisation age on an enderbitic pluton outcropping near Albany Pidgeon, 1990;
Fig. 1b suggests the existence of an earlier event. A recent U-Pb SHRIMP study by Clark
1995 identified 1304 9 5 and 1169 9 7 Ma meta- morphic zircon populations in a granulite facies
metasedimentary migmatite from near the c. 1289- Ma enderbite, indicating that the western part of
the Albany – Fraser Orogen did experience high- grade metamorphism and deformation at c. 1300
Ma.
K-Ar ages of 1160 – 1060 Ma obtained on horn- blende from metamorphosed basic rocks are inter-
preted to date late granite emplacement and post-metamorphic uplift and cooling of the west-
ern part of the orogen Stephenson et al., 1977.
3
.
2
. Metamorphic and structural context of the dated samples
Six rocks samples with well-defined relation- ships to the structuralmetamorphic history of the
Nornalup Complex were selected for age determi- nation Fig. 2, Table 2: 1 a post-D
3
, pre-D
4
aplite dyke from the Malcolm Gneiss; 2 a syn- D
4
pegmatite dyke from the Malcolm Gneiss; 3 a post-D
2
, pre-D
4
aplite dyke from a Recherche Granite pluton; 4 a syn-M
2a
leucosome layer from a granulite facies metapelite from the Salis-
bury Gneiss; and samples of quartzite 5 and schist 6 from the Mount Ragged metasedimen-
tary rocks. Zircon was chosen to date the igneous crystallisation ages for samples 1 and 3, leuco-
some formation in 4 and provenance ages for sample 5. Monazite was used to provide an
igneous age for sample 2 as zircon was unavail- able. Metamorphic rutile crystals were dated in
number 6. The morphological and internal char- acteristics of radiogenic minerals separated from
the samples are described together with the iso- topic results in Section 3.4.
3
.
2
.
1
. Post-D
3
, pre-D
4
aplite dyke
95091214
, Malcolm Gneiss
This metre-thick dyke cross-cuts F
3
folds defined by a pervasive S
1
gneissosity in granitic and metasedimentary rocks at Point Malcolm
Fig. 2. The dyke is linear but shows signs of tectonic attenuation resulting from D
4
deforma- tion. It contains an annealed assemblage of two
Table 2 Brief description of samples used for geochronology sample localities are shown in Fig. 2
a
Mineral assemblage Sample
Locality AMG coords
Lithology StructuralMetm
context Point Malcolm
WC704600 1 95091214
Aplite dyke Post-D
3
pre-D
4
Qtz-Bt-Kfs-Pl-Mag 2 9411112
Little Bellinger WC648573
Pegmatite dyke Syn-D
4
Qtz-Grt-Bt-Ms-Kfs-Pl-Mag Aplite dyke
WC148366 Cape Arid
3 9509243 Qtz-Bt-Kfs-Pl-Mag-Hbl-Ttn
Post-D
2
pre-D
4
Qtz-Grt-Spl-Crd-Sil-Bt-Kfs-Pl 4 9611201
Syn-M
2a
WB502982 Salisbury Island
Migmatitic paragneiss
Mt. Ragged WD437001
Quartzite Post-D
3
Qtz-Bt-Ms-Chl-Hem 5 9510101
Mt. Ragged WC431971
Mica schist Syn-M
2b
Qtz-Ms-Chl-Mrg-And-Hem-Rt 9 Ky 6 9510092
a
Mineral abbreviations after Kretz 1983.
feldspars, quartz, and biotite. Biotite is weakly oriented sub-parallel to the margins of the dyke,
which is oblique to the tectonic fabrics pre- served in the host rocks e.g. S
1
S
2
, S
3
, S
4b
. The fabric is interpreted to result from igneous flow.
Zircon and titanite occur as accessory minerals.
3
.
2
.
2
. Syn-D
4
pegmatite
9411112
, Malcolm Gneiss
Sample 9411112 is from a pegmatite dyke hosted by a large D
4
shear zone outcropping midway
between Point
Malcolm and
Cape Pasley Fig. 2. The shear zone contains an am-
phibolite facies mineral assemblage comparable in grade to an M
2b
overprint recognised in the metasedimentary rocks through which the shear
zone cuts. The dyke cross-cuts the S
4b
fabric in the shear zone and is boudinaged by later duc-
tile movement along the shear planes late-D
4b
or D
4c
. Its mineralogy comprises garnet, two feldspars, biotite, muscovite and quartz. Monaz-
ite occurs as an accessory mineral but zircon was not found in either thin section or sepa-
rates. The pegmatite contains a fabric parallel to S
4bc
defined by oriented biotite.
3
.
2
.
3
. Post-D
2
, pre-D
4
aplite dyke
9509243
, Recherche Granite
This aplite intrudes into a pluton of gneissic Recherche Granite outcropping at Cape Arid
Fig. 2. It comprises an annealed assemblage of two feldspars, quartz, hornblende, biotite, titan-
ite and zircon. The dyke is linear over several hundred metres of outcrop and cuts the S
2
and S
2b
fabrics in the host gneiss. F
3
folding in the area is at kilometre-scale so the timing of the
dyke relative to D
3
is unclear. A shear fabric is developed within the dyke oblique to its margins
consistent with deformation during D
4
.
3
.
2
.
4
. Syn-D
4
a
migmatitic leucosome
9611201
, Salisbury Gneiss
This sample was collected from an S
1S
concor- dant granitic leucosome outcropping on Salis-
bury Island Fig. 2. Leucosomes formed by extensive biotite dehydration partial melting of
metapelitic rocks during M
2a
and comprise mesoperthitic feldspars, quartz and minor gar-
net 9 cordierite. Thin restitic schlieren rich in garnet, biotite, sillimanite, spinel and cordierite
separate leucosome layers. Zircon and monazite are present in both leucosome and mesosome
layers. The leucosomes are everywhere concor- dant with S
1S
, and are locally disharmonically folded, suggesting that their formation occurred
synchronous with D
4a
. M
2a
garnets in leucosome areas proximal to mesosome schlieren are man-
tled by cordierite 9 spinel coronas, which con- tain abundant small zircon grains.
3
.
2
.
5
. Post-D
3
quartzite
9510101
, Mount Ragged
Sample 9510101 was taken from near the ex- posed base of the Mount Ragged metasedimen-
tary rocks at Mount Ragged. The quartzite consists almost entirely of a coarse-grained gra-
noblastic aggregate of recrystallised quartz. A weak annealed S
1R
foliation is defined by ori- ented haematite and minor muscovite, chlorite
and biotite. In section, haematite grains com- monly form rings up to several centimetres in
diameter enclosing many small quartz grains, suggesting
the recrystallisation
of originally
much coarser grains. Zircons are sporadically distributed along the boundaries of these relic
grains.
3
.
2
.
6
. Syn-D
4
mica schist
9510092
, Mount Ragged
This sample was taken from a thin pelitic lens intercalated with massive quartzite in the same
vicinity as sample 9510101. The rock contains the assemblage muscovite, chlorite, margarite,
andalusite, haematite and rutile. Ilmenite, spes- sartine garnet, gahnite-rich spinel, kyanite and
epidote occur as accessory phases. Small euhe- dral rutile needles post-date garnet growth and
form part of a near-peak retrograde-M
2b
meta- morphic paragenesis in this rock. Delicate knee-
bend twins are common, suggesting that rutile growth post-dates D
4b
shearing deformation in these layers. Rutile may have formed at the ex-
pense of ilmenite or could have recrystallised from previously detrital grains.
3
.
3
. Methodology and analytical procedures for isotopic analysis
The majority of U-Th-Pb isotopic measure- ments were made using the sensitive high resolu-
tion ion microprobe SHRIMP-II at Curtin University of Technology, Perth. Zircons from
sample 9611201
were analysed
using the
SHRIMP II facility at the Australian National University, Canberra, with the assistance of Dr
R. Armstrong. All sample minerals were ex- tracted from the disaggregated rock samples and
mounted in epoxy discs before being polished, gold coated and imaged. Before SHRIMP analy-
sis all zircon grains were imaged by cathodolu- minescence
CL using
a Cambridge
S360 scanning electron microscope, with an operating
voltage of 20 kV, equipped with a polychro- matic CL detector. The SEM is located in the
Electron Microscope Unit at the University of New South Wales.
Procedures for SHRIMP U-Th-Pb isotopic analysis of zircon follow those originally de-
scribed by Compston et al. 1984 and Williams et al. 1984, with subsequent modifications to
analytical routines and data reduction methods outlined
by Williams
and Claesson
1987, Compston et al. 1992 and Williams 1998.
For zircon analyses undertaken using the Perth SHRIMP-II, U-Pb ratios and U and Th concen-
trations were determined relative to Sri Lankan zircon standard CZ3 564 Ma,
206
Pb
238
U = 0.0914, Nelson 1997. For analyses undertaken
in Canberra, U-Pb ratios were determined rela- tive
to the
Duluth Complex
gabbroic anorthosite standard AS3 1099.1 Ma,
206
Pb
238
U = 0.1859, Paces and Miller 1989, whilst U and Th concentrations were determined rela-
tive to ANU zircon standard SL13. The
procedure for
monazite analysis
by SHRIMP followed the method outlined by
Kinny 1997, which differs somewhat from that of Williams et al. 1996, and Ireland and Gib-
son 1998 in that the calibration of PbU ratios is based upon a plot of ln
206
PbUO versus UO
2
UO calibration slope 0.7, with data for unknowns normalised to Madagascan monazite
standard MAD 514 Ma,
206
Pb
238
U = 0.0830, based on TIMS analyses of L.M. Heaman. An-
other difference in the monazite analytical pro- cedure of Kinny 1997 is that, prior to being
used for common Pb correction,
204
Pb counts are corrected for a background interference of
scattered ions the size of which is directly pro- portional to the Th content of the sample.
PbU ratios for rutiles were determined rela- tive to 2625 Ma-old rutile from the Windmill
Hill quartzite, Jimperding metamorphic belt, Western Australia
206
Pb
238
U = 0.5025, based on TIMS analyses by L.M. Heaman. Norm-
alisation of rutile unknowns was based on an
observed linear
covariation between
206
PbUO and UO2UO for the standard, slope 1.17.
Common Pb corrections were applied using the
204
Pb correction method Compston et al., 1984, assuming the isotopic composition of
Broken Hill ore Pb, except for zircon sample 96110201 which contained very low Th and so
was corrected via the
208
Pb method Compston et al., 1984, using a modelled common Pb iso-
topic composition appropriate to its age, and for the Mount Ragged rutile sample which con-
tained no detectable Th. For rutile data in which the measured
208
Pb peak is entirely non- radiogenic, a simplified
208
Pb correction method was applied, whereby the proportion of non-ra-
diogenic
206
Pb, denoted f206, is given by:
f206 = 100 ×
208
Pb
206
Pbmeasured
208
Pb
206
Pbcommon
For both monazite and rutile analyses, the composition of the common Pb component was
modelled upon that of contemporary terrestrial lead. Reproducibility of the U-Pb ratios of the
standards on both machines was better than 9
2.1 in all cases. Elemental concentrations in the monazite and rutile analyses were calculated
by assuming a similar sensitivity of ionising spe- cies for standards and unknowns, and are accu-
rate to
approximately 9
20. The
decay constants
used are
those recommended
by Steiger and Ja¨ger 1977.
3
.
4
. Isotopic results and age constraints on field relationships
The processed U-Pb data are presented in Ta- bles 3 – 6. Results are presented on conventional
concordia plots in Fig. 3a – f. Errors given on individual analyses in the data tables and on
concordia plots are at 1s level. They are based on counting statistics, uncertainty in the common Pb
correction and, in the case of PbU ratios, the uncertainties associated with normalisation to the
standards. Pooled ages quoted in the text are weighted means and their errors are given at ts or
95 confidence level. Brief descriptions of the morphology of the analysed grains and, in the
case of zircon, the internal characteristics as re-
Fig. 3. Concordia diagrams for dated samples; error boxes shown are 1s. Inset diagrams illustrate the structural context of the samples. a Sample 95091214, Point Malcolm. The hatched analysis has not been used to determine the crystallisation age of this
sample. b Sample 9411112, Malcolm Gneiss. Concordia diagrams for dated samples; error boxes shown are 1s. Inset diagrams illustrate the structural context of the samples. c Sample 9509243, Cape Arid. Hatched analyses have not been used in determining
the crystallisation age of this sample. The xenocrystic population is interpreted to be inherited from the Recherche Granite, and is quoted at 1s level. d Sample 9611201, Salisbury Island. The two groups main groups represent zircon growth under metamorphic
conditions. Analyses in black do not fall into either population and have been excluded from age calculations. Concordia diagrams for dated samples; error boxes shown are 1s. Inset diagrams illustrate the structural context of the samples. e Sample 9510101,
Mount Ragged. Both main populations are interpreted to be detrital. Discordant analyses have been included in the populations as they have suffered recent lead loss only. Analyses in black do not fall into either population and have been excluded from age
calculations. f Sample 9510092, Mount Ragged.
Fig. 3. Continued
vealed by CL imaging are provided before the results for each sample.
3
.
4
.
1
. Post-D
3
, pre-D
4
aplite dyke
95091214
, Malcolm Gneiss
Zircons extracted from this sample are colour- less, euhedral and squat to elongate. They range
in length from 150 to 200 mm and in lengthwidth ratio from 1.5 to 2.5. Delicate oscillatory
growth zoning is evident in most grains. CL inten- sity ranges from slightly darker cores to brighter
rims. Hourglass and sector zoning is prominent in many grains. Crystals are bounded by large prism
and pyramid faces notably {211}. Grains in this sample commonly have thin rims with dark CL
response. Rims, ranging in width from 5 to 20
m m, are typically concordant to the internal zona-
tion of the grains, but sometimes form embay- ments transgressive into core material, truncating
core zonation. Seventeen zircon analyses fall within error of a
mean
207
Pb
206
Pb age of 1313 9 16 Ma Fig. 3a. The remaining analysis 3.43 is significantly dis-
cordant 9 and so has been excluded from the age calculation. The analyses contain 79 – 204 ppm
U, 50 – 245 ppm Th Table 3 and ThU ratios clustering closely around an average of 0.8. The
ubiquitous presence of oscillatory and sector zonation, the abundance of {211} pyramid faces,
and the moderate ThU ratios strongly suggest that this zircon has a primary igneous origin. The
age recorded by this zircon population is therefore interpreted to date the crystallisation of the aplite,
thereby providing a lower age bound for D
3
. The
D .J
. Clark
et al
. Precambrian
Research
102 2000
155 –
183
Table 3 Geochronological results obtained on zircons from samples 95091214 and 9509243
207
Pb U ppm
9 1s
Label
208
Pb Pb ppm
f206
a
9 1s
206
Pb Th ppm
9 1s
207
Pb 9
1s conc.
b 207
Pb
206
Pb 9
1s
238
U
206
Pb Age
235
U
206
Pb
95091214
Malcolm Gneiss aplite dyke 0.00091
0.2909 0.0024
3.1 0.2236
185 0.0037
2.579 0.0536
101 1284
21 180
49 0.11
0.08363 0.00185
0.2595 0.0044
0.2238 0.0037
2.628 0.0762
0.08519 99
131 1320
42 3.2
0.11 34
116 0.08451
104 0.00144
0.1688 0.0031
0.2237 0.0038
2.607 0.0661
100 1304
33 60
25 0.31
3.3 0.08
0.08544 0.00093
0.1982 0.0021
0.2263 0.0037
2.665 0.0556
99 1326
21 3.8
180 121
45 0.00130
0.2323 0.0031
0.2209 0.0037
2.595 0.0622
0.08523 97
28 1321
29 0.10
3.1 111
87 0.08562
119 0.00152
0.1958 0.0035
0.2276 0.0038
2.687 0.0697
99 1330
34 78
30 0.31
3.14 0.08349
182 0.00102
0.2608 0.0025
0.2294 0.0038
2.640 0.0575
104 1281
24 157
49 0.21
3.19 0.00187
0.1763 0.0042
0.2237 0.0038
2.711 0.0780
0.08792 94
3.22 1381
41 0.26
24 59
99 0.18
0.08391 0.00142
0.1998 0.0032
0.2333 0.0039
2.698 0.0683
105 1290
33 3.28
118 83
31 0.00183
0.1834 0.0042
0.2262 0.0038
2.493 0.0751
0.07993 110
101 1195
45 3.37
0.34 25
64 0.08602
195 0.00128
0.3252 0.0032
0.2081 0.0034
2.468 0.0580
91 1339
29 244
50 0.57
3.43 0.08428
154 0.00140
0.2856 0.0035
0.2285 0.0038
2.656 0.0663
102 1299
32 150
42 0.28
3.46 0.00220
0.1791 0.0050
0.2242 0.0038
2.562 0.0852
0.08289 103
3.5 1267
52 0.59
20 50
80 0.00129
0.2794 0.0032
0.2266 0.0038
2.680 0.0639
3.54 99
119 1334
29 113
32 0.10
0.08580 0.00079
0.2552 0.0020
0.2287 0.0038
2.705 0.0536
0.08579 100
54 1333
18 0.04
3.62 203
171 0.08451
204 0.00098
0.2680 0.0024
0.2291 0.0038
2.670 0.0570
102 1304
23 188
55 0.09
3.63 0.08144
145 0.00125
0.2037 0.0029
0.2280 0.0038
2.560 0.0615
107 1232
30 103
37 0.37
3.66 0.00105
0.2694 0.0026
0.2294 0.0038
2.747 0.0598
0.08684 98
3.67 1357
23 0.09
34 115
127
9509243
Recherche Granite aplite dyke 0.00063
0.0877 0.0012
0.2266 0.0037
2.687 113
0.0500 492
98 1338
14 150
0.21 0.08600
1.1a 377
0.08685 0.00062
0.0175 0.0009
0.2286 0.0037
2.737 0.0509
98 1357
14 21
82 1.2b
0.03 154
0.09065 0.00103
0.2735 0.0025
0.2464 0.0041
3.080 0.0654
99 1439
22 175
45 1.2a
0.23 0.00145
0.3902 0.0040
0.2263 0.0038
2.663 0.0673
0.08536 99
1.8 1324
33 0.11
30 138
105 0.00133
0.1744 0.0029
0.2251 0.0037
2.609 1.15
0.0634 118
101 1294
31 70
29 0.30
0.08405 0.00120
0.4352 0.0033
0.2192 0.0036
2.526 0.0583
0.08358 100
69 1283
28 0.45
1.2 239
351 0.08112
129 0.00202
0.1887 0.0046
0.2277 0.0038
2.546 0.0805
108 1224
49 88
33 0.86
1.22b 0.08540
69 0.00211
0.1886 0.0048
0.2284 0.0039
2.689 0.0854
100 1325
48 46
18 0.46
1.23 0.00111
0.1908 0.0025
0.2281 0.0038
2.659 0.0597
0.08457 101
1.27 1306
26 0.00
32 80
130 0.00089
0.2793 0.0023
0.2251 0.0037
2.662 0.0547
98 1.31
1333 179
20 166
47 0.01
0.08576 0.00084
0.2209 0.0019
0.2261 0.0037
2.694 0.0541
0.08641 98
223 1347
19 1.33
0.06 57
166 0.08296
96 0.00144
0.1900 0.0033
0.2233 0.0037
2.555 0.0654
102 1268
34 63
24 0.26
1.41 0.08364
167 0.00102
0.3153 0.0027
0.2259 0.0037
2.606 0.0565
102 1284
24 176
46 0.15
1.44 0.00106
0.2347 0.0025
0.2250 0.0037
2.661 0.0579
0.08578 98
1.49 1333
24 0.14
50 154
196 0.00090
0.4004 0.0024
0.2320 0.0038
2.674 0.0546
105 1284
1.5 20
267 363
80 0.33
0.08362 0.00140
0.2610 0.0033
0.2087 0.0034
2.393 0.0595
0.08317 96
64 1273
33 1.17
1.52 254
236 0.08571
247 0.00082
0.3247 0.0022
0.2274 0.0037
2.688 0.0536
99 1332
18 275
68 0.05
1.54 0.00094
1.4 0.2111
181 0.0021
0.2245 0.0037
2.684 0.0560
96 1355
21 130
45 0.07
0.08673 0.00106
0.3062 0.0027
0.2194 0.0036
2.558 0.0560
98 1305
0.08456 24
1.6 180
192 48
0.32
a
f206 = 100×common
206
Pbtotal
206
Pb.
b
conc = 100×
206
Pb
238
U age
207
Pb
206
Pb age.
D .J
. Clark
et al
. Precambrian
Research
102 2000
155 –
183
169
Table 4 Geochronological results obtained on monazites from sample 9411112
9 1s
208
Pb
206
Pb 9
1s
206
Pb
238
U 9
1s
207
Pb
235
U 9
1s conc.
b 207
Pb
206
Pb Age Label
9 1s
U Th
Pb f206
a 207
Pb
206
Pb
9411112
Malcolm Gneiss pegmatite 0.00019
0.67741 0.00082
0.2094 0.0043
2.297 8.75
0.048 3.06
103 1186
5 3.09
dan.3 1.21
0.07955 0.00011
0.69294 0.00074
0.1998 0.0041
2.166 dan.4
0.045 3.69
101 1162
3 10.58
3.23 0.02
0.07860 0.00022
1.25280 0.00109
0.2011 0.0041
2.184 0.046
0.07877 101
1.22 1166
5 dan.5
3.59 21.71
3.42 0.00007
1.42859 0.00101
0.2089 0.0043
2.259 0.046
dan.6 106
3.12 1158
2 18.97
3.16 0.02
0.07842 0.00013
0.30047 0.00045
0.2020 0.0041
2.213 0.046
0.07947 100
dan.7 1184
3 0.74
3.83 5.09
3.92 0.00010
0.60103 0.00061
0.1999 0.0041
2.171 0.045
101 dan.8
1167 4.13
3 11.74
3.72 0.09
0.07879 0.00010
0.58745 0.00065
0.2086 0.0043
2.264 0.047
0.07870 105
dan.9 1165
3 0.22
2.88 7.63
3.13 0.00007
0.89487 0.00072
0.2073 0.0042
2.243 0.046
dan.10 105
3.48 1159
2 14.03
3.22 0.05
0.07848 0.00012
0.42808 0.00053
0.1979 0.0040
2.145 0.044
0.07863 100
dan.11 1163
3 0.03
3.71 8.05
4.44 0.00041
2.94523 0.00485
0.2109 0.0043
2.284 0.050
106 1161
dan.12 10
0.69 9.48
0.90 1.49
0.07855 0.00014
1.44305 0.00144
0.2011 0.0041
2.183 0.045
0.07872 101
0.28 1165
4 dan.13
2.57 16.90
2.62 0.00008
0.67851 0.00063
0.2051 0.0042
2.229 0.046
103 1168
dan.14 2
3.91 10.76
3.67 0.08
0.07884 0.00014
0.70253 0.00092
0.1962 0.0040
2.144 0.044
98 1179
0.07926 3
dan.15 2.32
8.05 2.01
0.24
a
f206 = 100×common
206
Pbtotal
206
Pb.
b
conc = 100×
206
Pb
238
U age
207
Pb
206
Pb age.
D .J
. Clark
et al
. Precambrian
Research
102 2000
155 –
183
Table 5 Geochronological results obtained on zircons from sample 96110201
f206
a
9 1s
208
Pb Pb ppm
9 1s
206
Pb Th ppm
9 1s
207
Pb U ppm
9 1s
Label conc.
b 207
Pb
207
Pb
206
Pb Age 91s
206
Pb
238
U
235
U
206
Pb
96110201
Salisbury Gneiss migmatitic leucosome 0.00073
– –
0.2104 96-1.2
0.0024 735
2.342 0.0364
101 1214
18 34
145 1.40
0.08071 0.00042
– –
0.2049 96-2.1
0.0024 611
2.282 0.0301
99 1216
10 22
117 0.10
0.08076 0.00048
– –
0.2023 0.0023
2.253 0.0306
0.08076 98
0.10 1216
12 96-3.1
687 27
130 0.00063
– –
0.1981 0.0025
2.190 0.0345
96-4.2 97
468 1202
16 20
87 0.17
0.08019 0.00048
– –
0.2069 0.0023
2.328 0.0308
0.08159 98
96-5.2 1236
12 2.69
167 48
854 0.00050
– –
0.2118 96-6.1
0.0025 854
2.404 0.0334
99 1253
12 31
170 0.05
0.08233 0.00028
– –
0.2107 0.0025
2.352 0.0295
0.08095 101
96-8.1 1220
7 0.08
144 28
726 0.00044
– –
0.2000 0.0022
2.209 0.0287
96-9.1 98
434 1199
11 20
82 0.20
0.08010 0.00056
– –
0.2075 0.0024
2.322 0.0334
0.08116 99
96-10.1 1225
14 0.15
99 18
508 0.00034
– –
0.2176 0.0026
2.455 0.0321
96-12.1 102
775 1242
8 23
158 0.03
0.08183 0.00053
– –
0.2064 0.0025
2.297 0.0327
0.08069 100
0.05 1214
13 96-13.1
565 20
109 0.00038
– –
0.2042 0.0023
2.253 0.0289
96-14.1 100
604 1198
9 9
115 0.10
0.08003 0.00054
– –
0.1993 0.0024
2.173 0.0311
0.07911 100
96-15.1 1175
13 0.23
91 17
488 0.07
0.08097 0.00042
– –
0.2077 0.0025
2.319 0.0312
100 1221
10 34
96-16.1 157
805 0.00074
– –
0.2144 0.0027
2.389 0.0396
0.08082 103
0.07 1217
18 96-17.1
533 21
107 0.00063
– –
0.2045 0.0026
2.300 0.0360
96-18.1 97
668 1236
15 32
129 0.32
0.08159 0.00102
– –
0.1875 0.0027
2.048 0.0419
0.07924 94
96-18.2 1178
26 6.22
87 15
497 0.00125
– –
0.1855 0.0105
2.101 0.1268
96-19.1 88
783 1248
30 34
137 6.71
0.08212 0.00058
– –
0.2165 0.0027
2.434 0.0371
0.08155 102
0.02 1235
14 96-20.1
704 47
144 0.00050
– –
0.2126 0.0054
2.351 0.0633
96-5.3 103
799 1202
12 26
159 0.08
0.08021 Rims
0.00032 –
– 0.2005
0.0023 2.182
0.0278 0.07894
101 96-1.1
1171 8
0.28 89
15 473
0.00036 –
– 0.2038
0.0022 2.232
0.0277 96-4.1
101 480
1182 9
15 92
0.05 0.07941
0.00046 –
– 0.2030
0.0027 2.206
0.0333 0.07882
102 96-5.1
1168 12
0.13 132
21 695
0.00034 96-7.1
– 488
– 0.2062
0.0023 2.269
0.0279 101
1192 8
27 95
0.11 0.07980
0.00033 –
– 0.2062
0.0023 2.270
0.0275 0.07987
101 0.06
1194 8
96-11.1 779
13 150
0.00065 96-16.2
– 574
– 0.2145
0.0027 2.326
0.0373 108
1164 16
19 115
0.17 0.07867
a
f206 = 100×common
206
Pbtotal
206
Pb.
b
conc = 100×
206
Pb
238
U age
207
Pb
206
Pb age.
D .J
. Clark
et al
. Precambrian
Research
102 2000
155 –
183
171
Table 6 Geochronological results obtained on zircons from sample 9510101 and rutiles from sample 9510092
207
Pb U ppm
208
Pb 9
1s Th ppm
9 1s
206
Pb 9
1s
207
Pb 9
1s conc.
b 207
Pb
206
Pb Age 91s Pb ppm
f206
a
Label
206
Pb
206
Pb
235
U
238
U
9510101
Mt Ragged Quartzite 0.00104
0.3236 0.0027
0.2193 1.1a
0.0055 253
2.639 0.077
94 1367
23 279
68 0.13
0.08727 0.00102
0.1998 0.0022
0.3207 0.0080
4.798 0.133
0.10853 101
361 1775
17 1.7a
0.82 134
257 0.08597
181 0.00104
0.2143 0.0024
0.2273 0.0057
2.694 0.079
99 1337
23 133
46 0.01
1.12 0.00221
0.5947 0.0063
0.3095 1.13
0.0080 87
4.643 0.161
98 1779
37 179
40 0.26
0.10879 0.00073
0.1967 0.0016
0.2018 0.0050
2.390 0.065
0.08592 89
135 1336
16 0.18
1.14 603
374 0.10803
187 0.00204
0.5124 0.0055
0.1688 0.0043
2.514 0.084
57 1767
34 269
45 0.72
1.15 0.00049
0.3696 0.0013
0.3097 0.0077
4.680 0.121
1.16 97
659 1793
8 846
260 0.04
0.10961 0.00173
0.2769 0.0040
0.3134 0.0080
4.550 0.146
0.10531 102
93 1720
30 1.2
0.34 35
93 0.10120
72 0.00252
0.3683 0.0063
0.2691 0.0070
3.754 0.143
93 1646
46 92
25 0.65
1.24 0.28
0.08392 0.00107
0.6240 0.0038
0.2224 0.0056
2.573 0.076
100 1291
25 1.25
483 231
76 0.00102
0.2097 0.0022
0.2177 0.0054
3.275 0.091
0.10911 71
346 1785
17 1.26
0.37 86
204 0.10749
244 0.00097
0.2771 0.0023
0.3145 0.0079
4.661 0.129
100 1757
16 239
92 0.22
1.29 0.00456
0.1554 0.0103
0.1096 0.0028
1.413 0.082
1.39 45
172 1498
92 71
23 3.80
0.09352 0.00131
0.2047 0.0030
0.2250 0.0057
2.564 0.080
0.08266 104
195 1261
31 1.49
0.28 49
139 0.08330
226 0.00155
0.1676 0.0035
0.1680 0.0042
1.930 0.064
78 1276
36 135
42 0.47
1.51 0.08590
49 0.00359
0.1835 0.0082
0.2209 0.0058
2.616 0.136
96 1336
81 29
12 0.23
1.52 0.00119
0.3026 0.0029
0.3199 0.0081
4.783 0.138
0.10844 101
60 1773
20 0.20
1.63 153
165 0.08768
502 0.00126
0.2844 0.0030
0.1412 0.0035
1.707 0.052
62 1375
28 451
86 0.95
1.68 0.08593
238 0.00122
0.2079 0.0028
0.2129 0.0053
2.522 0.077
93 1336
27 174
57 0.08
2.2 0.00234
0.4108 0.0058
0.2966 0.0076
4.692 0.162
0.11475 89
2.3 1876
37 0.36
42 130
107 0.08426
356 0.00090
0.1444 0.0019
0.2186 0.0055
2.539 0.072
98 1299
21 180
83 0.19
2.8 0.00235
0.1322 0.0051
0.2247 0.0058
2.595 0.105
0.08377 102
18 1287
55 0.33
2.3 74
34 0.10937
649 0.00115
0.2872 0.0027
0.1514 0.0038
2.284 0.065
51 1789
19 1139
121 0.89
2.36 0.00098
0.4184 0.0027
0.3159 0.0080
4.773 0.133
99 2.49
1793 180
16 261
75 0.18
0.10959
9510092
Mt Ragged mica schist 121
0.07797 0.00080
– –
0.1912 0.0044
2.055 0.054
99 1146
20 nd
24 mr1.1
0.62 116
0.07630 0.00085
– –
0.1931 0.0045
2.031 0.055
103 1103
22 nd
23 mr1.2
1.09 0.00100
– –
0.1962 0.0046
2.089 0.059
0.07723 103
21 1127
26 0.75
mr1.3 95
nd 0.07713
122 0.00079
– –
0.1926 0.0045
2.048 0.054
101 1124
20 nd
24 0.67
mr1.4 0.07955
77 0.00087
– –
0.2025 0.0047
2.221 0.060
100 1186
22 nd
24 2.83
mr10.1 0.00109
– –
0.2007 0.0047
2.191 0.063
0.07920 100
mr11.1 1177
27 5.25
25 nd
93 0.00077
– –
0.1958 0.0045
2.072 0.055
104 mr12.1
1115 83
20 nd
28 2.61
0.07676 0.00106
– –
0.1825 0.0043
1.976 0.056
0.07856 93
102 1161
27 mr2.1
3.50 23
nd 0.08033
94 0.00078
– –
0.1853 0.0043
2.052 0.054
91 1205
19 nd
24 2.39
mr3.1 0.00069
– –
0.1862 0.0043
2.010 0.052
mr4.1 96
80 1154
17 nd
29 1.85
0.07827 0.00141
– –
0.1983 0.0047
2.143 0.068
0.07840 101
86 1157
36 mr5.1
5.96 22
nd 0.07870
80 0.00086
– –
0.1864 0.0043
2.023 0.054
95 1165
22 nd
27 3.90
mr6.1 0.00098
mr7.1 –
121 –
0.1862 0.0043
2.015 0.056
95 1160
25 nd
25 3.06
0.07851 0.00110
– –
0.1919 0.0045
2.090 0.060
0.07903 97
23 1173
28 3.49
mr8.1 109
nd 0.07892
109 0.00108
– –
0.1910 0.0045
2.079 0.059
97 1170
27 nd
23 3.16
mr8.2 0.00143
mr8.3 –
76 –
0.1943 0.0047
2.100 0.067
99 1156
36 nd
20 4.73
0.07836 0.00119
– –
0.1904 0.0045
2.085 0.061
95 1183
0.07943 30
mr9.1 99
nd 22
3.54
a
f206 = 100×common
206
Pbtotal
206
Pb.
b
conc = 100×
206
Pb
238
U age
207
Pb
206
Pb age.
dark CL rims observed on these grains proved too thin to analyse.
3
.
4
.
2
. Syn-D
4
pegmatite
9411112
, Malcolm Gneiss
Honey-yellow monazites from this pegmatite are subhedral to anhedral and equant, with
lengthwidth ratios typically less than 2. Whole grains range from 100 to 250 mm in diameter and
are unzoned in transmitted and reflected light.
SHRIMP analyses define an approximately concordant population with excess scatter in
207
Pb
206
Pb around a mean age of 1165 9 5 Ma x
2
= 8.5. Although the data may be divided into
two statistically valid populations on the basis of counting statistics, we see no geological justifica-
tion to do so. The large x
2
of the population may be attributable to an underestimation of the errors
in the individual measurements in monazite analy- ses Ireland and Gibson, 1998. In the present
instance, the effect on the age uncertainty does not influence the geological significance of the age
Fig. 4.
Th contents of the population range widely, from 5.1 to 21.7, whilst ThU ratios range from
1.3 to 13.7, and average 4.4 Table 4. Typical Th contents in monazite range from 4 to 12 ThO
2
Watt, 1995 but Th-rich monazite up to 30 ThO
2
has been recorded from pegmatitic rocks Bowles et al., 1980. Th and U enrichment in
such rocks has generally been considered to be controlled, at least in part, by processes involving
magmatic fluids Watt, 1995. This suggests that the monazite grains from which the population of
analyses were derived crystallised from the host pegmatitic melt. Mineral assemblages preserved in
D
4
shear zones suggest the Malcolm Gneiss ter- rain was at upper greenschist to lower amphibo-
lite-facies temperatures at the time of intrusion of the 9411112 pegmatite. This corresponds to tem-
peratures much less than the estimated 725°C closure temperature for U-Pb diffusion in monaz-
ite Mezger et al., 1993. The pooled age of 1165 9 5 Ma therefore records the age of crystalli-
sation of the host pegmatite and provides an estimate for the timing of movement in D
4
shear zones in the Malcolm Gneiss.
3
.
4
.
3
. Post-D
3
, pre-D
4
aplite dyke
9509243
, Recherche Granite
Most zircons from this sample are uniform in morphology and range from 150 to 280 mm in
length. Lengthwidth ratios range from 2.5 to 4. Crystals are well-faceted, inclusion-free and
colourless. Prominent steep {211} pyramidal faces are common. CL imaging reveals bold oscillatory
growth zoning with dark cores grading into bright rims. Very thin B 10 mm dark rims are often
present. They are concordant with the oscillatory zonation of the cores. Two grains in this sample
are morphologically distinct from the remainder 1.1 and 1.2. They are squat and contain
rounded, irregular cores showing signs of metam- ictisation patchy CL response. The outer mar-
gins of core regions appear strongly resorbed. The cores are enveloped by thick 20 – 60 mm euhe-
dral, faceted rims, which show bold concentric zonation consisting of broad bands of bright and
dark CL response. Thin outer rims of dark CL response envelope the thicker inner-rims, without
truncation of zonation, and may represent contin- uous growth.
Fourteen zircon analyses define the main popu- lation in this sample and scatter about a mean
207
Pb
206
Pb ratio corresponding to an age of 1313 9 16 Ma Fig. 3c. Most analyses contain
95 – 270 ppm U and 60 – 370 ppm Th Table 3. ThU ratios range from 0.5 to 1.5 with a cluster-
ing of seven analyses around 0.7. All the analysed grains from this population are elongate, are
bounded by well-developed crystal faces, and show prominent oscillatory zoning. This habit is
consistent with their growth in a magma Vavra, 1994. The pooled age of 1313 9 16 Ma is there-
fore interpreted to represent the igneous crystalli- sation age for the host aplite dyke. Analyses 1.22b
and 1.52 are discordant 8 and 4, respectively and have a relatively high f206 0.86 and 1.17,
respectively and so were excluded from the age calculations.
Rim analyses 1.1a and 1.2b on the two mor- phologically distinct grains have much lower Th
U ratios 0.3 and 0.06, respectively than those in the main population, mainly due to significantly
higher U contents Table 3. This characteristic suggests that the rims of these grains formed in a
Fig. 4. Time-space diagram constructed for the Nornalup Complex. The four units compared are shown in Fig. 2. The height of an ‘event block’ is schematic; large boxes for intrusive events represent larger volumes of magma, cf. smaller boxes. Question marks
indicate uncertain interpretations. The S symbol represents the formation of bedding surfaces. SHRIMP error bars are at 95
confidence levels for pooled analyses and 1s for single analyses. Geochronological data from Nelson et al. 1995 and Myers 1995a included in the diagram are mentioned in the text. Four of the five single grain xenocrystic analyses shown for the Recherche Granite
were obtained from a gneissic sample of Recherche Granite located at Cape Arid Clark, D., unpublished data.
chemical environment of different ThU composi- tion to the main population of analyses. The two
analyses have a pooled
207
Pb
206
Pb age of 1345 9 10 Ma 1s, Fig. 3c which is consistent with the
1330 9 14 Ma crystallisation age obtained on the Recherche Granite pluton that the dyke intrudes
Nelson et al., 1995. Thin, dark CL rims on these grains too thin to analyse may represent a second
period of zircon growth within the aplitic magma. Analysis 1.2a from the irregular core of grain 1.2
contains 175 ppm Th, 154 ppm U and a ThU ratio of 1.1. A distinct
207
Pb
206
Pb ratio consistent with an age of 1442 9 22 Ma suggests that this
core is xenocrystic to the aplite.
3
.
4
.
4
. Post-D
3
migmatitic leucosome
9611201
, Salisbury Island
Two distinct morphological types of zircons occur in this sample. Elongate grains averaging
150 – 250 mm long occur dispersed throughout the leucosome portion of the migmatite. Length
width ratios range from 1.5 to 3.5. These zircons are clear, subhedral and are strongly oscillatorily
zoned. The second group consists of clear equant grains soccerballs averaging 150 – 200 mm in di-
ameter. The grains are bounded by many high-or- der facets. This group is also abundant in the
leucosome and shows strong oscillatory zoning. Grains from both groups are typically mantled by
unzoned rims 5 30 – 50 mm thick of slightly brighter CL-response than the cores. Core zona-
tion is truncated by rim material in rare instances. Very thin bright-CL outer rims truncate zonation
in some grains.
Eighteen zircon analyses of cores from both morphological groups in this sample define a pop-
ulation with a mean
207
Pb
206
Pb age of 1214 9 8 Ma Fig. 3d. Uranium contents range from 434
to 854 ppm and average 637 ppm Table 5. Thorium contents are low and range from 9 to 48
ppm, averaging 26 ppm. ThU ratios are ex- tremely low 0.01 – 0.07, which is consistent with
the coeval growth of monazite with this zircon. The well-developed planar boundaries and oscilla-
tory zonation exhibited by these grain cores sug- gests
that they
formed within
the granitic
leucosome. The 1214 9 8 Ma age is therefore in- terpreted to date the onset of crystallisation of the
leucosome and thus constrains the timing of M
2a
. Six rim analyses from both elongate and
equant grains fall within error of a mean
207
Pb
206
Pb age of 1182 9 13 Ma Fig. 3d. Uranium and thorium contents are lower but comparable
to the core material Table 5, while ThU ratios range from 0.02 to 0.06, averaging 0.03. Similar
chemistry and the absence of zonation is reported to be consistent with zircon growth under meta-
morphic conditions Williams et al., 1996. Fraser et al. 1997 demonstrated that zircon growth in
high-grade metamorphic rocks may be triggered by net transfer reactions involving the breakdown
of Zr-bearing phases such as garnet. Fluid-present D
4b
shearing, which resulted in the extensive re- placement of peak assemblages by M
2b
biotite + sillimanite + quartz, provides a likely candidate
for such a zirconium-liberating event. Hence, the 1182 9 13 Ma rim age is interpreted to record the
timing of D
4b
shearing, which then provides a lower age bound for high-grade activity and sub-
sequent decompression in the Salisbury Gneiss. Two analyses 6.1 and 12.1 fall statistically
outside the two major populations in this sample based on a x
2
-test and so were not included in age calculations.
3
.
4
.
5
. Post-D
3
quartzite
9510101
, Mount Ragged
Zircons in this sample range from subhedral elongate grains up to 320 mm in length exhibit-
ing strong concentric zonation in transmitted light and CL, to rounded grains \ 100 mm in length
filled with apatite inclusions. Many are metamict to varying degrees and are brown in colour, while
others are colourless. All show pitting and have irregular surfaces, consistent with detrital trans-
port. CL imaging reveals a surprising conformity of internal zonation patterns. Most grains pre-
serve concentric oscillatory zoning with no evi- dence of inherited cores. The zonation commonly
truncates against fracture surfaces. The intensity of the CL response varies markedly between
grains. No grains preserve evidence for more than one major period of zircon growth, although
some grains show evidence of small palaeofrac- tures having healed.
Zircon analyses from this sample fall into two main age groupings Fig. 3e. Several discordant
analyses were included in the age calculations for both populations. Their inclusion is justified be-
cause the lower intercept of the discordia they lie
upon is the present, indicating recent lead loss. Primary
207
Pb
206
Pb ratios are therefore retained. Percentages of common
206
Pb are generally less than 0.5 for these analyses Table 6.
The younger
population, comprising
nine analyses, has
207
Pb
206
Pb ratios, which are within error of a single value and indicate an age of
1321 9 24 Ma. Analyses contain between 54 and 600 ppm U, and 34 – 484 ppm Th Table 6. U
and Th contents average 257 and 207 ppm, re- spectively. Apart from a few outliers the ThU
ratios of analyses from this population cluster fairly closely around a mean value of 0.8. To-
gether with the oscillatory zonation noted in CL images, these data suggest that the analyses
sample zircon formed in an igneous rock.
The older population, comprising seven analy- ses, forms a discrete group with a pooled
207
Pb
206
Pb age of 1783 9 12 Ma. Two analyses 1.24 and 2.3 fall outside the older population. Both
are discordant and were not included in age cal- culations. Analyses from this population are
more heterogeneous with respect to chemistry. Uranium concentrations range from 71 to 660
ppm and thorium concentrations from 93 to 1126 ppm Table 6. The average U and Th
concentrations 261 and 322 ppm, respectively do not differ significantly from those of the
younger population. ThU ratios show no sig- nificant cluster and vary from 0.6 to 2.1. The
chemistry of the zircons does not provide con- clusive evidence as to their origin but as oscilla-
tory zonation is present in the majority of grains an igneous origin is most plausible.
The rounded, fractured and abraded surfaces of zircons from both populations indicates detri-
tal transport. The zircons are therefore inter- preted to be detrital grains in the sedimentary
precursor to the quartzite and have the U-Pb isotopic characteristics of their igneous source
rocks. The younger population of 1321 9 24 Ma sets a maximum age for the deposition of the
sediments that formed the protolith of the quartzite. The dominantly clean quartzitic na-
ture of the Mount Ragged metasedimentary rocks precludes a volcanic or volcanoclastic
origin and instead suggests granitic source rocks shed off the uplifted and eroding Albany – Fraser
Orogen. The
high oxidation
state of
the metasedimentary rocks, characterised by the sta-
bility of haematite, suggests deposition in a shal- low and oxygenated environment.
3
.
4
.
6
. Syn-D
4
mica schist
9510092
, Mount Ragged
Rutile crystals from this sample are a lustrous brown-red colour, euhedral in shape and vary
from elongate crystals lengthwidth 4 – 7 up to 500 mm in length to equant plates 250 mm
in length. The width of needles varies from
50 mm in the most elongate grains to several hundreds of micrometres. The grains show no
evidence of zonation in transmitted light. All 18 rutile analyses from this sample fall
within error of a mean
207
Pb
206
Pb ratio corre- sponding to an age of 1154 9 15 Ma Fig. 3f.
The percentage of common
206
Pb in the analyses is high ranging from 0.6 to 6.0, see Table 6
but is lower than usual for this mineral by virtue of atypically high uranium concentrations,
which range from 76 to 122 ppm and average 98 ppm Table 6. The metamorphic mineral as-
semblage in the schist, the abundance of planar crystal faces on rutile grains, and their tendency
to form delicate knee-bend twins suggests that they grew as a part of a post-kinematic parage-
nesis, which slightly post-dates peak metamor- phism.
Based on
considerations of
mineral assemblage, peak metamorphic temperatures are
unlikely to have far exceeded 500°C. The cool- ing rate for the metasedimentary rock is un-
known but can be assumed to be in the order of several degrees or more per million years. At
this cooling rate, and for rutile grains of the size analysed, the closure temperature must be in ex-
cess of 420°C Mezger et al., 1989. The rutiles therefore crystallised near to their closure tem-
perature, so it is expected that the age recorded is close to the actual crystallisation age. The
1154 9 15 Ma age therefore provides a minimum estimate for the timing of peak metamorphic
conditions in the Mount Ragged metasedimen- tary rocks.
4. Discussion