JOURNAL OF PETROLOGY
VOLUME 38
NUMBER 5
PAGES 625–649
1997
Evolution of the Darling Range Batholith,
Yilgarn Craton, Western Australia:
a SHRIMP Zircon Study
A. A. NEMCHIN∗ AND R. T. PIDGEON
SCHOOL OF APPLIED GEOLOGY, CURTIN UNIVERSITY OF TECHNOLOGY, GPO BOX U1987, PERTH, W.A. 6001, AUSTRALIA
RECEIVED JUNE 14, 1996 ACCEPTED DECEMBER 4, 1996
Primary and secondary internal structures observed in zircons from
granites from the Late Archaean Darling Range Batholith record
stages in the evolutionary history of the granites and provide a basis
for a SHRIMP U–Pb study on the timing of granite evolution.
Many granite zircons contain cores. A few low-U cores retain
concordant to slightly discordant U–Pb ages of ~2·8 Ga, some
show discordant intermediate ages, but most have nearly concordant
ages within the range 2690–2650 Ma. These ages are interpreted
as dating the source rocks of the Darling Range granites, or as
representing different degrees of isotopic resetting owing to recrystallization of protolith zircon during prograde metamorphism
and subsequent melting. The zircon cores are enclosed by inner rims
of oscillatory zoned zircon, which are interpreted as zircon growth
during the main crystallization phase of the granite magma.
SHRIMP ages of zoned zircon, of between 2648 and 2626 Ma,
suggest an extended period of granite crystallization. The oscillatory
zoned inner rims are surrounded by weakly zoned to unzoned outer
rims which transgress primary zoning of the inner rim, suggesting
corrosion followed by new zircon deposition. However, the preservation of weak zoning in the outer rim and the euhedral nature
of external zircon faces, which are identical to those developed in
the inner rim, suggests that the outer rims are in fact recrystallized
outer parts of inner rims. This conclusion is supported by the
younger ages (2628–2616 Ma) determined for outer rims. These
results indicate that formation of outer rims and accompanying loss
of radiogenic Pb occurred during or soon after granite crystallization
and before zircons had time to accumulate significant radiation
damage, suggesting that the recrystallization process is independent
of the degree of metamictization. The history of formation of the
Darling Range granites contained within the zircon crystals suggests
initial magma formation between 2690 and 2650 Ma, crystallization and emplacement of the granite magma at 2648–2626
∗Corresponding author. Telephone: (09)351-3710. Fax: (09)351-3153.
e-mail: [email protected]
Ma, and slow cooling, indicated by marginal recrystallization and
continued Pb loss from the zircons, until 2628–2616 Ma.
KEY WORDS:
Archaean granites; zircon U–Pb ages; zircon structures
INTRODUCTION
The origin and evolution of large granitoid masses is a
major problem in Archaean geology. Were they derived
exclusively from mantle material, from mixtures of mantle
and older sialic crust, or were they derived entirely from
older crustal rocks? Also, what were the processes leading
to melting and emplacement of these granitoids on a
scale of thousands of square kilometres? Were there a
series of granitoid-forming events with a systematic pattern of emplacement ages or was emplacement of granitoids essentially contemporaneous over wide areas? How
does generation of granitoids relate to high-grade regional
metamorphism and what was the cooling history of the
granitoids after crystallization?
The ability of the ion microprobe SHRIMP to make
micro-scale U–Pb isotopic analyses on the polished surfaces of sectioned zircon crystals provides a means to
explore some of the questions posed above. However,
the potential for a SHRIMP zircon study to provide
answers depends on a careful integration of the analytical
programme with structural information contained within
the zircon grains. Recent studies have shown that zircons
 Oxford University Press 1997
JOURNAL OF PETROLOGY
VOLUME 38
in granitoids can preserve information on their premagmatic history as well as providing a record of zirconium saturation and undersaturation during magma
evolution (Vavra & Hansen, 1991). There is also evidence
that zircons provide information on late- to post-crystallization processes during cooling (Pidgeon, 1992). The
present study represents a case history where we have
applied SHRIMP analysis to complex zircons from the
Archaean Darling Range Batholith using zircon structure
as a basis for analytical spot selection and interpretation
of results. We also make use of SHRIMP Th–U–Pb
concentration data to provide information on chemical
changes associated with events identified from the zircon
morphology. The results reveal a complex and extended
evolutionary history for the batholith.
SUMMARY OF THE GEOLOGY OF
THE BATHOLITH
The Darling Range Batholith is situated in the Southwest
Yilgarn Craton of Western Australia (Fig. 1). It is bordered
to the east by the Jimperding Metamorphic belt and to
the south by the Proterozoic Albany–Fraser Province.
The batholith is composed of granodiorites, alkali feldspar
granites and less extensive gneissic granitoids, and contains two volcanic belts, the Saddleback Greenstone Belt
and the Morangup Greenstone Belt (Wilde & Low, 1978,
1980; Wilde & Walker, 1982). In the present study we
report results on granites from the northern part of the
batholith, including the Logue Brook granite (Fig. 1).
Although extensive, the batholith is not well exposed,
and detailed mapping of relationships between different
types of granitoids forming the batholith is not possible.
Wilde & Low (1978) recognized massive and gneissic
granitoids in the batholith. These workers commented
that ‘the various textural types are irregularly interdeveloped’, suggesting that relative age relationships are
inconsistent. In our sampling programme we have identified three major granite types, a fine-grained grey
monzogranite to granodiorite, a medium- to coarsegrained light grey granite to adamellite and an alkali
feldspar porphyritic granite. Geochemical studies, which
will be reported elsewhere, support our textural subdivision of the granitoids into the above three types.
Because of poor exposure it is rare to observe contact
relationships between the granite types. However, in
Stathams Quarry near Perth the fine-grained granite
clearly transgresses and contains xenoliths of the coarsegrained granite. The fine-grained granite appears to form
a second pulse of granite magma injection, which may
reflect either the addition of magma from a new protolith,
or a reactivation of the initial magma chamber.
The three granite types are cut by aplite dykes and
pegmatite veins with aplite centres. The batholith is
NUMBER 5
MAY 1997
also cut by Proterozoic doleritic dykes. The locations of
granitoid samples are shown in Fig. 1.
PREVIOUS GEOCHRONOLOGY OF
THE BATHOLITH
A number of geochronological investigations of the Darling Range granites have been published, beginning with
the classic Rb–Sr whole-rock study by Compston &
Jeffery (1959), who reported an age of 2400 Ma for
granites from Canning Dam near the western margin of
the batholith. Rb–Sr studies have been made on whole
rock (Compston & Arriens, 1968; Arriens, 1971) and
biotite (Libby & de Laeter, 1979; de Laeter & Libby,
1993) from granites within the batholith. The biotites
record Early Palaeozoic Rb–Sr ages suggesting a Palaeozoic heating along the western margin of the batholith, whereas the whole-rock ages of ~2600 Ma have
large uncertainties and are not discussed further in this
paper. Relevant to the present study are the few zircon
U–Pb studies reported for the batholith. Conventional
multigrain zircon U–Pb ages of 2677 ± 50 Ma (Mortigup
granite) and 2642+242
–166 Ma (Monday Hill granite) were
reported by Nieuwland & Compston (1981) for undeformed granitoids from the eastern margin of the
batholith. More recently, Compston et al. (1986) reported
a SHRIMP zircon age of 2612 ± 5 Ma for the Logue
Brook granite from the western part of the batholith (Fig.
1) and argued that this age dates the crystallization of
the granite. These results raise the possibility of an
extended history for granite emplacement in the batholith. Fletcher et al. (1985) determined Sm–Nd CHUR
model ages of 2·9–3·0 Ga on four whole-rock samples,
including one sample from the same quarry in the Logue
Brook granite sampled by Compston et al. (1986), and
interpreted these ages as evidence that the granites were
derived from ~3·0 Ga crustal source rocks. Compston et
al. (1986) suggested that the variation of 0 to –4 in eCHUR
initially, calculated with the Logue Brook zircon age,
provides evidence that the granite formed by mixing
of older continental crust with younger mantle-derived
crust.
626
ANALYTICAL METHOD
Zircons were separated using conventional heavy liquid
and magnetic techniques. Grains were sized and +100
lm sieve size grains were mounted in epoxy resin,
polished and etched with HF to show internal structure,
and studied by reflected and transmitted light microscopy.
Zircon internal structures were also investigated using
cathodoluminescence ( JEOL 6400 at the University of
Western Australia). However, it was found that the granite
NEMCHIN AND PIDGEON
EVOLUTION OF DARLING RANGE BATHOLITH
Fig. 1. Simplified geological map of the Southwestern Yilgarn Craton.
zircons had very weak luminescence and this technique
did not give a clear picture of the internal structures of
the zircons except for some zircon cores. The weak
luminescence of these zircons was kindly confirmed by
Dr G. Vavra at the ETH in Zürich. Williams et al. (1995)
suggested that luminescence is suppressed by radiation
damage of the zircon lattice. This is supported by our
empirical observations that the more metamict the zircon
the greater its susceptibility to HF etching and the weaker
its cathodoluminescence. Backscattered electron images
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JOURNAL OF PETROLOGY
VOLUME 38
Fig. 2. Comparison of the cathodoluminescence image (a) with
photomicrograph of the etched zircon (b) from the coarse-grained
granite sample W332. Cathodoluminescence shows different features
of the complex zircon structure compared with the HF etching.
show similar features to those revealed by HF etching;
however, for the present zircons HF etching provides
excellent resolution of internal structural features, as
shown by the detail in photomicrographs (Figs 2, 3 and
4). Although the cathodoluminescence was unable to
resolve structural complexities in the zircon rims, it was
able to show, albeit weakly, internal structures in the
zircon cores that were not evident from etching (Fig. 2).
Most samples were etched by HF before SHRIMP
analysis, to identify exact locations for analysis spots.
However, all etched samples were repolished before the
analyses to remove the etched surface. Two samples
were analysed in separate SHRIMP sessions, one before
etching and the second after etching. For sample W330,
session one, made without etching, resulted in a high
number of analyses being located on boundaries between
the inner rims and the centres. During the second analytical session, after etching, for sample W390, an attempt
was made to locate several analytical spots exactly in
places where analyses were performed during the previous
session (see, e.g. analyses W390-6-1 vs W390-6-4 and
NUMBER 5
MAY 1997
W390-6-2 vs W390-6-3 in Table 1). Results of these
analyses indicate that the etching did not affect the
SHRIMP analytical results.
SHRIMP analytical procedures were similar to those
described by Compston et al. (1984). Characteristics of
the Curtin Consortium SHRIMP II have been reported
by Kennedy & de Laeter (1994) and also briefly by
Pidgeon et al. (1996), and the Sri Lankan gem zircon
CZ-3 used as our standard has been described by Pidgeon
et al. (1994). The age of this zircon, determined by
conventional analysis, is 564 Ma and the concentration
of U is 530–560 p.p.m. Uncertainties reported in tables
and figures are given at ±1r but final ages are quoted
at the 95% confidence level. Uncertainties in Th and U
concentrations are of the order of 20%, based on repeat
analyses of the standard. The O2– was used as a primary
beam. The SHRIMP output data for each spot analysis
were reduced initially using the PRAWN program developed at the Australian National University (ANU).
Calculations of element concentrations and isotopic ratios
were made using WALLEAD program modified by D.
Nelson from the LLEAD program used at ANU. Broken
Hill lead was used as the common lead correction,
assuming that most of the common lead was added in
the gold coat. This assumption is not correct for analyses
with high common lead concentrations. However, these
analyses were not included in final calculations of zircon
ages. The 204Pb corrected data were used for calculations.
Combined SHRIMP ages on unzoned centres, oscillatory zoned inner rims and weakly zoned outer rims
were calculated as the mean of radiogenic 207Pb/206Pb
ages for individual spots if the data are concordant within
the error. However, for zircon cores and outer rims
which have undergone various degrees of recrystallization
and resetting, it is unlikely that there is a single correct
age and the reported mean age and uncertainty indicates
an age range which embraces the ages of individual
SHRIMP analyses. An alternative way of reporting the
spread of 207Pb/206Pb ages as a median value with upper
and lower quartiles does not significantly alter the spread
of results indicated by the mean and 95% limits.
INTERNAL STRUCTURES OF THE
ZIRCONS
Zircons from the granites have complex internal structures involving zoned and unzoned zircon. Many grains
contain a distinct central core which consists, in various
proportions, of finely granular structured matrix which
reacts strongly with HF vapour, and irregular, rounded
patches of unzoned zircon which is inert to HF. The
granular etched zircon appears to be progressively replaced by unzoned, unetched zircon, beginning with the
development of irregular patches and embayments of
628
NEMCHIN AND PIDGEON
EVOLUTION OF DARLING RANGE BATHOLITH
Fig. 3. Photomicrographs of HF-etched zircons from the Darling Range granite showing different types of zircon cores (reflected light; length
of the photographs represents ~300 lm). (a) Zircon grain from the fine-grained granite W323. The granular, etched central core is surrounded
by an inner zoned rim and an outer weakly zoned rim. (b) Zircon grain from the fine-grained granite W323. The granular, etched central core
is partly replaced by clear unetched zircon and surrounded by an inner zoned rim and an outer weakly zoned rim. (c) Zircon grain from the
fine-grained granite W323. Etched zircon is preserved as remnants in the clear central core, which is surrounded by an inner zoned rim and
an outer weakly zoned rim. The [121] and [011] pyramids are developed in the inner part of the inner rim, whereas [121] pyramid is absent
in the outer part of the zoned rim. (d) Zircon grain from the fine-grained granite sample W323. The clear central core is surrounded by an
inner zoned rim and an outer weakly zoned rim. The boundary between core and inner rim is irregular but smooth, which can be interpreted
as corrosion (Vavra, 1994). The [121] and [011] pyramids are developed in the inner part of the inner rim. (e) Zircon grain from porphyritic
granite sample W382. This shows a remnant of oscillatory zoned zircon within clear core, which is surrounded by a thin zoned rim. (f ) Zircon
grain from the coarse-grained granite W327. Clear central core is surrounded by zoned rim. Boundary between core and zoned rim is rounded,
which can be attributed to the dissolution of the core (Vavra, 1994) before overgrowth by the oscillatory zoned rim.
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NEMCHIN AND PIDGEON
EVOLUTION OF DARLING RANGE BATHOLITH
clear zircon at the margins of the etched central areas
(Fig. 3a) and continuing (Fig. 3b and c) until the centres
are totally replaced by clear zircon (Fig. 3d). Another
explanation of the observed pattern (Fig. 3a, b and c) is
that the strongly etched (more structurally damaged)
parts of the zircon cores represent movement and concentration of some elements within the cores. This has
been discussed in detail by Pidgeon & Nemchin (in
preparation). Some zircons have unetched, apparently
unzoned cores, which, when subjected to cathodoluminescence (CL), show a residual internal structure (Fig. 2). We interpret this as indicating the presence
of residual zoning in crystalline cores, which are resistant
to HF etching but strongly luminescent. Conversely, in
some grains zoned structures are revealed by HF etching
but are not recorded by CL imaging (Fig. 2), reinforcing
our conclusions that these two techniques can reveal
different aspects of the internal structures of zircons. In
some grains unzoned zircon cores are bounded by a
rounded, irregular corrosion surface (Fig. 3e and f ), which
is interpreted as chemical corrosion under conditions of
mild zirconium undersaturation (e.g. Vavra, 1994).
An inner rim of oscillatory zoned zircon forms a mantle
around zircon cores and represents a sustained period of
zircon crystallization (Figs 2, 3 and 4). Growth begins
with initial plating of fine zoned zircon, with both 011
and 121 pyramid faces, at the corners of rounded cores
(Fig. 3c and d, and 4c). Once corners have formed,
growth of oscillatory zoned zircon proceeds with the
characteristic development of simple 011 faces. Inclusions
are concentrated in this zoned inner rim. We interpret
this rim as having formed during crystallization of the
granite magma (e.g. Vavra, 1994). The pattern of oscillatory zoning in this rim shows no evidence for a hiatus
during growth. Within a zircon population the inner rim
can vary in width, with respect to the core, from narrow
to very broad. The width of the inner rim can also
vary between zircon populations. For example, in some
populations the width of the inner rim is generally
<20 lm (e.g granite sample W323) and could not be
satisfactorily analysed using SHRIMP. In other populations, particularly those from the porphyritic granite
samples, the inner rims are strongly developed.
The inner rim is surrounded by an outer rim of weakly
zoned to unzoned zircon (resistant to HF vapour). The
inner rim is separated from this outer rim by an irregular,
serrated boundary which cuts into the zoned zircon and
sometimes extends into the zircon cores (Figs 3 and 4).
There are two possible interpretations for this boundary
and the genesis of the outer rim:
(1) It represents a corrosion boundary and marks a
profound change in conditions of crystallization involving
zircon dissolution followed by later magmatic crystallization of unzoned to weakly oscillatory zoned zircon
making up the outer rim. This is supported by apparent
breaking of crystals associated with the boundary in Fig.
4a, which can be taken as evidence for magma movement
during crystallization. In this explanation, the age of the
outer rim would represent the age of late-stage zircon
crystallization.
(2) It represents a recrystallization reaction front. This
explanation is supported by the delicate irregular structure of the boundary and the apparent conformity between the zoning in the inner and outer rims. Careful
inspection shows that the traces of oscillatory zoned inner
rim continue into the outer rim as weakly zoned zircon
(Fig. 4a). Patches of unzoned zircon in the inner and
outer rims can also be seen to cross-cut zoned zircon
and show disruption and removal of zoned zircon (Fig.
4). Different stages in the development of these unzoned
areas can be seen in zircons from the one zircon population as well as from different populations. The process
starts with the development of thin unzoned areas, several
microns long, disrupting a few zones from outside or
inside the zoned inner rim (Fig. 4b). This is followed by
the development of rounded 20–30 lm unzoned areas
which transgress the inner rim (Fig. 4c). Veinlets of
unzoned zircon are also seen to transgress the inner rim
(Fig. 4d). At a further stage the inner rim contains a
network of unzoned zircon (Fig. 4e). This patchwork
development of unzoned zircon resembles structures reported by Pidgeon (1992), who interpreted these to be
due to recrystallization. The zircons appear to have
largely retained their euhedral to subhedral shapes
developed during magmatic crystallization, confirming
Fig. 4. Photomicrographs of HF-etched zircons from the Darling Range granite showing different types of outer rims (reflected light; length
of the photographs represents ~300 lm). (a) Zircon grain from the porphyritic granite W427. The grain apparently was broken before the
overgrowth by the outer rim. However, in this grain a remnant of the inner zoned rim is evident within the outer weakly zoned rim. On the
other side of the grain, zoning of the inner rim is disrupted and rotated from its original orientation. (b) Zircon grain from the porphyritic
granite W427. The grain consists almost entirely of zoned inner rim. Only a small central part of clear zircon can be interpreted as the core.
An outer weakly zoned rim is developed around the grain. Small patches of clear zircon several microns in size are developed within the zoned
inner rim near the ends of the grain. Locally, these patches become larger (up to 10–20 lm in size). (c) Zircon grain from the fine-grained
granite W323. Rounded patches of unzoned zircon of 20–30 mm are developed within the inner zoned rim near the boundary with the outer
rim. (d) Zircon grain from porphyritic granite sample W427. The veinlet of clear unzoned zircon cuts the zoned inner rim and clear core and
is terminated by the boundaries with the outer rim on both sides of the crystal. (e) Zircon grain from porphyritic granite sample W427. The
internal structure of the zoned inner rim is extensively broken by patches of clear unzoned zircon. (f ) Zircon grain from the coarse-grained
granite W332. Weakly zoned zircon, developed on one side of the grain, deeply penetrates the zoned inner rim.
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NUMBER 5
MAY 1997
Table 1: SHRIMP data for the Darling Range granitoids
Labels
U
Th
Th/U Total
(p.p.m.) (p.p.m.)
206
Pb/204Pb
207
Pb/206Pb∗
208
Pb/206Pb∗
206
Pb/238U∗
207
Pb/235U∗
208
Pb/232Th∗
Pb
207
Pb/206Pb∗
age (Ma)
W323 fine-grained granite
clear cores
W323-1-1
272
270 0·99
173
4980
0·1822 ± 13
0·2786 ± 28
0·502 ± 15
12·60 ± 41
0·1411 ± 47
2673 ± 12†
W323-1-2
215
143 0·66
128
2380
0·1776 ± 17
0·1791 ± 35
0·500 ± 15
12·23 ± 41
0·1348 ± 51
2630 ± 16
W323-1-4
328
218 0·66
190
2260
0·1752 ± 14
0·1892 ± 29
0·482 ± 15
11·63 ± 38
0·1375 ± 48
2607 ± 13
W323-3-1
525
283 0·54
310
7940
0·1812 ± 7
0·1504 ± 13
0·511 ± 16
12·78 ± 40
0·1425 ± 46
2664 ± 7
W323-3-2
408
219 0·54
239
5640
0·1820 ± 10
0·1477 ± 19
0·506 ± 15
12·70 ± 40
0·1397 ± 47
2671 ± 9
W323-4-4
477
262 0·55
276
7450
0·1812 ± 8
0·1562 ± 15
0·500 ± 15
12·48 ± 39
0·1423 ± 46
2664 ± 7
W323-4-5
651
313 0·48
384
12600
0·1757 ± 6
0·1298 ± 9
0·523 ± 16
12·68 ± 39
0·1414 ± 45
2613 ± 6
W323-5-1
476
257 0·54
281
2350
0·1829 ± 10
0·1240 ± 19
0·512 ± 16
12·91 ± 41
0·1175 ± 40
2679 ± 9
W323-5-2
574
282 0·49
332
3640
0·1794 ± 8
0·1183 ± 15
0·510 ± 15
12·62 ± 39
0·1228 ± 41
2647 ± 7
W323-5-3
291
141 0·48
175
3320
0·1799 ± 11
0·1331 ± 20
0·524 ± 16
13·00 ± 41
0·1444 ± 50
2652 ± 10
W323-6-1
1774
1782 1·00
1123
12600
0·1823 ± 4
0·2864 ± 8
0·498 ± 15
12·53 ± 38
0·1421 ± 43
2674 ± 3
W323-9-2
1224
324 0·26
682
20700
0·1861 ± 4
0·0781 ± 5
0·511 ± 15
13·12 ± 40
0·1510 ± 47
2708 ± 4
W323-10-2 4767
512 0·11
2640
29600
0·1857 ± 2
0·0303 ± 2
0·528 ± 16
13·53 ± 41
0·1490 ± 46
2704 ± 2
W323-11-1
398
232 0·58
237
3040
0·1850 ± 10
0·1628 ± 20
0·506 ± 15
12·90 ± 41
0·1415 ± 47
2698 ± 9
W323-12-1 1421
355 0·25
809
9520
0·1817 ± 5
0·0722 ± 6
0·525 ± 16
13·16 ± 40
0·1517 ± 48
2668 ± 4
W323-13-1
543
230 0·42
290
2160
0·1800 ± 10
0·1210 ± 20
0·466 ± 14
11·58 ± 36
0·1334 ± 47
2653 ± 10
W323-14-1
666
357 0·54
382
3430
0·1877 ± 9
0·1498 ± 16
0·490 ± 15
12·69 ± 39
0·1371 ± 45
2722 ± 8
weakly zoned outer rims
W323-1-3
2095
117 0·06
1072
23300
0·1757 ± 4
0·0151 ± 3
0·499 ± 15
12·08 ± 37
0·1356 ± 51
2613 ± 3
W323-1-5
2267
147 0·06
1140
7430
0·1727 ± 4
0·0171 ± 5
0·488 ± 15
11·62 ± 35
0·1289 ± 54
2584 ± 4
W323-3-3
1511
1540 1·02
842
1360
0·1736 ± 8
0·0410 ± 15
0·512 ± 15
12·24 ± 38
0·0206 ± 10
2593 ± 7
W323-4-1
1797
125 0·07
937
5200
0·1765 ± 4
0·0152 ± 7
0·503 ± 15
12·25 ± 37
0·1094 ± 58
2620 ± 4
W323-4-6
1872
189 0·10
995
1880
0·1727 ± 6
0·0250 ± 10
0·501 ± 15
11·92 ± 36
0·124 ± 64
2584 ± 5
W323-5-4
2416
293 0·12
1249
7830
0·1764 ± 3
0·0177 ± 4
0·500 ± 15
12·15 ± 37
0·0730 ± 29
2619 ± 3
W323-6-2
2103
119 0·06
1070
16340
0·1751 ± 3
0·0144 ± 3
0·496 ± 15
11·96 ± 36
0·1257 ± 48
2607 ± 3
W323-6-3
1902
135 0·07
1037
22200
0·1750 ± 3
0·0161 ± 3
0·531 ± 16
12·81 ± 39
0·1201 ± 42
2606 ± 3
W323-9-1
2116
181 0·09
849
16200
0·1787 ± 3
0·0197 ± 3
0·388 ± 12
9·56 ± 29
0·0893 ± 31
2641 ± 3
W323-10-3 1813
83 0·05
921
7550
0·1793 ± 4
0·0113 ± 5
0·492 ± 15
12·17 ± 37
0·1210 ± 68
2646 ± 4
W323-11-2 2296
419 0·18
1193
2800
0·1777 ± 4
0·0306 ± 8
0·490 ± 15
12·00 ± 36
0·0821 ± 32
2631 ± 4
W323-13-2 1997
302 0·15
932
7110
0·1794 ± 4
0·0149 ± 6
0·451 ± 14
11·16 ± 34
0·0444 ± 21
2648 ± 4
metamict cores
3089
1494 0·48
1821
28800
0·1814 ± 3
0·1354 ± 4
0·519 ± 16
12·98 ± 39
0·1453 ± 44
2666 ± 3
W323-8-1 11142
2952 0·26
5013
36200
0·1590 ± 2
0·0811 ± 2
0·421 ± 13
9·23 ± 28
0·1289 ± 39
2445 ± 2
W323-7-1
W330 coarse-grained granite
clear cores
W330-7-1
930
434 0·47
543
63000
0·1811 ± 4
0·1262 ± 5
0·518 ± 4
12·94 ± 12
0·1400 ± 14
2663 ± 4
W330-7-2
907
405 0·45
523
275000
0·1809 ± 4
0·1202 ± 5
0·515 ± 4
12·84 ± 12
0·1385 ± 14
2661 ± 4
W330-7-3
960
655 0·68
549
3460
0·1812 ± 5
0·1600 ± 9
0·488 ± 4
12·20 ± 11
0·1146 ± 12
2664 ± 5
W330-13-1
639
112 0·17
339
159000
0·1820 ± 7
0·0471 ± 6
0·502 ± 8
12·59 ± 22
0·1354 ± 30
2671 ± 6
W330-13-2
529
73 0·14
233
34400
0·1845 ± 9
0·0376 ± 9
0·418 ± 7
10·62 ± 19
0·1146 ± 35
2694 ± 8
W330-10-1
737
207 0·28
412
15700
0·1798 ± 6
0·0758 ± 7
0·515 ± 8
12·78 ± 22
0·1395 ± 27
2651 ± 5
W330-16-1
513
251 0·49
220
1530
0·1831 ± 18
0·0746 ± 35
0·382 ± 6
9·65 ± 20
0·0582 ± 29
2681 ± 17
W330-15-2 1355
1884 1·39
992
45400
0·1865 ± 5
0·3752 ± 11
0·544 ± 9
13·97 ± 24
0·1466 ± 25
2711 ± 5
W330-5-1
1468
56 0·04
754
28600
0·1657 ± 4
0·0089 ± 3
0·508 ± 8
11·60 ± 20
0·1191 ± 51
2515 ± 4
W330-5-2
1217
55 0·04
604
34600
0·1705 ± 5
0·0128 ± 4
0·487 ± 8
11·45 ± 19
0·1388 ± 50
2563 ± 5
W330-5-3
2628
79 0·03
1411
69700
0·1641 ± 3
0·0070 ± 2
0·533 ± 9
12·05 ± 20
0·1230 ± 44
2498 ± 3
W330-6-2
661
972 1·47
410
508000
0·1808 ± 4
0·4049 ± 11
0·454 ± 7
11·33 ± 19
0·1251 ± 21
2661 ± 4
632
NEMCHIN AND PIDGEON
EVOLUTION OF DARLING RANGE BATHOLITH
Table 1: continued
Labels
U
Th
Th/U Total
(p.p.m.) (p.p.m.)
206
Pb/204Pb
207
Pb/206Pb∗
208
Pb/206Pb∗
206
Pb/238U∗
207
Pb/235U∗
208
Pb/232Th∗
Pb
207
Pb/206Pb∗
age (Ma)
cores mixed with inner rims
W330-15-1
960
1030 1·07
624
2140
0·1778 ± 8
0·2629 ± 16
0·512 ± 8
12·55 ± 22
0·1254 ± 23
2633 ± 8
W330-1-3
1366
1411 1·03
863
9300
0·1788 ± 6
0·2823 ± 13
0·500 ± 8
12·32 ± 21
0·1365 ± 24
2642 ± 5
W330-11-1
932
241 0·26
514
30500
0·1792 ± 5
0·0704 ± 5
0·512 ± 8
12·65 ± 21
0·1394 ± 26
2646 ± 5
W330-11-2
875
246 0·28
489
96000
0·1784 ± 5
0·0752 ± 5
0·518 ± 8
12·74 ± 21
0·1384 ± 25
2638 ± 5
W330-12-2 1527
1556 1·02
991
40800
0·1777 ± 4
0·2813 ± 7
0·516 ± 8
12·64 ± 21
0·1424 ± 24
2631 ± 4
W330-3-2
1930
3302 1·71
1364
9410
0·1770 ± 5
0·4590 ± 15
0·499 ± 8
12·18 ± 21
0·1339 ± 23
2625 ± 5
W330-12-1 2778
716 0·26
1621
763000
0·1769 ± 3
0·0702 ± 3
0·544 ± 9
13·26 ± 22
0·1481 ± 25
2624 ± 3
zoned inner rims
W330-9-2
1475
2744 1·86
1034
41000
0·1767 ± 4
0·4976 ± 10
0·485 ± 8
11·82 ± 20
0·1298 ± 21
2622 ± 4
W330-9-3
2412
4977 2·06
1746
90800
0·1762 ± 3
0·5493 ± 8
0·486 ± 8
11·82 ± 19
0·1295 ± 21
2618 ± 3
W330-2-3
1750
395 0·23
938
8800
0·1774 ± 5
0·0601 ± 7
0·500 ± 8
12·24 ± 21
0·1332 ± 28
2629 ± 5
W330-2-4
5099
2591 0·51
3221
5400
0·1730 ± 3
0·1385 ± 5
0·554 ± 9
13·22 ± 22
0·1511 ± 25
2587 ± 3
W330-10-2 2810
911 0·32
1393
22100
0·1657 ± 3
0·0886 ± 3
0·458 ± 7
10·47 ± 17
0·1252 ± 21
2514 ± 3
W330-15-3 2611
4630 1·77
2226
9370
0·1794 ± 4
0·4742 ± 9
0·596 ± 10
14·73 ± 24
0·1592 ± 26
2648 ± 4
W330-14-2 1192
1780 1·49
810
20300
0·1787 ± 5
0·4091 ± 11
0·495 ± 8
12·20 ± 21
0·1357 ± 23
2641 ± 5
W330-8-2
1068
1509 1·41
732
110000
0·1781 ± 4
0·3871 ± 8
0·508 ± 8
12·47 ± 21
0·1392 ± 23
2635 ± 4
W330-8-3
3454
4926 1·43
2314
386000
0·1776 ± 2
0·3847 ± 5
0·497 ± 8
12·18 ± 20
0·1342 ± 22
2631 ± 2
W330-2-1
2762
3908 1·41
1910
18300
0·1789 ± 4
0·3958 ± 10
0·509 ± 8
12·54 ± 21
0·1423 ± 24
2643 ± 3
W330-2-2
1296
1559 1·20
812
1590
0·1750 ± 9
0·3201 ± 22
0·472 ± 8
11·40 ± 21
0·1257 ± 23
2606 ± 9
W330-1-4
2531
5609 2·22
2062
10400
0·1780 ± 4
0·6084 ± 13
0·527 ± 8
12·93 ± 22
0·1446 ± 24
2635 ± 4
weakly zoned outer rims
W330-14-1
612
569 0·93
378
42800
0·1746 ± 7
0·2471 ± 11
0·505 ± 8
12·15 ± 21
0·1341 ± 24
2602 ± 6
W330-6-3
1123
89 0·08
541
5600
0·1760 ± 7
0·0162 ± 10
0·466 ± 7
11·29 ± 20
0·0952 ± 63
2615 ± 6
W330-1-1
698
99 0·14
380
1330
0·1613 ± 10
0·0302 ± 19
0·510 ± 8
11·33 ± 21
0·1089 ± 72
2469 ± 10
W330-1-2
671
112 0·17
351
4130
0·1782 ± 10
0·0295 ± 18
0·496 ± 8
12·19 ± 23
0·0876 ± 57
2636 ± 10
572
26000
0·1772 ± 4
0·0334 ± 4
0·520 ± 84
12·71 ± 21
0·1393 ± 29
2627 ± 4
3177 3220000
0·1813 ± 3
0·1102 ± 3
0·534 ± 86
13·34 ± 22
0·1382 ± 23
2665 ± 2
metamict cores
W330-8-1
1054
131 0·12
W330-16-2 5356
2279 0·43
W330-6-1
1048
1489 1·42
675
3520
0·1678 ± 8
0·3724 ± 19
0·479 ± 8
11·09 ± 20
0·1255 ± 22
2536 ± 8
W330-9-1
1480
3173 2·14
1155
180000
0·1769 ± 4
0·5803 ± 10
0·515 ± 8
12·56 ± 21
0·1394 ± 23
2624 ± 3
W332 coarse-grained granite
clear cores
W332-2-1
338
61 0·18
165
1120
0·1786 ± 16
0·1190 ± 33
0·417 ± 21
10·26 ± 53
0·2744 ± 160 2640 ± 15
W332-3-1
282
127 0·45
189
817
0·1805 ± 17
0·1539 ± 38
0·548 ± 28
13·63 ± 71
0·1882 ± 106 2657 ± 16
W332-8-1
1223
382 0·31
594
1110
0·1802 ± 8
0·1577 ± 17
0·403 ± 20
10·01 ± 51
0·2034 ± 105 2655 ± 8
W332-9-1
438
262 0·60
276
9080
0·1835 ± 7
0·1616 ± 13
0·541 ± 27
13·69 ± 70
0·1465 ± 5
2685 ± 7
W332-10-1
953
566 0·59
579
16500
0·1826 ± 5
0·1605 ± 8
0·524 ± 26
13·18 ± 67
0·1415 ± 72
2676 ± 5
W332-11-1
267
122 0·46
162
4160
0·1805 ± 11
0·1208 ± 21
0·535 ± 27
13·32 ± 69
0·1418 ± 76
2657 ± 10
W332-12-1
219
109 0·50
127
1570
0·1804 ± 15
0·1512 ± 32
0·489 ± 25
12·17 ± 63
0·1484 ± 82
2657 ± 14
W332-13-1
527
285 0·54
334
12300
0·1830 ± 6
0·1472 ± 11
0·551 ± 28
13·90 ± 70
0·1496 ± 76
2681 ± 6
W332-16-1
250
100 0·40
154
3050
0·1831 ± 12
0·1353 ± 22
0·534 ± 27
13·47 ± 69
0·1795 ± 96
2681 ± 10
W332-17-1
147
68 0·46
88
1740
0·1828 ± 17
0·1264 ± 34
0·518 ± 26
13·06 ± 69
0·1416 ± 82
2678 ± 16
W332-19-1
458
261 0·57
278
7720
0·1816 ± 7
0·1543 ± 12
0·525 ± 26
13·15 ± 67
0·1425 ± 73
2668 ± 6
633
JOURNAL OF PETROLOGY
VOLUME 38
NUMBER 5
MAY 1997
Table 1: continued
Labels
U
Th
Th/U Total
(p.p.m.) (p.p.m.)
206
Pb/204Pb
207
Pb/206Pb∗
208
Pb/206Pb∗
206
Pb/238U∗
207
Pb/235U∗
208
Pb/232Th∗
Pb
207
Pb/206Pb∗
age (Ma)
zoned inner rims
1618
859 0·53
995
14900
0·1771 ± 3
0·1488 ± 6
0·537 ± 27
13·12 ± 66
0·1506 ± 76
2626 ± 3
W332-14-3 4606
1393 0·30
2486
10700
0·1595 ± 2
0·0853 ± 3
0·501 ± 25
11·02 ± 55
0·1414 ± 71
2450 ± 2
W332-15-2 4270
2217 0·52
2322
14000
0·1628 ± 2
0·1396 ± 4
0·483 ± 24
10·85 ± 54
0·1300 ± 65
2485 ± 2
W332-24-1 1602
695 0·43
911
7400
0·1765 ± 3
0·1187 ± 6
0·507 ± 25
12·33 ± 62
0·1385 ± 70
2620 ± 3
W332-24-2 2746
1664 0·61
1620
18800
0·1777 ± 2
0·1648 ± 4
0·509 ± 25
12·47 ± 63
0·1385 ± 69
2631 ± 2
W332-4-2
weakly zoned outer rims
W332-1-1
1180
84 0·07
636
32400
0·1769 ± 4
0·0194 ± 3
0·523 ± 26
12·77 ± 64
0·1437 ± 76
2624 ± 3
W332-4-1
979
266 0·27
566
14300
0·1774 ± 5
0·0723 ± 6
0·536 ± 27
13·11 ± 66
0·1425 ± 73
2629 ± 4
W332-5-1
1022
42 0·04
577
19800
0·1777 ± 4
0·0121 ± 4
0·550 ± 28
13·48 ± 68
0·1608 ± 100 2631 ± 4
W332-6-1
862
61 0·07
472
52800
0·1772 ± 5
0·0202 ± 4
0·531 ± 27
12·98 ± 65
0·1509 ± 83
W332-7-1
1355
43 0·03
730
7800
0·1758 ± 4
0·0104 ± 5
0·524 ± 26
12·71 ± 64
0·1714 ± 122 2614 ± 4
W332-18-1 1767
156 0·09
947
9070
0·1778 ± 4
0·0236 ± 4
0·516 ± 26
12·64 ± 64
0·1377 ± 74
2632 ± 3
W332-20-2 1479
73 0·05
799
6850
0·1762 ± 4
0·0132 ± 5
0·524 ± 26
12·72 ± 64
0·1406 ± 90
2617 ± 4
W332-21-1 1428
431 0·30
712
630
0·1748 ± 9
0·0849 ± 20
0·422 ± 21
10·18 ± 52
0·1189 ± 66
2604 ± 9
W332-22-1 1542
220 0·14
758
2660
0·1767 ± 5
0·0253 ± 8
0·466 ± 23
11·35 ± 57
0·0829 ± 49
2622 ± 4
988
80 0·08
519
7170
0·1779 ± 4
0·0223 ± 6
0·505 ± 25
12·39 ± 62
0·1392 ± 80
2633 ± 4
W332-25-1 1321
63 0·05
689
13700
0·1777 ± 3
0·0133 ± 4
0·507 ± 25
12·42 ± 63
0·1403 ± 81
2632 ± 3
W332-25-2
975
87 0·09
634
473
0·1739 ± 11
0·1231 ± 23
0·522 ± 26
12·53 ± 64
0·7225 ± 390 2596 ± 10
W332-26-1 1402
64 0·05
722
8140
0·1768 ± 4
0·0115 ± 4
0·500 ± 25
12·20 ± 61
0·1262 ± 81
2623 ± 3
W332-23-1
2627 ± 4
metamict cores
W332-1-2
6291
4760 0·76
3572
71600
0·1536 ± 1
0·2074 ± 3
0·484 ± 24
10·26 ± 51
0·1327 ± 66
2387 ± 2
W332-1-3
3673
1739 0·47
1936
44800
0·1607 ± 2
0·1320 ± 3
0·473 ± 24
10·48 ± 53
0·1319 ± 66
2463 ± 2
W332-5-2
4021
1546 0·38
2187
65600
0·1641 ± 2
0·1099 ± 3
0·496 ± 25
11·22 ± 56
0·1416 ± 71
2498 ± 2
W332-14-1 3676
1231 0·33
2214
14100
0·1705 ± 2
0·0915 ± 3
0·552 ± 28
12·99 ± 65
0·1510 ± 76
2562 ± 2
W332-14-2 6487
3911 0·60
3704
95700
0·1572 ± 2
0·1641 ± 3
0·502 ± 25
10·88 ± 55
0·1366 ± 68
2426 ± 2
W332-15-1 1506
682 0·45
969
1920
0·1793 ± 6
0·1424 ± 12
0·551 ± 28
13·61 ± 69
0·1731 ± 88
2646 ± 6
W332-20-1 1608
810 0·50
967
7180
0·1791 ± 4
0·1460 ± 7
0·524 ± 26
12·93 ± 65
0·1519 ± 76
2644 ± 3
W332-20-3 3562
1670 0·47
2148
29500
0·1686 ± 2
0·1272 ± 3
0·540 ± 27
12·55 ± 63
0·1465 ± 73
2544 ± 2
100
7880
0·1823 ± 5
0·1265 ± 9
0·486 ± 16
12·22 ± 40
0·1363 ± 45
2674 ± 5
199
18860
0·1872 ± 3
0·1377 ± 5
0·487 ± 16
12·57 ± 41
0·1342 ± 43
2718 ± 3
W382 porphyritic granite
cores
W382-3-1
181
W383-5-1
358
82 0·45
W383-5-2
202
75 0·37
92
15000
0·2049 ± 5
0·1108 ± 7
0·402 ± 13
11·36 ± 37
0·1202 ± 40
2866 ± 4
W382-9-1
243
126 0·52
135
17800
0·1813 ± 4
0·1426 ± 7
0·487 ± 16
12·17 ± 39
0·1338 ± 44
2665 ± 4
W382-9-2
184
62 0·34
104
13100
0·1786 ± 5
0·0904 ± 8
0·517 ± 17
12·73 ± 41
0·1379 ± 46
2640 ± 5
W382-9-3
139
46 0·33
78
12700
0·1889 ± 6
0·0943 ± 9
0·503 ± 16
13·11 ± 43
0·1440 ± 49
2733 ± 6
W382-9-4
157
46 0·29
86
8620
0·1819 ± 6
0·0767 ± 8
0·500 ± 16
12·54 ± 41
0·1308 ± 45
2670 ± 5
W382-15-1
291
136 0·47
165
7720
0·1831 ± 7
0·1307 ± 7
0·501 ± 10
12·66 ± 26
0·1355 ± 30
2681 ± 6
W382-15-2
250
120 0·48
138
3720
0·1806 ± 8
0·1370 ± 8
0·483 ± 10
12·02 ± 25
0·1288 ± 30
2659 ± 7
W382-15-3
191
85 0·45
111
4120
0·1832 ± 10
0·1292 ± 10
0·509 ± 10
12·85 ± 27
0·1377 ± 36
2682 ± 9
W382-16-1
465
285 0·61
268
17100
0·1817 ± 5
0·1674 ± 7
0·496 ± 10
12·43 ± 25
0·1337 ± 27
2668 ± 4
W382-16-2
417
257 0·62
246
10400
0·1827 ± 6
0·1716 ± 8
0·505 ± 10
12·73 ± 25
0·1384 ± 29
2678 ± 5
W382-16-3
480
185 0·38
263
29400
0·1820 ± 5
0·1033 ± 6
0·496 ± 10
12·44 ± 25
0·1315 ± 28
2671 ± 5
W382-22-1
125
82 0·66
75
4100
0·1830 ± 2
0·1942 ± 15
0·502 ± 10
12·66 ± 28
0·1425 ± 36
2681 ± 11
W382-22-2
124
94 0·75
82
2770
0·1811 ± 1
0·2132 ± 16
0·544 ± 11
13·59 ± 30
0·1457 ± 37
2663 ± 12
179 0·5
634
NEMCHIN AND PIDGEON
EVOLUTION OF DARLING RANGE BATHOLITH
Table 1: continued
Labels
U
Th
Th/U Total
(p.p.m.) (p.p.m.)
206
Pb/204Pb
207
Pb/206Pb∗
208
Pb/206Pb∗
206
Pb/238U∗
207
Pb/235U∗
208
Pb/232Th∗
Pb
207
Pb/206Pb∗
age (Ma)
W382-23-1
122
53 0·43
67
3780
0·1863 ± 1
0·1300 ± 13
0·484 ± 10
12·42 ± 28
0·1348 ± 43
2710 ± 13
W382-25-1
232
103 0·44
134
6280
0·1826 ± 8
0·1270 ± 8
0·509 ± 10
12·81 ± 26
0·1400 ± 33
2676 ± 7
W382-25-2
98
35 0·36
55
2430
0·1812 ± 2
0·1125 ± 13
0·499 ± 10
12·46 ± 29
0·1364 ± 51
2664 ± 14
W382-28-1
192
104 0·54
113
4460
0·1818 ± 9
0·1586 ± 10
0·508 ± 10
12·73 ± 26
0·1414 ± 33
2670 ± 8
W382-28-2
489
148 0·3
282
8580
0·1819 ± 5
0·0870 ± 5
0·526 ± 10
13·21 ± 26
0·1440 ± 32
2670 ± 5
W382-28-3
170
79 0·46
103
2340
0·1813 ± 1
0·1418 ± 10
0·525 ± 10
13·13 ± 28
0·1445 ± 38
2665 ± 9
W382-8-2
328
197 0·60
187
14500
0·1821 ± 4
0·1644 ± 6
0·490 ± 16
12·31 ± 40
0·1342 ± 43
2672 ± 3
W382-12-1
548
284 0·52
307
90900
0·1227 ± 3
0·1441 ± 4
0·490 ± 16
12·35 ± 39
0·1318 ± 42
2682 ± 2
W382-13-2
108
61 0·56
64
7190
0·1846 ± 9
0·1520 ± 15
0·510 ± 16
12·99 ± 43
0·1375 ± 47
2695 ± 8
W382-20-3
674
212 0·32
395
9970
0·1816 ± 5
0·0859 ± 8
0·534 ± 10
13·38 ± 26
0·1456 ± 32
2668 ± 5
W382-20-1
205
86 0·42
120
2460
0·1795 ± 9
0·1125 ± 23
0·514 ± 10
12·73 ± 27
0·1371 ± 43
2648 ± 11
W382-4-1
433
246 0·57
241
60400
0·1832 ± 3
0·1549 ± 4
0·484 ± 16
12·22 ± 39
0·1318 ± 42
2682 ± 2
W382-4-2
1082
285 0·26
533
22800
0·1820 ± 2
0·0745 ± 2
0·455 ± 15
11·41 ± 37
0·1285 ± 41
2671 ± 2
W382-10-2 1546
911 0·59
904
150000
0·1813 ± 2
0·1600 ± 3
0·506 ± 16
12·65 ± 41
0·1374 ± 44
2665 ± 2
W382-17-1
528
203 0·38
299
9240
0·1804 ± 5
0·1072 ± 6
0·510 ± 10
12·67 ± 25
0·1372 ± 30
2657 ± 5
W382-17-2
371
161 0·43
211
12300
0·1810 ± 6
0·1218 ± 7
0·506 ± 10
12·63 ± 25
0·1389 ± 30
2662 ± 5
W382-18-1
309
176 0·57
187
6990
0·1826 ± 7
0·1579 ± 9
0·524 ± 10
13·19 ± 27
0·1407 ± 31
2677 ± 7
W382-18-2
277
153 0·55
162
4860
0·1812 ± 8
0·1572 ± 9
0·506 ± 10
12·64 ± 26
0·1374 ± 32
2664 ± 7
W382-19-1
205
131 0·64
126
6290
0·1788 ± 10
0·1824 ± 12
0·522 ± 10
12·87 ± 27
0·1448 ± 34
2642 ± 9
W382-19-2
267
189 0·71
164
4380
0·1810 ± 8
0·1973 ± 11
0·516 ± 10
12·86 ± 27
0·1382 ± 31
2662 ± 8
W382-14-1 1515
327 0·22
879
58800
0·1810 ± 2
0·0564 ± 2
0·525 ± 17
13·12 ± 42
0·1374 ± 44
2662 ± 2
zoned rims and clear grains
1068
354 0·33
546
30900
0·1792 ± 3
0·0870 ± 3
0·469 ± 150
11·59 ± 37
0·1231 ± 40
2645 ± 2
W382-11-1 1025
1006 0·98
616
22800
0·1779 ± 2
0·2676 ± 5
0·482 ± 154
11·81 ± 38
0·1313 ± 42
2633 ± 2
W382-11-2 1013
226 0·22
524
156500
0·1786 ± 2
0·0609 ± 2
0·485 ± 155
11·94 ± 38
0·1321 ± 43
2640 ± 2
W382-13-1 1852
130 0·07
938
85600
0·1792 ± 2
0·0191 ± 1
0·491 ± 157
12·14 ± 39
0·1337 ± 44
2645 ± 2
W382-14-2
294
186 0·63
170
75200
0·1788 ± 4
0·1725 ± 7
0·495 ± 159
12·21 ± 40
0·1351 ± 44
2641 ± 4
W382-14-3
650
309 0·48
375
29200
0·1779 ± 3
0·1317 ± 5
0·511 ± 164
12·52 ± 40
0·1415 ± 46
2633 ± 3
W382-14-4
851
323 0·38
466
46000
0·1787 ± 3
0·1050 ± 3
0·496 ± 159
12·21 ± 39
0·1369 ± 44
2641 ± 2
W382-21-1
410
294 0·72
267
5650
0·1790 ± 7
0·2033 ± 10
0·544 ± 106
13·44 ± 27
0·1497 ± 32
2644 ± 7
W382-21-2
608
194 0·32
351
6020
0·1779 ± 6
0·0920 ± 5
0·526 ± 102
12·9 ± 26
0·1424 ± 32
2633 ± 5
W382-24-1 1145
185 0·16
604
39900
0·1786 ± 3
0·0435 ± 2
0·502 ± 96
12·36 ± 24
0·1326 ± 27
2640 ± 3
W382-24-2
688
569 0·83
420
8980
0·1789 ± 4
0·2361 ± 7
0·500 ± 10
12·32 ± 24
0·1406 ± 28
2642 ± 4
W382-26-1 1391
434 0·31
672
19180
0·1676 ± 3
0·0863 ± 3
0·447 ± 8
10·33 ± 20
0·1212 ± 24
2534 ± 3
W382-26-2
530
228 0·43
297
24600
0·1772 ± 4
0·1182 ± 6
0·502 ± 10
12·27 ± 24
0·1361 ± 28
2627 ± 4
W382-6-1
208
160 0·77
119
22700
0·1786 ± 4
0·2131 ± 8
0·476 ± 15
11·72 ± 38
0·1323 ± 43
2640 ± 4
W382-7-1
800
354 0·44
430
47600
0·1774 ± 2
0·1203 ± 3
0·502 ± 10
12·27 ± 24
0·1361 ± 28
2627 ± 4
W382-7-2
1028
518 0·50
585
6670
0·1781 ± 4
0·1367 ± 7
0·499 ± 10
12·26 ± 24
0·1353 ± 27
2635 ± 4
W382-7-4
813
362 0·44
473
15900
0·1781 ± 4
0·1208 ± 6
0·490 ± 10
12·03 ± 24
0·1327 ± 27
2635 ± 4
W382-7-3
848
369 0·44
446
9580
0·1781 ± 4
0·1168 ± 6
0·497 ± 10
12·21 ± 23
0·1335 ± 27
2636 ± 4
W382-8-1
472
211 0·45
253
5650
0·1788 ± 3
0·1267 ± 5
0·474 ± 15
11·67 ± 38
0·1338 ± 43
2641 ± 3
W382-10-1 3300
749 0·23
1469
2070
0·1429 ± 2
0·1618 ± 4
0·385 ± 123
7·91 ± 25
0·2741 ± 88
2337 ± 2
W382-12-2 1030
310 0·30
530
111000
0·1741 ± 2
0·0826 ± 3
0·476 ± 152
11·42 ± 37
0·1305 ± 42
2597 ± 2
W382-2-1
metamict
635
JOURNAL OF PETROLOGY
VOLUME 38
NUMBER 5
MAY 1997
Table 1: continued
Labels
U
Th
Th/U Total
(p.p.m.) (p.p.m.)
206
Pb/204Pb
207
Pb/206Pb∗
208
Pb/206Pb∗
206
Pb/238U∗
207
Pb/235U∗
208
Pb/232Th∗
Pb
207
Pb/206Pb∗
age (Ma)
W427 porphyritic granite
cores
W427-1-1
213
61 0·29
97
9350
0·1830 ± 8
0·0723 ± 13
0·565 ± 23
14·24 ± 58
0·1429 ± 64
2680 ± 7
W427-1-3
287
98 0·34
105
15500
0·1805 ± 8
0·0916 ± 12
0·448 ± 18
11·15 ± 46
0·1207 ± 52
2657 ± 7
W427-1-5
362
175 0·48
153
3160
0·1838 ± 7
0·1318 ± 13
0·497 ± 20
12·60 ± 51
0·1356 ± 56
2688 ± 6
W427-3-1
216
281 1·30
102
1880
0·1818 ± 14
0·3711 ± 34
0·467 ± 19
11·70 ± 49
0·1329 ± 55
2669 ± 13
W427-3-2
335
691 2·06
208
2370
0·1847 ± 9
0·5340 ± 26
0·555 ± 22
14·14 ± 58
0·1438 ± 59
2696 ± 8
W327-3-3
86
40 0·47
60
19200
0·1845 ± 12
0·1287 ± 22
0·487 ± 20
12·39 ± 51
0·1345 ± 59
2694 ± 11
W327-3-4
220
153 0·70
141
11100
0·1765 ± 6
0·1940 ± 12
0·432 ± 17
10·52 ± 43
0·1201 ± 49
2621 ± 6
W327-3-5
112
102 0·91
82
10400
0·1824 ± 11
0·2518 ± 23
0·470 ± 19
11·82 ± 49
0·1298 ± 54
2675 ± 10
W427-6-1
480
188 0·39
410
10200
0·1840 ± 4
0·1142 ± 7
0·607 ± 24
15·39 ± 62
0·1768 ± 72
2689 ± 4
W427-9-1
120
93 0·78
91
1540
0·1876 ± 22
0·2361 ± 48
0·480 ± 19
12·42 ± 54
0·1459 ± 67
2722 ± 20
W427-9-2
289
207 0·72
186
1200
0·1829 ± 14
0·1449 ± 30
0·430 ± 17
10·85 ± 45
0·0871 ± 40
2679 ± 13
W427-10-1
80
44 0·55
87
912
0·1845 ± 20
0·1561 ± 43
0·708 ± 29
18·01 ± 78
0·2023 ± 100 2694 ± 18
W427-14-1 6044
727 0·12
4500
26200
0·1812 ± 1
0·0387 ± 1
0·708 ± 11
17·69 ± 28
0·2281 ± 37
2663 ± 1
W427-19-2
283
300 1·06
184
2900
0·1809 ± 6
0·2892 ± 13
0·505 ± 8
12·59 ± 21
0·1379 ± 24
2661 ± 5
W427-20-1
117
54 0·47
70
2800
0·1858 ± 11
0·1260 ± 21
0·520 ± 9
13·32 ± 24
0·1407 ± 34
2705 ± 10
W427-1-9
169
60 0·35
98
10700
0·1847 ± 7
0·0978 ± 11
0·523 ± 9
13·32 ± 23
0·1451 ± 30
2695 ± 6
W427-1-10
166
68 0·41
95
7120
0·1865 ± 7
0·1169 ± 12
0·504 ± 8
12·97 ± 22
0·1428 ± 29
2712 ± 6
zoned inner rims
W427-1-2
4191
634 0·15
1742
11600
0·1746 ± 2
0·0368 ± 2
0·536 ± 21
12·90 ± 52
0·1304 ± 53
2602 ± 2
W427-2-1
5291
933 0·18
1988
89600
0·1785 ± 1
0·0397 ± 1
0·484 ± 19
11·91 ± 48
0·1088 ± 44
2639 ± 1
W427-2-2
6952
1325 0·19
2828
7990
0·1754 ± 1
0·0384 ± 2
0·522 ± 21
12·63 ± 51
0·1052 ± 42
2610 ± 1
W427-4-1
5298
521 0·10
3404
198000
0·1733 ± 1
0·0276 ± 1
0·496 ± 20
11·86 ± 48
0·1394 ± 56
2589 ± 1
W427-4-2
2470
1280 0·52
1378
1440
0·1712 ± 3
0·0788 ± 6
0·399 ± 16
9·42 ± 38
0·0606 ± 25
2570 ± 3
W427-5-1
1501
359 0·24
956
39100
0·1732 ± 2
0·0726 ± 3
0·474 ± 19
11·31 ± 45
0·1438 ± 58
2589 ± 2
W427-6-2
2784
314 0·11
2163
18100
0·1787 ± 2
0·0245 ± 2
0·597 ± 24
14·71 ± 59
0·1298 ± 53
2641 ± 2
W427-7-1
1559
717 0·46
1293
5040
0·1754 ± 3
0·0805 ± 5
0·606 ± 24
14·65 ± 59
0·1062 ± 43
2609 ± 3
W427-8-1
1340
320 0·24
987
24000
0·1749 ± 3
0·0709 ± 4
0·548 ± 22
13·21 ± 53
0·1624 ± 66
2606 ± 3
W427-11-1 3882
606 0·16
2870
68800
0·1767 ± 1
0·0423 ± 1
0·562 ± 22
13·70 ± 55
0·1524 ± 61
2622 ± 1
W427-12-1 1689
389 0·23
995
4220
0·1748 ± 3
0·0579 ± 5
0·438 ± 18
10·54 ± 42
0·1100 ± 45
2604 ± 3
W427-13-1 3025
486 0·16
1546
27000
0·1746 ± 2
0·0424 ± 2
0·389 ± 16
9·36 ± 38
0·1026 ± 41
2602 ± 2
W427-15-1 1998
543 0·27
1135
25600
0·1768 ± 2
0·0739 ± 2
0·527 ± 8
12·84 ± 21
0·1433 ± 24
2623 ± 2
W427-16-1 2720
674 0·25
1615
75100
0·1763 ± 2
0·0676 ± 2
0·555 ± 9
13·48 ± 22
0·1514 ± 25
2618 ± 1
W427-17-1 2737
628 0·23
1448
18000
0·1714 ± 1
0·0715 ± 2
0·493 ± 8
11·66 ± 19
0·1537 ± 25
2572 ± 1
W427-18-1 1011
645 0·64
596
5800
0·1756 ± 3
0·1506 ± 5
0·512 ± 8
12·40 ± 20
0·1209 ± 20
2612 ± 3
W427-18-2 1242
367 0·30
577
2990
0·1737 ± 3
0·0257 ± 5
0·442 ± 7
10·58 ± 17
0·0385 ± 10
2594 ± 3
W427-19-1 2462
1043 0·42
922
772
0·1647 ± 4
0·0199 ± 8
0·342 ± 5
7·76 ± 13
0·0160 ± 7
2504 ± 4
W427-21-1 3165
1330 0·42
1813
1220
0·1748 ± 2
0·0672 ± 5
0·513 ± 8
12·36 ± 20
0·0821 ± 14
2604 ± 2
W427-1-7
2767
461 0·17
1610
14800
0·1765 ± 2
0·0342 ± 2
0·557 ± 9
13·55 ± 22
0·1144 ± 20
2620 ± 2
W427-1-8
2022
366 0·18
1130
67000
0·1763 ± 2
0·0483 ± 2
0·530 ± 9
12·88 ± 21
0·1414 ± 24
2618 ± 2
weakly zoned outer rims
W427-1-4
2686
99 0·04
1025
103000
0·1768 ± 2
0·0093 ± 1
0·505 ± 20
12·31 ± 49
0·1269 ± 53
2623 ± 2
W427-1-6
3116
81 0·03
1171
104000
0·1757 ± 2
0·0069 ± 1
0·498 ± 20
12·07 ± 48
0·1323 ± 56
2613 ± 2
W427-5-2
1170
29 0·02
849
15100
0·1749 ± 3
0·0067 ± 3
0·568 ± 23
13·70 ± 55
0·1542 ± 94
2605 ± 3
W427-17-2 2240
107 0·05
1236
71600
0·1764 ± 2
0·0134 ± 1
0·539 ± 9
13·10 ± 21
0·1514 ± 28
2619 ± 1
636
NEMCHIN AND PIDGEON
EVOLUTION OF DARLING RANGE BATHOLITH
Table 1: continued
Labels
U
Th
Th/U Total
(p.p.m.) (p.p.m.)
206
Pb/204Pb
207
Pb/206Pb∗
208
Pb/206Pb∗
206
Pb/238U∗
207
Pb/235U∗
208
Pb/232Th∗
Pb
207
Pb/206Pb∗
age (Ma)
W427-22-1 3625
152 0·04
2048
143000
0·1764 ± 1
0·0113 ± 1
0·553 ± 9
13·46 ± 22
0·1491 ± 26
2620 ± 1
W427-22-2 1765
138 0·08
898
4680
0·1762 ± 2
0·0083 ± 3
0·493 ± 8
11·99 ± 19
0·0527 ± 23
2618 ± 2
W427-1-11 2142
70 0·03
1111
43200
0·1767 ± 2
0·0084 ± 1
0·508 ± 8
12·38 ± 20
0·1308 ± 29
2622 ± 2
W390 aplite dyke
cores
W390-6-4
234
80 0·34
126
4960
0·1806 ± 5
0·0927 ± 8
0·485 ± 11
12·07 ± 29
0·1309 ± 33
2659 ± 5
W390-6-1
297
96 0·32
158
2660
0·1809 ± 6
0·0917 ± 11
0·476 ± 11
11·87 ± 28
0·1343 ± 35
2661 ± 5
W390-6-1a
443
82 0·18
244
23400
0·1826 ± 4
0·0493 ± 5
0·518 ± 11
13·04 ± 28
0·1388 ± 33
2677 ± 3
W390-11-2
864
237 0·27
372
10200
0·1810 ± 5
0·0750 ± 7
0·396 ± 9
9·89 ± 23
0·1083 ± 27
2662 ± 4
W390-1-1
624
120 0·19
332
25900
0·1814 ± 3
0·0545 ± 4
0·500 ± 11
12·50 ± 27
0·1412 ± 32
2666 ± 3
W390-1-2
987
252 0·26
544
27800
0·1813 ± 3
0·0695 ± 3
0·511 ± 11
12·78 ± 27
0·1392 ± 30
2665 ± 2
W390-1-3
897
169 0·19
494
17700
0·1813 ± 3
0·0513 ± 4
0·519 ± 12
12·96 ± 30
0·1414 ± 34
2665 ± 3
W390-2-2
951
43 0·04
489
17800
0·1815 ± 3
0·0111 ± 3
0·499 ± 11
12·50 ± 27
0·1226 ± 41
2667 ± 2
W390-6-2
464
168 0·36
274
23600
0·1944 ± 4
0·0991 ± 5
0·529 ± 11
14·19 ± 30
0·1447 ± 32
2780 ± 3
W390-6-3
442
174 0·39
256
9920
0·1950 ± 4
0·1067 ± 5
0·515 ± 12
13·84 ± 32
0·1394 ± 33
2785 ± 3
W390-12-1
642
366 0·57
407
1910
0·1953 ± 4
0·1586 ± 8
0·530 ± 12
14·27 ± 33
0·1474 ± 35
2787 ± 4
W390-12-2
184
57 0·31
112
4780
0·1956 ± 7
0·0832 ± 12
0·548 ± 13
14·76 ± 35
0·1464 ± 41
2789 ± 6
W390-9-2
483
261 0·54
337
7060
0·1960 ± 5
0·1504 ± 8
0·599 ± 14
16·19 ± 38
0·1668 ± 40
2794 ± 4
W390-3-3
382
443 1·16
270
9930
0·1981 ± 5
0·3098 ± 11
0·541 ± 11
14·79 ± 32
0·1447 ± 31
2810 ± 4
W390-9-1
233
219 0·94
174
737
0·1987 ± 11
0·2630 ± 25
0·557 ± 13
15·27 ± 38
0·1565 ± 40
2816 ± 9
W390-3-1
175
163 0·93
106
2980
0·2001 ± 8
0·2340 ± 17
0·485 ± 10
13·37 ± 30
0·1218 ± 28
2827 ± 7
W390-3-2
283
207 0·73
184
20300
0·1997 ± 5
0·1997 ± 9
0·538 ± 11
14·82 ± 32
0·1467 ± 32
2824 ± 4
old cores
zoned inner rims
W390-5-1
2606
701 0·27
1498
37900
0·1791 ± 1
0·0740 ± 2
0·533 ± 11
13·15 ± 28
0·1465 ± 31
2645 ± 1
W390-5-2
1708
461 0·27
930
78600
0·1789 ± 2
0·0723 ± 2
0·506 ± 11
12·48 ± 26
0·1356 ± 29
2643 ± 2
W390-4-1
530
103 0·19
280
72400
0·1791 ± 3
0·0539 ± 3
0·498 ± 11
12·29 ± 26
0·1383 ± 31
2644 ± 3
W390-5-4
1182
203 0·17
663
52400
0·1794 ± 2
0·0467 ± 2
0·532 ± 11
13·15 ± 28
0·1446 ± 31
2647 ± 2
W390-2-1
849
73 0·09
436
27300
0·1801 ± 3
0·0230 ± 3
0·496 ± 10
12·31 ± 26
0·1319 ± 33
2653 ± 3
W390-4-2
1288
431 0·33
747
42900
0·1802 ± 2
0·0904 ± 3
0·530 ± 11
13·16 ± 28
0·1431 ± 31
2655 ± 2
W390-10-2
612
335 0·55
343
3960
0·1805 ± 4
0·1310 ± 8
0·490 ± 11
12·19 ± 29
0·1170 ± 28
2657 ± 4
W390-10-3
488
210 0·43
290
19700
0·1796 ± 4
0·1191 ± 6
0·530 ± 12
13·13 ± 31
0·1471 ± 35
2649 ± 4
clear grains and outer rims
W390-10-1
448
196 0·44
261
1330
0·1764 ± 7
0·1194 ± 14
0·502 ± 12
12·22 ± 29
0·1369 ± 36
2619 ± 7
W390-11-1
658
91 0·14
328
6590
0·1745 ± 5
0·0333 ± 7
0·476 ± 11
11·45 ± 27
0·1149 ± 36
2601 ± 5
W390-7-2
293
50 0·17
155
5040
0·1766 ± 6
0·0457 ± 9
0·498 ± 12
12·12 ± 29
0·1339 ± 41
2622 ± 5
W390-7-3
243
47 0·19
132
4200
0·1771 ± 7
0·0532 ± 12
0·506 ± 12
12·35 ± 30
0·1394 ± 46
2626 ± 7
W390-7-4
176
28 0·16
87
2540
0·1779 ± 7
0·0387 ± 12
0·465 ± 11
11·41 ± 27
0·1131 ± 46
2633 ± 7
W390-14-1
788
263 0·33
446
27500
0·1778 ± 3
0·0909 ± 4
0·517 ± 12
12·67 ± 30
0·1410 ± 33
2633 ± 3
W390-14-2
717
256 0·36
410
10800
0·1774 ± 3
0·0986 ± 5
0·518 ± 12
12·68 ± 30
0·1430 ± 34
2629 ± 3
W390-9-3
514
129 0·25
320
11100
0·1780 ± 4
0·0696 ± 6
0·577 ± 13
14·16 ± 33
0·1596 ± 41
2634 ± 4
W390-13-1
348
54 0·15
187
6070
0·1782 ± 5
0·0412 ± 8
0·508 ± 12
12·49 ± 30
0·1354 ± 43
2636 ± 5
W390-13-2
271
140 0·52
103
487
0·1782 ± 17
0·0543 ± 35
0·320 ± 7
7·86 ± 20
0·0336 ± 23
2636 ± 15
W390-7-1
261
58 0·22
136
6870
0·1784 ± 5
0·0620 ± 8
0·485 ± 11
11·93 ± 28
0·1357 ± 37
2638 ± 5
metamict
W390-5-3
2570
733 0·28
1364
25400
0·1755 ± 1
0·0768 ± 2
0·491 ± 10
11·89 ± 25
0·1324 ± 28
2611 ± 1
W390-8-1
3129
956 0·30
1213
3000
0·1641 ± 2
0·0939 ± 4
0·352 ± 8
7·96 ± 18
0·1081 ± 25
2499 ± 2
W390-8-2
8686
2677 0·31
4337
2300
0·1595 ± 1
0·0993 ± 3
0·450 ± 10
9·90 ± 23
0·1451 ± 34
2451 ± 1
W390-8-3
2940
780 0·26
1485
3800
0·1741 ± 2
0·0837 ± 3
0·460 ± 11
11·04 ± 26
0·1453 ± 34
2597 ± 2
637
JOURNAL OF PETROLOGY
VOLUME 38
NUMBER 5
MAY 1997
Table 1: continued
Labels
U
Th
Th/U Total
(p.p.m.) (p.p.m.)
206
Pb/204Pb
207
Pb/206Pb∗
208
Pb/206Pb∗
206
Pb/238U∗
207
Pb/235U∗
208
Pb/232Th∗
Pb
207
Pb/206Pb∗
age (Ma)
W393 aplite dyke
clear cores
W393-12-1
83
46 0·56
49
2190
0·1796 ± 20
0·1459 ± 41
0·504 ± 9
12·49 ± 28
0·1318 ± 46
2649 ± 18
W393-7-1
577
58 0·11
311
7180
0·1796 ± 8
0·0253 ± 12
0·516 ± 8
12·78 ± 23
0·1299 ± 67
2650 ± 8
W393-12-2
101
63 0·63
61
2280
0·1801 ± 15
0·1675 ± 31
0·512 ± 9
12·72 ± 26
0·1368 ± 37
2654 ± 14
W393-10-2
467
271 0·58
274
6810
0·1801 ± 6
0·1498 ± 11
0·509 ± 8
12·64 ± 22
0·1313 ± 24
2654 ± 6
W393-11-2
779
360 0·46
461
8310
0·1812 ± 4
0·1214 ± 7
0·525 ± 8
13·11 ± 22
0·1381 ± 24
2664 ± 4
W393-1-3
370
155 0·42
213
7230
0·1813 ± 7
0·1142 ± 11
0·512 ± 8
12·82 ± 22
0·1394 ± 28
2665 ± 6
W393-9-1
355
138 0·39
211
4250
0·1815 ± 8
0·1002 ± 14
0·532 ± 8
13·30 ± 23
0·1369 ± 31
2666 ± 7
W393-18-2
407
182 0·45
238
6960
0·1821 ± 7
0·1183 ± 12
0·519 ± 8
13·03 ± 22
0·1373 ± 28
2672 ± 6
W393-8-1
1543
273 0·18
884
27600
0·1824 ± 3
0·0472 ± 3
0·541 ± 9
13·60 ± 22
0·1441 ± 26
2674 ± 3
W393-10-1
254
133 0·52
156
8450
0·1823 ± 8
0·1402 ± 13
0·538 ± 9
13·51 ± 24
0·1434 ± 29
2674 ± 7
W393-18-1
499
67 0·13
252
3830
0·1324 ± 8
0·0283 ± 14
0·498 ± 8
9·10 ± 17
0·1048 ± 56
2129 ± 10
W393-5-2
673
1480 2·20
896
73
0·1632 ± 27
0·2645 ± 62
0·587 ± 10
13·22 ± 32
0·0706 ± 20
2489 ± 28
W393-1-1
275
68 0·25
174
144
0·1710 ± 36
0·1519 ± 81
0·398 ± 7
9·33 ± 27
W393-11-1 1790
0·2416 ± 137 2568 ± 35
1412 0·79
1082
630
0·1718 ± 7
0·2436 ± 16
0·460 ± 7
10·89 ± 18
0·1419 ± 25
2576 ± 7
W393-14-1
411
408 0·99
268
133
0·1730 ± 33
0·1347 ± 74
0·401 ± 7
9·56 ± 26
0·0544 ± 32
2586 ± 32
W393-17-2
166
93 0·56
100
1920
0·1786 ± 14
0·1526 ± 29
0·512 ± 9
12·62 ± 25
0·1400 ± 37
2640 ± 13
zoned rims
W393-2-1
3492
1307 0·37
1463
6410
0·1470 ± 2
0·1060 ± 4
0·385 ± 6
7·81 ± 13
0·1091 ± 18
2311 ± 2
W393-2-2
5336
2117 0·40
2660
28600
0·1535 ± 15
0·1087 ± 2
0·458 ± 7
9·70 ± 16
0·1256 ± 20
2385 ± 2
W393-3-2
2003
132 0·06
763
5320
0·1546 ± 3
0·0252 ± 5
0·371 ± 6
7·91 ± 13
0·1427 ± 37
2397 ± 4
W393-5-1
3327
1220 0·37
1895
22300
0·1649 ± 2
0·0980 ± 3
0·523 ± 8
11·89 ± 19
0·1397 ± 23
2507 ± 2
W393-1-2
3147
1349 0·43
1523
20000
0·1659 ± 2
0·1160 ± 3
0·438 ± 7
10·01 ± 16
0·1185 ± 19
2517 ± 2
W393-17-1 2490
686 0·28
1130
7850
0·1666 ± 3
0·0758 ± 4
0·421 ± 8
9·68 ± 16
0·1160 ± 20
2523 ± 3
W393-3-1
1476
463 0·31
690
5670
0·1710 ± 4
0·0976 ± 6
0·424 ± 7
10·00 ± 16
0·1320 ± 23
2568 ± 4
W393-15-1 1251
703 0·56
846
397
0·1749 ± 9
0·0870 ± 20
0·545 ± 9
13·14 ± 23
0·0845 ± 24
2605 ± 9
W393-19-1
815
329 0·40
469
3890
0·1776 ± 5
0·1091 ± 9
0·513 ± 8
12·56 ± 21
0·1389 ± 26
2630 ± 5
W393-13-1 1809
656 0·36
1053
24400
0·1776 ± 3
0·0978 ± 4
0·529 ± 8
12·96 ± 21
0·1427 ± 24
2631 ± 2
W393-20-3 2008
530 0·26
1110
35800
0·1779 ± 3
0·0709 ± 3
0·514 ± 8
12·60 ± 20
0·1382 ± 23
2633 ± 2
W393-20-4 2287
667 0·29
1341
30100
0·1781 ± 3
0·0808 ± 4
0·540 ± 9
13·28 ± 22
0·1497 ± 26
2636 ± 3
W393-20-1 1379
314 0·23
742
13500
0·1783 ± 3
0·0611 ± 4
0·503 ± 8
12·35 ± 20
0·1349 ± 24
2637 ± 3
W393-12-3 1151
431 0·37
616
11600
0·1785 ± 3
0·0952 ± 5
0·486 ± 8
11·97 ± 20
0·1236 ± 21
2639 ± 3
W393-20-2 1663
689 0·41
937
1480
0·1789 ± 5
0·0740 ± 9
0·504 ± 8
12·43 ± 21
0·0900 ± 18
2642 ± 4
W393-19-2 1573
525 0·33
901
37500
0·1794 ± 3
0·0927 ± 4
0·523 ± 8
12·93 ± 21
0·1450 ± 24
2647 ± 3
clear grains
W393-4-1
961
136 0·14
517
17500
0·1751 ± 4
0·0391 ± 5
0·513 ± 8
12·39 ± 20
0·1422 ± 30
2607 ± 4
W393-16-1
713
60 0·08
394
10500
0·1771 ± 4
0·0211 ± 5
0·534 ± 9
13·04 ± 22
0·1332 ± 41
2626 ± 4
W393-6-1
128
49 0·38
75
1720
0·1775 ± 16
0·0970 ± 31
0·514 ± 9
12·58 ± 25
0·1312 ± 49
2630 ± 15
∗Corrected for Broken Hill common lead.
†All errors for individual analyses are given as 1r.
suggestions that recrystallization occurred after crystallization of the zircon, and that formation of the outer
rim is not due to corrosion and new zircon growth
during magmatic crystallization. In summary, the model
advocated here explains the zircon in the outer rim as
recrystallized inner rim material. Recrystallization takes
place after crystallization of the zircon in the completely
or almost completely crystallized granite magma at a
638
NEMCHIN AND PIDGEON
EVOLUTION OF DARLING RANGE BATHOLITH
temperature equal to or lower than the solidus
temperature. Similar outer rims were reported by Black
et al. (1986) for the zircons from Enderby Land in
Antarctica and explained as a result of granulite facies
metamorphism. However, the Darling Range granites
have not experienced metamorphism, or any visible
thermal overprint, since emplacement.
One objective of the SHRIMP study was to test the
validity and the significance of this interpretation of the
zircon structures, which contradicts present concepts of
zircon stability. Present models for the interpretation of
zircon ages in granites require that once zircon crystallizes
from a magma it retains its primary crystallization age
and can only be disturbed (to an older apparent age) by
the presence of inherited zircon or (generally to a younger
age) by a later isotopic disturbance related to weakening
of the zircon structure through time-integrated radiation
damage (e.g. Mezger & Krogstad, 1997). The high blocking temperature for the U–Pb system in crystallized
zircon, estimated as >900°C (Mezger, 1990), testifies to
the robustness of the U–Pb system in non-metamict
zircon.
SHRIMP II RESULTS ON INDIVIDUAL
SAMPLES
Space constraints prevent us from reporting all granite
zircon results. However, this section contains a description
of SHRIMP data and zircon morphology from samples
representing the following major rock types: one finegrained granite (W323), two coarse-grained granites
(W330 and W332), two porphyritic granites (W382 and
W427) and two aplite dykes (W390 and W393) (Table
1).
concordant. Cores show a spread in 207Pb/206Pb ages
from 2772 to 2607 Ma, compared with outer rim ages
which range from 2648 to 2584 Ma. Three analyses of
zircon cores with low 207Pb/206Pb ages can be explained
in terms of recent lead loss. Four analyses have high
207
Pb/206Pb ages compared with the other core analyses
and are interpreted as reflecting a separate population
of older zircons or as representing residual memory of a
much older event. These two possibilities cannot be
resolved from the present results. The mean age for most
of the analysed cores is 2665 ± 7 Ma. The 7 Ma
error is 1·5 times higher than the error expected from
uncertainties of individual analyses, suggesting that at
least some analyses were made on areas with mixed ages
or a disturbed U–Pb system. Nevertheless, 2665 ± 7
Ma is interpreted as the best estimate of the age of most
zircon cores from sample W323.
The 207Pb/206Pb ages for individual analyses of zircon
outer rims are evenly distributed within the range of
2584–2648 Ma with a mean of 2616 ± 13 Ma. The
error is much higher than expected from analytical
uncertainty. This difference can be explained on the
basis of our interpretation of the outer rim as recrystallized
inner rim. This interpretation suggests that differential
resetting of the U–Pb system results in a spread of
ages between the crystallization age and the age of
recrystallization.
The cores and outer rims have distinctly different
U–Th chemistry. U and Th concentrations in most cores
vary from 200 to 700 p.p.m. and from 140 to 500 p.p.m.,
respectively (Table 1, Fig. 6a). Two analyses on metamict
cores have very high U and Th contents. The outer rims
have a similar range of Th concentrations (80–400 p.p.m.)
to the clear zircon cores, but U concentrations are
significantly higher, ranging from 1500 to 2500 p.p.m.
Fine-grained granite sample W323 (32°18′S,
116°06′E)
Sample W323 is from an outcrop of medium-grained,
biotite granite with megacrysts of grey K-feldspar as well
as elongated areas, 2–3 cm by 0·5–1 cm, enriched in
biotite. Zircons have well-developed rounded cores and
complex rims. Most grains have cores of clear, possibly
recrystallized zircon with minor metamict areas, whereas
a few zircons contain largely metamict (high HF etched)
cores. Unfortunately, the inner rim of zoned zircon in
most crystals is relatively thin, <30 lm, and cannot be
analysed with SHRIMP. A well-developed highly serrated
unconformable boundary separates the oscillatory zoned
inner rim from the weakly zoned to unzoned outer rim.
SHRIMP analyses on clear cores and weakly zoned outer
rims are given in Table 1 and Figs 5a and 6a. On a
concordia plot (Fig. 5a) most data points are nearly
Coarse-grained granite sample W330
(31°57′S, 116°10′E)
Sample W330 is from an outcrop of coarse-grained,
biotite granite. Zircons from this sample contain wellpreserved central cores and inner rims. In contrast to
the previous sample, the outer zircon rims are usually
thinly developed along prismatic sections but are thicker
near crystal terminations.
SHRIMP results on cores, and inner and outer rims
are presented in Table 1 and Fig. 5b. In Fig. 5b, data
points are concordant or slightly discordant and show
small but consistent differences in age between the three
basic zircon subdivisions. The 207Pb/206Pb ages for the
most concordant analyses of cores are within the range
2694–2651 Ma (omitting one high uranium, reversely
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JOURNAL OF PETROLOGY
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NUMBER 5
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Fig. 5. SHRIMP concordia plots for zircons from the Darling Range Granite samples: (a) fine-grained granite W323; (b) coarse-grained granite
W330; (c) coarse-grained granite W332; (d) porphyritic granite W382; (e) porphyritic granite W427; (f ) aplite dyke W390; (g) aplite dyke W393.
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NEMCHIN AND PIDGEON
EVOLUTION OF DARLING RANGE BATHOLITH
Fig. 6. Th (p.p.m.) vs. U (p.p.m.) plots for zircons from the Darling Range Granite samples: (a) fine-grained granite W323; (b) coarse-grained
granite W332; (c) porphyritic granite W427; (d) aplite dyke W390.
discordant point on grain 15 and three high uranium
analyses on grain 5) with an average age of 2668 ± 10
Ma. After rejection of points 16-1 and 13-2, which are
statistical outliers, the average age is 2662 ± 5 Ma. This
error is similar to the expected error. SHRIMP spots
that were located on the boundary between core and
inner rim are characterized by higher U and Th concentrations and lower ages than analyses of pure zircon
cores (Table 1). Analyses of the zoned inner rims have
207
Pb/206Pb ages from 2643 to 2606 Ma (omitting analyses
for points 2-4 and 10-2) with an average age of 2629 ±
8 Ma. This error is also significantly higher than expected
from experimental error alone, suggesting the presence
of an additional factor contributing to the spread of the
SHRIMP ages. Three analyses of the outer rims show
207
Pb/206Pb ages of 2636, 2615 and 2602 Ma. The broad
‘non-normal’ spread of ages in the inner and outer rims
641
can be explained in terms of a continuum of 207Pb/206Pb
ages rather than two specific age events. It is suspected
that Pb loss from these zircon materials during recrystallization is incomplete and that it may be unrealistic
to expect the SHRIMP ages to define specific events.
The concentration of U in cores varies from 500 to
2600 p.p.m., which is generally higher than in zircon
cores from fine-grained granite sample W323 (Table 1).
Analyses representing mixed core–rim zircon (Table 1)
have U concentrations as high as 2780 p.p.m. Th concentrations of cores range from 55 to 655 p.p.m., with
one analysis (15-2) giving 1884 p.p.m., which is similar
to cores of zircons from the fine-grained granite sample
W323. The Th/U ratios and Th and U concentrations
in zoned inner rims (Table 1) are significantly higher
than cores, but U and Th contents of outer rims and
cores are similar.
JOURNAL OF PETROLOGY
VOLUME 38
Coarse-grained granite sample W332
(31°59′S, 116°05′E)
Sample W332 is from a medium-grained, biotite granite.
Zircons have cores composed of etched granular and
zoned zircon, unzoned (in HF) recrystallized zircon and
mixtures of the two. Zoned inner rims are well developed
in many grains but even so it was difficult to locate
SHRIMP spot-sized areas on zoned zircon that did not
show evidence of recrystallization. Outer rims of weakly
zoned zircon occur as thin borders, or as irregularly
shaped patches which strongly transgress zoned zircon
(Fig. 4f ). On a concordia plot (Fig. 5c) data points of
unzoned cores from nine grains are concordant and show
a spread in 207Pb/206Pb ages from 2649 to 2698 Ma
(omitting strongly discordant points for grains 2 and 8).
Two groups of ages can be distinguished within the
analyses of zircon cores. The younger group, which
includes analytical points 8-1, 3-1, 11-1, 12-1 and 19-1,
gives the average age of 2659 ± 5 Ma, whereas the
older includes the remaining five analyses and gives an
average of 2680 ± 3 Ma. The errors of these two
ages are in agreement with uncertainties from individual
analyses. Three measurements of zoned inner rims are
concordant and tightly grouped (Fig. 5c) with a 207Pb/
206
Pb age of 2626 ± 6 Ma. These data points overlap
those of the outer rims, which have a mean 207Pb/206Pb
age of 2622 ± 7 Ma. As previously described for results
from sample W330, data points for the inner and outer
rims of zircons from sample W332 appear to represent
an age continuum rather than an uncertainty distribution
around a single age. The concentrations of U and Th,
and the Th/U ratios of cores and inner rims are correlated
as shown in Fig. 6b. However, outer rims fall off this
trend, possibly owing to a lowering of the concentration
of Th.
NUMBER 5
MAY 1997
On a concordia plot (Fig. 5d) data points are concordant and grouped according to morphological subdivision. Most 207Pb/206Pb ages of cores are between
2650 and 2685 Ma (Table 1, Fig. 5d). However, some
conchoidally fractured cores, with relatively discordant
data points, have 207Pb/206Pb ages up to 2870 Ma. The
average 207Pb/206Pb age for the cores, excluding analyses
which show significant memory (5-1, 5-2, 9-3, 23-1), is
2668 ± 4 Ma. This error is similar to that expected
from analytical uncertainty alone. Zoned rims and clear,
transparent, weakly zoned grains show a spread of 207Pb/
206
Pb ages from 2625 to 2645 Ma, with the average 2638
± 3 Ma.
The range of concentrations of U and Th, and Th/U
ratios, in the cores is similar to that in zircon cores from
other analysed samples (Table 1). The zoned rims have
higher concentrations of both elements but a similar
range of Th/U ratios compared with the cores.
Porphyritic Logue Brook granite sample
W427 (32°52′S, 116°00′E)
Porphyritic granite sample W382 (31°51′S,
116°48′E)
Sample W427 is from a grey porphyritic granite with
1–2 cm megacrysts of K-feldspar. Internal structures of
zircons from the two porphyritic granite samples W427
and W382 are similar except that outer rims are much
better developed on zircons from sample W427. Data
points for zircons from sample W427, presented on a
concordia plot (Fig. 5e and Table 1), are more discordant
than those from sample W382. However, cores show a
similar spread of 207Pb/206Pb ages from 2657 to 2722
Ma, with a mean of 2680 ± 8 Ma. The 8 Ma error is 1·5
times higher than the error expected from the analytical
uncertainty. The inner rims in this sample are strongly
affected by recrystallization, and the geological significance of SHRIMP analyses of this inner rim zircon
is doubtful. More concordant data from zoned rims and
clear, transparent, weakly zoned rims show a spread of
207
Pb/206Pb ages from 2570 to 2641 Ma with an average
of 2613 ± 5 Ma, which is similar to the age determined
by Compston et al. (1986). However, seven analyses of
outer rims are concordant within the errors and have an
average age of 2618 ± 5 Ma. An average calculation
using concordant analyses gives an age of 2613 ± 8 Ma
for the inner rims, which is indistinguishable within error
from the outer rims. The extensive recrystallization of
zircon in this sample raises the possibility that ages
determined on the inner zoned rims are minimum ages
for crystallization of the inner rims. Nevertheless, the
distribution of Th and U in the cores and rims is similar
to that in zircons from other granite samples (Fig. 6c).
Cores are low in U and Th whereas inner rims have
higher concentrations of these elements. Outer rims have
Th concentrations similar to cores and U concentrations
similar to inner rims (Fig. 6c).
Sample W382 is from a medium-grained, biotite granite
with megacrysts of pink K-feldspar. Zircons have conchoidally fractured cores surrounded by generally thin rims
of oscillatory zoned zircon. Some cores contain remnants
of zoned zircon undergoing replacement by clear zircon
(Fig. 3e). Other cores consist of clear zircon broken into
irregular domains bordered by conchoidal fractures and
curved zones of finely striated zircon. Cores are commonly surrounded by an inner rim of zoned zircon.
However, some grains appear to be composed entirely
of fractured zircon identical to that present as cores in
other grains. Also present are a number of clear, weakly
zoned zircon grains that are not obviously related to
specific parts of more complex grains. Outer rims are
poorly developed on zircons from this sample.
642
NEMCHIN AND PIDGEON
EVOLUTION OF DARLING RANGE BATHOLITH
Aplite dyke sample W390 (32°22′S, 116°58′E)
Sample W390 is from an aplite dyke which intrudes
coarse-grained granite. Most zircons consist of clear, or
sometimes metamict, cores overgrown by a zoned inner
rim. Outer weakly zoned rims are rare and generally
developed as patches on only one side of the grains.
These grains are similar to those observed in zircons
from the major granite types and are interpreted as
inherited. Elongated grains, with length/breadth ratios
up to ten, and relatively simple morphology, are also
common. Some of these grains are metamict and apparently high in U, as revealed by HF etching. Most of
the elongated grains are unzoned or weakly zoned.
SHRIMP analyses are generally concordant (Fig. 5f ),
except for high-U metamict grains. Two ages for cores
are clearly identified. Nine data points show a distribution
of 207Pb/206Pb ages from 2780 to 2827 Ma (Table 1)
with an average of 2801 ± 12 Ma. Eight analyses are
distributed within the range 2659–2677 Ma (Table 1)
and have an average 207Pb/206Pb age of 2665 ± 4 Ma.
Strongly zoned inner rims show a narrow spread of
207
Pb/206Pb ages from 2643 to 2657 Ma with an average
of 2648 ± 4 Ma. Outer rims and clear elongate grains
are indistinguishable in age distribution and Th–U geochemistry (Fig. 6d), and were combined into one group
with a spread of 207Pb/206Pb ages from 2601 to 2638 Ma
with an average of 2628 ± 7 Ma.
DISCUSSION
Time span of events recorded in the
zircons
The proportion of cores to rims
The cores present in all zircons represent either inherited
restite from the granite precursor or zircon developed at
an early stage in the formation of the granite magma.
This is distinct from the oscillatory zoned zircon that
formed around cores during magma crystallization. We
have estimated the amount of core to rim in zircons from
the studied granites by measuring lengths and widths of
30 zircon grains and their cores from each sample.
Assuming zircon shapes are ellipsoidal and that polished
grains represent half crystals, the proportion of cores to
rims can be calculated as
V r/V c=R(34pAB 2 − 34pab 2)/R34pab 2
where V r and V c are the volumes of rim and core, A and
B are the length and width of a zircon crystal, and a b
are the length and width of the core.
Overall the ratio of core to rim volumes in any zircon
population is between 1:2 and 1:4; indicating that significant zircon existed in the granite magma before
crystallization. This zircon is considered to be restite
derived from the immediate granite source rocks. The
presence of zircon constrains the composition and conditions of the source, which must be saturated in Zr, but
is not suitable for redissolving zircon under conditions of
extreme metamorphism and melting (see later discussion
on constraints on the temperature of the granite magma).
Aplite dyke sample W393 (31°42′S, 116°35′E)
Unlike the previous aplite sample (W390), which was
collected from a dyke <2 m thick, sample W393 represents a relatively large body of at least 20–30 m width
where sampled in a road cutting. Zircons from this aplite
contain cores similar to those described for major phases
of the granite. Cores are overgrown by an oscillatory
zoned rim. The outer weakly zoned rim is not developed
as a continuous layer around the grains, but is represented
by irregularly shaped weakly zoned areas in the oscillatory
zoned rim.
SHRIMP analyses of clear cores show a distribution
of 207Pb/206Pb ages from 2649 to 2674 Ma (Table 1),
averaging 2663 ± 7 Ma (Fig. 5g). Nearly concordant
analyses of zoned rims show a spread of 207Pb/206Pb ages
from 2630 to 2647 Ma (Table 1) with an average age
of 2633 ± 8 Ma (Fig. 5g). Three analyses of clear grains
give ages of 2607, 2626 and 2630 Ma. Approximately
half of the zoned rim analyses are strongly discordant
(Table 1) and have been excluded from the calculation.
U concentrations in cores are generally <700 p.p.m.,
whereas in zoned rims they are 1000–5000 p.p.m. (Table
1).
Age distribution within cores
The 80 most concordant analyses of zircon cores were
combined and plotted on a concordia plot (Fig. 7a) and
a 207Pb/206Pb age histogram (Fig. 7b). These data show
a skewed distribution (Fig. 7b). Most points are distributed
between 2690 and 2650 Ma, although cores in some
grains show older 207Pb/206Pb ages (Fig. 7).
The geological significance of the core ages depends
on the stability of the U–Pb system in zircons that
have survived conditions of magma formation and a
subsequent residence period in granitic magma. This is
a different question from that of the stability of zircon
itself, which has been discussed in detail by Watson &
Harrison (1983). The robustness of the U–Pb systems of
zircon in granite magmas has been described by Gulson
& Krogh (1973) and also in a number of SHRIMP
studies. In a study of the origin of the S-type Cooma
granite, in the Lachlan Fold Belt of SE Australia, Williams
(1995) found that the age distribution of detrital zircons
from metasediments, proposed as a source of the granite,
was exactly reproduced by xenocryst zircon cores in the
granite itself. A SHRIMP study of zircon from four
643
JOURNAL OF PETROLOGY
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NUMBER 5
MAY 1997
Fig. 8. Zircon grain from the porphyritic granite sample W382. Four
SHRIMP analyses show a wide spread of 207Pb/206Pb ages within the
same core, which supports resetting of the U–Pb system in the cores.
Beam impact points appear as dark round spots.
Fig. 7. SHRIMP data for zircon cores from the Darling Range granite
samples: (a) concordia diagram; (b) histogram plot of 207Pb/206Pb ages.
samples of Scottish Caledonian granites (Pidgeon &
Compston, 1992) also found that cores have concordant
ages and provide a robust memory of their Proterozoic
source rocks. These studies suggest that zircon xenocrysts
in granite magmas will retain a strong memory of their
original age. Therefore, the simplest interpretation of the
spread of core ages obtained for the Darling Range
granite samples is that they reflect the different ages of
the granite source rocks. The age population of ~2·8 Ga
obtained for zircon cores in the granite dyke sample
W390 appears to be concordant, and suggests that the
age of one of the source rocks is close to 2·8 Ga. Ages
distributed in the interval of 2650–2690 Ma may date a
second and younger source of the granites. This interpretation is in agreement with the suggestion of
Compston et al. (1986) that the granite magma was
formed by mixing of older crust with younger mantlederived crust. However, the range 2650–2690 Ma can
be broadly subdivided into two groups. The first has
207
Pb/206Pb ages between 2650 and 2670 Ma with a
maximum at 2660 Ma, whereas the second shows the
range from 2670 to 2690 Ma with the maximum at
about 2680 Ma. This suggests the possible existence of
two younger sources for the Darling Range granite, with
ages of ~2660 and ~2680 Ma, respectively.
An alternative explanation for the core ages is suggested
by the observation that data points for zircon cores with
high 207Pb/206Pb ages are distributed along a reverse
discordia line (Fig. 7). This suggests that the U–Pb systems
in zircon cores have not remained closed, but represent
strongly isotopically disturbed zircons of an unknown but
older age. The observation that different 207Pb/206Pb ages
occur in the same core (see, e.g. W382 grain 9 in Fig.
8, and also W382 grain 5, W427 grains 1, 3 and 9, and
W390 grain 6 in Table 1) is a strong argument for this
interpretation. If the range of 207Pb/206Pb ages in zircon
cores reflects different source ages, different 207Pb/206Pb
ages should not be found in one zircon core because it
is hard to infer that two parts of the same core represent
different sources. Consequently, the occurrence of different ages within the same core in some zircon grains
suggests that the distribution of the zircon core ages
represents almost complete resetting of an original U–Pb
system. The two interpretations of the core ages presented
above propose significant differences in the ages of the
sources and early formation history of the granites. Present SHRIMP precision is not sufficient to resolve the
question of whether core ages can be explained as dating
the granite parent rocks, or reflect different degrees of
resetting of U–Pb systems from a single parent during a
younger event, such as a prograde metamorphism leading
to partial melting and formation of granite magma. In
this case, the actual time of resetting could be close to
the time of first zircon crystallization from the granite
melt, which is represented by the oscillatory zoned inner
rim.
644
NEMCHIN AND PIDGEON
EVOLUTION OF DARLING RANGE BATHOLITH
The inner rims
Metamict high-U areas in the zircons
The inner rims of oscillatory zoned zircon are interpreted
as magmatic zircon formed during crystallization of the
granite magma. The age of 2628 ± 7 Ma for clear
elongate grains from the aplite dyke which cuts the
coarse-grained granite (sample W388), is interpreted as
the emplacement age of the dyke and provides a minimum age for the crystallization of the inner rims in the
coarse-grained granite. As noted earlier, the 2613 ± 8
Ma age obtained for the inner rim of zircon from
porphyritic granite sample W427 is suspect in that it may
reflect late-stage recrystallization. Therefore, the most
probable range of crystallization of the Darling Range
granites is considered to be 2648–2626 Ma (age of inner
rim in sample W332). The precision of the data and the
possible influence of recrystallization Pb loss on some
samples limits our ability to refine the age of crystallization
any further, and to determine whether crystallization
occurred over the entire 20 Ma interval or whether it
took place over short stages within this interval. It is also
impossible to determine whether there is a difference in
crystallization age for different types of granitoids.
Areas of granular, highly HF etched zircon with concentrations of U of [3000 p.p.m. are found in cores
and zoned inner rims. The 207Pb/206Pb ages of this zircon
type are distributed between 2670 and 2400 Ma, with
most data points lying along the concordia curve. Some
of these areas of high-U, granular zircon have been
described by Pidgeon & Nemchin (in preparation) as
representing secondary movement and concentration of
U and Th in zircon during progressive recrystallization.
Outer rims
The average ages of outer rims of the Darling Range
granite zircons are distributed within the range 2628–
2616 Ma. Our interpretation from observations of the
internal structures is that this zircon recrystallized from
the magmatic inner rim and the younger age of this
zircon reflects this process which took place late in
the crystallization history of the granite. The U–Th
compositions of the outer rims provide further support
for this origin, as, compared with the inner oscillatory
zoned rims, the outer rims generally have slightly lower
U and significantly lower Th concentrations, which are
similar in range to the unzoned cores. A strong relative
decrease in Th was described by Pidgeon (1992) as
characteristic of changes in Th–U chemistry during zircon recrystallization. This contrasts with the expected
increase in Th and U concentrations of zircon crystallizing in the late stages of a granite magma. This
evidence, together with structural evidence discussed
above, supports our interpretation that the outer rims
formed by recrystallization resulting from a reaction that
progressed inwards from the grain margins. It is suggested
that this reaction was activated by the penetration of late
magmatic solutions into the zircon crystals during cooling.
As this occurred late in the crystallization history of the
granite the susceptibility of the zircons to recrystallization
and Th loss cannot be based on long-term radiation
damage, but is explained by the proposal of Sommerauer
(1974) and Pidgeon (1992) that the stability of zircon is
strongly dependent on the trace element content.
U–Th–Pb geochemistry of zircons
Although uncertainties in the determinations of U and
Th concentrations are of the order of ±20% it is still
possible to identify systematic relationships in the concentrations of U and Th between the different parts of
the zircons (Fig. 6). Unetched cores have the lowest
concentrations of U, from 200 to 600 p.p.m., with rare
cores over 1000 p.p.m., except for sample W330 where
U varies from 600 to 2000 p.p.m. Th concentrations
range from 35–285 p.p.m. in unzoned cores of zircons
from the porphyritic granite sample W382 to ~1500–
5000 p.p.m. in the metamict cores of zircons from coarsegrained sample W332. Th/U ratios of cores vary from
0·2 to 1·5, except for one low-Th–high-U and discordant
core (grain 5, W330). Zoned inner rims have 1000–4000
p.p.m. of U and 300–6000 p.p.m. of Th, with the
exception of one grain from W332 which had a Th
concentration <100 p.p.m. However, Th and U concentrations for zoned inner rims apparently lie on an
extension of the trend for unzoned cores, and Th/U
ratios in inner rims are similar to or slightly higher than
in cores. Metamict cores in fine- and coarse-grained
granite samples are characterized by even higher concentrations of U and Th and also lie close to the same
trend. Outer rims for samples W323, W388 and W332 are
definitely outside these trends and have concentrations
of U close to that of zoned parts of grains, whereas
concentrations of Th are similar to or even lower than
in unzoned cores.
The Th/U ratios in both metamict and recrystallized
cores are of the order of 0·5. A major change in chemistry
is observed in the surrounding zoned inner rims, which
contain higher concentrations of both elements relative
to clear cores and an increase in Th/U in some samples.
Further changes occur in the Th–U concentrations of
outer rims, where the Th concentration dramatically
decreases whereas U concentrations remain similar to
those of zoned zircon.
645
Constraints on the temperature of the
granite magma
Experiments reported by Harrison & Watson (1983) and
Watson & Harrison (1983) show that zircon saturation
JOURNAL OF PETROLOGY
VOLUME 38
NUMBER 5
MAY 1997
Table 2: Calculated M-values [M = (Na + K + 2Ca)/(Si × Al)] and temperatures of melt
saturation in zircon for Darling Range granites
Sample:
Si
Al
Ca
Na
K
Zr
ln D Zrzircon/melt
M
T (°C)
Coarse-grained granite
W317
69·9
14·84
2·05
3·69
4·05
232
7·67
1·45
816
W331
72·6
14·13
1·41
3·29
5·01
165
8·01
1·39
790
W332
70·3
14·74
1·96
3·47
4·08
269
7·52
1·39
834
W328
72·3
14·12
1·66
3·44
4·43
185
7·90
1·40
799
W330
70·7
14·84
2·13
3·70
3·89
240
7·64
1·44
820
W330
72·8
13·76
1·55
2·94
4·50
214
7·75
1·32
818
W386
73·8
14·01
1·22
3·40
4·79
93
8·58
1·34
745
W387
74·0
14·06
1·57
3·61
3·89
89
8·63
1·33
742
W388
74·1
13·71
1·23
3·39
5·05
143
8·15
1·39
777
mean
7·98
793
SD
0·40
33
Fine-grained granite
W325
69·9
14·35
2·59
3·55
4·00
145
8·14
1·58
765
W324
72·2
14·73
1·94
4·04
3·77
157
8·06
1·43
782
W335
72·0
14·05
2·27
2·97
4·14
237
7·65
1·41
820
W384
74·7
13·56
1·34
3·84
3·93
102
8·49
1·36
751
W380
73·8
14·05
1·46
3·86
3·97
116
8·36
1·36
761
W321
73·1
14·20
1·66
3·48
4·28
145
8·14
1·37
780
W323
72·9
13·58
1·76
3·74
4·09
121
8·32
1·48
757
W326
74·3
13·13
1·15
3·27
4·5
157
8·06
1·34
789
W337
72·3
14·34
1·84
3·60
3·67
165
8·01
1·35
792
mean
8·13
778
SD
0·23
20
Porphyritic granite
W382
68·5
15·67
3·38
4·31
3·23
152
is a function of both temperature and granite composition.
The cation ratio M = (Na + K + 2Ca)/(Al × Si) was
selected as characteristic of melt composition, and the
ratio of Zr in stoichiometric zircon to that of the melt
was related to absolute temperature and M as
8·09
1·45
778
ln D Zrzircon/melt = {–3·80–[0·85(M–1)]} + 12 900/T.
of 780°C, which is within the same range. These are
maximum temperatures for the granite magma, as the
calculations do not take into account zirconium in the
zircon cores. Nevertheless, these results suggest that the
temperature of the Darling Range granite melt did not
exceed ~800°C for a period of time sufficient to dissolve
zircon cores, otherwise rounded cores observed in zircon
grains would have been lost.
A temperature of 800°C may have been exceeded if
the H2O content of the magma was significantly less than
2% (Harrison & Watson, 1983). However, this is not
considered likely for the Darling Range granites, as there
is no evidence that this was originally a ‘dry’ magma.
The granitoids contain 3–8% biotite, and Clemens &
Vielzeuf (1987) and Clemens (1990) showed that, depending on water content in the source rock, the volume
of melt at 800°C can reach 30–40% at 5 kbar and 20–30%
at 10 kbar. This agrees with numerous experimentally
determined estimates of the temperatures of crustal melts
It is assumed in this model that zircon saturation is
not significantly dependent on pressure. Harrison &
Watson (1983) showed that where the H2O content in
the melt is >2%, there is no effect on the solubility of
zircon, but, where the H2O content of the melt is <1%,
a significant decrease in zircon saturation occurs. Calculated temperatures for Zr concentrations determined
in this study are shown in Table 2. Both the finegrained and coarse-grained granite have a similar range
of calculated temperatures averaging 778 ± 20°C and
793 ± 35°C (1 SD), respectively. The calculated temperature of the porphyritic granite (W382) has a value
646
NEMCHIN AND PIDGEON
EVOLUTION OF DARLING RANGE BATHOLITH
(e.g. Wyllie, 1977; Clemens & Vielzeuf, 1987; Le Breton
& Thompson, 1988; Rutter & Wyllie, 1988; Vielzeuf &
Holloway, 1988; Thompson, 1990; Vielzeuf et al., 1990)
and with our estimate of magma temperature from the
Watson & Harrison (1983) model.
Model for the evolution of the Darling
Range batholith
The source of the granites
The age or ages of the source is a primary issue regarding
the origin of granitoids in the batholith. Sm–NdCHUR
model ages of ~3·0 Ga on whole-rock samples of granites
from the batholith, and the reason the CHUR model
was selected to characterize the source of the granites,
have been discussed by Fletcher et al. (1994). The ~3·0
Ga CHUR age has been confirmed by unpublished
Sm–Nd results of A. Nemchin (1996). However, we have
not been able to confirm this age from our SHRIMP
zircon measurements, where the oldest zircon ages on
unzoned cores are near 2·8 Ga. This may indicate that
the Sm–Nd model ages are not meaningful, or that the
source rocks were in fact ~3·0 Ga old, but no zircons of
this age existed, or that isotopic memory of this age in
zircon cores has been lost. Whereas ages as high as 2·8
Ga are recorded in some zircon cores most core ages
fall in the range 2690–2650 Ma. One explanation is that
these are the true ages of the granite source material,
which is in accord with previous experience of the robustness of zircon core ages (e.g. Williams, 1992; Pidgeon
& Compston, 1992). A second explanation is that the
observed ages represent resetting of the zircon U–Pb
systems from an older source, which may be as old as
3·0 Ga. Almost complete resetting of core ages has not
been reported before but this would be in accord with
the Sm–Nd results. Unfortunately, uncertainties in the
SHRIMP analytical results are too high to permit a
resolution of these alternatives.
On the basis of the partition coefficients (Mahood &
Hildreth, 1983) and granite zircon cores with average
concentrations of U (from 200 to 600 p.p.m.) and Th
(from 30 to 300 p.p.m.), the concentrations of U and Th
in melt coexisting with these zircon cores are expected
to be between 0·7 and 2·1 p.p.m. for U and 0·4 and 3·8
p.p.m. for Th. These concentrations are 2–5 times lower
than the observed concentrations in the granite samples
(A. Nemchin, unpublished data, 1996), suggesting that
cores could not have been in equilibrium with the granite
melt. The low model U and Th concentrations, calculated
from the content of U and Th in zircon cores and the
reported partition coefficients, approximate those found
in tonalitic rocks (Martin, 1994), which may be close to
the composition of the granite source. The U–Th core
data therefore give support to the proposition that the
observed zircon core ages date the granite source rocks.
Magmatic crystallization
Magmatic crystallization of oscillatory zoned inner rims,
enclosing unzoned zircon cores, occurred between 2648
and 2626 Ma. This stage of granite evolution represents
a change of conditions from Zr undersaturation to oversaturation.
Oscillatory zoning commonly observed in magmatic
zircons reflects inhomogeneities in trace element concentrations within the zircon and absence of equilibrium
between the mineral and melt. Nevertheless, the average
U and Th concentrations of inner zoned rims may
still reflect the partition coefficients for these elements,
allowing estimates to be made of average partition coefficients for Th and U between the zircon and granite
melt. Average concentrations of U and Th in the coarsegrained granite are estimated from chemical analyses
(Nemchin, unpublished data, 1996) as 8·73 p.p.m. and
30·7 p.p.m., respectively, and calculated average partition
coefficients (for inner rims from zircons of samples W330
and W332) are 280 ± 140 (SD) for U and 78 ± 52
for Th. These results are comparable with partition
coefficients between zircon and felsic melt reported by
Mahood & Hildreth (1983) as 383 ± 4 for U and 91·2
± 0·6 for Th (early rhyolite), and 298 ± 4 for U and
62·4 ± 5 for Th (late rhyolite).
Late recrystallization of zircon and redistribution of U and
Pb
The last stage of the zircon evolution is characterized by
formation of weakly zoned to unzoned zircon outer rims.
These are marked by a decrease in Th concentration
but not U relative to the inner rims as discussed above.
The outer rim is interpreted to have formed from partial
recrystallization of existing zircon in the solid state. The
common occurrence of outer rims on the granite zircons
and the nature of its inner boundary suggests that the
outer rim formed by a recrystallization reaction proceeding inward from the surface. This was probably
facilitated by the penetration of late-stage granite fluids.
This conclusion has implications for models for the
evolution of the batholith and for interpretation of zircon
ages. The ages of outer rims from all granite samples are
younger than the emplacement of the aplite dykes (2628
± 7 Ma) into the solid granite, indicating that recrystallization of zircon to form outer rims continued in
the solidified granite.
Late recrystallization structures have been observed in
zircons from all granite samples. Pidgeon (1992) described
similar recrystallization structures in zircons from a late
granite in granitic gneiss and from a felsic dyke, elsewhere
647
JOURNAL OF PETROLOGY
VOLUME 38
in the Yilgarn Craton. However, the occurrence of latestage zircon recrystallization in granites throughout the
Darling Range Batholith has genetic implications that
have yet to be assessed. One explanation is that granitoids
of the Darling Range Batholith were deep-seated intrusions, emplaced at temperatures not greatly different
from that controlled by the geothermal gradient, and
that temperatures of ~600°C were maintained for a
sufficient time interval for partial recrystallization of
zoned zircon to occur on a regional scale.
NUMBER 5
MAY 1997
batholith, or are relatively common in Archaean batholiths, is an important avenue for future research.
CONCLUSIONS
Internal structures in zircons from the Late Archaean
Darling Range Batholith record complexities not previously recognized in granite zircons and provide a unique
opportunity for dating various stages in the evolution of
the granite using SHRIMP. Results reveal a history
of granite evolution, from magma formation to postcrystallization cooling, over a time span of ~60 Ma.
Many zircons have unzoned centres, which in some
cases represent rounded cores. Such cores have rare
discordant ages up to 2860 Ma and concordant U–Pb
ages of 2800 Ma (in zircon cores from a granite dyke),
but most ages fall within the range 2690–2650 Ma.
These ages may date the original granite source rocks or
alternatively they may represent the profound resetting
of U–Pb systems of zircons inherited from a single older
source, during formation of the granitic magma at ~2650
Ma.
Zircon cores are enclosed within an inner rim of
oscillatory zoned zircon which is attributed to growth of
zircon during cooling and crystallization of the granite
magma. Ages of inner rims fall within the range 2648–
2626 Ma. Inner rims are surrounded by outer rims of
weakly zoned zircon, which are of variable thickness and
have an irregular transgressive boundary with inner rims.
Formation of outer rims is attributed to recrystallization
of the outer parts of the magmatic zoned zircon by
inward penetration of granite fluids during late-stage
crystallization of the granite magma. Recrystallization is
accompanied by loss of radiogenic Pb and Th. SHRIMP
ages of 2628–2616 Ma for the outer rims demonstrate
continued fluid activity and date final closure of the
recrystallized zircon and consolidation of the granite
magma as the batholith cools. The formation of outer
rims indicates instability of the zircon structure under
conditions of granite cooling. One explanation is that
the granites were emplaced under deep-seated conditions
at temperatures not greatly different from the geothermal
gradient. The question of whether secondary structures
observed in zircons from the Darling Range Batholith
are unique, indicating a special cooling history for this
648
ACKNOWLEDGEMENTS
We thank Dr G. Vavra (ETH, Zurich) for cathodoluminescence imagery of a number of zircon samples.
We are also grateful to Drs N. Oliver, S. Wilde, D.
Nelson, F. Corfu and Yu. Amelin for discussion and
comments on versions of this manuscript. The paper
also benefited from constructive comments by Drs I. S.
Williams, S. D. Weaver and A. Ewart. This work was
supported by an Overseas Postgraduate Research Scholarship to A. Nemchin.
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