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Lead ages, reset rubidium-strontium ages and implications for the Archaean crustal evolution of the Diemals area, Central Yilgarn Block, Western Australia
0016-7037183105907.0X$03.00/0
Lead ages, reset rubidium-strontium ages and implications for the Archaean crustal evolution of the Diemals area, Central Yilgarn Block, Western Australia M. J. BICKLE, H. J. CHAPMAN, L. F. BETTENAY, D. I. GROVES Department of Geology, University of Western Australia, Nedlands. Western Australia 6009 and J. R. DE LAETER School of Physics and Geosciences, (Received
Western April
Australian
Institute
14, 1982; accepted
of Technology,
in revised Jtirm
South
February
Bentley,
Western
Australia
6 102
3, 1983)
Abstract-The Pb-Pb whole-rock geochronology of Archaean granitic and gneissic rocks from the Diemals area in the Central Yilgam granite-greenstone terrain provides important constraints on crustal evolution. The regionally extensive banded gneisses, previously considered as candidates for basement to the greenstones give a Pb-Pb whole-rock age of 2700 f 97 Ma (20 errors). This is within error of previously published Rb-Sr and Sm-Nd gneiss ages and also within error of the Sm-Nd ages on the greenstones in the Eastern Goldfields Province. Two synkinematic plutons give Pb-Pb whole-rock ages (2737 + 62 Ma and 2700 f 100 Ma) and Pb isotopic compositions consistent with the hypothesis, based on field and geochemical relations, that these plutons were derived by partial melting of the precursors to the banded gneisses. Assuming this, the combined data date the melting event at 2723 + 25 Ma with a model source p value of 8.18 * 0.02. This source fi value is close to the range postulated for mantle values and restricts the crustal history of the precursors to less than -200 Ma. A post-kinematic pluton with a whole-rock Pb-Pb age of 2685 f 26 Ma and p value of 8.26 * 0.02 puts a younger limit on this relatively short lived crustal accretion-differentiation event. Comparison of Pb-Pb and Rb-Sr whole-rock dates for the plutons suggests that the latter became closed systems up to 200 Ma after the Pb-Pb ages, and that the plutons gained or lost Rb or Sr at this time.
GEOLOGY
INTRODUCHON
AN UNDERSTANDING of the evolution of Archaean granite crust is fundamental for interpretation of Archaean tectonics. This in turn requires a knowledge of the sources, timing and mechanisms of addition of new plutonic material to the crust. In this paper we report whole-rock Pb-Pb isotopic analyses and some new Rb-Sr isotopic analyses for a variety of gneissic and plutonic bodies from the central part of the Archaean Yilgarn Block in Western Australia. This follows a Rb-Sr isotopic study of these rocks (CHAPMAN et al., 198 1). The Pb isotopic data provide relatively stringent tests for models of the genetic evolution of the granitic crust and are consistent with the hypothesis that syn-kinematic granodiorite-adamellite plutons were derived by partial melting of banded gneiss precursors (BETTENAY, 1977). The whole-rock Pb-Pb ages define a relatively short continental accretion-differentiation event between -2800 Ma and -2650 Ma consistent with recent Sm-Nd studies (MCCULLOCH et al., in press), but shorter than that inferred from Rb-Sr isotopic systems (COOPER et al., 1978; CHAPMAN et al., 1981). The new and previously published Rb-Sr ages are up to -400 Ma younger than the whole-rock Pb-Pb ages and often have relatively high initial 87Sr/86Sr ratios. The discrepancy between Pb-Pb and Rb-Sr ages is tentatively explained as due to later closure or resetting of Rb-Sr isotopic systems on the whole-rock scale. 907
The Diemals area (Figs. I and 2, discussed by CHAPMAN et al..1981), is situated centrally within the ca. 10” km2 Archaean Yilgam Block. It comprises a typical granitegreenstone terrain, similar in many respects to the better studied Norseman-Wiluna belt to the east (GEE e/al..198 1). Much of the granitic and gneissic crust within the Diemals area, and indeed the Yilgam Block, is rather poorly exposed and inferences on pluton boundaries and structural relationships require correlation of detailed observations made on small (
M. J. Bickle rr al.
908
q
Fractionated
++t ii+ 0+.+ :::: :::: Cl ElII + t+ + El 0
le”coadamell,,e
Post-klnematlc discrete
Syn-
granttads
k,“ema,lc
Banded
QranltoldS
gnmss
and
m~gmahle
Concealed banded gnens Sparse outcrop dom!“ated klnematlc phases
.’
formlng
plutons
and m,gmat,te by DOS,-
Fault
0
Sample
+
Sample
sate site
rthls
Study)
(McCulloch,
Compston
8
I
Froude)
N 0
100
km
FIG. I. Interpretative regional map ofgranitoids in the southern part of the Eastern Goldfields Province showing distribution of banded gneiss-migmatite complexes (from ARCHIBALD et al..1981). Geochronology sample sites for this study are: 1.Cockatoo Rocks (8 1898, 87922A. 87922B. 87927.87928, 87936) 2. N. Koolyanobbing (8 1885), 3. E. Koolyanobbing (81884), 4. W. Split Rocks (81962, 8 1975), 5, 6, 7 & 8 W. Lake Johnson-5. (82030), 6. (82023, 82024). 7. (82025) 8. (82026) 9. E. Lake Johnson (82034, 83035) and 10. Riverina (82167). Geochronology sample sites for the study of MCCULLKH et al (in press) are: I I. Pioneer Dome (3 samples), 12. Connolly Siding (2 samples), 13. Kirgella Rock-hole (3 samples).
**++.. .**t*. ..+**_ ..**+* */**t* ****t* ++.*+, **.. .**,+ .*++. .I++. .*+A..
1
+++.*.
*++a_*
COCKATO ROCK - 25 k yj
:::
, .A
-KEN :: ROC ..+...._,._ . ..+._ . ..*.. -..** . . . .
i... .. .. .. .. et.. 1... ,t... , *+.. et.. ,,.. **.
FIG. 2. Geological map of the Diemals area, Central Yilgarn Block compiled from unpublished University of Western Australia. Samples sites for Rb-Sr and Pb isotope studies shown.
data,
Pb-Pb
ages of Diemals
mellite in composition, was emplaced. These were followed by widely dispersed, but minor, post-kinematic plutons with distinctive fractionated compositions (BETTENAY, 1977). The prime objectives of the geochronology were to test the relative chronologies of plutonic suites as deduced from field relations, to test the synchroneity of plutonic events over large areas of the Central and Eastern Yilgam Block (700 km X 400 km) and to determine the duration of the tectonometamorphic evolution described above, especially given the possibility that multiple and unconnected events may be superimposed in any area. Critical and controversial topics include the status of structurally complex gneisses as candidates for a sialic basement to the greenstone sequences and the possibility of more than one cycle of greenstone belt evolution (GL~KSON, 1979). Detailed geochronological studies in the Yilgam are few. MCCULL~CH and COMPSTON (198 I) and MCCULL~CH et al. (in press) using Sm-Nd whole-rock methods dated greenstones in the Kambalda sequence at 2790 -C 30 Ma (2~ errors) and samples of banded gneisses from three nearby localities at 2800 + 100 Ma. OVERSBY (1975) reported whole-rock and mineral Pb-Pb dates on a number of synand post-tectonic plutons from the Kalgoorlie-Norseman area (ages between 2632 * 28 Ma to 2699 f 75 Ma). Two areas that have been studied in sufficient detail to provide a chronology based on Rb-Sr data are the AgnewLawlers-Perseverance area about 350 km north of Kalgoorlie (RODDICK et al., 1976; PLATO et al., 1978; CARPER et al., 1978) and the Diemals area (CHAPMAN et al.. 1981). The Rb-Sr chronologies and tectonic evolution in both areas are broadly comparable (Table 1). The greenstones in both areas and banded gneisses within and south-east of Diemals are thought to have been formed at cn. 2800-2700 Ma although this date is rather poorly constrained. Both areas were intruded by now highly foliated, extensive granodiorite-adamellite and lesser tonalite plutons (syn-kinematic granitoids of ARCHIBALD ef al., 1978) with Rb-Sr wholerock ages between 2670 to 2600 Ma and initial “Sr/*‘Sr ratios between 0.701 to 0.702. Intense deformation and regional metamorphic events occurred during and after the intrusion of the foliated plutons. These were followed in both areas by intrusion of largely unfoliated post-tectonic plutons with Rb-Sr ages of ca 2550 Ma. Late post-tectonic plutons with Rb-Sr ages of -2470 Ma are also recognised in both areas. In the Diemals area, some of the younger post-tectonic plutons have high initial 87Sr/86Sr ratios (0.7 l0.72). Although the sequence of ages in both areas is similar, it should be remembered that errors on the Rb-Sr ages are generally ca. ?50 to ? 100 Ma and that correlation of plutonic events based primarily on deformation state need not
Table
1.
Tectonic Event Late postKinematic Plutans
909
have chronological implications, especially between areas separated by several hundred kilometers. The Pb isotopic analyses reported in this study were initiated to investigate the limited resetting of Rb-Sr systematics proposed in the Diemals area on the basis of inconsistencies between geological relationships and the isotopic ages (CHAPMAN et al., 1981). As discussed below, discrepancies between wholerock Pb-Pb and Rb-Sr ages of up to 200 Ma are recorded and require substantial revision of the tectonic chronology. ANALYTICAL
TECHNIQUES
Most samples were those analysed and described by CHAPMAN ef al. (I 98 1). Sampling techniques and analytical methods for new Rb-Sr data quoted below are as described by CHAPMAN ef al. (198 1). Precision on Rb/Sr ratios (XRF analyses) is estimated at fl% (20) and on 87Sr/86Sr ratios as f.05%. Ages are calculated using XRb = 1.42 X lo-” yrr’ and quoted with 2~ errors. Lead was separated from -0.15 g of rock-powder by the electro-deposition technique (ARDEN and GALE, 1976). Lead isotopic analyses were made on one of two 30.5 cm radius of curvature, 90” magnetic sector solid-source mass spectrometers at the West Australian Institute of Technology, using the silica-gel loading technique on single rhenium filaments and run at 12OO”C- 1250°C. Hydrocarbons were monitored at mass 205 and generally burnt off very rapidly. On the alpha machine, signals from the collector were amplified by a vibrating-reed electrometer before being digitized and processed by on-line computer. For each sample -60 to 100 cycles were made using an automatic peak-switching cycle 208-207-206-204-206-207-208 with background measured half a mass unit below mass 204. Tailing measured at mass 205 was less than 1 part in 104. Fourteen measurements of NBS standard 98 I during 1980 and early 198 1 gave values of Zo8Pb/Z06Pb = 2.1487 f 0.10% (2 s.d.) “‘Pb/ 2”Pb = 0.9 113 f 0.08% and 204Pb/2”Pb = 0.05949 + 11%. Seven measurements of NBS 981 on the alpha machine during August I98 1, after the computer and peak-switching sequence had been updated to switch 204-208-204-207204-206-204-206-204-207 etc. gave isotopic ratios within error of these. On the beta machine signals were measured using a Faraday cup collector. The peak switching routine was similar to the later routine on the alpha machine as above. Tailing at half mass offsets was about 1 part in 104. Four measurements of NBS 98 1 gave mean values of *“Pb/ rmPb = 2.1622 f 0.08% 207Pb/z”Pb = 0.9 I34 -+ 0.04% and zo4Pb/206Pb = 0.059 16 -+ 0.17%. Duplicate analyses of samples implied a reproducibility of better than 20.15% (20) and after mass discrimination corrections no systematic bias
Rb-Sr Chronologies of the Diemals and Lawlers-Agnew Perseverence Areas Diemals Area Kelly5 Rocka
Rbl;; age 8;;;;gg;r
2467?72
0.719$?4
Late cataclasis over S.E. Diemals Area Post-kinematic Plutons
area
M"ky
Main Phasea 2557198 0.7099?80 Soak N. Outcrops 25501166 0.7040?39
Lawlers-Agnew-Perserverence Area Leucotonaliteb Pegmatite‘
Rb/Sr age lnitlal Ma 87~r/86sr 2474i14 2481'18
0.7019tl 0.7253*2
2576?14 2588118
0.7022~2 0.762t7
Lawlers Tonaliteb Perseverence Gneissb Mt Keith Granodiorite
2652?20 2625'34 2632~17
0.70152?12 0.7022t5 0.70149?15
Kathleen Valleyb Granophyre Gabbro
2718t50
0.7007'4
Deformation* 02 Lawlers L ucotonaliteb Pegmatite% First Deformatlond D1
Main Deformationsa 01(?).02,D3 Syn-kinematic Plutons
West Pigeon Rockse Plgeon Rockse Rainy Rock@
2475?76 2575256 2612'74
0.721t65 0.7123285 0.7014~22
Pre-kinematic Granitolds
South-east Yilgarn' banded gne~ss
2678'90
0.7021~10
Greenstones
Mat-da Felsic vo1can1csf
2635i80
0.703t2
Sources of data: (a) Chapman et al. (1981); (b) Cooper et al. (1978); (c) Roddick et al. (1976); (d) Platt et al (1978); (e) this work; (f) Hallberg et al. (1976); (g) Chapman et al. (1981) recalculated without sample L&25 from Cockatoo Rocks.
910
M. J. Bickle et (I/
was noted between samples analysed on both the alpha and beta machines. Total Pb blanks generally ranged from 510 ng, although in the early part ofthis work some Pb blanks were up to 25 ng. Duplicate analyses of USGS standard BCR- I gave 2osPb/204Pb = 38.665, 207Pb/204Pb = 15.613 and r”Pb/204Pb = 18.8 I I, within error of the values quoted by ARDEN and GALE(~ 976). Regression analysis was by the method of YORK (1969) with the quoted modification to allow for scatter of points outside analytical error. A correlation coefficient of 0.6 between errors of 0.15% (20) on both 2”7Pb/204Pb and z”6Pb/ 204Pb was used in the regression program. Ages were calculated with the decay constants of STEIGER and JAEGER (1977) and errors quoted at 20 level. The age and initial Pb isotopic composition of the Earth adopted were those of TA-~SLIMO~Oe/ a/ ( 1973). Errors on model source p values are estimated from the error on the intercept of the isochron with the geochron, calculated by the YORK (1969) program with the geochron as the ?’ axis.
LEAD
ISOTOPE RESULTS AND COMPARISON WITH Rb-Sr GEOCHRONOLOGY
(u) Banded grwisses
The banded gneisses represent the oldest granitic crust recognised within the Yilgarn granite-greenstone terrains (i.e. outside the Western Gneiss Terrain: GEE 6’1a/., 198 1) and enclaves of banded gneiss are found both in the foliated “syn-kinematic” plutons as well as the “post-kinematic” plutons (BETTENAY, 1977). Lead isotopic compositions of these rocks are listed in Table 2 and ages in Table 6. The whole-rock Pb isotopic analyses of the banded gneisses from the south-east Yilgarn (Fig. 3) show more scatter (mean squared weighted deviate, henceforth M.S.W.D., =8) than most of the other whole-rock isochrons, which is perhaps not surprising given the spread of outcrops sampled (Fig. 1) and their complex history. The whole-rock Pb age of 2700 * 97 Ma is similar to their whole-rock Rb-Sr age (2678 f 90 Ma, M.S.W.D. = 23, initial “Sr/“%r = 0.7021 f 10). This is within error of the Rb-Sr and Sm-Nd whole-rock ages of 2780 + 60 Ma, initial “Sr/*%r = 0.7007 I 5 and 2800 ? 100 Ma respectively on similar but geographically more restricted banded gneisses collected by L. F. Bettenay and N. J. Archibald (McCu~~oct~ pt al., in press: see Fig. 1). These ages
c
Banded
/ ,/'
"'Pb "'+Pb
,
*
,i'
16.
I :’ c
15
i*
.’
/’
South-east Age Jl
2700 8.17*
MSWD
8.
Yllgarn * 97 Ma 05
8
206Pb/*04Pb 16
16
20
22
FIG. 3. Whole-rock Pb-Pb isochron of banded from South-Eastern Yilgarn localities in Fig. I.
gneisses
are within error of the best available constraints on the ages of the greenstones (c.~.. MCCULLO(‘H and COMPSTON, I98 I) and. as discussed by MP&ILLO(‘H ct al., (in press), the geochronology is not sufficiently precise to establish if the gneisses are older than the greenstones. The model source 23R1J/204Pb ratio (henceforth fi value) for the gneisses is 8. I7 ? .OS and thus close to likely mantle values (see discussion in MOORBATH and TAYLOR, precursors
1981).
Derivation
of the gneisses more than
-200
of the
Ma prior
to 2700 Ma would seem improbable. (b) Foliated svn-kinematic
plutons
Two syn-kinematic plutons were studied in the RbSr geochronology program. Rainy Rocks is an intensely lineated granodiorite-adamellite outcrop marginal to a strongly deformed greenstone belt (Fig. 2). Ten whole-rock samples at Rainy Rocks define a Pb-Pb age of 2700 ? 100 Ma, M.S.W.D. == 12, model source fi value = 8.20 ? 0.07 (Table 4, Fig. 4) within error of its Rb-Sr age of 2612 t 74 Ma, initial *‘Sr/%r = 0.7014 + 22. M.S.W.D. = 13. The second pluton, Pigeon Rocks intrudes deformed greenstones in the central part of the green-
'04Pb
Table 2 Lead Isotonic Camnositions of Banded Gneisses
.,’
Rainy Rocks
17. 207pb
Collection NO. Sample
Gneisses
Age JJ
2700t100
/
Ma
8.20 * 07
MSWDIZ
LohPb/204Pb Lo7Pb/204Pb 2oxPb/'o"Pb
South East Yilgarn Banded Gneisses a1884 a1885 81962 81975 82023 82024 82025 82026 82030 82034 62035 82167
LB282 LB221 LBSP6 LB384 LB374 LB372 LB371 LB364 LB367A LB359 LB360 LB1676
14.428 14.459 15.876 17.800 16.822 16.363 15.463+79 14.534 15.471 20.234 16.369 22.859+69 _
14.983 14.981 15.193 15.529 15.416 15.296 15.118+76 14.980 15.150 16.077 15.343 16.439+49
37.470 33.954 37.202 37.196 38.661 37.976 35.3845248 35.718 35.563 38.663 44.155 35.968+72
ei-rors quoted where analytical precision is less good than + .15% (2 x standard error).
16.
//
,’ , 16
et. ’
7’
./*
/
i
206Pb1204Pb 18
FIG. 4. Whole-rock Rainy Rocks (Fig. 2).
20
22
Pb-Pb
isochron
2.
of samples
26
from
Pb-Pb
ages of Diemals
Table 4
Table 3 Rb and Sr Contents and isotopic Compositions of New Pigeon Rocks Samples Collection NO.
Field NO.
Rb
Sr
B7Rb/BbSr
B'Sr/BbSr
HC85 HC86 HC87A HC87B HC88 HC89 HC90 HC91 HC92 HC93 HC94
251 3M 325 293 266 264 270 269 298 279 296
79.5 63.6 52.9 60.1 74.1 75.7 80.7 66.4 80.3 81.6 86.3
9.44 14.46 19.19 14.91 10.79 10.57 10.05 12.20 11.15 10.26 10.35
1.06480 1.25442 1.42537 1.26613 1.12185 1.15620 1.08589 1.16654 1.12484 1.09210 1.10123
18 40 34 12 25 25 40 22 27 12 30
Lower Rb/Sr Samples - Western Outcrop 94163 94164
HC98 HC99
267 279
72.4 80.9
Higher Rb/Sr Samples 94160 94161 94162 94i65
HC95 HCY6 HC97 HClOO
388 330 316 336
15.5 13.9 25.6 22.4
11.12 10.27
1.13176 27 1.08866 24
Western Outcrop 97.62 91.57 41.52 51.16
4.21611 4.00504 2.21850 2.52853
18 43 17 28
Whole-rock Samples Between Pegmatites 94167 9416E 94165 9417c 94171
HCllAa HCllBb HCllCc HCllEe HCllFf
327 402 399 432 320
18.8 18.4 19.2 17.6 18.3
61.25 81.72 76.50 94.44 61.86
Lead Isotopic Compositions Collection NO.
2.92160 3.66329 3.48545 4.08689 2.96768
44 94 84 75 42
stone belt (Fig. 2). Samples were collected from relatively unfohated adamellite away from the contact zone. All outcrops at Pigeon Rocks are cut by prominent sets of pegmatites, but whole-rock samples were collected at least 1-2 m away from obvious pegmatite veins. The Pigeon Rocks Pluton comprises an earlier foliated medium- to coarse-grained granodiorite/adamellite intruded by a slightly finer grained granodiorite/adamellite. A sharp intrusive contact between these phases is evident at a few clean outcrops but over much of the area distinction between these phases is equivocal. Where contact relationships are observed Rb/Sr ratios are distinctive with the older suite characterised by lower Rb/Sr ratios and higher Sr contents (Table 3). Multiple intrusive phases showing variable deformation characteristics are a feature of synkinematic plutons (ARCHIBALD and BETTENAY, 1977). Both suites have similar Pb-Pb ages. Ten samples of the low Rb-Sr suite define a wholerock Pb-Pb isochron. age 2737 + 62 Ma M.S.W.D. = 2.9, with a model source F value of 8.16 f .10. Nine samples of the high Rb-Sr suite define a wholerock Pb-Pb isochron age 2682 k 53 Ma, M.S.W.D. = 2.0, with a model source p value of 8.36 +- .04. The Pb isotopic compositions of the two suites are clearly distinct (Fig. 5) and this is reflected by the difference in model source p values. CHAPMAN et al. ( 198 1) speculated that the Rb-Sr systematics in the Pigeon Rocks outcrop were perturbed after emplacement of the plutons. Twelve additional whole-rock samples of the low Rb-Sr suite (Table 3 excluding HC89) combined with the ten samples analysed by CHAPMAN et al. ( 198 1) define a Rb-Sr age of 2575 f 56 Ma, M.S.W.D. = 5.0 initial R7Sr/86Sr = 0.7 123 ?Z 85 (Fig. 6) significantly younger than the whole-rock Pb-Pb age of 2737 f 62 Ma on the same suite. Four samples of the high Rb-Sr suite
of Synkinematic
Sample 206Pb/204Pb Plgeon Rocks (Lowr
t2:r
Lower Rb/Sr Group - Main Outcrop 94149 94150 94151 94152 94153 94154 94155 94156 94157 94158 94159
911
area
87301 87902 87903 87904 67305 87906 87907 67908 87909 87910
tlc1* HCZ* tlc3* HC4* tlc5* HC6* HC7* HCa* tlc9* HClO*
207Pb/2"4Pb
Plutons
*08pb/L04pb
Rb-Sr samples)
23.096 23.253 21.775 28.192 23.148 24.137 24.520 25.763 24.380 22.667
lb.564 16.624 16.342 17.540 16.575 16.769 16.841 17.111 16.859 16.510
39,486 39.441 39.269 39.929 39.876 39.172 40.075 40.175 39.935 39.681
Pigeon Rocks (Higher Rb-Sr samples) 94160 94161 94162 94165
HC95 HC96 HC97 HCIOO
26.324 23.789 26.026 24.710
17.237 16.801 17.179 16.946
36.627 36.081 39.016 37.280
Plgeon Rocks Adjacent to Pegmatites and Pegmatite sample (HCllH) 879110 87911Gd 87911Gc 87911C 87911Gb 87911H
HCllD HCllGd HCllGc HCllC HCllGb HCllH
87963 87965 87967 87969 87971 67973 87975 a7377 87979 87981
HC63 HC65 HC67 HC69 HC71 HC73 HC75 HC77 HC79 HC81
28.410 22.694 23.339 25.032 22.371 24.645
17.606 16.557 16.676 16.983 16.498 16.853
36.823 35.806 36.156 35.909 35.723 34.934
15.676 16.007 15.544 15.766 15.648 15.728 15.820 17.057 16.196 16.021+24
40.564 35.406 38.317 39.650 35.747 40.122 39.481 36.924 39.574 40.099+62
Rainy Rocks 18.242 20.185 17.319 18.701 18.077 18.292 19.355 25.550 20.862 20.275+30
* analysis duplicated
define a Rb-Sr whole-rock age of 2475 2 76 Ma. M.S.W.D. = 4.5, initial 87Sr/86Sr = 0.721 f 65 (Table 3: Fig. 7). The other samples of the high Rb-Sr suite were collected within 0.5 m of two pegmatites (see Fig. 3 in CHAPMAN et al., 1981) to assess the influence of the pegmatites on the isotopic systems within these rocks. Nine whole-rock samples from between the pegmatites (Table 3) including the six samples analysed by CHAPMAN CJI al. ( 198 1) define a Rb-Sr age of 2268 f 143, M.S.W.D. = 3. I, initial 87Sr/8”Sr = 0.99 i 0.16 (Fig. 7). Two samples which fall below the 2268 Ma isochron lie very close to the line defined
Pigeon Plgeon Age
"'Pb
Jl
'04Pb
Rocks
2682r 8.36
MSWD
Rocks (higher RblSr) 53
Ma
f 04 2
17.0~ (lower Age
Rb/Sr)
2737'62
Ma
J/ 8.16i 10 MSWD 2.9
.
16.5.
Pigeon Rocks D Samples adiacent to pegmatltes A Pegmatlte 206Pb/204Pb
.? ./I /' 22
FIG.5. Whole-rock Pigeon
Rocks
(Fig. 2).
26
24
Pb-Pb
isochrons
28
of samples
from
M. J. Bickle et al. Pigeon
Age
Pb-Pb age of 2682 f 53 Ma is 0.53 + 0.05. The smaller spread of the Pb-Pb ages from all of the synkinematic plutonic suite is further evidence consistent with later closure or resetting of the Rb-Sr systematics.
Rocks
2575t56
Ma
lnttlal "Sr/"Sr MSWD 5
0.7123t85
;.’ /
.,-
/
* ,.’
.a’
(c) Post-kinematic
./
/’ 0.8
I r
/’
"FIblasS,
,’
5
10
20
15
FIG. 6. Rb-Sr isochron plot of whole-rock low Rb/Sr samples from Pigeon Rocks. Includes data from CHAPMAN et al. (198 1) and this study (Table 3). Sample HC89 not included in regression.
by the four high Rb-Sr samples and are apparently not affected by the alteration adjacent to the pegmatites. For the altered samples adjacent to the pegmatites the discrepancy between the Rb-Sr and PbPb age is plausibly a result of intrusion of the pegmatite or of fluid channelled along the pegmatite and it is of interest that this had little or no effect on the U-Pb systematics. The discrepancies between the whole-rock Pb-Pb and Rb-Sr ages on the suites collected away from the pegmatites may be due to later closure or resetting of Rb-Sr isotopic systems on the whole-rock scale. If so, this was accompanied by extensive gain of Rb and/or loss of Sr as initial s7Sr/86Sr ratios recalculated to the Pb-Pb ages, on the assumption that the average Rb and Sr contents of the plutons remained unchanged, are lower than possible mantle values. The inferred 87Sr/86Sr ratios of the low Rb/Sr suite at its Pb-Pb age of 2737 + 62 Ma is 0.687 + 0.0 13 and the inferred 87Sr/86Sr ratio of the high Rb-Sr suite at its
-8'Sr 86%
West Pigeon Rocks 8 Samples Adjacent to Pegmatites
.P
“A
0 Adjacent lo pegmatltes * West Pngeon Rocks
plutons
The Milky Soak outcrop is a weakly foliated postkinematic porphyritic granodiorite and adamellite. It contains distinctive enclaves of strongly lineated granodiorite very similar to that from the syn-kinematic Rainy Rocks outcrop. Two sample sites on the nothern margin of the outcrop contained such enclaves of deformed granodiorite within the main phase. The whole-rock Rb-Sr results on ten samples of the main phase of Milky Soak define a well-fitted isochron (M.S.W.D. = 3.0) of 2557 -t 98 Ma, initial 87Sr/86Sr = 0.7099 + 80 (Table I). Six samples from the two northern sample sites lie on a parallel isochron (M.S.W.D. = 13) of 2550 ? 166 Ma, initial 87Sr/86Sr = 0.7040 f 39 (Table 1). Fifteen whole-rock samples, incorporating both phases, define a well-fitted Pb-Pb isochron, M.S.W.D. = 1.9 (Table 5, Fig. 8) age of 2685 f 26 Ma with a source p value of 8.26 5 .02. The mean 87Sr/86Sr ratio at 2685 t 26 Ma, the Pb-Pb age, for the main phase would have been 0.699 f ,003 and for the lower Rb/Sr group of northern samples would have been 0.70 1 + ,001. These results are consistent with derivation of Milky Soak granitic rocks from a source isotopically similar to the mantle at 2685 -+ 26 Ma and final closure of Sr isotopes on the whole-rock scale at -2550 Ma. CONSTRAINTS (a)
ON CRUSTAL
EVOLlJTION
Tectonic chronology
The Pb-Pb ages require that granitic plutonism in the Diemals area took place within a period of - 150 Ma and possibly substantially less (Table 6). The relatively precise age of 2685 f 26 Ma for the postkinematic pluton sampled at Milky Soak places a minimum age on the main deformation events and metamorphic peak conditions of greenstones and syn-kinematic plutons. Rb-Sr mineral ages (biotite,
A Pegmatmte Ad,acent to pegmatltes Age
2268*143
Table 5
Ma
lnltla, 87Sr/%,
Lead Isotopic Compositions of Post-kinematic Plutons
0 99
Collection Sample 206Pb/zo4Pb 207Pb/204Pb zo8Pb/204Pb NO.
I’
Milky Soak
,'
*‘Rb/=Sr 2 40
60
80
100
FIG. 7. Rb-Sr isochron plot of high Rb-Sr samples from West Pigeon Rocks compared with high Rb-Sr samples adjacent to pegmatite. The two least radiogenic samples from the latter suite are not included in the regression calculation (see text).
87948 87949 87950 a7951 87952 87953 87954 87955 87956 87957 87958 87959 87960 87961 87962
HC48 HC49 HC50 HC5lA HC52 HC53 HC54 HC55 HC56 HC57 HC58 HC59 HC60 HC61 HC62
18.130 18.668 19.270 17.362 17.117 24.354 17.841 21.569 24.849 20.542 22.404 19.367 23.746 23.949 22.669
15.686 15.813 15.889 15.558 15.507 16.832 15.625 16.326 16.934 16.116 16.442 15.911 16.736 16.761 16.512
39.557 38.637 37.120 37.787 36.865 39.523 36.942 40.936 42.073 38.416 39.587 37.760 39.255 40.104 39.628
Pb-Pb ages of Diemals area Milky
Soak
Age 2665
well within error of the south-east Yilgam banded gneisses Pb-Pb isochron (Fig. 9). The combined south-east Yilgam gneiss/Pigeon Rocks low Rb-Sr suite/Rainy Rocks data give an age of 2723 ? 25 Ma (M.S.W.D. = 7) with fi = 8. I8 + .02 and it is plausible that this dates the major crustal melting event postulated by BINNS et ul. (1976) and ARCHIBALD et al. ( 1978). In addition, the relatively low Pb isotopic ratios of the gneisses are consistent with the low U
./ .I .’
f 26 Ma
)_I 626 * 02 MSWD 1 9
.*’
/” 9’
/
.,’
913
/’
.
contents
expected
if the gneisses
are depleted
in the
more mobile elements by removal of the melts parent to the syn-kinematic plutons. The model source p value of 8.18 +- .02 is close to the range thought typical of the mantle (e.g., MOORBATH and TAYLOR, FIG. 8. Whole-rock Milky Soak (Fig. 2).
Pb-Pb isochron of samples from
or a subsequent
thermal
isotopic
event
reset
mineral
and
systems.
Some of the younger Rb-Sr ages (2576 + 14 Ma and 2474 -t 14 Ma) from the Agnew-Lawlers-Perseverence region 250 km north-east of Diemals have low initial
87Sr/RhSr ratios systematics
would be of interest method. chrons
perturbation
in the Pigeon
of the to-
Rocks
pluton
it
to test these ages by an alternative
The Pb isotope of OVERSBY
(COOPER et ~1..
-0.702
1978). but given the apparent tal Rb-Sr
whole-rock
and mineral
iso-
( 1975) on a Series Of post-tectonic
plutons in the Kalgoorlie-Kambalda-Norseman region give ages between 2600 Ma and 2700 Ma synchronous with the Diemals plutons and suggesting that the plutonic episode occurred over a minimum extent of -500 km.
BEIXNAY (1977) inferred from the compositions and field relations of the banded gneisses that they may have been the source for both the syn-kinematic and post-kinematic plutons: such models are discussed by ARCHIBALD et al. (1978: 1981). The Pb isotopic data presented here provide confirmation of this hypothesis. The Pigeon Rocks low Rb-Sr suite and the Rainy Rocks Pb isotopic compositions lie
Table
Pb-Pb
6.
Chronology
of the
Pb-Pb age Fla
pvrnt
Pigeon Rocks Pigeon Rocks Rainy Rocks
(high Rb/Srl (low Rb/Sr)
Combined Gne~ss. Ra,ny Rocks, Plgeon South-East
to the banded short crustal
gneisses
must
residence
prior
to 2800
plagioclase and K-feldspar \‘ersus whole-rock) of -2540. -2460, -2360, -2350 and -2040 and - 1536 Ma are recorded by CHAPMAN et al. (198 1). Either the cooling and uplift history was protracted whole-rock
198 1) and the precursors have had only a relatively
Ylloarn
Rocks Gnelss
Ma as also suggested by McCulloch et al. (in press) on the basis of Sm-Nd and Rb-Sr data. The Pb isotopic compositions of the high Rb/Sr Pigeon Rocks suite and the post-kinematic Milky Soak pluton are clearly distinct from those of the gneisses and syn-kinematic plutons and this difference is reflected in the model source CCvalues of 8.36 f .04 and 8.26 t .02 respectively. The range in model source p values may result from heterogeneity in crustal or mantle sources. OVERSBY (1975) concluded from a study of five plutons in the KalgoorlieKambalda-Norseman area that crustal sources as old as 3300 Ma were necessary to explain initial Pb isotopic compositions of the plutons as inferred from K-feldspar Pb isotopic compositions. This possible heterogeneity of crustal sources may well be of significance to the evolution of this Archaean crustal segment but resolution of this problem will require further work. CONCLUSIONS Lead isotopic analyses of suites of plutonic samples from the Diemals area in the central Yilgam Block limits the formation of this segment of Archaean crust to one crustal accretion-differentiation event between about -2800 Ma and -2650 Ma. The Pb isotopic compositions of two suites of syn-kinematic plutons are consistent with their derivation from crustal sources of which banded gneisses represent residues of partial melting as suggested by BETTENAY ( 1977). The combined Pb isotope data date this event at 2723 +- 25 Ma. The model source p value of 8. I8
Olemals
MSWD
Area
Compared
Model source
L.
with
Rb-Sr
Ages
Rb-Sr age Ma
Initial 87 Sr/%
MSWD
0.7099'80 0.7040139
3.0 13
X85*26
19
8.X*.02
2557'98 2550'166
2682'53 2737~62 270nt100
2.0 2.9 11
8.36+.04 8.161.10 8.20'.07
2475‘76 2575'56 ?612'74
0.721?65 0.7123'85 0.7014?22
4.5 5.0 13
2723r25
7
8.18t.02
2700+97
8
F3.17+.05
2678!90
fl.7021'10
23
M. J. Bickle et al.
914
207Pb ?04Pb
ARCHIBALD N. J., BETTENAY L. F., BICKLE M. J. and GROVES D. I. (198 I) Evolution of Archaean crust in the
Banded Gneiss 8. Synkinematic Plutons
i
Eastern Goldfields Province of the Yilgarn Block, Western Australia. In Special Publ. tieol. Sot. Australia, No.
Age 2723
f 25 Ma pMSWD 8.18 7 * 0.02
7,491-504. ARDEN J. W. and GALE N. H. (1976) New electrochemical technique for the separation of lead at trace levels from natural silicates. Anal. Chem. 46, 2-9. BETTENAY L. F. (1977) Regional geology and petrogenesis of Archaean granitoids in the Southeastern Yilgarn Block,
I. 25
20
15
FIG. 9. Whole-rock
30
Pb-Pb regression
banded gneiss from south-east kinematic plutons at Rainy Growth curve for w = 8.18.
on samples of Yilgarn localities and synRocks and Pigeon Rocks.
is consistent
with a relatively limited prior crustal residence of the precursors to the gneisses in agreement with the Sr isotope evidence. Other plutons show small variations in model source p values. These may result from inhomogeneity in either crustal or mantle source regions. Comparisons of PbPb and Rb-Sr isotopic data indicate that the wholerock Rb-Sr systematics of rocks in the Diemals area did not become closed systems until up to 400 Ma after the U-Pb systems closed. K. J. R. Rosman provided considerable assistance with mass-spectrometric analytical methods; Dr. 1. Fletcher discussion on aspects of the Yilgam isotopic evolution; and Mr. D. Hosie and Mrs. M. Spaargaren technical assistance. C. M. Lesher and G. N. Phillips assisted with collection of some of the samples. This paper was partly prepared while M.J.B. was a visitor at the Department of Earth Sciences. University of Cambridge. The research was funded by ARGC grant No. E78- I5 194.
.4cknowl~d~ements-Dr.
REFERENCES
Western Australia. Unpublished Ph.D. thesis, Univ. Western Australia. BINNS R. A., GUNTHORPE R. J. and GROVES D. I. (1976) Metamorphic patterns and development of greenstone belts in the Eastern Yilgam Block, Western Australia. In The Eurly History of the Eurlh (ed. B. F. Windley). pp. 303-3 13, Wiley & Sons. CHAPMAN H. J., BICKLE M. J., DE LAETER J. R., BE ITENAY L. F., GROVES D. I., ANDERSON L. S., BINNS
R. A. and GORTON M. (1981) Rb-Sr geochronology of granitic rocks from the Diemals area. Central Yilgarn Block, Western Australia. In Spcciul Puhl. C;rwl. SW Australia, No. 7, 173-186. COPPER J. A., NESBITT R. W., PLATT J. P. and MORTIMEK G. E. (1978) Crustal development in the Agnew region.
Western Australia, as shown by Rb/Sr isotopic and geochemical studies. Precambrian Rec. 7, 3 l-59. GEE R. D., BAXTER J. L., WILDF. S. A. and WILLIAMS I. R. (198 1) Crustal development in the Archaean Y ilgarn Block, Western Australia. In Spec Pub/. Geol SK AU\truliu.
No. 7, 43-56.
GLIKSON A. Y. (1979) dhjemite sialic nuclei.
Precambrian
tonalite-tron-
Archaean
Marda
igneous
complex,
Western
Australia.
Precumhriun Re.5. 3, I 1 1-l 36. MCCULLOCH M. T. and COMPSTON W. (198 I) Sm-Nd
age of Kambalda and Kanowna greenstones and heterogeneity in Archaean mantle. Nature 294, 322-327. MCCULI.OC‘H M. T.. COMPSTON W. and FROLJDL D. (I 983) Sm-Nd and Rb-Sr dating of pre-grecnstone gneisscs, eastern Yilgarn Block, Western Australia. J. (ire/. .%x .lu.c/ (in press). M~~RBATH S. M. and TAYLOR P. N. (198 I) Isotopic e\ldence for continental growth in the Precambrian. In Pwc,umhriun Plate Tecronics L~c,vc,lo~~tncnl.s in Prcwm britrn G‘cw/o~J~ 4 (ed. A. KrBner). pp. 49 I-525 Elsevier. OVERSBY V. M. (1975) Lead isotopic systematics and ages of Archaean acid intrusives In the Kalgoorlie-Norseman area, Western Australia. (;cwhim C’osmochlm lc/u 39,
1107-I ANDERSON L. S., BI?~TENAY L. F., BINNS R. A., DE LAETER J. R., GORTON M. P. and GROVES D. I. (1976) Archaean crustal history of the Central Yilgarn Block, Western Australia (Abstr.) 25th Int. Geol. C‘ongr. Sydney. Ahstrucls 1, 3-4. ARCHIBALD N. J. (1979) Tectonic-metamorphic evolution of an Archaean terrain: a study of the Norseman-Widgiemooltha granitoid-greenstone belt, Eastern Goldfields Province. Western Australia. Unpublished Ph.D. Thesis, University of Western Australia. ARCHIBALD N. J. and BETTENAY L. F. (1977) Indirect evidence for tectonic reactivation of a pre-greenstone sialic basement in Western Australia. Eur:hrrhPlunc,t. Sci. L&t. 33, 370-378. ARCHIBALD N. J., BETTENAY L. F., BINNS R. A.. GROVES D. I. and GUNTHORPE R. J. (1978). The evolution of Archaean greenstone terrains, Eastern Goldfields Province, Western Australia. Precambrian Rex 6, 103- I3 I.
Early
Eurlh Sci. Rev 15, l-73. HALLBERG J. A., JOHNSTON C. and Bvri S. M. ( 1976) The
125.
PLATT J. P., ALLCHIJR~H P. D. and Rcr I I AND R. W. R. (I 978) Archaean tectonics in the Agnew supracrustal belt.
Western Australia. Prwumhriun Rec. 7, 3-30. RODDICK J.C.,COMPSTON W. and DURNFV D. W. (19715) The radiometric age of the Mount Keith Granodioritc. a maximum age estimate for an Archaean greenstone sequence in the Yilgarn Block. Weslern Australia. Pw cumhrian Res. 3, 55-78. STEIC;ERR. H. and JAEGER E. (1977) Subcommission on geochronology: convention on the use of decay constants in geo- and cosmochronology. Eunh P/urw/. .G,r L.ct/ 36,
359-362. TATSCJMOTOM., KNIC~HTR. J. and Al.1 &t C. J. (1973) Time differences in the formation of meteorites as de-
termined from the ratio of lead-207 to lead-206. ,Vc,ic~zcv, 180, 1279-1283. YORK D. (1969) Least squares fitting of a straight line with correlated errors. Ear/h Pkurw~ .Qi. L,cv/. 5. 3X-324.
Lead ages, reset rubidium-strontium ages and implications for the Archaean crustal evolution of the Diemals area, Central Yilgarn Block, Western Australia M. J. BICKLE, H. J. CHAPMAN, L. F. BETTENAY, D. I. GROVES Department of Geology, University of Western Australia, Nedlands. Western Australia 6009 and J. R. DE LAETER School of Physics and Geosciences, (Received
Western April
Australian
Institute
14, 1982; accepted
of Technology,
in revised Jtirm
South
February
Bentley,
Western
Australia
6 102
3, 1983)
Abstract-The Pb-Pb whole-rock geochronology of Archaean granitic and gneissic rocks from the Diemals area in the Central Yilgam granite-greenstone terrain provides important constraints on crustal evolution. The regionally extensive banded gneisses, previously considered as candidates for basement to the greenstones give a Pb-Pb whole-rock age of 2700 f 97 Ma (20 errors). This is within error of previously published Rb-Sr and Sm-Nd gneiss ages and also within error of the Sm-Nd ages on the greenstones in the Eastern Goldfields Province. Two synkinematic plutons give Pb-Pb whole-rock ages (2737 + 62 Ma and 2700 f 100 Ma) and Pb isotopic compositions consistent with the hypothesis, based on field and geochemical relations, that these plutons were derived by partial melting of the precursors to the banded gneisses. Assuming this, the combined data date the melting event at 2723 + 25 Ma with a model source p value of 8.18 * 0.02. This source fi value is close to the range postulated for mantle values and restricts the crustal history of the precursors to less than -200 Ma. A post-kinematic pluton with a whole-rock Pb-Pb age of 2685 f 26 Ma and p value of 8.26 * 0.02 puts a younger limit on this relatively short lived crustal accretion-differentiation event. Comparison of Pb-Pb and Rb-Sr whole-rock dates for the plutons suggests that the latter became closed systems up to 200 Ma after the Pb-Pb ages, and that the plutons gained or lost Rb or Sr at this time.
GEOLOGY
INTRODUCHON
AN UNDERSTANDING of the evolution of Archaean granite crust is fundamental for interpretation of Archaean tectonics. This in turn requires a knowledge of the sources, timing and mechanisms of addition of new plutonic material to the crust. In this paper we report whole-rock Pb-Pb isotopic analyses and some new Rb-Sr isotopic analyses for a variety of gneissic and plutonic bodies from the central part of the Archaean Yilgarn Block in Western Australia. This follows a Rb-Sr isotopic study of these rocks (CHAPMAN et al., 198 1). The Pb isotopic data provide relatively stringent tests for models of the genetic evolution of the granitic crust and are consistent with the hypothesis that syn-kinematic granodiorite-adamellite plutons were derived by partial melting of banded gneiss precursors (BETTENAY, 1977). The whole-rock Pb-Pb ages define a relatively short continental accretion-differentiation event between -2800 Ma and -2650 Ma consistent with recent Sm-Nd studies (MCCULLOCH et al., in press), but shorter than that inferred from Rb-Sr isotopic systems (COOPER et al., 1978; CHAPMAN et al., 1981). The new and previously published Rb-Sr ages are up to -400 Ma younger than the whole-rock Pb-Pb ages and often have relatively high initial 87Sr/86Sr ratios. The discrepancy between Pb-Pb and Rb-Sr ages is tentatively explained as due to later closure or resetting of Rb-Sr isotopic systems on the whole-rock scale. 907
The Diemals area (Figs. I and 2, discussed by CHAPMAN et al..1981), is situated centrally within the ca. 10” km2 Archaean Yilgam Block. It comprises a typical granitegreenstone terrain, similar in many respects to the better studied Norseman-Wiluna belt to the east (GEE e/al..198 1). Much of the granitic and gneissic crust within the Diemals area, and indeed the Yilgam Block, is rather poorly exposed and inferences on pluton boundaries and structural relationships require correlation of detailed observations made on small (
M. J. Bickle rr al.
908
q
Fractionated
++t ii+ 0+.+ :::: :::: Cl ElII + t+ + El 0
le”coadamell,,e
Post-klnematlc discrete
Syn-
granttads
k,“ema,lc
Banded
QranltoldS
gnmss
and
m~gmahle
Concealed banded gnens Sparse outcrop dom!“ated klnematlc phases
.’
formlng
plutons
and m,gmat,te by DOS,-
Fault
0
Sample
+
Sample
sate site
rthls
Study)
(McCulloch,
Compston
8
I
Froude)
N 0
100
km
FIG. I. Interpretative regional map ofgranitoids in the southern part of the Eastern Goldfields Province showing distribution of banded gneiss-migmatite complexes (from ARCHIBALD et al..1981). Geochronology sample sites for this study are: 1.Cockatoo Rocks (8 1898, 87922A. 87922B. 87927.87928, 87936) 2. N. Koolyanobbing (8 1885), 3. E. Koolyanobbing (81884), 4. W. Split Rocks (81962, 8 1975), 5, 6, 7 & 8 W. Lake Johnson-5. (82030), 6. (82023, 82024). 7. (82025) 8. (82026) 9. E. Lake Johnson (82034, 83035) and 10. Riverina (82167). Geochronology sample sites for the study of MCCULLKH et al (in press) are: I I. Pioneer Dome (3 samples), 12. Connolly Siding (2 samples), 13. Kirgella Rock-hole (3 samples).
**++.. .**t*. ..+**_ ..**+* */**t* ****t* ++.*+, **.. .**,+ .*++. .I++. .*+A..
1
+++.*.
*++a_*
COCKATO ROCK - 25 k yj
:::
, .A
-KEN :: ROC ..+...._,._ . ..+._ . ..*.. -..** . . . .
i... .. .. .. .. et.. 1... ,t... , *+.. et.. ,,.. **.
FIG. 2. Geological map of the Diemals area, Central Yilgarn Block compiled from unpublished University of Western Australia. Samples sites for Rb-Sr and Pb isotope studies shown.
data,
Pb-Pb
ages of Diemals
mellite in composition, was emplaced. These were followed by widely dispersed, but minor, post-kinematic plutons with distinctive fractionated compositions (BETTENAY, 1977). The prime objectives of the geochronology were to test the relative chronologies of plutonic suites as deduced from field relations, to test the synchroneity of plutonic events over large areas of the Central and Eastern Yilgam Block (700 km X 400 km) and to determine the duration of the tectonometamorphic evolution described above, especially given the possibility that multiple and unconnected events may be superimposed in any area. Critical and controversial topics include the status of structurally complex gneisses as candidates for a sialic basement to the greenstone sequences and the possibility of more than one cycle of greenstone belt evolution (GL~KSON, 1979). Detailed geochronological studies in the Yilgam are few. MCCULL~CH and COMPSTON (198 I) and MCCULL~CH et al. (in press) using Sm-Nd whole-rock methods dated greenstones in the Kambalda sequence at 2790 -C 30 Ma (2~ errors) and samples of banded gneisses from three nearby localities at 2800 + 100 Ma. OVERSBY (1975) reported whole-rock and mineral Pb-Pb dates on a number of synand post-tectonic plutons from the Kalgoorlie-Norseman area (ages between 2632 * 28 Ma to 2699 f 75 Ma). Two areas that have been studied in sufficient detail to provide a chronology based on Rb-Sr data are the AgnewLawlers-Perseverance area about 350 km north of Kalgoorlie (RODDICK et al., 1976; PLATO et al., 1978; CARPER et al., 1978) and the Diemals area (CHAPMAN et al.. 1981). The Rb-Sr chronologies and tectonic evolution in both areas are broadly comparable (Table 1). The greenstones in both areas and banded gneisses within and south-east of Diemals are thought to have been formed at cn. 2800-2700 Ma although this date is rather poorly constrained. Both areas were intruded by now highly foliated, extensive granodiorite-adamellite and lesser tonalite plutons (syn-kinematic granitoids of ARCHIBALD ef al., 1978) with Rb-Sr wholerock ages between 2670 to 2600 Ma and initial “Sr/*‘Sr ratios between 0.701 to 0.702. Intense deformation and regional metamorphic events occurred during and after the intrusion of the foliated plutons. These were followed in both areas by intrusion of largely unfoliated post-tectonic plutons with Rb-Sr ages of ca 2550 Ma. Late post-tectonic plutons with Rb-Sr ages of -2470 Ma are also recognised in both areas. In the Diemals area, some of the younger post-tectonic plutons have high initial 87Sr/86Sr ratios (0.7 l0.72). Although the sequence of ages in both areas is similar, it should be remembered that errors on the Rb-Sr ages are generally ca. ?50 to ? 100 Ma and that correlation of plutonic events based primarily on deformation state need not
Table
1.
Tectonic Event Late postKinematic Plutans
909
have chronological implications, especially between areas separated by several hundred kilometers. The Pb isotopic analyses reported in this study were initiated to investigate the limited resetting of Rb-Sr systematics proposed in the Diemals area on the basis of inconsistencies between geological relationships and the isotopic ages (CHAPMAN et al., 1981). As discussed below, discrepancies between wholerock Pb-Pb and Rb-Sr ages of up to 200 Ma are recorded and require substantial revision of the tectonic chronology. ANALYTICAL
TECHNIQUES
Most samples were those analysed and described by CHAPMAN ef al. (I 98 1). Sampling techniques and analytical methods for new Rb-Sr data quoted below are as described by CHAPMAN ef al. (198 1). Precision on Rb/Sr ratios (XRF analyses) is estimated at fl% (20) and on 87Sr/86Sr ratios as f.05%. Ages are calculated using XRb = 1.42 X lo-” yrr’ and quoted with 2~ errors. Lead was separated from -0.15 g of rock-powder by the electro-deposition technique (ARDEN and GALE, 1976). Lead isotopic analyses were made on one of two 30.5 cm radius of curvature, 90” magnetic sector solid-source mass spectrometers at the West Australian Institute of Technology, using the silica-gel loading technique on single rhenium filaments and run at 12OO”C- 1250°C. Hydrocarbons were monitored at mass 205 and generally burnt off very rapidly. On the alpha machine, signals from the collector were amplified by a vibrating-reed electrometer before being digitized and processed by on-line computer. For each sample -60 to 100 cycles were made using an automatic peak-switching cycle 208-207-206-204-206-207-208 with background measured half a mass unit below mass 204. Tailing measured at mass 205 was less than 1 part in 104. Fourteen measurements of NBS standard 98 I during 1980 and early 198 1 gave values of Zo8Pb/Z06Pb = 2.1487 f 0.10% (2 s.d.) “‘Pb/ 2”Pb = 0.9 113 f 0.08% and 204Pb/2”Pb = 0.05949 + 11%. Seven measurements of NBS 981 on the alpha machine during August I98 1, after the computer and peak-switching sequence had been updated to switch 204-208-204-207204-206-204-206-204-207 etc. gave isotopic ratios within error of these. On the beta machine signals were measured using a Faraday cup collector. The peak switching routine was similar to the later routine on the alpha machine as above. Tailing at half mass offsets was about 1 part in 104. Four measurements of NBS 98 1 gave mean values of *“Pb/ rmPb = 2.1622 f 0.08% 207Pb/z”Pb = 0.9 I34 -+ 0.04% and zo4Pb/206Pb = 0.059 16 -+ 0.17%. Duplicate analyses of samples implied a reproducibility of better than 20.15% (20) and after mass discrimination corrections no systematic bias
Rb-Sr Chronologies of the Diemals and Lawlers-Agnew Perseverence Areas Diemals Area Kelly5 Rocka
Rbl;; age 8;;;;gg;r
2467?72
0.719$?4
Late cataclasis over S.E. Diemals Area Post-kinematic Plutons
area
M"ky
Main Phasea 2557198 0.7099?80 Soak N. Outcrops 25501166 0.7040?39
Lawlers-Agnew-Perserverence Area Leucotonaliteb Pegmatite‘
Rb/Sr age lnitlal Ma 87~r/86sr 2474i14 2481'18
0.7019tl 0.7253*2
2576?14 2588118
0.7022~2 0.762t7
Lawlers Tonaliteb Perseverence Gneissb Mt Keith Granodiorite
2652?20 2625'34 2632~17
0.70152?12 0.7022t5 0.70149?15
Kathleen Valleyb Granophyre Gabbro
2718t50
0.7007'4
Deformation* 02 Lawlers L ucotonaliteb Pegmatite% First Deformatlond D1
Main Deformationsa 01(?).02,D3 Syn-kinematic Plutons
West Pigeon Rockse Plgeon Rockse Rainy Rock@
2475?76 2575256 2612'74
0.721t65 0.7123285 0.7014~22
Pre-kinematic Granitolds
South-east Yilgarn' banded gne~ss
2678'90
0.7021~10
Greenstones
Mat-da Felsic vo1can1csf
2635i80
0.703t2
Sources of data: (a) Chapman et al. (1981); (b) Cooper et al. (1978); (c) Roddick et al. (1976); (d) Platt et al (1978); (e) this work; (f) Hallberg et al. (1976); (g) Chapman et al. (1981) recalculated without sample L&25 from Cockatoo Rocks.
910
M. J. Bickle et (I/
was noted between samples analysed on both the alpha and beta machines. Total Pb blanks generally ranged from 510 ng, although in the early part ofthis work some Pb blanks were up to 25 ng. Duplicate analyses of USGS standard BCR- I gave 2osPb/204Pb = 38.665, 207Pb/204Pb = 15.613 and r”Pb/204Pb = 18.8 I I, within error of the values quoted by ARDEN and GALE(~ 976). Regression analysis was by the method of YORK (1969) with the quoted modification to allow for scatter of points outside analytical error. A correlation coefficient of 0.6 between errors of 0.15% (20) on both 2”7Pb/204Pb and z”6Pb/ 204Pb was used in the regression program. Ages were calculated with the decay constants of STEIGER and JAEGER (1977) and errors quoted at 20 level. The age and initial Pb isotopic composition of the Earth adopted were those of TA-~SLIMO~Oe/ a/ ( 1973). Errors on model source p values are estimated from the error on the intercept of the isochron with the geochron, calculated by the YORK (1969) program with the geochron as the ?’ axis.
LEAD
ISOTOPE RESULTS AND COMPARISON WITH Rb-Sr GEOCHRONOLOGY
(u) Banded grwisses
The banded gneisses represent the oldest granitic crust recognised within the Yilgarn granite-greenstone terrains (i.e. outside the Western Gneiss Terrain: GEE 6’1a/., 198 1) and enclaves of banded gneiss are found both in the foliated “syn-kinematic” plutons as well as the “post-kinematic” plutons (BETTENAY, 1977). Lead isotopic compositions of these rocks are listed in Table 2 and ages in Table 6. The whole-rock Pb isotopic analyses of the banded gneisses from the south-east Yilgarn (Fig. 3) show more scatter (mean squared weighted deviate, henceforth M.S.W.D., =8) than most of the other whole-rock isochrons, which is perhaps not surprising given the spread of outcrops sampled (Fig. 1) and their complex history. The whole-rock Pb age of 2700 * 97 Ma is similar to their whole-rock Rb-Sr age (2678 f 90 Ma, M.S.W.D. = 23, initial “Sr/“%r = 0.7021 f 10). This is within error of the Rb-Sr and Sm-Nd whole-rock ages of 2780 + 60 Ma, initial “Sr/*%r = 0.7007 I 5 and 2800 ? 100 Ma respectively on similar but geographically more restricted banded gneisses collected by L. F. Bettenay and N. J. Archibald (McCu~~oct~ pt al., in press: see Fig. 1). These ages
c
Banded
/ ,/'
"'Pb "'+Pb
,
*
,i'
16.
I :’ c
15
i*
.’
/’
South-east Age Jl
2700 8.17*
MSWD
8.
Yllgarn * 97 Ma 05
8
206Pb/*04Pb 16
16
20
22
FIG. 3. Whole-rock Pb-Pb isochron of banded from South-Eastern Yilgarn localities in Fig. I.
gneisses
are within error of the best available constraints on the ages of the greenstones (c.~.. MCCULLO(‘H and COMPSTON, I98 I) and. as discussed by MP&ILLO(‘H ct al., (in press), the geochronology is not sufficiently precise to establish if the gneisses are older than the greenstones. The model source 23R1J/204Pb ratio (henceforth fi value) for the gneisses is 8. I7 ? .OS and thus close to likely mantle values (see discussion in MOORBATH and TAYLOR, precursors
1981).
Derivation
of the gneisses more than
-200
of the
Ma prior
to 2700 Ma would seem improbable. (b) Foliated svn-kinematic
plutons
Two syn-kinematic plutons were studied in the RbSr geochronology program. Rainy Rocks is an intensely lineated granodiorite-adamellite outcrop marginal to a strongly deformed greenstone belt (Fig. 2). Ten whole-rock samples at Rainy Rocks define a Pb-Pb age of 2700 ? 100 Ma, M.S.W.D. == 12, model source fi value = 8.20 ? 0.07 (Table 4, Fig. 4) within error of its Rb-Sr age of 2612 t 74 Ma, initial *‘Sr/%r = 0.7014 + 22. M.S.W.D. = 13. The second pluton, Pigeon Rocks intrudes deformed greenstones in the central part of the green-
'04Pb
Table 2 Lead Isotonic Camnositions of Banded Gneisses
.,’
Rainy Rocks
17. 207pb
Collection NO. Sample
Gneisses
Age JJ
2700t100
/
Ma
8.20 * 07
MSWDIZ
LohPb/204Pb Lo7Pb/204Pb 2oxPb/'o"Pb
South East Yilgarn Banded Gneisses a1884 a1885 81962 81975 82023 82024 82025 82026 82030 82034 62035 82167
LB282 LB221 LBSP6 LB384 LB374 LB372 LB371 LB364 LB367A LB359 LB360 LB1676
14.428 14.459 15.876 17.800 16.822 16.363 15.463+79 14.534 15.471 20.234 16.369 22.859+69 _
14.983 14.981 15.193 15.529 15.416 15.296 15.118+76 14.980 15.150 16.077 15.343 16.439+49
37.470 33.954 37.202 37.196 38.661 37.976 35.3845248 35.718 35.563 38.663 44.155 35.968+72
ei-rors quoted where analytical precision is less good than + .15% (2 x standard error).
16.
//
,’ , 16
et. ’
7’
./*
/
i
206Pb1204Pb 18
FIG. 4. Whole-rock Rainy Rocks (Fig. 2).
20
22
Pb-Pb
isochron
2.
of samples
26
from
Pb-Pb
ages of Diemals
Table 4
Table 3 Rb and Sr Contents and isotopic Compositions of New Pigeon Rocks Samples Collection NO.
Field NO.
Rb
Sr
B7Rb/BbSr
B'Sr/BbSr
HC85 HC86 HC87A HC87B HC88 HC89 HC90 HC91 HC92 HC93 HC94
251 3M 325 293 266 264 270 269 298 279 296
79.5 63.6 52.9 60.1 74.1 75.7 80.7 66.4 80.3 81.6 86.3
9.44 14.46 19.19 14.91 10.79 10.57 10.05 12.20 11.15 10.26 10.35
1.06480 1.25442 1.42537 1.26613 1.12185 1.15620 1.08589 1.16654 1.12484 1.09210 1.10123
18 40 34 12 25 25 40 22 27 12 30
Lower Rb/Sr Samples - Western Outcrop 94163 94164
HC98 HC99
267 279
72.4 80.9
Higher Rb/Sr Samples 94160 94161 94162 94i65
HC95 HCY6 HC97 HClOO
388 330 316 336
15.5 13.9 25.6 22.4
11.12 10.27
1.13176 27 1.08866 24
Western Outcrop 97.62 91.57 41.52 51.16
4.21611 4.00504 2.21850 2.52853
18 43 17 28
Whole-rock Samples Between Pegmatites 94167 9416E 94165 9417c 94171
HCllAa HCllBb HCllCc HCllEe HCllFf
327 402 399 432 320
18.8 18.4 19.2 17.6 18.3
61.25 81.72 76.50 94.44 61.86
Lead Isotopic Compositions Collection NO.
2.92160 3.66329 3.48545 4.08689 2.96768
44 94 84 75 42
stone belt (Fig. 2). Samples were collected from relatively unfohated adamellite away from the contact zone. All outcrops at Pigeon Rocks are cut by prominent sets of pegmatites, but whole-rock samples were collected at least 1-2 m away from obvious pegmatite veins. The Pigeon Rocks Pluton comprises an earlier foliated medium- to coarse-grained granodiorite/adamellite intruded by a slightly finer grained granodiorite/adamellite. A sharp intrusive contact between these phases is evident at a few clean outcrops but over much of the area distinction between these phases is equivocal. Where contact relationships are observed Rb/Sr ratios are distinctive with the older suite characterised by lower Rb/Sr ratios and higher Sr contents (Table 3). Multiple intrusive phases showing variable deformation characteristics are a feature of synkinematic plutons (ARCHIBALD and BETTENAY, 1977). Both suites have similar Pb-Pb ages. Ten samples of the low Rb-Sr suite define a wholerock Pb-Pb isochron. age 2737 + 62 Ma M.S.W.D. = 2.9, with a model source F value of 8.16 f .10. Nine samples of the high Rb-Sr suite define a wholerock Pb-Pb isochron age 2682 k 53 Ma, M.S.W.D. = 2.0, with a model source p value of 8.36 +- .04. The Pb isotopic compositions of the two suites are clearly distinct (Fig. 5) and this is reflected by the difference in model source p values. CHAPMAN et al. ( 198 1) speculated that the Rb-Sr systematics in the Pigeon Rocks outcrop were perturbed after emplacement of the plutons. Twelve additional whole-rock samples of the low Rb-Sr suite (Table 3 excluding HC89) combined with the ten samples analysed by CHAPMAN et al. ( 198 1) define a Rb-Sr age of 2575 f 56 Ma, M.S.W.D. = 5.0 initial R7Sr/86Sr = 0.7 123 ?Z 85 (Fig. 6) significantly younger than the whole-rock Pb-Pb age of 2737 f 62 Ma on the same suite. Four samples of the high Rb-Sr suite
of Synkinematic
Sample 206Pb/204Pb Plgeon Rocks (Lowr
t2:r
Lower Rb/Sr Group - Main Outcrop 94149 94150 94151 94152 94153 94154 94155 94156 94157 94158 94159
911
area
87301 87902 87903 87904 67305 87906 87907 67908 87909 87910
tlc1* HCZ* tlc3* HC4* tlc5* HC6* HC7* HCa* tlc9* HClO*
207Pb/2"4Pb
Plutons
*08pb/L04pb
Rb-Sr samples)
23.096 23.253 21.775 28.192 23.148 24.137 24.520 25.763 24.380 22.667
lb.564 16.624 16.342 17.540 16.575 16.769 16.841 17.111 16.859 16.510
39,486 39.441 39.269 39.929 39.876 39.172 40.075 40.175 39.935 39.681
Pigeon Rocks (Higher Rb-Sr samples) 94160 94161 94162 94165
HC95 HC96 HC97 HCIOO
26.324 23.789 26.026 24.710
17.237 16.801 17.179 16.946
36.627 36.081 39.016 37.280
Plgeon Rocks Adjacent to Pegmatites and Pegmatite sample (HCllH) 879110 87911Gd 87911Gc 87911C 87911Gb 87911H
HCllD HCllGd HCllGc HCllC HCllGb HCllH
87963 87965 87967 87969 87971 67973 87975 a7377 87979 87981
HC63 HC65 HC67 HC69 HC71 HC73 HC75 HC77 HC79 HC81
28.410 22.694 23.339 25.032 22.371 24.645
17.606 16.557 16.676 16.983 16.498 16.853
36.823 35.806 36.156 35.909 35.723 34.934
15.676 16.007 15.544 15.766 15.648 15.728 15.820 17.057 16.196 16.021+24
40.564 35.406 38.317 39.650 35.747 40.122 39.481 36.924 39.574 40.099+62
Rainy Rocks 18.242 20.185 17.319 18.701 18.077 18.292 19.355 25.550 20.862 20.275+30
* analysis duplicated
define a Rb-Sr whole-rock age of 2475 2 76 Ma. M.S.W.D. = 4.5, initial 87Sr/86Sr = 0.721 f 65 (Table 3: Fig. 7). The other samples of the high Rb-Sr suite were collected within 0.5 m of two pegmatites (see Fig. 3 in CHAPMAN et al., 1981) to assess the influence of the pegmatites on the isotopic systems within these rocks. Nine whole-rock samples from between the pegmatites (Table 3) including the six samples analysed by CHAPMAN CJI al. ( 198 1) define a Rb-Sr age of 2268 f 143, M.S.W.D. = 3. I, initial 87Sr/8”Sr = 0.99 i 0.16 (Fig. 7). Two samples which fall below the 2268 Ma isochron lie very close to the line defined
Pigeon Plgeon Age
"'Pb
Jl
'04Pb
Rocks
2682r 8.36
MSWD
Rocks (higher RblSr) 53
Ma
f 04 2
17.0~ (lower Age
Rb/Sr)
2737'62
Ma
J/ 8.16i 10 MSWD 2.9
.
16.5.
Pigeon Rocks D Samples adiacent to pegmatltes A Pegmatlte 206Pb/204Pb
.? ./I /' 22
FIG.5. Whole-rock Pigeon
Rocks
(Fig. 2).
26
24
Pb-Pb
isochrons
28
of samples
from
M. J. Bickle et al. Pigeon
Age
Pb-Pb age of 2682 f 53 Ma is 0.53 + 0.05. The smaller spread of the Pb-Pb ages from all of the synkinematic plutonic suite is further evidence consistent with later closure or resetting of the Rb-Sr systematics.
Rocks
2575t56
Ma
lnttlal "Sr/"Sr MSWD 5
0.7123t85
;.’ /
.,-
/
* ,.’
.a’
(c) Post-kinematic
./
/’ 0.8
I r
/’
"FIblasS,
,’
5
10
20
15
FIG. 6. Rb-Sr isochron plot of whole-rock low Rb/Sr samples from Pigeon Rocks. Includes data from CHAPMAN et al. (198 1) and this study (Table 3). Sample HC89 not included in regression.
by the four high Rb-Sr samples and are apparently not affected by the alteration adjacent to the pegmatites. For the altered samples adjacent to the pegmatites the discrepancy between the Rb-Sr and PbPb age is plausibly a result of intrusion of the pegmatite or of fluid channelled along the pegmatite and it is of interest that this had little or no effect on the U-Pb systematics. The discrepancies between the whole-rock Pb-Pb and Rb-Sr ages on the suites collected away from the pegmatites may be due to later closure or resetting of Rb-Sr isotopic systems on the whole-rock scale. If so, this was accompanied by extensive gain of Rb and/or loss of Sr as initial s7Sr/86Sr ratios recalculated to the Pb-Pb ages, on the assumption that the average Rb and Sr contents of the plutons remained unchanged, are lower than possible mantle values. The inferred 87Sr/86Sr ratios of the low Rb/Sr suite at its Pb-Pb age of 2737 + 62 Ma is 0.687 + 0.0 13 and the inferred 87Sr/86Sr ratio of the high Rb-Sr suite at its
-8'Sr 86%
West Pigeon Rocks 8 Samples Adjacent to Pegmatites
.P
“A
0 Adjacent lo pegmatltes * West Pngeon Rocks
plutons
The Milky Soak outcrop is a weakly foliated postkinematic porphyritic granodiorite and adamellite. It contains distinctive enclaves of strongly lineated granodiorite very similar to that from the syn-kinematic Rainy Rocks outcrop. Two sample sites on the nothern margin of the outcrop contained such enclaves of deformed granodiorite within the main phase. The whole-rock Rb-Sr results on ten samples of the main phase of Milky Soak define a well-fitted isochron (M.S.W.D. = 3.0) of 2557 -t 98 Ma, initial 87Sr/86Sr = 0.7099 + 80 (Table I). Six samples from the two northern sample sites lie on a parallel isochron (M.S.W.D. = 13) of 2550 ? 166 Ma, initial 87Sr/86Sr = 0.7040 f 39 (Table 1). Fifteen whole-rock samples, incorporating both phases, define a well-fitted Pb-Pb isochron, M.S.W.D. = 1.9 (Table 5, Fig. 8) age of 2685 f 26 Ma with a source p value of 8.26 5 .02. The mean 87Sr/86Sr ratio at 2685 t 26 Ma, the Pb-Pb age, for the main phase would have been 0.699 f ,003 and for the lower Rb/Sr group of northern samples would have been 0.70 1 + ,001. These results are consistent with derivation of Milky Soak granitic rocks from a source isotopically similar to the mantle at 2685 -+ 26 Ma and final closure of Sr isotopes on the whole-rock scale at -2550 Ma. CONSTRAINTS (a)
ON CRUSTAL
EVOLlJTION
Tectonic chronology
The Pb-Pb ages require that granitic plutonism in the Diemals area took place within a period of - 150 Ma and possibly substantially less (Table 6). The relatively precise age of 2685 f 26 Ma for the postkinematic pluton sampled at Milky Soak places a minimum age on the main deformation events and metamorphic peak conditions of greenstones and syn-kinematic plutons. Rb-Sr mineral ages (biotite,
A Pegmatmte Ad,acent to pegmatltes Age
2268*143
Table 5
Ma
lnltla, 87Sr/%,
Lead Isotopic Compositions of Post-kinematic Plutons
0 99
Collection Sample 206Pb/zo4Pb 207Pb/204Pb zo8Pb/204Pb NO.
I’
Milky Soak
,'
*‘Rb/=Sr 2 40
60
80
100
FIG. 7. Rb-Sr isochron plot of high Rb-Sr samples from West Pigeon Rocks compared with high Rb-Sr samples adjacent to pegmatite. The two least radiogenic samples from the latter suite are not included in the regression calculation (see text).
87948 87949 87950 a7951 87952 87953 87954 87955 87956 87957 87958 87959 87960 87961 87962
HC48 HC49 HC50 HC5lA HC52 HC53 HC54 HC55 HC56 HC57 HC58 HC59 HC60 HC61 HC62
18.130 18.668 19.270 17.362 17.117 24.354 17.841 21.569 24.849 20.542 22.404 19.367 23.746 23.949 22.669
15.686 15.813 15.889 15.558 15.507 16.832 15.625 16.326 16.934 16.116 16.442 15.911 16.736 16.761 16.512
39.557 38.637 37.120 37.787 36.865 39.523 36.942 40.936 42.073 38.416 39.587 37.760 39.255 40.104 39.628
Pb-Pb ages of Diemals area Milky
Soak
Age 2665
well within error of the south-east Yilgam banded gneisses Pb-Pb isochron (Fig. 9). The combined south-east Yilgam gneiss/Pigeon Rocks low Rb-Sr suite/Rainy Rocks data give an age of 2723 ? 25 Ma (M.S.W.D. = 7) with fi = 8. I8 + .02 and it is plausible that this dates the major crustal melting event postulated by BINNS et ul. (1976) and ARCHIBALD et al. ( 1978). In addition, the relatively low Pb isotopic ratios of the gneisses are consistent with the low U
./ .I .’
f 26 Ma
)_I 626 * 02 MSWD 1 9
.*’
/” 9’
/
.,’
913
/’
.
contents
expected
if the gneisses
are depleted
in the
more mobile elements by removal of the melts parent to the syn-kinematic plutons. The model source p value of 8.18 +- .02 is close to the range thought typical of the mantle (e.g., MOORBATH and TAYLOR, FIG. 8. Whole-rock Milky Soak (Fig. 2).
Pb-Pb isochron of samples from
or a subsequent
thermal
isotopic
event
reset
mineral
and
systems.
Some of the younger Rb-Sr ages (2576 + 14 Ma and 2474 -t 14 Ma) from the Agnew-Lawlers-Perseverence region 250 km north-east of Diemals have low initial
87Sr/RhSr ratios systematics
would be of interest method. chrons
perturbation
in the Pigeon
of the to-
Rocks
pluton
it
to test these ages by an alternative
The Pb isotope of OVERSBY
(COOPER et ~1..
-0.702
1978). but given the apparent tal Rb-Sr
whole-rock
and mineral
iso-
( 1975) on a Series Of post-tectonic
plutons in the Kalgoorlie-Kambalda-Norseman region give ages between 2600 Ma and 2700 Ma synchronous with the Diemals plutons and suggesting that the plutonic episode occurred over a minimum extent of -500 km.
BEIXNAY (1977) inferred from the compositions and field relations of the banded gneisses that they may have been the source for both the syn-kinematic and post-kinematic plutons: such models are discussed by ARCHIBALD et al. (1978: 1981). The Pb isotopic data presented here provide confirmation of this hypothesis. The Pigeon Rocks low Rb-Sr suite and the Rainy Rocks Pb isotopic compositions lie
Table
Pb-Pb
6.
Chronology
of the
Pb-Pb age Fla
pvrnt
Pigeon Rocks Pigeon Rocks Rainy Rocks
(high Rb/Srl (low Rb/Sr)
Combined Gne~ss. Ra,ny Rocks, Plgeon South-East
to the banded short crustal
gneisses
must
residence
prior
to 2800
plagioclase and K-feldspar \‘ersus whole-rock) of -2540. -2460, -2360, -2350 and -2040 and - 1536 Ma are recorded by CHAPMAN et al. (198 1). Either the cooling and uplift history was protracted whole-rock
198 1) and the precursors have had only a relatively
Ylloarn
Rocks Gnelss
Ma as also suggested by McCulloch et al. (in press) on the basis of Sm-Nd and Rb-Sr data. The Pb isotopic compositions of the high Rb/Sr Pigeon Rocks suite and the post-kinematic Milky Soak pluton are clearly distinct from those of the gneisses and syn-kinematic plutons and this difference is reflected in the model source CCvalues of 8.36 f .04 and 8.26 t .02 respectively. The range in model source p values may result from heterogeneity in crustal or mantle sources. OVERSBY (1975) concluded from a study of five plutons in the KalgoorlieKambalda-Norseman area that crustal sources as old as 3300 Ma were necessary to explain initial Pb isotopic compositions of the plutons as inferred from K-feldspar Pb isotopic compositions. This possible heterogeneity of crustal sources may well be of significance to the evolution of this Archaean crustal segment but resolution of this problem will require further work. CONCLUSIONS Lead isotopic analyses of suites of plutonic samples from the Diemals area in the central Yilgam Block limits the formation of this segment of Archaean crust to one crustal accretion-differentiation event between about -2800 Ma and -2650 Ma. The Pb isotopic compositions of two suites of syn-kinematic plutons are consistent with their derivation from crustal sources of which banded gneisses represent residues of partial melting as suggested by BETTENAY ( 1977). The combined Pb isotope data date this event at 2723 +- 25 Ma. The model source p value of 8. I8
Olemals
MSWD
Area
Compared
Model source
L.
with
Rb-Sr
Ages
Rb-Sr age Ma
Initial 87 Sr/%
MSWD
0.7099'80 0.7040139
3.0 13
X85*26
19
8.X*.02
2557'98 2550'166
2682'53 2737~62 270nt100
2.0 2.9 11
8.36+.04 8.161.10 8.20'.07
2475‘76 2575'56 ?612'74
0.721?65 0.7123'85 0.7014?22
4.5 5.0 13
2723r25
7
8.18t.02
2700+97
8
F3.17+.05
2678!90
fl.7021'10
23
M. J. Bickle et al.
914
207Pb ?04Pb
ARCHIBALD N. J., BETTENAY L. F., BICKLE M. J. and GROVES D. I. (198 I) Evolution of Archaean crust in the
Banded Gneiss 8. Synkinematic Plutons
i
Eastern Goldfields Province of the Yilgarn Block, Western Australia. In Special Publ. tieol. Sot. Australia, No.
Age 2723
f 25 Ma pMSWD 8.18 7 * 0.02
7,491-504. ARDEN J. W. and GALE N. H. (1976) New electrochemical technique for the separation of lead at trace levels from natural silicates. Anal. Chem. 46, 2-9. BETTENAY L. F. (1977) Regional geology and petrogenesis of Archaean granitoids in the Southeastern Yilgarn Block,
I. 25
20
15
FIG. 9. Whole-rock
30
Pb-Pb regression
banded gneiss from south-east kinematic plutons at Rainy Growth curve for w = 8.18.
on samples of Yilgarn localities and synRocks and Pigeon Rocks.
is consistent
with a relatively limited prior crustal residence of the precursors to the gneisses in agreement with the Sr isotope evidence. Other plutons show small variations in model source p values. These may result from inhomogeneity in either crustal or mantle source regions. Comparisons of PbPb and Rb-Sr isotopic data indicate that the wholerock Rb-Sr systematics of rocks in the Diemals area did not become closed systems until up to 400 Ma after the U-Pb systems closed. K. J. R. Rosman provided considerable assistance with mass-spectrometric analytical methods; Dr. 1. Fletcher discussion on aspects of the Yilgam isotopic evolution; and Mr. D. Hosie and Mrs. M. Spaargaren technical assistance. C. M. Lesher and G. N. Phillips assisted with collection of some of the samples. This paper was partly prepared while M.J.B. was a visitor at the Department of Earth Sciences. University of Cambridge. The research was funded by ARGC grant No. E78- I5 194.
.4cknowl~d~ements-Dr.
REFERENCES
Western Australia. Unpublished Ph.D. thesis, Univ. Western Australia. BINNS R. A., GUNTHORPE R. J. and GROVES D. I. (1976) Metamorphic patterns and development of greenstone belts in the Eastern Yilgam Block, Western Australia. In The Eurly History of the Eurlh (ed. B. F. Windley). pp. 303-3 13, Wiley & Sons. CHAPMAN H. J., BICKLE M. J., DE LAETER J. R., BE ITENAY L. F., GROVES D. I., ANDERSON L. S., BINNS
R. A. and GORTON M. (1981) Rb-Sr geochronology of granitic rocks from the Diemals area. Central Yilgarn Block, Western Australia. In Spcciul Puhl. C;rwl. SW Australia, No. 7, 173-186. COPPER J. A., NESBITT R. W., PLATT J. P. and MORTIMEK G. E. (1978) Crustal development in the Agnew region.
Western Australia, as shown by Rb/Sr isotopic and geochemical studies. Precambrian Rec. 7, 3 l-59. GEE R. D., BAXTER J. L., WILDF. S. A. and WILLIAMS I. R. (198 1) Crustal development in the Archaean Y ilgarn Block, Western Australia. In Spec Pub/. Geol SK AU\truliu.
No. 7, 43-56.
GLIKSON A. Y. (1979) dhjemite sialic nuclei.
Precambrian
tonalite-tron-
Archaean
Marda
igneous
complex,
Western
Australia.
Precumhriun Re.5. 3, I 1 1-l 36. MCCULLOCH M. T. and COMPSTON W. (198 I) Sm-Nd
age of Kambalda and Kanowna greenstones and heterogeneity in Archaean mantle. Nature 294, 322-327. MCCULI.OC‘H M. T.. COMPSTON W. and FROLJDL D. (I 983) Sm-Nd and Rb-Sr dating of pre-grecnstone gneisscs, eastern Yilgarn Block, Western Australia. J. (ire/. .%x .lu.c/ (in press). M~~RBATH S. M. and TAYLOR P. N. (198 I) Isotopic e\ldence for continental growth in the Precambrian. In Pwc,umhriun Plate Tecronics L~c,vc,lo~~tncnl.s in Prcwm britrn G‘cw/o~J~ 4 (ed. A. KrBner). pp. 49 I-525 Elsevier. OVERSBY V. M. (1975) Lead isotopic systematics and ages of Archaean acid intrusives In the Kalgoorlie-Norseman area, Western Australia. (;cwhim C’osmochlm lc/u 39,
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