SmNd geochronology of greenstone belts in the Yilgarn Block, Western Australia

Precambrian Research, 26 (1984) 333--361

333

Elsevier Science Publishers B.V., Amsterdam -- Printed in The Netherlands

Sm-- N d G E O C H R O N O L O G Y O F G R E E N S T O N E YILGARN BLOCK, WESTERN AUSTRALIA

I.R. F L E T C H E R

BELTS IN THE

and K.J.R. R O S M A N

School of Physics and Geosciences, Western Australian Institute of Technology, Kent Street, Bentley, 6102, Western Australia (Australia) I.R. WILLIAMS, A.H. HICKMAN and J.L. BAXTER*

Geological Survey of Western Australia, 66 Adelaide Terrace, Perth, 6000, Western Australia (Australia) (Received September 22, 1983; revision accepted May 21, 1984)

ABSTRACT

Fletcher, I.R., Rosman, K.J.R., Williams, I.R., Hickman, A.H. a n d Baxter, J.L., 1984. Sm--Nd geochronology of greenstone belts in the Yilgarn Block, Western Australia. Precambrian Res., 26: 333--361. Sm--Nd geochronology has been used to date 3 widely spaced metavolcanic (greenstone) belts in the Yilgarn Block. The measured isochron ages are 2.78 -+ 0.03 Ga (initial end = 2.5 + 0.3) for greenstones at Kanowna in the Eastern Goldfields Province, 3.05 -+ 0 . 1 0 Ga (initial e n d = 0.9 -+ 0 . 7 ) for Diemals--Marda in the Southern Cross Province and 2.98 -+ 0.12 Ga (initial end = 0.7 -+ 1.2) for the Warriedar fold belt in the Murchison Province. These data show that substantial crustal components of age ~ 3,0 Ga exist in the Southern Cross and Murchison Provinces, and support published evidence that the Eastern Goldfields Province contains no crustal material ~ 2.8 Ga. These ages are all significantly younger than ages commonly observed in the Western Gneiss Terrain and so support the suggestion of a progressive crustal age trend across the Yilgarn Block. The measured age values depend strongly on the inclusion of data for felsic components of the greenstone belts; data for mafic and ultramafic units allow that eruption in the 3 localities could have been contemporaneous at ~ 2.8 Ga. The age range of metavolcanic rocks within each province is possibly comparable to the maximum possible age difference between study areas ( - 0.2 Ga), so the documented lithological variations between the greenstone successions of the 3 provinces cannot be attributed to depositional age differences. Leaching experiments on altered Warriedar samples indicate that both alteration and isotopic homogenisation occurred early in the evolution of the system, probably prior to the ~ 2.6 Ga regional metamorphic event. The initial e n d values do not conform to previously suggested mantle depletion curves. When considered with published data for greenstone belts they suggest complex mantle heterogeneity, with end diverging over time to both positive and negative values.

*Present address: School of Physics and Geoseiences, Western Australian Institute of Technology, Kent Street, Bentley, 6102, Western Australia, Australia.

0301-9268/84/$03.00

© 1984 Elsevier Science Publishers B.V.

334

INTRODUCTION

Occupying 700 000 km 2 of southwestern Western Australia (Fig. 1), the Yilgarn Block is one of the world's largest Archaean cratonic blocks, and displays the full range of terrains characteristic of such regions (Gee, 1979). I

I

I

117 °

1:20°

123 °

t3

- 28 °

PROVINCE

% SC, CROSS

PROVINCE

.31 °

PERTHle i~ ~

.34 °

~

WESTERN

~_~w

~

-v'~,,~

i ~" ~ ' ~ " -

~

0

I

I

I Domal and discordant granite

•

Study localities

1

Greenstone belts

2

Warriedar Diemals- Marda

Banded gneiss, migmatite, metasediment

3

Kanowna

--

3z

200 k m

I

114 °

117 °

120 °

123 °

I

I

I

I

Fig. 1. Generalised geology of the Yilgarn Block, showing study areas and Province boundaries (after Gee et al., 1981).

335

Medium and high grade metamorphic rocks in the western part of the craton constitute the Western Gneiss Terrain; the remainder is composed of classic granite--greenstone terrain. On the basis of structural, lithological and geochemical characteristics, the granite--greenstone part of the craton has been subdivided into distinct provinces, initially by Williams (1974) and subsequently somewhat differently by Gee et al. (1981), as shown in Fig. 1. Gee et al. (1981) suggested that the Eastern Goldfields, Southern Cross and Murchison Provinces correspond to 3 separate basins of deposition in which the greenstones originally accumulated as successions of volcanic and sedim e n t a r y formations. The evolution of the Yilgarn Block is now known to have spanned most of Archaean time, with crustal differentiation ages varying from 3.6 Ga (DeLaeter et al., 1981a) to at least 2.8 Ga (McCulloch and Compston, 1981), and it should provide a good testing ground for models of Precambrian crustal evolution (e.g., Fletcher et al., 1983a, b). As noted by Glikson and Lambert (1973), a general increase in metamorphic grade is evident from east to west across the craton, i.e., from greenschist facies around Kalgoorlie to amphibolite and granulite facies in the Western Gneiss Terrain. When all previous geochronological data are considered, it is apparent also that the oldest reported ages occur in the west. Glikson and Lambert (1973) explained these differences as due to an eastward crustal tilting causing deeper crustal levels to be exposed in the west. They regarded the differences in ages as a consequence of "(a) the failure of Rb--Sr isotopic dating to determine the original (igneous) ages of the greenstones, and (b) a geochronological stratification of granites whereby the ratio of K-granites to Na-granites increases towards shallow crustal levels." However, it is possible that the western part o f the craton is genuinely older than the east, and that some process of crustal accretion, such as that associated with laterally migrating "marginal basins" (Tarney, 1976; Burke et al., 1976) is responsible for the differences observed. Gee et al. (1981) noted this possibility, but did not favour it for various regional geological reasons. This paper presents the first results of a Sm--Nd isochron study of greenstone belts located across the Yilgarn Block: Analyses of samples from one greenstone belt in each of the 3 granite-=greenstone provinces have been completed. Prior to this, the only successful age determinations on greenstones were in the Eastern Goldfields Province (McCulloch and Compston, 1981). De Laeter et al. (1981b) have reviewed most of the geochronological data available prior to 1981 for the entire Yilgarn Block; some additional and more recent data are referred to below. A N A L Y T I C A L DETAILS

Chemistry Weighed samples (~ 0.35 g) were digested at 190°C using HF in steel-cased teflon digestion vessels, and converted to chloride solutions with HC104 and

336 HC1 acids. The resulting solution was split ~ 4:1 and the smaller split spiked with a calibrated lS°Nd + 147Sm tracer prior to separating Sm and Nd. The separation procedures were closely modelled on those developed by Lugmair and co-workers (Lugmair et al., 1975). Rare earth elements were separated from major elements in gravity fed columns of Dowex AG50W-X8, 200--400 mesh resin using 2M and 4M HCI. Sm and Nd were isolated in quartz-and-teflon columns, 20 cm high × 2 mm diameter, using specially prepared AG50W-X8, 200--400 mesh resin and 0.2M 2-methyllactic acid adjusted to pH 4.60. The column reservoirs were pressurised to give an elution rate o f ~ 2 ml h -1. After evaporation, the final eluted samples were treated with aqua regia and HNO3 to remove any NH4C1 and as much of the organic residue as possible. The 'S°Nd + 147Sm tracer was prepared from separated isotopes (ORNL; lS°Nd/> 9670, 147Sm ~ 9870) and calibrated against stoichiometric Sm and Nd solutions prepared from specpure (Johnson-Matthey) Nd2Os and Sm2Os. The oxides were ignited to constant weight at 900°C in quartz crucibles, and rehydration weight increases were monitored during weighings. The estimated uncertainty in calibration of the tracer is + 0.270 and this figure is incorporated into all data. Comparisons with the Caltech dual normal Sm+Nd standard (Wasserburg et al., 1981) indicate agreement within calibration uncertainty. Procedural blanks, including the chemistry and mass spectrometry, were < 1 ng for Nd and <0.1 ng for Sm. Strontium was isolated during the early elution of major elements from the first ion~exchange column.

Mass spectrome try Isotopic analyses were carried out in a modified AEI MS12 mass spectrometer. A deep Faraday cup was installed for ion collection and ion currents were measured using a vibrating reed electrometer (Cary Model 401MR), a voltage to frequency converter (Hewlett Packard Model 2212B) and a counter (Hewlett Packard Model 5301A). The linearity of the measurement system was checked using a voltage divider constructed from precision resistance boxes (Shellcross Model 6800) calibrated against a voltage standard (Fluke Model 335D). After software correction of the digitised o u t p u t based on these measurements, non-linearity errors were believed to be < 0.00370 of readings from 10 to 10070 of full scale for the 300 mV and 1 V ranges of the electrometer. A dynamic zero routine was incorporated into the data acquisition software to correct for time constant effects in the electrometer. When operating on the standard time-scale for peak switching (3 s delay, 3 s integration) the correction is 0.00870 of the change in reading. Measurements on natural barium samples using a range of ion currents from 10-1~. to 10 -1° A indicated that nonlinearity effects due to polarisation of the 1011~2 input resistor were

337 n o t detectable at ~< 1 0 - ' I A where the measurements of Nd and Sm were made, and insignificant for Sr data at currents of 3 × 1 0 - ' I A which was set as the upper limit for that element. Data acquisition was performed under computer control (PDP 11/10), using a Hall effect gaussmeter/controller (Bruker Model B - H l l D ) to control the magnetic field, and the remote range-switching facility of the Cary 401MR to select the appropriate electrometer range. All isotopes were measured on the same electrometer range. An investigation o f the o p t i m u m conditions for Nd (Fletcher et al., 1980) led to the adoption of a triple filament ion source, with measurements being made on metallic (Nd +) beams. Samples were loaded onto the side filaments of an outgassed rhenium filament assembly and taken to red heat in air prior to mounting in the mass spectrometer. To optimise data quality, most of the available sample was loaded in all cases except the few high-Nd samples, for which ~ 3 - - 5 pg was loaded. The centre filament was operated at 1870-1900°C and the temperature of the side filaments was increased until 142Nd÷ beams of 3 × 10-12A were achieved. Data collection was initiated at this time. The beam generally grew, with little or no increase in side-filament temperature, to ~ 10-11 A over a period o f ~ 1 h and was normally maintained at this level for the duration of the analysis. For some small samples, data were collected on the upper half of the 300 mV (3 X 10 -12 A) range. Isotopic ratio determinations were made using a peak-stepping sequence. The spectrum was stepped in a symmetrical mode, always returning to the reference isotope (142Nd) between measurements of data isotopes. The first half of a sequence measuring the isotopes 142, 143 and 146 would be 142

142 143

142 143

142 146

142 146

with the second half being the reverse pattern. After recording a set of 3 such data sequences (sweeps) the isotopic ratios were determined by linear interpolation between successive measurements, then averaged. The standard deviation of 143Nd/'42Nd ratios within a data set was typically 0.01%--0.03%. At least 10 sets of data constituted an analysis. The means of the data sets were individually corrected for mass fractionation, normalising to 146Nd/142Nd = 0.632265 (Lugmair et al., 1976) prior to grand averaging. This normalising ratio was chosen because it was central to the raw data ratios of typical analyses, and errors introduced in correcting for mass discrimination effects would therefore be minimised. It was anticipated from the data of Lugmair et al. (1976) that this normalisation would be equivalent to using the more c o m m o n 14~Nd/144Nd = 0.7219. Our measured value is higher by marginally more than analytical uncertainties at 143Nd/144Nd = 0.72192. Renormalising to 0.7219 would increase 14aNd/'44Nd data by 0.13 e-units. Final 143Nd/142Nd values were converted to 143Nd/144Nd using 144Nd/ '42Nd = 0.875811 + 0.000019, the value determined in our first compilation of measurements on the La Jolla standard (see below).

338

Spectral contamination from Ce was occasionally observed early in analyses. The interference was monitored at ~"°Ce and data were not considered acceptable until the contribution at ~"2Ce was confirmed to be <0.001% of 1,2Nd" Samarium was analysed under similar conditions to Nd, although slightly higher side filament temperatures were required. Measurements of STSr/86Sr were made on the same mass spectrometer as the Sm--Nd measurements, following the procedures described by DeLaeter et al. (1981c). The mean measured 87Sr/86Sr for the NBS-987 standard during this study was 0.71019 + 0.00004 (normalised to 88Sr/86Sr = 8.3752).

Interlaboratory Nd standards Measurements of ~"3Nd/l""Nd in interlaboratory Nd isotopic standards are compiled in Table I. Our primary standard has been the La Jolla Nd standard (e.g., Lugmair et al., 1976). The analyses of this standard have been grouped into 3 time blocks, each covering about 7 months and corresponding to major phases of the analytical programme. The Nd/3 and BCR-1 analyses span most of the analytical programme. TABLEI Measured 143Nd/144Nd values for i n t e r l a b o r a t o r y N d s t a n d a r d s , n o r m a l i s e d t o 14~Nd/~42Nd = 0 . 6 3 2 2 6 5 (see t e x t ) Sample

n

143Nd/144Nda

o eNd

14 12 12

0 . 5 1 1 8 4 3 ± 11 0 . 5 1 1 8 4 6 ± 11 0.511850± 9

- 1 5 . 1 5 ± 0.21 - - 1 5 . 1 1 ± 0.21 --14.97 ± 0.18

Caltech Ndl3

8

0.511894 ± 11

- 1 4 . 1 6 ± 0.22

BCR-1

5

0 . 5 1 2 6 3 0 +- 12

+0.20 ± 0.24

LaJollaNd

(1) (2) (3)

aUncertainties are 95% c o n f i d e n c e limits.

From the reproducibility of our La Jolla Nd measurements we have set the minimum uncertainty in any ~43Nd/144Nd result at 0.000025. Table II gives cumulative average measurements of all isotopes of the La Jolla Nd standard. T A B L E II Measured i s o t o p i c c o m p o s i t i o n o f La JoUa s t a n d a r d Nd, n o r m a l i s e d t o ~46Nd/142Nd =

0.632265 (see text) 143Nd/142Nda

144Nd/142Nd a

14SNd/142Nda

148Nd/14ZNda

lSONd/142Nda

0.448281 ± 5

0 . 8 7 5 8 0 6 _+ 8

0.305210 ± 8

0.211596 + 5

0.207103 ± 6

a U n c e r t a i n t i e s are 95% c o n f i d e n c e limits.

339

Data presentation The Sm--Nd data for the 3 greenstone belts are presented separately in subsequent sections. The Rb--Sr data d o n o t greatly expand on published information and the data are therefore recorded only as an addendum (Appendix II) although the resulting age values are discussed in the text. The decay constants used are k(147Sm) = 6.54 × 10 -12 a -1 and k(STRb) = 1.42 X 10 -11 a-1. Where necessary, quoted Rb--Sr data have been recalcu0 Nd lated to the latter value. Tabulated eNd and TCHUR values were calculated in the conventional manner using present-day CHUR parameters '47Sm/ 144Nd = 0.1967 and 143Ndf144Nd = 0.51262 (renormalised to 146Ndf144Nd = 0.632265 from Jacobsen and Wasserburg, 1980). Isochron line-fits were made using the programmes of McIntyre et al. (1966), with 147Sm/*44Nd data offset to optimise the initial eNd values (Fletcher and Rosman, 1982). Uncertainties are quoted at the 2am level'. All initial eNd values quoted for published isochrons have been re
Geological outline The Kalgoorlie (Kanowna--Kambalda) greenstone belt is the best known portion of the extensive greenstone terrains which dominate a large part of the Eastern Goldfields Province (Fig. 1). Outcrop in the region is poor, and despite extensive geological studies (Williams, 1970, 1976; Gemuts and Theron, 1975) there is considerable uncertainty concerning the stratigraphic relationships of mappable units, both within the study area north of Kanowna (Fig. 2) and between this and the Kalgoorlie--Kambalda section of the belt. Previous isotopic studies have indicated that the metavolcanic and metasedimentary rocks of the Kalgoorlie greenstone belt are between ~ 2 . 8 and ~ 2 . 6 Ga, and intrusive granitoids are ~ 2 . 7 5 - - - - 2 . 5 Ga (data reviewed b y DeLaeter et al., 1981b). McCulloch and Compston (1981) determined a S t a Nd isochron age of 2.91 -+ 0.17 Ga for metavolcanic rocks from Kambalda. Inclusion of two sodic granites and a felsic p o r p h y r y in the sample suite produced a much more precise age of 2.79 -+ 0.03 Ga b u t this is subject to the uncertainty of origin of the felsic samples relative to the basalts. McCulloch and Compston also analysed samples from the Kanowna area, but obtained only a relatively imprecise isochron age of 3.13 -+ 0.25 Ga. New data are presented below for 7 samples, ranging in composition from peridotite to rhyolite. They were selected from a large suite of specimens collected specifically for this study, selection being made on the basis of least alteration and greatest compositional spread. Figure 2 shows the locations and geological settings, and petrographic descriptions are summarised

340

0 I

10 km I

Kalpini Group : mafic and ultramafic volcanics with minor volcanogenic sediments Gundockerta Group : felsic to intermedmte voicanics and volcanogenic sediments Mulgabbie Group : mafic and ultramafic volcanics with minor volcanogenic sediments and felsic volcanics Ginoalbie Group: mentioned in text Morelands Group : mafic and ultramafic volcanics with minor volcanogenic sediments and felsic volcanics

~

Graniticrocks ~

""U'-" ®

Fault

Road

Unconformity

-----

Sampletocality

I

I

Track I

Railway

Fig. 2. Geological sketch map of the Kanowna study area, showing sample locations.

341

in Appendix I. Some of the felsic samples are from outcrops sampled by Turek and Compston (1971) for Rb--Sr analysis. Although all the samples in this suite are volcanics, they are subject to the same general uncertainties concerning a common mantle source as mentioned above for Kambalda felsic samples. As Fig. 2 shows, the samples are believed to come from the Mulgabbie and Gundockerta Groups (Williams, 1976), which are respectively the third and fourth oldest groups recognised in the area (Williams, 1970). However, alternative interpretations are possible which would place all but sample 63217 in the two lowest stratigraphic groups (Moorlands and Gindalbie Groups; Williams, 1970). Poor outcrop, faulting and the distance between 63217 and the other samples renders the relationship of this to the other samples particularly uncertain. All 7 samples have suffered static, low-grade metamorphism. Most show little evidence of weathering, though clouding of feldspars and matrix sericitisation in sample 63202 suggests some weathering or hydrothermal alteration, and samples 63209, 63210 and 63211 may be slightly weathered. Sample 63206 contains minor carbonate (<0.5%) which is separate from silica veins in the rock. Data

The Kanowna data (Table III) give a well defined isochron (Fig. 3) with only 1 of the 7 points deviating significantly from the line. The isochron age of 2.78 + 0.07 Ga (Model III) is statistically indistinguishable (at the 95% confidence level) from the 2.79 + 0.03 Ga measured at Kambalda and 3.13 + 0.25 at Kanowna by McCulloch and Compston (1981). However, the initial eNd of 2.3 + 0.5 differs from their Kambalda value of 3.4 + 0.3. Omitting the one discordant point (sample 63206, the only sample of this suite containing carbonates) improves agreement with the Kambalda age by reducing T A B L E III S m - - N d d a t a , K a n o w n a greenstones Sample

Sm (ppm)

Nd (ppm)

147Sm/~44Nda

~43Nd/~44Nda

eNd° a

63202 63206 63209 63210

4.0 3.3 0.4 0.4 0.4 3.2 2.3 4.8

15.3 12.7 1.0 1.1 1.1 12.0 8.6 30.3

0.16059 0.15645 0.22701 0.22684 0.22613 0.16003 0.16074 0.09636

0.512059 0.511948 0.513303 0.513311 0.513291 0.512079 0.512077 0.510918

-10.9 -13.1 +13.3 +13.5 +13.1 -10.6 -10.6 -33.2

63211 63213 63217

± ± ± ± ± ± ± ±

45 39 101 117 91 45 52 24

± ± ± ± ± ± ± ±

25 30 34 40 37 25 25 25

aUncertainties a r e 9 5 % c o n f i d e n c e l i m i t s . b U n e e r t a i n t i e s f r o m quoted limits f o r ~47Sm/~44Nd a n d 143Nd/144Nd.

TI~cduR(Ga) b

± ± ± ± ± ± ± ±

0.5 0.6 0.7 0.8 0.7 0.5 0.5 0.5

2.36 ± 0.13 2.53 ± 0.14

2.57 ± 0.04

342

the uncertainty at Kanowna to a preferred value of + 0.03 Ga (Model I), but the initial eNd value remains distinct at 2.5 + 0.3. The omission of 63206 is subjective; the strongest argument for doing so is that the remaining data are aligned so well. In fact the same age value results if all felsic samples are omitted, the mafic--ultramafic subset giving 2.80 + 0.07 Ga, but this observation is limited by the small number (4) of data points involved; to = 0.17 Ga. Very little change in the age estimate would result from combining these data with the McCulloch and Compston (1981) Kanowna data because of the greater internal scatter in the latter.

~--~/_/~.o/

KANOWNA Srn - Nd

0"513 143Nd

/ , ~ H U R

144N d

~

0"512

±007:Ga 0"511 /e I i 0"10

i

=

,

i . . . . 0-15 147S m

i , 0-20

,

, "25

144Nd

Fig. 3. S m - - N d i s o c h r o n p l o t for the Kanowna greenstones. The line p a r a m e t e r s i m p r o v e t o T = 2.78 -+ 0.03 Ga, initial eNd = 2.5 + 0.3 if t h e o n e d i s c o r d a n t p o i n t is o m i t t e d .

Initial eNd values are susceptible to interlaboratory calibration differences. In the specific case o f the Kambalda (ANU)--Kanowna (WAIT) comparison, 143Nd/144Nd data can be compared directly only through measurements on BCR-1, and the analytical uncertainties in these data would allow differences up to 0.3 e-units. Differences in 147Sm/144Nd calibrations would, within calibration uncertainties, permit isochron rotations giving 0.1--0.2 e-unit differences in measured initial e n d values. To eliminate this uncertainty, a direct-measurement comparison was made. Two samples, 72-19 and 72-916, from the ANU Kambalda suite, were re-analysed in this laboratory and the data are seen (Fig. 4) to be compatible with our Kanowna data. There is therefore no simple regional variation in eNd as suggested previously (Fletcher et al., 1982). Using these 2 samples as the basis of a full comparison between the data sets (e.g., setting Fig. 4 against Fig. 2a inset of McCulloch and Compston, 1981), a difference of ~0.4--0.5 remains between the Kanowna and Kambalda measured initial e n d values. It is most likely that there is real variation among the rocks at both localities and that the measured values are biased by the limited sampling, especially of ultramafic rocks. The cause of the variability is uncertain. It could be the result of

343 alteration, as seen to a greater degree in the Warriedar data presented below, or it may represent real variability in mantle magma sources or in degrees of crustal contamination of magmas {Carlson et al., 1981). Since there may be real source variability, the m e a s u r e d Kanowna and Kambalda initial eNd are treated as independent values in the discussion below, however, a best regional value for the Eastern Goldfields would be 3.0 + ~ 0.5.

63211

S(E)

72-916 |

63210

o

1

63213

72-19

63209

_ _ _ _ A _ _ 1 _ _ _ _ _ _ 2 _ _ _ £ _ _ ~ 0.16 0.20

0.24

147Sm//'144Nd Fig. 4. D e v i a t i o n p l o t s h o w i n g K a m b a l d a samples 7 2 - 9 1 6 and 7 2 - 1 9 against the best-fit line for four K a n o w n a mafic volcanics. D e v i a t i o n s ( 6 ) are in parts 10 -4 (e-units) and the u n c e r t a i n t y band s h o w n is -+ 0.3 e.

The Rb--Sr data (Appendix II) do not give a perfect line fit (MSWD = 5.5), Model II gives an unreasonably large age uncertainty (-+0.3 Ga) and Model III gives I.R. < 0.7. The best values, from Model I, are an age of 2.51 -+ 0.03 Ga and I.R. 0.7009 -+ 0.0003. This is in slight disagreement with Turek and Compston's (1971) 2.575--2.635 Ga model-age data for felsic volcanics from this area [association II in that paper, following Williams (1970), but later remapped as Gundockerta Group as in Fig. 2 (Williams, 1976)]. It is, however, indistinguishable from their value of 2.54 -+ 0.04 Ga for a felsic volcanic outcrop within association IV (Gundockerta Group). Interpretation of the Rb--Sr data is difficult and potentially ambiguous because of the ease with which Rb--Sr systems can be disturbed. This is reflected in the anomalously low model ages (<2.2 Ga) of samples 63209 and do provide some support for an association seen in the 63210, but the dat~Nd Sm--Nd data: the TCHUB for the felsic samples range from 2.36 -+ 0.13 to 2.57 -+ 0.04 and the 3-point Sm--Nd isochron for these samples gives a poorly defined age ~ 2 . 6 5 Ga. Since the Rb--Sr age value is almost entirely defined by the felsic samples, it is clearly possible that mantle fractionation and extrusion of the felsic volcanic units occurred substantially later than the 2.8 Ga indicated by the overall Sm--Nd data set and the mafic--ultramafic subset.

344 DIEMALS--MARDA

Geological outline The Diemals--Marda greenstone belt is located centrally in the Southern Cross Province of the Yilgarn Block between latitudes 29 ° ,30'S and 30 ° ,30'S and longitudes 119°E and 120°E (Fig. 1). Recent studies (Hallberg et al., 1976; Chin and Smith, 1981; Walker and Blight, 1981) have indicated that the belt has a relatively simple stratigraphy, unlike the cyclic or rhythmic stratigraphy inferred for the adjacent Eastern Goldfields Province (Williams, 1970; Gemuts and Theron, 1975). Three major stratigraphic divisions are seen: a thick mafic--ultramafic greenstone sequence, an epiclastic arenaceous sequence and a calc-alkaline complex (Fig. 5). The lower greenstone sequence consists of metamorphosed mafic and ultramafic volcanic rocks with concomitant intrusive rocks, intercalated with minor pelitic and graphitic metasediments and banded iron formation. Quartz-rich metasedimentary rocks are exposed at the base of the sequence along the southwestern margin of the Marda belt. Banded iron formation is increasingly prominent towards the top of the thick mafic--ultramafic pile. Above the banded iron formation the proportion of pelitic sedimentary and felsic tuffaceous rocks increases markedly. This sequence is unconformably overlain by the Diemals formation, an epiclastic arenaceous sequence which, in turn, underlies and intertongues with the Marda felsic volcanic complex. The Marda Complex contains the stratigraphically youngest rocks exposed in the Diemals--Marda greenstone belt, and has yielded a Rb--Sr isochron age of 2.63 -+ 0.08 Ga, I.R. = 0.7029 -+ 0.0015 (Hallberg et al., 1976). The Marda Complex is a calc-alkaline pile, petrographically ranging from andesite to rhyolite, which is chemically distinct from the underlying tholeiitic basalts. The complex is intruded by a granophyre granite which is chemically indistinguishable from the extrusive felsic pile. Metamorphism ranges from low--medium static at the base of the mafic-ultramafic pile to very low grade static for the stratigraphically high Marda Complex. Dynamic metamorphism and shearing is confined to discrete zones. Ten samples were collected from 2 main areas ~ 60 km apart (Fig. 5). Seven samples, spread over 16 km west of Diemals Homestead, were collected from tholeiitic and komatiitic basalt localities within the lower greenstone succession. A further 3 samples, spread over 5 km near Atkinsons Find, were collected from andesite, ignimbrite and granophyre localities of the Marda Complex. Appendix I gives detailed petrographic information. The mafic rocks from Diemals are metamorphosed uniformly at low grade. They show little evidence of weathering, apart from spotty limonitization in sample 63662, and rare limonite in samples 63667 and 63672. The Marda samples are rather more variable. All have been subjected to

345

low grade metamorphism, but only in sample 71178 has the original texture been modified. In this sample fine, synkinematic sericitization is pervasive. Minor carbonate is present in sample 71179 and heavy dusting of feldspar together with abundant limonite in sample 71180 suggest appreciable weathering.

-29o30

I

I

119O00 '

119 o 30'

i

120000

'

:) 3 0 ' "

i .30o00 '

oo0,.

0 I

•- 3 0 ° 3 ° ' ~

20 km I

Granite

.~uo30"-

Granophyre Felsic metavolcanics- Marda Complex Arenaceous metasediments - Oiemais Formation

Pelitic metasediments Banded iron formation Mafic - ultramafic metavolcamcs

Unconformity @

Samplelocality

1 2 3 4 5 6 7

63655 63656 63662 63667 63672 63673 63676 71178 71179 71180

Fig. 5. Geological sketch map of the Diemals--Marda greenstone belt, showing sample locations.

346

Data Data for the Diemals--Marda greenstone belt (Table IV) define a scattered i s o c h r o n ( F i g . 6) w h o s e a g e v a l u e o f 3 . 0 5 -+ 0 . 1 0 G a ( M o d e l I I I ) is s u b s t a n t i a l l y o l d e r t h a n t h e K a n o w n a - - K a m b a l d a a g e . T h e i n i t i a l eNd = 0 . 9 + 0 . 7 is a l s o d i f f e r e n t . W i t h o n l y a f e w e x c e p t i o n s t h i s a g e is s i g n i f i c a n t l y g r e a t e r t h a n a n y k n o w n r a d i o m e t r i c ages in e i t h e r t h e E a s t e r n G o l d f i e l d s o r S o u t h e r n Nd ages: 293 + 0.03 Ga for C r o s s P r o v i n c e s . T h e e x c e p t i o n s a r e a l m o s t all TCHUR Archaean sediments at Coolgardie (McCulloch and Wasserburg, 1978, recalculated to the CHUR parameters of Jacobsen and Wasserburg, 1980) and t w o v a l u e s o f 2 . 9 8 G a in g n e i s s a n d g r a n i t e t o t h e n o r t h o f D i e m a l s ( u n p u b l i s h e d d a t a , t h i s l a b o r a t o r y ) . O n t h e o t h e r h a n d , t h e age is i n d i s t i n g u i s h a b l e f r o m w i d e s p r e a d g n e i s s a g e s o f ~ 3 . 1 G a in t h e W e s t e r n G n e i s s T e r r a i n ( F l e t c h e r e t al., 1 9 8 3 a , b ; M c C u l l o c h e t al., 1 9 8 3 a ) . T h e i s o c h r o n a g e v a l u e is Nd of 3.00--3.02 c o n t r o l l e d l a r g e l y b y t h e 3 M a r d a f e l s i c s a m p l e s , w h o s e TCHUR Ga are indistinguishable from those of the nearby 2.98 Ga gneiss and granite samples. TABLE IV Sm--Nd data, Diemals--Marda greenstone belt Nd (ppm)

t4vSm/,44Nda

,43Nd/~44Nda

1.5

4.8

0.19414 ± 65

0.512604 ± 25

- 0 . 3 ± 0.5

1.5

4.9

0.18664 ± 53 0.18612 ± 49

0.512526 ± 26 0.512531 ± 32

- 1 . 8 ± 0.5 -1.7 ± 0.6

3.1

10.7

0.17675 -+ 40

0.512256 ± 25

-7.1 ± 0.5 - 9 . 4 ± 0.5 -8.6 ± 0.6

Sample

Sm (ppm)

63655 63656 63662

eNd°a

Nd b TCHuR(Ga)

63667

2.9

10.2

0.17057 ± 40 0.17000 ± 45

0.512137 ± 25 0.512177 ± 29

63672

1.5

3.8

0.23969 ± 62 0.23985 ± 58

0.513420 ± 25 +15.6 ± 0.5 0.513445± 26 +16.1± 0.5

63673

2.0

5.4

0.22766 ± 58 0.27771 ± 54

0.513313± 25 +13.5± 0.5 0.513300± 29 +13.3± 0.6

63676

1.7

5.2

0.19282 ± 49

0.512648 + 25

71178

7.2

0.10564 ± 25

0.510815± 25 - 3 5 . 2 ± 0.5

3.00 ± 0.05 3.02 ± 0.05 3.01 ± 0.05

32

+0.5 ± 0.5

71179

6.7

38

0.10744 ± 30

0.510838 ± 25 -34.8 ± 0.5

71180

6.6

37

0.10778 ± 27

0.510853± 25 - 3 4 . 5 ± 0.5

a Uncertainties are 95% confidence limits.

bUncertainties from quoted limits for t4~Sm/144Nd and '43Nd/~44Nd. T h e s c a t t e r in t h e d a t a ( M S W D = 2 2 ) p r e s u m a b l y r e s u l t s m a i n l y f r o m alteration/metamorphic e v e n t s a n d i t is s u f f i c i e n t t o r e n d e r i n t e r p r e t a t i o n s of the age ambiguous. The Diemals mafic--ultramafic volcanic suite, without t h e M a r d a f e l s i c s , gives a p o o r l y d e f i n e d a g e o f 2 . 8 5 +- 0 . 2 5 G a . O n t h e o t h e r

347

hand, the initial eNd is wholly determined b y the Diemals volcanics, the value changing only marginally to 1.1 + 0.7 if the Marda data are omitted. The 2.85 + 0.25 Ga age indicated by the Diemals volcanics allows the possibility that eruption o f this suite occurred at a time close to the eruption of the Eastern Goldfields mafic volcanics. However, at least the felsic portion of the Diemals--Marda volcanic pile represents material fractionated from the mantle at ~ 3.0 Ga. i

.

.

.

.

i

.

.

.

.

i

.

.

.

DIEMALS- MARDA 0.513

.

/"

Sm-Nd

143Nd

~CHUR

144Nd

°/°

0,512

f 0'511

T = 3-05+ 0-10 Ga

/ /#P h .... 0'10

~Na =0"9"-0"7 a .... n .... 0"15 147Sm 0"20 ____---144Nd

0"25

Fig. 6. Sm--Nd isochron plot for Diemals--Marda.

The Rb--Sr data (Appendix II) show statistically significant scatter, but give a well-defined age of 2.58 + 0.03 Ga, consistent with many Eastern Goldfields and Southern Cross Province Rb--Sr ages (Chapman et al., 1981; DeLaeter et al., 1981b). As with the Sm--Nd isochron, and the Kanowna Rb--Sr isochron, the age is controlled largely b y data for the Marda felsic volcanics which alone give ~ 2 . 5 Ga (n = 3), and it is indistinguishable from the age of 2.63 + 0.08 Ga measured for the Marda complex by Hallberg et al. (1976). This may be a reset age, b u t it could be the eruption age of the Nd felsic volcanics. The age contrast with the TCHUR of 3.0 for these samples would then imply derivation of the felsic volcanics from crustal material extracted from the mantle ~ 4 0 0 - - 5 0 0 Ma earlier. WARRIEDAR

FOLD BELT

Geological outline The Warriedar fold belt is the most westerly well-exposed large greenstone belt in the Yilgarn Block. Centred at 29°S, 117°30'E, it lies in the southwestern part of the Murchison Province (Fig. I). It is b o u n d e d to the west, east and south by granitoid batholiths, and to the southwest b y the Monger fault. To the north it connects, via a thin belt of greenstones, to another (unnamed) greenstone belt.

348

Stratigraphic and structural studies (Muhling and Low, 1977; Baxter and Lipple, 1978; Lipple et al., 1983; Baxter et al., 1983; Baxter, in prep.) have delineated a succession of supracrustal rocks and mafic subvolcanic sills within the fold belt. These are divided into 6 stratigraphic groups, defined primarily by petrogenetic association. There is established superposition for the lower 5 groups. Deformation within the belt is heterogenous, narrow zones of high strain being separated by relatively undeformed rocks. The entire sequence is metamorphosed to upper greenschist facies. Samples for this study were taken from 2 drill cores from Walgardy Well, located in the thin neck of greenstones joining the Warriedar fold belt to the greenstone belt to the north (25°21'20"8, 119°40'30"E and 25°21'11"S, 119 ° 40'31"E). The samples were taken between 81 and 350 m below surface level from a mixed sequence of felsic, mafic and ultramafic volcanic rocks within the lowest part of the stratigraphic sequence. There is no evidence of granitoid intrusions in the immediate vicinity of Walgardy Well and the material appears to be free of any effects of contact metamorphism from the batholiths to the east and west. Appendix I gives petrographic details. All 11 samples have been subjected to low grade metamorphism, the degree of destruction of primary grains ranging from nearly complete in the ultramafic rocks (e.g., sample 44484D) to minor in porphyritic felsities {e.g., sample 44484Q). Small amounts of carbonate are present in most samples b u t it is disseminated, n o t in veins. Weathering and hydrothermal alteration effects are not evident, b u t they could be present and obscured or merged with the more obvious effects o f metamorphism. Data

The effects o f the mineralogical alteration noted in the Warriedar samples are apparent immediately when the data (Table V) are presented on an isochron plot (Fig. 7). The ultramafic samples are clearly the most affected, 0.513

WARRIEDAR

143Nd /

Sm-Nd

$C/HUR

0'512

98 -+ 0"12 Ga 0-511

, /

Y,=g .... 0.10

~Nd= 0"7+- 1'2

, O-15 ~ '

' '0"120 ' ' = ~O-25 147Sm 144Nd

Fig. 7. Sm--Nd isochron plot for Warriedar. The data for two ultramafic samples (=)are not used in determining the line (see text).

349

sample 44484E falling well off the best-fit line, and b o t h it and 44484H exhibiting much lower 147Sm/144Nd than is c o m m o n for ultramafics. Sample 44484E is so clearly disturbed that it has been omitted from the isochron line-fit, and for consistency 4 4 4 8 4 H is also omitted. Several other points also fall significantly off the line, b u t there is no independent justification for omitting them. The 9 remaining points give an age of 2.98 + 0.12 Ga and initial eNd = 0.7 + 1.2, both values being indistinguishable from the corresponding Diemals--Marda values. The measured age is supported b y Pb isotopic data (unpublished, this laboratory) for galena from a major stratiform ore b o d y within the belt, ~ 2 5 km south of the Sm--Nd sample site. The Pb data conform closely to the Model III growth curve of Cumming and Richards (1975) and give a t7/s model age of 3.05 Ga. TABLE V Sm--Nd data, Warriedar fold belt 147Sm/144Ndb

143Nd/144Ndb

e~ld b

T~cduR(Ga) c

8.9

0.09680 ± 33

0.510741±

30

-36.7 ± 0.6

2.85 ± 0.05

44484E 2.4 HC1 l e a c h 0 . 3 residue 2.1

8.0 0.8 7.2

0.17959 ± 75 0 . 2 2 1 2 0 -+ 91 0.17559 ± 52

0 . 5 1 2 0 1 2 ± 37 0 . 5 1 2 8 6 2 ± 37 0.511912 ± 25

-11.9 + 0.7

44484G

1.6

9.8

0 . 1 0 1 9 7 ± 27

0.510818 ± 45

- 3 5 . 2 -+ 0 . 9

44484H

1.2 1.2

4.0 4.1

0.18280 ± 87 0 . 1 8 2 8 6 ± 51

0 . 5 1 2 2 9 4 -+ 5 8 0 . 5 1 2 3 4 3 + 34

-5.4 ± 0.7

Sample

Sm ( p p m a)

44484D

1.4

Nd ( p p m a)

44484M

2.0

6.6

0.18866 ± 72

0.512562 ± 65

-1.1 ± 1.2

44484N

2.3

7.4

0.18900 ± 59

0.512514 ± 53

-2.1 ± 1.0

2.88 ± 0.08

44484Q

1.7

10.1

0.10488 ± 29

0.510805

± 25

-35.4 ± 0.5

2.99 ± 0.05

44484S

1.4

8.8

0 . 1 0 1 5 3 ± 51

0.510773 ± 39

-36.0 ± 0.8

2.94 ± 0.08

44484U

3.1 3.0

17.6 17.1

0.10810 ± 69 0.10809 ± 34

0.510966 ± 60 0.510949 ± 36

-32.6 ± 0.7

2 . 8 6 -+ 0 . 0 7

49578 1.2 HC1 l e a c h 0 . 3 residue 1.0

4.0 1.1 3.0

0.18650 ± 43 0.13947 ± 43 0 . 2 0 2 7 5 ± 52

0.512451 ± 25 0.511630 ± 68 0.512757 ± 49

--3.3 ± 0 . 5 -19.3 ± 1.3 +2.7 ± 1.0

49585 2.2 HC1 l e a c h 1 . 3 residue 0.9

9.0 5.3 3.7

0.14749± 46 0.14669 ± 42 0.14520 ± 50

0.511623± 25 0 . 5 1 1 6 0 4 ± 27 0.511656 ± 65

-19.4± 0.5 -19.8 ± 0.5 -18.8 ± 1.3

a#g/g of w h o l e - r o c k f o r l e a c h - r e s i d u e d a t a . bUncertainties are 95% confidence limits. c U n c e r t a i n t i e s f r o m q u o t e d l i m i t s f o r 147Sm/~44Nd a n d ~43Nd/~44Nd.

In a simple a t t e m p t to gauge the nature of the disturbance to the Sm--Nd isotopic system, t w o discordant samples (44484E, a talc--chlorite rock and 49585, quartz dolerite) and one concordant basalt (49578, "dolerite") were

350

subjected to a 2
WARRIEDAR

HCI LEACH RESIDUE PAIRS

z

"%-=0 z
: (} : []

49585

: O

LEACHES :Open RESIDUES:Solid

- 0.001 - 0.05

0

+ 0.05

/~ 147Sm/44Nd Fig. 8. Scatter plot of Warriedar leach-residue data. Each point is shown as the deviation (~) from the corresponding (measured) whole-rock composition. Time-lines are for reference only.

351 the data (Appendix II) are highly scattered, though they are bracketed b y reference lines o f 2.3 Ga and 2.7 Ga with I.R. ~ 0 . 7 0 5 . DISCUSSION T h e ages measured at Diemals--Marda and Warriedar demonstrate the existence of appreciable volumes of ~ 3 Ga crust in the central regions of the Yilgarn Block and so support the suggestion (e.g., McCulloch et al., 1983a) of a progressive easterly decrease in radiometric ages across this craton. First hints of this age trend were seen in early Rb--Sr data (Compston and Arriens, 1968), and subsequent studies in the Western Gneiss Terrain (Nieuwland and Compston, 1981; DeLaeter et al., 1981c; Fletcher et al., 1983a, b; McCulloch et al., 1983a) and in the Eastern Goldfields Province ( R o d d i c k et al., 1976; Cooper et al., 1978; McCulloch and Compston, 1981; McCuUoch et al., 1983b) have confirmed a strong contrast in the ages of rocks from the east and west portions of the craton. The data presented here reinforce the concept of an age trend in that (1) rocks with Sm--Nd model ages i> 3.1 Ga are c o m m o n in the Western Gneiss Terrain, b u t no rocks of this antiquity have been identified in either the gneissic or granite--greenstone areas o f any other Province within the craton, the ~ 3 . 0 Ga values for Warriedar and Diemals--Marda being as old as any reliable ages reported previously and (2) the ages for Diemals--Marda and Warriedar, whether they are taken to indicate extrusion ages or mantle extraction ages of crustal precursors to the felsic volcanics, are distinctly older than any ages known for volcanic rocks in the Eastern Goldfields Province ( ~ 2 . 8 Ga maximum). The complete evolutionary sequence of the Yilgarn Block remains to be established fully, however, many areas having n o t yet been investigated. The c o m m o n granite ages of ~ 2.7 Ga in the Western Gneiss Terrain indicate that the development of this portion of the craton spanned at least 0.9 Ga and was partly contemporaneous with the formation of the more easterly Provinces. The present data do n o t provide a definitive age comparison between the mafic--ultramafic volcanics of the Murchison and Southern Cross Provinces and those of the Eastern Goldfields Province; eruption in the t w o regions could have been contemporaneous, at ~ 2 . 8 Ga. However, there must have been sialic basement present in the central provinces appreciably older (at ~ 3 . 0 Ga) than any known to underlie the greenstones of the Eastern Goldfield Province. This contrast reinforces the d o c u m e n t e d stratigraphic and lithological contrasts used to distinguish the Eastern Goldfields basin (= Province) from the others (Gee et al., 1981). Since depositional ages cannot be distinguished, and since deposition across the Yilgarn Block may have spanned an appreciable portion of the maximum possible age difference ( ~ 0 . 2 Ga), the lithological variations between the greenstone successions of the provinces must be attributed to lateral facies changes or differences in the character of depositional basins.

352 Published age data for the southern portion of the Southern Cross and Eastern Goldfields Provinces give a complex picture, many of the reported ages being difficult to reconcile. South and east of Diemals, in an area extending across the Southern Cross Province into the Eastern Goldfields Province, Pb--Pb data on gneisses and granites have been interpreted to indicate maximum crustal residence ages for these rocks and their gneissic precursors of ~ 2 . 8 5 Ga (Bickle et al., 1983). This is in accord with indications from earlier Rb--Sr data on the same rocks (Chapman et al., 1981), and with Sm-Nd data on gneisses in the Eastern Goldfields Province (McCulloch et al., 1983b). These ages are younger than the crustal ages indicated by the Diemals--Marda data, but at the same time they argue against a simple age margin corresponding to the structural boundary between the two provinces. Nd The overall picture is complicated by the 2.93 Ga TCHUR of a greywacke within the Kurrawang Formation, - 2 0 km west of Kalgoorlie (McCulloch and Wasserburg, 1978). This age is indirectly supported by an Rb--Sr isochron age of 2.82 + 0.16 Ga for selected granite and porphyry pebbles from conglomerate within the Kurrawang Formation (Compston and Turek, 1973). The Kurrawang Formation includes fragments of banded iron formation of a kind n o t found anywhere close to the present Kurrawang outcrop, the nearest known similar banded iron formations lying ~ 90 km to the west, on the eastern margin of the Southern Cross Province. It is therefore possible that the age measured on the Kurrawang Formation, as an indicator of mantle extraction age, more appropriately applies to the Southern Cross Province than to the Eastern Goldfields Province, the dated epiclastic material having been transported to the east at some later time. Glikson (1970) has reported palaeocurrent evidence for an easterly transport direction within the Kurrawang Formation. Further complication is caused by indicated crustal residence times of ~ 3 . 0 Ga for K-feldspars in the Norseman area of the Eastern Goldfields Province (Oversby, 1975). These ages depend on multi-stage modelling, other interpretations being possible. The whole-rock data reported with the K-feldspar data seem compatible with those of Bickle et al. (1983), so it is at least possible that the older ages calculated for the K-feldspars are overestimated. All available Pb--Pb and Sm--Nd data indicate crustal ages within the Southern Cross Province of ~ 2.8 Ga and ~ 3.0 Ga, and it is possible that both ages are represented in the Diemals--Marda volcanic suite. The 2.85 -+ 0.25 Ga isochron age of the Diemals mafic volcanic suite may indicate that these rocks were drawn from mantle sources ~ 1 5 0 Ma after the basement onto which they were extruded, the overlying Marda felsic suite (TCHUR Nd ~3.0 Ga) being derived from reactivated basement (Taylor and Hallberg, 1977). The Diemals--Marda belt thus may provide a useful testing ground for the general inclusion of felsic samples into Sm--Nd isochron studies of greenstone belts. However, because of the poor outcrop of ultramafic units a substantial drilling programme would be required to provide adequate samples to test this proposition.

353 The HC1 leach-residue data for Warriedar samples give a strong indication that the Sm--Nd system has been essentially undisturbed since early alteration and contamination. The data also suggest an early (diagenetic?) internal isotopic equilibration on the hand-specimen scale, followed by closed system evolution through the ~ 2.6 Ga metamorphic event which disturbed the R b - Sr system. If this is found to be generally true it m a y be practicable to derive useful geochronological data from internal isochrons even in cases where alteration precludes the determination of whole-rock isochrons. The measured initial eNd values provide further evidence for heterogeneity in the Archaean mantle. The Diemals--Marda and Warriedar values do n o t conform to the evolutionary trend suggested by McCulloch and Compston (1981) for the Kambalda portion of the Kanowna--Kambalda greenstone belt, implying mantle heterogeneity on at least an intra-cratonic scale. The differences between Kanowna (this paper) and Kambalda (McCulloch and Compston, 1981) initial eNd values suggest heterogeneity on a much finer scale, b u t it is more likely that these 2 measured values represent different samplings of the one scattered data set, n o t simple regional differences. It is impossible to tell whether the scatter results from variability in initial isotopic compositions or minor alteration effects. When the initial eNd are plotted with data available for other Archaean greenstone belts (Fig. 9) r

r

10 o

6

E'Nd 4

Kambalda Kan°wnat

~

{ "~

I

~ ~ e rwacht~",,.~j Zimbabwe ~ D~emals- Marda .... J. and Warriedar -2 __,.I_ I

i

I

2

3

4

Age (Ga) Fig. 9. Compiled initial end versus age values for greenstone belts. Published data from various sources as recalculated by Fletcher and Rosman (1982). The re-assessment of West Greenland by Hamilton et al. (1983) does not significantly affect the Isua value. The Kambalda and Kanowna data should possibly be combined (see text).

354 the overall picture is one of complex heterogeneity, possibly increasing with time, with initial eNd diverging to negative as well as positive values. Although it is possible that the more negative values may reflect contamination of the magmas by older crustal material prior to eruption this is considered unlikely, and the mantle evolution trends suggested by McCulloch and Compston (1981) and De Paolo (1981) for specific areas cannot be taken as global parameters generally applicable to mantle-derived rocks through geological time. CONCLUSIONS (1) The ages measured for the Diemals--Marda and Warriedar belts demonstrate the existence of significant portions of continental crustal material in the Murchison and Southern Cross Provinces with mantle extraction ages of ~ 3 . 0 Ga. This, combined with data from the Western Gneiss Terrain and the Eastern Goldfields Province, reinforces the suggestion that there was an easterly accretionary development of the Yilgarn Block. (2) The age differences between the 2 central Provinces and the Eastern Goldfields Province are n o t great (~< 0.2 Ga), and the present data allow that extrusion of the mafic and ultramafic volcanics in these 3 greenstone belts may have been contemporaneous. Thus the stratigraphic and lithological differences between the 3 Provinces cannot be attributed to their formation during distinct stages of Archaean evolution. (3) The inital eNd data for the 3 belts argue against the use of any single depleted-mantle Sm--Nd growth curve to represent the source of Archaean crustal rocks. Compiled data imply sustained complex heterogeneity in the mantle rather than a semi-regular progressive depletion of incompatible elements. (4) The Sm--Nd c o m p o n e n t of the Warriedar ultramafic samples has apparently evolved as a closed system since very early alteration and internal isotopic re-equilibration. This raises the hope of obtaining useful age information from internal Sm--Nd isochrons even when whole-rock characteristics preclude the formation of whole-rock isochrons. ACKNOWLEDGEMENTS We are indebted to the Minerals Division of Esso Pty Ltd. who provided the Warriedar drill-core samples, and to M.T. McCulloch who provided splits of the ANU Kambalda samples 72-19 and 72-916. W.G. Libby prepared the petrographic tables. A thorough review of the original manuscript by W. Compston was most valuable. This project was supported by the Australian Research Grants Scheme. The Director of the Geological Survey of Western Australia is thanked for his encouragement of this project.

355 APPENDIX I TABLE A1 Sample details, Kanowna greenstones Sample Locality

Lat. and Long. Field name

Petrological details

63202

N end of Four Mile 30 ° 3 4 ' 1 5 " S Hill, 100 m S of 121 ° 39'15nE road

Porphyritic rhyolite

Porphyritic felsite with quartz, K-feldspar and albite phenocrysts in an ultrafine matrix. Extensive sericitization present.

63206

On track 1 km NE of Harry Dam

30 ° 3 8 ' 2 0 " S 121 ° 38'50"E

Pale green rhyolite

Felsic cognate breccia. At least 70% of the rock is composed of angular fragments. Small, fragmented and/or partly resorbed phenocrysts of quartz, Kfeldspar and albite.

63209

1.5 km N of 'Marquis of Queensbury', 200 m E of track

30 ° 34'35"S 121 ° 36'10"E

Extrusive peridotite

Extrusive peridotite with serpentinized cumulate olivine in a feathery pyroxene matrix. Some fresh olivine is present.

63210

2 km N of 'Marquis of Queensbury', 200 m E of track

30 ° 3 4 ' 3 0 " S 121 ° 36'15"E

Extrusive peridotite

Similar to 63209 except that fresh olivine is absent and fresh clinopyroxene is present.

63211

Island 8 km S of Gordon

30°31'00"S 121 ° 34'00"E

Variolitic basalt

Rugged-texture basalt with varioles of chlorite and feldspar and/or zeolite.

63213

W side of lake, 7.5 km S of Gordon

30 ° 3 0 ' 5 0 " S 121°34'00"E

Komatiitic basalt

Metamorphosed mediumgrained komatiite or metadolerite. Largely consists of pale green amphibole with relict clinopyroxene, ~ 5% plagioclase, epidote and chlorite.

63217

300 m NNW from railway 12 km peg

30 ° 3 8 ' 3 0 " S 121 ° 2 6 ' 0 0 " E

Rhyolite

Trachyte. Consists of ~ 9C% lathlike and finely granular matrix K-feldspar with minor quartz, opaques, rutile and apatite. Quartz is present in aggregates and veins.

356 TABLE A2 Sample details, Diemals--Marda greenstone belt Sample Locality

Lat. and Long. Field name

Petrological details

63655

1.5 km S of Diemals Homestead

29 ° 40'48"S 119°17'59"E

Tholeiitic metabasalt

Fine grained, relict basaltic texture, tremolite--actinolite ( < 45%), plagioclase (<45%), minor opaques, sphene, secondary epidote, quartz, chlorite, some amygdales; low grade static metamorphism.

63656

1.5.km S of Diemals Homestead

29 °40'48"S 119 ° 17'59"E

Tholeiitic metabasalt

Similar to sample 63655.

63662

1 km WSW of Diemals Homestead

29 °40'27"S 119 ° 17'25"E

Tholeiitic metabasalt

Fine grained porphyritic texture, plagioclase (50%) tremolite--actinolite (45%), secondary quartz, chlorite, epidote, some minor amygdales; low grade static metamorphism.

63667

3 kmWSW of Diemals Homestead

29°40'47"S 119 ° 16'03"E

Komatiitic metabasalt

Randomly oriented needles tremolite--actinolite (<65%), interstitial finegrained tremolite--actinolite, plagioclase, trace opaques, secondary carbonate, low grade static metamorphism.

63672

3 km WSW of Diemals Homestead

29 °40'47"S 119 ° 16'03"E

Komatiitic metabasalt

Fine-grained tremolite-actinolite, minor interstitial p|agioclase, opaques, secondary chlorite, low grade static metamorphism.

63673

3 km WSW of Diemals Homestead

29 °40'47"S 119° 16'03"E

Komatiitic metabasalt

Similar to sample 63672.

63676

1.2 km SSW of Copper Mill Bore

29 °36'58''S 119 ° 08'35"E

Komatiitic metabasalt

Long thin needles hornblende (50%), fine-grained recrystallised groundmass of tremolite--actinolite, plagioclase, minor quartz, opaques, sphene, low medium grade static metamorphism.

71178

5.5 km NNE of Atkinsons Find

30°08'40"S 119 ° 19'15"E

Meta-ignimbrite

Altered feldspar phenocrysts set in swirled, devitrified cryptocrystalline felsic groundmass, scattered microspherulitic (in part)

357 TABLE A2 (continued) Sample Locality

Lat. and Long. Field name

Petrological details lithic fragments, secondary carbonates in groundmass and clasts, very low grade static metamorphism.

71179

8 km ENE of Atkinsons Find

30°09'57"S 119 ° 21'50"E

MetaAndesite

Meta-daci'te, altered plagioclase phenocrysts set in medium-grained devitrified quartzofeldspathic matrix with spherulitic growth of feldspar; common quartz, secondary epidote, chlorite, stilpnomelane and carbonate; very low grade static metamorphism.

71180

9 km ENE of Atkinsons Find

30°09'30"S 119 ° 22'30"E

Granophyre

Porphyritic dacite granophyre; euhedral altered plagioclase, quartz s e t i n granophyric matrix, minor blue-green fibrous amphibole, magnetite, very low grade static metamorphism,

TABLE A3 Sample details, Warriedar fold belt a Sample

Down-hole depth (m)

Petrological details

44484D

122

Heavily sericitized quartz--plagioclase--sericite rock. Much of the quartz is coarse, as is the feldspar.

44484E

127

Talc--chlorite rock with a little feldspar, opaques and coarse, euhedral sphene.

44484G

134

A mylonitic quartz--plagioclase---sericite rock with a small amount of carbonate.

44484H

81

Ultramafic chlorite--tremolite--talc rock with abundant coarse mafic relicts and minor carbonate.

44484M

193

Meta-dolerite. Medium-grained hornblende and saussuritized plagioclase predominate, with quartz and minor carbonate.

44484N

206

Metamorphosed dolerite, much like 44484M but with more carbonate.

44484Q

265

Deformed dacite with only minor alteration. Phenocrysts abundant (~ 20%); no fresh mafics but secondary minerals limited to epidote and chlorite.

44484S

291

Deformed and brecciated dacite. Heavily chloritized and sericitized, but carbonate is minor.

44484U

350

Altered porphyritic dacite with preserved texture. Heavily saussuritized and chloritized, but carbonate very minor.

358 T A B L E A3

(continued)

Sample

Down-hole depth (m)

Petrological details

49578

141

U n d e f o r m e d coarse meta-dolerite, mainly heavily saussuritized plagioclase a n d pale g r e e n a m p h i b o l e , a n d less b i o t i t e .

49585

279

U n d e f o r m e d coarse quartz meta-dolerite, mainly quartz, magnetite and chlorite with a fine-grained quartzofeldspathic matrix. Minor biotite and epidote.

a S a m p l e s 4 4 4 8 4 are f r o m E s s o drill c o r e DEW-2 ( 2 5 ° 2 1 ' 1 1 " S , 1 1 9 ° 4 0 ' 3 1 " E ) , 4 9 5 7 8 a n d 4 9 5 8 5 are f r o m DEW-1 ( 2 5 ° 2 1 ' 2 0 " S , 1 1 9 ° 4 0 ' 3 0 " E ) . A P P E N D I X II TABLE A4

Rb--Sr data, all samples Sample

Rb(ppm) a

Sr (ppm) a

S~Rb/S6Srb

STSr/8'Srb

K a n o w n a greenstones 63202

170

50

63206 63209 63210 63211 63213 63217

240 -1 -1 <1 2 300

20 5 8 190 110 65

10.1

± 0.1

1.06674 -+ 20

37.8 0.65 0.62 0.0052 0.035 14.3

± ± ± ± ± ±

0.4 0.15 0.07 0.0007 0.015 0.1

2.08105 ± 0.71731 ± 0.71985 ± 0.70115 ± 0.70212± 1.21134 ±

40 40 33 65 40 60 65 50 90

0.089 0.078 0.081 0.017 0.066 0.087 5.04 1.90 3.76

± ± ± ± ± ± ± ± ±

0.008 0.007 0.012 0.005 0.007 0.009 0.005 0.002 0.004

0 . 7 0 3 8 1 ± 21 0.70353 ± 7 0 . 7 0 4 8 3 -+ 10 0 . 7 0 3 2 6 ± 10 0 . 7 0 2 9 9 -+ 8 0.70612 ± 9 0.88934 ± 8 0 . 7 7 4 5 5 ± 19 0 . 8 4 1 3 2 ± 14

50 2 145 11 120 130 135 145 265 112 33

9.46 0.93 2.20 1.50 0.08 0.02 0.67 1.17 0.230 0.564 0.614

± 0.10 ± 0.25 ± 0.02 ± 0.10 ± 0.02 _+ 0.01 ± 0.01 ± 0.01 ± 0.005 ± 0.006 ± 0.010

26 20 25 26 20 16

Diemals--Marda greenstone belt 63655 63656 63662 63667 63672 63673 71178 71179 71180

1 1 1 <1 1 2 160 140 170

W a r r i e d a r fold b e l t 44484D 44484E 44484G 44484H 44484M 44484N 44484Q 44484S 44484U 49578 49585

160 <1 110 6 4 1 31 60 21 22 9

a B y X R F . U n c e r t a i n t i e s variable. b U n c e r t a i n t i e s are 95% c o n f i d e n c e limits.

1.01548 0.73340 0.77311 0.76714 0.70599 0.70395 0.72948 0.74793 0.71559 0.72205 0.73436

± 20 ± 60 ± 18 ± 23 ± 8 ±8 ± 12 ± 18 -+ 8 ± 8 ± 24

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