The age of the Kambalda greenstones resolved by ion-microprobe: implications for Archaean dating methods

Earth and Planetary Science Letters, 89 (1988) 239-259 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands

239

[2]

The age of the Kambalda greenstones resolved by ion-microprobe: implications for Archaean dating methods J.C. C l a o u r - L o n g 1, W. C o m p s t o n i a n d A. C o w d e n 2,. 1 Research School of Earth Sciences, Australian National University, Canberra, A. C. T. 2601 (Australia) 2 Kambalda Nickel Operations, Western Mining Corporation Ltd., Kambalda, W..A. 6442 (Australia) Received August 18, 1987; revised version accepted April 7, 1988 A "chert" horizon within the Archaean basalt pile at Kambalda, Western Australia, contains zircons from air-fall tufts erupted as the sediment was laid down. Ion-microprobe analyses of these magmatic zircons constrain the long-sought age of the Kambalda lavas to 2692 _+ 4 Ma (2a), and show that regional metamorphism and deformation of the greenstone pile occurred within 30 Ma of deposition. Rare zircon xenocrysts are evidence of pre-greenstone crust as old as 3450 Ma, exposed remnants of which are yet to be identified. Subsets of both the magmatic and xenocryst zircons show evidence of hitherto-unsuspected low-temperature U a n d / o r Pb mobility in the Proterozoic, long after stabilisation of the craton. The ion-microprobe data unravel complexities of isotopic inheritance and alteration, illustrating the hazards of using bulk or whole-rock samples in Archaean isotopic investigations. Identification of interflow "cherts" as a precisely dateable rock type opens the way to direct ion-probe dating of mafic-ultramafic piles, and close numerical age control of the complex, but ill-exposed, stratigraphic and structural relations in this richly-mineralised greenstone belt.

1. Introduction

Two widely important controversies have been sparked by recent studies of Archaean lavas at Kambalda, Western Australia. Debate began when an Sm-Nd isochron for the volcanic sequence yielded an age of > 3200 Ma [1], while concurrently published Pb isotope data [2] suggested a date close to 2700 Ma. This discrepancy has brought into question the use of the whole-rock samples for Sm-Nd isotopic dating, until now very widely employed for Archaean greenstones, and fuelled speculation about the roles of mantle heterogeneity and magma contamination in controlling Archaean Nd isotope systematics [3-5]. At the same time, attention has been focussed on the latter possibility by a theoretical treatment of thermal properties of komatiite [6], which calculated that such extremely hot magmas might have

* Present address: Geology Department, University of Auckland, Private Bag, Auckland, New Zealand. 0012-821X/88/$03.50

© 1988 Elsevier Science Publishers B.V.

a high propensity to become contaminated en route through the crust, and this has led to concern that mantle models based on komatiite chemistry may be spurious [7]. The twin problems of the age of these greenstones and the controls on komatiite composition are inter-related, since a reliable age of eruption is needed to calculate initial isotopic ratios and hence place constraints on the evolution of magma source regions. Defining this date for the Kambalda lavas therefore was, and remains, the fundamental problem to be resolved, and at the heart of this lies the well-known difficulty of dating mafic rocks. Most isotopic schemes are best adapted to dating felsic volcanics, an imbalance it was once hoped the Sm-Nd system would redress. This contribution briefly reviews the uncertainties remaining about the age of the greenstones, then describes the application of a new sampling approach tied with use of the ion-microprobe, that has provided a precise and unambiguous date for the eruption of the mafic lavas, and illustrates the sometimes unexpected complexities of isotopic in-

240

heritance and alteration that have been a cause of ambiguity in previous studies using whole-rock samples. The results will be used elsewhere in combination with new chemical data, to constrain the incidence of candidate processes, including contamination, in the petrogenesis of the lava pile.

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Correlations within the 850 km long Norseman-Wiluna greenstone belt are frustrated by extensive faulting, complex deformation and poor exposure. Indeed, the primary reason for pursuing isotope geochronology is to establish timing relations that field mapping is unable to provide with any confidence. Intensive drilling and exploration has afforded a detailed understanding of field relations in the vicinity of gold and nickel mines at Kambalda and Kalgoorlie, which together comprise one of the most richly mineralised greenstone terrains in the world [8-10]. The two mining areas are 60 km apart and separated by the major Boulder-Lefroy fault, across which correlation would normally be speculative. However, it has been shown that the lithologies and stratigraphic columns of the two areas are closely equivalent [10-13], and a common stratigraphic nomenclature has now been formalised that replaces most of the informal terms in use at Kambalda with existing names for the Kalgoorlie succession [13]. This revised terminology is adopted here. Gold and nickel deposits at Kambalda are concentrated in a pile of mafic and ultramafic lavas collectively known as the Kalgoorlie Group (formerly "Kambalda sequence" [10]), which is the lowermost stratigraphic unit and is overlain by felsic volcanic and sedimentary rocks. The map in Fig. 1 shows these units to be confined to a narrow, NNW-trending corridor bounded to east and west by major strike-parallel faults. Correlations within this fault block are possible, to either side they are hazardous. Between the strike faults the dominant structure is open, NNWtrending, and broadly anticlinal. Rocks of the Kalgoorlie Group, exposed by the Kambalda Dome and structurally repeated at St. Ives, comprise the Lunnon Formation (formerly "footwall basalts") of fine-grained pillow basalts with 6-9% MgO, interrupted by a sediment horizon ~ 200 m below the top; overlain by the Kambalda Komati-

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ite Formation of komatiite lava flows (liquid compositions 22-31% MgO), with sediments separating the lowest flow units; in turn overlain by pillowed and massive basalts with typically 10-15% MgO. Attention here is focussed on the complex basalt pile above the komatiite lavas, which is divided into the Devon Consols Basalt Formation (formerly "lower hangingwall basalts") and the Paringa Basalt formation ( " u p p e r hangingwall basalts") about a regionally persistent sediment horizon known as the Kapai Slate. The base of the Devon Consols Basalts is marked by a thick, internally fractionated basaltic unit, locally known as the Victory Dolerite, in which the zircon xenocrysts studied by Compston et al. [5] were found. The remaining Devon Consols Basalts comprise a pile of variolitic pilow basalts, with some interposed massive units. The Paringa Basalts comprise a thick basalt unit locally known as the Defiance Dolerite, overlain by both pillowed and massive high-Mg basalts with rare interflow sediments.

241

Subsequent to emplacement the rocks have been complexly deformed, a n d the area has been affected by metamorphism up to low amphibolite facies, locally intense metasomatism, and intrusion of Archaean granitoids, felsic-intermediate porphyries, minor mafic dykes and sills, and a Proterozoic dolerite dyke suite. Thus all Archaean rocks are now metamorphic, but local preservation of original textures and compositions merits consideration of the greenstone units as igneous and sedimentary rocks.

3. Previous isotopic studies Important minimum ages for the greenstones have been provided by isotopic studies of granites intruding them. Early Rb-Sr data [14,15] gave widespread granite ages at around 2600 Ma, and an isochron age of 2610 + 25 Ma for closure of the whole-rock Rb-Sr system in basalts. A mineral Pb-Pb isochron obtained for the Kambalda Granite gave an age of 2760 + 70 Ma [16]: an important aspect of the Pb isotope systematics was inference of crustal development in the region dating back to at least 3300 Ma, even though preserved remnants of the older rocks had not been identified. Subsequent conventional analysis of zircons in the Kambalda Granite gave an apparent precise age of 2820 + 15 Ma [17]. However, recent ion-microprobe study of the same body [18] has shown the conventional zircon "age" to be a misleading constraint relaxing to xenocrysts in the granite, whose crystallisation date is 2662 + 6 Ma. Controversy about the emplacement age of the Kambalda greenstones, and the sampling approaches being used to address the problem, began when McCulloch and Compston [19] applied the newly-developed whole-rock Sm-Nd dating scheme to the problem of dating the mafic-ultramafic lavas. An apparent isochron was obtained by combining data for the lavas with those for the granite, and the joint age so obtained, 2790 + 30 Ma, comfortably fitted in with regional geochronological data then available. However, doubts about the procedure of combining plutonic granite with erupted komatiites on an isochron led Claour-Long et al. [1] to revisit Sm-Nd systematics at Kambalda using only samples of the mafic-ultramafic lavas. A wide spread of S m / N d ratios was found among the lavas and defined a tight correlation on an isochron diagram without

recourse to felsic samples. This correlation did not extrapolate through the granite data, and gave an apparent age of 3262 + 44 Ma, more than 400 Ma older than the previously accepted date. By the standards of internal consistency applied to previous Sm-Nd studies (e.g. [19-26], the 3262 + 44 Ma age would normally have been accepted without question, but it was in immediate conflict with Pb isotopic data obtained by Roddick [2], whose Pb-Pb isochron for whole-rock samples of basalts gave a date of 2720 + 105 Ma, supported by a Pb model age of - 2 8 0 0 Ma for sulphides hosted by the komatiites. An isochron age of 2610 _ 30 Ma for closure of the whole-rock Rb-Sr system, and tight clustering of biotite Sr closure dates at 2555 + 10 Ma, also set important minima for the emplacement age of the greenstones. Chauvel et al. [4] confirmed the 400 Ma discrepancy between the Pb-Pb and Sm-Nd ages by analysing an independent sample set of komatiites and basalts. Their Sm-Nd data gave an apparent age of 3230 _ 120 Ma; their Pb-Pb data on the same samples gave a best age stated as 2 7 2 6 _ 30 Ma (we are unable to reproduce this apparent tight precision from their analyses; realistic error estimation [27] gives 2726 + 98 Ma); and their sulphide samples, colinear with the lavas on Pb-Pb plots, gave a Pb model age of 2700 Ma. In addition, a mineral Sm-Nd isochron for a wellpreserved sill intruding the greenstones at Ora Banda, 100 km north of Kambalda, gave an age of 2762 + 32 Ma and E Ni d = - - 1.05 and provided convincing evidence of the sort of long-lived magma source heterogeneity that could invalidate the whole-rock Sm-Nd approach. The difficulty of envisaging disturbance of Pb isotopes in Pb-rich, U-free sulphide ores, and the close similarity of the whole-rock Pb-Pb isochron age and sulphide Pb model age, were used as evidence that the whole-rock Pb isotopes faithfully recorded the crystallisation date of the lavas, and that the age derived from the whole-rock Sm-Nd system was therefore spurious [3,4]. However, it is important to realise that the whole-rock Pb isotopes do not alone raise a serious challenge to the older Sm-Nd isochron. Clearly, Pb model ages are only as good as the model used to describe the source of Pb, and this dating method provides no measures of internal consistency. Roddick [2] obtained a Pb model age

242 of 2800 Ma, whereas Chauvel et al. calculated 2700 Ma by substituting a different model of Pb evolution. The 100 Ma difference is an accurate measure of the uncertainties of this approach, being of similar order to the errors on the Pb-Pb isochrons, and the apparent close corroboration of the two Pb isotope methods must be viewed in this light. Also, it is not possible to rule out the possibility that sulphide Pb isotopic compositions were affected during the amphibolite facies metamorphism, during which the ores reverted (at least in part) to monosulphide solid-solution [28], and may therefore have been open to compositional exchanges with the host supracrustal pile. It must also be realised that the arguments about non-cogenetic whole-rock samples which invalidate the Sm-Nd isochron method equally disqualify the whole-rock samples defining the Pb-Pb "isochrons". As with the Sm-Nd system, it is necessary also for the Pb-Pb method to justify that the wide range in U / P b ratios observed among the whole-rock samples was brought about by igneous processes at the time of eruption. Wide fractionations of this ratio would not be expected among cogenetic basalts--precisely the same problem as lack of igneous fractionation of S m / N d ratios. The possibility that the ages registered by the whole-rock Pb-Pb and model Pb isotopic systems are too young cannot, therefore, be ruled out. Compston et al. [5] addressed this ambiguity with an ion-microprobe study of zircon xenocrysts found in the Victory Dolerite. The xenocrysts include grains with minimum ages between 3100 and 3400 Ma, while one mantle and massive grain are much younger at 2669 + 11 Ma, and a group of distinctive yellow grains are 2693 _+ 50 Ma old. The host dolerite must be younger than the youngest xenocrysts, but two interpretations were given of what this maximum age might be, depending on the relationship of the two youngest zircon groups to their host basalt. An analytically precise estimate is given by the youngest grains at 2669 _+ 11 Ma, but it was allowed that these sparse zircons could belong to later veins in the sample: this leaves the imprecise 2693 _+ 50 Ma age of the abundant yellow zircons as the best available constraint. Whichever maximum age is accepted, these data showed definitely that the - 3 2 0 0 Ma Sm-Nd age derived from whole-rock samples is spurious, and the existence of the xenocrysts requires that at

least one Kambalda basalt has suffered contamination with older felsic crust. 4. Zircons in the Kapai Slate

4.1. Sampling rationale Most available isotopic schemes have now been applied to the Kambalda greenstones in search of an emplacement age. All yield results either reflecting metamorphic resetting, or whose margins of error are large, or whose meaning is hotly debated. The S H R I M P ion microprobe has therefore been applied to the problem in the hope of obtaining precise zircon data that could differentiate between magmatic ages, metamorphic resetting and inheritances. Such studies are normally restricted to felsic volcanics in which the requisite concentrations of magmatic zircons are found. This is unfortunate because, in dating greenstone sequences, ideal targets are marker horizons which might be correlated over wide areas, and such targets are best presented by laterally extensive mafic lavas. Felsic volcanics in the NorsemanWiluna belt, as elsewhere, tend to occur as discrete centres, and structural complexity combines with poor exposure to render relations between adjacent felsic and mafic piles uncertain. So zircon dating of nearby felsic volcanics, while potentially accurate in an analytic sense, would suffer the hazard of uncertain extrapolation to the mafic lavas in question. The presence of zircon in interflow sediments at Kambalda was recorded by Bavinton [29]. A sediment sample has therefore been studied in the expectation of identifying two important zircon populations. A priori it seemed likely that all or most zircon grains would be granitic detritus, and so record the existence, and age, of felsic crustal rocks subaerially exposed at the time of greenstone deposition. A second component thought possible was a contribution from contemporaneous pyroclastic air-fall tufts, which would have the potential to record the date of deposition of the sediment, and therefore of the lava pile. Interflow sediments occur in several stratigraphic positions at Kambalda. Those studied by Bavinton [29,30] occur in the basal portion of the Komatiite Formation and are found throughout the area, although individual horizons have restricted lateral continuity. In contrast, the Kapai Slate separating

243

the Devon Consols and Paringa Basalts satisfies the criterion of being correlatable over a wide geographical area. For instance, it can be traced 40 km south of Kambalda at Tramways [10], and a sediment occurs in a stratigraphically similar position 60 km northwards at Kalgoorlie [8,9,13]. A 1-m-long sample was therefore chosen from a drillcore intersection of the Kapai Slate in a location south of Lake Lefroy (CD1342, 341-342 m; see Figs. 1 and 2). A deep drillcore sample was chosen in the hope of eliminating the problem of recent Pb loss commonly suffered by zircons in weathered rocks. The nature of the Kapai Slate has been little studied, but its gross appearance includes a range of compositions, or facies, similar to the variety of interflow sediments in the komatiite pile which were interpreted by Bavinton [29,30] as principally chemical precipitates. The most important facies are (a) massive, pale to dark "cherts" comprising mainly albite and quartz with up to 15% disseminated iron-sulphides, (b) laminated pale "cherts" with sulphide laminae, and (c) carbonaceous and sulphidic laminated sediments. Fig. 3 shows the sample collected for

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Fig. 3. Photograph of the drillcore sample of Kapai Slate. Down-core is from left to right, and the core is 5 cm in diameter. Interbedded lithologies include pale and dark "cherts", a laminated zone, and sulphide layers, disturbed by some large veins and ubiquitous microveining. Compare with fig. la in Bavinton's [30] description of interflow sediments in the komatiite pile. 90% of the zircons were contained in the laminated section (right).

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Before crushing, the sample was scrubbed, and sawn to remove sulphide layers and large veins. It is not possible to avoid the microveining, which is a pervasive feature at Kambalda, and this clear evidence of late hydrothermal alteration is important in interpreting isotopic data, as will become evident below. Of the remaining sample, an approximate division was made into three lithological fractions: (a) dominantly laminated sediment, (b) dominantly pale, cherty rock, and (c)

244 TABLE 1 Selected ion-microprobe U-Th-Pb data for zircons in the Kapai Slate Grain

U

Th

204pb

f206pb a

207pb/235 U

206pb/238 U

207pb/ZO6pb

Apparent age b

area

(ppm)

(ppm)

(ppb)

(%)

_+lo

_+lo

_+lo

(Ma) _+lo

(a) Type I magmatic zircons forming the principa12692 _+4 Ma age population (2 representatives of the 45 analyses) c 318-6-1 156 50 37 0.72 12.93 _+0.36 0.5156_+100 0.1819_+31 2669 _+29 0.28 13.17 _+0.35 2720 _+27 318-6-3 261 71 23 0.5094_+ 100 0.1875 + 30 (b) Zircon xenocrysts older than 2692 +_4 Ma 290-4-1 150 164 32 290-5-1 60 28 41 290-5-2 295 56 62 290-5-3 85 37 41 290-12-1 296 71 35 290-13-1 360 228 78 290-17-1 131 131 29 290-17-2 341 341 45 293-4-1 278 234 33 293-4-2 265 44 76 293-4-3 563 147 49 293-4-4 443 7 44 293-4-5 423 5 14 293-4-6 451 54 40 312-7-1 265 236 27 312-19-1 d 1841 102 16 312-19-2 889 75 18 312-19-3 d 1042 45 14 312-19-4 d 1577 70 37 312-24-1 291 273 35 312-24-2 d 1288 122 25 312-24-3 'l 1000 111 10 312-24-4 353 322 10 318-5-1 185 86 34 318-5-2 256 175 256 318-5-3 264 174 52 318-5-4 207 77 27 318-10-1 139 85 42 318-10-2 369 389 14 318-10-3 227 17 220

0.46 1.90 0.55 1.15 0.35 0.68 0.55 0.31 0.30 0.75 0.23 0.25 0.08 0.31 0.34 0.03 0.07 0.05 0.08 0.32 0.08 0.04 0.08 0.44 0.22 0.48 0.31 0.90 0.11 3.37

28.90-+0.75 22.13_+0.68 24.17_+0.62 27.18_+0.80 15.62_+0.40 18.06_+0.46 24.69 _+0.66 25.70 + 0.66 24.62-+0.48 21.40 _+0.28 22.84-+0.19 24.23_+0.22 25.17_+0.23 17.52_+0.19 13.02_+0.21 8.64_+0.11 12.75_+0.17 11.23_+0.15 8.94_+0.12 22.58_+0.32 8.02_+0.11 9.00_+0.12 20.84-+0.29 25.40+0.53 26.75 _+0.59 24.73-+0.54 25.11_+0.57 14.61 + 0.41 15.33 _+0.33 11.09_+0.39

0.7250_+180 0.5550_+140 0.6030_+150 0.6700_+170 0.5420_+130 0.5010_+120 0.6390 _+160 0.6780-+ 170 0.6120_+110 0.6061 _+ 50 0.5919-+ 41 0.6257-+ 45 0.6487_+ 47 0.4618_+ 33 0.4680_+ 61 0.4526_+ 57 0.4669_+ 59 0.4766_+ 60 0.4368_+ 55 0.5909_+ 76 0.4147+ 52 0.4272+ 54 0.5657-+ 73 0.6559-+120 0.7043 + 140 0.6478-+130 0.6634_+130 0.5349 _+110 0.5504 + 106 0.4424+ 90

0.2888_+16 0.2895_+39 0.2906_+12 0.2944_+34 0.2091_+10 0.2617_+12 0.2804 _+20 0.2751 -+ 11 0.2916_+16 0.2561 _+23 0.2799_+11 0.2809_+13 0.2814_+13 0.2751_+20 0.2017_+16 0.1385_+ 3 0.1980_+ 6 0.1709_+ 5 0.1484_+ 4 0.2771_+12 0.1403_+ 4 0.1528-+ 6 0.2672_+11 0.2809-+21 0.2755 -+ 22 0.2769_+20 0.2745_+24 0.1981 _+34 0.2021 _+15 0.1818-+49

3411_+ 9 3415_+21 3421_+ 6 3441_+18 2899_+ 8 3257_+ 7 3365 _+11 3335 -+ 6 3426_+ 9 3223 + 14 3362-+ 6 3368_+ 7 3371_+ 7 3335_+11 2840_+13 2209_+ 4 2810_+ 5 2566_+ 5 2328_+ 5 3347_+ 7 2231-+ 5 2377-+ 7 3290_+ 6 3368-+12 3338 _+13 3346_+11 3332_+14 2811 _+28 2843 _+12 2669_+45

(c) Zircon analyses younger than the 2692 -+ 4 Ma magmatic date 290-3-1 242 70 40 0.52 12.66_+0.34 290-8-1 383 254 31 0.26 11.94_+0.31 290-8-2 430 317 24 0.19 10.73_+0.28 293-1-1 c 416 153 37 0.28 12.03+0.23 293-3-2 ~ 555 313 35 0.19 12.36_+0.23 293-7-1 392 141 24 0.19 12.91_+0.14 293-10-1 915 704 123 0.44 10.73_+0.09 293-10-2 845 601 116 0.45 10.81-+0.09 293-11-1 1534 281 43 0.13 6.45_+0.05 293-11-2 1795 353 42 0.08 8.34_+0.06 312-3-1 1786 1796 363 0.82 8.16_+0.11 312-8-1 626 163 14 0.08 10.10_+0.14 312-9-1 500 223 19 0.16 9.44_+0.14 312-9-2 652 280 20 0.13 8.51_+0.12 312-11-1 1520 418 19 0.04 8.37-+0.11 312-18-1 376 179 35 0.33 10.92_+0.17 312-21-1 1289 492 55 0.21 6.32_+0.08 312-21-2 2118 746 15 0.06 3.08_+0.04

0.5080_+130 0.5010+120 0.4650_+110 0.5063-+ 90 0.5354_+ 95 0.5158_+ 37 0.4864_+ 32 0.4823-+ 32 0.3353_+ 21 0.4486_+ 28 0.3912_+ 49 0.4358_+ 55 0.3831+ 49 0.3711_+ 47 0.4475+ 56 0.4441 + 57 0.3199-+ 40 0.1896_+ 24

0.1809_+12 0.1729_+ 9 0.1673_+ 8 0.1723-+10 0.1674_+ 8 0.1815_+13 0.1600+ 7 0.1626-+ 8 0.1396+ 5 0.1348_+ 5 0.1513_+ 5 0.1682_+ 8 0.1787-+11 0.1663_+ 8 0.1357_+ 3 0.1784-+ 12 0.1433_+ 5 0.1177-+ 5

2661_+11 2586_+ 9 2531_+ 8 2580-+10 2532_+ 8 2667_+12 2413-+ 8 2483_+ 8 2222_+ 6 2161_+ 6 2361_+ 6 2540_+ 8 2641_+10 2521_+ 8 2173_+ 4 2638-+ 11 2267+ 6 1922-+ 8

245 TABLE 1 (continued) Grain area

U (ppm)

Th (ppm)

204pb (ppb)

f206pb a (%)

207pb//235U

206pb//238U

2°7pb/2°6pb

+_lo

+_lo

+_lo

Apparent age b (Ma) +_lo

318-2-1 318-7-1 318-7-2 318-7-3 318-15-1 318-17-1 318-19-1 318-20-1

284 1225 816 2580 436 550 363 408

104 302 218 518 163 161 152 143

69 33 30 19 34 27 40 38

0.77 0.11 0.14 0.09 0.36 0.16 0.35 0.34

12.07 ± 0.31 8.28±0.16 9.55±0.19 2.12+0.04 8.33±0.19 11.98+_0.25 12.49+_0.28 10.75+0.24

0.4966+ 100 0.3812± 69 0.4064± 78 0.1257+ 24 0.3456+ 67 0.4846± 93 0.5018+_97 0.4311+_83

0.1763± 22 0.1575± 7 0.1704+ 8 0.1226+ 6 0.1748+__18 0.1793+_11 0.1805+16 0.1808+_16

2618 ± 21 2429± 8 2562± 8 1994± 9 2604+_17 2646+_10 2657+_15 2660±15

a Denotes the percentage of c o m m o n 2°6pb in the total measured 2°6pb. b Apparent age is the 2°7pb//2°6pb age. c These two analyses of the same grain set both the upper and lower 95% confidence limits of the 2692:J:4 Ma group. d < 2692+ 4 Ma area on a Type B xenocryst. e Younger area on a Type I zircon of the 2692 ± 4 Ma magmatic population.

dominantly dark cherty material. Each fraction was processed in conditions designed to minimise particulate c o n t a m i n a t i o n in the laboratory. W a s h e d rock chips were crushed in a swing mill, the resulting powder screened and recrushed as necessary to < - 7 2 # mesh, then processed directly in heavy liquids and magnetically to separate the non-magnetic heavy mineral fraction. Sulphides were r e m o v e d using nitric acid, and the zircons were recovered and m o u n t e d by h a n d picking. Approximately 200 zircons were f o u n d in the final concentrates. 90% of these grains were contained in the laminated fraction, 24 grains only in the dark chert, and a single grain in the pale chert. The zircons were placed in four separatelyanalysed mounts, and sectioned in half by polishing to reveal interiors and rims. U - T h - P b isotopic compositions have been measured for 101 30-/~mdiameter areas on 68 sectioned zircons using the S H R I M P ion-microprobe. By this means, cracks and inclusions in the grains were avoided, and individual growth zones were separately analysed. Reproducibility of the P b / U ratio of our standard zircon SL3 was +2.45% during analysis of m o u n t Z290, +1.75% for Z293, +1.25% for Z312, and + 2 / 0 % for Z318, and these uncertainties are included in the quoted errors. Each analysis is the m e a n of 7 cycles through the mass stations, and data were reduced in the m a n n e r described elsewhere [31,32]. The ratios of P b / U and P b / T h in the samples are referenced to a value of 0.0894 for 2°rpb/238U (equivalent to an age of 552 Ma)

in standard SL3 [31]. Only some representative data are listed in Table 1: full analytical results are available from the first author on request. D u r i n g processing of this sample, the problem of laboratory c o n t a m i n a t i o n was encountered. A single zircon in m o u n t Z290 contrasted in appearance with all others and its ion-microprobe age confirmed it as a Palaeozoic contaminant. In view of this, we c a n n o t absolutely rule out the possibility that some A r c h a e a n grains studied m a y be laboratory c o n t a m i n a n t s also. However, we consider it extremely unlikely because the four analysed m o u n t s resulted f r o m four separate crushings and separations, each of which gave low yields of the zircon types described in detail below. Particulate c o n t a m i n a t i o n (which has been encountered before [5]), is distinct f r o m chemical c o n t a m i n a t i o n of a sample in that no allowance can be m a d e for it. A benefit of single grain analysis is that c o n t a m i n a t i n g grains m a y be identified, whereas conventional analysis of bulked zircon concentrates is liable to smother the evidence and u n k n o w i n g l y incorporate the alien composition as (a tiny fraction of) the bulked average.

4.3. Discordancy and correction for common Pb U - P b isotopic data for individual 30-/~m-sized areas of each zircon are illustrated on a Concordia diagram in Fig. 5. T h e most striking feature of the data is the extreme range in ages of the zircons. Individual 2°7pb/2°6pb ages range over a 1500 M a period, f r o m a m a x i m u m of over 3400 M a to less

246

than 2000 Ma. Within this wide range there is a single dominant group of zircons - 2700 Ma old. There are no other significant groupings, but a scatter of individual older and younger ages. The Concordia diagram also highlights the large pro-

A. g r a i n 318-6

transmitted

D. grain 293-10

G. grain 290-4

transmitted

transmitted

portion of analyses on, or very close to, the Concordia curve. The low incidence of recent differential movement of U and Pb greatly increases the confidence to be placed on obtained ages, and may be ascribed to a combination of the choice of

B. g r a i n 318-9

reflected

C. grain 312-23 transmitted

E. grain 318-7

transmitted

F. g r a i n 312-5

reflected

H. grain 318-5

transmitted

I. grain 312-24

reflected

247 0.80

0.60

0.40

020

,

0.00 0.0

I

I

10.0

I x).0

L

30.0

Fig. 5. Concordia diagram showing the entire range of zircon ages obtained. Individual 207Pb/2(16Pb ages range from < 2000 Ma to 32 3400 Ma, but the magmatic group at 2692 _+2 Ma contains 60% of the analyses. Enlargements of the shaded areas are in Figs. 7. 9 and 10.

a deep drillcore sample below the present weathering zone, and the ability of the ion microprobe to target the least flawed areas of zircon.

Almost all analysed grains have notably low contents of non-radiogenic *04Pb. The low concentrations of common Pb make it difficult to

Fig. 4. Photomicrographs illustrating structural features of zircons in the Kapai Slate. Grains have been mounted in epoxy and = transmitted light, sectioned in half to expose internal zoning. Field of view are approximately 120 pm X 120pm; “transmitted’ “reflected” = normal reflected light with Nomarski interference contrast. Polished surfaces were etched for IS s in HF after analysis to highlight structures by picking out radiation-damaged areas high in U and/or Th from less damaged zones (dark and light tones respectively in reflected light). A-C: Type I magmntic grains. A. The equant faceted form and strongly-developed euhedral zoning typical of Type I zircons, and interpreted as the product of magmatic crystallisation. Analyses of this grain coincidentally set both the lower and upper 95% confidence limits of the 2692+4 Ma magmatic age population. B. Reflected light view of a Type I grain similar to A. The simple internal structure is highlighted as oscillatory growth zoning of compositions with variable lJ contents, consistent with growth in a viscous felsic melt. The pit excavated during analysis is visible in the centre of the crystal. There is no textural evidence of an older core or younger overgrowth, and multiple analyses of similar grains failed to detect age differences between interior and exterior areas. C. This elongate Type I zircon shares the high clarity, oscillatory zoning, faceted shape, and age, of equant Type I grains. Despite its more fragile elongate form, the faceted termination is preserved and there is no indication of rounding or micro-pitting associated with the abrasion of sedimentary transport. D-F: Type M akered high-O zircons. D. A Type M zircon distinguished from unaltered magmatic grains such as A by its relative turbidity and blurring of the internal zoning. This zircon has an elevated U content ( - 900 ppm) and its 207Pb/206Pb age is - 250 Ma younger than the magmatic age population. E. A broken Type M zircon whose interior magmatic zoning is even further degraded than D, and which has both the highest U content and youngest apparent age in the sample. Zircons in A, E and F form a series of increasing degradation of original magmatic zoning, increasing U content, and decreasing apparent age. F. A Type M zircon outwardly similar to C. The strongly-developed oscillatory zoning is not pervasively degraded but instead is deeply embayed by sectors of annealed zircon, considered to be recrystallisation rather than new growth as *‘ghosts” of the overprinted zoning remain. G-I: Type B xemcty~ts. G. This elongate Type B xenocryst has a concordant U-Pb age of 3411-+ 9 Ma. Traces of fine-scale zoning suggest an igneous origin. Rounding of its termination is interpreted as metamorphic corrosion rather than sedimentary abrasion, as the outline is not pitted. H. This - 3350 Ma old xenocryst also preserves igneous-tvpe zoning and has a rounded form. Preservation of facets on some areas of the grain (top tight) suggests local metamorphic corrosion, rather than sedimentary abrasion, as the agent of rounding. I. A more complex Type B zircon with a finely zoned magmatic core and a thin overgrowth of different reflectance and colour. The core is > 3400 Ma old and has a moderate U content ( i 350 ppm). The overgrowth and the outer part of the core have elevated U contents ( 11000 ppm) and the same isotopic features and young age ( < 2200 Ma) indicated for the Type M group, and provide evidence for new zircon growth during the enigmatic zircon-resetting event at c 2200 Ma.

248 identify its isotopic makeup, but also mean that corrections for common Pb are insensitive to the precise composition used. In an attempt to characterise the isotopic composition of the common Pb, the measured 2°7pb/2°6pb and 2 ° 4 p b / 2 ° 6 p b compositions of all grains have been plotted in Fig. 6. Analytical errors are significant at such low concentrations, and only the - 2700 Ma age group shows sufficient coherency to constrain an approximate regression. However, the common Pb of this group closely resembles that of Pb in the Kambalda sulphide ores, when allowance is made for mass discrimination of 0.25%/amu which appears to accompany sputtering of Pb isotopes from zircon [31]. Consequently, radiogenic isotope -ompositions in Table 1 have been corrected on ~e basis that common Pb is Kambalda sulphide Pb fractionated by 0.25%. The correction is sufficiently insensitive to the common Pb composition 1.2

(a) Kambalda Sulphides

0.8

0.4

o

all z i r c o n s ]

2°4pb / 2°6pb 0.0

0.00

0.22

g.

i

i

i

0.02

0.04

0.06

(b)

0.08

o

o

[]

r~

o a

to Sulphide Pb j ~ , . "

0.18

. -

-

.

o

oo

[] []

o

0.14

o o

[]

o

magmatic population other zircons

o

2°4pb / 2°6pb 0.1C ~.0000

i 0.0002

i 0.0004

i 0.0006

Fig. 6. (a) Plot of measured zU~Pb/ZOOPb vs. measured 2°4pb/2°rpb for all analysed zircons, illustrating their low contents of common Pb, and the relative position of Kambalda sulphide Pb. (b) Greatly expanded view of the shaded area in (a), indicating the approximate regression defined by the magmatic zircon population (filled symbols) on a mixing line towards the composition of Kambalda sulphide Pb.

used that precisely similar results are obtained using Broken Hill Pb, which is the composition of Pb found in laboratory air-filters and so is presumed to be the composition of any Pb on mount surfaces.

4.4. The principal zircon population While a few zircons in the sample are texturally unique, the majority of the 200 grains obtained fall into one of three morphological groups, illustrated in Fig. 4, which have correspondingly distinct isotopic compositions. By far the majority of the zircons, over 60% of analysed grains, are grouped as Type I (Idiomorphic), which comprises small ( < 100 /~m) equant crystals with well-defined facets (Fig. 4A). These grains are transparent, pale pink, and distinguished by fine-scale interior zoning. In reflected light views of etched surfaces (Fig. 4B), the zoning is highlighted as euhedral oscillatory growth-zones of high and low U a n d / o r Th, interpreted as the result of crystal growth in a viscous (i.e. felsic) magma [33]. These is no textural evidence of a prehistory to these grains in the form of structural cores or "seed" grains, nor is there evidence for post-magmatic modification in the form of overgrowths, annealed zoning or resorption textures. Some elongate zircons (Fig. 4C) share the transparency, colour, zoning, and faceting of the equant crystals and are also grouped as Type I. Such tabular grains are associated with more rapid magmatic crystallisation than equant forms [33]. It is analyses of these Type I grains that form the single grouping of data at - 2700 Ma on the Concordia diagram in Fig. 5. This tight and concordant cluster, an expanded, view of which is shown in Fig. 7, contains only those zircons identified as magmatic on textural grounds. A large quantity of data of good measurement precision has been collected for this magmatic group. All analyses are of c o n s t a n t 2 ° 7 p b / 2 ° 6 p b age to within experimental error, and there is the potential to obtain a very precise mean date. To be confident about the validity of the mean age of this cluster, it is important to establish which analyses do, and do not, fall within the group. In Fig. 7 it can be seen that there is a clear separation of data between the magmatic group and older grains. Unfortunately, such a distinction

249

¢O

0~

~

~

~

Ka~i Slate

Magma~cPopulation -

[ ] 2692Jc4Mapopulation(45 datapoints) F

[ ] grainswithpartlyresetU-Pbsystems

' 0.400

/ 11.00

207 pb/23s u i

i

i

I

I

12.00

t

I

I 13.00

i

i

l

I 14.(30

Fig. 7. Expanded view of the 45 analyses forming the tight and concordant cluster of magmatic zircon data on the concordia diagram. Note the overlap of younger grains with partly-reset U-Pb systems. Compare this grouping with the pattern of data in Fig. 8b.

does not exist with grains younger than the magmatic group. The overlap of data is graphically illustrated in a "centipede" diagram in Fig. 8 (so named for the resemblance to venomous Australian wildlife), where the analyses have been sorted in age order and plotted as a series so that it is possible to view each analysis with its attendant error limits. The pattern highlights the magmatic group as a plateau of data at slightly under 2700 Ma, and shows that there is a continuum of younger data such that error limits on analyses in the main group overlap error limits on younger grains ranging in age down to 2600 Ma and less. The nature of these younger grains is dealt with in a later section, and attention is only focussed on them here for the purpose of separating them from the magmatic group. It suffices to mention at this stage that, while textural and chemical distinctions can usually be made between the magmatic and younger zircons, these criteria are not infallible. In the absence of definite geological criteria, a cutoff must be established on statistical grounds, and a large quantity of data has been obtained so that this can be performed with confidence. Using the fact that there is a clear separation of data between the magmatic group and older grains, it is taken that the oldest analysis in the plateau of data in Fig. 8 ( which is of the clearly magmatic

Type I grain 318-6 photographed in Fig. 4) belongs in the magmatic group. Following from this basis, the magmatic group is defined as all 2°vpb/Z°6pb ages younger than, and including, analysis 318-6-3, but excluding those younger analyses departing from the weighted population mean by more than the 95% confidence limits. So defined, both the upper and lower limits of the group are coincidentally set by the two analyses of grain 318-6 listed in Table 1, and the weighted mean age of the magmatic population of zircons is 2692 _+ 4 Ma (20). Inclusion of the next-youngest analysis slightly increases the scatter about the mean but leaves the group age and net uncertainty unchanged. Inclusion of still younger grains results in an apparent scatter of data in excess of analytical error.

4.5. Relationship of the magmatic zircons to the Kapai Slate The zircons forming the 2692 _+ 4 Ma age group are simple in form and of a type normally found in granites and felsic volcanics rocks (Fig. 4A-C). Given that they are hosted by a unit traditionally interpreted as a deep-water sediment, their most notable feature is the lack of the expected results of abrasion from sedimentary transport. The well-defined euhedral crystal facets have not un-

250 3500

3000 Q.

2000 2500 1000

1500

Serial

Age-order

0

2800 I

TT 2692M. . . . . . . . .

2700

.~I

2600

~ ~

2500

TTTTTIIzIII~I~I~fl

[ E~ analyses i outside ~ the 2

c~ error limits of the maclmatic group

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Serial Age-order

Fig. 8. (a) The age and U content of a]] analysed ~rcons sorted

in order of 2°7pb/2°6pb age and plotted as a series. Note the plateau of ages at - 2700 Ma, the obvious break with older data, and the continuum of younger ages. U contents of both magmatic and xenocryst zircon populations are uniformly moderate (< 1000 ppm), while < 2700 Ma grains have progressively higher U contents. (b) Expanded view of the age spectrum in the shaded portion of (a), showingthat error limits (lo) on data in the magmatic group overlap with error limits on the continuum of younger grains. Compare this pattern with the concordia diagrams in Figs. 7 and 10.

dergone rounding even in the more elongate Type I crystals, and abrasive micro-pitting of grain surfaces and edges is not observed. This important textural evidence indicates that the Type I magmatic zircons did not arrive in the Kapai Slate as erosion products washed in from the contemporary land surface. The only alternative mechanism by which unabraded igneous zircons could have been deposited in this horizon is as a contribution from pyroclastic air-fall tufts, and it is therefore interpreted that either (a) the Kapai Slate is not a sediment but a tuff horizon or, more conservatively (b) that the Kapai Slate is a deep water chemical sediment as previously interpreted [34,35] but includes a primary volcanogenic com-

ponent from air-fall tufts erupted as it was being laid down. The nature of the zircons described here is presently the only evidence for a tuff component in the Kapai Slate, since comprehensive data on the petrological and chemical nature of the unit are not available. Felsic volcanics and volcaniclastics are an important component of the greenstone pile, and overlie the Kambalda basalts, so it is not unrealistic to find evidence of felsic volcanism in the region contemporary with eruption of the Kambalda basic lavas. However, it seems improbable that the Kapai Slate as a whole is a tuff, since lithofacies within the unit include sulphide layers and carbonaceous sulphidic black shales inconsistent with such an origin. On the other hand, the "cherty" facies selected for this study could represent glass-rich felsic tufts, subsequently altered to the familiar albite + quartz composition, in which zircon is a remnant igneous phase owing to its resilience to metamorphism and alteration. Definitive textural evidence such as shard shapes have not been recognised in the sample used here, which metamorphism has rendered into a granoblastic texture in thin section. However, the existence of a single dominant zircon age group, the fact that it comprises only unmodified igneoustextured crystals which are the preponderant ( > 60%) morphological type in the sample, and the absence from the sample of any other clear age or textural groupings, are together powerful evidence for a felsic volcanic origin. It is therefore considered that the precise 2692 _+ 4 Ma date recorded by the Type I magmatic zircons is the crystallisation age of an air-fall tuff, and that this result accurately pins down the date of deposition of the Kapai Slate and the mafic lava pile of which it is part.

4. 6. Older zircon xenocrysts Rare zircons in the sample ( - 10% of analysed grains) are designated Type B (Brown), comprising a mixed bag of grains whose common difference with the Type I group is that they are brown in colour, instead of transparent and pale pink. Examples are photographed in Fig. 4. The brown grains have no single crystal form or internal structure; some are elongate with length:breadth ratios greater than 5 : 1 , others are equant. All preserve fine-scale igneous zoning, and most bear

251 either or both of metamorphically resorbed outlines, or metamorphic overprinting of the igneous zoning; none bear evidence of sedimentary abrasion. Data for zircons older than the Type I magmatic population are plotted in an expanded Concordia diagram in Fig. 9, and occur as two scattered populations. Among the oldest Type B zircons is 290-4 (Fig. 4G), an elongate, brown grain with traces of igneous zoning. Its rounded termination truncates the zoning and is interpreted as a result of metamorphic corrosion. The analysis of this grain is concordant and precise at 3411 + 9 Ma. Six other grains form a scattered group with minimum ages between 3200-3450 Ma. Of particular interest is grain 312-24 (Fig. 4I), which is a more complex Type B xenocryst with a dark brown, finely-zoned core overlain by a thin rim of different reflectance and paler colour. This zircon is remarkable for the range in ages of the different zones. Two analysed areas in the core have discordant 2°7pb/Z°6pb ages of 3290 + 6 Ma and 3347 _+ 7 Ma. In contrast, two areas to the outside of the grain have concordant 2°7pb/Z°6pb ages of 2377 + 7 Ma and 2231 +_ 5 Ma. The significance of this huge age range is explored in a later section; the clear evidence of metamorphism of

the grain suggests that the oldest age obtained for the core is a minimum age. There are only four other zircons older than the magmatic group, and all of these are younger than 2900 Ma. N o 2°Tpb/2°6pb bridge the wide divide between 2900 and 3200 Ma. Like the 3200-3450 Ma group, these each have distinct shapes, textures and compositions indicating individual magmatic and metamorphic histories. However, grain 312-19 is morphologically similar to 312-24 which, as mentioned above, has an extreme range of within-grain ages. However, metamorphic effects on 312-19 are more obvious than in 312-24: ghost zoning and the brown colour are confined to a small central kernel having a diffuse contact with an outer restructured area that is colourless and massive. The central region has a discordant 2°7pb/Z°6pb age of 2 8 1 0 + 5 Ma, which is a minimum age for the grain. Three analysed areas in the outer region all have ages younger than 2600 Ma, with the youngest determination being concordant at 2209 _+ 3 Ma. Like grain 312-24, this is a pre-2692 _+ 4 Ma zircon that contains metamorphically modified areas as young as 2200 Ma. Their widely varying morphologies and compositions suggest that the older zircons have un-

0.75

J

.

.

'2oo J / ~ r 0.35

" o.g/

-

2,

I I '~'''~J~'"~"

i

207pb/235U

high-U rim of 312-24 n

6.0

ooreot

n

i

~

I 16.0

~

n

~

J

I

t

i

26.0

Fig. 9. Expanded concordia diagram showing the distribution of zircon ages older than the eruption date. There are two groups of xenocryst ages at 2800-2900 Ma and 3200-3500 Ma. There is no firm evidence that any xenocryst U-Pb systems were affected by resetting at 2692+ 4 Ma, but areas of at least two grains have been reset at - 2200 Ma.

252

dergone individual histories of magmatic growth and metamorphic restructuring apparently independently from one another. How did this mixture of ancient zircons become deposited in a 2692 _+ 4 Ma old sediment? One of the few features that all of these zircons share in common is the lack of evidence for sedimentary abrasion (e.g. Fig. 4H). Most are subhedral or partly rounded, but this feature is interpreted as metamorphic corrosion. Despite their great age, these crystals are therefore unlikely to be eroded granitic detritus, and the only alternative explanation for their presence in the Kapai Slate is that they were deposited as a component of the 2692 + 4 Ma air-fall tuff. They are therefore interpreted as crustally-derived xenocrysts incorporated by the volcano that erupted the 2692 _+ 4 Ma magmatic zircons. The existence of these ancient zircons requires that crustal rocks as old as 3430 Ma existed in the region at the time that the Kambalda lavas were being erupted.

view of the complex post-emplacement processes to which the greenstone pile has been subjected, but recognition of the fact is important as it has been asserted [4] that the U-Pb system in the Kambalda greenstones was unaffected by such alteration. However, the extent and duration of post-eruption effects has been an unexpected discovery. The situation is illustrated in an expanded Concordia diagram in Fig. 10, where the younger data form a scatter of 2°7pb/2°6pb ages ranging from over 2600 Ma to less than 2200 Ma. 23 zircons, one-third of the analysed grains, have 2°7pb/2°6pb ages younger than the eruption date, but the data form no semblance of a discordia trend towards the age of a younger event. Instead, there is an amorphous scatter of both concordant and discordant zircon ages younger than 2692 + 4 Ma. This indicates complex evolution involving both early and late discordance, and at first sight leaves ambiguity as to whether the data record a series of zircon-modifying events, or the effects of a single early event complicated by zero-age Pb loss. The young zircons are of two morphological types. The majority are designated as Type M (Modified), and are I-type magmatic grains whose original textures have been metamorphically overprinted to some degree. Rarely, this restructuring

4. 7. Low-temperature alteration of zircon U-Pb systems Interpretations of isotopic data for the magmatic and xenocryst populations have had to take account of post-eruption resetting of the isotopic system in some grains. This is not surprising in

22y

/

.4OO

~x

"J~

Q

Reset Zircons •

/

C

J

n~mat~t~

[] gmlns wfthreset~

s ~

207 pb/23s u i

6.00

i

i

I

10.00

I

i

i

14.00

Fig. 10. Expanded concordia diagram showing the scatter of zircon data younger than the magmatic group. There is no semblance of a discordia chord, and it is interpreted that the pattern reflects second-order scatter from modern Pb-loss, superimposed on an early (probably ~ 2200 Ma) discordance pattern. See Fig. 11 and text for discussion.

253

takes the form of complete annealing of sectors of the crystal (Fig. 4F). More commonly, the only optical evidence of alteration is partial blurring of the fine-scale igneous zoning, accompanied by an increase in turbidity (Fig. 4D, E). Visible overprinting of the igneous textures increases with the degree of resetting of the 2°7pb/2°6pb age of each grain, and so is particularly subtle in zircons whose isotopic systems have been only slightly affected and which consequently plot close to the magmatic population on Concordia diagrams. For example, grains 293-1 and 293-3 were targetted as unmodified magmatic zircons, and analyses of two areas of each grain fall within the 2692 + 4 Ma age group, as expected. However, both grains have a third analysis significantly younger than the eruption age at, respectively, 2580 + 10 Ma and 2532 + 8 Ma, indicating local within-grain disturbance of the U-Pb isotopic system. Six of the zircons with < 2692 + 4 Ma ages are brown grains assigned to the Type B morphological group. Of these, 312-19 and 312-24 have been described as part of the xenocryst population as they also have areas with ages older than the eruption date. Both of these xenocrysts have outer areas with concordant 2°Tpb/2°6pb ages as young as - 2 2 0 0 Ma. It seems probable that the other four young Type B grains are similarly metamorphosed xenocrysts, and that their discordant 2°7pb/2°6pb ages reflect mixtures between their original (pre-2692 + 4 Ma) ages and the date of disturbance of their U-Pb systems. Despite the absence of a discordia trend, two features of the data suggest that the alteration recorded by the < 2692 + 4 Ma zircons was caused by a single event. The first is the absence of any clustering of the younger data about a particular 2°7pb/2°6pb age (Fig. 10), and the fact that no two analyses of a single Type M zircon have the same 2°7pb/2°6pb age, which is exactly the behaviour expected of zircons whose U-Pb systems have been variably and incompletely reset by a younger, non zero-age event. This holds even for those grains with the youngest apparent ages, which do not form a group of like age, suggesting that the event was younger than the 2 ° 7 p b / 2 ° 6 p b age of any of these. The second feature is illustrated in Fig. 11, where it is shown that there is a correlation between the 2°7pb/2°6pb age of the young zircons and their U content. This correla-

3'OOO u

mixingtrendtowards ~ high-Ucompositionat <2200Ma

~~ 2000

1000

~

e

m

m ° i

1500

2000

,

i

2500

,

o i

3000

,

Age/Ma

3500

Fig. 11. PlOt of U content vs. 2°Tph/2°6pb age for all analysed zircons. The approximate regression defined by analyses younger than the magmatic date is interpreted as a mixing line towards a high U ( > 2500 ppm) composition at ~< 2200 Ma.

tion applies regardless of the morphology of the zircons--it is obeyed by both the young areas of Type B xenocrysts and by modified magmatic grains that first grew at 2692 + 4 Ma. It is also independent of the degree of discordance of individual analyses, which suggests that the 2°7pb/2°6pb ages of the young grains are faithful measures of the degree of discordance caused by the metamorphic event, and that the failure of the data to form a simple discordia trend reflects only second-order scatter due to zero-age differential movement U and Pb. The relationship between U content and age could mean that the zircon-modifying event preferentially caused Pb-loss from existing high-U areas of affected grains, or it could mean that U was gained by these zircons, presumably from a high-U fluid, at a date equal to or younger than the youngest analysed areas. The youngest 2°Tpb/2°6pb age is a 1922 + 8 Ma area of grain 312-21, but this is a highly discordant composition. The youngest tolerably concordant analysis is of grain 293-11 at 2161 + 6 Ma. The zircons with ages younger than 2692 + 4 Ma are evidence for a hitherto unsuspected and very late metamorphism in these greenstones. The event was a weak one, variably and incompletely resetting the U-Pb system of approximately onethird of the zircons in the Kapai Slate, and leaving the remaining 70% unaffected. It is important to realise that this late event cannot be the amphibolite facies regional metamorphism of the greenstones, a minimum age for which is set by closure of the Rb-Sr system in biotites at 2550 + 10 Ma,

254

interpreted as recording cooling of the greenstones to below 300 ° C [2], which is the blocking temperature of Sr in biotite. The young zircon data must therefore record much later, low-temperature metamorphism during the Proterozoic. Perhaps a candidate process is the hydrothermal system presumably set up by intrusion of the unmetamorphosed mafic dykes cutting all Archaean rocks in the region. Dates so far obtained for these dykes are early Proterozoic [14,18,37] consistent with the resetting date inferred for the Kapai Slate zircons.

5. Geochronology of the Kambalda greenstones The most comprehensive survey of Kambalda's geological history is that of Gresham and LoftusHills [10], in which an attempt is made to fit the sequence of eruption, deformation and metamorphism into the time constraints that were then believed. The twin needs to place eruption before the then supposed granite emplacement age of 2820 _+ 15 Ma, and to accommodate the Rb-Sr ages at 2600 Ma and younger, resulted in an interpretation that spread the tectonic evolution of the greenstone belt over a 400 Ma period, from emplacement at - 2900 Ma to post-metamorphic retrograde effects at 2550 Ma. Principally through applications of the ion-microprobe, we are now in possession of timing

\

constraints that are both more precise and much more accurate. The available data now comprise: (1) The date of deposition of the Kapai Slate at Kambald~ is accurately set at 2692 _+ 4 Ma by the U-Pb age of magmatic zircons contributed to the sediment from air-fall tufts (this paper). This date must also apply to the immediately adjacent basalts of the Devon Consols and Paringa formations, and is the first unambiguous numerical age obtained for a stratigraphic level in the greenstone pile. The zircon age of the Kapai Slate is older than the one of the two estimates for the maximum age of the greenstones interpreted from zircon xenocrysts in the Victory Dolerite [5]. A conservative maximum age estimate of 2693 _+ 50 Ma for an abundant xenocryst population is not in conflict with 2692 _+ 4 Ma for deposition of the Kapai Slate but, as described above, a possible younger estimate comes from rare zircons found in the dolerite having a mean age of 2669 + 11 Ma [5]. The conflict of this younger estimate with the Kapai Slate deposition age may be explained in one of two ways. The Victory Dolerite is the only massive unit in the pile of otherwise pillowed Devon Consols Basalts (Fig. 2), and so could be a later sill not related to the basalts. However, the dolerite is nowhere seen to even locally transgress the stratigraphy. Alternatively, the discovery of post-magmatic alteration of zircons in the Kapai

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.

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.

.

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.

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Fig. 12. Plot of 2°7pb/2°rpb age vs. U content for the basalt-hosted zircons described by Compston et al. [5]. (a) The cores and rims of zircons in Mount Z129 which grew at, respectively, 3400 Ma and 3100 Ma, now lie on mixing lines towards a composition with 1500 ppm U at - 2700 Ma, which is the date of their incorporation into the basalt. Grain 3, which set an uncomfortably young limit to the emplacement date of the dolerite clearly does not belong to the main xenocryst population. (b) Data from Mount Z133, which contained the yellow 2693 _+50 Ma population (not shown for clarity), show no evidence of alteration at 2700 Ma. The only analysis bridging the 2900-3200 Ma gap is of grain 5, and this is a mixed age [5]. Young zircons found in a Lunnon Basalt with a similar age to grain 129-3 do not plot with the main xenocryst population.

255 Slate makes it likely that the younger Victory Dolerite zircons reflect in-situ modification of older grains, as was suggested as a possible interpretation by Compston et al. [5]. This interpretation is supported by Fig. 12, which shows that their compositions do not belong with the main xenocryst populations. Three aspects of the Kapai Slate age determination merit attention. The first is its accuracy. By virtue of obtaining numerous within-grain analyses of zircons, it has been possible to distinguish igneous grains from xenocrysts and to separate out the effects of metamorphism: the date is therefore both analytically precise and unambiguously magmatic. The second is the nature of the rock unit studied: the Kapai Slate is both thin ( < 10 m) and laterally extensive [8-10] and so closely approaches the ideal of a stratigraphic marker horizon. Thin but extensive "cherts" are a common occurrence in the Norseman-Wiluna belt, and identification of this as a precisely datable rock type opens the way to close numerical age control of the regional stratigraphy. At this level of accuracy it should be possible to resolve short stratigraphic intervals in the greenstone pile, and so correlate across strike faults and between greenstone belts with confidence for the first time. This possibility will shortly be tested in the KalgoorlieKambalda region and elsewhere. The third is the fact that this sampling approach, tied to use of the ion-microprobe, has directly dated the mafic lavas, and not geographically associated felsic volcanics from which extrapolation of age information would be highly uncertain. (2) Emplacement of the granitoid intruding the Kambalda Dome is accurately set at 2662 + 6 Ma [18]. Resolution of this date has also relied on ion-microprobe distinction of magmatic zircons from metamorphosed and inherited grains. This plutonic body post-dates ductile deformation of the greenstone pile and so constrains the folding

events to having occurred within 30 Ma of eruption of the lavas. Identification of the rapidity with which the greenstone pile was deformed is the principal difference between this, and previous, tectonic interpretations. The Kambalda Granite is one of several groups of felsic intrusives emplaced at discrete intervals in the post-emplacement history of the belt. The ability of the ion-microprobe

to accurately date these intrusives means that they could provide valuable time markers and so help to elucidate the detailed tectonic evolution of the greenstone belt. This possibility will shortly be tested in the Kambalda area where there is good field control on the relative timing of different intrusive suites. (3) The timing of regional metamorphism is less precisely constrained, in keeping with the less discrete nature of the process. If it is assumed that burial accompanied ductile deformation of the greenstones, then it can be inferred that the amphibohte facies peak was reached before intrusion of the granite at 2 6 6 2 _ 6 Ma, 30 Ma after deposition of the volcanic and sedimentary pile. This very rapid burial must have been followed by comparatively slow uplift, as registered by much later crossing of the 300 ° C blocking temperature of biotites at 2 5 5 0 _ 10 Ma [2,14]. There is no clear evidence for re-equilibration by zircons in the granite or Kapai Slate during regional metamorphic conditions. (4) Intrusion of the east-west-trending suites of dolerite dykes post-dated regional metamorphism. Isotopic data for these dykes are sparse, but include a Rb-Sr mineral isochron of 2420 _+ 30 Ma for the Celebration Dyke southwest of Kalgoorlie [14], identical mineral isochrons by Rb-Sr (2411 _+ 52 Ma [14]) and Sm-Nd (2411 + 55 Ma [37]) for the Jimberlana Dyke near Norseman, and a Rb-Sr mineral isochron of 2042 +_ 45 Ma for a dyke found in drillcore below the Kambalda Dome [18]. All are interpreted as intrusion dates, and it seems probable that these mafic dykes were emplaced over a lengthy period of time during the early Proterozoic. (5) An enigmatic event is registered by resetting of the U-Pb system in some zircons at ~< 2200 Ma. It is important to realise that these reset zircon U-Pb systems are recording a previously unsuspected low-temperature ( < 300 o C) metamorphism, presumably a hydrothermal system, and that this alteration occurred long after the metamorphic ages recorded by all other isotopic systems (including Rb-Sr) at Kambalda. The data indicate that geological activity in this part of the craton did not cease at the end of the Archaean, and this must be an important area for future research.

256

6. The nature of the basement

The ancient zircon xenocrysts identified by this study, and those previously discovered in a Kambalda basalt [5], are the only definitive evidence for the existence and nature of the crustal basement that existed when these greenstones were being emplaced, and will remain so until rocks comprising the basement are unambiguously identified. On structural grounds nearby banded gneisses have been advocated as candidate basement rocks [34]. Available isotopic dates for the gneisses at ca. 2800 Ma by whole-rock Rb-Sr and Sm-Nd [35,36], make it unlikely that they constitute the source of 3400 Ma zircons, but their probably composite origin warrants study using single zircon analysis. Can the zircon xenocrysts tell us anything about the nature of this elusive basement? It has already been noted that the Kapai Slate xenocrysts described above are representatives of a polymagmatic and polymetamorphic basement with components as old as 3450 Ma. Attention has also been drawn to a bimodal distribution of xenocryst ages, with one population in the range 3200-3450 Ma, and a younger population at < 2900 Ma, with a gap in data between 2900 and 3200 Ma. Is this apparent gap a statistical artefact of the small number of xenocrysts identified and analysed? The Victory Dolerite xenocrysts [5] also record a polymagmatic and polymetamorphic basement with a similar range of ages and a similar oldest component of 3450 Ma. At first appearance, there is a continuum of ages, apparently bridging the 2900-3200 Ma gap in the Kapai Slate data, but this is an artefact of alterations induced when the zircons were incorporated into the basalt. Compston et al. [5] speculated that the basalt-hosted xenocrysts resided in the magma within xenoliths; without such protection they would surely have dissolved rapidly. Supporting this notion is the fact that the two separate crushings of continuous lengths of drillcore through the Victory Dolerite yielded dissimilar zircon populations. For instance, the yellow 2693 _+ 50 Ma old population was present only in Mount Z133. Fig. 12 illustrates a systematic age-U relationship in both the cores and rims of basalt-hosted zircons in Mount Z129. The correlations strongly suggest that the apparently continuous range of

2°7pb/2°6pb ages of zircons from this crushing represent a single population with igneous cores grown at - 3400 Ma and metamorphic rims grown at - 3200 Ma, both of which have subsequently mixed with a high-U composition at - 2700 Ma - - t h e date of their incorporation into the basalt. No such relationship holds for zircons from the other crushing, in which there is a bimodal distribution of zircon ages similar to that of the Kapai Slate xenocrysts. The differences between the two sections of Victory Dolerite are best explained by the different zircon groups residing in discrete xenoliths rather than being evenly distributed throughout the dolerite, and zircons in only one of these suffered alteration when incorporated in the host basalt. The basalt data therefore support the existence of a gap in ages between 2900 and 3200 Ma. If this gap is real, then the xenocryst populations are samples of a basement having two distinct rock associations separated in time. The 3200-3450 Ma population might tentatively be identified with continental crust, and the xenocrysts younger than 2900 Ma with supracrustal rocks predating the Kambalda lavas. 7. Greenstone dating methods

Kambalda has been a focus of Archaean isotopic studies for 20 years and so is now an invaluable type area where information gained from previously applied methods can be reviewed with the hindsight of new techniques and interpretations.

7.1. Single-crystal zircon dating Single-crystal zircon dating by refinements of conventional methods has been developed for some years by Krogh (e.g. [38,39]) and applied most effectively to the Canadian greenstones. Similarly, most of the reliable timing constraints cited in this paper are single-crystal U-Pb dates, and these have revealed the composite origins of zircon populations in the Kambalda greenstone pile and associated rocks. The selective analytical method of the ion-microprobe illustrates the importance of establishing "geological" as well as "analytical" accuracy in assessing isotopic data. Analytical accuracy for state-of-the-art single zircon analyses by ion microprobe is typically a few tens of mil-

257 lions of years (e.g. Table 1), as compared with a few million years routinely obtained by conventional analyses. However, the spatial resolution and rapid analysis rate of the ion-probe allow the makeup of complex zircon populations to be unravelled, and so enable close control over exactly what is being dated. At the price of larger analytical errors, geological accuracy can be obtained. Moreover, analytical precision can be increased, as in this study, by making multiple replicate ion-probe analyses. The situation is graphically illustrated by the morphologically, chemically and isotopically diverse zircon population described above, in which there are several examples of multistage growth, and a 1500 Ma range of ages, even within single grains. In conventional zircon work, a composite sample is prepared by dissolution of a large number of grains, and the isotopic composition of the bulked concentrate is then analysed, often to high levels of analytical precision. However precise the analysis, it is unavoidable that the result of such a technique will be a meaningless average of all the zircons in the sample, except in rare cases where it might be demonstrated that only one age of zircon is present. The distribution of data in the Concordia diagram in Fig. 5 suggests that conventional analysis of the Kapai Slate zircon population reported here might yield an analytically precise age close to 2700 Ma, where the mode and mean of the population approximately coincide. This would be a fortuitous result to which should be attached a geological dispersion of _ 700 Ma. Even laborious conventional analysis of individual single zircons would be unlikely to overcome the problem of mixed ages, as this study has highlighted the incidence of variable U-Pb ages within optically homogeneous grains.

7.2. Mineral Sm-Nd isochrons The 2762 + 32 Ma Sm-Nd isochron obtained for the Ora Banda sill with mineral separates [4] is an important application of the Sm-Nd isotopic system that breaks none of the "cogenetic sampies" assumptions built into the isochron equation. The technique was applied to a sill in which igneous phases were preserved, but mineralogical preservation is rare in erupted rocks in this greenstone belt.

7.3. Whole-rock S m - N d The whole-rock Sm-Nd ages for the Kambalda lavas [1,4] are 500 Ma too old. That in which the isochron uses granite analyses [19] is 100 Ma too old. We can now see that the rocks were emplaced with a wide range of an initial 143Nd/144Nd ratios, thus invalidating their use on an isochron, but this was not predicted on petrographic or chemical evidence prior to analysis. All whole-rock Sm-Nd ages worldwide should be regarded now as suspect, and chronologies based on them need to be reinvestigated with other methods. This is not to detract from the validity of Sm-Nd ages based on (cogenetic) primary minerals such as those for the Ora Banda [4] and Jimberlana intrusions [37], or the usefulness of N d isotopes as petrogenetic tracers, but for the latter purpose calculation of initial Nd isotopic compositions requires reliable independent dates obtained by other means.

7.4. Whole-rock Pb-Pb Ambiguity surrounds the meaning of the whole-rock isochron and model Pb ages of the Kambalda greenstones [2,4]. Various authors have suggested that the data either (a) faithfully record eruption of the lavas; or (b) record post-eruption sea-floor alteration; or (c) reflect emplacement of the lavas with variable initial Pb isotopic compositions (as with the Sm-Nd system) and so give ages misleadingly old. To these permutations the present study has added the fact of mobility of U a n d / o r Pb long after emplacement and peak metamorphism, highlighted by Proterozoic effects on zircon U-Pb systems. While the Pb-Pb and model Pb ages reported are in the approximate area of the 2692 + 4 Ma emplacement date, their very large error limits overlap much older (preemplacement) and much younger (post-metamorphic) ages. The scatter of Pb-Pb compositions about the isochrons detectably exceeds analytical error and so demonstrates a "geological" component in the uncertainty. It is therefore not possible to rule out any of the candidate pre-, syn-, and post-emplacement processes outlined above, and it is most likely that the Pb data average differential adjustment of the whole-rock samples to a// of them. Facts provided by the whole-rock samples are therefore vague, and the inferred ages are not definitive but permissive.

258

8. Conclusion A general conclusion to be drawn is that no single isotopic system can hope to resolve the history of these, or other, greenstone belt lavas. The great promise of Nd isotopes, which appear resilient to metamorphic resetting, is now clouded by new sources of geological complexity. Wholerock Pb isotopes could record metamorphism or eruption, or even a meaningless date. Multi-stage zircons at Kambalda do not record all of the events in which the geologist is interested; but were apparently sensitive to previously unrecognised and very late low-temperature effects. These were not registered by mica Rb-Sr ages, which have hitherto been taken as the end of metamorphism. Recent advances in analytical methods (Nd isotopes; single zircon dating) have resulted in increased recognition of geological complexity in interpreting isotopic data, and replaced laboratory sources of error as the main uncertainty in dating Archaean rocks.

Acknowledgements Analytical assistance from R. Rudowski, J. Foster and I. Williams is greatly appreciated, and helpful reviews of an earlier draft were given by I. Campbell, C. Chauvel, C. Hawkesworth, R. Hill, P. Kinny, R. Page and C. Roddick. We thank Western Mining Corporation for access to the area, many fruitful discussions, and permission to publish the data.

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