The evolution of early precambrian crustal rocks at Isua, West Greenland — Geochemical and isotopic evidence

Earth and Planetary Science Letters, 27 (1975) 229-239 © Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands

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THE EVOLUTION OF EARLY PRECAMBRIAN CRUSTAL ROCKS AT ISUA, WEST GREENLAND GEOCHEMICAL AND ISOTOPIC EVIDENCE S. M O O R B A T H , R.K. O ' N I O N S and R.J. P A N K H U R S T

Department of Geology and Mineralogy, Oxford University, Oxford (Great Britain) Received May 9, 1975 Revised version received July 10, 1975

Petrographic and chemical evidence suggests that boulders from a conglomeratic unit in the Isua supracrustal succession were derived by the erosion of an acid volcanogenic sediment. Six samples of the boulders and surrounding matrix yield a R b - S r whole rock isochron with a slope corresponding to an age of 3860 -+ 240 m.y. (2 sigma error), but consideration of the initial 87Sr/86Sr ratio constrains the possible age of formation of 3710 ± ~o m.y. This is in general agreement with a published Pb/Pb age of 3760 ± 70 m,y. on Isua banded ironstones. Pb isotope compositions as well as highly fractionated, heavy element depleted, rare earth element abundance patterns for the boulders suggest that their igneous precursors were derived from a source region with a similar geochemical history to that of some components of the 3700-3800 m.y. old Ami'tsoq gneisses, involving fractionation of garnet during their evolution. A Pb/Pb whole-rock isochron for Amftsoq gneisses from lsua yields an age of 3800 -+ 120 m.y. (20), in good agreement with previously published Rb-Sr age data on the same rocks. The rock leads are highly unradiogenic and demonstrate substantial U depletion at least 3800 -+ 120 m.y. ago. A two-stage model for the U - P b system yields an average 238U/2°4Pb (~l) value of 9.3 -+ 0.2 for the source region, which is significantly different from the published value of 9.9 ± 0.1 for the Isua iron formation. This indicates the existence of U - P b heterogeneities between the source regions of plutonic and supracrustal rocks by about 3700-3800 m.y. ago. Attempts to apply U - P b whole-rock dating to the Amftsoq gneisses were unsuccessful because of geologically recent U loss, possibly due to groundwater leaching. A Rb-Sr whole-rock isochron on a suite of Amftsoq gneiss samples from a different locality in the Isua region has yielded an age of 3780 +- 130 m.y. In contrast to the Godthaab area, there is no geochronological evidence at Isua for major rock-producing or tectonothermal events after about 3700 m.y. ago. The entire gneiss-supracrustal system developed within the approximate interval 3900-3700 m.y. ago.

1. Introduction The Isua supracrustal succession is the oldest greenstone belt so far recognised in the Early Archaean of Greenland. A description and map o f the geology o f the area have been published recently by Bridgwater and McGregor [1], and Bridgwater et al. [2] whilst a structural analysis o f part o f the supracrustal belt has been made by James [3]. Banded ironstones f r o m the succession have yielded a Pb/Pb w h o l e - r o c k isochron age o f 3 7 6 0 -+ 70 m.y. [4]. Nearby A m i t s o q granitic gneisses have yielded a R b - S r isochron age of 3 7 0 0 -+ 140 m.y., which is indistinguishable within analytical error f r o m radiometric ages f r o m the type A m f t s o q

gneisses near G o d t h a a b [ 5 - 7 ] . The Isua supracrustals f o r m a belt o f low- to mediumgrade metavolcanics and m e t a s e d i m e n t s enclosed in the regional gneisses (Fig. 1). The assemblage is c o m p l e x and consists o f basic and ultrabasic igneous rocks, quartz. ires, various types o f schists, carbonate-rich rocks and banded ironstones. Thin units o f either clastic sedim e n t a r y or tuffaceous origin, as well as cherts, are interlayered with basic volcanic rocks [1 ]. The supraErustal belt has been folded into a semicircular arc, some 25 k m in diameter, around a core o f gneisses. The supracrustal belt reaches a m a x i m u m observed w i d t h o f app r o x i m a t e l y 3 - 4 km. The supracrustal succession contains a well-defined

230

CRATON

i

~f--'

~

r~

/

isuA

Cn • OC~rHAAB

~\(.';" ./ 2

.

Fig. 1. Locality map of lsua, showing supracrustal belt.

conglomeratic unit with pebbles and boulders ranging from a few centimetres to 2 m in diameter, set in a fine-grained carbonate-bearing matrix. Their state of deformation is very variable, but samples analysed in this work come from the least deformed parts. The conglomeratic unit can be traced for at least 33 km along strike, and can be traced westwards into a more massive unit of quartz-sericite schist. Bridgwater and McGregor [ 1], who discovered the conglomeratic unit in the summer of 1973, state that the boulders consist mainly of pale, fine-grained, muscovite-rich, quartzofeldspathic rocks, with high potassium contents. Most of the original K-feldspar has been replaced by muscovite. Secondary carbonate, probably derived from the sedimentary matrix, replaces parts of the original boulders. In some outcrops the boulders form distinct beds, whilst elsewhere they are scattered through the variably laminated matrix. An acid volcanic origin for many of the boulders was suggested by Bridgwater and McGregor (personal communication) on textural and chemical grounds. The Isua supracrustals appear to have suffered two major deformations [3]. The first phase ofheterogenous simple shear was caused by relative vertical movement of the inner and outer blocks, and imposed the main fabric of the rocks. The second deformation comprises a phase of major and minor folding and is reflected by the present arcuate nature of the greenstone belt. The contact relationships between the greenstone belt and the Amftsoq gneisses at Isua are complex. Bridgwater and McGregor [1] regard the gneisses as part of a

younger granitic suite on the basis of local intrusive contacts. James [3], however, states that both gneisses and supracrustals have undergone intense deformation together which could have destroyed any original intrusive relationship but could have caused local remobilisation of the acid gneisses after deformation. It is, therefore, still an open question whether part of the exposed Amftsoq gneisses represent a basement to the supracrustal sequence or not. The aim of the present study was to carry out Rb-Sr and Pb/Pb whole-rock age measurements on several of the boulders and the surrounding matrix, together with major element and rare earth abundance analyses, to further evaluate their petrogenesis. In addition, Pb/Pb and U-Pb whole-rock measurements are reported for the first time on Amftsoq gneisses from the lsua area to supplement both the previously published Rb-Sr whole-rock work [5], and the Pb/Pb whole-rock work on Amftsoq gneisses from the Godthaab area [6].

2. Analytical metfi6ds Si, A1, Fe, Mn, Ca, Mg, Na and K were determined by atomic absorption spectrophotometry using a PerkinElmer 306 instrument, and U.S.G.S. rocks as standards. The method is similar to that described by Abbey et al. [8]. Ferrous iron was determined volumetrically and P20s and Ti02 colorimetrically. Rb and Sr contents of the boulders and matrix were determined by mass-spectrometric isotope dilution with an accuracy of about 1% for the Rb/Sr ratio. Sr was extracted by conventional dissolution and cation exchange techniques. Rb was separated by a zirconium phosphate column technique. Isotopic analyses were carried out with either the Oxford 12-inch mass-spectrometer, or a VG Micromass 30. Full details of the chemical and mass-spectrometric techniques have been described elsewhere [9,10]. Rb/Sr ratios on Amitsoq gneisses were determined by a precise X-ray fluorescence technique fully described by Pankhurst and O'Nions [10]. Pb and U were determined by mass-spectrometric isotope dilution. Pb was extracted by a low-bank method involving total dissolution followed by sequential cathodic and anodic deposition. U was extracted by a low-blank reversed phase chromatography method.

231

These extraction techniques, as well as subsequent massspectrometric techniques for Uand Pb, have been fully described by Arden and Gale [ 11,121. Precision for U and Pb contents is about 1% (20). The concentrations of nine rare earth elements (La, Ce, Nd, Sm, ELI, Cd, Dy, Er and Yb) were determined by mass-spectrometric isotope dilution [34]. Accuracy is generally about +5% of the measured abundance, as determined by analysis of standard samples. Decay constants used in this work are as follows: “Rb = 1.39 X 10-l’ y -I, “‘U = 9.72 X lo-” y-l, 238U = 1S37 X IO-” y-l. The ratio 2 38U/235U is 137.8. All age values in this work are given at the 95% confidence level (2~).

3. Results and discussion 3.1. Boulders and matrix The boulders consist of a weakly foliated, metamorphic rock which, in thin section, shows a finegrained, recrystallised (mosaic texture) groundmass of quartz, K-feldspar, muscovite and biotite, traversed by numerous sub-parallel shreds, patches, bands and

lenses composed of coarser-grained aggregates of muscovite and biotite, with some calcite, epidote, clinozoisite and hornblende. Rounded, strained quartz grains occur throughout the rock. Some thin biotite-rich lenses and bands contain, in addition to the major minerals, significant concentrations of accessory minerals such as iron ore, apatite and very tiny euhedral to subhedral zircons. The fine-gramed, weakly foliated matrix surrounding the boulders is mineralogically very similar to the boulders themselves, except that it is more homogeneous in grain size and texture, and also contains major amounts of calcite. Major element analyses for four boulders and two matrix samples are given in Table 1. The overall chemical composition of the boulders and of the matrix is granitic. Highly variable CaO and COZ contents are due to calcite. Typical of granitic rocks is the exceptionally high and variable K20 content, ranging from ca. 5 to 11% for the boulders. The overall petrography, mineralogy, grain size and texture of the boulders, together with the uyiisual K20 contents and the localised concentratiy6f heavy accessory minerals, suggest that the source rock of these boulders could have been a volcanogenic sediment such as an acid tuff. It appears most unlikely

TABLE 1 Chemical analyses on boulders and matrix

sioz

158416

158534

158493

matrix

matrix

boulder

158536 boulder

158537 boulder

158538 boulder

MnO CO2 Hz0

60.23 12.80 3.44 0.66 2.37 5.37 0.68 7.71 0.54 0.18 0.16 4.37 0.81

55.66 8.13 3.84 0.56 5.58 10.34 0.54 3.98 0.25 0.07 0.41 10.25 0.64

67.26 16.24 1.04 0.76 1.61 3.10 0.64 4.95 0.54 0.15 0.15 1.89 1.08

69.08 15.10 0.21 0.56 0.75 1.24 0.56 10.62 0.46 0.35 0.02 0.56 0.56

68.57 15.00 0.63 0.27 0.95 2.56 0.35 8.64 0.47 0.18 0.04 1.54 0.80

69.44 15.31 0.70 0.32 0.82 1.28 0.53 10.64 0.49 0.16 0.03 0.70 0.55

Total (%)

99.32

100.25

99.41

100.07

100.00

100.97

A1203

Fe0 Fez03 MgO CaO NazO K2O

TiO2 pzos

Analysts: M. Hepher, P. Shreeve

232 TABLE 2 Analytical data on boulders and matrix from the Isua supracrustals Sample No.

Rock type

Rb*

St*

(ppm)

(ppm)

87Rb/S6Sr **

158493

boulder

54.2

17.13

9.58 ± 0.09

158536

boulder

117.9

21.17

17.62 ± 0.13

158537

boulder

90.8

23.80

11.72 ± 0.09

158538

boulder

106.7

20.18

16.58 ± 0.13

158534

matrix

74.3

25.92

8.66 ± 0.08

158416

matrix

151.4

46.13

9.98 ± 0.08

STSr/86Sr**

2°6pb/2°4pb**

2°Tpb/2°4pb**

2°spb/2°4pb**

1.1974 -+ 0.0005 1.6597 + 0.0008 1.3265 ± 0.0011 1.5561 ± 0.0011 1.1446 ± 0,0007 1.2351 -+ 0.0006

13.109 +- 0.016 12.301 ± 0.019 12.238 ± 0.008 12.146 ± 0.012 -

13.671 -+ 0.019 13.536 ± 0.022 13.541 ± 0.010 13.536 ± 0.015

33.550 ± 0.045 32.534 • 0.053 31.941 ± 0.023 32.476 ± 0.035

-

-

-

* Isotope dilution data. ** All errors are quoted at 2 sigma.

that the source rock of the boulders was either an acidic lava or pre-existing granitic basement. It could well represent a true intraformational horizon, derived from earlier members of the lsua supracrustal succession itself. Rb, Sr and Pb isotope analytical data on the four boulder and two matrix samples described above are presented in Table 2. The Rb-Sr data are presented on an isochron plot in Fig. 2. The data do not define a perfect isochron and the analytical errors have to be magnified by a factor of three to achieve a true fit. The slope thus obtained corresponds to an age of 3860 +240 m.y., with an intercept of 0.675 -+ 0.040. Since it is certain that the true initial 87Sr/S6Sr ratio of these rocks was greater than that of primordial Sr (about 0.699) only those solutions corresponding to ages in the range 3710-3620 m.y. may be regarded as realistic. This is within the regression error of all previous Rb-Sr and Pb/Pb whole-rock ages from Isua. It is possible that this represents homogenization of Sr isotopes during a post-formational metamorphism. If interpretation of these boulders as reworked sediments of ultimate volcanic origin is correct, it could be presumed that the R b - S r systematics of the parent igneous rocks would not have survived erosion and re-deposition. Thus the age should probably be regarded as a minimum for sedimentation. It is, therefore, concluded that both sedimentation and metamorphism occurred dur-

ing the interval 3710-3620 m.y. ago. The lack of true isochron behaviour suggests either that subsequent geological events have caused some degree of open system behaviour in the whole-rock systems, or that there was slight heterogeneity in age and/or initial 87Sr/a6Sr ratios perhaps as a result of incomplete homogenisation of Sr during the early metamorphism.

/

170

BO 1"60

aTS__r 1"4~

Bo~ ~ / 1"20

M

,~M

3710MY .

~

87Rb

~lb 10 tl 12 13 14 15 6 17 18 Fig. 2. R b - S r whole-rock isochron for boulders (B) and matrix (M) from the supracrustal belt. The best-fit line through all points yields an "age" of 3860 +_ 240 m.y., with an initial 87Sr/86Sr of 0.675 ± 0.040. Forcing the line through a minimum possible value of 0.699 (which is the line shown here) yields an age of 3710 -+90 m.y, 110

~

233

There is no whole-rock isotopic evidence at Isua for the major tectonothermal events which affected Godthaab and adjacent regions at ca. 2600-2800 m.y. and ca. 3000 m.y. ago [13-15]. However, R b - S r mineral dates from the Isua Amftsoq gneisses have demonstrated the occurrence of a thermal event at around 1600-1700 m.y. ago [ 14]. Preliminary R b - S r whole rock work (unpublished data) on four out of six fine-grained, altered, metabasites (amphibolites) from the Isua supracrustal succession with a wide range of Rb/Sr ratios (0.05-2.5) has yielded a linear array suggestive of an age of approximately 3500-3600 m.y. This again demonstrates the weakness of later geological events in the area, since R b - S r systematics are usually easily reset in such rocks. However, further collecting from Isua is essential to supplement the scanty available age and isotope data on these rocks, and, in particular, to determine their initial 87Sr/86Sr ratio. The 2°Tpb/2°4pb-2°6pb/2°4pb isotopic data for four boulder samples (Table 2) are plotted in Fig. 3, together with the Amftsoq gneiss data to be discussed below. The points clearly do not suffice to define an isochron, although they have similar Pb isotope compositions to the Arnftsoq gneisses, indicating a possibly similar overall source region and geochemical history for the precursors of the two rock types. The concentrations of nine rare earth elements (REE) have been determined in two boulders and one

IL~-~

~./

/o

........

t~o I

t

12@0

1~0

I

13'G0

I

14,00

I

'14'fiO

I

1500

Fig. 3. Pb/Pb whole-rock isochron for Amftsoq gneisses (1971 collection) (circles) and conglomeratic boulders from the supracrustal series (crosses). The slope o f the line is based on the Ami'tsoq gneiss data only. The Isua iron formation isochron [4] is shown for comparison. For discussion o f the difference between the two isochrons, see text. ( R b - S r data on the gneisses have been published previously [5]).

TABLE 3 Rare earth analytical data on boulders and matrix* 158534 matrix

158536 boulder

158493 boulder

La Ce Nd Sm

31.9 16.8 2.12

28.6 61.4 30.6 3.99

26.8 61.7 27.3 4.03

Eu Gd Dy Er

0.57 1.31 0.92 0.45

0.81 2.43 1.22 0.58

0.80 2.41 1.19 0.53

Yb

0.40

0.53

0.50

* All concentration data in ppm.

matrix sample (Table 3). The abundances are plotted in Fig. 4, relative to the chondrite meteorite average of Nakamura [16]. The chondrite-normalised REE distribution patterns for the two boulders are virtually identical and are characterised by a high degree of REE fractionation and light REE enrichment. The (La/Yb)N ratios are approximately 34. The matrix sample has somewhat lower overall REE abundances, but shows a similar degree of REE fractionation. None of the samples have significant Eu anomalies. The general similarity of the REE patterns from the boulders and matrix strongly suggests a genetic connection between them. It is probable that the distribution patterns of these samples do not differ greatly from their igneous precursors. Such highly fractionated, heavy REE-depleted patterns have previously been recorded from leucocratic components of the Amftsoq gneisses [ 17] and from trondhjemitic and tonalitic gneisses of Minnesota [18]. In both cases they were interpreted as indicating fractionation of garnet from the magmas of the igneous precursors or from their source regions. This interpretation is considered to be appropriate for the volcanic rock s already hypothesised as ultimately parental to the boulders in the Isua supracrustal series. The possibility of sufficient fractionation of garnet during the reworking of the volcanics into sediments is not favoured, since garnet is not evident either in the other igneous members of the series, or in the more acidic components of the Amftsoq gneisses at Isua. The absence of Eu anomalies suggests that either there was little plagioclase fractionation during evolu-

234 100"

",\ ---,, , , \

iIn

~

I

I

I

I

I

I

I

I

I

I

Fig. 4. Chondrite-normalised rare earth element distribution pattern for two boulders and one matrix from the lsua supracrustals.

tion of the magmatic precursors, or perhaps that it occurred under oxidising conditions [ 17,19-21 ].

3. 2. Isua Amftsoq gneisses Pb isotope data for nine samples of Amftsoq gneiss from Isua are presented in Table 4. This is part of the same set of twelve samples which gave a R b - S r wholerock isochron age of 3700 +- 140 m.y. [5]. The 2°Tpb/ 2°4pb-2°6pb/2°4pb isotopic data is plotted in Fig. 3. The linear array does not provide a perfect isochron and the analytical errors have to be magnified by a factor of 15 to achieve a fit. The age obtained is 3800 +- 120 m.y., which agrees within the regression error with previous R b - S r and Pb/Pb whole-rock ages from lsua [4,5]. The Pb isotope ratios are, on the whole, not as unradiogenic as some of the Amftsoq gneisses from the Godthaab area, although there is some overlap. The Godthaab area gneisses yielded a Pb/Pb age of 3620 +100 m.y., obtained by volatilisation analyses [6]. However, four total dissolution analyses in the earlier work yielded a Pb/Pb age of 3760 +- 150 m.y., in closer accord with the results on lsua gneisses obtained in the present work, in which all analyses were carried out by total dissolution techniques. The 238U/2°4Pb (/al) for the source region of the lsua Amftsoq gneiss Pb, assuming a two-stage model

of Pb evolution and using the primordial Pb parameters of Oversby [22], is 9.3 + 0.2. (It should be noted that use of recently published revised decay constants for 238U and 2aSU, as well as primordial Pb parameters, reduces the Pb/Pb ages discussed here by 1.5% and the calculated/ax values by nearly 15% (23)). Both this value, and the isochron age of 3800 + 120 m.y., are in close agreement with numerous total dissolution analyses recently obtained on Amftsoq gneisses from the Godthaab area (Oxford unpublished work). Tlae /a~ value is close to that of ca. 9 obtained for the conventional primary Pb growth curve as defined by conformable Pb ore deposits and some modern oceanic volcanic rocks [24]. For comparison, Fig. 3 also shows the Pb/Pb isochron obtained on the Isua iron formation, with an age of 3760 +- 70 m.y. and a two-stage model/a 1 value of 9.9 + 0.1 [4] which is significantly higher than the value for the lsua Amftsoq gneisses and for the boulders in the supracrustal succession. These two-stage model values must represent a timeintegrated value of a more complex multistage evolutionary history. It is not surprising, therefore, that supracrustal sediments such as ironstones, which are almost certainly chemical precipitates from an ancient sea, should have developed from a higher U - P b system than plutonic rocks, such as the Amftsoq gneiss precursors, or volcanic rocks, such as the precursors

235 TABLE 4 Analytical data on Ami'tsoq gneisses from Isua (1971 collection) Sample No.*

Description

U** (ppm)

Pb** (ppm)

238U/2°4pb 206pb/204pb*** 2°Tpb/2°4pb*** 2°spb/2°4pb***

155763

Leucocratic, poorly banded, granitic gneiss. Kf, Qz, PI, Myrm, Bi, Ep, Se, Chl

1.259

25.50

2.73

14.954 ± 0.006

14.462 ± 0.007

33.481 2.013

155764

Grey, homogeneous, almost 0.479 unbanded granodioritic gneiss. Pl, Qz, Kf, Myrm, Bi, Sph, Ep, Ap

-

-

14.350 ± 0.010

14.198 ± 0.010

33.542 ± 0.023

155766

Grey, speckled, poorly banded, granodioritic gneiss. PI, Qz, Kf, Bi, Sph, Chl, Ep, Ap

0.601

11.68

2.89

14.454 ± 0.003

14.274 ± 0.004

34.749 ± 0.007

155767

Leucocratic, poorly banded, 0.886 granitic gneiss KL PI, Qz, Bi, Ep, Sph. Feldspars sericitised and epidotised

22.09

2.37

13.878 ± 0.005

14.137 ± 0.004

39.039 ± 0.014

155768

Leucocratic, homogenem:s ,0.763 granitic gneiss. Kf, Qz, PI, Se, Ep, Chl, Sph

21.32

2.04

14.077 ± 0.003

14.270 ± 0.004

36.597 +- 0.008

155769

Grey, poorly banded, granitic 0.592 gneiss. Kf, Qz, PI, Bi, Sph, Ep, Se

19.41

1.66

13.108 ± 0.004

13.764 ± 0.004

34.875 ± 0.01l

155774

Grey, homogeneous, nearly 0.504 unbanded granitic gneiss. Kf, Qz, PI, Bi, Se, Ep, Sph, ore

-

-

12.627 ± 0.005

13.647 ± 0.007

33.203 ± 0.013

155775

Grey, poorly banded, granitic 0.468 gneiss. Kf, Qz, P1, Bi. Sericitised and epidotised

23.53

1.02

12.327 ± 0.003

13.547 ± 0.003

32.545 ± 0.008

155780

Grey, poorly banded, homo . geneous, granitic gneiss. Kf, Qz, PI, Bi, Ap, Ep, Mu

12.501 ± 0.005

13.576 ± 0.006

35.344 ± 0.014

.

.

.

* Rb-Sr data on these samples published previously by Moorbath et al. [5]. ** Isotope dilution data. *** All errors are quoted at 2 sigma. Abbreviations." Kf = potash feldspar, Qz = quartz, P1 = plagioclase, Bi = biotite, Hb = hornblende, Myrm = myrmekite, Mu = muscovite, Ep = epidote, Sph = sphene, Ap = apatite, Chl = chlorite, Se = sericite, ore = iron ore minerals.

o f the source r o c k o f the b o u l d e r s . A t a n y rate, b y ca. 3 7 0 0 - 3 8 0 0 m.y. ago, d i f f e r e n t U - P b source regions already existed in the e a r t h ' s crust. As p o i n t e d o u t previously [ 4 , 6 ] , the h i g h l y u n r a d i o g e n i c p r e s e n t - d a y

Pb i s o t o p e c o m p o s i t i o n s o b s e r v e d in the A m f t s o q , gneisses a n d in t h e Isua i r o n f o r m a t i o n d e m o n s t r a t e t h a t these rocks, or t h e i r source region, h a d u n d e r g o n e severe U d e p l e t i o n at or b e f o r e ca. 3 7 0 0 - 3 8 0 0 m.y.

236 ago. This has been interpreted as a metamorphic phen o m e n o n [4,6], although it could have occurred at any time between separation of the precursors of the rocks from their respective source regions and their transformation into the form in which they are seen today, provided that this interval did not exceed about 1 0 0 - 2 0 0 m.y. Similar constraints for fire age of the Amftsoq gneiss precursors are obtained from the observed initial 87Sr/86Sr ratio of about 0.701 at about 3750 m.y. ago. If this initial ratio is regarded as a metamorphic value, then the average Rb/Sr value for all analysed Amftsoq gneisses of 0.3 indicates that pre-

cursors with similar Rb/Sr ratios could not have been emplaced more than about 1 0 0 - 2 0 0 m.y. prior to metamorphism [5]. An attempt was made to date the Isua Am~tsoq gneisses by the U - P b whole-rock method. U and Pb analyses for most of the samples are presented in Table 4, together with the calculated 238U/2°4 Pb values in the rocks. A plot of 238U/2°4Pb versus 2°6pb/2°4pb (Fig. 5) produces a scatter which has no meaning whatever in terms of the age of the rocks. The results are closely comparable to those obtained by Rosholt et al. [25] on 2800-m.y.-old granitic gneisses (dated

TABLE 5 Analytical data (X-ray fluorescence data) on further Amftsoq gneisses from Isua (1973 collection) Sample No.

Description

Rb* (ppm)

Sr* (ppm)

87Rb/86Sr**

87Sr/86Sr**

171747

Grey, poorly banded, fine-grained gxanodioritic gneiss, PI, Qz, Kf, Bi, Se, Sph, Ep, Ap. Feldspars sericitised and epidotised

86

422

0.592 +-0.008

0.73131 ± 0.00006

171748

Grey, poorly banded, very fine91 grained granodioritic gneiss. PI, Qz, Kf, Bi, Sph, Ep, Se, Ap, Ca, ore

561

0.470 ± 0.007

0.72561 ± 0.00006

171749

Grey, banded, fine-grained, grano- 87 dioritic gneiss. PI, Qz, Kf, Bi, Chl, Ep, Se, Mu, Ap. Feldspars sericitised and epidotised

287

0.880 ± 0.013

0.74762 ± 0.00008

171750

Grey, poorly banded, fine-grained, granodiorific gneiss. PI, Qz, Kf, Bi, Ep, Chl, Sph, Ap

95

323

0.851 -+0.013

0.74561 ± 0.00010

171751

Grey, poorly banded, fine-grained, granodioritic gneiss. PI, Qz, Kf, Bi, Ep, Sph

92

269

0.988 ± 0.015

0.75290 ± 0.00006

171752

Grey, banded, fine-grained, grano- 81 dioritic gneiss. P1, Qz, Kf, Bi, Se, Ep, Sph. Feldspars sericitised and epido tised

410

0.574 ± 0.008

0.73058 ± 0.00006

171753

Medium-grained, poorly banded, granodioritic gneiss, P1, Qz, Kf, Bi, Ep, Sph

414

0.568 ± 0.008

0.72973 +-0.00006

81

* Concentration data +-10%;mass absorption coefficients estimated from background. ** All errors are quoted at 2 sigma. Abbreviations as in Table 4; Ca = calcite.

237

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Fig. 5. U-Pb plot for Amftsoq gneisses (1971 collection), showing divergence from a model 3700-m.y. isochron because of recent uranium loss from the samples. For further discussion, see text.

Fig. 6. Rb-Sr whole-rock isochron for a further suite of Amftsoq gneiss samples from Isua (1973 collection).

by R b - S r and Pb/Pb whole-rock methods) of the Granite Mountains, Wyoming. A material balance calculation, based on the Granite Mountains 238U/2°4pb versus 2°6Pb/2°4pb data, showed that an average of approximately 75% of the amount of U required to produce the radiogenic Pb was removed from the rocks during the Cenozoic, probably as the result of leaching by groundwater. Calculations for the analysed Isua Amftsoq gneisses reveal geologically recent (or nearrecent) U loss in the range 0 - 4 0 % (Fig. 5) probably attributable to the same causes as in the Granite Mountains. At any rate, U depletion from the Amftsoq gneisses (or their precursors) occurred at, or not very long before, 3700-3800 m.y. ago and then again in geologically recent times. In the Granite Mountains the depth of leaching has been shown to extend to a depth of 50 m in a drill core. In both areas, the leaching has been sufficiently recent not to affect the 2°6pb/2C4pb versus 2°TPb/2°4Pb systematics significantly, although minor open-system behaviour with respect to U and/or Pb would have occurred at any time during the history of the rocks, as shown by the lack of perfect isochron behaviour (cf. Fig. 3). It appears that geologically recent loss of U from whole-rock systems can provide a severe limitation for U - P b whole-rock dating. Similar behaviour to that described above has been found in the Amftsoq gneisses of the Godthaab area (Oxford unpublished data).

Consideration of the 2°8pb/2°4pb data in Table 4 shows no close relationship with the 2°6pb/2°4pb data. As has been observed previously [6,26] there is no close geochemical coherence between U and Th in gneisses. This variability in Th/U is probably controlled by the mineralogical composition of the rocks, which contain substantial amounts of sphene, apatite and epidote. Finally, as an indirect check on published R b - S r work [5], analyses are presented in Table 5 on a suite of seven gneiss samples collected in 1973. Unlike the earlier samples, these were collected from a single restricted area of less than 1 k m 2 , very close to the southern margin of the Isua supracrustal belt. The data are plotted in Fig. 6 and yield an almost perfectly fitted isochron of 3780 + 130 m.y., with an initial STSr/86Sr ratio of 0.6998 + 12. The relatively high error in the latter is due to the absence of samples with sufficiently low Rb/Sr ratios. The values of the age and initial ratio agree within error with previously published values for the Amftsoq gneisses from the Isua and Godthaab areas.

4. Concluding remarks In this and previous papers, the following facts have been established concerning the evolution of the crustal

238 rocks of the Isua area: (1) Two independent sets of Amftsoq gneisses have yielded R b - S r isochron ages of 3700 + 140 m.y. [5] and 3780 -+ 130 m.y. Some of these rocks have also yielded a Pb/Pb isochron age of 3800 + 120 m.y. In view of the high degree of deformation and metamorphism suffered by the gneiss complex both at Isua and Godthaab, it is possible that these ages refer to a regional metamorphic event rather than to the crystallization of the igneous precursors of the gneisses. (2) The low initial 87Sr/a6Sr ratios on the isochrons, as well as the extremely unradiogenic Pb isotope compositions (especially at Godthaab), constrain fractionation of the igneous precursors from a mantle system to not more than about 100-200 m.y. before the dated event. (3) Geochronological evidence from the lsua supracrustal series is very similar to that from the gneisses. The iron formation gave a Pb/Pb isochron age of 3760 +- 70 m.y. [4], although the significantly higher 2°TPb/ 2°4Pb ratios compared to those of the gneisses indicate a higher average U/Pb ratio in the source regions of the iron formation. Boulders from the conglomeratic unit give an R b - S r isochron age constrained to the interval 3710-3620 m.y. The mineralogy and geochemistry of both these rock units clearly identify them as sedimentary rocks. The iron formation is a chemical precipitate, whereas the boulders are composed of clastic sediments (possibly water-laid) derived from an acid igneous parent. The isochrons may relate to time of post-formational metamorphic homogenization. Nevertheless, consideration of the initial 87Sr/a6Sr ratios of the boulders shows that 3710 m.y. is also a maximum age for deposition of the sediment (or tuff) from which they were derived, if not also for eruption of the parent volcanic rock. The Isua supracrustal succession is thus at present the oldest dated greenstone belt on the earth. The major volcanic and sedimentary features are essentially indistinguishable from those of younger greenstone belts in North America and southern Africa [27-31 ]. (4) The fact that all isochron ages for both gneisses and supracrustals are within error of each other at around 3700 m.y. leads to the conclusion that they are all dating the same Sr isotope homogenization event. The entire sequence of crustal formation in the lsua area evidenced by presently exposed rock types must have occurred within an interval of not more

than ca. 200 m.y. prior to 3700 m.y. ago. This includes multi-stage fractionation of acid igneous rocks from the early mantle, their emplacement, deformation, and metamorphism, to form the presently exposed gneiss complex. It also includes the eruption of basic and acid volcanic rocks, erosion of pre-existing crustal rocks, and their deposition in an aqueous environment as chemica! and clastic sediments, followed by low-to-medium gra~e metamorphism of this supracrustal series to form a typical greenstone belt assemblage. The relative ages of the presently exposed gneisses and supracrustal rocks cannot be defined from the available age data. The most significant conclusion is that in the Isua area, the continental crust attained its present composition and metamorphic grade as a result of diverse processes during the interval of ca. 3 9 0 0 3700 m.y. ago. (5) Subsequent events in the lsua area, not yet dated, are confined to the emplacement of Ameralik dykes and the deformation associated with folding of the greenstones into an arcuate belt. Work is currently in progress to attempt to date the dykes where they have largely escaped the effects of this deformation [3]. The major post-Ameralik dyke rock-forming and tectonic events characteristic of the Godthaab region [13-15,32,33] are absent at Isua. Surprisingly little appears to have happened to the rocks of this area during the past 3700 m.y.

Acknowledgements We are grateful to D. Bridgwater and V.R. McGregor for supplying the samples of boulders and matrix from Isua, and to them and Dr. F. Kalsbeek for much help and discussion; the Geological Survey of Greenland for supporting two of us (S.M. and R.J.P.) in the field to collect samples; the Natural Environment Research Council for supporting age and isotope work at Oxfe,'d; R. Goodwin and M. Humm for skilled technical assistance; M. Hepher and P. Shreeve for the chemical analyses. The paper is published by permission of the Director of the Geological Survey of Greenland.

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