Extreme Nd-isotope heterogeneity in the early Archaean - fact or fiction? Case histories from northern Canada and West Greenland

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1 ELSEVIER Chemical Geology 135 (1997) ,NCL D,NG ISOTOPE GEOSCIENCE Extreme Nd-isotope heterogeneity in the early Archaean - fact or fiction? Case histories from northern Canada and West Greenland S Moorbath *, MJ Whitehouse, BS Kamber Department of Earth Sciences, Oxford Uniuersiry, Parks Road, Oxford, OXI 3PR, UK Received 30 June 1996; accepted 22 July 1996 Abstract Sm-Nd data on rock suites from early Archaean provinces in northern Canada and West Greenland clearly demonstrate that tectonothermal (ie igneous, metamorphic, tectonic) processes which affected the rocks long after their formation produced open-system behaviour leading to effective resetting of the Sm-Nd system accompanied by complete, or near-complete, Nd-isotope homogenisation This means that extreme caution is required in interpreting highly variable initial end values of ancient rocks in terms of long-standing regional mantle heterogeneity or of complex mantle-crust interaction processes In particular, calculated initial Nd values based on high-precision zircon U-Pb dates may be of little or no significance in terms of geochemical evolution of early mantle and crust source regions A striking example is provided by the Acasta gneisses of northern Canada, with published SHRIMP U-Pb zircon dates in the range Cia and apparent, initial end values in the range -48 to f36 (Bowring and Housh, 1995) A combination of 34 published and new Sm-Nd whole-rock analyses for a wide range of rock types yields a regression (error&on) age of 3371 f 59 Ma (MSWD = 921, with initial Nd = -56 k 07 Whilst the very negative initial Ed,, provides strong, independent support for the extreme age of the Acasta gneiss protolith, resetting of the Sm-Nd system at - 34 Ga renders calculation of initial Ed,, based on the zircon U-Pb dates geologically meaningless Analogous considl:rations for early Archaean Akilia enclaves and host Amitsoq gneisses of West Greenland suggest that their published range of initial Ed,, values of to + 45 at Ga (Bennett et al, 1993) may be unrealistically wide and, therefore, inappropriate for modelling upper-mantle heterogeneity In an attempt to determine a realistic initial end value, we have regressed 58 published and new Sm-Nd data for two major rock units of the Isua supracrustal belt (felsites and mica-schists), regarded as having a short-term crustal history combined with minimal Sm-Nd disturbance The Sm-Nd regression yields an age of 3776 &- 52 Ma (MSWD = 82), with initial end = + 20 k 06 This value is much closer to conventional depleted-mantle models (eg, DePaolo et al, 1991) than to the value of proposed by Bennett et al (1993) for this age range Our observations suggest that there may have been no major change in processes between early Archaean and more recent styles of depleted-mantle evolution Keywords: Archaean; Mantle: Crust; Nd isotopes; Geochronology * Corresponding author Present address: Laboratory for Isotope Geology, Swedish Museum of Natural History, Box 50007, S Stockholm, Sweden /97/$1700 Copyright Elsevier Science BV All rights reserved PI1 SOOO9-2541(96)

2 214 S Moorbath et al / Chemiwl Geology 135 (1997) Introduction Sm-Nd characteristics of precisely dated early Precambrian rocks show a wide range of initial Nd values, variously interpreted as evidence for transient, highly depleted mantle reservoirs during early Earth history (eg, Bennett et al, 19931, or as rock genesis from mixtures between mantle-derived and crustal melts derived from extremely old ( N 43 Ga), heterogeneous, depleted (high Sm/Nd) and enriched (low Sm/Nd) reservoirs (eg, Bowring and Housh, 1995) Such models are of great potential importance for studying the geochemical evolution of the earliest crust and mantle and could mean, for example, that the isotopic evolution of the Earth s mantle reflects progressive eradication of primordial heterogeneities related to early differentiation In addition, Bennett et al (1993) proposed a major change in processes between early Archaean (> 37 Ga) and later styles of depleted-mantle evolution, based on a Nd-isotope evolution curve with a pronounced end peak of f45 at 37 Ga This contrasts sharply with the conventional, smooth depleted-mantle Nd-isotope evolution curve of DePaolo et al (19911, which reflects removal of a light rare earth element (LREE)-enriched component through time, resulting in a LREE-depleted reservoir (see also Galer and Goldstein, 199 1) All these initial e,,-based models assume that the analysed rocks remained closed to disturbance of the Sm-Nd system since the measured age of the rock, most commonly obtained from SHRIMP U-Pb dating of zircons Here we question this assumption, because published Sm-Nd data clearly suggest that some critical sample suites from early Archaean terrains have been open systems to Sm and/or Nd during much later tectonothermal events, sometimes accompanied by nearly complete Nd-isotope homogenisation Such open-system behaviour would negate the validity of using initial en,, values for tracing early mantle/crust evolution, even when based on precise and valid age data for the metamorphic precursor, because the calculated initial Nd values would be apparent rather than real Numerous recent studies have documented open-system behaviour in the Sm-Nd system during subsequent metamorphism and metasomatism (eg, McCulloch and Black, 1984; Windrim et al, 1984; Black and McCulloch, 1987; Whitehouse, 1988; Bridgwater et al, 1989; Li et al, 1990; Tourpin et al, 1991; Gruau et al, 1992; Frost and Frost, 1995; Lahaye et al, 1995; Poitrasson et al, 1995; Gruau et al, 1996) Another line of evidence suggesting that the published range of initial end values for the early Archaean is much too wide comes from a comparison of Hf- and Nd-isotope systematics in zircons from ten early Archaean gneisses and supracrustal rocks from West Greenland (Vervoort et al, 1996) These authors conclude that the range of Nd compositions measured in these ancient gneisses do not faithfully represent primary isotopic variations in their source regions but rather have been produced, at least in part, by whole-rock geochemical disturbance Until these Nd data are examined more critically, they should not be used to constrain the details of early earth history Here we review published, and report new, Sm- Nd data on a wide range of early Archaean rocks from northern Canada and West Greenland Close analysis of their whole-rock Sm-Nd isotopic systematics demonstrates that, in some cases, open-system disturbance of the Sm-Nd system has affected these rock units long after their presumed age of rock formation We attach particular significance to wellcorrelated Sm-Nd regression lines (isochrons and errorchrons), interpreting them as resulting from nearly complete resetting of the Sm-Nd system during much later tectonothermal events In contrast, some previous workers (eg, Bennett et al, 1993; Bowring and Housh, 1995) do not present their Sm-Nd data on isotopic evolution diagrams because they appear to assume, a priori, that petrologically heterogeneous rock suites (with the same or different zircon U-Pb ages) cannot possibly have homogeneous initial Nd-isotope ratios and that a linear array on a Sm-Nd isotopic evolution diagram significantly post-dating rock formation automatically represents a mixing line with no age significance, rather than defining the age of a resetting event (eg, Bowring and Housh, 1995) We are very sceptical about this highly constrained approach to the interpretation of well-correlated Sm-Nd regressions and, particularly, to the suggestion that minor statistical scatter about well-correlated regression lines (but with MSWD > 1) necessarily results from initial end heterogeneity

3 S Moorbath et al / Chemical Geology 135 (1997) of a rock unit, rather than from subsequent minor open-system behaviour to Sm and Nd long after the age of rock formation Our overall philosophy to the interpretation of wihole-rock isochrons broadly follows that of Camleron et al (1981) who clearly distinguish between analytical and geological scatter and demonstrate (what many geochronologists intuitively realise and practise) that a realistic, though variably precise, assessment of the primary igneous or metamorphic age, as well as the initial isotope ratio, of a given rock suite is quite possible despite a small degree of statistical, geological scatter in excess of analytical error on an isochron diagram In other words, it is not necessary to obtain perfect isochrons (MSW:D = 1) to yield meaningful geochronological information Whilst Cameron et al (1981) dealt entirely with the Rb-Sr method, we believe that their general approach applies to the Sm-Nd method with equal validity Applying these general principles to published and new Sm-Nd data for rock units from northern Canada and West Greenland leads us to general agreement with the recent Hf-isotope work of Vervoort et al (1996), suggesting that the range of initial end values for the early Archaean may be considerably narrower than has been claimed in some of the recent work already quoted above 2 Acasta gneisses, Slave Province, NWT, Canada 21 Previous work The most detailed geological description so far of the Acasta gneisses has been given by Bowring et al (1990) They describe these rocks as a heterogeneous assemblage composed mainly of strongly foliated biotite-hornblende tonalitic-to-granitic orthogneisses commonly interlayered on a small scale with amphibolitic and chlorifc schlieren, boudins and layers Large areas of amphibolites also occur, together with less abundant litheologies of talc-silicate gneisses, quartzites, biotite schists and ultramafic schists, some of which presumably represent metamorphosed supracrustal rocks I(paragneisses) Metamorphic temperatures are regarded as somewhere between 400 and 650 C The above rocks are furthermore intruded by weakly foliated gabbroic-to-dioritic dykes and pods, as well as weakly-to-strongly foliated biotitebearing granitic rocks In general, the post-formational tectonic and metamorphic history of the Acasta gneisses is not well understood Zircon U-Pb dates for the Acasta gneisses have been reported by Bowring et al (1989a,b,1990) and by Bowring and Housh (1995) Zircons from two samples indicated that the tonalitic-to-granitic gneisses crystallised at 3962 & 3 Ma (Bowring et al, 1989a) Subsequently, Bowring and Housh (1995) reported further SHRIMP zircon U-Pb ages ranging from 40 to 36 Ga for ten Acasta gneiss samples, ranging in composition from amphibolitic to granitic Importance was also attached to a significantly later event at 348 Ga (Bowring et al, 1990): consistent with periods of intrusion, deformation and metamorphism at ca Ga A total of 13 Sm-Nd analyses has been reported by Bowring et al (1990) and Bowring and Housh (1995) For reasons stated in these papers, zircon U-W dates ranging from 36 to 40 Ga were used as the basis for calculating initial Nd values Bowring and Housh (1995) report that the Acasta gneisses exhibit a wide range of initial Ed,, ( + 35 to - 4 at 40 Ga, and + 4 to - 7 at 36 Ga) This wide range was then used as the basis of Bowring and Housh s (1995) complex model of heterogeneous mantle and crust evolution and interaction in the early Earth Our scepticism of the validity of the above approach was first aroused by plotting a Sm-Nd evolution diagram (143Nd/ IaNd vs 14 Sm/ 44Nd) for 7 Acasta gneiss samples based on Bowring et al s (1990) original Sm-Nd analyses, combined with preliminary work on a small suite of compositionally varied samples collected by MJ Bickle and by S Maruyama, and analysed for Sm-Nd at Oxford The pooled 7-point regression age (unpublished) of 3464 _t 97 Ma (MSWD = 49) suggested that the Sm-Nd system might have been reset at this time by an igneous or metamorphic event Furthermore, the negative initial end value of obtained from the Sm-Nd regression suggested that the Acasta gneisses had a low-sm/nd (enriched) crustal protolith, probably as old as the age obtained from zircon U-Pb analysis (This is discussed in much greater detail below) Nevertheless, the apparent re-

4 216 S Moorbnth et al/chemical Geology 135 ( setting of the Sm-Nd system at Ga precluded the use of samples with individually determined zircon U-Pb ages in the range of Ga for calculating valid initial Nd values 22 Samples and results mentioned in the previous section, are reported in Table 1 All 25 samples plot on a well-correlated Sm-Nd regression (Fig 11, yielding an age of 3348 k 65 Ma (MSWD = 88) with an initial end of - 56 f 07 All regressions in this paper were calculated by the method of Ludwig (19911, with errors quoted at 2 u In order to test and extend the above preliminary The Acasta sample suite comprises a very wide findings, one of us (SM) made a collection of 20 range of rock types from granitic, through intermedisamples of Acasta gneisses in summer 1995 (under ate, to amphibolitic gneisses, with ranges of SiO, the general guidance of W Padgham and S Bowring) from 74 to 43 wt%, K,O from 53 to 09 wt%, CaO from several of the type localities described previ- from 09 to 107 wt%, MgO from 06 to 156 wt%, ously (eg, Bowring et al, 1990; Bowring and Housh, and total Fe (as Fe,O,) from 06 to 196 wt% The 1995) Sm-Nd analyses on these 20 samples, to- range of mineralogical compositions varies accordgether with the previous 5 Oxford Sm-Nd analyses ingly The rock samples were collected from several Table 1 Sm-Nd analytical data for Acasta gneisses Sample Mineralogy SiO, Sm Nd (wt%) 43Nd/ 44Nd 14 Sm/ l Nd fsm,nd ENd <Nd (present) (I = 396 Gal SM/Acl PI, Hbl, Qtz, Chl 639 SM/Ac2 Pl, Hbl, Bt, Spn, Qtz 494 SM/Ac3 PI, Qtz, Ep, Chl, Grt, Bt 737 SM/Ac4 Hbl, PI, Bt, Qtz 592 SM/AcS Hbl, PI, Ilm, Spn, Bt, Qtz 455 SM/Ac6 Hbl, Pl, Ilm, Spn, Ep, Chl 457 SM/Ac7 Pl, Qtz, Kfs, Ep 665 SM/Ac8 Hbl, Chl, Pl, Ilm, Stp 487 SM/Ac9 Hbl, PI, Fe-ore, Chl, Qtz 434 SM/AclO Qtz, Pl, Kfs, Ep, Chl 613 SM/Acll Kfs, Qtz, Pl, Bt 698 SM/Acl2 PI, Qtz, Bt, Hbl, Grt 691 SM/Acl3 PI, Qtz, Hbl, Bt, Grt 696 SM/Acl4 Hbl, Pl, Bt, Qtz, Ilm 533 SM/AclS PI, Hbl, Qtz, Bt, Spn 554 SM/Acl6 Qtz, Kfs, Pl, Bt, Ep 715 SM/Acl7 PI, Qtz, Bt, Hhl, Kfs, 632 Ep, Spn SM/Ac18 Bt, Grt, Hbl, Ilm, Qtz, Pl 433 SM/Acl9 Qtz, Kfs, PI, Bt 708 SM/Ac20 Hbl, Pl, Bt, Fe-ore 433 Maruyama Kfs, Qtz, PI, Bt _ Bickle-A2 Qtz, Pl, Kfs, Bt Bickle-A3a Kfs, Qtz, PI Bickle-A3bP1, Hbl, Qtz, Bt, Chl, Spn - Bickle-A4 Qtz, PI, Kfs, Bt Analytical details refer only to previously unpublished data Sm and Nd were separated from whole-rock powders by dissolution using HF-HNO, at 170 C in pressurised containers, followed by standard ion-exchange techniques Sm and Nd concentrations determined by isotope dilution; error on 14 Sm/ 44Nd ratios is _ *Ol% Nd isotopic ratios were determined on a VG 54E mass spectrometer and corrected for within-run mass fractionation by normalization to a 46Nd/ 44Nd ratio of 07219; replicate analyses of La Jolla standard yield 14 Nd/ 44Nd f (00044%, 2 cr; n = 20) Data from other sources have been normalised to this value Decay constant (A) for 14 Sm = 654 X lo- I2 a- Nd parameters calculated relative to CHUR ( 43Nd/ l Nd = ; 14 Sm/ 44Nd = 01966; Hamilton et al, 1983); enrichment expressed as fsm,nd relative to CHUR

5 S Moorbath et al/ Chemical Geology 135 (1997) cogenetic or not in terms of Nd heterogeneities In our model, significant geological scatter (MSWD > 1) about the regression line could imply that either pre-337-ga end heterogeneities were not totally eradicated, or that post-metamorphic disturbance (eg, at N 19 Ga, see Hodges et al, 1995) caused limited open-system behaviour in the Sm-Nd system, or both In contrast, Bowring and Housh (1995) assume that each of their analysed samples remained a closed Sm-Nd system since time of rock formation, as estimated from the individually associated zircon U-Pb ages They state (Bowring and Housh, 1995, footnote 4) that Fig 1 Sm-Nd regression for Acasta gneiss samples from this paper (filled circles), and Bowring et al (1990) and Bowring and Housh (1995) (open circles) of the same localities for which Bowring and Housh (1995) quote SHRIMP zircon U-Pb dates ranging from 36 to 40 Ga Most of the Acasta gneiss Sm-Nd analyses of Bowring et al (1990) and Bowring and Housh (1995) fall on the same regression line defined by the Oxford analyses, with no significant change in parameters Thus a combined regression line based on 34 analyses (Fig 1) yields an age of 3371 k 59 Ma (MSWD = 921, with initial Nd = - 56 f 07 Three of the 36-Ga samples (granite 91-5, tonalitic gneiss 89-18, amphibolitic gneiss 91-38) of Bowring and Housh (1995) and one sample (amphibolitic gneiss BGXM) of Bowring et al (1990) have been omitted because of poor fit to the combined regression Their inclusion in the regression produces an age of 3329 f 110 Ma (MSWD = 1491, with initial Nd = However, i-n what follows, we rest our case on the well-correlated regression of 34 (out of 37) published and new Sm-Nd data points 23 Interpretation of the Sm-Nd regression Our main thesis is that the Acasta gneisses suffered a tectonothermal event at N 337 Ga (see above) which caused open-system behaviour for Sm-Nd and resulted in close approximation to Ndisotope homogenisation on the scale of sampling, regardless of whether the samples were originally the Acasta gneisses are compositionally and temporally heterogeneous and thus were not all derived from a single homogeneous reservoir at the same time and further claim that therefore, any linear array on an isochron diagram for these samples is a mixing line, and calculated ages and initial isotopic ratios have no geological significance Their view reflects a general scepticism towards whole-rock isochron dating Indeed, several studies have shown that linear arrays in isochron plots may reflect mixing relationships and have no direct geochronological significance With respect to Sm- Nd whole-rock dating, several attempts to date mafic-ultramafic rock units have produced false isochron ages (eg, Cattell et al, 1984; Hegner et al, 1984; Chauvel et al, 1985; Gruau et al, 1990) and were shown to result from two-component mixing (eg, contamination of a mantle-derived melt by continental crust) If the inverse of Nd concentration, l/nd, is plotted vs 143Nd/ 144Nd ratio (either present-day, or corrected for 14 Sm decay since the assumed geological age), simple two-component mixing produces a straight line, usually with a positive slope since the radiogenic end-member tends to have a lower Nd concentration Although straight lines in this diagram may also be obtained for true isochrons, a lack of correlation reasonably argues against mixing In Fig 2a, we plot data for matic-ultramafic whole-rock analyses by Cattell et al (19841, which define a provably false regression age A broad

6 218 S Moorbath et al /Chemical Geology 135 ( ~~,~1~~~~1~~,~1~~~~1~~~~'~~~~ VW * _ : - _ : -_ ;,,,, I,a,,,,,,,,,,,,,,,,,I VW 05100,, _,,, I C 19 Ge * "', '3"" """"""I Wdl G 40 Ge * * ' t * : f a'*' * : L ""'3" * 1 I Fig 2 Plots of inverse Nd concentration vs 14 Nd/ 44Nd: (a) present-day data from Cattell et al (1984) for late Archaean lavas from the Abitibi Belt, Ontario, define a positive, linear slope, illustrating binary mixing, (b) present-day values from Acasta gneisses from this study, Bowring et al (1990), and Bowring and Housh (1995); same data back-corrected to (c) 19 Ga, (d) 337 Ga, (e) 40 Ga The data scatter widely in (b), (c), and (e), indicating that binary mixing does not explain the tight correlation in the isochron plot (Fig 1) With the exception of three aberrant samples (squares), values back-corrected to 337 Ga define a linear, horizontal array, supporting the interpretation of a homogenisation event l/iw linear array is defined by these data, which supports the view that the regression is a mixing line, so that the resulting age and initial Nd isotope information is meaningless (This plot of 143Nd/ 44Nd versus l/nd is not actually given in Cattell et al, 1984) In contrast, when the Acasta gneiss samples are plotted in similar fashion (Fig 2b) they scatter randomly so that the tight linear regression plot is clearly not the

7 S Moorbath et al /Chemical Geology 135 (1997) result of binary mixing, thus refuting Bowring and Housh s (1995) explanation If 143Nd/ 144Nd ratios are calculated for various times in the past, such as the N 19-Ga metamorphic overprint (Hodges et al, 1995) the postulated 337-Ga homogenisation event, the protolitb formation age of 40 Ga, and plotted against l/nd (Fig 2c-e) the best-correlated linear fit is obtained at 337 Ga With few exceptions, the Acasta gneiss 143Nd,/ 144Nd ratios calculated back to 337 Ga broadly define a straight horizontal line (Fig 2d), showing that most analysed Acasta gneisses had approximately the same Nd isotopic composition regardless of their Nd concentration This further supports our thesis of a 337-Ga event which severely affected the Sm-Nd systematics of the Acasta gneisses A similar plot for 43Nd/ 44Nd ratios calculated back to 40 Ga shows significantly greater scatter and a negative slope (Fig 2e) It should be noted that Bowring and Housh s (1995) suggestion that the well-correlated linear array in the isochron plot represents two-component mixing actually conflicts with their postulate of a range of preserved Nd-isotopic heterogeneities in the Acasta gneisses In other words, the wide range in initial Nd ( - 48 to + 36) obtained by Bowring and Housh (1995) precludes binary mixing, if these val- 5 - MSWD=478 : age = 3661flW) Ma Nd MaI= B : i P :! I : : 8 I I : i o&ir~ 33~;lien: - 3 age = 37 umsu hia LM!;li-l _ mll*r - MsWD= 915 age = 3543%?50 Ma Nd initial = Nd initial = Sm/144NC age i: 363W60 Ma Nd initial = SO7618 : I gh?l~d~~3gliilx: age = 37E4%kz?O Ma Nd initial = I J i I t * Sml 4 Nd Fig 3 Model Sm-Nd isochron regressions for (a) data of Bowring and Housh (1995), assuming that each sample represents an entire rock suite with 14 Sm/ 44Nd values ranging from 007 to 016, calculated for corresponding zircon U-Pb age; (b), (c), Cd) isochron plots for three randomly selected data sets of 40 points from (a) Individual regressions were calculated firstly for all points, and secondly with the omission of the three most aberrant samples (see text)

8 220 S Moorbath et al/ Chemical Geology 135 (1997) ues are interpreted as reflecting undisturbed primary Nd-isotope systematics Nevertheless, because of the slow decay of 14 Sm, 40- and 36-Ga-old protoliths with variable initial 143Nd/ 44Nd compositions will tend to define a broadly linear array in an isochron diagram In order to explore the possible scatter associated with such an array, we have performed a simple numerical simulation Following the model of Bowring and Housh (1995) we have interpreted each of their data points to represent one sample of a cogenetic suite of rocks For each rock suite we have arbitrarily assumed a range of 14 Sm/ 44Nd ratios from 007 to 016 (note that the true range is not crucial to our calculations) Fig 3a shows isochrons for these suites, calculated for the appropriate U-Pb zircon age, starting with an initial 143Nd/ 144Nd ratio based on Bowring and Housh s (1995) initial end value Here we treat this data array as a random sampling pool, assuming that the Sm-Nd system remained undisturbed by any later overprint, as argued by Bowring and Housh (1995) We have randomly selected 40 data points from this array and calculated closeness of fit (indicated by MSWD), age and initial 143Nd/ 144Nd ratio in isochron plots for three simulations, displayed in Fig 3b-d Whilst computed ages and initial 43Nd/ 144Nd ratios do depend on the true 14 Sm/ 144Nd ratios of individual rock suites and should therefore not be regarded as predictive, the respective MSWD values of 478, 915 and 763 are much less sensitive to choice of 14 Sm/ 44Nd values and can be regarded as the expected closeness of fit of a linear data array If we regress all 37 measured Acasta gneiss Sm-Nd points (see previous section), we obtain an MSWD of 149 Even this is far better than predicted by the simulation More importantly, omitting the three worst samples from the simulation regression only improves the fit partially (ie 369 instead of 478, 682 instead of 915, 480 instead of 763) whilst omitting 3 out of 37 real samples lowers the MSWD to 92 (Fig l), almost two orders of magnitude smaller than predicted We regard this as strong evidence that our regression does not reflect random sampling from an inherently linear data pool, and we conclude that the obtained age and Nd-isotope information are of geological significance Whilst the slow decay of 14 Sm precludes using a whole-rock approach to resolve Nd-isotope heterogeneities resulting from age differ- ences of even several 100 Ma, the range of initial Nd-isotope ratios postulated by Bowring and Housh (1995) would, if genuine, provide sufficient dispersion to be easily detectable by whole-rock analysis In summary, inspection of the Acasta gneiss Sm-Nd regression shows that the obtained errorchron is neither the result of binary mixing nor of random sampling of an inherently linear original data pool The fact that the best correlation between l/nd vs 143Nd/ 144Nd is obtained for a horizontal line at N 337 Ga further supports our model of a geological event which nearly eradicated any pre-existing Nd-isotope heterogeneities There are several circumstantial lines of evidence for limited open-system disturbance of the Sm-Nd system in the Acasta gneisses post-dating the 337-Ga resetting event Hodges et al (1995) report U-Pb evidence for a Pb-loss event at 188 Ga, based on fine-grained titanites and metamorphic overgrowths on coarse titanite Support for this metamorphic event is provided by a 186-Ga 4o Ar/ 39Ar hornblende age Hodges et al (1995) further state that Nd isotopic data for garnets from the same rocks reveal complex systematics, probably in part related to multiple episodes of garnet growth, and provide no useful information regarding the age of either metamorphic event Turning again to the preservation of the pre-337- Ga record (other than zircon U-W ages) there is, of course, no a priori requirement that all the different Acasta gneiss rock types had an identical initial end value at time(s) of rock formation However, as explained later, if the observed Sm-Nd regression indeed represents a time of near-complete Nd-isotope homogenisation, then the record of any previous isotopic heterogeneities in initial E,,, may have been largely eliminated Interpretation of the regression age as resetting of the Sm-Nd system is strongly supported by the very negative initial end (337 Ga) value of (Fig l), pointing to the existence of an enriched (low Sm/Nd) crustal protolith with an age greatly exceeding 337 Ga Indeed, assuming an average continental crust Sm/Nd ratio of 017 (Taylor and McLennan, 1985) for the Acasta gneiss precursor, the observed initial end value of -56 at 337 Ga (Fig 1) extrapolates back to + 10 at 396 Ga, ie the oldest zircon U-Pb age obtained

9 S Moorbath et al/chemical Geology 135 (1997) ; I, t(w, Fig 4 Nd-isotope evolution diagram for all Acasta gneiss samples plotted in Fig 1 Individual evolution lines arc not shown, but all fall within the shaded jield The 2 u error polygon represents the parameters shown in the inset of Fig 1 The open squares represent the initial Nd values based on individual zircon U-Pb dates of Bowring et al (1990) and Bowring and Housh (1995) for the Acasta gnei:sses (Bowring et al, 1990) This is much closer to uniformitarian models of early Archaean Nd-isotope evolution in both the chondritic uniform reservoir (CHUR) and depleted-mantle models (eg, DePaolo et al, 1991) 24 Interpretation of individual, apparent initial end values If, as argued earlier, the Sm-Nd system in the Acasta gneisses was largely reset at N 337 Ga, then this would have destroyed all or most of the pre-337 Ga record of Nd heterogeneities, so that calculation of initial Nd values based on individual zircon U-W ages would be geologically meaningless This is illustrated in Fig 4 in a Nd-isotope evolution diagram The 34 individual Nd-isotope evolution lines are not plotted separately, but all fall within the shaded field, focusing on the 2u error polygon which represents the age and initial end value obtained from the Sm-Nd regression (Fig 1) Also shown (squares) are the zircon U-W ages of Bowring et al (1990) and IBowring and Housh (1995) Although there are no zircon U-Pb age measurements on the samples analysed for Sm-Nd at Oxford, they were collected frorn some of the same localities as those of Bowring and coworkers and therefore presumably occupy the same age range The corresponding values of apparent initial Ed,, can be read off from the vertical1 axis The greater the age differ- ence between the measured zircon U-Pb age and the Sm-Nd resetting age, the greater will be the range of apparent initial Nd values For example, at the maximum age of 40 Ga, the apparent range extends from _ -5 to +7, but at 36 Ga only from u - 65 to - 1 It is clear that the individual end values based on associated zircon U-Pb ages have no geological significance whatever The plot in Fig 4 rather simplistically assumes only Nd-isotope homogenisation in individual samples, so that individual whole-rock Nd-isotopic evolution lines pass straight through the 2cr error polygon region It is probable, given their similar chemistry, that both Sm and Nd would be mobile during a resetting event at 337 Ga, resulting in a change of Sm/Nd ratio for individual samples in addition to Nd-isotope homogenisation In that case, the overall field of Ndisotope evolution would be correspondingly less well constrained and could yield an even wider range of apparent pre-337 Ga mmal end values 25 Interpretation of the geological nature of the resetting event Earlier we saw that the observed MSWD of N 9 for the 34-point Sm-Nd regression (Fig 1) was very much smaller than can be accounted for by independent, undisturbed Nd-isotope evolution from different samples with different ages and initial Nd values The problem therefore is to propose a plausible geological process that could cause near-complete Nd-isotope homogenisation at 337 Ga in a petrologitally and geochemically wide variety of lithologies over an area of w km2 (for overall geological and locality map, see Bowring and Housh, 1995) It is unlikely that metamorphism and tectonism alone were responsible for near-complete Nd-isotope homogenisation (eg, Barovich and Patchett, 1992) Over the past decade, attention has been drawn to the fact that hydrothermal overprint, both at low metasom atic tern perature and at high metamorphic/anatectic temperature may severely disturb the REE distribution of adjacent rocks (eg, Windrim et al, 1984; Whitehouse, 1988; Frost and Frost, 1995; Poitrasson et al, 1995; Bau, 1996) Bowring and Housh (1995, footnote 4) state that the Acasta gneisses are a sequence of layered me&igneous rocks that have been subjected to

10 222 S Moorbath et al /Chemical Geology I35 (1997) deformation This geological history resulted in a series of layered rocks in which the layers reflect differences in primary igneous compositions From our own field and microscopic observations we interpret this layering to be the result of a powerful tectonometamorphic overprint accompanied by anatexis In addition, the rocks suffered a much younger low-grade metamorphic overprint at N 19 Ga (Hodges et al, 1995) which was clearly not responsible for the observed layering, but which may have caused some of the scatter around the 337 Ga regression line (Fig l), as pointed out earlier Thus the geological history of the Acasta gneisses requires a high-grade event involving melting, metamorphism, migmatisation and deformation which occurred between formation of the protolith (40 and 36 Ga) and the low-grade overprint at 19 Ga Here we postulate that this event may itself be recorded by the 337 Ga Nd-isotope homogenisation and that the anatectic nature of the Acasta gneiss layering provides a framework for possible REE redistribution Under these circumstances, preservation of initial Nd heterogeneity corresponding to rock type, as postulated by Bowring and Housh (19951, would be most improbable In our interpretation, therefore, it is probable that the age of the Acasta gneisses is only N 337 Ga, although both the zircon U-Pb ages (40 to 36 Ga) and the initial end value of N -56 obtained from the Sm-Nd regression (Fig 1) provide incontrovertible evidence for the existence of a substantially older precursor Such a precursor - which is well worth searching for in the field - may resemble the kind of stored mafic/ultramafic crust envisaged by Chase and Patchett (1988) to account for early Archaean mantle depletion Such mafic/ultramafic rocks alone, however, may be compositionally inappropriate as the source of ancient, inherited zircons in the Acasta gneisses The protolith would also have to comprise compositionally evolved rocks as a source for the zircons The only early Archaean terrain (_ 38 Ga) known to us which contains a major, essentially bimodal assemblage of ultramaficto-mafic/felsic volcanogenic rocks is that of the Isua supracrustal belt of West Greenland (eg, Nutman et al, 1984), which is discussed later Supracrustal and infracrustal assemblages with this compositional polarity may have been common during the early Ar- chaean and could have formed an appropriate protolith for the Acasta gneisses Finally we have to ask whether there is any evidence for contemporaneous rock-forming or tectonothermal events in the northern part of the Slave Province According to W Padgham (pers commun): 34 Ga dates in the Slave Province are rare, and none may have been published in refereed publications However, Yamashita et al (1995) have reported the existence of a N 34 Ga old tonalitic and gabbroic basement in the Hanikahimajuk Lake area at the northwestern margin of the Slave Province, N 425 km north of Yellowknife, NWT This is N 200 km northeast of the Acasta region, and we hope that further age and isotope work in this region may delineate the extent and intensity of the 34 Ga event, which we argue is so convincingly recorded in the Acasta gneisses 3 Early Archaean rocks of West Greenland 31 Previous age work An immense amount of geological, geochronological and related isotopic work has been published on the early Archaean rocks of West Greenland over the past 25 years and only the briefest summary is given here Most of the voluminous Rb-Sr and Pb-Pb whole-rock isochron work, together with conventional zircon U-Pb work, give ages in the range N Ga for both the earlier Isua supracrustal rocks and later Amitsoq gneisses (Moorbath et al, 1986, and references therein) Occasional excursions from this range can extend down to N 36 Ga (especially for the Amitsoq gneisses) and up to 38 Ga (especially for the Isua supracrustals) A great deal of additional Rb-Sr and Pb-Pb whole-rock data on both rock units (Oxford, unpublished data) gives essentially the same results In view of the close similarity, within limits of error, in Rb-Sr and Pb-Pb ages for both rock units, it is probable that the regional metamorphic heating event associated with the emplacement of the main bulk of the magmatic precursors of the Amitsoq gneisses partially or com-

11 S Moorbath et al/ Chemical Geology 135 ( pletely reset the Rb-Sr and Pb-Pb ages of the older Isua supracrustals to values characteristic of the Amitsoq gneisses, supposing that a significant age difference actually exists between the two rock units Almost all whole-rock regressions exhibit a degree of geological scatter exceeding analytical scatter (MSWD > 1) and this may be interpreted in the normal way as either incomplete eradication of primary Sr and Pb isotope heterogeneities or as opensystem isotopic disturbance superimposed by late Archaean or Proterozoic metamorphic events which have been documented in the region (eg, Pankhurst et al, 1973; Kalsbeek, 1981; Baadsgaard et al, 1986a; Gruau et al, 1996) The regressions therefore represent an averaging of data points, in the sense of Cameron et al (1981) Nonetheless, much useful information is provided by initial Sr- and Pb-isotope ratios obtained from Rb-Sr and Pb-Pb regressions, concerning the ultimate source region of the Isua supracrustals and the Amitsoq gneisses (Moorbath et al, 1986, and references therein) In particular, it appears that the immediate protolith of each of these rock units was itself derived from mantle-like source regions not more than N Ga before the age recorded by Rb-Sr and Pb-Pb whole-rock isochrons (eg, Moorbath and Taylor, 1981) The same general principles hold for the much smaller published Sm-Nd data base on these rocks The first Sm-Nd age determinations (Hamilton et al, 1978) on a combined suite of ten felsic and mafic Isua supracrustal rocks yielded a whole-rock regression age of Ma, with an initial Nd value of _ +2 A mixed suite of 11 Isua supracrustal rocks subsequently (Moorbath et al, 1986) yielded a Sm-Nd whole-rock regression age of Ma, with an initial Nd of + 20 f 08, whilst a suite of 7 Amitsoq gneiss sam,ples gave 3720 k 84 Ma, with an initial Nd of Some subsequent attempts to date the Isua supracrustals were not very successful, leading Jacobsen and Dymek (1988) to conclude that the stratigraphic age for the Isua rocks must lie in the rather large age range Ga, which is discomforting They further conclude that the Isua elastic metasediments show complex Rb-Sr and Sm-Nd systematics While much of the complexities in the Rb-Sr systematics can be attributed to disturbance(s) during post-36 Ga metamorphic events (ie at 28 and 18 Ga), the heterogeneities observed in initial end values appear to reflect real variability due to mixing of isotopically diverse source materials at the time of deposition This is relevant to what follows later In order to resolve finer details of the geochronological record in West Greenland, much recent work has been based on SHRIMP zircon U-Pb ages (eg, Nutman et al, 1996 and references therein) These workers obtain a spectrum of numerous precise dates, extending from N 3870 to N 3600 Ma for the Amitsoq gneisses and their enclaves (collectively renamed the Itsaq Gneiss Complex), although most of the ages appear to fall towards the lower end of the range For the Isua supracrustal belt, zircon U-Pb ages of 3807 f 2/ and Ma are reported for two units of felsic volcanic/volcanoclastic rocks, suggesting that the belt contains similar components differing in age by N 100 Ma No evidence was found for zircon derived from crust older than 3807 Ma 32 Initial Nd values Bennett et al (1993) report a wide range of initial end values from -46 to f45 in samples of Am?tsoq gneiss with zircon U-Pb dates in the range 3872 to 3729 Ma, as well as initial Nd values from I + 18 to for mafic inclusions within the Amitsoq gneisses (the so-called Akilia gabbros and leucogabbros) with ages constrained to the range to Ma by zircon U-Pb dates from the enveloping Amitsoq gneisses This wide range of initial end values was then used as a basis for complex models of early mantle and crust evolution In fact, Bennett et al (1993) postulate a LREE-depleted mantle reservoir with an initial end value of * +4 prior to 38 Ga They state that if the source region for the oldest Greenland gneisses was the prevalent upper mantle composition, it must have been an ephemeral feature later modified either by mixing with less-depleted mantle, or with recycled LREE-enriched, negative-e,, crust The generation of highly positive Nd values by 38 Ga requires differentiation of an extremely LREE

12 224 S Moorbath et al/ Chemical Geology 135 (1997) fractionated reservoir very early in Earth s history The Archaean Nd isotope data may record the isolation, depletion by crustal extraction and subsequent partial rehomogenisation of Eimited portions of the upper mantle, or alternatively may reflect transient large-scale differentiation processes unrelated to crustal extraction such as might occur in a terrestrial magma ocean Intrigued by the difference between initial end values of around +2 for the Isua supracrustal rocks and for the Am?tsoq gneisses obtained by Moorbath et al (1986) from Sm-Nd regressions, and the wide overall range from -46 to +45 reported by Bennett et al (19931, we decided to examine their Sm-Nd data more closely in an analogous manner to that discussed earlier for the Acasta gneisses of northern Canada We have plotted the seven Akilia enclave data points of Bennett et al (1993), which yield zircon U-Pb dates in the range f 10 to f 22 Ma and mlhal Ed,, values in the range I + 18 to 2 +37, on a Sm-Nd diagram (Fig 5) This yields an almost perfect isochron with a regression age of 3675 f 48 Ma and an initial end value of (MSWD = 21) We regard this as a r47sm/ 44Nd 0510 I I Fig 5 Sm-Nd regression for the Akilia suite of West Greenland from Bennett et al (1993) The inset shows Nd-isotope evolution lines for representative samples extrapolated back to the overall range of individual zircon U-Pb dates (shaded area) DM here (and in Fig 7) refers to the depleted-mantle model of DePaolo et al (1991) CHUR refers to the chondritic uniform reservoir (see Table 1) / Sm/ 44Nd I I I I Fig 6 Combined Sm-Nd regression (jilled circles) for Amitsoq gneiss samples from Baadsgaard et al (1986b), Moorbath et al (19861, and Shin-&u et al (1988) The Amitsoq gneiss data of Bennett et al (1993) are shown for comparison and omitted from the regression calculation (open squares), straightforward Nd-isotope homogenisation age, probably reflecting the age of the tectonothermal event associated with the emplacement of the magmatic precursors of the youngest of the nearby regional Amitsoq gneisses (eg, Nutman et al, 1996) The inset of Fig 5 is a Nd-isotope evolution diagram which shows a range of apparent initial end values within the constraining age bracket imposed by the Amitsoq gneiss zircon U-Pb dates, but without any specific geological significance Because of indeterminate complexities of the geochemical processes affecting the Sm-Nd system before and during the N 3675 Ga Nd-isotope homogenisation event, it is not possible at this stage to give a specific interpretation for the initial end value of + 26 f 04 obtained from the Sm-Nd regression (Fig 5) However, it may not have evolved far in terms of time and Sm/Nd ratio from its starting value(s) This needs much more work Turning now to the Amitsoq gneisses, in Fig 6 we plot the Sm-Nd data of Bennett et al (1993) for nine Amitsoq gneiss samples, which have U-Pb zircon ages ranging from 3872 f 10 to 3729 f 3 Ma, and initial Nd values ranging from -46 to +45 These data are far too scattered to fall on a meaningful Sm-Nd regression line, whilst most of them also scatter widely about a regression line (Fig 6) based

13 S Moorbarh et al / Chemical Geology 135 (1997) on published Sm-Nd data for varied suites of Amitsoq gneisses without individual zircon U-W age control (Baadsgaard et al, 1986b; Moorbath et al, 1986; Shimizu et al, 1988) This 26-point regression yields an age of Ma (MSWD = lo>, with an initial en,, value of + 09 * 14 Clearly, from this evidence alone, we cannot counter the claims of Bennett et al (1993) for gross heterogeneities in initial end values in their rocks However, it is at least possible that some of their samples were even more subject to later Sm-Nd open-system behaviour than the regressed samples in Fig 6, which themselves show significant geological scatter about the regression line The comparatively low initial nd value of _ +09 for the combined Amitsoq gneiss regression (even allowing for the large error of + 14) suggests the possibility of limited participation of a significantly older, enriched (ie low Sm/Ncl) crustal precursor, such as supracrustal or infracrustal rocks of the Isua/Akilia type, in the genesis of the magmatic precursors of the Amitsoq gneisses This problem is currently under investigation Whilst the Sm-Nd regression age of Ma for the Amitsoq gneisses is considered to be a reasonable estimate for the age of rock formation of the bulk of the younger Amitsoq gneisses (in broad agreement with the numerous published whole-rock Rb-Sr, Pb-Pb and zircon U-Pb dates, eg, Moorbath et al, 1986; Nutman et al, 1993, 19961, the scatter about the Sm-Nd regression line (Fig 6) is, in our view, most likely due to limited open-system behaviour during late Archaean and mid-proterozoic tectonothermal events, which are well documented in this region (for references, see later) In view of the above uncertainties, is it possible to determine a plausible initial Nd value for mantlelike source region:3 of crustal rocks at N Ga? In principle, this can be done only by using rock suites which have a short-term crustal history and which have remained a closed Sm-Nd system since time of deposition, We consider that some members of the Isua supracrustal belt offer this possibility Our interest was stimulated by finding that many published Sm-Nd data for the Isua supracrustal rocks (eg, Hamilton et al, 1978, 1983; Miller and O Nions, 1985; Moorbath et al, 1986; Jacobsen and Dymek, 1988; this paper) fell on a combined, well- correlated Sm-Nd regression line (not presented in full here) yielding an age of N Ga, and an initial Nd value of N -I- 15 to + 20 The best-correlated regressions were given by felsic, volcanogenic metasediments, and by mica-schists possible derived from (Nutman et al, 1984): either weathered basic rocks or from basic tuffs that interacted with water at the time of deposition These comprise two of the major rock units within tbe Isua supracrustals The worst-con-elated regression points were given by mafic and ultramafic metavolcanic rocks and by chemical sediments such as carbonates and banded iron-formation, many of which have very low Sm and Nd contents and are thus particularly sensitive to post-depositional chemical alteration and metasomatic processes, which are well documented in this area (eg, Rosing, 1990; Gruau et al, 1996) In this context, we disagree with the recent re-interpretation (Rosing et al, 1996) of the Isua supracrustal sequence, which regards the felsic rock units of Nutman et al (1984) as totally metasomatised, discordant Amitsoq gneisses, and the mica( + garnet)-schists as metasomatised amphibolites, which could, at face value, raise doubts about the validity of our approach We have seen no field or petrographic evidence as yet for such thorough and pervasive regional metasomatism, although both rock types sometimes exhibit secondary calcite infiltration, which is stronger in some areas than in others We regard metasomatism in the Isua supracrustal rocks as a localised, variably intense phenomenon which has not obliterated the integrity of the individual rock types, nor the original lithological and petrographic record of the provenance of any of the major rock units described by Nutman et al (1984) All Isua supracrustal rocks show medium-grade metamorphism (Boak and Dymek, 1982; Dymek and Klein, 19881, as well as locally highly variable degrees of deformation, frequently leading to local preservation of primary sedimentary and volcanic structures (eg, graded bedding, slump structures, true conglomerates, pillow-lava& without evidence for pervasive metasomatism In the Isua supracrustal rocks analysed for Sm-Nd at Oxford, we have concentrated on felsic vol-

14 226 S Moorbrrth et al / Chemicul Geology 135 C canogenic metasediments and on the mica secondary calcite infiltration Not unexpectedly, we ($gamet)-schists because they have the highest Sm have not been able to obtain any easily interpretable and Nd contents and generally appear to be least whole-rock Sm-Nd systematics from low-sm, lowaffected by later alteration and metasomatism This Nd mafic/ultramafic meta-igneous rocks or from is also evident from their relatively coherent whole- chemical metasediments, such as carbonates or rock Rb-Sr and Pb-Pb isotope systematics (see banded iron-formation previous section) which mostly give regression ages A suite of 32 samples (collected by SM in 1978 in the range N Ga We have avoided and 1993) from along 12 km of strike in the eastern analysing any samples with significant amounts of sector of the Isua supracrustal belt was analysed for Table 2 Sm-Nd data for lsua supracrustal rocks Sample Sm Nd 43Nd/ 14jNd Sm/ 44Nd fsm,nd ENd (wm) (present) (t = 3776 Ca) Felsic unit: la b le lg I j k m Schist unit: (i) Cii) SM/GR/ SM/GR/ SM/GR/ SM/GR/ A F L Touwwline boulder unit E E F J , X See Table 1 for analytical details