Precambrian Research 105 (2001) 93 – 114
On the scarcity of
\
3900 Ma detrital zircons in
]
3500
Ma metasediments
A.P. Nutman
a,baResearch School of Earth Sciences,The Australian National Uni6ersity,Canberra,ACT0200,Australia bDepartment of Earth and Planetary Systems Science,Hiroshima Uni6ersity,3-2Kagamiyama1-Chome,
Higashi-Hiroshima739,Japan
Received 8 October 1999; accepted 8 October 1999
Abstract
Constant volume models for the continental crust require a flux of crustal material back into the mantle (recycling), equal in volume to that of the juvenile igneous suites added to the continental crust throughout time. In growth of crustal volume models, there is not equilibrium between the volume of juvenile crustal additions and any recycling (destruction) of crust. By establishing the proportion of \3900 Ma detrital zircons in early Archaean sediments it might be possible to constrain the relative importance of crustal growth and recycling. Gneiss complexes in western Greenland, northern Labrador and northeastern China contain rare]3500 Ma detrital metasediments. In sediments deposited between 3500 and 3600 Ma, ]3900 Ma zircons have not been detected in a suite of 117 detrital grains. Based on statistical considerations, at the 95% confidence level any ‘missed’ ]3900 Ma component forms B3% of this suite. Likewise, \3900 Ma detrital grains do not occur amongst 54 detrital grains from (even rarer) 3700 – 3800 Ma sediments, arguing with 95% certainty that any ‘missed’]3900 Ma component forms B5% of this most ancient suite. If the age spectra of these detrital zircon suites are representative of the complexes in which they reside, then constant volume (recycling=new additions) models require that by 3500 Ma, \97% of \3900 Ma crust was destroyed by recycling. Such an extremely high recycling rate (:25% of the crust 100 Ma−1) is hard to reconcile with
the diversity of initial Nd and Sr isotopic ratios of well preserved early Archaean granitoid suites in the same complexes, because significant average crustal residence times are required to permit the radiogenic isotopic systems to evolve. The most likely interpretation of the detrital zircon record in the Greenland, Labrador and China sediments is that in their provenance areas the volume of continental crust was small at 3900 Ma, and that it grew significantly during the early Archaean. If the measured ]3500 Ma detrital sediment suites are globally representative, they support growth models for the continental crust in the early Archaean, rather than models involving recycling of a voluminous \3900 Ma sialic crust. Because of its global coverage and the dating of thousands of grains, the age spectra for detrital zircons from 3000 – 3200 Ma sediments provides a more reliable impression of crustal ages. However, as they were deposited 700 – 900 Ma after 3900 Ma, the globally small proportion of ]3900 Ma detrital grains in them (from Jack Hills, Mt. Narryer and Wyoming) can be accommodated in both crustal growth and moderate recycling models. © 2001 Elsevier Science B.V. All rights reserved.
Keywords:Archaean; Detrital sediments; Zircons; Crustal growth; Crustal recycling
www.elsevier.com/locate/precamres
A.P.Nutman/Precambrian Research105 (2001) 93 – 114 94
1. Introduction
There are two extreme viewpoints regarding the development of the continental crust throughout the history of the Earth. The first is that its volume grew by incremental extraction from the mantle, with return flux (recycling) of material from the crust back into the mantle being of second order importance. Variants of these growth models propose a smooth increase in vol-ume, or that growth was more rapid in the Ar-chaean, in some cases involving rapid spurts of
crustal growth (Dewey and Windley, 1981;
McLennan and Taylor, 1982; Nelson and De-Paolo, 1985; Patchett and Arndt, 1986). A second view is that the continental crust has remained approximately constant or even decreased in vol-ume since the early Archaean (Fyfe, 1976; Arm-strong, 1981, 1991). Early Archaean rocks form only a minute proportion of the current continen-tal volume and new crust is presently forming in volcanic arc systems. Therefore, models which propose large volumes of early Archaean conti-nental crust require a flux of conticonti-nental crustal material back into the mantle (i.e. recycling; Arm-strong, 1991). Some other models fall between these extremes. Thus Reymer and Schubert (1987) proposed that by 3900 Ma the continental crust
had :50% its current volume, and the rest was
added since then. Whole rock isotopic data, trace element geochemistry, the terrestrial heat budget and comparative planetology have been used as constraints in these models. The usefulness of some of these in distinguishing Archaean crustal growth versus recycling have been covered by several workers (e.g. Armstrong, 1991; McCulloch and Bennett, 1993, 1994; Bowring and Housh, 1995), and is also discussed below.
One constraint on crustal evolution models that has received less attention is the age spectrum of detrital zircons in very ancient sediments (e.g. Compston et al., 1987; Maas and McCulloch, 1991; Nutman et al., 1995). However, Stevenson and Patchett (1990) in a landmark paper demon-strated the power of detrital zircon data in chart-ing the evolution of the crust, by uschart-ing Hf isotope data from 2 – 15 mg zircon aliquots from Meso-proterozoic to mid Archaean sediments. By this
means they concluded that continental crust had grown in volume during the Archaean. Using the age spectrum of detrital zircons in very ancient sediments, the same conclusion is tentatively reached in this paper.
Zircons are predominantly of granitic (sensu lato) magmatic origin, although they do grow in some evolved gabbroic magmas and also in most lithologies during metamorphism. S-type granites (which form at least partially from older sedi-ments) and rocks that have undergone partial melting under granulite facies metamorphism or
eclogite (in some cases coesite9diamond grade)
facies metamorphism still preserve zircon derived from their source and protoliths (e.g. Schiøtte et al., 1989a; Claoue´-Long et al., 1991; Paterson et al., 1992), and Phanerozoic detrital zircon popula-tions commonly contain grains recycled from con-siderably older (sialic) crust (e.g. Kro¨ner et al., 1988; Ireland, 1992). This demonstrates the resis-tance of zircons to a wide range of surficial to deep-seated lithospheric processes. Thus detrital zircons can provide information of the history of their source region, particularly on the emplace-ment age of granitic (sensu lato) rocks.
Detrital zircon analyses have been undertaken on Archaean sediments throughout the world, to look at local provenance and to help in establish-ing regional geological history (e.g. Froude et al., 1983; Dodson et al., 1988; Kinny et al., 1988; Nutman et al., 1991; Liu et al., 1992; Mueller et al., 1992; Schiøtte et al., 1992; Nutman et al., 1996, 1997a; Mueller et al., 1998). In some of these studies, the likely amount of ancient crust in the local provenance region was discussed, but this data has never been pooled to form com-posite data sets for sediments of the same deposi-tional age. Combining data sets give the potential of seeing the age composition of the continental crust at any given time. In this paper, the age spectra of detrital zircons in sediments deposited before 3500 Ma are used to place constraints on
the volume of extremely ancient (\3900 Ma)
A.P.Nutman/Precambrian Research105 (2001) 93 – 114 95
2. Crustal growth versus recycling models and previous constraints
2.1. Growth and recycling
Both crustal growth and recycling models need to account for the formation of new material, in the form of subduction-related igneous rocks, added to the continental crust throughout the geological record. Where the models differ funda-mentally, is the volume of the continental crust in the early Archaean and the magnitude of the return flux of material from the continental crust back into the mantle. Characteristics of early to middle Archaean crustal recycling and crustal growth models are illustrated in Fig. 1, and fol-lows the representation used by McCulloch and Bennett (1998). Crustal evolution is charted every
200 Ma. Recycling is expressed as R%, the
vol-ume (relative to present day volvol-ume) of continen-tal crust recycled into the mantle every 100 Ma,
and growth is expressed asG%, the volume of the
continental crust formed every 100 Ma.
Cainozoic sediment accumulation has been
cal-culated at 2.491.2 km3per annum (Davies et al.,
1977). At this rate of accumulation the entire mass of the continental granitic (sensu lato) crust could be converted into sediment in 3600 Ma
(Armstrong, 1991), equivalent toR=2.8%.
How-ever, processes such as tectonic erosion in subduc-tion zones are needed to remove into the mantle the sediments that were ultimately produced from erosion of granitic rocks. Tectonic erosion in
sub-duction zones has been estimated at 1.1 km3 per
year (Scholl et al., 1990), equivalent to R=1.9%.
Allowing for a much more vigorous early Ar-chaean Earth by multiplying this value fivefold (approximating to the greater heat production of
those times; e.g. Dickinson and Luth, 1971),R:
10% could be postulated for the early Archaean. To be especially permissive to a recycling model,
this value is doubled here to R=20%. WithR=
20% and constant crustal volume (equal to that of the present day), approximately a third of the 3500 Ma continental crust should have consisted
of \3900 Ma material (Fig. 1). At this recycling
rate, \3900 Ma detrital zircons should be
globally abundant in 3500 Ma sediments of broad provenance. By 3100 Ma, they should still form
:5% of global detrital zircon populations (Fig.
1). Hence, unless both recycling and growth are small (which does not fit the geological record), then by the middle Archaean (3100 Ma) the
ob-served proportion of ]3900 continental crust will
be small. This illustrates the difficulty of using a younger (3100 Ma) detrital zircon record to provide robust constraints on the relative impor-tance of growth versus recycling in the earliest Archaean. Therefore, when using detrital zircon age spectra, the oldest parts of the geological
record (]3500 Ma here) are needed to help
assess the relative importance of crustal growth versus recycling.
2.2. Whole rock isotopic constraints on crustal e6olution
Nd, Sr and Pb isotope ratios measured on Archaean rocks have been the main constraints used in erecting models of Archaean crust-mantle evolution, with the importance of additional of
juvenile (low initial 87Sr
/86Sr, elevated initial
143Nd
/144Nd) material to the Archaean crust long
being recognised (e.g. Moorbath, 1978; Moorbath et al., 1977, 1997; Miller and O’Nions, 1985; McCulloch and Bennett, 1993, 1994; Bennett et al., 1994; Bowring and Housh, 1995). In these
A.P.Nutman/Precambrian Research105 (2001) 93 – 114 96
isotopic systems, there is marked fractionation of the parent – daughter isotopes between the mantle, oceanic crust and continental crust, allowing in principle the evolution of different reservoirs to be charted through time.
The closer chemical similarity of 147Sm –143Nd
compared with238,235U –206,207Pb,232Th –208Pb and
87Rb –87Sr parent – daughter pairs means that Nd
isotopic evolution should generally be less prone to disturbance by parent – daughter fractionation
in metamorphism/metasomatism after the rocks
formed. Therefore subsequent discussion concen-trates on Nd isotopic signatures. Even so, care should be taken in the choice of samples, because it is still possible to have secondary fractionation effects, as clearly demonstrated when ‘worst case scenario’ samples are studied (e.g. Bridgwater and
Rosing, 1995).143Nd/144Nd isotope ratios are
nor-mally expressed in oNd notation, where one oNd
unit is a deviation of one part in 10 000 from the
143Nd/144Nd of a chondritic uniform reservoir at
the time of interest. The depleted mantle and
MORB have higher Sm/Nd than CHUR, thus
their oNd values become more positive with time.
Continental crust and most arc rocks have lower
Sm/Nd than CHUR, thus theiroNdvalues become
increasingly negative with time. The evolution of the Archaean mantle has to a large degree been inferred from the Nd isotopic compositions of preserved Archaean mafic rocks (derived directly
from the mantle9crustal contamination) and
in-termediate rocks (generally interpreted to be
arc-related and ultimately incorporated into
continental crust).
Initial Nd isotopic values for early Archaean rocks (e.g. compilations in Bennett et al. (1993), Bowring and Housh (1995)) show considerable deviations from CHUR (analytical uncertainties
on individual determinations are B1 oNd), which
shows that the Earth had fractionated into
en-riched (low Sm/Nd) and depleted (high Sm/Nd)
reservoirs, by 3800 – 4000 Ma when the geological record starts in earnest (e.g. Hamilton et al., 1983; Jacobsen and Dymek, 1986; Collerson et al., 1991; Bennett et al., 1993; Bowring and Housh, 1995). Despite disagreement over the degree of fractiona-tions (Collerson et al., 1991; Bennett et al., 1993; Vervoort et al., 1996; Moorbath et al., 1997;
Bennett and Nutman, 1998; Kamber et al., 1998) there is general acceptance that by 3900 Ma there were already several fractionated crust and mantle reservoirs, with evidence of repeated additions of juvenile material to the crust in the early Ar-chaean. Nd isotopic studies of ancient detrital sediments (e.g. Miller and O’Nions, 1985; Jacob-sen and Dymek, 1987) and Hf isotopic studies of detrital zircons in Archaean sediments (Stevenson and Patchett, 1990) suggest juvenile components are dominant over recycled crustal components. However, it is shown by several workers that in isolation the Nd whole rock isotopic record does not provide a unique solution for the variables Archaean continental mass, mass of depleted mantle and degree of elemental fractionation of mantle (e.g. Bowring and Housh, 1995; McCul-loch and Bennett, 1997). It is for this reason that a different set of data, the age spectra of detrital zircons in early Archaean sediments, is examined in this paper in order to provide a new perspective on early Archaean crustal evolution.
2.3. Use of detrital zircon population age spectra to assess crustal e6olution
The validity of using detrital zircon population age spectra in order to place constraints on growth and recycling rates of (quartzofeldspathic) continental crust must first be assessed. Zircons are mostly grown in granitic (sensu lato) magmas (Poldervaart, 1956). Zircons may also grow within mafic magmas as magmatic phases (e.g. Paces and Miller, 1993), but the amount of zircon formed this way, relative to that grown from granitic
magmas, is very small. Furthermore, those
formed in mafic magmas tend to (but not always) have distinctive morphologies, such as centres rich in other silicate inclusions (sometimes to the
ex-tent of the zircons being hollow tubes/prismatic
grains), and also angular-anhedral forms that are the result of co-precipitation with other phases.
They also tend to have high Th/U (typically 0.8 –
2.0) and high (but not always) Th+U absolute
A.P.Nutman/Precambrian Research105 (2001) 93 – 114 97
1987; granitoids and gneisses, Kinny, 1986). Again, the amount of zircon grown this way is small relative to that grown from granitic magmas.
Suites of tonalite – trondhjemite – diorite – gran-odiorite are the most abundant rocks in the Ar-chaean geological record. They are most likely dominated by partial melting products of hy-drated mafic crust, which was heated when de-pressed into the mantle at Archaean convergent plate boundaries (e.g. Martin, 1986). Sr and Nd isotopic tracers show that these rocks are domi-nated by juvenile additions to the crust (e.g. Moorbath et al., 1977; Bennett et al., 1993; Moor-bath et al., 1997). Extensive experimental petrol-ogy studies of tonalite and diorite compositions (Rapp, 1997; Wyllie et al., 1997 for reviews), suggest that the observed whole rock composi-tions in Archaean tonalite – trondhjemite – diorite – granodiorite suites are close to that of precursor
magmas, rather than migmatites of magma+
abundant coexisting restite. Experimental studies (e.g. Rapp, 1997; Wyllie et al., 1997) also showed that these magmas are hot, with temperatures of
]850°C. Archaean tonalitic and dioritic rocks
typically have Zr abundances of ]200 ppm (e.g.
Wedepohl et al., 1991). Experimental studies of Zr solubility of granitic, sensu lato melts using a wide range of compositions, pressures and tempera-tures have been reported (Watson and Harrison, 1983). These studies show that if the Archaean tonalitic and dioritic rocks were close to melting
compositions, then at ]850°C solubility of Zr in
those melts would be ]400 – 500 ppm. This is
considerably higher than the B200 ppm Zr
present in these rocks. Thus the precursor melts were probably strongly undersaturated in Zr, and any zircon entrained in them would have had a strong potential to dissolve. Thus, it can be ar-gued that zircons from these rocks are predomi-nantly magmatic in origin. Zircons from these sources probably dominate early Archaean detri-tal suites, and consequently their detridetri-tal age spec-tra can be interpreted as predominantly (but not exclusively) controlled by the production of mag-matic zircon in granitoids that are juvenile addi-tions to the crust, rather than remelted older crust.
3. SHRIMP U – Pb zircon geochronology
Following coarse crushing and pulverisation, standard heavy liquid and isodynamic techniques were used to produce zircon concentrates. Min-eral separation was undertaken at the Research School of Earth Sciences, the Australian National University. The separates were then hand-picked using a binocular microscope, to produce a varied assortment least metamict and damaged grains for analysis. Together with chips of standard zircon, these were cast into epoxy resin discs and pol-ished. Assessment of grains and choice of sites for
analysis was based on transmitted/reflected light
imagery.
U – Th – Pb isotopic ratios and concentrations
were determined in zircon separates using
SHRIMP I and II and were referenced to the Australian National University standard zircon
SL13 (572 Ma; 206Pb/238U=0.0928). Descriptions
of analytical procedure and data assessment are given by Compston et al. (1984), Claoue´-Long et al. (1995). Comparative isotope dilution and SHRIMP analyses of zircons from several well-preserved Proterozoic and Archaean samples (Roddick and van Breemen, 1994; Ireland, 1995)
demonstrate that SHRIMP 207
Pb/206
Pb ratios are accurate within the stated errors. The decay
con-stants and present-day 238U
/235U value given by
Steiger and Ja¨ger (1977) were used to calculate the ages.
A SHRIMP produces a U – Pb zircon analysis in 10 – 20 min. The rapidity of analysis combined with adequate precision, measurement of concor-dancy and ability to correct for common Pb from
measurement of 204Pb makes SHRIMP an ideal
tool to collect age data on detrital zircon popula-tions, where large numbers of analyses are required.
4. Early Archaean sediments from Greenland and Labrador
4.1. Geological setting
A.P.Nutman/Precambrian Research105 (2001) 93 – 114 98
Schiøtte and Bridgwater, 1991 for summary). These rocks have been strongly modified by sev-eral early and late Archaean tectonothermal events. The late Archaean events include wide-spread granulite facies metamorphism and abun-dant intrusion of granitoids (Collerson et al., 1982; Schiøtte et al., 1989a). The early Archaean of the Nain complex is dominated by the Uivak Gneisses, whose protoliths consist of dioritic to granitic rocks of different ages and origins (Collerson, 1983; Schiøtte et al., 1989a). Within the Uivak Gneisses there are inclusions and inter-calations of the Nulliak assemblage, which is a
diverse suite of \3600 Ma metasedimentary,
mafic and ultramafic rocks (Nutman et al., 1989; Schiøtte et al., 1989b; Nutman and Collerson,
1991). A SHRIMP U – Pb zircon age of 377698
Ma (2s) was obtained on a Nulliak assemblage
metavolcanic rock (Schiøtte et al., 1989b). The
sediment (83/187) whose detrital zircon ages were
reported by Nutman and Collerson (1991) has a
deposition age of ]3600 Ma and possibly ca.
3800 Ma.
The early Archaean history of the Itsaq Gneiss Complex of southern West Greenland is as varied and protracted as that of the neighbouring Nain Complex in Labrador (Nutman et al., 1996 and references therein). The informal name Itsaq Gneiss Complex was introduced by Nutman et al. (1996) to include all early Archaean rocks of the
Godtha˚bsfjord area (Amıˆtsoq gneisses, Isua
supracrustal belt and akilia association, see Gregor, 1973; McGregor and Mason, 1977; Mc-Gregor et al., 1991), in order to emphasise that there are many groups of unrelated early Ar-chaean rocks formed over a 300 million year period.
Supracrustal rocks, metagabbros and ultramafic
rocks comprise B10% of the Itsaq Gneiss
Com-plex and range in age from ]3850 to3600 Ma
(Nutman et al., 1996, 1997a,b). Bodies of mafic rocks with layers of banded iron formation (BIF) and metachert associated with ultramafic rocks are the main lithologies (McGregor and Mason, 1977; Nutman et al., 1996). Units of supracrustal rocks occur as rafts and tectonic intercalations in dioritic, tonalitic and granitic gneisses which form
\90% of the complex (Nutman et al., 1996 and
references therein). Supracrustal units range in size from the 30 km long Isua supracrustal belt in
the north to bodies ]100 m across scattered
throughout the complex. The well-known Isua supracrustal belt contains volcanic and sedimen-tary rocks that are both ca. 3710 and 3800 Ma old (Nutman et al., 1996, 1997a), and elsewhere in the
complex some chemical sediments are ]3850 Ma
old (Nutman et al., 1997b). Only in the youngest supracrustal sequences of the Itsaq Gneiss
Com-plex (3600 Ma from Ameralik fjord, 150 km
south of the Isua supracrustal belt) are detrital sediments derived from mixed-provenance clastic sources an important lithology (Nutman et al., 1996).
4.2. ]3790 Ma metasediments,West Greenland
The \3790 Ma package dominated by
amphi-bolites forming the southern side of the Isua supracrustal belt contains rare detrital quartzites (Nutman et al., 1997a), on which data from two
samples G93/25 and MR81/318 are used in this
paper (Table 1). Both samples are found in associ-ation with mafic to felsic volcanic rocks, and are not part of a thick detrital sedimentary sequence. The nature of the provenance region is unknown, but as shown below it clearly contained pre-3800 Ma volcanism quartzofeldspathic rocks. A unit of
biotite9garnet bearing siliceous rock quartzite?
(MR81/318) from the southern central part of the
belt yielded approximately 100 zircons. All the
grains are very small (typically 30 – 50 mm across)
and are both equant and stubby-prismatic in habit, but somewhat rounded (Nutman and Collerson, 1991). These zircons have suffered vari-able loss of radiogenic Pb and also show a few
269696 Ma metamorphic overgrowths.
Nonethe-less, there are clearly two ]3800 Ma groups
present, which yielded weighted mean ages of
380895 Ma and 3847910 Ma. Another
mica-quartzite unit occurs in the southwest of the belt
(sample G93/25, Nutman et al., 1997a). This unit
is :1 m wide, and is bounded to the south by a
thick, partly carbonated, ultramafic schist unit and to the north by garnetiferous mafic schist,
metamylonite and felsic schists. G93/25 gave a
A
s) detrital zircon age data, after filtering according to methods described in texta
Deposited 3700–3800Ma Deposited 3500–3600 Ma
G93/25(G) Isua supra. G93/54(G) S. of Isua Nulliak 83/187(L) S. of VM90/10(G) S. of PDK quartzite(G) N. of CF89-26(C) Huangbaiyu ameralik quartzite
st. johns hbr. Quartzite ameralik graphite sch. paragneiss
385996 3828917 371396 3843915
3876932 387999
385998 3820912 3707958 3832911
3876932 3837916
3804913 382299
3697919
3872919 385996 3818915
3803914 3821912
3870925 385996 3804913 3677922
3866912 3859912 3794910 3676923 3793910 382096
3795916
3859920 3757926 3673910 3790930
3865910
3767927
MR81/318(G) Isua 3745942 3671914 3793918
3862919
381697 376197 364998 3669913
385398
3847922 3734911 3625912 364599
368096
380494 3726910 361698 3641912
384696
3842911 379197 369198 3601925 367497
3838912 368998 GGU221122(G) N. of 366996
ameralik graphite
3814913 3669910 3615917 361498
360499
366499 3590915
A
.
P
.
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/
Precambrian
Research
105
(2001)
93
–
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100
Table 1 (Continued)
Deposited 3500–3600 Ma Deposited 3700–3800Ma
G93/54(G) S. of Isua Nulliak 83/187(L) S. of VM90/10(G) S. of
G93/25(G) Isua supra. PDK quartzite(G) N. of CF89-26(C) Huangbaiyu
st. johns hbr. Quartzite
paragneiss ameralik quartzite
Belt quartzite ameralik graphite sch. quartzite
(V) (V/S) (V/S) (V/S)
(V) (?)
3603923 3649910
3807919
3551912 363598
362299 3603911 360199
aThree lines heading each data set are, (1) sample number (suffixes in parentheses are G, Greenland; L, Labrador; C, China); (2) locality and (3) lithology (suffixes
A.P.Nutman/Precambrian Research105 (2001) 93 – 114 101
size than those in quartzite? MR81/318. A group
at 384995 Ma is present, with possibly one older
grain at :3900 Ma and a few at ca. 3810 Ma
(Nutman et al., 1997a).
4.3. :3710Ma metasediments, West Greenland
A :3710 Ma package containing
amphibo-lites, felsic to intermediate schists and chemical sediments forms the northern side of the Isua supracrustal belt (Nutman et al., 1997a). Zirconif-erous rocks in this package are rare. Felsic
vol-canic rocks contain :3710 Ma magmatic zircon
and rare kyanite schists and some of the siliceous
(metachert?) units also contain :3710 Ma
zir-cons (Nutman et al., 1996, 1997a and unpublished data). So far, older (pre-3710 Ma) detrital zircons have not been detected in this package of the Isua
supracrustal belt. However, :15 km to the south
of the Isua supracrustal belt, there is a unit (B
200 m thick) of lithologically similar supracrustal rocks, interleaved with early Archaean tonalitic gneisses (Nutman, Bennett and Friend,
unpub-lished field observations and U – Pb zircon
SHRIMP geochronology). Felsic schists in this unit interpreted to be derived from intermediate volcanic rocks contain no protolith zircon.
How-ever, a thin lens of garnet+biotite9sillimanite
paragneiss (G93/54) of likely volcanic or
sedimen-tary origin on the southern margin of the unit
(64° 58%40%%N 50° 9%W) did yield zircons with early
Archaean ages. This sample belongs to a vol-canosedimentary sequence (possibly arc-related?), thus the provenance of the grains in it could be quite local.
The garnet+biotite9sillimanite paragneiss
G93/54 yielded ca. 30 zircons. Dominant are
small (575 mm) equant/multifaceted to anhedral
grains, with high U and low Th/U values (Table
2). Also present are prismatic grains, with rare overgrowths. The multifaceted to anhedral grains are generally structureless, whereas the prismatic to anhedral grains commonly display micron scale oscillatory zoning partly obliterated and cut across by domains of featureless recrystallised
zir-con. The equant/multifaceted to anhedral grains
yielded late to middle Archaean ages, with two or three groups apparent (Table 2, Fig. 2). The
Fig. 2.206Pb/238U –207Pb/235U concordia diagrams (analytical
errors depicted at the 1s level) and unfiltered 207Pb/206Pb age
histograms for zircons analysed from metasediments G93/54 and VM90/10.
youngest group, including analysis 18.2 of a rare
rim, has a weighted mean 207Pb/206Pb age of
268294 Ma whereas the oldest group has an age
of 2961911 Ma. In addition, two analyses (22.1
and 28.1) yield a mean age of 2827910 Ma and
may represent another group. These grains are interpreted to have grown in situ during middle to late Archaean thermal events. Non-zoned
irregu-lar/globular twinned grain 6 (6.1 and 6.2, Table 2)
A
Summary SHRIMP U–Pb zircondata for metasediments G93/54 and VM90/10a
Grain type U ppm Th ppm Comm.206Pb (%) 238U/206/Pb ratio 235U/207Pb ratio 207Pb/206Pb ratio 207/206 Age Disc (%)
Labels Th/U
G93/54garnet+biotite paragneiss,south of Isua supracrustal belt
0.68690.021
1.1 tw 260 79 0.31 0.26 29.49690.979 0.311990.0027 3530913 −5
0.57990.016 19.28490.560 0.241690.0012 313198 −6 0.01
tw
1.2 422 38 0.09
0.06
eq 1251 18 0.01 0.49890.016 12.59990.412 0.183590.0012 2685911 −3
2.1
0.01 0.00 0.51590.011 12.97490.270 0.182990.0004 267994 0 eq
2.2 980 11
0.70890.020 32.10291.008 0.329190.0030 3613914 −5 0.08
343 0.72
3.1 p 475
0.01
p 410 164 0.40 0.70890.015 30.50190.666 0.312690.0011 353495 −2
3.2
0.07
eq 1232 10 0.01 0.57390.016 17.12690.491 0.216890.0011 295799 −1
4.1
0.55590.012 16.02390.362 0.209290.0011 290098 −2 0.05
4.2 eq 1247 12 0.01
0.54 0.34 0.73790.022 35.12291.220 0.345690.0046 3688920 −4 p,f
5.1 214 116
0.68890.015 29.96690.733 0.315990.00217 3550913 −5 0.05
p,f
5.2 234 94 0.40
0.08
p,f 211 71 0.34 0.73890.020 32.85090.981 0.322890.0027 3583913 −1
5.3
0.31
p,f 243 54 0.22 0.74490.048 31.90092.208 0.311190.0052 3526926 2
5.4
0.70590.021 31.85091.007 0.327890.0027 3607913 −5 0.21
6.1 m,anh 193 128 0.67
0.68990.019 30.81390.865 0.324490.0012 359196 −6
6.2 m,anh 278 202 0.73 0.06
0.77890.023 37.91591.169 0.353690.0020 372299 0 0.31
92 0.64
7.1 m,p 145
0.08
e,p 156 121 0.77 0.78590.018 38.40790.923 0.354990.0019 372898 0
7.2
0.15
e,p 278 82 0.29 0.76690.023 36.52991.212 0.345990.0039 3689917 −1
8.1
0.77690.018 38.39390.935 0.358790.0013 374496 −1 0.01
8.2 e,p 260 111 0.43
0.82690.018 44.03490.955
9.1 p 252 142 0.56 0.03 0.386890.0008 385993 1
0.77690.017 40.86091.066 0.381990.0042 3839917 −4 0.02
227 0.54
9.2 p 417
0.18
m,p,f 266 318 1.19 0.79790.021 38.54491.021 0.350890.0013 371096 2
10.1
0.88790.049 42.90792.798 0.350890.0097 3711943 10
10.2 m,p,f 224 195 0.87 0.00
0.63990.014 25.13690.576 0.285390.0009 339295 −6 0.01
eq
11.1 377 113 0.30
0.01
e,p 851 8 0.01 0.58390.012 17.27990.413 0.214990.0021 2943916 1
12.1
0.04
c,p 182 67 0.37 0.69690.016 29.50590.781 0.307590.0031 3509916 −3
13.1
0.76790.020 37.29891.011 0.352790.0017 371997 −1 0.01
94 0.40
13.2 c,p 236
0.01
c,p 296 144 0.49 0.76290.031 37.10691.523 0.353090.0015 372096 −2
13.3
0.24
r+c,p 1100 220 0.20 0.70190.048 24.55191.773 0.253990.0036 3209923 7
13.4
0.65090.015 23.80490.566 0.265690.0010 328096 −2 0.01
14.1 p 515 108 0.21
0.01
m,anh,f 898 9 0.01 0.58190.013 17.52590.431 0.218990.0012 297299 −1
15.1
0.62390.016 24.16990.625 0.281290.0011 337096 −7 0.26
m,p
16.1 320 96 0.30
0.01
m,p,f 229 80 0.35 0.77890.019 37.48590.970 0.349390.0026 3704912 0
17.1
0.18
c,eq/an 324 274 0.84 0.72290.015 31.78990.702 0.319490.0013 356796 −2
18.1
0.50790.024 12.49990.651 0.178790.0031 2641929 0 0.39
18.2 r,eq/an 1029 20 0.02
0.53890.052 16.01791.613 0.215990.0041 2950931 −6
19.1 eq 903 14 0.02 0.02
0.51790.018 13.05390.458 0.183390.0004 268394 0 0.01
13 0.01
20.1 m,eq 1206
0.01
eq/an 857 226 0.26 0.72590.067 27.28792.551 0.272890.0017 3322910 6
21.1
0.00
eq/an 733 9 0.01 0.52890.015 14.63190.465 0.201290.0025 2836920 −4
22.1
0.76690.075 33.62493.375 0.318590.0036 3562917 3 0.04
A
Labels Grain type Th ppm
0.53490.014 13.49490.353
24.1 m,eq 1836 25 0.01 0.01 0.183490.0007 268497 3
0.52190.030 13.26790.780 0.184890.0014 2696913 0 0.01
m,eq
25.1 1574 20 0.01
0.23
m,p 187 50 0.27 0.61290.055 25.33192.319 0.300490.0033 3472917 −11
26.1
m,p/an 729 83 0.02 0.59890.013 21.11690.457 0.256290.0009 322396 −6
27.1 0.11
0.53890.012 14.84490.340 0.200190.0006 282795 −2 0.01
28.1 e,p/tw 969 18 0.02
VM90/10graphitic garnet quartzite,south coast of Ameralik
1.03 0.11 O.74790.036 34.43891.786 0.334590.0050 3638923 −1 99
1.1 mnp/ro 103
0.63 0.14 0.71690.039 32.86391.840 0.332790.0032 3629915 −4 118
2.1 m,p 74
0.72390.026 32.32091.205 0.324290.0017 359098 −2 0.09
3.1 m,eq 189 115 0.61
0.55 0.12 0.78990.049 36.09892.305 0.331690.0026 3625912 4 m,p/ro
4.1 145 79
0.54590.039 13.95891.024 0.185990.0013 2706912 4 0.10
eq
5.1 225 38 0.17
0.02
m,p/ro 836 566 0.68 0.76690.034 35.56991.609 0.336990.0018 364998 1
6.1
0.08
m,p/ro 482 257 0.53 0.78490.028 36.94191.404 0.341890.0032 3671914 2
7.1
0.78190.026 36.94991.389 0.343190.0050 3677922 1 0.05
8.1 m,p/ro 393 410 1.04
0.77390.035 36.50191.697 0.342490.0023 3673910 1
9.1 m,p 93 45 0.49 0.15
0.79190.041 37.11792.071 0.340490.0053 3665924 3 0.03
242 1.01
10.1 m,p 239
0.08
m,eq/tw 55 19 0.35 0.64590.027 27.21491.217 0.306090.0029 3501915 −8
11.1
0.05
m,p/ro 174 114 0.66 0.74790.024 33.96991.114 0.329790.0018 361698 −1
12.1
0.75790.026 35.79891.399 0.343090.0051 3676923 −1 0.07
13.1 m,ro 245 98 0.40
0.51 0.18 0.73790.059 32.77992.747 0.322590.0052 3582925 −1 m,ro
14.1 169 86
0.77490.050 37.35392.939 0.350190.0131 3707958 0 0.12
51 0.37
15.1 c,ro 140
0.18
r,ro 188 27 0.14 0.70690.063 31.04893.286 0.318990.0148 3564973 −3
15.2
0.12
eq 384 47 0.12 0.66490.019 29.08991.007 0.317790.0051 3559925 −8
16.1
0.78090.022 37.83291.166 0.351690.0030 3714913 0 0.02
17.1 m,p 259 168 0.65
0.56390.024 14.66890.639 0.189090.0015 2734913 5
18.1 eq 196 10 0.05 0.06
0.52190.039 14.43991.119 0.201190.0020 2835916 −5 0.04
26 0.14
19.1 r,eq 186
0.03
c,eq/ro 193 81 0.42 0.68990.054 31.02892.565 0.326590.0053 3601925 −6
19.2
0.08
m,ro 183 186 1.02 0.77290.028 36.36891.346 0.341690.0018 367098 1
20.1
0.78490.024 37.98491.203 0.351590.0014 371396 1 0.03
21.1 m,p 310 323 1.04
0.76090.024 34.86991.158 0.333090.0018 363198 0 22.1 m,p/ro 126 44 0.35 0.03
0.75290.026 36.05791.369 0.347690.0043 3697919 −2 0.08
m,p/ro
23.1 212 100 0.47
0.08
r,p/ro 141 55 0.39 0.69890.031 30.88691.576 0.321190.0067 3575932 −5
24.1
0.09
r,p/ro 283 106 0.38 0.68290.015 27.83090.649 0.295890.0011 344996 −3
25.1
0.73290.022 34.33591.111 0.340290.0026 3664912 −3 0.04
25.2 C.D 101 64 0.64
0.51090.016 12.88490.454 0.183190.0023 2681921 −1 26.1 eq/anh 386 135 0.35 0.10
0.49790.011 12.80590.286 0.186890.0008 271497 −4 0.03
52 0.09
27.1 Eq 598
0.02
r,eq/tw 241 21 0.09 0.69090.016 29.97790.775 0.315290.0023 3547911 −5
28.1
0.18
m,p 126 61 0.49 0.76790.030 35.71491.483 0.337690.0038 3652917 1
29.1
0.73390.024 34.38991.253 0.340390.0039 3664918 −3 0.11
A
.
P
.
Nutman
/
Precambrian
Research
105
(2001)
93
–
114
104
Table 2 (Continued)
238U/206/Pb ratio 235U/207Pb ratio 207Pb/206Pb ratio 207/206 Age
Labels Grain type U ppm Th ppm Th/U Comm.206Pb (%) Disc (%)
0.74090.026 34.72691.837 0.340690.0119 3665954
0.70 0.06 −3
150 216 m,p/ro
31.1
0.68090.015 28.85690.734 0.308090.0028
32.1 Eq 465 52 0.11 0.07 3511914 −5
0.06490.019 28.01590.869 0.306190.0021 3501911 −6 0.01
0.45 33.1 c?p/anh 223 100
0.71490.016 31.78690746 0.322790.0014 358397 −3 33.2 r,p/anh 686 237 0.34 0.02
0.70390.041 32.52091.965 0.335790.0034 3643916 −6 0.08
34.1 e,p 305 132 0.43
0.68890.026 28.38491.097 0.299190.0022 3466911 −3
35.1 r,p 262 29 0.11 0.09
0.74690.066 33.66893.165 0.327390.0076 3605936 0 0.03
c,p
35.2 94 53 0.57
0.05
r,p 297 34 0.11 0.69090.017 28.72490.772 0.302190.0029 3481915 −3
35.3
0.05
r,p 433 210 0.49 0.70390.041 30.86591.876 0.318490.0031 3562915 −4
36.1
0.56890.033 19.33691.287 0.247090.0061 3166940 −8 0.07
37.1 r,p 290 16 0.05
0.36 0.02 0.70290.050 32.66092.928 0.337690.0158 3652973 −6 c,p
37.2 241 87
0.52090.012 15.04490.369 0.209790.0013 2903910 −7 0.01
0.06
38.1 Eq 230 14
0.78190.030
39.1 m,p/ro 209 136 0.65 0.07 35.43491.483 0.329190.0040 3613919 3
ap, Prismatic; eq, equant; e, end; m, middle; r, overgrnwth; c, core; rex, rccrystallised; osr, euhedral finescale zoning; h, homogeneous; f, fragment; anh, anhedral;
turb, turbid; ro, rounded comm.206Pb% is percent of206Pb that is common, disc (%) is discordance of ages in percent. Errors are 1
A.P.Nutman/Precambrian Research105 (2001) 93 – 114 105
(Table 2; excluding composite core – rim analysis 13.4) yielded ages between 3700 and 3500 Ma, whereas with other grains duplicate analyses yielded consistent ages (e.g. grains 9 and 10). In other cases, recrystallised domains within pris-matic grains yielded ages of 3500 – 3600 Ma (e.g. 23.1, Table 2). In addition, some other sites
yielded 207Pb/206Pb ages intermediate between
3000 and 3500 Ma. These are generally either
higher U than the \3500 Ma sites or discordant.
Combined with ages of ca. 3600 Ma obtained on
globular/twinned grain 6, these results are
inter-preted to indicate that the prismatic zircons have
an age of ]3700 Ma, and that these grains were
subjected to Pb-loss and local recrystallisation in thermal events between 3600 – 3500 Ma (events well-known in the region; e.g. Nutman et al., 1996). The ten best analytical sites on 3700 – 3750
grains yielded a weighted mean207Pb/206Pb age of
372295 Ma (MSWD=3.3). The high MSWD
linked with apparent age differences between grains, suggest that grains ranging in age from ca. 3700 to 3740 Ma are present. In addition, two age determinations on grain 9 yielded ages of ca. 3840 Ma. The 3700 – 3740 Ma and ca. 3840 Ma grains are interpreted to be of volcano-sedimentary origin, giving a maximum age of deposition of ca. 3700 Ma. Best estimates on the ages of detrital grains are summarised in Table 1.
4.4. 3500–3600 Ma metasediments, West Greenland
Older sedimentary rocks of the 3700 – 3800 Ma Isua supracrustal belt and adjacent gneisses have been discussed in most detail (see Nutman et al., 1996 and references therein). A less studied part of the Itsaq Gneiss Complex is on the north side of Ameralik (fjord), where recent fieldwork and SHRIMP geochronology have shown that 3650 – 3600 Ma gneisses and granites as young as 3570 Ma are the dominant lithologies (Nutman et al., 1996). This unit may also crop out locally on the south side of the fjord. In this part of the complex are extensive units of graphite-bearing metasedi-ment, pelite, calc-silicate rocks, fuchsitic quartzite and amphibolites, some of which are intruded by
the :3570 Ma granites (Nutman et al., 1996) —
data from several samples of these sediments are
used in this paper (G91/55, VM90/10,
GGU221122, PDK quartzite, Table 1). The depo-sitional environment of these diverse supracrustal rocks is not certain, due to the reconnaissance nature of studies in this part of the Itsaq Gneiss Complex. However, the more widespread occur-rence of detrital sediments, including fuchsite quartzites suggests a closer association with sialic crust when deposited such as an amalgam of arc complexes formed in the period 3850 – 3600 Ma. As such, the detrital populations in these rocks might provide a better sampling of sialic crust than the 3700 – 3800 Ma sediments discussed above. Dating of zircons in a graphite-bearing metasediment and a fuchsite-bearing quartzite found that the youngest concordant analyses of
the detrital population have an age of :3600 Ma
(Kinny et al., 1988; Nutman et al., 1996). Hence
the age of deposition of these sediments is :3600
Ma, considerably younger than the ca. 3710 and 3800 Ma volcanic and sedimentary rocks of the Isua supracrustal belt and adjacent areas (Nut-man et al., 1996, 1997a). Age spectra for these sediments show that grains with ages of 3600 – 3750 Ma are most abundant, with a lesser number with ages of 3750 – 3900 Ma.
Zircon age data from an additional sample of
graphite-bearing garnet quartzite (VM90/10) are
presented here. VM90/10 is from the southern
shore of outer Ameralik. It is from a tabular body of metasediments no more than 100 m thick, broken-up by heterogeneous granitic gneisses. It yielded a large number of zircons, mostly of rounded prismatic to ovoid shape. Most analyses yielded close to concordant ages (Table 2, Fig. 2). Many of the grains partial or complete over-growths, interpreted to have grown during in situ metamorphism. Eight ‘old’ overgrowths yielded a
weighted mean 207
Pb/206
Pb age of 357799 Ma
(MSWD=1.8) and 3 others yielded a weighted
mean 207Pb
/206Pb age of 3455910 Ma
(MSWD=2.6). The 357799 Ma age of
meta-morphic overgrowths gives the minimum deposi-tional age of the sediment, and agrees well with
the 356796 Ma age obtained on granites
A.P.Nutman/Precambrian Research105 (2001) 93 – 114 106
present, such as a group with a has a weighted
mean 207Pb
/206Pb age of 2713911 Ma
(MSWD=1.8), indicating metamorphism of the
rock in the late Archaean. The analysed (n=25)
cores and whole grains of (rounded) prismatic and rounded morphology are interpreted to be detrital
in origin, and yield 207Pb/206Pb ages between ca.
3600 and 3715 Ma; no more ancient grains were detected.
4.5. ]3600Ma metasediments, northern Labrador
As in western Greenland, early Archaean quartzites and schists of likely detrital sedimen-tary origin are rare, with most metasediments of the (early Archaean) Nulliak assemblage being of chemical origin (e.g. Nutman et al., 1989). Nulliak
assemblage quartzite 83/187 was collected from a
discontinous, heterogeneous supracrustal unit, :
2 km south of St. John’s harbour, southern side of Saglek Bay. Unlike the sediments reported from western Greenland, there is less U – Pb zir-con age zir-constraint on the age of this supracrustal unit, beyond it is known to occur as an integral
part of an early Archaean (]3600 Ma) complex
(e.g. Schiøtte et al., 1989a,b and references therein). The depositional setting of this unit is uncertain. However, the general paucity of detri-tal sedimentary material and abundance of am-phibolites derived from volcanic rocks suggest that it belongs to a volcanosedimentary sequence. In which case the provenance of detrital grains is likely to be local rather than broad. Zircon results
on 83/187 were reported by Nutman and
Coller-son (1991), but are summarised here (Table 1).
The sample yielded abundant grains up to 300mm
across, and often consists of thin rims over
cor-roded cores. Most sites with 207
Pb/206
Pb ages of
]3600 Ma and ca. 2760 Ma plot close to
concor-dia. Most of the youngest analyses (8) are of
overgrowths and yield a weighted mean 207Pb
/
206Pb age of 276096 Ma (MSWD
=2.5) and are
interpreted to have grown in situ during late
Archaean regional metamorphism(s). For the ]
3600 Ma analyses, 6 grains have 207Pb/206Pb ages
of ca. 3800 – 3850 Ma. Interpretation of grains
with 207Pb/206Pb ages of 3600 – 3800 Ma is
prob-lematic. The preferred interpretation of Nutman and Collerson (1991) was that these grains are
]3800 Ma old, and had lost some radiogenic Pb
during early Archaean thermal events. Alterna-tively, the sediment could be as young as ca. 3600 Ma, the age of the youngest early Archaean
con-cordant zircon analyses. Analyses with207Pb/206Pb
ages of B3600 Ma are discordant and are
inter-preted as ]3600 Ma zircon that underwent loss
of radiogenic Pb in younger Precambrian events. In compilation of age data later in this paper, this latter more conservative interpretation of the sedi-ment being ca. 3600 Ma old is adopted.
5. Early Archaean sediments from China
The Sino – Korean craton in eastern Hebei province, northeast China is dominated by middle to late Archaean gneiss complexes, but early Ar-chaean rocks have been found at two widely-spaced localities (Liu et al., 1992; Song et al.,
1996). In the Anshan area, :500 km east
north-east of Beijing, a ca. 3800 Ma rocks have been found in two places. The ca. 3800 Ma component is associated with ca. 3300 Ma granite and migmatite components. The oldest proven vol-canosedimentary rocks in the Anshan area belong to the Chentaigou supracrustals, have an age of
336295 Ma (Song et al., 1996) and are devoid of
ancient pre-volcanic zircons. As this paper
con-centrates on \3500 Ma detrital sediments, these
rocks are not discussed further here.
Approximately 150 km to the east of Beijing in the Caozhuang area, small occurrences of fuch-site-bearing metaquartzite occur in association with some mica schists and mafic rocks, within polyphase high grade gneisses (Liu et al., 1992
and references therein). Amphibolite facies
supracrustal units on the slopes north of Huang-baiyu village include calc-silicate rocks interlay-ered with amphibolites of probably volcanic origin, metapelite, banded iron formation and fuchsitic metaquartzite. The percentage of sedi-mentary to volcanic rocks in this package is high
\50%? Thus there is a possibility that detrital
A.P.Nutman/Precambrian Research105 (2001) 93 – 114 107
to find early Archaean basement rocks in the amphibolite and retrogressed granulite facies or-thogneisses adjacent to the quartzites. So far, all samples dated by SHRIMP U – Pb geochronol-ogy, appear to be strongly deformed late
Ar-chaean granitoids (Song and Nutman,
unpublished data). Thus, presently these quartz-ites seem to be an isolated occurrence as early Archaean orthogneisses associated with them have not yet been detected nearby.
SHRIMP U – Pb zircon results from fuchsitic
metaquartzite sample CF89/26 were reported by
Liu et al. (1992) and are summarised here (Table 1). The youngest detrital zircon in the metaquartz-ites is ca. 3550 Ma, interpreted to be closed in age to the deposition of the quartzite, estimated at
3500980 and 34709107 Ma from Sm – Nd
iso-topic studies of associated amphibolites on nearby outcrops (Huang et al., 1986; Jahn et al., 1987). The detrital zircons obtained from fuchsite
metaquartzite mostly have 207Pb/206Pb ages
be-tween 3600 and 3850 Ma (Liu et al., 1992). The distribution of ages obtained fall into several ‘peaks’, which suggests that the geological history of the source region experienced several distinct
magmatic/thermal events in the 3600 – 3850 Ma
period, rather than a continuum of activity.
6. Ages of zircons in early Archaean sediments
6.1. Data filtering and interpretation of ages
Later disturbance to the grains can introduce common Pb into the zircons and also give rise to discordant ages. As a filter of the data, analyses
that are \10% discordant (contain unsupported
U), \5% reverse discordant (containing
unsup-ported radiogenic Pb) or clearly young in situ metamorphic growths have been rejected. Also analyses with a high component of common (non
radiogenic) Pb (f206
Pb \2%, where f206
Pb is the
proportion of non-radiogenic 206Pb) have been
rejected. Finally, where duplicate analyses of indi-vidual grains have been done, only the most
con-cordant (generally that with the oldest207Pb
/206Pb
age) has be included in this study. All these measures were undertaken to reduce distortion of age distributions in the detrital populations.
As only close to concordant ages are discussed,
the data are not presented in the206Pb
/238U versus
207Pb
/235U or 207Pb
/206Pb versus 238U
/206Pb
con-cordia plots. Instead, the filtered ‘best’ analyses are presented as composite histograms and rela-tive probability diagrams (Figs. 3 and 4). The histogram is constructed by binning data into 10 Ma intervals and the relative probability plots shown in the background are age spectra, where the analytical uncertainty on the age of each detrital grain is taken into account. Thus, the tails
in these diagrams to ages of \3900 Ma do not
indicate that detrital grains of these ages have actually measured. Instead, they indicate that the pooled data indicates that it is possible, but highly unlikely, that grains with these ages could occur.
6.2. Data sets
Data on most of the ]3500 Ma sediments
discussed in this paper have been published in isolation elsewhere (PDK fuchsite quartzite and GGU221122 — Kinny, 1987; Kinny et al., 1988,
MR81/318 and 83/187 — Nutman and Collerson,
1991, G91/55 — Nutman et al., 1996, G93/25 —
Nutman et al., 1997a; CF89 – 26 — Liu et al., 1992). Data on two additional samples have been
presented here (VM90/10 and G93/54, Table 2).
All the data are filtered according to the methods outlined above, and then collated into suites de-posited at 3500 – 3600 Ma (five samples, 117 grains) and 3700 – 3800 Ma (three samples, 54
A.P.Nutman/Precambrian Research105 (2001) 93 – 114 108
Fig. 4. 207Pb/206Pb age histograms (with relative probability
curves in background) for detrital grains in sediments de-posited at 3500 – 3600 and 3700 – 3800 Ma.
majority of its the zircons are markedly discor-dant or are metamorphic in origin (Schiøtte and Compston, 1990).
6.3. Estimation of the maximum contribution of
]3900 Ma detrital material in ancient sediments
The probabilityPof missing an age component
present at an abundance level xafter analysing n
grains is P=(1−x)n (Compston and Pidgeon,
1986; Dodson et al., 1988). For example, for a population present at the 5% level, the probability of missing it after analysing 20 grains is 0.36, but after 50 grains has fallen to 0.08 (Fig. 3). Thus,
for a low probability (P=0.05 is used in this
paper) of missing small components in detrital zircon populations, it is necessary to analyse as many grains as possible.
In both the suites deposited at 3800 – 3700 and
3600 – 3500 Ma, \3900 Ma grains were not
found (Fig. 5; Appendix A). For the 3800 – 3700 Ma suite where 54 grains are available, there is a
0.05 probability that \3900 Ma zircons form
55% of the detrital population, but were missed
during analysis. Likewise, for the 3600 – 3500 Ma suite where 117 grains are available, there is a
0.05 probability that \3900 Ma zircons form
53% of the population were missed (Fig. 3).
Older than 3900 Ma detrital zircons have so far only been encountered in some samples of the Jack Hills and of Mt. Narryer metasediments (deposited at ca. 3000 Ma; Kinny et al., 1990) of the Narryer Gneiss Complex in Western Aus-tralia, and in quartzites of the Wyoming Province, deposited at 3000 – 3200 Ma (Mueller et al., 1992).
Although \3900 Ma zircons form :6% of the
population in one Jack Hills conglomerate sample (Compston and Pidgeon, 1986), when all of the analysed sediments from the Narryer Gneiss
Complex are pooled, the proportion of \3900
Ma detrital grains is 51% (Nutman et al., 1991).
When the 3000 – 3200 Ma global sedimentary record is taken into account (now well over a thousand detrital grains analysed from many
lo-calities) the proportion of \3900 Ma detrital
grains is considerably less. However, because 700 – 900 million years had elapsed between 3900 Ma and the deposition of these sediments, the grains) for examination of detrital grain age
spec-tra (Table 1, Fig. 4).
Felsic rocks interpreted to be of volcanic origin
such as 377698 Ma Nulliak supracrustal
associa-tion sample DB82-15Z-A, (Schiøtte et al., 1989b),
371894, 371094 and 380692 Ma Isua
A.P.Nutman/Precambrian Research105 (2001) 93 – 114 109
observed proportion of \3900 Ma detrital grains
can be accommodated in a wide spectra of crustal growth and recycling models.
6.4. Early Archaean crustal growth6ersus recycling
Whereas there is now a substantial global data-base of detrital zircon ages from 3200 – 3000 Ma sediments from many localities, the database for older sediments is much more restricted. If the 3500 – 3600 Ma detrital sediment suites are repre-sentative of their source terranes, their detrital zircon age distributions indicate that constant
crustal volume (recycling=new additions) models
require that \97% of \3900 Ma crust was
destroyed by recycling by 3500 Ma (Figs. 1 and
3). The lack of detected \3900 Ma detrital
zir-cons in three 3700 – 3800 Ma detrital sediments supports this conclusion, and provide even older
constraints, requiring that \95% of \3900 Ma
crust was destroyed by 3700 Ma in constant vol-ume models.
Such rapid recycling rates are hard to envisage, unless the whole lithosphere was being ‘reworked’
by major impact/melting events as late as 3500
Ma. However, such a scenario may be ruled-out,
because since ]3850 Ma Earth has retained a
hydrosphere (Nutman et al., 1997a) which even then harboured life (Mojzsis et al., 1996). Further-more, such extremely high recycling rates might be hard to reconcile with the spread of initial Nd and Sr isotopic ratios of well preserved early Archaean granitoid suites (e.g. Fig. 2, Baadsgaard et al., 1986; Bowring and Housh, 1995), which require significant average crustal residence times of several hundred million years to permit radio-genic isotopic systems to evolve. Both these con-siderations would seem to rule-out extremely rapid recycling as the explanation of the low
abundance of \3900 Ma detrital zircons in the
3500 – 3800 Ma sedimentary record. Thus, from the detrital zircon perspective, it is concluded that in the crustal segments preserved in the early Archaean gneiss complexes in northern Labrador, western Greenland and northeastern China, the volume of continental crust at 3900 Ma was small and that it increased in the period 3900 – 3500 Ma.
7. Discussion
Initial lead isotopic values of early Archaean terrestrial rocks and minerals (e.g. Stacey and Kramers, 1975; Gancarz and Wasserberg, 1977; Appel et al., 1978; Richards and Appel, 1987)
suggest that the early Archaean mantle had m
values (238U/204Pb) of 7 – 9, which are very high
relative to the initial m values of carbonaceous
chrondrites. This suggests that the Earth’s core segregated soon after the planet formed, with siderophile Pb being more strongly partitioned into the core than U, leaving the mantle with
elevated m values (Oversby and Ringwood, 1971;
Gancarz and Wasserberg, 1977; Alle´gre et al., 1995). From thermal considerations as well, bod-ies the size and composition of Earth, Venus, Mars and parent bodies of meteorites must segre-gate cores by melting early (within the first 200 Ma) of their history (e.g. Elder, 1987).
With the evidence of core-mantle segregation within the first few hundred Ma of the Earth’s history, Armstrong (1991) argued that a continen-tal crust similar in volume to today’s also formed in the same early segregation event, perhaps as a differentiate out of a magma ocean. Lunar KREEP indicates that small volumes of felsic differentiate are possible from a magma ocean, and fractional crystallisation of terrestrial mafic magmas can produce small volumes of felsic ma-terial in the final stages. If such a crust did exist (e.g. Armstrong, 1991; Bowring and Housh, 1995) and it was either small in volume or disseminated through mafic crust, it could have been easily destroyed by convective overturn. If it was large in volume and segregated into discrete masses, the
results in this paper suggest that the few ]3500
A.P.Nutman/Precambrian Research105 (2001) 93 – 114 110
Marov, 1978; Arvidson et al., 1980) is because these planets did not retain their hydrosphere, necessary for the production of granites (Camp-bell and Taylor, 1983). Therefore, the argument that because the core and mantle separated very early, the same must apply for a wet granitic continental crust, has no substance.
Lack or rarity of ]3900 Ma detrital zircons in
very old sediments would dictate that early Ar-chaean crustal evolution models, which invoke constant volume rather than the growth, need very high recycling rates. Such high recycling rates could be achieved by a high recycling rate such as tectonic erosion in subduction zones, or by a surficial environment in the early Archaean that was more hostile to zircon than at present. For example, globally, more acidic waters surface might shorten the life of zircons in the sedimen-tary cycle. Not taking such a factor into account might give an erroneous impression of very rapid crustal recycling rates. However, the presence of carbonates in supracrustal sequences as old as ca. 3800 Ma (3800 Ma parts of the Isua supracrustal belt contain mobilised marbles, probably derived sedimentary rocks, e.g. Nutman et al., 1984) with precipitation of gypsum to at least ca. 3500 Ma, combined with the lack of a runaway greenhouse effect in the early history of the Earth, constrains
concentration of dissolved CO2in the oceans and
acidity to values only slightly higher than the present (Walker, 1983). Further limitations on a lower pH comes from suggestions from the chem-ical study of Palaeoproterozoic and late Archaean
palaeosoils for lower than expected CO2
concen-trations, with some of the mild greenhouse effect necessary to compensate for the lower luminosity of the sun coming from higher concentration of another gas, possibly methane (Rye et al., 1995). Furthermore, life was widespread and sophisti-cated by 3500 Ma and had certainly been estab-lished on Earth by 3800 Ma (Mojzsis et al., 1996). Combined, these lines of evidence point against a global, highly acidic ocean and surface environ-ment. Thus, paucity of extremely ancient zircons in the sediments can be taken to mean that they were never there in abundance, rather than their residence times in the sedimentary system were shortened by a more hostile environment.
A high recycling rate of sedimentary material could also be achieved by periodic catastrophic subduction of large amounts of sediments accu-mulated in continental margin rift basins upon slab failure (Hildrebrand and Bowring, 1999). Al-though, such a model is feasible for later epochs, it remains to be tested for the early Archaean: at the outset, such a model for recycling of crust requires the presence of a voluminous continental crust to be rifted. Such a recycling process is less likely to be of significance if the volume of conti-nental crust was small (predominantly as arcs and amalgams of arcs), particularly if distributed around the globe.
A critical question is whether the limited
cur-rent data on the age of detrital zircons in ]3500
Ma sediments are representative of the global average at their time of deposition, or whether they purely reflect the provenance of their local terrane. One way of addressing this problem is to
find more ]3500 Ma sediments, to increase the
presently small database. Within the last decade, four completely new early Archaean terranes were discovered in the Northwest Territories of Canada (Bowring et al., 1989), West Greenland (in addi-tion to the Itsaq Gneiss Complex, which was recognised in the late 1960s – early 1970s; Rosing, Løfqvist, Nutman and McGregor unpublished data) and in the Sino-Korean craton of northern China (Liu et al., 1992). Of these, early Archaean sediments have so far only been recognised at one locality in the Sino-Korean craton (Liu et al., 1992). Given the rate of discovery of new early Archaean terranes over the past decade, there is little doubt that further early crustal remnants, some with associated detrital sediments, will be found in forthcoming years.
A.P.Nutman/Precambrian Research105 (2001) 93 – 114 111
larger domains of sial would be a more useful tool than sediments deposited in arcs or on oceanic plateaux, which are more likely to be of local provenance. The LIPS (Large Igneous Provinces) model has been constructed to explain the pro-duction of large amounts of crust in a short time (summarised by Albarede, 1998). In this model, large volumes of mafic crust are produced by volcanism, particularly in oceanic plateaux. In study of the ca. 2100 Ma Birimian LIPS province in West Africa, Boher et al. (1992) have noted the lack of older detrital zircons in sediments found with voluminous mafic volcanic rocks. This was used to suggest that the Birimian province formed far away from any sialic crust. Such models might be relevant to interpretation of the ages of detrital grains found in thin sedimentary units interlay-ered with 3700 and 3800 Ma volcanic rocks in Greenland. However, the 3500 – 3600 Ma samples discussed in this paper might include some of broader provenance (e.g. fuchsite quartzite, CF89-26 in Table 1). In such samples, extremely ancient grains are still not important clearly in the zircon populations, still supporting a model of small continental crust volume in the early Archaean.
Study of samples of demonstrably broader provenance and from more, newly-discovered, ter-ranes will permit confirmation or refutation of the conclusion from the preliminary study of ages of zircons from very ancient detrital sediments (par-ticularly those deposited between 3500 and 3600 Ma, which might be more representative). At 3900 Ma the volume of continental crust was small, and that it grew in volume during the early Ar-chaean (but no doubt accompanied by some recy-cling). However, recalling the bulk zircon Hf isotope paper of Stevenson and Patchett (1990) more powerful method of charting early crustal evolution will be combination of single zircon U – Pb geochronology with precise single zircon Hf isotope studies.
Acknowledgements
I thank Richard Armstrong, Ian Williams, Vickie Bennett, Clark Friend and Ross Taylor for discussions on zirconology of old sediments. I
gratefully acknowledge support from my hosts Yuji Sano, Hiroshi Hidaka and Kentaro Terada in Hiroshima University, whilst drafting this pa-per. The paper was improved by constructive reviews by Paul Mueller and Ross Taylor.
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