The negative Eu anomaly in Archean sedimentary rocks: Implications for decomposition, age and importance of their granitic sources

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Earth and Planetary Science Letters 133 (199.5) 81-94

The negative Eu anomaly in Archean sedimentary rocks: Implications for decomposition, age and importance of their granitic sources S. Gao a*b,K.H. Wedepohl

a

a Geochemisches Institut, Uniuersitiit GSttingen, Goldschmidtstrasse I, 37077 Giittingen, Germany b Department of Geochemistry, China University of Geosciences, Wuhan 430074, People’s Republic of China Received 24 November 1994; accepted after revision 21 April 1995

Abstract A negative correlation between Eu anomaly (Eu/Eu * ) and chemical index of weathering (CIW) is found for Archean pelites in both greenstone belts and high-grade terrains from southern Africa, western Greenland, North America, Western Australia and China. A similar negative correlation between Eu/Eu* and chemical index of alteration (CIA) is also found for worldwide young deep-sea sediments from various tectonic settings. The increase in the indices of weathering and alteration coincide with both shifts in mineral composition and mineral decomposition from plagioclase/K-feldspar to mica/kaolinite and from amphibole to chlorite/smectite. Juvenile crustal materials from local sources are only slightly decomposed due to little or no transport. They are characterized by low CIW/CIA and negligible Eu anomalies. In contrast, crustal materials from large provenance areas have usually suffered long transport and recycling, which results in decomposition of high-temperature minerals and large CIW/CIA values that are negatively correlated with significant negative Eu anomalies. Thus, the observed trends are interpreted as mainly reflecting increasing contributions of recycled and decomposed materials from large provenances relative to juvenile crustal additions through arc magmatism at local sources. In this context, there is no obvious distinction between Archean greenstone and high-grade metasediments and young deep-sea sediments. The large provenance areas have contained a significant proportion of granitic and rhyolitic materials, which, in part, have not resisted decomposition and erosion due to their occurrence in shallow crustal layers. Granites received their distinctive negative Eu anomaly from equilibration with a feldspar-rich residuum. The negative Eu anomaly of Archean metaclastic sediments apparently increased with decreasing age. Trends of minimum and average Eu/Eu * values of metaclastic sediments evolving with time are calculated for the Archean and lead us to infer that the negative Eu anomaly (Eu/Eu * < 0.9) of the upper continental crust might have appeared before 3.8 Ga. Thus, the first granite responsible for intracrustal fractionation originated very early. The negative Eu anomaly had been a common feature at least for a time span of ca. 3.2 Ga. At 2.5 Ga, the upper continental crust and elastic sediments had almost acquired their present Eu signature (Eu/Eu* u 0.65). The Archean-Proterozoic transition is not a boundary in the evolution of the composition of the continental crust.

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1. Introduction The Eu anomaly is a measure of Eu*+ fractionation from Eu3+ relative to the neighbouring Sm3+ and Gd3+ or Tb3+ on the basis of the chondritic REE-normalized values. Mantle-derived rocks usually have no Eu anomalies, which means that primary mantle additions into the crust should induce no changes. Significant negative or positive Eu anomalies may occur in crustal rocks mainly as a result of intracrustal fractionation separating granitic melts from residues containing feldspar, which is the major host of Eu2+ in rocks. Fine-grained elastic sediments, such as shales, are the products of mixing of source materials from large provenances. Thus, they provide a representative sampling of exposed upper continental crust [l]. It is generally agreed that REE are very reliable indicators of sources. These elements have extremely low water-rock partition coefficients, which partly controls their nearly complete transfer from sources into elastic sediments, and allows us to assume that they are generally immobile during most of the diagenesis, alteration and low to medium grade metamorphism that occurs. Because of these important characteristics, the nature of the Eu anomaly in sedimentary rocks is of critical importance in our understanding of the formation, evolution and composition of the continental crust. Although post-Archean sediments are well established as being characterized by significant negative Eu anomalies, like the post-Archean upper continental crust, there is still controversy over the nature of Eu in Archean sedimentary rocks and accordingly in the Archean upper crust [l-lo]. Based on studies of metasediments from Archean greenstone belts, Taylor and McLennan suggested that there is an abrupt change in the chemical composition of the upper continental crust at the Archean-Proterozoic transition, with the Archean upper crust having no Eu anomaly [1,5,6,10]. Because Archean greenstone belts have rock associations like Phanerozoic island arcs, and Archean high-grade metasediments of usually quartzite-pelite associations show more similarities to Phanerozoic cratonic sediments, comparison needs to be made for sediments from comparable tectonic settings. This led other investigators [2,4,9,11,12] to conclude that there was little or no

change in Eu composition over the Archean-Proterozoic transition. A similar conclusion was reached from extensive sampling of the Canadian Precambrian Shield, Greenland, and some Chinese Archean terrains [3,7,8]. With the accumulation of more data, negative Eu anomalies (Eu/Eu* < 0.90) are apparently as common as the absence of Eu anomalies in metasediments of both Archean granulite facies terrains and greenstone belts. This contradicts the suggestion of McLennan and Taylor [6] that only rare Archean greenstone metasediments have been found with substantial negative Eu anomalies. Most of Archean greenstone metasediments from southern Africa and China have prominent negative Eu anomalies [12-151. For a better understanding of the Archean upper crustal composition from studying sedimentary rocks, the following four points will be considered: (1) It must be decided whether Archean greenstones, tonalites-trondhjemites-granodiorites (TTG) or granites have contributed large proportions to the metasediments of the Archean continental upper crust. Taylor and McLennan [1,5,6,10] prefer metasediments from greenstones as very abundant. They assumed for the Archean that before destruction by erosion and recycling, greenstone tectonic settings were initially more spread out than cratonic felsic terrains. In contrast, Condie et al. [2,4,9] support the view that Archean greenstones are not representative of the Archean continental upper crust because of their oceanic affinities. Reliable estimates of relative mass distributions of these two major types of Archean crust are required. (2) It must be explained why Archean greenstone belts have generally experienced low-grade metamorphism in contrast to the cratonic quartzite-pelite associations which have mostly undergone high-grade metamorphism. (3) The present-day extents of different types of Archean magmatic rocks are probably not sufficient to explain the significant negative Eu anomaly of Archean metasediments. Granites characterized by large negative Eu anomalies occur in small exposed areas relative to TTG gneisses which dominate Archean terrains and which usually do not show negative Eu anomalies [9,16]. This implies a former, greater abundance of early granitic sources that are no longer visible due to decomposition and erosion.

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(4) High-grade terrains are generally significantly older than surrounding greenstone belts. They contain the oldest known rocks on Earth and frequently record evidence of early ( > 3.4 Ga) intracrustal fractionation producing granitic rocks [17,18]. The Narryer Gneiss Complex (Western Australia), which contains the Earth’s oldest known (3.9-4.2 Ga) detrital zircons, exposes Archean metasediments that already have negative Eu anomalies similar to postArchean sediments [19,20]. These metasediments must be residual from a very early crustal fractionation. Our paper relates Eu anomalies, Sr contents and chemical indexes of weathering/alteration (defined as CIW = molecular ratios of Al,O,/(Al,O, + CaO + Na,O) X 100 [21], or CIA = molecular ratios of Al,O,/(Al,O, + CaO + Na,O + K,O) X 100 [22]) of worldwide sampled Archean metasediments and of young deep-sea sediments. CaO in both indices is restricted to that contained in the silicate fractions. We should point out that changes in the CIW/A between rock successions reflect changes in mineralogy. The latter is in most cases caused by decomposition, including weathering. Our procedure to recognize the degrees of decomposition should help in identifying decomposed granite and its proportion in Archean crustal evolution.

2. Data sources The data used in this study are from the literature and from analyses of Chinese Archean metasediments. The literature data include samples of various Archean ages from southern Africa, western Greenland, North America, Western Australia, and a few from China. The Malene metasediments of western Greenland require special attention. The data for these rocks are from McLennan et al. [23], Dymek and Smith [24] and Smith et al. 1251. These three sets were sampled in the Godthibsfjord region. The latter two, which are mostly quartz-cordierite gneisses, show distinct geochemical differences compared to the former, containing less than 10 ppm SC, Co, Cr and Ni each and less than 50 ppm Sr. The latter two sets of rocks are also high in MgO (2-lo%), Ga (20-50 ppm), Zr (300-700 ppm), Nb (20-50 ppm), Hf (lo-25 ppm),

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Ta (1.5-3 ppm), U (1.5-4 ppm) and Y and La (40-150 ppm), and have prominent negative Eu Even for postanomalies (Eu/Eu * = 0.3-0.7). Archean sediments these features are unusual, and so for Archean sediments they are highly unusual. The better area1 coverage and larger number of samples of the unusual metasediments compared to the sampling of McLennan et al. suggests that information including the new sets is more representative of the Malene supracrustal rocks. Because the Godthibsfjord region is regarded as a type locality of Archean high-grade terrains and has played an important role in our understanding of the Archean crustal evolution, these unusual metasediments are incorporated in our database. We consider them with caution, however. We selected only 18 samples with MgO < 6% from a total of 61 samples because the high MgO content of the other samples was attributed to hydrothermal alteration by hot seawater [24,25]. Sr contents of several of the 18 samples were reported as below the detection limit (< 25 ppm), and are taken for the plots in Fig. 2a. For the Chinese rocks from the Dengfeng, Luxi, Qianxi and Taihau terrains, CO1 was determined by volumetry, CaO, Na,O and K,O by AAS, and Sr and REE by ICP-AES. Details of the analytical methods, precision and accuracy have been published elsewhere [26]. For rocks of the Kongling, Wutai and Jining terrains, ICP-MS was used for determination of REE and Sr after acid digestion (HF, HNO, and H,SO,) in sealed Teflon screw cap beakers. XRF of glass discs was applied for determination of the major elements [27]. Analyses of international standard reference rocks indicate a precision that conforms with 10% deviation of our data from the recommended values for Sr, Sm, Eu and Gd and with 1% deviation for Al,O,, CaO, Na,O and K,O. Because clays are the products of better mixing and larger provenances than sands [5,28], our study concentrates mainly on pelites, unless otherwise indicated. For a better screening of the sample population, we used the following chemical criteria for pelites: (1) SiO, = 50-75%, (2) Al,O, = lo-23% and (3) CO, < 1% or LOI < 5%, or CaO < 4%. For normalization of our REE data we used the average chondritic REE composition reported in [l]. The detailed information on the data sources is included in the appendix.

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3. Results 3.1. Archean pelites

Fig. 1 illustrates the relationships between Eu/Eu * and the weathering index (CIW) for

Archean pelites from worldwide greenstone belts and high-grade terrains. These range in depositional age from Early (e.g., Isua and Akilia, western Greenland, 3.8 Ga [1,23]; Qianxi, North China) through Middle (e.g., Fig Tree and Moodies, southern Africa, 3.2-3.3 Ga [12]) to Late (e.g., Pongola and Witwatersrand, southern Africa [12]; Dengfeng, Wutai and Luxi, North China [29-311) Archean. The two sets include 120 samples from greenstone belts and 82 samples from high-grade terrains. Despite the great diversity in age and rock association, the two main types of Archean pelites show close similarities. Both exhibit negative Eu/Eu *-CIW correlations, with linear correlation coefficients (r) of - 0.50 and - 0.71 for the greenstone and high-grade pelites respectively. The samples with CIW > 80 almost invariably correspond to significant negative Eu anomalies (Eu/Eu * = 0.3-0.9). In contrast, samples with CIW < 80 show a wide range in Eu/Eu* ratios, from 0.4 to 1.5. This wide range reasonably reflects variable, lithologically dependent Eu/Eu * values of fresh magmatic rocks of mafic to felsic composition that correspond to CIW values ranging between 40 and 65. The rocks without Eu anomalies usually have lower CIW values ( < 80). The trend is best illustrated by the Fig Tree, Witwatersrand, Wutai, Limpopo, Malene, Jining and Kongling rocks (Fig. l), all of which are represented by a relatively large set of samples and show a considerable spread in CIW values and Eu/Eu * ratios. Although the greenstone data are biased towards southern Africa, the limited data from other areas show no deviation from the trend. Only one pelite from Mount Narryer is available, but according to Maas et al. [19] other pelites and semipelites from the region have CIA values of between 74 and 98, indicating CIW values higher than 74. Together with Eu/Eu* values from 0.59 to 0.81 for the six pelites and semipelites studied by these authors, this indicates that the result is consistent. Samples with Eu/Eu* ratios > 1.2, which are mostly from high-grade terrains, are exceptions in the sense that common magmatic and metamorphic rocks have Eu/Eu’ ratios of < 1.2. These rocks may have undergone post-depositional changes such as partial melting, as in the case of the Kapuskasing Structure Zone [32]. Even if these specific rocks and the previously mentioned unusual Malene samples are excluded, the trends of negative

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correlations (e.g., r = 0.72 for high-grade terrains) still hold. Comparison of Fig. la and b does not reveal obvious distinctions in sample distribution between greenstone and high-grade rocks, except that some of the latter samples tend to have more negative Eu anomalies. Excluding the Malene samples, pelites with Eu/Eu* > 0.9 account for only 22% in the total sample number of the two types of metasediments. There are only four (Moodies, Pontiac, Yellowknife and Kapuskasing) out of 21 regions/supergroups in Fig. 1 in which all samples have Eu/Eu* > 0.9. The negative Eu/Eu * -CIW correlation is accompanied by a positive Eu/Eu* -Sr correlation (Fig. 2a; I = 0.54 and 0.78 respectively for greenstone and high-grade pelites). Exclusion of the Malene data does not substantially influence the correlation (r = 0.76). This correlation indicates the close crystal chemical relationships between Eu2+ and Srzf , which is a result of comparable ionic size. Eu2+ and Sr2+ both partly substitute Ca2+ and Na+ in plagioclase, which is the major rock phase with high Eu/Eu *a The results for Archean graywackes (n = 58 and 19 for greenstone belts and high-grade terrains respectively) are comparable with those for pelites. Samples characterized by CIW > 80 have negative Eu anomalies (Fig. 3). Samples with low CIW and a small Eu anomaly are considerably more frequent, however.

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3.2. Young deep-sea sediments Data on relevant young deep-sea sediments come from (1) sand and associated mud samples from worldwide sampling in a variety of tectonic settings [5], (2) the Izu-Bonin arc turbidites (usually mediumto coarse-grained sands and sandstones) [ll], (3) the Sea of Japan [33] and (4) South Atlantic muds and mudstones [34]. Many of the sand and mud samples of McLennan et al. [5] have high CaO contents (4-22%), indicating the presence of variable amounts of carbonate minerals. For the CIW and CIA indices the silicate CaO fraction is required however. We have calculated this from the plagioclase composition and have reported CIA values for McLennan et als.’ samples. Because we did not know the plagioclase composition, we could not calculate the CIW

1.000

Fig. 2. Eu/Eu’ vs. Sr correlations for (a) worldwide Archean greenstone and high-grade metasediments and (b) young deep-sea sediments from various tectonic settings. Data are from [5,11,33,34].

indices. Therefore, we used the CIA values reported by McLennan et al. [5] instead and constructed Fig. 4a for young deep-sea sediments. Note that the only difference between the two indices is that CIA additionally incorporates K,O in the denominator. In this study, we prefer CIW rather than CIA because the former exclusively considers the consistent loss of Ca and Na at the expense of increasing Al during chemical weathering. K may behave differently [35]. The CaO contents from CaCO, in calcareous samples of the Izu-Bonin arc were subtracted from

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correlation with Sr is also present for the data of McLennan et al. and of the other suites (Fig. 2b). The CIA data for worldwide Archean metasediments have been calculated and added for comparison (Fig. 4b). This comparison emphasizes that (1) there exists a clear negative correlation between Eu/Eu * and CIA for Archean pelites and greywackes/sandstones, as in the case of CIW, and (2) young deep-sea sediments and Archean metasediments are comparable with respect to the investigated correlation. Both are characterized by significant negative Eu anomalies for samples having CIA 0.2

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the reported values [ll]. CaO is not corrected for carbonate fractions in the Sea of Japan sediments because most of the muds and mudstones have CaO < 1% and CaO/Na,O ratios of < 0.5. Of the South Atlantic sediments, only samples with CaCO, < 2% are used and corrections are made for both CaO and Na,O according to the reported CaCO, and Cl contents (341. Fig. 4a demonstrates that the data of McLennan et al. and those for the other three suites broadly overlap. A clear correlation exists between CIA and Eu/Eu* for sediments from all the investigated settings and areas (the Izu-Bonin arc, the Sea of Japan and the South Atlantic Ocean, r = -0.76). The decrease in Eu/Eu* and the increase in CIA corresponds to the sequence Izu-Bonin arc-Sea of Japan Sea-South Atlantic Ocean. The correlation for McLennan et als.’ samples is less significant, with r = -0.54 and - 0.33, respectively, for muds and sands (excluding one sand outlier from the back-arc setting). Samples from different tectonic settings do not exhibit considerable differences. This might be attributed to the fact that their sample set for each of the tectonic settings is too small and yet shows large variations in chemical composition. Additionally, it should be mentioned that a trend of positive Eu/Eu *

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> 70. Both sets contain numerous samples with CIA < 70 for samples which lack Eu anomalies. The CIA index especially considers potassium, which is especially abundant in granites. Therefore, high CIA values (correlated with negative Eu anomalies) indicate a large proportion of decomposed granite.

4. Discussion 4.1. Important sources for Archean metasediments

Both the CIW and the CIA indices were initially proposed as indicating the degree of rock weathering [21,22]. In fact, these indices depend on source compositions, felsic parent rocks having generally greater values than mafic ones. They also depend on processes causing mineral decomposition that may result in gain or loss of Al, Ca, Na and K in the rocks. These, apart from in-situ weathering, include transport accompanied by grain-size sorting, recycling, and post-depositional processes such as diagenesis, hydrothermal alteration and metamorphism. Because plagioclase is the most abundant mineral in the exposed unweathered continental crust, for a similar source composition the two indices are largely a measure of plagioclase decomposition. As shown at the top of Fig. 1, abundant plagioclase has a CIW of 50, while amphibole (20-501, K-feldspar (60-85) and mica, chlorite and illite (SO-1001 usually cover a larger range. Kaolinite, the most common weathering product of granitic rocks, has a CIW close to 100. Smectite, which is the major weathering product of mafic rocks and tonalite, has lower values (70-85). In-situ weathering is unlikely to cause specific loss of Eu*+ to form a negative Eu anomaly because weathering is usually connected with oxidation rather than reduction. REE behavior during post-depositional processes is not well understood. It is a common assumption that the REE are generally immobile during most diagenesis, alteration and low to medium grade metamorphism. Although there are a number of examples of REE mobility in these processes, Eu/Eu* usually shows no or only slight change [36-381. Post-Archean fine-grained elastic sediments from worldwide sampling on cratons, irrespective of age and possible difference in post-depositional pro-

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cesses, are characterized by quite similar REE patterns. Archean high-grade metasediments from felsic sources more frequently than their greenstone counterparts exhibit negative Eu anomalies comparable to post-Archean sediments. These observations also support the assumption that post-depositional processes exert no significant influence on Eu/Eu* ratios. The grain-size effect can be seen from a comparison of Fig. 1 with Fig. 3 and from Fig. 4b. Greywackes and sandstones tend to have lower CIW/CIA values compared to pelites, because the former contain more plagioclase and amphibole. This indicates that the mineralogically controlled grainsize effect must be taken into consideration for precise geochemical comparison. Nevertheless, comparison of sand-mud associations from young deep-sea sediments shows that their difference in Eu/Eu* ratios is usually less than 0.1 and never greater than 0.2 and their difference in CIA is mostly less than 15 [5]. On account of this and because our comparison is mainly based on pelites, the grain-size effect is unlikely to be a determining factor controlling the above correlations. In summary, the observed trend of correlations is best interpreted as reflecting a connection between source composition, distance of transport, and recycling effect. Transport and recycling are two closely related processes with similar effects and are heavily dependent on tectonic setting. The effect of transport is well illustrated by increasing CIW/CIA values from parent rocks, top soils and stream sediments developed on single batholiths of granite and granodiorite 1391. The Izu-Bonin arc, Sea of Japan and South Atlantic Ocean are good representatives of intra-oceanic island arc, backarc basin and passive margin settings respectively. The Izu-Bonin arc has been isolated from old recycled crustal debris. The turbidites from this arc are the most thoroughly studied sediments of intra-oceanic island arcs and consist almost entirely of volcanic debris from the arc, with little or no dilution by pelagic components. They provide a good compositional estimate of the juvenile arc upper crust [ll]. The Sea of Japan is a small, enclosed, marginal ocean basin in which elastic detritus is supplied by the bordering mature Japan arc and old continental crust. The South Atlantic sediments studied by Wang et al. [34] came from a site not far

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from southern Africa (DSDP Holes 530A and 530B) and the terrigenous provenance for these sediments is mostly from the exposed Archean African crust. The terrigenous detritus is well mixed with oceanic sediments. The trend of decreasing Eu/Eu* and increasing CIA from the Izu-Bonin arc through the Sea of Japan to the South Atlantic Ocean, as shown in Fig. 4a, thus reflects decreasing contributions from juvenile additions through arc magmatism and increasing proportions of old recycled materials from large provenances, in that sequence. The proportion of local and distant sources as a function of tectonic setting and recycling also controlled the composition of Archean metasediments. This will be explained with a few examples. Geological and geochemical studies indicate that Archean Superior and Slave metasediments were dominantly deposited in arc-related settings [40-451. The Abitibi metagreywackes are characterized by low CIW values without Eu anomalies (Fig. 3) and show Nd isotopic compositions similar to the associated volcanic rocks, with abundant depleted mantle model ages of 2.65 Ga that are close to the depositional age of 2.7 Ga. These ages, together with trace element characteristics, led Feng et al. [43,44] to conclude that the metasediments were derived from Abitibitype volcanic rocks in an oceanic island arc environment. The Pontiac metasediments, which more frequently exhibit negative Eu anomalies (Fig. 31, contain detrital zircon and have Nd depleted mantle model ages up to 3.0 Ga that are considerably older than the depositional age. Therefore, Fcng et al. [44] assumed that the Pontiac metasediments contain significant recycled contributions from older crustal rocks. The best examples showing the effect of recycled, old crustal material on the sediment composition are those of the 3.4-2.7 Ga old Dwalile, Pongola and Witwatersrand metapelites from southem Africa. The three suites are characterized by prominent negative Eu anomalies and high CIW values and fall into the lower right-hand area of Fig. la. Geological, geochemical and isotopic studies invariably indicate that these metapelites were deposited on and supplied by older evolved cratons with granitic materials as an important component [12,14,46,47]. One of the major observations used by Taylor and McLennan to support their model of crustal evolution is that young deep-sea sediments

from all tectonic settings show negative Eu anomalies [5,6,10]. This is not the case for the Izu-Bonin arc turbidites. The paucity of sediments with a major proportion of juvenile constituents formed in postArchean time reflects a sampling bias. Sediments from juvenile arcs are often diluted by recycled materials which are abundant on passive margins

Ml. Granitic melts are the major source of negative Eu anomalies because of their equilibration with plagioclase-rich (and Eu2+-rich) residuum. Archean felsic volcanic rocks also exhibit negative anomalies, which are, however, less significant compared to granites, both in terms of rock abundance and magnitude of the anomaly [9]. Archean mafic volcanic rocks typically have CIW values of _ 40, felsic volcanic rocks have values of _ 60, TTG has values of N 55, and granites have CIW values of N 65; the Eu/Eu* values for these rocks are, respectively, 1.0, 0.85, 1.0 and 0.5 [9]. A simple three-component mixing calculation using the average REE compositions of Archean granite, TTG and basalt [9] indicates that N 10% granite is needed to generate a Eu/Eu* value of 0.90 in the derived mixtures and N 65% granite is needed to cause a Eu/Eu* value of 0.60. Accordingly, metasediments with Eu/Eu* < 0.9 should be primarily related to significant amounts of granitic source materials. Because fresh magmatic rocks, including granite and felsic volcanic rocks, have CIW < 70, source materials of metasediments with Eu/Eu* < 0.9 and CIW > 70 must have undergone major to significant transport from a distant provenance, while those with CIW < 70 experienced little decomposition and suggest a local provenance. Mafic rocks subjected to moderate to severe weathering will have CIW > 80 [21]. In Figs. 1 and 3, there are almost no samples with CIW > 80 and Eu/Eu* > 0.9 however. This again supports the view that in-situ weathering is not a major factor controlling the high CIW values. Present-day exposed Archean terrains are dominated by TTG gneisses and volcanic rocks with small to negligible Eu anomalies. This makes it difficult to interpret them as the source provenance of Archean metasediments yielding the negative Eu anomalies. For example, the prominent negative Eu anomalies of the Malene metasediments in western Greenland must be derived from sources such as

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granites and their volcanic equivalents [24,25] in an area where tonalitic to granodioritic gneisses form ca. 70% of the exposed Archean gneiss terrain [16,24,25]. Maas and McCulloch [19] explain from detailed studies of trace elements and Nd isotopes and U-Pb ages that the source of elastic metasediments in the Narryer Gneiss Complex with prominent negative Eu anomalies was not dominated by the presently exposed orthogneisses. A similar conclusion is reached by our study of the 2.8 Ga Kongling granulite terrain, where metasediments can be classified into three groups based on mineralogical and chemical characteristics. The first major group with negative Eu anomalies is characterized by high CIW (67-87) and low Eu/Eu* (0.49-0.751, similar to post-Archean sediments. The second group, without a Eu anomaly, is characterized by lower CIW (60-62) and relatively high Cr (229-396 ppm) and Ni (108-151 ppm) concentrations. The third group is characterized by extremely variable REE distributions, possibly due to dehydration melting. The second group can be modelled by a mixture of N 0.60 average Kongling tonalitic trondhjemitic granodioritic granitic gneiss, * 0.30 average Kongling amphibolite and - 0.10 Archean komatiite as a source. It has to be assumed that the first group was caused by a more distant provenance because the Eu2+ deficit of the local granitic gneiss (Eu/Eu* = 0.59-0.86) is not sufficient. Similarly, the ubiquitous negative Eu anomalies of other Chinese Archean metasediments could not be modelled with locally exposed Archean magmatic suites. Only in southern Africa do there occur significant volumes of preserved granites ranging in age from 2.6 to 3.6 Ga which probably contributed to the ubiquitous negative Eu anomalies of the 3.2-3.4 Ga Dwalile and Moodies and later sediments [12,14,46,47]. Condie [9] estimated the age-dependent average compositions of the so-called juvenile upper continental crust from a combination of chemical analyses, geological maps, stratigraphic sections and isotopic ages. His juvenile crust was considered to be the upper crust extracted from the mantle during a certain time interval. The derived upper crust compositions for all ages from about 3.5 to < 0.2 Ga have a significant negative Eu anomaly (Eu/Eu* = 0.810.64). Important is a gradient of increasing negative Eu anomaly with decreasing age. This is also true for

89

his estimated cratonic shale compositions. These results are consistent with our conclusions. Condie [9] further noted a lack of HREE depletion and high Fe, V and SC contents for Archean cratonic shales. The presently exposed Archean crust is dominated by HREE-depleted TTG gneisses. This discrepancy led Condie to speculate that the shale sources lay in the uppermost Archean continental crust, which is mainly removed now by erosion. This crust was apparently composed of a major proportion of basalt ( + komatiite) and granite with a minor amount of ‘ITG that made up the deeper levels of the Archean crust. Such a model is able to explain the Eu anomaly discrepancy. Our results also suggest that such an Archean upper continental crust with significant amounts of granitic materials was not of only local importance: otherwise the Eu/Eu * -CIW relationships would not have been observed. The results require that granites from a shallow level of a vertically zoned Archean crust being destroyed by erosion have contributed a major fraction to the composition of the sediments. Our generalization is supported by evidence. Various exposures of Archean crust suggest that superacrustal rocks and granites decrease with depth at the expense of ‘ITG. In the Vredefort section, through a 14 km deep profile of the Archean Kaapvaal craton of southern Africa granitic gneisses are concentrated in the upper 3 km (with Eu/Eu* = 0.38-0.71) and granodioritic and charnockitic gneisses (with no or positive anomalies) occur at a lower level [48]. The Kapuskasing Zone in North America is another example of such an exposed Archean crustal section, in which granitic gneisses account for 24-28% in a higher level (amphibolite facies) and only less than 2% in a lower level (granulite facies) [49,50]. The Wutai-Henshan zone represents a cross section through the upper and middle crust of the Archean North China Craton. In this section, granites make up 18% of the total exposed area of Archean intrusive rocks in the greenschist facies and only < 5% in the amphibolite facies terrains. Source provenance studies of the Archean metapelites of southern Africa [12,46,47] allow us to infer that, from the deep Fig Tree Group through the Moodies Group up to the shallower Pongola Supergroup, the granite proportion increases in the erosion products. The granite fraction decreases considerably in the Parktown, Brixton and Promise Formations of the Witwater-

90

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and Planetary Science Letters 133 (1995) 81-94

*3 5 0.6 W

0.2 4.5

4

3.5

3

2.5

Minimum depositional age (Ga) Fig. 5. Eu/Eu’ vs. minimum depositional age for Archean metasediments with Eu/Eu’ 5 1.0. For sources of depositional age see the appendix. The evolving array for minimum Eu/Eu* values is represented by solid lines, which includes all samples with lowest Eu/Eu* at a given age (except two Malene samples at 2.75 Ga). Heavy plus signs represent average Eu/Eu* values for each age group. Shown for comparison are Eu/Eu * ratios of the post-Archean upper continental crust (PAUC), post-Archean Australian Shale (PAAS) [l], North American Shale Composite (NASC) [Sl], postkchean Qinling Shale (PAQS) [261, and average Proterozoic (PTGR) and Phanerozoic (PHGR) granites [9].

srand Supergroup because these successions represent an even deeper level where tonalite dominates. 4.2. Implications for the euoiution of negative Eu

anomalies in the upper continental crust The average Eu/Eu* value for the post-Archean upper continental crust is about 0.65 (see the box on the right-hand side of Fig. 5). This negative Eu anomaly has developed from a primordial crust without a Eu anomaly originating from the mantle. An attempt is made to estimate the evolution of the Eu/Eu * ratio in the Archean upper continental crust. This is done by relating values and group averages ratios with minimum depositional of the Eu/Eu* ages (see the appendix) of metasediments in the Archean. An extrapolation from younger to older Archean metasediments can only be tentative because of the small number of data for Early Archean sediments. In Fig. 5, only metasediments with Eu/Eu* ratios of I 1.0 are plotted, as constrained by the average compositions of Archean magmatic rocks described above. As shown in Fig. 5, the

average Eu/Eu * ratio decreases with decreasing age. The value of 0.64 approaches the post-Archean average Eu/Eu * at the end of the Archean. For a better understanding of the shift in the negative Eu anomaly with age we have arbitrarily separated seventeen plots which represent the lowest Eu/Eu* ratios per age group. These occur within the belt marked by the three lines in Fig. 5. The heavy line is constructed from equalising the group averages. We have no inclination to overestimate the importance of such a construction. The tentatively drawn line connects a point with absence of Eu anomaly at 4.5 Ga and a point with a Eu/Eu* ratio of 0.34 at 2.5 Ga. The Eu/Eu * ratio of 0.34 is surprisingly close to the ratios of 0.47 and 0.36 for average Proterozoic and Phanerozoic granites compiled, respectively, by Condie [9] and Wedepohl [7]. The evolution of the Eu anomaly can be observed in the shift of group averages and minimum values of Eu/Eu * with minimum age of the respective samples. An exception is the average Eu/Eu* value of the group at a minimum age of 2.7 Ga, which is considerably biased to the Abitibi metagraywackes. Because Eu anomalies in elastic sediments obviously reflect change of mixtures of juvenile crustal additions without Eu anomalies generated at active tectonic settings and of recycled crustal materials with significant Eu anomalies characterizing evolved stable cratons, the trend of decreasing Eu/Eu * with decreasing age suggests the progressively decreasing importance of active tectonic settings at the expense of growing cratons in the Archean. Our investigation indicates that granites formed fairly early in the Archean (not later than the registered sediments at 3.8 Gal.

5. Conclusions The observed trends in correlations of Eu/Eu* with CIW/CIA are explained as indicating that Archean pelites with high CIW and negative Eu anomalies were derived from distant, large provenances with a significant proportion of granite. In terms of the properties discussed, the sediments are representative of the Archean upper continental crust. The granitic source no longer existed in its original proportion due to its dominant occurrence in the upper crustal layers, where erosion and decomposi-

S. Gao, K.H. Wedepohl/ Earth and Planetary Science Letters 133 (1995) 81-94

tion were more prevalent. The negative Eu anomaly of the Archean upper continental crust seems to have evolved gradually, and probably was present as early as 3.8 Ga ago. Thus, the intracrustal fractionation producing granite must have occurred even earlier. The negative Eu anomaly has been a common crustal feature at least since 3.2 Ga. On average, by 2.5 Ga the upper continental crust and the elastic sediments had already acquired a negative Eu signature similar to that of post-Archean times. The Archean-Proterozoic transition is not a distinct boundary with respect to the Eu anomaly. A gradual evolution of the continental crust under the influence of a decreasing importance of active tectonics at the expense of growing cratons occurred during the Archean.

Acknowledgements We thank J. Hoefs for comments on this paper, S.M. McLennan for helpful discussion, and K. Si-

Appendix

mon and G. Hartmann for help in the analysis of the Archean sedimentary rocks from the Kongling, Wutai and Jining terrains. We also thank Y.-K. Hu, W.-L. Lin, Q.-D. Xue and Z.-D. Zhao for field assistance in the Kongling terrain and the North China Craton, and D.J. Wronkiewicz for providing unpublished individual chemical analyses of Archean sediments from southern Africa. Finally, we also thank B.-R. Zhang and T.-C. Luo for initiating projects on the crustal composition of the Qinling Orogen and the central North China Craton with funding provided by the National Natural Science Foundation of China (NSFC) (grant 49290100) and the Ministry of Geology and Mineral Resources of China (MGMRC) (grant 85022121, from which part of our Chinese Archean sediment data were derived. Constructive review comments by N.T. Arndt and two anonymous reviewers are greatly appreciated. S.G. was generously supported by an Alexander von Humboldt Research Fellowship, the MGMRC (grant 8505201) and the NSFC (grant 48900026). [UC]

1

Data sources for Archean

sediments Depositional

Greenstone belt Fig Tree, southern Africa Moodies, southern Africa Pongola, southern Africa Witwatersrand, southern Africa Dwaliie, southern Afica Abitibi, Canadian Shield Pontiac, Canadian Shield Yellowknife, Canadian Shield Wind River, Wyoming Pilbara, Western Australia Dengfeng, southern North China Craton Wutai, central North China Craton Luxi, eastern North China Craton High-grade terrain Isua and Akilia, western Greenland Malene, western Greenland Limpopo, southern Africa Kapuskasing, Canadian Shield Beat-tooth, Wyoming Mount Narryer, Western Australia Jining, northern North China Craton Qianxi, central North China Craton

91

age

3.2-3.3 Ga 3.2-3.3 Cia 2.92-2.96 Ga 2.7-2.8 Ga 3.4 Ga 2.7 Ga 2.7 Ga 2.7 Ga > 2.7 Ga 2.9-3.4 Ga 2.5 Ga 2.5-2.8 Ga 2.5 Ga 3.8 Ga > 2.75 Ga > 3.2 Ga - 2.7 Ga 2.7-3.0 Ga 2.7-3.1 Ga > 3.0 Ga > 3.4 Ga

Data source

[12,13,52,531 112,131 112,541 112,541 1141 1431 144,451 11,551 153,561 11,571 1291, a 1301, a 1311

[WI [23,24,251 1321 1321 121 119,321 158,591, a 1601

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and Planetary Science Letters 133 (1995) 81-94

Taihau, southern North China Craton Kongling, Yang&e Craton, South China a Unpublished

> 2.5 Ga > 2.5 Ga

data from the authors.

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