Geochemical and petrological significance of the Archaean- Proterozoic boundary. S. R. Taylor

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1 Geochemical and petrological significance of the Archaean- Proterozoic boundary S. R. Taylor SUMMARY: A significant change in the nature of igneous activity responsible for continental growth apparently occurred at the Archaean-Proterozoic boundary. The sedimentary record is consistent with an Archaean upper crust dominated by the bimodal basic-felsic suite, with both units derived by melting at mantle depths. In contrast, the Proterozoic (and younger) upper continental crust is dominated by K-rich granites, derived by intracrustal melting. This major episodic event produced an upper crust enriched in incompatible elements (e.g. Rb, U) as attested inter alia by the increase in 87Sr/S6Sr in marine carbonates in the early Proterozoic, and the abundance of early Proterozoic uranium deposits. This event seems to have been connected with the widespread initiation of subduction related calcalkaline volcanism, which, since that time, has been the principal contributor to continental growth. Although the change at the Archaean-Proterozoic boundary was the major episodic event in continental growth, minor episodes of cratonization occurred in the early Archaean, and some greenstone belt development lingered into the early Proterozoic. More recent episodes of continental growth (e.g Ma event) appear to have been on a smaller scale. The sedimentary record is transparent to them, and they do not represent any fundamental change in the igneous processes responsible for continental growth. The Archaean-Proterozoic boundary represents the major episodic change in the growth of the continental crust. It appears to separate two fundamentally distinct processes involved in deriving continental material from the mantle. Accordingly it appears that the uniformitarian doctrine does not apply beyond the base of the Proterozoic. Attempts to trace the evolution or change of igneous activity with time are beset by numerous problems, particularly since it is difficult to obtain an accurate idea of relative volumes and hence importance of specific volcanic or plutonic igneous activity as one proceeds back in time. Thus the amount of felsic volcanism in the Archaean may have been underestimated by a factor of about four, due to the fragmental nature of the deposits from Plinian-type eruptions (Thurston et al. 1985, p. 17). Clear evidence of calcalkaline volcanism exist in the Archaean in the Marda Complex ( Ma) for example (Taylor & Hallberg 1977). Does this imply that such volcanic activity was equally important at that time as now? An answer to such problems can be provided by studying the overall evolution of the continental crust as it is reflected in the sedimentary record. This is capable of providing a time-integrated average of the igneous processes responsible for the growth of the crust from the mantle. In this paper, the beginning of the Proterozoic is taken, not at the conventional figure of 2500 Ma, but at the tectonic change from Archaean style terrains to that of the extensive cratons, with their accompanying platform sedi- ments, which characterize the Proterozoic. This change was not isochronous on a world-wide scale, but proceeded over an interval from about 3.1 Ga in southern Africa to 2.5 Ga in the Canadian shield and Western Australia (Taylor & McLennan 1985, Chap. 8). The sedimentary record of crustal evolution This has been discussed extensively by Taylor & McLennan (1985) and only some salient points are noted here. In studying the chemistry of the sedimentary record as a tracer of crustal evolution, it is important to use those elements that are relatively insoluble, with short residence times in sea water. Experience has shown that elements such as the REE, Sc, Th, and occasionally Zr and Hf, are useful in this context, being transferred virtually quantitatively from the upper continental crust into clastic sediments. It should be noted that the sediments contain a record only of the evolution of the upper crust, and do not immediately distinguish between intra-crustal and mantle differentiation. Several significant changes are apparent at the Archaean-Proterozoic boundary. These include' 1 A change in REE patterns in clastic sediments involving the appearance of a significant depletion in Eu in chondrite-normalized REE patterns (Figs 1, 2). There is also a tendency for the total From PHARAOH, T. C., BECKINSALE, R. D. & RICKARD, D. (eds) 1987, Geochemistry and Mineralization of Proterozoic Volcanic Suites, Geological Society Special Publication No. 33, pp. 3-8.

2 4 S.R. Taylor 100 I I I I I I I I I I I I I ~ PinpAAS.X...~ - e Creek Geosyncline -_ I00 0O (D --I-- "(D C O..c 100 O E C).. ID.. I0 E C).. 10 shale - ~ I0 I I I I I I I I I I i l l Le Ce Pr Nd SrnEu Gd Tb Dy Ha Er Trn Yb FIG. 1. The contrast between Archaean and post-archaean REE patterns in clastic sedimentary rocks is illustrated by samples from the Pine Creek Geosyncline, Australia. Group I samples, low in the sequence, display patterns similar to those of average Archaean shales. Group II patterns, from the upper part of the sequence, have REE patterns similar to PAAS (post-archaean Australian shales). REE abundances to increase, as well as a relative enrichment in the light REE. 2 Parallel to the change in REE patterns is a significant increase in Th/Sc ratios (Fig. 3). These two elements are particularly useful indexes. Both are insoluble, with brief residence times in sea water. Thorium is a highly incompatible element, typically enriched in residual silicate melts, while scandium is concentrated in early crystallizing minerals such as pyroxenes. The abundances of these two elements thus provide an index for the relative proportions of acidic to basic material in the sources of the sediments. 3 There is a large increase in the 878r/S6Sr ratio in carbonates (Fig. 4) which reflects the increase in the supply of STRb to the upper crust exposed to weathering. Studies by Veizer (1983) illustrate this major change. This is consistent with a change in upper crustal composition from the mantle-derived Archaean bimodal basic-felsic suite to an upper crust dominated by intracrustal derived granites, granodiorites, rhyolites and other acidic K-rich igneous rocks, enriched in Rb and depleted in Eu. Like the REE patterns in sedimentary rocks, the 87 Sr/S6Sr record in carbonates shows only minor perturbations subsequent I 1 I I I I I I m i J -I.I Eu Eu* I ).6 I I I I I I Age (Ga) FIG. 2. The variation in europium depletion with time is shown in this plot of average Eu/Eu* values for Archaean and younger clastic sediments. There is a major shift at the Archaean-Proterozoic boundary. Eu/Eu* shows little variation from an average value of 0.65 in Proterozoic and younger sedimentary rocks. These data are interpreted as indicating a significant change in upper crustal composition between the Archaean and Proterozoic.

3 - ~o~a\e-'j Significance of Archaean-Proterozoic boundary 5 I I I I I I I 1 I I I I I I I ~.5 i.o 0.5 Age (Ga) ' I 1.0 Th 0.8 Sc 0.6 FIG. 3. Thorium and scandium ratios in fine grained clastic rocks show a major increase at the Archaean- Proterozoic boundary, consistent with a significant change in upper crustal composition between the Archaean and Proterozoic to the great break at the Archaean-Proterozoic boundary. This change is also reflected in the heat-flow data for crustal provinces (Fig. 5) (Morgan 1984). This shows that the Archaean crustal provinces have a lower heat production from crustal sources than do the Proterozoic and younger provinces (Pollack 1980). This effect does not appear to be due to differing levels of erosion, for many lowgrade Proterozoic rocks are still exposed at the present surface (Watson 1976), and there appears to be a step-like break at the Archaean-Proterozoic boundary, rather than an exponential decrease. There appears to be little change after the Lower Proterozoic until tectonothermal effects become apparent in the Lower Palaeozoic. These, due to residual heat from young tectonic processes, have a half-life of about 300 Ma. Archaean and Proterozoic continental crust The changes from Archaean to post-archaean upper continental crust are interpreted as due to a massive intra-crustal melting event which produced an upper crust dominated by K-rich siliceous igneous rocks. This event was preceded, by Ma, by a massive increase in crustal t '1 I I I I I I / River -I.712 water >l - S/.~~ea ~.710 _.~- - - ~ - ~ water Sr jj ~ Sr / Oc ~'~~ I I I I I I I Age (Ga) FIG. 4. Sedimentary carbonates, reflecting sea-water composition, show a sharp increase in 87Sr/S6Sr at the Archaean-Proterozoic boundary (Veizer 1983), consistent with a major increase in the concentration of 87Rb in the upper crust in late Archaean-early Proterozoic time.

4 6 S.R. Taylor I I I I 1 1 I 120 [ I00 8o Heat Flow 60 (mwm_2) 4O -20 i I i I I I I Tectonic Age (Go) FIG. 5. The variation in continental heat flow from regions of differing age (Morgan 1984) shows a peak in young tectonic regions due to orogenesis, and a distinct break between Archaean and Proterozoic terrains, consistent with a lower proportion of heat producing elements (K, U, Th) in the Archaean, compared to the Proterozoic crust. volume (McCulloch & Wasserburg 1978), an episodic event of derivation from the mantle still requiring a fundamental explanation. Of course, continental growth is not limited to a single steplike function. Many minor episodes will contribute to growth but no comparable event appears to have occurred earlier than the late Archaean, or later. The 1800 Ma event does not appear to influence the sedimentary record, and so presumably was similar in nature to the major late Archaean episode both of crustal growth and intra-crustal melting. In this context, the REE record in post-archaean sedimentary clastic rocks appears to be transparent to later crustforming events, indicating that these were both similar in nature to the major crust-forming event at the Archaean-Proterozoic boundary, or on a much smaller scale. The geochemical consequences of the intra-crustal melting event in the late Archaean were profound. The upper crust was enriched in incompatible elements (K, Rb, Cs, U, Th, LREE, Zr, Hf, Ba, etc.) resulting, for example, in an increased supply of 87Sr to the oceans, and providing an upper crust enriched in incompatible elements (Ba, Zr, Hf, Th, U, LREE, etc.) (Table 1). The Upper Archaean crust contrasts with this picture. The sedimentary evidence indicates a more basic crust (Table 1) dominated by the bimodal basic-felsic igneous suite (Taylor & McLennan 1985). The felsic members included both volcanics and intrusive tonalites and trondhjemites, and the Archaean sedimentary rocks reflect this chemistry. Although the average Archaean REE patterns superficially resemble those of modern island arc rocks, many samples reflect local provenance from basic or felsic rocks and the average pattern is consistent with mixing of these two diverse patterns. Further support for this view comes from the distribution of other TABLE 1. Comparison of the composition of the post-archaean upper crust (A ) with that of the Archaean upper crust (B). (Data from Taylor & McLennan 1985) Sc Ti V Cr Mn Co Ni Rb Sr Y Zr Ba La Ce SiO2 TiO2 A1203 FeO MgO CaO Na20 K20 Z A B A B % % O.5 O A Pr 7.1 Nd 26 Sm 4.5 Eu 0.88 Gd 3.8 Tb 0.64 Dy 3.5 Ho 0.80 Er 2.3 Tm 0.33 Yb 2.2 Lu 0.32 Hf 5.8 Th 10.7 U 2.8 B

5 Significance of Archaean-Proterozoic boundary 7 trace elements (e.g. Taylor & McLennan 1985, Figs 7.19, 7.20) which indicate that most Archaean sedimentary rocks are derived by weathering and mixing of the bimodal suite. Felsic volcanics, often erupted in Plinian-style eruptions, are underestimated in the Archaean volcanic record (Thurston et al. 1985, p. 17) but make a large contribution to the sedimentary record. Calcalkaline rocks are certainly present in the Archaean, as shown by studies of the 2635 Ma Marda Complex, which contains andesitic rocks indistinguishable from modern-day high-k calcalkaline rocks (Taylor & Hallberg 1977) except for higher Ni and Cr values. However, these are volumetrically insignificant, and represent localized occurrences. This particular example comes from the late Archaean, and possibly represents a precursor of the large calcalkaline volcanic sequences typical of the Proterozoic and younger eras. Many Archaean rocks which appear to be 'andesitic' in major element chemistry are in fact silicified basalts (MacGeehan & MacLean 1980). The dominating igneous activity in the Archaean thus appears to be that of the bimodal suite. Both basic and felsic members appear to be derived from mantle sources. The tonalites, trondhjemites and felsic volcanics all display steep LREE-enriched HREE-depleted patterns, consistent with the presence of garnet in the residue, implying derivation from mantle depths. Cratonic development in the Archaean Although major crustal evolution appears to have occurred in the late Archaean, minor examples of small-scale development of earlier cratonic areas appear to be preserved in some high-grade (granulite-facies) terrains. Thus some granulite facies metasediments from the Limpopo belt (Beit Bridge-Messina Complex) tentatively dated at 3.4 Ga (lying on the Sand River gneisses), have REE patterns indistinguishable from PAAS (Taylor et al. 1986). The sediments were laid down in a shallow shelf environment, and the pelitic sediments are interbedded with carbonates, quartzites and iron formations (Eriksson & Kidd 1985). The provenance of the clastic sediments must include K-rich granites with negative Eu anomalies and so represent a minicraton. However, this occurrence of K-rich granites must be of limited extent, for other samples from the complex display typical Archaean style REE patterns, with no significant Eu anomaly. Other examples occur in the Wind River Mountains, Montana, and in Greenland (McLennan et al. 1984). Metasediments from other high-grade Archaean terrains (e.g. Kapuskasing) show typical Archaean type REE patterns without Eu depletion (Taylor et al. 1986). This indicates that metamorphism is not changing the REE patterns or inducing Eu depletion. (The Kapuskasing region is a high-grade equivalent of the Wawa greenstone belt.) Accordingly, there were minor examples of cratonization in the early Archaean but these areas were not extensive enough to contribute an Eu depletion signature to the greenstone belt sedimentary record. They must have comprised less than about 10% of the provenance of typical Archaean sedimentary rocks. In the late Archaean, massive growth from the mantle of continental crust occurred. This was followed within Ma by intra-crustal melting to produce a granitic-granodioritic upper crust. The early Proterozoic sedimentary sequences record this event. Clastic sediments show REE similar to those of modern-day marine muds, and the upper continental crust assumed its present composition, although minor examples of greenstone belt terrains persisted into the early Proterozoic (Gibbet al. 1986). Consequences for volcanism and mineralization Present day crustal growth appears to be dominantly by accretion of island-arc calcalkaline volcanics and plutonics. This scenario differs from the Archaean bimodal basaltic-felsic mechanism which may have been responsible for contributing 80% of the continental crust. What was the cause of this change in style, from the dominating greenstone belts in the Archaean to island-arc calcalkaline volcanics of the Proterozoic and younger eras? This must be related to the presence of massive continental rafts after the late Archaean event. These unsubductible masses formed a barrier to sea-floor spreading, which had proceeded with less hindrance in the Archaean. This, coupled with the increased heat flow, led to the existence in the Archaean of many small plates (Pollack 1980). Oceanic basaltic crust was returned rapidly to the mantle, and a small proportion was melted to produce tonalites and trondhjemites, forming small continental nuclei. Occasional intra-crustal melting produced small cratons, preserved now in some high-grade Archaean terrains. The massive late Archaean period of continental growth, and the decline in

6 8 S.R. Taylor heat flow, led to the formation of a few large plates and the formation of linear subduction zones, present-day type plate tectonics and calcalkaline volcanism. The late Archaean massive intra-crustal melting event transported large amounts of incompatible elements (K, Rb, Cs, Th, U, Zr, Hf, LREE, etc.) upwards in the crust. This led to differences, noted earlier, in the supply of radiogenic Sr to the oceans, marked by the increase in STSr/S6Sr in marine carbonates at about the Archaean- Proterozoic boundary. The increase in the supply of incompatible elements in the upper crust, subject to erosion, accounts, for example, for the abundant Lower Proterozoic uraniferous conglomerates. References ERIKSSON, K. A. & KIDD, W. L Sedimentologic and tectonic aspects of the Archaean Limpopo Belt. Geological Society of America Program Abstracts, 17, 575. GIBB, A. K., MONTGOMERY, C. A., O'DAY, P. A. & ERSLEY, E. A The Archaean-Proterozoic transition: evidence from the geochemistry of metasedimentary rocks of Guyana and Montana. Geochimica et Cosmochimica Acta, 50, MACGEEHAN, P. J. & MACLEAN, W. H An Archaean sub-seafloor geothermal system, 'calcalkali' trends and massive sulphide genesis. Nature, 286, MCCULLOCH, M. T. & WASSERBURG, G. J Sm- Nd and Rb-Sr chronology of continental crust formation. Science, 200, MCLENNAN, S. M., TAYLOR, S. R. & MCGREGOR, V. R Geochemistry of Archean metasedimentary rocks from West Greenland. Geochimica et Cosmochimica Acta, 48, MORGAN, P. W The thermal structure and thermal evolution of the continental lithosphere. Physics and Chemistry of the Earth, 15, POLLACK, H. N The heat flow from the Earth: a review. In: DAVIES, P. A. & RUNCORN, S. K. (eds) Mechanisms of Continental Drift and Plate Tectonics, Academic Press, London, TAYLOR, S. R. & HALLBERG, J. A Rare-earth elements in the Marda calc-alkaline suite: an Archean geochemical analogue of Archean-type volcanism. Geochimica et Cosmochimica Acta, 41, & MCLENNAN, S. M The Continental Crust: its Composition and Evolution. Blackwell Scientific Publications, Oxford., RUDNICK, R. L., MCLENNAN, S. M. & ERIKSSON, K. A REE patterns in Archean high-grade metasediments and their tectonic significance. Geochimica et Cosmochimica Acta, 50, THURSTON, P. C., AYRES, L. D., EDWARDS, G. R., GI~LINAS, L., LUDDEN, J. N. & VERPAELST, P Archean bimodal volcanism. Geological Association of Canada Special Paper, 28, VEIZER, J Trace elements and isotopes in sedimentary carbonates. Reviews in Mineralogy, 11, WATSON, J. V Vertical movements in Proterozoic structural provinces. Philosophical Transactions of the Royal Society, Series A, 280, STUART ROSS TAYLOR, Research School of Earth Sciences, Australian National University, Canberra, Australia.