Chapter 22 How do we Recognize plate tectonics in very old Rocks?

Chapter 22

HOW DO WE RECOGNIZE PLATE TECTONICS IN VERY OLD ROCKS? W. S . FYFE ABSTRACT It is considered that Archaean volcanic and metamorphic patterns indicate more closely spaced, mantle magma penetration sites than for the present earth. If this is valid, lateral lithosphere motions may also have been smaller. It is considered that “hot-spots” may have been quantitatively more significant in the past. As there is mounting evidence that light materials can be subducted, models involving steady increase in the mass of continental crust must be questioned. In particular, the lack of aluminous blueschists may indicate massive recycling of pelagic sediments. It is suggested that the scale of mixing interactions between hot and heavy mantle magmas underplating crust and anatectic highlevel melts may have been underestimated and may have played a major role in the generation of the ancient grey gneiss complexes,

INTRODUCTION

The purpose of my brief contribution to this volume is to consider the question involved in the above title. Put another way we ask the question as to whether or not the presently operating convective style of the earth is adequate to explain the ancient geologic record. I think that when one approaches the problem of the nature of processes in the early earth, we must first remind ourselves of how recently we discovered plate tectonics, and of the types of observations that were vital t o the acceptance and quantification of this model. We must remember too, that when we go back in time, the sample of unmodified early surface is very small. Let us imagine for a moment that we are presented with a sample of only a few percent of the earth’s surface; that this surface has no topography, its rocks carry no reliable palaeomagnetic record and heat flow cannot be measured. Accept too that we do not have the tools of seismology, that we do not know the dimension of either continents or oceans, that there i s n o fossil record and that we can measure ages only with a precision of say 100Ma. Given such limitations of observation for the modern earth, would we have discovered plate tectonics? These are the problems we face when we consider ancient tectonics and we are forced to concentrate on only those parts of the record which are likely to be preserved and those parts of the record with unambiguous significance. It is clear that ancient tectonics must involve primarily the mineralogist,

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550 petrologist, geochemist and structural geologist; the full power of modern geophysics will play a smaller role, and quantification of ancient dynamic processes must be very difficult. I think there are certain fundamental starting points for our approach to the ancient earth. For me, one such point is John Elder’s (1976) concept of “roll-over time”, residence time or the time it takes t o modify old surface or form new surface. At the present, this time for oceanic crust is in the order of l00Ma. As stated by Uyeda (1978, p. 120) “since the Mesozoic era, nearly lOOOOkm of oceanic plate along with ocean ridges seems to have descended beneath Japan”. Given such a modern observation, it is possible that two fragments of Archaean surface now welded together, could have been “around the world” in the time contained in the errors of absolute dating. If so much new oceanic crust can form in something like 100Ma at present rates, what of the past? The early earth must have been much more energetic. Exactly how much depends on the model of accumulation and dissipation of energy (Runcorn et al., 1977; Runcorn, 1978). But prior t o 3Ga ago, a roll-over time in the order of 20Ma might have been possible. Thus, given the uncertainty of absolute age dating of old rocks, correlation of events across fragments of ancient surface may be misleading. I think there is another very vital consideration in our approach to the ancient earth. When objects cool at the surface by internal convective processes, these processes are more turbulent and temperature gradients are more erratic or noisy, if the body is hotter and is losing more heat (see Elder, 1976). This implies that events in the early earth would be less regular than for the present, and tectonic styles should also be more varied (see Young, 1978). The present earth operates in a rather smooth manner. Basalt produced in the hot mantle invades the crust at widely separated ocean ridges. Basaltcrust production leading to ocean-floor spreading proceeds at a rather constant rate. The new crust cools largely by seawater convection which may penetrate almost to the oceanic Moho (Lewis and Snydsman, 1977): As a result, thermal gradients and heat flow are drastically perturbed. Oceanic lithosphere thickens by cooling as it moves away from the ridges, eventually becoming heavier than the underlying asthenosphere and sinks at subduction zones. As the lithosphere bends at the trench, it cracks and these cracks may trap sediments and carry them .nto the mantle (Jones et al., 1978). Degassing of the descending slab triggers melting in or above the slab; the melts rise to form the andesite volcanoes. If the mantle melts rise under continental crust this crust may melt and produce granodiorites and the like. There is good evidence that mantle and crustal melts mix on a grand scale (Eichelberger, 1978) and produce hybrid melts. Above the subduction zone, the combination of cool descending plate and h.ot rising magma produce the characteristic paired metamorphic belts of Miyashiro (1961).

551 Our models of this process are quantified by geophysical observations and obvious topographic surface features. Petrological-geochemical arguments on the evolution and changes from basalt -+spilite + blueschist + ecologite + andesite + granite correlate with the tectonic model. All these phenomena ultimately root in concepts of heat production and transfer and the resulting heat-flow patterns. Modem plate tectonics involves vast areas of crust undergoing lateral motion only and small but very long zones where vertical motions dominate. The lateral motions in oceanic crust are quantified by measurements of age, magnetism and heat flow. The vertical motions are defined by heat flow, topography and seismic events. If we are to quantify ancient regimes similarly, then at present all we have to work with are approximate ages, petrological and structural patterns. If such patterns for the ancient surface are not similar to those of the present, then presumably the convective style must also differ. MAJOR QUESTIONS

When I consider the topic of this book certain questions come to my mind which must eventually be answered: (1)Were there vast areas of ocean crust of the modern type? (2) Was this oceanic crust conserved (more or less) as it is at present, i.e. did subduction balance formation at ocean ridges? (3) How were ocean ridges spaced globally if they existed? (4)If subduction occurred was the process similar and how were subduction zones spaced? (5) What was the rate of mantle magma production compared t o the present? (6) Was there great periodicity in magma production and continental growth as some writers have suggested? (7) Were the oceans similar in extent and volume? (8) Were there regions of oceanic and continental crust of similar type to the present? (9) If there was continental crust and oceanic crust, how much of each? It is easy to ask the questions but I doubt if we can adequately answer any at the present time. If we contemplate geological maps of regions of ancient crust (Rhodesia, Canada, Greenland, etc.) and if we assume that apparent age differences of a few Ma may mean synchronous formation, then it is clear that in relation to the modern situation the ancient crust-forming processes were quite different to those of the present. Does Goodwin’s (1978a) geotraverse map in the Superior Province of Canada showing a succession of greenstone and granite belts represent a single fragment of crust or the welded remnants of many mini-plates? If there were miniplates what was between the pieces and how did it move out of the way?

552 POSSIBLE ANSWERS

Perhaps there are a few features of Archaean geology from which some reasonable conclusions can be formed. If we look at typical volcanic successions (see Price and Douglas, 1972; Windley, 1977; Goodwin, 1978a, b, Fraser and Heywood, 1978) in the Archaean greenstone belts we seem to find something that is not modern. First, the successions often contain very high temperature, ultramafic lavas. Mantle which could produce both more or less normal basalts and komatiites must have been convectively turbulent and thermally erratic; large volumes of upper mantle could have been extensively molten. The problems of high-degree melting has been recently discussed by Arndt (1977). In the Archaean volcanic successions, there appears to be a sequential development (sometimes repeated) of volcanic rocks across the entire gamut of compositions from periodotite to rhyolite. All the types we now associate with an ocean ridge and its remote subduction zone are present and appear in the same stratigraphic sequence. And just as now, basalt dominates in volcanism, “granite” in plutonism (Goodwin, 1978b). Today it is common to use the strontium isotope systematics t o decide on a crust or mantle origin for the magmas. But recent observations (Armstrong et al., 1977; Eichelberger, 1978; Taylor and Silver, 1978; Elston, 1978) clearly show that many modern andesitic t o rhyolitic rock types are a product of varying degrees of mixing of crust and mantle. From these studies it becomes increasingly clear that large volumes of mantle melt products do accumulate beneath and in lighter crust, causing crustal melting, and that mixing of the anatectic melt with the mantle fusion product can lead t o hybrid magma types. Eichelberger (1979) has discussed possible mixing mechanisms (see also Fyfe, 1980). I t should be noted that, given the densities of some komatiitic liquids (ca 3,0), it is surprising that they appear at the surface at all. They may have been much more voluminous than indicated by surface manifestations. Magma underplating by basalt or komatiite liquids may well have contributed to the Moho complexity under continents discussed by Oliver (1978). Convective and diffusional mixing of light anatectic melt and dense mantle melt seems probable. Finally, I think it intriguing that, t o my knowledge, a modern type ocean-floor ophiolite has never been reported from an Archaean greenstone belt. I know many who have searched for such complexes. Further, time after time, late granites appear t o invade the greenstone belts. I think that until we find good examples of Archaean ophiolite complexes we cannot discuss modern type ocean-floor spreading or even the existence of modern type oceanic crust. If they exist, there are enough low-grade greenstone belts including rocks formed at the marine interface that we should be able to find them. The examination of Archaean lithologies suggests t o me that if ocean-floor spreading occurred, the scale was small. In Archaean terrains typical ridge

553 and subduction zone products appear almost in the same place at the same time and granitic basement never seems far away. What was the Archaean recycling process? I think that this is one of the greatest questions in Archaean geology. Petrologically we recognize modem subduction zones by features like andesite-granodiorite belts, blueschisteclogite belts and the like. I see nothing in an Archaean geological map to match the modem scale or assembly of rock types. There are no glaucophane schists, no eclogites in the Archaean. In general, low-Tlhigh-P metamorphic facies are not characteristic of the Archaean (see Lambert, 1976). Perhaps there are some clues as to the nature of the Archaean recycling process. First, the eruption of dense, hot magma types like the komatiites may indicate that the lithosphere was much thinner or even that parts of the upper mantle were largely molten. Such liquids would tend to underplate the crust. Second, the appearance of complex magma series from basalt to rhyolite all in one place may suggest that basaltic magma edifices were poured out on thin crust essentially uniquely Archaean, being a mixture of present oceanic and continental crusts. The edifices may have collapsed back into their own plumes. Goodwin ( 1 9 7 8 ~ )uses the term sag-subduction. During this process, more siliceous elements in the base of the crust would melt out t o form the less voluminous andesites, rhyolites and granitic types at the same time homogenizing with voluminous basaltic components. The picture might look more like a modern Hawaii (not on a moving plate), poured out on a granite-basalt sandwich (dominated by basalt), foundering as eclogite forms at the base. Perhaps the trigger for such vertical motions could be the settling of large volumes of deeply crystallized komatiite plus eclogite into hotter peridotite mantle. But fundamental t o any such argument is the probably transient nature of mantle plumes rising through thin lithosphere on a short wavelength pattern. PAN-AFRICAN SUTURES

Many modern studies of late Proterozoic regimes discuss possible evidence for plate collisions and the formation of ophiolite-decorated sutures during this period (Shackleton, 1979; Gass, 1977; Black et al., 1979) (see also Caby e t al., this volume, Chapter 16; Gass e t al., this volume, Chapter 15; Leblanc, this volume, Chapter 17, ed.). Certainly, the magmatic patterns in regions such as the Arabian Peninsula seem quite analogous to those above modern subduction zones. But in such Precambrian regions there is presently no evidence for the high-Pllow-T facies of metamorphism as seen in, say, the Alps or Urals. Most rocks in the vicinity of “sutures” seem to be quite normal greenschist-facies rocks (see Nawab, 1978). Why are the blueschists missing in these older zones where there is otherwise normal magmatic evidence for subduction? To form a massive zone of blueschist-eclogite metamorphic rocks, a wedge

5 54 of sediments and basaltic rocks must be dragged down to depths in the order of 30 km. By the time the lower members achieve a mineralogy dominated by phases like garnet-omphacite-amphibole-lawsonite-kyanite, they are much denser than normal continental crust with most of the minerals having densities of three or more. Why should they rapidly rebound into structural positions such as seen in the Coast Ranges of California? And they must rebound fast to preserve minerals such as metamorphic aragonite (Brown e t al., 1962). Recently Moore et al. (1979) have shown that trench sediments tend to underthrust the earlier members of an accretionary wedge. The potential for uplift of early members deeply depressed thus increases the length of time over which the subduction process operates. Thus failure t o produce observable blueschist zones may simply reflect the size of the oceans which closed. A highly controversial subject involves possible sediment subduction. Recent writings of Gilluly (1971), Sibley and Vogel (1976), Garrels and Lerman (1977) suffice t o indicate that many features of sedimentary balances seem t o point t o the possibility of massive subduction. I would stress that many crustal components, when moved into their blueschist and eclogite facies state, are dense materials and if we consider spilitic, andesitic (greywacke) or pelagic sediment starting materials, the final products may all have densities well above 3 gcm-3 which would aid in their subduction. Pelagic sediments (Sibley and Vogel, 1976) are abundant and are Al-rich, but kyanite blueschists are rare, implying that they must be subducted. Uyeda (1979) shows clearly the evidence for pelagic sediment removal and even erosion of the continental plate. CONTINENTAL GROWTH

One of the present great debates is concerned with the growth of continental crust. Perhaps the two extremes of the argument are t o be found in Hargraves (1976) who favours complete early segregation and Moorbath (1978) or McCulloch and Wasserburg (1978) who prefer a steady growth with views being more moderate in Shaw (1976). One strong line of evidence for continental growth involves the analysis of strontium and samarium-neodymium isotope systematics (see f o r example Moorbath and Taylor, this volume, Chapter 20, ed.). But more and more evidence from modern orogenic belts shows that the interpretation of these data is complex and that, at present, crust mantle mixing is more pervasive than previously thought (Eichelberger, 1978; Taylor and Silver, 1978). As I have discussed previously (Fyfe, 1978) we do know that some “dissolved” continental material delivered to the ocean (at a rate of about 4 x 10’’ ga-’ ) is fixed in spilites and recycled. If this material is reprocessed through the mantle it will show mantle isotopes. Further, the subduction of light crust is mechanically possible (Molnar and Gray, 1979; Hargraves, 1978; see also

555 Dimroth, this volume, Chapter 13; Kroner, this volume, Chapter 3, ed.). As many writers have stressed (see Windley, 1977) the high-grade basements of many ancient regions are dominated by grey tonalitic gneisses with varying contributions from more basic amphibolites. On the basis of their Sr isotope systematics many writers insist that these represent new additions to the crust from the mantle. But if some modern views on the origin of andesites have merit (Eichelberger, 1978; Fyfe, 1980) then these Archaean rocks may indicate vastly more pervasive mixing at the base of the crust. This might be no surprise if, on average, mantle melts were more basic and more dense. Underplating of scum crust and magmatic layering could produce the complex continental Moho’s described by Oliver (1978). I recall that in the 1960’s my former colleague John Verhoogen (see Verhoogen et al., 1970) was reflecting on continental growth and found that he could come t o no firm conclusion on the matter. In the modern plate-tectonic cycle we can roughly quantify continental growth by the addition of andesites and the like, we know something about Phanerozoic sedimentation rates, but we cannot yet quantify sediment subduction loss. I think that until we do, arguments about continental growth are likely to be to some extent futile. Present data indicate that continental erosion rates (Garrels and Lerman, 1977) far exceed magmatic additions above subduction zones (Brown and Hennessy, 1978). What is urgently needed is more refined seismic studies of the structure of descending slabs, particularly at their upper levels. Here I am most impressed by the recent work of Jones et al. (1978) on the Japan Trench. They report little evidence for massive sediment build-up which should occur if the sediment is scrapped off, and they also indicate cracking mechanisms that may have caused sediment removal. It would take very little cracking of the bending lithosphere at global subduction zones for significant removal of sediments; the cracks must fill with water or sediment (see also Fyfe, 1980). Cook et al. (1980) have shown clearly how thrust tectonics can build continental crust. RAYLEIGH NUMBERS

The convective state of a cooling object is often discussed in relation to the Rayleigh number of the system defined as R = apgh4 / K q ( h the thickness, Q coefficient of thermal expansion, 0 the thermal gradient, g the gravitational acceleration, K the thermal diffusivity and 77 the viscosity). As discussed by Elder (1976, p. 5 3 ) it is possible to classify the type of convection by the amount by which R exceeds a critical value needed for convection. If we look at estimates of R for the present earth (Elder, 1976; Uyeda, 1978) the values are so large that we might expect “chaotic eddying motions”(E1der) or turbulence. This does not appear t o describe the present, rather steady, convective patterns. It does appear more appropriate for the Archaean patterns. If we reflect on the form of R in the above equation, it

556 is clear that certain factors exert a major influence on changes. In particular larger values of the thermal gradient, and even more h , can change R by orders of magnitude. Thus, in the Archaean thicker and hotter (more molten) asthenosphere could greatly increase turbulence and create a high frequency of hot-spots. CONCLUDING STATEMENT

From a knowledge of heat production through time we can approximately quantify the rates of mantle magma production. At present our knowledge of crust destruction, particularly continental crust, is not quantified. We can only be sure that if crust mantle recycling occurs as the earth cools, we must tend to gradually lose continental crust and hydrosphere (Fyfe, 1976, 1978). There is good evidence from Pan-African terrains that the magmatic events of the late Precambrian were similar to those of modern style. But in the Archaean there is little evidence for the modern, large separation of constructive and destructive plate margins. The style of magmatic activity is more like that of the frequent small plumes as depicted by Shaw (1976, Fig. 2). Even given a greater magmatic intensity in the past (3-4 x present) if mantle penetration sites were numerous and closely spaced, lateral motions of the surface crust need not have been extensive. The Archaean recycling process is a mystery but some form of crustal foundering into cooling mantle plumes could be appropriate. Thus, in a sense, ancient tectonics could have been dominated by hot spot phenomena (see for example Lambert, this volume, Chapter 18, ed.) on an essentially stationary thin lithosphere (Fyfe, 1978). Obviously, as Ramberg’s (1967) studies have shown, the thickness of the lithosphere will exert a controlling influence on the frequency of mantle eruptive sites. Komatiites may be telling us that the degree of melting in the asthenosphere was much greater than at present and the viscosity of the mantle much lower; essentially solid mantle of today may not have been present till the Proterozoic (Schwarz and Fujiwara, 1977). Over the past five years or so we have just begun t o appreciate the geophysical and geochemical consequence of “crustal water cooling” (Wolery and Sleep, 1976; Lewis and Snydsman, 1977; Burke and Kidd, 1978; Fyfe, 1978). It is only now that petrologists are becoming aware that basalt erupted is not spilite subducted. Geochemists do not seem t o like massive return flow of “continental” elements (pace Heier, 1978; Moorbath, 1978) but there is no doubt that some does occur. If Aumento et al. (1976) and Sibley and Vogel (1976) are correct, there is massive feedback of uranium and potassium t o the mantle. Such things must be quantified. Thus, while geophysicists and sedimentologists seem to accept the possibility of sediment subduction, geochemists don’t like the added complexity. Again we are all happy about sinking basaltic eclogite, but a quartz eclogite from basaltic andesite will sink just as nicely.

557 The Archaean was dominated by submarine igneous activity and thus all we are learning about modern hydrosphere-crust interaction must be applied t o the Archaean on an even greater scale. We know the oceans were there 4Ga ago, and water cooling must have limited the outer thermal gradients (Burke and Kidd, 1978). If at present 30% of the entire earth’s surface contains presently active geothermal convection (Anderson e t al., 1979) then 3 G a ago, before massive continental emergence (as perhaps indicated by the date of Veizer and Jansen, 1979)’ this figure would easily have approached 100%. The crust of the earth would be modified by mantle underplating and hydrosphere convection over almost its entire surface. Finally, the problem of sialic crustal growth rests on the question of crust-mantle recycling. Here I think we must wait for more refined geophysical studies and drilling in the trench environments. It is with good reason that Uyeda (1978) lists subduction among the “remaining questions”. But if sediments and even andesites are subducted, then we must keep an open mind on the meaning of “age peaks”. These age peaks may correspond t o quantum jumps in convective style of the mantle as suggested by Runcorn (1965). But do we observe quantized creation or quantized preservation (Fyfe, 1978)? Perhaps in the next decades as Ion Microprobe Mass Analyzer techniques for dating reach their full potential (Hinthorne e t al., 1979), we may see a great increase in our appreciation of ancient events. REFERENCES Armstrong, R. L., Taubeneck, W. H. and Hales, P. O., 1977. Rb-Sr and K-Ar geochronology of Mesozoic granitic rocks and their Sr isotopic composition, Oregon, Washington and Idaho. Geol. SOC.Am. Bull., 8 8 : 397-411. Arndt, N. T., 1977. Ultrabasic magmas and high-degree melting of the mantle. Contrib. Mineral Petrol., 64: 205-222. Anderson, R. N., Hobart, M. A. and Langseth, M. G., 1979. Geothermal convection through oceanic crust and sediments in the India Ocean. Science, 204: 828-832. Aumento, F., Mitchell, W. S. and Fratta, M., 1976. Interaction between sea water and oceanic layer two as a function of time and depth. Can. Mineral., 1 4 : 269-290. Black, R., Caby, R., Moussine-Pouchkine, A. , Bayer, R., Bertrand, J. M., Boullier, A. M., Fabre, J. and Lesgner, A., 1979. Evidence for late Precambrian plate tectonics in West Africa. Nature, 278: 223-227. Brown, G. C. and Hennessy, J., 1978. The initiation and thermal diversity of granitic magmatism. Philos. Trans. R. SOC.London, Ser. A, 288: 6 3 1 4 4 3 . Brown, W. H., Fyfe, W. S. and Turner, F. J., 1962. Aragonite in California glaucophane schists, and the kinetics of the aragonite-calcite transformation. J. Petrol., 3 : 566582. Burke. K. and Kidd. W. S. F.. 1978. Were Archaean continental -geothermal gradients much steeper than’those of today? Nature, 272: 240-241. Caby, R., Bertrand, J. M. L. and Black, R., 1981. Pan-African closure and continental collision in the Hoggar-Iforas segment, central Sahara. In: A. Kroner (Editor), Precambrian Plate Tectonics. Elsevier, Amsterdam, pp. 407-434 (this volume). Cook, F. A . , Brown, L. D. and Oliver, J. E., 1980. The Southern Appalachians and the growth of continents. Sci. Am., 243: 156-168.

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