Encyclopedia of Marine Geosciences

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| Editors: Jan Harff, Martin Meschede, Sven Petersen, Jörn Thiede

Ancient Plate Tectonics

  • Stephen F. FoleyEmail author
Living reference work entry
DOI: https://doi.org/10.1007/978-94-007-6644-0_126-1

Keywords

Continental Crust Subduction Zone Ocean Crust Plate Tectonic Greenstone Belt 
These keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.

Definition

Plate Tectonics

Plate tectonics is a theory which attempts to explain all forms of geological and geophysical observations of the Earth through time in terms of the movement of rigid lithospheric plates that form the upper few tens of kilometers of the Earth. These observations include rock types and their spatial and temporal relationships to each other, their structures and the formation of mountains, the chemical and physical properties of rocks, as well as seismicity, volcanism, and even the distribution of fauna and flora. These lithospheric plates are made up of the crust and the uppermost layers of the mantle (the mantle lithosphere) that are mechanically coupled and can therefore move as a unit on top of convection currents within the mantle. Although moving at rates of only a few centimeters per year, these convection currents explain the movement of continents across the surface over geological time. The crust involved varies from thin ocean crust (around 7 km) to thick continental crust (25–100 km, averaging 35 km). Many plates, such as the South American plate, are topped by extensive tracts of both. When mantle lithosphere is included, some plates may reach total thicknesses of 250 km in the older cores of the continents.

The fact that all geological processes on the modern Earth are thought to be explainable by plate tectonics does not necessarily mean that this was the case for earlier periods in Earth history when the planet was significantly hotter. In deciding whether this was the case, we have a major problem concerning the preservation of rocks from the first half of Earth history. The production and consumption of ocean crust is axiomatic to the operation of plate tectonics, and yet the oldest ocean crust on the Earth’s surface today is less than 180 million years old, which is equivalent to only 4 % of the age of the Earth. Linked to this uncertainty is the growth rate of the continental crust, which is thought by most geoscientists to have been gradual or episodic over the 4.57 Ga age of the Earth, with perhaps less than half of it in existence at the end of the Archean period 2,500 myr ago.

History of Plate Tectonics

The term plate tectonics first emerged at the end of the 1960s, following a turbulent decade in which many branches of the geological sciences reorganized their understanding of geological processes to fit this single all-encompassing theory. Its forerunner was continental drift, which followed centuries-old observations such as the similarity of coastline form between Africa and South America, and hypothesized the lateral movement of continents over thousands of kilometers. Continental drift, most commonly associated with the work of Alfred Wegener, remained controversial for several decades owing to the lack of an explanatory mechanism. It received support from the then new science of paleomagnetism in the 1950s, which demonstrated the movement of continents around rotation poles (Runcorn, 1959), but it was the introduction of marine geology and geophysics in the late 1950s that led to the breakthrough. Magnetic stripes were discovered on the ocean floor (Raff and Mason, 1961) and eventually proved to be symmetrically distributed across mid ocean ridges (Pitman and Heirtzler, 1966). The concept of sea-floor spreading was born (Vine and Matthews, 1963), once again linked to rotation around poles that also explained the newly discovered transform faults between ridge segments (Wilson, 1965). However, if sea-floor spreading occurs commonly on a planet of constant size (there were a few who denied this; Carey, 1975), then the ocean crust produced must also disappear again, and this led to the concept of subduction zones (Le Pichon 1968), where sinking ocean crust is delineated by seismically active zones below island arcs (Isacks et al., 1968). Once the mathematical explanation of the rotation of rigid lithospheric plates around poles and the catalog of possible triple junctions was in place (McKenzie and Parker, 1967; McKenzie and Morgan, 1969), all that remained was to fit regional geology including mountains, rifts, and continental margins into the model to explain the geological history of Earth through time (Dickinson, 1970; Dewey and Bird, 1970). For the last 40 years, plate tectonics has been the single complete geological theory into which all geological and geophysical observations are expected to fit.

Mechanisms of Plate Tectonics

In order to decide whether plate tectonics functioned either differently or even at all early in Earth’s history, it is necessary to consider how plate tectonics is thought to work and what characteristics lead us to decide in favor of it. Some of the features described earlier in favor of continental drift, such as the similarity of coastlines between continents, the movement of continents across the surface of the globe, and polar wander curves defined by paleomagnetism, are compatible with plate tectonics but do not require it. The underlying mechanism lies in the distinction between the rigid upper lithosphere and the ability of the asthenosphere below it to flow and requires convection currents in the mantle below the lithosphere. Forces that act to move the plates are (1) the drag caused by partial coupling of the asthenosphere to the lithospheric plates above it, so that the plates follow the convection movements; (2) phase transformations in the subducting crust and lithosphere, which increase the density of the rocks and therefore cause them to sink through the mantle (known as “slab pull”); and (3) a lateral “ridge push” force imparted by the mountains of the mid ocean ridges where ocean crust is formed; new crust is formed at the top of the ridge and so causes a gravitational force pushing the slightly older crust to both sides. The slab pull force is due mainly to two reactions; the conversion of basalt and gabbro to eclogite (about 20 % denser) in the upper 30–40 km and the transformation of olivine to a high-pressure spinel structure at around 400 km depth (12 % density increase).

For the discussion of whether plate tectonics operated earlier in Earth history, mantle convection will certainly have occurred and volcanism may have been more abundant at any particular time, but the second-phase transformation of the slab pull force will only have functioned if subduction to depths of more than 400 km occurred.

The Evolution of Temperature in the Earth: The Background to Global Tectonic Regimes

Since mantle convection is an essential prerequisite for plate tectonics, and convection will occur once a given threshold is reached (a Rayleigh number of more than 2,380), the state of internal heat of the Earth as a function of time is the deciding factor. The blue curve in Fig. 1 shows the approximate expected heat evolution of the Earth. The colored background panels refer to the four Eons, the Hadean before the first preserved rocks, the Archean until 2,500 Ma, the Proterozoic, and the Phanerozoic, which started with the massive preservation of skeletal fossils at 542 Ma. The blue line begins with the growth of the planet during the accretion of the solar nebula at around 4,567 Ma. At least 85 % of the internal heat of the planet is derived from accretion energy (collisions between small planetary bodies and planetesimals) and the separation of the metallic core: if the moon was formed by a major impact at about 4,500 Ma as considered likely by most scientists (Lee et al., 1997), then this figure is much more than 90 %. The gradual loss of heat after planet formation and the end of collisions is due to two factors: the loss of heat to space due to conduction through the crust and the gradual radiogenic decay of isotopes, particularly K, U, and Th, which decreases with time leading to a flattening of the curve as the parent isotopes become rarer. The green bar shows a postulated “plate tectonic window” (Condie, 1989) in which plate tectonics may operate. Vigorous convection would follow the accretion process and the great loss of heat at this time is unlikely to have allowed the survival and orderly movement of plates. At the other end of the timescale, continued loss of heat and diminishing heat production from radioactive decay will cause the Earth to drop out of the plate tectonic window and form a one-plate planet sometime in the future; the time at which this may happen is not known.
Fig. 1

Evolution of planetary temperature (blue line) indicating entry into the plate tectonic window (green box) in the mid Archean from hotter conditions resulting from planetary accretion. Crustal formation in the period labeled “early plates?” may have occurred by melting of the base of thick crust without modern-style subduction. The red line indicates major episodes of continental crust formation deduced from the ages of igneous rocks. See text for further explanation

Note that there are many unknowns in the form of the curve and that it should only be taken as a conceptual illustration. Two important unknowns are the rate of decrease after the accretion peak and the degree to which episodic processes may cause humps in the curve (Davies, 1995). The height of the peak is poorly known, but accepting the giant impact model for the origin of the moon (Canup and Asphaug, 2001), the age of the moon at 4,500 Ma must correspond to the existence of a magma ocean in the Earth that may have been hundreds of kilometers thick, resulting in extremely hot conditions near the surface of the early Earth. However, this state may not have lasted for very long, as investigations of the oxygen isotope compositions of the earliest rocks and minerals (zircons) find that they formed in cool conditions in siliceous crustal rocks (that have since been destroyed) near the Earth’s surface (Valley et al., 2002). Since the oldest of these zircons are 4,404 Ma (Wilde et al., 2001), there appears to be only 100 million years for the Earth to progress from extremely hot to cool conditions. The other major simplification in the curve is that it represents the whole Earth and implies a gradual, smooth evolution without any episodic or catastrophic developments. This contrasts with the red line superimposed on Fig. 1, which shows the rate of production of new continental crust as gauged by the ages of igneous rocks on the same timescale (Hawkesworth and Kemp, 2006). Although there is some discussion as to whether this really indicates ages of crustal growth rather than merely its survival, there appear to be major periods of crustal formation in the late Archean (3,000–2,500 Ma) and in the early Proterozoic (2,000–1,750 Ma), with an additional pulse at around 1,100 Ma. There may have been a similar episodism in plate movements and subduction (O’Neill et al., 2007). These two uncertainties emphasize that the slope of the curve and its exact form are debatable, and therefore, the point marked for entry into the plate tectonic window at around 3,000 Ma can only be approximate, although there is increasing evidence for the onset of modern-style (i.e., steep and deep) plate tectonics at around 3,200 Ma (Shirey and Richardson, 2011; Van Kranendonk, 2011; Dhuime et al., 2012).

The other open question is that if plate tectonics did begin at about this time, then what happened before that? Was there some other form and movement of lithospheric plates that is distinguishable from the modern style (indicated by the light green band in Fig. 1; Brown, 2007), or could small plates tip towards each other like ice floes in the Antarctic, or was there a thick lid of crust that was recycled into the mantle by eclogite dripping from its base?

An Outline of Archean Geology

To adequately assess whether plate tectonics could have operated during the first half of Earth history, it is important not just to ascertain the presence or absence of indicators for plate tectonics but also to explain the Archean geological record. Archean rocks consist of about 80 % high-grade gneisses, mostly summarized as the tonalite-trondhjemite-granodiorite (or TTG) suite, and 20 % greenstone belts with lower metamorphic grade. The gneisses have igneous origins and can be explained by melting of basaltic protolith material (Foley et al., 2002; Rapp et al., 2003). Their deformation often indicates lateral compression, although in some regions, most notably Western Australia, vertical movements may have been important (Chardon et al., 1996). On the modern Earth, tonalitic melts are formed by melting of basaltic material at subduction zones, and so a subduction environment is the most commonly invoked option to explain them in the Archean. Greenstone belts consist principally of volcanic rocks and sediments wedged between domes of high-grade gneisses. Their most famous constituents are komatiites, which are magnesium-rich mafic volcanics that are almost exclusively Archean in age (Arndt and Nisbet, 1982). However, the majority of volcanics in greenstone belts are more “normal” basaltic to felsic rocks, and not komatiites. The sediments are dominantly texturally immature, whereas shelf sediments such as carbonates and mature sandstones, although present back to 3.5 Ga, are much less common (Eriksson et al., 1994). This would be expected on a planet on which island arc environments and/or volcanic plateaux are common but continents are rare; the first continents would be formed by the amalgamation of island arcs, and all sediments that are preserved would be formed close to these arcs. The rarity of shelf sediments may correspond to the rarity of continents. The relationship between the high-grade gneisses and greenstone belts ought, therefore, to be of paramount interest, but unfortunately, most contacts between them are the locations of later tectonic movements. Examples exist, however, that show basal unconformities, leading to the conclusion that greenstones commonly do not represent ocean crust but were deposited on continents (Bickle et al., 1994; Buick et al., 1995).

There are also changes in rock types towards the end of the Archean; following the peak in continental crust formation at 2.7 Ga (Fig. 1), more potassic granites that are typical for later periods in Earth history become most common, often appearing to seal large sections of the newly formed crust together into stable continents. Other rocks remain notably absent – eclogites and blueschists – which are indicators of high-pressure metamorphism at low pressures, and do not appear until around 2,100 Ma.

The Characteristics of Plate Tectonics

From theoretical expectations, we may consider how we can decide if plate tectonics operates on a planet and then deliberate about whether these features are relevant for the early Earth. Table 1 lists the main features by which the operation of either continental drift or plate tectonics can be recognized on the modern Earth. The problem in ascertaining the relevance of plate tectonics for the Archean and Proterozoic Eras is that most of these criteria depend either on geophysical measurements that only provide a picture of the Earth today or on good preservation of rock associations and geological structures. Much of the acceptance and detail of plate tectonics is due to seismological and other geophysical surveys that provide information on all levels of the deep Earth, documenting seismic discontinuities that delineate the size of core and mantle, that the outer core is molten (Fowler, 2005), and the densities of rocks that constitute these layers (Anderson, 1989).
Table 1

Criteria for continental drift or plate tectonics. Due to the young age of preserved ocean crust, the effects of metamorphism and deformation, and the restriction of geophysics to the present, most evidence for ancient plate tectonics is indirect

Feature

Evidence for

Applicable to Archean?

Geophysical, geomorphological, and geological indicators on the modern Earth

Matching coastlines

Continental drift

No

Same fauna and flora on different continents

Continental drift

No

Polar wander curves

Continental drift

Maybe, but very limited

Magnetic stripes/reversals

Sea-floor spreading

No

Earthquake zones under arcs

Subduction

No

Seismic tomography

Deep subduction

No

Geological indicators for plate tectonics in the past

Hadean zircons

Continental crust

Yes

Linear mountain ranges

Continental collision

Yes

Shelf sediments

Continents and their margins

Yes, but restricted

Continental rift rock associations

Continents and lateral movements

Yes, but continents restricted

Large transcurrent faults

Lateral plate movements

Yes, but controversial

Ophiolites

Ocean crust

Yes, but controversial

Geochemical signature of subduction in igneous rocks

Subduction

Yes, but controversial

Arc igneous rock associations

Subduction

Yes, but restricted

Eclogite

Subduction

Yes, but missing at the surface

Blueschist rock belts

Subduction (cold)

Yes, but missing

Sodic granites

Subduction

Yes, but controversial

Paired metamorphic belts

Subduction and collision

Yes, but restricted

Accretionary prisms

Subduction

Yes, but continents restricted

However, few of the criteria for either continental drift or plate tectonics can be successfully used for the Archean or early Proterozoic, which according to Fig. 1 is the time period where plate tectonics is questionable. Of geophysical measurements, seismological records are little more than 100 years old and cannot be carried out in retrospect; only the science of paleomagnetism, which first documented continental drift (Runcorn, 1959), can be used in the distant geological past (Strik et al., 2003), but even here, the accuracy and certainty of interpretations diminishes as we go back into the Archean and requires coupling to accurate age determinations for earlier times (Evans and Pisarevsky, 2008).

We are left with the indirect geological indicators in the second part of Table 1 that rely on the interpretation of the rock record. A cursory look shows that most of these are potentially applicable, but either their interpretation is to some extent controversial or their application is restricted. This restriction is mostly temporal, meaning that there is a time limit for each before which they cannot be unambiguously recognized. This may be because they did not exist or because of the effects of deformation and metamorphism (the large majority of early Archean rocks have been highly deformed and metamorphosed at medium or high temperatures) or even their complete destruction. Many make a first appearance between 3.3 and 2.6 Ga, but a first appearance must be taken to be a minimum age as they may have existed beforehand but no longer be recognizable. An example of differential preservation is the earliest eclogites, which are evidence for metamorphism of basalts or ocean crust at very high pressures. If we were to restrict our attention to those preserved in collision zones on the continents, then we would conclude that the oldest are younger than 2.9 Ga (Mints et al., 2010), whereas they are common as xenoliths trapped in the mantle lithosphere from 3.3 Ga, where they are interpreted to represent pieces of subducted ocean crust (Jacob, 2004).

The criteria in Table 1 address continental crust, ocean crust, and the subduction process, which raises an important point for discussion of plate tectonics. The hypsometric curve, which shows the distribution of crustal levels above and below sea level on the Earth, shows a bimodal distribution on the Earth, with most crust around 200 m above or 3–4 km below sea level (Fig. 2). This is equivalent to the distinction between continental and oceanic crust and can be taken to indicate the operation of plate tectonics over several hundreds of millions of years. Before plate tectonics began to function, the term “ocean crust” may have little meaning: there may have been just “crust,” broadly of mafic composition, and the huge variety of crustal elevations may not have been present.
Fig. 2

The hypsometric curve for the Earth today, indicating that most crust is slightly above sea level (continental crust) or 3–4 km below sea level (oceanic crust). This bimodality of elevations is a logical consequence of the operation of plate tectonics for several hundred million years

The time and rate of formation of the continental crust is a controversial topic, but currently most geologists accept episodic formation and accumulation of the continental crust through time (red curve in Fig. 1). This would mean that there was less than half the current volume of continental crust until a major phase of production between 3.0 and 2.5 Ga at the close of the Archean. The main controversy lies in the degree to which former continental crust may have been recycled into the mantle (Scholl and von Heune, 2007), such that the alleged crustal formation rate really illustrates the rate of crustal survival. Added to this uncertainty is the high degree of deformation and metamorphism in many Archean rocks, which hampers the interpretation of their formation. The existence of zircons dating from the Hadean (4.4–4.0 Ga), although preserved in younger rocks (Wilde et al., 2001), is thought to demonstrate the existence of some siliceous crust at the time, but does that have to be continental crust, or could it be merely siliceous scum on pre-plate tectonic crust? Small sections of felsic crust on large rafts of mafic crust may have been too insignificant volumetrically to prevent wholesale recycling of the crust and its remixing into the mantle.

In terms of structures, plate tectonics results in linear collisional belts of mountains and large strike-slip faults, but taken alone these are indications of lateral crustal movements, but not necessarily plate tectonics. Shelf sediments and rock associations typical of continental rifts are rare until the late Archean, but one cannot expect these before continental crust itself becomes abundant.

The best evidence for ocean crust takes the form of ophiolites, which are rock packages consisting of basalts and gabbros of the ocean crust underlying marine sediments and overlying ultramafic rocks of the uppermost mantle. Opinions differ greatly as to the first appearance of ophiolites; some scientists require the complete package to be present, whereas basalts with an oceanic geochemical signature suffice for others. The first relatively complete ophiolites date from around 2.0 Ga (Scott et al., 1991; Peltonen and Kontinen, 2004), whereas partial ones are claimed for the earliest Archean (de Wit, 1998). Due to the ephemeral nature of ocean crust, it is much more poorly preserved than continental crust, and some sections of mafic crust interpreted as ocean crust may be from the pre-plate tectonic mafic crust. The eclogite xenoliths already mentioned contain clear evidence of having been at the surface and were later returned to the mantle – but by which process? This may not be clear evidence for subduction in the modern plate tectonic style.

Most entries in Table 1 consider the subduction process, as this appears to be the best key to indicating that plate tectonics functioned. All of these either suffer from the preservation problem or are only indirect indictors. The high-pressure, low-temperature metamorphism characteristic of modern subduction produces eclogites and blueschists, and these ideally occur as part of paired metamorphic belts together with low-pressure, high-temperature metamorphism formed at the other side of the subduction suture (Miyashiro, 1961). Blueschists are not known from before 1 Ga (Stern, 2005). However, this time limit addresses large occurrences that can be recognized in the field. Evidence is found for much earlier high-pressure, low-temperature metamorphism as relict features preserved on a microscopic scale in rocks that have been re-equilibrated at lower pressures. These can be used to show that the duality of thermal indicators across subduction sutures can be recognized as far back as 3.26 Ga (Moyen et al., 2006), but at this time the temperature difference between the two sides was considerably smaller than in Phanerozoic times. Before 3.26 Ga, the pair of thermal indicators has not been found, and no relicts for high-pressure, low-temperature metamorphism are known (Brown, 2007; van Kranendonk, 2011).

The most widely used evidence for tectonic settings in old rocks is indirect and concerns the recognition of trace element geochemical signatures reminiscent of modern plate tectonic environments such as island arcs, mid ocean ridges, oceanic plateaux, and back arc basins (Smithies et al., 2005; Pearce, 2008). However, these are prone to overinterpretation, as they are not foolproof on the modern Earth: for example, continental flood basalts are known to have low-Ti characteristics normally associated with island arc volcanism and calc-alkaline rocks that are characteristic of magmatic arcs above subduction zones also been found in non-subduction settings (e.g., Hooper et al., 2002; Willbold et al., 2009) are known. There is a tendency to engrave interpretations from modern settings onto ancient rocks, although most Archean rocks give mixed signals (Pearce, 2008). But here again, the oldest arc-like greenstones appear to be between 3.2 and 3.1 Ga (Smithies et al., 2005). An additional option is to use rock types characteristic for subduction zones, which include andesites, boninites, shoshonites, and adakites. Most of these become frequent in the late Archean (Condie and Kroner, 2008), whereas adakite is a rock type from modern subduction zones (island arcs and Andean-type subduction beneath continental crust) that is similar to tonalitic and trondhjemitic gneisses that make up large tracts of the Archean continental crust. These are thought to indicate a subduction environment because they do not form in modern plume-related environments (Martin, 1999).

A Different Type of Plate Tectonics or No Plate Tectonics At All?

The Archean rock record consists of small continent-like blocks of high-grade gneisses of broadly tonalitic composition and subordinate greenstone belts that are composed mostly of volcanic rocks and texturally immature sedimentary rocks. These rocks are quite common from 3 Ga onwards, but similar rocks are present from the beginning of the Archean: tonalitic gneisses make up the bulk of the Itsaq complex of western Greenland, dated at 3,800 Ma (Nutman et al., 1999), and supracrustal rocks of similar age occur both here and in northern Quebec (Mloszewska et al., 2012). As we trace them back into the Archean, several of the best plate tectonic indicators, such as paired metamorphic belts, high-pressure, low-temperature metamorphism, and greenstone volcanics with arc-like geochemistry, seem to “pinch out” at about 3.1–3.3 Ga. For the beginning of plate tectonics, no single indicator is as convincing as the testimonial given by the accumulation of these lines of evidence at this time. Even so, the lack of eclogites and blueschists leads some to distinguish between Proterozoic plate tectonics (approximately 3,300–700 Ma) and modern plate tectonics (after 700 Ma; Brown, 2007). Before 3.3 Ga, we have only a limited number of indicators, but these tell us that TTG magmatism occurred from the beginning of the Archean, although with a tendency for MgO poorer, SiO2-richer compositions earlier on in time. This possibly indicates a type of shallow subduction, in which the subducting crustal plate directly underlies the overriding plate so that reaction of TTG melts with the mantle wedge could not occur. As time passed and the Earth cooled, subduction to deeper levels occurred and reaction with the mantle became more common.

However, the production of TTG melts does not necessarily denote subduction. The requirement from high-pressure experiments is that these melts must be derived by melting of basalt, but these experiments say nothing about how this basalt reaches its melting conditions. The main competitor hypotheses to subduction for explaining the origin of TTG melts are oceanic plateaux and stagnant lid tectonics. The oceanic plateau model uses the analogy of thickened ocean crust on the modern Earth, which are caused by extra heating from below by mantle plumes, resulting in crust up to 35 km thick (Neal et al., 1997). Thus, formation of some crust as oceanic plateaux could apply to an Archean Earth with plate tectonics, but a major uncertainty in this case becomes the behavior of this thick mafic crust in subduction zones. Can such thick crust be subducted as a unit, or does just part of it return to the mantle and the upper parts accumulate at the surface? In this scenario, the basaltic source rocks for TTG melts are derived from thick piles of volcanics that simply become buried to lower crustal levels because of continued volcanism.

The ocean plateau model is generally thought to work within plate tectonics and does not seem adequate for the early Archean if complete recycling of crust commonly occurred. Here a stagnant lid scenario may be more appropriate, in which the thick crust is not subducted, but the lower levels delaminate and melt back into the mantle. Only the upper sections of crust are preserved, a scenario known as “flake tectonics” (Hoffman and Ranalli, 1988). In the earliest Archean or Hadean, blocks of crust may have tipped like ice flows, thus returning extensive blocks of crust to the mantle without the mechanism of subduction as it occurs on the modern Earth.

It is probable that the large degree of crustal recycling implied by the paucity of surviving crust from the early Archean is incompatible with plate tectonics in its modern form. The transition to plate tectonics may have occurred during the period 3.3–3.1 Ga, followed by an immense increase in the survival of continental crust (Fig. 1). The Archean-Proterozoic boundary at 2.5 Ga is the much more publicized major disjuncture in the geological record, but these changes in geological style could only have occurred after abundant continental crust had collected for long periods. The middle Archean may have been a time in which plate tectonics operated on some parts of the Earth, but not on others, and it may have been intermittent (O’Neill et al., 2007).

Summary

Taking multiple indicators into account together, there is good evidence for the operation of plate tectonics on the Earth as far back as 3.3 Ga, but it may have been a modified form until the late Proterozoic without deep subduction and the production of ultrahigh pressure metamorphic belts. In the early Archean, the production of early SiO2-rich crust may have been caused by melting at the base of a uniformly thick global crust or oceanic plateaux, with or without subduction as on the modern Earth. In the past few years, the number of lines of evidence for cool conditions at the end of the Hadean is increasing, such as the stable isotope signatures of zircons and the composition of TTG gneisses. The importance and abundance of komatiites and exceptional mantle temperatures in the Archean may have been overestimated in the past.

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© Springer Science+Business Media Dordrecht 2014

Authors and Affiliations

  1. 1.Department Earth and Planetary Sciences, ARC Centre of Excellence for Core to Crust Fluid SystemsMacquarie UniversityNorth RydeAustralia