J.F. DEWEY, E.S. KISEEVA, J.A. PEARCE AND L.J. ROBB

Precambrian tectonic evolution of Earth: an outline

J.F. Dewey University College, Oxford, United Kingdom e-mail: [email protected]

E.S. Kiseeva School of Biological, Earth and Environmental Sciences, University College Cork, Ireland e-mail: [email protected]

J.A. Pearce School of Earth and Ocean Sciences, Cardiff University, United Kingdom e-mail: [email protected]

L.J. Robb Department of Earth Sciences, University of Oxford, United Kingdom DSI-NRF CIMERA, University of the Witwatersrand, South Africa e-mail: [email protected]

© 2021 Geological Society of South Africa. All rights reserved.

Abstract

Space probes in our solar system have examined all bodies larger than about 400 km in diameter and shown that Earth is the only silicate planet with extant plate tectonics sensu stricto. Venus and Earth are about the same size at 12 000 km diameter, and close in density at 5 200 and 5 500 kg.m-3 respectively. Venus and Mars are stagnant lid planets; Mars may have had plate tectonics and Venus may have had alternating ca. 0.5 Ga periods of stagnant lid punctuated by short periods of plate turnover. In this paper, we contend that Earth has seen five, distinct, tectonic periods characterized by mainly different rock associations and patterns with rapid transitions between them; the Hadean to ca. 4.0 Ga, the Eo- and Palaeoarchaean to ca. 3.1 Ga, the Neoarchaean to ca. 2.5 Ga, the to ca. 0.8 Ga, and the Neoproterozoic and Phanerozoic. Plate tectonics sensu stricto, as we know it for present-day Earth, was operating during the Neoproterozoic and Phanerozoic, as witnessed by features such as obducted supra-subduction zone ophiolites, blueschists, jadeite, ruby, continental thin sediment sheets, continental shelf, edge, and rise assemblages, collisional sutures, and long strike-slip faults with large displacements. From rock associations and structures, nothing resembling plate tectonics operated prior to ca. 2.5 Ga. Archaean geology is almost wholly dissimilar from Proterozoic-Phanerozoic geology. Most of the Proterozoic operated in a plate tectonic milieu but, during the Archaean, Earth behaved in a non-plate tectonic way and was probably characterised by a stagnant lid with heat-loss by pluming and volcanism, together with diapiric inversion of -trondjemite-granodiorite (TTG) diapirs through sinking keels of greenstone supracrustals, and very minor mobilism. The Palaeoarchaean differed from the Neoarchaean in having a more blobby appearance whereas a crude linearity is typical of the Neoarchaean. The Hadean was probably a dry stagnant lid Earth with the bulk of its water delivered during the late heavy bombardment, when that thin mafic lithosphere was fragmented to sink into the asthenosphere and generate the copious TTG Ancient Grey (AGG). During the Archaean, a stagnant unsegmented, lithospheric lid characterised Earth, although a case can be made for some form of mobilism with “block jostling”, rifting, compression and strike-slip faulting on a small scale.

SOUTH AFRICAN JOURNAL OF GEOLOGY 2021 • VOLUME 124.1 PAGE 141-162 • doi:10.25131/sajg.124.0019 141 PRECAMBRIAN TECTONIC EVOLUTION OF EARTH: AN OUTLINE

We conclude, following Burke and Dewey (1973), that there is no evidence for subduction on a global scale before about 2.5 Ga, although there is geochemical evidence for some form of local recycling of crustal material into the mantle during that period. After 2.5 Ga, linear/curvilinear deformation belts were developed, which “weld” together and palaeomagnetism indicates that large, lateral, relative motions among continents had begun by at least 1.88 Ga. The “boring billion”, from about 1.8 to 0.8 Ga, was a period of two super-continents (Nuna, also known as Columbia, and Rodinia) characterised by substantial magmatism of intraplate type leading to the hypothesis that Earth had reverted to a single plate planet over this period; however, orogens with marginal accretionary tectonics and related magmatism and ore genesis indicate that plate tectonics was still taking place at and beyond the bounds of these supercontinents. The break-up of Rodinia heralded modern plate tectonics from about 0.8 Ga. Our conclusions are based, almost wholly, upon geological data sets, including petrology, ore geology and geochemistry, with minor input from modelling and theory.

Introduction

Plate tectonics sensu stricto (Isacks et al., 1968; Le Pichon, 1968; present is the key to the past” perhaps with the notion that plate McKenzie and Parker, 1967; Morgan, 1968; Wilson, 1965) is the tectonics sensu stricto is too good a mechanism to waste. Since relative motion among torsionally rigid plates with narrow the advent of plate tectonics (Wilson, 1965), there has been a boundary deformation zones in the oceans but, commonly, naturally enthusiastic, but perhaps over-zealous, tendency to wider in the continents. Significant intra-plate deformation interpret the whole recorded history of Earth in these terms. in the oceans is rare, except very close to ridge axes, but Uniformitarianism has meant a variety of things to geologists. deformation and wide plate boundary zones are common in the The episodicity, periodicity, and catastrophic geologically- continents, except for the stiff Archaean cratons with a thick instantaneous nature of many, if not most, processes indicates lithosphere, which are almost earthquake-free and commonly that uniformitarianism is not strictly true, though this statement wrapped around by younger orogenic terrains. This definition depends upon the time period over which processes are of plate tectonics, involving torsional rigidity with substantial differentiated and integrated. It is clear that tectonic processes deformation and largely confined to relatively were not uniform through time and that there have been narrow boundary zones, is its very essence and that which we various forms of secular evolution, as shown in the compilation follow strictly in this paper. We acknowledge that other forms by Bradley (2011) of “everything through time”. As Bradley of lesser non-rigid mobilism as envisaged, for example, by demonstrates, heat production, the temperature of the Bédard (2006), probably took place on Archaean Earth and may asthenosphere, and heat flow have evolved with time whatever have involved block fragmentation and jostling. Subduction form of tectonics was operating, for example from three times zones involve the insertion of torsionally-rigid lithospheric slabs the heat production in the early Archaean compared to today, into the asthenosphere, but it is likely that other ways of with a corresponding progressive thickening and stiffening of transporting surface and shallow materials into the mantle were the lithosphere. prevalent during the Archaean; these have been called There is a plethora of opinions on the existence and origin sagduction, drip tectonics, or foundering. of plate tectonics, most of which do not adhere to the sensu Plate tectonics is a highly-efficient mechanism for global stricto definition of torsional rigidity and narrow boundaries. heat-loss by magmatism and hydrothermal convection at They tend to ignore that most Archaean geological patterns the oceanic ridges and cooling by subduction, with minor are blobby and rarely form narrow belts between older conductive heat-loss, supplemented, episodically, by intra-plate cratons. There are exceptions, however, such as the superb magmatism in large igneous provinces and hotspots above and challenging reviews of the Archaean by Bleeker (2002) and mantle plumes. If there was a time on Earth before plate Hamilton (1998). tectonics, neither conduction nor radiation can account for the The following is a partial list of opinions on the start time necessary heat-loss so that the only credible mechanism would of plate tectonics. At the ‘young end’, Hamilton (2003) and Stern have been pervasive plume upwelling and related massive mafic (2005; 2020) argue, from plate subductability, blueschists, ruby, magmatism with, possibly, some form of “oceanic” crustal- jadeite, and ophiolites, that modern-style plate tectonics began lithospheric fragmentation and foundering, to transport surface in the Neo-Proterozoic (0.8 Ga), preceded by a stagnant lid materials into the mantle. Mantle temperature and heat-loss have during the so-called ‘boring billion’ (1.8 to 0.8 Ga), itself been diminishing since the origin of Earth (Bickle, 1978; Bickle, preceded by a period of plate tectonics from about 2.1 to 1.8 Ga, 1986; Korenaga, 2013); therefore, one might expect changes in then a stagnant lid before that. Hamilton (2007) argues for a tectonic style, involving a stagnant non-segmented lithospheric 2.0 Ga start based upon the first large, narrow deformation belts lid changing to plate tectonics as Earth cooled. such as the Limpopo Belt. Burke and Dewey (1973), Dewey Planetary-scale plate tectonics in our solar system appears (2018, 2019), Brown et al. (2020) all argue for a start between to be restricted to Earth at the present time but may have taken 2.2 and 2.1 Ga based on a range of structural and metamorphic place in other silicate planets (especially Venus) in the past and criteria. Grieve (1980) argues for a Neoarchaean start. A large may be occurring now in moons of the giant gas planets. There group favour the Palaeoarchaean to Neoarchaean transition, has been a natural tendency to lean on the doctrine of “the from 3.2 to 3.0 Ga (Dhuime et al., 2011, 2012, 2015; Condie and

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Kröner, 2008; Cawood et al., 2006; Dewey, 2007; Pease et al., geochemistry and petrology, that define tectonic regimes 2008; Smithies et al., 2005a, 2007; Van Kranendonk et al., 2007). and involve mostly field observations and geological maps. In There are advocates for the Early Archaean mostly on the simple terms, the question is asked “do geological patterns, at all basis of uniformitarian magmatic and structural comparitors, scales, resemble those that we know were generated by plate including: de Wit (1991) for a 3.4 Ga start; Nutman et al. (2002) tectonics or do they differ?”. Changing heat production favours for 3.6 Ga); Polat and Kerrich (1999), Komiya et al. (1999) and progressive rather than instantaneous change, perhaps with Kusky et al. (2018) for 3.8 Ga; and Shirey et al (2008) for 3.9 Ga. tipping points. If we see a geological pattern like, for example, Advocates of the Hadean-Archaean boundary or Hadean, that of the Lower Palaeozoic of western Newfoundland, we may essentially based on characteristics of the oldest recovered rocks suppose that it was generated by an oceanic arc with an and minerals, include Bickle (1978, 1986) and Hastie et al. (2016) ophiolitic fore-arc colliding with a rifted continental margin. for 4.0 Ga, Hopkins et al. (2008) for 4.2 Ga), and Harrison et al. Colloquially, avoiding ducks, “if it looks and behaves like a cat, (2005) for 4.5 Ga. Clearly all cannot be correct! it probably is a cat”. If it has scales and two legs and swims, it There is little doubt, from palaeomagnetism (Evans and probably is not. Insufficient attention has been paid to rock Pisarevsky, 2008), that substantial relative horizontal motions suites and their structures and to arrangements that are seen in took place during the Proterozoic from about 1.9 Ga, also outcrop and across regions. For example, andesite, tonalite, and marked by an increase in the growth rate of the continental are not definitive indicators of plate tectonics unless they crust. The uniformitarian plate tectonic view was challenged by have arc chemistries and are in linear-arcuate belts that, in turn, Burke and Dewey (1973), who saw the Archaean as a indicate a related subduction zone. Also, too much reliance has “permobile” regime characterized by pervasive magmatism and been placed on numerical modelling at the expense of empirical deformation during density inversions, when no part of Earth field data: modelling should not trump observation, particularly consisted of torsionally rigid lithospheric plates sensu stricto on of tectonic style and geology. Obviously, however, this is not any scale. They argued that plate tectonics began during the possible for most of the Hadean for which we have only a early Palaeoproterozoic. handful of primitive zircons, and where modelling and inference If plate tectonics is the operator, one might expect to see are necessary. geological evidence of plates and plate boundaries such as the Plate tectonics can be dealt with in fully quantitative terms following: passive continental margin and shelf associations; from 160 Ma by sequential magnetic anomaly and continental oceanic arcs without continental basement; supra-subduction margin fitting from which poles and rates of relative zone ophiolite complexes sensu stricto indicating both fore-arc plate motion can be deduced. Semi-quantitative assessments plate accretion and the relative horizontal motion of obduction; are allowed by palaeomagnetism, faunas, facies, and long, linear and curvilinear deformation belts between widespread, paleoclimatology from the beginning of the Cambrian to the flat-lying, little deformed or undeformed epicontinental platform early Jurassic because oceanic crust with its magnetic anomalies sequences over thousands of square kilometres: transcurrent has been subducted and vanished along suture zones. faults with hundreds of kilometres displacements; paired In the Precambrian, there are no faunas of use so that metamorphic belts with adjacent high pressure-low temperature palaeomagnetism and facies allow only sketchy continental and high temperature-low pressure zones; adakites; subduction- reconstructions and analyses of relative motion. All those who accretion prisms; collision zones with ruby; foreland have written on the subject, and probably most geologists, fold-thrust-belts especially those of thin-skinned type; and would agree that plate tectonics sensu stricto can be traced back, palaeomagnetic evidence for the relative motion of continents. at least, to the break-up of the supercontinent Rodinia from These plate tectonic rock suite indicators might be expected to about 0.8 to 0.6 Ga. be arranged in belts and zones rather than in blobby patches. The existence of Rodinia requires that it was assembled from It is interesting to speculate on how we might approach older continental masses and fragments involving subduction the problem of Archaean tectonics were we to know nothing and collision. It is commonly considered that this was of plate tectonics. We know that Earth has evolved thermally completed at about 1.0 Ga when the Grenville and Namaqua and so perhaps it should not be surprising that the principal Orogens were formed (Figure 1). There is clear evidence from heat loss mechanisms of plumes and plate tectonics have also palaeomagnetism (Evans and Pisarevsky, 2008) that relative evolved. Many papers depend on one or a small number of motion of continental masses (continental drift) can be traced distinctive rock types, such as boninite and andesite, but these back to at least 1.9 Ga and is permissible back to about 3.0 Ga. are not definitive indicators of subduction because they can Extensive linear to arcuate zones of continental deformation, the also form, even in modern Earth, in intraplate settings. For oldest of which are the Limpopo and Ubendides of southern example, boninites have been found in the Manihiki Plateau Africa at about 2.0 Ga (Figure 2), are characteristic of continental (Golowin et al., 2017) and abundant calc-alkaline andesites collision zones as we know them today. in the Basin and Range (Hawkesworth et al., 1995), both Perhaps the finest and clearest example of a Proterozoic unrelated to active subduction. Thus, reliance upon certain collision zone is the Wopmay Orogen (Hoffman and Bowring, rock types and chemistries to give definitive solutions may 1984), which is part of an extensive network of late yield incorrect answers. Palaeoproterozoic orogens. These include the Trans-Hudson belt The views expressed in this paper focus on rock and Labrador Trough, the latter with the characteristic paired assemblages, patterns, and relationships, with accompanying positive-negative gravity anomaly indicating that the lithosphere

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was sufficiently strong to support it without relaxing non- Holder et al. (2019) made the key analysis and observation that elastically to perfect Airy isostasy. Moreover, there is no obvious paired metamorphic belts, distinctive of convergent plate horizontal strain in the cratons at the time the bounding boundary zones, become weaker in their bimodality back in Palaeoproterozoic orogens were developed, evidence that they time until the bimodality vanishes between 2.2 and 2.4 Ga, so were torsionally-rigid platforms. Care should be exercised with supporting the advent of plate tectonics at this time. this argument, however, because even modern continental plate Also, a particular key feature of plate tectonics, such as boundary zones may be very wide and complex with large, blueschists, may not have developed in a hotter regime. Periods torsionally rigid blocks - as seen in the Himalaya-Tarim and the with supercontinents will have an abundance of marginal arcs, Cordilleran-Rocky Mountain plate boundary zones. much intra-plate rifting and potassic magmatism, and A further problem is that, if oceans existed during the widespread continental sediment sheets, as during the so-called Precambrian, they have been subducted, except possibly in ‘boring billion’ from 1.8 to 0.8 Ga. Periods of distributed small protected patches such as the South Caspian. In continents will have abundant rifted margins, oceanic arcs with Phanerozoic suture zones, most ophiolites sensu stricto evolved ophiolite fore-arcs colliding with rifted margins, and Andean- in fore-arcs; therefore, although not themselves formed at any style orogens. Thus, we should not expect to be able, always, major divergent plate boundary, they mark the subduction of to tick off a convincing list of features that demand a plate oceanic lithosphere. It is fairly certain, from these general tectonic solution. arguments, albeit with caveats, that relative motion between and Bradley’s (2011) compilation shows that there are a number collision of continents extended back to at least 2.0 Ga, with of compelling changes in Earth history, both gradual and a lithosphere of some torsional strength suggestive of plate sudden, that are likely the results of changes from plume to plate tectonics (Figure1). It may thus be claimed, with some certainty, tectonics in a cooling earth. The time distribution of ‘almost that plate tectonics was operating in its present form since about everything’ shows rapid change from about 3.0 to 2.0 Ga, 0.8 Ga, and something resembling plate tectonics since about through the late Archaean and early Palaeoproterozoic, with the 2.0 Ga. Therefore, in this paper, we explore, mainly, the tectonic biggest switch in observed geology at the end of the Archaean history of Earth through much of the Precambrian. Eon. Dome-and-keel greenstone tectonics dominates, although Critical questions relate to the volume, composition, and not exclusively, the tectonic style of the Archaean, whereas areal distribution of the continental crust. For example, were mantled domes are a minor component of the Proterozoic there many early small continents - or was there a time when and younger Earth. The Archaean was a more mafic Earth continental crust covered the globe with a subsequent loss by with low-K tonalite-trondhjemite-granodiorites (TTGs) and massive tectonic erosion leaving stable deformation-resistant ; is known after 2.5 Ga only in a few cratons (Armstrong, 1968; Fyfe, 1978)? If plate tectonics was not Palaeoproterozoic localities and the Cretaceous Gorgona locality operating in a world of discrete continents, what lay between in Colombia. From 3.0 to 2.0 Ga, there was a progressive the continents and what was its composition? If there were increase in K2O/Na2O and decline in MgO and Cr of igneous discrete continents in a stagnant lid, what was happening in the rocks and a progressive increase of Rb/Sr in sediments, linked “oceans” between them? How can the rate of crustal addition to a surge in the growth of the continental crust (Dhuime et al., and growth be judged, and how much crustal subtraction 2015). The following sections deal in outline with processes and and loss took place (Dewey and Windley, 1981)? What are features that have changed over geologic time - Figure 1 shows the lithospheric conditions necessary for subduction? Without some key events and sequences in the tectonic history of Earth. subduction, what were the mechanisms for making continents? These questions are beyond the scope of this paper but are Hadean (4.54 to 4.0 Ga) related issues that deserve attention. Following the Gaia-Theia collision at 4.51 Ga, the 4.4 Ga Jack Plate tectonic indicators and trends in the rock record Hills zircons in Australia and the end-Hadean 4.03 Ga Acasta gneiss in Canada (Bowring and Williams, 1999) are the earliest There are a number of structural, geochemical, petrological, physical remnants of events on Earth. The nature of the Jack facies and stratigraphical features in the geological record that Hills zircons and the lithology from which they were derived is are characteristic of Phanerozoic plate tectonics. If many, or all, disputed. Harrison et al. (2005) have argued that the trace are present at a region in space and time, they can provide element geochemistry of the zircons points to plate tectonic confidence that plate tectonics was operating. A particular processes and the growth of continental crust for their origin. problem is that there are gaps in the time lines of some features. We believe that an opposite conclusion may be drawn from These may be genuine gaps because of episodicity (e.g. four simple geological reasoning. If there had been a substantial main episodes of ophiolite formation during the Phanerozoic) amount of Hadean granitic, island arc, or TTG crust, why did and periodicity, or false gaps caused by non-preservation none of it survive? The survivability of buoyant, weak at the surface by erosion, subduction, subduction-erosion continental crust is the reason for mountains and for the or temperature overprint. A good example is high P/T (pressure/ preservation of TTG rocks from 4.03 Ga. The absence of extant temperature) metamorphism, which is almost entirely younger Hadean continental crust implies that it never existed and that than 750 million years with just a single Mesoproterozoic and a the Hadean crust was largely mafic and later subducted. small number of Palaeoproterozoic outliers (Brown et al., 2020). Furthermore, the absence of granitoids implies the absence of

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water until the first TTG’s appear in the Eoarchaean, and hence partial melting of hot mantle (Debaille et al., 2013; Fischer and no plate tectonics. Gerya, 2016). Continuous bolide bombardment generated multi- We envisage the Hadean world to have been, like the ring impact basins with massive excavation, lithospheric Eoarchaean Moon, one of hot, anhydrous conditions with a fracturing, and mafic volcanism induced below impact sites with stagnant mafic lid, bombarded by bolides. Our reconstruction both shallow and deep plumes as the mechanisms of global of the Hadean is similar, in its essentials, to that of Grieve (1980), heat-loss and the origin of a probably komatiitic crust. Small summarized briefly as follows. Hadean Earth was a dry planet pools may have developed, from which the Jack Hills with heat production two to four times that of today (Bickle, zircons were derived, and there was likely reworking of intra- 1978; Bickle, 1986; England and Bickle, 1984). The stagnant lid basin volcanics, and impact lithologies. Impact craters, such as lithosphere must have been thin with a fifteen kilometre basaltic- those on the Moon, probably reflect the geometry and komatiitic plateau-like crust, generated by a high degree of topography of Earth’s surface in Hadean times, whereas the

Figure 1. Outline of domains, rock suites, sequences and events referred to in text.

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Vredefort crater in South Africa, for example, although younger In the Kaapvaal (Anhaeusser et al., 1969), the thick at 2 025 Ma, preserves the geological responses to cataclysmic Onverwacht komatiite-/chert/greywacke sequence is impacts upon both basement granitic crust and younger overlain by the Fig Tree argillite/chert and the Moodies sediments. (Bowring et al., 1989b). conglomerate/sandstone. The TTG’s are in domal structures, typically about 30 km in diameter. The structural fabric generally Eoarchaean (4.0 to 3.6 Ga) varies from almost isotropic in the cores of the domes, with increasing flattening fabrics outwards, to plane strain near and The main key to understanding the early tectonic behaviour of at the margins, typical of ballooning. In the greenstone keels, Earth is the TTG dominated early continental crust, the so-called strain is strong from more plane strain fabrics at their margins Ancient Grey Gneiss (AGG), exposed in the Kaapvaal to vertical stretching fabrics in their cores; in the Pilbara (Collins (Anhaeusser et al., 1969), Greenland (Moorbath et al., 1973), et al., 1998) and Yellowknife (Drury, 1977, Bleeker, 2003), Slave (Bowring et al., 1989a, b; Bowring and Williams, 1999), structures and fabrics are very similar. Zimbabwe (Dirks and Jelsma, 1998) and Pilbara (Collins et al, There is a substantial disagreement on the basic structure 1998) Cratons. The AGG is, in places, overlain unconformably and interpretation of greenstone belts. A quite different structural by Archaean komatiite sequences but, more commonly, has evolution for Palaeoarchaean greenstones has been advanced tectonic contacts at dome margins with mafic and ultramafic by geologists working in the Pilbara and Kaapvaal Cratons. They volcanic and sedimentary sequences (greenstone belts). TTGs invoke a pre-doming shortening, commonly with a plate- vary substantially from low-pressure derivatives, generated by tectonic interpretation. For example, Bickle et al. (1980) have the hydrous partial melting of garnet-free, plagioclase-rich, argued, for the Pilbara, that a pre-doming deformation event was amphibolites and low-magnesium at up to 900°C at responsible for shortening and early flat-lying structures along about 35 km, to high-pressure from garnet-amphibolites or the greenstone-granite contacts. We, however, interpret these eclogites at greater than 60 km depth (Adam et al., 2012). structures, commonly polyphase and concentrated along TTG Eoarchaean TTGs are low pressure. Therefore, the crust and border zones, as having formed along the contacts during mantle must have acquired water by about 4.0 Ga and there diapiric inversion. Jackson and Talbot (1989) and Van must have been either a very thick (>35 km) plateau-like basaltic Kranendonk et al., (2007) have highlighted the complex crust and/or a mechanism to transport mafic rocks into the polyphase structural complexities that can develop in TTG- shallow mantle by subduction or sagduction. Our favoured greenstone terrains along the margins of mushroom-shaped option is that a thick mafic crust on a thin lithosphere with an diapirs. Mechanisms include dome margin shear, compression enhanced geothermal gradient, perhaps aided by impact- by ballooning, horizontal shearing at the base of overhangs, and induced foundering, was the likely TTG source. One interesting compressional constriction in keel cores. Glazner (1994) has possibility is that Earth’s water was acquired from icy comets shown that mafic intrusions may also enjoy solid-state vertical during the late heavy bombardment, which also fractured the motions in the continental crust and cause polyphase lithosphere to allow the sinking and partial melting of giant slabs deformation with conflicting kinematics. His model, developed of mafic protocrust. Alternatively, Johnson et al. (2017) have for the Sierra Nevada in California, involves the ascent of mafic argued, from petrogenetic modelling, that Eoarchaean TTG’s magma to a position of neutral buoyancy. Upon cooling, the were derived from the bases of early basalt sequences with now denser mafic body sinks at rates of several kilometres per continuous and multiple cycles of volcanism, burial, partial million years to a new, deeper, level of neutral buoyancy. melting and remobilization of TTG’s in which there is no a priori Marginal upturns develop during ascent and downturns develop need for subduction, sagduction, or foundering. during sinking. These silicic and mafic buoyancy-driven inversions may be important in driving crustal stratification. Palaeo- and Mesoarchaean (3.6 to 2.8 Ga) de Wit (1982, 1991) and de Ronde and de Wit (1994) have argued that substantial horizontal shortening, with major thrust Palaeoarchaean rocks may be found in all continents and have repetitions, precede TTG diapirism, and that the Onverwacht- strikingly similar lithological assemblages and structures that are Kromberg section contains obducted ophiolite sequences. While wholly dissimilar from those in younger terrains. They form the we acknowledge that major deformation takes place in most earliest cratonic cores, such as the Kaapvaal and Pilbara. Today, greenstone belts, we have walked, with Carl Anhaeusser, across they are almost earthquake-free, except for mining-induced several thick komatiite sequences in the Onverwacht Group and seismic activity, have low heat-flow, and lithosphere perhaps saw only natural stratigraphic sequences with no evidence of up to 350 km thick. They have resisted subsequent regional thrust repetition. Meticulous structural mapping in the centre of deformation and commonly form hard knots around which the Barberton by Ramsay (1963) has shown younger orogens are moulded. They have, mostly and that the Barberton sequence, is shortened, folded and cleaved regionally, low-grade metamorphism and are little eroded; when with a style transition as the TTGs are approached and the formed, they were roughly the same crustal thickness as today. stratigraphy is thinned and distorted against and between, and Typically, they have the classic TTG domeandgreenstone keel wrapped around, the diapiric plutons. All of the shortening, structure (MacGregor, 1947; Anhauesser et al., 1969; Wilson, including thrusting, in the Barberton sequence can, we 1978; Collins et al., 1998). conclude, be ascribed to compression between rising and ballooning plutons. We believe that greenstone belt structures

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are best interpreted as having developed during TTG diapirism that these variations allow the definition of a number of discrete (Schwertner et al., 1979; 1983). Over many years, Lowe and his terranes that may have collided and jostled. We consider colleagues (e.g. Lowe, 1982 and 2019; Lowe et al., 1985) have collision to have been unlikely because the positions of the made very fine, detailed maps of the stratigraphy of the necessary sutures representing lost ocean-basins between these Barberton belt and shown that there are extremely complex terranes are not obvious and, while recognising the facies facies variations, especially in the Fig Tree Group. They propose variations, the gross tripartite stratigraphy of the Barberton

Figure 2. Distribution of major ore deposit types as a function of time and the supercontinent cycle (after Robb, 2020); IOCG – Iron Oxide-- /SEDEX – Sedimentary Exhalative/VMS – Volconogenic Massive Sulphide/MVT – Mississippi Valley Type.

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Supergroup is evidence that that it formed a blanket across the thrust stacks, is not oceanic (see arguments for the Neoarchaean whole region. Belingwe sequence of Zimbabwe below), and cannot be Grosch and Slama (2017) also argued for the presence of an considered to be part of an ophiolite complex as has been ophiolite-type sequence preserved in Kromberg Formation of proposed (de Wit, 1982, 1991). the Barberton greenstone belt. They combined new field The dome-and-keel structure was generated by vertical observations with detrital U-Pb zircon geochronology and inversion of a lighter, hotter TTG crust overlain by a heavier, geochemistry on drill-core material from the Kromberg type- cooler, komatiitic Palaeoarchaean volcanic sequence in a section. Grosch and Slama (2017) used trace element geothermal gradient greater than that of today, which would geochemistry, εNd values and Nd model ages to infer that the have decreased viscosity and facilitated inversion (Talbot, 1968). Kromberg metabasalts were derived from a depleted Archaean The Palaeoarchaean Isua Dome in west Greenland, with its mantle source similar to CHUR (chondritic uniform reservoir) flattened and stretched envelope of low-grade meta-volcanics with no continental (TTG) crustal contamination. They also and sedimentary rocks, is very similar to the Barberton TTG carried out U-Pb geochronology by laser ablation–ICPMS on domes and was probably also formed during crustal inversion. detrital zircons from an uppermost chert unit to demonstrate a Most greenstone terrains are predominantly anchi-metamorphic homogeneous age distribution and a gabbroic source in the or low grade. Much higher grade belts do occur, but are greenstone belt, in direct contrast to zircons from felsic probably the deeper parts of greenstone keels rather than conglomerates that structurally underlie the Kromberg sequence. separate discrete belts. The geology and evolution of Eo- and Their overall conclusion was that, collectively, the new data and Palaeoarchaean terrains is, in our opinion, wholly inconsistent field observations indicate that the 3.33 Ga Kromberg mafic- with having developed in a plate tectonic Earth. ultramafic sequence formed in a juvenile oceanic setting and represents a remnant of tectonically accreted oceanic crust, and Neoarchaean (2.8 to 2.5 Ga) that horizontal plate tectonic processes were operating on the Archaean Earth as early as 3.6 Ga. The principal problem in By the end of the Mesoarchaean, the Kaapvaal was regarding the Kromberg sequence as an ophiolite generated at established as the basement upon which the Pongola, an oceanic spreading ridge is that, although it is thrusted Witwatersrand, Ventersdorp, and Transvaal volcano-sedimentary (obducted) onto diamictites, turbidites and tuffs of the Noisy sequences were deposited (Figure1). The Pongola Supergroup Formation in the section described, it appears, elsewhere, to be was probably deposited in a rift, so indicating that the Kaapvaal part of the regular Onverwacht Group volcanic stratigraphy. lithosphere was sufficiently strong to crack and allowing Moreover, although there are some lithologies (dunites, gabbros, it, during deposition of the Ventersdorp Supergroup, to be and pillow basalts) that appear ophiolitic, they are not arranged penetrated by komatiite, basalt and intermediate magmas. The in anything resembling an ophiolite sequence and there is, little-deformed platform carbonates and banded iron formations critically, no sheeted dyke complex or tectonised . We of the Transvaal Supergroup were deposited across the Kaapvaal note that thick underwater flow units can have pillowed tops, Craton as three unconformable sequences in two basins progressively coarse-grained interiors with cumulate textures, separated by the Vryburg Arch indicating a relatively stable and picritic-dunitic basal cumulates; they are not necessarily platform and cratonization by this time. Similarly, in oceanic, however, and do not satisfy the criteria needed to be Northwestern Australia, the stabilized Pilbara Craton is overlain termed ‘ophiolites’. by the sedimentary rocks and volcanics of the Fortescue In summary, we favour the concept that the Onverwacht Formation and the banded ironstones of the Hamersley Basin. (3.56 to 3.33 Ga) and Fig Tree groups were laid down as a thick However, in the , the Superior Province sequence on a hot Palaeoarchaean TTG basement, which was and the Yilgarn Craton, the Neoarchaean record is quite different followed by crustal inversion such that the hot mobilized TTG as, within the period 3.1 to 2.5 Ga, a sub-linear TTG/greenstone basement rose as spreading/ballooning/inflating diapiric domes crustal fabric was developed. In Zimbabwe, the 2.9 to 2.55 Ga and the greenstones sank and were compressed between the Belingwe-Bulawayan sequence is made up of cycles of domes (e.g. Schwerdtner et al., 1983). The Moodies Group was komatiites, mafic- to silicic volcanics, and sediments, arranged probably mainly restricted to early depressions between the in synclinal tracts ranging from broad and open and shallow rising domes and represents the stripping of the Onverwacht (Belingwe) to tight and deep (Bulawayo). The distinctive and Fig Tree sequences, and some TTGs, from the rising Zimbabwe cover sequence, including thick komatiites, dome heads. These events were terminated by little-deformed, shows minor facies variations but is essentially a uniform late, high-level, sill-like sheets of potassic granite and small blanket (Wilson, 1978) and rests with a mapped unconformity syenite plutons. upon a Palaeoarchaean TTG-komatiite-basalt (Tokwe/Shibane/ Although commonly strongly-deformed in places, the Sebakwian) basement (Bickle et al., 1975; Orpen and Wilson, Onverwacht, Fig Tree, and Moodies groups have clear 1981; Prendergast, 2004) that is intruded by a 2.55 Ga potassic stratigraphies, with facies variations, in the main Barberton granite suite. This arrangement is similar to the Palaeoarchaean Synclinorium (Anhaeusser, 1969). This precludes their TTG/greenstone inversion in the . Thus, arrangement in assembled separate terranes as argued by Lowe Zimbabwe illustrates well the various stages of basement- cover (1982). Moreover, the Onverwacht Group is clearly a natural inversion from mild and gentle basins to deep and steep keels sequence of mainly komatiite and mafic flows, is not a series of (MacGregor, 1947; Wilson 1978).

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However, as for the Palaeoarchaean, interpretations of the are clear belts and some linearity, which Bédard et al, (2013) structure of Neoarchaean greenstone belts varies greatly from and Bédard (2018) explain by compression resulting from the the classic “gregarious batholith”, diapiric, TTG dome-sagducted convective drag of craton keels causing the collapse of weak greenstone keel model (MacGregor, 1947; Talbot, 1958; zones, sagduction, and compression between more rigid zones. Wilson, 1978) to pre-doming structural complexity, commonly Calvert et al. (1995) have interpreted shallow-dipping reflection interpreted in a plate tectonic framework (e.g. Kusky, 1998). images as evidence of a subduction zone, but Ji and Long (2006) Kusky and Kidd (1992) took an ultramobilist opinion of the argue that other reflectors, such as folded structures, could Belingwe greenstone belt, requiring most of the Ngezi sequence equally be responsible. Alternatively, the structures could be to be part of a huge allochthon transported on a shear zone thrusts developed under the compression inferred by the Bédard along the Jimmy Member of the basal Manjeri Formation. They (2018) mechanism. We prefer models in which Palaeoarchaean considered the supposedly transported komatiites of the through Neoarchaean terranes were built by the same, or very Reliance Formation and basalts of the Zeederberg Formation to similar, inversion of TTG basement and its basalt-komatiite be an obducted ophiolite suite. This is wholly at variance with cover, followed by a terminal, craton-sealing, high-level granite- the work of Bickle et al. (1975, 1994) and with our reading of syenite event, commonly in the form of thick sills (the GMS suite the outcrops during field excursions guided by Jim Wilson. The of Robb et al., 2021). Jimmy Member is part of the Manjeri stratigraphy, as are the The Yilgarn Craton has a north-northeast-trending komatiites and basalts. The Jimmy Member has some shear- TTG/greenstone belt linearity with TTG domes and greenstone related breccias, but they are more symptomatic of the flexural keels, and probably originated in the same way. Thus, all slip during folding of the Belingwe Syncline than massive these Neoarchaean terranes have an older basement upon displacement beneath a huge nappe. Also, there is no obvious which komatiite/basalt/calc-alkaline sequences rest, commonly suture from which a nappe could have been expelled in the unconformably, and are sealed by a late potassic phase of vicinity of the Belingwe Greenstone or, indeed, anywhere in granite/syenite magmatism. There are no obvious sutures Zimbabwe. Furthermore, Bickle et al, (1994) argue that the within them. They are arranged in more linear patterns than komatiites and basalts cannot represent oceanic crust because Palaeoarchaean-style terranes. Most of the volcanic rocks from they are part of the craton-wide Ngezi sequence that was these sequences have been interpreted by Smithies et al. (2018) deposited unconformably on an older continental crust and may as the product of interactions between plume-derived magmas represent a large igneous province (LIP). and continental crust leading them to ‘question any dominant Jelsma and Dirks (2000, 2002) and Dirks and Jelsma (1998, or significant role for modern style plate tectonics in the Archaean 2006) took a strongly mobilist stance in relating pre-TTG (at least to 2.65 Ga)’. Geologically, as well as geochemically, doming, major shortening, folding and thrusting to a plate we regard most of the volcanic components of Neoarchaean tectonic origin involving arcs and oceanic basins. This greenstone belts as pseudo-arcs, containing a significant dichotomy of interpretations resembles the similar divergence proportion of andesites but not of active subduction-derivation. of opinions for Palaeo-Archaean terrains. Our opinion is that The deep structure of Neoarchaean terranes is best shown these shortening deformations in the greenstones were in the Kapuskasing Belt (Percival and Card, 1983), along which developed between rising and ballooning TTG diapirs, and that the Superior Belt was turned up along its north-west margin. craton-wide stratigraphies prohibit interpretations that involve Here, almost the whole Superior crust is shown in cross-section either plate tectonics or another form of terrane assembly. as follows from top to base: up to 10 km greenstone belt The Superior Province has been interpreted, commonly, as metavolcanics underlain by 10 to 20 km of tabular and xenolithic having originated in a plate tectonic regime with volcanic gneissic tonalite and granodiorite, with a basal 20 to 25 km assemblages interpreted as volcanic arcs, seamounts, oceanic section of older gneissic granitoid assemblages. This does not plateaux, fore-arcs, and back-arc basins (Capdevila et al., 1982; favour an interpretation of multiple colliding arcs but rather Corcoran and Mueller, 2007; Davis et al., 1988, 1989; Desrochers a volcanic assemblage built upon an older granitoid crust in a et al., 1993; Dimroth et al., 1986; Hoffman, 1990; Jackson and stagnant lid. Cruden, 1995; Ludden and Hubert, 1986; Mueller et al., 1996; Many Archaean mafic-ultramafic associations have been Percival and Williams, 1989; Polat et al., 1998; Wyman, 1999). claimed to be ophiolites (Friend and Nutman, 2010; Furnes et al., The Abitibi and other belts in the Superior Province have ages 2014; Furnes et al., 2015; Komiya et al., 1999; Polat et al., 2002). of 2.8 to 2.7 Ga (Percival and Williams, 1989); tholeiite/komatiite However, they are scraps and slivers of, variably, pillow lava, (Pyke et al., 1973), calc-alkaline, and bimodal volcanics (Glikson, gabbro, and serpentinite that could be of any origin; none have 1979; Glikson and Derrick, 1978; Goodwin, 1982; Schwerdtner sheeted dyke complexes, tectonised harzburgites, or an ordered et al., 1979); an underlying 3.8 to 2.8 Ga TTG basement; and ophiolite sequence. late-stage 2.7 to 2.65 Ga “post-orogenic” potassic granites. Commonly, the volcanic sequences are concentric around Palaeoproterozoic (2.5 to 1.6 Ga) TTG domes (Goodwin, 1982; Schwerdtner, 1978, 1983), giving a regional blobby pattern reminiscent of patterns in the Pilbara, During the early Proterozoic, the first arcs appeared in the Dharwar, and Zimbabwe cratons. Volcanic sequences are Birimian from about 2.3 Ga (Combs, 2018). Narrow linear to commonly multi-cyclic from mafic to silicic (Goodwin, 1968), curvilinear deformation zones developed, bounded by cratonic unlike the volcanic stratigraphies in many modern arcs. There platforms, including the 1.8 to 1.6 Ga Capricorn Belt, the 2.0 Ga

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Limpopo Belt and Ubendides of Africa, the 1.9 to 1.75 Ga Trans- The (so-called) “boring billion” (1.8 to 0.8 Ga) Hudson, Coronation/Wopmay and Cape Smith Belts and the Labrador Trough in Canada, and the 1.8 to 1.7 Ga Mazatzal A stumbling block has been argued against plate tectonics orogeny in the US. The North American orogens weld together during the period 1.8 to 0.8 Ga, the so-called ‘boring billion’ seven Archaean provinces and have been interpreted as collision encapsulating, but extending both before and after, the zones (Hoffman et al., 1989). These belts and their margins have Mesoproterozoic Era. This was the period of the Columbia many of the hallmarks of Phanerozoic orogenic belts, and super-continent leading to the Rodinia supercontinent, which were clearly developed in a plate tectonic milieu. They include began to break up and disperse during the Neoproterozoic. the characteristic paired positive/negative gravity anomalies, Stern (2020) interpreted the boring billion as an interval of forelands with rifted margin and fore-deep sedimentary stagnant lid tectonics, with widespread potassic magmatism and sequences, continental shelf prisms, aulacogens, foreland fold- minor rifting in US granite/rhyolite terrains such as Pikes Peak and-thrust belts, pre- and syn-collisional magmatic zones 1.08 Ga, Tishomingo 1.375 Ga, Wolf River 1.468 Ga, and the including adakites and boninites (Wyman, 1999), transcurrent Wausau Syenite at 1.45 to 1.45 Ga and 1.34 to 1.08 Ga (Bickford structures, UHT metamorphism and deformation, and substantial and Van Schmus, 1985; Bickford et al., 1981). These are basement re-activation. Structures indicative of subduction zones characteristic of the interiors of supercontinents. However, a are seen in several places in these belts. This provides evidence planetary-scale stagnant lid is difficult to reconcile with the that some form of plate tectonics was operating on Earth horizontal extension in the Keneewenan Rift, the Grenville, between about 2.3 to 1.7 Ga, although it is difficult to gauge the Namaqua-Natal, and Sveco-Norwegian orogenies at about degree of plate torsional rigidity. There are no blueschists, 1.0 Ga, and the Gothian (1.5 to 1.4 Ga) and Hallandian (1.5 to although thermal conditions may have prevented the formation 1.4 Ga) orogenies (Stephens, 2020). of low temperature/high pressure metamorphism. Holder et al. There are also reasonably well-preserved, likely subduction (2019) have shown that paired metamorphic belts exist back to initiation, ophiolites of about 1.0 Ga age in the East Sayan the early Proterozoic, but the distinction between the hot, low- terrane in southern Siberia (e.g. Belyaev et al., 2017) and in the pressure and cold, high-pressure belts diminishes back in time Yangtse Craton in central China (e.g. Peng et al., 2012). to zero at 2.5 Ga. The oldest eclogites are outliers in time to the Ophiolites from less certain settings include those with reported dominant Neo-Proterozoic-Phanerozoic occurrences (Brown sheeted dykes and dated at ca. 1.4 Ga from the Kibaran Belt et al., 2020). The period, 2.3 to 1.7 Ga, also includes ophiolites of central, southern Africa (Johnson and Oliver, 2000) and indicative of sea-floor spreading, although no relics of normal ophiolitic fragments dated at ca. 1.33 Ga from a tectonic oceanic crust are preserved, except perhaps as accreted fragments. mélange in the Nellore Belt in India (Sain and Saha, 2018). The Jormua ophiolite appears to represent a slice of an ocean- Volcanic arc sequences and their plutonic equivalents have also continent transition (OCT) zone obducted about 100 km onto been described in a number of localities and at a number of an adjacent platform and has characteristics of modern OCT ages. These include the southeast Laurentian margin from 1.5 terranes such as the Galicia Margin (Peltonen et al., 1998). to 1.23 Ga (Rivers and Corrigan, 2000), the Namaqua Province, At the end of the Archaean, and in the Palaeoproterozoic, a South Africa at ca. 1.30 to 1.24 Ga (Bailie et al., 2010) and the global mafic outburst is witnessed by a number of LIPs with Southern Irumide Belt in central-southern Africa at ca. 1.10 to dykes, sills, and plutons, for example the 2.06 Ga Bushveld 1.04 Ga (Johnson et al., 2007). Thus, we see no reasons to deny Complex, the 2.45 Ga Matachewan dyke swarm (Hoffman, 1990) plate tectonics during the ‘boring billion’. and the 2.2 Ga Nipissing mafic dyke and sill complex, all evidence for vigorous plume activity accompanied by Igneous rock types extensional deformation in a brittle lithosphere. This vigorous mafic magmatism and a widespread coherent, non-continental Igneous rocks potentially provide the ‘smoking gun’ needed lithosphere may have been the harbinger and cause of the to establish the existence of plate tectonic processes in development of plate tectonics. At 2.5 Ga, there is some the Archaean. However, mid-ocean ridge basalts (MORB), as evidence for a massive high volatile flux from the mantle at least indicators of divergent plate margins, generally owe their ten times the present rate (Marty et al., 2019) based upon an compositions to shallow melting of slightly depleted mantle with Archaean 129Xe deficiency relative to modern compositions. moderate potential temperatures (Mckenzie and Bickle, 1988). Marty et al., argue that this massive outgassing could not have As these conditions would likely not apply to an ocean ridge in occurred in a plate tectonic regime but represents a 300 million an older and, hence, hotter Earth, the absence of clear examples year burst of mantle activity. There was also a global outbreak of Archaean MORB (e.g. Pearce, 2008) is insufficient reason to in sanukitoid (high Mg granitoid) intrusion at 2.5 Ga. We believe rule out plate tectonics. Much more robust indicators, therefore, it likely that, from 2.5 Ga, there began a phase of vigorous are rock types related to convergent plate margins, where convection pluming, mafic magmatism, and lithospheric subduction of cooler crustal materials beneath hotter mantle fragmentation that initiated true subduction and the should give distinctive compositions that are significantly establishment of plate tectonics, which has continued unabated independent of the age of the Earth at the time. These volcanic until today. arc rock types group are: • the products of crystallization of water-rich magmas, such as the calc-alkaline basalt-andesite-dacite-rhyolite (BADR) series,

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• the products of melting of hydrated, depleted mantle such and ‘low-Si boninites, LSB’ (with SiO2 = 52 to 57 at MgO = as the boninite-high-Mg andesite (HMA) series, and 8 wt.%) (Pearce and Reagan, 2019). The HSB group is • the products of subducted slab melting such as adakites. characteristic of the boninite type area (the Bonin forearc south of Japan), has been found in many ophiolites and basal arc Their respective plutonic equivalents include granodiorites, sequences in Phanerozoic and Proterozoic orogenic belts, gabbro-norites and TTGs. We examine these three groups in is characteristic of subduction initiation terranes, and is turn below. subduction-specific. The oldest example found so far on Earth Arc-like BADR sequences are rare but do exist in the marks the start of arc volcanism in the Trans-Hudson Belt at Archaean. Perhaps the best examples are the ca. 2.7 Ga Blake 1.9 Ga (Wyman, 1999). The LSB group co-exists with the HSB River and Confederation Assemblages of the Superior craton group in subduction initiation terranes but is found also at slab (Wyman and Hollings, 2006), part of the ca. 2.8 to 2.7 Ga edges and in a few oceanic arcs, forearc basins and back-arc Youanmi terrane of the western Yilgarn craton (Wyman and basins, and in the Manihiki intraplate setting. LSB have been Kerrich, 2012) and the 3.12 Ga Whundo Group on the western reported from several Archaean settings. edge of the Pilbara Craton (Smithies et al., 2005b). They differ In the detailed evaluations of proposed boninites in detail from Phanerozic arc assemblages, in particular by being by Smithies et al. (2004) and Pearce and Reagan (2019), three less porphyritic and having a complex range of rock types in sets of boninite localities emerged as most likely to be linked to addition to the BADR series which may include boninites, high- subduction, all classifying as LSB and all having subduction- Mg andesites (HMA), and Nb-enriched basalts (NEB), picrites like geochemical signatures. The first set (the ‘Whundo type’ of and adakites (e.g. Polat and Kerrich, 2006). The principal Smithies et al., 2004) comprises the boninites from within the proponents of Archaean plate tectonics have attributed this BADR sequences of the Yilgarn (Lowrey et al., 2019), Superior difference to the prevalence of hot and relatively flat subduction (e.g., Boily and Dion, 2002) and Pilbara (Smithies, 2002) cratons. in the Archaean, noting that these rock types are found on These are the most likely to have had an origin in an active present-day Earth in areas with hot slab-mantle interfaces such volcanic arc, and the same arguments for and against plate as ridge subduction and slab windows. They also highlight the subduction made in the above discussion of BADR series apply fact that these rock types typically have the chemical signatures to these boninites. of subduction such as negative Nb anomalies. The second set of boninites (the ‘Mallina type’) are Arguments against a subduction origin for Archaean BADR associated with silicic high-Mg basalts (SHMB) and their plutonic series were summarized by Bédard et al. (2013). A particular equivalents (high-Mg , or sanukitoids) and are found question raised by Bédard and others is why such BADR series within intracratonic rifted basins, notably the ca. 3.0 Ga Mallina are so rare in the Archaean and why they are typically bimodal Basin in the Pilbara terrane (Smithies, 2002; Sun et al., 1988) (basic-acid), with andesites relatively rare. To paraphrase, and and large igneous provinces (such as that hosting the 2.7 Ga extend their argument, surely 1.5 Ga of hot subduction, with Stillwater complex; Helz, 1985). These boninite-SHMB slab fusion adding silica to mantle wedges and abundant water associations are restricted to the period 3.0 to 2.0 Ga (Pearce (from subducted serpentinite in particular) available to promote and Reagan, 2019). As the authors cited above have all shown, calc-alkaline fractionation, should have produced many more high-Si, low-Mg compositions are indicative of depleted mantle BADR sequences and, in particular, much more andesite. In lithosphere refertilized by a subduction-like component. Thus, response to geochemical arguments, they point to evidence that although their geological settings show that they are not directly the negative Nb anomalies and related characteristics may be subduction-related, they do carry an inherited record of older generated by magma-crust interactions as well as subduction subduction events. Their depleted sources and restricted time (e.g. Pearce, 2008). They propose that Archean BADR sequences period are most consistent with a subduction-like refertilization are the product of interaction between hot, plume-derived event related to craton accretion. The final set of boninites is basic magma and Archaean crust rather than plate subduction. that reported for some of the oldest volcanic series, within the While accepting the present lack of consensus, we favour small, highly deformed and metamorphosed 3.8 to 3.7 Ga the conclusion of Smithies et al. (2018), based on detailed volcanic sequences at Isua, southwest Greenland (Polat et al., interpretation of the Nb anomalies, that most Archaean BADR 2002) and Nuvvuagittuq, Northern Quebec (O’Neil et al., 2011; series are indeed the products of magma-crust interaction, Turner et al., 2014). Pearce and Reagan (2019) found that the but that a small subset do involve some form of transient Isua ‘boninite like-rocks’ were most similar to the low-Ti basalt subduction or sagduction process not necessarily linked to global – boninite rocks from the Manihiki plateau, but that the Middle plate tectonics. Unit of the Nuvvuagittuq sequence did appear to have robust Of the high-Si, high-Mg rock types, boninites are one of the characteristics of low-Si boninites. More work is needed to principal rock types to have been linked to subduction. confirm the latter inference and investigate its possible link Boninites were once regarded as diagnostic of subduction, but to plate tectonic processes. the discovery of boninites in the Cretaceous Manihiki oceanic Adakite is another rock type that was once viewed as a plateau (Timm et al., 2011; Golowin et al., 2017), in particular, subduction indicator, but now has alternative interpretations. demonstrated that boninites can also form in plume terranes. It Adakites, and their supposed plutonic equivalents, TTGs, have is therefore useful to subdivide past boninite lavas into two long been explained in terms of subduction, specifically melting groups: ‘high-Si boninite, HSB’ (with SiO2 >57 at MgO = 8 wt.%); of subducted oceanic crust (e.g. Martin et al., 2005). There is no

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shortage of TTGs in the Archaean with the less-differentiated may well be the case for porphyry Cu-Mo and epithermal Au- members becoming deeper in origin (residual amphibole to Ag systems which appear to form dominantly in Mesozoic and residual garnet), more magnesian and less silicic from the Cenozoic times – older porphyry systems are known, such as earliest to latest Archaean. This is explained by Martin et al. Haib (1.885 Ga; Macey et al., 2017) in Namibia and Aitik (1.89 Ga; (2005) in the context of a plate tectonic Earth in terms of Wanhainen et al., 2005) in Sweden, but they are uncommon. increasing subduction dip, in which the high-Si adakites (HSA: It is nevertheless apparent that porphyry and epithermal ore >3Ga) are silicic slab melts that have ascended directly to the forming systems, as well as orogenic Au and MVT Pb-Zn surface, while low-Si adakites (LSA: 3.0 to 2.5Ga) are slab melts deposits, are typically associated with convergence and orogeny that have reacted with, or instigated melting of, a peridotite (Figure 2), and mineralized districts tend to form close to mantle wedge before reaching the crust. It is now, however, craton margins and paleo-sutures. By contrast, it is evident that clear that these rocks could be the product of crustal melting occurrences such as diamondiferous kimberlites and PGM-rich (of plume-derived basalts), in which case the changing layered mafic intrusions have an affinity with intracratonic characteristics could simply be the result of increasing depth of settings and tend, in many cases, to form preferentially during melting in a crustal geotherm that steepens with time. There periods of supercontinent stasis. The long-lived Mesoproterozoic are many arguments for why crustal melting is the more supercontinent Nuna, for example, is considered to have been viable interpretation, the ability of slab melting to generate in existence for up to 400 million years (between ca. 1.8 Ga and the observed volume of TTGs being one (Bédard, 2013). 1,4 Ga; Zhang et al., 2012), a period noted for an absence of Nonetheless, the much less voluminous adakite volcanic rocks orogenic ore deposit types, but with significant accumulations within BADR sequences could derive directly from either slab of continental sediment-hosted metal deposits (SedEx) as well melting, crustal melting or fractional crystallization involving as kimberlites and certain intrusion-related iron oxide-copper- amphibole and/or garnet, as is the case with recent arc-related gold (IOCG) ores (Figure 2). As noted previously, although adakites (e.g., Castillo, 2006). periods of the Mesoproterozoic were dominated by relative Overall, therefore, the direct evidence from igneous rock stasis, the margins of Nuna may have been tectonically active types for plate tectonics is small, with just a handful of localities so that ore deposits associated with plate margins and oceanic having rocks indicative of active subduction or sagduction and settings are still likely to have formed – their absence may, in no evidence for plate tectonics on a global scale. These localities part therefore, be a result of lack of preservation because of comprise a small number of BADR series that can better be subduction or erosion. explained by subduction than crustal assimilation together with a comparable number of intraplate boninite-SHMB series that Archaean metallogeny require refertilization of depleted cratonic lithosphere by subduction-like components prior to a second melting event. Evidence exists that the early atmosphere and the precursors to In our view, as with (Bédard, 2006; Bédard et al., 2013) and the present oceans formed only at the end of the Hadean era, Smithies et al. (2018) in particular, and as discussed further in once the main period of accretion and meteorite bombardment this paper, this can be achieved without global-scale plate had terminated at ca. 3.9 Ga (Kasting, 1993). The implications tectonics. If geological arguments for stagnant lid Archaean for metallogenesis are that sedimentary and hydrothermal tectonics are strong enough, as we believe they are, then process are likely to have been inconsequential in the Hadean, processes such as localized craton accretion could explain any and any ore deposits that did form were, therefore, largely subduction signatures so far. igneous in character. It is conceivable, for example, that oxide and sulphide mineral segregations accumulated from anorthositic Mineral deposits with time and basaltic magmas that did not require subduction for their formation. The Eoarchaean, 3.8 Ga, Isua supracrustal belt and Broad trends emerge when relating ore deposit types to associated Itsaq gneisses of western Greenland, for example, geologic age and tectonic setting. The mechanisms by which comprise largely mafic metavolcanics, as well as chemical the crust has evolved, and the amalgamation and dispersal of sedimentary rocks, and resembles younger greenstone belts from continents over time, have played critical roles in the formation elsewhere in the world, such as Barberton. The Isua belt contains and preservation of all mineral deposits. a major chert–magnetite banded iron-formation component, as Figure 2 illustrates the distribution of major ore deposit types well as minor occurrences of copper–iron sulphides in banded as a function of time, and also with respect to the existence of amphibolites and in iron-formations (Appel, 1990). The largest supercontinents – for completeness this diagram illustrates iron-formation contains an estimated two billion tons of ore at a patterns over the entire range of geologic time, although the grade of 32% Fe. Scheelite mineralization has also been found in focus of this paper is the Precambrian. The distribution of ore both amphibolite and calc-silicate rocks of the Isua belt, an deposit types over time reveals a broad association with the association which suggests a submarine-exhalative origin. periodicity of supercontinent assembly and break-up, with fewer Although the zones of known mineralisation in the Isua belt are deposits forming in periods of supercontinent stasis. In addition, sub-economic, at 3.9 billion years old they represent the oldest ore deposits that formed in a near-surface environment are known ore deposits on Earth. The coexistence of banded iron- prone to erosion and their preservation appears, therefore, to formations and incipient volcanogenic exhalative, massive be biased towards younger or better preserved orogens. This sulphide deposits points to circulation of fluids through ancient

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protocrust, but their formation is not in any way predicated on and the accompanying depletion in the soluble iron content of the existence of plate tectonic processes. the oceans, resulted in fewer BIFs forming after this time The Neoarchaean era represents a period of significant (Bekker et al., 2014; Bekker et al., 2010). The stability of easily crustal growth that was accompanied by a metallogenic ‘boom’ oxidizable minerals such as uraninite and pyrite is also and the proliferation of a wider variety of ore-forming processes dependent on atmospheric oxygen levels and it is, therefore, compared to earlier times. Well mineralized examples of relevant that major Witwatersrand- or Huronian-type placer continental crust that formed in the period 2.8 to 2.5 Ga are deposits did not form after about 2.0 Ga. typified by the granite-greenstone terranes of the Superior Metallogenic patterns during the Paleoproterozoic Era, from Province of Canada, as well as the Yilgarn, Zimbabwe, Tanzania 2.5 to 1.6 Ga, were the products of orogenic processes that are and many other cratons. Greenstone belts host important increasingly recognisable in terms of ore deposit formation in volcanogenic massive sulphide (VMS) Cu-Zn ore bodies, such more recent times. Although the recognition of Proterozoic plate as those at Kidd Creek and Noranda in the Abitibi greenstone movement is still fraught with difficulty, coherent patterns are belt of the Superior Province. A compilation of global VMS beginning to emerge with evidence for what might well have deposits by age (Figure 3) reveals that few deposits are known been the first supercontinent (Nuna) that existed between ca. prior to the Neoarchaean, but that a large number formed at 1.8 Ga and 1.4 Ga (Nance et al., 2014). Nevertheless, prior to around 2.7 Ga, a time that may have coincided with the onset the global tectonic stability created by the long-lived existence of cratonic movements on a more global scale. Off-shore, in of Nuna, the Palaeoproterozoic Era was tectonically active and more distal environments at this time, chemical sedimentation saw the formation of major orogenic gold and VMS base metal gave rise to Algoma type banded iron-formations, examples of deposits (Figure 2). The break-up of the Superia cratonic block, which include the Adams and Sherman deposits, also in the for example, was accompanied, between 2.0 Ga and 1.7 Ga, by Abitibi greenstone belt. Greenstone belts formed at this time also the creation of new oceanic crust and the widespread formation comprise komatiitic basalts that, under conditions favorable for of volcanogenic massive sulphide Cu-Zn deposits such as Flin magma mixing and contamination, exsolved immiscible Ni-Cu- Flon in Canada, Jerome in Arizona, and the Skellefte (Sweden) Fe sulphide fractions to form deposits such as Kambalda in – Lokken (Norway) ores of Scandanavia, all associated with Western Australia and Trojan in Zimbabwe. Also at this time, boninite or arc tholeiite sequences that have a subduction-like steeply-dipping zones of ductile deformation became the focus character. This event is evident in the plateau of VMS deposits of hydrothermal fluid flow derived either from metamorphic at around 1.8 Ga that immediately preceded the assembly of devolatilization or magmatism. These shear zones point to a sub- Nuna, as seen in Figure 3. Likewise, in West Africa for example, vertical style of tectonic displacement and are quite different to the period between 2.1 Ga and 1.9 Ga saw the development of the structural domains that host orogenic gold mineralization in substantial juvenile crust along the margins of the Man Craton later epochs, and in this context do not fit a plate tectonic during the Eburnean orogeny, accompanied by the formation paradigm. Neoarchaean examples of orogenic gold include of extensive orogenic gold mineralization. Like the Neoarchaean, major occurrences such as the St Ives deposit on the Yilgarn therefore, the latter part of the Palaeoproterozoic Era, especially Craton of Western Australia and the Hollinger-McIntyre deposits between 2.1 Ga and 1.8 Ga, was prolific for the formation of the Abitibi greenstone belt on the Superior Craton. As with mineral deposits associated with the assembly of the first VMS deposits, many old orogenic gold deposits formed at supercontinent, Nuna – an event that may have been catalysed around 2.7 Ga, suggesting that the Neoarchaean Era was by the onset of rigid plate movement and subduction. particularly prolific in terms of metallogeny. Subsequent to the amalgamation of Nuna, in late Palaeo- to early Mesoproterozoic times, most parts of the world were Proterozoic metallogeny characterised by a period of relative global stability that resulted in the deposition, between 1.8 Ga and 1.5 Ga, of marginal The broad transition over several hundred million years marine sedimentary basins that host the important SedEx Pb-Zn from the Archaean to the Proterozoic Eon witnessed major ores of eastern Australia (Mount Isa, Broken Hill, and McArthur changes in tectonic regime, crust-forming processes and River) and South Africa (Aggeneys and Gamsberg). Despite the surficial environment, many of which were associated with the fact that a significant proportion of continental crust might attainment of plate rigidity and the onset of subduction. From a have been connected at these times, local orogenic events metallogenic perspective, the Palaeoproterozoic is significant nevertheless occurred, such as the Isan in Australia and the because of atmospheric evolution and in particular the rise of Kheis in southern Africa, the latter events being implicated atmospheric oxygen levels at around 2.3 Ga (the Great in the formation of SedEx and other ore types at this time. In Oxidation Event - GOE). Prior to this, the major oxygen sink addition, anorogenic styles of magmatism appear to have been was the reduced ocean where any photosynthetically produced widespread in Nuna times – in South Australia, for example, the free oxygen was consumed by the oxidation of volcanic gases, 1.59 Ga Roxby Downs granite-rhyolite complex (Johnson and carbon, and ferrous iron. In this environment banded iron- Cross, 1995), host to the enormous magmatic-hydrothermal formations, as well as bedded manganese ores, developed, as Olympic Dam iron oxide-Cu-Au-U deposit, was emplaced. indicated by the widespread preservation of both Algoma- and Anorogenic granite magmatism at 1.88 Ga also gave rise to the Superior- type iron deposits. An increase in the ratio of later stages of IOCG style mineralization (such as Estrela; ferric/ferrous iron in the surface environment at circa 2.3 Ga, Volp, 2005) in the giant deposits of Carajas, Brazil.

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In summary, as more cohesive continents formed from the We suggest that and their inclusions cannot end of Archaean times and plate tectonic processes appear to provide a definitive answer to the time of onset of plate have developed, the range of ore deposit types appears to have tectonics. Sample bias is a serious hurdle to overcome. Dating broadened. Archaean metallogeny has an identifiable character inclusions (usually very small in size) is not a and the ore deposits and their host rocks, as well as the trivial task, while the lack of samples from a wide range of structures that control fluid flow, cannot be linked to processes locations and compositions may constrain attempts to obtain that are recognizably linked to subduction. The broader range representative ages. Thus, it is possible that eclogitic inclusions of magmatic-hydrothermal to hydrothermal deposit types that older than 3 Ga do exist, but have not been sampled yet. become evident in Proterozoic times, including porphyry Cu The carbon and nitrogen isotopic compositions of diamonds deposits such as Haib and Aitik, are more akin to those typifying themselves also cannot provide a definitive answer about their the Phanerozoic Eon when plate tectonic processes were source. Carbon isotopic compositions of mantle diamonds have fully extant. been extensively used to discriminate their origin. Among eclogitic and peridotitic diamond inclusions, the are Diamonds significantly more uniform isotopically (with the mean in δ13C = ~5‰) and are considered to have formed from mantle A key question is whether any biogenic surface carbon has been carbon, while the eclogites show a wide range of carbon isotopic taken into the mantle to depths of over 150 km to make values (δ13C = -41 to +5‰) attributed to a possible subduction diamond and, if so, how this relates to tectonic processes. origin. The organic carbon formed at the surface is isotopically Diamonds and their inclusions are the only natural samples much lighter (δ13C = -40 to 0‰) (Cartigny et al., 2014). brought to the surface from depths of >150 km. Most common The wide range of carbon isotopic compositions in eclogitic lithospheric inclusions in diamond can be subdivided into diamonds does not necessarily reflect a subduction origin (e.g. the eclogitic (e-type) and peridotitic (p-type) with garnet, Cartigny et al., 1998a; Cartigny et al., 1998b). First, to yield such clinopyroxene and sulphides being common for both sets. Less extremely light isotopic signatures (δ13C = <-25‰) the subducted commonly, olivine and orthopyroxene are reported in p-type material would need to consist of only organic carbon, with no inclusions. Because of the very small amount of impurities, it is input from surface carbonates that dominate surface sediments impossible to date diamonds; however, diamond inclusions (~80%) and are isotopically heavier (δ13C = -15 to -5‰). Second, make plausible material for dating. Over the last few decades a it is likely that eclogitic and peridotitic diamonds are produced number of studies have been undertaken on dating diamond by different mechanisms and involve different fluid speciation. inclusions. Shirey and Richardson (2011) compiled the silicate- For instance, it has been shown that, even at high temperatures, and sulphide dates from garnet (bulk), clinopyroxene (bulk) and significant isotopic fractionation could take place between the sulphide (single grain) inclusions worldwide and showed that oxidised (CO2) and the reduced (CH4) forms of carbon by means the oldest inclusions are peridotitic (up to 3.5 Ga), while of Rayleigh distillation (Javoy, 1972). This could lead to the light eclogitic inclusions are younger than 3 Ga. Based on this finding, δ13C in diamonds being precipitated from the methane-rich they concluded that the onset of plate tectonics should have fluids. Methane-related diamond crystallisation in the Earth’s occurred after 3 Ga. mantle was also detected by combined carbon-nitrogen isotopic This was contested by two recent studies on other sets of studies that do not support a recycled origin of carbon (Cartigny diamonds and their inclusions. Smart et al. (2016) analysed et al., 1998b; Javoy, 1972). Mikhail et al. (2014) have also shown carbon and nitrogen isotopic compositions of Archaean placer that diamonds in equilibrium with iron carbide are isotopically diamonds from the Kaapvaal Craton which formed between 3.1 lighter because of the high carbon isotopic fractionation during and 3.5 Ga, and concluded that diamonds must have crystallised the interaction between mantle carbon and native iron at very from a source previously exposed to the surface, arguing the reduced conditions. Thus, although the surface origin of carbon onset of subduction and plate tectonics between 3.1 and 3.5 Ga. cannot be ruled out, there are a number of mantle processes Smit et al., (2019) investigated mass-independent fractionation that cause carbon isotopic fractionation, triggering the formation (MIF) of sulphur isotopes, as recorded prior to 2.4 Ga (Farquhar of isotopically light diamonds. et al., 2000). Because of the change in atmospheric oxygen levels between 2.4 and 2.09 Ga, the style of sulphur isotope The lithosphere fractionation changed and only mass-dependent fractionation was recorded after 2.09 Ga. Smit et al., (2019) analysed sulphur Lithospheric density, thickness and strength are likely to isotopes in sulphide inclusions in diamonds from the West have increased in response to diminishing asthenospheric Africa, Zimbabwe, Kaapvaal and Slave cratons and reported that temperature during planetary history. Cooling increases in a MIF sulphur was not observed in the oldest, 3.5 Ga sulphide plate tectonic planet as slabs are injected deep into the hot inclusions from the Slave craton. Younger (<3 Ga) diamond mantle, ultimately causing the asthenosphere to cool, thin, inclusions from the other studied cratons, however, contained stiffen and, eventually to exterminate plate tectonics. Intra-plate MIF sulphur, consistent with the hypothesis that subduction alkalic basalts will increase in proportion as degrees of melting operated from around 3 Ga. Despite very different approaches, decrease. As the early mantle was hotter by 100 to 200°C; all studies on diamonds and diamond inclusions agree on an the lithosphere was thinner, weaker and more buoyant, so age of approximately 3.0 to 3.1 Ga for the onset of subduction. impeding the onset of plate tectonics. Lithospheric strength is

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Figure 3. Compilation of the ages of VMS deposits (data after Mosier et al., 2009). The majority of deposits appear to have formed over three periods of Earth history; in the Neoarchaean at around 2 700 Ma, in the Palaeoproterozoic at around 1 800 Ma, and in the Palaeozoic at around 400 Ma. Black vertical bars on the left indicate the approximate periods when supercontinents are believed to have existed (after Hawkesworth et al., 2016).

essential for plate tectonics to originate and continue. In that the first one may have been produced by interaction of a particular, oceanic lithosphere must be strong enough to remain large mantle plume head with old “oceanic” lithosphere (Gerya torsionally coherent during plate motion and subduction. et al., 2015) during the global outbreak of mafic magmatism Lithospheric strength increases with thickness, so lithospheric between 2.6 and 2.0 Ga. density and strength have increased together as the Earth has aged and cooled. Subduction and subductability There are three aspects of lithospheric weakness that are required for subduction and plate tectonics to happen. First, the Plate tectonics requires lithosphere that is dense enough to sink lithosphere-asthenosphere boundary must be weak enough for under its own weight. It must be strong enough to remain the lithospheric plates to move over it. This is accomplished by coherent during self-sustaining subduction, and weak enough the asthenosphere being hotter and weaker and aided by the to form localised plate boundaries. Such conditions are only concentration of volatiles at the interface. Such a weak zone is likely to happen in a mature silicate planet, as Earth is today. likely to have existed since lithosphere first formed, very early Special circumstances are required for plate tectonics: the planet in Earth history. The second essential weakness required must be dominantly silicate and conditions must be appropriate for establishing self-sustaining and asymmetric (one-sided) for self-sustaining subduction to occur. Because global plate subduction is evident in the two-dimensional (2-D) numerical motions are mostly powered by the sinking of the negatively experiments of Gerya et al. (2008) which show that the stability, buoyant oceanic lithosphere in subduction zones (although intensity, and mode of subduction require a zone of weak significant roles for ridge push and mantle convection drag for hydrated rocks above the subducted slab. The weak interface is modifying these motions has been repeatedly proposed), the maintained by the release of fluids from the subducted lithosphere must be denser than the underlying asthenosphere. sediments, oceanic crust and serpentinized upper mantle as the Self-sustaining subduction of the oceanic lithosphere, as the slab sinks and is pressurized and heated. The third weakness engine of plate tectonics, cannot take place on a constant-size required for subduction and plate tectonics is for the Earth without spreading ridges to balance the loss of area by development of large-scale, laterally extensive (~1 000 km long) subduction. Therefore, in a stagnant lid planet, MORB cannot weaknesses through the lithosphere where subduction zones be erupted and the non-continental crust must be formed can nucleate. Without such weaknesses, subduction and plate in some other way, probably by the vertical accumulation of tectonics are very unlikely to develop. Such trans-lithospheric basalts, komatiites, and minor differentiated silicic rocks. weaknesses are produced continuously today on Earth by plate Because of the higher temperature of the asthenosphere, the boundaries but it is less obvious how the first trans-lithospheric lithosphere would have been thinner and the mafic-ultramafic weakness was produced on a stagnant-lid Earth. It seems likely crust thicker. Thus, if Earth was a stagnant lid planet prior to

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2.5Ga, it seems unlikely that the pre-2.5 Ga crust was wholly primarily due to delamination; in a plate tectonic Earth, these continental because subduction would then have been unable processes would continue, though probably to a lesser extent, to start. Thus, the distribution of pre-2.5 Ga continental crust is while plate subduction and collision processes would provide particularly important. In this context, the present crustal further sources of additions (through arc and post-collision thickness of individual Archaean cratons was likely achieved magmatism) and subtractions (through tectonic erosion and immediately at the end of crustal inversion and the intrusion of collision-related delamination) (e.g., Dewey and Windley, 1981). late potassic granites and sanukitoids. Had this crust been Potentially, therefore, the crustal growth curve might be globally enveloping, some two thirds of this continental crust expected to contain a discontinuity that marks the transition would have had to have been lost by some process such as from a plume tectonic to a plate tectonic Earth. Dhuime et al. tectonic decretion by subduction erosion after 2.5 Ga. (2012) used a crustal growth curve derived from zircon Subduction, and hence plate tectonics, in a silicate planet, geochemical data to argue for a 3 Ga inception of plate needs lithosphere that is sufficiently dense, stiff, and coherent tectonics, thus contradicting the inference of a 2.5 to 2.3 Ga to sink at weak zones rather than float. The integrated density inception made by ourselves and others based on geological of present-day oceanic lithosphere younger than about criteria and discussed in earlier sections. 20 million years will float unless attached to older lithosphere, However, there is no single, consensus crustal growth curve whereas lithosphere older than 20 million years is unstable and and the Dhuime et al. (2012) discontinuity at 3 Ga is not a can sink. Stern et al, (2018) call this the crossover time, the time feature of other published curves. In a detailed evaluation of the at which the integrated density of the lithosphere equals the plethora of widely variable, published crustal growth curves, density of the asthenosphere (3 250 kg. m-3). A young stagnant Korenaga (2018) usefully subdivided existing curves into three lid of non-continental Archaean lithosphere will be more categories: crust-based, mantle-based and other non-crust-based. buoyant than young MORB lithosphere because of its thick The point is that, even after estimating and taking into account komatiitic crust and its thinner lithosphere resulting from a intracrustal reworking, the first category remains a combination hotter asthenosphere. Moreover, Archaean oceanic lithosphere of crustal recycling and crustal growth and so describes only the would likely not progressively thicken in the same way as lower bound of net crustal growth. In contrast, the second and present-day lithosphere moving away from a ridge because the third categories do describe net crustal growth itself. Korenaga lid would constantly be thermally eroded, by the hot underlying (2018) also criticised part of the numerical methodology used asthenosphere. Therefore, the crossover time of a stagnant lid to process zircon data that was developed by Belousova et al. lithosphere would have been correspondingly longer, so (2010) and used by Dhuime et al. (2012), to which (to our hindering the onset of subduction and plate tectonics. If plate knowledge) there has been no counter-response to date. Based tectonics did not initiate until after 2.5 Ga, this may be explained on this, the absence of corroboration from other crustal growth at least in part by a very long Archaean crossover time. curves, and the fact that the Dhuime et al. (2012) curve also fits Thus we concur that stagnant lid is the default mode of into the crust-based category, we do not, at present, regard the planetary tectonics (Stern et al., 2016; 2018). It is the potential 3 Ga discontinuity as reliable. energy resulting from denser lithosphere on weaker Even taking only Korenaga’s (2018) second and third asthenosphere that provides most of the energy for plate motion categories, growth curves vary significantly, essentially between (Forsyth and Uyeda, 1975; Lithgow-Bertelloni, 2014). Such a those requiring ca. 50% crustal growth by 2.5 Ga and several density inversion does not exist for continental lithosphere or others requiring 100% crustal growth within the Hadean and/or for oceanic plateaux, where low-density crust reduces the Eoarchaean followed by an approximate net balance between overall lithospheric density. It was also less likely early in creation and destruction of continental crust from then through Earth history, when hotter mantle resulted in thinner mantle 2.5 Ga until the present day. One outlier, that of Fyfe (1978), has lithosphere and thicker oceanic crust. crustal growth reaching a maximum of ca. 120% at ca. 2.5 Ga before declining to the present day, although this is based on Continental crust creation and destruction assumptions and estimates rather than data. Apart from the latter, there is no discontinuity in the early Palaeoproterozoic that One of the distinctive features of present-day Earth is its two might mark the change in global tectonics at that time. This lack types of crust: 40 km thick continental crust with an andesitic of a discontinuity need not negate the hypothesis for early bulk composition and 6 km thick oceanic crust with a basaltic Palaeoproterozoic plate tectonic initiation, however, as it could bulk composition, the former making up approximately 40% of be explained simply by gradually decreasing plume activity and Earth’s surface area and 70% of total crustal volume. It has been gradually increasing plate tectonic activity on either side of the argued that the pattern of growth of continental crust from Archaean-Proterozoic boundary. Alternatively, plume tectonics Hadean to Anthropocene is a function of Earth’s geodynamic and plate tectonics may have led to approximately the same net history and so potentially carries information on the age of crustal growth. inception of plate tectonics. Conceptually, crustal growth Korenaga (2018) also suggests that, as crustal recycling is represents the net balance between addition of material to the largely a product of subduction and hence plate tectonics, the crust from the mantle and subtraction of material from the crust difference between crust-based and mantle-based models to the mantle. Before the start of plate tectonics, additions would has the potential to determine when plate tectonics started. likely be primarily due to plume magmatism with subtractions At present, however, the crustal growth curves are insufficiently

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precise to test this hypothesis; moreover, we cannot be sure variably, adakites, miogeoclinal platforms, the classic paired that crustal recycling was negligible during periods of plume positive-negative gravity anomalies, suturing, and horizontal tectonics as delamination is an important part of many Archaean shortening, all of which support a plate tectonic origin back to pre-plate tectonic geodynamic models (e.g. Zegers and van at least 2.0 Ga. These orogenic belts weld together Archaean Keken, 2001; Bédard, 2006). cratons and are best-described as collisional orogens, themselves Also relevant to this study is the volume and distribution of an indication of the former subduction of some form of oceanic Archaean continental crust at the beginning of the Proterozoic, tract, either a stagnant lid lithosphere or lithosphere generated namely, at the time we infer plate tectonics to have started. One by sea-floor spreading. The 2.04 to 2.24 Ga Birimian in the end-member option is that there was a global continental Reguibat of Mauritania and the Man-Leo Shield of Cote d’Ivoire coverage of Archaean continental crust that was subsequently (Combs, 2018) is very similar to the Neoproterozoic Pan-African segmented and shortened into the extant Archaean cratons. This, and models for the Phanerozoic of central Asia (Sengor et al., however, is extremely unlikely given that there is insufficient post- 1993), comprising huge tracts of accreted volcanic arcs. Archaean shortening in the Archaean cratons. We therefore start While there are arguments (summarised by Stern, 2020) with the premise that Archaean continental crust was focused in for a hiatus in plate tectonic activity during the ‘boring and around the present cratonic nuclei with some form of once billion’ between the Neoproterozoic-Anthropocene and unsubductable non-continental crust between them. Palaeoproterozoic plate tectonic periods described above, this Based on the volume of present-day Archaean cratons, the is not supported by the geology. Although the ‘boring billion’ interstitial crust would have covered some 86% of the Earth’s was dominated by extensive intraplate magmatism and tectonics surface. However, crustal growth curves, including a correction within the Nuna and Rodinia supercontinents, there are many for intra-crustal recycling, reveal that Archaean continental examples of accretionary convergent margins, collision zones, crust was much more extensive than this. At the other extreme, ophiolites, island arcs and their related ore deposits at the models such as those of Armstrong (1968, 1991), Rosas and supercontinent margins. Thus, we have no geological reason to Korenaga (2018) and others have continental crust occupying believe that plate tectonics in some form was not continuous much the same volume (70%) and surface area (40%) as from the early Palaeoproterozoic onwards. present-day continental crust, in which case the interstitial non- The blobby to sub-linear TTG dome and greenstone keel continental crust would have occupied some 60% of the Earth’s pattern that dominates the upper crust from about 3.6 to 2.6 Ga, surface when plate tectonics started. There are obviously many and the stratigraphic sequences of the Archaean, show no caveats to such estimates, but there is still a high probability patterns that can be related to plate boundary zones, and no that interstitial non-continental crust was part of an Archaean, lithological assemblages like those of the plate tectonic world. plate tectonic Earth. That being the case, we cannot envisage The dome-and-keel patterns developed in response to crustal how a non-plate tectonic inversion process could have been inversion of a light, hot, TTG crust beneath a colder, heavier, creating the continental crust of the cratons while, in a volcanic crust, producing rising and ballooning TTG domes and surrounding ocean, plate tectonics was generating ridges, arcs greenstones sinking by sagduction and drip. The greenstone and collisions to produce rocks, none of which were keels are characterised by steep to vertical prolate fabrics while preserved. Thus, we incline towards massive tectonic decretion the TTG fabrics change from almost isotropic in dome centres from about 2.3 Ga, during which time any residual Hadean through increasing flattening to plane strain at the margins. and Archaean non-continental lithosphere was subducted, Strain patterns are, locally, more complex and polyphase, but balanced in terms of surface area by creation of new do not imply development prior to, or independent of, diapiric Palaeoproterozic oceanic lithosphere. ballooning (Snowden and Bickle, 1976). The common TTG gneisses may have been formed by flow and flattening beneath Conclusions the plutons. Komatiites, are mainly Archaean; they cannot be oceanic We consider 2.5 Ga, the end of the Archaean Eon, as the crust because they are associated with sedimentary rocks, basalts beginning of a time of fundamental change in Earth’s tectonic and silicic rocks in thick sequences on continental crust; they behaviour that led to the establishment of plate tectonics by were likely developed by peridotite partial melting in plumes at soon thereafter. Consensus is not a necessary prerequisite for mantle potential temperatures at least 200°C higher than present. truth, but there is general agreement, for all the reasons given The sedimentary rocks, usually water-lain, are commonly by Stern (2005) that plate tectonics has operated since at least volcaniclastic but do not show the structural patterns of 0.7 Ga. These reasons include the presence of ophiolites, accretionary prisms. Claims have been made that ultramafic- blueschists, UHP metamorphism, sutures and collision zones, basalt associations are parts of ophiolite complexes and paired metamorphic belts, zones of substantial crustal represent slices of obducted oceanic lithosphere (such as in the shortening, major strike-slip faulting, and linear calc-alkaline Kromberg Formation at Barberton; Grosch and Slama, 2017) but, magmatic zones. Palaeomagnetism shows that there was relative although they contain pillow lavas and are therefore sub- motion among continental cratons back to at least 1.88 Ga aqueous, they do not resemble, even remotely, the classic (Evans and Pisarevsky, 2008; Condie and Kroener, 2008), ophiolite sequences of the Phanerozoic and the characteristics The Wopmay and other Palaeoproterozoic orogens such as of their constituent components. For example, to our the Trans-Hudson, the Capricorn, and Limpopo, are belts of, knowledge, no reported Archaean peridotites exhibit the high-

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temperature ductile deformation seen in residual ophiolitic References mantle; instead they most closely resemble in texture and composition the cumulate bases of ultramafic flows or layered Adam, J., Rushmer, T., O’Neil, J. and Francis, D., 2012. Hadean greenstones from the Nuvvuagittuq fold belt and the origin of the Earth’s early intrusions. In other words, scraps of serpentinite, gabbro, and continental crust. Geology, 40, 363-366. pillow basalt do not necessarily constitute an ophiolite. Anhaeusser, C.R., 1969. The stratigraphy, structure, and gold mineralization of We emphasise also that one cannot use a simple shopping the Jamestown and Sheba Hills areas of the Barberton Mountain Land. list of features and characteristics to establish a plate tectonic Anhaeusser, C.R., Mason, R., Viljoen, M.J. and Viljoen, R.P., 1969. A reappraisal origin. In particular, no broad rock type name such as adakite, of some aspects of Precambrian shield geology Geological Society of America Bulletin, 80, 2175-2200. andesite, boninite, sanukite or tonalite can be treated as definitive; Appel, P.W.U., 1990. Mineral occurrences in the 3.6 Ga old Isua supracrustal these are merely names that conceal wide variations in petrology, belt, West Greenland. In: S.M. Naqvi (Edtor), Developments in Precambrian geochemistry and significance and can form in more than one Geology. 8. Elsevier, 593-603. setting. It is necessary to specify the precise petrology and Armstrong, R.L., 1968. A model for the evolution of strontium and lead chemistry in rock suites that show great variation among a variety isotopes in a dynamic earth. Reviews of Geophysics, 6, 175-199. Armstrong, R.L., 1991. The persistent myth of crustal growth. Australian Journal of tectonic situations. The same type of argument applies to of Earth Sciences. 38, 613-630. structural interpretations. For example, horizontal shortening is Bailie, R., Gutzmer, J. and Rajesh, H.M., 2010. Lithogeochemistry as a tracer common in Archaean rocks but could have developed at the of the tectonic setting, lateral integrity and mineralization of a highly margins of, and between, rising and ballooning plutons rather metamorphosed Mesoproterozoic volcanic arc sequence on the eastern than by plate tectonic processes. Moreover, as we have discussed, margin of the Namaqua Province, South Africa. Lithos 11, 345-362. Bédard, J.H., 2006. A catalytic delamination-driven model for coupled genesis even apparently robust geochemistry-based signatures such of Archaean crust and sub-continental lithospheric mantle. Geochimica Et as ‘organic’ carbon isotope ratios in diamonds, ‘subduction’ Cosmochimica Acta, 70, 1188-1214. signatures in volcanic rocks and discontinuities in crustal growth Bédard, J.H., 2018. Stagnant lids and mantle overturns: Implications for curves do have alternative explanations. Archaean tectonics, magmagenesis, crustal growth, mantle evolution, and Thus, we argue that, geologically, it is the pattern and the start of plate tectonics. Geoscience Frontiers, 9, 19-49. Bédard, J.H., Harris, L.B. and Thurston, P.C., 2013. The hunting of the snArc. arrangement of rock types in linear and curvilinear belts that Precambrian Research, 229, 20-48. separate older platforms and cratons and are indicative of extinct Bekker, A. et al., 2014. Iron formations: their origins and implications for plate boundary zones, just as for recent and extant ones. The ancient seawater chemistry. In: H.D. Holland and K.K. Turekian (Editors), difference is that, whereas today we can see oceanic lithosphere Treatise on geochemistry. Elsevier, 561-628. entering modern subduction zones, old subduction zones can Bekker, A. et al., 2010. Iron formation: the sedimentary product of a complex interplay among mantle, tectonic, oceanic, and biospheric processes. be inferred only from sutures, arc-volcanic belts, orogens and Economic Geology, 105, 467-508. palaeomagnetism and this is true for all types of plate tectonics, Belousova, E.A., Kostitsyn, Y.A., Griffin, W.L., Begg, G.C., O’Reilly, S.Y. and including ‘flat’ and ‘hot’ subduction. Such features are only really Pearson, N.J., 2010. The growth of the continental crust: constraints from clear back to the Palaeoproterozoic (including the ‘boring zircon Hf-isotope data. Lithos, 119, 457-466. billion’), to 2.0 Ga and, perhaps to 2.5 Ga. Opinion about the Belyaev, V., Gornova, M., Medvedev, A., Dril, S. and Karimov, A., 2017. Proterozoic Eastern Sayan ophiolites (Central Asian Orogenic Belt) record tectonics of the Archaean tends to be polarised between the subduction initiation in vicinity of continental block. EGUGA, 16079. ‘plate tectonic enthusiasts’ and a ‘stagnant lid vertical tectonics’ Bickford, M. and Van Schmus, W., 1985. Discovery of two Proterozoic granite- group. We lean strongly towards the latter. Of course, some rhyolite terranes in the buried midcontinent basement: The case for shallow method of taking crustal rocks back into the asthenosphere in drill holes, Observation of the Continental Crust through Drilling I. Springer, the Archaean is likely needed to explain some crustal recycling 355-364. Bickford, M., Van Schmus, W. and Zietz, I., 1981. Interpretation of Proterozoic models and geochemical features, but that remains well short basement in the midcontinent, Geol. Soc. Am. Abstr. Programs, 410pp. of requiring global-scale plate tectonic processes. Bickle, M., 1978. Heat loss from the Earth: a constraint on Archaean tectonics from the relation between geothermal gradients and the rate of plate Acknowledgements production. Earth and Planetary Science Letters, 40, 301-315. Bickle, M., 1986. Implications of melting for stabilisation of the lithosphere and heat loss in the Archaean. Earth and Planetary Science Letters, 80, 314-324 We are grateful for many discussions and field trips with Carl Bickle, M.J., Bettenay, L.F., Boulter, C.A., Groves, D.I. and Morant, P., 1980. Anhaeusser, Kevin Burke, Bill Collins, Andrew Glikson, Al Horizontal tectonic interaction of an Archean gneiss belt and greenstones, Goodwin, Warren Hamilton, Bob Stern, Alec Trendall, Steve Pilbara Block, Western-Australia. Geology, 8, 525-529. Moorbath, and Maarten de Wit., variously in the Barberton, Bickle, M.J., Martin, A. and Nisbet, E.G., 1975. Basaltic and peridotitic Superior, Pilbara, West Greenland, and Yilgarn Cratons, and komatiites, stromatolites and a basal unconformity in the Belingwe Greenstone Belt, Rhodesia, Earth Plan. Sci. Lett., 27, 155-162. many geologists working in Proterozoic and Phanerozoic Bickle, M.J., Nisbet, E. G. and Martin, A., 1994 Archaean greenstone belts are terrains. We dedicate this paper to Carl Anhaeusser whose not oceanic crust-J. Geol. 102, 121-138. superb field-work in southern Africa laid bare the essentials of Bleeker, W., 2003. The late Archaean record: a puzzle in ca. 35 pieces. Lithos, the Kaapvaal Craton, and John Ramsay, whose inceptive 71, 99-134. structural work in Barberton laid the groundwork for Blenkinsop, T.G., 2019.The geology of Zimbabwe. In: M. Predergast and J. Holloway (Editors), Mining in Zimbabwe, The Chamber of Mines of understanding its structural evolution, and whose recent death Zimbabwe, Harare. 1-38. is a great loss to geology. Finally, we are very grateful for the Boily, M. and Dion, C., 2002. Geochemistry of boninite-type volcanic rocks perceptive and incisive reviews by Paul Dirks and Jan Kramers in the Frotet-Evans greenstone belt, Opatica subprovince, Quebec: that greatly improved this paper.

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