Tag: Lid tectonics

Sagduction of greenstone belts and formation of Archaean continental crust

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Simplified geological map of the Archaean Yilgarn Craton in Western Australia. Credit: Geological Survey of Western Australia

Every ancient craton seen from space shows patterns that are unique to Archaean continental crust: elongated, ‘canoe-shaped’ greenstone belts enveloped by granitic gneisses, both of which are punctured by domes of younger, less deformed granites. The Yilgarn Craton of Western Australia is a typical granite-greenstone terrain. Greenstone belts contain lavas of ultramafic, basaltic and andesitic compositions, which in undeformed settings show the typical pillow structures formed by submarine volcanic extrusion. There are also layered mafic to ultramafic complexes, formed by fractional crystallisation, minor sedimentary sequences and occasionally more felsic lavas and ashes. The enveloping grey gneisses are dominantly highly deformed tonalite-trondhjemite-granodiorite (TTG) composition that suggest that they formed from large volumes of sodium-rich, silicic magmas, probably generated at depth by partial melting of hydrated basaltic rocks.

The heat producing radioactive isotopes of potassium, uranium and thorium in both the Archaean mantle and crust would have been more abundant before 2.5 Ga ago, because they decay over time. Consequently the Earth’s interior would have then generated more heat than now, gradually to escape by thermal conduction towards the cooler surface. The presence of pillow lavas and detrital sediments in greenstone belts indicate that surface temperatures during the Archaean Eon were below the boiling point of water; in fact probably much the same as in the tropics at present. Indeed there is evidence that Earth was then a water world. It may even have been so during the Hadean, as revealed by the oxygen-isotope data in 4.4 Ga zircon grains. The broad conclusion from such findings is that the Archaean geothermal gradient was much steeper; there would have been a greater temperature increase with depth than now and new crust would have cooled more slowly. Subduction of cool lithosphere would have been less likely than in later times, especially as higher mantle heat production would have generated new crust more quickly. Another likely possibility is that far more heat would have been moved by convection: there would have been more mantle-penetrating plumes and they would have been larger. Large mantle plumes of the Phanerozoic have generated vast ocean floor plateaus, such as the Kerguelen and Ontong Java Plateau.

A group of geoscience researchers at The University of Hong Kong and international colleagues recently completed a geological and geochemical study of the North China Craton, analysing their data in the light of recently emerging views on Archaean processes (Dingyi Zhao et al, A two-stage mantle plume-sagduction origin of Archean continental crust revealed by water and oxygen isotopes of TTGs, Science Advances, v. 11, article eadr9513  ; DOI: 10.1126/sciadv.adr9513).They found compelling evidence that ~2.5 Ga-old Neoarchaean TTG gneisses in the North China granite-greenstone terrain formed by partial melting of an earlier mafic-ultramafic greenstone crust with high water content. They consider this to support a two-stage model for the generation of the North China Craton’s crust above a vast mantle plume. The first stage at around 2.7 Ga was the arrival of the plume at the base of the lithosphere, which partially melted as a result of the decompression of the rising ultramafic plume. The resulting mafic magma created an oceanic plateau partly by underplating the older lithosphere, intruding it and erupting onto the older ocean floor. This created the precursors of the craton’s greenstones, the upper part of which interacted directly with seawater to become hydrothermally altered. They underwent minor partial melting to produce small TTG intrusions. A second plume arriving at ~2.5 Ga resulted in sinking of the greenstones under their own weight to mix or ‘hybridise’ with the re-heated lower crust. This caused the greenstones substantially to partially melt and so generate voluminous TTG magmas that rose as the greenstones subsided. . It seems likely that this dynamic, hot environment deformed the TTGs as they rose to create the grey gneisses so typical of Archaean granite-greenstone terranes. [Note: The key evidence for Dingyi Zhao et al.’s conclusions is that the two TTG pulses yielded the 2.7 and 2.5 Ga ages, and show significantly different oxygen isotope data (δ18O)].

Two stages of TTG gneiss formation in the North China Craton and the sinking (sagduction) of greenstone belts in the second phase. Credit: Dingyi Zhao et al., Fig 4)

Such a petrogenetic scenario, termed sagduction by Dingyi Zhao and colleagues, also helps explain the unique keel-like nature of greenstone belts, and abundant evidence of vertical tectonics in many Archaean terrains (see: Vertical tectonics and formation of Archaean crust; January 2002), Their model is not entirely new, but is better supported by data than earlier, more speculative ideas. That such processes have been recognised in the Neoarchaean – the North China Craton is one of the youngest granite-greenstone terrains – may well apply to far older Archaean continental crust generation. It is perhaps the last of a series of such events that began in the Hadean, as summarised in the previous Earth-logs post.

News about when subduction began

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Tangible signs of past subduction take the form of rocks whose mineralogy shows that they have been metamorphosed under conditions of high pressure and low temperature, and then returned to the surface somehow. Ocean-crust basaltic rocks become blueschist and eclogite. The latter is denser than mantle peridotite so that oceanic lithosphere can sink and be recycled. That provides the slab-pull force, which is the major driver of plate tectonics. Unfortunately, neither blueschists nor eclogites are found in metamorphic complexes older than about 800 Ma. This absence of direct proof of subduction and thus modern style plate tectonics has resulted in lively discussion and research seeking indirect evidence for when it did begin, the progress of which since 2000 you can follow through the index for annual logs about tectonics. An interesting new approach emerged in 2017 that sought a general theory for the evolution of silicate planets, which involves the concept of ‘lid tectonics’. A planet in a stagnant-lid phase has a lithosphere that is weak as a result of high temperatures: indeed so weak and warm that subduction was impossible. Stagnant-lid tectonics does not recycle crustal material back to its source in the mantle and it simply builds up the lithosphere. Once planetary heat production wanes below a threshold level that permits a rigid lithosphere, parts of the lid can be driven into the mantle. The beginnings of this mobile-lid phase and thus plate tectonics of some kind involves surface materials in mantle convection: the may be recycled.

Cartoon of possible Hadean stagnant lid tectonics, dominated by mantle plumes. (Credit: Bédard, J.H. 2018, Fig 3B, DOI: 10.1016/j.gsf.2017.01.005)

A group of geochemists from China, Canada and Australia have sought evidence for recycled crustal rocks from silicon and oxygen isotopes in the oldest large Archaean terrane, the  4.0 Ga old Acasta Gneiss Complex in northern Canada (Zhang, Q. and 10 others 2023. No evidence of supracrustal recycling in Si-O isotopes of Earth’s oldest rocks 4 Ga ago. Science Advances, v.9, article eadf0693; DOI: 10.1126/sciadv.adf0693). Silicon has three stable isotopes 28Si, 29Si, and 30Si. As happens with a number of elements, various geochemical processes are able to selectively change the relative proportions of such isotopes: a process known as isotope fractionation. As regards silicon isotopes used to chart lithosphere recycling, the basic steps are as follows: Organisms that now remove silicon from solution in seawater to form their hard parts and accumulate in death as fine sediments like flint had not evolved in the Archaean. Because of that reasonable supposition it has been suggested that seawater during the Archaean contained far more dissolved silicon than it does now. Such a rich source of Si would have entered Archaean oceanic crust and ocean-floor sediments to precipitate silica ‘cement’. The heaviest isotope 30Si would have left solution more easily than the lighter two. Should such silicified lithosphere have descended to depths in the mantle where it could partially melt the anomalously high 30Si would be transferred to the resulting magmas.

Proportions of 30Si in zircons, quartz and whole rock for Acasta gneisses (coloured), other Archaean areas (grey) and Jack Hills zircons (open circles. Vertical lines are error bars. (Credit: simplified from Zhang et al. Fig 1)

Stable-isotope analyses by Zhang et al. revealed that zircon and quartz grains and bulk rock samples from the Acasta gneisses, with undisturbed U-Pb ages, contain 30Si in about the same proportions relative to silicon’s other stable isotopes as do samples of the mantle. So it seems that the dominant trondhjemite-tonalite-granodiorite (TTG) rocks that make up the oldest Acasta gneisses were formed by partial melting of a source that did not contain rocks from the ocean crust. Yet the Acasta Gneiss Complex also contains younger granitic rocks (3.75 to 3.50 Ga) and they are significantly more enriched in 30Si, as expected from a deep source that contained formerly oceanic rocks. A similar ‘heavy’ silicon-isotope signature is also found in samples from other Archaean terranes that are less than 3.8 Ga old. Thus a major shift from stagnant-lid tectonics to the mobile-lid form may have occurred at the end of the Hadean. But apart from the Acasta Gneiss Complex only one other, much smaller Hadean terrane has been discovered, the 4.2 Ga Nuvvuagittuq Greenstone Belt. It occupies a mere 20 km2 on the eastern shore of Hudson Bay in Canada, and appears to be a sample of Hadean oceanic crust. It does include TTG gneisses, but they are about 3.8 Ga old and contain isotopically heavy silicon. So it seems unlikely that testing this hypothesis with silicon-isotope data from other Hadean gneissic terranes will be possible for quite a while, if at all.

Evidence for an early Archaean transition to subduction

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Modern plate tectonics is largely driven by slab-pull: a consequence of high-pressure, low-temperature metamorphism of the oceanic crust far from its origin at an oceanic ridge. As it ages, basaltic crust cools, become increasingly hydrated by hydrothermal circulation of seawater through it and its density increases. That is why the abyssal plains of the ocean floor are so deep relative to the shallower oceanic ridges where it formed. Due to the decrease in the Earth’s internal heat production by decay of radioactive isotopes, once oceanic lithosphere breaks and begins to descend high-P low-T metamorphism transforms the basaltic crust to a denser form: eclogite, in which the dense, anhydrous minerals garnet and sodium-rich pyroxene (omphacite) form. Depending on local heat flow, the entire oceanic slab may then exceed the density of the upper mantle to drag the plate downwards under gravity. Metamorphic reactions of any P-T regime creates minerals less capable of holding water and drive H2O-rich fluids upwards into the overriding lithosphere, thus inducing it to partially melt. Magmas produced by this create volcanism at the surface, either at oceanic island arcs or near to continental margins, depending on the initial position of the plate subduction.

A direct proof of active subduction in the geological record is the presence of eclogite and related blueschists. Such rocks are unknown before 2100 Ma ago (mid-Palaeoproterozoic of the Democratic Republic of Congo) but there are geochemical means of ‘sensing’ plate tectonic control over arc magmatism (See: So, when did plate tectonics start up? February 2016).  The relative proportions of rare-earth elements in ancient magmatic rocks that make up the bulk of continental crust once seemed to suggest that plate tectonics started at the end of the Archaean Eon (~2500 Ma). That method, however, was quite crude and has been superseded by looking in great detail at the geochemistry of the Earth’s most durable mineral: zircon (ZrSiO4), which began more than two decades ago. Minute grains of that mineral most famously have pushed back the geological record into what was long believed to be half a billion years with no suggestion of a history: the Hadean. Zircon grains extracted from a variety of ancient sediments have yielded U-Pb ages of their crystallisation from igneous magma that extend back 4.4 billion years (Ga) (see: Pushing back the “vestige of a beginning”;January 2001).  

Though simple in their basic chemical formula, zircons sponge-up a large range of other trace elements from their parent magma. So, in a sense, each tiny grain is a capsule of their geochemical environment at the time they crystallised. In 2020 Australian geochemists presented the trace-element geochemistry of 32 zircons extracted from a 3.3 Ga old sedimentary conglomerate in the Jack Hills of Western Australia, which lie within an ancient continental nucleus or craton. They concluded that those zircons mainly reveal that they formed in andesitic magmas, little different from the volcanic rocks that are erupted today above subduction zones. From those data it might seem that some form of plate tectonics has been present since shortly after the Earth’s formation. Oxygen-isotope data from zircons are useful in checking whether zircons had formed in magmas derived directly from partial melting of mantle rocks or by recycling of crustal magmatic rocks through subduction. Such a study in 2012 (see: Charting the growth of continental crust; March 2012) that used a very much larger number of detrital zircon grains from Australia, Eurasia, North America, and South America seemed, in retrospect, to contradict a subduction-since-the-start view of Earth dynamics and crust formation. Instead it suggested that recycling of crust, and thus plate-tectonic subduction, first showed itself in zircon geochemistry at about 3 Ga ago.

Detailed chemical and isotopic analysis of zircons using a variety of instruments has steadily become faster and cheaper. Actually finding the grains is much easier than doing interesting things with them. It is a matter of crushing the host rock to ‘liberate’ the grains. Sedimentary hosts that have not been strongly metamorphosed are much more tractable than igneous rocks. Being denser than quartz, the dominant sedimentary mineral, zircon can be separated from it along with other dense, trace minerals, and from them in turn by various methods based on magnetic and electrical properties. Zircons can then be picked out manually because of their distinctive colours and shapes. A tedious process, but there are now several thousand fully analysed zircons aged between 3.0 to 4.4 Ga, from eleven cratons that underpin Australia, North America, India, Greenland and southern Africa. The latest come from a sandstone bed laid down about 3.31 Ga ago in the Barberton area of South Africa (Drabon, N. et al. 2022. Destabilization of Long‐Lived Hadean Protocrust and the Onset of Pervasive Hydrous Melting at 3.8 Ga. AGU Advances, v. 3, article e2021AV000520; DOI: 10.1029/2021AV000520). The authors measured lutetium (Lu), hafnium (Hf) and oxygen isotopes, and concentrations of a suite of trace element in 329 zircons from Barberton dated between 3.3 to 4.15 Ga.

A schematic model of transition from Hadean-Eoarchaean lid tectonics to a type of plate tectonics that subsequently evolved to its current form, based on hafnium isotope data in ancient zircons (credit: Bauer et al. 2020; Fig 3)

The Hf isotopes show two main groups relative to the values for chondritic meteorites (assumed to reflect the composition of the bulk Earth). Zircons dated between 3.8 and 4.15 Ga all show values below that expected for the whole Earth. Those between 3.3 and 3.8 Ga show a broader range of values that extend above chondritic levels. The transition in data at around 3.8 Ga is also present in age plots of uranium relative to niobium and scandium relative to ytterbium, and to a lesser extent in the oxygen isotope data. On the basis of these data, something fundamentally changed in the way the Earth worked at around 3.8 Ga. Nadja Drabon and colleagues ascribe the chemical features of Hadean and Eoarchaean zircons to an early protocrust formed by melting of chemically undepleted mantle. This gradually built up and remained more or less stable for more than 600 Ma, without being substantially remelted through recycling back to mantle depths. After 3.8 billion years ago, geochemical signatures of the zircons start showing similarities to those of zircons derived from modern subduction zones. Hf isotopes and trace-element geochemistry in 3.6 to 3.8 Ga-old  detrital zircons from other cratons are consistent with a 200 Ma transition from ‘lid’ tectonics (see: Lid tectonics on Earth; December 2017) to the familiar tectonics of rigid plates whose basalt-capped lithosphere ultimately returns to the mantle to be involved in formation of new magmas from which continental crust stems. Parts of plates bolstered by this new, low density crust largely remain at the surface.

While Drabon et al. do provide new data from South Africa’s Kaapvaal craton, their conclusions are similar to earlier work by other geochemists based on data from other area (e.g. Bauer, A.M. et al. 2020. Hafnium isotopes in zircons document the gradual onset of mobile-lid tectonics. Geochemical Perspectives Letters, v. 14; DOI: 10.7185/geochemlet.2015), which the accompanying figure illustrates.

See also: Earliest geochemical evidence of plate tectonics found in 3.8-billion-year-old crystal. Science Daily, 21 April 2022. 3.8-Billion-Year-Old Zircons Offer Clues to When Earth’s Plate Tectonics Began. SciNews, 26 April 2022

Lid tectonics on Earth

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Geoscientists have become used to thinking of the Earth as being dominated by plate tectonics in which large, rigid plates of lithosphere move across the surface. They are driven mainly by the sinking of cold, densified lithosphere in slabs at subduction zones. The volume of recycled slabs is replaced by continual supply of mafic magma to form oceanic crust at constructive margins. Such a process has long been considered to have reached far back into the Precambrian past and there are lively debates concerning when this modus operandi first arose and what preceded it. Now that we know more about other rocky planets and moons it appears that Earth is the only one on which plate tectonics has occurred. The other, more common, behaviour is dominated by stagnancy, although some worlds evidence volcanism and resurfacing as a result of giant impacts. Their subdued activity has come to be known as ‘lid tectonics’, in which their highly viscous innards slowly convect beneath a rigid, stagnant lid through which thermal energy is lost by convection: they are ‘one-plate’ systems. Although Earth loses internal heat by conduction through plate interiors, a large amount dissipates by convection associated with constructive margins: the oceanic parts of its plates lose heat laterally, as they grow older. Six papers in an advance, online issue of the free-access journal Geoscience Frontiers are concerned with the issue of terrestrial lid-tectonics and whether or not it dominated the Earth repeatedly in its Precambrian history.

A model is emerging for a hot, early Earth that was dominated by a form of lid tectonics (Bédard, J.H. 2018 Stagnant lids and mantle overturns: implications for Archaean tectonics, magmagenesis, crustal growth, mantle evolution, and the start of plate tectonics. Geoscience Frontiers, v. 9, 19-49; https://doi.org/10.1016/j.gsf.2017.01.005). Bedard’s model centres on lithosphere that was so weak because of its temperature that its subduction was impossible. Density of the lithosphere rarely increased above that of the mantle because the necessary mineralogical changes were not achieved – those involved in plate tectonics require low-temperature, high-pressure metamorphism as oceanic lithosphere is driven down at modern subduction zones. Even if such reactions did happen, the lithosphere would have been too weak to sustain slab-pull force and dense lithosphere would have simply ‘dripped’ back to the mantle. Mantle convection in a hotter Earth would have been in the form of large, long-lived upwelling zones rather than the relatively ephemeral and narrow plumes known today. Low density materials resulting from magma fractionation, the precursors of continental crust, would have been shifted willy-nilly across the face of the planet to collide. accrete and undergo repeated partial melting. In Bedard’s view, plate tectonics arose as Earth’s heat production waned below a threshold that permitted rigid lithosphere, probably in the late Archaean, to dominate after 2.5 Ga.

Bédard’s impression of an early Archaean lid-tectonic scenario. (credit: Jean H Bédard 2018, Figure 3B)

A radically different view is that stagnant-lid episodes alternated with periods of limited subduction and plate tectonics in the Archaean. Some Archaean cratons – the so-called ‘granite-greenstone terrains – seems to provide geological evidence for lid tectonics (Wyman, D. 2018. Do cratons preserve evidence of stagnant lid tectonics? Geoscience Frontiers, v. 9, 19-49; https://dx.doi.org/10.1016/j.gsf.2017.02.001). Others, such as the famous Isua supracrustal belt in West Greenland hint at plate tectonics. John Piper, of Liverpool University in Britain, argues from a series of Archaean palaeomagnetic polar wander curves that in three periods – ~2650 to 2200 Ma, 1550 to 1250 Ma, and 800 to 600 Ma – the poles shifted comparatively slowly with respect to the cratons providing the magnetic data; a feature that Piper ascribes to dominant lid tectonics (Piper, J.D.A., 2018. Dominant Lid Tectonics behaviour of continental lithosphere in Precambrian Times: palaeomagnetism confirms prolonged quasi-integrity and absence of Supercontinent Cycles. Geoscience Frontiers, v. 9, p. 61-89; https://doi.org/10.1016/j.gsf.2017.07.009). Similarly, there is some evidence based on the geochemical variation of basaltic rocks derived from the mantle. Through the Archaean, geochemical changes roughly follow cycles in the abundance of zircon radiometric ages and other geological changes that may reflect plate- and lid-tectonic episodes (Condie, K.C. 2018. A planet in transition: the onset of plate tectonics on Earth between 3 and 2 Ga? Geoscience Frontiers, v. 9, p. 51-60; https://doi.org/10.1016/j.gsf.2016.09.001). Interestingly, the age-frequency plot of almost three thousand Archaean and Hadean zircons recovered from the famous 1.6 Ga old sandstones of the Jack Hills Formation in Western Australia reveals similar cycles that may reflect such tectonic fluctuations in the Hadean (Wang, Q. & Wilde, S.A. 2017. New constraints on the Hadean to Proterozoic history of the Jack Hills belt,Western Australia. Gondwana Research, v. 55, p. 74-91; https://doi.org/10.1016/j.gr.2017.11.008). Since zircons are most likely to crystallize from intermediate and felsic magmas – i.e. precursors of continental material – their abundance in the Jack Hills rocks suggests that their source must have been in the 3.7 to 3.3 Ga gneisses on which the younger sediments rest. That is, part of those Archaean gneisses may well be made up of Hadean continental material that was repeatedly reworked and maybe remelted since such crust first appeared (in the form of surviving zircons) around 4.4 to 4.5 Ga, perhaps during vigorous lid-tectonic regimes.

Possible evolution of magmatic and tectonic styles for large silicate planets. (Credit: Stern et al. 2018, Figure 3)

Based on their reassessment of tectonic activity revealed by 8 rocky planets and moons Robert Stern of the University of Texas (Dallas) and colleagues from ETH-Zurich suggest a possible evolutionary sequence of tectonics and magmatism that Earth-like bodies might go through (Stern, R.J. et al. 2018. Stagnant lid tectonics: Perspectives from silicate planets, dwarf planets, large moons, and large asteroids. Geoscience Frontiers, v. 9, p. 103-119 ; https://doi.org/10.1016/j.gsf.2017.06.004). In their scheme plate tectonics requires certain conditions of lithospheric density and strength to evolve and suggest that, depending on planetary characteristics, slab-pull driven tectonics is likely to be preceded and followed by stagnant lid tectonics, to give perhaps a cyclical geotectonic history.

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