Plate tectonics are a fundamental part of the Earth's geological system, providing a means by which the planet is able to shed heat from its interior. Under this system crustal deformation and magmatism are generally confined to the margins of the lithospheric plates, with the otherwise stiff plates are weakened, softened, and recycled into the the Earth's mantle along subduction zones, while new crustal material is formed at seafloor spreading zones. As far as we know, plate tectonics are only found on Earth, which has led to speculation that the process may have been essential for the creation of conditions for life, or at least complex life. As such when and how the plate tectonics came about is of great interest to scientists.
Potential dates for the origin of plate tectonics range from the Hadean Eon, more than four billion years ago, to the early Neoproterozoic, less than one billion years ago. However, finding evidence to support such hypotheses has proven difficult. The ideal geological evidence for plate tectonics would be metamorphic structures such as eclogites (associated with the shallow subduction of oceanic crust) or blueschists (associated with the deep subduction of crustal material) formed under a low temperature/pressure (thermobaric) regime, ideally less than 375 °C/GPa (apparently similar rocks formed under higher tthermobaric regimes are presumed to have formed due to different conditions). The oldest known ecglogites are found in Cameroon and the Democratic Republic of Congo, and are about 2.1 billion years old, while the oldest known blueschists are found in China, and are only 750-800 million years old.
Thus if plate tectonics was occurring during the Archaean (between 4 and 2.5 billion years ago), then there is a surprising lack of evidence for it.
In a paper published in the journal Geology on 1 January 2026, Meiyun Huang of the State Key Laboratory of Lithospheric and Environmental Coevolution at the Institute of Geology and Geophysics of the Chinese Academy of Sciences, and the College of Earth and Planetary Sciences at the University of the Chinese Academy of Sciences, Shujuan Jiao, also of the State Key Laboratory of Lithospheric and Environmental Coevolution at the Institute of Geology and Geophysics of the Chinese Academy of Sciences, Tim Johnson, Chris Clark, and Jie Yu, of the Curtin Frontiers Institute for Geoscience Solutions at Curtin University, Guangyu Huang, again of the State Key Laboratory of Lithospheric and Environmental Coevolution at the Institute of Geology and Geophysics of the Chinese Academy of Sciences, and Jinghui Guo, once again of the State Key Laboratory of Lithospheric and Environmental Coevolution at the Institute of Geology and Geophysics of the Chinese Academy of Sciences, and the College of Earth and Planetary Sciences at the University of the Chinese Academy of Sciences, present of a study of a section of the Lewisian Gneiss Complex on the northwest coast of Scotland, which they date to about 2.8 billion years ago, and which they suggest was formed under temperature and pressure conditions consistent with a subductive plate margin setting.
Gneisses are course-grained metamorphic rocks which show distinct banding, but which do not tend to cleave along those bands. They typically form at temperatures in excess of 300°C and pressures of between 0.2 and 1.5 GPa, and can be derived from both igneous and sedimentary source rocks. The Lewisian Gneiss Complex is a grey gneiss terrane which has yielded magmatic ages of between 3.1 and 2.8 billion years. This includes layers of ultramafic–mafic rock interlayered with a layered garnet-biotite rock known as the brown gneiss, which is thought to have originally been of sedimentary or volcanosedimentary origin.
Rocks from the Lewisian Gneiss Complex on the Scottish mainland have previously been shown to date from the Neoarchean (2.8-2.5 billion years ago) and to have been formed at temperatures in excess of 900°C, although the pressure at which these rocks formed has been harder to determine, with estimates for peak pressure ranging from about 0.8 GPa to more than 1.5 GPa. There are also two proposed interpretations on the timeline over which these rocks formed, with one proposing two distinct metamorphic episodes, one between 2.8 and 2.7 billion years ago, and the other at about 2.5 billion years ago, and the other proposing a single extended metamorphic event lasting about 200 million years.
(A) Simplified geological map of the Lewisian Gneiss Complex in northwest Scotland. Inset shows location of the Complex. (B) Geological map of the Scourie area in the Assynt terrane showing the location of the studied sample. (C) Outcrop of the studied sample (SC18-02; migmatitic aluminosilicate-bearing metasedimentary rock; 58°21′45″N, 5°09′49″W). Huang et al. (2026).
One possible interpretation is that the Garnet-rich fragments found within the ultramafic–mafic bodies represent retrogressed eclogites (metamorphic rocks which have recrystallised into their current form in response to the lowering of the temperature and pressure from the conditions in which they originally formed) which may have been heated to temperatures as high as 1040–1060°C and exposed to pressures as high as 2.2-2.4 GPa around 2.5 billion years ago, which would imply deep subduction during the late Neoarchaean. An alternative possibility is that they represent xenoliths, fragments of pre-existing rock which became incorporated into surrounding rock during the emplacement of a volcanic basalt or gabro.
Huang et al. analysed samples taken from a migmatitic aluminosilicate-bearing metasedimentary rock within the Lewisian Gneiss Complex, looking at the mineral phases formed by oxides of sodium, calcium, potassium, iron, magnesium, aluminium, silicon and titanium, all of which from different minerals under different temperature and pressure conditions, as well as examining the whole rock composition, occurrence of zirconium within the mineral rutile (which is temperature dependent), and the silica content of the mineral phengite, which is dependent on the pressure at which it formed.
The specimen is a migmatic 'brown gneiss' comprising brown layers rich in garnet and biotite, and white layers which are composed largely of feldspar. Examined in thin section, the material is about 50% plagioclase by volume, with about 18% garnet, 10% biotite, 10% alumminosilicates, 8% potassium feldspar, and small amounts of corundum, spinel, white mica, zircon, rutile, ilmenite, and pyrite.
(A) Distribution of minerals in the sample SC18-02 from the Lewisian Gneiss Complex in northwest Scotland. Alm, almandine; Pyr, pyrope. (B) Detailed Tescan Integrated Mineral Analyzer image showing mineral inclusions in garnet. Rt, rutile; Py, pyrite; Ky, kyanite; Phn, phengite; Crn, corundum; Spl, spinel. (C) Phengite, rutile, and kyanite inclusions within garnet (Grt). (D) Quartz and kyanite inclusions in garnet cores. (E) Spinel, corundum, and kyanite surrounded by biotite (Bt) in garnet. (F) Kyanite, pyrite, and biotite in the garnet. Pl, plagioclase. (G) Kyanite inclusions in garnet cores and sillimanite (Sil) inclusions in garnet rims. (H) An inclusion in corundum showing rutile replaced by ilmenite (Ilm). Huang et al. (2026).
Garnet grains within the sample were up to 12 mm in diameter, and rich in inclusions, including isolated and polymineralic spinel, white mica, corundum, rutile, ilmenite, plagioclase, potassium-feldspar, quartz, biotite, and/or aluminosilicate. White mica, which was largely confined to inclusions within garnet, had a high silicon content, with most grains being classifiable as phengite (high silicone mica). Kyanite occured as isolated inclusions or together with corundum, rutile, spinel, and/or biotite in polymineralic inclusions concentrated within garnet cores. Sillimanite was found in both the rims of garnets and the surrounding matrix, with no preferential orientation. Rare quartz grains were found within the cores of garnets, although they were otherwise largely absent. Corundum primarily occurs in the matrix in contact with sillimanite, spinel, and rutile, where it is commonly replaced at its margins by biotite, and rarely as polymineralic inclusions with spinel and kyanite within garnet. Rutile is present both as inclusions and within the matrix.Rarely, rutile inclusions are partially replaced by ilmenite.
Biotite found as inclusions within garnets has a different composition from biotite within the surrounding matrix, containing a lower proportion of both magnesium and titanium oxide. Spinel grains within the matrix contain slightly more zinc than those within garnet inclusions. Within garnet grains, these spinel inclusions tend to be associated with kyanite and corundum, and many are partially replaced by biotite. Spinel grains in the matrix are typically in contact with corundum and are commonly surrounded by sillimanite or biotite. However, the high zinc content of these spinel grains makes it hard to assess the temperature and pressure under which they formed, so they were excluded from the remainder of the study.
The presence of isolated inclusions of phengite, kyanite, rutile, quartz, and polymineralic inclusions including corundum, spinel, kyanite, rutile, phengite, and/or biotite within garnets is considered by Huang et al. as indicative of two phases of high-pressure metamorphism, with minerals formed by the first phase partially overwritten by the second. The presence of inclusions of sillimanite and plagioclase within the rims of garnet grains, as well as within the matrix assemblage, are taken as evidence for two later phases of high temperature metamorphism.
In order to determine the temperature and pressure conditions during the first phase of metamorphism, Huang et al. attempted to develop a phase equilibrium model for the whole rock. The presence of phengite, kyanite, and rutile within a quartz matrix suggests high pressure/low temperature conditions, with the silicon content of the phengites suggesting a pressure in excess of 2.4 GPa. Huang et al. estimate that during this first phase, temperatures reached 580-660°C and pressures between 1.5 and 2.5 GPa, giving a thermobaric ratio of between 230 °C/GPa (580 °C/2.5 GPa) and 440 °C/GPa (660 °C/1.5 GPa).
A second phase is deduced from the presence of an association of the minerals corundum, kyanite, biotite, plagioclase, and rutile/ilmenite. This association can only form at temperatures of between 830 and 880°C and pressures of between 1.1 and 1.7 GPa, consistent with the temperature deduced from the concentration of zirconium in rutile, approximately 740–960°C. This would correspond to a thermobaric ratio of between 490°C/GPa (830°C/1.7 GPa) and 800°C/GPa (880°C/1.1 GPa).
Another association present, comprising corundum, mesoperthite/antiperthite, and rutile, suggests a phase with higher temperatures but lower pressure. This association probably formed at a temperature of between 880 and 1000°C, but at a pressure of only 0.9.1.4 GPa. This is corroborated by zirconium-in-rutile data for this association, which suggests a formation temperature of about 800 to 1000°C. This would correspond to a thermobaric ratio of between 630°C/GPa (880°C/1.4 GPa) and 1110°C/GPa (1000°C/0.9 GPa).
Huang et al. were able to obtain a lutetium–hafnium from a garnet within the sample associated with the high pressure phase of 2.81 billion years (the isotope lutetium¹⁷⁶ decays to hafnium¹⁷⁶ at a predictable rate, with a half life of 37.1 billion years, enabling this system to be used for dating minerals which would not have contained hafnium at their time of formation).
Previous studies of the Lewisian Gneiss Complex have suggested formation under high pressure conditions, but not a precise age for the rocks nor any insight into the pressure-temperature relationship under which it formed. Huang et al. identify a low temperature/pressure event at about 2.8 billion years ago, associated with the formation of phengite, rutile, kyanite, and quartz inclusions within garnet. During this phase, temperatures reached 580–660°C and pressures reached 1.5–2.5 GPa, corresponding to thermobaric ratios of 230–440°C/GPa. This is consistent with the conditions associated with recent subductive margins where rock metamorphism occurs, but lower than is typical for Archean metamorphic terranes, which generally have thermobaric ratios of over 500°C/GPa.
A second phase is identified based upon the kyanite, biotite, and corundum inclusion complex, thought to have formed under conditions where the pressure had fallen by about 0.7 GPa, but the temperature had risen by about 200°C. This is consistent with the orogenic relaxation stage of a collisional cycle, where the horizontal movement of one plate into another has caused an episode of uplift, but that this horizontal movement has now stopped, causing the pressure within the rocks to slowly relax. This differs from the predominant system within Archean metamorphic environments, where the temperatures and pressures appear to have continued to climb, reaching levels far higher than seen in recent settings.
The low thermobaric ratios reported by Huang et al. are the lowest recorded for any Archean setting, and the only known example of low temperature/pressure metamorphism from the Archean. They propose the rocks or the Lewisian Gneiss Complex may preserve a record of a transition from a pre-plate tectonic regime to one in which subduction is beginning to occur, in a process that would eventually develop into plate tectonics. It has previously been suggested that this transition may have occurred during the Mesoarchean (between about 3.2 and 2.8 billion years ago) on the margins of the North Atlantic craton, as a result of the thickening and strengthening of the lithosphere, and long-term cooling of the mantle.
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