The emergence of continents above sea-level marks a pivotal junction in Earth’s evolution that fundamentally changed the chemistry of the atmosphere and oceans, which was critical to establishing a habitable planet. However, when and how the first subaerial continental landmasses formed remains contentious. Abrupt changes in proportion of submarine vs subaerial volcanism and in the oxygen isotopic ratios of shales and zircons at the Archean-Proterozoic transition (2.5 billion years ago, Ga) are invoked to argue for global continental emergence around that time (e.g., Kump and Barley, 2007; Bindeman et al., 2018). However, direct evidence for an earlier episode of continental emergence comes from ~3.0-2.7 Ga paleosols (like the Nsuze paleosol) and terrestrial sedimentary strata that formed atop stable cratons (cf. Eriksson et al., 2013). This attests continental emergence > 2.5 Ga, at a time when the operation of modern plate tectonics is debated.

To help resolve these issues, we focussed on the cratons like the Singhbhum and Kaapvaal cratons since they host widespread Mesoarchean terrestrial to shallow marine clastic strata and paleosols, which suggests early (> 2.5 Ga) continental emergence on Earth. We studied how crustal thickness and composition of these cratons evolved through time leading to their emergence, by linking the Paleo-to-Mesoarchean sedimentary and magmatic records of these cratons (Chowdhury et al., 2021). First, we studied the conglomerate-sandstone-shale successions that are uncomforably lying on the cratonic basement and determined their depositional ages to constrain the timing of the continental emergence. Then we analysed the chemistry of the tonalite-trondhjemite-granodiorite (TTG) suite of felsic rocks and performed petrogenetic modelling to quantify the evolution of crustal thickness and P-T conditions of crust formation, which elucidated the underlying mechanism and tectonic environment of emergence.

Our results show that the studied cratons became emergent between ca. 3.3-3.1 Ga due to progressive crustal thickening and maturation driven by granitoid magmatism. The cratonic crust  became chemically mature and extremely thick (45-50 km) by 3.2-3.1 Ga, such that isostatic compensation led to their rise about the sea level. Modelling of the TTG chemistry further elucidated that these TTGs formed at hotter thermal conditions characteristic of a thickened Archean crust atop a zone of rising mantle. Hence, we propose that emergence of stable continental crust began at least during the late Paleoarchean to early Mesoarchean and was driven by the isostatic rise of their magmatically thickened, SiO₂-rich crust without the help of plate tectonics (Chowdhury et al., 2021). We further surmise that such early episodes of emergence caused important changes in Earth’s early surficial environments including promoting transient atmospheric-oceanic oxygenation (O₂-whiffs) and CO₂ drawdown leading to glacial events.

Reference:

Bindman et al., 2018. Nature 557, 545–548.

Chowdhury et al., 2021. PNAS 118, e2105746118.

Eriksson et al., 2013. Gondwana Research 24, 468–489.

Kump and Barley, 2007. Nature 448, 1033–1036.

How to cite: Chowdhury, P., Cawood, P. A., and Mulder, J. A.: When and how did Earth’s earliest continents first emerge above the oceans?, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-4246, https://doi.org/10.5194/egusphere-egu23-4246, 2023.

Water plays a crucial role in the formation of new crust on modern Earth. Today, new continental crust is created through arc magmatism by fluid-fluxed mantle melting above subduction zones. The aqueous fluid is derived from the breakdown of hydrous phases in subducted oceanic crust as a result of a delicate interplay between phase stability and the cold thermal conditions in the slab. Hydrated and subducted ultramafic (mantle) rocks play a key role in supplying the water needed for wet mantle melting and provide an important link between the Earth’s deep water cycle and formation of crust with an average andesitic composition.

Archean felsic crust consists of the typical Tonalite-Trondhjemite-Granite (TTG) Series, which were likely produced from melting of altered basaltic precursors. Previous studies suggest that the water-present partial melting of metamorphosed basalt at temperatures of 750–950 °C is required to produce large volumes of partial melt with TTG compositions. However, the source of such water is unknown and exposed serpentinised mantle rocks likely played a negligible role in the early Earth’s water cycle.

We propose that hydrated komatiites played a vital role in TTG genesis. Using petrology, mineral chemistry and phase equilibria modelling of representative komatiite samples, combined with analysis of a global geochemical dataset of komatiites and basaltic komatiites, we show that during metamorphism hydrated komatiites can release at least 6 wt. % mineral-bound water. The great majority of this water is released by breakdown of chlorite and tremolite at temperatures between 680 and 800 °C. As the temperatures of komatiite dehydration are above the wet basalt solidus, the released water can trigger voluminous partial melting of basalt to ultimately create TTG batholiths. This considerable hydration potential of komatiites is due to their high X_Mg, which stabilises hydrous minerals during oceanic alteration on the seafloor, but also extends the stability of Mg-rich chlorite to high temperatures. During prograde metamorphism, the X_Mg, CaO and Al₂O₃ content of the reactive rock composition determines the proportion of chlorite vs amphibole, and therefore the volume of water which can be transported to temperatures of > 750 °C. Therefore, we suggest that water released from dehydrating komatiites - regardless of the prograde P–T path (i.e., tectonic scenario) they experienced - provided the free water necessary to partially melt large volumes of basalts to form the prominent and expansive TTG suits in the Archean. Even though komatiites make up moderate portions of greenstone belts, they thus likely played a key role in early crustal formation and the Earths’ early water cycle.

How to cite: Hermann, J., Tamblyn, R., Hasterok, D., Sossi, P., Pettke, T., and Chatterjee, S.: Hydrated komatiites as a source of water for TTG formation in the Archean, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-5476, https://doi.org/10.5194/egusphere-egu23-5476, 2023.

Earth is the only planet known to have continents, although how they formed and evolved is not well understood. Using the oxygen isotope compositions (SIMS) of dated magmatic zircon, we show that the Pilbara Craton in Western Australia, Earth’s best-preserved Archaean (4.0–2.5 Ga) continental remnant, was built in three stages. Stage 1 zircons (3.6–3.4 Ga) form two age clusters with one-third recording submantle δ¹⁸O, indicating crystallization from evolved magmas derived from hydrothermally-altered basaltic crust similar to that in modern-day Iceland. Shallow melting is consistent with giant meteor impacts that typified the first billion years of Earth history. Giant impacts provide a mechanism for fracturing the crust and establishing prolonged hydrothermal alteration by interaction with the globally extensive ocean. A giant impact at around 3.6 Ga, coeval with the oldest low-δ¹⁸O zircon, would have triggered massive mantle melting to produce a thick mafic–ultramafic nucleus. A second low-δ¹⁸O zircon cluster at around 3.4 Ga is contemporaneous with spherule beds that provide the oldest material evidence for giant impacts on Earth. Stage 2 (3.4–3.0 Ga) zircons mostly have mantle-like δ¹⁸O and crystallized from parental magmas formed near the base of the evolving continental nucleus. Stage 3 (<3.0 Ga) zircons have above-mantle δ¹⁸O, indicating efficient recycling of supracrustal rocks. That the oldest felsic rocks formed at 3.9–3.5 Ga, towards the end of the so-called late heavy bombardment, seems unlikely to be a coincidence.

How to cite: Johnson, T., Kirkland, C., Lu, Y., Smithies, H., Brown, M., and Hartnady, M.: Giant impacts and the origin and evolution of continents, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-1941, https://doi.org/10.5194/egusphere-egu23-1941, 2023.

A major contribution to the crustal growth and construction of the Congo Craton was the addition and preservation of the ≤ 45 000 km² Kunene AMCG Complex (KC), which straddles the international border between Angola and Namibia. KC magmatism encompasses dominantly juvenile anorthositic rocks (anorthosite, leuco-gabbro, -norite, -troctolite) and A-type granitoids (Red Granite Suite) of mixed crustal and juvenile signature. High-precision U-Pb dates of zircon and baddeleyite from the exposed western parts of the KC (~15 000 km²) in between 1500 and 1360 Ma indicate that both the anorthosites and Red Granites were pulsed and exceptionally long-lived. The remaining eastern portion of the KC can only be imaged using potential field geophysical methods as it is covered by a thin (≤ 300 m) cover of Cenozoic Kalahari sediments. Field mapping and recent remote sensing in the exposed part of the complex, together with airborne geophysics of the entire KC, indicate that the anorthosites were emplaced in up to 12 layered or massive batholiths, which are elliptical in a NNE-SSW or E-W direction. They are commonly separated by relatively thin and elongated KC granitoid bodies and are in tectonic or intrusive contact with Paleoproterozoic basement rocks.

Regional horizontal contraction in the Angolan portion of the KC is dated by U-Pb in zircon and Ar-Ar in micas at 1400-1370 Ma. Contraction formed N-S to NE-SW-striking, cm- to km-wide, discrete, syn- to post-magmatic thrust zones mainly localised in KC granitoids. The shear zones are parallel to magmatic foliation in the granitoids and magmatically layered anorthosites. A compilation of crystallisation ages (n = 60) suggests that the regional shortening triggered the magmatism that formed ~ 60% of the exposed KC by mobilising magmas from deep crustal mush zones. In contrast, the southern part of the KC in Namibia exhibits E-W- to ENE-WSW-striking magmatic layering, gneissic foliations and shear zones formed at amphibolite to greenschist facies conditions. These are compatible with north-directed ductile to brittle thrusting over the Angolan KC. Northward thrusting post-dates KC emplacement and is broadly constrained in between 1360 and 1330 Ma by Ar-Ar dating of micas. Airborne aeromagnetic and satellite gravimetric data indicate that the southern KC is parallel to and overlies a crustal and continental-scale geophysical lineament, which is interpreted as the relic of a linear Mesoproterozoic orogenic belt extending to the Kibaran Belt of Central Africa. The orogenic activity was terminated by 1127 Ma, which is the oldest age of a suite of mafic dykes crosscutting post-KC and undeformed capping siliciclastic units. U-Pb dates of detrital zircon and Hf-in-zircon data for these siliciclastic rocks overlap with those of the KC granitoids, indicating local recycling of KC rocks between 1360 and 1127 Ma.

Our results highlight that the 1500-1360 Ma period of the Congo Craton was a time of significant crustal growth in the form of voluminous Kunene Complex magmatism. The assembly of the entire KC magmatic edifice was facilitated by syn- to post-magmatic contractional deformation that juxtaposed distinct crustal domains during two newly defined Mesoproterozoic orogenic events.

How to cite: Lehmann, J., Bybee, G. M., Milani, L., Owen-Smith, T. M., Hayes, B., Ferreira, E., and Jelsma, H.: Modes of crustal growth and construction for the southwestern Congo Craton in the Mesoproterozoic, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-13221, https://doi.org/10.5194/egusphere-egu23-13221, 2023.

Whether Archean tectonics were horizontally or vertically dominated is controversially discussed because arguments bear on the kinematics and thermal state of the Archean mantle and constrain the mode of formation of the earliest continental crust. Highly deformed strata of Archean greenstone belts figure prominently in this debate because they record long periods of time and multiple deformation phases. Among the best-preserved greenstone belts counts the Barberton Greenstone Belt (BGB) of southern Africa. Geological mapping of part of the southern BGB in Eswatini (Swaziland), combined with U-Pb zircon dating, shows that the region preserves a tightly re-folded imbricate thrust stack in which metavolcanic and -volcaniclastic strata of the Onverwacht Group, deposited at 3.34–3.29 Ga, have been thrust on top of ca. 3.22 Ga siliciclastic strata of the Moodies Group. The structurally highest element, the Malolotsha Syncline, forms a tectonic klippe of substantial size and is >1,450 m thick. Forward modeling of a balanced cross section indicates that this thrust stack was part of a northwestward-verging orogen along the southern margin of the BGB and records a minimum horizontal displacement of 33 km perpendicular to its present-day faulted, ductily strained and multiply metamorphosed margin. Because conglomerate clasts indicate a significantly higher degree of prolate strain which extends further into the BGB than at its northern margin, late-stage tectonic architecture of the BGB may be highly asymmetrical. Our study documents that the BGB, and perhaps other Archean greenstone belts, preserves a complex array of both vertically- and horizontally-dominated deformation styles that have interfered with each other at small regional and short temporal scales.

How to cite: Heubeck, C., Thomsen, T. B., Heredia, B. D., Zeh, A., and Balling, P.: The Malolotsha Klippe: Large-Scale Subhorizontal Tectonics Along the Southern Margin of the Archean Barberton Greenstone Belt, Eswatini, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-2429, https://doi.org/10.5194/egusphere-egu23-2429, 2023.

The Earth is a dynamic planet that has been evolving ever since it was formed. The formation of protocontinents and their amalgamation to supercontinents and later dispersals are one of the fascinating geologic events during the course of the evolution of Earth. Studies on the assembly and dispersals, therefore, provide insights into the mechanisms of extraction of mantle materials at different time periods, the formation of mountain belts, the recycling of crustal materials, magmatism, metamorphism, etc. The recent supercontinent assembly, namely "Gondwanaland," took place during one of the most dynamic periods of the earth's history, and almost all of the existing continental fragments have records of this great geological event. The Southern Granulite Terrane (SGT) of South India is made up of a variety of crustal blocks and collisional sutures/shears that developed during the period of multiple orogenic cycles from the Mesoarchean to the late Neoproterozoic-Cambrian, including that of Gondwana period. Among this, the Palghat Cauvery Shear Zone (PCSZ) marks a major Neoproterozoic structure of crustal accretion, and it is considered the extension of major terrain boundaries identified in Madagascar and Sri Lanka in the final stages of the Gondwana assembly. Even though there have been plenty of studies carried out to understand the nature of the lower crust, terrain assembly, and shear sense indicators along the PCSZ, most of them are concentrated on the eastern side of the shear zone, and only a few have been carried out in the high-grade western terrain; therefore, unequivocal evidence showing collisional orogenesis is lacking from this terrain. The present study attempts to infer the geochemical characteristics of charnockites from the western parts of the PCSZ in terms of accretionary and/or collision tectonics. The geochemistry suggests that the charnockites are tonalitic to granodioritic in composition and have calc-alkaline affinity, indicating an origin related to collision tectonics. These are the products of granulite-facies metamorphism, most probably of an I-type granitic magma, with a low Rb/Sr ratio and a high Ba/Rb ratio suggesting resemblance with Archaean tonalites, and as a product of the remelting of protoliths of tonalite–trondhjemite–granodiorite (TTG) composition. The whole-rock major and trace element compositions indicate that charnockites are formed as the product of partial melting of garnet amphibolite or eclogite-facies basaltic crust during granulite-grade metamorphism at a lower crustal level during a collisional event.

How to cite: Nandan T, N. and Chettootty, S.: A geochemical perspective on the petrogenesis of charnockites from the western parts of the Palghat-Cauvery Shear Zone, southern India: implications for collisional geodynamics, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-11847, https://doi.org/10.5194/egusphere-egu23-11847, 2023.

Because Earth is the only known planet to host both plate tectonics and life it is sometimes concluded that the two phenomena are related. While life is thought to have originated by the Eoarchean (or earlier), the onset of plate tectonics remains unknown, with proposed initiation ages ranging as old as the Hadean. Paleomagnetism can be used to distinguish between mobile and fixed lithospheres, but studies have been impeded by the high-grade metamorphism and deformation that makes most rocks older than Paleoarchean in age unsuitable for analysis. However, select detrital zircons can preserve primary magnetizations, providing an opportunity to conduct direct tests. Here we examine the zircon paleomagnetic history recovered from Western Australia which provides evidence for near constant paleolatitudes between ca 3.9 and ca. 3.4 Ga. We further assess this record with select zircons bearing primary magnetic inclusions from South Africa, which yield magnetizations consistent with this history. The simultaneous recordings of the magnetic field by zircons from two continents with vastly different Phanerozoic geologic histories provide further support for the primary record of the zircon magnetizations, and for a pre-Paleoarchean stagnant lid regime of Earth. These data also indicate that life on Earth originated and was sustained without plate tectonic-driven geochemical cycling.

How to cite: Tarduno, J., Cottrell, R., Bono, R., Nimmo, F., and Watkeys, M.: Hadean to Eoarchean stagnant lid tectonics recorded by the paleomagnetism of zircons, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-10278, https://doi.org/10.5194/egusphere-egu23-10278, 2023.

The 3.33 Ga Josefsdal Chert in the Barberton Greenstone Belt, South Africa, records a sequence of sediments deposited under shifting energy conditions in a nearshore paleoenvironment (1, 2). At the base, volcanoclastic sediments were deposited under somewhat dynamic conditions on top of pillow basalt and hydrothermal chert. They grade gradually upwards into alternating deposits of chemical silica and very fine scale microbialites tabular phototrophic mats) formed under very quiet conditions frequently interrupted by storm currents, which then transitioned sharply into thinly bedded tuffs with much hydrothermal activity at the base. Growth faults permitted thick sequences of very shallow sediments to accumulate. While the REE data show the global, background Eu signature of hydrothermal influence throughout, local Sm/Yb:Eu/Sm ratios document local hydrothermal hot spots. Fluvial inflow is documented by flat REE patterns in the middle to upper sequences (2).

Within this environmental background, microbialites abound, their nature (phototrophic/chemotrophic), distribution and preservation being influenced by environmental factors, such as water depth (phototrophy), sedimentation flux, and hydrothermal vents and activity. Phototrophic activity was abundant during the middle, volcanically quiet period and was present also during the lower and upper volcanoclastic depositional periods, with biofilms and mats forming on the tops of individual fining upwards layers (3,4). Chemotrophic colonies were abundant in the vicinity of hydrothermal vents (5). Amost instantaneous silicification of both sediments and the microbialites resulted in excellent preservation, although the organo-geochemical signatures are heavily diluted (SiO₂ contents ranging from ~ 90-99.9%). Biogenicity of the different microbialites was evaluated on the basis of their morphology, interactions with the immediately surrounding sediment and environmental conditions (e.g.current flow), organic carbon and δ¹³C compositions, as well as their transition element compositions and the presence of minerals precipitated as by-products of microbial metabolism (e.g. aragonite, sulphate). Periodic exposure of some of the phototrophic biofilms, as indicated by desiccation and entrapped layers of pseudomorphed evaporite minerals (aragonite, calcite, gypsum, and halite)(3,4), as well as desiccation texture on certain bedding planes, indicates a littoral, on shore environment of formation.

(1) Westall, F. et al., 2015, Geology, 43, 615; (2) Westall, F., Bréhéret, J. et al. in prep.; (3) Westall, F. et al., 2006, Phil. Trans. Roy. Soc. Lond. B., 361, 1857; (4) Westall, F. et al., 2011, Earth Planet. Sci. Lett., 310, 468; (5) Hickman-Lewis, K., et al. 2020, Sci Rep 10, 4965.

How to cite: Westall, F., Bréhéret, J., Hickman-Lewis, K., Campbell, K., Giudo, D., Foucher, F., and Cavalazzi, B.: Environmental controls on the distribution of life in shallow seas on the early Earth in the 3.33 Ga Josefsdal Chert, Barberton Greenstone Belt, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-12866, https://doi.org/10.5194/egusphere-egu23-12866, 2023.

Earth is the only known inhabited world in our solar system. Criteria essential for planetary habitability include surface liquid water, a stable atmosphere, and a magnetic field. While the rock record suggests Earth has fulfilled these criteria for at least 4 billion years (Ga), both its environment and life have evolved over time. The Great Oxygenation Event (GOE), which occurred ~2.5 Ga ago, drastically altered the chemistry of the oceans and atmosphere. Decoding environmental and magnetic signals recorded in rocks prior to the GOE is essential for understanding the conditions under which life first emerged.

An ideal target for investigating surface conditions prior to the GOE are banded iron formations (BIFs), which precipitated directly from ancient oceans. However, BIFs have been significantly altered since their formation, and it is unclear whether a record of their depositional environment remains.  The present day mineralogy is dominated by magnetite, but it remains to be established how this relates to the precipitates deposited on the seafloor. Additionally, in spite of magnetite's ideal magnetic properties, BIFs are avoided for paleomagnetic analysis because the timing of magnetization is uncertain. It is vital to constrain the magnetic field record leading up to the GOE because it may have influenced atmospheric hydrogen loss, contributing to rapid surface oxidation.

We present paleomagnetic field tests from the Isua Supracrustal Belt that suggest a record of Earth’s 3.7-billion-year (Ga) old (Eoarchean) magnetic field is preserved in the banded iron formation in the northernmost northeast region of the belt. Our results are supported by radiometric Pb-Pb dating of magnetite from the same banded iron formation.  We show that the Pb-magnetite system has a closure temperature below 400 °C for the magnetite grain size range observed in the banded iron formation, suggesting the rocks have not been significantly heated since magnetization was acquired. This temperature range is well below the Curie temperature of magnetite (580 °C), suggesting Eoarchean magnetization has not been thermally overprinted by subsequent metamorphism.  Passed paleomagnetic field tests suggest the rocks have also avoided chemical overprints. We recover an ancient magnetic field strength, supporting previous studies that argue Earth’s magnetic field has been active throughout most of its history although variations in its strength remain poorly constrained.

How to cite: Nichols, C., Weiss, B., Eyster, A., Martin, C., Maloof, A., Kelly, N., Zawaski, M., Mojzsis, S., Watson, B., and Cherniak, D.: Using banded iron formations to understand habitable conditions on the early Earth, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-2404, https://doi.org/10.5194/egusphere-egu23-2404, 2023.

The impact that ancient Earth surface processes had on long-term thermal regimes remain uncertain despite their potentially important role in fostering craton stabilization and preservation. The distribution and redistribution of heat producing elements (HPEs) during craton development plays a major role in lithospheric cooling and strengthening. Whereas the redistribution of HPEs via erosion has theoretically been suggested to alter the long-term geotherm and contribute to Moho cooling, direct temporal constraints from the field are lacking to adequately assess the role that ancient Earth surface processes may have had on long-term thermal regimes. Here, we used apatite U-Pb thermochronology to assess the thermal evolution of the Archean Bundelkhand craton of central India immediately following its amalgamation and final phase of silicic magmatism at ~2.5 Ga. Apatite from both ~3.4 Ga granitic gneisses and ~2.5 Ga granitoids collected across the ~250 km-wide craton yielded near-uniform apatite U-Pb dates between ~2.4–2.3 Ga, indicating that the craton was broadly exhumed through mid-crustal depths shortly following shallow granitoid emplacement. Unroofing of the craton at this time is further corroborated by the presence of a distinct ~2.5 Ga detrital zircon U-Pb age peak obtained from ~2.2–2.3 Ga sandstones in direct non-conformable contact with Bundelkhand granitoids. We speculate that a two-step redistribution of HPEs largely contributed to the stabilization of the Bundelkhand craton. First, the concentration of HPEs within shallowly emplaced granitoids at ~2.5 Ga reduced the heat production of the lower-most crust. Second, post-emplacement exhumation of HPE-enriched Bundelkhand granitoids further modified the heat source distribution to a thermal regime that promoted cooling of the lower-crust. Although the mechanism driving exhumation through mid-crustal depths remains uncertain, temporal relationships from the Bundelkhand craton suggest that erosional processes may have had a significant role in promoting the craton’s stability and longevity.

How to cite: Colleps, C., McKenzie, N. R., Chen, W., and Sharma, M.: Did Earth surface processes promote stabilization of the central Indian Bundelkhand craton?, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-7623, https://doi.org/10.5194/egusphere-egu23-7623, 2023.

Isotopic proxies such as Hf, Nd and Pb are widely used to understand the evolution of Earth’s crust and mantle. Of these, Pb isotopes are particularly sensitive to crustal influences, and the extraction of mantle melts. We present a global compilation of Pb isotope data from syngenetic Volcanogenic Massive Sulphide (VMS) deposits, which allow us to track the evolution of Pb isotopes in deposits that are associated with dominantly back-arc and extensional oceanic settings through time.

Unradiogenic Pb isotope signatures, specifically low model source µ (²³⁸U/²⁰⁴Pb) values, in some Archean cratons have long been recognised, yet their origin remains elusive. For example, sulphides from the c. 2.7 Ga Abitibi Belt in the Superior Province of Canada require long-lived (> 500 my) evolution of a source component to generate the Pb isotope signatures observed. Other isotope systems, such as Lu-Hf and Sm-Nd, show relatively juvenile signatures for the Abitibi Belt, suggesting decoupling of the different systems. Low µ values are evident in ore deposits and rocks from the Archean to modern settings but are most prominent in Archean settings because of their associated low ²⁰⁷Pb/²⁰⁴Pb values, unlike for younger times.

Pb isotope data at a global and broad temporal scale show that periods with distinct low µ values have a marked cyclicity that coincides with the supercontinent cycle. We propose that during supercontinent assembly, portions of older unradiogenic, Pb-rich mantle are tapped and incorporated into VMS deposits. Pb, possibly enriched in sulphides, can explain the apparent decoupling of Pb from silicate-controlled isotope systems like Hf and Nd. We suggest that the source of this unradiogenic mantle component formed during the previous supercontinent cycle when large volumes are extracted from the mantle to form (radiogenic) crust and an unradiogenic residue, which most likely resides in the lithospheric mantle although some may also be present as discrete ‘pods’ in the circulating mantle. This process provides a mechanism to explain isolation of source regions for several hundred million years, as required to generate the low µ values, until later tapping during a subsequent supercontinent amalgamation cycle.

The low µ values in the c. 2.7 Ga Abitibi Belt represent the best-known Archean occurrence of this signature, indicating that their unradiogenic source relates to a major mantle extraction event that would have occurred at least 500 my earlier, i.e. at about 3.2 Ga.

How to cite: Armistead, S., Eglington, B., Pehrsson, S., and Huston, D.: Pb isotope heterogeneities in the mantle and links to the supercontinent cycle, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-4744, https://doi.org/10.5194/egusphere-egu23-4744, 2023.

Our understanding of the geological history of early Archean crust is limited by poor preservation of igneous features in rocks that have experienced multiple metamorphic and deformation events. Thus, regions with the best preserved Eoarchean rocks, as for example, the northern part of the Itsaq Gneiss Complex (IGC) of Greenland, have been the most intensively studied. The IGC underwent metamorphism at ca 3.6 and 2.7 Ga (Nutman & Bennett 2018). The grade of 2.7 Ga metamorphism varies from granulite facies in the southern part of the IGC (Fӕringehavn terrane) to lower amphibolite facies in the north (Isukasia terrane). This study compares the preservation of zircon in rocks from both terranes of the IGC.

Zircon grains from granitic gneisses in the Fӕringehavn terrane have rounded igneous cores with weak oscillatory zoning, surrounded by well-developed light-CL metamorphic rims. The ²⁰⁷Pb/²⁰⁶Pb zircon age obtained by in situ Secondary Ion Mass Spectrometry (SIMS) of these grains is ca 3.64 Ga for the cores, with metamorphic rims recording an age of ca 2.7 Ga. The Isukasia terrane extends either side of the Isua Supracrustal Belt (ISB), rock samples were collected from both the outer (SSE of the ISB) and inner (NNW of the ISB) Isukasia sub-terranes (Nutman & Bennett 2018). Zircon grains from the outer sub-terrane have well preserved igneous morphologies with evidence of metamictisation and fluid alteration but little to no metamorphic rims. The ²⁰⁷Pb/²⁰⁶Pb zircon ages are scattered towards 2.7 Ga, interpreted as the time of metamorphism, with a subgroup at ca 3.79 Ga that is interpreted as a minimum age for magmatic zircon. However, as the samples collected in the vicinity yielded an age of 3.82 Ga (Nutman et al. 1999, Kielman et al. 2018), the age of ca 3.79 Ga may have been disturbed by subsequent events. Zircon grains from the inner sub-terrane of Isukasia have well-preserved igneous cores with oscillatory zoning. Rounding of pyramidal terminations and thin rims are due to metamorphism. The age of crystalization of the protolith as recorded by igneous zircon is ca 3.71 Ga. 

The difference in the degree of the metamorphism at 2.7 Ga is visible in the structures and preservation of zircon grains. In this example, rounded cores and well-developed metamorphic rims characterize granulite facies, whereas well-preserved cores with oscillatory zoning and thin metamorphic rims represent lower amphibolite facies.

This research was funded by NCN grant UMO2019/34/H/ST10/00619 to MAK

References
Kielman, R., Whitehouse, M.,Nemchin, A., & Kemp, A., (2018). A tonalitic analogue to ancient detrical zircon. Chemical Geology, 499, 43-57.
Nutman, A.P. & Bennett, V.C., (2018). The 3.9-3.6 Ga Itsaq Gneiss Complex of Greenland. In: Van Kranendonk, M.J., Bennett, V.C. & Hoffmann, J.E., (Eds.). Earth’s Oldest Rocks (2^nd ed.), Elsevier, 375-399.
Nutman, A.P., Bennett, V.C., Friend, C.R. & Norman, M.D., (1999). Meta-igneous (nongneissic) tonalites and quartz-diorites from an extensive ca. 3800 Ma terrain south of the Isua supracrustal belt, southern West Greenland: constraints on early crust formation. Contrib. Mineral. Petrol. 137, 364–388.

How to cite: Mieszczak, M. J., Kusiak, M. A., Dunkley, D. J., Wilde, S. A., Whitehouse, M. J., Yi, K., and Lee, S.: Polymetamorphism and zircon preservation in the Itsaq Gneiss Complex, SW Greenland, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-13831, https://doi.org/10.5194/egusphere-egu23-13831, 2023.

Interpreting Archean geology is often challenging due to the rocks having obscure field relationships and polymetamorphic histories (Kusiak et al. 2019; Dunkley et al. 2020). In such circumstances, U-Pb isotopic analysis of zircon is crucial for revealing the geological history. This study investigates Archean gneisses from the Saglek Block in Canada, which record magmatic and metamorphic history between ca 3.9 Ga and 2.5 Ga. The predominant lithology is the Uivak gneiss which is primarily composed of tonalite-trondhjemite-granodiorite (TTG) with subordinate intermediate to mafic components. Uivak gneiss is traditionally divided into Uivak I and Uivak II, where Uivak I is grey gneiss and Uivak II is characterized by augen texture and Fe-rich geochemistry (Collerson and Bridgwater, 1979). Ages for the magmatic protoliths of Uivak I are >3.6 Ga, whereas Uivak II ages vary between ca 3.6-3.3 Ga (Sałacińska et al. 2019; Wasilewski et al. 2021 and references therein). 

This study presents geochemical and U-Pb zircon geochronology from Mentzel and Maidmonts Islands. Augen gneiss on Mentzel Island fits the definition of Uivak II augen gneiss and yield a U-Pb zircon age of ca 3.3 Ga. A similar age was reported for Maidmonts gneiss (Sałacińska et al. 2019) and Illuilik gneiss (Wasilewski et al. 2021). On Mentzel Island, granitic bodies intruded the augen gneiss at ca 2.7 Ga and 2.5 Ga during high-T metamorphism. New dating confirms that augen gneiss on Mentzel Island and elsewhere in the Saglek Block belongs to Uivak II gneisses of ca 3.3 Ga. Variations in rare earth element concentration between different ca 3.3 Ga rocks can be attributed to the involvement of different crustal components in the magmatic protolith. On Maidmonts Island, the augen gneiss intrudes grey gneiss with a protolith age of ca 3.7 Ga, which confirms deformation and metamorphism of Uivak I gneiss before ca 3.3 Ga. 

This research was funded by NCN grants UMO2019/34/H/ST10/00619 to MAK.                  

References:
Collerson, K.D. & Bridgwater, D. 1979. Metamorphic development of early Archaean tonalitic and trondhjemitic gneisses: Saglek area, Labrador. In: Barker, F. (Ed.), Trondhjemites, Dacites, and Related Rock. Elsevier, Amsterdam, 205–271.

Dunkley et al. 2020. Journal of the Geological Society, 177 (1), 31–49.

Kusiak et al. 2018. Chemical Geology, 484, 210–223.

Sałacińska et al. 2019. International Journal of Earth Sciences, 108, 753-778.

Wasilewski et al. 2021. Precambrian Research, 359, 106092.

How to cite: Keluskar, T., Kusiak, M. A., Dunkley, D. J., Whitehouse, M. J., Wilde, S. A., Yi, K., and Lee, S.: Uivak II augen gneiss from the Saglek Block, Labrador: the current state of play, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-9487, https://doi.org/10.5194/egusphere-egu23-9487, 2023.

The Acasta Gneiss Complex (AGC) in northwestern Canada is home to the oldest known evolved (felsic) rocks on Earth, dating back to around 4.03 billion years (Ga). These rocks preserve evidence for multiple episodes of magmatism, metamorphism, and deformation, offering insights into the geological processes that shaped the Earth's crust throughout the Archean and late Hadean. However, the metamorphic pressure–temperature (P–T) conditions of this complex remain poorly constrained. In this study, we use phase equilibria modelling and in situ garnet Lu-Hf geochronology to analyse two garnet-bearing tonalitic gneisses in the AGC, providing the first quantitative P–T constraints for a late Paleoarchean tectono-metamorphic event in the AGC. Our results indicate metamorphic peak conditions of approximately 725-780°C and 4.5-6.2 kbar, with limited partial melting (<7 vol.%) of the felsic gneisses at these crustal levels. In situ Lu-Hf garnet geochronology suggests that this metamorphic event occurred between 3.3-3.2 Ga, consistent with previous findings of high-grade metamorphism at that time. Isotopic disturbance of garnet at approximately 1.9 Ga is interpreted to reflect partial resetting of the Lu-Hf systematics in response to fluid-present re-equilibration during the Paleoproterozoic Wopmay orogeny. Our study extends the limited dataset of published P–T data for Mesoarchean and older metamorphic rocks and shows that tonalitic gneisses in the AGC evolved along a high apparent thermal gradient of 125-150°C/kbar.

How to cite: Kaempf, J., Johnson, T., Clark, C., Brown, M., and Rankenburg, K.: Pressure–temperature conditions and age of metamorphism in the Archean Acasta Gneiss Complex: constraints from phase equilibrium modelling and in situ garnet Lu-Hf geochronology, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-2207, https://doi.org/10.5194/egusphere-egu23-2207, 2023.

The Proterozoic orogenic belts incorporated in and around the present-day continents preserve complex magmatic, metamorphic, and geophysical signatures of the ancient supercontinents. One such orogenic belt, the Eastern Ghats Belt (EGB) is amalgamated with the Archean cratons of India along a crustal-scale suture zone known as the Terrane Boundary Shear Zone (TBSZ). The continental margin – orogenic belt interfaces, such as the TBSZ, are the black boxes of ancient tectonic processes, since they are rheologically weakened crustal discontinuities that undergo intense deformation and metamorphism recording the complete orogenic history. There have been two schools of thought on the age of final amalgamation of the EGB with the Bastar craton, as the TBSZ records two major tectonothermal events at ~950Ma and ~550Ma, coeval with the formation of supercontinents Rodinia and Gondwana, respectively. The age and mechanism of this amalgamation have implication on the crustal architecture of the Proterozoic supercontinents.

Recent studies confirmed the presence of felsic and mafic granulites of Archean Sm-Nd model ages (3.3 – 3.1 Ga) from the TBSZ that have undergone high-pressure granulite facies metamorphism. It is speculated that these rocks are of Bastar craton in origin and the underthrusting of the Bastar craton beneath the EGB, during the final collision, led to the high-pressure metamorphic conditions. In this communication, we have carried out a comparative petrological and geochemical investigation of the Archean felsic rocks (Grt-bearing charnockites) from the TBSZ and the Hbl-Bt granites from the adjacent regions of the Bastar craton to understand origin and tectonic significance of the charnockites. The garnet-bearing charnockites from the TBSZ are characterised by coarse grained Grt + Opx + Pl + Qz + Kfs + Hbl + Bt ± Ilm. The Hbl-Bt granites of the Bastar craton, adjacent to the TBSZ, are characterized by coarse grained Hbl + Bt + Qz + Kfs + Pl, with small Opx grains forming around Hbl in few places at the interface. The Grt-bearing charnockites and the Hbl-Bt granites are both ferroan and metaluminous to slightly peraluminous in nature. The high concentrations of trace elements, high Y/Nb (>1.2) ratio and pronounced negative anomalies of Eu, Sr and Ti in both the rocks are characteristic of A₂-type within plate granitoids, similar to the other reported granitoids from the Bastar craton. The strong similarity in the geochemistry of Grt-bearing charnockites and Hbl-Bt granites along with the available Archean model ages of the charnockites indicate that the Grt-bearing charnockites of the TBSZ are granulite-facies equivalents of the Hbl-Bt granites and hence represent the remnants of cratonic margin in the TBSZ. This geochemical study along with the Tonian ages (~950 Ma) from monazite cores and inclusions in garnet within the co-exposed metapelites in the suture zone indicate that the Bastar craton underthrusted beneath the EGB during the formation of Rodinia. The ~500 Ma ages reported from the strongly recrystallized monazite rims might represent the reactivation of the intracontinental suture zone due to the far-field stress from the Kuunga orogeny (~530 – 490 Ma) during the formation of East Gondwana.

How to cite: Padmaja, J., Sarkar, T., and Dasgupta, S.: Geodynamic significance of the Archean A-type granites exposed along the western margin of a Proterozoic orogenic belt: Insights on the final docking of the Eastern Ghats Belt with the Indian subcontinent, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-363, https://doi.org/10.5194/egusphere-egu23-363, 2023.

The present-day thermochemical structure of the subcontinental mantle holds key information on its origin and evolution and informs exploration strategies, natural hazard management and evolutionary model of the Earth system. As such, unravelling the nature of the continental lithosphere, its modification through time and its interactions with the sublithospheric mantle and the atmosphere/hydrosphere constitute some of the main goals of modern geoscience. Despite its fundamental importance, imaging the fine-scale thermochemical structure of the lithosphere using indirect (remote) data is plagued with difficulties, which has traditionally left the analysis of xenoliths and xenocrysts as the only reliable approach.

In recent years, however, ‘simulation-based’ inverse methods that integrate multiple geophysical and geochemical datasets within an internally- and thermodynamically-consistent platform have opened new and promising ways to address this ‘grand challenge’. In this presentation, I will discuss i) some recent progress, case studies and future directions on the mapping of the thermochemical structure of the continental lithosphere, and ii) their predictive power for the energy and critical minerals sectors and possible implications for planetary exploration in general.

How to cite: Afonso, J. C.: Unravelling the thermochemical structure and evolution of cratonic lithosphere with multi-observable probabilistic inversions, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-14694, https://doi.org/10.5194/egusphere-egu23-14694, 2023.

Archean cratons have thick, cold lithosphere that is remarkably stable, thanks to its compositional buoyancy and mechanical strength. Despite this stability, cratonic lithosphere can, sometimes, be modified and eroded, following the impact of a mantle plume, episodes of subduction and continental collision, or stretching and rifting. Although the chemical modification and removal of the Archean lithospheric material are permanent, there is intriguing evidence for re-growth in cratonic lithosphere’s thickness in some locations. In order to understand the enigmatic lithospheric evolution of cratons and continental blocks adjacent to them, we need the knowledge of the thermo-chemical structure of the lithosphere and of the dynamics of the lithosphere-asthenosphere interaction.

Seismic surface waves yield abundant evidence on the thermal structure and thickness of the lithosphere and on the temperature of the underlying upper mantle. Tomographic maps resolve in fine regional detail the boundaries between high-velocity (cold) cratons and lower-velocity (warm) neighbouring blocks. The radial structure and thickness of the lithosphere, however, are not resolved by tomographic models quite as well, due to their non-uniqueness. As a result, seismic-velocity profiles from tomographic models are normally incompatible with plausible geotherms. How, then, can we determine the structure and thickness of the lithosphere?

Recently developed methods for computational-petrology-powered inversion (e.g., Fullea et al. 2021) relate seismic, topography, heat-flow and other data directly to temperature and composition of the lithosphere and underlying asthenosphere. The misfit valleys in the surface-wave-dominated parameter space are still broad, and it is essential to have accurate measurements and low data-synthetic misfits. Here, we achieve remarkably low misfits of ~0.1% of the surface-wave phase-velocity values by precise tuning of the petrological inversion, its parameterisation and regularisation. The data are fit closely by models with depleted harzburgite mantle compositions within the lithosphere of cratons. The inversions tightly constrain the thickness of cratonic lithosphere, which we find to vary in the ~150-300 km range over different cratons. The plume-lithosphere interactions and the associated surface uplift and volcanism are controlled, to a large extent, by the lithospheric thickness  (e.g., Civiero et al. 2022), which, in turn, evolves with time, influenced by the processes. High-resolution seismic imaging and the petrological inversion of the resulting data yield exciting new discoveries on the evolution of continental lithosphere and its interactions with the underlying mantle.

References

Civiero, C., Lebedev, S., Celli, N. L., 2022. A complex mantle plume head below East Africa-Arabia shaped by the lithosphere-asthenosphere boundary topography. Geochemistry, Geophysics, Geosystems, 23, e2022GC010610.

Fullea, J., Lebedev, S., Martinec, Z., Celli, N.L., 2021. WINTERC-G: mapping the upper mantle thermochemical heterogeneity from coupled geophysical–petrological inversion of seismic waveforms, heat flow, surface elevation and gravity satellite data. Geophysical Journal International, 226(1), 146-191.

How to cite: Lebedev, S., Xu, Y., Davison, F., and Fullea, J.: Continental lithosphere and its interactions with the asthenosphere: New insights from seismic imaging and petrological inversion, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-7113, https://doi.org/10.5194/egusphere-egu23-7113, 2023.

The Northwest Indian shield (NWIS) comprises of Archean Bundelkhand, Marwar and Dharwar cratons, Proterozoic mobile belts of Aravalli Delhi fold belts (ADFB) and Central Indian tectonic zone (CITZ), and the basins such as Vindhyan (VB), Cambay (CR) and the Kutch (KR). The major area of the NWIS is covered by the Cretaceous Deccan Volcanic Province (DVP) that makes it difficult to assess the lithosphere structure in this region. Here we present the seismically constrained multi-scale geopotential field interpretation of  gravity, magnetic and geoid across the major Precambrian terrains of NWIS to delineate the lithosphere structure and further to understand the evolution of these terrains. The Bouguer gravity anomaly map shows overall high gravity values except the Bundelkhand and Dharwar cratonic parts over the NWIS region. The subsurface extension of the Precambrian  terrains of the NWIS are indicated by the distinct anomaly signatures in regional gravity anomaly map. The residual gravity anomaly map is able to delineate the shallow source bodies and boundaries between various terranes that correlat well with the surface geological expressions. The constrained geopotential modelling carried out along SW-NE trending profile across the region reveals that the Moho and  Lithosphre Asthenosphere Boundary (LAB) below the DVP and CR is relatively shallow as compared to the ADFB. It has also been noticed that a high density layer at the base of the lower crust, represents the presence of  underplated crust. The shallower lithosphere structure observed below the CR region might indicate the Cretaceous reworking. The imprints of the Deccan magmatism through intrusive bodies and the modelled structure below NWIS have implications on the lithosphere evolution in the region. 

How to cite: Sathapathy, S. K. and Radhakrishna, M.: Delineation of lithosphere structure below Northwest Indian Shield (India) through constrained geopotential field modelling : geodynamic evolution, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-11348, https://doi.org/10.5194/egusphere-egu23-11348, 2023.

We interpret the crustal and upper mantle structure along ~2500 km long seismic profiles in the northeastern

part of the Sino-Korean Craton (SKC). The seismic data with high signal-to-noise ratio were acquired with a nuclear

explosion in North Korea as source. Seismic sections show several phases including Moho reflections (PmP)

and their surface multiple (PmPPmP), upper mantle refractions (P), primary reflections (PxP, PL, P410), exceptionally

strong multiple reflections from the Moho (PmPPxP), and upper mantle scattering phases, which we

model by ray-tracing and synthetic seismograms for a 1-D fine-scale velocity model. The observations require a

thin crust (30 km) with a very low average crustal velocity (ca. 6.15 km/s) and exceptionally strong velocity contrast

at the Moho discontinuity, which can be explained by a thin Moho transition zone (< 5 km thick) with

strong horizontal anisotropy. We speculate that this anisotropy was induced by lower crustal flow during delamination

dripping. An intra-lithospheric discontinuity (ILD) at ~75 km depth with positive velocity contrast is

probably caused by the phase transformation from spinel to garnet. Delayed first arrivals followed by a long

wave train of scattered phases of up to 4 s duration are observed in the 800–1300 km offset range, which are

modelled by continuous stochastic velocity fluctuations in a low-velocity zone (LVZ) below the Mid-Lithospheric

Discontinuity (MLD) between 120 and 190 km depth. The average velocity of this LVZ is about 8.05 km/s, which

is much lower than the IASP91 standard model. This LVZ is most likely caused by rocks which are either partially

molten or close to the solidus, which explains both low velocity and the heterogeneous structure.

How to cite: Zhang, X., Thybo, H., Artemieva, I. M., Xu, T., and Bai, Z.: Upper Mantle Structure in the NE Sino-Korean Craton Based on Nuclear Explosion Seismic Data, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-16587, https://doi.org/10.5194/egusphere-egu23-16587, 2023.

We present the result of an integrated petrological and geophysical 3D modelling of the lithospheric mantle over the West and Central African rift system. For modelling, the integrated geophysical and petrological forward modelling software LitMod3D has been used. The initial geometry of the model is based on the Moho depth and base lithosphere of the global model WINTERC-G, and the sediment thickness from the global model Crust1.0 and the available seismic Moho depth have been used for validation. The model is fitted to satellite gravity gradients and the Bouguer anomaly calculated from the XGM2019e-2190 model. Different classes of mantle composition data have been considered and by iteratively trying to compute the best fitting between different modelled and observed signals, the final models of density, velocity and temperature distributions have been estimated. 

The model shows lateral transitions curved shape, extending horizontally for about 50km, between the West and Central African rift system, and the surrounding Congo craton and West African craton. More in detail, the results show the lateral and vertical variation of density, temperature and velocity in respect between the different lithospheric mantle domains. We notice the absence of a clear signature of the Saharan meta-craton, making this area more similar to the West and Central African rift system than the bordering cratons. Moreover, the modelled density profile shows a continuous depth dependent gradient under the rift system, but three steps in the depth profile under the cratons, suggest a layering of the lithospheric mantle with respect to its density gradient, which can be interpreted as metasomatism of the lower lithospheric mantle.

How to cite: Fosso Teguia M, E. E. and Ebbing, J.: Integrated 3D modelling of the lithospheric mantle under the West and Central African rift system and surronding., EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-3477, https://doi.org/10.5194/egusphere-egu23-3477, 2023.