To understand the earliest evolution of our planet, integrated and multidisciplinary approaches are essential. Isotope and elemental geochemistry, high-precision geochronology, petrology combined with geodynamic modelling will provide unique insights into the processes that shaped Earth’s earliest reservoirs. We welcome contributions from related disciplines that apply both established and innovative interdisciplinary approach towards addressing fundamental questions about pressing topics such as the differentiation and secular evolution of Earth’s crust and mantle, early reworking of the crust, transitionary stages of the ancient oceans and the nature of early tectonic regimes. These holistic studies will shed light on Earth's early formation, evolution, and transformation, revealing how initial habitable conditions were established and offering insights into ancient, possibly eroded, reservoirs.
Orals: Tue, 5 May, 08:30–12:30 | Room 0.51
The deep carbon cycle in the Archean is poorly constrained. Carbonate sedimentation only became an important reservoir for carbon from the late Archean onwards. It has been proposed that the transfer of carbon from the hydrosphere/atmosphere to the lithosphere mainly occurred during alteration of basalts of the oceanic crust [1]. While carbonation of ancient ultramafic rocks including komatiites has been described, it is often assumed that this carbonation occurred much later than the komatiite formation.
In this contribution, we investigate the role of carbonation of komatiites for the Archean deep carbon cycle. The Barberton Greenstone Belt of the Kaapvaal Craton, South Africa, provides a well-preserved pre-3 Ga terrestrial record and hosts hydrated komatiites that erupted ~3.48 Ga ago. We present data from samples of the ICDP drill core BARB1, transecting ultramafic lavas of the Komati Formation at depths of 108.77 to 112.73 meters. These komatiites have remained shielded from surface alteration throughout geological history and thus have not been affected by carbonate formation related to recent weathering.
The 3 m thick komatiite flow is covered by an andesitic volcaniclastic rock, where carbonate is intergrown with titanite in an albite-biotite-amphibole assemblage. A U-Pb age for titanite of 3266 ± 44 Ma demonstrates carbonate formation prior to this metamorphic overprint. In the uppermost 1.2 m of the komatiite flow only calcite is present, coexisting with chlorite, serpentine, tremolite, talc and magnetite. The volume of calcite decreases from 8-10 vol% in the first 40 cm to 2-6 vol% at 1 m depth where mainly spinifex textured komatiite is present. At 2-3 m depth, cumulate textures predominate, and the volume of calcite is always <2 vol%. Iron-bearing dolomite becomes the dominant carbonate with the volume increasing from 2 vol% to 9-13 vol% at the bottom of the flow. The carbonates display elevated Sr, Ba and B contents. The systematic change of carbonate minerals within the single komatiite flow and enrichment of these fluid-mobile elements indicate carbonation by seawater interaction directly after the emplacement of the flow rather than carbonate introduction during the later metamorphic event.
The intense carbonation of the komatiite lavas led to the incorporation of 1.5 to 6.5 wt% of CO2, illustrating that secondary carbonate is an important sink for carbon. We performed phase equilibria modelling on a komatiite + 5 vol% calcite composition to determine whether such carbon can be recycled back into the mantle during burial of oceanic crust along expected Archean geotherms. During prograde metamorphism, calcite is replaced by dolomite, which is stable up to temperatures of only 700-750 °C [2]. Therefore, carbonate incorporated into altered komatiites will be entirely released during burial/subduction of oceanic crust in the Archean. This efficient recycling of carbon suggests that ingassing of C into the mantle was likely insignificant and might have helped to keep atmospheric CO2 levels high in the Archean, an important aspect to explain the “faint young sun” paradox.
[1] Nakamura and Kato (2004): Geochem. Cosmochim. Acta 68 4595-4618.
[2] Tamblyn et al. (2023): Earth. Planet. Sci. Lett. 603, 117982
How to cite: Hermann, J., Vesin, C., Tamblyn, R., Hofmann, A., and Bolhar, R.: Carbonated komatiites and their importance for the Archean deep carbon cycle, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-6397, https://doi.org/10.5194/egusphere-egu26-6397, 2026.
A principal limitation of our understanding of the early Earth is the paucity of samples preserved from this time. Only around a dozen locations worldwide retain crust from the Eoarchaean period (>3.6 billion years, Ga), with controversy often surrounding the true age of these ancient vestiges of our planet. One of the best records of formation and alteration events is the mineral zircon. However, distinguishing between multiple igneous and metamorphic events experienced by a zircon population within an Archaean gneiss defines a critical question: how old is this rock?
Each new tract of ancient crust discovered carries a disproportionate significance in helping to shed light on the magmatic and geodynamic conditions shaping our planet's earliest evolution. Therefore, the utmost care must be taken when evaluating whether a terrane does host such ancient lithologies. We present a new U-Pb, Hf isotope and trace element dataset of zircons from 16 Archaean basement gneisses from the Eokuk Uplift, northwestern Slave Craton. Preliminary work from Eokuk showed one tonalitic gneiss with a U-Pb zircon crystallisation age of 3.813 Ga and an initial εHf value of -2.5, indicating the presence of Eoarchean crust derived from the partial melting of long-lived Hadean (~4.2 Ga) protocrust (Stoian 2023, Unpublished Thesis). Further investigation by depth profiling on 53 zircons to target the rim and outer mantle of this apparently Eoarchaean sample has revealed the presence of younger 3.150 Ga igneous crystallised zones (83% with Th/U> 0.3), with 60% of depth profiles yielding only this age. This compares with 13% of zircon depth profiles with only ~3.8 Ga ages and 6% drilling through both age domains. Whilst this complicates the argument that this sample represents an unambiguously Eoarchaean rock, younger igneous recrystallisation rims on Eoarchaean zircons are frequent in lithologies interpreted to be Eoarchaean from other terranes. Of the 114 spot analyses on grain interiors from this rock, 62% are ~3.8 Ga, with just 4% being ~3.15 Ga, and the rest being too discordant for age determination. We therefore conclude that this lithology records a dominant Eoarchean-aged zircon population, with depth profiling proving a robust tool to identify subsequent recrystallisation events.
Our geochronology study also reveals an additional four lithologies with Eoarchaean zircon cores (~3.6 – 3.7 Ga) from two distinct outcrops ~1km north of the preliminary study site. These rocks have additional igneous crystallisation ages at ~3.14, ~3.33, and ~3.43 Ga identified by combined textural and geochemical analysis. ~2.90 Ga metamorphic rims (Th/U <0.1) are identified in two of the four depth-profiled samples. These results further demonstrate that Eoarchaean crust in the Eokuk Uplift was continually reworked throughout the Palaeo- Mesoarchaean.
Previously, Eoarchaean-aged crust was only identified in the Slave Craton from the Acasta Gneiss Complex, some 275km south of the Eokuk Uplift. This new discovery provides the strongest case to date that larger packages of Eoarchaean crust exist beyond Acasta in the northwest Slave Craton. Further exploration and detailed mapping are required to determine the extent of Earth’s most recently discovered Eoarchaean terrane.
How to cite: Changleng, R., Schoonover, E. J., Stoian, C., Garber, J. M., Pearson, D. G., Luo, Y., and Reimink, J. R.: Beyond Acasta: A new Eoarchaean terrane identified in the Slave Craton , EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-20939, https://doi.org/10.5194/egusphere-egu26-20939, 2026.
Within the Aravalli-Delhi Fold Belt, NW India, sporadic granitic-gneisses outcrop are exposed in the Anasagar–Beawar region, whose age, origin, petrogenesis, tectonic setting, and stratigraphic status remain poorly constrained. Our zircon U–Pb geochronological data indicate that the protoliths of the Anasagar granite-gneiss (AGG) and Beawar granitic-gneiss (BGG) were emplaced at 1851 ± 20 Ma and 1130 ± 69 Ma, respectively. Field observations reveal that the AGG occurs as concordant, sheet-like bodies within the surrounding supracrustal rocks, giving a deceptive appearance of being the basement, whereas the BGG is exposed as isolated plutons surrounded by extensive soil cover. These relationships indicate that the Anasagar supracrustal sequence is older than 1.85 Ga—the crystallisation age of the AGG. Consequently, the AGG and its associated supracrustal rocks predate the majority of lithologies of the Delhi Supergroup. Therefore, these rocks are not part of the Delhi Supergroup but instead represent a small, isolated basin that developed during the closure of the Aravalli basin. Both AGG and BGG are megacrystic granitoids composed predominantly of quartz, K-feldspar, and plagioclase. They are calc-alkaline to shoshonitic, magnesian to ferroan, alkali-calcic to calcic, and are strongly peraluminous. The granitoids are characterised by high SiO₂, Na₂O + K₂O, low CaO and MgO, high FeOt/(FeOt + MgO), high Ga/Al ratios, and high average zircon saturation temperatures (~900°C for AGG and ~880°C for BGG), typical of A-type granitoids. Their enriched LREEs and LILEs, low Nb/Ta, and negative Eu anomalies indicate crustal sources. Both the granite-gneisses are classified as peraluminous A-type granitoids emplaced in a post-collisional extensional setting. The AGG exhibits εNd(t) values ranging from –6.4 to –4.6 and TDM ages between 2.57 and 2.78 Ga, whereas the BGG shows εNd(t) values from –11.2 to –10.4 and TDM ages between 2.53 and 2.81 Ga. Petrogenetic evidence suggests that both the granite-gneisses originated from the dehydration partial melting of Neoarchean crustal rocks, likely Banded Gneissic Complex (BGC) granitoids and Tonalite–Trondhjemite–Granodiorite (TTG) gneisses, under high-temperature and low-pressure conditions. The required heat flux was likely provided by large-scale mafic underplating or asthenosphere upwelling. Our findings suggest that the late Paleoproterozoic to early Mesoproterozoic tectonic evolution of the study area reflects two major events. The emplacement of the AGG represents the assembly of the Columbia supercontinent, whereas the BGG is not related to the assembly of Rodinia, as previously considered; instead, it more likely represents the pre-Rodinia assembly extension phase.
Keywords: NW India, Aravalli-Delhi Fold Belt, zircon U-Pb geochronology, A-type peraluminous granitoids, Sr-Nd isotopes, Crustal evolution
How to cite: Mohammed Khan, W. and Mondal, M. E. A.: Origin, petrogenesis, and tectonic implications of post-collisional A-type peraluminous granite-gneisses from the Aravalli–Delhi Fold Belt, NW India: Constraints from geochemistry, Sr–Nd isotopes, and zircon U–Pb geochronology, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-92, https://doi.org/10.5194/egusphere-egu26-92, 2026.
The Mid-Lithospheric Discontinuity (MLD) is a region of marked scattering of seismic waves which, according to several studies, is caused by mechanical weakness. It is located at an approximate depth of 100 km and has become a subject of intense research since it was identified through active-source seismology. The nature of the MLD is still a topic of intense debate. However, its contribution in destabilizing the continental lithosphere has often been invoked, and particularly in the destruction of cratons. Such a role is further enhanced when combined with other factors that may weaken the lithospheric mantle. Here we show the results of a 2D thermo-mechanical model, where we investigated the role of the MLD in the scenario of the interaction between long-lived mantle plumes and cratonic lithosphere. In this model, we implemented thermal and/or compositional plumes, with subsequent effects on their relative buoyancy with respect to the surrounding sublithospheric mantle. Our findings suggest that the combined effects of mantle plumes and MLD can effectively cause the destabilization and extensive delamination of cratonic lithosphere. However, mantle plumes must reach the MLD to trigger craton destabilization. For such a scenario, the presence of a weakened lithospheric mantle beneath the MLD is pivotal. This weak zone may be tectonic suture zone(s), or regions of melt and/or fluids percolation due to P-T variations in the plume during its ascent. We have verified that when plumes receive a constant material input from lower regions of the mantle, craton delamination can occur with very thin MLDs (< 10 km), and can be induced by cold and small compositional plumes, which are characterized by relatively low buoyancy.
How to cite: Lavecchia, A., Kovacs, I., Koptev, A., and Cloetingh, S.: Numerical modelling of plume-induced craton delamination: the role of the Mid-Lithospheric Discontinuity, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-2611, https://doi.org/10.5194/egusphere-egu26-2611, 2026.
The origin of Earth’s felsic protocrust remains enigmatic, with Iceland’s plume-thickened crust serving as a key analogue. We report Mg isotope systematics (δ26Mg) across Hekla volcano’s basalt-rhyolite suite, revealing unprecedented variations from -0.20‰ to +0.77‰, distinctly heavier than oceanic igneous rocks (-0.47‰ to -0.06‰). While fractional crystallization explains δ26Mg trends within basalt-andesite and dacite-rhyolite suites, the ~0.8‰ jump at intermediate compositions requires alternative processes. The exceptionally high δ26Mg in dacites, coupled with Th/U, and O-Li isotope systematics, fingerprints melting of hydrothermally altered mafic crust, likely recycled via plume-driven isostatic subsidence. These findings demonstrate that supracrustal signatures found in felsic magmas can emerge without plate tectonics, reshaping our understanding of early continental crust formation.
How to cite: Han, Y., He, Y., Wu, H., Sigmarsson, O., Williams, H., and Ke, S.: The presence of supracrustal Mg isotope signature in plume-derived felsic magmas, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-7240, https://doi.org/10.5194/egusphere-egu26-7240, 2026.
Tonalite-trondhjemite-granodiorites (TTGs) are a dominant lithology in many Archaean terrains and they retain a pivotal position in discussions of crust generation in the Archaean. Their major element compositions are consistent with partial melting of hydrated mafic source rocks, and yet their juvenile radiogenic isotope ratios indicate that they represent new continental crust.
This study links field evidence from the Lake Inari migmatite-granitoid terrain in northern Finland to petrogenetic models applicable to Archaean terranes. Lake Inari is part of the Meso- to Neoarchean TTG (tonalite-trondhjemite-granodiorite)-amphibolite terrains of Arctic Fennoscandia that form an extensive network of amphibolite metatexite-diatexite transitions controlled by melt proportions and syn-anatectic strain. The Lake Inari migmatite-granitoid terrain therefore provides a natural laboratory in which the bimodal association of felsic TTGs and their basaltic precursors are spatially and genetically linked, encouraging models in which the TTGs form directly through partial melting of the basalt. The zircons ages range over 300 Ma from 2.9-2.6 Ga (Joshi et al. 2024) and the geochemical data (Halla et al., 2024) confirm systematic trends supporting partial melting as the dominant TTG formation process. La/Sm increases from mafic rocks to TTGs, indicating progressive differentiation, but decreases at higher degrees of melting, defining a specific melting range. Th/Nb increases with La/Sm suggesting that negative Nb anomalies result from partial melting and differentiation. On average, Th/Nb increases from 0.17 in basalt to 0.96 in TTG (K2O/Na2O < 0.5). Co covaries with Ti in the TTG trending towards the mean Ti and Co values in the basalts, highlighting the role of ilmenite rather than rutile, and the REE variations indicate residual ampbibole rather than garnet. The average TTG was modelled as an 18% partial melt of basalt, assuming a bulk D-value of 0.01 for highly incompatible Th. The source mineralogy follows the thermodynamic model of Palin et al. (2016) for 20% melting at relatively shallow depths. While 20% represents an upper estimate, an 18% melting estimate yields bulk D-values of 0.4–0.5 for Rb, Sr, U, and Th; 1.37 for Nb and Ta; and 3.4–2.7 for Lu, Yb, and Y. Th/Nb increases with La/Sm in TTGs worldwide, highlighting its sensitivity to partial melting processes. The Lake Inari model is applied to other TTGs, allowing the distinction between TTGs derived from relatively high Th/Nb subduction-related sources and those formed in non-subduction settings, offering new insights into early continental growth. By linking field evidence with geochemical modelling, this study offers alternative insights into Archaean crustal evolution.
Halla et al (2024) Precam. Res. doi.org/10.1016/j.precamres.107407
Joshi et al (2024) Precam. Res. doi.org/10.1016/j.precamres.107418
Palin et al (2016) Precam. Res. doi.org/10.1016/j.precamres.11.001
How to cite: Hawkesworth, C., Halla, J., and Heilimo, E.: The Generation of Archaean TTG: insights from Lake Inari migmatites, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-9716, https://doi.org/10.5194/egusphere-egu26-9716, 2026.
Precambrian iron formations (IF), which typically contain more than 15% Fe, are important economic sedimentary rocks for iron resources and constitute an archive for understanding the geochemical evolution and processes of the early Earth. While iron deposits are found throughout the geological record, the origin, depositional conditions, biogeochemical cycling, and sources of geochemical components in Precambrian iron formations remain unclear.
Here, for the first time, we present mineralogical, geochemical, and Sm-Nd isotope data from two drill holes in the 2.8-2.7 Ga Baniaka iron deposits, south-eastern Gabon. The IFs are underlain by gneiss, consisting of metamorphosed silicate IF (amphibolitic facies) and oxide IF facies, from the base to the top, with minor occurrences of interbedded gneiss occurring within the amphibolitic facies. The basal silicate IFs essentially comprise biotite, stilpnomelane, magnetite, and actinolite, while the upper oxide facies are dominated by hematite and goethite with traces of magnetite. Quartz is a key component of all the samples while kaolinite and smectite are present in some of the oxide IF and the upper part of silicate IF samples, respectively. Traces of chlorite and siderite are locally observed in a few samples., The IF samples, except the kaolinite-rich ones, are rich in Fe (~20-60 wt.%) and relatively low in Al (0.1-4.0 wt.%). The Si content ranges from 1.3 to 30 wt.%, while Ca, Ti, Mg, Mn, K, and Na occur in trace amounts. The absence of a significant correlation between Fe and detrital proxies (Al and Ti) suggests that Fe enrichment is not controlled by detrital flux, indicating the involvement of Fe cycling.
The positive Eu anomalies in the chondrite- and shale-normalized rare earth element (REE) patterns, the slight depletion of light REE relative to heavy REE in the shale normalized patterns, and the chondritic to superchondritic Y/Ho values (27-48) demonstrate the influence of hydrothermal fluids and seawater mixing in an open ocean water depositional environment. The geochemical proxies of the incompatible elements (Th, Zr, Nb, and Sc), coupled with the positive εNd isotopic values (+0.34 to +9.75), are consistent with mantle-derived mafic materials from less-differentiated juvenile crust in a volcanic arc environment were significant source of the geochemical components during the deposition of the Archean Baniaka IF. Taken together, these results suggest that a significant proportion of the Fe likely derived from oceanic crust following hydrothermal alteration and seawater percolation within the Archean greenstone belts in southeastern Gabon.
How to cite: Aupissy, C. P. S., Bankole, O., Fontaine, C., and El Albani, A.: Mineralogical and geochemical constraints of Archean Baniaka Iron Formation: implication for origin and source of iron, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-12496, https://doi.org/10.5194/egusphere-egu26-12496, 2026.
Two contrasting end-members of Archean crustal growth can be envisaged: one dominated by juvenile additions from a depleted mantle, and the other by extensive reworking and remelting of older, chemically evolved continental crust. Archean cratons preserve the earliest record of crust–mantle differentiation and extreme tectonics, yet the relative contributions of these two processes remain debated. We address this issue through an integrated field, petrographic, whole-rock geochemical, zircon U–Pb geochronological, and Lu–Hf isotopic study of Neoarchean high-K I-type granites from the Bundelkhand Craton, northern India. These metaluminous to weakly peraluminous granites (SiO2 64–76 wt.%) intrude Palaeo- to Mesoarchean tonalite–trondhjemite–granodiorite (TTG) gneisses and display LILE-enriched, Nb–Ti–Sr-depleted, arc-like trace-element signatures, consistent with generation in a convergent-margin or arc-related geodynamic setting. New zircon U–Pb ages of 2559 ± 16 Ma, 2530 ± 12 Ma, and 2520 ± 26 Ma define a ~30 Myr-long episode of Neoarchean felsic magmatism, marking a protracted period of late Archean thermal and magmatic activity in the Bundelkhand Craton. Zircon εHf(t) values ranging from –9.3 to –1.9, together with two-stage Hf model ages of ca. 3.0–3.4 Ga, indicate that these granites were generated predominantly by partial melting of Meso- to Palaeoarchean TTG and mafic lower crust, with only limited input from juvenile, depleted-mantle–derived magmas. Zircon solubility in silicate melts and Ti in Zircon thermometry yield crystallisation temperatures of ~720–800 °C, while pressure estimates indicate emplacement at shallow to mid-crustal levels (~3–19 km), reflecting thickening of an already stabilised cratonic root.
The Lu–Hf signatures of the Bundelkhand granites provide an important basis for comparison with other Neoarchean cratons. Coeval granitoid suites in the North China Craton typically show more juvenile, near-chondritic to positive εHf(t) values, reflecting substantial additions of depleted-mantle–derived magmas during Neoarchean crustal growth. In contrast, the strongly negative εHf(t) values obtained for the Bundelkhand granites closely resemble those reported from Neoarchean granitoids in the Zimbabwe Craton, where crustal reworking and remelting of Neoarchean–Mesoarchean crust dominate over juvenile additions. The Bundelkhand high-K I-type granites, therefore, represent a crustal reworking-dominated end-member of late Archean continental growth. Placed within a Kenorland framework, our results emphasise that parts of the Indian shield evolved through prolonged reworking of older continental lithosphere rather than large-scale juvenile accretion, and they provide first-order constraints on the geodynamic regimes that governed late Archean continental assembly.
Keywords
Archean cratons; Bundelkhand Craton; Neoarchean granite magmatism; zircon U–Pb geochronology; Lu–Hf isotopes; crustal reworking; North China Craton; Zimbabwe Craton; Kenorland.
How to cite: Bisht, B. P. S., Xie, H.-Q., and Nasipuri, P.: The role of high-K I-type granites in Neoarchean craton stabilisation: insights from the Bundelkhand Craton, India, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-1020, https://doi.org/10.5194/egusphere-egu26-1020, 2026.
The Archean cratonic mantle formed as residues of extensive melt extraction, which is widely (but not universally) thought to have occurred in oceanic settings before being subducted and involved in the growth of the early continents. Despite its depleted nature, cratonic mantle peridotites, particularly from the Kaapvaal craton of southern Africa, often show geochemical and mineralogical evidence for intense secondary silica addition. Silica addition to the peridotitic cratonic mantle has been suggested to result from metasomatism by eclogite-derived silicic melts in subduction channels, serpentinization of oceanic protoliths, or, conversely, unrelated to oceanic protoliths and subduction tectonics, and results from interaction of the cratonic mantle with rising silica-enriched mantle-derived melts.
Here we use the Si-O isotopic compositions of cratonic mantle peridotites to constrain the source of silica enrichment, and thus improve understanding of processes that operated during the formation of Earth's first continental lithosphere. Mineral separates from coarse, low-T (<1000°C), orthopyroxene-enriched peridotite xenoliths from the Kaapvaal craton (South Africa) have δ30Si values from -0.56 to +0.40 ‰ and δ18O from +3.7 to +5.6 ‰. Silica-enriched peridotites with mantle-like δ18O-δ30Si indicate silica addition did not manifest in any isotopic change, in contrast to peridotites with high δ30Si at low δ18O. Rising mantle-derived silica-enriched melts (formed by hydrous fluxing of harzburgite or wall rock assimilation) could be the culprits of silica enrichment, based on recent oxygen isotope and geochemical studies of lithospheric mantle peridotites, which reconciles with the mantle-like Si-O isotopic signatures observed here. Post-3.8 Ga granitoids are characterized by elevated δ30Si, interpreted to be sourced from subduction-recycled Archean cherts with universally high δ30Si. Since Archean siliceous sediments are generally characterized by δ18O>>5‰, this is likely not a feasible method to produce the elevated δ30Si observed here. However, Precambrian ocean waters, with higher Si contents and δ30Si > 0‰ could instead have facilitated the high δ30Si with low δ18O observed for some Kaapvaal peridotites. However, both Si and O isotope disequilibrium observed in some of our samples raises questions regarding the timing of SiO2-addition, suggesting that the Si-addition, and isotopic signatures, may be a post-Arcehan feature related to Proterozoic subduction-driven metasomatism.
How to cite: Smart, K., Moynier, F., Deng, Z., Harris, C., and Tappe, S.: Silicon isotopic evidence for post-Archean silica enrichment of the cratonic mantle lithosphere, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-17565, https://doi.org/10.5194/egusphere-egu26-17565, 2026.
Emergence and formation of continental crust profoundly impacted Earth's evolution. Ensuing continental erosion enriched the oceans with terrestrial nutrients and set the stage for the evolution of complex life. Many crustal growth models indicate significant continental volume increases between 3.0–2.5 Ga, marking this period as critical for crustal evolution, possibly linked to the transition towards subduction tectonics.
Here we present a novel approach to trace continental emergence by reconstructing oceanic boron isotope composition from marine deposits (chert, iron formations, and shales). Boron enrichment in continental crust means runoff directly influences ocean boron concentration and isotopic composition, with continental runoff representing the largest modern source.
Our comprehensive marine B isotope record reveals a major compositional shift at 3.0 Ga: pre-3.0 Ga deposits show mean δ11B values of -23.1 ± 2.7 ‰, whereas post-3.0 Ga sediments are more variable with mean δ11B of -8.9 ± 3.1 ‰, projecting to seawater δ11B = +16 ‰ at 2.4 Ga. This change reflects enhanced continental emergence and subaerial erosion after 3.0 Ga, substantially increasing boron flux and driving seawater towards higher δ11B in the Proterozoic. A second elevation to modern values (δ11B = +39.6 ‰) occurred throughout the Phanerozoic due to increased chemical weathering following land plant appearance, paralleling the increase in seawater δ7Li.
How to cite: Abbo, A., Marschall, H., and Gerdes, A.: Tracing Craton emergence with Boron isotopes in Archean–Proterozoic marine deposits, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-7707, https://doi.org/10.5194/egusphere-egu26-7707, 2026.
Dyke swarms and sedimentary basins tend to appear in the late Archaean (Cawood et al., 2022), suggesting that the crust was cold and brittle enough to facilitate and preserve dyke swarms. However, the thermal and mechanical state of the Archaean lithosphere that facilitates extensive diking and fracturing remains unclear.
Continental crust seems to have been relatively mafic in the Archaean (Dhuime at el., 2015, Hawkesworth and Jaupart, 2021), but over less than one billion years, it underwent internal differentiation largely driven by in-situ radiogenic heat production (Perry et al., 2006, Michaut and Jaupart, 2007). Magmatic rocks in greenstone belts show bimodal silica distribution (Kamber, 2015), and bulk crust was composed of ~30% felsic crust and ~70% depleted or dehydrated mafic crust (Hawkesworth and Jaupart, 2021). Tonalite-trondhjemite-granodiorites (TTGs) are consistent with internal differentiation, and are thought to have been produced via partial melting of hydrous metabasalts (Moyen and Martin, 2012). A preferred model of the Archaean lithosphere is to generate mafic crust until it was thick enough to melt and form TTGs and then to have a more felsic crust that ultimately stabilised sufficiently for dyke swarms.
Here, we test the hypothesis that thickening and internal differentiation (felsification) of Archaean crust led to major cooling of the lithosphere allowing dyke swarms to be a feature of the late Archaean. We calculate thermal profiles for the Archaean lithosphere for different scenarios of internal differentiation between 3.5−2.5 Ga that are consistent with present-day observations (e.g., Michaut and Jaupart, 2007). These thermal profiles are then used to investigate melt transport in the lithosphere using a two-phase flow model that incorporates a new poro-viscoelastic–viscoplastic theory with a free surface (Li et al., 2023, Pusok et al., 2025), designed and validated as a consistent means to model dykes. Results show that a warmer, weaker crust facilitates formation of sills and smaller dikes, while a cold, brittle crust facilitates formation of larger dykes. Our results suggest that dyke swarms are evidence for a cooling geotherm and strengthening of crust, and that crustal differentiation was a necessary condition for crustal stability of Archean provinces. This threshold for dyke swarm formation could have implications for the onset of widespread subduction and plate tectonics.
References
Cawood et al. (2022) Rev. Geophys. DOI:10.1029/2022RG000789
Dhuime at el. (2015), Nat. Geosci. DOI:10.1038/ngeo2466
Hawkesworth and Jaupart (2021), EPSL DOI:10.1016/j.epsl.2021.117091
Kamber (2015), Precam. Res. DOI:10.1016/j.precamres.2014.12.007
Li et al. (2023), GJI DOI:10.1093/gji/ggad173
Moyen and Martin (2012), Lithos DOI:10.1016/j.lithos.2012.06.010
Michaut and Jaupart (2007), EPSL DOI:10.1016/j.epsl.2007.02.019
Perry et al. (2006), JGR DOI:10.1029/2005JB003893
Pusok et al. (2025), GRL DOI:10.1029/2025GL115228
How to cite: Pusok, A. E. and Hawkesworth, C.: Thickening and felsification of Archaean crust stabilised the geotherm allowing for dyke swarms in the late Archaean, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-5924, https://doi.org/10.5194/egusphere-egu26-5924, 2026.
Archaean lamprophyres and sanukitoids have been spatially and temporally linked to magmatic-hydrothermal gold deposits across the Yilgarn Craton of western Australia. Lamprophyres are considered to be the product of hydrous low-degree partial melting of metasomatic mantle source regions. Sanukitoids are relatively rare late Archaean mantle-derived hornblende-plagioclase porphyritic granitic complexes characterised by high MgO and relatively elevated concentrations of Ba and Sr. Therefore, sanukitoids exhibit mantle-derived (Mg, Ni, Cr) and incompatible element-enriched components (Sr and Ba) indicative of contributions from both mantle and crustal sources. Cognate xenoliths within the sanukitoids are amphibole-rich and/or biotite-rich metabasites. One model proposed for explaining sanukitoid formation is through amphibole-dominated fractional crystallisation of a lamprophyric parental melt[1].
Understanding whether the spatial relationship between gold systems and sanukitoid-lamprophyre magmatic systems is also genetic will be important for updating Archaean magmatic-hydrothermal gold deposit models. High precision trace gold analyses, with detection limits of 0.02 ppb, have been conducted on systematic samples of lamprophyres and sanukitoids to quantify the gold concentration variation during magmatic differentiation.
Here, we present mineral chemistry (amphibole, feldspar, mica) from sanukitoids associated with gold deposits and their cognate xenoliths in conjunction with whole-rock, and trace gold, geochemistry from the Yilgarn Craton. We present our results of fractional crystallisation modelling and our investigation into the behaviour of gold during mantle (primitive lamprophyres) and crust (evolved sanukitoid) transportation. We test whether the Archean lamprophyre-sanukitoid magmatic system is intrinsically elevated in gold or whether lamprophyre-sanukitoid magmas provide fertile fluid conduits for gold deposit formation.
[1] Smithies et al., 2019. Nature Communications, 10(1), p.5559.
How to cite: Nash, I. J., Hollis, S. P., Hastie, A. R., Smithies, R. H., Verbeeten, A., Holder, D., and Stock, E.: Origin of sanukitoid magmas linked to Archaean intrusion-related Au deposits: Insights from the Yilgarn Craton, Australia, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-13646, https://doi.org/10.5194/egusphere-egu26-13646, 2026.
The size and spacing of TTG batholiths is remarkably uniform within and amongst Archean granitoid-greenstone terrains. On craton-spanning maps, the average distance between centres of batholiths is, on average, twice the present-day depth to the Moho in the Zimbabwe, Ukraine, Superior and eastern Pilbara cratons, wherein the present erosional surface that reveals the granitoid-greenstone cellular pattern well corresponds to paleo-depths near the brittle-ductile rheologic transition (~520C). It is a fundamental principle of fluid dynamics that an array of convection cells in a horizontal layer convecting by Rayleigh-Bénard (temperature-induced) or Rayleigh-Taylor (composition-induced) density instabilities has convection cell radii (half the distance between centres of cells) that scale near 1:1 with the thickness of the convecting layer, as is the case in all these granite-greenstone terrains. Heavy oxygen isotopic compositions of mafic granulites (intercalated with paragneisses) in the Archean lowermost crust exposed in the Kapuskasing Zone (south-central Superior), the Vredefort structure (Kaapvaal craton), and as Archean xenoliths in Tertiary diatremes in western Wyoming craton show them to be seafloor-altered metavolcanics that migrated to the base of the crust during the Archean. Metamorphic peak P-T measurements in supracrustal rocks spanning the prehnite-pumpellyite to garnet granulite facies in the Wawa-Kapuskasing crustal section preserve a precisely resolved, steep, conductive geotherm of ~40°C/km to ~12 km paleodepth and a much flatter gradient (~11°C/km) deflected toward an adiabatic gradient in convecting ductile rocks at ~12-40 km paleodepth. Metamorphic fluids released from supracrustal rocks that migrated to the base of the crust and underplated hotter overlying rocks lowered the solidus temperature of the fluid-metasomatised overlying rocks and induced production of silicic partial melts (TTGs) having spidergram spikes in relative abundance of water-soluble elements like spidergrams of Phanerozoic arc magma. The arc-like spidergram patterns are absent in most coeval greenstones, which have spidergram patterns resembling MORB and OIB melts of asthenospheric mantle. Arc-like spidergram patterns in Archean TTGs can be well explained without plate tectonics. Craton-scale cellular arrays of greenstone belts and TTG batholiths are inconsistent with plate tectonics.
How to cite: Loucks, R.: Crustal Convection Turned Out a Superior Craton—and Zimbabwe, Pilbara and Ukraine Archean Cratons Too, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-15236, https://doi.org/10.5194/egusphere-egu26-15236, 2026.
Subaerial land today is mainly formed by continental crust, but before the stabilization of the first cratons, at ca. 3 Ga, volcanic structures (e.g., oceanic islands) may have been the first subaerial regions of the early Earth. Understanding the onset of felsic magmatism is crucial for constraining the formation of both continental crust and hypothetical early volcanic islands. Studies of ancient zircons suggest that subaerial land likely emerged at least by 3.5 Ga, but how long before that it began remains unknown. Our work examines circa 3.5 Ga old felsic volcanic rocks, the oldest known in the Kaapvaal (South Africa) and Singhbhum (India) cratons. We analyzed oxygen and Lu-Hf isotopes in zircon as they are effective proxies for distinguishing the melt source between mantle-derived and crustal (remelting of altered rocks and sediments). Oxygen isotopes ratios (δ18O) were measured by Secondary Ion Mass Spectrometry (SIMS) in coeval felsic units of the Kaapvaal Craton (i.e. Theespruit, Sandspruit, and Toggekry formations), and of the Singhbhum Craton (Daitari and Gorumahisani greenstone belts). This new data was compared with a newly compiled global Archean δ18O dataset (ca. 13,000 data points). Our felsic volcanic rocks display the averaged δ18O values ranging between 5.1 and 5.8 ± 0.24 ‰ (2 sd), which are purely mantle-like values. The only exception is a Toggekry formation sample (δ18O 3.9 ± 0.24 ‰), which reflects remelting of hydrothermally altered rocks. Published εHf values for the same rocks fall between CHUR and Depleted Mantle trends, implying juvenile melt signatures. In this context, we highlight the significance of the early Earth's first felsic rocks, whose formation is usually attributed to partial melting of a hydrated basaltic oceanic crust. In contrast, our data emphasizes the importance of purely mantle-derived felsic melts in the Archean. These felsic melts can be a result of extensive fractional crystallization (ca. 80%) of a stalled basaltic melt. Such relatively dry melting (possessing only juvenile water) requires elevated heat flow, and thick lithosphere. During the Archean, these conditions may have prevailed in a thick basaltic oceanic plateau setting. Reworking (i.e., melting) of such ancient oceanic plateaus could have led to the renewed generation of felsic melts producing buoyant silicic rocks and ultimately result in the consolidation and emergence of the earliest continental crust. The global Archean δ18O values compilation suggests that the mantle and seawater-altered rocks are both important sources of felsic melts during the Archean. This highlights the significance of global Archaean tectonic regimes that may have led to the formation of the first subaerial landmass in brief stints.
How to cite: Gromov, P., Jodder, J., Conrad, C. P., Torsvik, T. H., Agangi, A., Wiedenbeck, M., Couffignal, F., Glynn, S. M., and Gaina, C.: Widespread felsic volcanism as a possible step towards Archean subaerial landmass: Insights from combined oxygen and hafnium isotopes in zircon, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-17446, https://doi.org/10.5194/egusphere-egu26-17446, 2026.
Geochemical Investigation of Paleoproterozoic Siliclastic Rocks: Implications for Evolution of North Indian Craton.
Sadia Khanam1, Nurul Absar2, Mohammad Adnan Quasim1
- 1- Aligarh Muslim University
- 2- Pondicherry University
The Paleoproterozoic was the most significant era in geological history, during which crustal stabilization occurred, and Earth began to develop an atmosphere habitable to life after the Great Oxygenation Event. Paleoproterozoic cratonic rocks preserve chemical signatures that provide insight into the onset of plate tectonics, supercontinent assembly, and the Great Oxidation Event, offering a unique archive for reconstructing Earth’s first billion years and guiding mineral resource exploration. The Indian subcontinent has a unique cratonic nucleus comprised of rocks of the Paleoproterozoic, that is, the Aravalli craton. However, few studies have been conducted on the complete evolution of the Aravalli Craton. To this end, we investigated the Paleoproterozoic Rajgarh Formation of the Alwar sub-basin, North Delhi Fold Belt. This study provides critical insights into the provenance, depositional environment, and tectonic setting of the North Indian Craton (NIC) through petrographic, mineralogical, and geochemical analyses. The sandstones are quartzarenite to arkosic in nature and composed of monocrystalline quartz with undulose extinction, feldspars, micas, cordierite, and heavy minerals, including zircon, garnet, and tourmaline. Modal analysis and Qt–F–RF ternary plots indicate cratonic interior provenance. Chondrite-normalized REE patterns display fractionated LREE, flat HREE, and negative Eu anomalies (average Eu/Eu* = 0.76), consistent with felsic to intermediate sources such as granitoids, TTG gneisses, and granulites of the Banded Gneissic Complex and the Sandmata Complex. Trace element ratios (Th/Sc, La/Sc, La/Yb, and Cr/V) and discrimination diagrams (La–Th–Sc, Th–Sc–Zr/10, Ga with V and Sn) also suggest derivation from felsic crust in active to passive margin settings. Redox-sensitive proxies (U, V, Mo, Cd and EF of RSTE) and pyrite occurrence indicate deposition in oxic to sub-oxic shallow-marine environments with intermittent dysoxic phases. Collectively, the Rajgarh Formation of Alwar sub-basin records a complete tectonic cycle: intracratonic rifting and basin initiation, felsic-dominated sediment supply, shallow-marine rift deposition, and basin stabilization during the Mesoproterozoic Delhi Orogeny. Comparisons with other Purana basins (Vindhyan, Cuddapah, and Bayana) demonstrate that the NIC experienced widespread intracratonic extension during the Columbia supercontinent cycle, followed by stabilization during Rodinia accretion.
How to cite: khanam, S., Absar, N., and Quasim, M.: Geochemical Investigation of Paleoproterozoic Siliclastic Rocks: Implications for Evolution of North Indian Craton. , EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-1292, https://doi.org/10.5194/egusphere-egu26-1292, 2026.
Archean alkaline magmatism is exceptionally rare in the geological record, offering valuable insights into Archean tectonic processes, mantle evolution, and crustal growth. This study presents in situ titanite U-Pb geochronology, trace element geochemical data, and bulk-rock geochemical results for an adakitic syenite pluton from the Danduvaripalle area of the Eastern Dharwar Craton, southern India. The investigated pegmatoidal leucocratic syenite forms an undeformed, oval-shaped plug with an NNE-SSW orientation, emplaced in a regional extensional to transtensional crustal setting. The syenite is predominantly composed of pure end-members of alkali feldspar, aegirine-augite, diopside, titanite, actinolite, apatite, and magnetite.
Bulk-rock geochemical analyses indicate a metaluminous nature, exhibiting highly fractionated REE patterns with marked middle REE depletion, resulting in an overall spoon-shaped REE profile typical of melts affected by high-pressure amphibole and garnet fractionation. The rocks exhibit elevated La/Yb (>45) and Sr/Y (>100) ratios, with pronounced heavy REE depletion (Yb < 0.7 ppm, Y < 8 ppm), which confers a distinct adakitic geochemical signature. Extreme Lu/Gd ratios (<1) in titanite further confirm strong heavy REE depletion in the parental melt. Primitive mantle-normalized multi-element patterns display ‘crust-like’ signatures, notably with negative Nb-Ta-Ti anomalies. Titanite grains are exceptionally enriched in incompatible trace elements, reflecting the evolved nature of the melt from which they crystallized. Chondrite-normalized REE patterns for titanite show extreme LREE enrichment with minimal HREE, resulting in steeply fractionated trends. Additionally, low La/Ce (<0.4) and high Ce/Nd (>1) ratios in titanite indicate an oxidizing condition of the melt. Overall, the geochemistry (adakitic traits, high Th/U and low Nb/U) supports derivation from a mafic lower crust preconditioned by subduction- and accretion-related processes, which, upon partial melting, produced K- and SiO2-rich melts with adakitic characteristics and crust-like multi-element patterns.
LA-ICP-MS in situ U-Pb dating of unzoned titanite from two representative samples yields crystallization ages of 2526 ± 5 Ma (n = 19, MSWD = 2) and 2514 ± 7 Ma (n = 24, MSWD = 3.5). These ages likely correspond to the final stage of collision vis-à-vis slab breakoff, which facilitated extensional magmatism. This resulted in the emplacement of syenite in a post-collisional extensional regime, rather than an active subduction-related setting. Between ~2600 and 2500 Ma, the Eastern Dharwar Craton underwent widespread felsic plutonism, including tonalites, granodiorites, granites, and syenites, which were associated with the final stage of cratonization. The studied syenite is therefore interpreted to constitute a component of this terminal magmatic event during craton stabilization.
How to cite: Pandey, A., Chatterjee, A., Jiang, S.-Y., dos Santos, A. C., and Pandey, R.: Evidence of Neoarchean adakitic alkaline magmatism in the Eastern Dharwar Craton , EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-554, https://doi.org/10.5194/egusphere-egu26-554, 2026.
The Neoarchean era represents a pivotal transition in early Earth evolution and tectonic regime transformation, serving as a critical window for investigating continental origin and early geodynamic processes. The tectonic setting of the Neoarchean Eastern Hebei Complex in the North China Craton remains intensely debated. Ultramafic rocks provide key insights into deep Earth material circulation and dynamics, serving as petrological probes into early crust-mantle evolution, the onset of plate tectonics, crustal recycling, and deep geodynamic processes. We present an integrated study incorporating field investigations, petrology, whole-rock and mineral geochemistry, and zircon U-Pb geochronology on serpentinized lherzolites, pyroxenites, and metamafic rocks from the Zunhua-Shangying ophiolitic mélange belt. The field geological mapping shows that the ultramafic rock is mainly exposed in the Songling, Longwan, and Shangying regions. Serpentinized lherzolites exhibit refractory mantle characteristics with high Mg# (~85) and relatively flat to slightly depleted rare earth element patterns. High-Mg# (75.60-80.78) Songling-Longwan pyroxenites represent high-pressure cumulates derived from hydrous, subduction-modified basaltic magmas in the lower arc crust, whereas low-Mg# (24.6-41.6), high-Al₂O₃ (13.7-19.7 wt%) metamafic rocks constitute complementary evolved melts. Magmatic zircons from metamafic rocks yield a weighted average age of 2.52 Ga, interpreted as the crystallization age of the Songling-Longwan mafic-ultramafic suite. The Shangying garnet pyroxenites, showing typical N-MORB geochemical affinities, are identified as Archean oceanic crust remnants. Coexisting metagabbro yields an identical magmatic zircon age of 2.52 Ga, constraining the crystallization of the Shangying garnet clinopyroxenite. Metamorphic zircons in pyroxenites and granulites record subsequent tectonothermal events at 2.48 Ga and 1.85 Ga. Integrated geochronological and geochemical data demonstrate that the Eastern Hebei mafic-ultramafic suite developed in an intra-oceanic arc system, with distinct formation (2.55-2.52 Ga) and emplacement (2.52-2.47 Ga) stages. During the formation stage, the Songling-Longwan rocks originated at the arc root crust-mantle transition zone, while the Shangying garnet pyroxenites formed at greater depths within the subducted slab. Throughout the emplacement stage, both the N-MORB-type Shangying garnet clinopyroxenites and metagabbros and the arc-related Songling-Longwan mafic-ultramafic blocks were incorporated as coherent and elongated tectonic fragments into the Zunhua-Shangying forearc ophiolitic mélange. The late Neoarchean to early Paleoproterozoic metamorphism coincided with subduction of the Eastern Block beneath the Wutai/Fuping arc terrane and subsequent arc-continent collision, leading to the stabilization of the Eastern Block and the final accretion of the Zunhua-Shangying ophiolitic mélange belt.
How to cite: Wang, R., Wang, L., Ning, W., Deng, H., and Kusky, T.: Petrogenesis of mafic-ultramafic rocks in the Eastern Hebei Complex of the North China Craton: implications for the Neoarchean tectonic regime, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-2297, https://doi.org/10.5194/egusphere-egu26-2297, 2026.
The redox state of Earth’s mantle exerts a fundamental influence on volcanic degassing and the composition of the atmosphere, yet its long-term evolution remains uncertain. Here we use a machine-learning classifier to identify primitive arc basalts and reconstruct mantle wedge’s oxygen fugacity over time. Our results show that the redox state of mantle wedge raised in Earth’s middle-age. This Mesoproterozoic oxidation was asynchronous with surface oxygenation, suggesting that deep Earth processes, such as the enhanced fluxes of serpentinite-derived fluids, drove the oxidation of mantle wedge. The establishment of an oxidized mantle wedge may have reduced volcanic oxygen sinks and facilitated atmospheric oxygen accumulation in Mesoproterozoic, with implications for the rise of early eukaryotic life.
How to cite: Liu, C.-T., Ye, C.-Y., Xia, Q.-K., and Zhang, Z.: The oxidation of mantle wedge in Earth’s middle age, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-7331, https://doi.org/10.5194/egusphere-egu26-7331, 2026.
Cratons are thought to be the stable cores of continental lithosphere that have survived for 3000 Myr. Such long term survival is often attributed to the excess thickness and elevated viscosity of the cratonic lithosphere. Yet, the evolution of craton thickness during these 3000 Myr has remained highly debated. Several studies have explored three possible scenarios. First, cratons may have been thicker in the past and thinned to ~200 km in the present day. Second, cratons may have thickened slowly or third, they have maintained their current thickness since their origin. In this study we explore the evolution of craton thickness in the past 3000 Myr using 2-D thermo-mechanical numerical models. We initiate each model with a thick and compositionally lighter (1.5% less dense) craton of 200 km in a hot convecting mantle and let it run for 3000 Myr. We impose periodic compression and extension on the craton to mimic supercontinental cycles. We run a total of 24 models exploring a range of initial thicknesses, density contrasts, radioactive heating, and mantle cooling parameters, in order to test multiple evolutionary scenarios. The main results suggest that due to its lower density, the craton is initially flattened. As the craton cools, thermal density overcomes the compositional density, and the craton thickness increases. Viscosity increases concurrently and the mantle flow is diverted along the cratonic edges to self-compress the craton gradually. Due to periodic compression and extension in the model, craton topography varies within a few hundred meters, consistent with observations suggesting basin opening and erosion during and after the assembly and break up of supercontinents. However, the continental lithosphere remains stable. After 1500 Myr, the craton becomes thicker than 160 km depth, a crucial depth for generating kimberlites. Kimberlites are volatile-rich ultramafic rocks that are generated within a depth range of 160-250 km, and are only found above thick continental cratons. Importantly, most kimberlite ages cluster within the last 300 Myr, and available databases suggest that kimberlites were scarce between 3000 and 2000 Myr. Eruptions began occurring more continuously after ~1500 Ma, and accelerated after ~1100 Ma. This pattern is consistent with our models of a slowly growing craton thickness. We find that before 1500 Ma cratons were mostly thinner than the critical depth for kimberlite generation. After 1500 Myr, their thickness increased, allowing them to host more kimberlites. Although previous hypotheses emphasize mantle temperature and carbon availability as primary controls on kimberlite eruptions in the later part of Earth’s history, our results suggest that craton thickness also exerts a strong control on the eruption of kimberlite magmas.
How to cite: Paul, J. and Conrad, C. P.: Thickening of cratonic lithosphere: Implications for craton growth and kimberlite eruption trends, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-10266, https://doi.org/10.5194/egusphere-egu26-10266, 2026.
The Singhbhum Craton of eastern India comprises Archean TTG basement, granite-greenstone belt, and Neoarchean–Paleoproterozoic mobile belts, and records a major phase of Mesoarchean potassic to ferro-potassic granitoid magmatism (~3.12–3.05 Ga) following earlier TTG crust formation. Earlier literatures have largely focused on granitoids and associated mafic–ultramafic suites, with rhyolites remaining undocumented. The rhyolites identified in the vicinity of the Bangriposi area are spatially associated with the Mayurbhanj granite–granophyre suite. They are dark grey in colour, predominantly aphanitic and show rare primary flow banding. Some samples exhibit porphyritic textures, with quartz and feldspar phenocrysts (~1–2 cm) set within a fine-grained groundmass. Under the microscope they are extremely fine grained, and some relict feldspar grains are preserved despite alteration, indicating partial recrystallization. The primary mineral assemblage consists of quartz, plagioclase, and microcline, with accessory phases including muscovite, apatite, ilmenite, iron oxides, galena, zircon, and monazite. Zircon grains are fractured and display pitted texture. Quartz phenocrysts are typically anhedral, fractured, and show strong undulose extinction, with some grains forming polycrystalline aggregates indicating recrystallization.
Whole-rock XRF data show high SiO₂ (71-78 wt%) content with moderate to high total alkalis (Na₂O + K₂O ≈ 6.8–9.1 wt%). K₂O is always greater than Na₂O and their Na₂O/K₂O ratios lie within a restricted range varying between 0.52 and 0.95. The higher K₂O value compared to Na₂O suggests significant input from pre-existing crustal materials. On TAS diagrams, the samples plot in the rhyolite field. Harker variation diagrams show decrease in MgO, Fe₂O₃, CaO, and TiO₂ with increasing SiO₂ signifying magmatic differentiation, primarily through fractional crystallization. The rhyolites are metaluminous to weakly peraluminous, calc-alkaline to alkali-calcic, ferroan, and very low in Mg. Normative QAP compositions plot mainly in the monzogranite field. The data suggest that the rhyolites might have formed from partial melting of existing continental crust or from the fractional crystallization of a parental magma possibly combined with the assimilation of older crustal rocks. A comparative geochemical and petrogenetic evaluation of these rhyolites with the associated granite–granophyre suite will provide critical insights into crustal evolution and tectono-thermal history of the Singhbhum Craton, while further trace-element and isotopic studies are required to fully constrain their source characteristics and petrogenetic history.
How to cite: Mishra, S. and Pruseth, K. L.: Rhyolites from the northeastern margin of Singhbhum Craton: petrography and geochemistry, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-11766, https://doi.org/10.5194/egusphere-egu26-11766, 2026.
Northernmost Finland hosts two Archean tonalite-trondhjemite-granodiorite (TTG) complexes, the Inari complex in the east and the Rommaeno complex in the west. In our newly launched research project, “Birth of Continents”, these will serve as key areas in deciphering the mechanisms of the formation of buoyant, felsic crustal material from a basaltic precursor. Existing geochronological data are relatively sparse, but suggest a prolonged, semicontinuous zircon crystallization in excess of 200 Ma within spatially limited areas. Sampling in these extremely remote complexes has been skewed by the sparsity of road networks. Our sampling campaign with a more balanced grid will give a more thorough view into the age distributions within these TTG complexes.
Preliminary geochemical results as well as field evidence point to certain differences between the two complexes. These may reflect e.g. degrees of melting, variable metamorphic grade, or perhaps different present-day erosional levels. With help of Lu-Hf isotopes in zircon, we aim to constrain the pressures of zircon crystallization. Possible differences in the protoliths of the migmatized TTGs will be modeled with a melt reintegration procedure. Ultimately, we hope to develop a compelling model on how the earliest refractory felsic crust evolved from the transient basaltic one.
How to cite: Kurhila, M., Halla, J., Heilimo, E., and Joshi, K. B.: Archean Inari and Rommaeno complexes in Arctic Fennoscandia – windows into the evolution of early continental crust , EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-16086, https://doi.org/10.5194/egusphere-egu26-16086, 2026.
