This session will focus on new insights gained from studying Archean rocks using a blend of techniques, ranging from traditional fieldwork to high-precision drone imaging, and both established and novel in-situ analytical techniques.
We encourage submissions to this session that dive into the enigmas of Archean rocks by integrating metamorphic petrology with structural and microstructural analysis, in-situ petrochronology, thermodynamic modeling, geochemistry, geophysics, and geodynamic modeling. These techniques will facilitate the revelation of metamorphic and deformation histories, contributing new insights into the processes that influenced the early Earth.
Posters on site: Thu, 7 May, 10:45–12:30 | Hall X2
Earth’s tectonic mode during the Precambrian is controversial: one school argues that plate tectonics started in the Hadean and has probably operated continuously since, whereas another considers Earth's tectonic mode prior to the Proterozoic to have been different due to high ambient mantle temperature. Implicit in the second view is the necessity of transitions between different tectonic modes. Is there evidence of such transitions preserved in the petrology of the continental lithosphere? If so, does the petrological record allow us to constrain the timing of the transitions and differentiate between these two schools of thought? Whatever the tectonic mode was early on, there is clear petrological evidence of a change in tectonic mode after the first 700-1000 Myr of Earth evolution, which is marked by the onset of mantle depletion and the first appearance of substantial volumes of continental crust in the geological record. What then was the tectonic mode after this transition?
From an igneous perspective, based on geological relationships and Th–Nb systematics, purported Archean ‘ophiolites’ do not represent oceanic crust and subduction-related rocks are rare before the Proterozoic. Much of the extant Archean crust was likely generated by plumes, with limited lithospheric extension and convergence, and only short-lived episodic subduction (Brown et al., 2024, JGS). From a metamorphic perspective, Archean crust is characterized by a unimodal distribution of metamorphic T/P, consistent with plume-driven mantle dynamics (Brown et al., 2024, JGS). Further, the eclogite record provides an important constraint on Archean tectonics. Xenolithic (mantle) eclogites, scavenged by younger carbonated magmas as they rise through the lithospheric mantle roots of cratons, are mostly older than Mesoproterozoic and represent oceanic crust that was subducted to mantle depths during the later stages of craton formation (Brown et al., 2026, in review). By contrast, all reliably dated orogenic (crustal) eclogites are post-Archean and are generally found in sutures or accretionary complexes (Brown et al., 2026, in review). The presence of xenolithic eclogites in the mantle roots of Archean cratons suggests that moderate late-stage thickening was driven by subduction, whereas orogenic eclogites occur in sutures between cratons that form the composite continental fragments formed in the first supercontinent cycle.
Paleomagnetic data from Archean cratons require periods of lithospheric mobility at rates like those in the Phanerozoic (cm/yr), prolonged periods of stasis and brief periods of rapid mobility (up to tens of cm/yr), and differential motion between cratons, requiring active tectonic boundaries between them. This apparent contradiction between a dominantly plume origin for cratonic crust and periods of lithospheric mobility can be reconciled if tectonic units were larger than the preserved cratons, and subduction was off craton in an episodic mode. Thus, xenolithic and orogenic eclogites record complementary information about Archean subduction (mostly warmer and episodic, may result in soft collisions but does not generate orogenic eclogites) and Proterozoic subduction (mostly colder and continuous, may result in subduction of continental margins to eclogite facies conditions), where the change relates to the global emergence of plate tectonics.
How to cite: Brown, M.: The petrological record constrains Archean tectonics and transitions between Earth’s tectonic modes, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-1466, 2026.
The Barberton Greenstone Belt (BGB) is one of the best-preserved Paleo- to Mesoarchean crustal domains on Earth. Metapelites within the southern BGB are one of the few known occurrences of rutile-bearing Archean rocks and indicative of pressure dominated metamorphism. However, rutile is very often preserved as detrital grains in eroded, sedimentary equivalents of Precambrian rocks. Based on its trace element composition, detrital rutile can be used to identify formation conditions and -environment of metamorphic rocks. This makes in-situ rutile from the BGB a uniquely important case that can be used to test which proxies are reliable to infer formation conditions and source rock compositions.
The southern BGB records two amphibolite facies metamorphic events at ~3.44 Ga and ~3.23–3.19 Ga, with the younger event at relatively higher metamorphic grade [1]. The metapelites have the mineral assemblage plagioclase, clinozoisite, biotite, staurolite, quartz, kyanite, and k-feldspar with accessory rutile, ilmenite, zircon, apatite, monazite and tourmaline. The bulk rock major element composition is rich in SiO2 (69 wt%) and Al2O3 (15.5 wt%) typical for a metapelite, but is significantly enriched in Cr, Ni and V, consistent with an eroded greenstone source.
Rutile grew ~3.4 Ga contemporaneously with biotite and staurolite from the breakdown of muscovite. Dominant cooling ages ~3.14 Ga indicate diffusive resetting of the U-Pb system during the second metamorphic overprint. Zr-in-rutile temperatures are in a range of ~540–560 °C, recording prograde to peak temperatures of the first metamorphic event. In a detrital context, these signatures would provide accurate source rock information. Additionally, trace element classification diagrams based on rutile Zr-, H2O-, and/or Fe-contents would correctly rule out signatures related to low T/P and/or cold subduction.
Contrary, other typically used trace element signatures would give misleading results, if seen in a detrital context. Rutile shows unusually high Cr contents due to high Cr contents in the bulk rock. This results in mafic Cr-Nb signatures, that would lead to a false classification of such rutile grains in a detrital context. The extreme compatibility of Nb and Ta in rutile leads to a pronounced bell-shaped zoning. Partitioning of Ta into rutile and Nb into biotite additionally causes a high variability of Nb/Ta (~4–200). This spread in Nb/Ta is irreconcilable with bulk rock Nb/Ta, which is similar to the Bulk Silicate Earth (BSE) composition. Similarly, rutile Zr/Hf (~15–30) are clearly below the BSE-like bulk rock value due to the preferential incorporation of Zr over Hf into accessory zircon. In a detrital setting it would thus be virtually impossible to infer a realistic protolith composition based on the Zr-Hf-Nb-Ta signature of rutile.
Overall, rutile reliably records age and metamorphic conditions while the partitioning behaviour of Zr, Hf, Nb and Ta in zircon- and mica-bearing rocks, and significant contributions of Cr-rich Archean rocks might significantly limit the use of rutile trace element geochemistry as indicator for the protolith. This must be taken into consideration when using detrital rutile in the Archean to infer tectonic processes.
[1] Cutts K et al. (2014) GSA Bull. 126:251-270
How to cite: Lueder, M., Tamblyn, R., and Hermann, J.: Rutile from the Barberton Greenstone Belt – A petrogenetic indicator for Archean rocks?, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-6471, 2026.
Intrusions of large suites of late-stage Archean granitoids (ca. 3.0-2.5 Ga) are found across most Archean cratons globally, with their appearance marking the onset of potassic granitoid magmatism on Earth and reflecting a key stage in craton stabilization. Geochemical interrogation of these granitoids can inform on the characteristics of their more ancient precursors, including chronological fingerprints of the magmatic source and the nature of any supracrustal inputs. By reconstructing the chemical evolution of the ancient continental crust, we can further assess the timing of new crustal growth from the mantle versus crustal reworking processes.
The Slave craton (Northwest Territories, Canada) hosts a remarkable record of pulsed crustal formation throughout the Archean, including the oldest known zircon-bearing rocks in the Acasta Gneiss Complex (ca. 4.02 Ga). Notably, suites of 2.62-2.58 Ga granitoids occur across the craton which have largely intruded the Mesoarchean basement. Previous whole-rock analyses of these granitoids showed distinct geospatial trends in their Pb and Nd signatures which has led to the recognition of distinct isotopic boundaries. Where the western granitoids exhibit patterns suggesting a derivation from older and more evolved precursors, while the eastern granitoids contain patterns associated with derivation from a far more juvenile source [1]. These granitoids thus offer an unique opportunity to explore the nature of both ancient and juvenile crustal growth and/or reworking in the Archean.
We evaluated zircon Lu-Hf and U-Pb isotopes by laser-ablation split-stream (LASS) ICP-MS from 11 granitoid samples across the Slave craton. Additionally, we report O-isotopes from SIMS analysis of the same zircon crystals. In contrast to the patterns of whole-rock εNd, we found no significant geospatial trend in their zircon εHf or δ18O signatures. The εHf values of these samples are positive (+0.7 to +6.2), and fall just below the modeled 4.4 Gyr depleted mantle evolution. The δ18O values range from those of typical mantle zircon to slightly elevated (+4.5 to +6.9 ‰). The whole-rock elemental data for these granitoids are consistent with the global classification for late-stage Archean biotite- and two-mica granites and hybrid granites [2], however our εHf and δ18O values suggest the granitoids of this study represent juvenile magmatism with limited interaction with evolved supracrustal material. This is consistent with previous interpretations suggesting that these Slave craton granitoids formed in a convergent margin setting [1]. We will place these geochemical interpretations in a global context of other Archean crust-forming environments, exploring the implications regarding our understanding of crustal growth through time and the depleted mantle evolution.
[1] Davis, et al. (1996) Chem. Geo., 130 (3-4), 255-269. [2] Laurent, et al. (2014) Lithos, 205, 208-235.
How to cite: White, E., Davis, W., Pearson, D. G., Stern, R., Luo, Y., and Reimink, J.: Late-stage Archean continental growth: New insights from zircon Hf-isotopes of granitoids from the Slave Craton (Canada) , EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-14740, 2026.
The onset of plate tectonics on Earth remains controversial, with the ~3.8–3.7 Ga Isua supracrustal belt in SW Greenland often cited as evidence for Eoarchean subduction and intraoceanic arc magmatism. This model mainly relies on the reported absence of relatively old xenocrystic zircons in Isua rocks. We present new zircon U-Pb geochronology revealing ~3.97–3.75 Ga ages from supposedly ≤3.72 Ga Isua lithologies. These ages are associated with different zircon morphologies and textures, including inherited cores or individual grains with sector zoning and magmatic oscillatory zoning. Trace element signatures (enriched Pb, Th, U, and LREE) of relevant rocks indicate assimilation of more evolved crust. These findings suggest that Isua supracrustal rocks were emplaced onto an older, evolved, potentially polymetamorphosed crustal foundation, effectively ruling out juvenile intraoceanic arc origins.
How to cite: Zuo, J., Webb, A., Ganbat, A., Müller, T., Haproff, P., and Zhao, G.: Xenocrystic zircons challenge an intraoceanic arc origin of the Eoarchean Isua supracrustal belt, SW Greenland, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-15836, 2026.
The Isua Supracrustal Belt (ISB) encloses circularly an orthogneiss/metaplutonic complex comprising mainly ~3.7 Ga old metatonalite/tonalitic orthogneiss and slightly younger ~3.65 Ga old granite (Baadsgaard et al., 1986; Nutman et al., 1996). The central part of this metaplutonic complex has experienced weaker deformation during its evolution and based on field observations and cross cutting relationships 4 different metatonalite generations were identified. From oldest to youngest these are: (T1) rare occurrences of mesocratic fine grained tonalite boudins in (T2) biotite-rich coarse grained tonalitic augengneiss. (T3) Well foliated felsic tonalitc gneiss with medium sized plagioclase crysts. (T4) Weakly foliated felsic and fine grained metatonalite. The metatonalite commonly encloses centimeter to several meter sized boudins and bands of amphibolite/metagabbro. The slightly younger granites (G) occur as dikes and larger irregular shaped bodies/patches intruding the metatonalite (T2,T3,T4). Towards the contact with the ISB the metaplutonic complex develops a mylonitic to ultramylonitic fabric obscuring the above mentioned relationships. The youngest magmatic event is seen in ultramafic dikes (Ameralik dikes) aligning in 3 different strike directions. Some of them enclose several centimeter sized granitic xenoliths.
Detailed petrographic observations and fluid inclusions (FIs) studies revealed a later postmagmatic overprint in the metatonalites, probably related to one of the metamorphic episodes recorded in the ISB. Magmatic allanite crystals show frequently rims of newly grown epidote while the destabilized allanite forms partly numerous tiny U-Th rich phases.
The investigated metatonalites for our FIs study contain recrystallized equigranular quartz with lobate grain boundaries. FIs have been investigated using Raman spectroscopy combined with fluid inclusion microthermometry. Texturally and chemically, FIs can be distinguished into two major types: (1) polyphase aqueous high-saline FIs (46-49 mass% NaCl) arranged within central domains of quartz grains. (2) One phase FIs arranged as intragranular fluid inclusion planes (FIPs) within quartz grains or as intergranular FIPs along quartz grain boundaries. Type 1 inclusions are interpreted as primary inclusions characterizing an early stage of tonalite crystallization. Their textural occurrence is always restricted to clusters within the core domains of quartz aggregates. Beside halite, solid inclusions like ferropyrosmalite and calcite occur frequently. Type 2 inclusions are arranged along planes that occur inside quartz grains (internal), along grain boundaries (intergranular) or crossing grain boundaries (transphase) and are likely interpreted as resulting from type (1) during recrystallization/metamorphic overprint.
Additional petrographic, geochronologic, mineral chemical and geochemical analyses of the orthogneisse/metaplutonic complex north of the ISB are required to further elucidate their emplacement and metamorphic history for a better comprehension of tectonic processes during the Archean time.
References
Baadsgaard, H., Nutman, A.P., Bridgwater, D. (1986). Geochronology and isotope geochemistry of the early Archaean Amîtsoq gneisses of the Isukasia area, southern West Greenland. Geochimica et Cosmochimica Acta 50, 2173-2183.
Nutman, A. P., McGregor, V. R., Friend, C. R., Bennett, V. C., & Kinny, P. D. (1996). The Itsaq Gneiss Complex of southern West Greenland; the world's most extensive record of early crustal evolution (3900-3600 Ma). Precambrian Research, 78, 1-39.
How to cite: Hauzenberger, C. A., Müller, T., Krenn, K., Piazolo, S., Leung, C. Y. E., Sorger, D., Haproff, P., and Webb, A. G.: Evolution of the Eoarchean orthogneisse/metaplutonic complex north of the Isua Supracrustal Belt, Itsaq Gneiss Complex, SW-Greenland, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-17368, 2026.
The compositional maturation of continental crust is marked by increasing diversity in granitic rocks. However, whether fractional crystallization of felsic magmas contributes to crustal maturation remains contentious, primarily due to the scarcity of well-documented highly fractionated Archean granites. To address this gap, we present an integrated petrological and geochemical study of newly identified Mesoarchean garnet-bearing granites (highly fractionation granite), in conjunction with coeval sodic tonalite-trondhjemite-granodiorite (TTG) suites and potassic granites in the Kongling Complex of the Yangtze Craton.
Zircon U‒Pb dating results show that the studied TTGs, potassic granites, and garnet-bearing granites in the Kongling Complex were formed at 3.0–2.9 Ga. The TTGs have low K2O/Na2O ratios, high Sr/Y and (La/Yb)N ratios with negative zircon εHf(t) values and mantle-like zircon δ18O values, indicating they were originated from partial melting of thickened lower crust. Potassic granites have higher K content and K2O/Na2O ratios with negative zircon εHf(t) values and mantle-like zircon δ18O values, suggesting they were generated from anatexis of ancient felsic crust. Garnets in the garnet-bearing granite are euhedral and most of them are inclusion-free. These garnets are mainly composed of almandine and spessartine with homogeneous major elemental compositions, which are consistent with the characteristics of magmatic garnets. The garnet grains show decreasing trends of HREE and Y content from core to rim, indicating the fractional crystallization of garnet and zircon. The garnet-bearing granitic plutons show a blurred contact interface with the contemporaneous potassic granites and their zircon εHf(t) and δ18O values are similar to those of potassic granites, implying a congenic process between them. The Mesoarchean garnet-bearing granites have moderate whole-rock A/CNK values, high MnO content, MnO/FeOT ratios and 10000×Ga/Al ratios, but lower Zr content with lower zircon saturation temperature. These features of garnet-bearing granites suggest that they were formed from highly evolved K-rich granitic melts. The occurrence of highly fractionated granite in the Mesoarchean may imply that a mature continental nucleus was formed in the Yangtze Craton at that time. Furthermore, global detrital zircon records document a decreasing trend of Zr/Hf ratios during the Mesoarchean, with ultra-low zircon Zr/Hf values (<25) first appearing at the same time. This shift highlights the intra-crustal felsic magma fractionation as a significant mechanism driving crustal maturation since the Mesoarchean, coincident with global geodynamic transitions.
How to cite: Zhang, L. and Zhang, S.-B.: The differentiation of a continental nucleus: Implications from Mesoarchean garnet-bearing granite in the Kongling Complex of the Yangtze Craton, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-2892, 2026.
