The Mesoarchaean Coorg granulite block-Mercara Shear Zone (MRSZ) - Meso- to Neoarchaean Western Dharwar Craton (WDC) crustal section is ideally suited to study the Early Earth tectonics, and in particular, to establish collisional tectonics that led to their amalgamation. However, despite numerous petrological and geochronological studies, the nature of tectonic relationships among the three litho-tectonic domains is not well understood. Importantly enough, there is a dearth of metamorphic studies from the western part of the WDC, and in its absence, the amalgamation tectonics between the WDC and the Coorg block is poorly constrained. The south-western part of the Western Dharwar Craton (WDC), in contact with the Mesoarchaean Coorg Granulite Massif, consists of NNW-SSE trending mafic to ultramafic dyke swarm that is variably metamorphosed. Based on field observations and petrographic study, we have classified the metamafic dykes into two broad types: (a) Undeformed to foliated metagabbro, locally with coarse coronal Grt around Cpx, Pl, Ilm, and Hbl₁ (mineral abbreviations after Kretz, 1983) and differentially preserved igneous textures (Type-1) and (b) well-foliated and banded metamafites that lack magmatic textures and mineralogy, locally migmatitic with porphyroblastic Grt and Cpx and in others, garnetiferous amphibolite with porphyroblastic garnet (Type-2). Based on the degree of foliation development in these metamafites, we observe a south-westward increase in strain. Type-1 metamafites record a sequence of textural evolution, namely recrystallization of the magmatic Cpx±Opx+Pl assemblage→partial high-T hydration, producing Ti-Hbl (Ti=0.24-0.28)→growth of coarse coronal Grt [with a broad homogeneous magnesian core (X_Mg=0.25-0.23, X_Grs=0.20-0.22) and slightly ferroan rim (X_Mg=0.21-0.22, X_Grs=0.21-0.22), particularly in contact with Cpx] on the recrystallized matrix→a late Hbl-defined foliation (Ti=0.19-0.23). Type-2 metamafites show the development of a pervasive titaniferous Hbl-defined foliation (Ti=0.14-0.19), followed by the growth of compositionally homogeneous porphyroblastic Grt (X_Mg=0.21-0.22; Prp_14-15Grs_22-27) with or without Cpx (X_Mg=0.65-0.70, Al_total=0.03-0.08, and Na_total=0.01-0.03), and including localised crustal anataxis, producing tonalitic melt at the metamorphic peak. This was followed by the formation of late low Ti-Hbl (Ti=0.04-0.05). Using conventional thermobarometry, the peak P-T of type-1 metamafite has been estimated at ~ 8kb and 800°C.  In the type-2 metamafites, the peak and retrograde P-T is estimated at ~9kb and 800°C and ~7kb and 500°C respectively.  The effective bulk rock composition has been used to calculate the phase equilibria modelling of the type-2 metamafite, in which the intersections of compositional isopleths of X_Mg(Grt), X_Grs(Grt), X_An(Pl), Al(Cpx), Ti(Hbl) defines a peak P-T of 9.4 kb and 800°C which is similar to that calculated by conventional thermobarometry. The peak and retrograde P-T conditions together record the retrograde segment of a clockwise P-T path of evolution. We relate the textural sequence, results of thermobarometric computations and phase equilibria modelling, and strain patterns in the metamafic dykes to suggest a pervasive thermo-tectonic event that led to the prograde burial of the extended cratonised WDC beneath the Coorg Granulite Block to high-pressure upper amphibolite to granulite facies metamorphic conditions. We link this event with continental collisions between the WDC and Coorg Block at the dawn of the Proterozoic.

How to cite: Nayak, R., Das, B., Nayak, P., Dasgupta, A., Mukhopadhyay, D., Dasgupta, S., and Bhowmik, S. K.: Metamorphic evolution and thermo-tectonic history of Metamafic Dykes: Insights into Continental Collision between Coorg Block and Western Dharwar Craton, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-12964, https://doi.org/10.5194/egusphere-egu25-12964, 2025.

The inner and outer layers of the Earth can be connected by plate tectonics with exchange of material and energy, thus shaping the habitable Earth today. However, the existence of Archean plate tectonics has been controversial. One of the reasons is the lack of rock records that can best represent the presence of the convergent plate boundaries during that time, such as continental lithosphere with ultrahigh-pressure metamorphism (> 2.7 GPa or 80–100 km). Here we investigated the peridotites from the North China Craton, and conducted a systematic investigation involving field survey, mineralogy, petrology, geochronology and geochemistry on these peridotites. Temperature and pressure conditions for protoliths of these peridotites, as well as oxygen fugacity (fO₂), were also calculated, to constrain petrogenesis, tectonic setting, and characteristics of mantle fO₂.

In situ U-Pb dating on zircons from the peridotites yields metamorphic/altered age of 2535–2517 Ma and were intruded by the unmetamorphosed granite dykes at ~2500 Ma. Garnet pseudomorphs and pyroxene with exsolved textures were identified in these peridotites, suggesting that the original garnet and pyroxene were brought from high pressures and the breakdown was induced by decompression. Reintegrating the compositions of the original garnet and pyroxene and compositions of the original garnet and pyroxene indicate that these peridotites were brought up or once seated at mantle depths of 110–130 km. The calculated dT/dP thermal gradients is around 375 ^oC/GPa, close to those of modern collisional orogens.   The occurrence of phlogopite and amphibole in the studied peridotites and the enrichment of light rare earth elements in their bulk-rock and mineral trace elements, as well as the higher contents of magnesium and aluminum in the rim, and chromium and iron in the core of spinels in some samples, which further demonstrates that the studied peridotites experienced mantle metasomatism during plate subduction. Using Olivine-orthopyroxene-spinel oxybarometry, this dissertation obtained the fO₂ of these Archean metasomatized peridotites to range from ΔFMQ +1.0 to ΔFMQ +1.7, which are more oxidized than the Archean ambient mantle, but are similar to the modern sub-arc mantle.

The ultrahigh-pressure peridotites prove that some forms of plate tectonics have been operating at least since the Neoarchean, and also indicate that the continental deep subduction could have existed at least prior to 2.5 billion years ago. During this process, the Neoarchean mantle oxidation was increased, in which subducted crustal materials would have significantly metasomatized the mantle and increased its oxygen fugacity. This process may have contributed to the Archean atmospheric redox evolution and triggered the GOE in the early Proterozoic.

How to cite: Wu, Z.: Neoarchean Peridotites in the North China Craton and Implications for the oneset of Plate Tectonics, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-5404, https://doi.org/10.5194/egusphere-egu25-5404, 2025.

The Archean mantle is an invaluable source of information about the geodynamic processes active during the early Earth. However, our knowledge on Archean mantle compositions is still fragmentary due to its poor exposure on the present-day Earth. The Eoarchean Narssaq ultramafic complex within the Itsaq Gneiss Complex of southern West Greenland is one of the most extensively studied Eoarchean ultramafic sections. According to van de Locht et al. (2020), the ultramafic complex represents a remnant of the Archean mantle. Conversely, Zhang and Zilas (2024) proposed a cumulate origin for the same lithologies. Such contrasting interpretations have significant implications for the inferred geodynamic scenario active during the Eoarchean.

We carried out a new field survey on the Narssaq ultramafic complex collecting representative samples of dunites and hornblende peridotites. All samples include olivine, amphibole and phlogopite, as major rock-forming minerals. Accessory orthopyroxene, chromite and magnetite were in places also found. Mineral phases were characterized for major and trace elements concentration by electron microprobe and laser ablation ICP-MS, respectively. On selected amphiboles, images on trace element distribution were also carried out by LA-ICP-TOF-MS.

Preliminary data reveal that olivine is Fo_90-91 and is characterised by exceptionally low contents of incompatible trace elements and high contents of Ni (>3500 ppm) and fluid mobile elements, such as B and Li (up to 4 and 6 ppm, respectively). Amphibole is mostly tremolitic in composition, although, amphibole grains in equilibrium with olivine and phlogopite locally preserve core ghost domains consisting of Mg-hornblende. These domains are characterized by extremely high Cr contents (>8500 ppm), nearly flat chondrite-normalized REE pattern at about 10 times CI chondrite with a marked negative Eu anomaly, and relatively high contents of both crustal (e.g., Pb, Th, U) and fluid mobile elements (e.g., Li, Be and B). Phlogopite is exceptionally Cr- and Ni-rich and displays low contents in incompatible trace elements, including those for which the affinity is higher, such as Cs, Rb and Ba; Li and B are up to 6 ppm.

The typically null concentrations of fluid immobile incompatible trace elements in olivine and the constantly high and similar contents of fluid mobile elements in all mineral phases indicates that the entire ultramafic system underwent re-equilibration and re-crystallisation during the metamorphic evolution of the Narssaq ultramafic complex. The mantle or cumulate origin of the complex is thus difficult to be assessed. Notwithstanding, we propose that amphibole crystallisation was related to a melt with a significant crustal component that interacted with the pre-existing ultramafic system. Further trace element and stable isotope micro-analyses are in progress, to verify if amphibole crystallisation was related to the emplacement or partial melting of the host Itsaq gneisses.

van De Locht J., Hoffmann J.E., Rosing M.T., Sprung P., Munker C. Preservation of Eoarchean mantle processes in 3.8 Ga peridotite enclaves in the Itsaq Gneiss Complex, southern West Greenland. Geochimica et Cosmochimica Acta 280 (2020) 1–25.

Zhang L. & Zilas K. Eoarchean ultramafic rocks represent crustal cumulates: A case study of the Narssaq ultramafic body, southern West Greenland. Earth Planet. Sci. Lett. 625 (2024) 118508

How to cite: Tiepolo, M., Previti, V., Cannaò, E., Filippi, M., Tribuzio, R., Mariani, D., and Sessa, G.: New insights into the origin of the Eoarchean Narssaq ultramafic complex (southern West Greenland) from trace element mineral chemistry, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-6099, https://doi.org/10.5194/egusphere-egu25-6099, 2025.

One of the major and most impactful changes the Earth has experienced is the formation, evolution and volumetric accumulation of continental crust through geologic time. The transition of continental crust from being mostly submerged to subaerial greatly affected the evolution of complex life by inducing the enrichment of oceans with nutrients via erosion. The transition from stagnant/squishy-lid dynamics to modern-style plate tectonics was probably facilitated by the growth and accumulation of thick continental crust. Many ideas of crustal growth curves were previously suggested, each employing different types of constraints. Many of these curves point to the Archean–Proterozoic transition as an important point of infliction in the evolution and accumulation of continental crust. We adopted a new approach aiming to indirectly trace the growth in subaerial exposure of continental crust by observing the change in elemental and isotopic composition of boron in the ocean, preserved in Archean–Proterozoic marine deposits. Boron is a continental element that is concentrated in the continental crust over time. The oceanic boron isotopic composition is controlled by the balance between the different sources and sinks of boron in and out of the ocean, amongst which the largest source is continental runoff. The onset of widespread continental emergence initiated the largest boron influx into the ocean, thus greatly affecting the oceanic B concentration and B isotopic budget. Box modelling of oceanic B concentration over time employing different crustal growth scenarios also shows that the mode of crustal growth greatly affects the time dependant change in oceanic B concentration. We analyzed samples of Archean and Proterozoic cherts, iron formations and marine shales for B isotopes using in-situ LA-MC-ICP-MS in order to construct the oceanic B isotopic record across this critical period. We observe a significant increase both in B concentration and in the range and variation of B isotopic values across the Archean–Paleoproterozoic transition that may suggest a transition of oceanic boron towards modern values (modern ocean δ¹¹B = +39.6 ‰) at that time, suggesting a large increase in the area of exposed continental crust.

How to cite: Abbo, A., Marschall, H., and Gerdes, A.: Boron isotopes in Archean-Proterozoic marine deposits as a tracer of continental evolution and emergence, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-11470, https://doi.org/10.5194/egusphere-egu25-11470, 2025.

The Finero Complex in the northern Ivrea-Verbano Zone represents one of the most enigmatic cross-sections through the mantle and lower continental crust. The mantle peridotite unit (Phlogopite Peridotite, PP) consists of pervasively metasomatized spinel-harzburgite enriched with abundant phlogopite and amphibole, locally exhibiting Cr-diopside veins (Zanetti et al., 1999). This unit is in contact with a mafic complex of lower crustal origin and Permian age (Lu et al., 1997), composed of three distinct units (Siena and Coltorti, 1989): i) the Layered Internal Zone (LIZ), which includes lherzolites, garnet-hornblendites, pyroxenites, anorthosites, and garnet-gabbros; ii) the Amphibole Peridotite (AP); iii) the External Gabbro (EG), which borders the Kinzigite Formation. The genetic relationships among these units remain a subject of debate.

We carried out in-situ Sr-isotope analyses of clinopyroxene, amphibole, and plagioclase across all lithologies of the Finero Complex to identify isotopic disequilibria that might indicate multiple, genetically distinct igneous events. In-situ Sr-isotope measurements were performed using LA-MC-ICP-MS at the Geochemistry Laboratory (GL@M) of the University of Milan.

In the PP samples, clinopyroxene and amphibole are in isotopic equilibrium but ⁸⁷Sr/⁸⁶Sr is highly variable, ranging from around 0.7040 to 0.7080. In the LIZ lithologies, amphibole shows a narrow variation in ⁸⁷Sr/⁸⁶Sr (0.7020 - 0.7030) and it is in isotopic equilibrium with clinopyroxene. An exception is observed in the anorthosite, where clinopyroxene exhibits a significantly more radiogenic signature (⁸⁷Sr/⁸⁶Sr > 0.7040). Plagioclase in all LIZ lithologies is more radiogenic than amphibole, with ⁸⁷Sr/⁸⁶Sr values ranging from around 0.7030 to 0.7040. In the AP, clinopyroxene and amphibole are isotopically equilibrated, with values close to 0.7035 ± 0.0001 (2SD). In the EG, plagioclase displays a significantly less radiogenic ⁸⁷Sr/⁸⁶Sr values (around 0.7030) compared to amphibole and clinopyroxene (>0.7070), showing isotopic disequilibrium.

Amphibole and clinopyroxene from the PP show variable crustal signatures supporting differences in the metasomatic agents (Zanetti et al. 1999). In both the AP and LIZ, plagioclase consistently shows more radiogenic values than amphibole, which is in isotopic equilibrium with clinopyroxene, except in the anorthosites. The highly unradiogenic ⁸⁷Sr/⁸⁶Sr signature of plagioclase in EG, close to that found in LIZ, reveals that plagioclases are remnants of a former reacted lithology. These observed isotopic disequilibria suggest that present-day Finero Complex is the result of infiltration and reaction of melts with different ⁸⁷Sr/⁸⁶Sr signatures into a pre-existing igneous complex. The less reacted lithologies, preserving similar ⁸⁷Sr/⁸⁶Sr signatures are lherzolites, pyroxenites and wehrlites. This peculiar rock association, the layered structure and the extremely depleted initial ⁸⁷Sr/⁸⁶Sr signature (down to 0.7020) closely resemble that of some Proterozoic, or even older, layered mafic-ultramafic complexes.

Lu, M. Hoffman H.W., Mazzucchelli M., Rivalenti G. 1997. The mafic-ultramafic complex near Finero (Ivrea-Verbano Zone). Chem. Geo. 140, 207-222. 

Siena, F., and M. Coltorti. 1989. The petrogenesis of a hydrated mafic-ultramafic complex and the role of amphibole fractionation at Finero (Italian Western Alps)." Neues Jahrbuch für Mineralogie Monatshefte 6, 255-274.

Zanetti, A., Mazzucchelli, M., Rivalenti, G., & Vannucci, R. 1999. The Finero phlogopite-peridotite massif: an example of subduction-related metasomatism. Contributions to Mineralogy and Petrology, 134(2), 107-122.

How to cite: Previti, V., Tiepolo, M., Langone, A., Cannaò, E., Sessa, G., and D'Antonio, M.: Is the Finero Complex (Ivrea Verbano Zone) originally Proterozoic or even older? Inferences from in-situ Sr isotope disequilibria, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-709, https://doi.org/10.5194/egusphere-egu25-709, 2025.

Short-lived system chronometers provide evidence for the rapid and early differentiation of planet Earth. In particular, the hafnium¹⁸²Hf – tungsten ¹⁸²W system (t_1/2 = 8.9 Ma) has been used to constrain the age of core formation and silicate differentiation taking advantage of the geochemical differences between these two elements during geological processes. For example, during core formation, W, which is a siderophile element, was attracted into the Earth’s core, while Hf, being a lithophile element, was concentrated into the silicate portion of the Earth. This contrasting difference in the behavior of these elements resulted in distinct ¹⁸²Hf/¹⁸²W isotopic ratios in both reservoirs, which are now reflected in the ¹⁸²W/¹⁸⁴W isotopic compositions of the samples derived from these reservoirs. The purpose of this work is to try to develop a new method for obtaining W isotope data for silicate rocks. The current bulk-rock analysis requires the use and preparation of large amounts of rock powder due to the very low W concentrations in most samples. Here, we propose a new methodology that may generate less costs and optimize the current method by replacing bulk-rock isotopic analysis with that of mineralogical fraction. In this study, we determined W concentrations (and other trace elements) of individual mineral phases from different types of rock by LA-ICP-MS. Our preliminary results reveal that titanium (Ti) - rich minerals, such as rutile and ilmenite, have systematically more elevated W concentrations than the other minerals. Rutile, in particular, captures most of the W in the rock. Replacing bulk rock isotopic analyses with analyses of Ti-rich minerals would require significantly less material to process. It is expected to obtain and compare results of W isotopic compositions of both the bulk rock and the Ti-rich mineral fraction.

How to cite: Vera-Cedeño, D. and Mougel, B.: Geochemical and isotope composition of tungsten (u182W): comparative analysis between bulk rock and mineral phases in silicated rocks, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-14730, https://doi.org/10.5194/egusphere-egu25-14730, 2025.

Understanding lithospheric thickness during the Early Earth is crucial for unraveling tectonic modes (e.g., heat-pipe vs. stagnant-lid) and cooling mechanisms. The heat-pipe model likely produced a thick lithosphere early on, which thinned with declining volcanism before thickening again as stagnant-lid conduction became dominant. Conversely, the stagnant-lid mode involved a direct transition from a magma ocean to conduction, characterized by thin, weak lithospheres that gradually strengthened and thickened over time. Tracking the evolution of lithospheric thermal thickness provides a means to test these cooling mechanisms.

Basaltic rocks, the most abundant igneous rocks, offer critical insights into mantle conditions. Experimental evidence indicates that the oxides in primary basaltic melts are sensitive to melting pressure, making them effective proxies for lithospheric thickness. Using global geochemical datasets, such as GEOROC, we can infer lithospheric thickness from basaltic lithogeochemistry. This study evaluates lithospheric thickness during the Early Earth (4.0–3.0 Ga) using compiled basaltic lithogeochemical data. Despite their rarity, heat flow data, xenolith samples, and clinopyroxene thermobarometry were also used to validate findings. Basaltic lithogeochemistry indicates significant thinning from 120 km at ~3.65 Ga to 90 km by ~3.30 Ga, followed by subsequent thickening and eventual stabilization. Heat flow data, though craton-specific and with high age uncertainty, generally support a thinning trend from ~3.75 Ga to ~3.40 Ga, stabilizing and slightly thickening by ~3.20 Ga, with minimal fluctuations until ~3 Ga. Xenolith and clinopyroxene data, available only from ~3.60 Ga onward, indicate a stable lithospheric thickness between ~3.60 Ga and ~3.40 Ga, followed by thickening from ~3.40 Ga to ~3.20 Ga. These observations suggest an evolution from thick, cold lithospheres that initially thinned, likely transitioning to conductive cooling and thickening over time. This supports the probable viability of the heat-pipe model during the Early Earth and provides insights into the planet’s tectonic regimes and lithospheric evolution.

How to cite: Ganbat, A., Webb, A. A. G., Müller, T., Zuo, J., and Leung, E. C. Y.: Evolution of lithospheric thickness in Early Earth: Insights into tectonic regimes, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-19709, https://doi.org/10.5194/egusphere-egu25-19709, 2025.

The Archean mantle redox state played an important role in degassing of the Earth's interior and thus influenced atmospheric oxygen levels of the early Earth. But it is unclear if any parts of the uppermost mantle were significantly oxidized by a certain point in the Archean. Here, we investigate oxygen fugacity (fO₂) of Archean (> 2535–2517 Ma) peridotites in the North China Craton. Petrology and geochemistry reveal that they experienced strong Neoarchean subduction-related metasomatism. These Neoarchean subduction-metasomatized peridotites record fO₂ of ΔFMQ +1.3 ± 0.4 (SD) [relative to the fayalite-magnetite-quartz (FMQ) buffer], which are more oxidized than the Archean ambient mantle, but similar to the modern sub-arc mantle. We propose that this Neoarchean rise of mantle oxidation in the North China Craton was induced by plate subduction, during which the Neoarchean sub-arc mantle in the North China Craton could have been metasomatized and oxidized, and its oxygen fugacity was increased. This process may have had connections with the Great Oxidation Event in the Early Proterozoic.

How to cite: Wang, C., Wu, Z., Allen, M. B., Tang, M., Chen, Y., Jia, L., and Song, S.: Rise of mantle oxidation by Neoarchean subduction in the North China Craton, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-20570, https://doi.org/10.5194/egusphere-egu25-20570, 2025.

Ophiolitic mélange is a key geological component in ancient convergent plate margins. Understanding the block and matrix types, their igneous, sedimentary, and metamorphic P-T histories, and the structural and tectonic processes from formation to emplacement, is essential for deciphering paleo-subduction zone dynamics. Subduction channels, where some ophiolitic mélanges develop, are mechanically weak shear zones between the upper and lower plates, containing stacked oceanic sequences offscraped from the subducted plate, mixed with eroded crust and mantle materials from the forearc in arc-continent accretionary/collision zones. In this study, we present the results of detailed field mapping, structural analysis, geochemical, and geochronological investigations of the Neoarchean Zunhua and Shangying ophiolitic mélanges from the Eastern Hebei Complex in the Central Orogenic Belt, North China Craton. The Zunhua ophiolitic mélange consists of forearc-affinity ultramafic-mafic blocks (peridotite, podiform-chromite-bearing dunite, pyroxenite, metagabbro, metadiabase, metabasalt), with podiform chromitites containing inclusions of UHP TiO2(II) and remnants of UHP chenminigite, suggesting derivation from depths of 270–410 km prior to mélange formation. In contrast, the Shangying ophiolitic mélange to the east contains layered and isotropic N-MORB affinity metagabbro and garnet clinopyroxenite, metamorphosed under HP eclogite-facies conditions at 65–70 km. Both mélanges are characterized by a strongly sheared metasedimentary matrix. Zircon U-Pb dating of blocks and crosscutting dikes indicates that the ultramafic-mafic blocks in the Zunhua ophiolitic mélange formed at 2.55–2.52 Ga and were incorporated into the mélange between 2.52 and 2.50 Ga. The Shangying ophiolitic mélange formed between 2.53 and 2.52 Ga and was emplaced in the Eastern Block between 2.52 and 2.47 Ga. The Zunhua mélange preserves a nearly complete ophiolite sequence and records a Neoarchean subduction initiation and arc-continent collision event in a forearc supra-subduction zone (SSZ) setting. The Shangying mélange, composed of meta-basalts, gabbros, and garnet clinopyroxenites, formed at a mid-ocean ridge (MOR). The Zunhua SSZ ophiolitic mélange was emplaced over the forearc by accretionary thrusts, whereas the Shangying MOR ophiolitic mélange formed through subduction, mixing, and exhumation within a subduction channel. The juxtaposition of these two mélange belts with different tectonic affinities and emplacement styles during the Neoarchean arc-continent collision reveals a west-northwest-dipping subduction polarity, consistent with kinematic fabrics in the mélanges and their correlatives along the ~1,800 km paleo-subduction zone of the Central Orogenic Belt. The co-existence of SSZ and MOR ophiolitic mélanges in the Eastern Hebei Complex suggests that large-scale subduction/accretionary zones were active in the late Neoarchean. Geochronological data show that the processes of seafloor spreading, subduction initiation, forearc thrusting, and exhumation of subduction materials occurred over <80 million years, similar in duration to many Phanerozoic subduction-collision zones. These findings suggest that tectonic processes in the late Neoarchean were comparable to those of modern Earth.

How to cite: Ning, W., Kusky, T., and Wang, L.: The coexistence of Neoarchean SSZ and MOR ophiolitic mélanges in the North China Craton: dynamics of an Archean paleo-subduction zone, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-5303, https://doi.org/10.5194/egusphere-egu25-5303, 2025.

This research explores the geochemistry of Paleoproterozoic metasedimentary and metavolcanic units in the Alutaguse region of North Estonia and the South Svecofennian (SS) zones, including Ladoga, Saimaa, Häme Belt, and Uusimaa Belt, to better understand the tectonic evolution of the Svecofennian Orogeny in Eastern Fennoscandia. Metasedimentary units consist of micaceous gneisses (± Grt ± Crd ± Sil), while metavolcanics include amphibolites and pyroxenic gneisses. Historical and new data show that High-SiO₂ (>63 wt%) metasediments have felsic origins similar to the Upper Continental Crust (UCC), whereas Low-SiO₂ (≤63 wt%) metasediments, resembling graywackes and shales, indicate mafic to intermediate origins similar to post-Archean Australian Shale (PAAS). Various weathering indices, including CIA, PIA, CIW, and ICV for metasediments, and AI, CCPI, WIP, and SI for metavolcanics, were applied to reveal these geochemical trends. The metavolcanics are classified as sub-alkaline, with geochemical signatures pointing to asthenospheric mantle origins for Alutaguse and subducted oceanic crust origins for SS. Tectonic affinity analyses indicate a predominant oceanic arc setting across both regions. High CaO and MnO concentrations in Alutaguse and Uusimaa metasediments suggest a genetic link, positioning Alutaguse as a 1.90–1.89 Ga back-arc to the Uusimaa belt, followed by the accretion of Uusimaa and Häme belts around 1.87 Ga, marking the closure of the Svecofennian ocean. The Alutaguse zone likely developed as a back-arc to the Tallinn-Uusimaa belt after the accretion of the Bergslagen microcontinent. This interpretation is supported by geophysical anomalies correlated with Zn-Pb-Fe mineralisation. The assemblages found in Alutaguse province comprises high proportions of highly deformed sulphides (pyrite, pyrrhotite, arsenopyrite) and sphalerite disseminated in graphitic amphibolitic-gneisses, which shows similarities with Bergslagen's VMS (SEDEX?) provinces and warrants further investigation.

Figure 1. Crustal structure in the central and southern parts of the Svecofennian orogen as integrated across the Baltic Sea, after Bogdanova et al. (2015) and Geochemical relations from the Alutaguse and SS metasedimentary units include major elemental tectonic discriminant functions.

How to cite: Solano Acosta, J. D., Soesoo, A., Hints, R., and Graul, S.: Is the Estonian Alutaguse Section of Eastern Fennoscandia a continuation of the Southern Svecofennian Finnish Terranes, or is it akin to the Swedish Bergslagen region?, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-7212, https://doi.org/10.5194/egusphere-egu25-7212, 2025.