Continental collisional effects on magma source redox state and water content: Insights from thermodynamic modeling

Radiogenic isotopes serve as a crucial tool for investigating crustal evolution, playing a pivotal role in revealing magma sources and petrogenesis. However, they can be ineffective in distinguishing between distinct magmatic sources with similar radiogenic isotopic compositions, a common occurrence in nature. Here we addresse this challenge in the Ordovician igneous rocks from the West Kunlun orogenic belt (WKOB), aiming to distinguish between two potential magmatic sources (i.e. the Tarim Craton and the Tianshuihai terrane) with similar isotopic compositions using appropriate thermodynamic and geochemical modeling based on mineral and whole-rock geochemistry. Zircon U–Pb dating yields ages of 483 ± 3 Ma for the Pushou gabbros and 469 ± 2 Ma and 461 ± 2 Ma for the Datong monzogranites and syenites, respectively. The Pushou gabbros exhibit low SiO2 (47.4–49.1 wt %), high MgO (5.5–6.9 wt %), high large-ion lithophile elements (LILEs, e.g. Rb, Ba, Th, and K), and low high field-strength elements (HFSEs, e.g. Nb, Ta, Zr, Hf, P, and Ti), suggesting an origin in subduction-modified mantle. They display high whole-rock (87Sr/86Sr)i ratios (0.7156 to 0.7192), negative whole-rock εNd(t) values (−7.1 to −7.8), as well as high zircon δ18O values (7.6–7.9‰) and enriched zircon Hf isotopic compositions (εHf(t) = −5.3 to −7.7), which are consistent with 1–5% subducted sediments in an enriched mantle source. Trace element models further confirm that the gabbros are most likely derived from low-degree (~15%) partial melting of subduction-modified Tarim mantle in the spinel–garnet facies rather than from the Tianshuihai mantle. The Datong syenites belong to the shoshonitic series and are characterized by medium SiO2 (59.5–61.4 wt %), relatively low MgO (0.9–1.2 wt %) and Mg# (37–42), enrichment in LILEs and depletion in HFSEs. They have high whole-rock (87Sr/86Sr)i ratios (0.7103 to 0.7105) and negative whole-rock εNd(t) values (−3.8 to −4.3), along with negative to slightly positive zircon εHf(t) values (−3.8 to +2.6), similar to coeval mafic rocks. Thermodynamic and geochemical modeling suggest that the Datong shoshonitic rocks likely originated via crystal fractionation of shoshonitic basaltic magmas in the SW Tarim Craton. The Datong monzogranites have high SiO2 (69.7–72.6 wt %), low MgO (0.6–0.7 wt %), and a typical enrichment in alkalis, Zr, and Nb, with depletion in Sr, P, and Ti, consistent with A-type granites. They are characterized by high whole-rock (87Sr/86Sr)i ratios (0.7321 to 0.7323), negative whole-rock εNd(t) values (−11.3 to −11.8), negative zircon εHf(t) values (−11.0 to −16.5), and high zircon δ18O values (7.2–8.0‰), indicating derivation from the remelting of an ancient crustal source. Thermodynamic, major, and trace element modeling indicate that their parent magma may have been generated by water-deficient (~2 wt %) partial melting of ancient crustal material beneath the SW Tarim Craton rather than that of the Tianshuihai terrane, under high-temperature (T > ~950°C) and low-pressure (P = 5–8 kbar) conditions. Based on the tectonic framework of the WKOB, we propose that the original mantle and crust beneath the southern Kunlun terrane may have been modified or partially replaced by that beneath the SW Tarim Craton during the Ordovician. Therefore, this evidence for Tarim-derived magmatism, when combined with regional sedimentary and structural records, indicates that Ordovician magmatism in the southern Kunlun terrane is most consistent with episodic northward subduction of the Proto-Tethys Ocean, commencing at ~485 Ma. Middle Ordovician slab break-off can explain the formation of the A-type granites, but reinstated northward subduction is required for the generation of late Ordovician Datong syenites.

The Early Paleozoic Subashi ophiolite in the West Kunlun Orogenic Belt (northwestern Tibetan Plateau): Implication for the tectonic evolution of the Proto-Tethys

The West Kunlun Orogenic Belt (WKOB) in the northwestern Tibetan Plateau records key information about the tectonic evolution of the Proto-Tethys Ocean. To further constrain the Early Palaeozoic tectonic evolution of the WKOB, we report petrography, geochronology, whole-rock geochemistry, and Sr-Nd-Hf-O isotope data of the newly identified Ordovician Nanpingxueshan gabbros in the southern WKOB. Laser ablation-inductively coupled plasma-mass spectroscopy (LA-MC-ICP-MS) U-Pb zircon dating reveals that the gabbroic intrusion was emplaced during 476–471 Ma. Geochemically, the gabbroic rocks exhibit a tholeiitic signature and can be subdivided into two groups, i.e. normal arc-like gabbros (AGs) and Nb-enriched gabbros (NEGs). The AGs have TiO2 (1.20–1.64%), P2O5 (0.18–0.25%), and Nb (4.60–7.64 ppm) contents, fractionated REE patterns ((La/Yb)N = 2.64–4.11), and Nb-Ta depletion (Nb/La = 0.37–0.50). Compared with AGs, the NEGs have higher TiO2 (1.74–2.68%), P2O5 (0.34–0.55%), and Nb (10.79–14.08 ppm) contents and Nb/La ratios of 0.45–0.53, which are similar to typical Nb-enriched arc basalts. Isotopically, the AGs have depleted whole-rock εNd(t) values of 3.22 to 4.04 and negative to positive zircon εHf(t) values of −1.8 to 7.8, while the NEGs exhibit slightly lower whole-rock εNd(t) values of 0.69 to 3.18, zircon εHf(t) values of −7.8 to 4.6, and mostly δ18O values greater than those of normal mantle zircon. Integrated analyses reveal that the AGs and NEGs can be attributed to a similar mantle source metasomatized by different proportions of subducted sediment-derived melt during the subduction process. Our new data, combined with the results of previous studies, suggest that the southward subduction of the Proto-Tethys Ocean under the WKOB initiated at ca. 530 Ma. The long-term southward subduction led to the formation of the massive Early Palaeozoic accretionary complex in the South Kunlun Terrane, while trench retreat led to migration of the arc magmatism northward from the Mazar – Tianshuihai Terrane to the South Kunlun Terrane, as well as back-arc extension in the Mazar – Tianshuihai Terrane.

The origin of trachyte and pantellerite from Pantelleria, Italy: Insights from major element, trace element, and thermodynamic modelling

Early Mesozoic granitoids and microgranular enclaves (MEs) are widespread in the West Kunkun, northwestern Tibetan plateau, and record the tectonic evolution of Eurasia–Tethys in this area. This study reports geochemistry, zircon U–Pb and Hf isotopic data for a suite of granitoids and their MEs from the Middle Triassic Bulunkou pluton (BP) and the Late Triassic Akazishan pluton (AP) from the West Kunlun. LA-ICP-MS U–Pb zircon dating of a sample from the BP host monzogranite and an enclave, as well as a AP monzogranite, yielded ages of 236 ± 2, 230 ± 7 and 208 ± 1 Ma, respectively. The BP monzogranite and its enclaves from the northwestern part of the West Kunlun are mainly weakly peraluminous granites characterised by relatively high Rb, Th, Rb/Sr (2.64–9.03) and HREE contents, and low Mg#, Sr/Y and negative Eu anomalies. Zircons from the BP monzogranite have eHf(t) values from −5.7 to −1.6. Zircons from enclaves of the BP show more variable eHf(t) values from −4.1 to 3.8. We consider that the BP granites are likely to have been formed by partial melting of metasedimentary rocks at shallow crustal depth, and their enclaves, composed of quartz + biotite + plagioclase + garnet + K-feldspar, are relics from the melting of a source at middle crustal depths. The AP host and its enclaves from the southeastern part of the West Kunlun have low Rb, Rb/Sr (0.15–1.90) and weakly negative Eu anomalies but high HREE contents indicating limited fractionation of plagioclase without residual garnet in their source. The inferred protolith is an intermediate igneous rock in the middle or lower crust. MEs hosted in the AP have high Mg# (39.7–45.0) and Nb and weakly negative Eu anomalies, as well as high Sr, P and Ti, corresponding to a medium-K basaltic rock, which may have originated from mixing of partial melting of metasomatised mantle wedge that has been modified by upwelling asthenospheric mantle and crustal melting in the deep source. Post-collisional southeasternward younging of Mesozoic granitoids in the West Kunlun records a transition from shallow level, mid-crustal melting to deeper level, lower-crustal melting. The post-collisional granitoids show significant mantle input increasing from northwest to southeast, which was closely associated with mafic magma-driven partial melting of mantle and crustal sources as a consequence of slab break-off. Syn- to post-collisional Early Mesozoic granites emplaced along the Mazar–Kangxiwar suture zone suggest that these tectonomagmatic processes followed the closure of the Paleo-Tethys ocean during the Tarim and Karakorum–Qiangtang continental collision.

Melting of the Indarch meteorite (EH4 chondrite) at 1 GPa and variable oxygen fugacity: Implications for early planetary differentiation processes

Under the reducing conditions of the inner region of stellar nebulae (Cartier and Wood, 2019), thermodynamic calculations have shown that rocky planetary mantles could be dominated by pyroxenes instead of olivine (Cioria et al., 2024). This mineralogical composition would have geodynamic implications such as low mantle viscosity and reduced solidus temperature compared to peridotitic mantles, as on Earth and Venus. In this preliminary work, we perform high pressure- temperature experiments in a Paris-Edinburgh press to investigate the subsolidus and melting phase relations of CB (Bencubbin like) bulk silicate composition at conditions corresponding to the crust- mantle interface and core-mantle boundary in Mercury. The objective is also to experimentally constrain and anchor the previous thermodynamic calculations of Cioria et al. (2024), focusing particularly on the mineral constituents of the crusts and mantles of reduced bodies.Synthetic samples (CB1, CB2) have been prepared using the average bulk silicate fraction of CB chondrites as proposed by Malavergne et al. (2010) and Brown and Elkins-Tanton (2008). Starting powders were prepared from high purity oxide mixtures, according to Cioria et al., 2024 Model 2, with 67.9 wt% SiO2, 0.2 wt% TiO2, 3.1 wt% Al2O3, 1.1 wt% Cr2O3, 0.3 wt% FeO, 0.1 wt% MnO, 25 wt% MgO, 2.6 wt% CaO, 0.1 wt% Na2O and 0.01 wt% K2O. Paris-Edinburgh press experiments were conducted in standard 10/3.5mm (sample CB1) and 7/2.4 mm (sample CB2) cell assemblies, using boron epoxy transmitting medium and graphite furnace heaters. Powders were loaded into a double capsule with a graphite inner wall and boron nitride outer wall. Samples were subjected to 2 GPa–1300K (CB1) and 5 GPa–1700K (CB2), for durations ranging from 3hours to 24 hours, respectively; in order to achieve equilibrium conditions. These P-T ranges are the same used in Cioria et al. (2024) as boundary conditions in thermodynamic modelling. Recovered charges were mounted, polished, and optical observations shown that run-products have coarse-grained texture, where large, euhedral crystals are embedded in a fine-grained matrix. The detailed analysis is under progress and scanning electron microscopy (SEM) are planned to characterize textures, phase assemblages, major element distributions, and constrain oxygen fugacity conditions - fO2 - using relevant phase equilibria buffers. Results will be compared with thermodynamic models (Cioria et al., 2024) and experimental studies by Berthet et al., (2009) and Boujibaar et al. (2025), to understand any differences between products derived from CB and EH compositions, respectively. Further works will involve synthesis under larger P-T conditions as well as investigating sulfur-bearing compositions under various oxygen fugacity conditions in order to assess the building materials and differentiation processes of Mercury-like planets and small bodies in the early solar system.AcknowledgmentsG.M. and C.C. acknowledge support from the Italian Space Agency (2022-16-HH.1-2024). References Berthet, S., Malavergne, V., & Righter, K. (2009). Melting of the Indarch meteorite (EH4 chondrite) at 1 GPa and variable oxygen fugacity: Implications for early planetary differentiation processes. GCA, 73(20), 6402-6420.Boujibar, A., Righter, K., Fontaine, E., Collinet, M., Lambart, S., Nittler, L. R., & Pando, K. M. (2025). A Pyroxenite mantle on Mercury? Experimental insights from enstatite chondrite melting at pressures up to 5 GPa. Icarus, 116602.Brown, S., & Elkins‐Tanton, L. T. (2009). Compositions of Mercury's earliest crust from magma ocean models. EPSL , 286(3-4), 446-455.Cartier, C., & Wood, B. J. (2019). The role of reducing conditions in building Mercury. Elements,15(1), 39-45.Cioria, C., Mitri, G., Connolly, J. A. D., Perrillat, J.-P., & Saracino, F. (2024). Mantle mineralogy of reduced sub‐Earths exoplanets and exo‐Mercuries. JGR:Planets, 129.Connolly, J. A. D. (1990). Multivariable phase diagrams; an algorithm based on generalized thermodynamics. Am. J. Sci., 290(6), 666- 718.Malavergne, V., Toplis, M. J., Berthet, S., & Jones, J. (2010). Highly reducing conditions during core formation on Mercury: Implications for internal structure and the origin of a magnetic field . Icarus, 206(1), 199–209.

Late early Cretaceous peraluminous biotite granites along the Bangong–Nujiang suture zone, Central Tibet: Products derived by partial melting of metasedimentary rocks?

Revisiting the origin of the Carboniferous Oytag pluton in West Kunlun orogenic belt, northwest China

Granitoids play a critical role in understanding the maturation and reworking of continental crust within collisional orogenic systems. However, the diverse origins of magma sources, combined with the intricate dynamics of tectonic processes, present significant challenges for deciphering the genesis and evolutionary history of granitic magmas, particularly in geologically complex regions such as the Gangdese belt, southern Tibet. Here, we investigate the granitic plutons of the Eocene (dated at ca. 53 Ma) and Oligocene (dated at ca. 29 Ma) epochs in the eastern Gangdese belt, southern Tibet. Utilizing a multipronged approach combining whole-rock and mineral geochemistry, Sr-Nd-Hf-O isotopes, geochronology, and thermodynamic modeling, we aim to evaluate their sources, evolution, and the related tectonic processes. The Eocene granodiorite exhibits flat rare earth element (REE) patterns with weak heavy-REE depletion, enrichment in large ion lithophile elements (LILEs), and depletion in high field strength elements (HFSEs). Its isotopic signatures εNd(t) = +1.3 to +2.1; zircon εHf(t) = +0.7 to +7.4; δ18O = 5.2‰−7.0‰; and Sr-Nd mixing models suggest that the Eocene granodiorite primarily originated from the juvenile lower crust with the assimilation of the Nyingchi ancient basement (∼10%). Thermodynamic modeling further indicates that the magma formed through disequilibrium partial melting of mafic lower crustal rocks, leaving a pyroxene-rich residue. In contrast, the Oligocene granites display distinct adakitic geochemical features, including elevated whole-rock (87Sr/86Sr)i isotopic ratios (0.7060−0.7061), relatively enriched εNd(t) values (−3.0 to +0.8), and broad δ18O values (4.5‰ to 8.3‰). The significant variation in zircon εHf(t) values (+0.1 to +8.0) suggests a mixed source. Multiple lines of evidence suggest that the Oligocene granites were formed through fluid-fluxed melting of a thickened crust, with the involvement of ∼25% Indian continental materials. The trace element characteristics, combined with thermodynamic modeling, indicate that the Oligocene granites underwent limited fractional crystallization (∼15%), with amphibole, rutile, and garnet being the dominant phases segregated during magma ascent. Considering the regional geology and rock assemblages, we propose a detailed geodynamic framework for the formation of the Eocene and Oligocene granites in the Gangdese belt. This study provides new insights into the distinct magma sources and evolutionary pathways of Eocene and Oligocene granites, while also establishing a quantitative framework to constrain magma genesis in syn-collisional and postcollisional settings. These findings have broad implications for advancing our understanding of granite petrogenesis in continental collisional orogens on a global scale.

Strongly peraluminous granites (SPGs) form through the partial melting of metasedimentary rocks and therefore represent archives of the influence of assimilation of sedimentary rocks on the petrology and chemistry of igneous rocks. With the aim of understanding how variations in sedimentary rock characteristics across the Archean–Proterozoic transition might have influenced the igneous rock record, we compiled and compared whole-rock chemistry, mineral chemistry, and isotope data from Archean and Paleo- to Mesoproterozoic SPGs. This time period was chosen as the Archean–Proterozoic transition broadly coincides with the stabilization of continents, the rise of subaerial weathering, and the Great Oxidation Event (GOE), all of which left an imprint on the sedimentary rock record. Our compilation of SPGs is founded on a detailed literature review of the regional geology, geochronology, and inferred origins of the SPGs, which suggest derivation from metasedimentary source material. Although Archean and Proterozoic SPGs are similar in terms of mineralogy or major-element composition owing to their compositions as near-minimum melts in the peraluminous haplogranite system, we discuss several features of their mineral and whole-rock chemistry. First, we review a previous analysis of Archean and Proterozoic SPGs biotite and whole-rock compositions indicating that Archean SPGs, on average, are more reduced than Proterozoic SPGs. This observation suggests that Proterozoic SPGs were derived from metasedimentary sources that on average had more oxidized bulk redox states relative to their Archean counterparts, which could reflect an increase in atmospheric O2 levels and more oxidized sedimentary source rocks after the GOE. Second, based on an analysis of Al2O3/TiO2 whole-rock ratios and zircon saturation temperatures, we conclude that Archean and Proterozoic SPGs formed through partial melting of metasedimentary rocks over a similar range of melting temperatures, with both ‘high-’ and ‘low-’temperature SPGs being observed across all ages. This observation suggests that the thermo-tectonic processes resulting in the heating and melting of metasedimentary rocks (e.g. crustal thickening or underplating of mafic magmas) occurred during generation of both the Archean and Proterozoic SPGs. Third, bulk-rock CaO/Na2O, Rb/Sr, and Rb/Ba ratios indicate that Archean and Proterozoic SPGs were derived from partial melting of both clay-rich (i.e. pelites) and clay-poor (i.e. greywackes) source regions that are locality specific, but not defined by age. This observation, although based on a relatively limited dataset, indicates that the source regions of Archean and Proterozoic SPGs were similar in terms of sediment maturity (i.e. clay component). Last, existing oxygen isotope data for quartz, zircon, and whole-rocks from Proterozoic SPGs show higher values than those of Archean SPGs, suggesting that bulk sedimentary 18O/16O ratios increased across the Archean–Proterozoic boundary. The existing geochemical datasets for Archean and Proterozoic SPGs, however, are limited in size and further work on these rocks is required. Future work must include detailed field studies, petrology, geochronology, and constraints on sedimentary source ages to fully interpret the chemistry of this uniquely useful suite of granites.

Europium anomalies in zircon: A signal of crustal depth?

Distinction between S-type and peraluminous I-type granites: Zircon versus whole-rock geochemistry

Cretaceous magmatic migration and flare-up in Pamir–Karakoram

A large body of work has challenged the paradigm of carbonate stability at the forearc and subarc (> 80 km) conditions in subducted slabs and revealed a variety of complex processes that play an important role in the so-called slow C cycle. Serpentinite-hosted carbonate rocks (i.e., ophicarbonates) are an important rock type for the deep C cycle because they can occur either in the slab or in the mantle wedge. The question revolves around phase stability and metamorphic reactions upon subduction that can lead to a change in carbonate phase assemblage and fluid composition. Moreover, the phase relation between carbonates, silicates, oxide and sulfide minerals in ophicarbonates can be informative about the redox conditions during prograde metamorphism.We present a case study of ophicarbonate rocks from the Zermatt-Saas unit, Western Alps, that were subducted up to eclogite facies conditions at 2.5 GPa, 560° C. In the study area, ophicarbonates overlie a large body of partially dehydrated serpentinites. This allows us to understand whether fluids released from the serpentinites infiltrated the ophicarbonates or not, and to what extent decarbonation reactions occurred in an open or closed system. We investigated three carbonate-bearing rock types: ophicarbonates, olivine-carbonate veins, and a talc-magnesite reaction rind at the contact between ultramafic and mafic/felsic lithologies. Our petrological and geochemical investigation, as well as thermodynamic modelling, reveal that the metamorphic evolution of the ophicarbonate was in a closed system, where calcite/aragonite was replaced by metamorphic dolomite and diopside, and that this reaction is nearly CO2 conservative, with the released fluid composition close to pure water. In situ LA-ICP-MS trace element analyses also show that carbonate in olivine-carbonate veins was most likely sourced from the ophicarbonates. Our thermodynamic modelling indicates that the talc-magnesite reaction zone was most likely formed during early exhumation between 9-13 kbar and 530-460° C, at XCO2 between 0.007 and 0.009. Lastly, we will discuss how the silicate-oxide-sulfide redox buffering assemblage indicates that all three rock types were equilibrated at redox conditions < FMQ.In conclusion, our study demonstrates that in the absence of external fluid infiltration, carbonates in ultramafic lithologies are stable at subduction conditions. This suggests that ophicarbonate have a potential important role in the deep, long term, carbon cycle.