We invite contributions that integrate field observations, laboratory experiments, mineral-physics approaches, numerical models, and geochemical or microstructural analyses to quantify the timescales, conditions, and mechanisms of magmatic, metamorphic, and tectonic processes. Studies emphasizing textural and petrological quantification such as diffusion modelling, petrochronology, thermodynamic or mechanical simulations, and fabric development to bridge across spatial and temporal scales are particularly welcome.
The origin of Permian magmatism in the Chinese Altay Orogen is highly controversial, which is crucial for understanding the evolution of the Central Asian Orogenic Belt. Through a study of the petrology, temperature and/or pressure conditions, geochemistry, and geochronology of the Wuqiagou diorites, this study provides a genetic interpretation for the in the Chinese Altay Orogen. Estimates of crystallization temperatures and/or pressures for each sample were obtained using amphibole thermobarometer and zircon saturation thermometry, yielding results of 900-1000 ℃/0.6-0.8 Gpa and 803-914 ℃, respectively. Geochemical analyses show that these rocks belong to the calc-alkaline series with relatively low SiO2 contents (SiO2 =44.9-47.9 wt.%), low total alkali contents (K2O+Na2O=2.0-5.4 wt.%), high values of Mg# (54-74) and elevated concentrations of V, Cr, Co, and Ni. These diorites are enriched in large ion lithophile elements, but depleted in high field-strength elements. The negative εHf(t) values from -1.23 to -10.13 reflect an enriched mantle source of the diorites. Zircon U-Pb data show the Early Permian emplacement ages of ~280 Ma. The calculated crystallization temperatures are all consistently higher than those previously reported. Therefore, based on the above evidence, it can be inferred that the Early Permian mantle plume activity may have brought anomalous high heat flow, leading to a genetic links between the Wuqiagou diorites and the nearby ultrahigh-temperature granulites.
How to cite: Gao, Y.: A genetic link of Permian high-temperature magmatism with a mantle plume activity: a case study from the southern Chinese Altay Orogen, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-593, https://doi.org/10.5194/egusphere-egu26-593, 2026.
At (ultra)slow-spreading ridges, oceanic core complexes are commonly interpreted to form through dislocation creep in the lower crust, followed by strain localisation into evolved, oxide-rich domains along detachment faults due to fluid-assisted weakening. New observations from the Atlantis Bank (Southwest Indian Ridge) suggest that this framework underestimates the role of melt-present deformation in controlling lower-crustal rheology.
We investigate gabbroic samples from IODP Hole U1473A (Expedition 360), which penetrated ~800 m of the footwall of the Atlantis Bank detachment system. We developed a semi-quantitative microscale strain intensity classification (grades 0–IV, from undeformed to ultramylonite) and compared it with the IODP shipboard macroscopic fabric classification. We furthermore integrate electron backscatter diffraction, scanning electron microscopy, full thin-section chemical mapping, and in situ major and trace element analyses by laser ablation, allowing deformation, melt–rock interaction, and chemical evolution to be assessed from the macro- to the microscale.
Intracrystalline deformation is pervasive across all samples, including those classified as undeformed at the shipboard macroscopic scale, with no systematic relationship between strain intensity and bulk compositional evolution. Whole thin-section chemical maps reveal strong asymmetric zoning in porphyroclasts, with rims and neoblasts consistently enriched in more evolved compositions and preserving microstructural evidence of melt–rock interaction across all lithologies and strain classifications. In addition to neoblasts and overgrowths, evidence for the former melt presence is manifested by locally elevated modal proportions of secondary phases (pargasite, oxides, and enstatite), low apparent dihedral angles (<60°) between mineral phases, films or thin elongate grains interpreted as pseudomorphs after melt along grain boundaries, and cuspate grain boundaries that affect all phases. These microfabrics occur across the full range of microstructural gradients and rock types, but are most pronounced in higher-strain samples. In situ trace element profiles further confirm that rims and neoblasts are more evolved than their host minerals, marked by enrichment in light rare earth elements.
Although all mineral phases display well-defined crystallographic preferred orientations, these fabrics are not consistently related to known slip systems. Instead, we suggest that deformation is accommodated by stress-controlled precipitation and anisotropic growth, consistent with the observation of asymmetric zoning and neoblasts. Coupled rare earth elements and plagioclase–amphibole geothermometry indicates progressive cooling from ~1150 to ~850 °C, constraining the thermal conditions of melt-present deformation prior to brittle localization. This deformation is pervasive throughout the entire sampled hole interval, with protomylonites and mylonites comprising most of the deformed rocks and occurring in shear zones up to 10 m thick.
These results support a model in which the lower oceanic crust deforms and evolves predominantly by melt-assisted dissolution–precipitation creep, largely independent of bulk composition. Dislocation creep is interpreted as a secondary, local response to high strain rates imposed by melt-enhanced reactions. This process produces substantial rheological weakening at the base of the crust, promoting the initiation and long-term activity of detachment faults in oceanic core complexes.
How to cite: Taufner, R., Fagereng, Å., McLeod, C., Lissenberg, J., Faleiros, F., and Viegas, G.: Hot and juicy - Melt-assisted deformation controls the rheology of the oceanic crust beneath the detachment fault footwall at (ultra)slow-spreading ridges, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-15529, 2026.
Monazite is a critical petrochronometer for deciphering metamorphic histories, yet its formation in Archean terranes is restricted by the predominantly basaltic to ultramafic nature of early crust. These lithologies typically lack the bulk chemical compositions required for monazite stability, favoring allanite instead. We investigate rare monazite-bearing metapelites from the Isua Supracrustal Belt (ISB) to determine the geochemical drivers that enabled these rocks to host some of Earth’s oldest preserved monazite.
We utilized a supervised machine learning workflow to objectively identify the elemental ratios controlling monazite formation. By ranking an automated ratio library using Random Forest and optimizing feature selection by maximizing the Silhouette Score through iterative Linear Discriminant Analysis (LDA), we determined the geochemical drivers of group separation. This analysis highlights CaO/Al2O3, CaO/SiO2, CaO/Y2O3, CaO/Ce2O3, CaO/MnO, MgO/SiO2 and FeO/MgO as some of the most critical discriminants distinguishing monazite-bearing lithologies from typical Archean crust.
The LDA reveals that monazite-bearing rocks from Isua chemically overlap with modern monazite-bearing metasediments and S-type orthogneisses. This suggests that the weathering of mixed mafic-felsic sources to form clastic sediments, or the metasomatic alteration of a basaltic precursor, allowed specific Archean lithologies to evolve into compositions indistinguishable from modern crustal rocks. In-situ U-Th-Pb dating links this chemical evolution to the Eoarchean, yielding two monazite generations: ancient grains in garnet cores at ~3.6 Ga and younger grains in garnet rims and the matrix at ~2.7 Ga. The absence of other monazite occurrences in Isua and the in-situ formation of both generations suggest a metamorphic rather than detrital origin, indicating at least two metamorphic events affected the ISB. The ~3.6 Ga population represents one of the oldest monazite occurrences ever discovered on Earth. This finding establishes a minimum age for the emergence of "modern" high-Al/Ca crustal compositions, demonstrating that geological processes capable of stabilizing monazite were active in the ISB by 3.6 Ga.
How to cite: Sorger, D., Müller, T., and Webb, A. A. G.: Decoding the Origins of Eoarchean Monazite in the Isua Supracrustal Belt: A Machine Learning Approach to Crustal Evolution, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-17942, 2026.
Quartzofeldspatic rocks constitute a major component of the continental crust. Although their subduction to (U)HP conditions has been documented in multiple cases, the preservation of unequivocal evidence for (U)HP metamorphism in these lithologies remains rare.
The direct proof is the presence of the UHP index minerals, namely diamond and coesite. Where preserved, coesite typically occurs as inclusions in refractory phases as garnet, making its identification challenging, especially in the case of very small inclusions and/or the absence of the diagnostic back-reaction textures to quartz. Various methods can be used for the unequivocal identification of coesite. In addition to Raman spectroscopy and the EBSD, we propose the use of the cathodoluminescence (CL) spectrometry. Considering the distinct wavelengths emitted by coesite (c. 550 nm) and quartz (c. 660 nm), application of the CL spectrometry coupled with SEM-EDS enables rapid and unambiguous detection of coesite inclusions, even at submicron scale.
In the absence of UHP index minerals, quartzofeldspathic rocks commonly retain stable mineral assemblages over a wide P–T range, limiting their usefulness for peak pressure estimates. In such cases, other UHP indicators need to be searched, including trace-element substitutions in major minerals. We have investigated garnet with coesite inclusions from subducted metagranites of the Eger Crystalline Complex, Bohemian Massif, where garnet shows chemically distinct concentric domains with minor amounts of P, Na, and Li, that systematically coincide with coesite locations. From the correlation of these elements, we infer (Na,Li)1P1M2+−1Si−1 substitution. This coupled substitution is clearly connected to UHP conditions in natural samples and can be therefore considered as a tool indicating UHP conditions.
A further peculiarity of (U)HP quartzofeldspathic rocks is the frequent absence of jadeite-rich clinopyroxene, despite its predicted stabilityby experiments and thermodynamic modelling. This absence has been attributed either to complete retrograde decomposition or to its non-participation in the peak assemblage. Here we describe a quartzofeldspathic gneiss from Erzgebirge that is composed mainly of quartz, garnet, plagioclase, K-feldspar, muscovite, and kyanite and that contains relics of jadeite included in kyanite and garnet (the latter also contains coesite). Observed domains up to 3 mm in size of fine-grained plagioclase-muscovite symplectite, surrounded by plagioclase-muscovite mosaic and occasionally associated with small garnet grains are interpreted as pseudomorphs after jadeite. Peak P-T conditions estimated to >28 kbar and 600-800 °C are consistent with the coexistence of jadeite, coesite and Ca-poor garnet. The subsequent decompression led to jadeite breakdown into plagioclase-muscovite symplectite and Ca redistribution, reflected by increasing Ca content in newly formed garnet. Based on our observations, we propose that jadeite should be considered as a part of the HP mineral assemblage in quartzofeldspatic rocks, and the presence of muscovite-plagioclase assemblage associated with Ca-poor garnet may indicate its former presence.
This work was funded by the Czech Science Foundation grant GACR 24-12845S.
How to cite: Racek, M., Lexa, O., Jeřábek, P., Štípská, P., Paddon, B., Závada, P., Svojtka, M., and Hasalová, P.: Deciphering the (U)HP conditions in quartzofeldspatic rocks: mineral composition and phase preservation., EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-19633, 2026.
This study aims at reappraising the evolution of the crystal chemistry of metamorphic chlorite in metasediments along a well-constrained geothermal gradient, in order to challenge current chlorite semi-empirical thermometers and thermodynamic models, currently proposing contradictory views upon the chemical evolution of chlorite with temperature and its phase relations. For that purpose, we studied metasediments originating from the Schistes Lustrés complex (Western Alps). Metamorphic peak pressure and temperature were determined by Si-content in white mica and Raman Scattering on Carbonaceous Matter (RSCM) respectively, for the different units of the Liguro-Piemont domain1. Peak conditions increase from west to east, from blueschist- to eclogite-facies (1-2GPa, 350-550°C).
Chlorite and white mica crystallized in the successive deformation structures that record the peak and retrograde path of the rock. However, the link between structure and chlorite composition is not straightforward. Even chlorite in textural equilibrium with peak pressure mica (high celadonite content) displays significant local composition variation down to the micrometre scale, emphasising small scale re-equilibration of chlorite upon retrogression. After selection of chlorite on microtextural bases, the evolution of composition has been determined through exhaustive chemical analysis (major elements, minor elements, Fe3+/FeTot ratio and oxygen analysis).
Results highlight the importance of at least the homovalent Fe-Mg, Al-Fe, Tschermak (IVSi4+ + VIMg2+ ↔ IVAl3+ + VIAl3+) and di-trioctahedral (2Al3+ + vacancy ↔ 3Mg2+) substitution. The clearest signal is the increase of XMg (Mg/[Mg + Fe] atom per formula unit) with temperature, contrary to the suggestion of Bourdelle et al. (2013)2. The presence of an oxychlorite3 component is not observed. Variations in Fe3+ content are in the range Fe3+/FeTot = 10% to 25%. The sudoite content (chlorite with vacant octahedral sites) is significant, although it is neglected in the thermodynamic model of White et al 20144. The sudoite content appears increasing with decreasing temperature in agreement with the semi-empirical model of Bourdelle et al. (2013)2 but opposite to the thermodynamic model of Vidal et al. (2006)5 and Lanari et al. (2014)6.
Associated with further analysis, these data will allow to set new ground for the comprehension of metasediment chlorite composition evolution along a subduction metamorphic gradient, and to select the most suited formalism for thermodynamic modelling.
References
1. Herviou C. et al., Tectonophysics, 827, 0040-1951 (2022).
2. Bourdelle F. et al., Contribution to Mineral Petrology, 166:423–434 (2013).
3. Masci L. et al., American Mineralogist, 104, 403–417 (2019).
4. White R. W. et al., Journal of metamorphic Geology, 32, 261–286 (2014).
5. Vidal O. et al., Journal of metamorphic Geology, 24, 669–683 (2006).
6. Lanari P. et al., Contributions to Mineralogy and Petrology, 167, 968 (2014).
How to cite: Françoise, M., Dubacq, B., Bourdelle, F., and Verlaguet, A.: Reappraisal of chlorite composition along a subduction metamorphic gradient (Schistes Lustrés Complex, Western Alps), EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-19956, 2026.
The accurate representation of continental lower crust rheology is critical for modelling plate tectonic processes. However, limited observations and highly heterogenous composition make it difficult to describe the large-scale behavior of the lower crust. We aim to link local heterogeneities to large-scale behavior through a case study from the Pannonian Basin. Most of the available samples of the lower crust are rheologically very strong, dry, garnet-rich mafic granulites. In contrast, inferences from large-scale tectonics, such as the widespread extension, the formation of detachment systems, or the lack of lower crustal earthquakes, suggest a generally weak rheology for the Pannonian lower crust. Observations show that zones of garnet breakdown related to decompression and fluid percolation surround the intact, strong domains. Based on these observations, we designed visco-elastic numerical simulations to demonstrate that strain localization in the weak zones significantly decreases overall long-term stress magnitudes. Consequently, lower crust domains that mainly consist of strong lithologies may still behave as weak layers in the lithosphere due to reaction-induced long-term weakening. Strength increases significantly when weak zones are scarce or discontinuous.
Acknowledgements
This study was supported by the MTA FI FluidsByDepth Lendület (Momentum) project, provided by the Hungarian Academy of Sciences (LP2022-2/2022). K.P. was supported by the National Research, Development and Innovation Fund, Hungary (PD143377) and the János Bolyai Research Scholarship of the Hungarian Academy of Sciences.
How to cite: Porkoláb, K., Török, K., Spránitz, T., Kovács, I. J., Békési, E., and Berkesi, M.: Rheological and tectonic consequences of garnet breakdown in the lower crust: a case from the Pannonian Basin, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-6487, 2026.
The viscosity response of continental lithosphere to heating and partial melting has been a key focus of tectonics research over the past two decades. Much of this research has focused on the Himalayan orogen because it provides a canonical example of lithospheric behavior during continental convergence, involving widespread partial melting. The Himalayan mid-crust, now partially exposed as the Greater Himalayan Sequence, is widely interpreted to have been a low-viscosity, melt-bearing zone that accommodated ductile flow during the early Miocene. However, few studies have attempted to quantify the paleo-viscosity of Greater Himalayan Sequence rocks. To address this gap, we applied muti-mineral subgrain-size piezometry and titanium-in-quartz thermometry to specimens from across the Greater Himalayan Sequence. The specimens record stresses ranging from ~5–25 MPa and melt proportions ranging from ~0–20 modal percent, and include an example in which the final strain event occurred at supra-solidus conditions. By integrating the stress values and their corresponding temperatures with quartz flow laws, we calculate viscosities on the order of 1018Pa·s. Similarity in our stress and viscosity values, regardless of the proportion of melt present during final strain, indicate that the high metamorphic temperatures alone led to the effective crustal viscosities required for gravity-driven ductile extrusion of the Himalayan mid-crust, and decompressive melting was a merely a result of, rather than cause of, ductile flow.
How to cite: Norman, M., V. Dyck, B., and Larson, K.: Constructing a paleo-viscosity profile for the Himalayan middle crust using mineral microstructure, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-8234, 2026.
Mechanically twinned titanite in mylonites from the Sesia zone (strike-slip regime) and pseudotachylyte-bearing gneisses from the Silvretta basal thrust record the stress-strain histories at greenschist-facies conditions in the two different tectonic regimes. Twinned titanite in both fault rocks was investigated by analytical scanning electron microscopy, including electron backscatter diffraction (EBSD) and U-stage measurements. It highlights the similarities and differences in the recorded deformation history. Fine-lamellar (< 1µm) mechanical <110> twins in titanite from the Sesia mylonites with twin planes close to {221} show densities of 0.5 µm-1. Consistent with twinned jadeite, the differential stresses indicated are on the order of 0.5 GPa. In the Silvretta fault rocks, the twin density is higher, at 2.5 µm-1 and additionally, twin planes close to { ̅1 ̅1 2} occur, indicating higher stress/strain-rate conditions, consistent with twinned amphibole and ilmenite as well as the presence of pseudotachylytes. The Silvretta fault rocks do not record subsequent creep, indicating rapidly decreasing stresses. In contrast, in the Sesia mylonites, subsequent creep of the surrounding quartz matrix at decreasing stresses resulted in sets of subparallel intragranular fractures in titanite, garnet, jadeite and zircon oriented at angles between 60° and 80° to the mylonitic foliation. The similarities in high-stress crystal plasticity in both settings, with twinning at high differential stresses, as well as the differences with pseudotachylyte formation in the Silvretta fault rocks and creep at more slowly decreasing stresses in the Sesia zone mylonites, demonstrate the importance of deformation at transient high stresses for the subsequent stress-strain history.
How to cite: Trepmann, C., Beiers, L., and Dellefant, F.: Mechanically twinned titanite records stress-strain histories during strike-slip and thrust faulting at greenschist-facies conditions, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-7962, 2026.
Over the recent years, there has been a significant development in the numerical tools that are used in the thermodynamic and kinetic modelling of metamorphic assemblages. Inspired by the materials science community, different approaches of mineral growth have been suggested. Out of the multitude of approaches, two stand out as the most widely used: i) the phase field method and ii) the sharp interphase method. Both approaches have their advantages and disadvantages and can be used to tackle various problems.
In this presentation, I will present the main methods used in mineral growth modelling along with their distinctive features. I will focus on the sharp-interface method since it allows direct comparison with mineral chemistry and thermodynamic data. Recent development on that field (e.g. Stroh et al., 2025) allows the forward modelling of growing/consuming crystals along with their diffusional response. This approach allows the hypothesis testing for various geothermobarometry and isotope chronology systems. The direct comparison of the measured mineral compositions together with their mineral equilibria modelling offers a self-consistent framework of the timescales of metamorphic processes. Using this framework, various scenarios are investigated and the potential pitfalls are discussed.
References
Stroh, A., Aellig, P.S., Moulas, E., 2025. Numerical modelling of diffusion-limited mineral growth for geospeedometry applications. Geosci. Model Dev. 18, 10203–10220. https://doi.org/10.5194/gmd-18-10203-2025
How to cite: Moulas, E.: Thermodynamic Modelling of Mineral Growth – Implication for Metamorphic Rates, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-14459, 2026.
Developing coupled models of deformation and metamorphism is a key challenge in geodynamics, as these two processes are known to be closely intertwined. Here, we aim to contribute to this challenge by describing fundamental characteristics of deformation-induced phase transition models, such as the evolution of the pressure field and the time scale of the phase transition. We present an isothermal, compressible, Newtonian viscous numerical model for inclusion-host systems submitted to pure shear boundary conditions and use the quartz-coesite (SiO2) phase transition as an example. We derive the pressure-density relation via PerpleX (Connolly, 2005) using the (Holland and Powell, 1998) database. The simple geometry, involving an inclusion weaker than the matrix, enables direct comparisons of our results to the analytical solution of the incompressible case, as well as with numerical solutions for incompressible and compressible (but without phase transition) cases. We then tested the effects of key parameters (viscosity, applied background strain rate, and density difference between reactant and product) on the evolution of the model.
The applied background strain rate induces dynamic pressure variations around the weak inclusion, triggering the phase transition initiation in the matrix at zones of overpressure. The developing pressure field follows the prediction of the incompressible analytical solution, with a systematic shift (i.e. a pressure deficit where the phase transition occurs) controlled by physical properties (strain rate, viscosity). Pressure increases in small increments while the phase transition is taking place and increases rapidly to its steady state value once the transition is completed. Density is updated based on pressure via linear interpolation between the end-member quartz and coesite densities. The combination of this density update and elastic compressibility introduces an “internal” phase transition timescale in the model (where no kinetic law is prescribed): the faster the pressure increases, the less time the phase transition takes. This timescale strongly depends on the physical properties of the model, which is crucial to consider when dealing with compressible geodynamic models as well as when comparing results to natural cases. Results may also be used to interpret laboratory experiments and field observations and lay the basis for further comparisons of model timescales and real-world transformation kinetics.
Acknowledgements
B.K. was supported by the EKÖP-25 University Research Scholarship Program of the Ministry for Culture and Innovation from the source of the National Research, Development and Innovation Fund.
K.P. was supported by the National Research, Development and Innovation Fund, Hungary (PD143377) and the János Bolyai Research Scholarship of the Hungarian Academy of Sciences.
Connolly, J. A. D., 2005, Computation of phase equilibria by linear programming: A tool for geodynamic modeling and its application to subduction zone decarbonation: Earth and Planetary Science Letters, 236, 1-2, 524-541, DOI: 10.1016/j.epsl.2005.04.033.
Holland, T. J. B., and Powell, R., 1998, An internally consistent thermodynamic data set for phases of petrological interest: Journal of Metamorphic Geology, 16, 3, 309-343, DOI: 10.1111/j.1525-1314.1998.00140.x.
How to cite: Koszta, B., Porkoláb, K., Yamato, P., and Duretz, T.: Deformation-induced phase transitions: testing host-inclusion systems through 2D chemico-mechanical numerical models, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-714, https://doi.org/10.5194/egusphere-egu26-714, 2026.
The Kamieniec Metamorphic Belt (KMB), exposed in the Sudetes and forming part of the northeastern margin of the Bohemian Massif, represents one of the outermost exposures of the crystalline basement of the Variscan belt in Europe. It experienced an initial episode of high pressure low temperature (HP-LT) metamorphism reaching conditions of around 15 to 18 kbar and 470 to 570 °C, followed by a low pressure middle temperature (LP-MT) metamorphic event at pressures of 3 to 8 kbar and temperatures of 500 to 600°C (Szczepański et al., 2022a; Szczepański and Goleń, 2022). These metamorphic episodes were investigated using phase equilibria modelling, Raman barometry, and conventional geothermobarometry. Although, metamorphic ages of ca. 347 to 337 Ma have been reported from neighbouring units (Jastrzębski et al., 2020; Szczepański et al., 2022b), precise timing constraints for the metamorphic events within the KMB are still lacking.
In this study, we analysed zoned garnets from mica schists of the KMB. We used the GDIFF software (Moulas, 2023) to simulate compositional diffusion profiles and estimate the corresponding timescales related to the last thermal event experienced by the rocks. Our diffusion approach was supplemented by the use of Hamiltonian Monte Carlo (HMC) to rigorously estimate the statistical uncertainties of the inferred timescales. We successfully calculated the compositional profiles and were able to provide quantitative data on the time evolution of the KMB, which fit the overall known geodynamical history.
References
Jastrzębski, M., Żelaźniewicz, A., Budzyń, B., Sláma, J., and Konečny, P.: Age constraints on the Pre-Variscan and Variscan thermal events in the Kamieniec Ząbkowicki Metamorphic belt (the Fore-Sudetic Block, SW Poland), Ann. Soc. Geol. Pol., 90, 27–49, https://doi.org/10.14241/asgp.2020.05, 2020.
Moulas, E.: GDIFF: a Finite Difference code for the calculation of multicomponent diffusion in garnet, Zenodo[code], https://doi.org/10.5281/zenodo.8224137, 2023.
Szczepański, J. and Goleń, M.: Tracing exhumation record in high-pressure micaschists: A new tectonometamorphic model of the evolution of the eastern part of the Fore Sudetic Block, Kamieniec Metamorphic Belt, NE Bohemian Massif, SW Poland, Geochemistry, 82, 125859, https://doi.org/10.1016/j.chemer.2021.125859, 2022.
Szczepański, J., Zhong, X., Dąbrowski, M., Wang, H., and Goleń, M.: Combined phase diagram modelling and quartz‐in‐garnet barometry of HP metapelites from the Kamieniec Metamorphic Belt (NE Bohemian Massif), J. Metamorph. Geol., 40, 3–37, https://doi.org/10.1111/jmg.12608, 2022a.
Szczepański, J., Anczkiewicz, R., and Marciniak, D.: P-T conditions and chronology of the Variscan collision in the easternmost part of the Saxothuringian crust (Bohemian Massif, Fore-Sudetic Block, Poland), Mineralogia, Special Papers 50, 88, 2022b.
How to cite: Stroh, A., Moutzouris, D., Szczepański, J., and Moulas, E.: Metamorphic timescales for the Kamieniec Metamorphic Belt (NE Bohemian Massif) based on garnet diffusion modelling , EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-3751, 2026.
The Na-Ca partitioning between coexisting scapolite and plagioclase constitutes a potential geothermometer. The application is, however, complicated by the potential influence of additional parameters, including the bulk-rock and the fluid composition. The term scapolite refers to a solid solution between the marialite (3NaAlSi3O8·NaCl) and meionite (3CaAl2Si2O8·CaCO3) end-members. Thermodynamic mixing models for scapolite are available for Ca-rich scapolite with CO32- as the only anion (carbonate scapolite) and do not account for Ca-poor scapolite, where Cl- is an important anion, in addition to CO32-. Even for carbonate scapolite, the bulk rock composition, in particular, the alumina to silica and the Na2O to CaO ratios, may have an influence on the compositions of coexisting scapolite and plagioclase. We investigate the chemical compositions of scapolite and plagioclase from the southeastern part of the Bohemian massif in Austria. Scapolite is found within calcsilicate gneiss associated with marble layers of the Drosendorf unit. The layers can be traced more or less continuously for at least 75 km in a north-south direction with a slight temperature increase towards the south. We compare the results with predictions from Gibbs energy minimization. The thermodynamic calculations indicate a substantial temperature dependence of the Na-Ca partitioning between coexisting carbonate scapolite and plagioclase. At fixed pressure and temperature and in the presence of calcite, the compositions of scapolite and plagioclase are fixed. Under these specific conditions, the temperature dependence of the Na-Ca partitioning between Ca-rich carbonate scapolite and anorthite-rich plagioclase are a viable geothermometer. In the absence of calcite, the compositions of coexisting scapolite and plagioclase depend on the bulk rock composition and can produce a range of compositions for the same temperature. Still, the Na-Ca partitioning may be used for temperature determination if scapolite-plagioclase pairs of different compositions are available from one sample or one outcrop, so that several scapolite-plagioclase conodes can be determined simultaneously.
How to cite: Simian, L., Schuster, R., and Abart, R.: Applicability and Limitations of the Na–Ca Partitioning between Scapolite and Plagioclase as a Geothermometer, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-5748, 2026.
Garnet, a robust, rock-forming mineral, is often used to understand metamorphic processes through study of chemical zonation, mineral inclusions, and radiometric dating. Natural sample suites in which two distinct populations of garnet are found can be particularly insightful in unraveling the pressure-temperature-time (P-T-t) evolution of rocks through Earth’s crust. We studied two sample sets from the Smalls Falls formation (northern New Hampshire, USA) and the Hawley formation (western Massachusetts, USA) which both contain a high crystallization density coticule garnet quartzite (~106 crystals/cm3) and a lower crystallization density garnet schist (<102 crystals/cm3). Classical and elastic thermobarometry applied to both sample sets indicated that coticule and schist garnet may have crystallized at P-T conditions within uncertainty of each other. We collected ID-TIMS Sm-Nd isotopic data from these sample sets to better understand their nucleation history. In the Hawley formation, coticule garnet grew at 395.3±4.5 Ma (MSWD=1.6), while schist garnet from the same outcrop yielded an Sm-Nd isochron age of 376.6±4.0 Ma (MSWD=9.1). Smalls Falls formation coticule garnet yielded an isochron age of 372.4±2.1 Ma (MSWD=0.83), while schist garnet from the same formation crystallized at 367.0±1.4 (MSWD=1.3). These data indicate that spessartine (Mn) rich coticule garnets grew 18.7±6.1 Ma and 5.4±2.5 Ma before lower crystallization density schist garnets in the Hawley and Smalls Falls formations, respectively. We performed Mn diffusion modelling using FORTRAN program GarDiffMoveRim on Mn-rich small-radius garnets from the Hawley formation and found that at inferred temperatures of nucleation and growth (600°C), presently observed Mn zoning profiles persist for remarkably short time scales (<100 ky). Even at lower temperatures (500-550°C) Mn poor mantles in coticule garnets persist for <5 my. Preservation of bell-shaped Mn zoning profiles in Hawley formation garnets does not allow for residence at elevated T on the long timescales that may be inferred from our Sm-Nd isotopic data (~18 my). Instead, our Sm-Nd geochronology combined with Mn diffusion modelling suggests that heat sources during Acadian metamorphism may have been highly transient in nature, spurring garnet growth and followed by rapid cooling.
How to cite: Koch, M. M., Spear, F. S., Walker, S., Schroeder, K., Baxter, E. F., and Thomas, J. B.: Characterizing rates of Acadian metamorphism using Sm-Nd geochronology and major element diffusion in garnet, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-9478, 2026.
Mantle peridotite thermobarometry has been extensively used to establish the thermal state of the lithosphere. The pressure and temperature (P-T) information from these rocks is routinely compared to model geotherms and the goodness of fit can be used to identify critical parameters of the thermal model. This method implies that mineral thermobarometers do not have sufficient time to re-equilibrate during their transport to the surface and therefore have preserved the ambient geotherm of the source region.
In this work we performed a systematic analysis on garnet peridotite xenoliths from Hawai’i. We employed well-established and new thermometers based on Ca-Mg, Cr-Al, Fe-Mg exchange reactions between opx-cpx, opx-grt and cpx-grt mineral pairs. Pressure was determined using the Al solubility between opx and grt. Our results reveal that the Hawaiian xenoliths fit oceanic geotherms that span from 45 to 25 Ma. This apparent fit is at odds with the well-established age of the oceanic lithosphere in the region (90 Ma).
The discrepancy between the two age groups can be perfectly explained by the fact that the lithosphere beneath Hawai’i has experienced intense thermal perturbation during the ascent of the magmas to the surface. Our interpretation is in agreement with published plate flexure models that call on magma-assisted flexural weakening of the lithosphere.
How to cite: Kostopoulos, D., Moulas, E., Pomonis, P., and Papadopoulos, A.: Intense magmatic heating of oceanic lithosphere revealed by Hawaiian xenoliths, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-14917, 2026.
Granulite-facies metamorphism of the continental lower crust reflects not only the attainment of high peak temperatures but also prolonged residence at elevated thermal conditions, which is critical for equilibration and preservation of high-grade mineral assemblages. In granulite terranes of the Central Indian Tectonic Zone, peak metamorphic temperatures are widely attributed to externally driven tectono-thermal processes, such as crustal thickening, magmatic underplating, or mantle-derived heat input. In contrast, the factors governing post-peak thermal evolution remain less well constrained. This study evaluates the potential role of thorium-bearing granitoid lithologies in modifying post-peak thermal evolution during crustal cooling. Whole-rock trace element data from granitoids reveal pronounced heterogeneity in thorium contents, with several samples exhibiting substantial enrichment relative to surrounding granitoids and average lower crustal values. Although radiogenic heat production from thorium is insufficient to independently generate granulite-facies conditions or to act as a primary heat source, such enrichment represents localized zones of enhanced radiogenic heat production. Once high-temperature conditions are established by external heat sources, these Th-rich granitoid domains may modify post-peak thermal gradients by retarding isotherm relaxation during crustal cooling and exhumation.
This spatially limited thermal buffering effect, subordinate to tectonic heat sources, may nevertheless contribute to prolonged high-temperature residence of granulite-facies mineral assemblages. This study focuses on the role of heterogeneous radiogenic heat distribution in shaping the temporal evolution of crustal thermal regimes and suggests that thorium systematics constrain the duration, rather than the origin, of high-grade metamorphism in continental crust.
How to cite: Dixit, P. K. and Kumar C., I.: Role of Thorium-Bearing Granitoids in Thermal Buffering During Granulite Metamorphism in the Central Indian Tectonic Zone, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-16688, 2026.
Partial melting processes occurring in mafic lithologies at low pressures and medium-high temperatures are not well understood and therefore may conceal crucial aspects of crustal evolution. Here, we target a composite lens, 150 x 50 m in size, comprising massive amphibolites, banded amphibolites and levels of amphibole-bearing granulites, hosted in a highly deformed, migmatitic metasedimentary sequence belonging to the Variscan High-Grade Metamorphic Complex at Punta Scorno in the northern Asinara Island (Sardinia, Italy). These amphibolites and granulites record a complex history of prograde metamorphism, and possibly, multiple stages of partial melting.
The massive amphibolites do not display any evidence of partial melting and consist mostly of magnesio-hornblende (XFe = 0.26-0.36 and 0.46-0.51) and Ca-rich plagioclase (An41-96) with later-crystallised biotite (XFe= 0.36-0.51). Banded amphibolites show a similar assemblage, although hornblende and biotite are Fe-richer (XFe = 0.54-0.65 and XFe = 0.55-0.63, respectively) and plagioclase contains less Ca (An29-79). In addition, banded amphibolites are rich in quartz, and contain minor amounts of other amphiboles, in particular grunerite (XFe = 0.55-0.59) overgrown by tschermakite (XFe = 0.59-0.68), and rare garnet (Alm69-72Grs11-14Prp7-9Sps8-10). Cuspate edges of quartz and melt pseudomorphs of plagioclase within the banded amphibolites provide evidence of partial melting. Also, the amphibole-bearing granulites are characterised by a significant amount (ca. 10%) of grunerite (XFe = 0.51-0.64) with tschermakitic rims (XFe = 0.62-0.76), and garnet (Alm71-76Grs11-14Prp6-12Sps4-7). These rocks are more massive with respect to the banded amphibolites, and are much richer in quartz and plagioclase of variable composition (An49-86), suggesting that they might be the result of partial melting of the banded amphibolites. The former presence of melt is also supported by several crystallised melt inclusions found in the garnets.
We used a combination of single-element thermometry (Ti-in-Amp (Liao et al., 2021; Bartoli et al., 2024) and Ti-in-Qz (Osborne et al., 2022)) and phase equilibrium modelling (Connolly, 2005) to constrain the metamorphic evolution. Our results suggest prograde growth of hornblende in the massive and banded amphibolites, followed by crystallisation of grunerite in the partially molten rocks – possibly as a peritectic phase – subsequently replaced by garnet and overgrown by tschermakite. The early prograde conditions are recorded by small hornblende crystals yield ca. 650 °C, whereas the peak P-T conditions during which grunerite grew, are ca. 0.5 GPa/730 °C. Retrograde crystallisation of tschermakite occurred at ca. 600 °C. As the peak conditions occurred at much lower temperature than those required for dehydration melting of amphibole, it is likely that a fluid influx during the prograde part of the P-T path lowered the solidus temperature of the system (Weinberg and Hasalová, 2015).
This work was funded by Fondazione di Sardegna Progetto RAWEX and Horizon Europe programme, grant 101131765 (EXCITE2).
Bartoli, O. et al. (2024). Contributions to Mineralogy and Petrology, 179, 65
Connolly, J. A. D. (2005). Earth and Planetary Science Letters, 236, 524-541
Liao, Y. et al. (2021). American Mineralogist, 106(2), 180-191
Osborne, Z. et al. (2022). Contributions to Mineralogy and Petrology, 177, 31
Weinberg, R. F. and Hasalová, P. (2015). Lithos, 212-215, 158-188
How to cite: Turek, O., Ferrero, S., Casini, L., Idini, A., Dulcetta, L., Sorger, D., Mueller, T., Cruciani, G., and Buisman, I.: Partial melting evolution of the Variscan High-Grade Metamorphic Complex recorded in amphibolites from Asinara Island (Italy), EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-17251, 2026.
Mineral geothermometry is based on cation exchange between minerals and effectively reflects the closure temperature through kinetic processes. Each geothermometer is designed for a specific purpose and has its own merits. The use of different thermometers that employ cations of different diffusivities is extremely important in capturing “snapshots” of thermal events, which facilitate decoding cooling/heating pulses, ascent rates and residence times.
Here we compare the results of Fe-Mg exchange thermometers between coexisting mineral pairs in spinel peridotites from different geotectonic environments and discuss the most important implications that stem from this. Three mineral pairs were considered, olivine-spinel (ol-spl), orthopyroxene-spinel (opx-spl) and orthopyroxene-clinopyroxene (opx-cpx) which show progressively higher Fe-Mg closure temperatures in the order listed. The first two thermometers constitute new calibrations constructed by us whilst the third is the formulation of Brey & Köhler, 1990.
Application of the above thermometers to abyssal peridotites (abyssal) and peridotites exposed in oceanic forearc and backarc regions shows undisturbed cooling patterns for each setting, with higher mean opx-cpx temperatures followed by opx-spl and then by ol-spl temperatures. When these patterns are compared to those obtained for ophiolitic peridotites it becomes immediately apparent that temperature distributions in mantle peridotites from oceanic forearcs and oceanic backarcs exhibit great similarities with those from ophiolites for all 3 thermometers. Abyssal peridotites have distinctly higher mean temperatures suggesting that ophiolitic massifs have not been formed in major ocean basins but rather in oceanic forearc or backarc settings. Such a conclusion is also strongly supported by trace-element geochemistry of ophiolitic volcanic rocks.
The above order of the three thermometers is, nonetheless, reversed when they are applied to peridotite xenoliths found in volcanic rocks along potential continental rift zones. The subcontinental lithospheric mantle is expected to cool normally through time hence display a cooling pattern like that observed for abyssal peridotites. In the case of the xenoliths however, where mantle pieces are collected and transported by hot magma at temperatures much higher than the ambient geotherm, their exposure to high temperatures reactivates diffusion so that the thermometer containing the mineral pair with the fastest Fe-Mg diffusion (i.e., ol-spl) slides uphill faster and records the highest temperatures. The very fact that this (counter-intuitive) reverse order of temperature distributions has been preserved, places further constraints on the time scales of xenolith transport as protracted times of magma storage and fractionation in crustal chambers would have led to subsequent cooling and obliteration of the temperature patterns observed.
G. P. Brey & T. Köhler, 1990. Geothermobarometry in Four-phase Lherzolites II. New Thermobarometers, and Practical Assessment of Existing Thermobarometers. Journal of Petrology, Vol. 31, Part 6, pp. 1353-1378. DOI:10.1093/petrology/31.6.1353
How to cite: Giatros, V., Kostopoulos, D., Moutzouris, D., Moulas, E., Pomonis, P., and Papadopoulos, A.: Cooling vs. heating histories of mantle peridotites revealed by multiple Fe-Mg exchange thermometers , EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-18655, 2026.
Metasedimentary rocks in high-pressure complexes offer a complementary record of subduction processes by capturing mineralogical and chemical features that are highly sensitive to P-T evolution and strongly interact with fluids. Their variability helps constrain the nature and extent of fluid–rock interaction, provenance, and the mechanical and thermal structure of the subduction environment. The Raspas Complex (SW Ecuador), a well-preserved oceanic unit exhumed without continental collision, provides a unique opportunity to evaluate how sediments register burial and peak metamorphic conditions within a cold-subduction setting and how these records compare with those preserved in associated mafic lithologies.
Our integrated approach — combining petrography, bulk-rock and mineral chemistry, thermodynamic modelling, and Zr-in-rutile thermometry — defines a coherent prograde-to-peak metamorphic evolution. Garnet zoning shows strong decoupling between major and trace elements. Fe–Mn–Ca–rich cores and Mg-rich rims define two main stages (prograde, M1; and peak, M2), while HREE–Y distributions preserve a depleted inner core and limited diffusion during growth. Trace-element patterns (e.g., Sc following Mn; V showing the inverse trend; Cr decreasing outward; fracture-hosted enrichments in Zn) reflect episodic release from reacting phases and locally fracture-controlled modifications. Thermodynamic models that account for garnet fractionation constrain a clockwise P–T path from ~525 °C, 17–18 kbar (M1) to ~570 °C, ~21.5 kbar (M2). Zr-in-rutile temperatures of 483–630 °C (at 12–25 kbar) are consistent with the independently modelled P–T conditions. Retrograde chlorite at garnet rims and fractures marks subsequent cooling and decompression.
The results demonstrate that sedimentary slices can retain discrete growth stages and subtle overprints that complement the metamorphic information recorded in mafic blocks. Together, these data refine the thermal structure, fluid regime, and burial–exhumation dynamics of the Raspas subduction system. Forthcoming U–Pb in-situ dating of key phases, garnet oxygen-isotope analyses, diffusion-based modelling, and integration with parallel results from the metamafic rocks will further constrain the rates and conditions of subduction and exhumation, advancing reconstructions of deep-crustal recycling in cold-subduction settings.
How to cite: Paiva-Silva, G., Tedeschi, M., Lanari, P., Ganade, C., Silva, O. S., Beranoaguirre, A., and Gerdes, A.: A (meta)sedimentary window into ocean–continent subduction: the Raspas Ophiolitic Complex (SW Ecuador) , EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-625, https://doi.org/10.5194/egusphere-egu26-625, 2026.
