We seek interdisciplinary contributions integrating petrochronology, structural geology, and experimental / metamorphic petrology. Key approaches include diffusion chronometry; accessory-phase geochronology; geochemical tracers; microstructural analysis; thermodynamic modeling of phase equilibria; and geodynamic simulations. Studies quantifying metamorphic rates (e.g., burial/exhumation, fluid pulses), reconciling P-T-t path ambiguities through multi-method validation, or bridging microscale reactions with tectonic drivers are particularly encouraged.
We welcome innovative case studies from diverse settings—subduction zones, collisional orogens, and rift systems—alongside methodological advances that connect field observations with modeling.
We explore timescales and petrophysical responses to fluid–rock interaction processes associated with far-field differential stress that promoted mode I fracture opening during the development of E–W-striking shear zones. Our study focuses on a ~100 m-scale shear-zone system developed within the Kråkeneset gabbro (western Norway) during the Caledonian orogeny. Shear-zone formation induced brittle deformation of the gabbro, producing alternating N–S-trending mode I fractures with meter-scale spacing.
Fluid infiltration along these fractures resulted in the formation of decimeter-wide amphibolitized alteration zones, transforming an originally dry gabbro. Under amphibolite-facies conditions (~650 °C, 0.58 GPa), H₂O-rich fluids exploited the newly opened mode I fractures, which acted as efficient fluid pathways. Microstructural observations reveal that amphibolitization reactions preferentially occurred along mafic–felsic grain boundaries. These reactions proceeded via dissolution–precipitation mechanisms, generating transient porosity and thereby enhancing permeability and fluid transport.
In order to reveal chronometric and petrophysical constraints on the amphibolitization process, we applied reactive-transport modeling combined with lithium concentration and isotope data. The modeling results show that fluid and element transport was dominated by advection, whereas diffusion controlled local isotopic equilibration. From a tectonic perspective, the mode I fracture set most likely formed during a single deformation event. Such a brittle response of the gabbro to shear-induced stress buildup at elevated temperatures implies a rapid and sudden mode I fracture development.
Subsequent fluid infiltration was controlled by an externally imposed fluid-pressure gradient, which exerted first-order control on amphibolitization timescales. Modeling results suggest that the transient fluid overpressure at the wall rock interface generated short-lived porosity increases, accelerating hydration reactions. Outcrop observations show reaction zone widths along the mode I fractures clustering around ~30 ±15 centimeters. The wall rock adjacent to the fractures likely exhibited spatial variations in permeability within one order of magnitude prior to fluid infiltration. These pre-existing heterogeneities resulted in the development of reaction zones with variable widths during a single fluid infiltration event.
Modeled reaction-front propagation rates of decimeters to meters per year indicate brief, episodic brittle events that link rapid stress accumulation, fluid pressure relaxation, transient porosity-permeability relations, and metamorphic transformation in the lower crust. Together, these results provide a quantitative framework for understanding fluid-driven metamorphism and transient permeability in deep crustal environments.
How to cite: John, T., Grund, S., and Vrijmoed, J.: Timescales and petrophysical changes associated with amphibolitization of mafic crust, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-7039, https://doi.org/10.5194/egusphere-egu26-7039, 2026.
The plate interface at subduction zones represents one of Earth’s most dynamic environments in terms of deformation, metamorphism, and chemical exchange. The efficiency with which these processes affect rocks at the interface is strongly controlled by the presence of fluids. Accordingly, quantifying the rates of fluid–rock interaction is essential for understanding the pressure–temperature–deformation (P–T–d) evolution of rocks at plate interfaces.
In this study, we investigate a metasomatic reaction zone developed along the tectonic contact between serpentinite and metagabbro in the Voltri Massif (Ligurian Alps, Italy) under high-pressure conditions. The hydrated mantle-derived rocks were juxtaposed with the mafic oceanic crust at lower temperatures prior to the metasomatic process. A temperature increase led to dehydration of both lithologies, the serpentinite and the metagabbro, both of which liberated different amounts of aqueous fluids with very distinct fluid chemistries. This setting enabled Mg-rich fluids derived from serpentinite to infiltrate the adjacent mafic crust, triggering extensive metasomatic transformation. The aim of this study is to constrain the timescale of rock transformation and to explore the evolution of porosity and permeability within the modified system.
Our approach integrates a fully coupled Thermo–Hydro–Mechanical–Chemical (THMC) reactive transport model with thermodynamic phase equilibria calculations and diffusion chronometry. Phase equilibria calculations, validated by observed mineral assemblages, modal abundances, and mineral chemistry, are used to constrain the pressure–temperature conditions of the reaction zone formation. The estimated conditions correspond to pressures of 1.6 ±0.1 GPa and temperatures of 600 ±20 °C, with the maximum temperature being constrained by the serpentinite stability field. The profile across the reaction zone displays a continuous gradient in bulk MgO concentration from serpentinite (~40 wt.%) to metagabbro (~5 wt.%). This gradient is accompanied by a systematic Mg isotope fractionation, with δ²⁶Mg values decreasing from +0.09‰ in serpentinite to −1.1‰ within the reaction zone and an increase to −0.1‰ towards the least affected metagabbro. Such an Mg isotope profile indicates kinetic fractionation during Mg diffusion and provides the basis for Mg isotope diffusion chronometry.
Our THMC model results reproduce the observed major-element and isotopic profiles and suggest transient porosity generation localized at the reaction front. Calculated Peclet numbers (~0.01–0.1) indicate diffusion-dominated mass transport, with a minor advective component. The chronometric results of the modeling constrain the duration of metasomatic transformation to 10³–10⁴ years, highlighting the rapid nature of fluid-mediated processes at the subduction interface. This study shows how integrating diffusion chronometry with phase equilibria and reactive transport modeling helps bridge small-scale metamorphic processes and larger-scale subduction dynamics.
How to cite: Antonenko, B., John, T., Dragovic, B., Codillo, E., Scambelluri, M., and Vrijmoed, J.: Constraining timescales of fluid-driven metamorphic rock transformation at a subduction interface using THMC modeling combined with Mg isotope diffusion chronometry, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-12621, https://doi.org/10.5194/egusphere-egu26-12621, 2026.
Lower crustal terranes exposed at the Earth’s surface offer unique insights into the metamorphic conditions prevailing in the deep crust and at the crust-mantle boundary. In particular, the reconstruction of time-resolved pressure-temperature (P-T) gradients across terranes is essential for characterizing different tectonic settings and heat sources during metamorphism, with direct geodynamic implications. Additionally, physicochemical gradients along depth, such as variations in bulk rock composition and density, are also fundamental parameters intrinsically linked to crustal formation and evolution. However, a quantitative understanding of the deep Earth is strongly limited by the incomplete mineral record and inherent uncertainties in thermobarometry estimates. In practice, a standard approach involves sampling along a field gradient to retrieve punctual pressure-temperature-time information.
This contribution presents a natural case study of a lower continental crustal section, the Ivrea Verbano Zone (IVZ), northern Italy, where a metamorphic field gradient from amphibolite to granulite facies is exposed along the Ossola Valley. U-Pb dating of zircon from different lithologies and crustal depths constrains the high-temperature history and associated melting between 285–260 Ma. Multiple thermobarometers have been applied on mafic and felsic rocks along the section, including thermodynamic phase equilibria modelling and Zr-in-rutile thermometry. The Zr-in-garnet temperature dependence was also applied as a thermometer, revealing good agreement with Zr-in-rutile temperatures and successfully retaining peak temperatures in granulite facies metasedimentary rocks. The metamorphic gradient continuously evolves from ~ 5 kbar, 600 °C to 11 kbar, 1000 °C, and defines a present-day geobaric gradient of ~0.79 kbar/km, significantly higher than what is expected in a steady-state lower crust (0.28-0.3 kbar/km). Paleodepth reconstructions based on barometry and measured densities reveal that the lower crustal section experienced significant degrees of thinning (thinning factor β ~2.7). This result indicates that syn-to-post metamorphic extension has led to the modification of the geobaric gradient. Furthermore, it complements previous studies from the region, indicating that there is a lateral gradient in β along the axis of the current IVZ.
Lithological proportions and associated measured bulk rock compositions continuously evolve upgrade and define two distinct crustal endmembers. The amphibolite facies lower crust is volumetrically dominated by felsic metasediments and compositionally resembles typical upper continental crust, relatively enriched in heat producing elements. In contrast, the granulite facies lower crust is dominated by mafic lithologies, and its composition more closely resembles typical lower continental crust (Rudnick and Gao, 2014). Measured densities show significant variabilities (± 250 kg/m3, 2SD) within both felsic and mafic lithologies, with a linear increase from ~ 2750 to 3150 kg/m3 at the base of the section. Overall, our results reveal that the change in lithological proportions with paleodepth and high-temperature metamorphism play a primary role in controlling the physicochemical properties of the lower continental crust and its evolution.
How to cite: Pacchiega, L., Degen, S., Secrétan, A., Lemke, K., Bhattacharyya, A., Luo, Z., Hofer, A., Kurzen, S., Hetényi, G., Müntener, O., Hermann, J., and Rubatto, D.: Reconstruction of metamorphic gradients and thinning in the lower continental crust, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-17222, https://doi.org/10.5194/egusphere-egu26-17222, 2026.
Melt fractions higher than 10 vol.% in meta-igneous rocks are reported from many orogenic belts and commonly exert a strong control on strain localization within the crust. The production of this melt requires the addition of external water to increase the degree of partial melting. One potential source of this water is the subsolidus dehydration of adjacent metasedimentary rocks. To evaluate this hypothesis, we developed a path-dependent, multi-lithology phase equilibrium model that simulates the amount of water released by metasedimentary rocks between their solidus and the orthogneiss solidus. The released water is then transferred as external fluid influx to the orthogneiss and the resulting melt fractions simulated. We applied this model to ten prograde pressure–temperature (P–T) paths using metapelite–orthogneiss and metagraywacke–orthogneiss associations.
Our results show that metasedimentary rocks release less than 1.0 mol% H₂O, mainly through the breakdown of staurolite and paragonite, with minor contributions from muscovite and biotite consumption. Despite these limited quantities, the water significantly enhances melt fractions in orthogneiss by several percent, making orthogneiss the most melt-fertile lithology along most prograde paths at temperatures below 750 °C. If orthogneiss constitutes half or less of the crust, the melt fractions generated are sufficient to substantially weaken it and localize deformation. We propose that such strain localization may promote the development of preferential pathways for further fluid influx, thereby enhancing partial melting in meta-igneous rocks and establishing a positive feedback mechanism. These results indicate that this process is likely to operate along most prograde P–T paths in orogenic crusts composed of metasedimentary rocks and orthogneisses.
How to cite: Vanardois, J. and Lanari, P.: Assessing the budget of water-present melting in a heterogeneous continental crust, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-7128, https://doi.org/10.5194/egusphere-egu26-7128, 2026.
Understanding the role of H2O-fluxed melting in Fe-rich granites is crucial for evaluating their contribution to crustal differentiation. In this study, we examine the São Pedro da União Migmatite Unit (SE Brazil), where a Paleoproterozoic A-type granite was reworked during the Neoproterozoic, as part of the assembly of western Gondwana. New U-Pb and Lu-Hf zircon data indicate that the protolith crystallized at ca. 1.72 Ga, was derived from a quartz-feldspathic continental crustal source, and that magma generation was linked to Statherian continental rifting.
Phase equilibrium modelling suggests subsequent Ediacaran partial melting at ca. 600 Ma, which required an influx of externally derived H2O at 670–720 °C and 1.0 GPa. Despite the extensive anatexis, the migmatite preserves geochemical characteristics of the protolith, including high XFe in peritectic hornblende (hastingsite). The increase in leucosome proportion towards the Jacuí Shear Zone—from stromatic metatexite at the top to homogeneous diatexite at the base—suggests progressively greater H2O availability in a zone of syn-anatectic deformation.
These results reveal that, although A-type granites are typically considered hot and anhydrous, they can undergo significant reworking and generate substantial melt volumes when infiltrated by external H2O. In this scenario, shear zones likely acted as localized pathways for fluid ingress.
How to cite: Celis, E., Moraes, R., Beranoaguirre, A., Marschall, H., and Gerdes, A.: Water-fluxed melting of an A-type granite associated with shear zones: Insights from the São Pedro da União Migmatite Unit, Brazil, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-1597, https://doi.org/10.5194/egusphere-egu26-1597, 2026.
This study presents the first detailed investigation of garnet–scapolite-bearing calc-silicate rocks from the Pur–Banera Belt (PBB) in the Bhilwara region (NW India). These rocks preserve three stages of deformation (S1, S2, and S3) and typically comprise garnet + clinopyroxene + amphibole + biotite + calcite + scapolite bearing assemblages. T–XCO2 phase equilibrium modelling was carried out in MnO–Na2O–CaO–K2O–FeO–MgO–Al2O3–SiO2–TiO2–H2O–CO2 (MnNCKFMASTHc) to investigate the mineral paragenesis during metamorphic evolution of calc-silicates. The integration of petrological and phase equilibrium modelling results reveals that the calc-silicate rocks experienced prograde metamorphism from greenschist to upper-amphibolite facies conditions (~515–694 °C, ~9 kbar). Garnet compositional zoning, mineral inclusion patterns and trace element distributions collectively record a sequence of fluid-mediated reactions and episodic mineral growth under varying P–T–XCO2 conditions as observed from phase equilibria modelling. A comparison between observed and modelled mineral proportions, coupled with the presence of disequilibrium textures, indicates that externally derived fluids with significant hydrous infiltration would have controlled mineral growth in the PBB calc-silicates. U–Pb geochronology of inclusion-type titanite constrains peak metamorphism at ~1280 ± 4 Ma, corresponding to the D1–D2 deformation. In contrast, the recrystallized titanite, occurring along the matrix grain margins yielded the timing of retrograde re-equilibration at ~953 ± 7 Ma, which was synchronous with the D3 deformation. Overall, these results highlight the critical role of externally buffered fluids in driving mineral reactions and geochemical redistribution during the metamorphic evolution of the PBB, linked to basin closure, crustal thickening, and subsequent exhumation associated with the assembly of the Rodinia supercontinent.
How to cite: Praharaj, P. and Naraga, P.: Tectono-metamorphic evolution of garnet–scapolite-bearing calc-silicate rocks from the Pur-Banera Belt, Aravalli orogen (NW India) , EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-13547, https://doi.org/10.5194/egusphere-egu26-13547, 2026.
The Austroalpine Domain in the Eastern Alps consists of Mesozoic (meta)sedimentary cover units and polymetamorphic crystalline basements. The latter were heterogeneously overprinted by the Alpine metamorphism, with some of them still preserving traces of their Paleozoic magmatic and metamorphic evolution. One of the most prominent examples is the Ötztal–Stubai Complex, which extends over approximately 50 × 20 km across northern Italy and western Austria. Despite this complex experienced extensive Variscan high- to medium-grade metamorphism, some relicts of older events related to previous orogenies (i.e. Cadomian) are still preserved. As such, it represents a key natural archive for reconstructing the Paleozoic evolution of the Austroalpine Domain.
Within this regional framework, a small area of about 25 km² near Reschen Pass (South Tyrol, NE Italy) is of particular interest, as it hosts potential pre-Variscan geological features that escaped younger metamorphism and deformation. These include the Klopaier Pluton, its contact metamorphic aureole, and associated migmatites, all enclosed within country gneiss characterized by Variscan mineral assemblages and fabrics. The coexistence of these elements offers is key to investigate the timing and relationships between magmatism, metamorphism, and deformation during the early Paleozoic.
New detailed field mapping indicates that the Klopaier Pluton is largely undeformed, locally behaving as a rigid body and locally preserving primary intrusive contacts. Relicts of cordierite-bearing assemblages are locally found at the pluton margins and may represent remnants of a contact metamorphic aureole. The surrounding migmatites, predominantly metatexites, are interpreted as the result of syn-intrusion partial melting of the host rocks, later affected by Variscan metamorphism. Pegmatitic dikes are widespread both within the pluton and in the surrounding country rocks. These dikes are likely related to highly evolved fluid-rich melts derived from the main body of the Klopaier Pluton and have previously yielded U–Pb ages between ca. 490 and 413 Ma. These ages suggest that the observed structural configuration was already established during the Ordovician. New U–Pb zircon dating of the Klopaier Tonalite constrains its emplacement to approximately 460 Ma, in good agreement with the ages obtained from the associated pegmatites.
Despite these constraints, the relative timing of pluton emplacement and migmatite formation remains unresolved, posing a classic geological “chicken-or-egg” problem: did the pluton intrude into pre-existing, already cooled migmatites, or did its emplacement and associated heat supply trigger partial melting in the surrounding rocks? To address this question, new U–Pb zircon and monazite ages from the migmatites, combined with their geochemical characterization, indicate crystallization and melt-related processes between ca. 460 and 450 Ma. These new data provide crucial constraints on the early Paleozoic tectonometamorphic evolution of the Austroalpine Domain and contribute to a better understanding of pre-Alpine crustal processes in the Eastern Alps.
How to cite: Zanchetta, S., Favaro, S., Toffolo, L., Minopoli, L., Piccin, S., Poli, S., and Tumiati, S.: New U-Pb zircon and monazite ages on Ordovician magmatism and migmatization in the Paleozoic basement of the Eastern Alps (Ötztal Nappe, N Italy), EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-20881, https://doi.org/10.5194/egusphere-egu26-20881, 2026.
Massif-type anorthosites are one of the few rock-types on Earth that are restricted in time, occurring exclusively in the Proterozoic Eon. Understanding their unique quasi-monomineralic composition and temporal restrictions challenges current petrological and geodynamic models. It is generally accepted that mechanical accumulation of plagioclase from basaltic precursors begins at the Moho, but details regarding the crystallinity of ascending plagioclase-rich magmas and final level of emplacement remain uncertain. A major challenge is estimating the pressure conditions at which anorthosite is emplaced due to the absence of necessary phases. This uncertainty is particularly evident in the Kunene Complex (KC), the largest Proterozoic massif-type anorthosite on Earth (1.50-1.36 Ga), where pyroxene and amphibole thermobarometry suggest that the southern segment was emplaced at 7-9 kbar and the northern segment at 3-5 kbar. It is unclear if these differences are geological and/or methodological.
An alternative approach for determining emplacement depth of magmatic bodies involves examining the metamorphic conditions of magma-host rock interaction in the contact metamorphic aureoles. In this study, we focus on Paleoproterozoic (1.88-1.82 Ga) nebulite and stromatite migmatitic supracrustal rocks located at the margin of the northern KC anorthosite pluton (1.384-1.375 Ga). The stromatic foliation dips towards the pluton and is characterised by leucosomes of quartz, K-feldspar, plagioclase, cordierite, garnet, and sillimanite, while the mesosome is richer in cordierite, garnet and biotite. The presence of common, transgressive, and discontinuous nebulitic migmatite of similar mineralogy and in gradational contact with the stromatite attests to the segregation of partial melt outlasting the formation of the stromatic foliation. Mineral equilibria modelling indicates that the migmatite formed at P-T conditions of 4 kbar and 730°C.
In-situ monazite ages in nebulite as well as leucosome and mesosome of the stromatite span 500 million years, from 1.8 to 1.3 Ga. Two combined textural and chemical domains have been identified in monazite, helping to categorise the age data. Domain 1 shows embayed and cuspate BSE-dark grey areas and is Y-rich, has variable Eu/Eu* and Sr content, low Th/U and is typically found in monazite cores. Domain 2 is more common and shows BSE-bright rims or convex-inward mantles around Domain 1 or can encompass the entire grain, and has consistently low Y, Eu/Eu* and Sr content, and higher Th/U. U-Pb ages for Domain 1 cluster at 1.80-1.77 Ga, interpreted as the age of a prograde event, preceding the growth of peritectic garnet and cordierite. U-Pb ages for Domain 2 cluster at 1.41-1.36 Ga and correspond to the co-crystallisation of monazite, feldspar and garnet during the migmatitic event constrained at 4 kbar and 730°C. The age of Domain 2 monazite coincides with the emplacement of the northern KC anorthosite pluton.
Altogether, the fabric analysis, migmatitic metamorphic assemblage, and coeval age of Domain 2 monazite and KC anorthosite indicate that the emplacement of anorthosite caused significant heating, melting and hypersolidus ductile flow of the contact aureole at metamorphic pressures equivalent to a mid-crustal depth of ~15 km. Consequently, this study offers new insights into the length scale of ascent and emplacement levels of massif-type anorthosite magmas.
How to cite: Lehmann, J., Sharma, N. K., Vila, J., Owen-Smith, T. M., Belyanin, G., Bybee, G. M., MacRoberts, R. J., Ferreira, E., Milani, L., Hayes, B., and Elburg, M. A.: Mid-crustal emplacement of the northern Kunene Complex massif-type anorthosite revealed by monazite petrochronology and phase equilibria modelling, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-16538, https://doi.org/10.5194/egusphere-egu26-16538, 2026.
The Mogok Metamorphic Belt (MMB) consists of an interlayered suite of magmatic and metasedimentary rocks that record metamorphic conditions from low grade to upper amphibolite–granulite facies. Banded gneiss dominates the succession with subordinate quartzite and discontinuous marble layers. The Mogok area in the northern part of the MMB is a renowned gemstone tract notable for its world-class ruby, sapphire, and other gemstones. Marbles are major host rocks for best-quality pigeon blood ruby. The metamorphic rocks are associated with alkaline rocks (mostly sodic nepheline–syenite and syenite–pegmatite) which host exceptionally high-quality, royal blue sapphires. Searle et al. (2020) classified the alkaline host rocks as charnockite–syenite intrusions and identified multiple episodes of syenitic magmatism from the Jurassic to the Oligocene (170–168 Ma, ~68–63 Ma, and 44–21 Ma). U–Pb zircon dating of sapphire-bearing nepheline syenite from the Ondan district, approximately 40 km west of Mogok, yielded an age of 57.81 ± 0.5 Ma (Myo Min, 2016, per. com). U-Pb titanite ages from the ruby-bearing marbles and meta-skarns at Le Oo mine in the Mogok valley are 21 Ma, similar to titanite ages from an adjacent syenite (22 Ma) (Searle et al., 2020). Sutherland et al. (2019) considered that the Mogok rubies were formed at 32.4 Ma (U-Pb age of titanite inclusion in ruby) which is older than the 21 Ma U-Pb titanite age from the ruby-bearing marbles. Hence, the Mogok rubies were considered to have formed around 17-35 Ma. In comparison, the timing of the Mogok sapphires is debatable and not resolved yet (Akar Moe Myint et al., 2025 in press). The sapphires are associated with pegmatitic syenites of the Jurassic to Oligocene (Searle et al., 2020). Arkar Moe Myint et al. (2025, in press) dated zircon inclusions in Baw Mar (Mogok) sapphires using LA–ICP–MS U–Pb geochronology, yielding ages of 54.11 ± 1.6 Ma to 65.39 ± 1.77 Ma, which indicate the timing of sapphire formation in the Baw Mar area of Mogok. Precise LA-ICP-MS analysis of ruby and sapphire from Mook placer and in situ deposits reveal V can exceed 5000 ppm (Khin Zaw et al., 2015). Such values significantly exceed those elsewhere and are focused on a specific area, suggesting a geological control on V-rich ruby and sapphire distribution. These findings highlight vanadium as an important tracer for ruby, with vanadium and associated trace-element patterns, together with age dating, providing robust tools for geographic typing and fingerprinting.
References
Akar Moe Myint et al. 2025 (in press). Gem and Gemology.
Khin Zaw et al. 2015. Mineralium Deposita https://doi.org/10.1007/s00126-014-0545-0.
Searle et al. 2020. Tectonics https://doi.org/10.1029/2019TC005998.
Sutherland et al. 2019. Minerals https://doi.org/10.3390/min9010028.
How to cite: Zaw, K. and Moe Myint, A.: Insights into Mogok Ruby–Sapphire Genesis from U–Pb Geochronology and Trace Elements, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-1902, https://doi.org/10.5194/egusphere-egu26-1902, 2026.
Understanding the spatial distributions of peak metamorphic P-T-t conditions helps constrain models of orogenesis. In this study, we quantified P-T conditions in Himachal Pradesh in the NW Indian Himalaya, along the Sutlej River valley and nearby Pabbar valley. From structurally lowest to highest, these rocks consist of the Lesser Himalayan Sequence (LHS), the Munsiari Sequence (MS), the Greater Himalayan Sequence (GHS), and the Tethyan Himalayan Sequence (THS). The Munsiari thrust (MT) emplaces MS over LHS, while the Main Central thrust (MCT) emplaces GHS locally over MS or LHS. In areas along strike, the South Tibetan detachment system (STDS) drops the THS down onto GHS, but the STDS is not noticeably exposed along the Sutlej or Pabbar valleys.
In this study, we calculated P-T conditions using garnet-biotite thermometry and garnet-plagioclase barometry from the upper ~1 km thickness of the MS, through the GHS (~10 km), and into the basal ~9 km of the THS. P-T conditions increase abruptly at the MCT, from ~600 °C and ~9 kbar in the upper MS to ~750 °C and ~12 kbar in the lower 2 km of the GHS (c. 100 °C/km; 1 kbar/km). P-T conditions then increase to ~800 °C and ~14 kbar in the middle of the GHS (c. 15 °C/km; ~0.5 kbar/km), and then decrease consistently to ~500 °C and ~5 kbar in the highest level of the THS analyzed (c. 25 °C/km; 0.75 kbar/km). Upper GHS and THS data are sparsely distributed, so we cannot rule out a metamorphic discontinuity across the STDS. However, our data are equally consistent with flattening strain distributed through the upper GHS and lower THS without a distinct STDS, unlike all other transects in the Himalaya farther east for >1500 km. Previously published U-Pb titanite ages from the THS indicate peak metamorphism until ~22 Ma, while new zircon ages from a GHS migmatite indicate rapid cooling since ~21 Ma, similar to many transects to the east. Additional geochronology is needed to delineate prograde and high-temperature cooling patterns across the GHS and THS.
How to cite: Wuertemburg, A., Kohn, M. J., Robinson, D. M., Corrie, S. L., and Long, S. P.: Thermobarometry and geochronology of the Himachal Himalaya, NW India: Inverted metamorphism along the Sutlej and Pabbar Valleys, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-14631, https://doi.org/10.5194/egusphere-egu26-14631, 2026.
The Paleoproterozoic Sandmata Complex (SC) of the Aravalli Craton (NW India) is traditionally regarded as a reworked Archean crustal terrane. It also preserves exhumed granulite-facies rocks that record high-grade metamorphic events. In the westernmost part of the complex, the dominant lithology comprises garnet-bearing, migmatitic quartzo-feldspathic gneisses (MS30A) that host mafic microgranular enclaves (MME; MS30B) and leucosomes (MS30C). This study presents new insights based on integrated structural, metamorphic, geochemical, and geochronological analyses of rocks from the western Sandmata Complex. The migmatitic gneisses preserve evidence for at least two deformation events (D1 and D2). The early fabric (S1) is a centimetre-scale, gently dipping gneissic foliation, which is overprinted by a spaced, NNE-striking, steeply dipping axial-planar foliation (S2). The peak metamorphic assemblage in the migmatitic gneisses consists of Grt(core) + Pl2 + Kfs + Qz + Amp + Ilm + Ttn ± Bt1 ± Mt. In contrast, the MMEs record the peak assemblage Grt(core) + Pl2 + Kfs + Amp1 + Ep2 + Ttn2 + Qz + Ilm ± Mt, whereas the leucosomes contain Grt(core) + Pl2 + Qz + Amp1 + Ep2 + Ttn2 + Ilm ± Mt. The MME and leucosomes are characterized by symplectitic coronas around garnet, defined by Grt-rim + Pl3 + Qz + Amp2 + Ep3 + Ttn3 ± Mt ± Ilm ± Bt. Pseudosection modelling constrains peak metamorphic conditions at ~9.7 kbar and ~820 °C. The symplectite assemblages reflect post-peak re-equilibration at ~7.6 kbar and 580–610 °C. Prograde conditions, estimated using the melt-reintegration approach, indicate temperatures of ~600 °C at ~10.3 kbar. Together, the prograde, peak, and retrograde P–T estimates define a clockwise P–T path involving isobaric heating to peak conditions, followed by cooling and decompression. U–Pb dating of magmatic zircon cores from the migmatitic gneiss yields concordant age of 1720 ± 13 Ma (MSWD = 0.97; n = 15), which are interpreted as the crystallization ages of the parental magmatic protolith. In contrast, U–Pb analyses of titanite associated with the peak metamorphic assemblages produce a discordant age of 969 ± 7 Ma (MSWD = 1.7; n = 49). This younger age constrains the timing of partial melting and migmatization of the gneisses. These results provide the first robust evidence for ~0.97 Ga high-grade metamorphism in the Sandmata Complex, a metamorphic event not previously documented in this terrain. Combined with earlier records of 1.78–1.90 Ga granulite-facies metamorphism, the data indicate that the SC experienced two distinct high-grade metamorphic episodes during its geological history. Regionally, the ~0.95–1.0 Ga event corresponds to a major tectonothermal phase associated with the accretion and amalgamation of the Aravalli Orogen and the Central Indian Tectonic Zone. This younger metamorphic overprint, thus, reflects the development of a high-grade orogenic belt during the assembly of Rodinia, linking the evolution of the Sandmata Complex to broader Neoproterozoic continental-scale processes.
How to cite: Sahu, M., Dev, A., Naraga, P., and J Kallukalam, T.: Early Neoproterozoic (~0.95–1.0 Ga) HT-HP Metamorphism in the Sandmata Complex and Its Implications for Rodinia Assembly in the Aravalli Craton (NW India), EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-408, https://doi.org/10.5194/egusphere-egu26-408, 2026.
Although numerous studies have been conducted along the Mogok Metamorphic Belt (MMB), granulite facies and ultrahigh temperature (UHT) metamorphism have only been inferred in parts of the belt. This study documents typical UHT metamorphism in Momeik area, northeastern MMB, and its associated diagnostic mineral assemblages: (1) the Spl + Qz assemblage in khondalite (medium-grained garnet-sillimanite gneiss) formed by the reaction Grt + Sil = Spl + Qz, in which a partition coefficient between Grt and Spl shows 0.02 as quite good equivalent with the experimental result of Bohlen et al. (1986), and (2) Grt + Opx + Crd + Sil assemblage in sillimanite-garnet-orthopyroxene-cordierite granulite (GOC granulite) from melanosome layer intercalated with heterogeneous stromatic metatexite migmatite. The GOC granulite is very coarse- grained rocks in which garnet formed during prograde metamorphism through reactions involving Sil+Qz+Bt. At peak UHT metamorphism, Opx developed via biotite dehydration melting under strongly anhydrous conditions. During subsequent decompression, garnet became unstable and was replaced by symplectitic coronas composed of Opx, Crd, and Spl. The mineral assemblages and reaction textures surrounding garnet record multiple stages of deformation and metamorphism.
Mineral chemical characteristics, including Fe-rich garnet, Opx-Crd, Crd-Spl and Opx-Spl symplectites, Grt-Qz reaction textures, and high XFe values (up to 0.17) in residual F-rich biotite, indicate consistent peak UHT metamorphic conditions of ~6.5 ± 1.5 kbar and 900-1000℃. LA-ICP-MS U-Pb zircon data show detrital age populations of ~60-3100 Ma in khondalite, with metamorphic zircon growth at 26.56 ± 0.76 Ma, whereas GOC granulite record zircon ages of ~30-40 Ma. These results constrain Late Eocene to Early Oligocene prograde burial metamorphism in the Momeik area, subsequently overprinted by localized Oligocene UHT metamorphism during decompression and ductile extension, synchronous with collision-induced extrusion and slab-remnant thermal input during Indian-Asian collision.
Key words: Momeik; ultra-high-temperature metamorphism (UHT); Khondalite (garnet-sillimanite gneiss); sillimanite-garnet-orthopyroxene-cordierite granulite (GOC granulite); reaction textures
How to cite: Htay, K. N., Osanai, Y., Aung, L. T., Nakano, N., and Adachi, T.: Ultra-high Temperature (UHT) Metamorphism in the Momeik Area, Mogok Metamorphic Belt, Myanmar, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-17518, https://doi.org/10.5194/egusphere-egu26-17518, 2026.
Posters virtual: Thu, 7 May, 14:00–18:00 | vPoster spot 3
EGU26-4872 | Posters virtual | VPS25
Metamorphism records microcontinent accretion and subduction relocation: an example from early Paleozoic Qilian Orogenic Belt, NW ChinaThu, 07 May, 14:33–14:36 (CEST) vPoster spot 3