Mafic dykes can preserve near-peak metamorphic assemblages in granulitic terranes and often are key to deciphering their pressure-temperature-time-deformation (P-T-t-D) history. They are particularly important in high-grade polymetamorphic terranes, where younger metamorphic events may be poorly recorded in other units. The middle to lower crustal units of the Grenville Front Tectonic Zone (GFTZ) in western Québec, Canada, present a unique opportunity to address this issue, having experienced two metamorphic events separated by more than 1.6 billion years. The GFTZ exposes parautochthonous restitic orthopyroxene + garnet-bearing paragneisses and associated two-micas pegmatite dykes. These were formed in the Superior Craton during c. 2.6 Ga granulite-facies metamorphism (M1) and affected by a loosely constrained c. 1.0 Ga overprint during the Grenvillian orogeny (M2). Following M1 and prior to M2, the GFTZ have been intruded by Proterozoic gabbro dykes that can be used to monitor the conditions and timing of M2. In this contribution, we present new field relationships, whole rock geochemistry, in situ laser ablation U-Pb titanite and Lu-Hf garnet geochronology, along with isochemical phase equilibria modeling to provide quantitative P-T-t-D estimates of the metamorphism preserved in the mafic dykes.
Field evidence shows that ~10 m thick metagabbro dykes cross-cut foliated paragneiss and pegmatite dykes. Their immobile element geochemical signatures are consistent with those from regionally recognized Proterozoic mafic dyke swarms intruding the Superior Craton. Metagabbros are characterized by an assemblage of plagioclase + hornblende + clinopyroxene + garnet + orthopyroxene + quartz + titanite. Garnet coronas surrounding relict magmatic clinopyroxene in contact with plagioclase are common in dyke cores, contrasting with granoblastic and migmatitic assemblages in dyke margins. Five metagabbro samples returned in situ Lu-Hf garnet dates in the range of 1124 to 920 Ma and U-Pb titanite dates from 1022 to 985 Ma, interpreted as the timing of M2 metamorphism and consistent with documented Grenvillian metamorphism. Isochemical phase equilibria modeling of a granoblastic hornblende + plagioclase + clinopyroxene + quartz + garnet migmatitic assemblage in a dyke margin indicate equilibrium conditions of 833 ± 12 °C and 9.9 ± 0.4 kbar, corresponding to mid- to high-pressure granulite conditions.
Our results confirm a c. 1.0 Ga granulite-facies overprint attributed to the Grenvillian Orogen (M2). It is noteworthy that the high-grade assemblage is only expressed in the Proterozoic mafic dykes and that the host migmatitic paragneiss did not pervasively recrystallize during M2, potentially due to its restitic nature. This contrast in lithological reactivity resulted in a differential metamorphic record, emphasizing that the Grenvillian granulitic overprint could easily be overlooked if metagabbros are neglected. In conclusion, the results presented herein underscore the critical role of metamorphosed mafic dykes in disentangling the complex evolution of polymetamorphic granulitic terranes.
How to cite: Darveau, J., Guilmette, C., Godet, A., Jouvent, M., Côté-Roberge, M., and Larson, K.: Learning from mafic dykes in polymetamorphosed granulite terranes: an example from the Grenville Front Tectonic Zone, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-14676, https://doi.org/10.5194/egusphere-egu25-14676, 2025.
Where, when, and why large-scale shear zones nucleate and propagate into the continental lithosphere are critical issues that challenge the research in tectonics. The East Variscan shear zone is one of the crustal-scale strike-slip faults that shaped the Variscan orogenic crust during late Carboniferous time. Field-based structural analysis and petrological observations demonstrate that suprasolidus high-strain deformation zones and metagranite occurrences are spatially correlated. Among the three dominant lithologies forming this orogenic middle crust (metapelite, metagraywacke, and metagranite), petrological observations and phase equilibrium modeling indicate that the latter is the first lithology that melts during collision-induced heating, in response to H2O-fluid-saturated melting. Our field data and modeling suggest that the water-fluxed melting of metagranite has a primary rheological control on the localization, instigation, and growth of crustal-scale shear zones in the middle crust. Thus, the distribution and geometry of metagranite at the crustal scale could be regarded as critical parameters influencing the rheological inheritance governing the tectonic evolution and localization of bulk strain in the continental lithosphere.
How to cite: Vanardois, J., Trap, P., and Marquer, D.: Crucial role of water-present melting in metagranite: Implicationsfor the instigation of crustal-scale shear zones, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-2492, https://doi.org/10.5194/egusphere-egu25-2492, 2025.
Monazite, a key accessory mineral in high-grade metamorphic rocks, serves as both a robust geochronometer and a primary reservoir for rare earth elements (REE), Th, and U. This study investigates melt inclusions within monazite and garnet to elucidate the processes of crustal melting and element partitioning during granulite facies metamorphism in the Bohemian Massif.
Nanogranitoid inclusions, identified through Raman spectroscopy and electron microscopy, exhibit polycrystalline textures containing quartz and feldspar polymorphs, micas, and occasionally carbonate phases. These inclusions are found in chemically distinct domains of monazite and garnet, offering valuable insights into the interplay between melt entrapment, mineral growth, and metamorphic conditions. Compositional zoning in garnet, characterized by variations in major and trace elements, alongside the corresponding domains in monazite, provides a detailed record of the pressure-temperature (P–T) trajectory during high-grade metamorphism.
Our findings reveal two generations of garnet, each showing distinct chemical zoning and closely linked to monazite inclusions with different ages. Monazite grains dated to ~370 Ma are associated with the first garnet generation (Garnet1), while monazite grains dated to ~340 Ma are associated with the second garnet generation (Garnet2). These monazite inclusions, along with matrix-hosted monazite grains, display contrasting REE and Th/U patterns, reflecting diverse growth conditions and the impact of garnet breakdown during decompression.
The systematic investigation of these inclusions, alongside their textural and chemical context, enhances our understanding of monazite stability in melt-bearing systems and its role in recording the temporal evolution of crustal melting processes. This integrative approach establishes a robust framework for deciphering the intricate relationships between mineral chemistry, melt inclusions, and P–T paths. These findings underscore the critical role of accessory minerals like monazite as indispensable archives of crustal evolution.
How to cite: Sorger, D., Hauzenberger, C. A., Finger, F., and Linner, M.: Tales of Monazite, Garnet, and Melt: Linking Mineral Growth and Partial Melting to P–T Evolution, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-8442, https://doi.org/10.5194/egusphere-egu25-8442, 2025.
In zircon, transgressive textures across the oscillatory zoning and associated chemical modifications were mostly attributed to the process of fluid-mediated coupled dissolution-precipitation (CDP) over the past c. 20 years. Some works also admitted a possible role of melt in zircon CDP, expressing it for example by “fluid/melt”, but usually without explicit documentation of melt presence. The studies of melt-mediated zircon modification by CDP are thus extremely rare, and show an important but underrated process, with consequences on geological interpretation of zircon ages and chemical composition.
We choose one of the most common crustal rock types, a meta-granite, which undergone migmatization, and focused on modification of zircon textures and chemistry. To unravel primary and secondary zircon textures we use combination of high-resolution cathodoluminescence (CL), back-scattered electron (BSE) and secondary-electron (SE) images, because in CL most of the secondary textures were not visible. To relate zircon texture and date with trace- and rare earth element (REE) composition we use laser ablation–split-stream inductively coupled plasma–mass spectrometry (LASS). We relate the mineral inclusions with zircon textures, and for the secondary metamorphic inclusions we compare their assemblage and mineral chemistry with the rock assemblage and infer P–T conditions of their formation. Because the textures are typical of coupled dissolution-precipitation process and the P–T conditions of the secondary inclusions and matrix assemblage are above the wet solidus, we interpret zircon modification as caused by melt-mediated coupled dissolution-precipitation process.
Oscillatory zoning is commonly blurred to a variable degree and it is in places truncated by patchy, convolute or structureless embayments. The irregular and relatively sharp boundaries of the embayments may be spatially associated with micro-porosity located ahead, and are interpreted as modification fronts of dissolution-precipitation. Some modification fronts are superimposed and relative timing can be inferred. Micro-porosity and inclusions are arranged in trails along modified BSE-light grey channels and are spatially associated with depressions at the surface, indicating more pronounced dissolution over precipitation. Larger inclusions tend to be located at the joints of the channels. The metamorphic zircon domains are characterized by an overall decrease of HREE with large variation in Yb/Gd, increase or decrease in LREE, increase in U, and decrease of Th and Th/U, compatible with presence of melt, garnet and titanite. Inclusions of Ph−Grt−Ttn are compatible with the matrix assemblage of Grt−Ph−Bt−Ttn−Kfs−Pl−Qz±Rt±Ilm, and equilibrated at eclogite-facies, at 15−17 kbar and 690–740 °C. The melt-mediated zircon modification resulted in a smear of mostly concordant dates from protolith oscillatory zoned domains with Cambro-Ordovician age to c. 330 Ma.
How to cite: Stipska, P., Kylander-Clark, A., Racek, M., Zavada, P., and Hasalova, P.: Melt-assisted coupled dissolution-precipitation of zircon in metagranite, Bohemian Massif, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-18130, https://doi.org/10.5194/egusphere-egu25-18130, 2025.
The Caucasus is located at the convergence of the Eurasian and African-Arabian tectonic plates and represents a link between the European and Asian components of the Mediterranean (Alpine-Himalayan) collisional mobile belt. This region offers valuable insights into understanding collision tectonics and high-grade metamorphism. The presented study aims at investigating the high-grade metamorphism and crustal melting processes in the Greater Caucasian Main Range zone, which is subdivided into the Pass and the Elbrus sub-zones. They differ from each other in terms of geological structures, lithologies, metamorphism and magmatism. Migmatization and partial melting in the Caucasus refers to Cadomian (626±2 Ma) and Caledonian (461±5.3, 468±5, 471.7±4.6 Ma) stages of high-temperature regional metamorphism of Elbrus subzone infrastructure. The final, low-temperature stage of regional metamorphism occurred during the Variscan orogeny. The Elbrus subzone represents a migmatitic complex and its study is a key for understanding high-temperature metamorphic and anatectic processes in this region. This study combines geochronological data with detail mineralogical and textural analyses of migmatites from the Elbrus sub-zone to determine the conditions and mechanisms of partial melting and to shed light on the relationships between crustal anatexis and tectonics. The migmatites collected in the River Nenskra valley (Upper Svaneti region) are mostly fine-to medium-grained stromatic metatexites, characterized by light bands composed predominantly of quartz, potassium feldspar and plagioclase, and dark bands with biotite, garnet and sillimanite. Leucosomes contain euhedral feldspar crystals and thin quartz-feldspar films along grain boundaries, which can be interpreted as microstructures indicating the presence of former melt. Garnet crystals are often characterized by numerous tiny inclusions, giving them a cloudy appearance. MicroRaman investigation reveals the presence of cristobalite, graphite, phlogopite, biotite, chamosite, carbonate, CH4 and N2 in the vast majority of these inclusions, which are interpreted as former fluid inclusions and not as nanogranitoids. SEM analysis results show enrichments of Mn at the rims of some garnet crystals, which are related to local resorptions of the garnet crystals and replacement by biotite. In the highest grade samples muscovite is rare, displays a skeletal shape, is not in contact with quartz and is adjacent to crystals of K-feldspar and sillimanite. These observations suggest that partial melting conditions exceeded the stability of Ms+Qz and reached the stability of Kfs+Sil. In addition, inclusions of green spinel have been observed in some sillimanite clots, which represent the product of staurolite decomposition. These are the only microstructures that provide constraints on the pre-anatectic history of these migmatites. The proposed conditions of regional metamorphism suggest temperatures and pressures corresponding to upper amphibolite and up to granulite facies. These conditions support the occurrence of partial melting processes, followed by slow cooling. To further refine the understanding, new microstructural, microchemical, and geochronological data will be presented, providing deeper insights into the petrogenesis of migmatites in the Greater Caucasus. This work will also contribute to understanding the thermal regime of regional high-grade metamorphism and melting in other metamorphic structural zones of the Caucasus.
ACKNOWLEDGEMENTS: This work was supported by Shota Rustaveli National Science Foundation of Georgia (SRNSFG) [FR-22-11295].
How to cite: Tsutsunava, T., Javakhishvili, I., Cesare, B., Bartoli, O., Shengelia, D., Chichinadze, G., and Beridze, G.: New Insights on Partial Melting and Migmatization in the Greater Caucasus Main Range Zone , EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-6482, https://doi.org/10.5194/egusphere-egu25-6482, 2025.
A lot of emphasis has been given lately to SiO2 polymorphs in metamorphic rocks. A development of quartz-in-garnet elastic barometry and Ti-in-quartz thermometry refocused attention of metamorphic petrologists on this chemically simple and ubiquitous mineral in virtually all types of metamorphic rocks. A need for chemical equilibrium independent thermobarometric methods and a growing evidence for mineral reaction overstepping promoted in-depth studies of quartz behavior as inclusion in stiffer phases such as garnet and associated strain and stress development as well as fostered common usage of trace element thermometers. On the other hand, less common coesite became the primary target phase in metamorphic studies tackling a problem of deep subduction of Earth’s crust to mantle depths. A common routine to identify the latter mineral is to look for specific microtextures such as radial cracks around coesite inclusions in other minerals and/or characteristic pseudomorphs such as polycrystalline and palisade quartz. Subsequent confirmation of the presence of coesite with Raman spectroscopy and/or electron backscattered diffraction (EBSD) is needed. Therefore, we decided to look for (a) yet another way of quick identification of coesite, and (b) potential development of an alternative geothermobarometric technique applied to quartz and coesite. Here we report preliminary results obtained using acoustic microscopy, a vastly unknown technique in petrological community. Our preliminary tests on quartz monocrystals show that acoustic wave velocity depends on quartz orientation. However, the test on coesite and palisade quartz from Dora Maira shows that coesite reveals faster wave velocity than quartz regardless the crystallographic orientation. The latter is, in fact, not surprising since the method in question is primarily dependent on a density and elastic properties of the tested material. Nonetheless, to our knowledge an empirical test of this kind has not been done before. Thus, we can preliminarily conclude that the acoustic microscopy can be used as an alternative tool to quickly identify coesite. A development of a new thermobarometer would require a careful EBSD pre-study though. This obstacle together with limited access to acoustic microscopes in general may hinder this process. On the other hand, a use of acoustic impedance to image either growth and/or deformation zones in minerals or specific microtextures of multimineral systems appears to be especially advantageous.
Supported by the National Science Centre (Poland) grant no. 2021/43/D/ST10/02305.
How to cite: Majka, J., Potočný, T., Litniewski, J., Stepinski, T., Włodek, A., and Kośmińska, K.: Acoustic microscopy of quartz and coesite: yet another way to look at old good metamorphic fellas, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-8082, https://doi.org/10.5194/egusphere-egu25-8082, 2025.
Sulfur is transferred into the overlying mantle wedge during subduction of oceanic plates, with important ramifications for magmatic processes in volcanic arcs. Many studies have shown that arc magmas are more oxidized and enriched in 34S than mid-ocean ridge basalts, which has been linked to the transfer of slab-derived volatiles, such as sulfate, into the overlying mantle wedge and into arc magmas. However, the transfer mechanisms and the oxidation states of slab fluids is still under debate, and particularly the role of sulfur is currently widely discussed. Here we present bulk rock and in situ sulfur isotope results from metasomatic, eclogitic metagabbros from the Voltri Massif in Italy that are in contact with serpentinites (Schwarzenbach et al., 2024). Previous petrological work of this contact showed that metasomatism by fluid-mediated mass transfer of Mg from the serpentinite into the metagabbro caused formation of actinolite-chlorite schists and metagabbro rich in epidote and Na- and Na-Ca amphiboles along the contact (Codillo et al., 2022). Abundant euhedral to subhedral pyrite in the metasomatized metagabbro show distinct correlations between in situ sulfur isotope analyses and sharp Co and Ni growth zones documenting multiple generations of pyrite formation. In particular, a trend of increasing δ34S values from core to rim in pyrite associated with inclusions of distinct high pressure silicate minerals documents the input of 34S-enriched sulfur during metasomatism of the eclogitic metagabbros concurrent to subduction metamorphism. Using thermodynamic modeling our study shows that the infiltrating fluids equilibrated with the serpentinite before entering the metagabbro and that these fluids were HS--bearing. Infiltration of these HS--bearing and 34S-enriched fluids led to redox reactions in the Fe-Ti metagabbro involving sulfur, iron, and likely carbon, and the formation of euhedral pyrite with δ34S values of up to 12.5‰ in the metasomatized metagabbro. We argue that this process of 34S-enriched sulfur mobilization from serpentinites is pervasive along the plate interface in subduction zones and infer that melting of such metasomatic material can explain the 34S-enriched signatures observed in arc magmas.
References:
A. Codillo et al., Fluid-Mediated Mass Transfer Between Mafic and Ultramafic Rocks in Subduction Zones. Geochemistry, Geophysics, Geosystems 23, e2021GC010206 (2022).
M. Schwarzenbach et al., Mobilization of isotopically heavy sulfur during serpentinite subduction. Science Advances 10, eadn0641 (2024).
How to cite: Schwarzenbach, E., Dragovic, B., Codillo, E., Streicher, L., Scicchitano, M., Wiechert, U., Klein, F., Marschall, H., and Scambelluri, M.: Sulfur transfer during subduction of serpentinite: insights from the Voltri Massif, Italy, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-2498, https://doi.org/10.5194/egusphere-egu25-2498, 2025.
Fluid–rock interactions play a key role in the formation, evolution and recycling of the Earth’s crust. For fluids to infiltrate rocks and enable and sustain fluid-mediated mineral transformations, fluid pathways are required. Time scales of fluid-mediated rock transformations cover a wide range from several million years to several month in various geological settings. In this study, we examined the underlying mechanisms and timescales of amphibolitization of mafic crust. For this purpose we performed a detailed mineralogical, petrophysical and thermodynamic analysis of a dry, essentially “non-porous” gabbro that was hydrated and transformed into an amphibolite under amphibolite-facies conditions. The amphibolitization process was triggered by fluid infiltration through a newly opened N–S striking fracture network and allowed the fluid to pervasively infiltrate the rock. Thermodynamic modelling and petrological data show that the transition from gabbro to amphibolite was accompanied by densification and related porosity formation. The modes and compositions of minerals within partly-amphibolitized rocks indicate that besides the uptake of H2O, no significant mass exchanges were necessary for this transformation, at least on the thin-section scale. Once the gabbro was almost entirely amphibolitized, its mineral content and mineral chemistry no longer changed, so the progress of amphibolitization progress was controlled by fluid availability. To estimate the duration of the amphibolitization we set up a reactive transport model based on local equilibrium thermodynamics, mass balance and Darcy flow, which addresses the mineralogical and petrophysical changes of the rock along the sampled profile at constant ambient amphibolite-facies P–T conditions. Starting from a non-porous rock, the model calculated reaction-induced porosity, permeability, and fluid pressure evolution based on the local bulk composition and the evolving mineral paragenesis. We reproduced the extend of the reaction front by adjusting fitting parameters such as, initial fluid pressure, permeability or fluid viscosity. We repeated these calculations for different reaction front widths we measured in the outcrop to obtain a time estimate of the hydration process that resulted in the amphibolitization of the gabbroic crust.
How to cite: John, T., Grund, S., and Vrijmoed, J.: Amphibolitization of mafic crust: mechanism and time scales, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-18348, https://doi.org/10.5194/egusphere-egu25-18348, 2025.
Raman Spectroscopy of Carbonaceous Matter (RSCM) thermometry is a widely utilized method for estimating peak metamorphic temperatures based on the crystallinity and composition of carbonaceous matter (CM). In this study, we apply RSCM thermometry to samples from the contact aureole of the Torres del Paine Intrusive Complex (TPIC) and compare these temperature estimates with constraints from phase petrology and thermal modeling. While Raman spectra reveal a systematic increase in CM crystallinity toward the intrusion, peak temperatures estimated with RSCM are consistently lower than the modeled temperatures in the outer and middle part of the aureole, while both approaches give consistent temperatures close to the contact.
2-D thermal models suggest that the heating of the metasediments occurred over 2,000–10,000 years, while cooling occurs over some tens of thousands of years, depending on proximity to the intrusion, as well as the relative position of the sample with respect the intrusion (roof, side, or below). A review of contact metamorphic studies conducted around different sizes of intrusions underscores the critical role of the heating pulse duration. Hence, RSCM thermometry needs to take into account the thermal history, as was similarly shown for sedimentary basins and Anchizonal metamorphism (Sweeney and Burnham, 1990).
To address this limitation, we extend a first-order kinetic model originally developed for vitrinite reflectance by Sweeney and Burnham (1990) to describe CM crystallinity evolution by considering a total set of 34 parallel reactions that includes 14 new reactions with higher activation energies to account for the reactions that dominate maturation at higher temperatures, responsible for transforming amorphous CM into graphite. Our new kinetic model was calibrated by assuming that the maturations described by the thermometer for regional metamorphism were obtained by maintaining the temperature at the peak metamorphic temperature for at least 1Myr. Using this approach we find a better fit for maximum temperatures obtained in the Torres del Paine contact aureole. Our results highlight the importance of kinetic effects for RSCM thermometry. Hence, we strongly advise against the use of the RSCM thermometer for short-lived thermal pulses, unless an approximate knowledge of the temperature-time history is known.
References
Sweeney, J.J. and Burnham, A.K. (1990) ‘Evaluation of a Simple Model of Vitrinite Reflectance Based on Chemical Kinetics’, AAPG Bulletin, 74(10), pp. 1559–1570. Available at: https://doi.org/10.1306/0C9B251F-1710-11D7-8645000102C1865D.
How to cite: Ariza Acero, M. M., Baumgartner, L. P., and Foster, C. T.: The kinetics of organic maturation in the Torres del Paine contact aureole and lessons for RSCM thermometry, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-18266, https://doi.org/10.5194/egusphere-egu25-18266, 2025.
The Eastern Ghats Belt (EGB), along the eastern coast of the Indian peninsula, forms a key crustal element in reconstructing the Palaeoproterozoic and Neoproterozoic supercontinents, Columbia (~1.9-1.6 Ga) and Rodinia (~1.1-0.9 Ga), respectively. The connection of the south-eastern segment of the EGB with east Antarctica, Australia, and Laurentia in an accretionary orogeny during the assembly of the Columbia has been established. Additionally, the central and western segments of the EGB are believed to have accreted to the Rayner Complex of Western Australia and East Antarctica during the assembly of the Rodinia supercontinent.
In this contribution, we constrain the Proterozoic tectonothermal events recorded from the khondalites and granitoids occurring within the northernmost crustal segment of the EGB, as the major lithodemic units. In-situ monazite analysis from poly-metamorphosed garnet-sillimanite-biotite-quartz-feldspar bearing khondalites helps to establish two major metamorphic events: M1KH and M2KH. The earliest anatectic event (M1KH) was recorded at 1.1 Ga leading to the formation of peritectic garnet, potash felspar, and melt. A collisional tectonic setting is implied by a clockwise P-T path constrained for the mid-crustal partial melting event (M1KH; ~8-9 kbar, >760°C). The second metamorphic event (M2KH) occurred in a melt-absent solid-state condition (~7.8 Kbar, 675°C). The M2KH event occurred along an anticlockwise P-T trajectory with isobaric cooling. The monazite age ranges from 922±17 Ma and 909±8 Ma correlating with the isobaric cooling of the khondalites.
The granitoid bodies associated with the khondalites preserve evidence of multiple phases of deformation. Some of these granitoids are migmatites. Zircons from one of the samples of the undeformed granitoids yield a crystallization age of ca. 1700 Ma. The monazite ages yielded from the granitoids show crystallization ages between ~1041-997 Ma. Monazites from granitoids produce a second peak at ~934-884 Ma, indicating the litho units experienced a partial melting event which is correlated with Rodinia supercontinent formation when the belt was a part of the Rayner Complex. The creation of the extensional Mahanadi Shear Zone is linked to the third monazite population within the granitoids, which yields ages ranging from approximately 750 to 740 Ma.
The present study implies that signatures of both Columbia and Rodinia ages are stronger in the northernmost segment of the EGB. The Imprint of a 1700 Ma event significantly impacts our comprehension of the crustal domains of the EGB that formed during the accretionary orogeny as part of the Columbia assembly. A linkage of significant accretionary belts around preexisting cratons involving Laurentia, Antarctica, South Africa, and Australia can be established with the 1700 Ma events in the northern EGB. The entire EGB from the north to south thus can be added to a hypothetical correlation of orogenic belts in several continents that underwent orogenesis between 1800 and 1700 Ma, reflecting Columbia's expansion. The findings in this study further imply that all the crustal domains of the EGB were unequivocally part of the accretionary orogenies leading to the assembly of the Rodinia.
How to cite: Behera, S. R. and Saha, L.: Tectonothermal evolution of the northernmost crustal segment of the Eastern Ghats Belt, India, and its linkage to Columbia and Rodinia assembly, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-595, https://doi.org/10.5194/egusphere-egu25-595, 2025.
HP (High-pressure)/HT( high temperature) granulites can be considered as part of the thickened lower crust in overrding plate and may provide an important information for thermal evolutions of convergent plate margins. The Qilian orogenic belt is a typical early Paleozoic accretionary orogenic belt. The Qilian block, which is mainly composed of Precambrian basement, is located in the southern side of the North Qilian suture. In recent year, HP/HT granulites, subjects of this contribution, are identified on the northern margin of the Qilian block. Petrographic observations identified three-stage metamorphic assemblages for Grt–Cpx granulites: the first generation (M1) is Grt + Amp + Pl + Kfs + Ttn + Bt + Qz + Liq; the second generation (M2) is Grt + Pl + Amp + Cpx + Ttn + Bt + Qz + Liq; the final assemblage (M3) is Grt + Pl + Amp + Cpx + Ilm + Bt + Qz + Liq. Four stage metamorphic assemblages are identified from pelitic granulite: the first generation (M1) is Grt + St + Pl + Rt + Bt + Qz + Liq; the second generation (M2) is Grt + Ky + Pl + Bt + Qz + Liq; the third generation (M3) is represented by the occurrence of silimanite, rutile and ilmenite within plagioclase in matrix and the Bt + Qz + Sil + Crd + Pl + Ilm symplectites around the rim of garnet;the final generation (M4) is Grt + Crd + Bt + Pl + Qz + Ilm + Liq.
Zr-in-rutile thermometry, Zr-in-titanite thermometry and phase equilibria are applied for evaluating the P–T history and in situ U-Pb datings are used to define the timings of different metamorphic stages. The results indicate that the Pmax condition of Grt–Cpx granulite is at 12.8-13.7 kbar/ 735-760 °C and Tmax condition at 7.5-9.6 kbar/770-845 °C. The Pmax condition of pelitic granulite is at ~12 kbar/ 750-800 °C and Tmax condition at 5.2-6.8 kbar/780-840 °C. Both rocks show a similar decompressional heating P-T path. In-situ U-Pb datings (rutile, monanite, titanite, and zircon) indicate that the timing of prograde stage is at 500~470 Ma and decompressional heating stage is at 460~450 Ma. Our new data combined with the regional geological data show the early Paleozoic HP/HT metamorphism is related to terrane accretion and the southward subduction of Paleo-Qilian Ocean (Proto-Tethyan Ocean) during the early Paleozoic era.
How to cite: Mao, X. and Zhang, J.: Early Paleozoic HP/HT metamorphism on the northern Qilian block: insight on thermal evolution of the overrding plate in convergent margins, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-15506, https://doi.org/10.5194/egusphere-egu25-15506, 2025.
Granulite facies wollastonite-scapolite assemblage is commonly associated with grossular-andradite rich (granditic) garnet. Origin of grandite garnet has been variously attributed to iron metasomatism related to non-pervasive H2O-rich fluid infiltration from adjacent granitic or pelitic sources, or a result of vapour-deficient metamorphic reactions during near isobaric cooling, in addition to the influence of temperature and fO2.
The meta-supracrustal rocks of the Madukkarai Supracrustal Unit (MSU), within the granulite terrane of South India, expose an interlayered platformal sequence of metapelites, marbles, calcsilicates, meta-psammites and quartzites deposited during the Mesoproterozoic and metamorphosed during the. The present study records the occurrence of a wollastonite-free, scapolite-clinopyroxene-calcite- garnet-amphibole-epidote bearing calc-silicate granulite from the MSU near the Palghat Cauvery Shear Zone. The studied calc-silicates preserve grandite garnet, in close association with clinopyroxenes and scapolite.
Using the internally consistent thermodynamic dataset, quantitative topologies in P-T (for fixed XCO2) and isobaric T-XCO2 topologies in the CaO-MgO-Al2O3-SiO2-H2O-CO2 (CMASV) system were constructed, to trace the P-T-X (fluid) evolutionary history of the studied calc-silicate granulites. The effects of Fe2+, Fe3+ and Na+ on the CMASV topologies were analysed. Interpretation of the ‘frozen-in’ reaction textures and the findings from the activity adjusted CMASV topologies, integrated with the P-T evolutionary history of the intercalated pelitic granulites, supports the view that garnet-amphibole followed by epidote, formed as a result of temperature decrease, with the influx of moderately water-rich fluid (XCO2 =0.4-0.6, ~440-640 °C, at ~4.5-6 kbar). Reactions in the logfO2 vs logfCO2 topology in the system SiO2-FeO-CaO-CO2-O2 (SFC-CO2-O2) indicates the ‘essenite’ component in clinopyroxene to be a probable source of Fe+3 for grandite formation.
How to cite: Roy Choudhury, S. and Dey, A.: Metamorphic evolution of calc-silicate granulites from the Granulite Terrane of South India, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-712, https://doi.org/10.5194/egusphere-egu25-712, 2025.
Greenstone-TTG (Tonalite-Throndjemite-Granodiorite) belts and granitoid-gneiss complexes are typically described as two distinct geological assemblages. Greenstone-TTG belts, including volcano-sedimentary series, represent primitive crustal growth, while the significance of granitoid-gneiss complexes is more debated. The significance of the relationship between granitoid-gneiss complex and greenstone belts is thus key to document crustal growth and reworking. High-grade metamorphic rocks, variably designated as granitic gneisses, migmatitic gneisses or migmatites, are commonly found in the transition zones. Whether such high-grade rocks are related to the greenstone-TTG belts or the granitoid-gneiss complexes depends on whether they represent (i) a pre-existing basement for the volcano-sedimentary series, (ii) syntectonic intrusions, (iii) distinct tectonically accreted terrains, or (iv) the reworked equivalent of the volcano-sedimentary series as the result of intense deformation and metamorphism reaching partial melting.
In the northern part of the Paleoproterozoic Guiana Shield, in northeastern Suriname, high-grade metamorphic rocks of the Sara’s Lust Gneiss (SLG) complex mark the contact between the Marowijne Greenstone-TTG Belt (MGB) and the granitoid-gneiss complex. The structural and metamorphic record of these rocks has been attributed to the Transamazonian orogenic event. Their continuous exposure, provide a unique opportunity to study their geodynamic relationship. Here, we present a multidisciplinary approach combining field relationships, petrography, metamorphic P-T conditions and zircon petrochronology to investigate the significance of high-grade metamorphic rocks of the SLG. Field investigation indicate that the SLG consists of (i) mafic metatexite migmatite developed at the expense of amphibolite, characterized by plagioclase-rich leucosomes surrounding (peritectic) hornblende porphyroblasts; and (ii) felsic migmatitic biotite gneiss and metatexite migmatite derived from metagreywackes with calc-silicate and metapelite lenses, characterized by garnet-bearing quartz-plagioclase-biotite leucosomes. The leucosomes form a network of texturally continuous concordant and discordant veins relative to the synmigmatitic foliation. No tectonic contact was identified between the MGB and the SLG and the transition from the MGB to the high-grade rocks follows the same dominant NW-SE foliation and/or magmatic fabric, consistent with a metamorphic gradient. Moreover, trace element signatures of the mafic suite of the SLG are similar to the mafic volcanic formation of the MGB indicating that these may represent the same unit. In addition, trace element signatures of metagreywackes from the SLG are similar to the metasedimentary formations of the MGB, strengthening the correlation. Phase equilibrium modelling yields peak conditions of 760 (± 30) °C and 4.6 (± 1) kbar, consistent with a low- to medium-pressure / high-temperature metamorphic gradient. Zircon petrochronology enabled the distinction of inherited zircons with U-Pb dates between 2.36 to 2.10 Ga, which coincide with the age of the volcano-sedimentary rocks of the MGB at 2.26-2.15 Ga. Metamorphic ages of 2.08 ± 0.02 Ga agree with the collisional stage (2.11 – 2.08 Ga) of the Transamazonian Orogeny. This implies that the high-grade rocks and the volcano-sedimentary series of the greenstone-TTG belt share a common protolith and that the high-grade rocks are representative of a partially molten equivalent of the volcano-sedimentary series of the MGB. Accordingly, the high-grade rocks and granitoid-gneiss complex are attributed to crustal reworking of the Paleoproterozoic crust of the Guiana Shield.
How to cite: Sastrohardjo, F., Vanderhaeghe, O., Kriegsman, L., Kroonenberg, S., Van Der Molen, S., Goumans, J., and Eglinger, A.: High-grade rocks linking greenstone-TTG belts and granitoid-gneiss complexes; NE Suriname, Paleoproterozoic Guiana Shield, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-7539, https://doi.org/10.5194/egusphere-egu25-7539, 2025.
This work is part the GEMMAP project (Geological Mapping and Mineral Assessment of Malawi) conducted between 2016 and 2020 in Malawi to provide a national coverage of geological maps at 1:100 000 scale. The Unango Subdomain in southern Malawi is part of the Mesoproterozoic South Irumide Domain. Most of the Unango Subdomain was strongly reworked during the Neoproterozoic Pan African orogeny that led to the assembly of Gondwana. Our study focusses of the northwestern part of the Unango subdomain where it is possible to observe a mid- to lower crustal sequence with a well exposed contact between it and the overlying Lilongwe Subdomain. The Unango subdomain is characterized by various granulite facies lithodemic units (charnockitic gneisses, sillimanite garnet gneiss, garnet-pyroxene granulitic gneisses, marble, meta-anorthosite, syenite and syenitic orthogneisses and rare quartzite and calcsilicates) with intense penetrative deformation. Pressure-temperature estimates consistently show peak conditions around 1.0 Gpa and 860°C (orthopyroxene-garnet bearing gneiss; sillimanite-garnet gneiss and garnet-clinopyroxene metabasite) using thermodynamic modelling with Perple_X. These conditions are consistent with the observed partial melting (both at the outcrop scale and as melt inclusion entrapped within garnet). We also present the results of LA-ICP-MS U-Pb geochronology on zircon that constrain the age of the magmatism and high-temperature metamorphism. The magmatic rocks (syenite and syenite orthogneisses) were emplaced between 582 and 500 Ma, during the second phase of the East African orogen. Metamorphism is constrained between 551 and 545 Ma. The combination of alkaline magmatism and structural observation indicates a long-lasting extensional setting of ca. 100 Ma, in the Unango SD with possible intermittent thrusting event at ca. 570 Ma. The normal metamorphic gradient to the overlying Lilongwe SD suggests the interpretation of the Unango SD as a metamorphic dome.
How to cite: Plunder, A., Fullgraf, T., Le Bayon, B., Mtegha, J., and Thomas, R.: High to ultra-high temperature metamorphism from the Unango subdomain, Malawi, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-11421, https://doi.org/10.5194/egusphere-egu25-11421, 2025.
Here we report the first results of the petrological investigation performed on the high-grade, partially melted rocks belonging to the so-called High-Grade Metamorphic Complex (HGMC) cropping out at Punta Scorno area, north Asinara Island (Sardinia, Italy). We focus on banded amphibolites, ortho- and paragneisses and diatexite/granites. The gradual transition from amphibolites to gneisses to diatexite/granites is clearly visible on the field and makes this area an excellent target for tracing melt production processes in the medium-lower continental crust.
The amphibolite forms a large lens (200 x 50m) in the SE part of the studied area. The more massive and darker portions show under microscope observation a slightly foliated structure given by iso-oriented hornblende and elongated plagioclase. K-feldspar and Fe-sulphides are common throughout the rocks and align to the foliation. This rock also contains randomly oriented biotite, quartz, zoisite and white mica. Plagioclase is Ca-rich (An78-96), whereas biotite has XFe of 0.48-0.51. In the samples where the cm-scale banding is more visible, the amphibole is more randomly oriented. In these portions, biotite has higher XFe (0.55-0.60), the most abundant amphibole is grunerite (Fe-rich) which is overgrown by hornblende, and plagioclase is poorer in Ca (An43-58). Melt pseudomorphs of quartz are developed in contact with plagioclase and biotite. Strongly resorbed garnet porphyroblasts of almandine-rich garnet (Alm67-68Grs13-16Sps9-10Prp7-10) are associated with amphiboles and biotite.
Moving toward north the amphibolites transition to amphibole-bearing orthogneisses. The orthogneiss is foliated at the outcrop scale, although this feature is barely noticeable under the microscope. This rock has a similar assemblage to the banded portion of the amphibolite. However, the matrix contains more quartz and plagioclase, and garnet forms anhedral porphyroblasts. Locally garnet preserves a euhedral shape when in contact with biotite. Grunerite is present as anhedral crystals and it is overgrown by hornblende. Ca dominates the plagioclase composition (An48-81), whereas biotite and garnet are richer in Fe (Alm71-74Grs12-14Prp7-13Sps3-6) than in amphibolites.
Paragneisses occur both in the N and S parts of the studied area and show variable grain size and schistosity. They are characterized by iso-oriented biotite and white mica flakes, quartz-plagioclase elongated lenses, a large amount of apatite and melt pseudomorphs in the more felsic layers. Plagioclase has variable composition, and few grains of garnet (Alm~69Sps~22Prp~7Grs~3) are also present.
Leucogranite is composed of fine-grained quartz, feldspar and plagioclase with scattered biotite flakes and skeletal garnet richer in Mn (Alm66-76Sps10-26Grs2-11Prp3-8). The contact of leucogranite with coarser-grained paragneisses is marked by granophyric intergrowths and myrmekites. Preliminary geochronological data on magmatic zircons from leucogranites yielded an age of 295 ± 3.5 Ma. Coarse-grained granite contains euhedral crystals of feldspar, plagioclase, quartz and garnet with interstitial biotite and white mica. Garnet composition is similar to the one from leucogranite (Alm66-76Sps10-24Grs5-11Prp3-4).
This work provides the first petrological insights in the area and constitutes an introduction to a more detailed study of P-T evolution and re-melting processes in the HGMC at Asinara Island.
How to cite: Turek, O., Ferrero, S., Casini, L., Cruciani, G., Idini, A., Maino, M., and Langone, A.: Low-to-medium pressure crustal melting and granite formation in the Variscan High-Grade Metamorphic Complex of the Asinara Island (Italy), EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-12140, https://doi.org/10.5194/egusphere-egu25-12140, 2025.
Retrieving the thermodynamic properties of metamorphic rocks such as mineral/melt/fluid fractions, compositions, densities and thermal properties is essential for studying, quantifying, and modelling the reactive thermo-mechanical evolution of the lithosphere. These properties are derived from experimental data and are used to calibrate thermodynamic models, which can then predict melt-rock phase equilibria using a so-called Gibbs free energy minimization.
Here, we present recent advancements in modelling thermodynamic equilibrium achieved with the open-source parallel software package MAGEMin. These include the addition of a new thermodynamic database for dry alkaline magmatic systems and the continued development of our new Julia-based graphical user interface (MAGEMinApp), which greatly simplifies the calculation of phase equilibria.
MAGEMinApp’s functionality include the calculation of Pressure-Temperature-Composition diagrams (P-T, P-X, T-X, PT-X), modelling of Pressure-Temperature-Composition paths (fractional melting/crystallization with assimilation/extraction), trace-element and zirconium saturation predictive modelling, specific heat capacity calculation accounting for latent heat of reaction, mineral and magma classification (e.g., TAS diagram), as well as a new sensitivity analysis tool to investigate the control of bulk-rock composition on phase assemblage stability.
How to cite: Riel, N., Kaus, B., Weller, O., Green, E., and Moulas, E.: Advances in thermodynamic modelling tools for metamorphic rocks, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-12962, https://doi.org/10.5194/egusphere-egu25-12962, 2025.
Cratons form the stable nuclei of continents, built through the accretion of terranes along crustal-scale shear zones by the end of the Archean. These structures can channel fluids and facilitate fluid-rock interaction, often playing a critical role in forming major mineralization processes across Archean cratons. Despite their significance, the mechanisms and timing of potential Paleoproterozoic reactivation events remain poorly understood. This study examines the Neoarchean Storø crustal-scale shear zone in SW Greenland using in-situ Lu–Hf garnet geochronology, Rb–Sr biotite and U–Pb monazite dating of garnet- and sillimanite-bearing schists, garnet-bearing amphibolite and calc-silicate rocks. Garnet porphyroblasts in the schists record an initial metamorphism at c. 2.7 Ga, whereas Lu–Hf garnet ages of c. 2.63 Ga in metabasalt and calc-silicate rocks, along with robust U–Pb garnetite ages of c. 2.64 Ga, indicate a second Neoarchean metamorphic event. These results support a two-stage metamorphic evolution linked to the accretion of the Eoarchean Færingehavn and Mesoarchean Akia Terranes along the Storø shear zone involving lithospheric thickening and stabilization during the late Archean. In contrast, the biotite-defining foliation yields an age of c. 1.7 Ga, which may represent either the reactivation of the shear zone during the assembly of the supercontinent Columbia or the exhumation of the craton. Monazite grains included in garnet porphyroblasts (c. 2.7 Ga) and aligned parallel to the biotite-defining matrix (c. 2.5 Ga) support the interpretation that the biotite ages reflect craton exhumation rather than Paleoproterozoic reactivation of the shear zone. This study underscores that this region of North Atlantic Craton played the role of rigid block during the Paleoproterozoic assembly of the supercontinent Columbia and major Neoarchean tectonic boundaries were not reactivated. Instead, they represent well-preserved Archean shear zones, ideal for studying Archean tectonic processes.
How to cite: Volante, S., Glorie, S., Szilas, K., Tavazzani, L., and Basak, S.: One billion years later: Reactivation of an Archean shear zone or exhumation of a craton? , EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-17979, https://doi.org/10.5194/egusphere-egu25-17979, 2025.
One of the best preserved Archean igneous complexes in the world is located in the Fiskenæsset region in SW Greenland and is known as the Fiskenæsset Anorthosite Complex (FAC). The region hosts a variety of high-grade rock types ranging from anorthosites (clino-orthopyroxene / amphibole bearing), metaperidotites, garnetiferous amphibolites, garnetiferous pyroxenites and chromitites. Several studies have interpreted this region to represent a subduction zone setting with hydrous recycling of lithosphere and arc magmatism operating as early as the Mesoarchean, and leading to the formation of the anorthosites (Windley et al., 1973). This interpretations is, however, highly debated. The aim of our project is to constrain the metamorphic history of the FAC post their igneous emplacement, which started as early as the Neoarchean based on metamorphic U-Pb zircon ages (Polat et al. 2010, Keulen et al. 2010). This temporally constrained knowledge of Neoarchean P-T history can be used as fingerprints of the prevalent geodynamic setting in the region and thereby provide insights into the likely formation environment of the FAC, and, more broadly, information on the tectonic processes operating in the Neoarchean.
Here, we report geochemical and petrological results for a group of highly aluminous, sapphirine-bearing amphibolites dominated by orthopyroxene, corundum, phlogopite and anorthitic plagioclase, along with associated garnetiferous amphibolites and granulites from the FAC. These metamorphosed mafic rocks occur as individual bodies and as enclaves within anorthosites. Using an integrated approach of petrography, detailed elemental mapping, geothermobarometry and phase equilibria modelling, we constrained the metamorphic P-T-t history of the terrain. The rocks have been subjected to a multistage metamorphic history with the mafic rocks metamorphosed to amphibolite (M1 metamorphism) at ∼5-7 kbar and ∼700°C to granulite facies conditions (M2 metamorphism) at ca. ∼11-12 kbar and ∼900°C, forming corundum and eventually sapphirine during retrogression and cooling (M3). A K-rich fluid is further affecting these assemblages during retrogression leading to phlogopitic biotite (formation. Our results further show that the peak metamorphic event at lower crustal depths can be traced back to ~2.63 Ga from in-situ Lu-Hf garnet geochronology. The M3 event appears to be accompanied by metasomatism and a further objective of this study is to constrain the type and composition of the fluid(s) responsible through thermodynamic modelling. This will help to improve our understanding of Archean crustal metamorphic processes and, in particular, the role of element-redistributing fluids in the evolution of cratons.
References:
Windley (1973) Bulletin Grønlands Geologiske Undersøgelse, 106, 1-80
Polat A et al. (2010) Chem. Geol. 277(1-2), 1-20
Keulen A et al. (2010) GEUS Bulletin, 20, 67-70
How to cite: Basak, S., Szilas, K., and van Hinsberg, V.: Constraining the P-T-t history of sapphirine bearing granulites and associated rocks from the 2.9 Ga Fiskenæsset Anorthosite Complex, SW Greenland, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-18734, https://doi.org/10.5194/egusphere-egu25-18734, 2025.
Migmatites provide essential insights into the processes of partial melting and the rheological behavior of the middle and lower continental crust. This study investigates the extensively migmatized granite gneisses in the northern part of Tamil Nadu, within the Southern Granulite Terrane (SGT). The term transition zone refers to the metamorphic grade shift from lower to higher grades, observed south of the Dharwar Craton (DC) towards the SGT. The SGT is divided into several blocks, separated by crustal-scale shear zones, with this research focusing on the Krishnagiri area of the Shevaroy Block (SB), where the transition from greenschist to granulite facies is evident. Earlier studies proposed the Fermor Line, a hypothetical boundary marking the change from lower-grade to higher-grade metamorphic facies.
The entire Krishnagiri area has dome-shaped hills. The field evidence shows partial melting, including lensoidal leucosome patches and veins, migmatitic quartzo-feldspathic gneisses, and migmatites appearing as metatexites and diatexites. These rocks exhibit schollen, schlieren, and nebulitic structures, with greater migmatization observed near the Mettur shear zone, forming the western boundary of SB. Mafic enclaves in these rocks are deformed and stretched along the shear zone. Deformation features such as shear zones, mylonites, and shear sense indicators are also prominent. The stretching lineation within the granite gneisses trends at 015°.
Field and microscopic observations allow the Krishnagiri granite gneisses to be categorized into four distinct zones based on mineralogical assemblages. Zones 1 and 2 contain metamorphic minerals like epidote, amphibole, and biotite. Zone 3 lacks epidote but includes amphibole and biotite, with more pronounced foliation. Zone 4 features pyroxenes and garnet, with pyroxenes altered to chlorite and rimmed by amphiboles. Mafic enclaves are abundant in Zone 1 and occur within leucosomes, while Zone 4 contains charnockite as pods or lenses within the granite gneiss. K-feldspar veins, appearing from Zone 2 onward, cross-cut the gneisses and include mafic minerals such as amphiboles and pyroxenes.
All these observations indicate an increasing metamorphic grade from north to south across the Krishnagiri region and thus revealing progressively deeper crustal levels towards the south.
How to cite: Ganguly, A. and D'Souza, J.: Metamorphism of the Krishnagiri Granite Gneisses: The Transition zone between the Dharwar Craton and the Southern Granulite Terrane , EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-19868, https://doi.org/10.5194/egusphere-egu25-19868, 2025.
Although the Bundelkhand Craton (BuC) is a notable Archean region in northern India, little is known about its tectono-metamorphic evolutionary history. We present petrography, bulk composition modeling, and geochemical characterisation of garnet-bearing and garnet-absent amphibolites. Both basaltic and andesitic-basalt are the protoliths of the studied amphibolites of BuC. Chondrite normalized rare earth element (REE) patterns indicate enrichment of LREEs over HREEs coupled with negative Nb, Ta, and Ti anomalies that imply a typical subduction-related geochemical signature. Furthermore, our results show a basaltic protolith originated at the active edges of island arcs-type environment. Trace element geochemistry-based discrimination diagrams including Nb/Th vs. Zr/Nb, Zr vs. Zr/Y, and Th/Nb vs. Ce/Nb, as well as high Th/Yb and low Nb/Yb ratios further suggest an island arc setting for the genesis of our studied amphibolites. Our results such as petrography, mineralogy and pseudosection-modelling are consistent and invoke three phases of metamorphism experienced by studied amphibolites. The pre-peak metamorphic phase was characterized by pressure-temperature (P‒T) values of 6.25–6.5 kbar and 580–590°C for garnet-bearing amphibolites and 5.0–5.8 kbar and 400–450°C for garnet-absent amphibolites. Peak metamorphism took place in garnet-bearing amphibolites at 6.8–7.4 kbar and temperatures between 760 and 805°C, and in garnet-absent amphibolites at 7.0–7.4 kbar and temperatures between 785 and 810°C. P‒T values of 4.45–4.75 kbar and 585–615°C for garnet-bearing amphibolites and 3.1–4.0 kbar and 620–710°C for garnet-absent amphibolites were indicative of retrograde metamorphic processes. The mineral assemblages and P‒T trajectories delineate a clockwise P‒T path for both garnet-bearing and garnet-absent amphibolites from the Babina and Mauranipur regions. This suggests that the rocks were subjected to burial in a subduction tectonic setting within an arc-related environment, followed by a decompression stage that brought them to the surface.
How to cite: Pathak, P., Kumar, R. R., and Dwivedi, S. B.: Geochemical characterization and P-T trajectory of amphibolite enclaves from the Central Bundelkhand craton and its tectono-metamorphic evolution, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-525, https://doi.org/10.5194/egusphere-egu25-525, 2025.
Radiogenic isotope compositions of magmatic rocks have been widely used to infer the nature of their sources. However, updated studies have demonstrated that crustal derived granites can carry a large magnitude of isotope (Sr, Nd, and Hf) disequilibrium which could be used to infer the melting reactions as well as the nature of sources. The Himalayan Cenozoic granites are typical products from melting of crustal sources. Experimental results and theoretical calculations suggest that the Himalayan leucogranites are characterized by Sr, Nd, Hf isotope disequilibrium. Exception for the reported trace element compositions and ratios and the initial Sr isotopic ratios, radiogenic Hf and Pb isotopic ratios are heterogeneity. Leucogranites from Malashan-Gyirong area consist of two groups of granites formed fluid-absent melting of muscovite (Group-A) and fluid-fluxed melting of muscovite (Group-B), respectively. Except for substantial differences in key trace element compositions and their ratios, and Sr isotope compositions, follow-up studies show that as compared to Group-A granites, Group-B granites have much higher Th, Th/U, and 208Pb/204Pb ratios. However, their 206Pb/204Pb and 207Pb/204Pb ratios are similar. Such characteristics could be explained by enhanced solubility of monazite (high Th/U and 208Pb/204Pb phase) relative to zircon during fluid-fluxed melting of metasedimentary rocks. Our findings suggest that zircon and monazite play an critical role in shaping Pb isotope systematics in crustal-derived melts.
How to cite: Zeng, L., Gao, L.-E., and Yan, L.: Pb isotope disequilibrium in metasediment-derived granitic melts, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-4919, https://doi.org/10.5194/egusphere-egu25-4919, 2025.
