The occurrence of Ultrahigh Pressure (UHP) index minerals (i.e., coesite, microdiamond, overall) in tectono-metamorphic belts is of paramount importance to attest the depths attained during subduction of the lithosphere. Recently, many studies focuses on the importance of the inclusions stored in host garnet that preserve information on the trapping conditions (i.e., elastic properties). Detailed inclusions study allows to recognize UHP mineral phases that escaped re-equilibration. Locally, these minerals are no more present in rock matrix, making inclusions the unique evidence of early rock evolution. In this work, we perform a detailed inclusion study through a multi-technique approach to obtain new constraints on the peak metamorphic conditions of a deeply subducted oceanic slab (> 100km).

We focused on two tectono-metamorphic units outcropping in the Western Alps, characterized by a metamorphic peak in UHP conditions. These units belong to the meta-ophiolites of the Internal Piedmont Zone (IPZ) and are located in (i) the Susa Valley (Ghignone et al., 2021), and (ii) Pellice Valley (Colle del Baracun; Ghignone et al., 2023). The units consist of oceanic lithosphere (serpentinite, metagabbro) and metasedimentary cover (micaschist, calcschist). For each unit we selected three samples of Grt-bearing metasediments, making a detailed characterization of the garnet inclusions with optical microscope and micro-Raman spectroscopy. Measured Raman spectra allowed to identify coesite in the Susa Valley (previously unrecognized). We applied Raman-based elastic geothermobarometry (Angel et al., 2015) on quartz and zircon inclusions in garnet as well as Zr-in-rutile thermometry to define the metamorphic path of the units. Furthermore, we combined the distribution of inclusions in zoned garnet with multispectral compositional maps (SEM-EDS), for obtaining a detailed mineral assemblage related to each garnet growth stage.

Our results highlight substantial differences of inclusion mineralogy and their microstructural position within the garnet shells between the two studied units. The IPZ in the Susa Valley recorded (i) the peak-PT under UHP conditions, (ii) a second event under HP eclogitic conditions, and (iii) a late re-equilibration event under LP conditions. Instead, the IPZ in the Pellice Valley recorded i) an early event corresponding to the prograde stage, ii) a peak pressure in UHP conditions, iii) a late re-equilibration event under LP conditions. Despite these differences, the two studied meta-ophiolitic units show similar metamorphic evolutions down to UHP conditions. This evolution follows a similar gradient to that of the coesite-bearing Lago di Cignana unit (e.g., Groppo et al., 2009). This may point out that in the Western Alps three major ophiolite slices underwent UHP metamorphism, thus suggesting that a large volume of oceanic lithosphere was subducted at ca. 100 km depth.

Angel, R., J., Nimis, P., Mazzucchelli, M., L., Alvaro, M., Nestola, F. (2015). J. Metamorph. Geol. 33 (8), 801-813. [10.1111/jmg.12138]

Ghignone, S., Borghi, A., Balestro, G., Castelli, D., Gattiglio, M., Groppo, C. (2021). J. Metamorph. Geol. 39 (4), 391–416. [10.1111/jmg.12574]

Ghignone, S., Scaramuzzo, E., Bruno, M., Franz, A. L. (2023). Am. Mineral. 108, 1368–1375. [10.2138/am-2022-8621]

Groppo, C., Beltrando, M., Compagnoni, R. (2009). J. Metamorph. Geol., 27(3), 207-231. [10.1111/j.1525-1314.2009.00814.x]

How to cite: Boero, F., Ghignone, S., Bruno, M., Gilio, M., Borghini, A., and Scaramuzzo, E.: Detailed study of garnet inclusions from the UHP meta-ophiolites of the Western Alps: new petrological constraints on the deep history of a subduction zone, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-449, https://doi.org/10.5194/egusphere-egu24-449, 2024.

The area of Mt. Papuk in Croatia, which is protected as a UNESCO Global Geopark due to its exceptional geological diversity, is known for polycyclic metamorphic rocks. The study of metamorphic and igneous rocks demonstrate the occurrence of pre-Variscan, Variscan and Alpine orogenic events. Within the metamorphic lithologies (but also igneous ones, e.g., S-type granite), garnet is a key mineral to decipher details of the geological evolution even if it represents a rock volume of up to 2-3% only. The Variscan orogeny has been recognized here as the most imprinting one producing garnet-bearing medium- to high-grade rocks accompanied by partial melting. This is the only case in the Mt. Papuk area where garnet in metamorphic complex coexisted with melt, i.e. reached high temperatures.

The rock samples (mica schist, gneiss, migmatite) show a schistose fabric and a well-preserved medium to coarse-grained granolepidoblastic texture. Some of the rocks reached high-grade conditions discernible by partial melting. Schistosity is defined by the preferential orientation of elongated feldspar grains, micaceous (biotite and minor muscovite) domains and quartz bands. Feldspar (~40-50 vol%; oligoclase up to 28 mol% An) is the predominant phase, followed by biotite (20-30 vol%), quartz (20 vol%), white mica, reddish garnet and sillimanite (fibrolite). Zircon, apatite, monazite, ilmenite, rutile and titanite are accessory minerals. Monazite grains with high Ce₂O₃ content (approx. 28 wt%) were dated with the electron microprobe and yielded an age peak at 384.5±9.0 Ma (1σ).

Garnet occurs in almost all quartz- and mica-rich lithologies of the upper amphibolite facies and exhibits two size populations: 1) small, up to 100 μm large and chemically homogeneous (Alm=64, Prp=9, Grs=2 and Sps=25 mol%) garnets that are generally devoid of inclusions except some containing quartz, and 2) 1-2 mm large garnets with a slightly different chemical composition (Alm=66, Prp=12, Grs=2 and Sps=20 mol%) in inner zones containing numerous apatite, rutile, biotite, plagioclase and quartz inclusions. The thin rim of the large garnets corresponds chemically to the composition of the small garnet population.

Using Ti-in-biotite (average 645°C) and garnet-biotite thermometry (590-650°C at 500 MPa) and pseudosection modelling, a clockwise P-T path was reconstructed with maximum pressure conditions within the rutile P-T stability field where the garnet core grew at 800-900 MPa and a temperature of 590-600°C. These conditions were followed by a significant pressure drop, accompanied by melting, partial resorption of garnet and the formation of a thin garnet rim, to 450 MPa (~15 km depth) at temperatures of 680°C.

The geothermobarometric data, additionally constrained by microtextural evidences and mineral stability fields, suggest a convergence-related model reflecting burial, heating and finally rapid (tectonic?) exhumation of the moderately thickened crust. This model suggests a complex tectonometamorphic evolution of the crystalline terrain filling a gap in the Variscan belt between Central and Eastern Europe.

How to cite: Balen, D. and Massonne, H.-J.: Geochemistry and microtextural characteristics of garnet from the Variscan medium- to high-grade metamorphic complex of Mt. Papuk (Croatia), EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-3436, https://doi.org/10.5194/egusphere-egu24-3436, 2024.

The 3D microstructure and compositional zoning of garnet populations in micaschists from the Kolvik and Bekkarfjord nappes indicate the quasi-equilibration of their major components across the rock matrices during interface-controlled, size-independent garnet growth. There is microstructural evidence for foliation-parallel, small-scale resorption of garnet rims in the Kolvik Nappe, influencing the metamorphic peak conditions obtained from thermodynamic modelling. The local chemical compositions of rims less affected by resorption indicate a peak temperature of c. 630˚C, which is c. 40˚C higher than the temperature obtained from resorbed rims of the largest garnet crystal. The use of a diffusion geospeedometry approach that considers the geometry of the compositional zoning of the entire garnet population, and the higher, more realistic peak temperature, results in an estimated duration of 1 to 4.9 Myr for garnet growth in the Kolvik Nappe. This is approximately one order of magnitude faster than duration estimates calculated when using the apparent, lower temperature obtained from the resorbed garnet rims. Concomitantly to garnet growth in the Kolvik Nappe, garnet overgrowths formed in the Bekkarfjord Nappe for c. 2.5 Myr at metamorphic peak temperatures of c. 560˚C. The garnet growth durations obtained in this work are comparable with the uncertainty on the Lu–Hf garnet–whole-rock isochron ages published previously for the studied rocks (Gaidies et al., 2022). The results provide new insight into the timescales of repeated Barrovian-type metamorphic events experienced by the lower nappes of the Kalak Nappe Complex during the final stages of the Caledonian Orogeny in Arctic Norway.

Reference: Gaidies, F., Heldwein, O. K. A., Yogi, M. T. A. G., Cutts, J. A., Smit, M. A., Rice, A. H. N. (2022). Testing the equilibrium model: An example from the Caledonian Kalak Nappe Complex (Finnmark, Arctic Norway). Journal of Metamorphic Geology, 40(5):859–886.

How to cite: Yogi, M. T. A. G., Gaidies, F., Heldwein, O. K. A., and Rice, A. H. N.: Mechanisms and durations of metamorphic garnet crystallization in the lower nappes of the Kalak Nappe Complex (Arctic Norway), EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-4106, https://doi.org/10.5194/egusphere-egu24-4106, 2024.

The Western Gneiss Region (WGR) is a well-known UHP metamorphic terrane in SW Norway, where eclogite bodies crop out among gneisses. These rocks experienced high- and ultrahigh-pressure (HP-UHP) metamorphism during the Caledonian Orogeny and display pressure-temperature (P-T) gradient increasing from the south to the north (e.g., Cuthbert et al., 2000). Eclogites from nine localities along the entire length of WGR, namely Drøsdal, Vårdalsneset, Verpeneset, Halnes, Saltaneset, Grytting, Ulsteinvik, Solholm, and Juvika, have been chosen for P-T condition estimates. Here we present a reevaluation based on the new geothermobarometric techniques in the WGR regional study.

Eclogite varies in the amount of amphibole, kyanite, phengite, zoisite, and inclusions of quartz and rutile in garnet. Phengite occurs mainly in the eclogites in the south, whereas those from the north contain orthopyroxene. Garnet in the south is almandine-rich with the average composition of Alm_0.41–0.57Grs_0.15–0.31Prp_0.10–0.37Sps_0.01–0.07, with increasing Mg in the rims and often showing complex compositional zoning. Garnet from the other localities is homogenized and dominated by pyrope with the composition of Prp_0.41–0.56Alm_0.33–0.46Grs_0.09–0.13Sps_0.01–0.04. Clinopyroxene in the northern part is poorer in jadeite component with X_Na=0.20-0.29 and X_Fe=0.12-0.17 than clinopyroxene in the south with X_Na=0.40-0.52 and X_Fe=0.12-0.19. Silicon in phengite is up to 3.30 atoms per formula unit (apfu) in most localities, with a maximum of 3.48 apfu in the Verpeneset locality.

Coesite and polycrystalline quartz inclusions in garnet, evidence of UHP metamorphism, are typical of Saltaneset and Verpeneset localities, which is also validated by thermobarometric calculations. We estimated the peak P-T equilibration condition by Grt-Cpx-Phe thermobarometry, Ti-in-Quartz and Zr-in-Rutile thermometry, along with Quartz-in-Garnet elastic geobarometry. Preliminary estimates give slightly higher P values than previously reported in the two southernmost localities Drøsdal and Vårdalsneset [720-830°C and 1.9-2.1 GPa (Foreman et al., 2005), 635°C and 2.3 GPa (Engvik et al., 2007), accordingly] yielding the peak conditions of 680-740°C and 2.7-2.85 GPa. The obtained results from other localities give P conditions close to the boundary of quartz and coesite stability fields and T in the range of 650-750°C, with the highest P of 3.28 GPa in the Verpeneset locality. The highest T of 875°C has been obtained in Ulsteinvik locality. Those results may help refine previous studies and understand the history of the WGR.

This work was funded by the National Science Centre of Poland project no. 2021/43/D/ST10/02305.

References:

Cuthbert, S.J., Carswell, D.A., Krogh-Ravna, E.J., Wain, A. (2000). Eclogites and eclogites in the Western Gneiss Region, Norwegian Caledonides. Lithos, 52 (1–4), 165-195.

Foreman, R., Andersen, T.B., Wheeler, J. (2005). Eclogite-facies polyphase deformation of the Drøsdal eclogite, Western Gneiss Complex, Norway, and implications for exhumation. Tectonophysics, 398, 1-32.

Engvik, A.K., Andersen, T.B., Wachmann, M. (2007). Inhomogeneous deformation in deeply buried continental crust, an example from the eclogite facies province of the Western Gneiss Region, Norway. Norwegian Journal of Geology, 87, 373-389.

How to cite: Turek, O., Kośmińska, K., Majka, J., Gilio, M., Klonowska, I., Borghini, A., Włodek, A., Buczko, D., and Cuthbert, S.: P-T conditions of high- and ultrahigh-pressure metamorphism along the Western Gneiss Region, Norway, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-6669, https://doi.org/10.5194/egusphere-egu24-6669, 2024.

In the Grenville Province, high-P rocks that are discontinuously exposed along the margin of the allochthonous belt preserve the record of deep crustal processes and are an essential puzzle piece to understanding Mesoproterozoic geodynamics. The Manicouagan Imbricate zone (MIZ) is one of the three high-P domains from the Grenville Province. It is located in the Central Grenville between the Parautochthonous Belt to the North and the orogenic hinterland to the South, that were metamorphosed during the 1005–980 Ma Rigolet and 1080–1020 Ma Ottawan orogenic phases, respectively. In the western MIZ, the Lelukuau Terrane (LT) mostly consists of Labradorian-age (~1650 Ma) mafic suites with a fringe of aluminous rocks at its southern edge. Metamafic samples from the Western and Eastern parts of the MIZ display a peak assemblage of garnet, clinopyroxene, plagioclase, rare pargasite or edenite, and quartz ± kyanite. Pseudosection modelling suggests high-P granulite peak conditions at ca. 14 to 16 kbar and 800–900°C, with the scarcity of hydrous phases and quartz explaining the lack of evidence for partial melting. Zircon cores from the Western LT sample show a maximum magmatic age of ca 1.6 Ga. Lu–Hf and Sm–Nd dating on garnet from this sample yield ages of 1020 ± 7 Ma and 1005 ± 13 Ma, respectively, overlapping within error and inferred to represent peak metamorphic conditions followed by fast cooling. In the Eastern LT sample, garnet Lu–Hf dating yields two ages that are consistent with a petrographically preserved two stage growth, at 1033 ± 6 Ma and 1013 ± 6 Ma, while the Sm–Nd age indicates cooling at 1003 ± 8 Ma. The recorded high-P granulite facies conditions highlight a late Ottawan to Rigolet-age localized crustal thickening at the margin of the hinterland during the propagation of the orogen to the NW, with a possible younging of the high-P granulite-facies metamorphism from the Eastern to Western LT. These new results indicate that the high-P belt in the Central Grenville does not represent the exhumed base of an Ottawan age orogenic plateau, as previously proposed, and that no tectonic hiatus exists between the two orogenic phases, as generally thought. Finally, this publication highlights the diversity and diachronicity of the high-P domains in the Grenville Province.

How to cite: Lotout, C., Indares, A., Vervoort, J., and Deloule, E.: High-P metamorphism in the Mesoproterozoic: Petrochronological insights from the Grenville Province, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-8683, https://doi.org/10.5194/egusphere-egu24-8683, 2024.

Understanding the mechanisms of mountain building and its destruction poses significant challenges. Following subduction of a slab, an increase in the subduction rate and/or a decrease of the convergence rate, may lead to slab rollback. During slab rollback, segments of the middle-lower crust may undergo heating and partial melting due heating of thinned lithosphere by the asthenosphere and the migration of the magmatic arc. Alternatively, after accretion of radiogenic crust, the lower parts of the crust may relax through radiogenic decay. In both scenarios, the heating of the middle-lower crust reduces crustal strength, resulting in the development of metamorphic core complexes. The timing of lower crust heating is crucial for understanding the switchover from lithospheric shortening to extension.

The Hellenides in the eastern Mediterranean constitute an arcuate orogen located north of the present-day active Hellenic margin, marking the site of NNE-ward subduction of the African plate beneath Eurasia. The Aegean Sea region in the Hellenide is a world-class example of large-scale continental extension above a retreating subduction zone. In the Cyclades, the Hellenide orogeny began in the early Cenozoic, causing subduction and sustained high-pressure (HP) metamorphism between approximately 53 and 30 Ma. The timing of slab rollback is subject of intense debate. A decrease in the convergence rate was interpreted to suggest that slab rollback initiated at around 35–30 Ma, during or even after the waning stages of HP metamorphism. In contrast, the formation of extensional sedimentary basins and radiometric dating of extension-related mylonite place rollback at approximately 23 Ma, distinctly after the final stages of HP metamorphism.

Between about 30 Ma and 20 Ma, isobaric heating during the exhumation of some HP rocks has been proposed on some islands (Syros, Tinos, Andros, and Naxos). However, the precise timing and duration of this isobaric heating remains largely unconstrained. On Naxos, previous geothermobarometry estimates indicate isobaric heating occurred from 500 to 550°C from a middle segment of the nappe stack (Peillod et al., 2021). Garnet chemical zonation formed during exhumation allows for a heating timescale to be estimated via diffusion chronometry. We conducted Monte Carlo diffusion simulations to determine the best-fitting timescale based on Chi-square statistics, assuming three different heating paths. Diffusion model results for a heating path from 500 to 550°C indicate a 10 Myr timescale. A lower temperature path (from 400 to 450°C) requires heating over an unreasonable geological timescale (>100 Myr). Conversely, a higher temperature path (from 600 to 650°C) requires a timescale of <1 Myr. This higher temperature path corresponds to temperatures recorded near the migmatite core at the bottom of the Naxos nappe stack. At such temperatures and within a 10 Myr period, chemical heterogeneities in garnet would have relaxed, as observed in most garnets near the migmatite core. The results of this approach indicate that the heating of the lower crust occurs over a 10 Myr period, suggesting that the heating may result from radiogenic decay during the Oligocene before the slab roll during the Miocene.

How to cite: Peillod, A., Hess, B., and Ring, U.: Slab reequilibration and slab roll back timing in the Cyclades: Evidence from garnet diffusion and P-T estimates (Naxos, Greece), EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-9789, https://doi.org/10.5194/egusphere-egu24-9789, 2024.

The Koralpe-Saualpe-Pohorje Complex (KSPC) in the Eastern Alps stretches from SE Austria to NE Slovenia and hosts the type locality for eclogite. Although the KSPC has been studied for decades, some aspects of its tectonometamorphic evolution are still controversial. There is, for example, an ongoing discussion, if the Pohorje unit experienced ultra-high-pressure (UHP) conditions during the Eoalpine orogeny. The KSPC is part of the Austroalpine basement units and was interpreted to represent a coherent (U?)HP nappe consisting mainly of gneisses and metasedimentary rocks, with abundant eclogite lenses embedded. Some of the Austroalpine basement units, including the KSPC, experienced a long-lived Permian-Triassic tectonometamorphic event, where gabbros intruded into a thinned crust, which experienced eclogite facies conditions during the Late Cretaceous. A metamorphic field gradient with an increase in peak pressure-temperature (PT) conditions from NW to SE, and UHP conditions for Pohorje, has previously been proposed based on thermodynamic modelling, geothermobarometry and the discovery of diamond in fluid inclusions in garnet. To unravel the metamorphic evolution of the KSPC, we applied quartz-in-garnet elastic barometry, Zr-in-rutile thermometery and in-situ U-Pb dating of garnet and rutile from eclogite and metasedimentary rock samples along a NW-SE transect. This is the first application of quartz in garnet elastic barometry within the KSPC in order to determine the entrapment pressures of the quartz inclusions. The eclogite samples yielded maximum pressures of 1.9 GPa across the KSPC, indicating no pressure increase from the NW to SE. The metasedimentary rocks show overall lower pressures with a maximum of ca. 1.4 GPa. Zr-in-rutile thermometry yielded uniform temperatures of 640 (±30)°C, indicating no temperature gradient. The novel approach of in-situ garnet U-Pb dating was conducted to decipher potentially different metamorphic events. Garnet from the Koralpe, Saualpe and Pohorje metasedimentary rocks yielded Early Cretaceous dates ranging from ~95 to 105 Ma, similar to eclogitic garnet from the Koralpe with ~112 Ma. Additionally, garnet from a Saualpe micaschist yielded Late Triassic cores (~224 Ma) and Early Cretaceous rims (~115 Ma). Rutile throughout the KSPC yielded Late Cretaceous U-Pb dates of ~98–83 Ma (eclogites) and ~87–80 Ma (metasedimentary rocks).The results of this study suggest that the KSPC represents a coherent nappe. The recorded maximum pressures and temperatures are identical throughout the KSPC. The lower pressure for the metasedimentary rocks is interpreted to be the result of viscous relaxation in garnet due to the presence of fluids during metamorphism. The obtained garnet U-Pb dates from both eclogites and metasedimentary rocks are interpreted as Late Cretaceous bulk crystallization ages that reflect prograde to peak metamorphic garnet growth. The Late Triassic U-Pb dates from the Saualpe garnet cores are in line with the existing literature proposing a polymetamorphic cycle for the KSPC. The rutile dates are interpreted as cooling ages. The lack of a metamorphic field gradient may imply a different tectonical setting for the KSPC than previously proposed, and warrants further investigation.

How to cite: Wannhoff, I., Pleuger, J., Zhong, X., John, T., Millonig, L. J., Gerdes, A., and Albert, R.: New petrological and geochronological results from the Koralpe-Saualpe-Pohorje Complex (Eastern Alps), EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-9913, https://doi.org/10.5194/egusphere-egu24-9913, 2024.

Garnet is a common high-pressure mineral in the Earth’s lithosphere. Considered a high-strength mineral stable across a wide range of pressure and temperature conditions, it is generally accepted that garnets can retain their microstructures and chemical composition during deformation and metamorphism. Therefore, the trace and major element compositions of garnet are commonly used for geothermobarometers and geochronometers to provide the conditions and timing of metamorphic events. We combine electron backscatter diffraction (EBSD), electron channeling contrast imaging (ECCI) and atom probe tomography (APT) on garnet from an eclogite facies mylonite in the Musgrave Province (central Australia) to investigate the mechanisms of element mobility during high-strain deformation under dry (<0.002 wt% H₂O), lower crustal conditions. Previous investigations suggest that mylonitization occurred at 1.2 GPa and 650°C. Herein, we focus on two garnet clasts that are located within one common high-strain zone. EBSD and ECCI data from garnet clast 1 reveal a micro-shear zone characterized by recrystallized strain-free garnet crystals. APT analysis of one of the recrystallized high-angle grain boundaries shows Fe enrichment in the form of  equally spaced (4.5–6.5 nm), planar arrays of Fe-rich nanoclusters (3.0–7.5 nm). The combined data suggests that these nanoclusters formed as a result of enhanced Fe diffusion along high-angle grain boundaries of recrystallized garnet during high-strain deformation. Garnet clast 2 evinces crystal-plasticity associated with brittle deformation in the form of heterogeneous misorientation patterns and low-angle grain boundary development at the rim of the garnet porphyroclast. APT analysis of a low-angle grain boundary within the highly-strained clast shows Ca enrichment and Mg depletion along dislocations, suggesting crystal-plasticity enhances element mobility via 'pipe' diffusion, with dislocations acting as high-diffusivity pathways. Our data reveal the interaction of chemical and mechanical processes at the nanoscale through deformation-induced enhanced element diffusivity. Consequently, caution should be exercised when using deformed garnets as petrological tools because of this enhanced major element mobility during high-strain deformation. To evaluate this hypothesis, we modelled the Ca diffusion profiles across undeformed and deformed garnet rims within clast 2. Simulations based on the measured Ca concentration profiles of the undeformed rims yield time estimates of 0.8 to 1 Ma for the duration of high-strain deformation at eclogite facies along the shear zone, whereas simulations across the deformed rims yield longer time estimates of 5 to 22 Ma. Our study thus highlights the importance of conducting a thorough microstructural and geochemical analysis on garnet prior to utilizing the mineral for petrological applications.

How to cite: Dubosq, R., Camacho, A., Rogowitz, A., Schneider, D., and Gault, B.: Nanostructural and chemical evolution of garnet during high-strain deformation, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-11318, https://doi.org/10.5194/egusphere-egu24-11318, 2024.

This study analyses the H₂O content in nominally anhydrous minerals (NAMs) in 10 eclogites from W Norway. Each sample has oriented lamellar to acicular inclusions in clinopyroxene, which are either quartz with pargasite or quartz/albite without pargasite [1]. Low-Al orthopyroxene and polycrystalline quartz inclusions in several of these samples provide evidence for UHP metamorphism in the stability fields of diamond and coesite. The H₂O content is quantified using Fourier transform infrared spectroscopy (FTIR), unpolarised infrared radiation, spectra deconvolution, and the calibration of [2].

Preliminary data was obtained from the analysis of the first 5 eclogites yield for garnet 22-379 and 16-31 µg g^-1 structural H₂O for samples with and without pargasite lamellae, respectively. The highest value occurs in a zoisite-bearing eclogite. If regarded separately, then the variation in the 4 zoisite-free eclogites shrinks to 16-32 µg g^-1 H₂O. Absorption bands characteristic for molecular H₂O in garnet (centered at wavenumbers <3460 cm^-1) were not observed. The ranges for structural H₂O in clinopyroxene from these 5 samples are 125-380 and 183-564 µg g^-1, respectively. The highest value occurs in a sample with intense recrystallization of clinopyroxene (but not garnet) after peak metamorphism. If regarded separately, then the total range is 125-380 µg g^-1 H₂O. The obtained clinopyroxene–garnet H₂O partition coefficient has ranges of 1.0-11.0 and 11.6-18.2, respectively. The extreme values belong to the zoisite-bearing (1.0) and the strongly recrystallized (18.2) samples. If regarded separately, then the total range is reduced to 3.9-11.6.

Combining the preliminary data of the quantified structural water with petrological information tends to suggest the following relationships. (1) The current H₂O content in NAMs is affected by the presence of hydrous minerals during peak metamorphism and the retrogression history. (2) The peak UHP garnet is water-deficient unless zoisite forms part of the mineral assemblage. (3) The current H₂O content of clinopyroxene (containing oriented inclusions of quartz with and without pargasite) from "diamond-facies" UHP eclogite is lower compared to that of "graphite-facies" UHP eclogite from a similar tectonic setting that lacks such inclusion microstructures [3]. (4) Samples with oriented inclusions of pargasite in clinopyroxene tend to have lower clinopyroxene–garnet H₂O partition coefficients than those without pargasite, which suggests that pargasite lamellae formed by clinopyroxene dehydration during early decompression. Additional data will be presented to test these preliminary indications.

This study received funding from Norway Grants 2014–2021 operated by the National Science Centre (Poland) under project contract no. 2020/37/K/ST10/02784.

[1] Spengler et al., 2023, Eur. J. Mineral. 35:1125-1147

[2] Bell et al., 1995, Am. Mineral. 80:465-474

[3] Gose & Schmädicke, 2022, J. Metamorph. Geol. 40:665-686

How to cite: Spengler, D.: Water in eclogitic garnet and clinopyroxene with oriented quartz and pargasite inclusions, W Norway, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-21602, https://doi.org/10.5194/egusphere-egu24-21602, 2024.

Mafic–ultramafic lenses embedded in felsic granulites of the Gföhl Unit, Moldanubian Zone, are considered as mantle fragments incorporated into mid-crustal levels of the Variscan orogenic crust. We investigated a several 100 m-sized mafic lens mainly formed by garnet-pyroxenite. Several samples were collected from loose boulders. Petrographic features provide evidence for an early HP-HT eclogite-facies peak metamorphism overprinted to variable degrees by HT granulite-facies metamorphism at lower pressures.

The primary eclogite facies mineral assemblage comprises garnet, sodium-rich clinopyroxene, kyanite, rutile and quartz. The rocks are characterized by

compositional layering on the mm-scale, which is reflected by corresponding systematic variation of the compositions of garnet porphyroblasts. The garnets show homogeneous compositions in their internal domains defining plateaus, the compositional characteristics of which correlate with the compositional layering of the rocks and vary from Alm₁₉ Prp₅₅ Grs₂₇ to Alm_15-18 Prp_42-50 Grs_32-43. The systematic variation of garnet compositions with the bulk rock compositional layering testifies to lack of equilibration between the compositionally distinct layers on the mm-scale during HP-HT eclogite-facies metamorphism.

The HT-granulite-facies overprint is manifested by the breakdown of the eclogite facies mineral assemblage. This is evident, for example, from the formation of sapphirine–spinel–an-rich plagioclase symplectites in garnet supposedly replacing garnet-hosted kyanite and clinopyroxene inclusion. Another peculiar feature is represented by the partial resorption of garnet by plagioclase and clinopyroxene in the form of corrosion tubes penetrating the garnet in a worm-like fashion. Finally, garnet is partially or entirely replaced by plagioclase–spinel–orthopyroxene—clinopyroxene symplectite, where Grs-rich garnets are systematically more strongly affected by this replacement than Grs-poor garnets. Quartz is consumed during the decompression reactions and can only be found as rare relic grains. When a clinopyroxene matrix surrounds relic quartz, the clinopyroxene becomes successively more Si-rich due to inverse Tschermak substitution towards the relic quartz grain.

Throughout the samples and irrespective of the layer they pertain to, the garnets show similar pronounced secondary compositional zoning in the outermost 200 µm. The zoning is characterized by a strong decrease of the Grs content accompanied by an increase of the Alm and Prp contents towards the rim. The compositional changes in garnet are gradual suggesting diffusion-mediated re-equilibration at decreasing pressures, and the composition of the garnet at the interface to the rock matrix is the same throughout the specimen indicating that the rock equilibrated on the cm scale during the HT overprint.

Pressure and temperature were estimated based on equilibrium phase diagrams. They indicate peak pressures above 1.8 GPa and temperatures of around 950 °C for the primary mineral assemblages and the different garnet cores. In accordance with peak P-T, the garnet rims indicate pressures of around 1.3 GPa at the same temperature.

Considering the regional metamorphic setting of the Moldanubian Zone, the relatively localized secondary chemical zoning of garnet at its rim indicates that the granulite-facies metamorphism was remarkably short-lived and suggests rapid transport of the mafic–ultramafic lithologies from mantle depths to the mid-crustal level. Very likely incorporation of the relatively hot mafic lens into a supposedly cooler dominantly felsic environment led to immediate cooling of the mafic lens.

How to cite: Asenbaum, R., Abart, R., and Racek, M.: Decompression of high-grade metamorphic mafic rocks with small-scale compositional layering, Gföhl Unit, Moldanubian Zone, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-17060, https://doi.org/10.5194/egusphere-egu24-17060, 2024.

Garnets in the eclogites of Pfaffenberg, Granulitgebirge (Bohemian Massif, Germany) contain primary granitic melt inclusions with a continental crust signature. The inclusions are up to 30 µm in diameter and polycrystalline with a main mineral assemblage dominated by phlogopite/biotite, kumdykolite, quartz/cristobalite, two unknown phases with main Raman peaks at 412 and 430 cm^-1 respectively, osumilite and plagioclase. In minor amounts, the inclusions contain also white mica, K-feldspar, amphibole and kokchetavite with the local presence of a fluid phase composed of CO₂, CH₄ and N₂.

The inclusions were successfully re-homogenized at 975ºC and 2.7 - 3 GPa and the melt is from trondhjemitc to granitic, peraluminous and hydrous (average H₂O = 4.82 wt%). The melt trace elements patterns revealed similarities with melts produced by partial melting of metasediments part of the continental crust. The melt is in fact enriched in Cs, Pb, Rb, Th, U, Li and B and most likely it originated from the continental crust itself. Interestingly, in situ analyses of Cl and calculation of F partitioning between apatite and melt show that the melt is exceptionally halogens-rich with an average Cl content of 0.41 wt% and a calculated F content of 0.23 wt%.

Pfaffenberg eclogites occur as lenses in garnet peridotite and they are surrounded by continental rocks. They can be regarded as the product of crust-mantle interaction taking place during subduction at mantle depth with the agent of the interaction, i.e., the melt, now preserved as inclusions in the eclogite garnets. The melt is responsible for crustal material mobilization and transfer in the mantle and can be used to constrain and quantify the elements, especially volatiles, transported from the crust to the mantle.

This research is part of the project No. 2021/43/P/ST10/03202 co-funded by the National Science Centre of Poland and the European Union Framework Programme for Research and Innovation Horizon 2020 under the Marie Skłodowska-Curie grant agreement No. 945339.

How to cite: Borghini, A., Ferrero, S., O'Brien, P., Wunder, B., Tollan, P., Majka, J., Fuchs, R., and Gresky, K.: High-pressure eclogites preserve a halogen-bearing metasomatizing agent in primary melt inclusions (Bohemian Massif, Germany)., EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-11813, https://doi.org/10.5194/egusphere-egu24-11813, 2024.

Garnet in high grade rocks often contains primary melt (MI) and fluid inclusions (FI), fundamental to reconstruct the suprasolidus history of the rocks in which it occurs. MI and FI-bearing garnets have been recently found at Punta de li Tulchi, on the NE coast of Sardinia, Italy. They were identified in a single, decimetre-thick leucocratic body of overall granitic composition, hosted in migmatitic orthogneiss. Whereas in the host rock garnet is generally rare or absent, in the leucocratic body it reaches up to 5 vol% and occurs as subhedral grains up to several mm in size. Garnet composition is mainly almandine-spessartine (Alm_60-70, Sps_20-30 Pyr_~7 Grs_~3).

The inclusions occur in clusters in the inner part of the garnet, i.e., they are primary in nature and therefore trapped during garnet growth. A preliminary investigation via optical microscopy and MicroRaman Spectroscopy (MRS) shows the presence of (at least) two types of inclusions. Type 1 inclusions are polycrystalline, ≤30 microns in size, and contain cristobalite +white mica ±carbonate, an assemblage overall consistent with their interpretation as nanogranitoids, or crystallized melt inclusions of anatectic origin. Type 2 inclusions contain two phases, i.e., liquid and vapour (70-30 vol% approximately). MRS display the presence of liquid H₂O and a CH₄-rich bubble, with minor amounts of CO₂ and N₂ detected in some cases. Often type 2 inclusions are characterized by the presence of a carbonate – either ankerite (CaFe carbonate) or rhodochrosite (Mn carbonate), based on the features of its Raman spectrum.

Previous studies in the area suggest that these crustal rocks underwent high T metamorphism and re-melting in presence of fluid at 1.0 -1.1 GPa (Fancello et al., 2018) during the Variscan orogeny. The finding of melt-bearing garnets suggests that anatexis here also involved dehydration melting, at least locally, with formation of peritectic garnet. Moreover, this took place under conditions of primary fluid-melt immiscibility, as testified by the coexistence of MI and FI in the same cluster, a feature already found in many other nanogranitoid studies (see Ferrero et al., 2023). Finally, this is the first case study where nanogranitoids are found in almandine-spessartine garnet rather than in almandine-pyrope.

References

Fancello, D., et al. Trondhjemitic leucosomes in paragneisses from NE Sardinia: Geochemistry and P-T conditions of melting and crystallization. Lithos, https://doi.org/10.1016/j.lithos.2018.02.023

Ferrero, S., et al. H₂O and Cl in deep crustal melts: the message of melt inclusions in metamorphic rocks. EJM, https://doi.org/10.5194/ejm-35-1031-2023

How to cite: Ferrero, S., Cruciani, G., Franceschelli, M., Fancello, D., Rezaei, L., and Gresky, K.: Fluid-melt immiscibility during lower crustal melting recorded in the orthogneiss of Punta de li Tulchi,  Sardinia (Italy)., EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-8787, https://doi.org/10.5194/egusphere-egu24-8787, 2024.

Almandine-spessartine garnet in a Moldanubian peraluminous pegmatoid  (Bohemian Massif, Austria) shows asymmetric morphology, compositional zoning, and microstructural zoning, indicating directed crystal growth. Sector-specific variations in inclusion abundance and microstructures in {112} and {110} garnet sectors indicate facet-specific crystallization processes, associated with individual garnet surface configurations and a compositional boundary layer (CBL) present in the melt adjacent to growing garnet (Kohn et al., 2024). Rutile inclusions show distinct changes in abundance, aspect ratio, shape preferred orientations (SPOs) and crystallographic orientation relationships (CORs) between garnet growth zones. We quantified the SPOs of > 2400 rutile needles in two crystallographically equivalent {112}_Grt rim sectors, and recorded the COR, location, habit and SPO of > 350 rutile inclusions in a transect across core and rim zones within one {112} _Grt sector.

Rutile inclusions are elongated parallel to the four ⟨111⟩ _Grt directions, the three ⟨100⟩ _Grt directions and one ⟨112⟩ _Grt direction. The most frequent SPO for a given {112} _Grt sector is the one closest to the garnet growth direction (i.e. the facet normal), whereas the SPO lying in the facet is exceedingly rare. Sectioning effects cannot explain these frequency variations. Based on the facet-specific SPO and COR statistics, we infer rutile inclusions formed by nucleation at the advancing garnet surface and subsequent co-growth with the host.

Based on directly correlated SORs and CORs between elongate rutile inclusions and garnet host, specific CORs were pooled into three COR groups: 103R/111G (“one <103>Rutile direction one <111>Garnet direction”), 001R/111G and 001R/100G. Within one {112} _Grt sector, core domains exhibit lower aspect ratios and higher abundance of rutile inclusions, with COR group 103R/111G being predominant. Contrastingly, the rim domain exhibits highly elongate rutile needles with lower abundance. The dominant COR group changes to 001R/111G and COR group 001R/100G appears. We suggest the decrease in inclusion abundance signals a decrease in the ratio of rutile nucleation rate to rutile growth rate, while the increase in aspect ratio signals an increase in the growth rate of rutile compared to the garnet growth rate (normal to the facet). The needle-bearing rim supposedly crystallized from a melt with higher Na, Si and OH^– content compared to the core (Kohn et al. 2024). A corresponding increase in diffusion rates of components in the melt is hypothesized to have decreased supersaturation with respect to rutile in a CBL, decreasing rutile nucleation rates and affecting relative growth rates. The preference for particular SPO-COR combinations should be influenced by the garnet surface configuration upon heterogeneous nucleation of rutile. Radial and lateral variations of rutile CORs and SPOs are thus attributed to changes in the nature of the garnet/melt interface, and/or the garnet growth mechanism.

Based on comparison with previous studies, changes in rutile COR group frequencies associated with increasing Si- (and likely OH^–) content of the melt are a systematic feature of magmatic fractional crystallization in peraluminous pegmatitic systems containing rutile-bearing garnet.

Funded by Austrian Science Fund (FWF): I4285-N37 and Slovenian Research Agency (ARRS): N1-0115

Kohn et al. (2024), Lithos, DOI: 10.1016/j.lithos.2023.107461

How to cite: Griffiths, T., Kohn, V., Alifirova, T., Daneu, N., Libowitzky, E., Ageeva, O., Abart, R., and Habler, G.: Systematic variations in shape preferred orientation and crystallographic orientation relationships of rutile inclusions in garnet upon fractional crystallization of pegmatoid melt, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-11567, https://doi.org/10.5194/egusphere-egu24-11567, 2024.

How to cite: Schorn, S., Moulas, E., and Stüwe, K.: Constraining latent heat of hydration using combined thermal- and garnet diffusion modelling, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-5164, https://doi.org/10.5194/egusphere-egu24-5164, 2024.

In Kumaun Lesser Himalaya, North Ramgarh Thrust and South Ramgarh Thrust define the northern and southern boundaries of the Almora Nappe. The Almora Nappe consists of two tectono-stratigraphic units, viz. the Ramgarh Group and the Almora Group. The Ramgarh Group consists of mylonitised granite gneisses capped by low-grade metamorphic rocks and the Almora Group consists of an interbanded sequence of metapelites and metapsammites progressively metamorphosed from the greenschist to upper amphibolite facies conditions. The Almora Group rocks comprise quartzites, garnet-mica schists, and K-feldspar–sillimanite gneisses. The garnet-mica schists are coarse to medium-grained, with well-developed foliation defined by chlorite-biotite-muscovite-garnet-plagioclase-quartz mineral assemblage, and the accessory minerals are apatite and zircon. The pelitic gneisses consist of garnet, kyanite, cordierite, sillimanite, and K-feldspar. Garnet is a common and important mineral in these metamorphic rocks to constrain P-T conditions.  In this study, we specifically focus on the textural details of garnets from the schists and gneisses of the Almora Group. The most common garnet is type II garnet which is synkinematic with S-shaped inclusions of quartz and biotite, while the older type-I garnets occur as stretched out grains in the matrix with the stretch direction generally parallel to the foliation. The garnet porphyroblasts are euhedral to subhedral and consist of quartz, plagioclase, muscovite, and apatite inclusions, which are wrapped around by muscovite and biotite lepidoblasts. The hematite leachings are observed around the periphery of the garnet porphyroblasts that suggest high f_O2 in the last stages of garnet growth. The micas microfolded in a few samples, with the presence of quartz in the folded hinge region. The garnets can be classified into at least two generations based on textural and petrographic attributes. Moreover, the presence of an idioblastic rim in the garnets suggests that the later phase of metamorphism outlasted the deformation. The occurrence of two generations of garnet i.e. garnet within garnet documented by (Joshi & Tiwari, 2004; Joshi & Tiwari, 2009) suggests a hiatus in crystallization and the associated metamorphic processes are likely attributed to two generations of pre-Himalayan metamorphisms.

How to cite: Srivastava, T., Joshi, M., Kumar, A., Tiwari, A. N., and Patel, S.: Petrological and textural characteristics of Garnet mica schists and Gneisses from Almora Group: Insights into pre-Himalayan metamorphism from Kumaun Himalaya, India, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-10532, https://doi.org/10.5194/egusphere-egu24-10532, 2024.