GMPV6.2 | Garnet, the ultimate tool in petrology
Garnet, the ultimate tool in petrology
Convener: Silvio Ferrero | Co-conveners: Gabriele Cruciani, Lorraine TualECSECS, Aratz Beranoaguirre
| Thu, 27 Apr, 16:15–18:00 (CEST)
Room 0.14
Posters on site
| Attendance Thu, 27 Apr, 14:00–15:45 (CEST)
Hall X2
Posters virtual
| Attendance Thu, 27 Apr, 14:00–15:45 (CEST)
Orals |
Thu, 16:15
Thu, 14:00
Thu, 14:00
Garnet is likely the most useful mineral to understand the evolution of basement areas on Earth. Many aspects of the metamorphic evolution of rocks can be unravelled by studying its zoning and inclusion pattern. In addition, garnet can be a treasure chest of mineral, melt and fluid inclusions capable to provide insights into the often obscures prograde/peak metamorphic history of the host rock evolution. Garnets in peraluminous granitoids offer windows into their genetic processes when they are entrained material originating from the source, or on the magma evolution if magmatic in origin. Finally, recent analytical developments in garnet dating offers new possibilities to characterize and better constrain the temporal evolution of a wealth of deep processes, from partial melting to skarn formation to subduction zone dynamics.
We invite our colleagues geoscientists, being them petrologists, geochemists, petrochronologists and structural geologists to present their studies involving (but not limited to) garnet as a crucial petrological tool to better understand Nature. Studies of fluid, melt and mineral inclusions in garnets and application of new analytical techniques, methodological approaches and dating protocols are welcome!

Orals: Thu, 27 Apr | Room 0.14

Chairpersons: Silvio Ferrero, Gabriele Cruciani
On-site presentation
Hans-Joachim Massonne

Al-garnet is a common constituent of medium- to high-grade metamorphic rocks of sedimentary and basic to acidic igneous protoliths. Due to its compositional variability (main components: almandine, grossular, pyrope, spessartine), this mineral is very important to decipher the pressure-temperature (P-T) evolution of these rocks. In many cases, garnet occurs as compositionally zoned porphyroblast being the result of prograde metamorphism during which hydrous minerals such as chlorite or lawsonite were decomposed. The relevant temperature interval for corresponding mineral reactions is usually in the range 450-650 °C. Thus, garnet is a nearly perfect mineral to decipher P-T conditions experienced by medium-grade metamorphic rocks. In addition, it preserves minerals which were enclosed during its growth. Therefore, P-T conditions can also be derived for a metamorphic stage before garnet growth based on inclusion minerals in garnet cores.

Prograde metamorphism, characterized by slight heating but significant pressure increase at high temperatures, commonly leads to garnet by breakdown of dry minerals such as cordierite and plagioclase (anorthite component). Corresponding reactions mainly occur between 0.5 and 1.8 GPa. Above this pressure range, garnet cannot be used to precisely determine P-T conditions of rocks, which were subjected to metamorphism in the high-temperature eclogite- and high-pressure granulite-facies, unless melting reactions took place, for example, with participating hydrous minerals such as micas, (clino)zoisite, and amphibole resulting in the formation of peritectic garnet. But the derivation of the P-T conditions of peritectic garnet formation, concerning pressures also below 1.8 GPa, requires complex thermodynamic modelling as various parameters such as H2O content of the rock and possible melt loss have to be considered. In addition, intracrystalline cation diffusion, particularly of Mg, in garnet complicates this derivation by the perceptible change of the original garnet composition at metamorphic peak temperatures above 750-800 °C. As both complex thermodynamic modelling for peritectic garnet, if applicable at all, and modelling of this cation diffusion were very rarely applied in the past, published P-T paths through the realms of the high-temperature eclogite- and granulite-facies are fairly uncertain and sometimes even wrong.

The annoying intracrystalline cation diffusion, however, can be valuable, for example, in the case of a contact of two garnet generations with different chemical compositions within a single grain. Modelling of this contact feature, which is typical of polymetamorphic rocks, can yield a time interval for early cooling at high temperatures. In such rocks characterized by an early medium-temperature and garnet-free mineral assemblage, initial growth of garnet during burial can be delayed due to an energetic barrier deferring the formation of garnet seeds. This so-called “garnet overstepping” concerns a pressure range up to 1 GPa above the garnet-in curve and leads to garnet porphyroblasts with nearly homogeneous chemical composition. Therefore, this type of garnet was, so far, frequently mistaken for a high-temperature garnet homogenized by intracrystalline cation diffusion. Despite such pitfalls and the aforementioned limitations, garnet is essential to deduce the P-T evolution of metamorphic rocks of deep-seated crustal sections and, thus, to better understand geodynamic processes involving the Earth’s crust.

How to cite: Massonne, H.-J.: Garnet, a marvellous mineral for deriving P-T paths of metamorphic rocks, but what are the pitfalls and limitations?, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-1857,, 2023.

On-site presentation
Dražen Balen and Hans-Joachim Massonne

The area of the Slavonian Mountains in Croatia is considered as a natural laboratory for the study of metamorphic processes on polymetamorphic rocks formed during the pre-Variscan, Variscan and Alpine orogenies. Since most lithologies contain garnet, understanding of its growth is an important to unravel the polymetamorphic evolution. Garnet up to 200 µm in size with atoll texture were found in quartz- and mica-rich rocks that have experienced peak metamorphism at intermediate temperatures and pressures in the amphibolite facies. These atoll garnet-bearing rocks occur in the oldest metamorphic complex of the Slavonian Mountains at several localities and are restricted to highly foliated mica-schists and/or paragneisses composed of quartz (~20 vol. %), plagioclase (~40-50 vol. %), biotite (20-30 vol. %), garnet (2-3 vol. %) and opaque minerals.

The observed atoll structures are practically restricted to larger garnet comprising a core, an intermediate cloudy zone composed of garnet and the minerals of the rock matrix (quartz, biotite, plagioclase) and a nearly inclusion-free rim. Smaller garnet with uniform texture and composition, which corresponds to that of the rim of larger garnet, also occurs. The core of the atoll garnet shows higher Ca contents than the rim (12-16 vs. 4-5 mol.% of grossular component), lower Fe contents (68-69 vs. 75-76 mol.% of almandine component) whereas Mg and Mn contents are similar. Furthermore, the three domains of the atoll garnet show almost regular outlines or crystallographic forms. Pseudosection modelling was used to reconstruct a preliminary and simplified P-T path, which is clockwise with maximum pressure conditions at ~1.0-1.2 GPa and temperatures of ~650 °C. However, it is likely that these conditions were followed by a significant pressure decrease accompanied by resorbtion of garnet. Shape of garnet core and cloudy zone, but also mineral phase relations, suggest a second stage of garnet growth, forming the rim, during another pre-Variscan stage of amphibolite-facies metamorphism at P-T conditions of 0.5 GPa and 530 °C. This interpretation resulted from in-situ monazite dating, yielding two mean ages at 522±6 and 473±11 Ma (2σ).

Atoll-shaped garnets are uncommon and have been recognized in contrasting metamorphic environments worldwide. Several models have been proposed to explain their formation, including preferential dissolution of garnet cores by fluid infiltration, polymetamorphism, coalescence of subgrains, and kinetic control associated with rapid growth. Here, the formation of the atoll garnet is interpreted by the following processes: external fluid infiltration into the fine-grained rocks and element exchange between the core of primary small garnet grains and matrix, dissolution of the garnet core and later replacement by a new garnet during a subsequent metamorphic event.

How to cite: Balen, D. and Massonne, H.-J.: Zoned and atoll garnet from the Slavonian Mountains (Croatia) and their significance for the evolution of a complex polymetamorphic terrane, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-1991,, 2023.

On-site presentation
Francesca Piccoli, Daniela Rubatto, Leo J. Millonig, and Axel Gerdes

Metarodingites are metasomatized mafic dykes that are embedded within serpentinized mantle rocks and commonly contain garmet. Garnet in this rock type has the potential to preserve compositional and chronological information of the entire metamorphic-metasomatic evolution, from the ocean floor to deep subduction. In this study, we investigate the chemical and chronological record of garnet from chloritized metarodingites and garnet veins, from the Zermatt-Saas unit (Unter Theodulgletscher unit and Pfulwe pass). Compositional major and trace element maps reveal that a first generation of Ti-rich garnet is consumed during chloritization, while a second generation of Ti-poor garnet grows in textural equilibrium with chlorite and titanite. In both metarodingite and garnet vein, garnet rim displays an enrichment in Cr, suggesting that late garnet records the infiltration of fluids equilibrated with serpentinites. Fluids liberated from dehydrating serpentinites infiltrate the metarodingite leading to the first garnet generation dissolution, chloritization and titanite crystallization. To validate this hypothesis, we performed LA-ICPMS U-Pb dating of garnet, titanite and zircon. U-Pb dating of garnet core and rim, returned overlapping ages between ca. 44 and 46 Ma, which coincide with previous estimation of peak metamorphic conditions. Titanite from the metarodingite samples yields an age of ca. 45 Ma, which indicates that fluid release and chloritization occurred indeed at peak conditions. Garnet veins cutting across the foliation of the metarodingite and associated titanite are instead resolvably younger and yield ages of ca. 38-39 Ma and ca. 36 Ma, respectively. Zircon in chloritized mafic dykes from Pfulwe pass consists of a Jurassic magmatic core and a metamorphic rim of ca. 47 Ma, confirming that the major fluid-release event and related metasomatism occurred between 45-47 Ma. Rutile yields a younger age of ca. 34 Ma, probably linked to re-setting during exhumation. In conclusion, we show how garnet from metarodingites preserves the metamorphic-metasomatic history and can be used to gather information on (de)hydration reactions during subduction. The consistency of our multi-mineral geochronological data further indicates that petrochronology of garnet from metarodingites is a robust way to track in time metasomatic events in the subducted oceanic lithosphere.

How to cite: Piccoli, F., Rubatto, D., Millonig, L. J., and Gerdes, A.: Garnet in metarodingites: composition, chronology and link to dehydration/hydration reaction during subduction, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-4908,, 2023.

On-site presentation
Rene Asenbaum, Martin Racek, Tereza Zelinková, Vojtěch Janoušek, Elena Petrishcheva, and Rainer Abart

Mafic–ultramafic lenses embedded in felsic granulites of the Gföhl Unit, Moldanubian Zone, are considered to be 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. The primary mineral assemblage comprises calcium-rich garnet (XGrs = 0.4), kyanite, and sodium-rich clinopyroxene (XNa_M2 = 0.29) (± quartz), which indicates pressures above 1.8 GPa and temperatures around 1000 °C. Towards the margins of the mafic lens, the garnet pyroxenites were increasingly overprinted at lower pressures leading to the destabilization of kyanite, Na-rich clinopyroxene, and garnet. A first decompression phase is represented by garnet-hosted sapphirine–spinel–plagioclase symplectites supposedly replacing kyanite and clinopyroxene. A second stage is evident from the partial resorption of garnet by plagioclase and clinopyroxene in the form of a peculiar corrosion tubes penetrating the garnet in a worm-like fashion. Finally, the third stage decompression assemblage is represented by plagioclase–orthopyroxene–spinel symplectites partially replacing garnet. In all cases, garnet shows pronounced secondary compositional zoning towards the decompression products. The secondary zoning is qualitatively similar for the sapphirine–spinel–plagioclase symplectites and the plagioclase–clinopyroxene corrosion tubes and is characterized by a strong decrease of the Grs content accompanied by an increase of the Alm and Prp contents towards the decompression products. For the sapphirine–spinel–plagioclase symplectite, the garnet composition changes from Alm14Prp42Grs44 in the pristine garnet to Alm22Prp63Grs15 at the interface to the symplectite. The compositional change towards the corrosion tubes is from Alm19Prp40Grs41 to Alm30Prp54Grs16. The secondary zoning towards the plagioclase–orthopyroxene–spinel symplectites is characterized by an increase of XAlm from 0.19 to 0.27 and a concomitant decrease of XPrp from 0.55 to 0.49 at constant XGrs of 0.25. In all cases, the compositional changes are gradual suggesting diffusion-mediated re-equilibration of the garnet at decreasing pressures. Time scales for the duration of decompression were estimated by fitting a multicomponent diffusion model to the observed compositional patterns. Depending on the choice of the diffusion coefficients, the time scales vary from several hundreds to hundred thousands of years, whereby the earliest decompression features yield time scales that are five times longer than those obtained from the corrosion tubes and about ten times longer than those obtained from the plagioclase–orthopyroxene–spinel symplectites. These timescales reflect the duration from the onset of the different decompression-induced mineral reactions to the time when the rocks cooled below about 700 °C and the composition patterns of the garnet were effectively frozen. The longest timescales obtained from the early decompression reactions are on the order of 100,000 years and the shortest timescales obtained from the late-stage symplectites are on the order of 1,000 years. Considering the regional metamorphic setting of the Moldanubian Zone, such timescales are remarkably short and suggest rapid transport of the mafic–ultramafic lithologies from mantle depths to the mid-crustal level. Concomitant incorporation into a dominantly felsic environment led to immediate cooling.

How to cite: Asenbaum, R., Racek, M., Zelinková, T., Janoušek, V., Petrishcheva, E., and Abart, R.: Decompression time scales of mantle fragments constrained by secondary chemical zoning of garnet from the Gföhl Unit, Moldanubian Zone, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-14192,, 2023.

On-site presentation
Maria Rosa Scicchitano, Michael C. Jollands, Ian S. Williams, Jörg Hermann, Daniela Rubatto, Noriko T. Kita, William O. Nachlas, John W. Valley, Stephane Escrig, and Anders Meibom

Knowledge of oxygen diffusion in garnet is crucial for a correct interpretation of oxygen isotopic sig­natures in natural samples. Scicchitano et al. (2022) reported a series of experiments with pyrope and YAG at P-T of 1-atm to 2.5 GPa and 900 °C to 1600 °C, either under nominally-dry or water-saturated conditions, to better constrain the diffusivity of oxygen in garnet. Analysis of 18O/(18O+16O) profiles by Secondary Ion Mass Spectrometry (SIMS) shows that: (i) diffusivities in pyrope and YAG crystals annealed under similar conditions (P = 1 GPa and T = 900 °C) are comparable, suggesting a limited effect of chemical composition on oxygen diffusivity; (ii) diffusivity values calculated for water-saturated experiments at 900 °C fall on the Arrhenius curve described by nominally dry experiments performed at T = 1050-1600 °C; and (iii) several profiles deviate from the Gaussian error function, suggesting complex diffusion behaviour related to diffusion via interstitial (fast) and vacancy (slow) mechanisms. Modelling this process yields oxygen diffusion coefficients, D, that differ by approximately two orders of magnitude between the fast and slow diffusion mechanisms. The new experimental data suggest, however, that the slow mechanism is prevalent in natural garnet compositions and probably controls the retentivity of oxygen isotopic signatures in natural samples. Even though oxygen diffusivity in garnet is comparable to Fe-Mn and Ca diffusivity at high temperature (> 850 °C), oxygen diffusivity is slower than cation diffusivity at P-T conditions typical of crustal metamorphism due to its larger activation energy. Original oxygen isotopic signatures therefore can be retained in garnet showing zoning partially re-equilibrated by the diffusion of other major elements.                 



Scicchitano et al. (2022), American Mineralogist, 107, 1425-1441.

How to cite: Scicchitano, M. R., Jollands, M. C., Williams, I. S., Hermann, J., Rubatto, D., Kita, N. T., Nachlas, W. O., Valley, J. W., Escrig, S., and Meibom, A.: Experimental calibration of oxygen diffusion in garnet and implications for retention of primary oxygen isotopic signatures, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-1226,, 2023.

On-site presentation
Victoria Kohn, Taisia Alifirova, Nina Daneu, Thomas Griffiths, Rainer Abart, and Gerlinde Habler

Directed garnet growth in a peraluminous pegmatoid from the Moldanubian zone in the Bohemian Massif (AT) formed almandine-spessartine garnet that crystallized at the transition from melt to subsolidus stage. One side of the garnet crystal was in contact with a solidified matrix, the other with melt, resulting in asymmetric microstructure, crystal morphology and major element compositional zoning.

Based on major element composition, three garnet growth stages are inferred: (i) magmatic stage during which garnet grew as part of the coarse-grained assemblage Pl + Grt + Ky + Bt. This growth zone has the highest Mn content and P concentrations of 0.4 – 0.5 wt%, (ii) intermediate stage with decreasing Mn, and increasing Fe, Mg and Ca contents and Mg# (=Mg/(Mg+Fe)), (iii) subsolidus stage forming garnet reaction rims with highest Mg and Ca contents and Mg#.

Growth stage (i) involves the development of sector zoning, which is defined by the colour of garnet and the presence of < 1 micrometer sized inclusions observed in optical light microscope (OM). These are dominated by phosphates in Grt{110} growth sectors and by rutile in Grt{112} sectors. Additionally, nanoinclusions of 20 – 50 nm size were identified by scanning transmission electron microscopy of garnet even for zones that appear inclusion-free in OM.

Chemical compositions obtained from electron microprobe analyses integrate over garnet and the inclusions and thus also reflect the different nanoinclusion-contents of the two garnet growth sectors. Compared to the Grt{112} sector, the Grt{110} sector is c. 0.1 wt% higher in P, 100 – 150 ppm higher in Na, and 100 – 200 ppm lower in Ti, which is in line with the prevalence of Na-containing phosphates and the comparatively lower abundance of rutile in this sector.

The following scenarios are considered for the genesis of the nanoinclusions: As sector zoning has developed during magmatic conditions of growth stage (i), overgrowth of pre-existing accessory phases is implausible, as no mechanism for selective incorporation of different phases at different facets is known. Instead, facet specific minor and trace element partitioning during garnet growth and subsequent exsolution, or alternatively, sector specific nucleation and co-growth of accessory rutile and phosphates are both reasonable explanations for the observed distribution of inclusions in specific garnet sectors. These considerations indicate facet-selective processes likely related to the different crystal structure features exposed at the interfaces in contact with the melt.

We conclude that facet specific formation of nanoinclusions is an important factor controlling the trace element composition of pegmatoid garnet apart from bulk melt composition and pT-conditions. When interpreting P, Na and Ti contents in garnet, the potential presence of nanoinclusions that are invisible in the optical light microscope needs to be accounted for, as they may be more widespread in pegmatoid garnet than expected.

The study was funded by the Austrian Science Fund (FWF): I4285-N37 and the Slovenian Research Agency (ARRS): N1-0115.

How to cite: Kohn, V., Alifirova, T., Daneu, N., Griffiths, T., Abart, R., and Habler, G.: Nanoinclusions in apparently inclusion-free sector-zoned pegmatoid garnet – their impact on P, Ti and Na concentrations, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-4836,, 2023.

On-site presentation
Matthias Konrad-Schmolke, Ralf Halama, David Chew, Céline Heuzé, and Jan De Hoog

Metamorphic garnet can be the ultimate source of information about geodynamic and geochemical processes in the Earth’s crust. Compositionally zoned garnet porphyroblasts preserve records of the host rock’s reaction path, its element transport properties, the fluid-rock interaction during metamorphism as well as absolute ages and rates of these processes. Especially variations of rare earth element (REE) concentrations in garnet are reflecting many of these geodynamic and geochemical processes. In order to extract this information, the thermodynamic equilibrium and kinetic contributions of the REE uptake in garnet must be distinguished and quantified, for which high resolution major- and trace element mappings together with numerical growth simulations are an indispensable tool.   

Utilizing high resolution trace element and µ-Raman mapping together with combined thermodynamic-geochemical-diffusion models we discriminate the equilibrium and kinetic aspects of the heavy (H) REE uptake in a garnet porphyroblast from a high-pressure/low temperature blueschist from the Dominican Republic. Like many metamorphic garnets from different rock types and tectonic settings, the HREE zoning in the investigated garnet comprises an inner, bell-shaped part with a pronounced central peak and an overall decrease of all HREE from core to inner rim. The central peak in the garnet core decreases in intensity with decreasing atomic number of the REE. This trend is followed by a concentric zone of HREE enrichment and a drastic HREE decrease towards the outermost rim. Superimposed on this common trend is a concentric pattern of minor recurring fluctuations in the HREE concentrations with regularly spaced sets of peaks and troughs along the entire garnet radius.

Comparison of the trace element mappings and thermodynamic-geochemical models show that the inner, bell-shaped part results from fractional garnet growth in an unchanged mineral assemblage. The model results further show that the width of the central peak is controlled by the bulk permeability of the interconnected transport matrix and the fraction of matrix minerals that the garnet equilibrates with. The correlation of high resolution µ-Raman mappings of the inclusion suite and the trace element mappings indicate that the REE enrichment zone is caused by HREE redistribution during the titanite-rutile transition.

The superimposed REE fluctuations result from changing element transport properties of the host rock and mark recurring changes from equilibrium REE uptake to transport-limited REE uptake in garnet. Such fluctuating element transport properties can be best explained by pulse-like fluid fluxes that rhythmically change the interconnectivity of the intercrystalline transport matrix. Increasing numbers of published spatially highly resolved REE analyses show that such trace element fluctuations are common in metamorphic garnet indicating that recurring changes in rock permeabilities due to pulsed fluid fluxes are a common phenomenon during metamorphism.

How to cite: Konrad-Schmolke, M., Halama, R., Chew, D., Heuzé, C., and De Hoog, J.: Towards a comprehensive use of garnet as an indicator of geodynamic-geochemical processes, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-11106,, 2023.

On-site presentation
Jan Schönig, Carsten Benner, Guido Meinhold, Hilmar von Eynatten, and Keno Lünsdorf

The global onset and evolution of subduction-driven plate tectonics is one of the most debated topics in Earth sciences. Although very diverging views are hold, most observations indicate a transition from a stagnant- to a mobile-lid regime in the late Archean (e.g. Palin et al. 2020). Since then, geothermal gradients (T/P) of metamorphic rocks decreased, interpreted as an evolution from warm and shallow to cold and steep subduction (e.g. Brown et al. 2006), which may explain the oldest unequivocal evidence of UHP metamorphism at ~620 Ma (Jahn et al. 2001). By contrast, extreme UHP conditions of ~7 GPa at ~975 °C have been supposed for ~1.8 Ga crustal rocks in Western Greenland, mainly based on carbonaceous inclusions in garnet interpreted as diamond partially replaced by graphite as well as oriented inclusions of a hydrous phase interpreted as orthopyroxene exsolution from a majoritic precursor (Glassley et al. 2014).

In order to find mineralogical evidence for UHP metamorphism, like coesite, we used a detrital approach that has been demonstrated to be very powerful (Schönig et al. 2018, 2019, 2020; Baldwin et al. 2021). Modern sands from eight catchments draining the proposed UHP terrane in Western Greenland were extensively screened by semi-automated Raman heavy-mineral analysis (n = 52,130 grains) and electron microprobe analysis of garnet major-element chemistry as well as hyperspectral Raman imaging (>680 million spectra) of mineral-inclusion assemblages (n = 2,674 garnets). In all samples, amphibole, garnet, orthopyroxene, augitic clinopyroxene, and sillimanite represent the majority of heavy minerals, reflecting erosional material sourced from amphibolite- to granulite-facies rocks. Garnet chemistry and mineral inclusion assemblages, particularly the common co-existence of sillimanite and rutile inclusions, indicate a major garnet growth stage at MP to HP granulite-facies conditions. Though, lower garnet XMg and/or higher XMn and/or XCa as well as a more frequent occurrence of hydrous mineral inclusions (amphibole and phlogopite-biotite) of a smaller proportion (~20 %) imply garnet growth at lower temperature conditions, interpreted as relicts of prograde metamorphism. Garnets predicted to be grown at the highest P conditions (~6 %) commonly host inclusions of augitic clinopyroxene, amphibole, plagioclase, and quartz, mainly indicating HP amphibolite facies conditions that in maximum may have reached the transition zone between amphibolite- and eclogite-facies conditions. Furthermore, we show that neither the reported existence of diamond nor the interpretation of a majoritic precursor hold against a critical re-assessment. Overall, the total absence of minerals indicating UHP conditions (like coesite and diamond) and even HP conditions (like omphacite or glaucophane) in our large detrital dataset as well as alternative interpretations for reported UHP indicators strongly challenge the existence of a Paleoproterozoic UHP terrane in Western Greenland.

How to cite: Schönig, J., Benner, C., Meinhold, G., von Eynatten, H., and Lünsdorf, K.: Detrital garnet petrology challenges Paleoproterozoic UHP metamorphism in Western Greenland, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-12621,, 2023.

On-site presentation
Matthijs Smit

The wish to obtain age data from garnet became reality in 1980 when Griffin and Brueckner1 published the first-ever Sm-Nd garnet ages. The Silurian Sm-Nd ages that they obtained for eclogites, which until then had been considered Precambrian in age, showed the tremendous potential of this technique. Following analytical developments and constraints on chronometer systematics2, the technique became widely used in various metamorphic systems and settings, including in rocks from the upper mantle. Following the first successful attempt at Lu-Hf garnet dating3 and calibration of the 176Lu decay constant4,5, the Lu-Hf system was established as an alternative to Sm-Nd in garnet chronology, with possible advantages, including higher P/D, shorter half-life and lower daughter-element diffusivity.

Like other radiometric techniques that use isotope dilution, these techniques have the advantage of high precision, but are time-intensive and provide grain-averaged ages. Dating of individual garnet zones with either technique is possible, but only in specific cases6-8. Novel approaches to in-situ garnet chronology by combining laser ablation micro-sampling with U-Pb analysis by MC-ICP-MS9 or Lu-Hf analysis by ICP-MS/MS10 provide an exciting addition to garnet chronology, with advantages and disadvantages inverted compared to convention techniques: lower precision, but rapid throughput and high spatial resolution.

The field of garnet chronology is now at an exciting point, where applications of the technique have greatly diversified, new techniques are emerging, and improvements are ongoing – in sample-size requirements for conventional techniques, and accuracy and precision for in-situ techniques. How do these techniques compare, and which approach is best, or "good enough", in a given case? This presentation will focus on the status-quo anno 2023, and will explore frontier applications of Lu-Hf garnet dating in the crust and mantle.

1 Griffin, W.L., Brueckner, H.K. (1980) Nature 285, 319-321.
2 Mezger, K., Essene, E.J., Halliday, A.N. (1992) Earth Planet. Sci. Lett. 113, 397-409.
3 Duchêne, S., Blichert-Toft, J., Luais, B., Télouk, P., Lardeaux, J.-M., Albarède, F. (1997) Nature 387, 586-589.
4 Scherer, E.E., Münker, C., Mezger, K. (2001) Science 293, 683-687.
5 Söderlund, U., Patchett, P.J., Vervoort, J.D., Isachsen, C.E. (2004) Earth Planet. Sci. Lett. 219, 311-324.
6 Pollington, A.D., Baxter, E.F., (2010) Earth Planet. Sci. Lett. 293, 63-71.
7 Dragovic, B., Baxter, E.F., Caddick, M.J. (2015) Earth Planet. Sci. Lett. 413, 111-122.
8 Tual, L., Smit, M.A., Cutts, J.A., Kooijman, E., Kielman-Schmitt, M., Majka, J., Foulds, I. (2022) Chem. Geol. 607, 121003.
9 Millonig, L.J., Albert, R., Gerdes, A., Avigad, D., Dietsch, C. (2020) Earth Planet. Sci. Lett. 552, 116589.
10 Simpson, A., Gilbert, S., Tamblyn, R., Hand, M., Spandler, C., Gillespie, J., Nixon, A., Glorie, S. (2021) Chem. Geol. 577, 120299.

How to cite: Smit, M.: Garnet chronology: status quo, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-10218,, 2023.

On-site presentation
Harold Stowell, Elizabeth Bollen, Joshua Schwartz, and Keith Klepeis

Garnet in the lower crust may grow during sub-solidus and hyper-solidus heating, and in some cases during cooling. Equilibrium phase diagram sections for mafic orthogneiss predict that the greatest volume of garnet should grow over a narrow temperature range with heating above the solidus. This peritectic garnet can preserve a record of multiple magma injections into complex magma mushes that form plutons in continental magmatic arcs. The exhumed lower crustal section from Fiordland New Zealand provides an ideal laboratory for using garnet to track processes in the continental magmatic arc within the Gondwana margin during the Cretaceous.

The Western Fiordland Orthogneiss (WFO) dominates the inboard lower crustal section exposed in Fiordland. The three major plutons in the WFO preserve a range of igneous and metamorphic fabrics, with some of the oldest igneous foliations and magmatic contacts obvious in the central Misty Pluton and the youngest sub-solidus fabrics observed in the southernmost Malaspina Pluton which intruded c. 117 Ma based on zircon U-Pb ages. In the Malaspina Pluton, garnet Sm-Nd ages for ≥1 cm diameter peritectic garnet grains along cross-cutting trondhjemite veins range from 117.0±4.1 and 116.2±2.0 Ma for garnet cores (N=3), and from 115.8±2.6 to 108.0±2.0 Ma for bulk grains and rims (N=10). In contrast, garnet Sm-Nd ages for ≤0.5 cm diameter garnet grains from 3 locations In the Malaspina are 104.1±1.8, 106.2±2.1, and 103.6±2.2 Ma. Major and trace element zoning is compatible with insignificant volume diffusion in the large grains and significant compositional changes in the small grains. The Bloch et al. (2020) diffusion coefficients for Sm and Nd indicate that significant diffusion would cease at c. 700°C in small grains from the Malaspina Pluton; therefore, we calculate cooling rates of c. 21 (northeast) to c. 10°C (southwest) per m.y. The cooling ages combined with phase diagram sections and mineral thermobarometry indicate exhumation rates of 0.2 to 0.4 kbar per m.y. for the southwestern part of the Malaspina Pluton.

Our results indicate that initial garnet growth ranged from near-synchronous to c. 4 m.y. after construction of the Malaspina Pluton by injection of magma sheets. We speculate that faster cooling in the northeastern Malaspina may have been driven by extension during growth of sub-solidus L-S tectonites which are best developed in this area. Combining these data with structural analysis and detailed evaluation of magma source compositions for individual sheets in WFO plutons allows us to evaluate magma plumbing systems for the Fiordland Arc.

How to cite: Stowell, H., Bollen, E., Schwartz, J., and Klepeis, K.: Tracking magma intrusion and cooling in the lower crust with garnet Sm-Nd geochronology and phase diagram sections, Fiordland New Zealand, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-16751,, 2023.

Posters on site: Thu, 27 Apr, 14:00–15:45 | Hall X2

Chairpersons: Lorraine Tual, Aratz Beranoaguirre
Mineralogical and fluid inclusion studies of garnet and rutile quartz, Kashan, Iran
rahim Masoumi, Mohammadreza Rezapour, Azam Entezari Harsini, Roohangiz Masoumi, Mohammadamin Fozouni Sarghin, and Saeideh Aghamohammadi Tolon
Silvio Ferrero, Gautier Nicoli, Robert Darling, and Bernd Wunder

Garnet often traps droplets of anatectic melts (i.e. melt inclusions – MI), also called  nanogranitoids once crystallized, whose characterization allows us to clarify deep crustal melting processes. the Adirondacks (US), is an ideal location to investigate mafic melting and crustal growth . As a portion of the Grenville Province, this massif mainly consists of intrusive bodies metamorphosed during the Ottawan Orogeny (1090-1050 Ma). Nanotrondhjemites were previously reported in in the giant garnets of Barton mine, Gore Mountain area (Ferrero et al., 2021) in the central-southern part of the Adirondacks. Melting is, however, not limited to such location: MI-bearing garnets are also found in the mafic granulites at Hooper mine, approximately 5 km NW of the Barton Mine. Such garnets have been divided in two types based on size, chemical zoning, habitus as well as the composition of the trapped melt.

Type 1 garnets are large, euhedral porphyroblasts of diameter >5 cm, with a rather homogeneous composition similar to the Barton mine garnets. The nanogranitoids here are scattered randomly and contain a constant assemblage consisting of quartz, kumdykolite/albite, amphibole(s) and minor amounts of phlogopite. Re-melting experiments conducted via piston cylinder led to the complete re-homogenization of the inclusions at 940°C / 1.0 GPa with the generation of a hydrous trondhjemitic glass.

Type 2 garnets are instead significantly smaller, <1 cm in diameter, and xenoblastic in shape. Their composition resembles type 1 garnets with the exception of low Ca and Y in the MI-bearing domains. The nanogranitoids in type 2 garnets contain quartz, kokchetavite/K-feldspar, kumdykolite/albite and phlogopite. Such phase assemblage is remarkably different from the previous nanogranitoids, i.e., amphiboles are notably absent whereas kokchetavite is present. Such inclusions re-homogenize to a less hydrous granitic glass at lower T, 900°C, and same P conditions (1 GPa) with respect to the previous MI type.

LA-ICP-MS analyses show different signatures for the two melt types, hence suggesting different melt production mechanisms. The trondhjemitic melt trapped in type 1 garnets shows the same enrichment in Th, U, Zr and Hf observed in the Barton Mine, thus suggesting a similar genesis for this melt, i.e., a H2O-fluxed melting of a gabbro protolith (Ferrero et al., 2021). The granitic melt in type 2 garnets does not have such features, and we propose amphibole dehydration melting as the most likely genetic mechanism for this melt.

Altogether, microstructures, microchemistry and experiments indicate that the Adirondacks experienced multiple partial melting events at T≥ 900°C at in the deep crust. Moreover, the compositions of the melts generated at both Hooper mine and Barton mine defines a trend characteristic of primitive TTG melts or TTG embryos.


Ferrero S. Wannhoff I., Laurent O., Yakymchuk C., Darling R., Wunder B., Borghini A. & O’Brien P.J., 2021. Embryos of TTGs in Gore Mountain garnet megacrysts from water-fluxed melting of the lower crust. Earth Planet. Sci. Lett., 569, 117058.

How to cite: Ferrero, S., Nicoli, G., Darling, R., and Wunder, B.: Different embryos of TTGs in garnet at Hooper mine, Adirondacks (New York State, US), EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-6001,, 2023.

Jonathan Pownall and Kathryn Cutts

The Lapland Granulite Belt is an arcuate Paleoproterozoic (c. 1910–1880 Ma) granulite-facies metamorphic complex spanning arctic Finland, Norway, and the Kola Peninsula of Russia.  It has been interpreted as a core zone to the larger Lapland–Kola orogen that formed between the colliding Kola and Karelian Archean continents at 1930­­–1910 Ma (Daly et al., 2006; Lahtinen & Huhma, 2019).  The granulite complex comprises grt + crd + sill pelitic migmatites hosting centimetre- to kilometre-thick enderbitic (qtz + pl + opx ± cpx) and noritic (pl + opx ± hbl) sheets emplaced as mafic intrusions (Tuisku et al., 2006).  Along the granulite belt’s southwest margin is a large anorthosite body, the Angeli Anorthosite. 

The tectonic mechanism for granulite-facies metamorphism and melting is unresolved, with both the enderbite and anorthosite bodies as potential heat sources.  It is also unclear if the orogen formed in a modern-style arc setting, or in a more ‘Archean-type’ collision zone (Lahtinen & Huhma, 2019).  Previous pressure-temperature estimates for peak metamorphism are 850°C and 5­–9 kbar (Tuisku et al., 2006).  Spinel + quartz inclusions in garnet also hint at high (> 850°C) peak temperatures.  However, there has been relatively little published on the metamorphic history of the orogen in recent decades, and, until now, sample-specific phase equilibria modelling has yet to be applied to the region. 

Here we present preliminary fieldwork, petrographic observations, and geochemical data for a suite of garnet-bearing granulites from the Lapland Granulite Belt focussed on the Inari, Ivalo, and Kárášjohka valley regions.  Electron probe micro-analyser (EPMA) data have been acquired by the new JEOL JXA-iSP100 Superprobe at HelLabs, University of Helsinki.  Preliminary EPMA data analysis by XMapTools (Lanari et al., 2019) is introduced.



Daly, J. S., Balagansky, V. V., Timmerman, M. J. & Whitehouse, M. J., 2006. The Lapland-Kola Orogen: Paleoproterozoic collision and accretion of the northern Fennoscandian lithosphere. In: Gee, D., G. & Stephenson, R. A. (eds.), European Lithosphere Dynamics: Geological Society Memoirs 32, 579–598.

Lahtinen, R. & Huhma, H., 2019. A revised geodynamic model for the Lapland-Kola Orogen. Precambrian Research 330, 1­–19.

Lanari, P., Vho, A., Bovay, T., Airaghi, L., & Centrella, S., 2019. Quantitative compositional mapping of mineral phases by electron probe micro-analyser. Geological Society of London, Special Publications 478, 39–63.

Tuisku, P., Mikkola, P. & Huhma, H., 2006. Evolution of Migmatitic Granulite Complexes: Implications from Lapland Granulite Belt, Part I: Metamorphic geology. Bulletin of the Geological Society of Finland 78, 71–105.

How to cite: Pownall, J. and Cutts, K.: Garnet granulites from the Lapland Granulite Belt, arctic Finland, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-6293,, 2023.

Wen-Han Lo, Chin-Ho Tsai, and Dominikus Deka Dewangga

Two metamélange units in the Yuli belt contain high-pressure (HP) metamorphic rocks attesting to subduction zone metamorphism. These metamélange units are characterized by blocks of serpentinites, metaigneous, and metavolcaniclastic rocks in a matrix of garnet pelitic schist. Some of the blocks contain HP minerals (glaucophane, omphacite), but their peak metamorphic conditions are still poorly constrained. This presentation reports a compilation of garnet textures and compositions from these blocks and adjacent pelitic schists. Phase equilibrium modeling (Perple_X) with garnet isopleths is applied to estimate the P-T conditions quantitatively. We recognized two types of garnet compositional nature in these HP blocks. Garnet in the glaucophane schists (GlnS) displays a Mn-rich core and Fe-rich rim. By contrast, garnet in the garnet-paragonite-epidote amphibolites (GPEA) is nearly homogeneous and relatively Mg-rich. Garnet of the adjacent pelitic schists (PS) shows a similar zoning to that of the GlnS. The peak P-T conditions of these HP rocks constrained by garnet isopleths reveal a P-T range in 10–18 kbar and 500–580 ºC. The inferred P-T paths for the GlnS and PS are clockwise, whereas the one for the GPEA seems counter-clockwise. Our new P-T constraints suggest that these HP blocks and the metapelitic matrix likely formed in a paleo-subduction interface. The discrepancy in P-T data and paths among different rock types may reflect a kind of tectonic mixing.

Keywords: subduction zone, subduction interface, tectonic mélange, tectonic mixing, phase equilibrium modeling, garnet chemical zoning


How to cite: Lo, W.-H., Tsai, C.-H., and Dewangga, D. D.: On the peak metamorphic conditions of the Yuli belt, eastern Taiwan: new constraints using garnet isopleths, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-12855,, 2023.

Alessia Tagliaferri, Evangelos Moulas, Stefan Markus Schmalholz, and Filippo Luca Schenker

This contribution focuses on the timing of metamorphism within the Lepontine dome, located in the Penninic domain of the Central European Alps (Switzerland). The Lepontine dome is formed by crystalline basement nappes bent towards the south in the migmatites of the Southern Steep Belt. The Lepontine nappes are formed by metamorphic rocks, mainly ortho- and para-gneisses, whose foliation dip-direction together with the attitude of thrust sheets define a dome shape. The Lepontine dome is characterized by a widespread Barrovian metamorphism of Tertiary age whose expressions are: an asymmetric concentric zonation of mineral-zone boundaries, locally dissecting the tectonic nappe contacts, and a NW-SE directed mineral and stretching lineation developed during peak metamorphic conditions, which suggests non-coaxial deformation during thrusting.

In a recent work, we dated the upper amphibolitic non-coaxial deformation. We performed U-Pb zircon dating on multiple samples which resulted in two groups of ages at ca. 31 Ma and 22 Ma. We attribute the development of the amphibolite facies syn-kinematic metamorphism to the widespread-recorded event at 31 Ma. This time constraint still lacks of specific information on the duration of the temperature peak, the subsequent cooling and the nature of the cooling process. To solve the temporal character of the formation and evolution of the high-grade metamorphic rocks, we applied a method to determine cooling rates calculated using post-peak-T estimates as initial temperature in the metapelites of the Lepontine dome. We selected garnet-paragneisses from the core of the Lepontine dome at different levels in the nappe pile, being the structural lowest one at the base of the Simano nappe and the uppermost in the Cima Lunga unit. Their mineral assemblage is marked by quartz, feldspar, garnet, biotite, white mica, kyanite, local staurolite, rutile and minor phases. Garnets are pre- to syn-kinematic with respect to the amphibolite facies metamorphic foliation. Furthermore, in the migmatitic paragneisses of the Southern Steep Belt we analysed one sillimanite-rich sample, where we found textural evidences of the presence of melt and k-feldspar.

We exploited garnet compositional re-adjustment due to major-element diffusion at the borders of the crystal to extract cooling rates, whose estimates where constrained by temperatures obtained via geothermometry and phase equilibria modelling. The post-peak temperatures of re-equilibration were estimated at ca. 600 ± 50 °C at the border garnet-biotite, where a step in garnet major element composition was seen. The diffusion time necessary to fit garnet-rim profiles along short transects (less than 1 mm length) was calculated as a preliminary result, giving a value < 2 Ma for most of the samples.

Note that a cooling time < 1 Ma is typical of transient thermal regimes, however the type of thermal regime can be properly evaluated only with the calculation of the cooling rate. High cooling rates are consistent with high temperatures in a localized area developed in a small time frame, such in the case of thrust-related shear heating during metamorphism. Slow cooling rates indicate instead a regional thermal history. Our preliminary results suggest high cooling rates for the high-grade metapelitic rocks of the Lepontine dome.

How to cite: Tagliaferri, A., Moulas, E., Schmalholz, S. M., and Schenker, F. L.: Garnet compositional re-adjustment: cooling rate constraint in metapelites from the Lepontine dome (Central European Alps), EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-9052,, 2023.

Anna Hagen

Skarn type grandite garnet in oceanic lower crust of the Troodos Ophiolite, Cyprus. Grandite growth from a new view?


Anna Hagen 1, Romain Bousquet 1, Colin Devey 2, Thor Hansteen 2


1 Kiel University, Institute of Geosiences, Kiel – Germany    2 GEOMAR Helmholtz Centre for Ocean Research Kiel, Kiel – Germany


The Troodos ophiolite on Cyprus allows us to have a detailed view in the stratigraphy of a late cretaceous oceanic lithosphere, as this oceanic floor including a former spreading axis (mid oceanic ridge) is rotated and uplifted since then. Both structural and petrological, the Troodos ophiolite is a great study area to describe and understand former magmatic processes during the formation of new oceanic crust as well as for the comparison with today’s spreading ridges (Robinson et al., 2003).


Within the stratigraphy of oceanic crust as found in the Troodos ophiolite, several sequences of plagiogranites occurred from the lower crustal gabbroic complex until the sheeted dyke complex (Marien et al., 2019). In addition to these plagiogranites we now find a single plagiogranite complex incorporating a large amount of epidote and grandite, the later one up to fist size, that has not described yet. Epidote and grandite crystals show partially intergrown patterns within this special type of plagiogranite.


The grandite type garnets show features similar to other known skarn type associated grandites including an onion like, really fine and sharp oscillatory chemical zoning and both isotropic and anisotropic features when investigated under polarized light. The sieve texture observable at the outer rim of the grandite minerals is made up of mainly quartz inclusions, which itself incorporate many highly saline fluid inclusions. Due to the high content of Ca and Fe3+ in both epidote and grandite, we assume a highly oxidizing environment with a high involvement of saline fluids, boiling at some point and enhancing the mobility of iron.


In conclusion we can state that the grandite we find here in this plagiogranite in the Troodos ophiolite complex, looks like other skarn type associated grandite but here, no sedimentary rock type is included in the forming process. Further we assume the growth of this grandite to be associated with very high volatile activity, either during a late stage of hydrothermal alteration or metasomatic process or even earlier in a magmatic stage indicating the activity of magmatic volatiles in a system with very unique chemical composition.




Chris S. Marien, · J. Elis Hoffmann, · C.‐Dieter Garbe‐Schönberg and · Carsten Münker, 2019, Petrogenesis of plagiogranites from the Troodos Ophiolite Complex, Cyprus, Contributions to Mineralogy and Petrology, 174:35.


Paul T. Robinson, John Malpas and Costas Xenophontos , 2003, The Troodos Massif of Cyprus: Its role in the evolution of the ophiolite concept, Geological Society of America Special Papers, 373,  295-308.

How to cite: Hagen, A.: Skarn type grandite garnet in oceanic lower crust of the Troodos Ophiolite, Cyprus. Grandite growth from a new view?, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-8571,, 2023.

Leo J. Millonig, Elena V. Agasheva, Alexey M. Agashev, Richard Albert, Aratz Beranoaguirre, Horst Marschall, and Axel Gerdes

Garnet xenocrysts from kimberlites provide unique insights into the composition, structure and evolution of the subcontinental lithospheric mantle (SCLM). For example, different metasomatic events in the SCLM are reflected in compositional differences between garnet xenocrysts. As mantle metasomatism largely controls the physical and chemical properties of the SCLM, it exerts first order control over the genesis of kimberlitic magmas and diamond formation. However, dating mantle lithologies and processes is complicated by high ambient temperatures that allow the equilibration of most isotopic systems up to the time of kimberlite eruption. As a consequence, the temporal connection between metasomatic events in the mantle and kimberlite genesis is commonly ambiguous.

In this study, we applied LA-ICPMS U-Pb dating to 43 harzburgitic, lherzolithic and megacrystic garnet xenocrysts from the ~376 Ma diamondiferous V. Grib kimberlite, Russia, in order to investigate the link between different types of mantle metasomatism and kimberlite genesis.

Our results indicate that, with two possible exceptions, only harzburgitic garnet overlaps in age with the kimberlite eruption, whereas lherzolitic and megacrystic garnet crystals are ~20 to 130 million years older. Furthermore, garnet U-Pb ages and Ni-in-garnet temperatures of ~820 to 1200 °C do not correlate. This, and the high closure temperature of U-Pb in garnet (≥900 °C) suggests that the garnet U-Pb ages indeed reflect metasomatic events in the SCLM. However, the U-Pb ages could also reflect cooling ages. In this case, the metasomatic events recorded in the garnet crystals must still have occurred up to ~130 million years prior to the eruption of the V. Grib kimberlite.

These findings have far-reaching implications for the genesis of (diamondiferous) kimberlites, as they clearly show that the time lag between metasomatic events in the SCLM, as recorded in kimberlitic garnet xenocrysts, and kimberlite eruption may extend to tens of millions of years.

How to cite: Millonig, L. J., Agasheva, E. V., Agashev, A. M., Albert, R., Beranoaguirre, A., Marschall, H., and Gerdes, A.: Kimberlite genesis and mantle metasomatism: Insights from in situ U-Pb dating of single garnet xenocrysts, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-11252,, 2023.

Aratz Beranoaguirre, Jose Ignacio Gil-Ibarguchi, Leo J. Millonig, Richard Albert, and Axel Gerdes

The novel technique of U-Pb dating of garnet allows us to date rocks that were previously impossible to put into a temporal context, either because of the absence of traditional geochronometers such as zircon, monazite, and rutile or because of isotopic disequilibrium, e.g. Lu-Hf or Sm-Nd. The possibility to exclusively date garnet crystals can give us important information to understand the geological evolution of a particular region.
In this contribution, we present data from a garnet-quartz vein from the Cabo Ortegal complex, NW Iberian Massif. In the vein, andraditic garnet is associated with chlorite and serpentine in fissures, which cut the foliation of the surrounding lithologies. Field evidence thus indicates that vein formation postdates the formation of the regional foliation (ca. 390 Ma). However, until now, it is unknown, if vein formation is related to thrusting and fluid circulation during the uplift of the area (350-330 Ma) or as the consequence of igneous intrusions (ca. 300 Ma). 
Vein garnet was analysed in two analytical sessions. The analyses yielded unanchored lower intercept ages of 253.5 ± 7.6 Ma (MSWD = 1.1, n = 62/64) and 252.2 ± 9.5 Ma (MSWD = 1.33, n = 40/40). Those results are much younger than the basement rocks and structures in the Cabo Ortegal complex. The obtained ages coincide with the Permo-Triassic rifting widely documented from other geological domains of the Iberian Peninsula, but has never before been recognised in the Allochthonous Complexes of NW Iberian massif. The study of these types of samples may therefore provide clues to the geological history of the Cabo Ortegal Complex, which may not be available from other techniques and lithologies.

How to cite: Beranoaguirre, A., Gil-Ibarguchi, J. I., Millonig, L. J., Albert, R., and Gerdes, A.: Expanding our knowledge: Garnet U-Pb dating and the Permo-Triassic rifting of the Cabo Ortegal Complex (NW Iberia), EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-11486,, 2023.

Gabriele Cruciani, Marcello Franceschelli, and Aratz Beranoaguirre

The Punta de li Tulchi retrogressed eclogite crops out 50 km south from Olbia as an E–W oriented, 150 m long 20–40 m thick lens embedded within the Variscan migmatites of NE Sardinia. The lens is parallel to the E–W-oriented schistosity of the migmatite. The eclogite consists of an alternation of garnet-pyroxene-rich and amphibole-plagioclase-rich layers, striking E–W and dipping 50°N. The amphibole, plagioclase-rich layers show an EW-oriented foliation crosscut by a third retrograde S3 foliation defined by the occurrence of millimetric white pods consisting of plagioclase–amphibole kelyphites. Locally the S3 is crosscutted by centimetric to decimetric late shear zones. A pre-eclogite stage is documented by the occurrence of tschermakite and zoisite relics within garnet porphyroblasts. Four main metamorphic stages have been distinguished in the eclogite evolution: (1) eclogite stage, revealed by the occurrence of omphacite relics within garnet porphyroblasts; (2) granulite stage, producing orthopyroxene/clinopyroxene–plagioclase symplectites replacing omphacite; (3) amphibolite stage, leading to the formation of amphibole–plagioclase kelyphites between garnet and symplectite and to the growth of cummingtonite on orthopyroxene; (4) greenschist to sub-greenschist stage, defined by the appearance of actinolite, chlorite, and epidote. The P-T path is clockwise, with T =660-700 °C at the peak of pressure (1.7–2.1 GPa) and peak of temperature close to 800 °C at P=1.0–1.3 GPa, in the HP granulite facies. Palmeri et al. (2004, Neues J. Mineral. Monat. 6, 275–288) in the eclogite of Punta de li Tulchi found U–Pb zircon ages giving three weighted means of 453 ± 14, 400 ± 10 and 327 ± 7 Ma. The first one was interpreted as the gabbroid protolith age, the second was considered as the likely age of the HP eclogitic event or the result of Pb loss during the main Variscan event, while the third mean was referred to the final retrogression to amphibolite facies. With the aim to better define the ages of the different steps of the metamorphic evolution, an eclogite sample was selected and prepared to be investigated by LA-ICPMS U–Pb age dating on garnet. Although the majority of the analyses contain low uranium, an age of 380 ± 9.9/10.4 Ma was obtained based on 31 garnet spots. Besides, a less precise age of 340.9 ± 18.2/18.4 Ma was also calculated from 10 analysed points. The older age corresponds to the eclogite stage event, whereas we tentatively interpret the second age (340 Ma) as the possible age of the granulite event. The age of ca. 340 Ma is coeval to the 335-355 Ma high-temperature event recorded in other parts of the Variscan massif (e.g. Plešovice zircon, Slama et al., 2008 Chem. Geol. 249, 1–35). However, it cannot be excluded that such age, which was obtained with a limited number of spots, could be related to the Pb-loss of some areas of the garnet during the garnet breakdown that led to the corona/ kelyphite formation.

How to cite: Cruciani, G., Franceschelli, M., and Beranoaguirre, A.: Preliminary garnet dating in retrogressed eclogites from Punta de Li Tulchi, NE Sardinia, Italy, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-1776,, 2023.

Marcello Franceschelli, Gabriele Cruciani, Dario Fancello, and Daniela Rubatto

The relative slow diffusivity of trace elements in garnet is particularly suited to preserve garnet growth zones that results from complex metamorphic evolutions. As for major elements, the complexity of trace element distribution is best investigated by 2D mapping. This approach is applied to a garnet porphyroblast from a mylonitic micaschist along the Posada-Asinara Shear Zone in the Axial Zone of the Sardinia Variscan chain. The micaschist consists of quartz, white mica, biotite, garnet, staurolite, chlorite, plagioclase and chloritoid that reflect amphibolite-facies metamorphism. Accessory phases are ilmenite, rutile, zircon, monazite, apatite and tourmaline. Garnet porphyroblasts are enveloped by the S2 schistosity that is marked by the alternation of quartz-feldspathic and micaceous layers. They show a typical texture with distinct core and rim. The garnet cores contain numerous inclusions of quartz, rutile, apatite, monazite and zircon that define a rotated foliation with “snowball garnet” microstructure. In order to assess the relative behaviour of major and trace elements and gain insight into the garnet growth process, a large garnet crystal (ca. 6 mm in diameter) was investigated by LA-ICPMS mapping. Major element zoning evidences a wide core (ca. 4 mm; Alm45; Prp1;Grs25; Sps29) characterized by bell shaped zoning. The grossular and spessartine components progressively decrease, whereas almandine and pyrope increase, towards the 2 mm thick rim (Alm86; Prp11;Grs3; Sps1). Despite the relatively simple major element zoning, trace element distribution is more complex. The boundary between core and rim is marked by a thin and sharp annulus enriched in Y and HREE (Tb, Dy, Ho, Er, Tm, Yb and Lu). The annular enrichment supports a history of the garnet with partial resorption after the core growth. The garnet core consists of an inner and an outer zone where the maximum concentration of elements from Lu to Tb progressively moves outwards with decreasing atomic number. This trend continues in the rim outside the annulus, where a further distinction between a Sm-, Eu-, Tb-rich inner rim and a REE-poor outer rim is revealed by the maps. The REE total content (∑REE) of the inner core is significantly higher (200-400 ppm) than that observed in the outer rim (10 ppm). The strong REE fractionation between garnet core and rim results in distinct REE patterns with chondrite-normalised abundances up to 1000 for HREE in the core, whereas the outer rim show less fractionated REE patterns with HREE chondrite-normalised abundances up to 10. The progressive zoning of HREE to LREE from core to rim is in line with diffusion limited uptake of REE, as previously described in amphibolite and eclogite facies garnet. Superimposed on this core-rim pattern are additional complexities and fluctuations in trace element concentrations that are best explained by local availability of elements and variability in transport mechanisms (e.g. Konrad-Schmolke et al. 2022, J. metamorphic Geol., 10.1111/jmg.12703).

How to cite: Franceschelli, M., Cruciani, G., Fancello, D., and Rubatto, D.: Mapping trace-element zoning in garnet from mylonitic micaschist of NE Sardinia, Italy, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-7117,, 2023.

Alexandre Peillod, Clifford Patten, Kirsten Drüppel, Aratz Beranoaguirre, Armin Zeh, Dominik Gudelius, Simon Hector, Jarosław Majka, Barbara Kleine, Andreas Karlson, Axel Gerdes, and Jochen Kolb

Understanding the disruption of a tectonic nappe that experiences a subduction-related pressure temperature (P-T) loop is challenging. Thrust imbrications may disrupt single nappes during its subduction and/or exhumation which can be revealed by detailed petrological and geochronological work. Garnet commonly forms during subduction. It most likely hosts early prograde, peak high-pressure (HP) and subsequent metamorphic mineral inclusions making such assemblages a useful tool for detailed petrological and geochronological investigations. Multiple approaches were used to determine the detailed P-T loop of the Cycladic Blueschist Unit passive margin sequence (Greece) such as Zr-in-rutile thermometry coupled with quartz-in-garnet elastic barometry, average P-T and phase equilibrium thermodynamic modeling. U-Pb garnet and zircon geochronology age data were in addition determined to complement already existing age data.

The results of this approach reveal that the passive margin sequence in Thera (Santorini), Ios and Naxos was subducted as a coherent continental fragment at a subduction rate of ~2.1 km/My and a heating rate of ~12 °C/My. Prograde and peak HP metamorphism occurs at c. 50 and c. 40 Ma respectively.  Along Thera, Ios and Naxos, prograde and peak P-T condition increase from sub-blueschist to upper blueschist facies metamorphism. Subsequently, the sequence was disrupted by one or several thrust faults during its exhumation. The passive margin sequence of Naxos was thrust onto the Ios sequence during the Oligocene at c. 30 Ma. This imbrication is revealed by different exhumation rates of ~6 km/My for the passive margin sequence of Naxos and of ~3 km/My for the one of Ios. The passive margin sequence of Thera, Ios and the upper part of Naxos was exhumed to upper crustal levels, whereas the lower part of the Naxos passive margin sequence was exhumed to the lower crust leading to thermal relaxation of 9–96°C following tectonic accretion. This indicates that thermal relaxation following tectonic accretion in the Cyclades controlled the thermal evolution of the evolving Cycladic orogen during a tectonically quiet period before lithospheric extension.

How to cite: Peillod, A., Patten, C., Drüppel, K., Beranoaguirre, A., Zeh, A., Gudelius, D., Hector, S., Majka, J., Kleine, B., Karlson, A., Gerdes, A., and Kolb, J.: Disruption of a high-pressure unit during exhumation: petrology and geochronology of garnets within the Cycladic Blueschist Unit (Thera, Ios and Naxos islands, Greece), EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-1755,, 2023.

Posters virtual: Thu, 27 Apr, 14:00–15:45 | vHall GMPV/G/GD/SM

Chairpersons: Lorraine Tual, Aratz Beranoaguirre
Nefise Paksoy, Yunus Can Paksoy, and Gültekin Topuz

The Strandja Massif is a NW-SE trending, extensive polymetamorphic belt (300 km length and 100 km width) cropping out in the NW Turkey and SE Bulgaria. The last metamorphism is of upper greenschist- to lower amphibolite-facies and occurred during the Late Jurassic-Early Cretaceous. The massif constitutes a Paleozoic metamorphic basement cut by intrusions of various ages and overlain by a Mesozoic metasedimentary cover. The Mesozoic cover displays monotonous deformation and metamorphic history whereas the Paleozoic basement shows complex structural fabric and metamorphism. Age and metamorphic conditions of the Late Jurassic- Early Cretaceous metamorphism is well-constrained while the timing and P-T conditions of earlier metamorphism are hardly known. In this study, we deal with the timing and metamorphic conditions of the earlier metamorphism.

For this purpose, we studied the northwest part of the Strandja Massif where basement rocks are widely exposed. The basement rocks consist of biotite gneiss, metagranite, biotite garnet gneiss, amphibolite, quartzo-feldspathic schist, and metaperidotite. The most suitable rock type for the determination of P-T conditions and timing of the Paleozoic metamorphism is biotite garnet gneiss.

The biotite garnet gneiss is characterized by the development of mm- to 5 cm- thick leucosomes. The biotite garnet gneiss contains mineral assemblages involving biotite, garnet, staurolite, plagioclase, quartz, muscovite, and chlorite. Idioblastic garnet porphyroblasts, up to 1 cm across, contain inclusion- trails that define a curved internal foliation. Matrix foliation displays continuity with the internal foliation in the garnet, pointing to the syntectonic nature of the porphyroblast. A weak foliation which is defined by the parallel alignment of mainly muscovite crosscuts the main foliation. Based on these textural characteristics we infer that the migmatization and main foliation were developed probably at upper amphibolite facies conditions during the Paleozoic, and the relatively late weak foliation at upper greenschist- lower amphibolite- facies conditions during Late Jurassic-Early Cretaceous.

To constrain the timing and metamorphic conditions of the earlier and later metamorphism we separated monazite and rutile from biotite garnet gneiss and titanite from the amphibolite. Electron microprobe work and U-Pb dating on monazite, rutile, and titanite are in progress.

How to cite: Paksoy, N., Paksoy, Y. C., and Topuz, G.: Paleozoic Metamorphic History of Strandja Massif, NW Turkey, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-10115,, 2023.

Yanling Zhang, Changqing Yin, Donald Wayne Davis, Shun Li, Jiahui Qian, Jian Zhang, Peng Gao, Shangjing Wu, Wangchao Li, and Yanfei Xia

This study presents a comprehensive metamorphic study of geochronology, petrography, mineral chemical, and P-T path for late Cretaceous high-pressure garnet amphibolite from the southern Lhasa terrane of the Eastern Himalayan Syntaxis. Mineral textures and reaction relationships suggest that these rocks have experienced three metamorphic stages (M1-M3). The M1 stage is characterized by peak mineral assemblages of Grt + Hb + Ab + Ru + Ms + Qz, followed by the post peak (M2) assemblages of Grt + Hb + Pl + Ep + Bi + Ru + Qz in the matrix. Late retrograde stage (M3) is defined by Hb + Pl coronae surrounding garnet porphyroblasts, indicating a decompression process. These mineral compositions in combination with whole-rock phase equilibria modelling of high-pressure garnet amphibolite give P-T conditions of three metamorphic stages at 14-19 kbar/660-720 ℃ (M1), 8-10 kbar/650-660 ℃ (M2), and <7 kbar/<600 ℃ (M3), respectively. In summary, a P-T path involving a near-isothermal decompression process and late cooling accompanied by decompression has been reconstructed for high-pressure garnet amphibolite. Moreover, SIMS zircon U-Pb dating results show that metamorphic zircons yield a concordant age of ~90 Ma, suggesting a peak metamorphic age. The results indicate that the southern Lhasa terrane underwent a sequence of tectonometamorphic processes that were initiated by crustal thickening (M1) of up to 60 km at 90 Ma, followed by rapid exhumation along a subduction channel to the depth of 32-26 km (M1-M2) and later slow uplift (M2-M3).

How to cite: Zhang, Y., Yin, C., Davis, D. W., Li, S., Qian, J., Zhang, J., Gao, P., Wu, S., Li, W., and Xia, Y.: Metamorphic evolution of high-pressure garnet amphibolite from the Eastern Himalayan Syntaxis: Implications for the mechanism of crustal thickening and exhumation of southern Lhasa terrane during late Cretaceous, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-3782,, 2023.