Scientific programme (Posters)

Posters

Poster session 1: Petrology & Geochemistry

S1-P01 | Anthony Ramírez-Salazar | Garnet coronitic structures on the interface of felsic-mafic/restitic lithologies: possible evidence of fluid and/or melt movement during the ultra-high temperature metamorphism of the Oaxacan Complex, Southern Mexico

Ramírez-Salazar, A.1*, Almazán-López, M.M.2 and Ortega-Gutiérrez, F.1

The Oaxacan Complex is a well-exposed granulitic terrane located in Southern Mexico. It experienced ultra-high temperature metamorphism (T = 895–916°C and P = 0.83–0.99 GPa; Ramírez-Salazar et al., 2023) at the latest stages of the amalgamation of Rodinia (≈990 Ma; Solari et al., 2003). Melt microstructures and rare syn-tectonic pegmatites show that the ultra-high temperature metamorphism (UHTM) was contemporaneous to partial melting (Ramírez-Salazar et al., 2023), while the presence of fluid-rich minerals in calcsilicates suggests fluids were present at the metamorphic peak in some parts of the Complex (Ortega-Gutiérrez, 1984). Our recent fieldwork and petrographic observations indicate that infiltration and movement of fluids and melt might have occurred simultaneously in some areas. Meta-felsic rocks of the Anorthosite-Mangerite-Charnockite suite within the predominantly metasedimentary El Márquez Unit, display millimeter to meter size mafic and Al-rich “enclaves”. Reaction-like coronitic structures composed by garnet and biotite occur on the “enclave”-felsic host interface. Further petrographic inspection shows that some coronitic garnets are associated to spinel-corundum-plagioclase-ilmenite-garnet symplectitic microstructures within the Al-rich enclaves, pointing to its restitic nature. Moreover, biotite associated to the coronitic structure displays skeletal-like textures, and preliminary results indicate that the biotite grains are rich in Cl. These preliminary observations suggest that partial melting contemporaneous to Cl-rich fluids infiltration occurred during UHTM. The interpretations are non-conclusive yet, but the coronitic structures found in the Oaxacan Complex might provide insights into the melt-and fluid-driven reactions and processes occurring at extreme temperature and high-pressure conditions.

1 Instituto de Geología, Universidad Nacional Autónoma de México, Ciudad Universitaria 04510, Mexico City, Mexico; corresponding author email: r.s.anthonyy@gmail.com
2 Posgrado en Ciencias de la Tierra, Instituto de Geología, Universidad Nacional Autónoma
de México, Ciudad Universitaria, 04510 Coyoacán, Ciudad de México, México3

S1-P02 | Emond W.F. de Roever | UHT garnet in the Bakhuis Granulite Belt, Suriname, South America

De Roever, E.W.F.1 and Huizenga J.M. 2

The Paleoproterozoic Bakhuis Granulite Belt (BGB), ca. 3000 km2 in size, consists mainly of mafic and intermediate granulites, with minor metapelitic granulites. The BGB shows conventional Ultrahigh-Temperature (UHT) assemblages, with Al-rich orthopyroxene + sillimanite ± sapphirine in a small (50 km2) area of metapelites in the NE. Metapelite occurrences elsewhere in the BGB mostly show the for UHT metamorphism unique assemblage  cordierite + sillimanite ± Al-rich orthopyroxene (Nanne et al., 2020). This assemblage could be formed at UHT conditions because the cordierite is rich in CO2 (up to 2½ wt.% CO2), with little H2O. The assemblage was estimated to have formed at a pressure of ca. 11 kbar at 950-1050°C (De Roever et al., 2023). The high CO2 level would imply CO2 fluid phase saturation during UHT metamorphism. Fluid inclusions (FI) are common in quartz and feldspar. The FI contain pure CO2, which could be derived from a local source (which was not found) or an external source. Carbon isotope analysis of CO2 in FI indicates a possible mantle source. Without CO2 influx the cordierite assemblage could not have been formed.

Recently, the UHT assemblage garnet ± sillimanite was recognized locally in metapelite. The garnet cores show abundant exsolution of Ti-rich needles, suggesting that the cores consisted of Ti-bearing garnet before exsolution during retrograde metamorphism. EPMA analysis of garnet with exsolution using a wide beam suggests an average Ti level corresponding to a formation temperature of ca. 1000°C. The assemblage is not related to the orthopyroxene and cordierite assemblages mentioned above, because the garnet does not occur together with orthopyroxene and in cases contains orthopyroxene relics. Furthermore, the orthopyroxene and cordierite assemblages were formed at a high oxidation state, as they are accompanied by magnetite and titanohematite, whereas the garnet + sillimanite assemblage coexists with rutile. In the two main occurrences, areas of ca. 10 km2 in size, coinciding with an aeromagnetic anomaly, the garnet ± sillimanite assemblage is in part accompanied by accessory graphite flakes. Aeromagnetic anomalies are common in the BGB, in part due to the widespread occurrence of magnetite. However, geophysical investigation (with drilling) of the two anomalies with garnet showed that the anomalies are caused by a few % of graphite (and sulphide). The graphite is considered to have been formed during UHT metamorphism together with garnet but also occurs in later deformation zones and fractures.  The garnet grains frequently show fluid inclusions in their core. Analysis of one example showed the presence of pure CO2.

The dominant and widespread UHT assemblage in metapelites throughout the BGB is cordierite – sillimanite ± orthopyroxene, associated with CO2 fluid saturation. The UHT garnet ± sillimanite assemblage is rare and occurs only locally, i.e. in two small areas and at a few single locations, among the widespread assemblage with cordierite. The UHT garnet is not accompanied by contemporaneous cordierite or orthopyroxene but replaced a former orthopyroxene-bearing UHT assemblage, as indicated by remnants of orthopyroxene in garnet. The cordierite + sillimanite ± orthopyroxene assemblage is accompanied by magnetite and titanohematite, indicative of a high oxidation state. This is not in agreement with the occurrence of rutile in the garnet ± sillimanite assemblage, let alone with the reducing conditions during graphite formation. The cause(s) of this difference are being investigated.

1 Dept. of Earth Sciences, VU Amsterdam, De Boelelaan 1085, 1081 HV Amsterdam, the Netherlands – ederoever@ziggo.nl
2 Faculty of Environmental Sciences and Natural Resource Management, Norwegian University of Life Sciences, P.O. Box 5003, NO-1432, Ås, Norway

S1-P03 | Pierre Lanari | Deciphering elemental behaviour during high-grade metamorphism using multi-phase quantitative compositional mapping by LA-ICPMS

Lanari, P.1,2, Markmann T.A.1

Studying the behaviour of elements during high-grade metamorphism is a challenging task when partial melting is involved. Two fundamental limitations have restricted the use of natural samples for this purpose. The first limitation is related to the open system behaviour of partially melted rocks. In migmatites, the minerals present in a leucosome can be used to study the former melt composition at the time of crystallisation. Any further comparison with the residue to infer the melting history requires the assumption that the melt was immobile, which is unlikely due to the density and viscosity contrast between melt and solids. The second limitation is our ability to map the distribution of elements in domainal rocks. Minerals in migmatite are often chemically zoned, and large variations in major element compositions can cause significant matrix effects, making map calibration for trace elements challenging.

In this study we present a data reduction routine for multi-phase quantitative compositional mapping in chemically zoned minerals using LA-ICP-MS. Each mineral in the mapping area is individually calibrated using an independent element as internal standard with the option of a variable composition. This routine was implemented in the open-source software XMapTools (Markmann et al, 2024). Quantitative compositional mapping was applied to a migmatite sample from the El-Oro complex in Ecuador. This sample comes from a 30-m wide metasediment xenolith in the Marcabeli pluton and shows an exceptional record of partial melting in a closed system. The results allow the systematic behavior of these elements during partial melting to be retrieved and show that strong chemical potential gradients in major, minor and trace elements were established between the leucosome and the residuum during cooling. This dataset can also be used to identify domains of local equilibrium and to quantify disequilibrium gradients at the thin section scale. Implications for phase equilibrium modelling based on the bulk-rock composition will also be discussed.

This project has received funding from the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation programme (grant agreement No 850530).

1 Institute of Geological Sciences, University of Bern, Switzerland, pierre.lanari@unibe.ch
2 Institute of Earth Sciences, University of Lausanne, Switzerland

S1-P04 | Fernanda Torres Garcia | Constraints on the origin of Archean hornblendites: an example from the Lewisian Gneiss Complex, NW Scotland

Fernanda Torres Garcia1,2,*, Silvia Volante2,1, Tim E. Johnson3,  Johann Diener5, J. Elis Hoffmann4, Annika Dziggel1

The deep levels of Archean continental crust are dominated by felsic gneisses of the tonalite–trondhjemite–granodiorite (TTG) series, with ultramafic–mafic rocks constituting a volumetrically minor yet key component with potential for constraining TTG source rocks. Many of these ultramafic–mafic rocks contain amphibole (hornblende), of which rare hornblendites (>90 vol.% modal hornblende) are preserved as pods or lenses within TTG gneisses. However, the origin of these hydrated ultramafic rocks and their relationship to their felsic TTG host rocks remains unexplored, complicated by subsequent metamorphic processes. In this study, we combine field observations, bulk-rock major and trace element data, phase equilibrium modelling, and in-situ mineral chemistry to constrain the origin of hornblendites from the mainland Lewisian Gneiss Complex in NW Scotland. The studied hornblendites contain high MgO (up to 19 wt%), Cr (up to 5000 ppm) and Ni (up to 1000 ppm).  Previous studies interpreted that the hornblendites may have been cumulates of their host TTG magmas, modified by multiple hydration and partial melting events. Additionally, they show similar Cr, Ni, and major elements (SiO2, Al2O3, CaO and TiO2) values to Archean komatiites and ultramafic-mafic rocks from the Lewisian, indicating that they may have originated from a similar primitive mantle source. Amphibole compositions vary from pargasite to magnesio-hornblende cores to actinolite rims and actinolite-quartz symplectites, indicating different generations of amphibole. Phase equilibrium modelling illustrates that nearly monomineralic assemblages dominated by amphibole form under water-saturated conditions and at subsolidus or near-solidus temperatures. These lines of evidence suggest that hornblendite pods may have been fragments of ultramafic bodies containing primary hornblende, which acted as a water reservoir, retaining most of the water until temperatures reached 700- 750°C. Warmer geotherms during the Archean likely favoured dehydration of hydrous high-MgO ultramafic-mafic rocks at deep crustal levels, leading to fluid-present melting of overlying basaltic rocks to produce TTG magmas. Alternatively, hornblendites could have been igneous cumulates or ultramafic xenoliths that were later fully hydrated and re-equilibrated at low temperatures (700- 750°C), either in the magma source region or elsewhere within the magmatic system. In this scenario, external fluids may have played a crucial role in fully hydrating the ultramafic bodies. In either case, high-MgO hornblendite and ultramafic rocks appear to have been significant sources of fluids during the Archean, reflecting a pervasively hydrated crust. These ultramafic rocks carried water into the deep crust along cooler geotherms and released it during low-temperature dehydration. This process supplied essential water fluxes to the more enriched basaltic sources, facilitating crustal production.

1 Department of Tectonics and Resources, Institute of Geology, Mineralogy, and Geophysics, Ruhr-Universität Bochum, Bochum, Germany
2 Structural Geology and Tectonics Group, Geological Institute, Department of Earth Sciences, ETH Zürich, Zürich, Switzerland
3 School of Earth and Planetary Sciences, The Institute for Geoscience Research, Curtin University, GPO Box U1987, Perth, WA 6845, Australia
4 Institut für Geologische Wissenschaften, Freie Universität Berlin, Malteserstraße 74-100, 12249 Berlin, Germany
5 Department of Geological Sciences, University of Cape Town, South Africa

S1-P05 | Cindy Urueña | Tracking a polymetamorphic history: garnet trace element mapping in a Sveconorwegian granulite

Urueña, C.L.1, Rubatto, D.2 and Möller, C.3

Garnet trace element mapping has become an increasingly important tool for deciphering complex metamorphic histories, especially in high-grade and partially molten metamorphic rock. Trace elements (TE), especially rare earth elements (REE), are sensitive to variations in the physicochemical conditions during the garnet growth and preserve a growth stage event at high temperatures where major elements tend to homogenize. Compatible TE uptake in garnet such as Y and heavy REE often follows Rayleigh fractionation, recording a growth zoning pattern. However, different distribution patterns can be used to identify disequilibrium processes, diffusion during thermal overprinting, and fluid-induced modifications (e.g., Moore et al. 2013; Rubatto et al. 2020).

This study investigates the metamorphic evolution of high-grade polymetamorphic migmatitic gneisses from the Eastern Segment of the Sveconorwegian Province in southwestern Sweden. In the studied area, Sveconorwegian high-pressure granulite-facies metamorphism (0.98 Ga) overprinted the earlier medium-pressure amphibolite-facies metamorphism of the Hallandian event (1.45 Ga; Ulmius et al. 2015). U-Pb dating of zircon and monazite confirms the polymetamorphic history, revealing the age of each event in different crystal domains (Piñan-Llamas et al. 2015).

Trace element mapping of Mn-rich garnet from a sillimanite-bearing migmatitic gneiss has been used to document the crystal evolution during the two distinct tectonometamorphic events. The major elements Ca and Mn are homogeneously distributed, whereas Mg shows a patchy distribution with enrichment towards the inner parts of the crystal and depletion in the rim. Although Fe is mostly homogenized, the outermost rim displays a Fe-enriched zone (< 50 µm). Notably, the TE patterns do not reflect the observed Mg and Fe distributions. The Fe-rich and Mg-poor rim is therefore interpreted as a result of diffusional re-equilibration at high temperatures during granulite-facies metamorphism.

TE distribution suggests multi-stage garnet growth. The garnet mantle shows a broad concentric band (~400 µm) enriched in Y and Dy–Lu followed by a rim strongly depleted in HREE. The core exhibits a moderate depletion in Y + HREE relative to the mantle. Zr shows a weak enrichment in the mantle, whereas Eu and Sm are slightly enriched toward the rim. The uncommon enrichment of MREE and HREE in the mantle compared to the core may be attributed to garnet growth under peritectic conditions during partial melting in the later metamorphic event. The low REE rim would thus represent subsolidus garnet growth during cooling at conditions where Fe and Mg re-equilibration occurred.

1 University of Bern, Baltzerstrasse 1+3, Bern, Switzerland, cindy.uruena@unibe.ch
2 University of Bern, Baltzerstrasse 1+3, Bern, Switzerland.
3 Lund University, Sölvegatan 12, Lund, Sweden.

S1-P06 | Miisa Hakkinen | Two-stage metamorphism of the Sulkava granulite area, Southern Finland

Häkkinen, M.1, Hölttä, P.2, Whipp, D.1, Pownall, J.1, Cutts, K.2

How the temperature rose so high is a long-standing question in the tectonic interpretation of the Paleoproterozoic ca. 1.83 Ga granulite areas in southern Finland. In the Sulkava area in southeastern Finland, progressive metamorphic zonation increases in grade southwards from the tectonic boundary, appearing to have no spatial relationship with coeval magmatism. This is at odds with the idea of advective heating of shallow crustal levels by rising magmas (Korsman, 1984), which has been suggested as the cause of the late heating event either in an accretionary setting or as the result of radiogenic heating of thickened crust (e.g., Kukkonen and Lauri 2009). Decompression heating in an extensional setting or magmatic underplating models (e.g., Stephens and Andersson, 2015) may be consistent with conductive-type metamorphic zones, but do not adequately explain the structure of the crust, which remains 50 km thick with no evidence of major later tectonic events.

Two principal metamorphic events at ca. 1.88 Ga and 1.83 Ga have been well established in the Svecofennian domain in Finland. The younger event produced voluminous granitic magmatism and migmatization in the southern parts of the domain, where many areas record no evidence of an earlier metamorphic event. Recent monazite dating in the Sulkava area (Salminen et al., 2022) clearly shows that both metamorphic events affected this area.

We have studied the Sulkava granulite area in southeastern Finland by means of field work, petrography, and phase equilibrium modelling in an effort to identify, separate and characterize the two main metamorphic events and re-evaluate the meaning of the metamorphic zoning in relation to the tectonic models. The study area extends further south than in the original study of Korsman, 1984.

In the northern, lower-grade parts of the study area, the older metamorphism associated with a N-S foliation is dominant. The younger event caused crenulation of the old cleavage and growth of new metamorphic minerals of hotter assemblages in an E-W trending direction with poikiloblastic andalusite, k-feldspar and cordierite preserving evidence of earlier foliations. Going southwards only the younger event is seen until almost the middle of the granulite area where there are N-S foliated high-grade gneisses, folded and recrystallized migmatites and gt-opx-crd rocks. Even further south the area principally consists of igneous rocks or diatexites, however isolated occurrences of lower grade gneisses make the metamorphic pattern complicated. The interpretation proposed here is that the earlier 1.88 Ga event already produced the southwards increasing metamorphic zonation but is only seen now where the second event was weak (north) or where the rocks were already significantly dehydrated/melted during the first event (south). A collisional setting for the 1.88 Ga event between a juvenile microcontinent and the Archean Karelian continent (e.g., Nironen, 2017) can best produce the first conductive-type zonation HT-LP metamorphism in the Sulkava area and the second event may then be related to thickening induced heating in the crust, magmatic advection (e.g., Kukkonen and Lauri 2009) and gravitational uplift of the granulite areas (Korsman, 1984).

1 Department of Geosciences and Geography, P.O. Box 64, FI-00014 University of Helsinki, Finland, miisa.hakkinen@helsinki.fi
2 Geological Survey of Finland, P.O. Box 96, FI-02151 Espoo, Finland

S1-P07 | Francois Guillot | Pre-Variscan granulite under Alpine high-P: the Ruitor Massif, W-Alps

Guillot, F.1 and Lanari P.2

The Ruitor Massif in the Western Alps is part of the Grand-Saint-Bernard Nappe, across Savoy (F), Valle d’Aosta (I) and Valais (CH) (Sartori et al 2006). This pre-Carboniferous basement crops out between the internal border of the Zone Houillère (Upper Carboniferous), to the W, and the Combin-type Piemontese calcschists (~ Mesozoic), to the E. Along a W to E section from Tarentaise to Valgrisenche, the Alpine main foliation rotates from E-dipping, through vertical, to W-dipping at the Ruitor internal border, and is classically interpreted as backthrust-related (Platt et al 1989). Near the southeastern end of the Ruitor Massif in Valgrisenche, peak Alpine metamorphic conditions have recently been estimated at 493-560°C and 2 GPa, based on Raman spectra of carbonaceous matter and on phengite Si-content (samples RU of Mendes et al., 2023).

The Ruitor Massif consists of orthogneisses derived from granitic intrusions that yield U-Pb zircon ages close to 460 Ma (Guillot et al 2002, Bergomi et al 2017). The host rocks are a several kilometer thick stack of Al-rich, garnet-chloritoid-paragonite-phengite micaschist alternating with amphibolite and quartz-albitite layers. Preserved migmatite textures are visible in the field, with mineral relics such as andalusite pseudomorphs or muscovite attesting to mid- to lower crustal, pre-Alpine, HT conditions. Some amphibolite bands enclose garnet-blue amphibole boudins, suggesting an eclogitic episode of currently unknown age.

In this presentation, we will focus on the peculiar mineralogy inherited from the protracted, polyphased history, detailing the assemblage from dark ocelli of a variegated micaschist, sampled at the SE-border of the Massif near the Rifugio degli Angeli in Valgrisenche: chloritoid Fe83 Mn4 Mg13, garnet Alm58 Sps23 Grs15 Prp4, paragonite with Na/(Na+k)>0.95, minor phengite with Si=3.33 apfu, plus ilmenite, allanite and rare zircon. Albite and quartz are not observed in the ocelli, but instead make up large portion of the leucocratic inter-ocelli walls. This anomalously Al-rich assemblage is compatible with a restite chemistry, and the past occurrence of Al-silicates.

Overall, the detailed geology of the Ruitor Massif, although of interest, remains poorly understood, with little geochemical and lithostratigraphic data.

1 UMR 8187-LOG Univ. Lille –CNRS–Univ. Littoral Côte d’Opale-IRD, F-59000 Lille <francois.guillot@univ-lille.fr>
2 Institute of Geological Sciences, University of Bern, Switzerland.

S1-P08 | Mona Lueder | Granulite facies rutile: Exploring trace element zoning and homogenization

Lueder, M.1, Hermann J.1, Tamblyn R.1, Lanari P.1,2, Rubatto D.1,2 and Markmann T.A.1

Rutile is a common accessory mineral in granulite-facies rocks and frequently used as petrogenetic indicator, such as for Zr-in-rutile thermometry, indentifying source geochemical reservoirs through Nb/Ta-ratios, and U-Pb dating. However, intra-grain variabilities of trace elements in rutile and their capacity to retain primary zoning are rarely considered.

We present results from laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) mapping on granulite-facies rutile. To evaluate intra grain variabilities of trace elements and asses diffusive resetting of trace elements in rutile at high temperature conditions, we compare these results with rutile formed at lower temperatures or higher pressure.

Generally, trace elements in granulite facies rutile are mostly homogeneously distributed. Minor, patchy zoning is observed for Nb and Ta, resulting in relatively homogeneous Nb/Ta-ratios of 11–13. Additionally, rutile can contain zircon exsolutions. In a sample from the Ivrea zone, Western Alps, homogeneous, exsolution-free areas record cooling temperatures of 850 ± 10 °C (2370 ± 170 μg/g Zr), while areas with high density of zircon exsolution features records temperatures of up to 950 ± 110 °C (5580 ± 3240 μg/g Zr). We interpret that exsolution-free areas within the grain have diffusively lost Zr, recording cooling temperatures. Zircon exsolutions prevent Zr loss to the host matrix, and analyzing rutile with nano-scale zircon inclusions returns close to peak temperatures. Rutile from a high-T eclogite-facies samples from Alpe Capoli, Central Alps, shows core-rim zoning in multiple trace elements, with relatively flat concentration gradient. Nb/Ta-ratios vary more strongly with values between 13–25, while Zr-temperatures are constant within the uncertainty of the method (690–700 °C, 280–310 μg/g Zr). In contrast, rutile from amphibolite-facies samples from Val Malenco, Central Alps, show very pronounced primary zoning with sharp boundaries between zones. Nb/Ta-ratios vary within a similar range as observed in high-T eclogite facies rutile (12–21). However, Zr is zoned significantly (31 ± 2 μg/g to 48 ± 2 μg/g), leading to distinctly different calculated Zr-temperatures (463 ± 3 °C to 490 ± 2 °C), which might be related to temperature variations or Zr-undersaturation during rutile growth.

The systematically different zoning observed in these samples is interpreted as evidence of diffusive re-homogenization of trace elements in granulite-facies rutile, and consequently altered trace element signatures. To evaluate the implications for rutile as petrogenetic indicator mineral at granulite facies conditions, original low-T trace element zoning and diffusion processes in rutile need to be further investigated.

1 Insitute of Geological Sciences, University of Bern, Baltzerstrasse 1+3, 3012 Bern, Switzerland
2 Insitute of Earth Sciences, University of Lausanne, Géopolis, Quartier Mouline, 1015 Lausanne, Switzerland

S1-P09 | Caliméria Passos do Carmo | Zircon as a window on magmatic interaction between mantle-derived magmas and granulitic lower crust exposed in Ivrea-Verbano zone, northern Italy

Passos do Carmo C.1, Laurent O.1, Vanderhaeghe O.1, Galli, A.2, Tavazzani L.2

The significance of magmatic processes involved in the differentiation of the continental crust remain debated. Specifically, two endmember models are invoked to explain magmatic rocks that form the continental crust: fractional crystallization of partial melt from the mantle or melting of crustal lithologies. However, both processes generally operate together, such that a substantial fraction of continental silicic magmas are effectively hybrid in composition [1], but the respective contributions of both crustal and mantle endmembers remain difficult to address.

In the Permian Sesia Magmatic System of the Ivrea Verbano Zone (IVZ), granulite facies rocks are intruded by the Mafic Complex (MC) in the lower crust, and amphibolite to greenschist facies rocks are intruded by hybrid granites at upper crustal levels. Multiple lines of evidence, based on bulk rock composition, suggest that mantle- and crustal-derived magmas interact in the lower crust, which lead to formation of hybrid granites in the upper crust [2], [3]. However, the mechanisms by which these hybrid magmas were generated are not yet assessed at the outcrop, microscopic, or mineral scale. In this study, we present an integrated multi-scale sampling dataset, focused on zircon U-Pb dates, trace element and Hf isotopic compositions.

Within the MC at the Val Sessera locality, at a kilometre to metre scale, the contact between gabbro and migmatitic paragneiss is systematically marked by charnockite with various proportion of Px-Grt-Pla-Qz. Charnockite is intermingled with the gabbro and in textural continuity with leucosome veins concordant to discordant relative to the foliation of the migmatitic paragneiss. This association, along with bulk and mineral compositions, indicate that charnockites are intermediate hybrid magmas generated in the lower crust, from which melt was extracted to form granites in the upper crust [2], [4].

Zircons display four morphological textures, defined by core-rim relationships and textural positions (albeit in different proportions from a lithology to another). The first zircon generation corresponds to rare, highly corroded zircon cores in gabbros/charnockites interpreted as xenocrysts from metasedimentary lithologies. The oldest magmatic zircons from mafic lithologies are texturally and compositionally similar to those of typically granitic melts and interpreted as crystallized from hybrid liquids corresponding to a first large-scale hybridization step. Bright rims and individual CL-bright crystals are distinctly posterior, and, based on trace elements composition, may record re-injection of less evolved melt in the hybrid mush zone. The ca. 30 Ma span of U-Pb dates observed in these zircons and the complex relationships of their textures are indicative of a long-lived system possibly associated to the development of a deep crustal hot zone [5].

Overall, we propose that zircon is a key mineral for understanding the processes happening in long-lived high-temperature systems; furthermore, we suggest that the onset of a deep crustal hot zone in IVZ led to assimilation, hybridization, zircon crystallization, and ultimately generation of granitic magma.

1 CNRS, UPS, IRD, Géosciences Environnement Toulouse, 14 Av. E. Belin, 31400 Toulouse, France
2 Department of Earth Sciences, ETH Zurich, Sonnegstrasse 5, 8092 Zurich, Switzerland

S1-P10 | Samikshya Mohanty | Granulites- Energy source for future: Case study from beach placers of eastern coast of India and Sri Lanka

Mohanty, S.1

The growing worldwide demand for nuclear and green energy has generated considerable economic interest in the exploration of rare earth elements (REEs) and radionuclides. The delineation of REEs and radioactive elements and proper extraction and utilization of these elements for recent technological innovations will definitely improve the availability of indigenous resources for the economic development of any country. The present study reveals potential source of these REEs and radionuclides that are enriched within the granulite terranes, found as radioactive minerals such as zircon and monazite. These heavy minerals are extracted from the beaches associated with these granulite terranes. Two such high-grade terranes are the Eastern Ghats Mobile Belt (EGMB) and Wanni Complex (WC) from the east coast of India and Sri Lanka respectively. The EGMB and WC predominantly comprise of khondalite and charnockite, which serve as source rocks for REEs and radionuclides. Continued weathering and erosion in tropical climates facilitate the transportation and deposition of these detrital minerals as placer deposits. Using in-situ gamma-ray spectrometry, HPGe analysis, and INAA, the abundance of heat-producing elements such as uranium (U), thorium (Th), and potassium (K) was determined. Radioactive counts in the granulite terranes vary from 10 to 120 µR/h depending on the abundance of radioactive minerals in different lithologies. Analyses from river banks, acting as transportation mediums, and the berm region of associated beaches reveal total REE content varying from approximately 5.2 to 23.83 times and 20 to 90 times higher than the Universal Continental Crust, respectively. The Th/U ratio is higher in beach sediments, indicating the prevalence of radioactive heavy minerals throughout the beaches associated with granulite terranes. Concentrations of REEs in beach placers range from 79 to 4099.18 ppm, with a predominance of LREE over HREE. The concordant geochemical nature of radioactive minerals and REEs suggests that the heavy minerals are derived from charnockite granite gneisses, whereas khondalite are the secondary source rocks, as evidenced by the presence of higher Mg. Summarizing all the facts, the significant enrichment of radionuclides and REEs in granulite terrains designates them as a potential source for both nuclear and green energy.

1 Department of Geology and Geophysics, IIT Kharagpur, India. Email id: samikshyamohanty18@gmail.com

S1-P11 | Anuj Ghosh | Unravelling contrasting post-metamorphic peak tectonic evolutionary histories from a single high grade terrane: a case study

Ghosh, A.* 1, Samantaray, S. 1, Chatterjee, S. 1, Maitra, A.1, 2, Bose, S.1, 3 and Gupta, S. 1

The Eastern Ghats Province (EGP) is a granulite facies terrane in India that preserves evidence of ultra-high temperature (UHT) metamorphism correlatable with the Neoproterozoic assembly of the supercontinent Rodinia. There have been suggestions that the part of the EGP north of a major intra-province structural feature called the Mahanadi Shear Zone (MSZ) experienced a tectonic history that is separate from the rest of the belt. This study aims to evaluate if the tectonic evolution of the northernmost parts of the EGP differs from the rest of the terrane, how this relates to the rest of the EGP, and to explore the evidence for UHT metamorphism north of the MSZ that has not been previously reported. In recent years, the tectonometamorphic evolution of a high grade terrane has been reconstructed using Pressure-Temperature (P-T) pseudosections calculated at constant bulk composition. This has been applied in this study to rock samples collected from four regions, two each from the north and south of the MSZ. P-T pseudosections were calculated using PERPLE_X (v.7.1.3) after determining the effective bulk compositions (EBCs) of specifically chosen domains within thin sections of khondalites from the Khurda and Nayagarh regions respectively, south of the MSZ. The obtained results confirm that the rocks record near UHT peak conditions, based on the respective stable equilibrium mineral assemblages: melt+garnet+K-feldspar+plagioclase+ilmenite+quartz and melt+garnet+cordierite+K-feldapar+plagioclase+ilmenite+quartz, followed by isobaric cooling to below the solidus. Isothermal decompression is recorded in the subsolidus region. Based on previous studies, peak conditions were inferred to be either synchronous with or post-dating the development of the penetrative fabrics (S1/S2). North of the MSZ, three rock samples from the Angul region: a garnet-cordierite bearing metapelite, an augen gneiss and a mafic granulite were selected. The peak P-T conditions obtained for the garnet-cordierite bearing metapelite is 1100-1150°C at 5-7 kbar, based on the stable equilibrium assemblage melt+garnet+cordierite+plagioclase+ilmenite+quartz, is in the UHT regime. Based on textural evidence, post-peak isobaric cooling, followed by cooling associated with loading appears to be the most plausible retrograde path. The peak P-T conditions obtained for the augen gneiss are 800-900°C (lower than the former) at P> 7.5 kbar based on the stabilization of the peak assemblage melt+garnet+K-feldspar+plagioclase+ilmenite+quartz. Moreover, the garnet-plagioclase mass is wrapped by a later foliation and isoclinal folds defined by biotite (S1/S2) indicating that the precursor granite was emplaced close to peak pressures, and was also foliated and folded. Similarly, the peak P-T conditions obtained for a domain within a mafic granulite which clearly overprints the penetrative fabric (S1/S2) is around 800-850°C and 7 kbar. Garnet breakdown to a symplectite of orthopyroxene+plagioclase indicates decompression dominated cooling in the subsolidus regime following peak pressures. Thus, based on metamorphic studies, the overall post-metamorphic peak history for the northernmost EGP reported from the three samples are isobaric cooling from peak UHT followed by cooling and loading (consistent with the development of S1/S2 evident from the augen gneiss) and finally decompression dominated cooling. This indicates that there are significant differences in the post-peak temperature histories north and south of the MSZ signifying potential differences in the post-metamorphic peak tectonic evolution of the EGP north and south of the Mahanadi rift.

1 Department of Geology and Geophysics, Indian Institute of Technology, Kharagpur 721302, India
(*Corresponding author. Email address: anujghosh95@kgpian.iitkgp.ac.in)
2 School of Ocean and Earth Science, University of Southampton, Southampton, United Kingdom
3 Wadia Institute of Himalayan Geology, Dehradun 248001, India

S1-P12 | Emma Jordan Conway | Re-visiting Lower Crustal Xenoliths from Northeastern Australia

Conway, E.J.1, Murphy, D.T.1, Emo, R.B.2 and Kamber, B.S1

Cenozoic intraplate basaltic volcanism in eastern Australia has exhumed a rich suite of mantle peridotite and rarer lower crustal xenoliths (LCXs). They provide a great opportunity to explore deep-seated processes in the Australian mantle lithosphere, Moho and lower crust as direct, rapidly exhumed samples of otherwise inaccessible portions of the lithosphere. Past studies on LCXs from northeastern Australia (Queensland) have majorly featured in attempts at constructing the composition of the lower continental crust (LCC) (e.g. Rudnick & Taylor, 1987; Sutherland and Hollis, 1982).

The Queensland LCXs are overwhelmingly mafic and represent two granulite-facies assemblages: two-pyroxene-plagioclase-ilmenite and clinopyroxene-garnet-plagioclase-rutile. They have been revisited through combined geochemical and petrological modelling in work by our group (e.g., Emo & Kamber, 2022) confirming that the local LCC is very refractory, strongly depleted in incompatible and heat producing elements (HPE), having experienced melt extraction at very high temperatures (950-1050°C). The LCXs are sampled directly from the deep crust and differ markedly in mineralogical variety and HPE depletion from terrain granulites. They are not predisposed to the same tectonic complications and slow exhumation as lower crustal terrain granulites.

Our recent work has highlighted the role of melt-solid interaction in forming the LCXs, whereby basaltic under- or interplates apparently interacted with existing resident LCC near the Moho. The resulting process is a form of Assimilation-Fractional-Crystallisation (AFC), whereby both the resident LCC and the basalt are modified in mineralogy and chemistry, respectively. Thermodynamic modelling indicates that at P > 10 kbar, AFC leads to clinopyroxene-garnet-plagioclase-rutile residues that may potentially delaminate. The current work sets out to study a rarer xenolith population of ‘garnet websterites’ sometimes co-occurring with the granulites. Their mode is dominated by garnet and clinopyroxene (70-90%; see image), with some orthopyroxene, plagioclase, oxides and minor (secondary?) scapolite. These rocks have only received limited attention but have been compared to ‘pyroxenites’ from Salt Lake crater in Hawaii, even though they could also be mafic granulites.

In our ongoing study, we are determining the geochemistry and formation processes of these rocks. The aim is to establish whether they are cumulates (e.g., Lu et al., 2018), deep AFC products or foundered, originally more plagioclase-rich granulites to compare them with “arclogites” (Lee & Anderson, 2015). Constraining their origin is important for understanding whether garnet-pyroxenite delamination is also possible in intraplate settings as a tectonic environment of net continental growth.

1 School of Earth and Atmospheric Sciences, Queensland University of Technology, Brisbane, Australia; emmajordan.conway@hdr.qut.edu.au
2 Institute of Geology and Mineralogy, University of Cologne, Germany

2

Poster session 2: Partial melting and nanogranitoids

S2-P13 | Guibin Zhang | Partial melting of eclogite in Central Himalaya and its contribution to Miocene climate change

Guibin Zhang1

The presence of eclogite is a widely accepted indicator of the onset of modern-style plate tectonics. However, eclogite is rarely preserved in many heavily granulite-overprinted orogens, which are particularly common across ancient metamorphic terranes (i.e., Archean and Paleoproterozoic terranes). Here, we show that eclogite melting processes may be principally responsible for the poor preservation of high-pressure records. We demonstrate eclogite melting via detailed petrological, geochronological, and geochemical analyses on eclogites and separated centimetric leucosomes from the central Himalaya. The central Himalayan eclogites were overprinted by strong granulite-facies metamorphism, such that omphacite is only sparsely preserved. Thermodynamic modeling results indicate the eclogite experienced two types of anatectic reactions: phengite dehydration melting at high pressure in the presence of omphacite, and subsequent omphacite-dominated melting during exhumation. Omphacite-dominated melting is characterized by the jadeite breakdown, releasing Na and Al into the melts. This melting mechanism subsequently forms a less sodic clinopyroxene and high Na2O/K2O melts. These Himalayan findings appear relevant to early Earth explorations because of the high thermal gradients and intense granulite-facies overprints, implying that partial melting of eclogite dominated by omphacite breakdown could have erased early high-pressure records of modern-style plate tectonics.

Furthermore, we found calc-silicate zones produced from eclogite melt−carbonate interactions that resulted in significant CO2 emissions from the same area. The interactions occurred at high temperatures (685−828°C) in the lower crust (1.2−1.4 GPa) during the middle Miocene (ca. 17−14 Ma). Decoupled Sr−O isotope ratios suggest that calc-silicate zones were produced by eclogite-derived granitic melts infiltrated carbonates during exhumation. Estimates of the C budget indicate that 1.27 tons of CO2 was liberated during the formation of each cubic meter of calc-silicates, and <4% of the C remained at the reaction site. Map analysis suggests granite pluton areas vary from <10−1705 km2 in the Himalaya and each pluton contributed to carbon outfluxes of up to 1361 tons/yr. In this collisional orogeny, deep Earth degassing due to melt−carbonate interactions released 0.14−0.18 Pg/yr CO2 during the middle Miocene. The release of such a large CO2 reservoir into the atmosphere ultimately enhanced the global Miocene climate warming event.

1 The Key Laboratory of Orogenic Belts and Crustal Evolution, MOE, School of Earth and Space Sciences, Peking University, Beijing, China

S2-P14 | Luojuan Wang | Entrainment of peritectic garnet in granite petrogenesis: evidence from metapelitic enclaves in S-type garnet granitoids

Luo-Juan Wang1, Jing-Hui Guo2,3 and Guang-Yu Huang2,3

Entrainment of restite or peritectic assemblage has been proposed to account for the relatively mafic granites. In the eastern part of the Khondalite Belt of North China Craton, the parautochthonous, high-temperature (> 900℃), 1.93 Ga, S-type Liangcheng garnet granitoids contain a distinctive type of metapelitic enclave, which reflect melt-restite separation. The metapelitic enclaves display variable shapes and have diffuse or transitional contacts with garnet-rich facies of the Liangcheng garnet granitoids. They are composed of garnet-K-feldspar-rich leucosome and mesosome (K-feldspar + garnet + biotite + plagioclase + spinel + quartz ± sillimanite). The metapelitic enclave shows significant heavy rare earth elements (HREE) depletion, indicating the loss of garnet-bearing melt. All the garnets in the metapelitic enclaves have similar major-element compositions to those in the garnet granitoids, and show remarkable HREE depletion, further confirming that HREE-rich garnets have migrated away. Large garnet poikiloblasts (up to 2cm) in the surrounding garnet granitoids are intergrown with quartz. Some of the garnets in the garnet granitoids display amalgamation features. We infer that a significant quantity of HREE-rich peritectic garnets from the metapelitic enclaves were entrained to the melt, migrated out of the enclaves into the magma, and then amalgamated to form composite garnets in the garnet granitoids. The entrainment of peritectic assemblage is probably an important process in S-type granite petrogenesis.

This work was supported by research grants No.42072220 from the National Natural Science Foundation of China.

1 Chinese.Academy of Geological Sciences, Beijing 100037, China; wangluojuan@163.com
2 State Key Laboratory of Lithospheric Evolution, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China
3 University of Chinese Academy of Sciences, Beijing 100049, China

S2-P15 | Mikaela Krona | Experimental investigation of H2O and CO2 solubility in graphite- and fluid-saturated anatectic melt

Krona, M.1, Tumiati, S.2, Toffolo, L.2, Bartoli, O.1, Carvalho, B.B.1, Dingwell, D.B.3 and Cesare, B.1

Current models of the geological carbon cycle consider carbonates as the main carriers of carbon into the Earth and largely overlooks the contribution from graphite-bearing lithologies (Plank & Manning, 2019). However, the melting of graphitic metasedimentary rocks in the lower continental crust may remobilize carbon (Cesare et al., 2005). Thermodynamic modelling of graphitic anatectic systems is not possible because the models do not account for carbonic species at suprasolidus conditions. Previous experimental studies have been conducted with carbonates, considering highly oxidized conditions, and these results cannot be extrapolated to graphitic systems (Carvalho et al., 2023). Hence, obtaining new solubility data on melt-bearing graphitic systems in the presence of a ternary H2O-CO2-CH4 fluid is fundamental. The aim of this study is to improve our understanding of carbon mobility during anatexis in graphitic rocks.

In this study, experiments are carried out using a single-stage piston-cylinder apparatus (Univ. Milano). By utilizing the double capsule technique to control the oxygen fugacity of the experiment, the activity of water is maximized to simulate a fluid produced solely by dehydration (Connolly & Cesare, 1993). Haplogranitic glass is used as an analogue for an anatectic melt produced in the lower crust, and experiments are prepared with graphite and water as the source of the COH fluid. The speciation of the experimental COH fluid is quantified ex-situ by quadrupole mass spectrometry (Tiraboschi et al., 2016). At 5 kbar and 900°C for an experiment duration of 24 hours, a fluid with XCO2 = 0.57 is produced. Compared to the prediction made by thermodynamic models (XCO2 = 0.5), the fluid is slightly enriched in CO2. This could be attributed to the effect of silica dissolution (Tumiati et al., 2017).

Going forward, the amount of H2O dissolved in the silicate glass will be determined using microRaman, and an attempt to quantify the dissolved CO2 in the glass will be made. Moreover, the total H and C content of the silicate glass will be measured using nano secondary ion mass spectrometry to determine the solubility of H2O and CO2 in the haplogranitic glass. Later on, the experimental data will be integrated into a thermodynamic model.

1 Dipartimento di Geoscienze, Università degli Studi di Padova, Italy, mikaelaemma.krona@unipd.it
2 Dipartimento di Scienza della Terra “Ardito Desio”, Università degli Studi di Milano, Italy
3 Department of Earth and Environmental Sciences, Ludwig-Maximilian-Universität München, Germany

S2-P16 | Guangyu Huang | Partial melting mechanisms of peraluminous felsic magmatism in a collisional orogen: an example from the Khondalite Belt, North China Craton

Guangyu Huang1, *, Hao Liu1,2, *, Jinghui Guo1,2, Richard M. Palin3, Lei Zou1,2, Weilong Cui1,2

Sedimentary-derived (S-type) granites are an important product of metamorphism in a collisional orogen (Moyen et al., 2021), and a range of subtypes can be recognized by differences in field occurrence, mineralogy, and geochemistry. These subtypes can reflect variations of initial protolith composition, partial melting reactions, pressure and temperature of anatexis, or magmatic processes that occur during ascent through the crust (e.g. mineral fractional crystallization or crustal assimilation). Together, these diverse factors complicate geological interpretation of the history of peraluminous felsic melt fractions in orogenic settings. To assess the influence of these factors, we performed integrated field investigation, petrology, geochemistry, geochronology, and phase equilibrium modeling on a series of leucosomes within migmatite associated with different S-type granites within the Khondalite belt, North China Craton, which is an archetypal collisional orogeny (Zhao et al., 2005). Three types of leucosome are recognized in the east Khondalite belt: leucogranitic leucosome, K-feldspar (Kfs)-rich granitic leucosome, and garnet (Grt)-rich granitic leucosome. Phase equilibrium modeling of partial melting and fractional crystallization processes indicate that the leucogranitic leucosomes were mostly produced through fluid-present melting, Kfs-rich granitic leucosomes are produced through muscovite dehydration melting with 3 vol. % garnet fractional crystallization, and Grt-rich granitic leucosomes are produced through biotite dehydration melting with 20–40 vol. % K-feldspar fractional crystallization and up to 20 vol. % peritectic garnet entrainment. Mineral fractional crystallization and peritectic mineral entrainment occur in the source during melting, and play equally important roles in partial melting mechanisms in terms of affecting the geochemical compositions of granitic melts. Thus, we suggest that peraluminous felsic magmas preserved in collisional orogens are dominantly produced by fluid-absent melting in the middle to deep continental crust, although extraction of low-volume melt fractions from an anatectic source region at shallower depths during fluid-present melting can also generate small amounts of S-type granites that subsequently crystallize at high structural levels in the crust.

1 State Key Laboratory of Lithospheric Evolution, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China
2 University of Chinese Academy of Sciences, Beijing 100049, China
3 Department of Earth Sciences, University of Oxford, Oxford, OX1 3AN, United Kingdom

S2-P17 | Silvio Ferrero | Pfaffenbergite & "phase 430", new mineral phases from melt crystallization in nanorocks

Ferrero, S.1, Lorenzon, S.2, Borriello, R.2,3, Mugnaioli, E.2, Borghini, A.4, Fuchs, R.5, Wirth, R.6, Schreiber, A.6 and Grew, E.S.7

The study of nanorocks (crystallized inclusions of anatectic melts) has been delivering in the last 15 years many intriguing novel insights into anatectic processes at depth. Although the main focus has been on the geochemistry of the partial melts, the systematic use of MicroRaman Spectroscopy (MRS) on nanorocks has revealed that the mineral phases crystallizing from the melt inside the inclusions on cooling are rather distinctive, i.e., rare feldspar polymorphs (kokchetavite, kumdykolite, svyatoslavite and dmisteinbergite) and the SiO2 polymorphs cristobalite and tridymite (Ferrero & Angel 2018; Wannhoff et al., 2022) are very common findings in these crystalline aggregates.

In the last 6 years MRS data collected in samples from numerous localities worldwide have been showing the presence in nanorocks of two novel crystal phases. For operational purposes we first identified them as “Phase 412” (Borghini et al., 2024) and “Phase 430” (Gianola et al., 2021; Ferrero et al., 2021), based on their prominent MRS vibrational modes at 412 cm-1 and 430 cm-1, respectively. Here, we present a crystallographic study of these two new mineral phases. Their crystal structures have been solved ab-initio and refined through three-dimensional electron diffraction (3DED) data, collected by a TEM (Gemmi et al., 2019).

“Phase 412” has been recently approved by IMA-CNMNC with the name “Pfaffenbergite” (Bosi et al., 2024) based on its type locality Pfaffenberg in the Saxon Granulitgebirge (Bohemian Massif). This mineral has an ideal formula KNa3(Al4Si12)O32 and crystallizes in  P6/mcc space group. Pfaffenbergite is isostructural both with kokchetavite (KAlSi3O8) and wodegongjieite (KCa3(Al7Si9)O32, Mugnaioli et al., 2022), a mineral recently found as an inclusion in corundum in chromitite from the Luobusa ophiolite (Tibet, China). The second mineral, “phase 430”, has the ideal formula K2Ca3(Al8Si34)O84 and P6/mcc space group. Preliminary results on “Phase 430” instead show that such phase consists of a feldspathoid-like tetrahedral framework with a completely new topology, currently still under investigation.

1 Department of Chemical and Geological Sciences, University of Cagliari, I-09042 Monserrato (Italy);
*Corresponding author: silvio.ferrero@unica.it
2 Department of Earth Sciences, University of Pisa, I-56126 Pisa (Italy)
3 Department of Environmental Sciences, Informatics and Statistics, Ca’ Foscari University of Venice, I-30172 Mestre (VE) (Italy)
4 Faculty of Geology, Geophysics and Environmental Protection, AGH University of Krakow, 30-059 Krakow (Poland)
5 Institute of Geosciences, University of Potsdam, D-14476 Potsdam (Germany)
6 Helmholtz-Zentrum Potsdam, Deutsches GeoForschungsZentrum (GFZ), D-14473 Potsdam (Germany)
7 School of Earth and Climate Sciences, University of Maine, Orono, Maine 04469 (USA)

S2-P18 | Mahyra Tedeschi | Nanorocks in zircon from a garnet-free granulite: insights from petrography and major and trace elements compositions

Tedeschi, M.1, Ferrero, S.2, Wunder, B.3, Hermann, J.4, Borghini, A.5, Pettke, T.4, Lanari, P.4, Paiva-Silva, P.1, Rubatto, D.4, Van Schijndel, V.3, Tollan, P.6 and O’Brien, P.J.7

Partial melting and melt evolution and migration play an important role in the differentiation of continental crust. These processes affect metal transport and concentration, volatile cycling, and the mechanical properties of the crust. The study of nanorocks, i.e. crystallised melt inclusions (Bartoli & Ceasare, 2020), has emerged in recent decades as a key approach in the investigation of melt evolution, as these inclusions are considered to retain pristine melt information (Cesare et al., 2015). While, most research has been carried out on nanorocks in garnet, melt inclusions in zircon are of growing interest because (i) none of the nanorock-forming major elements except SiO2 are present in the host, (ii) zircon is formed by different (re)crystallization processes, and could therefore potentially record different stages of a metamorphic evolution, and (iii) the possibility of obtaining spatially resolved ages of the host.

Garnet-free granulites from the Guaxupé nappe (Brazil) record a complex history from protolith crystallization at ca. 2.55 Ga to ultra-high temperature (UHT) metamorphism at ca. 650-590 Ma (Tedeschi et al., 2018). The zircon crystals contain crystallized melt inclusions ranging in size from 1 µm to 15 µm distributed in cores and rims. MicroRaman spectroscopy combined with field emission gun electron probe microanalysis revealed that these inclusions consist of variable combinations of cristobalite/quartz, kokchetavite, kumdykolite, “phase 430”, carbonate, pyroxene and, biotite and/or white mica. The re-homogenized nanorocks have a metaluminous to peraluminous mainly granitic composition. These results were compared with the composition of stromatic granulite from quantitative micromaps and bulk rock of outcropping segregated leucosomes. Trace element (TE) data show an enrichment of Rb, Cs, Ba and LREE in the nanorocks relative to the leucosomes. The lower concentration of CaO, MgO, P2O5 and TiO2 and the TE signature suggest that the nanorocks represent melts formed at lower temperatures than those formed during UHT conditions recorded in the leucosomes. Thus, the nanorocks may have been trapped in zircon during prograde metamorphism. This hypothesis is consistent with the CL images and the U-Pb (LA-ICP-MS) data, which show that the nanorocks are hosted in zircon domains recording discordant dates in line with partial resetting of Archean ages, rather than being formed during UHT metamorphism in the Neoproterozoic. The homogenous composition of the inclusions and their location in cores and rims, disfavour the interpretation that the nanorocks record igneous crystallization.

1 CPMTC-IGC-Programa de Pós-graduação em Geologia, Universidade Federal de Minas Gerais, Belo Horizonte, Brazil; mtedeschi@ufmg.br
2 Dipartimento di Scienze Chimiche e Geologiche, Università di Cagliari, Monserrato, Italy
3 GFZ German Research Centre for Geosciences, Telegrafenberg, 14473, Potsdam, Germany
4 Institut für Geologie, Universität Bern, 3012 Bern, Switzerland
5 Faculty of Geology, Geophysics and Environmental Protection, AGH University of Kraków, Kraków, Poland
6 Eidgenössische Technische Hochschule, ETH, Zürich, Switzerland
7 Institut für Geowissenschaften, Universität Potsdam, Potsdam, Germany

S2-P19 | Jessie Shields | Experimental Melting of Eclogite Hosted Polycrystalline Melt Inclusions

Shields, J.E.1, Gordon, S.M.1, DesOrmeau, J.W.1 and Thomas, J.B.2

Melt inclusions trapped in peritectic minerals can record the earliest stages of melt generation and segregation during anatexis and provide insight into differentiation processes producing and modifying continental crust. Here we present a suite of polycrystalline melt inclusions, aka ‘nanogranites’, hosted in peritectic zircon from ultra-high pressure (UHP) eclogites in the D’Entrecasteaux Islands, southeastern Papua New Guinea (PNG). This terrane is the youngest known UHP terrane on Earth and is unique in that it was exhumed in an active rift. The PNG eclogites are exposed in quartzofeldspathic gneiss domes that preserve substantial evidence of partial melting, including abundant felsic leucosomes and dikes, and there are also large mappable granitoids. There is no field or petrographic evidence that the metabasite rocks melted; the only evidence is the nanogranite inclusions in the zircon. We focused on melt-inclusions in zircon from one eclogite to examine the relationship between the partial melting of eclogite and the geochemical evolution of the dikes and sills in this terrane.

To determine the composition of the melt that precipitated the zircons, we homogenized polycrystalline melt inclusions and measured their compositions using electron probe microanalysis. We performed two homogenization experiments at 775°C and 800°C and 1.5 GPa in piston cylinder devices at Syracuse University. In summary, we loaded ~25-30 mg of a zircon separate along with ~10 mg of distilled water into silver capsules that were run in 19-mm diameter NaCl-borosilicate glass-MgO assemblies.  Experiments quenched to <100°C in <20 seconds. After experiments, the capsules contained zircons and aqueous fluid.  During homogenization experiments, polycrystalline inclusions melted to form a homogeneous glass and a vapor bubble.  The melt inclusion glass has a granitic composition similar to the dikes and sills in the gneiss domes. Future work will focus on characterizing the now-homogenized melt inclusions using electron probe microanalysis and phase equilibria modeling to constrain the pressure and temperature conditions of partial melting of the eclogite within this unique tectonic setting.

1 Department of Geological of Sciences and Engineering, University of Nevada, Reno, NV, USA, jessieshields@unr.edu
2 Department of Earth and Environmental Sciences, Syracuse University, Syracuse, NY, USA

2

Poster session 3: Thermodynamic modelling

S3-P20 | Noralinde de Leijer | The sub- to supersolidus transition in the central Pyrenees, Lys-Caillaouas and Gavarnie-Héas

Kriegsman L.M.1,2*, De Leijer N.J.M.2, and Jordan D.A.A.1

Understanding partial melting is crucial when studying processes such as intracrustal differentiation and the mobility of critical metals within the continental crust. Within mountain belts, migmatites and deeply exposed granulites recorded the partial melting processes. In addition to being a location of melt production, migmatites also experience melt inflow from deeper sources. Furthermore, the interaction of crystallizing melts with restitic units leaves a retrograde overprint on peak mineral assemblages. This study focused on the interplay between these processes within a well-exposed transect in the Central Pyrenees.

A Variscan window is exposed beneath an Alpine nappe stack within the Gavarnie-Héas metamorphic dome and the western Lys-Caillaouas massif (Kilzi et al., 2016). This region features a transition from medium-grade metasediments to migmatites, progressing along a steep geothermal gradient of approximately 3-4 kbar. Fieldwork and detailed petrographic analysis reveal prograde metamorphic reactions, including the replacement of andalusite by sillimanite, subsolidus muscovite breakdown to K-feldspar, and the supersolidus breakdown of sillimanite, biotite, and quartz to form K-feldspar, garnet or cordierite, and melt. These reactions outline a clockwise pressure-temperature (P-T) path from low-grade to high-grade conditions. The Lys-Caillaouas and Gavarnie-Héas migmatites, both intruded by various magmatic rocks, exhibit notable differences in melt volume and composition, where the Gavarnie-Héas migmatites generally contain a higher melt concentration and more extensive biotite dehydration.

Geochemical analyses suggest wet prograde conditions with partial water loss during melting, followed by retrograde reintroduction of muscovite and chlorite. Peak metamorphic conditions were estimated at approximately 700°C and 3.5 kbar for Lys-Caillaouas and 730°C and 3.7 kbar for Gavarnie-Héas. Comparisons between leucosomes in migmatites and associated leucogranites highlight compositional similarities yet reveal distinct variations in element concentrations, indicative of differing melting degrees and fluid interactions. In certain Pyrenean samples, retrogression can be fully explained by fluids released from in-situ melts as investigated by mass balancing and trace element redistribution. However, other samples suggest the need for an external fluid source.

This project is part of the FluidNET research and training network on fluid-rock interaction, funded by EU’s Horizon 2020 Marie Sklodowska-Curie grant 956127.

1 Dept. of Research & Education, Naturalis Biodiversity Center, Darwinweg 2, 2333 CR, Leiden, Netherlands
2 Faculty of Geosciences, University of Utrecht, Heidelberglaan 8, 3584 CS, Utrecht, Netherlands

S3-P21 | Volker Schenk | A re-appraisal of metamorphic conditions in the lower crustal section of the Serre, Calabria

Schenk, V.1 and Karmakar, S.2

The application of conventional thermobarometry in combination with detailed mapping and an extensive petrographic study of the reaction history of meta-igneous and metasedimentary rocks of the Serre has led to the recognition of a ca. 7-8 km thick exposed lower crustal section of Variscan age in southern Calabria (Schenk, 1980, 1984, 1990). After a kinematic prograde evolution, peak conditions were reached under static conditions that led to the formation of symplectitic net-like Opx-porphyroblasts at the expense of Bt and Hbl in felsic and mafic granulites, and to Sil-Kfs symplectites at the expense of Ms in metapelites. Prograde Grt reaction rims around cordierite point to a late-stage increase in pressure. The main prograde Grt growth in metapelites occurred in the stability field of Sil, which constrains PT estimates and the prograde path. The results of conventional thermobarometry (Grt-Px; Grt-Crd; Grt-Bt) and the systematic shift of XMg in coexisting phases (Grt, Crd, Bt) has been interpreted as related to decreasing T and P in the lower crustal section from ca. 790 °C/7.5 kbar at the bottom and 690°C/5.5 kbar at the top. A petrographically mapped metamorphic bathograd in metapelites (Grt-Crd-Sil-Bt; XMg Crd 0.75) subdivides the metamorphic grade of the lower crustal section. The T at the top of the section (690°C) has been deduced from assumed St+Qz stability. However, the prograde nature of this assemblage is ambiguous. We have applied Zr-in-Rt thermometry (Kohn, 2020) to felsic granulites and metapelites of the whole section to improve temperature estimates. We obtained, in accordance with the results from conventional thermobarometry, a difference of about 100 °C for the base and the top of the lower crustal section. However, the Zr-in-Rt temperature estimates are about 60 °C higher (ca. 850 to 750°C) than estimates based on conventional thermobarometry. The slow isobaric cooling of the lower crust (2-3 °C/Ma at P of 5-3 kbar) that followed a near-isothermal decompression event during the Variscan orogeny is well constrained by consistent isotopic cooling ages (Hbl, Fsp, Bt) and breakdown textures of cordierite that led to Sil+And+Ky bearing assemblages. Isobaric cooling at elevated P is also documented by regrowth of Grt-Cpx-Qz assemblages after Grt breakdown in mafic granulites.

1 Heidelberg University, Germany; Volker.Schenk@geow.uni-heidelberg.de
2 Indian Statistical Institute Kolkata, India

S3-P22 | Suvankar Samantaray | Tectono-metamorphic evolution from ultrahigh temperature granulite to amphibolite facies metamorphism in the Central Domain of Assam-Meghalaya Gneissic Complex: NE India

Samantaray, S.1*, Ghosh, A.1, Gupta, S.1 and Mohanty W.K.1

Ultrahigh temperatures (UHT) during metamorphism testify to extreme and anomalous thermal conditions in the continental crust. The Assam-Meghalaya Gneissic Complex (AMGC), which constitutes the exposed basement to the foreland basin of the NE Himalayas, have not been known to host UHT rocks. The AMGC is part of an extended cratonic fragment of the Indian Peninsula, and is divided into three distinct domains based on geochronologic information: the Western Domain (WD), Central Domain (CD), and Eastern Domain (ED). The earliest metamorphic imprint in the region dates back to ~1.5 Ga, and peak P-T conditions of metamorphism were estimated by previous workers to be ~8 kbar, 850°C (Dwivedi et al., 2023). In this study, the metamorphic evolution was investigated from phase equilibrium modelling of spinel-bearing high-Mg metapelites in the system NCKFMASHTiO. The peak temperature assemblage is Opx + Spl + Kfs + Qtz + melt, which is estimated from the P-T pseudosection to stabilize at approximately 8-9 kbar, 950°C-1000°C. This UHT metamorphism (M1) is synchronous with the D1 deformation event that produces a planar fabric (S1) defined by the UHT assemblage Spl + Opx + Kfs + Qtz. The D2 event is associated with isoclinal folding (F2) of the S1 planar fabric, forming the S2 fabric axial planar to F2 folds. The D1 event is followed by cooling, leading to the breakdown of the Spl + Opx assemblage to form Grt1 and Crd1. Sillimanite is produced locally by the reaction Spl + Qtz = Grt1 + Sil, resulting in spinel grains rimmed by Sil enclosed within Grt1. Perthitic textures developed owing to the exsolution of high-temperature feldspar solid solutions during cooling. The S2 assemblage Grt1 + Crd1 + perthitic Kfs + Qtz + Bi + Opx ± Sil stabilized at < 750°C and 6-7 kbar (M2) under sub-solidus conditions. The entire metamorphic spectrum from M1 to M2 is considered a continuous event. A later D3 deformation event is associated with recumbent folding (F3) of S2, producing an S3 fabric axial planar to F3 folds under upper amphibolite-granulite facies conditions. During this event, the S2 assemblage was reworked, producing Grt2 through biotite dehydration melting through the reaction: Bi2 + Sil + Qtz = Grt2 + Kfs + Melt. The localized generation of melt, primarily around Grt2, is visible in outcrop, and is thought to have consumed all sillimanite in the matrix, leaving it only as inclusions within both garnet generations. Late-stage retrograde evolution involved the breakdown of Grt2 to form Crd2 via the reaction Grt2 + Melt = Crd2 + Bi3 + Qtz. The S3 assemblage Grt2 + Crd2 + Qtz + Kfs + Plg + Bi3 ± Opx evolved along a decompression dominated path to stabilize below ~650°C and 4-5 kbar.

Previously published geochronological data suggests that peak UHT metamorphism took place at ~1.5 Ga and was limited to the western part of the CD, with a ~500 Ma thermal overprint which operated in the upper amphibolite to granulite facies conditions (Chatterjee et al., 2007; Kumar et al., 2017; Yin et al., 2010). The peak UHT metamorphism at ~1.5 Ga metamorphism can be correlated with the assembly of the supercontinent Columbia. The Neoproterozoic-Cambrian metamorphism reflects the reworking of the CD during the Pan-African Orogeny.

1 Dept. of Geology and Geophysics, Indian Institute of Technology Kharagpur, India, 721302
* Email- samantarayshuvankar@gmail.com

S3-P23 | DingDing Zhang | Metamorphism of Chicheng HP and UHT granulites, northern Trans-North China Orogen and its implication for the initiation of the modern plate tectonics

Dingding Zhang1*, Patrick J. O’Brien2, Yi Chen3 and Jinghui Guo3

When did the modern-style plate start and how did it evolved is a significant and disputed scientific question in Earth Science. The deep subduction of ultra-high or high-pressure eclogites is a crucial indicator for the initiation of modern-style plate tectonics. A few Paleoproterozoic eclogites have been reported to have undergone deep subduction, and they show strong relationships with the evolution of old orogens, they can indicate the operation of modern-style plate tectonics. This study presents evidence of eclogite evidence for the evolution of the Trans-North China Orogen, through the further examination of Chicheng metapelite. Special multiphase solid inclusions are found in Chicheng, and confocal Raman Imaging shows that they are mainly composed of kumdykolite, kokchetavite, cristobalite and muscovite. They may experience near ultra-high pressure metamorphism with peak metamorphic PT condition of 2.5-2.7 GPa, 950-1050 ℃, re-calculated by THERMOCALC. The 1.9 Ga HP granulites in Trans-North China Orogen and the 0.34 Ga HP granulites in Bohemian Massif have the same kinds of metastable inclusions.  The result implies that the initiation of modern-style plate tectonics probably occurred around 1.91 Ga in the northern part of the Trans-North China Orogen.

1 State Key Laboratory of Earthquake Dynamics, Institute of Geology, China Earthquake Administration, Beijing, China
2 Institut für Geowissenschaften, Universität Potsdam, Karl-Liebknecht Str. 24-25, 14476 Potsdam, Germany
3 State Key Laboratory of Lithospheric Evolution, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China

S3-P24 | Juraj Butek | A PHREEQC model for the transformation of gabbro into rodingite

Butek J.1, Fabre S.2, Duchene S.3, Spišiak J.1 and Grégoire M.3

Rodingite is a pale Ca-rich and Si-poor metasomatic rock commonly occurring in association with serpentinites. This rock is characterized by specific mineral assemblages consisting of hydrated garnet, diopside, vesuvianite, epidote-zoisite, chlorite, or prehnite (Li et al., 2004). However, natural rodingites are significantly heterogeneous in mineral composition (Butek et al., 2022; Duan et al., 2022). Major factors controlling the mineral diversity as well as details on fluid-rock interactions leading to the evolution of mineral and chemical composition during rodingitization have not yet been fully constrained.

In this work, we use PHREEQC software (Parkhurst & Appelo, 2013) to present a geochemical model for the transformation of a mafic rock into vesuvianite-bearing rodingite at a temperature of 300°C. A series of thermodynamic calculations was run using the database SUPCRTBLT converted by SupPhreeqc (Zhang et al., 2020) into a format readable by PHREEQC. Fluid-rock interactions were modeled in a Na-K-Ca-Fe-Mg-Al-Si-H-O-C-S-Cl system without using solid solutions or kinetic factors. Through these simulations, we investigate the effect of fluid composition and progress of the metasomatic process on the formation of rodingites.

Our results show that significant fluid volume (high fluid-rock ratio) favours the formation of rodingites. We thus conclude that the metasomatic process requires an open system with a high input of hydrothermal fluid. Additionally, a decrease in the Si/Ca ratio in the metasomatized rock is correlated to an increase in the volume of incoming fluid and this ratio can eventually serve as a proxy to express the extent of transformation. Whole rock chemical and mineral composition in natural rodingites are well reproduced by the model. Furthermore, the diversity of mineral parageneses results mainly from different degrees of transformation and only to a lesser extent to the chemical composition of hydrothermal fluid or protolith. Concerning the fluid composition, the hydrothermal fluid doesn’t need to be especially rich in calcium to transform a mafic rock into rodingite, but it must be low in magnesium, silicon, and have a high pH, which is naturally controlled by serpentinization of surrounding ultramafic rocks.

1 Faculty of Natural Sciences, Matej Bel University, Tajovského 40, 97401 Banská Bystrica, Slovakia; juraj.butek@umb.sk
2 Institut de Recherche en Astrophysique et Planétologie (IRAP), CNRS, UPS, Observatoire Midi-Pyrénées (OMP), 14 Av. E. Belin, 31400 Toulouse, France
3 Géosciences Environnement Toulouse (GET), CNRS, UPS, IRD, CNES, Université de Toulouse, Observatoire Midi-Pyrénées (OMP), 14 Av. E. Belin, 31400 Toulouse, France

S3-P25 | Xia Teng | Metamorphic history of garnet pyroxenites from western Qaidam Block: Insights into the root of Andean-type arcs

Teng, X1, Wei CJ1 and Zhang JX2

Andean-type arcs form above subduction zones where the overriding plate is the continental lithosphere (Ducea et al., 2015). It is believed that the crustal thickness of Andean-type arcs is up to 60-80km, but the exposures of deep roots of thickening continental arcs are rarely reported.

The Qaidam block is one of the Asian blocks once located on the northern margin of East Gondwana. Previous studies on high-grade metamorphic rocks from the Huatugou region, the western Qaidam Block, revealed that the Qaidam Block experienced ultrahigh-temperature (UHT) metamorphism (>910–915℃/0.9–1.4GPa) during late Ediacaran to early Cambrian (Teng et al., 2020).

Here, we present the petrography, whole-rock and mineral chemistry, and results of phase equilibrium modeling and U-Pb dating for garnet pyroxenites that occur as lenses within granulite-facies sedimentary rocks in the Huatugou region. Geochemically, garnet pyroxenites are characterized by low SiO2 (43.98–44.64 wt.%), high FeO (17.33–17.72%) and TiO2 (1.49–2.22%), and enrichments in heavy REEs and HFSEs (Nb, Ta), with low Sr/Y ratios (<2). The least retrograde garnet pyroxenite comprises clinopyroxene (39 vol.%), garnet (37%), plagioclase (12%), amphibole (9%), ilmenite (2%), and orthopyroxene (1%), with minor amounts of apatite, rutile, and quartz. The peak mineral assemblage comprises garnet, clinopyroxene, and ilmenite, which are separated by coronal plagioclase and amphibole with local occurrences of fine-grained orthopyroxene. Garnets are Alm48-51Prp21-24Grs26-30Sps1 with inclusions of amphibole, clinopyroxene, quartz, rutile, and apatite. Clinopyroxenes are dominantly diopsides. Textures and chemistries of coarse-grained clinopyroxene vary systematically from core to rim: in the core zone, clinopyroxene contains decomposed fine-grained albite and quartz, and exhibits high XMg (0.74–0.77) and XJd (0.13–0.17) with Fe3+/Fe2+ <0.05; to mantle, the clinopyroxene contains abundant orthopyroxene lamellae with decreasing XMg and XJd; the rim of clinopyroxene lacks of orthopyroxene lamellae and is replaced by the clinopyroxene + plagioclase symplectite in some grains. Amphiboles are pargasite with 0.03–0.29 pfu Ti, plagioclases vary from albite to bytownite, and orthopyroxenes are hypersthene. Based on petrographic observation and phase equilibrium modeling, the prograde condition is 860℃/1.6GPa using inclusion assemblage, the peak metamorphic condition is constrained to 960℃/1.88GPa, and retrograde condition of 920–950℃/0.55–0.78GPa is constrained for the final assemblage of clinopyroxene, garnet, amphibole, plagioclase, ilmenite, and orthopyroxene. Therefore, the metamorphic P-T path of garnet pyroxene is clockwise crossing the kyanite-sillimanite transition under UHT granulite-facies conditions.

In the garnet pyroxenite, valid rutile U-Pb ages are dominated by 487±5Ma (MSWD=1.06), consistent with apatite U-Pb ages (480±4Ma, MSWD=0.79). They are interpreted as the cooling (ca. 500℃) ages for garnet pyroxenite. The rest of rutile yield ages of 536–532Ma, consistent with zircon metamorphic ages given by Mg-Al-rich granulites in the Huatugou region (Teng et al., 2020), and interpreted as the timing of decompression to significant cooling.

To sum up, the metamorphic history of garnet pyroxenites from the Huatugou region suggests that the Qaidam block, located on the northern margin of East Gondwana, experienced crustal thickening before ca. 530 Ma during southward subduction of the Proto-Tethys Ocean. The peak pressure recorded by garnet pyroxenite indicates a crustal thickness of ca. 70km beneath the western Qaidam block, the thickest continental arc ever reported in Gondwana’s peripheral orogens during late Ediacaran to Cambrian (Cawood et al., 2021).

1 School of Earth and Space Sciences, Peking University, Beijing, China, tengxia@pku.edu.cn
2 Institute of Geology, Chinese Academy of Geological Sciences, Beijing, China

2

Poster session 4: Petrochronology

S4-P26 | Fydji Sastrohardjo | U-Pb age constraints of migmatites in the Marowijne greenstone belt through textural analysis of zircons; NE Suriname, Paleoproterozoic Guiana Shield

Sastrohardjo, F.L.1, Kriegsman, L.M.2,3, Leisen, M.4, Vanderhaeghe, O.4 and Van der Molen, S.3

Investigations on crustal growth and involved processes are pivotal for understanding mineral systems. Part of the solution relies on the interpretation of the relationship between migmatites and granite-greenstone complexes. The Paleoproterozoic basement in north-eastern Suriname provides a moderately exposed transect from the Marowijne Greenstone Belt (MGB) to migmatites and a granitoid-gneiss complex, and allows examining their geodynamic relationship. Field relationships suggest that migmatites have developed at the expense of amphibolite and paragneisses (metapelites and metagreywackes), and that the migmatites grade into diatexite migmatite and subsequently heterogeneous granite. Phase equilibrium modelling of the migmatites resulted in peak conditions of 760 (± 30) °C and 4.6 (± 0.6) kbar, consistent with a low- to medium-pressure / high-temperature metamorphic gradient. Petrographic analysis shows interstitial quartz in the mesosome connected to leucosome and plagioclase-rich melt inclusions within amphiboles in the migmatitic amphibolite, peritectic garnet associated to retrogression in migmatitic paragneiss, all suggesting that the migmatites are issued from partial melting and that they share a common protolith with the greenstones.

To further test this hypothesis zircon U-Pb geochronology was applied. However, the interpretation of U-Pb ages in migmatites is not straightforward due to a combination of inheritance and high-temperature metamorphism. Concordia plots show a wide spread along the concordia line from early to late Rhyacian (2300 – 2050 Ma) and many points plot between the concordia and the origin due to Pb loss. In this study we demonstrate that detailed textural analysis of cathodoluminescence images from individual zircon grains by means of characterizing different core/rim textures in combination with their Th/U signature allows to distinguish among inherited and metamorphic-magmatic ages. A variety of zircons is obtained from migmatitic paragneiss (metatextite), diatexite migmatite and heterogenous granite which can be characterized in grains showing cores with oscillatory zoning or cores with convolute zoning, rims with a diffuse boundary attributed to in-situ dissolution-precipitation or rims with a sharp boundary surrounding a rounded core, discordant to internal zones suggesting crystallization of new zircon around an inherited core. All these grains are interpreted as detrital grains issued from erosion of a magmatic protolith and deposited with the other detrital sediments forming the protolith of the paragneiss, which have then been variously affected by high-grade metamorphism and partial melting. In contrast, some grains display continuous oscillatory zones from core to rim and are interpreted as magmatic grains crystallized in the melt during anatexis. As a result, a cluster of core measurements from the metatexite plot on the Concordia yield and average age of 2146 ± 13 Ma (n=11, MSWD= 1), which is interpreted as representing one of the sources of the sediment, protolith of the paragneiss. These inferred inherited ages coincide with the age of the greenstones of the Marowijne Belt (2.26 – 2.15 Ga). A cluster of metamorphic rims yields an age of 2087 ± 10 Ma (n=3; MSWD=0.34), which is assumed to represent the metamorphic age of the migmatite. Th/U ratios of these rims yield values between 0.08 – 0.41, consistently lower than their Th/U > 0.5 in cores. Among concordant analyses of the heterogenous granite, a cluster of analyses yields an age of 2072 ± 10 Ma (n=16, MSWD=3.1) which is interpreted as the age of the high temperature metamorphism and crystallization from the melt. Therefore, we argue that the migmatites in northeastern Suriname are indeed partially molten equivalents of the MGB.

1 Anton de Kom University of Suriname, Leysweg 86, Suriname fydji.sastrohardjo@uvs.edu
2 Utrecht University, Faculty of Geosciences, Princetonlaan 8a, 3584 CB Utrecht, Netherlands
3 Department of Research & Education, Naturalis Biodiversity Center, Darwinweg 2, 2333 CR, Leiden, Netherlands
4 Géosciences Environnement Toulouse, 14 Av. E. Belin, 31400 Toulouse, France

S4-P27 | Lucas Ramos Tesser | Assembly of the Riacho do Pontal orogenic wedge, NE Brazil: constraints from monazite petrochronology and phase equilibrium modeling

Tesser, L.R.1,2, Ganade C.E.3, Forshaw J. B 4, Basei, M.A.S. 1, Lanari, P. 2,4, and Cioffi, C.R. 1

The South Borborema Orogen (SBO) developed during the late Neoproterozoic closure of the Sergipano oceanic basin at the northern margin of the São Francisco Craton (Ganade et al., 2021). The geological evolution of the SBO, particularly its thermal-metamorphic history, remains a subject of debate. Discussion is focused on the western section of the SBO, known as the Riacho do Pontal belt (RPB), estimates for the timing of crustal thickening vary from 630 to 570 Ma [Brito Neves et al., 2015; Caxito et al., 2016]. These ages are based on zircon U-Pb dating of syn-tectonic granites, with no previously reported monazite ages in this region. In the RPB, three main tectonic zones, namely, external, central, and internal, compose a roughly ~200 km long east-west trending orogenic wedge characterized by a predominant top-to-the-south nappe stacking geometry [2,3]. To better understand the metamorphic history of this ancient collisional orogen, we have employed a combined study of X-ray mapping, phase equilibrium modeling, Zr-in-rutile thermometry, and in situ (LA-ICP-MS) monazite U–Th–Pb dating coupled with monazite–garnet trace element geochemistry on six metapelitic schist samples spatially-distributed along the main tectonic zones. St-Ky-bearing micaschist from the external nappes experienced a clockwise P-T path from 5.5 kbar and 525 °C to 11 kbar and 660 °C, followed by post-peak decompression to 8.5 kbar and 675 °C. In the higher-grade internal zone, Ky-bearing migmatitic micaschist recorded prograde to peak metamorphic garnet growth from 7.5 kbar and 560 °C to 13.5 kbar and 670 °C, followed by post-peak decompression above the H2O-saturated solidus to 10.5 kbar and 680 °C. Monazite in these rocks displays complex Y + HREE zoning, interpreted to vary alongside garnet in the equilibrium assemblage. Three distinct samples from the external zone yield weighted mean monazite U-Pb ages of 630, 610, and 590-580 Ma, interpreted as the timing of prograde, peak, and retrograde metamorphism, respectively. The Grt-Chl-Ms schist sample from the central zone yields weighted mean monazite U-Pb ages of ca. 680 Ma for cores and ca. 630 Ma for rims, interpreted as representing two distinct tectonic cycles, the first related to pre-collision accretion and the second as a result of regional metamorphism overprint. In the internal tectonic zone, monazite–garnet Y+HREE partitioning systematics are remarkably consistent across all samples and yield a U–Pb age range of ca. 590–580 Ma, interpreted as recording the timing of H2O-saturated melting and decompression. The overall results indicate that deformation and HP-amphibolite facies anatexis in the internal zone (590–580 Ma) are younger than the peak barrovian metamorphism (Ky-St zone) in the external and central zones (630–610 Ma). This observation, suggests a shift in the strength and mass transport of the orogenic wedge, leading to out-of-sequence thrusting propagation. We hypothesize that H2O-saturated melting may have weakened the upper nappes in the internal zone, allowing a localized deformation away from the lower units. Furthermore, our data highlight the importance of recognizing that monazite can (re)crystallize at various stages along the metamorphic P-T path and may not exclusively provide timing constraints on “peak” metamorphism. Therefore, it is imperative to use multiple petrochronologic approaches to robustly constrain the rates and timescales of metamorphic processes.

1 Institute of Geosciences, University of São Paulo, Brazil (ltesser.geo@gmail.com)
2 Institute of Earth Science, Université de Lausanne, Switzerland
3 Brazillian Geological Survey, Rio de Janeiro, Brazil
4 Institute of Geological Sciences, University of Bern, Switzerland

S4-P28 | Sampriti Basak | Tracing the evolution of granulites and associated rocks at 2.9 Ga Fiskenæsset Anorthosite Complex, SW Greenland

Basak, S.1, Szilas K.1

Fiskenæsset Anorthosite Complex (FAC), located in the Fiskenæsset region in SW Greenland is among the best preserved high-grade Archean complexes in the world. The region hosts a variety of rock types ranging from metaperidotites, garnetiferous amphibolites, garnetiferous pyroxenites, clino-orthopyroxene bearing anorthosites, amphibole bearing anorthosites, and chromitites. Several studies for decades have interpreted this region to be the result of a subduction zone setting which involved hydrous recycling of lithosphere and arc magmatism operating as early as Mesoarchean (Windley et al., 1973). While such interpretations are highly debated, the aim of our project is to constrain the detailed metamorphic history of the FAC rocks post its emplacement, which can be traced back as early as Neoarchean based on metamorphic U-Pb zircon ages from the region (Polat et al. 2010, Keulen et al. 2010). Therefore, knowledge of temporally constrained Neoarchean P-T history of the region can be used as fingerprints of the prevalent geodynamic and tectonic processes from at least Neoarchean and onward.

Here we report the result of studies on garnetiferous amphibolites and granulites and associated anorthosite rocks from FAC. The metamorphosed mafic rocks occur in addition to enclaves within regional anorthosites, as individual bodies in the region. It is also worth noting that the region also has the type locality and several occurrences of sapphirine bearing rocks that occurs east to these rocks. Using an integrated approach of petrography, detailed elemental mapping, geothermobarometry, phase equilibria modelling and Lu-Hf garnet ages, we constrained the metamorphic P-T-t history of the terrain. Our preliminary result shows that the rocks from Fiskenæsset have been subjected to multistage metamorphic events where the original mafic rocks were metamorphosed to an amphibolite (M1 metamorphism) at ∼5-7 kbar and ∼700°C   to high grade granulite facies conditions (M2 metamorphism) possibly at ∼11-12 kbar to temperatures ∼900°C, where they often crystallized corundum and orthopyroxenes (∼4 wt% Al2O3) first and eventually sapphirine during cooling, as substantiated by textural observations. A K+ rich fluid is further affecting these assemblages during retrogression in its evolutionary course (M3 metamorphism) leading to biotite crystallization. Our results corroborates that the peak metamorphic event at lower crustal depths can be traced back as early as ~2.63 Ga from in-situ Lu-Hf garnet geochronology. Compositional zoning formed by diffusion of Fe-Mg has been measured at garnet-biotite contact. These are being modelled currently to determine the actual cooling/decompression paths post M3 event and the timescales over which the individual processes described above occurred.

1 Department of Geosciences and Natural Resource Management, University of Copenhagen, Øster Voldgade 10, 1350 Copenhagen, Denmark (sbasak@ign.ku.dk)

S4-P29 | Alberto Zanetti | Geochemical Characterization of Quartz-Dioritic Leucosome Reveals Evidence of Amphibolite Partial Melting from Val Sabbiola, Central Ivrea-Verbano Zone (Alps): Insights from Mineral Chemistry, Zircon Geochronology, and Zircon Isotopes

Zanetti, A.1, Ogunyele A.C.1,2 and Bonazzi M.1,2

The Ivrea Verbano Zone (IVZ) is a fossil continental crustal section formed during Variscan and post-Variscan tectono-magmatic processes. This section is well-exposed on the surface due to the Alpine collision. The metamorphic sequence mainly comprises metasediments, metavolcanic rocks, and subordinate marble lenses, which have undergone multiple high-temperature (HT) metamorphic processes that locally modified the original chemical composition of these lithologies. Evidence of the HT metamorphism is observable in the field due to the presence of leucosomes intruding all metamorphic lithologies and often occurring sub-parallel to the metamorphic foliation (Schnetger, 1994; Kunz et al., 2014). These bodies serve as indicators of partial melting; however, this process is poorly constrained from a geochronological and petrological standpoint. However, the specifics of this partial melting, such as which lithologies were melted and the extent of melting, remain debated within the scientific community.

Here we report the occurrence of quartz-diorite leucosome intruding the lower crust’s metamorphic rocks in the central IVZ along Val Sabbiola. This bodie aligns sub-parallel to the foliation of the host rock and comprises andesine, quartz, biotite, K-feldspar (less than 5% by volume), apatite, and various accessory minerals including amphibole, zircon, ilmenite, titanite, allanite, pyrite, and magnetite. Biotite, as the primary hydrated ferromagnesian mineral, offers significant insights into the geochemical properties of the parental melts. Biotites in this dyke exhibit relatively high Mg# (0.57–0.61) and are relatively depleted in trace elements such as Nb (~34.9 ppm) and Ta (~0.9 ppm). U-Pb zircon dating predominantly indicates a Lower Permian emplacement age (288.7 ± 3.9 Ma), aligning with the main magmatic pulse of the IVZ (approximately 292 to 282 Ma; Peressini et al., 2007). Additionally, a few zircon crystals record peak granulite-facies metamorphism around 316 Ma (Ewing et al., 2013). The Hf isotope has an average value of -6.2, suggesting crustal lithology segregation, with only one zircon crystal showing positive values.

Preliminary findings from field relationships, mineral chemistry, as well as zircon U-Pb dating and Hf isotopes suggest that the quartz-diorite likely originated from crustal-derived calc-alkaline melts. These melts were generated by anatexis of mafic protoliths during post-collisional high-temperature metamorphism associated with the emplacement of the large Mafic Complex.

1 Institute of Geosciences and Earth Resources of Pavia (IGG-CNR), Via Ferrata 7, Pavia 27100, zanetti@crystal.unipv.it
2 Department of Earth and Environmental Sciences, University of Pavia, Via Ferrata 7, Pavia 27100

S4-P30 | Stefano Piccin | New constraints on the high temperature metamorphism of the Oetztal-Stübai Complex (Eastern Alps)

Piccin, S.1a, Favaro S.2, Minopoli L.1, Poli S.1, Sessa G.1, Tiepolo M.1, Toffolo L.1, Tumiati S.1 and Zanchetta S.2

The Oetztal-Stübai Complex (OSC) of the Eastern Austroalpine domain is one of the largest tectonic units recognized in the Alps extending between western Austria (Tyrol) and northern Italy (Autonomous Province of Bozen/Bolzano). The OSC is a polymetamorphic unit made of crystalline basement consisting of metasedimentary rocks (paragneisses and micaschists) hosting numerous bodies of metagranitoids and metabasic rocks, along with subordinate metacarbonates and ultramafics of igneous origin. The Alpine metamorphism affecting the OSC increases in temperature from 300°C in the north-west up to 500°C in the south-east (Purtscheller & Rammlmair, 1982).

Pre-Alpine evolution of the OSC is testified by the crystallization of Cambrian mafic to ultramafic cumulates with MORB-like signatures preserved in pods and layers within the Central Metabasite Zone (CMZ) (Miller & Thöni, 1995; Konzett et al., 2005); an Ordovician high temperature event resulting in widespread intrusions of granitoids and partial melting of metapelites (e.g. Winnebach migmatites); occurrence of eclogites of Variscan age within the CMZ and subsequent pervasive re-equilibration under amphibolite facies conditions. However, structural relationships in the field do not rule out the possibility of a pre-Variscan high pressure event.

Extensive field work, microstructural and petrological analyses, and radiometric dating are being carried out in two key areas, Längenfeld and Reschenpass/Passo Resia. The two areas are similar in the occurrence of two Ordovician intrusions, the Sulztal type S granite (Längenfeld) and the Klopaier Tonalite (Reschenpass), for which we obtained by U-Pb LA-ICP-MS dating of zircons ages of 482.4 ± 1.5 Ma and 460 ± 0.83 Ma, respectively.

In the Längenfeld area, rocks belonging to the CMZ show various degrees of metamorphic reactions progress often resulting in symplectitic relationships. Mafic to ultramafic rocks, characterized by exceptionally well preserved cumulitic textures, display the destabilization at high pressure conditions of anorthite-rich plagioclase to omphacite + corundum intergrowths bordered by garnet, which completely replaces plagioclase in some samples as confirmed by REE patterns determined by in-situ LA-ICP-MS. Within these cumulates, newly discovered layers of troctolitic composition show corundum-bearing coronas around olivine and granoblastic textures with increased anorthite content in plagioclase rims, implying a static phase of high temperature recrystallization. Associated metacarbonates are characterized by the presence of olivine (Mg/(Mg+Fe) = 0.95) and Fe-spinel.

Additionally, we found evidence of partial melting involving both metapelites and eclogites of the CMZ, resulting in corundum-bearing migmatitic gneisses and eclogite-derived melts. The Variscan age of CMZ eclogites has been assessed by Sm-Nd mineral and WR isochrons in Miller & Thöni (1995), but unpublished U-Pb zircon data (Sollner & Gebauer in Hoinkes & Thöni, 1993) points to an age of 497 Ma for this high pressure event. This hypothesis deserves further investigations on account of our field work and newly discovered field relationships, suggesting the existence of a pre-Variscan high temperature event post-dating an older eclogite facies metamorphism.

1 Dipartimento di Scienze della Terra, Università degli Studi di Milano, Via Botticelli 23, 20133 Milan, Italy
2 Dipartimento di Scienze dell’Ambiente e della Terra, Università degli Studi di Milano-Bicocca, Piazza della Scienza 4, 20126 Milan, Italy
a e-mail: stefano.piccin@unimi.it

S4-P31 | Marco Filippi | Timing and Metamorphic Evolution of the Deep Crust of Adria in the Valpelline Series

Filippi M.1, Caso F. 1, Farina F. 1, Ovtcharova M. 2, Piloni C.B. 1, and Zucali M.3

We have constrained the metamorphic evolution (P-T-t) of Permian granulites and migmatites of the Valpelline Series, a portion of the deep crust of Adria preserved in the Dent-Blanche nappe of the Western Alps (Austroalpine domain). This crustal section, compared to other preserved Permian deep crust sections in the Alps (e.g., Ivrea-Verbano zone, Malenco unit, etc.), lacks Permian mafic intrusions, whose emplacement is associated with significant transformations in the host rocks. This makes the Valpelline series suitable to constrain the regional tectonic evolution of these basements during Permian extension. In this study, we implemented a multidisciplinary approach combining petrography, multi-scale structural analysis, U-Pb zircon and monazite geochronology, and geochemistry. The metamorphic P-T conditions are estimated integrating thermobarometry, pseudosections, and Ti-in-zircon thermometry.

We identified two partial melting events in the metapelitic migmatites of the Valpelline Series: the first occurred at around 293 Ma under P-T conditions of 870 – 890 °C and 8 – 10 kbar, evidenced by the Pl + Grt + Opx + Qz + Bt + Kfs + Ilm assemblage. The T/depth ratio of this event (25 – 30 °/km) is interpreted as resulting from an initial phase of lithospheric thinning, which followed crustal thickening due to the Variscan event. The second event occurred at around 287 Ma under P-T conditions of 700 – 780 °C and 4 – 6 kbar, as shown by the Bt + Crd + Grt + Pl + Qz + Ilm ± Sill assemblage. This second event, characterized by a T/depth ratio of 40 – 50 °/km, corresponds to a more advanced stage of lithospheric thinning.

The presence of abundant granulite relicts within these rocks, locally with internal foliation discordant to the dominant foliation, further indicates that the structure and parageneses of the Valpelline Series were acquired during a retrograde exhumation path. The structural homogeneity of the Valpelline Series suggests a consistent deformation regime during the Permian, with deformation localized across the entire unit rather than along discrete fault zones.

1 Dipartimento di Scienze della Terra “A. Desio”, Università degli Studi di Milano, via Mangiagalli 34, 20133, Milano, Italy
Corresponding author: marco.filippi@unimi.it
2 Department of Earth Sciences, University of Geneva, rue des Maraichers 13, 1205, Genève, Switzerland

S4-P32 | Yaron Katzir | Granulite xenoliths from the Arabian plate margin: Reworking of the East African lithosphere?

Warhaftig, N.1, Elisha B.1, Haviv I.1, Boneh, Y.1 and Katzir Y.1

The Arabian Nubian Shield (ANS) exposes ~106 km2 of late Neoproterozoic (870-580 Ma) upper continental crust formed during the East African orogenesis. Mantle and lower crustal xenoliths brought to the surface by Neogene alkaline basalts of the vast volcanic fields of the Arabian plate yielded late Neoproterozoic Nd isochron and model ages, indicating coeval formation of the whole Arabian lithosphere (McGuire and Stern, 1993; Stein and Goldstein, 1996). The NW margin of Arabia was shaped by several Phanerozoic Tethyan rifting events and more recently by the tectonics of the Dead Sea Transform (DST), a young plate boundary, separating the Arabian plate from the Sinai sub-plate. These events are well recorded by the Phanerozoic sedimentary cover and more rarely by the upper igneous crust of the Levant margin, however, was the deeper Neoproterozoic Arabian lithosphere also modified and possesses younger additions?

Here we study granulite xenoliths associated with Pliocene basalt and scoriae erupted from the Qarnei Hittin volcano, located a few km west of the DST in the eastern lower Galilee, northern Israel.  The xenoliths are characterized by preferably oriented bands of plagioclase and clinopyroxene and minor orthopyroxene, and some include kelyphite (plagioclase + clinopyroxene + spinel) after garnet. Petrologically, they resemble previously studied granulite xenolith suites from this volcano (Gazit, 2005) and elsewhere in the Arabian volcanic fields (McGuire and Stern, 1993; Al-Mishwat and Nasir, 2004), considered as cumulates from mafic alkaline magma that crystallized and later recrystallized within the East African lower continental crust, which stabilized at ~600 Ma. One of our sampled mafic granulite xenoliths includes a granitic domain (quartz + feldspar) with sharp contact against the host granulite. In-situ U-Pb dating (LA-ICP-MS) of six zircon crystals from the granite domain yielded an age of 338 ± 34 Ma, overlapping an early Carboniferous thermal and magmatic event in the Levant arch (Kohn et al., 1992; Stern et al., 2014; Golan et al., 2018). Petrologic study to constrain the peak pressure-temperature conditions of granulite facies metamorphism and later decompression and U-Pb apatite geochronology of the host granulite are on their way, aiming to provide insight on possible post East African modifications of the Arabian lithosphere.

1 Dept. of Earth and Environmental Sciences, Ben Gurion University of the Negev, Be’er Sheva, Israel; ykatzir@bgu.ac.il

S4-P33 | Cerine Bouadani | Reconstruction of the Variscan history of the Texenna basement (Lesser Kabylia, Algeria)

Bouadani, C.1*, Chopin, F.1, Štípská, P.2, Bendaoud, A.3, Fettous, E.-H.3, Schulmann, K.1, 2, Miková, J.4, Bouzekria N.5

The Lesser Kabylia massif, situated within the internal zone of the Alpine Algerian Tell in the Maghrebides, hosts a basement of Precambrian to Paleozoic age (?). Despite the negligible tectono-metamorphic Alpine overprint, the pre-Alpine history of this basement remains poorly constrained.

To address this knowledge gap, we conducted a petrological and geochronological study on the basement in the Texenna area, which comprises two main units: (1) a high-grade metamorphic lower unit and (2) a low-grade metamorphic upper unit containing Cambrian-Ordovician to Silurian-Devonian strata. The high-grade metamorphic unit is dominated by mafic granulites and migmatites with a common assemblage of Cpx–Grt–Pl–Qz±Opx±Sp and Grt–Bi–Pl–KPl–Qz±Sill±Sp, respectively. Notably, the felsic granulites and migmatites occasionally contain garnet and sillimanite, which enabled us to infer peak P–T conditions of P > 8 kbar and T = 775 °C using pseudosection modeling in the Perple_X software. Zircon U-Pb dating by LA-ICP-MS revealed predominant Permian ages ranging from ca. 266–295 Ma, with notable Carboniferous populations.

Our findings demonstrate the presence of a Variscan basement in this part of the Lesser Kabylia. The observed medium-pressure high-temperature metamorphism may be attributed to the closure of the Paleo-Tethys Ocean or its intracontinental propagator tip near the edge of Gondwana and the southern part of the European Variscan belt, sealed during the Pangea formation.

1 Université de Strasbourg, CNRS, ITES UMR 7063, 5 rue René Descartes, 67084 Strasbourg Cedex, France; bouadani@unistra.fr
2 Centre for Lithospheric Research, Czech Geological Survey, Klárov 3, 11821 Prague, Czech Republic
3 Laboratory of Geodynamics, Geology of the Engineer and Planetology, Faculty of Earth Sciences, University of Sciences and Technology Houari Boumediene, BP32 El Alia Bab-Ezzouar, Algiers, Algeria
4 Laboratories of the Czech Geological Survey, Geologická 6, Prague, Czech Republic
5 Ecole Nationale Supérieure de Kouba, Algeria

S4-P34 | İnal Demirkaya | Unusual slow-cooling of amphibolite-facies rocks in an orogenic belt (Bolu Massif, NW Turkey)

Demirkaya, İ.1,2, Topuz, G.1, Wang, J.M.

The Bolu Massif within the Tethyan belt comprises two different tectono-metamorphic units separated from each other by crustal-scale fault. These are (i) an underlying amphibolite-facies unit, ca. 50 km long and 8 km wide, and (ii) a subgreenschist-facies metabasic to acidic rocks intruded by late Neoproterozoic tonalite-quartz diorite. The tectonic contact between them is sealed by latest Cretaceous limestones. Amphibolite-facies tectono-metamorphic unit is dominated by magmatic amphibolite with frequent leucocratic sills/dykes and stocks of trondhjemite-tonalite composition. Also, minor metapyroxenite and serpentinite are present within the amphibolite. Amphibolite contains hornblende, plagioclase ±biotite, ±epidote, titanite, ±rutile, ±quartz and ±pyrite. Rutile forms either discrete grains or resorbed grains within titanite. Accessory minerals are apatite and zircon.

U-Pb dating on zircons from amphibolite and trondhjemitic sills and dikes record ages of 250-260 Ma. These age values are interpreted as the age of peak metamorphic conditions and partial melting. The U-Pb dating on titanite and rutile yielded ages of 231 ± 6 and 181 ± 6 Ma (2s), respectively. Available K-Ar hornblende and Rb-Sr biotite data in the literature are 222-205 ± 8 and 161-155 ± 2 Ma, respectively (Bozkurt et al., 2013). Combination of all the age data with respective “commonly accepted” closure temperatures of isotopic systems suggests that cooling from the metamorphic peak to temperatures of 250-300 °C lasted ca. 100 Ma. Metamorphic peak conditions are constrained as 670-750 °C and pressures on 0-5-0.7 GPa.

The metamorphic peak (250-260 Ma) corresponds to the time of sporadic basic to acidic magmatism in the Istanbul Zone, and interpreted to have occurred in a back-arc rifting. On the basis of textural characteristics, we can rule out multiple processes of burial-exhumation. No intrusions of Triassic to Early Cretaceous age are known in the region. So, we can rule out the fast cooling followed by pulsated reheating due to younger intrusions. Overall, dates from the different chronometers are in close agreement with the commonly accepted closure temperatures for the respective isotopic system. On the basis of the field relations and late Palaeozoic-Mesozoic stratigraphy of the Istanbul Zone, we interpreted this case as a post-extensional static relaxation of disturbed geothermal gradient.

1 Istanbul Technical University, Eurasia Institute of Earth Sciences, TR34469 Maslak, Istanbul, Turkey
2 Istanbul Technical University, Mines Faculty, Geological Engineering Department, TR34469 Maslak, Istanbul, Turkey
3 State Key Laboratory of Lithospheric Evolution, Institute of Geology and Geophysics, Chinese Academy of Sciences, 19 Beitucheng West Road, Chaoyang District, 100029 Beijing, China

S4-P35 | Xiven Zhou | Metamorphism and formation age of pelitic granulite in the Jiapigou area, North China Craton

Zhou, X.W.1, Guo, J.L. 1 and Xu, Q. 1

The Jiapigou metamorphic terrane is an important part of the archean basement in the northeast of the North China Craton. It mainly consists of intensely foliated tonalitictrondhjemitic-granodioritic (TTG) gneisses, weakly foliated syn-tectonic potassium granites, charnockites and a minor amount of metamorphic supracrustal rocks(Ge et al., 2003).There have been significant controversies over its metamorphic evolution, and especially its tectonic attributes of the Neoarchean metamorphic events(Sun et al., 1992; Zhai and Liu, 2003; Zhao et al., 2005; Guo et al., 2016). Therefore, based on detailed field geological investigation, several representative pelitic granulite(sillimanite/kyanite garnet biotite monzogneiss) samples were systematically studied on petrology, zircon U-Pb chronology, and zircon Lu-Hf isotope geochemistry.

Petrographic observation and phase modeling indicate that the pelitic granulite experienced three stages of metamorphism. The temperature peak metamorphic stage (M1) is characterized by the presence of garnet+ sillimanite+perthite+plagioclase, with a probable PT condition of 0.7-0.8 GPa at ca. 820-850 ℃. The pressure peak metamorphic stage (M2) is characterized by overprinting of kyanite from sillimanite, with a probable PT condition of 1.0-1.2 GPa at ca. 780-820 ℃. The post-peak decompression and cooling stage (M3) is characterized by the presence of garnet+biotite+muscovite+plagioclase under PT conditions of 600-650℃、0.60-0.65 GPa. The PT path shows anti-clockwise pattern, indicating a sinking process of supracrustal rock blocks in the magma caused by large-scale magmatic activity.

Zircon U-Pb and Lu-Hf analytical results show that the protolith ages of the pelitic granulite range from 2521 Ma to 2506 Ma, the metamorphic ages range from 2489 Ma to 2399 Ma(average age is 2482±7 Ma).  The zircon Hf model ages are between 2.9 and 2.7 Ga with positive εHf(t) values (0.42-4.76), which is familiar to the zircon Hf isotope characteristics of the TTG gneiss in the area(Guo et al., 2016), indicating that the TTG gneiss might provide material for the pelitic granulite. Most evidence suggest that the Neoarchean tectonic thermal event in the eastern part of the North China Craton was likely related to lithospheric heating caused by mantle plume activity.

1 Institute of Geology, Chinese Academy of Geological Sciences

S4-P36 | Ping-Hua Liu | Discovery and its tectonic significance of Paleoproterozoic high temperature pelitic granulites in Alxa Block, North China Craton

Ping-Hua Liu1, Lei Zou2, Wei Wang1 and Lei Ji3

High-temperature (HT) pelitic granulites, a prominent feature of Paleoproterozoic orogenic belts, preserve a record of geodynamic processes during the early Precambrian (Neoarchean–Paleoproterozoic). Quantitative peak P-T conditions and metamorphic timing of these HT granulites can constrain the tectonic processes and metamorphic evolution in such a tectonic regime. Here, HT pelitic granulites are first reported in the Diebusige Complex in the eastern Alxa Block, western part of the Khondalite Belt (KB), North China Craton (NCC). The detailed petrographic studies show that one pelitic granulite sample preserve the peak middle-pressure granulite-facies mineral assemblage which is defined by garnet + biotite + perthite + plagioclase + ilmenite + sillimanite + melt, and another sample show corona textures around relict garnet. The “white-eye” structure of garnet indicates that the pelitic granulites probably have undergone a post-peak near-isothermal decompression. Phase equilibrium modelling constrain a peak conditions of ~875℃/0.72~0.84 GPa implying a high apparent geothermal gradient (33℃/km) for these pelitic granulites. Based on the corona textures of garnet and mineral assemblages, we identified a clockwise P–T path involving a near-isothermal decompression process for these HT pelitic granulites. In addition, metamorphic zircon and monazite LA–ICP–MS U–Pb dating yields two meaningful age groups at ~1945 Ma and 1878–1866 Ma, which are interpreted as representing the timing of the near peak HT granulite-facies metamorphism the retrogressive amphibolite-facies metamorphism. The new and published metamorphic data indicate that the HT metamorphic conditions of the Diebusige pelitic granulite from of the Alxa Block may be the result of long-term slow uplift with heating under a high geothermal gradient.

1 Institute of Geology, Chinese Academy of Geological Sciences, Beijing 100037, China, lph1213@126.com
2 Key Laboratory of Orogenic Belts and Crustal Evolution, MOE, School of Earth and Space Sciences, Peking University,
Beijing 100871, China
3 Chinese Academy of Geological Sciences, Beijing 100037, China

S4-P37 | Juan Hu | Eclogite and medium-grade metamorphic rocks from northeastern Hainan Island, South China

Juan Hu 1 Xiaochun Liu 2

The eclogites were firstly discovered at Chaotanbi on northeastern Hainan Island, South China (Liu et al., 2021a, 2021b; Xia et al., 2019). These rocks mainly of garnet, omphacite, hornblende, quartz and rutile/ilmenite, with or without zoisite or plagioclase. The rocks experienced a clockwise metamorphic evolution, with peak P-T condition of 820-860 °C and 17.0-18.2 kbar (Liu et al., 2021a).

Isotopic geochronology studies (Liu et al., 2021; Xia et al., 2022) have shown that the formation of Chaotanbi eclogite is at ca. 364 Ma, most of the metamorphic zircons have low U and Th contents, and metamorphic zircons give two cluster age, white and bright cores yielded at ca. 350-330 Ma, and grey homogeneous rims/grains gave ca. 310-300 Ma, combined with zircon internal structure and inclusion characteristics, the former is interpreted as the early metamorphic age, the later as the eclogite facies peak and retrograde age, the latter is consistently with the isochron ages of Lu-Hf and Sm-Nd obtained recently (Hu et al., in preparation), and subsequent pegmatite intrusion at 295 Ma.

Trace element geochemistry (Liu et al., 2021; Xia et al., 2022) shows that the protolith age of the Chaotanbi eclogite is mainly tholeiitic basalt. Based on geochemical data, the Chaotanbi eclogites can be divided into three groups. N-MORB-type Group 1 was originated from relatively high-degree partial melting of a depleted spinel iherzolite mantle source slightly modified by slab-derived fluids. E-MORB-type Group 2 was derived from a deeper, enriched spinel and spinel-garnet iherzolite transition mantle source. IAB-type Group 3 was generated by partial melting of the deepest spinel-garnet iherzolite mantle source modified by subduction-related recycled components.

The age, geochemical and Sr-Nd isotopic data suggest that the protoliths of the Chantanbi eclogites formed in a late Devonian mature back-arc basin setting, which was linked to the Jinshajiang-Ailaoshan-Song Ma-Hainan back-arc system between the South China and Indochina blocks in response to the evolution of the eastern Paleo-Tethyan Ocean.

The relatively high P-T metamorphic rocks from northeastern Hainan Island might have been generated by oceanic subduction leading to collision of the Hainan continental block, or at least part of it, with the South China Block during the Carboniferous. This scenario is similar to transitional eclogite-HP granulite facies rocks in the European Variscan orogen of the western Palaeo-Tethyan tectonic domain.

In addition, the Mulantou paragneisses in northeastern Hainan Island record amphibolite-facies metamorphism under conditions of 717-771°C and 4.4-5.8 kbar (Hu et al.,2022). Zircon and monazites in the paragneisses yield U-Pb metamorphic ages of 250-235 Ma. These data, combined with the geochemical characteristics of associated granitoids, suggest that the paragneisses were formed in a transitional tectonic setting from compression to extension.

1 Institute of Geomechanics, Chinses Academy of Geological Sciences, 11 Minzudaxue Nanlu, Beijing 100081, China.
Email: hujuan1314@163.com
2 Institute of Geomechanics, Chinses Academy of Geological Sciences, 11 Minzudaxue Nanlu, Beijing 100081, China.

S4-P38 | Jianxin Zhang | Late Ediacaran-Early Paleozoic HP/HT metamorphism in the northern Qilian block manifests a long-lived advancing accretionary orogeny along Northern Gondwana

Zhang, J.X.1, Mao X.H.1 and Teng X.2

During Ediacaran-Early Paleozoic, Gondwana was flanked by a system of peripheral accretionary orogens (Cawood et al., 2021). The previous data show that the retreating accretionary orogens characterized the Ediacaran-Early Paleozoic Gondwana margin, which is dominated by low pressure/ high-temperature metamorphism and relatively high geothermal gradients (Oriolo et al., 2021). Here, we present evidence of high pressure/high temperature (HP/HT) metamorphism with moderate geothermal gradients from the northern margin of Qilian block in northern Tibet, which is considered as a continental fragment from East Gondwana.

The HP mafic granulites and felsic granulite are identified along the northern margin of the Qilian block. The peak mineral assemblage of mafic granulite is garnet + plagioclase + clinopyroxene + hornblende + ilmenite + quartz. The phase equilibria indicate that the peak P-T condition is 11-14 kbar and 800-900°C. The peak mineral assemblage of the HP felsic granulites is garnet + plagioclase + K-feldspar + kyanite + rutile + quartz, recording a peak P-T condition at 13 kbar and ca. 800°C. The peak pressure conditions of HP granulites cor­respond to crustal depths of ∼40–45 km. The petrographic observation, mineral chemistry and phase equilibria indicate that the mafic and felsic granulites have experienced an Isothermal decompression P-T path after the peak stage. The HP mafic granulite and local garnet-cumulate represent mafic residues via dehydration melting involving breakdown of amphibole with anatectic garnet growth.

SHRIMP and LA-ICP-MS U-Pb dating results of zircons show that the protolith crystallization ages of mafic and felsic granulites are 1110- 1140 Ma and the metamorphic ages ranging between 460Ma and 550 Ma. Monazite U-Pb datings of felsic granulites yield metamorphic ages between 455Ma and 510 Ma, and rutile U-Pb datings give a cooling age at ca.450 Ma.

The Late Ediacaran-Early Paleozoic HP/HT metamorphism is interpreted to occur in continental arc lower crust (arc root), suggesting that the northern Qilian block experienced crustal thickening related to the southward subduction of the Qilian ocean (Proto-Tethyan ocean). Combined dextral transpressive deformation (Wu et al., 2024), we suggest that the northern Qilian block represents a long-lived advancing accretionary orogeny along the northern Gondwana. Our study indicate that the Ediacaran-Early Paleozoic orogeny along the periphery of Gondwana involved in both advance and retreat accretionary orogenesis in different domains.

1 Institute of Geology, Chinese Academy of Geological Sciences, Beijing 100037, China (zjx66@yeah.net)
2 School of Earth and Space Sciences, Peking University, Beijing 100871, China

S4-P39 | Alex De Vries Van Leeuwen | Identifying overprinting metamorphism in granulite-facies rocks via the application of LA–ICP–ToF–MS monazite U–Th–Pb age mapping

De Vries Van Leeuwen, A. 1,2,3, Payne, J.4,3, Hand M.1,3, Wade, C.2,1,3, and Morrissey, L. 5,3

Identifying overprinting metamorphic events in rocks which have previously attained suprasolidus conditions can prove troublesome. This is largely due to the refractory and thus unreactive nature of suprasolidus rocks when compared to their subsolidus counterparts. Consequently, the timing and nature of such overprinting metamorphic events can often be obscure and challenging to constrain. The Mulgathing Complex of the central-western Gawler Craton of South Australia is a prime example of a terrane exhibiting such behavior. The region attained suprasolidus conditions of ~7.0 kbar and 850 °C during the c. 2470–2420 Ma Sleafordian Orogeny (Halpin et al., 2016). Following this, between 1730 and 1690 Ma, the craton experienced a second amphibolite- to granulite-facies metamorphic event known as the Kimban Orogeny. In the Mulgathing Complex, the expression of this event is restricted to terrane-scale shear zones which yield Kimban-aged metamorphic monazite (e.g., Swain et al., 2005). This overprinting event has also been identified to the south-east in the Sleaford Complex which was once contiguous with the Mulgathing Complex. In the Sleaford Complex, the expression of overprinting Kimban-aged metamorphism is pervasive, being readily identified in rocks distal from major shear zones (e.g., Dutch et al., 2010). Given that the Sleaford Complex experienced similar suprasolidus P–T conditions during the Sleafordian Orogeny (~4.5–6.0 kbar and 750–780 °C; Dutch et al., 2010), it begs the question as to why the expression of the Kimban Orogeny is not more pervasive throughout the Mulgathing Complex.

Unfortunately, unlike the Sleaford Complex, the Mulgathing Complex lacks the luxury of extensive outcropping exposures. Instead, the majority of available sample inventory exists in the form of drillcore, severely impeding the interpretation of structural relationships and our ability to link these with grain-scale metamorphic features. An initial attempt to identify the presence of Kimban-aged metamorphism in the Mulgathing Complex (i.e., outside of major shear zones) was conducted using conventional monazite U–Th–Pb spot analyses attained via laser-ablation inductively coupled plasma mass spectrometry (LA–ICP–MS). This attempt proved inconclusive, with few spurious analyses alluding to the presence of Kimban-aged overprinting metamorphism. As such, to elucidate the presence of a Kimban-aged overprint we employed a novel approach to rapidly generate high spatial resolution U–Th–Pb age maps of monazite via laser-ablation inductively coupled plasma time-of-flight mass spectrometry (LA–ICP–ToF–MS). These data reveal the presence of thin, discontinuous, Eu-rich monazite rims (~1–10 µm) which yield Kimban ages and mantle complexly zoned Sleafordian-aged cores. As demonstrated, such small and irregularly shaped domains are difficult to identify and analyse using conventional spot dating techniques, often resulting in mixed analyses and inconclusive data. This method offers a promising new way to analyse such features which are common in complex metamorphic rocks.

1 Department of Earth Sciences, The University of Adelaide, Adelaide, SA, Australia
2 Department for Energy and Mining, Geological Survey of South Australia, Adelaide, SA, Australia
3 Mineral Exploration Cooperative Research Centre, Kensington, WA, Australia
4 UniSA STEM, University of South Australia, Mawson Lakes, SA, Australia
5 Future Industries Institute, University of South Australia, Mawson Lakes, SA, Australia

Corresponding author email: alexander.devriesvanleeuwen@adelaide.edu.au

2

Poster session 5: Timescales and isotopes

S5-P40 | Yiruo Xu | Did Archean Metamorphic Terranes Cool Slower? Garnet Diffusion Study of the Quetico Subprovince, Canada

Xu, Y.1 and Holder, R.M.1

Archean metamorphic terranes are traditionally suggested to have cooled significantly slower than their Phanerozoic counterparts. Many have argued that this contrast in metamorphic timescale reflects changes in Earth’s tectonic regime (Chowdhury et al., 2021; Brown et al., 2022). However, diffusion chronometry-based cooling rate data on Precambrian rocks are very limited. We present a case study of metamorphic timescales on the Neoarchean Quetico metasedimentary belt of the Superior Province, which has been hypothesized to represent a fore-arc accretionary prism. Metamorphic grade decreases from the center of the belt (upper-amphibolite to granulite facies) to the northern and southern margins (greenschist facies). We combine conventional thermobarometry and phase-equilibrium modeling to constrain the peak temperatures and pressures and estimate metamorphic cooling rates from major element diffusion in garnet. The resulting cooling rates from across the subprovince exhibit large variability, with the fastest estimates comparable to those from the Phanerozoic eon and the lowest rates slower by two orders of magnitude. We then discuss the uncertainties and potential biases in determining diffusion timescales. The results will contribute to the diffusion chronometry data available on Precambrian orogens for assessing any fundamental change in global tectonics.

1 Department of Earth and Environmental Sciences, University of Michigan. 1100 North University Avenue, Ann Arbor, USA 48109. xyiruo@umich.edu

S5-P41 | Sebastian "Batzi" Fischer | Variable Hf signatures in zircons of granitic bodies (can) already form by magma mixing in the source region

Fischer S.1, Prave A.R.1, Cawood, P.A.1,2 and Hawkesworth, C.J.1,3

The Hf isotopic composition of the zircons in evolved continental crustal rocks (i.e. granitoids) can vary by as much as 9 εHf units, both amongst cogenetic zircon from a single sample[1] or from different samples across the same pluton[2]. Proposed explanations for this include: magma mixing of crust- and mantle-derived magmas with different Hf isotopic composition[1,3], a heterogeneous (crustal) source[4], variable dissolution of pre-existing zircon during disequilibrium melting which causes variable amounts of zircon-derived (“old”) Hf to individual melt batches (“zircon effect”)[5], or localised dissolution-(re)precipitation resulting in transfer of variable Hf isotopic compositions from inherited zircon domains to new magmatic domains[2].

We have measured Hf isotopic composition, U-Pb age, O isotopes, and trace elements in zircons from a mafic migmatite as well as co-eval, unmelted mafic units of a Archaean crustal cross-section (Kapuskasing uplift, Superior Province, Canada). A ~20cm-wide felsic sheet (melt channel) which cross-cuts the mafic migmatite is in petrographic continuum with the leucosomes of the host migmatite. Zircons from the felsic sheet show a significant and continuous spread of ~4 εHf units. Based on zircon morphology and trace element data, the more radiogenic Hf analyses can be matched to the zircons in the mafic migmatite and the unmelted lithologies. Thus, these zircons were likely derived directly from the migmatite. The less radiogenic zircons in the melt channel are distinctly different in morphology and trace element composition and must have been inherited from a different, but presumably nearby, rock unit. Zircon domains with Hf isotopic compositions in between these two endmembers form a mixing trend.

Our findings show that hybridisation of intracrustal melts and formation of a highly variable zircon Hf isotopic record typical of evolved crustal rocks can be achieved entirely within small melt channels hosted in the melting region. They also highlight the power of, and necessity for, a careful and holistic analysis of zircon and its diverse range of proxies compared to restricting analytical campaigns to age and Hf data alone.

1 School of Earth & Environmental Sciences, University of St Andrews, St Andrews, UK (sf67@st-andrews.ac.uk)
2 School of Earth, Atmosphere & Environment, Monash University, Melbourne, Australia
3 School of Earth Sciences, University of Bristol, Bristol, UK

S5-P42 | Hugo Dominguez | A multidisciplinary investigation of pluton formation and melt production in the deep crust: case study from the El Oro Complex, Ecuador

Dominguez, H.1, Lanari, P.1, Tamblyn, R.1, Riel, N.2

The El Oro Complex, located in southwestern Ecuador, represents a tilted section of continental crust from the Ecuadorian forearc. The lower part of this complex experienced partial melting due to the intrusion of a gabbroic magma during a brief period of time in the Triassic. This process led to the formation of an S-type pluton, known as the Marcabelí granitoid. The brevity of this event offers a unique opportunity to explore the mechanisms of melt formation and transport within the continental crust, from deep to shallow levels.

In this study, we present a thermal model integrated with thermodynamics to simulate partial melting and melt extraction under various scenarios throughout the Triassic metamorphic history of the El Oro Complex. At each timestep, we simulate the evolution of temperature and mineral assemblages of the sequence, extracting melt from the model when a liquid percolation threshold is reached. The effects of fractional crystallisation and mineral entrapment on the melt composition are explored and the results are compared with new field and geochemical data. U-Pb geochronology and trace element analyses were also performed on zircons from the granitoid and migmatites, providing better constraints on the timing and duration of the pluton emplacement. The results of the numerical models show that contamination from the gabbroic melt is required to reproduce the natural data and could explain the observed spread in the bulk rock chemistry. This study shows that the combination of these different approaches offers new insights into the melting processes of the deep continental crust. 

This project has received funding from the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation programme (grant agreement No 850530).

1 Institute of Geological Sciences, University of Bern, 3012 Bern, Switzerland
2 Institute of Geosciences, Johannes Gutenberg University Mainz, J.-J.-Becher-Weg 21, D-55128, Mainz, Germany

S5-P43 | Miguel Angelo Stipp Basei | U-Pb ages and Hf isotopes in zircon from charnockites and mafic granulite enclaves of the Luis Alves Craton, Southern Brazil

Basei M.A.S.1, Tesser, L.1, Bhattacharya, S.2, Chatterjee, S.2 and Ghosh A.2

The Luis Alves Craton corresponds to a continental fragment that occurs between Ribeira (N) and Dom Feliciano (S) Neoproterozoic belts, in the southern part of Brazil. Its basement consists of gneisses and migmatites among which high-grade metamorphic associations characterized by the presence of orthopyroxene can be found. Two large outcrops (Volkswagen and Infrasul) expose massif-type charnockite-enderbite gneisses with two types of mafic enclaves. These two localities were examined in detail with both locations displaying excellent outcrops of orthoderived charnockitic rocks with clear evidences of magmatic differentiation as the main process to explain the compositional variation found. Centimetric to metric bands of amphibolites, enderbites, charnockites (predominant rock), and mafic granulite enclaves occurs. Pink leucogranite cross cutting the banding are also observed in both locations.

The predominant mineral assemblages of these rocks are:  1) Charno-Enderbitic gneiss with Plag-Qtz-KFls-Opx-Cpx-Ilm-Bio;  2) Coarse-grained mafic granulite enclave present Hbl-Plag-Opx-Cpx-Ilm-Qtz and 3) Opx-Cpx-Plag-Ilm-Hbl-Qtz-Bio characterise the fine-grained mafic  enclaves.  Zircon is an ordinary  accessory mineral.  Hornblende-mafic granulite enclave is characterised by reaction texture of prograde hornblende breakdown to produce Orthopyroxene and plagioclase, indicating its restitic character also indicated by marginal Mg enrichment in the hornblende. In addition quartzo-feldspathic films at hornblende margin may suggest in situ melt. The fine-grained enclave on the other hand, could represent peritectic segregates. Charnockite-enderbite gneisses with  garnet assemblages are observed only in the Infrasul quarry. In both locations but predominantly in Infrasul quarry there are common evidence of retrograde reactions involving orthopyroxene with presence of significant hydrous phases (Biotite and Amphibole) mainly distributed along a late foliation in the shear bands, with  no reaction relation with either garnet or orthopyroxene.

New SHRIMP and LAICPMS zircon analyzes held in rocks from both localities yield different ages. In the Volkswagen outcrop the values ​​are concentrated around 2.5Ga (enclaves of mafic granulites, enderbites and charnockites) while the leucogranite cutting the banding indicated 2.3Ga. On the other hand, in the Infrasul Quarry all ages of the five rocks analyzed are around 2.2Ga.  The Hf isotope of both locations are very similar, being impossible to establish a consistent difference between them. The vast majority of Epsilon Hf values ​​(T1) is slightly negative (-9.0 to 0.0) and the model ages (TDM) are all predominantly Archean with values ​​ranging from 3.0 to 3.2Ga proving the crustal origin of these rocks.  When considered the differences in U-Pb ages and the similarities in the Hf model ages, it can be proposed that the studied charnockitic rocks were formed and metamorphosed in deep crustal levels, representing the melting at 2.5 of the material accreted at the base of the crust around 3.1Ga.

Unlike what was observed at the Volkswagen outcrop, the relatively high-temperature shearing and hydrous fluid infiltration observed in the Infrasul quarry, might have resulted in complete resetting of U-Pb isotopic system, so that the 2.5 Ga anatectic event was erased in the Infrasul quarry, whereas the Hf isotopic system did retain the memory of crustal anatexis around 2.5 Ga.

1 Geoscience Institute, University of São Paulo, Rua do Lago 562, SP, Brazil Postcode: 05508080 (baseimas@usp.br)
2 University of Calcutta, Kolkata, (None) INDIA

S5-P44 | Daniela Rubatto | Timescales of felsic lower continental crust formation: Insights from U-Pb geochronology of detrital zircon (Malenco Unit, eastern Central Alps)

Ewing, T.E.1, Rubatto D.1,2, Lemke K2. and Hermann J.1

Burial of supracrustal sedimentary material is one of the processes that contribute to the formation of the lower continental crust. We investigate the rate of this process in Permian lower continental crust from the Malenco Unit (eastern Central Alps), which represents one of several fragments of preserved Permian lower continental crust with a significant metasedimentary component that are now exposed in the European Alps. Zircon U-Pb analysis by secondary ionization mass spectrometry is used to constrain the age of protolith deposition and subsequent high-grade metamorphism of felsic, garnet-rich granulites. Detrital zircon cores, many of which are small (<50 μm), yield concordant dates ranging from 387 ± 21 Ma to the Paleoproterozoic, with the majority of the dates falling in the range 450–1000 Ma. For the seven cores in each sample that gave the youngest concordant dates, replicate analyses were carried out. None of these cores had all measured U-Pb dates agree within error, indicating significant disturbance by the Permian granulite facies metamorphism. A 206Pb/238U age of 424 ± 6 Ma for a core with two overlapping replicate analyses is the best available constraint on the maximum depositional age of the sedimentary protolith. This maximum depositional age also coincides with the first significant population in the probability density plot of detrital core dates, which is a double peak at ~425 Ma and ~450 Ma.

Zircon detrital cores are separated from metamorphic overgrowths by a small seam of zircon that is riddled with tiny inclusions of quartz, biotite and muscovite. The first generation of metamorphic zircon rims has a 206Pb/238U age of 272.9 ± 2.8 Ma, displays steep heavy rare earth element (HREE) patterns and a moderate negative Eu-anomaly, and gives Ti-in-zircon temperatures of 740–780 °C. The second generation of metamorphic rims has a 206Pb/238U age of 263.8 ± 2.6 Ma, a flat HREE pattern and a pronounced negative Eu-anomaly, and gave Ti-in-zircon temperatures of 780–810 °C. Collectively these observations indicate that the first zircon rim formed on the prograde path at the onset of partial melting where muscovite was present but the rocks contained little garnet and no K-feldspar. Therefore, the metapelites resided at lower amphibolite facies, subsolidus conditions up until the intrusion of a Permian gabbro, which was emplaced shortly after the formation of the first generation of metamorphic zircon and caused heating to granulite facies conditions and widespread partial melting. There is no record of high-pressure metamorphism preceding granulite facies conditions, indicating that felsic lower crust in the Malenco unit was not formed by relamination.

The Malenco unit was situated at the north-eastern active margin of Gondwana 420 Ma ago. Subduction-related accretion of sediments at lower crustal levels shortly after their deposition could explain the formation of this felsic lower crust that resided at subsolidus conditions for up to 100 My prior to Permian extension, gabbro intrusion and granulite facies metamorphism.

1 Institute of Geological Sciences, University of Bern, Baltzerstrasse 3, 3012 Bern, Switzerland
2 Institut des Sciences de la Terre, University of Lausanne, 1015 Lausanne, Switzerland

S5-P45 | Aratz Beranoaguirre | The effect of ultra-high temperature (UHT) and recrystallisation events on garnet U-Pb ages

Beranoaguirre, A.1, Corvo, S.2, Millonig, L.J. 1, Langone, A. 2, Marschall, H. 1, Albert, R. 1, Shu, Q. 3, Brey, G.P. 1, and Gerdes, A. 1

U-Pb dating of garnet by LA-ICPMS has become a powerful tool in metamorphic studies (e.g. Millonig et al., 2020) as it provides valuable time constraints for metamorphic reactions/conditions, in a relatively fast and affordable way. However, the approach is still in its infancy and many challenges regarding the behaviour of U and Pb need to be addressed or further investigated. For this communication, we have studied garnet crystals from the Kaapvaal craton and the Central Alps, where the U and Pb isotope systematics responded differently to the respective metamorphic histories.

On the one hand, we analysed garnet from 16 UHT granulite xenoliths from the Star mine (Kaapvaal craton, South Africa). The samples contain garnet (up to 80 vol% in some samples) and variable amounts of sapphirine, plagioclase, quartz, sillimanite, and accessory phases. The estimated pressure-temperature conditions for the samples are 1 GPa and 1050 °C (Shu et al., 2024). The U-Pb dates obtained from the studied crystals span 400 million years with well-defined maxima at 3.09, 3.01 and 2.75 Ga. Regardless of the metamorphic interpretation of the ages (see discussion in Shu et al. 2024), the garnet U-Pb data records the UHT history of the garnet, which is distinctively older than the kimberlite eruption of ca. 124 Ma. Moreover, zircon U–Pb dates from previous studies show a single dominant age peak at 2.72 Ga.

On the other hand, we studied two samples from the Cima di Gagnone unit (Central Alps, Italy). The samples represent metasediments (plagioclase + quartz + biotite + muscovite + garnet + kyanite ± staurolite + accessories) that envelope UHP-HT ultramafic lenses. The rocks experienced synkinematic recrystallisation at pressure-temperature conditions of up to 1.2 GPa and 700 °C, although samples from the immediate proximity of the ultramafic lenses record significantly higher PT conditions of up to 1.7 GPa and 850 °C in the presence of fluids, and devoid partial melting (Corvò et al., 2021). The garnet-bearing sample in contact with the ultramafic lenses gave a more precise garnet U-Pb date of 40.7 ± 1.5 Ma, compared to the “country rock” garnet, which yielded 38.6 ± 6.0 Ma. In addition, the latter sample contained analyses that indicate a Jurassic component, which may represent a partial recrystallisation process of the garnet or a so far unrecognised geological event.

Our results indicate that the U–Pb system in garnet has a very high closure temperature (likely ≥ 1100 °C) and thus can retain its original isotopic composition even under conditions at the most extreme end of crustal metamorphism that are capable of resetting zircon. Such older U-Pb garnet ages can only be reset by partial or complete recrystallisation.

1 Frankfurt Isotope and Element Research Center (FIERCE) and Department of Geosciences, Goethe-University Frankfurt,
Altenhöferallee 1, 60438 Frankfurt, Germany. beranoaguirre@fierce.uni-frankfurt.de
2 Department of Earth and Environmental Sciences, University of Pavia, via Ferrata 1, 27100 Pavia, Italy
3 State Key Laboratory of Ore Deposit Geochemistry, Institute of Geochemistry, Chinese Academy of Sciences, Guiyang 550081, China

S5-P46 | Xiaofang He | Garnet ages, cooling rates and heat production in the Sri Lankan granulites

He X.1,2, Hand, M.2, Dharmapria, P.3

Sri Lanka lies at the conjunction of eastern and western Gondwana, linking the continents of India, Madagascar and East Antarctica via the East African and Kuunga orogens. The Precambrian basement of Sri Lanka underwent an extensive high temperature to ultrahigh temperature (HT-UHT) metamorphism during Gondwana assembly in Neoproterozoic-Cambrian transition. However, there has been ongoing debate on the heat sources as well as the duration of HT-UHT metamorphism and subsequent cooling.

The ambiguity surrounding the duration of high-T metamorphism stems from the potential complexity of multiple-stage growth and dissolution of accessary minerals during granulite facies metamorphism, and their compositional decoupling from the bulk silicate mineral assemblage. Consequently, Sri Lankan rocks show a continuous range of concordant accessory mineral U-Pb ages that span more than 100 Ma.  Despite a considerable number of studies, the significance of this age data and the thermobarometric evolution is unclear. Furthermore, little is known about the cooling history of the Sri Lankan granulites.

We have collected a suite of garnet Lu-Hf ages, together with biotite Rb-Sr and apatite U-Pb ages across the three high-grade tectonic units in Sri Lanka. In the Highland Complex, which has been the focus of the bulk of metamorphic research in Sri Lanka, ultrahigh temperature granulite garnet gives Lu-Hf ages of ca. 600-580 Ma, while lower temperature granulites have garnet Lu-Hf ages of ca. 560-550 Ma. Additionally in some samples, garnets record ages of ca. 1.8 Ga. This confirms previous suggestions that at least part of the Highland Complex comprises high-grade Paleoproterozoic metamorphic rocks. Importantly, the isotopic preservation of c. 1.8 Ga garnet suggests that despite the thermal intensity of Late Neoproterozoic to Early Cambrian metamorphism, garnet can retain useful age information, and therefore potentially illuminate the metamorphic structure of the Sri Lankan crust during Gondwana assembly.  In addition to outcrop samples, we have analysed garnet grains in beach sands from a number of locations. These show Lu-Hf ages peaks at about ca.1.8 Ga and 550 Ma, as well as much younger signal of ca. 500 Ma. The significance of the youngest age population is still unclear.

Biotite Rb-Sr ages in tectonic units in Sri Lanka cluster at ca. 490-480Ma. Apatite U-Pb ages span from ca. 480 to 460 Ma. Inferred cooling rates using the different closure temperature of those geochronometers range between 3.5–6.5 °C/Ma similar to the cooling rate of ≤ 7 °C/Ma from Southern India and Madagascar.

To better understand the hear sources for HT-UHT metamorphism we have collected regional-scale measurements of in-situ heat-producing elements to better characterise in-situ thermal sources. Volume averaging of rock types indicates terrain-scale U-Th concentrations would have generated around 2 micro Wm-3 at the time of median peak metamorphism. This suggests the generation of HT-UHT metamorphism in Sri Lankan required a significant contribution from mantle heat.

1 State Key Laboratory of Coal Resources and Safe Mining, College of Geosciences and Surveying Engineering, China University of Mining and Technology, Beijing 100083, China
2 Department of Earth Science, The University of Adelaide, Adelaide SA 5005, Australia
3 Department of Geology, Faculty of Science, University of Peradeniya, Peradeniya, Sri Lanka

S5-P47 | Martin Hand | Granulites record long-lived Meso-Neoarchaean orogenic plateau

Hand, M.1, He X.2,1, Morrissey, L.3, Glorie, S.1, Payne, J.3, Bockmann, M.4,1

The formation and rise of continental crust during the Archaean has been the focus of much attention, leading to the creation of various proxies and other arguments to evaluate crustal thickness. However where Archaean crust is well persevered, metamorphic data contributes useful information about crustal thickness and the potential duration of its thermomechanical stability.

Mesoarchean crust in southern India contains granulite assemblages that formed at 1.1 GPa and 900°C at around 3.1 Ga.  The granulites contain metapelitic rocks, implying the crust was appreciably thickened. Post peak mineral assemblages are typified by the formation of kyanite-biotite assemblages in metapelite and garnet in mafic rocks. Notably they lack evidence for reaction textures that denote significant pressure drop despite their thermobarometric sensitivity to such evolutions, implying the crust remained thick with respect to the thermal time scale.

Lu-Hf dating of garnet in mafic rocks that don’t appear to have experienced c. 3.1 Ga high-P granulite metamorphism, and therefore are probably deformed and recrystallized post c. 3.1 Ga intrusives, give ages around 2.5-2.6 Ga. These garnet-bearing assemblages formed at pressures similar to that of the c. 3.1 Ga metamorphism, and again record no reaction texture evidence for pressure drop.

Given the lack of evidence for exhumation of high-pressure c. 3.1 Ga assemblages and the similarity in pressures between c. 3.1 Ga and 2.5-2.5 Ga mineral assemblages, we speculatively interpret the data to reflect the long-term existence of thick Meso-Neoarchean crust. The inferred timeframe corresponds in part to the aggregation of Kenorland and encompasses development of the globally recognized c. 2.5 Ga granulite systems. We speculate the c. 3.1-2.5 Ga rocks in southern India are a fragment of a Meso-Neoarchean orogenic plateau.

1 Department of the Earth Science, the University of Adelaide, Australia
2 China University of Mining Technology, Beijing, China
3 Future Industries Institute, and STEM, University of South Australia, Australia
4 Geological Survey of South Australia, Australia

S5-P48 | Min Ji | Anatectic rocks witness thermal evolution and fluid action in the Himalayan orogen

Min Ji1, Xiao-Ying Gao1,2, and Yong-Fei Zheng1,2

Anatectic rocks (e.g., leucosome, leucocratic dike, and granite), formed through crustal anatexis at convergent plate margins, offer valuable insights into the chemical differentiation and compositional evolution of the continental crust. Crustal anatexis is triggered by an increase in temperature, a decrease in pressure (i.e. increasing geothermal gradient), or the addition of water, and proceeds via different anatectic mechanisms for different source rocks. Consequently, anatectic rocks exhibit diverse petrological and geochemical features, making them potentially powerful recorders of thermal evolution and fluid action at convergent margins.

The Cenozoic Himalayan orogen presents an excellent opportunity to test this hypothesis. The metamorphic evolution history of the orogen from the early to late Cenozoic has been extensively documented. The Higher Himalaya, generally referred to as its metamorphic core, exposes a suite of anatectic rocks with different petrogenesis (e.g., dehydration and hydration melting) alongside their potential source rocks (e.g., metapelite, metagraywacke, and granitic gneiss). In doing so, we conducted a comprehensive study of petrology, geochemistry, and thermodynamic modelling on both the source rocks and anatectic rocks in the Higher Himalaya.

The Higher Himalayan metamorphic rocks experienced metamorphism with increasing geothermal gradients from the early to late Cenozoic. In contrast, the anatectic rocks mainly formed in the late Cenozoic, indicating high geothermal gradients of the orogenic lithosphere at that time. Notably, the Nd isotope compositions of Higher Himalayan leucogranites exhibit a progressive decrease with time. This trend reflects an increasing contribution of materials with enriched Nd isotope signatures, such as the Higher Himalayan metapelites. Phase equilibrium modelling of metapelite, metagraywacke, and granitic gneiss shows that metapelite with enriched Nd isotope composition is the most fertile source rock, while granitic gneiss with relatively depleted Nd isotope composition is the least fertile. If the source of leucogranites is a mixture of these three rock types, then with increasing geothermal gradients, metapelite would generate a greater proportion of melts compared to other rock types. Consequently, the progressively decreasing Nd isotope compositions of leucogranites can be attributed to the increasing geothermal gradients.

Boron isotope geochemistry was employed to trace the fluid action, due to the enrichment of heavy boron isotope in fluids compared to coexisting minerals and melts. In a suite of metapelites and their derived leucocratic dikes, the decreasing δ11B values in tourmaline from low- to high-grade metapelites suggest that dehydration metamorphism results in the upward migration of metamorphic fluids enriched in heavy boron isotope in the early Cenozoic, and the higher δ11B values of leucocratic dikes compared to their source rocks can be ascribed to the addition of these dehydration fluids enriched in heavy boron isotope during anatexis in the late Cenozoic. This spatiotemporally coupled dehydration-hydration melting process highlights the role of early metamorphic fluids on later crustal anatexis.

This work was supported by funds from the National Science Foundation of China (42202044 and 42072070) and the China Postdoctoral Science Foundation (2023M733365).

1 School of Earth and Space Sciences, University of Science and Technology of China, Hefei 230026, China (minji@mail.ustc.edu.cn; minji@ustc.edu.cn)
2 Key Laboratory of Crust-Mantle Materials and Environments & Center of Excellence for Comparative Planetology, Chinese Academy of Sciences, Hefei 230026, China

S5-P49 | Shumpei Kudo | Development of U–Pb zircon dating using 2-4 μm spot size by LA-ICP-MS

Min Ji1, Xiao-Ying Gao1,2, and Yong-Fei Zheng1,2

Accessory minerals such as zircon, monazite, rutile and titanite crystalizes during igneous and metamorphic processes to form internal structures such as oscillatory zoning and sector zoning, which contains the age information of the geological events [1]. Combining the race element concentrations and formation age data in each growth domain with temperature and pressure conditions of minerals that coexisted in equilibrium is crucial to extract the information of the host rock histories [2]. However, conventional analytical methods with several tens of µm spot size often struggle to resolve the distinct growth domains with as narrow as 10 μm. For example, the zircon geochronology has been challenging and scarcely reported in the Himalayan region due to thin metamorphic overgrowths [e.g. 3,4,5].

Previous studies have employed Secondary Ion Mass Spectrometry (SIMS) and Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS) depth-profiling techniques to date several µm thick overgrowths [e.g. 6]. While these methods offer high depth resolution of 100 nm, several growth zones can be mixed within a single ablation volume, especially in minerals with intricate internal structures. In contrast, LA-ICP-MS analysis with several µm spots allows targeted analysis of individual growth zones based on internal structural observations. However, achieving sufficient analytical precision with reduced ablation volumes has been a challenge for practical applications in geology.

In our study, we aimed to achieve high-sensitivity U-Pb isotope ratio analysis by utilizing a newly developed in-house femtosecond laser ablation system with a focused laser beam (~2 µm) and a multi-collector ICP-MS. Femtosecond lasers produce fine particles during ablation, minimizing particle loss during ablation and transportation. In addition, the small particle size enhances ionization efficiency in the ICP, leading to improved sensitivity and reduced elemental fractionation.

U–Pb dating experiments were conducted with optimised conditions of the laser, totaling 20 points for zircon standards GJ-1 and Plešovice (PSV), with glass standards employed for Pb/U correction. The data were 608 ± 23 Ma (2SE) for GJ-1 and 288 ± 10 Ma (2SE) for PSV, respectively. The result of PSV data showed approximately 15% systematic younging from the reference value [7], while that of GJ-1 apparently matched with the literature value [8] within the error range. Pb contamination on the analysis surface was not removed before measurement by pre-ablation, thus GJ-1, which has a lower uranium concentration than PSV, possibly picks up the effects of lead contamination more strongly. In addition, high lead blank values during analysis may contribute to age underestimation.

In this presentation, we will further evaluate the precision and accuracy of the reference zircon age data obtained using the highly focused laser beam coupled with multiple-collector ICP-MS, and discuss the potential application of the present technique to metamorphic geochronology.

1 Department of Geology and Mineralogy, Kyoto University, Kitashirakawa-Oiwakecho, Sakyo, Kyoto, 606-8502 Japan
Email: kudo.shumpei.84x@st.kyoto-u.ac.jp
2 The University of Tokyo, Geochemical Research Center, School of Science,The University of Tokyo, 7-3-1 Hongo Bunkyo, 113-8654 Tokyo, Japan

2

Poster session 6: Deformation and structure

S6-P50 | Corey Flynn | Mechanisms of episodic brittle failure in deep continental crust; testing models in the Western Churchill Province, Canada

Flynn, C.1, Mahan, K.1 , Shinevar, W. J.2 , Holyoke, C.3, Lipper, C.1 , Sims, J.

Deformation mechanisms in continental crust predict a transition from brittle fracturing in the upper crust to viscous creep and diffusive processes in the middle and lower crust.  However, evidence for episodic brittle failure of an otherwise viscously deforming deep crustal shear zone in the form of variably mylotinized pseudotachylyte is preserved in the Athabasca granulite terrane in the Western Churchill Province of the Canadian shield (Regan et al., 2014; Orlandini et al., 2019). The ca. 1.89-1.88 Ga Cora Lake shear zone is 6 km wide and was active during a decompression and cooling path from 35-25 km paleodepths (1.1 GPa to 0.8 GPa) and from 800°C to 650°C, respectively. The shear zone is located along a domain boundary that juxtaposes the plagioclase-rich and mafic roots of a large tonalite pluton against granitoids, felsic granulites, and metasedimentary rocks with a high modal abundance of quartz. Recrystallized grain sizes range from 100 to 500 micrometers near the margins of the shear zone to less than 10 micrometers at the core (Orlandini and Mahan, 2020). Quartz and plagioclase recrystallized grain size paleopiezometry suggests that differential stresses were around 50 MPa during the development of mylonitic fabrics throughout most of the shear zone, but they locally exceeded 175 MPa near the highly strained core. Strength contrasts between discrete lithologic bodies near the domain boundary may have led to localized stress concentration and brittle failure. To test this, we are conducting detailed field mapping to better constrain the geometries of affected units, and we are characterizing microstructural variation in host mylonites and deformed pseudotachylyte veins to evaluate strain history and deformation mechanisms. We perform kinematic vorticity analysis on rigid porphyroclasts and recrystallized grains in mylonitized pseudotachylyte veins as well as mylonitic and ultramylonitic host rocks to determine the relative contributions of pure and simple shear and also the degree of strain partitioning between these parts of the shear zone.  We construct a 2D viscoelastic numerical model to determine under what conditions the observed lithological contrasts concentrate stresses enough to nucleate lower crustal earthquakes. Inputs to the model include field-constrained geometric relationships, observed mineral modes and grain sizes, and experimentally derived flow laws. Future work will also include fourier transform infrared spectroscopy measurements for water content that will be used to calculate water fugacity for better constraints on appropriate flow laws. 

1 Department of Geosciences, University of Colorado, Boulder, 2200 Colorado Ave., Boulder, CO 80309
2 CIRES, University of Colorado, Boulder, 1665 Central Campus Mall 216 UCB, Boulder, CO 80309
3 Department of Geosciences, University of Akron, 302 Buchtel Common, Akron, OH 44325

S6-P51 | Christina Lipper | Reconstructing strain patterns in a deep crustal shear zone in the Canadian Shield: Evidence from kinematic vorticity analysis of host mylonite and sheared pseudotachylyte

Lipper, C.1, Flynn C.1, Mahan K.H.1, Gestos, A.1, Holyoke, C. 2

Strain partitioning in shear zones is induced by strength heterogeneity due to thermal variations, lithology, and/or microstructural components such as grain size. The Cora Lake shear zone in the Athabasca Granulite terrane in the western Churchill Province of northern Saskatchewan, Canada, is an excellent natural laboratory to investigate strain partitioning in lower continental crust. This 4-6 km wide granulite- to upper amphibolite-facies shear zone was active over pressure and temperature ranges of 1.1-0.8 GPa and 800-650 °C, respectively, and serves as a major boundary between two lithologically distinct domains (Regan et al., 2014). The NW side of the shear zone is characterized by protomylonitic 2.6 Ga granitoids and felsic granulite whereas the SE side is dominantly 3.2 Ga anorthositic and tonalitic gneiss with scattered lenses of mafic granulite. Mylonite and ultramylonite near the domain boundary host pseudotachylyte that is commonly overprinted by kinematically compatible viscous deformation. This overprinting deformation suggests that the pseudotachylyte was intermittently generated while the shear zone was active in the lower crust (Orlandini et al., 2019; Orlandini & Mahan, 2020). In the mylonitized pseudotachylyte veins, rigid clasts of garnet, clinopyroxene and orthopyroxene up to several hundred microns in mean diameter derived from fractured ultramylonitic wallrock are set in a sheared, fine-grained (d=10 microns) matrix of neoblastic garnet, pyroxene, plagioclase, quartz, and iron oxides. Rigid grain net analysis was employed to determine kinematic vorticity of the host mylonite in a variety of lithologies and mylonitized pseudotachylyte. The observed range of mean kinematic vorticity is 0.43-0.66 in the host rock and 0.66-0.86 in the pseudotachylyte. Thus, deformation in the host mylonite appears to have had a strong component of pure shear, whereas the mylonitized pseudotachylyte was dominated by simple shear, perhaps reflecting a weaker rheology in the pseudotachylyte due to finer grain size and conditions more favorable for diffusive deformation mechanisms (Menegon et al., 2017). We will also use X-ray computed micro-tomography to explore 3-D clast geometric variations. Our results will be important for understanding the kinematic evolution of the Cora Lake shear zone and for efforts to better constrain the occurrence of and mechanisms for formation of pseudotachylyte in shear zones below the classic frictional-viscous transition.

1 University of Colorado Boulder. 2200 Colorado Ave, Boulder, CO 80309. Christina.lipper@colorado.edu
2 University of Akron. 302 E Buchtel Ave, Akron, OH 44325

S6-P52 | Jonas Vanardois | Crucial role of water-present melting in metagranite: Implications for the instigation of crustal-scale shear zones

Vanardois, J.1,2, Trap, P.2 and Marquer, D.2

Where, when, and why large-scale shear zones nucleate and propagate into the continental lithosphere are critical issues that challenge the research in tectonics. The East Variscan shear zone is one of the crustal-scale strike-slip faults that shaped the Variscan orogenic crust during late Carboniferous time. Field-based structural analysis and petrological observations demonstrate that suprasolidus high-strain deformation zones and metagranite occurrences are spatially correlated. Among the three dominant lithologies forming this orogenic middle crust (metapelite, metagraywacke, and metagranite), petrological observations and phase equilibrium modeling indicate that the latter is the first lithology that melts during collision induced heating, in response to H2O-fluid-saturated melting. Our field data and modeling suggest that the water-fluxed melting of metagranite has a primary rheological control on the localization, instigation, and growth of crustal-scale shear zones in the middle crust. Thus, the distribution and geometry of metagranite at the crustal scale could be regarded as critical parameters influencing the rheological inheritance governing the tectonic evolution and localization of bulk strain in the continental lithosphere.

1 UMR 7063 Institut Terre et Environnement de Strasbourg (ITES), Université de Strasbourg, 67084 Strasbourg Cedex, France
2 UMR 6249 Chrono-environnement, Université de Bourgogne-Franche-Comté, 25030 Besançon, France

S6-P53 | Fabiola Caso | Into the structure of the Permian deep continental crust: a multiscale approach to reconstruct migmatization in the Valpelline Series (Dent-Blanche Nappe, Western Italian Alps)

Caso, F.1, Filippi M.1, Piloni C.B.1, Roda, M.1, Farina F.1, Piazolo S2. and Zucali M.1

Melt production in the deep continental crust and migration towards shallow levels are processes that contribute to the chemical differentiation of the continental crust. The Valpelline Series cropping out for ca. 15 km along strike within the Dent-Blanche Nappe in the Western Italian Alps, provides a broad and continuous observation of the deep crust. For this reason, is a key target for investigating processes such as partial melting, granulitization, and melt migration during long-lasting lithospheric extension in Permian times (ca. 20 – 30 Ma, Kunz et al., 2018).

In this contribution, detailed mapping and structural analysis of the Valpelline Series allowed characterize the heterogeneity of the deep crust. The Valpelline Series consists of migmatitic metapelite with different mineral assemblages (i.e., garnet-, orthopyroxene- and cordierite-bearing), migmatitic amphibolite and marble intruded by aplite and pegmatite dykes (Caso et al., 2024a). Three superimposed foliations are recognized:  the S1 foliation occurs only within granulitic boudins across the whole unit and rarely in the metapelites. The dominant foliation S2, parallel to the alternation between leucosomes and melanosomes, is marked by different parageneses in different locations, recording different metamorphic conditions, consistent with exhumation from about 35 to 20 km depth within less than a 10 Ma interval. Lastly, the S2 is locally further transposed into a late S3 foliation extremely sillimanite-rich, localized in discrete metric-thick bands, and wrapping around garnet, cordierite, and orthopyroxene (Caso et al., 2024a, b).

The presence of several granitic plutons at shallow crustal levels along the entire margin of Adria suggests that melt extraction could have been a continuous and efficient process during the Permian, even in the Valpelline Series. Horizons extremely rich in sillimanite, where the S3 is the dominant structure, could testify to the interaction between the extracted melts and their host rocks.

1 Dipartimento di Scienze della Terra “A. Desio”, Università degli Studi di Milano, Via Luigi Mangiagalli 34, 20133 Milan, Italy; fabiola.caso@unimi.it
2 School of Earth and Environment, Institute of Geophysics and Tectonics, University of Leeds, Leeds LS2 9JT, United Kingdom

S6-P54 | Stefania Corvo | Coupled monazite and titanite petrochronology to unravel the shear zones activity from mid- to low continental crust (Ivrea-Verbano Zone; Italy)

Corvò, S.1,2*, Maino M.1,2, Bonazzi, M.1,2, Simonetti M.3, Montemagni C.4, Zanchetta S.4, Piazolo S.5, Langone A.1,2

Dating the time of shear zone activity remains still challenging. Here, we present the results of several studies from different mid/low-crustal shear zones exposed in the Ivrea-Verbano Zone (Southern Alps, Italy): the Anzola shear zone (ASZ), the Forno-Rosarolo shear zone (FRSZ) and the Premosello shear zone (PSZ) (Rutter et al., 1993; Simonetti et al., 2021; Corvò et al., 2023). Although the activity of these structures has been associated with the Mesozoic rifting phase (Petri et al., 2019), the age of their activities is still poorly constrained. In particular, we report an attempt to date the deformation by combining monazite and titanite U-(Th-)Pb data.

Studied shear zones share a sub-vertical attitude and a thickness between 100 and 500m. While the ASZ and the FRSZ formed at the amphibolite- to granulite-facies transition, the PSZ overprinted felsic and mafic granulites. The sheared rocks developed in a lithologically complex made of paragneisses, mafic rocks and minor calc-silicates. Paragneisses consist of Grt, Kfs, Plg, Bt and Sill; mafic rocks contain Cpx, Amph, Plg, Opx, and Grt whereas calc-silicates are made of calcite-rich and calcite-poor layers plus Amph, Px, Plg/Kfs and Grt. For in-situ U-(Th-)Pb dating purposes, we selected monazite from FRSZ and PSZ paragneisses and titanite grains from FRSZ and ASZ mafic rocks and calcsilicates (Corvò et al., 2022; Simonetti et al., 2023). Monazite from (ultra)-mylonitic paragneisses occurs in different microstructural positions (either included in porphyroclasts or along the mylonitic foliation) and commonly presents complex Th and Y chemical zoning. Titanite from mylonitic mafic rocks and calc-silicates are common along the foliation. Two types of titanite textures were identified: i) strongly zoned grains with LREE depleted rims/tips and ii) homogeneous grains. Both types show evidence for intracrystalline deformation (e.g., deformation twins and systematic crystal lattice bending).

U-(Th-)Pb dating across monazite and titanite revealed a contrasting behaviour of the two geochronometers. The Y-poor monazite domains give mostly Early Permian-Triassic ages, while the Y-rich domains provided late Triassic-Early Jurassic dates likely associated with fluid-assisted syn-deformational recrystallization. On the other hand, the titanite domains characterized by deformational features provide a lower intercept age at the Jurassic age. According to chemistry and isotopic data, while monazite recorded intense reactivity during Triassic ages, titanite locally developed syn-deformational rims or external domains providing Jurassic lower intercept ages.

This contrasting behaviour could be interpreted as the result of different response to changing P-T-composition and fluid availability conditions during the same deformation history. However, the combination of petrochronological and microstructural results and the contrasting behaviour of the two independent geochronometers lets to constrain a long-lasting deformation of the shear zones throughout the Triassic until Jurassic time. In particular, during this time interval, the activity of shear zones in the Ivrea-Verbano Zone was, within error, simultaneously at different crustal levels.

1 Department of Earth and Environmental Sciences, University of Pavia, Pavia, Italy; stefania.corvo@unipv.it
2 Institute of Geosciences and Earth Resources of Pavia, C.N.R., Pavia, Italy
3 Geological Survey of Italy, ISPRA, Roma, Italy
4 Dipartimento di Scienze dell’Ambiente e della Terra, Università degli Studi di Milano – Bicocca, Milano, Italy;
5 School of Earth and Environment, University of Leeds, Leeds, United Kingdom

S6-P55 | Maureen Gunia | The Chamrousse ophiolite, evidence of ductile deformation in amphibolitic facies during the pre-Variscan stage

Gunia, M.1, Cordier C.2 and Janots E.2

Chamrousse is located at the southern end of Belledonne massif in the External Crystalline Massifs (ECM) of the Western French Alps. The ECM are a privileged domain for understanding the construction of the Variscan basement (Guillot et al., 2009 ; Faure & Ferrière, 2022), as Alpine deformation rises only to a low degree of metamorphism (greenschist facies conditions (Rolland et al. 2003)) and is mainly localized in shear zones (Rossi and al., 2005). Identifying the ante-Variscan oceanic and continental domains whose amalgamation led to the construction of the belt remains a challenge.

The ophiolite of Chamrousse has been interpreted as the oldest and best preserved ophiolites in the Variscan terranes with an age of 496 Ma (Ménot et al., 1988). However new geochemical and geochronological data obtained in situ on zircon grains call into question the existence of a Cambro-Ordovician ocean at Chamrousse.  The base of the complex was interpreted as volcano-sedimentary and sheeted dyke units. They are, instead, made up of banded mafic and felsic units, now transformed into amphibolite and leptynite, emplaced in a continental setting, during the Early Paleozoic rifting of the Gondwana northern margin. The ophiolite, composed of gabbros and serpentinites, outcrops at the Chamrousse summit and is of Lower Carboniferous age.

All the Chamrousse units are affected by ductile deformation accompanied by metamorphic recrystallizations previously attributed to oceanic extension (Guillot et al., 1992). However, this model has difficulty in explaining the pervasive deformation and remobilization of continental crust in the amphibolite facies. U/Pb dating of metamorphic apatite in the Cambro-Ordovician lithologies suggests that their deformation occurred during the opening of the oceanic domain. In addition, the localized intense mylonitization, sometimes up to barely the migmatitic stage, coupled with U-Pb dating of gabbros on zircons and apatites suggests that the Chamrousse ophiolite may have been intensely deformed during the oceanic stage (pre-Variscan stage) and to a lesser extent during the Variscan collision. Field and microstructural observations, coupling geochemical, geochronology and thermobarometric data reveal a complex and multiphase tectonic-metamorphic history.

1 UMR5275 – Institut des Sciences de la Terre , Université Grenoble-Alpes – 1381 rue de la piscine 38610 Gières FRANCE – maureen.gunia@univ-grenoble-alpes.fr
2 UMR5275 – Institut des Sciences de la Terre , Université Grenoble-Alpes – 1381 rue de la piscine 38610 Gières FRANCE

S6-P56 | Leonardo Casini | Seismic deformation and water-fluxed melting of the upper continental crust (N Sardinia, Italy)

Casini, L.1, Corvò, S.2,3, Idini, A.1, Ferrero, S.4, Langone, A.2,3, Maino M.2,3

Recent works have demonstrated that water-fluxed melting is an effective mechanism to produces large volumes of low-temperature silicic melts at relatively low pressure, with important consequences for localization of deformation and the rheology of the continental crust (e.g., Collins et al., 2016; 2020; Tafur & Diener, 2020). Yet, several arguments have been used to question the commonness and the overall effectiveness of water fluxed melting, of which the most relevant is the low porosity of metamorphic rocks in the source region of migmatites.

Here we present field and micro-structural observations, geochemical data, and the results of preliminary pseudosection modelling from the Punta Bianca massif, a late Carboniferous  migmatitic complex exposed in N Sardinia, Italy (Casini et al., 2023; De Luca et al., 2023). Migmatites consist of quartz + biotite + plagioclase + K-feldspar metatexitic orthogneisses, garnet and cordierite-bearing diatexite and metatexites, derived from metasediments. Structural evidence and pseudosection modelling suggest that anatexis was episodic involving two main stages of partial melting. The first melting stage M1 is in the upper amphibolite facies (0.6-0.9GPa, 740°C). Cordierite overgrowths replacing sillimanite suggests decompression followed cooling below the solidus at low pressures of about 0.2-0.3 GPa (Casini et al., 2023). The M1 migmatitic fabric is offset by fractures and pseudotachylyte-bearing faults suggestive of cooling to greenschist facies conditions. Garnet/cordierite-bearing M2 diatexites incorporate fragments of metatexitic orthogneiss deformed by pseudotachylyte, and migmatitic metasediments. Monazite and zircon U-Pb dating of different leucosomes and pseudotachylyte veins confirm the field relationships and indicate that brittle deformation likely occurred shortly before the M2 melting stage.

The absence of peritectic minerals in diatexite, the substantial increase of the melt to protholith ratio, and the relatively low temperature suggest water-fluxed melting during isobaric re-heating up to 750°C. The dense network of fractures and faults cutting through the metatexitic orthogneiss may account for the fluid budget of M2 by increasing the dynamic porosity of the metamorphic crust. Beyond providing access to external fluids, flash heating during co-seismic deformation may be responsible for boiling of biotite yielding to instantaneous release of a large amount of fluids in situ.

1 Department of Chemistry, Physics, Mathematics and Natural Sciences, University of Sassari, Italy; casini@uniss.it
2 Department of Earth and Environmental Sciences, University of Pavia, Pavia, Italy;
3 Institute of Geosciences and Earth Resources of Pavia, C.N.R., Pavia, Italy
4 Department of Chemistry and Geology, University of Cagliari, Italy

>

Contact & Information: contact_granulite@sfmc-fr.org