The present study integrates petrochemical analyses and phase equilibria modeling of rare Garnet-Al-Silicates (Kyanite-Sillimanite-Andalusite) - Staurolite ± graphite bearing anatectic metapelites to comprehend its tectono-metamorphic evolution from the Eastern Himalayan Syntaxis (EHS), NE India. Extensive field studies reveal the occurrence of high-grade metapelitic rocks as enclaves in the Trans-Himalayan Lohit group of rocks (migmatitic gneisses), Dibang Valley, Arunachal Pradesh, India. Quartz-garnet-kyanite-plagioclase represents the peak metamorphic assemblage, in which garnets (Fe-rich) occur as porphyroblast and kyanite exhibit long crystals embedded in polygonal quartz and feldspar. The presence of peritectic kyanite and overgrowth of muscovite corona over kyanite (back reaction of melt with Al-silicates) indicates melting in the metapelitic rocks. Additionally, the presence of hydrous phases (e.g., muscovite and biotite), in the matrix points towards a wet melting condition. Furthermore, the post-peak metamorphic history of the rocks can be characterized by the presence of sillimanite and andalusite formed during exhumation. Thermobarometric studies estimate the peak metamorphic conditions for the metapelite at T =650±25°C, P~8 Kbar. The P-T pseudosection analysis predicts the peak metamorphic condition for the metapelites at T~ 650°C, P=7-8 Kbar; with clockwise PT paths. This suggests a wet melting condition at T~ 680°C (Solidus) and P~8 Kbar for the high-grade metapelites possibly related to the India-Asia collision from the Eastern Himalayan Syntaxis of Arunachal Himalaya.

How to cite: Saikia, D., Ozha, M., and Shankar, R.: Metamorphic Evolution of Rare Garnet-Al-Silicates (Kyanite-Sillimanite-Andalusite)-Staurolite ± graphite bearing Anatectic Metapelites from Eastern Himalayan Syntaxis, NE India, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-20042, https://doi.org/10.5194/egusphere-egu24-20042, 2024.

Accurately constraining the timing and tempo of specific metamorphic events is necessary to constrain the rates of important geodynamic processes, such as burial and exhumation in orogens and devolatilization in subduction zones. Accessory phase petrochronology is a popular tool employed to constrain the age and duration of metamorphic processes, including mineral nucleation and deformation fabric development. While accessory phases can produce robust dates, their thermodynamic data is often poorly constrained, requiring careful petrographic analysis to link dates to the pressure-temperature-deformation histories of metamorphic rocks. The dating of rock-forming minerals, like garnet and plagioclase, is typically more resource intensive than accessory phase petrochronology, but allows for directly linking dates to specific metamorphic events. Here, we consider the results of monazite accessory phase U-Th-Pb chemical dating with that of Sm-Nd dating of garnet from the Manhattan Schist, New York City, to explore how the synthesis of these two methods can be used to constrain robust tectonic histories.

The Manhattan Schist is a Laurentia-derived aluminous pelite historically thought to record polymetamorphism characterized by: 1) upper amphibolite facies metamorphism and anatexis associated with the Taconic Orogeny (~520-470 Ma) and 2) lower grade overprinting during the Acadian orogeny (~416-360 Ma). This interpretation, however, is based on correlation with seemingly similar units elsewhere in New England rather than direct study of the Manhattan Schist, and is called to question by the data presented here.

This work explores three samples, MAT-2017-01a, GWB-03, and CRT-06. All samples are grt-ky-bt-ms migmatites, that record a four-stage metamorphic history: 1) garnet growth starting at ~550°C and 4-6 kbar, 2) burial and heating to kyanite-grade anatexis at 700-750°C and 7-11 kbar, 3) ~1-2 kbar of isothermal exhumation to sillimanite (fibrolite) stability, and 4) continued exhumation to ~700°C and ~6 kbar. Monazite grains in all samples occur both in the matrix and as inclusions in garnet. Metamorphic monazite dates are dominated by a ~465 ±30 Ma (n=15) age consistent with growth during the Taconic Orogeny, with only a single date at ~383 ±25 Ma found in CRT-06. Bulk garnet Sm-Nd dating via TIMS in MAT-2017-01a, however, yields a late Acadian age of ~386 ±3.68 Ma (n=5, MSWD=0.94). The low MSWD coupled with the preservation of major element growth zoning in garnet, suggests that this value represents a single garnet age population reflective of growth at peak or near-peak conditions, entirely during the Acadian orogeny.

Taken as a whole, our results suggest that the Manhattan schist experienced: 1) early greenschist facies metamorphism during the Taconic orogeny recorded in metamorphic monazite, and 2) peak metamorphism associated with collision and burial in the Acadian orogeny recorded in garnet. Additionally, the sample-to-sample heterogeneity in monazite ages within the same lithology suggests that monazite growth is controlled by (sub)cm-scale processes. Therefore, future studies should investigate multiple samples of the same formation, if not the same outcrop, before making tectonic interpretations based on accessory phase petrochronology.

How to cite: Castro, A., Walker, S., Thomas, J. B., Jaret, S. S., Brunet, I., and Morin, K. D.: Insights from the Application of Accessory and Major Phase Petrochronology from the Manhattan Schist, NYC, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-12609, https://doi.org/10.5194/egusphere-egu24-12609, 2024.

How to cite: Demirkaya, İ., Topuz, G., and Wang, J.-M.: Unusually slow cooling of metamorphic rocks in an orogenic belt: A case from the Bolu Massif (NW Turkey) , EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-14, https://doi.org/10.5194/egusphere-egu24-14, 2024.

The Inthanon metamorphic core complex in northern Thailand comprises gneisses, schists, migmatites and calc-silicates which are intruded by a variety of granitoids of different age. New P-T estimates, U-Th-Pb total age and U-Pb age data of monazite indicate a multi stage history of the core complex.

Garnet mica-schists and gneisses are exposed in the western part of the Inthanon metamorphic core complex. Garnets show two episodes of growth, with some displaying garnet breakdown and corona formation containing plagioclase, quartz, biotite, and muscovite. The garnet core (grt1) records medium to high-grade metamorphism in the Late Triassic to the Early Jurassic. The second garnet growth (grt2) can be distinguished by a significantly lower Ca content and formed during a high-grade metamorphic event in the Late Cretaceous. Both garnet generations contain abundant inclusions of biotite, muscovite, and monazite, which are used for the reconstruction of the P-T-t history. The monazite included in grt1 yields an age of ~230 Ma. Metamorphic conditions during the first episode of garnet crystallization are 0.7–0.8 GPa at 530–570 °C. The second garnet growth occurred in the upper amphibolite facies at pressures of 0.4–0.5 GPa and temperatures of 640–670 °C. The monazite inclusions in grt2 and monazite within the garnet coronae yield an age of about 80 Ma. The monazite in the matrix, which exhibits complex chemical zoning indicative of recrystallization and re-precipitation during multiple stages of metamorphism, yields a main population at 230 Ma and a typical lead loss trend to a few clusters of 80 Ma dates.

Orthogneiss domains are characterized by a mylonitic texture with large K-feldspar augen and porphyroclasts surrounded by a fine-grained, foliated matrix of quartz and feldspar. This domain is mainly exposed in the core zone of the Inthanon complex and tends to become more mylonitic towards the east. Temperature conditions of 650–700 °C and 0.4 to 0.7 GPa were calculated by pseudosection modelling and using the Ti-in biotite-geothermometer. The monazite texture from the orthogneiss domain displays patchy zoning indicative of several resorptions and reprecipitations. The monazite exhibits an older age range of 230–210 Ma, with younger clusters yielding ca. 75 Ma, and shows a lead loss trend to ca. 30 Ma. The orthogneiss domain is extensively injected by concordant foliated biotite-garnet leucogranite with a monazite U-Pb age around 40 Ma. This age is considered as the upper age limit for the early stages of ductile shearing in the core complex.

Our data indicate that the Western Gneiss Belt in Thailand underwent several tectono-metamorphic events: (1) a medium P-T regional metamorphic phase in the Late Triassic to the Early Jurassic related to Sukhothai-Sibumasu collision (~200 Ma), followed by (2) a widespread younger overprint at upper amphibolite facies in the Late Cretaceous connected with local plutonic activity 75‑65 Ma. (3) Local Late Eocene–Oligocene magmatism and Chiang Mai basin development within deformation zones led to the emplacement of orthogneisses and local metamorphic overprint as seen e.g. in the Mae Ping and Three Pagoda shear zones.

How to cite: Santitharangkun, S., Hauzenberger, C. A., Skrzypek, E., Gallhofer, D., and Fritz, H.: P–T–t constraints on the Inthanon metamorphic core complex and implications for the evolution of the Western Gneiss Belt, northern Thailand, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-15425, https://doi.org/10.5194/egusphere-egu24-15425, 2024.

This study reports phase relations and U – Th – Pb_totalin-situ monazite geochronology of two-pyroxene granulite and high iron oxide bearing garnet-orthopyroxene granulite from the Karimnagar Granulite Belt, Eastern Dharwar Craton, India. The two-pyroxene granulite samples with granoblastic texture are composed of quartz, plagioclase, orthopyroxene, and clinopyroxene as major mineral phases, and biotite, zircon, ilmenite, and monazite occur as accessory phases. Small anhedral coronal orthopyroxenes surround the clinopyroxene megacryst and inclusions of clinopyroxene occur in orthopyroxene. Locally, biotites overgrow orthopyroxene and clinopyroxene along their rims. Garnet-orthopyroxene granulites are composed of orthopyroxene, garnet, spinel, quartz, plagioclase feldspar, hematite-magnetite, and ilmenite. Small, rounded orthopyroxene inclusions in garnet and anhedral coronal garnets surrounding orthopyroxene were observed. Spinels are hercynitic in composition. Ilmenite and tiny grains of spinel (~30 μm) occur as exsolve phases in magnetite. Magnetite separates spinel from garnet and orthopyroxene. The following metamorphic reactions can be proposed from the mineral assemblage:

• Cpx megacryst + Pl = metamorphic Opx + Cpx (Two-pyroxene granulite)
• (a) Opx ± Ilm + Pl → Coronal Grt + Qtz, (b) Al-rich magnetite→ Magnetite + Spinel (Hercynite), Ti-rich magnetite → Magnetite+ Ilmenite  (High iron oxide garnet-orthopyroxene granulite).

The result from petrography, phase equilibria, and thermometry of the two-pyroxene granulite predicts that metamorphic orthopyroxene becomes stable at 0.65 Gpa - 950^oC along a cooling path, and biotites overgrow orthopyroxene at 650 ^oC (0.65 GPa). Similarly, the result from grt-opx granulite implies a cooling path where coronal garnet becomes stable at 800 ^oC-0.65 Gpa.

The U-Th-Pb analysis of monazite grains in pyroxene (38 analysis) and feldspar (25 analysis) from two-pyroxene granulite shows the oldest and youngest peak at 2635 ± 90 Ma and 2449 ± 44 Ma, respectively. The monazite in pyroxene (46 analysis) and quartz (14 analysis) from garnet-orthopyroxene granulite exhibits the oldest peak at 2449 ± 14 Ma and a tiny youngest peak at 1987 ± 29 Ma. Another sample of garnet-orthopyroxene (40 analyses) displays the oldest and youngest peaks at 2574 ± 32 Ma and 2448 ± 25 Ma, respectively.

The late Neoarchean peak at ~2600 Ma retrieved from the pyroxene granulite and high iron oxide bearing garnet-orthopyroxene granulite is probably the protolith emplacement age of granulite, correlated with the accretion of Eastern Dharwar Craton and Bastar Craton as a part of extended Ur assembly. The early Paleoproterozoic peak at ~2450 Ma implies the formation of coronal garnet and metamorphic orthopyroxene during a cooling path.

How to cite: Satpathi, K. and Nasipuri, P.: Phase relations and monazite geochronology of two-pyroxene granulite and high iron oxide bearing garnet-orthopyroxene granulite from Karimnagar Granulite Belt: its importance in Neoarchean-Paleoproterozoic metamorphism, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-3113, https://doi.org/10.5194/egusphere-egu24-3113, 2024.

Subduction erosion present in oceanic subduction zones has been reported recently in continental subduction-collision zones, but the response of the upper plate remains enigmatic. The Dabie-Sulu orogen is considered to have formed by deep northward subduction of the Yangtze Block (YB) beneath the North China Block (NCB). However, the Haiyangsuo complex within the Northern Sulu ultrahigh-pressure (UHP) belt has intriguingly been accepted as Neoarchean–Paleoproterozoic metamorphic basement from the NCB. We examined petrography, mineral chemistry and geochronological data for the Haiyangsuo mafic granulites to decipher their multiperiod metamorphic evolution. The mineral assemblage from the first episode features coarse-grained Grt1 + Cpx1 ± Opx1 ± Amp1 + Pl1 + Ilm that formed under high-temperature (HT) granulite-facies conditions (7.3–8.3 kbar and 830–895 ℃). The second episode produced “red-eye socket” Grt2 + Cpx2 + Pl2 ± Amp2 + Qtz + Rut generated during high-pressure (HP) granulite-facies metamorphism (12.2–17.2 kbar and 800–875 ℃). Grt1 contains high heavy rare earth elements (HREEs) and oxygen isotopes in equilibrium with late Paleoproterozoic metamorphic zircons, while Grt2 has lower HREEs and oxygen isotopes, similar to Triassic metamorphic minerals in the Dabie-Sulu orogen. P-T estimates from pseudosection modeling and geothermobarometry show that Triassic HP granulite-facies metamorphism was followed by rapid decompression and slow cooling; these rocks resemble but mostly differ from the UHP eclogites due to non-UHP conditions. The above characteristics collectively imply that the metamorphic basement from the NCB was subducted to lower crustal to upper mantle depths (50–60 km) while the YB was subducted deeply; thus, tectonic erosion can indeed occur during continental subduction and collision.

How to cite: Liu, L.: Tectonic erosion along a continental subduction-collision zone revealed by compositions and metamorphism of mafic granulite in the Sulu orogen, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-18068, https://doi.org/10.5194/egusphere-egu24-18068, 2024.

Interaction between fluids and different rock types at the subducting slab interface directly influences deep volatile recycling processes. Recent studies [1] suggest that an influx of reduced fluids from graphite-bearing metapelites can promote deserpentinization at lower temperatures and modify the intrinsic redox capacity of the released fluid. Here we report a newly discovered high-pressure deserpentinization front ­– Cerro Pingano (Betic Cordillera, Spain), located 20 km north of the type locality of Cerro del Almirez [1] – where additional observations involving carbonate-bearing chlorite-harzburgites further support the influx of C-bearing fluids concomitantly to deserpentinization. This small body (<0.5 km²) is hosted within a metasedimentary sequence of graphite-bearing mica schists and marbles which together underwent an Alpine high-pressure metamorphism. Antigorite (Atg-)serpentinite (without carbonate) is separated from Chl-harzburgite (enriched in magnesite) through a sharp boundary of chlorite (Chl-)serpentinite that can be traced across a ~50 cm wide reaction front.

Atg-serpentinite shows an S-C fabric with a strong crystallographic preferred orientation (CPO) characterized by [001]_Atg perpendicular to the foliation. Although the transformation to Chl-harzburgite is associated with fabric weakening, the low-strain olivine, orthopyroxene, and tremolite show a remarkable distribution of the poles to their (010) perpendicular to the compositional layering. Magnesite [0001] axes are distributed perpendicular to the layering, and petrographic observations indicate textural equilibrium between Ol + Opx + Chl + Tr + Mag. The transformation of Atg-serpentinite to Chl-harzburgite is also associated with a decrease of Fe³⁺/ΣFe and a measurable increase of C, Na, and K content in bulk chemistry, suggesting an interaction with externally derived fluids.

We tested this hypothesis with thermodynamic modelling of the fluid-rock interaction between the host-rock derived fluids and Atg-serpentinites. Thermodynamic models of graphite-bearing mica schist predict dehydration due to chloritoid and chlorite breakdown, and release of highly reducing CH₄ + CO₂-bearing fluids. Open-system infiltration models show that host-rock derived fluids could effectively reduce the serpentinite bulk composition, consistently with the observed decrease of Fe³⁺/ΣFe between Atg-serpentinites and Chl-harzburgites. The infiltration-induced reduction shifts the stability of Chl-harzburgite mineral assemblage to temperatures ~50°C lower than predicted by closed-system models without external fluids input.

While the microstructural record suggests that Chl-harzburgite fabric was inherited from the Atg-serpentinite precursor, the changes in bulk chemistry and redox conditions result from interaction with highly reductive aqueous fluids derived from host graphite-bearing mica schists. Hence, we infer that the record of specific prograde metamorphic reactions may reflect changes in redox conditions, not necessarily associated uniquely with P-T changes. The redox contrast between the reduced fluids and prograde dehydrating serpentinites thus represents a new way to precipitate carbonates that can be further subducted and may devolatilized beyond subarc depths.  

[1] Padrón-Navarta, J.A. et al. (2023) Nat. Geosci.

Research funded by RUSTED project PID2022-136471N-B-C21 & C22 funded by MICIN/ AEI/10.13039/501100011033 and FEDER program, and “Juan de la Cierva” Fellowship JFJC2021-047505-I by MCIN/AEI/10.13039/501100011033 and CSIC (M. Bukała).

How to cite: Bukała, M., Padrón-Navarta, J. A., Menzel, M. D., Hidas, K., Sánchez-Vizcaíno, V. L., and Garrido, C. J. G.: The effect of modulated deserpentinization on the deep carbon cycle, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-12237, https://doi.org/10.5194/egusphere-egu24-12237, 2024.

Major- and trace-element zoning in metamorphic garnet can provide useful insights into the metamorphic evolution of the host rocks, in particular when it is coupled with microstructural investigation and thermodynamic modelling. Mylonitic micaschists along the Posada-Asinara Shear Zone in the Axial Zone of the Sardinia Variscan chain contain garnet porphyroblasts with four distinct growth zones (core, mantle, rim and outer rim). The porphyroblasts, enveloped by the S2 foliation, preserve an internal foliation defined by the orientation of the quartz, occasionally arranged in a sigmoidal pattern, suggesting rotation during growth. A large garnet porphyroblasts was investigated by laser ablation-inductively coupled plasma mass spectrometry (LA-ICPMS) mapping. The major element compositional variation follows a bell-shaped zoning, with Ca and Mn contents progressively decreasing, and Fe and Mg increasing, from the core (Alm₄₅Grs₂₅ Prp₁Sps₂₉) to the outer rim (Alm₈₆Grs₃Prp₁₁Sps₁). The trace element compositional profiles show a central enrichment area for the heaviest REE (Tm, Yb, Lu), corresponding to the garnet core. Elements with lower atomic number (Er, Ho and Y) show a similar enrichment in the mantle. Elements with even lower atomic numbers (Tb, Gd, Eu and Sm) are depleted in the core and mantle, but their content increases in broad shoulders in the garnet rim. The outer rim of the garnet is depleted in all REE. Trace element mapping also shows an annular enrichment zone in Y, Sc, Dy, Ho, Er and Tm at the mantle-rim boundary superimposed on the general compositional zoning. The REE distribution in the garnet growth zones reflects continuous growth controlled by diffusion-limited REE uptake. The Y + HREE annular enrichment zone is interpreted as a decrease in growth rate and  represents a short-lived episode in the garnet growth history. The clockwise P-T path, reconstructed by isochemical P–T phase diagrams and refined by the CZGM software (Faryad and Ježek, 2019, Lithos 332-333, 297-295) is divided into two distinct stages. The first, prograde stage is recorded by the compositional variation of garnet core and mantle, while the second stage reflects rim growth during exhumation. The core + mantle and the rim growth are separated by a decrease in garnet growth rate. The last part of the P-T path has been reconstructed from the rock mineral assemblage, which includes staurolite + biotite.

How to cite: Cruciani, G., Fancello, D., Franceschelli, M., and Rubatto, D.: Major- and trace-element zoning in garnet from mylonitic micaschists of NE Sardinia, Italy: implications for garnet growth and P-T history, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-4628, https://doi.org/10.5194/egusphere-egu24-4628, 2024.

Eclogites are a characteristic product of subduction-driven metamorphism and generally regarded as mineralogical evidence for modern-style plate tectonics. Consequently considerable effort has been expended to determine when eclogite appeared in the geological record, including claims of Archaean-aged eclogite.  One area of such contention is the Belomorian Orogenic Belt in NW Russia.  There are two schools of thought regarding Belomorian high pressure metamorphism.  One is that metamorphism occurred at c. 2.7 Ga, representing potentially the oldest occurrence of eclogite (eg Volodichev et al 2021; Minerals, 11, 1029).  The other view is that high pressure metamorphism occurred at around 2 Ga, forming part of the global set of c. 2.Ga eclogites (eg Yu et al., 2017; Journal of Metamorphic Geology, 35, 855-869). 

A combination of sample-scale X-ray mapping, laser ablation ICPMS and Time-of-Flight mapping has been used to delineate two generations of garnet in a sample from the well-studied Stolbikha Island locality in the Belomorian Belt.  The sample contains voluminous diopside-plagioclase symplectites whose re-integrated composition is consistent with the former presence of pyroxene with X_jd ~ 0.26-0.3. The symplectites overprint hornblende that probably formed during melt crystallisation.  The cores of the older generation garnet are comparatively HREE rich, and give an in-situ laser ablation Lu-Hf age of 2490 ± 40 Ma.  The second generation garnet is comparatively HREE depleted, and gives a Lu-Hf age of 1875 ± 86 Ma.  The comparatively large uncertainties reflect low Lu concentrations in the analysed garnets.  Both generations of garnet preserve partially relaxed major element zoning, and given the apparent diffusivities of major elements relative to Lu and Hf, it is probable both derived ages are meaningful.  An important difference between the two garnet generations is the presence of rutile inclusions in younger garnet and their absence in older garnet.   P-T modelling suggests c. 2.5Ga metamorphism occurred at c. 6-8kbar at temperatures > 750°C. For the younger mineral assemblage, P-T modelling taking into account the presence of the older garnet as well as the inferred clinopyroxene composition, combined with Zr-in rutile thermometry from rutile-qtz-zircon clusters in c. 1875 Ma garnet, gives c. 1.5 GPa and 700°C.   

The results indicate Belomorian Orogen high pressure metamorphism occurred at c.1875 Ma, confirming the conclusions of other workers (eg Yu et al 2017) that high pressure metamorphism is Palaeoproterozoic in age.  However we find no evidence for notably elevated pressures (> 2GPa) for the metamorphism at c. 1875 Ma as suggested by some previous workers, or evidence for high pressure metamorphism at c. 2.7 Ga. The Belomorian high-P rocks belong to the now globally distributed suite that records the emergence of eclogite in the initial stages of Nuna assembly. The general emergence of eclogite in the geological record at around 2 Ga probably reflects a combination of subduction into a cooler mantle and continental processes that facilitated preservation. 

How to cite: Hand, M., Brown, D., Glorie, S., He, X., Gilbert, S., and Payne, J.: In-situ garnet Lu-Hf geochronology from the contentious Belomorian high-pressure rocks., EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-4969, https://doi.org/10.5194/egusphere-egu24-4969, 2024.

Mineral reactions determine rocks' physical and rheological properties, but whether these reactions occur close to or far from equilibrium, and whether they are continuous or pulsed, is generally unclear. Garnet, the Rosetta Stone of metamorphic processes, presents an unparalleled potential to track the timing and rate of such reaction sequences often recorded and preserved in its growth zones. Here, we used a combination of major and trace-element mapping of garnet to identify single growth zones linked to specific mineral reactions. We extracted each zone by laser microsampling to perform high-precision Lu-Hf chronology and investigate whether and to what extent garnet keeps up with tectonic processes. The analyses were done on a single 1.3 cm-sized garnet grain from a mica schist from the Schneeberg Complex, Italy. The garnet grain was chemically characterised by major- and trace-element mapping (EPMA, LA-ICPMS), and five compositionally distinct micro-domains were extracted using a laser mill. Each single zone was divided into multiple garnet aliquots to enable multi-point isochrons. The four inner zones, corresponding to ~ 85% of the total garnet volume, yielded identical ages with a weighted mean of 98.4 ± 0.1 Ma (2σ). The outermost zone shows a strong chemical contrast with the rest of the grain, yielding a resolvably younger age of 97.8 ± 0.3 Ma. The data show that garnet growth in metapelites may take less than 1 Ma and, within that short time, progresses in several pulses that last less than c. 200 kyr. Our results demonstrate that garnet growth may occur much faster than required for changes in P–T conditions caused by tectonic processes. Pulsed, ultrafast garnet growth occurred instead at isobaric and isothermal conditions, far removed from equilibrium, and resulted in high-flux fluid production. While challenging the equilibrium paradigm, this example provides a rare glimpse into reaction overstepping and the rapid pushes of the system to attain equilibrium during periods of efficient matrix element transport.

How to cite: Tual, L., Smit, M. A., Cutts, J. A., Musiyachenko, K., Kooijman, E., Majka, J., and Foulds, I.: Garnet growth in a geological blink of an eye: evidence from high-precision Lu-Hf chronology, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-17461, https://doi.org/10.5194/egusphere-egu24-17461, 2024.

Subduction zones are critical sites for recycling of Li and B into the mantle. The way of redistribution of Li and B and their isotopes in subduction settings is debated, and there is a lack of detailed studies on Li and B partitioning between minerals of different types of eclogites and their host rocks. We present Li and B concentration data of minerals and Li and B whole-rock isotope data for low-T and high-T eclogites and their phengite schist host rocks from the Changning–Menglian suture zone, SW China. Omphacite controls the Li budget in both the low-T and high-T eclogites. Low-T eclogites have Li and δ⁷Li values (8.4–27.0 ppm, –5.5 to +3.2 ‰) similar to their phengite schist host rocks (8.7–27.0 ppm, –3.8 to +3.0 ‰), suggesting that Li was added to low-T eclogites from the phengite schists. In contrast, high-T eclogites have much lower δ⁷Li values (–13.2 to –5.8 ‰) than the phengite schists, reflecting prograde loss of Li or exchange with wall rocks characterized by low δ⁷Li values. Phengite and retrograde amphibole/muscovite are the major B hosts for low-T and high-T eclogites, respectively. The budgets and isotopic compositions of B in eclogites are affected by the infiltration of fluids derived from phengite schists, as indicated by eclogite δ¹¹B values (–15.1 to –8.1 ‰) overlapping with the values of the phengite schists (–22.8 to –9.5 ‰). Lithium and B in eclogites are hosted in different mineral phases that may have formed at different stages of metamorphism, implying that the contents and isotopic compositions of Li and B may become decoupled during subduction-related fluid-mediated redistribution. We suggest a mineralogical control on the redistribution of Li and B in eclogites during subduction and the exchange of Li and B with the immediate wall rocks. The observed contrasting Li and B isotopic signatures in eclogites are likely caused by a fluid-mediated exchange with different types of wall rocks during both prograde metamorphism and exhumation.

How to cite: Wang, D., Romer, R., Liu, F., and Glodny, J.: The behavior of Li and B isotopes in high-T and low-T eclogites enclosed by phengite schists, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-5469, https://doi.org/10.5194/egusphere-egu24-5469, 2024.

Eclogites have provided important constraints on the processes of plate subduction, collision and the overall tectonic evolution of orogens. The Gubaoquan eclogite is the only one identified in the Beishan Orogenic Belt, which constitutes the southernmost part of the Central Asian Orogenic Belt. The protolith age and tectonic setting of the eclogite and associated rock units remain ill-constrained. U–Pb dating of the eclogite indicates a Neoproterozoic age of 887 ± 7 Ma for its protolith age and an Ordovician age of 461 ± 15 Ma for the eclogite facies metamorphism. Detailed U-Pb ages of zircons, monazites, rutiles and apatites from the eclogite and country rocks suggest that they have experienced multiple metamorphic events in the Neoproterozoic and Paleozoic, which are inferred to correlate with retrograde metamorphism during the later exhumation. The protoliths of the Gubaoquan eclogite are of continental origin, and along with the extensive distribution of the Neoproterozoic arc-related rocks probably resulted from assembly of Rodinia. This work was financially supported by the National Natural Science Foundation of China (41730213 and 41890831), Hong Kong RGC GRF grants (17307918), HKU Internal Grants for Member of Chinese Academy of Sciences (102009906) and for Distinguished Research Achievement Award (102010100), Northwest University Excellent Doctoral Thesis Cultivation Program (YB2023003), and from the Australian Research Council (FL160100168).

How to cite: Wang, T., Cawood, P. A., Diwu, C., and Zhao, G.: Neoproterozoic-Early Paleozoic evolution of the southern Central Asian Orogenic Belt: constraints from eclogites in the Beishan Orogenic Belt (NW China), EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-9385, https://doi.org/10.5194/egusphere-egu24-9385, 2024.

Increasing evidence shows that, even where mid-ocean ridge serpentinization is widespread, significant volumes of fresh mantle are subducted. A part of these rocks may remain dry, or experienced subduction-related serpentinization. Once serpentinites are exhumed after a whole subduction cycle, it is challenging to determine timing and conditions of serpentinization. However, these may be constrained by investigating lithologies that are coupled with serpentinized peridotitic massifs from the mid-ocean ridge stage, such as gabbroic rocks. Here, we focus on variably transformed gabbroic rocks in the Monte Maggiore massif (Alpine Corsica, France). This body formed along a slow-spreading center of the Piemonte-Ligurian Basin in the Jurassic, and was subsequently subducted to blueschist-facies conditions. In the Monte Maggiore massif there is ample evidence for fluid–rock interaction processes. The most evident one is a serpentinization front of the peridotitic units where serpentinization degrees vary from < 10 to 100 %. Weakly serpentinized domains preserve rather fresh brown amphibole-bearing gabbroic rocks with only partial blueschist-facies metamorphic overprint. Instead, intensely serpentinized domains embed metasomatized gabbros such as rodingites.

To constrain timing and conditions of fluid circulation in the Monte Maggiore massif, we applied a multi-methodological approach to characterize the nature of metasomatic fluids and to date the timing of fluid–rock interaction in selected metasomatic and hydrated rocks. Low Cl contents (< 0.009 wt. %) measured by electron probe microanalyzer in late-magmatic brown and green amphibole in a Jurassic Ti-gabbro dike cutting the peridotitic units indicates that, after emplacement of the dike, this gabbro underwent alteration driven by magmatic water. This process was accompanied by titanite crystallization at the expenses of primary ilmenite and is dated at ca. 163 Ma by titanite LA-ICP-MS U–Pb geochronology. However, the formation of the Monte Maggiore serpentinization front recorded by rodingite-forming minerals and grossular-jadeite assemblage in partially re-equilibrated gabbros is dated at ca. 34 Ma and 47 Ma by LA-ICP-MS U–Pb geochronology of garnet.

Our data pinpoint at least two phases of fluid circulation in the Monte Maggiore unit, which span the entire evolution of the ophiolitic massif from rifting to high-pressure metamorphism during the Alpine orogeny. Notably, our geochemical and geochronological data rule out that any of these hydration phases were driven by seawater infiltrating the ocean floor, suggesting that mid-ocean ridge serpentinization contributed to a negligible extent to the overall hydration of the Monte Maggiore ophiolite. Conversely, our data suggest that serpentinization occurred primarily in subduction setting. Considering the role of subduction-derived serpentinization in transferring chemical species (e.g., C–O–H) feeding microbial activity in supra-subduction mantle regions, our evidence bears important implications for our understanding of the deep bio-geochemical cycle.

How to cite: Peverelli, V., Olivieri, O. S., Beranoaguirre, A., Cannaò, E., Ressico, F., Pastore, Z., Gerdes, A., and Vitale Brovarone, A.: Missing seawater in the hydration record of a peridotite massif from mid-ocean ridge to subduction (Monte Maggiore, Alpine Corsica, France), EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-879, https://doi.org/10.5194/egusphere-egu24-879, 2024.

In complex continental back-arc settings, such as the Cycladic realm of southeast Greece, polymetallic mineralization can provide information about past tectonic events and help to reconstruct the architecture of the metamorphic basement. In the Cyclades archipelago, slab rollback generated Miocene crustal extension and the formation of metamorphic core complexes, where hydrothermal systems occurring in the upper crust track one of the latest events in the geodynamic evolution. Lead isotope ratios of galena (PbS) and Sr isotope ratios of barite (BaSO₄) of the polymetallic ore deposits, which are exposed on most of the islands, reveal two distinct sources of lead and strontium in the underlying basement of the core complexes. A clear division exists between ²⁰⁶Pb/²⁰⁴Pb≤18.84 and ⁸⁷Sr/⁸⁶Sr≥0.711 in the north-central Cyclades and ²⁰⁶Pb/²⁰⁴Pb≥18.84 and ⁸⁷Sr/⁸⁶Sr≤0.711 in the west Cyclades. This regional pattern corresponds with the exposures of the high-pressure/low-temperature Upper and Lower Cycladic Blueschist nappes on the islands, respectively. Moreover, the pattern follows the surface trace of the proposed synorogenic Trans-Cycladic Thrust, a subduction-related structure that imbricated the high-pressure units. In addition, the isotopic composition of the hydrothermal minerals indicate that the structurally lower Cycladic Basement has a stronger influence on the metal sources of deposits occurring in the hanging wall of the Trans-Cycladic Thrust than in the footwall. Observations featuring the isotopic signature of hydrothermal minerals are complemented by a compilation of new and published isotopic and geochemical whole-rock data of the basement rock types. This allows us to further interpret the geodynamic significance of the apparent heterogeneities in the exposed rocks of the metamorphic core complexes and correlate the scattered exposures of the metamorphic basement. Furthermore, regional patterns in the isotopic and geochemical signature of the basement lithologies help to reconstruct the nature of the continental margin prior to Cretaceous-Eocene subduction.

How to cite: Wind, S., Schneider, D., Hannington, M., Hector, S., and Patten, C.: Reconstructing the nature of the metamorphic basement in the Cyclades, Greece, by the isotopic signature of hydrothermal minerals, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-16493, https://doi.org/10.5194/egusphere-egu24-16493, 2024.

Apatite, an abundant accessory mineral in igneous and metamorphic rocks, may accommodate appreciable amounts of trace elements including U, Pb and REE and is thus a useful U-Pb geochronometer and tracer of high-temperature metasomatism. Since apatite is susceptible to recrystallization, new growth and fluid-induced chemical changes, a good understating of the U-Pb system closure in apatite at variable geological conditions is required to correctly interpret apatite U-Pb ages. In this research, we delve into the behavior of apatite in marine phosphorites metamorphosed at sanidinite facies conditions to constrain U-Pb systematics in apatite under extreme high-T low-P conditions.

We sampled non-metamorphosed, metamorphosed, and hydrothermally altered marine phosphorites from the Mottled Zone pyrometamorphic complex exposures in the Hatrurim Basin, Israel. Petrographic study of the samples was followed by in -situ measurement of major (EMPA) and trace element compositions and U-Pb dating (LA-ICP-MS) of apatite.

Our results show that during the pyrometamorphic event apatite recrystallized and incorporated carbonate, silica, and sulphate into its crystal lattice. Thermally derived apatite recrystallization resulted in a submillimeter-scale Pb isotopic homogenization, inheritance of common lead of very low ²⁰⁷Pb/²⁰⁶Pb ratio from the U-rich, biogenic apatite precursor, and contemporaneous U-Pb system closure of apatite on a wider scale. The Miocene and Pliocene U-Pb ages of apatite coincide with previously known ⁴⁰Ar/³⁹Ar and K-Ar ages of the Mottled Zone pyrometamorphic event (Gur et al., 1995). Post-metamorphic interaction of high-T metamorphosed phosphorites with carbonate-, uranyl- and vanadate-rich fluids resulted in U and V enrichment of apatite, as well as redistribution of REE.

The results of our research shed new light on the timing and extent of the Mottled Zone pyrometamorphic event, contribute to understating the behavior of biogenic apatite under extreme high-T, low-P conditions, and exemplify the utilization of apatite in meta-sedimentary rocks for dating thermal events including combustion metamorphism.

How to cite: Segall, Z., Katzir, Y., Vapnik, Y., and Elisha, B.: U-Pb Dating of Apatite in Sanidinite-Facies Pyrometamorphosed Rocks from the Hatrurim Basin, Israel, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-11568, https://doi.org/10.5194/egusphere-egu24-11568, 2024.

Mineral host-inclusion systems can retain crucial information regarding their geologic history. For example, we can determine their formation conditions in terms of pressure (P) and temperature (T) from elastic geobarometry. In particular, it is possible to determine the strain acting on the inclusion when still entrapped in its host by measuring changes in the Raman-peak positions with respect to those of a free crystal, which are interpreted through the inclusion phonon-mode Grüneisen tensors (Grüneisen 1926).  The calculated inclusion strains can then be used in an elastic model to calculate the pressure and temperature conditions of entrapment. A simple case is when an anisotropic crystal is contained within a quasi-isotropic host, such as quartz in garnet. When this host-inclusion system is subjected to changes in P and T the host crystal will impose a uniform strain on the inclusion, which will in turn develop deviatoric stresses. In this scenario the symmetry of the inclusion mineral is preserved and the strains in the inclusion can be measured via Raman spectroscopy using the phonon-mode Grüneisen tensor approach.

             However, a more complex situation arises when the host-inclusion system is fully anisotropic, Such as when a quartz inclusion is entrapped in a zircon host, because symmetry breaking of the inclusion occurs as P and T change. In this case, the effect of symmetry breaking on the frequencies of phonon modes is not known and may be different from the case of structural phase transitions involving soft modes.

             Therefore, we calculated the expected deformations for a quartz inclusion in a zircon host in multiple orientations and at various geologically relevant P-T conditions. We then performed ab initio Hartee-Fock/Density Functional Theory simulations on α-quartz with the selected range of strains to (i) determine the role of the symmetry breaking strains in a completely anisotropic host-inclusion system and (ii) evaluate the possible application of the phonon-mode Grüneisen tensor when the symmetry is broken. Our results show the changes in the positions of the Raman modes produced by strains that are expected for symmetry broken quartz inclusions in zircon are generally similar to those that would be seen if the quartz inclusions remained truly trigonal in symmetry. Therefore, the Grüneisen components for trigonal alpha quartz can be used for Raman elastic geothermobarometry in anisotropic host-inclusion systems without introducing significant errors.

 Acknowledgements: This work has been partially supported by the PRIN-MUR project “THALES” Prot.2020WPMFE9_003 and the National Science Foundation under Award No. (1952698) to JPG.

References: Grüneisen, E., 1926. Zustand des festen K¨orpers. Handbuch der Physik 1, 1–52.

How to cite: Murri, M., Gonzalez, J. P., Mazzucchelli, M. L., Prencipe, M., Mihailova, B., Angel, R. J., and Alvaro, M.: Anisotropic host-inclusion systems: the role of symmetry breaking strains, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-1263, https://doi.org/10.5194/egusphere-egu24-1263, 2024.

Temperature predictably controls mineral stability during metamorphism, and exerts first-order influence on rock viscosity during deformation, but the co-evolution of temperature-dependent metamorphic mineralogy and rock rheology is poorly understood. Thermally-controlled metamorphic-mechanical feedbacks must lead to strain localization during plate boundary formation. However, geologic observations from horizontally-forced, newly-destructive, intra-oceanic subduction plate boundaries consistently present a rheological paradox: hot rocks are strong and cold rocks are weak. 

Observations from geo-/thermochronology and metamorphic thermobarometry demonstrate that following a stage of initially slow, forced convergence along a hot plate interface when the slab tip resists downward translation, the lower plate suddenly collapses into the mantle. After this catastrophic collapse, subduction becomes self-sustaining (i.e., slab pull is established), stable, and cold. The physical mechanism triggering collapse is unknown, though geological studies show that collapse occurs contemporaneously with rapid cooling of the nascent plate contact and depression of slab-parallel isotherms, or “refrigeration”. These observations imply that hot rocks are strong (i.e., viscously sticky), and cold rocks are weak (i.e., viscously lubricating). It is puzzling why the resistance phase would be characterized by the highest temperature metamorphism, since high-temperature rocks are commonly viscously weak; furthermore, plate boundary “unzipping” and lithosphere-scale localization only occur during the rapid refrigeration phase, when rocks should, to a first order, be stiffer and less deformable. What mechanism(s) overcome stiffening, such that a temperature decrease leads to rock weakening, strain localization, and successful subduction initiation? 

Here, we compare microstructures from metamorphic rocks that formed along ancient hot, warm, and cold plate interfaces that correspond to different stages in the evolution of subduction zone formation from initiation to self-sustaining. We conclude that refrigeration drives changes in metamorphic mineral stability, distributions of strain-accommodating minerals, fluid content, and deformation mechanisms, which together dramatically weaken the developing plate boundary. We use flow laws and paleopiezometry to bracket ductile rock strength and demonstrate that cooling at the time of inferred slab collapse causes a minimum 3 order of magnitude viscosity reduction from ~10²¹ to 10¹⁸ Pa-s—the most drastic change in viscosity over a subduction zone’s lifetime. The viscosity reduction leads to a series of dynamic feedbacks, namely: interface decoupling and accelerated plate rates; increased and sustained interface cooling; and stabilization of a wetter, weaker interface. Our ‘refrigeration weakening’ hypothesis embodies a coupled metamorphic-mechanical process that is inherent to the evolution of common oceanic crust, and successfully explains observed changes in upper plate stress state, inferred slab velocity, and timing of proto-forearc seafloor spreading. If true, this mechanism motivates several new testable hypotheses regarding the dynamics of subduction zone development as a function of temperature changes in the modern Earth and throughout Earth’s history.  

How to cite: Kotowski, A., Seyler, C., and Kirkpatrick, J.: From hot and strong to cold and weak: How ‘Refrigeration Weakening’ facilitates strain localization during subduction initiation, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-12666, https://doi.org/10.5194/egusphere-egu24-12666, 2024.

Based on petrological and geochronological studies of glaucophane-bearing schist in the Cenozoic Yuli metamorphic belt, Taiwan, as one of the world's youngest high-pressure (HP) terrane, we provide insights on the metamorphic ages, the metamorphic stages/microstructural fabrics, and the exhumation mechanisms of HP rocks.

In this study we conducted field surveys of well-preserved mesoscale amphibole facies ultramafic schist outcrops along the Mayuan River, located in the Juisui area. We also carried out microstructural analysis using a microscope and SEM-EDS on oriented thin sections to distinguish different deformation stages/fabrics, along with quantitative EPMA analysis of the chemical composition of amphibole and phengite mineral. Furthermore, two different grain sizes (0.18mm and 0.25mm) of phengite were used to conduct the ⁴⁰Ar/³⁹Ar step heating dating analysis to determine the metamorphic age.

Field observation identify S₂ as the main foliation, shallowly dipping to the northwest or northeast in the Mayuan River area. The amphibole schist represents the rim of the ultramafic blocks within the matrix of the albite-quartz-mica schist. We also find a right-lateral west dipping shearing fault zone occurred between these two schists.

By adopting main Phengite fabrics as S₂, in microstructure domain, the prograde to retrograde metamorphism (subduction to exhumation) of 5 stages were identified: 1) S_2-1, defined by rotated stubby subhedral phengite; 2) S₂, defined by Phengite ± Epidote ± Calcic-amphibole; 3) S₂₊₁, defined by Phengite ± Epidote ± Calcic-amphibole ± Chlorite; 4) S₂₊₂, defined by Phengite ± Calcic-amphibole ± Titanite ± Albite; 5) and post-S₂₊₂ phengite. The amphibole revealed varied zoned compositions and evidence of both “clockwise” (high-T to low-T) and “counterclockwise” (high-T low-P to high-P low-T) metamorphism from core to rim: from Pargasite to Edenite/Mg-Hb to Actinolite, Glaucophane to Winchite to Actinolite, and Pargasite to Winchite to Actinolite. Phengite grow in every 5 stages, and the ⁴⁰Ar/³⁹Ar dating yielded plateau ages of 10.7±1.6 Ma, 10.4±0.5 Ma, 8.8±0.6 Ma, 8.9±1.9 Ma, and 11.0±0.7 Ma, indicating cooling ages after the subduction stage with an estimated closure temperature of 410-420°C. One age spectrum showed a minimum age of 6.1 Ma, possibly representing a recrystallized age during exhumation.

This study reveals the P-T-t path of the amphibole schists in the rim of ultramafic rocks in the subduction channel from subduction to exhumation. The clockwise and counterclockwise P-T path of the zoned amphibole indicated the complicated exhumation mechanism, and the retrograde metamorphism occurred in 11~6 Ma.

How to cite: Pang, C.-H., Yeh, M.-W., Lee, J.-C., and Iizuka, Y.: Interpreted 11-6 Ma age of Retrograde Metamorphism in the subduction channel of the Yuli Belt, Taiwan, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-16362, https://doi.org/10.5194/egusphere-egu24-16362, 2024.