GMPV7.1

Garnet is one of the most robust and ubiquitous minerals that record element zoning during crustal metamorphism. In addition to major element distribution, zoning in trace elements can provide a wealth of information to document the changing conditions of garnet growth and modification. Trace element distribution in garnet grains was mapped in 2D in thin section with laser ablation inductively coupled plasma time of flight mass spectrometry (LA-ICP-TOFMS) and conventional LA-ICP-MS to achieve a lateral resolution of 15-5 µm and limits of detection for the heavy rare earth elements (REE) down to 0.2 µg/g (Rubatto et al. 2020).

In granulite-facies garnet, major elements show diffusional resetting, whereas trace elements still largely document the growth history. Enrichment of trace elements in the garnet mantle is attributed to the consumption of biotite (V, Cr) and the dissolution of zircon (Zr) and monazite (Y+REE) in the coexisting melt. Lu is notably enriched in the garnet mantle with implications for geochronology. The gradual zoning of Y+HREE between mantle and core is reconcilable with diffusion over ~200 µm in 10 My at temperatures of 750–800°C

In amphibolite facies garnet, Y+REE trace element zoning closely matches the growth zoning in Ca with no notable diffusive modification. Y+REE zoning is dominated by Rayleigh fractionation in the core and in the outer zones it shows annuli that mark the sporadic breakdown of accessory phases.

Garnet in eclogite facies samples that underwent fluid-rock interaction show growth zoning in major and trace elements, with local oscillations and sectors. In certain samples, the overall distribution of REE can be reconciled with diffusion-limited uptake. Where garnet displays fluid-related veinlets, visible in major elements, that cross-cut the primary growth zoning, the regular Y+REE and Cr growth zoning is not affected by the veinlets. This indicates that the veinlets did not form by a crack-seal mechanism but are rather related to a selective replacement process.

References

Rubatto D, Burger M, Lanari P, Hattendorf B, Schwarz G, Neff C, Keresztes Schmidt P, Hermann J, Vho A, Günther D (2020) Identification of growth mechanisms in metamorphic garnet by high-resolution trace element mapping with LA-ICP-TOFMS. Contrib Mineral Petrol 175:61 doi.org/10.1007/s00410-020-01700-5

How to cite: Rubatto, D., Pierre, L., Burger, M., Hattendorf, B., Schwarz, G., Günther, D., Hermann, J., Bovay, T., Vho, A., and Piccoli, F.: Growth, replacement and element diffusion in metamorphic garnet revealed by trace element mapping, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-6050, https://doi.org/10.5194/egusphere-egu21-6050, 2021.

Elastic geo-thermobarometry allows the retrieval of the pressure and temperature of entrapment of an inclusion within a host (Zhang, 1998; Angel et al., 2014; Angel et al., 2015). So far, quartz-in-garnet elastic geobarometry has mainly dealt with rocks with inclusions entrapped at high pressure and low temperature conditions, such as eclogite. This is because, at high-temperature (HT) and low-to-medium-pressure conditions (T > 700 °C and P < 1.0 GPa), the rock might cross the α–β quartz transition, changing the elastic properties of quartz inclusions. Here we will show some preliminary results of HT elastic geobarometry in quartz inclusions entrapped (or re-equilibrated) within the β–quartz stability field.
The analysed samples come from three HT-LP terranes: the Athabasca granulite terrane in Canada (Dumond et al., 2015), the Jubrique Unit in the Beltic Cordilliera in Spain (Barich et al., 2014), and the Aus granulite terrane from the Namaqua metamorphic complex in Southern Namibia (Diener et al., 2013). These terrains include crustal rocks such as garnet-bearing gneisses and felsic and mafic granulites that equilibrated at low pressures and high temperatures, near or within the β-quartz stability field. Within these samples, Cesare et al. (2020) described post-entrapment shape change of quartz inclusion in garnet. The quartz inclusions have Raman spectra with peaks shifted to lower wavenumbers with respect to the unstrained reference quartz crystal. The changes in Raman peak shifts of the inclusions were converted into strains using the software StRAinMAN (Angel et al., 2019) and have positive volume strains with ε1>0 and ε3<0. The quartz EoS by Angel et al. (2017), which includes the α–β quartz transition, allowed the entrapment isomekes crossing the phase transition to be calculated and the entrapment pressures of quartz inclusions at HT to be estimated. The results of elastic geobarometry for the set of samples in question are consistent with the PT estimates by classic geothermobarometry, suggesting entrapment or re-equilibration at HT within the β–quartz stability field.
This work was supported by ERC-StG TRUE DEPTHS grant (number 714936) to M. Alvaro

References
Angel et al. (2014) - Am. Mineral. 99, 2146-2149. Angel et al. (2015) - J. Metamorph. Geol. 33, 801-813. Angel et al. (2017) - Contrib. Mineral. Petr. 172, 29. Angel et al. (2019) - Z. Krist.-Cryst. Mater. 234, 129-140. Barich et al. (2014) - Lithos 206, 303-320. Cesare et al. (2020) - Earth Planet. Sc. Lett. 555, 116708. Diener et al. (2013) - Precambrian Res. 224, 629-652. Dumond et al. (2015) - J. Metamorph. Geol. 33, 735-762. Zhang (1998) - Earth Planet. Sc. Lett. 157, 209-222.

How to cite: Gilio, M., Campomenosi, N., Musiyachenko, K. A., Angel, R. J., Cesare, B., and Alvaro, M.: Elastic geobarometry of quartz inclusions in garnet at high temperature, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-1508, https://doi.org/10.5194/egusphere-egu21-1508, 2021.

In recent years, elastic thermobarometry has gained wider acceptance and utility within the petrologic community and beyond. In particular, quartz-in-garnet (qtz-in-grt) elastic barometry is widely used because of the ubiquity of garnet in metamorphic rocks. The technique is based on using Raman spectroscopy to quantify strains recorded by inclusions, and modeling the elastic evolution of the inclusion-host pair to constrain the initial conditions of inclusion entrapment. Recent studies have validated the technique experimentally by comparing pressures from the qtz-in-grt barometer with experimental conditions of garnet growth and entrapment of quartz, and have shown that the barometer can provide reliable pressure conditions of garnet growth. However, current experimental studies fail to capture the reliability of the technique under disparate pressure (P), temperature (T) and deformation conditions, and studies that systematically compare qtz-in-grt barometry and conventional thermobarometry are lacking. 

In this work, we compare P conditions from qtz-in-grt barometry and conventional thermobarometry from the following locations: spatially and temporally variant high P/T subduction zone eclogite blocks from the Franciscan Complex in California, high P/T subduction zone rocks of varying compositions from Syros, Greece, high P/T and low P/T rocks of varying compositions from the Betics system in Spain, low P/T schists from the Jajarkot and Karnali klippen in the Himalaya, high-P rocks from the Alps, and low P/T metapelites from northeast Nevada. Qtz-in-grt barometry constraints from the Franciscan and Syros show good agreement with some reference P-T conditions, but disagree with some thermodynamic equilibria constraints and subsets of multi-mineral thermobarometry calibrations. Qtz-in-grt barometry constraints from the Himalaya are in excellent agreement with reference P constraints. Measurements of samples from other localities are currently in progress. This set of quartz inclusion analyses further allows us to evaluate the effects of inclusion geometry, anisotropy, P and T conditions of garnet growth, and P and T paths on the ultimate P conditions recorded by the qtz-in-grt barometer. The data-set also provides insights into the possible limitations of other techniques (e.g., conventional thermobarometry).

How to cite: Cisneros, M. and Behr, W.: Quartz-in-garnet elastic barometry vs. conventional thermobarometers: a comparison across diverse tectonic settings, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-3822, https://doi.org/10.5194/egusphere-egu21-3822, 2021.

The Dizi Series is exposed within the Southern slope zone of the Greater Caucasus that occurs as a complex geological structure, which constitutes an integral part of the Mediterranean (Alpine-Himalayan) collisional orogenic belt. It is built up of terrigenous and volcanogenic-sedimentary rocks faunistically dated from the Devonian to Triassic inclusive (Somin, 1971; Somin, Belov, 1976; Kutelia 1983). Most of them are metamorphosed under conditions of chlorite-sericite subfacies of the greenschist facies of regional metamorphism (chlorite-phengite-albite±quartz, graphite-sericite-quartz phyllites and marbleized limestones), and only a minor part represented by clay-carbonaceous, phengite-chlorite-carbonaceous and prehnite-chlorite-carbonate schists underwent anchimetamorphism (Shengelia et al., 2015). The Dizi Series is intruded by numerous magmatic bodies of gabbro-diabases, diabases, diorites, diorite-porphyries, syenites, monzo-syenites and granitoids. The age of the intrusions was defined by K-Ar method at 176-165 Ma (Dudauri, Togonidze, 1998) and by U-Pb LA-ICP-MS zircon dating at 166.5 ± 4.6 Ma (authors` unpublished data) and corresponds to the Bathonian orogeny. The Middle Jurassic intrusions caused intense contact metamorphism of the rocks of the Dizi Series resulted in the formation of various hornfelses containing andalusite, cordierite, corundum, biotite, plagioclase, potassium feldspar, clinozoisite, hornblende, cummingtonite, clinopyroxene, wollastonite and scapolite. These rocks correspond to albite-epidote-hornfels, andalusite-biotite-muscovite-chlorite-hornfels and andalusite-biotite-muscovite-hornfels subfacies of the contact metamorphism (Javakhishvili et al., 2020). The analogues of the Dizi Series rocks have not previously been established either in the Greater Caucasus or in the neighboring regions. In our view, Paleozoic rocks similar to the Dizi Series occur under the Cretaceous and Jurassic deposits within the folded basement of the plain Crimea where they were recovered by wells. Most of these rocks, as in the Dizi Series, underwent metamorphism of chlorite subfacies of the greenschist facies and, to a lesser extent, deep epigenesis (clayey-carbonaceous, sericite-carbonaceous, actinolite-chlorite-prehnite, muscovite-albite-chlorite, epidote-actinolite-chlorite and graphite-talc-quartz schists) (Chernyak, 1969). These rocks are also intruded by Middle Jurassic igneous rocks, including gabbro-diabases, diabases, diorites, syenites, monzo-syenites, granite-porphyries, etc. (Shniukova, 2016; Shumlyanskyy, 2019). As a result of the contact metamorphism of the basement rocks, muscovite-quartz-cordierite and cordierite-quartz-feldspar micaceous hornfelses were formed. Quartz syenite yielded a K-Ar age of 158 Ma (Scherbak, 1981), while monzo-syenite was dated at 170 ± 5 Ma applying 40Ar/39Ar method (Meijers, 2010). Thus, based on the rock associations, the nature of metamorphism, the age of the metamorphic and igneous rocks, and on the spatial position of the Dizi Series and folded basement of the plain Crimea we assume that these units developed coevally in similar environment and geological conditions.

Acknowledgements.This work was supported by Shota Rustaveli National Science Foundation (SRNSF) [PHDF-19-159, Regional and Contact Metamorphism of the Dizi Series].

How to cite: Javakhishvili, I., Shengelia, D., Tsutsunava, T., Chichinadze, G., Beridze, G., and Shumlyanskyy, L.: On the possible analogy between the Dizi Series of the Southern slope zone of the Greater Caucasus and the folded basement of the plain Crimea: composition, metamorphism, magmatism and age, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-837, https://doi.org/10.5194/egusphere-egu21-837, 2021.

Detrital zircon records are prone to several sources of bias that can compromise sediment provenance investigations based on U-Pb ages. High-temperature metamorphism (>850 ºC) is herewith addressed as a natural cause of bias since U-Pb zircon data from rocks submitted to these extreme, often prolonged conditions, frequently display protracted apparent concordant geochronological U-Pb records. The resulting spectrum can originate from disturbance of the primary U-Pb zircon system, likewise from subsequent recrystallization and crystallization processes during multiple and/or prolonged metamorphic events. Consequently, a high-grade metamorphosed igneous rock can exhibit a zircon age spectrum similar to that produced by polymict sedimentary rocks, thereby inducing provenance misinterpretations if this rock becomes a source for a sediment. A polymict sedimentary source that undergoes such high temperatures could potentially generate an even more intricate spectrum. Archean, Neoproterozoic and Paleozoic metamorphic rocks from the literature, dated by different techniques (SIMS and LA-ICP-MS), are employed as examples to demonstrate the resulting complications.  The compilation shows that (1) high-temperature metamorphism may generate age peaks of unclear or lacking geological meaning, and (2) the interpretation of detrital zircon age spectra depends on the timing of the metamorphic event (pre- or post-depositional). When high-temperature metamorphic rocks are eroded in uplifted areas, the youngest population of a detrital spectrum represents the maximum depositional age through metamorphic zircon from the source. If a sedimentary succession was subjected to high-temperature metamorphic conditions after deposition, its youngest zircon population more likely records the metamorphism, and the maximum depositional age, as well as older sources cannot be directly accessed. To evaluate the presence of high-temperature metamorphism-related bias in a given detrital zircon sample, we suggest a workflow for data acquisition and interpretation, combining a multi-proxy approach with: in situ U-Pb dating coupled with Hf analyses to retrieve the isotopic composition of the sources, and the integration of a petrochronological investigation to typify fingerprints of the (ultra)high-temperature metamorphic event.

How to cite: Tedeschi, M., Reis, H., Stutenbecker, L., Kuchenbecker, M., Ribeiro, B., Barrote, V., Vieira, P. L., and Lana, C.: Unraveling potential biases in U-Pb detrital zircon record induced by high-temperature metamorphism (> 850 ºC), EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-13256, https://doi.org/10.5194/egusphere-egu21-13256, 2021.

The Zr content of rutile coexisting with zircon and quartz is mainly a function of the temperature condition and is calibrated as Zr-in-rutile geothermometers. Because of their robustness under high-temperature conditions, they have been applied to granulite facies rocks instead of the conventional Fe-Mg exchange type geothermometers to estimate more reliable temperature conditions. However, it is recently pointed out that in order for rutile to retain the primary Zr content, rutile must be chemically isolated from zircon and quartz during cooling. In this context, inclusion rutile separately enclosed in garnet can be considered to retain the primary Zr content at the time of entrapment, only if rutile, zircon, and quartz are all enclosed in a contemporaneous domain of the garnet.

In this study, we re-examined the pressure-temperature (P-T) conditions of high-grade pelitic gneisses from selected regions (Akarui Point, Skarvsnes, Skallen, and Rundvågshetta) of the Lützow-Holm Complex (LHC), East Antarctica. The LHC has been divided into the upper-amphibolite facies zone, the transitional zone, and the granulite facies zone, based on matrix mineral assemblages of mafic- to intermediate gneisses. Akarui Point is located in the transitional zone and others in the granulite facies zone.

While previous studies commonly applied the conventional Fe-Mg exchange type geothermometers, we applied the Zr-in-rutile geothermometer of Tomkins et al. (2007) to rutile grains enclosed in garnet that also encloses zircon, quartz, and Al₂SiO₅ minerals. By utilizing the phosphorus zoning in garnet, we defined contemporaneous domains of the garnet and identified coexisting inclusion minerals in each domain. In this way, coexisting Al₂SiO₅ minerals and rutile grains were utilized to constrain the P-T condition of each domain of the garnet.

As a result, samples from Akarui Point, Skarvsnes, and Skallen were shown to have experienced almost the same P-T conditions around the kyanite/sillimanite transition boundary (~ 830-850 °C/~ 11 kbar). This is significantly higher than the previously estimated peak condition of 770-790 °C/7.7-9.8 kbar based on the conventional garnet-biotite geothermometer in the case of Akarui Point. From Rundvågshetta, where ultrahigh-T metamorphism is reported by previous studies, higher-T condition (850 ± 15 °C/0.1 kbar to 927 ± 16 °C/12.5 kbar) than those of other three regions was confirmed from inclusion rutile in garnet enclosing sillimanite. Therefore, the traditional metamorphic zone mapping, which classified Akarui Point as belonging to the transitional zone, does not reflect the highest metamorphic grade attained. It should be noted that the regional P-T conditions estimated from inclusion minerals in this study is that of earlier higher-P metamorphic stage than the regional P-T conditions determined by the metamorphic zone mapping utilizing matrix mineral assemblages. This result indicates that the Zr-in-rutile geothermometer is a powerful tool to reveal the P-T evolution of high-grade metamorphic terrains, when combined with detailed microstructural observations focusing on the relationship between rutile, zircon, and quartz.

How to cite: Suzuki, K. and Kawakami, T.: Metamorphic pressure-temperature conditions of the Lützow-Holm Complex of East Antarctica deduced from Zr-in-rutile geothermometer and Al2SiO5 minerals enclosed in garnet, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-776, https://doi.org/10.5194/egusphere-egu21-776, 2021.

The Mt. Medvednica is located north of Zagreb, a capital of Croatia, reaching 1033 m in height. It belongs to a complex geological unit located in the border area between Alps, Tisia (crystalline basement of the Pannonian Basin) and Dinarides, that are separated with large and regionally significant tectonic zones. Such geological position inevitably resulted with preservation of characteristics inherited from those large tectonic units, as well as those related to the local scale geological processes. Despite the significant tectonism, the Cretaceous metamorphism of Mt. Medvednica did not exceed P-T conditions of a low-grade metamorphism, as a typical metamorphic rock present is greenschist originated from the mafic igneous rock protolith.

The investigated Mt. Medvednica greenschists are characterized with weak schistosity, granoblastic to granolepidoblastic texture and typically comprise chlorite (40 vol.%), albite (35 vol.%), opaques (up to 15 vol.%), epidote (5 vol.%) and quartz (5 vol.%) that do not exceed 0.5 mm in size, with accessory minerals like titanite, apatite, zircon and calcite, together with rare finding of pumpellyite. The pumpellyite was so far just sporadically reported in the greenschists and was not investigated in detail. On the contrary, pumpellyite was almost regularly reported in the basic rocks from Jurassic ophiolite mélange that tectonically overly greenschists. Pumpellyite can be found there as a secondary hydrous silicate occurring in the altered extrusive rocks that undergone low-temperature ocean floor hydrothermal metasomatism addressed to the ophiolite emplacement.

Since blasts of pumpellyite (ca. 0.2‒0.3 mm in size) that we have found in the greenschists are possible indicators for a polyphase metamorphic evolution, we have conducted microtextural analyses combined with a phase equilibrium modeling approach through the construction of P-T pseudosections. Chemical composition of greenschists suggested an origin from the altered calc-alkaline basalt. Therefore, P-T pseudosections in the range of 100‒1000 MPa and 250‒450 °C were constructed with PERPLEX software in the complex MnNCKFMASHTO chemical system, and contoured by isopleths for the mode and chemical composition of major rock-forming minerals.

Pumpellyite chemistry is characterized with SiO₂=36.77‒38.38 wt.%, Al₂O₃=18.56‒21.00 wt.%, CaO=20.69‒22.89 wt.% and FeO=14.50‒16.85 wt.% that classify this mineral as a pumpellyite-(Fe²⁺). Metamorphic P-T conditions for pumpellyite-(Fe²⁺) blasts in the assemblage with chlorite and albite were modeled to 500 MPa and 270°C. Those values correspond well with the theoretically expected values, as well as with previously obtained peak P-T values for greenschist metamorphism of Mt. Medvednica obtained on the metapelites and metabasites with aid of a classical geothermobarometry. For comparison, different pumpellyite chemistry and slightly higher P-T values obtained in this research with pressures (up to +300 MPa) and temperatures (approx. +40°C) point to metamorphic mineral different from pumpellyite related to Jurassic ophiolite mélange altered basic rocks. Microtextural relations between major mineral assemblage and assemblage with pumpellyite show that prograde part of Cretaceous metamorphism, as a consequence of closure of the Neo-Tethys oceanic crust, preceded the growth of pumpellyite that may be ascribed to the retrograde part of a clockwise P-T path.

How to cite: Balen, D. and Schneider, P.: Occurrence and significance of pumpellyite blasts from the greenschists of the Mt. Medvednica (Croatia), EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-10224, https://doi.org/10.5194/egusphere-egu21-10224, 2021.

Serpentine seamounts located in the forearc region of a subduction zone represent an excellent natural laboratory for studying the geochemical processes acting along convergent plate margins as well as the forearc structure and the related fault patterns. Active serpentinite mud volcanoes are currently restricted only to the Izu-Bonin-Mariana system, where old (presumably Cretaceous) oceanic lithosphere is subducting in the absence of an accretionary prism.

IODP Expedition 366 recovered cores from three serpentinite mud volcanoes at increasing distances from the Mariana trench (Yinazao, Fantangisña and Asùt Tesoro). Most of the material consists of serpentinite mud containing lithic clasts from the underlying forearc crust and mantle as well as from the subducting Pacific plate. Pelagic sediments and volcanic ash deposits underlying the mud volcanoes were also recovered. Recycled materials from the subducted slab are found at all three mud volcanoes and consist of metavolcanics, metamorphosed pelagic sediments including cherty limestone as well as fault rocks.

Preliminary investigation of lithic clasts from the furthest Asùt Tesoro Seamount revealed metavolcanics as well as serpentinized ultramafics with well-preserved primary mineral assemblages containing olivine, orthopyroxene and spinel.

Recovered clasts from the summit of the adjacent Fantangisña Seamount contain mainly sedimentary rocks of probable Pacific plate provenance. These consist of red cherty limestone breccia, red shale and mud-siltstone transected by a network of carbonate veins. In contrast, recovered material from the flank shows a wider variety including ultramafic rocks with various degrees of serpentinization and matrix composed of mesh and bastite textures, mafic metavolcanics as well as low-grade metasediments (cherty limestones). Interestingly, garnet with andradite composition occurs throughout the matrix of the ultramafics, indicating serpentinization temperatures of at least 225 °C.

Petrological analysis of metabasalt clasts from the flank of Fantangisña shows changes in the mineral composition within the different core intervals. The composition of clinopyroxene varies between aegirine-augite and omphacite, but augite and diopside are also present. The presence of phengite with Si content of up to 3.5 a.p.f.u. as well as the Na-content in pyroxene indicate minimum pressure of 0.7 GPa at ~250 °C. Additionaly, this estimation is supported by the presence of prehnite, chlorite and pumpellyite. 

Furthermore, providing a detailed characterization of the fluids composition and transport would allow the better constraining of the tectonic and metamorphic history as well as the physical properties of the subducting Pacific Plate. Additional data on that will be presented.

How to cite: Miladinova, I., Kurz, W., Del Gaudio, A. V., and Piller, W.: Serpentinite Mud Volcanism and Exhumation of Forearc- and Lower Plate Material in the Mariana Convergent Margin System (IODP Expedition 366), EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-4865, https://doi.org/10.5194/egusphere-egu21-4865, 2021.

The Tsäkkok Lens (northern Scandinavian Caledonides) represents the outermost part of the rifted passive Baltica margin and consists of sediments and pillow basalts of MORB affinity that were metamorphosed under eclogite facies conditions. Fieldwork and further multidisciplinary analytical approach (including e.g. X-ray and EBSD mapping, and μ-CT imaging) revealed that eclogites record brittle deformation on the μm-to-m scale. This deformation is expressed as a set of microfractures (single-grain rupture) and mesofractures (sealed by garnet- and omphacite-veins). Phase equilibrium thermodynamic modeling of phengite-bearing and phengite-free eclogites performed in NCKFMMnASHT and NCFMMnASHT systems predict profuse dehydration related to lawsonite and amphibole breakdown at ~2.35 GPa and ~600°C, close to the peak conditions of ~2.55 GPa and ~640°C. These estimates are in line with conventional thermobarometry and Zr-in-rutile thermometry results. The evidence for dehydration is also provided by the  occurrence of relic glaucophane in matrix and polyphase inclusions in garnet consisting of clinozoisite + quartz ± kyanite ± paragonite that are interpreted as pseudomorphs after lawsonite. Dehydration reactions were responsible for producing fluid, which facilitated brittle fracturing of the eclogites at HP conditions due to increased pore-fluid pressure (also promoted by the volume changes during eclogitization) on the microscale. Altogether, micro- and mesofracturing acted as migration pathways for released fluid, whereas the microfractures are likely precursors of the mesoscale fractures. Garnet-WR Lu-Hf geochronology provided ages of 487.7 ± 4.6, 486.2 ± 3.2, and 484.6 ± 4.5 Ma. LA-ICP-MS trace element profiles of garnet revealed a well-pronounced peak of Lu content in the garnet cores that decreased towards the rims, indicating these dates represent the age of prograde metamorphism. Therefore, the early Paleozoic Tsäkkok Lens eclogites constitute the oldest documented natural example of HP brittle deformation during eclogitization of blueschist.

Research funded by NCN project no. 2019/33/N/ST10/01479 (M. Bukała) and no. 2014/14/E/ST10/00321 (J. Majka), as well as the Polish National Agency for the Academic Exchange scholarship no. PPN/IWA/2018/1/00046/U/0001 given to M. Bukała.

How to cite: Bukała, M., Barnes, C., Klonowska, I., Hidas, K., Fassmer, K., and Majka, J.: Brittle deformation during eclogitization: a perspective from a cold, early Paleozoic subduction zone, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-2473, https://doi.org/10.5194/egusphere-egu21-2473, 2021.

High pressure garnet porpyhroblasts formed in subduction zones serve as a witness to an integrated history of fluid flow, deformation, metamorphic reaction, and exhumation processes. Seemingly ubiquitous within garnet from a heterogeneous suite of eclogite and blueschist units is primary oscillatory elemental zoning—rhythmic, short wavelength (< 10 µm) concentric fluctuations concentrated near the rims of porphyroblasts—which has been documented using a combined major element X-ray mapping and trace element LA-ICP-MS mapping approach. This oscillatory zoning must reflect some fundamental petrogenetic process operating during subduction zone metamorphism. While longer length scale (> 50 µm) oscillations have been interpreted to reflect rock-wide P–T changes during physical cycling through the subduction channel, these short wavelength oscillations have typically been interpreted to reflect changes in the effective grain boundary chemistry induced by fluid fluxing during mineral growth.

Here, we present secondary ion mass spectrometry (SIMS) O-isotope data across the oscillatory zoning in garnet from six subduction settings. A lack of spatial covariance between the elemental and δ¹⁸O records is inconsistent with the interpretation that oscillatory zoning is directly linked to infiltration of chemically and isotopically distinct fluids. However, in most samples, vascillations in δ¹⁸O of < 2 ‰ (over 20–50 µm) in the mantle and rim, coupled with < 1 ‰ net core-to-rim change may point to the predominance of: (a) an internally-controlled grain boundary fluid and relatively stagnant fluid conditions, with grain boundaries that may experience transient opening, heterogeneous and locally-derived fluid fluxing, and then re-sealing, or (b) a rock-buffered oxygen isotope composition during garnet growth between  450 ˚C and 550 ˚C. However, several samples exhibit a systematic 2.5–4 ‰ change in δ¹⁸O across oscillatory major and trance element zoning, accompanied by a 2–3 mol% decrease in andradite content. This change, outside that predicted via closed system crystallization and fractionation, is suggested to reflect the relatively uncommon and sudden transient passage of a reduced external fluid. While this dataset does not reveal the mystery of the oscillatory zoning, it demonstrates spatial and temporal heterogeneity of fluid transfer in subduction zones.

How to cite: George, F. R., Viete, D. R., Ávila, J., and Seward, G. G. E.: Decoupled oscillatory and O-isotope zonation in high pressure low temperature garnet: records of heterogeneous fluid transfer processes, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-3061, https://doi.org/10.5194/egusphere-egu21-3061, 2021.

Unravelling the timing and rate of subduction-zone metamorphism requires linking the composition of petrogenetic indicator minerals in blueschists and eclogites to time. Garnet is a key mineral in this regard, not in the least because it best records P-T conditions and changes therein and can be dated, using either Lu-Hf or Sm-Nd chronology. Bulk-grain garnet ages are the norm and can provide important and precise time constraints on reactions across both facies. Domain dating, i.e., dating of individual growth zones, moves beyond that. Domain dating by combining mechanical micro-milling and Sm-Nd chronology yielded important constraints on garnet-growth and fluid-release rates for blueschists (e.g., Dragovic et al., 2015). Developing this method for Lu-Hf chronology and, importantly, for "common-sized" garnet (≤1 cm) provides an important opportunity to further explore the potential of this approach.

We combined a low-loss micro-sampling technique in laser cutting with a refined Lu-Hf routine to precisely date multiple growth zones of a sub-cm-sized garnet in a blueschist. The targeted grain from a glaucophane-bearing micaschist from Syros Island, Greece, was chemically characterized by major- and trace-element mapping (EPMA, LA-ICPMS) and five zones were extracted using a laser mill. The three core and inner mantle zones are chemically comparable and identical in age within a 0.1 Myr precision (2σ). The outer two zones are chemically distinct and are resolvably younger (0.2-0.8 Myr). The timing of these two major garnet-growth episodes, together with the variations in trace-element chemistry, constrain important fluid-release reactions, such as chloritoid-breakdown. The data show that the integral history of garnet growth in subduction zones may be extremely short (<1 Myr), but may, even in that short timeframe, consist of multiple short pulses. Garnet-forming reactions clearly are localized and, thus, associated with focussed high-flux fluid flow. Beyond subduction-zone processes, our new protocol for zoned garnet Lu-Hf geochronology of "common-sized" garnet opens possibilities for constraining the causes and rates of garnet growth and in turn, the pace of tectonic processes in general.

_{Dragovic, B., Baxter, E.F. and Caddick, M.J., 2015. Pulsed dehydration and garnet growth during subduction revealed by zoned garnet geochronology and thermodynamic modeling, Sifnos, Greece. Earth and Planetary Science Letters, 413, pp.111-122.}

How to cite: Tual, L., Smit, M., Cutts, J., Kooijman, E., Kielman-Schmitt, M., and Foulds, I.: Rapid, paced metamorphism of blueschists from laser-based Lu-Hf garnet-domain geochronology and LA-ICMPS trace element mapping, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-8095, https://doi.org/10.5194/egusphere-egu21-8095, 2021.

In collisional orogens continental crust is subducted to (ultra-)high-pressure (HP/UHP) conditions as constrained by petrologic, tectonic and geophysical observations. Despite a wealth of studies on the subduction and exhumation of UHP rocks, the duration of prograde metamorphism during subduction is still not well constrained.

We plan to apply Lu-Hf and Sm-Nd geochronology on metamorphic rock samples to date the duration of garnet growth, which represents a major part of prograde metamorphism from the greenschist-facies onward. Micaschist samples from the Schneeberg and Radenthein Units in the Eoalpine high-pressure belt (Eastern Alps) will be used for dating as they contain cm- to dm-sized garnet blasts, which experienced only one subduction-exhumation cycle. With dating different parts of big garnet grains, we test whether (1) it is possible to resolve the duration of garnet growth within single crystals, and (2) Lu-Hf and Sm-Nd systems date the same events in the PT-path or yield complementary information. Additionally, we will perform U-Pb geochronology on titanite in order to obtain the age of the first stages of exhumation; in addition, dating of rutile inclusions as well as matrix rutiles will be used to test Eoalpine prograde age. We will also apply U-Th-Pb monazite dating (EPMA and LA-ICPMS) to some of the samples. Collectively, these data will allow us to compare the duration of subduction and the timing of initial exhumation in a single sample. We then will constrain the PT-path of the dated samples by pseudosection modeling combined with Zr-in-rutile, quartz-in-garnet, and carbonaceous material geothermo(baro)metry. We already have preliminary results for Zr-in-rutile thermometry of rutile inclusions in garnets and matrix rutiles for samples from both locations. We measured Zr content with an EPMA and used the calibrations of Tomkins et al. (2007) and Kohn (2020). The calibration of Kohn (2020) gives overall slightly lower temperatures, but all obtained temperatures lay in a range of c. 500-600 °C in accordance with previously published data. In addition, EPMA, µ-XRF, LA-ICPMS, and µCT will be used to control if garnets preserved major and trace elemental growth zoning and to provide spatial 3D information on inclusion patterns. µCT analyses were already successfully used to obtain the chemical centre of the garnet grains in order to be able to cut them directly through there center. This is important for all in-situ chemical analyses. With dating different parts of single garnet crystals separately with Lu-Hf and Sm-Nd geochronology, we will add tight time constraints to the PT-path and constrain the duration of garnet growth.

With this contribution we formulate the working hypothesis that prograde subduction together with exhumation is a fast process. The basis for testing the idea of fast prograde metamorphism is that many geochronological studies propose a prograde duration of < 10 Ma and studies using geospeedometry sometimes propose an even shorter duration, which is the impetus for this investigation.

References:

Kohn, M.J. (2020). A refined zirconium-in-rutile thermometer. American Mineralogist(105), 963-971.

Tomkins, H.S., Powell, R. & Ellis, D.J. (2007). The pressure dependence of the zirconium-in-rutile thermometer. Journal of Metamorphic Geology(25), 703-713.

How to cite: Fassmer, K., Tropper, P., Pomella, H., Angerer, T., Degenhart, G., Hauzenberger, C., Münker, C., Schmitt, A. K., and Fügenschuh, B.: Determining the speed of intracontinental subduction – preliminary results of zoned garnet geochronology in micaschists from the Schneeberg and Radenthein Complexes, Eastern Alps, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9515, https://doi.org/10.5194/egusphere-egu21-9515, 2021.

Inclusions of relic high pressure melts provide information on the fate of crustal rocks in the deep roots of orogens during collision and crustal thickening, including at extreme temperature conditions exceeding 1000°C. However, discoveries of high pressure melt inclusions are still a relative rarity among case studies of inclusions in metamorphic minerals. Here we present the results of experimental and microchemical investigations of nanogranitoids in garnets from the felsic granulites of the Central Maine Terrane (Connecticut, US). Their successful experimental re-homogenization at ~2 GPa confirms that they originally were trapped portions of deep melts and makes them the first direct evidence of high pressure during peak metamorphism and melting for these felsic granulites. The trapped melt has a hydrous, granitic, and peraluminous character typical of crustal melts from metapelites. This melt is higher in mafic components (FeO and MgO) than most of the nanogranitoids investigated previously, likely the result of the extreme melting temperatures – well above 1000°C. This is the first natural evidence of the positive correlation between temperature and mafic character of the melt, a trend previously supported only by experimental evidence. Moreover, it poses a severe caveat against the common assumption that partial melts from metasediments at depth are always leucogranitic in composition. NanoSIMS measurement on re-homogenized inclusions show significant amounts of CO₂, Cl and F. Halogen abundance in the melt is considered to be a proxy for the presence of brines (strongly saline fluids) at depth. Brines are known to shift the melting temperatures of the system toward higher values, and may have been responsible for delaying melt production via biotite dehydration melting until these rocks reached extreme temperatures of more than 1000°C, rather than 800-850°C as commonly observed for these reactions.

How to cite: Ferrero, S., Ague, J. J., O'Brien, P. J., Wunder, B., Remusat, L., Ziemann, M. A., and Axler, J.: High pressure, halogen-bearing melt in ultra-high temperature felsic granulites of the Central Maine Terrane, Connecticut (US), EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9657, https://doi.org/10.5194/egusphere-egu21-9657, 2021.

Numerous lenses of garnet amphibolite occur in the garnet-bearing biotite-plagioclase gneiss belt in the Baishan area of the Beishan Orogen, which connects the Tianshan Orogen to the west and the Mongolia-Xing’anling Orogen to the east. According to the microstructures, mineral relationships, and geothermobarometry, four stages of mineral assemblages have been identified as follows: (1) a pre-peak stage, which is recorded by the cores of garnet together with core-inclusions of plagioclase (Pl₁); (2) a peak stage, which is recorded by the mantles of garnet together with mantle-inclusions of plagioclase (Pl₂) + amphibole (Amp₁) + Ilmenite (Ilm₁) + biotite (Bt₁), developed at temperature-pressure (P-T) conditions of 818.9–836.5 °C and 7.3–9.2 kbar; (3) a retrograde stage, which is recorded by garnet rims + plagioclase (Pl₃) + amphibole (Amp₂) + orthopyroxene (Opx₁) + biotite (Bt₂) + Ilmenite (Ilm₂), developed at P-T conditions of 796.1–836.9 °C and 5.6–7.5 kbar; (4) a symplectitic stage, which is recorded by plagioclase (Pl₄) + orthopyroxene (Opx₂) + amphibole (Amp₃) + biotite (Bt₃) symplectites, developed at P-T conditions of 732 ± 59.6 °C and 6.1 ± 0.6 kbar. Moreover, the U-Pb dating of the Beishan garnet amphibolite indicates an age of 301.9 ± 4.7 Ma for the protolith and 281.4 ± 8.5 Ma for the peak metamorphic age. Therefore, the mineral assemblage, P-T conditions, and zircon U-Pb ages of the Beishan garnet amphibolite define a near-isothermal decompression of a clockwise P-T-t (Pressure-Temperature-time) path, indicating the presence of over thickened continental crust in the Huaniushan arc until the Early Permian, then the southern Beishan area underwent a continental crust tectonic thinning process.

How to cite: Kang, W. and Li, W.: Metamorphism and geochronology of garnet amphibolite from the Beishan Orogen, southern Central Asian Orogenic Belt: Constraints from P-T path and zircon U-Pb dating, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-2, https://doi.org/10.5194/egusphere-egu21-2, 2021.

A part of Palaeoproterozoic granite-gneiss complex, commonly known as Wangtu Gneissic Complex (WGC), exposed in Wangtu-Karcham-Akpa region along the Sutlej valley, northwest lesser Himalaya, India. The core part of this gneissic complex is exposed as the undeformed granitoid body. The basement of WGC is still more or less in its primeval condition. The Paleoproterozoic thermal evolution of the North Indian Continental Margin is uncertain as the Lesser Himalayan granites are viewed either as a subduction-zone volcanic arc or rift-related magmatism during the Columbia assembly or disintegration process. Integrated mineralogical, geochemical analyses, temperature calculations of Ti solubility in biotite and zircon, and computational phase equilibria modelling of the Wangtu Gneissic Complex (WGC), Himachal Himalaya show a peraluminous existence for most WGC rocks that crystallize at a temperature of ~650°C at a pressure of ~1.0-1.1 GPa. The WGC magmatic zircons' U-Pb ages indicate two significant age groups at 1867 Ma and 2487 Ma.

The U-Pb zircon data and model phase equilibria for metasedimentary rock show the generation of S-type peraluminous magma parental to the WGC, by melting pre-existing supracrustal rocks at ~ 1800 Ma, at temperature ~ 850-900 ° C and pressure 1.1-1.2 GPa, identical to P-T conditions found in modern-day subduction zone settings. Also, T_DM model ages vary between 3.07 Ga and 2.28 Ga, and f ^Sm/Nd values (-0.4930 to -0.3510) of the studied samples suggest a contribution of Achaean crust. This study shows that the North Indian Continental Margin was an active subduction zone during the Paleoproterozoic Columbia supercontinent assembly.

How to cite: Hifzurrahman, , Nasipuri, P., Raza, M. B., and Ganaie, A. M.: Evolution of Wangtu Gneissic Complex and its paleogeographic implications in Columbia assembly: insights from geochemistry, geochronology, and computational phase-equilibria study, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-14868, https://doi.org/10.5194/egusphere-egu21-14868, 2021.

  The Kuruqtag area, located at the northeastern margin of the Tarim Craton, where the Precambrian metamorphic basement exposed, is ideal for studying the Precambrian geological evolution of the Tarim Craton. Previous zircon U-Pb chronology studies revealed that the metamorphic basement recorded a Paleoproterozoic tectonothermal event and suggested it associates with the Paleoproterozoic Nuna/Columbian supercontinent convergence event. However, the extensive range of metamorphic ages obtained from different studies (ranging from 1750-2000 Ma) and the lack of detailed P-T path corresponding to different metamorphic ages make it difficult to constrain the evolutionary framework of the Tarim craton during the Paleoproterozoic, which in turn affects future comparative regional studies.

  To constrain the P-T path, in this study, we performed detailed petrography, mineral chemical, and phase equilibrium modeling of metapelite collected from the khondalite series in the western part of the Kuruqtag (a garnet-sillimanite-cordierite-biotite gneiss with metamorphic age ~1850 Ma) and obtained the following results.

  Through petrographic studies, at least three phases of mineral assemblages can be used to invert the P-T path experienced by the metapelite. They are    M1 (peak metamorphic stage):represented by fine-grained biotite remnant (Bi Ⅰ) + fine-grained plagioclase(Pl Ⅰ) and quartz+ Ilmenite + , occurring as inclusions within the metamorphic garnet, and equilibrated mineral assemblages is: Grt(core) + Bi Ⅰ + Sill + Kfs + Pl Ⅰ + Qz + Ilm. M2 (isothermal depression stage), represented by cordierite occurring in the garnet rim or with spinel in the matrix, inferred equilibrated mineral assemblages is Grt(rim)+Bi Ⅰ +Cd+Kfs+Pl ⅠⅠ+Ilm+Sp.M3 (isothermal depression stage), is marked by the appearance of new growth of biotite(Bi ⅠⅠ) and the conversion of Sill to And. 

The P-T conditions for the mineral assemblage evolution (M1 → M3) are constrained by a P-T pseudosection constructed in the Na2O -CaO-K2O-FeO-MgO-Al2O3-SiO2-H2O- TiO2-O2 chemical system. The resulting P-T path is clockwise from the M1 stage (840°C, 4 Kbar) through the isothermal depression path to M2 (840-850°C,5 Kbar) and then through the near-isobaric cooling path to the M3 stage (650°C, 3.5-4 Kbar).

How to cite: Guo, Y.: Petrography and phase equilibrium modeling of Paleoproterozoic metapelite in the Kuluktag area of Tarim Craton, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-11000, https://doi.org/10.5194/egusphere-egu21-11000, 2021.