Displays

Texturally complex monazite grains within two granulite-facies pelitic migmatites from southern Baffin Island, Arctic Canada, were mapped by laser ablation-inductively coupled plasma-mass spectrometry to quantitatively determine the spatial variation in trace element chemistry with a 4-5 μm resolution (with up to 1883 analyses per grain). The maps demarcate growth zones, some of which were cryptic with conventional imaging, highlighting the 3-D complexity of monazite grains that have experienced multiple episodes of growth and resorption during high-grade metamorphism. Associated monazite trace element systematics are highly variable, both within domains interpreted to have grown in a single event, and between samples that experienced similar metamorphic conditions and mineral assemblages. This result cautions against generalised petrological interpretations being made about monazite trace element signatures as it suggests sample-specific controls. Nevertheless, by quantifying monazite textures, a related U-Pb dataset is re-interpreted, allowing ages to be extracted from a continuum of concordant data. The results reveal a ~45 Myr interval between prograde metamorphism and retrograde melt crystallisation in the study region, emphasising the long-lived nature of heat flow in high-grade metamorphic terranes. Careful characterisation of monazite grains suggests that continuum-style U-Pb datasets can be decoded to provide insights into the rates of metamorphic processes.

How to cite: Weller, O., Jackson, S., Miller, W., St-Onge, M., and Rayner, N.: Decoding 3-D monazite textures using LA-ICP-MS raster mapping, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20617, https://doi.org/10.5194/egusphere-egu2020-20617, 2020.

Plagioclase-rich lower crustal granulites exposed on the Lofoten archipelago, N Norway, display pseudotachylytes, reflecting brittle deformation, as well as ductile shear zones, highlighting plastic deformation. Pristine pseudotachylytes often show no or very little difference in mineral assemblage to their host-rocks that exhibit limited, if any, metamorphic alteration. In contrast, host-rock volumes that developed ductile shear zones exhibit significant hydration towards amphibolite or eclogite-facies assemblages within and near the shear zones. We combine experimental laboratory results and observations from the field to characterize the structural evolution of brittle faults in plagioclase-rich rocks at lower crustal conditions. We performed a series of deformation experiments on intact granulite samples at 2.5 GPa confining pressure,  a strain rate of 5×10^-5 s^-1,  temperatures of 700 and 900 °C, and total strains of either ~7-8 % or ~33-36 %. Samples were either deformed ‘as-is’, i.e. natural samples without any treatment, or with ~2.5 wt.% H₂O added. Striking similarities between the experimental and natural microstructures suggest that the transformation of precursory brittle structures into ductile shear zones at eclogite-facies conditions is most effective when hydrous fluids are available in excess.

How to cite: Incel, S., Renner, J., and Jamtveit, B.: Evolution of brittle structures in plagioclase-rich rocks at high-grade metamorphic conditions – Linking laboratory results to field observations, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8367, https://doi.org/10.5194/egusphere-egu2020-8367, 2020.

The characterization of the pressure and temperature (P-T) histories of subducted rocks is of key importance to unravel geological processes at all scales. Conventional element-exchange geothermobarometers are challenged in ultra-high-pressure metamorphic terranes as the subduction temperatures may exceed their closure temperature and minerals may undergo re-equilibration along their path. Elastic geobarometry applied to host-inclusion systems is a complementary method to determine P and T conditions of metamorphism that does not rely upon chemical equilibrium. Recent development of elastic geobarometry (Angel et al., 2019; Campomenosi et al., 2018; Murri et al., 2018) allows us to retrieve entrapment pressures for host-inclusion pairs from the residual strains acting on the inclusion. Because only a single measurement, the inclusion strain, is made, only a line in PT space of possible entrapment conditions, the entrapment isomeke, can be determined. Thus, the entrapment pressure along an isomeke can only be determined if the entrapment temperature is known.

An alternative is to calculate entrapment conditions for two types of inclusions that are believed, from petrological evidence, to have been entrapped at the same time. In this study we performed micro-Raman measurements on quartz and zircon inclusions trapped in garnets from a garnet-kyanite gneiss and a quartz-garnet vein from the Fjørtoft UHP terrane, Norway. From the micro-Raman data, using the program stRAinMAN (Angel et al., 2019), we calculated the strains at room conditions (Murri et al., 2018) and thus the entrapment conditions. The intersection between the two sets of isomeke calculated on multiple quartz and zircon inclusions demonstrates that measuring different inclusion phases trapped inside a single host allows unique P-T conditions for the host rock to be determined.

This work was supported by ERC-StG TRUE DEPTHS grant (number 714936) to M. Alvaro

Angel R.J., Murri M., Mihailova B. & Alvaro M. (2019) - Stress, strain and Raman shifts. Zeitschrift für KristallographieCrystalline Materials, 234(2), 129-140.

Campomenosi N., Mazzucchelli M.L., Mihailova B., Scambelluri M., Angel R.J., Nestola, F., Reali A. & Alvaro M. (2018) - How geometry and anisotropy affect residual strain in host-inclusion systems: Coupling experimental and numerical approaches. American Mineralogist, 103(12), 2032-2035.

Murri M., Mazzucchelli M.L., Campomenosi N., Korsakov A.V., Prencipe M., Mihailova B.D., Scambelluri M., Angel R.J. & Alvaro M. (2018) - Raman elastic geobarometry for anisotropic mineral inclusions. American Mineralogist, 103(11), 1869-1872.

How to cite: Gilio, M., Alvaro, M., Angel, R., and Scambelluri, M.: Elastic geothermobarometry on multiple inclusions in a single host, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20670, https://doi.org/10.5194/egusphere-egu2020-20670, 2020.

The petrologic evolution of low-grade metamorphic rocks is essential for a coherent understanding of subduction- and exhumation-related processes during collisional orogeny. Retrieving useful P-T-t-d data from low-grade metamorphic units however is challenging as these rocks commonly lack suitable target minerals for geothermobarometry and/or geochronology. Herein we introduce a new geochronological method termed ‘bulk inclusion dating’ and present an example of a rock sampled at the base of the Stauffen-Höllengebirge Nappe (Austroalpine Unit, Eastern Alps, Austria) that witnessed an Eo-Alpine tectono-metamorphic event in the Late Cretaceous.

The investigated schist contains mm-scale chloritoid porphyroblasts in a foliated matrix consisting of chlorite, muscovite and quartz. Accessory minerals include ilmenite, hematite, rutile, zoned epidote with REE-rich cores, euhedral apatite and zircon. Thermodynamic modeling in the MnCNKFMASHTO system predicts the stability of the equilibrium assemblage in a narrow P-T field between 450–490°C and 5–7 kbar. Ilmenite, rutile and hematite inclusions in chloritoid cores indicate porphyroblast growth within this field, which is consistent with the observed chemical zoning of the chloritoid. The interpreted peak P-T conditions agree with the observation of garnet in a sample from the same outcrop and independent peak temperature constraints around 490°C derived from Raman spectroscopy of carbonaceous material.

Detailed petrographic investigations using high-resolution SEM imaging combined with EDX analysis revealed abundant minute (100 nm – 3 µm), idiomorphic zircons both included in chloritoid porphyroblasts and in the matrix. In the chloritoid rim, zircon comprises >95% of the inclusionary phases.  Based on grain size distribution, we interpret zircon growth during prograde metamorphism via dissolution-precipitation mechanism and progressive coarsening due to Ostwald ripening. In situ laser ablation ICP-MS analysis of the bulk zircon population included in the chloritoid rim using a 120 µm spot size yields a U-Pb age of 116.7 ± 6.4 Ma (MSWD: 1.5; n: 79). Combined with the results of thermodynamic forward modeling, we link the age to the late prograde part of the P-T evolution. The latest synorogenic sediments on top of the Stauffen-Höllengebirge Nappe were deposited at ca. 120 Ma, giving a consistent upper bound the late prograde age. An apatite U-Pb age from the same sample yields 429.3 ± 14.6 Ma (MSWD: 1.2; n: 60). Considering the protolith is an altered tuff and the apatite is likely magmatic, a Devonian protolith age is inferred. That the apatite age was not reset during Eo-Alpine metamorphism is in agreement with the inferred metamorphic conditions. We emphasize that the strength of the bulk inclusion dating approach lies in the improved link of P-T and age data and its relative ease of application compared to other geochronological methods.

How to cite: Hollinetz, M. S., Schneider, D. A., McFarlane, C. R. M., Huet, B., and Grasemann, B.: Bulk inclusion dating: a geochronological tool to date low-grade metamorphism, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-13967, https://doi.org/10.5194/egusphere-egu2020-13967, 2020.

The accretion of the Pearya terrane to the northern margin of Laurentia plays an important role in the paleogeographic reconstructions for the Arctic region. Earlier workers proposed a timing of its juxtaposition spanning from Late Silurian (Trettin, 1998) to Late Ordovician (Klaper 1992). In this study, we focus on the pressure-temperature-time (P-T-t) evolution of the Petersen Bay assemblage. This subduction related unit crops out between the crystalline basement of Pearya and volcano-sedimentary sequence of Clements Markham fold belt. The highest grade rocks, garnet-kyanite-bearing schist (sample 17-66) and garnet-kyanite-staurolite garbenschiefer (sample 17-64) were selected for P-T studies and in-situ monazite U-Pb dating by sensitive high resolution ion microprobe.

Thermodynamic modelling of sample 17-66 gives a P-T condition of 7.8-8.1 kbar and 590-610°C for garnet core formation, whereas a pseudosection calculated for the effective bulk composition indicates garnet rim growth at 8-9 kbar and 650-660°C. The QuiG Raman barometry coupled with Ti-in-biotite thermometry yield conditions of 6.5-7.5 kbar and 540-600°C for the garnet growth. The combination of QuiG barometry and Ti-in-biotite thermometry indicate garnet growth at 7.5-8 kbar and 500-550°C for the garbenschiefer sample.

Monazite shows distinctive zonation and 2, up to 3, domains were recognized based on textures and X-ray microprobe maps. For the sample 17-66, Monazite-I forms inclusions within garnet rims or cores of bigger matrix grains. It defines a weighted mean ²⁰⁶Pb/²³⁸U age of 397±2 Ma (n=18, MSWD=1.6). Monazite-II occurs in the matrix and gives an age of 385±2 Ma (n=19, MSWD=1.5). Monazite-I from sample 17-64 yields a weighted mean ²⁰⁶Pb/²³⁸U age of 394±2 Ma (n=11, MSWD=0.6). Monazite-II defines the age of 388±2 Ma (n=7, MSWD=0.8). Monazite-III was distinct only in garbenschiefer. It yields a younger age of 374±6 Ma (n=6, MSWD=3.1).

The P–T data coupled with monazite dating suggest a Middle Devonian metamorphism of the Petersen Bay assemblage under amphibolite facies conditions. These new results suggest that the juxtaposition of the Pearya terrane, Petersen Bay assemblage and the Clemens Markham fold belt is Middle Devonian or younger, i.e. much younger than previously thought.

References

Klaper E.M. 1992. The Paleozoic tectonic evolution of the northern edge of North America: A structural study of Northern Ellesmere Island, Canadian Arctic Archipelago Tectonics, 11, 854–870.

Trettin H.P. 1998. Pre-Carboniferous geology of the northern part of the Arctic Islands: Northern Heiberg Fold Belt, Clements Markham Fold Belt, and Pearya; northern Axel Heiberg and Ellesmere islands GSC Bulletin, 425, 401 p.

How to cite: Kośmińska, K., Gilotti, J., McClelland, W., and Coble, M.: Metamorphic evolution of the Petersen Bay assemblage, Ellesmere Island: What can we learn about Pearya - Laurentia accretion?, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-15931, https://doi.org/10.5194/egusphere-egu2020-15931, 2020.

In this contribution, we compare and test the reliability of zircon and monazite thermometers and suggest a new and independent method to constrain the H₂O content in granitic magmas from coeval zircon and monazite minerals. We combine multi-method single-mineral thermometry (bulk-rock zirconium saturation temperature (T_zr), Ti-in-zircon (T_(Ti-zr)) and monazite saturation temperature (T_mz)) with thermodynamic modelling to estimate water content and P–T conditions for strongly-peraluminous (S-type) granitoids in the Georgetown Inlier, NE Queensland. These granites were generated within ~30 km thick Proterozoic crust, and emplaced during regional extension associated with low-pressure high-temperature (LP–HT) metamorphism.

SHRIMP U–Pb monazite and zircon geochronology indicates synchronous crystallization ages of c. 1550 Ma for granitic rocks emplaced at different crustal levels—from the eastern deep crustal domain (P = 6–9 kbar), through the middle crustal domain (P = 4–6 kbar), to the western upper crustal domain (P = 0–3 kbar).

Bulk-rock T_zr and T_(Ti-zr) yielded magma temperature estimates for the eastern domain of ~800°C and ~910–720°C, respectively. Magma temperatures in the central and western domains were ~730°C (T_zr) and ~870–750°C (T_(Ti-zr)) in the central domain, and ~810°C (T_zr) and ~890–720°C (T_(Ti-zr)) in the western domain, respectively. These temperature estimates were compared with P–T conditions recorded in the host rocks to determine if the magmas had equilibrated thermally with the crust. Similar temperatures were obtained for the middle and lower crust suggesting that the associated magmas thermally equilibrated at their respective depths, whereas the sub-volcanic rocks were, as expected, significantly hotter than the adjacent crust.

By plotting the results on a P–T–X_H2O petrogenetic grid, and assuming adiabatic ascent through the crust, the sub-volcanic magmas appear to be drier (~3 wt% H₂O) than the granitic magmas (~7 wt% H₂O) which formed at greater depth. Monazite saturation temperatures (which depends on the water content, light–REE content and composition of the granitic melt), are in agreement with the zircon thermometers only if water values of ~3 wt% H₂O and ~7 wt% H₂O are assumed for the upper crustal magmas and deeper magmas, respectively. Moreover, melt compositions extracted from a modelled pseudosection of a sillimanite-bearing metapelite, which was interpreted to be the typical source rock for the surrounding granites (P=5 kbar and T=690°C–850°C), show comparable water content values.

The T_mz results provide independent evidence for the H₂O content in magmas, and we suggest that reconciling T_zr with T_mz is a new and independent way of constraining H₂O content in granitic magmas.

How to cite: Volante, S., Collins, W., Spencer, C., Blereau, E., Pourteau, A., Barrote, V., Nordsvan, A., Li, Z.-X., Evans, N., and Li, J.: Reconciling zircon and monazite thermometry constrains H2O content in granitic melts , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12038, https://doi.org/10.5194/egusphere-egu2020-12038, 2020.

Migmatite domes are common structures in orogens, and in some cases are comprised of deeply-sourced crust that experienced lateral and subsequent vertical flow, with ultimate emplacement in the mid/upper crust. The record of the deep-crustal history survives in layers and lenses of refractory rock types within the dominant quartzofeldspathic gneiss. These deep-crustal relics are typically the best archives of pressure-temperature-time-deformation conditions of crustal flow, although it can be difficult to extract information about the duration of deep-crustal residence – such as might accompany lateral flow of deep-crust – because intracrystalline diffusion at protracted high temperatures may erase much of the history and/or minerals may record only the timing of final emplacement and cooling. One possible indicator of deep-crustal history is the extent of recrystallization of zircon that experienced eclogite-facies conditions; the conditions of zircon growth/recrystallization are indicated by REE abundance and results of Ti-in-zircon thermometry. For example, in the eclogite-bearing Montagne Noire migmatite dome of the southern French Massif Central, zircon in eclogite from the core of the dome has been extensively recrystallized under eclogite-facies conditions. In contrast, zircon in eclogite from the margin of the dome experienced very little recrystallization and largely consists of inherited (magmatic) cores with very thin (<20 um) eclogite-facies rims. The two eclogites, which both contain garnet + omphacite + rutile + quartz, record the same age of protolith crystallization (~450 Ma) and high-P metamorphism (~315 Ma), and similar metamorphic conditions (700 ± 20°C, 1.4 ±0.1 GPa). Differences in extent of recrystallization of zircon in the two eclogites may relate to duration at high T and/or extent of interaction with aqueous fluid (ongoing work to obtain in situ oxygen isotope data for zircon and garnet will evaluate the latter for each eclogite). Deformation may have been involved in recrystallization of zircon, but is not the primary factor accounting for the differences in extent of recrystallization; both eclogites were deformed during eclogite-facies metamorphism, as indicated by crystallographic-preferred orientation of omphacite and shape-preferred orientation of rutile. Other variables that are also unlikely to explain differences in these eclogite zircons are differences in host rock chemistry, availability of Zr from decompression reactions involving Zr-bearing minerals, extent of radiation damage, and original crystal size. The two most likely explanations for variations in zircon recrystallization are duration at high-T and extent of fluid-rock interaction. In the case of the former, dome-margin eclogite may have had a shorter residence time in the deep crust and was more directly exhumed from a proximal source, whereas the dome-core eclogite may have had a more extended transit in the deep-crust before being exhumed in the steep, median high-strain zone of the migmatite dome.

How to cite: Whitney, D., Hamelin, C., Teyssier, C., Roger, F., and Rey, P.: Significance of variation in extent of recrystallization of zircon in orogenic eclogite, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10658, https://doi.org/10.5194/egusphere-egu2020-10658, 2020.

Keywords: HP-HT metamorphism, microstructures, U-Pb-Th dating, P-T-t-d path.

The occurrence of (ultra)high pressure and high temperature mineralogical assemblages developed during the Alpine phases makes the Cima di Gagnone area (Cima Lunga unit) one of the most studied area in the Central Alps. It consists of continental basement rocks (orthogneisses, paragneisses and metapelites) enveloping (ultra-) mafic bodies of oceanic crust (eclogite, amphibolites and peridotites) which record pressure and temperature up to 3 GPa and 800 °C, respectively (e.g. Nimis and Trommsdorff, 2001; Scambelluri et al., 2015). This high-grade metamorphism is constrained between 40 and 35 Ma by U-Pb dating from the ultra-mafic and mafic rocks (e.g. Gebauer, 1999). The metamorphism peak of the surrounding gneiss complex is instead constrained at considerably lower conditions (up to 0.8 GPa and 660 °C; Grond et al., 1995). The temperature peak in the felsic rocks is dated at ca. 32 Ma (Gebauer, 1996), coeval with the Bergell emplacement. Several models have been proposed to explain the coupling between ultrahigh- and middle- pressure rock pairs resulting in a large uncertainty in the adopted subduction-exhumation models.

We performed new petrological, micro-structural and geochronological data from the gneissic rocks, with the aim to investigate how the pressure and temperature conditions experienced by the felsic and mafic rocks are truly different. We explored the spatial variation of the metamorphic record through sample collection the structural control of the inclusion-matrix couples. Petrological and microstructural (SEM-EBSD) analyses are performed to define the deformation and metamorphic patterns of samples collected. Our results indicate that some portions of the gneissic matrix preserve relicts of higher pressure and temperature than previously suggested. The high-T conditions are temporally constrained by U-(Th)-Pb dating of monazite and zircon, which provides peak age estimations similar to the mafic rocks. The new data shed a light on heterogeneous metamorphism recorded by different rocks, providing new elements for the discussion on the most fitting geodynamic models.

REFERENCES

- Gebauer, 1996. A P-T-t Path for an (Ultra?-) High-Pressure Ultramafic/Mafic Rock-Association and its Felsic Country-Rocks Based on SHRIMP-Dating of Magmatic and Metamorphic Zircon Domains. Example: Alpe Arami (Central Swiss Alps). Earth Processes Reading the Isotopic Code, Geophysical Monograph 95, 307-329, AGU.

- Gebauer, 1999. Alpine geochronology of the Central Alps and Western Alps: new constraints for a complex geodynamic evolution. Schweiz. Mineral. Petrogr. Mitt., 79, 191-208.

- Grond, R., Wahl, F. and Pfiffner, M., 1995. Mehrphasige alpine Deformation und Metamorpshe in der nordlichen Cima Lunga-Einheit, Zentralalpen (Scweiz). Schweiz. Mineral. Petrogr. Mitt., 75, 371-386.

- Nimis, P. & Trommsdorff, V., 2001. Revised thermobarometry of Alpe Arami and other garnet peridotites from the central Alps. J. of Petrology, 42, 103-115.

- Scambelluri, M., Pettke, T., & Cannaò, E. (2015). Fluid-related inclusions in Alpine high-pressure peridotite reveal trace element recycling during subduction-zone dehydration of serpentinized mantle (Cima di Gagnone, Swiss Alps). Earth and Planetary Science Letters, 429, 45-59.

How to cite: Corvò, S., Maino, M., Langone, A., Schenker, F. L., Seno, S., and Piazolo, S.: Timing of HP/HT alpine metamorphism: new data from Cima di Gagnone (Central Alps), EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-351, https://doi.org/10.5194/egusphere-egu2020-351, 2020.

Ultrahigh-pressure (UHP) metamorphism is defined by achieving P–T conditions sufficient to transform quartz to coesite (~26–28 kbar at ~500–900 °C), which occurs at ~90-100 km depth within the Earth under lithostatic conditions. Thus, the occurrence of UHP metamorphism is often taken as being a diagnostic indicator of subduction having operated in the geological record, and hence plate tectonics. Yet, the oldest such coesite-bearing rocks belong to the Pan-African belt in northern Mali, and formed at 620 Ma, although there exist multiple lines of evidence to show that a global network of subduction had been operative on Earth for billions of years beforehand. Why, then, are these key geodynamic indicators missing from the majority of the rock record? Here, I show how secular cooling of the Earth's mantle since the Mesoarchean (c. 3.2 Ga) has affected the exhumation potential of UHP (and HP) eclogite through time due to time-dependent compositional variation of both oceanic and continental crust. Petrological modeling of density changes during metamorphism of Archean, Proterozoic, and Phanerozoic composite continental terranes shows that more mafic Archean crust reaches a point-of-no-return during transport into the mantle at shallower depths than less MgO-rich modern-day crust, regardless of whether this occurs via subduction of stagnant lid-like vertical 'drip' tectonics. Thus, while Alpine- and Himalayan-type (U)HP orogenic eclogites represented by metamorphosed mafic intrusions into continental crust may readily have formed during the Precambrian, they would have lacked the buoyancy required for exhumation and preservation in the geological record.

How to cite: Palin, R.: Changing exhumation potential of (U)HP eclogite through geological time, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2635, https://doi.org/10.5194/egusphere-egu2020-2635, 2020.

Reconstructing the tectonic history of metamorphosed terranes is a key step towards establishing a comprehensive model for collisional orogens such as the Alps. Single chronometers tend to record one specific component of such history—be it inheritance, reactions or cooling—or record several of these, without a clear indication of what each age datum means. Resolving the complex evolution of such terranes requires chronometric data of different minerals, which on the basis of their chemistry, may be linked to distinct stages. Here we present a multi-mineral geochronology of the Theodul Gletscher Unit (TGU; Western Alps). The tectonic unit is a metamorphic sequence containing a variety of pelitic and mafic rocks that mainly record Alpine low-temperature, high-pressure metamorphism. In addition, however, the rocks are known to host age components related to events and processes in the Permian and Jurassic; these could be attributed to inherited components and pervasive fluid-rock interaction during oceanic alteration and subduction. To investigate this, we subjected pelitic schists and mafic rocks from the TGU to a multi-method analysis, involving thermometry, oxygen isotope analysis in garnet, and zircon U-Pb and garnet Lu-Hf dating.

Zircon crystals in all rock types are Permian in age and have no significant record of Alpine metamorphism; they are interpreted as dating the source of the felsic and mafic sediments. Complex garnet textures in the schists reveal multiple growth stages: whereas the garnet rim reflects the subduction stage, the relict nature of the garnet core allows for speculation of an older, perhaps Permian age (Bucher et al., 2019). A distinct and abrupt rim-ward drop in δ¹⁸O coherent with major-element zoning in garnet from the schists indicates open system fluid-rock interaction. Rutile included in the different garnet zones as well as in the matrix of the schists provided consistent Zr-in-rutile thermometry results of 520–560 °C (calculated at 2.5 GPa). Similarly, Raman spectroscopy of carbonaceous material in the same textural positions indicates 540–580 °C. These results indicate a single Alpine metamorphic cycle. To look back beyond that stage, Lu-Hf data will be presented for garnet with and without seemingly inherited cores, as well as for cores that were physically isolated from the sample material. The results, together, provide new insight into the petrological and tectonic processes that affected rocks in the TGU during and prior to their Alpine history.

REFERENCES:

Bucher, K., Weisenberger, T. B., Klemm, O., Weber, S. (2019). Decoding the complex internal chemical structure of garnet porphyroblasts from the Zermatt area, Western Alps. Journal of Metamorphic Petrology, 37, 1151-1169

How to cite: Bovay, T., Smit, M. A., and Rubatto, D.: Multimineral chronology of a complex high pressure terrane: insights from the Theodul Gletscher Unit (Western Alps, Switzerland), EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-15877, https://doi.org/10.5194/egusphere-egu2020-15877, 2020.

In the Monte Duria area (Adula-Cima Lunga unit, Central Alps, N Italy) Grt-peridotites occur in direct contact with migmatised orthogneiss (Mt. Duria) and eclogites (Borgo). Both mafic and ultramafic rocks share a common HP peak at 2.8 GPa and 750 °C and post-peak static equilibration at 1.2 GPa and 850 °C (Tumiati et al., 2018).

Grt-peridotites show abundant amphibole, dolomite, phlogopite and orthopyroxene after olivine, suggesting that they experienced metasomatism by crust-derived agents enriched in SiO₂, K₂O, CO₂ and H₂O. Peridotites also display LREE fractionation (La/Nd = 2.4) related to LREE-rich amphibole and clinopyroxene grown in equilibrium with garnet, indicating that metasomatism occurred at HP conditions. At Borgo, retrogressed Grt-peridotites show low strain domains characterised by garnet compositional layering, cut by a subsequent low-pressure chlorite foliation, in direct contact with migmatised eclogites. Kfs+Pl+Qz+Cpx interstitial pocket aggregates and Cpx+Kfs thin films around symplectites after omphacite parallel to the Zo+Omp+Grt foliation in the eclogites suggest that they underwent partial melting at HP.

The contact between garnet peridotites and associated eclogites is marked by a tremolitite layer. Tremolitites also occur as variably stretched layers within the peridotite lens, showing a boudinage parallel to the garnet layering of peridotites, indicating that the tremolitite boudins formed when peridotites were in the garnet stability field. Tremolitites also show Phl+Tc+Chl+Tr pseudomorphs after garnet, both crystallized in a static regime postdating the boudins formation, suggesting that they derive from a Grt-bearing precursor. Tremolitites have Mg#>0.90 and Al₂O₃=2.75 wt.% pointing to ultramafic compositions but also show enrichments in SiO₂, CaO, and LREE suggesting that they formed after the reaction between the eclogite-derived melt and the garnet peridotite at HP. To test this hypothesis, we calculated a log aH₂O-X pseudosection at fixed P=3GPa and T=750°C to model the chemical interaction between the garnet peridotite and the eclogite-derived melt. Our results show that the interaction produces a Opx+Cpx+Grt assemblage + Amp+Phl, depending on the water activity in the melt, suggesting that tremolitites likely derive from a previous Grt-websterite with amphibole and phlogopite. Both peridotites and tremolitites also show a selective enrichment in LILE recorded by amphiboles in the spinel stability field, indicating that a fluid-assisted metasomatic event occurred at LP conditions, leading to the formation of a Chl-foliation post-dating the garnet layering in peridotites, and the retrogression of Grt-websterites in tremolitites.

The Monte Duria area is a unique case study where we can observe eclogite-derived melt interacting with peridotite at HP and relatively HT, and could thus represents a proxy for the crust-to mantle mass transfer at great depths in subduction zones.

Tumiati, S., Zanchetta, S., Pellegrino, L., Ferrario, C., Casartelli, S., Malaspina, N., 2018. Granulite-facies overprint in garnet peridotites and kyanite eclogites of Monte Duria (Central Alps, Italy): Clues from srilankite- and sapphirine-bearing symplectites. J. Petrol. 59.

How to cite: Pellegrino, L., Malaspina, N., Zanchetta, S., Langone, A., and Tumiati, S.: HP melting of eclogites and metasomatism of garnet peridotites in the Monte Duria area (Central Alps, N Italy): a proxy for the mafic crust-to-mantle mass transfer at subduction zones, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-716, https://doi.org/10.5194/egusphere-egu2020-716, 2020.

            Large earthquakes break the subduction interface to depths of 60 to 80 km. Current models hold that seismic rupture occurs when fluid overpressure builds in link with porosity cycles, an assumption still to be experimentally validated at high pressures. Porosities of subduction zone rocks are experimentally determined under pressures equivalent to depths of up to 90 km with a novel experimental approach that uses Raman deuterium-hydrogen mapping. Natural rocks (blueschists, antigorite serpentinites, and chlorite-schists) representing a typical cross-section of the subduction interface corresponding to the deep seismogenic zone are investigated. In serpentinite, and to a smaller extent blueschist, porosity increases with deformation, whereas chlorite-rich schists remain impermeable regardless of their deformation history[ 1]. Such a contrasting behavior explains the observation of over-pressurized oceanic crust and the limited hydration of the forearc mantle wedge. These results provide quantitative evidence that serpentinite, and likely blueschist, may undergo porosity cycles making possible the downdip propagation of large seismic rupture to great depths. 

[1] Ganzhorn, A.C., Pilorgé, H., Reynard, B., 2019, Earth and Planetary Science Letters, 522: 107-117.

How to cite: Reynard, B., Ganzhorn, A.-C., and Pilorgé, H.: Porosity of metamorphic rocks and fluid migration within subduction interfaces, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6986, https://doi.org/10.5194/egusphere-egu2020-6986, 2020.

The physical and mechanical processes rooted in the hydrated, serpentinized mantle above subduction zones (the “cold nose”) remain debated and poorly understood, despite fundamental consequences on the elastic loading of the seismogenic interface. The fluids crossing this interface are expected to generate nests of seismicity and at the same time weaken the interface hanging wall through serpentinization and metasomatic processes. Ultramafic and jadeitite samples from two natural laboratories where such fossil settings are now visible at the Earth’s surface are used here to document multi-scale deformation mechanisms and fluid-rock interaction processes. Field relationships enable tracking the pathways followed by the fluids during HP metamorphism. Petrographic, geochemical, geochronological and microstructural observations demonstrate the complex interplay between brittle and plastic deformation processes throughout the gradual hydration of the cold nose mantle over millions of years. Changes in bulk rock geochemical and paragenetic sequence also reveal the evolution of the composition of the fluid source through time. These results shed light on the geometry of the cold nose above the interface, with implications for volatile budget estimates, rheology of the plate interface (including the various types of seismicity) as well as the interpretation of Vp/Vs ratios from active subduction settings worldwide.

How to cite: Angiboust, S., Glodny, J., Cambeses, A., Raimondo, T., Monié, P., Deldicque, D., and Garcia-Casco, A.: The Fate of Subduction Fluids above the Subduction Interface: Implications for Mantle Wedge Deformation Processes, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9555, https://doi.org/10.5194/egusphere-egu2020-9555, 2020.

Garnet-epidote oxybarometry, major element mineral compositions, and textural analysis of eclogites from Syros, Greece reveal the presence of high fO₂ slab fluids. The four investigated samples from blocks hosted in the serpentinite matrix mélange on Syros fall into three categories: unmetasomatized eclogite (Type I, n = 1), heavily metasomatized garnet-clinopyroxene bearing rocks (Type II, n = 2), and an eclogite which hosts veins of Andr + Acm + Ep + Hem + Chl (Type III, n = 1). Type I samples of metagabbroic eclogite are characterized by a peak assemblage of Grt + Cpx + Rt with abundant clinozoisite after lawsonite. In addition to Grt + Cpx, Type II samples contain Chl + Ilm + Py + Ap ± Ep ± Cam. Clinopyroxene compositions within Type I samples display a prograde trend of increasing jadeite and decreasing acmite. In contrast, the matrix clinopyroxene in one Type II sample exhibits compositions up to 60 % acmite component. Alternatively, clinopyroxene in the second Type II sample investigated exhibit an increase in acmite component during metasomatism. Type II samples also contain epidote and hematite-rich ilmenite as opposed to clinozoisite and rutile. The association of Fe³⁺-rich phases with sulfides, such as inclusions of acmitic pyroxene in pyrite, in Type II samples suggests a temporal link between sulfide deposition and oxidation of Fe²⁺ to Fe³⁺. In contrast, the Adr + Acm + Ep + Chl + Hem veins in the Type III sample are sulfide absent and suggest fluids with fO₂ above the Hem-Mag and Hem-Py buffers.

Garnet-epidote oxybarometry revealed elevated fO₂ in metasomatically altered samples. Calculations were performed using a new oxybarometry Matlab code written by the authors. Our code utilizes the latest thermodynamic database, A-X models, and equations of state implemented in THERMOCALC. The code was also implemented in the XMapTools software package for quantitative visualization of fO₂ using EPMA X-ray maps. Fugacity calculations were conducted at 550 °C, 2.0 GPa, and an aH₂O of unity, unless otherwise stated. Oxygen fugacities for clinozoisite-garnet pairs in the Type I sample were calculated in XMapTools and fall within 0.5 log units of the quartz-fayalite-magnetite (QFM) buffer. Inclusions of garnet in epidote and epidote overgrowths on garnet were selected for fO₂ calculation in the Ep-bearing Type II sample. These garnet-epidote pairs exhibit fO₂ of QFM+1.9 to +2.0. A minimum fO₂ of QFM+4 calculated from the Hem-Mag buffer is estimated for Type III veins. The remarkably high fO₂ of Type III veins contrasts with prograde fO₂conditions of QFM+1 to +2 estimated for epidote inclusions in garnet cores from the same sample at 400-450 °C and 1.0-1.5 GPa. While elevated fO₂ and acmite inclusions in pyrite are consistent with a SO_x(aq)-Fe²⁺ redox pair in Type II samples, fO₂ above the Hem-Mag buffer require the bulk addition of Fe³⁺ or Mn³⁺ as a carrier of oxidation. These data demonstrate that slab fluids may impose fO₂ well above the sulfur-sulfur oxide buffer.

How to cite: Walters, J., Marschall, H., Lanari, P., and Cruz-Uribe, A.: Oxidized slab fluids revealed in metasomatized eclogites: A case study from Syros, Greece, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20622, https://doi.org/10.5194/egusphere-egu2020-20622, 2020.

Plate tectonics is responsible for shaping the Earth’s surface, influencing the geological, hydrological and atmospheric cycles. However, there is no consensus on when plate tectonics initiated: was it fully operational during the Archean or did it not develop until the Proterozoic?

Much of what is currently known about the secular evolution of Earth’s continental crust and its links to plate tectonics has been recovered from detrital minerals. This is related to the incomplete rock record; the detrital record allows access to information from eroded and unexposed terrains. Most studies have relied on the detrital zircon record, but it is still unclear if the coincidence in age peaks with periods of supercontinent assembly reflects episodic continental growth or bias due to selective preservation of new crust within collisional orogenic belts. Furthermore, because zircon mostly grows in high-temperature conditions, it mostly calibrates magmatic cycles. To understand the evolution of plate tectonics and to assess its influence on continental crust preservation, we developed a new proxy, relevant to a range of metamorphic conditions, including HP-LT.

We investigate the U-Pb distribution ages of detrital rutile, from a range of modern stream sediments and siliciclastic units at sub-amphibolite facies metamorphic grade. Rutile mostly forms in collisional orogens and, by comparison with the zircon record, we can test the existence of a preservation bias. Zircon and rutile age distributions from our sample sets show a significant correlation, both peaks and troughs, that can only be reconciled if the detrital zircon record reflects a preservation bias that occurred during supercontinent assembly.

We further present new U-Pb and trace element data from detrital rutile within two clastic sedimentary units, preserved at sub-greenschist facies conditions in NW Scotland. These are the Torridon (Tonian) and the Ardvreck (Cambrian) groups, whose detrital zircon ages span a significant period between 3 and 1 Ga. By applying Zr-in-rutile thermometry and comparing it to the preserved metamorphic record, we show that both low and high dT/dP conditions can be inferred since at least 2.1 Ga.

Combining the existence of paired metamorphism up to 2.1 Ga with the periodic preservation of the continental crust throughout most of the Earth’s history implies that one-sided subduction, a hallmark of plate tectonics, has operated since at least the late Paleoproterozoic, and that supercontinent assembly during and after this period has been driven by plate tectonic mechanisms.

How to cite: Pereira, I., Storey, C. D., Strachan, R., Moreira, H., Darling, J., and Cawood, P. A.: Fitting a new piece into the Precambrian puzzle: the detrital rutile record and its links to modern plate tectonics, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6664, https://doi.org/10.5194/egusphere-egu2020-6664, 2020.

Incipient charnockites are orthopyroxene-bearing granitic gneisses that are commonly considered to be a product of infiltration of CO₂-rich fluids during high temperature dehydration in the granulite terrane. Greenish patches of incipient charnockite are locally present and hosted by granitic gneiss in the Sancheong-Hadong anorthosite complex, southern Yeongnam Massif. Both lithologies are foliated and show a variety of field evidence for partial melting and melt crystallization. Granitic leucosomes and biotite or garnet-rich residua are ubiquitous along ductile shear bands and/or penetrative foliations in the gneiss. These melt-related features are consistent with mineral assemblages and reaction textures, characterized by biotite-breakdown melting. Based on phase equilibria modeling, P-T conditions of peak metamorphism are constrained at 3.5–8.5 kbar and 770–840 °C. Sensitive high-resolution ion microprobe U-Pb analyses of inherited cores and overgrowth rims of zircon from a charnockite yielded the weighted mean ²⁰⁷Pb/²⁰⁶Pb ages of 1880 ± 5 Ma and 1861 ± 4 Ma, which are interpreted as the times for magmatic crystallization and subsequent anatexis of granitic protolith, respectively. This timeline is consistent with that determined from the host granitic gneiss. In contrast, monazite grains from the charnockite and granitic gneiss yielded the weighted mean ²⁰⁷Pb/²⁰⁶Pb ages of 1842 ± 8 Ma and 1838 ± 18 Ma, respectively, suggesting that an influx of aqueous fluid took place ~20 m.y. after the crystallization of granitic melt. Both charnockitic and granitic gneisses underwent high-temperature metamorphism and partial melting at ~1.86 Ga, and were followed by fluid influx at ~1.84 Ga, apparently characterized by monazite recrystallization in association with the retrogression of orthopyroxene to ferromagnesian amphibole-rich aggregates in the former. Thus, the timing and conditions of high-temperature metamorphism, charnockite formation, and fluid flow suggest that the granulite-facies metamorphism and fluid-rock interaction is linked to the waning stage of Paleoproterozoic hot orogenesis in the Yeongnam Massif.

How to cite: Lee, Y., Cho, M., and Kim, T.: Linking high-temperature metamorphism, charnockite formation, and fluid-rock interaction during the waning stage of Paleoproterozoic hot orogeny in the Yeongnam Massif, Korea, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4405, https://doi.org/10.5194/egusphere-egu2020-4405, 2020.

Dating the onset of the continental collision and amalgamation of crustal blocks is at the basis of the reconnaissance of orogenic cycles and yields time constraints for the estimate of rates of accretionary processes over the last 4.5 Gyrs. The Paleoproterozoic Southeastern Churchill Province (SECP) represents the easternmost branch of the Trans-Hudson Orogen, squeezed between the Superior and North Atlantic Cratons (NAC). It comprises a collage of Archean to Paleoproterozoic crustal blocks (Core Zone), and two transpressive orogenic belts (New Quebec and Torngat Orogens), for which crustal amalgamation and associated collisional events are largely undated. We apply a multi-chronometer approach coupled with trace elements geochemistry on supracrustal sequences from the granulitic Tasiuyak Complex accretionary prism and the occidental margin of the NAC (upper plate) to estimate the timing of prograde, peak and retrograde metamorphism in the core of the Torngat Orogen. Our results yield to prograde garnet growth at 1885 ± 12 Ma (Lu-Hf), peritectic prograde monazite growth at 1873 ± 5 Ma (U-Pb), retrograde zircon growth during melt crystallization at 1848 ± 12 Ma, and rutile closure during slow exhumation at 1705 ± 5 Ma in the Tasiuyak Complex. Garnet from the NAC are dated at 2567 ± 4.4 Ma (Lu-Hf) and suggest that the granulite facies metamorphism in the NAC margin largely predates the Torngat Orogeny. We integrate the metamorphic record throughout the SECP to decipher its Paleoproterozoic tectonometamorphic evolution and propose a sequential collisional evolution from ~1.9 to 1.8 Ga.

How to cite: Godet, A., Guilmette, C., Labrousse, L., Smit, M. A., Davis, D. W., and Vanier, M.-A.: The metamorphic record of crustal assembly in the Paleoproterozoic Southeastern Churchill Province, Trans-Hudson Orogen, Canada, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-18718, https://doi.org/10.5194/egusphere-egu2020-18718, 2020.

In collisional orogens continental crust is subducted to (ultra-)high-pressure (HP/UHP) conditions as constrained by petrologic, tectonic and geophysical observations. These (U)HP rocks are exhumed by an extremely fast process (few Ma) as numerous rocks still preserve their high-pressure metamorphic assemblages, which would not be the case if they had time to re-equilibrate at lower pressure conditions. 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 do 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 on. 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 garnets, which experienced only one subduction-exhumation cycle with P-T conditions reaching 600 °C and up to 1 GPa. With dating different parts of big garnet grains we test (1) if it is possible to resolve the duration of garnet growth within single crystals, (2) if both systems, Lu-Hf and Sm-Nd, are needed for better age-constraints, and (3) whether both systems date the same events in the PT-path or give differing information. Additionally we will perform U-Pb geochronology on titanite in order to obtain the age of the first stages of exhumation and on rutile inclusions as well as matrix rutiles to confirm the Eoalpine prograde age with this additional method. Therefore, we will be able 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 samples that will be dated by pseudosection modeling combined with Zr-in-rutile geothermometer, quartz-in-garnet geobarometer, and carbonaceous material geothermometer. 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. 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.

How to cite: Fassmer, K., Tropper, P., Pomella, H., Angerer, T., Degenhard, 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 Complex, Eastern Alps., EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-21505, https://doi.org/10.5194/egusphere-egu2020-21505, 2020.

The most significant consequence of prograde metamorphism for orogenic evolution is the melting of high-grade metamorphic rocks, resulting in a dramatic decrease in their mechanical strength, the activation of shear zones and consequent exhumation. Granitic bodies emplaced within the highest metamorphic grades of the Himalayan orogen form by the melting of amphibolite-grade pelitic rocks, either due to the presence of aqueous fluid or through the dehydration of hydrous phases such as muscovite. Across the Himalayas, these granites, and partially melted source migmatites, are found in the Greater Himalayan Sequence (GHS), bounded by the Main Central Thrust (MCT) and the South Tibetan Detachment (STD). Many of these granites formed during the Miocene when decompression of the unit during rapid exhumation triggered melting; however, exact timings and reaction pathways appear to vary laterally across the orogen. The timescales of anatexis, amalgamation, migration, and emplacement are the focus of active research and have implications for orogenic tectonic development. Recent studies of granite pluton formation suggest a series of pulsed melting events with protracted periods of crystallisation under low melt-fraction conditions. These studies show that grain-scale variations in age can be linked with trace element data in both monazite and zircon, spanning millions of years of crystallisation. It is, therefore, important to recognise the geochemical signatures that these processes leave in granites, migmatites, and melt-extracted restite and to delineate more precisely the relevant processes and timescales leading to magma genesis. We present a preliminary dataset that aims to constrain the source, melt reactions, and timescales of melting episodes that form the migmatites and leucogranites of the upper GHS. We sampled leucogranites, migmatites, and their host metasediments along the Rishi Ganga (Badrinath) and Alaknanda valleys in the Garhwal region of the Indian Himalaya. Zircon from these samples were analysed for their crystallisation age (U-Pb), Hf-isotopic ratios, oxygen isotope and trace element composition using LA-ICPMS.  Rim domains identified using cathodoluminescence (CL) imaging were preferentially targeted, with the aim of collecting data that related to Himalayan melting processes. Preliminary findings suggest that the leucogranites crystallised from 22 Ma to ~13 Ma, with punctuated zircon crystallisation occurring throughout this timespan. Zircon rim ages from migmatites are generally older, ranging from 34 Ma to ~15 Ma. Integration of Hf-isotopic and trace elemental data, combined with petrographic observations allow mineral age data to be linked to changes in geological processes.

How to cite: Oldman, C., Warren, C., Spencer, C., Argles, T., Harris, N., and Hammond, S.: Finding a Pulse: Melt Formation and Timing in the Garhwal Himalaya, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-1576, https://doi.org/10.5194/egusphere-egu2020-1576, 2020.

During subduction, devolatization reactions within the downgoing slab release significant volumes of fluid. Once released, the fate of such fluids remains unclear; they may either stagnate such that local rock systems remain undrained, or fluids may be mobilized over large length scales, draining the dehydrating rock volume. The fact that there is evidence from the metamorphic rock record to support both open- and closed-system fluid behavior demonstrates that permeability in deep crystalline metamorphic rock is both spatially and temporally heterogeneous. Prograde eclogitic veins greater than cm-scale are volumetrically scarce in the high pressure–low temperature (HP–LT) rock record, suggesting that either transient channelized flow is incredibly efficient and thus necessitates negligible grain boundary transfer and a low intact rock permeability, or that a large proportion of fluid migration to the subduction interface may be via more elusive grain boundary mechanisms.

Major element electron microprobe maps of HP–LT garnets from metabasic rocks of the Urals, Russia, As Sifa, Oman, and Syros, Greece, variably reveal short-wavelength and concentric oscillatory zoning in the outer rim region. Oscillatory zoning in most garnets is accompanied by homogeneous core-to-rim aluminum content. However, in samples from As Sifa and Syros, the onset of near-rim major element oscillatory zoning is concomitant with a rimwards step increase in Al content. Secondary ion mass spectrometry (SIMS) O-isotope analyses across rhythmic zoning in samples from each setting are used to assess the hypothesis that this sharp, stepwise change in garnet chemistry reflects a period of localized open system fluid-fluxing behavior, superimposed on a history of an otherwise stagnant fluid within an impermeable grain boundary network. In such a case, coupled oscillatory zoning in major and trace elements—as revealed by laser ablation–inductively coupled plasma­–mass spectrometry (LA–ICP–MS) mapping—may point to pulsed P–T fluctuations, variable partitioning behavior, local kinetic effects associated with metamorphic reaction/dehydration, or changes in redox state serving as a driver for the development of this characteristic HP–LT geochemical garnet zoning.

How to cite: George, F. R., Viete, D. R., Ávila, J., and Seward, G. G. E.: Oscillatory and stepwise compositional zoning in high pressure–low temperature garnets: records of transient and spatially-variable fluid-fluxing during subduction?, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9077, https://doi.org/10.5194/egusphere-egu2020-9077, 2020.

The nucleation of subduction zone remains a widely discussed topic in the global tectonics. The prevalent view is that subduction starts within an oceanic plate. However, there is strong evidence that subduction can also be initiated within a continent. To test this hypothesis, we combine petrology, isotope geochronology and thermodynamic phase equilibrium modelling on eclogites from the Austroalpine Nappes of the Eastern Alps.

The high- and ultrahigh-pressure rocks occur in a ~400 km long belt from the Texel Complex in the west to the Sieggraben Unit in the east without remnants of Mesozoic oceanic crust. Garnet growth during pressure increase was dated using Lu-Hf chronometry. The results range between c. 100 and c. 90 Ma, indicating a short period of subduction. Combined with already published data, our estimates of metamorphic conditions indicate a field gradient with increasing pressure and temperature from northwest to southeast, where the rocks experienced ultrahigh-pressure metamorphism. The oldest Cretaceous eclogites (c. 100 Ma) are found in the Saualpe-Koralpe area which comprises widespread gabbros formed during Permian to Triassic rifting. This supports the hypothesis that subduction initiation was intracontinental and localized by a Permian rift. In the Texel Complex two-phased garnets yielded a Variscan-Eoalpine mixed age indicating re-subduction of Variscan eclogite-bearing continental crust during the Eoalpine orogeny. Jurassic blueschist-facies metamorphism at Meliata in the Western Carpathians and Cretaceous eclogite-facies metamorphism in the Austroalpine are separated by a time gap of ~50 Ma and therefore do not represent a transition from oceanic to continental subduction but rather separate events.

How to cite: Miladinova, I., Froitzheim, N., Nagel, T., Janák, M., Fonseca, R., Sprung, P., and Münker, C.: Constraining the process of intracontinental subduction: implications from petrology and Lu-Hf geochronology of eclogites from the Austroalpine Nappes, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8749, https://doi.org/10.5194/egusphere-egu2020-8749, 2020.

Pressure estimated from metamorphic rocks is one of the main tools for geodynamic reconstructions. The pressure-temperature path of UHP metamorphic rocks typically shows a linear increase of P and T followed by a rapid drop of Pressure at near-constant temperature. The geological history can be reconstructed by using the metamorphic pressure as a proxy for depth. Researchers often base their geodynamic reconstruction on a simple linear mapping of pressure to depth, by considering that the pressure is the weight of the overlying column of rock or lithostatic pressure. In recent years, an increasing corpus of evidence demonstrates that rocks can experience pressures that deviate from the lithostatic state on the order of GPa. These deviations can be at the scale of the orogen (Petrelli and Podladchikov, 2002), the outcrop (Jamtveit et al., 2018; Luisier et al., 2019); or even at the grain-scale (Tajcmanova, 2015). Thus, these studies raise the concern that metamorphic pressures may not be reliable proxies for depth, and therefore could not be used for geodynamic reconstructions. The objective of this contribution (1) to review the various models proposed in the literature for metamorphic pressure, (2) to formulate analytical models with simple assumptions that can be used to convert metamorphic pressure to depth even in the case where pressure deviates significantly from the lithostatic pressure. We use our pressure-to-depth conversion models to estimate the depth of ~60 samples from various orogens worldwide. The prediction of the different models varies widely. Some models predict depth as deep as 160km for specific samples, while other models predict depth $<75$ km (i.e. deepest depth of the Moho) for all data points.  We discuss the limits of applicability and the geodynamic implications of each model. 

How to cite: Bauville, A. and Yamato, P.: Pressure-to-depth conversion models for (ultra-)high-pressure metamorphic rocks: review and application, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6539, https://doi.org/10.5194/egusphere-egu2020-6539, 2020.

Serpentinites can significantly modulate the carbon fluxes in subduction zones because they occasionally host substantial concentrations of carbonate formed during the oceanic stage of subducting oceanic lithosphere (ophicalcite; [1]) or during metasomatic reaction with CO₂-bearing fluids at the subduction plate interface (e.g. hybrid carbonate–talc rocks; [2]). At subarc depth, fluid-mediated carbon release from lithologies like serpentinite-hosted carbonates is critical to understand the global carbon balance and magnitude of carbon fluxes from the subducting plate into the deep mantle. However, the solubility of carbon and the open-system metasomatic reactions during fluid-rock interactions in carbonated serpentinites at high P are not fully understood. In line with previous studies of prograde devolatilization [3], newer models show that the carbon release during prograde devolatilization reactions of serpentinite-hosted carbonate rocks is limited even if accounting for the higher carbon solubility of electrolytic fluids compared to molecular fluid models [4]. Therefore, devolatilization reactions driven by infiltration of Atg-serpentinite dehydration fluids into serpentinite-hosted meta-carbonate rocks determines how much carbon in the mantle lithosphere subducts deep into the mantle. Here we present the results of thermodynamic modelling – using the implementation of the DEW aqueous database in Perple_X [5] – to explore subduction fluid compositions and metasomatism of serpentinite-hosted carbonate rocks during prograde and infiltration-driven devolatilization reactions. The chemical system of serpentinite + carbonate is ideal to understand the interplay of changes in fluid composition, pH, bulk chemical modification and mineral assemblage during open-system fluid infiltration metamorphism. Our models provide new insights into the interaction of carbon-bearing subduction fluids with the cold hydrated mantle wedge, and the carbon release from serpentinite-hosted carbonates related to infiltration of serpentinite dehydration fluids at subarc depths. Our results further show that even though high fluid fluxes from serpentinite dehydration will transform meta-ophicalcites and talc-carbonate rocks into carbonate-garnet-clinopyroxene-olivine rocks and carbon-bearing orthopyroxenites, these rocks can subduct carbon beyond subarc depths into the deeper mantle where they may be related to the formation of deep diamonds, carbonatites and kimberlites.

REFERENCES

[1] Menzel et al., 2019, JMG 37, 681– 715.

[2] Spandler et al., 2008, CMP 155, 181-198.

[3] Kerrick & Connolly, 1998, Geology 26, 375-378.

[4] Menzel et al., 2020, EPSL 531.

[5] Connolly & Galvez, 2018, EPSL 501, 90-102.

How to cite: Menzel, M., Garrido, C. J., and López Sánchez Vizcaíno, V.: Fluid-mediated carbon release by infiltration of serpentinite dehydration fluids during subduction: insights from thermodynamic models of serpentinite-hosted carbonate rocks, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11027, https://doi.org/10.5194/egusphere-egu2020-11027, 2020.

We studied monazite behaviour in UHP diamond-bearing gneiss from Saxnäs in the Seve Nappe Complex of the Scandinavian Caledonides (Petrík et al., 2019). Although the rock has been re-equilibrated under  granulite facies and partial melting conditions, the UHP stage is recorded by the presence of diamond. Microdiamonds occur in situ as inclusions in garnet, kyanite and zircon, either as single-crystal or polyphase inclusions with Fe-Mg carbonates, rutile and CO₂. Two garnet types have been recognised: dominant Grt I  with inclusions of diamond found mostly in the garnet rims, which suggests that originally the bulk of Grt I grew at UHP conditions. Grt II, forming small crystals, overgrowths on, or domains within Grt I originated by dehydration melting reactions involving breakdown of phengite and clinopyroxene during decompression. Monazite occurs in the rims of Grt I close to microdiamond, where garnet shows the highest pyrope content and a secondary peak of yttrium. Such a position indicates thermally activated diffusion under high temperature at the end of prograde metamorphism. Based on such textural relations, we argue that monazite formed at UHP conditions.

Monazite composition shows negative Eu anomalies and moderate Y contents, which is not in agreement with common interpretation that UHP conditions necessarily lead to the absence of Eu anomaly and low Y content due to absence of plagioclase and high garnet content. We explain this by the effect of whole-rock composition. LA ICP MS analyses show that whole-rock budget is controlled by monazite, apatite and garnet, all having negative Eu anomalies. Whole rock composition is successfully modelled by (wt. %) garnet 16, apatite 3, monazite 0.06. We conclude that the Eu anomaly is inherited from the source rock, not reflecting the coexistence with plagioclase and/or K-feldspar, which are unstable at UHP conditions. Uniform garnet abundance (16 vol. %) above 20 kbars predicted by pseudo-section modelling explains the lack of Y decrease due to the increase of garnet content at UHP conditions. Our results suggest that the effect of the whole-rock composition may be more important than that of coexisting phases.

U-Th-Pb chemical age dating of monazites yields an isochron centroid age of 472 ±3 Ma. We interpret this age as monazite growth under UHP conditions related to subduction of the Baltican continental margin in Early Ordovician time.

This work was supported by the projects APVV-14-0278 and APVV-18-0107, National Science Center “CALSUB” 2014/14/E/ST1/00321

Reference: Petrík, I., Janák, M., Klonowska, I., Majka, J., Froitzheim, N., Yoshida, K., Sasinková, V., Konečný, P., Vaculovič, T. 2019. Journal of Petrology doi: 10.1093/petrology/egz051

How to cite: Petrík, I., Janák, M., Klonowska, I., Majka, J., Froitzheim, N., Yoshida, K., Sasinková, V., Konečný, P., and Vaculovič, T.: Monazite Behaviour during Metamorphic Evolution of a Diamond-bearing Gneiss , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4906, https://doi.org/10.5194/egusphere-egu2020-4906, 2020.

Serpentinization is the process of hydroxylation of olivine-rich ultramafic rocks to produce minerals such as serpentine, brucite, magnetite, and may release H₂. The hydrogen produced through serpentinization reactions can be involved in abiotic reaction pathways leading to the genesis of abiotic light hydrocarbons such as methane (CH₄). Examples of this phenomenon exist at the seafloor, such as at the serpentinite-hosted Lost City hydrothermal field, and on land in ophiolites at relatively shallow depths. However, the possibility for serpentinization to occur at greater depths, especially in subduction zones, raises new questions on the genesis of abiotic hydrocarbons at convergent margin and its impact on the deep carbon cycle. High-pressure ultramafic bodies exhumed in metamorphic belts can provide insights on the mechanisms of high-pressure serpentinization in subduction zones and on the chemistry of the resulting fluids. This study focuses on the ultramafic Belvidere Mountain complex belonging to the Appalachian belt of northern Vermont, USA. Microstructures show overgrowth of olivine by delicate antigorite crystals, suggesting olivine serpentinization at high-temperature consistent with the subduction evolution of the Belvidere Mountain complex.  Fluid inclusion trails cross-cutting the primary olivine relicts  suggest their formation during the antigorite serpentinization event. MicroRaman spectroscopy on the fluid inclusions reveals a CH₄-rich gaseous composition, with trace of N₂, NH₃ and S-H compound. Moreover, the precipitation of daughter minerals of lizardite and brucite in the fluid inclusions indicate the initial presence of H₂O in the fluid. Secondary olivine is observed at the rim of pseudomorphosed primary pyroxenes (bastite), and has higher forsterite (Fo₉₅) content with respect to the primary olivine (Fo₉₂), suggesting either a syn-serpentinization olivine precipitation in the subduction zone, or a successive partial dehydration of the antigorite during metamorphism. Decreasing oxygen fugacity during serpentinization and related abiotic reduction of carbon at high-pressure conditions is proposed at the origin of methane in the fluid inclusions. This potentially places the Belvidere Mountain complex as an example of deep serpentinization related to high-pressure genesis of abiotic methane.

How to cite: Boutier, A., Vitale Brovarone, A., Martinez, I., Sissmann, O., and Mana, S.: High pressure serpentinization and abiotic methanogenesis in metaperidotite from the Appalachian subduction, northern Vermont, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5720, https://doi.org/10.5194/egusphere-egu2020-5720, 2020.

The Tsäkkok lens (northern Scandinavian Caledonides) represents the outermost part of the rifted Baltica passive margin and consists of sediments and pillow basalts of MORB affinity that were metamorphosed in eclogite facies. The Tsäkkok eclogites underwent metamorphism in a cold subduction regime (~8 °C/km) at the onset of the Iapetus Ocean closure. These rocks record pervasive high-pressure, fracturing during prograde dehydration at eclogite-facies conditions reaching up to 2.2 GPa and 590 ºC. Locally, the omphacite-dominated groundmass is transected by fractures sealed either by omphacitite or garnetite veins. Garnetite veins form a dense network that disrupt intact eclogite blocks, whereas omphacitite is found in rare, single veins. The garnetite veins are dominated by dense, poikiloblastic garnet clusters and display two chemically different zones, i.e., a high-Mn inner zone and a low-Mn outer zone. Detailed microstructural and geochemical mapping by EDS-EBSD SEM revealed that the high-Mn inner zone is disrupted and sealed by the low-Mn garnet zone. Garnets in the vein usually show little elongation and moderate intracrystalline substructure that is dominated by slightly changing misorientations without clear subgrain boundaries. By contrast, garnets of the sealed domain display an abrupt grain size reduction and anomalously high density of sharp intracrystalline misorientations in equant grains. The interstitial space between garnet grains in both of the inner and outer zones of the vein is infilled by omphacite + rutile + quartz + phengite + glaucophane.

The textural relationship between the inner- and outer zones of the garnetite vein implies syn-deformation growth of the outer zone, while the mineral assemblage attests for high-pressure conditions of the vein formation. Considering the lack of significant offset along the vein, we interpret the observed microstructures as formed during the sudden opening and closing of a brittle fracture, typical of hydrofracturing, and fast crystal growth assisted by high-pressure fluids. Presumably, these fractures constitute a fluid escape pathway during dehydration at prograde/peak conditions.

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 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., Hidas, K., Garrido, C. J., Barnes, C., Klonowska, I., and Majka, J.: Deciphering the brittle failure of eclogites at high-pressures: hydrofractures as fluid-escape pathways , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-13756, https://doi.org/10.5194/egusphere-egu2020-13756, 2020.

In subduction zones, aqueous fluids derived from devolatilization processes of the oceanic lithosphere and its sedimentary cover, are major vectors of mass transfer from the slab to the mantle wedge and contribute to the recycling of elements and to their geochemical cycles. In this setting, assessing the mobility of redox sensitive elements, such as iron, can provide useful insights on the oxygen fugacity conditions of slab-derived fluid. However, the amount of iron mobilized by deep aqueous fluids and melts, is still poorly constrained.

We experimentally investigate the solubility of magnetite-hematite assemblages in water-saturated haplogranitic liquids, which represent the felsic melt produced by subducted eclogites. Experiments were conducted at 1 GPa and temperature ranging from 700 to 900 °C employing a piston cylinder apparatus. Single gold capsules were loaded with natural hematite, magnetite and synthetic haplogranite (Na_0.56K_0.38Al_0.95Si_5.19O_12.2). Two sets of experiments were conducted: one with H₂O-only fluids and the second one adding a 1.5 m H₂O–NaCl solution. The capsule was kept frozen during welding to ensure no water loss. After quench, the presence of H₂O in the quenched haplogranite glass was checked by Raman spectroscopy, while major elements were determined by microprobe analysis.

Preliminary results indicate that a significant amount of Fe is released from magnetite and hematite in hydrous melts, even at relatively low-pressure conditions. At 1 GPa the FeO_tot quenched in the haplogranite glass ranges from 0.60 wt% at 700 °C, to 1.87 wt% at 900 °C. In the presence of NaCl, we observed an increase in the amount of iron quenched in the glass (e.g., at 800 °C from 1.04 wt% to 1.56 wt% of FeO_tot). Our results suggest that hydrous melts can effectively mobilize iron even at low-pressure conditions and represent a valid agent for the cycling of iron from the subducting slab to the mantle wedge.

How to cite: Tiraboschi, C. and Sanchez-Valle, C.: Solubility of magnetite-hematite assemblages in slab-derived saline fluids, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-16224, https://doi.org/10.5194/egusphere-egu2020-16224, 2020.

Phengite is the most common metamorphic mineral in HP-UHP metasedimentary rocks, which can convey H₂O, LILEs (especially K, Ba, Cs and Rb), Li, B and N in their structure formed at depths up to 300 km. The breakdown of phengite in a downgoing oceanic slab would cause fluid-induced element transport into the overlying mantle wedge. We have investigated the ²H/¹H (D/H) and ¹⁸O/¹⁶O ratios of twenty-four phengite separates from pelitic schists of the Devonian–Carboniferous Renge Belt (SW Japan), Permian Shaiginsky Complex (Far East Russia) and Cretaceous Sambagawa Belt (SW Japan).

We found the presence of the very light hydrogen isotope (δD < –95‰) in blueschist-facies phengites in the three different metamorphic belts. For example, phengite from the lawsonite- and epidote-grade metasedimentary schists of the Osayama Serpentinite Mélange (OSM) of the Renge Belt are characterized by negative hydrogen isotope compositions (δD values relative to VSMOW) ranging from –113 to –93.9‰ and oxygen isotope compositions (δ18O values relative to VSMOW) ranging from +12.9 to +14.6‰.

High-Si features and K–Ar ages of the investigated phengites deny the possibility of meteoric-hydrothermal alteration to have caused the low δD values. The light values might be attributed to isotopic fractionation during progressive metamorphic dehydration.Assuming a meamorphic temperatures range of 250–350°C for the OSM schists, the inferred metamorphic fluid compositions in blueschist-facies depth for that fossil slab had a range of δD = ~–40 to –75‰ and δ18O = ~+13 to +15‰. These values are significantly lighter than the slab-fluid induced from the Arima hot spring water in a forearc region of modern SW Japan subduction zone. Our study suggests that slab-derived fluids in ancient Pacific-type subduction zone are characterized by light hydrogen isotope and that the phengite breakdown can affect hydrogen isotope of nominally anhydrous minerals (NAMs) in the deep mantle.

How to cite: Tsujimori, T., Pastor-Galán, D., and Álvarez-Valero, A.: Slab-derived fluid evolution induced from oxygen and hydrogen isotopes compositions of blueschist-facies phengites, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-13604, https://doi.org/10.5194/egusphere-egu2020-13604, 2020.

The spatial, temporal, and pressure-temperature (P-T) relationships among high-pressure metamorphic rocks from within subduction complexes have key implications for their exhumation mechanisms and the rheological properties of the subduction interface. Structural, age, and P-T relationships among exhumed rocks may indicate, for example, (1) melange-style mixing during subduction and exhumation or (2) progressive underplating and coherent exhumation.  Melange-style subduction ‘channels’ should exhibit a range of peak metamorphic grades in incorporated blocks, whereas coherent underplating may result in similar peak P-T conditions among blocks, especially from similar structural levels. Determining P-T conditions of high grade blocks is key for understanding these subduction zone endmembers, but constraining formation pressures of high grade blocks such as eclogites has historically been challenging for petrologists due to the lack of suitable barometers.

In this study, we compare pressure conditions recorded by spatially and temporally variant high-grade eclogite blocks from the Franciscan Complex in California. We use quartz-in-garnet elastic barometry, a technique that can reliably provide growth P conditions of garnets, to determine formation pressures of eclogites from sections of the northern (Jenner Beach, Ring Mountain, and Junction School) and the southern Fransican Complex (Santa Catalina Island). Multiple eclogite blocks from Jenner Beach are analyzed, and single eclogite blocks from the other localities. By comparing  garnet growth conditions from within a single outcrop and between distinct outcrops, we evaluate the local and regional spatial distribution of P conditions recorded by eclogites. Preliminary data from > 100 quartz-in-garnet inclusion pressures suggests that eclogites from the northern Franciscan record similar garnet growth conditions (~1.5 - 1.9 GPa), and pressures recorded on Santa Catalina Island differ slightly (~1.2 - 1.3 GPa). We use these results to address spatio-temporal variations of peak P recorded by eclogites and its implications for exhumation of the Franciscan complex, and further discuss how quartz-in-garnet pressures compare with conventional thermobarometry techniques.

How to cite: Cisneros, M., Behr, W., and Platt, J.: Formation pressures of eclogites from the Franciscan complex, California, from quartz-in-garnet elastic barometry, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-16048, https://doi.org/10.5194/egusphere-egu2020-16048, 2020.

    Subduction zone is a distinct activity structure of hypocenter distribution of earthquakes. Hydrous minerals are involved in the chemical and physical activities in subduction zones. As a widely distributed hydrous mineral in shallow depths, talc has potential significance in various fault activities, and its dehydration reaction may be an important cause of the earthquake. Iron is a main element of the earth's crust, and the iron contents of hydrous minerals have a large impact on melting point, the rheological strength physical and chemical properties of the rocks. As a common hydrous mineral, the iron content of talc is not uniform; therefore, it is very important to study the dehydration kinetics of talc with different iron content.

    The dehydration reaction of three different iron contents talc was studied by means of synchronous thermal analysis, high temperature and high pressure differential thermal experiment and in-situ synchrotron X-ray diffraction experiment. Data of synchronous thermal analysis was calculated by Flynn-Wall-Ozawa (FWO). The activation energies of different iron content talc were calculated as 359.8 kJ/mol（FeO：0.4wt%），368.2.0 kJ/mol（FeO：2.0wt％），belonging to the second-order reaction. Data of in-situ synchrotron X-ray diffraction experiment was fitted by Avrami equation, E=350 kJ/mol（FeO：2.0wt％），n=1.67. The dehydration of talc followed random nucleation and growth mechanism. High content of iron obviously resulted in lower dehydration temperature.

  The release rate of talc dehydration fluid was 2.3E-05 to 6.1E-06 obtained by in-situ synchrotron X-ray diffraction experiment，it could lead to local overpressure induced rock brittle fracture. The supercritical fluid produced by the dehydration of talc in the subduction zone further attenuates the rock, resulting in local overpressure, which eventually leads to rock failure. The results suggested that the dehydration of different iron contents of talc may occur at the different depth around hundreds of kilometers, so the study was significant to our understanding of the genetic mechanism of earthquakes in the subduction zone.

How to cite: Yi, L., Zhang, R., and Yang, S.: The effect of iron content on dehydration kinetics of talc, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6258, https://doi.org/10.5194/egusphere-egu2020-6258, 2020.

The Turvo-Cajati Formation (TCF) is a metasedimentary unit composing the Curitiba Terrane, a major segment of the southern Ribeira Belt, SE Brazil. It is composed of rocks of greenschist (Low-TCF), amphibolite (Medium-TCF) and granulite (High-TCF) facies conditions. Previous studies in High-TCF indicates that the unit underwent extensive partial melting under high-pressure conditions (670-810 °C and 9.5-12 kbar), within the kyanite stability field. New data on the metamorphic zoning within Low and Medium-TCF were collected using petrography and thermodynamic modeling in the MnNCKFMASHTO system. Four metamorphic zones were recognized for Low-TCF and Medium-TCF: biotite, garnet, staurolite and sillimanite zones where sillimanite zone prevails. The pressure regime is estimated to be below 8 kbar, as staurolite breaks down straight to sillimanite. Thermodynamic modeling yielded metamorphic peak conditions of ~530-560 °C ,~6-7 kbar (garnet zone) and ~660-690 °C ,~6-7 kbar (sillimanite zone). The metamorphic field gradient is flat and of low to medium pressure, below the typical barrovian-type baric regime. It is inferred that Low and Medium-TCF were metamorphosed in a tectonic setting different from the High-TCF. Probability density plots(pdp) from detrital zircon indicate late-Cryogenian-Ediacaran arc-related and Rhyacian sources for all TCF sub-units, where High-TCF presents forearc depositional setting and Low-Medium-TCF back arc depositional setting. This scenario suggests that the TCF is made up of a collisional juxtaposition of an accretionary wedge (High-TCF) and a back-arc basin (Medium-TCF and Low-TCF) on the border of a microplate that includes a Rhyacian basement microcontinent (Atuba Complex). Available petrological and geochronological data suggest that the TCF comprises a paired low-P and high-P belt, associated with a major Ediacaran suture zone in the southern Ribeira Belt. The high metamorphic gradient recorded in the Medium-TCF and Low-TCF was related to asthenospheric upwelling in the back-arc region, which also produced extensive partial melting in the Atuba Complex basement. Metamorphic ages where previously obtained in High-TCF with ages around 589 ± 12 Ma and 584 ± 4 Ma. Petrochronology will be used to obtain the age of metamorphic events, using monazite and apatite grains from Low and Medium-TCF and compare them to available High-TCF data to understand and adjust the proposed model.

How to cite: Ricardo, B., Moraes, R., Faleiros, F., Siga Júnior, O., Campanha, G., and Mottram, C.: Investigation of juxtaposed high- and low-pressure metamorphic field gradients rocks and its tectonic implications, a case study of Turvo-Cajati Formation, Ribeira Belt, Brazil, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6078, https://doi.org/10.5194/egusphere-egu2020-6078, 2020.

More than a dozen deposits of corundum-bearing rocks are known within the Belomorian mobile belt (references in Serebryakov, Rusinov, 2004); their genesis remains debatable. Some authors consider corundum-bearing rocks to be normal metamorphic rocks (for example, Lebedev et al., 1974), others suggest the metasomatic genesis of rocks with corundum: 1 – corundum-bearing rocks were formed as a result of high-temperature high-pressure (600 - 700ºC, 7 - 8 kbar) metasomatism which was accompanied by desilification and the introduction of Ca and Na (Serebryakov, Rusinov, 2004); 2 – these rocks are a product of hydrothermal alteration of gneisses by fluids associated with basic intrusions (Bindeman et al., 2014). All these assumptions were made without a detailed physicochemical analysis of the mineral parageneses of corundum-bearing rocks.
The Perple_X software package (Connolly, 2005) is discussed in some recent works as an effective tool for the thermodynamic modeling of the open systems (Goncalves et al., 2012, Manning, 2013). Using the Perple_X software package (version Perple_X 6.8.7, updated 04.07.2019) we constructed P-T, T-μ (SiO₂), and μ(SiO₂)-μ(Na₂O) pseudosections for a given chemical composition of kyanite-garnet-biotite gneiss of the Chupa sequence. The hp02ver.dat thermodynamic database was used, the diagram μ(SiO₂) - μ(Na₂O) was calculated for P = 8 kbar, T = 650ºC, in the presence of a carbonic-aqueous fluid with X(CO₂) = 0.3. Selected solid solution models are Ca-Amph(D) for hornblende, Gt(HP) for garnet, St(HP) for staurolite, Bi(HGP) for biotite, feldspar for feldspar, Sp(HP) for spinel.
The results show that the majority of corundum-bearing rocks varieties (amphibole-free corundum-bearing rock, amphibole-bearing rock with corundum, altered quartz-free kyanite-garnet-biotite gneiss, kyanite-garnet amphibolite) could be formed by metasomatic alteration of kyanite-garnet-biotite gneisses of the Chupa sequence. This process was characterized by a significant decrease in µ(SiO₂) and a slight increase in µ(Na₂O). Our conclusion is partly consistent with the hypothesis that corundum-bearing rocks were formed as a result of metasomatism, which was accompanied by desilification of Ky-Grt-Bt gneisses and the introduction of Na and Ca (Serebryakov, Rusinov, 2004).

The study was conducted according to the IPGG project 0153-2019-0004.

Bindeman I.N., Serebryakov N.S., Schmitt A.K. et al. (2014) Field and microanalytical isotopic investigation of ultradepleted in ¹⁸O Paleoproterozoic “Slushball Earth” rocks from Karelia, Russia. Geosphere. V. 10. P. 308-339.

Connolly J.A.D. (2005) Computation of phase equilibria by linear programming: A tool for geodynamic modeling and its application to subduction zone decarbonation.  Earth and Planetary Science Letters, 236, p. 524–541.

Goncalves P., Oliot E., Marquer D., Connolly J.A.D. (2012) Role of chemical processes on shear zone formation: an example from the Grimsel metagranodiorite (Aar massif, Central Alps). J. metamorphic Geol., 30, p. 703–722.

Lebedev V.I., Kalmykova N.A. & Nagaytsev Yu.V. (1976) Corundum-staurolite-hornblende schists of the Belomorskiy complex, International Geology Review, 18:6, 653-662.

Manning C.E. (2013) Thermodynamic modeling of fluid-rock interaction at conditions of the earth's middle crust to upper mantle. Reviews in Mineralogy & Geochemistry, 76, p. 135-164.

Serebryakov, N.S., Rusinov, V.L. (2004) High-T high-pressure Ca, Na metasomatism and formation of corundum in the precambrian Belomorian mobile belt. Dokl. Earth Sci. 395, pp. 549–533.

How to cite: Akimova, E. and Kol’tsov, A.: Thermodynamic modeling of the formation of corundum-bearing rocks within the Belomorian mobile belt using Perple_x software, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-327, https://doi.org/10.5194/egusphere-egu2020-327, 2020.

Thermobarometric data and fluid inclusions data of conditions of interaction between mafic granulite xenoliths and plagiogranites in the Lotta river area, Lapland Granulite Belt, confirm the conclusion that leucocratic garnet-bearing plagiogranites of the Lapland complex are associated with the anatexis of country khondalites during peak of metamorphism.

The formation of plagiogranitic magmas, probably, occurred at depths of about 25-30 km. As they ascended, they captured numerous xenoliths (Kozlov, Kozlova, 1998). The most remarkable of them are two-pyroxene-plagioclase granulite xenoliths (orthopyroxene ± clinopyroxene + plagioclase ± quartz + magnetite + ilmenite + pyrrhotite). The xenoliths show extensive amphibole formation, which is manifested as coronas of K-bearing pargasite-edenite amphibole and coarse-grained amphibole-quartz symplectites in contacts of pyroxenes, magnetite, ilmenite and pyrrhotite with plagioclase.

The more calcic composition of plagioclase and the lower Mg-number of pyroxenes in the amphibolized portions of xenoliths correspond to the amphibole formation via reaction: Opx + Ilm + Mt + Pl = Amph ± Qtz. Amphibole formation is locally accompanied by biotite, indicating the addition of potassium into the xenoliths.

A pressure of 6.0-6.4 kbar was estimated from the equilibrium of clinopyroxene + orthopyroxene + plagioclase + quartz in non-amphibolized portions of xenoliths. The corresponding temperatures 800-860°C are within the range of temperatures estimated for the plagiogranite crystallization (Kaulina et al., 2014) as well as peak temperatures of the M2 tectonic-thermal event in the Lapland complex (Mints et al., 2007). Amphibole-plagioclase equilibrium (Blundy, Holland, 1990) recorded the temperatures of the amphibole formation 740-780°C at a pressure of 5.0-5.5 kbar. Compositional variations of amphibole toward tremolite indicate further cooling. It was, probably, due to the interaction of an essentially aqueous fluid issued from plagiogranitic magma with xenoliths as they were captured and transported.

Indeed, xenoliths are crossed by plagiogranitic veins. Abundance of aqueous-salt (17-20 wt. % NaCl eq.) inclusions and the subordinate amount of carbon dioxide inclusions in plagiogranite minerals confirm this assumption. Thus, plagiogranites of the Lapland complex and associated fluids were formed inside the complex at P-T parameters comparable to the peak conditions of granulite metamorphism. During ascension, these granite magmas could only produce fluid effects on the country rocks including xenoliths.

How to cite: Butvina, V., Golunova, M., and Safonov, O.: Fluid inclusion and thermobarometric study of interaction between mafic xenoliths and plagiogranites in the Lotta River Area, Lapland Granulite Belt, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-294, https://doi.org/10.5194/egusphere-egu2020-294, 2020.

The widespread occurrence of high to ultra-high temperature (HT-UHT) metamorphism in continental crust has been widely documented worldwide. However, there has been ongoing debate on the heat sources responsible for generating these HT-UHT conditions.

Generating HT-UHT temperatures is thought to require either singularly, or in combination, long-lived crustal thickening (e.g. orogenic systems) with high radioactive heat production and low erosion rates, or large supplies of heat from the mantle either through conduction within thinned lithosphere (e.g. back-arc) or by advective heating linked to large-scale mantle-derived  magma’s. Distinction between these two major thermal sources can made on the crustal heat generation rates and timescales of the HT-UHT metamorphism and the volumes of externally derived high-temperatures magmas. Therefore, a detailed understanding of the terrain-scale heat generation rates, and the metamorphic P-T-­t path inferred from the integration of petrochronology and phase equilibria modelling can provide important information.

The Paleoproterozoic Khondalite (metasedimentary) rock system in the North China Craton is thought to represent a typical Paleoproterozoic HT metamorphic belt with local areas reaching UHT conditions and it has been extensively studied. In terms of the thermal drivers, most workers suggest advective heating from the emplacement of mantle related mafic magma, although the apparent volume of clearly-mantle derived magma appears generally insufficient to account for the regional extent of HT-UHT conditions.

To better understand the mechanisms leading the HT-UHT conditions, we need (1) regional-scale measurements of in-situ heat producing elements and (2) a better understanding of the duration of HT-UHT conditions on a regional scale. To better characterise in-situ thermal sources we have determined heat generation rates using quantitative in-field gamma ray spectrometer (GRS) analysis. Volume averaging of rock types indicates terrain-scale U-Th concentrations would have generated around 3mWm^-3 at the time of metamorphism.  Given that U-Th would have been lost from the metamorphic system during extraction of high-temperature crustal melts, simple modelling shows the crustal U-Th concentrations would have contributed substantially to the generation of the high-temperature thermal regime. Furthermore, a preliminary compilation of concordant zircon and monazite metamorphic ages from published literature shows a range of ca. 1950-1850 Ma in both western and eastern Khondalite Belt, suggesting possible long-lived metamorphism. Therefore, we argue that the role of the mantle derived advective heat in generating the UHT regime in the North China Khondalite Belt may have been overestimated.

Key words: heat generation, HT-UHT metamorphism, Khondalite Belt, North China Craton

How to cite: He, X., Hand, M., and Hasterok, D.: Crustal heat generation rates in the North China Khondalite Belt high to ultra-high-temperature metamorphic rock system, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12688, https://doi.org/10.5194/egusphere-egu2020-12688, 2020.

Based on differences in metamorphic grade and isotope model ages, the basement rocks of Sri Lanka can be subdivided from NW to SE into the Wanni Complex (WC), the Highland Complex (HC) and the Vijayan Complex (VC) (Milisenda et al. 1994). The UHT conditions of the HC were studied extensively and are well constrained whereas data from the WC and VC are less abundant. Only few recent petrological and geochemical work has been done especially along the WC–HC boundary which is still ill-defined (Kitano et al. 2018; Wanniarachchi & Akasaka 2016). Due to the common occurrence of migmatites, pyroxene bearing gneisses, and cordierite bearing metapelites/paragneisses, the WC clearly experienced granulite facies metamorphism. However, PT conditions are lower compared to the HC. In this study, U-Th-Pb monazite dating combined with a petrological study including phase equilibria modelling and thermobarometry was conducted focusing on cordierite bearing migmatic biotite gneisses located at the WC–HC boundary in the West of Sri Lanka. The HC underwent UHT metamorphism at 580-570Ma (Sajeev et al. 2010), the main metamorphic phase of the VC is dated with 580Ma. (Kröner et al., 2013). With U-Th-Pb monazite ages of around 530 Ma, the cordierite bearing assemblages from the WC are significantly younger (Wanniarachchi & Akasaka 2016). The predominantly felsic but also mafic peraluminous migmatic ortho- and paragneisses comprising the mineral assemblage cordierite + garnet + biotite + plagioclase + k-feldspar + quartz + ilmenite + magnetite + spinel + sillimanite ± orthopyroxene and contain monazite (+ zircon ± xenotime) as garnet inclusions (Group1) and in the matrix (Group2). Group1 monazite ages cluster around 575±5 Ma and 561±5 Ma whereas ages of Group 2 cluster at 550±3 and 527±3. Based on ages and textural occurrence of monazite we suggest that two thermal events at ca. 550-575 Ma and ca. 530-550 Ma are recorded in this rock type indicating a complex evolution during the late stage of the Pan-African orogeny. PT conditions range from 700–900°C and from 5–8 kbar with a decreasing north-south gradient. Further geochronological investigations are needed to relate either to the older or the younger overprint to the main metamorphic phase of the WC.

Kitano, I., Osanai, Y., Nakano, N., Adachi, T., & Fitzsimons, I. C. W. (2018). Journal of Asian Earth Sciences, 156, 122–144.

Kröner, A., Rojas-Agramonte, Y., Kehelpannala, K. V. W., Zack, T., Hegner, E., Geng, H. Y., … Barth, M. (2013). Precambrian Research, 234, 288–321. 

Milisenda, C. C., Liewa, T. C., Hofmanna, A. W., & Köhler, H. (1994). Precambrian Research, 66(1–4), 95–110.

Sajeev, K., Williams, I. S., & Osanai, Y. (2010). Geology, 38(11), 971–974.

Wanniarachchi, D. N. S., & Akasaka, M. (2016). Journal of Mineralogical and Petrological Sciences, 111(5), 351–362.

How to cite: Lechner, N., Hauzenberger, C., Masten, M., Sorger, D., and Fernando, G. W. A. R.: Petrology and geochronology of cordierite bearing assemblages from the Wanni Complex – Sri Lanka, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9956, https://doi.org/10.5194/egusphere-egu2020-9956, 2020.

The Wanni Complex is found in the northwestern part of Sri Lanka. The boundary to the Highland complex occurring to the south is partly ill defined. Differences in isotopic model ages were used to seperate both units (Kitano et al. 2018; Milisenda et al. 1994). While the Highland Complex has gained a lot of attention due to the UHT metamorphic overprint (up to 1150°C and 8-12kbar)(Sajeev and Osanai 2004) detailed petrological and geochronological work in the Wanni Complex is missing. Only a few studies focus on the border area between the Wanni Complex and the Highland Complex (Kitano et al. 2018; Wanniarachchi and Akasaka 2016).

Large areas of the Wanni Complex are covered by biotite gneisses, mostly migmatic, partly with occurrences of arrested charnockites or displaying potassium metasomatism (Cooray 1994; Kröner et al. 2003). However, charnockitic gneisses, garnet bearing gneisses and in the southwestern part cordierite bearing gneisses and metapelites occur which can be used for obtaining the PTt history of this complex. PT conditions of the Wanni Complex obtained from garnet bearing rocks place the metamorphic overprint clearly into the granulite facies and partly into the UHT field. Compared to the Highland Complex, temperatures are somewhat lower at 800-1000°C at 7-9kbar.

LA-ICP-MS U/Pb dating was performed on zircons from different locations of the Wanni Complex and shows igneous protolith ages of 855-963Ma. The ages were obtained from felsic hornblende-biotite gneisses and charnockitic gneisses. The wide range of ages could be a result of resetting shortly after magmatic crystallisation. CL images of some zircons show dark zones separated from oscillatory zoned cores by thin bright fronts. Taken together with core/rim dating of these zircons, this could be a sign of transgressive recrystallization (Hoskin and Black 2000).

Cooray, P.G. 1994. Precambrian Research 66(1–4):3–18.

Hoskin, P.W. and Black L.P. 2000. Journal of Metamorphic Geology 18:423–39.

Kitano, I., Osanai, Y., Nakano, N., Adachi, T. and Fitzsimons, I.C.W. 2018. Journal of Asian Earth Sciences 156:122–44.

Kröner, A., Kehelpannala, K.V.W. and Hegner, E. 2003. Journal of Asian Earth Sciences 22(3):279–300.

Milisenda, C.C., Liew, T.C., Hofmann, A.W. and Köhler, H. 1994. Precambrian Research 66:95–110.

Sajeev, K. and Osanai, Y. 2004. Journal of Petrology 45(9):1821–44.

Wanniarachchi, D.N.S. and Akasaka, M. 2016. Journal of Mineralogical and Petrological Sciences 111(5):351–62.

How to cite: Masten, M., Hauzenberger, C. A., Lechner, N., Gallhofer, D., and Fernando, G. W. A. R.: Metamorphic evolution and geochronology of gneisses from the Wanni Complex, Sri Lanka, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-18403, https://doi.org/10.5194/egusphere-egu2020-18403, 2020.

South Delhi orogeny is constrained by correlating the deformational fabric with geochronology of the granites and metasediments around Beawar- Rupnagar-Babra, Rajasthan, NW India. The area consists of metaconglomerate, calcareous schist, mica schist and amphibolite. These were deformed by three stages of deformation(D_1-3) and intruded by four types of granite plutons (G_1-4). The D₁ deformation produced F₁, reclined/recumbent folds with S₁ axial planar fabric in greenschist facies metamorphic condition. The D₂ deformation produced NE-SW trending F₂ folds coaxial with F₁(type 3 interference pattern), crenulations and F₂-axial parallel ductile shear zones.  The D₃ deformation produced NW-SE F₃ folds, which superimposed on F₁ and F₂ to create type 1 and 2 interference pattern. Granites carry pervasive S₁ fabric. In G_1-3 granites, the S₁ is characterized by low temperature deformation fabric marked by bulging recrystallization of quartz. The G₄ granite (namely Sewariya granite) contains magmatic to submagmatic fabric and the S₁ fabric in it is a high temperature deformation fabric and lies parallel to magmatic fabric in the rock. Plagioclase is dynamically recrystallized by subgrain rotation and grain boundary migration and quartz shows chess board twinning. We interpret that the G₄ granite is syntectonic and G_1-3 were pre-tectonic to D₁ deformation.  U-Pb data (SHRIMP method) of G₁, G₂ and G₄ granites yield Concordia age calculated with ²⁰⁶Pb/²³⁸U and ²⁰⁷Pb/²³⁵U ratio at ~982 Ma, ~992 Ma and ~878 Ma respectively. Thus the South Delhi orogeny is constrained by the age of G₄ granite at ~ 878 Ma (~ 870 Ma).  The G_1-3 granites are pre- Delhi orogeny and probably constrain the age of rifting of the Delhi basin. EPMA Th-U-total Pb monazite geochronology of the garnet-staurolite-quartz-feldspar-biotite schist from the basal conglomerate zone shows three distinct ages, ca. 1611 Ma, 864 Ma and 718 Ma.  Correlating with granite SHRIMP age, the ~ 864 Ma corresponds to Delhi metamorphism and D₁ deformation (~ 870 Ma). The event ca. 1611 Ga probably belongs to pre-Delhi age, which is observed in nearby pre-Delhi localities like Sandmata terrane.

Keywords: Deformational fabric, geochronology, metaconglomerate, granite and geochronology.

How to cite: Singh, S. and Biswal, T. K.: 870 Ma age of South Delhi Orogeny: A study on Geochronology of the granites and the metasediments, NW India., EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3259, https://doi.org/10.5194/egusphere-egu2020-3259, 2020.

Ductile shear zones usually record mineralogical and isotopic changes that are not apparent in the surrounding host rocks and thus may preserve a complete evolutionary record in a single locale from relatively undeformed to highly deformed rocks. The Zhujiafang ductile shear zone is situated in the central Hengshan Complex, a key area for understanding the Paleoproterozoic tectonic evolution of the Trans-North China Orogen, North China Craton. Detailed metamorphic and geochronological analyses were carried out on metapelite and garnet amphibolite from the Zhujiafang ductile shear zone. The metapelite preserves two phases of mineral assemblages: early kyanite-rutile-bearing assemblage and late chlorite-staurolite-bearing assemblage in garnet–mica schist, and inclusion-type muscovite (high-Si) + kyanite assemblage and late sillimanite-bearing assemblage in sillimanite–mica gneiss. Garnet in the metapelite occasionally exhibits pronounced two-stage zoning characteristic of a diffusion core with irregular pyrope (X_py) and grossular (X_gr) contents and a growth rim with X_py and X_gr increasing outwards. The isopleths of the maximum X_gr in garnet core and Si content in inclusion-type muscovite in the P–T pseudosections suggest that the early mineral assemblages underwent medium-high-pressure type metamorphism with pressures up to 12–14 kbar at 700–750 °C. The late assemblages and the growth zoning of garnet rim predict a late separated clockwise P–T path with peak conditions of 6.5 ± 0.2 kbar/620 ± 10 °C (medium-low-pressure type). The garnet amphibolite is mainly composed of garnet, hornblende, plagioclase, ilmenite and quartz, without overprinting of late mineral assemblages except for localized corona textures. Phase modeling suggests that the rock has experienced high-amphibolite facies metamorphism with peak conditions of 10.5 ± 0.8 kbar/770 ± 50 °C, which is broadly consistent with the early-phase metamorphism of metapelite. Zircon U–Pb dating on metapelite yields two metamorphic age groups of 1.96–1.92 Ga and 1.87–1.86 Ga which are interpreted to represent the timing of the two separated phases of metamorphism. Two separated orogenic events may have occurred respectively at ~1.95 Ga and ~1.85 Ga in the Hengshan–Wutai area. The older orogeny was resulted from continental collision and the younger one may be caused by within-plate deformation. The final exhumation of the high-grade rocks formed in the older (i.e. 1.95 Ga) orogeny should be related with the younger deformation/metamorphic event. For more details, please refer to https://doi.org/10.1016/j.lithos.2019.02.001.

How to cite: Qian, J.: Two Paleoproterozoic metamorphic events in the Zhujiafang ductile shear zone of the Hengshan Complex: Insights into the tectonic evolution of the North China Craton, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-13282, https://doi.org/10.5194/egusphere-egu2020-13282, 2020.