GMPV7.2

Here we studied metapelites from the ultrahigh-pressure (UHP) Brossasco-Isasca unit in the Dora-Maira Massif, Western Alps, combining zircon-in-garnet elastic geo-thermobarometry and phase equilibria modelling. We determined the residual strain and pressure of zircon inclusions via micro-Raman spectroscopy and the dedicated softwares available online such as stRAinMAN [1] and EntraPT [2]. The entrapment isomekes obtained for 28 zircon inclusions in garnet from metapelites (Alm_67-79-Py_9-30-Grs_1-6-Sps_0-6) were combined with thermodynamic modelling to constrain the P-T range of garnet growth, assuming purely elastic behaviour.

The presence of chloritoid and/or staurolite inclusions at the garnet core-mantle and the presence of coesite inclusions only at the garnet rim suggest that most of the garnet volume formed during an early prograde path and only a small portion under UHP conditions. Most of the selected inclusions, however, come from the rim of the garnet. Since the rim is limpid, we could localize and target those inclusions that are spaced enough to be used reliably for elastic thermobarometry without corrections. The entrapment pressures obtained for most zircon inclusions do not match the previously published results obtained from conventional petrologic methods [3]. For example, combining our results with the available retrograde P-T paths of the UHP unit [3], we bracket the apparent entrapment conditions of zircon inclusions at 0.5 GPa and 600-650 °C, below the expected conditions in the coesite stability field. The same discrepancy between the elastic and chemical barometric methods has been documented for the pyrope-bearing whiteschists from the same metamorphic unit [4]. The observed misfit has been tentatively attributed to post-entrapment viscous relaxation of the garnet–zircon inclusion system, which cannot be accounted for by purely elastic models. These results provide further evidence of a general post-entrapment elastic resetting of the zircon-in-garnet pairs along the retrograde path at temperatures near 600-650°C.

This work was supported by ERC-StG TRUE DEPTHS (grant number 714936) to Matteo Alvaro. Nicola Campomenosi and Mattia L. Mazzucchelli are supported by the SIMP PhD Thesis Award and by the Alexander von Humboldt research fellowship. [1] Angel et al. (2019) Zeitschrift für Kristallographie, 234, 219. [2] Mazzucchelli et al. (2021) American Mineralogist, 106, 830. [3] Groppo et al. (2019) European Journal of Mineralogy, 31, 665. [4] Campomenosi et al. (2021) Contrib Mineral Petrol 176, 36.

How to cite: Mingardi, G., Campomenosi, N., Mazzucchelli, M. L., Chopin, C., Scambelluri, M., and Alvaro, M.: Elastic thermobarometry on Zircon-in-Garnet (ZiG) from the Brossasco-Isasca unit (Dora-Maira Massif, Western Alps), EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-186, https://doi.org/10.5194/egusphere-egu22-186, 2022.

Polymorphic transformations are key tracers of metamorphic processes, also used to estimate the pressure and temperature conditions reached by a rock. In particular, the quartz-coesite transition is commonly used to define the lower boundary of the ultrahigh-pressure (UHP) metamorphic field. The partial preservation of coesite included in garnets from UHP rocks bring considerable insights into the burial and exhumation mechanisms of the continental crust involved in convergent zone. Coesite was first described in the Western Alps by Chopin^[1], in the Dora-Maria whiteschist, one of the most emblematic UHP rock worldwide. Although the partial preservation of coesite inclusions in garnet has long been attributed to the pressure vessel effect, the interrelationship and relative timing between fracturing and retrogression is still contentious.

Here we study the reaction-deformation relationships of coesite inclusions initially enclosed in garnet and transforming into quartz during the decompression process. We combine 2D numerical thermo-mechanical models constrained by pressure-temperature-time (P-T-t) estimates from the Dora-Maira whiteschist. The model accounts for a compressible visco-elasto-plastic rheology including a pressure-density relationship of silica based on thermodynamic data. This allows us to study the effect of reaction-induced volume increase during decompression. Our results capture the typical fracture patterns of the host garnet radiating from retrogressed coesite inclusions and can be used to study the relative role of volume change associated with a change of P-T conditions on the style of deformation during decompression.

The mechanisms of the coesite-quartz transformation and geodynamic implications are presented and validated against geological data. The effect of fluids on the phase transition and the conditions of access of fluids during the transformation are discussed in the light of the results of the thermo-mechanical models.

This study demonstrates the high potential of thermo-mechanical modelling in enhancing our understanding of the processes involved in the formation and evolution of metamorphic minerals.

[1]Chopin (1984) Contributions to Mineralogy and Petrology 86, 2, 107-118

How to cite: Luisier, C., Duretz, T., Yamato, P., and Marquardt, J.: Decompression of host-inclusion systems in UHP rocks: insights from observations and models, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-7573, https://doi.org/10.5194/egusphere-egu22-7573, 2022.

The Western Gneiss Region in Norway is constituted by a crustal nappe stack that comprises some of the best-preserved exhumed ultra-high pressure (UHP) terranes on Earth. The UHP rocks result from the subduction of the western edge of the Baltica craton beneath Laurentia during the Caledonian orogeny. Mafic eclogites form lenses within granitoid orthogneisses and show the best record of the pressure and temperature evolution. Their exhumation from the UHP conditions has been largely studied, but the prograde evolution has been rarely quantified in the eclogites although it constitutes an important constraint on the tectonic history of this area. This study focused on an unaltered eclogite sample from Vågsøy in the Nordfjord region. This sample was investigated using a large panel of methods including phase-equilibria modelling, trace-element analyses of garnet, trace- and major-element thermo-barometry and quartz-in-garnet barometry by Raman spectrometry. The eclogite comprises omphacite, garnet, white mica, epidote and amphibole and accessory rutile, quartz, zircon, carbonates and kyanite. Garnet shows a grossular-rich core with inclusions of quartz, epidote, white mica and amphibole, while grossular-poor rims are enriched in pyrope and middle rare-earth elements and include omphacite and rutile. Inclusions in garnet core point to crystallisation conditions in the amphibolite facies at 550–600 °C and 11–15 kbar, while chemical zoning in garnet suggests growth during isothermal compression up to the peak pressure of 28 kbar at 600 °C, followed by near-isobaric heating to 640–680 °C. Isothermal decompression to 8–13 kbar is recorded in fine-grained clinopyroxene-amphibole-plagioclase symplectites. The absence of a temperature increase during compression seems incompatible with the classic view of crystallization along a geothermal gradient in a subduction zone and may question the tectonic significance of eclogite-facies metamorphism. Two main tectonic scenarios are discussed to explain such an isothermal compression: (1) either the mafic rocks were originally at deep level within the lower crust and were then buried along the isothermal part of the subducting slab, or (2) the mafic rocks recorded significant tectonic overpressure at constant depth and temperature conditions during the collisional stage of the orogeny. A multi-chronometer geochronological study is currently performed and expected to bring additional, discriminant constraints on this P–T evolution. 

How to cite: Simon, M., Pitra, P., Yamato, P., and Poujol, M.: Isothermal compression of an eclogite from the Western Gneiss Region (Norway): a multi-method study, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-11750, https://doi.org/10.5194/egusphere-egu22-11750, 2022.

We report calcic-ferroan-metaluminous garnetiferous magmatic charnockites that are extremely rare in nature and hence interesting to study. The garnetiferous porphyritic granite pluton of the Tilaboni area of Chhotanagpur Gneissic Complex of Eastern Indian shield contains older enclaves of enderbite-charnoenderbite-charnockite (charnockitic suite). Garnetiferous metagabbro are spatially associated with charnockitic rocks. Plagioclase, K-feldspar, quartz, ortho-, and clinopyroxene, garnet, biotite ± amphibole, ilmenite ± magnetite are major mafic phases. Biotite is sub-alkaline to alkaline. Plagioclase compositions vary from andesine to oligoclase. Garnet is rich in almandine (70.28–74.04 mol%) and grossular (17.77–21.41 mol%) but contains low pyrope (2.83–7.67 mol%) and spessartine (4.09–4.59 mol%). Amphibole formed through the hydration of hypersthene, clinopyroxene, and garnet.

Garnet-clinopyroxene and orthopyroxene-clinopyroxene geothermometry and garnet-orthopyroxene-plagioclase-quartz geobarometry give granulite-facies (750-850°C; 7.5-8.0 kb) of metamorphism of the charnockitic rocks. Amphibole-plagioclase thermobarometry yields temperature and pressure (733−795 °C; 5−6 kbar) that suggest amphibolization of the mafic minerals at a relatively shallower level. Pseudosection modeling shows that the garnets and orthopyroxene finally equilibrated at around 560°C temperature and 5.8 kb.

Primary ilmenite and high Fe/(Fe+Mg) ratios of amphibole-biotite indicate these charnockites metamorphosed under reduced conditions (ΔNNO −2).

These charnockites are dominantly calcic and ferroan to slightly magnesian (Fe-number: 0.74–0.97); dominantly metaluminous to weakly peraluminous (A/CNK: 0.84–1.08); high- and medium-K calc-alkaline and shoshonite series.

These exhibit moderate variations of Al2O3 (12.44–18.19 wt.%), K2O (1.16–5.7 wt.%), and CaO (1.01–5.72 wt.%) contents. Na2O (3.71–3.89 wt.%) show a slight variation in concentration. Abundances of Fe2O3(total) (2.45–7.88 wt.%) and TiO2 (0.21–1.11 wt.%) are generally moderate, whereas the concentration of MgO (0.08–1.99 wt.%) remains low.

These rocks show enrichments of the Rb, Ba, Th, K, Zr, and Hf but depletion in Nb, Ta, and Ti relative to the primitive-mantle composition. They also show strong depletions in Sr and P, whereas enrichment in Pb. LaN/SmN (2.68–12.95) and GdN/YbN ratios (1.57–2.89) of these rocks are high. Five of the six samples show negative Eu-anomalies (0.29–0.91), one sample shows pronounced positive Eu-anomaly (3.09).

These rocks exhibit similar multicationic trace-element and REE patterns and a nearly collinear array of sample plots in Harker diagrams. Further, these samples follow a calcic to alkali-calcic trend in SiO2 vs. MALI diagram. These factors are the result of magmatic differentiation. Decreases in CaO and Fe2O3t with increasing SiO2 but increasing agpaitic index with increasing silica alkalis are due to fractional crystallization from a common parental magma. Decreasing modal plagioclase following the calc-alkaline trend also supports magma differentiation. High Nb/U (av. 22.48) and Ce/Pb (av. 12.64) ratios but low Th/U (average 7.76) ratios suggest mantle source of the magma parental to these charnockites.

Their ferroan and reduced characters resulted from intense fractionation of early-formed allanite, magnetite, etc. Geochemical modeling shows the calcic charnockites evolved by fractionation of garnet and clinopyroxene from basaltic magma derived from a depleted mantle.

How to cite: Goswami, B., Das, S., Basak, A., Bhattacharyya, C., and Goswami, C.: Petrology, geochemistry, and petrogenesis of calcic-ferroan-metaluminous garnetiferous magmatic charnockites from eastern Chhotanagpur Gneissic Complex, Eastern Indian Craton, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-6214, https://doi.org/10.5194/egusphere-egu22-6214, 2022.

In a combined geological, petrological and isotopic study from the Lofoten-Vesterålen Complex, Norway, graphite is documented formed in the deep Proterozoic crust. Graphite schist is hosted in sequences of banded gneisses dominated by orthopyroxene-bearing quartzofeldspatic gneiss, interlayered with horizons of marble, calcsilicates and amphibolite. The schist displays a strong foliation and has a major content of graphite up to a modality of 39%. Quartz and plagioclase (Ab_47-93An_5-52), pyroxenes, biotite (Mg# = 0.67-0.91; Ti < 0.66 a.p.f.u.), and K-feldspar (Ab_1-8Kfs_92-99) or perthite (Ab_35-64An₃Kfs_50-62) are additional major phases. Pyroxene is present either as orthopyroxene (En_69-74Fs_26-29; Mg#=0.70-0.74), as clinopyroxene (En_33-53Fs_1-14Wo_44-53; Mg#=0.70-0.97), or both. Pseudosection modeling of the plagioclase + orthopyroxene (Mg#-ratio = 0.74) + biotite + quartz + rutile + ilmenite + graphite-assemblage constrains its stability field to pressure-temperature conditions of 810-835 °C and 0.73-0.77 GPa. Zr-in-rutile also supports a temperature of formation of 740-870°C.

Stable isotopic δ¹³C in graphite schist shows values from -38 to -17‰ while δ¹³C values of marbles range from +3‰ to +10‰. Mixed graphitic and calcite carbon samples give lighter values for the calcite (δ¹³C_calcite = -8.65‰ to -9.52‰) and heavier values for graphite (δ¹³C_grapite = -11.50‰ to -8.88‰) compared to the “pure” samples. δ¹⁸O for marble shows relatively light values for calcite ranging from -15.44‰ to -7.53‰ reflecting metamorphic and hydrothermal processes. From the stable C-isotopes we interpret the graphite origin as organic carbon accumulated in sediments contemporaneous with the Early Proterozoic global Lomagundi-Jatuli isotopic excursion.

From petrography and mineral composition, we deduce the reaction equations producing and consuming H₂O- and CO₂-fluids leading to the stabilisation of graphite and orthopyroxene. The high Mg#-ratio of biotite and pyroxenes is an indication of metasomatism, and together with a high Cl-content of apatite up to 2 a.p.f.u. show the importance of fluids during the high-grade formation of graphite.

The enrichment of graphite resulted in zones with strong schistosity and a sharp strain gradient towards host massive granulite gneiss; High-ordered graphite occurs as euhedral “flakes” (i.e., flake graphite) of fine- to medium grain size, with a strong preferred crystal orientation forming the well-developed foliation together with the crystal preferred orientation of biotite. The presence of graphite reduces crustal strength and causes strain localisation in the granulite facies crust.

How to cite: Engvik, A. K., Gautneb, H., Mørkved, P. T., Knezevic, J., Erambert, M., and Austrheim, H.: Graphite in granulite - characterization, origin, role of fluids and consequences for rheology, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-1633, https://doi.org/10.5194/egusphere-egu22-1633, 2022.

Corona microstructures comprised of garnet (grt) and clinopyroxene (cpx) were observed at the contacts between plagioclase (pl) and Fe-rich orthopyroxene (opx) in meta-gabbroic rocks in a several 100 m sized (ultra-)mafic lens embedded in felsic granulite of the Gföhl unit (Moldanubian zone, Lower Austria).

The corona microstructures are formed around monomineralic aggregates of opx and they are comprised of two layers, an inner about 100 μm thick
layer of polycrystalline cpx and an outer, about 800 μm thick layer of polycrystalline garnet. The corona structures are surrounded by the pl-rich rock matrix. The cpx layer shows a weak but systematic chemical zoning characterized by increasing Mg and decreasing Na and Al contents from the contact with grt towards the contact with opx. The grt layer shows a pronounced and complex chemical zoning. There is a consistent trend of decreasing Mg and increasing Ca contents from the contact with the cpx layer, where the composition is Alm₂₂ Prp₆₇ Grs₁₁ towards the contact with the rock matrix, where we observe Alm₂₅ Prp₄₈ Grs₂₈. This pattern is interpreted as a primary growth zoning. Superimposed on the growth zoning there is a secondary zoning, which is evident from a decrease of the Ca content and a concomitant increase of the Mg content from the interior of the individual grains
of the grt polycrystal forming the grt layer towards the grt grain boundaries. The secondary zoning is most pronounced in the outermost portions of the garnet layer, where the primary growth zoning shows the highest Ca and the lowest Mg contents. Locally the garnet grains contain abundant primary melt inclusions. In most segments of the corona, secondary opx and pl form layers along the contact between the primary cpx and grt layer, where the opx partially replaces the cpx layer and the pl partially replaces grt. The secondary opx has higher Mg and lower Na, Al, and Ca contents than the opx
in the core of the corona structure. The secondary pl has the same composition as the matrix pl. At its outer edge, the garnet layer is locally replaced by spinel bearing cpx-pl symplectites. The primary compositional zoning of the garnet layer could be reproduced in equilibrium assemblage diagrams (pseudosections). Calculated equilibrium phase relations indicate that the grt-cpx corona formed at the contacts between opx and pl at supersolidus HP − HT conditions of P > 1.8 GPa and T > 900 °C and low H₂O content. Growth of coronal grt and cpx requires the diffusive transport of Fe and Mg from the opx to the pl and concomitant transport of Ca and Al in the opposite direction. The secondary zoning of garnet, the back reaction forming secondary opx and pl at the contact between the primary grt and cpx layer and the spinel bearing pl-cpx symplectites locally replacing garnet at the outer edge of the grt layer are related to different decompression stages. Preservation of the secondary garnet zoning indicates relatively rapid cooling during late
stages of or immediately after decompression.

How to cite: Asenbaum, R., Portenkirchner, J., Racek, M., Petrishcheva, E., and Abart, R.: Formation of garnet-clinopyroxene coronas at orthopyroxene–plagioclase contacts during high-pressure granulite facies metamorphism, Gföhl unit, Moldanubian zone, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-11826, https://doi.org/10.5194/egusphere-egu22-11826, 2022.

Compared to the well-studied upper continental crust, the composition of the lower crust is much more poorly constrained. Geophysical constraints and geochemical data from granulite xenoliths indicate that the lower crust is, on average, mafic and depleted in most incompatible elements, including the heat-producing elements (HPE). However, the extent of this depletion is not well known. The large uncertainties associated with lower crustal estimates have important implications for the Earth’s evolution, as the lower crust is often proposed to be a “hidden reservoir” (e.g., for unradiogenic Pb) needed to close mass balance discrepancies for the Bulk Silicate Earth.

In this study, we analysed granulite xenoliths from Queensland, eastern Australia, and the Kola Peninsula, northwest Russia, using a reconstitution approach that corrects for host magma contamination. This method also provides detailed insight into which minerals control elemental distribution and concentrations of the xenoliths. The major element compositions of both suites of granulite xenoliths highlight their mafic nature, with SiO₂ contents similar to previously published estimates. However, the concentrations of the most incompatible elements, including the large ion lithophile elements (LILE) and HPE, are very low. Some elements are more depleted by an order of magnitude than the most popular composites used in the literature. Zircon and monazite are rare in these mafic granulites, while apatite and rutile have relatively low Th and U concentrations. The absence of hydrous silicates (e.g., mica and amphibole) and the relatively high anorthite contents of feldspar in the xenoliths is a controlling factor in the low LILE concentrations, particularly for Rb and Cs. If this composition is representative of typical lower continental crust, then such highly refractory compositions limit the ability of the lower crust to act as a significant contributor for planetary mass balance considerations because it does not contain enough Pb, Nb, Ta, Cs and Rb to balance other inventories of the differentiated bulk silicate Earth.

How to cite: Emo, R., Kamber, B., Downes, H., and Caulfield, J.: A new compositional estimate for refractory lower continental crust, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-8700, https://doi.org/10.5194/egusphere-egu22-8700, 2022.

Precambrian high-pressure (HP) granulites can provide crucial information for reconstructing ancient continental nuclei. Here we report the pelitic granulites from Qingyuan terrane, eastern North China Craton (NCC), which are archean supracrustal rocks occurred as enclaves in gneisses. Two samples from the pelitic granulites both record clockwise P-T paths involving prograde stage (M₁), peak stage (M₂) and post-peak stage (M₃). Prograde stage is represented by biotite, plagioclase, quartz, rutile and ilmenite, preserved as mineral inclusions whthin garnet porphyroblasts, formed at P-T conditions of 8-9 kbar/670-700 ℃ constrained by mineral assemblages within garnet porphyroblasts and Ti-in-quartz geothermometer. The peak stage (M₂) can be represented by the garnet cores, matrix rutile, kyanite, K-feldspar and the P-T conditions are constrained to be ~12 kbar/800-820 ℃ by the isopleths of X_Py and X_Grs from the core of garnet grains. The followed post-peak stage (M₃) can be represented by matrix minerals assemblages including garnet, biotite, K-feldspar, sillimanite, ilmenite, quartz and plagioclase, revealing isothermal decompression process to ~9 kbar constrained by the isopleths of X_Py and X_Grs from inner rims of garnet grains. Monazite age dating suggests that the pelitic granulites possibly reached the peak metamorphic stage at ~2.47 Ga, slightly later than TTG magmatic events. The clockwise P-T paths including sequential isothermal decompression (ITD) segments recorded by the pelitic granulites may be caused by a subduction-collision event during the late Neoarchean in the eastern NCC.

How to cite: Liu, G., Lu, J., Kong, X., Feng, Q., Li, Y., and Zhang, Y.: Metamorphic P-T-t paths of Neoarchean pelitic granulites from the Qingyuan terrane, eastern North China Craton, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-9177, https://doi.org/10.5194/egusphere-egu22-9177, 2022.

A prominent natural laboratory to deduce the interplay of seismic and aseismic deformation in the lower continental crust is exposed on the Lofoten archipelago (northern Norway). A key feature to unravel its tectonic history is the ~600 m thick Ramberg-Flakstad shear zone (RFS) that is interpreted as a retrogressed eclogite-facies shear zone. However, the rest of the lower crustal section preserves evidence of cyclicity between seismic rupture (pseudotachylytes) and viscous shear at amphibolite-facies conditions, while the record of high-pressure deformation and metamorphism is less clearly preserved. The RFS is thus a key structure to understand the subduction-exhumation history of the Lofoten crustal section, providing insight into the localization of metamorphism and strain during orogenesis. Here we report field observations combined with mineral chemical, microstructural, and textural observations of this long-lived multistage shear zone. The shear zone is heterogeneous with the main foliation wrapping around weakly to non-foliated blocks. These blocks are dissected by millimeter to centimeter-thick shear zones. The RFS is hosted by Paleoproterozoic gabbroic rocks that were intruded by anorthositic and charnockitic plutons at ~1.8 Ga. Granulite-facies metamorphism, indicated by the crystallization of garnet, recrystallization of orthopyroxene, and a locally preserved migmatitic fabric is likely related to pluton emplacement. Later eclogite-facies metamorphism (age disputed) is evidenced by inclusions of omphacitic clinopyroxene in garnet and clinopyroxene + plagioclase symplectites after omphacite within the main foliation. Inclusion distributions in garnet are patchy and electron backscatter diffraction (EBSD) analysis reveals that individual garnet grains can be divided into multiple domains, indicating various growth phases. The main foliation is dominantly formed by the preferred orientation of amphibole and plagioclase, consistent with amphibolite-facies P-T conditions reported from shear zones and pseudotachylytes elsewhere in Lofoten. The symplectites after omphacite are aligned with this main foliation but internally preserve a vermicular microstructure indicating that retrogression actually occurred statically after alignment. Additionally, plagioclase within the symplectites is more albitic than in the matrix, precluding that significant element redistribution occurred during or after retrogression. Lastly, the main fabric is crosscut by undeformed (to locally weakly folded) pegmatite dykes of Caledonian age which provides a lower age boundary on RFS deformation at ~413 Ma. These observations indicate that the RFS is long-lived (~1.4 Ga), established during Proterozoic granulite-facies metamorphism and repeatedly exploited as a site of metamorphism at varying P-T conditions, hydration/dehydration reactions, and deformation. Key minerals and mineral assemblages reveal these modifications through a history of stable lower continental crust, subduction, and exhumation.

How to cite: Zertani, S., Menegon, L., Pennacchioni, G., Corfu, F., and Jamtveit, B.: Repeated metamorphism and deformation localized in a shear zone recording the formation-subduction-exhumation history of the continental crust, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-7532, https://doi.org/10.5194/egusphere-egu22-7532, 2022.

The tectonometamorphic evolution of the Kalak Nappe Complex in the northernmost Scandinavian Caledonides is currently uncertain; at least two pre-Caledonian events have been locally recognised within the complex, as well as Caledonian events. To help clarify the evolution of the complex, we document here the P-T-t paths of garnet growth, which represent the peak metamorphic conditions within this relatively unstudied external part of the complex.

Metamorphic P-T paths for the lower part of the Kalak Nappe Complex were obtained using the THERIA_G model of Gaidies et al. (CMP 2008). In the model, equilibrium in the MnNCKFMASHT system was established across the entire rock-volume during prograde metamorphism, except for garnet, which developed growth zoning preserved at levels controlled by the kinetics of intracrystalline diffusion. The mass and composition of material used in successive increments of garnet growth is cumulatively subtracted from the matrix bulk-rock composition before calculating the P-T conditions of the next increment of garnet growth.

There is some latitude with regards to the absolute metamorphic conditions determined using this model, due to the inherent uncertainty of the thermodynamic data and the approximation of the reactive volume composition. However, the slopes of the determined P-T paths, together with lithological, geochemical and Lu-Hf garnet whole-rock isotopic data and garnet crystal size frequency distributions, enabled the identification of three nappes in the study area; from lowest upwards, the Bekkarfjord, Veidnes and Kolvik nappes.

An early, low-pressure Barrovian-type metamorphic event at ∼464 Ma is preserved in the Veidnes Nappe, where garnet cores (Grt 1V) give a P-T gradient of ∼15 bar/°C, with peak conditions of ∼560 °C and 4.5 kbar. That was followed by moderate-pressure metamorphism in the Bekkarfjord Nappe at ∼423 Ma, resulting in garnet crystallization (Grt 1B, core growth) along a gradient of ∼20 bar/°C, with peak conditions of ∼570 °C and 6.0 kbar. All three nappes then experienced Barrovian-type metamorphism at ∼420 Ma on a steep P-T gradient of ∼40 bar/°C, with peak conditions of ∼560 °C and 6.7 kbar in the Bekkarfjord and Veidnes nappes (Grt 2B, V, rim growth), while the overlying Kolvik Nappe was metamorphosed at peak conditions of ∼590 °C and 7.5 kbar (Grt 1K, core growth). We consider the latter two episodes (423, 420 Ma) to be different stages of the Scandian phase of the Caledonian Orogeny.

The juxtaposition of the three nappes, with the youngest event having occurred in the structurally highest unit and the oldest event now being sandwiched between the two younger events indicates out-of-sequence thrusting associated with the final continent-continent collision. This has been modeled in “balanced” cross-sections of the ductile thrusting.

How to cite: Rice, A. H. N., Gaidies, F., Heldwein, O. K. A., Yogi, M. T. A. G., Cutts, J. A., and Smit, M. A.: Using P-T-t estimates to identify and restore out-of-sequence thrusting in the lower part of the Kalak Nappe Complex (Nordkinnhalvøya, Sværholthalvøya), internal Scandinavian Caledonides, Finnmark, N. Norway, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-6127, https://doi.org/10.5194/egusphere-egu22-6127, 2022.

The Sanandaj-Sirjan zone (SaSZ) on the northern edge of the Arabia-Eurasia suture in Iran includes a significant high-pressure (HP) metamorphic suite exposed along the upper Zayanderud River north of Shahrekord. Phengitic micas from eclogite in the Zayanderud metamorphic complex (ZMC) yielded ⁴⁰Ar/³⁹Ar dates ranging from 184 to 173 Ma ^[1], whereas zircon from an associated anatectic pegmatite gave an average U-Pb age of 176 ± 3 Ma ^[2]. These data are consistent with a subduction channel metamorphism and rapid exhumation during the Early to Middle Jurassic. To constrain the timing of high-pressure conditions, we have conducted Lu-Hf mineral-whole rock dating on two eclogite samples. The resulting garnet-controlled isochron dates of 171.4 ± 0.4 (MSWD = 1.2) and 175 ± 1 (MSWD = 0.43) Ma have important geodynamic implications as the Jurassic initiation of the Neotethyan subduction in Iran has recently been disputed ^[3][4]. The metamorphic ages of the ZMC eclogite now leave no doubt that subduction was ongoing along the SaSZ peri-Tethyan margin during the Middle Jurassic.

[1] Davoudian et al., 2016 Gondwana Research 37: 216-240; [2] Jamali Ashtiani et al., 2020 Gondwana Research 82: 354-366; [3] Azizi & Stern, 2019 Terra Nova 31: 415-423; [4] Lechmann et al., 2018 Contrib. Mineral. Petrol. 173 (12): 102

How to cite: Jamaliashtiani, R., Scherer, E., K. Schmitt, A., and Hassanzadeh, J.: Lu-Hf dating of Jurassic eclogites of the Zagros hinterland, Iran: Implications for the timing of Neotethyan subduction initiation, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-4832, https://doi.org/10.5194/egusphere-egu22-4832, 2022.

The Lepontine Dome is a structural and metamorphic dome formed by crystalline basement nappes belonging to the Penninic domain of the European Alps (Switzerland). The mineral-zone boundaries of the Barrovian Tertiary metamorphism show an asymmetric concentric zonation not coinciding with the dome shape defined by the regional attitudes of foliation and thrust sheets. The related Barrovian isogrades locally dissect the tectonic nappe contacts suggesting a post-thrusting thermal event. However, the extremely pervasive and NW-SE directed mineral and stretching lineation, also developed during the upper amphibolite facies metamorphism, suggests non-coaxial deformation during thrusting at peak metamorphic conditions. This apparent paradox may be explained with several geodynamic scenarios that are still debated by the scientific community. One crucial element helping to evaluate the different scenarios is the timing of the upper amphibolitic, non-coaxial deformation along the tectonic contacts, which is still poorly constrained. Hence, the goal of our work is to date this deformation with a multidisciplinary approach that aims to solve the relation between the geologic structures and the distribution of heat in the nappe pile.

In the studied domain, the lower unit (the Simano nappe) is formed by metagranitoids and by minor paragneiss. The upper thrusted unit (the Cima Lunga/Adula nappe) is made of metasediments, mainly quartz-rich gneiss intercalated with amphibole-gneiss, peridotitic lenses and, locally, calcschist and/or marble. The alternation of lithotypes is mostly parallel to the nappe boundary, and constant over its kilometer-scale length. Below the Cima Lunga/Adula, the transition to the Simano nappe is marked by a progressive change in gneiss texture: more stretched towards the top of the sequence, indicating a strain increase. Migmatitic leucogneisses have been found parallel to the tectonic contacts. Field observations indicate that their deformation is syn-tectonic, hence suggesting partial melting conditions during nappe emplacement. Their foliation is locally crosscut by granitic dikes of aplitic and pegmatitic texture.

To define the temporal duration of melting, U-Pb zircon dating with LA-ICP-MS (Laser Ablation Inductively Coupled Plasma Mass Spectrometry) has been performed on migmatites, paragneiss, gneiss, and granitic dikes. The results show two main groups of (metamorphic) ages centring at ca. 31 and 22 Ma. The younger ages date the intrusion of the post-tectonic dikes found exclusively in the southernmost area, proximal to the roots of the Lepontine nappes, likely related to the melt production along the Southern Steep Belt which lasted until ca. 22 Ma (according to U-Pb zircon dating by other authors). Ages indicating ca. 31 Ma are widespread from north to south, representing the nappe emplacement stage, coeval with migmatization.

Our results suggest the existence of two main heat sources: one related to thrusting and the other to fluid advection and/or diffusion of heat from the bottom along the Southern Steep Belt. Which heat source is responsible for the regional Barrovian metamorphism remains unclear. Our future studies will focus on the comprehension of the mechanisms of heat transfer and the relative roles of diffusion, advection and production to understand how these events are responsible for the net Barrovian heat budget of the Lepontine Dome.

How to cite: Tagliaferri, A., Schenker, F. L., Schmalholz, S. M., Ulianov, A., and Seno, S.: LA-ICP-MS U-Pb dating on zircons from the Lepontine Dome (Central European Alps), EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-9282, https://doi.org/10.5194/egusphere-egu22-9282, 2022.

In low-grade metamorphic units, precise thermobarometric and geochronologic data are often ambiguous or entirely lacking, thus complicating the temporal interpretation of metamorphism and hampering the identification of complex polymetamorphic histories. We present new P-T-t-D data from samples collected in two Austroalpine nappes exposed in the Eastern Alps, Austria: the structurally upper greenschist-facies Schöckel Nappe (“Graz Paleozoic,” Drauzug-Gurktal Nappe System) and the structurally lower amphibolite-facies Waxenegg Nappe (Koralpe-Wölz Nappe System). Although polymetamorphism was previously inferred from garnet zonation indicating multiphase growth in the Waxenegg Nappe, the timing of metamorphism is poorly resolved and only limited geochronology exists in the Schöckel Nappe.

Detailed petrographic investigations revealed that the chloritoid-bearing phyllite and micaschist of the Schöckel Nappe contain allanite that occasionally show partial replacement by small (<10 µm) monazite and thorite. Large (up to 500 µm) monazite exhibiting distinct core-rim chemical zoning were observed in the garnet-bearing micaschist of the Waxenegg Nappe. Careful documentation of the microstructural phase relations, thermodynamic modeling in the MnCNKFMASHT system, Raman spectroscopy of carbonaceous matter and in-situ LA-ICPMS U-(Th)-Pb dating of the accessory phases allow us to reconstruct a first metamorphic imprint at ~560°C and 4 kbar in the Waxenegg Nappe at c. 270 Ma (Permian event). Overprinting occurred at ~540°C and 8-10 kbar at c. 90 Ma (Eo-Alpine event). In the Schöckel Nappe, peak metamorphic conditions of ~470°C and 3-4 kbar existed during the Permian event at c. 260 Ma and the Eo-Alpine event in the upper part of the nappe did not exceed lower to middle greenschist-facies conditions.

Our results provide unequivocal evidence for Permian metamorphism in the Schöckel Nappe, which was hitherto unknown in this part of the Austroalpine Unit. Moreover, it demonstrates that the main metamorphic signature in this unit occurred during the Permian event and that the Eo-Alpine overprint is relatively lower grade than previously proposed. Combined with the data from the Waxenegg Nappe, there is an obvious marked increase in the Eo-Alpine peak conditions of ~130°C and 5 kbar across the nappe contact with higher grade in the footwall compared to the hanging wall. This is consistent with the existence of a major normal fault between the Drauzug-Gurktal Nappe System and the Koralpe-Wölz Nappe System in the easternmost part of the Austroalpine Unit, as already identified in its central and western parts. Modern thermobarometric analytical approaches coupled with high spatial resolution geochronology on accessory minerals is allowing a more thorough assessment of the subtle metamorphic histories recorded in the fundamentally important low-grade units of orogens.

How to cite: Hollinetz, M. S., Huet, B., Schneider, D. A., McFarlane, C. R. M., and Grasemann, B.: Coupling pressure-temperature and time constraints in greenschist- and amphibolite-facies polymetamorphic rocks: a case study from the Austroalpine Unit (Eastern Alps, Austria), EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-9763, https://doi.org/10.5194/egusphere-egu22-9763, 2022.

Zircon (U-Th)/He (ZHe) dating has seen rapid growth and widespread application among low-temperature thermochronological methods. Complex diffusion kinetics, primarily due to radiation damage density, may substantially influence the diffusivity of He and cause a wide temperature range from ca. 220 to <25 °C for the transition from an open to a closed system. Complexities may augment for (meta-)sedimentary rock samples containing minerals of different initial ages with highly variable uranium content leading to differences in accumulated radiation damage and thus annealing behaviours. In such cases, individual grains may only share their postdepositional thermal path. Current diffusion models predict inheritance to play a role for those samples that remained at diagenetic temperatures below 200 °C during burial.

In this contribution, we address the question whether ZHe dates from anchizonal to very low-grade metamorphic units may be transformed into geologically meaningful age information and as such may enhance thermal history reconstructions. We applied ZHe dating on 37 samples from Austroalpine and Southalpine basement-cover series adjacent to the eastern part of the Periadriatic fault line. In an attempt to quantify maximum thermal overprint during Alpine burial we compiled evidence from paleothermal indicators (e.g. vitrinite reflectance, illite crystallinity, CM Raman spectroscopy), geological field observations, and geochronological dates. These data suggest overprint at diagenetic conditions up to low-grade metamorphism in our study area. According to current ZHe diffusion models anchizonal and higher thermal conditions should have harmonized the samples’ age response and thus should have reset the ZHe system leading to concordant Alpine ages.

However, our new thermochronological dataset is characterized by a large variability in intra- and intersample age dispersion. Most of our single grain ages ranging from 12 to 305 Ma are much older than predicted by forward modeling. Such mismatch may be explained either by an underestimate of He retention resulting from a still incomplete understanding of He diffusivity. In this scenario, metasedimentary samples with an overprint up to lower anchizonal conditions (≤270°C) are likely to preserve inherited detrital information and cooling ages will reflect both the previous and most recent thermal histories. Alternatively geothermal data compiled from the literature may have overestimated peak temperatures reached during Alpine burial.

Both alternatives will be discussed in detail as they bring up challenging methodical issues. We underline the need for combining thermal maturity studies with ZHe low-temperature thermochronology in order to extract thermal history information for such complex detrital datasets.

How to cite: Heberer, B., Dunkl, I., Neubauer, F., Schulz, S., Guenthner, W., Pomella, H., and von Eynatten, H.: "Too old" zircon (U-Th)/He ages in Austro- and Southalpine units of the European Alps: an overestimate of temperature or an underestimate of helium retention?, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-9332, https://doi.org/10.5194/egusphere-egu22-9332, 2022.

As an important component of the lower crust, mafic granulites can provide a great deal of information about orogens’ metamorphic and tectonic evolution, and thus are studied extensively. According to the previous studies, the Wuhe Complex experienced the Late Paleoproterozoic metamorphic event. Here, we report the newly discovered Mesozoic metamorphic titanite age from the mafic granulites of Wuhe Complex and provide some clues to the Mesozoic metamorphic event of the southeastern NCC. Mafic granulite (sample 20BB44) is composed of garnet (12–15 vol.%), clinopyroxene (30–35 vol.%), hornblende (3–6 vol.%), plagioclase (40–50 vol.%), and quartz (1–2 vol.%) with minor ilmenite, pyrite, apatite, zircon, and titanite. Titanite grains are subhedral, euhedral, or homogeneous with grain sizes of 50–300 μm, and have inclusion minerals of hornblende, plagioclase, quartz, and ilmenite. Titanites have variable contents of U (1.0–17.2 ppm), Th (0.2–29.6 ppm), Pb (2.2–6.5 ppm), and Zr (20–259 ppm) with Th/U ratios of 0.07–4.53. According to the Zr-in-Titanite thermometer (Hayden et al., 2008: the estimated pressure was assumed as 0.5 GPa, and the activity of SiO2 (αSiO2) and TiO2 (αTiO2) were assumed as 1 and 0.8, respectively), the titanites may form at the temperature of 607–725 ℃ (689 ℃ on average). Thirty analysis spots on 29 titanite grains yield a lower intercept U–Pb age of 163 ± 28 Ma (MSWD = 1.17). Titanite U–Pb age of 163 Ma may represent the Mesozoic metamorphic event of southeastern NCC and may relate to the subduction of the Paleo-Pacific plate.

How to cite: Kong, X., Lu, J., Liu, G., Feng, Q., Li, Y., and Zhang, Y.: Mesozoic titanite U–Pb age from mafic granulites of the Wuhe Complex, southeastern North China Craton (NCC), EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-9119, https://doi.org/10.5194/egusphere-egu22-9119, 2022.