GMPV7.2

Metamorphic minerals as windows into a dynamic lithosphere

Metamorphic minerals provide unique records of the tectonic processes that have shaped Earth through the ages. Innovative new approaches in metamorphic petrology, chemical and isotope micro-analysis, and geochronology provide exciting new avenues to let these minerals tell their story of deformation, reaction and fluid flow. The insights from such research provide key means of testing long-standing concepts in petrology and tectonics, and shifting paradigms in these fields.

This session will highlight integrated metamorphic petrology, with application to tectonics and development of collisional orogens, cratons and subduction zones. We welcome contributions, from petrology, (petro-)chronology, to trace-element and isotope geochemistry. Through these diverse insights, the session will provide an exciting overview of current research on metamorphic and metasomatic processes, as well as the avenues for future innovation.

Co-organized by GD6/TS10
Convener: Matthijs Smit | Co-conveners: Daniela Rubatto, Lucie Tajcmanova, Tom Raimondo
Presentations
| Wed, 25 May, 08:30–11:47 (CEST)
 
Room D2

Presentations: Wed, 25 May | Room D2

Chairpersons: Daniela Rubatto, Matthijs Smit
08:30–08:33
08:33–08:40
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EGU22-186
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ECS
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On-site presentation
Giulia Mingardi, Nicola Campomenosi, Mattia Luca Mazzucchelli, Christian Chopin, Marco Scambelluri, and Matteo Alvaro

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 (Alm67-79-Py9-30-Grs1-6-Sps0-6) 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.

08:40–08:50
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EGU22-7573
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ECS
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solicited
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On-site presentation
Cindy Luisier, Thibault Duretz, Philippe Yamato, and Julien Marquardt

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.

08:50–08:57
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EGU22-11750
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ECS
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Virtual presentation
Martin Simon, Pavel Pitra, Philippe Yamato, and Marc Poujol

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.

08:57–09:04
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EGU22-8678
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On-site presentation
Giovanni Toffol, Giorgio Pennacchioni, Alfredo Camacho, and Neil Mancktelow

The Woodroffe Thrust (WT) in the Musgrave Ranges (central Australia) is a shallowly south-dipping crustal-scale mylonitic zone extending E-W for over 600 km. The WT, developed during the intracontinental Petermann Orogeny (630-520 Ma), placed hanging wall lower-crustal granulite to upper-amphibolite facies rocks of the Fregon Subdomain (FS) over footwall amphibolite-facies mid-crustal gneisses and granitoids of the Mulga Park Subdomain (MPS). The WT mylonites largely affect the MPS and to a minor extent the FS. Towards the WT, the hanging wall hosts the largest volumes of supposedly deep-seated, tectonic pseudotachylytes (pst) worldwide, also partially involved in mylonitization adjacent to the WT. The WT has been inferred to have only a very small difference in pressure (depth) over the ca. 60 km of N-S exposure along the transport direction, from 1.0 – 1.3 GPa to 0.8 – 1.1 GPa, thus representing effectively a very shallowly dipping structure[1]. However, it was noted that these pressure estimates had to be considered with some caution due to not always ideal mineral compositions. Here we present new pressure constraints in northern outcrops from the eastern segment of the thrust suggesting a more complex geometry than previously inferred, with significant variation in depth along the structure.

Pseudotachylyte-bearing peraluminous gneisses, from two localities ca. 80 km apart (Sentinel Bore, SB, to the east and Kelly Hills, KH, to the west) in the immediate hanging wall of the WT, were investigated to establish the ambient conditions during seismic faulting. The gneisses display mm-thick alternation of quartz-feldspar and cordierite-sillimanite-rich layers, including sparse garnet, magnetite, ilmenite, and biotite. Along microfractures of the pst damage zone (i) sillimanite was fractured and remained unaltered; (ii) cordierite broke down to either an andalusite + quartz + biotite symplectite overgrown by kyanite (SB), or just kyanite (KH); and (iii) K-feldspar developed flame perthites. The pst at SB and KH also show a different mineralogy. At SB, pst assemblages include (i) andalusite (pseudomorphosed by biotite) + quartz intergrowths rimmed by plagioclase and K-feldspar; (ii) sillimanite microlites overgrowing sillimanite clasts; (iii) microlitic kyanite, and (iv) poikilitic garnet as the latest grown phase. At KH, pst assemblages include (i) cordierite + quartz intergrowths; (ii) sillimanite microlites overgrowing sillimanite; (iii) microlites of kyanite, and (iv) poikilitic garnet. Andalusite is absent at KH.

The newly identified andalusite, stable in pst, sheared pst and along microfractures in the host rock at SB indicates pressures ≤ 0.5 GPa during seismic faulting, i.e. significantly lower than in the more southern portion close to Mount Woodroffe (ca. 60 km to the SW of SB)[2]. The absence of andalusite at KH implies a complex undulating geometry for the WT.

 

 

1: Wex et al., 2017, Geometry of a large‐scale, low‐angle, midcrustal thrust (Woodroffe Thrust, central Australia). Tectonics36(11), 2447-2476.

2: Hawemann et al., 2018, Pseudotachylytes as field evidence for lower-crustal earthquakes during the intracontinental Petermann Orogeny (Musgrave Block, Central Australia). Solid Earth, 9, 629-648

How to cite: Toffol, G., Pennacchioni, G., Camacho, A., and Mancktelow, N.: Geometric complexity of the Woodroffe Thrust (Musgrave Ranges, central Australia) recorded in hanging wall Al-silicate-bearing peraluminous gneisses and hosted pseudotachylytes, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-8678, https://doi.org/10.5194/egusphere-egu22-8678, 2022.

09:04–09:11
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EGU22-9426
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ECS
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On-site presentation
Saskia Bläsing, Timm John, Johannes C. Vrijmoed, Michael J. Henehan, and Daniel A. Frick

To understand numerous geological processes, like element recycling or plate dynamics, the quantification of fluid-induced reactions in the Earth’s crust and mantle is an important but challenging subject, especially for short-lived events including substantial mass exchange. Lithium can serve as a powerful tool to quantify timing and fluid-flow mechanisms that happen on short geological timescales, because it is a very fast diffusing element and usually appears as a trace element in both fluid and rock.

The Kråkenes Gabbro is part of a fossil continent-continent collision zone, located in the Western Gneiss Region in Norway, and shows the effects of fluid-rock interaction perfectly.  The low permeability gabbro is cross-cut by strictly N-S-trending fractures, which opened during exhumation, serving as a pathway for an aqueous fluid to infiltrate the rock. Metasomatism occurred under amphibolite-facies conditions, resulting in a sharp amphibolite-generating reaction front propagating on dm-scale into the magmatic gabbro. This reaction is driven by strong chemical gradients between the reactive fluid and the dry, metastable gabbro. Samples were taken as continuous profiles (~ 30 cm length) perpendicular to the vein and analyzed using a) SEM automated quantitative mineralogy mapping to quantify evolving mineral assemblages during amphibolite-facies metamorphism and b) MC ICP-MS to determine variations in bulk rock lithium concentrations and isotope compositions along the profile.

To understand fluid-flow mechanisms, reactive flow-based diffusion models were created, and model accuracy was checked by integrating measured mineral and lithium data. Mass balance calculations and recalculations of the gabbro and amphibolite mineral assemblages give information on the fluid composition and its transported elements, showing that the fluid-induced reaction is not diffusion-limited only. Furthermore, these models portray the evolving reaction front and the evolution of physical parameters such as mineral assemblage, density or porosity within it. Our investigations into lithium concentrations and δ7Li values show that lithium is transported by the fluid into the formerly almost dry system and thus propagated into the gabbro. Reaction-induced variations in e.g. porosity and partition coefficients are included into lithium-diffusion models to find the minimum misfit between measured and modelled lithium data to estimate the duration of the fluid-induced reaction.

How to cite: Bläsing, S., John, T., Vrijmoed, J. C., Henehan, M. J., and Frick, D. A.: Fluid-rock interactions and amphibolitisation of the lower continental crust (The Kråkenes Gabbro, Western Gneiss Region, Norway), EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-9426, https://doi.org/10.5194/egusphere-egu22-9426, 2022.

09:11–09:18
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EGU22-6214
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Presentation form not yet defined
Bapi Goswami, Susmita Das, Ankita Basak, Chittaranjan Bhattacharyya, and Chandreyee Goswami

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.

09:18–09:25
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EGU22-1633
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Virtual presentation
Ane K. Engvik, Håvard Gautneb, Pål Tore Mørkved, Janja Knezevic, Muriel Erambert, and Håkon Austrheim

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 (Ab47-93An5-52), pyroxenes, biotite (Mg# = 0.67-0.91; Ti < 0.66 a.p.f.u.), and K-feldspar (Ab1-8Kfs92-99) or perthite (Ab35-64An3Kfs50-62) are additional major phases. Pyroxene is present either as orthopyroxene (En69-74Fs26-29; Mg#=0.70-0.74), as clinopyroxene (En33-53Fs1-14Wo44-53; 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 (δ13Ccalcite = -8.65‰ to -9.52‰) and heavier values for graphite (δ13Cgrapite = -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.

09:25–09:32
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EGU22-11826
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ECS
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Virtual presentation
Rene Asenbaum, Julian Portenkirchner, Martin Racek, Elena Petrishcheva, and Rainer Abart

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.

09:32–09:39
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EGU22-8722
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ECS
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On-site presentation
Sandeep Thapa, Frédéric Girault, Damien Deldicque, Jabrane Labidi, Jana Börner, Christian France-Lanord, Pierre Agrinier, Élodie Muller, Lok Bijaya Adhikari, Mukunda Bhattarai, Kabi Raj Paudyal, Sudhan Singh Mahat, Rémi Losno, and Frédéric Perrier

The Nepal Himalayas result from the India-Eurasia collision and the actual shortening is accommodated by a detachment ramp, the Main Himalayan Thrust (MHT). Separating high-grade metamorphic rocks from the Greater Himalayan Sequence to the north and low-grade metamorphic rocks from the Lesser Himalayan Sequence to the south, the Main Central Thrust (MCT) shear zone, is related to the MHT at depth where large Himalayan earthquakes nucleate. The MCT zone occurs from Far-Western to Eastern Nepal, associated at mid-crustal depth with active seismicity and high electrical conductivity; it exhibits carbon-rich rock layers and numerous active hydrothermal systems. Here, based on a multidisciplinary approach that includes geology, geochemistry and geophysics, we study the various sulphur and carbon signatures in the MCT zone in the Nepal Himalayas. First, we characterise the upper LHS rocks that include alternation of graphite-rich mica-schists (the so-called “black schists”) and carbonates (mainly siliceous dolomite). In the laboratory, we determine organic and inorganic carbon contents, as well as complex electrical conductivity. Second, we concentrate on numerous thermal springs in which we measure dissolved carbon and sulphur concentrations and their isotopic compositions (δ13C and δ34S). Third, we study the surface gaseous emissions, directly observed in the vicinity of hot springs, with the measurements of carbon dioxide (CO2) and hydrogen sulphide (H2S) fluxes and isotopic compositions. By comparing the signatures of carbon and sulphur sequestration and carbon and sulphur release at a large spatial scale, our work provides insights into the carbon source-to-sink duality of large orogens, the metamorphic processes and the carbon and sulphur geochemical cycles.

 

How to cite: Thapa, S., Girault, F., Deldicque, D., Labidi, J., Börner, J., France-Lanord, C., Agrinier, P., Muller, É., Adhikari, L. B., Bhattarai, M., Paudyal, K. R., Mahat, S. S., Losno, R., and Perrier, F.: Sulphur and carbon signatures of metamorphic processes in the Nepal Himalayas, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-8722, https://doi.org/10.5194/egusphere-egu22-8722, 2022.

09:39–09:46
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EGU22-8736
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ECS
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On-site presentation
Shashi Tamang, Chiara Groppo, Franco Rolfo, and Frédéric Girault

Calcite-bearing sediments (calcareous pelites, marls, impure limestones) are among the most investigated sources of carbon in collisional settings (e.g. Groppo et al., 2017, 2021, 2022; Rapa et al., 2017). Dolomite- and magnesite-bearing sediments, however, can also be important constituents of evaporitic sequences deposited along passive margins and involved in collisional orogenic processes. So far, decarbonation reactions in dolomite- and magnesite-bearing rocks have been rarely investigated, and their contribution to the orogenic carbon cycle substantially neglected.          

As a contribution to the understanding of the influence of dolomite- and magnesite-bearing lithologies on the global Earth's carbon cycle, a petrologic study was focused on the Lesser Himalayan Sequence (LHS) in central Nepal. The LHS is a thick Proterozoic sedimentary sequence originally deposited on the northern margin of the Indian plate, metamorphosed during the Himalayan orogeny. Abundant dolomite- and magnesite–bearing lithologies occur in the Upper-LHS, whose protoliths can be grouped in: (1) a dolomitic series (dolostones, dolomitic marls, dolomitic pelites), and (2) a magnesitic series (sparry magnesite ores, magnesitic pelites). The magnesite deposits associated to dolomitic lithologies are interpreted as the evidence of evaporitic environments during the Proterozoic.

The schists derived from dolomitic pelites show mineral assemblages similar to those of normal metapelites, but with significant amounts of Ca-rich minerals (e.g. plagioclase) and with biotite anomalously enriched in Mg. The schists derived from magnesitic pelites are, instead, characterized by uncommon assemblages such as orthoamphibole + kyanite + garnet + phlogopite. Thermodynamic forward modelling (P/T-X(CO2) pseudosections) applied to these schists allowed to: (1) understand the nature of the main decarbonation reactions; (2) constrain the P-T conditions at which these reactions occurred, and (3) estimate the amounts of dolomite/magnesite consumed during prograde metamorphism, and the correspondent amounts of released CO2. The main results are:

  • the observed assemblages formed during a heating decompression stage, at P-T conditions of 620 ± 20°C, 8.5 ± 0.2 kbar, consistent with those registered by the associated metapelites;
  • the observed peak assemblages are predicted to be stable in equilibrium with a CO2-bearing fluid, even in those samples where carbonates are no more preserved;
  • the overall results point to an internally buffered P/T-X(CO2) evolution. The amount of carbonates consumed during prograde metamorphism varies in the range 7-20 vol%, corresponding to 3-10 wt% of CO2 These CO2 amounts are nearly double the CO2 released by calcareous pelites (Groppo et al., 2021).

The main consequence of this study is that the CO2 productivity of dolomitic and magnesitic pelites is significant and that these lithologies could be relevant sources of CO2, possibly contributing to the diffuse Himalayan CO2 degassing (e.g. Girault et al., 2014, 2018).

 

References

Girault et al. (2014). Geoph. Res. Lett. 41, 6358–6366

Girault et al. (2018). Nat. Comm. 9, 2956

Groppo et al. (2017). J. Petrol. 58, 53-83.

Groppo et al. (2021). J. metam. Geol. 39, 181-207.

Groppo et al. (2022). Comm. Earth Environ, doi: 10.1038/s43247-022-00340-w

Rapa et al. (2017). Lithos, 292–293, 364–378.

How to cite: Tamang, S., Groppo, C., Rolfo, F., and Girault, F.: Dolomite-and magnesite-bearing pelites: poorly investigated, yet significant, sources of CO2 in collisional orogens., EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-8736, https://doi.org/10.5194/egusphere-egu22-8736, 2022.

09:46–09:53
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EGU22-8700
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ECS
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On-site presentation
Robert Emo, Balz Kamber, Hilary Downes, and John Caulfield

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.

09:53–10:00
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EGU22-28
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ECS
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Virtual presentation
Chaouki Djallel Eddine Bendimerad, Abderrahmane Bendaoud, Julien Berger, Renaud Caby, and Nachida Abdallah

The mafic-ultramafic Ougda magmatic complex is located in the west part of Tuarge Shield, in Algeria, between Tassendjanet terrane in the east and Ahnet terrane in the west. It is composed of three successive generations of magmatic rocks (Dostal et al., 1996). The first generation located in the north, includes ultramafic rocks cut by dikes of cumulate garnet-bearing mafic rocks and quartz diorite sheets. It records high-temperature metamorphic conditions, granulite facies. The second and third generation located in the south, includes undeformed cumulate and non-cumulate gabbros and intermediate to mafic dikes. The three generations record a geochemical evolution from tholeiitic to calco-alkaline magmatism with subduction-related oceanic environment (Dostal et al., 1996). The age of the first generation is around 800 Ma and the second generation is dated at 680 Ma, considered as the ages of the inception to demise of the oceanic lithosphere (Dostal et al., 1996; Caby and Monié, 2003). Here, we focus on garnet-bearing rocks that show particular interest, as they are affected by high-grade metamorphism in this area. Understanding the pressure-temperature (P-T) evolution of those garnet-bearing rocks allow a crucial constrain of the evolution of the oceanic crust in this area during the Panafrican orogeny.

Petrographical investigation shows that all samples share similar mineralogical assemblages with garnet, plagioclase, amphibole, clinopyroxene, ilmenite and rutile. It is interpreted as typical of granulite facies. Garnet is the most dominate phase and show different textural types: Pokioblastic garnet with inclusions of amphibole, clinopyroxene, plagioclase, ilmenite and rutile. In some samples, garnet is very large (~2 cm), ilmenite is observed in garnet core and rutile appears with ilmenite in garnet rims. Clinopyroxene in garnet is a primary phase as it is surrounded by amphibole, which indicate a reaction with garnet. Garnet corona is around clinopyroxene and plagioclase and both are not in contact with each other. Modeling phase relationship using P-T pseudosections was calculated to constrain the P-T conditions and mineralogical evolution. For garnet growth, modal calculations with observed mineral assemblages are more consistent with a solid-state reaction where clinopyroxene and plagioclase are consumed to produce garnet. The PT path manifest with either cooling at high pressure or pressure increase stage, linked to garnet growth, 14-7 Kbar and 1000-700 °C. The P-T conditions are limited by the appearance of biotite at low temperature, solidus at high temperature and olivine at low pressure. The maximum pressure being recorded by rutile-ilmenite-bearing assemblage. This granulitisation stage is followed by a decompression in subsolidus conditions, amphibolites facies, where amphibole appears either as the product of clinopyroxene transformation or reaction between primary clinopyroxene and garnet through hydration. Lastly, hydration in low grade, greenschist facies, is recorded in garnet- and clinopyroxene-free domains with hydrous phases, chlorite, epidote and amphibole. Hence, P-T evolution recorded in garnet-bearing rocks of Ougda shows an anticlockwise PT path with granulitisation stage showing P-T peak recorded by rutile-ilmenite-bearing assemblage in garnet. Followed by a decompression in amphibolite facies with production of amphibole and ended up with late hydration in geenschist facies.

How to cite: Bendimerad, C. D. E., Bendaoud, A., Berger, J., Caby, R., and Abdallah, N.: Preliminary investigation on PT path of garnet-bearing mafic rocks in the Neoproterozoic Ougda magmatic complex, Tuareg Shield, Algeria, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-28, https://doi.org/10.5194/egusphere-egu22-28, 2022.

Coffee break
Chairpersons: Daniela Rubatto, Matthijs Smit
10:20–10:27
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EGU22-9177
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ECS
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Virtual presentation
Gang Liu, Jun-sheng Lu, Xu Kong, Qiang Feng, Yu-ting Li, and Yi-yi Zhang

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 XPy and XGrs 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 XPy and XGrs 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.

10:27–10:37
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EGU22-6734
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solicited
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Presentation form not yet defined
Besim Dragovic, Victor Guevara, Mark Caddick, Jeremy Inglis, Tom Raimondo, and Andrew Kylander-Clark

High-grade metamorphic rocks can record the dynamic processes that lead to crustal heating and a departure from normal crustal geothermal gradients. High temperatures in the Archean crust led to particularly significant melt generation and cratonic stabilization, and understanding the depths, temperatures and rates of Archean metamorphism may reflect our clearest window into possible tectonic styles at this time. However, several Archean metamorphic terranes record polymetamorphism, and unravelling the pressure-temperature-time (P-T-t) histories of such terranes has proven difficult, with complexity inherent in both chronologic and petrologic data.

Here we synthesize results of a multi-analytical study in which garnet and monazite petrochronology, coupled with thermodynamic and diffusion modeling, were applied to Archean granulites from the Beartooth Mountains in the northern Wyoming Province, U.S.A. The data reveal two phases of garnet growth and high-temperature metamorphism. Garnet cores grew coeval with emplacement of a granitoid batholith at ~2.78-2.76 Ga. This was followed by a distinct, second phase of peritectic garnet rim growth at ~2.71 Ga, during biotite breakdown melting at peak temperatures of ~750˚C. Diffusion modeling of chemical zoning in garnet rims shows that this second event was brief: near-peak temperatures were maintained for < 1 Myrs. In contrast, core and rim dates of garnet from a meta-granitoid from the same outcrop record only the initial phase of growth, most likely because a lack of grain boundary fluids inhibited further crystallization in these rocks. Evidence for this second event is cryptic in other granitoid samples, such that this period of heating to at least 750˚C, ~50-100 Myrs after initial batholith emplacement, is poorly recorded in the broader rock record of the Beartooths.

The results of our study show that different parts of the metamorphic history of a rock may be recorded differently between garnet and accessory phases. Lastly, while field and petrologic evidence for polymetamorphism may be cryptic, direct dating of distinct garnet growth zones with preserved major and trace element zonation allows for a clear interpretation between isotopic dates and the metamorphic history of the rock.

How to cite: Dragovic, B., Guevara, V., Caddick, M., Inglis, J., Raimondo, T., and Kylander-Clark, A.: Deciphering Neoarchean polymetamorphism and crustal melting in the northern Wyoming Province using garnet petrochronology, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-6734, https://doi.org/10.5194/egusphere-egu22-6734, 2022.

10:37–10:44
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EGU22-3348
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ECS
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Virtual presentation
Kota Suzuki, Tetsuo Kawakami, and Shuhei Sakata

The duration of anatexis in high-grade metamorphism is essential to understand the crustal melting processes and the tectonic settings. In the case of Rundvågshetta, Lützow-Holm Complex, East Antarctica, the linkage between the U-Pb zircon ages and the metamorphic pressure-temperature (P-T) evolution is still unclear. Only the melt crystallization age of ca. 520 Ma is constrained. In this study, we aim to constrain the duration of anatexis by using petrochronological approaches to an ultrahigh-temperature (UHT) granulite sample from Rundvågshetta.

Garnet in the studied sample consists of the P-poor core, P-rich mantle and P-poor rim. Based on the detailed petrography of inclusion minerals in garnet, we interpret that the garnet core was formed as a peritectic product of biotite dehydration melting during prograde metamorphism, and that the garnet mantle and rim were formed in the peak and retrograde stages, respectively, in a clockwise P-T evolution.

Zircon in the rock matrix shows four microstructural domains; oscillatory-zoned inherited core, dark-CL annulus, slightly bright-CL inner rim and bright-CL outer rim. The inner rim was too thin for the LA-ICP-MS U-Pb zircon dating with 20 µm spot size. The inherited cores are always truncated by the dark-annulus with low Th/U ratios below 0.04. The dark-annulus includes muscovite, biotite, rutile, quartz and melt inclusions and yielded weighted mean age of 564.0 ± 4.9 Ma (2σ error, n = 4, MSWD = 1.8). The dark-annulus is further truncated by the outer rim with higher Th/U ratios (0.08-1.13). The outer rim includes sillimanite, K-feldspar and rutile and yielded weighted mean age of 530.5 ± 4.9 Ma (2σ error, n = 13, MSWD = 1.5).

The microstructures of inclusion zircon vary systematically with the phosphorus zoning of the host garnet. Zircon in the garnet rim show four microstructural domains that are common to the matrix zircon. Meanwhile, zircon in the garnet core always lacks the inner and outer rims. The dark-annulus and outer rim of zircon respectively showed steeply positive-sloping and negative-sloping heavy rare earth elements (HREE) patterns. Meanwhile, the garnet core, mantle and rim showed positive, flat and negative HREE patterns, respectively. Based on these systematic microstructures of inclusion zircon and on the partitioning of HREE between zircon and garnet, it is revealed that the outer rim of zircon grew simultaneously with the garnet rim during the retrograde metamorphism, and that the dark-annulus of zircon grew prior to the garnet core during the prograde metamorphism.

Inclusion minerals in the dark-annulus of zircon suggest the possible occurrence of muscovite dehydration melting at ca. 560 Ma. Therefore, microstructural observations of zircon enabled us to deduce the prograde anatexis prior to the attainment of UHT condition that is not recorded in garnet. Taking the melt crystallization age of ca. 520 Ma into account, the duration of anatexis in Rundvågshetta is constrained to be at least ~40 Myr. Further U-Pb dating of the thin inner rim of zircon may reveal the duration of the UHT itself precisely.

How to cite: Suzuki, K., Kawakami, T., and Sakata, S.: Duration of anatexis in a Neoproterozoic-Cambrian UHT terrane: constraints from prograde melt inclusions in zircon, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-3348, https://doi.org/10.5194/egusphere-egu22-3348, 2022.

10:44–10:51
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EGU22-7532
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ECS
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On-site presentation
Sascha Zertani, Luca Menegon, Giorgio Pennacchioni, Fernando Corfu, and Bjørn Jamtveit

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.

10:51–10:58
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EGU22-6127
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On-site presentation
A. Hugh N. Rice, Fred Gaidies, Olivier K. A. Heldwein, M. Thereza A. G. Yogi, Jamie A. Cutts, and Matthjis A. Smit

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.

10:58–11:05
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EGU22-4832
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On-site presentation
Rezvaneh Jamaliashtiani, Erik Scherer, Axel K. Schmitt, and Jamshid Hassanzadeh

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.

11:05–11:12
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EGU22-11626
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ECS
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Virtual presentation
Laser ablation Lu–Hf dating reveals Laurentian garnet in subducted rocks from southern Australia
(withdrawn)
Dillon Brown, Alexander Simpson, Martin Hand, Laura Morrissey, Sarah Gilbert, Renée Tamblyn, and Stijn Glorie
11:12–11:19
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EGU22-6405
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On-site presentation
Chris Mark, Laura Stutenbecker, Sergio Andò, Marta Barbarano, Gary O'Sullivan, Stijn Glorie, Alexander Simpson, and J. Stephen Daly

Detrital geochronology is a powerful tool to interrogate the sedimentary archive of (paleo-)hinterland tectonic, metamorphic, and climatic processes, and can also be applied to modern river sediment as a first-pass tool to establish regional bedrock ages. The popular zircon U-Pb detrital geochronometer has seen widespread adoption for these tasks (3,626/4,471 results for the search term detrital geochronology also contain the term zircon U-Pb; Clarivate Analytics Web of Science). However, zircon fertility is strongly biased to intermediate to felsic source rocks. Moreover, zircon crystallization is volumetrically limited in metamorphic terranes which do not achieve anataxis (e.g., Moecher & Samson, 2006), and is typically restricted to rim overgrowths which are vulnerable to mechanical destruction during fluvial transport, and which are challenging to detect and analyse (e.g., Campbell et al., 2005).

Therefore, it is desirable to develop complementary provenance tools for metamorphic settings. Garnet group minerals are rock-forming in several common metamorphic lithologies, and garnet is therefore a common constituent of clastic detritus from orogens. Moreover, single-grain in-situ dating of garnet by LA-ICPMS is possible using the U-Pb (e.g., Seman et al., 2017) and, by use of an online reaction cell, the Lu-Hf radioisotope systems (Simpson et al., 2021).    

Here, we present results from U-Pb and Lu-Hf double-dating, acquired by LA-ICPMS for detrital garnet recovered from the Oligo-Miocene pro-foreland basin of the European Alps, as well as modern Alpine river sediment. We integrate these data with compositional data acquired by Raman spectroscopy, and energy and wavelength-dispersive X-ray spectroscopy (Stutenbecker et al., 2019). We discuss the implications for Alpine tectonics and metamorphism, and future scope of detrital garnet geochronometry.   

Campbell, I., et al., 2005. Earth Planet. Sci. Lett. 237, 402-432,  doi: 10.1016/j.epsl.2005.06.043

Moecher, D., & Samson, S., 2006, Earth Planet. Sci. Lett. 247, 252–266, doi: 10.1016/j.epsl.2006.04.035

Seman, S., et al., 2017. Chem. Geol. 460, 106–116. doi: 10.1016/j.chemgeo.2017.04.020

Simpson, A., et al., 2021. Chem. Geol. 577, 120299. doi: 10.1016/j.chemgeo.2021.120299

Stutenbecker, L., et al., 2019, Solid Earth 10, 1581–1595, doi: 10.5194/se-10-1581-2019

How to cite: Mark, C., Stutenbecker, L., Andò, S., Barbarano, M., O'Sullivan, G., Glorie, S., Simpson, A., and Daly, J. S.: Detrital garnet Lu-Hf and U-Pb geochronometry coupled with compositional analysis: Possibilities and limitations as a sediment provenance indicator, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-6405, https://doi.org/10.5194/egusphere-egu22-6405, 2022.

11:19–11:26
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EGU22-9282
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ECS
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On-site presentation
Alessia Tagliaferri, Filippo Luca Schenker, Stefan Markus Schmalholz, Alexey Ulianov, and Silvio Seno

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.

11:26–11:33
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EGU22-9763
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ECS
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Virtual presentation
Marianne Sophie Hollinetz, Benjamin Huet, David A. Schneider, Christopher R. M. McFarlane, and Bernhard Grasemann

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.

11:33–11:40
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EGU22-9332
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Virtual presentation
Bianca Heberer, István Dunkl, Franz Neubauer, Sina Schulz, William Guenthner, Hannah Pomella, and Hilmar von Eynatten

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.

11:40–11:47
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EGU22-9119
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ECS
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Virtual presentation
Xu Kong, Jun-sheng Lu, Gang Liu, Qiang Feng, Yu-ting Li, and Yi-yi Zhang

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.