GMPV6.1
Orals: Mon, 24 Apr | Room N1
Metamorphic rocks record the imprint of the tectonic processes that shaped the lithosphere and record the effects of their journey through time and space. The record can be interrogated by using a number of different geochronological techniques. The 40Ar/39Ar geochronology method is particular useful when it comes to extracting information from the major rock-forming minerals such as mica and feldspar, commonly filling the temporal gap between the ages obtained by U–Pb dating of accessory minerals and the application of low-temperature thermochronometers. Here we present a case study illustrating a novel and innovative way to investigate metamorphic processes across tectonic settings and geologic time, involving metamorphic petrology, geochronology, geochemistry, numerical modelling and tectonics.
The method involves quantitative modelling of 40Ar/39Ar age spectrum morphologies, constrained by conjointly using information from white mica, biotite and potassium feldspar from a single Proterozoic gneiss. Temperature-controlled step-heating diffusion experiments provide estimates of the relevant diffusion parameters using Multi-Domain Diffusion (MDD) models to invert Arrhenius data. Computer modelling and simulation then allows the production of admissible temperature-time paths for all three minerals used in this study, allowing the identification of previously unrecognised episodes of mineral growth and/or periods of cryptic metasomatism. In this way, 40Ar/39Ar geochronology enables estimates for the timing of a sequence of mineral growth events and the veriation of ambient temperature through time.
Two examples are provided from Palaeoproterozoic gneisses from northern Australia. Typically, the morphology of each age spectrum (for biotite, white mica, and potassium feldspar) required a minimal two-component microstructure to explain the mixing pattern. In each mineral, a MDD model is needed to explain the pattern of gas release during furnace step-heating. Estimates of the diffusion parameters using the Arrhenius data allow the inference that both phengite-poorer muscovite and phengite-richer muscovite existed in the white mica aliquot. Quantitative modelling of the age spectrum morphology allowed constraints to be placed on possible temperature-time-growth (T-t-Δ) paths followed by the rock sample in the natural environment, spanning a duration of more than a billion years.
How to cite: Wu, Y., Forster, M., Fraser, G., Kelsey, D., and Lister, G.: Constraining fluid-rock alteration and temperature history using multi-mineral argon spectra and conjoint T–t-Δ inversion, EGU General Assembly 2023, Vienna, Austria, 24–28 Apr 2023, EGU23-3045, https://doi.org/10.5194/egusphere-egu23-3045, 2023.
The emerging field of “in-situ beta decay dating” has enormous potential for Earth Sciences. Here, the Rb-Sr system is the most advanced, although other systems (e.g., K-Ca, Lu-Hf, Re-Os) promise exciting opportunities as well. In this contribution, I want to first highlight several analytical and conceptual advances made with regard to in-situ Rb-Sr geochronology, and in particular utilizing the mica group (mostly biotite, muscovite and glauconite): (1) the community (e.g., Redaa et al, 2022) has made important progress characterizing the reference material Mica-Mg (from CRPG) for Rb-Sr ratios and Sr isotope composition, used as a nanopowder pellet, it currently serves in most laboratories as a primary reference material; (2) several new natural mica samples have been distributed to several laboratories to serve as secondary reference materials (Rösel & Zack, 2022). Both these activities serve not only to improve precision and accuracy of this technique, but in general allows better comparison of results of different studies. Furthermore, (3) many micas are almost devoid of Sr when forming, which allows treating them similar to zircon in the U-Pb system, meaning that the common Sr can simply be estimated, making the isochron approach obsolete (Rösel & Zack, 2022). This has important practical implication; so-called single spot ages can be utilized to map out age distribution within single crystals, target crystals of different textural context or even used in provenance studies of detrital mica (Rösel et al., this conference). Finally, (4) as most analytical facilities where in-situ beta decay dating is possible employ a quadrupole ICP-MS, selecting isotopes for spot analysis are not limited to Rb and Sr isotopes, but can set to cover all elements of interest from Li to U. With sufficient care in the choice of calibration material, it is possible to not only couple age information with trace element signatures, but even calculate mica mineral formula with surprising accuracy. In my presentation I want to illustrate how in-situ Rb-Sr mica geochronology can be utilized in the field of metamorphic petrology. For further applications in metamorphic settings, please also see Barnes et al. (this session).
How to cite: Zack, T.: Prospects for in-situ Rb-Sr mica geochronology in metamorphic petrology, EGU General Assembly 2023, Vienna, Austria, 24–28 Apr 2023, EGU23-11635, https://doi.org/10.5194/egusphere-egu23-11635, 2023.
GARNET LU-HF SPEED DATING: A NOVEL METHOD TO RAPIDLY RESOLVE POLYMETAMORPHIC HISTORIES
Alexander Simpsona,, Stijn Gloriea,, Martin Handa,, Carl Spandlera, Sarah Gilberta,
aDepartment of Earth Sciences, School of Physical Sciences, The University of Adelaide, Adelaide SA-5005, Australia
Abstract
Garnet is a remarkably useful mineral in the study of crustal and lithospheric-scale tectonic systems. A vast body of knowledge exists on its thermodynamic properties, meaning garnet is widely used for determining pressure-temperature conditions. Traditional methods to date garnet (e.g. Lu-Hf and Sm-Nd) are time consuming and commonly volumetrically imprecise. Consequently, garnet is generally now dated indirectly using co-existing minerals such as zircon and monazite that can be efficiently analysed using U-Pb age-dating, with their chemical compositions linked to garnet via inferred equilibrium elemental relationships. The disadvantage of this approach is the age of garnet is inferred, rather than directly determined, and it can be difficult to prove garnet grew contemporaneously with the dated U-Pb minerals. In situ Lu-Hf laser ablation dating of garnet is a comparatively recent development that embodies the efficiency of LAICPMS U-Pb dating. Here, we explore the use of laser ablation garnet Lu-Hf dating to rapidly reveal the metamorphic history of polymetamorphic terranes, producing accurate dates (in excellent agreement with published ages) with precisions as low as 1.5%. For instance, garnets analysed from the Moine supergroup, NW Scotland produce ages that coincide with all major orogenic evets (e.g. ~950 Ma Renlandian Orogeny, 810 - 740 Ma Knoydartian orogeny, and 470 – 420 Ma Caledonian Orogeny). In certain cases, two of these orogenic events are preserved in a single garnet grain, and the high spatial resolution of the laser method allows to accurately resolve such multi-growth histories. In addition, we demonstrate that the laser ablation Lu-Hf technique has the potential to evaluate micron-scale isotopic disturbances (such as diffusion) in garnet.
How to cite: Simpson, A., Glorie, S., Hand, M., Spandler, C., and Gilbert, S.: Garnet lu-hf speed dating: a novel method to rapidly resolve polymetamorphic histories, EGU General Assembly 2023, Vienna, Austria, 24–28 Apr 2023, EGU23-10990, https://doi.org/10.5194/egusphere-egu23-10990, 2023.
The only accepted evidence in the rock record for fossil earthquakes are pseudotachylytes, quenched frictional melts produced during seismic slip. Specifically, earthquakes in the lower continental crust recently have received increased attention, because they occur at depths where the lower continental crust is expected to flow rather than fracture. Nevertheless, lower crustal seismicity is also reported from active settings, for example, below the Himalaya. In order to properly address how and why they occur, pseudotachylytes exhumed from lower-crustal terranes are used as analogues. However, in order to fully understand lower-crustal seismicity, it is important to constrain the tectonic setting in which pseudotachylytes formed, which requires determining their age. Rapid melting and quenching, re-crystallization, and extremely fine grain sizes make age dating difficult. In this context, apatite may provide useful information, as it is known to quickly reset U-Pb ages during recrystallization.
We present the first reported in-situ U-Pb ages from lower crustal pseudotachylytes. The analyses were performed on samples from the Lofoten archipelago (Northern Norway) that exposes a block of lower continental crust with only minor overprint from the Caledonian orogeny. Field observations indicate that some of the exposed amphibolite-facies pseudotachylytes in the area have been overprinted by amphibolite-facies ductile shear zones. We couple in-situ U-Pb analysis (SIMS) with cathodoluminescence (CL) and electron backscatter diffraction (EBSD) to ensure full microstructural control of the ages. Analysis was conducted on variably mylonitized pseudotachylytes. All apatites originated from the Paleoproterozoic host rock and are either preserved in the immediate damage zone within the host rock or as survivor clast within the pseudotachylytes. Our analysis reveal that apatite in pristine pseudotachylytes deformed only by fragmentation and was subsequently annealed. Apatite in mylonitized pseudotachylytes displays evidence that deformation occurred dominantly by grain-boundary sliding after fragmentation, while grains in the host rock show evidence of crystal-plasticity and recrystallization. SIMS analyses yield a bimodal age distribution at ~450 and ~350 Ma. Combination of the ages with the microstructural evidence shows that the former captures the age of the earthquake, while the latter is related to late fluid infiltration, which was localized in the pseudotachylyte-bearing faults embedded in an otherwise dry and impermeable lower-crustal block.
How to cite: Zertani, S., Menegon, L., Whitehouse, M., and Jamtveit, B.: Dating fossil lower-crustal earthquakes by in-situ apatite U-Pb geochronology, EGU General Assembly 2023, Vienna, Austria, 24–28 Apr 2023, EGU23-3230, https://doi.org/10.5194/egusphere-egu23-3230, 2023.
Within the Earth crust metamorphic reactions strongly participate to strain partitioning and localization. However, the timing of metamorphism relative to viscous deformation, the spatial scale of metamorphic processes and mineral re-equilibration remain elusive, with metamorphic reactions and associated fluid percolation generally considered as syn-kinematic. We investigate how, where and when (relative to viscous deformation) metamorphic reactions occurred in deformed gabbros of the Poroshiri Ophiolite of Hokkaido (Japan), in the core of a plate-boundary dextral shear zone. In these rocks, low and high strain areas preserve evidences of amphibolitization that occurred at 850-950°C (~5 kbar), triggered by fluid influx during fracturing (active in supra solidus conditions) and predating the viscous deformation. The abundance, composition heterogeneity of amphibole and the location of amphibole nucleation sites were regulated by water availability and by different reaction mechanisms as epitaxial growth or dissolution-reprecipitation observed at the nanoscale which controlled the magnitude and pathways of element supply (especially Fe and Mg). Pre-shearing metamorphism was accompanied by the local partial melting at grain boundaries and along crystallographic discontinuities of igneous clinopyroxene and resulted in grain size reduction of two orders of magnitude and formation of a patchwork of domains with different composition, where local chemical equilibria prevailed at the scale of 100-500 µm. Shearing occurred along the retrograde path, at 650-750°C and was coeval with amphibole and plagioclase recrystallization in high strain areas and in late fractures. Although fluid influx and amphibolitization reactions continued during shearing as attested by variations in major element content between high and low strain areas, mineral composition heterogeneities inherited from the pre-shearing metamorphic stage were largely preserved despite high strain and temperature, indicating in mylonites equilibrium scales shorter than 500 µm. Minor variations in amphibole modal abundance between inside and outside shear zones indicate that amphibolitization largely predated shearing and was controlled by fluid availability (through fracturing) rather than being strain-driven, with shearing mainly reworking the size and chemistry of amphibole grains. While throughout tectonic evolution, fluid infiltration primarily resulted from brittle fracturing active before and during viscous deformation, areas of pre-shearing amphibolitization appeared as preferential loci for strain localization and mineral re-equilibration during shearing. Pre-shearing metamorphism influenced strain localization and mineral re-equilibration during shearing also by controlling (i) the grain size reduction, (ii) the degree of phase mixing, (iii) the distribution of hydrated phases (and therefore of stored fluid) and (iv) the strain partitioning among the inherited metastable mineralogical domains.
How to cite: Airaghi, L., Raimbourg, H., Tsuyoshi, T., Jolivet, L., Bévillard, B., Arbaret, L., and Richard, G.: Metamorphic reactions in deformed mafic rocks: timing, fluid percolation and equilibrium scales from undeformed gabbros to mylonites, EGU General Assembly 2023, Vienna, Austria, 24–28 Apr 2023, EGU23-9034, https://doi.org/10.5194/egusphere-egu23-9034, 2023.
When continents collide, the Earth’s crust experiences structural and metamorphic transformations that control the geodynamic evolution of the orogen. Metamorphism of dry, lower crust requires fluid supply and produce mechanically weaker rocks. Metamorphism is often localized in shear-zones, which provide the available fluid pathways. Several field-based studies show that shear zone development is preceded by brittle faults, frequently portraying evidence for seismic slip rates and introduction of externally derived fluids. However, despite the extensive documentation of lower crustal metamorphism and associated deformation features, a unifying model coupling deformation to fluid migration and metamorphic reactions does not exist. Here, we present a visco-elasto-plastic model where the most pertinent features observed in transformed lower crust emerge from basic mechanical principles during the deformation of a coherent rock volume with associated fluid introduction. Characteristic features include a strikingly dynamic and heterogeneous pressure distribution in the reacting and deforming rock volumes. Lower crustal pressure variations may reach 1 GPa at any given depth. This will have first order effects on the pattern of fluid migration in the lower crust, and may also explain the apparent discrepancies between the relevant tectonic settings and petrologically-inferred burial depths. An additional petrological consequence of the positive pressure variations is the generation of fluid-undersaturated high-pressure assemblages. For common bulk-rock compositions that are observed in the Bergen Arcs (Norway), and for finite amounts of fluid, phase equilibria modelling results suggest that the quasi-isothermal pressurization will lead to the formation of H2O-undersaturated metamorphic rocks. These results highlight the importance of coupling between metamorphic reaction progress and deformation at high-grade conditions.
Acknowledgements:
This project was supported by a research award from the Alexander von Humboldt foundation to BJ, by ERC Advanced Grant Agreement n°669972 to Jamtveit and ERC Consolidator Grant Agreement n°771143 to Kaus from the European Union’s Horizon 2020 Research and Innovation Programme. Parts of this research were conducted using the supercomputer MOGON2 and/or advisory services offered by Johannes Gutenberg University Mainz (hpc.uni-mainz.de), which is a member of the AHRP (Alliance for High Performance Computing in Rhineland Palatinate, www.ahrp.info) and the Gauss Alliance e.V. Andrew Putnis and Håkon Austrheim are acknowledged for numerous discussions.
References:
Moulas, E., Kaus, B., Jamtveit, B., 2022. Dynamic pressure variations in the lower crust caused by localized fluid-induced weakening. Communications Earth & Environment 3, 157. https://doi.org/10.1038/s43247-022-00478-7
How to cite: Jamtveit, B., Moulas, E., and Kaus, B.: Dynamic Pressure Variations in the Lower Crust Caused by Localized Fluid-Induced Weakening, EGU General Assembly 2023, Vienna, Austria, 24–28 Apr 2023, EGU23-4230, https://doi.org/10.5194/egusphere-egu23-4230, 2023.
The process of peridotite hydration, or serpentinization, is known to generate reducing conditions through the production of H2-CH4-rich fluids. The release of these abiotic energy sources has attracted a broad scientific attention spanning natural energy research, carbon cycling, and deep subsurface microbiology and astrobiology. Serpentinization is documented at various geological settings including sub-seafloor hydrothermal systems and at much higher pressures and temperatures in subduction zones. Determining the conditions at which serpentinization and H2 release occur is crucial to comprehensively understand the geochemical cycle of life-essential, redox-sensitive elements such as C in subduction zones and the potential supply of energy to the deep subsurface biosphere. However, especially at convergent margins, ultramafic rocks may record multiple serpentinization events ranging from seafloor to subduction metamorphic conditions, which challenges the study of this key geological process. Petrographic and geochemical tracers, such as δ11B, have been used to disentangle multiple serpentinization events taking place at different geodynamic settings and/or from different fluid sources. However, petrographic features may be of ambiguous interpretation, and boron isotope data may show significant overlap among different serpentinization conditions.
To tackle these open questions, we adopted a high-resolution approach at the massif scale within the blueschist-facies Monte Maggiore ultramafic body, Alpine Corsica, France. This massif recorded the critical conditions of the lizardite/antigorite transition, which makes it an ideal case to study preserved and structurally controlled serpentinization events. We collected more than 150 samples of partially to fully serpentinized peridotites over an area of about 1Km2. The samples were selected and processed for petrographic analysis, Raman Spectroscopy, major and trace elements and δ11B with the aim of reconstructing a massif-scale distribution of multiple serpentinization events. Four main serpentine generations were identified: lizardite/chrysotile, lizardite/antigorite, sole antigorite, and late chrysotile. These generations show characteristic and systematic features, and their association defines a limited number of sample types at the massif scale. Bulk δ11B analyses show a wide range of values, from -2.51 to 23.33 ‰, which overlap with both slab and ocean derived fluids. When compared with petrographic data, it appears that samples belonging to the same sample type, therefore sharing common mineralogical and microstructural features characteristic of a specific serpentinization process, show substantially different boron isotopic values.
Our results indicate that large petrographic and δ11B variability may exist within a single serpentinized ultramafic massif, and also among samples plausibly belonging to the same serpentinization event. This high-resolution study of serpentinization events at the massif scale calls for caution while interpreting large-scale serpentinization processes through the study of individual samples or small sample sets inferred to represent large geodynamic contexts.
How to cite: Ressico, F., Vitale Brovarone, A., Agostini, S., Malaspina, N., Cannaò, E., and Olivieri, O. S.: Disentangling serpentinization events at the massif scale through microstructural and B isotope characterization., EGU General Assembly 2023, Vienna, Austria, 24–28 Apr 2023, EGU23-771, https://doi.org/10.5194/egusphere-egu23-771, 2023.
Metamorphic fluids have been central in the evolution of our planet and may also control the evolution and habitability of other planetary bodies. Although a large body of literature has focused on metamorphic carbon dioxide (CO2), from its sources to its emissions into the atmosphere, methane (CH4) may also be a fundamental species in metamorphic fluids in a large variety of rock systems and produced through multiple processes. However, the geology of metamorphic methane is still largely unexplored.
This study centers on metamorphic methane formation and transformation through a variety of processes and chemical systems from literature data and unpublished results, including open and closed systems in meta-sedimentary, meta-basic, and meta-ultrabasic rocks. Particular attention will be given to the types of methane that may be formed in metamorphic rocks and their classification, their distribution and abundance, and their abiotic or biotic interpretations.
This contribution highlights the importance of metamorphic methane – it is more common than generally considered – and identifies a series of fundamental open questions on the topic that still need to be addressed by future work.
This work is part of project that has received funding from the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation programme (Grant agreement No. 864045).
How to cite: Vitale Brovarone, A.: Metamorphic methane degassing: questions and challenges, EGU General Assembly 2023, Vienna, Austria, 24–28 Apr 2023, EGU23-9369, https://doi.org/10.5194/egusphere-egu23-9369, 2023.
‘Equilibration volume’ (EV) is the part of a rock volume over which the chemical potential of its components is spatially equivalent and thus the minerals present within that rock volume is presumed to be in equilibrium with each other. With metamorphism, the size of the EV for each component changes spatially and temporally as a function of a number of parameters (e.g. diffusivity of components, temperature, time, presence/absence of fluid/melt, grain size etc.) leading to a continuous evolution of the chemical potential landscape (CPL). The micro-textures present in a metamorphic rock bear the first-hand testimonies of its CPL evolving through time and space. Thus, unless the dynamic evolution of the EV with changing P-T path is taken into account, complete understanding on the generation and preservation of many mineral textures, like corona, may remain elusive.
Here we study a suite of Mg-Al rich ortho-amphibole-cordierite gneiss from the Cauvery Shear System in the Granulitic Terrane of South India. The rock features aluminosilicate porphyroblasts successively surrounded by an inner symplectic corona of sapphirine + cordierite, and an outer mono-mineralic corona of cordierite. Locally, corundum + cordierite grow along the interface of aluminosilicate and the inner symplectic corona. This double corona separates the aluminosilicate grains from a matrix of ortho-amphibole ± quartz. Based on detailed petrography and composition of individual minerals, the following corona-forming reactions were identified:
R1: Ortho-amphibole + aluminosilicate + quartz = cordierite
R2: Ortho-amphibole + aluminosilicate = sapphirine + cordierite
R3: Sapphirine + aluminosilicate = corundum + cordierite
We calculated quantitative petrogenetic grids within the MgO-Al2O3-SiO2-H2O (MASH) system taking pressure (P), temperature (T), and chemical potential (µ) of multiple diffusive components as variables to constrain the physico-chemical conditions of the corona formation. The results show that the formation of the corona-bearing assemblage in the studied rock occurred in response to decompression (at lower granulite facies conditions) and continuously changing µMgO- µSiO2 gradients around the primary aluminosilicate crystals. The calculated grid quantitatively models the evolution path of the CPL for the corona-bearing micro-domain in the P-µMgO-µSiO2 (isothermal) space. The path demonstrates that during retrogression, a sequential change of equilibrium mineral assemblage occurred through a series of reactions (R1-R3) in response to the continuously changing µMgO- µSiO2 gradients around the primary aluminosilicate crystals. Those equilibrium assemblages were preserved in typical spatial arrangement in the form of multiple layers of corona due to the progressively shrinking EV around the central aluminosilicate. The path quantifies the formation of corona-bearing assemblage and their typical spatial arrangement as a function of decompression and decreasing mobility of diffusing elements during retrogression.
How to cite: Dey, A., Roy Choudhury, S., and Sengupta, P.: Corona texture: a complex interplay of evolving P-T conditions, equilibration volume and chemical potential landscape, EGU General Assembly 2023, Vienna, Austria, 24–28 Apr 2023, EGU23-9403, https://doi.org/10.5194/egusphere-egu23-9403, 2023.
Within and near lower crustal shear zones, plagioclase grains frequently exhibit a peculiar compositional zonation: Albite-rich single crystals contain anorthite-rich lamellae and smaller, polygonal grains show an increase in anorthite-content from core to rim. This is the opposite of the zonation that develops during fractional crystallization in magmatic systems. Both the changes in plagioclase compositions and associated grain size reductions may affect rock rheology. Therefore, these microstructures may potentially provide valuable information about shear zone development and the behaviour of plagioclase-rich lower crustal rocks during an orogeny.
Next to shear zones in gabbronorites of the Ramberg section (Lofoten, Northern Norway), we observe both endmember microstructures (anorthite-rich inclusions in larger single crystals and zoned polygonal grains) as well as transitions between them. These were investigate in detail with scanning electron microscopy, including electron backscatter diffraction, and transmission electron microscopy.
The microstructures range from isolated, anorthite-rich lamellae in the host albite-richer plagioclase, via connected networks of anorthite-rich plagioclase within plagioclase single-crystals, to polygonal plagioclase grains with anorthite-rich rims close to the shear zones. These grains occur in clusters of similar orientation (presumably representing pre-existing larger grains). Preliminary work suggests that the plagioclase experienced an overall enrichment in Ca, which implies that fluid introduction played an important role during the reaction. The orientations of the anorthite-rich lamellae do not appear to be influenced by the crystallography of the host grain. Additionally, the density of the lamellae is highest in areas between grains of other phases than plagioclase, suggesting a stress-control on the reaction.
Ongoing transmission-electron microscopy work will help to understand whether the transition between the different microstructures is only spatial, or also temporal: Did the polygonal microstructure develop from the lamella-type microstructure, or are they expressions of the same event at different stress levels and/or fluid contents?
How to cite: Dunkel, K. G. and Jamtveit, B.: Formation and evolution of inversely-zoned “complex feldspar” in the lower crust, EGU General Assembly 2023, Vienna, Austria, 24–28 Apr 2023, EGU23-4953, https://doi.org/10.5194/egusphere-egu23-4953, 2023.
Posters on site: Mon, 24 Apr, 14:00–15:45 | Hall X2
The granulite terrane of southern India exposes Precambrian granulites that are dismembered into several crustal blocks with intervening shear/suture zones. Madurai Block forms the largest crustal block, preserving rock record from the Neoarchean to Neoproterozoic, acting as a time capsule to understand the Precambrian crustal evolution in southern India. In this context, magmatic and metamorphic signals acquired from various rock suites in the terrane are crucial as they serve as proxies of changing tectonic style and thermal evolution of crust with time. There is protracted evidence of multiphase magmatism (~2.8, 1.0, 0.8, 0.5Ga) from the metaigneous rocks in Madurai Block. The Cryogenian (~0.8Ga) magmatism specifically is characterized by voluminous A-type granite and anorthosite magmatism reported throughout the block and beyond. Temporally spaced multiphase metamorphic events (~2.5, 1.0, 0.8, 0.6, 0.5Ga) are reported from Madurai Block, wherein, the Ediacaran-Cambrian (~0.5Ga) high-grade metamorphism dominates the metamorphic signals, with sporadic evidence of older metamorphic events. Further, the ~0.5Ga metamorphism has completely obliterated the signals of older metamorphic events in the supracrustal rocks, rendering characterization of older metamorphic events difficult. In this study, we provide the evidence of Cryogenian metamorphic event from a charnockite and metapelites from the Dindigul region in Madurai Block, and for the first time characterize the Cryogenian metamorphism from the lower crustal rocks of southern India.
The charnockites preserve peak assemblage of Grt+Opx+Qz+Kfs+Pl, wherein the garnet has been replaced by Opx+Pl corona. The associated metapelites preserves peak assemblage of Grt+Sil+Bt+Kfs+Pl+Qz±Ilm±Spl, which has been replaced by retrograde biotite. The peak P-T conditions for both the rock types are constrained at 850±30℃ at 8±0.3kbar, using exchange thermometry and phase-equilibria modeling. The P-T estimates for the corona formation yield similar temperatures (820±30℃) at relatively lower pressures (7.4±0.2kbar). Conventional thermometry utilizing the garnet and retrograde biotite yields P-T condition of 650±30℃ at 6.5±0.3kbar for the retrograde metamorphism.
The timing of different stages of metamorphism has been constrained using in-situ monazite geochronology from the metapelites. BSE imaging and X-ray elemental maps obtained from individual monazite grains display patchy zonation with three distinct compositional domains which corresponds to distinct age clusters. The Th-poor and HREE enriched dark cores of matrix monazite, yield a weighted mean age of 844±35Ma, dating the prograde evolution prior to or during the initial stages of garnet growth. On the other hand, relatively Th-enriched and HREE poor, cuspate mantles replacing the core, yield a weighted mean age of 815±30Ma, dating garnet growth due to biotite dehydration melting at the peak stage. In contrast, Th and HREE poor fine rims, cross cutting both core and mantle, yield a weighted mean age of 615±35Ma, dating the timing of Ediacaran-Cambrian overprinting. Obtained ages are suggestive of a Cryogenian granulite facies metamorphism in the Madurai Block. The spatially and temporally associated A-type granite and anorthosite magmatism were most likely the heat source. Similar ages obtained from metaigneous rocks from several places within the block shows that Cryogenian granulite facies metamorphism was widespread, although its signal has been substantially overprinted by a higher grade Ediacaran-Cambrian metamorphism.
How to cite: Singha, A., Tiwari, A. K., Sarkar, T., Sorcar, N., and Mukherjee, S.: Petrological characterization of the Cryogenian lower crust of southern India: Evidence from charnockites and metapelites from the northern part of Madurai Block, EGU General Assembly 2023, Vienna, Austria, 24–28 Apr 2023, EGU23-610, https://doi.org/10.5194/egusphere-egu23-610, 2023.
High temperature (HT) processes culminating in granulitization and partial melting significantly contribute to the growth and internal differentiation of the continental crust. These processes may be activated in different geodynamic contexts, under both extensional and compressional regimes. The exhumed HT metamorphic rocks are thus crucial to unveil the P–T–d–t and compositional evolution of the lowest crustal levels, which are not accessible in any other way. Permian lithospheric extension led to an HT regime that affected the Variscan crust, which is nowadays fragmented and widespread worldwide, and within the Alpine belt, and is not always well-preserved. The Valpelline Unit (Dent-Blanche Tectonic System, Western Italian Alps) represents a spectacular exposure of a pre-Alpine lower continental crust section; it has almost totally escaped the Alpine-age metamorphic imprint perfectly preserving Permian HT metamorphic assemblages and structures. This unit comprises migmatitic gneiss displaying heterogeneous mineral assemblages (i.e., Grt-Bt-Crd, Grt-Bt-Opx, Grt-Sil-Bt) and complex structural relationships, together with minor migmatitic amphibolites, basic granulites and marbles. Therefore, the Valpelline Unit represents a rare opportunity to explore the evolution of the lower crustal levels during the Permian lithospheric extension. Mostly for these reasons, several works (Diehl et al., 1952; Nicot, 1977; Gardien et al., 1994; Manzotti & Zucali, 2013) have dealt with the HT evolution of the Valpelline Unit in the past decades, but a full description of the rock types and structures is still lacking. This kind of information, coupled with a clear overview of the melt-present deformation and its resulting fabric relationships, is necessary to start an extensive multidisciplinary study (e.g., P–T–d paths, geochronology and geochemical surveys) aimed to unveil the processes of crustal differentiation and make interpretations regarding the Permian HT tectonics affecting these deep continental fragments. This contribution provides (i) a detailed litho-structural overview of the rocks exposed in the Valpelline Unit and (ii) preliminary thermometric and barometric estimations (e.g., by combining Zr–in–rutile and Ti–in–biotite geothermometers with quartz–in–garnet elastic geobarometry) related to HT metamorphism and melt production stages to check pressure and temperature variations among different types of migmatites (e.g., Crd– vs. Opx–bearing) in different sectors of the studied area.
Diehl E.A., Masson R. & Stutz A.H. (1952). Contributo alla conoscenza del ricoprimento della Dent Blanche. Memorie degli Istituti di Geologia e Mineralogia dell’Università di Padova, 17, 1-52.
Gardien V., Reusser E. & Marquer D. (1994). Pre-Alpine metamorphic evolution of the gneisses from the Valpelline series (Western Alps, Italy). Schweiz. Minerla. Petrogr. Mitt., 489-502.
Manzotti P. & Zucali M. (2013). The pre-Alpine tectonic history of the Austroalpine continental basement in the Valpelline unit (Western Italian Alps). Geol. Mag., 150, 153–172.
Nicot E. (1977). Les roches meso et catazonales de la Valpelline (nappe de la Dent Blanche, Alpes italiennes). (Doctoral dissertation, éditeur inconnu).
How to cite: Caso, F., Zucali, M., Strambini, A., Piloni, C. B., and Filippi, M.: Structural and metamorphic features of a Permian lower crust section from the Western Italian Alps (Valpelline Unit, Valle d’Aosta), EGU General Assembly 2023, Vienna, Austria, 24–28 Apr 2023, EGU23-3529, https://doi.org/10.5194/egusphere-egu23-3529, 2023.
Nephrite had long been mined as resources of gemstone in eastern Taiwan. It outcrops in the orogenic mountain, the Central Range, where the black schist dominates and the ultramafic serpentinites distribute sparsely. The orogeny has occurred when the subduction (South China Sea subducted to the Philippine Sea Plate) ceased and collision began at about 5 Ma. Observations shows the nephrite occurred at the interface of serpentinite and the Clinozoisite schist, enriched in Cr, Ni, but also Ca. The genesis of nephrite had been thought as a result of a series of complex reactions include the metasomatism of ultramafic rock and its surroundings and succeeding fluid interactions. This study conducts B isotopes, Sr isotopes and trace elemental measurement to give further geochemical constraints on the genesis of nephrite. Samples include rocks--black schist, clinozoisite schist, serpentinite, and associated minerals—nephrite, diopside, calcite, tremolite asbestos, cat’s eye nephrite and talc. The Sr element are enriched in clinozoisite schist, calcite, black schist (1005 ppm, 285 ppm~545 ppm, 150 ppm, respectively), and rather depleted in nephrite, diopside, cat’s eye nephrite, tremolite asbestos, serpentinite (4.3 ppm, 5.6 ppm, 3.2 ppm, 2.5 ppm, 2.0 ppm, respectively). Despite the huge difference in Sr contents, the 87Sr/86Sr ratios of all the samples are in the range of 0.71424 ~ 0.71815, with the highest in serpentinite (0.718151) and lowest in clinozoisite schist, nephrite and clacite (0.714240, 0.714788, 0.714951~ 0.715925, respectively), indicate the Sr source from continental crust majorly. The B concentrations and δ11B values are: in serpentinite ~21 ppm and -0.5 ‰, in nephrite ~5 ppm and -6.1 ‰, in clinozoisite schist ~2.5 ppm and -5.9 ‰. The B isotopes characterize the serpentinite as of “subduction zone type”. The isotopes study provides constraints to the genesis of nephrite and thus a possible viewpoint: although the immobile elements, e.g. Cr, Ni, shows the nephrites origin from serpentinite, its different 87Sr/86Sr ratios from serpentinite indicates later flushing by fluids which are similar to those in clinozoisite schist and calcite. And the nephrite’s lower B concentrations and δ11B values than in serpentinite may result from the flushing (replacement) of later fluids or dehydration processes, or both. Further discussions combining the viewpoints of mineralogy would be necessary to make more comprehensive interpretations.
How to cite: Pi, J., Chen, H.-F., and Chao, H.-C.: The Genesis of Nephrite— Geochemical Constraints by B isotopes, Sr isotopes and Trace Elements, EGU General Assembly 2023, Vienna, Austria, 24–28 Apr 2023, EGU23-4889, https://doi.org/10.5194/egusphere-egu23-4889, 2023.
Over the last one decade, it has become increasingly clear that a distinct tectonothermal event has affected the entire Southern Granulite Terrane of India during the Cryogenian (850-635 Ma), however, the evidence is more predominant from the Madurai Block. Characterization of this tectonothermal event through multi-dimensional petrochronological studies is crucial in understanding the Proterozoic crustal evolution of southern India in particular, and the thermal evolution of continental crust, in general.
In the Madurai Block, the oscillatory-zoned elongated magmatic zircon grains, with unzoned metamorphic rims, from the porphyritic charnockites, intruding the massive mafic rocks and enderbites, yield a Cryogenian (~800 Ma) magmatic emplacement age and an Ediacaran-Cambrian metamorphic overprint (~570 Ma). Detailed geochemical study reveal that the precursors of these charnockites were ferroan A-type granite plutons that were most likely emplaced in a riftogenic setting. Texturally controlled in-situ dating of monazite grains from the associated garnet-biotite-sillimanite bearing metapelitic granulites, occurring north and west of the Sirumalai Hills near Dindigul city, yield weighted mean ages of 845-815 Ma from the core and mantle, dating the age of peak metamorphism. The chemically distinct, recrystallized thin rims, sometimes cutting across both core and mantle, yield a weighted mean age of ~615 Ma, signifying Ediacaran-Cambrian metamorphic overprint. Detailed petrological and thermobarometric study, complemented by thermodynamic modelling, constrain the peak P-T conditions of these rocks at ~800-850°C, 7.5-8.0 kbar. The age of the peak metamorphism, obtained from the monazite cores and mantles, is coeval with the extensive A-type felsic magmatism in the Madurai Block, suggesting that the metamorphic event was linked to the enhanced heat input through rift related felsic magmatism. However, the trigger behind the widespread Cryogenian thermal events needs to be ascertained to place them in context of the global tectonic framework.
The Mesoproterozoic supercontinent Rodinia, which assembled between 1300 and 900 Ma, broke apart during the Cryogenian between 830 and 650 Ma. The Indian continent, being an integral part of all Rodinia reconstructions, was largely affected by the magmatic and metamorphic events related to Rodinia breakup, and the Southern Granulite Terrane is no exception. In summary, we suggest that the pre-Cryogenian crust of the Madurai Block has been affected by widespread and voluminous A-type magmatism and associated granulite facies metamorphism in response to rifting and crustal extension during the breakup of the Rodinia supercontinent. Subsequent compression and crustal thickening related to Gondwana amalgamation during Ediacaran-Cambrian resulted in high- to ultrahigh-temperature metamorphism. This metamorphic event was long and strong enough to overprint, and sometimes obliterate, the signals of the Cryogenian thermal event.
The Cryogenian thermal events have also been recorded from the Nilgiri-Namakkal Block, north of the Palghat Cauvery Shear Zone. The strikingly similar geochemical characteristic and close spatial association of the Cryogenian rocks across the perceived terrane boundary, i.e. the Palghat Cauvery Shear Zone, negates the hypothesis of Cambrian amalgamation of the Southern Granulite Terrane with the Dharwar craton along the Palghat Cauvery Shear Zone.
How to cite: Sarkar, T., Tiwari, A. K., and Singha, A.: Cryogenian tectonothermal events in the Madurai Block of the Southern Granulite Terrane, India: Characterization and implications., EGU General Assembly 2023, Vienna, Austria, 24–28 Apr 2023, EGU23-8405, https://doi.org/10.5194/egusphere-egu23-8405, 2023.
The Cryogenian gabbros of the Ambatondrazaka region belong to the Imorona-Itsindro plutonic suite that originated from an upper mantle source after the eastward subduction of the Mozambican oceanic lithosphere beneath the Precambrian Malagasy basement from 0.8–0.7 Ga. These gabbros exhibit a particular coronitic texture where each corona consists of a core of forsteritic olivine surrounded by three successive rims. The first rim is formed by clinoenstatite, the second is formed by the clinoenstatite-diopside intergrowth with some exsolutions of pleonaste and pyrope garnet. However, the last is formed by symplectites of pargasite with exsolutions of pleonaste. Assuming that the temperature gradually decreases and that the pressure remains constant or also gradually decreases, the coronitic texture is the result of three successive stages of mineral reactions. In the first stage at rim one, the crystallization of clinoenstatites was favored by the diffusion of Fe2+ and Mg2+ from the forsteritic olivine being rich in Mg2+ while the supply of Si and Al comes from the surrounding labradorite. During the second stage in rim two, the formation of the clinoenstatite-diopside intergrowth follows the same crystallization process as that in rim one, but the calcium input from the surrounding labradorite favored the crystallization of diopside. Additionally, the supply of Mg and Fe from olivine and Al from labradorite resulted in the formation of pleonaste and pyrope garnet exsolutions. In the last stage at rim three, the formation of pleonaste exsolutions is identical as in stage two, while the supply of H2O favored the crystallization of pargasite symplectites. Overall, the coronitic texture is the result of a solid-state metamorphic reaction due to orogenic uplift related to the Pan-African Orogeny (0.58 – 0.51 Ga). The anhydrous phases of the reaction in the upper mantle formed the pyroxenes, spinels, and garnet in rims one and two, while the hydrous phase in the continental crust favored the formation of pargasites in rim three.
How to cite: Rarivoarison, H.: Petrographic and mineralogical studies of the formation of coronitic gabbros in the Ambatondrazaka region, central Madagascar , EGU General Assembly 2023, Vienna, Austria, 24–28 Apr 2023, EGU23-5499, https://doi.org/10.5194/egusphere-egu23-5499, 2023.
The Pan-African Damara Belt in southern Africa is a trench-trench-trench triple junctions orogen that formed at 590-470 Ma during the Gondwana Supercontinent assembly. The Damara Belt records up to granulite facies HTLP metamorphism in the core, upper plate of the orogen. However, the cause of this metamorphism is not well understood. To tackle this problem, we focus on the ENE-WSW-trending Namibfontein-Vergenoeg (NV) migmatitic domes. We use P-T-t-d data to investigate the temporal relationships of deformation fabrics, metamorphism and melting.
The NV domes formed through the superposition of four folding events. We use LA-(Q/MC)-ICP-MS U-Pb dating of monazite from structurally controlled granitoids and leucosomes to define the relative timing of the deformation phases. These include 1) an early phase of E-W shortening forming upright F1 folds and steep N-S-striking S1 deformation fabrics. D1 was active between ~559 and 530 Ma. 2) N-S shortening followed, forming dome-scale F2 anticlines with steep E-W-striking deformation fabrics at ~527 Ma. 3) Local inclined folding of S1 and S2 fabrics formed shallow NW-dipping S3 fabrics that was active before ~520 Ma. Lastly, 4) NE-SW shortening produced F4 folds and associated moderately NE-dipping S4 deformation fabrics at ~520-500 Ma.
Rocks of the NV domes are metamorphosed to upper amphibolite facies. Melt (up to 10%) exists within and defines structures of all four deformation phases. All deformation fabrics show similar mineral assemblage; cordierite + sillimanite + biotite + K-feldspar + quartz + melt ± garnet and plagioclase with accessory amounts of apatite, monazite, zircon, ilmenite, and magnetite. Matrix consists of sillimanite, garnet, cordierite, biotite, quartz, k-feldspar, plagioclase, ± ilmenite, magnetite, monazite, zircon, and apatite. Two distinct garnet porphyroblasts occur, i) an earlier large (1-2 mm) poikiloblastic garnet (with sillimanite, biotite, and quartz inclusions) partly replaced by cordierite occurring mostly in D1 and D2 samples, and ii) smaller (up to 1 mm sized), peritectic garnet. Pseudosection modelling shows that rocks of the NV domes record HTLP conditions (740-760 °C, 4-4.5 kbar). The overgrowth of cordierite on early garnet in the presence of melt supports the HTLP conditions along the retrograde path.
The rocks at the NV domes were deformed, in the presence of melt, four times over at least ~60 Ma under the same HTLP amphibolite facies conditions, during which granitic magmatism was prevalent. The absence of HP inclusions in porphyroblasts (either not preserved or never developed) and deformation structures supporting orogenic collapse, exclude decompression melting as a mechanism for crustal anatexis. Rather these data suggest the rocks continuously melted during crustal shortening, likely during the collisional phase of the orogen.
How to cite: Ormond, R., Lehmann, J., Hasalová, P., and Elburg, M.: Pressure-Temperature-time-deformation (P-T-t-d) constraints on dome formation in the HTLP Pan-African Damara Belt, Namibia, EGU General Assembly 2023, Vienna, Austria, 24–28 Apr 2023, EGU23-6324, https://doi.org/10.5194/egusphere-egu23-6324, 2023.
The consideration of mass balance to loss of elements from metamorphic rocks during devolatilization and anatexis reveals some principal constraints that must be considered in any model of element redistribution in metamorphic processes. During devolatilization, the changes in rock composition with the increase of metamorphic grade are a result of loss of fluid, produced by devolatilization reactions. Fluid, characterised by low viscosity and density, can be effectively extracted from a rock. Metamorphic devolatilization on average results in loss of 1-4 wt. % of the rock mass to the fluid and typically the average loss is <2 wt. %. This relatively small mass fraction mandates that in order to decrease the content of an element significantly (small percentage loss will not be visible on sediment heterogeneity) the concentration of an element in fluids must be much greater than in the protolith. For example, for 50% extraction of an element by 2% fluid, the fluid should have 25 times higher content than the protolith and loss of 50% of element with 0.5% of fluid require fluid with 100 times enrichment (Stepanov 2021).
Anatexis produce granitic melt with high viscosity and density lower than restite. The experimental data suggest that melt extraction could occur when melting degrees >10%. For a completely incompatible element enrichment by 10 times relative to protolith could is maximum achievable in anatectic process. Many elements are concentrated in residual phases and completely incompatible behaviour is rarely observed, hence reducing the efficiency of enrichment. The closes examples of incompatible behaviour during anatexis are restites produced by high-T anatexis, when accessory minerals experienced complete dissolution in melt, such as restites of the Kokchetav complex and septa from Ivrea Verbano Zone (Ewing et al., 2014). However, higher melting degree produce less enriched melt even for incompatible elements. For compatible element melt loss increase content in restite, but loss of 10% melt increase only by 11%, and 50% of melt loss (which could be considered as maximum) increase incompatible element by factor of 2. The mass balance constraints show limits of the possible effect of fluid/melt loss on rock composition and suggests that fluid loss could produce higher enrichment factors than melt loss.
References
Stepanov A.S., A review of the geochemical changes occurring during metamorphic devolatilization of metasedimentary rocks. Chemical Geology, 568 v, 120080, 2021.
Ewing, T.A., Rubatto, D., Hermann, J., 2014. Hafnium isotopes and Zr/Hf of rutile and zircon from lower crustal metapelites (Ivrea–Verbano Zone, Italy): Implications for chemical differentiation of the crust. Earth and Planetary Science Letters 389, 106–118.
How to cite: Stepanov, A.: The mass balance constraints on the depletion of elements during metamorphic devolatilization and anatexis, EGU General Assembly 2023, Vienna, Austria, 24–28 Apr 2023, EGU23-7897, https://doi.org/10.5194/egusphere-egu23-7897, 2023.
A robust temporal constraint on the timing and duration of metamorphism is paramount for correctly interpreting the geodynamic evolution of orogenic belts. The Madurai Block of the Southern Granulite Terrane, India has garnered much attention on account of regional-scale ultrahigh-temperature metamorphism. Although there has been a comprehensive characterization of the conditions of metamorphism from various rock types, the timing and especially the duration of metamorphism remain ambiguous, resulting in diverse geodynamic interpretations. Here, we investigate the charnockites and associated sapphirine-bearing semipelitic granulites from the eastern part of Madurai Block by integrating texturally controlled in-situ monazite geochronology with petrology, thermobarometry and phase-equilibria modelling. The integrated petrochronological approach provides a petrographic context for the monazite ages, which enables obtaining a detailed chronological-metamorphic evolution of the rock suites to confidently constrain the P-T-t evolution and timescale of metamorphism.
Conventional exchange thermobarometry yields peak P-T conditions of 970-950°C at 10-11kbar pressure for both rock types. Peak ultrahigh-temperatures are further confirmed by feldspar solvus thermometry (950-980°C at 10kbar) in the semipelites and P-T pseudosection (MnNCKFMASHTO) contoured for compositional and modal isopleths of major minerals phases in both the rock types. Subsequent decompression-cum-cooling has led to the formation of coronal Opx+Pl in the charnockite and symplectic Opx±Crd±Spr±Pl in the semipelite, at the P-T range of 950-820°C and 9.0-6.5kbar. This was followed by cooling to sub-solidus conditions. Based on the obtained P-T estimates, preserved reaction textures, and phase equilibria modelling, a clockwise P-T evolution with decompression-cum-cooling is inferred for both rock types.
The in-situ U-Th-Pb ages and compositional characteristics of monazite grains are strongly correlated to their textural association, providing a temporal control on the obtained P-T path. The core of the matrix monazite in the charnockite and semipelite, having low Th, Y and extreme HREE depletion, yielding weighted mean ages of 590-582 Ma, date the prograde evolution. The rim of matrix monazite in charnockite and mantle in the semipelite, having relative Th enrichment than core, yielding weighted mean ages of 557-552 Ma, date extensive dissolution-reprecipitation from melt at the peak stage. The relatively Th and Y enriched and moderately HREE depleted rim of matrix monazite in the semipelite, yielding weighted age of 516 Ma, date initial garnet breakdown during post peak melt-crystallization. In contrast, the Th-poor and Y- and HREE-rich symplectic monazite, yielding weighted mean age of 490 Ma, date extensive garnet breakdown during final stages of melt crystallization. Our findings point to a collision initiation at ~590 Ma, where the peak conditions were attained at ~550 Ma followed by extensional collapse at ~520-490 Ma, resulting in rapid upliftment of lower crustal rocks to mid-crustal levels in sustained UHT conditions, followed by cooling to reach a stable geotherm. Our results suggest a long-lived hot orogeny in the Madurai Block, where the UHT conditions were sustained for at least 60 MYr. The UHT conditions were most likely attained in the core of a long-lived hot orogen by the combined effect of conductive heating through radioactive decay and mantle heat supply, with the former being the primary driver.
How to cite: Tiwari, A. K., Sarkar, T., Sorcar, N., and Mukherjee, S.: Petrochronological appraisal on the timing and duration of ultrahigh-temperature metamorphism in southern India: Insights from charnockite and sapphirine bearing semipelitic granulites from the Madurai Block , EGU General Assembly 2023, Vienna, Austria, 24–28 Apr 2023, EGU23-472, https://doi.org/10.5194/egusphere-egu23-472, 2023.
In-situ laser ablation ICP-MS/MS is becoming a widespread approach for white mica Rb/Sr geochronology. This technique allows determination of single-spot dates using an initial 87Sr/86Sr composition measured from Ca-bearing phases (Rösel & Zack 2022; GGR 46). The dates can be correlated with microstructural position and chemistry of white mica to discern complex tectonic histories. To demonstrate the power of in-situ white mica Rb/Sr geochronology, the technique was applied to four high-pressure/low-temperature (HP/LT) lithologies of the Cycladic Blueschist Unit (CBU) on Syros, Greece, which reached ~22 kbar and ~550°C at c. 53-45 Ma (e.g., Laurent et al. 2018; JMG 36). The CBU along the southern coast contains foliated eclogitic blocks that are wrapped by retrograde, foliated blueschists. At the western coast, the CBU possesses non-foliated HP skarn blocks similarly surrounded by retrograde, foliated blueschists. In the eclogite and blueschists, alignment of white mica defines the foliation along with glaucophane, epidote, and titanite. The southern blueschist also bears white mica grains with mineral cleavage oblique to the foliation. In the skarn, white mica are undeformed and sometimes exhibit a radial habit. White mica chemistry is relatively homogeneous in the eclogite (XCel: 0.33-0.39) and skarn (XCel: 0.36-0.50) compared to the blueschists from the western (XCel: 0.26-0.50) and southern (XCel: 0.33-0.57) exposures. Single-spot Rb/Sr dates are not correlated with microstructure nor chemistry for the eclogite and skarn, yielding weighted averages of 58.1 ± 4.3 Ma (MSWD: 1.3; n: 38) and 43.8 ± 2.8 Ma (MSWD: 1.1; n: 30), respectively. The blueschists show dispersions of dates that correlate with chemical variations, proxied by high-Ti (>1300 µg/g) and low-Ti (<1000 µg/g) domains. For the western blueschist, high-Ti domains yield a weighted average of 44.8 ± 3.4 Ma (MSWD: 0.93; n: 14), whereas low-Ti zones are 35.5 ± 2.9 Ma (MSWD: 1.4; n: 22). For the southern blueschist, high-Ti regions yield dispersed Cretaceous to Eocene dates, predominantly defined by the oblique white mica. The low-Ti domains gave a weighted average of 39.8 ± 2.1 Ma (MSWD: 0.99; n: 19). Altogether, white mica Rb/Sr geochronology records the timing of HP/LT metamorphism in the eclogitic block, followed by HP metasomatism in the skarn, and subsequent retrograde deformation events recorded by the low-Ti mica domains in both blueschist samples. The dates from high-Ti zones of the western blueschist reflect partial retention of the metasomatic history. The dates from high-Ti domains from the southern blueschist are older than HP/LT metamorphism and are interpreted as partial retention of 87Sr from the blueschist’s protolith. The older events in the blueschist, and the metamorphic record of the eclogite, were not recorded by white mica 40Ar/39Ar geochronology on the equivalent rocks from the same exposures, which instead preserve the retrograde events (Laurent et al. 2021; GCA 311). These results demonstrate that Rb/Sr geochronology is a dynamic tool when coupled with structural and chemical data to extract metamorphic, metasomatic, deformation, and possibly detrital/magmatic records of white mica in rocks metamorphosed below ~600°C.
Funding provided by the National Science Center of Poland project nr. 2021/40/C/ST10/00264
How to cite: Barnes, C. J., Zack, T., Bukała, M., Rösel, D., and Schneider, D. A.: White mica Rb/Sr geochronological records of high-pressure/low-temperature rocks in the Cycladic Blueschist Unit (Syros, Greece), revealed by in-situ laser ablation ICP-MS/MS, EGU General Assembly 2023, Vienna, Austria, 24–28 Apr 2023, EGU23-9493, https://doi.org/10.5194/egusphere-egu23-9493, 2023.
The Barrovian type metamorphism affecting the Precambrian microcontinents of peri-Siberian tract of the Central Asian Orogenic Belt is mostly dated indirectly on zircon from (syn-tectonic) magmatic rocks as Late Proterozoic – Ordovician. However, in-situ monazite geochronology in micaschists and migmatite gneisses at the northern part of the Precambrian Baidrag block, central Mongolia, revealed that the Baikalian Late Proterozoic – Early Cambrian cycle overprints an earlier Tonian phase of metamorphism. The apparent Barrovian-type zoning ranging from garnet, staurolite, kyanite to kyanite/sillimanite migmatitic gneisses is thus false and points to hidden metamorphic discontinuities and mixed metamorphic histories from different times. Therefore, to decipher and interpret the record of different tectono-metamorphic events it is necessary to unravel complete P–T–t paths from individual samples. Two localities with Tonian-age monazite show anticlockwise P–T paths: 1) Grt−Sil−Ky gneiss records burial to the sillimanite stability field (~720°C, 6.0 kbar) followed by burial to the kyanite stability field (~750°C, 9 kbar) and, 2) The Grt−St schist records burial to the staurolite stability field (~620°C, 6 kbar), further followed by almost isothermal burial (~590°C, 8.5 kbar). Based on monazite textural position, internal zoning, and REE patterns, the time of prograde burial under a thermal gradient of 27–32°C/km is estimated at c. 890−853 Ma and further burial under a geothermal gradient of 18–22°C/km is dated at c. 835−815 Ma. On the other hand three localities with Late Proterozoic to Cambrian monazite ages show clockwise metamorphic paths at variable P–T gradients: 3) P–T conditions of the Grt schist reaches ~5 kbar and 500 °C and 4) the Grt−St−Ky schist reaches conditions of 9 kbar and 670 °C, indicating burial under a geothermal gradient of 20–26 °C/km. 5) Grt–Sil gneiss shows peak of 6–7 kbar and 700–750 °C, indicating melting conditions at 30–32 °C/km gradient. Monazite included in porphyroblasts and in the matrix indicate that these P–T conditions reached under variable geothermal gradient were semi-contemporaneous and occurred between 570 and 520 Ma. By correlation with published zircon ages of 600–530 Ma from granitoid magmatic rocks we suggest that the areas with higher geothermal gradient may be explained by closer vicinity of magmatic intrusions. These P−T and geochronology data from a continuous Barrovian metamorphic section suggest that anticlockwise P−T evolution from c. 930 to 750 Ma can be interpreted as a result of thickening of peri-Rodinian supra-subduction extensional and hot edifice. This metamorphic event was followed by a clockwise P−T evolution from c. 570 to 520 Ma possibly related to shortening of the northern Baidrag active margin and incipient collision with with peri-Siberian continental mass further north.
How to cite: Stipska, P., Peřestý, V., Soejono, I., Schulmann, K., Collett, S., Kylander Clark, A. R. C., and Aguilar, C.: Hidden metamorphic discontinuities in the NE Baidrag block, Mongolia, reveal anticlockwise metamorphic paths at c. 890−790 Ma indicating peri-Rodinian back-arc compression followed by c. 560–520 Ma burial , EGU General Assembly 2023, Vienna, Austria, 24–28 Apr 2023, EGU23-15332, https://doi.org/10.5194/egusphere-egu23-15332, 2023.
Precise thermobarometric and geochronologic data are crucial to correctly interpret the timing of metamorphism and identify 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). In the latter, polymetamorphism was previously inferred. However, the timing of metamorphism is poorly resolved and only limited geochronology exists in the Schöckel Nappe.
Detailed petrographic investigations of chloritoid-bearing phyllite and micaschist samples collected at two localities at the base and in a higher structural level of the Schöckel Nappe revealed complex phase relations of REE-minerals, involving multiple REE-epidote generations that may be associated with monazite, xenotime, apatite and zircon. In garnet-bearing micaschist of the Waxenegg Nappe, we observed large (up to 500 µm) monazite exhibiting distinct core-rim chemical zoning. From careful documentation of the microstructural phase relations, thermodynamic modeling, Raman spectroscopy of carbonaceous matter and in-situ LA-ICPMS U-(Th)-Pb dating of REE-epidote and monazite we show that rocks in all three localities were affected by LP metamorphism (0.3 – 0.4 GPa) during the Permian event (250 – 282 Ma) with peak temperatures decreasing from 560°C in the lower to 475°C in the upper nappe. During the Eo-Alpine event, overprinting at c. 90 Ma occurred under conditions of ~550°C and 1.0 – 1.1 GPa in the Waxenegg Nappe. At the base of the Schöckel Nappe, peak metamorphism at ~450 – 470°C and 0.4 – 0.7 GPa and cooling below 300°C likely took place before 110 Ma. Towards higher structural levels, only limited Eo-Alpine overprinting at low P-T conditions (<400°C, 0.3 – 0.5 GPa) is evident, thus the observed mineral assemblage reflects mostly Permian metamorphism.
Our results demonstrate that the main metamorphic signature in the Schöckel Nappe can be resolved as the Permian event and that the Eo-Alpine overprint is relatively lower grade than previously proposed. We observe a marked increase in Eo-Alpine peak conditions (~80 – 100°C, 0.3 – 0.5 GPa) across the nappe contact with higher grade rocks in the footwall compared to the hanging wall. The metamorphic pattern is consistent with the existence of a major normal fault between the Drauzug-Gurktal Nappe and Koralpe-Wölz Nappe systems in the easternmost part of the Austroalpine Unit, as already identified in its central and western parts. Finally, our study highlights that coupling modern thermobarometric analytical approaches with high spatial resolution geochronology on accessory minerals is critical to improve our understanding of the fundamentally important low-grade units of orogens.
How to cite: Hollinetz, M. S., Huet, B., Schneider, D. A., McFarlane, C. R. M., Rantitsch, G., and Grasemann, B.: Unravelling polymetamorphism in greenschist- and amphibolite-facies rocks using thermodynamic modeling and in situ U-Pb dating of REE-minerals (Austroalpine Unit, Eastern Alps, Austria), EGU General Assembly 2023, Vienna, Austria, 24–28 Apr 2023, EGU23-11972, https://doi.org/10.5194/egusphere-egu23-11972, 2023.
The Southern Granulite Terrane (SGT) of southern India being a regional granulite-facies terrane with exposed mid- to lower-crustal rocks has been the attention of several studies focusing on amalgamation of Gondwana supercontinent. It comprises of a collage of several crustal blocks bisected by crustal scale shears [1]. Among these, the Madurai Granulite Block (MGB) forms the central and largest block in SGT, bounded by Palghat-Cauvery Shear Zone (PCSZ) to the north and Achankovil shear zone (AKSZ) in the south. Within the MGB, a V-shaped shear zone extending towards SW direction from Karur to Kambam, then taking a sharp NW turn at Painavu Shear Zone (KKPTSZ) in the central region of the MGB. Previous studies, however, contradict on the nature and evolution of the KKPTSZ [2,3]. The lithological makeup north of the shear zone is more comparable to the counterparts of Dharwar Craton, while the rocks south of the KKPTSZ are more akin to those of the Eastern Ghats. A recent tectonic model suggests the extension of Karur–Kambam lineament up to the AKSZ, and demarcated it as Kambam ultrahigh-temperature (UHT) belt [2] This has been interpreted to mark a fundamental collisional crustal boundary between eastern and western MGBs. Though, the newly suggested eastern and western crustal block model has greatly aided in understanding the evolution of the HP-UHT belt in north-central MGB, it suffered with inadequate data in identifying basement characteristics and age variations in southern part of the MGB. The present study attempts to synthesize multifarious geological information across the terrain integrated with new petrological, geochemical data for a comprehensive understanding of tectonic and metamorphic processes and thereby crustal evolution in the central Madurai block. The petrological and geochemical characteristics of the granulite-facies rocks suggest igneous origin of the protolith by partial melting of the source region. They are enriched in Na2O over K2O, thus the K2O/Na2O ratio is less than one suggesting it is Tonalitic charnockite [4]. The K/Rb values of the charnockite vary between 81 and 400 with an average of about 245. Ba/Rb ratios in the charnockites are high, between 3.95 and 27.58 (average 12.23) indicating that they are not derived directly from a mantle melt, rather suggesting the role of internal differentiation of a pre-existing TTG-type crust through intra-crustal melting [5]. The result gives similarity to arc granitoid, while from the major and trace element data it is inferred that the formation is during a collisional event. With limited isotope geochronology data and field evidence, the argument of KKPTSZ as a possible terrain boundary is withered. Therefore, more convincing field-based data, integrated with petrological, geochronological, and phase equilibria models are required from this belt for a comprehensive understanding of the crustal evolution in Madurai Block.
[1] Braun & Kriegsman (2003) Spec. Publ., Geol. Soc., London, 206:169–202.
[2] Brandt et al (2014) Precambrian Research, 246: 91–122.
[3] Plavsa et al (2014) Geol. Soc. of America Bulletin, 126: 791–811.
[4] Ravindra Kumar & Sreejith (2016) Lithos, 262: 334–354.
[5] Elis Hoffmann et al (2014) Earth & Planetary Sciences Letters, 388: 374-386.
How to cite: Mohan Sheela, P. and Chettootty, S.: Karur–Kambam–Painavu–Trichur Shear Zone (KKPTSZ) as a possible terrane boundary in Madurai Granulite Block, Southern India: Current understanding and future perspectives, EGU General Assembly 2023, Vienna, Austria, 24–28 Apr 2023, EGU23-11846, https://doi.org/10.5194/egusphere-egu23-11846, 2023.
The Inthanon Zone is regarded as the main suture between the Indochina and Sibumasu blocks and comprises ultramafic rocks, marine sediments and crystalline basement rocks. The gneissic basement is exposed in two different structural domains: (1) the Inthanon core complex located to the west of the Chiang Mai basin, and (2) the Mae Ping shear zone located to the south of the core complex. Here, we present new petrological and geochronological results from gneisses and schists of the Inthanon zone. Four different mineral assemblages can be recognised in gneisses and schists: (1) garnet–muscovite–biotite±sillimanite±chlorite schist, (2) garnet–muscovite–biotite–plagioclase–K-feldspar gneiss, (3) tourmaline-bearing muscovite–biotite– orthogneiss, and (4) migmatitic biotite gneiss. These rocks typically contain accessory ilmenite, pyrite, apatite, tourmaline, monazite, xenotime, and zircon. In-situ Th-U-total Pb dating of monazite reveals at least two metamorphic events, one in the Early Jurassic and another one in the Early Paleocene. A garnet–muscovite–biotite–sillimanite schist sample shows matrix micas and fibrolithic sillimanite wrapped around garnet porphyroblasts. Multi-equilibrium thermobarometry using Tweequ (Berman, 1996) yields metamorphic peak conditions of 0.5 GPa and 570 °C. Monazite dating yields two age populations at 189±5 and 61±7 Ma. A second sample belonging to this group contains chlorite instead of sillimanite and has a main schistosity with tightly folded relicts of a former fabric. Garnet porphyloblasts exhibit pressure shadows with quartz and mica. Monazite dating gives a single age population of 65±6 Ma. Garnet–muscovite–biotite–plagioclase–K-feldspar gneiss samples show corona textures with plagioclase, quartz, biotite, and muscovite around garnet porphyroblasts, indicative of pressure decrease. P–T conditions of 0.6–0.7 GPa and 680–700 °C were calculated using the garnet-biotite-plagioclase-quartz and garnet-biotite geothermobarometers. The formation of coronae around garnet occurred during exhumation at slightly lower conditions of 0.4–0.5 GPa and 640–660 °C. Monazite dating yields a main population at 189±5 Ma with few 50-70 Ma dates. Tourmaline-bearing muscovite–biotite–plagioclase–K- feldspar gneiss samples are characterized by an ultramylonitic texture. Large K-feldspar augen and tourmaline porphyroclasts are surrounded by a fine-grained, foliated matrix of quartz, and feldspar. The mineral assemblage indicates middle amphibolite grade. Monazite dating of this sample yields two populations at 192±3 and 58±4Ma. Migmatitic biotite–gneiss samples preserve a biotite–plagioclase–K-feldspar–quartz assemblage in both the melanosome and leucosome. Monazite dating provides a single population of 61±2 Ma. Two tectono-metamorphic events are revealed by our data: a widespread medium P-T regional metamorphic phase, and a younger overprint of unclear grade (low to high T assemblages are found) but significant spatial extent. While the first event was coeval with abundant plutonism during Sukhothai-Sibumasu collision (~185 Ma), the second one (~60 Ma) does not appear to be connected with regional plutonic activity and might be related to large scale shearing as seen in the Mae Ping and Three Pagoda shear zones.
How to cite: Santitharangkun, S., Hauzenberger, C., Skrzypek, E., and Gallhofer, D.: Petrology and Th-U-Total Pb Monazite Ages from The Inthanon Core Complex, Thailand, EGU General Assembly 2023, Vienna, Austria, 24–28 Apr 2023, EGU23-13576, https://doi.org/10.5194/egusphere-egu23-13576, 2023.
The Dacia megaunit in the Eastern part of Serbia comprises Getic and Supragetic nappe systems and corresponds to E-W striking Balkan Mountains (Sredna Gora and East Balkan units; sensu Schmid et al., 2020). The area of our study is located between Danube River to the East and Mlava to the West (Homoljske Mts., a part of Balkan Mts.) and consist of low to medium grade metamorphic rocks of Late Proterozoic to Early Paleozoic ages.
Two different metamorphic units were sampled:
(1) northern, low-grade metamorphic sequence is characterized by numerous types of chlorite sheets containing chlorite, epidote, muscovite, actinolite, hornblende and garnets together with quartz, albite and secondary calcite and fine-grained illite. Accessory minerals are titanite, rutile, ilmenite and apatite.
The sampled schists were recognized as belonging to low and lower part of medium grade Barrovian metamorphic assemblages, characterized by zonal distribution of the index–minerals: chlorite, epidote, biotite, amphibole and garnet.
(2) southern, medium-grade metamorphic sequence is characterized by different amphibolite rocks, with amphiboles (28-60 vol.%) ranging from tchermakite and magnesiohornblende to actinolite. Additionally, these rocks contain 17 – 40 vol.% of oligoclase, 5-22 vol% of quartz; 5 – 13 vol% chlorite (ripidolite), 0,4 – 13 vol% of Al-Fe epidote and 0,1-0,7 vol% of andradite garnet.
Multielement diagrams normalized to N-MORB of low-grade metamorphic sequence show enrichment of LILE relative to HFSE with negative Nb and positive K, U and Pb anomalies, while medium-grade metamorphic sequence shows a disturbed pattern with LILE >> HFSE, positive Pb anomaly and in some cases U, Th, while Nb, Ti and Sr are negative. Both sequences show significant crustal influence.
Medim-grade metamorphic sequence originate from an igneous precursor (andesite-subalkaline basalt protolith). Using Zr-Ti plot after Pearce, these rocks belong to volcanic arc basalts and within plate tholeiites. According to Meschede (1986) Zr/4-2Nb-Y and Wood (1980) Th-Hf/3-Ta plots, they display normal to enriched MORB characteristics similar to basalts from volcanic arc setting.
Geothermobarometric calculations were made for garnet-amphibole-plagioclase assemblage from medium-grade metamorphic sequence using values of titanium in amphibole and aluminum in chlorites. Obtained temperature range between 600 and 750 °C while pressure range between 7 and 9 Kb, corresponding to the recognized amphibolite facies of medium grade metamorphism. A direction of increase of pressure and temperature conditions within the prograde metamorphic sequence towards the south is proposed.
References:
Schmid SM, Fügenschuh B, Kounov A, Maţenco L, Nievergelt P, Oberhänsli R, Pleuger J, Schefer S, Schuster R, Tomljenović B, Ustaszewski K, van Hinsbergen DJJ (2020) Tectonic units of the Alpine collision zone between Eastern Alps and western Turkey. Gondwana Res 78:308–374.
Meschede, M. (1986) A Method of Discrimination between Different Types of Mid-Ocean Ridge Basalts and Continental Tholeiites with the Nb-Zr-Y Diagram. Chemical Geology, 56, 207-218.
Wood, D.A. (1980) The Application of a Th-Hf-Ta Diagram to Problems of Tectonomagmatic Classification and to Establishing the Nature of Crustal Contamination of Basaltic Lavas of the British Tertiary Volcanic Province. Earth and Planetary Science Letters, 50, 11-30.
How to cite: Borojević Šoštarić, S. and Anzulović, A.: Getermobarometry of the late Proterozoic to Paleozoic Barrovian metamorphic sequence in the Dacia megaunit: case study Eastern Serbia, EGU General Assembly 2023, Vienna, Austria, 24–28 Apr 2023, EGU23-16783, https://doi.org/10.5194/egusphere-egu23-16783, 2023.
Posters virtual: Mon, 24 Apr, 14:00–15:45 | vHall GMPV/G/GD/SM
Mafic granulites occur as enclaves within host mylonitized felsic rocks along the WNW-ESE trending, northerly dipping (40°-80°) Mahanadi Shear Zone (MSZ) of the Eastern Ghats Province (EGP), eastern India. Mafic granulite enclaves are characterized by the mineral assemblages Grt+Cpx+Pl+Qtz±Opx±Hbl±Bt (type-1) and Opx+Cpx+Pl+Hbl±Bt (type-2). The type-1 mafic granulite is the focus of the present study and this rock occurs as small enclaves (up to a few tens of meters in maximum size) within mylonitic augen gneiss, finer grained felsic gneiss (Qtz+Kfs+Pl+Bt±Grt), and type-2 mafic granulite. The type-1 mafic granulite is partially to completely recrystallized, massive to crudely foliated rock containing the peak metamorphic assemblage of coarse granoblastic garnet (Grt), clinopyroxene (Cpx), plagioclase (Pl) and quartz (Qtz). Coarse Grt contains inclusion of hornblende (Hbl) which suggests that the peak assemblage was formed by Hbl-dehydration melting. While the peak assemblage is stable in most of the samples, coarse Grt shows partial decomposition to a symplectic intergrowth of Cpx+Pl±Opx (orthopyroxene) in a few samples. Phase chemical data suggest that the rim compositions of coarse Grt show small but significant drop in pyrope content (ΔPrp = 2-3 mole%) from the core, while the coarse Cpx shows more magnesian core (XMg = 0.76) than the rim (XMg=0.68). Plagioclase core is more albitic (XAb = 0.40) compared to the rim composition (XAb=0.16). Geothermobarometric calculations show that the peak pressure of metamorphism was 14-12 kbar at a temperature of ~760-840°C, whereas the rim compositions of Grt in association of coarse Cpx+Pl+Qtz and symplectic Cpx+Pl±Opx yield pressure of 8-9 kbar at ~700-750°C. This suggest a near-isothermal (ΔT=60-90°C) decompression (ΔP=3-6 kbar) of the thickened lower crust indicating exhumation related to thrusting. This regional-scale thrusting was followed by an event of cooling that produced Hbl- and Bt-bearing assemblages. Combining the inferred prograde and retrograde histories, we reconstruct a clockwise P-T path from the studied type-1 mafic granulites. Identification of such clockwise P-T path with characteristic high-temperature decompression from the MSZ is a first of its kind from the interior of the EGP which is otherwise characterized by ca. 1000-900 Ma ultrahigh temperature metamorphism (UHTM; T>900°C) at 7-8 kbar pressure. This study thus shows convincing evidence of a hitherto unrecognized early (> 1000-900 Ma) collisional tectonometamorphic history of the MSZ vis-à-vis the EGP, and hints that the former could represent a fossilized suture zone linked with possible terrane accretion and collision between India and East Antarctica.
How to cite: Karmakar, S., Bose, S., Ghosh, G., Sorcar, N., and Mukherjee, S.: Evidence of high-pressure metamorphism along the Mahanadi Shear Zone in the Eastern Ghats Province, eastern India: implications on tectonics and continental assembly involving India and East Antarctica., EGU General Assembly 2023, Vienna, Austria, 24–28 Apr 2023, EGU23-2040, https://doi.org/10.5194/egusphere-egu23-2040, 2023.
Formation and Evolution of different rock types during growth of Indian shield and mobile belts gives us opportunity to understand tectono-metamorphic implications of Indian subcontinent in Precambrian time. CGC is one such well-preserved fold belt formed during Proterozoic time period which serves valuable knowledge about the evolutionary history of Peninsular India through its rock record. It is located in the eastern part of Indian subcontinent and vastly occupied by Precambrian granite gneiss. From our field observation along Purulia shear zone (PSZ) and published data from different parts of CGC, we observed six stages of deformational and metamorphic evolution based on overprinting relationship of deformation, metamorphic and igneous intrusions.
During stage-I, oldest 1870 Ma Ultra High Temperature (UHT) Metamorphic event (M1) happened and it is observed in form of granulite enclaves in E and SE regions of CGC. In stage-II, high-grade metamorphism (M2) defines by regional UHT metamorphism and partial melting of supracrustals during collisional orogeny that causes formation of migmatitic charnockite gneiss by intrusion of granitoid into older M1 granulites. In Northern part of CGC gray granites (porphyritic) intruded into unknown felsic basement with pelitic/calc-silicate supracrustals at 1750-1660 Ma. In this stage S1 gneissic band developed in the regionally extensive gneisses during D1 deformation. Stage-III is defined by post-D1 magmatism where gabbro-anorthosite, porphyritic granitoid, syenite within ∼1650 Ma high grade basement gneiss intruded at ~1550-1500 Ma. In Stage-IV, Paleoproterozoic basement along with the post D1 intrusive deformed under granulite facies metamorphism (M3) in continent-continent collisional setting causes development of regional thin gneissic banding (S2) along E-W related to D2 and D3 deformations during 1000–950 Ma. Stage-V is defined by Post- D3 Intrusion of nepheline syenite, alkali syenite, porphyritic granite and mafic dyke during rifting stage of Grenvillian basement crosscutting all the preexisting fabrics during 950-900 Ma. Stage-VI is defined by upper amphibolite-facies metamorphism (M4) to produce amphibolite, foliated granite and augen gneiss. Pegmatite & leucogranite emplaced parallel to the axial planes of F1-F3 folds interpreted from the mafic dykes in the eastern part of CGC. This causes development of the S3 fabric in N-S orientation overprinted early granulite fabrics because of dominant F2 folding indicates strong E-W compression during 850-780 Ma & 870-780 Ma.
How to cite: Kundu, S. and Tiwari, S. K.: Deformation and metamorphic evolution of Chotanagpur Gneissic Complex (CGC), East Indian Shield, EGU General Assembly 2023, Vienna, Austria, 24–28 Apr 2023, EGU23-8984, https://doi.org/10.5194/egusphere-egu23-8984, 2023.
