Articles | Volume 35, issue 4
https://doi.org/10.5194/ejm-35-589-2023
© Author(s) 2023. This work is distributed under
the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/ejm-35-589-2023
© Author(s) 2023. This work is distributed under
the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
Research article
| 
02 Aug 2023
Research article |
| 02 Aug 2023
| 02 Aug 2023
Petrological study of an eclogite-facies metagranite from the Champtoceaux Complex (La Picherais, Armorican Massif, France)
Université de Rennes, CNRS, Géosciences Rennes UMR 6118,
35000 Rennes, France
Philippe Yamato
Université de Rennes, CNRS, Géosciences Rennes UMR 6118,
35000 Rennes, France
Institut Universitaire de France, 75000 Paris, France
Gaston Godard
Université Paris Cité, Institut de Physique du Globe de Paris, 75005 Paris, France
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At the evocation of ‘icy landscapes’, Africa is not the first place that comes to mind. The modern relief of Southern Africa is generally considered as resulting from uplift and counteracting erosion. We show that many modern reliefs of this region are fossil glacial landscapes tied to an ice age that occurred 300 million years ago: striated pavements, valleys, fjords. We emphasise how these landscapes have escaped being erased for hundreds of millions of years, generally considered improbable.
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Petrological and mineralogical studies of mica schists, orthogneisses and glaucophane eclogites from Dumet Island (Armorican Massif, NW France) indicate that this occurrence, which has undergone high-pressure metamorphism up to 16 kbar and 620 °C, is similar to that of Groix Island. There are about 10 similar occurrences within the Ibero-Armorican Arc, forming a discontinuous high-pressure belt, but most of them have remained unnoticed due to a high degree of retrogression.
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Despite the fact that rock textures depend on the 3D spatial distribution of minerals, our tectono-metamorphic reconstructions are mostly based on a 2D visualisation (i.e. thin sections). For 2D a thin section scan has been combined with chemical X-ray maps, whereas for 3D the X-ray computerised axial microtomography (μCT) has been applied. This study corroborates the reliability of the thin section approach, still emphasising that 3D visualisation can help understand rock textures.
Taehwan Kim, Yoonsup Kim, Simone Tumiati, Daeyeong Kim, Keewook Yi, and Mi Jung Lee
Eur. J. Mineral., 36, 323–343, https://doi.org/10.5194/ejm-36-323-2024, https://doi.org/10.5194/ejm-36-323-2024, 2024
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The manganese-rich siliceous metasediment in the Antarctic Ross orogen most likely originated from Mn-nodule-bearing chert deposited not earlier than ca. 546 Ma. Subduction-related metamorphism resulted in the production of highly oxidized assemblages involving Mn3+ and rare-earth-element-zoned epidote-group mineral and Mn2+-rich garnet. A reduced environment was responsible for the Mn olivine-bearing assemblages from silica-deficient composition.
Ane K. Engvik and Johannes Jakob
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The paper documents sillimanite gneiss in the Western Gneiss Region (WGR) and its presence, composition, formation and metamorphic evolution. Peak metamorphism is modelled to T = 750 °C and P around 0.6 GPa. Subsequent retrogression consumes garnet and shows mineral replacement and melt crystallization involving sillimanite, white mica, K-feldspar and quartz. The petrological evolution is in accordance with the investigated eclogites and HP granulites in the northwestern part of WGR.
Alessia Borghini, Silvio Ferrero, Patrick J. O'Brien, Bernd Wunder, Peter Tollan, Jarosław Majka, Rico Fuchs, and Kerstin Gresky
Eur. J. Mineral., 36, 279–300, https://doi.org/10.5194/ejm-36-279-2024, https://doi.org/10.5194/ejm-36-279-2024, 2024
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We studied primary granitic and halogen-rich melt inclusions trapped in mantle rocks in the Bohemian Massif (Germany) in order to retrieve important information about the nature of the melt and the source rock. The melt was produced by the partial melting of metasediments during the deepest stages of subduction and interacted with the mantle. This work is an excellent example of transfer of crustal material, volatiles in particular, in the mantle during the subduction of the continental crust.
Gaston Godard, David C. Smith, Damien Jaujard, and Sidali Doukkari
Eur. J. Mineral., 36, 99–122, https://doi.org/10.5194/ejm-36-99-2024, https://doi.org/10.5194/ejm-36-99-2024, 2024
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Petrological and mineralogical studies of mica schists, orthogneisses and glaucophane eclogites from Dumet Island (Armorican Massif, NW France) indicate that this occurrence, which has undergone high-pressure metamorphism up to 16 kbar and 620 °C, is similar to that of Groix Island. There are about 10 similar occurrences within the Ibero-Armorican Arc, forming a discontinuous high-pressure belt, but most of them have remained unnoticed due to a high degree of retrogression.
Dirk Spengler, Adam Włodek, Xin Zhong, Anselm Loges, and Simon J. Cuthbert
Eur. J. Mineral., 35, 1125–1147, https://doi.org/10.5194/ejm-35-1125-2023, https://doi.org/10.5194/ejm-35-1125-2023, 2023
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Rock lenses from the diamond stability field (>120 km depth) within ordinary gneiss are enigmatic. Even more when these lenses form an alternating exposure pattern with ordinary lenses. We studied 10 lenses from W Norway and found that many of them have a hidden history. Tiny needles of quartz enclosed in old pyroxene cores are evidence for a rock origin at great depth. These needles survived the rocks' passage to the surface that variably obscured the mineral chemistry – the rocks' memory.
Hafiz U. Rehman, Takanori Kagoshima, Naoto Takahata, Yuji Sano, Fabrice Barou, David Mainprice, and Hiroshi Yamamoto
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Zircon preserves geologic rock history. Electron backscatter diffraction (EBSD) analysis is useful to visualize deformed domains in zircons. Zircons from the Himalayan high-pressure eclogites were analzyed for EBSD to identify intra-grain plastic deformation. The U–Pb isotope age dating, using Nano-SIMS, showed that plastic deformation likely affects the geochronological records. For geologically meaningful results, it is necessary to identify undisturbed domains in zircon via EBSD.
Silvio Ferrero, Alessia Borghini, Laurent Remusat, Gautier Nicoli, Bernd Wunder, and Roberto Braga
Eur. J. Mineral., 35, 1031–1049, https://doi.org/10.5194/ejm-35-1031-2023, https://doi.org/10.5194/ejm-35-1031-2023, 2023
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Garnet often entraps small droplets of deep melts generated during mountain building processes. Using high-resolution techniques, we studied these droplets in order to provide hard numbers for the quantification of volatile budgets during crustal evolution, show how even melts formed at >1000°C contain water, and clarify how water behaves during metamorphism and melting at the microscale. Moreover, we provide the very first data on chlorine in natural melts from crustal reworking.
Benoît Dubacq, Guillaume Bonnet, Manon Warembourg, and Benoît Baptiste
Eur. J. Mineral., 35, 831–844, https://doi.org/10.5194/ejm-35-831-2023, https://doi.org/10.5194/ejm-35-831-2023, 2023
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Minerals in a vein network from the Aravis limestone (Haute-Savoie, France) include carbonates, quartz, fluorite and phyllosilicates, crystallized at around 7 km depth and 190 °C. The mineralogy has been studied with emphasis on the chlorite types: chamosite (iron-rich), cookeite (lithium-rich) and sudoite. The presence of the three chlorite types sheds light on their phase diagrams, and observed cationic substitutions confirm the need for more systematic measurement of lithium in chlorite.
Simon Schorn, Anna Rogowitz, and Christoph A. Hauzenberger
Eur. J. Mineral., 35, 715–735, https://doi.org/10.5194/ejm-35-715-2023, https://doi.org/10.5194/ejm-35-715-2023, 2023
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We investigate rocks called eclogite, which are related to subduction and the collision of continents. Our samples show evidence of limited melting at high pressure corresponding to about 70 km depth, which may play an important role in the exhumation of these rocks and the differentiation of the crust. However, due to their composition and metamorphic evolution, melt production is limited, suggesting that similar rocks are unlikely to contribute strongly to subduction-related magmatism.
Pan Tang and Shun Guo
Eur. J. Mineral., 35, 569–588, https://doi.org/10.5194/ejm-35-569-2023, https://doi.org/10.5194/ejm-35-569-2023, 2023
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In this study, unusual corundum- and spinel-bearing symplectites after muscovite were found in ultrahigh-pressure eclogites from the Dabie terrane, China. The results indicate that these symplectites formed by the low-pressure partial melting of muscovite during slab exhumation. We stress that the occurrence of corundum- and spinel-bearing symplectites after muscovite in eclogites provides important implications for fluid and melt actions in exhumed slabs.
Michael Brown
Eur. J. Mineral., 35, 523–547, https://doi.org/10.5194/ejm-35-523-2023, https://doi.org/10.5194/ejm-35-523-2023, 2023
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The past 40 years have been a golden age for eclogite studies, supported by an ever wider range of instrumentation and enhanced computational capabilities, linked with ongoing developments in the determination of the temperatures and pressures of metamorphism and the age of these rocks. These data have been used to investigate the spatiotemporal distribution of metamorphism and secular change but not without controversy in relation to the emergence of plate tectonics on Earth.
Larry Tuttle and Darrell J. Henry
Eur. J. Mineral., 35, 499–522, https://doi.org/10.5194/ejm-35-499-2023, https://doi.org/10.5194/ejm-35-499-2023, 2023
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Quartz inclusions in garnet are used to constrain the metamorphic pressure–temperature history of multiple ~2.8 Ga metasedimentary rocks from Montana, USA. Inclusion studies along with mineral and whole rock chemistry suggests that the rocks of interest experienced a clockwise metamorphic P–T history that included isobaric heating to peak metamorphic temperatures once inclusions were entrapped. These findings place fundamental constraints on the P–T evolution of this important geologic setting.
Jan Schönig, Carsten Benner, Guido Meinhold, Hilmar von Eynatten, and N. Keno Lünsdorf
Eur. J. Mineral., 35, 479–498, https://doi.org/10.5194/ejm-35-479-2023, https://doi.org/10.5194/ejm-35-479-2023, 2023
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When and how modern-style plate tectonics initiated is a matter of debate. Although the earliest unequivocal evidence for ultrahigh-pressure metamorphism is Neoproterozoic, similar processes have been proposed for Paleoproterozoic rocks of western Greenland. We intensely screened the area by studying detrital heavy minerals, garnet chemistry, and mineral inclusion assemblages in garnet. Our results raise considerable doubts on the existence of Paleoproterozoic ultrahigh-pressure rocks.
Sara Nerone, Chiara Groppo, and Franco Rolfo
Eur. J. Mineral., 35, 305–320, https://doi.org/10.5194/ejm-35-305-2023, https://doi.org/10.5194/ejm-35-305-2023, 2023
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The coexistence of chloritoid and biotite in medium-pressure Barrovian terranes is uncommon, with chloritoid usually occurring at lower temperatures than biotite. A petrologic approach using equilibrium thermodynamic modelling points out how metapelites can attain H2O-undersaturated conditions even at medium pressure and amphibolite-facies conditions and consequently can be affected by kinetic barriers, which need to be taken into account.
Gerhard Franz, Martin Kutzschbach, Eleanor J. Berryman, Anette Meixner, Anselm Loges, and Dina Schultze
Eur. J. Mineral., 33, 401–423, https://doi.org/10.5194/ejm-33-401-2021, https://doi.org/10.5194/ejm-33-401-2021, 2021
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Metamorphic rocks contain information about their original rocks and thus provide insight into the earlier stages of a region of interest. Here, we used the whole-rock chemical composition and stable boron isotopes of a suite of rocks from the Alps (Italy–Austria), which were deposited in a restricted intramontane basin before the Alpine orogeny. It is possible to reconstruct the depositional conditions for these sediments, which are now common metamorphic rocks such as schists and gneisses.
Annette Süssenberger, Susanne Theodora Schmidt, Florian H. Schmidt, and Manuel F. G. Weinkauf
Eur. J. Mineral., 32, 653–671, https://doi.org/10.5194/ejm-32-653-2020, https://doi.org/10.5194/ejm-32-653-2020, 2020
Wentao Cao, Jane A. Gilotti, and Hans-Joachim Massonne
Eur. J. Mineral., 32, 405–425, https://doi.org/10.5194/ejm-32-405-2020, https://doi.org/10.5194/ejm-32-405-2020, 2020
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Zoisite eclogites from the Sanddal area, North-East Greenland, contain numerous textures, such as cusps and neoblasts, which are interpreted as melt-related textures. Mineral chemistry and thermodynamic modeling demonstrate that they were partially melted through the breakdown of hydrous minerals, phengite, paragonite and zoisite. Pressure–temperature phase diagrams show that the eclogites reached a maximum depth of ∼70 km and were partially melted near that depth and during exhumation.
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Short summary
The La Picherais metagranite is a key example of undeformed high-pressure quartzofeldspathic rock from the Armorican Massif. Through petrological observations and thermodynamic modelling, this study determines that the metagranite was pressured above 1.7 GPa and the associated mafic lenses at ~ 2.1 GPa. This metagranite provides an opportunity to study the degree of transformation of quartzofeldspathic rocks at high pressure, which may have a significant impact on the dynamics of subduction.
The La Picherais metagranite is a key example of undeformed high-pressure quartzofeldspathic...
