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A new UHP unit in the Western Alps: First occurrence of coesite from the Monviso Massif (Italy)

  • Stefano Ghignone ORCID logo , Emanuele Scaramuzzo , Marco Bruno and Franz A. Livio
Published/Copyright: July 10, 2023
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American Mineralogist
From the journal American Mineralogist Volume 108 Issue 7

Abstract

The Western Alps are one of the most studied exhumed subduction-accretionary complexes worldwide. Ultrahigh-pressure (UHP) metamorphism has been documented there since the 1980s. We now report the first discovery of coesite in the meta-ophiolitic suite of the Monviso Massif, corresponding to the fourth UHP unit defined on the Western Alps. Previous petrographic studies and results from thermodynamic modeling already suggested that these Alpine units may have experienced UHP metamorphism, but no occurrences of index minerals, such as coesite, have been reported to date. The newly discovered coesite inclusions from the Monviso Massif occur as intact single crystals (10–60 μm) hosted by garnet. The observations suggest that they have escaped re-equilibration and maintained all the original features from the trapping time. The reduced size of the crystals and the lack of re-equilibration significantly differ from the typical textural features described in past findings (i.e., radial cracks, palisade texture of quartz surrounding coesite relicts). Detailed garnet inclusions analysis and thermodynamic modeling constrained the metamorphic peak conditions at P = 2.8–2.9 GPa and T = 500–520 °C within the coesite stability field.

The Lago Superiore Unit represents the fourth UHP unit discovered on the Western Alps. The UHP metamorphism on the Western Alps was considered rare due to the escape process of unusual units from mantle depths. In our view, the implication of our discovery provides new insight into UHP processes that seem to be more common than expected. Further tectonic reconstructions should take into account the common features observed in the UHP units to better constrain the subduction- and exhumation-related mechanisms that drove the actual stacking of mountain belts.

Keywords: Coesite; UHP; ophiolite, Monviso; Western Alps

Funding statement: This work was financially supported by PRIN 2017 (2017L83S77), of the Italian Ministry for Education, University and Research (MIUR) of Marco Bruno and by Insubria University Research Fund (FAR) of Franz A. Livio.

Acknowledgments

We thank the editor Claire Bucholz, P. Manzotti, and an anonymous reviewer for their critical and constructive reviews. We thank the insightful comments by Samuel Angiboust on an earlier version of the manuscript. Alessia Borghini and Mattia Gilio are warmly thanked for their contribution in the UHP discussion. We are thankful to G. Roberts for his first critical revision of the manuscript and language editing. Special thanks goes to Cinzia and all the staff of the Barbara Lowrie refuge for their hospitality and availability during our fieldwork.

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Received: 2022-06-03
Accepted: 2022-08-10
Published Online: 2023-07-10
Published in Print: 2023-07-26

© 2023 by Mineralogical Society of America

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https://doi.org/10.2138/am-2022-8621
Keywords for this article
Coesite; UHP; ophiolite, Monviso; Western Alps

Articles in the same Issue

  1. On the origin of fluorine-poor apatite in chondrite parent bodies
  2. Fluorine behavior during experimental muscovite dehydration melting and natural partitioning between micas: Implications for the petrogenesis of peraluminous leucogranites and pegmatites
  3. Telescoped boiling and cooling mechanisms triggered hydrothermal stibnite precipitation: Insights from the world’s largest antimony deposit in Xikuangshan China
  4. MSA Distinguished Lecturer Series Correlations between cathodoluminescence intensity and aluminum concentration in low-temperature hydrothermal quartz
  5. Behavior of hydrogen defect and framework of Fe-bearing wadsleyite and ringwoodite at high temperature and high pressure
  6. What is mineral informatics?
  7. Metal source and hydrothermal evolution of the Jiaoxi quartz vein-type tungsten deposit (Tibet): Insights from textural and compositional variations of wolframite and scheelite
  8. Geochemical processes and mechanisms for cesium enrichment in a hot-spring system
  9. Identifying xenocrystic tourmaline in Himalayan leucogranites
  10. Contrasting alteration textures and geochemistry of allanite from uranium-fertile and barren granites: Insights into granite-related U and ion-adsorption REE mineralization
  11. Feiite: Synthesis, stability, and implications for its formation conditions in nature
  12. Thermal equation of state of Fe3O4 magnetite up to 16 GPa and 1100 K
  13. UHP eclogite from western Dabie records evidence of polycyclic burial during continental subduction
  14. CO2 quantification in silicate glasses using μ-ATR FTIR spectroscopy
  15. Local structure determination of Zn-smectite
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  17. Mineral evolution and mineral niches of ammonium sulfates: The case of Pastora mine, Aliseda, Spain
  18. Discrete late Jurassic Sn mineralizing events in the Xianghualing Ore District, South China: Constraints from cassiterite and garnet U-Pb geochronology
  19. Ryabchikovite, CuMg(Si2O6), a new pyroxene group mineral, and some genetic features of natural anhydrous copper silicates
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