Startseite A new UHP unit in the Western Alps: First occurrence of coesite from the Monviso Massif (Italy)
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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 und Franz A. Livio
Veröffentlicht/Copyright: 10. Juli 2023
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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.

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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