Skip to main content

Twinning and partial melting as early weakening processes in plagioclase at high pressure: insights from Holsnøy (Scandinavian Caledonides, Norway)


Eclogitization reactions of plagioclase-bearing rocks in water-limited environments are often incomplete. In such domains, metamorphism and strain localization interact, yielding complex strain-transformation patterns in the field. This is the case in the granulite facies anorthosites of Holsnøy, where partial eclogitization proceeds along digitations in the least strained domains. These areas are among the rare examples of preserved early eclogitization textures. The aim of this study is to assess the mechanical alteration induced by plagioclase breakdown reactions, through a detailed petrological study of the very first increments of the eclogitization process, expected to be preserved at the tip of a digitation in an apparently undeformed granulite block. We show that (i) the zoisite-forming reaction within plagioclase grains occurs unbalanced and initiates mechanical twinning, and subgrains individualization, and (ii) kyanite-analcime association at plagioclase grain boundaries gives evidence of partial melting, which might have affected the rock near the peak PT conditions in response to early water infiltration. This transient partial melting stage results in the re-crystallization of μm-scale plagioclase grains. Intra-grain and grain boundary transformations therefore induce an effective grain size reduction. It constitutes an alteration of the overall aggregate properties of the burying continental rocks, prior to any significant deformation, at potentially low differential stress. Partially transformed granulites are eventually weaker than initial granulites and are prone to later strain localization.

This is a preview of subscription content, access via your institution.

Fig. 1
Fig. 2
Fig. 3
Fig. 4
Fig. 5
Fig. 6
Fig. 7
Fig. 8
Fig. 9
Fig. 10
Fig. 11
Fig. 12

Data availability

The data underlying this article will be shared on request to the corresponding author.


Download references


We thank E. Bras for his help in sampling and mapping in the field. M.B. thanks D. Deldicque and O. Boudouma for the EBSD analyses performed at ENS Paris and Sorbonne Université, respectively. We thank W. Cao, S. Schorn and an anonymous reviewer for their constructive suggestions that truly helped improve that manuscript. J. Hermann and L. Menegon are especially thank for fruitful discussions on petrological aspects around melting and thermodynamic equilibrium, and mechanical considerations, respectively. A.S. acknowledges the European Research Council grant REALISM (2016-grant 681346). P.Y. acknowledges the Institut Universitaire de France for financial support.

Author information

Authors and Affiliations


Corresponding author

Correspondence to Marie Baïsset.

Ethics declarations

Conflict of interest

There is no conflict of interest.

Additional information

Communicated by Timm John.

Publisher's Note

Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

Supplementary Information

Below is the link to the electronic supplementary material.

Supplementary file1 (PDF 3416 KB)

Rights and permissions

Springer Nature or its licensor (e.g. a society or other partner) holds exclusive rights to this article under a publishing agreement with the author(s) or other rightsholder(s); author self-archiving of the accepted manuscript version of this article is solely governed by the terms of such publishing agreement and applicable law.

Reprints and Permissions

About this article

Check for updates. Verify currency and authenticity via CrossMark

Cite this article

Baïsset, M., Labrousse, L., Yamato, P. et al. Twinning and partial melting as early weakening processes in plagioclase at high pressure: insights from Holsnøy (Scandinavian Caledonides, Norway). Contrib Mineral Petrol 178, 19 (2023).

Download citation

  • Received:

  • Accepted:

  • Published:

  • DOI: