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

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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.

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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.

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Correspondence to Marie Baïsset.

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Communicated by Timm John.

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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).

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