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

Abstract

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 P–T 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.