Mechanisms of shear zone development in Krossøy, the Bergen Arcs, Norway

The island of Krossøy in the Bergen Arcs, southwestern Norway, provides an ideal geological setting to study the mechanisms of grain size reduction in plagioclase during shear zone development. The transition from a granulite facies anorthosite to mylonitic textures under amphibolite facies conditions is investigated through field observations, petrographic analysis, electron microprobe analysis, and electron backscattered diffraction (EBSD) techniques. Initial ductile deformation, evidenced by complex twinning and intragrain misorientation, leads to strain hardening and subsequent brittle fracturing. This results in grain size reduction primarily through fragmentation, forming a bimodal grain size distribution. Syntectonic recrystallization produces compositionally similar neoblasts. Subsequent interactions with fluids induce compositional recrystallization, leading to further grain size reduction and the formation of zoned plagioclase grains. These processes highlight the critical role of fluid-mediated recrystallization in shear zone development and strain localization. The spatial variation in deformation and hydration stages across Krossøy is attributed to both mechanical anisotropy and chemical reequilibration driven by fluid pathways. These findings enhance our understanding of the interplay between mechanical anisotropy, brittle failure and fluid infiltration in the lower crust, providing insights into the conditions that facilitate the formation and evolution of shear zones in the Bergen Arcs. • Misorientation in large, less hydrated plagioclase suggests strain hardening before brittle fracturing. • Fragmentation and twinning of plagioclase grains create pathways for fluid infiltration. • Syntectonic recrystallization forms neoblasts along cracks, inheriting parent grain orientation. • Fluid infiltration drives further recrystallization, producing zoned plagioclase in finer-grained, altered samples.