Reaction‐Driven Development of Fine‐Grained Plagioclase Networks Influences the Rheology of the Crust in the Bergen Arcs, Norway

ABSTRACT In the middle to lower continental crust, mineral reactions can play a key role in controlling rheological behaviour by generating fine‐grained, mechanically weak domains. In granulite‐facies rocks from Krossøy (Bergen Arcs, Norway), we document how garnet breakdown produced interconnected networks of orthopyroxene + plagioclase ± spinel that formed along grain boundaries and reaction fronts. We interpret the reaction as being initiated during early Caledonian decompression and cooling, which changed from Grenvillian high‐pressure granulite‐ to lower pressure granulite‐facies. The networks are characterized by fine‐grained Ca‐enriched plagioclase (relative to the surrounding coarse‐grained plagioclase) and orthopyroxene symplectite (10–50 μm) surrounded by larger feldspar grains (1–2 mm). Phase‐equilibrium modelling indicates that the formation of fine‐grained Ca‐enriched plagioclase networks occurred under lower pressure granulite‐facies conditions and predates the main amphibolite‐facies overprint. These reaction products form mm‐ to cm‐scale bands that align under stress and are reactivated during amphibolite‐facies conditions. Our observations show that reaction‐driven grain size reduction, even when limited to 5%–10% of the rock volume, produces mechanically weak plagioclase networks that are potential sites for later strain localization. This suggests that the early formation of fine‐grained reaction products can locally weaken the crust, forming distinct rheological domains and controlling where deformation and later metamorphic evolution are concentrated.