Migmatites and deep crustal shear zones: a comparative review of their relationship and evolution

The presence of melt significantly weakens the continental crust and promotes strain localization, establishing an intimate relationship between migmatites and deep shear zones. Here, we review four migmatite case studies developed within major crustal-scale shear zones under distinct tectonic settings, from collisional to extensional regimes: the Kinawa migmatite (Brazil), Opatica migmatite (Canada), Saint-Malo migmatite (France), and Øksfjord Shear Zone (Norway). Our main goal is to examine the relationship between migmatites and shear zones, evaluate the role of deformation during partial melting, and assess how these interactions influence shear zone evolution. The migmatites formed at mid- to lower-crustal levels (4–9 kbar) and temperatures between 650–820°C, under both fluid-present and fluid-absent regimes. Field observations, microstructural analysis, and phase equilibrium modelling show that melt productivity depends strongly on protolith fertility and H2O availability, providing a quantitative framework for interpreting the contrasting behaviour of the four systems. Kinawa and Opatica preserve predominantly magmatic microstructures indicating that shearing stopped once the melt had crystallised. In contrast, in Saint Malo and Øksfjord record significant post-anatectic solid state overprinting of magmatic microstructures, demonstrating that deformation continued after melt crystallization. Together, these examples show that shear zones localize melting, promote melt segregation and crustal differentiation, and, in some cases, facilitate melt redistribution within the crust. Rather than supporting a single evolutionary model, the case studies illustrate that the relationship between partial melting and shear zone evolution depends on the timing of deformation relative to melt crystallization, the fertility of the protolith, and the tectonic setting. These case studies collectively illustrate the fundamental role of shear zones as dynamic crustal domains where deformation, metamorphism, and partial melting are closely connected.