This paper presents a detailed field characterization of boudinage in a high-strain zone several kilometers wide in Northern China to establish relationships between boudin types and rheological contrasts between different parts of migmatites during the migmatization process. This zone contains nearly all types of boudins: foliation boudins, block-torn boudins, pinch-and-swell structures, tapering boudins, object boudins, and modified boudins. These boudinage structures record the different stages of melt-involved and solid-state deformation.
The boudinage of migmatites is significantly controlled by the evolving rheological contrasts between the leucosome and melanosome. During the melting stage, the deformation and boudinage of rocks are controlled by the melt fraction. Migmatite strength progressively decreases with increasing melt fraction. The occurrence of melt-filled foliation boudins and melanosome block boudins suggests that the residuum and melanosome are more competent than the leucosome. During solid-state deformation after crystallization, the existence of recrystallized solid-state leucosomes and the intrusion of pegmatites cause the migmatite strength to increase. The relationship is reversed: the leucosome is much more competent than the melanosome. The type and geometry of boudins and pinch-and-swell structures are correlated to the fraction of leucosome in the migmatites. The mechanical strength and strain localization of migmatites after crystallization depend on the presence and volume fraction of the different mineral phases, as well as the mineral grain size. The type and geometry of boudins suggest that the effective viscosity of migmatite can be ranked, from high to low, as: quartz veins; coarse-grained, thick pegmatite; coarse-grained, diatexite migmatite; medium-grained leucosome; and fine-grained melanosome.
While experiencing partial melting and migmatization, a rheologically homogeneous protolith is turned into two dominant lithologic domains: a competent diatexite migmatite domain and an incompetent melanosome migmatite domain. Spatially, with the increasing leucosome fraction in migmatites, the migmatite rheology of rock changes from homogeneous to heterogeneous and anisotropic, and then back to homogeneous. The strain distribution likewise changes from uniform to partitioned, and then back to uniform. This evolutionary process of strength and rheological properties of rocks during migmatization may promote strain localization at mid crustal conditions.