Crenulation cleavage formation: Evolving diffusion, deformation and equilibration mechanisms with increasing metamorphic grade

An integrated microstructural and chemical study of crenulation cleavages developed in chlorite- to garnet-zone schists from the Wenchuan-Maowen shear zone, central China, has resulted in the recognition of the deformation and equilibration mechanisms operating during their formation at a range of temperatures. Deformation during crenulation cleavage formation at chlorite- to biotite-zone temperatures (~520 °C) is essentially a solution-deposition process, in which transport occurred by a combination of localized advection and grain-boundary diffusion. Recrystallization and hence equilibration of muscovite in discrete cleavages is recognized by distinct compositional differences between muscovite from different domains. Crenulation cleavages developed in garnet-zone schist are characterized by homogeneous muscovite compositions across domains and microstructures related to intracrystalline deformation, indicating an increasing importance of volume diffusion and diffusive equilibration of mica at temperatures greater than ~520 °C.