Superplastic deformation inside the knife-sharp shear bands in mid-crustal granites

Abstract

We examined the microstructures of three ductile shear bands of granitic composition that deformed under mid-to-upper greenschist facies. These shear bands, from the Bundelkhand Craton, India, are 0.8–4.4 mm thick with 20–30 μm median grain sizes and share sharp contacts with the host granite. Comminution of the rheologically stronger feldspars increased permeability, promoted fluid influx, and allowed strain localisation in the incipient shear bands. The major components, quartz and feldspars, deformed via phase-specific crystal plastic deformation mechanisms. Quartz in the monomineralic bands deformed predominantly via dislocation creep, while the feldspars deformed via diffusion creep. Finer quartz grains, which occur at plagioclase triple junctions, resulted from myrmekitisation – also evident from the anti-clustered distribution of quartz-plagioclase phase boundaries and CPO inheritance. The weak CPOs and high mean misorientation angles of the fine plagioclase and K-feldspar grains and yet a high shear strain (γ = 16–37) suggest a significant contribution of grain boundary sliding (GBS) in strain localisation inside the ductile shear bands. Since intracrystalline slip alone cannot explain the high ductile strain in the shear bands, we suggest that GBS controlled the deformation and maturation of the shear bands and promoted superplasticity to accommodate further shearing.