Syn-Shearing Deformation Mechanisms of Minerals in Partially Molten Metapelites

Abstract

We investigated an experimentally sheared (γ = 15, (Formula presented.) = 3 × 10−4 s−1, 300 MPa, 750°C) quartz-muscovite aggregate to understand the deformation of parent and new crystals in partially molten rocks. The scanning electron microscope and electron backscatter diffraction analyses along the longitudinal axial section of the cylindrical sample suggest that quartz and muscovite melted partially and later produced K-feldspar, ilmenite, biotite, mullite, and cordierite. Quartz grains became finer, and muscovite was almost entirely consumed in the process. With increasing γ, melt and crystal fractions decreased and increased, respectively. Among the new minerals, K-feldspar grains (highest area fraction and coarsest) nucleated first, whereas cordierite and mullite grains, finest and least in number, respectively, nucleated last. Fine grain size, weak crystallographic preferred orientations, low intragranular deformation, and equant shapes suggest both initial and new minerals deformed dominantly by melt-assisted grain boundary sliding, which is further substantiated by higher misorientations between adjacent grains of quartz, K-feldspar, and ilmenite.