Feldspar fabrics in a greenschist facies albite-rich mylonite from electron backscatter diffraction

摘要

The current database of plagioclase CPOs in naturally deformed rocks is small and is not supported by CPOs from experimentally deformed rocks or by numerical modelling so that the current understanding of plagioclase CPOs is limited. Electron backscatter diffraction (EBSD) has the potential of measuring the full crystallographic orientation of small domains of any crystalline material, including plagioclase. We generate and test a file of the relative electron diffraction intensities for lattice planes in the albite structure. This enables rapid and accurate individual orientation measurements of albite, using EBSD. EBSD is used to collect 360 data points from 7 areas across a 2 cm by 1 mm pure albite band in a greenschist-facies, albite mylonite from a gabbro protolith. The CPOs are unlike any published. The [a]-, [b]- and [c]-axes of albite are not symmetrically distributed with respect to the kinematic axes of deformation and do not possess an internal monoclinic or orthorhombic symmetry. There is some microstructural evidence for the operation of dislocation creep, although this interpretation conflicts with the generally held view that crystal plastic deformation does not occur in plagioclase below about 500 deg C. An alternative mechanism is one of granular flow accommodated by solution transfer. However, this mechanism would be expected to give a random CPO. For either mechanism we need to explain the triclinic nature of the CPO. It is possible that the CPO represents the modification by deformation of pre-existing CPO, probably defined by a small number of original plagioclase grains.