Microstructural evidence for the deep pulverization in a lower crustal meta-anorthosite

Abstract

We report on the evidence of the pulverization in a deep-seated meta-anorthosite in the Eidsfjord shear zone, Vesterålen, northern Norway. Some plagioclase porphyroclasts comprise a few large relict clasts and many fine grains that preserve the outlines of the original grains. The fine-grained plagioclase does not show any plastic-deformation microstructures and has strong crystallographic preferred orientations, which are inherited from the twinned porphyroclast. Misorientation-axis distributions indicate that the grains have rotated randomly, so that the misorientation axes are not aligned with either the crystallographic or kinematic axes. The observed grain-size distribution has a fractal dimension, suggesting their fracturing/fragmentation origin. The microstructures are characterized by the fracturing/fragmentation with a very low shear strain, indicating that it may be associated with pulverization at ~20–25 km depth.