Crystallographic fabric development along a folded polycrystalline hematite.

Abstract

Detailed analyses of microstructures and crystallographic preferred orientations (CPOs) of hematite rocks were conducted in samples of polycrystalline hematite around a tight fold in the Quadrilátero Ferrífero region, southeastern Brazil. Grain size is dominantly very fine in all samples (z15–30 mm), with a slight increase toward the hinge. Grain aspect ratio increases substantially toward the hinge, from an average of 2 in the limb to 9 in the hinge. Distribution patterns of [0001] axes and poles of planes {1010}, {1120} and {1014} suggest that intracrystalline slip operate on the basal plane along the <a> direction. The distribution of the poles of prismatic planes parallel to the foliation of reference frame indicate that the all the symmetric <a>-slip directions of hematite crystals were equally efficient during activation of basal intracrystalline slip. Increasing aspect ratio is accompanied by CPO intensification toward the hinge, where intensification was aided during deformation by bulk rotation of the hematitic plates and possibly by some grain boundary sliding. Such CPO intensification with the increase of aspect ratio toward the hinge is used to infer that the fold developed by flexural flow.