The development of slaty cleavage across the Nackara Arc of the Adelaide Geosyncline

Abstract

The macro-, meso- and micro-structures associated with various intensities of slaty cleavage development across a slate belt are described. The deformation history of this slate belt was such that initially flexural slip folds were produced. However, as the deformation continued folding by flexural slip gave way to folding by inhomogeneous flattening. The slaty cleavage was produced during the latter stage of the deformation and formed slightly oblique to the axial planes of macroscopic folds due to a noncoaxial strain path. The slaty cleavage is defined mainly by preferred orientations of (001) of mica and chlorite. In rocks where slaty cleavage is more intensely developed it is also defined by alignment of the long and intermediate axes of ellipsoidal quartz, feldspar and calcite grains. These grain shapes are a product of pressure solution with dissolution and growth in directions respectively normal and parallel to the slaty cleavage. The mica preferred orientation is dominantly due to crystallization of mica with (001) parallel or at a low angle to the slaty cleavage direction. This involved pressure dissolution of bedding oriented mica with nucleation and growth of slaty cleavage mica on sites commonly dissociated from relic mica, for example other silicate grain boundaries. However, a small but important contribution from rotation of bedding micas towards the slaty cleavage direction accompanied the inhomogeneous flattening.