Cleavages in deformed psammitic rocks from southeastern Australia: Their nature and origin
Cleavages in deformed psammitic rocks from southeastern Australia range from spaced rough cleavages (morphological equivalents of slaty cleavage in psammites) to crenulation cleavages. The microstructure of these cleavages suggests that solution transfer processes play an important part in their development, and that their morphology is dependent on features of the pre-existing fabric (the fabric existing prior to cleavage formation). Former mechanical hypotheses (Leith, 1905; Wilson, 1946) do not explain the observed cleavage microfabrics. There is a general lack of cataclastic textures and of any significant internal deformation of shape-modified grains. These grains, however, have corroded, irregular boundaries typical of dissolution. Rough cleavage development requires dissolution about individual grains, whereas the development of “zonal” and “discrete” crenulation cleavages requires dissolution on existing rough cleavage seams along the limbs of developing microfolds. Cleavage differentiation in each case is due to the dissolution of quartz and feldspar and the passive concentration of phyllosilicates, opaques, and iron oxides along the cleavages. However, crystallization and/or recrystallization of mica may also contribute to the final cleavage fabric, particularly in rough cleavage fabrics where mica beards are well developed.