Development of a layered crenulation cleavage in mica schists of the Kanmantoo Group near Macclesfield, South Australia

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Abstract
Kanmantoo Group metasedimentary rocks are folded by a large, dextral, second-generation fold near Macclesfield, South Australia. In the hinge regions of this fold, pelitic schists are crenulated, which gives rise to a variably developed, layered, axial-plane crenulation cleavage. The layered cleavage is produced by different microstructural, mineralogical, and chemical changes on alternate limbs of asymmetric crenulations. The long limbs (mica or M domains) become enriched in muscovite at the expense of biotite, quartz, and feldspar with a consequent large increase of Al2O3 and a smaller increase in K2O at the expense of SiO2, MgO, and FeO + Fe2O3. The compositional changes in the short limbs (quartz-feldspar or QF domains) are somewhat complementary, but comparison with uncrenulated rock within 20 mm of these crenulations shows that the layering development involves a bulk chemical change, primarily a depletion in MgO and FeO + Fe2O3. All mineral grains are finer and less equidimensional in M domains and coarser and more equidimensional in QF domains than in the equivalent uncrenulated rock. In addition, very little evidence of intracrystalline deformation, recovery, or partial re-crystallization was found in a wide range of variably intensely crenulated rocks. The crenulation cleavage probably developed by a combination of (1) rotation of existing grains accompanied by modification of their shape and size by diffusive processes, (2) migration of material, on the scale of grains and domains, controlled by the deformation path and microstructural anisotropies, and (3) nucleation and growth of grains with an orientation and shape compatible with the strain history in their vicinity during nucleation and growth. It is shown that a pressure solution mechanism driven solely by differences in stress magnitude will not explain the range of micro-structural and mineralogical changes. More important controls on diffusive mass transfer are likely to be available from the chemical reactions, strain history, volume changes, and microstructural anisotropies.

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