Deformation volume and cleavage development in metasedimentary rocks from the Ballarat slate belt

Psammites from the Ballarat slate belt in SE Australia exhibit well-developed differentiated or spaced cleavage defined by alternating phyllosilicate- and quartz-rich domains (termed P- and Q-domains, respectively). Strain estimates derived from independent microstructural and chemical observations suggest that the P-Q fabrics developed in response to plane-strain deformation dominated by solution transfer with the principal finite shortening in the P-domains approximately twice that in the adjacent Q-domains. Significant finite extensions are indicated by ubiquitous quartz-albite-chlorite overgrowths in both P- and Q-domains, while pressure-shadow development around syntectonic pyrite porphyroblasts suggest finite extension of at least 100%. This estimate is comparable with the extension predicted for constant-volume deformation and consequently there appears to have been no significant bulk material loss or gain on the hand-specimen scale. These observations are consistent with solute transfer scales as little as a few centimetres via diffusion in a stationary fluid and do not require, although do not necessarily preclude, large-scale advective fluid transport through the slate belt during cleavage formation as suggested in previous studies.