ANISOTROPY OF MINERAL ELASTICITY AND ITS ROLE IN DEFORMATION AND FABRIC DEVELOPMENT IN METAPELITES

Slaty and crenulation cleavages are common in deformed metapelitic rocks. These fabrics are characterized by phyllosilicate-rich (P) domains, in which the phyllosilicates define the overall cleavage of the rock, separated by quartz and feldspar-rich (QF) domains. This characteristic mineralogical differentiation is thought to be the result of dissolution of quartz and feldspar in the P-domains, and precipitation of this dissolved material in the QF-domains. Pressure solution is most commonly invoked as the driving force for this mass transfer.

In this study the driving forces for mass transfer and for development of preferred orientation in the P-domains are explored using finite element numerical experiments. Each mineral is assigned its own 3D stiffness tensor, and we investigate the influence of elastic interactions among minerals of different orientation on grain-scale stress and strain distributions. These stress and strain distributions play an important role in driving mass transfer. The anisotropic compressibility of phyllosilicates may also influence the development of a preferred orientation in the P-domains. A preferred orientation in which the elastically most compressible direction (c-axes) of the phyllosilicates is perpendicular to the cleavage plane is commonly found in P-domains of deformed pelitic rocks. The hypothesis being tested is that the elastic anisotropy of minerals, and phyllosilicates in particular, makes certain crystallographic orientations energetically more favorable than others.