A POLYMINERALIC APPROACH TO THE MODELING OF CRENULATION CLEAVAGE DEVELOPMENT

Crenulation cleavage is a common fabric in multiply-deformed metapelitic rocks and is characterized by phyllosilicate-rich (P) domains, in which the phyllosilicates define the overall cleavage, separated by quartz- and feldspar-rich (QF) domains. This mineralogical differentiation is the result of dissolution of quartz and feldspar in the P-domains, and precipitation of the dissolved material in the QF-domains. The driving force for this mass transfer remains a poorly understood combination of stress- and or strain- induced dissolution and precipitation.

The finite element modeling program OOF2 is used to investigate how the elastic interactions of quartz and muscovite minerals affect the grain-scale stress and strain distributions within a rock at different stages of crenulation cleavage development. The models contain hundreds of individual grains, each assigned their own 3D stiffness tensor and orientation. Results of the modeling reveal bulk gradients in mean stress and volumetric strain between the different structural domains (e.g. P- and QF-domains) within the crenulation cleavage fabric. These gradients are consistent with a pressure-solution mechanism of fabric formation, but they are also consistent with a formulation that emphasizes volumetric strains as opposed to stresses. The volumetric strain gradients are used as a proxy for how fluids will flow within the samples, providing a better understanding of mass transfer during crenulation cleavage development.

Crenulation cleavage development represents a near-complete reorientation of phyllosilicates throughout a rock. This reorientation has important implications for rock strength owing to easy slip along their (001) crystallographic planes. It also has important implications for crustal seismic anisotropy owing to the highly anisotropic compressibility of these minerals.