Journal of the Virtual Explorer

Internal foliations within porphyroblasts provide key information on relative timing of metamorphism with respect to regional deformation and relative rates of porphyroblast growth with respect to the rates of deformation [e.g., Zwart, 1962; Spry, 1963; Rosenfeld, 1970; Vernon, 1978, 1989; Bell and Johnson, 1989; Barker, 1994; Bell and Forde, 1995]. Much of these studies using the geometry of internal foliations has been carried out for garnet porphyroblasts [e.g. Rosenfeld, 1970; Gray and Busa, 1994; Jung et al., 1999] because internal foliations within garnets have relatively simple geometries. The basis of the simple geometries of internal foliations can be found in the rotation theory of Jeffery (1922) which predicts fixed rotation axis for spherical rigid objects with respect to the local kinematic reference frame (e.g. shear zone boundary). However, for porphyroblasts having non-spherical shape (e.g. andalusite and staurolite), there have been relatively few studies. This is because non-spherical or prismatic porphyroblasts do not have fixed rotation axis [Jeffery, 1922]. Therefore, the resulting patterns of internal foliations are complex [e.g. Jezek et al., 1999]. In this study, we present a computer model for simulating development of internal foliations in rotating prismatic porphyroblasts. It will also be shown how difficult or problematic it is to interpret internal foliations within prismatic porphyroblasts with the conventional two dimensional observation of thin sections.