Discussion and Conclusions

One of the major limitations of our model is that no dragging of external foliation occurs. Since dragging of external foliation (before being incorporated into internal foliation) by porphyroblast-matrix interaction does not occur in our model, the resulting geometry of the internal foliation will be simpler than the natural conditions where dragging occurs. Therefore, it is difficult to apply the model results for the interpretation of natural microstructures. Models that incorporates dragging effects [e.g. Jezek et al, 1999] will be more suitable for natural rocks, but these models will also have limitations because matrix rheology is assumed to be Newtonian. A multiple process model such as Elle [Jessell 2001] that allows crystal growth and local displacement calculations by fully mechanical codes may be the only solution for this type of modelling.

Although the model results can not be directly compared with the natural rocks, we feel that the model results may have some implications when interpreting the pattern of internal foliation within porphyroblasts. As demonstrated on the serial sections (Figure 5), the perfect monoclinic geometries are observed only on the central sections of a porphyroblast. Some of the sections lacking the monoclinic symmetry (Figure 5) can be misinterpreted. For example, they can be misinterpreted to develop when the porphyroblast post-tectonically captures a fold hinge that developed prior to porphyroblast growth. Therefore, it is suggested to observe microstructures on the central sections when interpreting kinematics from natural rocks.

During pure shear flow or general shear flow, prismatic porphyroblasts with larger axial ratio may have stable orientations (Figure 4). In this case, the syntectonic porphyroblasts can have straight internal foliations that are usually interpreted to be pre- or post-tectonic. Thus, it is suggested to be cautious when interpreting straight internal foliations within prismatic porphyroblasts. The question of syntectoic vs pre- or post-tectonic timing relation can be answered by combined examination of the distribution of the longest axis of porphyroblasts as well as the patterns of internal foliations. For example, straight internal foliations within porphyroblasts of a certain longest-axis orientation group and curved internal foliation within other orientation groups can be interpreted to synteconic growth at stable orientation with the flow geometry of general shear or pure shear.

In conlusion, our model, in spite of major limitations, produced results that can be helpful for microstructural examination of porphyroblasts. Two general statements for petrographic analyses of internal foliation can be made.

  1. For the correct observation of internal foliations, observation plane should contain the central section of porphyroblast. Otherwise, the patterns of internal foliations may contain misleading microstructures for interpretation.

  2. Straight inclusions within prismatic porphyroblasts do not always represent pre- or post-tectonic growth. These microstructures need to be interpreted with caution because syntectonic straight inclusions can be formed at stable orientations when flow geometry is general or pure shear.