Page 202 - Microtectonics

192 7 · Porphyroblasts and Reaction Rims

Fig. 7.3. Example of the process visualised in Fig. 7.1. Post-tectonic porphyroblast of biotite (centre) and staurolite (below) grew over a
layered structure in a fine-grained schist. The structure is mimicked within the porphyroblasts. South Africa. Width of view 5.5 mm. PPL

Fig. 7.4. Diagrammatic sketch of the evolution of complex porphyroblast structures as shown in Fig. 7.5a,b. a S 2 crenulation cleavage develops,
overprinting an older foliation S (Sect. 4.2.6). b A porphyroblast overgrows the structure and mimics it in its inclusion pattern. c Continued
1
deformation and/or recrystallisation and grain growth (transposition: Box 4.9) destroys the folds in the matrix where a more or less continu-
ous foliation (S 2 ) develops. Only the relict structure included in the relatively rigid porphyroblast records the structural evolution in the rock
net can be expected to grow at Al-rich sites such as mica quartz-rich layers or strain shadows. They are unable to
layers and to have difficulty in replacing minerals that build a complete lattice at these sites, but rather follow
lack Al by an intact lattice: such minerals are included grain boundaries. In many cases a compositional layer-
instead. This explains why Al-silicate porphyroblasts ing is included as inclusion-poor and inclusion-rich ar-
commonly contain numerous inclusions when they grow eas in porphyroblasts (Figs. 7.1, 7.3, 7.5b). This happens
in quartz and graphite-bearing pelitic rocks. Such por- by overgrowth of mica-rich and quartz-rich domains (e.g.
phyroblasts may even obtain a skeletal shape (Fig. 7.6) a differentiated crenulation cleavage). In extreme cases
when they grow into Al-poor domains, for instance a porphyroblast grows only along layers of a specific com-