Page 208 - Microtectonics

198 7 · Porphyroblasts and Reaction Rims

Fig. 7.12. Intertectonic biotite porphyroblast in biotite phyllite with approximately straight S (S ) surrounded by a crenulation cleavage (S )
i
1
2
that wraps around the crystal. This porphyroblast is labelled intertectonic since it grew after D 1 (including S 1 ) and before D 2 . Its growth
period may, however, overlap with both these phases since crystals grown very late during D or very early during D would show similar
1
2
structures. Note mineral cleavage (001) within the biotite, almost perpendicular to S i . Example of case d in Fig. 7.9. Concepción. Central-
south Chile. Width of view 4 mm. PPL

Fig. 7.13. Intertectonic garnet porphyroblast in garnet-mica schist. The garnet may be classified as intertectonic since it overgrew a straight
secondary fabric (S i =S n ) that has been deformed in the matrix by later deformation resulting in crenulation and relative rotation of about 90°
of the garnet with respect to S . Since the continuity between S and S is almost completely destroyed by post-garnet deformation and grain
n
i
e
growth in the matrix, an alternative explanation, considering S and S in the matrix as generated by entirely different phases, is also possi-
i
n
ble. Example of case d in Fig. 7.9. Tärnaby, Norrbotten, Sweden. Width of view 20 mm. Polars at 45°