Page 230 - Microtectonics

220 7 · Porphyroblasts and Reaction Rims

Fig. 7.42. Elongate garnet porphyroblast with an inclusion pattern that looks like helicitic folds with two complete wave-lengths; close
inspection shows that the garnet is probably the result of amalgamation of two porphyroblasts along a black line close to the centre.
Considering the two porphyroblasts separately they show similar patterns as in Fig. 7.41, with relative dextral rotation of about 90°.
Itumirim, southern Minas Gerais State, Brazil. Width of view 6 mm. PPL

mation of two individual garnet crystals, both with an
S-shaped inclusion pattern (Fig. 7.43).
Another example, in this case of amalgamated albite
porphyroblasts, was observed in an albite-garnet schist
from Elephant Island, South Shetland Islands, Antarctica.
This schist contains numerous albite porphyroblasts that
are frequently in mutual contact (Fig. 7.44). Along sev-
eral of these contacts an abrupt change in angle of the
inclusion pattern shows how the porphyroblasts ap-
Fig. 7.43. Two rotational garnets that amalgamate may show an inclu- proached each other in the deforming matrix until they
sion pattern of apparent helicitic folds with more than one wavelength touched. In a few cases the contacts were modified, ap-
parently by dissolution, leading to the penetration of one
porphyroblasts with relatively elongated shape contain porphyroblast into the other along interlobular contacts
inclusion patterns that look like helicitic folds with two with stylolite-like appearance (Fig. 7.45). The inclusion
complete wavelengths (Fig. 7.42). At first sight this could patterns of the intergrown porphyroblasts are almost
be interpreted as growth during or after a crenulation orthogonal showing the provenance of the porphyro-
phase that was erased from the matrix since it clearly blasts from parts of the matrix with different cleavage
does not mach the crenulation that postdates garnet orientation. Locally this process can even produce iso-
growth described above. However, detailed observation lated parts (in 2D) of a former porphyroblast within an-
reveals that the structure is in fact the result of amalga- other (Figs. 7.40, 7.45).