Page 206 - Microtectonics

196 7 · Porphyroblasts and Reaction Rims
tectonic (some texts use kinematic instead of tectonic), tions; schemes of regional deformation phases have to be
to describe the time relation between porphyroblast- evaluated from schemes for individual thin sections
growth and one or two specific phases of deformation, (Fig. 7.10) combined with field observations (Sect. 7.9).
normally represented by a foliation or by folding in the For a correct interpretation of inclusion patterns it is de-
matrix. As a shorthand notation, we also use symbols to sirable to determine their approximate three-dimensional
show the time relationship of deformation and metamor- shape by comparison of several parallel or orthogonal
phism as outlined in Box 7.1 and Fig. 7.10. Deformation sections (Sect. 12.7). This applies specifically to inclusions
phases (Sect. 1.2) are determined for individual thin sec- with complicated patterns such as spiral-S garnets.
i

Fig. 7.9. Schematic representation of pre-, inter-, syn-, and post-tectonic porphyroblast growth. The upper part of the diagram refers to
deformation resulting in a single foliation or deformation of an earlier foliation without folding; the lower part considers deformation
resulting in crenulation of an older foliation. Pretectonic porphyroblasts (a and b) show strong foliation deflection and randomly ori-
ented inclusions. Intertectonic porphyroblasts (c and d) grew passively over a fabric in absence of deformation, and protect the resulting
inclusion pattern from later deformation. Inclusion patterns are usually straight but more complex situations (c3) are also possible.
Syntectonic porphyroblasts (e and f) have grown during a phase of deformation. Inclusion patterns are usually curved and continuous
with the fabric outside the porphyroblast, and show evidence of having been modified during porphyroblast growth. The distinction of
syn- and intertectonic porphyroblasts is usually difficult since transitions occur and differences are subtle (cf. c1 and e1; c2 and e3;
c3 and f1). Post-tectonic porphyroblasts (g and h) have grown after cessation of deformation. The inclusion pattern is identical to and
continuous with the external fabric. No strain shadows, strain caps or deflection of foliation occur

Fig. 7.10.
Example of a relative age dia-
gram for a single thin section of
a micaschist. Chlorite is pre-D ;
1
garnet is intertectonic between
D and D . Staurolite is pre-syn
1
2
D and biotite has two growth
2
phases, one syn-D 1 and the other
syn-post-D . Albite grows post-
2
D . In the diagram, abbrevia-
2
tions are given at right