Page 214 - Microtectonics

204 7 · Porphyroblasts and Reaction Rims
7.4.5 Box 7.2 Distinction between syn- and
Post-Tectonic Porphyroblast Growth intertectonic growth
The distinction between syntectonic and intertectonic por-
This group is easy to define by the absence of deflection
phyroblasts is not a sharp boundary (Barker 1994; Fig. 7.9).
of S , strain shadows, undulose extinction or other evi- Figures B.7.1 and B.7.2 show how a gradual transition can ex-
e
dence of deformation, which is common to pre- syn- and ist between syn- and intertectonic structures. In practice, the
intertectonic porphyroblasts (g and h in Fig. 7.9). If in- distinction can be difficult; in Fig. 7.9, the difference in ge-
clusions are present, S is continuous with S (g in Fig. 7.9), ometry between the intertectonic case c1 with straight inclu-
e
i
sion pattern, and the syntectonic case e1 is rather small. Nev-
even if folded (h in Fig. 7.9, ×Video 7.9h). Some care is
ertheless, c1 may involve porphyroblast growth between two
needed with apparently post-tectonic porphyroblasts. It deformation phases 100 m.y. apart, while e1 may have formed
is not uncommon to find weak deformation effects, in- during a single deformation phase 20 m.y. in length. In the
cluding strain shadows, in or around some crystals in a same way, d can be formed syntectonically if the porphyro-
population of apparently post-tectonic porphyroblasts. In blast grew rapidly, early during the later deformation phase
(Bell and Rubenach 1983) or late during the early deforma-
fact, there are no reliable criteria to distinguish between
tion phase.
very late syntectonic porphyroblasts and post-tectonic The scarce available data on growth rate of porphyroblasts
ones (Figs. 7.3, 7.20, 7.21, ×Photos 7.20, 7.21). indicate that straight inclusion patterns can indeed be formed
–14 –1
in small synkinematic porphyroblasts at strain rates of 10 s
7.4.6 or less (Sect. 7.2; Barker 1994). At faster strain rates or slow
Complex Porphyroblast Growth radial growth, curved patterns typical of synkinematic growth
can develop (Barker 1994). However, since accurate establish-
ment of the growth rates of porphyroblasts and foliation de-
A large number of combinations of the categories of por- flection is presently still difficult, we prefer to classify all
phyroblast-deformation relations mentioned above are geometries shown in Fig. 7.9c,d as intertectonic and reserve
possible, especially if a mineral has several growth phases. the term syntectonic for those cases with clear indications of
growth during a deformation phase (Fig. 7.9e,f).
Relatively common are syntectonic crystals with post-tec-
tonic rims, but combinations of porphyroblasts with
pretectonic cores and syntectonic rims also occur (Fig. 7.17, or abrupt changes in the geometry of the inclusion pat-
×Photo 7.17). These complex relations are easily over- tern, especially if this is associated with zoning in the
looked but may be recognised by an unusual geometry porphyroblast (Sect. 7.6.8).

Fig. 7.22. a Millipede structure and b deflection fold structure for two inter- to syntectonic porphyroblasts. c Development of a millipede struc-
ture by coaxial flattening. d Three alternative ways to develop a deflection fold structure