Page 220 - Microtectonics

210 7 · Porphyroblasts and Reaction Rims

Fig. 7.29. Large clinopyroxene crystal showing substitution by hornblende in an exsolution-like structure that may be mistaken for
passive inclusions. The crystallographic control of this type of false inclusions reveals their true nature. Retrograded granulite. Southern
Minas Gerais State, Brazil. Width of view 1.8 mm. PPL

7.6.5 direction, or even better, if they trace structures such as
Deformed Porphyroblasts layering or folds in connection with similar structures
in the matrix, they are most probably passive inclusions.
The relations in Fig. 7.9 can also be recognised if pre-, Other inclusions that are probably included passively are
inter- or syntectonic porphyroblasts show evidence of quartz grains, which form part of overgrown strain shad-
intracrystalline deformation such as microboudinage, ows (Figs. 7.6, 7.39a) and inclusions with identical size,
undulose extinction and formation of subgrains. How- shape and distribution as grains of the same mineral in
ever, special attention is needed for crystals with helicitic the matrix. Using these passive inclusions, the age rela-
folds; if an entire crystal is folded with the same wave- tion of deformation and porphyroblast growth can be
length and amplitude as the inclusion pattern, S may have established (Sect. 7.4). However, in some cases it may be
i
been originally straight. Such crystals, recognisable by difficult to determine the precise growth sequence of the
their undulose extinction, must not be mistaken for mineral species that form porphyroblasts and inclusions
undeformed porphyroblasts containing true helicitic (cf. Flood and Vernon 1988; Barker 1994). If isolated
folds (Sect. 7.4.4). grains of mineral A are included in grains of mineral B,
they may either have grown in B (e.g. as sericite in feld-
7.6.6 spar), or they may have been included during growth of B.
Uncertain Age Relation of Host and Inclusions The latter situation, however, does not necessarily mean
that mineral B formed entirely later than mineral A. It is
In the previous sections, the recognition of passive in- possible to establish from such relations that minerals A
clusions, that is the inclusion of mineral grains that to- and B were stable or growing together for a period of time,
tally predate their host porphyroblast, has been presented but it is very difficult or even impossible to decide which
as a rather straightforward procedure. If elongate inclu- mineral started and stopped growing first (e.g. garnet and
sions of a certain mineral occur in groups in a porphy- blue amphibole in Fig. 6.33). The only possible exceptions
roblast and are not oriented parallel to a crystallographic to this rule occur if all grains of A are enclosed in the