Page 205 - Microtectonics

7.4 · Classification of Porphyroblast-Matrix Relations 195

Fig. 7.8. Garnet biotite schist. The garnet crystal (lower part of the photograph) is very rich in inclusions except for an almost rectangular area in its
upper part where only a few inclusions of opaque minerals are visible. The garnet apparently overgrew a biotite crystal in this area, the fringes
of which are still visible at the outer contact. Since biotite has a similar Al concentration as garnet, the garnet crystal could substitute the biotite
without incorporation of extra inclusions. Note also the more idiomorphic outline of the garnet crystal in this area. Once the biotite is com-
pletely overgrown its shape remains visible as a ghost structure of inclusion-poor garnet. Grenville Province, Canada. Width of view 2 mm. PPL

position and obtains an elongate shape, although its nor- Box 7.1 Terminology of porphyroblast-matrix relations
mal crystal habit may be more equidimensional. Fig-
ure 7.7 shows an example of elongate staurolite growing The terminology of porphyroblast-matrix relations as used
in this book can be abbreviated by mathematical symbols,
along pelitic layers in a metasediment (×Photo 7.7). The
which can be useful in complex diagrams. These symbols are
shape of pre-existing but now substituted minerals may also illustrated in Fig. 7.9:
also remain visible as inclusion-poor areas within some
porphyroblasts. These structures are known as ghost Growth interval of mineral P
structures (Fig. 7.8, ×Photo 7.8a,b). post-tectonic with respect to D n D n < P
Growth interval of mineral P
Since the diffusion rate of ions is a function of tem-
pretectonic with respect to D 1 P < D 1
perature, inclusions become increasingly rare, more Growth interval of mineral P
coarse-grained and less well defined at higher metamor- syntectonic with respect to D n D ⊃ P
n
phic grade (upper amphibolite and granulite facies), and Growth interval of mineral P
are seldom frequent enough to define S surfaces. syn- to post-tectonic with respect to D n D n ≤ P
i
Growth interval of mineral P
intertectonic between D n and D n+1
7.4 Growth interval of mineral P D n < P < D n+1 7.4
Classification of Porphyroblast-Matrix Relations post D and pre to syn D n+1 D < P ≤ D n+1
n
n
7.4.1
Introduction tals may be older, younger or of the same age as specific
deformation phases (Sect. 1.2). Zwart’s scheme has long
Porphyroblasts with inclusion patterns contain informa- been used for basic reference, but, as pointed out by
tion on the nature of early deformation and metamor- Vernon (1978), several criteria are ambiguous and rela-
phic events, and on the relative age of mineral growth tions have to be studied with great care. We use a modi-
and deformation. Zwart (1960, 1962) elaborated a scheme fied version of the Zwart scheme (Fig. 7.9) in which
with nine diagnostic relations based on the idea that crys- porphyroblasts are classified as pre-, syn-, inter- and post-