Page 201 - Microtectonics

7.3 · Inclusions 191
by diffusion to the surface of the porphyroblast. Miner-
als adjacent to the growing grain that do not or only
partly participate in the mineral reaction have to be re-
moved by dissolution and diffusion. In some cases, espe-
cially at high-grade metamorphic conditions, diffusion
rates have been high enough to allow complete removal
of reaction products and non-participating material, and
clear, ‘gem-quality’ porphyroblasts result. In most cases,
and especially at low to medium-grade metamorphism,
minerals that do not participate in the reaction are not
removed completely but are overgrown and enclosed by
porphyroblasts as passive inclusions. If the rock adjacent
to the growing porphyroblast had a compositional lay-
ering or a grain shape preferred orientation of grains,
this fabric may be partly preserved when grains are in-
Fig. 7.2. a A large number of nuclei in a rock will lead to development
of a large number of small porphyroblasts, each with few inclusions. cluded in the porphyroblast; an inclusion pattern results
b Few nuclei lead to few large porphyroblasts, which may contain a that mimics the original fabric (Figs. 7.1–7.3). In this way,
clear inclusion pattern and are therefore useful in fabric analysis straight foliation traces can be included, but also more
complex patterns such as folds or even complete crenu-
ble than large ones (Poirier 1985). This unstable stage can lation cleavages (Figs. 7.4, 7.5). Opaque minerals and
be overcome at specific sites controlled by small irregu- quartz are most commonly included in this manner, but
larities such as strongly deformed grains or microfrac- zircon, monazite, apatite, rutile, sphene and epidote-group
tures (Yardley 1989). If many suitable sites are available, minerals are also common. Mica inclusions are rare but
many small porphyroblasts may form; if few suitable do occur in some Al-silicate porphyroblasts; they may
nucleation sites are present, isolated large porphyroblasts have been included as excess phases of reactants. How-
develop (Fig. 7.2). Thus, nucleation rate and growth rate ever, care is needed, since mica overgrowths may resem-
are competing processes. Matrix grain size also influences ble inclusions (see below).
the size of porphyroblasts (Carlson and Gordon 2004). Microstructural observation of inclusions in porphyro-
Some minerals nucleate on the crystal lattice of an- blasts by numerous workers has led to the conclusion that
other in a particular orientation, e.g. sillimanite on mus- they are mostly included in a passive manner, without be-
covite, amphibole on pyroxenes. This relationship is ing significantly displaced by the growing porphyroblast
known as epitaxy. The special situation where the crystal (Zwart 1962; Spry 1969; Vernon 1975, 1976, 1989; Zwart and
lattices of both minerals are parallel is known as syntaxy. Calon 1977; Bell 1981, 1985; Bard 1986; Yardley 1989; Yardley
Once a stable porphyroblast has formed, its radial et al. 1990; Barker 1990, 1998). Deflection of matrix folia-
growth rate is likely to decrease with time if its growth tion around porphyroblasts (Fig. 7.5a, ×Photo 7.5a,b) is
rate in terms of added mass is constant. However, in a therefore thought to form by deformation of the matrix
study based on backscatter images and X-ray maps of around a rigid pre-existing porphyroblast (Zwart 1962;
garnets, it was found that multiple nuclei formed simulta- Vernon 1976; Barker 1990, 1998; Yardley 1989) and not by
neously and grew amoeba-like, to coalesce later to a sin- mechanical displacement of the matrix by the growing por-
gle garnet with a constant radial growth rate, regardless phyroblast as earlier proposed by Spry (1969) and Misch
of size (Spear and Daniel 1998). Although little is known (1971). However, growing porphyroblasts can displace
about the absolute growth rate of porphyroblasts, theo- graphite and white mica in rare cases, as explained in
retical considerations and available radiometric dating give Sect. 7.7. Surfaces of aligned elongate inclusions within por-
conservative estimates for growth of a garnet 2 mm in phyroblasts are referred to as S (i for internal) whereas the
i
diameter of less that 0.1 m.y. to 1 m.y. (Cashman and Ferry foliation outside the porphyroblasts is called S (e for exter-
e
1988; Christensen et al. 1989; Burton and O’Nions 1991; nal; Fig. 7.1b). If deformation occurs after porphyroblast
Paterson and Tobisch 1992; Barker 1994; Williams 1994). growth, S may have a different orientation from S .
i
e
The abundance of inclusions in Al-silicate porphyro-
7.3 blasts has been attributed to the limited mobility of Al- 7.3
Inclusions ions (Carmichael 1969). At greenschist and lower am-
phibolite facies conditions, Al-ions are far less mobile
The growth process of porphyroblasts is mainly control- than Si, Fe, Mg, K or Ca-ions, unless the pH is extremely
led by diffusion, either in the solid state or through flu- high or low, or if salinity is extremely high (Slack et al.
ids present along grain boundaries. Elements necessary 1993). This means that porphyroblasts of Al-silicates such
for growth that are not present have to be transported as andalusite, cordierite, staurolite, chloritoid and gar-