Page 200 - Microtectonics

190 7 · Porphyroblasts and Reaction Rims
7.1 7.1 Porphyroblasts are not equally common in all rock
Introduction types and under all metamorphic conditions. Most com-
mon and most informative are Al-silicate porphyroblasts
A volume of rock involved in deformation and metamor- like garnet, biotite, staurolite, chloritoid, andalusite etc.
phism will continuously undergo changes in structure grown under upper greenschist to amphibolite facies
and mineral content. This chapter treats mineral growth conditions in metapelites. Garnet, plagioclase, epidote
and replacement structures and the way in which their and hornblende may form interesting porphyroblasts in
geometry can be used to reconstruct tectonic history. Two metabasites. Since porphyroblasts are such informative
types of informative structures are treated: porphyro- structures, it is usually advantageous to sample and study
blasts and reaction rims. microstructures in available metapelites (and metabasites
Relatively large single crystals, which formed by meta- especially if they are garnet-bearing) in any area for large-
morphic growth in a more fine-grained matrix, are known scale tectonic studies. In order to evaluate the significance
as porphyroblasts (Box 5.2). Porphyroblasts are a valu- of porphyroblasts, we first describe why and how por-
able source of information on local tectonic and meta- phyroblasts grow and how they acquire inclusions. Im-
morphic evolution. Inclusion patterns in porphyroblasts portant texts on the tectonic significance of porphyro-
can mimic the structure in the rock at the time of their blasts are Zwart (1962), Zwart and Calon (1977), Schone-
growth (Fig. 7.1) and allow a reconstruction of the rela- veld (1979), Spry (1969), Vernon (1975, 1976, 1989), Bell
tive timing of mineral growth, reflecting metamorphic and Rubenach (1983), Bell (1985), Bard (1986), Bell et al.
conditions, and deformation. As such they can play a key (1986, 1992c), Yardley (1989), Yardley et al. (1990), Barker
role in the determination of pressure-temperature-defor- (1990, 1998), Shelley (1993), Passchier et al. (1992),
mation-time (P-T-D-t) paths experienced by metamor- Johnson (1993a,b, 1999b), Williams and Jiang (1999), Ilg
phic rocks. Apart from their inclusion patterns, porphy- and Karlstrom (2000) and Kraus and Williams (2001).
roblasts may also record the metamorphic evolution from
7.2 core to rim; either a growth zoning may be present, or 7.2
inclusions of certain minerals may show P-T conditions Porphyroblast Nucleation and Growth
different from the matrix. Porphyroblasts can also be used
in the study of the kinematic significance of their rota- The distribution and size of porphyroblasts in a meta-
tion or non-rotation with respect to specific reference morphic rock depend on the amount of nucleation sites
frames, both with relation to folding mechanisms and to and the rate at which the nuclei grow. The nucleation and
determine sense and minimum amount of shear in shear initial growth stage of a new mineral in a metamorphic
zones. A review of current trends in the study of porphy- rock is hampered by the fact that small grains have a rela-
roblast microstructures is given in Johnson (1999b). tively high surface free energy and are therefore less sta-
Fig. 7.1.
a Diagram illustrating how an
Al-silicate porphyroblast may
grow in a mica-rich matrix by
substitution reactions involving
minor volume change. Opaque
minerals (right hand side) and
quartz are taken up as inclu-
sions and their preferred orien-
tation and distribution is mim-
icked by the inclusions. Later
deformation may affect the ma-
trix but will not change the in-
cluded structure if the porphy-
roblast remains undeformed.
The inclusion patterns may un-
dergo rigid body rotation but
will retain a record of the struc-
ture in the rock at the time of
porphyroblast growth. b Com-
monly used terminology for
porphyroblasts