Page 243 - Microtectonics

7.8 · Reaction Rims 233
7.8.5 subsequent stability of at least two metamorphic miner-
Pseudomorphs als, they are important for the reconstruction of P-T-t
evolution. Guidotti and Johnson (2002) provide an ex-
If a crystal is largely or completely replaced by another ample of how pseudomorphic replacement can be used
mineral or an aggregate of minerals, the new minerals in the study of superposed contact metamorphism on
may preserve the shape of the original grain (Fig. 7.54g,h). regional metamorphism.
Such an aggregate is known as a pseudomorph; for ex-
ample, if chlorite replaces garnet we speak of a ‘chlorite 7.8.6
pseudomorph after garnet’. Pseudomorphs can be rec- Relation with Deformation
ognised if relicts of the old mineral grain are still present
(Fig. 7.54g,h), or if the old grain was euhedral with a char- As mentioned above, some symplectites may form in re-
acteristic crystal shape. It is sometimes difficult to de- sponse to deformation. In general, however, it may be dif-
cide in an aggregate of minerals which ones are involved ficult to decide whether pseudomorphs or coronas have
in a reaction and in which direction the reactions were replaced a mineral before, during or after a phase of de-
proceeding. Figure 7.56 shows an example of a partial formation. In principle, the criteria mentioned in Sect. 7.4
muscovite pseudomorph after staurolite, and Fig. 7.60 a can be used, but unfortunately reaction rims rarely con-
complete pseudomorph of plagioclase and orthopyrox- tain passive inclusions. Intracrystalline deformation of the
ene after garnet. new minerals in the aggregates can be used to decide about
A distinction can be made between prograde and ret- relative age. For example, if undeformed chlorite replaces
rograde pseudomorphs according to the minerals in- strongly deformed biotite grains, the chlorite is likely to
volved. For example, a staurolite pseudomorph after be post-tectonic. Figures 7.57 and 7.59 show interesting
chlorite is prograde, while a chlorite pseudomorph after examples where the relative age of intracrystalline frac-
garnet is retrograde. Since pseudomorphs show the tures and symplectite can be established.

Box 7.5 Reconstruction of tectono-metamorphic evolution

Porphyroblast-matrix relationships and reaction rims may At this stage it is clear that the reconstruction that can
help to unravel the tectono-metamorphic evolution in an be reached with porphyroblast analysis alone is insufficient
area (see also Chap. 1). Figure B.7.1 shows a schematic evolu- for a complete understanding of the tectonic evolution; the
tion in space and time to illustrate the different relationships scheme of Fig. B.7.2 is only a coarse approximation of the
that may be expected between mineral growth and deformation true pattern in Fig. B.7.1b. Additional data related to the
(×Video B.7.1). The following course of action could be under- P-T-t evolution of each sample site, absolute age dating, nature
taken in an area with a history as shown in Fig. B.7.1. and chemistry of igneous intrusions, and sedimentary environ-
ment of protoliths all play an equally important role in the final
1. Define in each outcrop or tectonic domain (Fig. B.7.1α–ε, reconstruction. Particularly promising is the absolute age dat-
×Video B.7.1) the sequence of deformation phases that ing of mineral grains in thin section as outlined in Sect. 10.4.7
can be recognised in thin sections from overprinting rela- which may provide the approximate age of porphyroblast growth
tions (Sect. 4.2.10.2). The phases have to be properly de- and of deformation phases (Montel et al. 1996; Williams and
fined, for example as ‘responsible for slaty cleavage, for crenu- Jercinovic 2002).
lation cleavage, relative rotation of porphyroblasts, kink Many examples could be cited of the use of porphyroblast-
bands’ etc. The phases may be labelled D 1 , D 2 , D 3 if field matrix relations for the better understanding of the tectonic
relations indicate that no earlier deformation phases were evolution of certain areas (Johnson 1999b). The following are
present. If this is not clear, labels like D , D n+1 etc. are more only a few recent ones amongst many others: Williams (1994),
n
appropriate. Aerden (1995), Johnson and Vernon (1995b), Kraus and Williams
2. Plot the deformation phases on a horizontal relative time axis, (1998), Solar and Brown (1999), Ilg and Karlstrom (2000).
leaving some space in between for possible intertectonic growth, An interesting example of how porphyroblast-matrix rela-
unless there is evidence for continuity (Figs. 7.10, B.7.2a). tionships can help to constrain the intrusion model of a pluton
3. Determine the growth period for each mineral phase and plot is given in Morgan et al. (1998). They demonstrate how a sill
it with horizontal bars or dashed lines where in doubt caused initial andalusite growth in the adjacent contact aure-
(Figs. 7.10, B.7.2a). ‘Pillows’ may be used to indicate main ole, including the regional cleavage at a small angle to bed-
growth versus subsidiary growth (Fig. 7.10). ding as S 1 inclusion trails. Later inflation of the sill, trans-
4. The terms pre-, syn- and post-tectonic have to be clearly de- forming it to a laccolith-like body, produced vertical uplift and
fined with respect to a specific deformation phase or episode. translation accompanied by strong attenuation of the country
For example, in Fig. B.7.2 mineral C in locality ε grew syn- to rocks, and rotation of bedding, including the andalusites.
post D and pre D . During this stage a second foliation, S , was created in the host
2
2
3
5. Use field data and geometrical arguments to correlate defor- rock, parallel to bedding and to the contact. Andalusite contin-
mation phases in different thin sections and from different ued to grow over the curved transition of S 1 into S 2 in the rims
sample locations (Fig. B.7.2). of the crystals.