Page 226 - Microtectonics

216 7 · Porphyroblasts and Reaction Rims
Fig. 7.37.
a Diagram traced from photo-
graphs of an experimental model
of the development of spiral-S i
garnet inclusions from Schone-
veld (1977). Four stages in the
development of a porphyroblast
are shown. The lines outside and
inside the porphyroblast indicate
S e and S i . The foliation surface
marked s is shown in three di-
mensions in b. A double spiral
develops in the blast, one of
densely spaced lines where the
porphyroblast overgrows the
mica-rich strain caps (usually
formed by opaque inclusions),
and a second one where the blast
overgrows the strain shadow
(marked in grey; usually formed
by quartz). The amount of rota-
tion (in degrees) is shown under
each diagram (cf. Fig. 7.33). After
Schoneveld (1977). b Stereoscopic
representation of a non-central S i
surface in a model garnet with a
o
rotation angle of 400 , marked
as s in lower part of a. (Schone-
veld 1979)

cluded without rotation of the porphyroblast with respect Williams and Jiang 1999; Kraus and Williams 1998, 2001;
to geographical coordinates. However, the truncation Ilg and Karlstrom 2000; Jiang 2001; Jiang and Williams
planes can also be explained by overgrowth of strain caps 2004). The implications are important since inferred
by the porphyroblast (Fig. 7.39; Sect. 6.5; Passchier et al. shear sense is opposite for the rotational and non-rota-
1992). Since strain caps in metapelites are mica-rich, tional models respectively and non-rotation of porphy-
Al-silicate porphyroblasts tend to overgrow their own roblasts would imply that these structures retain infor-
strain caps (Fig. 7.32). Pulsating growth of a rotating mation on the orientation of foliations at early stages of
porphyroblast, alternating with periods during which deformation (Bell et al. 1997). Several studies (e.g. Williams
mica-rich strain caps develop can cause development of 1994; Aerden 1995; Bell et al. 1997; Ilg and Karlstrom 2000)
sharp deflection planes (Figs. 7.32, 7.39a). If porphyro- have demonstrated that porphyroblast inclusion patterns
blast growth is temporarily alternating with local disso- of oblique-S or sigmoidal porphyroblasts may be remark-
i
lution, truncation planes as reported by Bell and Johnson ably constant in their spatial orientation over large areas,
(1989) can also develop during progressive deformation irrespective of relatively heterogeneous post-porphyro-
(Fig. 7.39b). blast deformation. A number of authors (Bell 1985, 1986;
The models of Bell (1985, Fig. 7.25) and Bell and Bell and Johnson 1989, 1990, 1992; Steinhardt 1989; Hay-
Johnson (1989) have led to extensive debate (e.g. Bell et al. ward 1990, 1992; Johnson 1990; Bell et al. 1992a–c, 1997,
1992a–c; Busa and Gray 1992; Passchier et al. 1992; Visser 2003; Aerden 1994, 1995; Bell and Forde 1995; Bell and
and Mancktelow 1992; Wallis 1992; Lister 1993; Johnson Hickey 1997; Stallard 1998; Hickey and Bell 1999; Bell
1993a,b, 1999b; Gray and Busa 1994; Williams 1994; and Mares 1999; Jung et al. 1999; Stallard and Hickey