Page 224 - Microtectonics

214 7 · Porphyroblasts and Reaction Rims
Fig. 7.35.
a Syntectonic albite porphyro-
blasts with sigmoidal S pattern
i
and asymmetric strain shadows
of quartz. The asymmetry of the
strain shadows indicates dextral
shear sense, which implies that
the albite rotated in a clockwise
sense with respect to the kin-
ematic frame during its growth.
b Detail of a. Sanbagawa Belt,
Japan. Width of view a 2.5 mm;
b 1 mm. PPL. (Photographs
courtesy S. Wallis)

tions would, of course, give an entirely different tectonic rals can in some cases be explained as helicitic folds which
significance to the structure shown in Fig. 7.34a. Bell (1985) have been overgrown, but this is unlikely for porphyro-
and Passchier and Speck (1994) have shown that these blasts with apparent rotation angles exceeding 180°; such
alternative explanations are indeed feasible in at least structures have traditionally been interpreted as syntec-
some cases. Careful investigation of the geometry of ob- tonic porphyroblasts, growing in non-coaxial flow, that
lique-S porphyroblasts is therefore needed before an at- were rotating with respect to S and ISA of bulk flow (and
i
e
tempt is made to use them as shear sense indicators. also with respect to geographic coordinates) during por-
Spiral-S porphyroblasts may occur in garnet, stauro- phyroblast growth (refs. cited). Data consistent with this
i
lite (Busa and Gray 1992; Gray and Busa 1994), albite and latter interpretation were presented by Powell and Treagus
several other minerals, but well-developed spirals of S (1969, 1970), Busa and Gray (1992), Schoneveld (1977,
i
with a relative rotation angle exceeding 180°, also known 1979), Powell and Vernon (1979), Passchier et al. (1992),
as snowball structures (Figs. 7.16, 7.33), seem to be re- Johnson (1993a,b), Williams and Jiang (1999).
stricted to garnet (e.g. Rosenfeld 1970; Schoneveld 1977, Powell and Treagus (1969, 1970) have shown that in-
1979; Powell and Vernon 1979; Johnson 1993a,b, 1999b; clusion patterns in spiral-S garnets are not cylindrical
i
Moore and Johnson 2001). Porphyroblasts with open spi- in three dimensions, but commonly mirrored around a