Page 221 - Microtectonics

7.6 · Problematic Porphyroblast Microstructures 211
centre of grains of B, or if single grains of A are rimmed 7.6.7
by coronas of B (Sect. 7.8). Figure 7.30 shows an example Discontinuous S and S e
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where either staurolite or garnet start growing. In the fi-
nal stages of both situations, the garnet includes a stauro- If S is discontinuous with S (Figs. 7.5, 7.13, 7.31), defor-
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lite grain in its rim, giving the impression that garnet is the mation must have taken place after porphyroblast
younger mineral (Fig. 7.30, ×Video 7.30a,b). The inclusion growth. However, it is not correct to attribute S in such
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of one mineral by the other is therefore not always in- cases always to the last preceding deformation phase,
dicative of different age, but may result from different nu- since it may also represent any earlier phase or an ear-
cleation and growth rates of both minerals (Fig. 7.2). Min- lier stage in the progressive development of the external
erals with low nucleation rate and rapid growth rate en- foliation, S (e.g. Figs. 7.5, 7.34e). In multiple deformed
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gulf those with high nucleation rate (Fig. 6.33). regions with discontinuous S and S no reliable relation
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As a conclusion, the relation between periods of min- can be established (Johnson and Vernon 1995a) by tex-
eral growth and deformation can be established in most tural criteria only and several possible interpretations
cases, using the relations shown in Figs. 7.9 and 7.26; should be tested by other means, such as absolute age
however, the relative age of growth periods of different dating of minerals.
mineral species is more difficult to establish.
7.6.8
Rotation of Porphyroblasts
According to theory, rigid objects suspended in a homo-
geneously deforming matrix are expected under certain
circumstances to rotate with respect to the ISA of flow
(Box 5.4); this applies to elongate objects oriented ob-
lique to shortening axes in coaxial progressive deforma-
tion, and to equidimensional and many elongate objects
in non-coaxial progressive deformation (Fig. 5.23; Ghosh
and Ramberg 1976; Masuda and Ando 1988). Equidimen-
sional inter- and syntectonic porphyroblasts with ob-
lique, sigmoidal and spiral-shaped S patterns (Figs. 7.14–
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7.16, 7.32, 7.33, ×Photo 7.32a,b) have therefore intrigued
the imagination of many geologists as natural examples
Fig. 7.30. Sequence of events to show that it is difficult to estimate of objects that rotated with respect to ISA, capable of
the relative age of two minerals from inclusion relations. In indicating sense of shear (e.g. Zwart 1960; Rosenfeld 1968,
a staurolite (s) starts to grow and becomes included in garnet (g) 1970, 1985; Cox 1969; Powell and Treagus 1969, 1970; Spry
because the latter mineral has a higher growth rate; in b garnet
starts to grow, including staurolite that starts to grow later, leading 1969; Wilson 1971; Trouw 1973; Schoneveld 1977, 1979;
to the same inclusion relationship Powell and Vernon 1979; Olesen 1982; Lister et al. 1986;
Vernon 1988; Miyake 1993; Johnson 1993a,b, 1999b;
Williams and Jiang 1999). Two basic types can be distin-
guished: those with a straight or slightly curved S that
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makes an angle of up to about 90° with S (oblique-S or
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sigmoidal-S porphyroblasts; Sect. 7.4.3), and those with
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a spiral-shaped S showing apparent rotation angles in
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excess of 90° (spiral-S porphyroblasts). The sequence of
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events leading to the development of oblique-S porphy-
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roblasts (Figs. 7.14, 7.34, Box 7.4; ×Video 7.34; Passchier
and Speck 1994) can be as follows: (1) development of a
foliation S ; (2) growth of the porphyroblast, including
n
S as S ; (3) deformation that either causes relative rota-
n
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tion of the porphyroblast with respect to S or the devel-
n
opment of a new foliation S n+1 without rotation. Such
porphyroblasts are intertectonic, or syntectonic if the
porphyroblast growth rate exceeded strain rate signifi-
Fig. 7.31. If all connection is lost between S i and S e , they may either
represent two different foliations S and S (left), or early and late cantly. An intuitive explanation of oblique-S porphyro-
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1
2
stages of development of a single foliation (right) blasts (Fig. 7.34a, ×Video 7.34) is dextral rotation of the